Advanced Materials Research Vols. 750-752

Abstract: ZnS quantum dots (ZnS QDs) synthesized in aqueous solution is easy to gather a mass, which always results in low quantum yield. So, in this study benzene was used as reaction medium in which ZnS QDs with different hues were synthesized, and the hues of ZnS QDs were depend on the molar ratio of Zn (CH₃COO)₂ and Na₂S·9H₂O. The results show the emission spectra of ZnS QDs shifts with the change of the precursor molar ratio but the absorption peak at 310nm is not. The emission peaks centered at 430nm and 580nm at higher molar ratio [Zn²⁺]/[S^2-] with blue-violet emitting phosphors; but, ZnS QDs synthesized at higher molar ratio [S^2-]/[Zn²⁺] have orange-red emission at 580nm only. The X-ray diffraction analysis shows the crystallinity of ZnS QDs is better at [Zn²⁺]/[S^2-]=1:10, which are typical zinc blend with nanorod structure.

Abstract: A nano-particle array grating is proposed to achieve fine polarized output and transmittance on GaN-based LED. This nanostructure enables simple fabrication, since the novel arrays can be directly coated on the surface of GaN. In this letter, Aluminum nano-particle array gratings are optimized at blue and green band respectively. Extinction ratio up to 26dB and a TM polarized transmittance of 76% are achieved. Theoretical results show the nano-particle grating can be a guidance in designing the integrated GaN based and polarized photonic devices. Numerical simulations are performed using the Finite-difference time-domain (FDTD) solutions.

Abstract: The performance of the switched reluctance motor (SRM) greatly depends on magnetic properties of core material since it consists of only laminated-core and windings. This paper analyses the performance of an SRM with amorphous stator, which has very low iron losses. Copper loss, iron loss and efficiency of the SRM are estimated by finite element method. Furthermore, a suitable structure for the SRM with amorphous stator is examined on the optimization design.

Abstract: The conductive properties of donor doped BaTiO₃ semiconductor ceramic was studied with a quantitative model in this paper. Considering the influence of acceptor grain boundary segregation, a differential equation of electron level was established. According to a four order Runge-Kutta formula, the energy level of the grain boundary barrier was obtained at different temperatures numerically. Further, the PTCR effect of BaTiO₃ semiconductive ceramics was calculated.

Abstract: Ten kinds of terbium doped inert yttrium complexes with 2,3-pyrazinedicarboxylate (2,3-pzdc^2-) have been synthesized. Characterization results indicate that the complexes have the compositions of Tb (pzdc)_1.55H₂O and Tb_xY_y(pzdc)_1.55H₂O (x:y=0.10:0.90; 0.20:0.80; 0.30:0.70; 0.40:0.60; 0.60:0.40; 0.70:0.30; 0.80:0.20; 0.90:0.10). IR spectra show that the lanthanide ions coordinate with the carboxylic oxygen atoms and nitrogen atoms of the ligands. Luminescence spectra show that the Y(III) ions can remarkably increase the luminescent intensities of terbium complexes. And Tb_0.7Y_0.3(pzdc)_1.55H₂O exhibits the strongest luminescent emission. Furthermore, the doped lanthanide complexes show longer luminescence lifetimes and higher quantum yields. The enhanced luminescence efficiencies of Tb³⁺ ions in the doped complexes may result from intramolecular energy transfer as well as the decrease of the self-quench of the Tb³⁺ ions induced by the doped Y(III) ions.

Abstract: We have successfully formed Fe-nanodots with an areal dot density as high as ~2.4×10¹¹ cm^-2 on thermally grown SiO₂ by exposing a 3-nm-thick Fe layer to a remote plasma of pure H₂ without external heating. During remote H₂ plasma exposure, the surface temperature rising up to ~500 °C was caused by surface recombination of atomic H, which enhanced surface migration of Fe atoms and promoted self-assembling nanodots. Electrical separation among Fe-nanodots was also verified from the changes in surface potential after charge injection using an AFM/Kelvin probe technique. The magnetic characterization by using magnetic force microscopy suggests that Fe-nanodots act as not only charge storage nodes but also spin-dependent active elements.

Abstract: Based on experimental characteristics of the iso-field heat capacity of the working substances Gd_0.45Tb_0.55 and Gd, the magnetic entropy change with temperature is calculated. The regenerative Brayton refrigeration cycles employing these materials as the working substances are established. By means of thermodynamic analysis and numerical calculation, the effects of the non-perfect regeneration on the performance characteristics of the refrigeration cycles are revealed. Furthermore, the cyclic performances employing Gd_0.45Tb_0.55 and Gd as the working substances are evaluated and compared. The results obtained may provide some useful information for the optimal design of the room temperature magnetic refrigerators.

Abstract: Ceramic powders with chemical formula Eu₄Ti₃O₁₂ were fabricated by sol-gel method. A milk-white homogeneously mixed gel was prepared, and transferred into a porous resin intermediate through charring. Eu₄Ti₃O₁₂ powders were prepared by calcination of the resin intermediate, and ceramic samples with two phases of Eu₂TiO₅ and Eu₂Ti₂O₇ were got by sintering. Meanwhile, the crystal structure was determined by X-ray diffraction and grain character was analyzed by scanning electron microscope. Eu₄Ti₃O₁₂ ceramics has good microwave dielectric characteristics: ε_r=27.51, Q_f=9450 GHz, τ_f=211 ppm/°C.

Abstract: The thin films of CoFe₂O₄ spinel ferrite were prepared on the substrates of monocrystalline silicon(100) at low temperature by means of a developed citrate processing.Structural properties and surface morphology of deposited films were examined using an X-ray diffractometer (XRD) and a field emission scanning electron microscope (FESEM). XRD patterns of all deposited films consisted of a single phase of cobalt ferrite and they did not have any preferred orientation. FESEM micrographs of CoFe₂O₄ films showed that the film calcined at 400°C had a surface morphology different from the ones at 700°C and 800°C. The surface morphology of the samples was very different with various alcined temperatures. Magnetic properties measured at room temperature showed that the films did not have any magnetically preferred orientation. The maximum values of the coercivities measured at perpendicular and in-plane directions were 3.873 and 2.70 kOe, respectively.

Abstract: The present paper prepared a structural epitaxial film of gallium nitride multiple-quantum-well (GaN MQW) blue light-emitting diode (LED) on Si (111) substrates with different conduction types using the metal-organic chemical vapor deposition method. The method prevented the interdiffusion of GaN and Si to achieve high-quality film growth by introducing aluminium nitride (AlN)-interposed layer and rich-Gallium GaN layer with low V/III ratio double buffer layers. Surface analysis shows that the GaN LED epitaxial film on the Si (111) substrate with different conduction types presented an entirely different appearance. The surface roughness of all the samples was less than 3 nm. A much smoother surface of the epitaxial film on the N-type substrate had less roughness, whereas a layered stack surface of the epitaxial film on the P-type substrate had larger surface roughness. The full width at half maximum of the XRD Omega rocking curve with (002) and (102) planes of GaN film grown on the N-type substrate was less than that of the GaN film on the P-type substrate. Furthermore, the film was superior to the samples on the P-type substrate in terms of crystal quality. There was little difference in the peak position of the PL of the epitaxial film on the N-type substrate, but the peak position of the PL of the epitaxial film on the P-type substrate was long and had a large half-peak width. The tensile stress of the GaN film on the P-type substrate was higher. The above results show that the N-type Si (111) substrate with high resistivity is more suitable for the growth of GaN MQW LED epitaxial film.