Thermal Metastability Research Articles

Superconductivity in Monolayer Carbon Allotropes with High Thermal Stability.

Intrinsic superconductivity is rarely discovered in sp2-hybridized monolayer carbon allotropes. Here we design a carbon monolayer configured of pentagon, heptagon, and hexagon rings with p2 plane group symmetry. Full-sp2 hybridization is proposed to favor thermal metastability on a low Gibbs free energy. The extremely small thermal expansion coefficient is predicted to the turn negative value to positive with elevating temperature. Carbon polygon structures remain intact at a high thermal temperature of 3,000 K. The high specific surface area is found to approach 2,700 m2/g, with O2-adsorption being advantageous over pristine graphene. We reveal electronic Fermi surfaces mediated by phonon modes of carbon out-of-plane vibrations. By calculating the Eliashberg equation, we evaluate intrinsic superconductivity with a large electron-phonon coupling coefficient. The superconducting transition temperature is estimated to reach 20 K under a high logarithmic average frequency. These first-principles calculations shall stimulate experimentalists' interest in exploring low-dimensional carbon superconductors with gas sensitivity.

Design of a novel non‐volatile hybrid spintronic true random number generator

AbstractTrue random number generators (TRNGs) are mainly used in cryptographic applications for the generation of various keys. A standard CMOS‐based TRNG utilizes physical phenomena such as thermal noise, metastability, and oscillator jitter to generate fully non‐deterministic random outputs but has high power dissipation and a large overhead area. In this work, we develop a novel low power non‐volatile TRNG circuit that utilizes the inherent stochastic switching nature of magnetic tunnel junction (MTJ) as a randomness source. Thanks to the capabilities of MTJ and carbon nanotube field‐effect transistors (CNTFETs), the proposed TRNG circuit consumes low power when compared with conventional CMOS‐based TRNGs. The reliability of the proposed circuit is checked by running the corner case tests that considers all the process and environmental variations. The quality of the output bitstream is evaluated by running the twelve statistical randomness tests by the U.S National Institute of Standards and Technology (NIST). The circuit passes all the tests, meaning the output bit‐stream is truly random. The circuit is designed in such a way that, even if the power is turned off, the quality of the randomness is not affected. The proposed TRNG structure has a less complex design and generates truly random output.

Thermal Metastability of Pearlite in Iron–Carbon Alloys

Thermal Metastability of Pearlite in Iron–Carbon Alloys

Glassy magnetic ground state in layered compound MnSb2Te4

As a sister compound of MnBi2Te4, the high-quality MnSb2Te4 single crystals are grown via solid-state reaction where prolonged annealing and narrow temperature window play critical roles on account of its thermal metastability. Single-crystal X-ray diffraction (SCXRD) analysis on MnSb2Te4 illustrates a crystal model that is isostructural to MnBi2Te4, consisting of Te-Sb-Te-Mn-Te-Sb-Te septuple layers (SLs) stacking in an ABC sequence. However, MnSb2Te4 reveals a more pronounced cation intermixing in comparison with MnBi2Te4, comprising 28.9(7)% Sb antisite defects on the Mn (3a) site and 19.3(6)% Mn antisite defects on the Sb (6c) site, which may give rise to novel magnetic properties in emerging layered MnBi2Te4-family materials. Unlike the antiferromagnetic (AFM) nature in MnBi2Te4, MnSb2Te4 exhibits a glassy magnetic ground state below 24 K and can be easily tuned to a ferromagnetic state under a weak applied magnetic field. Its magnetic hysteresis, anisotropy, and relaxation process are investigated in detail via static and dynamic magnetization measurements. Moreover, anomalous Hall effect as a p-type conductor is demonstrated with transport measurements. This work grants MnSb2Te4 a possible access to the future exploration of exotic quantum physics by removing the odd/even layer number restraint in realizing quantum transport phenomena in intrinsic AFM MnBi2Te4-family materials, as a result of the crossover between its magnetism and potential topology arising from the Sb-Te layer.

The Hydrates of TEMPO: Water Vibrations Reveal Radical Microsolvation.

An organic radical monohydrate complex is detected in vacuum isolation at low temperature by FTIR supersonic jet spectroscopy for the first time. It is shown to exhibit a rich conformational and vibrational coupling dynamics, which can be drastically reduced by appropriate isotope substitution. Its detection with a new gas recycling infrared spectrometer demonstrates the thermal metastability of the gaseous TEMPO radical even under humid gas conditions. Compared to its almost isoelectronic and isostructural, closed shell ketone analogue, the hydrogen bond of the solvating water is found to be less directional, but stronger and more strongly downshifting the bonded water OH stretch vibration. A second solvent water directs the first one into a metastable hydrogen bond position to solvate the nitrogen center and the first water at the same time.

The Hydrates of TEMPO: Water Vibrations Reveal Radical Microsolvation

AbstractAn organic radical monohydrate complex is detected in vacuum isolation at low temperature by FTIR supersonic jet spectroscopy for the first time. It is shown to exhibit a rich conformational and vibrational coupling dynamics, which can be drastically reduced by appropriate isotope substitution. Its detection with a new gas recycling infrared spectrometer demonstrates the thermal metastability of the gaseous TEMPO radical even under humid gas conditions. Compared to its almost isoelectronic and isostructural, closed shell ketone analogue, the hydrogen bond of the solvating water is found to be less directional, but stronger and more strongly downshifting the bonded water OH stretch vibration. A second solvent water directs the first one into a metastable hydrogen bond position to solvate the nitrogen center and the first water at the same time.

Jahn–Teller Disproportionation Induced Exfoliation of Unit‐Cell Scale ϵ‐MnO2

AbstractExfoliation of non‐layered (structurally) bulk materials at the nanoscale is challenging because of the strong chemical bonds in the lattice, as opposed to the weak van der Waals (vdW) interactions in layered materials. We propose a top‐down method to exfoliate ϵ‐MnO2 nanosheets in a family of charge‐ordered La1−xAExMnO3 (AE=Ca, Sr, Ba) perovskites, taking advantage of the Jahn–Teller disproportionation effect of Mn3+ and bond‐strength differences. ϵ‐MnO2 crystallized into a nickel arsenide (NiAs) structure, with a thickness of 0.91 nm, displays thermal metastability and superior water oxidation activity compared to other manganese oxides. The exfoliation mechanism involves a synergistic proton‐induced Mn3+ disproportionation and structural reconstruction. The synthetic method could also be potentially extended to the exfoliation of other two‐dimensional nanosheet materials with non‐layered structures.

Jahn-Teller Disproportionation Induced Exfoliation of Unit-Cell Scale ϵ-MnO2.

Exfoliation of non-layered (structurally) bulk materials at the nanoscale is challenging because of the strong chemical bonds in the lattice, as opposed to the weak van der Waals (vdW) interactions in layered materials. We propose a top-down method to exfoliate ϵ-MnO2 nanosheets in a family of charge-ordered La1-x AEx MnO3 (AE=Ca, Sr, Ba) perovskites, taking advantage of the Jahn-Teller disproportionation effect of Mn3+ and bond-strength differences. ϵ-MnO2 crystallized into a nickel arsenide (NiAs) structure, with a thickness of 0.91 nm, displays thermal metastability and superior water oxidation activity compared to other manganese oxides. The exfoliation mechanism involves a synergistic proton-induced Mn3+ disproportionation and structural reconstruction. The synthetic method could also be potentially extended to the exfoliation of other two-dimensional nanosheet materials with non-layered structures.

Preparation and characterization of phosphate-stabilized amorphous calcium carbonate nanoparticles and their application in curcumin delivery

Nano-sized amorphous calcium carbonate (ACC) have a great potential for drug transport and drug delivery because of their high drug loading capacity, excellent biocompatibility and biodegradability. However, ACC is the thermodynamically least stable phase of calcium carbonate, and such thermal metastability often transforms it into more stable vaterite or calcite phases, so the inhibition of the phase transformation of ACC is of great significance for its application in drug delivery systems. Herein, phosphate as a morphology controlling additive was introduced to stabilize the metastable ACC. The results showed that the composite nanoparticles (named here the ACCP) with narrow size distribution located at approximately 128 nm (measured by dynamic light scattering) were obtained, when Ca2+ concentration and C/P were 10 mM and 7/3 respectively, prepared at 30 °C for 10 min. The powdered X-ray diffraction (XRD) results showed that phosphate has an excellent ability to stabilize ACC, and even after storage for 60 days at room temperature, the structure of ACCP remains amorphous. The in vitro drug release tests showed that ACCP nanoparticles had a high curcumin (Cur) loading capacity and a sustained drug release property. Moreover, the 1, 1-diphenyl-2-picrylhydrazyl radical (DPPH) radical scavenging activity assay showed that ACCP-encapsulating strategy could effectively prevent the decomposition of Cur. In vitro cytotoxicity tests demonstrated that the resultants have excellent biocompatibility. Therefore, the as-prepared ACCP nanoparticles are promising for drug delivery applications.

Controllable Electrode Stochasticity Self-Heats Lithium-Ion Batteries at Low Temperatures.

Cold start is an ineluctable stipulation for electric vehicle operation under low-temperature extremes. It has typically been addressed through cell-level heating strategies. We advocate an electrode-level strategy leveraging pore-scale manifestation of thermal metastability that promotes self-heating. Appropriate controllable stochastic characteristics of porous electrodes are delineated, which contribute to the proposed solution at low temperatures. This approach is most conducive to high-energy-density Li-ion cells and devoid of extrinsic overheads.

Probing Signatures of Thermal Metastability in Lithium-Ion Batteries

With advances in battery technology, lithium-ion cells have continued to grow in energy density. This scale-up in electrochemistry implicitly fosters coupled thermal effects and a higher propensity for undesired thermal events. Such thermal metastability is directly related to self-heating behavior, however, the understanding of which in conventional calorimeter testing is limited to specific thermal operation. In this work, we present an in operando approach to characterize thermal signatures based on ambient test data (without a calorimeter). Thermal metastability of Li-ion cells is especially examined in the context of electrochemical fatigue (i.e., cycling), electrochemical abuse (i.e., external short) and thermal extremes (e.g., low temperature).

In operando thermal signature probe for lithium-ion batteries

Thermal metastability is an inescapable trait of lithium-ion batteries. However, canonically, only electrochemical signatures are studied as calorimetry imposes a controlled environment to isolate the self-heating signal. We propose an in operando approach for characterizing the thermal signatures. Using an inverse heat transfer formulation, we deconvolve the self-heating signature from other simultaneous heat transfer modes. Temporal variation of heat generation is subsequently estimated. This approach does not presuppose a particular electrochemical operation and is agnostic to materials used in the Li-ion cells. The generality and simplicity of this experimental approach rely on inverse thermal analysis and concurrent calibration of ambient natural convection response.

Insights into the Hydrothermal Metastability of Stishovite and Coesite

Hydrothermal experiments aiming at the crystal growth of stishovite near ambient pressure and temperature were performed in conventional autoclave systems using 1 M (molar) NaOH, 0.8 M Na2CO3, and pure water as a mineralizing agent. It was found that the hydrothermal metastability of stishovite and coesite is very different from the thermal metastability in all mineralizing agents and that because of this fact crystals could not be grown. While stishovite and coesite are thermally metastable up to 500 and >1000 °C, respectively, their hydrothermal metastability is below 150 and 200 °C, respectively. The thermally induced conversion of stishovite and coesite leads to amorphous products, whereas the hydrothermally induced conversion leads to crystalline quartz. Both stishovite and coesite are minerals occurring in nature where they can be exposed to hydrothermal conditions. The low hydrothermal stability of these phases may be an important factor to explain the rarity of these minerals in nature.

Superluminal travel, UV/IR mixing, and turbulence in a (1+1)-dimensional world

We study renormalizable Lorentz invariant stable quantum field theories in two space-time dimensions with instantaneous causal structure (causal ordering induced by the light "cone" time ordering). These models provide a candidate UV-completion of the two-dimensional ghost condensate. They exhibit a peculiar UV/IR mixing - energies of all excitations become arbitrarily small at high spatial momenta. We discuss several phenomena associated with this mixing. These include the impossibility to reach a thermal equilibrium and metastability of all excitations towards decay into short wavelength modes resulting in an indefinite turbulent cascade. In spite of the UV/IR mixing in many cases the UV physics can still be decoupled from low energy phenomena. However, a patient observer in the Lineland is able to produce arbitrarily heavy particles simply by waiting for a long enough time.

Microstructure formation during the annealing of iron nitrides

The thermal metastability of iron nitride was exploited to fabricate thin-film microstructures applicable to left-handed metamaterials. A granular thin film containing dispersed iron particles was prepared by post-annealing an rf-sputter-deposited Fe0.5Al0.5N thin film with a wurtzite-type structure in a nitrogen atmosphere at 450 °C. The obtained samples showed magnetic resonance in the microwave region. Laser direct writing was also applied in a nitrogen atmosphere to an rf-sputter-deposited Fe0.7Al0.3N thin film. 200 μm-wide, 300 Ω cm iron metal lines were drawn in a host nitride film of approximately 2 kΩ cm. Combining and optimizing the above negative permeability and permittivity values should provide left-handed materials with a negative refractive index.

Spin-phonon coupling and magnetoelectric properties:EuMnO3versusGdMnO3

Correlation between spin-phonon coupling and the magnetoelectric properties of EuMnO3 and GdMnO3 were investigated using ceramic samples in this work. The experimental results evidence a stronger spinphonon coupling in GdMnO3 than in EuMnO3, as well as a clear anomaly in the phonon contribution for the dielectric constant. This anomaly is located at the paramagnetic-incommensurate antiferromagnetic phasetransition temperature of GdMnO3, corroborating the existence of a magnetoelectric effect in this system. The absence of the magnetoelectric effect in EuMnO3 is due to the existing disorder revealed in this compound by the striking thermal metastability and the large decrease in the activation energy associated with the dielectric relaxation processes. This kind of behavior has no counterpart in GdMnO3. In fact, it has been confirmed that the existence of spin-phonon coupling is not a sufficient condition for the existence of magnetoelectric property, and other mechanisms have to be considered in order to understand the magnetoelectric properties.

Relaxation of thermally induced defects in LPCVD amorphous silicon

Thermally induced metastability has been studied on P doped LPCVD a-Si (low pressure chemical vapour deposited amorphous silicon) films as a function of the concentration of hydrogen in these films. The hydrogen concentration has been varied over two orders of magnitude, from 0.06 at.% in as-deposited films to 15 at.% in heavily implanted films. We find that thermal metastability effects are present even at the lowest concentrations of hydrogen. These become larger as hydrogen concentration increases. Our studies indicate that hydrogen plays an important role in thermal equilibration of a-Si:H and supports the hydrogen glass model.

Xe ions irradiation on bcc-Fe/Tb multilayers

Two bcc-Fe/Tb MLs with t Fe far from and close to the limit of crystallization of Fe layers were irradiated with Xe ions at various fluences and investigated by 57Fe Mössbauer spectrometry at 300 K. Two structural effects were evidenced: the usual mixing of layers and a demixing of the Fe and Tb atoms at the interfaces. The first process, observed in the two samples, destroys progressively the layered structure and leads to the formation of the corresponding amorphous alloy at high fluence. The second process, observed only at low fluence before the step of mixing for the multilayer close to the limit of crystallization of Fe layers, features the thermal metastability of this multilayer which relaxes towards a minimal energy state. It increases the pure iron part in the center of the iron layers, thus sharpening the interfaces.

Epitaxy and thermal behaviour of metastable metal films

We demonstrate the complexity of metal-on-metal epitaxy. Low-energy ion scattering and medium-energy electron diffraction were used to study the growth, structure and thermal stability of iron deposited on a Cu(001) surface. The system exhibits as a function of film thickness a rich variety of morphological and structural phases. At smallest coverages (< 2 ML) iron does not grow layer-by-layer at room temperature. Iron is even partially incorporated into the copper substrate. Near 2 ML the substrate is covered for the most part (90%) with Fe and at even higher coverages layer-by-layer growth occurs, leading to well-ordered fcc iron films. Above 10 ML a structural phase transition into the bcc equilibrium modification is observed. All of the deposited films exhibit additional thermal metastability. Heating the samples causes enrichment of the surface with copper, resulting in a Cu/Fe/Cu sandwich morphology with a Cu overlayer of initially monoatomic height on top of the iron layers, which remain essentially intact. The onset temperature of Cu diffusion depends on the film thickness, but is always far below the values for regular bulk diffusion.

Excitation morphology of short range Ising spin glasses

We numerically examine Ising spin-glass models, with lattice sizes up to 32 3 in 3D and 256 2 in 2D. A good local minimum Ψ is initially found, and used as a reference state. The phase-space of the system in a neighborhood of Ψ is sampled by first generating an excited state with energy b measured from Ψ and by then quenching the system to a new local energy minimum Ψ'. Connected clusters of spins which differ in the two configurations are identified. The dependence of their number, size and energy on b is shown to be systematic. It can be described in terms of simple functions for sufficiently low b. We find in addition a transition to a high b regime, where the behavior of the above quantities is qualitatively different. The dynamical consequences of our results, in particular the thermal metastability properties, are discussed in some detail.