Dependence of the preexponential factor on temperature

The dependence of the preexponential factor on the temperature has been examined and the errors involved in the activation energy calculated from isothermal and non-isothermal methods without considering such dependence have been estimated. It has been shown that the error in the determination of the activation energy calculated ignoring the dependence of Aon Tcan be rather large and it is dependent on x=E/RT, but independent of the experimental method used. It has been also shown that the error introduced by omitting the dependence of the preexponential factor on the temperature is considerably larger than the error due to the Arrhenius integral approach used for carrying out the kinetic analysis of TG data.Peer reviewe

Limitations of model-fitting methods for kinetic analysis: Polystyrene thermal degradation

In this paper, some clarifications regarding the use of model-fitting methods of kinetic analysis are provided in response to the lack of plot linearity and dispersion in the activation energy values for the thermal degradation of polystyrene found in the literature and some results proposing an nth order model as the most suitable one. In the present work, two model-fitting methods based on the differential and integral forms of the general kinetic equation are evaluated using both simulated and experimental data, showing that the differential method is recommended due to its higher discrimination power. Moreover, the intrinsic limitations of model-fitting methods are highlighted: the use of a limited set of kinetic models to fit experimental data and the ideal nature of such models. Finally, it is concluded that a chain scission model is more appropriate than first order. © 2013 Elsevier B.V.Peer Reviewe

AC VS. DC flash sintering: Influence of field frequency on flash processes

Please click Additional Files below to see the full abstract

Thermal characterization of Montmorillonite clays saturated with various cations

Emanation thermal analysis (ETA), thermogravimetry and high temperature XRD were used to characterize the thermal behavior during dehydration of natural Na montmorillonite (Upton Wyoming, USA) and homoionic montmorillonite (MMT) samples saturated with different cations, i.e. Li+, Cs+, NH 4 +, Mg2+ and Al3+. ETA results characterized radon mobility and microstructure changes that accompanied the mass loss of the samples due to dehydration on heating in air. A collapse of interlayer space between the silicate sheets after water release from the MMT samples was characterized by a decrease of the radon release rate, ΔE. Decreases in c-axis basal spacing (d 001) values determined from XRD patterns for the different montmorillonite samples follow the sequence: Mg−MMT>Al−MMT>Li−MMT>Na−MMT>NH4−MMT>Cs−MMT The decrease of the radon release rate (ΔE) determined by ETA that characterized microstructure changes due to collapse of interlayer space corresponded well to differences in the c-axis basal spacing (Δd 001) values determined from the XRD patterns before and after samples dehydration.Peer Reviewe

Multi-Phase Flash Sintering: The Next Natural Step in Flash Sintering Evolution

Please click Additional Files below to see the full abstrac

Influence of ball milling on CaO crystal growth during limestone and dolomite calcination: Effect on CO2 capture at Calcium Looping conditions

The multicycle CO2 capture performance of CaO derived from the calcination of ball-milled limestone and dolomite have been tested under high temperature and high CO2 concentration environment for the first time. Here it is shown that the CO2 capture capacity of CaO is inversely related to the milling power applied to the starting mineral and the size of nascent CaO nanocrystals. In situ X-ray diffraction analysis used to follow the average crystallite size of CaCO3 and CaO during the calcination process as a function of temperature demonstrates that crystal growth is notably enhanced in a CO2-rich atmosphere for milled sorbents. Contrary to early reports suggesting improved reactivity towards carbonation of CaO from milled sorbents, promoted agglomeration, and crystal growth under these more “realistic” conditions lead to a severe deterioration of both capture capacity and recyclability, as observed from the multicyclic carbonation/calcination experiments. Yet the negative effect of milling is less pronounced in dolomite due to the constrained sintering effect of the inert MgO grains that results in smaller CaO crystallite sizes, reduced crystal growth rate, and improved performance. These results provide insight on the role of CaO crystallinity on the carbonation reaction, useful for devising strategies to improve sorbents performance.Peer reviewe

A comparative study of magnetic and electrical properties of BiFeO3 mutiferroic ceramics sintered by electric field assisted-methods: Spark plasma sintering and flash sintering

Please click Additional Files below to see the full abstract

Effect of reactive flash sintering on the magnetic and hyperfine parameters of SrFe12O19 ceramic permanent magnets

Please click Additional Files below to see the full abstract

Electrical properties of stoichiometric BiFeO3 prepared by mechanosynthesis with either conventional or spark plasma sintering

Phase-pure powders of stoichiometric BiFeO3 have been prepared by mechanosynthesis. Ceramics sintered by either conventional heating in air or spark plasma sintering (SPS) followed by oxidative anneal in air are highly insulating with conductivity e.g. ~10–6 Scm–1 at 300 °C and activation energy 1.15(2) eV, which are comparable to those of a good-quality BiFeO3 single crystal. By contrast, the as-prepared SPS sample without the post-sinter anneal shows higher conductivity e.g. ~10–6 Scm–1 at 225 °C and lower activation energy 0.67(3) eV, indicating some reduction of the sample by the SPS process. The reason for the high conductivity observed in some ceramic samples reported in the literature appears to be unclear at presentGobierno de España CTQ 2011-27626 MAT 2008-06619Junta de Andalucia TEP-0300

Reactive Flash Sintering of High Entropy (Mn0.2Co0.2Ni0.2Cu 0.2X 0.2)Fe2O4 (X=Fe, Mg) Spinel Oxides

Please click Additional Files below to see the full abstrac