External Temperature Field Research Articles

Remotely tuned multistate resistive switching in MoS2/NiMnIn thin film heterostructure for highly flexible ReRAM application

In this article, the Cu/MoS2/NiMnIn memory structure was fabricated over stainless steel substrate for flexible electronics applications. The bipolar resistive switching characteristics were observed in two different fabricated devices (i) without sulfur vacancies (D1) and (ii) with sulfur vacancies (D2) in MoS2 thin film. Three different resistance states such as the high resistance state (HRS), intermediate resistance state (IRS) and low resistance state (LRS) have been detected for the D2 memory device. It could be ascribed to the two filamentary models based on the movement of sulfur vacancies and the formation of Cu metallic filament with applied bias voltage. Moreover, the Ohmic and modified space charge limited conduction mechanisms clearly explain the current conduction in different resistance states of device D2. The fabricated MoS2 thin film-based memory structure exhibits stable resistive switching behavior with a high OFF/ON ratio of ∼3.6 × 103, good consistency of ∼3600 endurance cycles and excellent data retention capability up to 3000 s. Moreover, the remote tuning of device D2 was thoroughly investigated and an appreciable change in SET voltage was detected with external temperature and magnetic field. Additionally, the external magnetic field altered the switching states and enhanced the multi-bit data storage capability of the memory device. This can be ascribed to the effect of the Lorentz force on ionic movement in the presence of the external magnetic field. The mechanical flexibility of the memory structure was tested for 1000 bending cycles at various bending angles in both the tensile and compressive bending modes. Hence, the present study opens up new ways for the futuristic flexible device for high data storage and neuromorphic computing applications.

External temperature field test and leakage fault diagnosis for SOFC stacks

The uniformity of Solid Oxide Fuel Cell (SOFC) stacks largely affects the stability and performance of the SOFC power generation system with multiple stacks. However, the health conditions of the stacks are usually assessed by the long-time electrochemical performance test, and some small faults are not able to be quickly identified when the I–V performances have small differences. In this paper, a non-contact temperature sensor array constructed by thermocouples is proposed for NDE (non-destructive evaluation) application in SOFC stacks. The temperature sensor array is fixed near the outer surfaces of the 1 kW-class stack to measure the external temperature field of the stack. Based on the experimental results, it is found that the proposed method is feasible for the examination of the external temperature field of stacks under different operation conditions, which helps to screening the stacks before assembling to the system. At the same time, a real-time online monitoring of fuel leakage is performed using the temperature sensor array, and stacks with unusual temperature distribution due to the leakages can be quickly recognized by comparing with the healthy stack.

Thermochromic Insulation Materials for Thermal Sensing and Overheat Prewarning

The real-time monitoring and scientific prewarning of local overheating for electrical equipment and electronic devices are of great significance but remain challenging. Therefore, in this work, a temperature-responsive microcapsule is prepared to endow insulating materials with the ability to thermally sense external temperature fields. Specifically, in the microcapsule/epoxy composites, the epoxy resin served as the polymer matrix was doped with reversible thermochromic microcapsules, acting as a temperature indicator whose color transition temperature is centralized between 45 °C and 55 °C. The test results indicate that the color of the prepared composites can sensitively vary in response to the fluctuations in external ambient temperature, which was illustrated by the behavior of isothermal melting. Further analysis reveals that composites are able to precisely locate multiple tiny overheated areas and present remarkable thermal durability undergoing 480 h of aging at 70 °C. The thermochromic materials designed in this article provide a new technological route for large-scale thermal sensing applications of electrical equipment.

Characteristics of droplet exudation on slippery liquid-infused porous surfaces considering thermal effect

The self-healing property of slippery liquid-infused porous surfaces ensures the continuity and stability of the liquid film on the surfaces. Therefore, how to promote the exudation and spreading of the lubricant is the key to realize the self-healing. In this study, a numerical model of lubricant exudation and spreading in micropores was established under the stimulation of the external temperature field. The morphological evolution of the meniscus and the distribution characteristics of the internal pressure during the exudation process were investigated. The influence mechanism of the thermal effect on the exudation and film formation on the porous surface was revealed. Results show that the formation process of lubricating film includes the lubricant exudation in the pores, the growth and spreading of microdroplets on the porous surface. In the lubricant exudation stage, the internal pressure of the fluid is negative. When the lubricant exudes to the edge of the pore, the internal pressure of the liquid changes from negative to positive. The three-phase contact wire is pinned at the edge of the pore and droplets are formed on the porous surface. When the droplet begins to spread, the internal pressure of the droplet decreases for the increasing of the meniscus curvature radius. The spreading velocity decreases gradually in the form of fluctuation under the combined action of the time-varying resistance and driving force. At higher wall temperature, the droplet surface tension and viscous resistance decrease, whereas the thermal-capillary force and driving force increase. Therefore, the higher the wall temperature, the greater the average velocity of the droplet spreading front, and the better the droplet spreading performance. Therefore, higher wall temperature can promote the droplet exudation and spreading on the surface, which is conducive to the self-healing of the lubricating film.

BOUNDARY CONTROL OF ROD TEMPERATURE FIELD WITH A SELECTED POINT

In this paper, we study the issue of boundary control of the temperature field of a rod with a selected point. The main purpose of the work is to clarify the conditions for the existence of a boundary control that ensures the transition of the temperature field from the initial state to the final state. Relations connecting the boundary controls with the initial and final states, as well as with the external temperature field are found. Such boundary controls, generally speaking, constitute an infinite set. For an unambiguous choice of the boundary control, a strictly convex objective functional is chosen. We are looking for a boundary control that minimizes the selected target functional. To do this, we first investigate the existence and uniqueness of solutions to the initial boundary value problem and the conjugate problem. And also, we present the derivation of a system of linear Fredholm integral equations of the second kind, which are satisfied by an optimal boundary control that minimizes a strictly convex target functional on a convex set. Along the way, the linear part of the increment of the target functional is highlighted. Necessary and sufficient conditions for the minimum of a smooth convex functional on a convex set are established. The difference between the results of this work and the available ones is that in the proposed work, the temperature field is given by the heat conduction equation with a loaded term. As a result, the conjugate problem has a slightly different domain of definition than the domain of the conjugate problem in the case of no load.

Thermodiffusion of ions in nanoconfined aqueous electrolytes

Understanding of thermal effects on ion transport in porous media is very important for environmental applications. The movement of ions along a temperature gradient is named thermophoresis or thermodiffusion. In nanoporous media, where the interaction of ions with solid–liquid interfaces has a significant influence on their migration, the theoretical understanding of thermodiffusion is still incomplete. Herein, we present experimental results for the thermodiffusion of cations in saturated nanoporous silica by the through-diffusion method. Both the experimental data and theoretical analysis indicate that the temperature-induced polarization of surface charges strongly influences ionic transport. Stated simply, the electric field in a liquid electrolyte confined in nanopores changes when the applied temperature gradients are altered, thereby affecting the motion of the nanoconfined ionic species. By applying an external temperature field, the gradient of the surface charge density leads to the charged aqueous species exhibiting strong temperature gradient-dependent electrophoretic mobility. When the thickness of the electrical double layer is comparable to the size of the nanopores, the theory used herein indicates that this kind of nonisothermal ionic mobility is up to one order of magnitude larger than classical thermophoretic mobility. This study improves the understanding of the underlying mechanisms that govern the transport of ions in nanoporous media, which could set the stage for diffusional metamaterials induced by specific thermal fields.

Thermal resistance modeling of flat plate pulsating heat pipes

Flat plate pulsating heat pipes (FP-PHP) represent a future technology, enabling a more efficient thermal management of high heat flux devices. Whereas established thermal resistance models exist for wicked heat pipes, there are no such comprehensive models for pulsating heat pipes. In this paper, a conduction-based approach for pseudo-steady heat transfer is presented which can be used for 3D thermal simulations. For the calculation of the internal thermal resistance, multi-fluid correlations for flow boiling and condensation are utilized. A saturated homogenized state in-channel is assumed which allows the temperature-dependent determination of thermophysical fluid properties. A good agreement for the external temperature field was achieved compared to two independent experiments. The simulations show a significant sensitivity to the condenser conditions. Interestingly no adiabatic zone exists along the channels. Furthermore, it is shown that consideration of thermal spreading effects is key for the interpretation of the overall thermal resistance, effective heat fluxes and the design of optimal FP-PHPs.

Determination of Thermomechanical Stresses in Elements of Vehicles' Braking Systems

The main objective of the study was to develop a model and analyze the thermomechanical behavior of the hub material of the vehicle brake disk. The simulation strategy was based on the solution of the three-dimensional problem of the theory of elasticity for the case of effect of external loads and temperature fields on the metal structure element of the vehicle brakes. To solve this type of task of the theory of elasticity, the differential equations of the second order were used for the first time. Adaptation of the proposed model, completed in the article, has proved the correctness of use of these equations in modeling the thermomechanical processes with determination of stresses and displacements in unevenly heated rotary cylinders of the final length. The proposed method can be applied with high efficiency in stress strain state simulation of individual parts of vehicles.

Near-infrared range tuneability in GaAs-based type-II nanoscale heterostructure emitter

We present the investigation of energy wavefunctions, dispersion profile, dipole momentum behavior, and optical gain for a nanoscale heterostructure formed with material layers of InAlAs/InGaAs/GaAsSb/InGaAs/InAlAs on a GaAs substrate. The designed structure is simulated with a 6 band k p formalism for the investigation. At room temperature, the 1548-nm wavelength has an optical gain of 2352/cm, making it suitable for NIR applications. The response of the designed heterostructure is also investigated with external temperature, strain, charge density, and electric field variations to report the tunability of the optical gain and wavelength. Due to its tunability in the NIR wavelength region, this structure can be used for different NIR applications without changing the material system.

Analysis of the direct steam generation, thermal stresses, and deformations in a tubular receiver of a solar tower

In solar tower systems, the thermal receiver is a crucial element. Solar receivers may be exposed to high non-uniform heat fluxes, which requires consideration during their structural design. This paper presents a numerical study of the direct steam generation, thermal stress, and total deformations on a representative tube of an external solar receiver. A non-uniform distribution of solar heat flux on the receiver was considered. The sub-cooled boiling and the direct steam generation were analyzed in terms of the temperature field, the volumetric vapor fraction distribution, and steam quality. Under this configuration, the system can produce a steam quality of 0.113%. To develop the structural analysis of the tube, the internal and external temperature fields of its walls were introduced as boundary conditions. Numerical methodologies for direct steam generation, temperatures, and deformation were validated with experimental data. Since the base case presented high deformation (78.63 cm), the use of clips was proposed. Besides, the number, size, and location of the clips were tested. It was found that deformation can be reduced by about 99.1% by placing clips at different positions along the tube. However, the von-Mises stresses increased, but they did not reach the ultimate tensile strength.

Numerical evaluation of the exhaust-direction effects on plume flow and helicopter infrared radiation under hover and cruise statuses

Purpose This study aims to investigate the effects of exhaust direction on exhaust plume and helicopter infrared radiation in hover and cruise status. Design/methodology/approach Four exhaust modes are concerned, and the external flow field and fuselage temperature field are calculated by numerical simulation. The infrared radiation intensity distributions of the four models in hovering and cruising states are computed by the ray-tracing method. Findings Under the hover status, the exhaust plume is deflected to flow downward after it exhausts from the nozzle exit, upon the impact of the main-rotor downwash. Besides, the exhaust plume shows a “swirling” movement following the main-rotor rotational direction. The forward-flight flow helps prevent the hot exhaust plume from a collision with the helicopter fuselage generally for the cruise status. In general, the oblique-upward exhaust mode provides moderate infrared radiation intensities in all of the viewing directions, either under the hover or the cruise status. Compared with the hover status, the infrared radiation intensity distribution alters somewhat in cruise. Originality/value Illustrating the influences of exhaust direction on plume flow and helicopter infrared radiation and the differences of helicopter infrared radiation under hover and cruise statuses are identified. Finally, an appropriate exhaust mode is proposed to provide a better IR signature distribution.

Thermotaxis of Janus particles

The interactions of autonomous microswimmers play an important role for the formation of collective states of motile active matter. We study them in detail for the common microswimmer-design of two-faced Janus spheres with hemispheres made from different materials. Their chemical and physical surface properties may be tailored to fine-tune their mutual attractive, repulsive or aligning behavior. To investigate these effects systematically, we monitor the dynamics of a single gold-capped Janus particle in the external temperature field created by an optically heated metal nanoparticle. We quantify the orientation-dependent repulsion and alignment of the Janus particle and explain it in terms of a simple theoretical model for the induced thermoosmotic surface fluxes. The model reveals that the particle’s angular velocity is solely determined by the temperature profile on the equator between the Janus particle’s hemispheres and their phoretic mobility contrast. The distortion of the external temperature field by their heterogeneous heat conductivity is moreover shown to break the apparent symmetry of the problem.Graphic abstract

A large birefringence and high nonlinearity liquid crystal photonic crystal fiber with low confinement loss

Through filling the liquid crystal E7 into the defect position of the photonic crystal fiber (PCF) which possesses full circular air hole structure, a PCF with adjustable optical performance is designed. The effect of external temperature and magnetic field on its optical properties is analyzed by adopting the finite element method. The results reveal that its confinement loss is reduced, and the dispersion performance is also improved. Besides, the high birefringence bandwidth can covers all the S, C, L and U communication bands and the it reaches to the order of 10−1 at the wavelength of 1.55 μm; a high nonlinear coefficients (about 3 × 105 W−1km−1 and 9 × 105 W−1km−1 in X, Y polarization directions respectively) are also obtained. Most importantly, the optical performance can be adjusted without changing its structural parameters. It provides a new method for obtaining PCFs with good optical performance, and it has potential applications in the fields of polarization control, dispersion management, super-continuum generation and optical fiber sensing.

Designed band-pass frequency selective surfaces with thermal transparency

Band-pass frequency selective surfaces (FSSs) have been wildly used to achieve stealth performance for radar electromagnetic wave detection. However, the existence of air slots will lower the effective thermal conductivity of the FSS structure that distorts the external temperature field and makes the FSS infrared visible. An interesting and practical challenge for the development of FSSs is the feasibility of empowerment of both microwave frequency and thermal transparency. Based on the mechanism of periodic interparticle interaction, a periodic filled region is introduced to achieve thermal transparency. The design principle is discussed in detail, and typical Jerusalem cross shape aperture samples are selected for modeling simulation and experimental verification. The results of frequency response and heat transfer characteristics confirm the designed band-pass FSSs have less-distortion frequency response and thermal transparency, which provide an effective approach for the design and fabrication of microwave and infrared camouflage with easy implementation in practical applications.

METHOD OF LINES IN DISTRIBUTED PROBLEMS OF EXPERIMENTAL DATA PROCESSING

In many cases, the mathematical support of non-stationary thermal experiments is based on methods for solving the inverse heat conduction problem (IHCP), which include boundary thermal conditions determination, identification of heat and mass transfer processes, restoration of external and internal temperature fields, etc. However, at present, the main field of the IHCP application remains the processing and interpretation of the results of the thermal experiments. It was here where the most considerable theoretical and applied successes were achieved in methods' effectiveness and the breadth of their practical use. This paper highlights the issues of mathematical modeling of multidimensional non-stationary problems of metallurgical thermophysics.The primary research purpose aims at solving problems associated with identifying parallel structures of algorithms and programs and their reflection in the computers’ architecture in solving a wide range of applied problems. Supercomputers are currently inaccessible due to the enormous cost and service price. In this regard, a real alternative is cluster-type computing systems by which the simulation results are covered in this paper.Being a relatively new technology, cluster-type parallel computing systems are useful in solving a large class of non-stationary multidimensional problems, while allowing to increase the productivity and quality of computations. The software developed in this paper can be used to plan and process the results of a thermophysical experiment. The algorithms developed in the application program package are simply reconstructed to solve other coefficient and boundary problems of thermal conductivity. The developed algorithms for solving thermophysical problems are highly accurate and efficient: the test solution for IHCP with accurate input data coincides with the thermophysical features of the sample material. The developed software for processing the results of a thermophysical experiment is self-regulating. Moreover, it is quite merely tuned to the solution of others and, in particular, of boundary IHCP.

Effect of Temperature on Formaldehyde Diffusion in Cellulose Amorphous Region: A Simulation Study

A formaldehyde-cellulose amorphous region model at the micro-level was established using the molecular dynamics software Materials Studio to simulate the change of cellulose and formaldehyde molecules in an external temperature field. The diffusion coefficients of formaldehyde molecules increased as the temperature increased. Moreover, the total number of hydrogen bonds decreased, and the interaction energy in the formaldehyde-cellulose model was reduced, which confirmed this conclusion and indicated that temperature increase could enhance the diffusion of formaldehyde in cellulose. The mechanical parameters of cellulose were analyzed in terms of Young’s modulus, shear modulus, bulk modulus, Poisson’s ratio, and the ratio of bulk modulus to shear modulus (K/G), which were affected by the temperature. The elastic modulus (E, G, and K) of cellulose decreased as the temperature increased, while the Poisson’s ratio V and K/G values increased. The results of the research explain how elevated temperature can promote the release of formaldehyde in furniture from a microscopic perspective, which supports each other with the results of previous experimental data and practical applications in production.

Numerical simulation study on band gap characteristics of surface phononic crystal with spherical composite column

In the study of acoustic characteristics of micro-scale surface phononic crystal, the band gap characteristics below 100 MHz need to be further optimized. In this work, a piezoelectric surface phononic crystal with a composite column composed of nickel balls and epoxy backing is proposed. The finite element method is used to calculate the band gap characteristics and displacement vector field of the model. The influence of column radius on the band structure is studied, and meanwhile, the effect of the multi-layer composite column structure on the band gap is discussed via increasing the number of elements in the composite column, while the reason for the opening of the high-order band gap is analyzed in detail by combining the vibration mode. Furthermore, the temperature adjustability of the band gap is further studied. The results show that the spherical composite column deposition structure can open a wider complete band gap of surface acoustic wave in a lower frequency range than the existing inverse conical surface phononic crystal structure with the same lattice constant (Hsu J C, Lin F S <ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://doi.org/10.7567/JJAP.57.07LB01">2018 <i>Jpn. J. Appl. Phys.</i> <b>57</b> 07LB01</ext-link>). The restricted cavity mode is easily formed between the hard boundaries with the increase of column radius, which provides a possible way for low-order vibration modes to open high-order band gaps. There exist mode inheritance and energy coupling between adjacent modes, which leads the band gap to flatten and anti-flatten. Moreover, the real-time adjustment of band gap frequency by external temperature field can be realized via introducing the temperature-sensitive material epoxy resin into the structure. The band gap frequency range can be effectively reduced by increasing the number of composite cylinder layers, while the multi-vibrator structure can generate multipole resonance coupling with traveling wave and finally open a complete band gap between high-order frequency bands. This work provides a theoretical reference for analyzing the low-frequency band gap mechanism of micron-scale surface phononic crystal.

Nonlinear superharmonic and subharmonic resonances of piezoelectric elliptic thin plates under thermoelastic coupling effect

This paper aims at investigating the nonlinear vibration characteristics of the piezoelectric elliptic thin plate under the combined action of external harmonic excitation and temperature field. Based on the Von-Karman plate theory of large deflection, the nonlinear governing equation is derived by using the Bubnov-Galerkin method. Then, the amplitude-frequency response equations are further obtained using the multi-scale method, and the stability conditions of the steady solution are determined according to the Routh-Hurwitz criterion. The effects of the temperature difference at the plate center, the effective damping coefficient, the external excitation amplitude, and the ratio of the semi-major axis to the semi-minor axis on superharmonic and subharmonic resonances behaviors of the piezoelectric elliptic thin plate are analyzed. It is found that the coexistence region of multiple solutions and the resonant amplitude decrease for superharmonic resonance, as the temperature difference at the plate center increases. For subharmonic resonance, with the increase in the temperature difference at the plate center, the response amplitude increases, while the distance between two branches of the amplitude-frequency response curve has no obvious change. The softened/hardened nonlinear characteristics of the system are determined by the ratio of the semi-major axis to the semi-minor axis of the piezoelectric elliptic thin plate.

Cloaking: Controlling Thermal and Hydrodynamic Fields Simultaneously

Hydrodynamic and thermal cloaking effects have attracted interest among researchers using metamaterials. This paper explores an analytical solution using only isotropic materials to achieve cloaking under forced convective heat transfer. It is shown that, under a background of uniform flow and linearly varying temperature, a circular bilayer cloak with an inner layer made of impenetrable and insulated material and an outer layer made of a porous medium having a constant permeability and effective conductivity does not disturb the external flow and temperature fields. Moreover, the cloaked region is shielded from the external flow and temperature fields. Analytical temperature and velocity distributions within the porous outer layer are presented to demonstrate both the hydrodynamic and thermal cloaking effects of such a bilayer. Finally, numerical results, based on practically attainable material properties, are given to confirm the analytical model.

Environmental Response of 2D Thermal Cloak under Dynamic External Temperature Field.

As a typical representative of transformation thermodynamics, which is the counterpart of transformation optics, the thermal cloak has been explored extensively while most current research focuses on the structural design instead of adaptability and practicability in a dynamic environment. The evaluation of energy processes involved in the thermal cloak under dynamic conditions are also lacking, which is essential to the engineering application of this functional structure. In this paper, based on the dynamic environment of a sinusoidal form with ambient amplitude, distribution density, phase, and temperature difference as variables, we evaluated the cloaking performance and environmental response of a 2D thermal cloak. Considering the heat dissipation and energy loss in the whole procedure, local entropy production rate and response entropy were introduced to analyze the different influences of each environmental parameter on the cloaking system. Moreover, we constructed a series of comprehensive schemes to obtain the fitting equation as well as an appropriate scope to apply the thermal cloak. The results are beneficial to the novel use of the concept of entropy and valuable for further improving the working efficiency and potential engineering applications of the thermal cloak.