Hydriding and dehydriding reaction rate of FeTi intermetallic compound

Hydriding and dehydriding reaction rate of FeTi intermetallic compound

Im. 1. Hydrogen Energy, Vol. 10, No. l/8, PP. 531442,lQ~. Primed in Great Britain. HYDRIDING 03s31QQp35 $3.00 + 0.00 Pergamon Press Ltd. @ lQg5 Inte...

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Im. 1. Hydrogen Energy, Vol. 10, No. l/8, PP. 531442,lQ~. Primed in Great Britain.

HYDRIDING

03s31QQp35 $3.00 + 0.00 Pergamon Press Ltd. @ lQg5 Intemational Association for Hydrogen Energy.

AND DEHYDRIDING REACTION INTERMETALLIC COMPOUND

RATE OF FeTi

H. S. CHUNG and JAI-YOUNG LEE Department

of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Cheongyang, Seoul, Korea

P.O. Box 131,

(Receiued for publication 4 October 1984) Abstract-It is observed how the hydriding and dehydriding reaction rates of FeTi approach to the intrinsic ones as the heat effect is reduced sequentially by mixing the sample with Mn powder in a stainless steel reactor or using a specially designed Cu tube reactor with the mixtures. Nearly intrinsic hydriding rate data are obtained by mixing the sample with Mn powder in the Cu reactor. An empirical rate equation is deduced from the results and the equation is helpful in the expectation and analysis of the heat effect for a given set of experimental data. The dependences of the hydriding reaction rates on the hydrogen pressure and the hydrogen concentration is analysed, and it is verified that the kinetic behavior changes with temperature. Heat effect is more pronounced in the dehydriding reaction than the hydriding. Dehydriding rate data with a minimum heat effect are presented.

NOMENCLATURE (Y B

R &l

metal phase in which hydrogen is dissolved as a solid solution monohydride phase of FeTi of which the composition is FeTi H-l applied hydrogen pressure equilibrium hydrogen pressure between (Y and /I phases. gas constant temperature atomic ratio of hydrogen to metal

The hydriding and dehydriding reaction rate and the temperature changes in the reactor are measured during the reaction, heat effect being sequentially reduced by mixing the sample with an inert metal powder (Mn in this work) and using a highly heat conductive Cu tube reactor with the mixture. It is observed how the heat effect influences the reaction rate at each test condition and how the kinetic data approach to the intrinsic ones as heat effect is reduced sequentially. It will be examined whether the experimental data in the Cu-tube reactor can be assumed to be the isothermal one. EXPERIMENTAL

INTRODUCTION attention has been paid to the FeTi intermetallic compound as a hydrogen storage alloy [l]. For the practical application of the alloy for hydrogen storage medium, understanding of the kinetic properties of the hydriding and dehydriding reaction is needed. It is most important to reduce the heat effect efficiently and thus to obtain the intrinsic rate data in order to study the hydriding and dehydriding kinetics. The reaction of (Y* /3transformation of FeTi generates heat of about +23 (hydriding) or -29 (dehydriding) kJ (molHr)-l [ 1,2,3], which is enough to cause the temperature of the sample to change over 200 or 300°C in an adiabatic condition (heat capacities of FeTi and FeTiH are 0.468 and 0.518 J (g”C)-’ (41). Because the reaction rate is fast (usually over half of the reaction is finished within 1 min) and the heat conductivity of the powder bed is poor, it is impossible to get rid of the heat effect thoroughly in an ordinary test condition, therefore the heat effect should be expected for a given set of kinetic data and it must be determined whether the assumption of isothermal condition is reasonable or not as a preliminary step towards the understanding of the intrinsic kinetics. Much

PROCEDURES

Details of the experimental procedures are found in ref. [S] . The compositions of the specimen are FeTi and FeTi1.w. It is verified that almost all of the samples are in a single phase range of FeTi and a small amount of oxide phase exists by X-ray diffraction and optical microscopy. PCI curvesof FeTi1.w are similar to those of FeTi except that the plateau pressure is somewhat lowered and sloped. The reaction rate is nearly the same with each other. Two types of reactors used in the experiment are shown in Fig. 1. In the reactor (a), S.S. reactor, about 1 g of FeTir.or and 1 g of FeTil.m mixed with 6- 7 g of Mn powder, is hydrided and dehydrided, respectively, temperature sweep in the reactor being measured. 0.8 g of FeTil.04, mixed with about 30g of Mn, is hydrided and dehydrided in the reactor (b), Cu tube reactor. The size of FeTi1.w particles is below 200 mesh and that of Mn, -100, +200 mesh. Smaller size of Mn particles may inhibit gas flow, while increasing the heat conductivity by raising the contact area. The sample was mixed with Mn powder after Mn had been fully baked at 4OO’C under vacuum in order to remove impurities on Mn particles. Kinetic experiment was carried aut after the 537

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H. S. C H U N G A N D J A I - Y O U N G L E E

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sample had been fully activated. Hydrogen pressure was in the range of ~ ~ ~ transformation. RESULTS A N D DISCUSSION

Fig. 3. Changes of the dehydriding reaction rate and the temperature in the reactor with (H/M). FeTil.04, mixed with Mn powder, reacted at 30°C of bath temperature in the S.S. reactor. The reaction started from HIM = 0.50:1 and A: p~ = 0.40 arm, 2 and O: P~ = 0.99 aim, 3 and &: P~ = 1.56 arm, 4 and O: P~ = 2.13 atm. Pn/M - 0 - P, = 0.56 arm.

Figures 2 and 3 represent the variations of the hydriding and dehydriding reaction rates and the temperature in the reactor with HIM(atomic ratio of hydrogen to metal) when FeTit.04, mixed with Mn, reacted in the

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Fig. 2. Changes of the hydfidmg reaction rate and the ternperature in the reactor with HIM. FeTiL04, mixed with Mn powder, reacted at 30°C of bath temperature in the S.S. reactor. 1 and O: P~ = 31.65 arm, 2 and Z~: P~ = 25.4 atm, 3 and O: P~ ffi 19.6 arm; P, is the initially applied hydrogen pressure; and P~ - PH/M- 0.50 = 4.58 atm.

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Fig. 4. Maximum temperature changes in the reactor vs initially applied hydrogen pressure.

HYDRIDING AND DEHYDRIDING REACTION OF FeTi S.S. reactor. Figure 4 shows the relation between the maximum temperature changes in the reactor and the initially applied hydrogen pressures when the sample reacts in the S.S. reactor and Cu tube reactor, respectively. The higher the initially applied hydrogen pressure, the faster the reaction rate and the bigger the maximum temperature increase in the reactor. The temperature change is lowered significantly by mixing the sample with Mn powder in the S.S. reactor. When the sample reacts in the Cu reactor, the temperature change is detected to be very small, less than 2"C. It cannot be excluded that the temperature change of the sample itself might be somewhat bigger than the detected value because the ratio of Mn/FeTi is very high. Since the hydriding reaction is an exothermic reaction and a thermally activated process, the heat effect results in the increase of the mobility and in the decrease of the driving force of the reaction by raising the plateau pressure. Thus the hydriding reaction rate may increase or decrease by the heat effect. In the dehydriding reaction, however, the heat effect decreases the driving force and the mobility of the reaction at the same time, resulting in drastic decrease of the reaction rate. It can be known in Fig. 3 that the initial abrupt decrease of the reaction rate is at least partially due to the heat effect. Figures 5 and 6 represent the dependences of the hydriding reaction rate on the hydrogen pressure in the Cu reactor. The hydriding reaction rate in the S.S. reactor, which is influenced significantly by the heat effect, shows linear dependence on the hydrogen pressure in the whole range of the reaction. The results of Figs 5 and 6, which show linear dependence of the rate on the applied pressure at the high temperatures and

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Fig. 6. Hydriding reaction rate vs hydrogen pressm'e. FeTil.0,, mixed with Mn powder, reacted in the Cu tube reactor. HIM = 0.40. Dotted lines represent the results of rbest fittings among the linear regressions of rate vs Pro, VP.2 and In P.2 (In PH2at 0, 25 and 40°C and 1~ 2 at 50 *C).

low HIM and parabolic dependence at the low temperatures and high H/M, are obtained by reducing the heat effect and this represents the intrinsic kinetic behavior. In order to compare the heat effect at each test condition, linear dependence of the reaction rate on the hydrogen pressure is assumed in the whole range of the reaction at all test conditions and the apparent rate constant is defined as the slope of the linear regression of the reaction rate vs PH2. The apparent rate constant is only meaningful as a parameter which H I M :0.12 50"C enables us to compare the reaction rates at different test conditions when (P,2- P~) is identical, where Peq O is the plateau pressure when the temperature is the same as the bath temperature. Therefore the apparent *.trate constant is a useful parameter to compare the heat 40"C O ~ effect at each test condition with one another. Figures OeA 7, 8 and 9 show how the kinetic behavior of the hydriding reaction approaches to the intrinsic one as the heat o. effect is diminished sequentially. Figure 7 implies that 2. negative activation energy is observed when the temperature change is very big. This phenomefion is due 25"C 4 to the increase of the width of the change 0f plateau -,c pressure per unit temperature change with tile temperature. For FeTi, the values of dP~/dT are O.147, 0.297, 0.53 and 0.716 atm/°C -] at 0, 25, 50 and 65°C, respeci 101 i 210 , 310 * tively [2]. Thus the reduction of the driving fOrce of the Hydrogen pressure (,',';m) reaction due to the heat effect dominates over the Fig. 5. Hydriding reaction rate vs hydrogen pressure FeTid.0,, increase of the mobility at the higher temperatures when mixed with Mn powder, reacted in the Cu-tube reactor. H/M the temperature change is large, resulting in a negative apparent activation energy. = 0.12.

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Fig. 9. Variations of the apparent rate constants with HIM. Fe.Til.04, mixed with Mn powder, reacted in the Cu-tube reactor.

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Fig. 7. Variations of the apparent rate constants with FeTil.04 reacted in the S.S. reactor.

HIM.

Figure 8 indicates that the reaction rate increases at high temperature and decreases at low temperatures as heat effect is reduced. At 65"C, the heat effect still dominates over the intrinsic kinetics. Comparing Fig. 9 with Fig. 8, the largest difference appears in the middle range of the reaction, which agrees well with that the heat effect is the largest in that region (Fig. 2). Considering that the result of Fig.



8 is obtained when the temperature change in the reactor is less than about 10°C, it is concluded that Fig. 9 represents nearly the intrinsic rate data. It was mentioned previously that the temperature change in the Cu reactor was measured to be very small (less than 2°C). Arrhenius plot of the data in Fig. 9 yields the average value of 23.8 kJ (tool H2) -~ of the apparent activation energy. The following empirical rate equation is given. Rate = A (Pro - Peq) exp (-23800/RT)

(1)

where A is a constant depending on HIM and R is gas constant. Equation (1) is plotted in Fig. 10, which relates

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Fig. 10. Variations of the hydrogen absorption rate with the temperature in the reactor at constant hydrogen pressure.

HYDRIDING AND DEHYDRIDING REACTION OF FeTi the reaction rate to the temperature at constant hydrogen pressures. Van't Hoff relation of Pcq is quoted from ref. [2]. Figure 10 is helpful in the qualitative expectation of the heat effect for a given experimental condition. The dependence of the hydriding reaction rate must be observed in the direction of A in Fig. 10, however the heat effect causes the observation to be done in the directions of B's, which is determined by the heat transfer capabilities of the reactor. When the temperature change is so large that heat transfer is rate controlling, the reaction rate should be deduced from heat transfer equations rather than Fig. 10. At low temperatures of 0 and 25"C, the reaction rate (the apparent rate constant) can be increased by the heat effect. At the higher temperatures, the reaction rate cannot be increased by the heat effect. The higher the temperature, the more drastic the decrease of the reaction rate due to the heat effect. Initial increase of the rate constant with HIM at high temperatures, 40 and 50°C, must be the intrinsic properties. At low temperatures, the reaction rate increases by the heat effect at higher hydrogen pressures more than at low pressures. The intrinsic parabolic like dependence of the reaction rate on the hydrogen pressure is appeared to be linear due to the heat effect in the S.S. reactor. Both the dependence of the reaction rate on the hvdrolzen pressure and the dependence of the rate constant on HIM changes with temperature in Figs 5, 6 and 9. These results indicate that the overall reaction rate is mixed rate controlled by two simple reaction steps and that the contribution of each step reaction to the overall reaction rate is competitive with each other and is determined with temperature. Figure 11 shows that the dehydriding reaction rate is linearly proportional to X/-Pm at HIM = 0.20. Dehydriding run is started from HIM = 0.52. Arrhenius plot of the results in Fig. 11 yields the following empirical rate equation. Rate = m' (X/-P-~q- V'P"~.~)exp

(-19870/RT),

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Fig. 11. Dehydriding reaction rate vs hydrogen pressure. Stoichiometric FeTi, mixed with Mn powder, reacted in the Cutube reactor. HIM = 0.20, (H/M)i,,ti,l= 0~52.

(2)

where A ' is a constant. Equation (2) is plotted in Fig. 12. The heat effect causes the observation of the dependence of the dehydriding reaction rate on the hydrogen pressure to be done in the directions of B's rather than A. It can be known that the dehydriding reaction rate is reduced drastically by the heat effect even for a relatively small temperature change in the reactor.

|

CONCLUSION Nearly isothermal intrinsic kinetic data are obtained by mixing FeTi with Mn powder in a Cu tube reactor. Intrinsic hydriding reaction rate shows parabolic like dependence on the hydrogen pressure at low temperature and high H/M, and linear dependence at high temperature and low HIM. The hydriding rate constant increases initially and then decreases with HIM at high

0

20

40

temperoture ('C }

60

Fig. 12. Variations of the dehydriding reaction rate with temperature at constam hydrogen pressure.

542

H. S. CHUNO AND JAI-YOUNG LEE

temperatures of 40 and 50"C, while it decreases mono,tonieally at the lower temperatures. The higher the temperature, the bigger the heat effect and excessively high temperature must be avoided in order to study intrinsic kinetics. In the dehydriding reaction, heat effect is more drastic than in the hydriding reaction. REFERENCES 1. J. J. Reilly and R. H. Wiswall, Jr., Inorg. Chem. 13, 218-222 (1974).

2. P. D. GoodeU and G. Sandrock, Metallurgical Studies of Hydrogen Storage Alloys, Final Report for U.S. Department of Energy, under BNL Contract No. 451117-S, (April 1980). 3. M. H. IVfintz et al., I. appl. Phys. 52, 463-467 (January 1981), 4. H. Wenzl and E. Lebsan~, J. Phys. F: Metal Physics, 10, 2147-56 (1980). 5. C. N. Park and Jai-Young Lee, J. Less Common Metals, 18, No. 2 (1983). 189-201