Pressure Dependence Of Thermal Conductivity Research Articles

Pressure-dependent thermal conductivity transient measurement of graphene

The thermal properties of graphene determine the thermal damage evaluation of graphene diaphragm and the lifetime of graphene-based devices. The conventional transient measurement method tends to result in the low-precision thermal conductivity (κ) for suspended monolayer graphene and complex interferometry setup. Hence, in order to improve κ measurement of graphene, a simple, miniaturized and portable Fabry-Perot (F–P) resonator with 10-layer graphene is proposed to perform the actuation and vibration detection of the suspended graphene. Then, the κ value can be calculated by means of laser-heated graphene's thermomechanical response, along with the established model for thermal time constant (τ) of a uniformly heated circular film. A quadratic fit is applied to the measured κ at different air pressures and an average fitting curve is obtained, thus determining that the measured κ relatively decreases by 13.32% with the increasing air pressure outside the F–P cavity from ∼22 kPa to ∼101 kPa. Subsequently, the COMSOL heat transfer simulation confirms that convective heat transfer is the primary reason for the external air pressure affecting transient measurements of κ of graphene. Hence, after taking the intercept of the average fitting curve, the predicted κ at 0 Pa of 10-layer suspended graphene is calculated to be 194.48 W/(m·K), which is close to the fitting curve calculated from previously reported κ of graphene with different layers. This study contributes to accurate transient measurement of κ in graphene, and is also instructive for investigating transient thermal process in other two-dimensional materials.

Seasonal Variations of Soil Thermal Conductivity at the InSight Landing Site

AbstractThe heat flow and physical properties package measured soil thermal conductivity at the landing site in the 0.03–0.37 m depth range. Six measurements spanning solar longitudes from 8.0° to 210.0° were made and atmospheric pressure at the site was simultaneously measured using InSight's Pressure Sensor. We find that soil thermal conductivity strongly correlates with atmospheric pressure. This trend is compatible with predictions of the pressure dependence of thermal conductivity for unconsolidated soils under martian atmospheric conditions, indicating that heat transport through the pore filling gas is a major contributor to the total heat transport. Therefore, any cementation or induration of the soil sampled by the experiments must be minimal and soil surrounding the mole at depths below the duricrust is likely unconsolidated. Thermal conductivity data presented here are the first direct evidence that the atmosphere interacts with the top most meter of material on Mars.

Ab initio study of pressure-dependent phonon heat conduction in cubic boron nitride

Phonon transport in cubic boron nitride (c-BN) at elevated pressure in several tens GPa is potentially of interest for cooling high-power electronics. In this work, we study the pressure-dependent thermal conductivity (up to 20 GPa) of isotope-engineered c-BN by solving the Boltzmann transport equation with inputs only from ab initio calculations. Four boron isotope compositions are analyzed, including natural abundance, enriched 10B and 11B, and a roughly equal mixture of the two. In all cases, the thermal conductivity of c-BN increases linearly with pressure. Compared to isotope-enriched c-BN, the thermal conductivity of isotopically mixed c-BN is less sensitive to pressure variations since phonon-isotope scattering causes substantial thermal resistance yet is pressure independent. Based on mode-resolved analysis, the thermal conductivity enhancement is mainly attributed to the suppression of the aao absorption processes with transverse-acoustic phonons in the frequency range from 300 to 600 cm–1, due to the synergistic effect between decreased coupling strength and phonon hardening. In addition, suppression of the aaa processes is also responsible for a slight increase in thermal conductivity. Our results are further supported by the frequency- and mean-free-path-dependent spectra of the thermal conductivity. This work provided a fundamental interpretation of pressure-dependent phonon transport in c-BN from a microscopic perspective.

Response of vibrational properties and thermal conductivity of perovskites to pressure

We study the response of SrTiO3 and KTaO3's vibrational properties and thermal conductivity Λ to pressurization. Our goal is to improve the understanding of the relationship between crystal structure, vibrational dynamics, and thermal conductivity in perovskites. We measure the thermal conductivity of SrTiO3 and KTaO3 up to 28 GPa by time-domain thermoreflectance. We also perform Raman scattering and stimulated Brillouin scattering measurements of SrTiO3 and KTaO3 to characterize changes in vibrational dynamics with pressure. The thermal conductivity of SrTiO3 increases under pressure with a slope comparable to that of other perovskites whose thermal conductivity has been measured versus pressure. Alternatively, the thermal conductivity of KTaO3 has a stronger pressure dependence than that of other materials with similar crystal structure. We correlate pressure-induced changes in Raman and Brillouin spectra with pressure-induced changes in thermal conductivity. We show that pressure-induced changes in phonon lifetimes dominate the pressure dependence of thermal conductivity. This study provides benchmark knowledge of why Λ depends on pressure and improves understanding of structure/thermal-property relationships.

Pressure-dependent thermal conductivity in Al, W, and Pt: Role of electrons and phonons

Understanding the pressure dependence on thermal transport of metals is crucial for high-pressure applications and fundamental research in earth science. As high-pressure thermal measurements are challenging, the theoretical approach and first-principles methods are widely adopted to investigate the pressure-dependent thermal transport in metals. However, these approaches are generally limited to the free-electron metals and the phonon contributions are neglected. The mechanisms behind the pressure effect on thermal transport of metals are not fully addressed. In this work, we implement rigorous mode-level first-principles calculations to reveal different mechanisms behind the pressure-dependent electronic and phonon thermal conductivity in aluminum (Al), tungsten (W), and platinum (Pt). While the overall thermal conductivity values of the three metals all increase with pressure, the mechanisms are different. For the electronic thermal conductivity part, the main contribution for the positive pressure effect on free-electron metal Al is the decrease of the electron-phonon scattering rate, while the dominant contribution in Pt and W, whose $d$ state electrons are abundant around the Fermi energy, is the increase of electron group velocity. Phonon thermal conductivity of Pt and Al is found to increase with pressure, but the rates of increase are different. In contrast, phonon thermal conductivity of W is nearly independent with pressure. Such pressure invariance is due to the competing effect between phonon lifetime decrease and phonon group velocity increase under pressure.

Methodical selection of thermal conductivity models for porous silica-based media with variation of gas type and pressure

If the effective thermal conductivity of a silica powder in any gas atmosphere is to be calculated analytically, one is faced with a whole series of decisions. There are a lot of different models for the gas thermal conductivity in the pores, the thermal accommodation coefficient or the effective thermal conductivity itself in the literature. Furthermore, it has to be decided which input parameters should be used. This paper gives an overview and recommendations as to which calculation methods are best suited for the material classes of precipitated silica, fumed silica, silica gel and glass spheres. All combinations of the described methods result in a total of 2800 calculation models which are compared with pressure-dependent thermal conductivity measurements of 15 powdery materials with 7 different gases using Matlab computations. The results show that with a model based on a spherical unit cell, which considers local Knudsen numbers, the measuring points of all powder-gas combinations can be determined best with an average variance of about 18.5%. If the material class is known beforehand, the result can be predicted with an average accuracy of about 10% with the correspondingly determined methods.

Impact of the Backbone Connectivity on the Gas Pressure-Dependent Thermal Conductivity of Porous Solids

Impact of the Backbone Connectivity on the Gas Pressure-Dependent Thermal Conductivity of Porous Solids

Gap conductance modeling II: Optimized model for UO2-Zircaloy interfaces

The model conventionally used to calculate heat transfer across the fuel-cladding gap in light water nuclear reactors is a modified version of the Ross-Stoute model. The model was modified to include gap distance in the formulation, which introduced additional uncertainties because the model parameters were not adjusted after the modification. In this study, this conventional model is optimized for uranium dioxide-Zircaloy interfaces using experimental data at high pressure for single- and multi-component gases. First, a calibration is performed for single-component gases. Second, the calibration is extended to multi-component gases, which allows for a demonstration of sources of uncertainty in the model. Third, a general form of the gap conductance model is optimized by combining both data sets. Difficulties arise due to: (i) inaccurate estimation of contact characteristics (e.g., number of solid contacts, deformation mechanism of surface irregularities, contact shapes) that are different for each experimental setup; (ii) the non-physical ratio of temperature jump distance to the gap distance for postulated model function form; (iii) an insufficient description of the appropriate heat transfer regime; and (iv) the pressure dependence of thermal conductivity for inert gases aside from helium. Lastly, a general model is optimized by setting the temperature jump distance at the wall to zero, which reduces possible uncertainties. This final analysis results in a more accurate prediction of the available experimental data. The Associated parameter uncertainty of the model is estimated by performing uncertainty propagation. Overall, the optimized model results in a larger gap conductance with significantly reduced error.

Numerical study on the self-heating effects for vacuum/high-k gate dielectric tri-gate FinFETs

The self-heating effects (SHEs) for Vacuum Gate Dielectric (VGD) tri-gate FinFET compared with High-k (HK) Dielectric tri-gate FinFET were investigated here based on 3D numerical electro-thermal TCAD simulation method. Simulation results show that VGD device suffers from a fiercer SHEs than HK device which will result in larger saturation current degradation. It is due to the low thermal conductivity of the gas around the channel which impedes the heat diffusion to the sinks and increases the peak temperature in the hot spot region. The SHEs can be suppressed by increasing the width of gas gap and gas pressure, based on the analysis of the pressure-dependent thermal conductivity of the gas in Nano-gap. However, the increased gas gap width entails the loss of the gate control ability over the channel resulting in the severer short channel effects.

Non-monotonic pressure dependence of the thermal conductivity of boron arsenide

Recent experiments demonstrate that boron arsenide (BAs) is a showcase material to study the role of higher-order four-phonon interactions in affecting heat conduction in semiconductors. Here we use first-principles calculations to identify a phenomenon in BAs and a related material - boron antimonide, that has never been predicted or experimentally observed for any other material: competing responses of three-phonon and four-phonon interactions to pressure rise cause a non-monotonic pressure dependence of thermal conductivity, κ, which first increases similar to most materials and then decreases. The resulting peak in κ shows a strong temperature dependence from rapid strengthening of four-phonon interactions relative to three-phonon processes with temperature. Our results reveal pressure as a knob to tune the interplay between the competing phonon scattering mechanisms in BAs and similar compounds, and provide clear experimental guidelines for observation in a readily accessible measurement regime.

Measurements of thermal conductivity across the B1-B2 phase transition in NaCl

We present measurements of the pressure dependence of thermal conductivity for high pressure phases of KCl and NaCl using the laser-heated diamond anvil cell (LHDAC) and a 3D finite element model for heat flow. Temperature measurements are made in the LHDAC of KCl in the B2 phase, from 15 GPa to 24 GPa, and of NaCl from 14 GPa to 43 GPa, across the B1-B2 phase transition. The measurements are forward modeled, using the geometry and material properties of the cell as inputs, solving for the change in thermal conductivity between pressure steps. The results for B2 KCl indicate increasing thermal conductivity over the experimental pressure range and give dlnκdlnρ=3.75 ± 0.9. For NaCl, thermal conductivity increases in the B1 and B2 phases, dlnκdlnρ=1.6 ± 0.5 and dlnκdlnρ=2.9 ± 0.8, respectively. Our results constrain the reduction in thermal conductivity across the NaCl B1-B2 transition to 37% ± 7%.

Pronounced low-frequency vibrational thermal transport in C60 fullerite realized through pressure-dependent molecular dynamics simulations

Fullerene condensed-matter solids can possess thermal conductivities below their minimum glassy limit while theorized to be stiffer than diamond when crystallized under pressure. These seemingly disparate extremes in thermal and mechanical properties raise questions into the pressure dependence on the thermal conductivity of ${\mathrm{C}}_{60}$ fullerite crystals, and how the spectral contributions to vibrational thermal conductivity changes under applied pressure. To answer these questions, we investigate the effect of strain on the thermal conductivity of ${\mathrm{C}}_{60}$ fullerite crystals via pressure-dependent molecular dynamics simulations under the Green-Kubo formalism. We show that the thermal conductivity increases rapidly with compressive strain, which demonstrates a power-law relationship similar to their stress-strain relationship for the ${\mathrm{C}}_{60}$ crystals. Calculations of the density of states for the crystals under compressive strains reveal that the librational modes characteristic in the unstrained case are diminished due to densification of the molecular crystal. Over a large compression range (0--20 GPa), the Leibfried-Schl\omann equation is shown to adequately describe the pressure dependence of thermal conductivity, suggesting that low-frequency intermolecular vibrations dictate heat flow in the ${\mathrm{C}}_{60}$ crystals. A spectral decomposition of the thermal conductivity supports this hypothesis.

Pressure dependence of thermal conductivity and specific heat in CeRh2Si2 measured by an extended thermal relaxation method

We have developed a new technique for measuring thermal conductivity and specific heat under pressure by improving a thermal relaxation method. In this technique, a cylindrical sample with a small disc heater is embedded in the pressure-transmitting medium, then temperature variations of the sample and heater were directly measured by thermocouples during a heating and cooling process. Thermal conductivity and specific heat are estimated by comparing the experimental data with temperature variations simulated by a finite element method. The obtained thermal conductivity and specific heat of the test sample CeRh2Si2 exhibit a small enhancement and a clear peak arising from antiferromagnetic transition, respectively. The observation of these typical behaviors for magnetic compounds indicate that the technique is valid for the study on thermal properties under pressure.

Effect of Greenhouse Gases Dissolved in Seawater.

A molecular dynamics simulation has been performed on the greenhouse gases carbon dioxide and methane dissolved in a sodium chloride aqueous solution, as a simple model of seawater. A carbon dioxide molecule is also treated as a hydrogen carbonate ion. The structure, coordination number, diffusion coefficient, shear viscosity, specific heat, and thermal conductivity of the solutions have been discussed. The anomalous behaviors of these properties, especially the negative pressure dependence of thermal conductivity, have been observed in the higher-pressure region.

Competing mechanism driving diverse pressure dependence of thermal conductivity ofXTe(X=Hg,Cd, and Zn)

Effectively engineering the lattice thermal conductivity of materials is a key interest of the current thermal science community. Pressure or compressive strain is one of the most worthwhile processes to modify the thermal transport property of materials, due to its robust tunability and flexibility of realization. While it is well documented in the literature that the application of hydrostatic pressure normally increases the thermal conductivity of bulk materials, little work has been performed on abnormal pressure-dependent thermal conductivity and the governing mechanism has not been fully understood yet. In this paper, taking bulk telluride systems $X\text{Te}$ $(X=\mathrm{Hg},\phantom{\rule{0.16em}{0ex}}\mathrm{Cd},\phantom{\rule{0.16em}{0ex}}\mathrm{Zn})$ as examples, we show, by combining first-principle calculation and the phonon Boltzmann transport equation, that the thermal conductivity presents diverse pressure dependence although they belong to the same group. The thermal conductivity of ZnTe is independent of pressure, while abnormal negative pressure dependence of thermal conductivity is observed in HgTe. As for CdTe, the trend falls in between HgTe and ZnTe and relies largely on the temperature. By comparing the key contributors of the lattice thermal conductivity, we find that the diverse pressure dependence of the lattice thermal conductivity is governed by the competition between the enhancement of group velocity of longitudinal acoustic and optic modes and the reduction of phonon relaxation time of transverse acoustic modes, with both effects being fully quantified by our calculation. Comparison with traditional bulk systems such as silicon further underpins the governing mechanism. The correlation between the diverse thermal transport phenomena and the nature of the atomic bonding is also qualitatively established. These findings are expected to deepen our understanding of manipulating phonon transport of bulk materials via simple compressive strain and are also helpful for related applications, such as optimizing thermoelectric performance.

Density Scaling of the Thermal Conductivity of a Jet Oil

The application of the new quantitative elastohydrodynamics, which employs real properties of the liquid lubricant, to practical tribology problems obviously requires property measurements at high pressure to parameterize the correlations. The thermal conductivity under elevated pressure has been shown to be essential for nonisothermal situations. A new transient hot-wire instrument for pressure to 600 MPa is described and validated. New measurements are presented of the temperature- and pressure-dependent thermal conductivity of a synthetic oil for jet aircraft engines. The density scaling law, , with exponent approximately equal to the standard value of 3 gives an excellent accounting of the results. The temperature dependence of the conductivity may be ignored in elastohydrodynamic lubrication (EHL) simulations without great loss of accuracy and it may be sufficient to describe the pressure dependence with a general expression, .

Controlling Thermal Conductivity of Few-Layer Graphene Nanoribbons by Using the Transversal Pressure

We study the thermal transport of few-layer graphene nanoribbons in the presence of the transversal pressure by using molecular dynamics simulations. It is reported that the pressure can improve the thermal conductivity of few-layer graphene nanoribbons. This improvement can reach 37.5% in the low temperature region. The pressure dependence of thermal conductivity is also investigated for different length, width and thickness of few-layer graphene. Our results provide an alternative option to tuning thermal conductivity of few-layer graphene nanoribbons. Furthermore, it maybe indicate a so-called pressure-thermal effect in nanomaterials.

An integration to optimally constrain the thermal structure of oceanic lithosphere

[1] The evolution through time of the oceanic lithosphere is a substantial, incompletely resolved geodynamical problem. Consensus remains elusive regarding its thermal structure, physical properties, and the best model through which to unify observational constraints. We robustly reevaluate all three of these by (i) simultaneously fitting heat flow, bathymetry, and temperatures derived from a shear velocity model of the upper mantle, (ii) using the three main thermal models (half-space, plate, and Chablis), and (iii) analyzing five depth-age curves, wherein contrasting techniques were used to exclude anomalous features from seafloor depths. The thermal models are updated to all include a temperature-dependent heat capacity, a temperature- and pressure-dependent thermal conductivity, and an initial condition of adiabatic decompression including melting. The half-space model, which lets the lithosphere thicken indefinitely, cannot accurately fit the subsidence curves and requires mantle potential temperatures, Tm, that are too high. On the other hand, the models including a mechanism of basal heat supply are able to simultaneously explain all observations within two standard errors, with best-fitting parameters robust to the choice of the filtered bathymetry curve. For the plate model, which imposes a constant temperature at a fixed depth, Tm varies within 1380–1390°C, the equilibrium plate thickness a within 106–110 km, and the bulk thermal expansivity α¯ within 2.95−3.20 ⋅ 10−5 K−1. For the Chablis model, which prescribes a fixed heat flow at the base of a thickening lithosphere, the best-fitting values are Tm = 1320−1380°C, a = 176−268 km, α¯=3.05−3.60⋅10−5 K−1. Driven by more accurate ocean depths, the plate model provides better joint-fittings to the observations; however, it requires values of α¯ lower than experimentally measured, which can be explained by a reduction of the apparent expansivity due to elastic rigidity of the upper lithosphere. The Chablis model better fits the data when α¯ is set close to or above the experimental values. Although statistically consistent within two standard errors, a tendency toward incompatibility between observed depth-age curves and seismically derived temperatures is revealed with new clarity, because the latter do not exhibit a clear steady state whereas the former flatten; further work is needed to identify the origin of this apparent discrepancy.This work opens the way to investigations fully independent of particular solutions of the heat equation.

Testing the minimum thermal conductivity model for amorphous polymers using high pressure

Pressure dependence of thermal conductivity provides a critical test of the validity of the model of the minimum thermal conductivity for describing heat transport by molecular vibrations of an amorphous polymer. We measure the pressure dependence of the thermal conductivity \ensuremath{\Lambda}($P$) of poly(methyl methacrylate) using a combination of time-domain thermoreflectance and SiC anvil-cell techniques. We also determine \ensuremath{\Lambda}($P$) from a computational model of amorphous polystyrene. In both cases, \ensuremath{\Lambda}($P$) is accurately predicted by the minimum thermal conductivity model via the pressure dependence of the elastic constants and density.

Thermal conductivity of CaGeO3perovskite at high pressure

[1] The pressure dependence of thermal conductivity () in Earth's lower mantle is poorly understood, resulting in very uncertain values in the D″. Using orthorhombic CaGeO3 perovskite as an analogue for MgSiO3 perovskite, we measured in a multi-anvil apparatus using the Angstrom method at pressures of 8, 10, 14 and 18 GPa, and temperatures of 573–1073 K. We find the pressure dependence of thermal conductivity for orthorhombic perovskite shows a somewhat higher slope than that predicted by Debye theory, and account for this by relating phonon velocities to the bulk modulus, rather than to the Debye temperature. From this relation, we estimate the thermal conductivity of MgSiO3 perovskite at the top and bottom of the D″ to be 3.3 W/m K and 2.2 W/m K, corresponding to values of 6.4 W/m K and 4.5 W/m K for the mantle.