Godfrey Sauti

The AC and DC conductivity of nanocomposites

The microstructures of binary (conductor-insulator) composites, containing nanoparticles, will usually have one of two basic structures. The first is the matrix structure where the nanoparticles (granules) are embedded in and always coated by the matrix material and there are no particle-particle contacts. The AC and DC conductivity of this microstructure is usually described by the Maxwell-Wagner/Hashin-Shtrikman or Bricklayer model. The second is a percolation structure, which can be thought to be made up by randomly packing the two types of granules (not necessarily the same size) together. In percolation systems, there exits a critical volume fraction below which the electrical properties are dominated by the insulating component and above which the conducting component dominates. Such percolation systems are best analyzed using the two-exponent phenomenological percolation equation (TEPPE). This paper discusses all of the above and addresses the problem of how to distinguish among the microstructures using electrical measurements.

Boron nitride nanotube: synthesis and applications

Scientists have predicted that carbon’s immediate neighbors on the periodic chart, boron and nitrogen, may also form perfect nanotubes, since the advent of carbon nanotubes (CNTs) in 1991. First proposed then synthesized by researchers at UC Berkeley in the mid 1990’s, the boron nitride nanotube (BNNT) has proven very difficult to make until now. Herein we provide an update on a catalyst-free method for synthesizing highly crystalline, small diameter BNNTs with a high aspect ratio using a high power laser under a high pressure and high temperature environment first discovered jointly by NASA/NIA/JSA. Progress in purification methods, dispersion studies, BNNT mat and composite formation, and modeling and diagnostics will also be presented. The white BNNTs offer extraordinary properties including neutron radiation shielding, piezoelectricity, thermal oxidative stability (> 800°C in air), mechanical strength, and toughness. The characteristics of the novel BNNTs and BNNT polymer composites and their potential applications are discussed.

Densification, Microstructure and Properties of Liquid-Phase Sintered Silicon Carbide

Liquid-Phase Sintered Silicon Carbide (LPSSiC) materials were produced with different Y2O3:Al2O3 sintering additive ratios. Different densification techniques were employed (hot pressing (HP) and gas pressure sintering (GPS)) and densification kinetics studied. The results show significant increase in densification with decreasing Y2O3: Al2O3 ratio. Microstructure and phase evolution during sintering and post-sintering heat treatment was studied. The ratio of the SiC polytypes and the amount and crystalline composition of grain boundary phases was determined using Rietveld analysis. Microstructures of the materials were related to mechanical (hardness, fracture toughness and strength) and electrical properties. Electrical properties were studied using Impedance Spectroscopy. By this method, the resistivities of the grain and grain boundaries could be separated. Introduction LPSSiC as a structural ceramic has only been studied extensively for the past 10 years. Although much work has been done on the Y2O3 -Al2O3 sintering additives system and its mechanical properties [1-3], there is a lack of microstructure-properties relationships established in the literatureespecially in terms of quantitative analysis of the SiC polytypes and binder phases in the materials, and relating these microstructural aspects to the mechanical and electrical properties of the materials. SiC is a well-known semiconductor material, of which a lot is known in terms of the electrical properties of single crystals of SiC [4]. Extensive knowledge of the electrical behaviour of polycrystalline SiC, and in particular LPSSiC, is lacking. The aim of this work was to investigate some relationships between composition, microstructure formation and properties of hot pressed and gas pressure sintered LPSSiC materials. Experimental Commercially available powders alpha-SiC (UF-15, H.C. Starck), Y2O3 (fine grade, H.C. Starck) and Al2O3 (AKP-50, Sumitomo) were used as the starting materials. Compositions with 10wt% sintering additives and varying mole ratio (Y2O3: Al2O3: 1:4, 3:5, 1:1, 4:2, 4:1) were milled in a planetary ball mill for 2 hours, using triethylene glycol (Sigma-Aldrich) (0.5 wt%) and Triton X-100 (Sigma Aldrich) (3wt %). Materials were hot pressed (HP) at 1925 o C, 30 minutes and gas pressure sintered (GPS) at 1925 o C for 60 minutes at final Ar gas pressure of 80 bars, in graphite crucibles. Additional heat treatments were carried out at 1925 o C for 1.5, 5 and 8 hours, in graphite crucibles. Density was measured using the Archimedes method. Phase analysis was carried out using RD7 (Seifert-FPM) and calculated quantitatively using Rietveld analysis (Autoquan). Hardness, indentation toughness and 4-point bending strength measurements were carried out on selected materials (3.0 x 4.0 x 45 mm bars). Four-point impedance spectroscopy was done using a 1610 Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 957-960 doi:10.4028/www.scientific.net/KEM.264-268.957

Nanostructured solar irradiation control materials for solar energy conversion

Tailoring the solar absorptivity (αs) and thermal emissivity (ƐT) of materials constitutes an innovative approach to solar energy control and energy conversion. Numerous ceramic and metallic materials are currently available for solar absorbance/thermal emittance control. However, conventional metal oxides and dielectric/metal/dielectric multi-coatings have limited utility due to residual shear stresses resulting from the different coefficient of thermal expansion of the layered materials. This research presents an alternate approach based on nanoparticle-filled polymers to afford mechanically durable solar-absorptive and thermally-emissive polymer nanocomposites. The αs and ƐT were measured with various nano inclusions, such as carbon nanophase particles (CNPs), at different concentrations. Research has shown that adding only 5 wt% CNPs increased the αs and T by a factor of about 47 and 2, respectively, compared to the pristine polymer. The effect of solar irradiation control of the nanocomposite on solar energy conversion was studied. The solar irradiation control coatings increased the power generation of solar thermoelectric cells by more than 380% compared to that of a control power cell without solar irradiation control coatings.