Note: Descriptions are shown in the official language in which they were submitted.
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ZI~C ALLOY POWDER FOR ALKALINE CELL
AND METHOD TO PRODUCE THE SAME
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BACKGROUND OF THE INVENTION
[Field of the Invention]
The present invention relates to a zinc alloy
powder for use in an alkaline cell and a method to
produce the same. More particularly, the present
inventioll is concerned with a non-amalgamated zinc
alloy~powder for use in an alkaline cell, which
comprises zinc containing iron in an amount of not more
than 1 ppm as an inevitably accidental impurity, and
specific elements added so as to suppress the evolution
of hydrogen gas and to improve the leaktightness of a
cell without the use of mercury and lead which are
toxic elements, and a method to produce the same.
[Prior Art]
The mercury contained in an amalgamated zinc
powder used as an anode active material in an alkaline
ZO cell has been known to be an essential component -for
such an active material from the viewpoint of
suppressing the evolution of hydrogen gas due to the
corrosion o~ zinc and preventing a liquid from leaking
from the cell as a result of the evolution of hydrogen
gas.
In light o-f environmental protection, howe~er,
a reduction in the mercury content is required in this
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field. In line with this requirement, it has become
possible to suppress the evolution oY hydrogen gas -
through the addition of not only lead but also
aluminum, bismuth, indium and the like as additional
elements to ~inc so that the mercury content is
remarkably reduced from 10 % by weight to about 1 % by
weight.
As further social needs, in recent years, it
is required to decrease the mercury content of the
anode active material to O % by weight, in other words,
to effect non-amalgamation. This non-amalgamation
greatly changes the situation, and it has been
dif~icult to decrease the evolution of hydrogen gas to
a desired level even when the above-described
additional elements are added. That is, although zinc
alloy powders as an anode active material having
various types of elements added thereto have been
proposed {see, for example, Japanese Patent Appln.
Publication Gazette No. (H~i.) 2-22984 (22984/1990) and
Japanese Patent Appln. Laid-Open Gazette No. (Sho.~ 61-
; 153950 (153950/1986)}, it has been impossible to attain
the desired suppression of hydrogen gas evolution when
the mercury content is 0 % by weight, though such
suppression has been possible even when the mercury
content is 1 % by weight or less.
RecentlY~ in a trend to minimize the contentof mercury, the effect of lead to inhibit the ~inc
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corrosion has become increasingly important.
Accordingly, anode active materials of low-mercury
alkaline cells which have been commercially available
generally consist of alloy compositions such as zinc-
lead, zinc-aluminum-lead, zinc-aluminum-indium-lead and
zinc-bismuth-lead. It has been generally believed that
the minimization of the content of mercury is largely
attributed to the effect of lead being added, and that
non-amalgamation of the anode active material can never
been achieved if the lead is not used at all.
By the way, it is known that lead is also as
harmful to the human body as mercury is. Accordingly,
in view of social demand to a clean environment, the
intentional addition of the lead is also not desirable.
As mentioned above, however, it has been failed up to
date to realize the productlon of a lead-free active
anode material, even in the case of the low
amalgamation.
Meanwhile, attempts have been made to suppress
the evolution of hydrogen gas and to improve the
discharge performance by reducing the impurity content
of ~inc. For example, Japanese Patent Appln. Laid-Open
Gazette No. (Sho.) 62-123653 (123653/1987) describes a
reduction in the content o-f impurities such as iron and
chromium. Table 1 on page 4 o~ the published
specification shows that an improvement in the
discharge performance while suppressing the evolution
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of hydrogen gas is attained by reducing the iron
content to about 10 ppm in an anode active ma-terial
which comprises an amalgamated zinc alloy powder
containing predetermined amounts of lead, indium and
aluminum and containing 1 ~ by weight of mercury.
However, a zinc alloy powder having a mercury
content of O % by weight could not attain the desired
effect of suppressing the evolution of hydrogen gas
even when the content of iron contained as an impurity
was reduced to about 10 ppm and additional elements
such as lead were incorporated.
Thus the non-amalgamation of an anode active
material and the freeing the material from lead are
accompanied by a difficultY which is fundamentally
different from that encountered in the low amalgamation
leading to a mercury content of 0.6 to 1 % by weight,
and there has not been developed any alkaline cell
wherein a non-amalgamated and lead-free zinc alloy
powder is used as an anode active material, the
evolution of the hydrogen gas is suppressed, and the
leaktightness is improved.
- SUMMARY OF THE INVENTION
The present invention has been made to solve
the above~described problems of the prior art. It is
an object of the present invention to provide a zinc
alloy powder for use in an alkaline cell which
substantially suppresses the evolution of hydrogen gas
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in non-amalgamated and lead-free cells, and a method to
produce the same. The final object of the prese~t
invention is to improve the leaktightness of a mercury-
free alkaline cell.
The present inventors have made intensive
studies in line with the above-described objects. As a
result, they have found that the objects can be
attained by the synergistic effect of the use of a zinc
having an extremelY low content of iron as an impurity
and the addition of specific elements thereto. The
present invention is based on the above finding.
The zinc alloy powder ~or use in an alkaline
cell according to the present invention consists
of elements component selected essentially from among
the following compositions (1) to (4);
(1) 0.001 to 0.5 % by weight of aluminum and 0.01
to 0.5 % by weight of bismuth,
(2) 0.001 to 0.5 % by weight of alumlnum, 0.01 to
0.5 % by weight of bismuth and not more than 1.0 % by
weight of indium,
(3) 0.001 to 0.5 % by weight of aluminum, 0.01 to
0.5% by weight of bismuth and not more than 0.5 % by
weight of lithium, and
(4) 0.001 to 0.5 % by weight of aluminum, 0.01 to
0.5 % by weight of bismuth, not more than 1.0 % by
weight of indium and not more than 0.5% by weight of
calcium or not more than 0.5 % by weight of lithium:
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and the balance being zinc and containing iron as an
inevitably accidental impurity in an amount of not more
than 1 ppm.
In the present invention, it is a requisite
that the content of iron as an inevitably accidental
impurity in zinc be not more than 1 ppm. When it
exceeds 1 ppm, the e~fect of suppressing the evolution
of hydrogen gas is lowered. The expression "the iron
content is not more than 1 ppm" used herein means that
the iron content is not greater than the limiting
analytic value as measured by the conventional
analytical methods, such as ICP or atomic absorption
spectrometry, without separating iron from zinc. ~;
Neither attempt has hitherto been made to use a zinc or
zinc alloy powder having such a low iron content as an
anode active material, nor there has been any report
describing such use. A hi~h-purity metallic zinc can
be prepared for use in special applications such as a
semlconductor by special methods such as zone meltlng.
Such a metallic zinc is so expensive that it cannot be
used as the raw material of dry cells. Also there is
no example wherein such a metallic zinc has been used
as an alloy powder. In rectified zinc which is
regarded as having the highest purity out of the zinc
ingots obtained by industrial mass production, the iron
content prescribed in Japanese Industrial Standards is
20 ppm or less. Among the varieties of the rectified
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zinc, even the one having a particularly low impurity
level generally has an iron content of not less than 2
ppm. Eurther, the iron content of electrolytic zinc is
on the same level.
In the present invention, the zinc alloy
contains an eleme~t component selected from among the
above compositions (1) to (4). When the content of
each component element falls outside the above-
described range, there occur problems such as the
failure to attain the desired effect of suppressing the
evolution of hydrogen gas or the failure to maintain a
practical discharge performance. If elements other
than the above-described combinations are added, for
example, if aluminum, bismuth, calcium or the like,
generally incorporated in a zinc alloy powder used as
an anode active material, is added alone, the above-
described effects of the present invention cannot be
attained.
The production method of the present inventlon
will now be described.
In the present invention, a zinc containing
not more than l ppm of iron as an inevitably acciden$al
impurity is used. Examples of the zinc having such a
low iron content include a deposited zinc obtained by
electrolysis and a zinc ingot prepared from zinc
obtained by vacuum distillation. A zinc ingot prepared
by melting deposited zinc together with a flux, such as
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ammonium chloride, and casting the melt into a mold has
hitherto been used as a starting zinc material of an
anode active ma-terial. In such zinc ingot, it is
impossible to decrease the iron content to not more
than 1 ppm. This is because, in general, zinc is
contaminated with iron originatin~ in a separator in
the step of removing dross formed on the surface of
molten zinc and returning partially recovered zinc to
the melt. Further, the contamination with iron may
occur from a melt pump, a mold or an atmosphere. ~ -
Elements listed in the compositions (1) to (4)
described above are dissolved ln the melt of a zinc
having a low iron content so as to be within
predetermined ranges of contents. Then, pulverization
is performed by atomization, followed by sifting, to
thereby give a zinc alloy powder. It is preferred from
the viewpoint of further improving the suppressing
effect of hydrogen gas evolution to limit the iron
content of the atmosphere for each of the melting and ~ -
atomization steps to not more than 0.009 mg/m3. From
.
the same viewpoint, it is also Preferred to
magneticall~ separate the obtained zinc alloy powder.
In this manner, the difference between
conventional method and method according to the present
invention to produce a zinc alloy powder is shown in
Table 1 as flow sheet.
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Table 1
l~low Sheet
Conventional Method Method according to
the present ;nverit;on
Outlet of rec~fierElectrolytic Starting
(Gaseous zinc)dep~sit z~nc material
Electrol~rtic
depos;t zmc ~eS lppm
C-- n~ issollred (Fe--sorted out)
(L~quld zmc) ~additive
mP.nts
C~st ~ st l
~hfi~ted Elec~31~ic
z~nc ~Dgot ~ 3ngot
Fe 2 2ppmFe 2 2ppm
Star~g m~t~
Z3nc iD~ot
t additive PlP.mPnt.s
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Dissol~ed ~ Dissol~ed
~tmosphe;re >Atmosphere
~e ~ 1m~/D~ Fe < 0,OO9mgim3
Atomized Atomized
Zinc alloy powder Zinc alloy powder
re 2 2ppm F'e s lppm
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I'he iron content of the resultant zinc alloy
powder is not more than 1 ppm as described above. This
~owder can suppress the evolution of hydrogen gas to
not more than about 300 uQ/day-cell (type LR6) which is
an allowable upper limit of leaktightness.
With respect to the ~echanism of evolving
hydrogen gas, macroscopic measurements of amount of
hydro~en gas evolved and presumptive relations between
the gas evolution and the structure of crystals have
heretofore only been discussed; but, to elucidate said
mechanism, there have been made no practical researches
into as far as the sites at which the hydrogen gas is
evolved. This would be the cause for the fact that
various techniques so far applied for a patent were
practically unuseful for mercury-free cells, the
present inventors thought. Thus, they carefully made
microscopic observations and EPMA (Electron Probe X-ray
Microanalyzer) analyses of the sites where hydrogen gas
was evolved and found that iron, its oxides, alloys and
; 20 the like in particulate form inevitably contained in
zinc powder were sources or causes for evolving
hydrogen gas when these particles were present between
the zinc particles and/or on the surfaces of the zinc
particles.
~5 More specifically, the present inventors
microscopically observed that there were specific sites
where hydrogen gas was continuously evolved when the
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zinc powder was immersed in an aqueous solution of
potassium hydroxide which was similar to the
electrolyte for an alkaline cell. Then, they likewise
observed how hydrogen gas was evolved using zinc in the
form of comparatively large particles, slender rods or
plates to confirm that the gas was evolved at the same
sites for a long period of timer after which said sites
were marked with a sharp tool. Then, the zinc having
said marked sites was analyzed for its composition by
EPMAo
As the result of this analysis, it was found
that said gas continuous evolution sites necessarily
had fine particles of 0.5 - 5 ~m in particle size
mainly containing iron localized therein. As elements
other than iron, there were detected chromium, nickel,
silver, sulfur and oxygen in some cases. Thus, it was
found that the gas evolution was effected in the
presence of a very minute amount of particulate iron
and/or iron oxide scattered in the zinc body.
As in~icated in the Table 2, particles of
various solid materials (other than zinc) having an -
average particle si~e of 0.1 - several mm were attached
to zinc powder or zinc plates in such an amount that
the particles so attached had a concentration of 1 -
several ppm, after which the particles - attached zinc
samples were immersed in an aqueous solution of
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potassium hydroxide to observe how hydrogen gas was
evolved with the resul~s being shown in Table 2.
Table 2
Gas Evolution Caused by Various ;~
Kinds of Particles Added -to Zinc
Particles added S~ate of gas-evolution
Fe2 ~3 (particles) Vigorous and contlnuous
Fe3 04 ~part1cles~ ~igorous ~nd continuous
Fe ~~~)2 (particles~ Slow and gradual
M~ ~2 (particles) Slow and gradual ~:
NiS (particles) Slow and gradual
Stainless steel piece ~lrgorous and continuous
; Al2 ~3 (particles3 No evolution
: 20 Ca O ~particles3 No evolution .
;~ SiO~partlles3 No evolution
~e ~partlcles) Vigorous and continuous
Cr (particles3 Slow and gradual
. Ni (Particles) Vigorous and contlnuous
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It was -found from the results that the sites
where the gas was evolved were those where particulate
iron, iron oxides or stainless steel was present.
The above experiment indicated that the
sources of gas evolution were the fine particles of
mainly iron type.
It has also been found b~ the present
inventors that the effect of the addition of lead is
more prominently seen in the inhibition of the
corrosion through a local cell reaction which could be
brought about by the omnipresence of iron in zinc,
rather than in the inhibition of a simple corrosion
that can be brought about between zinc and electrolyte,
and that if the content of iron as an impurity in the
zinc is extremely minimized, the rate of hydrogen gas
evolution can be controlled, even in the absence of
lead, to an amount lower than the allowable upper limit
of leaktightness.
In the present invention, therefore, the
content of iron as an inevitably accidental impurity in
zinc is minimized and predetermined amounts of specific
elements other than mercury and lead are added, whereby
a synergistic effect works to suppress the evolution of
hydrogen gas.
~s described above, a zinc alloy powder for
use in an alkaline cell and having an iron content of
not more than 1 ppm is produced by melting a zinc
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containing not more than 1 ppm of iron as an inevitablY
accidental impurity, together with specific elements to
give a melt and directly atomizing the melt.
Although this zinc alloy powder is non-
amalgamated and lead--free, it can greatly suppress the
evolution of hYdrogen gas and maintain the discharge
performance on a practical level when it is used as an
anode active material of an alkaline cell. Further,
since neither mercury nor lead is contained, the
alkaline cell comprising this zinc alloy powder as an
anode active material satisfies social needs.
BRIEF ~ESCRIPTION OF THE DRAWINGS
Fig. 1 is a cross-sectional side view of an
alkaline manganese cell according to the present
invention.
Fig. 2 is a graph showing the relationship
between the mercury content of a zinc alloy powder and
the rate of hydrogen gas evolution.
Fig. 3 is a graph showing the relationship
between the iron content of a zinc alloy powder and the
rate of hydrogen gas evolution.
PREFERRED EMBODIMENTS OF THE PRESENT INVENTION
The present invention will now be described in
more detail with reference to the following Examples
and Comparative Examples.
Examples 1 to 15 and Comparative Examples 1 to
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A ~elt of zinc alloy was prepared by meltiIIg
an electrolytically deposited ~inc containing not more
than 1 ppm of iron as an inevitably accidental impurity
at about 500~C and adding thereto predetermined amounts
of elements listed in Table 3 in an atmosphere having
an iron content of 0,005 mg/m3 in a room.
The melt was directly powdered using high-
pressure argon gas (ejection pressure: 5 kg/cm~) ln the
same atmosphere to give zinc alloy powders, which were
sifted to give S0- to 150-mesh powders.
Using a magnet, magnetic separation was
performed to thereby remove free iron powder. All of
the obtained zinc alloy powders had an iron content of
not more than 1 ppm.
An electrolyte was prepared by adding about
1.0 % of carboxymethylcellulose and polysodium acrylate
as a gelating agent to a 40 % aqueous potassium
hydroxide solution saturated with zinc oxide.
3.0 g of the above-described zinc alloy powder
as an anode active material was mixed with 1.5 g of the
electrolyte to thereby give a gel. Using the gel as an
anode material. the alkaline manganese cell as shown in
Fig. 1 was produced.
This alkallne manganese cell was partially
discharged by 25 %, and the rate of hydrogen gas
evolution due to the corrosion of the zinc alloy powder
was measured. The results are given in Table 3. The
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reason for performing the 25 % partial discharge is
that the rate of hydrogen gas evolution is maximized
around 25 % partial dischar~e supposing that the time
necessary for discharge to 0.9 V is 100 % for a
separatelY prepared non-mercury alkaline manganese
cell. It was supposed that dischargè conditions o~ 1
and 11 minutes represent 25 % partial discharge.
The alkaline manganese cell of Fig. 1 is
composed of cathode can 1, cathode 2, anode (~elled
zinc alloy powder) 3, separator 4, opening sealant 5,
anode bottom plate 6, anode electricity collector 7,
cap 8, heat-shrinkable resin tube 9, insulating rings
10 and 11 and exterior can 12.
Comparative Examples 9 to 11
A zinc ingot, as a starting material, prepared
by casting according to the conventional procedure an
electrolytically deposited zinc having an iron content
of not more than 1 ppm, was melted at about 500~C in an
atmosphere having an iron content of 5 mg/m3. Added to
the melt were predetermined amounts of elements listed
in Table 3 to give zinc alloy melts.
Each of the melts directly powdered using a
high-pressure argon (ejection pressure: 5 kg/cm2) in
the same atmosphere to give zinc alloy powders, which
were sifted to give 50- to 150-mesh powders.
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All of the obtained zinc alloy powders had an
iron content of 3 ppm. No magnetic selection was
performed.
Using each of the zinc alloy powders, an
alkaline cell as shown in Fig. 1 was produced in
substantiallY the same manner as tha't of Example 1 and
25 % partial discharge was per~ormed to measure the
rate of hydrogen gas evolution. The results are given
in Table 3,
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Table 3
Example Added elements (~t%) Fe content Gas evolution rate
Comp. Ex. AQ Bi Ca In L,i (ppm) IJQ/cell-day
Example 1 0.002 0.05 - - - < 1 186
Example 2 0.010.05 - - - ~ 1 116
Example 3 0.4 0.05 - - - < 1 252
Example 4 0.010.02 - - - < 1 157
Example 5 0.010.4 -~ - - - < 1 230
Example 6 0.010.05 - 0.05 - < l 51
Example 7 0.010.05 - 0.4 - < 1 110
Example 8 0.010.05 - 0.8 - < 1 83
Example 9 0.010.05 - - 0.01 < 1 92
Example 10 0.010.05 - - 0.08 < 1 113
Example 11 0.010.05 - - 0.3 < 1 141
Example 12 0.010.05 0.0250.05 - < 1 49
Example 13 0.010.05 0.08 O.OS - < 1 47
Example 14 0.010.05 0.3 0.05 - < 1 96
Example 15 0.010.05 - 0.05 0.01 < 1 26
Comp. Ex.l - - - - - < 1 1728
Comp. Ex.2 0.01 - - - - < 1 1117
Comp. Ex.3 - 0.05 - - - < 1 581
Comp. Ex.4 - - 0.025 - - < 1 1489
Comp. Ex.5 - - - 0.05 - < 1 1767
Comp. Ex.6 - - - - 0.01 < 1 1467
Comp. Ex.7 0.7 0.05 - - - < 1 1331
Comp. Ex.8 0.01 0.6 - - - < 1 580
Comp. Ex.9 0.010.05 - - - 3 810
Comp. Ex.10 0.010.05 - 0.05 - 3 45I
Comp. Ex.ll 0.010.05 0.025 0.05 - 3 483
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As shown in Table 3, in all of the zinc alloy
powders of Examples 1 to 15 having an iron content of
not more than 1 ppm and a speci-fic composition, the
rate of hydrogen gas evolution is smaller than about
300 uQ/day-cell (type LR6) which is an allowable upper
limit of leaktightness. By contrast, in the zinc alloy
powders of Comparative Examples 1 ~o 8, the composition
falls outside the scope of the present invention and
hence, although the iron content is not more than 1
ppm, no effect of suppressing the evolution of hydrogen
gas is recognized. ~oreover, in the zinc alloy powders
of Comparative Examples 9 to 11, the iron content is 3
ppm and hence, irrespective of whether or not the
composition falls within the scope of the present
invention, no effect of suppressing the evolution of
hydrogen gas is recognized.
Examples 16 to l9
A ~inc alloy powder (Example 16) was produced
according to the same composition and conditions as
those of Example 2, except that no magnetic separation
was performed. A zinc alloy powder ~Example 17) was
produced according to the same composition and
conditions as those of Example 2, except that the
melting and the atomization were conducted in an
atmosphere of 5 mg/m3.
Similarly, a zinc alloy powder (Example 18)
was produced according to the same composition and
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conditions as those of Example 6, except $hat no
magnetic separation was performed. Eurther, a zinc
alloy powder (Example 19) was produced according to the
same composition and conditions as those of Example 6,
except that the melting and the atomization were
conducted in an atmosphere of 5 mg/m~.
All of the zinc alloy powders thus obtained
had an iron content of not more than 1 ppm. Using each
of the zinc alloy powders, an alkaline cell shown in
Fig. l was produced in substantially the same manner as
that of Example 1 and 25 % partical discharge was
performed to measure the rate of hydrogen gas
evolution. The results are given in Table 4.
.
Table 4
Exam? e Gas evolut_~- rate ~l/cell day
Ex. _
Ex. _ _
Ex.
Ex.
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As is apparent from Table 4, substantially the
same results as those o-f example 2 were obtained ln
examples 16 to 17, and substantially the same results
as those of Example 6 were obtained in Examples 18 to
19 .
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Experiment 1
Zinc alloy powders o-f Example 2 and
Comparative Example 9 were amalgamated so as to have a
mercury content of 1 and 10 % by weight, respectively,
thereby producing amalgamated zinc alloy powders.
Using each o-f the amalgamated zinc alloy
powders, an alkaline cell shown in Fig. 1 was produced
in substantially the same manner as that of Example 1
and 25 % partial discharge was conducted to measure the
rate of hydrogen ~as evolution. The results were
plotted together with the values of Example 2 and
Comparative Example 9 as shown in Fig. 2.
As indicated in Fig. 2, when the iron content
is 3 ppm, the evolution of hydrogen gas is below the
allowable upper limit of.leaktightness at a mercury
content of 1 % by weight or greater. By contrast, when
the iron content is not more than 1 ppm, the evolution
of hydrogen gas is below the allowable upper limit of
leaktightness irrespective of the presence or absence
of mercurY-
Experiment 2
Various kinds of zinc alloy powders which
consist of (1) 0.01 % by wei~ht of aluminum, 0.05 % by
weight of bismuth and the balance being zinc; (2) 0.01
% by weight of aluminum, 0.05 % by weight of bismuth,
0.05 % by weight of lead and the balance being zinc; or
(3) 0.01 % by weight of aluminum, 0.05 % by weight of
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bismuth, 0.05 % by weight of lead, 0.15 % by weight of
mercury and the balance being zinc, respectively, and
further contain various amounts of iron were produced
in substantially the same manner as that of Example 2.
Using each of these zinc alloy powders, an
alkaline cell shown in Fig. 1 was produced in
substantially the same manner as that of Example 1 and
25 % partial discharge was conducted to measure the
rate of hydrogen gas evolution. The results were
plotted as shown in Fig. 3.
As indicated in Fig. 3, when the content of
iron as an inevitablY accidental impuritY is not less
than 2 ppm, the e~olution of hYdrogen gas is above the
allowable upper limit of leaktightness irrespective of
the presence or absence of mercury and lead. It is
found that the iron content must be not more than 1 ppm
to maintain the evolution of hydrogen gas below the
allowable upper limit of leaktightness irrespective of
the presence or absence of mercury and lead.
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