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Differential thermo-analysis (DTA) method in strip casting AB5-type hydrogen storage alloys
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International Journal of Hydrogen Energy 28 (2003) 409 – 414 www.elsevier.com/locate/ijhydene
Di&erential thermo-analysis (DTA) method in strip casting AB5-type hydrogen storage alloys Chaoling Wua; ∗ , Yungui Chena , Feng Lia , Dingxiang Tangb , Chuan Zhouc , Mingjing Tua a The
Department of Metallic Materials, School of Materials Science and Engineering, Sichuan University, Chengdu 610065, China b Research Centre of Rare Earth Materials and Applications, Chengdu 610065, China c The Department of Chemical Engineering and Mechanism, School of Chemical Engineering, Sichuan University, Chengdu 610065, China
Abstract In this work, strip casting was introduced, and the necessity of heat treatment was explained from the viewpoint of DTA thermo-analysis. The results showed that: Five exothermic peaks appeared in the DTA curves of LPCNi3:55 Co0:75 Mn0:4 Al0:3 alloy in which the intensity of the second and third peak decreased strikingly with the decrease of cooling rate. It was sensitive to the grain sizes. The intensity of the fourth and =fth peak showed less correlation with cooling rate, and they located in the range of theoretical recrystallization temperature. It was possibly introduced by recrystallization. In addition, segregation and internal stress also contributed to exothermic peaks. All heat variation had =nished during the =rst temperature-rising period of DTA test, which implied that there was no irreversible phase transformation in the hydrogen storage alloys during the whole heated-cooled process. Four kinds of AB5 -type hydrogen storage alloys in our work showed similar exothermic characteristics. ? 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. Keywords: Di&erential thermo-analysis; Strip casting; Hydrogen storage alloys; Heat treatment; Rare earth
1. Introduction LaNi5 -type hydrogen storage alloys have been deeply investigated [1–3]. They drew most researchers’ attention, owing to the environmental friendship, comparatively low cost, and relatively superior comprehensive performances. However, there were still some problems to be solved, including lowering the cost further [4,5], improving the high-rate capabilities [6 –9], low-temperature performances and cyclic stabilities [9 –11]. Strip casting was expected to be a method for hydrogen storage alloys to obtain superior comprehensive properties, including high activation property, high discharge capacity ∗ Corresponding author. Tel.: +86-28-85407335; fax: +86-28-85407442. E-mail address: [email protected] (C. Wu).
and cyclic stability, etc. while lowering the cost. Furthermore, heat treatment was unavoidable in the manufacturing process of commercial hydrogen storage alloys. But few theories on it have been found at present. In this paper, strip casting was introduced, and the necessity of heat treatment was explained from the viewpoint of thermo-analysis. 2. Experimental Four kinds of hydrogen storage alloys were investigated, see Table 1. Alloys #1; #2 and #3 were cast into a copper mould. Their thickness were 1; 3; 6 and 10 mm respectively. The as-cast three samples were grinded into both 200 and 400 meshes, respectively. Then, the above three kinds of 200-mesh alloys with #4 commercial alloy were tested by
0360-3199/02/$ 30.00 ? 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 0 2 ) 0 0 0 8 6 - 1
410
C. Wu et al. / International Journal of Hydrogen Energy 28 (2003) 409 – 414
Table 1 The alloys and the thickness of their ingots Alloys
#1
#2
#3
#4
Contents
LPCNi3:55 Co0:75 Mn0:4 Al0:3
LPCNi3:55 Co0:4 Mn0:4 Al0:3 Cu0:15 Fe0:1 Si0:1
MmNi3:55 Co0:75 Mn0:4 Al0:3
Thickness (mm)
1
1
1
Commercially used alloy (Shen Jiang, made in China) —
3
6
10
3
6
10
3
6
10
Statement: LPC = 77–84% La + 5–10% Ce + 9–12% Pr; Mm = 27–32% La + 43–56% Ce + 5–7% Pr.
T/˚C
0.4
0.2 d
0.1 a
0.0 _ 0.1 (A) 0.3
b
c de
T/˚C
0.2 0.1 a
0.0 _ 0.1 (B) 0.3 T/˚C
0.2
cd
b
0.1
e
a
0.0 _ 0.1 (C) 0.3
3. Results and discussion The exothermic peaks are possibly caused by disequilibrium in the crystals [12], such as the crystal defects, recrystallization, and=or the phase transformation, etc. Fig. 1 is the DTA curves of #1 alloy with di&erent thickness in temperature rising process. The curves show =ve exothermic peaks a–e in turn. Peak a shows relatively lower intensity than others. And in Fig. 1B and C, it is lapped by peak b. It is assumed that peak a is introduced by unavoidable mechanical stress during powder making process. It is obvious that the intensity of peaks b and c decreases strikingly with the increase of the thickness of alloy ingots. In other words, they are rather sensitive to cooling rates. In our experiment, the cooling rate of 1 mm sample is 103◦ C=s in order, and that of 10 mm one is 102◦ C=s.
c
b
0.3
T/˚C
DTA method. In the test, DuPont 2100-type DTA instruments were used. The sample cup was platinum crucible (D = 3 mm). The thermocouples were platinum–platinum ◦ 13% rhodium. The LT sensitivity was 0:001 C. And the ◦ temperature precision was ±2 C. Simultaneously, #1; #2 and #3 alloys (400 mesh) were tested by D=max-rA X-ray di&raction instrument, with Cu K di&raction. The former three ingots were also tested by Hitachi-650 scanning electronic microscopy (SEM) and EDAX9100 energetic dispersive spectrum instrument (EDS). The mixture of 200- and 400-mesh powder was mixed with copper powder (wt% = 1:2) and made into a pellet ( = 10 mm; thickness = 1 mm) to make the cathode of a half-cell, whose anode was Ni(OH)2 , the electrolyte was 6 N KOH, and the reference electrode was Hg=HgO. Then, the half-cells were tested by a DC-5 battery testing instrument. First, the electrodes were charged at a current of 11 mA for 7:5 h, then discharged at the same current to 0:6 V vs. Hg=HgO. After several charge–discharge cycles, the cathode reached its highest discharge capacity, i.e. it =nished the activation process. Second, the electrodes were charged at the current of 55 mA for 1:3 h, and then discharged at the same current to 0:6 V vs. Hg=HgO. The DC-5 instrument recorded all the data during the whole charge–discharge cycles.
de
0.2 b
0.1
c
a
0.0 _ 0.1 (D)
0
200
400 600 800 1000 1200 T/˚C
Fig. 1. DTA curves of #1 alloy with di&erent thickness in temperature-rising process: A—1 mm, B—3 mm, C—6 mm and D—10 mm.
Fig. 2 shows the microstructures of #1 alloy ingots with di&erent thicknesses. It is seen clearly that the grain sizes of the ingots become larger with the increase of the thickness. Thus, it is concluded that peaks b and c are sensitive to the grain sizes, i.e. the areas of the grain boundaries. The larger the areas of the grain boundaries, the higher the intensity of the exothermic peak will be.
C. Wu et al. / International Journal of Hydrogen Energy 28 (2003) 409 – 414
411
Fig. 2. The SEM pictures of #1 alloy with di&erent thickness: (a) 1 mm, (b) 3 mm, (c) 6 mm and (d) 10 mm.
◦
Fig. 3. The SEM pictures of the section of #1 alloy (as-quenching samples): (a) as-cast, (b) annealed at 900 C [13].
The intensity of peaks d and e varies less in the curves, and they seem to be determined by temperatures rather than cooling rates. Peak d is lapped by peak e in Fig. 1A, and it tends to go left with the increase of the thickness of ingots. They are possibly introduced by recrystallization mainly. Fig. 3 shows that the as-quenching #1 sample recrystallized ◦ after heat-treated at 900 C. It indicates that recrystallization
does exist during temperature-rising period. And according to the well-known equation: Tr = (0:35–0:45)Tm
(1)
in which Tr represents recrystallization temperature, and Tm represents melting temperature, it =ts quite well that peaks d and e locate in the range of recrystallization temperature,
412
C. Wu et al. / International Journal of Hydrogen Energy 28 (2003) 409 – 414
Table 2 ◦ Distribution of elements of 700 C annealed and as-cast 10 mm LPC Ni3:55 Co0:75 Mn0:4 Al0:3 hydrogen storage alloys Main elements
Mn
◦
Annealed at 700 C as-cast
Al
Co
Ni
La
Ce
Pr
m
b
m
b
m
b
m
b
m
b
m
b
m
b
6.36 2.79
8.39 10.55
8.08 4.49
4.86 3.36
13.13 13.03
13.96 12.36
56.36 64.58
53.85 55.01
4.99 4.51
5.67 4.34
8.55 8.29
10.21 12.83
1.90 2.09
1.31 1.49
Statement: m—within the grains, b—at the boundaries. Units: at%.
Intensity (CPS)
900 ˚C
as-cast
20
30
40
50
60
70
80
90
Diffraction Angle 2θ / (°) ◦
Fig. 4. The di&erence of XRD curves between as-cast and 900 C annealed 10 mm LPCNi3:55 Co0:75 Mn0:4 Al0:3 hydrogen storage alloys. ◦
448–512 C, given that the melting temperature of #1 alloy ◦ is 1280 C. There exist other factors to cause the exothermic peaks in the DTA curves besides grain sizes and recrystallization. Table 2 shows the EDS results of distribution of main elements in as-cast and annealed 10 mm #1 alloy. It is seen that Mn has rather distinguished segregation in as-cast ingots, and other elements, such as Ce, also show a little ◦ segregation. After being annealed at 700 C, all elements tend to become homogenous except Al. The decrease of segregation must be one factor to introduce exothermic peaks in DTA curves. The exothermic peaks in DTA curves may also be caused by the diminishing of internal stress in the grains, see Fig. 4. It is seen that the XRD peaks of 10 mm #1 alloy which is an◦ nealed at 900 C (FWHM=0:188, FWHM means full-width at half-maximum of the main peak) is sharper than those of as-cast sample (FWHM=0:376). This is mainly contributed to the decrease of internal stress within the grains. In fact, the above processes may occur during annealing, which do good to the comprehensive performances of hydrogen storage alloys, for example, improving the cyclic stabilities of the alloys, as is popularly accepted by most researchers. Moreover, the characteristic temperature trend is to become lower, i.e. the curves go left with the decrease of the cooling rate. It is helpful to determine the annealing process in manufacturing. Fig. 5 is the comparison of DTA curve of commercially used alloy to those of 10 mm hydrogen storage alloys. From Fig. 5A and B, it is seen that the sample of 10 mm #1 alloy
has a similar exothermic characteristic with the commercially used alloy (#4). It is consistent with the result that the two alloys have similar electrochemical performances, see Fig. 6. Fig. 6 gives the cyclic stability of the four kinds of alloys. It can be seen that the curves of #1 and #4 alloys get very close. The two alloys can be activated completely within seven charge–discharge cycles. They show similar maximum discharge capacity (276 mAh=g for #1 alloy and 285 mAh=g for #4 alloy) under 55 mA charge– discharge current, and the capacity decay seems very close. So it is presumed that the exothermic characteristic is one of the factors to a&ect the electrochemical performances of LaNi5 -type hydrogen storage alloys. In fact, the La-side of #3 alloy is also a commercially used material. From Fig. 5C and D, it is seen that #2 alloy has a similar DTA curve with that of #3 alloy. While their electrochemical properties are little consistent, see #2 and #3 in Fig. 6. Although both alloys can be activated within seven cycles, they show great di&erence in maximum discharge capacity (249 mAh=g for #2 alloy and 286 mAh=g for #3 one). This is attributed to the great di&erence of compositions between the two kinds of alloys. Moreover, the temperature characteristics of the peaks seem to be similar to #1 and #4 alloys, but the intensity is di&erent from the latter two. Fig. 7 is the DTA curves of 10 mm #1 alloy. The sample is heated, cooled and heated again. Fig. 7a shows the exothermic peaks during the temperature-rising process, while in Fig. 7b, all the peaks disappear after the alloy is cooled down and heated again. Then, the annealed sample is tested to demonstrate the above phenomenon, and the
C. Wu et al. / International Journal of Hydrogen Energy 28 (2003) 409 – 414
0.3
0.3 cd e
0.2
0.2 T/˚C
T/˚C
b 0.1 a
0.1
_ 0.1
b
_ 0.1
(A)
0 0.3
T/˚C
0.2 b
0.1
same result is obtained. It could be concluded that no irreversible phase transformation happened to the alloys during the whole heated–cooled period, because the curves verify an irreversible process.
_ 0.1 0.3 c
T/˚C
0.2
4. Conclusion
b
0.1
d
a 0.0 _ 0.1 (C) 0.3 c
T/˚C
0.2 b
0.1
d
a 0.0 _ 0.1 (D)
200 400 600 800 1000 1200 T/˚C
Fig. 7. DTA curves of 10 mm #1 alloy: a—the =rst temperature-rising curve and b—the second temperature-rising curve.
d e
c
a
0.0
0
200
400
600
800 1000 1200
T/˚C Fig. 5. DTA curves of 10 mm hydrogen storage alloys: A—#4 alloy, B—#1 alloy, C—#2 alloy, D—#3 alloy.
300 discharge capacity/mAh.g-1
a
0.0
0.0
(B)
413
In the DTA test of LPCNi3:55 Co0:75 Mn0:4 Al0:3 , =ve exothermic peaks appear in the DTA curves, in which the intensity of peaks b and c decreases strikingly with the decrease of cooling rate. It is straight related to the grain sizes. The smaller the grain sizes, the higher the intensity of the peaks will be. The intensity of peaks d and e shows less correlation with cooling rate, and they locate in the range of theoretical recrystallization temperature. It is possibly introduced by recrystallization. In addition, the elimination of segregation and the decrease of internal stress in the grains may also cause the exothermic peaks in DTA curves. There is no irreversible phase transformation in the hydrogen storage alloys during the whole heated–cooled–heated process, because no exothermic peaks appear during the second temperature-rising period. Four kinds of AB5 -type hydrogen storage alloys in our work show similar exothermic characteristics.
References
250 200
1#(10mm) 2#(10mm) 3#(6mm) 4#
150 100 0
50
100
150
200
250
Cycles
Fig. 6. The cyclic stability curves of hydrogen storage alloys under 55 mA charge–discharge current.
[1] Sakai Tetsuo, Uehara Ituki, Ishikawa Hiroshi. R& D on metal hydride materials and Ni-MH batteries in Japan. J Alloys Compounds 1999;293–295:762–9. [2] Sandrock G. A panoramic overview of hydrogen storage alloys from a gas reaction point of view. J Alloys Compounds 1999;293–295:877–88. [3] ZUuttel A, GUuther V, Otto A, BUartsch M, KUotz R, Chartouni D, NUutzenadel Ch, Schlapbach L. About the mechanism and the rate limiting step of the metalhydride electrode reaction. J Alloys Compounds 1999;293–295:663–9. [4] Okada M, Kuriiwa T, Tamura T, Kamegawa A, Takamura H. Crystal structure and protium absorption properties of La-rich
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C. Wu et al. / International Journal of Hydrogen Energy 28 (2003) 409 – 414 La(Ni; M)x (x = 3 − 4:7) (M = Al, Co, Mn, Si) melt-spun ribbons. J Alloys Compounds 1999;293–295:130–4. Sakai T, Yoshinaga H, Migamura H, Kuriyama N, Ishikawa H. Rechargeable hydrogen batteries using rare-earth-based hydrogen storage alloys. J Alloys Compounds 1992;180: 37–54. Nakamura H, Nakamura Y, Fujitani S, Yonezu I. InVuence of annealing on hydrogenation characteristics and microstructure of LaNi4:55 Al0:45 alloy. J Alloys Compounds 1995;218:216–20. Guo JH, Jiang ZT, Duan QS, Liu GZ, Zhang JH, Xia CM, Guo SY. InVuence of Ce and Nd contents of Ml(NiCoMmAl)5 alloy on the performances of Ni-MH batteries. J Alloys Compounds 1999;293–295:821–4. Ye H, Zhang H, Cheng JX, Huang TS. E&ect of Ni content on the structure, thermodynamic and electrochemical properties of the non-stoichiometry hydrogen storage alloys. J Alloys Compounds 2000;308:163. Li Rong, Wu Jian-min, Wang Xin-lin. E&ects of AB5 -type hydrogen storage alloy prepared by di&erent techniques on
[10]
[11]
[12] [13]
the properties of MH=Ni batteries. J Alloys Compounds 2000;311:40–5. Notten PHL, Einerhang REF, Daams JLC. On the nature of the electrochemical cycling stability of non-stoichiometric LaNi5 -based hydride-forming compounds. Part I. Crystallography and electrochemistry. J Alloys Compounds 1994;210: 221–32. Li Chuan-Jian, Wang Feng-Rong, Cheng Wen-Hao, Li Wei, Zhao Wen-Tong. The inVuence of high-rate quenching on the cycle stability and the structure of the AB5 -type hydrogen storage alloys with di&erent Co content. J Alloys Compounds 2001;315:218–23. Obe Hakutalo, editor. Liu Zhenhai, translator. Thermoanalysis. Beijing: Chemical Engineering Publishing House, China, 1982. p. 114 (in Chinese). Li Quan’an. The study and development of LPC hydrogen storage alloy for MH electrode. PhD thesis, Sichuan university, 2000 (in Chinese).
International Journal of Hydrogen Energy 28 (2003) 409 – 414 www.elsevier.com/locate/ijhydene
Di&erential thermo-analysis (DTA) method in strip casting AB5-type hydrogen storage alloys Chaoling Wua; ∗ , Yungui Chena , Feng Lia , Dingxiang Tangb , Chuan Zhouc , Mingjing Tua a The
Department of Metallic Materials, School of Materials Science and Engineering, Sichuan University, Chengdu 610065, China b Research Centre of Rare Earth Materials and Applications, Chengdu 610065, China c The Department of Chemical Engineering and Mechanism, School of Chemical Engineering, Sichuan University, Chengdu 610065, China
Abstract In this work, strip casting was introduced, and the necessity of heat treatment was explained from the viewpoint of DTA thermo-analysis. The results showed that: Five exothermic peaks appeared in the DTA curves of LPCNi3:55 Co0:75 Mn0:4 Al0:3 alloy in which the intensity of the second and third peak decreased strikingly with the decrease of cooling rate. It was sensitive to the grain sizes. The intensity of the fourth and =fth peak showed less correlation with cooling rate, and they located in the range of theoretical recrystallization temperature. It was possibly introduced by recrystallization. In addition, segregation and internal stress also contributed to exothermic peaks. All heat variation had =nished during the =rst temperature-rising period of DTA test, which implied that there was no irreversible phase transformation in the hydrogen storage alloys during the whole heated-cooled process. Four kinds of AB5 -type hydrogen storage alloys in our work showed similar exothermic characteristics. ? 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. Keywords: Di&erential thermo-analysis; Strip casting; Hydrogen storage alloys; Heat treatment; Rare earth
1. Introduction LaNi5 -type hydrogen storage alloys have been deeply investigated [1–3]. They drew most researchers’ attention, owing to the environmental friendship, comparatively low cost, and relatively superior comprehensive performances. However, there were still some problems to be solved, including lowering the cost further [4,5], improving the high-rate capabilities [6 –9], low-temperature performances and cyclic stabilities [9 –11]. Strip casting was expected to be a method for hydrogen storage alloys to obtain superior comprehensive properties, including high activation property, high discharge capacity ∗ Corresponding author. Tel.: +86-28-85407335; fax: +86-28-85407442. E-mail address: [email protected] (C. Wu).
and cyclic stability, etc. while lowering the cost. Furthermore, heat treatment was unavoidable in the manufacturing process of commercial hydrogen storage alloys. But few theories on it have been found at present. In this paper, strip casting was introduced, and the necessity of heat treatment was explained from the viewpoint of thermo-analysis. 2. Experimental Four kinds of hydrogen storage alloys were investigated, see Table 1. Alloys #1; #2 and #3 were cast into a copper mould. Their thickness were 1; 3; 6 and 10 mm respectively. The as-cast three samples were grinded into both 200 and 400 meshes, respectively. Then, the above three kinds of 200-mesh alloys with #4 commercial alloy were tested by
0360-3199/02/$ 30.00 ? 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 0 2 ) 0 0 0 8 6 - 1
410
C. Wu et al. / International Journal of Hydrogen Energy 28 (2003) 409 – 414
Table 1 The alloys and the thickness of their ingots Alloys
#1
#2
#3
#4
Contents
LPCNi3:55 Co0:75 Mn0:4 Al0:3
LPCNi3:55 Co0:4 Mn0:4 Al0:3 Cu0:15 Fe0:1 Si0:1
MmNi3:55 Co0:75 Mn0:4 Al0:3
Thickness (mm)
1
1
1
Commercially used alloy (Shen Jiang, made in China) —
3
6
10
3
6
10
3
6
10
Statement: LPC = 77–84% La + 5–10% Ce + 9–12% Pr; Mm = 27–32% La + 43–56% Ce + 5–7% Pr.
T/˚C
0.4
0.2 d
0.1 a
0.0 _ 0.1 (A) 0.3
b
c de
T/˚C
0.2 0.1 a
0.0 _ 0.1 (B) 0.3 T/˚C
0.2
cd
b
0.1
e
a
0.0 _ 0.1 (C) 0.3
3. Results and discussion The exothermic peaks are possibly caused by disequilibrium in the crystals [12], such as the crystal defects, recrystallization, and=or the phase transformation, etc. Fig. 1 is the DTA curves of #1 alloy with di&erent thickness in temperature rising process. The curves show =ve exothermic peaks a–e in turn. Peak a shows relatively lower intensity than others. And in Fig. 1B and C, it is lapped by peak b. It is assumed that peak a is introduced by unavoidable mechanical stress during powder making process. It is obvious that the intensity of peaks b and c decreases strikingly with the increase of the thickness of alloy ingots. In other words, they are rather sensitive to cooling rates. In our experiment, the cooling rate of 1 mm sample is 103◦ C=s in order, and that of 10 mm one is 102◦ C=s.
c
b
0.3
T/˚C
DTA method. In the test, DuPont 2100-type DTA instruments were used. The sample cup was platinum crucible (D = 3 mm). The thermocouples were platinum–platinum ◦ 13% rhodium. The LT sensitivity was 0:001 C. And the ◦ temperature precision was ±2 C. Simultaneously, #1; #2 and #3 alloys (400 mesh) were tested by D=max-rA X-ray di&raction instrument, with Cu K di&raction. The former three ingots were also tested by Hitachi-650 scanning electronic microscopy (SEM) and EDAX9100 energetic dispersive spectrum instrument (EDS). The mixture of 200- and 400-mesh powder was mixed with copper powder (wt% = 1:2) and made into a pellet ( = 10 mm; thickness = 1 mm) to make the cathode of a half-cell, whose anode was Ni(OH)2 , the electrolyte was 6 N KOH, and the reference electrode was Hg=HgO. Then, the half-cells were tested by a DC-5 battery testing instrument. First, the electrodes were charged at a current of 11 mA for 7:5 h, then discharged at the same current to 0:6 V vs. Hg=HgO. After several charge–discharge cycles, the cathode reached its highest discharge capacity, i.e. it =nished the activation process. Second, the electrodes were charged at the current of 55 mA for 1:3 h, and then discharged at the same current to 0:6 V vs. Hg=HgO. The DC-5 instrument recorded all the data during the whole charge–discharge cycles.
de
0.2 b
0.1
c
a
0.0 _ 0.1 (D)
0
200
400 600 800 1000 1200 T/˚C
Fig. 1. DTA curves of #1 alloy with di&erent thickness in temperature-rising process: A—1 mm, B—3 mm, C—6 mm and D—10 mm.
Fig. 2 shows the microstructures of #1 alloy ingots with di&erent thicknesses. It is seen clearly that the grain sizes of the ingots become larger with the increase of the thickness. Thus, it is concluded that peaks b and c are sensitive to the grain sizes, i.e. the areas of the grain boundaries. The larger the areas of the grain boundaries, the higher the intensity of the exothermic peak will be.
C. Wu et al. / International Journal of Hydrogen Energy 28 (2003) 409 – 414
411
Fig. 2. The SEM pictures of #1 alloy with di&erent thickness: (a) 1 mm, (b) 3 mm, (c) 6 mm and (d) 10 mm.
◦
Fig. 3. The SEM pictures of the section of #1 alloy (as-quenching samples): (a) as-cast, (b) annealed at 900 C [13].
The intensity of peaks d and e varies less in the curves, and they seem to be determined by temperatures rather than cooling rates. Peak d is lapped by peak e in Fig. 1A, and it tends to go left with the increase of the thickness of ingots. They are possibly introduced by recrystallization mainly. Fig. 3 shows that the as-quenching #1 sample recrystallized ◦ after heat-treated at 900 C. It indicates that recrystallization
does exist during temperature-rising period. And according to the well-known equation: Tr = (0:35–0:45)Tm
(1)
in which Tr represents recrystallization temperature, and Tm represents melting temperature, it =ts quite well that peaks d and e locate in the range of recrystallization temperature,
412
C. Wu et al. / International Journal of Hydrogen Energy 28 (2003) 409 – 414
Table 2 ◦ Distribution of elements of 700 C annealed and as-cast 10 mm LPC Ni3:55 Co0:75 Mn0:4 Al0:3 hydrogen storage alloys Main elements
Mn
◦
Annealed at 700 C as-cast
Al
Co
Ni
La
Ce
Pr
m
b
m
b
m
b
m
b
m
b
m
b
m
b
6.36 2.79
8.39 10.55
8.08 4.49
4.86 3.36
13.13 13.03
13.96 12.36
56.36 64.58
53.85 55.01
4.99 4.51
5.67 4.34
8.55 8.29
10.21 12.83
1.90 2.09
1.31 1.49
Statement: m—within the grains, b—at the boundaries. Units: at%.
Intensity (CPS)
900 ˚C
as-cast
20
30
40
50
60
70
80
90
Diffraction Angle 2θ / (°) ◦
Fig. 4. The di&erence of XRD curves between as-cast and 900 C annealed 10 mm LPCNi3:55 Co0:75 Mn0:4 Al0:3 hydrogen storage alloys. ◦
448–512 C, given that the melting temperature of #1 alloy ◦ is 1280 C. There exist other factors to cause the exothermic peaks in the DTA curves besides grain sizes and recrystallization. Table 2 shows the EDS results of distribution of main elements in as-cast and annealed 10 mm #1 alloy. It is seen that Mn has rather distinguished segregation in as-cast ingots, and other elements, such as Ce, also show a little ◦ segregation. After being annealed at 700 C, all elements tend to become homogenous except Al. The decrease of segregation must be one factor to introduce exothermic peaks in DTA curves. The exothermic peaks in DTA curves may also be caused by the diminishing of internal stress in the grains, see Fig. 4. It is seen that the XRD peaks of 10 mm #1 alloy which is an◦ nealed at 900 C (FWHM=0:188, FWHM means full-width at half-maximum of the main peak) is sharper than those of as-cast sample (FWHM=0:376). This is mainly contributed to the decrease of internal stress within the grains. In fact, the above processes may occur during annealing, which do good to the comprehensive performances of hydrogen storage alloys, for example, improving the cyclic stabilities of the alloys, as is popularly accepted by most researchers. Moreover, the characteristic temperature trend is to become lower, i.e. the curves go left with the decrease of the cooling rate. It is helpful to determine the annealing process in manufacturing. Fig. 5 is the comparison of DTA curve of commercially used alloy to those of 10 mm hydrogen storage alloys. From Fig. 5A and B, it is seen that the sample of 10 mm #1 alloy
has a similar exothermic characteristic with the commercially used alloy (#4). It is consistent with the result that the two alloys have similar electrochemical performances, see Fig. 6. Fig. 6 gives the cyclic stability of the four kinds of alloys. It can be seen that the curves of #1 and #4 alloys get very close. The two alloys can be activated completely within seven charge–discharge cycles. They show similar maximum discharge capacity (276 mAh=g for #1 alloy and 285 mAh=g for #4 alloy) under 55 mA charge– discharge current, and the capacity decay seems very close. So it is presumed that the exothermic characteristic is one of the factors to a&ect the electrochemical performances of LaNi5 -type hydrogen storage alloys. In fact, the La-side of #3 alloy is also a commercially used material. From Fig. 5C and D, it is seen that #2 alloy has a similar DTA curve with that of #3 alloy. While their electrochemical properties are little consistent, see #2 and #3 in Fig. 6. Although both alloys can be activated within seven cycles, they show great di&erence in maximum discharge capacity (249 mAh=g for #2 alloy and 286 mAh=g for #3 one). This is attributed to the great di&erence of compositions between the two kinds of alloys. Moreover, the temperature characteristics of the peaks seem to be similar to #1 and #4 alloys, but the intensity is di&erent from the latter two. Fig. 7 is the DTA curves of 10 mm #1 alloy. The sample is heated, cooled and heated again. Fig. 7a shows the exothermic peaks during the temperature-rising process, while in Fig. 7b, all the peaks disappear after the alloy is cooled down and heated again. Then, the annealed sample is tested to demonstrate the above phenomenon, and the
C. Wu et al. / International Journal of Hydrogen Energy 28 (2003) 409 – 414
0.3
0.3 cd e
0.2
0.2 T/˚C
T/˚C
b 0.1 a
0.1
_ 0.1
b
_ 0.1
(A)
0 0.3
T/˚C
0.2 b
0.1
same result is obtained. It could be concluded that no irreversible phase transformation happened to the alloys during the whole heated–cooled period, because the curves verify an irreversible process.
_ 0.1 0.3 c
T/˚C
0.2
4. Conclusion
b
0.1
d
a 0.0 _ 0.1 (C) 0.3 c
T/˚C
0.2 b
0.1
d
a 0.0 _ 0.1 (D)
200 400 600 800 1000 1200 T/˚C
Fig. 7. DTA curves of 10 mm #1 alloy: a—the =rst temperature-rising curve and b—the second temperature-rising curve.
d e
c
a
0.0
0
200
400
600
800 1000 1200
T/˚C Fig. 5. DTA curves of 10 mm hydrogen storage alloys: A—#4 alloy, B—#1 alloy, C—#2 alloy, D—#3 alloy.
300 discharge capacity/mAh.g-1
a
0.0
0.0
(B)
413
In the DTA test of LPCNi3:55 Co0:75 Mn0:4 Al0:3 , =ve exothermic peaks appear in the DTA curves, in which the intensity of peaks b and c decreases strikingly with the decrease of cooling rate. It is straight related to the grain sizes. The smaller the grain sizes, the higher the intensity of the peaks will be. The intensity of peaks d and e shows less correlation with cooling rate, and they locate in the range of theoretical recrystallization temperature. It is possibly introduced by recrystallization. In addition, the elimination of segregation and the decrease of internal stress in the grains may also cause the exothermic peaks in DTA curves. There is no irreversible phase transformation in the hydrogen storage alloys during the whole heated–cooled–heated process, because no exothermic peaks appear during the second temperature-rising period. Four kinds of AB5 -type hydrogen storage alloys in our work show similar exothermic characteristics.
References
250 200
1#(10mm) 2#(10mm) 3#(6mm) 4#
150 100 0
50
100
150
200
250
Cycles
Fig. 6. The cyclic stability curves of hydrogen storage alloys under 55 mA charge–discharge current.
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