Reversible thermochromic coloration was detected in BiO1.5-CdO-AlO1.5 system glasses. The relevant parameters were strongly dependent on the BiO1.5 content. It was suggested that the weakened and more polarized Bi-O bond due to increased BiO1.5 content enhanced the thermochromism of the glasses. The redshift of absorption edges at room temperature and the increase in the temperature coefficient of the shift with increasing BiO1.5 content were analyzed. The thermochromic mechanism was explained in terms of the temperature dependence of the optical energy gap caused mainly by electron-phonon interaction.
Effects of CaO and Al2O3 on the reaction between MgO and C have been investigated and corresponding mechanisms have been proposed. A small amount (within the solubility limit) of Al2O3 slightly inhibited the MgO-C reaction, but a small amount (within the solubility limit) of CaO had little effect on the MgO-C reaction. Large amounts (above the solubility limits) of Al2O3 or CaO accelerated the MgO-C reaction.
The effect of a rising R-curve behavior on the high-temperature lifetime and flaw-tolerance of Si3N4 ceramics has been systematically investigated. Extensive mechanical testing was pursued at 1400°C, typical temperature at which the selected materials fractured under the elastic slow-crack-growth (SCG) regime (i.e., with negligible plastic strain). Hot-isostatically pressed (HIP) Si3N4, its WC-platelet, SiC-platelet and SiC particulate composites were selected for this basic investigation. All the composites investigated contained the same volume fraction (i.e., 25%) of reinforcing phase and were fully dense after the HIP process without external addition of densification agents. This latter important processing circumstance has made available materials which, besides having the same grain-boundary structure and, hence, the same intrinsic thermo-mechanical response of the grain boundaries at high temperature, showed different R-curve behaviors. It was systematically found that the rising R-curve, especially pronounced in composites reinforced with high-aspect-ratio platelets, provided a positive effect on lifetime and flaw-tolerance. From the theoretical side, both the lifetime elongation and the improved flaw-tolerance were well explained by an algorithm incorporating into the mechanical driving force acting on the crack tip, the closure-field contribution due to the rising R-curve behavior of the material.
Ti-Al-C ternary composites were produced by combustion synthesis using titanium, aluminum and graphite powders. Microstructure observations, X-ray diffraction experiments and micro Vickers hardness test were performed for the obtained porous specimens. With low carbon contents, a few intermetallic compounds of titanium and aluminum were obtained. Ti4Al2C2 and TiC phases were produced as the carbon content increased. It is considered that formation of these phases depends on the quantity of heat generated during combustion synthesis. It was suggested that Ti4Al2C2 phase was formed by direct reaction between TiC and Ti-Al system intermetallic compounds. Ti4Al2C2 phase indicated the same hardness value (about 4.5GPa) as γ-TiAl phase. As the carbon content increased, the hardness value increased remarkably because of dispersion strengthening by TiC particles. The maximum hardness was about 8.6GPa. Microstructure observations for low carbon contents showed that large TiAl3 particles were covered with γ-TiAl particles with a grain size of approximately 1μm. On the other hand, for high carbon contents, it was found that microcomposites which TiC particles with a grain size of approximately 1μm were dispersed homogeneously in the Ti4Al2C2 phase with rod-like structures. This microcomposite will be expected to have more excellent elevated temperature strength than that of TiAl intermetallic compounds.
The formation process of the 2223 phase synthesis via conventional calcining condition has been systematically investigated using XRD, SEM/EDX, TG/DTA and TMA. The calcining powder prepared at 1073K for 18ks consists of 2201 phase, 2212 phase, Ca2PbO4, CaO, CuO and Sr14-xCaxCu24O41. With increasing a holding time in firing process at 1133K, 2212 phase, Ca2PbO4 and Sr14-xCaxCu24O41 did decrease, and 2223 phase, Ca2-xSrxCuO3 and Bi0.5Pb3Sr2.5Ca2CuOx increased. It was considered that 2223 phase, Ca2-xSrxCuO3 and Bi0.5Pb3Sr2.5Ca2CuOx were formed by reaction with 2212 phase, Ca2PbO4 and Sr14-xCaxCu24O41. The liquid phase was formed from 1113 to 1133K in firing process, and then Sr14-xCaxCu24O41 was drastically formed reaction with the liquid phase and CuO. The reason that needs long term-heat treatment to synthesize 2223 phase with conventional calcining condition is regarded as formation of Sr14-xCaxCu24O41.
Semiconducting ceramics of p- and n-type bismuth telluride fabricated from the milled powders of (Bi2Te3)0.25(Sb2Te3)0.75+Te 2.0mass% and (Bi2Te3)0.95(Bi2Se3)0.05+HgBr2 0.07mass% with low impurity oxygen having figures of merit were as large as 2.6×10-3K-1 (p-type) and 2.0×10-3K-1 (n-type) at 25°C, respectively, have been developed by hot press. The figure of merit showed a maximun when the grain size was 5 and 10μm because thermal conductivity apparently became the lowest possibly due to enhanced phonon scattering at grain boundaries. The compressive strength of p- and n-type obeyed Hall-Petch type which was directly proportional to (-1/2) the power of grain size, and showed 8.8 and 6.1MPa whose figure of merit was maximum, respectively.
The grindability of various silicon nitride ceramics ground using different diamond grinding wheels was evaluated in terms of specific grinding energy and strength degradation. Main results are summarized below. (1) Specific grinding energy increases with increasing hardness of the workpiece material using a grinding wheel of small grain size under the condition of small wheel depth of cut. (2) Strength degradation increases with a grinding wheel of large grain size. Degradation increases with a smaller material constant of the critical indentation radius: E⋅Kc2/Hv3 (E: Young's modulus, K1c: fracture toughness, Hv: Vickers hardness). (3) The degradation increases with increasing mean value of maximum grain depth of cut. The strength in the region of a large mean value of maximum grain depth of cut decreases with a larger material constant of crack extension length: Hv1/4/Kc.
The article describes the indentation-induced surface damage and cracks in ceramic materials under quasi-static loading. Three different materials, Al2O3, Si3N4 and SiC were prepared. The polished surface was indented with a conical diamond indenter. After indentation, the indented surface was examined and analysed. It is confirmed from the present experiments that the initiation and growth of indentation-induced cracks follow the Weibull statistics. The loading condition influences on the depth and diameter of indentation. The mean crack length around indentation can be expressed by an experimental formula.
Nine kinds of spraying solutions with a Ca/P ratio of 1.67 were prepared by mixing calcium sources (Ca(NO3)2, CaCl2 and Ca(CH3COO)2) and phosphorus sources ((NH4)2HPO4, H3PO4 and PO(OCH3)3). Apatites powders were obtained by spray-pyrolysing the resulting solutions at 600°C; some properties of the obtained powders were examined. The stoichiometric hydroxyapatite (HAp) was obtained by spraying the solutions containing Ca(NO3)2-(NH4)2HPO4 and Ca(NO3)2-H3PO4 systems. As the solutions containing CaCl2 were sprayed, chloroapatite (CAp) and Ca2PO4Cl were formed; Ca2PO4Cl was changed into CAp during heating at 600°C. The carbonate-containing HAp was formed when the solutions containing Ca(CH3COO)2-(NH2)HPO4 and Ca(CH3COO)2-H3PO4 systems were sprayed. When PO(OCH3)3 was used as a phosphorus source, the Ca/P ratio of the apatites was below 1.67. The powder derived from Ca(CH3COO)2-(NH4)2HPO4 system showed the best sinterability among the powders examined; the relative density of the sintered body fired at 1150°C for 5h reached-96%.
Immersion liquid technique was quantitatively evaluated by measuring the size and amount of artificial pores in ceramic green bodies. A fixed amount of artificial pores with various known sizes was introduced into green bodies of various particle sizes using standard polymer microspheres, and then the size and amount of the artificial pores were examined by the immersion liquid technique. The artificial pores ranging from 5-40μm could be observed clearly. By an appropriate choice of thickness of sample and refractive index of immersion liquid in order to suit for the size of the particles and artificial pores, the size and amount of artificial pores were accurately evaluated. Thicker samples can be quantitatively evaluated when the particle size is smaller and index of immersion liquid is closer to that of powder.
Ultrasonic spray-pyrolysis method was applied to the preparation of fine particles of solid solution in the system ZnS-CdS. An aqueous solution of zinc nitrate, cadmium nitrate and thiourea was used as a starting solution. Morphology and crystallinity of the produced particles were examined as a function of preparation conditions and their composition. Spherical particles of a hexagonal phase were obtained by the pyrolysis at around 700°C. The size of the obtained particles was distributed in the range from 0.3 to 1.5μm; each particle was found to consist of much finer crystallites. Lattice constants of the particles varied linearly with the composition, showing that solid solution particles were formed in the whole composition range of ZnS-CdS.
New layered perovskite compounds, LiLnTiO4 (Ln=La and Eu), have been synthesized by an ion-exchange reaction from parent sodium compounds, NaLnTiO4. The crystal structures of LiLaTiO4 and LiEuTiO4 were determined by Rietveld analysis of the powder X-ray diffraction (XRD) patterns. The lithium ion-exchanged compounds retain the crystal symmetry of the parent phases. Although the framework of the perovskite layer of these compounds is the same as that of the parent compounds, there exists a fundamental difference. The lithium ions within the interlayer have a tetrahedral coordination because of small ionic radius for a rock-salt coordination. The structure of LiLaTiO4 and LiEuTiO4 consist of intergrowths of the perovskite sheets with the layers of Li-O in a tetrahedral coordination and those of Ln-O in a distorted rock-salt coordination. These compounds exhibit new phases analogous to the Ruddlesden-Popper phase.
Aluminum nitride (AlN) was prepared by pyrolysis of poly (isopropyliminoalane) under Ar and NH3/N2 atmospheres. X-ray powder diffraction analysis revealed that the only crystalline phase was AlN, whose lattice constants showed that possible amounts of carbon and oxygen in the lattice should be very small. Chemical analyses showed the presence of carbon only in the product pyrolyzed in Ar; almost all carbon was removed by pyrolysis in NH3/N2. The ceramic yield for two-step pyrolysis in Ar (1000°C for 2h, 1600°C for 2h) was 32%, while that for pyrolysis in NH3/N2 (600°C for 2h in NH3, 1350°C for 8h in N2) was 47%.
The hydration and hardening characteristic of materials in the system CaO-MgO-SiO2 have been investigated by XRD, SEM, FT-IR and calorimetry. 2CaO⋅MgO⋅2SiO2 glass hydrated and calcium silicate hydrate gels (C-S-H) were formed after 90min of curing. Diopside did not hydrate. The compressive strength of the hardened cement (at 37°C for 6h) was 27MPa. The setting time for the crystalline sample in ammonium phosphate solution was 4-6min and that for the glass sample was less than 3min. It is expected that CaO-MgO-SiO2 cement is useful as a biomaterial.
Polycrystalline isotropic perovskite-like La-AM-Mn-O compounds (AM=K, Rb) with trivalent La3+ partially substituted for pseudo-divalent cations (a half of trivalent La3+ plus a half of monovalent K+ or Rb+) were fabricated by conventional ceramic techniques, and their magnetoresistance effects and magnetic/electric properties were studied. All studied samples show ferromagnetism and high electrical conductivity at low temperature of 77K. As sintering temperature increases, Curie temperature, saturation magnetization and resistivity decrease. Polycrystalline isotropic La-K-Mn-O sample shows a large negative magnetoresistance effect of -25% at H=1.5T and T=280K. This value is comparable to the value previously reported in La1-xMexMnO3 (Me=divalent cation) single crystal.
Two magnesia ceramics were joined by the microwave heating method. The joining was easily accomplished in a short time by covering the joining part with a mullite insulating fire brick during microwave irradiation. The bending strength of the joint increased with increasing joining temperature and pressure. The hardness of the joint was higher than that of the matrix. The maximum bending strength of the sample joined at 2150K and 0.5MPa for 4min was 105MPa and was about 70% that of the matrix.