Exact Transition Temperature Research Articles

Preparation of Shape Memory NiTiNOL Filaments for Smart Textiles

A nickel-titanium alloy (NiTiNOL, nitinol) fi lament with a diameter of 200 μm was used for preparing a smart knitted textile fabric with a shape memory eff ect within the range of human body temperatures. The annealing of the fi lament at 500 °C for 30 minutes was followed by air cooling at 20 °C to achieve a suitable transition temperature from martensite to austenite phase within the range between room temperature and 75 °C. The tensile properties of fi laments before annealing and after it were analysed on Instron 6022 dynamometer. The measurements of exact transition temperatures from soft martensite state at room temperature to hard austenite state at heating were made on a dynamic mechanical analysis instrument. From the annealed fi lament a left-right knitted fabric was hand made. The fabric was trained into a selected 3D form by cyclic heating in a strained form at 75 °C for 10 minutes and then cooled at room temperature. For a stable two-way memory eff ect, the nitinol fabric needed to make fi fteen cycles of heating and cooling.

Pressure-induced ferromagnetic quantum criticality in the low-carrier system CeP

We have already obtained the tentative pressure (P)-temperature (T) magnetic phase diagram of the antiferromagnetic (AF) semimetal CeP with strongly localized 4f electrons by measurements of the only T-dependence of the electrical resistivity, ρ(T), up to 8 GPa. In order to get further reliability, we performed the simultaneous measurements of the T- dependent real part of the AC magnetic susceptibility, χ'AC(T), and ρ(T) at P ≲ 2.5 GPa. As a result, the temperature which give the resistive anomaly was found to be the exact transition temperature from the paramagnetic (PM) phase to the magnetically ordered state, even in P > 2.5 GPa. The detailed magnetic structures (MSs) is already clarified by the neutron diffraction (ND) experiments under high pressures up to 1.7GPa by Kohgi et al. and up to 5.6 GPa by Goncharenko et al. The careful comparison between our macroscopic P-T magnetic phase diagram and the microscopic MSs elucidated the new facts: in the high-P region, the induction of the pure ferromagnetic (FM) phase for P ≳ 4 GPa is revealed to cause a rapid decrease in the Curie temperature TC and the Ce localized magnetic moment (LM); and furthermore, the TC is observed to vanish at a critical pressure PC ≈ (5.5±0.1) GPa together with the LM. We report the convincing basis for the presence of a P-induced FM quantum critical point (FM- QCP) and the possibility of localized-delocalized transition of the Ce-4f states approximately at the FM-QCP in the semi-metallic LM system CeP with the extremely low-carrier density.

Study on the thermoresponsive two phase transition processes of hydroxypropyl cellulose concentrated aqueous solution: from a microscopic perspective

In this paper, it was discovered that during the heating process from 35 to 63 °C, hydroxypropyl cellulose (HPC) concentrated aqueous solution (20 wt%) would first go through coil-to-globule transition and then sol–gel transition with temperature elevation. The microdynamic mechanisms of the two phase transitions were thoroughly illustrated using mid and near infrared spectroscopy in combination with two-dimensional correlation spectroscopy (2Dcos) and perturbation correlation moving window (PCMW) technique. Mid infrared spectroscopy is an effective way to study the hydrophobic interactions in HPC molecules. And near infrared spectroscopy is a potent method to study hydrogen bonds between HPC molecules and water molecules. Boltzmann fitting and PCMW could help determine the exact transition temperatures of each involving functional groups in the two processes. Moreover, 2Dcos was used to discern the sequential moving orders of the functional groups during the two phase transitions. Depending on the structure of HPC and the thermodynamic conditions, the dominating associative elements in either process might vary. During the coil-to-globule transition, HPC molecules precipitated to form an opaque system with mobility.It was discovered that the driving force of the coil-to-globule transition process in microdynamics could only be the dehydration and hydrophobic interactions of C–H groups. However, in the sol–gel transition, the system crosslinked to form a physical network with no mobility. The driving force of this process in microdynamics was primarily the self-assembly behavior of O–H groups in HPC “active molecules”.

First-Order Studies of Nanometric Biferroic

Magnetoelectric biferroic nanocomposite with composition 0.5Ni0.5Zn0.5Fe2O4 + 0.5BaTiO3 was synthesized by ceramic technique. The structural and electrical characterizations of the investigated nanocomposite are discussed and reported. The formation of nanosized composite with two separate phases was confirmed by X-ray diffraction, scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). The variation of dielectric constant (ε'), dielectric loss factor (ε") and the ac conductivity (σac) of 0.5Ni0.5Zn0.5Fe2O4 + 0.5BaTiO3 was investigated as a function of both frequency and temperature. Thermal hysteresis (first-order transition) was obtained during heating (300 - 830 K) and cooling runs (830 - 300 K). The exact transition temperature and the amount area of the thermal hysteresis depend on applied ac electric field. The delay (lagging) time between heating and cooling processes was esti-mated from the hysteresis loop area versus frequency. The conduction mechanism in the investigated samples was explained according to different models. This study enhances the use of this prepared system in memory applications.

Phase diagram and structural evolution of Ag–Au bimetallic nanoparticles: molecular dynamics simulations

We studied the structural evolution of a 270-atom Ag-Au bimetallic nanoparticle (2 nm in size) with varying composition and temperature. The liquid to solid transition region and the solid-state structure were investigated using molecular dynamics simulations. To determine the exact transition temperature region, we applied the mean square displacement and structure deviation methods, as well as the generally used caloric curve of potential energy versus temperature. The results showed that a complete solid-solution phase diagram of the binary Ag-Au system was obtained. Irrespective of the composition, the freezing temperature of a Ag-Au bimetallic nanoparticle was lower than that of the bulk state by a margin of several hundred degrees, and three different solid-state structures are proposed in relation to the Au composition. Our phase diagram offers guidance for the application of Ag-Au nanoparticles.

AFM study of the thermotropic behaviour of supported DPPC bilayers with and without the model peptide WALP23

Temperature-controlled Atomic Force Microscopy (TC-AFM) in Contact Mode is used here to directly image the mechanisms by which melting and crystallization of supported, hydrated DPPC bilayers proceed in the presence and absence of the model peptide WALP23. Melting from the gel Lβ′ to the liquid-crystalline Lα phase starts at pre-existing line-type packing defects (grain boundaries) in absence of the peptide. The exact transition temperature is shown to be influenced by the magnitude of the force exerted by the AFM probe on the bilayer, but is higher than the main transition temperature of non-supported DPPC vesicles in all cases due to bilayer–substrate interactions. Cooling of the fluid Lα bilayer shows the formation of the line-type defects at the borders between different gel-phase regions that originate from different nuclei. The number of these defects depends directly on the rate of cooling through the transition, as predicted by classical nucleation theory.The presence of the transmembrane, synthetic model peptide WALP23 is known to give rise to heterogeneity in the bilayer as microdomains with a striped appearance are formed in the DPPC bilayer. This striated phase consists of alternating lines of lipids and peptide. It is shown here that melting starts with the peptide-associated lipids in the domains, whose melting temperature is lowered by 0.8–2.0°C compared to the remaining, peptide-free parts of the bilayer. The stabilization of the fluid phase is ascribed to adaptations of the lipids to the shorter peptide. The lipids not associated with the peptide melt at the same temperature as those in the pure DPPC supported bilayer.

Decomposition and phase transition of NiS/Ni1−x S in the temperature range 323–973 K

Although the phase relations in the Ni–S system have been studied by many researchers, an accurate decomposition temperature range of the near equimolar composition α-NiS/α-Ni1−x S has not been established due to the complexity brought by the nonstoichiometic nature of α-NiS/α-Ni1−x S. In the current study, a series of α-NiS/α-Ni1−x S synthetic schedules were adopted to study the phase relations and variations in cell parameters of various phases in the temperature range 323–643 K. The decomposition of NiS started above 773 K according to the reaction: where Ni3−x S2 is high-temperature heazlewoodite. The Ni3S2 (low-temperature heazlewoodite) and a possible phase, Ni7S6, (the uncertainty is due to its low diffraction intensity in XRD profiles) observed in the current study are the products of the instantaneous decomposition of Ni3−x S2 during quenching from 973 K. The exsolutions of Ni3S2 and Ni7S6 during the quenching are less significant for compositions Ni0.97S and Ni0.92S compared with NiS. For equimolar composition α-NiS, the exact transition temperature of α-NiS↔β-NiS (millerite) lies between 623 K (derived from a cooling process) and 670 K (derived from a heating process). For nickel-deficient composition Ni0.97S, α- and β-phases coexist in the temperature range 573–613 K, and the onset temperature for α- to β-phase transition is lowered by around 50 K (for cooling processes) compared with that of NiS.

Optical Observation of Heterophase and Domain Structures in Relaxor Ferroelectrics Pb(Zn1/3Nb2/3)O3/9%PbTiO3

Phase boundary and domain structure in relaxor ferroelectrics Pb(Zn1/3Nb2/3)O3/9%PbTiO3 were observed using a (111) plate under a polarization microscope. The dielectric constant was measured simultaneously to determine exact transition temperatures and to connect the dielectric behavior with phase boundary movements. Movement of the phase boundary between cubic and tetragonal phases was observed at the temperature Tc where the dielectric constant changed abruptly. Below Tc, tetragonal and rhombohedral phases coexisted over a wide temperature range. The complicated domain/phase patterns are classified into two blocks according to extinction position. The difference in extinction positions between the two blocks deviated from 30° and manifested temperature dependence. This fact indicates that the symmetry-lowering occurs in the heterophase. A small, but the distinct temperature hysteresis of the dielectric constant was related to the nucleation and growth of one phase into another phase.

The effects of isotopes on structural phase transition in ammonium chloride

The effects of the isotopes 14N 15N and/or HD on the structural CsCl⇄NaCl type phase transition temperature of ammonium chloride using 14NH 4Cl, 15NH 4Cl, 14ND 4Cl, and 15ND 4Cl have been studied by thermal analysis, TG-DTA and DSC. The TG-DTA curves revealed that these ammonium chlorides exhibit an endothermic peak in the range 183.8–197.3°C with a good reproducibility in spite of sublimation. The exact transition temperatures of these four ammonium chlorides during heating and cooling were measured by DSC. The endothermic peak temperature became 0.4°C lower by substituting 14N with 15N in 14ND 4Cl, and 2.3 and 8.7°C lower by substituting H with D in 14NH 4Cl and 15NH 4Cl, respectively, while the temperature became 6°C higher by substituting 14N with 15N in 14NH 4Cl. The exothermic peak temperature was reduced by 15.7 and 14.4°C by substituting 14N with 15N and H with D in 14NH 4Cl, respectively.

Properties of the gap energy in the van Hove scenario of high-temperature superconductivity.

An exact transition-temperature (${\mathit{T}}_{\mathit{c}}$) formula is derived within the van Hove scenario of BCS phonon-mediated pairing theory consisting of a logarithmic singularity in the density of states (DOS) at the Fermi energy. Also evaluated are a near-exact zero-temperature gap-energy expression along with an exact (numerical) temperature-dependent gap energy. Though ${\mathit{T}}_{\mathit{c}}$ is greatly enhanced over the conventional BCS (constant DOS) value for given (weak) coupling, the zero-temperature gap-to-${\mathit{T}}_{\mathit{c}}$ ratio and the temperature-dependent gap energy both differ little from the BCS values.

Thermal analysis of polyaniline free standing films

We report on Dynamic Mechanical Thermal Analysis (DMTA) studies of free standing films of polyaniline. These films were cast from chemically synthesised emeraldine base dissolved in the solvent N-methyl-2-pyrrolidinone (NMP). Thermal scans show a softening ‘T g process’ for such films at ∼100°C. This occurs as solvent molecules are being removed from the samples. The exact transition temperature is dependent on the weight fraction of the NMP solvent and increases with decreasing solvent content. Above 180°C a crosslinking of the polymer chains occurs resulting in a hardening of the films. This crosslinked material is characterised by a new transition at 265°C. A low temperature transition is also seen at −80°C. This is interpreted as a torsional motion of the aromatic rings in the polymer chains.

DC Conductivity and Peierls instability in the quasi-one-dimensional organic charge density wave conductor (fluoranthene)2X

Due to the high anisotropy of the dc conductivity (σ|/σ⊥ ≈ 104) the organic conductor (fluoranthene)2X can be regarded as a model system for studying the Peierls instability in quasi-one-dimensional systems. The temperature dependence of the dc conductivity σ| (T) along the highly conducting crystal axis exhibits the typical behaviour of a quasi-one-dimensional metal with a Peierls transition at about 180 K to a charge density wave (CDW) ground state. As expected for a highly one-dimensional conductor the exact transition temperature depends on three-dimensional coupling effects and therefore on the size of the counterion X− = PF, AsF, SbF. Above the Peierls transition σ| (T) can be described quantitatively within a model of CDW fluctuations leading to a pseudo gap in the electronic density of states. Below, the existence of a real energy gap at the Fermi level with a BCS-like temperature dependence determines the charge transport over more than eight orders of magnitude in the electrical resistance. For the intrinsic energy gaps 2 Δ (0), which characterize the ground state of the Peierls semiconductor, values of 120-180 meV have been found for different crystals.

Exact transition temperature for an Ising model in three dimensions

For the most general vertex model respecting spin-flip symmetry the authors obtain on two- and three-dimensional SC lattices all fixed points of the generalised weak-graph transformation. From the result, a phase transition of a constrained Ising model on a three-dimensional lattice is conjectured to occur at coupling strength 1/4 In(1/2+2-1/2). They find this value to be consistent with Monte Carlo simulation results, which indicate a first-order phase transition.

Thermally induced crystallization of amorphous-titania films

Three types of amorphous-titania films prepared by ion- and electron-beam techniques have been annealed thermally. An amorphous-crystalline transformation is found in each of these film types at around 350 °C. Its resulting microcrystalline structure and the exact transition temperature appear to be dictated by the rutile microcrystalline seed present in the as-deposited films under different deposition conditions. An amorphous film with a weak rutile seed crystallizes at a lower temperature into the anatase structure, while a film with a relatively strong rutile base crystallizes into the rutile structure at a somewhat higher temperature. It is demonstrated that Raman spectroscopy is a simple and effective tool for characterization of these submicron-thick amorphous films and for the dynamical study of such a phase transformation. Accompanying this amorphous-crystalline transformation, a two-order increase in elastic light scattering is noted implying optical degradation associated with microcrystalline boundaries. In addition, results of the anatase–rutile transformation at a temperature near 900 °C are presented.

Exact Results for Two- and Three-Dimensional Ising and Potts Models

The exact transition temperature, as well as some of the critical exponents, of certain three-dimensional Ising and Potts models is presented. With the analytic results supplemented by assumptions concerning the validity of universality, the critical point is identified as a Liftshitz point. On the basis of hyperscaling, the dynamic exponent $z$ for two-dimensional Ising and Potts model is predicted to be given by $z\ensuremath{\nu}=2+\ensuremath{\alpha}$.

Eight-vertex model on the honeycomb lattice

The most general vertex model defined on a honeycomb lattice is the eight-vertex model. In this paper it is shown that the symmetric eight-vertex model reduces to an Ising model with a nonzero real or pure imaginary magnetic field H. The equivalent Ising model is either ferromagnetic with e2H/kT real or antiferromagnetic with e2H/kT unimodular. The exact transition temperature and the order of phase transition in the former case are determined. As an application of the result we verify the absence of a phase transition in the monomer-dimer system on the honeycomb lattice.

Exact transition temperature of the Potts model with q states per site for the triangular and honeycomb lattices

By the use of duality and star-triangle transformations together with a uniqueness assumption, the exact transition temperature of the Potts model with q states per site is obtained for the triangular and honeycomb lattices.

Phase Transition in a Vertex Model in Three Dimensions

The exact transition temperature and the order of phase transition are determined for a vertex model in three dimensions. The transition is in general of first order with a latent heat and occurs in a limited region in the parameter space. Details of the region depend on the underlying lattice and differ significantly between lattices with high (fcc and bcc) and low (simple cubic and diamond) coordination numbers.