Heat Capacity Peak Research Articles

Crossover and splitting of the heat capacity peak

The fine structure of the peak of heat capacity C(p), which arises at the crossover point upon changing an external parameter p, is considered theoretically. If the hybridization of the states does not occur in the neighborhood of this point, then C(p) has the form of a double Schottky peak. The structure of the C(p) peculiarity becomes more complicated in the presence of the off-diagonal matrix elements and strongly depends on the relationship between temperature and the magnitude of the interaction between the levels. The experimental study of the phenomenon can give information about the relatively weak interatomic interactions in solids or molecules, e.g. about weak exchange interaction in paramagnets with an anisotropy of the “easy plane” type.

Persistent Medium-Range Order and Anomalous Liquid Properties ofAl1−xCuxAlloys

The development of short-to-medium-range order in atomic arrangements has generally been observed in noncrystalline solid systems such as metallic glasses. Whether such medium-range order (MRO) can exist in materials at well above their melting or glass-transition temperature has been a long-standing important scientific issue. Here, using ab initio molecular dynamics simulations, we show that a novel, persistent MRO exists in liquid Al-Cu alloys near the composition of CuAl3. The correlated atomic motions associated with the MRO give rise to a substantially enhanced viscosity in the vicinity of the composition. The component of the MRO liquid state gradually decreases with increasing temperature, and it disappears above a crossover temperature T(LLC). The continuous liquid-liquid crossover through a percolationlike transition leads to a pronounced heat capacity peak at T(LLC).

Phase Behavior of a Lipid Bilayer System Studied by a Replica-Exchange Molecular Dynamics Simulation

A replica-exchange molecular dynamics (REMD) simulation of a lipid bilayer system has been performed in order to study the sol–gel phase transitions. The REMD method enhances conformational sampling efficiency, and one can study the system in a wide temperature range at once. A coarse-grained model MARTINI was used. The results show sudden changes in enthalpy, bilayer thickness, and area of bilayer around 296 K. A peak in heat capacity was also observed around this temperature. These results suggest sol–gel phase transitions. We also investigated temperature dependences of composite energy terms. According to potential of mean force maps and tilt angle distributions, the bilayer is found to have two states in the gel phase. One is a tilted gel state and the other is an un-tilted gel state. A previous work with MARTINI force field reported only the un-titled gel state. Another previous work with MARTINI force field reported titled gel state only with externally applied tension. This suggests that enhanced ...

Comment on “Rapid chemical and topological ordering in supercooled liquid Cu46Zr54”

In their recent publication, Wessels et al. [Phys. Rev. B 83, 094116 (2011)] present evidence of rapid ordering in a supercooled Cu${}_{46}$Zr${}_{54}$ liquid. A part of their supporting argument is based on an experimentally observed peak in the specific heat capacity of a Cu${}_{46}$Zr${}_{54}$ melt as a function of temperature. The experimental observations are supported by classical molecular dynamics (MD) simulations for which the phase trajectory is modeled using an embedded-atom model. We believe that (i) the hypothesis that has led to the determination of the specific heat capacity as a function of temperature, (ii) the MD general simulation procedure, and (iii) the interpretation of the experimental evidence are not fully justified, and this alters some of the conclusions of their paper.

Tunneling Defect Nanoclusters in hcp 4He Crystals: Alternative to Supersolidity

A simple model based on the concept of resonant tunnelling clusters of lattice defects is used to explain the low temperature anomalies of hcp 4He crystals (mass decoupling from a torsional oscillator, shear modulus anomaly, dissipation peaks, heat capacity peak). Mass decoupling is a result of an internal Josephson effect: mass supercurrent inside phase coherent tunnelling clusters. Quantitative results are in reasonable agreement with experiments.

Multibaric–Multithermal Ensemble Study of Liquid–Solid Phase Transition in Lennard-Jones Particles

Molecular dynamics simulations in a multibaric–multithermal ensemble have been carried out for Lennard-Jones systems of 108 and 256 particles. Using reweighting techniques, the temperature dependence of the isobaric heat capacity is calculated, and a liquid–solid coexistence curve on the pressure-temperature plane is also determined from heat capacity peaks. The curve is in good agreement with those obtained by other methods in previous reports.

Performance characterization of PCM impregnated gypsum board for building applications

Previous research studies conducted on building components containing a phase-change material (PCM) have shown a great potential for direct and indirect energy and cost savings in the building envelopes. In particular, PCM impregnated gypsum boards, one of the most popular application of PCMs in buildings, have been reported to reduce building cooling loads by 7−20%. However, in order to best design and optimize the PCM-enhanced building materials, it is critical to accurately characterize the dynamic thermal properties such as enthalpy curve, volumetric heat capacity, sub-cooling, hysteresis − of these PCM-enhanced components. In addition, test data on these dynamic characteristics is necessary for whole-building simulations, energy analysis, and energy code work. In the past, the only existing readily-available method of thermal evaluation of PCMs utilized the Differential Scanning Calorimeter (DSC) methodology. Unfortunately, this method required small and relatively uniform test specimens. This requirement is unrealistic in the case of many PCM-enhanced building envelope products. Small specimens are not representative of PCM-based blends with gypsum, concretes, fiber insulations, plastic foams etc., since these materials are often not homogeneous. In this paper, dynamic thermal properties such of a ½” thick PCM impregnated gypsum board are analyzed based on a novel dynamic experimental procedure: using the conventional HFMA. The gypsum board tested in this work contained 20−25% by weight of a microencapsulated PCM with latent heat of ∼120kJ/kg. First, the theoretical details of the dynamic HFMA (DHFMA) are described. In essence, top and bottom plates of the HFMA are set to the same temperature and heat flow signals from the corresponding heat flux meters are integrated over time to compute the enthalpy changes during a temperature step change. Volumetric heat capacity profile is determined by taking the slope of the enthalpy curve. A negligible sub-cooling and hysteresis is observed for the PCM impregnated gypsum board. In addition, thermal properties such as onset of melting and solidification, and sensible heat of the specimen when PCM was in solid and liquid state were also determined. Dynamic properties such as heat capacity profiles and peaks of melting and solidification cycles, and amount of sub -cooling as measured by DHFMA were found to be relatively close to the DSC results on the same microencapsulated PCM.

Neutral and charged gallium clusters: structures, physical properties and implications for the melting features

We report the putative Global Minimum (GM) structures and electronic properties of Ga(N)(+), Ga(N) and Ga(N)(-) clusters with N = 13-37 atoms, obtained from first-principles density functional theory structural optimizations. The calculations include spin polarization and employ an exchange-correlation functional which accounts for van der Waals dispersion interactions (vdW-DFT). We find a wide diversity of structural motifs within the located GM, including decahedral, polyicosahedral, polytetrahedral and layered structures. The GM structures are also extremely sensitive to the number of electrons in the cluster, so that the structures of neutral and charged clusters differ for most sizes. The main magic numbers (clusters with an enhanced stability) are identified and interpreted in terms of electronic and geometric shell closings. The theoretical results are consistent with experimental abundance mass spectra of Ga(N)(+) and with photoelectron spectra of Ga(N)(-). The size dependence of the latent heats of melting, the shape of the heat capacity peaks, and the temperature dependence of the collision cross-sections, all measured for Ga(N)(+) clusters, are properly interpreted in terms of the calculated cohesive energies, spectra of configurational excitations, and cluster shapes, respectively. The transition from "non-melter" to "magic-melter" behaviour, experimentally observed between Ga(30)(+) and Ga(31)(+), is traced back to a strong geometry change. Finally, the higher-than-bulk melting temperatures of gallium clusters are correlated with a more typically metallic behaviour of the clusters as compared to the bulk, contrary to previous theoretical claims.

Characterization of Magnetic Transformation at Curie Temperature in Finemet-type Microwires by DSC

ABSTRACTFinemet-type amorphous alloys are good soft magnetic materials due to their amorphous-nanocrystalline structure with close to zero magnetostriction. A very sensitive method for controlling relaxation and crystallization processes in such alloys is proposed. It consists in precise DSC measurements of heat capacity peak in the vicinity of the Curie temperature TC. Time-temperature dependencies of TC for microwires in comparison with ribbon shaped amorphous alloys were studied. Relaxation of atomic structure of amorphous phase during annealing was accompanied by an increase of TC. σ-shaped time dependencies are characterized by at least two relaxation processes, corresponding apparent values of activation energy were estimated. Decomposition of amorphous phase and redistribution of components between amorphous phase and growing nanocrystals affect the shape and position of the DSC peak at TC as well.Study of microwires with glass coating revealed the influence of internal stress on the shape and position of the Curie peak on DSC curve: increase of internal strain tensions leads to suppression of the TC peak.

Magnetism of Ho1−xTbxAl2alloys: Critical dependence of a first-order transition on Tb concentration

HoAl${}_{2}$ exhibits a first-order spin reorientation transition at 20 K, which is manifested as a sharp peak in the heat capacity. When Ho is partially replaced by only 5$%$ of Tb, the sharp heat-capacity peak in ${\mathrm{Ho}}_{1\ensuremath{-}x}{\mathrm{Tb}}_{x}{\mathrm{Al}}_{2}$ ($x$ = 0.05) disappears, and then reappears again for $x$ \ensuremath{\ge} 0.07. For $x$ = 0.05, the anomaly corresponding to the spin reorientation transition is barely seen in the heat capacity, but as $x$ exceeds 0.07 the weak anomaly transforms to a sharp peak. The spin reorientation transition temperature increases to 29 K for $x$ = 0.05, and as $x$ increases further the transition shifts to lower temperature and returns to \ensuremath{\sim}20 K for $x$ = 0.25. The transition is no longer observed when $x$ exceeds 0.60. Temperature-dependent x-ray powder-diffraction data confirm the first-order nature of the spin reorientation transition for the alloy with $x$ = 0.40, and indicate that the compound retains the room-temperature cubic structure within the sensitivity of the technique. Experimental observations are discussed considering the easy magnetization directions of HoAl${}_{2}$ and TbAl${}_{2}$.

The effect of Er doping on the spin reorientation transition in Ho1−xErxAl2

The magnetothermal properties of pseudo binary Ho1−xErxAl2 alloys have been investigated by heat capacity measurements. Two anomalies are observed in the heat capacity of HoAl2. A sharp peak at 20K represents the first order spin reorientation transition, and a second order anomaly occurs in the vicinity of the ferromagnetic transition at 32K. As Ho is partially replaced by Er in Ho1−xErxAl2 the sharpness of the first order heat capacity peak diminishes with increasing Er concentration, while the temperature of this transition remains practically unaffected. The second order ferromagnetic transition shifts to higher temperature region with increasing Er concentration. The observed behaviors are explained considering the geometry of 4f charge densities of Ho3+ and Er3+ and the easy magnetization directions of HoAl2 and ErAl2.

Is There a Liquid–Liquid Transition in Confined Water?

Water confined in narrow nanopores that prevent ice crystallization is usually studied as a means to understand the anomalous behavior of bulk liquid water. Nevertheless, there is no agreement on the similarity of the thermodynamics of bulk and nanoconfined liquid water. In this work, we use molecular dynamics simulations with the mW water model to investigate the phase behavior of liquid water in bulk and confined in a 1.5 nm cylindrical pore with water-surface interactions identical to water-water interactions. Through analysis of the isochors of bulk liquid water V(T,p) we extrapolate the locus of a putative liquid-liquid critical point (LLCP) in bulk mW water at 190 K and 1215 atm. This is a "virtual LLCP", as it would lie in a region of the phase diagram where fast crystallization of water impedes equilibration of the liquid. We find that confinement has a weak effect on the loci of the thermodynamic anomalies: the maxima in density and heat capacity of confined water occur at T,p similar to that in bulk. The heat capacity peak of confined water is due to a transformation within the confined liquid; we verify that ice does not form in the pores. Confined mW water presents a heat capacity maximum C(p)(max)(p) up to the highest pressure we investigated, 4000 atm. The magnitude of the heat capacity peak C(p)(max)(p) has a nonmonotonous dependence with pressure, attaining a maximum at conditions close to those of the locus of the bulk water's virtual LLCP. We do not find, however, direct evidence of a first-order liquid-liquid transition in confined water for pressures above or below the locus of the maximum response function. The extreme value of the response functions of confined water could be a rounded manifestation of an equivalent feature in the free energy surface of bulk water.

Low-temperature anomalies of 4He crystals

Low-temperature anomalies of hcp 4He crystals (mass decoupling from a torsional oscillator, shear modulus anomaly, dissipation peaks, and heat capacity peak) are explained. A simple model based on the concept of resonant tunneling systems in imperfect crystals is proposed. Mass decoupling is caused by an internal Josephson effect: the mass supercurrent inside phase coherent tunneling systems. Quantitative results are in reasonable agreement with experiments.

Bose-Einstein condensation and heat capacity of two-dimensional spin-polarized atomic hydrogen

The static fluctuation approximation (SFA) is used to study the condensate fraction and the specific heat capacity of finite two-dimensional spin-polarized atomic hydrogen. It is found that Bose-Einstein condensation occurs in this system. The transition temperature at different densities decreases as the number of particles of the system increases. At low density, a sharp peak in the specific heat capacity is observed at the transition temperature. On the other hand, as the density of the system increases, the transition temperature becomes no longer well-defined, and a hump is observed in the specific heat capacity around the transition temperature. A qualitative comparison of our results to published results for finite Bose systems shows good agreement.

The NMR line shape of a system of nuclear spins with equal spin-spin coupling constants

We obtain an expression for the NMR line at low temperatures for a system of nuclear spins described by a Hamiltonian with equal spin-spin coupling constants. We show that in the case of “easy axis” anisotropy, the line has a logarithmic low-frequency singularity and an exponentially decreasing high-frequency asymptotic behavior at the temperature of an anomalous peak of heat capacity. In the case of “easy plane” anisotropy, the line has the traditional Gaussian form. We discuss the possibility of using NMR data to discover specific thermodynamic and magnetic properties of the considered model system.

Curie temperature behaviour at relaxation and nanocrystallization of Finemet alloys

Dependence of Curie temperature (Tc) on time and temperature of annealing for Finemet-type amorphous alloys is studied by DSC. Structural relaxation of an amorphous phase at annealing is accompanied by increase of Tc. The σ-shaped time dependencies, however, are not characterized by the definite final value of Tc: long-term annealing of amorphous phase results in continuation of slow increase of Tc for every temperature. Apparent values of activation energy of relaxation were evaluated from the Tc time–temperature dependencies. It is found that the average value of activation energy increases with progress of the relaxation process. Correlation of the Tc data with structural alterations reflects redistribution of components in amorphous phase during annealing. Further increase of the annealing temperature results in precipitation of α-Fe(Si) nanograins in an amorphous matrix, that accompanied by gradual degradation of corresponding to Tc heat capacity peak. DSC measurements of Tc allow to detect the starting moment of amorphous phase crystallization.

Thermal Properties of Ferrimagnetic Systems

The heat capacity of some ferrimagnets has additional structures like a shoulder in the Schottky-like peak, or emergence of a second peak when an external magnetic field is applied. It is shown here that the ferromagnetic and anti-ferromagnetic elementary excitation spectra give rise to two independent heat capacity peaks, one enveloped by the other, which add up to give the peak for the total system. Taking this into account helps understand the additional structures in the peaks. Moreover, the classification of ferrimagnets into predominantly antiferromagnetic, ferromagnetic, or a mixture of the two is shown to be validated by studying them under additional influences like dimerization and frustration. Because these two are shown to influence the ferromagnetic and antiferromagnetic dispersion rela tions—and hence the quantities like heat capacity and magnetic susceptibility—by different amounts, the characterization of ferrimagnetic systems (1,1/2), (3/2,1) and (3/2,1/2) is brought out more clearly. Both these influences enhance antiferromagnetic character. PACS numbers: 75.10.Jm, 75.50.Ge.

Property of 4He Confined in 1D Nano-porous Medium FSM16 with 2.8-nm Channel Above Bulk Freezing Pressure

We have performed the freezing pressure and heat capacity measurements for 4He confined in a nano-porous medium FSM16 which possesses a one-dimensional 2.8-nm straight channel, in the pressure region above the bulk freezing pressure. At 2.8 MPa, where 4He in the channel remains a non-superfluid liquid down to the lowest temperature, the heat capacity decreases monotonically with decreasing temperature. It is in contrast to the heat capacity at 0.03 MPa with a broad bump at the temperature higher than the superfluid onset. At around the freezing onset in the channel, no clear heat capacity peak was observed. It suggests that the latent heat of the liquid–solid transition in the channel is quite small.

Calorimetric study of the nematic to smectic-Aphase transition in octylcyanobiphenyl-hexane binary mixtures

The continuous nematic to smectic-A (N-SmA) phase transition has been studied by high-resolution ac-calorimetry in binary mixtures of the liquid crystal octylcyanobiphenyl (8CB) and a nonmesogenic, low-molecular weight, solvent n-hexane (hex) as a function of temperature and solvent concentration. Heating and cooling scans about the N-SmA transition temperature were repeatedly performed on pure and six 8CB+hex samples having hexane molar concentration ranging from x(hex)=0.02 to 0.12. All 8CB+hex samples in this range of x(hex) remain macroscopically miscible and exhibit an N-SmA heat capacity peak that shifts nonmonotonically to lower temperature and evolves in shape, with a reproducible hysteresis, as x(hex) increases. The imaginary part of heat capacity remains zero up to x(hex)(TCP) ≃ 0.07 above which the distinct peak is observed, corresponding to a jump in both the real and imaginary enthalpy, indicating the onset of first-order behavior. A simple power-law analysis reveals an effective exponent that increases smoothly from 0.30 to 0.50 with an amplitude ratio A-/A+→1 as x(hex)→x(hex)(TCP). This observed crossover of the N-SmA toward a tricritical point driven by solvent concentration is consistent with previous results and can be understood as a solvent softening of the liquid crystal and concument promoting of nematic fluctuations.

Non-Debye excess heat capacity and boson peak of binary lithium borate glasses

The non-Debye excess heat capacities of binary lithium borate glasses with different Li 2O compositions of x = 8, 14 and 22 (mol%) are investigated to understand origin of the boson peak. The low-temperature heat capacities are measured between 2 and 50 K by a relaxation calorimeter. The experimental non-Debye heat capacities with x = 14 is successfully reproduced using the excess vibrational density of states measured by inelastic neutron scattering. This finding indicates that the non-Debye heat capacities of lithium borate glasses originate from the excess vibrational density of states measureable by inelastic neutron scattering. Moreover, it is demonstrated that all of the excess heat capacity spectra lie on a single master curve by the scaling using boson peak temperature and intensity.