Etching and morphology of poly(vinylidene fluoride)

The lamellar structures within spherulites of melt-crystallized poly(vinylidene fluoride) have been examined following the development of an etching technique which allows the study of representative morphologies in this polymer. The banded -spherulites, which predominate at crystallization temperatures below 165°C, are found to be made up of densely packed lamellae with an intrinsically planar habit, whilst the -spherulites which develop preferentially at higher temperatures, have a curious architecture in which the lamellae adopt a highly curved scroll-like morphology. These observations are discussed in terms of existing models for spherulite banding and non-planar lamellar habits.     

Fracture of Ni-Fe base metallic glasses

The fracture toughnesses of specimens of three transition metal base metallic glasses, Ni₄₈Fe₂₉P₁₄B₆Al₃, Ni₃₉Fe₃₈P₁₄B₆Al₃ and Ni₄₉Fe₂₉P₁₄B₆Si₂ are reported. Each alloy was tested in a characteristic thickness, i.e., 25m (Ni₄₈), 43m (Ni₃₉) and 72 m (Ni₄₉) andK _C values of 120, 62 and 30 kg mm^–3/2, respectively, were observed. It is suggested that this variation is associated primarily with a transition from plane strain (K _IC 30 kg mm^–3/2) toward plane stress conditions as sample thickness is decreased. The fatigue crack propagation rate in the Ni₃₉ alloy is also reported;da/dn (mm/cycle) 2×10^–8 K ^2.25, whereK has units of kg mm^–3/2. When the respective data are plotted in terms of (K/E), whereE is Young's modulus, the crack growth behaviour for the Ni-Fe glasses approximates that for crystalline ferrous alloys. A classical chevron pattern, macroscopically at 90° to the tensile axis, is observed when amorphous metallic alloy strips fracture under plane strain conditions. On a finer scale, the chevrons exhibit a sawtooth structure, and the sawtooth surfaces show a fine scale, equi-axed vein pattern. This indicates that local failure occurs by shear rupture.     

Grain refinement of a TiAl alloy by heat treatment through near gamma transformation

Casting of TiAl alloys is receiving more and more research because of its relatively low cost. One problem with this technology is that the coarse microstructure developed during solidification is harmful to material properties. Thus it is essential to seek an approach to refining the cast microstructure and this approach may also be applicable to cast components. In this study, a novel heat treatment route is proposed to obtain a fine fully lamellar structure from a cast TiAl alloy with a grain size of 1000 m. This route consists of three steps, namely pretreatment to have a feathery structure, annealing in the + region to have a fine near gamma structure and solution treatment to develop a fully lamellar structure with a grain size of 30 m.     

Non fermi liquid ground states in strongly correlated f-electron materials

Experimental efforts to characterize and develop an understanding of non Fermi liquid (NFL) behavior at low temperature in f-electron materials are reviewed for three f-electron systems: M_1–xU_xPd₃ (M = Sc, Y), U_1–xTh_xPd₂Al₃, and UCu_5–xPd_x. The emerging systematics of NFL behavior in f-electron systems, based on the present sample of nearly ten f-electron systems, is updated. Many of the f-electron systems exhibit the following temperature dependences of the electrical resistivity p, specific heat C, and magnetic susceptibility for T T₀, where T_o is a characteristic temperature: P(T) 1 –aT/T ₀, where a < 0 or > 0, C(T)/T (-1/T _o) In (T/bT ₀), and (T) 1 –c(T/T_o)^1/2. In several of the f-electron systems, the characteristic temperature T_o can be identified with the Kondo temperature T_k.     

Vortex imaging in superconducting films by scanning hall probe microscopy

We have used a low noise Scanning Hall Probe Microscope (SHPM) to study vortex structures in superconducting films. The microscope has high magnetic field (2.9×10^–8T/Hz at 77K) and spatial resolution, 0.85m. Magnetic field profiles of single vortices in High T_c YBa₂Cu₃O_7– thin films have been successfully measured and the microscopic penetration depth of the superconductor has been extracted as a function of temperature. Flux penetration into the superconductor has been imaged in real time (8s/frame).     

Some observations on the deformation characteristics of bulk polycrystalline zirconium hydrides

The compressive deformation of polycrystalline -zirconium hydride has been investigated within the temperature range 22 to 400° C and over the composition range ZrH_1.71 to ZrH_1.92. The observed deformation modes included slip, creation of new lamellae of different orientations to those of the main transformation lamellae, and movement of lamellar boundaries. The results are considered in terms of the contribution of these three processes to the overall deformation behaviour.     

Lamellae deformation and structural evolution in an Al-33%Cu eutectic alloy during equal-channel angular pressing

Equal channel angular pressing was successfully applied on a lamellae eutectic alloy of Al-33%Cu at 400°C, up to an equivalent strain of 8. A homogeneous fine equiaxed duplex microstructure with an average grain size of 1.1 ± 0.3 m was obtained. Deformation accommodation is realized in various form of periodical shear banding: periodical bending, periodical shear banding, shear switching, and periodical shear cutting in the eutectic. The shear essence of the strain mode involved in ECAP determines the very different behavior, from that in drawing, of lamellae structural evolution.     

Synthesis and characterization of new precursors to nearly stoichiometric SiC ceramics: Part II A homopolymer route

An original route to SiC ceramics possessing low free carbon(3.8 at%) and oxygen (1 at%) contents is reported. It consists of the polycondensation of 2,4-dichloro-2,4-disilapentane, prepared by Grignard reaction of dichloromethane on methyldichlorosilane. The undesired remaining chlorine was eliminated via a subsequent reduction using lithium aluminum hydride. The synthesis of the ceramic precursor was achieved by thermolysis under argon at atmospheric pressure, with the evolution of hydrogen and methane. The pyrolysis of the precursor provided SiC ceramics in an excellent yield (79%).     

Absorption Spectrum of ZnO Precipitates in ZnSe

The absorption spectrum of ZnO precipitates in ZnSe saturated with oxygen is studied in the spectral range 500–2000 cm^–1 and is shown to correlate with the transmission spectrum of single-crystal ZnO. Saturation of ZnSe with oxygen in the course of growth leads to ZnO precipitation during cooling. The precipitates give rise to narrow absorption bands in the range 5.8–7.1 m, which correlate with the components of the multiphonon absorption spectrum of ZnO, formed by combinations of LO and TO phonons. The 2LO mode defines the long-wavelength transmission edge of ZnO. In addition, the spectrum shows strong absorptions due to the LO + TO(10.2 m) and 2TO (11.3 and 12.7 m) modes. The possible role of SeO₂ is discussed. It is suggested that ZnO with a high carrier concentration may act as a gray filter in the spectral range 2–9 m.     

Equal-channel angular pressing of an Al-6061 metal matrix composite

An Al-6061 metal matrix composite, reinforced with 10 vol % Al₂O₃ particulates, was subjected to equal-channel angular (ECA) pressing at room temperature to a total strain of 5. It is shown that the intense plastic straining introduced by ECA pressing reduces the grain size from 35 m to 1 m and this leads to an increase in the microhardness measured at room temperature. Inspection revealed some limit cracking of the larger Al₂O₃ particulates as a consequence of the ECA pressing. Tensile testing after ECA pressing gave a maximum ductility of 235% at a temperature of 853 K when testing at strain rates from 10^–4 to 10^–3 s^–1. It is suggested that high strain rate superplasticity is not achieved in this material after ECA pressing due to the presence of relatively large Al₂O₃ particulates.     

Deformation mechanisms during uniaxial drawing of melt-crystallized ultra-high molecular weight polyethylene

The crystal deformation mechanisms during solid-state uniaxial drawing of melt-crystallized ultra-high molecular weight polyethylene (UHMW-PE) film have been studied as a function of draw ratio. At higher draw ratios (3) the fine slip processes during uniaxial drawing of melt-crystallized UHMW-PE result in a single-erystal-like (1 0 0) [0 0 1] texture, whereas the normals to the lamellae are inclined by more than 45° with respect to the applied force. It is postulated that in melt-crystallized UHMW-PE the coarse slip process is predominantly restricted due to the fold plane restraints, preventing lamellae from breaking up and rotating with their normals towards the draw direction. The inclination of lamellar normals with respect to the draw direction prohibits further drawing because shear stresses act perpendicular instead of parallel to the lamellar normals.     

Deposition of thermal barrier coatings using the solution precursor plasma spray process

The solution-precursor plasma-spray (SPPS) process is capable of producing highly durable thermal barrier coatings. In an effort to improve the understanding of the deposition mechanisms in this novel process, a series of specific experiments, where the substrate is held stationary and the plasma torch is programmed to scan a single pass across the substrate, were conducted and the resulting deposits were carefully characterized. In addition to the deposition mechanisms identified previously in the stationary torch experiments, the deposition mechanisms of two other types of deposits, thin film and fine spherical particles, were identified in this study. The melting of inflight formed 7YSZ particles and their rapid solidification to form ultra-fine splats on the substrate was found to be the dominant deposition mechanism. The characterization of actual SPPS coatings confirmed that the various coating-deposition mechanisms identified in the model experiments occur in concert during the actual coating process. Adherent deposits (ultra-fine splats, deposits from gel-like precursor and film formed via chemical vapor deposition), unmelted particles (spherical particles, deposits from non-decomposed precursor) and porosity were estimated to constitute 65, 19 and 16 vol%, of the coating, respectively.     

Effect of alloying elements and dendrite arm spacing on the microstructure and hardness of an Al-Si-Cu-Mg-Fe-Mn (380) aluminium die-casting alloy

The microstructure of aluminium alloys based on 380 die-casting alloy was studied in detail, as a function of the alloying elements iron, magnesium, copper and manganese, and the solidification rate. Three methods of solidification were employed to simulate cooling rates obtained from investment, permanent, and die-casting processes, corresponding to 0.4, 12 and 260 °C s^–1, respectively, with emphasis on the highest cooling rate. Hardness measurements were carried out on samples obtained from the latter, in the as-cast and T5 tempered conditions (4 h at 25, 155, 180, 200 and 220 °C). The results have been discussed and the correlation between the hardness and microstructure as a function of alloying elements is presented. The effect of solution heat treatment on the variations in the microstructure and hardness has also been discussed.     

On crystal alignment in polycrystalline YBCO materials based on TEM observation

Previously, we have observed that parallel lattice fringes show up regularly in MTG samples, indicative of good alignment of unit cells in strips of size 100 nm × a few times 100 nm. We have prepared films ( 80 nm) for TEM observation using YBCO material fabricated by the conventional solid-state reaction method in order to study the crystal texture in a length scale from a few 100 nm down to 3 Å. Parallel lattice fringes are observed to stretch across a cross-section 0.3×0.5 m². Cross lattice fringes intersecting at 90° and 45° are observed as rare events. This result suggests that the degree of alignment in the crystal texture is higher than expected inside a grain of size a few m.     

In situ X-ray Diffraction Studies of a Multilayered Membrane Fluid under Confinement and Shear

The structure of a fluid membrane system composed of surfactant-co-surfactant-oil-water mixtures has been investigated under confinement and shear conditions. Small angle x-ray scattering (SAXS) was employed with a second generation x-ray surface forces apparatus (XSFA-II) to study the time evolution of the orientational structure of the lamellar fluid under oscillatory shear. In a regime of relatively big confinement gap (800 m) and small shear amplitude (40 m), direct evidence of an orientational phase separation behavior, where a surface boundary layer adopts different orientation and separates from the bulk region, was observed for the first time. Under continuous shearing, the surface boundary layer grows in thickness and aligns towards a shear-favored (low friction) state while the bulk orientation remains unchanged. To further investigate the effects of surface confinement, we spatially mapped, in 1 m sections, the orientation structure of the lamellar fluid sample confined between two glass surfaces using a micro-focused x-ray beam produced by a linear Bragg–Fresnel lens at the Advanced Photon Source. The data confirmed the expected trend that the smectic domains align progressively better with respect to the surface as they approach the surface.     

CO/pH2: A Molecular Thermometer

We utilize reversible temperature dependent changes in the IR absorption spectrum of CO molecules isolated in solid parahydrogen (pH₂) to probe bulk temperature changes during rapid vapor deposition. The intensity of a well resolved feature near 2135 cm^–1 increases monotonically with temperature over the 2 to 5 K range. The thermally populated initial state of this transition lies 12 K above the CO/pH₂ ground state. During the deposition of 100 ppm CO/pH₂ samples, we detect temperature gradients 10 K/cm in 0.1 cm-thick samples subjected to heat loads 10 mW/cm². The resulting estimated thermal conductivity (TC) is 3(±2) mW/cm-K, averaged over the 2 to 5 K region. This value is 1000 times lower than the TC of single crystal solid pH₂, and 10 times lower than previously measured for pH₂ solids doped with 100 ppm concentrations of heavy impurities [Manzhelii, Gorodilov, and Krivchikov, Low Temp. Phys. 22, 131 (1996)]. We attribute this abnormally low TC to the known mixed fcc/hcp structure of the rapid vapor deposited solids.     

Characterization of the spinel phase in a diphasic mullite gel using dynamic X-ray diffraction

Dynamic X-ray diffraction (DXRD) has been used in an effort to identify the specific phase changes which are responsible for observed thermal events at 980 °C in mullite gel precursors. Specifically, changes in the evolution of the common and strongest diffraction peak (d = 0.139 nm) corresponding to both transient alumina phases and the Al-Si spinel were followed in order to descriminate between these two phases. Results which compare the DXRD results for a diphasic mullite gel and a boehmite gel are presented and suggest that the Al-Si spinel phase forms at 980 °C in diphasic gels along with - and/or -Al₂O₃. These results are corroborated by separate TEM measurements which indicate the presence of both phases in samples quenched from 1000 °C.     

Low-temperature synthesis,pyrolysis and crystallization of tantalum oxide gels

Tantalum oxide gels in the form of transparent monoliths and powders have been prepared from hydrolysis of tantalum pentaethoxide under controlled conditions using different mole ratios of Ta(OC₂H₅)₅C₂H₅OHH₂OHCl. Alcohol acts as the mutual solvent and HCl as the deflocculating agent. For a fixed alkoxide water HCl ratio, the time of gel formation increased with the alcohol to alkoxide molar ratio. Thermal evolution of the physical and structural changes in the gel has been monitored by differential thermal analysis, thermogravimetric analysis, X-ray diffraction, and infrared spectroscopy. On heating to 400 °C, the amorphous gel crystallized into the low-temperature orthorhombic phase -Ta₂O₅, which transformed into the high-temperature tetragonal phase -Ta₂O₅ when further heated to 1450 °C. The volume fraction of the crystalline phase increased with the firing temperature. The -Ta₂O₅ converted back into the low-temperature phase, -Ta₂O₅, on slow cooling through the transformation temperature of 1360 °C, indicating a slow but reversible transformation.     

High resolution TEM study of ceramic BaO·Pr2O3·4TiO2

Ceramic specimens of BaO·Pr₂O₃·4TiO₂ (Ba_4.5Pr₉Ti₁₈O₅₄) were prepared by the mixed oxide-route. The single phase products were examined by High Resolution Transmission Electron Microscopy (HRTEM). Orthorhombic symmetry, with cell parameters a22.2 Å, b12.2 Å and c7.6 Å, was confirmed. On the basis of space group pnam (No. 62) and the crystal structural data of Rawn et al., good agreement was obtained between the experimental HRTEM images and images simulated by the multislice method.     

Electromagnetic radiation emitted during friction process

A series of studies of electromagnetic radiation (EMR) emitted during fracture of materials, enabled us to obtain a relationship between the width of a fracture and the wavelength of the emitted EMR. Applying this relation to friction we could check one of its suggested mechanisms, namely the Bowden-Tabor model, which states that during a friction process, asperities on the two contacting surfaces are welded together (at a microscopic level) and fractured. A uniaxial tension machine was used, whereby two half cylinders of chalk (CaCO3) bound together, were moved one against the other, generating friction. Calculations based on EMR observations showed that the average width of the fractured asperities was 26.3 m, while mechanical profilograph measurements of the average width of the total number of long asperilies before and after the experiment, yielded values of 15.6 and 18.4 m, respectively, implying that 25% of long asperities were fractured during a single friction process.