Phase Transition Hysteresis Research Articles

Large phase-transition hysteresis for nanostructured VOx film prepared on ITO conductive glass by DC reactive magnetron sputtering

Nanostructured vanadium oxide (nano-VOx) films were prepared on indium-tin oxide (ITO) glass substrate at low temperature by means of direct current (DC) reactive magnetron sputtering from pure vanadium target in Ar + O2 atmosphere. Field emission scanning electron microscope (SEM) reveals that the VOx film is composed of spheroidal nanoparticles whose diameters are in the range of 20–40 nm. This nano-VOx film shows a broad hysteresis loop whose width is as large as 41.6 °C. Moreover, the metal-insulator transition (MIT) can also be triggered by Joule heat produced by electrical current through the ITO sublayer. Compared to traditional heating of the sample by heating plate, this Joule heating is more efficient and convenient, which enables potential applications of this nano-VOx film on ITO conductive glass in compact storage devices.

Temperature dependent morphological evolution and the formation mechanism of anisotropic nano-aggregates from a crystalline-coil block copolymer of poly(p-dioxanone) and poly(ethylene glycol)

The morphological evolution and phase transition of a branched crystalline-coil multi-block copolymer, poly(p-dioxanone)-block-poly(ethylene glycol) (PPDOstar-b-PEG), in aqueous solution under heating and cooling were investigated. The changes in size and morphology of the nano-aggregates were monitored by dynamic light scattering (DLS), transmission electron microscopy (TEM) and atomic force microscopy (AFM). A semitransparent and uniform dispersion of nano-aggregates with star anise-like morphology was obtained from PPDOstar-b-PEG at room temperature. The dispersion gradually turned transparent during heating to 80 °C because of the melting of the crystallized PPDO blocks. The crystals with low regularity melted first leading to dissociation of the star anise nano-aggregates to flake-like particles. The copolymer formed sphere-like micelles when the temperature was high enough for melting all PPDO crystals. During the cooling run, a hysteresis of phase transition was observed because of the supercooling of crystallization. The morphological evolution of the copolymer micelle suggested that the formation of the star anise-like nano-aggregates was a hierarchical assembly process. A “crystallization induced hierarchical assembly” mechanism was therefore proposed to explain the formation of the star anise-like nano-aggregates. Metastable flake-like nano-particles formed at the initial stage of crystallization of PPDO blocks. The hydrophobic core of the flake was composed of several crystal lamellae or plates piled up in a layer-by-layer fashion. With further crystallization of PPDO blocks, the flakes tended to aggregate because of the variation of the hydrophilic–hydrophobic balance. The active edge of crystalline lamellae in the hydrophobic core of one flake may induce two different growth modes: epitaxial growth with amorphous spherical micelles and interparticle interpenetration crystallization in the amorphous region of other flakes. The branched structure of the nano-particles was therefore formed driven by interparticle interpenetration crystallization and epitaxial crystallization simultaneously.

Hysteresis of Field-Induced Ferroelectric Transition in Pb(Mg1/3Nb2/3)O3 Relaxor Ferroelectrics

The electric field induced ferroelectric transition was studied by polarization and quasi-isothermal measurements in relaxor Pb(Mg1/3Nb2/3)O3 (PMN) single crystal oriented in [110] direction. The polarization hysteresis loops and isothermal response of PMN exhibit a hysteresis at glassy to ferroelectric transition. Hysteresis effects are much stronger at lower temperatures indicating that the slowing glassy dynamics is mainly responsible for the phase transition hysteresis, and that the nonlinear effects related to electric-field induced ferroelectric domain switching become important only at higher temperatures close to the critical point.

Liquid–solid–liquid phase transition hysteresis loops in the ionic conductivity of ten imidazolium-based ionic liquids

This paper extends previous ionic conductivity measurements to the phase transition between liquid to solid states, where we present measurements for increasing and decreasing temperatures. Data show the existence of hysteresis loops for some ILs, while others do not present any transition (at least it is not measurable). Seven of the studied ILs have 1-ethyl-3-methyl imidazolium (EMIM) as a common cation, which allows us to observe the anion influence, and four IL compounds have tetrafluoroborate (BF4−) as common anion and the 1-alkyl-3-methyl imidazolium as cation with four different alkyl chains (ethyl, decyl, dodecyl and hexadecyl). The hysteresis loop differs in shape and amplitude for the different IL compounds presented here, thus some of them present a closed hysteresis loop, without any jump in the value of ionic conductivity, while other ILs have a sharp decrease of the conductivity at a given temperature, and so they present open hysteresis loops. Those hysteresis loops are presented here for the first time and it is shown that they are different for the studied ILs. These measurements demonstrate the capacity of the ILs to maintain its liquid state characteristics (being so in a super cooled liquid state) when temperature decreases well below its melting point (up to 60K for some compounds). We explain it as resilience of the pseudolattice structure already present in liquid state to become a rigid crystalline structure, typical of these ILs at solid state.

Phase transition hysteresis of ferroelectric Sr5EuTi3Nb7O30 ceramics with tetragonal tungsten bronze structure

The first-order ferroelectric transition of Sr5EuTi3Nb7O30 tungsten bronze ceramics was investigated together with the thermal history dependence of dielectric properties and differential scanning calorimetry (DSC) analysis. Moreover, the diffuse dielectric maximum appeared at a temperature above the ferroelectric transition, and this phenomenon had been reported in ferroelectrics with perovskite structure, but had never been observed in tungsten bronzes. Large hysteresis in the ferroelectric transition and severe depression of T0 were observed in both the permittivity-temperature and DSC curves, and were attributed to the complex nature of the tetragonal tungsten bronze structure. The DSC measurements were also used to get better insight into the gradual stability of the ferroelectric phase after cooling from the high temperature paraelectric phase. The abnormal splitting of the endothermic peaks was observed during the DSC measurements with lower start temperature or longer aging time. Two possible mechanisms were suggested. One was the separation of ferroelectric phase due to the compositional inhomogeneity. In another possible mechanism, the polar nanoregions coexisting with the ferroelectric domains were suggested and would possibly transit into the paraelectric phase at temperature below the ferroelectric transition.

Tuning the upper critical solution temperature behavior of poly(methyl methacrylate) in aqueous ethanol by modification of an activated ester comonomer

A statistical copolymer of methyl methacrylate (MMA) and pentafluorophenyl methacrylate (PFPMA, 6 mol%) exhibits upper critical solution temperature (UCST) behavior in aqueous ethanol solutions tunable by post-polymerization modification with different amines. The phase transition behavior of the obtained copolymers in aqueous ethanol was evaluated in detail. As expected, all copolymers reveal an upper critical solution temperature with 55 vol% or higher ethanol content. Furthermore, the solubility in aqueous ethanol of the copolymer can be increased by the introduction of hydrophilic moieties. When hydrophobic substituents are introduced a decrease in solubility was observed with low content of ethanol and an increase in solubility when adding more ethanol. As such, the thermoresponsive behavior of PMMA can be significantly altered by post-modification of 6 mol% of the reactive comonomer units. The hysteresis of the polymer phase transitions between heating and cooling was found to be strongly dependent on the polarity of the amine substituent and the ethanol/water ratio. The metastability of the hysteresis is addressed based on isothermal turbidity studies.

Effects of chemical composition and stereoregularity on phase-transition behaviors of aqueous solutions of copolymers composed of N-isopropylacrylamide and N- n-propylacrylamide

Radical copolymerizations of N-isopropylacrylamide (NIPAAm) and N- n-propylacrylamide (NNPAAm) in various ratios were carried out in toluene at −40 °C in the presence of 3-methyl-3-pentanol to prepare syndiotactic copolymers with racemo dyad contents of ca. 70%. It was revealed that copolymers containing more than 92.5 mol% NNPAAm units exhibited large phase-transition hysteresis of their aqueous solutions. Sequence analysis suggested that intramolecular hydrogen-bonding of contiguous NNPAAm units in syndiotactic stereosequences in the dehydrated state were responsible for induction of the large hysteresis.

Time-dependent phase behavior of 5CB-DDAB-water microemulsions monitored by means of dielectric spectroscopy

The phase behavior of an inverse microemulsion composed of 4-cyano-4-n-pentylbiphenyl (5CB), didodecyl dimethyl ammonium bromide, and water was intensively studied recently, but controversy still remains on the existence of a "transparent nematic" (TN) phase. Previously, we studied the temperature-dependent dielectric behavior of this microemulsion [Z. Chen and R. Nozaki, J. Chem. Phys. 134, 034505 (2011)]. The results suggested that a superstructure composed of water droplets and confined 5CB molecules exists in the coexistence phase region, which may correspond to the notion of a TN phase. In this study, the time-dependent dielectric behaviors of this microemulsion in the isotropic, coexistence, and supercooled phase regions were investigated. In the coexistence and supercooled phase regions, the dielectric behaviors showed strong dependence on time, due to the hysteresis of phase transition and the upward diffusion of the water-droplet-rich isotropic phase. After the isotropic phase has been totally excluded out of the effective measurement area, the slow process was still observed in these two phase regions, which suggests that water droplets can stably exist in the liquid crystalline phase. A local microscopic phase evolution was observed in the coexistence phase region, which is similar to the temperature-induced phase evolution previously proposed by us. The dielectric behavior of the slow process during the phase evolution also suggests the existence of the superstructure. In addition, the mechanism of the slow process in different phase regions proposed in our previous study is also confirmed.

Preparation of Ta-Ti Co-Doped VO<sub>2</sub> Polycrystal Thin Film with High Resistance Temperature Coefficient and without Hysteresis

Using vanadyl acetylacetonate (C10H14O5V) as precursor, use Tantalum Ethoxide (Ta(OC2H5)5) and Tetrabutyl titanate （C16H36O4Ti）as doper, by sol-gel method fabricate Ta, Ti mono doping and Ta-Ti co-doped V1-x-yTaxTiyO2 thin film. XRD spectrum indicated that the film was oriented in (011) direction. XPS results indicated the valence state of V, Ta, Ti in the film is +4, at all. While Ta mono doping, single 1at.%Ta can deduce the phase transiton temperature (Tt) by 7.8°C, phase transition hysteresis (ΔT) by 1°C. When the doping rate is 6at.%, Tt=22°C, ΔT=1°C. Ti dopings has little affection to Tt but deduce ΔT obviously. Ta-Ti co-doped V0.93Ta0.06Ti0.01O2 film thin films without phase transition hysteresis were also fabricated, and its TCR is as high as -7.58%/K at 25°C.

The ferroelectric properties of films with defect layers

Using the transverse Ising model and the effective field theory based on the probability distribution technique, which accounts for the single-site spin correlations, the phase transition temperature, polarization, susceptibility and hysteresis loops for a ferroelectric (FE) film with defect layers are studied. It is shown that the defect layers in the FE film can induce a strong increase or decrease of the critical temperature of the phase transition due to different exchange interactions in the defect layers, which is connected with the values of the exchange interactions that depend on the distance between the spins. Considering the fact that the interactions can be different in defect layers compared to layers without defects, we have shown that the coercive field decreases in the case of different numbers of defect layers. The results obtained are in qualitative agreement with experimental data on ferroelectric films with different thicknesses.

Significant changes in phase-transition hysteresis for Ti-doped VO 2 films prepared by polymer-assisted deposition

This study is one of a series of researches on polymer-assisted deposition (PAD) of thermochromic VO 2 film. This paper describes the synthesis of Ti-doped VO 2 films (V 1− x Ti x O 2, x=0–0.167) and a systematic investigation of the effects of Ti doping on morphology, crystalline phase and optical properties (visible transmittance, transmittance changes in near infrared (NIR) light across the metal–insulator transition (MIT), absorption edge, MIT temperatures and sharpness) of V 1− x Ti x O 2 films. The films showed excellent visible transmittance and large changes (∼50% at 2000 nm) in the NIR-light transmittance before and after MIT for samples with a wide range of Ti contents (0–10%). The width of the hysteresis loop was severely reduced, which is in agreement with a qualitative analysis using the nucleation theory, while the MIT sharpness was not obviously influenced. An increase in the MIT temperature was observed for all samples with Ti doping, in particular 18.5 °C for 2% Ti doping, which is quite large compared with those in the literature. However, the MIT seemed saturated at around 80 °C. The morphology evolution of Ti-doped VO 2 films was investigated for the first time. Interestingly, Ti doping could remarkably reduce the particle size of VO 2 films due to Ti enhanced heterogeneous nucleation process of VO 2 particles, indicating a new method for the growth of nanostructured VO 2 films.

Constitutive modeling of shock response of phase-transforming and porous materials with strength

The paper analyzes constitutive behavior of phase-transforming materials and metal powders loaded by shock waves. A two-phase material model developed earlier is extended to the case of materials with strength. Available experimental free surface velocities of iron samples under the α-ε phase transition and shock velocity data for porous aluminum and copper are analyzed numerically with the present model. The phase transition hysteresis is demonstrated by numerical analysis of experimental free surface velocities. Possible role of the martensitic mechanism in the hysteresis is emphasized. The calculated anomalous Hugoniots as well as pressure equilibrium (PE) and pressure-temperature equilibrium (PTE) Hugoniots are compared with available experiments for aluminum and copper powders. It is argued that a large nonequilibrium interphase heat transfer zone for aluminum powders results in a scatter of experimental Hugoniot points between the PE and PTE Hugoniots in low pressure region. In turn, in high pressure region, the effect of strength on porous Hugoniot may increase with porosity due to an extra dissipation, which improves description of experiments for an extremely porous aluminum.

Modeling the competing phase transition pathways in nanoscale olivine electrodes

Recent experimental developments reveal that nanoscale lithium iron phosphate (LiFePO 4) olivine particles exhibit very different phase transition behavior from the bulk olivine phase. A crystalline-to-amorphous phase transition has been observed in nanosized particles in competition with the equilibrium phase transition between the lithium-rich and lithium-poor olivine phases. Here we apply a diffuse-interface (phase-field) model to study the kinetics of the different phase transition pathways in nanosized LiFePO 4 particles upon delithiation. We find that the nucleation and growth kinetics of the crystalline-to-crystalline and crystalline-to-amorphous phase transformations are sensitive to the applied electrical overpotential and particle size, which collectively determine the preferred phase transition pathway. While the crystalline-to-crystalline phase transition is favored by either faster nucleation or growth kinetics at low or high overpotentials, particle amorphization dominates at intermediate overpotentials. Decreasing particle size expands the overpotential region in which amorphization is preferred. The asymmetry in the nucleation energy barriers for amorphization and recrystallization results in a phase transition hysteresis that should promote the accumulation of the amorphous phase in electrodes after repeated electrochemical cycling. The predicted overpotential- and size-dependent phase transition behavior of nanoscale LiFePO 4 particles is consistent with experimental observations.

Ferroelectric phase transitions in ionic conductors based on bismuth vadanate

Ceramic solid solutions (Bi1 − x La x )4V2O11 − z (I), Bi4(V1 − x Fe x )2O11 − y (II), and (Bi1 − x La x )4(V0.96Fe0.04)2O11 − y (III) (x = 0–0.3, step Δx = 0.02) are prepared using solid-phase synthesis. The concentration and temperature ranges of stabilization of different polymorphic modifications, including the ranges of concentrations x corresponding to the stabilization of the ferroelectric phase, are established. It is revealed that an increase in the concentration x in the region of existence of the pseudoorthorhombic phase α of the solid solutions studied leads to a decrease in the transition temperature, smearing of the transition, and an increase in the width of the thermal hysteresis of the ferroelectric phase transition. The effect of compressing of the domain walls by oxygen vacancies was revealed in the samples from the region of existence of the ferroelectric α phase, and the effect of dielectric relaxation was detected in the samples from the region of existence of the orthorhombic phase β.

The 1-by-3 Tandem Differential Mobility Analyzer for Measurement of the Irreversibility of the Hygroscopic Growth Factor

A new instrument, namely the 1 × 3 tandem differential mobility analyzer (1 × 3-TDMA), has been developed. Its primary measurement is the irreversibility of the hygroscopic growth factor of aerosol particles. The instrument uses the hysteresis of phase transitions to infer the solid or aqueous state of the particles. A first DMA passes particles of a selected electric mobility at relative humidity RH0. Exiting this DMA, the particles are split into three separate flows. The first flow is exposed to RH 0 → (RH 0 + δ ) → RH 0 in a deliquescence test before passing through a second DMA that is set to the same electric mobility as the first DMA. The second flow passes directly to a third DMA without change in RH, thereby serving as a reference arm. This DMA is also set to the same electric mobility as the first DMA. The transmission ratio of the 1 × 3-TDMA is defined as the particle concentration passing the deliquescence test divided by that passing through the reference arm. The transmission ratio is unit...

Nucleation and hysteresis of vapor-liquid phase transitions in confined spaces: Effects of fluid-wall interaction

In this work, we propose a method to stabilize a nucleus in the framework of lattice density-functional theory (LDFT) by imposing a suitable constraint. Using this method, the shape of critical nucleus and height of the nucleation barrier can be determined without using a predefined nucleus as input. As an application of this method, we study the nucleation behavior of vapor-liquid transition in nanosquare pores with infinite length and relate the observed hysteresis loop on an adsorption isotherm to the nucleation mechanism. According to the dependence of hysteresis and the nucleation mechanism on the fluid-wall interaction, w , in this work, we have classified w into three regions ( w>0.9 , 0.1< or =w< or =0.9 , and w<0.1 ), which are denoted as strongly, moderately, and weakly attractive fluid-wall interaction, respectively. The dependence of hysteresis on the fluid-wall interaction is interpreted by the different nucleation mechanisms. Our constrained LDFT calculations also show that the different transition paths may induce different nucleation behaviors. The transition path dependence should be considered if morphological transition of nuclei exists during a nucleation process.

Semiconductor-metal Phase Transition in Doped Ion Beam Synthesized VO2 Nanoclusters

AbstractWe have synthesized W and Mo doped VO2 nanoclusters embedded in 200 nm thick thermally grown SiO2 on 4-inch silicon wafers by sequential ion implantation of the elements V, W, Mo and O. The implantation energies have been chosen to locate the mean projected range in the centre of the SiO2 thin film. A post implantation rapid thermal annealing step in flowing Ar at 1000°C for 10 min leads to the growth of doped VO2 nanoclusters. The optical properties of the nanoclusters were analyzed by temperature dependent spectral ellipsometry in the spectral range of 320 to 1700 nm. It will be shown, that the semiconductor-metal phase transition hysteresis width starting at 50K in the undoped case can be systematically closed by increasing dopand concentration.

Sensitivity of sulfate direct climate forcing to the hysteresis of particle phase transitions

The effects of solid‐aqueous phase transitions on sulfate direct climate forcing (SDCF) are investigated by using both a column model and a global chemical transport model. Aqueous particles have a larger mass extinction efficiency but a smaller backscattered fraction than their solid counterparts. The column model shows that the hysteresis of the phase transition can result in an uncertainty in the SDCF of 20%. The global chemical transport model explicitly accounts for the relative humidity processing of particles and the associated hysteresis. The model also treats the extent of sulfate neutralization by ammonia. For the anthropogenic sulfate, the base case simulation finds that solid particles contribute 41% of the global burden, 26% of the clear‐sky optical thickness, 31% of the clear‐sky SDCF, and 37% of the full‐sky SDCF, a trend that reflects the correlation of solid particles with clear skies. A perturbation to the model, omitting hysteresis by assuming that all particles are aqueous, results in an overestimate of the SDCF by +8% compared to the base case. A converse assumption that crystallization occurs at the deliquescence relative humidity underestimates the SDCF by −8%. A case that assumes that aqueous particles occur whenever the ambient relative humidity exceeds the crystallization relative humidity biases the SDCF by +5%. A case that includes hysteresis but omits the difference in the fraction of radiation backscattered to space by aqueous compared to solid particles changes the SDCF by +15%. Seasonal and regional differences can be much larger. We recommend that the ratio of the sulfate aerosol optical thickness calculated with versus without consideration of particle hygroscopicity be reported as a standard output of SDCF models to facilitate meaningful intercomparisons among different models.

Global distribution of solid and aqueous sulfate aerosols: Effect of the hysteresis of particle phase transitions

The partitioning between solid and aqueous phases of tropospheric sulfate‐ammonium particles is simulated with a global 3‐D chemical transport model (CTM). The simulation explicitly accounts for the hysteresis of particle phase transitions by transporting aqueous sulfate and three solid sulfate forms (namely, ammonium sulfate, letovicite, and ammonium bisulfate). Composition‐dependent deliquescence relative humidities (DRH) and crystallization relative humidities (CRH) are based on recent laboratory data. We find that the solids mass fraction on a sulfate basis is 0.34, partitioned as 93% ammonium sulfate, 6% letovicite, and 1% ammonium bisulfate. The fraction increases with altitude from 0.10 to 0.30 in the boundary layer to 0.60–0.80 in the upper troposphere. The dominance of solids in the upper troposphere arises in part from high sulfate neutralization, reflecting in our simulation a low retention efficiency of NH3 upon cloud freezing. High sulfate neutralization is consistent with the few available observations in the upper troposphere. High acidity with a dominant aqueous phase, however, can occur following volcanic eruptions. Seasonal variation of the solids mass fraction in both the lower and upper troposphere is modulated by emissions of NH3 from the terrestrial biosphere and biomass burning as well as by emissions of dimethylsulfide from the ocean biosphere. The timescale of phase transitions, as driven by changes in relative humidity, varies from 10 to 50 h in the boundary layer to 150–400 h in the upper troposphere. Omission of the hysteresis effect in the CTM by assuming that particle phase follows the lower side of the hysteresis loop increases the solids mass fraction from 0.34 to 0.56. An upper side assumption decreases the fraction to 0.17. Lower and upper side assumptions better approximate particle phase for high and low altitudes, respectively. Fluctuations in the CRH, which can be induced by other constituents in sulfate particles such as minerals or organic molecules, strongly affect the solids mass fraction in the boundary layer but not at higher altitudes. Further studies are needed to determine the effects of large solids mass fraction on heterogeneous chemistry and cirrus cloud formation.

Accurate measurements of high pressure resistivity in a diamond anvil cell

A new technique incorporating a diamond anvil cell with photolithographic and film deposition techniques has been developed for electrical resistivity measurement under high pressure. Molybdenum was sputtered onto a diamond anvil facet and patterned to the desired microcircuit. A sputtered Al2O3 (alumina) layer was then fabricated onto the Mo-coated layer to insulate the thin-film electrodes from the metallic gasket and to protect the electrodes against plastic deformation under high pressure conditions. For better insulation, Al2O3 was also sputtered onto the metallic gasket. The regular shape of the microcircuit makes it convenient to perform an electric current field analysis, hence, accurate resistivity data can be obtained from the measurement. We performed the measurement of nanocrystalline ZnS to 36 GPa and determined its reversibility and phase transition hysteresis.