Journal of Alloys and Compounds 766 (2018) 759e768 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom Rapid synthesis of ZrB2eB4C composite powders via induction heating and its effect on the properties of Al2O3eSiCeC castables Huan Xu, Xitang Wang*, Zhoufu Wang, Yan Ma, Hao Liu, Yulong Wang The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China a r t i c l e i n f o a b s t r a c t Article history: Received 3 May 2018 Received in revised form 28 June 2018 Accepted 30 June 2018 Available online 3 July 2018 ZrB2eB4C composite powders (ZBCs) were synthesized by B4C reduction method via induction heating in an argon atmosphere using ZrO2, B4C, and pitch powder as raw materials. The effect of ZBCs addition on the performance of Al2O3eSiCeC (ASC) castables was evaluated by considering their mechanical properties, oxidation resistance, and slag resistance. The results showed that high-purity ZBCs with a median particle diameter of 3.07 mm could be easily obtained by induction heating at 1400 C for 30 min. The bulk density, cold modulus of rupture (CMOR), hot modulus of rupture (HMOR), and anti-oxidation performance of ASC castables can be synchronously improved via the introduction of synthesized ZBCs. The isothermal oxidation test at 1450 C revealed that the oxidation behavior of the samples with ZBCs addition follows a parabolic law. The slag corrosion resistance of ASC castables was slightly impaired, but the slag penetration resistance was obviously enhanced due to improved oxidation resistance. © 2018 Elsevier B.V. All rights reserved. Keywords: Al2O3eSiCeC castables ZrB2eB4C composite powders B4C reduction method Oxidation resistance Slag resistance 1. Introduction Al2O3eSiCeC (ASC) castable refractories have been widely used in blast furnace trough linings due to their excellent physical and chemical characteristics including high strength, good anti-slag performance, and good resistance to both abrasion and thermal shock [1e4]. Despite developments in blast furnaces and smelting, the properties of ASC castables must still be improved to meet the new challenges. Approaches for improving the service life of ASC castables mainly focus on enhancing their anti-oxidation performance, mechanical properties and slag resistance . In the Al2O3eSiCeC system, carbon and SiC offer high thermal conductivity and non-wettability with liquid slag. However, the high susceptibility to oxidation always leads to a porous material with diminished properties. Antioxidants have been extensively investigated to improve their oxidation resistance of these materials. Examples include metal-powders (such as Al and Si) and their alloys, Al4SiC4 [6,7], as well as boron-based non-oxide ceramics [8e10]. Of these, boron-based non-oxide ceramics (such as B4C and ZrB2, etc) [11e13] display excellent anti-oxidization performance when incorporated into the carbon-containing refractories due to * Corresponding author. E-mail addresses: [email protected], [email protected] (X. Wang). https://doi.org/10.1016/j.jallcom.2018.06.375 0925-8388/© 2018 Elsevier B.V. All rights reserved. their self-repairing ability. It is well known that B4C can react with O2 or CO to form a B2O3 liquid phase. This can further form a compound silicate glass layer containing multi-component oxides including SiO2, Al2O3, and CaO at elevated temperatures . The resulting liquid phases can ﬁll up the pores and cracks and hinder oxygen diffusion. Meanwhile, the B4C action can result in the deposition of C and reduce the loss of C [15,16]. Wu et al.  recently indicated that the addition of B4C could greatly improve the anti-oxidation performance of ASC castables and promote the nucleation of SiC whiskers. However, excessive addition of B4C into the refractories is detrimental to the hot modulus of rupture (HMOR) and slag corrosion resistance . Thus, the composite additive of antioxidants is a promising method to solve these problems [19e21]. Many studies have shown that the addition of ZrB2 can signiﬁcantly improve oxidation resistance by modifying the formed glass protective layer without impairing the hot mechanical properties [22e24]. This is because the ZrB2 belongs to the family of ultra-high temperature ceramics (UHTCs). It has outstanding mechanical and thermo-chemical properties [25,26]. During oxidation, the ZrO2 product can react with SiO2 to form a thermally stable ZrSiO4 phase . Corral et al. improved the ablation of CeC composites at high temperature via ZrB2eB4C composite powders (ZBCs). The ablation rates decreased by 30% when the CeC composites were ﬁlled with a combination of ZrB2eB4C particles over carbon black and B4C ﬁlled CeC composites 760 H. Xu et al. / Journal of Alloys and Compounds 766 (2018) 759e768 . The ZBCs might be an appropriate candidate antioxidant for ACS castables, but applications of ZBCs are limited by the high synthetic cost of ZrB2 and B4C powders. Moreover, the resulting powders are usually relatively coarse, and the subsequent ZBCs machining process is time-consuming and costly. Furthermore, the ZBCs obtained from directly mechanical mixing of ZrB2 and B4C powders suffers from agglomeration. To address these problems, it is critical to develop an alternative technique to one-step ZBCs synthesis. Induction heating is a new rapid heating method recently developed for the synthesis of ceramics and composites, including HfB2 , LaB6 , NbSi2eSiC , and HfSi2eSiC . This process is fast and effectively inhibits grain growth. Another important advantage of induction heating is the role of rapid heat transfer to the product via electromagnetic waves . Consequently, small and well-dispersed B4C and ZrB2 particles can be produced in situ via rapid induction heating. These materials enhance the antioxidation performance. Here, ZrO2, B4C, and pitch powder were used to synthesize ZBCs by a simple B4C reduction method via rapid induction heating for the ﬁrst time. The effect of different contents of the ZBCs on the anti-oxidation properties of ASC castables was evaluated. Simultaneously, the analysis of physical properties and slag corrosion resistance as well as thermodynamic calculations were also performed in order to better understand the effect of ZBCs introduction on the performance of ASC castables. 2. Experimental methods 2.1. Synthesis of ZBC ZrO2 (98.5 wt%, 3 mm, Macklin Biochemical Co., Ltd.), B4C (98 wt %, 5 mm, Macklin Biochemical Co., Ltd.) and pitch powder (C: 51.3 wt %, Vd: 48.55 wt%, Ad: 0.15 wt%, 200 mesh, Handan, China) were used as the starting materials. According to the following reaction (1), weighed quantities of ZrO2, B4C and pitch powder with a molar ratio of 2:2:3 were mixed thoroughly in a polythene bottle for 3 h in order to obtain ZBCs. The powder mixture was then pressed into cylinders with 20 mm in diameter under a pressure of 100 MPa. The pellets were then loaded in a graphite crucible and heated in an induction furnace. At the start of the experiments, the furnace chamber was sealed, evacuated and purged with argon (99 wt% purity). The samples were then held at a ﬁxed temperature (1400 C, 1500 C, 1600 C, and 1700 C) for 30 min in an argon atmosphere at a rate of 2.5 L/ min. The temperature was measured using a pyrometer with an accuracy of ±10 C. At the end of the process, the reacted pellets were cooled in the furnace until it reached room temperature and was removed. The black products were ﬂuffy and easily ground into powders with a corundum mortar and a pestle for 5 min. 2ZrO2 ðsÞ þ B4 CðsÞ þ 3CðsÞ/2ZrB2 ðsÞ þ 4COðgÞ (1) 2.2. Preparation of Al2O3eSiCeC castable specimens The raw materials used in this work were brown alumina with different particle grades (95 wt%, D 8), silicon carbide (98 wt%, 3-1 mm, 1-0 mm and 0.075 mm), white fused alumina (99 wt%, 200 mesh), reactive alumina powder (99 wt%, CL370, Almatis), ball pitch (60 wt% C), metallic Al (98 wt%, 120 mesh) and metallic Si (97 wt%, 200 mesh) and calcium aluminate cement (70 wt% of Al2O3, Secar 71, Kerneos) as a binder. Several groups of ASC castable samples with different synthesized ZBCs contents (0 wt%, 0.5 wt%, 1 wt%, 1.5 wt%, and 2 wt%) were designed with these raw materials, and the details of composition of ASC castables are listed in Table 1. All the raw materials were ﬁrst dry-mixed in a mixer, and then 4.8 wt% of extra water containing 0.1% of a polycarboxylate dispersant (FS20, BASF, Germany) was added to the dry mixed batches. After 5 min of mixing, the batches were cast into rectangle shaped molds with dimensions of 40 mm 40 mm 160 mm on a vibratory table. Castables were cured at about 20 C ambient temperature and 70% relative humidity within the mold for 48 h. They were then dried at 110 C for 24 h. Finally, the samples were ﬁred in air at 600 C, 800 C, 1100 C and 1450 C respectively for 3 h at a heating rate of 5 C/min below 1000 C and then 3 C/min to the target temperature. 2.3. Test and characterization methods The phase compositions of the synthesized ZBCs and the ASC castables samples were identiﬁed by X-ray diffraction (XRD, X0 Pert pro). The purity of the obtained ZBCs were examined by (XRF, ARL 9900 series, Thermo Scientiﬁc) and chemical analysis. The morphology of the obtained powders and microstructure of the ASC castables samples were examined in ﬁeld emission scanning electron microscopy (SEM, Nova NanoSem400) supported with energy dispersive spectroscopy (EDS, Phoenix). The median particle diameter and particle size distribution of ZBCs were measured with laser particle size analysis. The bulk density and apparent porosity of the ASC castable samples were tested via the Archimedes Principles. The linear change rate was tested according to ISO 2477:2005, MOD. CMOR was determined using the three-point bending test at ambient temperature. HMOR of the sintered samples was tested via threepoint bending tests at 1400 C for 30 min. The standard deviations quantiﬁed the amount of variation of data values. The oxidation resistance properties of the samples were evaluated by calculating the oxidation index (Id) using the Image-Pro Plus 6.0 software. The oxidation index Id was obtained via equation (2): . Abefore Id ð%Þ ¼ 100 Abefore Aafter (2) Here, Abefore and Aafter are the cross-sectional area of the samples before and after oxidation, respectively. The isothermal oxidation kinetics of ASC castables was also evaluated by analyzing the mass change vs time curve at a heating rate of 10 C/min, from room temperature to 1450 C, followed by thermal insulation for 8 h. The slag corrosion resistance experiments used crucible-shaped specimens inserted into the lining of an induction furnace (21WGJL0.025-100-2.5P). Subsequently, the steel and slag were added to the lining. The samples ﬁred at 1450 C for 3 h were used for slag corrosion resistance experiments. The blast furnace slag with a basicity (CaO/SiO2) of 1.28 was selected, and the details of the chemical composition are provided in Table 2. After testing, the corroded samples were cut, and the corrosion rates were calculated to measure the corrosion damage using Image-Pro Plus 6.0 software. Thermodynamic simulations of the matrix used FactSage 6.2 software with an excessive amount of O2 for oxidation. This software could predict the castables' phase compositions as a function of the amount of ZBCs. The Fact53 and FToxid databases were used along with Equilib modules. 3. Results and discussion 3.1. Synthesis of ZBCs Fig. 1(a) shows well-deﬁned peaks for ZrB2 and B4C. These can be identiﬁed in the XRD patterns at all temperatures ranging from H. Xu et al. / Journal of Alloys and Compounds 766 (2018) 759e768 761 Table 1 Batch composition of the prepared ASC castables. Raw materials Content, wt% Brown fused alumina aggregate Silicon carbide White fused alumina powder Ball pitch Binder (fumed silica and calcium aluminate cement) Reactive alumina powder Metallic Si Metallic Al ZBCs FS20 ZBC0 ZBC0.5 ZBC1 ZBC1.5 ZBC2 52 23 8 3 4 8 2 0.1 0 0.1 52 23 7.5 3 4 8 2 0.1 0.5 0.1 52 23 7 3 4 8 2 0.1 1 0.1 52 23 6.5 3 4 8 2 0.1 1.5 0.1 52 23 6 3 4 8 2 0.1 2 0.1 prepare ZBCs. Table 2 Chemical composition of blast furnace slag (wt%). Constituent SiO2 CaO Al2O3 MgO Fe2O3 K2O Na2O S Others Percentage 32.75 39.16 16.62 8.75 0.36 0.18 0.06 1.37 0.75 1400 C to 1700 C; no other phases were found. We used the XRF analysis combined with chemical analysis to identify the purity of ZBCs. The total content of ZrB2 and B4C is more than 98%. The impurity content is less than 1.6% (residual C: 0.45 wt%, Ce:0.08 wt%, Mo: 0.41 wt%, Nd: 0.12 wt%, Ca: 0.23 wt%, Fe: 0.06 wt%, Al: 0.06 wt%, Si: 0.05 wt%; Ti: 0.02 wt%; O:＜0.1 wt%). This indicates that highpurity ZBCs can be easily synthesized at temperatures as low as 1400 C using this induction heating method. Fig. 1(b) shows the backscattered electron image of the ZBCs synthesized at 1400 C for 30 min. There are clearly two different color phases in the productdthe EDS analysis proved that the white phases are ZrB2, and the black ones are B4C originated from the raw materials. The ZrB2 and B4C particles are well distributed with no obvious agglomeration. The laser particle size analysis results at the topright corner of Fig. 1(b) show the particle size distribution of the samples synthesized at various temperatures. The ZBCs synthesized at 1400 C has a narrow particle size distribution with a median particle diameter of 3.07 mm. As the temperature increases from 1400 C to 1700 C, the median particle diameter slightly increases from 3.07 mm to 4.60 mm. The preparation temperature used to synthesize ZBCs here is much lower than that used for preparing single powders via conventional heating methods (about 1500e2000 C) [34,35]. These results indicate that this induction heating method is a fast, low energy, and capable tool to 3.2. Physical properties of ASC castables upon the introduction of ZBCs The linear change rate of the prepared ASC castables containing different amounts of ZBCs are depicted in Fig. 2(a). The linear change rate gradually increases with temperature. At 800 C and 1100 C, the linear change rate slightly varies with increasing ZBCs content. However, all samples have a volumetric expansion after heat treatment at 1450 C. The linear change rate tends to increase with increasing ZBCs content. The ZrB2 and B4C can easily react with oxygen to form B2O3 via reactions (3) and (4). These are accompanied by a volume expansion. At the same time, many new phases such as mullite and ZrSiO4 are formed in the matrix at this temperature. This can lead to a volumetric expansion. 2B4 C þ 7O2 ðgÞ/4B2 O3 ðlÞ þ 2COðgÞ (3) 2ZrB2 ðsÞ þ 5O2 ðgÞ/2B2 O3 ðlÞ þ 2ZrO2 ðsÞ (4) The variation of bulk density and apparent porosity of the samples heat treated at various temperatures as a function of ZBCs content are illustrated in Fig. 2(b) and (c), respectively. Fig. 2(b) shows that the bulk density strongly depends on the heat treatment temperature and the ZBCs content. The bulk density ﬁrst decreases and then increases with the increase of the temperature. In all cases, the highest bulk density ranges from 2.94 to 2.99 g cm3. This can be obtained after drying at 110 C for 24 h. The sample then sharply decreases when the temperature reaches Fig. 1. (a) XRD patterns and (b) backscattered electron image, EDS, and laser particle size analysis of the synthesized ZBCs. 762 H. Xu et al. / Journal of Alloys and Compounds 766 (2018) 759e768 ﬁne powder (3.95 g cm3). Furthermore, the B2O3 liquid phase produced by the oxidation of ZrB2 and B4C improves the bulk density. However, the sample containing 1.5% ZBCs presents the highest bulk density when ﬁring at 1450 C. It is generally accepted that an appropriate extent of volume expansion was proved to be advantageous as the pores were ﬁlled. However, excessive volume expansion is harmful to the density of the castables with a tremendous permanent linear change rate of 0.57% (shown in Fig. 2(a)). The apparent porosity indicates an opposite tendency versus bulk density (Fig. 2(c)). The CMOR of the ASC castable samples after heat treatment at different temperatures are shown in Fig. 3. The samples dried at 110 C for 24 h introduce ZBC but do not signiﬁcantly inﬂuence CMOR. All the samples have a considerable strength that mainly originates from the hydration of the calcium aluminate cement. Meanwhile, the softening of some ball pitch at this temperature also improves CMOR. However, the samples heat treated above 800 C show that CMOR is greatly enhanced via ZBCs. The CMOR increases by 5 times from 4.2 MPa to 20.6 MPa and 2.5 times from 7.4 MPa to 18.7 MPa for the samples ﬁred at 800 C and 1100 C respectively, versus the unmodiﬁed samples. This signiﬁcant enhancement is likely due to B2O3 glass phase . When heating over 1450 C, the sample containing 1.5% ZBCs has the largest CMOR value of 23.1 MPa. This slightly decreases to 21.2 MPa for the sample ZBC2. This phenomenon might be associated with the excessive volume expansion resulting in destruction of the microstructure. Fig. 4 shows the HMOR of the ASC castables with different ZBCs contents. It is widely accepted that the HMOR is markedly affected by their ceramic bonding phases in the matrix . Fig. 4 shows that the HMOR value tends to increase with increasing ZBCs content. This signiﬁcant enhancement is because the samples containing a higher amount of ZBCs have a higher bulk density and a lower apparent porosity. Second, the introduction of ZBCs can greatly improve their anti-oxidation properties, and this can minimize the structural damage induced by the oxidation of C and SiC. Furthermore, the addition of ZBCs can lead to the formation of ceramic bonding phases such as columnar mullite and ZrSiO4. This can improve the strength at high temperature . These observations were conﬁrmed via XRD and SEM analysis (as shown in Figs. 5 and 6). Next, the phase composition and microstructure evolution of the samples with various ZBCs contents were analyzed to better Fig. 2. (a) Linear change rate, (b) bulk density, and (c) apparent porosity of the ASC castable samples containing different contents of ZBCs after heat-treating at different temperatures. 800 C due to the pyrolyzation of ball pitch and the oxidation of C and SiC. This leaves numerous cavities inside the castables. Next, the bulk density again begins to increase when the temperature increases to 1100 Cdboth B4C and ZrB2 start reacting with oxygen at temperatures below 800 C. This leads to pore blocking due to the formation of B2O3 liquid phases. At 1450 C, the bulk density further increases due to the sintering process of castable composites and the formation of new phases in the matrix. This makes the castables denser. In addition, the bulk density of the samples all increase with increasing ZBCs content at the temperatures ranging from 110 C to 1100 C. This is because the microsized ZBCs have an excellent capability to ﬁll the voids between aggregates. This leads to a higher packing density. The ZBCs also has a much higher density (z4.74 g cm3) than the white fused Al2O3 Fig. 3. CMOR of the ASC castable samples after heat-treating at different temperatures. H. Xu et al. / Journal of Alloys and Compounds 766 (2018) 759e768 763 Fig. 4. HMOR of the ASC castables with different ZBCs contents. understand the effect of ZBCs addition on the physical properties of the ASC castables. Fig. 5(a) depicts the XRD patterns of the oxidized Fig. 6. Typical microstructures of the non-oxidized zone of samples with different ZBCs contents after ﬁring at 1450 C: (a), (b): 0%; (c), (d): 0.5%; (e), (f): 1%; (g), (h): 1.5%; and (i), (j): 2%. Fig. 5. XRD patterns of the oxidation layer of (a) the samples with different ZBCs contents after heat-treating at 1450 C (b) the sample ZBC2 after heat-treating at different temperatures. region of samples with different amounts of ZBCs after ﬁring at 1450 C. The Al2O3, SiC, and mullite are the major crystal phases in all the samples. ZrO2 is generated when ZBCs is added. This is attributed to the oxidation of ZrB2. The relative diffraction peak intensity of mullite/Al2O3 is gradually enhanced with increasing ZBCs content. This suggests that the incorporation of ZBCs into ASC castables can promote the formation of mullite. This improves the high temperature strength. Signiﬁcant diffraction peaks from ZrSiO4 are observed in the samples with 1.5% and 2% ZBCs, but these are not seen detected in the samples with a ZBCs content below 1.5% (the concentration is under the XRD detection limit). Previous studies reported that ZrO2 and ZrSiO4 can act as an “embedding structure” on the surface of the silicate glass layer. This increases the viscosity of the glass layer, and reduces its porosity, improves the anti-oxidation ability, and promotes the mechanical strength . 764 H. Xu et al. / Journal of Alloys and Compounds 766 (2018) 759e768 Fig. 5(b) shows the XRD patterns of the oxidized region in the ZBC2 sample coked at various temperatures (600 C, 800 C, 1100 C, and 1450 C). At 600 C, both ZrB2 and m-ZrO2 are identiﬁed in the XRD spectrum suggesting that ZrB2 was partially oxidized at this temperature. When the temperature increases to 800 C, the diffraction peaks of ZrB2 fully convert into ZrO2. The accompanying oxidation scale of B2O3 is ﬂuid. It ﬁlls the pores and cracks, and this accounts for the high CMOR above 800 C. When the temperature is elevated to 1100 C, the Si intensity obviously decreases, and the t-ZrO2 transforms into m-ZrO2. Up to 1450 C, many mullite phases can be observed along with ZrSiO4. The FESEM micrographs of the non-oxidized zone of samples at 1450 C with different ZBCs contents are shown in Fig. 6. Many SiC whiskers are formed in the pores of the samples with 0% and 0.5% ZBCs. The formation mechanism has been depicted previously . However, a few of SiC whiskers were found in the ZBC1 sample and the SiC whiskers almost disappear when the ZBCs content increases to 2%. Instead, a large amount of the columnar structure mullite crystals were observed in the pores, and the crystals are oriented and interlocked with each other to form an intertexture that might improve the mechanical strength of the ASC castable. The decrease in SiC whiskers might be associated with a change in CO partial pressure in the castables. The formation of SiC ﬁbers is usually dependent on the amount of CO and SiO gases with a high amount of these gases considered favorable for this transformation. However, we found that the addition of ZBCs led to a dense protective layer on the surface of castables. This layer could inhibit oxygen inﬁltration. Moreover, the CO gases in the matrix could be reduced to C by ZrB2 and B4C through reaction (5) and (6). Both of these can decrease the CO partial pressure in the inner parts leading to the disappearance of SiC whiskers. The mullite is generated through two paths: First, the active alumina particles directly react with SiO2 derived from the additive microsilia, the oxidation of Si, and SiC, and the CaAl2Si2O8. This product is mullite based on reactions (7) and (8). Second, mullites are easily formed by the Al18B4O33 at high temperatures due to the similar crystal structure (9). Besides, the higher B2O3 liquid content is also favorable for the formation of Al18B4O33 and mullites. B4 CðsÞ þ 6COðgÞ/2B2 O3 ðlÞ þ 7CðsÞ (5) ZrB2 ðsÞ þ 5COðgÞ/ZrO2 ðsÞ þ B2 O3 ðlÞ þ 5CðsÞ (6) 2SiCðsÞ þ 3O2 ðgÞ/2SiO2 ðsÞ þ 2COðgÞ (7) 2SiO2 ðsÞ þ 3Al2 O3 ðsÞ/Al6 Si2 O13 ðsÞ (8) Al18 B4 O33 ðsÞ þ 6SiO2 ðl; sÞ/3Al6 Si2 O3 ðsÞ þ 2B2 O3 ðlÞ (9) Thermodynamic calculations were also made using FactSage software to predict the phase evolution. Fig. 10 shows that alpha stands for the ratio of ZBCs to the matrix of the ASC castable. The mass of the matrix was 100 g according the experimental formula. The N2 and O2 were set as 395 g and 105 g, respectively, according to their ratios in the air. With increasing ZBCs contents, the partial pressure of oxidizing gases O2 and CO2 decreases, while the partial pressure of B2O3 and CO increase. The content of liquid phase increases with increasing ZBCs; mullite decreases. The trend is different from our present observations. Because the prediction presumed that the ZrB2 and B4C are completely oxidized in an adequate O2 atmosphere, but that is impossible in reality. 3.3. Oxidation tests Fig. 7 shows the oxidation cross area of the specimens heattreated at different temperatures. All the samples without ZBCs addition undergo signiﬁcant oxidation. The sample is completely oxidized by 1100 C. After the introduction of ZBCs, the oxidation resistance properties are greatly enhanced at all temperatures. The decarbonized areas calculated via Image-Pro Plus software are shown in Fig. 8. The decarbonized areas all obviously decrease as the ZBCs content increases. The carbonized area decreases by 64% from 100% to 36% at 1100 C, and by 17% from 85% to 68% at 800 C, respectively. The decarbonized area of ASC castables with ZBCs at 1100 C and 1400 C was much lower than that at 800 C. Because ZBCs can provide oxidation protection through the formation of continuous B2O3 layer at 800 C. When the temperature is above 1100 C, metal Si and SiC begin to sharply oxide to form SiO2. This can further react with B2O3 to form borosilicate glass layer with good oxidation resistance. The synergistic effect of ZBCs and Si signiﬁcantly improves the oxidation resistance at 1100 C and 1400 C. Fig. 9 shows the mass change of the tested samples as a function of ZBCs content. The mass loss for the sample without ZBCs undergoes a sequential increment before 1450 C for 200 min as a result of the oxidation of C and the removal of crystal water and organic compositions. However, for the samples with ZBCs addition, there was a signiﬁcant mass increment phenomenon at 890 C. The mass change rate became sharper with increasing ZBCs content. The ZBCs oxidation leads to weight gain, and somewhat counteracts the weight loss. The generated liquid protective layer also restricts the diffusion of oxygen into the inner castables. This protects the C and SiC against oxidation. These results suggest that the introduction of ZBCs powders can signiﬁcantly improve the oxidation resistance of ASC castables at middle temperature (800e1100 C) and elevated temperatures. Isothermal oxidation was also performed at 1450 C for 8 h to better understand the effect of ZBCs on the oxidation mechanism. The plot of (Dm/m)2 as a function of time (Fig. 9(b)) shows that for the unmodiﬁed sample, the mass change rate continues to decrease until 1400 C for 250 min. It then linearly increases with increasing oxidation time. At the beginning of oxidation, C directly touches O2, and their chemical reaction is the main control condition. After oxidation, SiC begins to oxidize leading to a SiO2 ﬁlm and a mass increment. Samples with ZBCs addition, however, have a linear (Dm/m)2 with oxidation time indicating that the oxidation mechanism is parabolic. Previous work [36,41] has shown that ZrB2 and B4C Fig. 7. Oxidation proﬁles of the evaluated ASC castables after heating in air. H. Xu et al. / Journal of Alloys and Compounds 766 (2018) 759e768 765 Fig. 10. Phase changes of the ASC castables containing different amounts of ZBCs at 1450 C. Fig. 8. Calculated decarbonized area of the castables as a function of ZBCs content. Fig. 9. Isothermal oxidation weight gain vs. time curve: (a) from room temperature to 1450 C and then thermal insulation for 8 h, and (b) isothermal process (the mass at the beginning of thermal insulation was zero). initially react with O2 or CO to form C and B2O3 at temperatures above 450 C and 540 C, respectively. Due to the expansion reactions, the open porosity of castables is markedly reduced (shown in Fig. 2). The reacts not only provides carbon protection, but also generates extra C. The B2O3 glass can further react with other components (such as SiO2, Al2O3, and CaO) to form a protective layer. Oxidation layer is formed on the surface, and O2 needs to overcome a large diffusion resistance in order to react with C and SiC. As the diffusion path becomes longer, the rate of diffusion 766 H. Xu et al. / Journal of Alloys and Compounds 766 (2018) 759e768 Table 3 Values of rate constants kd. Content of ZBCs (wt%) Kd (%▪min1) 0 0.5 1 1.5 2 e 1.11 105 1.26 104 8.35 104 2.05 103 becomes the limiting factor. Thus, the rate of oxidation is equal to the rate of oxygen diffusion as shown in Equation (10). 2 Dm m ¼ kd $t (10) The values of kd were obtained by curve ﬁtting and are summarized in Table 3. The oxidation rate constant obviously increases with increasing amounts of ZBCs. This implies that introducing ZBCs into ASC castables obviously retards the oxidation rate compared to the unmodiﬁed sample. 3.4. Slag corrosion resistance Slag corrosion tests were conducted to evaluate the effect of ZBCs on the corrosion resistance of ASC castables. Fig. 11 presents the samples' cross-sections after corrosion experiments in an induction furnace for 1 h. All the samples have an excellent slag corrosion resistance. With increasing amounts of ZBCs, the erosion layer become thinner, but the erosion region becomes wider. The results suggest that the introducing of ZBCs has a signiﬁcant inﬂuence on the slag penetration and slag corrosion. The corrosion resistance of the castables can be characterized by corrosion area per minute as shown in Fig. 11(b). The corrosion rate initially increases and then decreases with increasing ZBCs. The calculated corrosion rate for the sample without ZBCs is 1.8 mm2 min1. This increases to 3.5 mm2 min1 and 5.9 mm2 min1 for ZBC0.5 and ZBC1, respectively. It then slightly decreases to 5.72 mm2 min1 for ZBC2. In general, the slag corrosion process of ASC castables is mainly described via the following three processes: (1) the surface of ASC castables is directly dissolved into the melt or oxidized at high temperatures to form a decarburized layer under the action of molten steel; (2) The liquid slag penetrates into decarburized layer through open pores, and the Al2O3, SiC, and ZrO2 aggregates are isolated and subsequently eroded into the slag along the crystal boundary; (3) The ASC castables are gradually eroded under the alternation of slag and molten steel. Any process above that controls erosion will somewhat increase the erosion of the materials, Fig. 11. (a) Cut-section view photographs and (b) slag corrosion rates of slag-corroded samples. Fig. 12. Typical microstructures of the corroded samples with different ZBCs contents. and the erosion rate is controlled by the slowest one of these steps. Fig. 12 shows the microstructures of the corroded samples with different ZBCs contents. Fig. 12(a) shows a signiﬁcant decarburized layer with a thickness of 1.6 mm. There are many coarse pores in ZBC0, which is responsible for the bad slag penetration resistance. An obviously thick penetration layer can be observed in ZBC0. The slag penetration dominates the corrosion mechanism. Samples with ZBCs addition have a markedly reduced decarburized layer. This is only 0.40 mm and 0.25 mm for ZBC1 and ZBC2, respectively, due to improvements in the anti-oxidation property. The penetration layer also becomes thinner versus ZBC0. This is because the generated B2O3 liquid phase ﬁlls the open pores and blocks the channels for molten slag. Hence, the samples exhibit excellent slag penetration resistance. However, the higher ZBCs content always generates more liquid phase containing B at elevated temperature. The formed liquid phase can easily dissolve into the alkaline molten slag that in turn deteriorates the slag corrosion resistance. Here, the chemical corrosion is the main control factor affecting the samples H. Xu et al. / Journal of Alloys and Compounds 766 (2018) 759e768 containing ZBCs. This analysis shows that the proper ZBCs content has a positive effect on corrosion penetration resistance, but excess addition would impair slag corrosion resistance. Thus, we concluded that ZBCs addition can effectively improve the oxidation resistance performance and the high temperature mechanical properties of the ASC castables. The incorporation of ZBC increases slag penetration resistance but decreases slag corrosion resistance against alkaline molten slag. If the synthesized ZBCs are used in ASC castables, then the total ZBCs content should be comprehensively considered. 4. Conclusion The ZrB2eB4C composite powders were rapidly synthesized by a simple B4C reduction method via induction heating. The effect of ZBCs additives on the behavior of ASC castables was systematically investigated. Based on the obtained results, the following conclusions can be drawn: (1) High-purity ZrB2eB4C composite powders with a median particle diameter of 3.07 mm were obtained via induction heating at 1400 C for 30 min in an argon atmosphere. This method is more efﬁcient than conventional heating methods. (2) ZBCs addition effectively improved the anti-oxidation property and the thermal mechanical strength. As the addition amount of ZBCs increased from 0 wt% to 2 wt%, the oxidation index reduced by 19%, 60%, and 38% at 800 C, 1100 C, and 1450 C respectively; the HORM increased by 41%. The isothermal oxidation kinetics at 1450 C indicated the oxidation behavior of the samples with ZBCs addition follows a parabolic law. The substantial enhancement in the thermomechanical properties was mainly attributed to the formation of more ceramic bonding, such as ZrSiO4 and columnar mullite crystals. 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