Materials with tunable thermal properties enable on-demand control of temperature and heat flow, which is an integral component in the development of solid-state refrigeration, energy scavenging, and ...thermal circuits. Although gap-based and liquid-based thermal switches that work on the basis of mechanical movements have been an effective approach to control the flow of heat in the devices, their complex mechanisms impose considerable costs in latency, expense, and power consumption. As a consequence, materials that have multiple solid-state phases with distinct thermal properties are appealing for thermal management due to their simplicity, fast switching, and compactness. Thus, an ideal thermal switch should operate near or above room temperature, have a simple trigger mechanism, and offer a quick and large on/off switching ratio. In this study, we experimentally demonstrate that manipulating phonon scattering rates can switch the thermal conductivity of antiferroelectric PbZrO
bidirectionally by -10% and +25% upon applying electrical and thermal excitation, respectively. Our approach takes advantage of two separate phase transformations in PbZrO
that alter the phonon scattering rate in different manners. In this study, we demonstrate that PbZrO
can serve as a fast (<1 second), repeatable, simple trigger, and reliable thermal switch with a net switching ratio of nearly 38% from ~1.20 to ~1.65 W m
K
.
Abstract
Integrated nanophotonics is an emerging research direction that has attracted great interests for technologies ranging from classical to quantum computing. One of the key-components in the ...development of nanophotonic circuits is the phase-change unit that undergoes a solid-state phase transformation upon thermal excitation. The quaternary alloy, Ge
2
Sb
2
Se
4
Te, is one of the most promising material candidates for application in photonic circuits due to its broadband transparency and large optical contrast in the infrared spectrum. Here, we investigate the thermal properties of Ge
2
Sb
2
Se
4
Te and show that upon substituting tellurium with selenium, the thermal transport transitions from an electron dominated to a phonon dominated regime. By implementing an ultrafast mid-infrared pump-probe spectroscopy technique that allows for direct monitoring of electronic and vibrational energy carrier lifetimes in these materials, we find that this reduction in thermal conductivity is a result of a drastic change in electronic lifetimes of Ge
2
Sb
2
Se
4
Te, leading to a transition from an electron-dominated to a phonon-dominated thermal transport mechanism upon selenium substitution. In addition to thermal conductivity measurements, we provide an extensive study on the thermophysical properties of Ge
2
Sb
2
Se
4
Te thin films such as thermal boundary conductance, specific heat, and sound speed from room temperature to 400 °C across varying thicknesses.
Four high-entropy metal diborides have been synthesized and densified by borocarbothermal reduction of metal oxides with boron carbide and graphite and subsequent spark plasma sintering. Three of ...them, (Hf0.2Zr0.2Ti0.2Ta0.2Nb0.2)B2, (Hf0.2Zr0.2Ti0.2Ta0.2Mo0.2)B2, and (Hf0.2Zr0.2Ti0.2Ta0.2Cr0.2)B2, possess single high-entropy phases and have been sintered to >99% of the theoretical densities. The fourth (Hf0.2Zr0.2Ti0.2Mo0.2W0.2)B2 specimen contained a Ti–Mo–W rich secondary phase in addition to the primary metal diboride phase. The specimens made by borocarbothermal reduction exhibit improved hardnesses in comparison with those samples previously fabricated via high energy ball milling and spark plasma sintering. Interestingly, the single-phase (Hf0.2Zr0.2Ti0.2Ta0.2Mo0.2)B2 and (Hf0.2Zr0.2Ti0.2Ta0.2Cr0.2)B2 (both of which have Vickers hardness values of ~25 GPa) are substantially harder than (Hf0.2Zr0.2Ti0.2Ta0.2Nb0.2)B2 (20.5 GPa), despite MoB2 and CrB2 being typically considered as softer components. These single-phase high-entropy metal diborides were found to have low thermal conductivities of 12–25 W/mK, which are ~1/10 to ~1/5 of the reported values of HfB2 and ZrB2.
Performance of the commonly-used transient hot wire method for measuring the thermal conductivity of fusible materials near solid-liquid phase change is reported with the focus placed on explaining ...the observed “anomalous” increase of thermal conductivity in relation to the solid-solid transition. Utilizing a 1-D transient formulation and considering a one-step thermal conductivity model, the improved computational methodology captured the monotonic dependence of the predicted thermal conductivity value on the initial temperature of the solid medium more effectively. Hypothesizing that the reported measurements of increase of the thermal conductivity near the solid-liquid phase change temperature are linked to the solid-solid transition, a two-step thermal conductivity model was adopted. This model featured a higher thermal conductivity over a narrow temperature range before a sharp drop upon melting. The predicted values of the thermal conductivity in relation to the initial solid-state temperature were discussed. A rising trend for the predicted thermal conductivity values was observed followed by a smooth decline once the initial solid-state temperature was increased. This predicted trend based on the piecewise thermal conductivity vs. temperature model closely resembled the reported “anomalous” thermal conductivity measurements observed in experimental studies utilizing transient techniques.
•High-entropy boride-carbide two-phase ultrahigh temperature ceramics are made.•This work further extends the emerging field of high-entropy ceramics.•A novel reactive spark plasma sintering route ...produces > ∼99 % dense specimens.•A thermodynamic relation governing the equilibrium phase compositions is discovered.•The hardness is higher than the weighted average of the two high-entropy phases.
A series of dual-phase high-entropy ultra-high temperature ceramics (DPHE-UHTCs) are fabricated starting from N binary borides and (5-N) binary carbides powders. > ∼99 % relative densities have been achieved with virtually no native oxides. These DPHE-UHTCs consist of a hexagonal high-entropy boride (HEB) phase and a cubic high-entropy carbide (HEC) phase. A thermodynamic relation that governs the compositions of the HEB and HEC phases in equilibrium is discovered and a thermodynamic model is proposed. These DPHE-UHTCs exhibit tunable grain size, Vickers microhardness, Young’s and shear moduli, and thermal conductivity. The DPHE-UHTCs have higher hardness than the weighted linear average of the two single-phase HEB and HEC, which are already harder than the rule-of-mixture averages of individual binary borides and carbides. This study extends the state of the art by introducing dual-phase high-entropy ceramics (DPHECs), which provide a new platform to tailor various properties via changing the phase fraction and microstructure.
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.
Grain boundaries (GBs) are a prolific microstructural feature that dominates the functionality of a wide class of materials. The functionality at a GB results from the unique atomic arrangements, ...different from those in the grain, that have driven extensive experimental and theoretical studies correlating atomic‐scale GB structures to macroscopic electronic, infrared optical, and thermal properties. In this work, a SrTiO3 GB is examined using atomic‐resolution aberration‐corrected scanning transmission electron microscopy and ultrahigh‐energy‐resolution monochromated electron energy‐loss spectroscopy, in conjunction with density functional theory. This combination enables the correlation of the GB structure, nonstoichiometry, and chemical bonding with a redistribution of vibrational states within the GB dislocation cores. The new experimental access to localized GB vibrations provides a direct route to quantifying the impact of individual boundaries on macroscopic properties.
Localized measurements of vibrations in the dislocation cores of a complex grain boundary are correlated with structure and nonstoichiometry using monochromated electron energy‐loss spectroscopy in a scanning transmission electron microscope. The experimental results are correlated with density functional theory calculations to understand the atomistic influence of the unique structure and chemistry of grain boundary dislocations and their influence on atomic vibrations.
We experimentally show that the ballistic length of hot electrons in laser-heated gold films can exceed ∼150 nm, which is ∼50% greater than the previously reported value of 100 nm inferred from ...pump–probe experiments. We also find that the mean free path of electrons at the peak temperature following interband excitation can reach upward of ∼45 nm, which is higher than the average value of 30 nm predicted from our parameter-free density functional perturbation theory. Our first-principles calculations of electron–phonon coupling reveal that the increase in the mean free path due to interband excitation is a consequence of drastically reduced electron–phonon coupling from lattice stiffening, thus providing the microscopic understanding of our experimental findings.
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