Generally, ceramics are made from powders through shape forming and then sintering. However, the sintering method is not always effective in material forming method. If ceramics can take casting method, ceramics can be formed more effectively. Especially, when we try to make nano-structured bulk ceramics by sintering method, it will require a lot of special technique and much time. In this research, in order to show that ceramics can also choose the casting method as one of the forming methods, we took the rapid solidification method and got the amorphous products without glass component. Furthermore, we got successfully nano-structured bulk ceramics by annealing the amorphous products.

View full abstract

Gas pressure sintering conditions of silicon nitrides mixed with lutetium oxide (lutetia, Lu203) as sintering additive was investigated. Silicon nitrides with 9wt%Lu2O3-1wt%SiO2 and 8wt%Lu2O3- 2wt%SiO2 were successfully densified, but that with 10wt%Lu203 was not. Microstructures of the densified silicon nitrides consisted of rod-like grains. They exhibited strength of about 600MPa and 400MPa at R. T. and 1500oC, respectively.

View full abstract

It is well-known that thermal conductivities of Si3N4 are strongly influenced by the oxygen contents in the Si3N4 clystal lattice. Recently, it reported that the sintered Si3N4 with MgSiN2 additive exhibited high thermal conductivity due to its low lattice oxygen content. In this work, Si3N4 with a series of nitride (TiN, ZrN, Li3N and YN) and carbide (TiC, ZrC) additives were fabricated by the pressureless sintering method. Paying attention to the lattice oxygen content, the effect of sintering additives on thermal conductivity was investigated.

View full abstract

The dielectric properties (ε' and tanδ) of Si₃N₄ ceramics were measured from 1 to 10GHz using the cavity resonant method. Effects of porosity and lanthanide oxide sintering aids on them were investigated. The dependence of &epsilonr' on porosity can be described simply by Maxwell's relationship, although tan δ showed no dependency on porosity. Highly porous Si₃N₄ ceramics show very low ε' values around 3 and reasonably low tanδ values around 10×10^-4.Si₃N₄ ceramics did not show the dielectric dispersion observed for AlN ceramics.

View full abstract

We were successful to develop to the sintered silicon nitride with high thermal conductivity. In order to use these materials in applications such as heat sinks, good mechanical properties are also required. The purpose of this study is to investigate the mechanical properties of high thermal conductivity Si3N4 ceramics. Thermal conductivity of sintered silicon nitride increased with sintering time. But strength and fracture toughness of that decreased with sintering time.

View full abstract

We investigated preparation conditions of Y₂Si₃O₃N₄ for the solid-state reaction, using Si₃N₄ and Y₂O₃ as the starting powders; after a mixture of Si₃N₄ and Y₂O₃ powder was pressed uniaxially, the compact was heated at a temperature between 1500°C and 1700°C for 2h or more. A single phase of Y₂Si₃O₃N₄ could be obtained by heating the compact at 1650°C for 2h. When the Y₂Si₃O₃N₄ compact was fired at 1800°C for 4h or more, the relative densities were as high as 95%. The maximum Vickers hardness reached 14.9 GPa.

View full abstract

Grain aligned porous silicon nitride was synthesized by adding Lu2O3 and SiO2 as sintering aid. Fracture testing was performed at room temperature and at elevated temperature in order to evaluate heat resistance. Effects of the anisotropy and pores on the mechanical properties were investigated and discussed.

View full abstract

Silicon nitride/tungsten compound composites were fabricated by thermal decomposition of Tungsten Boride (W2B) under hot press sintering. The fabricated composite consisted of β-Si3N4, W, W2B and W2C. The Si3N4 matrix grains were composed of an elongated and bimodal structure similar to conventional Si3N4. Wear properties of the Silicon Nitride/ W composite was evaluated under dry sliding conditions. The composites had high wear resistance compared with a conventional silicon nitride, attributed to the formation of protective compounds, which consisted of W, Si and O. These protective compounds restricted the adhesion of Si-0-H debris, which was formed by tribo-reaction due to the moisture vapour in the atmosphere.

View full abstract

Yttrium α-Sialon ceramics have been fabricated by reaction-bonding method, which offers low sintering shrinkage and is a low cost process. The green compacts fabricated from starting materials of Si, Y₂O₃, Al₂O₃ and AlN were nitrided at 1350°C and post-sintered at 1700-1900°C in nitrogen atmosphere. In the samples sintered at 1900°C, relative density was high (98%) and the grains became elongated. The mechanical tests showed that yttrium α-Sialon ceramics with elongated grains produced by reaction-bonding process exhibit high hardness (17GPa) and high fracture toughnes (5 Mpam^1/2).

View full abstract

Sialon has high hardness, high strength and high oxidation resistance to be expected as an engineering ceramics in a high temperature or severe environment use. In our previous work, we synthesized Ca-α sialon nano powders by carbothermal reduction-nitridation of SiO2-Al2O3-CaO system, and fabricated Ca-α sialon nano ceramics using it by spark plasma sintering method. As a result of corrosion test, it was found to be better corrosion resistance in H2SO4 35mass% than in H2SO4 5mass% and better in HNO3 5mass% than in H2SO4 5mass%.

View full abstract

Ca-Ce-codoped α-sialon ceramics were prepared using the powder mixture of Si3N4-AlN-CeO2-CaCO3 system by gas-pressure sintering at 2000°C for 2h. Co-doping of Ca ion is effective for stabilizing the Ce-α-sialon.

View full abstract

β-SiAlON is possible to use as high-temperature materials and bearing ball, it has excellent thermal and mechanical properties However it is necessary for it to sinter densely. In this study , the dense β-SiAlON body was obtained by sintering its powder compact with various additives in PECS.

View full abstract

β-sialon ceramics was fabricated using the nano-sized β-sialon powder by spark plasma sintering method. The β-sialon nano ceramics was found to have much higher corrosion resistance than commercialized Si₃N₄ ceramics. As a result of SPM observation of the surface, it was confirmed that the corrosion of β-SiAlON nano ceramics in acid made very slow progress because glass phase is hardly found in the grain boundary.

View full abstract

Zeolite and carbon powders were mixed in ethanol for 1hour. After the slurry was dried, the powder mixture was fired at the temperature of 1450°C for 2hours in 0.5l/minN2 flow, followed by heat-treatment at the temperature of 700°C for 2hours in air in order to remove residual carbon. As a result, it was confirmed that the main phase of synthesized powder was β-SiAlON from XRD analysis and it was composed of ultra fine particles from SEM observation and measurement of specific surface area. Consequently, zeolote is very useful raw material to synthesige very fine β-SiAl0N powders.

View full abstract

In composite materials containing solid lubricant, the distribution as well as its characteristics are expected to affect the tribological properties. In this investigation, multilayered Si3N4 composites with h-BN as a solid lubricant containing Si3N4 interlayers were fabricated by alternate stacking of tape cast sheets, followed by hot-pressing. The tribological properties on the side plane of the composites were evaluated. The properties were difference by the directions of samples. It was considerer that these phenomena were caused by the orientations of Si3N4 grains.

View full abstract

Gas-pressure sintering condition of silicon nitride with additives of 8%-lutetium oxide and 2%-silica was studied under 0.9 MPa of nitrogen pressure. Density of the sintered body was around 3. 4 g/cm3 when the sintered temperature was 2050°C.It was suggested that there is an optimum silica content for the powder bed.

View full abstract

DV-Xα molecular orbital calculations were performed for alkali diborate crystals, R2OB2O3(R=Li,Na,K) to investigate the influence of the structural variety on the electronic states and XPS spectra. The dependence of the sort of alkali ions was found in the bond overlap population (BOP) of R-O bonds, and as for B-O bonds,however, the alkali dependence was confirmed neither BOP nor bond length.

View full abstract

Si2p binding energy of α-SiC was smaller than that of β-SiC by an XPS measurement. In this study, the Si2p orbital energies of several SiC clusters were calculated by a DV-Xα method. For a ground state calculation, the Si2p orbital energy ranged from 85 to 89 eV. These data were not depend on the cluster size and were over 10 eV smaller than the Si2p binding energy by the XPS measurement. This difference was considered that a relaxation energy of the Si2p orbital.

View full abstract

Thermal stability of the amorphous-like structure in high-energy grain boundaries of Si and SiC systems is studied using empirical and tight-binding molecular dynamics method. The time evolution of thickness of the disordered grain boundaries is compared for different empirical interatomic potentials for Si such as Stillinger-Weber model and Tersoff model. The comparison is also made with tight-binding model for Si by Kwon et al.. We find all models give same results on stable thickness Σ=5 of the grain boundary. The detailed analyses for several types of boundaries of Si and SiC grain boundaries will also be presented.

View full abstract

We conducted the Monte Carlo simulation of sintering and grain growth in order to design the microstructure of the grain-oriented ceramics. It was found that our simulation method would be useful to optimize the initial conditions of the templated grain growth processing, which is widely applicable to the fabrication of the functional ceramics (e.g., thermoelectrics and piezoelectrics).

View full abstract

Monte Carlo simulations of liquid phase sintering were performed to determine the influence of cofiring. It was found that cofiring influenced sintering rates and the microstructure. The results closely match the behavior of real materials and demonstrate possibility of material design using MC simulation.

View full abstract

We discuss a fine structure in elastic properties distribution of ceramics material. α-Al₂O₃ film on a glass substrate, which was fabricated by the aerosol deposition (AD) method, was observed using ultrasonic atomic force microscopy (UAFM) that is a novel technique to examine the elastic property of hard materials with high spatial resolution. The observation successfully displayed the elastic property distribution of the α-Al₂O₃ film with nano-scale resolution, and revealed some regions which have smaller resonance frequency and lower Q value in the surface vibration characteristics than the surrounding area. The specific regions in resonance characteristics corresponded to the protrusions, 20∼40 nm in height and 300∼500 nm in diameter, observed by atomic force microscopy (AFM). This result suggests that the bond under the protruded regions be incomplete locally.

View full abstract

Bulk density and Young's modulus of the Si₃N₄ doped β-spodumene ceramic significantly improve compared to those of the un-dopedβ-spodumene ceramic. The averaged thermal expansion coefficient between 293K and 300K is -0.032×10^-6/K for the ceramic doped with 23.1 mass% Si₃N₄.

View full abstract

A new method to evaluate the distribution as well as the average of crystallite size from powder X-ray diffraction data is presented. The Kα2-subpeak and instrumental aberrations are simultaneously removed from the experimental data by a method based on deconvolution of the instrumental function. Evaluation of the size distribution is achieved by fitting of theoretical peak profile for lognormally distributed spherical particles to the observed data. The method has been applied to the powder X-ray diffraction data of a SiC powder sample.

View full abstract

X-ray diffraction method was demonstrated on the inside structure analysis of ceramic layer. The independent peak patterns of inside layer were obtained by an extension of thin film X-ray diffraction method. By this method, the diffraction patterns of the inside structure of ceramic layer was determined by the subtraction those of the shallow layer. In the bio-functional calcium phosphate coatings, the crystallinity of the inside layer were clearly revealed.

View full abstract

The layered tungstic acid H₂W₂O₇ was prepared through selective leaching of bismuth oxide sheets from an Aurivillius phase, Bi₂W₂O₉. The transmission electron microscopic (TEM) images along [001] indicate ReO3-like slabs were maintained after leaching of bismuth oxide sheets. The layers consist of double octahedral sheets, consistent with Bi₂W₂O₉ structure. The arrangement of WO₆ octahedra, however, slightly changed after acid treatment, leading to the contraction along a and b axes.

View full abstract

Ceria-zirconia solid solution play an important role as an oxygen supplier as well as absorber in automotive exhaust emission control systems. Structures of Ce₂Zr₂O_7+x with an ordered oxygen vacancies were studied using a HRTEM. On the basis of direct observation with HRTEM and with the aid of computer simulation, the ordered oxygen vacancies in Ce₂Zr₂O₇ and Ce₂Zr₂O_7.5was found to be identified as bright-spot in HRTEM image in the [001] orientation.

View full abstract

Structural change of bismuth oxide (Bi₂0₃) was studied in situ by high-temperature neutron and synchrotron powder diffraction techniques. Crystal structure was refined by Rietveld analysis of neutron diffraction data. The nuclear density distribution of δ-Bi₂O₃ was obtained by maximum entropy method/Ritveld analysis. The position of oxygen atoms distributed forming tetrahedra.

View full abstract

In the present work, the crystal structures of perovskite-type lanthanum gallate compounds, LaGaO₃ and La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.8 at room temperature were determined using neutron powder diffraction. The structures of LaGaO₃ and La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.8 were in orthorhombic (Pbnm) and cubic (Pm-3m) phases, respectively. In the LaGaO₃ structure, the GaO₆ octahedron was tilted along the b and c axes. The tilt of GaO₆ octahedron was not observed in the La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.8 which showed superior oxide ion conduction.

View full abstract

We have investigated in situ the phase transition of strontium zirconate by neutron and high-resolution synchrotron powder diffraction techniques. The unit-cell parameters increased with temperature. The sequence of existing phases was confirmed to be Pnma, Imma, I4/mcm and Pm-3m. The tilt angle decreased with increasing of temperature.

View full abstract

We have investigated in situ an orthorhombic (o)-tetragonal (t) phase transition of double perovskite-structured La0.68(Ti0.95,Al0.05)O3 by high-resolution (Δd/d≈0.0002) synchrotron powder diffraction technique. The unit-cell parameters increased continuously with temperature also near the o-t transition point. Good agreement was obtained in the unit-cell parameter values between heating and cooling. These results indicate that the o-t phase transition occurs reversibly and continuously at 349.3±0.6°C. We demonstrate that the present synchrotron X-ray powder diffraction technique is very powerful to determine both the transition temperature of a continuous transformation and precise unit-cell parameters even near the continuous transition point.

View full abstract

We synthesized a novel barium niobium copper oxide, Ba₄Nb_3.16Cu_0.13O₁₂by high-temperature reaction, which was isostructural with Ba₄M₃LiO₁₂(M=Nb or Ta). The single crystal X-ray diffraction analysis indicated that this compound crystallize in the hexagonal space group P6₃/mmc(#194) with a=5.827(3)Å and c=19.068(5)Å, and the final R factors were R=0.050 and Rw=0.065 for 371 unique reflections.

View full abstract

K2NiF4 type (Ca2-xNdx)CoO4 (1.0≤x≤1.3) was synthesized from calcium acetate, neodymium oxide, and cobalt acetate. The increase in the cell volume is explained by the difference in the ionic radius of the Ca and Nd ions. (Ca2-xNdx)CoO4 is the semiconductor and the electrical resistivity at room temperature increases with increasing x.

View full abstract

The crystal structure of fluorescence property of BaCa₂MgSi₂O₈:Eu²⁺ were investigated in order to use as a blue phosphor for PDP. This alkaline earth silicate had polymorphism of the high temperature form with the hexagonal cell and the low temperature one with the orthorhmbic cell. The crystal structure of the high temperture form was similar to the merwinite-type structure. The intensity of fluorescence for the high temperature form was stronger than that for the low temperature one.

View full abstract

10 mol% SrO and 20 mol% MgO doped LaGaO3 (LSGM1020) was directly synthesized via a gel combustion process. The properties of the powder as-obtained were characterized by XRD, Raman, BET and SEM. It has rhombohedral structure even without high temperature calcinations. The powder has porous morphology and its specific surface area reaches 122 m2/g.

View full abstract

Dielectric and dc measurements have been carried out on La_0.8Sr_0.2GaO_3-δ, which has a high potential for the electrolyte-material in SOFC, so as to elucidate the correlation between O^2--diffusion and electric conduction. A dielectric relaxation shows up in loss tangent. The relative relation of activation energies as to dielectric relaxation and dc conduction suggests the ionic conduction due to O^2--diffusion assisted by free single oxygen vacancies taking place.

View full abstract

YSZ single crystals were compressed at 1300°C to introduce the aligned dislocations. The electric conductivity of the deformed YSZ single crystals was measured using the AC impedance measurement. In this case, we set up two current directions to investigate the anisotropic conductivity of the deformed samples. One direction was parallel to the dislocation lines, the other was perpendicular to dislocation lines. The conductivities of deformed samples were higher than that of undeformed samples in both current directions. Plastic deformation enhanced the conductivity independent on the dislocation lines alignment and the crystallographic orientation.

View full abstract

P(O₂) or oxygen chemical potential variation in Ce_0.8M_0.2O₂-δ (M; Sm and Y) under SOFC operating conditions was determined using Raman spectroscopy, by which the information of oxygen vacancies can be obtained. Under open-circuit conditions, the P(O₂) variation revealed that oxygen chemical potential was quite low in most part of the sample and thus only a thin layer remained as electronically insulating near the sample/air interface.

View full abstract

Phosphate hydrogels with high viscosity were easily derived for a brief period from the reaction of water with the metaphosphate glass powders. The hydrogels showed high conductivity of 20~40 mS/cm at 30∼70°C. The high conductivities were regarded as the fast proton transfer promoted by coexistence of large amounts of acidic POH groups and water molecules.

View full abstract

Fast proton conducting calcium phosphate hydrogels were successfully prepared utilizing hydrogelaiotn of the calcium phosphate glasses. The conductivity was improved by preparation of the hydrogels with high P₂O₅ content and / or by heating them at a temperature above 70°C.

View full abstract

Hexagonal mesoporous silica was synthesized from tetraethylorthosilicate and hexadecyltrimethylammonium bromide under alkaline conditions and its structure was characterized using low-angle X-ray diffraction. The conductivities of mesoporous silica mixed with phosphoric acid and that heat-treated after mixing were measured under various temperatures and water vapor pressures. The conductivity decreased with increasing temperature above 80°C. As for the heat-treated mesoporous silica mixed with phosphoric acid, the conductivity was higher than that of silica mixed with phosphoric acid at above 110°C.

View full abstract

Raman spectra of SrZrO₃ have been successfully obtained between 600 and 1050°C. The Pnma to Imma and Imma to I4/mcm phase transitions were observed by measuring the variation of energies of phonon modes as a function of temperature. The results can be accounted for by terms of coupling between a zone-center optical mode and an elastic strain associated with a soft q≈0 acoustical phonon.

View full abstract

Solid-Liquid composite ion conductors were prepared by impregnationg liquid electrolyte into silica mesoporous thin film synthesized via self-assemble method. Hexadecyltrimethylammonium Chloride(C16TAC) and triblock copolymer(Pluronic F127)were used as surfactants for silica mesoporous thin films. Silica thin films of the cubic(Pm3n) structures with pre size of 3nm were obtained when C16TAC was used and (lm3m)sutructure with pore size of 5nm were obtainded when Pluronic F127 was used, respectively. From impedance measurements Li ion conductivity of the composite film prepared using C16TAC was estimated at 1.1 × 10^-4S/cm.

View full abstract

Lithium ionic conductor, (La,Li)TiO₃ ceramics (LLTO), was synthesized by sintering La₂O₃-Li₂O-TiO₂ amorphous spheres at various temperatures. Single crystalline LLTO ceramics required calcination at 700°C and sintering at temperatures higher than 1200°C because of decomposition of La₂CO₅. Lithium ionic conductivity of LLTO ceramics derived from the amorphous spheres showed 4.0 ×10^-3 S cm^-1 at room temperature. In addition, Li₄Ti₅O₁₂ particles were obtained by firing Li₂O-TiO₂ amorphous spheres at 800°C for 1 min in air. Both LLTO and LTO spheres prepared with amorphous spheres can be applied to all-solid-state lithium micro-batteries.

View full abstract

Hybrid polymers combining P₂S₅ moieties with oligoether spacers such as PEG₄₀₀ have been prepared, and the conductivities and glass transition temperatures of the hybrids with Li₂S have been examined. Although those electrolytes exhibited low glass transition temperatures below -40°C, the electrolytes showed low conductivities of 10^-6 S cm^-1 at room temperature. The decoupling of Li⁺ motion from the polymer segmental motion is demonstrated in the sulfide-PEO matrix.

View full abstract

A conduction mechanism of Li ions in lanthanum lithium tantalate was investigated by impedance spectroscopy and ⁷Li NMR. Conductivity spectra of La_1/3-xLi_3xTaO₃ exhibited Jonscher' s universal dynamic response behavior. A scaling analysis showed that this conductor is expected to behave as a solid electrolyte. Correlation times of ionic motion were estimated from NMR spin-lattice relaxation times. Activation energy (E_NMR) of the correlation time was confirmed to relate to that (E_σ ) of dc conductivity through the following equation : E_NMR = (1-n) E_σ (n, power-law exponent).

View full abstract

Preparation of ionic conducting inorganic-organic hybrid membranes has been investigated by sol-gel method. Conductivity of the membrane was exhibited 4.20-3(S/cm) at 70°C, 95%RH.

View full abstract

We have already reported changes in strength of barium titanate/zirconia composite on polarization treatment. Bending strength of this composite increased on longitudinal poling and decreased on transverse one. However, the strength change ratio was less than 15%. In the present study, we prepared sodium β-alumina ceramics and evaluated the change in mechanical properties on ion exchange of Na⁺ for K⁺. Vickers hardness increased with ion-exchange time up to 30 min, then gradually decreased.

View full abstract

ZnO and CaO do hardly form solid solution. By pulsed laser deposition CaO dissolved into ZnO in a few percent in film on substrate. It is demonstrated with the change of lattice constant of ZnO measured by XRD. 266 nm, 5 ns laser pulses were irradiated on a ceramic target in 6 Pa in the chamber.

View full abstract

LnCuOS (Ln=La,Ce,Pr and Nd) oxysulfides crystallize in the same layered structure. However, only CeCuOS shows an anomalous shrinkage of the unit cell and unique electrical and optical properties among these oxysulfides: black color, high electrical conductivity and no photoluminescence. The origin of the unique features in CeCuOS is discussed from the viewpoint of the crystal structure, especially interatomic distances.

View full abstract