Influence of YbP on the thermoelectric properties of n-type P doped Si95Ge5 alloy

Article abstract of DOI:10.1016/j.jallcom.2018.02.184

Since the report of high zT in Si₉₅Ge₅ there has been significant interest in low Ge alloy compositions for thermoelectric applications. The application of YbP was explored as a means to lower thermal conductivity. A series of 3% pho

Full text of DOI:10.1016/j.jallcom.2018.02.184

Accepted Manuscript
Influence of YbP on the thermoelectric properties of n-type P doped Si Ge alloy
95
5
Fan Sui, Sabah K. Bux, Susan M. Kauzlarich
PII:
S0925-8388(18)30635-2
10.1016/j.jallcom.2018.02.184
JALCOM 45060
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 13 November 2017
Revised Date: 13 February 2018
Accepted Date: 14 February 2018
Please cite this article as: F. Sui, S.K. Bux, S.M. Kauzlarich, Influence of YbP on the thermoelectric
properties of n-type P doped Si Ge alloy, Journal of Alloys and Compounds (2018), doi: 10.1016/
95
5
j.jallcom.2018.02.184.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Influence of YbP on the thermoelectric properties of n-type P doped Si Ge alloy
9
5
5
1
,2
3
1*
Fan Sui , Sabah K. Bux and Susan M. Kauzlarich
Department of Chemistry, University of California, One Shields Avenue, Davis,
1
California 95616
2
Center for Information Photonics and Energy Materials, Shenzhen Institutes of
Advanced Technology, Chinese Academy of Sciences, 518055, China
3
Thermal Energy Conversion Technologies and Advancement Group, Jet Propulsion
Laboratory, California Institute of Technology, 4800 Oak Grove Drive, MS 277-207,
Pasadena
*
Email: smkauzlarich@ucdavis.edu
Keywords: Thermoelectric materials; mechanochemical processing;, thermoelectric
transport; Silicon
Abstract:
Since the report of high zT in Si Ge there has been significant interest in low Ge
95
5
alloy compositions for thermoelectric applications. The application of YbP was
explored as a means to lower thermal conductivity. A series of 3 % phosphorus (P)
doped n-type Si Ge (SiGe) alloy was reacted with YbH (0, 1, 2%). YbP was formed
9
5
5
2
in the SiGe alloy matrix from the reaction between YbH and P during the Spark
2
Plasma Sintering (SPS) process. Thermoelectric property measurements were
performed on sintered pellets from room temperature to 1273 K. X-ray diffraction
patterns were collected from the ground powder samples and confirmed the main
ꢀ
phase possessed diamond structured Si Ge (space group: Fd3m) as well as the
95
5
ꢀ
presence of YbP (space group: Fm3m). The carrier concentration of the sample was
controlled by the amount of YbH added, removing some of the phosphorus to form
2
YbP. n-type Si Ge alloy samples with higher YbP amounts showed higher electrical
9
5
5
resistivity and lower thermal conductivity attributed to loss of the P dopant. Another
composite series of n-type Si Ge with YbP were synthesized with additional P
9
5
5
compositions (1, 2 %). The thermoelectric properties were characterized from room
temperature to 1273 K, and the samples possess electrical resistivity, carrier
concentrations, and thermal conductivity as expected from the additional P dopant.
The presence of YbP lowered lattice thermal conductivity when the sample was
appropriately doped. The Seebeck coefficients were measured with both off-axis and
uniaxial axis experimental configurations. These results show that the off-axis
measurements overestimate the Seebeck coefficients of the Si Ge alloy samples.
9
5
5
This is attributed to a cold finger effect and therefore only the uniaxial data are
combined for zT calculations. The n-type Si Ge samples with YbP and less than 3 %
9
5
5
P dopant show similar zT compared with the Si Ge sample with no YbP inclusions
9
5
5
and 3 % P dopant with a peak zT of 0.6 at 1200 K.
1

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1
. Introduction:
Large amounts of heat are generated as a by-product from the energy consumption of
gasoline, coal and natural gas combustion. Thermoelectric materials, defined by the
Seebeck Effect, can enhance the energy efficiency ratio by the direct conversion of
1
electricity from waste heat. The challenge for thermoelectric devices to achieve wide
applications is to convert waste heat into electricity efficiently. Thermoelectric
2
efficiency is related by the figure of merit zT, which is defined by α T/ρκ. zT will be
maximized along with the thermoelectric conversion efficiency for materials with a
high Seebeck coefficient (α, ꢀV/K), low electrical resistivity (ρ, mꢁ·cm) and low
thermal conductivity (κ, mW/cm·K); however, these parameters are interrelated and
2
to decouple them is challenging.
There are a variety of energy consumption processes that generate high temperature
waste heat above 1000 K, for example, power plants and vehicle engines.
Thermoelectric materials for high temperature applications need to be thermally stable
in the high temperature range and also provide good thermoelectric performance at
the corresponding operating temperature. So far, SiGe alloys are one of the most
2
, 3
studied thermoelectric materials for high temperatures. Si Ge alloys have been
80
20
applied in Radioisotope Thermoelectric Generators (RTGs) which generate electricity
3
for space system from the heat released by the nuclear decay of radioactive isotopes.
n-type bulk SiGe alloys achieved a peak zT of about 1.0 in the temperature range of
◦
4
2
9
00 ~ 950 C in the 1990s. Properly doped SiGe alloys have high powder factors (α σ)
but also high thermal conductivity of around 5 W/m·K. Thermoelectric properties of
SiGe alloys can be greatly enhanced if the thermal conductivity can be further
reduced. High energy ball milling processes have been employed to prepare
nanostructured and doped SiGe alloys. The nano-size grain boundaries were shown to
decrease lattice thermal conductivity to below 1 W/m·K and enhance the zT of n-type
5
, 6
Si Ge alloy to 1.84 at 1073 K characterized by off-axis Seebeck measurement.
80
20
While the reported thermal conductivity decrease is convincing, the zT value is
suspicious because the work failed to correct the off-axis Seebeck measurement error
introduced by the cold-finger effect, which will be discussed in this work. This
method also successfully enhanced the zT of boron doped p-type Si Ge to 0.95 at
9
5
5
◦
7
9
00 C, 90 % higher than the currently used RTG thermoelectric leg. Si Ge with
95 5
low Ge content, has also been studied. Low Ge content makes SiGe alloys more
likely for wide-spread industrial applications, as Si is much more earth abundant than
8
, 9
8
Ge. However, the lattice thermal conductivity of Si Ge alloy is higher than Si Ge
95
5
80
20
alloy because there is less alloy scattering from Ge substitution. Nanostructured
n-type Si Ge alloy synthesized from high energy ball mill route was studied and
9
5
5
2

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showed increased short range phonon scattering and therefore achieved a comparable
8
zT of 0.95 to the Si Ge used in RTG but with significantly less Ge.
80
20
Mingo et al. proposed an “Nanoparticle-in-Alloy” strategy to enhance zT’s in
semiconductors; the basic idea is that nanoparticles embedded in an alloy matrix can
1
0
lead to a lattice thermal conductivity reduction and thereby enhanced zT.
Calculations showed the coexistence of scattering by individual atoms and
nanoparticles can effectively decrease phonon mean free path and lead to a potential
1
0
5
-fold increase in zT. One experimental example of this idea is seen in
11
Yb-substituted Mg Si which showed lower thermal conductivity than plain Mg Si.
2
2
Yb was alloyed into the crystal structure of Mg Si to form Mg Yb Si and excess Yb
2
2-x
x
1
1
introduced Yb Si inclusions in Mg Si matrix. Therefore, the samples showed
3
5
2
combined scattering effect at both low and high phonon frequencies from individual
atoms and embedded clusters. Among the 0.2, 0.5, 1 % Yb substituted samples, 1 %
1
1
Yb sample showed lowest thermal conductivity. Besides silicide nanoparticles, other
properly chosen nanoparticles embedded in a SiGe matrix have been studied as well.
1
2,
Modulation-doping has also been reported to increase thermoelectric power factor.
1
3
It was proposed that the good band alignment of the Si Ge P nanoparticle and the
7
0
30 3
SiGe matrix can promote the flow of charge carriers from nanoparticle to SiGe matrix
1
3
and increase mobility as well as electrical conductivity. It was proposed that
modulation doping of Si Ge P could increase carrier mobility and decrease the
70
30 3
lattice thermal conductivity of Si Ge matrix. Overall, Si Ge P in SiGe matrix lead
9
5
5
70
30 3
◦
13
to a zT of 1.3 ± 0.1 at 900 C. This provides incentive for further research, as these
data were obtained with the off-axis Seebeck measurement and should be validated.
n-type Si Ge alloys multi-doped with GaP and P were reported to possess enhanced
9
5
5
9
electrical conductivity and lower thermal conductivity than alloys doped with P only.
We present our work of synthesizing n-type Si Ge alloys with YbP introduced by
9
5
5
the decomposition and chemical reaction of YbH with P as well as the thermoelectric
2
properties from room temperature to 1273 K. The ball milled metal hydride precursor
14
allows for homogeneous distribution of the Yb element in the SiGe alloy matrix.
Metal hydrides decompose and generate hydrogen which can be vented during either
annealing or sintering processes in SPS (Spark Plasma Sintering), and provides a
15
clean and convenient synthesis method. The thermoelectric properties of Si Ge
5
9
5
with YbP composite were characterized to examine the influence of YbP on the alloys’
transport properties.
2
. Experiments:
2
.1 Synthesis:
Samples were synthesized by arc-melting of Si lump (Alfa Aesar, 99.99999+ %) and
Ge pieces (Alfa Aesar, 99.999 %) with the molar ratio of 95 : 5 in an argon filled arc
melting apparatus, where a pellet of pure Zr is initially melted to get rid of any
oxygen before the sample being arc-melted. The arc-melted ingot was then ball milled
3

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with 3 at % phosphorus (Johnson Matthey Chemicals, 99.9999%) with a SPEX
SamplePrep 8000M Mixer/Mill for 16 hours (15 min on/off mode to prevent over heat)
inside a 55 mL steel-jacketed tungsten carbide vial with tungsten carbide inserts. Two
8
mm and one 15 mm tungsten carbide balls were used for the ball milling process
and the ball mill container was sealed in air-tight plastic bag filled with Ar to prevent
any oxidation. A typical sample size was about 6 g in total. YbH powder of 0, 1, 2 at %
2
respectively, was added to the n-type Si Ge alloy and ball milled for 30 min inside a
95
5
5
mL polymer ball mill container, sealed in an air-tight plastic bag. The YbH powder
2
was purchased as Yb powder (American Elements, 5N) and determined to be phase
pureYbH from powder X-ray diffraction. The sample sizes were ~ 1 gram each and
2
the four samples were synthesized from the same batch of n-type Si Ge (3 at % P),
95
5
shown in Figure 1.
Another series of samples were synthesized in order to compensate for the loss of P
from the SiGe matrix from the formation of YbP. Equal amounts of P and YbH (1, 2
2
at %) were added to the n-type Si Ge alloy (3 at % P, prepared as described above)
9
5
5
in the ball mill container in order to investigate the addition of YbP with an excess
amount of P dopant. In principle, these samples should result in 3% P dopant with
additional YbP. These samples are labeled with * to distinguish from the first series of
samples.
2
.2 Spark Plasma Sintering (SPS):
Ball milled powders were sieved with 200 mesh filter inside an Ar filled glove box to
prevent oxidation and loaded into a 12.7 mm diameter graphite die with graphite foil
wrapped around the inner surface in glove box. The graphite foil prevents the pellets
from cracking when removing them from the die after sintering. Samples were
◦
sintered using a Dr Sinter Lab Jr. at 950 C for 15 mins and 47.4 MPa. All the pellets
were fully sintered and greater than 98 % dense.
2
.3 X-ray Diffraction (XRD):
Sintered pellets were polished with 400 grit SiC paper to remove graphite foil
residues from the surface and ground into powder. X-ray diffraction patterns were
collected on the ground powder samples utilizing a Bruker D8 diffractometer with Cu
Kα radiation (λ = 1.5418 Å). XRD patterns were also collected from ground powder
samples of the pellets after thermoelectric property characterizations.
2
.4 Electron Microprobe Analysis:
Fragments of the SPSed pellets were polished for microprobe analysis utilizing a
Cameca SX-100 electron microprobe. The elemental composition and YbP inclusions
were verified by X-ray elemental mapping of a typical 200 ꢀm x 200 ꢀm area with an
electron accelerating voltage of 15 keV.
2
.5 Thermoelectric property characterization:
4

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Temperature dependent Hall coefficient and resistivity were measured with Van der
Pauw 4-point tungsten probe measurement from room temperature to 1273 K at 0.8 T
◦
magnetic field. Samples were rapidly annealed in air to 1050 C and then quenched to
16
freeze the high solubility of P dopant in the matrix. Pressure was applied on the
tungsten probes to assure good contact with the sample. The Seebeck coefficient of
sintered 12.7 mm SiGe pellets was measured from 275 K to 1200 K with tungsten–
1
7
niobium thermocouples under high vacuum in a custom apparatus. Thermal
diffusivity was measured utilizing Netzsch LFA 404 system on the graphite coated
pellets, which sat on SiC 12.7 mm sample holders.
Approximately 12 mm x 1.5 mm x 1.5 mm bars were cut from the center of the pellet
with diamond saw. The sample bars were polished and their electrical resistivity and
Seebeck coefficient were measured with a LSR-3 Linseis Seebeck and Electrical
Resistivity Unit in the temperature range of 323 K to 1073 K. This measurement is a
four probe off-axis measurement, e.g. the sample is clamped between upper and lower
Pt electrodes and a pair of Pt thermocouples with a distance of 8 mm (total distance
with the mid-distance being half the length of the bar) are placed off-axis.
3
. Results and discussions:
3
.1 Synthesis and sintering:
The X-ray diffraction (XRD) pattern collected from the 3 at % P doped Si Ge
5
9
5
powder after 16 hours ball milling is presented in Figure 1. All the peaks can be
ꢀ
indexed with Si diamond structure (Fd3m). XRD peaks are significantly broadened.
Based on the pattern’s peak FWHM, sample particle size is calculated to be 141 nm
according to Williamson-Hall plot method, indicating the presence of nanocrystallites
from 16 hours’ ball milling process. After sintering the pellets were polished and their
density was measured in toluene according to the Archimedes principle and listed in
Table 1.
5

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Figure 1. Powder XRD pattern of n-type Si Ge (with 3 at % P) powder after 16
9
5
5
hours ball milling, used as starting material for all samples.
3
1
Table 1. Measured density (g/cm ) of sintered Si Ge pellets at room temperature.
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5
5
3
at % P
No
YbH2
1 %
YbH2
2 %
YbH2
* 1 %
YbH2
* 2%
YbH2
doped
Si Ge
5
95
No
YbP
1 %
YbP
2 %
YbP
* 1 %
YbP
* 2 %
YbP
Sample I.D.
Measured
Density
2.48 (1) 2.56 (1) 2.62 (1) 2.59 (1) 2.67 (1)
3
(g/cm )
Theoretical
Density
2.50
2.55
2.60
2.55
2.60
3
(g/cm )
1
All samples were prepared with 3 at % P dopant (Figure 1). The samples indicated with * had
additional P added in an equivalent at % as the YbH2.
The powder XRD patterns are collected from the polished pellets of the first series of
Si Ge samples and results are shown in Figure 2. The samples with additional P
95
5
added give similar XRD patterns. After the samples are sintered, the peak broadening
is not seen in the XRD pattern, indicating that the grain size is significantly larger.
Besides the Si Ge main phase, the XRD pattern from YbP phase are present,
9
5
5
indexed with the drop line in Figure 2. No YbSi or any other Yb-Si phases are
2
apparent in the sintered pellets. Yb reacts with the phosphorus dopant instead of SiGe
alloy matrix during the SPS process, presumably due to the high mobility of P and the
large formation energy of the rock salt structure. The relative diffraction intensity of
6

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YbP increases with increasing amount of YbH . Therefore, Table 1 provides the YbH
2
2
starting percentages with their sample identification (I.D.) according to the following
compositions: Si Ge with 3 at % P (No YbP) 1 at % YbP and 2 at % P (1 % YbP), 2
9
5
5
at % YbP and 1 at % P (2 % YbP), 1 at % YbP and 3 at % P (*1 % YbP) and 2 at %
YbP and 3 at % P (*2 % YbP).
Figure 2. XRD pattern of sintered n-type Si Ge pellets with the calculated pattern
9
5
5
for the YbP phase indicated by the drop lines (PDF#47-1624). The diffraction peaks
for YbP are indicated with *.
3
.2 Electron microprobe analysis:
To understand the YbP phase distribution, electron microprobe analysis was
performed on the polished sample surface. Yb element mapping images are shown in
Figure 3. YbP particles are characterized by the Yb signal and are uniformly
distributed throughout the samples.
Sample
1
% YbP
2 % YbP
composition
7

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Yb Lα
00ꢀm
*
1 % YbP
* 2 % YbP
1
Figure 3. Yb elemental mapping of the Si Ge samples with YbP and P doping with
9
5
5
1
5 keV electron accelerating voltage. The color corresponds to the relative elemental
composition level according to the color bar shown at the right.
3
.3 Transport properties:
Electrical resistivity of the n-type Si Ge samples were measured from 275 K to 1273
9
5
5
K and plotted in Figure 4. The electrical resistivity of sintered Si Ge samples show a
95
5
similar trend in general as the electrical resistivity increases with increasing
temperature and bends over when temperatures exceed 1000 K. In the samples of YbP
with lesser amounts of P (1 % YbP and 2 % YbP), YbH reacted with electron donor
2
P during sintering process and as a result, samples with higher amounts of YbP
possess higher electrical resistivity, which is not beneficial for thermoelectric
conversion efficiency. 2 % YbP sample has a sharp transition, which may be
attributed to YbP pinning the grain boundary at high temperature. In the case of the
samples indicated with * (*1 % YbP, *2 % YbP), the YbH content was equivalent to
2
the additional P amount of 1 % and 2 %, thereby leaving the P dopant amount at 3 %.
These samples show lower electrical resistivity, as expected. Unlike the multi-doped
9
SiGe alloy with GaP and P, the presence of YbP does not reduce the electrical
resistivity significantly and it is the P content that determines the measured resistivity.
The electrical resistivity of the * samples also increases with temperature below 750
K but show an unusual sharp increase in the temperature range of 750 ~ 800 K and
fluctuating in the temperature range of 800 ~ 1100 K. These temperature effects are
also seen in the plain Si Ge sample doped with 3 % P, but not the samples with less
95
5
than 3 % P (1 % and 2 % YbP labeled samples). The fluctuation in electrical
1
8
resistivity is attributed to P dopant precipitation kinetics at high temperature.
Electrical resistivity decreases at temperatures exceeding 1100 K.
8

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Figure 4. Electrical resistivity of n-type Si Ge samples measured from 275 K to
9
5
5
1
273 K. Electrical resistivity of Si Ge samples are labeled according to YbP and P
95 5
content with 3 at % P (No YbP), 1 at % YbP and 2 at % P (1 % YbP), 2 at % YbP and
at % P (2 % YbP), 1 at % YbP and 3 at % P (*1 % YbP) and 2 at % YbP and 3 at %
1
P (*2 % YbP).
Hall measurements were performed on the Si Ge samples to determine their carrier
9
5
5
concentrations as well as mobility from 275 K to 1273 K (Figure 5). The values of the
carrier concentration confirmed that the samples are heavily doped n-type
2
0
-3
semiconductors with electron carrier concentration in the range of 2 – 5 x 10 cm at
room temperature. The sample with no YbP (Si Ge with 3 % P) has an electron
9
5
5
20
-3
carrier concentration of about 5 x 10 cm from room temperature to about 700 K,
above that temperature the carrier concentration decreases slightly and then increases
above 900 K. The sample with 1 % YbP (with 2 % P) has a carrier concentration of 4
2
0
-3
20
-3
x 10 (cm ). The sample with 2 % YbP (1 % P) is below 3 x 10 (cm ). These
results are consistent with the amount of P dopant. The samples with YbP and 3 % P
(
2
0
20
-3
*1 % YbP and *2 % YbP) show carrier concentration of 4.5 x 10 ~ 5 x 10 cm ,
similar to the 3 % P doped Si Ge (No YbP) sample. The samples’ mobility are in the
95
5
2
same range, about 30 cm /Vs at room temperature, and all the samples possess
linearly decreasing mobility with increasing temperature.
9

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Figure 5. Charge carrier concentrations and mobility from Hall measurements in the
temperature range of 275 K to 1275 K. Si Ge samples are labeled according to YbP
9
5
5
and P content with 3 at % P (No YbP), 1 at % YbP and 2 at % P (1 % YbP), 2 at %
YbP and 1 at % P (2 % YbP), 1 at % YbP and 3 at % P (*1 % YbP) and 2 at % YbP
and 3 at % P (*2 % YbP).
3
.4 Seebeck coefficient measurement:
Sintered samples were measured twice with a custom uniaxial set up and a
commercial four probe off-axis set up. In the uniaxial set up, absolute Seebeck
coefficient was measure from 275 K to 1200 K under vacuum.
Figure 6 shows the Seebeck coefficient for the two series of samples with both
experimental set-ups with the off-axis data indicated. Samples showed large negative
Seebeck coefficients from about – 60 ꢀV/K at 275 K and their absolute Seebeck
coefficients increased with temperature until around 1000 K above which the values
decreased which is attributed to the dopant precipitation kinetics. The Seebeck
coefficients of the two series of samples are shown in the upper and lower figures
separately. The Si Ge sample with 2 % YbP content (1 % P dopant) from the first
9
5
5
series of samples showed a maximum absolute Seebeck coefficient value of over
-200│ꢀV/K at about 1000 K, while no YbP (3 % P dopant) and 1 % YbP content (2 %
P dopant) samples’ Seebeck coefficient are in similar range and stay below
│
1
0

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│
-200│ꢀV/K. In the second series of samples with the same amount of P dopant
(3 %), the samples with 1 % and 2 % YbP content possess similar Seebeck
coefficients. The Seebeck coefficient measurements showed that the off-axis
measurement gave overestimated Seebeck coefficient of up to 25 % higher at 1000 K
compared with the uniaxially measured Seebeck coefficient. This over-estimation has
1
8
been described in the literature. One of the main differences between the two
measurements is the position of thermocouples which measure temperature and
voltage simultaneously. The Seebeck coefficient is obtained from ꢂV/ꢂT, therefore,
the accurate reading of temperature from thermocouples is critical for Seebeck
1
7
coefficient measurements.
a.
-
60
80
No YbP
1
2
% YbP
% YbP
-
-
-
-
-
-
-
-
-
-
100
120
140
160
180
200
220
240
No YbP-off axis
1 % YbP-off axis
2
% YbP-off axis
260
2
00
400
600
800
1000
1200
1400
Temperature (K)
b.
-
60
80
No YbP
*1 % YbP
2 % YbP
-
*
-
-
-
-
-
-
-
-
-
100
120
140
160
180
200
220
240
No YbP-off axis
*
*
1 % YbP-off axis
2 % YbP-off axis
260
2
00
400
600
800
1000 1200 1400
Temperature (K)
1
1

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c.
Figure 6. Comparisons of Seebeck coefficients measurements results from off-axis
instrument set up and uniaxial set up on samples with no YbP and x % YbP (x = 1, 2)
(
a) with changing P dopant amounts and (b) 3 % P dopant. Si Ge samples are
95 5
labeled according to YbP and P content with 3 at % P (No YbP), 1 at % YbP and 2 at %
P (1 % YbP), 2 at % YbP and 1 at % P (2 % YbP), 1 at % YbP and 3 at % P (*1 %
YbP) and 2 at % YbP and 3 at % P (*2 % YbP); (c) Illustration of off axis and
uniaxial instrument setup.
In the off-axis set up, the sample bars are clamped in between the Pt electrodes, and
two Pt thermocouples are pressed against the sample off-axis. The measurement was
performed under helium and graphite foils were used in between the sample and Pt
electrodes to prevent a reaction at high temperature. The thermocouples can act as
cold fingers (since they extend out of the furnace) at the contact position with the
sample and underestimate the temperature gradient reading and therefore the Seebeck
1
9, 20
coefficients are overestimated.
In the uniaxial set up, the thermocouples are
embedded through the heater and cold finger effect can be resolved, but as the
thermocouples are in contact with the samples by constant pressure independently, the
strength of the two thermocouples towards the sample can still affect the temperature
2
0
reading. This disagreement of Seebeck coefficient measurement results from
different instrumental set up and increases with measurement temperature Compare
these two thermoelectric studies, Yb MnSb with a large complex unit cell, possess
.
1
4
11
2
1
thermal conductivity about ten times smaller than SiGe alloys. It is possible that the
cold finger effect is more prominent on compounds with high thermal conductivity
than the compounds with low thermal conductivity. As the off-axis set up of Seebeck
coefficient measurements are common in commercial instruments (ZEM, Linseis,
etc.), researchers should be aware of this temperature reading deviation in the Seebeck
1
9, 20
coefficient measurements of the SiGe alloys.
.5 Thermal transport properties:
The thermal conductivity of the Si Ge samples were characterized from 275 K to
3
95
5
1
275 K and plotted in Figure 7. The samples possess moderate thermal conductivity
in the range of about 60 ~ 70 mW/cmK. This value of thermal conductivity is lower
than what is reported for bulk Si Ge , but higher than Si Ge alloys used in RTG
9
5
5
80
20
1
2

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6
, 9
(
below 50mW/cmK). Samples with YbP inclusions and less than 3 % P dopant (1%
YbP and 2% YbP) show lower total thermal conductivity but the two * samples with 3%
P possess higher thermal conductivity, likely contributed from the over doping and P
at the grain boundaries.
To examine the influence of YbP composites on the lattice thermal conductivity, the
electrical thermal conductivity were calculated with the Wiedemann-Franz equation, κ
=
LT/ρ, in which the measured electrical resistivity at the same temperature range was
-8
−2 21
used and L was 2.22x10 W ꢁ K . The lattice thermal conductivity was shown in
Figure 7, estimated by subtracting electrical thermal conductivity from the total
thermal conductivity.
Figure 7. Thermal conductivity vs. temperature from laser flash measurements and
calculated lattice thermal conductivity. Si Ge samples are labeled according to YbP
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and P content with 3 at % P (No YbP), 1 at % YbP and 2 at % P (1 % YbP), 2 at %
YbP and 1 at % P (2 % YbP), 1 at % YbP and 3 at % P (*1 % YbP) and 2 at % YbP
and 3 at % P (*2 % YbP).
After estimating the electron and phonon contributions to the total thermal
conductivity, the YbP composites with less than 3 % P were found to influence the
thermal conductivity by decreasing the lattice thermal conductivity. The 3% P doped
samples (indicated with *) with YbP showed higher lattice thermal conductivity than
the no YbP phase reference sample (no YbP and 3 % P doping), which is likely to be
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attributed to the P precipitation behavior with higher P level at high temperature.
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Figure 8. The figure of merit of n-type Si Ge composite samples with YbP. Si Ge
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95
samples are labeled according to YbP and P content with 3 at % P (No YbP), 1 at %
YbP and 2 at % P (1 % YbP), 2 at % YbP and 1 at % P (2 % YbP), 1 at % YbP and 3
at % P (*1 % YbP) and 2 at % YbP and 3 at % P (*2 % YbP).
The figure of merit of n-type Si Ge composite samples with YbP is provided in
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Figure 8 and were calculated from the polynomial fits of measured electrical
resistivity, Seebeck coefficient (from the uniaxial instrumental set up) and thermal
conductivity. As discussed above, more exciting data could be presented by applying
off-axis Seebeck data, but this type of measurement provides high Seebeck values due
to a cold-finger effect. The Si Ge sample with 3 % P (No YbP), 1 % YbP with 2 % P
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(1% YbP) and 2 % YbP with 1 % P (2% YbP) show similar values of zT increasing
with temperature. As the thermoelectric properties are interrelated by carrier
concentration, the similarity of results of the YbP containing samples to the (No YbP)
sample is attributed to the improved absolute Seebeck coefficient and lower thermal
conductivity, mainly from decreased carrier concentration. Compared to n-type
Si Ge alloys which have a reported peak zT of 1 in the temperature range of 900 to
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20
◦
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50 C, the low Ge content of Si Ge samples studied herein possess lower figures of
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merit. The YbP inclusions in the matrix provide a smaller effect than the amount of
dopant on the experimental transport properties. The zT of the * samples containing
both YbP and 3 % P bend over at temperatures exceeding 1200 K and show lower zT
due to higher thermal conductivity.
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. Conclusion:
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n-type Si Ge alloy with 3% P dopant was synthesized though 16 hours of ball
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milling with P. YbP was introduced to the alloy matrix during SPS sintering process
with YbH as a starting material. The reaction of YbH with P to form YbP tunes the
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carrier concentration of the composite of Si Ge as YbH reacts with the P, which is
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the electron donor. The sintered composite samples are dense pellets with the
diameter of 12.7 mm. To understand the influence of YbP on the samples’
thermoelectric properties, two series of samples were synthesized in the same manner:
one with decreasing P content (P removed by addition of YbH to form YbP), the
2
second one with constant 3 % P (P added stoichiometrically with YbH to compensate
2
the amount of YbP formed in situ). The samples’ thermoelectric properties were fully
characterized. The deviation of up to 25 % between uniaxial and off-axis configured
instruments that measure Seebeck coefficients are documented, consistent with a cold
finger effect observed in off-axis measurements. Because of the combined effect of
increased electrical resistivity and decreased lattice thermal conductivity, the YbP
with less than 3 % P samples remain consistent with the modest zT observed in 3% P
doped Si Ge . The efforts to compensate P loss by additional P amount during ball
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milling process failed to lead to zT enhancement, because the excess P dopant in the
matrix leads to higher thermal conductivity.
5
. Acknowledgements: The authors thank Gregory Baxter and Nick Botto for sample
polishing and the electron microprobe measurements. Financial support from the DOE
Office of Nuclear Energy's Nuclear Energy University Programs and NSF
DMR-1405973 is gratefully acknowledged. Part of this research was carried out at the
Jet Propulsion Laboratory, California Institute of Technology, under a contract with
the National Aeronautics and Space Administration. This work was supported by the
NASA Science Mission Directorate’s Radioisotope Power Systems program.
6
. Reference:
1
. Slack, G. A., CRC Handbook of Thermoelectrics, edited by D. M. Rowe. CRC
press: Boca Raton, FL, 1995.
2
2
3
. Snyder, G. J.; Toberer, E. S., Complex thermoelectric materials. Nature Materials
008, 7 (2), 105-114.
. Schmidt, G. R.; Dudzinski, L. A.; Sutliff, T. J., Radioisotope power: A key
technology for deep space exploration. INTECH Open Access Publisher: 2011.
. Koumoto, K.; Mori, T., Thermoelectric Nanomaterials: Materials Design and
Applications. Springer Heidelberg, 2013; Vol. 182.
. Basu, R.; Bhattacharya, S.; Bhatt, R.; Roy, M.; Ahmad, S.; Singh, A.;
4
5
Navaneethan, M.; Hayakawa, Y.; Aswal, D. K.; Gupta, S. K., Improved thermoelectric
performance of hot pressed nanostructured n-type SiGe bulk alloys. J. Mater. Chem. A
2
014, 2 (19), 6922-6930.
6
. Wang, X. W.; Lee, H.; Lan, Y. C.; Zhu, G. H.; Joshi, G.; Wang, D. Z.; Yang, J.;
Muto, A. J.; Tang, M. Y.; Klatsky, J.; Song, S.; Dresselhaus, M. S.; Chen, G.; Ren, Z.
1
5

ACCEPTED MANUSCRIPT
F., Enhanced thermoelectric figure of merit in nanostructured n-type silicon
germanium bulk alloy. Appl. Phys. Lett. 2008, 93 (19), 193121.
7
. Joshi, G.; Lee, H.; Lan, Y.; Wang, X.; Zhu, G.; Wang, D.; Gould, R. W.; Cuff, D.
C.; Tang, M. Y.; Dresselhaus, M. S.; Chen, G.; Ren, Z., Enhanced Thermoelectric
Figure-of-Merit in Nanostructured p-type Silicon Germanium Bulk Alloys. Nano Lett.
2
008, 8 (12), 4670-4674.
8
. Zhu, G. H.; Lee, H.; Lan, Y. C.; Wang, X. W.; Joshi, G.; Wang, D. Z.; Yang, J.;
Vashaee, D.; Guilbert, H.; Pillitteri, A.; Dresselhaus, M. S.; Chen, G.; Ren, Z. F.,
Increased Phonon Scattering by Nanograins and Point Defects in Nanostructured
Silicon with a Low Concentration of Germanium. Phys. Rev. Lett. 2009, 102 (19),
1
9
96803.
. Xu, Y.-d.; Xu, G.-y.; Ge, C.-c., Improvement in thermoelectric properties of
n-type Si95Ge5 alloys by heavy multi-dopants. Scripta Mater. 2008, 58 (12),
1
1
070-1073.
0. Mingo, N.; Hauser, D.; Kobayashi, N. P.; Plissonnier, M.; Shakouri, A.,
“
Nanoparticle-in-Alloy” Approach to Efficient Thermoelectrics: Silicides in SiGe.
Nano Lett. 2009, 9 (2), 711-715.
1. Janka, O.; Zaikina, J.; Bux, S.; Tabatabaifar, H.; Yang, H.; Browning, N. D.;
1
Kauzlarich, S. M., Microstructure Investigations of Yb-substituted Mg2Si for
Thermoelectric Applications. In J. Mater. Chem. C, 2016.
1
2. Liao, B.; Chen, G., Nanocomposites for thermoelectrics and thermal engineering.
MRS Bull. 2015, 40 (9), 746-752.
3. Yu, B.; Zebarjadi, M.; Wang, H.; Lukas, K.; Wang, H.; Wang, D.; Opeil, C.;
1
Dresselhaus, M.; Chen, G.; Ren, Z., Enhancement of Thermoelectric Properties by
Modulation-Doping in Silicon Germanium Alloy Nanocomposites. Nano Lett. 2012,
1
2 (4), 2077-2082.
1
4. Zaikina, J. V.; Batuk, M.; Abakumov, A. M.; Navrotsky, A.; Kauzlarich, S. M.,
Facile Synthesis of Ba1–xKxFe2As2 Superconductors via Hydride Route. J. Am.
Chem. Soc. 2014, 136 (48), 16932-16939.
1
5. Sui, F.; He, H.; Bobev, S.; Zhao, J.; Osterloh, F. E.; Kauzlarich, S. M., Synthesis,
Structure, Thermoelectric Properties, and Band Gaps of Alkali Metal Containing Type
I Clathrates: A8Ga8Si38 (A = K, Rb, Cs) and K8Al8Si38. Chem. Mater. 2015, 27 (8),
2
1
812-2820.
6. Bux, S. K.; Blair, R. G.; Gogna, P. K.; Lee, H.; Chen, G.; Dresselhaus, M. S.;
Kaner, R. B.; Fleurial, J.-P., Nanostructured Bulk Silicon as an Effective
Thermoelectric Material. Ad. Func. Mater. 2009, 19 (15), 2445-2452.
1
7. Wood, C.; Chmielewski, A.; Zoltan, D., Measurement of Seebeck coefficient
using a large thermal gradient. Rev. Sci. Instrum. 1988, 59 (6), 951-954.
8. Dunham, S. T., MODELING OF THE KINETICS OF DOPANT
PRECIPITATION IN SILICON. J. Electrochem. Soc. 1995, 142 (8), 2823-2828.
9. Joshua, M., Protocols for the high temperature measurement of the Seebeck
coefficient in thermoelectric materials. Meas. Sci. Technol. 2013, 24 (8), 085601.
0. Borup, K. A.; de Boor, J.; Wang, H.; Drymiotis, F.; Gascoin, F.; Shi, X.; Chen, L.;
Fedorov, M. I.; Muller, E.; Iversen, B. B.; Snyder, G. J., Measuring thermoelectric
1
1
2
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ACCEPTED MANUSCRIPT
transport properties of materials. Energy Environ. Sci. 2015, 8 (2), 423-435.
2
1. Grebenkemper, J. H.; Hu, Y. F.; Barrett, D.; Gogna, P.; Huang, C. K.; Bux, S. K.;
Kauzlarich, S. M., High Temperature Thermoelectric Properties of Yb14MnSb11
Prepared from Reaction of MnSb with the Elements. Chem. Mater. 2015, 27 (16),
5
2
791-5798.
1. Kumar, G. S.; Prasad, G.; Pohl, R. O., Experimental determinations of the Lorenz
number. J. Mater. Sci. 1993, 28 (16), 4261-4272.
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Highlights
•
•
•
A new composite phase of YbP with phosphorus (P) doped Si Ge (SiGe) alloy
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YbP lowers lattice thermal conductivity when the sample is appropriately doped
Off-axis measurements overestimate the Seebeck coefficients of Si95Ge alloys
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