Thermal behaviour of polystyrene/silica composites

The particle-size distribution in silica powder prepared by the sol–gel method has been determined by dynamic light scattering analysis. The average diameter of the particles was found to be 250?nm. Using a low-temperature nitrogen adsorption–desorption technique, it was found that the synthesised powder may be referred to as mesoporous materials. Polystyrene/silica composite films were fabricated by casting from o-xylene solutions. It was found, using thermogravimetry, that incorporation of silica leads to an increase in both the onset temperature of polymer degradation and in the temperature at which the maximum rate of weight loss occurs. Using differential scanning calorimetry, the phase transitions from the glassy state to the elastic one were studied for the polymeric materials. New data relating to the effect of silica on the glass-transition temperature, T_g, of composites with a low weight fraction of SiO₂ were found. Specifically, we found a non-monotonic concentration dependence of the value of T_g. The present results advocate for employing silica as an effective filler for producing polymer composites with enhanced thermal properties.     

Effect of network connectivity on thermal properties of As30Te70– x Tl x glasses

Alternating differential scanning calorimetry (ADSC) studies were undertaken to investigate the effect of Tl addition on the thermal properties of As₃₀Te_{70– x} Tl _x (6?≤?x?≤?22 at%) glasses. These include parameters such as glass-transition temperature (T _g), changes in specific heat capacity (ΔC _p) and relaxation enthalpy (ΔH ^NR) at the glass transition. It was found that T _g of the glasses decreased with the addition of Tl, which is in contrast to the dependence of T _g in As–Te glasses on the addition of Al and In. The change in heat capacity ΔC _p through the glass transition was also found to decrease with increasing Tl content. The addition of Tl to the As–Te matrix may lead to a breaking of As–Te chains and the formation of Tl⁺Te^?AsTe_2/2 dipoles. There was no significant dependence of the change of relaxation enthalpy, through the glass transition, with composition.     

On the evaporation kinetics of C60/toluene solutions

The increasing interest in fullerene nanoparticles makes the question regarding their effect on liquid evaporation behavior unavoidable. In this study, we used a gravimetric technique to investigate the effect of C₆₀ fullerene nanospheres on the evaporation kinetics of toluene over their entire range of solubility. Saturated solutions showed the highest evaporation rate. The increase in the evaporation rate as a function of concentration is not monotonic, but rather exhibits a maximum and a minimum. The results strongly support the notion of molecular structuring within the liquid toluene depending on fullerene mole fractions.     

Formation of icosahedral quasicrystals in an annealed Zr65Al7.5Ni10Cu12.5Ag5 metallic glass

The formation and thermal stability of an icosahedral quasicrystalline phase in an annealed Zr₆₅Al_7.5Ni₁₀Cu_12.5Ag₅ metallic glass have been investigated by X-ray diffraction and transmission electron microscopy analyses. It was found that the quasicrystalline phase can precipitate from the glassy state and the supercooled liquid of the alloy over a wide range of annealing temperatures. After optimizing the heat-treatment conditions, the volume fraction of the quasicrystalline phase in the alloy can reach as high as about 80%. Investigation of the thermal stability of the quasicrystalline phase demonstrates that it is very stable when the annealing temperature is below the glass transformation temperature T _g of the alloy.     

Synthesis of a La-based bulk metallic glass matrix composite

A bulk metallic glass matrix composite, that is a crystalline phase in an amorphous matrix, has been successfully synthesized by chill casting a La-based quaternary La₆₆Al₁₄Cu₁₀Ni₁₀ alloy. A composite rod as large as 12?mm in diameter was obtained. The reinforcing crystalline phase was identified as α-La, which was uniformly distributed and well developed throughout the sample. The critical cooling rate for the formation of the composite was determined by the Bridgman solidification process to be below 15?K?s^?1. Increased thermal stability was observed for the composite compared with the fully glassy rods, the reason for which remains unclear.     

Heterophase polydomain structure and metal-semiconductor phase transition in vanadium dioxide thin films deposited on (1010) sapphire

Vanadium dioxide (VO₂) thin films deposited on ₍1010₎ sapphire are composed of two mixed monoclinic phases, namely M1 and M2. The M1 phase is unstable because of the existence of a larger misfit strain in the ₍102₎ VO₂ film. The reduction of misfit strain in the film favours the formation of the M2 phase. The X-ray diffraction and pole figure results show that both M1 and M2 phases are well aligned with the substrate and both contain twinned structures. Therefore, the microstructure of the film can be regarded as being a transversely modulated heterophase polydomain. A higher electrical resistivity ratio of the semiconductor phase to the metallic phase (rho_s/rho_m) can be achieved only in single-phase VO₂ thin films, either the M2 or M1 phase. Phase mixing degrades the ratio of rho_s/rho_m. The film with a single M2 phase exhibits a lower transition temperature of 58 C without any degradation of the rho_s/rho_m ratio.     

Nanocrystallization in a Zr 57 Ti 5 Cu 20 Al 10 Ni 8 bulk metallic glass

The first stages of thermal relaxation towards equilibrium in a Zr 57 Ti 5 Cu 20 Al 10 Ni 8 bulk metallic glass have been studied by differential scanning calorimetry, X-ray diffraction, transmission electron microscopy and high-resolution electron microscopy. These coupled experiments rule out for this glass the existence of a phase separation on the nanometric scale preceding the onset of crystallization. The first step of crystallization is of the primary type, that is it consists of the nucleation of nanocrystallites in the amorphous matrix with which they coexist in metastable equilibrium. The very high nucleation rate leads to a nanometric composite structure with a number density of about 7 2 10 24 m -3 of 3-4nm crystallites, occupying a volume fraction of about 15%. These features, as well as the crystallization kinetics observed during isochronal or isothermal heating, are discussed.     

Electron spin resonance of hydrogenated amorphous silicon-carbon alloy films prepared by magnetron sputtering

Abstract

New electron spin resonance (ESR) lines with g¹ = 2·0017 and g₂ and g₃ = 2·0006 have been found in the ESR spectra of as-deposited a-Si_1–x C_x:H films prepared by magnetron sputtering of silicon in the gas mixtures of methane and argon. Similarities between the observed spectra and those for the E′ centre in glassy SiO₂ are discussed.     

Structure of twins in GaAs nanowires grown by the vapour–liquid–solid process

Semiconducting crystalline materials that are poor conductors of heat are important as thermoelectric materials and for technological applications involving thermal management. A combination of neutron scattering, lowtemperature ultrasonic attenuation and thermal conductivity measurements are reported on single crystals of the semiconductors Sr₈Ga₁₆Ge₃₀ and Ba₈Ga₁₆Ge₃₀. Taken together, these measurements suggest specific structural features that result in a crystal with the lowest possible thermal conductivity, namely that of a glass with the same chemical composition. Weakly bound atoms that 'rattle' within oversized atomic cages in a crystal result in a low thermal conductivity, but the present data show that both 'rattling' atoms and tunnelling states are necessary to produce a true glass-like thermal conductivity.     

Phase transitions of Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass at pressures up to 27 GPa

The phase transitions of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk metallic glass have been investigated under high pressures and at room temperature. Direct resistance measurements in a diamond anvil cell provide evidence of a reversible transition between amorphous and crystalline phases; crystallization events occurred at 24 and 26.2 GPa on uploading, and crystalline-to-amorphous phase transitions were observed at 16 and 10.6 GPa on downloading. The phase transitions were confirmed by transmission electron microscopy observations.     

On defining necessity in terms of entailment

In their book Entailment, Anderson and Belnap investigate the consequences of defining Lp (it is necessary that p) in system E as (pp)p. Since not all theorems are equivalent in E, this raises the question of whether there are reasonable alternative definitions of necessity in E. In this paper, it is shown that a definition of necessity in E satisfies the conditions { _E Lpp, _EL(pq)(LpLq), _E pLp} if and only if its has the form C ₁.C₂ .... C_np, where each C _iis equivalent in E to either pp or ((pp)p)p.     

Dynamic and quasi-static mechanical properties of fibre-reinforced metallic glass at different temperatures

We have investigated the dynamic and quasi-static mechanical properties of a tungsten fibre-reinforced Zr_41.25Ti_13.75Ni₁₀Cu_12.5Be_22.5 bulk metallic glass composite (W/BMG) at temperatures ranging from 77 to 473?K. It was found that the yield stress and elastic modulus of the composite increase while strain-to-failure decreases with decreasing temperature. Tests under uniaxial dynamic compression showed that the W/BMG has a higher yield stress than under quasi-static compression. It was shown that yielding of the composite material is dominated by a shear banding process in the glass matrix, which is essentially a viscous flow as a consequence of local adiabatic heating. The interval between the nominal testing temperature and the temperature at which shear bands initiate is a key factor affecting the shear banding process, rendering the properties of the W/BMG very sensitive to temperature.     

Microstructure and superconductivity of bulk BPSCCO/LPMO composite

Bulk superconducting (SC) ceramics containing BPSCCO and LPMO (Lanthanum/Lead – manganite phase) have been produced. The initial components were prepared by a low-temperature Pechini method. The submicron powders in weight ratio 90/10 Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_z/La_0.6Pb_0.4MnO₃ (nominal composition) were heat treated at 840°C. The duration of the heat treatment effect (60 and 100?h) of the composite on the transformation of the microstructure was studied. The obtained composites were analyzed by scanning electron microscopy (SEM) and by the method of energy-dispersive X-ray spectroscopy (EDX). They contain several phases. It was established that the SC 2212 phase is predominate in the composite. The phase La_0.6Pb_0.4MnO₃ transforms in solid solution with preliminary composition La_0.5(Sr?+?Ca)_0.5Mn_{1? z} Cu _z O₃, which after full replacement of the La and Mn ions leads to the appearance of phases with nominal composition Sr_{1? x} Ca _x CuO _y . AC and DC magnetization measurements were used to study the SC and magnetic properties of the samples. Both samples are SC with critical temperatures 75 and 77?K, respectively. It was concluded that the SC and magnetic phases stably coexist in the composite sintered at 60?h heat treatment at 840°C.     

Crystallization behaviour of Al-based amorphous alloy and nanocomposites by rapid quenching

The microstructure and crystallization behaviour of melt-spun Al₈₈Ni₉Ce₂Fe₁ amorphous alloy and nanophase composites have been studied by means of X-ray diffraction, transmission electron microscopy and scanning and isothermal calorimetry. The diffraction patterns from Al₈₈Ni₉Ce₂Fe₁ amorphous alloys are diffuse, indicating a basically amorphous structure but contain two rings presumed to be associated with quenched-in nuclei. In the cases of Al₈₈Ni₉Ce₂Fe₁ nanophase composites, nanoscale precipitated particles are homogeneously dispersed in an amorphous matrix, and the crystallite diameter and volume fraction are sensitive to quenching conditions. During thermal crystallization, a two-step phase transformation occurs in the amorphous alloy and nanocomposites, which is characterized by a diffusion-controlled precipitation of nanoscale Al particles and the growth of a Al₃(Ni, Fe) nanophase prior to a Al₁₁Ce₃ nanophase. This study gives insight into structure-control for obtaining nanophases dispersed in an amorphous matrix by rapid quenching.     

Commensurate-incommensurate phase transition in muthmannite,AuAgTe2: first evidence of a modulated structure at low temperature

To study the temperature-dependent structural changes and to analyze the crystal chemical behavior of silver as a function of temperature, a crystal of muthmannite, AuAgTe₂, has been investigated by X-ray single-crystal diffraction methods at 300 K and 110 K. At room temperature, muthmannite was confirmed as belonging to the space group P2/m, while at low temperature (110 K) it undergoes a reversible commensurate–incommensurate phase transition with a modulation wave vector q = 0.215(1)a* + 0.379(2)c*. Muthmannite reconverts to the commensurate type upon returning to room temperature, thus indicating that the phase transition is completely reversible in character. The average structure of the low-temperature muthmannite remains monoclinic, space group P2/m, and shows only normal thermal compression over the entire temperature range investigated. Crystal-chemical characteristics are compared with published data on the other members of the system Au–Ag–Te. Speculations on the possible origin of the modulated structure at low temperature are also given.     

Density-functional and shell-model calculations of the energetics of basal-plane stacking faults in sapphire

We report the generation of fullerenes and nanotubes using an arc discharge on graphite in a high-pressure nozzle. The vapour from the arc is quenched via collisions with helium gas, forming carbon clusters within a localized highpressure region. The carbon molecules are entrained in the gas jet as it expands into the vacuum and deposited onto a silicon substrate in a low-pressure (6 mT) environment. Mass spectroscopy measurements of the plasma reveal the presence of C molecules in the expanding plume. Microstructural examination of films deposited using this method revealed clustered regions of larger fullerenes and nanotubes surrounded by an amorphous matrix. Films containing fullerenes and nanotubes were found to be significantly harder and more elastic than amorphous carbon films deposited under identical parameters but without conditions for fullerene and/or nanotube formation.     

Mechanical behaviour of an Al–matrix composite reinforced with nanocrystalline Al-coated B4C particulates

In metal–matrix composites (MMCs), interfacial bonding between the metal matrix and the ceramic reinforcement plays a crucial role in their mechanical performance. In the present study, B₄C particles were cryomilled with an Al alloy to produce a composite powder, in which the B₄C was uniformly distributed in nanocrystalline Al. The cryomilling developed a strong bond between the B₄C and the Al, allowing the nanocrystalline Al to act as a coating with a strong ceramic–metal interface. This cryomilled composite powder was then introduced, as a reinforcement, into a conventional Al alloy to strengthen the material. After consolidation, the result was a bulk Al–matrix composite reinforced with B₄C particles encapsulated in nanocrystalline Al. This composite exhibits greatly improved strength and stiffness.     

Dielectric relaxation study of mixtures of alkyl methacrylates and 1-alcohols using time-domain reflectometry

Dielectric relaxation measurements on alkyl methacrylates (methyl methacrylate, ethyl methacrylate and butyl methacrylate) with 1-alcohols (1-propanol, 1-pentanol, 1-heptanol, 1-octanol and 1-decanol) have been carried out using time-domain reflectometry (TDR) over the frequency range 10?MHz to 20?GHz at 303?K for different concentrations of alcohols. The dielectric parameters, namely the static dielectric constant (ε ₀), the dielectric constant at microwave frequencies (ε _∞) and the relaxation time (τ) were determined. The Kirkwood correlation factor, which contains information regarding solute–solvent interaction and corresponding structural information, the excess permittivity and the excess inverse relaxation time were also determined. The values of the static dielectric constant and the relaxation time increase with the percentage of alkyl methacrylates in the alcohol, whereas the static dielectric constant decreases and the relaxation time increases with an increase in the alkyl chain length of both the methacrylates and the alcohols.     

Paraconsistent algebras

The prepositional calculiC _n , 1 n introduced by N.C.A. da Costa constitute special kinds of paraconsistent logics. A question which remained open for some time concerned whether it was possible to obtain a Lindenbaum's algebra forC _n . C. Mortensen settled the problem, proving that no equivalence relation forC _n . determines a non-trivial quotient algebra.The concept of da Costa algebra, which reflects most of the logical properties ofC _n , as well as the concept of paraconsistent closure system, are introduced in this paper.We show that every da Costa algebra is isomorphic with a paraconsistent algebra of sets, and that the closure system of all filters of a da Costa algebra is paraconsistent.     

Sol-gel-hydrothermal growth of oxide thin films at low temperatures

Lead zirconate titanate (PZT) (Pb(Zr_0.52Ti_0.48)O₃) and barium titanate (BaTiO₃) thin films have been successfully prepared using a novel sol-gel- hydrothermal (SG-HT) technique at low temperatures, which involves a combination of the conventional sol-gel process and a hydrothermal method. Highly (111)-oriented PZT thin films with a single perovskite phase and polycrystalline BaTiO thin films with well developed crystallites were obtained at a processing temperature as low as 160⁰C. The microstructural characteristics demonstrate that the SG-HT-derived PZT and BaTiO₃ thin films with good crystallinity and surface morphology are converted from the amorphous phase to the desired perovskite phase on platinum-coated and bare silicon substrates at a low processing temperature of 100-200 C. These results suggest that the SGHT technique, which is of great significance because of its low processing temperature, will become a potential and promising process for fabricating PZT, BaTiO₃ and other oxide thin films.