Continuous Improvement for Manufacturing Techniques: Thin Film Coating and More (original) (raw)

Synthesis and characterisation of CZTSe bulk materials for thermoelectric applications

Nanosystems: Physics, Chemistry, Mathematics, 2020

Quaternary Copper Zinc Tin Selenide (CZTSe) is a preferred candidate as an absorber layer in solar cells due to its non-toxicity and the abundancy of its constituents. This material also has thermoelectric properties suitable for solar thermal energy conversion and waste heat recovery. The preparation of bulk thermoelectric materials is a tedious, multistep task and requires considerable time and energy consumption for tuning of desired properties. Here one step solid state reaction has been used for synthesis of bulk CZTSe materials in five different ratios of elemental precursors: Cu, Zn, Sn and Se. Atomic Force Microscopy (AFM), X-Ray Photoelectron Spectroscopy (XPS) and X-ray diffraction (XRD) techniques have been used for structural and compositional analysis of the materials. AFM analysis shows significant difference in roughness parameters and grain size with respect to Cu/Zn variations. The XRD spectra of various samples show the formation of CZTSe materials. Raman spectra verifies absence of secondary phases. XPS analysis reveals constituent atoms display chemical valences of +1, +2, +4, and −1 for Cu, Zn, Sn, and Se, respectively. The stoichiometric sample, Cu 2 ZnSnSe 4 , exhibited the maximum power factor 0.30 mW•m −1 K −2 , having carrier concentration in the range of 10 18-10 19 cm −3 and resistivity in the range of 0.21 to 0.24 Ω•cm. Keywords: thermoelectric devices, thermoelectric effects in semiconductors and insulators, Hall effect in semiconductors, Raman spectroscopy in chemical analysis, photoelectron spectroscopy in chemical analysis, powder diffraction X-ray, transport properties (electric and thermal conductivity, thermoelectric effects, etc.).

Enhancement of Thermoelectric Figure-of-Merit by a Bulk Nanostructuring Approach

Advanced Functional Materials, 2010

where T h and T c are the hot-end and cold-end temperature of the thermoelectric materials, respectively, and T is the average temperature of T c and T h. Thus, it is important to use materials with a high ZT value for practical applications. The low ZT value of commercially available thermoelectric materials limits the applications of thermoelectric devices. Metals and metal alloys whose ZTs are very low (ZT (1) can only be applied in thermocouples to measure temperature and radiant energy. [2] Semiconducting thermoelectric materials, such as Bi 2 Te 3 and SiGe alloys with ZT % 1, [6] are used commercially in low-power cooling and low-power thermoelectric power generators, such as beverage coolers and laser diode coolers, and power generators in space missions. To make thermoelectric devices competitive in large-scale and high-power commercial applications, materials with significantly higher ZT values in the application temperature range are required. [1,3,4,10,14] Since the 1960s, much research has been devoted to identifying thermoelectric materials which could satisfy this requirement. The traditional method to improve ZT is to discover new thermoelectric materials. Since the thermoelectric effect was discovered, many thermoelectric materials have been identified, such as Bi 2 Te 3 , skutterudites Co 4 Sb 12 , SiGe alloys, PbTe, CsBi 4 Te 6 , [15] Tl 9 BiTe 6 , [16] clathrate (Ba,Sr) 8 (Al,Ga) 16 (Si,Ge,Sn) 30 , [17] PbTe-PbS, [18] lead antimony silver tellurium based materials [19,20] (such as AgPb m SbTe 2þm (LAST), [19a] Ag(Pb 1Àx Sn x) m SbTe 2þm (LASTT), [19b] Na 1Àx Pb m Sb y Te mþ2 (SALT), [19c] and NaPb 18Àx Sn x-SbTe 20 (SALTT) [19d]), and In 4 Se 3Àd. [21] Many of these materials are alloys which help in reducing the phonon thermal conductivity. More thermoelectric materials can be found in other review papers

Nanocomposites as thermoelectric materials

2010

Thermoelectric materials have attractive applications in electric power generation and solid-state cooling. The performance of a thermoelectric device depends on the dimensionless figure of merit (ZT) of the material, defined as ZT = S 2 o-T / k, where S is the Seebeck coefficient, o is the electrical conductivity, k is the thermal conductivity, and T is the absolute temperature. In recent years, the idea of using nanotechnology to further improve the figure of merit of conventional thermoelectric materials has triggered active research and led to many exciting results. Most of the reported ZT enhancements are based on thin films and nanowires in which the thermal conductivity reduction plays a central role. We pursue the nanocomposite approach as an alternative to superlattices in the quest for high ZT materials. These nanocomposites are essentially nano-grained bulk materials that are synthesized by hot pressing nanoparticles into a bulk form. The interfaces inside a nanocomposite strongly scatter phonons but only slightly affect the charge carrier transport. Therefore, we can significantly reduce the lattice thermal conductivity and even somewhat increase the power factor S 2 U, resulting in higher ZT than for bulk materials. Compared with expensive thin-film superlattices, nanocomposites will have significant advantages in mass production, device construction and operation. This thesis covers my studies on bismuth antimony telluride nanocomposites and some recent work on Co 4 Sb 12-based nanocomposites. In bismuth antimony telluride nanocomposites, we have achieved a peak ZT of 1.4 at 100 'C, a 40% increase in ZT over the bulk material. This is the first significant ZT increase in this material system in fifty years. The same approach has also yielded a peak ZT around 1.2 in Yb filled Co 4 Sbi 2 nanocomposites. During the process, great efforts were dedicated to assuring accurate and dependable property measurements of thermoelectric nanocomposites. In addition to comparing measurement results between the commercial setups and a homebuilt measurement system, the high ZT obtained in bismuth antimony telluride nanocomposites was further confirmed by a device cooling test. To better understand the measured thermoelectric properties of nanocomposites, theoretical analysis based on the Boltzmann transport equation was performed. Furthermore, frequency-dependent Monte Carlo simulations of the phonon transport were conducted on 2D periodic porous silicon and 3D silicon nanocomposites. In the thermoelectrics field, the latter one provided the first accurate prediction for phonon size effects in a given nanocomposite. For charge carriers in thermoelectric nanocomposites, The process of writing this thesis provided me an opportunity to revisit many of the exciting moments of my Ph.D. studies. During my stay at MIT, I feel deeply grateful for many people who have taught and inspired me in their own ways. Among these, I would first thank my advisor, Prof. Gang Chen, who not only set an example of conducting first-class research but also helped me to improve myself in many aspects. In addition, I highly appreciate the enthusiastic help and support of my other Ph.D. committee members, including Prof. Mildred S. Dresselhaus, Prof. Zhifeng Ren, and Prof. Borivoje B. Mikic. I benefited a lot from their constructive criticisms of my research and encouragement along the way. With this chance, I would acknowledge the support and guidance of my Master's advisor, Prof. Li Shi at the University of Texas at Austin, who led me into the nanotechnology field from a traditional thermal engineering background. I am also indebted to Prof. Yinping Zhang at Tsinghua University for his advice over the years. Over the past six years, I was fortunate to work with many brilliant and diligent students and visiting scholars in both Boston College and MIT. Among many ex-and current MIT/BC members, I would especially thank the follows for their friendship and many useful discussions:

Cold Spray Deposition of Thermoelectric Materials

JOM, 2020

Thermoelectric materials convert heat flux to electricity (or vice versa as Peltier coolers); however, their application to harvest waste heat is limited by challenges in fabrication and materials optimization. Here, cold-spray deposition is used as an additive manufacturing technique to fabricate p-and ntype Bi 2 Te 3 , on substrates ranging from quartz to aluminum. The sprayed material has a randomly oriented microstructure largely free from pores (> 99.5% dense), and deposition is achieved without substantial compositional changes. The Seebeck coefficient and thermal conductivity are largely preserved through the spray process, but the defects introduced during deposition significantly increase electrical resistivity. Defects can be removed, and compressive strain relaxed by a post-deposition anneal, which leads to Bi 2 Te 3 blocks with a typical ZT of 0.3 at 100°C. Generators fabricated on sheets or pipes made of copper compare favorably with similar designs constructed using bulk Bi 2 Te 3 , displaying a wider operating temperature range. These results demonstrate the power and versatility of cold-spray additive manufacturing and provide a pathway toward fabrication of thermoelectric generators in complex geometries that are inaccessible to generators made by traditional approaches.