He Zhuo Miao

Novel Low Thermal Conductivity Ceramic Materials for Thermal Barrier Coatings

The increase of the efficiency for gas turbines leads to the increasing combustion-chambertemperatures. Rapid degradation of the conventional yttria-stabilized zirconia coatings does not fulfill therequirements at these temperatures for a reliable thermal barrier coatings (TBCs) due to the phasetransformation of zirconia and the sintering behaviour. Therefore, it is very important to develop novelceramic materials for TBCs with low thermal conductivity and long-term stability at high temperatures.In this paper, the developments of potential novel ceramic materials for TBCs with low thermalconductivity are reviewed.

Preparation and Characterisation of Gd2Zr2O7 Ceramic by Spark Plasma Sintering

Rare earth Gd2Zr2O7 ceramic was prepared by spark plasma sintering from Gd2O3 and ZrO2 powders. The powders were sintered at 1400°C for 10min. The synthesized ceramic was annealed at 800°C for 2h under air atmosphere. XRD structural and SEM microstructural characterization showed the formation of a single phase material with pyrochlore crystal structure. The relative density of Gd2Zr2O7 ceramic was measured by the Archimedes method with an immersion medium of water and the results revealed that the relative density of the ceramic was 92%. The thermal conductivity of the ceramic was tested by laser flash method from room temperature to 700°C. The result shows the thermal conductivity of Gd2Zr2O7 ceramic is lower than that of 7YSZ.

Structure and Ionic Conductivity of Ln2Zr2O7-Type Rare Earth Zirconates

Three rare earth zirconates (Sm2Zr2O7, Gd2Zr2O7 and Er2Zr2O7) were prepared by solid state reaction. The crystal structure and ionic conductivity of these zirconates were characterized by X-ray diffraction (XRD) and complex impedance spectroscopy. The results show that Sm2Zr2O7 exhibits single-phase pyrochlore structure and Er2Zr2O7 exhibits single-phase fluorite structure, while Gd2Zr2O7 has pyrochlore and fluorite structure. Among three zirconates, the ionic conductivity of Sm2Zr2O7 is highest, while that of Er2Zr2O7 is lowest.

Synthesis and growth mechanism of silicon nitride nanostructures

A new method to synthesize Si3N4 nanostructures via catalyst-assisted polymeric precursor pyrolysis is present in this article. The as-prepared nanobelts are single crystals with a uniform thickness and width along the entire length, and contain no detectable defects such as dislocations or stacking faults. The thickness and width of Si3N4 nanobelts range from 40 to 60 nm and 600 to 1200 nm, respectively, and the lengths can be up to several millimeters. The growth directions of a-Si3N4 nanobelts are [101] and [100]. A solid-liquid-solid and gas-solid reaction/crystallization is proposed for the growth of S3N4 nonastructures.

Erosive Wear Behavior of Reaction Sintered Si3N4-SiCp Composite Ceramic in Liquid-Solid Flow

Si3N4-SiCp composites were prepared using fine Si powders as the starting materials for Si3N4, and SiCp as the aggregates. The erosive wear behavior of reaction sintered Si3N4-SiCp composite ceramic was investigated in liquid-solid flow. The results display that the composite ceramic hold a better capability of erosive wear resistance than 92 Al2O3 ceramics. The SEM pictures of the worn surface indicate that the main erosive wear mechanism of the composite ceramic is coexistence of micro-cutting and chisel-cutting.

Roles of Dopants in Sintering Behavior of ZnO-Based Varistor

Three series of ZnO-based materials with different doping levels were prepared. The correlation between the composition and microstructure, and the roles of main dopants, Bi2O3 and Sb2O3, in the sintering behaviors were proposed. Both Bi2O3 and Sb2O3 evaporated at 1115°C, but the amount of them, in which bismuth is the majority, is not significant. Bi2O3 functioned mainly as liquid during sintering to promote the sintering of ZnO, but it doesn’t mean the materials will be denser. The bismuth-rich phase retracted into small pores during cooling, leaving the big pores as voids at room temperature. More Bi2O3 added would result in less increase in material densities and dramatic decrease in relative densities, and a little bit increase in grain sizes of matrix ZnO. Sb2O3 would react with ZnO matrix into spinel, Zn7Sb2O12, which will pin at the grain boundary of ZnO to control the ZnO grain growth. The more Sb2O3 added, the smaller the grain sizes of ZnO. Appropriate amount of Sb2O3 added will yield denser materials.

Kinetics for Supercritical CO2 Debinding of Injection Molded ZrO2

Mercury porosimeter and scanning electron microscope (SEM) were used to analyze the pore structures evolution and distributions for supercritical CO2 debinding of injection molding ceramics. Classical diffusion equation was used to describe the mass transfer of supercritical CO2 debinding of the injection molded ZrO2 ceramics. The behavior and kinetics of the debinding were studied and analyzed. Results show that the solubility and diffusivity of soluble binder are the key factors in supercritical CO2 debinding, while the diffusivity is a dominant factor. The calculation data from the theoretical model are consistent with the experiment under the condition of enough long debinding time. It is shown that the diffusivity can be obtained by simple theoretical model combined with experimental data. The extraction rate and the extraction kinetics of the process can be predicted using the theoretical model.

Bending Strength of Ceramics Implanted by Titanium, Zirconium and Chromium Ions with MEVVA Source

ZTA (alumina toughened by 20 wt.% zirconia), hot-pressed silicon nitride (with totally 10 wt.% Y2O3 and Al2O3 as additives) and TZP (pressureless-sintered yttria stabilized zirconia) ceramics were implanted by various doses (5 × 1016 ions/cm2 ~ 1 × 1018 ions/cm2) of Ti, Zr, and Cr ions with a MEVVA (metal vapor vacuum arc) source implanter. The bending strength of these ceramics was investigated. It was discovered that, for different ceramics, different behaviors were presented with the same doses of implantation ions. For alumina and zirconia ceramics, the bending strength increased with increasing implantation doses of Ti and Zr ions, but decreased with high dose of Cr ions. For silicon nitride ceramics, however, the bending strength originally increased with smaller doses of metals implanted, and decreased with higher doses of metals of Ti, Zr, and Cr ions. The different behaviors are correlated to the different variations in compositions and microstructures of ceramics after ion implantation.

Indentation Size Effect in the Nanohardness of Soda-Lime Glass

The nanoindentation load-displacement curves of soda-lime glass were analyzed with the widely adopted Oliver-Pharr method. The resultant nanohardness exhibits a significant indentation size effect (ISE). An empirical relationship between the peak load and the contact depth, which was established to analyze the ISE in microhardness tests, was suggested to be suitable for describing the load-dependence of the nanohardness. By comparing the best-fit values of the parameters included in this empirical relationship, the origin of the load-dependence of nanohardness was analyzed briefly.

Characterization of Superhard Ternary (Ti,Al)N Coatings Prepared by Pulsed High Energy Density Plasma

Traditional TiN coatings cannot meet the requirements in many applications under extreme conditions because of their relatively low hardness (similar to2000Hv), high friction coefficient (>0.4), inade- quate thermal stability and insufficient corrosion. (Ti,Al)N coatings are a promising alternative to TiN coatings, which have recently been obtained by some groups. However, The deposition of coatings on ceramic substrates prepared by traditional physical vapor deposition was puzzled by low adhesions. In this work, (Ti,Al)N deposition was realized by a new technique namely pulsed high energy density plasma. The ratio of Ti and Al atoms was kept constant at 1:1, since this was known to be the best ratio for (Ti,Al)N synthesis. However, the process allows many other deposition parameters to be varied. In this study, the influences of the discharge voltage between the inner and outer electrode, the distance between the sample and the pulsed plasma gun, and the shot number of the pulsed plasma on coating formation and properties were investigated. Under the optimum deposition conditions, the adhesive strength of (Ti,Al)N coating to Si3N4 ceramic substrate was satisfactory with the critical load up to more than 80 mN. In addition, the coatings were investigated with respect to mechanical and wear behavior, including nanohardness, Youngs modulus and cutting performances. The (Ti,Al)N coatings possess resulted very high values of nanohardness and Youngs modulus, which are near to 40 GPa and 680 GPa, respectively. Because of the deposition of (Ti,Al)N coatings the wear resistance and edge life of the coated tools were improved dramatically evaluated by the cutting perforinances of the coated tools in turning HT250 (HB230) steel under industrial conditions. These improvements were attributed to a combination of four effects of deposition, ion implantation, quench and solid solution strengthening in terms of the structural analyses.