Thermoelectric effects, such as the Seebeck and Peltier effects, have been widely used for temperature sensing and precise temperature control. However, thermoelectric generation has not been commonly used except for special purposes, such as an electrical source used in a spacecraft traveling far from the sun. Since the theoretical framework of thermoelectric energy conversion was established in the 1950's, enormous efforts, over two decades, have been devoted to developing high efficiency thermoelectric materials. However, they did not lead to satisfactory improvements, and the research slowed down. That changed when the newly introduced concepts and materials brought out in the early 1990's as well as the strong demand for energy saving pulled thermoelectric generation back under the spotlight again. In this article, the basic concepts of thermoelectric energy conversion and the current status of thermoelectric generation are presented.

The spin Seebeck effect refers to the conversion of a heat current into a spin voltage in a ferromagnet/paramagnet junction. Since the spin Seebeck effect appears not only in metals and semiconductors but also in magnetic insulators, it enables the construction of “insulator-based thermoelectric generators” in combination with the inverse spin Hall effect, which would be impossible if only conventional thermoelectric technology were used. In this article, we briefly report on the experimental observation of the spin Seebeck effect in magnetic insulator/metal junctions and discuss the potential for novel thermoelectric technologies based on spin currents.

Energy harvesting is a set of technologies that harvest ambient energies including light, wind, vibration, heat and radio waves to power small electronic devices e.g. wireless autonomous devices, like those used in wearable electronics and wireless sensor networks. In order to commercialize energy harvesting technologies, many activities to combine them with related technologies-such as energy storage, power management, low-power sensors and low-power wireless transmission-are being performed by the public and private sectors in Japan as well as overseas.

Most of the high-ZT materials developed in the past 15 years contain rare and/or toxic elements and their practical applications are limited. In contrast, materials composed of earth-abundant and non-toxic elements still show very low ZT and they have not yet been commercialized. In order to break out of this situation, the thermoelectric performance of non-rare & non-toxic cheap materials needs to be further enhanced by a nanostructure engineering strategy, which has been successfully applied for developing high-ZT materials. This article will briefly describe the recent progress in developing bulk superlattice materials based on SrTiO₃ and TiS₂ through nanostructure engineering, and a PV/TE hybrid device will be introduced as an example of future thermoelectric applications.

In the context of increasing energy prices and climate change, thermo-electric conversion is of the highest interest for producing electrical power from waste heat. The automotive industry is keenly awaiting the installation and development of thermoelectric generators (TEG) because of the strict fuel consumption regulations in the EU. Since ～70% of the gasoline that is used in cars is emitted as waste heat, if some percentage of the discarded heat could be reused then fuel consumption would be improved. An on-board TEG system is one possible technique for conserving fuels and supplying electricity. Magnesium silicide (Mg₂Si) has emerged as the most promising thermoelectric (TE) material for automotive applications. This is mainly due to the light weight of Mg₂Si, the relative abundance of its constituent elements (which have no associated national risk from the material supply), and to its good TE properties with sufficient durability at elevated operational temperatures. We have been engaging in the establishment of fundamental fabrication techniques for the manufacturing of Mg₂Si TEGs, including the use of all-molten poly-crystalline source materials, plasma activated sintering with tunable pulse wave current operation, sintering scalability with a metal binder, and monobloc sintering or a modified plating method for metal electrode termination to Mg₂Si.

Thermophotovoltaic (TPV) generation systems are the systems which convert thermal radiation into electric power using photovoltaic (PV) cells. In TPV systems, the performance can be improved by matching between the emitter's thermal radiation spectrum and the PV cell's spectral response. A large-area fabrication technology of spectrally selective emitters for high-temperatures, called “High-temperature photonic materials” is required to investigate high-efficient TPV systems. The fabrication technology of high-temperature photonic materials is presented using the self-organization of nickel-based super-alloys. Combustion TPV systems have been developed in this study. The enhancement of efficiency is observed in the system with spectrally selective emitters.

Radiowaves and electromagnetic fields from broadcasting, wireless communication and electrical devices can be converted to DC energy using a rectifier. The energy density of such ambient electromagnetic fields has been too weak to be used for electronics devices. However, due to the improvement of rectifying efficiency and the reduction of the power consumption of electronics devices, a small amount of energy such as below 100µW can be effectively used to operate low power microprocessors. In this paper, the feasibility of ambient energy harvesting technologies and its demonstration as a means to supply power to a battery-less sensor network are introduced.

When thermoelectric power generation is performed under the condition of a low energy density, organic materials have the advantage of producing flexible and large area devices. They also have the advantage of the easy design of thinner devices by applying their 1-2 order smaller thermal conductivities than those of inorganic materials. In this article, the author presents an overview of the broad survey results of thermoelectric properties of organic and organic/inorganic composite materials using an original instrument. Some examples where the Seebeck effects by unusual mechanisms emerged are also introduced.

A large area X-ray image sensor called a Flat panel detector (FPD) is widely used for diagnoses such as with radiography and fluoroscopy. The outlines of the detector including the device structure, working principles and newly developed clinical applications using this detector are described. To reduce the X-ray exposed dose in X-ray diagnoses, new approaches to the detector are also mentioned. To maintain the signal to noise ratio in the low dose region, the introduction of active pixel technology in the TFT matrix and a photoconductor material which has low electron-hole pair creation energy such as CdZnTe are effective.