Kinetic analysis of the α-β HgI2 phase transition

The kinetics of the α-β phase transition of HgI₂ were investigated by isothermal and non-isothermal differential scanning calorimetry. The effective activation energy of the transition, 415±40 kJ mol^-1, was determined applying the methods of Kissinger and Ozawa. The transition kinetics were described by the Johnson-Mehl-Avrami model and the value of the Avrami exponent n was found to range from high values (n>3) at the early stages to lower values at later stages of the transformation, with an average value of 2. This revised version was published online in August 2006 with corrections to the Cover Date.     

A combined study of the oxidation mechanism and resistance of AISI D6 steel exposed at high temperature environments

In this work it is thoroughly examined the oxidation performance of D6 tool steel under isochronal and isothermal oxidations. Isochronal oxidation tests, from ambient temperature to 1000 °C, revealed the oxidation rate of the coupons at different temperatures. Four different temperatures were selected for the isothermal oxidation test, which correspond to different oxidation rates. The oxidation and the examination of the samples were accomplished by thermogravimetric analysis (TG) in air with which the mass gain of the samples due to oxidation was simultaneously acquired. The samples were, also, examined by scanning electron microscopy (SEM), in order to observe their surface before and after the oxidation tests. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were used for the accurate identification of the as formed oxides. The results revealed that in every case two distinct layers of oxides were formed while their composition was different, depending on the temperature of oxidation. Furthermore, the thickness of the as formed oxides is increased when the oxidation is performed at higher temperatures.     

Multicyclic study on the carbonation of CaO using different limestones

Different samples of limestones, with small differences in their stoichiometry, have been studied comparatively. The carbonation reaction has been studied for a large area of isothermal temperatures. The conditions for the multicyclic experiments of calcination/carbonation were: isothermal temperature 670°C, heating time 60 min and carrier gas CO₂. The final carbonation conversion depends mainly on the isothermal temperature of the carbonation reaction and the heating time. The final temperature of the calcination reaction depends on the percentage of CaO that it has not been conversed to CaCO₃ in the repeated carbonation experiments. The quantity of CaO that has not been carbonated, in the same sample, affects the values of the coefficients of the kinetic model that fit the calcination reaction. In the multicyclic experiments the carbonation conversion for two of the four studied samples, was high enough in comparison to other samples of calcite. At sample A the reduction of the carbonation conversion during the first five cycles is less than it is at other samples from the literature. Under the above experimental conditions — isothermal temperature and heating time — specific samples consisted mainly of calcite can absorb larger quantities of CO₂ than samples consisted mainly of dolomite.     

Thermal degradation kinetics of in situ prepared PET nanocomposites with acid-treated multi-walled carbon nanotubes

A series of PET/acid-treated multi-walled carbon nanotubes (MWCNTs) nanocomposites of varying nanoparticles’ concentration were prepared, using the in situ polymerization technique. TEM micrographs verified that the dispersion of the MWCNTs into the PET matrix was homogeneous, while some relatively small aggregates co-existed at higher filler contents. Intrinsic viscosity of the prepared nanocomposites was increased at low MWCNTs contents (up to 0.25 wt%), while at higher contents a gradual reduction was observed. The surface carboxylic groups of acid-treated MWCNTs probably reacted with the hydroxyl end groups of PET, acting as chain extenders at smaller concentrations, while at higher concentrations, on the other hand, led to the formation of branched and cross-linked macromolecules, with reduced apparent molecular weights. From the thermogravimetric curves, it was concluded that the prepared samples exhibited good thermostability, since no remarkable mass loss occurred up to 320 °C (<0.5%). The activation energy (E) of degradation of the studied materials was estimated using the Ozawa, Flynn, and Wall (OFW), Friedman and Kissinger’s methods. Pure PET had an E = 223.5 kJ/mol, while in the PET/MWCNTs nanocomposites containing up to 1 wt% the E gradually increased, indicating that MWCNTs had a stabilizing effect upon the decomposition of the matrix. Only the sample containing 2 wt% of MWCNTs exhibited a lower E due to the existence of the aforementioned cross-linked macromolecules. The form of the conversion function for all the studied samples obtained by fitting was the mechanism of nth-order auto-catalysis.     

DSC study of the deposition reactions of zinc pack coatings up to 550°C

Zn pack coating formation takes place in three steps as differential scanning calorimetry shows. The initial step (at 193.9°C) is endothermic and involves the transformation of α-NH₄Cl to β-NH₄Cl and the NH₄Cl decomposition to NH₃ and HCl. During the second step (at 248.6°C), which is exothermic, Zn²⁺ salts are formed and most probably ZnCl₂. Finally at 264.1°C (endothermic reaction) it seems that ZnCl2 is decomposed to form Zn that is deposited on the ferrous substrate. The as-cast Zn diffuses in the iron substrate forming the gamma and delta phase of the Fe–Zn phase diagram. Al₂O₃ is not involved in the above-mentioned mechanism and acts only as filler.     

LaSrMnCoO6: a new cubic double perovskite oxide

Single-phase polycrystalline powder samples of the double perovskite oxide LaSrMnCoO₆ were synthesized by the Pechini (citrate-gel) technique. The structural, magnetic and electrical properties of the obtained powders were investigated by X-ray diffraction, electron microscopy, dc magnetization, ac susceptibility and dc resistivity measurements. The crystal structure of the new compound was found to be cubic of space group at room temperature. Below 225 K, the samples exhibit ferrimagnetic behavior with a spin-glass-like character. Resistivity measurements indicate semiconducting behavior with two different conductivity mechanisms: thermally activated behavior below 190 K and variable range hopping above 190 K.     

Characterization and phase transformation study of TlSbSe2 crystals

TlSbSe₂ monocrystals were grown using the modified Bridgman–Stockbarger method and were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Reflectivity spectra have been registered in the range 50 to 4000 cm^–1 for E parallel to a and E parallel to b polarizations, on the cleavage plane. A remarkable anisotropy at two directions was verified. With regard to previous observations, additional peaks were discriminated and the fundamental phonon parameters were determined using classical dispersion relations. The material presents a complex phase transformation – with two thermal effects – that was examined using differential scanning calorimetry (DSC). Non-isothermal measurements, at different heating and cooling rates (β), were used to study the thermal phenomena. The main effect is attributed to a structural displacement and the second one to a cation exchange procedure. The phase transformation temperature depends strongly on the cooling rate and the peaks are shifted by 30 K with the increase of this rate, on the contrary to the increase of the heating rate that has a smaller effect. Phenomena related with the influence of the previous, repeated heating and cooling cycles on the transformation are also examined and analytically discussed.     

Effect of molecular weight on thermal degradation mechanism of the biodegradable polyester poly(ethylene succinate)

A series of aliphatic polyesters, in particular poly(ethylene succinate), having different molecular weights, were synthesized from succinic acid and ethylene glycol, following the melt polycondensation process. Intrinsic viscosities (IV), GPC, DSC, ¹H NMR and carboxylic end group measurements were used for their characterisation. From thermogravimetric analysis, it was concluded that the molecular weight of polyesters achieved during polycondensation are strongly related to thermal stabilities of initial oligomers. In order to synthesise high molecular weight polyesters, the number average molecular weight of oligomers must not be lower than 2300–3000 g/mol, since thermal decomposition begins at temperatures lower than 200 °C. However, even in that case, polycondensation temperatures must not exceed 230–240 °C. From TGA studies, it was found that sample having different molecular weights could be divided into two groups characterized by different thermal stability. In the first group, belong samples with intrinsic viscosity of IV = 0.08 dL/g and in the second one all the other samples (IV > 15 dL/g). From kinetic analysis of thermal degradation, it was found that degradation of all polyesters takes place in three stages, its one corresponding to a different mechanisms. Degradation of samples with low molecular weight is more complex that that of polyesters having high molecular weights. The values of the activation energy and the exponent n for the two groups of samples—with different molecular weight—are similar, regarding the first two mechanisms, while there is an alteration in the case of the third mechanism.     

Crystallization kinetics of amorphous Fe75-xCuxSi9B16

In this work the influence of Cu admixtures on the crystallization process of amorphous Fe-Si-B alloys is studied, based on measurements of differential thermal calorimetry of the series Fe_75-xCu_xSi₉B₁₆ (x=0, 1, 2, 2.8 and 3.5) during their heating with different heating rates. The first crystallization stage can not be traced for any of the amounts of Cu content examined, while the second stage is observed only when the Cu content is 1 at%. The activation energy as estimated with Kissinger's method for the third crystallization stage has a mean value of 326 kJ mol^-1 and with the isoconversional Flynn, Wall and Ozawa method is almost constant when 0.05<a<0.6 and exhibits a small monotical decrease when a>0.6. The main crystallization peak can not be described by means of a single JMA-type function. This revised version was published online in August 2006 with corrections to the Cover Date.     

Melt‐crystallization mechanism of poly(ethylene terephthalate)/multi‐walled carbon nanotubes prepared by in situ polymerization

A series of poly(ethylene terephthalate)/multi‐walled carbon nanotubes (PET/MWCNTs) nanocomposites were prepared by in situ polymerization using different amounts of multi‐walled carbon nanotubes (MWCNTs). The polymerization of poly(ethylene terephthalate) (PET) was carried out by the two‐stage melt polycondensation method. The intrinsic viscosity (IV) of the composites is ranged between 0.31 and 0.63 dL/g depending on the concentration of the MWCNTs. A decrease of IV was found by increasing MWCNTs content. This is due to the reactions taking place between the two components leading to branched and crosslinked macromolecules. These reactions are, mainly, responsible for thermal behavior of nanocomposites. The melting point of the nanocomposites was shifted to slightly higher temperatures by the addition till 0.55 wt % of MWCNTs while for higher concentration was reduced. The degree of crystallinity in all nanocomposites was, also, reduced by increasing MWCNTs amount. However, from crystallization temperature, it was found that MWCNTs till 1 wt % can enhance the crystallization rate of PET, whereas at higher content (2 wt %), the trend is the opposite due to the formation of crosslinked macromolecules. From the extended crystallization analysis, it was proved that MWCNTs act as nucleating agents for PET crystallization. Additionally, the crystallization mechanism due to the existence of MWCNT becomes more complicated because two mechanisms with different activation energies are taking place in the different degrees of crystallization, depending on the percentage of MWCNT. The effect of molecular weight also plays an important role. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1452–1466, 2009