Kenya Hamano

Boron-rich crystals in A1-M-B (M = Li, Be, Mg) systems grown from high-temperature aluminum solutions

Abstract Crystals of icosahedral B12 compounds grown from high-temperature A1 solutions have been studied. The crystals grown are A1LiB14, A1MgB14 and A1~1.1Be~0.6B22. They are obtained as platelets or irregularly shaped polyhedra with maximum dimensions of 5 to 7 mm. The crystallography and the mechanical properties of these materials are presented.

Crystal growth of the new compound Nb2B3, and the borides NbB, Nb5B6, Nb3B4, and NbB2, using the copper‐flux method

Crystals of NbB, Nb5B6, Nb3B4, Nb2B3 (new phase), and NbB2 were prepared by the high temperature copper solution method using niobium and boron powder as starting materials in an argon atmosphere. The experimental conditions for obtaining relatively large crystals of niobium borides were established. NbB and NbB2 crystals were obtained as a single phase, however, Nb5B6, Nb3B4, and Nb2B3 crystals were invariably obtained as a phase mixture for various atomic ratio in the starting material. The optimal conditions for growing NbB, Nb5B6, Nb3B4, Nb2B3, and NbB2 crystals were found to include soaking temperature 1700 °C, soaking time 5h, cooling rate 25 °C/h and the atomic ratios of starting materials: B/Nb=1.00, Cu/Nb=1.83∼29.24 for NbB; B/Nb=1.25, Cu/Nb=1.83∼3.66 for Nb5B6; B/Nb=1.40, Cu/Nb=1.83 for Nb3B4; B/Nb=1.70, Cu/Nb=1.83 for Nb2B3; B/Nb=2.50, Cu/Nb=29.24 for NbB2. The unit cell dimensions of niobium borides are as follows. NbB: a=3.2961(6), b=8.7224(8), c=3.1653(4) (A); Nb5B6: a=3.1567(2), b=22.767(2)...

Effects of TiO2 on sintering of alumina ceramics

The effects of TiO2 addition on the sintering process and microstructure of alumina ceramics were studied. In air, the solid solubility of TiO2 in α-Al2O3 was too small to be determined by the lattice parameter shift of α-Al2O3. Then, the relative amounts of titanium compounds remaining in fired bodies were measured by X-ray diffractometry using a step-scanning technique which can detect less than 0.1wt% rutile or Al2TiO5 in α-Al2O3, and were compared with the amount of TiO2. The solid solution of TiO2 in α-Al2O3 was found above 1150°C, and the solubility was estimated to be 0.27wt% at the temperature range from 1300° to 1700°C. Beyond the solubility limit, excess TiO2 coexisted with α-Al2O3 as rutile below 1350°C and as Al2TiO5 above 1450°C. The sintering of α-Al2O3 was markedly promoted when TiO2 was added up to the solubility limit and the fired density higher than 97% of the theoretical was obtained at 1400°C. The addition of TiO2 also promoted the grain growth of α-Al2O3. But beyond the solubility limit, the grain size decreased with an increase of Al2TiO5. Therefore it is inferred that Al2TiO5 existing as a second phase retards the grain growth of α-Al2O3. The lattice parameters of Al2TiO5 in fired bodies considerably differed from the those of a single crystal. It is explained as due to the difference of thermal expansion coefficient between Al2TiO5 and α-Al2O3.

Properties of mullite powder prepared by coprecipitation and microstructure of fired bodies

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 200A, 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.

Crystal growth of icosahedral B12 compounds from high-temperature metal solutions

Crystal growth of icosahedral B 12 compounds from high-temperature Al or Cu solutions has been studied. The crystals grown are BeB 9 , BeB 12 , α-tetragonal boron-type Al x Cu y B 25 , β-rhombohedral boron-type Al x Cu y B 105 and γ-AlB 12 -type Al ∼1.4 Mg ∼0.5 B 22 . The crystallography and the chemical compositions of these materials are described in detail.

Crystal growth of vanadium silicides from high-temperature metal solutions and some properties of the crystals

Abstract Crystals of vanadium silicides were grown from high-temperature tin and copper solutions in an argon atmosphere using vanadium metal chips and silicon powder as starting materials. The crystals grown were V 3 Si, V 5 Si 3 and VSi 2 . The growth conditions for obtaining single crystals of relatively large size were established. The as-grown V 3 Si, V 5 Si 3 and VSi 2 crystals were used for chemical analysis and measurements of unit cell dimensions. Densities, microhardness and electrical resistivity were determined on V 5 Si 3 crystals, and oxidation at high temperature in air was studied for V 5 Si 3 and VSi 2 crystals. The microhardness value on (110) planes of V 5 Si 3 is 12.3 (±0.1) GPa. The electrical resistivity determined on V 5 Si 3 crystal is 17.6 (± 0.7) μω cm. The oxidation of V 5 Si 3 and VSi 2 crystals begins to proceed at a measurable rate in the temperature range of about 355 and 515 °C, respectively. The final oxidation product is V 2 O 5 . In all cases, non-crystalline SiO 2 seemed to be formed during the oxidation reaction.

Dehydration of Magnesium Hydroxide and Sintering Behaviour of its Compressed Bodiy

Structural changes of magnesium hydroxide and its decomposition product, magnesia, with heat treatments were examined by means of powder X-ray diffraction, and the sintering behaviour of the compacts of magnesium hydroxide was also studied.Magnesium hydroxide began to decompose by 2hrs soaking at 300°C. During the decomposition, the crystallite size of magnesium hydroxide initially decreased, then increased to a certain maximum and then decreased rapidly, with raising temperature or prolonging soaking time. Its lattice parameter, c, also changed similarly, i.e. increased initially to a certain maximum, and then decreased rapidly. These changes could be explained clearly by ionic diffusion processes proposed by Taylor et al. From the results thus obtained, a detailed mechanism of the decomposition process was inferred.Magnesia formed by the decomposition of magnesium hydroxide at low temperatures showed a larger lattice parameter than that of the usual magnesia. With raising temperature, the parameter decreased gradually and approached to 4.211A at about 1400°C. Above that temperature, no further change was observed. Magnesia formed at low temperatures also showed a distorted cubic lattice, whose [111] direction was somewhat contracted compared with any other directions. The distortion was not released, even when the magnesia was fired to considerably high temperatures.Small tablets were prepared by compressing magnesium hydroxide, and fired according to various schedules. Bulk density of the compacts decreased rapidly at about 500°-700°C, due to the decomposition of the hydroxide, but little change was observed in the temperature range 700°-900°C. Above 900°-1000°C, sintering of the compacts progressed gradually with firing temperatures. From the results obtained, it is inferred that the sintering behaviour of the compacts consisting of magnesium hydroxide is not governed by the structural factors of magnesia formed, except during the period of the decomposition of the hydroxide, but is rather greatly governed by other external factors such as firing temperature, soaking time etc.

Grain Orientation of Aluminum Titanate Ceramics during Formation Reaction

A microstructural change during the formation reaction of aluminum titanate from a mixture of rutile and corundum powders has been studied. The characterization was carried out using a polarization microscope, a scanning electron microscope and a micro-focus X-ray diffractometer. The formation of aluminum titanate was controlled by a nucleation step. The formation reaction proceeded to form spherically oriented regions of aluminum titanate grains among the matrix of rutile and corundum. At the end of the reaction, the specimen was entirely filled with the oriented region of consisting several hundred micrometers. The oriented region was composed of primary aluminum titanate grains of several micrometers and pores. Large cracks due to a thermal expansion anisotropy were formed at the boundaries of the orientated regions. The formation of the oriented region was caused by a small change in free energy, increasing elastic energy, and the endothermic nature of the reaction.

Microstructures and mechanical properties of TiO2-added alumina ceramics

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.

Electrical Resistivity of ZnO Ceramics Doped with Sm2O3

ZnOに2mol%のSm2O3を添加した焼結体の電気的性質について検討した.ZnO単味に電圧を印加すると, 低密度焼結体 (750℃, 1h以下) では, 電流値は時間とともに減少した. しかし, Sm2O3を添加したものでは, このような経時変化は認められなかった. これらの試料の比抵抗値は相対密度が低くなると著しく増大した.焼結体の相対密度と粒径を用いて, 初期焼結モデルとKakerによる立方体モデルより粒子間のネック半径を算出した. このネック半径による比抵抗値の変化量は二つの直線に分かれており, その直線の交点からデバイ長さを見積もった. 得られたデバイ長さは, ZnO単味で0.31μm, Sm2O3を添加したもので0.53μmとなった.