Isothermal Variation Research Articles

Experimental verification of a common assumption in the use of concentration-dependent interdiffusion coefficient

There has generally been a view that concentration-dependent interdiffusion coefficient, D(C), obtained at any diffusion temperature and time is a material constant and can be reliably used to predict diffusion effects at all other isothermal diffusion times. This notion which is implicitly predicated on the assumption that diffusion-induced stress (DIS) rapidly and completely relaxes once formed and does not influence D(C) is experimentally verified in the present work. The results of the study show that reliably computed D(C) from concentration profile obtained at long diffusion time fails to correctly predict concentration profiles at shorter diffusion periods, due to isothermal variation of the D(C) with time. Accordingly, instead of continuing to assume that D(C) is time-independent and can be reliably used to predict diffusion effects at any isothermal diffusion time, without appropriate experimental support, proper experimental verification of this crucial concept in other alloy systems, as done in the present work, is imperative.

Study on the magnetothermal properties of the [formula omitted] series of compounds

In this work, we developed a theoretical model Hamiltonian, in the mean field approximation, to describe the magnetic and magnetocaloric behavior of the series of compounds Dy1-xTbxAl2(x=0.00,0.15,0.25,0.30,0.35,0.40 and 0.75). We adjusted the exchange parameters λDyDy,λTbTb, and λDyTb to obtain the spin reorientation temperatures TSR and the critical temperature TC for each compound in the series. The results obtained by the Hamiltonian model agree satisfactorily with the experimental results. The heat capacity curves with and without an applied magnetic field, adiabatic temperature variation and isothermal entropy variation were modeled and compared with the experimental data. As the experimental results show, our model was also able to reproduce the change in the spin reorientation process: a first order spin reorientation transition appears for concentrations x = 0.15, 0.25, 0.30, 0.35, and no spin reorientation transitions after x = 0.40.

Isothermal magnetization and magnetoresistive variations in trilayer (La2/3Sr1/3MnO3/γ-Fe2O3/La2/3Sr1/3MnO3) hetero-structure

We report the magnetic field dependent magnetization and magnetoresistance (MR) properties of La2/3Sr1/3MnO3 (LSMO)/ γ -Fe2O3/LSMO trilayer heterostructure and single layer LSMO film grown on SiO2/Si (100) substrates utilizing Pulsed Laser Deposition technique. The metal- insulator-metal configuration is a magnetic tunnel junction topology, which is a parallel network of two metallic layers (LSMO) and one insulating layer ( γ -Fe2O3) in current-in-plane (CIP) geometry. The intrinsically inhomogeneous polycrystalline trilayer film shows much lower (almost half ) coercivity, compared to single layer LSMO film. The MR-H [MR = (ρ (H)-ρ (0))/ρ (0)] behavior of the films is studied under two regimes namely, Low Field Magnetoresistance (LFMR) and High Field Magnetoresistance (HFMR) at 5 K and 300 K in the field range of 0–7 T. Several equations were developed to simulate the experimental MR-H data of the studied samples. For both the films, in the LFMR region (0 T < H ≤ 1 T @ 5 K), the variation in MR exhibits a linear increase with H, followed by a logarithmic behavior in the HFMR region (1 T < H ≤ 7 T @ 5 K). For the trilayer film in CIP geometry, a negative MR of 16% is achieved at 5 K and 1 T field conditions, while LSMO single layer shows a negative MR of 13% at same temperature and field conditions. The MR obtained at high field (7 T) for both the films is quantitatively equal with a value of 38% @ 5 K and 15% @ 300 K. We obtained significantly better MR-H results for the trilayer in current-perpendicular-to-plane (CPP) configuration, where the two metallic layers and an insulating layer are in series. In the CPP configuration, MR is 21% @ 5 K and 1 T field, while it reached to 44% at 5 K and 7 T field. At room temperature, the trilayer heterostructure shows MR of 17% at 7 T field condition. At a higher field of 9 T, trilayer film in CPP mode exhibits MR value of 48% @ 5 K and 23% @ 300 K. Further, the anti-parallel and parallel magnetization states of the MIM device are well manifested in CIP and CPP geometries.

Thermo-magnetic radiative flow in porous enclosure with deep-learning parameter estimation

This study numerically investigates convective heat transfer, fluid flow, and entropy generation within an inverted T-shaped porous enclosure filled with a hybrid nanofluid. The interplay between thermal radiation and a magnetic field is considered, as these factors significantly influence thermal acquisition processes in renewable and thermal engineering applications, particularly solar power collectors with the selected physical domain. By analyzing various combinations of flow parameters, this work aims to identify favorable conditions for maximizing heat transfer efficiency in such systems. A Darcy–Forchheimer–Brinkman-based mathematical model has been incorporated and numerically simulated using the penalty finite element method (FEM). Moreover, the variation of fluid flow, isotherms, and entropy generation is thoroughly analyzed for the variation of non-dimensional permeability parameter (Darcy number, Da), buoyancy force (Rayleigh number, Ra), magnetic field (Hartmann number, Ha), porosity value (ϵ), and thermal radiation parameter (Rd). The comprehensive result demonstrates that increasing values of Ra, Da, ϵ, and Rd intensify entropy generation, convective heat, and fluid flow phenomena, whereas the increasing value of Ha substantially weakens these phenomena. The thermal efficiency of the model attends at the lower buoyancy forces (Ra≤105) for each flow parameter (except for Ha). However, the increasing magnetic forces (Ha) substantially improve the thermal efficiency of the developed model. Additionally, this study employs a feed-forward neural network (FNN) based deep learning (DL) approach to predict the parametric influence on convective heat transport rate (Num), total entropy generation rate (Stot), and total Bejan number due to heat transport irreversibility (Beθ,tot) based on the minimally supplied experimental data for FNN training process. The trained FNN model predicts the results very quickly with 98.6% accuracy compared to FEM results, demonstrating its efficiency in approximating the parametric influence of complex mathematical model results. Moreover, this approach significantly reduces computational costs and efforts compared to traditional numerical methods, making it a valuable tool for forecasting the influence of an unknown set of flow parameters in complex mathematical models.

Numerical study of entropy generation in magneto-convective flow of nanofluid in porous enclosure using fractional order non-Darcian model

The present numerical work examines the effect of fractional order parameter on heat transfer and entropy generation for a thermo-magnetic convective flow of nanofluid (Cu-water) in a square porous enclosure that contains semi-circular bottom wall. The Darcy–Brinkmann–Forchheimer model is utilized to evaluate the momentum transfer in porous media, and the Caputo-time fractional derivative term is introduced in momentum as well as in the energy equation. Further, non-dimensional governing equations are simulated through the penalty finite element method, and the Caputo time derivative is approximated by L1-scheme. The study is carried out for various parameters, including Rayleigh number (Ra), Darcy number (Da), radius of the semicircle (r), fractional order (α), and Hartmann number (Ha). The comprehensive results are presented by the contour variation of isotherms, streamlines, and total entropy generation at the selected range of parameters. In addition, thermal transport and irreversibilities due to heat transfer, fluid friction, and magnetic field have been accounted through the numerical variation of mean Nusselt number (Num) and Bejan number due to heat transfer (Beht), fluid friction (Beff), and magnetic field (Bemf), respectively. The key findings of the present study reveal that during the initial evolution period, the Num value increases as α→1. Additionally, time taken to achieve the steady state condition varies and depends on fractional order α. Furthermore, in the absence of Ha, the heat transfer and entropy generation intensifies with augmentation of Ra and Da for all α, while, the increasing value of Ha shows an adverse impact on the heat transfer rate.

Simulation of recoverable strain variation during isothermal holding of the Ni51Ti49 alloy under various regimes

The aim of the present paper is to simulate the strain variation on isothermal holding of Ni-rich NiTi alloy under various regimes. The modified Likhachev-Volkov microstructural model and a new Nelder-Mead algorithm for the determination of the model parameters were used. To determine the model parameters, the experimental data obtained during holding of the Ni51Ti49 alloy under a stress of 200 MPa were chosen. Using these parameters, the strain variation on holding of the NiTi alloy under a stress in two regimes was simulated. It was shown that the modified Likhachev-Volkov microstructural model allowed one to calculate the isothermal strain variation on holding after cooling under a stress (Regime 1) and a good correlation between the theoretical and experimental results were found. At the same time, the simulation of the strain variation on holding after active deformation (Regime 2) did not fit to experimental data because the model did not consider the difference in the stored elastic energy in two regimes. It was shown that a decrease in the elastic energy stored during the transformation increased the strain during holding under a stress after active deformation and made the simulated curves to be close to the experimental.

Energy dissipation in a rapid filling vertical pipe with trapped air

An energy-dissipation model is developed to simulate rapid filling having an entrapped air pocket within a vertical pipe. Both convective heat transfer and transient wall shear stresses are considered. The resulting predictions are compared both to those obtained via a conventional empirical polytropic model and to experimental data. The comprehensive model accurately reproduces the experimental pressure oscillations. Results reveal that the dynamic behaviour of air pockets in all tested cases approaches a purely adiabatic process over the first two oscillations, but also that the pressure variation then gradually evolves to an isothermal variation. Moreover, the high air temperatures predicted in the numerical simulations account for the observed phenomenon of white mist in the pipe as well as the notably hot pipe wall. Significantly, the conventional empirical polytropic model associated with the adiabatic assumption for the gas phase was sufficient to reproduce the pressures and temperatures during the first two oscillations.

Statistical thermodynamics in reversible clustering of gold nanoparticles. A first step towards nanocluster heat engines

A statistical thermodynamics variational criterion is propounded to study thermal hysteresis in reversible clustering of gold (Au) nanoparticles. Experimentally, a transient equilibrium mapping analysis is employed to characterize it thermodynamically, further measurements being performed at the nanostructural and electrochemical levels (UV-Vis-NIR spectra, SLS/SAXS, zeta potential). Theoretically, it is successfully interpreted as a thermodynamic cycle, prompting that nanoclusters has potential to produce useful work from heat and paving the way to nanoclustering heat engines. By taking into account the virial expansion of hysteretic pressure, an entropy measure is deduced for a dilute system with given virial coefficients. This allows us to figure out the role of relevant interparticle potential parameters (i.e. surface potential, nanoparticle size, Debye’s length, Hamaker energy) in both isothermal and isochoric variations at the onset of hysteresis. Application to spherical Au nanoparticles in watery salt solution (NaCl) is developed when an ad-hoc (DLVO) pairwise potential governs the second virial coefficient at the nanoscale. In particular, the variational criterion predicts a pressure drop between heating and cooling paths which is likely at the base of some energy redistribution (e.g. ordering/restructuring of electric double layers). We found an integrating factor that is able to numerically predict the existence of a critical value for the initial salt concentration maximizing the hysteretic area, and the effect of nanoparticle size on the cycle extent.

Modifications in the magnetocaloric effect owing to composition changes in Gd2In1−x Ge x (0 ≤ x ≤ 0.2) system of compounds

In this paper, we report the variations observed in magnetothermal, magnetocaloric, and magnetic properties of the Gd2In1−xGex (0 ≤ x ≤ 0.2) system of compounds concerning the changes in substitution of Ge. Changes have been recorded in a temperature sweeping range of 2-300 K while the magnetic field was increased from 0 to a maximum of 5 T. All the compositions set into the Ni2In-type hexagonal structure and no structural changes are observed with the change in the value of x. Though the transition temperature values, which are TC ≈ 191 K and TN ≈ 100 K in case of x = 0, have only subtle shifts with the Ge substitution yet promising changes have been observed in the magnetocaloric effect. At x = 0.2 the isothermal magnetic entropy variation peak touches the value of 2.5 J/kg − K around TN in a field change from 0 to 5 T. This showcases that the compounds reported have a worthy place within the bracket of magnetocaloric compounds.

Theoretical investigation of crystalline electric field influence on the magnetocaloric effect in the cubic praseodymium system PrNi2

We report a theoretical investigation of the magnetocaloric effect in paramagnetic intermetallic $\mathrm{Pr}{\mathrm{Ni}}_{2}$. We choose this particular compound because it presents high cubic symmetry, and two sets of crystalline electrical field parameters were reported, allowing a systematic study. In one of these sets, the magnetic entropy increases upon applied magnetic field in the low temperature region $(T&lt;10\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ for $H=5\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. This anomaly was investigated through a model Hamiltonian which includes the magnetic, exchange, and crystalline electric field interactions. The simulated curves of the temperature dependence of isothermal entropy variation and adiabatic temperature change present the inverse magnetocaloric effect, which is not expected for a paramagnetic system.

Theoretical investigation of crystalline electric field influence on the magnetocaloric effect in PrRh2

We investigate the magnetocaloric effect in the intermetallic ferromagnetic PrRh2 compound in which two possible crystalline electrical field level schemes were reported, from inelastic neutron scattering. The levels schemes parameters (set A and set B) were considered in our model Hamiltonian and anomalous effect were predicted in the isothermal entropy variation and in adiabatic temperature change magnetocaloric quantities. These anomalies were physically understood. The set A leads to a strong quenching in the Pr3+ magnetic moment and the set B leads to a first order magnetic phase transition.

Interesting magnetic response of the nuclear fuel material UO2

ABSTRACT We report results of dc magnetization measurements on UO2. Our study reveals a deviation from standard Curie-Weiss (CW) paramagnetic behavior below 280 K, and that is followed by a transition to antiferromagnetic state with marked thermomagnetic irreversibility below TN = 30.6 K. The zero field cooled (ZFC) magnetization exhibits distinct structures not usually observed in the antiferromagnets, whereas field cooled (FC) magnetization looks more like a ferromagnet. The thermomagnetic irreversibility continues to exist in a subtle way even in the paramagnetic regime well above TN . This behavior along with the deviation from CW law highlight non-standard nature of the paramagnetic state. Magnetic response below TN changes significantly with the increase in the applied magnetic field. In the isothermal magnetic field variation of magnetization, a subtle signature of a magnetic field-induced phase transition is observed. All these experimental results highlight the non-trivial nature of the magnetic state in UO2.

The strain variation during the isothermal B2→B19’ martensitic transformation in the quenched or annealed Ni51Ti49 alloy

The strain variation during the isothermal holding under constant stress was studied in the quenched or annealed Ni51Ti49 alloy samples. The isothermal strain variation was found in both samples and this strain was completely recovered on subsequent unloading and heating. This allowed to conclude that the strain variation on holding was caused by the isothermal martensitic transformation. It was found that the maximum value of isothermal strain depended on the alloy heat treatment. This value was equal to 0.5 % in annealed sample and it was equal to 6 % in quenched sample. It was assumed that the formation of the Ni4Ti3 phase during annealing led to a decrease in concentration of substitutional Ni atoms in NiTi phase that were responsible for the isothermal transformation. As a result, the less volume fraction of the martensite formed during holding that supresses the strain variation in annealed samples.

Magnetotransport and magnetocaloric effect in Gd3In

The electrical transport and magnetic properties of the rare-earth based binary intermetallic compound Gd3In are reported here. The sample has orthorhombic crystal structure with space group Pnma. At TC = 190 K, the sample orders ferromagnetically, however, a reentrant antiferromagnetic-like state is observed below TN = 100 K where a sharp drop in magnetization is noticed. Clear signature of meta-magnetic transition is present in the isothermal variation of magnetization for an applied field of HC = 11.5 kOe at 2 K. Due to the presence of field induced transition, we have calculated the magnetocaloric effect (MCE) around the magnetic transitions using our magnetization data, which turns out to be quite eventful. MCE, expressed in terms of change in entropy (ΔS) due to the change in magnetic field (H), is found to be negative around TC with a maximum value of ΔS = −5.3 J kg−1 K−1 for H = 50 kOe. On the other hand, it is positive up to H = 10 kOe around TN, and turns negative for higher values of H with a maximum value of ΔS = −2.6 J kg−1 K−1 for 50 kOe applied magnetic field. This is clearly due to the metamagnetic transition observed around 11.5 kOe. In the temperature variation of electrical resistivity, we observe clear signatures of magnetic transitions occurring at TC and TN, and the sample shows negative magnetoresistance throughout the temperature range (6–300 K) with a value of −12% at around 6 K for H = 50 kOe.

Numerical investigation of convective heat transfer in a rectangular vented cavity with two outlets and cold partitions

The convective flow in a ventilated cavity having two outlets and one inlet is numerically investigated. Introducing cold partitions inside the cavity modifies heat and fluid flow characteristics. It is analyzed by placing a single and double partition inside the cavity. Three configurations of the rectangular cavity are analyzed in the laminar flow range, with Rayleigh numbers varying between 103 and 106. The Navier Stokes equation, which governs the flow, is solved using the finite difference method. The medium of flow is considered as air with Prandtl number 0.71. The result shows that the fluid entering the inlet splits into two to enter the two outlets, and this cold air occupies most of the central place of the cavity at higher Ra when there is no partition in the enclosure. With the introduction of cold sections, a part of fluid gets attracted towards it and creates fluid circulation. It decreases the velocity of the fluid at the bottom of the cavity at the midsection of outlet 2. The fluid velocity at outlet 1 is not much affected. The average heat transferred to the right wall gets significantly decreased due to the cold partitions at lower Rayleigh numbers, but there is an increase in the overall average Nusselt number of all outer walls. A correlation equation that gives the relationship between Nusselt number and Rayleigh Numbers is developed. The study presents the variation of isotherms, streamlines, velocity, rate of heat transfer and correlation of Nu for the considered problem.

Action and Entropy in Heat Engines: An Action Revision of the Carnot Cycle.

Despite the remarkable success of Carnot’s heat engine cycle in founding the discipline of thermodynamics two centuries ago, false viewpoints of his use of the caloric theory in the cycle linger, limiting his legacy. An action revision of the Carnot cycle can correct this, showing that the heat flow powering external mechanical work is compensated internally with configurational changes in the thermodynamic or Gibbs potential of the working fluid, differing in each stage of the cycle quantified by Carnot as caloric. Action (@) is a property of state having the same physical dimensions as angular momentum (mrv = mr2ω). However, this property is scalar rather than vectorial, including a dimensionless phase angle (@ = mr2ωδφ). We have recently confirmed with atmospheric gases that their entropy is a logarithmic function of the relative vibrational, rotational, and translational action ratios with Planck’s quantum of action ħ. The Carnot principle shows that the maximum rate of work (puissance motrice) possible from the reversible cycle is controlled by the difference in temperature of the hot source and the cold sink: the colder the better. This temperature difference between the source and the sink also controls the isothermal variations of the Gibbs potential of the working fluid, which Carnot identified as reversible temperature-dependent but unequal caloric exchanges. Importantly, the engine’s inertia ensures that heat from work performed adiabatically in the expansion phase is all restored to the working fluid during the adiabatic recompression, less the net work performed. This allows both the energy and the thermodynamic potential to return to the same values at the beginning of each cycle, which is a point strongly emphasized by Carnot. Our action revision equates Carnot’s calorique, or the non-sensible heat later described by Clausius as ‘work-heat’, exclusively to negative Gibbs energy (−G) or quantum field energy. This action field complements the sensible energy or vis-viva heat as molecular kinetic motion, and its recognition should have significance for designing more efficient heat engines or better understanding of the heat engine powering the Earth’s climates.

Unraveling the Guest-Induced Switchability in the Metal-Organic Framework DUT-13(Zn)*.

The switching mechanism of the flexible framework Zn4O(benztb)1.5 (benztb=N,N,N’,N’‐benzidine tetrabenzoate), also known as DUT‐13, was studied by advanced powder X‐ray diffraction (PXRD) and gas physisorption techniques. In situ synchrotron PXRD experiments upon physisorption of nitrogen (77 K) and n‐butane (273 K) shed light on the hitherto unnoticed guest‐induced breathing in the MOF. The mechanism of contraction is based on the conformationally labile benztb ligand and accompanied by a reduction in specific pore volume from 2.03 cm3 g−1 in the open‐pore phase to 0.91 cm3 g−1 in the contracted‐pore phase. The high temperature limit for adsorption‐induced contraction of 170 K, determined by systematic temperature variation of methane adsorption isotherms, indicates that the DUT‐13 framework is softer than other mesoporous MOFs like DUT‐49 and does not support the formation of overloaded metastable states required for negative gas‐adsorption transitions.

Innovative method to estimate state of charge of the hydride hydrogen tank: application of fuel cell electric vehicles

ABSTRACT Significant attention has been paid to metal hydrides (MH) as an environmentally friendly and safe way to store hydrogen. This technology has considerable potential for the application of embedded hydrogen storage in fuel cell electric vehicles, but its widespread application faces a major problem in terms of estimating the remaining hydrogen amount in the tank. In this work, a new method is proposed for estimating the state of charge (SoC) of the hydrogen hydride tank (HHT) by application of piezoelectric material. The idea is to cover the entire inner wall of the metal-hydride tank with a layer of piezoelectric material. During the process of hydrogen absorption, the pressure inside the tank increases, resulting in an increase in the mechanical stress applied to the piezoelectric layer, thus generating a voltage. This effect can therefore be used as an effective means of estimating the amount of hydrogen absorbed. The COMSOL Multiphysics software was used to implement the model and solve the differential equation of energy, mass, and moment. The simulation results showed that the voltage evolution as a function of hydrogen concentration follows the same behavior as the variation of the Pressure Concentration Isotherm (PCI).

Entropy, enthalpy and volume of perfect crystals at limiting high pressure and the third law of thermodynamics

According to the third law of thermodynamics, the entropy of a perfect crystal, Scryst, at ambient pressure is zero at 0 K, or (∂Scryst/∂T)P = 0 at T = 0 K. Here we consider isothermal variations of its entropy and enthalpy as the applied pressure P is varied, employing the Maxwell relation and estimating the respective limit of the Boltzmann equation. We deduce that as P is increased, (i) Scryst at a fixed T and (∂Scryst/∂P)T would asymptotically (progressively more slowly) decrease to zero, (ii) the enthalpy and the Gibbs free energy would asymptotically increase to the same limiting values, and (iii) the volume would asymptotically decrease to some constant limiting value. The approach of Scryst and (∂Scryst/∂P)T to zero with increase in P at a fixed T may be considered as complementary to the third law of thermodynamics. Variations of these thermodynamic functions in response to isothermal change of pressure are compared against their variations in response to isobaric change of temperature.

A Thermophysical Perspective of the Inter-relationship between Debye Temperature and Electron Density

A comprehensive thermophysical analysis of the linkage between the Debye temperature θD and electron density ne has been presented. Assuming that the Debye cut-off frequency ωD is related to the effective oscillation frequency ω of screened ion cores of a metallic material, a simple analytical relationship could be established between θD, bonding electron density ne, and materials density ρm. Alternately, it is possible to establish a consistent set of electron density n e values using θD, which in the case of pure metals is shown to exhibit a linear scaling with Miedema’s thermochemical electron density n e M values. The temperature, pressure, and composition dependencies of θD are addressed uniformly in terms of isobaric and isothermal volume variations of ne; besides, invoking the subregular solution formalism for expressing composition-induced molar volume variation simple approximations for melting temperature Tm and vibrational entropy $$ S_{T}^{\text{o}} $$ have been suggested in terms of electron density. Estimates of θD, $$ S_{298}^{\text{o}} $$ , and average sound velocity have been obtained for metastable bcc U1−xZrx solid solution.