Bing Bing Fan

Properties and Processing of Porous Si3N4 Ceramics

Porous silicon nitride (Si3N4) ceramics green were prepared by sol-gel and freeze drying processing. Al2O3 and MgO were selected as sintering additives. Porous Si3N4 ceramics were sintered at 1200~1300 °C. The porosity of porous Si3N4 ceramics reached 60~80%, the pore size of porous Si3N4 ceramics dried by freeze drying is less than 5μm. Two kinds of pores were formed, including open pores with pore size of 1~5μm and closed pores pore size with the nanometer level. The compression strength of porous Si3N4 ceramics was 15~25MPa. Thermal conductivity of porous Si3N4 ceramics was 0.08-0.1 W/m·K.

Microwave-Assisted Synthesis and Gas-Sensing Performance of Hollow-Spherical WO3 Nanocrystals

Hollow-spherical WO3 nanocrystals were obtained by calcining an organic-inorganic W-C precursor containing C and W in a microwave oven or in a conventional muffle furnace, and the W-C precursor hybrid precursor was synthesized via a hydrothermal method. The samples were characterized by XRD, TG-DTA, FTIR and SEM. The morphologies of the WO3 samples obtained by conventionally heating (C-WO3) and microwave-assisted heating (M-WO3) were compared. The average diameter and shell thickness of C-WO3 hollow spheres is about 450 and 200 nm, respectively. The average diameter and shell thickness of M-WO3 hollow spheres is about 500 and 50 nm, respectively. The M-WO3 has a loose and multilayered shell, and their nanoparticles are smaller than those of C-WO3. The improved structure of M-WO3 is due to shorter heating time and the unique heating style in a microwave oven. The gas-sensing performances of the WO3 sensors were investigated. The M-WO3 sensor has better response to ethanol vapors than the C-WO3.

Processing and Properties of (Zr,Hf)B2-SiC Ceramic Composites

Different molar ratio of HfB2 and ZrB2 had been mixed, and 30 vol.% SiC was selected as sintering additives. The mixing powders were sintered by hot pressing at 1900 °C for 1 h under a pressure of 20 MPa in Ar atmosphere. X-ray diffraction, scanning microscopy and Archimedes’s method were used to characterize the phase, microstructure and density of the sintered composites. Meanwhile, the hardness, the fracture toughness and flexural strength of the obtained composites were considered too. It can be found that the (Zr,Hf)B2 solid solutions were formed by HfB2 and ZrB2 during the sintering. The flexural strength of (Zr,Hf)B2-SiC composites increased with the amount of HfB2 increasing, which reached (332±40) MPa for the composites content of 70% HfB2. Which fracture toughness was (2.22±0.25) MPa·m1/2. The highest Vickers’ harness of was (24.8±3.4) GPa for the composites content of 50% HfB2.

Processing and Properties of ZrB2-Cu Composites Sintered by Hot-Pressing Sintering

ZrB2-Cu composite is a new electrical contact materials in the integration of high conductivity, high wear resistance and good mechanical strength. In this paper, ZrB2-Cu composites were prepared by hot-pressing sintering at 800~900 °C under a pressure of 20 MPa.The densification of ZrB2-Cu composites was improved by the addition of nickel using an electroless metal plating technique. X-ray diffraction and scan electron microscopy were used to analyze the phase and microstructure of ZrB2-Cu composites. The results showed that ZrB2-Cu composites with 60 vol % Cu which was sintered at 900 °C had a higher relative density, highest flexural strength of 381 MPa and higher hardness of 2.16 GPa(HV). ZrB2-Cu composites with 50 vol % Cu which was sintered at 900 °C had higher flexural strength of 297 MPa and the highest hardness of 2.66 GPa.

Preparation of Industrial Watchcase of 5Y-Tetragonal ZrO2 by Microwave Sintering

The industrial watchcases of 5Y-Tetragonal ZrO2 were prepared by microwave sintering. Samples were sintered in a microwave chamber with TE666 resonant mode at 2.45GHz. The sintering temperature was from 1250°C to 1400°C. XRD and SEM techniques were used to characterize the samples. It is found that microwave sintering improved the phase change from t-ZrO2 to m-ZrO2. Dense and homogeneous microstructure was obtained within the microwave-sintered samples and the average grain size was about 500nm. Compared to conventional sintering, microwave-sintered samples show higher density and hardness. And microwave-sintered samples were performed 130°C lower temperature. Optimized sample with the density of 6.1 g/cm3 and Vickers Hardness of 16.2MPa is microwave-sintered at 1350°C for 30min.

Effect of CNTs on Mechanical Properties of SiCp/Cu Composites

SiCp/Cu composites were prepared by vacuum hot-pressing at 770°C for 1.5 h under the pressure of 30MPa. The composites were enhanced by CNTs with different volume fractions from 0 vol. % to 4 vol. %. Three-step approach wrapping process was introduced to prepare composite powders. XRD and SEM techniques were used to characterize the samples. It is found that the core-shell structure composed of SiC core and Cu shell was formed in the composite particles. Optimized volume fraction of CNTs is 1 vol. %. Minimum Porosity and maximum Hardness is about 0.84% and 2.31GPa, respectively. But maximum Flexural strength was measured as 248MPa for samples containing 2 vol. % CNTs. Flexural strength was improved by the bridge effect caused by the increased CNTs.

Effect of Amorphous Phase on Mechanical Properties of SiC/Cu Composites

In this study, glassy phase is formed by SiO2-K2O addition to serve as amorphous grain boundary transition layer. SiC (SiO2-K2O) / Cu composite material were prepared by two-step coating method and hot pressing sintered below 770°C, 30MPa for 1.5h, using α-SiC as main reinforced phase, SiO2-K2O as grain boundary and Cu as matrix. The Cu-SiC volume ratio was 75:25. The SiO2 contents were 5vol.%, 10vol.%, 15vol.%, 20vol.% and 25vol.% of the total volume of the SiC / Cu. XRD and SEM techniques were used to characterize the composite particles and the sintered compacts; Archimedes method, Vickers hardness tester, universal testing machine to test the apparent porosity of the composite materials, the Vickers hardness and the bending strength, respectively. The results showed that with increasing of glassy phase contents, the Vickers hardness and the bending strength first rise and then drop, at the same time, it shows the opposite tendency for the apparent porosity. The sintering samples with the SiO2 content of 15vol.% have the optimum mechanical properties, the Vickers hardness reached 1.49 GPa, and the bending strength was close to 235 MPa.

Effect of Potassium Carbonate Addition on the Sintered Properties of Coal Fly Ash

In this project, coal fly ash was transformed into ceramic materials by adding a certain amount of fusing agent. Ash samples were compacted and sintered with the addition of potassium carbonate (K2CO3·1/2H2O) under a suitable sintering temperature range. Mineralogy and microstructure of the obtained products were characterized by means of X-ray diffraction and field emission scanning electronic microscope techniques respectively. The results indicate that K2CO3·1/2H2O facilitates the transformation of mullite and quartz phases above 800°C, and the mineralogy phases of the product is leucite (KAlSi2O6) and potassium aluminum silicate (KAlSiO4). In the process, K+ interacts with oxygen atom and destroys the original lattice. The regular morphology of the sintered samples was confirmed by the observation under SEM, which reveals a uniform dense ceramic is formed at 900°C with the 40wt% addition of K2CO3·1/2H2O.

Preparation and Characterization of Nanocrystalline CaO-ZrO2 Powders by Microwave Pyrolysis

CaO(15%)-ZrO2 nano-powders were prepared by microwave pyrolysis in a multi-model chamber at the temperature ranging from 650°C to 800°C, with the precursor processed at different reaction temperature from 0°C to 80°C by chemical co-precipitation method. XRD and SEM techniques were used to characterize the phase transition and micrograph of powders. It is found that the content of m-ZrO2 phase decreased with the increasing of reaction temperature and pyrolysis temperature. The high dispersed and superfine nano-powders were obtained at the pyrolysis temperature of 750°C for 20 min at 80°C. And only cubic ZrO2 phase were detected in CaO (15%)-ZrO2 powders and the average size of the powders is about 41 nm.

Preparation of α-Al2O3 Powder by Microwave Pyrolysis

A fast method of microwave pyrolysis was provided to prepare α-Al2O3 powders. Aluminum hydroxide and Aluminum ammonium sulfate doclecahydrate were used as raw materials to obtain α-aluminum oxide powder by microwave pyrolysis, respectively. Thermo-Gravimetric/Differential Thermal Analyzer (TG/DTA) and X-ray Diffraction (XRD) analysis were employed to investigate pyrolysis process and the transformation of metastables Al2O3 in the process of heating different precursors. Meanwhile, Flied Emission Scanning Electron Microscopy (FESEM) was applied to observe microstructure and grain growth, and the phase composition was characterized by XRD. The results indicated that the high purity α-Al2O3 was obtained which met the demands of market, and the sample obtained from aluminum hydroxide performed high purity, small particle size and, while the sample from ammonium aluminum sulfate showed lower purity and larger grain size.