Studies on the hydrogen absorption properties of LaNi - Springer Link

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Also, a thermodynamic expression of equilibrium is applied ... The PCT diagrams of ..... Plot of equilibrium hydrogen plateau pressure (in atm) as function.
METALS AND MATERIALS International, Vol. 11, No. 3 (2005), pp. 241~248

Studies on the Hydrogen Absorption Properties of LaNi Sn and La Ni Sn Alloys Using Magnetic and Thermoelectric Power Measurement 5-x

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Yeong-Do Park * and Brajendra Mishra

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Materials Research Team, Hyundai·Kia Motors, 772-1 Jangdeok-dong, Hwaseong-si, Gyeonggi 445-706, Korea Department of Metallurgical and Materials Engineering, Colorado School of Mines Golden, Colorado 80401, USA 1

2

Magnetic and thermoelectric power (TEP) measurements are used to quantify hydrogen availability, absorption, and desorption in materials for nickel-hydride batteries and hydrogen storage. The effect of the concentration of conduction electrons is introduced and used to investigate hydride properties of these alloys. To investigate hydrogenation properties the calculated average number of d-band electrons in hydrogen storage material is related to the measured thermoelectric power. Also, a thermodynamic expression of equilibrium is applied to demonstrate the relationship between the TEP and the activity of hydrogen for hydrogen adsorption. Magnetization decreases with increasing absorbed hydrogen in the stoichiometric LaNi Sn alloy. However, magnetization increases with increasing hydrogen content in the nonstoichiometric La Ni Sn alloy. The TEP for the stoichiometric LaNi Sn alloy monotonically increases as a function of hydrogen content. However, the TEP of the nonstoichiometric La Ni Sn alloy decreases as hydrogen content increases. hydrogen storage alloys, cell volume, magnetic measurement, TEP measurement, electronic band structure 4.78

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Keywords:

1. INTRODUCTION

Metal hydride technology has been extensively studied because of its potential future impact on energy economics. Rapid development of metal hydride alloys started in 1970 after the discovery of two high density hydrogen intermetallic compounds (LaNi5 and FeTi) [1,2]. Numerous types of metal hydride alloys have been found and developed since then and extensive research efforts characterized and created a collection of property databases for each metal hydride family [3-5]. The relationship of the hydride formation to the electronic band structure has been considered and reviewed by Gelatt . [6,7]. Hydrogen addition changes the Fermi energy level and the band structure of metals, and as such this property has motivated many studies on the electronic properties of metal hydrides. Termsuksawad [8] and Niyomsoan [9] introduced an electronic measurement technique to investigate hydrogenation properties for AB5 and AB2 type hydrogen storage alloys. Bernauer [10] showed that the amount of absorbed hydrogen in a transition metal directly relates to the -band electron concentration of the host metal. It is believed that et al

et al.

d

*Corresponding author: [email protected]

the alloying elements play an important role of forming metal hydride and creating a bonding band with hydrogen. Also, the electronic structure and electronic properties of intermetallic compounds have been studied and reviewed by several scientists [11-13]. Useful room temperature metal hydride materials must easily absorb and desorb hydrogen. The PCT diagrams of many systems were studied to identify the absorption properties of metal hydrides. Structural approaches have been employed to predict hydrogen absorption and desorption properties. It was found that there is also a geometric factor that affects the hydrogen absorption properties. Mendelsohn . [14] reviewed several studies and established a relationship between the cell volume and the plateau pressure on the PCT diagram for most AB5 compounds. The study of the cell volume, therefore, can provide useful information in identifying better candidates for hydrogen storage materials. Bernauer . [10] proposed that the capacity for hydrogen absorption of transition metal alloys is related to the -band electron concentration. It is determined by: =5(1) where is the hydrogen to metal ratio. is the -electron concentration, which is calculated by: et al

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3d

d

H/M

H/M

DEC

DEC

d

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of d – band electron----------------------------------------------------------DEC= Sum Number of metal atoms

(2)

From this model, the absorbed hydrogen will be stored until the 3d-band is half full. Consequently, the model is valid only when the d-band electron concentration of the host metal is less than half filled. A review of previous studies [8,9] indicates that electronic properties, such as d-band electron concentration, affect the hydrogen absorption properties. Therefore, the electronic measurement of metal hydrides can be an effective tool to classify hydrogen storage materials. Two electronic property measurement techniques, thermoelectric power (TEP) and magnetic measurement, were applied in the present study. The magnetic measurements were used to determine the effect of absorbed hydrogen on saturation magnetization, magnetic susceptibility, and magnetic hysteresis. Absorbed hydrogen in transition-metal alloys is present in both metal hydride and soluble hydrogen atoms [7]. Hydrogen, either an electron acceptor or donor, can be correlated to whether a specific material has a hydride-forming tendency or hydrogen solubility behavior. If the electrons from hydrogen have the same spin as the majority of the electrons in the d-band in the alloy, the magnetic moment will increase. The magnetic moment decreases when the electrons have the opposite spin. Expansion of the crystal lattice on the order of 6 % with increasing hydrogen content may also affect magnetic properties [16]. The presence of hydrogen in transition-metal alloys alternatively has been detected by changes in electrical resistivity [17]. Also, the thermoelectric power, an intrinsic property of a material, can characterize the electronic density of states at the Fermi energy level: the amount of electric charge and the type of charge carrier. The TEP (Z) is a material property and is defined as the ratio of the voltage difference and the temperature difference: Z = ΔV/ΔT (3) From the spherical parabolic band model, the TEP is determined by: k2B T 2 * ( 27.1 ) ⋅ ( r + 3 ⁄ 2 ) ⋅ ( m ⁄ h ) Z = ± -----------2 3 n e ⁄

(4)

where, n, r, m*, and T are the number of electrons per unit volume, the scattering parameter, the effective mass, and the absolute temperature, respectively. The TEP is not only a function of electron concentration, but also a function of geometry and cell volume. The cell volume of alloys is well known to affect the stability of hydrogen absorption of AB5 hydrogen storage [14,15]. The cell volume is also related with the Fermi energy, which is directly related to the number of electrons per unit volume; i.e., the higher the cell volume, the lower the hydrogen plateau pressure. Therefore, the TEP can serve as a measurement to identify sound hydrogen storage alloys. The objectives of the present study are 1) to find the theoretical relationship between d-band electron concentration and hydrogen absorption using DEC number and TEP, 2) to assess TEP and magnetic properties with the amount of hydrogen absorbed in AB5 type hydrogen storage alloys using TEP and magnetic measurements, and 3) to correlate hydrogen absorption behavior and TEP and magnetic properties.

2. EXPERIMENTAL PROCEDURES Five alloys, listed in Table 1, were fabricated by arc melting in a helium atmosphere. The AB5 stoichiometry was normalized with respect to the rare earth component(s) and the structural formula as determined from the crystal structure. The melting process was repeated several times followed by annealing at 1173 K for three days. The post-anneal alloys were characterized by X-ray diffraction, and the cell volume of each alloy was calculated. All measurements of asreceived condition were performed in powder size less than 45 m to eliminate any surface effects for XRD, TEP, and magnetic measurements. The hydrogen content of the alloys was determined by PCT isotherms in a calibrated-volume system. Gas-phase charging involved five activation (hydrogenation) cycles of full charge and discharge. After five cycles, the hydrogen charged alloys had particle sizes of 2 to 5 µm. After charging, alloys were “poisoned” by carbon monoxide to prevent hydrogen from diffusing out at room temperature. LECO hydrogen determinator (RH404) was used to measure amounts of absorbed hydrogen. The amount of μ

Lattice parameters and unit cell volume measured by XRD Sample ID Alloy Composition a (nm) c (nm) Cell Volume (nm 10− ) LaNi Sn 0.5039 0.4000 87.97 S10 S22 LaNi Sn 0.5051 0.4027 89.01 LaNi Sn 0.5089 0.4057 90.99 S40 LaNi Sn 0.5108 0.4074 91.91 S50 A2349 La Ni Sn 0.5062 0.4047 89.81 Lattice parameter previously determined–[17] Alloy prepared by Nuclear Research Center – Negev. Alloy prepared by Brookhaven National Laboratory. Table 1.

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Normalized Stoichiometry N/A N/A N/A N/A LaNi Sn 4.84

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Studies on the Hydrogen Absorption Properties of LaNi Snx and La Ni Sn Alloys Using Magnetic and Thermoelectric 5-x

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Schematic diagram of the TEP measurement system.

hydrogen was recorded in parts per million and later converted to hydrogen per formula unit (H/M) ratio. For magnetic measurement, magnetization hysteresis loops were measured with a transverse-field vibrating-sample magnetometer at room temperature in maximum applied fields of 0.8 T. Magnetization was computed as magnetic moment per unit mass of metal, not including hydrogen, and expressed in units of A·m /kg (equivalent to emu/g). The exclusion of hydrogen causes an uncertainty of 0.1 to 2 % in the mass determination. Uncertainty arising from systematic effects is estimated to be less than five percent of the reported values. A simple and flexible experimental apparatus (Fig. 1) consisting of two massive copper blocks maintained at different temperatures measured with thermocouples mounted inside the blocks was employed/fabricated to measure differential temperature. The powder sample is enclosed in a plastic tube between the ends of the copper blocks. The potential difference is measured across the two copper probes. Generally, good electrical and thermal contacts are needed between the sample surface and probes. TEP measurements on powder are a new advance in materials characterization. The measurement system was calibrated using a standard Constantan (−40 μV/K) specimen. The absolute TEP (Seebeck coefficient, Z ) of the alloy material was determined as (6) a Δ Δ Cu where Δ is the voltage difference measured between probes, is the temperature difference, and Cu is the Seebeck coefficient for copper, 1.83 µV/K at 300 K. The temperature difference was maintained around 10 K. The reported TEP results for hydrogen storage alloys are the average of the three measurements.

Relationship between DEC number and equilibrium pressure at H/M=0.5 for LaNi Sn hydrides, based on the data from Luo . [18]. Fig. 2.

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3. RESULTS AND DISCUSSION 3.1. DEC model

Using the DEC ( -band electron concentration) model, the d

absorbed hydrogen will be stored until the -band is half full. Consequently, the model is valid only when the -band electron concentration of the host metal is less than half filled. Therefore, the DEC number can be used to predict the stability of hydrogen absorption for LaNi Sn . Fig. 2 shows the relation between the known equilibrium pressure of LaNi Sn alloys at H/M=0.5, based on equilibrium pressure data from Luo . [18], are the alloys’ calculated DEC numbers. Lnear relationships between DEC and the natural logarithm of equilibrium pressure were found. The selection of H/M=0.5 places an alloy at about the middle of the plateau of the PCT diagram. The equilibrium hydrogen pressure (lnP) decreases with DEC number. Equilibrium hydrogen pressure (lnP) linearly decreased and cell volume increased with increasing amounts of tin. Therefore, the equilibrium pressure is inversely proportional to the cell volume, as shown in Fig. 3. According to Adzic . [19], the equilibrium hydrogen pressure of La Ce B hydrides decreases directly with the cerium content. Additionally, Mendelsohn . [14] pointed out that the cell volume is inversely proportional to the natural logarithm of pressure. Results of correlation between equilibrium hydrogen pressure and cell volume for LaNi Sn alloy show good agreement with other studies. TEP measurements of LaNi Sn alloys before hydrogen absorption in powder form were conducted. The error of the TEP measurement was found to vary inside five percent. Fig. 4 shows the relationship between the TEP, composition, and cell volumes of LaNi Sn . This figure shows that TEP and cell volume is proportional to the composition of the 3d

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Plot of equilibrium hydrogen plateau pressure (in atm) as function of cell volume for LaNi Sn alloy (x=0.1, 0.22, 0.4, and 0.5). Fig. 3.

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The Equilibrium hydrogen plateau pressure (in atm) versus the TEP of LaNi Sn (x=0.1, 0.22, 0.4, and 0.5) at 300 K. Fig. 5.

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attempted using thermodynamic models [9], and further modification has been made in this paper. A thermodynamic expression of equilibrium can be applied to demonstrate the relationship between the TEP coefficient ( ) and the activity of hydrogen. Beginning with the first law of thermodynamics, Z

dE

=

dq



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–δ

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where is heat, is pressure, is volume, and ext is the external work done by the system. Assuming a reversible process: q

P

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w

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dq=TdS

Hence, the first law becomes: dE

The relationship of the compositions, TEP, and cell volumes for LaNi Sn . Fig. 4.

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alloys. As shown in Fig. 3, cell volume is inversely proportional to the lnP (equilibrium hydrogen pressure) for LaNi5 Sn alloys; therefore, the TEP is inversely proportional to lnP, as shown in Fig. 5. -x

3.2. Thermodynamic relationship

=

TdS



PdV

–δ

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w ext

At constant pressure ( =0), enthalpy is introduced as the sum of the internal energy and the product of pressure and volume ( ). Differentiating enthalpy and incorporating into Eq. 7 gives: dP

H=E+PV

x

It is important to determine the thermodynamic relationship between the thermoelectric power coefficient and equilibrium hydrogen pressure. Attempts to identify/ elucidate the relationship between TEP and lnP have been

dH

=

TdS

–δ

(10)

w ext

Eq. 8 is inserted into the differential Gibb’s free energy ( − − ), given as: dG=dH

dG

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= –δ

w ext

TdS



SdT

+ ∑ μi

ni

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where Σµi i is the stoichiometric constant accounting for the n

Studies on the Hydrogen Absorption Properties of LaNi Snx and La Ni Sn Alloys Using Magnetic and Thermoelectric 5-x

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addition of alloying elements (hydrogen) and µ =µ +RT lna. In a reversible process, the external work term is equivalent to the product of the number of moles of transported electrons, Faraday’s constant (96.487 Coulombs per electron equivalent), and electric potential. The thermoelectric power coefficient (Z) involves the generation of an electromotive force (external work) under an applied temperature difference (ΔT), given as: o

i

ΔV = Z ΔT

i

(12)

Now assume that the external work is electric work, and hence the external work term becomes: δwext = nF( ZΔT )

(13)

Eq. 11 then becomes: dG = –nF( Z ΔT ) – SdT + ∑ μini

(14)

Assuming equilibrium conditions, ΔG = 0, and solving Eq. 14 for the thermoelectric power coefficient gives: –S + ------------∑ μin-i (15) Z = -----nF nFΔT Because the thermoelectric power coefficient measures the change from the reference TEP, it is important to note that the reference thermoelectric power coefficient (Z ) as a function of entropy before the addition of hydrogen is given as: 0

–S Zo = -----nF

(16)

Then, when hydrogen is added to the metal, the thermoelectric power technique measures the change in the thermoelectric power coefficient, which is ΔZ=Z−Z , so that the actual thermoelectric power coefficient measurement is: ∑ μin-i ΔZ = ------------(17) nFΔT Eq. 17 is then considered for two regions of the pressurecomposition-temperature diagram (PCT). There are two different regions making up the PCT diagram: (1) alpha-phase region, which is interstitial hydrogen, (2) beta-phase region, which is formed hydrides. (Alpha-Phase Region) First, consider the alpha-phase region, where the appropriate reaction is given as: 0

H2 ( g ) = 2H ( M )

(18)

H(M) is a hydrogen atom in solid solution in the metal matrix, M. The thermoelectric power coefficient for the alpha-phase region derived from Eq. 17 is: ΔGo- + ------------RT - ( ln [ a ]2 – ln [ p ] ) ΔZα = ------------(19) H H nF ΔT nFΔT 2

Desorption isotherms for alloys shown. The hydrogen content for nonstoichiometric was calculated using the respective normalized formulae shown (see Table 1). Fig. 6.

(Beta-Phase Region) The thermoelectric power coefficient must be determined in the beta-phase region in order to find the thermoelectric power coefficient for the (alpha+beta)phase region. The reaction for the beta-phase region (hydride formation) is derived from Eq. 17 and is given as: 2--- M + H ( g) → 2--- MH (20) 2 x x x The thermoelectric power coefficient is given as: ΔGo- + ------------RT - ( ln [ p ]–1 ) (21) ΔZβ = ------------H nF ΔT nFΔT 2

3.3. Hydrogen isotherms for LaNi Sn and La Ni Sn 4.78

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Absorption isotherms obtained after five activation cycles of the LaNi Sn and La Ni Sn alloys are presented in Fig. 6. The data from these measurements show that the gas phase hydrogen absorption capacities of La Ni Sn alloy are marginally lower than those of the LaNi Sn alloy in that the equilibrium plateau pressure of the non-stoichiometric alloy (La Ni Sn ) is less than one atm. Despite its small loss of hydrogen absorption capacity, the non-stoichiometric composition of the La Ni Sn alloy is known to yield good life cycle performance. It decreases the tendency of the alloy to corrode, thereby increasing its lifespan. 4.78

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3.4. Magnetic measurement

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The magnetization and the susceptibility curves of LaNi Sn with the amount of absorbed hydrogen after 4.78

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netic moment decreases. As the magnetic moment decreases in transition alloys, the magnitude of magnetic susceptibility will decrease. For AB5 type hydrogen storage alloys, additional electrons from hydrogen have the opposite spin and reduce the magnetic moment and lower the magnitude of magnetic susceptibility, as seen in Fig. 8. The other explanation for the reduction of the magnetization with the absorbed hydrogen can be the reduction of the number of electrons in the conduction band. According to Nakamura . [20], the soluble hydrogen gives an electron to the conduction band of the host AB5 metal. They found that the Fermi energy level increases when hydrogen diffuses into the LaNi Sn alloy as diffusible hydrogen and decreases after hydrogen forms metal hydrides. Therefore, forming hydrides reduces the number of electrons in the conduction band and then decreases the magnetization. For the LaNi Sn alloy, the calculated number of the conduction electrons, 6.87, is above the number of electrons to half-fill the band. Therefore, adding an electron to the conduction band will reduce the magnetic moment of the material. Above a hydrogen-to-metal ratio of about one, hydride is formed. Fig. 9 shows the magnetization curves and Fig. 10 shows the susceptibility (not corrected for demagnetizing factor) of the nonstoichiometric La Ni Sn alloy as a function of hydrogen content. The susceptibilities increase when the hydrogen-to-metal ratio is greater than two. This trend is precisely opposite to that of the stoichiometric LaNi Sn alloy. This result suggest that nonstoichiometric composition alloy may change chemical bonding and -band electron filling, and these changes play a crucial role in controlling the hydrogen storage properties of the alloys. et al

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Magnetization as a function of field for LaNi Sn alloy after five activation cycles. (H/M is the hydrogen to metal ratio)

Fig. 7.

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Effect of hydrogen content on magnetic susceptibility of LaNi Sn after five activation cycles. Fig. 8.

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five activation cycles are shown in Figs. 7 and 8. Increasing absorbed hydrogen content decreases the magnetization of the alloy. The decreased magnetization can be explained with magnetic moment theory. The absorbed hydrogen in transition-metal alloys is present as metal hydride and hydrogen in solution. Each free hydrogen atom carries one electron, which can either increase or decrease the magnetic moment of the alloy. The magnetic moment will increase when the electrons donated from hydrogen have the same spin as the majority of the electrons in the conduction band in the alloy. If the electrons have the opposite spin, the mag-

Magnetization as a function of field for La Ni Sn alloy after five activation cycles. (H/M is the hydrogen to metal ratio)

Fig. 9.

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Studies on the Hydrogen Absorption Properties of LaNi Snx and La Ni Sn Alloys Using Magnetic and Thermoelectric 5-x

Effect of hydrogen content on magnetic susceptibility of La Ni Sn after five activation cycles.

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Fig. 10. 0.95

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In a structural analysis using high-intensity X-ray for the [21] found that “dumbbells” La Ni Sn alloy, Vogt made of two nickel atoms replaced some of the lanthanum atoms on the cube corners. These nickel dumbbells make the structure more compact. This caused a change in the Fermi energy, which is directly related to the number of electrons per unit volume. 0.95

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Correlation between TEP at hydrogen content for LaNi Sn after five activation cycles. Fig. 11.

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3.5. TEP measurement

The TEP measurement of LaNi Sn alloy at various hydrogen content is shown in Fig. 11. As seen in this figure, the TEP of the alloy increases with an increase in absorbed hydrogen until reaching a plateau region where the H/M ratio is about one. From Eq. 4, the TEP depends on the scattering parameter, electron concentration, and effective mass of the electron (or the curvature of the electronic structure). However, the effect of the scattering parameter can be neglected because the measurement was conducted at near room temperature. In the single a phase, electrons from the absorbed hydrogen fill the conduction band and increase the electron concentration and the curvature of the energy at the Fermi energy level. In the plateau region (H/M higher than one) lattice expansion due to hydride formation may hinder the effect on the Fermi energy with hydrogen absorption. Also, it is known that the Gibb’s free energy of hydride formation is almost constant when a material is in the plateau region. The free energy is related to the chemical potential, which is directly related to the Fermi energy, as shown in Eq. 22. /6)( (ln (ε))/ ε) (22) μ εF (π The chemical potential is also related to the shape of the 4.78

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Correlation between TEP at different hydrogen content for La Ni Sn after five activation cycles. Fig. 12. 0.95

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energy band. Therefore, the constant free energy may be interpreted as a small change in the Fermi energy level. For nonstoichiometric La Ni Sn alloys, the room temperature TEP decreases as hydrogen content increases, as shown in Fig. 12. The markedly different dependence of the TEP on hydrogen content between stoichiometric and nonstoichiometric alloys appears to be related with Ni dumbbells in the crystal structure, as mentioned in reference to the magnetic measurement results. 0.95

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4. CONCLUSIONS

Mater. -Int. , 609 (2000). 6. C. D. Gelatt, H. Ehrenreich, and J. A. Weiss, Phys. Rev. B. , 4 (1978). 7. C. D. Gelatt, Theory of Alloy Phase Formation, p. 451, TMS, Warrendale, PA (1980). 8. P. Termsuksawad, Ph. D. Thesis., Colorado School of Mines, Golden, Colo. (2003). 9. Niyomsoan, Ph. D. Thesis., Colorado School of Mines, Golden, Colo. (2003). 10. O. Bernauer, J. Topler, D. Noreust, R. Hemplemann, and D. Richter, Int. J. Hydrogen Energ. , 187 (1989). 11. M. Gupta, J. Less-Common Met. , 219 (1987). 12. K. Nakatsuka, M. Yoshino, H. Yukawa, and M. Morinaga, J. Alloy. Compd. , 222 (1999). 13. M. Morinaga, H. Yukawa, K. Nakatsuka, and M. Takagi, J. Alloy. Compd. , 20 (2002). 14. M. H. Mendelsohn, D. M. Gruen, and A. E. Dwight, J. Less-Common Met. , 193 (1979). 15. C. Y. Seo, Z. L. Zhang, J. H. Kim, P. S. Lee, and J. Y. Lee, Met. Mater. -Int. , 341 (2002). 16. B. D. Cullity, Introduction to Magnetic Materials, p. 134, Addison-Wesley, Reading, MA (1972). 17. F. Ishikawa, H. Tega, I. Yamamoto, and M. Yamaguchi, J. Alloy. Compd. , 182 (1995). 18. S. Luo, J. D. Clewley, T. B. Flanagan, R. C. Bowman Jr., and L. A. Wade, J. Alloy. Compd. , 171 (1998). 19. G. D. Adzic, J. R. Johnson, J. J. Reilly, J. MaBreen, S. Mukerjee, M. P. Sridhar Kumar, W. Zhang, and S. Srinivason, J. Electrochem. Soc. , 3429 (1995). 20. H. Nakamura, D. Nguyen-Manh, and D. G. Pettifor, J. Alloy. Compd. , 81 (1998). 21. T. Vogt, J. J. Reilly, J. R. Johnson, G. D. Adzic, and J. McBreen, Electrochem. Solid St. , 111 (1999). 22. R. D. Barnard, Thermoelectricity in Metals and Alloys, Taylor and Francis LTD., London, (1972). 6

The following conclusions can be derived from the present study on the characterization and prediction of hydrogen absorption behaviors for AB5 hydrogen storage alloys: Application of the DEC model is found to be proportional to the equilibrium hydrogen plateau pressure for LaNi5 Sn hydrogen storage alloys. TEP measurement is a potential tool to predict the absorption property of the alloys. Changes in magnetization and TEP were observed with increasing absorbed hydrogen for the La Ni Sn alloy. These results indicate that TEP and magnetic measurement are very sensitive to the amount of absorbed hydrogen in these alloys and can be used as a hydrogen sensor. -x

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ACKNOWLEDGEMENTS

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This work was supported by the National Institute of Standards and Technology (NIST) and the U.S. Army Research Office. We thank Dr. R. Goldfarb of NIST and Dr. V. I. Kaydanov of CSM for helpful discussion, and also thank Dr. J. Johnson of BNL and Dr. Z. Gavra of the Nuclear Search Center for providing samples. We thank Professor C. N. Park and his student J. H. You of Chonnam University for experimental assistance.

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