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Sept. 6, 1949-
Filed July 2, 1947'
0770 J?
Heréer/A’. Dela/75
“Hqgo A. Baré/an- .
Patented Sept. 6, 1949
Herbert K. De Long and Hugo A. Barbian, Mid
’ land, Mich., assignors to The Dow Chemical
Company, Midland, Mich., a corporation of
Application July 2, 1947, Serial No. 758,518
4 Claims. (0]. 136-100)
This invention relates to improvements in pri- ' i'voltage and low capacity, but the e?iciency is
good. Those employing solutions of chromlc acid
mary cells employing a magnesium anode, in
containing a ?uoride have relatively high initial
which the electrolyte is an aqueous solution com
prising chromic acid.
It has been heretofore disclosed that solutions
of chromic acid may be employed as electrolyte
for magnesium primary cells. With such an elec
trolyte, if free from deleterious impurities, the
Voltage and moderate to good e?iciency, but low
It is an object of this invention to provide a
magnesium primary cell with a chromic acid elec
trolytc, which has a high initial voltage and
greater capacity than those of the prior art, ac
open circuit attack of the aqueous electrolyte on
the magnesium anode is reduced to a low order. 10 companied by good e?iciency.
Our invention is based on the discovery that
A disadvantage of such a primary cell, when the
the addition of phosphoric acid to a chromic acid
electrolyte is a solution of chromic acid alone, is
electrolyte in a magnesium primary cell increases
that the initial closed circuit voltage is a low
the voltage of the same. Within the range
' of_
value of the magnitude of about 0.6 volt, varying
little with concentrations of chromic acid up to
electrolyte in a magnesium primary cell gives a
about 50 per cent by weight.
susbtantial increase in voltage over solutions of
As a means of increasing the initial closed cir
chromic acid alone, combined with high capacity
cuit voltage of such a magnesium primary cell, it
and e?iciency.
has been proposed to add a ?uoride to the
chromic acid electrolyte. In this way the ini 20 We have found that the rate of increase of volt
age obtainable by addition of phosphoric acid to
tial voltage is said to be raised materially, e. g.
a chromic acid electrolyte depends upon the con
to a value of over 1 volt. However, this initially
centration of CrOs in the solution. In general,
high voltage falls rapidly when the cell is in use,
the higher the concentration of the chromic acid
and a serviceable voltage can be maintained for
25 solution, the higher the ratio of HaPOr/CrOa re
only a short period of operation.
quired to produce an equal increase in voltage.
The utility of primary cells for industrial uses
On the other hand, in order to prevent the deposit
depends not only on their voltage, but also on.
of a polarizing ?lm on a magnesium anode in a
their useful life in service and on the efficiency
chromic acid electrolyte containing phosphoric
of the electrochemical system employed to gen
the concentration of CrOa should be 30 per
erate the current. The useful life of a cell is 30
cent by weight or more. The range of propor
measured by its capacity, which term is used
tions, within which a substantial increase in volt
herein to mean the ampere-hours delivered upon
age is obtained without producing a ?lm on the
discharge from- the initial voltage to a prescribed
- minimum usable voltage, which is taken usually
surface of ‘the magnesium anode, is from 30 to
as 1.0 volt or 0.5 volt, depending on the type of 35 50 per cent by weight of CrOa and 5 to 25 Per cent
by weight of I-I3P04. Optimum results are ob
service required for the cell. Capacity varies di
tained in the range of 40 to 45 per cent of CrOs
rectly with the volume of electrolyte in a cell.
and 20-25 per cent of H3PO4, when considering
Thus, capacity in a particular case is de?ned in
not only the increased voltage but also the capac
terms of ampere-hours delivered on discharge of
a cell of de?nite volume to a de?nite minimum 40 ity and e?iciency of the cell. Within the broad
percentage range above stated, the limit of solu
voltage. The efficiency of a cell is determined by
bility will be slightly exceeded, if the maximum
the ratio of the theoretical weight loss of the
stated amounts of the two solutes are employed’
consumable anode to the actual weight loss of
together. A solution containing 45 per cent CrOa
the same in generating current in ampere-hours
and 25 per cent H3PO4 is saturated, and for solu
_ equal to the capacity of the cell. In the case of
magnesium, the electrochemical equivalent is ap 45 tions containing more than 45-per cent and up to
50 per cent of CrOs, the saturating amount of
proximately 1,000 ampere-hours per pound.
H3PO4 is less than 25 per cent by weight. In any
Thus, in a primary cell having a magnesium an
case the two solutes are to be employed within
ode, a weight loss of 1 pound of magnesium by
dissolution is equivalent at 100 per cent efficiency 50 the limit of solubility thereof.
In the following Table I are shown represen
to 1,000 ampere-hours of current.
In the magnesium primary cells of the prior
art, those employing solutions of chromic acid
alone as electrolyte have the disadvantages of low
tative data-that have been obtained in experi
mental cells having a volume oi‘ 500 cc. of electro
lyte. In each case the anode was of a mag
nesium alloy and the cathode was or graphite.
_ the dimensions oi’ the anodes being '
capacity and higher emciency.
The purity of the electrolyte is of importance
The magnesium alloy was a commercial alloy 5 ‘and for best performance, particularly eii‘iciency. _
it should be free iromanions other than Cros
identi?ed as Dowmetal .14, heat treated at 040'
and PO_, with the exception that a very small
P. for 8 hours and quenched. This is a so-called
concentration of S04-- ions has certain advan
high purity alloy having the composition: Al, 8.13
tages. it a tolerance limit of about 0.25 per cent
per cent; Mn, 0.2 per cent: _Zn. 1.0 per cent: lie.
the CIO; content oi’ the solution is not ex
less than 0.002 per cent; Cu. less than 0.002 per 10
ceeded. Sulphate is a common impurity oi’ the
cent; Ni, less than 0.001 per cent; balance, Hg.
usual technical grades of chromic acid and phoe
The tests were run at normal room temperature, phoric acid, which should be removed. at least to
at a current density of from 1 to 3 amperes per
below the tolerance limit, when the technical
square feet. In the table the ?rst two columns
show the per cent by weight oi’ the active com-v 15 acids are used for preparing the electrolyte.
This may. be done by treating the acid solution
ponents of the aqueous electrolyte; the third
with barium carbonate to precipitate the sul
column shows the initial closed circuit voltage;
phate as barium sulphate, boiling to decompose
the fourth and iiith columns show the ampere-,
any excess carbonate, and filtering oi! the pre
hours delivered on discharge to 1.0 volt and 0.5
volt, respectively; and the last column shows the 20 cipitate. small additions of sulphuric acid or a
soluble sulphate within the tolerance limit have
electrochemical eillciency measured by the loss in
the e?ect 01' further increasing the initial closed
weight of the magnesium anode.
circuit voltage. although at the expense of low
Table I
ering the capacity and e?lciency of the cell. At
the higher CrOa concentrations the negative
E1, 25 e?ect of sulphate on capacity and emciency is
Volt I
less pronounced than at lower CrO: concentra
tions, The higher voltage attainable by addition
of sulphate increases the power of the cell at the
beginning of discharge, but shortens its useful
30 life; In cases where higher power at the start
is of more importance than long life, the addi
tion of sulphate to the electrolyte, within the
tolerance limit, is indicated.
Table II shows the e?ect oi’ sulphate upon the
35 initial closed circuit voltage, capacity and cm
ciency of the cell, with electrolytes containing
varying proportions of CrOa and H1PO4. The
tests shown in Table II were made under the
same condition as those in Table I, except for .
40 the addition of sulphate to the electrolyte.
Table II '
B04‘ "
The ?rst three lines of the table show the re
sults with an electrolyte of CrOa alone in the per
Amp-Hrs. Amp-Hrs.
to 1 .
A comparison of the results shown in Table II
with those of Table I brings out the voltage in
centages stated. The succeeding groups of lines
crease with increase of sulphate in the electrolyte.
4-7, 8-11, 12-15 and 16 show the eifect of addi 70 With a sulphate content below the tolerance
tions of HzPOl, progressively increasing voltage
limit of 0.25 per cent of the weight 01' CrO: in the
with increase of H.1P04 content. The greatest
electrolyte, a materially higher initial voltage is
increase in initial voltage is shown in the solu
produced with good capacity and a not excessive
tions containing 30 per cent of CrOa, but the
loss of e?iciency, particularly at (310: concentra
lower voltages of the groups containing 40 and ‘5 ‘It tions from about 40 to 45 per cent.
' 8.481.904,
The performance tests shown in Tables rand
invention. ll‘he'containerisaiar I madeofan
insulating material such as glass or molded plas
tic, having a cover 2 of similar material. ‘.Jar I
' H. as above indicated, were made with cells hav
ing a volume of 500 cc. of electrolyte. The ca
pacity of a cell being proportional to its volume.
larger sizes of cells have correspondingly larger
capacity, although the increase in capacity ‘is not
exactly proportional to increase in size, due to the
is ?lled with a liquid electrolyte I 01' the type
herein described to a level as shown. The anode
. 4 is a plateof magnesium or vmagnesium alloy,
preferably submerged beneath‘the surface of the
electrolyte, and provided with a lead-in rod or
effect of operating for a longer time at less than
wire {secured thereto, the latter being covered
theoretical e?lciency. As a comparison, a'500 cc.
cell with electrolyte containing 40 per cent CrO:_
.and 23 per cent H3PO4. by weight, had-a capacity
of 28.4 ampere-hours on continuous discharge to
or coated‘ with a layer of insulating material 0
in the parts exposed to contact with the electro
lyte. ‘The cathode I is a plate of graphite or
1.0 volt, while a 10,000 cc. cell with the same elec
trolyte and at the same current density produced
may extend upwardly through a slot in the cover,
carbon. disposed parallel to anode 4. Cathode ‘I
560 ampere-hours on continuous discharge to 1.0 16 as ‘shown, and is provided with a lead-in wire I
secured'thereto in any usual manner.
volt, i. e. 98.6 per cent of the expected capacity
We claim:
1. A primary cell comprising an anode consist
as extrapolated from the capacity of the 500 cc. '
ing essentially of magnesium and an electrolyte
' As anode for the primary cell either magnesium .
or the usual commercial magnesium alloys may 20 consistinghessentially of an aqueous solution of
chromlc acid and phosphoric acid, in which the
be used, with no signi?cant di?erences in cell
performance. Suitable commercial alloys have. content of 'CrO: is from 30 to 50 per cent by
weight and the content of HzPOs is from 5 to 26
the following composition:
per cent by weight, but within the limit of solu
FB-l ............................... --
Mn‘ Zn
‘B '
bility of the two solutes.
2. A primary cell as claimed. in claim .1, in
_ which the electrolyte contains sulphate expressed
' as Sor- ion in amount-by weight not exceeding
0.25 per cent of the weight of CrO: therein.
,3. A primary cell comprising an anode consist
The magnesium or magnesium alloy used for '
ing essentially of magnesium and an electrolyte
consisting essentially of an aqueous solution of
the purpose should conform to standard speci?
cations for purity, particularly as to the heavy
chromic acid and phosphoric acid, in which the
metals Fe, Cu, Ni. The anodes may be formed
content of CrO: is from 40 to 45 per cent by
by casting or from wrought'metal, such as plates 35 weight and the content of HJPO4 is from 20 to 25
of suitable thickness. In service the magnesium
per cent by weight.
anodes are dissolved smoothly and uniformly on
'4. A primary cell as claimed in claim 3, in
which the electrolyte contains sulphate expressed
closed circuit in accordance with the electro
as SO4— ion in amount by weight not exceeding
chemical reaction, and are subject to very slight,
I-l ................................. _.
if any, corrosion while standing on open circuit. (l 0.25 per cent of the weight of CrO; therein.
0n discharge, a current density up to about 6
amperes per square foot may be employed, with
optimum performance at densities up to about 3
amperes per square foot.
For the cathode of the primary cell either 45 The following references are of record in the
graphite or hard carbon may be used, although
?le of this patent:
graphite is preferred, with current densities of
about the same order as for the anodes.
In the drawing, the single ?gure is a sectional
view of a primary cell for use according to the
Kugel ___________ __ Feb. 25, 1930
Potent No‘. 2,481,204 ;
' , '_
Certi?c‘ate of Correction
_ September a, 1949
It is hereby certi?ed that emit appears in the printed speci?cation of theabove
' numbered patent
correction as follows:
Column 4; line 6, for “PO"“” read P0“";
and that the said Letters Patent should be read with this correction therein that the
same may conform to the record of the case in the Patent O?ice.
Signed and sealed this 21st day-of February, A. D. 1950.
moms F. MURPHY,
Assistant Oommiaaioner of Patents.
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