Papers by Author: Zhi Hao Jin

Abstract: The porous SiC ceramics were prepared by two methods of pore-forming. The gas permeability and flexural strength of the porous SiC ceramics prepared by different ways were investigated. The porous SiC ceramics prepared by particle stacking have evenly pore distrubution; the gas permeability was greatly increased with the particle size increasing while the porous SiC ceramics were kept a higher flexural strength. The pore-former also can be used to form large pores to increase the gas permieability. However, these unevenly distribution large pores lead to a deterioration in flexural strength.

Abstract: The B4C/BN composites were fabricated by hot-pressing process. The microstructure, mechanical properties and oxidation resistances of the B4C/BN composites were investigated. It was shown that the h-BN particles were distributed in the B4C ceramics matrix. The mechanical properties of the B4C/BN microcomposites and the B4C/BN nanocomposites decreased gradually with the increasing content of h-BN. The mechanical properties of the B4C/BN nanocomposites were significantly improved in comparison with the B4C/BN microcomposites. The oxidation processes were performed at 1000oC, 1100oC, 1200oC, 1300oC for 20h. The oxidation curves of the B4C monolith, the B4C/BN microcomposites and the B4C/BN nanocomposites decreased gradually with the increase of oxidation temperature and oxidation time. The specimen’s weight and the oxidation resistance decreased gradually with the increase of oxidation temperature and oxidation time. The specimens remained good oxidation resistance at 1000oC; the oxidation resistance decreased remarkably at 1300oC. The decreasing specimen’s weight was attributed to the evaporation of B2O3 which produced by oxidation process of B4C and h-BN. The phase composition and microstructure of specimen’s surface after oxidation process were investigated by XRD and SEM.

Abstract: C/C-SiC brake materials were prepared by improved chemical liquid vaporized infiltration (CLVI) combined with liquid silicon infiltration (LSI) process, which needed less than thirty hours. The microstructure and frictional properties of the material were investigated. The density and porosity of the C/C-SiC brake material were 2.05 g/cm3 and 4.8%, respectively. The average dynamic friction coefficient of the materials was about 0.36, and the friction coefficient was stable. The average linear wear rate was less than 4.7 µm cycle-1 for rotating and stationary disk. The matrix composition and microstructure resulted in the high frictional performances.

Abstract: Machinable SiC/graphite composites were fabricated by Plasma Activated Sintering (PAS), which characterized by high heating rate, short time sintering and high efficiency process. The staring powder mixture included SiC, graphite power and a few sintering aids. They were sintered by PAS under 30 MPa Pressure in vacuum at 1500°C and 1550°C, respectively. The density, bending strength and hardness were measured. The phase and the microstructure of the composites were also investigated. The results indicated that the relative density of SiC/C(graphite) composite sintered at 1550°C was more than 99.5%, the bending strength was 336.6MPa, but surface Vickers hardness was 5.8GPa far lower than SiC ceramics. The machinability of SiC/Graphite sintered by PAS in 1500°C was good because the continuous weak microstructure played a crucial role in the machining process, including a lot of weak interface between the grains of SiC and graphite partials and nearly 20vol% pores in the composite ceramics.

Abstract: C/C-SiC composites were rapidly fabricated using C/C with four different porosities in the range of 12.4%~45.7% and silicon by reactive-melt-infiltrated (RMI) method. The influence of the C/C porosity on the Si infiltration during the processing and on the microstructure and mechanical properties of the resulting C/C-SiC were investigated. The results show that β-SiC was formed by Si/C reaction and free Si remained in the composites. A higher porosity of C/C leads more Si infiltrating to the preform and produces higher density of C/C-SiC with lower porosity. The flexural strength of the composites was strong influenced by the matrix content and the interface between different phases. C/C-SiC derived from C/C with 24.8% porosity has the highest flexural strength (325.1MPa).

Abstract: The relationship between the processing technology and the dielectric property of the laminated AlN/h-BN ceramic composites has been studied. The results showed that the main polarization mechanism of laminated ceramic composites in low frequency range is space charge polarization at the interface of AlN layer and BN layer. Due to the affection of porosity, the permittivity increases and the dielectric loss decreases with increasing the hot pressing temperature. The permittivity increases and the dielectric loss decreases with increasing thickness ratio of different layer., Both permittivity and dielectric loss increase with increasing the content of AlN doping in BN layer.

Abstract: Glass-ceramics are especially useful for the dental restorations because of their good biocompatibility, chemical stability, aesthetic, mechanical strength and wear resistance. The aim of this work was to obtain one mica glass-ceramic, which can be easily used for rapid machining into all-ceramic tooth with computer assisted design/computer assisted manufacture (CAD/CAM) devices. In the study, on the base of low melting machinable fluorosilicic mica glass ceramics, the effects of CeO2 and Fe2O3 in SiO2-B2O3-K2O-Na2O-Li2O-Al2O3-MgO-F system on color were studied. By orthogonal experimental design, the effects of crystallized parameters on the color, three point flexural strength and machinability of the glass ceramics were obtained, and the samples were analyzed by differential thermal analysis(DTA), X-ray diffraction (XRD) and scanning electron microscopy(SEM), respectively. Experimental results showed that the glass-ceramics with color close to the tooth can be obtained by adjusting the percentage of CeO2 and Fe2O3, and the glass-ceramics crystallized at 680°C for 2h have excellent mechanical properties and machinability.

Abstract: In this article, the microstructure, mechanical properties and machinability of the B4C/BN microcomposites and the B4C/BN nanocomposites were investigated. Homogenous distribution of the h-BN particles in the B4C ceramics appeared. The mechanical properties decreased gradually with the increasing content of h-BN for the both composites, while the nanocomposites demonstrated high performance. Machinability increased gradually with the increasing content of h-BN, and excellent machinability exhibited for both composites with more than 20wt% h-BN. The weak interface between the B4C matrix grains and the h-BN particles as well as the cleavage behavior of the laminate structured h-BN particles significantly attributed to the machinability of the B4C/BN composites.

Abstract: In this paper, porous Si3N4 ceramics were fabricated by carbothermal reduction between silicon dioxide and carbon. The influences of four types of sintering additives on the microstructure and mechanical properties of the porous Si3N4 ceramics were investigated. XRD analysis proved complete formation of a single-phase β-Si3N4. SEM analysis showed that the resultant porous Si3N4 ceramics occupied fine microstructure and a uniform pore structure. The sintered sample with Lu2O3 as sintering additive showed finer, a higher aspect ratio β-Si3N4 grains. The addition of Eu2O3 accelerated the densification of porous Si3N4 ceramics, decreased the porosity and increased the flexural strength.

Abstract: Polyacrylonitrile (PAN) based carbon fiber felt which contains abundant various SiC microthreads and some other microstructures was prepared through sintering the pretreated felt at high temperature at low nitrogen pressure. XRD, SEM, TEM, HRTEM analyses for the sintered felt were carried out to study its components and microstructures. There are SiC nanothreads, SiC submicron threads, SiC micron threads and a few SiO2 two- or three-dimensional microstructures (possibly intermix with the similar form of SiC) existed within the inner hollow spaces of the felt. The complex permittivity, complex permeability of the sample in the X-band frequency range were obtained.