ARTICLE IN PRESS

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Materials Letters 4460 (2003) 1–7

Chemical alloying and characterization of nanocrystalline

bismuth telluride

Muhammet Toprak*, Yu Zhang, Mamoun Muhammed*

The Royal Institute of Technology, Materials Chemistry Division, SE 100 44, Sweden

Received 10 February 2003; accepted 25 February 2003

Abstract

A novel chemical alloying method has been developed for the fabrication of nanocrystalline thermoelectric alloy Bismuth

telluride, Bi2Te3. The method consists of a combination of solution chemical method and thermal processing under controlled

heating conditions. The components have been coprecipitated from a solution and the precursor consists of a solid solution of

the different intermediate compounds and exhibits high reactivity. Calcination and hydrogen reduction of the precursor at

moderate temperature, 350 jC, for 2 h resulted in the alloying of these elements to obtain the pure phase of the thermoelectric

material in fine powder. The method is simpler than conventional melt processing and produced a 95–98% yield in laboratory

scale. High concentration of grain boundaries provided by nanostructuring is expected to lower the thermal conductivity of

thermoelectric material and further increase the performance.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Nanomaterials; Powder technology; Thermoelectric; Bi2Te3; Coprecipitation; Chemical alloying

1. Introduction idified alloys tend to be fragile. In order to offer

Bismuth telluride is the most commonly used

thermoelectric material at ambient temperature

mainly for cooling applications. The most conven-

tional technology used for the production of Bi–Te

alloys is fusion alloying, where pure elements are

co-melted at temperatures above 600 jC, kept at

elevated temperature for a prolonged period of time

for homogeneous mixing, and then cooled gradually

to form Bi–Te alloys [1]. This process is labor

intensive, and thermoelectric elements cut from sol-

0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All righ

doi:10.1016/S0167-577X(03)00250-7

* Corresponding authors.

E-mail addresses: mtoprak@matchem.kth.se (M. Toprak),

mamoun@matchem.kth.se (M. Muhammed).

improved structural integrity and the polycrystallin-

ity, powder metallurgy was used to fabricate the

thermoelectric elements through a route of pulveriz-

ing/crushing the solidified alloy and sieving the

resultant particles and sintering the polycrystalline

powders [2].

The figure-of-merit, ZT, of the currently available

Bi–Te thermoelectrics is around unity at 300 K, and

there is a possibility of increasing ZT by nanostruc-

turing the material and reducing more the thermal

conductivity. Theory suggests that for semiconduc-

tors, arrays of dislocations at phase boundaries, sub-

microscopic precipitates, and distortions in the lattice

structure would be effective in scattering phonons but

ineffective in scattering the electrons which have

relatively longer wavelengths [3,4]. This selective

ts reserved.

ARTICLE IN PRESSM. Toprak et al. / Materials Letters 4460 (2003) 1–72

scattering can be used to improve the figure-of-merit

for thermoelectric materials by reducing the thermal

conductivities while leaving electrical resistivity

unchanged.

An alternative to the conventional route of fusion

alloying and pulverization of solidified alloy is

chemical alloying that readily produces nanosize

polycrystalline powder of the desired alloy at a much

lower temperature. This chemical method is a simple

process consisting of two major steps, hydrochemical

coprecipitation and thermochemical alloying, which

has been successfully developed for the synthesis of

thermoelectric Skutterudites [5–8]. The coprecipita-

tion is carried out in an aqueous solution at ambient

temperature to produce the precursor powder of an

oxalate/oxide mixture or composite, while the alloy-

ing is done by the thermochemical processing of the

precursor under hydrogen atmosphere at moderate

temperature, much lower than the fusion temperature.

Ritter [9] has reported the preparation of fine particle

(100 nm) bismuth telluride via a chemical route of

(1) coprecipitation of a mixture of bismuth and

tellurium oxides at room temperature and (2) reduc-

tive alloying of the coprecipitate at 275 jC in

flowing hydrogen for 12 h [9]. The process is based

on the precipitation of bismuth oxalate–tellurium

oxide composites. The oxalate is more reactive in

the decomposition, thus shorten the reaction time as

well as the particle size. In the present work, the

chemical alloying approach is developed for the

synthesis of the nanocrystalline powder of Bi2Te3.

Fig. 1. Computer modeling: the fraction o

The process conditions and features of this new

approach as well as the properties of the synthesized

products are particularly investigated.

2. Experimental

2.1. Materials and solutions

Bismuth trioxide (Bi2O3), tellurium dioxide (TeO2),

oxalic acid (H2C2O4), sodium hydroxide (NaOH),

and nitric acid (65% HNO3) were of reagent grade

from Merck. The stock solutions of Bi and Te were

prepared, respectively, by dissolving Bi2O3, and

TeO2 in 4 M HNO3. The concentration of each

metal ion in the stock solution was assigned to be

1 M and was precisely determined by Atomic

Absorption Spectroscopy. A solution of 0.5 M

H2C2O4 as precipitant, and a solution of 3 M

NaOH as neutralizer were prepared by dissolving

required amount of respective chemicals in distilled

water.

2.2. Hydrochemical coprecipitation

In order to design the coprecipitation experiments

for the synthesis of the precursor, thermodynamic

modeling was performed. Fig. 1 shows the results

from the computer modeling of a system of H+–

Bi3 +–Te4 +–C2O42�NO3

�. As can be seen, a complete

coprecipitation of bismuth oxalate Bi2(C2O4)3 and

f precipitates of Bi and Te vs. pH.

ARTICLE IN PRESSM. Toprak et al. / Materials L

tellurium dioxide TeO2 can be expected at pH range =

1–4, to obtain a composite precursor with a stoi-

chiometry of Bi and Te for the final product

Bi2Te3.

Based on the modeling results, the following

precipitation procedure was adopted for the synthesis

experiment.

(1) A certain amount of the stock solutions of Bi and

Te were mixed in a molar ratio of 2:3 (solution A),

while solution B contained oxalic acid (where

[C2O4]/[Bi] is more than 3/2) and NaOH (solution

C) was used for pH adjustment.

(2) An aqueous medium in the pH range 2–4 was

prepared by adding oxalic acid to distilled water

in a well-stirred reactor.

(3) Solutions A and B were added simultaneously into

the reactor under rapid stirring, and NaOH solution

was added to maintain the reaction mixture at a pH

range 2–4. By this simultaneous addition, the

coprecipitation is achieved and thus high compo-

sition homogeneity with respect to bismuth and

tellurium is obtained in the composite.

(4) When the reaction is completed, the suspension

was left under stirring for 15 min to ensure a

complete precipitation.

(5) The precipitates were filtered off, washed with

water and ethanol and dried at 80 jC overnight.

For comparison, the precipitation of individual

compounds, i.e. bismuth oxalate and tellurium

oxide, were carried out (in two different experi-

ments) using a procedure similar to that described

above.

2.3. Thermochemical alloying

Dried precursors were heated in a tube furnace.

The following procedure was used: (1) the temper-

ature of the sample was ramped up at a rate of 5 jC/min. During this step, the powder was exposed to air,

(2) the temperature of the powder was maintained at

300 jC (350 jC) for some samples for 2 h, while

hydrogen gas (100%) was allowed to pass over the

sample, (3) after the reduction step, the furnace was

shut down and the sample was allowed to cool to

room temperature under hydrogen gas flow. This step

required 3 h.

2.4. Characterization

The samples obtained at different processing steps

were characterized using several techniques. The

chemical composition of the samples (the solid was

dissolved in acidic solution) was determined by

Atomic Absorption Spectroscopy (AAS), using Spec-

tra AA-20 Varian spectrometer. The crystalline phase

identification was made by X-ray powder diffraction

analysis (XRD) using a Philips PW 1012/20 diffrac-

tometer with Cu Ka and PCDFWIN JCPDS-ICDD

database. Thermogravimetric analysis (TGA) was

performed in air with a heating rate of 10 jC/min

by using Setaram TGA92 system to characterize the

thermal decomposition of the precipitated precursors.

Surface area analysis of the resultant TE alloy was

performed using nitrogen adsorption according to

BET technique using a Micromeritics Gemini 2370

system. The microstructure has been examined by

scanning electron microscopy JEOL840 SEM with

elemental analysis, EDS.

etters 4460 (2003) 1–7 3

3. Results and discussion

3.1. Coprecipitation

According to the results from thermodynamic

modeling shown in Fig. 1, the respective precipita-

tions of bismuth oxalate and tellurium oxide proceed

as follows:

2Bi3þ þ 3C2O2�4 þ xH2O ! Bi2ðC2O4Þ3 � xH2O ð1Þ

Te4þ þ 2H2O ! TeO2 þ 4Hþ ð2Þ

where x stands for the number of crystalline water. In

the literature, xV 7 is reported for bismuth oxalate

[10].

The addition of oxalic acid solution also produces

H+, which is neutralized by the addition of NaOH

solution to maintain the pH range 2–4. The obtained

precipitates, obtained from the different experiments

(coprecipitation and individual metal precipitate)

consisted of very fine powders and white in color.

As shown in the XRD data in Fig. 2, the main

crystalline phase in the coprecipitated powder corre-

sponds only to that of bismuth oxalate while the

ARTICLE IN PRESS

Fig. 3. TGA patterns of (a) Bismuth oxalate, (b) Bi–Te coprecipitate.

Fig. 2. X-ray powder diffraction patterns of (a) Bi–Te coprecipitate, (b) Bismuth oxalate, (c) Tellurium oxide, and (d) Bi–Te coprecipitate

calcined.

M. Toprak et al. / Materials Letters 4460 (2003) 1–74

amorphous phase is expected to be the tellurium

oxide, which is different from the crystalline phase

in its pure precipitate. The amorphous nature of

TeO2 in the coprecipitated powder is probably an

advantage of being more reactive in the subsequent

thermal treatment of the precursor, especially when

the reduction of TiO2 is a control step of the over

alloying process. The molar ratio of Bi and Te in the

solid powder was determined by AAS, and was

found to be 2:3.

3.2. Decomposition and calcination

Fig. 3 represents the TGA curves sample obtained

from the precipitation of bismuth oxalate and the

sample obtained from the coprecipitation of Bi–Te

precursor. As can be seen, the two TGA curves are

similar with an apparent decomposition starting at 220

jC and are completed at 350 jC for the bismuth

oxalate precipitate and at 300 jC for the coprecipi-

tated Bi–Te composite. The decomposition is de-

scribed by the following reaction:

Bi2ðC2O4Þ3 � xH2O ! Bi2O3 þ 3COðgÞþ 3CO2ðgÞ þ xH2OðgÞ ð3Þ

Based on the calculation of the weight losses

according to the TGA data, x was found to be 7 for

the bismuth oxalate and x = 1 for the coprecipitated

Bi–Te composite. It seems that the coprecipitation of

bismuth oxalate and tellurium oxide made the former

crystallize with much less water and the latter became

amorphous. Therefore, it is easier for bismuth oxalate

to decompose in the coprecipitate than in its single

precipitate.

The calcination of the different precipitates was

performed in air at a temperature of 300 jC to ensure

ARTICLE IN PRESSM. Toprak et al. / Materials Letters 4460 (2003) 1–7 5

a complete decomposition of the carbon content. As

shown in Fig. 2, the XRD pattern reveals a mixture of

several phases of Bi and Te oxides. Besides Bi2O3 and

TeO2, two other phases containing double oxides, i.e.

Bi4TeO8 and Bi32TeO50, are also formed to appreci-

able extent. The formation of these compounds can be

expressed by the following reactions:

2Bi2O3 þ TeO2 ! Bi4TeO8 ð4Þ

16Bi2O3 þ TeO2 ! Bi32TeO50 ð5Þ

The formation of these double oxide compounds

has probably an important influence on the subse-

quent reduction and alloying process.

3.3. Reduction and alloying

Fig. 4 shows the XRD patterns of the samples that

have been calcined for 1 h and heated in hydrogen for

2 h. The Bi–Te sample reduced at 300 jC, showedthat Bi4Te3 is formed and is more dominant than

Bi2Te3 phase. Neither Bi nor Te metals could be

detected. On the other hand, XRD data showed that

TeO2 is present in the reduced sample. The Bi–Te

sample reduced at 350 jC, however, showed a pure

phase of Bi2Te3 and no significant indication of other

phases. It is important to note that the reduction of

individual metal oxide samples cannot produce metal

Fig. 4. XRD patterns of samples reduced (a) at 300 jC from Bi–Te coprec

Bismuth oxalate, (d) at 350 jC from Bi–Te coprecipitate.

Bi or Te metal under the conditions used. As shown in

Fig. 4, the Bi sample consists of a dominant metallic

phase and minor oxide unreduced, while the Te

sample is almost in its oxide form with very little

elemental phase reduced.

For the chemistry of the reduction and alloying in

hydrogen, the possible reactions can be expressed as

follows:

Bi2O3 þ 3H2ðgÞ ! 2Biþ 3H2OðgÞ ð6Þ

TeO2 þ 2H2ðgÞ ! Teþ 2H2OðgÞ ð7Þ

2Biþ 3Te ! Bi2Te3 ð8Þ

3Bi4TeO8 þ 24H2ðgÞ ! 6Biþ Bi4Te3 þ 24H2OðgÞð9Þ

3Bi32TeO50 þ 150H2ðgÞ ! 92Biþ Bi4Te3

þ 150H2OðgÞ ð10Þ

Bi4Te3 þ 3Te ! 2Bi2Te3 ð11Þ

By comparing the XRD data in Fig. 4, it is

apparent that the reduction of the double oxide com-

pound (as given by Reactions (9) and (10)) should be

much faster than that of the individual metal oxides as

ipitate, and (b) at 350 jC from Tellurium oxide, (c) at 350 jC from

ARTICLE IN PRESSM. Toprak et al. / Materials Letters 4460 (2003) 1–76

given by Reactions (6) and (7). Reaction (7) seems to

be the controlling step for the overall process of

reduction and alloying. On the other hand according

to Reactions (9) and (10), metallic Bi should exist in a

much higher concentration than Bi4Te3. As seen from

the X-ray data, Bi metal cannot be detected. There-

fore, we assume that metallic Bi phase likely exists

either as an amorphous phase or as a nanocrystalline

which is not reflected in the XRD pattern. Consider-

ing that the reduction of TeO2 in the Bi–Te sample of

mixed oxides is much faster than that in the Te-

sample, one may expect that the elemental Bi must

have an important role in increasing the reaction rate

of TeO2. It is likely that, instead of a path through

Reactions (7) and (8), the alloying may undergo the

bypath through Reactions (6) and (12):

6Biþ 3TeO2 ! 2Bi2O3 þ Bi2Te3 ð12Þ

In other words, Bi, due to its nanosize, may

catalyze the reduction of TeO2. In the same way,

Reactions (7) and (11) may be replaced by a bypath

via Reactions (6) and (13):

2Bi4Te3 þ 3TeO2 ! 2Bi2O3 þ 2Bi2Te3 ð13Þ

From the XRD data, average size of the crystallite

grains of the Bi2Te3 powder was calculated to be 40

nm. The SEM study revealed that individual grains

exist almost always as nanocrystalline spheres though

many of them were agglomerated, as shown in Fig. 5.

The particle size was also calculated as 40 nm from the

surface area of the Bi2Te3 powder, assuming a spherical

distribution. The particle sizes obtained from the two

methods are in a good agreement. EDS analysis

Fig. 5. SEM micrographs of Bi2Te

revealed the homogeneity of the prepared Bi2Te3powder.

4. Conclusions

A novel chemical alloying route to prepare nano-

crystalline powder of bismuth telluride has been

developed that consists mainly of two steps, copreci-

pitation, and thermochemical alloying. Hydrogen

reduction of the precursors produces the alloys in fine

powder, polycrystalline form. The method is simpler

than conventional melt processing and produced 95–

98% yield in laboratory-scale. The new method

reduces equipment, materials, and labor costs by

producing fine powders directly, thus eliminating the

crushing and sieving steps necessary after melt-pro-

cessing. Precursor synthesis occurs at room temper-

ature in aqueous solution from commonly available

chemicals. Alloy synthesis at 300–400 jC lower than

melt-processing temperatures yields more than 95%

product compared with theory. Scale up to continuous

production is possible using common chemical flow

reactor technology.

The coprecipitation step is quite simple and pro-

duces an oxalate/oxide Bi–Te composite precursor at

ambient temperature. The composition of bismuth

oxalate– tellurium dioxide coprecipitate is easy to

control and insensitive to the excessive oxalate addi-

tion in the acidic region. A main advantage of this

precursor is that its TeO2 phase is amorphous and

therefore is more reactive during the further process-

ing, especially during the reduction step. Besides, the

present route has the advantage of producing fine

homogenous powders to further decrease the time

for heat treatments.

3 reduced at 350 jC for 2 h.

ARTICLE IN PRESSM. Toprak et al. / Materials Letters 4460 (2003) 1–7 7

Although the thermochemical alloying needs two

sub-steps of treatments, calcination in air for 1 h and

reduction in hydrogen for 2 h, the overall heating time

including that of heating-up is 3 h, much shorter than

12 h, needed for the coprecipitated powder of the

mixed oxides as reported by Ritter [9]. This is

important for the preparation of nanoparticles with

size less than 50 nm. Due to the higher reactivity of

the present precursor, several Bi–Te double oxide

compounds can be formed during calcination, which

are more reactive allowing a fast process of reduction

and alloying. The overall reaction rate of reduction

(and alloying) from the Bi–Te precursor was much

faster than the reduction of the single oxide, especially

TeO2. Besides the amorphous nature of TeO2, the

existence of amorphous or nanosize Bi obtained

during the reduction of double oxides may play a

catalytic role to promote the reduction and alloying.

Due to the advantages of relatively low temperature

and short time for heating treatments, the nanocrystal-

line (40 nm) powder has been produced.

Acknowledgements

This work is a part of a program that was supported

by European Community 5th Framework Programme

under The contract number G5RD-CT2000-00292,

NanoThermal project, and in part by Swedish Research

Council for Engineering Science (VR). The authors

would like to say thanks to all the partners in this

project, i.e. to K. Billquist (KTH), B. Iversen and M.

Christiansen (University of Aarhus, Denmark), M.

Rowe, S. Williams (NEDO Laboratories, Cadiff

University, UK), C. Gatti and L. Bertini (Instituto di

Scienze e Technologie Molecolari, Milano, Italy),C.

Stiewe, D. Platzek and E. Muller (DLR, Germany), A.

Palmqvist, A. Saramat (Chambers Tekniska Hogkola,

Sweden), G. Noriega (CIDETE, Spain), and L.

Holmgren (Termo-Gen AB, Sweden).

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