We report a new finding that CaSi reversibly absorbs and desorbs hydrogen. Its hydriding and dehydriding properties were investigated both theoretically and experimentally. Before experiment, first-principles calculation was performed with the ultrasoft pseudopotential method based on the density functional theory, and it was predicted that CaSi hydride is thermodynamically stable. In actual measurement, the pressure-composition isotherms clearly had plateau pressures in a temperature range of 473-573 K, and the maximum hydrogen content was 1.9 wt% under a hydrogen pressure of 9 MPa at 473 K, which means CaSi forms CaSiH_1.3. The theoretical and experimental values of the enthalpy of CaSiH_1.3 were estimated as -42 kJ/molH₂ and -62 kJ/molH₂, respectively. They are almost in agreement, within a typical error of 20 kJ/molH₂. From the X-ray diffraction profiles, it is confirmed that the crystal structure of CaSi changes reversibly with the absorption and desorption of hydrogen.

Naoyuki Yamada

This paper describes the results of our evaluation of a pedestrian's radio wave reflection characteristics. Recently, the demands made for safety systems, such as the Forward Collision Avoidance Assistance System (FCAAS), have been increasing. The radar sensors of such safety systems have to be capable of detecting not only vehicles but also pedestrians, bicycles, and roadside objects. Furthermore, such a safety system should be able to make a decision without any mistake as to whether a collision occurs. Therefore, the radar sensor must be capable of detecting objects with 100% certainty up to a point immediately before a collision would otherwise occur.

The reflection characteristics of radio waves from a pedestrian were measured as part of the effort to improve the pedestrian detection performance of the radar sensor. A pedestrian's radio wave reflection intensity is low, at about 15- 20 dB less than that of the rear of a vehicle, and can vary by as much as 20 dB. Evaluating these characteristics in detail is a prerequisite to the development of a radar sensor that is capable of detecting pedestrians reliably.