Materials Science Forum Vols. 514-516

Abstract: Thermal treatment of ceramics composed of two or more oxides under controlled reducing conditions selectively volatilizes the less stable components, leaving microstructured surfaces consisting of the most stable of the original oxides. Volatility diagrams showing equilibrium partial pressures allow interpretation of this process and the calculation of equilibrium conditions for the different reactions involved. In this communication we present results for ceramics composed of mullite (3Al2O3·2SiO2), zircon (SiO2·ZrO2) and cordierite (2Al2O3·2MgO·5SiO2).

Abstract: Elemental powder mixtures of Fe and Si were mechanically alloyed with a ball-mill. Mössbauer spectroscopy and X-ray diffraction were used to characterise the microstructural changes of these mixtures which are induced by high-energy ball-milling. Mössbauer spectra are discussed in terms of two main spectral components (corresponding to FeSi alloy and α-Fe) and the time dependence of the alloy formation follows a Johnson-Mehl type law. Calorimetry measurements show that the formed alloy is stable up to 800°C as no crystallisation or phase transformation peaks are observed. From X-ray diffraction, a crystallite size of 9 nm is obtained. Magnetic measurements at low temperature were carried out on the final alloy and the saturation magnetic moment at 4.2K is 0.44 μB/Fe.

Abstract: The influence of uranium doping up to 2 at. % on grain size, electrical resistivity and dielectric constant of barium titanate ceramics has been investigated. The samples were prepared by the conventional ceramic technique using pure raw materials. The mixed oxides were calcined at 1100 oC. The pressed pellets were sintered at temperatures between 13001500 oC in controlled atmosphere of argon and oxygen. The electrical resistivity drastically decreased, from 109 to 103 cm, with increasing uranium content up to 1.5 at. % and increased thereafter to about 107 cm, for 2 at. % U. The dielectric constant slightly increased with increasing U-content, up to about 1 at. % U and then shows a sudden increase with about two orders of magnitude, having a sharp maximum at 1.5 at. % U, when sintered in argon atmosphere compared to only half an order of magnitude when sintered in oxygen atmosphere. The results are discussed in terms of the possible A and B sites occupied by uranium as well as the boundary layer enriched with vacancies.

Abstract: Ni-Ti thin films where the R-phase transformation occurs between 55°C and 30°C, the peak temperature being 40°C, have been produced. These thin films have been grown using a magnetron assisted system of dc sputtering, with a Glow-Discharge Optical Emission Spectroscopy device. The OES technique has been used to investigate the spatial distribution of sputtered atoms from the cathode to the substrate in different operating conditions: Argon pressure of 5 and 9x10 – 4 Torr, without polarization and with – 60 V bias. Structural characterization of the thin films has been made by XRD and the transformation temperatures associated to the shape memory effect have been determined by DSC. A discussion of the optimization of the processing parameters (Argon pressure and polarization) is then presented.

Abstract: Aluminium powder has been mechanically milled using different amounts of process control agent (PCA). Mechanically alloyed aluminium powder (MA Al) was prepared by attrition milling in the presence of 1.5 and 3wt.% of an EBS wax. Milling was carried out in vacuum during 10 h. Milled powders were consolidated by a press-and-sintering method. This consolidation method is not usually employed with MA Al powders. The amount of dispersed carbides formed in the Al powder increases with the percentage of PCA. These carbides restrain Al grain growth during sintering, resulting in consolidated compacts with a grain size of about 550 nm. Thus, these PM materials can be considered ultrafine grained materials. Due to grain refinement and dispersion strengthening, the tensile strength of MA Al specimens is increased remarkably.

Abstract: The thermal stability of Ni,Al-CO3 hydrotalcite-like compounds synthesized by the coprecipitation method and aged upon microwave-hydrothermal treatment for different periods of time was studied. The samples prepared were characterized by Elemental Analysis, PXRD, Thermal analyses (DTA and TG) and Temperature Programmed Reduction (TPR). The results show that the use of microwave radiation as a source of heating during the ageing treatment leads to an increase in the crystallinity of the solids, which determines their thermal stability.

Abstract: The powders in the micrometer scale of Sm(Co0.86Fe0.1B0.04)7.5, Sm(Co0.82Fe0.1Zr0.04B0.04)7.5, Sm(Co0.715Fe0.1Cu0.12Zr0.04B0.025)7.5, Sm(Co0.69Fe0.1Cu0.12Zr0.04B0.05)7.5 compounds have been prepared by using a high-energy planetary ball miller. The X-ray diffraction patterns have an amorphous state after 42 hours milling while the average grain size is in the range of 0.5-10 μm, as measured by using a scanning electron microscope. Amorphization has been achieved at shorter milling times for samples with higher boron content. Depending on the annealing temperature, the major phases are 1:7, 2:17 hexagonal or fcc-Co as revealed from the Xray patterns. The annealing at temperature over 900 oC for 1h helps the formation of the fcc-Co which becomes the dominant phase, mostly on the higher boron doped samples. From the hysteresis curve of Sm(Co0.715Fe0.1Cu0.12Zr0.04B0.025)7.5, a coercive field of ~9 kOe has been determined. For this sample there is difference between the two coercive fields, Ηc(χirr=max) and Ηc(M=0). The zirconium doped sample has higher coercive field than the Sm(Co0.86Fe0.1B0.04)7.5 one. The coercive field is significant increases to the copper doped samples. In the thermomagnetic curves of the asmilled powders, it has been evidenced that there is crystallization, connected to the increase of the magnetization under the constant applied field. The initial and demagnetization curves exhibit slope variations that are connected to the soft fcc-Co phase. Almost zero hydrogen absorption was found for the Cu-doped samples at room temperature.

Abstract: This study aims the definition of a new material that may act as a robust and yet cost effective biosorbent for treatment of wastewater with low concentration of heavy metals. A comparative study was made between two biosorption systems composed of an Arthrobacter viscosus biofilm supported on Cuban natural zeolites and on prepared NaY and NaX, in terms of their ability to retain ionic chromium. The bacterium is able to reduce Cr(VI) to Cr(III) and, only then, this smaller and positive ion may be entrapped in the zeolite cages by ion exchange. The first support was tested in a continuous flow semi-packed bed column. The highest removal ratio, 42%, was achieved for initial chromium concentration of 10 mg/L, but the best up-take, 5.5 mg/gzeolite, was obtained for initial concentration of 70 mg/L. Biosorbents prepared with the same biofilm supported in NaY and NaX zeolites were also considered in batch studies, with a typical kinetics of biosorption processes, reaching 20% of initial chromium removal within an initial range of Cr(VI) concentration between 50 and 250 mg/L. These last structures were characterized by spectroscopic methods (FTIR and ICP-AES), surface analysis (DRX) and thermal analysis (TGA). All these techniques indicated that the biosorption process does not modify the morphology and structure of the FAU-zeolites.

Abstract: In Lead-cadmium fluorogermanate glasses (PbF2-CdF2-PbGeO3) the addition of metal fluorides to the base PbGeO3 glass leads to a decrease of the glass transition temperature (Tg) and to an enhancement of the ionic conductivity properties. Based on different spectroscopic techniques (19F NMR, Ge K-edge X-ryas absorption and Raman scattering) an heterogeneous glass structure is proposed at the molecular scale, which can be described by fluoride rich regions permeating the metagermanate chains. The temperature dependence of the 19F NMR lineshapes and relaxation times exhibits the qualitative and quantitative features associated with the high fluoride mobility in these systems. Eu3+ emission and vibronic spectra are used to follow the crystallization process leading to transparent glass ceramics.

Abstract: Novel polynuclear tungsten and molybdenum (VI) complexes with 3-hydroxypicolinic acid (HpicOH) and europium(III) [M4O12Eu(picOH)3] (M(VI) = W, Mo) were prepared by hydrothermal methods. The solid compounds were characterized by FT-IR and FT-Raman spectroscopy and elemental analysis. The photoluminescent properties of the materials were investigated, showing sensitization of the europium(III) luminescence by both the 3- hydroxypicolinate ligand and the polyoxometalate moiety