Crystal structure and thermoelectric properties of YAl3C 3

Article abstract of DOI:10.1111/j.1551-2916.2007.01855.x

The crystal structure of YAl₃C₃ was refined from laboratory X-ray powder diffraction data (CuKα₁) using the Rietveld method. The crystal structure is hexagonal (space group P6 ₃mc, Z=2) with lattice dimensions a=0.342157(4) nm, c=1.72820(1) nm, and V=0.175217(3) nm³. The final reliability indices were R _wp=9.94% (R_wp/R_e=1.18), R_p=7.36%, R_B=1.77%, and R_F=1.03%. The compound shows an intergrowth structure with electroconductive [YC₂] thin slabs separated by Al₄C₃-type [AlC] layers. This material had thermoelectric properties superior to those of the layered carbides Zr₂[Al _3.56Si_0.44]C₅, Zr₂Al ₃C₄, and Zr₃Al₃C₅ in the temperature range of 500- 1073 K, with a maximal power-factor value of 1.96 × 10^-4 W·(m·K²)^-1 at 1073 K.

Full text of DOI:10.1111/j.1551-2916.2007.01855.x

J. Am. Ceram. Soc., 90 [10] 3299–3302 (2007)
DOI: 10.1111/j.1551-2916.2007.01855.x
r 2007 The American Ceramic Society
ournal
J
Crystal Structure and Thermoelectric Properties of YAl3C3
,w
Koichiro Fukuda* and Miyuki Hisamura
Department of Environmental and Materials Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan
The crystal structure of YAl C was refined from laboratory
3
m 3 2
moelectric properties slightly superior to those of (ZrC) Al C
3
X-ray powder diffraction data (CuKa ) using the Rietveld
(m 5 2 and 3).
1
method. The crystal structure is hexagonal (space group
P6 mc, Z 5 2) with lattice dimensions a 5 0.342157(4) nm,
3 3
In the present study, we have prepared YAl C (T5 Y and
m 5 1) and refined the crystal structure from X-ray powder
diffraction (XRPD) data using the Rietveld method. Because
the electroconductive layer of the crystal is thinner than those of
the layered carbides with m 5 2 and 3, we expected a superior
absolute value of S, and consequently a prominent power-factor
value.
3
3
c 5 1.72820(1) nm, and V 5 0.175217(3) nm . The final reliabil-
ity indices were Rwp 5 9.94% (R /R 5 1.18), R 5 7.36%,
wp
R 5 1.77%, and R 5 1.03%. The compound shows an inter-
e
p
B
F
growth structure with electroconductive [YC ] thin slabs
2
separated by Al C -type [AlC] layers. This material had ther-
4
3
moelectric properties superior to those of the layered carbides
Zr [Al3.56Si0.44]C , Zr Al C , and Zr Al C in the temperature
range of 500– 1073 K, with a maximal power-factor value of
2
5
2
3
4
3
3 5
II. Experimental Procedure
ꢁ
4
2 ꢁ1
.
.
1
.96ꢀ 10 W (m K ) at 1073 K.
(
1) Synthesis
A powder specimen of YAl
ric amounts of reagent-grade chemicals of YC
C
3 3
was prepared from stoichiomet-
and Al . Be-
2
4 3
C
cause both reagents are highly hygroscopic, they were mixed in a
dry box to prevent hydrolysis as much as possible. The mixture
was pressed into compacts (10 mm ꢀ 10 mm ꢀ 3 mm), heated
under vacuum at 1873 K for 1 h, followed by cooling to ambient
temperature by cutting furnace power. The reaction product was
a slightly sintered polycrystalline material, mainly consisting of
I. Introduction
HE ternary carbides consisting of one transition metal T
( 5 Zr, U, and Sc), aluminum, and carbon form a homol-
ogous series, whose general formula is (TC) Al C (m 5 1, 2,
3 2
T
m
1
,2
and 3). The crystal structures can be regarded as intergrowth
structures where the Al -type [AlC] layers are the same, while
C
4 3
3 3
YAl C , together with small amounts of amorphous carbon,
the [T Cm11] layers increase in thickness with increasing m val-
ue. The previous structural determinations of the layered car-
m
Y O , and Y Al O . The amorphous carbon was produced con-
2
3
4
2
9
comitantly by the reaction
bides Zr
and UAl
group P6 /mmc.
2
Al
3
C
4
(T 5 Zr and m 5 2), ScAl
C (T5 U and m 5 1) were all carried out in the space
3 3
C (T5 Sc and m 5 1),
4
YC2 þ 3Al4C3 ! 4YAl3C3 þ 5C
(1)
3 3
3
–5
All of these crystals, including Zr Al C
3 5
3
3
The two types of oxides were formed because the hydrolysis
occurred to some extent for the starting materials during weigh-
ing and mixing.
In order to obtain a dense sample, the polycrystalline material
was finely ground to obtain a powder specimen. A part of the
powder specimen was subsequently sintered by a pulse electric cur-
rent sintering method (SPS-1030, Sumitomo Coal Mining Co.,
Tokyo, Japan). Sintering was conducted at a uniaxial pressure of
about 40 MPa and at 1873 K for 5 min in vacuum. Dense sintered
bodies in the form of disks (15 mm in diameter and 4 mm in length)
(
T5 Zr and m 5 3), are now considered to belong to the lower
1,2,6
3
symmetry space group, P6 mc.
Thermoelectric materials with a high efficiency of energy con-
version are of interest for applications as heat pumps and power
generators. Low-dimensional materials that consist of, for ex-
ample, conducting two-dimensional (2D) layers are promising
7
–12
for thermoelectric energy conversion.
The advantage of the
low dimensionality can be interpreted in terms of the carrier
confinement effect in the 2D layers, which leads to an enlarged
absolute value of the Seebeck coefficient (S) compared with the
materials with 3D conducting paths. The crystal structures of
with m 5 2 and 3 are comprised of electroconduc-
Cm11] layers separated by less conductive [AlC] lay-
ers. Hence, the homologous compounds demonstrated good
performance of thermoelectricity, which is quantified by a power
3 3
were thus obtained. The compound YAl C is also highly hygro-
scopic. However, the coexistence of impurity phases (probably
amorphous carbon) seems to suppress effectively the occurrence of
a hydrolysis reaction in both powder and sintered specimens.
(
tive [Zr
ZrC)
m
Al
3
C
2
m
1
3
2
13
(2) Structural Characterization and Thermoelectric
Properties
factor S s, where s is the electrical conductivity. Recently,
Fukuda et al. have successfully prepared a new quaternary
1
3
carbide Zr [Al3.56Si0.44]C (space group R3m). The crystal struc-
2
The XRPD intensities were collected for the powder specimen
on a PANalytical X’Pert PRO Alpha-1 diffractometer (Almelo,
the Netherlands) equipped with a high-speed detector (X’Cel-
erator, PANalytical) in Bragg–Brentano geometry using mono-
5
ture is made up of electroconductive [Zr C ] slabs separated by
2
3
4 3
Al C -type [Al0.89Si0.11C] layers. This material exhibited ther-
chromatized CuKa radiation (45 kV, 40 mA) in a 2y range from
H. P. Beck—contributing editor
1
20.01541 to 148.49161 (an accuracy in 2y of 70.00011). A di-
vergence slit of 0.51 was used to collect the quantitative profile
intensities over the whole 2y range. The other experimental con-
ditions were: continuous scan, a total of 7689 datapoints, and a
total experimental time of 2.1 h. The structure data were stan-
Manuscript No. 23022. Received April 2, 2007; approved April 23, 2007.
This work was supported by a Grant-in-Aid for Scientific Research (No. 18560654)
from the Japan Society for the Promotion of Science, and a grant from the Research Foun-
dation for the Electrotechnology of Chubu.
1
4
dardized using the computer program STRUCTURE TIDY.
The crystal-structure models were visualized using the computer
program VICS.
*
Member, American Ceramic Society.
w
Author to whom correspondence should be addressed. e-mail: fukuda.koichiro@
1
5
nitech.ac.jp
3
299

3300
Communications of the American Ceramic Society
Vol. 90, No. 10
3 3 2 3 4 2 9
Fig. 1. Comparison between observed (1marks) and calculated (upper solid line) patterns of YAl C , Y O , and Y Al O . The difference curve is
shown in the lower part of the figure. Vertical marks indicate the positions of possible Bragg reflections.
Samples in the form of rectangular bars (10 mm ꢀ 4 mm ꢀ 3
mm) were cut from the disks using a diamond saw for thermo-
electric measurements. The X-ray diffraction intensities from the
cut surface revealed that the sintered samples were free from pre-
ferred orientation of the YAl C crystal grains. The thermoelectric
phases was found to be 88.8 mass% YAl C , 9.2 mass% Y O ,
3 3 2 3
and 2.0 mass% Y
assumption that 15.2% of the YC
ture was subjected to hydrolysis to finally produce Y
another 2.8% of the YC reagent was hydrated and further re-
4
Al
2
O
9
. These values can be derived from the
reagent in the starting mix-
, and
2
2 3
O
3
3
2
2
properties were evaluated by the S s value, which is determined in
a manner similar to that described in the previous study.
4 2 9
acted with an aluminum component to form Y Al O . Taking
13
into consideration the amount of amorphous carbon that should
be produced by Eq. (1), the phase composition of the present
specimen is estimated to be 84 mass% YAl C , 6 mass% amor-
3
3
phous carbon, 8 mass% Y
3 3
Figure 2 shows the crystal structure of YAl C , which can be
regarded as an intergrowth structure consisting of two types of
layers: one comprised of [YC ] and the other of [AlC] in an atom
4 3
arrangement very similar to that of Al C . The Al1 and Al2 at-
oms are tetrahedrally coordinated with mean distances of 0.212
nm for /Al1–CS and 0.203 nm for /Al2–CS (Table III). The
Al3–C1 and Al3–C3 distances ranged from 0.198 to 0.247 nm.
2
O
3
, and 2 mass% Y Al
4
2 9
O .
III. Results and Discussion
(
1) Structure Refinement and Phase Composition
Structural parameters were refined by the Rietveld method using
2
1
6
the computer program RIETAN-FP. In the initial stage of the
refinement process, the structural parameters of YAl were
C
3 3
1
17
identical to those of UAl C . The structure models of Y O
3
3
3
2
1
9
8
and Y Al
4
2
O
were added to the refinement as additional
phases. A Legendre polynomial was fitted to the background
intensities with 12 adjustable parameters. The pseudo-Voigt
function was used to fit the peak profile. All of the isotropic
These Al–C distances in YAl
Al–C distances of the AlC
0.194 to 0.218 nm (the mean 5 0.206 nm). Accordingly, the
[AlC] layers in YAl would be structurally comparable with
the Al compound. With Al3 atoms in YAl , the coordi-
3
C
3
are nearly comparable with the
4
tetrahedra in Al , ranging from
4 3
C
1
9
6
atomic displacement (B) parameters of carbon atoms were
constrained to have the same value. The reliability indices
3 3
C
2
0
C
4 3
3 3
C
for the final result were Rwp 5 9.94% (Rwp/R 5 1.18),
e
nation number is five if C2 atoms at a relatively long distance of
0.293 nm are included in the [AlC ] unit. The Al3 atoms are
therefore located in trigonal bipyramidal voids formed by five
R 5 7.36%, R 5 1.77%, and R 5 1.03% (Fig. 1). Crystal
p
B
F
5
data are given in Table I, and the final positional and B param-
eters of atoms are given in Table II. The selected interatomic
distances, together with their standard deviations, are given in
Table III.
carbon atoms. This is also the prominent feature of the Al
4 3
C -
type [AlC] layers in Zr Al C and Zr Al C , as pointed out in
2
3
4
3
3 5
1
,2
previous studies. Accordingly, the crystal structures of the
ternary layered carbides with the general formula (TC) Al
Quantitative X-ray analysis with correction for microabsorp-
m
3 2
C
2
1
tion according to Brindley’s procedure was implemented in
the program RIETAN-FP. The composition of the crystalline
(m 5 1, 2, and 3) would all belong to the space group P6
where T 5 Y, Zr, U, and Sc.
3
mc,
Table II. Structural Parameters for YAl C3
3
2
Site
Wyckoff position
x
y
z
100 ꢀ B (nm )
Table I. Crystal Data for YAl C3
3
Chemical composition
Space group
a (nm)
c (nm)3
V (nm )
YAl C3
3
P6 mc
3
0.342157(4)
1.72820(1)
0.175217(3)
2
Y
2a
2b
2b
2b
2b
2b
2b
0
0
0
0.12(1)
0.2(1)
0.3(1)
0.32(4)
0.21(8)
0.21
Al1
Al2
Al3
C1
C2
C3
1/3
1/3
1/3
1/3
1/3
1/3
2/3
2/3
2/3
2/3
2/3
2/3
0.1324(5)
0.3640(5)
0.7582(3)
0.256(1)
0.5885(9)
0.901(1)
Z
D (Mg/m )
3
3.90
0.21
x

October 2007
Communications of the American Ceramic Society
3301
Table III. Interatomic Distances (nm) in YAl C3w
3
Y–C2
Y–C3
0.2499(9) ꢀ 3
0.261(1)ꢀ 3
0.3022(7) ꢀ 3
0.3070(6) ꢀ 3
0.2116(5) ꢀ 3
0.213(2)
Y–Al1
Y–Al2
Al1–C2
Al1–C1
Al1–Al3
Al2–C1
Al2–C3
Al2–Al3
Al3–C1
Al3–C3
Al3–C2
C1–C3
w
0.2938(5) ꢀ 3
0.187(2)
0.2076(5) ꢀ 3
0.2691(4) ꢀ 3
0.19760(5)ꢀ 3
0.247(1)
0.293(1)
0.320(2)ꢀ 3
All distances shorter than 0.31 nm (metal–metal) and 0.33 nm (metal–carbon)
are given.
(2) Thermoelectric Properties
The temperature dependence of the s, S, and the power-
factor values of the present sample, together with those of
1
3
Zr
Fig. 3. The present sample was composed mainly of YAl
with small amounts of amorphous carbon, Y O , and Y Al O .
2
[Al3.56Si0.44]C
5
, Zr
2
Al
3
C
4
, and Zr
3
Al
3
C
5
,
are shown in
3 3
C
2
3
4
2
9
The amorphous carbon has a high electrical resistance, with the
ꢁ
2
ꢁ5
22,23
s value ranging from 10 to 10 S/cm.
continuous conduction pass is formed by YAl
specimen. The only material in the sample that shows thermo-
electricity is YAl . Accordingly, the electric and thermoelec-
This implies that a
3 3
C
in the sintered
3 3
C
tric properties of the sintered specimen are principally
attributable to the constituent YAl C compound.
3
3
The s value of the present sample was about one order of
magnitude lower than the other layered carbides throughout the
temperature region measured (Fig. 3(a)). The relatively low s
value could be attributable in part to the coexistence of impurity
phases. All of the s values steadily decreased with increasing
temperature, representing a conductor-type behavior. When
samples were compared at the same temperature, Zr
3 3 5
Al C al-
ways showed the largest s value, followed by Zr [Al3.56Si0.44]C5
2
2 3 4
and Zr Al C , suggesting that the intergrowth structure with the
thicker electroconductive layer appears to be more advanta-
geous for the conduction of electricity.
Fig. 3. Temperature dependences of (a) electrical conductivity s, (b)
2
Seebeck coefficient S and (c) thermoelectric power factor S s for (n) the
present specimen, (J) Zr
Zr Al . The data of the latter three materials were taken from a pre-
vious study.
2
[Al3.56Si0.44]C
5
, (
ꢃ
2 3 4
) Zr Al C , and (&)
3
3 5
C
13
The signs of S were always negative for all the samples re-
gardless of temperature, indicating predominant negative mo-
bile charge carriers (Fig. 3(b)). With YAl C in the present
3 3
sample, the absolute value of S was much larger than those of
the other layered carbides and steadily increased with increasing
temperature, with a maximal value of 112 mV/K at 1073 K.
Thus, the intergrowth structure with the thinner electroconduc-
tive layer would be more favorable for thermoelectric power
generation.
3 3
The power-factor value for YAl C steadily increased from
ꢁ
5
ꢁ5
2 ꢁ1
4
.2 ꢀ 10 to 19.6 ꢀ 10 W ꢂ (mꢂ K ) with an increase in tem-
3 3
Fig. 2. Crystal structure of YAl C .
perature from 373 to 1073 K (Fig. 3(c)). On the other hand, the

3
302
Communications of the American Ceramic Society
Vol. 90, No. 10
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