Thermoelectric properties of the Bi-doped Mg2Si system
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Spark Plasma Sintering
Atmospheric temperature range
Figure of Merit
Dimensionless quantity
Stoichiometry
Thermoelectric materials are electronic materials that can exhibit noticeable voltage under temperature gradient and high electrical conductivity. The conventional thermoelectric materials are inorganic semiconductors or semimetals. Recently, flexible thermoelectric materials including conducting polymers and polymer composites gained great attention. The thermoelectric performance is usually characterized by the dimensionless figure of merit ( ZT ), ZT = S 2 σT / κ , where S being the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity. S 2 σ is called the power factor. To achieve high thermoelectric performance, it is important to possess the fundamental knowledge of thermoelectric materials, particularly the physics of the Seebeck coefficient, Peltier coefficient, electrical conductivity, and thermal conductivity. The Seebeck coefficient is related to the dependence of charge carrier density on temperature. The Seebeck coefficient of conducting polymers is usually lower than that of their inorganic counterparts by about one to two orders of magnitude. The electrical conductivity depends on the charge carrier density and charge carrier mobility. But the Seebeck coefficient and electrical conductivity are interdependent. Decreasing the charge carrier density can increase the Seebeck coefficient while decreasing the electrical conductivity. Thus, there is an optimal power factor in terms of the charge carrier density. Low thermal conductivity is required for high thermoelectric performance. The thermal conductivity includes the contributions by phonons and charge carriers. The thermal conductivity of conducting polymers is usually significantly lower than the inorganic thermoelectric materials. The thermoelectric materials have important application in harvesting heat, cooling, or sensing. The efficiency of thermoelectric generators depends not only on the ZT values of the thermoelectric materials but also their electrical and thermal contact resistances. The operation mechanism of thermoelectric coolers is basically the opposite process of thermoelectric generators. The temperature-sensitive voltage due to the Seebeck effect of thermoelectric materials is the basic principle of their application in sensors.
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A relationship between the dimensionless thermoelectric figure of merit, ZT, and the ratio of the open-circuit temperature difference, ΔTo, and the short-circuit temperature difference, ΔTs, is reported. Equivalent relationship between ZT and the open circuit voltage, Vo, and short-circuit current, Is, are also obtained. Both relationships provide a theoretical basis for the development of a novel, user-friendly and low cost technique for rapid and accurate measurement of the thermoelectric dimensionless figure of merit.
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In this study, FeSi2 bulk specimens were prepared by mechanical alloying, spark plasma sintering, and subsequent annealing. The annealed FeSi2 bulk specimens consisted of the β-FeSi2 phase and exhibited high Seebeck coefficient values. The maximum Seebeck coefficient of 356 μVK−1 was achieved in the FeSi2 bulk specimen annealed at 1173 K for 6 h. However, the power factor of the FeSi2 bulk specimen was quite small due to its high electrical resistivity, and a drastic improvement is required. Therefore, Mn- and Co-substituted specimens, Fe1−xMnxSi2 (x = 0.2–0.8) and Fe1−xCoxSi2 (x = 0.2–0.8), were produced, and their thermoelectric properties were evaluated. The Mn- and Co-substituted specimens exhibited lower electrical resistivity and a higher power factor than the FeSi2 bulk specimen. The Fe1−xMnxSi2 (x = 0.2–0.8) bulk specimens were p-type thermoelectric materials, and a Seebeck coefficient of 262 μVK−1 and a power factor of 339 μWm−1K−2 were achieved in the Fe0.94Mn0.06Si2 bulk specimen. On the other hand, the Fe1−xCoxSi2 (x = 0.2–0.8) bulk specimens were n-type thermoelectric materials, and a Seebeck coefficient of −180 μVK−1 and a power factor of 667 μWm−1K−2 were achieved in the Fe0.96Co0.04Si2 bulk specimen.
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Abstract YbAl specimens were prepared using a hot-pressing technique and the 3 Seebeck coefficient and electrical resistivity measured over the temperature range 150-700 K in order to assess their potential as thermoelectric materials. The preliminary results show that YbAl possesses an electrical power factor 3 double those of the state-of-the-art thermoelectric materials based on Bi Te 2 3 alloys which are employed over the range around room temperature and is a promising candidate material for thermoelectric generation using 'lowtemperature' waste heat.
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Zinc oxide and many other materials have long been used in thermoelectric devices for Waste heat recovery to convert heat into electricity. The performance of these materials depends on the figure of-merit ZT (= S2σ T/κ), where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the temperature. The study focused on the doping mechanisms of ZnO with Aluminum, and then the thermoelectric properties of ZnO doped with aluminum by spark plasma sintering technique. For Al-doped ZnO, the spark plasma sintering technique used to prepare samples from precursors canceled at various temperatures. I take these samples for study for doping mechanism in different material. For Al, doped ZnO, the spark plasma sintering conditions together with the micro structural evolution and thermoelectric properties of the samples were studied in detail. Moreover, nano composite approaches have been used to study the thermoelectric properties of other material systems such as Zinc oxide and half-Heusler phases. We observed a significant improvement in peak ZT of nano structured n-type HH compound 0.8 to 1.0 respectively. Here we studied only n-type HH compounds. The improvement of figure of merit is mainly due to the reduction of thermal conductivity. This nanostructure approach is applicable to many other thermoelectric materials that are useful for automotive, industrial waste heat recovery, space power generation and many other fields.
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Thermoelectric generator
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Thermoelectric power factor and figure of merit of Nd2-xCexCuO4 (x=0-0.1) sintered bodies were estimated from the Seebeck coefficient, electrical resistivity, and thermal conductivity measured in temperature range of 300-673 K. Temperature dependence of Seebeck coefficient and electrical resistivity showed n-type semiconducting behavior. Thermal conductivities, which decreased with increasing temperature for each Cc concentration, were in range of 3.7-7.5 Wm-1K-1. The power factor and the figure of merit for x=0.01 and 0.05 increased with decreasing temperature. Their maxima were 9.2⋅10-5 Wm-1K-2 and 1.7⋅10-5K-1, respectively, at 320 K for x=0.05. The figure of merit would be improved to some extent by reduction of lattice thermal conductivity.
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Electrical conductivity and seebeck coefficient at different temperatures,and thermal conductivity at room temperature for various doped polyaniline (Pan) samples were measured,and the thermoelectric figure of merit ZT was calculated.The effects of preparation methods and temperature on thermoelectric properties were discussed.The results show that the electrical conductivity and the seebeck coefficient of Pan are strongly dependent on the preparation conditions and temperature.The electrical conductivity becomes larger and the seebeck coefficient becomes smaller as Pan molecular weight increases.Redoping by organic acid and HCl results in an increase in both electrical conductivity and Seebeck coefficient of Pan,and therefore ZT value.The electrical conductivity increases and the seebeck coefficient decreases as the temperature increases when T＜Td (dedoping temperature).The decreasing of the electrical conductivity and increasing of the seebeck coefficient take place by dedoping when T＞Td.The thermal conductivity is lower,and insensitive to the sample preparation conditions.
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Measurements of the electrical conductivity σ, the thermal conductivity K, and the Seebeck coefficient (thermoelectric power) α, of InAs1−xPx have been made at high temperatures with x varying from 0 to 0.4. Between 450° and 800°C, the average thermoelectric figure of merit (z = α2σ/K) increases moderately with x for small values of x, reaching a maximum near x = 0.1, and subsequently decreasing. Between 450 and 800°C, the average z for x = 0.1 is equal to 0.63 × 10−3(°K)−1, a 15% improvement over InAs. For the same figure of merit, the Seebeck coefficient and electric resistivity is higher in the ternary than in the binary InAs; this is advantageous for the design of thermoelectric devices. Previous values for the thermoelectric figure of merit of InAs are revised.
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