Effects on the Thermal Properties and Bioactivity of Substitution of CaO by M2O3 (M=La, Y, Al) in Wollastonite Glass

The effects on the thermal properties and bioactivity of the substitution of CaO by La₂O₃, Y₂O₃ and Al₂O₃ in a glass of composition CaO·SiO₂ were studied and compared. The trivalent metal oxides were all effective in raising the glass transformation and softening temperatures when they replaced CaO in the glass of composition CaO·SiO₂. The experimental results suggest that Al₂O₃ plays the role of a glass-former, while La₂O₃ and Y₂O₃ behave as glass-modifiers. The tendency to devitrify appears to be the lower, the farther the glass composition is from those of the crystalline phases, owing to the need for diffusion over longer distances, the greater the composition difference. The substitution with the trivalent metal oxides is detrimental to the bioactivity, which is preserved only in the event of very small degrees of substitution. The most negative role appears to be played by Al₂O₃. This revised version was published online in July 2006 with corrections to the Cover Date.     

Utility of kinetic analysis in the determination of reaction mechanism

Thermal analysis has a long and prominent role in the characterization of materials, including polymeric materials. Kinetic studies in one form or another have often been employed in an attempt to assess stability, predict lifetime, establish degradation pathway, or project suitable processing conditions. The results of such studies have often formed the basis for the proposal of the ‘mechanism’ of reaction. This despite the fact that the reaction being observed is often unknown or is not a single process but rather several parallel or consecutive events. This latter is particularly true for ‘variable temperature kinetics’. The utility/value of such exercises is marginal at best and contributes nothing to an understanding of the mechanism of any of the reactions involved.     

Investigation of surface and thermogravimetric characteristics of carbon-coated iron nanopowder

Demand for high-density press and sinter components is increasing day by day. Of the different ways to improve the sinter density, the addition of nanopowder to the conventional micrometer-sized metal powder is an effective solution. The present investigation is aimed at studying the surface chemistry of iron nanopowder coated with graphitic carbon, which is intended to be mixed with the conventional iron powder. For this purpose, iron nanopowder in the size range of 30 nm to submicron (less than 1 micron) was investigated using thermogravimetry at different temperatures: 400°C, 600°C, 800°C, 1000°C, and 1350°C. The X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and high-resolution scanning electron microscopy (HR-SEM) were used for characterizing the powder as well as samples sintered at different temperatures. The presence of iron, oxygen, carbon, chromium, and zinc were observed on the surface of the nanopowder. Iron was present in oxide state, although a small metallic iron peak at 707 eV was also observed in the XPS spectra obtained from the surface indicating the oxide scale to be maximum of about 5 nm in thickness. For the sample treated at 600°C, presence of manganese was observed on the surface. Thermogravimetry results showed a two-step mass loss with a total mass loss of 4 wt.% when heated to 1350°C where the first step corresponds to the surface oxide reduction.     

Dynamic thermal analysis of solid-state reactions

There are many reactions of interest in which one or more of the reactants belong to some solid phases. Modern thermoanalytical instruments can conveniently provide reaction kinetic data of high precision and accuracy, from which the underlying activation energyE may be derived in principle. Unfortunately, no best method yet exists for the derivation when the data have been collected with a programmed linear increase in sample temperature, unlike the case of isothermal measurements, which however suffer from experimental limitations [1]. Here we propose a method for extractingE from non-isothermal data, that promises general validity.     

Non-isothermal kinetics in solids

The integral methods, which are obtained from the various approximations for the temperature integral, have been extensively used in the non-isothermal kinetic analysis. In order to obtain the precision of the integral methods for the determination of the activation energy, several authors have calculated the relative errors of the activation energy obtained from the integral methods. However, in their calculations, the temperature integral at the starting temperature was neglected. In this work, we have performed a systematic analysis of the precision of the activation energy calculated by the integral methods without doing any simplifications. The results have shown that the relative error involved in the activation energy determined from the integral methods depends on two dimensionless quantities: the normalized temperature θ=T/T ₀, and the dimensionless activation energy x ₀=E/RT ₀ (where E is the activation energy, T is the temperature, T ₀ is the starting temperature, R is the gas constant).     

Model-fitting and model-free analysis of thermal decomposition of palladium acetylacetonate [Pd(acac)2]

The non-isothermal decomposition process of the powder sample of palladium acetylacetonate [Pd(acac)₂] was investigated by thermogravimetric (TG) and the X-ray diffraction (XRD) techniques. Model-free isoconversional method of Tang, applied to the investigated decomposition process, yield practically constant apparent activation energy in the range of 0.05≤α≤0.95. It was established, that the Coats-Redfern (CR) method gives several statistically equivalent reaction models, but only for the phase-boundary reaction models (R2 and R3), the calculated value of the apparent activation energy (E) is nearest to the values of E obtained by the Tang’s and Kissinger’s methods. The apparent activation energy value obtained by the IKP method (132.4 kJ mol⁻¹) displays a good agreement with the value of E obtained using the model-free analysis (130.3 kJ mol⁻¹). The artificial isokinetic relationship (aIKR) was used for the numerical reconstruction of the experimental integral model function, g(α). It was established that the numerically reconstructed experimental function follows R3 reaction model in the range of α, taken from model-free analysis. Generally, decomposition process of Pd(acac)₂ starts with initial nucleation which was characterized by rapid onset of an acceleratory reaction without presence of induction period.     

Dynamic and controlled rate thermal analysis of hydrozincite and smithsonite

The understanding of the thermal stability of zinc carbonates and the relative stability of hydrous carbonates including hydrozincite and hydromagnesite is extremely important to the sequestration process for the removal of atmospheric CO₂. The hydration-carbonation or hydration-and-carbonation reaction path in the ZnO-CO₂-H₂O system at ambient temperature and atmospheric CO₂ is of environmental significance from the standpoint of carbon balance and the removal of green house gases from the atmosphere. The dynamic thermal analysis of hydrozincite shows a 22.1% mass loss at 247°C. The controlled rate thermal analysis (CRTA) pattern of hydrozincite shows dehydration at 38°C, some dehydroxylation at 170°C and dehydroxylation and decarbonation in a long isothermal step at 190°C. The CRTA pattern of smithsonite shows a long isothermal decomposition with loss of CO₂ at 226°C. CRTA technology offers better resolution and a more detailed interpretation of the decomposition processes of zinc carbonate minerals via approaching equilibrium conditions of decomposition through the elimination of the slow transfer of heat to the sample as a controlling parameter on the process of decomposition. The CRTA technology offers a mechanism for the study of the thermal decomposition and relative stability of minerals such as hydrozincite and smithsonite.     

Effect of surfactant agent upon the structure of montmorillonite

The modification of sodium montmorillonite (MMT) through the incorporation of amphiphilic octadecylammonium cations in various concentrations (10–200% CEC) into the clay’s interlayer spaces has been studied. High resolution thermogravimetric analysis shows that the thermal decomposition of modified montmorillonite occurs in three steps. The first step of mass loss is related to dehydration of adsorbed water and water hydrating metal cations such as Na⁺. The second step of mass loss is attributed to the decomposition of surfactant. The third step is due to the loss of OH units during the dehydroxylation of the montmorillonite. The conformation of the surfactant cations in the confined space of the silicate galleries is investigated by X-ray diffraction analysis. These analyses are very important for any attempt to incorporate the organomodified MMT particles into different media for various applications such as polymer nanocomposite preparation.     

Thermoanalytical and microscopical investigation of the microstructure of emulsions containing polymeric emulsifier

Polymeric emulsifiers provide exceptional stability to oil-in-water, water-in-oil or multiple emulsions by their steric stabilization. Pemulens as polymeric emulsifiers are able to stabilize o/w type emulsions because their short lipophilic part integrates into the oil droplets while their long hydrophilic part forms a micro gel around the droplet. In our present study the microstructure and integration of the polymeric emulsifier at the water-oil interface was investigated with thermogravimetric and microscopical methods. It was established that depending on the amount of both of the polymeric emulsifier and added coemulsifier the microstructure of the system changes.     

Deduction of channel-length distribution from isothermal thermogravimetry

A possible deduction is proposed of channel length distribution in one-dimensional porous materials from the kinetic data obtained in isothermal thermogravimetry (TG). The method utilizes the absorption/desorption of small molecules into one-dimensional nano-channel. In the surface-controlled absorption/desorption, the second derivative with respect to time is directly proportional to the channel-length distribution function. Even in the diffusion-controlled case, the second derivative with respect to the square root of time gives rough information on the distribution function.