T. Sekino

Titania Nanotubes Prepared by Chemical Processing

We report a new method for the synthesis of titanium oxide (TiO2) nanotubes. When anatase-phase- or rutile-phase-containing TiO2 was treated with an aqueous solution of 5–10 M NaOH for 20 h at 110 °C and then with HCl aqueous solution and distilled water, needle-shaped TiO2 products were obtained (diameter ≈ 8 nm, length ≈ 100 nm). The needle-shaped products are nanotubes with inner diameters of approximately 5 nm and outer diameters of approximately 8 nm. The formation mechanism of titania nanotubes is discussed in terms of the detailed observation of the products by transmission electron microscopy: the crystalline raw material is first converted to an amorphous product through alkali treatment, and subsequently, titania nanotubes are formed after treatment with distilled water and HCl aqueous solution.

Fabrication and mechanical behaviour of Al2O3/Mo nanocomposites

Two types of Al2O3/Mo composites were fabricated by hot-pressing a mixture of γ- or α-Al2O3 powder and a fine molybdenum powder. For Al2O3/5 vol% Mo composite using γ-Al2O3 as a starting powder, the elongated molybdenum layers were observed to surround a part of the Al2O3 grains, which resulted in an apparent high value of fracture toughness (7.1 Mpa m1/2). In the system using α-Al2O3 as a starting powder, nanometre sized molybdenum particles were dispersed within the Al2O3 grains and at the grain boundaries. Thus, it was confirmed that ceramic/metal nanocomposite was successfully fabricated in the Al2O3/Mo composite system. With increasing molybdenum content, the elongated molybdenum particles were formed at Al2O3 grain boundaries. Considerable improvements of mechanical properties were observed, such as hardness of 19.2 GPa, fracture strength of 884 MPa and toughness of 7.6 MPa m1/2 in the composites containing 5, 7.5, 20 vol% Mo, respectively; however, they were not enhanced simultaneously. The relationships between microstructure and mechanical properties are also discussed.

Microstructural characteristics and mechanical properties for Al2O3/metal nanocomposities

Abstract High density Al 2 O 3 /W nanocomposite has been successfully fabricated by using controlled reduction and hot-pressing of Al 2 O 3 /WO 3 mixtures. For realizing desirable microstructures, chemical processing was used to prepare the mixtures by using an inorganic salt as a starting material of W03. TEM investigation revealed that the very fine WO 3 particles surrounded Al 2 O 3 particles. During the sintering process under the reductive atmosphere, WO 3 was reduced to metal particles. TEM observation indicated that the nanometer-sized W particles were mainly located within the A1 2 O 3 matrix grains. The mechanical properties were improved by dispersing small amount of nanometer-sized metal particles and resultant internal stresses.

New Nanocomposite Structural Ceramics

The new materials design was established for the structural ceramics to break through the present monolithic and composite ceramics. In this concept, the ceramic-based composites were divided into two groups: micro and nanocomposite. The microcomposite is the so-called composite in which the micro-sized particles, whiskers, platelets and fibers are dispersed at the grain boundaries of matrix grains. The nanocomposites, on the other hand, have three types of structures: intra and intergranular nanocomposites and nano/nano composite. The intra and intergranular nanocomposites were found to show the tremendous improvement of mechanical properties even at elevated temperatures compared with those of monolithic and microcomposite ceramics. The nano/nano composites gave the new functions such as machinability and superplasticity like metals. Our final goal for developing supertough and strong ceramics was achieved by hybridizing the intra and intergranular nanocomposites with the micro-meter sized fibers. This new concept was found to be also applicable for ceramic-metal composite systems.

Deformation of sapphire induced by a spherical indentation on the (101̄0) plane

This work clarifies the origin of the characteristic discontinuities registered during the deformation of the (1010) plane of sapphire, by means of depth sensing indentation experiments with a spherical indenter. The sudden increase of the plasticity of the crystal was found to be caused by a twinning process. The calculation and analysis of the distribution of resolved shear stresses under a spherical indenter predicted the features of the surface deformation. The high resolution microscopic examination of the residual impressions confirmed the theoretical prediction.

Fabrication process and electrical properties of BaTiO3/Ni nanocomposites

Abstract A new capacitor, based on tetragonal BaTiO 3 /Ni micro- and nanocomposites, was made from BaTiO 3 and NiO. The NiO powders were reduced to micro- and nano-sized metal particulates during early stages of sintering, and the mixtures were pressureless-sintered in flowing argon gas atmosphere. TEM observation revealed that the nano-sized Ni particulates were homogeneously dispersed within matrix grains. The fracture toughness was improved 2.5 times by the Ni dispersion. The dielectric constant was also improved extremely by these Ni dispersion.

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.

Non-linear surface deformation of the (101̄0) plane of sapphire: identification of the linear features around spherical impressions

Abstract The work aims to clarify the origin of the characteristic linear features which appear in the vicinity of the spherical indentation impressions on the (10 1 0) plane of sapphire crystal. Since the location of the features was already predicted by the Resolved Shear Stress Model which pointed towards twinning as a source of the surface defects, in the present study the electron and atomic force microscopy were employed to verify the results of earlier numerical calculations. In order to identify the deformation systems which contributed to the formation of the linear features, the precise measurement of the surface slope in the defected region has been accomplished, which led to the conclusion that the observed eruptions cannot be caused by the rhombohedral twinning exclusively. The presented results indicate that the basal twinning contributed to the process of linear features formation. It is concluded that the observed surface defects may result from the cooperative basal twinning and rhombohedral slip or twinning—something predicted in an earlier model. The present study supports the conclusion that the high-load pop-ins registered during the loading cycle of the indentation into the (10 1 0) plane of sapphire are due to twinning—the process being largely overlooked in the recent studies of non-linear indentation deformation mechanisms.

Effects of ZrO2 addition on microstructure and mechanical properties of MoSi2

Fine 3Y-ZrO{sub 2} added MoSi{sub 2}-based composites were fabricated by the conventional hot-pressing method. These composites mainly consisted of matrix MoSi{sub 2}, well-crystallized ZrSiO{sub 4}, Mo{sub {<=}5}Si{sub 3}C{sub {<=}1} and non-reacted tetragonal ZrO{sub 2}. The intergranular glassy SiO{sub 2} phase could be changed into the thermodynamically stable ZrSiO{sub 4} crystalline phase by the 3Y-ZrO{sub 2} addition. Mechanical properties at room temperature such as fracture toughness and strength were enhanced by the 3Y-ZrO{sub 2} addition. The enhancement of fracture toughness in the MoSi{sub 2}-3Y-ZrO{sub 2} system was attributed mainly to the decrease of the brittle glassy SiO{sub 2} phase at grain boundaries and the residual compressive stresses induced by the mismatch of thermal expansion coefficient between MoSi{sub 2} and ZrSiO{sub 4}. These results indicate that in-situ crystallization of the glassy SiO 2 phase by the addition of 3Y-ZrO{sub 2} has great advantage to fabricate MoSi{sub 2}-based composites with well-balanced mechanical properties. Further optimization of fabrication conditions will enable one to improve the mechanical properties of MoSi{sub 2}-ZrO{sub 2} system.

Peculiar surface deformation of sapphire: Numerical simulation of nanoindentation

This report addresses the origin of peculiar anisotropic deformation of sapphire. The three-dimensional finite element simulation of the contact between spherical indenter and elastically anisotropic solid allowed us to analyze stress under the tip that penetrates in the (1010) and (0001) planes, and consequently, to localize those regions in which particular deformation mechanisms are likely to be activated. This approach contrasts the available models of “hardness anisotropy,” which routinely apply a modified uniaxial-stress approach to this essentially three-dimensional, nonisotropic contact problem. The calculated results are in agreement with the microscopic inspection of impressions; that is, the surface features reflect the distribution of stress. The computations made it also possible to evaluate the actual radius of the tip (nominally 5 μm ball).