Journal of ELECTRONIC MATERIALS, Vol. 44, No. 8, 2015
DOI: 10.1007/s11664-015-3708-6
Ó 2015 The Minerals, Metals & Materials Society
Nanostructuring of Undoped ZnSb by Cryo-Milling
X. SONG,1,4 K. VALSET,1 J.S. GRAFF,2 A. THØGERSEN,1,2
A.E. GUNNÆS,1 S. LUXSACUMAR,2 O.M. LØVVIK,1,2
G.J. SNYDER,3 and T.G. FINSTAD1,5
1.—Department of Physics, University of Oslo, Blindern, P.O. Box 1048, 0316 Oslo, Norway.
2.—SINTEF Mat and Chem, 0314 Oslo, Norway. 3.—Materials Science, California Institute of
Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA. 4.—e-mail: xins@fys.uio.no.
5.—e-mail: tgf@fys.uio.no
We report the preparation of nanosized ZnSb powder by cryo-milling. The
effect of cryo-milling then hot-pressing of undoped ZnSb was investigated and
compared with that of room temperature ball-milling and hot-pressing under
different temperature conditions. ZnSb is a semiconductor with favorable
thermoelectric properties when doped. We used undoped ZnSb to study the
effect of nanostructuring on lattice thermal conductivity, and with little contribution at room temperature from electronic thermal conductivity. Grain
growth was observed to occur during hot-pressing, as observed by transmission electron microscopy and x-ray diffraction. The thermal conductivity was
lower for cryo-milled samples than for room-temperature ball-milled samples.
The thermal conductivity also depended on hot-pressing conditions. The
thermal conductivity could be varied by a factor of two by adjusting the process conditions and could be less than a third that of single-crystal ZnSb.
Key words: Nanostructuring, ZnSb, thermal conductivity, thermoelectric
materials
INTRODUCTION
Thermoelectric energy conversion has the potential to be reliable, scalable, and environmentally
friendly. Research performed to develop materials
with increased conversion efficiency has flourished in
the last two decades.1,2 Improved efficiency has
resulted from synthesis of new materials, from
nanostructuring of known materials, and from combinations thereof.3,4 Many different nanostructuring
methods have been used, with a variety of intended
effects. These methods, which include growth of
superlattices, precise control of three-dimensional
heteroepitaxial systems,5–7 and nano-segregation in
bulk materials,7–10 have been proposed for reducing
thermal conductivity, by increasing phonon scattering, and the Seebeck coefficient, by altering the
electronic density of states11 or by energy filtering.12
These approaches have also been proposed for
increasing the electric conductivity, by modulation
(Received September 9, 2014; accepted February 18, 2015;
published online March 6, 2015)
2578
doping13 or band engineering.14 In some cases several
of the mentioned effects could be occurring simultaneously and ascertaining a particular nanostructure
feature or benefit is not trivial.
The purpose of this work was to prepare nanograins of the thermoelectric material ZnSb with
different grain size. In particular we investigated
cryo-milling as a technique for creation of
nanopowders of ZnSb for pressing into pellets.
There were no intended secondary phases and no
doping was performed. ZnSb has been known for a
long time and used for thermoelectric applications.15 Lately there has been a renewed interest in
the material.16–24 ZnSb has the advantages of being
environmental friendly, abundant, and with
relatively high hole mobility and Seebeck coefficient, thus yielding a good power factor. It could be
an attractive material for the temperature range
370–720 K if combined with a matching n-type
thermoelectric material, for example Mg2Si1 xSnx.15
Nanostructuring of ZnSb might be mainly beneficial
for reducing the thermal conductivity.
Nanostructuring of Undoped ZnSb by Cryo-Milling
Introducing grain boundaries to scatter phonons
has been used a long time. Rowe et al.25 reported
reduction of the thermal conductivity of SiGe alloys
prepared by milling and hot pressing rather than
use of solidified ingots. More recently Zhu et al.26
reported for nanostructured Si and SiGe alloys that
nanosized interfaces are not as effective as point
defects at scattering phonons with wavelengths
shorter than 1 nm. They also found that replacing
Si with 5 at.% Ge results in very efficient scattering
of phonons with wavelengths shorter than 1 nm; the
figure-of-merit of nanograin Si95Ge5 was similar to
that of large-grain Si80Ge20 alloys. Poudel et al.27
reported an increase in the figure of merit of up to
40% as a result of hot pressing of BiSbTe
nanopowders compared with an ingot of the alloy.
They attributed this to reduced thermal conductivity as a result of phonon scattering at grain
boundaries and defects. The electrical significance
of grain boundaries is not well known, especially
because the electronic structure of the grain
boundary may not be known for all materials. One
should expect interplay between dopants and grain
boundaries. For polycrystalline Si(polySi), as used
for gate and conducting paths in microelectronics
devices, the effects of grain boundary trap states,
doping level, and grain size on electrical properties
have been much studied and modeled.28 This analysis was also found useful for analysis of thermoelectric behavior,29 although many more details
than are generally known for most thermoelectric
materials may be required. The state of the art has
been reviewed by Schierning.30
In this paper we report synthesis of undoped
ZnSb by ball-milling of powders at room and cryogenic temperatures, followed by different types of
hot-pressing. We also report the microstructure of
the ZnSb and correlate this with thermal transport
measurements.
EXPERIMENTAL
ZnSb ingots were prepared by melting the elements in a vacuum sealed quartz ampule. The
ingots were crushed then ball-milled in two different mills with the important difference being the
temperature during milling—room temperature or
cryogenic (liquid nitrogen temperature). We call
these methods ‘‘RT-milling’’ and ‘‘cryo-milling’’
respectively. After ball-milling the powders were
hot-pressed into pellets by use of one of two different
hot-presses, the main differences between them
being the temperature conditions. We call these
processes ‘‘slow hot-press’’ (SHP) and ‘‘rapid hotpress’’ (RHP), respectively. The set temperature was
470°C for 30 min for both. The nominal uniaxial
pressure was 20 MPa and 12 MPa for SHP and
RHP, respectively. The cool time for SHP from
470°C to room temperature was>20 h whereas that
for RHP was approximately 1.5–2 h. The setup for
RHP is described in detail by LaLonde et al.31
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Four different synthetic procedures were used,
depending on the nature of ball-milling and
hot-pressing:
1.
2.
3.
4.
RT + SHP;
RT + RHP;
cryo + SHP; and
cryo + RHP.
These will be compared. After hot-pressing the pellets were sawn into smaller pieces then ground into
disk shaped samples suitable for characterization.
The size distribution profile of the ZnSb particles
in the cryo-milled powders was measured by
dynamic light scattering (DLS; Malvern Zetasizer),
by suspending the powder particles in alcohol then
using factory-preinstalled routines for data analysis. The structures of the milled powders and the
hot-pressed samples were characterized by transmission electron microscopy (TEM) and x-ray
diffraction (XRD). Sizes of nanoparticles were measured by high-resolution TEM. Simple mean grain
size or particle size were determined by measuring
the full width at half maximum (FWHM) of the
diffraction peaks and use of the Scherrer formula.32
The thermal conductivity of disk-shaped samples
was determined by measuring the thermal diffusivity by use of the laser-flash technique (Netzsch
Laser Flash LFA 457) and an experimental value
for the specific heat capacity of ZnSb of 0.3 J/(gK).
The absolute measurement uncertainty may be
approximately 10% but the repeatability is much
better, making all comparisons between samples
significant. The mass density of each sample was
measured by use of the Archimedes method,
adapted to ISO5017 standard.
RESULTS AND DISCUSSION
The size-distribution profile of the ZnSb particles
in the cryo-milled powders was measured by DLS.
Figure 1 shows a profile obtained by measuring for
64 h. One strong peak corresponds to particle sizes
<10 nm and another to sizes of approximately
1000 nm. The profile below 1 nm is regarded as
erroneous and is omitted. The contributions to the
distribution profile change substantially with measurement time, because of sedimentation. Because
the scattered light intensity for Rayleigh scattering
decreases with the inverse of the sixth power of the
particle size the contribution from large particles
completely dominates at the beginning of the measurement whereas during the last 32 h of measurement the large and medium sized particles have
sedimented. The profile is thus taken to indicate
that the powders contain particles of many different
sizes down to below 10 nm. The peak at 5 nm could
have some significance.
We analyzed the powders by TEM by drying
droplets of suspended powders on electron-transparent carbon films. Figure 2a and b show images of
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Song, Valset, Graff, Thøgersen, Gunnæs, Luxsacumar, Løvvik, Snyder, and Finstad
the cryo-milled powders; the particle-size distribution in the area indicated is shown in Fig. 2c. We
noticed in the TEM that in addition to the many
small (10 nm) particles there were many larger
lumps; one is seen in the corner of Fig. 2a. By use of
high-resolution TEM we found that these large
lumps consisted of many smaller ZnSb particles or
grains.
Some of the medium-sized cryo-milled particles
were revealed as resulting from agglomeration of
smaller particles, are shown in Fig. 3. A Moiré
pattern originating from grain overlap was
observed, and is indicated in Fig. 3. Thus TEM
confirms the existence of many small nanoparticles,
as inferred from DLS measurement, and many
medium and larger particles, many of which are the
result of agglomeration of smaller particles.
XRD analysis of the powders identified the main
phase as ZnSb, and a very small amount of Sb.
(Amounts of Sb [1% are regarded as normal for
ZnSb. The main reasons for excess Sb are believed
to be oxidation of Zn to form ZnO and evaporation of
Zn at high temperatures. At these low levels Sb
seems to have an insignificant direct effect on the
changes studied in this paper) The FWHM of the
ZnSb diffraction peaks corresponds to a mean grain
Fig. 1. DLS particle-size profile accumulated for 64 h. Sample: cryomilled ZnSb suspended in alcohol. The curve is affected by
sedimentation.
size of 19 nm. Considering the different averaging
process and uncertainties, this can be regarded as
compatible with the TEM observation. Figure 4
shows how the grain size, as determined by XRD,
increases with milling temperature. The temperature was ramped up to the next measurement
point in 10 min. The temperature then stabilized
and the measurement was obtained. The sample
was kept at each temperature for approximately
20 min for stabilization and measurement. The
XRD peak width remained at the highest value after
cooling, which excludes the possibility that the XRD
peak broadening was because of inhomogeneous
stress which could be caused by temperature gradients. Figure 4 shows that grain growth occurs in
the powder without application of hydrostatic
pressure. This indicates that many of the
nanocrystals in the powder are in good physical
contact and the larger particles may be classified as
‘‘hard agglomerates’’.33 We also note from Fig. 4
that the grain size of the RT milled powder is larger
than that of the cryo-milled powder, which is
expected, but that the grain-growth behavior is very
similar.
Considering the strong grain growth observed in
Fig. 4 we would expect substantial grain growth during hot-pressing, because of increased diffusivity at
the high temperature (470°C) of the hot-presses.
Table I shows results from XRD peak FWHM
examination after hot-pressing the powders to pellets;
It is apparent that RT + SHP results in the largest
grains and cryo + RHP results in the smallest grains.
The results of Table I are qualitatively as
expected, given the initial rational ordering of the
grain sizes of the powders. There should be less
emphasis on the exact values than on their ordering, because the simple FWHM used here is a crude
value for samples which can have wide distributions
of crystallite sizes.32
Fig. 2. TEM study of cryo-milled powder. (a), (b) Images of nanoparticles; (c) size distribution in an area without lumps such as that seen in the
right hand corner of (a).
Nanostructuring of Undoped ZnSb by Cryo-Milling
2581
Fig. 3. TEM images of cryo-milled ZnSb powders containing many lumps. (a), (b) The lumps were recognized as the result of agglomeration of
small nanograins and nanoparticles. The starry cross in the middle of (b) indicates a Moiré pattern arising from overlapping grains of different
crystal orientations. The two images have different magnification. The area focused on is indicated by the frame.
Fig. 5. TEM micrograph of pellets synthesized from cryo-milled
powders of ZnSb by different hot pressing: (a) cryo + SHP; (b)
cryo + RHP.
Fig. 4. Grain size of ZnSb powders determined by from XRD peak
FWHM, measured at different temperatures for two differently prepared ZnSb powders: RT ball-milled powder and cryo-milled powder.
Table I. XRD FWHM mean grain size of ZnSb pellets
SHP
RHP
RT-milled (nm)
Cryo-milled (nm)
292
178
199
68
Figure 5 shows TEM micrographs of the local
microstructure for pellets hot-pressed from cryomilled powders. For the area we observed, the grain
size of RHP is smaller than that of SHP, on average,
which can be seen by comparing Fig. 5a with
Fig. 5b. The bright spots seen in the TEM micrograph at triple points of adjoining grains are simply
because of perforation of the sample, and can be
ignored. Many small ([10 nm) particles are
revealed in the TEM micrographs of Fig. 5 and can
be identified. It has been reported that ZnO
nanoparticles can form at the grain boundaries of
ZnSb material.34 To identify the particles here we
used high resolution TEM. Figure 6a shows part of
a TEM high-resolution micrograph of ZnSb prepared by cryo + RHP. Several particles of
approximately 10–20 nm are seen, and their lattice
fringes are clearly visible. The fringes and diffraction patterns identify the nanoparticles as ZnSb
which effectively have ‘‘survived’’ the RHP process.
Figure 7 shows the grain size increase, as a
function of temperature, of pellets synthesized by
cryo + RHP, measured by XRD. We see that the
behavior of the powders is similar to that in Fig. 4.
Figure 7 shows the hot-pressed sample started with
a grain size of approximately 70 nm, which agrees
with the value for the cryo + RHP pellet in Table I.
Even though the sample at the start of the
experiment of Fig. 7 has been subjected to the
temperatures of the hot-press, grain growth continues
at temperatures as low as 200°C and the change of
the FWHM at 300°C in Fig. 7 is similar to that in
Fig. 4. We take this as an indication of relatively
strong grain growth and that the RHP process
results in a sample that is not temperature stable in
terms of grain growth.
Figure 8 shows the thermal conductivity of
pellets synthesized by the processes indicated.
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Song, Valset, Graff, Thøgersen, Gunnæs, Luxsacumar, Løvvik, Snyder, and Finstad
Fig. 6. (a) High-resolution TEM image of ZnSb pellets synthesized by cryo + RHP. (b) Fourier transform of the areas indicated. All are identified
as ZnSb nanocrystals.
Fig. 7. The grain size of ZnSb pellets synthesized by cryo + RHP,
determined by XRD at different temperatures.
Fig. 8. Thermal conductivity of ZnSb pellets made by ball-milling
either at RT or at cryogenic temperature followed by either rapid hotpress RHP or slow hot-press SHP.
These values are lower than the value of 3.5 W/(mK)
reported for pure ZnSb single crystals35 at room
temperature. So ball-milling reduces the thermal
conductivity and the values are ordered in accordance with the methods used for synthesis, as can
be read directly from the legend in Fig. 8. For
practical applications, the process cryo + SHP
seems the most promising for reasons we will
discuss. It yields a thermal conductivity below 1 W/
(mK) at 500 K.
The thermal conductivity values are approximately
the same a those reported by others for ZnSb.17,18,20
Ekløf et al.,20 reported an RT thermal conductivity
of 1.4 W/(mK) for undoped ZnSb. Okamura et al.24
reported a value of 1.4 W/(mK) for ZnSb with no
doping ballmilled at RT in a process similar to
our RT-SHP, although they reported a mass density
much higher than that obtained in our work
(Table II).
The density of the pellets was affected by the
synthetic process, as is apparent from Table II; SHP
yields the most compact samples and RHP the least
compact samples. The compactness of the samples
might be indicative of greater pore volume and more
scattering from voids and pores. We believe this
contributes to the effect observed. It is also reasonable that SHP pellets are more dense than RHP,
because they have been densified for longer. The
longer thermal treatment in the SHP process also
leads to more grain growth, as we have observed.
Thus, there are fewer grain boundaries that can
scatter phonons for SHP than for RHP samples. It is
also apparent from Fig. 8 that cryo-milling results
in lower thermal conductivity than RT ball-milling
when the other conditions are the same. This is
consistent with the observation that cryo-milled
samples have smaller grains than the RT ballmilled samples when hot-pressed identically, as
Nanostructuring of Undoped ZnSb by Cryo-Milling
Table II. Density of ZnSb pellets (±3%)
SHP
RHP
RT-milled (g/cm3)
Cryo-milled (g/cm3)
6.0
5.8
6.0
5.6
indicated by the XRD. We also believe it is plausible
that other defects, as a result of stacking faults,
dislocations, etc., are introduced with a density
increasing with decreasing grain size, and these
defects will also contribute to reducing the thermal
conductivity.
The thermal conductivity, j, is usually split into
two parts, j = je + jL, where, jL is the lattice
thermal conductivity and je is the electronic contribution from the carriers. In this work the measured carrier concentrations at room temperature
were so low that jL dominated. (The room temperature resistivity is 2–6 9 10 2 Xcm, giving
je < 2 9 10 2 W/mK > 2 W/mK.) This means that
at room temperature the reduction of the thermal
conductivity as a result of nanostructuring (Fig. 8)
is because of reduction of the lattice thermal conductivity. At higher temperatures the thermally
generated electrons and holes cause je to increase so
much that j also increases. Because the samples are
intrinsic (n = p) at the higher temperatures, it becomes nontrivial to extract jL at the highest temperatures because the nature of the bipolar thermodiffusion effect36 is unknown for ZnSb.
Although details of how the electrical properties
change are beyond the scope of this paper, it is
important to consider the stability of the material,
with regard to both the reliability of the measurements reported and the use of these synthetic procedures for practical applications. A few details of
our electrical measurements (of resistivity, mobility,
carrier concentration, and Seebeck coefficient) are
therefore included. Most of the synthetic procedures
yield samples with stable and reproducible electric
properties that can be cycled between room temperature and 350–400°C. However, it is clear that
for undoped ZnSb the process cryo + RHP yields
samples that are not thermally stable—we found
that resistivity and Seebeck coefficient were changed by heat treatment at 400°C, and that other
properties changed during temperature-dependent
measurements. For sample category cryo + RHP
the thermal instability even affects the thermal
conductivity. For example, after the thermal conductivity had been measured for the cryo + RHP
sample, its electric properties were measured from
RT up to 425°C. The thermal energy received by the
sample during these measurements changed its
thermal conductivity; it was increased by 20% and
became similar to that of cryo + SHP. We believe
this is connected with the atomic mobility in the
2583
material. We can deduce from the XRD results in
Fig. 7 that diffusion in the material must be
appreciable at temperatures above 300°C, considering the grain growth. The large differences
between the stability and behavior of samples produced by use of the different synthetic processes is
surprising and will be addressed in future work.
It seems that the process of cryo-milling has the
potential to make nanostructured materials with
enhanced properties. Even if there is much grain
growth in ZnSb during hot-pressing many nanostructures do survive and this leads to the low thermal conductivity. It is not known whether possible
reduction of grain growth will have an overall
positive effect. It is believed that addition of other
elements to the material may lead to segregation in
grain boundaries and prevent some of the grain
growth, or that addition of foreign nanoparticles in
the cryo-milling process may have a similar effect.
For good thermoelectric performance ZnSb has to be
optimally doped; dopants and dopant segregation
may also affect grain growth.
CONCLUSION
We synthesized ZnSb by cryo-milling and rapid
hot-pressing. The products were compared with
those obtained by ball-milling at room temperature
and by hot-pressing with longer temperature
treatment. We observed that nanosized grains can
be produced by cryo-milling, and that some nanosized grains remain after the hot-press process.
Broad grain-size distribution, confirmed by use of
different techniques, may promote scattering of
phonons of different wavelengths. We studied the
effect of temperature on grain growth during both
hot-pressing and annealing. Thermal conductivity
at room temperature was reduced by approximately
66% by use of cryo-milling and RHP, because of
reduction of the lattice thermal conductivity. However, cryo-milling and RHP of undoped ZnSb under
the conditions used in this work resulted in properties that were changed by cycling to 400°C,
accompanied with grain growth. Cryo-milling combined with longer heat treatment during hotpressing resulted in less reduction of the thermal
conductivity than did RHP, but resulted in good
robustness to temperature cycling.
ACKNOWLEDGEMENT
We thank Jaya Nolt of UCSB for performing XRD
at different temperatures. X.S. is grateful for help
with the rapid hot-press at California Institute of
Technology (c/o Jeff Snyder) and use of high-temperature Hall setup. This work was supported by
the Norwegian Research Council under contract
NFR11-40-6321 (NanoThermo) and the University
of Oslo. X.S. acknowledges financial support by a
Kristine Bonnevie stipend from University of Oslo,
a travel grant, and infrastructure grants from the
Norwegian Nano-Network and from NorFab.
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Song, Valset, Graff, Thøgersen, Gunnæs, Luxsacumar, Løvvik, Snyder, and Finstad
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