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Fabrication and Characterization of Nano-crystalline Bi 2 Te 3 by Ball Milling
A. Tsiappos [1] , Th. Kyratsi [1]* and P. Trikalitis [2]
[1]
Dept of Mechanical & Manufacturing Engineering, University of Cyprus, 1678 Nicosia, Cyprus
[2]
Dept of Chemistry, University of Crete, 71409 Heraklion, Greece
*e-mail address: kyratsi@ucy.ac.cy
Introduction Thermoelectric materials can directly convert heat into electricity and vice versa. These materials
have many attractive applications in solid-state cooling (e.g. small refrigeration) and electric power generation
(e.g. space applications Voyager I-II and Cassini missions to Saturn [1].
The semiconducting bismuth telluride material and its alloys are the best known bulk thermoelectric materials
available today with the higher performance (ZT~1) near room temperatures. These materials are widely used in
cooling operations [2]. Much research has been done in the recent years to improve the performance of these
materials by structural modifications. Kanatzidis et al. prepared the alkali bismuth chalcogenides compounds
CsBi 4 Te 6 [3] which is the best known bulk thermoelectric material for temperatures below the room temperature.
Superlattices p-type Bi 2 Te 3 /Sb 2 Te 3 was fabricated by deposition of thin films of Bi 2 Te 3 and Sb 2 Te 3 layers indicate
the biggest ZT values [4]. Moreover, theoretical investigations of the thermoelectric properties of low dimensional
systems have been pursued extensively for various materials. The enhancement in thermoelectric properties is
anticipated to be more pronounced as the dimensionality decreases [5]. Nanostructured morphologies including
nanorods, nanotubes, polygonal nanosheets, and polyhedral nanoparticles of Bi 2 Te 3 based alloys have recently
been prepared by solvothermal or hydrothermal synthesis [6].
Ball milling is another approach for producing nanostructured and nano-composite thermoelectric materials [7].
Bulk materials are deformed plastically by means of repeated mechanical impacts. The shape and size of these
materials gradually change and a degree of disorder is being formed in their lattice. In this study nano-crystalline
Bi 2 Te 3 powders were fabricated via high energy ball milling of Bi 2 Te 3 bulk. Powder X-ray diffraction patent
analysis and SEM were used to investigate the morphological and microstructural changes during the ball milling
operation.
Experimental Bi 2 Te 3 starting material was synthesized from melt by stoichiometric reaction of elements at
temperatures higher than 800 o C. Subsequently, powdered samples were prepared via high energy planetary ball
mill with grinding speed of 400rpm. Bulk material was placed in a tungsten carbide jar partially filled with balls
and the powder to ball weight ratio was held constant at 1:20 throughout the experiments. The powder was ball
milled using 5mm grinding balls for 120h and followed by grinding process using 0.6mm grinding balls. The jar
with the balls was sealed under high purity nitrogen atmosphere so that no contaminants from the air would affect
the milling powder. Due to high grinding speed and raising temperatures, the system was allowed to cool for
10min after 1h ball milling operation. Samples were collected every 20h ball milling.
Results and Discussion Figure 1 shows the X-ray powder diffraction patterns of the starting material and of the
ball milled samples at various times (80h, 140h and 160h). The X-ray powder diffraction patterns clearly reveal
that the peak intensity decreased with increasing the milling time. Moreover, peak broadening occurs and the
degree of overlapping of neighbouring reflections increases. The peak broadening of the milled powder increased
mainly due to the combined effect of the crystallite size reduction and of the lattice strain within the crystallites
that is introduced during the ball milling procedure [8]. In this work, we considered only (015) reflection
(2θ=27.61 o ) because other reflections are much less significant as well as overlapping appears at higher milling
time. The instrumental broadening was corrected using silicon reference sample. In order to determine the
crystallite size and the strain two distributions were used; the Cauchy (Lorentzian) and the Gausian distributions.
The crystallite sizes were calculated [7] from the Lorentzian equation (1) where FWHM (Lorentzian) =FWHM (Total) -
FWHM (instr.) and the strain by the Gausian equation (2) where FWHM 2 (Gausian)=FWHM 2 (Total)-FWHM 2 (instr.).
FWHM (Gausian)
λ
t
hkl
= (1)
e = (2)
FWHM
(Lorentzian)
cosθ
4tanθ
The crystallite size and the strain calculations that resulted from the (015) reflection are shown in figure 2 as a
function of the milling time. The crystallite size reduces from ~30 to ~25nm during the first 60h milling and then
attains a saturation value up to 120h milling. On the other hand, reduction of the crystallite size to ~15nm was
achieved using grinding balls (0.6mm). In contrary, the lattice strain was increased reaching a certain value up to
120h milling time. Using smaller balls, the strain increases further and it finally reaches the value of ~7x10 -3 .
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