J. Am. Ceram. Soc., 92 [11] 2687–2692 (2009)
DOI: 10.1111/j.1551-2916.2009.03244.x
r 2009 The American Ceramic Society
ournal
J
Thermal Conductivity of Monazite-Type REPO4
(RE 5 La, Ce, Nd, Sm, Eu, Gd)
Aibing Du, Chunlei Wan, Zhixue Qu, and Wei Panw
State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering,
Tsinghua University, Beijing 100084, China
Low-thermal conductivity ceramics in monazite-type REPO4
(RE 5 La, Ce, Nd, Sm, Eu, Gd) ceramics are expected to have
potential in structural (refractories, thermal insulator) and nu-
clear applications. To this end, the present study determines
their thermal conductivities and examines how differences of the
rare earth ions change their thermal conductivity at different
temperatures. The results show that their conductivities are re-
markably low from 251 to 10001C. In addition, different con-
ductivity variation mechanisms exist that change gradually upon
altering from LaPO4 to GdPO4 at low and high temperatures.
At relatively lower temperatures (r4001C), the thermal con-
ductivities of all the REPO4 ceramics decrease nearly at first,
reach a minimum value, and then rise with gradual altering from
LaPO4 to GdPO4. It may be due to the combined effects of the
increase of both the anharmonicities in lattice vibrations and the
bond strength. As the temperature increases, the conductivity
trends become obscure, and the conductivities of the monazite-
type REPO4 approach their minimum thermal conductivities
when the temperature is above 8001C.
garnet (Y3AlxFe5ꢀxO12),14 and other candidates with low ther-
mal conductivity1,15 have also been investigated. However, there
is still great motivation for developing new candidate ceramics
with lower thermal conductivities.
Rare earth phosphates (REPO4), as another class of candi-
dates with excellent thermal phase stability and high melting
point,16 are expected to have low thermal conductivity because
they have high mean atomic mass and their structures are sim-
ilar to the network silicates with various arrangements of corner
and edge-sharing PO4 tetrahedra rather than SiO4 tetrahedra.17
They exist in nature as two distinct, but closely related, crystal
structures, the monazite and the xenotime structures, depending
on the ionic size of the lanthanide ions. The monazite, which is
monoclinic with four formula units in the P21/n unit cell and
consists of REO9 polyhedra (shown in Fig. 1), preferentially in-
corporates the larger lanthanide ions (RE 5 La–Gd) while the
xenotime, which is isostructural with zircon (space group I41/
amd) and composed of REO8 polyhedra, tends to incorporate
the smaller lanthanide ions (RE 5 Tb–Lu,1Y).17 Therefore,
monazite-type REPO4 is expected to have lower thermal con-
ductivity, owing to the lower crystal symmetry and weaker
bonds than that of xenotime-type REPO4.
I. Introduction
Several investigations on the thermal behaviors of monazite-
type REPO4 have been made in recent years. The melting point
and linear thermal expansion coefficients of several monazite-
type REPO4 (RE 5 La, Ce, Nd, Sm) have been confirmed to be
in the range of 19161–20721C and 9.7ꢀ10 ꢁ 10ꢀ6 Kꢀ1 at 10001C,
respectively.16,18 Konings and colleagues have reported the heat
capacities and thermodynamic functions of a series of monazite-
type REPO4 (RE 5 La, Ce, Nd, Sm, Eu, Gd).19–22 However, as
far as we know, few results have been systematically reported on
the thermal conductivity of monazite-type REPO4.1,23,24 The
objective of the present study is to determine their thermal con-
ductivities and examine how differences of the rare earth ions
change the thermal conductivity of the monazite-type REPO4 at
different temperatures. The phonon mean free path and
Young’s modulus of REPO4 were calculated and measured in
order to clarify the effect on the thermal conductivity.
HERE is a growing demand for high-temperature structural
ceramics with low thermal conductivity for a variety of
T
thermal insulation applications such as thermal insulator and
nuclear reactor components.1–3 Although identifying such ma-
terials is difficult, several new strategies for seeking possible ma-
terials with low thermal conductivity at high temperatures
appear to be promising. These strategies suggest that candidate
materials can be identified by searching for compounds with a
combination of high atomic mass, flexible ionic structures, non-
directional bonding, and extensive site disorder.1,4 In light of
these selection guidelines, a large number of possible candidate
materials have been found and investigated. For instance, yttria-
stabilized zirconia (YSZ), which has been widely used, relies
mostly on its high concentration of point defects (oxygen va-
cancies and substitutional solute atoms) to scatter heat-conduct-
ing phonons (lattice waves), and much of its physics have been
investigated.5,6 Recently, another class of materials with a for-
mula of RE2Zr2O7 (RE 5 La, Nd, Gd, Sm, Er, Dy, Yb, etc.),
which crystallizes in pyrochlore or fluorite structure that in-
volves a large content of structural oxygen vacancies in the unit
cell, giving rise to strong phonon scattering and significantly re-
duced thermal conductivity. Furthermore, it has been shown
that the doped fluorite- and pyrochlore-structured oxide mate-
rials can further reduce the thermal conductivity.7–10 In addi-
tion, many materials such as La2Ce2O7,11 mullite,12 silicates,13
II. Experimental Procedure
(1) Preparation of Monazite-Type REPO4 (RE 5 La, Ce,
Nd, Sm, Eu, Gd) Samples
The REPO4 rhabdophane samples were synthesized by a con-
trolled precipitation method from lanthanide-citrate chelate and
(NH4)2HPO4.18 The process was as follows: RE2O3 (RE 5 La,
Nd, Sm, Eu, Gd) (purity 99.9%, Rare Earth Chem. Co.,
Baotou, China) or Ce(NO3)3 ꢂ 6H2O (purity 99.9%, Tianjin
NO.3 Chemical Reagent Factory) were used as starting materi-
als. The RE2O3 was calcined at 10001C for 6 h before weighting
in order to remove moisture and other volatile impurities. The
RE(NO3)3 solution was obtained by dissolving the Ce(N-
O3)3 ꢂ 6H2O in deionized water or the RE2O3 in diluted
HNO3. Anhydrous citric acid was added to the solution of
RE(NO3)3 with a mixing mole ratio (citric acid/RE(NO3)3) of 5,
and stirred for 2 h at 201C to form the lanthanide–citrate chelate
K. Faber—contributing editor
Manuscript No. 25597. Received December 7, 2008; approved June 2, 2009.
This work was supported by the National Natural Science Foundation of China (Grant
No. 50572042).
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