SrB4O7:Bi2VO5.5 - A novel nanocomposite

Glass composites of strontium tetraborate, SrB4O7 (SBO) with bismuth vanadate, Bi2VO5.5 (BiV) of the composition (1-x) SBO-x BN (0 less than or equal to x less than or equal to 0.75), have been synthesized. X-ray powder diffraction and electron microscopy indicate as-quenched composites to be amorphous and the annealed samples showed the presence of nanometer sized particles of BN dispersed in the glassy matrix of SBO. The dielectric constant of these composites increases with increase in the volume fraction of BiV, at 300 K. The measured dielectric constant of the composite very nearly obeys the Maxwell's relationship. Optical transmission studies confirm a steady shirt in the optical absorption edge towards longer wavelengths with increase in x

Large deformation finite element analysis of laminated circular composite plates

A 48 d.o.f., four-noded quadrilateral laminated composite shell finite element is particularised to a sector finite element and is used for the large deformation analysis of circular composite laminated plates. The strain-displacement relationships for the sector element are obtained by reducing those of the quadrilateral shell finite element by substituting proper values for the geometric parameters. Subsequently, the linear and tangent stiffness matrices are formulated using conventional methods. The Newton-Raphson method is employed as the nonlinear solution technique. The computer code developed is validated by solving an isotropic case for which results are available in the literature. The method is then applied to solve problems of cylindrically orthotropic circular plates. Some of the results of cylindrically orthotropic case are compared with those available in the literature. Subsequently, application is made to the case of laminated composite circular plates having different lay-up schemes. The computer code can handle symmetric/unsymmetric lay-up schemes. The large displacement analysis is useful in estimating the damage in composite plates caused by low-velocity impact.

Al2O3---ZrO2 composite coatings for thermal-barrier applications

Graded alternate layers of Al2O3 and 8% Y2O3-ZrO2 and their admixtures were plasma sprayed onto bond-coated mild steel. They were evaluated for thermal-shock resistance, thermal-barrier characteristics, hot corrosion resistance (molten NaCl corrodant) and depth of attack, adhesion strength and the presence of phases. Although front-back temperature drops of 423-623 K were observed, some of the coatings showed good adherence even after 100 thermal shack cycles. In the sequence of the graded layers, the oxide which is directly in contact with the bond coat appears to influence the properties especially in coatings of 150 and 300 mu m thickness. Molten NaCl readily attacks the films at high hot-face temperatures (1273 K for 1 h) and the adhesive strength falls significantly by 50-60%. Diffusion of alkaline elements is also found to depend on the chemical composition of the outer coating directly facing the molten corrodant. (C) 1997 Elsevier Science Limited.

Stochastic Aeroelastic Analysis of Composite Helicopter Rotor

The effect of uncertainty in composite material properties on the aeroelastic response, vibratory loads, and stability of a hingeless helicopter rotor is investigated. The uncertainty impact on rotating natural frequencies of the blade is studied with Monte Carlo simulations and first-order reliability methods. The stochastic aeroelastic analyses in hover and forward flight are carried out with Monte Carlo simulations. The flap, lag, and torsion responses show considerable scatter from their baseline values, and the uncertainty impact varies with the azimuth angle. Furthermore, the blade response shows finite probability of resonance-type conditions caused by modal frequencies approaching multiples of the rotor speed. The 4/rev vibratory forces show large deviations from their baseline values. The lag mode damping shows considerable scatter due to uncertain material properties with an almost 40% probability of instability in hover.

Covalent Grafting of Polydimethylsiloxane over Surface-Modified Alumina Nanoparticles

The synthesis of ``smart structured'' conducting polymers and the fabrication of devices using them are important areas of research. However, conducting polymeric materials that are used in devices are susceptible to degradation due to oxygen and moisture. Thus, protection of such devices to ensure long-term stability is always desirable. Polymer nanocomposites are promising materials for the encapsulation of such devices. Therefore, it is important to develop suitable polymer nanocomposites as encapsulation materials to protect such devices. This work presents a technique based on grafting between surface-decorated gamma-alumina nanoparticles and polymer to make nanocomposites that can be used for the encapsulation of devices. Alumina was functionalized with allyltrimethoxysilane and used to conjugate polymer molecules (hydride-terminated polydimethylsiloxane) through a platinum-catalyzed hydrosilylation reaction. Fourier transform infrared spectroscopy, X-ray-photoelectron spectroscopy, and Raman spectroscopy were used to characterize the surface chemistry of the nanoparticles after surface modification. The grafting density of alkene groups on the surface of the modified nanoparticles was calculated using CHN and thermogravimetric analyses. The thermal stability of the composites was also evaluated using thermogravimetric analysis. The nanoindentation technique was used to analyze the mechanical characteristics of the composites. The densities of the composites were evaluated using a density gradient column, and the morphology of the composites was evaluated by scanning electron microscopy. All of our studies reveal that the composites have good thermal stability and mechanical flexibility and, thus, can potentially be used for the encapsulation of organic photovoltaic devices.

Synthesis of nanodispersed phases during rapid solidification and crystallization of glasses in Ti75Ni25 alloys

The formation of the metallic glass and crystalline phases and related microstructures and the decomposition behavior of rapidly solidified Ti75Ni25 alloys obtained under different processing conditions have been investigated in detail. The competition between glass transition and nucleation of beta-Ti during rapid solidification leads to the possibility of synthesizing the nanocomposites of beta-Ti and glass. Additionally, it is shown that the presence of a small amount of Si also promotes simultaneous nucleation of fine Ti2Ni intermetallic compound. Thermodynamic calculation of the metastable phase diagram indicates the presence of a metastable eutectic reaction between alpha-Ti and Ti2Ni. Evidence of this reaction at lower cooling rates has been presented. On heating, the glass decomposes through this reaction. Finally, on the basis of understanding of the microstructural evolution during decomposition, a new approach has been adopted to synthesize a nanodispersed composite of alpha-Ti in the crystalline Ti2Ni matrix with a narrow size distribution by controlling the devitrification heat treatment of the metallic glass.

Local environmental structures of Mo in the superionic conducting glass (AgI)(0.6)(Ag2MoO4)(0.4) by anomalous X-ray scattering

The anomalous X-ray scattering (AXS) method using Mo K absorption edges has been employed for obtaining the local structural information of superionic conducting glass having the composition (AgI)(0.6)(Ag2MoO4)(0.4). The possible atomic arrangements in the near-neighbor region of this glass were estimated by coupling the results with the least-squares variational analysis so as to reproduce the differential intensity profile for Mo as well as the ordinary scattering profile. The coordination number of oxygen around Mo is found to be about 4 at the distance of 0.180 mn. This implies that the most probable structural entity in the glass is the MoO4 tetrahedral unit which has been proposed based on infrared spectroscopy. The value of the coordination number of I- around Ag+ is estimated as 4.4 at 0.287 nm, suggesting an arrangement similar to that of crystalline or molten AgI.

Regarding the effect of aging heat treatment on the failure behaviour of a SiC reinforced Al based metal matrix composite

In the present study, 6061 Al metallic matrix was reinforced by 12.2 wt% df SiC particulates using liquid metallurgy route. The composite material thus obtained was extruded and characterized in the as-solutionized and peak aged conditions in order to delineate the effect of aging associated precipitation of secondary phases on the tensile fracture behavior of the composite samples. The results' of microstructural characterization studies carried out using scanning electron microscope revealed the increased presence of precipitated secondary phases in the metallic matrix and a more pronounced interfacial segregation of alloying elements in case of peak aged samples when compared to the as-solutionized samples. The results of the fractographic studies conducted on the as-solutionized samples revealed that the failure was dominated by the SiC particulates cracking while for the peak aged samples the fracture surface revealed a comparatively more pronounced SiC/6061 Al debonding and reduced SiC particulates cracking. This change in the failure behavior was rationalized in terms of embrittlement of the interfacial region brought about by the aging heat treatment and is correlated, in addition, with the mechanical properties of the composite samples in as-solutionized and peak aged conditions.

Microstructure and properties of squeeze cast Cu-carbon fibre metal matrix composite

There have been reported attempts of producing Cu based MMCs employing solid phase routes. In this work, copper was reinforced with short carbon fibres by pressure infiltration (squeeze casting) of molten metal through dry-separated carbon fibres. The resulting MMC's microstructure revealed uniform distribution of fibres with minimum amount of clustering. Hardness values are considerably higher than that for the unreinforced matrix. Addition of carbon fibres has brought in strain in the crystal lattice of the matrix, resulting in higher microhardness of MMCs and improved wear resistance. Tensile strength values of MMCs at elevated temperatures are considerably higher than that of the unreinforced matrix processed under identical conditions. (C) 1999 Kluwer Academic Publishers.

Role of corundum-adapted strains of Bacillus polymyxa in the separation of hematite and alumina

Strains of Bacillus polymyxa, preadapted and grown in the presence of corundum, were found to be capable of the efficient separation of hematite from alumina. Results of rests peformed using binary hematite-corundum and ternary hematite-quartz-corundum mixtures in the presence of cells and metabolic products separated from the adapted bacterial culture indicated that more than 99% of the hematite could he efficiently separated through selective flocculation after desliming. It was found that alumina-specific bioproteins and other nonproteinaceous compounds were secreted by bacterial cells after adaptation to the mineral. The utility of this bioprocessing is demonstrated in the removal of iron from bauxite ores through selective flocculation in the presence of the adapted bacteria.

Sensing of delaminations in composite laminates using embedded magnetostrictive particle layers

This is an exploratory study to illustrate the feasibility of detecting delamination type of damage in polymeric laminates with one layer of magnetostrictive particles. One such beam encircled with excitation and sensing coils is used for this study. The change in stress gradient of the magnetostrictive layer in the vicinity of delamination shows up as a change in induced voltage in the sensing coil, and therefore provides a means to sense the presence of delamination. Recognizing the constitutive behavior of the Terfenol-D material is highly nonlinear, analytical expressions for the constitutive relations are developed by using curve fitting techniques to the experimental data. Analytical expressions that relate the applied excitation field with the stress and magnetic flux densities induced in the magnetostrictive layer are developed. Numerical methods are used to find the relative change in the induced voltage in the sensing coil due to the presence of delamination. A typical example of unidirectional laminate, with embedded delaminations, is used for the simulation purposes. This exploratory study illustrates that the open-circuit voltage induced in the sensing coil changes significantly (as large of 68 millivolts) with the occurrence of delamination. This feature can be exploited for device off-line inspection techniques and/or linking monitoring procedures for practical applications.

Sliding wear of copper against alumina

OFHC copper pins with 10 ppm oxygen were slid against alumina at a load of 50 N and sliding speeds of 0.1 ms(-1) to 4.0 ms(-1) The wear characteristics of copper were related to the strain rate response of copper under uniaxial compression between strain rates of 0.1 s(-1) and 100 s(-1) and temperatures in the range of 298 K to 673 K. It is seen that copper undergoes flow banding at strain rates of 1 s(-1) up to a temperature of 523 K, which is the major instability in the region tested. These flow bands are regions of crack nucleation. The strain rates and temperatures existing in the subsurface of copper slid against alumina are estimated and superimposed on the strain rate response map of copper. The superposition shows that the subsurface of copper slid at low velocities is likely to exhibit flow band instability induced cracking. It is suggested that this is the,reason for the observed high wear rate at low velocities. The subsurface deformation with increasing velocity becomes more homogeneous. This reduces the wear rate. At velocities >2 ms(-1) there is homogenous flow and extrusion of thin (10 mu m) bands of material out of the trailing edge. This results in the gradual increase of wear rate with increasing velocity above 2.0 ms(-1).

Structural, dielectric and optical properties of lithium borate-bismuth tungstate glass-ceramicsc

Glasses in the system (1 - x)Li2B4O7-xBi(2)WO(6) (0.1 less than or equal to x less than or equal to 0.35) were prepared by splat quenching technique. Powder X-ray diffraction (XRD) and differential thermal analysis (DTA) were employed to characterize the as-quenched glasses. High-resolution transmission electron microscopy (HR TEM) revealed the presence of fine, nearly spherical crystallites of Bi2WO6 varying from 1.5 to 20 nm in size, depending on x in the as-quenched glasses. The glasses (corresponding to x = 0.3) heat-treated at 723 K for 6 h gave rise to a clear crystalline phase of Bi2WO6 embedded in the Li2B4O7 glass matrix, as observed by X-ray studies. The dielectric constants of the as-quenched glasses as well as the glass-ceramics decreased with increase in frequency (40Hz-100 kHz) at 300 K, and the value obtained for the glass-ceramic (x = 0.2) is in agreement with the values predicted using Maxwell's model and the logarithmic mixture rule. The dielectric constants for both the as-quenched glass and the glass-ceramic increased with increase in temperature (300 - 873 K) and exhibited anomalies close to the onset of the crystallization temperature of the host glass matrix. The optical transmission properties:of these glass-ceramics were found to be compositional dependant. (C) 2000 Elsevier Science Ltd.