Preparation of PbTe-based materials through thermal decomposition of lead acetate

Article abstract of DOI:10.1134/S002016851206009X

This paper examines the possibility of producing microstructured lead telluride with a high grain- boundary density through the thermal decomposition of lead acetate in the presence of tellurium under a reducing atmosphere, followed by cold compaction of the powder and hot pressing of the green compacts. The formation of fine lead telluride particles was followed using differential thermal analysis and thermo-gravimetry. Electron microscopy, X-ray microanalysis, and specific surface area measurements were used to assess the conductivity and size of the powder particles and the structure and composition of the grains in the hot-pressed samples in relation to the preparation conditions and starting-mixture composition. Pleiades Publishing, Ltd., 2012.

Full text of DOI:10.1134/S002016851206009X

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ISSN 0020ꢀ1685, Inorganic Materials, 2012, Vol. 48, No. 6, pp. 563–568. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © V.G. Leontyev, L.D. Ivanova, K. Bente, V.F. Gremenok, 2012, published in Neorganicheskie Materialy, 2012, Vol. 48, No. 6, pp. 654–660.
Preparation of PbTeꢀBased Materials
through Thermal Decomposition of Lead Acetate
V. G. Leontyeva, L. D. Ivanovaa, K. Benteb, and V. F. Gremenokc
a Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Leninskii pr. 49, Moscow, 119991 Russia
b Fakultät für Chemie und Mineralogie, Universität Institut für Mineralogie,
Kristallographie und Materialwissenschaft, Leipzig, Germany
c Scientific–Practical Materials Research Centre, Belarussian Academy of Sciences, ul. Brovki 19, Minsk, 220072 Belarus
eꢀmail: leontiev_v_g@mail.ru
Received November 18, 2011; in final form, December 12, 2011
Abstract—This paper examines the possibility of producing microstructured lead telluride with a high grainꢀ
boundary density through the thermal decomposition of lead acetate in the presence of tellurium under a
reducing atmosphere, followed by cold compaction of the powder and hot pressing of the green compacts.
The formation of fine lead telluride particles was followed using differential thermal analysis and thermoꢀ
gravimetry. Electron microscopy, Xꢀray microanalysis, and specific surface area measurements were used to
assess the conductivity and size of the powder particles and the structure and composition of the grains in the
hotꢀpressed samples in relation to the preparation conditions and startingꢀmixture composition.
DOI: 10.1134/S002016851206009X
INTRODUCTION
The purpose of this work was to examine the possiꢀ
bility of producing microstructured lead telluride with
a high grainꢀboundary density from a powder prepared
through the thermal decomposition of a Pb salt in the
presence of tellurium in a reducing atmosphere, folꢀ
lowed by cold compaction and hot pressing.
Leadꢀtellurideꢀbased materials are used in various
devices for direct thermoelectric energy conversion at
temperatures from 500 to 1000 K.
According to phase diagram data, lead telluride has
a narrow homogeneity range, within a fraction of a
percent. In this range, it can be both nꢀ and pꢀtype,
EXPERIMENTAL
depending on tellurium content relative to the stoichiꢀ
ometric composition. The performance of PbTeꢀbased
materials can be improved by doping with various
impurities (e.g., Na, Tl, and Sn) [1, 2]. The thermoꢀ
electric figure of merit (ZT) of such materials at their
working temperatures (around 800 K) can reach 1.8
[2]. A conventional procedure for the fabrication of
leadꢀtellurideꢀbased materials is to melt an elemental
mixture in evacuated silica ampules and homogenize
the resultant ingot by prolonged annealing, followed
by crushing and hot pressing. This procedure yields
Lead telluride was prepared through the thermal
decomposition of a mechanical mixture of lead aceꢀ
tate trihydrate and tellurium powder at temperatures
from 200 to 400
mixture was slowly heated and held at temperature
250–400 ) for 1 h, which was sufficient for the
°
С in a flowing reducing gas (Н2). The
(
°
С
decomposition and reaction processes to reach comꢀ
pletion. The flow rate of the reducing gas was
100 mL/min. The starting materials used were lead
acetate trihydrate, Pb(CH3COO)2 3H2O (analytical
⋅
grade: nitrates, 0.005; chlorides, 0.0005; Fe, 0.001;
K + Na + Ca + Sr, 0.01; Cu, 0.0005 wt %), and telluꢀ
rium powder (99.999% purity, particle size under
coarseꢀgrained (above 100 m) materials. According
μ
to recent theoretical and experimental studies [3–5],
the ZT of thermoelectric materials can be raised furꢀ
ther by reducing their grain size to tens of nanometers.
This is related to specific features of the electronic
structure of nanometerꢀsized grains, which reduce the
electrical conductivity of the material but may
increase its Seebeck coefficient and considerably
reduce its thermal conductivity. A large grainꢀboundꢀ
ary surface effectively scatters phonons but has little
effect on carrier transport, which leads to a sharp drop
in thermal conductivity. As a result, the ZT of nanoꢀ
structured thermoelectric materials exceeds that of
coarseꢀgrained materials [6, 7].
56 m). The mixture was prepared by mechanical
μ
grinding with a pestle in an agate mortar. The telluꢀ
rium to lead atomic ratio in the starting mixture was
either stoichiometric or corresponding to Te excesses
of up to 20 at %.
Some of the samples were prepared in covered
graphite crucibles. The heating rate, temperature, gas
flow rate, and sample weight were controlled directly
in a Netzsch STA 409 thermogravimetric system. The
heating rate was 2.5 K/min, and the initial sample
weight was 0.5–2 g. To prepare larger samples, we used
563

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564
LEONTYEV et al.
DTG, %/min
[1.1]
0
ТG, %
100
2.2
2.1
95
90
85
80
1.
1
–0.2
–0.4
–0.6
–0.8
[2.1]
1.2
Weight
change
–14.59%
3
4
75
70
65
60
–1.0
–1.2
–1.4
–1.6
2
[1.1]
1
[2.1]
Weight
change
5
–14.86%
350
50
100
150
200
250
300
400
Temperature, °C
Fig. 1. TG and DTG heating curves of Pb(CH COO) and a Pb(CH COO) + Te mixture (heating rate, 2.5°C/min; hydrogen
3
2
3
2
atmosphere): (1, 4) double peaks due to dehydration and water boiling, (2, 3) peaks due to the decomposition of anhydrous lead
acetate, (5) single peak due to the decomposition of the reaction products of anhydrous lead acetate and tellurium. The solid lines
1
.
1
and
1
.
2
show the weight change and derivative curve for the decomposition of a mixture of lead acetate and tellurium.
and show the weight change and derivative curve, respectively, for the decomposition of lead acetate.
The dashed lines
2.1
2.2
a quartz boat, which was placed in a tubular furnace followed using thermogravimetry (TG/DTG) on a
with a hermetically sealed quartz reactor. The heating Netzsch STA 409pc system and differential thermal
rate in that furnace was higher: 10–20 K/min. The analysis on a Netzsch DSC 204 F1 Phoenix. The elecꢀ
holding time at temperature was 1 h. The temperature trical conductivity and Seebeck coefficient of the
was maintained with an accuracy of
released gas was not analyzed.
5
K. The pressed samples were determined at room temperature
by standard techniques [8].
To prepare compact samples for thermoelectric
measurements, Xꢀray microanalysis, and transmission
electron microscopy (TEM), the synthesized powder
was coldꢀcompacted and then hotꢀpressed in a steel die
in air at 50 MPa. The die was heated to the hotꢀpressing
RESULTS AND DISCUSSION
Figure 1 presents thermogravimetry data for the
decomposition of lead acetate (dashed lines 2.1 and
2.2) and a mixture of tellurium powder and lead aceꢀ
temperature (350–370 С) over a period of 1 h.
°
The crystal structure of the particles was examined tate (solid lines 1.1 and 1.2). The charge was heated at
by Xꢀray diffraction on a Siemens D 5000 diffractomeꢀ a rate of 2.5 C/min in flowing hydrogen, and the
ter with Cu radiation ( = 1.5418 Å). The internal weight changes due to lead acetate decomposition and
°
K
λ
α
standard used was silicon or germanium powder susꢀ vaporization of volatile reaction products were continꢀ
pended in acetone. The PbTe powders were characterꢀ uously recorded. The decomposition of lead acetate triꢀ
ized by scanning electron microscopy (SEM) (Hitachi hydrate involved three distinct steps. We observed a peak
Sꢀ806) and transmission electron microscopy (Philips due to the dehydration of the acetate and the boiling of
CMꢀ200 STEM, accelerating voltage of 200 kV). The the released water at temperatures from 60 to 100 С
°
STEM was equipped with a calibrated Xꢀray (Fig. 1, line 2.2, peak 1) and two peaks due to the
microanalysis system, which was used to determine decomposition of the anhydrous lead acetate (Fig. 1,
the elemental composition of the grains in the pressed line 2.2, peaks 2, 3). Peaks 2 and 3 occurred at 248 and
samples. The specific surface area of the particles was 374 С. The weight change of the acetate (Fig. 1,
°
determined on a TriStar 3000 analyzer (BET measureꢀ line 2.1) was roughly the same in each decomposition
ments). The synthesis of lead telluride particles was step (14.6–14.9%). It is therefore reasonable to assume
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2012

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PREPARATION OF PbTeꢀBASED MATERIALS THROUGH THERMAL DECOMPOSITION
565
0.30
0.25
0.20
0.15
0.10
0.05
0
500
450
400
350
300
250
416°C
4
321.9°C
3
199.1
°
C
t
200
150
100
–0.05
–0.10
–0.15
2
266.6°C
50
0
–0.20
–0.25
1
204.7
80
°
C
0
20
40
60
100
120
140
160 180
Time, min
Fig. 2. Heating curve of a stoichiometric Pb(CH COO) + Te mixture in argon: (1) melting of the anhydrous lead acetate,
3
2
(2) triple decomposition peak, (3) melting of the lead, (4) melting of the PbTe–Te eutectic.
that the thermal decomposition of lead acetate and forꢀ the temperature range 220–300
°
С
; (3) a very small
mation of a pyrophoric metallic lead powder involved peak at 321
°
С
(melting point of Pb); and (4) a small
the following consecutive steps: peak at 416
°
С, which corresponds to the melting point
of the PbTe–Te eutectic. The last two peaks indicate
that, at the heating rate used in this study, the reaction
between the lead acetate and tellurium does not reach
completion before the decomposition onset. The lack
of thermal events during cooling is due to the rather
high rate of the reaction between the residual tellurium
and the lead formed during heating.
⋅
3H2O = Pb(CH3COO)2 +3H2O
↑
,(1)
(2)
(3)
Pb(CH3COO)2
Pb(CH3COO)2 = Pb(CH3COO) + gas
Pb(CH3COO) = Pb + gas
↑,
↑.
The temperature of the last decomposition step was
340–380
°С.
These results strongly suggest that the melting of
the anhydrous lead acetate, the reaction of the lead
acetate with Te, and the decomposition of the reaction
products can be represented by the following thermoꢀ
chemical schemes:
The lead acetate + tellurium mixture decomposed
in two steps. As seen in Fig. 1 (solid line 1.2), the peak
due to the dehydration of lead acetate trihydrate perꢀ
sists (Fig. 1, peak 4), and instead of two decomposition
peaks (line 2.2, peaks 2, 3) there is only one, stronger
liquid
Pb(CH3COO)s2olid Pb(CH3COO)2
decomposition peak at 280
°
С (Fig. 1, line 1.2, peak 5),
=
at 199 C,
°
that is, 100 below the onset of the last lead acetate
°
С
Pb(CH3COO)2liquid
decomposition step (line 2.2, peak 3). Thus, the addiꢀ
tion of tellurium powder has a significant effect on the
lead acetate decomposition process. This indicates
that there is active reaction between lead acetate and
tellurium below the decomposition onset temperature.
+ Te + H2
liquid
→
+ {gaseous products
}↑ at
PbTe(CH3COO)2
200–220
°C,
PbTe(CH3COO)2liquid
+ H2
for
→
PbTesolid + {gaseous
C.
Figure 2 presents the differential thermal analysis
results for the decomposition of the tellurium + lead
acetate mixture (mixture with a Pb/Te atomic ratio of
1). The DSC curve shows four distinct thermal events:
products
}
↑
t
> 220
°
In addition, the following reactions took place
above 220 C:
°
(1) the melting of the anhydrous lead acetate at 199 С;
°
Pb(CH3COO)2liquid
Teunreacted + Pb = PbTesolid
→
Pb + { gaseous products}↑,
(2) a triple peak due to the reaction of the lead acetate
with tellurium and the decomposition of the latter in
.
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566
LEONTYEV et al.
of the telluriumꢀenriched and telluriumꢀdeficient
samples coincided to the third decimal, confirming
that lead telluride has a narrow homogeneity range.
Figure 3 shows SEM images of powder particles.
The lead telluride powder had a coralꢀlike polydisꢀ
perse structure with a large surface area. It contained
large particles up to several microns in size and small
particles less than 100 nm in size.
The specific surface area of the powders depended
significantly on synthesis temperature (Fig. 4). It was
typically from 6 to 10 m2/g at a synthesis temperature
in the range 300–350
when the synthesis temperature was lowered to 220–
250
°
С and dropped by a factor of 2
200 nm
(а)
°С.
Figures 5a and 5b show darkꢀfield TEM images of
grains in pressed samples prepared from a mixture
containing 20% excess Te. The samples are seen to
contain both rather large grains (several microns in
size) and grains several tens of nanometers in size. The
small grains reside between the large grains. Figure 5c
presents the elemental compositions of 15 grains. It is
worth noting the presence of excess tellurium in all of
the grains studied. That lead telluride may contain a
considerable excess of tellurium in the form of defects
or microprecipitates was reported earlier by Zayachik
et al. [9]. According to their findings, the concentraꢀ
tion of excess tellurium may vary widely and has a sigꢀ
nificant effect on the electrical and magnetic properꢀ
ties of lead telluride. The grains were rather uniform in
composition, which showed little or no grainꢀtoꢀgrain
variation. In their measurements, tellurium content
was probably overestimated. The DTA curve (20–
3 μm
(b)
Fig. 3. SEM images illustrating the microstructure of lead
telluride particles synthesized at 300°C.
450
°
С
) of our hotꢀpressed samples showed a single
endothermic peak, at 414–416
°
С, due to the melting
of the PbTe–Te eutectic. This is consistent with availꢀ
able Pb–Te phase diagram data. It is, therefore, reaꢀ
sonable to assume that the excess Te resides within
grains in the form of nanoprecipitates [9], which form
16
14
12
10
8
6
4
2
0
250
the PbTe–Te eutectic on heating to 416 С.
°
According to Xꢀray microanalysis data for polished
sections of the hotꢀpressed lead telluride samples, the
tellurium content of the grains was 54–55 at %
(Fig. 6a). When the starting mixture contained 20%
excess tellurium, we observed individual tellurium
grains and PbTe–Te eutectic at grain boundaries
(Fig. 6b). Because hot pressing was performed in air
and heating to the pressing temperature was rather
slow, we observed accumulations of oxygenꢀrich (up to
30 at %) nanoparticles between large PbTe grains. In
the large grains, no oxygen was detected (Fig. 6). This
points to significant oxidation of the smallest lead telꢀ
300
350
400
450
°
500
550
Temperature, C
Fig. 4. Specific surface area of lead telluride particles as a
function of decomposition temperature.
luride particles during pressing in air at 350–370
The roomꢀtemperature Seebeck coefficient of the
hotꢀpressed (350–370 ) samples prepared using a
°С.
The Xꢀray diffraction patterns of the synthesized
powders showed reflections from only one phase,
°С
3
which had the NaCl structure (sp. gr. Fm m) with a mixture containing 20% excess tellurium varied from
lattice parameter = 6.462 0.003 Å. This value corꢀ 340 to 400 V/K, depending on the decomposition
a
µ
responds to the crystal lattice of bulk PbTe. The lattice temperature and the specific surface area of the powꢀ
parameter was independent of the synthesis temperaꢀ ders (Fig. 7). Reducing the amount of excess tellurium
ture in the range 250–400 С. The lattice parameters in the starting mixture led to a decrease in Seebeck
°
INORGANIC MATERIALS Vol. 48
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2012