Materials Science and Engineering A 391 (2005) 264–271
The relationship between the thermoelectric power and phase
structure in AB
2
hydrogen storage materials
S. Niyomsoan
a
, P. Termsuksawad
a
, D.L. Olson
a
, B. Mishra
a,∗
,
V.I. Kaydanov
a
, Z. Gavra
b
a
MaterialsScienceProgram,ColoradoSchoolofMines,Golden80401,CO,USA
b
NuclearResearchCenter—Negev,BeerSheva84190,Israel
Received 4 March 2004; received in revised form 30 August 2004; accepted 2 September 2004
Abstract
The phase structure during the hydrogenation of AB
2
-type hydrogen storage intermetallic alloys has been investigated using the compound
of Zr
0.9
Ti
0.1
Cr
x
Fe
2- x
as a typical representative type of AB
2
system. The as-received compounds of Zr
0.9
Ti
0.1
Cr
x
Fe
2- x
with x = 0.6, 0.8 and
1.0 are all pure C-14 Laves phase, a hexagonal structure. With an increasing amount of the stored hydrogen, the thermoelectric power (TEP)
of these compounds increase.
The amount of stored hydrogen alters the electronic states in the alloys, which can be distinguished from the variation of the results of the
thermoelectric power measurements. The corresponding TEP is found to have the relationship with phase structure of the alloy hydrides under
hydrogenation. The relationship of the Seebeck coefficient and the thermodynamics of hydrogenation are derived by considering the Seebeck
effect as the external work. The variation in alloy composition has been represented by the d-shell electronic concentration (DEC) number.
© 2004 Elsevier B.V. All rights reserved.
Keywords: AB
2
hydrogen storage; Phase structures; Thermoelectric power; Seebeck coefficient
1. Introduction
1.1. Thermodynamicsofhydrogenation
Description of the fundamental thermodynamics pertain-
ing to hydrogenation reactions has been included to estab-
lish the relationship between the thermoelectric power and
the molar free energy change. A schematic phase diagram
of hydrogenation is presented in the pressure–composition–
temperature diagram (PCT diagram) [1] as shown in Fig. 1
for an alloy. The diagram is the relationship between the hy-
drogen equilibrium partial pressure and the hydrogen content
in the material at a given temperature. The solid line shows
the variation of the equilibrium hydrogen partial pressure of
absorption as a function of the hydrogen content. Hydro-
gen atom initially dissolves in a metal forming a solid solu-
tion phase (-phase). The hydrogen partial pressure increases
∗
Corresponding author. Tel.: +1 303 273 3893; fax: +1 303 384 2189.
E-mailaddress: bmishra@mines.edu (B. Mishra).
with increasing hydrogen concentration until there is the pre-
cipitation of a higher concentration hydride phase (-phase).
The dash line in Fig. 1 indicates the region of two-phases
where the isotherms show an apparent constant pressure. For
the equilibrium coexistence, at the constant temperature and
pressure, it is required that the partial Gibbs free energy of
hydrogen in -phase, G
α
H
, and -phase, G
β
H
, are equal.
Considering that hydrogen gas dissolves to form atoms in
solid solution, the equilibrium would be written as follows
[2]:
H
2
(g) → 2H (M) (1.1.1)
where H (M) denotes the solution of a hydrogen atom in the
metal, M, phase. The thermodynamic relation of the chemical
reaction (1.1.1) is expressed as
K =
(a
H
-
(M)
/a
0
H
)
2
P
H
2
/P
0
H
2
(1.1.2)
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.msea.2004.09.001