Showing papers by "Beijing University of Technology published in 2004"

Effect of MnO2 Addition on the Structure and Electrical Properties of Pb(Zn1/3Nb2/3)0.20(Zr0.50Ti0.50)0.80O3 Ceramics

Abstract: Pb(Zn1/3Nb2/3)0.20(Zr0.50Ti0.50)0.80O3 ceramics of pure perovskite structure were prepared by the two-stage method with the addition of 0–3.0 wt% MnO2 and their piezoelectric properties were investigated systematically. The MnO2 addition influences in a pronounced way both the crystal structure and the microstructure of the materials. The materials are transformed from the tetragonal to the rhombohedral structure, and the grain size is enhanced when manganese cations are added. The distortion of crystal structure for samples with MnO2 addition can be explained by the Jahn–Teller effect. The values of electromechanical coupling factor (kp) and dielectric loss (tan δ) are optimized for 0.5-wt%-MnO2-doped samples (kp= 0.60, tan δ= 0.2%) and the mechanical quality factor (Qm) is maximized for 1.0-wt%-MnO2-doped samples (Qm= 1041), which suggests that oxygen vacancies formed by substituting Mn3+ and Mn2+ ions for B-site ions (e.g., Ti4+ and Zr4+ ions) in the perovskite structure partially inhibited polarization reversal in the ferroelectrics. The ceramics with 0.50–1.0 wt% MnO2 addition show great promise as practical materials for piezoelectric applications.

Preparation and characterization of nitrogen doped SrTiO3 photocatalyst

Abstract: Nitrogen doped SrTiO 3 powders were prepared by the mechanochemical reaction of SrTiO 3 and 20 wt.% of three kinds of doping sources, hexamethylenetetramine, urea, and ammonium carbonate using a high energy planetary ball mill, followed by calcination at 400 °C for 1 h. The photocatalysis of nitrogen doped SrTiO 3 powders under the irradiation of wavelength λ >400 nm and near ultraviolet light regions ( λ >290 nm) greatly changed depending on particle size, specific surface area and porosity. The absorption of visible light increased with increasing nitrogen content doped in SrTiO 3 lattice. Nitrogen monoxide elimination ability of SrTiO 3 could be greatly improved by nitrogen doping. The photocatalytic activity of nitrogen doped SrTiO 3 made by 20 wt.% hexamethylenetetramine-SrTiO 3 under light λ >400 nm irradiation was about 3.5 and 1.4 times higher than those of pure SrTiO 3 and commercial titania powder (Degussa P-25).

Photo-oxidation properties of nitrogen doped SrTiO3 made by mechanical activation

Abstract: Nitrogen doped SrTiO 3 powders were prepared by mechanochemical reaction of SrTiO 3 and 20 wt.% of three kinds of doping sources, hexamethylenetetramine, urea and ammonium carbonate using a high energy planetary ball mill. The effects of several factors such as the kinds of nitrogen doping source, milling time, content of nitrogen source in the raw material, post-grinding heat treatment temperature and post-grinding heat treatment atmosphere on the photocatalysis of the samples have been studied. The nitrogen doped SrTiO 3 had yellow color and possessed two absorption edges around 390 and 470 nm. The sample prepared by grinding the mixture of 20 wt.% hexamethylenetetramine-80 wt.% SrTiO 3 followed by heat treatment at 400 °C in air showed the best performance in the elimination of NO. The value of NO destruction under visible light ( λ >400 nm) irradiation attained to 47.8%, which was about 3.5 and 1.4 times higher than that of pure SrTiO 3 and commercial titania (Degussa P-25).

Syntheses, Thermal Stability, and Structure Determination of the Novel Isostructural RBa3B9O18 (R = Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb)

Abstract: A novel borate compound YBa3B9O18 has crystallized in a melt of BaYB9O16. Single-crystal X-ray diffraction measurements reveal that YBa3B9O18 adopts a hexagonal space group P(3 m with cell parameters of a = 7.1761(6) Angstrom and c = 16.9657(6) Angstrom. The structure is made up of the planar B3O6 groups parallel to each other along the (001) direction, regular YO6 octahedra, and irregular BaO6 and BaO9 polyhedra to form an analogue structure of beta-BaB2O4. A series of isostructural borate compounds RBa3B9O18 (R = Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) were prepared by powder solid-state reactions. The DTA and TGA curves of YBaB9O16 show an obvious weight loss at about 955 degreesC associated with a decomposition into YBO3, B2O3, and YBa3B9O18 due to its incongruent melting behavior. The DTA and TGA curves of YBa3B9O18 show that it is chemically stable and a congruent melting compound. A comparison of the structures of YBa3B9O18 and beta-BaB2O4 is presented.

Constitutive relations on creep for SnAgCuRE lead-free solder joints

Abstract: Taking the most promising substitute of the Sn-3.8Ag-0.7Cu solder as the research base, investigations were made to explore the effect of rare earths (REs) on the creep performance of the Sn-3.8Ag-0.7Cu solder joints. The SnAgCu-0.1RE solder with the longest creep-rupture life was selected for subsequent research. Creep strain tests were conducted on Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints in the intermediate temperature range from 298 K to 398 K, corresponding to the homologous temperatures η=0.606, 0.687, 0.748, and 0.809 and η = 0.602, 0.683, 0.743, and 0.804, respectively, to acquire the relevant creep parameters, such as stress exponent and activation energy, which characterize the creep mechanisms. The final creep constitutive equations for Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints were established, demonstrating the dependence of steady-state creep rate on stress and temperature. By correcting the apparent creep-activation energy of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints from the experiments, the true creep-activation energy is obtained. Results indicated that at low stress, the true creep-activation energy of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints is close to the lattice self-diffusion activation energy, so the steady-state creep rates of these two solder joints are both dominated by the rate of lattice self-diffusion. While at high stress, the true creep-activation energy of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints is close to the dislocation-pipe diffusion activation energy, so the steady-state creep rates are dominated by the rate of dislocation-pipe diffusion. At low stress, the best-fit stress exponents n of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints are 6.9 and 8.2, respectively, and the true creep-activation energy of them both is close to that of lattice self-diffusion. At high stress, it equals 11.6 and 14.6 for Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints, respectively, and the true creep-activation energy for both is close to that of the dislocation-pipe diffusion. Thus, under the condition of the experimental temperatures and stresses, the dislocation climbing mechanism serves as the controlling mechanism for creep deformation of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints. The creep values of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints are both controlled by dislocation climbing. Dislocation glide and climb both contribute to creep deformation, but the controlling mechanism is dislocation climb. At low stress, dislocation climbing is dominated by the lattice self-diffusion process in the Sn matrix and dominated by the dislocation-pipe diffusion process at high stress.

Complex mode superposition algorithm for seismic responses of non-classically damped linear mdof system

Abstract: This paper deals with a complex mode superposition method for the seismic responses of general multiple degrees of freedom (MDOF) discrete system with complex eigenvectors and eigenvalues. A delicate general solution, completely in real value form, for calculating seismic time history response of the MDOF system which cannot be uncoupled by normal modes, is deduced based on the algorithms of the complex superposition method. This solution comprises of two parts which are in relation to the Duhamel integration to sine and cosine function respectively. The related term of the Duhamel integration to sine function is actually the displacement response of the oscillator with corresponding modal frequency and the damping ratio. The other can be transferred into a combination of the displacement and velocity responses of the same oscillator. In order to meet the practical needs of seismic design based on code design spectra for various kinds of structures equipped by viscous dampers, the complex complete quadratic combination (CCQC) method is deduced following similar procedures such as the well-known CQC method, in which a new modal velocity correlation coefficient, together with a new modal displacement-velocity correlation coefficient are involved besides the modal displacement correlation coefficient in normal CQC formula. The new algorithm of CCQC is not only as concise as that of the normal CQC but also has explicit physical meaning. The results obtained from complex mode superposition approaches are discussed and verified in some examples through step by step integration computation under a prescribed earthquake motion input. From these examplary analyses, it may be pointed that the CCQC algorithm normally yields conservative outcome and that the forced mode uncoupling approach has good approximation even the discussed examplary structures are strongly non-proportional.

Giant Magnetoresistance Effect of Two-Dimensional Electron Gas Systems in a Periodically Modulated Magnetic Field

Abstract: We investigated a giant magnetoresistance (MR) effect of two-dimensional electron gas systems subjected to a periodically modulated magnetic field. It is found that the MR ratio of such a periodically modulated system shows strong dependence on the space between the magnetic potentials. With the increase in the number of periods, the maximal MR ratio tends to be enhanced and the peak of the MR ratio locates at a specific relative Fermi energy for the given space between magnetic potentials. Moreover, the maximal MR ratio of odd-period configurations is always larger than that of even-period configurations.

Structure and electrical properties of PMZN–PZT quaternary ceramics for piezoelectric transformers

Abstract: Pb(Mn1/3Nb2/3)O3–Pb(Zn1/3Nb2/3)O3–PbZrO3–PbTiO3 (PMZN–PZT) quaternary ceramics with various contents of Pb(Mn1/3Nb2/3)O3 from 5 to 20 mol% were prepared by the columbite two-stage process and the effects of composition on the microstructure and piezoelectric properties were investigated. The results revealed that the quaternary ceramic can be densified at 1250–1300 °C, and its crystal structure was transformed from the tetragonal to the cubic with increasing amount of Pb(Mn1/3Nb2/3)O3. The optimized results of kp (0.55), tan δ (0.003) and Qm (2528) were obtained at 10 mol% Pb(Mn1/3Nb2/3)O3, which is a new promising material for piezoelectric transformer application.

Synthesis of manganese titanate MnTiO3 powders by a sol-gel-hydrothermal method

Abstract: Manganese titanate (MnTiO 3 ) powders have been synthesized by a sol–gel–hydrothermal method at low temperature below to 473 K through using NaOH as mineralizer. The structural character of as-synthesized powders were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), Fourier transformed infrared spectroscopy (FTIR) and Raman spectroscopy. The results indicate that the as-obtained powders are phase pure pyrophanite MnTiO 3 with pie-shape morphology. The electronic properties of as-synthesized powders was characterized by UV–vis diffuse reflectance spectroscopy (UV–vis), which shows the as-obtained sample has a significant absorption in the visible region.

Low-temperature preparation of Na0.5Bi0.5TiO3 nanowhiskers by a sol–gel–hydrothermal method

Abstract: Na0.5Bi0.5TiO3 (NBT) nanowhiskers with diameters of 20 nm and lengths of 300 nm have been prepared at low temperature by using a sol–gel–hydrothermal technique, which combined the conventional sol–gel process and the hydrothermal method. The reaction conditions such as alkali concentration and temperature were investigated. The powders were characterized by x-ray diffraction (XRD), Fourier transform infrared (FTIR) absorption spectroscopy, transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM). It is believed that the gel precursor and hydrothermal environment play an important role in the formation of the nanowhiskers.