Thermal Conductivity Contrast Research Articles

Thermal diode using controlled capillary in heterogeneous nanopores

The development of a compact, efficient, reliable thermal diode is crucial to improve advanced thermal management efficiency and controllability, and to enable brand new applications such as thermal logic gates and computers. In this study, we examine a nanoscale and efficient capillary-controlled thermal diode mechanism in Ar-filled Pt-based heterogeneous nanoporous structures, using Grand Canonical Monte Carlo (GCMC) simulation combined with Non-Equilibrium Molecular Dynamics (NEMD) simulation at the temperature range of 70–150 K and the pressure of 1.66 atm. Results show that the large thermal conductivity contrast between the controlled adsorption and capillary states using the structural heterogeneity (nanopillars on only one surface) and/or material heterogeneity (two different materials for nanogap surfaces) allows for the maximum thermal rectification ratio, Rmax ∼ 140 with minimal hysteresis under the cyclic operating temperatures −40 K < ΔT < +40 K. It is also found that the material heterogeneity is equivalent to the structural heterogeneity for minimizing the hysteresis in adsorption-capillary transition, but the heat flux across the nanogap with the material heterogeneity reduces due to weaker Ar-solid interaction. The coupled structural-material heterogeneity for the capillary-driven thermal diode is also discussed. The obtained results pave pathways for advanced thermal management systems such as thermal transistors, thermal logic gates, and computers.

Conductive thermal diode based on the thermal hysteresis of VO2 and nitinol

We theoretically demonstrate that the junction between a phase-change material with a phase-invariant one can efficiently function as a conductive thermal diode. Analytical expressions for the heat flux and optimal rectification factor are derived and analyzed for junctions whose operations are driven by the thermal conductivity hysteresis of both VO2 and nitinol. It is shown that phase-change materials with higher thermal conductivity contrast, smaller thermal hysteresis, and faster phase transitions yield a conductive thermal diode with higher rectification of heat currents. Rectification factors of up to 19.7% and 18.8% are found for thermal diodes based on VO2 and nitinol, operating with a temperature difference between their terminals of 369.5 − 300 = 69.5 K and 388.2 − 273 = 115.2 K, respectively. These similar rectification factors could be enhanced by increasing the thermal conductivity variations of the diode terminals, and hence, the results obtained will be useful for guiding the development of phase-change materials capable of optimizing the rectification of conductive heat fluxes.

Computational Thermal Model of Unidirectional Composites with Random Fiber Array

The purpose of this work was to study the influence of microstructure on effective transverse thermal behavior of unidirectional fiber reinforced composites. Three types of microstructures are taken into account, including square periodic, hexagonal periodic and random arrangements of circular fibers. Unlike classical results at low fiber volume fractions and low thermal conductivity contrast between fibers and matrices, results provided by finite elements simulations for copper matrix composite reinforced with Carbon T-300 fibers have shown that random microstructures strongly affect the effective thermal properties of unidirectional composites for both high volume fractions and thermal conductivity contrast and can give closer predictions to the experimental results than the regular microstructures and the theoretical model.

Nano-Localized Thermal Analysis and Mapping of Surface and Sub-Surface Thermal Properties Using Scanning Thermal Microscopy (SThM).

Determining and acting on thermo-physical properties at the nanoscale is essential for understanding/managing heat distribution in micro/nanostructured materials and miniaturized devices. Adequate thermal nano-characterization techniques are required to address thermal issues compromising device performance. Scanning thermal microscopy (SThM) is a probing and acting technique based on atomic force microscopy using a nano-probe designed to act as a thermometer and resistive heater, achieving high spatial resolution. Enabling direct observation and mapping of thermal properties such as thermal conductivity, SThM is becoming a powerful tool with a critical role in several fields, from material science to device thermal management. We present an overview of the different thermal probes, followed by the contribution of SThM in three currently significant research topics. First, in thermal conductivity contrast studies of graphene monolayers deposited on different substrates, SThM proves itself a reliable technique to clarify the intriguing thermal properties of graphene, which is considered an important contributor to improve the performance of downscaled devices and materials. Second, SThM's ability to perform sub-surface imaging is highlighted by thermal conductivity contrast analysis of polymeric composites. Finally, an approach to induce and study local structural transitions in ferromagnetic shape memory alloy Ni-Mn-Ga thin films using localized nano-thermal analysis is presented.

Thermal conductivity switch: Optimal semiconductor/metal melting transition

Scrutinizing distinct solid/liquid ($s/l$) and solid/solid ($s/s$) phase transitions (passive transitions) for large change in bulk (and homogenous) thermal conductivity, we find the $s/l$ semiconductor/metal (S/M) transition produces the largest dimensionless thermal conductivity switch (TCS) figure of merit ${Z}_{\text{TCS}}$ (change in thermal conductivity divided by smaller conductivity). At melting temperature, the solid phonon and liquid molecular thermal conductivities are comparable and generally small, so the TCS requires localized electron solid and delocalized electron liquid states. For cyclic phase reversibility, the congruent phase transition (no change in composition) is as important as the thermal transport. We identify $X\mathrm{Sb}$ and $X\mathrm{As}$ ($X=\text{Al}$, Cd, Ga, In, Zn) and describe atomic-structural metrics for large ${Z}_{\text{TCS}}$, then show the superiority of S/M phonon- to electron-dominated transport melting transition. We use existing experimental results and theoretical and ab initio calculations of the related properties for both phases (including the Kubo-Greenwood and Bridgman formulations of liquid conductivities). The $5p$ orbital of Sb contributes to the semiconductor behavior in the solid-phase band gap and upon disorder and bond-length changes in the liquid phase this changes to metallic, creating the large contrast in thermal conductivity. The charge density distribution, electronic localization function, and electron density of states are used to mark this S/M transition. For optimal TCS, we examine the elemental selection from the transition, basic, and semimetals and semiconductor groups. For CdSb, addition of residual Ag suppresses the bipolar conductivity and its ${Z}_{\text{TCS}}$ is over 7, and for ${\mathrm{Zn}}_{3}{\mathrm{Sb}}_{2}$ it is expected to be over 14, based on the structure and transport properties of the better-known $\ensuremath{\beta}\ensuremath{-}{\mathrm{Zn}}_{4}{\mathrm{Sb}}_{3}$. This is the highest ${Z}_{\text{TCS}}$ identified. In addition to the metallic melting, the high ${Z}_{\text{TCS}}$ is due to the electron-poor nature of II-V semiconductors, leading to the significantly low phonon conductivity.

Exploiting length-dependent effects for the design of single-material systems with enhanced thermal transport properties

Topology optimization approaches for thermal transport problems generally rely on the presence of high contrast in thermal conductivity between constituent materials. Even though this might result in topological configurations that are optimal according to some desired thermal metric, the resulting systems might be impractical from a mechanical standpoint: high contrast in thermal conductivity generally translates into disparate ranges of thermal expansion, which in turn may lead to mechanical failure. In this paper, we focus on the combination of multi-scale modeling and topology optimization techniques for the design of polycrystalline single-material systems with enhanced thermal transport properties. Spatial variation of the length-dependent thermal conductivity is achieved by grading the characteristic grain size of the microstructure. We adopt a multi-scale approach to compute the attainable range of thermal conductivity due to grain-size effect, given maximum and minimum grain sizes and an operating temperature range. Then, we utilize an adaptive topology optimization technique to obtain an optimal grain size distribution, while accounting for manufacturing constraints. Through a series of specific examples, we demonstrate the extent to which thermal transport properties can be optimized without resorting to the adoption of multi-material systems.

Thermal Management in Long-Wavelength Flip-Chip Semiconductor Disk Lasers

We address the thermal management of flip-chip semiconductor disk lasers (SDLs) emitting at wavelengths 1.3–1.6 μm. The emphasis of the study is on fabricating thin SDL structures with high thermal conductance. An essential part of this task is to use GaAs-based materials in the distributed Bragg reflector (DBR), because they can provide a combination of high thermal conductivity and high refractive index contrast. Furthermore, the reflectivity of the GaAs-based DBR should preferably be enhanced using a thin dielectric layer and a highly reflecting metal layer. Such a configuration enables very thin mirror structures with a reduced number of DBR layer pairs without compromising the reflectivity. The concept is demonstrated experimentally with a 1.32-μm flip-chip SDL, where the GaAs-based DBR is finished with a thin Al2O3 layer and a highly reflective Al layer. In addition, the design principles, thermal management, and the development issues related to semiconductor–dielectric–metal mirrors in 1.3–1.6-μm flip-chip SDLs are discussed.

Methods for topography artifacts compensation in scanning thermal microscopy

Thermal conductivity contrast images in scanning thermal microscopy (SThM) are often distorted by artifacts related to local sample topography. This is pronounced on samples with sharp topographic features, on rough samples and while using larger probes, for example, Wollaston wire-based probes. The topography artifacts can be so high that they can even obscure local thermal conductivity variations influencing the measured signal. Three methods for numerically estimating and compensating for topographic artifacts are compared in this paper: a simple approach based on local sample geometry at the probe apex vicinity, a neural network analysis and 3D finite element modeling of the probe–sample interaction. A local topography and an estimated probe shape are used as source data for the calculation in all these techniques; the result is a map of false conductivity contrast signals generated only by sample topography. This map can be then used to remove the topography artifacts from measured data or to estimate the uncertainty of conductivity measurements using SThM. The accuracy of the results and the computational demands of the presented methods are discussed.

3ω Measurements for Tracking Freezing Fronts in Biological Applications

ABSTRACTOne approach to treating atrial fibrillation relies on freezing tissue of the heart wall. This surgical technology requires sub-millimeter spatial resolution when dynamically tracking the freezing of pulmonary vein; conventional techniques such as ultrasound lack the necessary precision. Here we use an electrothermal “3ω” method to track propagating freezing fronts in nearly real time. The heater line is excited with multiple frequencies simultaneously, and the freezing front detected as it passes through the various penetration depths due to the contrast between thermal conductivities on either side of the front. Comparison of water freezing experiments with video images further suggests the accuracy of the method. Analysis and experiments show how the uncertainty, time response, and measurement range depend on the frequencies and thermal conductivity contrast. Finally, the method is demonstrated on biological tissue as further proof of principle for medical applications.

Tunable Electrical and Thermal Transport in Ice-Templated Multilayer Graphene Nanocomposites through Freezing Rate Control

We demonstrate tunable electrical and thermal conductivities through freezing rate control in solution-based nanocomposites. For a prototypical suspension of 1 vol % multilayer graphene suspended in hexadecane, the solid-liquid electrical conductivity contrast ratio can be tuned from 1 to 4.5 orders of magnitude for freezing rates between 10(2) and 10(-3) °C/min. We hypothesize that this dramatic variation stems from ice-templating, whereby crystal growth drives nanoparticles into concentrated intercrystal regions, increasing the percolation pathways and reducing the internanoparticle electrical resistance. Optical microscopy supports the ice-templating hypothesis, as these dramatic property changes coincide with changing crystal size. Under the same range of freezing rates, the nanocomposite solid-liquid thermal conductivity contrast ratio varies between 2.3 and 3.0, while pure hexadecane's varies between 2.1 and 2.6. The nanocomposite's thermal conductivity contrast ratios and solid phase enhancements are greater than effective medium theory predictions. We suggest this is due to ice-templating, consistent with our electrical measurements, as well as nanoparticle-induced molecular alignment of alkanes.

Anomalous Thermal Conduction Characteristics of Phase Change Composites with Single-Walled Carbon Nanotube Inclusions

We report strikingly large contrasts in the thermal conductivity enhancement of phase change alkane in liquid and solid state with single-walled carbon nanotube (SWCNT) inclusions. With a small SWCNT loading of 0.25 wt % a strikingly high, 250% enhancement is achieved in the solid state and a nominal enhancement of 10% is achieved in the liquid state. The thermal conductivity contrast between solid and liquid state was found to increase with increasing SWCNT loading. The thermal conductivity contrast was more pronounced in the presence of SWCNTs compared to the presence of exfoliated graphite nanoplatelets reported in the literature.

Effect of solid thermal conductivity and particle–particle contact on effective thermodiffusion coefficient in porous media

Transient mass transfer associated to a thermal gradient through a saturated porous medium is studied experimentally and theoretically to determine the effect of solid thermal conductivity and particle–particle contact on thermodiffusion processes. In this study, the theoretical volume averaging model developed in a previous study has been adopted to determine the effective transport coefficients in the case of particle–particle contact configurations. The theoretical results revealed that the effective thermodiffusion coefficient is independent of the thermal conductivity ratio for pure diffusive cases. In all cases, even if the effective thermal conductivity depends on the particle–particle contact, the effective thermodiffusion coefficient remains independent of the solid phase connectivity. We also found that the porosity can change the impact of dispersion effects on the thermodiffusion coefficients. For large values of the thermal conductivity contrast, dispersion effects are negligible and the effective thermal conductivity coefficients are the same as the ones for the pure diffusion case. Experimental results obtained for the purely diffusive case, using a special two-bulb apparatus, confirm the theoretical results. These results also show that, for non-consolidated porous media made of spheres, the thermal conductivity ratio has no significant influence on the thermodiffusion process for pure diffusion. Finally, the particle–particle contact also does not show a considerable influence on the thermodiffusion process.

Measurement and Analysis of Thermal Conductivity of Rocks in the Tarim Basin, Northwest China

Abstract:As a parameter that describes heat transmission properties of rocks, thermal conductivity is indispensable for studying the thermal regime of sedimentary basins, and retrieving high‐quality data of thermal conductivity is the basis for geothermal related studies. The optical scanning method is used here to measure the thermal conductivity of 745 drill‐core samples from the Tarim basin, the largest intermontane basin with abundant hydrocarbon potential in China, and water saturation correction is made for clastic rock samples that are of variable porosity. All the measured values, combined with previously published data in this area, are integrated to discuss the distribution characteristics and major controlling factors that affect the thermal conductivity of rocks in the basin. Our results show that the values of thermal conductivity of rocks generally range from 1.500 to 3.000 W/m·K with a mean of 2.304 W/m·K. Thermal conductivity differs considerably between lithological types: the value of a coal sample is found to be the lowest as being only 0.249 W/m·K, while the values for salt rock samples are the highest with a mean of 4.620 W/m·K. Additionally, it is also found that the thermal conductivity of the same or similar lithologic types shows considerable differences, suggesting that thermal conductivity cannot be used for distinguishing the rock types. The thermal conductivity values of mudstone and sandstone generally increase with increasing burial depth and geological age of the formation, reflecting the effect of porosity of rocks on thermal conductivity. In general, the mineral composition, fabric and porosity of rocks are the main factors that affect the thermal conductivity. The research also reveals that the obvious contrast in thermal conductivity of coal and salt rock with other common sedimentary rocks can induce subsurface temperature anomalies in the overlying and underlying formations, which can modify the thermal evolution and maturity of the source rocks concerned. This finding is very important for oil and gas resources assessment and exploration and needs further study in detail. The results reported here are representative of the latest and most complete dataset of thermal conductivity of rocks in the Tarim basin, and will provide a solid foundation for geothermal studies in future.

Optical Measurement of Thermal Conductivity Using Fiber Aligned Frequency Domain Thermoreflectance

Fiber aligned frequency domain thermoreflectance (FAFDTR) is a simple noncontact optical technique for accurately measuring the thermal conductivity of thin films and bulk samples for a wide range of materials, including electrically conducting samples. FAFDTR is a single-sided measurement that requires minimal sample preparation and no microfabrication. Like existing thermoreflectance techniques, a modulated pump laser heats the sample surface, and a probe laser monitors the resultant thermal wave via the temperature dependent reflectance of the surface. Via the use of inexpensive fiber coupled diode lasers and common mode rejection, FAFDTR addresses three challenges of existing optical methods: complexity in setup, uncertainty in pump-probe alignment, and noise in the probe laser. FAFDTR was validated for thermal conductivities spanning three orders of magnitude (0.1–100 W/m K), and thin film thermal conductances greater than 10 W/m2 K. Uncertainties of 10–15% were typical, and were dominated by uncertainties in the laser spot size. A parametric study of sensitivity for thin film samples shows that high thermal conductivity contrast between film and substrate is essential for making accurate measurements.

Heat flow in salt-dome basins of Eurasia: A comparative study

The geothermal fields in the Pericaspian, Pripyat, and North German basins are considered. These basins are characterized by widespread Upper Paleozoic evaporite sequences, which underwent halokinesis with the formation of salt domes and plugs owing to tectonic and gravity instability. Heat flow refraction occurs at the boundaries of the domes with country rocks due to the contrast in thermal conductivity of evaporites and terrigenous rocks between the domal zones. This is the main cause of heat flow variation in the lateral and vertical directions in the salt-dome basins. Close correlation between zones of elevated temperature in the sedimentary rocks and petroleum occurrences is confirmed by the results of 2D and 3D modeling of the geothermal field. The previously noted relations of oil and gas fields to the deep faults in the studied basins create prerequisites for consideration of the geothermal field as a genetic factor controlling the tectonic features and petroleum resources of the salt-dome basins.

Role of thermal refraction in localizing intraplate deformation in southeastern Ukraine

The relative importance of the various factors that lead to intraplate deformation has been difficult to establish. Modelling results show that compressional deformation of a thick sedimentary basin in southeastern Ukraine was facilitated by strain localization resulting from the contrast in thermal conductivity between the basin and the surrounding crystalline crust. Although tectonic plates are deformed mostly at their boundaries, plate interiors can also show considerable non-rigid behaviour1,2,3. The deformational response of plate interiors to tectonic forces depends on composition and texture, temperature and confining pressure, strain rate, presence of fluids and pre-existing structure. However, the relative importance of these factors has been difficult to establish4. Here we use numerical modelling constrained by geological and geophysical data to assess the factors controlling intraplate deformation in southeastern Ukraine. The model’s starting point was the steady-state thermal structure in an otherwise tectonically stable but heterogeneous lithosphere. Our results show that compressional deformation and uplift of the thick Dniepr–Donets sedimentary basin was facilitated by strain localization resulting from temperature effects (thermal refraction) produced by the contrast in thermal conductivity between the sedimentary fill of the basin and the surrounding crystalline crust. We suggest that in settings where thick sedimentary basins occur in cold lithosphere, intraplate deformation can occur simply because of thermal conductivity contrasts, and the reactivation of inherited mechanical weaknesses may not be necessary.

Effect of interface scattering on phonon thermal conductivity percolation in random nanowire composites

The effect of phonon-interface scattering on the percolation of the phonon thermal conductivity in random nanowire composites is investigated by solving the phonon Boltzmann transport equation using the Monte Carlo simulation. The forming of a geometric percolating network does not lead to dramatic increase of the thermal conductivity in nanowire composites because of the dominance of the phonon-interface scattering. The thermal conductivity of nanocomposites is much less sensitive to the thermal conductivity contrast ratios of the constituent materials than that of bulk composites, especially when the volumetric concentration of the high thermal conductivity constituent is below the percolation threshold.

Thermal conductivity contrast measurement of fused silica exposed to low-energy femtosecond laser pulses

Femtosecond laser irradiation has various noticeable effects on fused silica. Of particular interest, pulses with energy levels below the ablation threshold can locally increase the refractive index and the material etching selectivity to hydrofluoric acid. The mechanism responsible for these effects is not yet fully understood. In this letter, the authors report on local thermal conductivity mapping of laser-affected zones. It is found that these zones exhibit a lower thermal conductivity at room temperature.

Improving strength by controlling segregation in liquid phase sintered aluminium nitride ceramics

SEM investigations on liquid phase sintered AlN ceramics showed segregation of the liquid phase. The liquid phase concentrated in clusters of some tens of micrometers in diameter. Crack deflection was observed in the vicinity of the clusters indicating high thermal stresses which could impair strength and reliability of the ceramics. Therefore, a thermodynamic model based on interface and grain boundary energies was established. With it the parameters were identified which caused segregation of the liquid phase during sintering. Due to the high contrast in thermal conductivity between liquid phase and AlN, wetting properties of the liquid phase could be determined indirectly by measuring the thermal conductivity of the ceramics at temperatures between 1600 and 1850 °C. In situ measurements of shrinkage and thermal conductivity were performed using a Thermooptical measuring device (TOM). With it a sintering process was developed which led to a homogeneous microstructure without segregation. An increase of flexural strength by 25% and of Weibull modulus higher than 80% was achieved.

Nature of a regional dogleg pattern in maturity profiles from Thrace basin, northwestern Turkey: A newly discovered unconformity or a thermal anomaly?

Vitrinite reflectance (%Ro) profiles of the Thrace basin are characterized by a regional dogleg pattern corresponding to the depths of Ceylan and Hamitabat formations. We investigated the nature of this dogleg pattern using a one-dimensional thermal-maturity model by simulating three possible geological scenarios: (1) two possible newly discovered unconformities, one between the Hamitabat and Soucak or Hamitabat and Ceylan formations and the other within the Hamitabat Formation; (2) an abnormal paleoheat-flow regime (paleothermal kick) that developed in the Oligocene and extended until the end of the early Miocene; and (3) a sharp thermal-conductivity contrast in the stratigraphic column caused by thermally low conductive deposits of the Ceylan Formation, which act as a regional thermal blanket. The sensitivity of each suggested scenario within the model and their effect on the observed dogleg pattern have been examined. A comparison of the measured and calculated (modeled) %Ro profiles from the proposed scenarios shows that the dogleg pattern can be best explained by a combination of the second (paleoheat flow) and third scenarios (thermal conductivity) instead of the unconformity scenario. However, the discovery of these two newly defined discordance surfaces with the help of well and seismic data requires a significant modification to the chronostratigraphy of the Thrace basin. The Hamitabat, Karaaa, Ftepe, and Gaziky formations and the lower sections of the Kean Formation are suggested to be time-correlative counterparts in a newly constructed chronostratigraphy. Thus, the Hamitabat Formation can be evaluated as a part of the early–middle Eocene sequence similar to the Gaziky Formation. In this article, we suggest that the Hamitabat Formation be classified as a group instead of a formation.