Effect of Thermoelectric and Electrical Properties on the Cooling Performance of a Micro Thermoelectric Cooler

Active cooling has been studied to prevent microprocessor temperature rise due to hot spots, and a micro thermoelectric cooler is a promising candidate for this spot cooling since it can be used to effectively cool the small area near the hot spot. Numerical analysis has been conducted to determine the effect of thermoelectric and electrical properties on the cooling performance of such a micro thermoelectric cooler. In the cooler considered herein, Bi₂Te₃ and Sb₂Te₃ were selected as the n- and p-type thermoelectric materials, respectively. The thermoelectric column considered was 20 μm thick. The coefficient of performance (COP) and cooling rate were the primary factors used to evaluate the performance of the cooler. Although cooling performance varies with thermal conditions such as thermophysical properties and temperature difference, the present study only focuses on the effect of thermoelectric and electrical properties such as the Seebeck coefficient and electrical resistivity.     

Thermoelectric and mechanical properties of Bi0.5Sb1.5Te3 solid solution prepared by melt solidification in liquid

The influence of the conditions for preparing samples from granules of Bi_0.5Sb_1.5Te₃ solid solution obtained by melt solidification in liquid on the mechanical and thermoelectric properties of these samples is investigated. The microstructure and surface morphology of sample cleavages are analyzed by optical and scanning electron microscopy. The mechanical properties of the samples are investigated by compression tests at temperatures from 300 to 620 K. The thermoelectric characteristics (Seebeck coefficient and electrical and thermal conductivities) are measured both at room temperature and in the temperature range of 100–700 K. The samples with the highest thermoelectric figure of merit, (ZT)_max ≈ 1.3 at 370 K, are obtained.

     

Synthesis and Characterization of New Ceramic Thermoelectrics Implemented in a Thermoelectric Oxide Module

Novel thermoelectric oxides were developed, produced, and characterized to demonstrate their promising thermoelectric conversion potential in a thermoelectric converter. Four-leg thermoelectric oxide modules were fabricated by combining p- and n-type oxide thermoelements made of pressed polycrystalline GdCo_0.95Ni_0.05O₃ and CaMn_0.98Nb_0.02O₃, respectively. In these modules, the p- and n-type thermoelements were connected electrically in series and thermally in parallel. The materials were joined by electrical contacts consisting of a Ag/CuO composite material. Fairly good thermal contacts were ensured by pressing the thermoelements between alumina substrates. Cross-sections of the alumina/Ag–CuO mixture/thermoelement interface were investigated by scanning electron microscopy. The temperature distribution across the module was monitored using K-type thermocouples and a micro-infrared (IR) camera. The open-circuit voltage and the load voltages of the module were measured up to a temperature difference of ΔT = 500 K while keeping the temperature of the cold side at 300 K. The output power and internal resistance were calculated. The characteristics of the module evaluated from electrical measurements were compared with respective values of the p- and n-type leg materials. An output power of 0.04 W at ΔT = 500 K led to a power density of ~0.125 W/cm³, where the volume of thermoelectric material was determined by a cross-section of 4 mm × 4 mm and a leg length of 5 mm.     

Three-Stage Thin-Film Superlattice Thermoelectric Multistage Microcoolers with a Δ<Emphasis Type="Italic">T</Emphasis><Subscript>max</Subscript> of 102 K

Thin-film Bi₂Te₃- and Sb₂Te₃-based superlattice (SL) thermoelectric (TE) devices are an enabling technology for high-power and low-temperature applications, which include low-noise amplifier cooling, electronics hot-spot cooling, radio frequency (RF) amplifier thermal management, and direct sensor cooling. Bulk TE devices, which can pump heat loads on the order of 10 W/cm², are not suitable in these applications due to their large size and low heat pumping capacity. Recently, we have demonstrated an external maximum temperature difference, ΔT _max, as high as 58 K in an SL thin-film p–n couple. This state-of-the-art couple exhibited a cold-side minimum temperature, T _cmin, of −30.9°C. We regularly attain ΔT _max values in excess of 53 K, in spite of the many significant electrical and thermal parasitics that are unique to thin-film devices. These measurements do not use any complex thermal management at the heat sink to remove the heat flux from the TE device’s hot side. We describe here multistage SL cooling technologies currently being developed at RTI that can provide useful microcooling cold-side temperatures of 200 K. This effort includes a three-stage module employing independently powered stages which produced a ΔT _max of 101.6 K with a T _cmin of −75°C, as well as a novel two-wire three-stage SL cascade which demonstrated a T _cmin of −46°C and a ΔT _max of nearly 74 K. These RTI modules are only 2.5 mm thick, significantly thinner than a similar commercial three-stage module (5.3 mm thick) that produces a ΔT _max of 96 K. In addition, TE coolers fabricated from these thin-film SL materials perform significantly better than the extrapolated performance of similar thickness bulk alloy materials.     

Iron Disilicide as High-Temperature Reference Material for Traceable Measurements of Seebeck Coefficient Between 300 K and 800 K

Thermoelectric generators (TEGs) convert heat to electrical energy by means of the Seebeck effect. The Seebeck coefficient is a central thermoelectric material property, measuring the magnitude of the thermovoltage generated in response to a temperature difference across a thermoelectric material. Precise determination of the Seebeck coefficient provides the basis for reliable performance assessment in materials development in the field of thermoelectrics. For several reasons, measurement uncertainties of up to 14% can often be observed in interlaboratory comparisons of temperature-dependent Seebeck coefficient or in error analyses on currently employed instruments. This is still too high for an industrial benchmark and insufficient for many scientific investigations and technological developments. The TESt (thermoelectric standardization) project was launched in 2011, funded by the German Federal Ministry of Education and Research (BMBF), to reduce measurement uncertainties, engineer traceable and precise thermoelectric measurement techniques for materials and TEGs, and develop reference materials (RMs) for temperature-dependent determination of the Seebeck coefficient. We report herein the successful development and qualification of cobalt-doped β-iron disilicide (β-Fe_0.95Co_0.05Si₂) as a RM for high-temperature thermoelectric metrology. A brief survey on technological processes for manufacturing and machining of samples is presented. Focus is placed on metrological qualification of the iron disilicide, results of an international round-robin test, and final certification as a reference material in accordance with ISO-Guide 35 and the “Guide to the expression of uncertainty in measurement” by the Physikalisch-Technische Bundesanstalt, the national metrology institute of Germany.     

Thermoelectric properties of the (Bi,Sb)2Te3-based material obtained by spark plasma sintering

The dependence of the thermoelectric properties of the nanostructured bulk (Bi,Sb)₂Te₃ material on the composition and the spark plasma-sintering (SPS) temperature T _SPS has been studied. It has been revealed that the Bi_0.4Sb_1.6Te₃ solid solution sintered at a temperature of 450–500°C has a thermoelectric figure of merit ZT = 1.25–1.28. The dependence of thermoelectric properties on the sintering temperature T _SPS above 400°C is correlated to the transformation of the fine structure of the material due to the rearrangement of point vacancy-donor defects in the process of repeated recrystallization. It has been established that point structural defects make a considerable contribution to the formation of the thermoelectric properties of nanostructured material.     

Synthesis and Characterization of <Emphasis Type="Italic">p</Emphasis>-Type Pb<Subscript>0.5</Subscript>Sn<Subscript>0.5</Subscript>Te Thermoelectric Power Generation Elements by Mechanical Alloying

A mechanical alloying (MA) process to transform elemental powders into solid Pb_0.5Sn_0.5Te with thermoelectric functionality comparable to melt-alloyed material is described. The room-temperature doping level and mobility as well as temperature-dependent electrical conductivity, Seebeck coefficient, and thermal conductivity are reported. Estimated values of lattice thermal conductivity (0.7 W m⁻¹ K⁻¹) are lower than some reports of functional melt-alloyed PbSnTe-based material, providing evidence that MA can engender the combination of properties resulting in highly functional thermoelectric material. Though doping level and Sn composition have not been optimized, this material exhibits a ZT value >0.5 at 550 K.     

Finite element analysis of miniature thermoelectric coolers with high cooling performance and short response time

The miniature thermoelectric cooler (TEC) is a promising device for microelectronics applications with high cooling performance and short response time. In this paper, a comprehensive numerical analysis focusing on the cooling performance and response time of the TEC is performed by finite element methods (FEMs). The effects of load current, geometric size, ratio of length to cross-sectional area and substrate's thermal resistance on the performance of the TEC are studied. The results show that the performance of TECs has been improved by reducing the TEC's size and ratio of length to cross-sectional area, resulting in a maximum cooling temperature difference of 88 °C, a cooling power density of 1000 W cm⁻² and a short response time on the order of milliseconds. Furthermore, the substrate, which hinders the circulation of heat between the TEC and the atmosphere, also has a significant influence on the performance of the TEC.     

Thermoelectric Performance and High-Temperature Creep Behavior of GeTe-Based Thermoelectric Materials

New, efficient thermoelectric materials (GeTe) _x (Mn_0.6Sn_0.4Te)_1−x (0.8 ≤ x ≤  1.0) were prepared by hot pressing, and the effect of MnTe and SnTe contents on thermoelectric and mechanical properties of GeTe was investigated. The maximum dimensionless figure of merit ZT of the prepared materials is 1.57 in the temperature range from 720 K to 770 K for x = 0.15. Niobium was added to the quasiternary GeTe-based materials to suppress creep without degradation of thermoelectric properties. The distortion of the material with added Nb was less than 0.4% under experimental conditions of 100 N load at 873 K for 100 h. The favorable thermoelectric properties of these materials are accompanied by their stability in long-term use and the possibility of widening the service temperature range as a result of decreasing their phase-transition temperature T _c.     

Thin-Film Thermoelectric Module for Power Generator Applications Using a Screen-Printing Method

A new process for fabricating a low-cost thermoelectric module using a screen-printing method has been developed. Thermoelectric properties of screen-printed ZnSb films were investigated in an effort to develop a thermoelectric module with low cost per watt. The screen-printed Zn _x Sb_1−x films showed a low carrier concentration and high Seebeck coefficient when x was in the range of 0.5 to 0.57 and the annealing temperature was kept below 550°C. When the annealing temperature was higher than 550°C, the carrier concentration of the Zn _x Sb_1−x films reached that of a metal, leading to a decrease of the Seebeck coefficient. In the present experiment, the optimized carrier concentration of screen-printed ZnSb was 7 × 10¹⁸/cm³. The output voltage and power density of the ZnSb film were 10 mV and 0.17 mW/cm², respectively, at ΔT = 50 K. A thermoelectric module was produced using the proposed screen-printing approach with ZnSb and CoSb₃ as p-type and n-type thermoelectric materials, respectively, and copper as the pad metal.