Cross-plane Thermal Conductivity Research Articles

Interfacial electric field and cross-plane thermal conductivity of GaN/InxGa1-xN superlattices (x = 0.1 and 0.3)

Superior thermoelectric property requires high electrical conductivity ( $$\sigma$$ ), Seebeck coefficient (S) but low thermal conductivity (k). GaN/InxGa1-xN/GaN superlattice (SL) has a strong interfacial polarization electric (IPE) field ~ 1 MV/cm at hetero-interfaces. Experiments reported that IPE field enhances S and $$\sigma$$ of the SL. In this paper, IPE field on thermal boundary resistance (TBR) and cross-plane thermal conductivity (kcp) of the SL is explored for indium contents x = 0.1 and 0.3 theoretically. IPE field revises phonon velocity causing enhanced boundary/interface scattering. Our result shows that TBR is enhanced (2.10–5. 30) ✕ 10–9 m2 KW−1 due to unequal changes in specific heat and phonon velocity on both sides of interface leading to decreased phonon transmission and acoustic properties of material become more dissimilar under IPE field. This reduced kcp. For GaN (10 nm)/InxGa1-xN (5 nm) SL, room-temperature (RT) kcp in the presence (absence) of IPE field is 4.652 (5.720) and 4.282 (5.221) Wm−1 K−1, respectively, for x = 0.1 and 0.3 proving more than 20% reduction. This work demonstrates that desired value of k can be achieved by tailoring interfacial polarization field of SL for optimum power production at RT and above.

Low-cost synthesis of high quality graphene oxide with large electrical and thermal conductivities

A simple and cost-effective method for synthesizing high quality thermally reduced graphene oxide (TrGO) thin films using Shellac, is presented. The synthesis temperature ranges from 550 °C to 900 °C, and is the sole control parameter to obtain a carbon content as high as 97 atomic percentage. For TrGO synthesized at 900 ˚C, its XPS, Raman, and TEM data indicate a large portion of sp2 hybridized carbon atoms and large crystal sizes (~1 µm), which resulted in very high electrical and cross-plane thermal conductivities, 2488 S/cm and 1.3 W/m-K, respectively. Furthermore, this high electrical conductivity of TrGO was observed to remain unaffected under mechanical stress 10 times higher than that completely broke solution-processed reduced graphene oxide (rGO) films, promising its potential for robust, transparent, and cost-effective conductive coating.

Polarization electric field effect on cross-plane thermal conductivity of GaN/InxGa1-xN/GaN superlattice (x ≤ 0.3)

GaN/InxGa1-xN/GaN superlattice (SL) has electric field at interfaces termed as interfacial polarization electric (IPE) field. In this work, effect of this electric field on cross-plane thermal conductivity (kcp) of the SL is investigated for Indium content x ≤ 0.3. IPE field revises phonon velocity which enhances interfacial scattering and thermal boundary resistance (TBR). This is due to unequal changes in specific heat and phonon velocity which leads to decrease in phonon transmission and more mismatches of acoustic properties of material. This reduces kcp. Room temperature kcp in presence (absence) of IPE field for GaN (10nm)/InxGa1-xN (5nm) SL are 4.652 (5.720) and 4.282 (5.221) Wm−1K−1 respectively, for x=0.1 and 0.3. This work demonstrates that electric field of nitride SL can be utilized to reduce k for optimum thermoelectric power production.

Anisotropic thermal conductivity measurement of organic thin film with bidirectional 3ω method.

Organic thin film materials with molecular ordering are gaining attention as they exhibit semiconductor characteristics. When using them for electronics, the thermal management becomes important, where heat dissipation is directional owing to the anisotropic thermal conductivity arising from the molecular ordering. However, it is difficult to evaluate the anisotropy by simultaneously measuring in-plane and cross-plane thermal conductivities of the film on a substrate because the film is typically as thin as tens to hundreds of nanometers and its in-plane thermal conductivity is low. Here, we develop a novel bidirectional 3ω system that measures the anisotropic thermal conductivity of thin films by patterning two metal wires with different widths and preparing the films on top and extracting the in-plane and cross-plane thermal conductivities using the difference in their sensitivities to the metal-wire width. Using the developed system, the thermal conductivity of spin-coated poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) with thickness of 70nm was successfully measured. The measured in-plane thermal conductivity of PEDOT:PSS film was as high as 2.9Wm-1K-1 presumably due to the high structural ordering, giving an anisotropy of 10. The calculations of measurement sensitivity to the film thickness and thermal conductivities suggest that the device can be applied to much thinner films by utilizing metal wires with a smaller width.

High-Throughput Computations of Cross-Plane Thermal Conductivity in Multilayer Stanene

Computational materials science based on data-driven approach has gained increasing interest in recent years. The capability of trained machine learning (ML) models, such as an artificial neural network (ANN), to predict the material properties without repetitive calculations is an appealing idea to save computational time. Thermal conductivity in single or multilayer structure is a quintessential property that plays a pivotal role in electronic applications. In this work, we exemplified a data-driven approach based on ML and high-throughput computation (HTC) to investigate the cross-plane thermal transport in multilayer stanene. Stanene has attracted considerable attention due to its novel electronic properties such as topological insulating features with a wide bandgap, making it an appealing candidate to ferry current in electronic devices. Classical molecular dynamics simulations are performed to extract the lattice thermal conductivities (κL). The calculated cross-plane κL is orders of magnitude lower than its lateral counterparts. Impact factors such as layer number, system temperature, interlayer coupling strength, and compressive/tensile strains are explored. It is found that κL of multilayer stanene in the cross-plane direction can be diminished by 86.7% with weakened coupling strength, or 66.6% with tensile strains. A total of 2700 κL data are generated using HTC, which are fed into 9 different ANN models for training and testing. The best prediction performance is given by the 2-layer ANN with 30 neurons in each layer.

Cross-plane thermal conductivity of GaN/AlN superlattices

Heterostructures consisting of alternating GaN/AlN epitaxial layers represent the building blocks of state-of-the-art devices employed for active cooling and energy-saving lightning. Insights into the heat conduction of these structures are essential in the perspective of improving the heat management for prospective applications. Here, the cross-plane (perpendicular to the sample's surface) thermal conductivity of GaN/AlN superlattices as a function of the layers' thickness is established by employing the 3ω-method. Moreover, the role of interdiffusion at the interfaces in the phonon scattering is taken into account in the modeling and data treatment. It is found that the cross-plane thermal conductivity of the epitaxial heterostructures can be driven to values as low as 5.9 W/(m·K) comparable with those reported for amorphous films, thus opening wide perspectives for optimized heat management in III-nitride-based epitaxial multilayers.

Above-room-temperature giant thermal conductivity switching in spintronic multilayers

Thermal switching provides an effective way for active heat flow control, which has recently attracted increasing attention in terms of nanoscale thermal management technologies. In magnetic and spintronic materials, the thermal conductivity depends on the magnetization configuration: this is the magnetothermal resistance effect. Here, we show that an epitaxial Cu/Co50Fe50 multilayer film exhibits giant magnetic-field-induced modulation of the cross-plane thermal conductivity. The magnetothermal resistance ratio for the Cu/Co50Fe50 multilayer reaches 150% at room temperature, which is much larger than the previous record high. Although the ratio decreases with increasing the temperature, the giant magnetothermal resistance effect of ∼100% still appears up to 400 K. The magnetic field dependence of the thermal conductivity of the Cu/Co50Fe50 multilayer was observed to be about twice greater than that of the cross-plane electrical conductivity. The observation of the giant magnetothermal resistance effect clarifies the potential of spintronic multilayers as thermal switching devices.

Innocuous, Highly Conductive, and Affordable Thermal Interface Material with Copper-Based Multi-Dimensional Filler Design.

Thermal interface materials (TIMs), typically composed of a polymer matrix with good wetting properties and thermally conductive fillers, are applied to the interfaces of mating components to reduce the interfacial thermal resistance. As a filler material, silver has been extensively studied because of its high intrinsic thermal conductivity. However, the high cost of silver and its toxicity has hindered the wide application of silver-based TIMs. Copper is an earth-abundant element and essential micronutrient for humans. In this paper, we present a copper-based multi-dimensional filler composed of three-dimensional microscale copper flakes, one-dimensional multi-walled carbon nanotubes (MWCNTs), and zero-dimensional copper nanoparticles (Cu NPs) to create a safe and low-cost TIM with a high thermal conductivity. Cu NPs synthesized by microwave irradiation of a precursor solution were bound to MWCNTs and mixed with copper flakes and polyimide matrix to obtain a TIM paste, which was stable even in a high-temperature environment. The cross-plane thermal conductivity of the copper-based TIM was 36 W/m/K. Owing to its high thermal conductivity and low cost, the copper-based TIM could be an industrially useful heat-dissipating material in the future.

Tacticity-dependent cross-plane thermal conductivity in molecularly engineered amorphous polymers

The present study demonstrated that not only chain extension but also torsion of repeating units and steric hindrance should be considered when designing a thermally conductive polymer by extended chain conformation.

Epoxy composites with high cross-plane thermal conductivity by constructing all-carbon multidimensional carbon fiber/graphite networks

In the new era of 5th generation mobile networks (5G), electron devices need to reach high power, frequency and transmission rate (Gbit/s) so as to dedicate to the 4K video, Virtual Reality, low latency communication, Internet of Things and so on. However, excess heat generated in devices has become a critical bottleneck to influence reliability and performance. Polymer composites with high thermal conductivity (K), low cost and light weight are urgently required for thermal management application. In this paper, all-carbon networks composed of 1D carbon fibers and 2D graphite sheets were constructed to reinforce epoxy at low addition by air flow technology and solution dipping strategy. The all-carbon networks show anisotropic structure, polymer composites have high cross-plane K up to 6.2 W/(m·K) with 17.48 vol% CFs and only 6.34 vol% graphite adhesive. The all-carbon networks could also improve the thermostability and electrical conductivity. Furthermore, the polymer composites with all-carbon networks display strong thermal management capability as observed by thermal infrared imaging. The epoxy composites demonstrate strong potential as interface materials integrated in electronic devices for heat dissipation and composites for aerospace and electromagnetic shielding.

Wide bandwidth frequency-domain thermoreflectance: Volumetric heat capacity, anisotropic thermal conductivity, and thickness measurements.

A frequency-domain thermoreflectance (FDTR) system with a frequency range of 1 Hz to 75 MHz is presented. The wide bandwidth of pump modulation frequency enables enhanced simultaneous thermophysical property characterization of bulk and thin-film materials. The wide bandwidth FDTR system is demonstrated with simultaneous measurements of in-plane and cross-plane thermal conductivities of sapphire and muscovite mica, thickness and thermal conductivity of gold/titanium thin films, and isotropic thermal conductivity and volumetric heat capacity of lithium niobate and silicon. Thermophysical properties measured with FDTR are compared to literature values or independent measurements for verification. Finally, at low frequencies, a negligible error was achieved in the numerical integration of the heat diffusion equation with a Gauss-Legendre quadrature method solved with 400 integration points and an upper integration limit of 16weff, where weff is the effective pump-probe 1/e2 radius.

Anisotropic thermal conductivity of AlGaN/GaN superlattices

High thermal conductivity is an important parameter for nitride-based power electronic and deep-UV light emitters. Especially in the latter case short period superlattices and multicomponent alloys are used and the knowledge of the thermal properties of the binary compounds is sufficient. In-plane and cross-plane thermal conductivity of AlGaN/GaN superlattices were measured by differential two-wire 3ω method in the temperature range from 147 to 325 K. Samples were grown by metalorganic vapor phase epitaxy; the structure quality and accuracy of superlattice structures preparation were verified by means of HRXRD and transmission electron microscopy. It was observed, that value of thermal conductivities decrease with decreasing period thickness, while temperature dependencies differ from each other—in-plane thermal conductivity decreases, and cross-plane—increases with increasing temperature. Callaway method was used for thermal conductivity calculation; dependence of boundary scattering rate on the phonon wavelength was taken into account. Minimum thermal conductivity was added to calculated values to include the influence of high frequency acoustic phonons and optical phonons on the heat transport. Calculations are in good agreement with experimental results.

Chain-Length Dependence of Thermal Conductivity in 2D Alkylammonium Lead Iodide Single Crystals.

In 2D organic-inorganic hybrid perovskite materials, layers of conducting inorganic material are separated by insulating organic spacers whose length and composition can be tuned. We report the heat capacity and cross-plane thermal conductivity of 2D alkylammonium lead iodide single crystals with increasing chain length, (CnH2n+1NH3)2PbI4 (n = 4-7). The measured thermal conductivities are some of the lowest ever recorded for single crystals, with averages in the range k = 0.099-0.125 W/m K. Although a model based on independent interface resistances between adjacent layers predicts an increase in thermal conductivity with a chain length of more than 30%, experimentally we find that the thermal conductivity is nearly independent of chain length and possibly decreases. We hypothesize that phonons carry an appreciable portion of the heat across the interface coherently, rather than being limited by individual weak interfaces.

Thermal-Dynamics Optimization of Terahertz Quantum Cascade Lasers with Different Barrier Compositions

The interplay of high operating temperatures and good heat dissipation is crucial for high-performance terahertz quantum cascade lasers. We therefore study the influence on the cross-plane thermal conductivity of different aluminum concentrations in the barrier of ${\mathrm{Ga}\mathrm{As}/\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ active regions. The thermal conductivity is decreasing from $30\phantom{\rule{0.1em}{0ex}}\mathrm{W}\phantom{\rule{0.1em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}\phantom{\rule{0.1em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}$ to $12\phantom{\rule{0.1em}{0ex}}\mathrm{W}\phantom{\rule{0.1em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}\phantom{\rule{0.1em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}$ if the aluminum concentration is increased from 15% to 24%. The temperature during pulsed-laser operation is obtained by measuring the variation of the emission frequency for different laser pulse lengths. This shows, that besides the thermal conductivity, the amount of electric input power has a strong influence on the temperature reached internally during laser operation and is critical for creating high-power devices operating at high temperatures. We show that active regions with thin but high ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ barriers fulfill this need and are well suited for high-temperature operation. A thermal model of the devices allows prediction of the active-region temperature increase for very short pulse durations. For the structure with 24% Al barriers and a starting temperature of 10 K, the model shows an increase by 24 K for a pulse length of only 300 ns.

Experimental Control and Statistical Analysis of Thermal Conductivity in ZnO-Benzene Multilayer Thin Films.

We have fabricated a model system of precisely layer-engineered inorganic–organic thin-film structures using atomic/molecular-layer deposition (ALD/MLD). The samples consist of nanoscale polycrystalline ZnO layers and intervening benzene layers, covering a broad range of layer sequences. The samples characterized in this study combined with previous publications provide an excellent sample set to examine thermal transport properties in inorganic–organic thin films. The cross-plane thermal conductivity is found to depend on multiple factors, with the inorganic–organic interface density being the dominating factor. Our work highlights the remarkable capability of interface engineering in suppressing the thermal conductivity of hybrid inorganic–organic materials, e.g., for thermoelectric applications.

Anisotropic thermal conductivity of the nanoparticles embedded GaSb thin film semiconductor

The prior theoretical model shows that GaSb is one of the few non-alloy semiconductors showing phonons ballistic effect in the thermal conductivity. However, no previous literature had been reported on the experimental measurements on the quasi-ballistic thermal transport of the GaSb thin film. In this paper, we employed the time-domain thermoreflectance (TDTR) to study the thermal transport of nanoparticles embedded GaSb thin film. Our measurements results provide first experimental evidence to verify the quasi-ballistic effect in the thermal transport of the GaSb thin film. The apparent cross-plane thermal conductivity of pure GaSb sample drops ∼15% when the pump laser modulation frequency is increased from 0.8 MHz to 10 MHz at room temperature. To further understand the thermal transport mechanism, Tempered Lévy analysis is employed to study the quasi-ballistic effect of the GaSb thin film. The model shows that GaSb thin film thermal transport has a superdiffusion exponent, = 1.51 ± 0.23 and Lévy-Fourier transition length, r LF = 0.19 ± 0.13 µm. Both obtained values via Tempered Lévy indicates the quasi-ballistic transport phenomena in GaSb thin film. However, this frequency dependence of the cross-plane thermal conductivity will disappear in the presence of the 3%–20% ErSb nanoparticles. Another thermal transport mechanism, i.e. anisotropic thermal transport, can be observed in GaSb thin film. The ratio of in- to cross-plane thermal conductivity varies from ∼0.2 to ∼0.7 in the 0%–20% ErSb nanoparticles volume concentrations. Detailed temperature dependence of the in-plane thermal conductivity of ErSb:GaSb samples with 0%–20% are also included in the paper for the understanding of the scattering mechanism in the thin film thermal transport. With enhanced understanding of the quasi-ballistic and anisotropic thin film thermal transport, our results might improve the thermal management efficiency of the GaSb devices.

In situ time-domain thermoreflectance measurements using Au as the transducer during electrolyte gating

Understanding the relationships between the thermal conductivity and carrier density in thin films is of great importance for the thermal management of flexible thin film electronics. Here, we report a robust measurement technique to tune the carrier density in thin films and to evaluate their cross-plane thermal conductivities simultaneously. We employed the time-domain thermoreflectance method using an Au transducer and evaluated the thin film thermal conductivity in situ using electrolyte gating with an ionic gel. The robust measurement technique proposed in this study elucidated the relationships among the above-mentioned parameters in semiconducting single-walled carbon nanotubes.

Intercalated ion tuning of the cross-plane thermal transport properties of graphite

The effects of the intercalated ion concentration on the cross-plane thermal conductivity and the thermal boundary conductance in the graphite/lithiated graphite interface are investigated from molecular dynamics simulations. At low ion concentration, the cross-plane thermal conductivity of the lithiated graphite is lower than that of the pristine graphite. However, as the intercalated ion concentration increases, the cross-plane thermal conductivity increases rapidly, even exceeding that of the pristine graphite at high ion concentration. By analyzing the variations of the cross-plane elastic constants and phonon dispersion relation with the intercalated ion concentration, it is found that the intercalated ions significantly increase the phonon irradiation heat flux along the cross-plane direction. Our study further shows that the variation of the intercalated ion concentrations can also modulate the thermal boundary conductance in the graphite/lithiated graphite interface. The non-equilibrium molecular dynamics simulations show that the thermal boundary conductance between graphite and lithiated graphite decreases as the lithiation level increases, which would worsen the thermal performance of Li-ion batteries. A one-dimensional atomic chain model is proposed to elaborate on how the effective spring stiffness of material influences the interfacial transmission of phonons with different frequencies. This work provides a quantitative calculation of the cross-plane thermal conductivity and thermal boundary conductance in intercalated graphite samples and is also extremely important for the thermal management and structural design of lithium-ion batteries.

Influence of stacking disorder on cross-plane thermal transport properties in TMPS3 (TM = Mn, Ni, Fe)

We investigated the thermal transport properties of magnetic van der Waals materials, TMPS3 (TM = Mn, Ni, and Fe), using the time-domain thermoreflectance technique. We determined the cross-plane thermal conductivity, which turns out to be relatively low, i.e., about 1 W m−1 K−1 for all TMPS3 investigated. When compared with previous results of graphite and transition metal dichalcogenides (TMDs), thermal conductivity becomes smaller as it goes from graphite to TMDs to TMPS3, and the difference is larger at low temperature, e.g., around 50 K. From the Callaway model analysis, we could attribute the large thermal conductivity reduction for TMPS3, particularly at low temperature, to the phonon scattering from the boundary. We actually confirmed the existence of the large population of the stacking faults with the cross-sectional transmission electron microscopy image of MnPS3. This suggests that intrinsic or extrinsic stacking faults formed in van der Waals materials and their heterostructures can play an important role in reducing the cross-plane thermal conductivity as a source of the boundary scattering.

Measurement of thermal transport properties of selected superlattice and thin films using frequency-domain photothermal infrared radiometry

Thermal transport properties in multi-layered semiconductor samples are reported using modulated photothermal infrared radiometry (PTR). The cross-plane thermal conductivity and diffusivity of thin AlxGa(1-x)As layers and AlAs/GaAs superlattices were determined by fitting solutions of the heat equation for multi-layered systems to PTR data. The thermal conductivity of an AlxGa(1-x)As film with x = 0.5 was found to be lower than an AlxGa(1-x)As film with x = 0.33 which is expected as scattering from alloy disorder is maximized for x = 0.5. In addition, it was found that thermal conductivities of AlAs/GaAs superlattices decrease when the superlattice period is decreased for a constant total thickness which is expected since the number of AlAs/GaAs interfaces (which impede thermal transport) increases as the period decreases. The maximum PTR amplitude signal was found to occur when the diffusion length of the thermal wave is on other order of the thickness of the semiconductor layer. The accuracy and applicability of photothermal infrared radiometry to the study of semiconductor multilayer structures are further discussed in the paper.