Keiji Daimon

Porous ceramics prepared by mimicking silicified wood

Abstract Porous titania, alumina and zirconia ceramic woods with wood-like microstructures, analogous to that of silicified wood, were prepared from natural woods as templates. The production of these ceramic woods was performed by the following process: (1) infiltration of metal alkoxide into wood specimens, (2) hydrolysis of the alkoxide in the cell structure to form titania, alumina or zirconia gels, (3) firing in air to form titania, alumina or zirconia ceramic woods. The resulting titania, alumina and zirconia ceramic woods were studied by means of an X-ray diffractometer, a mercury porosimeter and a scanning electron microscope. The microstructure of these ceramic woods retained the same structure as that of the raw wood: with the pore sizes corresponding to those of the original wood, and the major pores being unidirectionally connected.

Thermal and mechanical properties of sintered LaPO4-Al2O3 composites

Abstract Mixtures of (1 − x)LaPO 4 and xAl 2 O 3 (x = 0 to 1 mass) were dry-pressed to disks or bars. Relative densities larger than 94% and apparent porosities less the 6% were achieved when the specimens were sintered at 1600°C for 5 hours in air. The sintered ceramics (x = 0 to 0.7) were found to be machinable — they could be drilled easily using conventional metallic WC drills. The thermal and mechanical properties of the sintered (1 − x)LaPO 4 - xAl 2 O 3 composites had the following ranges: 10 × 10 −6 /°C (x = 0) to 9.0 × 10 −6 /°C(x = 1) (linear thermal expansion coefficient at 200°–1000°C); 5.0 (x = 0) to 43 W/(m·K)(x = 1) (thermal conductivity at 25°C); 140±19 (x=0) to 352±23 MPa (x = 1)(bending strength at room temperature); 5.7 (x = 0) to 16.5 GPa (x = 1) (Vickers hardness); and 155 (x = 0) to 400 GPa (x = 1) (Young’s modulus).

Morphology of corundum crystallized by heating mixture of ≠-Al2O3 and AlF3

Abstract The morphology of corundum crystals, synthesized from a mixture of ≠-Al2O3 prepared from well-crystallized hydrated aluminum sulfate and reagent grade AlF3, was studied. The average size of the largest crystal was about 35 ≠m in diameter, which was obtained at 900°C for 20 h. The crystal size in diameter decreased and the thickness increased with elevating the heating temperatures from 900 to 1200°C. The crystal habit was composed of hexagonal {0001} faces, and of {10 1 2} and {10 12 } faces at the side of the hexagonal planes. A number of trigonal pyramids were observed on the {0001} planes of the corundum crystallized at lower heating temperatures.

Synthesis and thermal reactions of rhabdophane-(Yb or Lu)

Abstract Poorly crystalline rhabdophane-(Yb or Lu) was synthesized by precipitation from mixed aqueous solutions of Ln(NO 3 ) 3 (Ln = Yb or Lu), (NH 4 ) 2 HPO 4 , and citric acid with the mixing mole ratios of (NH 4 ) 2 HPO 4 /Ln(NO 3 ) 3 = 4 and citric acid/Ln(NO 3 ) 3 = 5 to 30, at pH 7 and 90°C for 7 to 30 days. However, unknown XRD peaks and C–O bond FTIR absorption peaks were observed for the precipitates. These peaks disappeared with increasing heating temperature up to 400°C in air, and single phase rhabdophane-(Yb or Lu) was obtained when heated at 400 to 800°C in air. Chemical formulas of the single phase rhabdophane were YbPO 4 ·0.4H 2 O and LuPO 4 ·0.5H 2 O, respectively. The water corresponding to 0.4H 2 O (or 0.5H 2 O) was zeolitic water. The lattice constants are a = 0.676 and c = 0.626 nm for Yb, and a = 0.674 and c = 0.630 nm for Lu. Rhabdophane-(Yb or Lu) changed into the xenotime structure above 860°C, which was stable even at 1800°C in air.

Formation and thermal change of monoclinic zirconia polycrystalline thin films by sol-gel process

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 400A thick after firing at 1000°C.

LaPO 4 -Containing Machinable Al 2 O 3 Ceramics

The melting temperature and thermal expansion coefficient of monoclinic LaPO4 were almost identical to those of alumina. Davis et al.1 reported that LaPO4-Al2O3 composite was found to be machinable. However, there have been a few systematic studies on the (1-x)LaPO4xAl2O3(x= 0 to 1 mass) composite ceramics, including fabrication methods and their material properties. Several of these are discussed in this article. Mixtures of (1-x)LaPO4 and xAl2O3 (x = 0 to 1 mass) were drypressed to disks or bars. Relative density larger than 94% was achieved when the specimens were fired at 1600◦C for 5 h in air. The sintered ceramics (x = 0 to 0.7) were found to be machinable; that is, they could be drilled using WC drill. Thermal and mechanical properties of the sintered composites were in the ranges; 10.0 × 10−6/◦C (x = 0) to 9.0 × 10−6/◦C (x = 1) (linear thermal expansion coefficient at 200–1000◦C); 5.0(x= 0) to 43 W/(m·K)(x= 1) (thermal conductivity at 25◦C); 100(x= 0) to 350 MPa (x= 1) (bending strength), 5(x= 0) to 17 GPa (x= 1) (Vickers hardness); and 1.8 (x = 0) to 3.8 MPa·m0.5(x = 1) (fracture toughness).

Machinable Dy 2 O 3 -Containing HAp Ceramics

The machinable ceramics are the ceramics that can be drilled by machine. It has reported before. But they are low strength.1,2 In our study, Dy2O3-containing HAp ceramics were found to be machinable. The mixed powder of Dy2O3 and hydroxyapatite (Ca10(PO4)6(OH)2, HAp) with molar ratios ((Ca+Dy)/P) from 1.67 (HAp only) to 1.89 were uniaxially dry-pressed at 50 MPa to disks. The test specimens were sintered at 1200–1500◦C for 3 h in air. The phases observed mainly by XRD in the sintered body were HAp and Dy2O3. The relative density of the test specimens were almost above 95%. Three-point bending strength were 50∼90 Mpa, and fracture toughness were 0.8∼1.7 MPa·m0.5 of the test specimens sintered at 1250◦C. The test specimens presented machinability in the range of molar ratios 1.78–1.89.

Effect of Spark Plasma Sintering on Densification and Mechanical Properties of LaPO 4 -ZrO 2 Composites

It recently has been found that when LaPO4 with a high melting point of 2072 ± 20◦C1 was added to other refractory oxides, such as Al2O3 (melting point, 2050◦C) or ZrO2 (melting point, 2700◦C), the machinable composites with high strength and high melting temperature could be obtained. In this work the spark plasma sintering (SPS) technique was used to manufacture xLaPO4-(1-x)ZrO2 (x= 0 to 1) composites. The mixture of LaPO4 and ZrO2 powder was sintered in a graphite die at 1300◦C, 40 MPa for 2 to 3 min in a vacuum. Usual pressureless-sintering technique was also used to compare the properties. The sintered composites were found to be machinable. The SPS method gives dense sintered bodies with the relative densities above 96% whereas the relative densities of composites sintered by pressureless sintering method is above 90%. The mechanical properties obtained by SPS had higher strength (387 MPa, x = 0.3) and fracture toughness (4.3 MPa·m1/2, x = 0.3) than those obtained by pressureless-sintering (148.6 MPa, 3.1 MPa·m1/2). The results show that the SPS method increases density and bending strength of the composites; it, however, weakens the machinability.

Kinetics for Thermal Decomposition of Anhydrous Aluminum Sulfate Powder Composed of Single Crystal Rhombohedrons in Shape.

The anhydrous aluminum sulfate powder composed of rhombohedral single-crystal grains was prepared by precipitation from a concentrated sulfuric acid solution. The average grain size and the size distribution was 3.8±0.7μm. Thermal decomposition behavior of the anhydrous aluminum sulfate was investigated by thermogravimetric measurements. The kinetics of the thermal decomposition fitted to a contracting volume model with an apparent activation energy of 360 kJ/mol. The mobility of the reaction boundary of the thermal decomposition at 760°C was estimated to be about 7.0×10-5μm/s. The temperature dependence of the mobility was expressed by an Arrhenius type equation.

Effects of ZrO2 ultrafine particle additive on the formation of Al2O3 from sulfate

An aqueous solution of aluminum sulfate containing ultrafine ZrO2 particles was dehydrated at 300°C to prepare the starting material of ZrO2-Al2O3 composite. The effects of ZrO2 additive on the decomposition of anhydrous sulfate into η-Al2O3, on the η→α phase transformation and on the sinterability of produced ZrO2-Al2O3 composite powder were studied. The apparent crystallite size of the anhydrous sulfate decreased with increasing ZrO2 content from 42nm (no additive) to 18nm (20vol% ZrO2). The composite oxide powder, in which aggregated particle (50-100nm in diameter) of tetragonal ZrO2 dispersed in the secondary grains of η-Al2O3, was obtained by decomposing the anhydrous sulfate. In the composite powder containing 20vol% ZrO2, α-Al2O3 was found above 1100°C, while it formed above 1200°C for the single component Al2O3 powder. The grain growth of Al2O3 was prevented by ZrO2 particles dispersed in the η-Al2O3 grains; that is, the primary particle size of Al2O3 decreased with increasing ZrO2 content from 120nm (no additive) to 60nm (20vol% ZrO2) after calcination at 1250°C for 1h. The composite powders had very low green densities and they were difficult to sinter. But ball-milling of the composite powders yielded high densities up to 94% for 10vol% ZrO2 bodies fired at 1600°C for 1h in air. The ZrO2 particles were located at the boundary of Al2O3, and the grain size of Al2O3 decreased from 3.0μm (no additive) to 1.2μm (10vol% ZrO2) as the content of ZrO2 increased.