Mechanism Of Thermal Instability Research Articles

The saturation mechanism of thermal instability

The literature on thermal instability (TI) reveals that even for a simple homogeneous plasma, the nonlinear outcome ranges from a gentle reconfiguration of the initial state to an explosive one, depending on whether the condensations that form evolve in an isobaric or nonisobaric manner. After summarizing the recent developments on the linear and nonlinear theory of TI, here we derive several general identities from the evolution equation for entropy that reveal the mechanism by which TI saturates; whenever the boundary of the instability region (the Balbus contour) is crossed, a dynamical change is triggered that causes the comoving time derivative of the pressure to change the sign. This event implies that the gas pressure force reverses direction, slowing the continued growth of condensation. For isobaric evolution, this “pressure reversal” occurs nearly simultaneously for every fluid element in condensation and a steady state is quickly reached. For nonisobaric evolution, the condensation is no longer in mechanical equilibrium and the contracting gas rebounds with greater force during the expansion phase that accompanies the gas reaching the equilibrium curve. The cloud then pulsates because the return to mechanical equilibrium becomes wave mediated. We show that both the contraction rebound event and subsequent pulsation behavior follow analytically from an analysis of the new identities. Our analysis also leads to the identification of an isochoric TI zone and makes it clear that unless this zone intersects the equilibrium curve, isochoric modes can only become unstable if the plasma is in a state of thermal non-equilibrium.

Rayleigh–Bénard instability in nanofluids: effect of gravity settling

Understanding the mechanism of thermal instability in nanofluids is of fundamental importance to explore the reasons behind the enhancement of heat transfer efficiency. Since Buongiorno (ASME J. Heat Transfer, vol. 128, 2006, pp. 240–250) proposed his theoretical model of nanofluids, most studies focusing on the thermal instability analysis exclusively considered Brownian motion and thermophoresis as the main diffusion mechanisms of nanoparticles. All the analyses concluded that a nanofluid layer is much more unstable than its pure counterpart as it is heated from below. However, a recent experimental observation on Rayleigh–Bénard convection appears to contradict the theoretical prediction, implying that some mechanisms neglected in the previous model may have a significant impact on the onset of thermal convection. In the present study, we revise the convective transport model of nanofluids proposed by Buongiorno and find that the gravitational settling of nanoparticles is a crucial factor influencing the thermal instability behaviour of nanofluids. By performing a linear stability analysis based on the novel model, the effect of gravity settling exhibits a stabilizing mechanism to resist the destabilizing effect of thermophoresis. Furthermore, the onset of instability can be delayed once the nanoparticle diameter exceeds a certain threshold, which explains the phenomenon observed in experiments. Particularly, the oscillatory mode is found to emerge and dominate the flow instability when the gravity settling effect is competitive with the effect of thermophoresis.

Statistical temperature coefficient distribution in analog RRAM array: impact on neuromorphic system and mitigation method

Emerging analog resistive random access memory (RRAM) based on HfO x is an attractive device for non-von Neumann neuromorphic computing systems. The differences in temperature dependent conductance drift among cells hamper computing accuracy, characterized by the statistical distribution of temperature coefficient (Tα ). A compact model was presented in order to investigate the statistical distribution of Tα under different resistance states. Based on this model, the physical mechanism of thermal instability of cells with a positive Tα was elucidated. Furthermore, this model can also effectively evaluate the impact of conductance distribution of different levels under various temperatures in artificial neural networks. A current compensation scheme and hybird optimization method were proposed to reduce the impact of the distribution of Tα . The simulation results showed that recognition accuracy was improved from 79.8% to 91.3% for the application of Modified National Institute of Standards and Technology handwriting digits classification with a two-layer perceptron at 400 K after adopting the proposed optimization method.

Instability and energy budget analysis of viscous coaxial jets under a radial thermal field

Temporal linear instability of viscous coaxial jets under a radial thermal field is carried out by considering axisymmetric and non-axisymmetric disturbances. The interfacial tensions of different fluids are taken to be temperature dependent. The para-sinuous, para-varicose, and helical unstable modes are identified in the Rayleigh regime. The energy budget is also employed to explore the relative importance of thermal-induced stresses on the jet instability at the most unstable wavenumber by changing the dimensionless parameters. It is shown that decreasing the temperature ratio of inner fluid to surrounding fluid (T13) promotes the jet instability. For coaxial jets at T13 > 1, the Marangoni flow makes coaxial jets more stable, and increasing the fluid thermal conductivity suppresses the jet instability. For coaxial jets at T13 < 1, however, their influences on the jet instability are opposite. Compared with the thermal-induced stresses at the inner and outer interfaces, the inner interfacial tension is the main factor dominating the flow. Increasing either inner interfacial tension or outer surface tension and decreasing viscosity of any fluid can promote the instability of coaxial jets. The variations of thermal conductivity and specific heat capacity of either inner or surrounding fluids apparently influence the jet instability of the para-varicose mode, but hardly influence that of the para-sinuous mode. This work would provide great insight into the physical mechanism of thermal jet instability in various applications.

Thermal instability and characteristics of donkey casein micelles

The thermal stability of donkey milk, which largely depends on the stability of casein and whey proteins, plays a critical role in the manufacture of donkey dairy products. However, the thermal stability of donkey casein micelles is poorly understood. Our study first found that donkey milk exhibited poor thermal stability, with sedimentation observed after heating at 75 °C for 10 min. Moreover, electrophoresis results indicated that donkey casein micelles were more sensitive to heat treatment than whey protein. The characteristics of donkey casein micelles were then studied to determine the thermal instability mechanism. Donkey casein micelles had a larger diameter (295 ± 11 nm) and lower absolute zeta potential (−15.4 ± 0.5 mV) than bovine casein micelles, mainly due to their very low κ-casein levels. The higher proportion (51%) of β-casein in donkey casein micelles made them less hydrated. Furthermore, <40% of calcium in donkey milk was associated with donkey casein micelles. To eliminate the influence of whey protein, micellar casein was isolated from skim donkey milk by ultracentrifugation and then redispersed in its ultrafiltrate. Transmission electron microscopy and particle size analysis showed the extensive aggregation of isolated casein micelles after thermal treatment, which confirmed the thermal instability of casein micelles observed in donkey milk. Overall, poor colloidal stability and a high‑calcium environment largely contributed to the thermal instability of donkey casein micelles.

Numerical studies on thermal shock crack branching instability in brittle solids

The new weakly coupled thermal mechanical bond-based peridynamic model enriched with shear deformation is developed to study crack propagation and branching instability in brittle solids subjected to thermal shocks. First, the proposed numerical model is validated via comparisons with the classical finite element method (FEM) results. Next, thermal shock crack branching instability in brittle ceramic disks under infrared radiation heating is investigated using the proposed numerical model. Three different types of thermal shock cracking patterns, including no crack branching, crack branching inside heating region and crack branching outside heating region, can be captured by using the proposed numerical model, which are consistent with the previous experimental and numerical results. Effects of heat flux density, heating size, pre-existing notch length, horizon size and nonlocal ratio on thermal shock cracking patterns, crack propagation speed, strain energy distributions around crack tips and crack branching angle are also systematically investigated. The mechanism of thermal shock crack branching instability is revealed. Finally, some characteristics of thermal shock crack branching instability in brittle solids are discussed and summarized.

Investigation of the relevant kinetic processes in the initial stage of a double-arcing instability in oxygen plasmas

A numerical investigation of the kinetic processes in the initial (nanosecond range) stage of the double-arcing instability was developed. The plasma-sheath boundary region of an oxygen-operated cutting torch was considered. The energy balance and chemistry processes in the discharge were described. It is shown that the double-arcing instability is a sudden transition from a diffuse (glow-like) discharge to a constricted (arc-like) discharge in the plasma-sheath boundary region arising from a field-emission instability. A critical electric field value of ∼107 V/m was found at the cathodic part of the nozzle wall under the conditions considered. The field-emission instability drives in turn a fast electronic-to-translational energy relaxation mechanism, giving rise to a very fast gas heating rate of at least ∼109 K/s, mainly due to reactions of preliminary dissociation of oxygen molecules via the highly excited electronic state O2(B3Σu−) populated by electron impact. It is expected that this fast oxygen heating rate further stimulates the discharge contraction through the thermal instability mechanism.

Thermal Stability of the Eutectic Composition in NaCl-CaCl2-MgCl2 Ternary System Used for Thermal Energy Storage Applications

The ternary eutectic chloride salt (NaCl-CaCl2-MgCl2) was designed and prepared for thermal energy storage over 550°C in a concentration solar power system. The melting temperature and fusion enthalpy of the eutectic salt were measured experimentally using the Differential Scanning Calorimeter (DSC) technique which were determined to be 420.83°C and 201.50 J/g, respectively. The thermal stability including the short-term and the long-term thermal stability of the ternary system was investigated by the Thermo-gravimetric Analyzer (TGA) technique and the DSC technique. The thermal properties of the eutectic mixture kept stable during the short-term thermal cycling test. The weight loss of the eutectic salt was less than 5%, and the melting temperature and fusion enthalpy changed slightly as the temperature increased below 650°C in the long-term isothermal stability study. The thermal stability test revealed that the ternary eutectic system demonstrated excellent thermal stability below 650°C which was taken as the upper limit temperature of the ternary system. The eutectic composition was characterized by the X-Ray Diffraction (XRD) technique before and after thermal treatment to provide preliminary analysis on the mechanism of thermal instability. The results suggested that the ternary eutectic chloride salt was a potential candidate for applying in high-temperature thermal energy storage.

Thermal properties and thermal stability of the ternary eutectic salt NaCl-CaCl2-MgCl2 used in high-temperature thermal energy storage process

The ternary eutectic chloride salt (NaCl-CaCl2-MgCl2) was designed and prepared for thermal energy storage over 500°C in a concentrated solar power system. The thermal properties of the eutectic salt were measured experimentally using the Differential Scanning Calorimeter (DSC) technique, and the thermal stability including the long-term and short-term thermal stability of the ternary system was investigated by the Thermo-gravimetric Analyzer (TGA) technique and the DSC technique. The results indicated that the melting temperature and fusion enthalpy were 420.83°C and 201.50J/g, respectively, and the specific heat capacity was 1.49J/(g·°C). In the long-term isothermal stability study, the total weight loss of the eutectic salt was less than 5% below 650°C, and the melting temperature and fusion enthalpy changed slightly as the temperature increased. The ternary eutectic system demonstrated excellent thermal stability below 650°C, which was defined as the upper temperature limit of the ternary system. During the short-term thermal cycling test, the stability and recyclability of the ternary salt were verified in future during the working temperature, generally 50°C lower than the upper temperature limit. The eutectic composition before and after thermal treatment was characterized by the X-ray Diffraction (XRD) technique to analyze the mechanism of thermal instability. The mechanism of thermal instability turned out to be the thermal decomposition of the generated magnesium chloride hexahydrate at high temperature. It was proved that the ternary eutectic chloride salt was a potential candidate for high-temperature thermal energy storage applications up to 650°C.

Stability of two-layer fluid flow

The problem of two-layer convective flow of viscous incompressible fluids in a horizontal channel with solid walls in the presence of evaporation is considered in the Oberbeck–Boussinesq approximation assuming that the interface is an undeformable thermocapillary surface and taking into account the Dufour effect in the upper layer which is a mixture of gas and liquid vapor. The effects of longitudinal temperature gradients at the boundaries of the channel and the thicknesses of the layer on the flow pattern and the evaporation rate are studied under conditions of specified gas flow and the absence of vapor flow on the upper boundary of the channel. It is shown that the long-wavelength asymptotics for the decrement is determined from the flow characteristics, the longwavelength perturbations occurring in the system decay monotonically, and the thermal instability mechanism is not potentially the most dangerous.

High-energy x-ray scattering studies of battery materials

Abstract

Origin of thermal instability of CH3NH3PbI3−xClx films for photovoltaic devices

The rapid improvements in perovskite solar cells have led to high power conversion efficiencies of over 20% in recent years. Despite this remarkable performance, the mechanism of thermal instability for those films is still one of the problems that we have to face. In this work, we systematically measure the thermal stability of CH3NH3PbI3−xClx films on substrates of ITO, PEDOT:PSS/ITO, TiO2/ITO, at various temperatures. Comparing their counterparts on Glass, PEDOT:PSS/Glass, TiO2/Glass, we found that the films on substrates with ITO are always less thermal stable, resulting from charge transfer between perovskite and ITO substrates, in particular at higher annealing temperatures. Therefore, it was believed that more thermal degradation can occur in CH3NH3PbI3−xClx film with less neutral electrical environment.

An interface facet driven Rayleigh instability in high-aspect-ratio bimetallic nanolayered composites

One limitation hindering the structural and electrical applications of nanostructured metals is the loss of their nanostructure and strength under elevated temperatures. Nanostructured metals often have grain structures that contain a high density of triple junctions, where thermally induced instabilities commonly initiate. Prior work has resulted in fabrication of nanolayered two-phase composites that possess high-aspect ratio grains, a scarcity of triple junctions, and a thermally stable microstructure. In this work, transmission electron microscopy is used to investigate how these composites could eventually breakdown during heating. We reveal an unconventional thermal instability mechanism in this class of materials, which operates without the assistance of triple junctions. The mechanism can be rationalized by that thermally induced pinch off occurs as the result of bimetal interface faceting and can trigger a classic Rayleigh instability.

Thermal instability induced by high current densities in high-Tc coated conductors

By using a 1 ms pulse technique, the current–voltage curves (CVC) under zero-applied magnetic field have been measured in practical high-Tc superconducting wires based on coated (RE)BCO. The measurements extend up to the so-called supercritical current intensity, I*, at which superconductivity disappears accompanied by an abrupt jump in the voltage. The resulting CVC, including the temperature dependence of I*, were analysed at a quantitative level in terms of a thermal instability model proposed recently. An attractive aspect of our thermal instability approach is that in spite of the complexity of the coated layered structure, in its simpler formulation all the geometric and thermal variables are encapsulated into a single characteristic parameter r with units of thermal resistance. All the experimental results for I* from 76 to 86 K are quantitatively, within 2%, accounted for by the thermal instability analysis. Furthermore, the best-fit value of r is found to be quite consistent with the inner geometry and composition of the wires. Another remarkable result of our analysis is that the temperature T* reached by the wire at I*, i.e., the temperature triggering the thermal runaway, is less than one kelvin above the bath temperature. These results further support the universality of the thermal instability mechanism of superconductivity quenching by high current densities.

Spheroidization Effects on the Electrical and Magnetic Properties of Cu-Nb Composites

We report on the microstructure evolution in Cu-15% Nb multifilamentary wires upon annealing and the corresponding effects on their electrical and magnetic properties. During annealing at temperatures higher than 800 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$^circ C$</tex> thermal instability mechanisms take place in the microstructure of the Cu-15%Nb composite, leading to spheroidization, followed by coarsening of niobium filaments. Such microstructural changes exert considerable influence on the magnitude of the normal electrical resistivity for the Cu-Nb composite. In addition, the annealing process leads to a double-peak structure in the isothermal DC magnetization curves of the composite, which is strongly dependent on the annealing time. This behavior suggests that the spheroidization kinetics can be obtained from M(H) curves.

Restabilizing mechanisms after the onset of thermal instability in bipolar transistors

The electrothermal behavior of single- and two-finger bipolar transistors at medium- and high-current operations is studied through theoretical modeling, experimental measurements, and computer simulations. Bias conditions that border thermally stable and unstable operation regimes are described by novel analytical formulations, which for the first time include simultaneously all relevant parameters that weaken the electrothermal feedback at high currents such as ballasting resistors, current dependence of the base-emitter-voltage temperature coefficient, and high-injection effects. Hence, besides giving a correct description of thermal instability mechanisms, the developed formulations also allow the prediction and physical understanding of restabilization phenomena. The models are supported by measurements on silicon-on-glass n-p-n bipolar junction transistors and by simulation results from a novel SPICE-based electrothermal macromodel for bipolar transistors. Furthermore, the models are employed to analyze the influence of the germanium percentage in the base of SiGe heterojunction bipolar transistors on the thermal ruggedness of the device.

Stability of an evaporating thin polymer film

The paper demonstrates existence of a purely thermal instability mechanism in a thin evaporating film that can be responsible for formation of mesoscopic patterns in thin polymer films. The instability is caused by the increasing evaporation with the film thinning due to higher heat flux to the film surface.

The evolution of Nb recrystallization in a Cu matrix: morphology of Nb and modulation ofits critical fields in a wire configuration

We report on the microstructural evolution in Cu15%Nb multifilamentary wires upon annealing andthe corresponding effects on their magnetic properties. During annealing at temperatures higher than800 °C thermal instability mechanisms take place in the microstructure of the Cu15%Nbcomposite, leading to the spheroidization of niobium filaments. Anotherimportant consequence of annealing is the recrystallization of Nb, that has astraightforward influence on the magnetization data. When compared to the as-drawncomposite, in the annealed Cu15%Nb samples the bulk upper critical fieldHc 2(T) is reduced significantly, while the lower critical fieldHc 1(T) is increased as a function of the annealing temperature and time. Thisindicates that we may employ this process as an efficient method to modifythe basic superconducting parameters of Nb, namely the coherence lengthξ(T) and thepenetration depth λ(T).

Ab initiosimulations of accretion disc instability

We show that accretion discs, both in the subcritical and supercritical accretion rate regime, may exhibit significant amplitude luminosity oscillations. The luminosity time behaviour has been obtained by performing a set of time-dependent two-dimensional smoothed particle hydrodynamics simulations of accretion discs with different values of a and accretion rate. In this study, to avoid any influence of the initial disc configuration, we produced the discs injecting matter from an outer edge far from the central object. The period of oscillations is 2-50 s for the two cases, and the variation amplitude of the disc luminosity is 10 38 -10 39 erg s -1 . An explanation of this luminosity behaviour is proposed in terms of limit cycle instability; the disc oscillates between a radiation pressure dominated configuration (with a high luminosity value) and a gas pressure dominated one (with a low luminosity value). The origin of this instability is the difference between the heat produced by viscosity and the energy emitted as radiation from the disc surface (the well-known thermal instability mechanism). We support this hypothesis showing that the limit cycle behaviour produces a sequence of collapsing and refilling states of the innermost disc region.

Linear analysis of an aerothermal instability occurring in diffusion-controlled premixed catalytic combustion

The onset of the unstable temperature distribution which may appear in plane axisymmetric catalytic burner is studied. The instability mechanism is assumed to be related with the temperature dependence of the viscosity, thus with the pressure drop in the porous combustor, governed by Darcy’s law. An area e.g., of lower temperature (dark zone) characterised by a smaller value of the kinetic viscosity gives rise to a locally increased gas flow mass velocity, the pressure drop remaining constant over the burner cross-section. The locally increased mass velocity produces an enhanced cooling of the area, whereby heat conduction from the hotter surrounding area tends to restore a homogeneous temperature distribution. A linear analysis of this thermal instability mechanism is carried out and yields a simple analytic solution for the state of neutral stability.