Temperature Dependence Of Time Research Articles

Insight into Pyroelectricity and Phase Transitions in Ferroelectrics from Nonequilibrium Approach: The Case of PbTiO3

AbstractComputational methodology is proposed to simulate time dependent temperature variations via thermostat approach. The methodology is applied to study pyroelectricity under nonequilibrium conditions in prototypical ferroelectric PbTiO3. It is found that at room temperature the nonequilibrium effects in pyroelectricity begin manifesting above 1.0 THz and originate from the overlap with the intrinsic soft mode frequency. Therefore, the proposed computational methodology can be used to predict equilibrium pyroelectric response by choosing a frequency of temperature variation below the intrinsic polar mode frequencies for the material. Probing the dynamics of ferroelectric phase transition with the proposed methodology revealed that it takes about a nanosecond for the computational supercell to undergo phase transition. It is concluded that computational techniques capable of simulating picoseconds can be used to compute pyroelectric response from nonequilibrium dynamical approach while those that can reach nanoseconds are suitable for simulating phase transitions in ferroelectrics.

Multitemperature Modeling of Thermal Transport across a Au–GaN Interface from Ab Initio Calculations

With device dimensions shrinking toward nanoscale sizes, an accurate understanding and modeling of the thermal energy transfer mechanisms at metal–nonmetal interfaces becomes highly desirable. To model the thermal transport across the Au–GaN interface, we developed a general multitemperature model (MTM) that accounts for both electron and phonon transport and allows for a nonequilibrium phonon population described as a sum of thermal distributions of the phonon branches. The model is populated with material parameters computed from ab initio calculations and used to describe the temperature profile across a Au–GaN junction 400 nm in length subjected to constant temperature differential, continuous-wave laser heating of Au, and constant heat flux on GaN. The calculated steady-state and time-dependent temperature profiles reveal strongly nonequilibrium electron–phonon and phonon–phonon interfacial regions created through energy carrier coupling. Our results indicate that a multitemperature-based model is required for the accurate resolving of the thermal energy flow across metal–nonmetal interfaces. Our MTM will advance the theoretical tool to investigate nonequilibrium thermal transport across metal–nonmetal interfaces.

Molecular dynamics in amorphous double active ionic liquid developed by chemical structural modification of ibuprofen

Ibuprofen is a non-steroidal anti-inflammatory drug categorized as a Class II Biopharmaceutics Classification System (BCS) pharmaceutical with low solubility and high permeability. Though the drug and its salt form are glass-forming, but not physically stable in their amorphous state, which limits their use as amorphous drugs. In this study, a dual-active pharmaceutical ingredient, metforminium ibuprofenate (MtHIb), was prepared by chemical structural modification of sodium ibuprofen (NaIb) with metformin hydrochloride (MtHCl) by a stochiometric metathesis reaction. Information on translational and rotational mobility of both daughter and parents has been obtained from broadband dielectric spectroscopy (BDS) covering a wide temperature and frequency regimes in supercooled and glassy states in conjunction with quantum mechanical calculations. The temperature dependence of α-relaxation times was described by Vogel–Fulcher–Tammann equation. The glass transition temperature for MtHCl and MtHIb were found to be 269.45 and 227.24 K, respectively, which is in good agreement with the DSC thermogram with a slight discrepancy. The fragility index of ibuprofen is found to decrease upon the modification and attained 50.39 for MtHIb indicating its strong glass-forming nature which is well evidently supported by broad diffraction peaks in X-ray diffraction patterns of MtHIb. These results demonstrate that chemical structural modification of ILs can be considered a proven strategy for inhibiting the devitrification of disordered pharmaceuticals to control or bypass the secondary relaxations in the glassy state by modification of its structure. These remarkable outcomes had an impact on the fundamental understanding of amorphous pharmaceuticals on the industrial front.

Trefftz-Type Functions Applied for Subcooled Flow Boiling Heat Transfer Calculations in a Minichannel of Different Spatial Orientation

ABSTRACT The paper describes the results of subcooled flow boiling heat transfer in an asymmetrically heated group of five minichannels 1 mm deep. The heated element for the cooling fluid (FC-72) flowing in the minichannels was a foil made of Haynes-230 alloy. The time-dependent temperature distribution of the outer heated foil surface was recorded as a result of infrared thermography. The test section was oriented in several spatial orientations, that is, 30°, 45°, 60° and 75° inclinations to the horizontal plane. The objective of the numerical calculations, referring to the experimental results, was to determine the time–dependent heat transfer coefficient on the wall-fluid contact surface from the Robin boundary condition. Both the foil and the fluid temperatures were the result of solving the inverse and direct two-dimensional heat transfer problem in two neighboring domains: the heated foil and flowing fluid. The problem was solved using the Trefftz method and the FEM with Trefftz type basis functions. The heat transfer coefficient obtained by two numerical methods appears to be similar. During subcooled flow boiling, at higher minichannel inclination angles, higher values of the heated foil temperature were obtained, as well as higher values of the heat transfer coefficient.

A study on cylindrical moving boundary problem with variable thermal conductivity and convection under the most realistic boundary conditions

The melting of a phase change material is the most applicable process in thermal energy storage system to capture heat transfer phenomena arising in a class of moving boundary problem. Demand of present technology motivates researchers to develop new theories and techniques for thermal management of a material. Experimental work on melting of a material may be difficult and development of robust theoretical formulation in cylindrical geometry with convection is critical. While there is already available study on cylindrical moving boundary problem, but still insufficient modeling of a size-dependent thermal conductivity and convection effect is not addressed properly, which is being considered in this paper and is expected to improve the previous understanding. In this work, a one-dimensional moving boundary problem with size-dependent heat conductivity and convection effect is analyzed in cylindrical geometry. In the mathematical model, we have considered a time-dependent temperature boundary condition which later assumed in periodic form, and a convective boundary condition at the outer surface of the body. The numerical result of the problem is obtained successfully via heat-balance integral method. Our numerical result is compared with a previous work and found in good acceptance. From mathematical framework, it is found that convection delayed melting process. With a size-no independent thermal conductivity, the rate of moving front decreases more in comparison to the fixed thermal conductivity.

Shock ignition and deflagration growth in plastic-bonded TATB (1, 3, 5-trinitro-2, 4, 6-triaminobenzene) microstructures

TATB (1,3,5-triamino-2,4,6-trinitrobenzene) plastic-bonded explosives (PBX) were shocked with laser-launched flyer plates. The spectral radiance of the emitted light from a small portion of the microstructure (a “microenvironment”) containing a small number of TATB particles with an estimated mass of 150 ng was measured every 0.8 ns from 1 ns to 200 μs and was analyzed to give radiance and time-dependent graybody temperatures. By fabricating an array with 186 PBX charges, we could obtain ≥15 shots at each of 12 velocities between 1.8 and 4.7 km/s. We found that every microenvironment generated a unique radiance fingerprint. Some of these microenvironments were much more reactive than average. The radiance has two peaks around 20 ns and 5 μs, associated with shock ignition and deflagration growth. In our interpretation, the shock creates an ensemble of hot spots of various sizes and temperatures. Of those hot spots that ignite, only a small portion, at about 2200 K, was large enough and hot enough to survive long enough (>100 ns) to ignite individual TATB particles, leading to deflagration. Integrating various time intervals of the radiance can quantify the strength of the shock–PBX interaction, and the decay and growth of the hot spot ensemble and the deflagration.

Behaviour of vertically loaded steel beams under a travelling fire

As the structural response to a time and space dependent non-uniform temperature field is challenging to manage by code-based design, the behaviour of steel beams exposed to a travelling fire was simulated numerically in this paper. The finite element (FE) models for both thermal and mechanical analyses were validated by two benchmark tests and used to study the effect of uniform and non-uniform thermal exposures and different steel grades on the response of steel beams. The developed five-stage mechanism well characterized the response of the beams exposed to the travelling fire. For the beam in the elastic range, the heating-cooling cycles of gas temperatures induced cyclic axial forces, but the cyclic nature of structural responses was cushioned by material plasticity. For the beams stressed into the plastic range, the repetitions of cycles in axial response created residual deformations that induced catenary action in the beams at lower temperatures. The local heating of the critical sections made the beams more prone to a runaway failure than the uniform heating of the whole beam. Compared to the mild steel beams, the axial force response of the HSS beams showed higher axial compression forces and fluctuated more. The critical temperatures based on the deformation or strength criterion were consistent for the beams with the load ratio of 0.3. However, for designing the beam with the load ratio of 0.5, critical temperature of 350 °C or the strength criteria based on the proportional limit are recommended as the catenary action was activated at low temperatures. These recommendations can be used as a guide for practical design.

Temperature dependence of aging effect in Pb(Mg1/3Nb2/3)O3–PbTiO3 single crystal

The temperature dependence of the aging effect of permittivity in the paraelectric phase of 70.5%Pb(Mg1/3Nb2/3)O3–29.5%PbTiO3 (PMN–29.5%PT) is investigated. Time dependences of permittivity due to the aging effect at constant temperatures without DC biasing field can be empirically analyzed with the Williams–Watts relaxation function. Using the distribution function of relaxation frequency for the Williams–Watts relaxation function, we discuss the temperature dependence of the characteristic time of the aging effect. We clarify that the distribution width of the characteristic time markedly increases with decreasing temperature.

Gravure Printing for PVDF Thin-Film Pyroelectric Device Manufacture

Pyroelectric energy harvesting is one of the more recent and promising solid-state approaches for directly converting time-dependent temperature fluctuations into electric energy. Conventional printing technologies can offer many advantages for the production of pyroelectric thin-film-based devices, such as low cost, low temperature, the use of flexible substrates and shaping at the same time as deposition. Nevertheless, some issues related to low printed thickness and film-forming microstructure control need to be addressed. In this exploratory study, the possibility of exploiting the highly attractive gravure printing process for the potential industrial manufacture of flexible polyvinylidene fluoride (PVDF) thin-film pyroelectric devices was investigated. By the use of corona pre-treatment of the printing substrate and low-temperature polar solvent evaporation, multilayer gravure-printed PVDF pyroelectric devices were successfully manufactured for the first time, achieving a maximum generated current of 0.1 nA at 2.5 K/s from a device with an active area of 1 cm2. Considering the very low thermal inertia and performance scaling by the area expected for pyroelectric thin-film-based devices, combined with the upscaling potential of roll-to-roll gravure printing, our results provide new opportunities for on-demand, low-cost pyroelectric device manufacture and their integration in hybrid harvesters.

Temperature dependence of radio- and photoluminescence and scintillation properties of Y0.6Gd2.4Al2Ga3O12:Ce,Mg single crystal

Mg2+-codoped Y0.6Gd2.4Al2Ga3O12:Ce (YGAGG:Ce,Mg) single crystal was grown by the Czochralski method and its luminescence and scintillation properties were investigated. Temperature dependence of radioluminescence (RL) yield and photoluminescence (PL) decay time was measured, and activation energy of thermal ionization process was determined from temperature-dependent PL decay time measurement. YGAGG:Ce,Mg exhibited high X-ray excited RL yield almost 5 times higher than BGO reference scintillator. Its light yield of 41,500 ph/MeV and scintillation decay times of 12 ns (2%) + 52 ns (65%) + 207 ns (33%) were measured under excitation with 662 keV γ rays from 137Cs source. Afterglow and thermally stimulated luminescence characteristics are also investigated.

Semi-Analytical Method for 3D Transient Heat Transfer in Thermally Interacting Fields of Ground Heat Exchangers

The study of heat transfer in ground heat exchangers (GHEs) considering the fluid advection inside the pipes; the heat transfer between the fluid and the ground through the grout material; and the thermal interaction between GHEs is a challenging task. The present paper presents a new semi-analytical method that takes into account the aforementioned effects to consider both the short- to long-term effects of GHEs. The heat transfer between the fluid and grout was studied by a transient multipole expansion considering time-dependent fluid temperatures and an advection model for the pipes obtained from an energy balance on the heat carrier fluid. Thermal interactions were analyzed using an equivalent borehole method while penalizing the transient multipole expansion to include thermal interaction effects. Validation of the short-term predictions was performed by comparing the proposed model to experimental data found in the literature and to an FEA model. The proposed model was then compared with a FEA model in long-term simulations of a geothermal field comprised of 24 GHEs for multi-annual simulation. The method resulted in substantial reduction in computational time while preserving good accuracy when compared with the FEA model.

Effect of heating–cooling imbalance on slow mode with time-dependent background temperature

Effect of heating–cooling imbalance on slow mode with time-dependent background temperature

Understanding temperature and dwell time dependence of liquid metal embrittlement in austenitic stainless steel by liquid zinc and copper

Austenitic stainless steels are known to be susceptible to LME-induced cracking when exposed to various liquid metals, including zinc and copper. However, the conditions under which LME cracking occurs are only vaguely defined. The effect of temperature and dwell time are investigated in this study using Gleeble hot tensile testing. Liquid zinc embrittlement is most severe at 800 °C and decreases towards lower or higher test temperatures. The lower temperature limit for copper embrittlement is defined by the melting temperature of copper (1085 °C); significant embrittlement is observed up to a test temperature of 1300 °C, at which ductility largely recovers. The observed zinc and copper induced LME behavior in Type 304L austenitic stainless steel are discussed based on the thermodynamics of the 304-Zn and 304-Cu systems which suggest that liquid metal embrittlement occurs in a temperature range where the phase diagrams display a two phase liquid zinc (or copper) + solid region that extends over a wide compositional range. Diffusion calculations across the stainless steel to liquid zinc interface and results from electron microscopy analysis indicate that liquid zinc availability due to isothermal solidification of BCC ferrite at the stainless steel to liquid zinc interface is a limiting factor for LME in the 304-Zn system during Gleeble hot tensile testing.

Study of transient characteristics of BAW filters under high power levels

The improvement of power capacity would benefit greatly from the analysis of transient characteristics of bulk acoustic wave (BAW) devices. For this purpose, a measurement system was established, which could capture the temperatures and the insertion loss (IL) of the BAW devices simultaneously. Based on the obtained testing data of four different commercial BAW filters, we clarified the time dependence of device temperature and IL. The results showed that the BAW filters would exhibit higher power capacity with higher threshold frequency and a lower slope of the characteristic curves. This prompts us to propose an evaluation method for BAW filters under high power levels, which could improve the efficiency of device selection and performance prediction.

Unmoderated molten salt reactors design optimisation for power stability

The critical region of unmoderated molten salt reactors consists in a cavity filled with a liquid fuel. The lack of internal structure implies a complex flow structure of the circulating fuel salt. A preliminary core shape optimization has been performed during the EVOL European project to limit recirculation and hotspots. This optimization was based on a Reynolds Averaged Navier Stokes (RANS) approach, but the latter only provides time-averaged values for velocity and temperature. However, the power stability is sensitive to thermal fluctuations induced by the flow turbulence itself, even at steady state without pump flow rate or heat extraction variation. This phenomenon is studied using a Detached Eddies Simulation approach to solve the turbulence in the reactor and get a time dependent temperature distribution and then the reactivity fluctuations. A new geometry is proposed to limit the total power fluctuations from 7.5% for the preconceptual EVOL geometry down to 1.2%.

Local Plastic Response and Slow Heterogeneous Dynamics of Supercooled Liquids.

We demonstrate, via numerical simulations, that the relaxation dynamics of supercooled liquids correlates well with a plastic length scale measuring a particle's response to impulsive localized perturbations and weakly to measures of local elasticity. We find that the particle averaged plastic length scale vanishes linearly in temperature and controls the super-Arrhenius temperature dependence of the relaxation time. Furthermore, we show that the plastic length scale of individual particles correlates with their typical displacement at the relaxation time. In contrast, the local elastic response only correlates with the dynamics on the vibrational timescale.

Partially disordered pyrochlore: time-temperature dependence of recrystallization and dehydration

Abstract The comparison of the evolution of the mechanical properties (elastic modulus and hardness) after step-wise thermal annealing for 1 and 16 h up to 900 K of a radiation-damaged pyrochlore (∼35% amorphous fraction; 1.8 wt% ThO2) provides insights to the time-temperature dependence of the recrystallization behavior. Especially the elastic modulus, directly related to interatomic bonding, enables the correlation with the amount of amorphous fraction. From this a pronounced effect of the annealing time on percolation behavior could be deduced. Evolved gas analysis indicate dehydration in the course of the structural reorganization process.

Parametric sensitivity analysis of the transient adsorption-diffusion models for hydrocarbon transport in microporous materials

The complex nature of hydrocarbon transport in microporous materials offers unique challenges for the description of transient kinetic data in catalysis by e.g. acidic zeolites and zeotypes. Currently, very few models in the literature capture such phenomena as surface barriers or hindered microporous diffusion - processes that are known to impact gas transport during the kinetic studies of zeolite catalyzed reactions. In this work we provide extended models that reflect the aforementioned phenomena during pulse-response experiments in the Temporal Analysis of Products (TAP) reactor. Systematic studies of models’ parametric sensitivity are presented to assist the design of future experiments. Numerical simulations are used to investigate how thermodynamic adsorption properties of realistic zeolites can affect the pulse-response shapes. For certain combination adsorption/diffusion parameters, microporous diffusion is shown to result in a characteristic bend in the mean residence time temperature dependency, which can be used as a fingerprint in model discrimination.

Data-driven analysis of process, structure, and properties of additively manufactured Inconel 718 thin walls

In additive manufacturing of metal parts, the ability to accurately predict the extremely variable temperature field in detail, and relate it quantitatively to structure and properties, is a key step in predicting part performance and optimizing process design. In this work, a finite element simulation of the directed energy deposition (DED) process is used to predict the space- and time-dependent temperature field during the multi-layer build process for Inconel 718 walls. The thermal model results show good agreement with dynamic infrared images captured in situ during the DED builds. The relationship between predicted cooling rate, microstructural features, and mechanical properties is examined, and cooling rate alone is found to be insufficient in giving quantitative property predictions. Because machine learning offers an efficient way to identify important features from series data, we apply a 1D convolutional neural network data-driven framework to automatically extract the dominant predictive features from simulated temperature history. Very good predictions of material properties, especially ultimate tensile strength, are obtained using simulated thermal history data. To further interpret the convolutional neural network predictions, we visualize the extracted features produced on each convolutional layer and compare the convolutional neural network detected features of thermal histories for high and low ultimate tensile strength cases. A key result is the determination that thermal histories in both high and moderate temperature regimes affect material properties.

Solar-driven melting dynamics in a shell and tube thermal energy store: A numerical analysis

The numerical modeling of solar-driven melting in a shell-and-tube accumulator using paraffin RT70HC as PCM is presented. The coupling between an array of evacuated-tube solar collectors and the TES system is implemented in ANSYS Fluent, allowing for the proper evaluation of the time-dependent heat transfer fluid inlet temperature. A simplification of the TES geometry to a 2D section is accomplished, and an algorithm for the continuous evaluation of the heat transfer fluid temperature across the tubes in a serpentine-type configuration is developed. Validation with experimental results obtained in an outdoor test rig is carried out in terms of PCM temperature evolution. The model provides simulation results in a realistic configuration to be used in low-temperature applications of thermal energy storage for the residential sector. Energy performance is discussed, and the influence of the physics of melting and buoyancy forces leading to stratification are also analyzed, using temperature and liquid fraction contours during the solar irradiance cycle. The numerical results confirmed that natural convection plays an important role during melting of PCM: the motion of the melting front from the bottom to the top of the accumulator is described. Moreover, the results show that an increase of 50% of the solar surface improves the thermal storage by 28.8%, while doubling the solar surface increases the stored energy by 46.2%.