High-temperature Relaxation Research Articles

Effects of vanadium doping on the charge ordering and low-temperature spin-glass phase in Pr0.45Ca0.55MnO3

We report structural, magnetic, and dielectric properties of the Pr0.45Ca0.55Mn1-xVxO3 (x = 0.05 and 0.1) polycrystalline compounds, prepared by solid state reaction method. Pnma space group and orthorhombic crystal structure of the compounds are confirmed by the Rietveld refinement of X-ray diffraction data. Magnetic study reveals a multiple-phase-separated metastable magnetic behavior with charge ordered phase followed by a spin-glass state at the low-temperature region. For a single change of vanadium content from x = 0.05–0.1, the charge ordering temperature decreases from 245 K to 239 K, the antiferromagnetic ordering temperature decreases from 131 K to 127 K, and the ferromagnetic cluster-glass ordering temperature increases from 41 K to 44 K. This indicates the weakening of charge ordering strength and development of ferromagnetic components in the antiferromagnetic background, which is also confirmed by isothermal hysteresis. Colossal dielectric constant is observed in both samples. In the measured temperature range, an implication of a ferroelectric phase transition around 253 K is observed in Pr0.45Ca0.55Mn0.90V0.10O3 compound. The calculated results indicate that the low-temperature relaxation follows variable range hopping behavior, whereas the high-temperature one takes place according to the Arrhenius model. The fitting parameters ascribe the low-temperature relaxation process to the hopping of polaron charge carriers at localized sites, whereas the high-temperature relaxation is related to Maxwell-Wagner relaxation, caused by blocking of charge carriers at grain boundaries.

Colossal dielectric permittivity in CuCrO2 ceramics

CuCrO2 ceramics were prepared by solid-state reaction method. Low-temperature (10–320 K) dielectric properties were investigated in the frequency range of 102–106 Hz. Colossal dielectric constant behavior composed of two thermally activated relaxations was observed. The low-temperature relaxation (R1) occurring around 125 K with an activation energy of 0.21 eV was ascribed to be a dipolar relaxation caused by defect dipoles of OHO•−Cr3+. The high-temperature relaxation (R2) appearing around 200 K with an activation energy of 0.26 eV was argued to be a Maxwell-Wagner relaxation originating from humidity sensitivity.

Experimental study on high-temperature relaxation behavior of carbon fiber reinforced polymer cable

In order to fully investigate the relaxation behavior of carbon fiber reinforced polymer (CFRP) cable exposed to high temperature, both steady state-constant displacement and temperature (ST-CDT), and transient state-constant displacement and temperature (TR-CDT) tests were conducted on specimens. The failure modes, stress loss laws and rupture times of high-temperature relaxation specimens were determined under ST-CDT and TR-CDT conditions. After high-temperature relaxation tests, high-temperature tensile tests were conducted on samples without rupture to obtain its residual mechanical properties. Final stress losses and stress loss rates of CFRP cable specimens determined by ST-CDT tests were compared with these of TR-CDT tests to clarify the effect of temperature-load path. A high-temperature relaxation model was developed to characterize the variation law of stress losses for CFRP cables under ST-CDT and TR-CDT conditions. A correlational model between stress loss and creep strain rates in steady stages of CFRP cables under high temperature was calibrated. The obtained results demonstrated that high-temperature stress losses of CFRP cables were closely correlated with target temperature, initial stress level and relaxation duration. The application of initial pre-tension load before heating accelerated the stress loss rate in transition relaxation stage and final stress loss of CFRP cable under high temperature.

Kinetic and Continuum Modeling of High-Temperature Air Relaxation

Fully kinetic, vibrationally kinetic, and continuum solvers with varying model fidelity are used in this work to model the high-temperature relaxation of air in 7230 and 15,000 K adiabatic heat baths and a hypersonic flow over a cylinder. The results show significant impact of uncertainties in vibrational relaxation times and reaction rate constants on thermal and chemical relaxation, in particular, on gas temperature and species mole fractions. Most notably, these uncertainties need to be reduced for collisions that include nitric oxide. Order-of-magnitude differences in the nitric oxide dissociation and recombination rates have a large impact on the peak NO mole fraction immediately behind the shock and surface-distributed heat flux, respectively. High-fidelity kinetic and continuum approaches are found to have different reaction channels having the largest effect on species mole fractions and gas temperature: exchange and dissociation in the former, and and dissociation in the latter.

High-Performance Luminescence Thermometer with Field-Induced Slow Magnetic Relaxation Based on a Heterometallic Cyanido-Bridged 3d-4f Complex.

The 1:1:1 reaction of DyCl3·6H2O, K3[Co(CN)6] and bpyO2 in H2O has provided access to a complex with formula [DyCo(CN)6(bpyO2)2(H2O)3]·4H2O (1) in a very good yield, while [DyFe(CN)6(bpyO2)2 (H2O)3]·4H2O (2) was also precipitated (also in a high yield) using K3[Fe(CN)6] instead of K3[Co(CN)6]. Their structures have been determined by single-crystal X-ray crystallography and characterized based on elemental analyses and IR spectra. Combined direct current (dc) and alternating current (ac) magnetic susceptibility revealed slow magnetic relaxation upon application of a dc field. μ-SQUID measurements and CASSCF calculations revealed high-temperature relaxation dynamics for both compounds. Low-temperature magnetic studies show the relaxation characteristics for 1, while for compound 2 the dynamics corresponds to an antiferromagnetically coupled Dy···Fe pair. High-resolution optical studies have been carried out to investigate the performance of compounds 1 and 2 as luminescence thermometers. For 1, a maximum thermal sensitivity of 1.84% K-1 at 70 K has been calculated, which is higher than the acceptable sensitivity boundary of 1% K-1 for high-performance luminescence thermometers in a broad range of temperature between 40 and 140 K. Further optical studies focused on the chromaticity diagram of compound 1 revealed a temperature shift from warm white (3200 K) at 10 K toward a more natural white color near 4000 K at room temperature.

Oxygen vacancies-related high-temperature dielectric relaxation and pyroelectric energy harvesting in lead-free Ba(Zr0.2Ti0.8)O3 ceramics

Oxygen vacancies-related high-temperature dielectric relaxation and pyroelectric energy harvesting in lead-free Ba(Zr0.2Ti0.8)O3 ceramics

Influence of La3+ ion doping on electrical conduction and thermal stability of NiO nanostructures

Ni1-xLaxO (x = 0.00, 0.01, and 0.03) nanostructures have been synthesized by the conventional sol-gel method, and further the electrical conduction and thermal stability have been investigated. It is observed that ε′ decreases with increasing frequency and does not exhibit any significant change with temperature T < 150 °C. It attains a high value ε′ ∼103-104 (75 kHz and 300 °C) for NiO sample. The high value of ε′ in pure and LNO samples might be explained via electron relaxation–mode-coupling model, associated with oxygen vacancies (OVs) which become dominant at high temperature. The high-temperature dielectric relaxation behavior was also observed in all the samples. The calculated activation energies (Ucond and Uac) from DC and AC conductivities, revealing that the relaxation behavior associated with the migration of OVs as confirmed by the EPR spectroscopy. On the other hand, a highly doped (x = 0.03) LNO sample shows high thermal stability than pure NiO, which needs a much higher degradation temperature, i.e., above 500 °C. The thermal activation energy was also determined in the temperature range of 80–150 °C.

Activation free energy gradient controls interfacial mobility gradient in thin polymer films.

We examine the mobility gradient in the interfacial region of substrate-supported polymer films using molecular dynamics simulations and interpret these gradients within the string model of glass-formation. No large gradients in the extent of collective motion exist in these simulated films, and an analysis of the mobility gradient on a layer-by-layer basis indicates that the string model provides a quantitative description of the relaxation time gradient. Consequently, the string model indicates that the interfacial mobility gradient derives mainly from a gradient in the high-temperature activation enthalpy ΔH0 and entropy ΔS0 as a function of depth z, an effect that exists even in the high-temperature Arrhenius relaxation regime far above the glass transition temperature. To gain insight into the interfacial mobility gradient, we examined various material properties suggested previously to influence ΔH0 in condensed materials, including density, potential and cohesive energy density, and a local measure of stiffness or u2(z)-3/2, where u2(z) is the average mean squared particle displacement at a caging time (on the order of a ps). We find that changes in local stiffness best correlate with changes in ΔH0(z) and that ΔS0(z) also contributes significantly to the interfacial mobility gradient, so it must not be neglected.

Organosilica-Modified Multiblock Copolymers for Membrane Gas Separation.

Organosubstituted silica derivatives were synthesized and investigated as modifiers of block copolymers based on macroinitiator and 2,4-toluene diisocyanate. A peculiarity of the modified block copolymers is the existence in their structure of coplanar rigid polyisocyanate blocks of acetal nature (O-polyisocyanates). Organosubstituted silica derivatives have a non-additive effect on high-temperature relaxation and α-transitions of modified polymers and exhibit the ability to influence the supramolecular structure of block copolymers. The use of the developed modifiers leads to a change in the gas transport properties of block copolymers. The increase of the permeability coefficients is due to the increase of the diffusion coefficients. At the same time, the gas solubility coefficients do not change. An increase in the ideal selectivity for a number of gas pairs is observed. An increase in the selectivity for the CO2/N2 gas pair (from 25 to 39) by 1.5 times demonstrates the promising use of this material for flue gases separation.

Sintered Ti/Al core/shell nanoparticles: computational investigation of the effects of core volume fraction, heating rate, and room-temperature relaxation on tensile properties

Molecular dynamics simulations were performed to roughly imitate the conditions of selective laser sintering during additive manufacturing. The role of core volume fraction on the resultant uniaxial tensile properties of sintered Ti/Al bimetallic core/shell nanoparticles (NPs) was investigated during various sintering states. A chain model was created from five single thermally equilibrated Ti/Al NPs with weak neck connections by a solid-state sintering process at room temperature (298 K). The chains were heated to 800 K with two heating rates (0.04 and 0.2 K ps−1), underwent high-temperature relaxation, and were cooled to 298 K with a cooling rate of 0.08 K ps−1. They were then relaxed at 298 K for different periods (i.e. 1, 4, and 10 ns). In a follow-up procedure, those sintered NPs were subjected to uniaxial tension at different strain rates (i.e. 0.001, 0.01, and 0.1% ps−1). The thermodynamic properties and the structural evolutions of atomic configurations were investigated during the sintering process. The tensile responses were also obtained to examine the final product quality. The results indicate a strong correlation between the tensile strength of the final sintered chain product and the Ti core volume fraction. A larger Ti core volume fraction yields a stronger chain structure, resulting in higher tensile strength. The effect of heating rate on the tensile strength of final products with larger core volume fraction is more pronounced. The effect of room-temperature relaxation is not obvious on the tensile strength except for two products, which were sintered with the fast heating rate and tested under the lowest/highest strain rate. Also, high strain rates improve the tensile strength, and low strain rates will lead to enhanced ductility of the final products, especially with residual single atomic chain.

A comparative study on the grain boundary internal friction peak of pure iron

The high-temperature relaxation behavior of single-crystal and polycrystalline iron is systematically investigated using the internal friction (IF) technique. A relaxation peak located at ∼0.45Tm (Tm, the melting temperature) is observed in the IF spectrum of polycrystalline iron as reported in conventional studies, but it is absent for single-crystal iron, implying that the appearance of this peak is related to grain boundaries, which provides a direct and important evidence for the origin of the peak located at ∼0.45Tm from grain boundary relaxation. At the same time, parameters of grain boundary relaxation of polycrystalline iron are carefully evaluated combined with a coupling model. This study will provide an important reference for the study of defect relaxation behavior in pure iron and ferrite steels.

Study on sealing performance of packer rubber based on stress relaxation experiment

In the last few decades, rubber seal elements have been extensively used in the oil and gas industry. However, mechanical properties of rubber decrease at high temperatures, and stress relaxation can lead to sealing performance degradation, even sealing element failure. Therefore, how high temperature and stress relaxation affect the sealing performance of rubber is an important issue in oil and gas production. According to the oilfield feedback information, hydrogenated nitrile butadiene rubber (HNBR) was chosen for sample tension test at different temperature (20 °C and 120 °C) and the stress relaxation tests at 120 °C. The constitutive model of rubber was fitted by the test data. At the setting pressure of 14 MPa, the influence of temperature and stress relaxation on the packer rubber seal system was investigated by finite element analysis (FEA) software. The results indicated that temperature and stress relaxation had an appreciable impact on the sealing performance of packer rubber seal system. The average contact stress at 120 °C between the rubber and the borehole wall, compared with the calculation results at 20 °C, decreased by more than 15% and the maximum contact stress decreased by 20%. Similarly, the average contact stress before the relaxation decreased by more than 35% after stress relaxation behavior at 120 °C. In addition, the maximum contact stress decreased by 50% due to the stress relaxation. Our experimental results and discussion enable us to provide some advice to improve the sealing performance of a packer seal system, and present a basic experimental and theoretical foundation for future research into the optimum design of a packer sealing system.

Mechanisms of the relaxations in (In + Nb) co-doped TiO2 ceramics

(In0·5Nb0.5)0.1Ti0·9O2 ceramic sample was prepared by sol-gel method. The dielectric properties of this sample were investigated in a temperature range of 5–320 K. Two thermally activated relaxations were found. The low-temperature relaxation (P0-relaxation) appearing around 50 K follows the Vogel-Fulcher law and is ascribed to be a low-temperature Maxwell-Wagner relaxation caused by frozen electrons. The intermediate-temperature relaxation (P1-relaxation) occurring around 150 K obeys the Arrhenius law. Thermally activated depolarization current (TSDC) investigations reveal that it contains two relaxation processes (P1’ and P1 peaks). This relaxation was argued to be a polaronic relaxation caused by electrons hopping between Ti3+ and Ti4+ ions. TSDC also reveals a high-temperature relaxation (P2 peak) near room temperature, which is related to humidity sensing property.

Effect of diesel and microwave on the properties of crumb rubber and its modified binders

Some novel methods are introduced to improve thermal storage stability and pavement performance of crumb rubber modified binder. In this paper, the objective was to investigate the effects of compound activation on the molecular structure and physical properties of crumb rubber, as well as rheological properties and thermal storage stability of crumb rubber modified binders. Two activation methods, with and without diesel pre-swelling, and six microwave radiation intensities were designed in this study. Fourier transform infrared spectroscopy (FTIR) test, swelling equilibrium test and dynamic mechanical thermal analyzer (DMA) test were performed to evaluate the characteristic functional groups and physical properties of crumb rubbers before and after activation. Additionally, rheological properties test at various service temperatures and thermal storage stability test were carried out on the crumb rubber modified binders. The results show that certain microwave radiation intensity would break the C-S and C=S bonds and reduces the cross-linking density of crumb rubber. Activation mode was the main factor affecting high temperature stability, fatigue life and low temperature relaxation property rather than microwave radiation intensity compared with control one. In addition, thermal storage stabilities of activated crumb rubber modified binders were better than control one. The more radiation intensity, the improvement effect of thermal storage stability is. Synthesizes each kind of property, diesel pre-swelling and microwave radiation intensity of 1.2 kJ/g was a preferable combination in this study.

Temperature and frequency dependence of dielectric relaxations in YFeO3

We report a dielectric study in wide frequency and temperature ranges of yttrium orthoferrite, YFeO3, in order to obtain the temperature-frequency dependence of the dielectric relaxation processes. A mixture of oxide powders (Y2O3 and Fe2O3) was activated by high-energy ball milling. The mechanically activated powders were pressed and sintered at 1073 K. The cylindrical test samples were characterized at room temperature by X-ray diffraction (XRD) and vibrating sample magnetometry (VSM). Broadband Dielectric Spectroscopy (BDS) studies were carried out at several temperatures to the best understanding of dielectric properties. The X-ray diffraction patterns show an orthorhombic YFeO3 single phase. The magnetic hysteresis loop shows a weak ferromagnetic behavior, characteristic of the YFeO3. Dielectric spectroscopy analysis allows identifying two relaxation processes, the first at temperatures between 193 and 253 K and second above 333 K, defined as low-temperature dielectric relaxation (LTDR) and high-temperature dielectric relaxation (HTDR), respectively. It was determined the activation around 0.4 eV and 1.0 eV for the LTDR and HTDR, respectively. It indicates a single and doubly ionized oxygen vacancy. Moreover, it was proved that a hopping model is the dominating mechanism in the studied material, due to the conduction mechanism follows the Jonscher power law.

High‐temperature dielectric properties and impedance spectroscopy of PbHf1−xSnxO3ceramics

PbHf1−x Sn x O3 (PSH) ceramics were synthesised by a conventional solid-state reaction method. Dielectric properties were investigated in the temperature range of 20–650°C. As the Sn4+ content goes up, the phase transition temperatures of an antiferroelectric (AFE1) to another intermediate antiferroelectric (AFE2) phase and AFE2 to the paraelectric (PE) phase decrease gradually. When x≥0.1 for PSH ceramics, the ferroelectric (FE) phase appears around 225°C, and phase transition temperature from FE phase to PE phase goes up with the increasing concentration of Sn4+. Moreover, high-temperature dielectric relaxation (HTDR) phenomenon can be seen from all samples. Mechanism of HTDR was discussed from impedance spectroscopy and conductivity for PSH ceramics. It was found that three dielectric responses were observed in complex impedance plots and HTDR was involved with the movement of oxygen vacancies. Activation energy calculated from dielectric data suggested that the HTDR was governed by the hopping conduction process.

Giant dielectric constant and high-temperature dielectric relaxation properties in La-doped SrTiO3 ceramics

[Formula: see text] (SLTO) ceramics ([Formula: see text] = 0.05, 0.10, 0.15) were prepared with giant dielectric constant by the traditional solid-state method at 1350[Formula: see text]C. The temperature dependence of the dielectric constant was obtained at the temperature ranging from 29[Formula: see text]C to 500[Formula: see text]C and the frequency ranging from 2 kHz to 2 MHz. Two sets of relaxation peaks appear in the low temperature (region I) and the high temperature (region II), respectively. For region I, we conclude that the relaxation behavior is related to the oxygen vacancy migration. For region II, the two relaxation processes are caused by grain boundary for high frequency and Sr or Ti defects at grain interior for low frequency. With the doping amount reaching 0.15, the relaxation peaks disappear and become a common phase transition because of the aggravation of lattice distortion. These possible physical mechanisms of two sets of relaxation peaks are briefly discussed.

Significantly improved dielectric properties of TiO2 ceramics through acceptor-doping and Ar/H2 annealing

The acceptor-doped rutile TiO2 ceramics, x mol% M2O3-(1-x) mol% TiO2 (M = Al3+, Ga3+, and In3+), were prepared by solid state reaction method. The influence of Ar/H2 annealing on the structural and dielectric properties of the ceramics were systematically investigated. Our results reveal that the dielectric properties of the ceramics can be significantly improved by the Ar/H2 annealing. Ga3+ is found to be the most suitable dopant with the best doping level of 5 mol%. Excellent dielectric properties of colossal and flat dielectric permittivity (~1.2 × 105 (@1 kHz and 25 °C), low dielectric loss (~0.1), and good frequency stability were achieved over the temperature range of -70–150 °C in the Ar/H2-annealed 5 mol% Ga2O3-95 mol% TiO2 ceramic. This approach of acceptor-doping and Ar/H2 annealing leads to two thermally activated relaxations in the sample. The low-temperature relaxation is argued to be a Maxwell-Wagner relaxation caused by frozen electrons, while the high-temperature relaxation is a glass-transition-like relaxation associated with the freezing process of the electrons. This work highlights that engineering low-temperature Maxwell-Wagner relaxation paves a new way other than the frequently used acceptor-donor dual doping to design superior dielectric properties in the TiO2 system.

High-Temperature Dielectric and Relaxation Behavior of Tantalum-Doped Sodium Bismuth Titanate-Barium Titanate Ceramics

Ta-doped 0.94Bi0.5Na0.5TiO3–0.06BaTiO3 [(1 − x)(0.94BNT–0.06BT) − xTa (x = 0.00–0.02)] ceramics were prepared by a solid-state reaction process. X-ray diffraction (XRD) shows a rhombohedral symmetry with space group R3c for all samples. The introduction of Ta moves the low-temperature dielectric anomaly to lower temperature while the high-temperature dielectric anomaly becomes broader, although the dielectric permittivity is depressed slightly. Dielectric permittivity (ɛr) as a function of temperature for the sample x = 0.02 shows a stable and broad relative permittivity ∼ 3500 from 50 °C to 400 °C with low dielectric loss (tanδ) ≤ 0.02 from 80 °C to 340 °C (at 1 kHz). A Debye-like relaxation was found in the paraelectric region. The activation energy of relaxation frequency (Ea) of the samples x = 0, 0.005, 0.01, 0.02 is 0.62 eV, 0.84 eV, 1.15 eV and 1.20 eV and the activation energy of conduction (Econ) is 0.56 eV, 0.72 eV, 0.89 eV and 0.90 eV for x = 0.00, x = 0.005, x = 0.01 and x = 0.02, respectively, which are derived from isothermal impedance spectroscopy and admittance spectroscopy, respectively. The dielectric relaxation behavior is related to the migration of oxygen vacancies and cation vacancies dependence on the concentration of Ta.

Improved Analysis of Ultraviolet Absorption Spectra Under Nonequilibrium Shock Conditions

A landmark set of measurements of high-temperature relaxation under strong shock heating conditions, conducted in 2013 by Ibraguimova et al. (“Investigation of Oxygen Dissociation and Vibrational Relaxation at Temperatures 4000–10800 K,” Journal of Chemical Physics, Vol. 139, No. 3, 2013, Paper 034317) using ultraviolet (UV) absorption converted to time-dependent vibrational temperature profiles, have been receiving considerable attention in the literature as a test of nonequilibrium thermochemical models. A number of attempts by computational fluid dynamics modelers to reproduce the published results were unsuccessful in predicting the peak vibrational temperature, the downstream slope, or both. Our recent analysis has shown that some of the assumptions used in spectra-to-temperature conversions, such as the two-temperature approximation, are violated in the early region of the shock, and a more detailed analysis and direct comparison of UV absorption, and not temperatures, is necessary. This work presents such an analysis where a gas number density, translational temperature, and distribution of rotational and vibrational level populations, obtained using a kinetic gas dynamic approach, are converted into UV in-band absorption for each temporal or spatial location in the shock using a detailed spectroscopic model. The computed UV absorption is then directly compared with the raw measured time-dependent UV absorption curves at several wavelengths. Such a comparison is shown to have more validity than the previous ones.