Jiehua Wu

Preparation and properties of SiO2 matrix composites doped with AlN particles

SiO2 matrix composites doped with AlN particles were prepared by hot-pressing process. Mechanical properties of SiO2 matrix composites can be greatly improved by doping with AlN particles. Flexural strength and fracture toughness of 30 vol%AlN-SiO2 composite sintered at 1400°C reached 200 MPa and 2.96 MPa·m1/2. XRD analysis indicated that, up to 1400°C, no chemical reaction occurred between SiO2 matrix and AlN particles suggesting an excellent chemical compatibility of SiO2 matrix with AlN particles. The influences of hot-pressing temperature and the content of AlN particles on dielectric properties of SiO2-AlN composites were studied. The temperature and frequency dependency of dielectric properties of SiO2-AlN composites were also studied. Residual flexural strength of SiO2-AlN composites decreased with increasing temperature difference. The critical temperature difference was estimated about 600°C.

Large refrigerant capacity of RGa (R = Tb and Dy) compounds

The magnetic properties and magnetocaloric effects (MCEs) of RGa (R = Tb and Dy) compounds are investigated. The TbGa compound exhibits two successive magnetic transitions: spin-reorientation (SR) transition at TSR = 31 K and second-order ferromagnetic (FM)–paramagnetic (PM) transition at Curie temperature TC = 154 K, while the DyGa compound undergoes a SR transition with TSR=25 K and a FM–PM transition with TC = 113 K. It is noteworthy that a broad distribution of the magnetic entropy change peak is observed. The values of the refrigerant capacity (RC) for TbGa and DyGa are found to be 620.6 and 381.9 J/kg for a field change of 0–5 T, respectively. And for a field change of 0–7 T, the values are 900 and 584.2 J/kg, respectively. The large value of RC for TbGa and DyGa originates from the combined contribution from SR and FM–PM transitions, which enlarges the temperature span of large MCE.

The influence of addition of AlN particles on mechanical properties of SiO2 matrix composites doped with AlN particles

Abstract SiO 2 –AlN composites were prepared by hot-pressing process. Mechanical properties of SiO 2 matrix materials can be greatly improved after introducing the second-phase particles AlN. Flexural strength and fracture toughness of SiO 2 –AlN composites with 30 vol.% AlN sintered at 1400°C reached 200 MPa and 2.96 MPa m 1/2 respectively. XRD analysis indicated that the precipitation of cristobalite can be restrained by introducing AlN particles and in the temperature range of 1250 to 1400°C no chemical reaction occurred between SiO 2 and AlN, which suggested that SiO 2 had excellent chemical compatibility with AlN. Microstructure of SiO 2 –AlN composites was investigated through SEM and TEM analysis.

Building a High-Efficiency and Compact-Sized Thermoacoustically-Driven Pulse Tube Cooler

Thermoacoustic systems have attracted lots of attention in recent years due to their structural simplicity, high reliability, and potential for very high efficiency. There has been extensive research on standing wave and traveling wave systems, including both compressors and refrigerators. This article introduces our efforts towards building a high-efficiency and compact-sized thermoacousti-cally driven pulse tube cooler for temperatures below 80 K. Firstly, we have improved the heat exchangers in the thermoacoustic systems. By using Electrical Discharge Machining (EDM) cut heat exchangers, pressure ratios of 1.15/Helium and 1.22/Nitrogen have been obtained on a 1/4 wavelength standing wave system. Coupling the thermoacoustic compressor with a miniature pulse tube cooler has led to a lowest no-load temperature of 105.4 K. To reduce the size of the systems, we plan to use spring-mass resonators. The design and manufacture of two types of spring-mass resonators has just been completed, and tests will be done soon.

The influence of calcium oxide on the 60Co-γ ray irradiation resistance of Ce-doped (alumino-) borosilicate glass

Irradiation-induced UV–VIS absorptance spectra and electron paramagnetic resonance X-band spectra of some cerium (Ce)-doped borosilicate glasses and alumino-borosilicate glasses were studied, and it was found that the addition of calcium oxide could reduce irradiation resistance of Ce-doped (alumino-)borosilicate glass. As a result, the concentration of calcium oxide in the Ce-doped irradiation-resistant (alumino-)borosilicate glass should be reduced.

Reflectivity and absorption coefficient of a borosilicate glass during 60Co-γ irradiation calculated from data measured by an integrating sphere

Before and after 11 kGy 60Co-γ irradiation as well as after 150°C bleaching, reflectivity and absorption coefficient of a borosilicate glass were calculated from the intensity of transmitted light and the intensity of reflected light measured using an integrating sphere. The irradiation induced absorption in the studied wavelength range, while it only made the reflectivity change obviously in the range 4.5–5.6 eV (275–221 nm). The absorption coefficient spectrum and the reflectivity spectrum of the irradiated glass both recovered to a degree after 150°C bleaching. The relative error of irradiation-induced absorption coefficient calculated neglecting reflectivity change was studied, and the relative errors in the studied wavelength range theoretically increase when the thickness of glass sheet decreases, so if irradiation-induced absorption coefficients are studied using a thin glass sheet, reflectivity changes should be considered. The band gap of the glass was studied using Mott and Daviss theory, and it was found to reduce after irradiation, then after 150°C bleaching, it increased but still smaller than the one before irradiation.

Comparative studies of sol-gel SiO2 thin films prepared by spin-coating and dip-coating techniques

Silicon dioxide (SiO2) film is an important material for semiconductor industry as the gate dielectric material, and optical industry as antireflective coatings, and so on. The sol-gel method has been widely used for the deposition of oxide thin films due to several advantages such as simple and low-cost equipment, normal atmospheric conditions, and ease control of the precursor composition doping. In this work, SiO2 thin film was deposited on BK7 glass substrate by spin-coating and dip-coating techniques, respectively. Three precursor concentrations were investigated, i.e., tetraethylorthosilicate (TEOS): ethanol: water= 1:7:7, 1:10:10 and 1:15:15 (molar ratio). X-Ray Diffraction spectrometry (XRD), scanning electron microscopy (SEM), and UV-VIS-NIR scanning spectrophotometry were used to characterize the crystallinity, morphological and optical properties of deposited SiO2 thin films. Results show that all the SiO2 thin films deposited by the spin-coating and dip-coating are amorphous. SEM analysis confirms that the deposited SiO2 thin films have high surface quality and tight adherence with the substrate. The transmittance measurements indicate that dip-coated SiO2 thin film from gel of low precursor concentration represents better transmittance than that from high concentrations at the range of 600-1200nm. SiO2 thin film spin-coated from gel of high precursor concentration has better transmittance than that by dip-coating, but of opposite results for the films deposited from gels of low precursor concentrations.

An experimental research on a small flow vortex tube at low temperature ranges

Publisher Summary This chapter investigates the characteristics of a vortex tube at low temperature. The performance of the vortex tube depends on many geometrical and physical parameters. To understand the influence of the absolute gas temperature and to apply the vortex tube in low temperature conditions, it is necessary to carry out some experiments at low temperature range. A small flow vortex tube operating in low temperature below 80 K is developed. Experimental results show that there is still significant separation effect of vortex tube below 80 K for some inert gases. With LN 2 pre-cooling, the lowest temperature of 60.2 K and 3.3 W refrigeration at 67.5 K has been first achieved using a closed-cycle vortex tube refrigeration system. And an interesting inverse temperature effect is found when the cold mass fraction is less than 0.38. These achievements can help thoroughly understand the mechanism of energy separation in the vortex tube and use the vortex tube in some potential situations.

A novel wet chemistry/two-step calcining process for the synthesis of ultrafine α-Al2O3 particles

Abstract Ultrafine α-Al 2 O 3 particles have been synthesized via a wet chemistry/two-step calcining process. X-ray diffraction (XRD) analysis shows that the as-synthesized particles are of single-phase α-Al 2 O 3 . The grain size of the particles is examined by TEM analysis and is found to be around 80–100 nm. The study demonstrates a novel process for the synthesis of ultrafine α-Al 2 O 3 particles.

Al2O3 films with ni-based buffer layer prepared by plasma-ion assisted deposition on Cu substrate

In this study, Al2O3 films with an Ni-based buffer layer were prepared on a Cu substrate by plasma-ion assisted deposition (PIAD). The main purpose of this study is to develop a novel electrical insulating film to be used at high temperature. X-ray diffraction (XRD) spectra show the Al2O3 films prepared by this method are amorphous. The results of atomic force microscopy (AFM), scanning electron microscopy (SEM), and auger electron spectroscopy (AES) analyses reveal that the Al2O3 films perfectly adhere to the substrate through the buffer layer, no visible defects were observed, and no impurity from Ni or Cu was detected. The diffusion of Cu into the Al2O3 film at high temperature is suppressed. Al2O3 films with an Ni-based buffer layer exhibit excellent resistivity (>1010Ω·cm) even after experiencing a high temperature environment as high as 600°C 10 times.