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Tianli Zhang - One of the best experts on this subject based on the ideXlab platform.
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The effect of temperature on the crystallization of Zr55CU30Al10Ni5 bulk metallic Glass in the Glass Transition Region
Journal of Alloys and Compounds, 2005Co-Authors: Lilin Liu, Kang Cheung Chan, Tianli ZhangAbstract:Abstract In this paper, the effect of temperature on the isothermal crystallization of Zr55Cu30Al10Ni5 bulk metallic Glass in the Glass Transition Region was investigated by differential scanning calorimetry, X-ray diffraction and scanning electron microscopy. It was found that the crystallization behaviour of Zr55Cu30Al10Ni5 bulk metallic Glass strongly depends on the annealing temperature. The crystallization follows a diffusion-controlled growth mechanism with the formation of a dendritic-like structure in the high temperature Region from 743 to 753 K. However, it follows two different mechanisms, i.e. diffusion-controlled and interface-controlled, with the formation of both dendritic-like and spherical structures, respectively, in the low temperature Region from 723 to 739 K. Furthermore, the activation energy for crystallization from supercooled liquid is also temperature-dependant. The apparent activation energy is about 212 kJ/mol in the high temperature Region, but 314 kJ/mol in the low temperature Region. The different crystallization behaviour is believed to be due to the different structures that the metallic Glass possesses at different temperatures.
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The effect of temperature on the crystallization of Zr55Cu30Al10Ni5 bulk metallic Glass in the Glass Transition Region
Journal of Alloys and Compounds, 2005Co-Authors: Lilin Liu, Kang Cheung Chan, Tianli ZhangAbstract:In this paper, the effect of temperature on the isothermal crystallization of Zr55Cu30Al10Ni5 bulk metallic Glass in the Glass Transition Region was investigated by differential scanning calorimetry, X-ray diffraction and scanning electron microscopy. It was found that the crystallization behaviour of Zr55Cu30Al10Ni5 bulk metallic Glass strongly depends on the annealing temperature. The crystallization follows a diffusion-controlled growth mechanism with the formation of a dendritic-like structure in the high temperature Region from 743 to 753 K. However, it follows two different mechanisms, i.e. diffusion-controlled and interface-controlled, with the formation of both dendritic-like and spherical structures, respectively, in the low temperature Region from 723 to 739 K. Furthermore, the activation energy for crystallization from supercooled liquid is also temperature-dependant. The apparent activation energy is about 212 kJ/mol in the high temperature Region, but 314 kJ/mol in the low temperature Region. The different crystallization behaviour is believed to be due to the different structures that the metallic Glass possesses at different temperatures.Department of Industrial and Systems Engineerin
Salvador Montserrat - One of the best experts on this subject based on the ideXlab platform.
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The application of temperature-modulated DSC to the Glass Transition Region: II. Effect of a distribution of relaxation times
Thermochimica Acta, 2001Co-Authors: John M. Hutchinson, Salvador MontserratAbstract:Abstract An analysis of temperature-modulated differential scanning calorimetry (TMDSC) in the Glass Transition Region is presented. It extends an earlier and simpler model by introducing a distribution of relaxation times, characterised by a Kohlrausch–Williams–Watts (KWW) stretched exponential parameter β, in addition to the usual kinetic parameters of relaxation, namely the Tool–Narayanaswamy–Moynihan (TNM) non-linearity parameter x and the apparent activation energy Δh∗. The present model describes, more realistically than did its predecessor, all the characteristic features of TMDSC in the Glass Transition Region, and it has been used to examine the effects of the important experimental variables, namely the period of modulation and the underlying cooling rate. It is shown that, for typical experimental conditions in practice, it is likely that there well be an interaction between the vitrification process, due to the underlying cooling rate, and the dynamic Glass Transition whereby the complex heat capacity C p ∗ shows a sigmoidal decrease in a temperature range dependent on the modulation frequency. Accordingly, care must be exercised in the quantitative evaluation of TMDSC data in the Glass Transition Region, and suggestions are made regarding the optimum procedures in this respect. Also, by comparing the cooling rate and modulation period required to define the same Transition temperature for conventional DSC and C p ∗ , respectively, a correspondence between them is obtained which allows the magnitude of temperature fluctuations in Donth’s fluctuation dissipation theorem to be evaluated. Finally, it is shown that β and x have similar effects on conventional DSC cooling curves, but have very different effects on C p ∗ , whereby there is little effect of x but a significant broadening of the Transition as β decreases. It is argued that the breadth of the C p ∗ Transition therefore provides a measurement of β independent of the value of x, thus resolving a problem that has existed for some years.
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A theoretical model of temperature-modulated differential scanning calorimetry in the Glass Transition Region
Thermochimica Acta, 1997Co-Authors: John M. Hutchinson, Salvador MontserratAbstract:Abstract A new analysis of an earlier theoretical model of the enthalpic response of Glasses to temperature-modulated differential scanning calorimetry in the Glass Transition Region has been made. This yields quasi-continuous values of the complex specific heat capacity, C∗ p , and its in-phase and out-of-phase components, C′p and C″p, respectively, as well as those of the phase angle,f. This analysis has been used to predict the effects of three parameters: the period, the amount of annealing, and the non-linearity parameter. Increasing the period reduces the mid-point Transition temperature for C′p in a quantitatively similar way to the effect of cooling rate on Tg, from which a relationship between period and cooling rate has been derived and compared with a fluctuation model for the Glass Transition. The amount of annealing is shown to have no effect on the area under the C″p peak, confirming that this out-of-phase component does not provide information about the enthalpic state of the Glass. Finally, the non-linearity parameter has opposite effects on the cooling and heating stages of intrinsic thermal cycles, with reducing x causing the C″p peak to broaden on cooling and to sharpen on heating. For these same intrinsic cycles, the area difference under the C″p peaks on heating and cooling may be either positive or negative, depending on the value of x. This last observation may have implications for the interpretation of C″p in terms of entropy changes occurring during the modulation periods.
Bernhard Wunderlich - One of the best experts on this subject based on the ideXlab platform.
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Reversing and Nonreversing Heat Capacity of Poly(lactic acid) in the Glass Transition Region by TMDSC
Macromolecules, 2005Co-Authors: Marek Pyda, Bernhard WunderlichAbstract:A study of the Glass Transition of an amorphous and a semicrystalline poly(lactic acid) (PLA) is performed with adiabatic calorimetry, differential scanning calorimetry (DSC), and temperature-modulated DSC (TMDSC). The reversing, total, and nonreversing apparent heat capacities of samples with different contents of l- and d-lactic acid and with various thermal histories were evaluated. Different modes of TMDSC analyses of amorphous and semicrystalline PLA were compared to the total heat capacity from standard DSC. The enthalpy relaxation and the cold crystallization in the Glass Transition Region are largely irreversible. The melting is largely irreversible, but a 100% reversing fraction is observed at low temperatures from 375 to 420 K, which becomes small inside the major melting peak at about 440 K. From the TMDSC of amorphous PLA, the combined information on endothermic and exothermic enthalpy relaxation and Glass Transition were deconvoluted into the reversing and nonreversing components. The Glass t...
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Frequency dependence of the heat capacity of polystyrene in the Glass Transition Region measured by multi-frequency light-modulated DSC
Thermochimica Acta, 2002Co-Authors: P. Kamasa, Marek Pyda, A. Buzin, Bernhard WunderlichAbstract:Abstract The reversing heat capacity of polystyrene has been measured in the Glass Transition Region as a function of frequency by temperature-modulated DSC using infra-red light for the modulation. The mean temperature was controlled by the calorimeter operating in the standard DSC mode. The additional power transferred to the specimens is obtained by infra-red diodes with their mean power controlled by a pulse-width modulation method. This method allows to synthesize sinusoidal temperature-profiles with negligible distortion with frequencies of up to 0.125 Hz. The experiments were carried out using multi-frequency temperature-profiles composed by superposition of four sinusoidal modulations of periods of 243, 81, 27, and 9 s, which cover the frequency range from the 1st to the 27th harmonic. To study the kinetics in the Glass Transition Region, the thermal history was produced by cooling the samples at different rates. The results and the advantages of the new method are discussed and compared to a series of less-precise separate single-frequency analyses.
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Modulated differential scanning calorimetry in the Glass Transition Region
Journal of thermal analysis, 1997Co-Authors: Bernhard Wunderlich, Iwao OkazakiAbstract:Temperature-modulated calorimetry (TMC) allows the experimental evaluation of the kinetic parameters of the Glass Transition from quasi-isothermal experiments. In this paper, model calculations based on experimental data are presented for the total and reversing apparent heat capacities on heating and cooling through the Glass Transition Region as a function of heating rate and modulation frequency for the modulated differential scanning calorimeter (MDSC). Amorphous poly(ethylene terephthalate) (PET) is used as the example polymer and a simple first-order kinetics is fitted to the data. The total heat flow carries the hysteresis information (enthalpy relaxation, thermal history) and indications of changes in modulation frequency due to the Glass Transition. The reversing heat flow permits the assessment of the first and higher harmonics of the apparent heat capacities. The computations are carried out by numerical integrations with up to 5000 steps. Comparisons of the calculations with experiments are possible. As one moves further from equilibrium, i.e. the liquid state, cooperative kinetics must be used to match model and experiment.
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Modulated differential scanning calorimetry in the Glass Transition Region
Thermochimica Acta, 1995Co-Authors: A. Boller, Christoph Schick, Bernhard WunderlichAbstract:Modulated differential scanning calorimetry (MDSC) presents two time scales to the measurement of the Glass Transition, a fast one, fixed by the modulation time period, and a slow one, fixed by the average, underlying heating or cooling rate. The same situation arises in dynamic mechanical and dielectric measurements. The various time effects on the Glass Transition are discussed and documented with experiments on the reference material polystyrene. It will be shown that MDSC can measure the Glass Transition temperature largely independent of the thermal history (and the heating or cooling rates), but dependent on the modulation frequency. This distinguishes MDSC from standard calorimetry that can measure accurate data on the Glass Transition only on cooling, a mode of operation often neither convenient nor very precise.
Lilin Liu - One of the best experts on this subject based on the ideXlab platform.
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The effect of temperature on the crystallization of Zr55CU30Al10Ni5 bulk metallic Glass in the Glass Transition Region
Journal of Alloys and Compounds, 2005Co-Authors: Lilin Liu, Kang Cheung Chan, Tianli ZhangAbstract:Abstract In this paper, the effect of temperature on the isothermal crystallization of Zr55Cu30Al10Ni5 bulk metallic Glass in the Glass Transition Region was investigated by differential scanning calorimetry, X-ray diffraction and scanning electron microscopy. It was found that the crystallization behaviour of Zr55Cu30Al10Ni5 bulk metallic Glass strongly depends on the annealing temperature. The crystallization follows a diffusion-controlled growth mechanism with the formation of a dendritic-like structure in the high temperature Region from 743 to 753 K. However, it follows two different mechanisms, i.e. diffusion-controlled and interface-controlled, with the formation of both dendritic-like and spherical structures, respectively, in the low temperature Region from 723 to 739 K. Furthermore, the activation energy for crystallization from supercooled liquid is also temperature-dependant. The apparent activation energy is about 212 kJ/mol in the high temperature Region, but 314 kJ/mol in the low temperature Region. The different crystallization behaviour is believed to be due to the different structures that the metallic Glass possesses at different temperatures.
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The effect of temperature on the crystallization of Zr55Cu30Al10Ni5 bulk metallic Glass in the Glass Transition Region
Journal of Alloys and Compounds, 2005Co-Authors: Lilin Liu, Kang Cheung Chan, Tianli ZhangAbstract:In this paper, the effect of temperature on the isothermal crystallization of Zr55Cu30Al10Ni5 bulk metallic Glass in the Glass Transition Region was investigated by differential scanning calorimetry, X-ray diffraction and scanning electron microscopy. It was found that the crystallization behaviour of Zr55Cu30Al10Ni5 bulk metallic Glass strongly depends on the annealing temperature. The crystallization follows a diffusion-controlled growth mechanism with the formation of a dendritic-like structure in the high temperature Region from 743 to 753 K. However, it follows two different mechanisms, i.e. diffusion-controlled and interface-controlled, with the formation of both dendritic-like and spherical structures, respectively, in the low temperature Region from 723 to 739 K. Furthermore, the activation energy for crystallization from supercooled liquid is also temperature-dependant. The apparent activation energy is about 212 kJ/mol in the high temperature Region, but 314 kJ/mol in the low temperature Region. The different crystallization behaviour is believed to be due to the different structures that the metallic Glass possesses at different temperatures.Department of Industrial and Systems Engineerin
Christoph Schick - One of the best experts on this subject based on the ideXlab platform.
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Phase angle correction for TMDSC in the Glass-Transition Region
Thermochimica Acta, 1997Co-Authors: S. Weyer, A. Hensel, Christoph SchickAbstract:Abstract Temperature-modulated DSC (TMDSC) allows the determination of frequency (time) dependent heat capacities. If heat capacity is frequency dependent, for example, in the Glass-Transition Region, one get a phase angle between heating rate and heat-flow rate which is related to material intrinsic properties. Because heat does not propagate but flows, the time-consuming heat transfer into the sample yields an additional phase angle. This phase angle has to be taken into account if one wants to calculate the imaginary part of the complex heat capacity of the sample from the measured phase angle. The heat transfer related phase angle depends on frequency, heat capacity and heat conductance. In the case of Glass-Transition, heat capacity and heat conductance may change in the respective temperature interval. A correction for the corresponding phase angle is suggested.
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Broad band heat capacity spectroscopy in the Glass-Transition Region of polystyrene
Thermochimica Acta, 1997Co-Authors: S. Weyer, A. Hensel, J. Korus, E. Donth, Christoph SchickAbstract:Abstract The combination of results from temperature modulated DSC (TMDSC) and 3ω-method (HCS) in the Glass-Transition Region allows broad band heat capacity spectroscopy in a frequency range of seven orders of magnitude. In an Arrhenius diagram, the curve for calorimetric data is close to that of dielectric data, except from some remaining problems of temperature calibration. The dynamic Glass-Transition temperature and the width of the Transition interval are determined in the frequency range 10 −4 ν 3 Hz and compared with DSC measurements on linear cooling (vitrification). The dynamic calorimetric data are obtained in the liquid equilibrium state, where the dispersion of the Glass-Transition interval is related to the mean temperature fluctuation of cooperatively rearranging Regions (CRR). As expected from the fluctuation approach to Glass Transition, this temperature fluctuation increases with increasing temperature. This indicates decreasing size of CRRs with increasing temperature. The a = q δT∗ω parameter is obtained as a = 6±2 from a comparison of thermal and dynamic Glass Transition.
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Temperature modulated calorimetry and dielectric spectroscopy in the Glass Transition Region of polymers
Journal of Thermal Analysis and Calorimetry, 1996Co-Authors: A. Hensel, J. Dobbertin, J.e.k. Schawe, A. Boller, Christoph SchickAbstract:The results from temperature modulated DSC in the Glass Transition Region of amorphous and semicrystalline polymers are described with the linear response approach. The real and the imaginary part of the complex heat capacity are discussed. The findings are compared with those of dielectric spectroscopy. The frequency dependent Glass Transition temperature can be fitted with a VFT-equation. The Transition frequencies are decreased by 0.5 to 1 orders of magnitude compared to dielectric measurements. Cooling rates from standard DSC are transformed into frequencies. The Glass Transition temperatures are also approximated by the VFT-fit from the temperature modulated measurements. The differences in the shape of the curves from amorphous and semicrystalline samples are discussed.
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Modulated differential scanning calorimetry in the Glass Transition Region
Thermochimica Acta, 1995Co-Authors: A. Boller, Christoph Schick, Bernhard WunderlichAbstract:Modulated differential scanning calorimetry (MDSC) presents two time scales to the measurement of the Glass Transition, a fast one, fixed by the modulation time period, and a slow one, fixed by the average, underlying heating or cooling rate. The same situation arises in dynamic mechanical and dielectric measurements. The various time effects on the Glass Transition are discussed and documented with experiments on the reference material polystyrene. It will be shown that MDSC can measure the Glass Transition temperature largely independent of the thermal history (and the heating or cooling rates), but dependent on the modulation frequency. This distinguishes MDSC from standard calorimetry that can measure accurate data on the Glass Transition only on cooling, a mode of operation often neither convenient nor very precise.