Key Engineering Materials Vols. 368-372

Abstract: MnZn ferrite powder was modified by SrTiO3 sol and then filled into PTFE matrix to form MnZn ferrite/SrTiO3/PTFE composite. The effect of modification and heat-treatment on the electromagnetic property of the composite was investigated. The results revealed that the as-prepared composite possessed high permeability, high permittivity and low loss. The modification and heat-treatment could improve the electromagnetic property of the composites. The frequency dependence of the dielectric property of the composite is accorded with Debye relaxation theory.

Abstract: A new systematic study of the formation and structure of solid solutions Gd (5-x)HoxSi4 (x = 1.0-5.0) has been performed. By X-ray powder diffraction method, it was found that these compounds all crystallize in orthorhombic structure with space group Pnma and the cell parameters a, b, c and the volume of unit cell are correlated with the holmium content.

Abstract: Nanocrystalline Y2BaCuO5 was prepared by a co-precipitation method with aid of ultrasonic irradiation using Y2O3, CuCl2 and BaCl2 as source materials. The crystallization and morphology of the prepared nanoparticles were characterized by X-ray diffraction and transmission electron microscopy. Results showed that Y2BaCuO5 monophase can be prepared at 900°C with NaOH + NaCO3 mixture as precipitator. Particle size of Y2BaCuO5 crystallites decreases with the increase of sonicating power. Around 30 nm Y2BaCuO5 crystallites could be achieved when sonicating power increased to 300 W.

Abstract: The effect of pre-treatment on the microstructure and critical current of (Bi,Pb)-2223/Ag tapes has been investigated. Experimental results show that pre-treatment has important influences on the formation and the grain size of the (Bi,Pb)-2223 phase as well as the performances of Bi-2223/Ag tapes. During the pre-treatment process, the decomposition of the 3321 phase enhances the growth of (Bi,Pb)-2212 grains, which promotes the formation of (Bi,Pb)-2223 phase because it followed the size and orientation of the original (Bi,Pb)-2212 phase. As a result, the properties of Bi-2223/Ag tapes were remarkably improved.

Abstract: The effect of lead oxide compounds in (Bi,Pb)2Sr2Ca2Cu3Ox ((Bi,Pb)-2223) precursor powder on the microstructure and properties of Bi-2223/Ag tapes was investigated. Firstly, precursor powders containing different types of lead-rich phases were prepared by controlling the calcination temperature and atmosphere. Then monofilament (Bi,Pb)-2223/Ag tapes were fabricated using these powders. Lastly, samples scissor from these tapes were heat treated under different conditions. The phase composition and microstructure of these powders and tapes were analyzed using XRD and SEM. Experimental results show that the lead-rich phases in BSCCO precursor powders played an important role on the formation of the (Bi,Pb)-2223 phase, the microstructure and critical current of Bi-2223/Ag tapes.

Abstract: Different opening and modifying methods of MWNTs are studied and compared in this paper. MWNTs are dipped in a refluxed HNO3 solution. The influence of temperature, concentration and holding time on the opening and modifying of MWNTs is studied. The compared process is that MWNTs are treated by O3. The solvent that can highly dissolve O3 and disperse MWNTs is crucial. In order to prompt the reaction, 1M CF3COOH is elected as reactant medium and catalysis. The opening of MWNTs is tested by BET and confirmed by TEM. The surface structure of MWNTs treated by different processes is analyzed by FT-IR. The optimum conditions of opening and modifying of MWNTs are discussed.

Abstract: Hollow nanostructures have important applications in the field of gas adsorption, separation and storage due to their large specific surface areas. Silicon carbide nanotubes (SiCNTs) are one kind of such materials. In this paper, SiCNTs were synthesized from multiwalled carbon nanotubes (MWCNTs) via chemical vapor reaction (CVR) and purification. The SiCNTs were characterized by XRD, SEM and TEM. The results revealed that C-SiC nanotubes were obtained after CVR and most carbon were removed after purification with some traces retained inside SiCNTs. Hydrogen storage capacities measurements indicated that SiCNTs are superior to MWCNTs.

Abstract: Orthorhombic boron nitride (o-BN) films with various thickness (150, 220 and 300nm etc.) are prepared on Si(100) substrate by radio frequency plasma enhanced pulsed (Nd:YAG) laser deposition (RF-PEPLD) in Ar-N2 gas system. The films are characterized by Fourier transform infrared spectroscopy and atomic force microscopic. The field emission characteristics of the BN thin films are measured in an ultrahigh vacuum system. A threshold electric field of 8V /6m and the highest emission current density of 157.5mA/cm2 at an electric field of 20V/6m are obtained for the 150nm-thick BN film and a threshold electric field of 18V / 6m and the highest emission current density of 332.9mA/cm2 at an electric field of 52 V/6m are obtained for the 220nm-thick BN film. The results show that the threshold electric field increases with increasing thickness of the films, while the withstand voltage characteristic of the BN films also increases with increasing thickness of the films. The Fowler-Nordheim plots show that electrons emitted from BN to vacuum by tunneling through the potential barrier at the surface of BN thin films.

Abstract: Sb-doped BaPbO3 ceramics were prepared by sol-gel method. The influence of Pb/Ba ratio and Sb concentration on the electrical properties of BaPb1+x-ySbyO3 (x=0.0, 0.1, 0.2 and 0≤y≤0.2) compositions were investigated. Holes and Pb vacancies were the major defects in lightly donor-doped BaPbO3, where the increase of donor concentration resulted in decrease of charge carriers (holes), leading to resistivity increasing. In the highly donor-doped conditions, the microstructure or solid solubility substituted defect structure as the main factor affecting the variety of resistivity. The lowest electrical resistivity of Sb-doped BaPbO3 was 2.69 × 10-4 /·cm when the Sb concentration y=0.12~0.13. Excess of Pb causes the born of barium vacancies. And, the observed PTCR behavior of BaPb1.2O3 involves the Barium vacancies in grain boundaries. 0.5 mol% Sb-doped BaPbO3 showed the best PTCR behavior and its Curie temperature was about 850°C.

Abstract: An aqueous photocrosslinkable binder system for tapecasting ceramic substrates was investigated based on chitosan (CS) and polyvinyl alcohol (PVA) by adding UV photoinitiator 2-hydroxy-1- [4-(hydroxyethoxy)phenyl]-2-methyl-1-propanone (Darocur 2959). Alumina green sheets were prepared by using the binder and exposed to UV irradiation. SEM, FT-IR, XRD and DSC verified that these CS/PVA polymeric films or green sheets obtained using the binder were homogeneous, which indicated that CS was compatible with PVA in the binder, polymeric films and green sheets. Blending CS and PVA may created a semi-interpenetratin structure with good mechanical properties enough for handling and machining, and the process may not have oxygen retarding effect in arcylamide system.