Phase Transition Region Research Articles

Tertiary pyroelectric effect in TGS group crystals

In this paper, the study of the dynamic pyroelectric effect under different powers of modulated heat flux in deuterated triglycine sulfate (DTGS) near the Curie point is presented. It was found that pyroelectric coefficient was essentially dependent on the power of the modulated radiation, especially in the phase transition region. It is shown that a tertiary pyroelectric effect makes an additional contribution to the pyroelectric response observed in TGS group crystals near phase transition region. The tertiary pyroelectric effect is caused by a nonuniform distribution of piezoelectric coefficients in the samples as a result of temperature gradient presence.

Pretransition Phenomena in the Region of a Structural Phase Transition in Potassium Perchlorate

Molecular relaxation processes in potassium perchlorate KClO4 are studied using Raman spectroscopy. The first-order structural phase transition in crystalline perchlorate KClO4 is shown to be temperature-extended. The presence of a pretransition region in the studied perchlorate KClO4 is reported.

Characterization of high-temperature PCMs for enhancing passive safety and heat removal capabilities in nuclear reactor systems

Suitable high temperature PCMs for thermal energy storage are characterized and proposed as a promising candidate for nuclear reactor safety applications in the containment structure in the event of LOCA accidents. PCM with phase transition temperature in the range 70 °C to 90 °C and latent heat of around 200 kJ/kg or higher can be utilized to reduce temperature and pressure in the reactor containment during accidents. Reviewing the available data for high-temperature PCMs indicated significant deficiencies in published data and further investigations revealed that a material misrepresentation has occurred in some cases. Octadecanoic acid was identified as the most promising PCM for this application and the temperature-dependent thermo-physical properties and phase change behavior of octadecanoic acid were studied in detail. The results closed the gaps in the existing experimental database, studied the feasibility of utilizing several PCM candidates, measured the currently unknown temperature-dependent properties for optimum PCM, measured the specific heat capacity in the overlap and phase transition regions, and provided a valuable database for future design concepts, experiments and predictive simulation and modelling models.

Precision Measurements of the AC Field Dependence of the Superconducting Transition in Strontium Titanate

Strontium titanate has resurfaced as a material prompting vigorous debate about the origin of its superconductivity in the extremely low carrier concentration regime. Here, we used simultaneous AC susceptibility and transport methods to explore the superconducting phase transition region in this material. We determined that strontium titanate is extremely sensitive to even small AC fields, which also influence the resistive transition; we suggest that extreme vortex sizes and mobilities contribute to this large effect. Our findings will be of importance for accurately determining transition temperature, informing the debate about the pairing mechanism in strontium titanate, for which even millikelvin errors may be critical.

Quarks and light (pseudo-)scalar mesons at finite chemical potential

We investigate the properties of light scalar and pseudoscalar mesons at finite (light) quark chemical potential. To this end we solve a coupled set of (truncated) Dyson-Schwinger equations for the quark and gluon propagators in Landau-gauge QCD and extend earlier results for $ N_{\mathrm{f}} = 2+1$ dynamical quark flavors to finite chemical potential at zero temperature. We then determine the meson bound state masses, wave functions, and decay constants for chemical potentials below the first-order phase transition from their homogeneous Bethe-Salpeter equation. We study the changes in the quark dressing functions and Bethe-Salpeter wave functions with chemical potential. In particular, we trace charge-conjugation parity breaking. Furthermore, we confirm the validity of the Silver-Blaze property: all dependencies of colored quantities on chemical potential cancel out in observables and we observe constant masses and decay constants up to and into the coexistence region of the first-order chiral phase transition.

Joint Activity Detection and Channel Estimation for IoT Networks: Phase Transition and Computation-Estimation Tradeoff

Massive device connectivity is a crucial communication challenge for Internet of Things (IoT) networks, which consist of a large number of devices with sporadic traffic. In each coherence block, the serving base station needs to identify the active devices and estimate their channel state information for effective communication. By exploiting the sparsity pattern of data transmission, we develop a structured group sparsity estimation method to simultaneously detect the active devices and estimate the corresponding channels. This method significantly reduces the signature sequence length while supporting massive IoT access. To determine the optimal signature sequence length, we study the phase transition behavior of the group sparsity estimation problem. Specifically, user activity can be successfully estimated with a high probability when the signature sequence length exceeds a threshold; otherwise, it fails with a high probability. The location and width of the phase transition region are characterized via the theory of conic integral geometry. We further develop a smoothing method to solve the high-dimensional structured estimation problem with a given limited time budget. This is achieved by sharply characterizing the convergence rate in terms of the smoothing parameter, signature sequence length and estimation accuracy, yielding a tradeoff between the estimation accuracy and computational cost. Numerical results are provided to illustrate the accuracy of our theoretical results and the benefits of smoothing techniques.

Unambiguous determining the Curie point in perovskite manganite with second-order phase transition by scaling method

The accurate determination of the Curie temperature (TC) is particularly important in describing the magnetic behavior close to the paramagnetic-ferromagnetic (PM-FM) phase transition. In this paper, we studied the magnetic phase transition and accurately predicted the Curie point of perovskite manganite La0.825Sr0.175MnO3. We find the compound shows a second-order PM-FM transition and has a large magnetic entropy change (MEC) in vicinity of phase transition region. Based on the scaling law and the correlation between magnetic field and MEC, the precise and magnetic-independent Curie temperature was determined to be 281.7 K, obviously lower than 285.4 K decided from the magnetization versus temperature. The reliability of new Curie temperature can be well confirmed by the modified Arrott plot together with the critical exponents. Our results not only open up a new pathway for precise definition of Curie point but also facilitate the efficient exploitation of spontaneous magnetic bubbles in perovskite manganite.

Measurement and analysis of thermophysical parameters of the epoxy resin composites shape-stabilized phase change material

To obtain the phase change materials (PCMs) with good stability in the asphalt mixture, the epoxy resin composites shape-stabilized phase change material (EPC-PCM) was prepared by encapsulating the pentadecane microcapsule, which was prepared with pentadecane, nano-silica, and ethyl cellulose by physical compounding, with epoxy resin (EP). The prepared EPC-PCM had good thermal storage performance, thermal stability and mechanical properties. The specific heat capacity and thermal conductivity of EPC-PCM were respectively measured by heat exchange system and Kemtherm QTM-D3 thermal conductivity meter. Studies showed that the specific heat capacity of EPC-PCM varied significantly with temperature due to the phase change properties of pentadecane and it could be divided into solid-state region, fuzzy region (phase transition region) and liquid-state region. According to the correlation between specific heat capacity and temperature, the regression equation between the specific heat capacity of EPC-PCM with temperature was established. The thermal conductivity of EPC-PCM decreased slightly with the increase of temperature, which was similar to that of pentadecane, but opposite to that of epoxy resin.

Experimental Study of the Insulator-to-Metal Phase Transition in VO2 Thin Films in the Microwave Range

Temperature dependence of insulator-to-metal phase transition in VO2 epitaxial thin films was studied in the microwave range. Biaxially textured VO2 thin films were obtained on 3-inch sapphire substrates by the method of chemical vapour deposition (MCVD). The change in the electrical resistance with temperature reached approximately 3 orders of magnitude for the obtained films, measured in the metal-insulator phase transition region. Using a high-temperature quasi-optical setup, the microwave features of the phase transition in VO2 thin films were studied at temperature. The hysteresis of microwave properties of the studied samples was observed in the temperature range of VO2 phase transition.

Predicted model of magnetocaloric effect in BiFeO3-based multiferroics

Structure, magnetic properties and magnetocaloric effect in synthesized Bi-based multiferroics with a partial isovalent substitution of bismuth (III) cations for rare-earth elements (R = La, Nd, Sm, Eu, Gd, Dy, Er) have been studied. The samples have been prepared by the solid-state reaction method under cold pressing at high pressure (4 GPa). Proposed, for the first time, has been a modified phenomenological model for describing temperature dependences of magnetization and investigating a magnetocaloric effect within “weak ferromagnetic-antiferromagnetic” and “antiferromagnetic-paramagnetic” magnetic phase transition regions. Systematic changes of thermodynamic characteristics have been found out to depend on the degree of replacement and the type of a substituted cation.

Pretransitional phenomena in the region of structural phase transition in sodium sulfate

Pretransitional phenomena in the region of structural phase transition in sodium sulfate

A novel bio-based polyurethane/wood powder composite as shape-stable phase change material with high relative enthalpy efficiency for solar thermal energy storage

A novel bio-based polyurethane (PU)/wood powder (WP) composite is prepared as a shape-stable phase change material (SSPCM) for solar thermal energy storage (TES) applications. Fourier transform infrared (FTIR) spectra, X-ray diffraction (XRD), polarizing microscope (POM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and accelerated thermal cycling testing were performed to examine the chemical composition and structure, crystalline performances and thermal performances. The results indicate that, with only 3.0 wt% WP content, the PU/WP composites presented remarkable performance on phase change behaviors and thermal stability during PEG solid-liquid phase transition region. The maximum latent heat for PU/WP SSPCMs during melting and freezing process are 134.2 J/g and 132.4 J/g, respectively, and the relative enthalpy efficiency reaches 98.7%. At the same time, the obtained PU/WP SSPCMs exhibited reliable light-thermal conversion and temperature-regulated performance. Thus, the novel bio-based PU/WP SSPCMs displayed a high potential application as solar light absorber in the field of solar thermal energy storage and temperature regulation.

RMF models with $\sigma$-scaled hadron masses and couplings for the description of heavy-ion collisions below 2 A GeV

Within the relativistic mean-field framework with hadron masses and coupling constants dependent on the mean scalar field we study properties of nuclear matter at finite temperatures, baryon densities and isospin asymmetries relevant for heavy-ion collisions at laboratory energies below 2$A$ GeV. Previously constructed (KVORcut-based and MKVOR-based) models for the description of the cold hadron matter, which differ mainly by the density dependence of the nucleon effective mass and symmetry energy, are extended for finite temperatures. The baryon equation of state, which includes nucleons and $\Delta$ resonances is supplemented by the contribution of the pion gas described either by the vacuum dispersion relation or with taking into account the $s$-wave pion-baryon interaction. Distribution of the charge between components is found. Thermodynamical characteristics on $T-n$ plane are considered. The energy-density and entropy-density isotherms are constructed and a dynamical trajectory of the hadron system formed in heavy-ion collisions is described. The effects of taking into account the $\Delta$ isobars and the $s$-wave pion-nucleon interaction on pion differential cross sections, pion to proton and $\pi^-/\pi^+$ ratios are studied. The liquid-gas first-order phase transition is studied within the same models in isospin-symmetric and asymmetric systems. We demonstrate that our models yield thermodynamic characteristics of the phase transition compatible with available experimental results. In addition, we discuss the scaled variance of baryon and electric charge in the phase transition region. Effect of the non-zero surface tension on spatial redistribution of the electric charge is considered for a possible application to heavy-ion collisions at low energies.

Substituted (Nd,Pr)2Fe14B alloys: structural features and magnetic properties

In this work the magnetic properties of (Nd,Pr)2Fe14B alloys have been investigated in a wide range of temperatures (4.2 – 700 K) and magnetic fields (up to 14 T). The features of the structure were studied using two methods: SEM and AFM/MFM. Magnetic phase transition diagram was constructed. In the region of magnetic phase transitions (Curie and spin- reorientation transition temperatures) the value of the magnetocaloric effect was determined as an entropy change. The constancy of the magnetocaloric effect value for (Nd,Pr)2Fe14B alloys has been established.

Experimental study on spray and atomization characteristics under subcritical, transcritical and supercritical conditions of marine diesel engine

In this research, detailed characteristics of diesel spray and atomization based on marine engine scale have been studied under subcritical, transcritical and supercritical conditions. A large constant volume chamber with an inner diameter of 300 mm and a 300 μm single-nozzle injector are used to mimic the in-cylinder thermodynamic condition of marine diesel engine. Sub/trans/supercritical conditions are composed of nine cases by means of different diesel phase transition regions. Optical diagnostic measurements are conducted through backlight illumination technology and schlieren imaging technology in an inert atmosphere. The images are processed with multi-threshold technique to obtain quantitative spray parameters, such as jet and liquid penetrations, averaged lengths, liquid steady time, spray cone angles and R-parameter. Moreover, some new finding about the structure of spray under supercritical condition is also discussed. The results show that the spray liquid periphery is quite smooth and stable with shorter penetration under supercritical condition, meanwhile, the jet of spray behaves as thinner periphery near the injector nozzle region and shows lower density at downstream region. Higher ambient pressure produces a negative effect on the increase of the tip velocity and development of spray. With the transition from transcritical condition to supercritical condition, the liquid length decreases by 32 mm and there exists an obvious decrease of 35 mm from jet length to liquid length. A trade-off relationship between the jet cone angle and jet penetration is also found. Generally, this work is helpful to understand the detailed process of diesel atomization under transcritical and supercritical conditions.

Electrocaloric effect and pyroelectric performance in (K,Na)NbO3‐based lead‐free ceramics

AbstractThe issue of how to achieve an electrocaloric effect (ECE) and pyroelectric effect in a material simultaneously remains to be a challenge for developing practical solid‐state cooling devices and RF‐detectors. Here, we structure a polymorphic phase transition (PPT) region by doping modification in KNN‐based ceramics, which are developed to achieve the ECE. The direct measured ECE and pyroelectric properties are investigated in lead‐free (1‐x)K0.5Na0.5NbO3‐xBi0.5Na0.5ZrO3 (KNN‐xBNZ) ceramics. The adiabatic temperature change (∆T) of 0.22 K at 100°C, 0.14 K at 70°C and 0.16 K at 30°C can be obtained under an electric field of 35 kV cm–1 for x = 0.03, 0.04 and 0.05, respectively. In addition, the temperature dependence of pyroelectric coefficient (p) is established for all compositions via the Byer‐Roundy method. A large p of 454.46 × 10–4 C m–2 K–1 is detected at Curie temperature (TC) in the ceramics with x = 0.03. Achieving electrocaloric effect and pyroelectric performance simultaneously may shed light and provide a feasible design scheme for developing practically useful electrocaloric and pyroelectric materials.

Squeezed ensemble for systems with first-order phase transitions

All ensembles of statistical mechanics are equivalent in the sense that they give the equivalent thermodynamic functions in the thermodynamic limit. However, when investigating microscopic structures in the first-order phase transition region, one must choose an appropriate statistical ensemble. The appropriate choice is particularly important when one investigates finite systems, for which even the equivalence of ensembles does not hold. We propose a class of statistical ensembles, which always give the correct equilibrium state even in the first-order phase transition region. We derive various formulas for this class of ensembles, including the one by which temperature is obtained directly from energy without knowing entropy. Moreover, these ensembles are convenient for practical calculations because of good analytic properties. We also derive formulas which relate statistical-mechanical quantities of different ensembles, including the conventional ones, for finite systems. The formulas are useful for obtaining results with smaller finite-size effects, and for improving the computational efficiency. The advantages of the squeezed ensembles are confirmed by applying them to the Heisenberg model and the frustrated Ising model.

Relation of the phase transition and electrical, photoluminescence properties in (1 − x) Na0.5K0.5NbO3–xLiSbO3:0.006Dy3+ lead free ceramics

The (1 − x) Na0.5K0.5NbO3–xLiSbO3:0.006Dy3+ were synthesized by a solid state reaction method. The phase structure, microstructure and electric properties were systematically investigated. The orthorhombic–tetragonal (O–T) phase was identified in the composition of x = 0.04 with the help of Raman spectra and photoluminescence response. The ceramic with x = 0.04 mol% LiSbO3 demonstrates optimal properties: Pr = 32.2 μC/cm2, Ec = 12.26 kV/cm, TC = 325 °C, tan δ ≤ 1.5% (250–400 °C). We believe that this work can provide an effective method to investigate the phase transition near polymorphic phase transition (PPT) region. The advantages of simple operation, quick response and pollution-free make it potential to be used in investigating the phase transition near the PPT region of lead-free ceramics.

Structure, microstructure, dielectric and piezoelectric properties of (1-x-y)BiFeO3-xPbFe0.5Nb0.5O3-yPbTiO3 ceramics

Abstract Ceramics of seven quasi-binary concentration sections of the ternary solid solution system (1- x-y )BiFeO 3 - x PbFe 0.5 Nb 0.5 O 3 - y PbTiO 3 were prepared by the conventional solid-phase reaction method in the range of 0.05 ≤ x ≤ 0.325; 0.05 ≤ y ≤ 0.325. By using x-ray diffraction technique, the phase diagram of the system was constructed which was shown to contain the regions of tetragonal and rhombohedral symmetry and the morphotropic phase boundary between them. Grain morphology, dielectric and piezoelectric properties of selected solid solutions were investigated. The highest piezoelectric coefficient d 33 = 50 pC/N was obtained. Dielectric characteristics of ceramics revealed ferroelectric relaxor behavior and region of diffuse phase transition from the paraelectric to ferroelectric phase in the temperature range of 600–800 K.

Thermal Properties of a Eutectic InSb + MnSb Composite

We have studied the thermal conductivity κ(T) and thermal diffusivity a(T) of a eutectic InSb + MnSb composite in the range 80–700 K with the heat flux parallel and perpendicular to MnSb inclusions. The phonon thermal conductivity data have been analyzed in terms of the Holland model. The weaker temperature dependence of thermal conductivity in the range 250–450 K is due to resonance transfer of the energy of ionized impurity states. The observed anisotropy in κ(T) in the range 80–250 K is associated with the scattering of long-wavelength phonons by interfaces. The anomalies in κ(T) and a(T) observed in the phase transition region are due to phonon scattering by magnetic order parameter fluctuations.