Characteristic Transition Temperature Research Articles

Composition driven structural transition in La2−xSrxCuRuO6 (0 ≤ x ≤ 1) double perovskites

We report results of detailed structural investigations on the system La2−xSrxCuRuO6 (0 ≤ x ≤ 1) as a function of composition using neutron powder diffraction. Our results reveal that this series of compounds exhibit an interesting structural transition manifested through Sr doping; with Ru in single valency, in end members, and mixed valency otherwise. The end members La2CuRuO6 and LaSrCuRuO6 crystallize in monoclinic structures with space group P21/n. However, the compositions with intermediate Sr doping (x = 0.2 and 0.8) crystallize in triclinic structure with space group P-1, albeit with minute distortions. The detailed structural studies by neutron powder diffraction at room temperature and at 10 K, for compositions, x = 0.2, 0.8 and 1 are presented. While magnetic measurements are indicative of ferro and antiferromagnetic like transitions for x = 0 and x > 0 respectively, neutron diffraction patterns collected at 10 K and 30 K across the characteristic transition temperature suggested by the magnetic data, do not show any extra peaks or extra intensity at lower temperature as compared to the high temperature, thus, establishing the absence of any long-range magnetic ordering in these samples.

Second-order magnetic critical points at finite magnetic fields: Revisiting Arrott plots

The so-called Arrott plot, which consists in plotting $H/M$ against ${M}^{2}$, with $H$ the applied magnetic field and $M$ the magnetization, is used to extract valuable information in second-order magnetic phase transitions. Besides, it is widely accepted that a negative slope in the Arrott plot is indicative of a first-order magnetic transition. This is known as the Banerjee criterion. In consequence, the zero-field transition temperature ${T}^{*}$ is reported as the characteristic first-order transition temperature. By carefully analyzing the mean-field Landau model used for studying first-order magnetic transitions, we show in this work that ${T}^{*}$ corresponds in fact to a triple point where three first-order lines meet. More importantly, this analysis reveals the existence of two symmetrical second-order critical points at finite magnetic field $({T}_{c},\ifmmode\pm\else\textpm\fi{}{H}_{c})$. We then show that a modified Arrott plot can be used to obtain information about these second-order critical points. To support this idea we analyze experimental data on ${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_{3}$ and discuss an estimate for the location of the triple point and the second-order critical points.

Determination of microplastic polyethylene (PE) and polypropylene (PP) in environmental samples using thermal analysis (TGA-DSC)

Microplastics are increasingly detected in the environment and the consequences on water resources and ecosystems are not clear to date. The present study provides a cost-effective and straightforward method to determine the mass concentrations of polymer types using thermal analysis. Characteristic endothermic phase transition temperatures were determined for seven plastic polymer types using TGA-DSC. Based on that, extracts from wastewater samples were analyzed.Results showed that among the studied polymers, only PE and PP could be clearly identified, while the phase transition signals of the other polymers largely overlap each other. Subsequently, calibration curves were run for PE and PP for qualitative measurements. 240 and 1540mg/m3 of solid material (12µm to 1mm) was extracted from two wastewater effluent samples of a municipal WWTP of which 34% (81mg/m3) and 17% (257mg/m3) could be assigned to PE, while PP was not detected in any of the samples. The presented application of TGA-DSC provides a complementary or alternative method to FT-IR analyses for the determination of PE and PP in environmental samples.

Glass transition and state diagram for freeze-dried Lentinus edodes mushroom

Glass transitions of freeze-dried Lentinus edodes mushroom were determined using differential scanning calorimetry (DSC) as a function of water contents. Freeze-dried L. edodes containing unfreezable and freezable water were examined to explore the state diagram of L. edodes. The state diagram included the freezing curve, glass transition line, and ultimate maximal-freeze-concentration condition. The freezing curve and the glass transition line were fitted according to Clausius-Clapeyron model and Gordon-Taylor model, respectively. The ultimate maximal-freeze-concentration conditions were calculated as solid content of 0.81g solid/g (w.b.) with the characteristic glass transition temperature (Tg′) being −53.2°C, and characteristic temperature of end point of freezing (Tm′)u being −33.4°C. The other characteristic glass transition temperature Tg″ was −42.4°C. The state diagram of Lentinus edodes can be used in determining the storage stability as a function of temperature and water content, and optimizing drying and freezing processes.

Engineering Green Lubricants II: Thermal Transition and Flow Properties of Vegetable Oil-Derived Diesters

Six homologous series of linear aliphatic diesters were prepared from commonly available fatty acids (chain lengths 10–22 carbons) and diols (chain lengths, n, 2–10 carbons). The thermal transition and flow properties are presented as functions of their molecular structures, namely chain length, symmetry, end group interactions, and saturation. Predictive relationships between the total chain length of the diesters and their characteristic thermal transition temperatures were obtained. The thermal transition temperatures were affected by intramolecular steric repulsion of the ester groups at small diol chain lengths (n ≤ 4) and by the odd–even effect associated with large diol chains (n > 4), allowing for further refinement of the crystallization and melting prediction models. All of the diesters presented Newtonian flow behavior above their melting points, making them particularly suitable for use in lubricant formulations and other flow-dependent applications. The influence of mass on the viscosity was ...

Infrared reflectivity investigation of the phase transition sequence in Pr0.5Ca0.5MnO3

This work reports an infrared reflectivity study of the phase transition sequence observed in Pr0.5Ca0.5MnO3. The need to measure over an extended spectral range in order to properly take into account the effects of the high frequency polaronic absorption is circumvented by adopting a simple approximate method, based on the asymmetry present in the Kramers Kronig inversion of the phonon spectrum. The temperature dependence of the phonon optical conductivity is then investigated by monitoring the behavior of three relevant spectral moments of the optical conductivity. This combined methodology allows us to disclose subtle effects of the orbital, charge and magnetic orders on the lattice dynamics of the compound. The characteristic transition temperatures inferred from the spectroscopic measurements are compared and correlated with those obtained from the temperature dependence of the induced magnetization and electrical resistivity.

Light scattering and computer simulation studies of superionic pure and La-doped BaF2

A combination of both Raman and Brillouin scattering experiments as well as Molecular Dynamics (MD) was used to study the superionic behaviour of BaF2 doped with a wide range of LaF3 concentrations (0⩽x⩽50mol%). Raman spectroscopy reveals that for undoped BaF2 and those doped with 5% and 10% LaF3, the room temperature spectra show the usual T2g symmetry mode at 241cm−1 whereas for those doped with 20%, 30% and 50% LaF3, the dominant Raman mode is of the Eg symmetry situated at ∼263, 275 and 286cm−1, respectively. The Raman linewidths show near linear increases with temperature followed by rapid increases above the characteristic transition temperatures (Tc), being at 1200, 850, 800, 975, 950 and 920K for LaF3 concentrations of 0, 5, 10, 20, 30 and 50; respectively. The temperature dependence of the squares of the Brillouin frequencies (ΔωB)2 of the LA and TA acoustic modes respectively related to elastic constants C11 and C44 showed linear decreases followed by significant deviations around the same temperatures (Tc), at which the Raman linewidths start to show substantial increases. The complementary studies using MD simulations show that the diffusion coefficients increase markedly above the same temperatures observed experimentally. The extrinsic fluorine ion trajectories were also determined from the MD simulations to better understand the mechanisms of diffusion.

Study of low temperature rf-sputtered Mg-doped vanadium dioxide thermochromic films deposited on low-emissivity substrates

Undoped VO2 was fabricated by RF sputtering from metallic target on low emissivity glass substrates at a record low growth temperature. The structure of the films was examined by X-Ray Diffraction whereas temperature dependence transmittance was employed for monitoring the film's thermochromic properties. Thermochromic VO2 could be achieved at the substrate temperature of 300°C which is the lowest growth temperature of VO2 phase by sputtering reported in literature without any additional post-deposition annealing treatment of the films. The thermochromic properties obtained for undoped VO2 were: luminous transmittance of 36.2%, metal-to-insulator transition temperature at 55.7°C and infrared transmittance modulation, ΔTIR, (transmittance difference in the infrared, at λ=2000nm, when heated from 25°C to 90°C) by 20.5% and solar transmittance modulation ΔTsol=5.2%. Subsequently, magnesium (Mg) was introduced in the structure of thermochromic VO2 up to 2.4at.%, and the Mg-induced changes in thermochromisity were examined. By increasing the amount of Mg in VO2 structure the visible transmittance increased up to 70%. Improvement in thermochromic characteristics was observed for atomic percentage of Mg up to 0.3% above which the films tend to be non-thermochromic. The optimum atomic percentage of Mg (0.3at.%) leads to thermochromic VO2 with the characteristic lowest phase transition temperature at 49.2°C, luminous transmittance ΤIum=46.6% and solar transmittance modulation ΔΤsol=2.8% and ΔΤIR=13%. Overall, 70nm thick thermochromic VO2 films deposited at the low substrate temperature of 300°C and low deposition rate 0.75nm/min when doped with Mg up to 0.3at.% resulted in vanadium dioxide with improved thermochromic properties is regarded as particularly promising for smart windows applications on flexible substrates.

The effect of low temperature thermal annealing on the magnetic properties of Heusler Ni–Mn–Sn melt-spun ribbons

We report the effect of low temperature vacuum annealing (823K; 550°C) on the elemental chemical composition, structural phase transition temperatures, phase structure, and magnetic properties of Ni50.6Mn36.3Sn13.1 as-solidified ribbons. Their elemental chemical composition, highly oriented columnar-like microstructure and single-phase character (L21-type crystal structure for austenite) remain unchanged after this low temperature annealing. Annealed ribbons show a reduction of interatomic distances which lead to a small change in the characteristic phase transition temperatures (~3–6K) but to a significant rise of ~73 and 63% in the saturation magnetization of the martensite and austenite phases, respectively, that can be strictly ascribed to the strengthening of ferromagnetic interactions due to the change in interatomic distances.

Premartensite to martensite transition and its implications for the origin of modulation inNi2MnGaferromagnetic shape-memory alloy

We present here results of temperature dependent high resolution synchrotron x-ray powder diffraction study of sequence of phase transitions in Ni2MnGa. Our results show that the incommensurate martensite phase results from the incommensurate premartensite phase, and not from the austenite phase assumed in the adaptive phase model. The premartensite phase transforms to the martensite phase through a first order phase transition with coexistence of the two phases in a broad temperature interval (~40K), discontinuous change in the unit cell volume as also in the modulation wave vector across the transition temperature and considerable thermal hysteresis in the characteristic transition temperatures. The temperature variation of the modulation wave vector q shows smooth analytic behaviour with no evidence for any devilish plateau corresponding to an intermediate or ground state commensurate lock-in phases. The existence of the incommensurate 7M like modulated structure down to 5K suggests that the incommensurate 7M like modulation is the ground state of Ni2MnGa and not the Bain distorted tetragonal L10 phase or any other lock-in phase with a commensurate modulation. These findings can be explained within the framework of the soft phonon model.

Framework for Analysis of Irreversible Fuel Cell Heat Engine Hybrids

A thermodynamic and electrochemical framework for the analysis of the degrading fuel cell heat engine hybrid is developed and explored for various working reactions. Fuel cell hybrids are a combination of energy conversion sub-systems – fuel cells and heat engines. Fuel cell hybrids are important for the future for they are the most efficient devices when converting chemical energy of methane from natural gas and renewable fuels to electricity. While the perfect fuel cell would undergo no degradation, practical fuel cells with irreversibilities will degrade. The common practice is to linearize degradation. However, experimental evidence shows that ASR(t) is commonly an ohmic parabolic function. By the principle of superposition one can easily develop the equations for degrading hybrids from those developed for hybrids. The path of maximum power can be developed for degrading hybrids as a function of ASR and temperature. The maximum power for a degrading fuel cell hybrid is given by the Gaussian hypergeometric function. The range of the operation of the SOFC is likely most conducive to hybrid operation for hydrogen oxidation as the working reaction. Each working reaction has a characteristic transition temperature and a characteristic optimal operating temperature.

Framework for Analysis of Irreversible Fuel Cell Heat Engine Hybrids

A thermodynamic and electrochemical framework for the analysis of the degrading fuel cell heat engine hybrid is developed and explored for various working reactions. Fuel cell hybrids are a combination of energy conversion sub-systems – fuel cells and heat engines. Fuel cell hybrids are important for the future for they are the most efficient devices when converting chemical energy of methane from natural gas and renewable fuels to electricity. While the perfect fuel cell would undergo no degradation, practical fuel cells with irreversibilities will degrade. The common practice is to linearize degradation. However, experimental evidence shows that ASR(t) is commonly an ohmic parabolic function. By the principle of superposition one can easily develop the equations for degrading hybrids from those developed for hybrids. The path of maximum power can be developed for degrading hybrids as a function of ASR and temperature. The maximum power for a degrading fuel cell hybrid is given by the Gaussian hypergeometric function. The range of the operation of the SOFC is likely most conducive to hybrid operation for hydrogen oxidation as the working reaction. Each working reaction has a characteristic transition temperature and a characteristic optimal operating temperature.

On the magnetostructural transition in MnCoGeBx alloy ribbons

The magnetostructural transition in the Mn0.96Co1.04GeB0.02 ribbon alloy was investigated. Chemical, structural, microstructural, and magnetic studies were performed on the samples which were annealed at different temperatures. The resulting samples underwent a first-order phase transition in which the characteristic structural transition temperature shows a near-linear and inversely proportional dependence to the annealing temperature. The magnetostructural transition occurs through a simultaneous ferroelastic–magnetic transition between the ferroelastic–paramagnetic and paraelastic–ferromagnetic phases. Our results suggest that the crystal structure of the hexagonal high temperature phase allows the formation of ferroelastic domains, and the domain walls act as the natural nucleation site of the low temperature paraelastic–ferromagnetic phase.

Application of Thermal Analysis Tests Results in the Numerical Simulations of Continuous Casting Process

Abstract Measurement of thermophysical properties of steel is possible by using different thermal analysis techniques. In the field of metallurgy the most relevant methods are Differential Thermal Analysis (DTA) and Differential Scanning Calorimetry (DSC). The paper presents the results of thermophysical properties which are necessary to carry out numerical simulation of continuous casting of steel. The study was performed for two steel grades S320GD and S235JR. The main aim of the research was to determine the dependence of specific heat on temperature. On the basis of obtained results the thermal effects of phase transformations and characteristic transition temperatures were also identified. Both the specific heat of steel and thermal effects of phase transformations are included in the Fourier-Kirchhoff equation, as the material properties necessary to obtain the numerical solution. The paper presents the research methodology, analysis of results and method of determining the specific heat of steel based on the results of DSC analysis.

Thermal Properties, Conductivity, and Spin-lattice Relaxation of Gel Electrolyte Based on Low Molecular Weight Gelator and Solution of High Temperature Ionic Liquid

Gel electrolyte based on low molecular weight organic gelator methyl-4,6-O-(p-nitrobenzylidene)-α-D-glucopyranoside was formed by the self-assembly phenomena in aqueous solution of high temperature ionic liquid tetramethylammonium bromide. The solidification process was based on sol-gel technique with controlled gelation temperature. When the temperature was below the characteristic gel-sol phase transition temperature, Tgs, the gel electrolyte was solid-like. The gel electrolytes showed enhanced ionic conductivity to those of the pure electrolyte in liquid state in whole temperature range below Tgs. The thermal stability, ionic conductivity and molecular dynamics investigated as a function of temperature and concentration of the gelator, together with the gel microstructure were performed to get some insight in to the origin of the enhanced conductivity properties. Intermolecular interaction between ion complexes and gelator aggregates was implicated by the data obtained and suggested as the origin of the conductivity enhancement effect.

Structure transition of Sn57Bi43 melt and its thermodynamic and kinetic characteristics

In this paper, the temperature dependence of electrical resistivity (ρ–T), the thermodynamic and kinetic characteristics of Sn57Bi43 eutectic alloy melt structure transition were investigated; the activation energy of structure transition (Ea) and the continuous heating transformation curve (CHT curve) were calculated by a thermal analysis method. The results showed that, both resistivity and thermal parameters of Sn57Bi43 alloy changed abnormally at the range of 850–1100K, and their characteristic transition temperatures matched well each other, which verifies the existence of liquid–liquid structure transition(L–LST) in Sn57Bi43 alloy and the effectiveness of the resistivity and DSC experimental results, and the method of Kissinger and Ozawa can be applied to study the kinetic characteristics of Sn–Bi alloy melt structure transition.

Operational Performance Envelope for Irreversible Fuel Cell Heat Engine Hybrids

The performance envelope for the fuel cell heat engine hybrid (FCH) with irreversibilities is developed and explored. Fuel cell hybrids are combination of energy conversion sub-systems – fuel cells and heat engines. Fuel cell hybrids are important for the future for they are the most efficient devices when converting chemical energy of methane from renewable fuels to electricity. While the perfect fuel cell and heat engine would have no irreversibilities and undergo no degradation, practical fuel cells and heat engines exhibit irreversibilities and will degrade. Various working reactions (WR) can be used in a FCH, for example, hydrogen oxidation, carbon monoxide oxidation, etc. For the FCH each working reaction has a characteristic transition temperature and a characteristic optimal operating temperature for maximum power. The characteristic optimal operating temperature of 1070 K for the FCH with an ideal heat engine and hydrogen oxidation WR is not a function of ASR for ASR not a function of temperature. The path of maximum power can be pursued as a FCH operational strategy. In the case of degradation, or changing ASR, the path can be followed by varying the fuel flow rate (fuel cell current) at constant fuel utilization. This is a continuation of work previously published by the authors in the ECS Transactions.

All-atom simulation study of protein PTH(1-34) by using the Wang-Landau sampling method

We perform simulations of the N-terminal 34-residue protein fragment PTH(1-34), consisting of 581 atoms, of the 84-residue human parathyroid hormone by using the all-atom ECEPP/3 force field and the Wang-Landau sampling method. Through a massive high-performance computation, the density of states and the partition function Z(T), as a continuous function of T, are obtained for PTH(1-34). From the continuous partition function Z(T), the partition function zeros of PTH(1-34) are evaluated for the first time. From both the specific heat and the partition function zeros, two characteristic transition temperatures are obtained for the all-atom protein PTH(1-34). The higher transition temperature T 1 and the lower transition temperature T 2 of PTH(1-34) can be interpreted as the collapse temperature T θ and the folding temperature T f , respectively.

Continuous tuning of W-doped VO2 optical properties for terahertz analog applications

Vanadium dioxide (VO2), with its characteristic metal-insulator phase transition, is a prospective active candidate to realize tunable optical devices operating at terahertz (THz) frequencies. However, the abrupt phase transition restricts its practical use in analog-like continuous applications. Incorporation of tungsten is a feasible approach to alter the phase transition properties of thin VO2 films. We show that amplitude THz modulation depth of ∼65%, characteristic phase transition temperature of ∼40 °C, and tuning range larger than 35 °C can be achieved with W-doped VO2 films grown on sapphire substrates. W-doped VO2 films can also be used to suppress Fabry-Perot resonances at THz frequencies but at temperatures much lower than that observed for undoped VO2 films. The gradual phase transition temperature window allows for precise control of the W-doped VO2 optical properties for future analog based THz devices.

Thermally Sprayed Y2O3-Al2O3-SiO2 Coatings for High-Temperature Protection of SiC Ceramics

The suitability of certain glass compositions in the Y2O3-Al2O3-SiO2 (YAS) system as protecting coatings for silicon carbide components has been prospected. One particular YAS composition was formulated considering its glass formation ability and subsequent crystallization during service. Round-shaped and homogeneous granules of the selected composition were prepared by spray drying the corresponding homogeneous oxide powder mixture. Glassy coatings (197 µm thick) were obtained by oxyacetylene flame spraying the YAS granules over SiC substrates, previously grit blasted and coated with a Si bond layer (56 µm thick). Bulk glass of the same composition was produced by the conventional glass casting method and used as reference material for comparative evaluation of the characteristic glass transition temperatures, crystallization behavior, mechanical, and thermal coating properties. The mechanical properties and thermal conductivity of the coating were lower than those of the bulk glass owing to its lower density, higher porosity, and characteristic lamellar structure. The crystallization of both bulk glass and coating occurred during isothermal treatments in air at 1100-1350 °C. Preliminary data on ablation tests at 900 °C using the oxyacetylene gun indicated that the YAS glassy coating was a viable protective shield for the SiC substrate during 150 s.