Oxygen Content in ß-Si3N4 Crystal Lattice Kitayama, Mikito; Hirao, Kiyoshi; Tsuge, Akira ...
Journal of the American Ceramic Society,
11/1999, Letnik:
82, Številka:
11
Journal Article
Recenzirano
ß‐Si3N4 crystals were obtained through the heat treatment of alpha‐Si3N4 powder with additives, Y2O3:SiO2= 1:2 and 2:1, and subsequent acid treatments that removed the secondary phases. The lattice ...oxygen contents of these crystals were determined by the hot‐gas extraction method to be 0.258 ± 0.006 and 0.158 ± 0.003 wt% for the additive compositions of Y2O3:SiO2= 1:2 and 2:1, respectively. The oxygen dissolved in the ß‐Si3N4 crystal lattice as much as in the alpha‐Si3N4 crystral lattice prepared by the chemical vapor deposition process and in the AlN crystal lattice that exhibited high thermal conductivity.
β‐Si
3
N
4
ceramics sintered with a series of rare‐earth (RE = La, Nd, Gd, Y, Yb and Sc) oxide additives were fabricated by hot pressing and subsequent annealing. Their microstructures, lattice ...oxygen contents, and thermal conductivities were evaluated. Mean grain size increased, while lattice oxygen content decreased, and hence, thermal conductivity increased with decreasing ionic radius of the rare‐earth element. In all cases, a marked change was observed in the order of ionic radius from La to Nd to Gd, and a little change was observed below them. Rare‐earth oxide additives significantly influenced the thermal conductivity of β‐Si
3
N
4
, unlike in the case of AlN.
Microstructure designing of silicon nitride Kanzaki, Shuzo; Brito, Manuel E.; Valecillos, M.Cecilia ...
Journal of the European Ceramic Society,
1997, 1997-1-00, 19970101, Letnik:
17, Številka:
15
Journal Article, Conference Proceeding
Recenzirano
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.
In their recent paper, Wang et al. proposed a new growth mechanism, and developed an analytical solution for the steady state to explain their experimental observation of liquid-grown silicon nitride ...(Si sub 3 N sub 4 ) crystals with a concave depression in the center of the (001) end face. The key to their analysis is that the chemical potential at the center is lower than the edge as evidenced by the negative curvature at the center. From consideration of continuity, it follows that the chemical potential at the (100) side surface is higher than that at the end tip. Two growth modes of the end face--(i) liquid diffusion from the surroundings and (ii) surface diffusion from the edge--are additive to each other. The liquid diffusion mode has a tendency to smooth out the curvature difference. If the source of atoms is primarily from the edge, then the curvature difference must continue to sustain the driving force for surface diffusion. Thus, they claimed that the surface mode is the dominant form of atmoic transport. Although their analysis successfully explained the phenomena, we found that there was a misunderstanding in the formulations for the chemical potentials of the side and end surfaces of beta -Si sub 3 N sub 4 crystals presented in the beginning.
The influence of a strong/weak interface ratio on the mechanical properties of Si3N4/BN‐based layered composites was studied. The ratio was controlled by the number of BN spots between the adjacent ...Si3N4 layers. By increasing the BN interface area from 0% to 72%, fracture toughness increased from 7.7 to 10.9 MPa·m1/2, and bending strength decreased from 1275 to 982 MPa. Fracture toughness was improved from 8.6 to 10.1 MPa·m1/2 by additional heat treatment of samples containing 2 vol%β‐Si3N4 seed particles. The bending strength of samples with 35% weak BN interfaces, measured perpendicular and parallel to layer alignment, was 1260 and 1240 MPa, respectively. This confirmed the two‐directional isotropy of layered samples.
