De Liang Chen

Carbothermal Reduction Synthesis of Zirconium Diboride Powders Assisted by Microwave

ZrB2 powder has been prepared through carbothermal reduction boronization of zirconia/boron carbide/carbon mixtures heating assisted by microwave. The powder characteristics were investigated by X-ray diffraction (XRD), X-ray fluorescence (XRF), nitrogen absorption (BET model) and scanning electron microscope (SEM). The experiments indicated that excessive B4C is necessary and the carbothermic reaction reacts severely at a higher temperature and complete at 1600oC. The crystallite size has ranged from 50-100 nm, according to the calculated surface area. Highest purity of ZrB2 powder, which was synthesized at 1600oC, is 99.67 wt%. The surface area of ZrB2 powder synthesis at 1600oC is 18.33 m2/g. Vibration of temperature should affect the purity of ZrB2, as the sub reaction acted.

Preparation and Characterization of Polycrystalline Cubic Boron Nitride (PcBN) Compacts by a Pressureless Sintering Process

A novel cost-effective pressureless sintering method has been developed to prepare polycrystalline cubic boron nitride (PcBN) compacts. The effect of feldspar as sintering aids was analyzed in this paper. Various amounts of feldspar from 5 to 15 wt% were added to cBN powders, and the pressureless sintering was conducted at temperatures ranging from 900°C to 1200°C under an air atmosphere. The microstructure, phase, density and microhardness of the as-obtained PcBN compacts were measured and correlated to amounts of Si added and to sintering temperatures. The sample showed superior sintering behavior in comparison to those fabricated using hot pressed sintering. The results of X-ray diffraction (XRD) and scanning electron microscopy (SEM) showed that feldspar diffused homogeneously and tightly bonded with cBN. But hBN appeared when the sintering temperature even at 900°C, which dramatically affected the property of PcBN compacts. A PcBN compact with a relative density of 91% was obtained at 1100°C and its microhardness was as high as 1000HV.

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.

Hydrothermal Synthesis and Ethanol-Sensing Properties of MoO3 Nanobelts

Uniform MoO3 nanobelts were synthesized through a fast and simple hydrothermal route without any other agents. The hydrothermal reaction was performed at 180 °C for 12 h using a HNO3 aqueous solution as the solvent. The phases and microstructures were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results indicated that the sample obtained was an orthorhombic MoO3 phase, and had a belt-like morphology with lengths of 510 μm and apparent widths of about 220 nm. The MoO3 nanobelts obtained were used as the sensing materials to fabricate chemical sensors for detection of some volatile organic compounds (VOCs) (including ethanol, methanol, isopropanol, acetone, methanal, and benzene). The gas-sensing results indicated that the sensor of the α-MoO3 nanobelts has enhanced ethanol-sensing performance, e.g., with the highest sensitivity of Sr =144 for 500 ppm ethanol vapor operating at 300 °C.

Fabrication of Hierarchical SnO2 Nanocrystals and their Sensing Properties to Volatile Organic Compound Vapors

Hierarchical SnO2 (H-SnO2) and particulate SnO2 (P-SnO2) nanostructures were synthesized by a hydrothermal method with and without the aid of sodium 1-dodecanesulfonate (SDS), respectively. X-ray diffraction and scanning electron microscopy were used to characterize the products obtained. The sensing properties of the H-SnO2 and P-SnO2 nanostructures to volatile organic compound gas (VOCs) were measured. The H-SnO2 sensors show better gas-sensing performance than the P-SnO2 sensors due to the hierarchical microstructure.

Preparation of Fe Coating Al2O3 Nanometer Composite Powder and its Mechanical Properties after Hot Press Sintering

Nano-Fe particles coating Al2O3 composite powders were prepared by heterogeneous precipitation method with nanometer -Al2O3 and Fe(NO3)3•9H2O as raw materials. The composite powders were analyzed by DSC-TG, XRD,SEM and Zeta potential. Results showed that Fe coating Al2O3 nanometer composite powders were obtained in the condition of being sintered at 500°C for 30min and reduced at 700°C for 1h in H2. The coating Fe nanometer particles are in the shape of sphericity with diameter about 30nm and the dispersion of the powders is uniform. Al2O3/Fe composite ceramics were obtained by hot-pressing (30MPa). The mechanical properties of the composite were investigated after hot press at different temperatures. With the increasing of Fe content in composite ceramics, the hardness of the composite is decreased. Fracture toughness of 10mol%Fe content is 5.62MPa after sintered at 1400°C, which is increasing 57% high than that of monolithic Al2O3 ceramics.

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.

Visible-Light-Driven Photocatalytic Performance of Ag@TiO2 Nanocrystals: The Effects of Phases and Morphologies

Silver decorated TiO2 nanoparticles (Ag@syn-TiO2, Ag@P25) were synthesized by photoreducing Ag+ species in the presence of the synthetized TiO2 (syn-TiO2) nanocrystals or P25. The samples obtained were characterized by XRD, XPS, UV-vis and TEM. The XRD results showed the syn-TiO2 nanoparticles were pure anatase, and the P25 nanoparticles were the mixture of antase and rutile. The TEM observations and XPS spectra indicated that Ag species anchored on the surfaces of syn-TiO2 nanoparticles as Ag0 and AgO nanoclusters (NCs). The Ag NCs have a size range of 13 nm and the AgO NCs have a size range of 1015 nm. The Ag NCs on the surface of P25 have a size range of 57 nm, and no AgO species were found. The photocatalytic performance of the Ag@syn-TiO2 and Ag@P25 samples was evaluated by degradating Rhodamine B (RhB) under visible-light irradiation (λ ≥ 420 nm). The results indicated that the phases and morphologies of TiO2 nanocrystals have effects on their photocatalytic properties, and that the syn-TiO2 nanocrystals wiht the pure anatase are more active than P25 with the mixture of antase and rutile in RhB degradation. The possible mechanisms were discussed.