Characteristic Temperature Tc Research Articles

Effect of conformation of interfacial adsorbed chains on physical aging of polymer nanocomposites.

The dynamics of polymer nanocomposites varies depending on the physics and chemistry at the polymer-nanoparticle interface. The physical aging of the nanocomposites is accelerated or retarded based on interfacial interactions and the state of polymer adsorption at the interfaces. In this study, we investigated the aging kinetics of silica-polystyrene nanocomposites using differential scanning calorimetry, focusing on the effect of local conformations of chains adsorbed on the nanofiller surface. The results show that the temperature dependence of the aging rate follows a Vogel-Fulcher-Tammann relationship at high temperatures, whereas it exhibits an Arrhenius-like behavior below a characteristic temperature (Tc). Notably, at T < Tc, the aging rate decreases with increasing loop height of the chains adsorbed on the filler surface, but the activation energy remains unchanged. We proposed that the suppression of the aging rate at T < Tc is likely related to an increase in the length scale over which the slow interfacial dynamics can propagate due to the increased topological interactions between the chain loops of a larger size and the free chains in the matrix. The increased packing frustration occurring at the filler surface occupied by the larger loops might also contribute to the decreased aging rate.

Effects of Activation Barriers on Quenching to Stabilize Prebiotic Chemical Systems.

We have previously shown in model studies that rapid quenches of systems of monomers interacting to form polymer chains can fix nonequilibrium chemistries with some lifelike properties. We suggested that such quenching processes might have occurred at very high rates on early Earth, giving an efficient mechanism for natural sorting through enormous numbers of nonequilibrium chemistries from which the most lifelike ones could be naturally selected. However, the model used for these studies did not take account of activation barriers to polymer scission (peptide bond hydrolysis in the case of proteins). Such barriers are known to exist and are expected to enhance the quenching effect. Here, we introduce a modified model which takes activation barriers into account and we compare the results to data from experiments on quenched systems of amino acids. We find that the model results turn out to be sensitive to the width of the distribution of barrier heights but quite insensitive to its average value. The results of the new model are in significantly better agreement with the experiments than those found using our previous model. The new parametrization of the model only requires one new parameter and the parametrization is more physical than the previous one, providing a chemical interpretation of the parameter p in our previous models. Within the model, a characteristic temperature Tc emerges such that if the temperature of the hot stage is above Tc and the temperature of the cold stage is below it, then the 'freezing out', in a quench, of a disequilibrium ensemble of long polymers is expected. We discuss the possible relevance of this to models of the origin of life in emissions from deep ocean rifts.

Length Scale of Molecular Motions Governing Glass Equilibration in Hyperquenched and Slow-Cooled Polystyrene.

Polymer glasses attain thermodynamic equilibrium owing to structural relaxation at various length scales. Herein, calorimetry experiments were conducted to trace the macroscopic relaxation of slow-cooled (SC) and hyperquenched (HQ) polystyrene (PS) glasses and based on detailed comparisons with molecular dynamics probed by dye reorientation, we discussed the possible molecular process governing the equilibration of PS glasses near the glass transition temperatures (Tg). Both SC and HQ glasses equilibrate owing to the cooperative segment motion above a characteristic temperature (Tc) slightly lower than the Tg. In contrast, below the Tc, the localized backbone motion with an apparent activation energy of 290 ± 20 kJ/mol, involving approximately six repeating units, assists equilibrium recovery of PS glasses on the experimentally accessible time scales. The results possibly indicate the presence of an alternative mechanism other than the α-cooperative process controlling physical aging of materials in their deep glassy states.

Horizontal flux growth as an efficient preparation method of CeRh2As2 single crystals.

We report an efficient method to obtain CeRh2As2 single crystals with the use of a bismuth flux growth method in a horizontal configuration. Based on our numerous attempts, we found this technique to be scalable and repeatable. The crystals thus obtained are characterized by much sharper phase transitions and distinctly higher characteristic temperatures Tc and T0, compared to previous reports. Moreover, based on our specific heat studies of the obtained single crystals, we also indicate a clear connection between both transition temperatures.

Two distinctive temperature dependences of the London penetration depth in high-Tc cuprate superconductors as support for the theory of Bose-liquid superconductivity

In the framework of the theory of Bose-liquid superconductivity, we show that in high-Tc cuprate superconductors the magnetic field penetration depth (known as the London penetration depth λL(T)) decreases first exponentially below the superconducting transition temperature Tc with decreasing the temperature T and then λL(T) decreases like a certain power of T at lower temperatures due to the vanishing of the energy gap Δg(T) in the excitation spectrum of a superfluid Bose-liquid at a characteristic temperature Tc⁎ lower than Tc. Below Tc, the London penetration depth λL(T) has an exponential temperature dependence down to Tc⁎, which is different from a BCS- like exponential temperature dependence, and then λL(T) will follow a power law temperature dependence down to T=0, which is also different from power-law temperature dependences predicted by some BCS-like (d-wave) pairing theories. In unconventional (bosonic) cuprate superconductors characterized by the strong interboson coupling constant (γB≳1), λL(T) exhibits the distinctly different temperature dependences in different temperature intervals 0≤T≤Tc⁎<<Tc and Tc⁎<T<Tc when the energy gap Δg(T) vanishes at a temperature Tc⁎ well below Tc. In the weak interboson coupling (γB<<1) the energy gap Δg(T) vanishes at a temperature Tc⁎ close to Tc and the reduced London penetration depth λL(T)/λL(0) has an unusual Tn power-law temperature dependence in a wide temperature range 0<T<Tc⁎ below Tc, i.e. the expression λL(T)/λL(0)≃[1−aL(T)(T/Tc)n]−1/2 (where n≳4 and aL(T)≲1) is valid only below Tc⁎ and is similar to the well-known empirical Gorter-Casimir law. Our theoretical predictions of the distinctive power-law and exponential temperature dependences of λL(T)/λL(0) are in good agreement with the experimental results reported for GdBa2Cu3O7−δ ceramics and YBa2Cu3O7−δ films. The peculiar temperature dependences of λL(T)/λL(0) observed in these high-Tc materials at low and high temperatures do not follow the predictions of the s- and d-wave BCS-like theories of Fermi-liquid superconductivity and give evidence for the predictions of the theory of Bose-liquid superconductivity.

Comparing the shock sensitivity of insensitive energetic materials

We present a tabletop method to study the shock sensitivity of plastic-bonded explosives that are considered shock insensitive using high dynamic range optical emission spectroscopy with laser-launched km/s flyer plate impacts (2, 3, and 4 km/s), which measures the spectral radiance (the emission spectrum vs a calibrated intensity standard) every 0.8 ns in the nanosecond and microsecond regimes. The explosives were TATB (1,3,5 trinitro, 2,4,6 triamino benzene), FOX-7 (1,1-diamino-2,2-dinitroethylene), LLM-105 (2,6-diamino-3,5-dinitropyrazine-1-oxide), and NTO (nitrotriazolone), all with 20% Sylgard 182 binder. Time resolving emission from shocked explosives produces a unique fingerprint at each shock pressure, and the emission temporal profile can be used to understand each material's shock response and the underlying mechanisms within the framework of the well-known shock ignition and growth models. Ignition was characterized by the emission intensity during nanosecond hot spot ignition. The growth process, which occurs after the shock has unloaded, was characterized by the time derivative of microsecond emission intensity. Singular-value decomposition was used to determine a characteristic spectral radiance over the entire time range, and this radiance was fit to a graybody model to determine a characteristic temperature Tc. Even though these insensitive explosives have similar shock sensitivities, our method can clearly distinguish their shock response. FOX-7 and NTO were the most sensitive, with FOX-7 having the greatest shock ignition sensitivity, growth rate, and Tc. LLM-105 was much less sensitive than FOX-7 and NTO, and TATB consistently showed the lowest sensitivity by any measure.

Non-isothermal crystallization of LiNaGe4O9 glass

The glass of lithium-sodium tetragermanate LiNaGe4O9 is crystallized on heating under the control of differential scanning calorimetry and thermal gravimetric analysis. The measurements were prepared in the temperature range 300-870 K and showed the relatively weak endothermic DSC anomaly and 40-50 K above the single exothermic peak. The endothermic anomaly observed at Tg testified to softening the glass structure whereas the exothermic peak at TC manifested crystallization of the amorphous phase. Studying of TGA demonstrated smooth nearly linear dependences without any visible anomalies. Varying the heating rate from 1.2 up to 40 K/min resulted in noticeable increase of the characteristic temperatures Tg and TC. Lower limit of the glass transition temperature Tg0 was estimated with the help of the existing model. The mechanism of the LiNaGe4O9 glass crystallization is discussed.

The study of Si/Ge interdiffusion using molecular dynamics simulation

The coefficients of mutual diffusion of silicon into germanium (and vice-versa) at temperatures of 900-1300K were found based on molecular dynamics simulation. For the characteristic temperature Tc=1100 K, which is close to the melting point of Ge, we determined the thickness of the diffusing layer for each type of atom. It is shown that the depth of diffusing significantly depends on the sintering time, and silicon atoms penetrate 1.5 times deeper into bulk germanium than germanium into silicon.

Fastest frozen temperature for a thermodynamic system

For a thermodynamic system obeying both the equipartition theorem in high temperature and the third law in low temperature, the curve showing relationship between the specific heat and the temperature has two common behaviors: it terminates at zero when the temperature is zero Kelvin and converges to a constant value of specific heat as temperature is higher and higher. Since it is always possible to find the characteristic temperature TC to mark the excited temperature as the specific heat almost reaches the equipartition value, it is also reasonable to find a temperature in low temperature interval, complementary to TC. The present study reports a possibly universal existence of the such a temperature ϑ, defined by that at which the specific heat falls fastest along with decrease of the temperature. For the Debye model of solids, below the temperature ϑ the Debye’s law manifest itself.

Temperature-dependent dielectric response of antiferroelectric (Pb0.97La0.02)(Zr0.50Sn0.40Ti0.10)O3 ceramic

In practical applications, the operating temperature was an important performance of antiferroelectric (AFE) material. Temperature dependent dielectric response of (Pb0.97La0.02)(Zr0.50Sn0.40Ti0.10)O3 ceramics were studied in detail, the results showed that dielectric behavior were closely related to the two characteristic temperatures T0 and Tc. The hysteresis (P-E) loops exhibited ‘square’ double loops below T0, and changed to ‘slanted’ loops between T0 and Tc, and finally linear loops above Tc, simultaneously three kinds of nonlinear dielectric behavior were observed in the corresponding temperature regions. The tunability first increased and then decreased with increasing temperature, and exhibited best performance was between T0 and Tc. The results indicated that (Pb0.97La0.02)(Zr0.50Sn0.40Ti0.10)O3 AFE ceramics was an excellent dielectric tunable performance, and provided an easy way to optimize the operating temperature of AFE tunable devices.

Crystallization kinetics of tellurium-rich Se–Te–Sn glassy alloys

Differential scanning calorimetry (DSC) has been used to study the non-isothermal crystallization kinetics of Se30Te70−xSnx (for x = 0, 1.5 2.5 and 4.5) glassy alloys. Characteristic kinetic parameters say glass transition temperature (T g), peak crystallization temperature (T p) and melting temperature (T m) are determined from the DSC curves, recorded at four different heating rates, i.e., 9, 12, 15, 18 K min−1. The characteristic temperatures T g, T c and T m are found to increase with an increase in Sn concentration as well as with the heating rate (α). The increase in T g, T p and T m may be ascribed to the increase in cross-linking of amorphous network due to the dispersion of Sn into the Se chains. The activation energy of glass transition (E g), crystallization activation energy (E c) and Avrami exponent (n) have been evaluated from the heating rate dependence of T g, T c and T p using Kissinger, Mahadevan, Augis-Bennett and Moynihan methods. Thermal stability and glass-forming ability of amorphous Se30Te70−xSnx alloy have also been examined. Our results show that Se30Te70−xSnx composition, with thermal stability parameter H R > 0.5, is thermally stable and has characteristics of good glass former. Further, the composition corresponding to x = 1.5 is found to be thermally more stable as compared to the other compositions.

Light scattering and computer simulation studies of superionic pure and La-doped BaF2

A combination of both Raman and Brillouin scattering experiments as well as Molecular Dynamics (MD) was used to study the superionic behaviour of BaF2 doped with a wide range of LaF3 concentrations (0⩽x⩽50mol%). Raman spectroscopy reveals that for undoped BaF2 and those doped with 5% and 10% LaF3, the room temperature spectra show the usual T2g symmetry mode at 241cm−1 whereas for those doped with 20%, 30% and 50% LaF3, the dominant Raman mode is of the Eg symmetry situated at ∼263, 275 and 286cm−1, respectively. The Raman linewidths show near linear increases with temperature followed by rapid increases above the characteristic transition temperatures (Tc), being at 1200, 850, 800, 975, 950 and 920K for LaF3 concentrations of 0, 5, 10, 20, 30 and 50; respectively. The temperature dependence of the squares of the Brillouin frequencies (ΔωB)2 of the LA and TA acoustic modes respectively related to elastic constants C11 and C44 showed linear decreases followed by significant deviations around the same temperatures (Tc), at which the Raman linewidths start to show substantial increases. The complementary studies using MD simulations show that the diffusion coefficients increase markedly above the same temperatures observed experimentally. The extrinsic fluorine ion trajectories were also determined from the MD simulations to better understand the mechanisms of diffusion.

Vitreous and Crystalline Phosphates : elaboration and electrical properties

Interest in mixed metals titanium phosphates, both in glass and crystalline forms, for their properties as ionic conductors or non-linear optical materials, has led to large number of studies of these materials [1-3]. Our investigation of Na2O – Li2O – CaO – TiO2 – P2O5 system led to the discovery of NaxLiyCazTit(PO4)3 solid solutions which exist in both glassy and crystalline forms. Here the preparation of NaxLiyCazTit(PO4)3 will be described. The compounds were structurally characterized by PXRD, and their chemical and physical characterization were completed using DTA, XRD, UV-visible, Raman spectroscopy and ionic conductivity measurements. NaxLiyCazTit(PO4)3 glasses have been prepared by heating stochiometric amounts of Na2CO3, Li2CO3, CaCO3, TiO2 and NH4H2PO4 in a platinum crucible at 10500C. The microcrystalline samples were obtained by crystallization of the corresponding glasses at 6400C or by standard solid state preparation at 7000C. The values of the characteristic temperatures Tg (glass transition), Tc (crystallization) and Tf (fusion) of the glasses are found in the following ranges: 400 – 5000C for Tg, 440 – 6500C for Tc and 780 – 8300C for Tf. Rietveld refinements of powder X ray diffraction data collected on crystalline samples of NaxLiyCazTit(PO4)3 shows the existence of a solid solution in the domain 0 ≤ X ≤ 0.5 and the presence of a mixture of phases in the domain 0.5 &lt; X ≤ 1.5. The crystalline phases belong to the Nasicon family, space group R32. Their structure, determined from refinement of powder diffraction patterns , consists of a 3D network of AO6 (A = Ti, Na, Li, and Ca) octahedra and PO4 tetrahedra linked by corners. Sodium atoms fully occupy the M(1) sites and partially the M(2) sites.

Temperature memory effect in amorphous shape memory polymers.

Temperature memory effect (TME) refers to the ability of shape memory polymers (SMPs) to memorize the temperature at which pre-deformation was conducted. In the past few years, this TME was experimentally demonstrated by comparing the applied programming temperature (Td) with a characteristic recovery temperature (Tc), which corresponds to either the maximum recovery stress or free recovery speed. In these well-designed experiments, Tc was observed to be close to Td, which is consistent with the intuitive understanding of 'memorization'. However, since the polymer recovery behavior has been proved to be strongly dependent on various programming and recovery conditions, a new question that whether Tc is always equal to Td in any thermo-temporal conditions remains to be addressed. In this paper, we answered this question by examining the free recovery profile of an acrylate based amorphous SMP. The recovery Tc, which is the temperature with the maximum recovery speed, versus the recovery temperature is shown to be strongly dependent on both programming and recovery conditions. Their detailed influence could be explained by using the reduced time. During a thermomechanical working cycle of SMPs, in addition to the Td, any other thermo-temporal conditions, such as the holding time (th), cooling rate, recovery heating rate (q), etc., can affect the observed Tc by changing the reduced programming or recovery time. In this manner, the relationship between Tc and Td is not uniquely determined. Besides, the TME in SMPs can only be achieved within a given temperature range. Both onset and offset of this temperature range are shown to be influenced by the programming history, but are independent of the recovery conditions.

Biodegradable poly(L‐lactic acid)/TiO2 nanocomposites: Thermal properties and degradation

AbstractPoly(L‐lactic acid)‐titanium dioxide nanocomposites (with various loadings of TiO2: 0.5, 1, 2, 5, and 10 wt %) were produced by solution casting method. The influence of TiO2 on thermal properties and crystallinity of PLA was investigated by DSC and FTIR spectroscopy. The TiO2 nano filler has no significant influence on the characteristic temperatures (Tg, Tc, and Tm), but has high impact on the crystallinity of these systems. The degree of crystallinity Xc significantly increases for PLA nanocomposites loaded with up to 5 wt % of TiO2, while for 10 wt % load of TiO2 it drops below Xc of the pure resin. The degradation of the prepared composites was evaluated hydrolytically in 1N NaOH, enzymatically in α‐amylase solutions, and under UV irradiation. The catalytic effect of TiO2 nano particles on the degradation processes under UV light exposure (λ = 365 nm) and hydrolytic degradation was confirmed with the increase of the filler content. The opposite effect was identified in enzymatic degradation experiments. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Preparation and Thermal Properties of P&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt; Doped Li&lt;sub&gt;2&lt;/sub&gt;O-Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;-SiO&lt;sub&gt;2&lt;/sub&gt; Glass-Ceramics

Using TiO2 and ZrO2 as compound nucleation agent, serials Li2O–Al2O3–SiO2 (LAS) glasses containing different content of P2O5 were prepared by conventional melt quenching technique and subsequent converting to glass–ceramics. The influence of P2O5 content on viscosity of LAS glasses was investigated in the temperature range from the softening point to melting temperature. DSC was undertaken to analyze the change of characteristic temperature Tg, Tc and Tp. Thermal expansion coefficient (TEC) of matrix glass and glass-ceramic containing different P2O5 were measured. The results show introducing P2O5 effectively decreased the melting temperature and viscosity of LAS glasses. TECs of the matrix glass and glass-ceramic range from 4.1×10-6 °C -1 to 5.3×10-6 °C -1 and from -0.2×10-6 °C -1 to 1.2×10-6 °C -1, respectively. XRD analysis indicates that β-spodumene is the main crystalline phase in the P2O5 doped LAS glass-ceramics, resulting in the low thermal expansion coefficient of the glass-ceramics.

Ferromagnetic ordering and weak spin-glass-like effect inPr2CuSi3 and Nd2CuSi3

We present the results of the temperature dependences of ac and dc susceptibilities,high-field magnetization, magnetic relaxation, specific heat, and electrical resistivity ofPr2CuSi3 and Nd2CuSi3, compounds previously shown in the literature to exhibit interesting properties. It is observed thatthe investigated compounds undergo a ferromagnetic phase transition at a characteristic temperatureTC (= 9.8 K forPr2CuSi3 and5.6 K for Nd2CuSi3), where zero-field-cooled dc susceptibility shows a rapid increase followed by a sharp peak just belowTC.Below TC, the magnetization curve displays an open hysteresis loop and a steep rise at low fields, whileirreversible magnetism and long-time magnetic relaxation effects can be observed. Furthermore, nearTC both the real and imaginary components of the ac susceptibility show a large peak with asmall frequency shift of the peak position, and a sharp anomaly appears in thespecific heat and electrical resistivity curves. These unusual features observed forPr2CuSi3 and Nd2CuSi3 strongly suggest the formation of huge ferromagnetic clusters accompanied by a very weakspin-glass-like effect in both samples. The obtained results are discussed by comparingthem with the data reported for other 2:1:3 intermetallic compounds and some alloyedcompounds with different stoichiometry.

Thermal behaviour of chitosan and chitin thin films studied by spectroscopic ellipsometry

AbstractIn‐situ spectroscopic ellipsometry was used to evaluate the temperature behaviour of chitin and chitosan thin films. For both biopolymers it was found that film thickness decreases with temperature. Two different linear rates were clearly identified by a characteristic temperature Tc, of 51 and 59 °C for chitin and chitosan, respectively. However, the refractive index for chitin decreases with temperature and a more complex behaviour was found for chitosan. The results of this work show that a more complex transformation than water desorption should be claimed to explain the observations. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Charge transport through conducting organic poly(2-methoxy-5- (2-ethylhexyloxy)-1, 4-phenylenevinylene)

Current–voltage (J–V) characteristics of poly(2-methoxy-5- (2-ethyhexyloxy)-1, 4-phenylenevinylene)(MEH-PPV) have been studied in the hole-only devices ITO/PEDOT : PSS/MEH-PPV/Au, as a function of temperature from 300 to 98 K. Hole conduction in MEH-PPV has been well explained by an exponential trap-controlled drift model. In a considerable range of applied voltages, current obeys the power law J ∝ Vl+1, where l = Tc/T and Tc is the characteristic temperature of the exponential trap distribution. Assuming the effective density of states Nν = 2.7 × 1018 cm−3, the holes trap density (Hb) and characteristic temperature (Tc) have been calculated to be 1 × 1018 cm−3 and 550 K, respectively. As the voltage further increases current deviates and becomes smaller than J ∝ Vl+1. At low temperatures and sufficiently high-applied voltages, current follows the V2 law. At higher temperatures slopes less than 2 of the log J–log V plots have been observed. The possible reasons for this behaviour are mentioned.

Protein dynamics on different timescales

Structure and dynamics determine the function of proteins. This contribution discusses two aspects of protein dynamics, the structural fluctuation and the structural relaxation connected with conformational changes. Myoglobin and haemoglobin were investigated. To cover a wide time range different experimental techniques had to be used. Moreover, measurements in a large temperature regime were used to separate contributions from different modes of motions. Phonon assisted Mössbauer effect using synchrotron radiation allowed the study of the harmonic vibrations which have characteristic times of 1fs to 0.6ps. They are present in the whole temperature range from cryogenic to room temperature. With a combination of neutron structure analysis and incoherent neutron scattering it was possible to distinguish three types of hydrogen mean square displacements which are present only above a characteristic temperature Tc: These are the backbone-like (slower than about 100ps), methyl-like (partly slower partly faster than about 100ps) and lysine-like (faster than about 100ps) displacements. The exceptional high energy resolution of Mössbauer absorption on 57Fe allowed the measurement of quasi diffusive modes of molecular segments which have characteristic times slower than 1ns and are present only above Tc. Conformational changes from the ligated to the unligated structure of myoglobin and haemoglobin were investigated by creating a metastable intermediate and observing the relaxation into the equilibrium conformation. A metastable state was obtained by X-ray irradiation. Structural relaxation was investigated as a function of time and temperature using the Mössbauer hyperfine interactions as indicator. Furthermore it was possible to measure intermediates created by photolysis of a ligand with temperature dependent X-ray structure analysis or time dependent X-ray structure analysis with the Laue technique. It was shown that the quasi diffusive structural fluctuations above Tc strongly facilitate structural relaxations.