Frequency Dependence Of Heat Capacity Research Articles

Absolute accuracy in membrane-based ac nanocalorimetry

To achieve accurate results in nanocalorimetry a detailed analysis and understanding of the behavior of the calorimetric system is required. There are especially two system-related aspects that should be taken in consideration: the properties of the empty cell and the effect of the thermal link between sample and cell. Here we study these two aspects for a membrane-based system where heater and thermometer are both in good contact with each other and the center of the membrane. Practical, analytical expressions for describing the frequency dependence of heat capacity, thermal conductance, and temperature oscillation of the system are formulated and compared with measurements and numerical simulations. We finally discuss the experimental conditions for an optimal working frequency, where high resolution and good absolute accuracy are combined.

Dynamics of reversible melting revealed from frequency dependent heat capacity

Heat capacity of semi-crystalline polymers shows frequency dependence not only in the glass transition range. Also above glass transition and below melting temperature a frequency dependent heat capacity can be observed. The asymptotic value of heat capacity at high frequencies equals base-line heat capacity while the asymptotic value at low frequencies yield information about reversing melting. For polycarbonate (PC), poly(3-hydroxybutyrate) (PHB) and syndiotactic polypropylene (sPP) the asymptotic value at high frequencies can be measured by temperature-modulated DSC (TMDSC). For polycaprolactone (PCL) and sPP the frequency dependence of heat capacity can be studied in quasi-isothermal TMDSC experiments. The heat capacity spectra were obtained from single measurements applying multi-frequency perturbations (spikes in heating rate) like in StepScan™ DSC or rectangular temperature–time profiles. Actually, the dynamic range of commercial TMDSC apparatuses is limited and only a small part of the heat capacity spectrum can be measured by TMDSC. Nevertheless, comparison of measured base-line heat capacity with expected values from mixing rules for semi-crystalline polymers yield information about the formation (vitrification) and disappearance (devitrification) of the rigid amorphous fraction (RAF). For PC and PHB the RAF is established during isothermal crystallization while for sPP only a part of the RAF is vitrified during crystallization. Devitrification of the RAF seems to be related to the lowest endotherm.

Frequency dependence of heat capacity of thePd40Ni10Cu30P20amorphous alloy by temperature-modulated calorimetry

The newly developed temperature-modulated differential scanning calorimetry (DSC) has been used to investigate the frequency dependence of heat capacity in the glass transition region for the ${\mathrm{Pd}}_{40}{\mathrm{Ni}}_{10}{\mathrm{Cu}}_{30}{\mathrm{P}}_{20}$ alloy upon heating and cooling. In contrast to conventional DSC results, the present work showed a dissipative behavior of the heat-flow response of the deeply supercooled ${\mathrm{Pd}}_{40}{\mathrm{Ni}}_{10}{\mathrm{Cu}}_{30}{\mathrm{P}}_{20}$ liquid in the glass transition region, qualitatively similar to the results obtained by specific heat spectroscopy on glycerol. A strong dependence of the temperature modulation period on the temperature of the peak imaginary part of complex heat capacity, ${T}_{\mathrm{max}},$ was found indicating a slowdown of the supercooled liquid dynamics as temperature decreased. This frequency dependence of ${T}_{\mathrm{max}}$ can be well described by either the Arrhenius law or the Vogel-Fulcher-Tamman (VFT) equation. Furthermore, the VFT fit to the experimental data showed that the VFT temperature ${T}_{0}$ was coincident with the thermodynamically determined Kauzmann temperature ${T}_{K}.$ The average characteristic time of enthalpy relaxation was determined to be approximately 50 s at 579 K and the apparent activation energy of glass transition was estimated to be $577\ifmmode\pm\else\textpm\fi{}22\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}.$

Characteristic and applicability of a temperature-modulated differential scanning calorimeter for steady state measurement of heat capacity

Temperature-modulated calorimetry, in which an apparatus for temperature-modulated differential scanning calorimetry (DSC) is employed, has a variety of advantages in heat capacity measurement. Namely, the absolute value of heat capacity can be obtained easily, by using periodic signal we can exclude the effects of external noises and drifts, the frequency dependence of heat capacity can be measured, etc. However, it is quite different from differential scanning calorimetry when we consider the principle of its performance, although the apparatus developed for DSC is generally used for temperature-modulated calorimetry as well. In this paper, the difference between them is clarified and based upon it the possibility of the application is discussed.

Simultaneous measurement of heat capacity and thermal diffusivity in solid–solid and solid–liquid phase transitions of n-alkane

A new technique analyzing Fourier spectrum of higher harmonics of temperature wave is proposed, which is called as Fourier Transform Temperature Wave Analysis (FT-TWA). It enables a simultaneous measurement of thermal diffusivity, heat capacity per unit volume, and thermal conductivity at a time by temperature scanning under multiple frequencies. This method was applied to solid–solid and solid–liquid phase transitions of n-alkane, which is usually used as a model of molecular crystal. The frequency dependence of heat capacity and thermal diffusivity in the phase transitions was observed.