K. Niihara

Aluminium composites reinforced with a new aluminium borate whisker

This is the first report on the metal matrix composite reinforced with a new aluminium borate whisker

Hot-pressed Si3N4-32% SiC nanocomposite from amorphous Si-C-N powder with improved strength above 1200 °C

On montre que la resistance a la rupture du manocomposite Si 3 N 4 - 32% SiC est amelioree de facon significative si on remplace Y 2 O 3 /Al 2 O 3 par Y 2 O 3 seul comme additif de frittage

PZT nanocomposites reinforced by small amount of oxides

Pb(Zr,Ti)O 3 (PZT) nanocomposites were prepared from high purity PZT powder and small amount (0.1-1.0 vol%) of oxides, i.e. Al 2 O 3 , MgO and ZrO 2 . Effects of the additives on mechanical and piezoelectric properties of the nanocomposites were investigated. The fracture strength and hardness of PZT nanocomposites with 0.5 vol% Al 2 O 3 or 0.1 vol% MgO additives were significantly improved. The reduced grain size of the nanocomposites is considered to be responsible for the excellent mechanical properties. Though the dielectric constants of the composites were decreased with an addition of Al 2 O 3 or MgO, the planar electromechanical coupling factor, K p , of MgO-added composites was higher than that of Al 2 O 3 -added. As a consequence, the PZT/0.1 vol% MgO nanocomposite showed good mechanical and suitable piezoelectric properties. Addition of ZrO 2 had little effect on mechanical and electrical properties of the composites.

AA6061 composite reinforced with potassium titanate whisker

Recent works reported the reduction in strength of the AA6061 matrix composite fabricated by powder metallurgy by T6 treatment. Thus, reactions between whisker and aluminium matrix seems to restrict the application of the aluminium matrix composites with potassium titanate whisker. The aim of the present work was to examine whether or not any reaction occurs at the whisker/matrix interface caused by high-pressure infiltration or by T6 heat treatment. This was carried out using carried out using transmission electron microscopy (TEM)

Perovskite-type BaTiO3 ceramics containing particulate SiC: Part II Microstructure and mechanical properties

Microstructure and mechanical properties of BaTiO3 and BaTiO3-based composites containing nanosized SiC particulates were investigated. Fracture behaviour and fractography were also discussed from the fracture surface observation by scanning electron microscopy and by the controlled surface flaw method. The added SiC particulates were uniformly distributed within the matrix BaTiO3 grains, with some larger particulates located at the BaTiO3 grain boundaries. The microstructure of BaTiO3 was modified by incorporation of the SiC particulate. Mechanical properties, particularly the fracture strength, were remarkably improved by adding the SiC particulate, owing to grain-size reduction and/or crystal structure change by incorporating the SiC. From the fracture surface observations, it was confirmed that the subcritical crack growth phenomenon of BaTiO3 was also improved by the nanosized SiC dispersions.

Microstructure and mechanical behaviour of 3Y-TZP/Mo nanocomposites possessing a novel interpenetrated intragranular microstructure

Yttria stabilized tetragonal zirconia polycrystal (Y-TZP)/0-100 vol % molybdenum (Mo) composites were fabricated by hot-pressing a mixture of Y-TZP powder containing 3 mol % yttria (Y2O3) and a fine Mo powder in vacuum. This composite system possessed a novel microstructural feature composed of an interpenetrated intragranular nanostructure, in which either nanometer sized Mo particles or equivalent sized zirconia (ZrO2) particles located within the ZrO2 grains or Mo grains, respectively. The strength and toughness were both greatly enhanced with increasing Mo content for the 3Y-TZP/Mo composites thus breaking through the strength-toughness tradeoff relation in transformation toughened ZrO2 and its composite materials. They exhibited a maximum strength of 2100 MPa and a toughness of 11.4 MPa·m1/2 for the composite containing 70 vol % Mo. These simultaneous improvements in strength and toughness were determined to be the result of a decrease in flaw size associated with the interpenetrated intragranular nanostructure, and a stress shielding effect created in the crack tip by the elongated Mo polycrystals bridging the crack tip in addition to the stress induced phase transformation.

TiO2 coating photocatalysts with nanostructure and preferred orientation showing excellent activity for decomposition of aqueous acetic acid

TiO2 coatings are attractive materials for use as photocatalysts, photoelectrodes and solar cells. Structural modification, quantum size effects, and large surface-to-volume make the coatings interesting. We have described the effects of crystal strucmre [1], crystallinity [2] and morphology [3] of TiO2 coatings on photocatalytic properties, and shown that a TiO2 coating prepared from a chemically-modified alkoxide solution had characteristic structur.es [4]. Here we report the photocatalytic properties of TiO2 coatings prepared by the chemically modified alkoxide method, which have nanostructure and show preferred orientation along the c axis. We also report the effect of oxygen vacancies in the TiO2 coatings on the photocatalytic properties. Decomposition of acetic acid is a very important reaction because during decomposition acetic acid usually generates organic compounds containing more than two carbons. Decomposition of aqueous acetic acid by use of oxygen and photocatalysts is lcnown to proceed differently from the photo-Kolbe reaction [5] with a product of ethane. During the photocatalytic decomposition of aqueous acetic acid with oxygen, formaldehyde is generated as an intermediate compound and this oxidizes readily to carbon dioxide. Chemical oxygen demand (COD) decreases to zero characteristically in the process. Therefore, the photocatalytic decomposition of aqueous acetic acid with oxygen is a key reaction for purification and treatment of water. T iQ coatings were prepared by the chemically modified alkoxide method. The method has been described in detail elsewhere [4]. The precursor solution for the coatings was prepared from titanium tetraisopropoxide, anhydrous el:hanol, diethanol amine, water and polyethylene glycol (molecular weight; Mw = 2000). The concentration of titanium tetraisopropoxide in ethanol was 0.5 mol dm -3. The molar ratios of water and diethanolamine to titanium tetraisopropoxide were 1 and 2, respectively. The concentration of polyethylene glycol to titanium tetraisopropoxide was 20 wt%. Gel coating, which was prepared on a quartz glass plate by a dip-coating method, crystallized to anatase during elevation on

Perovskite-type BaTiO3 ceramics containing particulate SiC

BaTiO3-based composites with nanosized SiC particulates were successfully fabricated by a hot-pressing technique in an argon atmosphere. Crystal structure and phase transformation behaviour were investigated by X-ray diffraction analysis, linear thermal expansion analysis and internal friction measurement. It was confirmed that the added SiC particulates were uniformly distributed within the matrix BaTiO3 grains, with some larger particulates located at the BaTiO3 grain boundaries. In addition, there were no reaction phases between BaTiO3 matrix and SiC particulates. The crystal structure gradually changed from tetragonal to cubic phase with respect to the SiC content. The Curie temperature, Tc, was lowered as the SiC content increased. Moreover, the transformations in the low-temperature range almost disappeared above 1 vol% SiC. The diffused phase transformation phenomenon was observed as the SiC content increased up to 3 vol%. The results were associated with the grain-size reduction, the existence of oxygen vacancies and the residual stresses associated with the thermal expansion mismatch between matrix and SiC particulate. The influence on the domain structure development of SiC particulates dispersed within the matrix grains was also discussed.

Mechanical properties and microstructures of machinable silicon carbide

Mechanical properties and microstructures of machinable silicon carbide, fabricated by pressureless sintering of silicon carbide fine powder with the aid of polysilastyrene, have been examined. Drastic changes in microstrucyure and in mechanical properties between specimens sintered at below 1773 K and at above 1873 K were observed. By sintering at above 1883 K the macinable silicon carbide had a good strength of more than 200 Mpa with high reliability, which was maintained beyond 1773 K. Polysilastyrene was converted in β-phase silicon carbide and ribbon carbon in the pores. The (001) plane of carbon is parallel to the (111) planes of β-phase silicon carbide.

Influence of SiC particle size and drying method on mechanical properties and microstructure of Si3N4SiC nanocomposite

Abstract Si 3 N 4 20 vol% SiC nanocomposites with different SiC particle sizes (mean particle size of 30 nm and 270 nm, which were made by plasma CVD and carbothermal reduction, respectively) were fabricated by hot pressing. The effects of second-phase particle size and drying methods for mixed slurry (microwave drying and rotary evaporator) on microstructure and mechanical properties were analyzed. The nanocomposite with ultra-fine (30 nm) SiC dispersoid had a high average strength of 1680 MPa. This value was 26% higher than that of the nanocomposite with coarser (270 nm) SiC dispersoid and corresponded to the strength of nanocomposite made from amorphous Si-C-N precursor powder. Fracture strength and microstructure of the nanocomposite were significantly influenced by the drying methods as well as the SiC particle size. Refinement of microstructure and deagglomoration of the SiC particles were the main reasons for strengthening of the nanocomposite. Furthermore, consolidation of grain boundary phase caused by free-carbon impurity in ultra-fine SiC powder could also be an important factor in the improvement of strength.