Sadahiro Tsurekawa

The control of brittleness and development of desirable mechanical properties in polycrystalline systems by grain boundary engineering

Grain boundaries can be effectively controlled to produce or enhance their beneficial effects and also to diminish or reduce their detrimental effects on bulk properties in polycrystalline materials. Particular attention has been paid to the control of intergranular brittleness which remains a serious problem of material processing and development. Recent studies are presented and discussed, which have been successfully performed to control intergranular brittleness of “intrinsically brittle” materials such as the refractory metal molybdenum and the ordered intermetallic alloy Ni3Al and to produce superplasticity in an Al–Li alloy, by grain boundary engineering through controlling a new microstructural factor termed the grain boundary character distribution (GBCD). The optimization of GBCD and the grain boundary connectivity has been found to be a key to produce desirable bulk mechanical properties in both structural and functional polycrystalline materials.

Electron-beam-induced current study of grain boundaries in multicrystalline silicon

The effects of grain boundary (GB) character and impurity contamination on the recombination activity of grain boundaries (GBs) in multicrystalline silicon (mc-Si) were systematically studied through an electron-beam-induced current (EBIC) technique. First, clean GBs of various characters were checked at 300 and 100K. The EBIC contrasts of these GBs were in the same range of 0%–2% at 300K and 2%–4% at 100K, suggesting that the recombination activity of clean GBs is weak and the GB character has no significant effect on it. Second, the effect of impurities was studied by comparing the EBIC contrasts of the same type of the GBs in mc-Si with different Fe contamination levels. The recombination activity of GBs became stronger as the contamination level rose. The variation in the recombination activity related to the GB character was also observed in these specimens. The random or high-Σ GBs showed a stronger EBIC contrast than the low-Σ GBs. Moreover, we found that the EBIC contrast was not uniform along one...

Piezoelectric properties of BaTiO3 ceramics with high performance fabricated by microwave sintering

Hydrothermally synthesized BaTiO3 powders with nanoscale-sized particles were densified by microwave sintering. A sintered sample of the nanopowder fabricated by hydrothermal synthesis has a high piezoelectric constant d33 due to fabrication by microwave sintering. The maximum value of the piezoelectric constant d33 of a specimen fabricated by microwave sintering was approximately 350 pC/N for a small grain size of 2.1 µm. Detailed microstructures of the samples were observed by transmission electron microscopy (TEM) and scanning electron microscopy/electron backscattered diffraction analysis/orientation imaging microscopy (SEM/EBSD/OIM). The size of ferroelectric domains in the samples showing superior piezoelectric properties was less than 50 nm. SEM/EBSD/OIM observations revealed that the fraction of random boundaries was higher by approximately 10% in microwave sintered samples than in conventionally sintered ones. It is suggested that the small size of domain and the higher fraction of random boundaries might be responsible for the excellent piezoelectric properties of small grains, which can partially be attributed to domain size.

Lead-free barium titanate ceramics with large piezoelectric constant fabricated by microwave sintering

A lead-free barium titanate (BaTiO3) ceramics with a high density and a large piezoelectric constant, d33, as manufactured at 1320°C by microwave sintering, using a pure fine powder with a particle size of 100 nm produced by hydrothermal synthesis. The density of the ceramic with a 3.4 µm grain size was more than 98.3% of the theoretical value. The ceramic after poling had a dielectric constant of e33T/e0 =4200, an electromechanical coupling factor planar mode of kp =36% and d33 =350 pC/N. The value of d33 is the largest one ever reported for lead-free BaTiO3 ceramics.

Grain boundary structure, energy and strength in molybdenum

Abstract Molybdenum has many attractive properties for high-temperature structural applications. However, its usefulness as a structural material is impaired by brittleness at and below ambient temperature, resulting from its intrinsically weak grain boundary strength. According to our previous study, the fracture strength in molybdenum depends markedly on the grain boundary character and, in particular, on the orientation relationship between the two adjacent crystals and the orientation of the grain boundary plane. However, our present knowledge is still far from a complete understanding. In this study, the relationship between the strength, grain boundary structure and grain boundary energy has been investigated for purified biccrystals of molybdenum with various symmetric tilt boundaries, by means of transmission electron microscopy and optical interferometry. The main results obtained are as follows. (1) There is a relatively good correlation between the fracture strength and the grain boundary energy. The energy cusps, for instance, are observed for (112) and (111) Σ3 coincidence boundaries, which are high in fracture strength, while the energy is higher for near (114) and (122) Σ9 boundaries, which are low in strength. (2) Σ1 small angle and near (112) Σ3 coincidence boundaries have a good coherence, which agrees well with the result of the boundary energy measurement. Grain boundary dislocations are observed and they can be described by the boundary dislocation model. (3) On the (114) Σ9 coincidence boundary with low fracture strength, the structure is poor in coherence compared with Σ1 and Σ3 boundaries. However, the grain boundary structure can also be described by the grain boundary dislocation (displacement shift complete (DSC) dislocation) model. (4) It is considered that the grain boundary structure in molybdenum with a high covalency in bonding is not greatly different from that in normal metals.

Relationship between Electrical Activity and Grain Boundary Structural Configuration in Polycrystalline Silicon

Temperature dependent electron beam induced current (EBIC) technique has been applied to investigate the electrical activities of grain boundaries (GBs) in polycrystalline silicon. The GB character, misorientation and orientation of GB plane, were analyzed using a FE-SEM/EBSP/OIM system prior to the EBIC measurements. The EBIC contrasts were found to depend on GB character; low ΣGBs showed weak contrasts compared with general GBs at any temperatures, and also demonstrated to vary at GB irregularities such as boundary steps. These results indicate that electrical properties depend on the orientation of the GB plane as well as the misorientation. On the other hand, there existed less differences in temperature dependence of EBIC contrast irrespective of GB characters. The EBIC contrast decreased with increasing temperature, showed a minimum around 250 K, then increased again with further increasing temperature. The resulting temperature dependence of EBIC contrast probably comes from the combination of two types of recombination processes of carriers. One is related to a shallow level associated with an inherent GB structure, though the exact energy levels also would probably depend on GB structures, and the other to a deep level associated with impurities segregated at GBs, which acts as recombination center.

Enhancement of homogeneity of grain boundary microstructure by magnetic annealing of electrodeposited nanocrystalline nickel

Abstract The effect of a magnetic field on grain growth in nanocrystalline nickel produced by electrodeposition was investigated. Magnetic annealing was carried out at 573 K for 120 s to 1.8 ks under a direct current magnetic field of 1.2 MA/m (15 kOe). A magnetic field enhanced grain growth at the early stage of annealing was observed, which produced a homogeneous grain boundary microstructure.

Domain Properties of High-Performance Barium Titanate Ceramics

It has been known that barium titanate (BaTiO3) causes abnormal grain growth during conventional sintering utilizing resistance heating. The grain growth of barium titanate induced using by a hydrothermal synthesis method is controlled using microwave sintering having effects on internal overheating and rapid sintering and relationships between domain size and the piezoelectric constant d33 are studied. The domain size of a sample having a grain size of 2.5 µm was 76 nm and the d33 was 350 pC/N; however, the domain size of a 10 µm sample was 156 nm and the d33 was 300 pC/N. Furthermore, with the sample having a higher d33, the continuity of the domain structure across grain boundaries was confirmed.

Considerations for BaTiO3 Ceramics with High Piezoelectric Properties Fabricated by Microwave Sintering Method

A decrease in the domain size of barium titanate (BaTiO3) improves the piezoelectric constant (d33). Other considerations suggest the high d33 of BaTiO3 may be caused by the enhanced high dielectric constant (e33T/e0) at room temperature due to the size effect. However, we have clarified that even a sample with a low e33T/e0 shows high d33 according to the optimization of microwave sintering conditions. We have verified that the d33 of BaTiO3 ceramics fabricated by microwave sintering is as high as 370 pC/N despite the e33T/e0 of 2100. Mechanisms affecting d33 in microwave sintering and in conventional sintering of BaTiO3 are thought to be different. The relationship between the grain size, the mechanical quality factor Qm, and the frequency constant Np, which was obtained from microwave sintering, has assumed that a decrease in grain size causes a decrease in internal stress. We suggest that a decrease in internal stress induces continuity of strain in a grain boundary, resulting in continuity of domains across a grain boundary.

Measurements of potential barrier height of grain boundaries in polycrystalline silicon by Kelvin probe force microscopy

In recent years, the importance of polycrystalline silicon has been recognized in electronic device technology, although grain boundaries present in the material often exert a detrimental influence on the electrical properties because of the potential barriers associated with them. However, it is not true that all grain boundaries have similar properties, since they have their own character depending on the orientation relationship between two adjoining grains. We report here the first determination of the potential barrier height for well-characterized grain boundaries in polycrystalline silicon, using Kelvin probe force microscopy. The observed barrier height of the grain boundaries was found to vary in the range 10 to 100 meV depending on the grain boundary character. The most important finding is that the potential barrier height is approximately twice as high at random boundaries as at low-energy coincidence boundaries.