Materials Science Forum Vols. 654-656

Abstract: Ba0.6Sr0.4TiO3 powder was synthesized by a citrate method. The powder with a pure pervoskite phase and superfine particle morphology was produced at low calcining temperature of 550°C by controlling heating rate during the calcination. Microstructure and nonlinear dielectric properties of the ceramic specimens were investigated with respect to sintering temperature. It was found that the superfine morphology of the powder effectively reduced the sintering temperature of the ceramic specimens. The ceramic specimen sintered at 1260 °C attained about 95% of the theoretical density. At room temperature, the ceramic specimen showed a dielectric constant of 3010 and a dielectric loss of 0.48% at 10 kHz together with a tunability of 32.4% at 10 kHz and 20 kV/cm.

Abstract: Temperature-dependent Raman scattering, dielectric properties, and Impedance spectra have been carried out on Ba(Ti0.8Sn0.2)O3 ceramic sample. The number of modes observed for the compositions of cubic symmetry is more than that predicted by group theory, and the A1(TO1) and E(TO2) modes become Raman active below the critical temperature (around 300K) in the lower frequency region. This gives evidence that there are some deviations of the local structure from the average global symmetry. Moreover, The characteristic of A1(TO3) mode is confirmed be associated with the diffused dielectric properties. Impedance/modulus analyses indicate that a third resistance-capacitance (RC) response is present in the sample, in addition to the grain and grain-boundary RC elements above 573K. Thus, it probably inferred that the existence of inter- and/or intra-granular surface layers associate with the abnormal dielectric properties, which is different from the normal ferroelectrics. Variation of bulk ac conductivity as a function of temperature shows that Ba(Ti0.8Sn0.2)O3 follows the classical Arrhenius relation.The results show that the activation energy of grains and grain boundaries are 0.61 and ~1.08 eV, respectively.

Abstract: Millimeter-wave sintering of ceramic laser host materials has been under investigation for high-energy laser (HEL) applications. Advantages of polycrystalline, compared to single-crystal, laser host materials include lower processing temperature, higher gain from higher dopant concentration, cheaper fabrication, and larger devices. We are currently investigating the solid-state reactive sintering of neodymium-doped yttrium aluminum garnet (Nd:YAG) using a high power millimeter-wave beam as the heat source. The 83 GHz beam is generated in the Naval Research Laboratory (NRL) High Frequency Materials Processing Facility that is powered by a 15 kW, CW, 83 GHz GYCOM gyrotron. The starting powder is a mixture of commercially available alumina, yttria, and neodymia powders. Near transparency and over 99% theoretical density have been achieved with grain sizes of 5 to 10 µm. The fluorescence lifetime of the Nd+3 1.06 µm lasing transition was measured to be about 200 µs, in good agreement with other work. SEM studies of the sintered microstructure show residual porosity caused by trapped pores that must be eliminated to produce fully transparent material.

Abstract: Lithium-based ceramics have been recognized as promising tritium breeding-materials for D-T fusion reactor blankets. Lithium silicates, Li4SiO4 and Li2SiO3, are recommended by many ITER research teams as the first selection for the solid tritium breeder. The solid-state reaction method is the most important way to synthesize lithium silicates. In present study, the processes of solid-sate reaction between amorphous silica and Li2CO3 powders was investigaed by TGA/DSC; the lithium silicate powders were synthesized at 700~900°C with different Li:Si molar ratio using solid-state reaction method. The optimized synthesis temperature and the solid-state reaction mechanism were derived on the base of experimental results.

Abstract: This research presented the kinetics of mullite formation in fly ash-bauxite reactants couples. Experiment on isothermal conversion of fly ash-bauxite at 1100°C, 1300°C, 1500°C has been carried out. XRD was used to quantitatively measure the content of mullite specimens sintered at different temperatures and times. The kinetics curve of conversion ratio versus time has been drawn. The results obtained showed that the full transformation of fly ash-bauxite to mullite takes place between 1300 °C and 1500 °C. The activation energy of secondary mullite formation is deduced to be 151 kJ/mol in the range of 1100-1500°C. The growth morphology parameter, n, is about 1.14 and 0.45 at 1100°C and 1500°C, respectively, indicating that bulk nucleation is dominant in mullite crystallization followed by three-dimensional growth of mullite crystal controlled by diffusion, but at 1500 °C diffusion process dominates mullite formation process.

Abstract: BaCe1-xYxO3-α (x = 0.1, 0.15) solid electrolytes were prepared via ball milling raw powders of BaCO3, CeO2 and Y2O3, and pressing the powder mixtures into tablets followed by calcining in air. X-ray diffraction analysis indicated that the BaCe0.9Y0.1O3-α and BaCe0.85Y0.15O3-α solid solutions were synthesized at 1250°C and 1400°C, respectively. The solid electrolyte disks were made by grinding the calcined tablets into fine powders, cold isostatic pressing into the shape and sintering at 1550°C in air. The electrochemical impedance spectroscopy measurements revealed that they show useful protonic conduction, up to 2.7×10-2 S/cm at 900°C.

Abstract: Mo particles have been introduced into Ag-Cu-Ti brazing alloy for the joining of Si3N4 ceramic. Effects of Ti content on microstructure and mechanical properties of the joints were investigated. The results present that a compact reaction layer which is composed of TiN and Ti5Si3 was formed at the Si3N4/solder interface. The central part of the joint was composed of Ag based solid solution, Cu based solid solution, Mo particles and Cu-Ti intermetallics. By increasing Ti content in the composite filler, both the thickness of reaction layer and the amount of Cu-Ti intermetallics in the joint increased, which is beneficial for the joint strength. However, the reaction between Ti and Si3N4 proceeded more excessively. Simultaneously, more Cu-Ti intermetallics were precipitated while elevating Ti content to 6wt.%, leading to deterioration of the bonding strength. The maximum bending strength (429.4MPa) was obtained when Ti content is 4wt.% in the composite filler.

Abstract: Magnesium carbonate whisker was prepared by thermal decomposition of Mg(HCO3)2 solution that was prepared through hydration and carbonation of light burnt magnesia derived from magnesite tailings. The effects of thermal decomposition conditions on the morphology of magnesium carbonate crystal were investigated. Magnesium carbonate whiskers were successfully prepared when a kind of soluble magnesium salt was added, and magnesium carbonate whiskers with the length of 20 to 60μm and aspect ratio of 10~20 were obtained under the condition of 50 °C thermal decomposition temperature and 200 rpm stirring intensity.

Abstract: (PO4)3- -doped red phosphors, Na5Eu(MoO4)4-x(PO4)x (x ≤ 0.10), were prepared by the conventional solid-state reaction method, and their luminescent properties were studied. Under the excitation of near-UV 395 nm, the phosphors show intense red emission. In particular, the relative emission intensity of Na5Eu(MoO4)3.96(PO4)0.04 sample reaches about 5.0 times in comparison with that of the commercial red phosphor Y2O2S:Eu3+. The phosphor could be suitable for the application of white light-emitting diodes.

Abstract: Due to the great potentials of tantalite and niobate materials in ferroelectric and photocatalytic applications, development of proper tantalum or niobium precursors is urgently need. In this work, a simple novel route to synthesize environmentally friendly aqueous tantalum and niobium precursors has been developed using cheap and stable Nb2O5 or Ta2O5 as starting source. Using home-made Ta and Nb precursors, several photocatalytic nanopowders such as BiNbO4 and BiTaO4, and ferroelectric ceramics such as 0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3 (PMN-PT), have been prepared by a modified PC method. These powders and ceramics from polymeric precursors have low crystalline temperature and transformation temperature with uniform grain distribution, compared to conventional solid phase reaction (SSR). Pure triclinic phase of BiNbO4 powders can be obtained at 700°C instead of 1020°C of SSR, with excellent photodegradation activity of methyl violet. PC-derived PMN-PT ceramics also exhibit better ferroelectric and piezoelectric properties. All these indicate that this is an attractive and flexible approach using environmentally friendly water-soluble tantalum and niobium precursors for fabrication of tantalate and nibonate functional materials.