Hideo Awaji

Nanocomposites—a new material design concept

Abstract Plasma-gas condensation cluster deposition systems have been introduced and applied for preparation of Co/CoO and Co/Siclusters assemblies. In Co/CoO cluster assemblies prepared by the single source PGC system with introduction of O2 gas into the deposition chamber, fcc Co cores are covered with NaCl type CoO shells, showing marked enhancement of unidirectional and uniaxial magnetic anisotropy and a clear cross-over phenomenon in the magnetic relaxation from the high temperature thermal regime to the low temperarure quantum tunneling regime. In Co/Si cluster assemblies prepared by the double source PGC system, fcc Co cores are also covered with amorphous Si rich shells, showing rather small magnetic coercivity. Since Co/CoO and Co/Si core–shell clusters are stable in ambient atmosphere, they will be used as building blocks for novel nano-structure-controlled materials.

Mechanisms of toughening and strengthening in ceramic-based nanocomposites

Toughening and strengthening mechanisms in ceramic-based nanocomposites were explained based on Griffiths energy equilibrium and residual stresses around second-phase nanoparticles dispersed in matrix grains. The residual thermal stresses around a spherical dispersed particle within a concentric sphere of a matrix grain were specifically analyzed to clarify the effects of residual stresses on the toughening mechanism in the frontal process zone involving nanocracking and on the strengthening mechanism caused by nucleation of dislocations. The analytical results revealed that the ratio of the thermal expansion coefficients of the particle and matrix has a marked effect on residual stresses and the estimated residual stresses were of sufficient magnitude to generate lattice defects such as dislocations around the particle even in ceramics, and that the nanosize particle within a matrix can only create dislocations around the particle. The toughening and strengthening mechanisms for nanocomposites were also discussed based on the analytical results and explained schematically.

Powder characteristics, sintering behavior and microstructure of sol-gel derived ZTA composites

Abstract A series of alumina/zirconia composites of varying compositions of zirconia were prepared through the sol–gel technique. Precursors were calcined at different temperatures ranging from 300 to 1400°C and sintered at 1530°C for 3 h. Compacts made from the powder calcined at 950°C yielded density up to >99% of theoretical density by pressureless sintering. Pore size distribution and the densification behavior were explained with respect to calcination temperature. Microstructural analysis of the sintered compacts revealed the uniform distribution of the zirconia grains in the alumina matrix. It is also observed that the faceted intergranular zirconia grains are at the grain junctions and the corners of the alumina matrix.

Pressureless sintering of sol-gel derived alumina–zirconia composites

Abstract Alumina–zirconia (pure zirconia, 12 mol % ceria stabilized zirconia and 3 mol% yttria stabilized zirconia) composites containing 5, 10, 15, 20 and 25 vol.% zirconia were prepared by sol-gel technique. The procedure involved the following steps: preparation of stable (hydrous) alumina and zirconia sols, mixing of sols in proper ratio, to obtain the final precursor with the desired composition and finally stabilizing the mixed sols. Thermal analysis was carried out for the dried precursor. The sol-gel derived precursors were calcined at different temperatures and their crystallization behavior was studied using X-ray diffraction (XRD) technique. Samples calcined at 950°C sintered well than those calcined at other temperatures. All the samples were sintered at 1530°C for 3 h by pressureless sintering, which yield upto 99.8% of the theoretical density of the composite.

Fracture toughness measurements of ceramics by V notch technique

Abstract It has been made clear that the values of the fracture toughness of structural ceramics can be accurately obtained by applying the highly skilled technique of cutting a V-shaped notch sharply into the single edge notched beam (SENB) and chevron notch (CN) methods. These improved methods are referred to as the SEVNB and CVN methods, respectively, which are made possible by using a V-shaped slicing diamond wheel in cutting. Several kinds of ceramics were evaluated by these methods and the values were compared with those obtained by the single edge precracked beam (SEPB) and conventional CN methods. The values of the fracture toughness scattered only slightly and agreed quite well with one another, except for in the conventional CN method.

Molecular dynamics — simulations of the fracture toughness of sapphire

Abstract A new method for the estimation of the fracture toughness K IC by using the Molecular Dynamics method is presented. The Al 2 O 3 single crystals were deformed under constant displacement conditions with an initial crack of length 1 nm. The strain was applied in the a axis, which was set perpendicular to the crack plane, so that the mode I crack deformation was achieved. The total energy, which is a result of MD simulations, was analyzed in the elastic, plastic and surface energy part. From the structural plots the crack propagation was measured and the analysis using the Griffith criterion was applied. From the critical condition the fracture toughness was estimated. The experimental values for the fracture toughness in the four crack orientations (plane and direction) (11.0)[001], (11.-2)[12-1], (00.1)[100], and (11.0)[010] were 2.84, 2.71, 5.62, 2.67 MPa m −1/2 , respectively, and the calculations are in good agreement. Also, a heterogeneous alumina–zirconia nanocomposite material was analyzed by this method. Although the particle had only a diameter of 1 nm, the toughness increased because the interface was found to be quite strong.

Fabrication of Alumina-Based Toughened Nanocomposites

Highly toughened nickel dispersed alumina-based nanocomposites were fabricated using our developed soaking method, based on our previously proposed toughening mechanism of nanocomposites. Commercially available γ-alumina agglomerate with high porosity was used as a starting material. Nickel nitrate solution was penetrated into the nano-pores of the γ-alumina agglomerate using this new technique. The alumina-nickel composite powder with α-alumina seeds was then sintered using a pulse electric current sintering technique. To disperse dislocations generated around the nano-particles into alumina grains, the sintered materials were annealed. The results showed that the maximum fracture toughness was 7.6 MPam1/2, which was about two times higher than that of dense alumina.

Influence of MgO on microstructure and properties of mullite–Mo composites fabricated by pulse electric current sintering

Abstract Mullite–Mo (10 vol.%) composites and monolithic mullite were fabricated using a pulse electric current sintering technique. Both monolith and composites of mullite were sintered up to theoretical density at 1500°C within few minutes. MgO of 0.25 wt.% was added as a sintering aid to both the mullite and composites. Addition of MgO significantly increased the bending strength of the monolithic mullite and mullite/10 vol.% Mo composites to 441 and 634 MPa respectively. The apparent increase in the bending strength of the composites was attributed to the combinational effect of Mo and MgO present in the composites. The fracture toughness of the composites also increased from 2 to 3.9 MPa.m 0.5 for the mullite/10 vol.% Mo composites, which was nearly twice that of the mullite. Crack-bridging and frontal process-zone elongation were expected to be the toughening mechanisms operated in these composites. The addition of Mo having high thermal diffusivity slightly increased the thermal diffusivity of the composites, because the 10 vol.% Mo particles were well dispersed and discontinuous in the matrix. Elongated mullite grains were observed for the composites without MgO, whereas the composites with MgO have a controlled microstructure.

Sintering and microstructure of mullite-Mo composites

Abstract Mullite–Mo composites of different compositions (0–100 vol.% Mo) were sintered to near theoretical density by pulse electric current sintering (PECS). The densification behaviour and the microstructure of mullite–Mo composites as a function of Mo content were studied. The addition of 10 vol.% Mo significantly enhanced the strength and toughness of monolithic mullite to 556 MPa and 2.9 MPa m 1/2 , respectively. SEM observations revealed the modification of discrete isolated Mo particles to continuosly interconnected network with the increase in the Mo content. Mo grains were located at the grain boundaries as well as inside the mullite grains. The addition of Mo to monolithic mullite led to a change in the fracture mode.

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.