Electron thermal EMF for Na x Cu 2-x S

Abstract

In the present study, the temperature dependences of the thermoelectromotive force (thermo–emf) in copper selenide, substituted in a small concentration, were studied. The results of the measurements showed that the thermo–emf coefficient of the samples increases, and the conductivity decreases with increasing silver concentration in its composition. These results allow – with optimal selection of the doping regime and protective coatings – to develop on the basis of nanostructured copper selenide an effective thermoelectric for use at temperatures of 20–500°C as p–type semiconductors suitable for increasing the efficiency of thermoelectric generators.

Full text

Electron thermal EMF for NaxCu2-xS
Kairat Kuterbekov1,*, Malik Balapanov2, Rais Ishembetov 2, Marzhan Kubenova 1, Talgat Baitasov 1,
Asset Kabyshev1, Aidos Azhіbekov1, Bekmyrza Kenzhebatyr1, and Temirulan Alіbay1
1L.N. Gumilyov Eurasian National University, Astana, Kazakhstan
2 Bashkir State University, Ufa, Russia
Abstract. In the present study, the temperature dependences of the
thermoelectromotive force (thermo-emf) in copper selenide, substituted in
a small concentration, were studied. The results of the measurements
showed that the thermo-emf coefficient of the samples increases, and the
conductivity decreases with increasing silver concentration in its
composition. These results allow - with optimal selection of the doping
regime and protective coatings - to develop on the basis of nanostructured
copper selenide an effective thermoelectric for use at temperatures of
20–500оС as p-type semiconductors suitable for increasing the efficiency
of thermoelectric generators.
1 Introduction
Modern thermoelectric converters have a number of advantages over traditional electric
generators: simplicity of design, absence of moving parts, noiselessness of operation, high
reliability, possibility of miniaturization without loss of efficiency.
In order for thermoelectric generators to become more competitive than conventional
power sources, thermoelectric materials should achieve high performance ZT ≥ 4 [1]. This
indicator is a guide in the search and synthesis of new promising materials that can become
the basis for industrially produced thermoelectric devices in the foreseeable future (20 to
25 years).
The dimensionless thermoelectric figure of merit, characterizing the efficiency of
materials in thermoelectric devices, is determined by the formula

/
2TZT 
(1)
where α – is the coefficient of thermal emf (Seebeck coefficient); σ – is electrical
conductivity; χ – is the thermal conductivity of the material.
Achieving the optimal combination of all three properties at the same time to obtain
a high thermoelectric figure of merit is a complex task.
At present, among the industrially produced thermoelectric materials, the most common
is doped bismuth telluride (Bi1-xSbx)2(Se1-yTey)3, which has a Q-factor of about 1 (ZT≈1) at
* Corresponding author: kkuterbekov@gmail.com
© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons
Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/).
E3S Web of Conferences 22, 00096 (2017) DOI: 10.1051/e3sconf/20172200096
ASEE17

room temperature [2]. The highest values of Z are strongly doped semiconductors or
semimetals with an electron concentration of the order of 1019–1020 см-3 [2].
2 Experiment
The results of experimental studies of the dependence of the Thermo-electromotive force
(emf) of nanocomposite and microcrystalline copper and silver chalcogenides differing in
chemical composition, the synthesis method and the particle size, as well as thermo-emf of
nanocomposite and microcrystalline copper and silver chalcogenides in the temperature
range 20–500°С are presented.
The choice of the objects of research NaxCu2-xS (x = 0.05, 0.1, 0.15, 0.2) is due to their
use as p-type semiconductors suitable for increasing the efficiency of thermoelectric
generators [1].
To study the thermoelectric properties of solid alloys of copper selenides, they were
prepared by solid-state synthesis in an inert atmosphere and by low-temperature chemical
synthesis. To obtain nanocrystalline powders, a technique was used, described in detail in a
number of papers [3, 4].
The registration of thermoelectric properties was carried out with the aid of an
experimental setup for studying the Seebeck coefficient and electrical resistance (Model
ZEM-3) shown in Fig. 1.
Fig. 1. Experimental setup for studying the Seebeck coefficient and electrical resistance.
The values of the electronic thermal-emf coefficient for the solid solution NaxCu2-xS
(x = 0.05, 0.1, 0.15, 0.2) were measured as a function of temperature. The sign of the
coefficient e is positive, which, taking into account the rule of choice of the sign for all
samples, corresponds to the motion of electron holes from the hot end of the sample to the
cold one.
Figures 2–5 show the dependence of the coefficient е of the electronic thermo-emf for
the composition Na0,05Cu1,95S, Na0,1Cu1,9S, Na0,15Cu1,85S and Na0, 2Cu1,8S in the range from
room temperature to 750 K. For all samples, With an increase in temperature, an increase in
the electronic thermal-emf coefficient е is observed.


2
E3S Web of Conferences 22, 00096 (2017) DOI: 10.1051/e3sconf/20172200096
ASEE17

room temperature [2]. The highest values of Z are strongly doped semiconductors or
semimetals with an electron concentration of the order of 1019–1020 см-3 [2].
2 Experiment
The results of experimental studies of the dependence of the Thermo-electromotive force
(emf) of nanocomposite and microcrystalline copper and silver chalcogenides differing in
chemical composition, the synthesis method and the particle size, as well as thermo-emf of
nanocomposite and microcrystalline copper and silver chalcogenides in the temperature
range 20–500°С are presented.
The choice of the objects of research NaxCu2-xS (x = 0.05, 0.1, 0.15, 0.2) is due to their
use as p-type semiconductors suitable for increasing the efficiency of thermoelectric
generators [1].
To study the thermoelectric properties of solid alloys of copper selenides, they were
prepared by solid-state synthesis in an inert atmosphere and by low-temperature chemical
synthesis. To obtain nanocrystalline powders, a technique was used, described in detail in a
number of papers [3, 4].
The registration of thermoelectric properties was carried out with the aid of an
experimental setup for studying the Seebeck coefficient and electrical resistance (Model
ZEM-3) shown in Fig. 1.
Fig. 1. Experimental setup for studying the Seebeck coefficient and electrical resistance.
The values of the electronic thermal-emf coefficient for the solid solution NaxCu2-xS
(x = 0.05, 0.1, 0.15, 0.2) were measured as a function of temperature. The sign of the
coefficient e is positive, which, taking into account the rule of choice of the sign for all
samples, corresponds to the motion of electron holes from the hot end of the sample to the
cold one.
Figures 2–5 show the dependence of the coefficient е of the electronic thermo-emf for
the composition Na0,05Cu1,95S, Na0,1Cu1,9S, Na0,15Cu1,85S and Na0, 2Cu1,8S in the range from
room temperature to 750 K. For all samples, With an increase in temperature, an increase in
the electronic thermal-emf coefficient е is observed.


Fig. 2. Temperature dependence of the coefficient of electron thermal emf Na0,05Cu1,95S.
Fig. 3. Temperature dependence of the coefficient of electron thermal emf Na0,1Cu1,9S.
Fig. 4. Temperature dependence of the coefficient of electron thermal emf Na0,15Cu1,85S.
0
0,2
0,4
0,6
0,8
1
1,2
1,4
250 350 450 550 650 750
αе,мB/K
Т,К
0
0,05
0,1
0,15
0,2
0,25
0,3
0,35
0,4
0,45
250 350 450 550 650 750
α
е
, мВ / К
T,K
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1
250 350 450 550 650 750 850
α
е
,мВ/К
T,K
3
E3S Web of Conferences 22, 00096 (2017) DOI: 10.1051/e3sconf/20172200096
ASEE17

Fig. 5. Temperature dependence of the coefficient of electron thermal emf Na0,2Cu1,8S.
Fig. 6 shows the temperature dependence of the emf of the electrochemical cell
Cu/CuBr/Na0.2Cu1.8S/C. The observed dependence is linear in nature, which indicates the
constancy of the entropy of the metal atoms in the compound within the temperature range
630–680 K. From the values of the cell emf, one can judge the degree of non-stoichiometry
of the compound, in this case the deviation from the stoichiometric composition is δ ≈ 0.01
in the chemical formula of the sample Na0.2Cu1.8-δS.
Fig. 6. Temperature dependence of the emf of an electrochemical cell Cu/CuBr/Na0.2Cu1.8S/C.
3 Conclusion
Thus, results of measurements have shown that temperature dependences of coefficient of
electronic copper of thermo-eds nanocrystal sulfides which cationic sublattice has been
alloyed by sodium, with the general chemical formula NaхCu2-хS (x = 0.05; 0.1; 0.15; 0.2)
in the field of temperatures from 20°C to 500°C. The sign of coefficient thermo-eds

е for
all samples was positive that taking into account the rule of the choice of a sign for all
samples corresponds to the movement of electronic holes since the hot end of a sample on
cold. In general, the coefficient thermo-eds with temperature increase from room
temperature to about 650–660 To grows. In the neighborhood of temperature 660 K there is
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
350 450 550 650 750
αе,мВ/К
T,K
4
E3S Web of Conferences 22, 00096 (2017) DOI: 10.1051/e3sconf/20172200096
ASEE17

Fig. 5. Temperature dependence of the coefficient of electron thermal emf Na0,2Cu1,8S.
Fig. 6 shows the temperature dependence of the emf of the electrochemical cell
Cu/CuBr/Na0.2Cu1.8S/C. The observed dependence is linear in nature, which indicates the
constancy of the entropy of the metal atoms in the compound within the temperature range
630–680 K. From the values of the cell emf, one can judge the degree of non-stoichiometry
of the compound, in this case the deviation from the stoichiometric composition is δ ≈ 0.01
in the chemical formula of the sample Na0.2Cu1.8-δS.
Fig. 6. Temperature dependence of the emf of an electrochemical cell Cu/CuBr/Na0.2Cu1.8S/C.
3 Conclusion
Thus, results of measurements have shown that temperature dependences of coefficient of
electronic copper of thermo-eds nanocrystal sulfides which cationic sublattice has been
alloyed by sodium, with the general chemical formula NaхCu2-хS (x = 0.05; 0.1; 0.15; 0.2)
in the field of temperatures from 20°C to 500°C. The sign of coefficient thermo-eds

е for
all samples was positive that taking into account the rule of the choice of a sign for all
samples corresponds to the movement of electronic holes since the hot end of a sample on
cold. In general, the coefficient thermo-eds with temperature increase from room
temperature to about 650–660 To grows. In the neighborhood of temperature 660 K there is
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
350 450 550 650 750
αе,мВ/К
T,K
a maximum, probably, the nature of which, probably, is connected with the happening
phase transition. The highest values of coefficient electronic thermo-eds in 1.2 mV/To are
reached at the same time at structure of Na0,05Cu1,95S.
The results of the measurements showed that the thermo-emf coefficient of the samples
increases, and the conductivity decreases with increasing silver concentration in its
composition. These results allow – with optimal selection of the doping regime and
protective coatings – to develop an effective thermoelectric on the basis of nanostructured
copper selenide for use at temperatures (400–600) K as p-type semiconductors suitable for
increasing the efficiency of thermoelectric generators.
This work was supported by the Ministry of Education and Science of the Republic of
Kazakhstan within the scientific-technical program for 2015–2017 Development of
Hydrogen Energy and Technology in the Republic of Kazakhstan.
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5
E3S Web of Conferences 22, 00096 (2017) DOI: 10.1051/e3sconf/20172200096
ASEE17