Broad Phase Transition Research Articles

Mechanism of broadened phase transition temperature range in LaFeCoSiC compounds prepared upon high-pressure annealing

The cobalt-carbon co-doped NaZn13-type compound LaFe10.95Co0.65Si1.4C0.15 (LFCSC) is one of the most promising candidates for room-temperature working substance in magnetic refrigerator due to its many excellent properties such as large reversible entropy, low cost, and short annealing time. However, owing to the narrow temperature regions of magnetic phase transition in LFCSC, the operation-temperature window for magnetic refrigeration is limited, which restricts its actual application to some extends. In this paper, it is shown that the application of high-pressure to LFCSC during annealing can tailor atomic environment and magnetic transition, which leads to a strongly expanded phase transition temperature range in LFCSC. This broadening behavior can be well understood by importing the magnetoelastic interaction of localized magnetic moments into a microscopic model. The refrigeration performance of the high-pressure annealed sample with wide phase transition temperature range is enhanced according to the relative cooling power (RCP). On the contrary, temperature averaged entropy change (TEC) exhibits a weakened value in the high-pressure annealed sample, which suggests that the magnetic cooling performance could not be effectively improved by simply expanding the phase transition temperature range in the second-order phase transition materials. However, high-pressure annealing would be helpful to the magnetic refrigeration performance for the first-order phase transition materials.

Structural State Preceding the First-Order Phase Transition of Li2SO4

Molecular relaxation processes in lithium sulfate, Li2SO4, have been studied by Raman spectroscopy. Crystalline Li2SO4 has been shown to undergo a broad first-order structural phase transition, with a pretransition region.

On the lipid flip-flop and phase transition coupling.

We measured the passive lipid flip-flop of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in solid supported lipid bilayers across their main gel to fluid (Lβ → Lα) phase transition. By performing time and temperature resolved neutron reflectometry experiments, we demonstrated that asymmetric systems prepared in the gel phase are stable for at least 24 hours. Lipid flip-flop was found to be intrinsically linked to the amount of lipid molecules in the fluid phase. Moreover, the increase of this amount during the broad phase transition was found to be the main key factor for the timing of the flip-flop process. By measuring different temperature scan rate, we could demonstrate that, in the case of supported bilayers and for the temperature investigated, the lipid flip flop is characterised by an activation energy of 50 kJ mol-1 and a timescale on the order of few hours. Our results demonstrate the origin on the discrepancies between passive flip-flop in bulk systems and at interfaces.

Potassium sodium niobate ceramics with broad phase transition range: Temperature-insensitive strain

Excellent piezoelectric properties have been achieved in (K,Na)NbO3 (KNN)-based ceramics in the past few years. However, temperature-sensitive strain restricts the use of these ceramics in practical applications. In this work, we report improved piezoelectricity and temperature-insensitive strain in KNN-based ceramics. A broad temperature range for phase boundaries is formed by composition optimization, accounting for the observation of a highly improved strain stability. Interestingly, the strain for a ceramic with x = 0.025 varies by less than 5% and 9%, even if the temperature reaches 150 °C and 170 °C, respectively, which is comparable to that found for textured-LF4T ceramics. Furthermore, the stability of the original domain configuration with increasing temperature is tested to understand the underlying mechanism for the temperature-insensitive strain. The huge number of original domains retaining a piezoresponse at high temperature due to the extended phase boundary contributes to the temperature-insensitive strain. The present results open a new window for KNN-based ceramics to be used in applications demanding temperature-insensitive piezoelectric properties.

Enhanced refrigerant capacity and Curie temperature of amorphous Gd60Fe20Al20 microwires

Gd60Fe20Al20 microwires with an average diameter of ∼44 μm were fabricated by the melt-extraction method. XRD, TEM and HRTEM confirmed the amorphous nature of the microwires. The dimensional and chemical uniformity of the microwires over their length were confirmed by SEM and EDS, respectively. Magnetization measurements revealed a broad paramagnetic to ferromagnetic phase transition at TC ∼202 K. For μ0ΔH = 5 T, the microwires exhibit a broad magnetic entropy change with its maximum value ΔSMmax of ∼4.8 J kg−1 K−1 and a large refrigerant capacity (RC) of ∼687 J kg−1 over a large temperature interval (150 K). This RC value of the microwires is larger than those of the previously reported bulk and ribbon counterparts. An analysis of critical exponents reveals that γ = 1.246 ± 0.017 is close to that predicted by the 3D Ising theoretical model, while β = 0.723 ± 0.011 does not agree with any of the critical exponents predicted by the existing models. This can be attributed to the antiferromagnetic coupling between the rare earth (Gd) and the transition metal (Fe). The origins of the broad magnetic phase transition, the broad magnetic entropy change, and the large refrigerant capacity are discussed.

Experimental investigation on a novel phase change material composites coupled with graphite film used for thermal management of lithium-ion batteries

Thermal management system is a key component to maintain the performance of lithium-ion batteries in electric vehicles. Cooling technology based on phase change materials with single-phase transition range has been proposed by many researchers but the temperature control of batteries is not ideal for high charge/discharge rates and cycle tests. Herein, we designed a novel phase change material consisting of paraffin with dual-phase transition ranges (around 34 °C and 48 °C), expanded graphite (thermal conductivity = 40 W/m K), and epoxy resin with a mass ratio of 5:2:3. This material was combined with graphite film (in-plane thermal conductivity of 1400 W/m K) to prepare a thermal management module. Graphite film and expanded graphite form an excellent heat conduction structure. Epoxy resin endows this composite with satisfactory mechanical properties, even at 70 °C. The paraffin provides a double buffer effect that minimizes the temperature increase of the batteries and the temperature differences between the batteries in the pack due to the broad phase transition range. As a result, the maximum temperature of the batteries is 33 °C and the maximum temperature difference between the batteries is only 1.4 °C, even at the highest 4C discharge rate. In addition, the maximum temperature of the batteries is only 44.8 °C after six extreme cycles.

Thermochromic [formula omitted] nanocomposite smart window coatings with narrow phase transition hysteresis and transition gradient width

Thermochromic vanadium dioxide (VO2) window coatings hold the promise of reducing the energy consumption of the built environment by passively regulating solar heat gain in response to changing conditions. Composite materials with embedded VO2 particles have shown greatly improved optical performances compared with thin films, however they typically exhibit broadened phase transition hysteresis and gradient widths, which negatively impacts the overall performance. Here, we present a scalable one-step solution based synthesis for a thermochromic smart window coating based on a vanadium dioxide sol-gel containing silica (SiO2 nanoparticles. We compare the performance of our nanoparticle composite with thin film VO2 along with composites formed by mixing VO2 and SiO2 sol-gels and find that both composites achieve an acceptable visible transmittance (∼ 50%) along with a comparable and competitive solar modulation (12.5% and 16.8% respectively), roughly double that of the plain VO2 film (6.7%). However, our SiO2 nanoparticle containing composite also benefits from a narrow transition hysteresis and gradient width (9.4 ∘C and 2.9 ∘C respectively). We predict that this method may subsequently be combined with metal ion doping to control both the optical and phase transition characteristics to achieve composite films with high overall energy saving performances.

Broad Phase Transition of Fluorite‐Structured Ferroelectrics for Large Electrocaloric Effect

Field‐induced ferroelectricity in (doped) hafnia and zirconia has attracted increasing interest in energy‐related applications, including energy harvesting and solid‐state cooling. It shows a larger isothermal entropy change in a much wider temperature range compared with those of other promising candidates. The field‐induced phase transition occurs in an extremely wide temperature range, which contributes to the giant electrocaloric effect. This article examines the possible origins of a large isothermal entropy change, which can be related to the extremely broad phase transitions in fluorite‐structured ferroelectrics. While the materials possess a high entropy change associated with the polar–nonpolar phase transition, which can contribute to the high energy performance, the higher breakdown field compared with perovskites practically determines the available temperature range.

Ferromagnetic ordering along the hard axis in the Kondo lattice YbIr3Ge7

Ferromagnetic Kondo lattice compounds are far less common than their antiferromagnetic analogs. In this work, we report the discovery of a new ferromagnetic Kondo lattice compound, YbIr3Ge7. Like almost all ferromagnetic Kondo lattice systems, YbIr3Ge7 shows magnetic order with moments aligned orthogonal to the crystal electric field (CEF) easy axis. YbIr3Ge7 is unique in that it is the only member of this class of compounds that crystallizes in a rhombohedral structure with a trigonal point symmetry of the magnetic site, and it lacks broken inversion symmetry at the local moment site. AC magnetic susceptibility, magnetization, and specific heat measurements show that YbIr3Ge7 has a Kondo temperature TK = 14 K and a Curie temperature TC = 2.4 K. Ferromagnetic order occurs along the crystallographic [100] hard CEF axis despite the large CEF anisotropy of the ground state Kramers doublet with a saturation moment along [001] almost four times larger than the one along [100]. This implies that a mechanism which considers the anisotropy in the exchange interaction to explain the hard axis ordering is unlikely. On the other hand, the broad second-order phase transition at TC favors a fluctuation-induced mechanism.

Thermomagnetic properties and magnetocaloric effect of FeCoNiCrAl-type high-entropy alloys

In this work, we investigate the effects of substituting Ni/Al for Cr on the thermomagnetic and magnetocaloric properties of FeCoNiCrAl-type high entropy alloys (HEAs). Ni and Al appear to prefer the BCC phase, and increases in the Al composition appear to stabilize the BCC phase. In contrast to Al, Ni content yields an increase in the FCC phase fraction, resulting in a drop off in magnetization. The phase transformation from BCC to FCC was intensified at annealing temperatures of 800 °C and higher due to increased diffusion rates and the resulting spinodal decomposition. A magnetic phase transition around 150 K was found in the FeCoNi1.5Cr0.5Al annealed alloy potentially corresponding to the FCC phase, and a very broad magnetic phase transition was observed in the annealed FeCoNiCrAl alloy, resulting in a high refrigerant capacity of RCFWHM = 242.6 J⋅kg-1 near room temperature. A peak magnetic entropy change of −ΔSM = 0.674 J⋅kg-1⋅K-1 was also obtained at applied fields of ∼70 kOe at 290 K in the FeCoNiCrAl HEA. These magnetocaloric values are comparable to Fe-based metallic glasses such as Fe-Tm-B-Nb and Fe-Zr-B-Co alloys, with a similar transition near room temperature.

Improvement of Energy Storage Characteristics of (Ba0.7Ca0.3)TiO3 Thick Films by the Increase of Electric Breakdown Strength from Nano-Sized Grains

Lead free (Ba0.7Ca0.3) TiO3 thick films with nano-sized grains are prepared using an aerosol deposition (AD) method at room temperature. The crystallinity of the AD thick films is enhanced by a post annealing process. Contrary to the sharp phase transition of bulk ceramics that has been reported, AD films show broad phase transition behaviors due to the nanosized grains. The polarization-electric hysteresis loop of annealed AD film shows ferroelectric behaviors. With an increase in annealing temperature, the saturation polarization increases because of an increase in crystallinity. However, the remnant polarization and cohesive field are not affected by the annealing temperature. BCT AD thick films annealed at 700 °C/2h have an energy density of 1.84 J/cm3 and a charge-discharge efficiency of 69.9%, which is much higher than those of bulk ceramic with the same composition. The higher energy storage properties are likely due to the increase in the breakdown field from a large number of grain boundaries of nano-sized grains.

Dielectric and piezoelectric properties of Sm3+ doped lead barium niobate (PBN) ceramics

Rhombohedral samarium doped Lead based ceramic materials were fabricated by a high temperature conventional solid state mixed oxide ceramic process. The ceramic composition Pb1-x-3y/2BaxRey3+ Nb2O6 (x = 0.35 and y = 0.00, 0.02, 0.04 and 0.06, Re3+ = Sm3+). The pure and Sm3+ doped lead barium niobate ceramic materials are single phase with rhombohedral(hexagonal) symmetry obtained from powder x-ray diffraction (XRD). The grain size has been measured for all the compositions and lying between 1.60 and 2.90 μm. Broad dielectric peak phase transitions observed with temperature in pure and Sm3+ ion modified lead barium niobate ceramic materials. It is observed that the high values of piezoelectric charge coefficients in d31 and d33 in Sm3+ doped lead barium niobate ceramics. The mechanical quality factor Qm and d33 increases with increase of Sm3+ ion concentration and indicating a highly polarisable lead ion contributing to high Curie temperature dielectric constant.

Interpenetrating thermophobic and thermophilic dual responsive networks

ABSTRACTPoly(N‐acryloyl glycinamide) (PNAGA)/poly(N‐isopropyl acrylamide) (PNIPAAm) interpenetrating network (IPN) hydrogels were made by UV‐light initiated radical polymerization in two‐steps. The IPN hydrogels showed a double thermoresponsive behavior due to the combination of PNIPAAm (thermophobic) and PNAGA (thermophilic) networks. Increasing the content of the thermophobic PNIPAAm network leads to a change from a broad thermophilic volume phase transition temperature of PNAGA to a thermophilic–thermophobic‐type dual transition for the prepared IPN. Due to the double thermoresponsive character of the IPN gels, the mechanical properties are dependent upon temperature as demonstrated by performing tensile tests in water at 15 and 50 °C. Furthermore, the IPN hydrogels were characterized using turbidity measurements, SEM, and the determination of the equilibrium swelling ratio. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 539–544

Synthesis, structural, magnetic and magnetocaloric properties of La0.8Sr0.2MnO3 nanoparticles

In this work, we reported a detailed study on the synthesis, structural and magnetic properties of nanocrystalline La0.8Sr0.2MnO3. The synthesized nanoparticles were prepared using a sol–gel method and characterized using X-ray diffraction and high-resolution transmission electron microscope. The average particle size was found in the range from 40 to 45 nm. The magnetization versus temperature M(T) measurements as well as magnetization field dependence M(H) have been investigated using vibrating-sample magnetometer. The magnetization as a function of temperature M(T) indicated a broad second-order magnetic phase transition from ferromagnetic state to paramagnetic state in the Curie temperature region (320–340 K). The magnetocaloric effect of the sample has been estimated and presented a maximum magnetic entropy change |ΔSM|max = 0.86 J kg−1 K−1 with relative cooling power = 62.12 J kg−1 at magnetic field (H) = 2T. Based on the result of magnetocaloric properties, the investigated sample could be considered as a good refrigerant material for near room temperature magnetic refrigeration.

Dielectric and optical properties of Ni doped Na0.5Bi0.5TiO3

Polycrystalline Ni doped Na.5Bi0.5TiO3 samples (Na0.5Bi0.5)Ti1-xNixO3, (x = 0.5, 0.10, 0.15) have been prepared by solid state reaction. The appearance of the additional peak in X-ray diffraction pattern indicates the ordering of Ti4+ and Ni2+ ions. Polygonal grains are converted into flakes with an increase of Ni concentration. Replacement of Ti4+ by Ni2+ strongly modifies the relative contribution of two peaks in the Raman bands within 200–400 cm−1. Oxygen vacancy is observed in X-ray photoelectron spectrum to maintain charge neutrality due to aliovalent doping. Broad diffuse phase transition centered at the dielectric constant maximum indicates relaxor behaviour. Comparison between impedance and electric modulus spectrum suggests non-Debye relaxation. The ac conductivity follows the power law with the frequency exponent lies 0.52 − 0.72. The generation of holes by divalent Ni dopant at tetravalent Ti sites enhances optical band gap.

Multitemperature Memory Actuation of a Liquid Crystal Polymer Network over a Broad Nematic-Isotropic Phase Transition Induced by Large Strain.

The shape change of a polymer actuator based on liquid crystal network (LCN) generally occurs over a relatively sharp LC-isotropic phase transition. Reported herein is the discovery of an unusual phenomenon and the enabled actuation control for LCN. The smectic phase of a LCN with mesogenic moieties on the chain backbone can be suppressed by high elongation of the specimen, which gives rise to a broad nematic-isotropic phase transition. Consequently, the actuation force and related shape of the actuator can be activated to a given degree by easily varying the temperature over a wide range (35 K for LCN prepared with 500% strain) to adjust the proportion of the order-disorder phase transition. This reversible multitemperature memory actuation can translate into many stable and interconvertible shapes with one single LCN actuator.

Controlling the phase transition in nanocrystalline ferroelectric thin films via cation ratio.

Traditionally, the ferroelectric Curie temperature can be manipulated by chemical substitution, e.g., in Ba1-xSrxTiO3 as one of the archetypical representatives. Here, we show a novel approach to tune the ferroelectric phase transition applicable for nanostructured thin films. We demonstrate this effect in nano-grained BaTiO3 films. Based on an enhanced metastable cation solubility with Ba/Ti-ratios of 0.8 to 1.06, a significant shift of the phase transition temperature is discovered. The transition temperature increases linearly from 212 K to 350 K with increasing Ba/Ti ratio. For all Ba/Ti ratios, a completely diffused phase transition is present resulting in a negligible temperature sensitivity of the dielectric constant. Schottky defects are identified as the driving force behind the off-stoichiometry and the shift of the phase transition temperature as they locally induce lattice strain. Complementary temperature dependent Raman experiments reveal the presence of the hexagonal polymorph in addition to the perovskite phase in all cases. Interestingly, the hexagonal BaTiO3 influences the structural transformation on the Ba-rich side, while on the Ti-rich side no changes for the hexagonal polymorph at the ferroelectric transition temperature are observed. This concerted structural change of both polymorphs on the Ba-rich side causes a broad phase transition region spanning over a wide range up to 420 K including the transition temperature of 350 K obtained from dielectric measurements. These findings are promising for fine adjustment of the phase transition temperature and low temperature coefficient of permittivity.

Combined effect of oxygen annealing and La-doping in broadening the phase transition of Ba(Zr0.2Ti0.8)O3 ceramics

The present study focuses on a novel approach to broaden the phase transition in Ba(Zr0.2Ti0.8)O3 by the combined effect of oxygen annealing and La-doping. The (Ba1-xLax)(Ti0.8Zr0.2)O3, Pb-free ceramics, with nominal compositions (x = 0.00, 0.01, 0.02 and 0.03) are synthesized in oxygen annealing atmosphere at 1673 K for 11 h using solid-state reaction method. While the X-ray diffraction confirmed the single phase formation, the secondary electron images revealed inhibited grain growth at a low concentration of La-doping but intermediate 2-dimensional grain growth at higher La-doping. The Raman peak appearing at 836 cm−1 indicates La-substitution at Ba-site and vacancies at Ti-sites. The temperature dependent dielectric permittivity exhibits a broad diffuse phase transition at Tm = 230 K with a full width at half maxima (FWHM) of 289 K for 3 atomic % La-doped sample. The combined effect of oxygen annealing and La-doping results in an extremely broad diffuse phase transition. Such an extremely broad phase transition close to room temperature due to La-doping and oxygen annealing is useful for capacitor applications.

Development of Novel N-isopropylacrylamide (NIPAAm) Based Hydrogels with Varying Content of Chrysin Multiacrylate.

A series of novel temperature responsive hydrogels were synthesized by free radical polymerization with varying content of chrysin multiacrylate (ChryMA). The goal was to study the impact of this novel polyphenolic-based multiacrylate on the properties of N-isopropylacrylamide (NIPAAm) hydrogels. The temperature responsive behavior of the copolymerized gels was characterized by swelling studies, and their lower critical solution temperature (LCST) was characterized through differential scanning calorimetry (DSC). It was shown that the incorporation of ChryMA decreased the swelling ratios of the hydrogels and shifted their LCSTs to a lower temperature. Gels with different ChryMA content showed different levels of response to temperature change. Higher content gels had a broader phase transition and smaller temperature response, which could be attributed to the increased hydrophobicity being introduced by the ChryMA.

Volume Phase Transition Mechanism of Poly[di(ethylene glycol)ethyl ether acrylate]-Based Microgels Involving a Thermosensitive Poly(ionic liquid).

The microdynamic volume phase transition mechanism of poly[di(ethylene glycol)ethyl ether acrylate] (PDEGA)-based microgels with newly developed thermoresponsive polyionic liquid (PIL) (poly(tetrabutylphosphonium styrenesulfonate) P[P4,4,4,4][SS]) moieties was studied by applying temperature-variable Fourier transform infrared (FTIR) spectroscopy in combination with two-dimensional correlation spectroscopy (2Dcos) and the perturbation correlation moving window (PCMW) technique. It can be found that the content of hydrophilic PIL moieties plays a significant role in the thermally induced phase transition behavior of microgel systems; namely, the microgels containing fewer PIL moieties present a sharp transition behavior and a gel-like state (10%, w/v) in water whereas the microgels with more PIL moieties undergo a slightly broad phase transition process and a flowable solution state. Herein, the C═O···D2O-PIL hydrogen bonds as the interaction between PDEGA and P[P4,4,4,4][SS] moieties result in a complete dehydration process for the microgels with fewer PIL moieties and the dehydrated behavior of SO3- groups acts as the driving force during the phase transition. As for the microgels with more PIL moieties, the whole transition process is dominated by the hydrophobic interaction of C-H groups. Even though the intermolecular hydrogen bonds (C═O···D2O-PIL) appear as well, the more remarkable effect of the Coulombic repulsive force of PIL restrains the water molecules from breaking away, thus causing a gradual and incomplete dehydration process during heating.