Cold Spray Deposition of Thermoelectric Materials (original) (raw)
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Thermoelectrics and its Energy Harvesting, 2 Volume Set, 2012
Technique Advantages Disadvantages CVD Very good materials: High ZT. Low deposition rate. Expensive and complicated equipment. ECD Very high deposition rate. Patterning can be done during deposition (LIGA). Low ZT. Sputtering Good deposition rate. Average ZT value. Composition can be controlled with cosputtering. Annealing (or substrate heating) improves ZT. Composition difficult to control (depends on power). Thermal co-evaporation High ZT. Good deposition rate. Control of film composition. Simple/inexpensive equipment. Needs substrate heating. Needs precise control of deposition rate and crucible temperature. The conventional techniques used in the fabrication of macro thermoelectric devices cannot be used in the micro devices. Few pattern techniques have been demonstrated in the fabrication of thermoelectric micro devices, with feature size bellow tens of micrometers. These techniques were imported from the fabrication of MEMS (micro-electro-mechanical systems) techniques, namely wet-etching, lift-off (with SU-8 photoresist), reactive ion etching (RIE) and lithography-electroplating-molding (LIGA). The wet etching patterning process assisted with UV photolithography is presented in this chapter, using thermoelectric Bi/Sb/Te films, in a planar device structure. Applications for micro cooling and energy harvesting are also presented in the end of the chapter. 2. Thin film deposition The figure of merit of Bi/Sb/Te thermoelectric materials is always related with composition and crystalline structure of materials, despite the technique used. And these properties are related with deposition variables. Previous work [1-3] showed that the optimum composition to maximize the figure of merit is obtained with Te content in the range 60%-65% (atomic). Some of the deposition techniques presented in table 5.1 allow the composition control of the film during growth, thus the figure of merit can be improved. When evaporating directly either Bi 2 Te 3 , Sb 2 Te 3 or Bi x Sb 2-x Te 3 the materials decompose and the final composition of the film does not match the initial composition of the evaporant, due to the different vapour pressure of the elemental substances Bi, Sb and Te. When heated, these materials decompose, the tellurium evaporates faster than bismuth or antimony, and a composition gradient is expected when thicker films are deposited. The composition differs from the surface layers (Te rich in the first fabricated layers) into the bulk and the last fabricated layers (Bi and Sb rich). A detailed study of composition along thickness was presented by Silva [1]. To overcome this problem, a composition control during film growth is necessary, mainly in thermal evaporation techniques, but also in sputtered films. Using co-evaporation, the deposition rate of each element (Bi, Sb or Te) is controlled independently, and an optimal composition can be achieved [1-4]. The same technique is also used in co-sputtering [5,6].
2017
A method for directly bonding thermoelectric elements onto copper electrodes without applying a solder paste was developed in this study. A tin coating of thickness approximately 50 m was deposited via electroplating onto the surface of a Bi 2 Te 3-based thermoelectric element, which had a nickel diffusion barrier layer. The resulting structure was subsequently subjected to direct thermocompression bonding at 250 C on a hotplate for 3 min at a pressure of 1.1 kPa. Scanning electron microscopy imaging confirmed that a strong and uniform bond was formed at the copper electrodethermoelectric element interface, and the melted or solidified tin layer remained defect-free. The thermoelectric module fabricated using tin plating had an average bonding strength similar to that fabricated using soldering.
Optimization of thermoelectric properties on Bi2Te3 thin films deposited by thermal co-evaporation
Thin Solid Films, 2010
The optimization of the thermal co-evaporation deposition process for n-type bismuth telluride (Bi 2 Te 3 ) thin films onto polyimide substrates for thermoelectric applications is reported. The influence of deposition parameters (evaporation rate and substrate temperature) on film composition and thermoelectric properties was studied for optimal thermoelectric performance. Energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy confirmed the formation of Bi 2 Te 3 thin films. Seebeck coefficient (up to 250 μVK -1 ), in-plane electrical resistivity (≈10 μΩm), carrier concentration (310 19 -2010 19 cm -3 ) and Hall mobility (80 -170 cm 2 V -1 s -1 ) were measured at room temperature in selected Bi 2 Te 3 samples.
Flexible micro thermoelectric generator based on electroplated Bi2+xTe3−x
2008 Symposium on Design, Test, Integration and Packaging of MEMS/MOEMS, 2008
We present and discuss the fabrication process and the performance of a flexible micro thermoelectric generator with electroplated Bi 2+x Te 3-x thermocouples in a SU-8 mold. Demonstrator devices generate 278µWcm -2 at ∆T meas =40K across the experimental set up. Based on model calculations, a temperature difference of ∆T G =21.4K across the generator is assumed. Due to the flexible design and the chosen generator materials, the performance stays high even for curved contact surfaces. The measurement results correlate well with the model based design optimization predictions.
Heat Treatment Effects on Electrochemically Grown Bi2Te3 Thin Films for Thermoelectric Applications
MATERIALS TRANSACTIONS, 2012
Bi 2 Te 3 thin films were grown on a large area of n-Si/Ti/Au substrate by an electrochemical process. The synthesized film sample was cut into four different pieces and each piece underwent different heat treatments for 1 h to optimize their carrier concentration. Heat treatment experiments were performed in an inert atmosphere to prevent oxidation of the films during the treatment. X-ray diffraction showed an increase in the crystallite size with increasing annealing temperatures, which affected the thermoelectric performances of the films. At room temperature, the Seebeck coefficient and electrical resistivity were measured using a custom-built setup. Initially, the measured conductivity appeared to be ntype for all films backed by the metal buffer layer and Si substrate. A simple model that could classify the substrate contribution on the overall transport properties was then developed. The model confirmed that the actual conductivities of the films were p-type, and this was supported by their elemental analysis.
Thermoelectric Generation Using Industrial Grade Low-Cost Materials
2015
Thermoelectric effect or Seebeck effect provides a means by which thermal energy can be converted into electricity directly. Voltage appears between two different types of materials (Conductor or Semiconductor) that are joined together and its junction is heated. While most efficient materials are semiconductors, these are expensive, thus, research into thermoelectric properties of industrial grade materials is very useful for fabrication of low cost devices. Using materials such as Iron, Aluminum and Brass, cheap TEGs were constructed and several parameters were tested for three separate combinations made using these materials. An output voltage up to 0.5mV per couple at a ZT of 0.08 was achieved. It was apparent that using active cooling methods such as ice water or heat sinks with cooling fans increased the output values to around 0.7mV per couple. Although the efficiencies are not high, these TEGs provide low-cost devices that can produce small-scale sustainable energy through co-generation and energy-scavenging.
Electrochemical deposition of Bi 2 Te 3 for thermoelectric microdevices
Journal of Solid State Electrochemistry, 2003
The electrolyses of solutions of bismuth oxide and tellurium oxide in nitric acid with molar ratios of Bi:Te=3:3-4:3 lead to cathodic deposits of films of bismuth telluride (Bi 2 Te 3 ), an n-type semiconductor. Current densities of 2-5 mA/cm 2 were applied. Voltammetric investigations showed that Bi 2 Te 3 deposition occurred at potentials more negative than )0.125 V (Ag/ AgCl, 3 M KCl). The deposit was identified as bismuth telluride (c-phase) by X-ray analysis. Hall-effect measurements verified the n-type semiconducting behaviour. The films can be deposited in microstructures for thermoelectric microdevices like thermoelectric batteries or thermoelectric sensors.
Bismuth telluride has already been utilized as a thermoelectric material near room temperature for a long time. Higher mechanical strength is still needed in order to prevent unexpected cleavage during manufacturing process of modules, and a further improvement in thermoelectric performance is also longed. For a high-performance thermoelectric material, a fine-grained microstructure is desirable owing to low thermal conductivity as well as high strength and toughness. We focused on the effectiveness of a combination of mechanical alloying (MA) process of starting materials and subsequent hot-extrusion process. The bulk extrudates of p-type Bi0.4Sb1.6Te3 compound have sound appearances, high density, fine-grained microstructures with single phase and also the enhanced thermoelectric performance. As a result, a significant improvement in both thermoelectric and mechanical performances of p-type Bi0.4Sb1.6Te3 compounds was obtained by the combination of MA process of starting materials...