Influence of Topical Fluorides on corrosion behaviour of silver amalgam alloys

Potentiodynamic polarization and cyclic voltametry measurements were employed to study the effect of topical fluoride preparations on corrosion of silver amalgam alloy in synthetic saliva and aqueous fluoride solutions. Results support that small amounts of copper and indium alloyed to silver – tin amalgam enhance corrosion resistance considerably. Indiloy shifts both the open circuit potential and the anodic polarization curve to more positive potential values, this lead to a decreases in the current density. The weak activity of Indiloy as compared with Spheraloy is attributed to the fact that the amount of γ₂ phase (tin–mercury) present in the former is much lesser than that present in the latter. Findings also show that fluoride solutions (NaF, SnF and SnF + HCI) enhance the dissolution rate of the investigasted alloys and the polarization curves illustrate the appearance of an active – passive transitions. This behaviour was discussed on the basis of complex formation. Cyclic voltammograms confirm the results obtained from the potentiodynamic polarization measurements and elucidate the anodic oxidation peaks of silver oxide and copper verifying the proposed mechanism.     

Thermal expansion behaviour of sodium-beta-alumina

The thermal expansion of sodium -and '-alumina in thea andc directions has been determined by high temperature X-ray diffractometry using polycrystalline samples. The measured values show that internal stresses as a result of anisotropy in the thermal expansion of the two phases should have little adverse effect on the application of beta-alumina in electrochemical devices.     

Thermal expansion behaviour of thermoplastic composites

The thermal-expansion behaviours of a variety of thermoplastic composites based on the ICI Victrex polymers PEEK, ITA, HTA and ITX are examined. It is shown that when each of the composites is raised above the glass-transition temperature, T _g, of the polymer matrix considerable permanent distortion occurs in the small samples used, though the effect is not evident below T _g. This is attributed to relaxation of process-induced residual stresses generated in the larger plates from which the samples were prepared. A number of models are used to calculate the thermal-expansion behaviour of the fibres transverse to their long axes; the results are shown to be inconsistent, and in poor agreement with directly measured properties. Finally, the utility of the data for calculation of thermal-expansion behaviour parallel to the fibres in the composites is considered.     

Thermal behaviour of dendritic methacrylated polyesters

A dendritic polyester based on pentaerythritol and 1,2,4 – benzenetricarboxylic acid anhydride followed by modification with glycidyl methacrylate was synthesised by using a divergent approach. The dendrimer (D-2) thus obtained had approximately eight double bonds. Several blends of D-2 with varying proportions of bismethacryloxy derivative of diglycidyl ether of bisphenol-A having styrene as a reactive diluent (VE resin) were prepared. The Brookfield viscosity of VE resin increased with an increase in D-2 content in the blends. Curing behaviour of these resins in the presence of AIBN was investigated by DSC. No significant change in curing exotherm was observed by addition of D-2 to the VE resin. Isothermal curing of resin samples in the presence of AIBN was done at 60 °C for 5 h and then at 150 °C for 90 mins. A significant improvement in char yield at 600 °C in nitrogen atmosphere was observed in samples containing D-2. Mechanical properties of glass fiber reinforced laminates with 31 ± 3% of resin content (w/w) were also determined. Smoke density of laminates increased with decrease in D-2 content in the resin, whereas only a marginal effect on limiting oxygen index was observed. Electronic Publication     

Thermal behaviour of polymeric Langmuir-Blodgett multilayers

Langmuir-Blodgett multilayers of hydrocarbon and fluorocarbon polymers with hydrophilic spacer, lipid-polyelectrolyte complexes and mesogenic polymers have been prepared. The thermal behaviour of the multilayers was studied by small angle X-ray scattering, IR and UV—visible spectroscopy. Good thermal stabilities were found for the various classes of polymers. In addition, for both complexed multilayers and mesogenic polymer films, reorientation processes were observed.     

Thermal behaviour of bioactive alkaline-earth silicophosphate glasses

The thermal behaviour (sinterability and first crystallization) of a series of alkaline-earth silicophosphate glasses has been studied by differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The samples were prepared from a base bioactive glass of the system CaO–P₂O₅–SiO₂–CaF₂, by (a) slightly changing the F/O ratio; (b) replacing part of the CaO by SrO or MgO; and (c) increasing the total alkaline earth concentration by MgO additions. The results show that the addition of MgO is the most effective way of improving sinterability. In these samples, a decrease of the glass transition temperature, together with an increase in the temperature of the first crystallization, is observed. The difference between both temperatures is proposed to be an adequate indicator of the sinterability. The initial stages of the first crystallization (which produces an oxo-fluorapatite), and its composition dependence, are discussed in terms of the results of sinterability, and the classical theory of nucleation.     

Thermal behaviour of high performance conducting composites

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Thermal stability of silver thin films on zirconia substrates

The thermal stability of silver thin films between 100 nm and 820 nm thick deposited onto single crystal yttria-stabilised zirconia (YSZ) substrates by evaporation was investigated by annealing the films between 250 °C and 550 °C for different durations. Films approximately 100 nm thick were thermally unstable at temperatures as low as 250 °C. A dewetting process occurred in which grain boundaries ruptured, to uncover the substrate and reduce the overall energy of the system, by a combination of grain boundary grooving at the outer surface and void growth at the Ag-YSZ interface. The surface self diffusion coefficient of Ag was determined from the kinetics of the process to be 2.6 ± 0.3 × 10^− 5 cm²s^− 1 at 500 °C. The resulting silver morphology ranged from ‘self-organised’ interconnected silver network structures to completely isolated silver islands. A structure predominance map of the rearrangement process is presented.     

Thermal behaviour kinetic study of dihydroglyoxime and dichloroglyoxime

Glyoxime derivatives have increasingly gained attention because of their potential for generation of energetic materials. In this paper, the thermal behaviours of dihydroglyoxime (DHG) and dichloroglyoxime (DCG) in a nitrogen atmosphere were studied under non-isothermal conditions by thermogravimetry (TG) and differential scanning calorimetry (DSC) techniques. The results showed that the decomposition of DCG occurs in two continuous steps, in the 160-230 °C temperature range. Each thermal degradation stage for DCG results an exothermic peak in the DSC curve. On the other hand, applying TG-DSC techniques indicates that DHG sublimates about 178 °C before it decomposes. Also, the influence of the different heating rates such as 5, 10, 15 and 20 °C min^− 1 on the thermal behaviour of both compounds was investigated. The results of DSC experiments indicate that the peak temperature of DCG decomposition and sublimation temperature for DHG were increased, as the heating rate was increased. The kinetic parameters such as activation energy and pre-exponential factor for each compound were found by means of the Kissinger method and were verified by the Ozawa-Flynn-Wall method. Based on the values of activation energy obtained by the Kissinger method, the first stage of DCG decomposition has an activation energy about 100 kJ mol^− 1 and the sublimation process for DHG has an activation energy about 259 kJ mol^− 1. Finally, the thermodynamic parameters (ΔG^#, ΔH^# and ΔS^#) for decomposition of DCG and sublimation of DAG were determined.     

Thermal expansion behaviour of nanocrystalline titanium powder compacts

This investigation is an attempt to understand the expansion behaviour of compacts of nanocrystalline titanium powder during thermal treatments. Entrapped gases cause excessive expansion in the green compacts. The dilatometric study reveals that there is no significant difference in the expansion coefficient of sintered nanocrystalline titanium samples, compared to their micron-size counterpart.     

Thermal expansion behaviour of unidirectional carbon-fibre-reinforced copper-matrix composites

Continuous carbon-fibre-reinforced copper-matrix composites prepared by diffusion bonding technology have been used for investigation of the coefficient of thermal expansion. For reasons of economy and ease of availability, continuous Torayca T300 fibres have been used for sample preparation. They were coated continuously with copper (galvanically and then chemically) and unidirectional composites were prepared by diffusion bonding in vacuum at 873 K for 30 min. The linear coefficient of thermal expansion (CTE) of samples with different volume fractions of carbon fibres was measured in directions parallel and perpendicular to the fibre direction. The samples were heat-treated for one temperature cycle in the range 293–573 K or cycled three times in a temperature range from 253 to 573 K. Measured CTE values are compared with those predicted by the well-known Schapery model and the model derived by Kural and Min. Better agreement was achieved with the predictions of the longitudinal CTE of the composite. Prediction of the transverse CTE was more difficult because of a lack of knowledge of the transverse CTE of carbon fibres. Models including the transverse CTE of carbon fibres (Kural–Min) gave better results for prediction of the transverse CTE of the unidirectional composite.     

Thermal expansion behaviour of hydrostatically extruded linear polyethylene

The thermal expansion behaviour of a series of oriented linear polyethylenes has been measured over the temperature range –180 to + 70° C. With increasing deformation ratio the axial thermal expansion coefficient approaches that of the crystal unit cell, and the transverse expansion coefficient approaches that of the average in the crystal a- and b-axis directions. The implications of these results are discussed in terms of present understanding of the structure of these materials.     

Thermal behaviour of hydroxides, hydroxysalts and hydrotalcites

Mass spectrometric analysis of gases evolved during thermal decomposition of divalent metal hydroxides, hydroxysalts and hydrotalcites show that all these compounds undergo dehydration in the temperature range 30 <T < 220°C followed by decomposition at temperatures above 250°C. The latter step involves simultaneous deanation and dehydroxylation of the layers. Our observations conclusively prove that alternative mechanisms which envisage CO₂ evolution due to deanation at lower temperatures proposed by Kanezaki to be wrong.     

Thermal lensing in silver gallium selenide parametric oscillator crystals

We performed an experimental investigation of thermal lensing in silver gallium selenide (AgGaSe(2)) optical parametric oscillator crystals pumped by a 2-μm laser at ambient temperature. We determined an empirical expression for the effective thermal focusing power in terms of the pump power, beam diameter, crystal length, and absorption coefficient. This relation may be used to estimate average power limitations in designing AgGaSe(2) optical parametric oscillators. We also demonstrated an 18% slope efficiency from a 2-μm pumped AgGaSe(2) optical parametric oscillator operated at 77 K, at which temperature thermal lensing is substantially reduced because of an increase in the thermal conductivity and a decrease in the thermal index gradient dn/dT. Cryogenic cooling may provide an additional option for scaling up the average power capability of a 2-μm pumped AgGaSe(2) optical parametric oscillator.     

Thermal precipitation of silver nanoparticles and thermoluminescence in tellurite glasses

Silver metal and/or oxide precipitation of nanoparticles in thermally treated Ag-doped tellurite glasses was studied by optical absorption (OA) and transmission electron microscopy (TEM). The Lorentzian adjusted silver nanoparticles plasma resonance OA band was compared to the Drude model approach. The silver nanoparticles size distribution on the surface rather than in the bulk was determined by TEM. A model for the metallic silver precipitation is proposed. The characterization of the formation of silver nanoparticles was carried out with differential thermal analysis (DTA) to determine the glass transition temperature (Tg) and of crystallization (Tc). Previously γ-irradiated samples exhibited thermoluminescence (TL) peaks and the defect centers TeOHC, NBOHC and TeEC were identified by electron paramagnetic resonance (EPR), but no Ag⁰ signal was detected. The silver nanoparticles are known to introduce desired third-order optical nonlinearities in the composites, at wavelengths close to the characteristic surface-plasmon resonance of the metal precipitates. An increase of the glass density and refractive index with increasing AgNO₃ content was observed.