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Recent studies of chemical properties of amorphous alloys
22
Materials Science and Engineering, A133 (1991 ) 22-25
Recent studies of chemical properties of amorphous alloys K. Hashimoto, N. Kumagai, H. Yoshioka, H. Habazaki, A. Kawashima, K. Asami and B.-P. Zhang Institute for Materials Research, Tohoku University, Sendai, 980 (Japan)
Abstract This is a review of studies performed by the authors after RQ6 on the subjects in the field of the chemical properties of rapidly quenched metals. This includes corrosion-resistant alloys, the effects of alloy homogeneity and structural relaxation on corrosion behavior, electrocatalytic materials for electrolysis of seawater and fuel cells, and catalysts for the oxidation of carbon monoxide.
1. Corrosion resistance 1.1 Corrosion-resistant alloys
Of particular interest is the production of new alloys consisting of low boiling point elements and high melting point elements which cannot be prepared by conventional casting. From this point of view, AI-Ta [1], A1-Nb [1], AI-W [2-4], A1-Mo [2, 3], AI-Zr [2, 3], AI-Ti [3, 5-7], Cu-Ta [8, 9] and Cu-Nb [8, 9] alloys were prepared by magnetron sputtering. Figure 1 shows the structure of various aluminum alloys [3]. In the aluminum-rich alloys a single a-A1 phase re#on is significantly extended. The single a-Al phase is supersaturated with alloying elements and its grain size becomes smaller with increasing alloying additions [6]. Single amorphous phase alloys are formed in wide middle composition ranges, which often correspond to the intermetallic compound regions at equilibrium. • Amorphous
o
,~t-r,
Al-Zr
•
"
• Amor.* Crys.
o Crystalline
"Z'/////','////',
~' ®
Except A1-Ti alloys the corrosion resistance of these alloys in 1 M HCI increases with increasing alloying additions, as shown in Fig. 2 [3]. Amorphous A1-Ta and A1-Nb alloys are especially corrosion-resistant. Even if amorphous alloys are not formed, the corrosion resistance increases with increasing alloying elements. It is already known [10] that the a-Al phase is most susceptible to corrosion and that supersaturation of the a-Al phase with corrosion-resistant alloying elements is quite effective in enhancing the corrosion resistance. Amorphous Cu-Ta and Cu-Nb alloys also have extremely high corrosion resistance even in 12 M HCI due to the formation of the passive film consisting exclusively of tantalum or niobium oxides [8, 9]. They are immune to pitting by anodic polarization in 12 M HCI. In contrast, amorphous A1-Ta, A1-Nb, A1-Zr and A1-Ti alloys suffer pitting by anodic polarization in 1 M HCI. XPS analyses have revealed that the fraction of cations in the passive films formed on these
,,~,o r~
101
k
•D~////q;.y/!2~/////.e~//////~,'l~//////~l t O/ • • •
A, I . I
to°
E .. 10-~ AL- ro
o
AI-Mo
o
Al-w
•
O~/ / ' , ~O.0' Z/ / / Z * / /•/ / / / ~
~
o
.;/////Z*/////~y/////., O~
.//////.Z/////S.
~o
•
o
o •
,b
Concentration of A l l o y i n g
p-to ( Tetoragonal )
do Element
=~
~b I
•
MO
o ~ 10-4
w
,oo at'/,
Fig. 1. Structure of sputter-deposited aluminum alloys identified by X-ray diffraction [3]. 0921-5093/91/$3.50
• "~ 10 J
b~c Mo o
I0 2
i0~[
[,
Zr
I,/;J
20 40 60 80 100 Concentration of Alloying Element / at%
Fig. 2. Corrosion rates of sputter-deposited aluminum alloys in 1 M HCI at 30 °C [3].
© Elsevier Sequoia/Printed in The Netherlands
23 alloys are almost the same as the fraction of elements in the alloys. This is in contrast to copperbased [8, 11] and nickel-based [11, 12] alloys containing tantalum and niobium on which the passive films are remarkably enriched in tantalum and niobium cations. Because aluminum has a high affinity to oxygen, the formation of the films enriched in alloying elements is difficult. The formation of the film containing a large amount of aluminum, which readily suffers pitting, is responsible for pitting of the amorphous aluminum-valve metal alloys.
1.2. Dual function of structural relaxation When amorphous Ni-19P alloys are prepared by rapid quenching of the white and red heat melts, the former shows a higher anodic dissolution current than the latter by potentiodynamic polarization from the open circuit potential to about 400 mV (SCE) in 1 M HC1 [13]. After structural relaxation by heating up to the temperature just below the crystallization temperature, these two alloys show the same potentiodynamic polarization curves as each other which locate just between the polarization curves of the two as-quenched alloys. The alloy formed by quenching of the white heat melt includes a higher density of quenchedin defects than the alloy quenched from the red heat melt. This is responsible for a higher anodic dissolution current of the former alloy. Structural relaxation decreases the density of quenched-in defects and hence decreases the anodic dissolution current of the alloy quenched from the white heat melt. In contrast, the structural relaxation also results in atomic rearrangement and regrouping. This increases alloy heterogeneity, and hence the structural relaxation of the alloy quenched from the red heat melt leads to a rise in the anodic dissolution current. On the other hand, all amorphous Ni-19P alloys exhibit the same polarization curves in the steady state obtained by potentiostatic polarization, regardless of structural relaxation. The potentiostatic polarization at potentials lower than 400 mV (SCE) gives rise to selective dissolution of nickel forming an elemental phosphorus layer on the alloy surface, and hence the dissolution current in the steady state is controlled by diffusion of nickel through the phosphorus layer. The high corrosion resistance of the binary amorphous Ni-P alloys in strong acids is due to the
high stability of the elemental phosphorus layer covering the alloy surfaces. When the amorphous alloys have high passivating ability, such as F e - C r - W - P - C alloys, the structural relaxation affects the current density not in the passive region but only in the active region [14]. Because the effect of the structural relaxation appears in the alloy surface, when the surface does not directly contact with the electrolyte by being covered by the passive film or other film such as the elemental phosphorus layer, the current density is affected by the differences in the structure and composition of the alloy but not by the difference in the structural relaxation.
1.3. Effects of alloy homogeneity Chromium-based alloys were prepared by rapid quenching of the melts in which the ratio of Cr-13P to Ni-19P was changed. As shown in Fig. 3 [15], the corrosion rate of the Cr-27Ni-15P alloy in 6 M HC1 is higher than that of the 15.14P alloy by a factor of 2000 in spite of the fact that both alloys are amorphous by X-ray diffraction. Detailed examination by TEM and XPS analyses reveals that the Cr27Ni-15P alloy is composed of microcrystalline (Cr, Ni)3P and b.c.c. (Cr, Ni) phases in the amorphous matrix, whereas the Cr-29Ni-15.14P alloy consists of a single amorphous phase [16]. The surface film formed on these two alloys by air exposure is exclusively composed of hydrated chromium oxyhydroxide, which is the same as the passive film. Nevertheless, the presence of alloy heterogeneity of the Cr-27Ni-15P alloy is responsible for the passivity breakdown by pro10,~ lot
r ~
- ~
I
I
[
--
Cr-13P
[
i
I
6 M HC{ 3 0 ° C
m=55, /_m=4 m=3 m=6 D~ z~-
'~ 10' --
I
Pure Cr Pure Ni
( C r - 1 3 P ) / ( N i - 19P)=rn
l
m=2.2 ' 1
I O " --
Ni- t9 (re=O)
tO-~ _
"~o
0 Amor ~ Amor+Cryst [3 Cryst
10-'-0
I 0
~, i /3
L 14
[
~:~ m=l.8
"-S
[ I I 15 16 17 18 Phosphorus Content / o t ~
I 19
"[ 20
Fig. 3. Corrosion rates of melt-spun Cr-Ni-P alloys prepared by chan@ng the ratio of C r - ] 3 P to Ni-19P (m) measured in 6 MHCI at 30 °C [15].
24 i
e.o
? E
P t / Ti
~o
5
i
i
i
i
Ni-4OZr-I.5Ru-I.5Pt
£o 2v7;~ ~ . ~ ~
to;
"6.0
~
1@
i
~I4e/ tMNp~O~~J'C
,, ,.~o,2,.c
\ .
~
4.0 -
~
3.0
0
-
10t
-
to ~
N l - N b ' / ° ~
~2.o
Ni
1
TO
0.5M I~CI
0
500
I I000 /500 CURRENT DENSITY I
I
-200 A T 30"C
[ 2000 A.m-a
Fig. 4. Specific electric power consumption of amorphous alloy electrodes and currently used electrodes for production
of 1 kg of chlorine by electrolysisof 0.5 M NaCI solution [18]. longed immersion, and hence for the low corrosion resistance in the aggressive solution. 2. Electrode materials
2.1. Anodes for the electrolysis of seawater Amorphous nickel-valve metal (Ti, Zr, Nb, Ta) alloys containing a few at.% platinum group elements have a high electrocatalytic activity for chlorine evolution. They have been prepared as the surface alloys covering conventional corrosion-resistant bulk metals such as niobium by laser and electron beam processing. The electron beam processing is particularly effective since the processing time is about 1/70 in comparison with CO2 laser processing because of the high reflectivity of the CO2 laser beam on the solid metal [17]. As shown in Fig. 4 [18], the electricity required to produce 1 kg of chlorine by the amorphous alloy electrodes is about one-third of the currently used most active Pt-Ir/Ti electrode at 1000 A / m 2 which is the industrially used current density for electrolysis of seawater. 2.2. Electrodes for fuel cells Fine powder catalysts have been prepared from the amorphous nickel-valve metal alloys containing a few at.% platinum group elements by selective dissolution of nickel and valve metals in hydrofluoric acids. The porous gas-diffusion electrodes were prepared by coating the pastes consisting of these alloy catalysts, PTFE and carbon black powder, followed by baking in a nitrogen atmosphere. The performance of these electrodes for electrochemical oxidation of methanol [19] and hydrogen [20] and the electro-
I
t
0
200
~
400
I /
600
I
800
Potenfiol / mV vs SCE
Fig. 5. Polarization curves of the porous gas-diffusion electrodes prepared from the amorphous Ni-40Zr-l.5Ru1.5Pt alloyfor hydrogenoxidationand oxygenreductionin 1 MH2SO4 at 25 °C [20]. chemical reduction of oxygen [19, 20] was examined. These electrodes show a very high activity and durability for the oxidation of fuels and the reduction of oxygen. As shown in Fig. 5 [20], their activity for oxidation of hydrogen is very high, and hence hydrogen is oxidized at potentials very close to the equilibrium potential. A large power can, therefore, be obtained even at ambient temperature. 3. Catalysts Attempts to utilize amorphous alloys in catalysis have been performed for purification of the earth's atmosphere without using a large amount of energy for catalytic reactions, because most of the pollutants inducing the greenhouse effect and depletion of the ozonosphere are formed as a result of energy conversion or consumption. Catalytic reactions must, therefore, take place at low temperatures which can be easily attained by solar light heating. When ribbon-shaped amorphous nickel-valve metal alloys containing a few at.% platinum group elements were activated by immersion in hydrofluoric acid, they showed high activities for conversion of CO to CO 2 at 100 °C and of CO and NO to CO2 and N2 at 170 °C [21]. 4. Concluding remarks The fact that the amorphous alloys form a single-phase solid solution supersaturated with various elements provides almost infinite potential in developing new materials having unknown useful characteristics. In particular, the further challenge to purify the earth's atmosphere by using new amorphous alloys is of interest.
25
References 1 H. Yoshioka, A. Kawashima, K. Asami and K. Hashimoto, in R. B. Diegle and K. Hashimoto (eds.), Corrosion, Electrochemistry and Catalysis of Metallic Glasses, The Electrochemical Society, Pennington, 1988, p. 242. 2 H. Yoshioka, A. Kawashima, K. Asami and K. Hashimoto, Proc. MRS International Meeting on Advanced Materials, Materials Research Society, Pittsburgh, Vol. 3, 1988, p. 429. 3 H. Yoshioka, Q. Yan, H. Habazaki, A. Kawashima, K. Asami and K. Hashimoto, Corros. Sci., 31 (1990) 349. 4 H. Yoshioka, Q. Yan, H. Habazaki, A. Kawashima, K. Asami and K. Hashimoto, Corros. Sci., 30 (1990) in press. 5 Q. Yan, H. Yoshioka, H. Habazaki, A. Kawashima, K. Asami and K. Hashimoto, Corros. Sci., 31 (1990) 401. 6 Q. Yan, H. Yoshioka, H. Habazaki, A. Kawashima, K. Asami and K. Hashimoto, Corros. Sci., 30 (1990) in press. 7 Q. Yan, H. Yoshioka, H. Habazaki, A. Kawashima, K. Asami and K. Hashimoto, J. Non-Crystalline Solids, 125 (1990) 25. 8 K. Shimamura, K. Miura, A. Kawashima, K. Asami and K. Hashimoto, in R. B. Diegle and K. Hashimoto (eds.), Corrosion, Electrochemistry and Catalysis of Metallic Glasses, The Electrochemical Society, Pennington, 1988, p. 232. 9 K. Shimamura, A. Kawashima, K. Asami and K. Hashimoto, Proc. MRS International Meeting on Advanced Materials, Materials Research Society, Pittsburgh, Vol. 3, 1988, p. 335. 10 H. Yoshioka, S. Yoshida, A. Kawashima, K. Asami and K. Hashimoto, Corros. Sci., 26 (1986) 795.
11 K. Shimamura, K. Miura, A. Kawashima, K. Asami and K. Hashimoto, in R. B. Diegle and K. Hashimoto (eds.), Corrosion, Electrochemistry and Catalysis of Metallic Glasses, The Electrochemical Society, Pennington, 1988, p. 201. 12 A. Mitsuhashi, A. Kawashima, K. Asami and K. Hashimoto, in R. B. Diegle and K. Hashimoto (eds.), (brrosion, Electrochemistry and Catalysis of Metallic Glasses, The Electrochemical Society, Pennington, 1988, p. 191. 13 H. Habazaki, S.-Q. Ding, A. Kawashima, K. Asami, K. Hashimoto, A. Inoue and T. Masumoto, Corros. Sci., 29 (1989) 1319. 14 H. Habazaki, A. Kawashima, K. Asami and K. Hashimoto, Corros. Sci., 31 (1990) 343. 15 A. Kawashima, B.-E Zhang, H. Habazaki and K. Hashimoto, Corros. Sci., 31 (1990) 355. 16 B.-P. Zhang, H. Habazaki, A. Kawashima and K. Hashimoto, Corros. Sci., 30 (1990) in press. 17 N. Kumagai, S. Jikihara, A. Kawashima, K. Asami and K. Hashimoto, Proc. MRS International Meeting on Advanced Materials, Materials Research Society, Pittsburgh, Vol. 3, 1988, p. 429. 18 K. Hashimoto, N. Kumagai, H. Yoshioka and K. Asami, Materials and Manufacturing Processes, Laser Processing (1990) in press. 19 T. Kanda, A. Kawashima, K. Asami and K. Hashimoto, in R. B. Diegle and K. Hashimoto (eds,), Corrosion, Electrochemistry and Catalysis of Metallic Glasses, The Electrochemical Society, Pennington, 1988, p. 411. 20 Y. Hayakawa, A. Kawashima, H. Habazaki, K. Asami and K. Hashimoto. Submitted to J. AppL Electrochem. 21 K. Teruuchi, H. Habazaki, A. Kawashima, K. Asami and K. Hashimoto, to be submitted.
Materials Science and Engineering, A133 (1991 ) 22-25
Recent studies of chemical properties of amorphous alloys K. Hashimoto, N. Kumagai, H. Yoshioka, H. Habazaki, A. Kawashima, K. Asami and B.-P. Zhang Institute for Materials Research, Tohoku University, Sendai, 980 (Japan)
Abstract This is a review of studies performed by the authors after RQ6 on the subjects in the field of the chemical properties of rapidly quenched metals. This includes corrosion-resistant alloys, the effects of alloy homogeneity and structural relaxation on corrosion behavior, electrocatalytic materials for electrolysis of seawater and fuel cells, and catalysts for the oxidation of carbon monoxide.
1. Corrosion resistance 1.1 Corrosion-resistant alloys
Of particular interest is the production of new alloys consisting of low boiling point elements and high melting point elements which cannot be prepared by conventional casting. From this point of view, AI-Ta [1], A1-Nb [1], AI-W [2-4], A1-Mo [2, 3], AI-Zr [2, 3], AI-Ti [3, 5-7], Cu-Ta [8, 9] and Cu-Nb [8, 9] alloys were prepared by magnetron sputtering. Figure 1 shows the structure of various aluminum alloys [3]. In the aluminum-rich alloys a single a-A1 phase re#on is significantly extended. The single a-Al phase is supersaturated with alloying elements and its grain size becomes smaller with increasing alloying additions [6]. Single amorphous phase alloys are formed in wide middle composition ranges, which often correspond to the intermetallic compound regions at equilibrium. • Amorphous
o
,~t-r,
Al-Zr
•
"
• Amor.* Crys.
o Crystalline
"Z'/////','////',
~' ®
Except A1-Ti alloys the corrosion resistance of these alloys in 1 M HCI increases with increasing alloying additions, as shown in Fig. 2 [3]. Amorphous A1-Ta and A1-Nb alloys are especially corrosion-resistant. Even if amorphous alloys are not formed, the corrosion resistance increases with increasing alloying elements. It is already known [10] that the a-Al phase is most susceptible to corrosion and that supersaturation of the a-Al phase with corrosion-resistant alloying elements is quite effective in enhancing the corrosion resistance. Amorphous Cu-Ta and Cu-Nb alloys also have extremely high corrosion resistance even in 12 M HCI due to the formation of the passive film consisting exclusively of tantalum or niobium oxides [8, 9]. They are immune to pitting by anodic polarization in 12 M HCI. In contrast, amorphous A1-Ta, A1-Nb, A1-Zr and A1-Ti alloys suffer pitting by anodic polarization in 1 M HCI. XPS analyses have revealed that the fraction of cations in the passive films formed on these
,,~,o r~
101
k
•D~////q;.y/!2~/////.e~//////~,'l~//////~l t O/ • • •
A, I . I
to°
E .. 10-~ AL- ro
o
AI-Mo
o
Al-w
•
O~/ / ' , ~O.0' Z/ / / Z * / /•/ / / / ~
~
o
.;/////Z*/////~y/////., O~
.//////.Z/////S.
~o
•
o
o •
,b
Concentration of A l l o y i n g
p-to ( Tetoragonal )
do Element
=~
~b I
•
MO
o ~ 10-4
w
,oo at'/,
Fig. 1. Structure of sputter-deposited aluminum alloys identified by X-ray diffraction [3]. 0921-5093/91/$3.50
• "~ 10 J
b~c Mo o
I0 2
i0~[
[,
Zr
I,/;J
20 40 60 80 100 Concentration of Alloying Element / at%
Fig. 2. Corrosion rates of sputter-deposited aluminum alloys in 1 M HCI at 30 °C [3].
© Elsevier Sequoia/Printed in The Netherlands
23 alloys are almost the same as the fraction of elements in the alloys. This is in contrast to copperbased [8, 11] and nickel-based [11, 12] alloys containing tantalum and niobium on which the passive films are remarkably enriched in tantalum and niobium cations. Because aluminum has a high affinity to oxygen, the formation of the films enriched in alloying elements is difficult. The formation of the film containing a large amount of aluminum, which readily suffers pitting, is responsible for pitting of the amorphous aluminum-valve metal alloys.
1.2. Dual function of structural relaxation When amorphous Ni-19P alloys are prepared by rapid quenching of the white and red heat melts, the former shows a higher anodic dissolution current than the latter by potentiodynamic polarization from the open circuit potential to about 400 mV (SCE) in 1 M HC1 [13]. After structural relaxation by heating up to the temperature just below the crystallization temperature, these two alloys show the same potentiodynamic polarization curves as each other which locate just between the polarization curves of the two as-quenched alloys. The alloy formed by quenching of the white heat melt includes a higher density of quenchedin defects than the alloy quenched from the red heat melt. This is responsible for a higher anodic dissolution current of the former alloy. Structural relaxation decreases the density of quenched-in defects and hence decreases the anodic dissolution current of the alloy quenched from the white heat melt. In contrast, the structural relaxation also results in atomic rearrangement and regrouping. This increases alloy heterogeneity, and hence the structural relaxation of the alloy quenched from the red heat melt leads to a rise in the anodic dissolution current. On the other hand, all amorphous Ni-19P alloys exhibit the same polarization curves in the steady state obtained by potentiostatic polarization, regardless of structural relaxation. The potentiostatic polarization at potentials lower than 400 mV (SCE) gives rise to selective dissolution of nickel forming an elemental phosphorus layer on the alloy surface, and hence the dissolution current in the steady state is controlled by diffusion of nickel through the phosphorus layer. The high corrosion resistance of the binary amorphous Ni-P alloys in strong acids is due to the
high stability of the elemental phosphorus layer covering the alloy surfaces. When the amorphous alloys have high passivating ability, such as F e - C r - W - P - C alloys, the structural relaxation affects the current density not in the passive region but only in the active region [14]. Because the effect of the structural relaxation appears in the alloy surface, when the surface does not directly contact with the electrolyte by being covered by the passive film or other film such as the elemental phosphorus layer, the current density is affected by the differences in the structure and composition of the alloy but not by the difference in the structural relaxation.
1.3. Effects of alloy homogeneity Chromium-based alloys were prepared by rapid quenching of the melts in which the ratio of Cr-13P to Ni-19P was changed. As shown in Fig. 3 [15], the corrosion rate of the Cr-27Ni-15P alloy in 6 M HC1 is higher than that of the 15.14P alloy by a factor of 2000 in spite of the fact that both alloys are amorphous by X-ray diffraction. Detailed examination by TEM and XPS analyses reveals that the Cr27Ni-15P alloy is composed of microcrystalline (Cr, Ni)3P and b.c.c. (Cr, Ni) phases in the amorphous matrix, whereas the Cr-29Ni-15.14P alloy consists of a single amorphous phase [16]. The surface film formed on these two alloys by air exposure is exclusively composed of hydrated chromium oxyhydroxide, which is the same as the passive film. Nevertheless, the presence of alloy heterogeneity of the Cr-27Ni-15P alloy is responsible for the passivity breakdown by pro10,~ lot
r ~
- ~
I
I
[
--
Cr-13P
[
i
I
6 M HC{ 3 0 ° C
m=55, /_m=4 m=3 m=6 D~ z~-
'~ 10' --
I
Pure Cr Pure Ni
( C r - 1 3 P ) / ( N i - 19P)=rn
l
m=2.2 ' 1
I O " --
Ni- t9 (re=O)
tO-~ _
"~o
0 Amor ~ Amor+Cryst [3 Cryst
10-'-0
I 0
~, i /3
L 14
[
~:~ m=l.8
"-S
[ I I 15 16 17 18 Phosphorus Content / o t ~
I 19
"[ 20
Fig. 3. Corrosion rates of melt-spun Cr-Ni-P alloys prepared by chan@ng the ratio of C r - ] 3 P to Ni-19P (m) measured in 6 MHCI at 30 °C [15].
24 i
e.o
? E
P t / Ti
~o
5
i
i
i
i
Ni-4OZr-I.5Ru-I.5Pt
£o 2v7;~ ~ . ~ ~
to;
"6.0
~
1@
i
~I4e/ tMNp~O~~J'C
,, ,.~o,2,.c
\ .
~
4.0 -
~
3.0
0
-
10t
-
to ~
N l - N b ' / ° ~
~2.o
Ni
1
TO
0.5M I~CI
0
500
I I000 /500 CURRENT DENSITY I
I
-200 A T 30"C
[ 2000 A.m-a
Fig. 4. Specific electric power consumption of amorphous alloy electrodes and currently used electrodes for production
of 1 kg of chlorine by electrolysisof 0.5 M NaCI solution [18]. longed immersion, and hence for the low corrosion resistance in the aggressive solution. 2. Electrode materials
2.1. Anodes for the electrolysis of seawater Amorphous nickel-valve metal (Ti, Zr, Nb, Ta) alloys containing a few at.% platinum group elements have a high electrocatalytic activity for chlorine evolution. They have been prepared as the surface alloys covering conventional corrosion-resistant bulk metals such as niobium by laser and electron beam processing. The electron beam processing is particularly effective since the processing time is about 1/70 in comparison with CO2 laser processing because of the high reflectivity of the CO2 laser beam on the solid metal [17]. As shown in Fig. 4 [18], the electricity required to produce 1 kg of chlorine by the amorphous alloy electrodes is about one-third of the currently used most active Pt-Ir/Ti electrode at 1000 A / m 2 which is the industrially used current density for electrolysis of seawater. 2.2. Electrodes for fuel cells Fine powder catalysts have been prepared from the amorphous nickel-valve metal alloys containing a few at.% platinum group elements by selective dissolution of nickel and valve metals in hydrofluoric acids. The porous gas-diffusion electrodes were prepared by coating the pastes consisting of these alloy catalysts, PTFE and carbon black powder, followed by baking in a nitrogen atmosphere. The performance of these electrodes for electrochemical oxidation of methanol [19] and hydrogen [20] and the electro-
I
t
0
200
~
400
I /
600
I
800
Potenfiol / mV vs SCE
Fig. 5. Polarization curves of the porous gas-diffusion electrodes prepared from the amorphous Ni-40Zr-l.5Ru1.5Pt alloyfor hydrogenoxidationand oxygenreductionin 1 MH2SO4 at 25 °C [20]. chemical reduction of oxygen [19, 20] was examined. These electrodes show a very high activity and durability for the oxidation of fuels and the reduction of oxygen. As shown in Fig. 5 [20], their activity for oxidation of hydrogen is very high, and hence hydrogen is oxidized at potentials very close to the equilibrium potential. A large power can, therefore, be obtained even at ambient temperature. 3. Catalysts Attempts to utilize amorphous alloys in catalysis have been performed for purification of the earth's atmosphere without using a large amount of energy for catalytic reactions, because most of the pollutants inducing the greenhouse effect and depletion of the ozonosphere are formed as a result of energy conversion or consumption. Catalytic reactions must, therefore, take place at low temperatures which can be easily attained by solar light heating. When ribbon-shaped amorphous nickel-valve metal alloys containing a few at.% platinum group elements were activated by immersion in hydrofluoric acid, they showed high activities for conversion of CO to CO 2 at 100 °C and of CO and NO to CO2 and N2 at 170 °C [21]. 4. Concluding remarks The fact that the amorphous alloys form a single-phase solid solution supersaturated with various elements provides almost infinite potential in developing new materials having unknown useful characteristics. In particular, the further challenge to purify the earth's atmosphere by using new amorphous alloys is of interest.
25
References 1 H. Yoshioka, A. Kawashima, K. Asami and K. Hashimoto, in R. B. Diegle and K. Hashimoto (eds.), Corrosion, Electrochemistry and Catalysis of Metallic Glasses, The Electrochemical Society, Pennington, 1988, p. 242. 2 H. Yoshioka, A. Kawashima, K. Asami and K. Hashimoto, Proc. MRS International Meeting on Advanced Materials, Materials Research Society, Pittsburgh, Vol. 3, 1988, p. 429. 3 H. Yoshioka, Q. Yan, H. Habazaki, A. Kawashima, K. Asami and K. Hashimoto, Corros. Sci., 31 (1990) 349. 4 H. Yoshioka, Q. Yan, H. Habazaki, A. Kawashima, K. Asami and K. Hashimoto, Corros. Sci., 30 (1990) in press. 5 Q. Yan, H. Yoshioka, H. Habazaki, A. Kawashima, K. Asami and K. Hashimoto, Corros. Sci., 31 (1990) 401. 6 Q. Yan, H. Yoshioka, H. Habazaki, A. Kawashima, K. Asami and K. Hashimoto, Corros. Sci., 30 (1990) in press. 7 Q. Yan, H. Yoshioka, H. Habazaki, A. Kawashima, K. Asami and K. Hashimoto, J. Non-Crystalline Solids, 125 (1990) 25. 8 K. Shimamura, K. Miura, A. Kawashima, K. Asami and K. Hashimoto, in R. B. Diegle and K. Hashimoto (eds.), Corrosion, Electrochemistry and Catalysis of Metallic Glasses, The Electrochemical Society, Pennington, 1988, p. 232. 9 K. Shimamura, A. Kawashima, K. Asami and K. Hashimoto, Proc. MRS International Meeting on Advanced Materials, Materials Research Society, Pittsburgh, Vol. 3, 1988, p. 335. 10 H. Yoshioka, S. Yoshida, A. Kawashima, K. Asami and K. Hashimoto, Corros. Sci., 26 (1986) 795.
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