Porous 2H-Silicon Carbide Ceramics Fabricated by Carbothermal Reaction between Silicon Nitride and Carbon

Porous SiC ceramics were synthesized by sintering pressed and pressed/CIPed powder compacts of α-Si₃N₄, carbon (Si₃N₄:C = 1:3 mol as ratio), and sintering aids, at 1600°C for few hours to achieve a reaction, and subsequently sintering at a temperature range of 1750°–1900°C, in an argon atmosphere. High porosities from 45%–65% were achieved by low shrinkage with large weight loss. Formation of pure 2H-SiC phase via a reaction between Si₃N₄ and carbon can be demonstrated by X-ray diffractometry. The resultant porous SiC samples were characterized by SiC grain microstructures, pore-size distribution, and flexural strength. This method has the advantage of fabricating high-porous SiC ceramics with fine microstructure and good properties at a relatively low temperature.     

Mullite–Boron Nitride Composite with High Strength and Low Elasticity

Mullite–boron nitride (BN) composite with high strength, low Young's modulus, and highly improved strain tolerance was prepared by reactive hot pressing (RHP) using aluminum borates (9Al₂O₃·2B₂O₃ and 2Al₂O₃·B₂O₃) and silicon nitride as starting materials. Compared with the monolithic mullite, the composite RHPed at 1800°C showed 1.64 times (540 MPa) the strength, 70% (153 GPa) the Young's modulus, and 2.34 times (3.53 × 10⁻³) the strain tolerance. Transmission electron microscopy observation revealed that the composite had an isotropic microstructure with a fine mullite matrix grain size of less than 1 μm and nanosized hexagonal BN (h-BN) platelets of about 200 nm in length and 60–80 nm in thickness. The high strength was suggested to be from the reduced matrix grain size and the small toughening effect by the h-BN platelets.     

Tribological Properties of Silicon Carbide and Silicon Carbide–Graphite Composite Ceramics in Sliding Contact

A monolithic SiC ceramic and two SiC–C composite ceramics containing 10 and 20 vol% graphite were fully densified with Al₄C₃ and B₄C as additives. The tribological properties of these materials were evaluated by sliding against sintered silicon carbide under dry conditions using two tribometers, block-on-ring and pin-on-disk, where wear occurred under low and high contact stresses, respectively. For all three materials, under low stress, worn surfaces were smooth and wear processes were dominated by tribochemical reaction; under high stress, worn surfaces were rough and wear processes were dominated by fracture and three-body abrasion. A lubricating effect of the graphite particles in the SiC–C composites was observed in all sliding tests. However, while the addition of graphite could concurrently result in a reduction in friction and an increase in wear resistance in the block-on-ring tests, the addition of graphite led to sharply enhanced wear rates despite the lowered coefficients of friction in the pin-on-disk tests. The cause for that difference was attributed to the effect of both the hardness of the materials and the contact stresses.     

High-Strength Porous Silicon Carbide Ceramics by an Oxidation-Bonding Technique

Porous silicon carbide (SiC) ceramics were fabricated by an oxidation-bonding process in which the powder compacts are heated in air so that SiC particles are bonded to each other by oxidation-derived SiO₂ glass. Because of the crystallization of amorphous SiO₂ glass into cristobalite during sintering, the fracture strength of oxidation-bonded SiC ceramics can be retained to a relatively high level at elevated temperatures. It has been shown that the mechanical strength is strongly affected by particle size. When 0.6 μm SiC powders were used, a high strength of 185 MPa was achieved at a porosity of ∼31%. Moreover, oxidation-bonded SiC ceramics were observed to exhibit an excellent oxidation resistance.     

Realistic imitation of mosquito's proboscis: Electrochemically etched sharp and jagged needles and their cooperative inserting motion

Aiming at the use in low-invasive medical treatments, this paper proposes a realistic imitation of mosquito's proboscis. A silicon needle is electrochemically etched, making the three-dimensionally sharp tip with finely smooth surface. The jagged shank shape is machined by a deep reactive ion etching (DRIE). The combined needles comprising a central straight needle and two outer jagged needles are fabricated, imitating a labrum and two maxillas of the mosquito, respectively. The cooperative motion of the three needles imitating the mosquito's motion is realized by applying PZT actuators independently to all the needles. The effectiveness of inserting these needles cooperatively was experimentally confirmed. Considering practical medical application, a biodegradable polymer needle with three-dimensionally sharp tip is also developed. The fabrication process based on micromolding is as follows: a nickel negative cavity is made by electroplating on a silicon sharp needle, to which melted polymer is injected, and it is finally released using a lost molding technique. The effectiveness of sharp tip for easy insertion was experimentally proven.     

Magnetic Properties of a Nitronyl Nitroxide Triradical as a Model for Single-Component Molecule-Based Ferrimagnets

IR and Raman spectra of neutral iodine substituted electron donors DIET, DIEDO and DIETS were measured. Their fundamental vibrational modes were calculated and compared with the experimental data. Additionally, we studied IR and Raman spectra of 2:1 charge-transfer salts formed by these donors with paramagnetic [Fe(bpca)(CN)₃] anions. Polarized IR reflectance spectra of DIET and DIEDO salts were also recorded.     

Sintering and Mechanical Properties of Stoichiometric Mullite

Stoichiometric mullite powder (3Al₂O₃·2SiO₂) prepared by spray pyrolysis and sintered at 1650°C attained 95% of theoretical density. The flexural strength was 360 MPa at room temperature and decreased slightly at 1400°C. A fairly high K_Ic value (2.8 MN/m^3/2) was obtained. These mechanical properties can be attributed to the highly homogeneous stoichiometric composition of the raw powder.     

Pulse Electric Current Sintering of Al2O3/3 vol% ZrO2 with Constrained Grains and High Strength

The pulse electric current sintering technique (PECS) was demonstrated to be effective in rapid densification of fine-grained Al₂O₃/3Y-ZrO₂ using available commercial powders. The composites attained full densification (>99% of TD) at 1450°C in less than 5 min. The composites sintered at a high heating rate had a fine microstructure. The incorporation of 3 vol% 3Y-ZrO₂ substantially increased the average fracture strength and the toughness of alumina to as high as 827 MPa and 6.1 MPa·m^1/2, respectively. A variation in the heating rate during the PECS process influenced grain size, microstructure, and strength, though there was little or no variation in the fracture toughness.     

Thermal Conductivity of ß-Si3N4: I, Effects of Various Microstructural Factors

Calculations based on a simple modified Wiener's model for thermal conductivity of a composite material predict that the thermal conductivity of ß-Si₃N₄ decreases quickly as the grain-boundary film thickness increases within a range of a few tenths of a nanometer and also that it initially increases steeply with increased grain size, then reaches almost constant values. Because of the faceted nature of the ß-Si₃N₄ crystal, the "average" grain-boundary film thickness is much greater than that in equilibrium. The present study demonstrates both theoretically and experimentally that grain growth alone cannot improve the thermal conductivity of ß-Si₃N₄.