Kohei Morishita

Thermally induced residual strain accumulation in Bi2223/Ag/Ag alloy composite superconductor

A method to estimate the thermally induced residual strain accumulation under varying temperature in a Bi2223/Ag/Ag alloy composite superconductor was presented, in which the mechanical property values measured from the stress?strain curves of the samples with different residual strain states, the residual strain value of Bi2223 filaments in the composite tape at room temperature measured by x-ray diffraction and the reported coefficients of thermal expansion of the constituents (Bi2223, Ag and Ag alloy) in the relevant temperature range were incorporated. This method was applied to estimate the change of the residual strain of all constituents of the high critical current type composite tape fabricated by American Superconductor Corporation as a function of temperature. The residual strain value at 77?K estimated by this method and the reported fracture strain of Bi2223 filaments accounted well for the measured strain tolerance of the critical current at 77?K.

Shape and quality of Si single bulk crystals grown inside Si melts using the noncontact crucible method

The noncontact crucible method enables production of Si bulk single crystals without crucible contact by intentionally establishing a distinct low-temperature region in the Si melt. In this contribution, we correlate crystal growth conditions to crystal material properties. The shape of the growing interface was generally convex in the growth direction. The quality of the Si ingots was determined by the spatial distributions of dislocations, resistivity, oxygen concentration, and minority-carrier lifetime. In an ingot with a convex bottom, swirl patterns with higher resistivity are present in the top, middle, and bottom of the ingot. The dislocation density decreased from the top (first to solidify) to the bottom of the ingot because dislocations in the ingot moved to the periphery from the center of the ingot during crystal growth owing to the convex growing interface. The oxygen concentration was concentrically distributed on the seed axis owing to the convex growing interface. The lifetime was as high as 1.8 ms after phosphorus diffusion gettering (PDG) and 205 µs before PDG at an injection level of 1 × 1015 cm−3. The lifetime was not strongly affected by the dislocation density, which was as low as 102–103 cm−2.

Large-aperture focusing of x rays with micropore optics using dry etching of silicon wafers

Large-aperture focusing of Al K(α) 1.49 keV x-ray photons using micropore optics made from a dry-etched 4 in. (100 mm) silicon wafer is demonstrated. Sidewalls of the micropores are smoothed with high-temperature annealing to work as x-ray mirrors. The wafer is bent to a spherical shape to collect parallel x rays into a focus. Our result supports that this new type of optics allows for the manufacturing of ultralight-weight and high-performance x-ray imaging optics with large apertures at low cost.

Growth of square Si single bulk crystals with large side-face widths using noncontact crucible method

The noncontact crucible method was used to prepare square Si single bulk crystals. The size of the square part of the ingots was determined by the side-face width of the four-cornered pattern that appeared on the top surface. We obtained square Si single crystals with sizes of 9.4 × 9.7 and 10.9 × 11.0 cm2 that had no fan-shaped {110} faces and had diagonal lengths of up to 91% of the crucible diameter. To obtain large square Si single bulk crystals with a large side-face width using the present method, the importance of establishing a larger low-temperature region in the Si melt while maintaining a smaller initial temperature reduction was considered.

Optical Image Analysis of the Novel Ultra-Lightweight and High-Resolution MEMS X-Ray Optics

We have been developing novel microelectromechanical systems X-ray optics for future satellites. It can be ultra-lightweight and of high-resolution. For the first time, we fabricated a spherical test optics made of silicon. We used the dry etching and hot plastic deformation method. We conducted imaging tests to examine whether it can focus a parallel beam of light. Visible light was selected instead of X-rays because of the convenience of testing. The focusing was confirmed with a full-width at half-maximum focal size of 2 arcmin. Since the focus is affected by optical diffraction, a smaller focus can be expected in future X-ray imaging tests.

A Monte Carlo–shear lag simulation of tensile fracture behaviour of Bi2223 filament

The damage evolution in Bi2223 filaments and its influence on critical current was described by a Monte Carlo–shear lag simulation method. The experimentally observed zigzag crack propagation across aligned Bi2223 grains under tensile strain was effectively modelled by including transverse and longitudinal failure modes for individual grains. From the simulated stress–strain curve, the survival parameter (slope of the stress–strain curve normalized with respect to the original Youngs modulus) was estimated with increasing applied strain. With this parameter combined with the strain sensitivity of the critical current, the measured change of critical current of the composite tape with applied strain could be described well.

Novel ultra-lightweight and high-resolution MEMS x-ray optics

We have been developing ultra light-weight X-ray optics using MEMS (Micro Electro Mechanical Systems) technologies.We utilized crystal planes after anisotropic wet etching of silicon (110) wafers as X-ray mirrors and succeeded in X-ray reflection and imaging. Since we can etch tiny pores in thin wafers, this type of optics can be the lightest X-ray telescope. However, because the crystal planes are alinged in certain directions, we must approximate ideal optical surfaces with flat planes, which limits angular resolution of the optics on the order of arcmin. In order to overcome this issue, we propose novel X-ray optics based on a combination of five recently developed MEMS technologies, namely silicon dry etching, X-ray LIGA, silicon hydrogen anneal, magnetic fluid assisted polishing and hot plastic deformation of silicon. In this paper, we describe this new method and report on our development of X-ray mirrors fabricated by these technologies and X-ray reflection experiments of two types of MEMS X-ray mirrors made of silicon and nickel. For the first time, X-ray reflections on these mirrors were detected in the angular response measurements. Compared to model calculations, surface roughness of the silicon and nickel mirrors were estimated to be 5 nm and 3 nm, respectively.

Generation mechanism of dislocations and their clusters in multicrystalline silicon during two-dimensional growth

The generation mechanism of dislocations and their clusters during the two-dimensional growth of multicrystalline Si was studied by in situ observation of a growing interface and subsequent analysis of dislocations. Dislocations were frequently generated at impingement points of the growth of crystal grains where Si melt was enclosed by crystal grains when it solidified. The generation of dislocations was accompanied by the formation of a new twin boundary. On the other hand, no dislocations were observed at impingement points of the growth of crystal grains where Si melt was open when it solidified. We herein present a scheme for dislocation generation with the formation of a new twin boundary on the basis of the results of our former study on dislocation generation in the unidirectional growth of multicrystalline Si ingots.

Computational Investigation of Relationship between Shear Stress and Multicrystalline Structure in Silicon

We performed computational calculations by three-dimensional finite element analysis to investigate the relationship between the shear stress on slip planes and multicrystalline structural properties in Si, such as the grain orientation and the structure of the grain boundary. In our calculations, the change in multicrystalline structure is defined as the change in anisotropic elastic coefficient, which depends on the grain orientation. As a result, it becomes clear that the shear stress on slip planes depends on the grain orientation and concentrates near the grain boundary. Calculations in various multicrystalline structures reveal that controlling the growth direction in or has a great advantage for decreasing the shear stress on slip planes and resulting in the suppression of dislocation occurrence.

Identification of lifetime limiting defects by temperature- and injection-dependent photoluminescence imaging

Identification of the lifetime limiting defects in silicon plays a key role in systematically optimizing the efficiency potential of material for solar cells. We present a technique based on temperature and injection dependent photoluminescence imaging to determine the energy levels and capture cross section ratios of Shockley–Read–Hall defects. This allows us to identify homogeneously and inhomogeneously distributed defects limiting the charge carrier lifetime in any silicon wafer. The technique is demonstrated on an n-type wafer grown with the non-contact crucible (NOC) method and an industrial Czochralski (Cz) wafer prone to defect formation during high temperature processing. We find that the energy levels for the circular distributed defects in the Cz wafer are in good agreement with literature data for homogeneously grown oxide precipitates. In contrast, the circular distributed defects found in NOC Si have significantly deeper trap levels, despite their similar appearance.