Thermal stability of solid solutions obtained by mechanical alloying

Summary The heats of ordering for mechanochemically synthesized nano-sized supersaturated solid solutions were demonstrated for Ni-In, Ni-Sn, Ni-Al, Cu-Sn, Cu-Hg systems. It is shown that increasing concentration of doping element leads to decreasing of decomposition temperature and to increasing of decomposition enthalpy. The concentration heterogeneity of doping elements in mechanochemically synthesized supersaturated solid solutions was found. The reactivity of alloys Cu 20 mass % Sn for commercial powder and MA powder was investigated.     

The effect exerted by temperature on the phase formation of titanium oxide layer on silica surface at different stages of molecular layering

The effect exerted by thermal conditions at the different stages of the molecular layering synthesis on the structure and phase composition of a coating being formed was studied for the example of the system silica matrix: titanium dioxide surface layer.     

Thermal decomposition of lighter lanthanide selenate hydrates and their solid solutions

The thermal dehydration-decomposition of Ln₂(SeO₄)₃·nH₂O (wheren=12 forLn=Pr, Nd andn=8 forLn=Sm) and Pr_xLn_2−x(SeO₄)₃·nH₂O (wheren=12 forx=1.0 andLn=Nd;n=8 forx=0.2 and 1.0 in case ofLn=Sm) have been reported.

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Zusammenfassung Die thermische Dehydratation-Zersetzung von Ln₂(SeO₄)₃·nH₂O (mitn=12 fürLn=Pr, Nd undn=8 fürLn=Sm) und Pr_xLn_2−x(SeO₄)₃·nH₂O (mitn=12 fürx=1.0 undLn=Nd;n=8 fürx=0.2 und 1.0 in Falle vonLn=Sm) wurde beschrieben.

     

The effect of temperature on the enthalpy of mixing of polyelectrolyte solutions

The effect of temperature on the enthalpies of mixing of sodium dextran sulphate and sodium alginate with sodium chloride has been determined and the results compared with the predictions based on the line charge theory. Better agreement between experimental and calculated values is obtained at the higher reaction temperature, as previously found for the enthalpies of dilution. Measurement of the additional enthalpy changes occurring when different alkali metal chlorides are mixed with the sodium salt of the polyanion gives the sequence Li⁺ > Na⁺ > K⁺ in order of decreasing overall endothermic enthalpies of reaction. The same sequence is observed with polysulphates and polycarboxylates at different temperatures.     

The effect of temperature on the enthalpy of dilution of polyelectrolyte solutions

The integral enthalpies of dilution of sodium dextran sulphate and sodium alginate have been measured at 15, 25 and 40°. The results are compared with the predictions of the line charge theory. The experimental values show a much greater decrease with temperature than the calculated values and this difference is attributed to a contribution from solute/solvent interactions.     

On the stability of extremely diluted solutions to temperature

Conductometric and calorimetric titrations of extremely diluted solutions (EDS) were performed by adding HCl or NaOH solutions. The aim of this work was to confirm the hypothesised presence of molecular aggregates of water molecules in EDS stored for 15 months at 5 different temperatures. Conductometric and calorimetric measurements on the EDS samples revealed significant differences compared to the solvent. The experimental data can be interpreted as indicating a favourable interaction between the H₃O⁺ and OH^? ions and the dissipative structures, leading to the formation of complexes between the two ions deriving from probes and the molecular aggregates of water molecules. These results suggest that storing EDS at different temperatures does not alter the non-equilibrium changes in the supramolecular structure of water in the studied solutions.     

Chemical composition and acid-base properties of the surface of GaAs-CdS solid solutions

Chemical composition and acid-base properties of the surfaces of GaAs-CdS solid solutions and the constituent binary compounds exposed to air, vacuum, CO, and NH₃ were analyzed using mass spectrometry, IR spectrometry, isoelectric-point pH (pH_iso) analysis, and mechanochemical methods. Powders and films were prepared from the solid solutions by isothermal diffusion and discrete vacuum evaporation, respectively. XRD and Raman analysis revealed that a solid solution has a sphalerite or wurtzite structure, depending on its composition. The initial surface of all the systems studied exhibited acidic properties: the pH value ranged from 4.9 to 6.45. The surfaces contained Lewis (electron accepting) and Brönsted (adsorbed H₂O and CO₂ molecules, OH^? groups) sites. The acidity of the surface increased upon treatment with CO and decreased upon treatment with NH₃. For all types of treatment, the concentration dependence of the acid-base properties of the surface exhibited an extremum. The strongest acidic sites were revealed on the surface of the (GaAs)_0.95 (CdS)_0.05 solid solution. The adsorption activity of this solution, a promising material for manufacturing gas sensors, was evaluated.     

Thermal stability of solid and aqueous solutions of humic acid

The effects of temperature on the stability of a soil humic acid were studied in the present work. Solid samples of Gohy-573 humic acid (HA) and dissolved ones in aqueous solution (pH 6.0, 0.1 mol L⁻¹ NaClO₄) were investigated in order to understand the impact of temperature on the chemical properties of the material. The methods applied to solid samples in the present investigation were thermogravimetric analysis (TGA), temperature-programmed desorption coupled with mass spectrometry (TPD-MS), and in situ diffuse reflectance infrared Fourier transformed spectroscopy (in situ DRIFTS). Humic acid samples were studied in the 25-800 °C range, with focus on thermal/chemical processes up to 250 °C. The reversibility of the changes observed was investigated by cyclic changes to specified temperature ranges (40-110 °C). All measurements were conducted under inert-gas atmosphere in order to avoid samples combustion at increased temperatures. Aqueous solutions were analyzed by UV-vis absorption spectroscopy after storage at temperatures up to 95 °C, and storage times up to 1 week. For temperatures below 100 °C experiments on solid and aqueous samples have shown results which were consistent to each other. The amount of water desorbed is temperature dependent and up to 70 °C this process was totally reversible. Above 70 °C an irreversible loss of water was also observed, which according to UV-vis spectroscopy corresponds to water produced by condensation leading to more condensed polyaromatic structures. The water released up to 110 °C was about 7 wt% of the total mass of the dried humic acid, where less than 50% corresponded to reversibly adsorbed water. At higher temperatures (>110 °C), gradual decomposition resulting in the formation of carbon dioxide (110-240 °C), and carbon monoxide (140-240 °C) takes place. Hence, thermal treatment of Gohy-573 humic acid above 70 °C results in irreversible structural changes, that could affect chemical properties (e.g., complex formation) of the material.     

Analysis of solid solutions stability in scheelite-type molybdates and tungstates

Mutual solubility of bivalent metal molybdates and tungstates with scheelite structure was theoretically estimated by calculating formation enthalpies and the maximal decomposition temperatures of solid solutions at different temperatures. The theoretical stability of continuous solid solutions in binary systems of bivalent metal molybdates and tungstates was found to be higher than reported literature data. After cooling down continuous substitution solid solution should remain in following systems: CaMoО₄–CdMoО₄, SrMoО₄–MMoО₄ (M=Ba, Pb), BaMoО₄–PbMoО₄, SrWO₄–MWO₄ (M=Ca, Pb), and BaWO₄–PbWO₄. There is a probability that at room temperature in systems CaMoО₄–SrMoО_4, CaWO₄–PbWO₄, and BaWO₄–SrWO₄ the single homogeneity region may decompose to limited solid solutions. It was shown experimentally that a continuous series of scheelite-structure solid solutions M_1−xM^I_xТO₄ can be formed via citrate synthesis at temperatures below 500°С.     

Investigation of the surface composition of NiOMgO solid solutions by X-ray photoelectron spectroscopy

The surface composition of NiOMgO solid solutions has been investigated by XPS. It has been found that no appreciable deviation from bulk composition is present, or, if present, it goes in the direction of a slight depletion in Ni²⁺ ions at the surface. The approximations involved and the problems encountered in the determination of the surface composition of air-exposed oxide samples are discussed.     

Thermal stability of Ba1-xCaxF2 solid solutions

Ba_1-xCa_xF₂ samples with fluorite-type structure were prepared by high-energy ball milling. DSC measurements revealed exothermic decomposition at temperatures of about 450 °C. Decomposition was also observed by high temperature X-ray experiments. As a product, two phases with compositions close to pure BaF₂ and CaF₂ were obtained. The decomposition temperature observed by X-ray measurements is lower than in the DSC experiments. This might indicate that X-ray irradiation could foster the decomposition.     

Study of the temperature effect on the process of rubidium hexachlorotellurite decomposition

Thermal dissociation of rubidium hexachlorotellurite in inert atmosphere (argon) and in atmosphere with air access was studied. The DTG and DTA curves for the process of rubidium hexachlorotellurite thermal dissociation in the argon atmosphere are given. Their character is defined by the behavior of TeCl₄, which is liberated on the decomposition of samples.     

The decomposition of hypobromite solutions

The stability of 10⁻¹–10⁻³N hypobromite solutions, stored in brown or colourless bottles at room temperature, has been investigated. A reaction mechanism of decomposition has been proposed and some practical conclusions have been drawn concerning the optimal method of preparation and the required frequency of control of hypobromite solutions used as an analytical reagent.     

The effect of γ-irradiation and polymer composition on the stability of PLG polymer and microspheres

The effects of lactide:glycolide ratio and γ-irradiation on the stability characteristics of poly(d,l-lactide-co-glycolide) (dl-PLG) co-polymer and microspheres were studied. Samples containing 50:50, 65:35, 75:25, 85:15 and 95:5 lactide:glycolide were studied as both the raw polymer and microspheres. The samples were characterised and degradation was monitored using a variety of spectroscopic, chromatographic, thermal and particle analysis techniques. The lactide:glycolide ratio was shown to be an important parameter in controlling the degradation of PLG co-polymer, especially in the microspheres, and the raw polymer samples were observed to be less stable than the microspheres. γ-Irradiation was shown to reduce the molecular weight of the PLG samples without significantly affecting the polydispersity, suggesting a random chain scission degradation mechanism. The detrimental effects of γ-irradiation were shown to continue on storage of the samples for 4 weeks in the solid state at ambient and accelerated conditions.     

The effect of composition of solid silver amalgam electrodes on their electrochemical response

The main purpose of this work was to evaluate the effect of the silver to mercury ratio on the voltammetric responses of silver solid amalgam electrodes (AgSAE’s). For this, the AgSAE were prepared by mechanical mixing the metals in the following mass ratios of silver to mercury: 30/70, 40/60, 50/50, 60/40, and 70/30. The resulting AgSAE’s were physically characterized by energy dispersive X-ray analysis, X-ray diffraction and scanning electron microscopy, confirming the mass percentages of the silver and mercury, the total absence of liquid mercury and a globular structure of all AgSAE’s. Furthermore, it was observed that the AgSAE 30/70 contained only one single phase (Ag₂Hg₃), and no metallic silver or mercury oxides. Additionally, the resulting AgSAE’s were chemically characterized with respect to the influence of the electrode composition on the reproducibility and electrochemical signals of a hexamine-ruthenium (III) chloride solution by use of electrochemical impedance spectroscopy and cyclic voltammetry. The separation between anodic and cathodic peaks, and consequently, the charge transfer resistance across the electrode/solution interface, and the electroactive area were calculated demonstrating that the 30/70 composition is the best surface for practical applications. Finally, square-wave voltammetry experiments were performed in 4-nitrophenol solution, with a previous optimization of the experimental and voltammetric parameters. The calculated detection limit shows that the AgSAE 30/70 is suitable for determining any contamination by p-nitrophenol, minimizing the toxic residues in case of using liquid mercury electrodes.     

Application of programmed temperature decomposition to the study of solid decomposition reactions taking place through the prout and tompkins mechanism

A method has been developed to allow kinetic analysis of the Prout and Tompkins mechanism of solid decomposition reactions using a single TG diagram obtained in a linear heating program. On the other hand, provided that the nickel formate decomposition follows such a mechanism, this reaction has been used for checking the kinetic equation obtained.