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Biomedical subjects

N Shiokawa

Publications and source records attributed to N Shiokawa.

At least 19 recordsLinked to original sources
PubMed1993

Skin holes of titanium casting.

The present study describes refractories which contribute to formation of skin holes of Ti casting. Plates consisting of quartz (Qu) and cristobalite (Cr) powders and Qu- and Cr-MgO mixtures were fired at high temperatures and used as phosphate-free molds. The cross-section of the castings was investigated using an optical microscope and an electron probe microanalyzer. Whereas the castings into the Qu molds were almost complete, those into the Cr molds were incomplete. The surface reaction zone of the Qu molds formed a typical layered structure with fewer skin holes. That of the latter extended deeply into the interior with a net-shaped Ti-Si phase, cracks, and holes. Addition of MgO to Cr effectively improved the castability and changed the reaction zone to the typical layered structure. However, slender holes occurred along the mold wall. The overall area around the holes was contaminated by Si and Mg. Probably, this resulted from a reaction of forsterite with Ti. The reactivity difference between Qu and Cr was not related to the fired strength of mold.

Dental Casting Investment↗
PubMed1991

Molten titanium flow in a mesh cavity by the flow visualization technique.

The tracer element molten (TEM) method, which provides titanium flow visualization in a mold cavity, was applied to investigate the flow in a mesh pattern. When a pressure casting machine was used, most of the mesh rods were composed of many solidification layers, which implied that the flow was laminar and almost steady. When a centrifugal casting machine was used, the flow was almost irregular and a stack of solidification layers was observed in only a few segments. These results indicate that the flow was turbulent. In a complicated cavity, such as a mesh pattern, pressure rather than centrifugal force acts as an effective casting force. Therefore, in such a cavity, favorable castability can be achieved with high pressure and laminar flow.

Dental Casting Technique↗
PubMed1990

[Grinding of titanium. 1. Commercial and experimental wheels made of silicon carbide abrasives].

Cast titanium was ground with commercial and experimental wheels made of silicon carbide abrasives, and their grinding performance was investigated. With the vitrified wheels made of the GC abrasive, at a higher the wheel circumferential speed and heavier the grinding pressure, the cutting rate was greater, accompanied by violent wear of the wheel. Being independent of the wheel speed, the grinding ratio reached about 1 under pressure heavier than 100 gf. The MgO-MgCl2-bonded wheels of the C abrasive exhibited a similar tendency. The manner in which the wheel was moved over the work during grinding proved to be very important, compared with the Ni-Cr alloy as reported previously. Only depression of the wheel against the work resulted in chemical attrition of the abrasive and discoloration of the work surface, or grinding burn, due to oxidation of titanium. Even when the wheel was moved over the work, chip-formation process of the cutting edge was far from ideal, and the work surface was contaminated due to reaction of titanium with the abrasive. At a higher wheel circumferential speed, more chips were loaded or built-up in the wheel and strongly rubbed the work surface, resulting in violent wear of the wheel; loading and dislodging of such chips were repeated.

Carbon↗
PubMed1990

[Grinding of titanium. 2. Commercial vitrified wheels made of alumina abrasives].

Cast titanium was ground with commercial vitrified wheels made of alumina abrasives, and their grinding performance was investigated. For cutting, the appropriate circumferential speed of the alumina wheels was about 700 m/min. A speed lower or higher than this yielded unfavorable grinding results, which were attributed to wheel loading or chemical attrition of the abrasive, respectively. The hard wheel made of the A abrasive was suitable for grinding of titanium, and moreover, the wheel of the WA abrasive was more suitable than that made of the A abrasive. Generally, the cutting rate of the alumina wheels was inferior to that of the silicon carbide ones investigated previously. Depression of the wheel against the work yielded unfavorable grinding results; the manner in which the wheel was moved over the work during grinding was very important, compared with the silicon carbide wheels. Although the wheel was moved over the work, the high circumferential speed of the wheel resulted in chemical attrition of the abrasive and discoloration of the work surface, or grinding burn. The grinding burn layer mainly consisted of a few microns-thick titanium oxide.

Aluminum Oxide↗
PubMed1990

[Determination of titanium flow by the tracer element molten method in the dental precision casting. 1. The principle of flow visualization and application to simple castings].

The tracer element molten (TEM) method has been developed for flow visualization of molten Titanium in dental precision casting. The principle of this technique is as follows. When Titanium is cast, the tracer element wire inserted previously into the selective point of the sprue is molten little by little and distributed according to the molten Titanium flow in the mold cavity. After solidification, to observe the flow pattern, the tracer element needs to be analyzed on a section of the casting by EPMA equipped with the stage scan mapping system. This technique using Ag, Au, Pd or Pt as a tracer has been applied to some simple castings in shape and has been confirmed to be a very powerful technique for Titanium flow visualization in dental castings.

Dental Casting Technique↗
PubMed1990

[Reactions at the nonprecious metal-ceramic interface during porcelain firing. (3) A commercial alloy forming much Cr oxide during firing].

To examine the interface reaction between porcelain and a commercial Ni-Cr dental alloy (containing higher Cr and Mo) during porcelain firing, electron probe microanalyzer (EPMA) and X-ray diffraction have been used to characterize the interface which has emerged by means of selective dissolution of the alloy caused by a bromine-methanol solution. Much Cr oxide was formed at the early stage of firing, and remained in the reaction layer even if the holding time at 960 degrees C was up to 8 minutes. As a result, many Cr atoms diffused into the porcelain layer, most of which was found to form oxides through X-ray diffraction. Moreover, the porcelain layer nearest to the oxide layer was deduced to be modified seriously.

Chromium↗
PubMed1989

Layered structure of cast titanium surface.

The present study concerns the surface layered structure of the cast Ti. A commercial Ti was cast into a mold which was made of a phosphate-bonded Al2O3/SiO2 investment. Elemental analyses of the interfacial zone of the casting were made under an electron probe micro-analyzer. The interfacial zone was composed of four layers: the outermost reaction or casting burn layer, the second layer of an O-and Al-stabilized alpha case, the third layer in which Si, P, O, and C were inhomogeneously concentrated, and the fourth layer which consisted of acicular or plate-like crystals. It was observed that the larger the cast volume and the higher the mold temperature, the thicker became each layer and the coarser became the acicular grains. Probably, the layered structure was formed through decomposition of reducible species in the burnout investment and diffusion of the resulting elements into the casting.

Dental Casting Investment↗
PubMed1989

[Study on grinding of base metal alloys. 4. Constant pressure grinding of a Ni-Cr alloy with vitrified wheels].

The grinding techniques and the constituent element of vitrified wheels suitable for a 13% Cr-Ni dental casting alloy were determined. The lever-type grinding test machine used in the previous study was modified so that a work might be ground under a constant pressure as it moved reciprocally within a short stroke along the tangential direction of a rotating wheel. The grinding performance of two marketed wheels and eleven experimental wheels with different constituent elements was tested. Abrasive grains on the working surface of alumina wheel wore extremely due to abrasive attrition. Carborundum wheels proved to be more suitable for grinding of the comparatively soft Ni-Cr alloy. Not only depressing a wheel against a work but also moving the wheel over it with heavier pressure should be desired for the maximal grinding efficiency. The experimental carborundum wheels exhibited much the same performance as the marketed carborundum wheel under a less grinding pressure that 100 gf. Only the wheel of grain size #150 bonded with 19% binder wore obviously under the pressure of 150 or 200 gf and provided about two times the performance of the marketed wheel.

Aluminum Oxide↗
PubMed1989

[Study on grinding of base metal alloys. 5. Constant pressure grinding of a Ni-Cr alloy with electro-deposited wheels].

The grinding techniques and electro-deposited wheels suitable for a 13% Cr-Ni dental casting alloy were determined. The lever-type grinding test machine modified in the previous study was used to investigate the grinding performance of experimental wheels of CBN- and diamond-particles. Depression of the diamond wheel against the work yielded unfavorable grinding results. Not only depression of the wheel against it but also moving the wheel over it with a heavier pressure is desired for higher grinding efficiency and its durability. Probably, the undurability of this wheel is associated with abrasive attrition due to oxidation or some chemical reaction with the work. The CBN wheel had not been suitable for the Co-Cr alloy tested in the third paper, but it exhibited very excellent performance for grinding of the Ni-Cr alloy, even without being moved over the work. Although being high in cost, the CBN wheel may pay for the very high performance and its durability.

Chromium Alloys↗
PubMed1989

[Reactions at the nonprecious metal-ceramic interface during porcelain firing (2). A commercial alloy containing higher Cr and Si].

To clarify the interface reaction between porcelain and a commercial Ni-Cr dental alloy (containing higher Cr and Si) during porcelain firing, electron probe microanalyzer (EPMA), X-ray diffraction and X-ray fluorescence analysis have been used to characterize the interface which has emerged by means of selective dissolution of the alloy caused by a bromine-methanol solution. At the early stage of firing, tin oxides (SnO2), contained in the porcelain powder, were reduced to Sn metal and the resultant reduction layer was about 10 microns in thickness from the interface. Although in the beginning of firing some Cr oxides were formed at the interface, with increasing retention time (within a few minutes) at 960 degrees C, extremely low oxygen partial pressure caused reduction of even such products.

Air Pressure↗
PubMed1985

A new method for finite element simulation of orthodontic appliance-teeth-periodontium-alveolus system.

This paper describes a new simulation method to analyze the initial behavior of the total system comprising orthodontic appliance, teeth, and their supporting structures. It is based on a finite element method which additionally takes account of a rotational degree of freedom. Beam and rod elements are used for finite element idealization of orthodontic appliance. Through spring elements it is connected with the teeth supported by the alveolar structures. The technique of 'initial strain' is introduced so as to analyze the effects of a gable bend and activation on the force system which is delivered by the orthodontic appliance. As compared with the photoelastic technique hitherto used, this method serves to investigate systematically and quantitatively the initial aspect of orthodontic tooth movement.

Alveolar Process↗
PubMed1983

Structure of high-temperature oxidation zones of gold alloys for metal-porcelain bonding containing small amounts of In and Sn.

Changes in the morphology of the oxidation zone and the quantities of oxide for gold alloys containing two base metals (In and Sn) were investigated by exposing alloys at 1000 degrees C for one h in air. The contents of In and Sn were varied in the range of from 0-1.5 wt%. In the range of from 1.5-0.35 wt% In (from 0-1.15 wt% Sn), In 2O3 was formed predominantly at the alloy surface. The electron probe x-ray micro-analyzer detected no uniform external oxidation zone. A mixture composed of In2O3 and SnO2 precipitated internally in the alloy matrix. An external oxidation zone composed of SnO2 was formed on the alloys containing over 1.2 wt% Sn, and no internal oxidation zone was observed.

Crystallography↗
PubMed1982

Effects of phase transformations of silicas and calcium sulfates on the compressive strength of gypsum-bonded investments at high temperatures.

The effects of transformations of silicas and calcium sulfates on high temperature compressive strength were investigated in commercial and experimental investment materials containing fused quartz as silica. The strength is only slightly affected by the alpha leads to beta transformation of cristobalite and not by the quartz alpha leads to beta transformation. The state of the calcium sulfates is the main factor influencing variations in the strength at a particular temperature.

Calcium Sulfate↗
PubMed1982

The structure of oxide formed by high-temperature oxidation of commercial gold alloys for porcelain-metal bonding.

For gold alloys containing only In as a base metal, an external In2O3 layer forms uniformly on the alloy surface. However, when the alloy contains Sn and In, no external oxide layer can be detected by electron probe micro-analysis, and oxide particles composed of In2O3 and SnO2 precipitate in the alloy. For alloys containing Ni, in addition to In and Sn, the external oxide is composed of NiO; there is little development of internal oxide. For alloys containing Fe and Sn, an oxide layer of only Fe2O3 forms on the alloy surface, and the internal oxidation zone shows a band-like structure containing SnO2 and a small amount of Fe2O3.

Chemical Phenomena↗