Room Temperature Phase Transition Research Articles

Structural and magnetic properties of morphotropic phase boundary involved Tb1−xGdxFe2 compounds

In the present paper, structural, magnetic and magnetostrictive properties of Tb1−xGdxFe2 (0 ≤ x ≤ 1.0) were studied. Synchrotron x-ray diffraction (XRD) results show the non-cubic symmetry of Tb1−xGdxFe2 at room temperature and composition-induced crystallographic phase transition from rhombohedral phase to tetragonal phase. The Gd concentration dependent lattice parameters, lattice distortion and change of easy magnetic direction were detected by synchrotron XRD. With the Gd concentration increases, Curie temperature Tc increases while room temperature magnetization and magnetostriction coefficient λ111 and the anisotropy of TbFe2 decrease. The decrease in spontaneous magnetostriction coefficient λ111 with increasing Gd substitution can be understood on the basis of the single-ion model; the corresponding decrease of magnetostriction for Tb1−xGdxFe2, and the large magnetostriction value occurs on the Tb-rich side, are ascribed to decrease of λ111.

Variable emittance behavior of smart radiative coating

Smart radiative coating on yttria stabilized zirconia (YSZ) substrate was prepared by the sol-gel LaSrxMnO3 (x = 0.125, 0.175 and 0.2) nanoparticles and the binder composed of terpineol and ethyl cellulose. The crystallized structure, grain size, chemical compositions, magnetization and the surface morphology were characterized. The thermal radiative properties of coating in the infrared range was evaluated from infrared reflectance spectra at various temperatures. A single perovskite structure is detected in sol-gel nanoparticles with size 200 nm. Magnetization measurement reveals that room temperature phase transition samples can be obtained by appropriate Sr substitution. The influence of surface conditions and sintering temperature on the emittance of coating was observed. For rough coatings with root-mean-square roughness 640 nm (x = 0.125) and 800 nm (x = 0.175) , its emittance increment is 0.24 and 0.26 in in the temperature range of 173–373 K. Increasing sintering temperature to 1673 K, coating emittance variation improves to 0.3 and 0.302 respectively. After mechanical polishing treatment, the emittance increment of coatings are enhanced to 0.31 and 0.3, respectively. The results suggested that the emittance variation can be enhanced by reducing surface roughness and increasing sintering temperature of coating.

Holmium induced enhanced functionality at room temperature and structural phase transition at high temperature in bismuth ferrite nanoparticles

Ho-doped sample simultaneously exhibits high-coercivity and enhanced remnant magnetization with a polar R3c symmetry at room temperature. The onset of R3c to Pnma phase transition is observed at high temperatures in the Ho-doped samples.

Phase selective synthesis of quantum cutting nanophosphors and the observation of a spontaneous room temperature phase transition.

Oxygen-free Eu(3+)-doped NaGdF4 nanocrystals with high quantum cutting efficiency are accessible at low temperatures (room temperature to 80 °C) using task-specific ionic liquids (ILs) as structure directing agents and only water as solvent. Selective tuning of the shape, morphology and, most importantly, the crystal phase of the host lattice is achieved by changing the alkyl side length, the H-bonding capabilities and the concentration of 1-alkyl-3-methylimidazolium bromide ILs, [C(n)mim]Br. When using [C2mim]Br, hexagonal NaGdF4 nanoparticles are obtained. In the case of methylimidazolium bromides with longer pendant alkyl chains such as butyl (C4), octyl (C8) or decyl (C10), extremely small nanoparticles of the cubic polymorph form, which then convert even at room temperature (RT) to the thermodynamically favored hexagonal modification. To the best of our knowledge, this kind of spontaneous phase transition is not yet reported. The hexagonal nanomaterial shows a substantial quantum cutting efficiency (154%) whilst in the cubic material, the effect is negligible (107%). The easy yet highly phase selective green synthesis of the materials promises large scale industrial application in environmentally benign energy efficient lighting.

Vanadium Dioxide Nanoparticle-based Thermochromic Smart Coating: High Luminous Transmittance, Excellent Solar Regulation Efficiency, and Near Room Temperature Phase Transition.

An annealing-assisted preparation method of well-crystallized VxW1-xO2(M)@SiO2 core-shell nanoparticles for VO2-based thermochromic smart coatings (VTSC) is presented. The additional annealing process reduces the defect density of the initial hydrothermally prepared VxW1-xO2(M) nanoparticles and enhances their crystallinity so that the thermochromic film based on VxW1-xO2(M)@SiO2 nanoparticles can exhibit outstanding thermochromic performance with balanced solar regulation efficiency (ΔTsol) of 17.3%, luminous transmittance (Tlum) up to 52.2%, and critical phase transition temperature (Tc) around 40.4 °C, which is very promising for practical application. Furthermore, it makes great progress in reducing Tc of VTSC to near room temperature (25.2 °C) and simutaneously maintaining excellent optical properties (ΔTsol = 14.7% and Tlum = 50.6%). Such thermochromic performance is good enough to make VTSC applicable to practical architecture.

A review on the structure of cold-compressed graphite phase

The room-temperature structural phase transition of graphite at elevated hydrostatic pressure has drawn considerable scientific interest for its fundamental importance in condensed matter physics and materials science over the past few decades. A pressure-induced phase transition has been demonstrated in previous experiments by the measurement of electrical resistance, optical spectrum, X-ray diffraction spectrum, inelastic X-ray scattering and Raman spectroscopy. However, the nature of the cold-compressed graphite phase has been puzzling the experts and pioneers in the field of high pressure research due to some inherent factors, until recently a monoclinic structure, i.e. [Formula: see text]-carbon, stands out from the other structure candidates and successfully accounts for the crystal structure of the cold-compressed graphite phase both in theory and experiment that eventually putting an end to this long-lasting controversial issue. This paper reviews the recent progress on the pressure-induced phase transitions of graphite at room temperature especially for the theoretical investigations. The review will focus on the recent proposed novel carbon allotropes as candidate structures of the cold-compressed graphite phase by using different crystal structure prediction methods. The history of structure determination of cold-compressed graphite phase is discussed.

High Solubility of Hetero-Valence Ion (Cu2+) for Reducing Phase Transition and Thermal Expansion of ZrV1.6P0.4O7

Large thermal expansion at room temperature and high phase transition temperature of ZrV2O7 limit its practical applications and are reduced by the high solubility of hetero-valence ion (Cu2+) on the basis of an equal-valence substitution of P5+ for V5+. The temperature-dependent Raman spectra show that Zr0.9 Cu0.1 V1.6 P0.4 O6.9 maintains a normal parent cubic structure till 173K and transforms to a 3 × 3 × 3 cubic superstructure below 173K. Temperature dependent x-ray diffraction patterns of Zr0.9 Cu0.1 V1.6 P0.4 O6.9 show near zero and negative thermal expansion. High solubility of lower valence Cu2+ relates to an equal-valence substitution of smaller P5+ for V5+, which extends the bond angle of V(P)—O—V in ZrV1.6 P0.4 O7 close to 180°. The change of microstructure is considered to be responsible for reduced phase transition temperature and thermal expansion.

Switchable dielectric behaviour associated with above room-temperature phase transition in N-isopropylbenzylammonium dichloroacetate (N-IPBADC)

An above room-temperature phase transition material, N-isopropylbenzylammonium dichloroacetate, undergoes a phase transition and exhibits switchable dielectric behaviour.

Effect of polytetrafluoroethylene (PTFE) phase transition at 19°C on the use of Spectralon as a reference standard for reflectance

Sintered polytetrafluoroethylene (PTFE) is highly reflective and is widely used as a reference standard in remote sensing, radiometry, and spectroscopy. The relative change in output flux from a PTFE integrating sphere over the room temperature phase transition at 19°C has been measured at a monochromatic wavelength of 633 nm as 1.82±0.21%. The change in output flux was attributed to a small change of 0.09±0.02% in the total hemispherical reflectance of PTFE, caused by a change in its material density as a result of the phase transition. For the majority of users, this small change measured in total hemispherical reflectance is unlikely to impact significantly the accuracy of PTFE flat panel reflectors used as reference standards. However, owing to the multiple reflections that occur inside an integrating sphere cavity, the effect is multiplied and remedial action should be applied, either via a mathematical correction or through temperature stabilization of the integrating sphere when high accuracy (<5%) measurements of flux, irradiance, or radiance are required from PTFE-based integrating spheres at temperatures close to the phase transition at 19°C.

Tungsten-doped vanadium dioxide thin films on borosilicate glass for smart window application

Tungsten-doped VO2 (W–VO2) thin films with low metal–semiconductor transition temperature (Tt) of 34°C were grown on borosilicate glass substrates by reactive ion beam sputtering at room-temperature (RT) followed by a post annealing process. In order to investigate the thermal process effect, four samples S1–S4 were annealed at different temperatures of 490, 530, 570 and 610°C, respectively. X-ray photoelectron spectroscope and Raman measurements demonstrate that the tungsten atoms have been successfully doped into VO2 films. Unlike VO2 thin films which should be annealed at critical temperature, well crystallized W–VO2 films can be annealed in a wide temperature range, i.e. 530–570°C, as revealed by both electrical and optical property investigations. Besides, the infrared (IR) transmittance measured below/above Tt illustrates that W–VO2 films possess excellent switching efficiency, i.e. 33% at 2500nm for S3. Above all, the near RT phase transition, RT deposition, easy control of annealing process and high IR switching efficiency make the W–VO2 thin film a promising material for application of smart windows.

Phonon-Limited Transport in Graphene Pseudospintronic Devices

A predicted room-temperature phase transition from Fermi liquid to dissipationless Bose-Einstein exciton superfluid suggests that graphene pseudospin devices may have the potential to far outperform traditional CMOS devices. When examining the possibility of a room-temperature exciton condensate, it is important to consider scattering of charge carriers by phonons in each of the constituent graphene monolayers. Using the nonequilibrium Green's function formalism, we examine the effect that carrier-phonon scattering has on device performance. We find that the effect of carrier-phonon scattering has strong dependence on the device coherence length. As such, for large gate voltages, the effect of phonons on interlayer transport is negligible.

LARGE ELECTROCALORIC EFFECT IN RELAXOR FERROELECTRICS

Organic and inorganic relaxor ferroelectrics used for electrocaloric effect (ECE) applications are introduced. Relaxor ferroelectrics offer several advantages for ECE devices, e.g., infinite states without applying electric field, field-induced large polarization, no-hysteresis of heating and cooling, small-hysteresis polarization loss, room temperature phase transition, and broad temperature range. The ECE in relaxor ferroelectrics under a high electric field can be described using a theory similar to that for first-order phase transition materials. Large ECE was observed directly in high-energy electron irradiated poly(vinylidene fluoride–trifluoroethylene) (P(VDF–TrFE)) 68/32 mol% copolymers, P(VDF–TrFE–CFE) (CFE-chlorofluoroethylene) 59.2/33.6/7.2 mol% terpolymers, P(VDF–TrFE–CFE)–P(VDF–CTFE) (CTFE-chlorotrifluoroethylene) 95/5 wt% terpolymer blended films, and (PbLa)(ZrTi)O3 (PLZT) ceramic thin films. ECE reported in Pb(Sc1/2Ta1/2)O3 (PST), Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN–PT) thin films is also summarized. Finally, the perspective of ECE devices is illustrated.

Role of room-temperature phase transition in the electrical properties of (Ba, Ca)(Ti, Zr)O 3 ceramics

In order to shift the tricritical point to room temperature, Ba 0.85 Ca 0.15 Ti 0.90 Zr 0.10 O 3 (BCTZ) ceramics were prepared by normal sintering with the addition of ZnO. The sintering behavior of BCTZ ceramics is also improved by using ZnO as a sintering aid. The room-temperature tricritical point results in improved electrical properties of ZnO-modified BCTZ ceramics. BCTZ ceramic with 0.06 mol.% ZnO demonstrates good piezoelectric properties : d 33 ∼ 521 pC N −1 and k p ∼ 47.8%.

Thermal stability of habit modified ammonium nitrate: Insights from isoconversional kinetic analysis

Compared to the conventional ammonium perchlorate based solid propellants, burning of ammonium nitrate (AN) based propellants produce environmentally innocuous combustion gases. However, the near room temperature phase transition limits the application of AN as a propellant ingredient. A method to produce phase modified ammonium nitrate (PMAN) was reported by means of habit modification route using potassium ferrocyanide. In the present study, the thermal stability and moisture absorption characteristics of PMAN were evaluated to understand the viability of its application in the environmentally benign propellant systems. The energy of activation of decomposition reaction of AN, PMAN and an admixture of AN-potassium ferrocyanide trihydrate were computed by Friedman's differential and Vyazovkin's non-linear integral isoconversional methods and used to ascertain their thermal stability. The moisture absorption characteristics were determined at 53 and 62% RH and the cyclic DSC analysis, combination of non-isothermal and isothermal, was used to trace the phase behaviour of the PMAN. The study indicates that PMAN and AN has comparable properties suggesting similar processing and handling conditions.

Above room temperature heat capacity and phase transition of lithium tetrahydroborate

New calorimetric determinations of molar heat capacity C p,m of lithium tetrahydroborate (LiBH 4) were performed in order to analyze the origin of the previously observed “anomaly” before the polymorphous transition at 386 K. The above room temperature dependence of LiBH 4 C p,m was measured until approaching the melting point of the compound at 553 K and the abnormal behaviour was attributed to lattice defects independent of the main crystalline phase transition. As a result, lower entropy of the transition Δ trs S = 13.11 ± 0.23 J K −1 mol −1 than that proposed in literature was obtained, which is in agreement with the observed anisotropy and crystal density decrease before the transition from recent X-ray diffraction indexing. An estimate of the liquid C p,m of LiBH 4 “molten salt” is proposed.

Raman Spectroscopy Measurements of the Pressure−Temperature Behavior of LiAlH4

The pressure/temperature phase diagram of LiAlH4 has been constructed by using Raman spectroscopy data. In situ high pressure−temperature experiments were carried out using resistively heated diamond anvil cells up to 150 °C and 7 GPa. Room temperature phase transitions of monoclinic α-LiAlH4 → δ-LiAlH4 were observed at ∼3.2 GPa. As the temperature is increased to ∼100 °C, both the α and δ phases transform to β-LiAlH4 and remain stable up to 5.5 GPa. At temperatures greater than 300 °C, a new γ-LiAlH4 phase forms. Data of Konovalov (1995) has been used to define the phase boundary between β- and γ-LiAlH4 phases. We present a pressure−temperature phase diagram of LiAlH4 based using diamond anvil cells coupled with Raman spectroscopy.

Synthesis of a metal oxide with a room-temperature photoreversible phase transition

Photoinduced phase-transition materials, such as chalcogenides, spin-crossover complexes, photochromic organic compounds and charge-transfer materials, are of interest because of their application to optical data storage. Here we report a photoreversible metal-semiconductor phase transition at room temperature with a unique phase of Ti(3)O(5), lambda-Ti(3)O(5). lambda-Ti(3)O(5) nanocrystals are made by the combination of reverse-micelle and sol-gel techniques. Thermodynamic analysis suggests that the photoinduced phase transition originates from a particular state of lambda-Ti(3)O(5) trapped at a thermodynamic local energy minimum. Light irradiation causes reversible switching between this trapped state (lambda-Ti(3)O(5)) and the other energy-minimum state (beta-Ti(3)O(5)), both of which are persistent phases. This is the first demonstration of a photorewritable phenomenon at room temperature in a metal oxide. lambda-Ti(3)O(5) satisfies the operation conditions required for a practical optical storage system (operational temperature, writing data by short wavelength light and the appropriate threshold laser power).

Effects of polymorphism on charge transport in organic semiconductors

The increasing interest in fluorinated 5,11-bis(triethylsilylethynyl)anthradithiophene is motivated by the demonstrated high-performance organic field-effect transistors and circuits based on this organic semiconductor, complemented by reduced complexity processing methods that enable this performance. We identify two polymorphs of this material and report on their crystal structure, formation, and the effect of the different molecular packings on the electronic properties. The polymorphs are interconvertible through a phase transition that occurs at $T=294\text{ }\text{K}$. We study the variations in the electrical properties as a response to the structural changes induced by the phase transition in both single crystals and thin films, and discuss the technological implications that a room-temperature phase transition has on the performance and stability of devices fabricated with this organic semiconductor.

Phase Characteristics of a U-22Pu-4Am-2Np-40Zr Metallic Alloy Containing Rare Earths

Metallic fuel alloys consisting of uranium (U), plutonium (Pu), and zirconium (Zr) with minor additions of americium (Am) and neptunium (Np) are under evaluation for potential use to transmute long-lived transuranic actinide isotopes in fast reactors. The current irradiation test series design, designated Advanced Fuel Cycle-2 (AFC2), includes minor additions of rare earth (RE) elements to simulate expected fission product carryover from the electrochemical molten salt reprocessing technique. The as-cast fuel alloys have been investigated for phase and thermal properties; specifically, enthalpies of transition, transition temperatures, and room temperature phase characteristics. Results and observations related to these characteristics for the “fresh” fuel alloys are provided. The alloy compositions are based on a U-22Pu-4Am-2Np-40Zr alloy, along with additions of 1.3 and 1.9 at.% RE (at the expense of uranium where RE denotes rare earth alloy of cerium, lanthanum, praseodymium, and neodymium). Phase behavior and associated transitions have been compared to available U-Pu-Zr ternary diagrams with acceptable agreement. Enthalpies of transition were deconvoluted from heating and cooling thermal traces for relatively reliable values. The RE additions to the base alloy have a minimal influence on the room temperature phases present and phase transition temperatures, but the room temperature phases present did impact the enthalpies of transition.

Phase transitions and corresponding magnetic entropy changes in Ni2Mn0.75Cu0.25−xCoxGa Heusler alloys

Detailed studies of room temperature crystal structures, phase transitions, and related magnetic entropy changes (ΔSm) in shape memory alloys Ni2Mn0.75Cu0.25−xCoxGa (x=0.0, 0.01, 0.02,0.025 0.03, 0.05) have been carried out by x-ray diffraction, magnetization, and thermal expansion measurements in magnetic fields of up to 5 T and in a temperature interval of 5–400 K. The high temperature austenitic cubic phase passes through a magnetic transition to ferromagnetic state and a structural transition to martensitic phase at the same temperature for all samples of the Ni2Mn0.75Cu0.25−xCoxGa system. The first order magnetostructural transition temperature increases from 308 to 345 K with increasing Co concentration. All of the alloys in the Ni2Mn0.75Cu0.25−xCoxGa system were found to possess large magnetic entropy changes. The maxima in the magnetic entropy changes ranged from ΔSmmax=−48 J/kg K to −64 J/kg K in a temperature range of 308–345 K.