K. Komeya

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.

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.