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Results for “SPECIFIC HEAT”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

The Incredible Effect of Heat Load Increase Rate on the Specific Heat Flux at Boiling of Some Nanofluids

Boiling curves for different heat-load increase rates (HLIRs) are obtained for distilled water, tap water, and two nanofluids (NFs) based on a natural mixture of montmorillonite and attapulgite (AlSi-7) and titanium dioxide (NF-8). An unusually strong influence of the HLIR on the thermal parameters of NFs is detected upon boiling of AlSi-7 based on a mixture of highly anisometric nanoparticles of aluminosilicates of various morphologies capable of forming nanostructured openwork “architectures” on the heater surface. An explanation of the reasons for the observed effect is proposed. The data obtained indicate the existence of an optimal HLIR at which the structure of the deposit appearing on the heating surface during the boiling of an NF is most favorable for heat exchange.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

Negative specific heat from non-planar interactions and small black holes in AdS/CFT

The gravity side of the gauge/gravity duality predicts the existence of small black holes with negative specific heat. A free theory of strings has a Hagedorn behavior, but it does not lead to negative specific heat. To understand such states one needs to consider a theory of interacting strings. In the dual gauge theory, the string interactions are related to non-planar diagrams. In this paper the simplest gauge matrix model of two free matrices, that has Hagedorn behavior is analyzed in detail. A simple double trace deformation of the Hamiltonian, proportional to square of the free Hamiltonian square with a negative sign that mimics a gravitational attraction is enough to produce states with negative specific heat perturbatively and one can still compute the equation of state relating the entropy and the energy. A more general argument based on non-planar interactions that are random and that grow faster in strength than the energy suggests that states with negative specific heat appear generically.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Impact of anharmonicity on the vibrational entropy and specific heat of UO 2

The impact of anharmonicity on the vibrational entropy and heat capacity of UO 2 has been discovered from 10 to 1200 K using inelastic neutron-scattering measurements of the phonon density of states (PDOS). Small changes in the PDOS are observed from 10 to 295 K, with more noticeable changes appearing in the 750- and 1200-K data. The specific heat determined from the PDOS measurements is in agreement with macroscopic specific heat measurements, and the overall impact of nondilation anharmonicity on the specific heat has been shown to be less than 2%. An analysis of the phonon measurements shows that the softening of acoustic phonons with temperature is consistent with the quasiharmonic approximation. The optical phonons deviate from the quasiharmonic prediction, with the low-energy optical phonons between approximately 20 and 50 meV softening more than expected, while the higher-energy optical phonons between approximately 50 and 80 meV have no appreciable softening over the temperature range measured. The observation of a small anharmonic specific heat contribution has been demonstrated to be the result of relatively large energy-dependent anharmonic effects which have opposite sign, leading to a total contribution near zero.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Development and Study of Nb 3 Sn Wires With High Specific Heat

Recently, a new type of Nb 3 Sn conductors were developed, which, by incorporating substances with high specific heat at 2-8 K in a proper design, have demonstrated significant improvement of minimum quench energy (MQE). This method to improving energy margins of conductors is promising to reduce Nb 3 Sn magnet training. Here, we continue studying this type of conductors. Voltage-current (V-I) measurements from 15 T to 0 T and voltage-field (V-B) sweeps at various currents were conducted to investigate the influence of increased specific heat on the intrinsic stability of Nb 3 Sn conductors. Normal zone propagation velocity (NZPV) was also studied. Lastly, a new scheme was put forward to introduce a new type of substances with not only high specific heat but also high thermal diffusivity to Nb 3 Sn conductors, which is expected to work more effectively.

43 PARTICLE ACCELERATORS↗
DOE OSTI2017

Transport characteristics of phonons and the specific heat of Y{sub 2}O{sub 3}:ZrO{sub 2} solid solution single crystals

The temperature dependences of the specific heat and transport characteristics of phonons in single crystals of yttrium-stabilized zirconium dioxide Y{sub 2}O{sub 3}:ZrO{sub 2} solid solutions have been studied. It has been shown that the temperature dependences of the specific heat at T > 5 K are almost identical at the degree of stabilization of a solid solution with an Y{sub 2}O{sub 3} content of 5–20 mol %. Differences in the temperature dependences of the specific heat of samples from different sources at T < 5 K are due to the presence of low-energy two-level systems. The features of the transport characteristics of thermal phonons at liquid helium temperatures reflect not only the presence of two-level systems but also the scattering of phonons on low-dimensional domains of another phase coherently conjugate to the main phase of the Y{sub 2}O{sub 3}:ZrO{sub 2} solid solution.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Prediction of nonlinear specific heat during single crystal HMX phase transition

Here, we develop a thermodynamically consistent chemo-thermo-mechanical model for the β → δ phase transition of energetic HMX crystals. In contrast to previous models, which either considered specific heat to be a constant or utilized a calibrated function, this model provides novel expressions for the specific heats at constant volume and constant elastic strains derived directly from continuum mechanics. In addition, the model provides a novel prediction for the critical temperature at which the chemical heating rate achieves its extremum for Arrhenius kinetics. The numerical solution predicts highly nonlinear specific heat behavior including order of magnitude changes.

42 ENGINEERING↗
DOE OSTI2016

Neutron diffraction, specific heat and magnetization studies on Nd{sub 2}CuTiO{sub 6}

Structural and physical properties of a double-perovskite compound, Nd{sub 2}CuTiO{sub 6} have been studied using neutron diffraction, magnetization and specific heat measurements. The compound crystallizes in an orthorhombic structure in space group Pnma. The interesting observation we make here is that, though no long range magnetic order is observed between 2 and 300 K, the low temperature specific heat and magnetic susceptibility behavior exhibits non-Fermi liquid like behavior in this insulating compound. The magnetization and specific heat data are presented and discussed in light of these observations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Method for Accurate Determination of the Electron Contribution: Specific Heat of Ba 0.59 K 0.41 Fe 2 As 2

Twenty two years after the opening of the field of high temperature superconductivity by Georg Bednorz and Alex Müller a new addition to this class of materials, namely the iron-pnictides and chalcogenides, perplexes the scientific community. Making use of the accumulated wisdom, a great deal of their physics has been “mapped” in record time using all available measurement techniques. Among these, specific-heat has been proven a powerful and unique technique in the study of physical properties of bulk materials, especially of superconductors. We report here specific-heat measurements on the nearly optimally doped iconic iron-arsenide superconductor Ba 0.59 K 0.41 Fe 2 As 2 . We use a new method, based on direct comparisons of α-model expressions for the electron contribution with the measured total specific heat, to extract the electron contribution. It circumvents the need in the conventional analyses for an independent, necessarily approximate, determination of the lattice contribution, which is subtracted from the total to obtain the electron contribution, and it eliminates the consequent uncertainties in the electron contribution. For Ba 0.59 K 0.41 Fe 2 As 2 the electron density of states is comprised of contributions from two electron bands with superconducting-state energy gaps differing by a factor 3.8, with 77% coming from the band with the larger gap. The vortex-state specific heat suggests nodeless gaps. Comparison of the normal-state density of states with band-structure calculations shows an extraordinarily large effective mass enhancement, for which there is no precedent in simple metals and no theoretical explanation.

Conventional Analysis↗
DOE OSTI2018

Specific Heat and Thermal Expansion of Triglycine Sulfate–Porous Glass Nanocomposites

The effect of restricted geometry on specific heat capacity and thermal expansion of the triglycine sulfate (TGS)–borosilicate glass composites have been studied first. A decrease in the entropy and temperature of the P2{sub 1} ↔ P2{sub 1}/m phase transition in the TGS component with decreasing the glass matrix pore diameter at the invariable specific heat and thermal expansion coefficient has been observed. The estimates are indicative of the minor effect of internal pressure on the TGS pressure coefficient dT{sub C}/dp in the composites.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Free energy and specific heat near a quantum critical point of a metal

We analyze the free energy and the specific heat for fermions interacting with a gapless boson at a quantum-critical point (QCP) in a metal. We use the Luttinger-Ward-Eliashberg formula for the free energy in the normal state, which includes contributions from bosons, fermions, and their interaction, all expressed via fully dressed fermionic and bosonic propagators. The sum of the last two contributions is the free energy F γ of an effective low-energy model of fermions with boson-mediated dynamical 4-fermion interaction V(Ω m ) ∝ 1/|Ω m | γ (the γ model). This purely electronic model has been used to analyze the interplay between non-Fermi liquid behavior and pairing near a QCP, which are both independent of the upper energy cutoff Λ. However, the specific heat C γ (T), obtained from F γ , does depend on Λ. We argue that this dependence is spurious and cancels out, once we include the contribution from bosons. We further argue that the full C(T) is the sum of the contribution from free fermions and the one from a critical boson, with the fully dressed propagator, other terms cancel out. We compare the full C(T) with the C γ (T), obtained using recently proposed regularization of F γ . We argue that for γ < 1, the full C(T) and the regularized C γ (T) differ by a γ-dependent prefactor, while for γ > 1, the full C(T) and C γ (T) differ by the positive contribution from free massless fermions (a positive constant for the electron-phonon case γ = 2). For these γ, C γ (T) is negative, but the full C(T) is positive. We argue that only the full C(T) matters as the positive and the negative contributions originate from the term in C(T) which contains the fully dressed bosonic propagator. Here, we then argue that the normal state remains stable until the pairing instability develops.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

FEM Modeling and Simulation of 2-D High Specific Heat Nb 3 Sn Wires

In the past few years, new high specific heat Nb 3 Sn wires have gained much focus at FNAL. Indeed, they have proved to be more stable against thermal perturbations with respect to standard wires. Nevertheless, a trade off exists between their thermal efficiency and production feasibility. In this report I describe the thermal and structural models that I have developed by exploiting ANSYS Mechanical APDL ® , which I got acquainted with at the beginning of my training. The aim has been to optimize the location of high specific heat elements in order to obtain an optimal thermal stability, while minimizing the risk of wire breakage during drawing. FEM results are compared with experimental ones made beforehand. Other minimum quench energy (MQE) thermal models have been developed for expected new experimental results. New data may improve the understanding of the embedded physical uncertainties in the model.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Specific heat in strongly hole-doped iron-based superconductors

We compute specific heat C(T) in a strongly hole-doped Fe-based superconductor, like KFe 2 As 2 , which has only hole pockets. We model the electronic structure by a three-orbital/three-pocket model with two smaller hole pockets made out of d xz and d yz orbitals and a larger pocket made out of d xy orbital. We use as an input the experimental fact that the mass of d xy fermion is several times heavier than that of d xz /d yz fermions. We argue that the heavy d xy band gives the largest contribution to the specific heat in the normal state, but the superconducting gap on the d xy pocket is much smaller than that on d xz /d yz pockets. We argue that in this situation the jump of C(T) at T c is determined by d xz /d yz fermions, and the ratio (C s –C n )/C n is a fraction of that in a one-band BCS superconductor. At T < T c ,C(T) remains relatively flat down to some T*, below which it rapidly drops. This behavior is consistent with the data for KFe 2 As 2 and related materials. We use one-parameter model for the interactions and fix this only parameter by matching the experimental ratio of the gaps on the two d xz /d yz pockets. Furthermore, we argue that the resulting parameter-free model reproduces quantitatively the data on C(T) for KFe 2 As 2 . We further argue that the very existence of a finite T* < T c favors s +– gap structure over d-wave, because in the latter case T* would almost vanish.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Electronic specific heat capacities and entropies from density matrix quantum Monte Carlo using Gaussian process regression to find gradients of noisy data

In this work, we present a machine learning approach to calculating electronic specific heat capacities for a variety of benchmark molecular systems. Our models are based on data from density matrix quantum Monte Carlo, which is a stochastic method that can calculate the electronic energy at finite temperature. As these energies typically have noise, numerical derivatives of the energy can be challenging to find reliably. In order to circumvent this problem, we use Gaussian process regression to model the energy and use analytical derivatives to produce the specific heat capacity. From there, we also calculate the entropy by numerical integration. We compare our results to cubic splines and finite differences in a variety of molecules in which Hamiltonians can be diagonalized exactly with full configuration interaction. We finally apply this method to look at larger molecules where exact diagonalization is not possible and make comparisons with more approximate ways to calculate the specific heat capacity and entropy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Critical Behavior of the Specific Heat of Pr{sub 0.6}Sr{sub 0.4}Mn{sub 1−x}Fe{sub x}O{sub 3} Manganites

The specific heat of Pr{sub 0.6}Sr{sub 0.4}Mn{sub 1−x}Fe{sub x}O{sub 3} (x = 0 and 0.1) manganites was measured in the temperature range 80–350 K. A nature of the ferromagnetic phase transition is studied via the analysis of a specific heat critical behavior near the Curie temperature with account of correction to the scaling. The critical indices calculated with the account of correction to the scaling α = −0.0981 and −0.1208 for x = 0 and 0.10, correspondingly, are close to those of the 3D Heisenberg model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Correlations for the specific heat capacity of ( U x Pu 1 - x ) 1 - y Gd y O 2 - z derived from molecular dynamics

We report UO 2 is the primary conventional fuel used in most nuclear reactors with Gd 2 O 3 commonly added as a burnable absorber to produce a more level power distribution in the reactor core at the beginning of operation. It can also be mixed with other actinide oxides to produce mixed oxide (MOx) fuel. In this study, molecular dynamics simulations were used to predict the specific heat capacity of Gd-doped PuO 2 , UO 2 and (U, Pu)O 2 MOx accommodating Gd 3+ substituted at cation sites via two charge compensation mechanisms - oxygen vacancy formation and the oxidation of U 4+ to U 5+ . The specific heat capacity values for PuO 2 and UO 2 are in good agreement with other studies showing a distinct peak at high temperatures - above 1800 K. As Gd 3+ is added, the peak height reduces for each composition considered. An analytical fit was applied to the data where Gd 3+ was fully charge compensated by either oxygen vacancies or U 5+ . The expression was then validated by predicting the specific heat capacity for three compositions of (Ux Pu 1-x ) 1-y Gd y O 2-z containing both oxygen vacancies and U 5+ , and compared to molecular dynamics data.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2024

Specific heats for quantum BTZ black holes in extended thermodynamics

It was shown recently that extended black hole thermodynamics, where the cosmological constant is a dynamical variable, giving rise to a pressure p and its conjugate volume V , can be given a natural setting in the context of braneworld models. We study the specific heat capacities C p ( T ) and C V ( T ) of the quantum version of the Bañados, Teitelboim, and Zanelli (BTZ) black hole that lives in the induced gravity theory on the brane. There are multiple branches of solutions, and we explore and characterize key features of the possible behavior. We identify and study a critical point in the space of solutions where both specific heats diverge. In the regime of weak backreaction where we are close to an ordinary theory of gravity, the black hole is “subentropic,” but as backreaction is increased we note that there are parts of parameter space that has regions where it is “superentropic.” While a study of the sign of the specific heats does not always show a corresponding instability (conjectured in the literature), the presence of strong backreaction makes interpretation unclear. Published by the American Physical Society 2024

Johnson, Clifford V. (ORCID:0000000189645830)↗
DOE OSTI2017

Thermophysical Properties of Cold- and Vacuum Plasma-Sprayed Cu-Cr-X Alloys, NiAl and NiCrAlY Coatings II: Specific Heat Capacity

Part I of the paper discussed the temperature dependencies of the electrical resistivities, thermal conductivities, thermal diffusivities and total hemispherical emissivities of several vacuum plasma-sprayed (VPS) and cold-sprayed (CS) copper alloy monolithic coatings, VPS NiAl, VPS NiCrAlY, extruded GRCop-84 and as-cast Cu-17(wt.%)Cr-5%Al. Part II discusses the temperature dependencies of the constant-pressure specific heat capacities, C{sub P}, of these coatings. The data were empirically regression-fitted with the equation: C{sub P}=AT{sup 4}+BT{sup 3}+CT{sup 2}+DT+Ewhere T is the absolute temperature and A, B, C, D and E are regression constants. The temperature dependencies of the molar enthalpy, molar entropy and Gibbs molar free energy determined from experimental values of molar specific heat capacity are reported. Calculated values of C{sub P} using the Neumann–Kopp (NK) rule were in poor agreement with experimental data. Instead, a modification of the NK rule was found to predict values closer to the experimental data with an absolute deviation less than 6.5%. The specific molar heat capacities for all the alloys did not agree with the Dulong–Petit law, and C{sub P} > 3R, where R is the universal gas constant, were measured for all the alloys except NiAl for which C{sub P} < 3R at all temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2016

The effect of Ca doping on specific heat of YCoO{sub 3} cobaltate

We have investigated the thermodynamic properties of Y{sub 1-x}Ca{sub x}CoO{sub 3} (0.0≤x≤0.1) perovskites by means of a modified rigid ion model (MRIM). The variations of specific heat at wide temperatures 1 K ≤ T ≤ 1000 K are reported. Also, the effect of lattice distortions on the elastic and thermal properties of pure and Ca doped cobaltates has been studied by an atomistic approach. Besides, we have reported bulk modulus (B), cohesive energy (ϕ), molecular force constant (f), Reststrahlen frequency (υ), Debye temperature (θ{sub D}), Gruneisen parameter (γ) and specific heat (C). It is found that the present model has a promise to predict the thermodynamic properties of other perovskites as well.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗