Research

The research in this group is focused on the physical properties of grain boundaries and interfaces, the influence of grain boundary microstructure on the function and performance of polycrystalline materials and the development of processing methods to control the grain boundary microstructure. Illustrative examples of our research are given below.

Improvement of the efficiency of polysilicon solar cells using a materials science approach

Fig.1: Kelvin probe force (KFM) micrograph measurement of potential barriers at silicon grain boundaries: (left) Σ9 boundary, (right) random boundary (S. Tsurekawa et al., Phil. Mag. Lett. 85 (2005) pp. 41-49.)
Fig.1: Kelvin probe force (KFM) micrograph measurement of potential barriers at silicon grain boundaries: (left) Σ9 boundary, (right) random boundary (S. Tsurekawa et al., Phil. Mag. Lett. 85 (2005) pp. 41-49.)

　The energy conversion process in solar cells is subject to certain energy loss factors including the material-related phenomena of bulk recombination loss and resistance loss. It is known that in polycrystalline silicon (polysilicon) solar cells, the many grain boundaries inevitably present in the polycrystal act as preferential sites for bulk recombination loss, in which electrons and holes reunite and carriers are lost. In addition, grain boundaries form potential barriers against the flow of carriers, causing electrical energy to be lost as Joule heat.

　However, it is not necessarily the case that all grain boundaries contribute equally to these loss phenomena. To improve the conversion efficiency of polysilicon solar cells, examination of the relationship between the electronic properties of grain boundaries and their character and structure is necessary to clarify the physical origin of recombination losses at grain boundaries. The results of these investigations should indicate the way in which microstructural design and control could be used to increase conversion efficiency. Towards this aim, our group is researching the electrical characteristics of grain boundaries in polysilicon and the II-VI compound CdTe.

Microstructural control of metallic materials using magnetic fields

Fig.2: microtexture control by magnetic crystallisation in Fe-Si-B: (a) no applied magnetic field, (b)applied field（H=6T）(H. Fujii et al., Phil. Mag. Lett. 86 (2006) pp.113-122.)
Fig.2: microtexture control by magnetic crystallisation in Fe-Si-B: (a) no applied magnetic field, (b)applied field（H=6T）(H. Fujii et al., Phil. Mag. Lett. 86 (2006) pp.113-122.)

　Recently, many studies have been carried out on the effect of applied magnetic fields on metallic materials, and research on electromagnetic processing of materials (EPM) is continuing both in Japan and elsewhere. In our group, we are carrying out basic research on the influence of magnetic fields on diffusion in the solid state and on the physical phenomena controlling grain boundary segregation, grain boundary energy and the relationship between magnetic properties and phase stability.

Study of microstructural dynamics using in-situ SEM/EBSD

Fig. 3: In-situ SEM-EBSD image of late-stage abnormal grain growth in nanocrystalline nickel
Fig. 3: In-situ SEM-EBSD image of late-stage abnormal grain growth in nanocrystalline nickel

　Since the first commercial appearance in 1994, the automated Scanning Electron Microscope - Electron Backscatter Diffraction (SEM-EBSD) technique been widely adopted in both industry and academia, and now forms an important part of the microstructural analysis toolkit. Thanks to advances in CCD camera and image processing technology, the analysis speeds of the latest EBSD systems are as much as 100-300 greater than those of the original systems from 1994. The time taken by today's EBSD systems to analyse data from a single point is only about 0.01 seconds, so a 10,000-point dataset taking over eight hours to acquire in the original models can be acquired in only three minutes today. This increase in speed has permitted the use of in-situ EBSD, in which microstructural evolutions are studied as they occur. In our laboratory, using a heating stage (max. temperature 1000K) made in-house, the dynamics of changes such as phase transformations and grain growth are being observed, with the aim of clarifying thus far unresolved questions in the field of materials microstructures.

Control of creep fracture in ferritic heat-resistant steels using grain boundary engineering

Fig. 4: EBSD image of a creep crack (in black) and surrounding microstructure. The differently coloured areas above and below the crack represent different prior austenite grains.
Fig. 4: EBSD image of a creep crack (in black) and surrounding microstructure. The differently coloured areas above and below the crack represent different prior austenite grains.

　9-12 wt.%Cr steel is one of the heat-resistant alloys developed for fossil-fired power plant. Under operating conditions, one of the possible mechanisms of failure is by creep cracking. Previous results indicate that prior austenite boundaries act as a preferential path for creep cracks, in particularly when the prior austenite grains are large. One possible approach to designing against creep cracking is by manipulating the grain boundary microstructure in the austenite phase before transformation to martensite. Our current research aims to develop a method of processing in the austenite phase stability region in order to increase the creep crack resistance of the product martensitic phase.