Thermal conductivity reduction of crystalline silicon by high-pressure torsion

We report a dramatic and irreversible reduction in the lattice thermal conductivity of bulk crystalline silicon when subjected to intense plastic strain under a pressure of 24 GPa using high-pressure torsion (HPT). Thermal conductivity of the HPT-processed samples were measured using picosecond time domain thermoreflectance. Thermal conductivity measurements show that the HPT-processed samples have a lattice thermal conductivity reduction by a factor of approximately 20 (from intrinsic single crystalline value of 142 Wm⁻¹ K⁻¹ to approximately 7.6 Wm⁻¹ K⁻¹). Thermal conductivity reduction in HPT-processed silicon is attributed to the formation of nanograin boundaries and metastable Si-III/XII phases which act as phonon scattering sites, and because of a large density of lattice defects introduced by HPT processing. Annealing the samples at 873 K increases the thermal conductivity due to the reduction in the density of secondary phases and lattice defects.     

Effect of substrate on the microstructure and thermoelectric performances of Sr-doped Ca3Co4O9 thick films

Ca₃Co₄O₉ thermoelectric materials in form of thick films are very promising in practical applications due to their low costs and relatively high performance. In this work, two different suspensions have been used to produce different coatings on Al₂O₃ polycrystalline substrates with theoretical green thickness of 360 and 2000?µm. Moreover, the effect of substrate has also been investigated using Al₂O₃ monocrystalline substrates and a 360?µm green thickness. Sintering procedure at 900?°C for 24?h has drastically decreased coating thickness. XRD performed on the coatings surface has shown the formation of small amounts of Ca₃Co₂O₆ secondary phase on the polycrystalline substrates, while it was more abundant, and accompanied by Ca₂Co₂O₅ on the monocrystalline substrates. In spite of the higher secondary phases content, monocrystalline substrates produced a slight grain orientation which led to the highest thermoelectric properties between the samples (0.38?mW/K²m at 800?°C), and very close to the best reported values in the literature.     

Electrodeposition of PbTe thin films from acidic nitrate baths

Electrodeposition of PbTe thin films from an acidic nitric bath was systematically investigated to understand the kinetics and the effect of electrodeposition conditions on film composition, crystallographic structure, texture and grain size. The electroanalytical studies employed initially with a rotating disk electrode to investigate the kinetics associated with Te, Pb and PbTe electrodeposition. The results indicated that the PbTe thin films were obtained by the underpotential deposition (UPD) of Pb atoms onto the overpotentially deposited Te atoms on a substrate.Based on these studies, PbTe thin films were potentiostatically electrodeposited using e-beam evaporated gold thin films on silicon substrate to investigate the effect of various deposition conditions on film composition and microstructure. The data indicated that the microstructure, composition and preferred film growth orientation of PbTe thin films strongly depended on the applied potential and electrolyte concentration. At −0.12 V, the film was granular, dense, and preferentially oriented in the [1 0 0] direction. At potentials more negative than −0.15 V, the film was dendritic and preferentially oriented in the [2 1 1] direction. A smooth, dense and crystalline film with nearly stoichiometric composition was obtained at −0.12 V from a solution containing 0.01 M HTeO₂⁺, 0.05 Pb²⁺ and 1 M HNO₃.     

Thermoelectric performance of Mg2−xCaxSi compounds

Thermoelectric materials Mg_2−xCa_xSi (x = 0, 0.01, 0.03, 0.05, 0.07, 0.1) compounds have been prepared by vacuum melting followed by hot-pressing. Effects of the substitution of Ca for Mg on phase structures and the thermoelectric properties of the hot-pressed compounds were investigated. It was found that the alloying of Ca in Mg₂Si based compounds increases the electrical conductivity and decreases the Seebeck coefficient of the compounds, due to the electronegativity difference between Ca and Mg. The dimensionless figures of merit of Mg₂Si and Mg_1.99Ca_0.01Si reach, respectively, 0.41 and 0.34 at 660 K.     

High-temperature thermoelectric properties of late rare earth-doped Ca3Co4O9+δ

Misfit-layered oxides Ca_3−xLn_xCo₄O_9+δ with Ln = Dy, Er, Ho, and Lu were synthesized using solid state reactions. The resulting samples were hot-pressed (HP) at 1123 K in air for 2 h under a uniaxial pressure of 60 MPa. Thermoelectric properties of Ca_3−xLn_xCo₄O_9+δ were investigated up to 1200 K. Both the Seebeck coefficient and electrical resistivity increase upon Ln substitution for Ca. Among the Ln-doped samples, the magnitude of Seebeck coefficient tends to increase with decreasing ionic radius of Ln³⁺. The Ln-doped samples exhibit a lower thermal conductivity than the non-doped one due to a decrease of their lattice thermal conductivity. The dimensionless figure of merit, ZT, reaches 0.36 at 1073 K for the Ca_2.8Lu_0.2Co₄O_9+δ sample, which is about 1.6 times larger than that for the non-doped counterpart.     

Studies on surface preparation and smoothness of nanostructured Bi2Te3-based alloys by electrochemical and mechanical methods

Significant improvements in the dimensionless thermoelectric figure-of-merit (ZT) for nanostructured bismuth telluride, Bi₂Te₃, and its alloys have been demonstrated. In designing high-performance thermoelectric devices, variations in the thermal and electrical contact resistances due to interfacial effects between the nanostructured alloy and the metallic electrodes remain a significant issue. Smooth scratch-free surfaces should provide a baseline for contact resistance studies. In this paper, the root mean square roughness over a 10 μm² of nanostructured bismuth tellurium based alloys was reduced from 133 nm to 1.9 nm by a procedure consisting of electrolysis, mechanical polishing, and chemical mechanical polishing (CMP). Post-CMP cleaning was also developed to yield a wettable surface for the subsequent conformable metallization.     

Fabrication of bismuth telluride nanoparticles using a chemical synthetic process and their thermoelectric evaluations

Bismuth telluride nanoparticles for thermoelectric applications were successfully prepared via a water-based chemical reaction. In this process, we used both a complexing agent (ethylenediaminetetraacetic acid) and a reducing agent (ascorbic acid) to stabilize the bismuth precursor (Bi(NO₃)₃) in water and to favor the reaction with the reduced source of tellurium. The resulting powder was confirmed to range in size below ca. 100 nm with the crystalline structure corresponding to the rhmobohedral Bi₂Te₃. We sintered the nanocrystalline powder via a spark plasma sintering process, thus we obtained the sintered body composed of nano-sized grains. Then, we measured some important transport properties (electrical resistivity, Seebeck coefficient, and thermal conductivity) of the sintered body to calculate its thermoelectric performance, the figure of merit. Finally, we discussed the effect of the nanostructure in the sintered body on the thermal conductivity.     

High-pressure synthesis of phonon-glass electron-crystal featured thermoelectric LixCo4Sb12

Li-filled CoSb₃, which is inaccessible under ambient pressure, was successfully synthesized with a high-pressure synthesis technique, demonstrating a fast and effective way to broaden elemental species that can be filled into voids of skutterudites. The optimized Li_0.36Co₄Sb₁₂, with a greatly enhanced thermal power factor and much reduced thermal conductivity, has a ZT value of 1.3 at 700 K, the highest among all single elemental filled CoSb₃ materials at this temperature. In addition, an instructive linear relationship between the Einstein temperatures of the distinct rattling fillers and their ionic radii is revealed, which as a reference can easily be applied to the multiple elemental filling strategy for selecting suitable filling elemental species to reduce the lattice thermal conductivity more effectively.     

Off-Center Guest Vibrations and Their Effect on Lattice Thermal Conductivity in <Emphasis Type="Italic">n</Emphasis>- and <Emphasis Type="Italic">p</Emphasis>-Type β-Ba<Subscript>8</Subscript>Ga<Subscript>16</Subscript>Sn<Subscript>30</Subscript>

We report on thermoelectric and Raman scattering studies of single crystalline samples of a type I clathrate Ba₈Ga₁₆Sn₃₀. The n- and p-type samples have a thermopower of −300 μV/K and +270 μV/K, and electrical resistivity of 20 mΩ cm and 40 mΩ cm at room temperature, respectively. Regardless of the charge carrier type, the lattice thermal conductivity κ _L shows typical glass-like behavior. The low-energy vibration of Ba(2) in the tetrakaidecahedron is manifested in the Raman scattering spectrum as a peak at 15 cm⁻¹. This energy agrees with the characteristic vibrational temperature of 20 K derived from the analysis of the specific heat data with a soft-potential model. The fact that both the vibrational energy and the magnitude of κ _L for Ba₈Ga₁₆Sn₃₀ are lowest among the type I clathrates is evidence that off-center vibrations are responsible for the strong reduction of κ _L. Analysis of κ _L(T) revealed that the positional disorder associated with the off-center vibrations acts as the Rayleigh scattering center, and the coupling between guest vibrational modes and acoustic phonons is stronger than that in Sr₈Ga₁₆Ge₃₀ and Eu₈Ga₁₆Ge₃₀.     

Facial fabrication of an inorganic/organic thermoelectric nanocomposite based gas sensor for hydrogen detection with wide range and reliability

A novel method for fabrication of a thermochemical hydrogen (TCH) gas sensor composed of platinum (Pt)-decorated graphene sheets and a thermoelectric (TE) polymer nanocomposite was investigated. The hydrogen sensing characterization for the device included gas response, response time (T₉₀), recovery time (D₁₀), and reliability testing, which were systematically conducted at room temperature with a relative humidity of 55%. Here, the Pt-decorated graphene sheets act as both an effective hydrogen oxidation surface and a heat-transfer TE polymer nanocomposite having low thermal conductivity. This property plays an important role in generating output voltage signal with a temperature difference between the top and bottom surfaces of the nanocomposite. As a result, our TCH gas sensor can detect the range of hydrogen from 100 ppm to percentage level with good linearity. The best response and recovery time revealed for the optimized TCH gas sensor were 23 s and 17 s under 1000 ppm H₂/air, respectively. This type of sensor can provide an important component for fabricating thermoelectric-based gas sensors with favorable gas sensing performance.