Shuzo Kanzaki

Effective Sintering Aids for Low-temperature Sintering of AlN Ceramics

A disappearing sintering aid was used to promote densification during the initial and middle stages of sintering and to be removed in gaseous form from the specimens during the final stage of sintering. From thermodynamic consideration such as assessment of Gibbs free energy change of formation of Al 2 O 3 compounds including metal-oxide and evaluation of the vapor pressure of metal-oxide, Li 2 O is expected to become a disappearing sintering aid for AlN sintering. Doping with Li 2 O resulted in densification of AlN ceramics with Y 2 O 3 and CaO additives by sintering at a firing temperature of 1600 °C. The amount of Li 2 O in the specimens decreased by volatilization at temperatures higher than 1300 °C, and its amount was at a level of several ppm after firing at 1600 °C for 6 h. Low-temperature densification of AlN specimens by addition of Li 2 O also caused the improvement of thermal conductivity and mechanical strength of sintered specimens. Present results indicate that a Li 2 O addition is effective for AlN sintering. Furthermore, LiYO 2 was also used as a new sintering aid instead of Li 2 O and Y 2 O 3 , and the results of thermal conductivity and mechanical strength are shown.

Hot Isostatic Pressing to Increase Thermal Conductivity of Si 3 N 4 Ceramics

Highly anisotropic Si 3 N 4 ceramics were successfully fabricated by tape-casting of raw α–Si 3 N 4 powders with β–Si 3 N 4 single-crystal particles as seed particles and Y 2 O 3 as an effective sintering aid, followed by hot isostatic pressing at a temperature of 2773 K for 2 h under a nitrogen gas pressure of 200 MPa. The microstructure consists of very large elongated grains (diameter ~10 μm; length of ~200 μm), highly oriented in the tape-casting direction. The thermal conductivity along this direction reaches 155 W m -1 K -1 at room temperature, but varies significantly between room temperature and 1273 K. This thermal conductivity is closely related to (1) formation of extremely large elongated β–Si 3 N 4 grains with a reduced amount of crystal defects due to the high-temperature firing and to (2) orientation of β–Si 3 N 4 grains due to addition of seed particles and to tape-casting.

Modeling and simulation of grain growth in Si3N4—I. Anisotropic Ostwald ripening

Abstract The anisotropic Ostwald ripening model has been developed for completely faceted crystals. This model has been applied to the simulation of grain growth in β -Si 3 N 4 with a highly anisotropic rod-like grain shape developed in the liquid phase. The reduction of aspect ratio after the phase transformation observed by previous studies is proved to be a consequence of the anisotropic Ostwald ripening. This model predicts a growth exponent n =3 for totally interfacial reaction controlled kinetics, and higher values when the diffusion constant approaches the interfacial reaction constants. This would explain the puzzling results reported by previous works that growth exponents n =3 or higher have been observed in the grain growth of faceted crystals. While the length distribution becomes wider with time, the reduced radius distribution approaches the shape that is known as the asymptotic distribution function derived from the LSW theory.

Microstructure designing of silicon nitride

A new concept of materials design that allows simultaneous control of the morphologies and distribution of the structural elements at plural scale levels to create a new family of advanced ceramics was proposed. The validity of the concept was experimentally demonstrated using silicon nitride ceramics as model materials. Controlling anisotropic grain growth by seeding of small amounts of morphologically regulated, β-silicon nitride single crystals (micro-scale level control), combined with alignment of the seed particles by tape casting followed by stacking of laminates (macro-scale level control) allows compatibility of high strength and high fracture toughness in this material, with a high degree of reliability for the mechanical strength.

Modeling and simulation of grain growth in Si3N4—II. The α–β transformation

Abstract A model for the α – β transformation has been developed for completely faceted crystals as an extension of the anisotropic Ostwald ripening model developed in the companion paper. Si 3 N 4 grain growth simulations have been performed using various relationships between diffusion and interfacial reaction constants. It has been found that length growth is dominant, and that its growth rate is independent of width and does not change with time in the totally interfacial reaction controlled case during the α – β transformation. Simulation predicts that a time–length relationship deviates from a straight line as the growth kinetics in the length direction shifts from interfacial reaction controlled to diffusion controlled. It has been confirmed that the ratio of the interfacial reaction constants of the (100) and (001) interfaces and the α – β ratio are the key factors for determining the aspect ratio of β -Si 3 N 4 grains.

Effect of additives on some properties of silicon oxynitride ceramics

Silicon oxynitride ceramics are formed by reaction sintering of silicon nitride and silica with certain metal oxide additives. The reaction rate during sintering and the subsequent properties of silicon oxynitride are affected by the quantity and kinds of additives. The reaction rate increased for addition of equal molar amounts of ZrO2, ZrO2 (+2.8 mol % Y2O3), AlO1.5, LnO1.5, CeO2, MgO, in that order (where Ln=Nd, Sm, Gd, Dy, Er, Yb and Y). The lanthanide oxide (1.5 mol %)-doped silicon oxynitride ceramics had a high fracture toughness, because crack deflection occurred due to the precipitation of an intergranular crystalline phase with a high thermal expansion coefficient compared with silicon oxynitride. The oxidation rate was higher with an increasing quantity of additive. In samples containing an intergranular crystalline phase, stability of the crystalline phase is an important factor and could impair the oxidation resistance of silicon oxynitride ceramics.

Factors affecting mechanical properties of silicon oxynitride ceramics

Abstract The effect of additives and impurities on the mechanical properties of silicon oxynitride ceramics was investigated. The toughening of the ceramics was affected by three factors: (1) the thermal tensile stress in the intergranular glassy phase and the compressive stress in Si2N2O grains, developed by a thermal expansion mismatch between Si2N2O grains and the intergranular glassy phase; (2) the large grain size of Si2N2O; and (3) the concentration of impurities in the intergranular glassy phase. The degradation of high-temperature strength was primarily dependent on the basic chemical composition (the kind of additives (MeOx) and Me Si ratio) of the intergranular phase and impurities.

Joining of silicon nitride ceramics by hot pressing

The joining of hot-pressed silicon nitride ceramics, containing Al2O3 and Y2O3 as sintering aids, has been carried out in a nitrogen atmosphere. Uniaxial pressure was applied at high temperature during the joining process. Polyethylene was used as a joining agent. Joining strength was measured by four-point bending tests. The effects of joining conditions such as temperature (from 1400 to 1600°C), joining pressure (from 0.1 to 40 MPa), holding time (from 0.5 to 8 h) and surface roughness (Rmax) of the joining couple (about 0.12, 0.22 and 1.2μm) on the joining strength were examined. The joining strength was increased with increases in joining temperature, joining pressure and holding time. Larger surface roughness caused lower joining strength. The higher joining strength was attributed to a larger true contact area. The area was increased through plastic deformation of the joined couple at elevated temperatures. The highest joining strength attained was 567 MPa at room temperature, which was about half the value of the average flexural strength of the original body. The high temperature strength measured at 1200° C did not differ very much from the room-temperature value.

Multifunctional Si3N4/(β-SiAlON+TiN) layered composites

Abstract Layered multifunctional ceramic composites on the base of Si 3 N 4 and TiN have been prepared by tape casting. The reaction conditions for in situ preparation of β-SiAlON + TiN composite were optimised and dense Si 3 N 4 /(β-SiAlON + TiN) layered materials were prepared by hot pressing. The bending strength and fracture toughness of layered materials measured in the direction perpendicular to the layer alignment were remarkably higher (1184 MPa and 9.75 MPa m 1/2 ) in comparison to the “monolithic” β-SiAlON + TiN composite (647 MPa and 4.71 MPa m 1/2 ). High anisotropy was achieved for the electrical resistance of the layered materials in parallel (6.10 −2 Ω cm) and perpendicular (5×10 11 Ω cm) direction to the layer alignment.

Preparation of translucent mullite ceramics

Des ceramiques de mullite hautement translucides ont ete preparees par filtrage sans pression ni additif, avec elimination des phases secondaires et agrandissement approprie de la taille des grains de mullite