Premelting Region Research Articles

Distinctive features of the crystal-chemical, thermal and luminescence properties of (Zn0.94Mg0.06)2SiO4:Mn phosphor

The effect of magnesium introduction on the structural, thermal, and luminescent characteristics of Zn2SiO4:Mn at the fixed magnesium concentration of 6 at% has been studied. Zn1.88–2xMn2xMg0.12SiO4 phosphors were synthesized by the solid phase method. The structural properties were studied using X-ray diffraction. The luminescent properties were characterized by the PLE and PL spectra. The charge state of manganese ions was determined by the XPS method. It was found that the Zn1.88–2xMn2xMg0.12SiO4 samples at x ≤ 0.14 were single-phase. The unit cell parameters of the solid solutions changed nonmonotonically with increasing x. The change in the linear trend of the parameters occurred at x = 0.06, and was caused by the oxidation of a part of the Mn2+ ions to Mn3+. The introduction of 6 at% magnesium into the Zn2–2xMn2xSiO4 structure caused an inversion of the crystallographic positions in which manganese was predominantly oxidized. It was shown that the melting temperatures of the Zn2–2xMn2xSiO4 and Zn1.88–2xMn2xMg0.12SiO4 solid solutions decreased with increasing manganese concentration, but were practically independent of the presence of magnesium. The structural phase transition α→β was recorded in the premelting region for all Zn2–2xMn2xSiO4 compositions. For the Zn1.88–2xMn2xMg0.12SiO4 solid solutions, the α→β transition was registered only for x˂0.06. The optimal photoluminescence intensity of Zn2–2xMn2xSiO4 shifted to the region of lower values of x with the introduction of magnesium and turned out to be maximum for Zn1.76Mn0.12Mg0.12SiO4. The introduction of 6 at% magnesium reduced the decay time by almost a factor of two; for the sample with x = 0.06 the decay time was 0.654 ms.

Extended Defects in Graphene and Their Contribution to the Excess Specific Heat at High Temperatures.

The recent experiments on fast (microsecond) pulse heating of graphite suggest the existence of sharp maximum (6500K at 1-2GPa) on its melting curve. To check the validity of these findings, we propose to investigate the accumulation of extended in-plane defects in graphene. Such defects would contribute to thermodynamic properties of graphene and impose the upper limit on its melting temperature. We propose a type of extended defect of graphene, consisting of pentagonal and heptagonal rings with record low formation energy, whose accumulation leads to the loss of shear rigidity of graphene at temperatures above 6400K, thus setting the upper limit on its melting temperature. We found that this model satisfactorily explains the increase of specific heat observed in the premelting region of graphite in slow (millisecond) pulse heating experiments. However, in fast (microsecond) pulse heating experiments such an increase of specific heat was not observed, which is a strong indication of overheating that takes place in these experiments.

Experimental investigation of thermal expansion of niobium at high temperatures

Experimental study of thermal expansion of niobium by millisecond pulse heating technique was performed. Using this technique, an experimental determination of the temperature dependence of thermal expansion coefficient, electrical resistivity, enthalpy and specific heat capacity at premelting region for niobium have been carried out. The results obtained allow calculating the density of Nb in the wide range of high temperatures up to the melting point.

Thermal shrinkage and heat capacity of monolithic polymeric physical aerogels

Monolithic physical aerogels based on semicrystalline linear polymers are suitable for molecular separation, catalysis, and thermal insulation. These low-density materials generally exhibit thermal shrinkage in their pre-melting region. Differential Scanning Calorimetry (DSC) shows that aerogel thermal shrinkages occur with relevant step decreases of heat capacity (ΔCp < 0). For physical aerogels based on the thermodynamically stable β form of syndiotactic polystyrene (sPS), a positive heat capacity step associated with glass transition (Tg) and a negative heat capacity step associated with thermal shrinkage are well separated. As for poly (2,6-dimethyl-1,4-phenylene)oxide (PPO), which exhibits the rather unique closeness of Tg and Tm, the two phenomena are superimposed and a regular increase of heat capacity with porosity, going from positive to progressively more negative values is observed. Negative heat capacity steps are not present for thermal shrinkage of polymer foams, which exhibit high porosity but the low surface area. On the basis of the present study, simple and fast DSC scans allow accurate evaluation of temperatures of thermal shrinkage of monolithic physical polymer aerogels.

Experimental study of thermophysical properties of W-Re alloy at high temperatures

This article presents data on thermophysical properties of a tungsten-rhenium alloy W-Re (27 at. % of Re) in the temperature range of 2400–3100 K. Data on the thermal expansion and specific electrical resistance of this substance at premelting region are obtained, and the emission spectra of the alloy are obtained at high temperatures.

First‐Principles Molecular Dynamics Studies of Oxygen Sublattice Melting in Thoria

We apply first‐principles molecular dynamics to investigate the order–disorder transition in the oxygen sublattice, observed experimentally in thoria. The mean‐square displacements were calculated at high temperatures (2700 K–3660 K) and we demonstrate that oxygen atoms become mobile at 3000 K, while they stay confined at 2700 K. The rate of diffusion of oxygen atoms was calculated while for thorium atoms the mean‐square amplitudes of vibrations were evaluated. Thoria is predicted to exhibit a higher increase in thermal expansion with temperatures in the premelting region, similar to urania.

Experimental investigation of refractory metals in the premelting region during fast heating

This work demonstrates experimental possibility of investigation of high refractory materials around its melting point, particularly in premelting region with high accuracy. In this article authors describe the developed experimental setup based on rapid resistive self-heating of a sample by a large current pulse generated by a capacitor discharge circuit that allow fast pulse interruption by temperature feedback signal. The sample temperature was measured with a two-channel microsecond radiation pyrometer. Preliminary experiments were conducted on tantalum and molybdenum at heating speed of 108 K/s. The method allows investigating thermophysical properties of refractory conductive materials such as melting temperature, melting heat, specific resistivity, specific enthalpy and specific heat capacity in solid and liquid phase, especially in premelting area.

Phase field crystal simulation of subgrain boundary annihilation and dislocation rotation mechanism under strain at high temperature

In this work, the annihilation process of subgrain boundary (SGB) and splitting of low-angle symmetric till grain boundaries(GBs) during plastic deformation in the representatively system of these materials near but below the melting point and the temperature far from liquid-solid coexistence line are simulated using the phase filed crystals (PFC) model, respectively. The results show that premelting GBs at the temperature far from liquid-solid coexistence line will split into two SGBs and glide under strain. With the strain increasing, these SGBs of dislocations with opposite sign Burgers vectors are close to each other, and generate new grains to swallow up the old one. Finally these SGBs meet each other and annihilate, and then the SGBs and premelting regions disappear. The system with two grains transforms into that of one grain. The GBs at the temperature close to melting point under external exerting strain split into two SGBs with opposite sign dislocation, and move to approach other SGB split from other GB. The SGBs with opposite sign move close to each other at certain distance to form a dipole of dislocation pair in relative stability. Owing to the shear stress, the dislocation pair in the SGBs changes its direction two times, about 120°. After that, the SGBs move reverse to approach other opposite SGB, and appear to dislocation dissociation and transform into Zigzag shape of SGBs, and then the dissociated dislocation annihilates each other. For the premelting GB of the dislocation pair dipole under strain, it will start to extend and consolidate together, then decompose and separate from each other. In this process, the interaction of dislocation of different types occur, and also the generation of new dislocations and annihilation of dislocation by gliding, which will result in the change of Burgers vector of dislocation and the exchange of types of dislocation. The nature of this process is that the atomic lattices are softening inside the premelting region, which lead to the atomic array easily to gliding and torsional deformation under applying strain, and result in dislocation reaction in different types inside the premelting region and the change of Burgers vector of dislocation.

PHASE FIELD CRYSTAL SIMULATION OF STRAIN EFFECTS ON DISLOCATION MOVEMENT OF PREMELTING GRAIN BOUNDRIES AT HIGH TEMPERATURE

The properties of modern materials, especially superplastic, nanocrystalline or composite materials, depend critically on the properties of internal interfaces such as grain boundaries(GBs) and interphase boundaries(IBs). All processes which can change the properties of GBs and IBs affect drastically the behaviour of polycrystalline metals and ceramics. In this work, the annihilation processes of low-angle symmetric tilt GBs and dislocations during plastic deformation in the representative system of these materials near but below the melting point and the temperature at liquid-solid coexistence line were simulated using the phase-field crystal model, respectively. The results show that local premelting occurs around lattice dislocations near the melting point but the dislocation structure in the premelting region does not change, while the region become significantly larger when the system reaches the melting temperature. After premelting, deformation to the system causes dislocations in the premelting GB to begin to glide then annihilate with opposite Burgers vectors via the movement, finally the GB and the premelting region disappear. The annihilation mechanisms of dislocations are similar to those for premelting conditions. The more the temperature is closer to the melting point, the more obvious the atomic lattice around the premelting region is softened leading to the atomic binding strength around the dislocations being lowered. Only at this moment, the lattice atoms enable to reduce the resistance of the dislocation motion and accelerate its velocity during deformation. At the temperature reaching to the liquid-solid coexisting region in the simulation, the original premelting regions are induced to develop into bigger ones by the external strain acting. During this process, it can be seen some interactions including the multiplication dislocation pairs, the rotation of dislocation pairs and their annihilation. Furthermore, the shape of the premelting region changes with the variation of the interaction of dislocations inside the region, it is observed that the premelting regions approach each other and consolidate together,then decompose and segregate from each other. Although the shape of the premelting region changes with the applied strain, these regions do not disappear at the end of the simulation, totally different those in lower premelting temperature.

Phase field crystal simulation of grain boundary movement and dislocation reaction

The phase field crystal (PFC) model is used to simulate the premelting dislocation movement of the symmetric tilt grain boundary (STGB) under strain action when the system temperature is at far from the melting point and close to the melting point, respectively. The results show a local premelting occurs surrounding the dislocations as the premelting temperature is approached to from below temperature. The premelting dislocations of the STGB can glide under strain action, and the premelting region is a companion for dislocation gliding. The process of STGB decay is very similar at the two high temperature conditions. As premelting presents, it diminishes the gliding resistance for the dislocations and leads to a faster movement of dislocations, and also brings about more energy reduction of the system during the decay process of STGB. In spite of applying strain to these premelting samples in whole decay processes of STGB, the premelting dislocation region does not obviously develop and extend. This indicates that the external strain action does not promote the premelting at the high temperature, and cannot induce more premelting dislocation, which can be owed to the premelting phase around the dislocation exhibit fluid-like properties and to the premelting dislocation easily gliding and relaxing the strain energy; this is in agreement with the results of experiments and molecular dynamics.

Volume properties and spectroscopy: A terahertz Raman investigation of hen egg white lysozyme

The low frequency depolarized Raman spectra of 100 mg/ml aqueous solutions of hen egg white lysozyme (HEWL) have been collected in the 25-85 °C range. Short and long exposures to high temperatures have been used to modulate the competition between the thermally induced reversible and irreversible denaturation processes. A peculiar temperature evolution of spectra is evidenced under prolonged exposure of the protein solution at temperatures higher than 65 °C. This result is connected to the self-assembling of polypeptide chains and testifies the sensitivity of the technique to the properties of both protein molecule and its surrounding. Solvent free spectra have been obtained after subtraction of elastic and solvent components and assigned to a genuine vibrational contribution of hydrated HEWL. A straight similarity is observed between the solvent-free THz Raman feature and the vibrational density of states as obtained by molecular dynamics simulations; according to this, we verify the relation between this spectroscopic observable and the effective protein volume, and distinguish the properties of this latter respect to those of the hydration shell in the pre-melting region.

Thermal stability and oxidizing properties of mixed alkaline earth-alkali molten carbonates: A focus on the lithium-sodium carbonate eutectic system with magnesium additions

A comparative study on thermal behavior and oxygen solubility properties of eutectic 52/48 lithium/sodium carbonate salt containing minor additions of magnesium up to 10mol% has been made in order to determine whether a general correlation between these two properties can be found or not. Consecutive TG/DSC heating/cooling thermal cycles carried out under alternating CO2 and N2 gas flows allowed to assign thermal events observed in the premelting region to a partial decarbonation process of the magnesium-alkali mixed carbonates. The observed decarbonation process at 460°C is believed to come from initial stage of thermal decomposition of magnesium carbonate resulting in the metastable formation of magnesium oxycarbonate-like phases MgO·2MgCO3, in a similar manner as previously reported for lanthanum. Reversible formation and decomposition of the magnesium carbonate phase has been observed under a CO2 gas atmosphere. The intensity of the decomposition process shows a maximum for a 3mol% MgO addition that gives also the highest oxygen solubility, suggesting therefore that instability thermal analysis in the premelting region can be considered as providing an effective measure of the basicity/oxidizing properties of alkali carbonate melts with magnesium or, in more general terms, with cations that are strong modifiers of the carbonate melt basicity through formation of soluble oxycarbonate phases.

Heat capacity of solid tantalum: Self-consistent calculation

A review is given of the results of calculations of the heat capacity C(T) of solid tantalum in a wide temperature range from helium temperatures to the melting point of metal T m . Calculation of the heat capacity of the metal within the framework of a self-consistent thermodynamic model (SCTDM) of a solid body versus the experimental (reference) data on C(T) is performed. In the range from the Debye temperature θ0 ∼ 250 K to the beginning of the premelting region T ∼ 0.8T m , the standard SCTDM ensured an adequate description of the tantalum heat capacity with a mean square error of less than 0.5 J K−1 mol−1. At lower and higher temperatures, extra models are needed. A number of thermodynamic characteristics of the element are determined whose values were previously absent or needed verification.

Pippard Relations Close to the Melting Point in the Premelting Region of Hexadecane

We examine here the validity of the Pippard relations close to the melting point in the premelting region of hexadecane. For this verification, we analyze the observed data for the thermal expansivity obtained for various pressures at constant temperatures of 302 and 325 K in this system. By calculating the isothermal compressibility and the specific heat in the same pressure region of the premelting region of hexadecane, we obtain that varies linearly with and also that varies linearly with for this system. This indicat es that some molecular organic compounds, such as solid hexadecane studied here, can exhibit -phase transitions prior to melting which are expected to verify the Pippard relations.

Ultrafast molecular rotor: an efficient sensor for premelting of natural DNA

Structural changes in nucleic acids in the premelting region (T < melting temperature, T(m)) play an important role in the biological activity of DNA at physiological temperature. In the present communication we report the use of an ultrafast molecular rotor as an extrinsic fluorescence sensor to monitor the structural changes in natural DNA at T < T(m), which could not be detected even by circular dichroism spectroscopy. Further, the fluorescence sensor used in the present study is superior than most commonly used DNA stains.

On Relaxation of point defects in metals

Methods for studying the phenomenon of relaxation of point defects (thermal vacancies) and their influence on properties of metals are reviewed. For refractory metals, an equation for the temperature dependence of the relaxation time of an equilibrium concentration of thermal vacancies and calculation of this time at the melting temperatures of metals is proposed. The possible methods for determining the contribution of vacancies to the enthalpy are considered in the case of using a pulsed electric current for bulk heating of metals. The heating rates reached in the pulsed method, at which overheating of the metal-melting onset is observed and the relaxation phenomenon in the premelting region can be studied, are estimated.

Premelting phenomena in pseudo-binary ionic crystals

The theory of the premelting phenomena in ionic crystals on the basis of theconcept of the heterophase fluctuation has been applied to the pseudo-binary ioniccrystals, KCl–NaCl, AgBr–AgCl and AgBr–CuBr systems. Molecular dynamicssimulations (MD) have been performed to examine the ionic configurations in theirpremelting region in the vicinity of their melting points. Liquid-like clusters havebeen observed in the results of MD utilizing the Lindemann instability condition.The sizes of liquid-like clusters have been estimated by theory and MD. Thecharacteristics of the dynamical behavior of ions in the premelting region have beenexamined by the mean square displacement and the velocity correlation functions.

Calculation of the compressibility and heat capacity of ice I in the pre-melting region

The isothermal compressibility and the heat capacity are calculated here using the experimental data for the heat expansion of ice I in the pre-melting region. By analysing the data at various pressures, compressibility and the heat capacity are predicted as functions of temperature and pressure near the melting point (p m = 202.4 MPa, T m = 252.3 K) in ice I. Our predicted compressibility and heat capacity exhibit anomalous behaviour as the heat expansion in the pre-melting region of ice I.

Melting and Crystallization Behaviors of an Ionic Liquid, 1-Isopropyl-3-methylimidazolium Bromide, Studied by Using Nanowatt-Stabilized Differential Scanning Calorimetry

Abstract We studied the melting and crystallization behaviors of 1-isopropyl-3-methylimidazolium bromide ([i-C3mim]Br) using a laboratory-made differential scanning calorimeter (DSC) with nanowatt stability and sensitivity. In addition to common behaviors for many ionic liquids such as premelting over a wide temperature range and excessive supercooling, we observed unique thermal phenomena: reversible phase transitions of domains in the premelting region, rhythmic crystallization and melting, and intermittent crystallization. The latter two phenomena were observed in the returning experiments from the premelting region at a cooling rate as slow as 0.05 mK s−1. It is considered that these phenomena occurred because an [i-C3mim]+ ion is capable of taking plural stable conformations and the conformational changes occurred linking with melting and crystallization. We interpret these curious phenomena as the phase changes of the locally melted domains composed of different conformers, which significantly hindered the overall phase changes.

Effect of structural perturbations on helix-coil transitions in covalently closed supercoiled dna

The role of base-pair cooperativity in causing the helix-coil transition or the “melting” of a linear hydrogen-bonded macromolecule like DNA is well known. In the present paper, the melting curve for the covalently closed supercoiled DNA is studied, on the basis of statistical mechanics, by assuming the existence of supercoil-induced cruciforms as significant structural perturbations over the usual regular helical sequence and the bubbles in the premelting region of the macromolecule. In comparison, the nicked circular DNA, like the linear macromolecule, is assumed to consist of the regular helical sequences and the bubbles only. The partition functions are obtained on the basis of the appropriate sequence-generating functions, so that the number of helical units at a given temperature can be determined. The results are compared with the available experimental data in tetraethylammonium bromide (TEA) solution where the effect of the base-pair heterogeneity is minimized. The higher melting point and the flatter melting curve for the supercoiled DNA, as obtained from theory, are found to be in close agreement with the corresponding experimental observation.