In this report, we analysed quantitatively the effects of stress distribution, Weibull's shape parameter and volume of a specimen on the flaw-size distribution. We also analysed the effect of grain-size dependence of the fracture toughness on the flaw-size distribution. Furthermore, we presented a new method for estimating Weibull's shape and scale parameters by linear regression using only flaw-size data and the fracture toughness in the uniaxial tensile test.

View full abstract

TZP's containing from 1.5 to 3.0mol% Y₂O₃ were prepared by hot-pressing and their microstructure was analyzed by TEM and electron diffraction.
Y-TZP containing 3.0mol% Y₂O₃ showed a microstructure of about 0.5μm grains. Two mol% Y-TZP showed microstructures of grains, plate-shaped crystals and repeated twins, or their mixtures. Y-TZP containing 1.5mol% Y₂O₃ showed the completely plate-typed structure with many cracks in the radial direction.
Twin structure was found on the (100) plane and the orientational relation between tetragonal (t) and monoclinic (m) crystals was determined to be (010)_m//(010)_t and [001]_m//[001]_t.

View full abstract

The superplastic deformation properties of 3mol% Y₂O₃-stabilized tetragonal ZrO₂ polycrystals (Y-TZP) were studied by uniaxial compression tests in air at temperatures up to 1500°C with strain rates ranging from 10^-3 to 10^-5s^-1. The effect of microstructure on the superplasticity was also examined, Fine-grained Y-TZP with an average grain size of 0.3-0.4μm was deformed to the true strain of more than -1.5. The macroscopic strain rate (ε) in superplasticity was expressed phenomenologically by the following equation, ε=2×10^-5exp(-380000/RT)σ^2.1/d^1.8, where T, σ, and d are temperature, stress, and grain size, respectively, and R is the gas constant. Although the microstructure of the deformed specimen showed slight grain growth and slight anisotropic deformation of each grain, the large strain was mainly caused by the grain boundary sliding. The cavitation was not observed in the compressed specimens. ZrO₂ particles maintained tetragonal or cubic phase after the superplastic deformation. The deformation properties and the observed microstructure in the superplasticity of Y-TZP were explained by the Gifkins's model.

View full abstract

The phase stability of the cubic and tetragonal phase in ZrO₂-2-6mol% Y₂O₃ is studied by X-ray diffraction method and transmission electron microscopy. Two kinds of tetragonal phase, t and t', and a cubic (t'') phase, are identified consistently. The t-phase is the equilibrium tetragonal phase, which transforms martensitically to the monoclinic phase on further cooling. The t'-phase has a thin plate or lamellar structure, and is formed in the decomposed structure of the cubic (t'') phase. The high resolution electron microscopy reveals that the fine domains (t''-phase) due to displacement of oxygen atoms appear dispersedly in the initial stage of decomposition, resulting in the tweed contrast.

View full abstract

The effects of grain boundaries and their movement on the self-diffusion of oxygen ions were studied in polycrystalline solid solutions of fluorite-type 90mol% CeO₂-10mol% ZrO₂ and tetragonal 14mol% CeO₂-86mol% ZrO₂. Oxygen self-diffusion coefficients were determined using a gas-solid isotope exchange technique and measuring the time dependence of the ¹⁸O concentration in the gas phase. The oxygen self-diffusion coefficients were determined at 1570 and 1670K.
The oxygen self-diffusion was not enhanced by the presence of grain boundaries. However, when rapid movement of grain boundaries due to rapid grain growth occurred, the oxygen self-diffusion was enhanced. The cause of this was interpreted as being due to the creation of a fast diffusion layer in the vicinity of the grain boundaries. The fast diffusion layer was considered to be effective in enhancement of the diffusion not only of the oxygen ions but also of the cations in the oxides.

View full abstract

The distribution of monoclinic phase in Y₂O₃-PSZ, induced by the stress field around Vicker's indents, was examined by using a new system of Raman microprobe. This system is equipped with a step scanning X-Y stage and a multichannel detector and was used to obtain the monoclinic distribution, i. e. the 2-dimensional map of a monoclinic fraction. The results obtained for the PSZ's with 2 and 3mol% Y₂O₃, 2Y-PSZ and 3Y-PSZ, are described as follows: (1) The monoclinic fraction at the center of indents, where large plastic deformation has occurred, is smaller than that around the edges of each indent. (2) The size of the transformed zone and the monoclinic fraction of 2Y-PSZ around indents were found to be greater than those of 3Y-PSZ. (3) The size of the transformed zone and the monoclinic fraction reach a maximum respectively around 200°C, at which thermal degradation occurs greatly.

View full abstract

Partially stabilized zirconia is a desirable material as high toughness ceramics. On the other hand, Al₂O₃-ZrO₂ composite is also a desirable material for engineering ceramics because of chemical stability, high hardness, high wear resistance and high toughness. Some properties of Al₂O₃-ZrO₂ composites such as critical grain size for tetragonal ZrO₂ and mechanical behabior have been reported, but studies on the transformation behavior are scarce.
In the present research, the microstructure and transformation behavior of zirconia in Al₂O₃-ZrO₂ composites were studied. Specimens containing 5 to 15vol% ZrO₂ were hot-pressed and analyzed mainly by laser Raman spectroscopy and microstructure was observed by SEM.
The transformation of zirconia in Vickers-indented areas was more pronounced near the edge (stress concentrated area) than the center area. On the fracture surface after the bending test, the transformation proceeded in the tensile stress area, i.e., the fracture origin. Therefore, it was thought that the stress-induced transformation is closely related to tensile stress.

View full abstract

Al₂O₃-ZrO₂ composites have been extensively investigated with a 3MV electron microscope on the effect of oxide additives such as ZrO₂, MgO and Y₂O₃ on microstructures and determination of the most suitable heat treatment conditions to increase the toughness. The results obtained are: (1) MgO and ZrO₂ additive powders generally suppress the growth of Al₂O₃ grains, (2) voids are almost completely eliminated by ZrO₂ addition due to not only enhancement of sintering around very fine ZrO₂ powders but also good affinity between Al₂O₃ and ZrO₂, (3) grain sizes of both Al₂O₃ and ZrO₂ in Al₂O₃-ZrO₂ composites are not affected with a small amount of MgO, (4) ZrO₂ in Al₂O₃+ZrO₂ composites is monoclinic at room temperature, but ZrO₂ in Al₂O₃+YTZ (Yttria Toughened Zirconia) composites is tetragonal in general, (5) grain sizes of both Al₂O₃ and ZrO₂ in Al₂O₃+YTZ composites are not sensitive to the addition of a small amount of MgO. Furthermore, in the present experiment, using a very-high temperature stage, annealing and mechanical phenomena of Al₂O₃-ZrO₂ composites have been studied up to 2300K by in situ experiments with a 3MV electron microscope.

View full abstract

Relatively large Al₂O₃ disc particles (16μm, 41μm) with high aspect ratios (5-20) were incorporated into a fine-grain Al₂O₃ matrix (-1μm) in order to improve the mechanical properties of sintered Al₂O₃. The flexural strength depends on the disc size and the microstructure of composites. For a composite containing 5 vol% 16μm Al₂O₃ disc particles, the maximum flexural strength was 604MPa, 28.5% larger than that without disc particles.

View full abstract

The microstructure and bending strength of alumina ceramics containing 0-16wt% TiO₂ fired from 1400° to 1700°C were investigated. The addition of TiO₂ markedly promoted the sintering of alumina at comparatively low temperatures in air. The addition of TiO₂ within the solid solubility limit of TiO₂ (0.27wt%) also promoted the grain growth of α-Al₂O₃ at 1600°-1700°C. An excess amount of TiO₂ over the solid solubility limit reacted with alumina to form aluminum titanate above 1500°C and dispersed in alumina ceramics. Al₂TiO₅ as a second phase prevented the grain growth of α-Al₂O₃, and the grain size of α-Al₂O₃ decreased with increasing amount of TiO₂. The bending strength of specimens fired at lower temperatures increased with increasing amount of TiO₂, i. e., with an increase in fired density, In the specimens fired above 1500°C, the bending strength was lowest for 0.2wt% TiO₂ addition and increased with further increase in the amount of TiO₂, i. e., with a decrease in grain size of α-Al₂O₃, but the specimens containing a large amount of Al₂TiO₅ had relatively low bending strengths. In the specimens containing Al₂TiO₅, some strain appeared in both α-Al₂O₃ and Al₂TiO₅ grains due to the difference in their thermal expansion coefficients.

View full abstract

The effects of SiC dispersions with average particle sizes of 2 and 8μm on the microstructure and mechanical properties were investigated for Al₂O₃-SiC composites, which were fabricated at 1500° to 1800°C for 1h under the applied pressure 27MN/m². The sintering temperature increased with increasing the SiC content. However, the grain growth of Al₂O₃ was remarkably inhibited by incorporating SiC particles, especially those of 2μm. The Vickers hardness of composites increased with increasing the SiC content. The maximum toughness and strength, 4.5MN/m^3/2 and 490MN/m², were observed for the composite containing 10mol% 2μm SiC prepared at 1600°C, approximately 40% higher than those with Al₂O₃ only. The increases may be attributed to the increase in toughness by crack deflection and/or microcracking caused by the SiC dispersions.

View full abstract

Alpha-Si₃N₄ whiskers prepared by the gas-solid reaction have been investigated by transmission electron microscopy (TEM). The correlations among morphology, growth direction and structural defect have been studied. The growth directions of whiskers have been determined accurately by observing the TEM images and corresponding diffraction patterns taken with two different orientations by large angle tilting of the specimens with a double-tilting device in an electron microscope. Defects in whiskers have been observed in detail using various TEM techniques, particularly by high resolution electron microscopy. Two distinct types of the whisker have been found. One is a triangular prism-like shape having the {1010}* growth direction, and the other is a thin tape-like shape having the {1011}* growth direction. The former includes only a small number of defects, whereas the latter has a lot of planar defects only in one side of whiskers. The planar defects in the latter whisker are found to consist of the {0001} and {1012} planar defects. The most probable displacement vector of the {0001} planar defect is determined to be a/2 ‹1010›.

View full abstract

The high temperature mechanical properties of chemical vapor deposition CVD-SiC are superior to those of hot pressed- and normally sintered-SiC. In order to have an insight into the origin of the superiority of CVD β type SiC, transmission electron microscopy (TEM) observation has been done around the areas indented with a Vickers diamond pyramid at room temperature and 1000°C. At room temperature, two types of microstructure were distinguished in the region around indentation. Around a hole of indentation, there was a plastically deformed region (a) containing a number of stacking faults formed by motion of Shockley partial dislocations on the close packed planes and micro-cracks orientated randomly from the hole. Surrounding this region, there exists a circular region (b) with extensive elastic lattice strain. In region (a) of indentation at 1000°C, stacking faults formed by dislocation movement were seen more clearly. But no micro-cracks were observed and macro-cracks occurred from two of the fonr corners of indentation. It is concluded that the formation and the movement of partial dislocations play an important role for strain relaxation in CVD-SiC indented at room temperature as well as at 1000°C.

View full abstract

The plastic behavior of 6H SiC single crystals was studied by constant-rate compression tests between 1150°C and 1750°C and Knoop indentation tests between room temperature and 1400°C. Defects introduced by deformation were investigated with using a transmission electron microscope (TEM) operating at 1000kV. SiC crystals deformed mainly via glide of basal dislocations which were dissociated to Shockley partials. Owing to a low formation energy of basal stacking faults (evaluated to be 1.6mJ/m² from the dissociation width), partials with high mobility can glide almost freely even when the mobility of their counterparts is limited. At temperatures below -800°C, glide of mobile partials leaving wide stacking faults behind them causes limited plasticity. Above -1000°C partial dislocations with lower mobility become mobile enough for dislocation multiplication and normal ductility is gained. The flow rate above 1150°C measured as a function of applied shear stress τ and temperature T was found to be proportional to (τ-τ_i)^m+2 exp(-U/κT), where m=1.1±0.4, U=3.4±0.7eV and τ_i=3MPa. It was concluded from the stress exponent m-1 and dislocation morphology observed by TEM that dislocation motion is controlled by the Peierls mechanism.

View full abstract

Studies were made of effects of fabrication methods on the properties and microstructure of sintered silicon carbides. The specimens used in this investigation were three kinds of commercially available SiC bodies which were fabricated by reaction bonding, pressureless sintering and hot-pressing. The hot-pressed SiC contained a small amount of BeO. Measurements were carried out on density, the polytype by X-ray diffraction method and 4-point bend strength. Microstructural observation was also carried out using an optical microscope, a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The results of density measurement showed that the open porosities of three specimens were negligibly small and that the density of the hot pressed SiC had nearly the theoretical density. The measurement of 4-point bend strength indicated that the reaction bonded SiC had the highest value and the hot-pressed SiC the lowest. The analysis of the polytype indicated that all the specimens consisted mainly of α-SiC of 6H type. In the reaction bonded SiC, about 11% of 3C type (β-SiC) and 9% of free Si were recognized. The average grain diameter and fracture mode of each specimen were determined from observation with an optical microscope and SEM. In the hot-pressed SiC, the fracture occurred mainly at grain boundaries, whereas it occurred mostly in grains in the reaction bonded and pressureless sintered SiC. A lot of stacking faults were observed in all the specimens with a TEM. In addition, small closed pores were often recognized in the pressureless sintered SiC. In the hot-pressed SiC, a contrast originated from strain field within grains was observed, and dislocations near grain boundaries were a characteristic feature of this material. Small short partial dislocations accompanied by stacking fault were often observed in the reaction bonded SiC.

View full abstract

Bicrystals with (110) tilt boundaries and (100) twist boundaries were prepared by joining two single crystals of Mgo, using a graphite heating element at 2480°C without pressure. The joined boundaries were observed with an optical microscope and TEM. The evaporation/condensation mechanism was operative in joining. The boundary energies were measured at 1500°C by the thermal grooving method. The development of boundary grooves was controlled by surface diffusion. The ratio of the boundary energy to the surface energy (σ_b/σ_s) for small angle (110) tilt boundaries below 17° varies according to the Read-Shockley model, which suggests the boundary being constructed by edge dislocation arrays. TEM observation suggested that the small angle twist boundaries below 10° were formed by screw dislocation arrays. The (110) tilt boundaries had low σ_b/σ_s at about 40°, 70° and 130° which correspond to twin boundaries of (221), (111) and (113), respectively. σ_b/σ_s for high angle boundaries at 1500°C was around 0.4, lower than that of (100) tilt boundaries. Twist boundaries above 10° had also low energies at 22°, 29° and 37° which correspond to Σ of 13, 17 and 15 of the CSL theory, respectively. Hence, it is concluded that the high angle twist boundaries on (100) are explained by the CSL theory.

View full abstract

Precipitates of Mg(OH)₂ containing various amounts of phosphate were prepared by coprecipitation. The phase changes in the coprecipitates during heating were investigated by X-ray diffractometry and DTA-TG; MgO powders obtained by calcination were characterized by crystallite size and specific surface area, and the sinterability was evaluated by relative density. Mg(OH)₂ was the only crystalline phase in the coprecipitate and the phosphate may be in amorphous state. The amorphous phase crystallized into Mg₃(PO₄)₂ at about 750°C by thermal decomposition and the solid-state reaction with the matrix. Rapid growth of primary-particle was observed, corresponding to an increase in the amount of Mg₃(PO₄)₂; Mg₃(PO₄)₂ may act as a bonding material of MgO crystallites. A highly-densitied MgO body was obtained when MgO powder containing 0.50mol% of P₂O₅ was prepared by calcining its original coprecipitate at 900°C for 1h, and sintered at 1400°C for 5h.

View full abstract

Changes in microstructure of magnesium oxide compact containing magnesium phosphate equivalent to 0.50mol% P₂O₅ were investigated at a heating rate of 10°C·min^-1 up to 1400°C and by isothermal 5 h-heating at 1200°, 1300° and 1400°C. The microstructure changed with densification, which was caused by an increase in the liquid volume formed above 1100°C. Between 1100° and 1300°C, the densification took place along with rapid grain growth. Thus, Ostwald ripening is operative in the presence of a liquid phase. From 1300° to 1400°C, the grain growth continued, although the densification was slowed down.

View full abstract

Spherical fine particles of SrTiO₃ were prepared by ultrasonic spray pyrolysis of a methanol solutoin containing Sr and Ti iso-propoxides. The microstructure and composition of particles were studied by TEM and SEM. TEM observation revealed that the particles prepared at 1200°C have a hollow around the center of the particle and consist of small primary particles of 0.02-0.05μm. This primary particle size was comparable to both the particle size calculated from the specific surface area and crystallite size derived from XRD line broadening. While the particles prepared at 600°C contained some unreacted alkoxide and gave a broad XRD pattern and had no distinct particle shape in TEM photograph, particles calcined at 1000°C consisted of well developed crystalline primary particle of 0.02-0.05μm.
The mechanism of particle formation and microstructure development were discussed based on partial hydrolysis and complex alkoxide polymer formation.

View full abstract

A mullite powder was prepared from aluminum isopropoxide and methyl silicate by coprecipitation using a dilute ammonium aqueous solution. Properties and microstructure of fired bodies were examined. The results were compared with those from a sol mixture (SM) and oxide mixture (OX). The coprecipitated powder (Cp), the primary particle size of which was about 200Å, was amorphous by X-ray diffraction analysis. On heating, the amorphous phase changed into spinel phase at about 980°C, and then into mullite phase at about 1250°C. Mullite specimens calcined at 800° or 1000°C and fired at 1780°C for 1h showed a high bulk density of 3.14g/cm³ (relative density 99%). Lattice parameters, a₀ and b₀, of mullite decreased with increasing firing temperature from 1500° to 1690°C, but a₀ increased at 1780°C. Cp specimens contained the smallest amount of glass phase in the three specimens fired at 1780°C.

View full abstract

A highly dispersed monoclinic zirconia hydrosol of rod-shaped ultrafine particles was prepared by hydrolysis of a concentrated zirconium oxychloride solution. Transparent gel films were formed by drying the sols set in a petri dish of glass. The particles in the films have a tendency to orientate the monoclinic b-axis perpendicularly to the film plane. The orientation increased as gelation was performed more slowly and increased with increasing firing temperature up to 1000°C. The firing shrinkage of films was greater in the thickness direction than in lateral directions. A polycrystalline thin film was obtained as an isolated monograin layer of about 400Å thick after firing at 1000°C.

View full abstract

In our previous study, it was found that BaSO₄ powders, a sparingly soluble salt, was able to dissolve in water suspended with a strong acidic ion exchange resin (abbreviated as H-R). This paper presents a method to elucidate the surface activity of BaSO₄ powders which are heat-treated up to 800°C. Some factors responsible for the dissolution tendency of BaSO₄ such as H-R/H₂O ratio, soaking temperature and the amount of H-R were examined to determine the optimum condition. The average particle diameter was compared with an electron microscope. A remarkable change in solubility was observed for the BaSO₄ powders calcined at 450°-500°C without any significant change in particle size. Calcination at higher temperatures resulted in a slight increase in solubility in spite of the enhanced grain growth.

View full abstract

The cation-exchange characteristics between Sn²⁺ ions in aqueous solutions containing various counter-anions (Cl^-, F^-, BF₄^-) and Ca²⁺ ions in synthetic hydroxyapatite (samples) [Ca₁₀(PO₄)₆(OH)₂] have been investigated in the pH range below 3 at 25°C by a normal batch method.
It was found that the hydroxyapatite samples are transformed into amorphous states by the reactions between Ca²⁺ ions in the samples and Sn²⁺ ions in the SnCl₂ and Sn(BF₄)₂ acidic aqueous solutions with pH of 3.0 or below with a molar ratio of Sn²⁺/Ca²⁺≅1.0. These results are interesting because the Sn²⁺ uptake phenomena appear to be an ion-exchange reaction between Sn²⁺ ions in solution and Ca²⁺ ions in the apatite even in an acidic atmosphere. However, the hydroxyapatite was decomposed into Sn₃F₃PO₄ in a SnF₂ acidic aqueous solution.
The apatite samples are quickly transformed into the amorphous state in the SnCl₂ acidic aqueous solutions, while they are slowly transformed into the amorphism in Sn(BF₄)₂ acidic aqueous solutions even with high concentrations of Sn²⁺ ions (ex. 44.0×10^-3mol/200cm³).
The existence of hydroxyapatite as amorphism in acidic aqueous solutions such as SnCl₂ and Sn(BF₄)₂ is quite interesting because in general the hydroxyapatite has been found to be dissolved in acidic aqueous solutions. Moreover, the obtained amorphism are found to be stable up to at least 500° to 600°C but to be unstable in alkaline solutions. The characteristics of Sn²⁺ ions are found to be quite different from those of homologous Pb²⁺ ions which have been found to form crystalline Pb²⁺ apatite even in such an acidic atmosphere mentioned above.

View full abstract

It was found by the IR spectra that absorption by OH stretching (3600cm^-1) decreased with increase of yttrium content and weak absorption peaks of oxyhydroxyapatite appeared in Y_x-Ca-HAp. It was confirmed by XRD analysis that structure of Y_x-Ca-HAp consisted of only apatitic phase.
Electrical conductivity increased with yttrium concentration in the region x=0-0.7, and it decreased abruptly with yttrium content to x=1.0. In the region btween x=1.0 and 2.0, it increased slightly with content of Y³⁺. Calculated activation energies of conduction of the samples in the region between x=1.0 and 2.0 were larger than that of ones in the other region (x≤0.7).
The above results suggests that the conduction mechanism of the samples in the low Y³⁺ concentration region (x≤0.7) is different from that of the other (x≥1.0).

View full abstract

Two fundamental methods have been used for X-ray structure analysis of amorphous materials, i.e., the intensity comparison method and radial distribution analysis. There is an opinion that Fourier conversion from raw intensity data to a radial distribution function (RDF) introduces some errors and a comparison on intensity curves is, therefore, more exact. The present calculations have indicated that an RDF obtained from an intensity curve can be exactly re-transformed to the original intensity curve, if the upper range of Fourier-integral is sufficiently high. This means that no information disappears or any other spurious information is not added in the process of transformation between intensity data and an RDF. On the other hand, direct calculation of intensity with the Debye equation inevitably introduces truncation errors, unless some approximations are used. When the comparison on intensity curves is requested, it is desirable to transform an RDF obtained from a structure model to an intensity curve. Some distortions introduced in RDFs are offset each other in an observed and calculated RDFs and usually need not to be considered in the pair function method. There is, however, another opinion that some distortions introduced in RDFs difuse the part of small atomic distances and a comparison in intensity curves is, therefore, necessary. The present calculations indicate that atomic pairs with small inter-atomic distances can be equally compared in RDFs as well as in intensity curves as for oxide, chalcogenide or halide glasses. In conclusion, the present calculations have proved that errors are not intermixed in transformation process between an intensity curve and an RDF and that the same level of assessment of the structural models is possible either in intensity curves or in RDFs, although the intensity comparison method by use of the Debye equation can not avoid the intermixture of truncation errors, unless some approximations are used.

View full abstract

The molar volume, optical basicity, X-ray fluorescence spectra of Al Kα and X-ray photoelectron spectra of O1s, B1s and Al2p core electrons were measured for alkaline earth aluminoborate (CaBAl and BaBAl) glasses as a function of molar ratio MO/(MO+B₂O₃)=X (M=Ca and Ba). Inflections and discontinuities in the property vs. X curves, for instance, in compositions containing 20mol% Al₂O₃ were discussed in terms of formation of [BO₄]^- and [AlO₄]^- units and partition of MO to B₂O₃ and Al₂O₃. Each of these anomalies was attributed to: the increase in the electron density on bridging oxygens (X=0.25), disappearance of AlO₆ units (X=0.35), formation of significant amount of non-bridging oxygen (X=0.40) and decomposition of the BO₄ units into O₂BO^- ones (X=0.44). On this basis, the triclusters were concluded to be absent in the aluminoborate glass network.

View full abstract

In order to elucidate the influence of TiO₂ on the structure of sodium silicate glasses and melts, a MD (molecular dynamics) simulation using the ionic pair potential function, Born-Mayer-Huggins form, has been performed for 0.25Na₂O⋅0.47SiO₂⋅0.28TiO₂ glass and the corresponding melt. The calculation pair correlation functions were in good agreement with a RDF (radial distribution function) curve obtained from the previous structural analysis by TOF (time-of-flight) pulsed neutron scattering. The computer-simulated configuration for the glass indicated that most of Ti ions were in 4-fold coordination, whilst the remainder were 6-fold coordination. The frequency spectrum of the glass obtained from MD simulation was in good agreement with the Raman spectra measured on glasses in the Na₂O-SiO₂-TiO₂ system. As a result, the weak Raman band near 700cm^-1 can be assigned to Ti ion in 6-fold coordination, and the strong Raman band near 900cm^-1 reflects a Ti-O stretching band, which is characteristic of TiO₄ tetrahedra corner-linked with SiO₄ tetrahedra. The Raman spectra for glasses and melts in Na₂O-SiO₂-TiO₂ system were measured by the high temperature Raman spectroscopic technique. A slight difference in the obtained Raman spectra was observed between the glasses and the corresponding melts, implying that the structure of glass should be similar to that of the corresponding melt.

View full abstract

The chemical structures of fluorine doped into two types of quartz glasses synthesized by CVD method were examined with a high resolution ¹⁹F-NMR spectrometer. Longitudinal relaxation times (T₁) of the fluorine in SiO₂: F glasses were seven times longer than those in the glasses doped with boron and fluorine. Though the difference of the chemical shifts was slight (-3ppm), fluorine nuclei in SiO₂: F glasses were shifted downfield from those in SiO₂: B: F glasses. Therefore, the environments of fluorine nuclei in the two types of glasses were considerably different. It was suggested that the fluorine was probably bonded to the boron in SiO₂: B: F glasses, while the fluorine was bonded to the silicon in SiO₂: F glasses.

View full abstract

The present study was concerned with the effect of “soft” inhomogeneities such as pores on the fracture process of brittle materials. Model inhomogeneous glass specimens were prepared by introducing a single groove or hole in flat plates of commercial soda lime silica glass. The specimens were pre-cracked and then loaded in a double torsion (DT) configuration using a constant cross-head speed. The crack was driven towards the artificial inhomogeneity in such a way that the direction of propagation is perpendicular to the groove or hole axis. Acoustic emission (AE) during the crack propagation was measured using the ring-down counting method. Successive changes in morphology of the crack front interacting with the inhomogeneity were observed by using TV camera and recorded on the video-tape. Fractographic observations were also made on fractured surfaces of each specimen using the Nomarski interference microscopy. Experimental results indicated that the groove or hole clearly disturbs the crack propagation process and can lead to local changes in the crack shape. It was confirmed that the crack-groove or crack-hole interaction process involves the initial attraction of a crack to the inhomogeneity and the subsequent impedance to crack extension. This impedance may be attributed to the localized crack blunting.

View full abstract

In order to obtain highly conductive glass-ceramics containing vanadium bronze, M_xV₂O₅ (M=alkali metal, etc), which is known to have high electronic conductivity, Na₂O-V₂O₅-P₂O₅ glasses were heat treated to perform crystallization. The dc conductivities of the glasses containing 60 to 77mol% V₂O₅, in which [Na₂O]/[V₂O₅] molar ratio was 1/6 to 1/3, were about 10^-4 to 10^-2ohm^-1cm^-1 at 150°C, and the activation energies were about 0.4eV. Exothermal peaks accompanied by weight losses were observed by TG-DTA in the temperature range from 250° to 300°C. Broad peaks of β-Na_xV₂O₅ were observed by X-ray diffraction after 2 to 5 h-heat treatment at 260° to 300°C. Glass blocks and disks of glass powders were heat-treated in two steps, at 300°C for 2h and at 500°C for 1 to 4h in air. After the heat treatment at 500°C, sharp diffraction peaks of β-Na_xV₂O₅ developed. The dc conductivity of the crystallized glasses was more than 3 orders of magnitude higher than that of as-cast glasses. The activation energy decreased to less than 0.1eV, similar to that of vanadium bronze. The microstructure of the crystallized glasses was observed by SEM and optical microscopy.

View full abstract

Sintered silicon nitride ceramics were joined with CaO-SiO₂-TiO₂ glass solder without applying pressure. The properties of the glass solders and Si₃N₄-glass reactions were studied in detail. The bond strength obtained from the Si₃N₄ joints was strong enough for practical applications.

View full abstract

ZrO₂-3mol% Y₂O₃ was rapidly quenched from melts, and annealed at 1700°C for 48h in air. X-ray diffraction analysis and TEM observation revealed that the tetragonal phase in as-quenched samples was nontransformable and the phase in annealed samples was transformable under external streesses. The tetragonal phase in as-quenched samples was formed by diffusionless transformation of the cubic phase. Upon annealing, the microstructures did not change discontinuously, but the tetragonal twins coarsened.

View full abstract

Firing of zirconia (Y₂O₃: 3mol%) powder-carbon fiber compact in air at 1500°-1600°C gave a zirconia ceramics with straight channels of uniform diameter.

View full abstract

Alumina ceramics, the most universal modern ceramic materials, generally require a grinding process. There are so many kinds of alumina ceramics of which proper grinding conditions have not yet been established. In this study, test pieces of alumina ceramics were made by sintering at various temperatures and ground by a diamond grinding wheel in order to investigate the relation between the microstructure and the grindability. The results showed that alumina ceramics with 4μm-grain were the most difficult to grind because of the maximum toughness and that those of larger grains were easy to grind because of the ease of the chip removal.

View full abstract

Electronic and structural properties of SiC powders and whiskers are characterized by means of Raman spectroscopy from the microscopic point of view. The Raman spectroscopy is a very convenient method to determine the polytype of SiC particles. A high density of free carriers with a very short mean free path is present in SiC powder made from polycarbosilane and commercial SiC whiskers. An appreciable amount of strain may exist in SiC whiskers produced commercially.

View full abstract

β-SiC whiskers were grown at about 1300°C by the reaction of silicon powder and propylene in a flowing hydrogen atmosphere containing a few percent of hydrogen sulfide. The whiskers with the growth direction [111] were examined by TEM and SEM. Stacking faults observed in the whisker are classified into two types; one has the (111) fault plane perpendicular to the growth direction and the other has fault planes parallel to the (111), (111) and (111) which are not perpendicular to the growth direction. It is likely that these two types of faults have a correlation with the difference in morphology of whiskers. Their fault vector was shown to be 1/3‹111› by the g⋅b=0 criterion.

View full abstract

Si₃N₄ composites reinforced with SiC whiskers were fabricated using injection molding and HIP treatment. The strength and fracture toughness were significantly improved over monolithic Si₃N₄. Microstructure of composites before and after HIP treatment was examined by transmission electron microscopy. No reaction layer was observed between whiskers and matrices. HIP treatment eliminated voids at the interface. The interfacial bonding was excellent and no pull-out of whiskers occurred on fracture. Factors controlling the improvement of properties are discussed based on the microstructural analysis.

View full abstract

A MgO-monticellite (CaMgSiO₄) ceramic has been observed with high resolution transmission electron microscopy (HR-TEM) and scanning transmission electron microscopy (STEM). Two samples were quenched at different rates, yielding different microstructures. The quenched sample (1.5×10⁴°C/s) exhibits complete wetting of the grain boundaries by the liquid phase. The rapidly cooled sample (120°C/s) has monticellite or forsterite crystals growing from the MgO grains into the liquid phase. The growing crystallites are magnesium rich relative to the liquid composition. They also appear to grow rapidly with a columnar microstructure.

View full abstract

Preferred orientation and microstructure of MgO films prepared by RF-sputtering have been investigated. Transparent MgO films (1μm in thickness) were obtained with a deposition rate of 5-9nm/min in Ar atmospheres (1.33-13.3Pa) at 150W (RF power). The (200) orientation was confirmed under the various conditions of 6.66-13.3Pa and 15°-250°C at 150W. In the present study, (200) orientation was strongly dependent on the gas pressure rather than on the substrate temperature. A rounded-grain (isotropic) texture was observed on the surface of the MgO film with (200) orientation in contrast to the angular-grain (anisotropic) texture of the film without orientation. The rounded grain (below 300nm in diameter) in the MgO films was found to be consisted of small crystallites about 10nm by X-ray diffraction and TEM observation.

View full abstract

Ceramic coatings of MgO⋅Al₂O₃ and ZrO₂ (8wt% Y₂O₃) sprayed by water plasma were examined. They showed rough texture with especially large grains, large pores of over 20μm and layered structure resulting from excessive fusion of powder materials. On the other hand, ceramic coatings of the same chemical composition sprayed by gas plasma had relatively homogeneous texture with grains of about 30μm and pores of 5-10μm. Results of thermal spalling tests showed that it was necessary to have a microstructure with fine grains and fine pores distributed homogeneously in order to decrease the exfoliation due to temperature change. Two kinds of powders in the system Al₂O₃-Cr₂O₃ were applied to gas plasma spraying in order to study the effect of powder materials on the properties of sprayed coatings. The sprayed coating with an electrofused powder was a solid solution, while the sprayed coating with a mixed powder of Al₂O₃ and Cr₂O₃ consisted of layered structure of γ-Al₂O₃ and Cr₂O₃ crystals. The solid solution coating seemed to be more resistant to exfoliation, since AE counts of the coating with the electrofused powder at the temperature change were one tenth of those with the mixed powder.

View full abstract

Precipitation of mullite from the spherical kaolinite fired at 900°-1400°C has been studied by means of electron microscopy and powder X-ray diffractometry. The starting kaolinite which has spherical shape with concentric layering has been synthesized under hydrothermal condition. The following results were obtained:
(1) The reaction sequence was observed as: metakaolinite→γ-Al₂O₃+SiO₂ (amorph.)→Al rich mullite+SiO₂ (amorph.)→3:2 mullite+cristobalite. (2) The initially formed mullite was Al-rich and a and b axes were almost the same as those of tetragonal mullite. (3) The morphology of mullite grains changed from elliptical through lath-like to prismatic shape. Acicular or needle-like mullite grains which have been reported on the fired platy kaolinite were rarely observed. (4) Some prismatic mullite grains were formed along the relict surface of spherical kaolinite. However, no mullite with 120° configuration were observed as reported on a hexagonal platy kaolinite. (5) These differences in morphology and orientation of mullite were attributed to the difference in lattice deformation of the starting kaolinite structures.

View full abstract

Aluminum-i-propoxide, Al (Oi-C₃H₇)₃, was dissolved in benzene and its solution was freeze-dried. Scanning electron microscopy of resultant powder revealed a unique microstructure compared with that of salt derived from an Al sulfate aqueous solution; its microstructure was thought to be apparently honeycomb type. It was then suggested that this structure, with its large surface area, is effective in releasing gas during thermal decomposition, and that the powder could therefore decompose at lower temperature. Thermal analysis also showed that the transformation temperature of γ- to α-alumina was about 50°C lower in this powder than that in alumina derived from sulfate. Finally, the alumina powder obtained by freeze-drying of Al-i-propoxide is expected to be sinterable.

View full abstract

The influence of thermal stress in joints, which is induced during cooling by the difference in thermal expansion coefficient between the joined brittle materials, on the failure probabilities of the joints was estimated. A numerical analysis by the finite-element method was carried out on SiC/Si₃N₄ joints. It was assumed that the strength of each material obeys the Weibull distribution function and that the joining strength is high enough to prevent rupture at the joining interface. Since the tensile stress develops predominantly at the side of SiC which has a larger thermal expansion coefficient than that of Si₃N₄, the Weibull modulus of SiC was taken into consideration for calculating the failure probability of the joint. The dimension of the joint and the Weibull modulus of SiC member were shown to have great influence on the failure probability. Therefore, these two factors must be taken into consideration in addition to the fracture strength of material to be joined for the evaluation of the joining strength of various brittle materials.

View full abstract