Thermal stability of alkali metal borates and alkaline earth metal borates

The correlation between the enthalpies of formation of alkali metal and alkaline earth metal borates and the composition of the vapor in equilibrium with their melts is considered. The thermal stabilities of the studied borates have been estimated. A method is suggested for determination of the relative composition of the vapor over borate melts on the basis of their enthalpies of formation.

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Zusammenfassung Es wurde die Beziehung zwischen den Bildungsenthalpien von Alkali- (Erdalkali-)metallboraten und der Zusammensetzung der sich mit der Schmelze im Gleichgewicht befindlichen Gasphase betrachtet. Die thermische Stabilität der untersuchten Borate wurde geschätzt. Es wurde ein Verfahren entwickelt, die relative Zusammensetzung der Gasphase über Boratschmelzen auf Grund ihrer Bildungsenthalpien zu bestimmen.

     

Thermal and structural properties of sodium,potassium and carbonate doped strontium hydroxyfluorapatite

Ion-doping in hydroxyapatite bioceramics has attracted a lot of interest particularly for biomedical applications in repairing and replacing failure parts of musculoskeletal systems. Thus the multiple doping aims to mimic and resemble the chemical composition of the bone mineral component. Herein strontium hydroxyapatites bioceramics containing sodium Na⁺ and potassium K⁺ as cationic substituent and carbonate CO₃^2? and fluoride as anionic substituent were synthesized and characterized by several analysis techniques. Therefore the chemical assays indicated that obtained compounds were less stoichiometric comparably to bone tissues. The X-ray diffraction diagrams and the infrared spectra revealed that pure phases of hydroxyfluorapatite containg the cited ions were obtained. The triple insertion of sodium, potassium and carbonate into the apatite structure leaded to the B-type carbonate apatite. The FE-SEM micrographs of the powders were formed by agglomerates. Moreover, the particles' morphology strongly depends on the ions nature and amount. The D-GTA curves indicated that the heating of the powders from the room temperature to 1000 °C didn't affect the structural and thermal stability of the materials apart from a partial decomposition of the apatite inducing the formation of the β-tristrontium phosphate phase and enhancing the biomaterial character of the materials.     

Thermal reactions of solid-state amorphized borates and borate glasses

The properties and course of phase transition of amorphous borates obtained by solid-state thermal and mechanical treatment of hydrated borates and melt quenched glasses with the compositions of these borates were studied. Processes of structural relaxation and crystallization were considered.It was found that different methods of preparation of amorphous borates and the differences in their structural ordering resulting from these methods affect the rate and the mechanism of transformations occurring during their heating.Support for this work was provided by the Polish Committee of Scientific Research (KBN) — grant No. 3 P407 034 06.     

Thermal decomposition of strontium hydroxide

The thermal behaviour of strontium hydroxide octahydrate was studied by thermal analysis, an X-ray diffraction technique and optical microscopy. Up to 210° this compound transforms to the hexahydrate and then to the monohydrate. Anhydrous strontium hydroxide crystallizes in the tetragonal system and a polymorphous transformation may occur at 480°. From 530° on it decomposes in three stages. Amorphous strontium oxide is obtained at 700°.

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Zusammenfassung Das thermische Verhalten von Strontiumhydroxyd-Oktahydrat wurde röntgendiffraktometrisch, thermoanalytisch und durch optische Mikroskopie untersucht. Die Verbindung geht bis 210° in das Hex- und daraufhin ins Monohydrat über. Die Anhydroverbindung kristallisiert tetragonal. Bei 480° erfolgt eine polymorphe Umwandlung. Über 530° verläuft eine Dehydroxylation in drei Stufen. Amorphes Strontiumoxyd wurde bei 700° erhalten.

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Résumé On a étudié le comportement thermique de l'hydroxyde de strontium octohydraté par analyse thermique, diffraction de rayons X et examen microscopique. Ce composé donne l'hexahydrate jusqu'à 210° puis le monohydrate. L'hydroxyde de strontium anhydre cristallise dans le système tétragonal et présente une transformation polymorphique à 480°. La déshydroxylation commence à 530° et s'effectue en 3 étapes. L'oxyde de strontium amorphe s'obtient à 700°.

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, . 210° , . 480°. 530° . 700°.

     

Thermal stabilities of isoelectronic,isostructural nitrates,carbonates and borates

The effects of temperature changes on an isoelectronic, iso-structural group of compounds containing an iso-electronic, iso-structural complex of ions were studied. The stability of the nitrates is discussed on the basis of their temperature of fusion and solid state transitions; the carbonates from their decomposition temperatures and solid state transitions; and the borates on solid state transitions. The order of thermal stabilities is found to be ABO₃ > A′ CO₃ > A″ NO₃ .     

Thermal decomposition kinetics of strontium oxalate

The thermal decomposition behavior in air of SrC₂O₄ · 1.25H₂O was studied up to the formation of SrO using DTA-TG-DTG techniques. The decomposition proceeds through four well-defined steps. The first two steps are attributed to the dehydration of the salt, while the third and fourth ones are assigned to the decomposition of the anhydrous strontium oxalate into SrCO₃ and the decomposition of SrCO₃ to SrO, respectively. The exothermic DTA peak found at around 300°C is ascribed to the recrystallization of the anhydrous strontium oxalate. On the other hand, the endothermic DTA peak observed at 910°C can be attributed to the transition of orthorhombic-hexagonal phase of SrCO₃. The kinetics of the thermal decomposition of anhydrous strontium oxalate and strontium carbonate, which are formed as stable intermediates, have been studied using non-isothermal TG technique. Analysis of kinetic data was carried out assuming various solid-state reaction models and applying three different computational methods. The data analysis according to the composite method showed that the anhydrous oxalate decomposition is best described by the two-dimensional diffusion-controlled mechanism (D₂), while the decomposition of strontium carbonate is best fitted by means of the three-dimensional phase boundary-controlled mechanism (R₃). The values of activation parameters obtained using different methods were compared and discussed.     

Thermochemical studies of group iiib borates and mixed borates

The changes in energy, mass and structure of Sc, Y, La and In borate were studied at temperatures up to 1400°C with and without the solid diluents, alumina and magnesia. These compounds did not decompose, but based on the energetics of solid state transitions, the stability of the crystal lattice was found to be: ScBO₃ ～- LaBO₃ ～- InBO₃ > YBO₃. YBO₃ was found to be dimorphic. The high temperature form being the pseudohexagonal vaterite structure. LaBO₃ formed a solid solution with this structure. The reverse transition took place with considerable hysteresis.     

Thermal expansion of polyoxymethylene

The thermal expensivities of polyoxymethylene crystals in the direction parallel (α_|^c) and perpendicular (α_‖^c) to the chain axis have been measured from 160 to 400 K using wide-angle x-ray diffraction. Although polyoxymethylene has a helical chain structure, it exhibits a thermal expansion behavior similar to that of polymer crystals with planar zigzag chains, namely that α_‖^c is negative while α_|^c is positive and larger by an order of magnitude. The negative α_‖^c arises from the shortening along the chain axis caused by the torsional and bending motions of the chain, whereas the large and positive α_|^c reflects the weak interaction across the chains. Combining the crystal data with dilatometric measurements on semicrystalline samples, the thermal expansivity is found to vary linearly with crystallinity, thus allowing the expansivity of the amorphous phase to be derived by extrapolation. With the thermal expansivities of the crystalline and smorphous phases known, the draw ratio dependence can be calculated in terms of existing models and is found to agree reasonably with experimental data.     

Thermal decomposition of calcium and strontium peroxotitanates to metatitanates

Methods of DTA, TG, DSC, IR spectroscopy and X-ray phase analysis were used to study the thermal dehydration and decomposition of Ca²⁺ and Sr²⁺ peroxotitanates to the corresponding metatitanates. The stages of the process and the intermediate phases were identified. The information obtained was utilised to determine the optimum temperatures of heating of the initial peroxotitanates to yield metatitanates with a fairly high degree of crystallinity (for CaTiO₃ 680°C, and for SrTiO₃ 650°C).     

Thermal expansion of irradiated polyoxymethylene

The thermal expansion coefficient of gamma irradiated polyoxymethylene has been measured in the temperature range 80–340 K by using a three terminal capacitance technique. The radiation induced changes are measured by recording the i.r. spectra of the irradiated samples. The change in crystallinity caused by irradiation is measured by an X-ray technique. The thermal expansion coefficient increases with radiation dose below 170 K due to the predominant effect of degradation. Above 170 K, this trend reverses and the expansion coefficient decreases with radiation dose due to the increased crystallinity caused by irradiation.     

Thermal expansion of ultradrawn polystyrene

Measurements on the linear thermal expansivities α∥ and α⊥ parallel and perpendicular to the extrusion direction, respectively, have been carried out between ?160 and 30°C for atactic polystyrene (aPS) with extrusion ratio 1 ≤λ≤ 15 and isotactic polystyrene (iPS) with 1 ≤λ≤ 7.5. For both aPS and iPS, α∥ decreases sharply with increasing λ whereas α⊥ shows only a slight increase. Below λ = 5 the anisotropy α⊥/α∥ is nearly the same for both, but α⊥/α∥ for iPS becomes much larger at higher λ. This is accompanied by an abrupt rise in crystallinity and probably results from the increase in the number and tautness of intercrystalline tie molecules. The birefringences of aPS and iPS increase with λ and have nearly the same values for λ < 5. At higher λ, however, there is a sharp rise in the birefringence of iPS which is probably associated with the sudden increase in crystallinity. With an aggregate model, the chain orientation function for aPS calculated from thermal expansivity is found to be in reasonable agreement with the corresponding value obtained from birefringence.     

Thermal expansion of irradiated polytetrafluoroethylene

The thermal expansion coefficient of gamma-irradiated Polytetrafluoroethylene (PTFE) has been measured in the temperature range 80–340 K by using a three-terminal capacitance technique. The samples are irradiated in air at room temperature with gamma rays from a Co⁶⁰ source at a dose rate of 0.26 Mrad/h. The change in crystallinity is measured by an x-ray technique. The expansion coefficient is found to increase with radiation dose below 140 K owing to the predominant effect of degradation. Above 140 K, the expansion coefficient is found to decrease with radiation dose because of the enhanced crystallinity. The temperatures of the two first-order phase transitions are also found to shift to lower temperatures owing to degradation of PTFE.     

Thermal expansion of structure-H clathrate hydrates

The temperature dependence of the unit cell parameters of two newly identified hexagonal structure clathrate hydrates of hexamethylethane (HME) and 2,2-dimethylbutane (DMB) have been measured by X-ray powder diffraction. The thermal expansion of the two distinct crystallographic axes was found to be inequivalent. However, the coefficients of cubic expansion are comparable to that in the cubic structure I and II hydrates. The larger thermal expansivity in the clathrate hydrates relative to ice is attributed to the weakening of the host lattice due to the internal pressure generated by the rattling motions of the encaged guests.Dedicated to Dr D. W. Davidson in honor of his great contributions to the sciences of inclusion phenomena.     

Thermal investigation of strontium acetate hemihydrate in nitrogen gas

The thermal decomposition of strontium acetate hemihydrate has been studied by TG-DTA/DSC and TG coupled with Fourier transform infrared spectroscopy (FTIR) under non-isothermal conditions in nitrogen gas from ambient temperature to 600°C. The TG-DTA/DSC experiments indicate the decomposition goes mainly through two steps: the dehydration and the subsequent decomposition of anhydrous strontium acetate into strontium carbonate. TG-FTIR analysis of the evolved products from the non-oxidative thermal degradation indicates mainly the release of water, acetone and carbon dioxide. The model-free isoconversional methods are employed to calculate the E _a of both steps at different conversion α from 0.1 to 0.9 with increment of 0.05. The relative constant apparent E _a values during dehydration (0.5<α<0.9) of strontium acetate hemihydrate and decomposition of anhydrous strontium acetate (0.5<α<0.9) suggest that the simplex reactions involved in the corresponding thermal events. The most probable kinetic models during dehydration and decomposition have been estimated by means of the master plots method.     

Thermal studies of fly ashes expansion

Journal of Thermal Analysis and Calorimetry - The object of this study is to investigate the thermal properties of fly ashes from the last, farthest dedusting zone in terms of their use as ceramic...     

Thermal expansion of a viscoelastic gel

It was previously shown [J. Non-Cryst. Solids, 130, 157 (1991)] that the permeability of a saturated gel can be determined from a measurement of its rate of expansion during a change in temperature. The existing analysis assumes that the solid network of the gel behaves elastically, but in many cases the gel is likely to be viscoelastic, especially when the gel exhibits syneresis. In this paper, the expansion kinetics are determined for a gel with a viscoelastic (VE) network. Sample calculations for a silica gel containing ethanol indicate that a naive application of the elastic analysis could result in errors exceeding a factor of two in the estimate of the permeability of a VE gel. However, in such cases, there are qualitative features in the experimental data that alert the experimenter to the existence of significant VE relaxation. Therefore, it is possible to avoid errors by careful examination of the data.