Thermoelectric properties of p-type (Bi2Te3) x(Sb2Te3)1-x single crystals doped with 3 wt. % Te

This is the final version of the article. Available from AIP via the DOI in this record.In the present work, thermoelectric properties of p-type (Bi 2Te3)x (Sb2Te3) 1-x single crystals doped with 3 wt. % Te are theoretically explored for various chemical compositions (x = 0.18, 0.19, 0.20, 0.22, 0.24, 0.26) in the temperature range of 290-500 K. The influence of the chemical composition in enhancing the thermoelectric figure of merit (ZT) is discussed in detail. Using the nearly-free electron approximation and the Fermi-Dirac statistics, the temperature dependences of Fermi level (E f), Seebeck coefficient (S), and electrical conductivity (σ) are successfully reproduced as reported in the experimental study of Li [Intermetallics 19, 2002 (2011)]. The thermal conductivity contributions from phonons (κph), acceptor holes (κh), and electron-hole pairs (κbp) are included by employing Srivastava's scheme, Wiedemann-Franz law, and Price's theory, respectively. By combining all three contributions of the thermal conductivity we successfully explain the experimental measurements of the total thermal conductivity as reported by Li Furthermore, it is theoretically found that among all the compositions the p-type 20%(Bi2Te3)-80%(Sb 2Te3) sample has the maximum ZT value of 1.31 at 390 K, which is also in good agreement with the experimental results obtained by Li © 2013 American Institute of Physics.Övgü Ceyda Yelgel is grateful for financial support from The Republic of Turkey Ministry of National Education through the Recep Tayyip Erdog̃an University in Rize/Turkey (Recep Tayyip Erdog̃an Üniversitesi Rektörlüg̃ü, Fener Mah. Merkez Kampüs 53100/RİZE/TÜRKİYE)

Effects of Dimensionality Reduction for High-Efficiency Mg-Based Thermoelectrics

Over the past decade, there has been significant interest in the field of thermoelectric materials (TEs) owing to their use in clean and sustainable energy sources for cooling and/or power generation applications. Especially, Mg2XIV (XIV = Si, Ge, Sn) based TEs are promising candidates for middle-temperature range energy conversion due to their high thermoelectric performance, environmentally harmless, abundant raw materials, non-toxicity, and relatively inexpensive cost of modules. In this book chapter, we present an overview of the theoretical background of the thermoelectric transport properties (Seebeck coefficient, electrical conductivity, thermal conductivity, and thermoelectric figure of merit ZT) of magnesium-based bulk and low dimensional systems (i.e., quantum wells and quantum wires). A detailed description of the temperature-dependent Fermi level both in extrinsic and intrinsic regimes will be provided whereby it is the primary step in deriving the thermoelectric transport parameters of materials. Following the linearized Boltzmann transport equations temperature-dependent electronic transport properties (Seebeck coefficient, electrical conductivity, and electronic thermal conductivity) of materials under the energy-dependent relaxation time approximation will be defined. By employing Debye’s isotropic continuum model within the single mode relaxation time approximation including various phonon relaxation rates contributed by different scattering mechanisms the lattice contribution to the thermal conductivity will be included

Thermoelectric Properties of V-VI Semiconductor Alloys and Nanocomposites

Thermoelectric materials are materials which are capable of converting heat directly into electricity and vice versa. They have long been used in electric power generation and solid-state cooling. The performance of a thermoelectric device determined by the dimensionless figure of merit (ZT) of the material, defined as ZT = (S2 σ/κ)T, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the total thermal conductivity, and T is the absolute temperature. The total thermal conductivity consists of contribution from electrons, electron-hole pairs and phonons. Since the 1960s, the best thermoelectric material has been Bi2Te3 alloys, with a ZT of 1.0 at room temperature. In recent years, the idea of using nanotechnology has opened up the possibility of engineering materials at nanoscale dimensions to achieve higher values of ZT in other words to have more efficient thermoelectric devices. This thesis starts with a broad introduction to thermoelectricity including various thermoelectric effects and their applications. The state-of-the-art thermoelectric materials and the optimisation methods to enhance the value of ZT have also been reviewed. A systematic theoretical modelling of the thermoelectric properties of three dimensional bulk semiconductors has been presented in Chapter 2. Electronic properties (Fermi level, Seebeck coefficient, and electrical resistivity) and thermal conductivity contribution from carriers (donor electrons or acceptor holes) have been derived by using the nearly-free electron approximation and the Fermi-Dirac statistics. Other thermal conductivity contributions originated from electron-hole pairs and phonons have also been described in detail. In Chapter 3, this theoretical study is extended to two dimensional semiconducting quantum well structures bearing in mind that the Fermi level should change with the temperature as well as the quantum well width and additional interface scattering mechanisms (interface mass-mixing and interface dislocation scatterings) should be included for the definition of anharmonic scattering rate. Thermoelectric properties of n-type (Bi2Te3)0.85(Bi2Se3)0.15 single crystals doped with 0.1 wt.% CuBr and 0.2 wt.% SbI3 and p-type (Bi2Te3)x(Sb2Te3)1−x single crystals doped with 3 wt.% Te (0.18 ≤ x ≤ 0.26) have been explored in Chapter 4 and 5, respectively. It has been found that p-type Bi2Te3 based alloys showed higher values of ZT due to their larger power factor (S2σ) and smaller thermal conductivity values. These calculations have concluded that the influence of the composition range of semiconductor alloys together with its type and amount of dopant plays an important role in enhancing the ZT. In Chapter 6, a detailed theoretical investigation and comparision of the thermal conductivities of these single crystals have been reported including frequency dependence of the phonon thermal conductivity for different temperatures. In Chapter 7, based on temperature and well width dependent Fermi level, a full theory of thermoelectric properties has been investigated for n-type 0.1 wt.% CuBr doped Bi2Se3/Bi2Te3/Bi2Se3 and p-type 3 wt.% Te doped Sb2Te3/Bi2Te3/Sb2Te3 quantum well systems. Different values of well thicknesses have been considered for both types of quantum well systems to study the effect of confinement on all thermoelectric transport coefficients. It has been found that reducing the well thickness has a pronounced effect on enhancing the ZT. Compared to bulk single crystals studied in Chapter 4 and 5, significantly higher thermoelectric figure of merits have been estimated theoretically for both n- and p-type semiconducting quantum well systems. For the n-type Bi2Se3/Bi2Te3/Bi2Se3 quantum well system with taking 7 nm well width the maximum value of ZT has been estimated to be 0.97 at 350 K and for the p-type Sb2Te3/Bi2Te3/Sb2Te3 quantum well with well width 10 nm the highest value of the ZT has been found to be 1.945 at 440 K. Chapter 8 briefly recapitulates the results presented in this thesis and outlines possibilities for future work

Thermoelectric transport behaviours of n-type Mg-2 (Si,Sn,Ge) quaternary solid solutions

WOS: 000487287600015Mg2X (X=Si, Sn, and Ge) based systems have attracted widespread attention owing to their various benefits in thermoelectric applications. in particular, to date, ternary Mg2X based solid solutions have become one of the most widely investigated thermoelectric systems. However, the investigation of temperature varied thermoelectric properties of Mg2X based quaternary systems is rather limited both theoretically and experimentally. Therefore, here, we report a rigorous theoretical work of thermoelectric properties for n-type Mg2Si0.55-zSn0.4Ge0.05Biz quaternary solid solutions (z=0.02, 0.025, 0.03, and 0.035) from 300 K to 850 K. By using nearly-free-electron model together with Fermi-Dirac statistics we define Fermi level both in extrinsic and intrinsic regimes as a function of temperature. We follow Hicks and Dresselhaus' approach to calculate electronic transport properties. By performing the Debye's isotropic continuum model a detailed theoretical investigation of lattice thermal conductivity is presented among with various phonon relaxation rates. From our theoretical analysis the highest ZT is attained for Mg2Si0.53Sn0.4Ge0.05Bi0.02 solid solution as 1.14 at 850 K. (C) 2019 Published by Elsevier B.V. on behalf of Chongqing University.TUBITAK (Scientific and Technical Research Council of Turkey)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [115F387]Ovgu Ceyda Yelgel wishes to acknowledge financial support from TUBITAK (Scientific and Technical Research Council of Turkey) (Project number: 115F387)

Theoretical study of thermoelectric properties of p-type Mg-2 Si1-chi Sn-chi solid solutions doped with Ga

WOS: 000386227900020Mg-2 Si-Mg-2 Sn solid solutions are a promising class of thermoelectric materials. the thermoelectric properties of p-type Mg-2(Si-0.3 Sn-0.7)(1-y) Gay solid solutions with the doping levels as y = 0.05 and y = 0.07 are investigated in the temperature range of 300 K-800 K. By using the nearly-free hole approximation and the Fermi-Dirac statistics, the temperature dependences of Fermi level (E-f), Seebeck coefficient (S), and electrical conductivity (sigma) are calculated theoretically and compared with related experimental measurements. the thermal conductivity contributions from carriers (namely acceptor holes in this present work, k(c)), electron-hole pairs (k(bp)), and phonons (k(ph)) are included by employing Wiedemann-Franz law, Price's theory, and Srivastava's scheme, respectively. A maximum thermoelectric figure of merit (ZT) of 0.355 is theoretically achieved for the Mg-2(Si-0.3 Sn-0.7)(0.95) Ga-0.05 sample arising from a high Seebeck coefficient of 175.71 mu V/K and low total thermal conductivity of 1.82 W m(-1) K-1 at 650 K where its experimental ZT value was reported as 0.356 at 620 K. (C) 2016 Elsevier B.V. All rights reserved.Scientific and Technical Research Council of Turkey (TUBITAK)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [115F387]This work was supported by the Scientific and Technical Research Council of Turkey (TUBITAK) with the grant number of 115F387

Theoretical study of thermoelectric properties of n-type doped Mg2Si0.4Sn0.6 solid solutions

WOS: 000372097300004In this work, a systematic theoretical investigation of thermoelectric properties of n-type doped [GRAPHICS] solid solutions with [GRAPHICS] is presented in the temperature range [GRAPHICS] K. Electronic transport properties ( [GRAPHICS] , S, and [GRAPHICS] ) are calculated using the nearly-free-electron approximation and the Fermi-Dirac statistics. Thermal transport properties including contributions from carriers ( [GRAPHICS] ), electron-hole pairs ( [GRAPHICS] ) and phonons ( [GRAPHICS] ) computed using the Wiedemann-Franz law, Price's theory and Srivastava's scheme, respectively. in a very good agreement with available experimental measurements, among with [GRAPHICS] samples, the highest value for thermoelectric figure of merit ZT is found to be 1.41 at 800K for [GRAPHICS] sample owing to its highest electrical conductivity and the lowest lattice thermal conductivity values. Additionally, by theoretically considering the doping levels as [GRAPHICS] , we suggest that at 800K ZT goes up by 30% for [GRAPHICS] sample with the value of [GRAPHICS] compared to [GRAPHICS] sample due to increment in the electrical conductivity and additional mass defect effects to the phonon thermal conductivity.Scientific and Technical Research Council of Turkey (TUBITAK)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [115F387]This work was supported by the Scientific and Technical Research Council of Turkey (TUBITAK) [grant number 115F387]

Theoretical Study of Thermal Conductivities of n- and p-type Doped Mg2Si1-xSnx Thermoelectric Solid Solutions

Mg2Si1-xSnx katı alaşımları yüksek termoelektrik verimlilikleri sebebiyle 500 K’den 800 K’e kadar olan orta sıcaklılık aralığı için umut vaadeden termoelektrik materyallerdir. Bu çalışmada hem n- hem p-tip katkılı Mg2Si1-xSnx katı alaşımlarının termal iletkenlikleri teorik olarak detaylıca incelenmesi sunulmuştur. Taşıyıcılardan (elektronlar yada holler), elektron-hole çiftlerinden ve fononlardan kaynaklanan termal iletkenlik katkıları ayrı ayrı göz önüne alınarak ve sırasıyla Wiedeman-Franz kanunu, Price’in teorisi, ve Debye’nin izotropik sürekli modeli uygulanarak hesaplanmıştır. Bütün fonon çarpışma mekanizmaları, kaynağı kristal sınırlarından, kütle bozukluklarından, bozunum potansiyellerinden ve anharmoniklikten olan katı alaşımların hepsi için eksiksiz bir şekilde incelenmiştir. En düşük toplam termal iletkenlik değerleri n-tip katkılı Mg2(Si0.4Sn0.6)0.98Bi0.02 katı alaşım için 700 K’de 2.431 WK-1 m-1 olarak, p-tip katkılı Mg2(Si0.3Sn0.7)0.95Ga0.05 katı alaşım için 600 K’de 1.843 WK-1 m-1 olarak bulunmuştur buda açıkca öneriyor ki p-tip katkılı Mg2Si1-xSnx tabanlı katı alaşımlar n-tip katkılı katı alaşımlarından termoelektrik cihazlar için daha iyi adaylardır.Mg2Si1-xSnx solid solutions are a promising class of thermoelectric materials due to their high thermoelectric efficiencies at intermediate temperature range from 500 K to 800 K. Present study presents a theoretical work of the thermal conductivities of both n- and p-type doped Mg2Si1-xSnx solid solutions. The thermal conductivity contributions arising from carriers (electrons or holes), electron-hole pairs, and phonons are taken into account separately by employing the Wiedemann-Franz law, Price's theory, and Debye's isotropic continuum model, respectively. All phonon scattering mechanisms originate from crystal boundaries, mass-defects, deformation potentials, and anharmonicity are investigated rigorously for all solid solutions. The lowest total thermal conductivity values are obtained as 2.431 WK-1 m-1 at 700 K for n-type doped Mg2(Si0.4Sn0.6)0.98Bi0.02 solid solution and 1.843 WK-1 m-1 at 600 K for p-type doped Mg2(Si0.3Sn0.7)0.95Ga0.05 solid solution which clearly suggest that p-type doped Mg2Si1-xSnx based solid solutions are better candidates for the thermoelectric devices than their n-type doped solid solutions

Thermoelectric properties of Mg2X (X = Si, Ge) based bulk and quantum well systems

WOS: 000393476900018Mg2X (X = Si, Ge) compounds are promising thermoelectric materials for middle temperature applications due to good thermoelectric properties, nontoxicity, and abundantly available constituent elements. So far, these materials used in applications have all been in bulk form. Herein we report a full theory of thermoelectric transport properties of 3D bulk and 2D quantum well systems. the main aim of this present work is to show the effect of quantum confinement on the enhancement of the thermoelectric figure of merit theoretically. Results are given for n-type Mg2Si0.5Ge0.5 solid solutions and n-type Mg2Si/Mg2Ge/Mg2Si quantum well systems where the values of well widths are taken as 10 nm, 15 nm, and 20 nm, respectively. the n-type doping is made by using Sb- and La-elements as dopants. Experimental results for solid solutions are included to provide demonstration of proof of principle for the theoretical model applied for 3D bulk structures. the maximum thermoelectric figure of merits of LaxMg2-xSi0.49Ge0.5Sb0.01 solid solutions are obtained to be 0.64 and 0.56 at 800K for x = 0 and x = 0.01 sample, respectively. While, at the same temperature, due to the relatively low phonon thermal conductivity the state-of-the-art ZT values of 2.41 and 2.26 have been attained in the Mg2Si/Mg2Ge/Mg2Si quantum well samples with 0.01 wt. % Sb-doped and 0.01 wt. % Sb- and 0.01 wt. % La-doped, respectively. Published by AIP Publishing.Scientific Research Projects Coordination Unit of Recep Tayyip Erdogan University [373, 2016.53007.109.06.01]; Scientific and Technical Research Council of Turkey (TUBITAK)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [115F387]; Scientific Research Projects Coordination Unit of Recep Tayyip Erdogan UniversityThis work was supported both by the Scientific Research Projects Coordination Unit of Recep Tayyip Erdogan University with the Project ID: 373 and Project Code: 2016.53007.109.06.01 and by the Scientific and Technical Research Council of Turkey (TUBITAK) with the Grant No. 115F387. Thanks to Dr. Raif Kandemir for his technical support to get the project grant from Scientific Research Projects Coordination Unit of Recep Tayyip Erdogan University. Finally, thanks to Dr. Celal YELGEL for his support on the DFT calculations

Theoretical and experimental evaluation of thermoelectric performance of alkaline earth filled skutterudite compounds

BALLIKAYA, Sedat/0000-0002-0588-2212WOS: 000516130200019Skutterudite compounds are one of the most promising thermoelectric materials that can be used for intermediate power generation applications because of their excellent thermoelectric and mechanical properties. in spite of extensive research efforts during the past two decades, there is still a lack of full understanding of the electronic and thermal transport in these compounds. in the present study, we carry out a combined experimental and theoretical investigation of the electronic and thermal transport properties of alkaline earth-filled (Ca,Sr,Ba)(0.2)-Co4Sb12 skutterudites in the temperature range from 300 K to 800 K. the experimental work includes structural properties probed by PXRD and EMPA for phase purity and composition analysis. Theoretical values of the temperature dependent electronic transport parameters were determined using the nearly-free electron approximation, and various phonon thermal conductivity contributions were studied using the Debye isotropic continuum model and the theory of Price for the bipolar contribution. the theoretical and experimental transport parameters were found in good agreement with each other. They confirm that both Ba and Sr filled skutterudites are highly degenerate semiconductors, while the heat transport in the Ca filled compound is dominated by the bipolar contribution at temperatures above 300 K. the Sr filled skutterudite shows the highest Seebeck coefficient, likely due to its lowest donor ionisation energy. From our detailed theoretical investigations, it is seen that the key mechanism to control the phonon thermal conduction is originated from the interaction between mass defects and phonons. the maximum ZT value obtained 0.14 for Sr0.2Co4Sb12 compound at 800 K likely indicate that Sr is more effective filler among the alkaline earth metals.Scientific and Technological Research Council of Turkey (TUBITAK)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [216M254]; Scientific Research Projects Coordination Unit of Istanbul UniversityIstanbul University [21890, 32641]This work is supported by the Scientific and Technological Research Council of Turkey (TUBITAK) under the Project Number 216M254 and Scientific Research Projects Coordination Unit of Istanbul University with project number of 21890, 32641