Materials Science Forum Vols. 492-493

Abstract: The changes of phase composition and structure evolvement of Fe and Si powders with the ratio of 6.5wt%Si to 93.5wt%Fe were mainly studied. It is found that, the local graded structure, Fe-Fe(Si)-Fe3Si-(FeSi)-Si, forms due to the obvious diffusion and the alloying reaction between Fe and Si powders when sintering at 900-975oC. The graded structure, in which the high silicon phase based on Fe-Si alloys is packed by the low silicon solution based on Fe, exhibits a graded concentration of Si distribution. And most of the Fe-containing phase remains a state of lower Si content, less than that of 3wt%Si-Fe alloy, thus provides the good deformation behavior of rolling and cutting for the compacts. Therefore, it is feasible for the high silicon iron sheets to be produced by the powder metallurgy method.

Abstract: Experimental results on the preparation of functionally graded TiC-based cermets obtained by combustion synthesis (also known as Self-Propagating High-Temperature Synthesis, SHS) followed by quasi-isostatic (QIP) pressing in a granulate medium are presented. Pellets of TiC-Fe graded cermets are produced by stacking layers of Ti and C powder mixtures in which the content of a NiFe alloy (50 wt% Ni and 50 wt% Fe) is varied from 5 up to 25 vol %. X-ray diffraction showed that the NiFe alloy did not react with the TiC, thus preserving its special properties. Scanning electron microscopy results show a graded material with pores increasing in size towards the side with the highest ceramic fraction.

Abstract: The fabrication of Al2O3-ZrO2~Ni functionally graded pipes has been investigated by slurry coating and pressureless sintering process. Each slurry of Al2O3-ZrO2~Ni mixture and Ni was coated in order on the Al2O3-ZrO2 pipes formed by slip casting method with those slurries. The obtained laminar green pipes were 30 mm in diameter and approximately 90 mm in length. The laminar green pipes were sintered for 2 hours at 1430°C in a vacuum. The structure of pipes fabricated by this method was optically and microscopically examined and the graded distribution was examined by an EPMA analysis of Al and Ni. The pipes with 2 layers of Al2O3-ZrO2 and Ni had crevices in the bonding interface. Some functionally graded pipes with 3 layers were free from cracks and warps without porosity, and each interface had the complete bonding.

Abstract: An entirely new functional material named photonic fractal has been developed. It can strongly localize electromagnetic waves in a dielectric fractal cube called Menger sponge without reflection and transmission. The wavelength and frequency of the localized mode can be predicted using a simple equation associated with the fractal geometry and the spatially averaged dielectric constant of the Menger sponge structure. A wide variety of applications to communication, information, energy, sensing, medical care, and other fields are considered. Design and fabrication of Menger sponge fractals with epoxy resin and ceramics, their electromagnetic wave responses, integration of photonic fractals as well as potential applications are reported.

Abstract: In order to provide higher driving forces for densification of Indium-Tin oxide(ITO) sputtering targets, nano-sized ITO powders with uniform size distribution were prepared by solvothermal process. In this study, sintered bodies were densified by a spark plasma sintering (SPS) process at 950 oC. The experiments conduct the SPS method in order to achieve the highest density of sputtering targets. The phase and relative density of ITO sputtering targets were identified by X-ray diffractometry_(XRD) and Archimedes method, respectively. The surface microstructure and quanitative analysis of the targets were observed by scanning scanning electron microscopy (SEM), and ITO powders particle were measured by SEM, XRD-LB methods.

Abstract: In the paper, firstly self-consistent model (SCM) is used for establishing the predicted formula which can predict the effective permittivity of the composite materials with core-shell type inclusions. Then the factors on the effective permittivity of this kind of composite are investigated on the base of this predicted formula. The researching results indicate the effective permittivity of composite depends on not only the volume fraction of inclusions, but also the permittivity of matrix and inclusions (core and shell). According to our works, the optimal design of composite with core-shell type inclusion must be done if we want to make this composite get better effect of absorbing wave.

Abstract: Yb3+, Ho3+ co-doped Y2O3 nanocrystalline powders were synthesized via a reverse-strike co-precipitation method using nitrates and ammonia as raw materials. The as-prepared powders are cubic Y2O3 after fired at 7000C for 2 h. Field emission scanning electronic microscopy revealed that most of the particles are nearly spherical and the average diameter of the particles is in the range of 60-80 nm. Under the excitation with a 980 nm continuous wave diode laser, two main emission bands, i.e., green emission centered at 549 nm and red emission centered at 666 nm is observed, respectively. The green emission is attributed to the transition of (5S2, 5F4-5I8) levels of Ho3+; while for the red emission, the transition of 5F5-5I8 levels is suggested. In addition, near infrared emission centered at 755 nm is also detected, corresponding to the transition of (5S2, 5F4-5I7) levels. The dependence of the intensity of upconversion emissions upon the pump power of the diode laser suggests that two-photon absorption mechanism be involved in these processes.

Abstract: Preparation of nanosized (Y,Gd)2O3:Eu powders by co-precipitation processing is reported. Phase transformation during calcination was studied by means of DTA-TG FT-IR, SEM and XRD in detail. It was found that the morphologies and X-ray excited luminescence of (Y,Gd)2O3:Eu powders were greatly influenced by the precipitants used. The (Y,Gd)2O3:Eu powder resulted from the complex precipitants of NH3•H2O and NH4HCO3 possessed best microstructural features with grain size of about 30nm and specific surface area of 38m2/g after being calcined at 8500C for 2hr, showing a much finer grain and less agglomeration than those from other two precipitation processing. It was demonstrated that nanosized (Y,Gd)2O3:Eu powder from complex precipitant exhibited much higher luminescence intensity at 611nm under excitation of X-ray (70Kev). which might be attributed to less degree of agglomeration among primary grains, decreasing the probability of non-radiation relaxation.

Abstract: Several pellets of general formula Ba1-xMx(Ti1-xLix)O3-3xF3x were prepared from BaTiO3, MF2 ( M = Ca, Sr, Ba or Pb ) and LiF powders then sintered by the conventional solid state route at 900 or 1000°C for 1, 2 or 4 hours. X-Ray powder diffraction patterns were collected at room temperature and SEM observations were carried out. Dielectric measurements were performed between 150 and 450K in the frequency range 50Hz - 40MHz. All the ceramics exhibit a very broad ferroelectric displacive phase transition in the temperature range 255K ≤ TC ≤ 328K.The ε' r - T curves and the values of tanδ are compatible with the Z5U class of capacitors. Furthermore, a Debye dielectric relaxation occurs in these oxifluorids in the frequency range 4MHz ≤fr ≤ 38.5MHz.

Abstract: In the 21st century, the optical computation is likely to be the basic technology for processing lots of information at high speed. The aim of the present research work is to develop optical logic gates or memory chips. For this purpose, we have examined the suitability of organic nonlinear optical dye material Vanadyl-phthalocyanine (VOPc). Large single crystals of this material have been fabricated by using Molecular Beam Epitaxy (MBE) technique. The epitaxial films were formed on the substrate under optimum operating conditions. However, the epitixial growth is observed only up to a limited thickness. Above this thickness, the films become non-epitaxial, which can be improved by annealing. The reformation of the epitaxial films has been confirmed. We have also reported the effects of the environment of high temperature on the multilayered tetra-tert-butyl-Vanadyl-phthalocyanine ((t-bu)4VOPc) films, formed by Langmuir-Blodgett (LB) method. The solvent used to dissolve ((t-bu)4VOPc) was 1,2-dichloroethane. The monolayer on the surface of the water was transferred to a glass substrate by the vertical dipping method. If the multilayered stack is too thick, the molecular arrangement of the film may get disturbed. The improvement in the molecular arrangement of the LB films was examined and confirmed by measuring it’s nonlinear optical susceptibility, using Maker Fringe Method. Monolayer formation on water surface depends on the surface pressure-area isotherm. If this monolayer formation is not perfect, multiplayer stacks cannot be formed. The molecular films were aligned almost perpendicular to the substrate, as estimated from the limiting molecular area of surface pressure-area isotherm. The molecular organization of the monolayer on the substrate and the molecular structure of the multilayered ((t-bu)4VOPc) films are discussed.