Xue Jian Liu

Upconversion luminescence in Er-doped γ-AlON ceramic phosphors

γ-AlON:Er3+ phosphors with different Er3+ concentration for upconversion luminescence were prepared by a combination of carbothermal reduction and nitridation and solid-state synthesis methods. The effect of Er3+ content on emission spectra was investigated. Under the excitation of 980 nm diode laser, green emission centered at 548 nm and red emission centered at 666 nm were observed, which are ascribed to the transition of S43/2/H211/2→I415/2 and F49/2→I415/2 of Er3+ ions, respectively. The pump power dependence indicates a two-photon process involved in the upconversion luminescence. The ratio of red emission relative to green emission was enhanced with the increase in Er3+ concentration. The possible mechanisms were discussed.

Pressureless Sintering of ZrB2-SiC Ceramics Incorporating Sol-Gel Synthesized Ultra-Fine Ceramic Powders

Ultra-high temperature ceramic composites of ZrB2-SiC were densified by pressureless sintering. Ultra-fine ZrB2-SiC composite powders synthesized by sol-gel method were mixed with commercial ZrB2 and SiC powders. The sintered body of hybrid powders (combined commercial and synthesized composite powders) showed excellent properties not only in the relative density but also the flexure strength. Dry-pressed compacts using 4wt% Mo as a sintering aid were sintered to nearly full density at 2200°C/2h. The average strength was ~560MPa and the maximum was ~632MPa. SEM and TEM showed that SiC particles were distributed homogenously in the ZrB2 matrix and the average particle size was ~5μm. From HRTEM observations, the grain boundaries were apparently free of glassy phases and no intermediate phases existed.

Wetting Behaviors of Nickel-Based Alloys on Sintered Silicon Carbide Ceramics

Wetting behaviors of pure nickel and nickel-based alloys on sintered silicon carbide ceramics and their interfacial microstructures were investigated in this presentation. The nickel-based alloys were mainly the commercial nickel-molybdenum-chromium products. The wetting and spreading properties were observed by a real-time thermal optical measurement system under flowing argon-5%hydrogen atmosphere. As temperature increased, the pure nickel cylinder sample had few changes before 1356°C except for the thermal expansion in size and changed to liquid drop-shape when the temperature reached 1366°C. The contact angle was about 90o. And nickel could not contact sintered silicon carbide ceramics well. However, the introduction of molybdenum and/or chromium in the pure nickel was good for the wetting properties of pure nickel on sintered silicon carbide ceramics. The contact angles of nickel-based alloys (B-1, C-1 and C-2) on sintered silicon carbide ceramics after holding 15min at 1400°C were 25o, 12.5o, 11.5o, respectively. And they hardly reduced as temperature increased. The SEM images indicated that the interfacial microstructures of B-1 and C-1 on sintered silicon carbide ceramic substrates were uniform and the dissolved interface near silicon carbide ceramics indicated that they had a good bonding. At the same time, the reciprocal diffusion was clear in the intermediate layer. Besides that, the introduction of chromium was supposed to reduce the diffusion rate of molybdenum from the alloy to the ceramic substrate.

The Effect of TiC on the Liquid Phase Sintering of SiC Ceramics with Al2O3 and Y2O3 Additives

Low fracture toughness of SiC ceramics limits their applications for the low reliability. Inclusion of second ceramic phase improves the toughness of SiC ceramics. In this presentation, SiC ceramics with 5wt% TiC was pressureless liquid phase sintered (LPS) with the additives of Al2O3 and Y2O3 to ~98% theoretical density at the temperature of 1920 °C for 1 hour. The TiC grains were well distributed and good for the uniform distribution of the liquid phase YAG in the SiC matrix, which resulted in the homogeneous microstructure with fine SiC grains. The existence of TiC benefited the formation of elongated α-SiC, which favored the bridging and deflection of cracks so that the fracture toughness was improved to some extent. Because the amount of TiC was so small that the inherent properties of SiC was not degraded. The flexural strength and Vickers hardness maintained similar with the LPS SiC without TiC. The phase compositions detected in the sample were SiC, YAG and TiC. And no solid-solution of SiC and TiC was revealed by the mapping of EDS.

Effects of Particle Size on Densification Behavior of Si3N4 Ceramics

Silicon nitride (Si3N4) ceramics with Y2O3-Al2O3 as sintering additives were prepared by pressureless sintering technology using two kinds of Si3N4 powders as raw materials. The particle size of Si3N4 powders ball-milled with various times were characterized and the effects of particle size on the densification behavior of Si3N4 ceramics were investigated. The relative density and microstructure of the sintered bodies at different sintering temperatures were determined. The experimental results show that the powders with longer milling time have finer particle size and the samples with fine particle display higher shrinkage-rate at lower sintering temperature than the one with coarse particle. By extending milling time, the density gap between the sintered ceramics using different Si3N4 powder become narrower. The main factor resulting in the difference of sintering densification behavior is the particle size of Si3N4 powders rather than metallic oxide impurity of raw powders.

Wetting Behaviors of Pure Nickel and Nickel-Based Alloys on Sintered ZrB2-SiC Ceramics

The wetting and spreading behaviors of pure nickel and nickel based alloys on sintered ZrB2-SiC ceramics and their interfacial microstructures were investigated in this presentation. The nickel-based alloys were mainly the commercial nickel-molybdenum-chromium products. The wetting and spreading properties were observed by a real-time thermal optical measurement system under flowing argon-5%hydrogen atmosphere. As temperature increased, the pure nickel cylinder sample on ZrB2-SiC substrate had few changes before 1228°C except for the thermal expansion in size. After that, liquid phase formed and spread gradually on the ceramic substrate. The contact angle was about 15o after holding 15min at 1600°C. Therefore, pure nickel could contact sintered ZrB2-SiC ceramics well. Meanwhile, the introduction of molybdenum and/or chromium in the pure nickel was beneficial for the wetting of nickel on sintered ZrB2-SiC ceramics. The contact angles of Ni-28Mo and Ni-16Mo-23Cr alloys on sintered ZrB2-SiC ceramics after 1600°C/15min were 13o and 2o, respectively. In addition, the temperatures of the liquid drop formed rose obviously in contrast to the pure nickel. The SEM images indicated that the interfacial microstructures of Ni-based alloys on sintered ZrB2-SiC ceramic substrates were uniform and the dissolved boundaries showed that they had a good bonding. However, some cracks were found in the Ni/ZrB2-SiC system for their high thermal mismatch. On the other hand, the Ni-Mo (-Cr)/ZrB2-SiC interface had few defections and evident elemental diffusion between the ceramic substrates and the alloys were found at the interface.

Friction and Wear Behavior of Pressureless Solid State Sintered SiC

In the present work, SiC was pressureless solid state sintered with 3 wt% C and 0.6 wt% B4C as sintering additives. The friction and wear behavior of the PSSS SiC ceramics was investigated by using a block-on-ring tribometer. The wear volume and friction coefficient was measured. It is as expected that the friction coefficient increased with the elevation of the normal load and sliding speed. The microstructure of the worn surface was observed, based on which the wear mechanism was analyzed. Different degrees of oxidation during the friction process was found and the degree of oxidation was related to the severity of wear. The normal load was found to exert great influence on the wear of the SSiC ceramics.

Effect of Microstructure on Removal Mechanism of Solid State Sintered Silicon Carbide in Polishing Procedure

Solid state sintered silicon carbide (S-SiC) ceramic is one of the top optical materials for high space reliability and other excellent properties. Two microstructures were produced by sintering under different conditions. The effects of microstructure on removal rates of SiC ceramics during polishing processes were studied. The material removal mechanisms during polishing were analysed and modeled. With the increase of the aspect ratio and grain diameter size during polishing, grain pull-out is more difficult in elongated grains than in exquiaxed grains. The SiC ceramic with high hardness has high removal resistance leading to get bad surface quality under the same mechanical procedure. The samples with elongated microstructure have low hardness and surface toughness.

Preparation and Characterization of Stable ZrB2-Based Ultra-High Temperature Ceramics Slurry by Aqueous Gelcasting

Stable ZrB2-based ultra-high temperature ceramic slurries were prepared and characterized with a solid content 40 vol% by aqueous gelcasting which was suitable to form high quality and complexshaped ceramic parts. In the present work, Ammonium Citrate Tribasic (ACT) was used as the dispersant. The properties of ZrB2 slurry, and the influencing factors were investigated by conventional techniques such as sedimentation tests, particle size distribution measurements, electrokinetic measurements and rheological analysis. High solids loading and low viscosity slurry was obtained by controlling the optimal conditions for the ZrB2-based ceramic powders.

CNTs/Si3N4 Composites Fabricated by Reaction Bonded Processing

The CNTs/Si3N4 ceramic matrix composites were prepared by the reaction bonded processing. The phase compositions, chemical compatibility, mechanical properties, and microwave attenuation properties of the composites were investigated. XRD analysis shows the composites consist mainly of the α- and β-Si3N4, with a trace of unreacted silicon. The SEM micrograph displays the fractured surface of the composites studs with intact CNTs, indicating that CNTs and Si3N4 are chemically compatible. The composites with 1.0wt.% CNTs have a strength of 280 MPa, hardness of 8.2 GPa and toughness of 2.3 MPa·m0.5. The average value of the transmission attenuation reaches 6 dB at X band, indicating the composites have a potential for application in electromagnetic adsorbing or shielding.