Papers by Author: Shigeo Asai

Abstract: A static magnetic field and an alternating current are imposed on a metallic alloy during solidification for a crystal alignment of the primary phase. A Sn-10%Pb is selected as a sample because its primary phase is expected to have an anisotropic nature in magnetic susceptibility. In the x-ray diffraction pattern of the sample solidified without the magnetic field, the first and second highest peaks are (101) and (211) planes. On the other hand, those solidified with the magnetic field are (200) and (220) planes which are magnetically preferred planes. That is, the primary phase crystals in the sample solidified with the magnetic field are aligned to the specific direction.

Abstract: Free surface motion of a liquid metal submerged in an alternating magnetic field has been examined. A copper vessel filled with a liquid gallium is set in a coil for the imposition of the alternating magnetic field. The alternating magnetic field penetrates into a liquid gallium only from an upper free surface because thickness of the copper vessel is larger than the electromagnetic skin layer of copper. Time variation of displacement of the standing wave loop excited on the free surface is detected by a laser level sensor. The standing wave was suppressed not only by intensification of the magnetic field magnitude but also increase of the magnetic field frequency.

Abstract: A high magnetic field is a useful tool to control the crystal alignment of ceramic materials. In this study, a horizontal 10T static magnetic field was imposed on slurry containing hydroxyapatite (HAp) crystals under the horizontal mold rotation during slip casting process so as to introduce uni-axial alignment for some amount of crystals in the sample, and then it was sintered in atmosphere without the magnetic field. From X-ray diffraction, it has been found that the HAp crystals in the sample treated with the mold rotation under the magnetic field were aligned its c-axis to a particular direction.

Abstract: Formation of crystallographically orientaed hydroxyapatite (HAp) is one of the promising ways to utilize their anisotropic nature of chemical and biological properties. On the other hand, the development of super conducting magnet technology enables to introduce a high magnetic field which can control crystal orientation of non-magnetic materials with magnetic anisotropy. In this study, a high magnetic field and sample rotation are simultaneously imposed on the hydroxyapatite during a slip casting process in order to align its c-plane within a horizontal plane. From X-ray diffraction, it has been found that the HAp crystals in the sample treated with the magnetic field and the sample rotation were oriented to a particular direction in the slip casting process and it was enhanced by the subsequent sintering process, while the c-axis crystal orientation of the sample treated without the magnetic field and with the sample rotation was not observed before and after the sintering.

Abstract: We have reported that CaSiO3 ceramics show very fast bone-like apatite formation in simulated body fluid (SBF). However, CaSiO3 ceramics have disadvantages in their mechanical properties and shapability. It is therefore more effective to develop composites of CaSiO3 particles dispersed in a matrix of polymer or metal. Such composites are usually prepared by homogeneously blending the ceramic powder with the matrix component. This method is, however, ineffective for the preparation of biocompatible polymers or metals because only the surfaces containing accidentally-exposed ceramic particles play a role in generating apatite in SBF. It is therefore necessary to add a large volume of ceramic powder and also to abrade the surface to expose more of the ceramic particles. In this study, CaSiO3/high-density polyethylene (HDPE) composites were prepared by hot-pressing to introduce surface CaSiO3 particles and their biocompatibilities were evaluated under in vitro conditions using SBF. CaSiO3 powders were spread on a HDPE plate and hot-pressed at 140oC and 4.9-14.7 MPa for 2 min. The composite sample (about 10×10×1 mm3 in size) was immersed in 30ml SBF (sample/solution ratio of 2.5 g/l) at 36.5oC. After 14 days soaking, the apatite product particles covered most of the composite surface and formed apatite layers. Bone-like apatite particles were formed only on the surface regions containing exposed CaSiO3 particles but no apatite was formed on the CaSiO3 particles buried in the HDPE matrix. The results show that this surface deposition method is very effective in developing biocompatibility in the composites using very small amounts of CaSiO3 powder (about <1 %v) compared with results reported for hydroxyapatite and AW glass-ceramic powders (requiring about 40 %v). It is also found that the inhomogeneous state of the CaSiO3 particles in the surface of the present composites strongly influences their biocompatibility. It will be necessary to improve the homogeneity of CaSiO3 dispersion in the surface of the composites to achieve a more uniform surface apatite layer.

Abstract: A high magnetic field is a useful tool to control the crystal alignment of non-magnetic materials such as ceramics and polymers. In the case of Hydroxyapatite crystal, the a,b-axis is aligned parallel to the direction of an imposed magnetic field. This fact implies that the alignment of the c-axis is not controllable only using a high static magnetic field due to the freedom of the c-axis in a plane perpendicular to a magnetic field direction. In this study, a high static magnetic field and mold rotation was simultaneously so applied during a slip casting process as to align the c-axis of HAp poly crystals.

Abstract: A new technology relating to crystal orientation and structure alignment has emerged by the development of superconducting technologies. Now, a high magnetic field covering a rather large space is available even in small-scale laboratories. Under this circumstance it has been found that the crystal orientation in materials can be controlled by imposition of the high magnetic field. This principle due to a magnetization force can be applied not only to magnetic materials but also to non-magnetic materials with asymmetric unit cells. In this paper, three novel processes for the crystal orientation of ceramics and metals are described.

Abstract: The controlled development of texture microstructure in ceramics is one effective way to improve their properties, such as electrical, mechanical properties and biocompatibility. A bioceramics with oriented crystal structure has attracted great interest. In bone reparations, Hydroxyapatite (HAp)-based biomaterials were frequently used. And HAp is the main mineral constituent of the hard tissue of human bodies, which occurs with a hexagonal crystal. A HAp crystal turns out to have different surface properties in a- (or b-) plane and c-plane. In this regard, to get highly oriented HAp is very important before using HAp as a biomaterial. And the crystal orientated HAp is useful not only as biomaterials but also as protein absorbents. In this research, two different kinds of HAp-based biomaterial with oriented structure (HAp bioceramics and HAp-coated titanium composite) were studied.