Effect of mechanical activation of reagents’ mixture on the high-temperature synthesis of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–TiB<Subscript>2</Subscript> composite powder

A powder mixture of Al/TiO₂/H₃BO₃ = 10/3/6 in molar ratio was used in this study to form the Al₂O₃–TiB₂ ceramic composite via thermite reactions (combustion synthesis). As no combustion synthesis occurred for an unmilled sample in a furnace, the mixture was milled in a planetary ball-mill for various milling times, and the as-milled samples were in situ synthesized in the furnace at a heating rate of 10 °C/min. The differential scanning calorimetry (DSC) measurements were performed with the same heating rate on the unmilled and the as-milled samples to evaluate the influences of the milling on the mechanisms and efficiencies of reactions. Although no combustion synthesis occurred for the unmilled sample in the furnace, two exothermic peaks were detected in its DSC curve after the melting of the Al. For the as-milled samples, significant changes revealed in the DSC curves, suggest that the milling process before the combustion synthesis changed the mechanisms and efficiencies of reactions. In addition, the intensity and the temperature of the exothermic peaks in the DSC curves changed by increasing the milling time. According to the XRD analyses, by enhancing the milling time, the purity of the final products would increase, confirming that the efficiency of the reactions increased. Finally, the microstructures of the as-milled and as-synthesized samples were examined by a SEM, and it was shown that the morphology of the reactant powders was altered by increasing the milling time.     

Evaluation of thermodynamic properties of irreversible protein thermal unfolding measured by DSC

Summary We assessed the applicability of the extrapolation procedure at infinite scanning rate to differential scanning calorimetry (DSC) data related to irreversible protein unfolding. To this aim, an array of DSC curves have been simulated on the basis of the Lumry-Eyring model N&harr;U&rarr;F. The results obtained confirmed that when the apparent equilibrium constant K_{app (T=T1/2)} is lower than 3, the application of the extrapolation procedure provides accurate thermodynamic parameters. Although this procedure applies only to monomeric proteins for which the Lumry-Eyring model is a reasonable approximation, it will hopefully contribute to increase the potential of DSC in obtaining reliable thermodynamic information regarding the folding/unfolding equilibrium.     

Origin of double melting peaks in drawn nylon 6 yarns

The differential scanning calorimetry (DSC) melting curves of drawn nylon 6 were studied from the standpoint of reorganization of the crystals during the heating process. A new method was presented to obtain the DSC curve associated with the growth and melting of the original crystals, and that with the recrystallization and final melting process, separately. The results obtained show that, in the case of a heating rate of 10°C/min, the original crystals in the sample start perfecting themselves at temperatures far below their initial melting temperature and melt out below 222°C, recrystallization starts at about 210°C, and the newly emerged crystals melt out at 228°C. The superposition of two such constructed DSC curves reproduces the observed DSC curve well. Therefore, the double melting peaks of the sample are considered to be the result of superposition of three processes which occur successively during heating; perfection of the original crystals, melting of the perfected crystals concurrently with recrystallization, and melting of the recrystallized crystals.     

The calculation of heat capacity curves and phase diagrams based on regular solution theory

Summary The intermolecular interactions of molecules within the bilayer are responsible for the lipid organisation, e.g. domain formation, and the interaction and stabilisation of proteins within the lipid matrix. The mixing behaviour of lipids, which reflects the intrinsic molecular interactions, can be deduced from the shape of the phase diagram (temperature vs. mole fraction diagram), which is constructed from the analysis of heat capacity curves obtained by DSC. However, there are no objective procedures to determine the temperatures corresponding to the border lines of the coexistence region, i.e. the liquidus and solidus curves of the phase diagram. The main challenge to overcome is to develop an objective method for the correct determination of the onset and offset temperatures of the melting curve for every single transition curve in a standardized manner. The presented paper describes a procedure for the simulation of heat capacity curves. In a second step, based on the results from the heat capacity curve simulation, a phase diagram is calculated using a non-ideal, non-symmetric mixing model. The non-ideality parameters obtained from the calculation describe the intermolecular interaction of both components in a single phase region. Using this procedure, examples of the mixing behaviour of various binary phospholipid systems are analysed and it is shown how the mixing behaviour is influenced by external factors like e.g. the pH or ionic strength.     

Some methodological problems concerning nonisothermal kinetic analysis of heterogeneous solid–gas reactions

Isoconversional methods, those using only one curve α = α(T) (α is the conversion degree and T is the temperature), and invariant kinetic parameter method were applied to estimate the kinetic parameters from the following nonisothermal data: (1) simulated TG curves for a single reaction; (2) TG curves for thermal degradation of PVC; and (3) TG curves for the dehydration of CaC₂O₄·H₂O. The results obtained by applying various methods for the same system are compared and discussed. Finally, a procedure of kinetic analysis is suggested. Its application could lead to kinetic parameter values that can be used to predict either α = α(t) curves for other heating rates or α = α(T) curves for isothermal conditions. © 2001 John Wiley & Sons, Inc. Int J chem Kinet 33: 564–573, 2001     

Compatibility of tire elastomers using derivative heat flow traces in the glass transition region

Derivative heat flow curves give much more information about the phase heterogeneity of binary blends composed of NR, SBR and BR elastomers thanT _g. In blend compositions, the areas under the derivative heat flow curves appear to be an additive function of the concentration of elastomers in the case of incompatible blends (NR/BR, NR/SBR). They are less than additive for either a partially compatible blend (uncured SBR/BR) or a compatible blend (covulcanized SBR/BR). In the case of 60/40 SBR/BR blends, a DSC (T _0.5) reveals a singleT _g, in conformity with the earlier investigators, whereas the derivative heat flow curve shows two peaks (T _p) indicating incomplete homogenization of the phases. This is a new observation not mentioned in the published literature. Thus, derivative heat flow traces are likely to provide a unique tool to determine compatibility of elastomers. The study also reveals the importance of sample contact with the DSC pan in quantitative determinations.     

Enthalpy Relaxation in Glasses: Regression analysis of integral DSC data

A new method of calculation of parameters of enthalpy relaxation models is proposed. Regression analysis treatment compares the experimental and calculated values of relaxation enthalpy. The experimental values of relaxation enthalpy are obtained by numerical integration of the difference between the two DSC curves. Contrary to the overall shape of the DSC curve the integral values are not affected by particular heat flow conditions during the DSC experiment. The Narayanaswamy's numerical model based on the Kohlrausch—William—Watts relaxation function was used to calculate the theoretical values of relaxation enthalpy. The application of the proposed method on the DSC experimental data of enthalpy relaxation of As₂Se₃ is shown.This revised version was published online in November 2005 with corrections to the Cover Date.     

Enhancing signal-to-noise ratio and resolution in low-field NMR relaxation measurements using post-acquisition digital filters

The traditional way to enhance signal-to-noise ratio (SNR) of nuclear magnetic resonance (NMR) signals is to increase the number of scans. However, this procedure increases the measuring time that can be prohibitive for some applications. Therefore, we have tested the use of several post-acquisition digital filters to enhance SNR up to one order of magnitude in time domain NMR (TD-NMR) relaxation measurements. The procedures were studied using continuous wave free precession (CWFP-T₁) signals, acquired with very low flip angles that contain six times more noise than the Carr–Purcell–Meiboom–Gill (CPMG) signal of the same sample and experimental time. Linear (LI) and logarithmic (LO) data compression, low-pass infinity impulse response (LP), Savitzky–Golay (SG), and wavelet transform (WA) post-acquisition filters enhanced the SNR of the CWFP-T₁ signals by at least six times. The best filters were LO, SG, and WA that have high enhancement in SNR without significant distortions in the ILT relaxation distribution data. Therefore, it was demonstrated that these post-acquisition digital filters could be a useful way to denoise CWFP-T₁, as well as CPMG noisy signals, and consequently reducing the experimental time. It was also demonstrated that filtered CWFP-T₁ method has the potential to be a rapid and nondestructive method to measure fat content in beef and certainly in other meat samples.     

Development of master curves for discriminating the mechanism of solid state reactions from experimental data obtained by using the cyclic and controlled rate thermal analysis

The cyclic and Controlled Rate Thermal Analysis method (CRTA) has been used. The two rates automatically selected in the cyclic curve are small enough to allow the two states of the sample to be compared have nearly the same reacted fraction. Thus, the activation energy can be calculated without previous knowledge of the actual reaction mechanism. Provided that the activation energy,E, is known, a procedure has been developed for determining the kinetic law obeyed by the reaction by means of master curves that represent the values of the reacted fraction, α, as a function of?E/R(1/T-1/T _0.5),T _0.5 being the temperature at which α=0.5. This procedure has been tested by studying the thermal decomposition reaction of BaCO₃.     

Thermal analysis of the double-melting behavior of poly(L-lactic acid)

The melting and crystallization behavior of poly(L -lactic acid) (PLLA; weight-average molecular weight = 3 × 10⁵) was studied with differential scanning calorimetry (DSC). DSC curves for PLLA samples were obtained at various cooling rates (CRs) from the melt (210 °C). The peak crystallization temperature and the exothermic heat of crystallization determined from the DSC curve decreased almost linearly with increasing log(CR). DSC melting curves for the melt-crystallized samples were obtained at various heating rates (HRs). The double-melting behavior was confirmed by the double endothermic peaks, a high-temperature peak (H) and a low-temperature peak (L), that appeared in the DSC curves at slow HRs for the samples prepared with a slow CR. Peak L increased with increasing HR, whereas peak H decreased. The peak melting temperatures of L and H [T_m(L) and T_m(H)] decreased linearly with log(HR). The appearance region of the double-melting peaks (L and H) was illustrated in a CR–HR map. Peak L decreased with increasing CR, whereas peak H increased. T_m(L) and T_m(H) decreased almost linearly with log(CR). The characteristics of the crystallization and double-melting behavior were explained by the slow rates of crystallization and recrystallization, respectively. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 25–32, 2004     

A differential scanning calorimetry method to determine the isothermal crystallization kinetics of cocoa butter

This research aimed to reduce the variability on the data obtained from differential scanning calorimetric (DSC) analysis of the isothermal crystallization kinetics of cocoa butter.

To enable transformation of the DSC crystallization peak to a sigmoid crystallization curve, the DSC peak area has to be integrated. Usually, the start and end points of the crystallization peak are determined visually. The result of this visual determination appeared to be very much dependent on the operator, but also differed considerably when the same operator performed the integration several times. By proposing an objective calculation algorithm to determine the start and end points of integration, the variability caused by the operator during the integration procedure could be eliminated. Furthermore, sample preparation and the DSC heating protocol to melt the sample prior to crystallization were studied. Three heating protocols (65 °C for 15 min, 65 °C for 30 min and 80 °C for 15 min) were compared and it was shown that holding at 65 °C for 15 min was sufficient to eliminate any influence of sample history. Two different sample preparation procedures were compared and it appeared that a change in sample preparation procedure had a significant influence on the measured crystallization process. It is thus important to keep this method constant to eliminate the variability caused by it.     

Radiothermoluminescence of surface layers of n-tetracosane crystals

A radiothermoluminescence (RTL) curve of surface layers of n-tetracosane crystals has been obtained, which appear to be similar but not identical to the RTL of the evacuated sample or the sample with CCl₄. Since CCl₄ does not penetrate the crystal, the difference between the RTL curves of the evacuated sample and the one with CCl₄ represents the RTL of crystal surface layers. The RTL curve of tetracosane with the scavenger is that of RTL of intracrystalline areas of the alkane. The difference in shape between the surface and bulk RTL curves is manifested well in the ratio of the RTL curves for the surface and the evacuated sample.     

Thermal analysis of polymer networks consisting of different homopolymers

TG and DSC were carried out on PMAA-net-POTM in order to elucidate the relation between the heterogeneity and the thermal changes; moreover, TG and DSC were carried out on PPOTMDM and PMAA to compare the results. The onset temperature in the last stage in the TG curve of PMAA-net-POTM increases as the concentrations of DMF and POTMDM in the polymerization decrease. This is explained in terms of plasticizers effects. A single peak due to fusion of POTM chains appears in the DSC curves of most copolymers. However, it is not seen in the DSC curves of copolymers with high heterogeneity. This is explained in terms of the freezing of POTM chains by frozen heterogeneous moieties.     

Functional nonparametric classification of wood species from thermal data

In this study, thermogravimetric (TG) and differential scanning calorimetry (DSC) curves, obtained by means of a simultaneous TG/DSC analyzer, and statistical functional nonparametric methods are used to classify different wood species. The temperature ranges, where the highest probability of correct classification is reached, are also computed. As each observation is a curve, a nonparametric functional discriminant technique based on the Bayes rule and the Nadaraya–Watson regression estimator is used. It assigns a future observation to the highest probability predefined class (supervised classification). The smoothing parameter needed in this nonparametric method is selected according to the cross-validation technique. The method proposed is applied to a sample of 49 wood items (7 per wood class) and also to classify between hardwoods and softwoods. In all the cases, the samples have been successfully classified, obtaining better results with the TG curves. The results are compared with those obtained with other nonparametric methods based on boosting algorithm. A discussion about the relation of the obtained results with the referenced wood component degradation temperature ranks is presented.     

Dehydration studies of Co(II), Cu(II) and Zn(II) methanesulfonates

The dehydration process of Co(II), Cu(II) and Zn(II) methanesulfonates was studied by thermogravimetry/derivative thermogravimetry (TG/DTG) and differential scanning calorimetry (DSC) techniques in dynamic N₂ atmosphere. The TG/DTG curves show that all of them contain four crystallization water molecules, which are lost in two steps. The peak temperature and dehydration enthalpies ΔH were measured from DSC curves for each compound. The effect of procedural variables on the TG and DSC curves was investigated. In this work, the procedural variables included heating rate, Al pan state (unsealed and sealed) and sample mass.     

Serial filter method for fluorescence evaluation

A method has been developed to obtain the reflective spectral data, β_R(λ), for a fluorescent sample by making a simple modification to a conventional poly-chromatically illuminated spectrophotometer. To use this procedure, a series of sharp cut-off filters are inserted successively into the illumination beam of the instrument to obtain a number of spectral curves from which the complete reflectance curve is constructed piece-wise. The calculation of the complete reflectance curve is done wavelength-by-wavelength, starting at about 30 nm above the peak emission of the sample and proceeding toward the shortest wavelength cut-off filter, selecting the spectral data that are the lowest value. When the data from each filter are assembled, the long wavelength portion above the peak emission is made equal to the unmodified spectrum. Examples are given of the method of computation and comparison with bi-spectral data. The serial filter technique is equally applicable to instruments having either 45/0 or d/8 integrating sphere optical geometry.     

Melting of constrained drawn nylon 6 yarns

A differential scanning calorimeter (DSC) was used to study the melting behavior of drawn nylon 6 yarns which were prevented from shrinking during heating. The DSC curves exhibit a single melting peak at a higher temperature instead of the double peaks which, as reported previously, were observed in the unconstrained state. The curve is explained quantitatively in terms of the perfecting of the original crystals followed by monotonic melting of these crystals during heating. The single peak results from the absence of the partial melting–recrystallization process which plays an important role in the appearance of double peaks. The temperature of the melting peak for the constrained sample increases linearly with draw ratio, and is unaffected by drawing temperature and by annealing at constant length after drawing. The elevation of the melting temperature is discussed on the basis of the entropy effects predicted theoretically by Zachmann. Thermal analysis of constrained samples has proved to be useful for detecting oriented crystals which coexist with unoriented ones.     

The influence of mass transfer phenomena on the kinetic analysis for the thermal decomposition of calcium carbonate by constant rate thermal analysis (CRTA) under vacuum

The influence of the mass transfer phenomena on the thermal decomposition of calcium carbonate powders under vacuum was investigated through a detailed kinetic analysis by the constant transformation rate thermal analysis (CRTA). Reliable kinetic curves, free from the mass transfer problems, can be obtained by CRTA under vacuum, but within a restricted range of small sample sizes, <10 mg. The influence of mass transfer phenomena on the apparent kinetic parameters is discussed in relation to the distribution of fractional reaction α of the individual particles in a sample assemblage. Only when the distribution of α is maintained constant among a series of experimental kinetic curves, can a reliable activation energy, E, be obtained by one of the isoconversion methods. In this respect, a single cyclic CRTA permits the α distribution to be maintained constant between the two adjacent data points with different decomposition rates. In the present study, an apparent E value of about 223 kJ mol⁻¹ was obtained by the Friedman method from a series of CRTA curves with sample sizes less than 10 mg and by the rate jump method from a single cyclic CRTA curve with sample size of about 40 mg. The first-order (F₁) law was determined to be the most appropriate kinetic model function, from a series of CRTA curves, instead of the ideal contracting geometry (R₃) law formalized for the three-dimensional shrinkage of the reaction interface in the respective particles. The particle size distribution of the sample particles is suggested to be one possible reason for the apparent agreement with the F₁ law. A kinetic exponent n of the nth-order law that deviated from unity was obtained from the CRTA curves with sample sizes larger than 10 mg, due to an additional distribution of α produced by mass transfer phenomena. Because the α distribution due to the mass and heat transfer phenomena cannot be expressed practically in an analytical function, a meaningful kinetic model and preexponential factor are difficult to estimate from kinetic data that are influenced by the transfer phenomena. © 1998 John Wiley & Sons, Inc. Int. J Chem Kinet: 30: 737–744, 1998     

Comparison of the Thermal Desorption Spectra and Exoemission Spectra of BeO:TiO2 and BeO:Li Thermally Stimulated Exoelectron Emission Detectors

In thermally stimulated exoelectron emission (TSEE) dosimetry the irradiation is generally executed in air while the read out is in methane. Under these conditions adsorption/desorption effects appear which influence the exoemission. BeO:Li and BeO:TiO₂ ceramics were compared from this point of view. The effect of adsorption/desorption was studied in such a way that in the cycles of irradiation/read-out the cooling down was executed successively in a methane atmosphere or in air. The TSEE curves of the BeO:TiO₂ ceramic were more influenced by the cooling atmosphere than those of the BeO:Li ceramic. In order to explain this result the thermal desorption spectra of these materials was taken. © 1997 John Wiley & Sons, Ltd.