Shinobu Hashimoto

SYNTHESIS OF ALPHA -AL2O3 PLATELETS USING SODIUM SULFATE FLUX

When powder mixtures of {Al 2 (SO 4 ) 3 + Na 2 SO 4 }or {γ–Al 2 O 3 [obtained by heating Al 2 (SO 4 ) 3 at 900 °C for 3 h] + Na 2 SO 4 } were heated in an alumina crucible at 1100 °C for 1 h, α–Al 2 O 3 platelets were formed. The powder mixture of {Al 2 (SO 4 ) 3 + 2Na 2 SO 4 } yielded aggregations of platelets that were less than 5 μm in size. The size of the aggregations increased in proportion to the amount of Na 2 SO 4 , and aggregations of 120 μm were obtained using a mixture of {Al 2 (SO 4 ) 3 + 6Na 2 SO 4 }. The powder mixture of {γ–Al 2 O 3 + 2Na 2 SO 4 yielded hexagonal platelets having an average diameter of 3.7 μm and an average thickness of 0.3 μm. In addition to aggregation size, the size of the hexagonal platelets also increased in proportion to the amount of Na 2 SO 4 , and platelets having an average diameter of 5 μm were obtained using a mixture of {γ–Al 2 O 3 + 6Na 2 SO 4 }.

Synthesis of needlelike mullite particles using potassium sulfate flux

Abstract When a powder mixture consisting of Al 2 (SO 4 ) 3 , K 2 SO 4 and SiO 2 was heated in an alumina crucible covered with a lid, aggregations of needlelike mullite particles were obtained. The effects of the mixing ratio of the raw materials and of the heating temperature on the synthesis and morphology of the needlelike mullite particles were investigated. When the powder mixture consisting of 24.5 mol% of Al 2 (SO 4 ) 3 , 48.0 mol% of K 2 SO 4 and 27.5 mol% of SiO 2 was heated at 1100°C for 3 h, the mullite particles were 0.2–0.5 μm in width and 2–5 μm in length, resulting in a specific surface area of the mullite particles of 136 m 2 /g. The formation process of these mullite particles was discussed.

Characterization of Zeolite in Zeolite-Geopolymer Hybrid Bulk Materials Derived from Kaolinitic Clays

Zeolite-geopolymer hybrid materials have been formed when kaolin was used as a starting material. Their characteristics are of interest because they can have a wide pore size distribution with micro- and meso-pores due to the zeolite and geopolymer, respectively. In this study, Zeolite-geopolymer hybrid bulk materials were fabricated using four kinds of kaolinitic clays (a halloysite and three kinds of kaolinite). The kaolinitic clays were first calcined at 700 °C for 3 h to transform into the amorphous aluminosilicate phases. Alkali-activation treatment of the metakaolin yielded bulk materials with different amounts and types of zeolite and different compressive strength. This study investigated the effects of the initial kaolinitic clays on the amount and types of zeolite in the resultant geopolymers as well as the strength of the bulk materials. The kaolinitic clays and their metakaolin were characterized by XRD analysis, chemical composition, crystallite size, 29Si and 27Al MAS NMR analysis, and specific surface area measurements. The correlation between the amount of zeolite formed and the compressive strength of the resultant hybrid bulk materials, previously reported by other researchers was not positively observed. In the studied systems, the effects of Si/Al and crystalline size were observed. When the atomic ratio of Si/Al in the starting kaolinitic clays increased, the compressive strength of the hybrid bulk materials increased. The crystallite size of the zeolite in the hybrid bulk materials increased with decreasing compressive strength of the hybrid bulk materials.

Fabrication and properties of novel composites in the system Al-Zr-C

Composite bodies in the system Al–Zr–C, with about 95% relative density, were obtained by heating the compact body of powder mixture consisting of Al and ZrC (5 : 1 mol %) in Ar at 1100–1500°C for various lengths of time. Components of the material heated at more than 1200°C were Al, Al3Zr, ZrC and AlZrC2. The Al3Zr exhibited plate-like aggregation, and its size increased with increasing temperature. In the material heated at 1500°C for 1 h, the largest plate-like Al3Zr aggregation was 2000 μm long and 133 μm thick. Then the AlZrC2 was present as well-proportioned hexagonal platelet particles with a 8–9 μm diameter and a 1–2 μm thickness in the interior of the plate-like Al3Zr aggregation and Al matrix phase. The average three-point bending strength of the bodies was 140–190 MPa, and the maximum strength was 203 MPa in the body heated at 1300°C for 1 h. The body heated at 1500°C for 1 h showed high oxidation resistivity to air up to 1000°C.

Synthesis and Crystal Structure of a New Layered Carbide ZrAl4C4

A new ternary layered carbide, ZrAl4C4, has been synthesized and characterized by X-ray powder diffraction. The crystal structure was successfully determined using direct methods and further refined by the Rietveld method. The crystal is trigonal (space group P3m1, Z=2) with lattice dimensions a=0.332471(3) nm, c=2.19717(2) nm, and V=0.210330(3) nm3. The final reliability indices calculated from the Rietveld refinement were Rwp=6.56% (S=1.58), Rp=4.92%, RB=1.90%, and RF=0.98%. The compound shows an intergrowth structure with NaCl-type [Zr2C3] thin slabs separated by Al4C3-type [Al8C7] layers.

Thermo-elastic behaviour of a natural quartzite: itacolumite

Itacolumite is a particular type of sandstone constituted of quartz grains, with an interlocked microstructure and large intergranular decohesions. This article is devoted to the study of the thermo-elastic behaviour of this material during thermal cycles between 20 and 800 °C. This was made by using an ultrasonic pulse-echo measurement technique of Young’s modulus E, coupled to thermal expansion experimentation and to acoustic emission. An unusual evolution of elastic properties versus temperature is found, with strong irreversible effects around the temperature of the α–β transition of quartz. A damage parameter, representing the fractional number of cracks, is calculated with a Kachanov type formulation, by using the experimental data for itacolumite and the values of E, obtained from literature for an hypothetic ideal quartz polycrystal. The evolution of this parameter versus temperature is explained by internal stresses that are developed by both thermal and elastic effects in quartz grains.

Sintering and Characterization of Zr2Al3C5 Monolith

Zr2Al3C5 has been successfully synthesized via solid state reaction between Al, ZrC and carbon powder at 1600 in vacuum. This complex carbide has very strong bond between metal atoms and carbon atoms. Thus, this material has a potential to be utilized as structural materials. Some properties of Zr2Al3C5 powder from solid-state reaction in vacuum had been tested. It was found that this powder was completely oxidized in air at 900 1 h, and can be hydrated in moist air. These drawbacks might come from the high reactivity of the powder due to synthesis in vacuum. Zr2Al3C5 powder from solid state reaction in vacuum was sintered at various temperatures from 1500 to 2000 under vacuum with pulse electric current sintering (PECS) and pressureless sintering. Zr2Al3C5 started to sinter at 1500 and got partially dense from 1700. Physical properties and mechanical properties of this material were investigated and discussed.

Growth of MgCr2O4 whiskers

A powder mixture compact composed of MgO and Cr2O3 was embedded in carbon powder in an alumina crucible which is covered with a lid with a hole. On heating to above 1600°C, MgCr2O4 whiskers were grown on the compact. The whiskers grown at 1700°C for 2 h were several mm to about 6 mm in length and several μm to about 220 μm in diameter. Most of them grew in the direction of the a-axis. The whiskers apparently grew by the vapor-solid (VS) mechanism.

Fabrication and characterization of TiC/Al composites

Abstract TiC/Al composites were fabricated by heating the powder mixture compacts consisting of Al and TiC in Ar. The effects of particle size of TiC as starting materials and cooling rate on microstructure of the composites were investigated. Further, the relationship between bending strength and microstructure of the composites was discussed. In case of heating the powder mixture compacts of 5:1 (mol ratio)=Al (10 μm): TiC (1–3 μm) at 1000–1500°C for 1 h, they became 95–97% in relative density. When the powder mixture compact was heated at 1100°C for 1 h and cooled to room temperature at a rate of 2°C/min, threadlike parts at which Al concentrated were formed in the compact. Then the average bending strength of the compact decreased to 270 from 400 MPa of the compact without Al concentrate parts, that was obtained by rapid cooling after the heating.

Growth and characterization of needle-like β-CaCr2O4 crystals

Abstract A mass of residue including needle-like β-CaCr 2 O 4 crystals was obtained by heating a powder mixture consisting of CaCO 3 and Cr 2 O 3 on graphite powder in an alumina crucible with holes. The needle-like β-CaCr 2 O 4 crystals were separated from the mass by hydrochloric acid. Yield of the crystals increased with increasing temperature above 1500 °C and reached about 41 wt% when heated at 1700 °C for 4 h. Then the average length and diameter of the crystals were 2.5 mm and 118 μm, respectively; the largest crystal was 3.8 mm in length and 187 μm in diameter. Needle axis of the crystals was the c axis.