Electrodeposition and characterization of nano-crystalline antimony telluride thin films

Electrodeposition is a promising low-cost method to fabricate nanostructured thermoelectric thin films such as Sb₂Te₃. However, electrodeposition of crystalline Sb₂Te₃ without the need for additional processing and with good compositional control has presented a challenge. Here we report on the electrodeposition of crystalline Sb₂Te₃ thin films at room temperature from a tartaric-nitric acid electrolyte using a pulsed, potentiostatic process. The effects of synthesis conditions on the resulting microstructure and compositional homogeneity are investigated using x-ray diffraction, electron diffraction, electron microscopy, and energy dispersive x-ray spectroscopy. The composition of the Sb-Te films was found to be dependent on the interval between pulses, a result that is likely due to the slow kinetics associated with Sb₂Te₃ formation at the surface. We also observed a change in texture and microstructure with varied applied pulse duration: for short pulse durations a lamellar microstructure with a {000?} texture forms, whereas for longer pulse durations a more equiaxed and randomly oriented microstructure forms. The thermal conductivities of the pulsed electrodeposited films are surprisingly low at less than 2 W/K·m and are found to systematically decrease with reduced pulse time.     

Enhanced power factor of Indium co-doped ZnO:Al thin films deposited by RF sputtering for high temperature thermoelectrics

An improvement in the thermoelectric power factor of Al doped ZnO has been achieved by means of co-doping with indium using a dual magnetron sputtering system. The concentration of indium in the film was varied from 0 to 10 atomic % by varying the RF power of the In target, with the ZnO:Al target fixed at 100 W. It has been found that the films with In concentrations at or below 5 at.% have no significant change in microstructure, and yet a marked improvement in thermopower. At higher doping levels, the Seebeck coefficient continues to increase, however poly-crystallinity is induced in the ZnO matrix which results in a considerable decrease in electrical conductivity. This factor ultimately has a negative impact on the materials power factor. Taking into account the films studied, (ZnO)Al_.03In_.02 exhibited the best thermoelectric properties with an electrical conductivity of 5.88 × 10² S/cm and a Seebeck coefficient of −220 μV/K at 975 K, resulting in a power factor is 22.1 × 10⁻⁴ Wm⁻¹ K⁻², which is three times greater than for the film with no In doping. Film microstructure, composition, and thermal stability were investigated using X-ray diffraction, scanning electron microscopy, and Auger electron spectroscopy.     

Effect of heat exchanger material and fouling on thermoelectric exhaust heat recovery

Thermoelectric devices are being investigated as a means of improving fuel economy for diesel and gasoline vehicles through the conversion of wasted fuel energy, in the form of heat, to useable electricity. By capturing a small portion of the energy that is available with thermoelectric devices can reduce engine loads thus decreasing pollutant emissions, fuel consumption, and CO₂ to further reduce green house gas emissions. This study is conducted in an effort to better understand and improve the performance of thermoelectric heat recovery systems for automotive use. For this purpose an experimental investigation of thermoelectrics in contact with clean and fouled heat exchangers of different materials is performed. The thermoelectric devices are tested on a bench-scale thermoelectric heat recovery apparatus that simulates automotive exhaust. It is observed that for higher exhaust gas flowrates, thermoelectric power output increases from 2 to 3.8 W while overall system efficiency decreases from 0.95% to 0.6%. Degradation of the effectiveness of the EGR-type heat exchangers over a period of driving is also simulated by exposing the heat exchangers to diesel engine exhaust under thermophoretic conditions to form a deposit layer. For the fouled EGR-type heat exchangers, power output and system efficiency is observed to be 5-10% lower for all conditions tested.     

Thermoelectric Characterization of (Ga,In)<Subscript>2</Subscript>Te<Subscript>3</Subscript> with Self-Assembled Two-Dimensional Vacancy Planes

The key properties for the design of high-efficiency thermoelectric materials are a low thermal conductivity and a large Seebeck coefficient with moderate electrical conductivity. Recent developments in nanotechnology and nanoscience are leading to breakthroughs in the field of thermoelectrics. The goal is to create a situation where phonon pathways are disrupted due to nanostructures in “bulk” materials. Here we introduce promising materials: (Ga,In)₂Te₃ with unexpectedly low thermal conductivity, in which certain kinds of superlattice structures naturally form. Two-dimensional vacancy planes with approximately 3.5-nm intervals exist in Ga₂Te₃, scattering phonons efficiently and leading to a very low thermal conductivity.     

Characterization of a β-FeSi2 p-n junction formed by the PECS method

We have fabricated semiconducting β-FeSi₂ bulks without doping and with Mn and Co doping by using a pulse electric current sintering (PECS) method, and explored the possibility of a direct bonding of n-type and p-type β-FeSi₂ bulks to form a p-n junction structure. P-type Mn-doped and n-type Co-doped β-FeSi₂ bulks were obtained by an annealing process at 800-850 °C for 100 h. The PECS was applied to bond the n-type and p-type bulks together for forming a p-n junction structure. We confirmed that the bonding was processed without any change in the β-FeSi₂ phase and was strongly joined with each other. Although we could not obtain the electrical characteristics of the p-n junction, Seebeck coefficients for n-type and p-type β-FeSi₂ in the bonded sample were determined to be −356 and 778 μV/K, respectively. We propose that these results should lead to an expanded use of the sintered β-FeSi₂ bulks in thermoelectric devices.     

Combined hot extrusion and spark plasma sintering method for producing highly textured thermoelectric Bi2Te3 alloys

Hot extrusion is a promising method for producing high-performance thermoelectric bismuth telluride alloys because of its ability to create textured microstructures. However, hot extrusion is less favourable for scaling-up because of temperature and strain gradients along the radial direction, and only <110> -textured thermoelectric legs can be obtained because of the fibre-like texture. We suggest a way to overcome these disadvantages by implementing an additional spark plasma sintering process on a stack of extrudates. Using this combined process, we demonstrate the fabrication of 12 × 15 × 13 mm³ p-type (Bi_0.2Sb_0.8)₂Te₃ samples from extrudates that had originally been 3 mm in diameter. The evolution of sheet-like texture revealed by SEM, XRD, and EBSD allows us to obtain both <110> - and <001> -textured thermoelectric legs from a single specimen that are desirable for low- and high-temperature applications, respectively. Our results demonstrate the combined method as an industry-friendly process for fabricating high-performance thermoelectric materials.     

Nanostructured Thermoelectric Materials and High-Efficiency Power-Generation Modules

For thermoelectric applications, the best materials have high electrical conductivity and thermopower and, simultaneously, low thermal conductivity. Such a combination of properties is usually found in heavily doped semiconductors. Renewed interest in this topic has followed recent theoretical predictions that significant increases in performance are possible for nanostructured materials, and this has been experimentally verified. During exploratory synthetic studies of chalcogenide-based bulk thermoelectric materials it was discovered that several compounds spontaneously formed endotaxially embedded nanostructures. These compounds have some of the best known properties for bulk thermoelectric materials in the 500–800 K temperature range. Here we report our continued efforts to better understand the role of the nanostructures while concurrently furthering the development of these new materials (for example n-type lead–antimony–silver–tellurium, and p-type lead–antimony–silver–tin–tellurium) into thermoelectric power-generation devices.     

Al-doped ZnO and N-doped CuxO thermoelectric thin films for self-powering integrated devices

With the use of a thermoelectric material, terrestrial heat can be harvested then converted to electrical power. The advent of these devices has led to the idea of self-powering wherein devices are driven by heat from their working environment. The focus of this study is to fabricate low cost thermoelectric materials, such as aluminum-doped ZnO (ZnO:Al) and nitrogen-doped Cu_xO (Cu_xO:N) that can effectively harvest heat for power generation.ZnO:Al (n-type) and Cu_xO:N (p-type) thin films with nanocrystallites were deposited in (1.27×0.64) cm² glass substrates via spray pyrolysis technique. These materials exhibit significantly high thermoelectric properties, which is comparable to previous works on thermoelectric materials. ZnO:Al showed to have a maximum Seebeck coefficient (S) of 448 μV/K ranging from 300 to 330 K. Cu_xO:N exhibited a significantly much larger |S| of 1002 μV/K at the same temperature range. A prototype of a thermoelectric device was constructed based from these grown thin films and showed to generate a maximum of 32.8 mV at 28 K temperature difference.     

Thermoelectric properties and power generation characteristics of sintered undoped n-type Mg2Si

Fabrication of sintered undoped n-type Mg₂Si, initiated from pre-synthesized all-molten polycrystalline Mg₂Si powder, was carried out using a Plasma Activated Sintering technique. The thermoelectric properties were evaluated from room temperature up to 861 K. The maximum dimensionless figure of merit was estimated to be 0.60 at 861 K. For output power measurements, the length and cross-sectional area of Mg₂Si elements equipped with Ni electrodes were varied from 6 to 15 mm and 2 × 2 to 4 × 4 mm², respectively. For the 3 × 3 × 7.5 mm³ element, the maximum output power density was 1.43 Wcm^-2 with the temperatures of the cool and hot sides being 373 K and 873 K (ΔT = 500 K), while the highest output power was 203 mW for the sample of 4 × 4 × 7.5 mm³ at ΔT = 500 K. The results of aging tests for 11,000 h with the hot side at 873 K and the cool side at 373 K under atmospheric conditions showed that the fabricated device elements possess sufficient durability at high power-generation operating temperatures.