Dependence Of Glass Transition Research Articles

Kinetics of Phase Transformation in Nanostructured Ga–Se–Te Glasses

Bulk samples of Ga25Se(F5-x)Te(x) chalcogenides were prepared by melt quenching technique. The glassy nature of these alloys was verified by X-ray diffraction. The morphology of these as-prepared alloys was studied using field emission scanning electron microscopy. Kinetics of crystallization in these glassy alloys was studied under non-isothermal conditions at different heating rates (5, 10, 15, 20 and 25 K/min) using differential scanning calorimetry. The value of glass transition and crystallization temperature was found to be composition and heating-rate dependent. The value of order parameter indicates that the crystallization was due to volume nucleation with two dimensional growths. The activation energy of crystallization and activation energy of glass transition were determined from the heating rate dependence of glass transition and crystallization temperature. Both the activation energy for glass transition and the activation energy of crystallization were found to decrease from 130.05 to 102.99 kJ/mole and from 130.05 to 102.99 kJ/mole respectively with increasing Te content in Ga-Se system.

Phase transformation and crystallization kinetics of melt-spun Ni60Nb20Zr20 amorphous alloy

The glass transition and crystallization kinetics of melt-spun Ni60Nb20Zr20 amorphous alloy ribbons have been studied under non-isothermal and isothermal conditions using differential scanning calorimetry (DSC). The dependence of glass transition and crystallization temperatures on heating rates was analyzed by Lasocka's relationship. The activation energies of crystallization, Ex, were determined to be 499.5kJ/mol and 488.6kJ/mol using the Kissinger and Ozawa equations, respectively. The Johnson–Mehl–Avrami equation has also been applied to the isothermal kinetics and the Avrami exponents are in the range of 1.92–2.47 indicating a diffusion-controlled three-dimensional growth mechanism. The activation energy obtained from the Arrhenius equation in the isothermal process was calculated to be Ex=419.5kJ/mol. The corresponding three dimensional (3D) time–temperature–transformation (TTT) diagram of crystallization for the alloy has been drawn which provides the information about transformation at a particular temperature. In addition, the intermetallic phases and morphology after thermal treatment have been identified by X-ray diffraction (XRD) and scanning electron microscope (SEM).

Heating rate dependence of glass transition and primary crystallization of Al88Gd6Er2Ni4 metallic glass

Abstract The kinetics of glass transition and the primary crystallization of Al 88 Gd 6 Er 2 Ni 4 metallic glass have been investigated by differential scanning calorimetry. The crystallization kinetics have a stronger dependence on the heating rate than the glass transition does and hence T g is only detectable at heating rates above 0.67 K/s. The activation energies for the primary crystallization are different depending on the state of the matrix phase, i.e. amorphous solid or supercooled liquid.

Polyetherimides for Gas Separation Membranes

A series of polyetherimides were synthesized by polycondensation reaction at high temperature of 2,2-bis[4,4–(3,4–dicarboxyphenoxy)phenyl]propane dianhydride with various aromatic diamines. These polymers are easy soluble in polar aprotic solvents such as N-methylpyrrolidinone, dimethylformamide or dimethylacetamide or even in less polar liquids such as chloroform. They show high thermal stability, with decomposition temperature being above 400°C and glass transition temperature in the range 200–275°C. Polymer solutions in chloroform were processed into thin films which were tested as gas separation membranes. Transport parameters for light gases were measured. The dependence of glass transition and decomposition temperature on conformational rigidity parameters was calculated.

Electrical and thermal properties of a-(Se70Te30)100−x(Se98Bi2)x (0⩽x⩽20) alloys

Abstract In electrical properties, the dc conductivity and photoconductivity measurements have been made in vacuum evaporated thin films of a-(Se 70 Te 30 ) 100− x (Se 98 Bi 2 ) x system, in the temperature range (308–355 K). It has been observed that dc conductivity and activation energy depend on the Bi concentration. Photocurrent dependence on incident radiation has also been observed which follow the power law ( I ph ∝ F γ ). Transient photocurrent exhibits the non-exponential decay time. All these parameters show that the recombination within the localized states is predominant. In crystallization kinetics, the heating rate dependence of glass transition and crystallization temperatures is studied to calculate the activation energy for thermal relaxation and activation energy for crystallization. The composition dependence of the activation energy for thermal relaxation and activation energy for crystallization is discussed in terms of the structure of Se–Te–Bi glassy system.

The Noncrystalline State Within PET Fibers—Meaning and Characterization by Mechanical-Relaxation Measurements

The performance of synthetic fibers cannot be completely understood without any knowledge of the structure in the noncrystalline regions. X-ray diffraction measurements led to the conclusion that the noncrystalline regions in PET fibers can be divided into isotropic noncrystalline (amorphous) and oriented noncrystalline fractions, the amount of which is dependent on the thermal and mechanical history. But x-ray diffraction data, thermal analysis, and dyeing experiments left many effects unexplained: the dependence of glass-transition and dye uptake on thermal history, the real order-disorder situation within the noncrystalline regions, certain mechanical properties, and their time-dependence such as relaxation. The glass-transition temperature Tg of PET fibers, as determined by viscoelastic measurements, runs through a maximum depending on the heat-setting conditions. This can be explained by the assumption that the average size of crystallites reaches a minimum, and therefore the number of crystallites per unit volume shows maximum values at a heat-setting temperature of about 190°C. By raising the heat-setting temperature above 190°C, the crystal lites become larger. Segregation of the crystallites in the noncrystalline matrix occurs. If the network formed by the crystallites has its highest density, Tg shows maximum, and dye uptake, minimum values due to low molecular mobility. For the same reason the storage modulus in the rubber state has maximum values after heat-setting at temperatures around 190°C.