Papers by Author: Hai Yang Dai

Abstract: Ca_1-xLa_xCu₃Ti₄O₁₂ (x=0-0.05) ceramics are prepared by the solid-state reaction method. The effects of La doping on the microstructure and non-Ohmic properties of Ca_1-xLa_xCu₃Ti₄O₁₂ system are investigated by XRD, SEM and Positron annihilation technique. The results show that no phase transition occurs in the doping range (x=0-0.05), while the significant changes in grain size can be observed as x≥0.02. The positron lifetime results prove that the average lifetime is strongly affected by La doping. The correlation of the nonlinear behavior of CaCu₃Ti₄O₁₂ system and the grain size, the local electron density in defect was discussed.

Abstract: The material of sulfur-hyperdoped silicon was prepared by ion implantation and then annealed by heat treating furnace and pulse laser respectively. The effects of annealing process on the absorption, raman shift, carrier concentration and mobility ratio of this material were researched. Furthermore, the doppler broadening spectrum of crystal silicon substrate and sulfur-hyperdoped silicon before and after annealing was presented by using a slow positron technique. And the relationship between the microstructure and photoelectric properties of the material was explored. The experimental results indicate that the absorption and carrier concentration of the sulfur-hyperdoped silicon annealed by pulse laser were higher than that of the samples annealed by heat treating furnace. And the doppler broadening spectrum shows that the photoelectric properties of this material were closely related to the defects in the material.

Abstract: To specify the effects of modulating mechanism of Gd substitution on the dielectric properties for Ca_1-xGd_xCu₃Ti₄O₁₂ system, the XRD, SEM and positron positron annihilation technique have been implemented. The results display that both grain size and vacancy-type defect concentration play important roles in controlling the dielectric properties of CaCu₃Ti₄O₁₂. The dielectric properties are improved by adding an appropriate amount of Gd additives (x=0.01) but weakened by higher Gd doping content since this is closely related to the grain size and concentration of vacancy-type defect in the samples.

Abstract: Hydrogenated amorphous carbon (a-C:H) films on silicon wafers were prepared by middle frequency pulsed unbalanced magnetron sputtering technique (MFPUMST) at different substrate bias under the methane-argon mixed gases. Raman spectra show that the sp3 fraction in a-C:H films increases with increasing substrate bias voltage from 0 to 100 V, and then decreases when the substrate bias above 100 V. Nano-hardness for these films prepared under different substrate bias voltage show that nano-hardness increase with increasing substrate bias voltage from 0 to 100 V, and then decrease from 100 up to 200 V. The results above indicate that the sp3 fraction in the prepared a-C:H films is directly related to nano-hardness, therefore, substrate bias voltage is an important factor for influence on the bonding configuration of the deposited a-C:H films. The related mechanism is discussed by sub-plantation model in this paper.

Abstract: Hydrogenated amorphous carbon (a-C:H) films on silicon wafers were prepared by middle frequency pulsed unbalanced magnetron sputtering technique (MFPUMST) at different substrate bias under the acetylene-argon mixed gases. These films were characterized with Raman spectroscopy, atomic force microscopy (AFM) and nanoindentation. Raman spectra show that the sp3 fraction in a-C:H films increases with increasing substrate bias voltage from 0 to 100 V, and then decreases when the substrate bias above 100 V. AFM and nanoindentation results reveal that the surface roughness and nano-hardness of the films increase with increasing substrate bias voltage from 0 to 100 V, and then decreases when the substrate bias above 100 V. The mechanism of sputtering current on the sp3 fraction is discussed in this paper.

Abstract: The effect of Fe doping on the structure, local electron density and electronic transport properties of DyBa₂Cu_3-XFe_xO_7-δ (x = 0.00- 0.30) superconductors have been investigated by X-ray diffraction (XRD), positron annihilation and other experimental methods. XRD results indicate that the compound undergoes an Orthorhombic–Tetragonal (O-T) phase transition at about x = 0.10. The electrical transport measurements show that the superconductivity was strongly suppressed by Fe doping, and the superconducting transition temperature decreases with the increase of Fe doping content. The results of local electron density calculated using positron annihilation data show that the local electron density of the systems is strongly changed by the Fe doping, and the local electron density vary noticeably at the range of O-T phase transition. From the results of local electron density and the superconductivity, it can be found that the superconducting transition temperature decreases with decreasing the local electron density. These investigations reveal that the crystal phase transition and the local electron density could strongly affect the superconductivity in the Fe doped DyBa₂Cu_3-XFe_xO_7-δ systems.

Abstract: Multiferroic ceramic samples of Bi_1-xGd_xFeO₃ (x = 0, 0.05, 0.1 and 0.15) have been prepared by the rapid liquid phase sintering technique. The effect of Gd substitution on ferroelectric properties of Bi_1-xGd_xFeO₃ ceramics has been investigated. The results of x-ray diffraction (XRD) patterns show that the single-phase BiFeO₃ sample has a rhombohedral structure and Gd³⁺ substitution for Bi³⁺ has not affected the perovskite structure. Experimental results suggest that Gd³⁺ substitution for Bi³⁺ has markedly affected the Raman scattering spectra, and the ferroelectric properties of BiFeO₃ are improved by Gd³⁺ doping. When x = 0.15, saturated ferroelectric hysteresis loop is observed at room temperature with the maximal 2Pr = 1.62 μC/cm².