Tiberius C. Vaimakis

Preparation of hydroxyapatite via microemulsion route.

Hydroxyapatite (HAp) was prepared using a microemulsion route in combination with the pH-shock wave method. The samples as received consisted of amorphous aggregated particles, which had remarkable mesoporosity with a narrow pore size distribution. After being heated at 650 degrees C, the A-type carbonate hydroxyapatite was crystallized at 635 degrees C in particles of similar size (40--120 nm) with no internal porosity. At a higher temperature (900 degrees C) a sintering process took place, resulting in network of a larger particles, consisting of HAp and beta-tricalcium phosphate (beta-TCP). The crystallization of HAp occurs at 635 degrees C with an activation energy of 62.7--72.2 kcalmol(-1).

Variation of surface properties and textural features of spinel ZnAl2O4 and perovskite LaMnO3 nanoparticles prepared via CTAB-butanol-octane-nitrate salt microemulsions in the reverse and bicontinuous states.

Two binary oxides, a spinel, ZnAl2O4, and a typical perovskite, LaMnO3, have been prepared via CTAB-1-butanol-n-octane-nitrate salt microemulsion in the reverse and bicontinuous states. The exact point of the reverse and bicontinuous states of the microemulsion used in the synthesis was determined by conductivity experiments. The materials obtained after heating at 800 degrees C were characterized by XRD analysis for their crystal structure, N2 porosimetry for their surface area and porosity, and SEM and TEM photography for their texture. The ZnAl2O4 spinel obtained via the reverse microemulsion appears in SEM in a more fragmented form and with a higher specific surface area (143.7 m(2)g(-1)), compared to the corresponding solid prepared via the bicontinuous microemulsion, which appears more robust with lower surface area (126.7 m(2)g(-1)). Nevertheless both materials reveal in TEM a sponge-like structure. The perovskite materials LaMnO3 prepared via the reverse microemulsion showed in SEM a peculiar doughnut-like texture, each doughnut-like secondary particle having a diameter of 2 microm. The corresponding sample developed via the bicontinuous microemulsion showed in SEM uniform secondary particles of size approximately 0.2 microm. Both perovskite samples LaMnO3 appear well crystallized with relative low surface areas, 23.7 m(2)g(-1) for the reverse sample and 10.9 m(2)g(-1) for the bicontinuous one. The TEM photographs reveal that both of them, of reversed and bicontinuous origin, are made up of primary nanoparticles in the size range 40-100 nm. In SEM those materials showed a different secondary structure.

Threshold limits and kinetics of the non-isothermal decomposition of ammonium nitrate catalysed by chromium ions.

Abstract Ammonium nitrate (AN) samples, containing varying amounts of Cr 3+ cations were prepared and their mode of thermal decomposition was analysed under non-isothermal conditions in a thermogravimetric balance. Pure AN decomposes endothermically, but addition of Cr 3+ in amounts larger than one mole of Cr 3+ per 3000 moles of AN acts catalytically, changing the decomposition to an exothermic mode. At concentrations lower than this, the diffusion limitations prevent the condensation of Cr 3+ species into Cr 2 O 7 2− which are the catalytically active species. The rates of decomposition, described by a typical Coats-Redfern kinetic equation, show two stages. Satisfactory results were obtained using a modified Coats-Redfern equation and by taking into account the continuous increment of chromium catalyst in the melt. The second stage of the decomposition shows an increased activation energy compared with the first. This is probably caused by a change in the rate-determining step of the reaction path, which is most likely the decomposition of dichromates.

The kinetics of the thermal dehydration of the system Ca(H2PO4)2·H2O-CaHPO4·2H2O

Abstract The thermal dehydration of an equimolar mixture of the Ca(H 2 PO 4 ) 2 · H 2 O-CaHPO 4 · 2H 2 O system was studied using a thermobalance up to 650°C. The weight loss takes place in two main steps, the first occurring at 90–160°C and the second at 270–450°C with weight losses of 13% and 8%, respectively. The dehydration was studied at heating rates of β = 2.2, 3.7, 5.8, 8.0, 9.0 and 10.5°C min −1 . From the experimental results the kinetic parameters of dehydration were studied by the methods of Friedman (F), Ozawa-Flynn-Wall (OFW), Kissinger (K) and Coats-Redferm (CR) as well as by the formal kinetics (FK). The results obtained with the first two methods (F and OFW) show that both the first and the second step occur in two stages. The second stage of the first step shows an activation energy substantially less than that of the first stage. This fact might be due to diffusion limitations of water removal during the late stages of dehydration. The same fact seems to be true also for the first and second stages of the second step treated by the F and OFW methods. The other three methods (K, CR and FK) do not distingish between the two stages noted above and the results are similar to those found for the first stage of each step. The calculated pre-exponential factors show a compensation effect of the activation energies. The results are discussed in relation to their physical meaning and in terms of the internal consistency of the methods used.

The effect of procedural variables and mechanical activation on the thermal decomposition of calcite. An approach by experimental design

Second-order orthogonal experimental design has been used to examine the effect of mechanical activation, mass sample, heating rate and molar fraction of carbon dioxide in the environmental atmosphere on the thermal decomposition of calcite. The mathematical model of the activation energy (Ea) for various mechanisms was examined by the Coats-Redfern equation using the best-fit procedure and the method of the shape of the TG/DTG curves. The best-fit procedure gave the second-order reaction mechanism (F2) as the predominant mechanism while the shape method gave the contacting area geometrical mechanism (R2) and the two-dimensional diffusion mechanism (D2) as the predominant mechanisms. The mathematical models for the change of enthalpy (Δ H and the temperature of the maximum rate of decomposition (Tmax) were found, but unfortunately only the Tmax model was of significance.

Thermoanalytical study of La2NixCu1-xO4 preparation from mixed nitrates of lanthanum, nickel and copper

Abstract The thermal decomposition of La(No3)3 · 6H2O, Ni(NO3)2 · 6H2O and Cu(NO3)2· 3H2O, and their mixtures with molar ratios such as are required for the formation of the perovskite series La2NixCu1−xO4(x = 0, 0.2, 0.4, 0.6, 0.8 and 1), was studied using a thermogravimetric continuous mode method and a batch method, the mixtures being heated at appropriately chosen temperatures. The batches after cooling were examined by X-ray diffraction (XRD) and IR spectrometry to identify the intermediate and final products. From the thermogravimetric results the kinetic parameters of decomposition were calculated by the Coats-Redfern method using various mechanisms. The experimental results for the mixtures show that the production of the perovskite phases takes place through the formation of the basic nitrate-hydrate salts at first, and subsequently the oxynitrate salts. The predominant mechanism of formation of the oxynitrate salts is chemical reaction with activation energies substantially less than that of the raw materials. It should be noted that the compensation effect is observed.

A detailed study of the condensation of the Ca(H2PO4)2·H2O-CaHPO4·2H2O system under thermal treatment

Abstract The thermal condensation of an equimolecular mixture of Ca(H2PO4)2-H2O-CaHPO4· 2H2O has been studied from room temperature up to 620°C by a thermogravimetric continuous mode method, and by a batch method, heating the mixture at 140, 190, 240, 300, 360, 420 and 620°C. The batches were cooled after heating at each temperature, and examined by X-ray diffraction (XRD) and IR to identify the intermediate products. The thermal condensation towards the final products, which were identified as β-Ca(PO3)2 Ca4P6O19 and 7CaO·5P2O5 (tromelite), proceeds through initial removal of the crystal water molecules (140–190°C) and subsequent condensation and removal of additional water (190–360°C). A detailed reaction pathway has been constructed, made up of parallel and consecutive chemical transformations, taking into account the intermediate products observed by XRD and IR, and also maintaining the mass balance of the system. It should be noted that the second constituent of the starting mixture Ca(H2PO4)2-H2O-CaHPO4· 2H2O appears to suffer initially a kind of mechanochemical dehydration at room temperature towards CaHPO4.

Thermochemical effects in the development of mesoporous alumino-phosphoro-vanadates from ammonium nitrate precursors

Abstract Mesoporous alumino-phosphoro-vanadate solids were prepared by heating precipitates of the corresponding metals containing ammonium nitrate salts. The materials obtained contained the elements Al : P : V = 100 : X : Y , where X , Y = 0, 5, 10, 20 (16 samples overall). Thermogravimetric analysis showed that the addition of P or V in the solids influences the endoand exothermicity of the NH 4 NO 3 decomposition, driving the net result from highly endothermal at no addition of P and/or V to almost neutral at high addition (20%) of those heteroatoms. The activation energies, calculated according to the Coats-Redfern procedure, were found to increase on adding 5% of P or V and then to drop at higher addition. The order of the decomposition reaction drops from ≈0.5 to 0 at 20% of P and/or V heteroatoms. The materials obtained had a high surface area (BET) which increases from 200 m 2 g −1 for pure alumina to almost 400 m 2 g −1 on addition of 5–10% of P and/or V, while further addition of those heteroatoms decreases the surface area to its original values. The maximum of the surface area appears in samples where the NH 4 NO 3 decomposition follows almost equal endothermic and exothermic routes.

Structure and Thermal Stability of Polystyrene/Layered Silicate Nanocomposites

Polystyrene/layered silicates nanocomposites were prepared by intercalation in solution method, using CHCl3 and CCl4 as solvents. The clay used was organically modified by hexadecyltrimethyl–ammonium bromide (CTAB) at various surfactant loadings. It was found that intercalated nanocomposite structure was obtained using CHCl3 as solvent while exfoliated or partially exfoliated was the predominant form in the case of CCl4. X-ray diffraction and thermogravimetric analysis were used to characterize the nanocomposite morphology and thermal stability, respectively. Enhancement in thermal stability was observed for PS-nanocomposites compared to that of pristine polymer as indicated by TGA measurements. This increment was more prevalent for exfoliated nanocomposites prepared with carbon tetrachloride as solvent.

The thermal dehydration of the Ca(H2PO4)2·H2OSiO2 system. Part 1. Mechanism

Abstract The thermal dehydration of the Ca(H2PO4)2·H2OSiO2 system has been studied by a continuous thermogravimetric mode method, and by a batch method, heating the mixtures in the molar ratios SiO 2 Ca(H 2 PO 4 ) 2 · H 2 O = 0.5, 1, 2 and 3 at 150, 185, 245, 275, 320, 430 and 780°C. The batches, after cooling, were examined by X-ray diffraction (XRD) and infrared spectroscopy (IR) to identify the intermediate and final products. The final products were mainly Ca(PO3)2 and SiP2O7, while pyro- and triphosphates and Si(HPO4)2 were detected as intermediate products. The quantity of SiO2 present affects the dehydration which takes place in five stages. The removal of water molecules in the first, second and fourth stages was increased, and in the third and fifth stages was decreased on increasing the amount of SiO2.