Temperature Scan Rate Research Articles

Latent heat in the thermoelastic martensitic transformation of Co

In this paper differential scanning calorimetry (DSC) methods were used to study the kinetics of phase transformation in Co. In the authors' experiment measurements performed at constant temperature scanning rates are first done, for they allow both the determination of important thermodynamic parameters, such as enthalpy differences between phases, as well as the characteristic of the kinetics of the transformation considered. The dependence of the latent heat on the scanning rate can be shown in a general power law, with different scaling exponents on heating and cooling. Furthermore, in order to show the dependence of latent heat to thermal history, another designed experiment is introduced. Discussions of the experimental results are also present.

Determination of phase stability in a bulk amorphous alloy by differential scanning calorimetry

Phase transformations in a bulk amorphous alloy have been studied by differential scanning calorimetry (DSC). DSC scans exhibited increasingly complex structure as the temperature scan rate was reduced. A Kissinger analysis yielded transformation time constants and activation energies for several peaks and also for the glass transition. The fact that the Kissinger method fits the glass transition data was surprising but justifiable. Several higher temperature peaks departed from Kissinger behavior, particularly for very low scan rates. The DSC results indicate that the amorphous phase should be stable at temperatures near and above 200°C.

Two-Photon Fluorescence Microscopy Observation of Shape Changes at the Phase Transition in Phospholipid Giant Unilamellar Vesicles

Using the sectioning effect of the two-photon fluorescence microscope, we studied the behavior of phospholipid giant unilamellar vesicles (GUVs) composed of pure diacylphosphatidylcholine phospholipids during the gel-to-liquid crystalline phase transition. We used the well-characterized excitation generalized polarization function (GP ex) of 6-dodecanoyl-2-dimethylamine-naphthalene (LAURDAN), which is sensitive to the changes in water content in the lipid vesicles, to monitor the phase transition in the GUVs. Even though the vesicles do not show temperature hysteresis at the main phase transition, we observed different behaviors of the vesicle shape, depending on how the GUV sample reaches the main phase transition. During the cooling cycles, we observed an increase in the vesicle diameter at the phase transition (∼0.5–1%), followed by a decrease in the diameter when the vesicle reached the gel phase. During the heating cycles and close to the phase transition temperature, a surprising behavior is observed, showing a sequence of different vesicle shapes as follows: spherical-polygonal-ellipsoidal. We attribute these changes to the effect of lipid domain coexistence on the macroscopic structure of the GUVs. The “shape hysteresis” in the GUVs is reversible and largely independent of the temperature scan rate. In the presence of 30 mol% of cholesterol the events observed at the phase transition in the GUVs formed by pure phospholipids were absent.

A method to determine the kinetics of a supercooled liquid by temperature scanning measurements applied to (Li,Na)acetate and GeO 2

Temperature scanning techniques are frequently used to measure glass transitions. For such a measurement, the width of the glass transition regime depends on the fragility of the substance. This paper discusses the degree to which the transition regime becomes smaller when the temperature scan-rate is decreased. Based on the general properties of supercooled liquids it is shown that this decrease may vary by a factor of two, from the most strong to the most fragile substances. To test this prediction, scanning calorimetry data are reported for germanium oxide, a strong glass-forming system, and (Li,Na)acetate, a fragile system. By considering the scanning rate dependence of the width, the second derivative of the glass temperature with respect to the relaxation time can be determined from the data. The parameters of the Vogel–Fulcher equation can be specified with this result from temperature scanning data alone, and the numerical results are compared with equilibrium viscosity measurements. The method demonstrated here to determine the kinetic parameters of a supercooled liquid is well suited for a regime of longer relaxation times, whereas in the case of a constant temperature experiment it is difficult to ensure that a sample is in thermal equilibrium.

Precipitation kinetics in solutionized aluminum alloy 2124: Determination by scanning and isothermal calorimetry

Kinetics and energetics of precipitation in solutionized (SOL) aluminum alloy 2124 have been determined by differential scanning calorimetry (DSC) and differential isothermal calorimetry (DIC). DSC experiments at several temperature scan rates were analyzed by the Kissinger method to give activation energies and rate constants. From the DIC experiments, we obtained kinetics information using a 2-exponential fit, a rate-averaged time constant, and (for GP formation) an Avrami model. It appears that the 2-exponential fit is applicable when two distinct processes contribute to precipitation, while the rate-averaged time constant is appropriate when one process is dominant. Criteria are established for choosing the proper analysis. Activation energies and time constants from DSC and DIC agree fairly well for both GP zone formation and precipitation. Kinetics results for GP zone dissolution in SOL 2124 were obtainable only from DSC experiments. Both calorimetric methods indicate that, after GP zones have formed and dissolved, two mechanisms are involved in precipitation. The results are compared to DSC studies of other workers for similar alloys. TEM studies indicate that the two precipitation mechanisms in alloy 2124 involve formation of S′ (CuMgAl 2) and θ′ (CuAl 2) phases. Δ Q, the heat evolved during GP zone formation and precipitation, was measured isothermally over the 30–300°C range. At the temperature of maximum GP zone formation rate (∼70°C), Δ Q≈−14.7 J/g; at the precipitation maximum (∼270°C) Δ Q≈−27.2 J/g.

X-Ray Diffraction Study of the Lamellar–Hexagonal Phase Transition in Phospholipid/Surfactant Mixtures

The lamellar-to-hexagonal phase transition of a phospholipid/ surfactant mixed system of 1-palmitoyl-2-oleoyl-sn-glycero-3phosphocholine (POPC) and oligo(ethylene oxide)dodecyl ethers of type C12H25O(CH2CH2O)2H(C12E2) in molar surfactant/phospholipid ratio (RS/L) of 2 at low hydration driven by temperature has been studied by X-ray diffraction. The Lβ–HIIphase transition is a reversible two-state process showing hysteresis at fast temperature scan rates. The obtained hexagonal phase exhibits a temperature dependent structural change. The numbers of bound water molecules per composite particle (WS+L) absorbed in the lamellar and hexagonal phases are nearly the same, changing fromWS+L= 5.0 to 4.7 during the phase transition. The fluidity of the alkyl chains on increasing the temperature and the close packing of the hydrophilic molecular parts are the driving parameters of the lamellar-to-hexagonal transformation.

Precipitation kinetics in an air-cooled aluminum alloy: A comparison of scanning and isothermal calorimetry measurement methods

The purpose of this work is to establish for the first time the equivalence of determinations of both precipitation time constants, τ, and activation energies, E act, by two calorimetric techniques: differential scanning calorimetry (DSC) and differential isothermal calorimetry (DIC). To accomplish this, kinetics and energetics of precipitation in air-cooled (ACO) aluminum alloy 339 were determined by both methods, using Perkin–Elmer instruments. The ACO alloy was chosen as the subject of the study because of its calorimetric simplicity: a single precipitation exotherm dominates each DSC scan. The DSC data were analyzed using a modified Kissinger equation, from which both time constants and activation energies were derived. From differential isothermal calorimetry experiments, we determined precipitation time constants by fitting the almost exponential decay of DIC heat release curves, using analyses developed in our laboratory. Arrhenius plots of the time constants then yielded values of E act. Both activation energies and time constants from the DSC/Kissinger analysis agreed rather well with those from DIC provided DSC temperature scan rates were slow compared to the calorimeter's instrumental equilibration time (lag time). Thus, the equivalence of the DSC and DIC techniques has been established, at least for this test case.

Irreversible thermal denaturation of uridine phosphorylase from Escherichia coli K-12

Thermal denaturation of uridine phosphorylase from Escherichia coli K-12 has been studied by differential scanning calorimetry. The excess heat capacity vs. temperature profiles were obtained at temperature scanning rates of 0.25, 0.5, and 1 K/min. These profiles were analysed using three models of irreversible denaturation which are approximations to the whole Lumry–Eyring model, namely, the one-step model of irreversible denaturation, the Lumry–Eyring model with the fast equilibrating first step, and the model involving two consecutive irreversible steps. In terms of statistics the latter model describes the kinetics of thermal denaturation of uridine phosphorylase more satisfactorily than the two other models. The values of energy activation for the first and second steps calculated for the model involving two consecutive irreversible steps are the following: E a,1=609.3±1.8 kJ/mol and E a,2=446.8±3.2 kJ/mol.

Analysis of differential scanning calorimetry data for proteins Criteria of validity of one-step mechanism of irreversible protein denaturation

We consider in this work the analysis of the excess heat capacity C p ex versus temperature profiles in terms of a model of thermal protein denaturation involving one irreversible step. It is shown that the dependences of ln C p ex on 1 T ( T is the absolute temperature) obtained at various temperature scanning rates have the same form. Several new methods for estimation of parameters of the Arrhenius equation are explored. These new methods are based on the fitting of theoretical equations to the experimental heat capacity data, as well as on the analysis of the dependence d(ln C p ex ) d( 1 T ) on 1 T . We have applied the proposed methods to calorimetric data corresponding to the irreversible thermal denaturation of Torpedo californica acetylcholinesterase, cellulase from Streptomyces halstedii JM8, and lentil lectin. Criteria of validity for the one-step irreversible denaturation model are discussed.

Application of the half-width kinetic method on the amine-initiated cross-linking of an epoxy resin with cyclic anhydrides

The utility of a non-isothermal kinetic method—the half-width method—is demonstrated in the determination of the activation energy for amine-initiated cross-linking reactions of the epoxy resin diglycidylether of bisphenol-A with three different cyclic anhydrides: succinic, maleic and cis-1,2-cyclohexanedicarboxylic anhydrides. Triethylamine was employed as an initiator. The cross-linking reactions were kinetically followed by differential scanning calorimetry at various temperature scanning rates. The half-width method furnished values for the energy of activation in the range 60–100 kJ mol −1. These values are similar to those obtained by applying the well-known non-isothermal multiple-scanning rate methods of Kissinger and Osawa but differ significantly from the ones obtained by applying the single-rate method of Barret.

Precipitation Kinetics in an Air-Cooled Aluminum Alloy: A Comparison of Scanning and Isothermal Calorimetry Measurement Methods

ABSTRACTThe purpose of this work is to study precipitation kinetics in air-cooled aluminum alloy 339 by both differential scanning calorimetry (DSC) and differential isothermal calorimetry (DIC) and thereby establish the equivalence of the two methods. To accomplish this, precipitation time constants, τ and activation energies, Eact, were determined by both techniques. Kissinger analysis of the DSC data yielded Eact and τ values. Using DIC we measured precipitation time constants, Arrhenius plots of which gave Eact. Activation energies and τ values from both methods agree provided DSC temperature scan rates are slow compared to the instrumental lag time. Thus, DSC and DIC have been shown to be equivalent (at least for this test case).

Conformational stability of simusan—an exocellular lipopolysaccharideAcenitobacter sp.

By the heating of diluted solutions of simusan—an anionic exocellular lipopolysaccharide of Acenitobacter sp.—an endothermic, presumably, conformational transition occurs. Specific features of this transition are its high cooperativity and the reduction of the transition heat by the decrease of the temperature scanning rate. The temperature of the transition is a linear function of the logarithm of the concentration of NaCl or KCl. The conformational transition appears to be accompanied by an athermic transition that manifests itself in the additional change of the specific heat. Divalent metal ions (Mg2+ and Ca2+) increase the temperature and the enthalpy of the transition to a greater extent than monovalent ions do. The cooperativity of the transition in the presence of divalent metal ions is higher than in the presence of monovalent metal ions. The removal of hydrophobic groups leads to the decrease both of the temperature and the cooperativity of the conformational transition of simusan. © 1996 John Wiley & Sons, Inc.

Conformational stability of simusan—an exocellular lipopolysaccharide Acenitobacter sp.

By the heating of diluted solutions of simusan—an anionic exocellular lipopolysaccharide of Acenitobacter sp.—an endothermic, presumably, conformational transition occurs. Specific features of this transition are its high cooperativity and the reduction of the transition heat by the decrease of the temperature scanning rate. The temperature of the transition is a linear function of the logarithm of the concentration of NaCl or KCl. The conformational transition appears to be accompanied by an athermic transition that manifests itself in the additional change of the specific heat. Divalent metal ions (Mg2+ and Ca2+) increase the temperature and the enthalpy of the transition to a greater extent than monovalent ions do. The cooperativity of the transition in the presence of divalent metal ions is higher than in the presence of monovalent metal ions. The removal of hydrophobic groups leads to the decrease both of the temperature and the cooperativity of the conformational transition of simusan. © 1996 John Wiley & Sons, Inc.

Dynamic mechanical analysis of epoxy composites plasticized by water: Artifact and reality

AbstractThe plasticization of a glass/epoxy unidirectional composite by water has been investigated by Dynamic Mechanical Thermal Analysis (DMTA). The analysis of the thermomechanical spectra of the water aged material revealed the apperance of a splitting of the tan δ peak associated with the α transition. This splitting was found to be dependent on the experimental conditions, i.e., the temperature scan rate, the straining frequency, and the initial water content on the composite. A thermogravimetric analysis (T.G.A) of the wet specimens has shown that this effect could be attributed to an enhanced drying of the specimens above the glass transition, during the DMTA temperature scan. A differential plasticization of the epoxy network at the aging temperature can therefore not be invoked from the appearance of a splitting of the DMTA spectra. In conditions where the sample drying was minimized, plasticization effects on the thermomechanical properties of the composite were found to be temperature independent and reversible after re‐drying of the aged material. Owing to the filling of hygrothermally induced cracks with water, the composite sensitivity to plasticization can be underestimated significantly at elevated aging temperature.

Scaling of thermal hysteresis with temperature scanning rate.

We address several problems concerning thermal hysteresis and its characterization. Based on the time-dependent Ginzburg-Landau theory of the (${\mathrm{\ensuremath{\Phi}}}^{2}$${)}^{3}$ model with O(N) symmetry in the large -N limit, we obtain numerically familiar spindle-shaped thermal hysteresis loops under linearly varying temperature. These hysteresis loops can be presented in the coordinate plane formed by the reduced temperature and its conjugate variable, so that the enclosed areas A of the loops represent directly the dissipation in the cycles. Moreover, this dissipation scales with the rate of temperature scanning R as A=${\mathit{A}}_{0}$+${\mathit{aR}}^{\mathit{n}}$, with n approaching two-thirds, universal for both the mean-field and field theoretical models concerned. The scaling results from the nature of the equilibrium transition point involved.

Potentiodynamic estimation of sensitization of 347 grade SS superheater tubes Part I: Effect of surface finish, scan rate and solution temperature

A nondestructive electrochemical method for estimating the degradation of 347 stainless steel superheater tubes has been investigated using basic electrolytes. It was found that Cr/Fe concentration ratio and the anodic peak current density corresponding to dissolution of precipitates (carbides and sigma phases) in 1m NaOH increased with exposure time at elevated temperatures. The peak current density was observed to increase with electrode surface finish, electrolyte temperature and potential scan rate. A scan rate of 45 mV min−1, temperature below 40° C and surface finish using 1200 grade SiC paper or a mirror finish (3 μm diamond paste) is recommended.

Phase behavior and arrangement of molecular species in mixtures of a mixed chain and a symmetric phosphatidylethanolamine in the gel and fluid phases

1-Octadecanoyl, 2-decanoylphosphatidylethanolamine (C(18:0)C(10:0)PE) has been reported to exhibit mixed interdigitated gel-phase packing of the phospholipid acyl chains (Mason, J.T. and Stephenson, F.A. (1990) Biochemistry 29, 590–598). In contrast, ditetradecanoylphosphatidylethanolamine (C(14:0)C(14:0)PE) packs without significant interdigitation of the phospholipid acyl chains across the bilayer center. In this report, the gel-fluid transition temperatures of C(18:0)C(10:0)PE and C(14:0)C(14:0)PE in multimellar dispersion were determined by fluorescence anisotropy of cis-parinaric acid and trans-parinaric acid with a descending temperature scan rate of 0.67°C/min. The transition mid-points detected for C(18:0)C(10:0)PE with cis-parinaric acid were 19°C in water, 18°C at pH 8.1, and 14°C at pH 10. The phase diagram for C(14:0)C(14:0)PE and C(18:0)C(10:0)PE at pH 10 suggests complete mixing in the fluid phase and considerable immiscibility in the gel phase. Cross-linking of equimolar mixtures of C(14:0)C(14:0)PE and C(18:0)C(10:0)PE with dimethylsuberimidate at pH 10 revealed a random arrangement of the two species in the fluid phase, confirming the notion that C(18:0)C(10:0)PE and C(14:0)C(14:0)PE are miscible in the fluid phase, as determined from the phase diagram. In contrast, cross-linking of the equimolar mixture of C(18:0)C(10:0)PE and C(14:0)C(14:0)PE in the gel phase at 0°C revealed a non-random arrangement, demonstrating and confirming immiscibility in the gel phase.

Modified sample holder for low-temperature deep-level transient spectroscopy, current-voltage and capacitance-voltage measurements

This note describes the design, construction, and test results of a modified sample holder used during low-temperature deep-level transient spectroscopy, current-voltage and capacitance-voltage measurements. This improved sample holder allows temperature scan rates of up to 6 K/min with a temperature shift of less than 1 K. High electrical isolation makes this sample holder also suitable for low-temperature current-voltage and capacitance-voltage measurements.

The Dynamics of the Domain Boundaries and Soft Lattice Mode in SbSI Crystals

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Lamellar/inverted cubic (L.alpha./QII) phase transition in N-methylated-dioleoylphosphatidylethanolamine

Inverted cubic (QII) phases form in hydrated N-methylated dioleoylphosphatidylethanolamine (DOPE-Me). Previous work indicated that QII phases in this and other systems might be metastable structures. Whether or not QII phases are stable has important implications for models of the factors determining the relative stability of bilayer and nonbilayer phases and of the mechanisms of transitions between those phases. Here, using X-ray diffraction and very slow scan rate differential scanning calorimetry (DSC), we show that thermodynamically stable QII phases form slowly during incubation of multilamellar samples of DOPE-Me at constant temperature. The equilibrium L alpha/QII phase transition temperature is 62.2 +/- 1 degree C. The transition enthalpy is 174 +/- 34 cal/mol, about two-thirds of the L alpha/HII transition enthalpy observed at faster scan rates. This implies that the curvature free energy of lipids in QII phases is substantially lower than in L alpha phases and that this reduction is substantial compared to the reduction achieved in the HII phase. The L alpha/QII transition is slow and is not reliably detected with DSC until the temperature scan rate is reduced to ca. 1 degrees C/h. At faster scan rates, the HII phase forms at a reproducible temperature of 66 degrees C. This HII phase is metastable until ca. 72-79 degrees C, where the equilibrium QII/HII transition seems to occur. These results, as well as the induction of QII phases in similar systems by temperature cycling (observed by others), are consistent with a theory of L alpha/QII/HII transition mechanisms proposed earlier (Siegel, 1986c).