Calorimetric measurements
have been performed in glassy Se90M10
(M=In, Te, Sb) alloys to study the effect
of In, Te and Sb additives on the kinetics of glass transition and crystallization
in glassy Se90M10 system. Kinetic
parameters of glass transition and crystallization such as the activation
energy of glass transition (Eg),
the activation energy of crystallization (Mc),
the order parameter (n), the rate constant
(K), etc. have been determined using different
non-isothermal methods. The composition dependence of the activation energies
of glass transition and crystallization processes is also discussed. 相似文献
Summary: Isoconversional kinetic analysis involves evaluating a dependence of the effective activation energy on conversion or temperature and using this dependence for making kinetic predictions and for exploring the mechanisms of thermally stimulated processes. The paper discusses major results obtained by the authors in the area of the isoconversional analysis of polymer kinetics over the past decade. It provides a brief introduction to isoconversional methods and surveys the impact made by isoconversional analysis in several application areas that include kinetic predictions, thermal degradation, crosslinking (curing), glass transition, and glass and melt crystallization. It is concluded that isoconversional analysis has been used broadly and fruitfully because it presents a fortunate compromise between the single‐step Arrhenius kinetic treatments and the prevalent occurrence of processes whose kinetics are multi‐step and/or non‐Arrhenius.
An isoconversional method applies the Arrhenius equation to a narrow temperature region, ΔT related to a given extent of conversion. 相似文献
The properties and course of phase transition of amorphous borates obtained by solid-state thermal and mechanical treatment of hydrated borates and melt quenched glasses with the compositions of these borates were studied. Processes of structural relaxation and crystallization were considered.It was found that different methods of preparation of amorphous borates and the differences in their structural ordering resulting from these methods affect the rate and the mechanism of transformations occurring during their heating.Support for this work was provided by the Polish Committee of Scientific Research (KBN) — grant No. 3 P407 034 06. 相似文献
Crystallization and glass-transition phenomena were studied for amorphous chlorobenzene (CB)/toluene (TL) binary systems as
the function of composition. Samples were prepared by vapor-deposition onto cold substrates, and their structural changes
due to temperature elevation were monitored with Raman scattering and light transmission. It was found that the crystallization
temperature (Tc) of CB-rich amorphous samples increases as the TL concentration is increased. This is similar to the linear dependence of
glass-transition temperatures (Tg) of many organic compounds on the concentration of additive. Also found was that Tc of TL-rich supercooled-liquids decrease as the CB concentration is increased. Issues related to the two kinds of Tc are discussed briefly.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
The nature of the crystallization water in MgSO4·7H2O, Mg(NO3)2·6H2O and MgCl2·6H2O has been studied with the nonisothermal methods of thermogravimetry (TG), derived thermogravimetry (DTG) and differential thermal analysis (DTA). Analysis of the characteristic thermogravimetric data (TM,W∞) and the kinetic parameters (n, Ea), together with the DTA results, with CuSO4·5H2O as control sample, provided evidence of the existence of coordinated water and of the nature of the anions in these hydrates. The results are confirmed by the observation of a real compensation effect. For the compensation effect, the following equation is proposed: InA=0.220E-0.8 Structures explaining the presence of the coordinated water and the nature of the anions in these hydrates are also proposed. 相似文献
Precipitation of trehalose dihydrate in water is observed at room temperature for trehalose concentrations higher than 47.5%w/w. Direct observations of crystal melting in water and measures of the solution density determine the thermal variations of trehalose saturationS(T) (mM) in water: ln(S(T))=ln(0.1223)-(1330/T) withR2=0.9982. The glass transition (Tg) curve measured by DSC is lower at low concentrations and higher at high concentrations than previously reported.Tg is also measured as a function of the cooling/warming rates. Analysis of specific heat changes atTg and associated activation energy leads to identify a most stable glassy state around the second eutectic concentration. 相似文献
The crystallization kinetics of Cu55Zr45 (at%) glassy alloy is studied under isothermal condition using differential scanning calorimetry (DSC). The plot of correlation between the crystallized volume fraction α and annealing time t shows a sigmoid-type curve, which is steeper with higher annealing temperature. Furthermore, in isothermal crystallization condition, local activation energy Eα values, determined using the Arrhenius equation, range from 181.1 to 187.8 kJ/mol, which is nearly a constant. The local Avrami exponent n(α) values, obtained by the Johnson-Mehl-Avrami equation, which range from 2.2 to 4.0 at different annealing temeperatures, which indicates that the crystallization mechanism is diffusion-controlled transformation. Moreover, n(α) becomes greater with increasing annealing temperature, which indicates that annealing temperature can affect nucleation rate and growth type. 相似文献
The soft segment crystallinity and morphology of poly(ester-urethanes) (PEUs) based on poly(ε-caprolactone) (PCL) as a soft
segment and an aliphatic diisocyanate in the hard segment were studied. It was found that the restriction of the crystallization
of the PCL soft segment depends on the hard segment concentration, the length of the soft segment, and the total molecular
mass of the PEUs. The PEU based on a low molecular mass PCL (M=2000) is an amorphous elastic material during a long time after
casting from solution or after melt crystallization. A soft-hard segment endothermal mixing transition (Tmix) of about 70-80°C is observed in the DSC curves of this PEU sample.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
Lithium germanate gels, whose compositions are expressed by the general formula Li2O·xGeO2 with x=3; 4; 7, were synthesized by hydrolytic polycondensation of germanium ethoxide with lithium methoxide (x=3 and 7) or lithium hydroxide monohydrate (x=4) in alcoholic medium. The values of glass transition temperature of the gels exhibit a maximum at x=4. Crystallization behaviour of the gels, examined by differential thermal analysis and X-ray diffraction, is reported and discussed. The terms x=4 and 7 crystallize in two steps. Microcrystallites of the same composition of the gel are initially formed in an amorphous matrix and are then converted at higher temperatures into well shaped crystals. In term x=3, Li2GeO3 and Li2Ge4O9 crystals are directly formed. The values of activation energy for each crystallization step are consistent with the crystallization mechanisms and comparable with those reported with conventional melt glasses from oxides. 相似文献
This work present comparative results on powder milk storage quality, obtained from analytical methods. Protein content was determined conventional (Kjeldahl) and colorimetric with biuret reagent at 540 nm and integral quality by thermogravimetric and biological methods. A method was developed for the protein separation of powder milk. Powder milk was submitted to degradation processes at 45, 60 and 80°C for 20 days. The results indicated that protein content values were inconsistent if determinations by Kjeldahl and colorimetric methods and biological tests were compared. There is evidence of thermal decomposition of powder milk as detected by biological and thermogravimetric methods.The authors thanks CNPq/PADCT for financial support. 相似文献
Summary The present paper reports the Differential Scanning Calorimetric (DSC) study of some Ag doped Se-Te chalcogenide glasses.
DSC runs were taken at different heating rates. Well-defined endothermic and exothermic peaks were obtained at glass transition
and crystallization temperatures. The variation of glass transition temperature Tgwith Ag concentration has been studied. It has been found that Tgdecreases with increase in Ag concentration. The heating rate dependence of Tgis used to evaluate the activation energy of glass transition (DEt). The value of<span style='font-size:10.0pt; font-family:"SymbolProp BT";mso-bidi-font-family:"SymbolProp BT"'>DEthas been found to increase with increase in Ag concentration followed by nearly constant value at higher concentrations of
Ag. 相似文献
The formation of nano-quasicrystals on isothermal annealing of melt-spun ribbons of Zr69.5Al7.5Ni11Cu12 metallic glass has been investigated using transmission electron microscopy (TEM). The crystallization study of this metallic glass has been carried out using differential scanning calorimetry (DSC) in non-isothermal (linear heating) mode. It exhibits two-stage crystallization where the first stage corresponds to the precipitation of icosohedral nano-quasicrystalline phase. This has been confirmed with the help of TEM investigations. The crystallization parameters like the activation energy (Ec) and frequency factor (k0) have been derived using the Kissinger peak shift analysis. The activation energies for the first and second crystallization peak are found to be 278 and 295 kJ mol–1, respectively. The frequency factors obtained for the two peaks are respectively 7.16·1019 and 1.42·1020 s–1. Ec, k0 and the Avrami exponent (n) have also been derived by fitting the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation for the transformed volume fraction (x) to the crystallization data. JMAK results of Ec for the first and second crystallization peak turn out to be 270 and 290 kJ mol–1 respectively. However, k0 and n are found to be heating rate dependent as reported in similar studies. The values of n for the first crystallization stage ranges between 1.66 and 2.57 indicating diffusion-controlled transformation in agreement with earlier reports. 相似文献
Summary: Amorphous syndiotactic polystyrene (sPS) was crystallized at room temperature in Norbornadiene (bicyclo[2,2,1]-hepta-2,5-diene), Mesitylene (1,3,5- Trimethylbenzene), 3-Carene (3,7,7-trimethyl bicyclo[4,1,0]hept-3-ene) and DMN (1,4-Dimethylnaphthalene) to form the sPS-solvent complex (δ form) with respective solvent molecules. In situ HTFTIR studies showed that the δ form to γ form transformation temperature occurs well below the glass transition temperature of sPS, which is depressed due to the presence of solvent in the amorphous phase; higher the solvent content in the complex, lower the transition temperature. Glass transition temperatures determined by Modulated differential scanning calorimetry (MDSC) coincide with the transition temperatures, indicating that the δ form transforms into γ form at the glass transition temperature for these complexes. Such a behavior is very different from the behaviour of the sPS- solvent complexes formed by dichloromethane, chloroform, toluene, o-dichlorobenzene, decalin (cis-trans) etc. and for these complexes the transition occur well above the Tg. 1 相似文献