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УДК 621.362
The Research of Temperature Dependences of Electrical Conductivity and Thermopower of WS2 and WSe2 with Partial Replacement of W on Nb
Galina E. Yakovleva*
Nikolaev Institute of Inorganic Chemistry Lavrentiev, 3, Novosibirsk 630090, Russia
Anatoly I. Romanenko^
Nikolaev Institute of Inorganic Chemistry Lavrentiev, 3, Novosibirsk, 630090 National Research Tomsk State University Lenina, 36, Tomsk, 634050, Russia
Alexander S. Berdinsky*
Novosibirsk State Technical University Marksa, 20, Novosibirsk, 630073, Russia
Vitalii A. Kuznetsov§
Nikolaev Institute of Inorganic Chemistry Lavrentiev, 3, Novosibirsk, 630090 Novosibirsk State Technical University Marksa, 20, Novosibirsk, 630073, Russia
Alexandra Yu. Ledneva^ Vladimir E. Fedorov^
Nikolaev Institute of Inorganic Chemistry Lavrentiev, 3, Novosibirsk, 630090, Russia
Received 12.05.2017, received in revised form 10.11.2017, accepted 20.04.2018 The temperature dependences of electrical conductivity and thermopower of tungsten disulfide and tungsten diselenide have been investigated in this paper. These materials have the low value of electrical conductivity. The main idea of this paper is the increase of electrical conductivity for obtaining high thermoelectric efficiency. Niobium have served as an acceptor impurity. It has been revealed that the replacements of metal atoms have increased of electrical conductivity of W0.s5Nb0.-i5Se2 by 4 order of magnitude. While the thermopower has decreased with increasing of impurity concentration. The thermoelectric power factor has been calculated for materials. The figure of merit ZT of the best composition Wo.9БNbo.обSe2 has been estimated and has had a value of 0.02 at room temperature.
Keywords: thermopower, tungsten disulfide, tungsten diselenide, power factor. DOI: 10.17516/1997-1397-2018-11-4-459-464.
* galina.yakovleva.91@mail.ru
t air@niic.nsc.ru
^ berdinsky.alexander@gmail.com
§ vitalii.a.kuznetsov@gmail.com ^ ledneva@niic.nsc.ru II fed@niic.nsc.ru © Siberian Federal University. All rights reserved
Introduction
The thermoelectricity is based on the Seebeck effect. The Seebeck effect is the direct conversion of temperature difference to electric voltage. The efficiency of thermoelectric material is characterized by figure of merit ZT = S2 ■ a ■ T/k, where S is thermopower, a is electrical conductivity, k is thermal conductivity, T is temperature. Consequently, high-performance thermoelectric materials have to possess high value of electrical conductivity, thermopower and low value of thermal conductivity. In this paper tungsten disulfide (WS2) and tungsten diselenide(WSe2) have been investigated. These materials belong to the layered transition metal dichalcogenides. The properties of WS2 and WSe2 have been researched at high temperature [1-6]. It has been found that these materials has a high value of thermopower and low value of electrical conductivity and thermal conductivity. The aim of this paper is to research the influence of partial replacement tungsten atom on niobium atom in WS2 and WSe2. The Nb has one less electron in electron shell, as a consequence the uncompensated charge arises. As a result, p-type carriers are created. Nb acts as acceptor impurity. In this article temperature dependence of electrical conductivity and thermopower of Wi_KNbKS2 and W1_KNbKSe2 (where x = 0, 0.05, 0.10 and 0.15) are presented.
1. Sample preparation and measurement procedure
A series of the samples W1_KNbKSe2 and W1_KNbKS2 (x = 0, 0.05, 0.10 h 0.15) have been synthesized by high-ampoule method. The powders obtained have been pressed. The sample researched have been cut to the size 2x2x8 mm3. The temperature dependences of electrical conductivity have been researched by four-contact technique from 4.2 to 300 K in helium atmosphere. The temperature dependences of thermopower have been investigated by two-gradient technique from 80 to 300 K in helium atmosphere.
2. Temperature dependences of electrical conductivity
The measurement results of electrical conductivity and thermopower are presented in the Fig. 1. The WS2 and WSe2 have the exponential increase of electrical conductivity with increase of temperature. Such behavior is typical for semiconductors. The W1_KNbKSe2 samples have a linear decrease of electrical conductivity with increase of temperature. Such behavior is typical for metals. The W1_KNbKS2 have a linear increase of electrical conductivity with increase of temperature. It should be noted that addition of Nb atoms increases the electrical conductivity. Thus we have succeed in increasing of electrical conductivity by 4 orders of magnitude.
3. The temperature dependences of thermopower
The WS2 and WSe2 are the semiconductors. The electron transport of these materials occurs within the hopping conductivity with a variable hopping length according to the formula [7, 8]:
a(T ) = ac exp[-(T0/T )1/4]. (1)
With such conductivity model thermopower is described by the formula [9]:
S (T) = ^ [-(Tc/T)1/4j, (2)
50 100 150 200 250 300 T ,K
Fig. 1. The temperature dependences of electrical conductivity of Wi_KNbKS2 and W1_KNbKSe2
where T0 is parameter, = 1/4. The temperature dependence of electrical conductivity and thermopower of WS2 and WSe2 within the hopping model are presented in the Fig. 2. With decrease of thermopower the electrical conductivity increases.
0,24 0,25 0,26 0,27 0,28 0,29 0,30 0,31
T-14jK-14
Fig. 2. The temperature dependences of electrical conductivity a and thermopower S of WS2 and WSe2
The temperature dependences of thermopower of W1_KNbKSe2 and W1_KNbKS2 are presented on the Fig. 3.
In the W1_KNbKSe2 samples the linear increase of temperature dependence of thermopower are observed. With Nb addition material have a metallic behavior. It can be seen on temperature dependence of termopower. The metals are characterized by linear temperature dependence of
T, K T, K
Fig. 3. The temperature dependences of thermopower of Wi_KNbKSe2 and Wi_KNbKS2 samples
the thermopower according to the formula[10]:
5 (T ) = - £ "-f, (3)
6 e EF
where -B is Boltzmann constant, e is electron charge, EF is Fermi energy.
With addition of Nb thermopower of the samples decreases. Respectively, the more concentration of Nb the less thermopower is observed. TheW1_KNbKS2 samples had a loose structure. Perhaps this fact affected on the measurement. As a result, the temperature range is small in comparison withW1_KNbKSe2 samples. In these samples the increase of electrical conductivity is accompanied by increase of thermopower.
Such behavior of thermopower and electrical conductivity can be explained by the presence of n- and p-type carriers in the materials. The n- and p-type carriers make different contributions at various temperatures. Just as in the case of WSe2, with addition of Nb the thermopower of W1_KNbKS2 samples decreases.
4. Power factor
In this paper the power factor has been calculated in order to estimate the replacement of W on Nb according to the formula P = S2 ■ a. The results of power factor calculation are presented on the Fig. 4.
According the data obtained, the best value of power factor has W0.95Nb0.05Se2 samples. The maximum value of power factor is 72 yU,W/m-K2 at room temperature. The power factor cannot fully characterize the thermoelectric efficiency of materials. Because the thermal conductivity is not taken into account. Using literature data (- = 1 W/m-K) the figure of merit ZT of W0.95Nb0.05Se2 has been estimated. This value is 0.02 at room temperature.
5. Conclusion
In this paper the temperature dependences of electrical conductivity and thermopower of W1_KNbKSe2 and W1_KNbKS2 samples have been investigated. It has been found that the
80n -1-r
75-
70- m W Nb Se 0.85 0.15 2
65- • W Nb Se 0.90 0.10 2
60- ▲ W Nb Se 0.95 0.05 2
55 ♦ WSe2
50- □ Wa^A
45- O Wo.»Nb0,oS2
40- A Wo
35- O ws2
30-
25
20-
15-
10- ▲ ▲
5 A •
0- t. ♦ ♦ ■
Cj
• <A<>
T,K
Fig. 4. The temperature dependences of power factor of W1_KNbKSe2 and Wi_KNbKS2 samples
increase of Nb concentration has led to change from semiconductors to metallic behavior. The figure of merit ZT has been estimated for the Wo.95Nb0.05Se2 sample and has had a value 0.02 at room temperature. This value is to small in order to compete with modern thermoelectric materials. The modern thermoelectric materials have a value of figure of merit ZT about 1 and above. Probably, W1_KNbKSe2 and W1_KNbKS2 will have a higher thermoelectric efficiency at high temperature.
The study was supported by the Russian Science Foundation (grant 14-13-00674).
References
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Исследование температурных зависимостей электропроводности и термоЭДС WS2 и WSe2 с частичным замещением W на Nb
Галина Е. Яковлева
ИНХ СО РАН
Лаврентьева, 3, Новосибирск, 630090, Россия
Анатолий И. Романенко
ИНХ СО РАН Лаврентьева, 3, Новосибирск, 630090 Томский государственный университет Ленина, 36, Томск, 634050, Россия
Александр С. Бердинский
Новосибирский государственный технический университет Маркса, 20, Новосибирск, 630073, Россия
Виталий А. Кузнецов
ИНХ СО РАН Лаврентьева, 3, Новосибирск, 630090 Новосибирский государственный технический университет Маркса, 20, Новосибирск, 630073, Россия
Александра Ю. Леднева Владимир Е. Федоров
ИНХ СО РАН
Лаврентьева, 3, Новосибирск, 630090, Россия
В данной работе исследованы температурные зависимости электропроводности и термоЭДС дисульфида вольфрама и диселенида вольфрама. Так как данные материалы имеют низкую электропроводность, то основной целью данной работы было увеличение этого параметра, необходимого для высокой термоэлектрической эффективности. В качестве акцепторной примеси выступал ниобий. Было выявлено, что такое замещение увеличивает электропроводность материала на четыре порядка в образцах состава Wo.s5Nbo.i5Se2. При этом термоЭДС уменьшается в зависимости от концентрации примеси. Измеренные параметры использовались для оценки термоэлектрического фактора мощности. Для наилучшего состава Wo.95Nbo.o5Se2 был оценен фактор добротности, который составил 0.02 при комнатной температуре.
Ключевые слова: термоЭДС, дисульфид вольфрама, диселенид вольфрама, фактор мощности.
