Threshold Stress for Crack-Healing of Si3N4/SiC and Resultant Cyclic Fatigue Strength at the Healing Temperature

Si₃N₄/SiC composite ceramics were hot-pressed in order to investigate the crack-healing behavior under stress. Semi-elliptical surface cracks of 100 μm in surface length were made on each specimen. The pre-cracked specimens were crack-healed under cyclic or constant bending stress, and the resultant bending strength and cyclic fatigue strength were studied. The threshold stress for crack-healing was investigated at healing temperatures of 1000° and 1200°C. The cyclic fatigue strengths of crack-healed specimens were also investigated at healing temperatures of 900° and 1000°C. The main conclusions are as follows: (1) The threshold cyclic and constant stresses for crack-healing, below which pre-cracked specimens recovered their bending strength, were 300 MPa, which was 75% of the bending strength of the pre-cracked specimens and (2) the crack-healed specimens exhibited quite high cyclic fatigue strength at crack-healing temperatures of 900° and 1000°C.     

Lattice Strain and Dislocations in Polished Surfaces on Sapphire

Polishing of ceramics, surface defects and strain are introduced that influence the mechanical properties in the final products. In this study, the sapphire surfaces with a -plane and c -plane were polished using several kinds of diamond slurry, and the introduced damaged zones were investigated using transmission electron microscopy (TEM), high-resolution TEM, and TEM-CBED (convergent beam electron diffraction). It was found that the types of slip systems and strain distribution were different between the a -plane-polished and the c -plane-polished specimens. That is, the basal slip and the prism slip were mainly activated in the c - and a -plane-polished specimens to form the different damaged zones. This difference was probably due to the difference in the direction and angle from the slip plane and applied shear stress during the polishing. On the basis of these observations, the types of introduced dislocations and distribution of strain were discussed to clarify the mechanism of polishing.     

Self-ordering of anodic porous alumina formed in organic acid electrolytes

New self-ordering porous alumina films were fabricated in organic acid electrolytes. Highly ordered cell arrangements of porous alumina films were realized in malonic acid at 120 V and tartaric acid at 195 V having 300 nm and 500 nm pore intervals, respectively. Self-organization was achieved at the maximum voltage required to induce high-current-density anodization while preventing burning, i.e., an extremely high-current flow concentrated at local points. The cells of the film grown at a high field must be pressed against each other, so that the self-ordering proceeds with the porous layer growth. When the self-ordering of cell arrangement proceeds, the cells became smaller. To improve the regularity of the cell configuration, a low electrolyte temperature and a relatively high electrolyte concentration were effective for maintaining a high-current-density to prevent burning. Surface flatness was an essential factor for self-ordering, however, the surface oxide film produced by electropolishing an aluminum substrate prevented quick pore growth in the organic acids having a low dissociation constant. It is confirmed that electropolishing followed by alkaline treatment was most appropriate as the pre-treatment in preparing flat surfaces.     

Non-destructive Measurement of Moisture Distribution in Wood during Drying Using Digital X-ray Microscopy

The objective of this study was to develop a nondestructive method by which moisture distribution in wood during drying could be predicted. A newly developed digital X-ray microscope was used to measure the moisture content of wood and its accuracy and resolution was evaluated compared to the classic oven-dry method.

Small green wood specimens of Sugi (Cryptomeria japonica D. Don) were cut and dried under constant temperature and humidity. As the weight was decreasing during drying, X-ray microscope images of cross section were obtained. From these digital images and specimen weight, the moisture content during drying was measured by the two methods. After the shrinkage of the specimen was canceled, the standard error achieved finally was about 1% moisture content within the experimental range. As the image was divided into small subimages, the clear moisture distribution can be seen. It was found that the image divided into 32 × 32 subimages in each size of 0.625 × 0.625 mm might be valid to determine the moisture distribution, and that the drying rate in early wood is larger than in late wood.     

Microstructure design of lead-free piezoelectric ceramics

Computational and experimental methodologies are integrated into a novel combined technique to define microstructure design criteria and maximize the properties of rhombohedral Bi_0.5Na_0.4K_0.1TiO₃, from untextured (1 MRD), d₃₃ = 155 pC/N, to textured (4.41 MRDs), d₃₃ = 227 pC/N. Two-dimensional orientation maps obtained using electron backscatter diffraction on sequential parallel layers are used to computationally reconstruct three-dimensional samples, simulate the local piezoelectric grain interactions, and thus demonstrate that superior lead-free piezoelectric microstructures can be fabricated by engineering its associated crystallographic and polarization texture. Computer-generated material representations, based on the experimentally determined microstructures, were used to simulate the crystallographic orientation of each grain, as function a macroscopic polarization and crystallographic texture. Computer-generated material representations, based on the experimentally determined microstructures, were used to simulate the crystallographic orientation of each grain, as function a macroscopic polarization and crystallographic texture. The method takes advantage of the anisotropy of the properties of the underlying single-crystal phases and delivers a guide to search for material anisotropy |microstructure parameters that are optimal in piezoelectric performance and reliability, and thus establish practical links between structure and macroscopic length scales.     

Analysis of abnormal grain growth of oriented LiCoO2 prepared by slip casting in a strong magnetic field

The relationship between the development of the crystallographic orientation and the grain growth behavior were studied. The degree of orientations of the green compacts and sintered samples were evaluated by the Lotgering factor. The f(0 0 l) of all the samples were drastically increased with the increasing applied magnetic field strength. The f(0 0 l) of the samples sintered at 1223 K were improved in comparison to those of the green compacts. However, the f(0 0 l) value of the samples sintered at 1273 K were not increased at 4 T or lower. To characterize the grain growth process, these samples were analyzed using electron backscatter diffraction (EBSD). The sintered samples prepared in the magnetic field at 4 T or lower showed abnormal grain growth. The samples with an applied magnetic field of 8 T or higher had no abnormal grain growth. It was revealed that the orientation angle of the particles has an effect on the grain growth.     

Synthesis of polyguanamines from 2-N,N-dibutylamino-4,6-dichloro-1,3,5-triazine with aromatic diamines

Solution polycondensation of 2-N,N-dibutylamino-4,6-dichloro-1,3,5-triazine (BDCT) with various aromatic diamines, including 4,4′-oxydianiline (ODA), p-phenylenediamine (pPDA), m-phenylenediamine (mPDA), o-tolidine (oTD), 4,4′-(9-fluorenylidene)dianiline (BAFL), and 2,4-diamino-6-(N,N-dibutylamino)-1,3,5-triazine (DABT), was investigated. High molecular weight (>10,000) polyguanamines (PGs) were obtained via the polymerization of BDCT with ODA, pPDA, oTD, and BAFL at 150–180 °C in N-methylpyrrolidone (NMP) for 6 h. The polymerizability with BDCT was determined on the basis of the molecular weights of the polymers and the chemical shifts of the NMR data as follows: ODA > oTD > BAFL > pPDA > mPDA ? DABT. PGs were obtained from the reaction of BDCT with ODA and mPDA in only 55–77% yield, which may be attributed to the formation of cyclic oligomers. All of the polymers showed high thermostability (5% weight-loss temperature in N₂ greater than 444 °C), and the polymers generated from reaction of BDCT with ODA, mPDA, and BAFL exhibited good solubility in tetrahydrofuran and polar aprotic solvents such as NMP.     

Doping dependence of low-energy quasiparticle excitations in superconducting Bi2212

The doping-dependent evolution of the d-wave superconducting state is studied from the perspective of the angle-resolved photoemission spectra of a high-T_c cuprate, Bi₂Sr₂CaCu₂ O_8+δ (Bi2212). The anisotropic evolution of the energy gap for Bogoliubov quasiparticles is parametrized by critical temperature and superfluid density. The renormalization of nodal quasiparticles is evaluated in terms of mass enhancement spectra. These quantities shed light on the strong coupling nature of electron pairing and the impact of forward elastic or inelastic scatterings. We suggest that the quasiparticle excitations in the superconducting cuprates are profoundly affected by doping-dependent screening.     

Temperature dependence of tack and pulse NMR analysis of polystyrene block copolymer/tackifier system

The influence of tackifier structure on the temperature dependence of tack for a polystyrene block copolymer/tackifier system was investigated. A blend of polystyrene-block-polyisoprene-block- polystyrene triblock and polystyrene-block-polyisoprene diblock copolymers was used as the base polymer. Four different tackifiers were used: special rosin ester resin (RE), rosin phenolic resin (RP), hydrogenated cyclo-aliphatic resin (HC), and aliphatic petroleum resin (C5). Tack at 20?°C increased with the tackifier content for both RE and HC tackifier systems. Tack is affected by two factors: the work of adhesion at the adherend interface and the viscoelastic properties of the adhesive. The good balance of these two factors brought high tack. The adhesive with 10 wt.% tackifier exhibited the highest tack at 20?°C, whereas those with 30 and 50 wt.% tackifier were lower than those systems with 10 wt.% of the RP or C5 tackifiers. The adhesive with overly high hardness lowered the work of adhesion and the tack was not improved with more than 30 wt.%. A compatibility test in toluene solution and in solid state showed that tackifier RE has good compatibility with both polyisoprene and polystyrene, whereas tackifier RP has lower compatibility. Tackifiers HC and C5 had good compatibility with polyisoprene, but poor compatibility with polystyrene, and that of C5 was poorer. Pulse nuclear magnetic resonance (NMR) analyses indicated that tackifiers RE and HC effectively restrict the molecular mobility of polyisoprene phase.