The mass transfer phenomenon of contained impurities causes differences in the morphologies, densification processes, and heat resistance of ceramics. Of these, in this paper, differences in the heat resistance of ceramic fibers are discussed. Third-generation SiC polycrystalline fibers demonstrated excellent heat resistance. However, at temperatures above 1800 °C, sintered fiber (Tyranno SA) and non-sintered fiber (Hi-Nicalon Type S) showed remarkable differences in heat resistance. At temperatures above 1800 °C, the non-sintered fiber underwent structural changes, including the formation of a surface carbon layer and abnormal SiC grain growth, whereas the sintered fiber maintained its stable polycrystalline structure. Until now, these differences and a detailed description of them have not been discussed. Here, we first explain the dramatic differences in heat resistance that occurred at high temperatures in relation to the mass transfer of excess carbon. Our findings should be widely used for the development of much more stable structures and for the long-term use of materials at higher temperatures in applications such as airplane engines and turbines.

This paper provides an innovative controlling process of surface morphology. The contact area between a liquid and a solid is strongly affected by the critical surface tension (cft) of both materials. Using this phenomenon, a wide range of morphologies, from flat surface layers to sea-grape-like surface structures, were created. Due to the bleed-out phenomenon, low-molecular-mass additive (liquid) oozed out of a precursor polymer (solid), leading to different surface morphologies depending on the cft values of the liquid (Mcft) and solid (Pcft). When Mcft < < Pcft, a flat surface layer was obtained; however, in the case of Pcft < < Mcft, the formation of a sea-grape-like surface layer was created. Tetra-butoxy-titanium and polydimethylsiloxane were used as low-molecular-mass additive and precursor polymer, respectively. After coating on titanium metal and calcination, sea-grape-like materials composed of titanium oxide and silicon oxide were obtained. Furthermore, unique characteristics (bioactivity, photocatalytic activity, and prevention of atomic diffusion) were observed. A remarkable increase in the wettability, bioactivity, and catalytic activity of materials was achieved using our simple process to create unique surface morphologies. Our proposed process is applicable to a wide range of materials and morphologies, and can be used in catalysts, biomaterials, and environmental barrier coatings.

Because of the formation of a surface passive film (rutile TiO2) on its surface layer, titanium metal shows adequate corrosion resistance. As the surface layer (passive film) of titanium metal is very stable, any functionalization of the titanium metal has been generally performed using relatively complicated methods. This is because any direct oxidation of titanium metal only leads to the formation of rutile TiO2 over the entire temperature range. Chemical reactions using titanium chemicals can easily produce anatase TiO2 at temperatures of <= 600 degrees C. Using precursors is one of the ways of producing an anatase TiO2 coating on titanium metal. However, in previous studies, anatase TiO2 layers easily peeled off when they were used in flowing water. Herein, we describe a simple process for obtaining an anatase TiO2 coating layer strongly bonded to the titanium metal surface. In our process, titanium metal was pretreated with a reducing agent to create a surface TiH2 layer, whose condensation reaction easily proceeds with a precursor (composed of oxalic acid and tetra-butoxy titanium). Subsequently, the treated titanium metal was calcinated at 550 degrees C in air to achieve strong bonding between the anatase TiO2 coating layer and titanium metal surface. The treated titanium metal exhibited excellent photocatalytic activity.

Hexagonal boron nitride (h-BN) particles exhibit high thermal conductivity and are promising fillers for resin filling. However, h-BN particles are plate-like particles with thermal anisotropy in the planar and thickness directions. Therefore, their applications are limited due to low thermal conductivity in the direction of the thickness of a resin sheet filled with h-BN particles. In this study, we control the size and thickness of h-BN particles using carbothermal reduction nitridation (CRN), which involves the carbothermic reduction of boric oxide in an N-2 gas atmosphere and develop them into resin sheets. In CRN using a CaO promoter, a novel method is developed to control the shape, size, and thickness of h-BN particles. Using h-BN particles grown in the thickness direction, we have successfully provided resin sheets with high thermal conductivity.

Silica-titania mixed oxides have excellent properties, such as a low thermal expansion coefficient and a refractive index that can be adjusted by changing the Ti content. However, when the Ti content increases, silica and titania phases in silica-titania mixed oxides can separate. This phase separation leads to the precipitation of the titania component as rutile or anatase crystals. When silica-titania mixed oxides undergo phase separation, their properties become unstable; for example, the refractive index of the particles becomes non-uniform. Therefore, it is preferable to synthesize silica-titania mixed oxides in an amorphous state without causing phase separation.Based on our previous studies on particle size control in silica synthesis, we employed a dry process using organosilicon compounds to synthesize silica-titania mixed oxides. In this study, spherical amorphous silica-titania mixed oxide particles were obtained via flame synthesis using organosilicon and organotitanium compounds.The purpose of this study was to characterize the obtained powder and explore the possibility of controlling particle size during synthesis. By studying the dry process synthesis of spherical silica-titania mixed oxide particles, we confirmed the relationships: between the Si/Ti molar ratio and the obtained crystal structure and between the adiabatic flame temperature and the particle size.

Highly purified and crystallized hexagonal boron nitride (h-BN) powder is suitable as thermally conductive filler in resins. To obtain h-BN powder with large particle size, as well as high purity and crystallinity, high-temperature heat treatment over 1800 degrees C in a N-2 gas atmosphere is effective. The carbothermal reduction nitridation (CRN) involves the carbothermic reduction of boric oxide in a N-2 gas atmosphere. In CRN using a CaO promoter, h-BN particles with high crystallinity can be obtained by a simple heat treatment process. CaO prevents the evaporation of boron oxide and aids in h-BN particle growth at high temperatures. However, CaB6 is formed as byproduct or impurity when CRN using the CaO promoter is performed at temperatures higher than 1800 degrees C. In this study, the relationship between the products and the reaction temperature was clarified via thermodynamic considerations and experimentation. The results clarified the ideal reaction process of CRN using a CaO promoter to obtain highly purified and crystallized h-BN powder.

Surface functionalization of titanium metal is very attractive for bio- and environmental applications. This is because titanium metal is very stable and has a good biocompatibility. In this case, surface roughness and crystalline structure are important factors for obtaining effective characteristics. Titanium metal is usually covered with a surface passive film of thermodynamically stable rutile-TiO2 that grows as the heat treatment temperature in air increases. On the other hand, to obtain an anatase-TiO2 surface layer on titanium metal, we must employ specific treatments such as our previous method, which uses a silica-coexisting heat-treatment process. In this paper, the relationship between the fine structure formed on the titanium metal and the surface hydrophilic property was clarified, and the potential for the bio-application was discussed. The formed anatase-TiO2 coexisting with silica exhibited improved biocompatibility with good apatite formation.

In this study, synthesis of yttrium aluminum garnet (YAG):Ce3+ phosphor powders for white light emitting diodes was investigated by mechanical method using the attrition-type mill with no external heating and no flux in dry phase. High mechanical energy input to the starting powder mixture of Y2O3, Al2O3 and CeO2 achieved the synthesis of YAG:Ce3+ without any flux materials. X-ray diffraction patterns of the processed powders after 5 min processing revealed the peaks of YAG were clearly identified. The maximum temperature of the mill chamber during the processing was 240celcius. The YAG phosphor obtained by the mechanical method revealed the internal quantum yield of 65% in the case of the sample mechanically processed under a reducing atmosphere. The synthesized powder showed granule structure consisting of submicron size of YAG particles, which is better handling for the fabrication of light emitting diode devices.

To synthesize silica particles, a dry process consisting of burning an organosilicon compound as raw monomer has been widely studied. Previously, we used six starting materials in the dry process and investigated the effects of monomer species on the particle shape and size of silica. Here, we fixed the raw monomer species and amount of gas supplied from the burner and focused on the relationship between the gas environment around the flame and silica particle size. Specifically, we focused on quaternary gas flowing around the flame as a factor that affected the temperature around the flame. Its flow rate and velocity were expected to directly affect the temperature around the flame. We established the relationship between the temperature environment around the flame, specifically the amount of heat absorbed by the hot water for cooling the reactor and silica particle sizes. The temperature environment around the flame was determined by the balance between the reactor's size and amount of quaternary gas. Simultaneously, it was emitted as the amount of heat absorbed per unit heat transfer area of hot water. Consequently, we concluded that the environment around the flame determined the size of spherical silica particles and successfully controlled the particle size.

We describe a general photoelectrical deposition process for the priority deposition of noble metals inside surface mesopores, which are selectively formed in the interstitial regions between crystal arrays composed of sintered photoactive nanocrystals. The basis of our approach is to prepare continuous coherent bonds between photoactive nanocrystals aligned toward the surface accompanied by the formation of interstices (2-20 nm) filled with a heterogeneous insulator phase between the crystal arrays; the subsequent removal of part of the heterogeneous phase creates surface mesopores. The coherent crystalline structure causes a smooth electron flow from the outside to the inside of the array through the coherent boundaries of the nanocrystals during photodeposition using an ultraviolet lamp, accompanied by outside-hole consumption and noble metal deposition in the recesses of the surface mesopores. The size and morphology of the deposited noble metals are restricted to the shape of the formed mesopores, which can be easily controlled. This approach is applicable to a wide range of functional materials and is expected to be applicable to the preparation of excellent photocatalysts, redox catalysts, and photochromic materials. Using our new process, we also achieved a remarkable increase in the photocatalytic activity of the obtained Pd-deposited TiO2/SiO2 photocatalytic fiber.

Hexagonal boron nitride (h-BN) powder is a promising thermally conductive filler in resins and controlling its particle size is important. To obtain h-BN powder with a large particle size as well as high purity and crystallinity, high-temperature heat treatment over 1800 oC in N2 gas atmosphere is effective. Carbothermal reduction nitridation (CRN) involves carbothermic reduction of boric oxide in N2 gas atmosphere. In CRN using a CaO promoter, h-BN particles with large sizes can be produced through a simple heat-treatment process. During CRN, CaO forms calcium borate (CaO-B2O3), which prevents the vaporization of B2O3 and promotes h-BN particle growth at high temperatures. Up to now, the effect of CaO-B2O3 on the growth of h-BN particles during CRN is unclear. In this study, the role of CaO-B2O3 in the growth of h-BN particles during CRN was clarified via experimentation using various approaches. The results led to the ideal reaction process of CRN using a CaO promoter to produce large-particle-size h-BN powder.

We clarified the controlling factors of the particle size of the amorphous silica synthesized by wet and dry processes. In the wet process using methyl-trimethoxy-silane as a starting monomer, the obtained particle size can be easily controlled by changing the reaction time appropriately. However, to obtain larger particles, a relatively long time is needed. After the condensation reaction was conducted for 50h, the silica particles (D50: 3?m) were synthesized by calcination at 550oC in air. To synthesize larger silica particles, we used silica-seed particles (8?m) to obtain very large spherical silica particles (D50: 20?m). Thus, although the wet process needs a relatively long reaction time, it is very useful for synthesizing spherical silica particles with a wide range of particle size. In the dry process, we used methyl-trimethoxy-silane (MTMS), tetra-ethoxy-silane (TEOS), and octamethyl-cyclotetrasiloxane (OMCTSO) as the starting materials. In this process, the size of the silica particles was dominated by the molecular structure of the monomer, in particular, the number of silicon atoms contained in the monomer and the bulkiness of the substituent group. The largest silica particles were synthesized from OMCTSO, which contains the largest number of silicon atoms.

Si-Al-C-O materials are well-known as a good precursor material for obtaining a dense, excellent heat-resistant SiC polycrystalline materials. Although it has been found that the aluminum contained in Si-Al-C-O materials plays an important role in obtaining dense SiC materials, its actual behavior during heat-treatment processes has not been investigated. In this study, we investigated the behavior of aluminum contained in the SiC crystals, during the densification of SiC crystals produced from the Si-Al-C-O material. A part of the aluminum contained in the SiC crystals plays an important role in creating a thermodynamically stable grain boundary by the diffusional transportation of aluminum to the SiC grain boundaries at temperatures above 1700 degrees C. Subsequently, most of the aluminum disappeared during the heat treatment at higher temperatures (similar to 1900 degrees C); as a consequence, a dense SiC solid solution with uniformly distributed residual aluminum (0.15wt%) was formed without a secondary phase at the grain boundary.

AbstractSiC‐polycrystalline fiber (Tyranno SA) had been developed using a conversion process from amorphous Si‐Al‐C‐O fiber to SiC‐polycrystalline fiber via carbothermal reduction (~1700°C) and sintering processes (~2000°C). In the conversion process, the carbothermal reduction (SiO + 2C → SiC + CO and SiO2 + 3C → SiC + 2CO) of the oxide phase and residual carbon contained in the raw Si‐Al‐C‐O fiber and the subsequent sintering proceed inside each filament. Here, we describe a new densification process of SiC powder that optimally uses the conversion process from Si‐Al‐C‐O fiber to SiC‐polycrystalline fiber. In this new process, we adopted the densification process, that occurred in each filament of the Si‐Al‐C‐O fiber, for the development of a dense SiC sintered body using partially oxidized SiC‐powder with surface oxide layer and polymer‐derived pyro‐carbon containing a considerable quantity of aluminum as sintering aid for SiC crystals. Herein, the reactions and sintering, which occurred in each filament of Si‐Al‐C‐O fiber, proceeded in the interstices of SiC powder to create a dense SiC body. In this paper, we will describe the new densification system of SiC powder.

MgO is industrially produced from seawater or dolomite as the raw material. MgO synthesized from seawater has a relatively low impurity concentration. However, these impurities strongly affect the fine structure and physical properties (especially the thermal conductivity) of MgO. In this research, the influence of impurity concentration on the MgO grain growth and its thermal conductivity was investigated. The processing conditions for MgO powder synthesis were optimized by a polymer complex method using magnesium nitrate hydrate, citric acid, ethylene diamine, and chemical compounds containing B, Ca, or Si, which are the main impurities of MgO produced from seawater. The morphology and phase composition of the MgO powders were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS). The morphological changes and differences in the thermal conductivity of MgO crystalline systems containing impurities (B, Ca, and/or Si) were clarified, and the relationship between the fine structure of MgO crystals containing impurities and their thermal conductivities was described in detail.

炭化ケイ素は共有結合性が強く難焼結材料と して知られており,焼結助剤の添加なしでは緻 密な焼結体を得ることが困難である(Khodaei M. et al, 2019).これまで,炭化ケイ素焼結体 の作製方法として,ホウ素と炭素を焼結助剤と した固相焼結法が報告されているが,焼結温度 が 2000°Cを超える上,破壊靭性値が低い問題 点がある(Prochazka S. and Scanlan R.M., 1975). 一方,Al2O3 などを用いた液相焼結法は, ~1900°Cの焼結温度で高い破壊靭性値を得るこ とができるが,焼結助剤が粒界第二相を形成す るため,高温特性が焼結助剤の種類や添加量 に強く依存する(Rasouli S. and Taheri-Nassaj E., 2010).また,いずれの方法も,炭化ケイ素粉 末と焼結助剤を物理的に混合するため均一混合 が困難である. 当研究室ではこれまでに,有機ケイ素ポリ マーに焼結助剤としてアルミニウムが均一固溶 した前駆体を用いて,粒界第二相を形成しない 緻密な結晶質炭化ケイ素繊維を粒界固相焼結に より合成することに成功している(Ishikawa T., et al., 1998).本研究では,同様に焼結助剤とし てアルミニウムを採用し,炭化ケイ素粉末の前 駆体段階で含有させることで,元素レベルで均 一混合し,繊維内で起きた粒界固相焼結現象を 用いて,粒界第二相を形成しない焼結体を作製 することを最終目的する.本報告では,前段階 として原料であるアルミニウムが含有した炭化 ケイ素粉末の合成を目的とする.

To date, several polymer-derived ceramic fibers have been developed all over the world, out of which SiC fibers synthesized from polycarbosilane and their derivatives have achieved highest heat resistance and show excellent mechanical properties. Their use in ceramic matrix composite materials has resulted in high-temperature stability and light weight, which show great promise in next-generation applications, such as aerospace engines. This book presents polymer-derived ceramic fibers from a historical viewpoint; basic information about them, such as production process, fine structures, and physical properties; their applications; and prospects of future inorganic fibers.

This paper relates to the Bridge Building Award, which was presented to the author (Toshihiro Ishikawa) by the American Ceramic Society on 27 January 2020. We have developed many types of functional ceramics using polycarbosilane as a raw material. Since 1983, several grades of SiC-based fibers have been produced from polycarbosilane by Ube Industries, Ltd. Of these grades, we developed the highest heat-resistant SiC-polycrystalline fiber (Tyranno SA), which can withstand up to 2000 degrees C, using an organic silicon polymer (poly-aluminocarbosilane) containing a small amount of aluminum as a precursor material. By employing curing (in air) and firing (in nitrogen atmosphere at 1300 degrees C) processes using the precursor fiber, an amorphous fiber (Si-Al-C-O fiber) containing a small amount of aluminum was obtained; subsequent heat treatment at higher temperatures (similar to 2000 degrees C) in argon atmosphere led to carbothermal reduction (SiO2 + 3C SiC + 2CO(g)) and a sintering process, producing the abovementioned SiC-polycrystalline fiber (Tyranno SA). In the same year, using the same raw precursor fiber (Si-Al-C-O fiber), we also developed a new type of tough, thermally conductive SiC composite (SA-Tyrannohex) with high strength up to 1600 degrees C in air. This ceramic consists of a highly ordered, close-packed structure of very fine hexagonal columnar SiC-polycrystalline fibers with a thin interfacial carbon layer between them. Further, by using the polycarbosilane as a starting material, we successfully developed a strong photocatalytic fiber (TiO2/SiO(2)fiber) with a gradient surface layer composed of TiO2-nanocrystals, making the best use of controlled phase separation (bleed-out) of additives (titanium (IV) tert-butoxide) contained in polycarbosilane. In this paper, the story of the development of these materials and the subsequent progress will be described along with the historical background.

Most of the present production processes of SiC sintered bodies require some powder mixing using a mechanical milling process (ball milling, and so on). In this case, relatively long hours are required, and there is the problem of contamination during the preparation process. To avoid these problems, we developed a new process for obtaining a self-sinterable, stoichiometric SiC powder, whose precursor material is water-soluble; the precursor material was synthesized from aqueous silica and citric acid containing a small amount of aluminum compound. In order to obtain the stoichiometric SiC composition, the above aqueous precursor material was adequately cured in air (200 degrees C-400 degrees C); subsequently carbonization reaction (similar to 800 degrees C) in nitrogen atmosphere, carbothermal reduction (similar to 1600 degrees C) in argon atmosphere, and pressureless sintering (similar to 1900 degrees C) were performed. Among these processes, the curing process (cross-linking process) is very important for obtaining the equivalent composition (silica and carbon) for the subsequent carbothermal reduction. In this study, the adequate curing temperature and suitable preparation condition for the carbothermal reduction were investigated for the production of stoichiometric self-sinterable SiC powder. The pressureless sintered body achieved using the obtained SiC powder demonstrated a desirable trans-crystalline fracture behavior.