Transition Temperature Ttr Research Articles

First Phase Nucleation And Growth Of Titanium Disilicide With An iPhasis On The Influence Of Oxygen

ABSTRACTThe existence of a TiSi2phase in the C49 (ZrSi2) structure prior to formation of the stable low resistivity C54 TiSi2structure has been justified for Ti-Si thin film diffusion couples after low temperature (500°C to 650°C) anneal. The transition temperature (Ttr) to the C54 phase is shown to be dependent on layer thickness, thinner films yielding higher Ttr. We could not detect an influence of oxygen on Ttr. Moreover the growth rate of C49 TiSi2is independent of oxygen contamination in the Ti layer up to a mean concentration of 23 at.%. It is postulated that the transition from the C49 to the C54 phase actually is a recrystallization process.

Tb2(Mo O4)3 spontaneous shear strain measurements and free energy expression

The spontaneous shear strain uxy versus the temperature is measured by the y ray diffractometry technique in the (300 K-430 K) range. The first-order type of the ferroelectric-ferroelastic transition is clearly demonstrated (discontinuity of some 100 for uxy at the transition temperature Ttr) : our data are in accordance with phenomenological descriptions corresponding to the free energy expressions with four adjustable parameters.

Far‐infrared and Raman spectroscopy of the phase transition in CsCuCl3

AbstractFar‐infrared reflectivity (30–400 cm−1), submillimetre and millimetre transmissivity (5–30 cm−1) and Raman scattering measurements on CsCuCl3 single crystals have been performed for several temperatures below and above the phase transition temperature Ttr = 423 K. The spectra above Ttr indicate the non‐polar D structure (CsNiCl3‐type) with a high degree of disorder. The millimetre permittivity is temperature independent in both phases in contrast to previous static measurements. Assignment of most modes is done by means of symmetry considerations. The A2 mode at 50 cm−1 which is connected with the Cs vibrations along the hexagonal axis, undergoes an increase of damping at Ttr which may be connected with the increased conductivity above Ttr. Well‐underdamped, very low‐frequency modes are observed in the A1 spectrum at 27 cm−1, A2 spectrum at 17 cm−1 and E2 spectrum at 24 cm−1. In accordance with the group‐theoretical expectation, they are believed to correspond to fluctuations of the amplitude and phase of the helicoidal modulation in the ordered phase and to fluctuations of the enantiomorphic modulation, respectively. CsCuCl3 presents the first example of a transition into a non‐polar phase where the symmetry analysis shows the soft mode (here the phason component) to be infrared‐active.

Studies on conformation of soluble and immobilized enzymes using differential scanning calorimetry. 1. Thermal stability of nicotinamide adenine dinucleotide dependent dehydrogenases

The technique of differential scanning calorimetry (DSC) has been applied to the study of temperature-induced irreversible denturation and thus to the heat stability of soluble and Sepharose-bound liver alcohol dehydrogenase (LADH, EC 1.1.1.1) and lactate dehydrogenase (LDH, EC 1.1.1.27) in the presence of various coenzymes or coenzyme fragments. The transition temperature (Ttr) of 82.5 degrees C obtained for soluble LADH was increased by 12.5 degrees C in the presence of a saturating concentration of NACH. In the presence of NAD+, Ttr increased by 8.5 degrees C, whereas ADP-ribose and AMP caused an increase in Ttr of only 2 and 1 degree C, respectively. The Ttr of 85.5 degrees C obtained for Sepharose-bound LADH was increased by about 12 degrees C after the addition of free NADH. However, when the enzyme was immobilized simultaneously with a NADH analogue (which also binds to the matrix), a broad endotherm with a Ttr of 91.5 degrees C was obtained, indicating the presence of immobilized enzyme molecules both with, and without, associated NADH. Corresponding increases in heat stability were observed for LDH in solution in the presence of NADH, NAD+, and AMP, leading to increases in Ttr from 72 to 79.5 and 74 and 73 degrees C, respectively. The addition of pyruvate and NAD+ to the enzyme to form an abortive ternary complex led to the same stabilization as that observed with NADH, attendant with a large increase in the enthalpy of transition, deltaHtr. In these studies the technique of DSC was utilized because it is applicable both to soluble and immobilized enzymes and (1) provides rapid information about Ttr and thus thermal stability of enzymes, (2) different energetic states of an enzyme molecule can be identified, and (3) an overall picture of the thermal process is rapidly obtained.

Calorimetric measurement of enthalpy change in the isothermal helix-coil transition of poly(L-ornithine) in aqueous solution.

AbstractThe enthalpy change associated with the isothermal pH‐induced uncharged coil‐to‐helix transition ΔHh° in poly(L‐ornithine) in 0.1 N KCl has been determnined calorimetrically to be −1530 ± 210 and −1270 ± 530 cal/mol at 10° and 25°C, respectively. Titration data provided information about the state of charge of the polymer in the calorimetric experiments, and optical rotatory dispersion data about its conformation. In order to compute ΔHh°, the observed calorimetric heat was corrected for the heat of breaking the sample cell, the heat of dilution of HCl, the heat of neutralization of the OH− ion, and the heat of ionization of the δ‐amino group in the random coil. The latter was obtained from similar calorimetric measurements on poly(D,L‐ornithine). Since it was discovered that poly(L‐ornithine) undergoes chain cleavage at high pH, the calorimetric measurements were carried out under conditions where no degradation occurred. From the thermally induced uncharged helix–coil transition curve for poly(L‐ornithine) at pH 11.68 in 0.1 N KCl in the 0°–40°C region, the transition temperature Ttr and the quantity (∂θh/∂T)Ttr have been obtained. From these values, together with the measured values of ΔHh°, the changes in the standard free energy ΔGh° and entropy ΔGh°, associated with the uncharged coil‐to‐helix transition at 10°C have been calculated to be −33 cal/mol and −5.3 cal/mol deg, respectively. The value of the Zimm–Bragg helix–coil stability constant σ has been calculated to be 1.4 × 10−2 and the value of s calculated to be 1.06 at 10°C, and between 0.60 and 0.92 at 25°C.

Ferroelectric transition in Co-I-Boracite under pressure

The effect of hydrostatic pressure up to 103 atm has been measured on the ferroelectric phase transition temperature of Co3B7O13l. It was found that δTtr/δp = -11.1 × 10-3 deg/atm on cooling while δTtr/δp = -8.1 × 10-3 deg/atm on heating so that thermal hysteresis which reflects the 1st order nature of the transition increases with increasing pressure. Experimental data have been related to the thermodynamic theory developed by Dvorˆak which accounts for the fact that polarization is not the parameter of the transition in boracites. It is shown that the pressure dependence of the ideal transition temperature Ttr is governed by the interaction of stress with nonhomogeneous lattice modes and co-governed by the electrostrictive interaction. On the other hand, maximum thermal hysteresis. permittivity above Ttr and spontaneous polarization depend on pressure due to electrostriction only.

Visco-elastic properties of some homopolymers and copolymers in the glass-rubber transition region

In experiments on polyvinyl acetate and a number of copolymers of ethylene and vinyl acetate with different ethylene contents it was found that the frequency at which the dielectric loss factore″ is maximum on thee″-frequency curve decreases very strongly at first to about 10−4 c/s and then less strongly with decreasing temperature. In addition, the area of this dielectric dispersion peak is strongly dependent on temperature in a temperature region near the glass temperatureTg determined dilatometrically. In this temperature region the width of the peak is virtually constant. The dielectric transition temperatureTtr is defined as the temperature at which the area of the dispersion peak is equal to half the sum of the (small) area at low temperature and the maximum area above the transition temperature. This definition of a transition temperature is consequently not based on a given measuring frequency. For polyvinyl acetateTtr≃Tg, as obtained from dilatometry and refractometry. Interference with the measuring results of a strong dependence of the time scale of temperature variations on the transition was not observed.

A MELTING POINT FOR THE BIREFRINGENT COMPONENT OF MUSCLE

The A filament of the striated muscle sarcomere is an ordered aggregate of one or a few species of proteins. Ordering of these filaments into a parallel array is the basis of birefringence in the A region, and loss of birefringence is therefore a measure of decreased order. Heating caused a large decrease in the birefringence of glycerinated rabbit psoas muscle fibers over a narrow temperature range ( approximately 3 degrees C) and a large decrease in both the birefringence and optical density of the A region of Drosophila melanogaster fibrils. These changes were interpreted as a loss of A filament structure and were used to define a transition temperature (T(tr)) as a measure of the stability of the A region. Since the transition temperature was sensitive to pH, ionic strength, and urea, solvent conditions which often affect protein structure, it is an experimentally useful indicator for factors affecting the structure of the A filament. Fibers from glycerinated frog muscle were less stable over a wide pH range than fibers from glycerinated rabbit muscle, a fact which demonstrates a species difference in structure. Glycerinated rabbit fibrils heated to 70 degrees C shortened to about 40% of their initial length. The extent of shortening was not correlated with the loss of birefringence, and phase-contrast microscopy showed that this shortening occurred in the I region as well as in the A region. This response may be useful for studying the I filament and actin in much the same way that the decrease in birefringence was used for studying the A filament and myosin. The observations presented show that some properties of muscle proteins can be studied essentially in situ without the necessity of first dispersing the structure in solutions of high or low ionic strength.