Thermoelectric Conversion Elements Research Articles

(Invited) Structural Control of Molybdenum Silicide By Electrolytic Silicification of a Mo Substrate

Metal silicides are attracting attention as environment-friendly functional materials because they are composed of abundant elements in the earth’s crust. However, conventional processes for preparing silicides from SiO2 as the starting material consume a lot of energy and utilize hazardous materials through the reduction process of SiO2. Against this background, the electrochemical process in molten salt become key technology for fabricating silicon materials in an environmentally friendly, simple, and inexpensive process. We have already reported the electrodeposition of metallic silicon films from SiO2 in high temperature molten salt. This process has the potential to develop environmentally friendly and sustainable systems for fabricating silicon materials by combining non-consumable oxygen-evolving anodes. In this study, we focused on the electrochemical formation of molybdenum silicide from SiO2 in molten chloride because the molybdenum silicide are promising materials for ultra-high temperature protective coating, heating element, and thermoelectric conversion element. We report here the electrochemical formation of molybdenum silicide in CaCl2-based molten chloride containing SiO2 system. A Mo substrate was used as the working electrode. From the results of characterization by XRD, SEM, and EDS analysis, a metastable β-MoSi2 phase and stable α-MoSi2 phase were obtained by controlling applied potentials during potentiostatic electrolysis. In addition, since the nucleation and growth process of the silicide is very important to control its structure, composition and morphology, the electrolytic time transient between molybdenum substrate/molten chloride interface were carefully observed by SEM analysis. The fundamental research on formation process of silicides in molten salt will greatly contribute to materials science in high temperature environment and developing the novel rout for fabricating silicon materials.

Effect of the Deposition Time and Heating Temperature on the Structure of Chromium Silicides Synthesized by Pack Cementation Process

Transition metal silicides have attracted great interest for their potential use in optoelectronic devices, photovoltaic cells, and thermoelectric conversion elements because of their high melting point, high oxidation resistance, and satisfactory thermoelectric properties. This study focuses on the effect of the deposition time and the heating temperature on the morphology and structure of the chromium silicides synthesized by the pack cementation method. A series of experiments were carried out at various temperatures (1000–1150 °C) with different deposition times (15–120 min). The morphology and the chemical composition of the samples were determined using SEM with an EDS analyzer. The structure determination and phase identification were performed by XRD analysis. The examination of the as-formed materials was completed by performing thermal stability tests. The most suitable conditions for producing CrSi2 sample with satisfactory properties and simultaneously minimizing the cost and production time are listed. It was found that the sample synthesized at 1000 °C for 15 min during the chromizing step, in combination with the siliconizing step at 1000 °C for 60 min, presents the best thermal stability and these selected temperatures offer appropriate, economical, and repeatable results.

ヘマタイトの熱電変換特性に及ぼす陽極酸化条件の影響

The purpose of this study is to investigate the effect of fabrication conditions for α-Fe2O3 based thermoelectric conversion elements. It was demonstrated that the open circuit voltage of α-Fe2O3 thermoelectric conversion elements could be enhanced by decreasing the load current density used in production of the film by anodic oxidation. α-Fe2O3 films were fabricated under various oxidizing condition in an aqueous solution of FeCl3, KF, KCl and H2O2 at the temperature of 50 °C. The load current density and oxidation time were varied. The amount of α-Fe2O3 in the α-Fe2O3 film, the crystal quality and the crystal grain size of the α-Fe2O3, were evaluated. In terms of the maximum value of the open circuit voltage, the best film thickness of the α-Fe2O3 film was found to be about 40 ~ 50 μm. α-Fe2O3 films with greater crystal quality and larger crystal grain size can be fabricated by decreasing the load current density. These improvement on α-Fe2O3 thermoelectric conversion elements lead to more powerful thermoelectric conversion element, i.e., higher open circuit voltage.

Heat Transfer Characteristics of Work Fluid Including Phase Change Material That Flow into Heating Surface from Narrow Path

Use of the low temperature (less than 100°C) energy contributes to effective use of heat resources. The cost recovery by power generation is difficult by using an existing system (the binary cycle or the thermoelectric conversion element), because the initial investment is large. The final purpose of this research is development of the low temperature difference drive engine supposing use in a hot-springs resort as a power source for electric power generation. In order that a traveler may look at and delight a motion of an engine, it is made to drive at low-speed number of rotations. An engine cycle of this study is aimed at the development of Stirling cycle engine which can maintain high efficiency in small size. This kind of engine has simple structure; it brings low cost, and it is easy to perform maintenance. However, it is difficult to obtain enough output by this type of engine, because of its low temperature difference. This paper deals with the heat transfer characteristic that the working fluid including a phase change material flows into the heating surface from the narrow path. In order to increase the amount of the heat transmission, Diethylether is added to the working fluid. Diethylether is selected as a phase change material (PCM) that has the boiling point which exists between the heat source of high temperature and low temperature. The parameters of the experiment are additive amount of PCM, rotational speed of the displacer piston and temperature of heat transfer surface. It is shown that it is possible to make exchange of heat amount increase by adding phase change material. The result of this research shows the optimal condition of the difference in temperature in heat processing, number of revolutions, and addition concentration of PCM.

20806 熱電発電素子を用いた温泉水の廃熱エネルギー利用システムに関する研究

This study is to investigate of a utilization of thermal energy by slight temperature difference. An experiment has been carried out using the Peltier elements.We focus on an electric power generation system using thermal energy of hot spring water and Seebeck effect of thermoelectric conversion elements. In order to set a suitable temperature difference on both sides of the Peltier element, an apparatus consists of hot water and cold water. This paper describes a relationship between the electric voltage and the temperature difference of the Peltier element. This system will be applied to the continuous power generation and to the LED for 24 hours.

G060052 熱電変換素子による温度差発電を伴う温泉の温度制御

The temperature control of hot spring water is currently attained by mixing hot water with cold water or by a simple heat exchanger. However, in these techniques, the huge amount of available energy is being thrown away during the cooling process. In this paper, we propose a new heat exchanging system using thermoelectric conversion elements, in which the exit temperature is automatically controlled while generating electricity. The portable test apparatus is manufactured, next the performance test is conducted by varying temperature and flow rate of hot water. In conclusion, the amount of electricity generated and the response of temperature control is clarified experimentally, and the validity of this system is verified.

Integrated dual-cycle energy recovery using thermoelectric conversion and an organic Rankine bottoming cycle

Hot engine exhaust represents a resource that is often rejected to the environment without further utilization. This resource is most prevalent in the transportation sector, but stationary engine—generator systems also typically do not utilize this resource. Engine exhaust is considered high-grade heat and can potentially be utilized by various approaches to produce electricity or to drive heating and cooling systems. One idea for this application is to combine an organic Rankine cycle and thermoelectric conversion. This approach is being developed to more fully utilize the thermal energy contained in hot exhaust streams. The model developed here is composed of a high-temperature heat exchanger which extracts thermal energy for driving the thermoelectric conversion elements and a closely integrated bottoming cycle to capture the large amount of remaining thermal energy in the exhaust stream. Many interacting parameters that define combined system operation are employed in the model to determine the overall system performance including output power, efficiency, and total energy utilization factors. In addition, the model identifies a maximum power operating point for the combined system. The model can identify the optimal amount of heat to remove from the exhaust flow to drive the thermoelectric elements for maximizing the combined cycle output. The model has been developed such that heat exchanger UAH (heat transfer coefficient multiplied by heat transfer area) values, thermal resistances, and the thermoelectric figure of merit ( ZT) can be investigated in the context of system operation. The model also has the ability to simultaneously determine the effect of each cycle design parameter on the performance of the overall system, thus giving the ability to utilize as much waste heat as possible. Results of exercising the model give system performance and inter-relationships between various design parameters as they affect overall performance.

Ｒｕｄｄｌｅｓｄｅｎ‐Ｐｏｐｐｅｒ Ｃａｎ＋１（Ｍｎ１−ｘＶｘ）ｎＯ３ｎ＋１（ｎ＝２）化合物粉体の調製と焼結体の熱電特性

Ruddlesden-Popper type compounds, being presented as (ABO3)n-AO or An+1BnO3n+1 (n=1, 2, 3…∞), have a layer structure where perovskite layer ABO3 and rock salt type layer AO are alternately in place; Can+1(Mn1-xVx)nO3n+1 (0≤x≤1) belongs also to this compound. As CaO layer shows low thermal conductivity whereas CaMnO3 layer reveals high electrical conductivity and high Seebeck coefficient, therefore, Can+1(Mn1-xVx)nO3n+1 seems to become a promising module as a thermoelectric conversion material. However, there was no report concerning about the thermoelectric conversion characteristic of this compound. In this study we have investigated electrical conductivity, Seebeck coefficient, and thermal conductivity of Can+1(Mn1-xVx)nO3n+1 and discussed about the possibility of the application as thermoelectric conversion elements from dimensionless performance index ZT.

The Maximum Power Operating Point for a Combustion-Driven Thermoelectric Converter With Heat Recirculation

A model is developed for determining the ideal operating point, based on maximum power output, for a thermoelectric conversion (TEC) element coupled to a combustor. In the analysis, heat recirculation from the combustor exhaust is included. Results presented here are relevant to the operating characteristics of small, combustion-driven energy systems. The model is composed of a TEC element, a combustor, a counterflow heat exchanger, and a thermal shunt resistance to the surroundings. Including the shunt is necessary due to the increased importance of this effect in small-scale thermal systems. From this combination of components, an optimal combustor operating temperature is found giving maximum power output and efficiency. The model is used to determine ideal performance figures as a function of system parameters such as the effectiveness of heat regeneration, loss of heat by conduction, and the parameters describing the thermoelectric conversion element (the so-called ZT parameter). Although a high degree of idealization is employed, the results show the importance of heat recirculation and the significance of thermal losses on system operation.

Development of a superconducting magnet excited with thermoelectric conversion element for quasi persistent mode

Some superconducting magnets are generally operated in persistent mode. The persistent-mode operation of these magnets is necessary to maintain field decay rates below a certain level, typically 0.01 ppm over a 1-hr period. These magnets used to be physically uncoupled from the external power source. In the case of HTS magnets, it is very difficult to keep the magnetic field stable without a high-stability external power source for large flux flow losses. A superconducting magnet is a high current and low voltage system except for excitation voltage. From this point of view, a thermoelectric conversion element is a very suitable power supply for a quasi persistent-mode magnet system. Moreover, this element seems to be particularly useful for HTS magnets. As the first step of this study, the authors tried to design a superconducting magnet excited with a thermoelectric conversion element and carried out an excitation test of a small HTS magnet by using the elements.

Development of a thermoelectric power generation system using reciprocating flow combustion in a porous FeSi2 element

A thermoelectric power generation system using reciprocating flow combustion in a porous thermoelectric conversion element has been developed and examined in its performance. Mn- (n-type) and Co- (p-type) doped FeSi2 powders were molded into the cylindrical element via a spark plasma sintering process, in which Mn- and Co-doped parts were separated by a thin insulator sheet exclusive of a terminus. The porous element consisted of two semicylindrical p/n couples, arranged electrically in series but thermally in parallel. A thermopower of 1.0–1.2 mV/K at 295–624 K and an apparent internal resistivity of 1.6×10−1 Ω cm at 556–624 K have been obtained for the element. A power generation system was then made using a pair of the elements, which were arranged lengthwise in a cylindrical combustion chamber. A reciprocatory flow of dilute fuel gas was introduced into the element, and it was ignited between the element. A steep temperature gradient of about 200 K/cm was formed lengthwise in both elements. The energy density has reached as much as 7 kW/m3 (excluding combustion chamber and mounting clamp) by the reciprocating flow combustion with a dilute fuel gas which may not even be normally flammable.

Development of a Porous FeSi<SUB>2</SUB> Thermoelectric Conversion Element

Heavily porous FeSi 2 bulky materials for thermoelctric conversion elements have been developed via a spark plasma sintering process. FeSi 2 bulky materials with a porosity ratio of about 45% were molded into U-type thermoelectric conversion elements in which the p-type and n-type FeSi 2 were separated by insulating glass fiber sheets. The Doping elements used were Co and Mn. Ag mesh electrodes were firmly attached, simultaneously by the sintering process, to the surface of the sintered products. A thermoelectric power of 0.5-0.7 mV/K at 300-500 K and an apparent internal resistivity of 3.99 x 10 -3 Ωm at 448-479 K have been obtained, which shows a potential for use in power generation by gas-combustion and heat-exchange type thermoelectric processes.