Dynamic Specific Heat Research Articles

Specific Heat and Transformations of Water in 1.4 and 1.8 nm Pore-MCMs

Thermodynamic changes on transformations of water in MCMs of 1.4 and 1.8 nm diameter cylindrical pores were investigated by measuring the specific heat, Cp,app, during cooling and during heating at 12 K/h rate in the 160–280 K range. The nature of the slow transformation was further investigated by measuring the dynamic specific heat by using temperature-modulated calorimetry. It led us to establish whether or not contributions to Cp,app are thermally reversible. We found that a thermally reversible, monotonic change in Cp,app of water occurs in 1.4 nm pore-MCM and a sigmoid-shape change occurs in 1.8 nm pore-MCM. Cp,app measured during cooling and during heating is found to be the same at T < 227 K for 1.4 nm pore-MCM, and at T < 232 K for 1.8 nm pore-MCM, as is the real component of the complex specific heat, Cp′. This shows that the sigmoid-shape change in Cp,app is not due to glass–liquid transition of water in the pores. This is further confirmed by showing that the Cp,app features observed for water...

Non-equilibrium phase transition in the kinetic Ising model driven by a propagating magnetic field wave

The two-dimensional ferromagnetic Ising model in the presence of a propagating magnetic field wave (with a well-defined frequency and wavelength) was studied by Monte Carlo simulation. This study differs from similar earlier studies, because in earlier studies the oscillating magnetic field was considered to be uniform in space. The time averaged magnetization over a full cycle (the time period) of the propagating magnetic field acted as the dynamic order parameter. The dynamical phase transition was observed. The temperature variation of the dynamic order parameter, the mean square deviation of the dynamic order parameter, the dynamic specific heat and the derivative of the dynamic order parameter were studied. The mean square deviation of the dynamic order parameter and the dynamic specific heat show sharp maxima near the transition point. The derivative of the dynamic order parameter shows a sharp minimum near the transition point. The transition temperature is found to depend also on the speed of propagation of the magnetic field wave.

Numerical investigation of acceleration effect on heat transfer deterioration phenomenon in supercritical water

It is important to understand the heat transfer deterioration (HTD) phenomenon for specifying cladding temperature limits in the fuel assembly design of supercritical water-cooled reactor (SCWR). In this study, a numerical investigation of heat transfer in supercritical water flowing through vertical tube with high mass flux and high heat flux is performed by using six low-Reynolds number turbulence models. The capabilities of the addressed models in predicting the observed phenomena of experimental study are shortly analyzed. Mechanisms of the effect of flow structures and fluid properties on heat transfer deterioration phenomenon are also discussed. Numerical results have shown that the turbulence is significantly suppressed when the large-property-variation region spreads to the buffer layer near the wall region, resulting in heat transfer deterioration phenomenon. The property variations of dynamic viscosity and specific heat capacity in supercritical water can impair the deterioration in heat transfer, while the decrease of thermal conductivity contributes to the deterioration.

Spectral responses in granular compaction

The slow compaction of a tapped granular packing is reminiscent of the low-temperature dynamics glasses. Here, I study the dynamics of granular compaction by means of a volumetric spectroscopy. While the specific packing volume v displays glassy aging and memory effects at low tapping amplitudes Gamma, the dynamic volumetric susceptibility chi(v)= partial differentialv/ partial differentialGamma displays minimal glassy effects, and its frequency spectrum gives no indication of a rapidly growing time scale. These features are contrast sharply with that found in the dielectric and magnetic susceptibilities of structural and spin glasses. Instead, chi(v) appears to exhibit the behavior of a dynamic configurational specific heat, such as that obtained from computer simulations of spin-glass models. This suggests that the glassy dynamics of granular compaction is controlled by statistically rare processes that diverge from the typical dynamics of the system. From modifications of the dynamical spectrum by finite system size, I suggest that these glassy processes derive from large-scale collective particle rearrangements.

Dynamic and apparent specific heats during transformation of water in partly filled nanopores during slow cooling to 110 K and heating

The specific heat of water in partially and completely filled 4 nm pores of Vycor has been studied by static and dynamic calorimetry. Water was transferred to the pores in controlled amounts of 4.4–22 wt% (i.e., 20–100% filling of pores) isothermally via the vapour phase in a closed system, and measurements were made on cooling from 280 to 110 K at 12 K/h and thereafter on heating from 110 K at the same rate. On cooling, a marginally small exothermic peak appeared at ∼233 K for 20% filling, and the peak grew in intensity with more water in the pores. But it vanished for 100% filling. For 46 and 100% filling of the pores, a sharp crystallization peak appeared at ∼250 K. On heating from 110 K, a broad endothermic peak appeared for all samples, the peak became narrower with increase in the pore-filling and its onset temperature increased. The effect also appeared in the dynamic specific heat but was less pronounced. We envisage that in partially filled pores, H 2O molecules form bulkier clusters attached to the silica wall. The crystallization peak is at ∼233 K and the melting peak at 255 K. The H 2O clusters grow with increase in filling, but vanish for 100% filling. For 46% filling some regions of the pores also contain closely packed H 2O that crystallize at 248 K and melt at ∼260 K (all references to peak). Water in 100% filled pores crystallizes at 250 K and the solid melts at 260 K. Crystallization of 46% filled sample over different T ranges showed broadened endotherm on heating, which was due to merging of two peaks from two-step melting. The rise in the onset temperature and the narrowing of the endotherm with increase in the pore water, are remarkably similar to those observed on increasing the size of 100% filled pores. It is necessary to use both cooling and heating to reveal the structural transformations in nanopores.

Validating the modelling of a gas-jet quenched carburised gear

The extensive Computational Fluid Dynamics (CFD) modelling of cooling using nitrogen jets showed that an optimised array of high-velocity gas jets close to its surface could cool the part at a speed similar to that of oil. When these optimised conditions were applied to an idealised gear form, the model suggested that it could be fully hardened if a nitrogen/hydrogen mixture was used. Physical experiments under exactly the same conditions were carried out to try and validate the model. Unfortunately, although close to the physical results, the model results had some important differences. When the model was modified to correct radiation and dynamic-specific heat effects, there was a closer, but by no means perfect, agreement. The finite-element model used at this stage was thought to model heat transfer accurately, but not steel. Therefore, the heat transfer data were applied to a metallurgical model. The results from this model were superior in some respects, particularly regarding the occurrence of phase changes, although it is - again - not perfect.

Dynamic Specific Heat, Glass Transition, and Non-Debye Nature of Thermal Relaxation in Lithium Borate Glasses

The physical properties of lithium borate glasses, x Li 2 O·(100- x )B 2 O 3 , vary anomalously with the lithium oxide content, known as “borate anomaly”. In this study, the compositional variations of the dynamic glass transition temperature T g ω and the non-Debye nature (or stretched exponentiality β KWW ) in the vicinity of T g have been investigated over a wide composition range. Since T g ω depends on the frequency of the temperature modulation, a more precise composition dependence can be discussed. The trend of T g ω shows a single broad maximum at about x = 26–28. Moreover, the composition dependence of the non-Debye behavior has been presented. It shows a broad maximum at about x = 40 mol %. It is found that β KWW is systematically controlled using the fraction of each intermediate structural unit, particularly the tetrahedron composed of the 4-coordinated boron atom. Both the composition dependences of T g ω and β KWW can be explained using the structure-superposition model.

Interplay between Kondo-like behavior and short-range antiferromagnetism inEuCu2Si2single crystals

The static and dynamic magnetic properties, electrical resistivity, specific heat, and magnetoresistance have been studied in ${\text{EuCu}}_{2}{\text{Si}}_{2}$ single crystals grown by a floating zone method. The magnetic susceptibility exhibits a considerable anisotropy and a steep rise below 10 K for external fields parallel to the $c$ axis but with no evident magnetic ordering in the temperature range of 2--350 K. The data imply a gradual change in the Eu valence as a function of temperature. Electron spin resonance (ESR) measurements reveal a sizeable fraction of stable ${\text{Eu}}^{2+}$ magnetic moments that interact with conduction electrons and develop quasistatic antiferromagnetic correlations on the ESR timescale. The electrical resistivity and specific heat demonstrate the presence of spin fluctuations and Kondo-like behavior, which apparently competes with the antiferromagnetic order. The analysis of experimental data enables to conclude that the remarkable diversity of the physical properties of ${\text{EuCu}}_{2}{\text{Si}}_{2}$ results from the variation of lattice parameters as well as of local crystal chemistry as a consequence of the particular preparation route employed for the growth of single crystals and polycrystals.

NONEQUILIBRIUM PHASE TRANSITIONS IN MODEL FERROMAGNETS: A REVIEW

The thermodynamical behaviors of ferromagnetic systems in equilibrium are well studied. However, the ferromagnetic systems far from equilibrium became an interesting field of research in last few decades. Recent exploration of ferromagnetic systems in the presence of a steady magnetic field are also studied by using standard tools of equilibrium statistical physics. The ferromagnet in the presence of time-dependent magnetic field, shows various interesting phenomena. An usual response of a ferromagnet in the presence of a sinusoidally oscillating magnetic field is the hysteresis. Apart from this hysteretic response, the nonequilibrium dynamic phase transition is also a very interesting phenomenon. In this chapter, the nonequilibrium dynamic phase transitions of the model ferromagnetic systems in presence of time-dependent magnetic field are discussed. For this kind of nonequilibrium phase transition, one cannot employ the standard techniques of equilibrium statistical mechanics. The recent developments in this direction are mainly based on numerical simulation (Monte Carlo). The Monte Carlo simulation of kinetic Ising model, in presence of sinusoidally oscillating (in time but uniform over space) magnetic field, is extensively performed to study the nonequilibrium dynamic phase transition. The temperature variations of dynamic order parameter, dynamic specific heat, dynamic relaxation time etc. near the transition point are discussed. The appearance and behaviors of a dynamic length scale and a dynamic time scale near the transition point are also discussed. All these studies indicate that this proposed dynamic transition is a nonequilibrium thermodynamic phase transition. The disorder (quenched) induced zero temperature (athermal) dynamic transition is studied in random field Ising ferromagnet. The dynamic transition in the Heisenberg ferromagnet is also studied. The nature of this transition in the Heisenberg ferromagnet depends on the anisotropy and the polarisation of the applied time varying magnetic field. The anisotropic Heisenberg ferromagnet in the presence of elliptically polarised magnetic field shows multiple dynamic transitions. This multiple dynamic transitions in anisotropic Heisenberg ferromagnet are discussed here. Recent experimental evidences of dynamic transitions are also discussed very briefly.

A simple model of entropy relaxation for explaining effective activation energy behavior below the glass transition temperature

Strong changes in relaxation rates observed at the glass transition region are frequently explained in terms of a physical singularity of the molecular motions. We show that the unexpected trends and values for activation energy and preexponential factor of the relaxation time tau, obtained at the glass transition from the analysis of the thermally stimulated current signal, result from the use of the Arrhenius law for treating the experimental data obtained in nonstationary experimental conditions. We then demonstrate that a simple model of structural relaxation based on a time dependent configurational entropy and Adam-Gibbs relaxation time is sufficient to explain the experimental behavior, without invoking a kinetic singularity at the glass transition region. The pronounced variation of the effective activation energy appears as a dynamic signature of entropy relaxation that governs the change of relaxation time in nonstationary conditions. A connection is demonstrated between the peak of apparent activation energy measured in nonequilibrium dielectric techniques, with the overshoot of the dynamic specific heat that is obtained in calorimetry techniques.

Multiple dynamic transitions in an anisotropic Heisenberg ferromagnet driven by polarized magnetic field.

A uniaxially (along the Z axis) anisotropic Heisenberg ferromagnet, in the presence of time-dependent (but uniform over space) magnetic field, is studied by Monte Carlo simulation. The time-dependent magnetic field was taken as elliptically polarized where the resultant field vector rotates in the X-Z plane. The system is cooled (in the presence of the elliptically polarized magnetic field) from high temperature. As the temperature decreases, it was found that in the low anisotropy limit the system undergoes three successive dynamical phase transitions. These three dynamic transitions were confirmed by studying the temperature variation of dynamic "specific heat." The temperature variation of dynamic specific heat shows three peaks indicating three dynamic transition points.

Dynamics of Random One-Dimensional ±J Systems

Finite chains of a two-state random Potts spin model with periodic boundary conditions are studied within Glauber dynamics. The spin exchange is assumed random with frustration between ferro and antiferromagnetic values (±J). Time-dependent fluctuations are induced by periodic temperature oscillations. Master type differential equations for spin correlation functions are solved within linear response theory. The spectrum of relaxation times are calculated at different temperatures. The ±J Potts glass chains undergo a zero temperature phase transition. The barriers against inversion of the spin chain take only two values; 0 and 2|J|. The temperature behaviour of specific heat is characterized by rounded peaks. The frequency dependence displays two plateaus for the real part of specific heat and two corresponding peaks for the imaginary part. The dynamic specific heat is not affected by the longest relaxing mode like susceptibility. The time separation of the modes is demonstrated by the Cole-Cole plots.

A Discussion of the Dynamic Specific Heat during Solidification

State-of-the-art solidification theory shows that the apparent latent heat of fusion decreases with increasing cooling rates, due to the creation of lattice defects. A portion of the defects conden ...

Two-phase coexisting state of n-hexatriacontane in the first-order phase transition

The small crystal of n-hexatriacontane was observed by a polarizing microscope in the rotator phase transition temperature region. In the temperature region, the rotator phase coexists with the solid phase (low-temperature ordered phase). The boundaries of two phases move reversibly with the temperature change. The area fractional change of the rotator phase can be described by the Debye relaxation. The relaxation time decreases and the relaxation strength increases as the sample temperature is raised. The relaxation time agrees well with that of the dynamic specific heat, which was measured in the frequency range of 0.0003≤ f/Hz≤0.05.

Method of low-frequency dynamic specific heat measurement in millimetre-scalesamples

We propose a method to obtain the dynamic specific heat at hundreds ofmicrohertz and higher. This low-frequency measurement is needed in order togather the data covering the wide frequency dispersion of the dynamicspecific heat in the temperature regions of the first-order phase transitions.By using the method, we can use medium-size cylindrical samples, theradius and the thickness of which are several millimetres. Although asample of this size has the advantage of being easily handled, systematicmeasurement and analysis using a millimetre-size sample to obtain thelow-frequency dynamic specific heat has not been attempted before. Weintroduce the results of the dynamic specific heat and thermal conductivity atthe frequencies from 0.0003 to 0.05 Hz obtained by using the method.

The static and dynamic specific heat of undercooled metallic liquids

Experimental data on the equilibrium (static) specific heat of undercooled metallic liquids are presented that are representative for different classes of constitutive behavior ranging from pure noble metals to alloy systems that are characterized by strong tendencies to form self-associates or associates of unlike atoms in the liquid state. Deviations from a monotonous course of the specific heat that are expected to occur as a characteristics for an accelerated evolution of ordering processes in the undercooled melt excluding the crystalline state are discussed and selected examples are compared to the results of theoretical model calculations. Besides these measurements of the static heat capacity, modulation calorimetry provides the possibility to detect characteristic times of slow relaxation modes of the deeply undercooled melt that are coupled to the occurrence of the glass transition upon continued cooling. Nevertheless, such data are only scarcely available for metallic glass formers and have not yet been quantitatively analyzed with respect to the intrinsic relaxation times of the deeply undercooled liquid state. In this contribution, modulated temperature calorimetry measurements on bulk glass forming Pd alloys are reported. The results that reveal the typical behavior known for dissipative systems are evaluated with respect to the temperature dependence of the mean relaxation time as well as the shape of the relaxation time spectrum in the vicinity of the structural glass transition.

Relaxation phenomenon measured as dynamic specific heat in the first-order phase transition of a molecular crystal

We found that the specific heat becomes a frequency-dependent complex quantity at and around the two first-order phase transition points of n-hexatriacontane $(n\ensuremath{-}{\mathrm{C}}_{36}{\mathrm{H}}_{74}).$ One is the rotator phase transition point, and the other is the melting point. The dynamic specific heat was measured by using temperature-modulated calorimetry in the low-frequency range of $0.0003l~f/\mathrm{Hz}l~0.05.$ The frequency range is sufficient to discuss the dispersion of the dynamic specific heat. The dynamic specific heat can be described by the Debye relaxation. The relaxation parameters show different trends in the lower-half temperature region and a higher half-temperature region in each phase transition. We label the former by ${I}_{R}$ and ${I}_{M},$ and the latter as ${\mathrm{II}}_{R}$ and ${\mathrm{II}}_{M}$ ${(I}_{R}$ and ${\mathrm{II}}_{R}$ are in the rotator phase transition, and ${I}_{M}$ and ${\mathrm{II}}_{M}$ are in the melting). The values of the relaxation time are about 600 s in ${I}_{R}$ and ${I}_{M}.$ The values reduce to about 40 s in ${\mathrm{II}}_{R}$ and about 120 s in ${\mathrm{II}}_{M}.$ The relaxation strengths in ${I}_{R}$ and ${I}_{M}$ are smaller than those in ${\mathrm{II}}_{R}$ and ${\mathrm{II}}_{M}.$ We found that the thermal conductivity is a real and frequency-independent quantity in the entire temperature region including the phase transition points. The relaxation time of the dynamic specific heat measured in the end of ${\mathrm{II}}_{R}$ agrees with the characteristic time measured by using a polarizing microscope. This agreement reveals that the origin of the obtained dynamic specific heat in the temperature region is the weight fractional fluctuation of the rotator phase.

A new light driven spectrometer for the determination of complex heat capacities and conductivities by combination of effusivity and diffusivity measurements

We describe the construction, performance and first application of a spectrometer for measurements of frequency-dependent complex thermal diffusivities and effusivities. Knowledge of these quantities enables the complex dynamic specific heat and the thermal conductivity to be calculated. The technique uses the properties of thermal waves initiated at the surface of plate-like samples by the absorption of light with an oscillating intensity. A registration of the resulting temperature oscillations in the absorbance layer at the upper surface of the sample yields the effusivity, an analogous measurement at the bottom gives the diffusivity. The detection of both amplitudes and phases of the signals leads to complex quantities. Measurements encompass a frequency range from 10 −2 to 10 2 Hz. The apparatus was used in a study of the reversible surface melting of poly(ethylene oxide). Both the magnitude and the characteristic time range of this process could be determined.

OFF-AXIAL DYNAMIC SYMMETRY BREAKING IN UNIAXIALLY ANISOTROPIC HEISENBERG FERROMAGNET

The dynamics of uniaxially (along the z direction) anisotropic Heisenberg ferromagnets (in three dimensions) in the presence of a magnetic field varying sinusoidally in time (along the x-direction only) is studied by Monte Carlo method using Metropolis rate. The time averaged (over a complete cycle of the oscillating field) value of z component of the magnetization continuously vanishes at a particular transition temperature associated with a dynamic symmetry breaking of mz-hx loop. The temperature variation of dynamic order parameter and the dynamic specific heat for different values of the anisotropy show that the transition temperature increases as the strength of anisotropy increases.

Dynamic Specific Heat of a Metallic Glass-Former

AbstractModulated temperature calorimetry measurements on the bulk glass forming Pd40Ni40P20 alloy are reported in the frequency range between 1/30 s−1 and 1/1000 s−1. These measurements represent the first experimental assessment of the slow relaxation modes of a metallic liquid over a substantial frequency interval. The results reveal the typical behavior known for dissipative systems. The frequency-dependence of the dynamic specific heat is compared to results of dynamic rheology measurements that have been performed on the same material at similar modulation frequencies to compare the dynamic response of the respective modes.