Heat Capacity Peak Research Articles

Phase change alloy viscosities down to Tg using Adam-Gibbs-equation fittings to excess entropy data: A fragile-to-strong transition

A striking anomaly in the viscosity of Te85Ge15 alloys noted by Greer and coworkers from the work of Neumann et al. is reminiscent of the equally striking comparison of liquid tellurium and water anomalies documented long ago by Kanno et al. In view of the power laws that are used to fit the data on water, we analyze the data on Te85Ge15 using the Speedy-Angell power-law form, and find a good account with a singularity Ts only 25 K below the eutectic temperature. However, the heat capacity data in this case are not diverging, but instead exhibit a sharp maximum like that observed in fast cooling in the Molinero-Moore model of water. Applying the Adam-Gibbs viscosity equation to these calorimetric data, we find that there must be a fragile-to-strong liquid transition at the heat capacity peak temperature, and then predict the "strong" liquid course of the viscosity down to Tg at 406 K (403.6 K at 20 K min−1 in this study). Since crystallization can be avoided by moderately fast cooling in this case, we can check the validity of the extrapolation by making a direct measurement of fragility at Tg, using differential scanning calorimetric techniques, and then comparing with the value from the extrapolated viscosity at Tg. The agreement is encouraging, and prompts discussion of relations between water and phase change alloy anomalies.

Heat capacity peak at the quantum critical point of the transverse Ising magnet CoNb2O6.

The transverse Ising magnet Hamiltonian describing the Ising chain in a transverse magnetic field is the archetypal example of a system that undergoes a transition at a quantum critical point (QCP). The columbite CoNb2O6 is the closest realization of the transverse Ising magnet found to date. At low temperatures, neutron diffraction has observed a set of discrete collective spin modes near the QCP. Here, we ask if there are low-lying spin excitations distinct from these relatively high-energy modes. Using the heat capacity, we show that a significant band of gapless spin excitations exists. At the QCP, their spin entropy rises to a prominent peak that accounts for 30% of the total spin degrees of freedom. In a narrow field interval below the QCP, the gapless excitations display a fermion-like, temperature-linear heat capacity below 1 K. These novel gapless modes are the main spin excitations participating in, and affected by, the quantum transition.

Monte Carlo Calculations of Magnetic Heat Capacity of La0.7Sr0.3-x MnO3-d

In this work, Monte Carlo method is used with Ising model to simulate magnetization versus temperature of La0.7Sr0.3-xMnO3-d. The simulated magnetization variation versus temperature is in a good agreement with the available experimental data. The calculations showed that the sample with x = 0.10 has the smallest amplitude of magnetic heat capacity peak. Moreover, the magnetic heat capacity increases with increasing x when x > 0.10.

The impact of elastic and plastic strain on relaxation and crystallization of Pd–Ni–P-based bulk metallic glasses

The effects of deformation and subsequent heat treatment on the low-temperature heat capacity, enthalpy relaxation rate and mechanical losses of two Pd–Ni–P-based bulk metallic glasses of slightly different compositions and different thermal stabilities have been investigated. It was found that the crystallization temperatures decreased significantly with imposed strain and the effect was more pronounced for the alloy with a higher thermal stability. The boson heat capacity peak increases with increasing strain in both alloys. However, after annealing treatments above room temperature, it relaxes to a lower enthalpy state as compared to that of the as-quenched state for the alloy with a lower thermal stability. The existence of two counteracting processes that might be related to different shear band structures within one homogeneously deformed sample is suggested. These results agree with the internal friction data, which indicate different regimes of mechanical damping as a function of the strain amplitude, while the critical amplitude of a transition between the regimes depends on the imposed strain. The results are interpreted within the energy landscape approach and advocate that the composition-dependent local atomic configurations affect significantly the response of the glass to an applied strain.

The structural and magnetic properties of Pr1−xErxAl2

We report on the effect of Er addition to PrAl2 on the lattice parameters, magnetic behavior, heat capacity, and magnetocaloric effect by using x-ray diffraction, magnetization, and heat capacity measurements. Unlike Pr0.6Er0.4Al2, other alloys we studied in the pseudobinary (Pr1−xErx)Al2 system do not exhibit a sharp peak in heat capacity with the application of magnetic field. Both the cubic lattice parameter and the Curie temperature decrease with increasing Er concentration. The nuclear specific heat coefficient decreases from 660 mJ K mol−1 for x = 0.05 to a nearly negligible value for x = 0.95. The magnetic entropy and adiabatic temperature change varies from 2 to 4 J mol−1 K−1 and 2.5 to 5 K at ΔH = 20 kOe for x = 0.05 to 0.95, respectively. These values of the magnetocaloric effect are comparable to those of the other rare-earth dialuminides systems.

Investigating the magnetovolume effect in isotropic body-centered-cubic iron using spin-lattice dynamics simulations

The understanding of the magnetovolume effect lacks explicit consideration of spin-lattice coupling at the atomic level, despite abundant theoretical and experimental studies throughout the years. This research gap is filled by the recently developed spin-lattice dynamics technique implemented in this study, which investigates the magnetovolume effect of isotropic body-centered-cubic (BCC) iron, a topic that has previously been subject to macroscopic analysis only. This approach demonstrates the magnetic anomaly followed by the volumetric changes associated with the effect, each characterized by the corresponding field-induced inflection temperature. The temperature of the heat capacity peaks is useful in determining the temperature for retarding the atomic volume increase. Moreover, this work shows the correlation between the effects of temperature and field strength in determining the equilibrium atomic volume of a ferromagnetic material under a magnetic field.

Multiple melting behavior of poly(lactic acid)-hemp-silica composites using modulated-temperature differential scanning calorimetry

Abstract Poly(lactic acid) (PLA)-hemp-nanosilica (PHS) composites were prepared by impregnation of hemp woven fabric with PLA solution. Nanosilica was dispersed in the PLA solution to introduce a matrix reinforcing nanophase within the composite. The melting behavior of PLA composites was obtained by using differential scanning calorimetry (DSC) and modulated-temperature DSC (mT-DSC). Multiple melting which appeared in the non-isothermal heating curve showed that the temperature of a low melting peak increased when using a slower scanning rate. The incorporation of nanosilica in PLA composites affected the melting temperature (Tm) and sufficiently formed nucleation sites that promoted the growth of PLA crystals. Composites analyzed by a temperature-modulated program showed a broad exothermic peak before the melting peak in the non-reversing heat capacity and endothermic melting in the reversing heat capacity curve. This behavior was explained by a process of partial melting, recrystallization and remelting (mrr). The mT-DSC resolved that hemp fiber induced recrystallization and nanosilica acted as an effective nucleating agent, which promoted small and imperfect crystals that changed successively into more stable crystals through a melt-recrystallization process.

Relationship between boson heat capacity peaks and evolution of heterogeneous structure in metallic glasses

The dependence of boson heat capacity peaks of a typical Zr52.5Ti5Cu17.9Ni14.6Al10 metallic glass on different annealing time and quenching rates is studied. It is found that the boson heat capacity peak moves to higher temperatures and reduces intensity when the metallic glass is isothermally annealed or slowly quenched. We show that the intensity and position change of the boson heat capacity peak are associated with the evolution of heterogeneous structure and inelastic regions in metallic glasses. The results might help in understanding the structural features and evolution as well as their effects on boson peak of metallic glasses.

Impact of plastic deformation and shear band formation on the boson heat capacity peak of a bulk metallic glass.

The effect of annealing on the low-temperature heat capacity of a bulk Pd38.5Ni40P21.5 metallic glass is investigated for as-quenched and deformed (rolled) states. Although the boson heat capacity peak increases with increasing strain, it relaxes faster and to a lower level compared to that of the as-quenched state after annealing treatments both below and above the glass transition temperature Tg. The glass is found to retain a certain "memory" on the room-temperature plastic deformation even after annealing above Tg. Indications for two counteracting processes that might be related to different types of shear bands are observed.

Dispersion of γ-Alumina Nano-Sized Spherical Particles in a Calamitic Liquid Crystal. Study and Optimization of the Confinement Effects.

We report an experimental study on confined systems formed by butyloxybenzylidene octylaniline liquid crystal (4O.8) + γ-alumina nanoparticles. The effects of the confinement in the thermal and dielectric properties of the liquid crystal under different densities of nanoparticles is analyzed by means of high resolution Modulated Differential Scanning Calorimetry (MDSC) and broadband dielectric spectroscopy. First, a drastic depression of the N-I and SmA-N transition temperatures is observed with confinement, the more concentration of nanoparticles the deeper this depression is, driving the nematic range closer to the room temperature. An interesting experimental law is found for both transition temperatures. Second, the change in shape of the heat capacity peaks is quantified by means of the full width half maximum (FWHM). Third, the confinement does not noticeably affect the molecular dynamics. Finally, the combination of nanoparticles and the external applied electric field tends to favor the alignment of the molecules in metallic cells. All these results indicate that the confinement of liquid crystals by means of γ-alumina nanoparticles could be optimum for liquid crystal-based electrooptic devices.

Characterization of the Glass Transition of Water Predicted by Molecular Dynamics Simulations Using Nonpolarizable Intermolecular Potentials

Molecular dynamics simulations allow detailed study of the experimentally inaccessible liquid state of supercooled water below its homogeneous nucleation temperature and the characterization of the glass transition. Simple, nonpolarizable intermolecular potentials are commonly used in classical molecular dynamics simulations of water and aqueous systems due to their lower computational cost and their ability to reproduce a wide range of properties. Because the quality of these predictions varies between the potentials, the predicted glass transition of water is likely to be influenced by the choice of potential. We have thus conducted an extensive comparative investigation of various three-, four-, five-, and six-point water potentials in both the NPT and NVT ensembles. The T(g) predicted from NPT simulations is strongly correlated with the temperature of minimum density, whereas the maximum in the heat capacity plot corresponds to the minimum in the thermal expansion coefficient. In the NVT ensemble, these points are instead related to the maximum in the internal pressure and the minimum of its derivative, respectively. A detailed analysis of the hydrogen-bonding properties at the glass transition reveals that the extent of hydrogen-bonds lost upon the melting of the glassy state is related to the height of the heat capacity peak and varies between water potentials.

Heat capacity of paramagnetic nickelocene: Comparison with diamagnetic ferrocene

Nickelocene [bis(η5-cyclopentadienyl)nickel: Ni(C5H5)2, electron spin S=1, the ground state configuration 3A2g] is paramagnetic and belongs to a typical molecule-based magnet. Heat capacities of nickelocene have been measured at temperatures in the 3−320K range by adiabatic calorimetry. By comparing with those of diamagnetic ferrocene crystal, a small heat capacity peak centered at around 15K and a sluggish hump centered at around 135K were successfully separated. The low-temperature peak at 15K caused by the spin is well reproduced by the Schottky anomaly due to the uniaxial zero-field splitting of the spin S=1 with the uniaxial zero-field splitting parameter D/k=45K (k: the Boltzmann constant). The magnetic entropy 9.7JK−1mol−1 is substantially the same as the contribution from the spin-manifold Rln3=9.13JK−1mol−1 (R: the gas constant). The sluggish hump centered at around 135K arises from rotational disordering of the cyclopentadienyl rings of nickelocene molecule. The enthalpy and entropy gains due to this anomaly are 890Jmol−1 and 6.9JK−1mol−1, respectively. As the hump spreads over a wide temperature region, separation of the hump from the observed heat capacity curve involves a little bit ambiguity. Therefore, these values should be regarded as being reasonable but tentative. The present entropy gain is comparable with 5.5JK−1mol−1 for the sharp phase transition at 163.9K of ferrocene crystal. This fact implies that although the disordering of the rings likewise takes place in both nickelocene and ferrocene, it proceeds gradually in nickelocene and by way of a cooperative phase transition in ferrocene. A reason for this originates in loose molecular packing in nickelocene crystal. Molar heat capacity and the standard molar entropy of nickelocene are larger than those of ferrocene beyond the mass effect over the whole temperature region investigated. This fact provides with definite evidences for the loose molecular packing in nickelocene crystal.

Unusual magnetic hysteresis and the weakened transition behavior induced by Sn substitution in Mn3SbN

Substitution of Sb with Sn was achieved in ferrimagnetic antiperovskite Mn3SbN. The experimental results indicate that with an increase in Sn concentration, the magnetization continuously decreases and the crystal structure of Mn3Sb1-xSnxN changes from tetragonal to cubic phase at around x of 0.8. In the doping series, step-like anomaly in the isothermal magnetization was found and this behavior was highlighted at x = 0.4. The anomaly could be attributed to the magnetic frustration, resulting from competition between the multiple spin configurations in the antiperovskite lattice. Meantime, Hc of 18 kOe was observed at x = 0.3, which is probably the highest among those of manganese antiperovskite materials reported so far. With increasing Sn content, the abrupt change of resistivity and the sharp peak of heat capacity in Mn3SbN were gradually weakened. The crystal structure refinements indicate the weakened change at the magnetic transition is close related to the change of c/a ratio variation from tetragonal to cubic with Sn content. The results derived from this study indicate that the behavior of Mn3Sb1-xSnxN could potentially enhance its scientific and technical applications, such as spin torque transfer and hard magnets.

Investigation of Size Effects on the Al Nanoclusters Physical Properties via Molecular Dynamics Simulations

Molecular dynamics (MD) simulation has been carried out for study of size effects on physical properties of Al nanoclusters with different sizes (N = 256, 500, 864, 1372, 2048 and 4000) in the temperature range 300 K ≤ T ≤ 1200 K for both free and periodic boundary conditions. Energy per site for periodic and free boundary conditions was calculated for the mentioned sizes and temperature. By increasing size of Al nanocluster, the energy per atom was convergence to that of the bulk. By use of molecular dynamics some physical properties such as melting point, surface energy, sublimation energy, thermodynamic limit and pair distribution function have been obtained. Melting point of bulk state is 875 K which proportional the jump of energy and sharp peak in heat capacity. Our calculated sublimation energy (ΔHs-calculated = 330 kJ/mol) was in good agreement with experimental result (ΔHs-experimental = 318 kJ/mol).

Thermodynamics of Protein Aggregation

Abstract Amyloid protein aggregation characterizes many neurodegenerative disorders, including Alzheimer's, Parkinson's, and Creutz- feldt-Jakob disease. Evidence suggests that amyloid aggregates may share similar aggregation pathways, implying simulation of full-length amyloid proteins is not necessary for understanding amyloid formation. In this study we simulate GNNQQNY, the N-terminal prion-determining domain of the yeast protein Sup35 to investigate the thermodynamics of structural transitions during aggregation. We use a coarse-grained model with replica-exchange molecular dynamics to investigate the association of 3-, 6-, and 12-chain GNNQQNY systems and we determine the aggregation pathway by studying aggregation states of GN- NQQNY. We find that the aggregation of the hydrophilic GNNQQNY sequence is mainly driven by H-bond formation, leading to the formation of /3-sheets from the very beginning of the assembly process. Condensation (aggregation) and ordering take place simultaneously, which is underpinned by the occurrence of a single heat capacity peak only.

CdSe nanoparticles dispersed in ferroelectric smectic liquid crystals: Effects upon the smectic order and the smectic-Ato chiral smectic-Cphase transition

Spherical CdSe nanoparticles, surface-treated with oleylamine and tri-octylphosphine, dispersed in ferroelectric liquid crystals, can efficiently target disclination lines, substantially altering the macroscopic properties of the host compound. Here we present an ac calorimetry and x-ray diffraction study demonstrating that for a large range of nanoparticle concentrations the smectic-A layer thickness increases monotonically. This provides evidence for enhanced accumulation of nanoparticles at the smectic layers. Our results for the Smectic-A (SmA) to chiral smectic-C (SmC) phase transition of the liquid crystal S-(+)4-(2'-methylbutyl)phenyl-4'-n-octylbiphenyl-4-carboxylate (CE8) reveal that the character of the transition is profoundly changed as a function of the nanoparticle concentration. Large transition temperature shifts are recorded. Moreover, the heat-capacity peaks exhibit a crossover trend to a step-like anomaly. This behavior may be linked to the weakening of the SmA and SmC order parameter coupling responsible for the observed near-tricritical, mean-field character of the transition in bulk CE8. At lower temperatures, the presence of nanoparticles disrupts the phase sequence involving the tilted hexatic phases most likely by obstructing the establishment of long-range bond-orientational order.

New Computational Approach to Determine Liquid-Solid Phase Equilibria of Water Confined to Slit Nanopores.

We devise a new computational approach to compute solid-liquid phase equilibria of confined fluids. Specifically, we extend the multibaric-multithermal ensemble method with an anisotropic pressure control to achieve the solid-liquid phase equilibrium for confined water inside slit nanopores (with slit width h ranging from 5.4 Å to 7.2 Å). A unique feature of this multibaric-multithermal ensemble is that the freezing points of confined water can be determined from the heat-capacity peaks. The new approach has been applied to compute the freezing point of two monolayer ices, namely, a high-density flat rhombic monolayer ice (HD-fRMI) and a high-density puckered rhombic monolayer ice (HD-pRMI) observed in our simulation. We find that the liquid-to-solid transition temperature (or the freezing point) of HD-pRMI is dependent on the slit width h, whereas that of HD-fRMI is nearly independent of the h.

Thermodynamic analysis of structural transitions during GNNQQNY aggregation

Amyloid protein aggregation characterizes many neurodegenerative disorders, including Alzheimer's, Parkinson's, and Creutzfeldt-Jakob disease. Evidence suggests that amyloid aggregates may share similar aggregation pathways, implying simulation of full-length amyloid proteins is not necessary for understanding amyloid formation. In this study, we simulate GNNQQNY, the N-terminal prion-determining domain of the yeast protein Sup35 to investigate the thermodynamics of structural transitions during aggregation. Utilizing a coarse-grained model permits equilibration on relevant time scales. Replica-exchange molecular dynamics is used to gather simulation statistics at multiple temperatures and clear energy traps that would aversely impact results. Investigating the association of 3-, 6-, and 12-chain GNNQQNY systems by calculating thermodynamic quantities and orientational order parameters, we determine the aggregation pathway by studying aggregation states of GNNQQNY. We find that the aggregation of the hydrophilic GNNQQNY sequence is mainly driven by H-bond formation, leading to the formation of β-sheets from the very beginning of the assembly process. Condensation (aggregation) and ordering take place simultaneously, which is underpinned by the occurrence of a single heat capacity peak.

Crystal structure, magnetic properties, and the magnetocaloric effect of Gd5Rh4 and GdRh

The crystal structures of Gd5Rh4 and GdRh have been studied by powder and single crystal x-ray diffraction. The results show that Gd5Rh4 is isotypic with Pu5Rh4 and the bond length of the short Rh-Rh dimer is 2.943(4) Å. According to heat capacity measurements in zero magnetic field, the magnetic ordering temperature of Gd5Rh4 is 13 K, in agreement with magnetization measurements. Both the heat capacity peak shape and the positive slope of the Arrott plots at Curie temperature (TC) indicate the second-order nature of the magnetic transition. The temperature dependence of magnetization of Gd5Rh4 measured in 1 kOe applied field indicates noncollinear magnetic ordering that may change into nearly collinear ferromagnetic ordering by increasing the magnetic field. GdRh is ferromagnetic below TC = 22 K. Moderate magnetocaloric effects and relatively high refrigerant capacities are observed in Gd5Rh4 and GdRh.

Preliminary Heat Capacity and Vapor Pressure Measurements of 2D 4He on ZYX Graphite

We report preliminary heat capacity and vapor pressure measurements of the first and second layers of 4He adsorbed on ZYX graphite. ZYX is known to have much better crystallinity than Grafoil, the most commonly-used exfoliated graphite substrate, such as a ten-times larger platelet size. This allows us to distinguish different phases in 2D 4He much more clearly and may provide qualitatively different insights into this system. We found a significantly asymmetric density-dependence of the heat-capacity peak associated with the $\sqrt{3}\times\sqrt{3}$ phase formation comparing with that obtained with Grafoil. The 2nd-layer promotion density is determined as 11.8±0.3 nm−2 from the heat-capacity measurement of low density samples in the 2nd layer and vapor pressure measurement.