Phase study and thermoelectric properties of Cu2SnSe3 for green energy harvesting

Abstract

TE energy harvesting is one of the approaches to address the global environmental challenges, utilizing waste heat for useful electric power generation. This technology recently attracted attention of the commercial sectors, due to the possible applications of TE materials in wearable electronic devices, solid state Peltier cooling, remote area sensors, etc. Currently available commercial TE materials such as Bi2Te3, Sb2Te3 and PbTe consist of either toxic or expensive elements. Therefore, exploration of TE materials with less toxic, less expensive and more earth-abundant elements is the focus of the TE community these days. The ternary chalcogenide Cu2SnSe3 is expected to be one of these auspicious TE materials, owning large flexibility to tune its physical and electronic properties. The elements of Cu2SnSe3 are less toxic, abundant in nature and relatively cheap compared to Te and Pb. Previous studies reported that Cu2SnSe3 can be found in various allotropic forms, which include orthorhombic, monoclinic and cubic structures. Phase transformation in solid state compounds involves rearrangement of atoms, which can be induced by the change of temperature, pressure and/or composition. It is important to understand the thermodynamic conditions of these diverse phases since different synthesis conditions and stoichiometry can lead to the formation of different phases. Phase transitions can lead to different band gap and lattice parameters, which can further affect the physical properties. The effect of annealing temperature on the phase transition of Cu2SnSe3 was investigated, and the TE properties of the monoclinic and cubic phases of Cu2SnSe3 were compared for the first time. Stoichiometric composition of Cu2SnSe3 was synthesized by melt solidification and heat treatment at various temperatures followed by water quenching. XRD analyses reveal that the samples annealed at 720 and 820 K have mostly monoclinic phase along with small amount of cubic phase. The Cu2SnSe3 annealed at 960 K is mostly cubic. It was found that the mostly cubic phase has ZT much higher than the mostly monoclinic phase sample. Better TE performance of high temperature cubic phase can be attributed to the smaller band gap (~0.92 eV) which can lead to higher electrical conductivity than monoclinic (~1.0 eV) at room temperature. Although the cubic phase Cu2SnSe3 has slightly higher thermoelectric efficiency than the monoclinic phase but still the ZT of both the phases is much lower compare to commercialized PbTe thermoelectric material. It can be realized that both have nearly similar thermoelectric properties but still the ZT of the Cu2SnSe3 is much lower than the PbTe due to the lower Seebeck coefficient of the Cu2SnSe3. To improve the Seebeck coefficient of the Cu2SnSe3 we synthesized CTSe-SnS composites by mechanical alloying followed by spark plasma sintering. An optimized improvement in TE properties was achieved by adding 3% SnS to the CTSe matrix, which led to enhanced ZT in the medium temperature range. It was found that SnS addition to Cu2SnSe3 (CTSe) can effectively enhance the Seebeck coefficient from 30 to 490 μVolt/K at room temperature, but the power factor was not much improved due to the loss of electrical conductivity. But still, a prominent improvement in ZT value from 0.06 to 0.18 at 570 K for the CTSe was achieved due to disorder scattering, which can lead to a lower thermal conductivity of the composites than the pristine CTSe. The results show that the thermoelectric properties of CTSe can be engineered by the presence of SnS phase in the CTSe matrix. Despite of large improvement enhancement in Seebeck coefficient in the Cu2SnSe3 –SnS composites, the improvement in the ZT value was not that impressive due to loss of electrical conductivity. It is a big challenge to decouple Seebeck coefficient and electrical properties in alloying and composites, where increasing one value will lead to compromise another. Doping and solid solution can be one of the promising approaches to control the Seebeck coefficient without largely compromising the electrical conductivity for improved ZT of the various thermoelectric materials. Here we introduced a strategy to control the Seebeck coefficient without losing large part of electrical conductivity of the CTSe by making its solid solution with Cu2SnS3 (CTS). Suppressing Se vacancies by introducing S in the CTSe lattice is one of the way to control the Seebeck coefficient. The analogous crystal and band structure of Cu2SnSe3 (CTSe) with Cu2SnS3 (CTS) was the motivation behind the study of TE performance of Cu2Sn (SX, Se1-X)3 (0≤ X ≤0.8) solid solution. CTSe and Cu2Sn (SX, Se1-X)3 (0≤ X ≤0.8) solid solution were prepared using mechanical alloying (MA) followed by spark plasma sintering. Rietveld refinement of the powder X-ray diffraction (XRD) data was performed to determine the variation in the lattice constants of the Cu2Sn (SX, Se1-X)3 (0≤ X ≤0.8) solid solution with gradual replacing of Se with S. Here we found that the band gap and TE properties optimization as well as reducing in thermal conductivity can be efficiently achieved by careful optimizing of the composition of Cu2Sn (SX, Se1-X)3 (0≤ X ≤0.8) solid solution for enhanced TE performance of the CTSe. Finally making thin films and 2D structures of the Cu2SnSe3 can be another way to explore the its commercial potential due to its higher power density, light weight and compact size. Due to high conductive nature of the Cu2SnSe3 it is expected that the Cu2SnSe3 thin film can give us much better TE properties compare to thin films of conventional TE compounds like SnSe, due to incorporation of defects during thin film deposition. CTSe thin films were successfully deposited by PLD for the first time, and the effects of substrate temperature on the microstructure and TE properties of the films were investigated. A mmonoclinic CTSe target and Pt-coated silicon substrates were used to deposit films at substrate temperatures of 300, 350 and 400 °C. The TE properties of the films were studied, and it was found that 350 °C is the optimized substrate temperature in which the highest power factor of 2.8*10-4 (W/m2K) was achieved among the studied samples. Increasing substrate temperature further up to 400 °C degraded the microstructure and electrical properties of the deposited film, and hence the power factor was decreased compared to the other films deposited at 300 and 350 °C. The power factor of 2.8*10-4 (W/m2K) of the sample deposited at 350 °C is very close to the reported value for bulk CTSe. A high power factor which is comparable with bulk CTSe, along with other benefits like higher energy density, light weight and compact size, makes CTSe film a potential candidate for various TE applications.

TE 에너지 수확은 유용한 전력 생산을 위해 폐열을 활용하여 지구 환경 문제를 해결하기위한 접근법 중 하나입니다. 이 기술은 최근 웨어러블 전자 장치, 고체 상태 펠티어 냉각, 원격 영역 센서 등의 TE 재료의 응용 가능성으로 인해 상업 분야에서 주목을 받고 있습니다. 현재 Bi2Te3, Sb2Te3 및 PbTe와 같은 상업용 TE 물질은 독성 또는 비싼 요소. 그러므로 독성이 적고 값이 싸지 않으며 지구에 풍부한 원소가 함유 된 TE 재료를 탐구하는 것이 TE 공동체의 요즘입니다. 삼원 칼 코겐화물 Cu2SnSe3는 물리적 및 전자적 특성을 조정할 수있는 큰 유연성을 지니고있어 이러한 상서로운 TE 재료 중 하나 일 것으로 예상됩니다. Cu2SnSe3의 원소는 독성이 적고 자연이 풍부하며 Te 및 Pb에 비해 상대적으로 저렴합니다. 이전의 연구들은 Cu2SnSe3가 사방 정계, 단사 정계 및 입방 형 구조를 포함하는 다양한 동족체 형태로 발견 될 수 있다고보고했다. 고체 상태 화합물의 상 변환은 원자의 재 배열을 수반하며, 이는 온도, 압력 및 / 또는 조성의 변화에 ​​의해 유도 될 수있다. 상이한 합성 조건 및 화학량 론이 상이한 상 형성을 유도 할 수 있기 때문에 이러한 다양한상의열역학적 조건을 이해하는 것이 중요하다. 상전이는 상이한 밴드 갭 및 격자 파라미터를 유도 할 수 있으며, 이는 물리적 특성에 더 영향을 미칠 수있다. 어닐링 온도가 Cu2SnSe3의 상전이에 미치는 영향을 조사하였고, Cu2SnSe3의 단사 정계 및 입방 형상의 TE 특성을 처음으로 비교 하였다. Cu2SnSe3의 화학량 론적 조성은 다양한 온도에서의 용융 응고 및 열처리와 수처리에 의해 합성되었다. XRD 분석에 따르면 720 및 820K에서 어닐링 된 샘플은 소량의 입방 단계와 함께 대부분 단사 정계 (monoclinic phase)를 나타냅니다. 960K에서 어닐링 된 Cu2SnSe3는 대부분 입방체이다. 대부분 큐빅상은 주로 단사 정계 상 샘플보다 훨씬 높은 ZT를 갖는 것으로 밝혀졌다. 고온 입방 위상의 TE 성능은 실온에서 단사 정계 (~ 1.0eV)보다 높은 전기 전도도를 유도 할 수있는보다 작은 밴드 갭 (~ 0.92eV)에 기인 할 수있다. 큐빅 상 Cu2SnSe3은 단사 정계 상보다 약간 더 높은 열전기 효율을 갖지만, 여전히 상용화 된 PbTe 열전 재료와 비교하여 두 상 모두의 ZT가 훨씬 낮다. 둘 다 거의 비슷한 열전자 특성을 가지지 만 Cu2SnSe3의 ZT는 Cu2SnSe3의 낮은 제벡 계수 때문에 PbTe보다 훨씬 낮다는 것을 알 수있다. Cu2SnSe3의 제벡 계수를 개선하기 위해 우리는 기계적 합금화와 스파크 플라즈마 소결에 의한 CTSe-SnS 복합체를 합성했다. TE 특성의 최적화 된 향상은 CTSe 매트릭스에 3 % SnS를 첨가함으로써 달성되었으며, 이로 인해 중간 온도 범위에서 ZT가 향상되었습니다. Cu2SnSe3 (CTSe)에 대한 SnS 첨가는 실온에서 제 베크 계수를 30에서 490 μV / K까지 효과적으로 향상시킬 수 있음을 발견하였으나 역률은 전기 전도성의 손실로 인해 크게 향상되지 않았다. 그러나 CTSe의 570K에서 ZT 값이 0.06에서 0.18로 현저한 향상을 보였으 나 무질서한 산란으로 인해 원래의 CTSe보다 복합체의 열전도율이 낮아질 수 있습니다. 결과는 CTSe 매트릭스의 SnS 상 존재로 CTSe의 열전기 특성이 조작 될 수 있음을 보여줍니다.