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Combining the superior thermal conductivity of graphene and the outstanding heat storage of paraffin, micro-nanocavity graphene/paraffin nanocomposites (MNGPNs) have recently served as promising thermal management materials in high-power microelectronic devices. However, current evaluations of the thermal management performances of MNGPNs are restricted in the lab condition, deviating from the complex mechanical–thermal coupling environment in practical scenarios. Here, we have investigated the structural and thermal management properties of MNGPNs with varying mechanical loads by in situ electron microscopy and in situ thermal characterizations. Our results reveal distinct mechanical–thermal coupling effects along in-plane and out-of-plane directions of MNGPNs. Specifically, mechanical loading reduces the porosity and enhances the heat transfer capacity of MNGPNs in the out-of-plane direction, while mechanical loading along the in-plane direction causes local damage to the graphene structure and weakens the heat transfer capacity of MNGPNs. Since the heat management performance of MNGPNs is dominated by the in-plane thermal transport, MNGPNs with mechanical loading show a delayed phase transition response time and unchanged phase transition enthalpy. This work provides in situ mechanical guidance for the practical application of MNGPNs for heat management. [ABSTRACT FROM AUTHOR]

Here, we report the temperature-dependent electrical resistivity and thermopower of hole (Sr) and electron (Hf) doped LaCoO3 in the range of 303–753 K. With increasing temperature, the insulating behavior (303–503 K) with dominance of small polaron hopping to metallic transition (>503 K) is observed. The electron doped sample shows an insulating behavior (19.5 Ω cm) and positive thermopower (139 μV K−1) value due to the spin state blockade, i.e., electron hopping from high spin Co2+ to low spin Co3+ is strongly inhibited. The calculated weighted mobility ( μ W ) of 0.01 to 0.96 cm 2 V − 1 s − 1 validates the observed spin blockade mechanism in electron doped LaCoO3. The fluctuation of spin/orbital ordering and point defect scattering results in the low thermal conductivity of 0.5 W m − 1 K − 1 for Hf doped LaCoO3. The spin state blockade observed in the electrical resistivity and low lattice thermal conductivity reveals that spin state transition drives the thermoelectric response in Mott insulator LaCoO3. [ABSTRACT FROM AUTHOR]

Accurate measurement of thermal properties in thin films is crucial for optimizing devices and deepening our understanding of heat transfer at nano and micro scales. This study presents a combined experimental and computational investigation on a chip-integrated technique for the assessment of in-plane thermal properties of thin films. This method stands out by incorporating inherent error cancelation to lessen the impact of radiative heat loss and allows simultaneous, independent determination of both thermal conductivity and diffusivity through straightforward linear fittings from the same dataset, reducing error propagation. We examine an 84 nm thick SiNx membrane over a temperature range from 100 K to nearly 500 K, aligning with previous studies. Further investigations into a conducting polymer film post-doping demonstrate a notable increase in both thermal conductivity and diffusivity, corroborating scanning thermal microscopy observations, confirming the technique's efficacy and reliability. [ABSTRACT FROM AUTHOR]

Strain provides an additional mechanism in tuning the thermal/electrical properties of nanostructures and therefore has triggered lots of interest in recent years. However, experimental research about the strain effect on the thermal conductivity of nanowires is still limited, especially in the low-temperature range, which is important in understanding the physics of strain-induced regulation in thermal conductivity. Here, we present thermal transport measurements of bent silicon carbide nanowires at temperatures ranging from 20 to 300 K. Reduction in thermal conductivity compared to their straight counterparts is observed. More specifically, the relative change is up to 55% at 20 K and descends with temperature, which is due to the inhomogeneous strain-induced phonon scattering. This study will deepen the understanding of thermal properties in nanostructures with strain. [ABSTRACT FROM AUTHOR]

This study investigates spatially resolved phonon-mediated thermal transport across nano-sized grains and grain boundaries (GBs) in 3C-SiC using molecular dynamics (MD) simulations. This investigation involves controlling the complete range of inter-grain misorientation tilt angles (θ = 0°–90°) and nanoscale grain sizes (d = 2.18–130.77 nm). The grain boundary energy and interfacial thermal transport are found to be highly θ -sensitive and asymmetric with respect to θ = 45° due to the low symmetry associated with two interpenetrating diatomic SiC fcc lattices. When adjacent grains are tilted at θ = 14.25°, the interfacial heat conduction is highly suppressed compared to other θ values, especially for larger grains. The most stable atomic configuration of the GB region associated with the minimal GB energy results in the highest suppression of heat conduction across the GB interface. Spatially resolved thermal anisotropy reveals a strong GB-mediated nanoscale hydrodynamic phonon Poiseuille effect when heat flows parallel to the GB planes, as shown by our perturbed MD study. With the reduction of d , the intra-grain and inter-GB thermal conductivities decrease due to the enhanced phonon scattering from interfaces, but the difference between these conductivities becomes negligible for the heat flow normal to the GB planes. It is envisioned that nanoscale spatially resolved control of thermal energy will provide useful guidance to engineer nanocrystalline ceramics with tunable interfacial thermal properties. [ABSTRACT FROM AUTHOR]

In the context of the extensive application prospect of two-dimensional (2D) chalcogenides, we synthesized layered SnSe2+δ bulks with defects employing a hybrid chemical vapor transport-melt approach. Both the Eg and A1g Raman characteristic peaks in SnSe2+δ are dominated by cubic anharmonicity, coupled with nonlinear temperature dependencies below 140 K. Notably, the reduction in phonon energy observed in these vibrational modes can be ascribed to defect-mediated Raman scattering, irrespective of deficient or excess Se defects. However, the lower consistency in the Raman shifts of the in-plane Eg vibrations compared to the out-of-plane A1g modes suggests that the defects predominantly entail the absence of Se atoms and the substitutions of Sn by Se, delineating a continuum of Se-deficient and Se-enriched compositions. Furthermore, Se defects induce the contraction of the indirect bandgaps, facilitating a transition from medium to narrow bandgap semiconductors in SnSe2+δ, which underscores the tunable nature of the bandgaps through the incorporation of Se defects. These discoveries present an avenue for bandgap engineering and foster a deeper comprehension of the phonon and thermal properties of layered chalcogenides for further advanced technologies. [ABSTRACT FROM AUTHOR]

Thermal transport properties of metallic nanowires are crucial to the near-junction heat dissipation of transistors. However, the understanding of the size-dependent thermal conductivity of these advanced interconnect metals is still limited. In this work, we select nine candidate metals and investigate the size effect on thermal transport properties by the mode-level first-principles method combining with the Boltzmann transport equation. Their thermal conductivity, the phonon contribution, and the Lorenz ratio in nanowires with characteristic size from 3 to 30 nm are analyzed. While all these metals have lower bulk thermal conductivity than Cu, we find some of these metals have larger thermal conductivity with characteristic size smaller than 10 nm. We identified that their smaller electron mean free path is the key factor. Moreover, the contribution of phonon thermal conductivity is smaller than 25% to total thermal conductivity. The Lorenz ratio is found to be slightly larger than the Sommerfeld value, mainly due to the phonon contribution. This work can provide important guidance for selecting advanced interconnects in the development of next-generation integrated circuits. [ABSTRACT FROM AUTHOR]

Due to the unique structures, van der Waals (vdW) materials have advantages over traditional magnetothermal materials in manipulating magnetothermal properties through structural modification and in cooling applications in nanodevices. Here, we study the magnetothermal properties of vdW ferromagnet Fe5GeTe2 with Curie temperature around room temperature. The results show that Fe5GeTe2 is a second-order magnetic phase transition material, and its in-plane and out-of-plane values of relative cooling power are large up, respectively, to 299.3 and 269.2 J/kg for a field change of 5 T. Compared to other vdW materials reported, Fe5GeTe2 has the greatest potential for room-temperature magnetic cooling applications. [ABSTRACT FROM AUTHOR]

Due to the unique structures, van der Waals (vdW) materials have advantages over traditional magnetothermal materials in manipulating magnetothermal properties through structural modification and in cooling applications in nanodevices. Here, we study the magnetothermal properties of vdW ferromagnet Fe5GeTe2 with Curie temperature around room temperature. The results show that Fe5GeTe2 is a second-order magnetic phase transition material, and its in-plane and out-of-plane values of relative cooling power are large up, respectively, to 299.3 and 269.2 J/kg for a field change of 5 T. Compared to other vdW materials reported, Fe5GeTe2 has the greatest potential for room-temperature magnetic cooling applications. [ABSTRACT FROM AUTHOR]

Although anisotropic phononic crystals (PnCs) could be utilized to control the phonon dispersions and thermal transports, rapidly discovering their properties presents a significant challenge due to the enormous consumption of traditional computational methods. In this study, we have developed machine learning techniques to forecast the thermal conductance of anisotropic PnCs (GPnC and GPnC/Gmem) based on the elastic constants, taking conventional inorganic and halide perovskites as examples for their thermoelectric applications. Our findings suggest that predicting GPnC/Gmem is more challenging than predicting GPnC attribute to the complex influence factors and spatial distribution patterns of the former. The GPnC and GPnC/Gmem of the weakest thermal anisotropic materials—all hexagonals are invariants in the (0 0 1) plane, because the velocities in this plane are direction-independent. The GPnC and GPnC/Gmem of the strongest thermal anisotropic material FAPbI3 reaches the minimum and maximum values in [1 1 0] and [1 0 0] directions, respectively. Ultimately, our machine learning models can map the hidden complex nonlinear relationships between target thermal properties and mechanical features to provide valuable insight for accurate and efficient prediction and analysis of the thermal behaviors of PnCs at a mesoscopic level under low temperatures. [ABSTRACT FROM AUTHOR]

NbOCl2 is an emerging layered ferroelectric (FE) material with unique photoelectric properties. However, the phonons and thermal properties of NbOCl2 have remained elusive. Here, we report measurements of Raman and second harmonic generation spectrum of NbOCl2 flakes as a function of temperature (80–500 K). All observed Raman modes exhibit temperature-dependent shift. The first-order temperature coefficients of P1, P2, P3, P4, and P5 modes are 0.001 58, −0.005 21, −0.006 44, −0.012 56, and 0.036 75 cm−1 K−1, respectively. Interestingly, the frequency of the P1 mode changes directly from 161 to 156 cm−1 with increasing temperature, which is attributed to the switching between two independent Raman modes (P1H and P1L). The P1H mode is mainly contributed by the FE phase, while the P1L mode is mainly contributed by the antiferroelectric phase. Our results provide a fundamental understanding and analysis of the thermal–phonon coupling properties of few-layer NbOCl2 and promote the deep exploration of the basic physical properties of few-layer NbOCl2, which is crucial for the design and application of NbOCl2-based thermal and optoelectronic devices in the future. [ABSTRACT FROM AUTHOR]

NbOCl2 is an emerging layered ferroelectric (FE) material with unique photoelectric properties. However, the phonons and thermal properties of NbOCl2 have remained elusive. Here, we report measurements of Raman and second harmonic generation spectrum of NbOCl2 flakes as a function of temperature (80–500 K). All observed Raman modes exhibit temperature-dependent shift. The first-order temperature coefficients of P1, P2, P3, P4, and P5 modes are 0.001 58, −0.005 21, −0.006 44, −0.012 56, and 0.036 75 cm−1 K−1, respectively. Interestingly, the frequency of the P1 mode changes directly from 161 to 156 cm−1 with increasing temperature, which is attributed to the switching between two independent Raman modes (P1H and P1L). The P1H mode is mainly contributed by the FE phase, while the P1L mode is mainly contributed by the antiferroelectric phase. Our results provide a fundamental understanding and analysis of the thermal–phonon coupling properties of few-layer NbOCl2 and promote the deep exploration of the basic physical properties of few-layer NbOCl2, which is crucial for the design and application of NbOCl2-based thermal and optoelectronic devices in the future. [ABSTRACT FROM AUTHOR]

The importance of higher-order anharmonic effects on thermal transport has recently been demonstrated in highly symmetrical 2D materials with large acoustic–phonon (A–O) gap. However, the phonon scattering and the thermal transport properties in low-symmetry structures remain ambiguous. In this work, we employed moment tensor potential and Boltzmann transport equation to investigate phonon thermal transport properties of 1T′-ReS2 and 2H-WS2. We show that the mechanism of four-phonon scattering in 1T′-ReS2 is quite different from that in 2H-WS2. In 1T′-ReS2, the four-phonon scattering can reduce the thermal conductivity by up to 40.29%, even in the absence of an acoustic-optical phonon bandgap. The strong four-phonon scattering in 1T′-ReS2 is attributed to A–O phonon scattering mediated by abundant flattening optical phonon modes. However, in 2H-WS2, the strong four-phonon scattering is attributed to the presence of a large A–O phonon bandgap. Our work suggests that considering the four-phonon scattering is essential for calculating the thermal conductivity of 2D materials, even in the absence of an A–O phonon gap. [ABSTRACT FROM AUTHOR]

Thermal transport within nanostructures is highly confined by interfaces, and non-trivial physics can emerge at boundaries. Theoretical studies have shown that different phonon modes can exhibit varying thermal resistances at interfaces. Experimental observation of these variations, however, is lacking. Using the steady-state Raman thermometry, the E 2 g 1 and A1g vibrational modes of MoS2 were utilized to characterize the thermal transport properties across the MoS2/Si interface. Our results revealed distinct temperature rises associated with different modes, indicating various mode contributions in the interfacial thermal conductance. Combining experimental and numerical simulations, the out-of-plane mode in MoS2 was found to contribute less to the interfacial transport, by 21.5%, attributed to the less variational mode mismatch of the in-plane phonon, compared to the in-plane mode. Furthermore, our results confirmed a 26.9% higher thermal conductivity from the out-of-plane mode than the in-plane one. [ABSTRACT FROM AUTHOR]

The thermal management has been widely focused due to its broad applications. Generally, the deformation can largely tune the thermal transport. The main challenge of flexible electronics/materials is to maintain thermal conductance under large deformation. This work investigates the thermal conductance of a nano-designed Si metamaterial constructed with curved nanobeams by molecular dynamics simulation. Interestingly, it shows that the thermal conductance of the nano-designed Si metamaterial is insensitive under a large deformation (strain ∼ −41%). The new feature comes from the designed curved nanobeams, which exhibit a quasi-zero stiffness. Further calculations show that, when under large deformation, the average stress in nanobeam is ultra-small (<151 MPa), and its phonon density of states are little changed. This work provides valuable insight on the multifunction, such as both stable thermal and mechanical properties, of nano-designed metamaterials. [ABSTRACT FROM AUTHOR]

As an ultrawide bandgap (∼4.1 eV) semiconductor, single crystalline SrSnO3 (SSO) has promising electrical properties for applications in power electronics and transparent conductors. The device performance can be limited by heat dissipation issues. However, a systematic study detailing its thermal transport properties remains elusive. This work studies the temperature-dependent thermal properties of a single crystalline SSO thin film prepared with hybrid molecular beam epitaxy. By combining time-domain thermoreflectance and Debye–Callaway modeling, physical insight into thermal transport mechanisms is provided. At room temperature, the 350-nm SSO film has a thermal conductivity of 4.4 W m−1 K−1, ∼60% lower than those of other perovskite oxides (SrTiO3, BaSnO3) with the same ABO3 structural formula. This difference is attributed to the low zone-boundary frequency of SSO, resulting from its distorted orthorhombic structure with tilted octahedra. At high temperatures, the thermal conductivity of SSO decreases with temperature following a ∼T−0.54 dependence, weaker than the typical T−1 trend dominated by the Umklapp scattering. This work not only reveals the fundamental mechanisms of thermal transport in single crystalline SSO but also sheds light on the thermal design and optimization of SSO-based electronic applications. [ABSTRACT FROM AUTHOR]

In this study, we employ a machine-learning potential approach based on first-principles calculations combined with the Boltzmann transport theory to investigate the impact of lithium-ion de-embedding on the thermal conductivity of LiFePO4, with the aim of enhancing heat dissipation in lithium-ion batteries. The findings reveal a significant decrease in thermal conductivity with increasing lithium-ion concentration due to the decrease in phonon lifetime. Moreover, removal of lithium ions from different sites at a given lithium-ion concentration leads to distinct thermal conductivities, attributed to varying anharmonicity arising from differences in bond lengths and bond strengths of the Fe-O bonds. Our work contributes to a fundamental understanding of the thermal transport properties of lithium iron phosphate batteries, emphasizing the pivotal role of lithium-ion detachment and intercalation in the thermal management of electrochemical energy storage devices. [ABSTRACT FROM AUTHOR]

In this study, we employ a machine-learning potential approach based on first-principles calculations combined with the Boltzmann transport theory to investigate the impact of lithium-ion de-embedding on the thermal conductivity of LiFePO4, with the aim of enhancing heat dissipation in lithium-ion batteries. The findings reveal a significant decrease in thermal conductivity with increasing lithium-ion concentration due to the decrease in phonon lifetime. Moreover, removal of lithium ions from different sites at a given lithium-ion concentration leads to distinct thermal conductivities, attributed to varying anharmonicity arising from differences in bond lengths and bond strengths of the Fe-O bonds. Our work contributes to a fundamental understanding of the thermal transport properties of lithium iron phosphate batteries, emphasizing the pivotal role of lithium-ion detachment and intercalation in the thermal management of electrochemical energy storage devices. [ABSTRACT FROM AUTHOR]

We investigate the electrical and thermal conduction properties of low- (ON) and high-resistance (OFF) states in Pt/NiOx/Pt based unipolar ReRAM devices during cooling and warming cycles between 300 and 180 K. The conduction electron-trap activation energy was found to decrease upon warming. Although thermal cycling did not significantly affect the average resistance-temperature coefficient of the Pt diffused conductive filaments in the system, the ON-state resistance fluctuations increase at high temperatures, indicating that ambient temperature significantly affects the sizes of the formed filaments. The mechanism behind these thermally activated changes is discussed. [ABSTRACT FROM AUTHOR]

This paper studies thermal transport in nanoporous silicon with a significant specific surface area. First, the equilibrium molecular dynamics approach was used to obtain the dependence of thermal conductivity on a specific surface area. Then, a modified phonon transport kinetic theory-based approach was developed to analyze thermal conductivity. Two models were used to evaluate the phonon mean free path in the porous materials. The first model assumes that the dependence of the mean free path only relies on the specific surface area, and the second one also considers the mean free path variation with the porosity. Both approaches approximate molecular dynamics data well for the smaller porosity values. However, the first model fails for highly porous matrixes, while the second one matches well with molecular dynamics simulations across all considered ranges of the porosities. This work illustrates that the phonon mean free path dependence with the porosity/volume fraction of composite materials is essential for describing thermal transport in systems with significant surface-to-volume ratio. [ABSTRACT FROM AUTHOR]

Normally, the introduction of porous structures into materials can tune their thermal conductivity, showing great applications in thermal management and thermoelectric energy harvesting. However, the ability of disorder changing the thermal conductivity of porous materials has seldom been explored. In this work, we show that an introduction of disorder into the macroscopic porous materials with a certain porosity can lead to a desired effective thermal conductivity over a large range, where an abnormal enhancement of ∼7.9% and a normal reduction of ∼44% at room temperature are predicted by the machine-learning-optimized algorithm. All of these theoretical calculation results are further verified by our experiments performed in the current work by using the steady-state thermal flux method. Moreover, when these periodic units are artificially connected, a structural anisotropy up to 40 is achieved, which can be further used to adjust the direction of the thermal flux in a well-controlled way. Our work provides an efficient and convenient approach for designing high-performance porous materials with specific thermal conductivity and high structural anisotropy for various applications in thermal management. [ABSTRACT FROM AUTHOR]

Characterizing spatially varying thermal conductivities is significant to unveil the structure–property relation for a wide range of thermal functional materials such as chemical-vapor-deposited (CVD) diamonds, ion-irradiated materials, nuclear materials under radiation, and battery electrode materials. Although the development of thermal property microscopy based on time/frequency-domain thermoreflectance (TDTR/FDTR) enabled in-plane scanning of a thermal conductivity profile, measuring depth-dependent profiles remains challenging. This work proposed a machine-learning-based reconstruction method for extracting depth-dependent thermal conductivity K (z) directly from pump–probe phase signals. We demonstrated that the simple supervised-learning algorithm kernel ridge regression can reconstruct K (z) without requiring pre-knowledge about the functional form of the profile. The reconstruction method can not only accurately reproduce typical K (z) distributions such as the exponential profile of CVD diamonds and the Gaussian profile of ion-irradiated materials but also complex profiles artificially constructed by superimposing Gaussian, exponential, polynomial, and logarithmic functions. In addition to FDTR, the method also shows excellent performance of reconstructing K (z) of ion-irradiated semiconductors from TDTR signals. This work demonstrates that combining machine learning with pump–probe thermoreflectance is an effective way for depth-dependent thermal property mapping. [ABSTRACT FROM AUTHOR]

This study investigated the impact of a Ge2Sb2Te5 (GST) thermal barrier on the performance of NbO2-based selector devices. Our findings showed that the GST barrier could significantly decrease the off-state leakage current from 3 μA to 300 nA without increasing the threshold switching voltage owing to its insulation properties and high thermal resistance. We also found that the GST barrier can effectively contain the Joule heat within the NbO2 switching region, as confirmed through a cryogenic analysis of the thermal resistance of GST. The results showed that the GST/NbO2 device had a thermal resistance 3.48 times higher than that of a single-layer NbO2 device. Our results provide design guidelines for utilizing a barrier layer to reduce the leakage current in low-power threshold switching devices. [ABSTRACT FROM AUTHOR]

A method to reveal low temperature absolute internal quantum efficiency of an InGaN-based light-emitting diode (LED) is reported. Structural (via scanning electron and atomic force microscopies, x-ray diffraction, and Raman spectroscopy) and optical (via temperature- and power-dependent photoluminescence) properties of InGaN-based blue LEDs are coupled with a channel-based recombination model, which reveals the low temperature absolute internal quantum efficiency of the LED on traditional sapphire and emerging Si (111) substrates as 27.5% and 71.1%, respectively. High low-temperature absolute internal quantum efficiency of the former is attributed to its lower defect density and less strain. Our results show assuming perfect (i.e., 100%) internal quantum efficiency in an InGaN-based LED at low temperatures could be misleading, and using negative thermal quenching properties of defect luminesce bands with a channel-based recombination model can be a method to quantify LED's less than ideal low temperature internal quantum efficiencies. [ABSTRACT FROM AUTHOR]

Here, we report the thermal transport properties of lead telluride (PbTe)1−x(AgBiTe2)x (x = 0.05 and 0.15) alloys. It is found that the prominent peak in lattice thermal conductivity at 15 K for PbTe disappears after forming solid solution with AgBiTe2, exhibiting a typical glass-like thermal transport behavior. The high energy phonons are scattered by the point defects induced by cationic disorder, while the appearance of soft vibrational modes arises from Ag atoms acting like Einstein oscillators (ΘE1 = 5.4 K, ΘE2 = 67.3 K), which substantially affects the lattice thermal conductivity. Further with nanostructuring, the mid-frequency phonons are scattered by the high-density disc-like Ag2Te precipitates. As a result, an ultralow lattice thermal conductivity (≤1.0 W m−1 K−1) for (PbTe)0.85(AgBiTe2)0.15 is obtained, which is the lowest value ever reported to date for the PbTe-based TE materials. Our work highlights a synthetic route to achieve glass-like lattice thermal conductivity in PbTe over the entire temperature range. [ABSTRACT FROM AUTHOR]

As the pseudo-binary alloys between GeTe and Sb2Te3, GeSbTe-based compounds are promising thermoelectric materials. Although Ge2Sb2Te5 and GeSb2Te4 have widely been studied, the thermoelectric properties of GeSb4Te7 have not been well understood yet. In this work, we design a series of GeSb4-xBixTe7 solid solutions and systematically study the variation in the crystal structure, electrical, and thermal transport properties. Alloying Bi effectively reduces the carrier concentration and, thus, enhances the Seebeck coefficient. Meanwhile, the thermal conductivity is greatly reduced via large mass fluctuations. A maximum zT of 0.69 at 550 K and an average zT value of 0.52 between 300 and 700 K have been achieved for GeSb1.5Bi2.5Te7. These findings will promote the understanding and development of GeSbTe-based thermoelectric materials. [ABSTRACT FROM AUTHOR]

The layered solid electrolyte Li2ZrCl6 and Li metal electrodes have a very good contact stability, but the thermal transport properties of Li2ZrCl6 are still unclear. Here, we systematically study the intrinsic lattice thermal conductivity (κp) of Li2ZrCl6 using the machine-learning potential approach based on first-principles calculations combined with the Boltzmann transport theory. The results show that the κp of Li2ZrCl6 at room temperature is 3.94 W/mK along the in-plane (IP) direction and 1.05 W/mK along the out-plane (OP) direction, which means that the κp is significantly anisotropic. In addition, under the compressive stress in the OP direction, the κp evolution along the IP and OP directions exhibits completely different trends, because the stress has a significant regulatory effect on the contribution of optical phonons to κp. With the increase in stress, the κp in the IP direction monotonically decreases, while the κp in the OP direction increases by a factor of 2.2 under a compressive strain of 13%. This is because the contribution of low-frequency optical phonons to κp in the IP direction is as high as 58% when no stress is applied, and this contribution is significantly suppressed with increasing compressive strain. However, the contribution of optical phonons in the OP direction to the κp increases with the increase in stress. Our results reveal the thermal transport properties of Li2ZrCl6 and the effect of the compressive strain on the κp of Li2ZrCl6, thereby providing a reference for the use of Li2ZrCl6 in Li-metal batteries. [ABSTRACT FROM AUTHOR]

Black arsenic (b-As) with extreme in-plane anisotropy, incredible ambient stability, and excellent transport performance has unlimited potential for device applications. Uncovering phonon–phonon (ph–ph) interactions is a critical step in understanding its transport, especially thermal transport properties. Here, we report on the phonon anharmonicity in an exfoliated b-As flake using temperature-dependent polarized Raman spectroscopy from 80 to 300 K. Polarization of A g 2 mode helps identifying the crystalline orientation of the flake. A nonlinear redshift of peak frequencies of A g 1 , B 2 g , and A g 2 modes is observed along the given crystalline orientation. Contributions from intrinsic ph–ph interactions are extracted: cubic-phonon decay dominates the ph–ph scattering processes at low temperature, while cubic- and quartic-phonon decays together do at high temperature. It is further confirmed by the nonlinear full-width-at-half-maxima broadening. Our findings disclose the phonon anharmonicity in b-As and give insight for better understanding and utilizing its thermodynamic properties. [ABSTRACT FROM AUTHOR]

The allotrope of carbon, biphenylene, was prepared experimentally recently [Fan et al., Science 372, 852–856 (2021)]. In this Letter, we perform first-principles simulation to understand the bonding nature and structure stability of the possible in-plane heterostructure built by graphene and biphenylene. We found that the graphene–biphenylene in-plane heterostructure only exhibits along the armchair direction, which is connected together by strong covalent bonds and energetically stable. Then, the non-equilibrium molecular dynamics calculations are used to explore the interfacial thermal properties of the graphene/biphenylene heterostructure. It is found that the graphene/biphenylene in-plane heterostructure possesses an excellent interfacial thermal conductance of 2.84 × 109 W·K−1·m−2 at room temperature. Importantly, the interfacial thermal conductance presents different temperature dependence under opposite heat flux direction. This anomalous temperature dependence results in increased thermal rectification ratio with temperature about 40% at 350 K. This work provides comprehensive insight into the graphene–biphenylene heterostructure and suggests a route for designing a thermal rectifier with high efficiency. [ABSTRACT FROM AUTHOR]

High-temperature processing steps are frequently used in manufacture of AlGaN/(AlN/)GaN high electron mobility transistors (HEMTs). The thermal budgets drive Al diffusion into the GaN channel, increase alloy disorder scattering (ADS) of the two-dimensional-electron-gas (2DEG), and degrade the 2DEG mobility. By correlating the ADS to device thermal budget, we propose an analytical model to calculate the ADS limited mobility as a function of Al diffusion lengths (LD,Al) in AlGaN/(AlN/)GaN HEMTs. The simulation indicates that the ADS is a significant mobility degradation mechanism whose scattering rate increases with the 2DEG density and the LD,Al. The findings explain the generally observed trends in the literature that the 2DEG mobility and sheet resistances of GaN heterostructures degrade with increased thermal budgets, especially after >900 °C processing. [ABSTRACT FROM AUTHOR]

Direct bonded Si-on-SiC is an interesting alternative to silicon-on-insulator (SOI) for improved thermal management in power conversion and radio frequency applications in space. We have used transient thermoreflectance and finite element simulations to characterize the thermal properties of direct bonded Si-on-4H–SiC samples, utilizing a hydrophobic and hydrophilic bonding process. In both instances, the interface has good thermal properties resulting in TBReff values of 6 + 4/−2 m2 K GW−1 (hydrophobic) and 9 + 3/−2 m2 K GW−1 (hydrophilic). Two-dimensional finite element simulations for an equivalent MOSFET showed the significant thermal benefit of using Si-on-SiC over SOI. In these simulations, a MOSFET with a 200 nm thick, 42 μm wide Si drift region was recreated on a SOI structure (2 μm buried oxide) and on the Si-on-SiC material characterized here. At 5 W mm−1 power dissipation, the Si-on-SiC was shown to result in a >60% decrease in temperature rise compared to the SOI structure. [ABSTRACT FROM AUTHOR]

One of the key challenges of phase change memory (PCM) is its high power consumption during the reset operation, when the phase change material (typically Ge2Sb2Te5, i.e., GST) heats up to ∼900 K or more in order to melt. Here, we study the temperature-dependent behavior of PCM devices by probing the reset power at ambient temperatures from 80 to 400 K. We find that different device structures exhibit contrasting temperature-dependent behavior. The reset power in our confined-type PCM is nearly unchanged with ambient temperature, corresponding to a temperature-dependent thermal resistance, whereas results for mushroom-type PCM from the literature show a linear relation between power and temperature, suggesting a more constant thermal resistance. This discrepancy is ascribed to different temperature distributions and thermal properties of the dominant components of the PCM cell thermal resistance, as shown by electro-thermal modeling. In the confined cell, the thermal boundary resistance of the GST and the thermal conductivity of the bottom electrode dominate the thermal resistance, while for the mushroom cell, the GST thermal conductivity plays a greater role. These findings can help to design more power- and energy-efficient PCM devices by better focusing thermal management efforts on the key components of the device. [ABSTRACT FROM AUTHOR]

We blend n-type Bi2Te3 with an inexpensive abundant conducting polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), to gain a bulk-structured composite, in which energy filtering and phonon scattering effects should occur at the interface of two components. The composite records somewhat higher electrical resistivity than pristine Bi2Te3, because the interface possibly interrupts carrier transport. However, the composite completely compensates for the resistivity increment with a significant increase in the Seebeck coefficient, which is caused by energy filtering effects at the interface; thus, it exhibits the improved power factor. The composite also records a much lower thermal conductivity than the pristine Bi2Te3 because of phonon scattering effects at the interface. The composite induces significant decoupling of electrical and thermal properties, thus affording the remarkably enhanced figure of merits (ZTmax ∼ 1.19 at 132 °C, ZTave ∼ 1.14 at 50–150 °C), which are approximately double those of the pristine Bi2Te3. The ZT values are not only predominant among the performance of n-type binary Bi2Te3, but they are also as competent as the excellent performance of n-type ternary Bi2(Te,Se)3 previously reported. [ABSTRACT FROM AUTHOR]

Compared to extensive studies of thermal transport in two-dimensional materials, very limited attention has been paid to the corresponding phenomenon in quasi-one-dimensional van der Waals crystals. Here, we show that Ta2Se3 can be easily exfoliated into thin nanowires, indicating strong anisotropy in the bonding strength within the basal plane. Systematic thermal property measurements disclose signatures of one-dimensional phonons as the nanowire hydraulic diameter reduces below 19.2 nm with linearly escalating thermal conductivity as temperature increases and size dependence inconsistent with the classical size effect. We further show that these unusual transport properties are induced by elastic stiffening occurring for wires of <30 nm diameter. [ABSTRACT FROM AUTHOR]

Independently controlling electronic and thermal transport in solids is a challenge, because these properties are coupled. Here, we show that disordered nanoporosity in Ca3Co4O9 thin films can decrease the thermal conductivity without significantly hampering electronic transport. Scanning thermal microscopy was used to determine the out-of-plane thermal conductivity and estimate the in-plane values. Nanoporous Ca3Co4O9 films exhibit a thermal conductivity of 0.82 W m−1 K−1, which is nearly twofold lower than that obtained from nonporous Ca3Co4O9 films. Nanoporous Ca3Co4O9 exhibit a room-temperature electrical resistivity of 4 mΩ cm, which is comparable to polycrystalline Ca3Co4O9 and twice that reported for single-crystal Ca3Co4O9. Our results suggest that controlling nanoporosity and their degree of disorder can offer a means of decoupling electrical and thermal properties in materials. [ABSTRACT FROM AUTHOR]

Dispersion of surface acoustic waves (SAWs) caused by a dynamic surface heating–cooling cycle has been applied for evaluation of thermal properties. A laser-generated broad-band SAW pulse is propagated through the material whose elastic properties and density were modified by the dynamic inhomogeneous temperature field produced by laser heating and subsequent cooling. A sample with depth-dependent material properties introduces a phase velocity dispersion in SAW propagation. Dispersion was measured by a differential technique, which helped to isolate the thermally induced dispersion and eliminate the contributions of other possible sources of dispersion. The dispersion curves were measured at several time instants, which allowed evaluation of the following parameters: thermal diffusivity, temperature dependence of the shear modulus, and the temperature magnitude. The inverse problem was solved by fitting the experimental dispersion curves into the theoretical ones by means of differential evolution techniques. [ABSTRACT FROM AUTHOR]

Germanium (Ge) plays a crucial role in setting up important functionalities for silicon-compatible photonics. Diamond cubic germanium is an extensively studied semiconductor, although its other exotic forms, like BC8, ST8, ST12 phases, may possess distinct electronic properties. We have fabricated stable ST12-Ge nanowires via a self-seeded bottom-up three phase growth in a confined supercritical toluene environment. Here, we report on the direct evidence of the presence of the ST12 phase by a combination of Raman spectroscopy and first-principles calculations using density functional theory (DFT). It is important to remark that the DFT calculation predicts all the Raman active optical phonon modes of the P 4 3 2 1 structure, and it is in very good agreement with the experimental results. The phonon dynamics as a function of temperature is investigated through Raman measurements at temperatures varying from 80 to 300 K. First-order temperature coefficients for all the observed Raman modes are estimated from the linear temperature dependence of the phonon shifts. A complete set of isobaric Grüneisen parameters is reported for all Raman modes of ST12-Ge nanowire, and the values are lower compared to the same for Si, dc-Ge bulk, and Ge nanowire. These results have important implications for understanding thermal properties of ST12-Ge nanowire. [ABSTRACT FROM AUTHOR]

In the last decade, several works have focused on exploring the material and electrical properties of GeTe/Sb2Te3 superlattices (SLs), in particular because of some first device implementations demonstrating interesting performances such as fast switching speed, low energy consumption, and non-volatility. However, the switching mechanism in such SL-based devices remains under debate. In this work, we investigate the prototype GeTe/Sb2Te3 SLs to analyze fundamentally their electronic and thermal properties by ab initio methods. We find that the resistive contrast is small among the different phases of GeTe/Sb2Te3 because of a small electronic gap (about 0.1 eV) and a consequent semi-metallic-like behavior. At the same time, the out-of-plane lattice thermal conductivity is rather small, while varying up to four times among the different phases, from 0.11 to 0.45 W m−1 K−1, intimately related to the number of Van der Waals (VdW) gaps in a unit block. Such findings confirm the importance of the thermal improvement achievable in GeTe/Sb2Te3 superlattices devices, highlighting the impact of the material stacking and the role of VdW gaps on the thermal engineering of the phase-change memory cell. [ABSTRACT FROM AUTHOR]

Scanning Thermal Microscopy (SThM) provides efficient thermal property measurement with micro- or nanoscale spatial resolution. However, the sensitivity and accuracy of state-of-the-art thermal probes have been limited by excessive thermal contact resistance between the probe and sample. Introduced herein is a robust thermal microprobe that can increase the probe-sample contact force by more than two orders of magnitude, thereby reducing the probe-sample thermal contact resistance by as much as 96% and increasing measurement sensitivity by more than 240% compared to a commercial thermal probe with the same dimensions and measurement principle. The relationship between the probe-sample thermal contact resistance, thermal exchange radius, and sample thermal conductivity is determined experimentally. Simultaneous measurement of thermal conductivity and Seebeck coefficient with unprecedented sensitivity is demonstrated using the enhanced scanning thermal microprobe on samples of an extended range of thermal conductivity up to 18 W/m K, increasing the range of samples applicable to SThM when compared to the conventional commercial probe with diminished measurement sensitivity above ∼10 W/m K. The probe is further demonstrated by simultaneously mapping thermal conductivity and Seebeck coefficient as a function of depth from the irradiated surface of an ion-irradiated bulk nanostructured thermoelectric material. In addition to enabling microscale thermal conductivity and Seebeck coefficient measurement of materials previously not applicable to SThM, the probe can also facilitate high-throughput characterization of combinatorial materials to aid the rapid discovery of compositions and processing conditions that yield highly desired thermal and thermoelectric properties. [ABSTRACT FROM AUTHOR]

Low-dimensional hybrid organic–inorganic metal halide perovskites are rapidly emerging as a fascinating sub-class of the three-dimensional parent structures, thanks to their appealing charge and thermal transport properties, paired to better chemical and thermal stabilities. Extensive investigations of the thermal behavior in these systems are of paramount relevance to understand their optoelectronic and thermoelectric applications. Herein, we present a complete thermophysical characterization of imidazolium lead iodide, (IMI)PbI3, a 1D pseudo-perovskite with chains of face-sharing octahedra, and histammonium lead iodide, (HIST)PbI4, a 2D layered perovskite with corner-sharing octahedra. Upon heating, the two compounds show highly anisotropic thermal expansion effects and high thermal stability until 250–300 °C. The thermal diffusivity of pelletized powders was measured with the laser flash technique from room temperature up to 225 °C. To account for the reduced density of the pelletized powders with respect to the bulk, the diffusivity data in different atmospheres were modeled as a function of the volume fraction and dimensionality of the pores, allowing to extrapolate the thermal conductivity of the bulk materials. The two compounds exhibit an ultralow thermal conductivity of 0.15 W/m K, two to three times lower than that reported on 3D MAPbI3 using the same technique. This finding suggests the primary role of the organic molecules within the hybrid systems, regardless of the octahedra connectivity and dimensionality. [ABSTRACT FROM AUTHOR]

Long-range spatial coherence can be induced in incoherent thermal emitters by embedding a periodic grating within a material supporting propagating polaritons or dielectric modes. However, only a single spatially coherent mode is supported by purely periodic thermal emitters. While various designs have been proposed for the purpose of allowing arbitrary emission profiles, the limitations associated with the partial spatial coherence of thermal emitters are not known. Here, we explore superstructure gratings (SSGs) to control the spatial and spectral properties of thermal emitters. SSGs have long-range periodicity but employ a unit cell that provides multiple Bragg vectors to interact with light. These Bragg vectors allow simultaneous launching of polaritons with different frequencies/wavevectors in a single grating, manifesting as additional spatial and spectral modes in the thermal emission profile. However, SSGs still have a well-defined period, which allows us to assess the role that finite spatial coherence plays in thermal emitters. We find that the spatial coherence length defines the maximum possible SSG period that can be used. This provides a fundamental limit on the degree of spatial coherence that can be induced in a thermal emitter and has broader implications for the use of techniques such as inverse design for structure optimization. [ABSTRACT FROM AUTHOR]

Hematite (α-Fe2O3) has several attractive properties such as corrosion resistance, catalytic activity, sensing properties, and magnetic features but also a room-temperature stable thermal conductivity of about 16 W/m K. Its use in polymer-matrix composites as a thermal performance enhancer is rather uncommon. In this study, hematite and graphene nanoplatelet (GnP) hybrids in a rubbery latex matrix were prepared and their thermal properties were characterized. The hybrids were mechanically stabilized into freestanding films by hot-pressing them into a porous cellulosic membrane. Optimization of the total filler concentration and the α-Fe2O3/GnP ratio yielded thermal interface material (TIM) films with a thermal conductivity of 8.0 W/mK. Infrared measurements showed that the TIMs significantly improved heat sink cooling and demonstrated rapid heat transfer in a system simulating stacked up electronic packing. [ABSTRACT FROM AUTHOR]

Unlike many superlattice structures, Ruddlesden–Popper phases have atomically abrupt interfaces useful for interrogating how periodic atomic layers affect thermal properties. Here, we measure the thermal conductivity in thin films of the n = 1–5 and 10 members of the (SrTiO3)nSrO Ruddlesden–Popper superlattices grown by molecular-beam epitaxy and compare the results to a single crystal of the n = 1 Ruddlesden–Popper SrLaAlO4. The thermal conductivity cross-plane to the superlattice layering (k33) is measured using time-domain thermoreflectance as a function of temperature and the results are compared to first-principles calculations. The thermal conductivity of this homologous series decreases with increasing interface density. Characterization by x-ray diffraction and scanning transmission electron microscopy confirms that these samples have a Ruddlesden–Popper superlattice structure. [ABSTRACT FROM AUTHOR]

It is expected to develop the measurement system to obtain physical/chemical information with nanoscale space resolution related to the non-thermal equilibrium phenomena. In this study, we developed controlled temperature-gradient kelvin force microscopy (T-KFM) to measure the temperature difference (ΔT)-induced vacuum level variation under non-thermal equilibrium. Therein, the biggest issue, difficulty in applying the large ΔT in narrow space (∼100 μm), was solved by introducing "heating and cooling systems" in T-KFM; one sample side is heated using a ceramic heater and the other side is cooled using liquid nitrogen. Using T-KFM, the space distribution of ΔT-induced vacuum level variation was well observed on the scale of hundreds of nanometers in a polycrystalline ZnO film with nanostructures. The obtained image of the ΔT-induced vacuum level variation can reflect a distribution of the thermal properties such as the thermal resistance and thermoelectromotive force. This pronounced technique for obtaining surface potential under T-gradient helps us to comprehend the non-thermal equilibrium phenomena. [ABSTRACT FROM AUTHOR]

Layered indium selenide (InSe) has emerged as a promising two-dimensional semiconductor due to its high electron mobility and direct optical bandgap in the few-layer limit. As InSe is integrated into high-performance electronic and optoelectronic systems, thermal management will become critical, thus motivating detailed characterization of intrinsic thermal properties. Here, we report the room-temperature thermal conductivity of exfoliated crystals of InSe along the through-plane and in-plane directions using conventional and beam offset time-domain thermoreflectance (TDTR), respectively. InSe crystals with varying thicknesses were prepared by mechanical exfoliation onto Si(100) wafers followed by immediate encapsulation with a 3-nm-thick AlOx passivation layer to prevent ambient degradation prior to coating with metal films for TDTR measurements. The measured thermal conductivity in the in-plane direction, Λin ≈ 8.5 ± 2 W/m K, is an order of magnitude higher than that in the through-plane direction, Λthrough ≈ 0.76±0.15 W/m K, which implies a high thermal anisotropy ≈11 ± 3. These relatively high anisotropy and low thermal conductivity compared to other layered semiconductors imply that InSe will require unique thermal management considerations when implemented in electronic, optoelectronic, and thermoelectric applications. [ABSTRACT FROM AUTHOR]

Cellulose nanofibers (CNF) are abundant biomaterials that have attracted significant attention in the thermal management field for a wide range of applications including flexible heat dissipation materials and thermal insulators. While thermal transport properties of individual CNFs are significant for the fundamental understanding and design of advanced materials, experimental studies of the thermal transport properties of CNFs are limited to bulk scales and thermal measurement on individual CNFs has not been reported to date. We report here the experimental study on the thermal conductivity (κ) of individual CNFs using the well-established thermal bridge method. The κ of individual CNFs is found to be approximately 2.2 (±1.2) W/m K at 300 K, and the temperature dependent data from 40 to 320 K indicate that the phonon transport of CNFs is dominated by boundary scattering. Theoretical simulation results on κ of individual CNFs and cellulose bulk crystal support the experimental results and suggest that intermolecular interaction also impedes the thermal transport. [ABSTRACT FROM AUTHOR]

In order to improve the performance of thermoelectric materials, nanoinclusions are often used to enhance phonon scattering. In this study, activated carbon, which is porous and thus has a large surface area, was incorporated in GeTe-based materials to cause increased boundary phonon scattering. Carbon dispersed in grain boundaries resulted in improved thermal properties without significant deterioration in electrical properties. Due to the extrinsic addition of activated carbon, the lattice thermal conductivity decreased by 13.8% on average. A maximum dimensionless figure of merit of 1.66 was achieved at 723 K for the Ge0.9Sb0.1Te composition with additional activated carbon. [ABSTRACT FROM AUTHOR]

Due to the high-porosity structure, the low thermal transport property of graphene foam (GF) is expected. However, the interconnected skeleton can still act as excellent thermal conductor branches if phonon scattering is not severely affected in the structure of graphene flakes. Such a property has not been validated experimentally due to the difficulty in sample manipulation and the fragility of the structure. In this work, we report the characterization results of thermal properties of the free-standing skeleton in GF. Three individual skeleton samples from one GF piece are prepared under the same condition. The thermal diffusivity of GF skeletons is characterized in the range of 3.26–3.48 × 10−4 m2/s, and the thermal conductivity is determined to be 520–555 W/(m K), which is two orders of magnitude larger than the value of bulk GF. These high thermal conductivity values originate from the intrinsic thermal property of graphene, while the contact interfaces, wrinkled structures, and defects induced in the synthesis process do not affect the phonon transport property significantly, which proves that the three-dimensional hierarchical graphene structure can still be implemented in energy-intensive applications. [ABSTRACT FROM AUTHOR]

We study the thermal conductivity distribution of hypothetical graphene-like materials composed of carbon and heavy carbon atoms. These materials are representative of alloys and disordered materials, which are relatively unexplored for thermal properties owing to their large configuration spaces. Since the full thermal conductivity calculations using the Boltzmann transport equation based solutions are computationally prohibitive for each of the 232 considered configurations, we employ regularized autoencoders, a class of generative machine learning models that transform the configuration space to the latent space in which materials are clustered according to the target property. Such conditioning allows selective sampling of high thermal conductivity materials from the latent space. We find that the model is able to learn the underlying thermal transport physics of the system under study and is able to predict superlattice-like configurations with high thermal conductivity despite their higher mass. [ABSTRACT FROM AUTHOR]

Thermoelectric colusites, one of the most recently identified and most promising family of complex Cu–S materials, have quickly attracted significant attention based on their outstanding performance. Herein, we investigate the effect of zinc for copper substitution on the thermoelectric properties of the high-performance Cr–Ge colusite, Cu26Cr2Ge6S32. We discuss the striking impact of the aliovalent Zn/Cu substitution on the charge carrier mobility and effective mass and the consequences on the electrical and thermal transport properties. The investigation is supported by first-principles calculations of the electronic density-of-states of doped colusites. The theoretical study reveals the removal of the sharp features at the top of the valence manifold with the incorporation of Zn in the conductive network, with a strong reduction in the estimated relaxation time. These theoretical and experimental observations confirm the importance of disorder within the conductive network and the high sensitivity of the Cu-S tetrahedral framework toward defects in high-performance thermoelectric colusites. [ABSTRACT FROM AUTHOR]