Takeshi Meguro

Preparation and Electrical Properties of Sintered Bodies Composed of Monophase Spinel Mn(2-X)Co2XNi(1-X)O4 ( 0 \LeqX \Leq 1) Derived from Rock-Salt-Type Oxides

Preparation of spinel-type oxides, Mn(2-X )Co2X Ni(1-X )O4 (0X1), and their electrical properties were investigated. Die-pressed oxides (25 mm diam. ×4 mm), containing metals with desired molar ratios, were heated to 1400° C, and held at that temperature for 3 h in nitrogen. The sintered specimens were quenched to 800° C and then oxidized for 95 h in air to completely convert them into spinel-type oxides. The lattice constants of the spinels decreased with increasing X. Two turning points, at X=0.5 and 0.84, were observed. Based on these results, the cation distributions were estimated. Calculation of the Seebeck coefficient proved that the carriers of the oxides with X=0 and 0.25 are electrons and those of the oxides with X=0.5 to 1 are holes. Activation energies calculated from mobility were 0.34 to 0.36 eV. The conduction was considered to be due to the small polaron hopping mechanism.

Synthesis of Mg-α SiAlON powders from talc and halloysite clay minerals

Abstract Carbothermal reduction and nitridation (CRN) of SiO 2 is an attractive method to manufacture Si 3 N 4 powders with controlled grain morphology. Some inexpensive raw materials were previously used to synthesize β-sialon powder, resulting in cheaper products to be useful for some engineering applications. In this work, Mg-α sialon (Mg x Si 12− m − n Al m + n O n N 16− n ) powders were achieved by CRN of mixtures of talc (Mg 3 (Si 2 O 5 ) 2 (OH) 2 ) and NZ halloysite clay (Al 2 Si 2 O 5 (OH) 4 ) minerals. The final products consisted mainly of α-sialon and β-sialon phases. Small amounts of 15R AlN-polytypoid and SiC were also identified in the synthesized powder. By adding a small amount of α-Si 3 N 4 powder as seeds to the starting composition, the conversion rate of α-sialon phase was significantly enhanced and powders with up to 90 wt% of α-sialon were produced.

Morphology-retaining synthesis of AlN particles by gas reduction-nitridation

Abstract Al 2 O 3 raw materials with characteristic morphologies were converted to AlN, utilizing an NH 3 –C 3 H 8 gas mixture as a reduction–nitridation agent. A high conversion to AlN was achieved at low temperatures of 1400–1500 °C in a reaction time of 0.5 h. Observation by SEM clearly indicated that the product AlN maintained the original particle morphology of the initial Al 2 O 3 and, thus, a morphological control of the final product was found to be possible. Morphology-retaining syntheses of AlN fibres, spherical AlN particles and granules were demonstrated.

Inhomogeneous grain growth and elongation of Dy-α sialon ceramics at temperatures above 1800°C

Abstract A series of samples with dysprosium α-sialon compositions and different amounts of sintering additives has been fabricated from α-Si 3 N 4 , AlN, Al 2 O3 and Dy 2 O 3 starting powders, using pressureless sintering (PLS) at 1800°C plus gas pressure sintering (GPS) at the same and higher temperatures under a relatively low gas pressure of 0.9 MPa N 2 . The resultant α-sialon grains show significantly different features, such as regularly fine and equiaxed, elongated, and even a few extraordinarily large with high aspect ratio grains in the fine matrix, which has rarely been observed in α′ ceramics. It is suggested that the temperature strongly influences the grain morphologies of α-sialon, playing an important role particularly in the latter stage of the “nucleation-growth” process. Such a kind of microstructural morphology as mixed with equiaxed and elongated grains toughens the α-sialon ceramics and leaves them still hard as usual. Overly high temperature treatment leads to the preferential growth of a few grains which become excessively large and long, and do not significantly improve the materials toughness.

Control of grain morphology in Ca-α sialon ceramics by changing the heating rate

Abstract A Ca α-sialon (α′) ceramic with an overall composition of Ca 0.8 Si 8.8 Al 3.2 O 1.6 N 14.4 was fabricated from α-Si 3 N 4 , AlN, Al 2 O 3 and CaCO 3 starting powders using pressureless sintering (PLS) at 1800°C for 1 h. The microstructural morphology of the resultant Ca-α′ depended greatly on firing patterns, i.e., heating rate. Typical fine and equiaxed α′ grains could only be found in samples fired at a regular heating rate of 20°C/min. In contrast, aciculate grains with a high aspect ratio of 5 to 10 dominated the Ca-α′ sample sintered at a heating rate of up to 60°C/min. The number of α′ nuclei in the Ca-α′ sample during heating was found to be significantly reduced by rapidly increasing the temperature because the yield of α′ through reactions of Si 3 N 4 , AlN, Al 2 O 3 and CaO was somewhat retarded. Fewer nuclei could then grow into elongated grains during the subsequent isothermal holding process at higher temperature. Furthermore, much liquid phase existed in the system until higher temperatures were reached due to rapid heating, and this was another key factor in enhancing the elongation of α′ grain.

Formation region of monophase with cubic spinel-type oxides in Mn–Co–Ni ternary system

This study was performed to find the composition area of cubic spinel-type monophase oxides composed of the Mn–Co–Ni ternary system. Starting materials were prepared by mixing Mn, Co, and Ni nitrates then evaporating to dryness. Each starting oxide was fired at 700, 800, 900, 1000, and 1100 °C in air. The regions of cubic spinel monophase (CSM) were confirmed to spread with decreasing firing temperatures. The region of CSM at 1000 °C was seen near the line connecting the points of Mn : Co : Ni = 2 : 4 : 0 and 4.5 : 0 : 1.5. The area at 800 °C spread toward Co and Ni, as compared to the results at 1000 °C. In the region containing more Mn above the area of CSM at 800 °C, the phase had tetragonal spinel or α-Mn2O3 besides cubic spinel structure. Below this area, the phase contained rock-salt-type crystal besides cubic spinel structure. This tendency at 1000 °C was the same as that at 800 °C.

Oxidation behaviour of the sintered Si3N4-Y2O3-Al2O3 system

The oxidation behaviour of silicon nitride composed of Si3N4, Y2O3, Al2O3, AlN and TiO2 was investigated in dry and wet air at 1100–1400 °C. The oxidation rates were confirmed to obey the parabolic law. An activation energy of 255 kJ mol−1 was calculated from the Arrhenius plots of the results of oxidation in an air flow. In still air the oxidation rate was larger than that in an air flow, but the oxidation rate in flowing air was not affected by the air flow rate. α-cristobalite and Y2O3·2SiO2 were formed in oxidized surface layers. These crystal phases increased with increasing oxidation temperature. In particular, a higher content of α-cristobalite was obtained in still air oxidation. The existence of water vapour in flowing air greatly promoted the oxidation.

Preparation and electrical properties of monophase cubic spinel, Mn1.5Co0.95Ni0.55O4, derived from rock salt type oxide

The purpose of this study was to prepare a sintered body consisting of monophase cubic spinel type oxide, Mn1.5Co0.95Ni0.55O4, and to evaluate its electrical properties. It was found that cooling from 1400 to 1000 °C in nitrogen did not affect the preservation of the sintered rock salt type oxide formed at 1400 °C. A crack free sintered body of monophase cubic spinel may be obtained by heat treatment at 1000 °C in air, using a specimen cooled from 1400 °C at a rate of 500 °C min −1. A heat treatment time in air at 1000 °C of more than 48 h was required to convert the rock salt type structure into a perfect cubic spinel structure. The electrical conductivity, δ, of the sintered cubic spinel oxide synthesized in this work was found to be stable at 100 and 200 °C in air and at 100, 200 and 300 °C in nitrogen. The sintered spinel oxide was a p-type semiconductor, based on small polaron hopping conduction. The intrinsic hole concentration, n, was estimated to be constant, with a value of 1.6–1.8×1028m−3. The mobility, μ, increased exponentially with increasing annealing temperature in both atmospheres, suggesting that the change in δ is dependent on μ.

Control of Shrinkage during Sintering of Alumina Ceramics Based on Master Sintering Curve Theory

The Master Sintering Curve (MSC) is quite useful for analyzing the shrinkage behavior of ceramics. It is possible to compare shrinkage behavior using MSCs that are obtained from different firing profiles. In this study, shrinkage behavior during sintering of green bodies of several kinds of Al2O3 based ceramics were evaluated, using an electric furnace equipped with a dilatometer to be controlled based on the MSC theory. Although all of the samples shrank monotonically, shrinkage behavior depended on the additive and heating rate. The MSC theory was applied to analyze shrinkage behavior. As a result, a different MSC could be obtained in Al2O3 with and without the addition of MgO. In the pure Al2O3, a single MSC could be obtained from shrinkage curves by firing at a heating rate of 7.5-20oC/min, though the shrinkage curve at a heating rate of 3-5oC/min did not correspond with the MSC. In contrast, shrinkage curves at heating rate of 5-20oC/min were converged in the case of the MgO doped Al2O3 to obtain a unique MSC independent of firing profile. Apparent activation energy for sintering was estimated as 555 kJ/mol in the pure Al2O3 and 880 kJ/mol in the MgO doped Al2O3. The firing profile to obtain a requested sintering shrinkage curve was predicted from the resultant MSC. A comparison between the predicted and the experimental shrinkage curves, showed good consistency, thus confirming that it is possible to control shrinkage behavior using the MSC.

An improvement of thermal conduction of activated carbon by adding graphite

This study was performed to improve thermal conduction of granular activated carbon by adding graphite powder. Granular activated carbons were fabricated from anthracites and graphite powders using a binder pitch, and then activated in a nitrogen-carbon dioxide gas at 850°C. Additions of graphite from 20 to 30 wt% were considered to be suitable for thermal conductivity and n-butane adsorption power. The thermal conductivities of such granular activated carbons were 20 times those of activated carbons without graphite. A fixed bed filled with the granular or powder activated carbons containing graphite were never ignited even though the inside of the bed was directly heated.