3-Dimensional Tyranno fiber reinforced SiC composites were fabricated by infiltration of molten polycarbosilane into a 3-dimensional textile preform (volume fraction: 39%) in an autoclave followed by pyrolysis in an argon atmosphere up to 1200°C. An attempt was made to improve the density of the composite by repeating the infiltration and pyrolysis process. Ten times repetition increased the density of the composite to 2.4×10³kg·m^-3 (relative density: 83%, open porosity: 5%). The bending strength and fracture toughness of the composite were 420 MPa and 11.5 MPa·m^1/2, respectively. The composite showed semistable fracture behavior because of bridging of fibers parallel to the tensile direction.

View full abstract

Aluminium nitride (AIN) powders with 0-1.2 mass% of carbon and 5 mass% of Y₂O₃ addition were sintered at 2023 and 2073K under a nitrogen atmosphere, and the sintered samples were characterized by their oxygen content, composition of the secondary phase, microstructure, density, and thermal conductivity. With an increase of carbon addition, the oxygen content decreased linearly and the secondary phase changed from aluminium-rich to yttrium-rich. The microstructure, density and thermal conductivity of the sintered samples were also found to depend strongly on the oxygen content. The correlation between the oxygen content and the sintering behavior of AIN with Y₂O₃ sintering aid has been discussed.

View full abstract

The humidity sensitive electrical properties of ZnO dense sintered compacts with two, one, or no ohmic contact are reported. Sintered ZnO discs were prepared by traditional ceramic processing. Ohmic contacts were obtained with Al electrodes. Non-ohmic contacts were made by mechanically pressing Au sheets on the uncoated sides of sintered discs. Dark current-voltage (I-V) characteristics of ZnO pellets were measured at different relative humidity (rh) values and at room temperature. The I-V curves for pellets with one ohmic contact showed a diode behaviour, with a rectifying character, attributed to the presence of a Schottky barrier. The rectifying characteristics of ZnO pellets were enhanced in the presence of humidity, and the rh-sensitivity was bias-dependent. The rh-sensitivity of dense ZnO pellets with a Schottky barrier can be related to the variation of the barrier height due to the presence of surface states. Their behaviour resembles that of p-n junctions, with a sensing mechanism which is different from those already proposed in the literature for ceramic materials.

View full abstract

The Curie temperature and the maximum dielectric constants increase as the amount of additives increases in PMN-based ferroelectric ceramics. The T_c of PMN, 0.9MN-0.1PT, and 0.8PMN-0.2PZT is ca. 0°C, 40°C and 65°C, respectively. Bipolar-induced strains rarely change with temperature in the ferroelectric phase below T_c. The plots of polarization vs. electric field are consistent with the S-E plots. The coercive field and remanent strain of PMN-based materials increase as the temperature decreases below T_c. Consequently, unipolarly induced strains become smaller than bipolarly induced strains as the temperature decreases. The temperature of maximum strain is lower than T_c by ca. 30°C in 0.8PMN-0.2PZT and 0.9PMN-0.1PT. The temperature of maximum strain is ca. 10°C and 30°C, in 0.9PMN-0.1PT (T_c=40°C) and 0.8PMN-0.2PZT (T_c=65°C), respectively, and is expected to be -30°C in PMN (T_c=0°C).

View full abstract

The synthesis process and the formation mechanism of Al₄SiC₄ were investigated using Al, Si, and C as starting materials. Properties such as hydration resistance of the synthesized Al₄SiC₄ were examined. SiC and Al₄C₃ begin to form from about 800°C and 900°C, respectively. When the temperature is above 1300°C, both products further react with each other to form Al₄SiC₄. With increasing temperature and time, the formation ratio of Al₄SiC₄ increases. The addition of Al₂O₃ can facilitate the formation of Al₄SiC₄. The hydration resistance of Al₄SiC₄ is excellent and sufficient for the practical use.

View full abstract

Many metals show the steep increase of their specific resistivity on melting. This phenomenon was applied for producing new PTCR materials in the Pb-TiO₂ and Sn-TiO₂ systems. The xPb-(100-x)Ti_0.99Nb_0.01O₂ samples with x>15 showed the steep increase of resistivity (PTCR properties) around 327°C, and it was noted that the temperature coefficient of resistivity below the PTCR temperature changes from negative to positive with increasing Pb content. The xSn-(100-x)Ti_0.99Nb_0.01O₂ samples showed PTCR properties around the melting point of Sn (232°C), but PTCR properties are less significant than those observed in the Pb-TiO₂ system.

View full abstract

Fe₂O₃-Sb₂O₃-TeO₂ glasses were prepared by the pressquenching method and their do conductivities were measured. The glass formation region was found to be 0≤Fe₂O₃≤15mot%, 0≤Sb₂O₃≤18mol%, and 78≤TeO₂<100mol%. Seebeck coefficient measurements showed that these glasses were n-type semiconductors. The glasses gave conductivities σ ranging from 1.62×10^-6 to 1.86×10^-7 S·cm^-1 at 473K. The dc conductivity increased with an increase in Fe₂O₃ content. Evaluated carrier mobility and concentration ranged from 7.5×10^-10 to 5.3×10^-3cm²·V^-1·s^-1 and from 1.5×10²¹ to 1.9×10¹⁵cm^-3 at 473K, respectively. The conduction of these glasses was due to non-adiabatic small polaron hopping.

View full abstract

The temperature dependence of piezoelectric constant (d₃₁) and the strain characteristics of the (Pb_0.845Ba_0.105Sr_0.05) ((Zr_xTi_1-x)_0.95Sn_0.045Nb_0.005)O₃ series (x=0.49, 0.52, 0.55), in which the grain size was controlled by hot-pressing (HP), were investigated between -196 and 90°C. The piezoelectric constant (d₃₁) was measured using an impedance analyzer (YHP4192A). The strain characteristics were measured by applying DC electric field (1.0kV/mm) and the durability under repetitious DC electric field (1.0kV/mm, 1Hz). The results showed that the piezoelectric constant (d₃₁) and the actually found strain (δ) under DC electric field increase with increasing temperature. Between -196 and 90°C, the actually found strain (δ) was larger than calculated value (δ_cal), obtained by multiplying the d₃₁ with DC electric field. This difference (δ-δ_cal) was caused by the strain (δ_p) of domainial reorientation alone the intensity of electric field. It is found that the δ_p was nearly zero at -196°C (liquid nitrogen temperature), and increases with increasing temperature. Furthermore, it was clarified that the temperature dependence of δ_p varied with the composition and grain size. It is shown that the temperature dependence of δ_p is related to that of the binding force of domain (σ_b). In the durability test between -196 to 90°C, the δ became small after the test. The change of δ increased with increasing temperature. It is found that the decrease of δ_p led to that of δ. We found that the change of δ_p is dependent on the composition and grain size.

View full abstract

To evaluate the wettability of phosphate ceramics by water, contact angles of hydroxyapatite-tricalcium phosphate composite ceramics were measured by the sessile drop and adhesion tension methods. It was difficult to obtain an equilibrium contact angle by the sessile drop method. On the other hand, reproducible values were obtained by the adhesion tension method if the sample was ground to less than 0.6mm in thickness. The contact angles of hydroxyapatite and β-tricalcium phosphate ceramics were 30.7° and 23.5°, respectively. The contact angle of the composite ceramics obtained by mixing of hydroxyapatite with tricalcium phosphate increased suggesting that the hydrophilicity of the surface decreased. The wettability of the composite ceramics increased with increasing surface hydroxyl group density measured by X-ray photoelectron spectroscopy.

View full abstract

A. C. electrical properties of defect-perovskite LaTa₃O₉ were measured in the temperature range between 423 and 1073 K under oxygen partial pressures (P_o2) from 10Pa to 0.1 MPa. Complex impedance spectra showed three dispersions associated with bulk, surface layer and a diffusion (Warburg) process. The conductivity of bulk was independent of P_o2 and its activation energy. was 0.40 to 0.48 eV. The capacitance of bulk was 5 to 8 pF and independent of temperature. The conductivity of surface layer (σ₂) increased with increasing P_o2, as expressed by σ₂∝P_o2ⁿ (n=0.14 to 0.18). The capacitance of surface layer increased from 3 to 20 nF and increased with increasing temperature. The ionic transport number measured with an oxygen concentration cell was nearly 1 at 750 K, and decreased with increasing temperature.

View full abstract

Electrophoretic coating by a constant current deposition method (CCDM) was conducted to form alumina insulation layers on the tungsten double helical filaments of a CRT heater. Alumina particles (average diameter of about 4μm) were dispersed in an ethanol-water suspension containing dehydrated Al(NO₃)₃+Mg(NO₃)₂⋅6H₂O. Positively charged alumina particles in the suspension (43 mass% solid) were deposited on tungsten double helical filaments by passing a direct current from 0.015 to 0.32A. The thickness and packing ratio of Al₂O₃ film formed within the spaces between the coils of the double helical filaments were measured after deposition under various CCDM conditions. A gel-like substance with a uniform thickness was deposited on the overall surface of the electrode of the tungsten helical heater by passing the constant current. Better current conditions of the CCDM for achieving Al₂O₃ packing ratio above 55% were 0.05 A<I<0.12A. The thickness of the Al₂O₃ film did not reach 75μm because a long range deposition time for the CCDM caused surface roughness above 20μm. Use of a double-step CCDM for a practical process was examined to give an Al₂O₃ film thickness over 100μm and Al₂O₃ packing ratio of over 55%. A typical condition for double-step CCDM was I=0.08A for 10s as the first step and I=0.5A for 3.0s as the second step.

View full abstract

The effects of IV group element (Zr, Ti, Ge, Si) oxides addition on the grain growth of SnO₂ thin films at elevated temperature (600-1000°C) were investigated. It was found that addition of 10mol% GeO₂ controlled the grain growth of SnO₂ effectively without changing the resistivity. Addition of GeO₂ decreased the response time for 2-methyl-2-butene by the formation of SnO₂ thin films with uniform crystallite size.

View full abstract

Amorphous regions and thermal stability of sputtered films in the SiO₂-CaO system were determined. The results are summarized as follows. (1) The amorphous region in the SiO₂-CaO system by the sputtering method (100W, 25h) was found in the composition range CaO=0-89mol%, which is slightly larger than that of the SiO₂-MgO system. (2) Crystallization temperature T_C was determined from the DTA measurements of the sputtered amorphous films and melt-quenched glasses. (3) Thermal stability of the amorphous films and glasses was evaluated from the T_C/T_L ratio. T_C/T_L is highest 0.8 at CaO=0mol%. It passes maxima and minima as CaO content is increased and reaches the lowest value 0.44 at CaO=71.5mol%. Melt-quenched glasses showed similar compositional dependence of T_C/T_L ratio to that of sputtered films. (4) According to the empirical relationship between T_C/T_L and critical cooling rate for glass formation Q*, the T_C/T_L ratio of 0.44 corresponds to Q*=10^8.5°C/s which is well within the estimated quenching rate by the sputtering method.

View full abstract

The effect of methanol on the morphology of hydroxyapatite (HAp) crystals has been studied under hydrothermal conditions. A precursor HAp slurry prepared from CaCO₃ and CaHPO₄⋅2H₂O was synthesized hydrothermally in the presence of methanol. The products obtained from slurries containing varying amounts of methanol were found to be carbonate-containing HAp by X-ray diffraction and IR spectrometry. TEM showed that the morphology of the products was influenced by the amount of methanol added to the slurry. The products obtained from slurries without methanol were rod-like or granular crystals, about 20 to 100nm in size. Addition of methanol to the slurries caused an increase in the ratio of plate-like crystals to rod-like crystals or granular crystals. When the weight of methanol added was equal to the weight of the slurry, only plate-like crystals 20 to 200nm in size were obtained. X-ray diffraction patterns of oriented products indicated that the plane grown selectively was the a-plane of the plate-like crystals. The results by DTA suggested that the variation of the morphology can be attributed to the adsorption of methanol on the surface of the HAp nuclei inhibiting the crystal growth along a particular axis.

View full abstract

Surface morphology of MoO₃ was observed with an atomic force microscope. The array of (MoO₆)^6- octahedra was clearly observable for the oxidized surface of a thin crystal prepared by the evaporation-condensation process. Similar high resolution AFM image was also obtained for the powder sample which was coated and dried on the substrate using an ethanol suspension. The high resolution image of the (MoO₆)^6- array disappeared after heating the thin crystal in reducing atmosphere at 350°C, but restored after reoxidation in air at 500°C. Vacuum deposited MoO₃ film on silica glass was amorphous and its surface was smooth and flat after heating at or below 300°C. The film crystallized on heating at 400°C accompanying a slight microstructural change at the film surface. This film exhibited sharp diffraction lines from (0k0) planes. The thin film morphology was destroyed and changed into randomly oriented particles after heating at 500°C.

View full abstract

Thick silica films were prepared on a stainless steel sheet by the electrophoretic sol-gel deposition method in the presence of sodium dodecyl sulfate (SDS). Electrophoretic deposition was carried out for the silica sols prepared from tetraethoxysilane under the basic conditions. SDS was added as a dispersant in sol preparation. While deposited films were peeled off during drying process in the absence of SDS, thick films were prepared in the presence of SDS; the maximum thickness of the films was about 20μm at an applied voltage of 30V and an SDS content of 0.05 mass%.

View full abstract

ZrO₂-MgO-Al₂O₃ powder composites were prepared by calcining gelled mixtures of Mg(OH)₂, Zr-n-butoxide and Al-sec-butoxide (or Al₂O₃) at 1000°C for 2h. The calcined powder was cold isostatic pressed at 200MPa and sintered at 1500 to 1650°C for 1h. Fine and homogeneous tetragonal-ZrO₂ grains were dispersed in the sintered composite ceramics. ZrO₂-dispersed MgO-Al₂O₃ ceramics had higher bending strength and fracture toughness than ZrO₂-free MgO-Al₂O₃ ceramics due to the formation of finer spinel grains and stress-induced-transformation of the dispersed tetragonal-ZrO₂.

View full abstract

A new etching method using organic solvents instead of acids was developed for ZnO films as well as MgO films. ZnO films prepared by normal pressure chemical vapor deposition (NP-CVD) and R. F.-magnetron sputtering were etched at 30-60°C in a methanol solution of acetylacetone (2, 4-pentandione). The etching rate of the film prepared by R. F.-magnetron sputtering was smaller than that prepared by NP-CVD. The activation energy of etching for the film prepared by R. F.-magnetron sputtering was 59kJ·mol^-1.

View full abstract

Semiconductive carbon fibers, materials for infrared sensors, were prepared with final heat treatments at 470 to 800°C in pure N₂ (O₂ concentration<10^-10%), and in O₂-mixed N₂ (O₂ concentration=0.2%). Changes in electrical resistivity and thermistor constant by applying high-voltage pulses (about 10²-10⁴ V during 20-100μs) were measured on those carbon fibers. Decreases in resistivity and thermistor constant by the pulses were observed only in carbon fibers heat treated in O₂-mixed N₂. Plots of resistivity versus thermistor constant for fibers treated in pure N₂ at various temperatures gave a straight line, and data for pulseapplied fibers treated in O₂-mixed N₂ were confirmed to lie on the same line. It is assumed that the effect of resistivity decreases by high-voltage pulse application has a close relation to the erosion on surface of carbon fibers caused by O₂ attack when heat-treated.

View full abstract

Zeolite A (NH₄-A) exchanged with NH₄⁺ was used as a precursor of mullite. NH₄-A was dehydrated and deammoniated by heating up to 300°C and it entered an amorphous state. Mullite crystallized directly from the amorphous phase at above 950°C without forming a spinel phase as an intermediate phase. Since the starting material is zeolite A, which has the tetrahedral framework structure with homogeneous arrangement of Si, Al, and O atoms, these experimental results supported the previous hypothesis proposed for the formation process of mullite.

View full abstract

Electrostrictive behavior responding to reducing gas (CO, H₂) atmospheres was investigated on a piezoelectric actuator consisting of a dense PZT plate (Pb_0.995 Nb_0.01(Zr_0.53Ti_0.47)_0.99O₃) and a porous ZnO plate (ZnO+0.2mol% Li₂O+10mol% NiO). The dense PZT and press-formed ZnO disks were bonded by hot-pressing using glass frits. The PZT side of the actuator showed a shrinkage by a d.c. bias at 250°C, and its value was larger in reducing gas atmosphere than in air. When measured on the separated plates, the PZT and ZnO plates had high resistivities in the same level in air but only the ZnO showed a decrease in resistivity by introduction of reducing gas. It is assumed that the applied voltage is distributed to both plates in air, but concentrated to the PZT part in reducing gas atmospheres due to a decrease in resistance of the ZnO part. The bending displacement of the actuator was also observed in reducing gas atmosphere, although the precise value was not obtained due to the high temperature in measurements.

View full abstract