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Further reproduction prohibited without permission. SURFACE TEM PERING OF DENTAL CERAM ICS BY INTERNAL HEATING USING M ICROW AVE ENERGY By Karen Joan Thom pson A Thesis Subm itted to the Faculty o f the U niversity o f Louisville Speed Scientific School as Partial Fulfillment o f the Requirements for the Professional Degree M ASTER OF ENGINEERING Departm ent o f Chemical Engineering May 2000 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . UMI Number: 1400448 UMI UMI Microform 1400448 Copyright 2000 by Bell & Howell Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. Bell & Howell Information and Learning Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, Ml 48106-1346 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . SURFACE TEMPERING OF DENTAL CERAM ICS BY INTERNAL HEATING USING C O NTROLLED M ICROW AVE ENERGY Submitted by: ■ ' '/ C c s * v Karen J. Thompson • A Thesis Approved on ll» f Zooo (Date) by the Following Reading and Examination Committee: % Dr. Jam es C. Watters Thesis Co-D irector Dr. Lawrence Gettleman Thesis Co-D irector lJ Dr. John Naber Dr. Dean O. Harper ^ ii R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . ACKNOWLEDGMENTS I sincerely thank Dr. James C. Watters. Professor o f Chemical Engineering, and Dr. Lawrence Gettleman. Professor of Prosthodontics and Biomaterials, School o f Dentistry, for all o f the time, education and support that they gave me as co-directors o f this thesis. I also acknowledge Dr. Dean O. Harper. Professor o f Chemical Engineering and Dr. John Naber, Assistant Professor of Electrical Engineering for their lime and assistance as members o f the evaluation committee. I extend my gratitude to Dr. James C. W atters, Dr. Dean O. Harper and Dr. Alan Johnson o f the Chemical Engineering Department for sharing their expertise in the area o f ceramic processing. I also thank Dr. John Naber and Dr. Donald J. Schcer o f the Electrical Engineering Department for their advice concerning the feasibility of this project and the modification o f microwave ovens. Finally. I thank Dr. Lawrence Gettleman for educating me about dental ceramics and the fabrication o f dental restorations. I thank Ms. Rodica McCoy for her assistance in procuring the necessary chem icals for the experimental work. I also thank Mr. M ark Paul o f the Kersey Library for his guidance in the preliminary literature search. I thank Brian Knopf, Director o f Research and Allen Steinbock, President o f Whip Mix, Corporation o f Louisville, Kentucky for their contribution to this project, including refractory material, a dental firing oven, and advice concerning the future direction o f this work. Finally I thank my parents and family for their unwavering support o f all my endeavors. iii R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . ABSTRACT Dental ceramic restorations such as crowns are fabricated by layering and firing ceramic powders to create esthetic reproductions o f natural teeth. The firing sequences alternate between controlled heating and cooling phases. The final firing step is performed in air with heat supplied by electrical resistance coils. During the fabrication process, the restorations are supported on refractory die materials or other refractory oven furniture. This project proposes to modify the final firing step by heating the refractory die materials using microwave energy while the ceramic restorations are cooling. It is anticipated that the restorations w ill be strengthened by a thermal tempering effect, keeping the outside cooler than the inside and controlling the rates o f cooling simultaneously. The outer convex surface o f the restoration will be under compression as it is rapidly cooled, while the heated inner concave surface will be under tension as it cools more slowly and is pulled to the cooler surface. The focus o f this work w as a review o f related literature and preliminary experimental work, performed in a 650-watt domestic microwave oven operating at 2.45 GHz. Experiments showed that commercial refractory die materials alone do not absorb microwave energy adequately, but the addition o f silicon carbide whiskers or copper powder enhanced the microwave absorption o f the refractory die material and allowed it to be heated to the required firing temperatures in less than five minutes. iv R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . TABLE OF CONTENTS Bags APPROVAL PAGE............................................................................................................................ ii ACKNOWLEDGMENTS................................................................................................................iii ABSTRACT.........................................................................................................................................iv NOMENCLATURE........................................................................................................................... vi LIST OF TABLES............................................................................................................................ vii LIST OF FIGURES.......................................................................................................................... viii INTRODUCTION...........................................................................................................................1 BACKGROUND............................................................................................................................ 3 REVIEW OF THE LITERATURE.............................................................................................5 MICROWAVE HEATING.....................................................................................................5 SILICON CARBIDE A S A SUSCEPTOR........................................................................ 17 TEMPERATURE M EASUREM ENT IN MICROWAVE O V E N S ............................25 MICROWAVES IN THE DENTAL INDUSTRY.......................................................... 26 TEMPERING M EC H A N ISM S.......................................................................................... 28 EXPERIMENTAL A PPA R ATU S............................................................................................32 PROCEDURE................................................................................................................................ 34 RESULTS AND D ISSC U SSIO N .............................................................................................38 CONCLUSIONS........................................................................................................................... 44 RECOMMENDATIONS............................................................................................................ 45 REFERENCES..............................................................................................................................47 APPENDIX I - DENTAL REFRACTORY PRODUCT SPECIFICATIONS.....................50 VITA....................................................................................................................................................55 v R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . NOM ENCLATURE E = Electric field intensity P = M icrowave power absorbed Tc = Critical temperature above which a material absorbs microwave energy Tg = G lass transition temperature V = V olum e o f sample s' = Dielectric constant e" = Dissipation factor = Dielectric loss = M icrowave frequency tan a) 6 vi R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . LIST OF TABLES TABLE I: M ICROW AV E AND CONVENTIONAL SIN TER IN G OF A12 0 3 .................11 TABLE II: PHYSICAL PROPERTIES OF SILICON C A R B ID E ....................................... 18 TABLE III: PHYSICAL PROPERTIES OF DENTAL R EFRA CTO RIES........................ 35 TABLE IV: SILICON CARBIDE WHISKERS SPEC IFIC A T IO N .................................... 36 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . LIST OF FIGURES Figure 1 - Dental Restorations Supported by Refractory M aterial.......................................... 4 Figure 2 - Average Power Absorbed by A lum ina Samples Versus S intering Tem perature....................................................................................................................11 Figure 3 - CEM Patented M icrowave A shing Furnace............................................................ 13 Figure 4 - CEM Patented M icrowave A shing C rucible........................................................... 14 Figure 5 - Dielectric Constant and Loss Tangent o f Silicon Carbide H exaloy SA at Varying Frequencies and T em peratures................................................................... 18 Figure 6 - Power and Temperature D istributions for a 5 cm Slab o f S ilico n Carbide Exposed to Microwaves from Both Sides................................................................ 19 Figure 7 - Power and Temperature D istributions for a 5 cm Slab o f S ilico n Carbide Exposed to Microwaves from Both Sides................................................................ 19 Figure 8 - Surface Temperature Versus Tim e for SiC Irradiated with D ifferent M icrowave Frequencies...............................................................................................21 Figure 9 - Surface Temperature o f SiC with 900 Seconds o f M icrow ave Heating at 35 W /cm 2 Followed by C ooling.......................................................................................21 Figure 10 - M icrowave Heating Rates for Silicon Carbide Dispersed in AI 2 O 3 C em ent...........................................................................................................................24 Figure 11 - Stress Distribution in Fully Tem pered G lass.........................................................28 Figure 12 - Forces Present in Tempered G lass........................................................................... 29 Figure 13 - Experimental A pparatus.............................................................................................33 Figure 14 - Silicon Carbide W hiskers M agnified 400 Tim es.................................................36 Figure 15 - Initial Stages o f Microwave Heating o f Dental Refractory Material Using Silicon Carbide as a Susceptor................................................................................. 39 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . Figure 16 - Later Stages o f M icrowave Heating o f Dental Refractory Material Using Silicon Carbide as a Susceptor................................................................................40 Figure 17 - Microwave Heating o f Dental Refractory Material Using Silicon Carbide as a Susceptor...................................................................................................................... 41 ix R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . INTRODUCTION This project investigates a novel means o f strengthening dental porcelain or ceramic restorations by m eans o f microwave heating o f the refractory die that supports a crown in the firing furnace. The experimental procedure involves adding materials that absorb microwave energy readily (such as silicon carbide or copper) to the refractory die to enhance m icrow ave absorption, as well as changing the geometry o f the oven cavity to channel the m icrow ave energy to the sample. Proper control o f the temperature o f the exterior and interior o f the ceramic piece during cooling should result in residual compressive forces on the outer surface o f the restoration. It is anticipated that a thermal tempering effect w ill occur, which should produce stronger structures. Traditionally, ceram ic crowns have been heated using thermal heat from electrical resistance coils. Therm al methods rely on the thermal conductivity o f ceramics, and long processing times are required to heat up the specim ens. Slowly heating and cooling is very time consum ing, and subjects the crowns to repeated therm al stresses, which could weaken them. M icrow aves heat more uniformly, o n the surface and internally, in a shorter amount o f tim e, and are more energy efficient than traditional heating methods. Thus microwave heating could prove to be prom ising in this application. M icrowave energy is gaining popularity in a variety o f processing areas beyond the traditional applications in the food and com m unications industries. In research laboratories, m icrow aves are used in analytical processes, ashing and in acid digestion. Microwave energy is now being commercially used to cure polymers, sinter ceram ics, 1 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . sterilize medical equipm ent, cure structural wood products, and to remediate nuclear and hazardous wastes. The ultim ate goal o f this project is to achieve a superior product through increased strength o f the finished crow n by controlled tempering o f the ceramic. While the feasibility o f this idea has been confirmed by initial experim ental work, further investigation is necessary before a commercial apparatus can be proposed. The majority o f this thesis is a review o f related literature in the areas o f microwave heating, the use o f silicon carbide as a susceptor, tem perature measurement in microwave ovens, the role o f microwaves in the dental industry and tempering mechanisms. The prelim inary experimental apparatus and results are also described. O f particular interest are the recom mendations offered as to the future goals o f this project. 2 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . BACKGROUND Dental ceramics are com posed o f silica, alumina, kaolin, and o th er oxides. They are used to make permanent dental restorations such as jacket crow ns, ceramic inlays, castable ceramics and denture teeth and w hen fused to noble or base m etal alloys. The fabrication process, which takes place at a dental laboratory, involves layering ceramic powders and firing them in a vacuum to fuse the powders an d build up a shape that simulates natural tooth structure. In the deeper layers, the ceram ic is opaque, transitioning to translucent and then alm ost transparent to mimic the shading o f natural teeth. The final high firing is usually open to the atmosphere, which collapses remaining pockets o f air in the ceramic structure, oxidizes metallic additions used for coloring and staining, and produces a surface glaze. The result is a ceramic that is esthetic, minimizes the attachment o f biofilm to the surface, and will not excessively w ear the opposing dentition while chewing. During the firing procedure, the restorations are supported either on fire clay furnace furniture called sagger points, or on refractory die m aterials, as illustrated in Figure 1. The refractory material is dim ensionally stable at high tem peratures. During firing, the combination o f refractory die and ceramic are h eated relatively slowly (100°C/minute) to a prescribed tem perature o f the order o f 1000°C, depending on the m etal and/or ceramic product in use. The furnace is then cooled at a sim ilar controlled rate to prevent unequal contraction betw een and within layers until the glass transition (Tg) temperature is crossed. The process involves cooling from the outside to the inside. 3 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . This is due to the circumferential fum ace coil and its enclosure retaining heat longer than the floor or ceiling o f the cham ber w hich typically do not contribute heat to the fumace. FIGURE 1 - Dental Restorations Supported by Refractory Material This project proposes to m odify the final air-firing step to incorporate microwave heating. When the restoration reaches the prescribed temperature, the resistance coils will be shut off to begin the cooling stage, as usual. While the finished crown is cooling, microwave energy will be used to heat the refractory die that is supporting the restoration. The microwaves will be absorbed by the refractory die material, but not by the dental ceramic. Thus the outer convex surface o f the restoration will be cooler than the concave inside surface. Thermal tempering strengthens materials by creating surface stresses as a hot material is rapidly cooled. The surface becomes rigid, since the core o f the material tends to shrink as it cools more slowly. The surface is then under com pression while the internal portion is under tension, being pulled to both surfaces. R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . REVIEW OF THE LITERATURE MICROWAVE HEATING When exposed to microwaves, all materials either reflect, absorb or transmit the energy. Those materials that absorb microwaves are termed “lossy.” As a material absorbs microwaves, it converts the microwave energy to thermal energy, resulting in volumetric heating. Rapid, yet controlled and selective heating can be achieved. The use o f m icrowave energy for heating was first commercialized in 19S0 when Raytheon’s Radarrange® developed microwave ovens for cooking in the home. The market for domestic microwave ovens is now saturated, as more than 90% o f homes in the United States have a microwave oven. These ovens account for 75,000 megawatts o f energy consumption annually, compared to 100 m egawatts for all uses o f industrial microwave heating (Clark, 1997). Conversion o f fo ssil energy to microwave energy is only 30 - 40 % efficient. The original microwave research in the 1950s and 1960s w as driven by the prediction that fossil fuels would increase dramatically in cost, but this incentive has not materialized (Clark 1997). Microwaves w ere not integrated into industrial heating and drying processes until 1962 when practical choke systems were developed to prevent microwave energy from escaping the oven w h ile specimens pass through on a conveyor belt. Chokes also prevent leakage o f microwave energy in places where thermocouples or other monitoring equipment enter the microwave cavity. Currently 90% o f industrial microwave processing is meat tempering, bacon cooking, and rubber vulcanization (Clark 1997). 5 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . In the developm ent o f new applications o f m icrowave heating, the focus should be on tailoring parameters such as com position and additives specifically to facilitate m icrow ave heating and to receive the full benefits o f microwave energy rather than to try to use microwave processing with established parameters (Clark. 1997). In any m icrowave system, safety o f the operator is an im portant issue as the polar m olecule water is the m ajor com ponent o f the human body. In 1966. a standard o f 10 m W /cm 2 was established as the exposure limit for microwave radiation based on onetenth the am ount o f radiation required to heat human tissue one degree Celsius (Katz, 1992). This standard was revised in 1982 to give specific exposure limits for com mon frequencies. The current exposure limit is 0.5 m W /cm 2 at 2.45 G Hz (Katz. 1992). All materials absorb, reflect or transm it microwaves. This interaction is dependent on the dielectric properties or rotation o f dipoles in the material. The dielectric loss, tan 6 , can range from 10 “* at room tem perature to values approaching one at high tem peratures (M athis. 1993). M aterials with high dielectric loss are called "lossy" and absorb microwaves readily. Tan 5 is the ratio o f the dissipation factor, e". to the dielectric constant, s', as shown by the equation: tan 8 —~ t ( 1) £ w here e \ the real part, is in phase with the electric field and e", the imaginary part, is out o f phase (Newnham. 1991). The dielectric constant, e', is the perm ittivity o f a material divided by permittivity in a vacuum or free space. The dissipation factor, e", measures a 6 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . m aterial's ability to dissipate absorbed m icrow ave energy as heat. dissipated through oscillation o f dipoles in the material. The energy is The dipoles align w ith the electric field portion o f the electrom agnetic field and then relax. This oscillation causes frictional heating in the material (M athis. 1993). The energy absorbed by m aterials is represented by the following equation: P= 10 e' E 2 V tan 5 (2) w here e 1 is the dielectric constant and V is the volume of the sample (Jian. 1998). If one holds the microwave frequency (co) and m icrow ave electric field intensity (E) constant, the power absorbed by the samples (P) is proportional to the dielectric loss, tan 5. Jian (1998) states that it may therefore be necessary to adjust the microwave pow er during the sintering process to compensate for the variations in tan 5 with temperature and density. It is difficult to measure the lossiness o f materials because their interaction with m icrow aves is dependent on many factors. T he dielectric constant increases linearly with tem perature, and the loss tangent increases exponentially with temperature (K atz. 1992). M icrow ave power absorption increases w ith increasing field intensity, frequency, loss factor and dielectric constant (N ewnham , 1991). Frequently the dielectric properties o f a specific material are unknown functions o f tem perature and density, and the electric field in the m icrow ave cavity is difficult to quantify in terms o f its interaction w ith the sam ple being heated. Tabulated values o f the loss tangent o f materials are therefore rare. Product literature from ORPAC (O ak Ridge, TN), a chemical supplier, gives a qualitative listing o f the microwave absorption o f some ceramics. Poor absorbers include 7 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . Y 2 O 3 , AI2 O 3 , Si0 2 , S 13 N 4 , AIN, BN, M g 0 »Al2 0 3 , 3 AI2 O 3. 2 S 1O 2 , and CaFi. M oderate absorbers are Z 1O 2 (unstabilized), and T 1O 2 . The following are listed as good absorbers: Z r 0 2 with 4% CaO, H tD 2, S n 0 2, '/2 Y 20 3 . 2 Ba0 . 3 Cu 0 , TiC, SiC (alpha and beta). TaC, ZrB 2 . M oSi 2 . and TiN. Sturcken offers the following list o f lossy ceramics: SiC . Z 1O 2 , ZnO, U 0 2, U 3 0 8 and P u 0 2 (1991). N ewnham . et al. provides another listing o f lossy materials. The following materials can be heated to 1000°C after a few minutes at one kW power: m ixed valent oxides such as Fe 3 0 .|. CuO. C 0 2 O 3 , and NiO; sulfide semiconductors such as FeS 2 , PbS. C uFeS 2 ; and carbon and graphite. M etals reflect microwave energy because electric fields do not penetrate beyond the surface. The skin or penetration depth o f copper is less that one pm at microwave frequencies. The lossiness o f materials also changes as the frequency o f the m icrow ave energy changes. Borides and carbides are highly lossy; therefore, at 2.45 GHz, the m icrowave heating is a surface phenom enon because the skin depth is only millimeters (K atz. 1992). Despite the difficulty in obtaining reliable values o f the variables associated with m icrowave interactions, microwave heating is used extensively in the ceram ic industry. Levinson was the first to use microwaves to fire ceramics in 1969. lie patented a “ M icrow ave Kiln” in 1969 and wrote “ Methods o f Firing Ceramic A rticles Utilizing M icrowave Energy” in 1971 (Sturcken, 1991). Using conventional firing methods, the poor thermal conductivity o f ceram ics can result in the center o f a sam ple being at significantly lower temperatures than the surface. If the tem perature difference is too great, cracking and distortion occurs. T he rates o f heating and cooling m ust therefore be limited and firing is a slow process. Traditional 8 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . tiring kilns are not energy efficient, as the specific energy consum ption o f ceramic com ponents is often more than ten tim es the theoretical m inim um energy required to fire the product (W roe, “ Improving E nergy...” 1993). M icrow ave heating can provide several advantages over conventional heating, such as faster heating rates, improved energy efficiency, and reduced thermal stresses. Heating rates up to 1000°C per minute can be achieved in MW sintering (Rahaman, 1995). Many ceram ics do not absorb microwaves readily at room temperature, however. The low dielectric constant o f ceramics at low tem peratures leads to low initial heating rates and inefficiency (Hamlyn, 1997). As the tem perature rises, heat is lost to the surroundings by radiation to the cooler surroundings, creating an inverted parabolic temperature distribution across the specimen. As the critical temperature (Tc) is approached, the ceram ic begins to absorb microwave energy readily, and thermal runaway can occur. The reason for the change in tan 5 above T c is unclear (Rahaman, 1995). Thermal runaw ay is a direct result o f non-uniformity o f the M W field and o f the properties o f the sample (Raham an, 1995). The surfaces lose heat by radiation, and the center o f the sam ple (which is at a higher temperature) absorbs m icrowave in preference to the surfaces, causing the thermal runaway to be self-propagating (W roe, "Improving E nergy...” 1993). Therm al runaway and cracking can be avoided by changing the properties o f the sam ple to im prove the therm al conductivity and by varying the input pow er to control the heating rate. A dding SiC or ZrCh o r another highly therm al conductive ceramic not only im proves the microwave absorption o f the sample, but also elim inates thermal runaway (Tian, 1991). A ny com pound that is added to im prove the absorption o f the sam ple is 9 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . called a “ susceptor.” Boron carbide (B4C) has been used successfully as a susceptor. Silicon carbide w hiskers, 10 vol% have been used as susceptors to heat alumina at 60 G H z (Katz, 1992). The use o f susceptors, however, may ch an g e the properties o f the m aterial being heated. It is also difficult to predict and control the temperature that results from the heating o f the susceptor and, ultimately, th e entire sample (Hamlyn, 1997V The advantages o f both conventional and microwave heating can be exploited in a hybrid oven, which com bines radiant heating and m icrow ave heating. Gas burners or electric radiant elem ents provide the radiant heat at ceramic firing temperatures. At lower tem peratures, hot air or infra-red can be used as the radiant h eat source (Hamlyn, 1997). Low-loss insulation should be used, in quantities com parable to conventional ovens. Sam ples are usually conveyed through the oven autom atically for operator safety so that m inim al handling o f hot sam ples occurs. The main advantage o f hybrid heating is that therm al gradients can be m inim ized. The radiant heating also helps to bring the ceramic sam ples up to their critical tem peratures so that microwave heatin g occurs more readily. At the U niversity o f W uhan, China, researchers com pared m icrowave sintering o f AI2O3 w ith conventional sintering (Jian, 1998). The m icrow aved sam ples have superior strength and density to those sintered conventionally, as show n in Table I. In this experim ent, the average pow er absorbed by the alum ina sam ples w as also measured, and is graphed in Figure 2 as a function o f time. The pow er absorbed increases rapidly at 900°C until the m axim um value o f the loss tangent o f the alu m in a sam ples is reached at about 1050°C. 10 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . TABLE 1 M ICRO W AVE AND CONVENTIONAL SIN T E R IN G OF Al20 3 Jian, et a/., University o f W uhan, C hin a, 1998 Microwave Sinterinu Conventional Sintering Rate o f tem perature increase (°C/m in) Sintering tem perature (°C) Sintering time (min) Relative density (%) Bending strength (M Pa) 15 to 20 1450 60 94 300 150 to 250 1250 15 98 380 200 150 100 S. Il 800 900 1000 1100 1200 1300 Temperature (°C) FIGURE 2 - Average M icrowave Power A bsorbed by Alum ina Samples versus Sintering Tem perature EA Technology, Inc. also combined conventional gas firing and electric resistance elem ents with m icrow ave heating to develop a hybrid heating system. The energy costs for conventional electric tiring are seven times m ore than conventional gas firing. The study concluded that energy costs for m icrowave-assisted electric firing were 3.5 times less than conventional electric firing and one-fifth o f m icrow ave firing alone. The energy 11 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . costs for microwave-assisted gas firing were h alf o f the cost o f conventional gas firing (W roe, “ Scaling u p ...” 1993). A nother type o f hybrid heating system involves the placement o f lossy furniture inside the microwave cavity to induce convective heating o f a non-lossy sample. In one such exam ple, samples o f Ti+C pow der m ixtures are placed in a fused quartz crucible and this crucible is placed inside a larger crucible containing silicon carbide granules. Both crucibles are then placed inside non-lossy insulation in the cavity o f a simple domestic 700-W m icrowave oven. Ignition o f the samples occurs within several minutes. A study o f this system concluded that the greater the mass for a given density, the shorter time for the sample to ignite. Similarly, an increase in packing density led to a shorter time for ignition. Also, increasing the am ount o f SiC in the crucible increased the rate o f com bustion (Ahmad. 1991). A patent assigned to CEM Corporation, M atthews. N C, describes a microwave ashing apparatus that incorporates both microwave and radiative heating in a hybrid arrangem ent. O f primary interest to the current application is the description o f the furniture used in the microwave oven cavity, shown in figure 3. The patent (Collins et al.. 1986) describes a chair-shaped insulating block made o f a refractory material such as firebrick. This insulation stands on legs to m inim ize contact with the metal floor o f the oven cavity. Sitting on the “chair” is a block o f silicon carbide in non-particulate form. The silicon carbide is used as a susceptor to absorb m icrowave energy and to heat the sam ple by radiative heating. According to the authors, silicon carbide is the “most useful and most preferred” o f the variety o f possible susceptor materials. Atop the silicon carbide block sits a thin (0.08 - 0.2 m m ) support plate with a surface area o f 20 - 25 cm 2 12 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . made o f non-woven fibers o f fused quartz. If a circular plate is used, a 5.4-cm diam eter is recommended. This plate h o ld s the sample, and an identical plate covers the sample. The quartz is transparent to m icrow ave, and serves only to contain the sam ple. quart/ supports / silicon carbide block FIGURE 3 - C E M Patented Microwave Ashing Furniture An alternative arrangem ent involves a thin-walled quartz crucible, such as Vycor crucibles 2-mm thick, surrounded by tightly packed blocks o f silicon carbide, which are contained in a box made o f refractory material, as shown in figure 4. T he authors state that the slabs o f silicon carb id e may be in any non-particulate shape, but that particles, powders or granules o f silico n carbide do not provide sufficient heating as susceptors. Ideal blocks o f silicon carb id e are 0.5 - 1.5 cm thick with sides 6.5 - 8.5 cm in length, such as “finishing sticks” so ld by N orton Company. It is expected th at the silicon carbide blocks will deteriorate and w ill need to be replaced after every 1,000 to 5,000 uses. 13 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . quartz crucible refractory material silicon carbide \ blocks FIGURE 4 - CEM Patented M icrowave Ashing Crucible Using the furniture described above, samples tested by the authors heated to glowing red in three to four m inutes (600 - 800 °C). The microwave applicator was a 600-watt oven capable o f being adjusted from 0 to 100% power in one-percent increments. The authors state that increasing the am ount o f insulation or increasing the pow er to the m icrowave will allow the sample to reach higher temperatures. In the experiments cited, the turntable w as removed from the oven, but a fan (rotating baffle or paddle) was utilized to evenly distribute the microwave energy. The design o f the m icrowave applicator should be considered in microwave heating systems. processing. An applicator is a device into which a material is inserted for The mode defines the way in which the electric and magnetic fields are distributed within the cavity. Transverse electric (TE) describes the situation in which the electric field is transverse to the w ave propagation. Transverse magnetic (TM ) refers to situations in which the magnetic Held is transverse to the wave propagation. In a single m ode cavity, superposition o f the incident and reflected waves gives a standing wave 14 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . pattern that is w ell-defined in space. In a m ultim ode cavity, the microwave signal is coupled through a slot or launcher and suffers m ultiple reflections, giving rise to a standing w ave pattern so that the applicator w ill support a number o f resonant m odes (M etaxas, 1993). Either single mode applicators o r m ultim ode applicators may be used in the m icrow ave sintering o f ceramics (Tian, 1991). T he single mode applicator is used m ore often because it offers several benefits. It yields higher electric field strength, couples m ore energy into ceram ic samples, and allow s heating rates o f 100-1000 °C per m inute. A single m ode applicator also allows for on-line electric feedback to precisely control the tem perature and heating rate. Single m ode applicators are often used to m easure the changes in m aterial properties with changing electrom agnetic field (Tian, 1991). The use o f m icrowave energy for heating m ay also provide econom ic benefits in ceram ic processing. The faster heating rates ach iev ed through microwaves should lead to m ore efficient use o f capital equipment and larger throughput rates (Katz, 1992). The overall conversion o f a fossil fuel to m icrow aves is only about 15% efficient, as conversion o f fossil fuel to electricity is 30% efficient and conversion o f electricity to m icrow aves is 50% efficient. Conversion o f m icrow aves to heat is 80% efficient, so conversion o f a fossil fuel to heat during a typical sintering process is 40% efficient (K atz, 1992). H am lyn adds that the conversion o f electric energy to microwave energy is 50% efficient at 2.45 G Hz, but is increased to 80-90% efficiency at 915 M Hz (1997). The cost o f microwave ovens using the standard 2.45 G H z is becom ing increasingly m ore attractive. At 2.45 G H z, a 2 0 kW experimental apparatus co sts less than $100,000 (in 1992 dollars), while at h ig h er frequencies (28 - 60 G H z), a 2 0 kW 15 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . apparatus costs $1,000,000 (Katz, 1992). For small batch processes, simple domestic microwave ovens can be purchased for a few hundred dollars. In the design o f new microwave heating processes, it is therefore not advisable (from an economic standpoint) to deviate far from the dim ensions and operating parameters o f simple domestic microwave ovens. 16 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . SILICON C A R B ID E AS A SUSCEPTOR Silicon carbide has been used extensively in research as a susceptor to promote the microwave heating o f non-lossy materials. Some ceram ics such as Fe^Ch, Cr 2 0 3 and SiC absorb m icrow ave radiation efficiently at room tem perature. Others must be heated to a critical tem perature, above which they will absorb M W radiation (Rahaman, 1995). Silicon carbide is a covalent compound with a decom position temperature o f 2500°C at atm ospheric pressure (Swain, 1994). It can ex ist as a (hexagonal) or p (cubic) crystal structures. Silicon carbide has high hardness, excellent high temperature creep resistance, high therm al conductivity, good sem iconducting properties and excellent oxidation/corrosion resistance. It has applications at high tem peratures under corrosive conditions and in areas where wear must be prevented. O ne o f the critical problems with SiC is low fracture toughness (3-4 MPa m l/2). Table II lists som e physical properties o f SiC. The dielectric constant and loss tangent are representative values. The actual values depend on the tem perature and density o f the sample, and on the measurement technique used in the experim ental set up. Iskander reports a value o f e' = 29.36 at 2.45 GHz for silicon carbide (1991). The dielectric constant and loss tangent for silicon carbide are plotted as a function o f frequency from 8 to 40 GHz from 25 to 150°C in Figure 5. Both parameters decrease with increasing frequency and increase with increasing temperature. 17 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . T A B L E II PHYSICAL PROPERTIES O F SILIC O N CARBIDE Rahaman, 1995 a n d S w ain, 1994 Crystal Structure a (hexagonal) or p (cubic) Density P 3100 kg/m3 Heat Capacity Cp 3300 J/kg-K Therm al Conductivity k 40 W/m-K Dielectric Constant s’ (at 2,450 M Hz) 26.66 Loss tangent tan 5 (at 2,450 MHz) 27.99 17* Silicon 100 Carbide 1!rjioli>v S A a & 25 140 . 4 ------ 0 .3 ------ I oo 3 0 . 2 ------ 0.1 70 75 t) I ’r e ^ i * e i » 4 . ' y ( C i l ! / . ) FIGURE 5 - Dielectric Constant an d L oss Tangent of Silicon C arbide H exaloy SA at Varying Frequencies and T em peratures (from Hollinger, 1991). 18 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . Using a m athem atical m odel, Chatteijee graphed the pow er and tem perature distributions in SiC slabs for a 5-cm slab (Figure 6) and a 2-cm slab (Figure 7). 1300 7 456 mm x m d 3 (X -i i I 1200 1100 i 3 728 mm 1000 tu Q. 5 900 UJ 1.863 m! O 20 H I 800 700 0.5 0 1 FRACTIONAL DISTANCE FRACTIONAL DISTANCE FIGURE 6 - Power and Tem perature Distributions for a 5 cm Slab o f Silicon Carbide Exposed to M icrowaves from Both Sides (Chaterjee, 1998). 1800 125 1 SiC x 7.456 min 1600 7 100 i UJ £T 3.728 min 3 1400 HI r __ 75 i £ u 1200 - £ 1.863 min 50 / - \ - UJ •“ 1000 800 E 25 - i 0 0.5 --------------------------- _ 0 1 FRACTIONAL DISTANCE .................. - 0 - ■ 0.5 1 FRACTIONAL DISTANCE FIGURE 7 - Pow er and Tem perature Distributions for a 2 cm Slab o f Silicon Carbide Exposed to M icrowaves from Both Sides (Chaterjee, 1998). 19 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . A mathematical m odel w as developed by Singh et al. to describe the microwave heating o f slabs o f highly lossy ceram ics (1993). The model incorporates the temperature dependence o f the dielectric properties o f the material, th e non-linear radiative and convective losses from th e slab, and the geometric considerations o f the sample. The model shows that m icrow ave energy penetrates four centim eters into silicon carbide slabs at room temperature. At elevated temperatures, however, this penetration depth decreases to 0.35 cm at 700° C due to the changes in the dielectric properties o f silicon carbide as the temperature is increased. A s the penetration depth decreases, the heating becomes a surface phenomenon, an d large temperature gradients develop in the slab. This model also shows that the tem perature profile as samples are heated is initially linear with time. Once the sample reaches high tem peratures o f approximately 730°C to 1230°C, however, the heat losses from radiation becom e significant, and the surface tem perature reaches a constant value, know n a s th e steady state temperature. For a given material, the steady state temperature can be increased by increasing the m icrow ave pow er input, as shown in Figure 8. W hen the m icrow ave pow er is turned off, the surface o f the sample cools rapidly by radiation d u e to the fourth power dependence o f the radiative losses on temperature. For exam ple, a slab o f silicon carbide heated to 1180°C cools to less than 900°C in the first 2.5 m inutes o f cooling as shown in Figure 9. Joining o f ceram ic com ponents is another area in w hich m icrowave energy has been used to supply the required heating duty. Sintered silicon carbide (SSiC) is used as the material o f construction for ceram ic heat exchangers because it is able to withstand high temperatures and corrosive environments. Solid SSiC is difficult to manufacture, 20 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . 1500 / / Temperature (K) 1200 / / / 000 / / 35 W/cm2 / - - - 20 W/cm2 / / / / 600 ~“ - 10 W/cm2 V /, 300 o 300 600 400 1200 1500 Time (s) FIGURE 8 - Surface Tem perature Versus Tim e for SiC Irradiated with Different M icrowave Intensities I 500 1200 Healing 600 C oolin g 300 300 400 600 1200 1500 Time (s) FIGURE 9 - Surface T em perature o f SiC with 900 Seconds o f M icrowave Heating at 35 W /cmx Followed by C ooling 21 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . however, using conventional means such as slip casting, pressure casting and injection molding. Reaction bonded silicon carbide (R B S C ) is much easier to fabricate, but it is not an acceptable alternative to SSiC due to its low melting temperature, 1410°C (the m elting point o f silicon) (Ifikhar, 1993). Research in this application involved tu b es o f SSiC (sintered silicon carbide) and sockets o f RBSC (reaction bonded silicon carbide), heated to facilitate bonding along the joint. The sam ples were placed inside a hybrid heating apparatus o f refractory brick surrounded by alum ina insulation with a th in layer o f SiC on the inside. The hybrid heating apparatus was wrapped in alum ina insulation and heated inside a 900-watt multim ode m icrow ave oven. The tubes and sockets reached a maximum temperature o f 1530°C, and the joints were leak-tight, indicating that microwave energy was a good source o f heat for this application (Ahmad. 1993). Experim ental work by Binner (1995) and (1998) in the microwave joining o f ceram ics revealed an interesting phenom enon concerning the microwave absorption characteristics o f silicon carbide. Despite all attempts, hot pressed silicon carbide (H PSC) could not be heated to temperatures g reater than 1312°C using microwave pow er up to 600 W (0.6 °C per second) for 30 m inutes. Binner uses the fact that silicon carbide is a good electrical conductor to explain th is behavior. It is believed that, as the tem perature o f HPSC is raised, the electrical conductivity o f the material increases, resulting in a decrease in the penetration dep th o f the microwave energy. Thus the ceram ic begins to reflect microwaves rather than absorb them at elevated tem peratures. The result is that when the ceramic reaches 1 3 10°C, increases in power do not result in an increase in temperature. This tem perature plateau may be advantageous in some 22 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . applications, but can be overcome using a hybrid heating system to contribute conductive heat to further increase the temperature. In contrast, reaction bonded silicon carbide (RBSC) heated easily to 1376 °C at 300 W pow er (2.0 °C per second) in 15 m inutes total time. Heating was done in a TE 102 rectangular tunable cavity. Control o f the R B SC heating was difficult, how ever, as a rapid increase in the com plex permittivity occurred at 1150°C which resulted in a large shift in the tuning o f the cavity (Binner, 1998). During the period o f 1991 to 1997, the U niversity o f Florida and Atomic Energy o f Canada formed a collaboration to measure the dielectric properties o f various m aterials (Clark, 1997). Silicon carbide was identified as a good microwave susceptor. In one exam ple, varying am ounts o f SiC particles w ere added to alumina/calcia cem ent (A liO j/C aO ) which has good strength and form ability. With the addition o f 40 w t.% fine SiC particles, the sam ples heated to nearly 1200°C in 270 seconds. Figure 10 gives the m icrowave heating rates for this system using varying weight percent o f coarse (1000 pm diam eter) silicon carbide and fine (85 pm diam eter) silicon carbide (Clark. 1997). Various experim ents have shown that the am ount and arrangement o f silicon carbide added as a susceptor is critical. Silicon carbide has high electrical conductivity; thus excessive use o f silicon carbide may hinder the penetration o f the m icrowave energy to the ceramic sam ples. In this case, the SiC absorbs all the energy and heating is by radiation only (from the SiC to the sam ple), negating any benefits o f m icrow ave processing. The am ount o f susceptor should therefore be kept to a minimum, depending on the size o f the sam ple to be heated. 23 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . HHnr •nm% 1 •m i m m m m iiiiimiim!! MOmwactiMiaMMmi) ai FIGURE 10 - Microwave Heating Rate* for Silicon Carbide Dispersed in AljOj/CaO Cement 24 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . TEMPERATURE M EA SU R EM EN T IN M ICRO W AVE O V EN S Measurement o f the temperature o f specim ens that are heated in a microwave oven leads to unique problem s. The temperature-measuring device m ust be properly grounded to the oven cavity to prevent arcing. The most accurate tem perature o f the sample is that m easured directly on the surface. According to G rellinger, the tem perature on the surface o f the sample can be measured using a therm ocouple, radiation pyrometer, or optical fiber probe (1993). Proper shielding with a platinum/rhodium (Raham an, metallic protective tube, such as m olybdenum or 1995) must be provided to avoid arcing with the thermocouple and “ shine through” with the optical fiber probe. The measured tem perature will be significantly lower if the m easurem ents are not taken directly on the surface or the inside o f the samples due to the fact that microwave heating begins internally at the m olecular level (Grellinger, 1993 and Raham an, 1995). Hamlyn suggests that temperature measurements be taken with K-type thermocouples or w ith optical fibre probes, such as Accufiber M l00 with a black body sensor (1997). Raham an states that optical pyrometers are more practical, and must be focused directly on the sam ple and calibrated using a heated blackbody source (1995). 25 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . M IC RO W AV ES IN THE DENTAL INDUSTRY The use o f microwave energy in the dental industry is not a novel concept. M icrow aves are used for curing o f denture polymers and drying o f gypsum products. T he m icrow ave sterilization o f dental implements has also been attempted, with lim ited success. G ettlem an el al. (1977) compared the m icrow ave curing o f polym ethyl m ethacrylate (PM M A) with other curing methods. PM M A is used in a variety o f dental im plants because o f its rapid curing time and m echanical strength. A microwave curing tim e o f six minutes resulted in PMMA with a tensile strength o f 8.610 psi. but the m echanical properties decreased at a curing time o f seven minutes. Steam autoclave curing resulted in better tensile strength (9,130 psi), but a curing time o f 30 m inutes w as required. The authors noted that the exact curing tim e m ust be determined for each polym er to avoid over-curing or under-curing in the m icrow ave oven. A related study found that PMMA cured using microwaves resulted in superior bond strength to denture teeth compared to PM M A cured by conventional m ethods (G eerts, 1993). A study at the University o f Istanbul com pared the properties o f m icrow ave-dried gypsum products (used for refractory casts in the dental industry) with air-dried products. T he three gypsum products tested were M oldano dental stone, Glastone dental stone and M ulti-vest partial denture investment. The study concluded that microwave drying o f M ulti-vest at low power resulted in a significant tim e savings over the air-dried m ethod, w ithout a loss o f com pressive strength (Tuncer, 1993). T he surfaces o f the specim ens 26 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . should appear dry before they are microwaved, however, because water may escape violently from very w et specim ens as they are heated in the m icrowave oven. The other microwaved gypsum products in the study had lower com pressive strength than air-dried specimens, regardless o f the pow er level o f the microwave energy. Researchers at the National Institute o f Standards and Technology (NIST) developed a m icrowave-sterilization method for dental and medical instruments. Sterilization is traditionally done w ith dry-heat or steam in an autoclave, a process which can take up to two hours to com plete. Repeated heating can dull the cutting edges and damage the rubber seals and gaskets o f metal instruments. In the microwave method, the instruments were placed in a ja r inside the microwave and a vacuum is drawn. W hen the ja r is irradiated with m icrowaves, a gas plasm a forms as the atmosphere ignites. The plasm a destroys the bacteria and prevents the metal instrum ents from arcing (Hemenway 1986). A similar study conducted at the University o f Adelaide used a simpler experimental set up without the vacuum conditions. The researchers concluded that microwave sterilization under the tested conditions was not effective, as microorganisms survived irradiation for up to 64 m inutes (Hume, 1975). 27 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . TEM PERING M EC H A N ISM S Thermal tempering strengthens materials by creating surface stresses as a hot material is rapidly cooled. T he tem pering o f all-ceramic m aterials is sim ilar to the tempering o f glass, which is discussed in detail. All information contained in this section regarding tempering o f glass is attributed to “Tempered Glass: M ore Than You Want to Know” by C. Bay in http://newsuroup.sci.aqm iria.rcc.aQ uaria.alt.aquaria.htm . In glass tempering processes, a sheet o f glass is heated to a m alleable state (red hot) in a furnace. The surfaces are then quickly cooled by blasting cold air on both sides (Bay, 1995). The cooled surfaces rem ain rigid as the core o f the m aterial shrinks as it cools more slowly. The surfaces are then under very high com pression while the internal portion is under tension, being pulled by both surfaces. This is advantageous as the surfaces o f glass objects, w hen exposed to external forces, are susceptible to surface flaws which lead to cracks, crack propagation and failure. The opposing forces strengthen the material (Bay, 1995). D iagram s o f the stress distribution and the forces present in tempered glass are given in Figures 11 and 12. --------- 10.000 psi (mm) 02 Compression Zone Neutral Zone — ► 1-02 Tension Zone 02 Neutral Zone H m H l o 2 Compression Zone FIGURE 11 - Stress Distribution in Fully Tem pered G lass (from Saflex Technical Information) 28 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . / surfaces in compression compression neutral center in tension tension neutral compression FIGURE 12 - Forces Present in Tempered Glass (from Saflex Technical Info.) Tempered glass is resistant to cracking because cracks will only propagate under tensile stress. Any cracks or flaws on the surface o f tempered glass are trapped by the compressive forces and can not extend into the internal regions o f the sheet o f glass. The surfaces o f fully tempered glass are under compressive forces o f 15,000 pounds o f force per square inch (psi). In order to shatter the glass, one must exceed 15,000 pounds o f force per square inch at one point on the surface. Thus a hit from an object with a small surface area such as a needle or ice pick is more likely to shatter the glass than a hammer or a baseball, which has a larger surface area. The pieces o f shattered fully tempered glass ate one-eighth to one-sixteenth o f an inch long. Curved glass, such as automobile windows, can not be fully tempered because curved glass will not cool uniformly. Automobile glass is tempered to 10,000 psi. The shattered pieces measure approximately one-fourth o f an inch. The degree o f tempering 29 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . is m easured by shattering a sheet o f tem pered glass and counting the num ber an d sizes o f the pieces. A non-destructive method o f m easuring the degree o f tempering is to shine light through the surface and measure the polarization. This is an effective m ethod o f m easurem ent because the process o f tem pering also polarizes the glass. O nce a sheet o f glass has been tem pered, it can not be cut or shaped. The only way to remove the tempering is to reh eat the glass to molten tem peratures. Tem pered glass is non-uniform, and has ripples, warps and twists formed by the opposing com pressive and tensile forces. Non-tempered (annealed) glass is under lower com pressive surface force, approxim ately 400 psi. Annealed glass will not shatter, but it will crack o r break more easily. Tem pering glass does not change its density, but tempered glass is m ore than ten tim es stronger than annealed glass d u e to the opposing com pressive and tensile forces present. Tem pering can also be accom plished using ionic forces. A patent issued in 1989 to LaCourse describes a process for strengthening glass through ion-exchange using m icrow ave radiation at frequencies from 0.9 to 22.1 GHz. This strengthening process involves exchanging smaller ions in th e glass with larger ions contained in a coating on the surface o f the glass. Thus the su rface o f the glass is placed in com pression (w ith the larger ions occupying space meant for sm aller ions) while the internal portion o f the glass is under tension. The microwave radiation heats th e glass to a temperature below its strain point to facilitate the ion exchange. The auth o rs state that conventional ion exchange results in 30 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . shallow penetration o f the larger ions, which causes the glass to lose its strength over time. Longer processing tim es can compensate for this, but are impractical and subject the internal portion o f the glass to extreme tension, often resulting in violent breakage o f the glass. This patent claim s that glass strengthened using microwave radiation produces deeper penetration depth o f the larger ions in a shorter am ount o f time than conventional m ethods o f ion exchange (LaCourse, 1989). 31 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . EXPERIM ENTAL APPARATUS The experimental apparatus is a standard 650-watt domestic microwave oven. The turntable has been removed, and the oven is inverted so that the microwaves enter through the floor and com e in direct contact with the sample, rather than first dissipating into the oven cavity. To further direct the microwaves tow ards the sample, a copper cham ber has been fabricated over the trapezoidal opening o f the waveguide as shown in Figure 13. A copper baffle, grounded with five machine screws, couples the cham ber to the floor o f the m icrow ave oven. The cham ber consists o f a soldered copper structure, which channels the microwave energy upward into a trapezoidal copper tower (low er chamber). A fire clay table was cut to shape and placed in the lower cham ber a few centimeters above the baffle to hold the furnace furniture and the ceramic restorations to be fired. Above this a second copper cham ber was constructed to simulate the enclosure that a resistance furnace firing muffle would occupy in the com pleted device. The upper cham ber hinges open and latches shut in order to place and remove objects on the table. A copper apron was attached at the junction where the upper and low er cham bers jo in, to prevent arcing between the metallic components. C opper screening was inserted in the front wall o f the upper cham ber to view the firing as it proceeds. A second copper screen was placed at the top o f the upper cham ber to vent hot gases during firing. 32 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . FIGURE 13 - E sp ertaeatal Apparatus 33 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . PROCEDURE The preliminary experim ental procedure involves testing the microwave- absorbing capabilities o f the dental refractor)' materials with and w ithout lossy additives. The first step prepared sm all sam ples o f dental refractory casting investm ent material. Five refractory die materials, provided by Whip M ix Corporation o f Louisville, KY, include Polyvest and V H T (phosphate-bonded refractory die m aterials). Hi-Temp (carbon-free refractory m aterial). Beauty Cast (gypsum-bonded refractory material), and Ceramigold (carbon-bonded refractory material). The physical properties o f these materials are summarized in T ab le III, and additional information from the Whip Mix Product Director)’ is included in A ppendix I. The refractory m aterial is m ixed with water to form a paste. The paste hardens chemically and dries into a so lid in about 30 minutes. The samples are then placed in the microwave oven. The refractory m aterials heated up only enough to drive o ff the water they contained. One way to improve the ability o f a material to absorb m icrow aves is to use lossy additives as susceptors. Based o n a thorough survey o f the literature, different forms of silicon carbide and copper p o w d er were used to try to improve the heating. Powdered silicon carbide was mixed w ith the refractor)' casting material in varied proportions. The susceptors that were tested include tine (320 grit) SiC gray p o w d er from Fischer Scientific Company ( a o r (3, h o t pressed or reaction-bonded SiSiC ), coarse SiC from a grinding wheel, green SiC w h isk ers and 99.7% copper pow der (3 m icron) from Aldrich 34 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . Chemical Com pany. C om bination o f the above were also tested. The physical properties o f silicon carbide w hiskers are tabulated in Table IV. Figure 14 shows a 400% magnification by of silicon carbide whiskers, provided Advanced Refractory Technologies. TA BLE III PH Y SIC A L PR O PERTIES O F DENTAL REFR AC TO R IES f '’ Color Thermal Expansion Compressive Strength Compressive Strength After Firing Maximum Furnace Temperature Compatibility t (W HIP MIX CORPORATION) v i .i t : r , , . . . .. Polyvcsti Beauty VHT Hi-Temp Cast while while while blue 1.2% 0.65 % 0.80 % 0.55 % to 1.20% 6.000 psi 1.500 psi 700 psi 2.500 psi 10 M Pa 5 MPa 42 M Pa 17 M Pa 6,500 psi 4,800 psi N/A N/A 46 M Pa 34 M Pa Ceramigold while 1.2 % 1.500 psi 10 MPa N/A 1.200 °C 1.200 °C N/A N/A N/A use with m edium expanding porcelains use with highexpanding porcelains use with nonprecious metal alloys use with low fusing alloys use with ceramic gold alloys In the proposed com mercial apparatus, heating will occur not only by microwave energy, but also by resistance coils. This was tested by first heating the samples in simple convective ovens, then quickly placing the hot sam ples into the microwave oven. Most materials absorb m icrow aves more readily at elevated temperatures, so this should allow the samples to reach a much higher temperature as they absorb microwaves. 35 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . TABLE IV SILICON CARBIDE WHISKERS SPECIFICATION # ART-PS-2003-04 Advanced Refractory Technologies, Inc., Buffalo, NY M ean W hisker Length M ean W hisker Diam eter Mean W hisker Aspect Ratio Particulate Content Particulate Size Surface Area Free Carbon Free Silica M inim um S.00 pm M inim um 0.60 pm 6-15 < 30 vol. % < 50 pm 2 - 5 m2/g < 0.2 wt. % < 6.0 w t % FIGURE 14 - Silicon Carbide Whiskers Magnified 400 Times (Advanced Refractory Technologies) 36 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . The 650-watt microwave oven is not equipped with a temperature-measuring device. The temperature has been estim ated by observing the color o f the fired samples and by feeling the radiation from the surfaces. An optical pyrom eter was also used to m easure the temperature, but the 400 °C m axim um range o f this device was not sufficient to accurately gauge the tem peratures o f the hotter samples. The temperature o f the sam ples was therefore measured only qualitatively in the preliminary experiments. Control o f the microwave energy was achieved by cycling the power on and o ff manually when the samples reached the desired temperature range. 37 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . RESULTS AND DISSCUSSION it was determined that the dental refractory materials alone do not absorb appreciable am ounts o f microwave energy at room temperature. All sam ples heated only enough to drive o ff the water that they contained; some exploded from steam pressure if heated before their green strength w as established. It is apparently necessary to com bine the refractory materials with lossy susceptors before microwave heating can occur. The first success encountered in this experiment involved silicon carbide from intact pieces o f a grinding wheel as a susceptor. In less than three m inutes, these samples heat to a glowing orange, as can be seen in the photographs in figures 15, 16 and 17. In figure 17. the temperature was so high that the solder in the upper cham ber o f the copper tow er melted. The upper cham ber w as rem oved for the photograph in figure 17. The particles o f silicon carbide from the grinding wheel are not uniform, and som e arcing was observed between the components after heating began. The arcing is possibly due to the electrical conductivity o f organic com pounds such as the matrix binder, w hich became carbonized after burning. Arcing is a form o f heating that is unpredictable and difficult to control, so it is undesired in this project. Silicon carbide whiskers have also been successful as an additive. uniform heating was observed. Rapid and T h e use o f silicon carbide w hiskers may not be econom ical, however, as they cost approxim ately $900 per kg, but their density is low and surface area is high. 38 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . FIGURE 15 - laitial Stages of Microwave Heattag of Deatal Refractory Material asiag Silieoa Carbide as a Sasceptor 39 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . FIGURE 16 - L ater Stages of Microwave Heating of Dental Refractory Material Using Silicon Carbide aa a Snaceptor 40 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . FIGURE 17 - Microwave Heatiag of Deotal Refractory Material Uaiag Silkoa Carbide aa a Saaceptor 41 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . Copper pow der is the m ost successful susceptor tested in term s o f rapid heating. These samples consistently heat to a glow ing orange in less than two minutes. Copper may not be the optim al choice, how ever, because the copper could fuse with metal structures in the restoration and/or cause greening o f the ceramic layers in the restoration. The preliminary experim ental investigation and literature search revealed several advantages and possible disadvantages o f the proposed hybrid heating system. The heating provided by the resistance coils m ight bring the refractory die material to a tem perature exceeding the critical tem perature (Tc) for microwave absorption. This should improve the efficiency o f the hybrid heating. It w as determ ined that tuning the w avelengths o f the m icrowave energy may not be necessary, as successful heating was achieved using 2.45 GHz only. Custom-made m icrowave applicators utilizing tuned cavities are considerably more expensive than the sim pler domestic m icrowave units w hich use 2.45 GHz. In general, increasing the w avelength o f the microwave energy increases the pow er absorbed by the material. Since silicon carbide is such a strong absorber o f m icrowave energy, how ever, higher wavelengths reflect from the surface o f silicon carbide sam ples rather than penetrate into the samples. This is another reason that a sim ple 2.45 G Hz oven seem s to be preferred over a more com plex tuned cavity for this application. Control o f the m icrow ave energy entering the hybrid oven could be accomplished using a) tem poral control (off/on cycles), b) variation o f the am perage to the magnetron tube, or c) a hot pressed SiC iris heat sink choke driven by a linear motor. Temporal control was found to be a sufficient controlling m ethod by switching the magnetron on 42 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . and off. More investigation o f the later two possibilities is necessary, as the prelim inary experimental work used only temporal control. Another challenge presented by the proposed apparatus is that the tem pering effect could lead to undesirable mechanical properties in the finished all-ceram ic restoration. Thermal tem pering not only leads to com pressive forces on the external convex surface, but may also generate tensile forces on the internal concave surface. This could cause the interior o f the ceramic to be w eak and brittle, trading high external strength for low internal strength. The proposed com m ercial apparatus will not achieve faster processing o f dental restorations despite the m ore rapid heating. The production o f crowns and bridges is a multi-step batch process, so the time saved by m icrow aves at one step may not save time in the overall process because material-in-progress ju st backs up at the next processing step. To avoid excessive therm al stresses, the heating and cooling stages should n ot be rushed. Therefore, the success o f this project relies on the improved properties that should be achieved through thermal tempering. 43 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . CONCLUSIONS 1. Dental refractory materials do not absorb microwave energy appreciably. The addition o f lossy susceptors allow s m icrowaves to influence the rate and direction o f heating o f the refractory materials. 2. Silicon carbide whiskers are successful susceptors for dental refractory materials. 3. A sim ple 2.45 GHz microwave applicator with temporal control is sufficient for the firing o f dental refractory materials using susceptors. It is not necessary to use a tunable cavity nor is it advisable to use higher wavelengths, as silicon carbide is no longer a good m icrowave absorber at higher wavelengths. 4. The use o f furniture in the m icrowave oven to channel the energy to the sample is critical to ensure sufficient heating for firing the dental restorations. 5. The use o f m icrowave energy resulted in faster heating tim es o f dental refractory materials (w ith proper susceptors) than are possible through conventional heating m ethods using resistance coils. 44 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . RECOMMENDATIONS Before this idea can be converted into a commercially viable unit, more experim entation is needed. A ceramic oven m uffle w ith resistance coils should be obtained from a dental equipm ent manufacturer. This oven should be disassembled and the resistance coils fitted inside the upper portion o f the experim ental chamber. Some m odification o f the existing cham ber design may be required. The experim ental oven should be further modified to incorporate a device to evenly distribute the microwaves w ithin the cham ber. An alum inum paddle with four vanes should be installed in the lower chamber. The paddle will be held in place by a non-m etallic support, and the paddle will rotate on plastic needle bearings. T he incoming m icrowave energy will rotate the vanes o f the paddle, resulting in m ore even attenuation o f the microwave energy into the chamber. Tem perature-m easuring devices and feedback control systems sh o u ld be designed and incorporated into the experim ental apparatus to quantify the heating effects and control the heating and cooling o f the exterior and interior o f th e ceram ic dental restoration. This new experim ental apparatus will allow for the o p tim ization o f the geom etry o f the oven furniture and susceptor m aterials to get the best p o ssib le heating. A m ethod m ust be developed to m easure and quantify the strength gained through therm al tem pering o f the dental restorations to show that the hybrid m icrow ave heating 45 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . method is an im provem ent over current heating methods. A dditional sam ples should be prepared to match exactly the crow ns and that will be heated in the com m ercial device. Additional experim entation may be warranted to test other possible susceptors. In cooperation with a dental laboratory, the exact chemical com position o f the refractory die materials should be determ ined to ensure that the susceptors are com patible with dental ceramics. 46 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . 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F. 1991. “The U se o f ‘S elf Heating’ Ceram ics as C rucibles o f Microwave Melting M etals and N uclear Waste Glass.” M icrowaves: theory and application in materials processing. Ed. Clark, D. E. et al. Cincinnati: The American Ceramic Society. 433-440. Swain, M. V. vol. ed. 1994. Structure and Properties o f Ceram ics. M aterials Science and Technology. Eds. C ahn, R. W., Haasen, P. and Kram er, E. J. New York: VCH. Vol. 11. Tian, Yong-Lai. 1991. “ Practices o f Ultra-Rapid Sintering o f C eram ics Using Single Mode Applicators.” M icrow aves: theory and application in m aterials processing. Ed. Clark. D. E. et al. Cincinnati: The American Ceram ic Society. 283-300. Tuncer, N., Tufek<;ioglu, H. B. and Qalikkocaoglu, S. 1993. “ Investigation on the compressive strength o f several gypsum products dried by m icrowave oven with different program s.” Journal o f Prosthetic Dentistry. 6 9 ,3:333-339. Wroe, F. C. R. 1993. “Scaling up the microwave firing o f ceram ics.” Microwaves: theory and application in materials processing II. Ed. Clark, D. E. et al. Cincinnati: The A m erican Ceramic Society. 449-458. Wroe, F. C. R. 1993. “Im proving Energy Efficiency in Firing o f C eram ics.” Materials World. Aug. 1993: 446-448. http://newseroup.sci.aquaria.rec.aquaria.alt.aquaria.htm. 9/2/99, 12:35 p.m. http://www.saflex.com/Technicah'Structure/ StrCh3b.htm. 9/2/99, 2:00 p.m. 49 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . APPENDIX I DENTAL REFRACTORY PRODUCT SPECIFICATIONS This section contains product inform ation from the W hip Mix Products Directory, distributed by W hip M ix Corporation, Louisville, Kentucky. The product specifications are current as o f May 2000. 50 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . R e fr a c to r y I n v e st m e n t s F o r D irect-F iring C e r a m i c s Polyvcst Refractory rO L 'iU M K tlia .'.fn P it M .iltr.il i- I rr ..lalec w nt' i i . f l l i i tin u i , I ti N 'i v rr.ilct; I i h I. i i nc:rv..il tx p .iiw .'r. newer p o r e .ur- W ith lb n 'n tr i’lli\l i-\p:iibum :v i ) \ ) - i .u .m .iif i t u i i i i f :r>i, i.in..r.r r . •; • 'llti'. in.in> .itita ri.i^ * iim w i- -lit :i .i• A m ple n o ik in g u n it V v t i r niivlc • I'.r.e t ; r . i . . ; f t l m . r . t i ; . i v . i : i : l l u i i i . i t i.-i t'-Ct- ■ 1“ 11■i>’ ; ' . . ‘'-H i 'i.-i: • H :i'.:ii 'i ' i-.:i..;l' i t rc -M : r u ire ir.J .I'M - fii • i ui'i'. Hr.; . \:'.;r.i., :i v in.i:.:i •:..-i 'i ii.fl.iri? V.H.T. Industrial Investment lit- -i h - . 'i i w u i l \ Jf.h .jh lc ' v rf" i i ' i .i'i til i e - e l i - i i >: ; l , r c 'r.u to r. H I .11 . v l u ' h .i c b A i M p f T . i L i n r . - ' i . . , i. r . i n i .. r . . ri.ii f i.i.i- 'iii m m , ' p f . i l f . . 1: r - ■ • . i r i . i t . . ;■ .■ L i'v u ■ .umLm.;';iiif !-• " nii;iu:iv . I'.t- t k ; m 1 V • . ^ M." 11 i.;:i . 1,'i.iJui; .••• i .in- :. " . . . , . i ■■■ m : -.ft: i» i!l in i t u m .mi. u'.ii'i1.' -ir... :::■ ■ ,t‘ ii v l ' Prm nighed Envelope* . I mi -i N«-i Item No. D escription 18473 P m * * r p / i * d E nvefopn 24 60 gram Box 11-340 ml) .■ , : n p ' . . L i : , Item No. Description 24929 25062 24-60 giam Box 1 1 340 mi) 340 mi Pnlyves! Liquid IK ee p Irorn Iroa/mg) •• i- '. Physical Properties Color Liquid/Powder Ratio Working Time Setting Time Thermal Expansion 500-C. 2nd firing Compressive Strength (1 hr.) Compressive Strength (after firing) Maximum Furnace Temperature V.H.T. Blue 19 ml’100 g 4-5 minutes 0 30°. P o ly v M t White 0 80“. 2.500 psi (17 MPa) 4.800 psi(34 MPa) 2,200'F(1.200'C) 0 6 5 °. 6.000 psi (42 MPa) 6.500 psi (46 MPa) 2.200 F t 1.200 C) 22 mo 1 0 0 g \ 2 minutes 0 8 0 °. Compatibility Is The Key l<ac V.HJ. with bl^i npseil^ p iH ttiH i Ducerapild Degussa Denial. Inc Optec-HSP Jenenc/Pcntmn !nc Oprcc-VP tenenc/Pentron Inc Wii-Centn ivudtr ISA/Williams Denial Co fortune Iiociat USA/Wllluim Denul Co. Execko Ney Denul [nttmanonal. Inc Excrko inlay/Onlay Ney Dental International. Inc Microbond Auaenal Dental Inc7 Nobdpharma Creations Jensen Indusnts. Inc. fortress Myron IntcmanunaL Inc. UcroUfVESTwtth Finesse Ceramco O'lorlogtc Cetamco II Ceramco Veneer Spectrum Blobond Ttu-es Ducetam Certotn-PVS ChameleonAtira# S'ltaWIK 68 DentsplyOnnkci Inc Der.isply/Ceramco. Inc DrnupKyCrtamco. Inc DenuplyvCeratnco. Int. Dciusply International. Inc. Demsplv Intetnauonal. Inc. Cenpac Corporation Degussa Dental. Inc. fetenko Dental Health Products Mvton International, Inc. Vidtnt/Ylu ZahnFahnk Vita Omcya Ssr.-Spar Pencralt Ftancet Vintage Crystat Silhouette G-Cera VidentAita Zahniabnk JenetK/Ptntron Inc. JenenoTemrcn Inc. Eno/Suisut SA 3M/Tab Products 3M/tjb Products Leach & Dillon G-C International Corp All ol the above poirelam brand names are trademarks oi their respective manufacturers 51 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . H i-T emp Carbon-Free Investment for High Fusing Alloys \ v>'. . i.i, i . v H ' . u - u ; : ; u i i ’Y .ii'.l iy :i -(.m i p r t i h Y j u i v t - > . . . n t.ii'b r i ''. i r 'n ’i' V r . p r c u j'.b T iit.i. .illov- -i ijiun- .u: .rvi-'fn-t i'-'v. i.i;.'. i. ihjm.ru:<-i: in • , fn i|v risiti- :«>• jlinv -i.cckjqc • Wound :..r-n4-.;Tip-:ar..':-> • Pn»v iL‘ .i :K f.-.i> i'.ij:r..n jt:n c a tm . s p K r t • \ l i l ' V . . . M i l l ) . I ., I l j l l r ' - l i l ' l ’ - l l . II 1 - ’ I k ' • v . l- i n i n : a 'lid ;- i ' K ' v . - . . r . k - .if I. in it !' .'.i'.."- j -m bix Mf a>t:r.>:.’! "Af rejJih-.niti.-vcr.i*. I.t ■ni- itit.a l n : ir i r / n - .u .i. ! ,1-. lll.ji - - if i .v l - v i:.i u :r.L .e ; ’he -I.i !T. v .n n u > -im rli 4 ''rru.ii L...ui.l ;• .to .; .1 ' . u v r i ' 1' i . t i r . ' . l fk a r . k v . j y : i I tr.s. jg'.i.d :ri ■ r.c iilutt.-n "h I'.--- lit. i - v p . i r K i ' l ' . Physical P roperties Liquid"Powder Ratio Working Time Setting Expansion Thermal Expansion Compressive Strength, wet C ir - 16 ml 100 g 7-8 minutes I ' l . ' l ' i HI 0 7*0 12*o Ik' ' . ' , i - ' m i ' ' "i.i '1 ' . l ^ . i - k [ j . " - v j : . r J v ’ Tm rr .*■ : i i c -• ' " ' l I r k l l " ' ,il I'ri.'-'.i' it i ti 'tlx t'A tx ' '.:k- tc .I '.r .i..; I 500 psi 1.10 MPa I . - - .n t iu l: \ :h c M r * D escription .b f u n it. r a u r : v .. ir 'i r it in 1, i - i—; r t T v . l u r • 00450 00477 ..iii.l: rc ’-.'ii'i,r: ' r lull i n n > ..r » .• :-i..|v n s; 'Suggested concentration ol Special Liquid is 75% i,3 parts liquid to I part water) Item No. WithLiquid i I '■ \ ’m : - . 'i ' n i • p.i -tu-vi-itu- ' . i r . • '! i i l - . L m i ..! . 'n n r i M t . n - ■ its- '|Y < .il I t.u .l 2 kg (4 12 lb) Jar (1-340 mi) 11 kg (25 lb. I Carton ( 1 1,ter & 1-340 ml | PrmnigtMdemWoptt 00493 00507 00515 24-60 gram Box 11-340 ml) 24-90 gram Box 11-340 ml) 144-60 gram Package (1 Liter & 1-340 ml) 00523 144-90 gram Package (2 Liters) !• .i: i: «i -j ij KKEkT UOUO COKBfltUlIM 52 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . i. >. '.c* C eram igold * Investment with Carbon fo r Crown and Bridge and Ceramic Gold Alloys T his outstanding phosphate investm ent has th ese advantages C o m a iM u r k s tor bnght. gold-colored castings F ip a a a l a a c a n b e c o a ln U c d to ht your n eed s - yet technic is easy to follow E v c a u l w t ly th in scctfcm s w ill be cast w ith sharp, d ea n m argins |ust like lowerfu sin g golds A c c u r a te ly f h tt a g c a s t t a f i w ith sm ooth an d clean surfaces - without gn n d in g U niform results tim e after tim e. The chart below show s the effective expansion figures obtainable by varying the con cen t ration ol special liq u id A 7 V k concentranor.. i parts special Liquid and 1 p an Water, is generally prelerreu (set en d ed expansion tigures on chan I Thermal and also setting Physical Properties e x p n s io n will increase by greater Special 16 m l/1 0 0 g U q w d'/P ow dw Ratio 6 - 7 m in u tes Working Tima 0.7% Sotting E xpantion 12% Thermal Expansion 1 ,5 0 0 p si (1 0 M Pa) C om p reM M Strangto, w et ’S u gg stM d concentration al S p e d e i Liquid is 75% (3 parts k qu d (o 1 pert w eler). Liquid concentration, these expansions w ill decrease by dilunon with m ore water C onsequently, expansion can be controlled to lit individual needs There is no additional inermal expansion item No. b etw een TiXR! and IcXX'^C 113 00s)-' and Description 1 8 0 0 °h l The curve tn the ch a n show s d if- H W lU j u t f lerent figures for the thermal e x p n s io n m erely 00 3 4 5 00361 2 kg (4 1/2 to.) Jar (1 -3 4 0 m l) 11 kg (2 5 to ) C arton (1 U tar & 1-340 ml) as a result from higher or low er concentration A O M lf c M t f f lM t o p M o l Special Liquid. The setting e x p n s io n (or a 00396 00418 00 4 2 6 00434 2 4 -4 0 gram Bern (1 -3 4 0 ml) 2 4 -9 0 gram B o a (1 -3 4 0 m l) 1 4 4 -6 0 gram P K * a g a ( 1 L M r t 1 -3 4 0 ml) 1 4 4 -9 0 gram P a ck a g a (2 U la n ) b ench -set m old is show n in the low est curve ■- i . » X ’tfl f IC 20 * 53 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . •• . m B ea u ty -C a s t For Low Fusing Crown a n d Bridge Alloys \ n u r u .1 s 11.1d i i i v * 1 1 \ I i i •i-siim-i'.i • I he pruun standard :or the Hvcrosiopt. It-ihnic • Al'O e.uelletil lur ihe hij^h Heat Relink rite lechnc » a ta iiLU l)-iA > ; .5 as simple and a iu a as the I lid'. Heat tuhnu hat '.lie n-s-.dl' ate ttuun mere ur.ili'mi Hie Jse ol a water bath itluKi'K.UM' at 1' 3l . 100T ’ auti'matk.iilv I'rinqs all parents to atiaotiri teni|vral'u:e. sutten.nu ihe '.vax etuui^h let even expansion ol die imesineiit l orovt’e l die wax cxp.ir:->..'ii .a.U the laeii hm u'viipi. expansion „.iii. pensate lor most ol the eek: snrnkaee an: ihib n e low lenqvrjiurc I'tiituuit at 4H0 t W 'h i proMdes ihe habnec ol die required mold expansivm Hp 1 shout; hi .qroseopie expansion ol ?FAl~'t a.s7 .ii | iV jin iF '£ d slums Thetrna! Expansion at 0 is Jt a a o x i - W xie unhesiutinid) reeemmend me iIv^n* -eopn Teelmie with Hr U l i t A 'I > liest suitniit :odi\ s needs lor a ptceise m simple rotiune le-.nnx Ihe mnercni htdl I nermal I xpar.sion i.i Hl-.x. P . as : at hsp'V U lX 'T 'r e ituhes n ixAsihie to use die standard I lid: Iteal lee m u xttinqexpan- einar. iv oat roiled nv .aniiiq die uumnei n nnc liners in me Inlai Kme ■l ij’ I resuams in varvnc ■leqn'es ol i.na! expatis on xu'tinga'. t-1(1 i IJi'd 'h ' nueisap.'lor the acoitioral : J'. Ihemul I xp.itiHon to ..unpensjie to' o ld 'liriulsaei' In the IPA '•pet .nulior 'so . :«>t Investments the term. Ivpe I. tnlav I dermal re!ors to inxesiinents used die I lid* I le.;l Vtnn.i, Kpell s.alVdlnbx Hsqrosvopn With I'Hl IVi .‘.-I either tee r,nu - tie llvevseopiv o r t h e llipl Ilea! -svill priellue .■'Uenien 'iiuoil' vusimo Ma u.h voider. I’h v s u u i r ‘t t ' | H ' l t l v ' s Water. Powder Ratio Working Time Setting Time (ADA Method) Reedy (or Burnout (minimum) Setting Expansion Hygroscopic Expansion Thermal Expansion 480'C Thermal Expansion 650JC Compressive Strength, wet ..nor that seldom require fUKiine 30 ml/100 g 3 minutes 16 minutes 30 minutes 0.35% 1 50% 0.55% 1.20% 700 psi (5 MPa) Ill'll! \t> 00019 00035 2 kg (4 1/2 lb.) Jar 11 kg (25 lb) Canon 00078 00094 00108 00124 24-50 gram Box 24-75 gram Box 144-50 gram Package 144-75 gram Package Ir r w c sr a w : OT*#SJ w i*TW»- j I i - / ------ 1' iS * j j y I J l ' S l T I p I It 111 / / s Pisae|p(iid Ertvafopee 1 !.J -ja r tr w is c o p c E X M r c w i _ I 1 i i * ,ta II i ^ n XPAftSX N Ml TMINMNUtU KS 1 II.*. U'll FI,. I 4 ! I; f» rTTTTl TYPE 1fc II i ;x is re IK) «: toco itod t«k> m m m m m n rc Fl«. 1 54 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n . VITA T he author, Karen Joan Thom pson, is the daughter o f Kenneth an d Kathleen Thompson o f Louisville, KY. She was bom in Louisville on February 7, 1976 and has two younger sisters, Katie and Kelly. Karen is a graduate o f Assumption H igh School, where she w as the secretary o f the N ational H onor Society and a m em ber o f the French Honor Society, Beta Club, Ensemble Choral Group, Red Cross Club, A cadem ic Team and yearbook staff. She received a Commonwealth Scholarship from the University o f Louisville as well as the L & N Federal Credit Union Scholarship. Kentucky Grocers A ssociation Scholarship, Asparagus Club Scholarship and ValuM arket Scholarship. She began attending classes at the University o f Louisville in 1994 as a biology m ajor, and transferred to the engineering school in 1995. While a student at U o f L, Karen participated in the Collegiate Chorale, the Speed School Student Council, and the Society o f W omen Engineers. Her senior year, she was the president o f A lpha O m icron Pi sorority and as a graduate student, was the secretary o f the student ch ap ter o f the American Institute o f Chemical Engineers. K aren com pleted three co-op term s w ith E.I. DuPont de N em ours, Inc. in Louisville, KY. She received a B achelor o f Science in Chemical E ngineering in December o f 1999. graduating w ith honors. She was awarded second place for her Chemical Engineering Exhibit in Engineer’s W eek 2000, for this thesis w ork. She will receive a M aster o f Engineering in Chem ical Engineering in May o f 2000. 55 R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .