Weiwu Chen

Estimation of the phase diagram for the ZrO2-Y2O3-CeO2 system

Abstract Comprehensive descriptions of the thermodynamic properties and experimental information in three oxide systems ZrO 2 –Y 2 O 3 , ZrO 2 –CeO 2 and Y 2 O 3 –CeO 2 are given and thermodynamic models for the calculation of these systems are discussed. The phase diagrams of the quasi-ternary ZrO 2 –Y 2 O 3 –CeO 2 system in the zirconia-rich corner are estimated at different temperatures with a substitutional model and Muggianus extrapolation. The equilibrium phase diagram calculation is extended to low temperatures, as well as the Gibbs free energy of the tetragonal, monoclinic and cubic phases of zirconia doped with yttria and ceria.

Synthesis of (Ca,Mg)-α-sialon from slag by self-propagating high-temperature synthesis

The synthesis of (Ca,Mg)-α-sialon powders using slag as a starting material by self-propagating high-temperature synthesis (SHS) is reported for the first time. The only crystalline phase detected in the synthesized products was α-sialon. The characters of the synthesized powders were observed and compared with that of conventionally hot-pressed materials. More metal cations were absorbed into the α-sialon structure of the SHS synthesized powders. Fully densified sialon ceramics can be obtained using the SHS powders by hot-pressing. The hardness and fracture toughness of the hot-pressed α-sialon ceramics reached 16 GPa and 5.1 MPa m−1/2 respectively.

Effect of processing on the morphology of α-sialon grains

Abstract In order to explore the effect of processing on the formation of elongated α-sialon grains, a mixture of α-Si 3 N 4 and β-Si 3 N 4 (5–40 wt.%) powders combined with variant firing cycles was used for preparing an α-sialon doped with Dy 2 O 3 . The results showed that the morphology of α-sialon grains was more closely relevant to the firing conditions than to the composition of the starting powder. The specimen sintered by gas pressure sintering (GPS) or through heat treatment at 1800°C or1900°C (for hot-pressed (HP) specimen) possessed a bimodal microstructure. The mechanism of forming elongated α-sialon grains is discussed in the present work.

Optical properties of SPS-ed Y- and (Dy,Y)-α-sialon ceramics

Y0.67Si9Al3ON15 and Dy0.4Y0.3Si8.85Al3.15O1.05N14.95 ceramics were prepared by Spark Plasma Sintering (SPS) with and without heat treatment at 1700°C for 7 h or 17 h, and their optical transmittance were investigated over the wavenumber range 4000–1500 cm−1. The results showed that the assemblages of the SPS-ed samples consisted of single crystallized α-sialon phase in both compositions. EDS analysis indicated that α-sialon was mainly stabilized by Dy3+ in the multi-cation (Dy,Y)-α-sialon composition. The SPS-ed specimens showed relatively high optical transmission properties, and the maximum transmittance reached around 65% at 2800 cm−1 for 0.5 mm thick specimens of both compositions. The 7 h heat treatment caused the formation of small amount of melilite phase, resulting in non-uniform microstructure and decrease in optical transmittance. Extended heat treatment for 17 h led to more homogenous microstructure and increased the transmittance to some extent. Less melilite was formed in the multi-cation (Dy,Y)-α-sialon composition than in the single cation Y-α-sialon after heat treatment, and the transmittance of (Dy,Y)-α-sialon was also higher than that of Y-α-sialon.

Phase formation and microstructural evolution of Ca α-sialon using different Si3N4 starting powders

Abstract Silicon nitride has two polymorphous structures, α-Si 3 N 4 and β-Si 3 N 4 . In this study three different Si 3 N 4 starting powders (∼100%α, 40%α+60%β, ∼100%β) were used to prepare Ca α-sialon with the composition Ca 1.8 Si 6.6 Al 5.4 O 1.8 N 14.2 by pressureless sintering. Comparison was made concerning the densification process, reaction sequence and microstructure of the corresponding materials. The sluggish reactivity of β-Si 3 N 4 resulted in poorer densification during sintering. All the three starting powders produced a similar final phase assembly, namely α-sialon together with a small amount of AlN and AlN polytypoid except that traces of unreacted β-Si 3 N 4 remained until 1800° in samples prepared with ∼100%β-Si 3 N 4 powders. Elongated α-sialon grain morphology has been identified in the samples prepared using all the three different Si 3 N 4 starting powders. Coarser elongated α-sialon grains with lower aspect ratio were found in samples using higher β phase starting powders.

Effect of additives on microstructure of Ca α-sialon

Abstract Three kinds of additives were added into Ca α-sialon, (Ca 0.6 Si 10.2 Al 1.8 O 0.6 N 15.4 ). It was found that the additives resulted in more of the liquid phase during sintering, which promoted the completion of the α→α′ transformation and the development of the elongated morphology characteristic of α-sialon grains. Comparison was made concerning the effect of the amount and type of additives on the microstructural development of Ca α-sialon. Phase and property analysis was also carried out on the hot-pressed samples.

Formation behavior, microstructure and mechanical properties of multi-cation α-sialons containing calcium and neodymium

Abstract Dual modifying cation (Ca+Nd) α-sialons were hot pressed for the x=1.0 composition ((Ca,Nd)xSi12-3nAl3nOnN16-n) with mixtures of CaO:Nd2O3 in the molar ratios of 0:1, 0.3:0.7, 0.5:0.5, 0.7:0.3 and 1:0. Experimental results showed that except for pure Nd- and Ca-α-silaon compositions, the main crystalline phase in samples hot-pressed at 1750°C for 1 h was α-sialon together with a small amount of melilite solid solution and AlN polytypoid. The higher CaO:Nd2O3 ratio in the starting composition, the higher α-sialon content and the lower melilite solid solution content in the final phase assembly. The α-sialon lattice also expanded with increasing substitution of Ca for Nd. Elongated grain morphology was observed in (Ca+Nd) α-sialon. EDS analysis revealed higher solubility of Ca than Nd in the α-sialon lattice and lower solubility of Ca than Nd in the intergranular phase. The hardness increased, ranging from 15.87 to 18.79 MPa, when the amount of α-sialon phase in the material became higher. The fracture toughness varied from 4.2 to 5.1 MP am1/2 for (Ca+Nd) α-sialon compositions, in which the composition with CaO:Nd2O3=0.5:0.5 possessed the highest value.

Anisotropic grain growth of R-α-sialon (R = Nd and Er)

Two R-α-sialon (R0.6Si9.3Al2.7O0.9N15.1, R=Nd and Er) compositions were first fired at 1750°C/25 min and 1650°C/2 h respectively for completion of the α → α′ phase transformation. Elongated α-sialon grain morphology was developed in both samples after being re-fired at 1800°C for different periods of time. The growth in width of R-α-sialon grains is controlled by diffusion in the liquid, while the length growth tends to be interfacial reaction controlled. The anisotropic growth of R-α-sialon is attributed to the large difference in the growth rate constant between the length and the width directions of the grain.

Effect of processing on toughness of Ca α-sialon ceramics

The influence of processing on microstructural development of Ca α-sialon composition (CaxSi12−3xAl3xOxN16−x) with x = 1.8 was studied by dwelling at different intermediate temperatures before reaching the final sintering temperature. The microstructural observation results have showed the different aspect ratios of elongated grains obtained by the various processing conditions, reflecting the effect of the number of nuclei of α-sialon on morphology of grains during sintering. Improved toughness was achieved by applying low temperature dwelling for Ca α-sialon compositions with low x values. The toughness showed an increase of 33% and 16% for x = 0.6 and 1.0 compositions respectively with middle dwelling processing at 1350°C for 3 h before reaching 1750°C for 1 h by hot-pressing.

Oxidation behavior of N-containing melilite solid solutions Nd2Si3−xAlxO3+xN4−x

Abstract Four compositions of N-containing Nd-melilite solid solution (Nd-M′) Nd2Si3−xAlxO3+xN4−x (x = 0,0.3, 0.6, 1.0) prepared by hot-pressing technique were oxidized at 1000 °C or 1250 °C (only for x = 1.0) for 20 h in air in a TG unit. The phase compositions, densities, oxidation products, oxidation curves and microstructures of the samples were compared. The oxidation curves obtained indicate that the weight gains per unit area decrease obviously with increasing the Al substitution in Nd-M′, which is in good agreement with the corresponding changes in morphologies of the oxide-scale and cross-section of the samples, i.e. the cracks vary with the increase of Al content of Nd-M′, from large to small, and transit to pores. The oxidation mechanism of Nd-M′ was also discussed.