Zhao Hui Chen

Dielectric and Ferroelectric Properties of Ba1-xSrxTiO3 Thin Films Prepared by Pulsed Laser Deposition

In this work, BST (x=0.7, 0.5, 0.3) films have been deposited onto Nb-SrTiO3 substrate with pulsed laser deposition. The crystal structure and surface morphology have been characterized by X-ray diffraction (XRD) and atomic force microscopy, respectively. XRD results revealed that in certain elaboration condition the films were aligned along (00l) direction, normal to the substrate surface. The dielectric loss, relative dielectric permitivity, and polarization of BST films strongly depended on Sr content at room temperature. The tunability of relative dielectric permitivity of BST films exhibited strongly dependence on Sr content, and BST-0.5 shows the maximun K (K = tunability/loss) value.

Effects of Adding SiC Powder on the Properties of Carbon Fiber Reinforced Si-O-C Composites Fabricated via Precursor Pyrolysis

Three dimensional carbon fiber reinforced silicon oxycarbide (3D Cf/Si-O-C) composites with low cost silicon resin as precursors and 3D Cf as reinforcement. Effects of adding SiC powder (SiCP) on the microstructure, mechanical properties and anti-oxidation properties of 3D Cf/Si-O-C composites were investigated. The results showed that adding SiCP filler could reduce the porosity and improve the interface bonding, therefore the properties of composites increased. But when the SiCP content was excessive, it was difficult to dense the matrix of composites at the further cycles and pores existed in the matrix. As a result, the mechanical properties of the composites decreased. It was found that when fabricated with 18.2 weight percent SiCP the composites exhibited highest mechanical properties, and the flexural strength and fracture toughness reached 421.3MPa and 13.0 MPa•m1/2, respectively. And the anti-oxidation properties were improved with the increase of the SiCP content. When fabricated with 25.0 weight percent SiCP the composites exhibited best oxidation resistance properties, and the composites retained 89.5% of original flexural strength.

Mechanical Properties and Microstructure of Partially Sintered Zirconia Ceramics

Partially sintered zirconia ceramics (PSZCs) for dental uses were prepared from zirconia nanopowder via isostatic pressing and partially sintering. The open porosities, pore diameters, grain sizes and mechanical properties of the ceramics with different densities were studied. The results show that the pores formed in the PSZCs are all open pores, with a diameter distribution of 60nm~130nm and a grain size distribution of 120~170nm. The machinability becomes worse when the density of PSZC is higher than 75% of the theoretical density, so a ceramic named PSZC-70% with density of 70%TD was selected as the target material. Its bending strength is 168 MPa and fracture toughness is 1.8 MPa·m1/2. A dental restoration framework can be obtained via machining the PSZC-70% on a dental CAD/CAM system.

Effects of Pyrolysis Temperatures on the Microstructure and Mechanical Properties of 2D-Cf/Si-O-C Composites

Two-dimensional carbon fiber cloth reinforced silicon oxycarbide (2D-Cf/Si-O-C) composites were fabricated with silicone resin (SR) as precursors, ethanol as solvent and SiC as inert fillers by precursor infiltration pyrolysis (PIP). Effects of the pyrolysis temperatures in the first cycle and the last but third cycle on the microstructure and mechanical properties of 2D-Cf/Si-O-C composites were investigated. The results showed that, when the pyrolysis temperature of the first cycle was 1200°C, 2D-Cf/Si-O-C composites exhibited good mechanical properties, which can be attributed to the better fiber/matrix interfacial bonding. When the pyrolysis temperature of the last but third cycle was 1400°C, the mechanical properties of 2D Cf/Si-O-C composites were further enhanced. The flexural strength and fracture toughness of the composites reached 263.9MPa and 12.8 MPa·m1/2, respectively.

Raman Spectra Study on Multilayered Compositional Graded (Ba0.8Sr0.2)(Ti1-xZrx)O3 Thin Films

Compositional graded thin films of (Ba0.8Sr0.2)(Ti1-xZrx)O3 (BSTZ) are grown on MgO by pulsed laser deposition technique with four BSTZ ceramic targets. Gradients of composition are achieved by artificially tailoring composition in multilayered thin films to form compositional graded layers (CGL). In each CGL four individual layers of BSTZ with x = 0.36, 0.18, 0.08 and 0 are grown^in series with equal thickness. Three kinds of CGL samples comprising one, two or four CGLs have been elaborated with the same total thickness by varying the thickness of each CGL. Raman spectra show existence of tetragonal structure in all the multilayered BSTZ thin films. Raman peak at 535 cm-1 shifts to high frequency with increasing of compositional gradient, and the peak at 750 cm-1 also shows a small shift to high frequency. Moreover, other Raman peak is observed at about 830 cm-1, which is associated with phonon mode of cubic phase, and such peak shifts towards lower frequency with increasing of compositional gradient. The shift of Raman peak is related to variation of internal stress in BSTZ thin film due to increasing compositional gradient.

Effect of Heat-Treatment in Ar Atmosphere on Pore Structure of Carbonaceous Materials from Polysiloxane

Nanoporous carbonaceous materials derived from polysiloxane were first prepared by pyrolysis at 1300°C followed with hydrofluoric acid (HF) etching treatment. Their thermal stability of pore structure in inert condition was investigated in this paper by nitrogen adsorption technique in detail. The specific surface area (SSA) and pore volume (total pore volume, micropore volume, mesopore volume) decreased continually in the heat-treatment temperature range of 1000~1400°C. The average pore size almost kept the same with the raw sample. However, when the temperature exceeded 1400°C, the micropore interconnection began transforming to mesopore structure, which led to the decline of SSA and the increase of average pore size. Furthermore, the pore size distributions (PSDs) curves showed that heat-treatment had an advantage on the transition process of pore structure from disorder to regularity to some extent when heat-treated in the range 1000~1400°C for the most possible reason of relief of residue strain in the carbonaceous materials.

Effect of Pore-Forming Agent on Porous Mullite Ceramics from Pyrolysis of Alumina Powders Filled Silicone Resin

Mesoporous and macroporous mullite ceramics were fabricated from pyrolysis of alumina nanopowders filled silicone resin. The effect of pore-forming agent (carbon nanopowders) on the pore structure and mechanical property of the porous mullite ceramics were investigated. By varying the carbon content, the porosity and flexural strength of the obtained ceramics could be easily tuned. Both of the open porosity and average pore size of the obtained samples increased at elevated carbon content, and reached the maximum value of 61.0%, 225.5 nm respectively when the carbon content got the highest point (40 Vol.%). The flexural strength of nanoporous mullite ceramics changed almost with an opposite trend of open porosity, with the biggest value of 36 MPa when 10 Vol.% carbon was adopted. The microstructure of porous mullite ceramics consists of dense region and loose region when the added carbon content was at relatively low level. However, the dense area gradually disappeared with the increasing of carbon content.

Evaluation of the Microstructure of 3D-Cf/SiC Composites Fabricated by Precursor Infiltration and Pyrolysis

The microstructure and its evolution of 3D-Cf/SiC composites derived from organic precursor are studied by using scanning electronic microscopy, mercury intrusion porosimetry, isothermal N2 sorption and bubble point method, etc. As the results shown, MIP is preferable to N2 sorption for the characterization of pore size distribution (PSD) because of its wider effective probing ranges. The typical porosity of fabricated 3D-Cf/SiC composites is 10-15vol.%, and all the pores distribute in a quite wide size ranging from some dozens of nanometers to hundreds of microns and can be divided into three groups, according to their sizes, contents and locations: the inter-bundle macro-pores/paths, the intra-bundle pores and the micro-pores/cracks around the interfaces or in the matrixes. The macro-pores/paths constitute a porous network, which is partially open throughout the composites.

Preparation and Properties of Hot Pressed 2D C/SiC Composites

In this paper a pseudo-HP process, which uses solid powder to transfer pressure to prepare complex shape articles, was adopted to prepare 2D C/SiC composites. Nano-SiC powder was used to lower the sintering temperature, thus decreasing the damage to carbon fibers. The optimum processing parameters are as follows: sintering temperature 1850°C, holding time 60min, sintering pressure 10MPa. The maximum strength is over 300MPa, and toughness is around 8MPa·m1/2. BN powder is more lubricous than carbon powder, and thus is more convenient for demoulding and pressure transferring. Precursor Infiltration and Pyrolysis (PIP) was used to further densify the composites after HP process, and strength and toughness of the samples were slightly enhanced.

Effects of Molding Pressure on the Microstructure and Mechanical Properties of 2D-Cf/SiC Composites by PIP Routes

Carbon fiber cloth reinforced silicon carbide (2D-Cf/SiC) composites were prepared through polycarbosilane(PCS) /divinylbenzene(DVB) pyrolysis with SiC as inactive filler. Effects of the molding pressure on the microstructure and mechanical properties of 2D-Cf/SiC composites were investigated. With increasing molding pressure from 0MPa to 3MPa, the fiber volume fraction of the composites was increased. As a result, the strengths of the composites were enhanced. But when the molding pressure exceeded 3MPa, SiC particles would damage the carbon fibers seriously. Therefore, although the fiber fraction of the composites was increased further, the flexural strengths of the composites were decreased. It was found that the composites fabricated with the molding pressure of 3 MPa exhibited highest flexural strength, reached 319.4 MPa.