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Due to their diverse bonding character and corresponding property repertoire, carbides are an important class of materials regularly used in modern technologies, including aerospace applications and extreme environments, catalysis, fuel cells, power electronics, and solar cells. The recent push for novel materials has increased interest in high entropy carbides (HECs) for such applications. The extreme level of tunability alone makes HECs a significant materials platform for a variety of fundamental studies and functional applications. We investigate the thermal conductivity of high entropy carbide thin films as carbon stoichiometry is varied. The thermal conductivity of the HEC decreases with an increase in carbon stoichiometry, while the respective phonon contribution scales with elastic modulus as the excess carbon content increases. Based on the carbon content, the HECs transition from an electrically conducting metal-like material with primarily metallic bonding to a primarily covalently-bonded crystal with thermal conductivities largely dominated by the phononic sub-system. When the carbon stoichiometry is increased above this critical transition threshold dictating bonding character, the electronic contribution to thermal conductivity is minimized, and a combination of changes in microstructure, defect concentration and secondary phase formation, and stiffness influence the phononic contribution to thermal conductivity. Our results demonstrate the ability to tune the thermal functionality of high entropy materials through stoichiometries that dictate the type of bonding environment.

Despite the exceptional thermal and mechanical functionalities of diamond, its superlative properties are highly subject to the presence of point defects, dislocations, and interfaces. In this study, polycrystalline diamond is ion implanted with C3+, N3+, and O3+ ions at an energy of 16.5 MeV, producing an amorphous layer at the projected range and a damaged crystalline region between the surface and amorphous layer. Using time-domain thermoreflectance in combination with thermal penetration depth calculations based upon the multilayer heat diffusion equation, it is determined that reductions in the thermal conductivity can span nearly two orders of magnitude while still maintaining a polycrystalline structure within the regions thermally probed. Dynamical diffraction simulations of high-resolution x-ray diffraction measurements demonstrate the formation of a strained layer localized at the end of range, with much lower levels of strain near the surface. Furthermore, within the polycrystalline region above the amorphous layer, the average number of displacements-per-atom from the ion irradiation is found to be 1 % , with mass impurity concentrations much less than 1%. These low defect concentrations within the thermally probed region demonstrate the remarkably large impact that dilute levels of defects from the ion implantation can have on the thermal conductivity of diamond.

In this work, the high entropy oxide A2CuO4 Ruddlesden-Popper (La0.2Pr0.2Nd0.2Sm0.2Eu0.2)2CuO4 is explored by charge doping with Ce+4 and Sr+2 at concentrations known to induce superconductivity in the simple parent compounds, Nd2CuO4 and La2CuO4. Electron doped (La0.185Pr0.185Nd0.185Sm0.185Eu0.185Ce0.075)2CuO4 and hole doped (La0.18Pr0.18Nd0.18Sm0.18Eu0.18Sr0.1)2CuO4 are synthesized and shown to be single crystal, epitaxially strained, and highly uniform. Transport measurements demonstrate that all as-grown films are insulating regardless of doping. Annealing studies show that resistivity can be tuned by modifying oxygen stoichiometry and inducing metallicity but without superconductivity. These results in turn are connected to extended x-ray absorption fine structure (EXAFS) results indicating that the lack of superconductivity in the high entropy cuprates likely originates from a large distortion within the Cu-O plane ({\sigma}2>0.015 {\AA}2) due to A-site cation size variance, which drives localization of charge carriers. These findings describe new opportunities for controlling charge- and orbital-mediated functional responses in Ruddlesden-Popper crystal structures, driven by balancing of cation size and charge variances that may be exploited for functionally important behaviors such as superconductivity, antiferromagnetism, and metal-insulator transitions, while opening less understood phase spaces hosting doped Mott insulators, strange metals, quantum criticality, pseudogaps, and ordered charge density waves.

Vanadium oxide films are widely employed as thermal detectors in uncooled infrared detection systems due to their high temperature coefficient of resistance near room temperature. One strategy toward maximizing detectivity and reducing the thermal time constant in these systems is to minimize the system platform dimensions. This approach necessitates thinner film thicknesses ([Formula: see text]100 nm), for which there is little information regarding thermal sensing performance. Herein, we report on the sensitivity of reactively sputtered vanadium oxide thin film resistive thermometers nominally ranging from 100 to 25 nm and assess the influence of thermal annealing. We demonstrate that films in this minimum limit of thickness maintain a high temperature coefficient while additionally providing an enhancement in characteristics of the noise equivalent power.

Visibly transparent and electrically conductive oxides are attractive for a wide array of applications. Indium tin oxide (ITO) and fluorine tin oxide (FTO) are the subset of the larger transparent conducting oxide family and possess transmittance in the visible spectrum as well as high electrical conductivity. Even though their unique optical and electrical properties have been thoroughly examined, the thermal transport properties, namely, thermal conductivity in the cross-plane direction, have received much less attention. In this paper, using a series of ITO and FTO thin films comprising a range of thicknesses and grain sizes, we characterize the cross-plane thermal conductivity using time-domain thermoreflectance. We determine the heat capacity of the FTO films from simultaneous measurements of volumetric heat capacity and thermal conductivity on an ~396-nm-thick FTO film. We show that the size effects have a considerable influence on the thermal conductivity from both the perspective of grain boundary and thin film scattering.

Disorder enhances desired properties, as well as creating new avenues for synthesizing materials. For instance, hardness and yield stress are improved by solid-solution strengthening, a result of distortions and atomic size mismatches. Thermo-chemical stability is increased by the preference of chemically disordered mixtures for high-symmetry super-lattices. Vibrational thermal conductivity is decreased by force-constant disorder without sacrificing mechanical strength and stiffness. Thus, high-entropy ceramics propel a wide range of applications: from wear resistant coatings and thermal and environmental barriers to catalysts, batteries, thermoelectrics and nuclear energy management. Here, we discuss recent progress of the field, with a particular emphasis on disorder-enhanced properties and applications., Comment: 6 pages, 2 pics, super-short review

Due to their diverse bonding character and corresponding property repertoire, carbides are an important class of materials regularly used in modern technologies, including aerospace applications and extreme environments, catalysis, fuel cells, power electronics, and solar cells. The recent push for novel materials has increased interest in high entropy carbides (HECs) for such applications. The extreme level of tunability alone makes HECs a significant materials platform for a variety of fundamental studies and functional applications. We investigate the thermal conductivity of high entropy carbide thin films as carbon stoichiometry is varied. The thermal conductivity of the HEC decreases with an increase in carbon stoichiometry, while the respective phonon contribution scales with elastic modulus changes as the excess carbon content increases. Based on the carbon content, the HECs transition from an electrically conducting metal-like material with primarily metallic bonding to a primarily covalently bonded crystal with thermal conductivities largely dominated by the phononic sub-system. When the carbon stoichiometry is increased above this critical transition threshold dictating bonding character, the electronic contribution to thermal conductivity is minimized and remains at a constant, and a combination of changes in film morphology and phase precipitation systematically lower the phononic contribution to thermal conductivity. This trend in thermal conductivity with carbon content in the HECs is opposite to those observed in binary metal carbide systems. Our results demonstrate the ability to tune the thermal functionality of high entropy materials through stoichiometries that dictate the type of bonding environment.

This paper explores thin films of the entropy-stabilized oxide (ESO) composition MgxNixCoxCuxZnxScxO (x~ 0.167) grown by laser ablation in incremental gas pressures and O2/Ar ratios to modulate particle kinetic energy and plume reactivity. Low pressures supporting high kinetic energy adatoms favor the kinetic stabilization of a single rocksalt phase, while high pressures (low kinetic energy adatoms) result in phase separation. The pressure threshold for phase separation is a function of O2/Ar ratio. These findings suggest large kinetic energies facilitate the assembly and quench of metastable ESO phases that may require immoderate physical or chemical conditions to synthesize using near-equilibrium techniques.

We investigate the effect of mass disorder on the thermal conductivity of Lennard-Jones based multi-component solid solutions via classical molecular dynamics simulations. In agreement with Klemens' perturbation theory, the thermal conductivity reduction due to mass scattering alone is found to reach a critical point, whereby adding more impurity atoms in the solid solution does not reduce the thermal conductivity. A further decrease in thermal conductivity requires a change in local strain-field, which together with mass defect scattering can lead to ultralow thermal conductivities in solid solutions, surpassing the theoretical minimum limits of the corresponding amorphous phases.

Carbon coated aligned multi-walled carbon nanotube (AMWCNT/C) sheets were used for studying the controlled radial growth of MWCNTs. Pyrolytic carbon (PyC) was deposited on the surface of nanotubes using multiple cycles of chemical vapor infiltration. Morphological and structural characterization showed that when graphitization was done in one step, after the deposition of multiple cycles of PyC, the presence of a large amount of disordered carbon on the surface of nanotubes led to a poorly graphitized coating structure that did not resembled nanotube walls anymore. Graphitization of the AMWCNT/C sheets after each deposition cycle prevented the development of disordered carbon during the subsequent PyC deposition cycles. Using the cyclic-graphitization method, thick PyC coating layers were successfully graphitized into a crystalline structure that could not be differentiated from the original nanotube walls. TEM observation and X-ray data confirmed radial growth of nanotubes, while spectra collected from Raman spectroscopy revealed that radially grown CNTs had the same quality as graphitized pristine nanotubes. The focus of this study was to compare the effect of cyclic graphitization with a one-step graphitization method to gain insight on the necessary parameters needed to radially grow high quality CNTs.

Heterogeneous nanosystems offer a robust potential for manipulating various functional material properties, beyond those possible from their individual constituent materials. We demonstrate the formation of a class of materials with a homogeneous lattice but spatially heterogeneous electrical functionality; specifically, we develop epitaxial modulation-doped thin films in which the spatial separation of electronic charge densities is achieved without perturbing the parent crystal's compositional or structural homogeneity. Unlike the previous realizations of modulation doping in crystals, our materials demonstrate periodic layering of spatially segregated, varying electronically donor-doped regions in a single compositionally and structurally homogenous single-crystalline lattice. We demonstrate the formation of ``modulation-doped epitaxial crystals'' (MoDECs) using alternating layers of doped cadmium oxide, and the ability to spatially confine regions of variable carrier concentration via low potential-energy barriers in a spatially homogeneous, epitaxial crystal with a chemically and structurally homogenous lattice (i.e., no chemical or structural lattice interfaces). The low potential energy that confines electrons within the doped layers coupled with the crystalline nature of the MoDECs and lack of lattice interfaces presents a platform to study the electron thermal boundary resistances at low-energy electronic barriers. We find that the electron interfacial density does not impede thermal conductivity, despite evidence that the doped layers retain their carrier concentrations. Thus, the negligible thermal boundary resistances at the electronic interfaces result in the thermal conductivities of the MoDECs being related to only a series resistance sum of the thermal resistances of each of the individual layers, with no thermal resistances from the electronic boundaries that maintain charge separation. This is in stark contrast with other nanoscale multilayer materials, where thermal boundary resistances at the internal material interfaces reduce the thermal conductivity of the multilayer compared to that of the parent materials. The ability to modulation dope epitaxially grown films with no structural heterogeneity in the lattice will further enable unique platforms for mid-IR photonics, such as hyperbolic metamaterials, optical filters with spatially discrete optical absorption, or energy harvesting based on charge injection across modulation-doped interfaces.

Twelve different equiatomic five-metal carbides of group IVB, VB, and VIB refractory transition metals are synthesized via high-energy ball milling and spark plasma sintering. Implementation of a newly developed ab initio entropy descriptor aids in selection of candidate compositions for synthesis of high entropy and entropy stabilized carbides. Phase formation and composition uniformity are analyzed via XRD, EDS, S/TEM-EDS, and EXAFS. Nine of the twelve candidates form true single-phase materials with the rocksalt (B1) structure when sintered at 2473 K and can therefore be investigated as high entropy carbides (HECs). The composition (V0.2Nb0.2Ta0.2Mo0.2W0.2)C is presented as a likely candidate for further investigation as an entropy stabilized carbide. Seven of the carbides are examined for mechanical properties via nanoindentation. The HECs show significantly enhanced hardness when compared to a rule of mixtures average of the constituent binary carbides and to the highest hardness of the binary constituents. The mechanical properties are correlated to the electronic structure of the solid solutions, offering a future route to tunability of the mechanical properties of carbide ceramics via exploration of a new complex composition space.

Fundamental theories predict that reductions in thermal conductivity from point and extended defects can arise due to phonon scattering with localized strain fields. To experimentally determine how these strain fields impact phonon scattering mechanisms, we employ ion irradiation as a controlled means of introducing strain and assorted defects into the lattice. In particular, we observe the reduction in thermal conductivity of intrinsic natural silicon after self-irradiation with two different silicon isotopes, $^{28}\mathrm{Si}^{+}$ and $^{29}\mathrm{Si}^{+}$. Irradiating with an isotope with a nearly identical atomic mass as the majority of the host lattice produces a damage profile lacking mass impurities and allows us to assess the role of phonon scattering with local strain fields on the thermal conductivity. Our results demonstrate that point defects will decrease the thermal conductivity more so than spatially extended defect structures assuming the same volumetric defect concentrations due to the larger strain per defect that arises in spatially separated point defects. With thermal conductivity models using density functional theory, we show that for a given defect concentration, the type of defect (i.e., point vs extended) plays a negligible role in reducing the thermal conductivity compared to the strain per defect in a given volume.

Sheets of aligned multi-walled carbon nanotubes (AMWCNTs) were used to study the structural annealing of pyrolytic carbon (PyC) coatings with various thicknesses on MWCNTs. PyC was deposited using chemical vapor infiltration and the thickness was controlled via the infiltration time. Structural annealing of the PyC coated AMWCNT (AMWCNT/C) sheets at 2150 °C provided different results for different thickness coatings. Transmission electron microscopy images showed that the carbon deposited from acetylene formed laminar PyC coatings, resembling rough tube walls, on the CNT surfaces. Following the high temperature heat treatment, coatings from short PyC deposition times changed their structure, resulting in radial growth of the MWCNTs. Raman and X-ray diffraction measurements also revealed that the radially grown MWCNTs had graphitic quality very close to pristine nanotubes after annealing. Electrical conductivity of AMWCNT/C sheets after high temperature heat treatment was twice that of pristine AMWCNT sheets. The focus of this study was to determine the PyC coating thickness at which a rough PyC coating would no longer change its structure into new CNT walls. The samples treated longer than 30 min had much more disordered PyC deposited on the surface and the additional material did not form additional tube walls after thermal annealing.

It is shown using classical molecular dynamics simulations that phonon scattering from disorder in the interatomic forces introduced by charge transfer and not from mass disorder is needed to explain the thermal conductivity reduction experimentally measured that accompanies the addition of a sixth cation to the entropy stabilized oxide J14 [(Mg0.1Co0.1Ni0.1Cu0.1Zn0.1)O0.5]. The simulations were performed on five entropy-stabilized oxides, J14, and J14 plus Sc, Sn, Cr, or Ge in equi-molar cation proportions. Comparing the simulation results to predictions from the Bridgman equation using properties from the simulations suggests that despite phonon scattering from disorder in both atomic forces and mass, the thermal conductivity for these systems is still above an analytical limit for an amorphous structure.

We report on the thermal resistances of thin films (20 nm) of hafnium zirconium oxide (Hf1–xZrxO2) with compositions ranging from 0 ≤ x ≤ 1. Measurements were made via time-domain thermoreflectance and analyzed to determine the effective thermal resistance of the films in addition to their associated thermal boundary resistances. We find effective thermal resistances ranging from 28.79 to 24.72 m2 K GW−1 for amorphous films, which decreased to 15.81 m2 K GW−1 upon crystallization. Furthermore, we analyze the heat capacity for two compositions, x = 0.5 and x = 0.7, of Hf1–xZrxO2 and find them to be 2.18 ± 0.56 and 2.64 ± 0.53 MJ m−3 K−1, respectively.

Manipulating a crystalline material's configurational entropy through the introduction of unique atomic species can produce novel materials with desirable mechanical and electrical properties. From a thermal transport perspective, large differences between elemental properties such as mass and interatomic force can reduce the rate at which phonons carry heat and thus reduce the thermal conductivity. Recent advances in materials synthesis are enabling the fabrication of entropy-stabilized ceramics, opening the door for understanding the implications of extreme disorder on thermal transport. Measuring the structural, mechanical, and thermal properties of single-crystal entropy-stabilized oxides, it is shown that local ionic charge disorder can effectively reduce thermal conductivity without compromising mechanical stiffness. These materials demonstrate similar thermal conductivities to their amorphous counterparts, in agreement with the theoretical minimum limit, resulting in this class of material possessing the highest ratio of elastic modulus to thermal conductivity of any isotropic crystal.

GaN surface and near-surface chemistry influence on band offsets of oxide overlayers is demonstrated through X-ray photoelectron spectroscopy measurements using epitaxial (111)-oriented MgO films on (0001)-oriented Ga-polar GaN as a case study. For identical cleaning and MgO growth conditions, GaN subsurface oxygen impurities influence the GaN bare surface band bending and the ultimate band offset to MgO heterolayers. As the GaN surface oxygen concentration increases from an atomic concentration of 0.9% to 3.4%, the valence band offset to MgO decreases from 1.68 eV to 1.29 eV, respectively. This study highlights the sensitivity of the oxide/nitride interface electronic structure to GaN epilayer preparation and growth conditions.

Time domain thermoreflectance (TDTR) and frequency domain thermoreflectance (FDTR) are common pump-probe techniques that are used to measure the thermal properties of materials. At elevated temperatures, transducers used in these techniques can become limited by melting or other phase transitions. In this work, time domain thermoreflectance is used to determine the viability of HfN thin film transducers grown on SiO2 through measurements of the SiO2 thermal conductivity up to approximately 1000 K. Further, the reliability of HfN as a transducer is determined by measuring the thermal conductivities of MgO, Al2O3, and diamond at room temperature. The thermoreflectance coefficient of HfN was found to be 1.4 × 10−4 K−1 at 800 nm, one of the highest thermoreflectance coefficients measured at this standard TDTR probe wavelength. Additionally, the high absorption of HfN at 400 nm is shown to enable reliable laser heating to elevate the sample temperature during a measurement, relative to other transducers.

Innovation in new mixtures of constituents can lead to discover exciting new materials with unexpected properties and revolutionary applications [1,2]. It is known, the Gibbs energy needs to be minimized, as the main requirement, to achieve a stable single phase compound. Conventional approach to minimize the total energy of system is searching for a large and negative enthalpy. However, in this work, we show that the phase stability can be reached where the configurational entropy is maximized with mixing as many diverse elements as possible.

Density functional theory calculations were carried out for three entropic rocksalt oxides, (Mg0.1Co0.1Ni0.1Cu0.1Zn0.1)O0.5, termed J14, and J14 + Li and J14 + Sc, to understand the role of charge neutrality and electronic states on their properties, and to probe whether simple expressions may exist that predict stability. The calculations predict that the average lattice constants of the ternary structures provide good approximations to that of the random structures. For J14, Bader charges are transferable between the binary, ternary, and random structures. For J14 + Sc and J14 + Li, average Bader charges in the entropic structures can be estimated from the ternary compositions. Addition of Sc to J14 reduces the majority of Cu, which show large displacements from ideal lattice sites, along with reduction of a few Co and Ni cations. Addition of Li to J14 reduces the lattice constant, consistent with experiment, and oxidizes some of Co as well as some of Ni and Cu. The Bader charges and spin-resolved densi...

The thermal conductivity of n- and p-type doped gallium nitride (GaN) epilayers having thicknesses of 3–4 μm was investigated using time domain thermoreflectance. Despite possessing carrier concentrations ranging across 3 decades (1015–1018 cm–3), n-type layers exhibit a nearly constant thermal conductivity of 180 W/mK. The thermal conductivity of p-type epilayers, in contrast, reduces from 160 to 110 W/mK with increased doping. These trends—and their overall reduction relative to bulk—are explained leveraging established scattering models where it is shown that, while the decrease in p-type layers is partly due to the increased impurity levels evolving from its doping, size effects play a primary role in limiting the thermal conductivity of GaN layers tens of microns thick. Device layers, even of pristine quality, will therefore exhibit thermal conductivities less than the bulk value of 240 W/mK owing to their finite thickness.