Temperature Dependence Of Activation Energy Research Articles

Sintering Simulation for Porous Material by Integrating Molecular Dynamics and Master Sintering Curve

In this study, we propose a combination of molecular dynamics (MD) and the master sintering curve (MSC) approach to analyze the activation energy of sintering for porous materials. MD calculations were performed by using a porous structure with various initial densities, and the change of relative density with simulation time was analyzed. To relate the MD results with long-term sintering behaviors of porous materials, we established a method to obtain the MSC, which is able to determine the activation energy of sintering, on the basis of these MD simulation results. We have successfully obtained sintering behavior and activation energies of sintering depending on temperature range and particle diameter. These activation energies obtained by our approach are in agreement with experimental observations. In addition, the temperature dependence of activation energy of sintering is also in good agreement with that of surface diffusion, which indicates that surface diffusion is the dominant sintering mechanism.

Enhancement of magnetic and electrical properties in Sc substituted BiFeO3 multiferroic

Polycrystalline BiFe1−xScxO3 (x=0, 0.05, 0.1 and 0.15) compounds are prepared using solid state reaction. The XRD patterns show that all compounds are crystallized in rhombohedral structure with R3c space group. An induced weak ferromagnetism in Sc substituted BiFeO3 due to suppression of spiral modulated spin structure is revealed. In addition, a spin glass like behaviour is observed from the zero field cooled (ZFC) and field cooled (FC) magnetization curves in the low temperature region. Further, the coupling between the ferroelectric and (anti) ferromagnetic orders is evident from the appearance of anomaly in the dielectric data near the magnetic Néel temperature (373°C). The reduction of oxygen vacancies due to Sc substitution is evident from the ac conductivity data and the suppressed anomaly in dielectric data at 220°C. The temperature dependence of ac conductivity is consistent with correlated barrier hopping (CBH) model. The temperature dependent ac conductivity and activation energies indicate that electronic conduction, oxygen vacancies movement and creation of defects are the prime contributors to the ac conductivity in measured temperature regions. The improved magnetic and electrical properties due to the structural modification are prominent for novel device applications.

Effect of anionic surfactant concentration on the variable range hopping conduction in polypyrrole nanoparticles

The mechanism of charge transport in polypyrrole (PPy) nanoparticles prepared with different concentrations (5 to 30 mM) of anionic surfactant (sodium dodecyl sulfate) is reported. Transmission electron microscopy technique confirms the formation of PPy nanoparticles of sizes ∼52 to 28 nm under surfactant directed approach. The room temperature electrical conductivity of the prepared nanoparticles found to increase from 3 to 22 S/cm with surfactant concentration. The temperature dependent activation energy rules out the possibility of band conduction mechanism in the prepared PPy nanoparticles and thus the synthesized nanoparticles are analyzed under variable range hopping (VRH) model for conduction mechanism. The PPy nanoparticles, reduced with liquid ammonia, hold 3D VRH conduction mechanism for the charge transport. However, in the doped samples, some deviation from 3D VRH conduction behavior at higher temperatures (>150 K) has been observed. This may be attributed to the presence of anionic surfactant in these samples. The doping of anionic surfactant causes rise in conducting islands, which may lead to the change in the shape/distribution of density of states governed by Gaussian or exponential type near Fermi level.

Functional properties of human ferroportin, a cellular iron exporter reactive also with cobalt and zinc

Iron homeostasis is achieved by regulating the intestinal absorption of the metal and its recycling by macrophages. Iron export from enterocytes or macrophages to blood plasma is thought to be mediated by ferroportin under the control of hepcidin. Although ferroportin was identified over a decade ago, little is understood about how it works. We expressed in Xenopus oocytes a human ferroportin-enhanced green fluorescent protein fusion protein and observed using confocal microscopy its exclusive plasma-membrane localization. As a first step in its characterization, we established an assay to detect functional expression of ferroportin by microinjecting oocytes with (55)Fe and measuring efflux. Ferroportin expression increased the first-order rate constants describing (55)Fe efflux up to 300-fold over control. Ferroportin-mediated (55)Fe efflux was saturable, temperature-dependent (activation energy, Ea ≈ 17 kcal/mol), maximal at extracellular pH ≈ 7.5, and inactivated at extracellular pH < 6.0. We estimated that ferroportin reacts with iron at its intracellular aspect with apparent affinity constant < 10(-7) M. Ferroportin expression also stimulated efflux of (65)Zn and (57)Co but not of (64)Cu, (109)Cd, or (54)Mn. Hepcidin treatment of oocytes inhibited efflux of (55)Fe, (65)Zn, and (57)Co. Whereas hepcidin administration in mice resulted in a marked hypoferremia within 4 h, we observed no effect on serum zinc levels in those same animals. We conclude that ferroportin is an iron-preferring cellular metal-efflux transporter with a narrow substrate profile that includes cobalt and zinc. Whereas hepcidin strongly regulated serum iron levels in the mouse, we found no evidence that ferroportin plays an important role in zinc homeostasis.

Cooperativity and the Freezing of Molecular Motion at the Glass Transition

The slowing down of molecular dynamics when approaching the glass transition generally proceeds much stronger than expected for thermally activated motions. This strange phenomenon can be formally ascribed to a temperature-dependent activation energy E(T). In the present work, via measurements of the third-order nonlinear dielectric susceptibility, we deduce the increase of the number of correlated molecules N(corr) when approaching the glass transition and find a surprisingly simple correlation of E(T) and N(corr)(T). This provides strong evidence that the noncanonical temperature development of glassy dynamics is caused by a temperature-dependent energy barrier arising from the cooperative motion of ever larger numbers of molecules at low temperatures.

Percolation conductivity of single-crystal manganite Pr1 − x Sr x MnO3 (x = 0.22; 0.24) in the phase-separation state

This paper presents an experimental study of the temperature dependences of the electrical resistivity, thermopower, and magnetoresistance of single-crystal manganite Pr1 − xSrxMnO3 (x = 0.22, 0.24). The results obtained have been analyzed in terms of the theory of hopping electrical conductivity of manganites in the phase-separation state and the main postulates of percolation theory. In the 2–400-K interval, hopping conductivity with temperature-dependent activation energy has been observed. Near the Curie temperature, the monotonic exponential growth of electrical resistivity with decreasing temperature becomes distorted by formation of finite metallic clusters. For T ≤ 36 K, the conductivity of Pr0.8Sr0.22MnO3 follows the Mott law.

Study of the thermo-magnetic fluctuations in carbon nano-tubes added Bi-2223 superconductors

We have investigated the effect of addition of carbon nano-tubes (CNT) on the properties of Bi1.6Pb0.4Sr2Ca2Cu3Oy superconductors. The samples are prepared from commercial powders with addition of 0.1wt% of CNT. They are characterized by XRD, SEM and magneto-resistivity in the transition region. As it was reported for CNT added Y-123 compounds, the refined cell parameters are practically independent of this kind of addition. Under applied magnetic field, a large broadening of resistive transition is observed. The dissipative behavior of resistivity can be explained using the well known expression, ρ=ρ0(T/Tg−1)S. The modified vortex-glass to liquid transition theory is used to calculate the values of the glass-transition temperature Tg and the temperature and magnetic field dependent activation energy U0(B,T). These parameters are seen to decrease with CNT addition and applied magnetic field. Also, we have found that the undoped sample proves a better transition width, residual resistivity (ρ0) and a higher onset critical transition temperature of about 117.75K. Also, the appearance of a double resistive transition for both samples is a confirmation of the existence of a secondary phase which plays the role of the weak links at the grain boundaries.

Dielectric Properties of PbNb2O6 up to 700°C from Impedance Spectroscopy

Piezoelectric materials have wide band gap and no inversion symmetry. Only the orthorhombic phase of lead metaniobate (PbNb2O6) can be ferroelectric and piezoelectric below Curie temperature, but not the rhombohedral phase. High temperature piezoelectric applications in current decades have revived international interest in orthorhombic PbNb2O6, synthesis of which in pure form is difficult and not well documented. Second problem is that its impedance spectroscopy (IS) data analysis is still incomplete. Present work attempts to fill up these two gaps. Presently found synthesis parameters yield purely orthorhombic PbNb2O6, as checked by X-ray Rietveld analysis and TEM. Present 20 Hz to 5.5 MHz IS from room temperature to 700°C shows its ferroelectric Curie temperature to be one of the highest reported, &gt;574°C for 0.5 kHz and &gt;580°C for 5.5 MHz. Dielectric characteristics and electrical properties (like capacitance, resistance and relaxation time of the equivalent CR circuit, AC and DC conductivities, and related activation energies), as derived here from a complete analysis of the IS data, are more extensive than what has yet been reported in the literature. All the properties show sharp changes across the Curie temperature. The temperature dependence of activation energies corresponding to AC and DC conductivities has been reexamined.

Time-Varying Flexible Least Squares for Thermal Desorption of Gases

Time-varying linear regression via flexible least squares is used to determine temperature-dependent kinetic parameters during low-pressure, steady-state, temperature-programmed desorption from catalytic surfaces. The flexible least squares approach optimizes time-varying parameters by minimizing dynamic and measurement discrepancies between a linear theoretical model and experimental data using linear regression. The effectiveness of this approach is demonstrated by calculation of accurate temperature-dependent activation energies, preexponential factors, and differential conversion functions for the evolution of 3-methyl-2-oxetanone (β-lactone) during the selective oxidation of isobutane over aluminum phosphomolybdates.

Electrical Properties Of Barium Titanate Dispersed Silver Sulphate

Composite materials which are heterogeneous mixtures of two or more solid phases offer value added properties for device applications. When ion conducting Ag2SO4 is dispersed with insulating BaTiO3, enhanced electrical properties are observed along with improved surface morphology. The electrical properties have been derived from the Complex Impedance Spectroscopic studies. Arrhenius Plot, Concentration and Temperature dependent conductivity and activation energy have also been studied. The ionic transference number of the composite system remains unchanged on dispersion thereby retaining its category as Solid Ionic Conductor (SIC). Moreover, 30 Wt% BaTiO3 dispersed in Ag2SO4 offers promisingly enhanced ionic conductivity and reduced activation energy of ion migration. This material has further scope for its utilization as a silver ion conductor in electrochemical applications like solid state batteries, gas sensors etc. The results obtained fit well in the established theories of composites.

Thermodynamic and Energetic Effects on the Diameter and Defect Density in Single-Walled Carbon Nanotube Synthesis

Single-walled carbon nanotube (SWCNT) ensembles are characterized by their defect density and diameter distribution. Here, SWCNTs are grown using chemical vapor deposition with acetylene as the carbon source and cobalt as the catalyst and analyzed ex situ, without any modification or processing, using Raman spectroscopy. The defect density shows an activated temperature dependence (activation energy ∼0.8 eV or ∼80 kJ/mol) with fewer defects at high growth temperatures for a wide range of experimental parameters. This is consistent with a single activated mechanism, such as the catalytic healing of defects, possibly a single simple defect. Consistent with previous reports, we see that low growth temperatures produce smaller diameter SWCNTs than high growth temperatures. Elementary thermodynamic considerations of the strain energy in the lattice constrain the SWCNT diameter distribution and its temperature dependence and appear consistent with our observations. A “phase diagram” for SWCNT growth is construc...

Broad-band conductivity and dielectric spectroscopy of composites of multiwalled carbon nanotubes and poly(ethylene terephthalate) around their low percolation threshold

Composites of multiwalled carbon nanotubes with poly(ethylene terephthalate) (PET–MWCNT) with up to 3 vol% MWCNTs were prepared and characterized by broad-band AC conductivity and dielectric spectroscopy up to the infrared range using several techniques. A very low electrical percolation threshold of 0.07 vol% MWCNTs was revealed from the low-frequency conductivity plateau as well as from DC conductivity, whose values show the same critical power dependence on MWCNT concentration with the exponent t = 4.3. Above the plateau, the AC conductivity increases with frequency up to the THz range, where it becomes overlapped with the absorption of vibrational modes. The temperature dependence down to ∼5 K has shown semiconductor behaviour with a concentration-independent but weakly temperature-dependent small activation energy of ∼3 meV. The behaviour is compatible with the previously suggested fluctuation-induced tunnelling conductivity model through a thin (∼1 nm) polymer contact layer among the adjacent MWCNTs within percolated clusters. At higher frequencies, deviations from the simple universal conductivity behaviour are observed, indicating some distribution of energy barriers for an electron hopping mechanism.

Temperature Dependent Relaxation Behavior of Polycrystralline LuFe2O4 Studied By Dielectric Spectroscopy

Polycrystalline LuFe2O4 ceramics was synthesized by solid state reactions at 1200°C with a mixed gas flow of CO2 and H2. The temperature dependence of zero-field cooled and field-cooled magnetization measurements indicates a clear paramagnetic to ferromagnetic transition in our LuFe2O4 samples. The temperature relaxation behavior of LuFe2O4 ceramics was investigated by an impendence analyzer in the frequency range from 20 Hz to 1 MHz. The contributions of grain boundaries, bulks, and electrodes to the ac conductivity and relaxation processes were identified by fitting the impendence data with an equivalent circuit model. When the temperature is lower than around 220 K, the bulk effect dominates over the contributions of grain boundaries and electrodes on the dielectric relaxation process. However, when the temperature is higher than 220 K, the electrode effect becomes dominant. The polycrystalline LuFe2O4 also show a temperature dependent activation energies for ac conductivities which may relate to non-stoichiometric oxygen deficiency in our samples.

Entropic vs. elastic models of fragility of glass-forming liquids: Two sides of the same coin?

The two most influential atomistic models that have been proposed in the literature to explain the temperature dependent activation energy of viscous flow of a glass-forming liquid, i.e., its fragility, are the configurational entropy model of Adam and Gibbs [J. Chem. Phys. 43, 139 (1965)] and the elastic "shoving" model of Dyre et al. [J. Non-Cryst. Solids 352, 4635 (2006)]. Here we demonstrate a qualitative equivalence between these two models starting from the well-established general relationships between the interatomic potentials, elastic constants, structural rearrangement, and entropy in amorphous materials. The unification of these two models provides important predictions that are consistent with experimental observations and shed new light into the problem of glass transition.

Distinctive Features of Catalytic and Transport Mechanisms in Mammalian Sarco-endoplasmic Reticulum Ca2+ ATPase (SERCA) and Cu+ (ATP7A/B) ATPases

Ca(2+) (sarco-endoplasmic reticulum Ca(2+) ATPase (SERCA)) and Cu(+) (ATP7A/B) ATPases utilize ATP through formation of a phosphoenzyme intermediate (E-P) whereby phosphorylation potential affects affinity and orientation of bound cation. SERCA E-P formation is rate-limited by enzyme activation by Ca(2+), demonstrated by the addition of ATP and Ca(2+) to SERCA deprived of Ca(2+) (E2) as compared with ATP to Ca(2+)-activated enzyme (E1·2Ca(2+)). Activation by Ca(2+) is slower at low pH (2H(+)·E2 to E1·2Ca(2+)) and little sensitive to temperature-dependent activation energy. On the other hand, subsequent (forward or reverse) phosphoenzyme processing is sensitive to activation energy, which relieves conformational constraints limiting Ca(2+) translocation. A "H(+)-gated pathway," demonstrated by experiments on pH variations, charge transfer, and Glu-309 mutation allows luminal Ca(2+) release by H(+)/Ca(2+) exchange. As compared with SERCA, initial utilization of ATP by ATP7A/B is much slower and highly sensitive to temperature-dependent activation energy, suggesting conformational constraints of the headpiece domains. Contrary to SERCA, ATP7B phosphoenzyme cleavage shows much lower temperature dependence than EP formation. ATP-dependent charge transfer in ATP7A and -B is observed, with no variation of net charge upon pH changes and no evidence of Cu(+)/H(+) exchange. As opposed to SERCA after Ca(2+) chelation, ATP7A/B does not undergo reverse phosphorylation with P(i) after copper chelation unless a large N-metal binding extension segment is deleted. This is attributed to the inactivating interaction of the copper-deprived N-metal binding extension with the headpiece domains. We conclude that in addition to common (P-type) phosphoenzyme intermediate formation, SERCA and ATP7A/B possess distinctive features of catalytic and transport mechanisms.

Photoconducting state and its perturbation by electrostatic fields in oxide-based two-dimensional electron gas

The two-carrier transport model as proposed for the two-dimensional electron gas formed at the interfaces of oxide heterostructures is investigated by means of a combined perturbation of near ultra-violet radiation and electrostatic field, applied both separately and simultaneously. Comparison of the photo-response of the prototype systems such as the band insulator LaAlO3 and Mott insulator LaTiO3 films on TiO2 terminated SrTiO3 show remarkably similarities. Two types of non-equilibrium carriers are generated in each system, having a signature of a particular type of perturbation characterized by distinctly different relaxation process. While, the photo-conducting state diminishes in a stretched exponential manner, with a temperature dependent activation energy varying from few tens of meV to ~ 1 to 2 meV on lowering the temperature, and a relaxation time of several hours, the recovery from electrostatic gating occurs in milli-seconds time scale. An attempt is also made to explain the experimental observations using the ab-initio density functional calculations. The calculations show that the electronic transitions associated with near ultra-violet radiation emerge from bands located at ~ 2 eV above and below the Fermi energy, which are the Ti 3d states of the SrTiO3 substrate, and that from the AlO2 (TiO2) layers of the LaAlO3 (LaTiO3) films, respectively. The slow decay of the photo-current to the unperturbed state is explained in terms of the closely spaced Ti 3dxy states in the lower conduction band, which are manifested as flat bands (or localized states) in the band structure. Such localization leads to increased carrier life-times, through the energy-time relationship of the uncertainty principle.

Estimation of temperature effects on autogenous shrinkage of concrete by a new prediction model

To accurately estimate the temperature effects on autogenous shrinkage of concrete, a new prediction model based on time–temperature dependent activation energy is proposed in this study. For this purpose, a series of autogenous shrinkage tests are performed considering different water to cementitious materials ratios and curing temperatures. Results revealed that the temperature dependency of autogenous shrinkage cannot be described solely by the simple time shift of the existing maturity method. In particular, maturity cannot incorporate the cross-over effect of autogenous shrinkage by which later age autogenous shrinkage of concrete, subjected to low curing temperature, usually exceeds the autogenous shrinkage of concrete subjected to relatively high curing temperature. Therefore, to overcome the shortcomings of the maturity method, a new prediction model based on time–temperature dependent activation energy is proposed. Reasonably good agreement is observed between experimental and predicted autogenous shrinkage, thus indicating that the proposed prediction model can be successfully used to estimate the time–temperature effects in the development of autogenous shrinkage. However, the absolute value of autogenous shrinkage strain still cannot be obtained from the proposed model, because the model gives only the development of autogenous shrinkage. Therefore, a prediction model for the absolute value of autogenous shrinkage is needed in order to obtain predicted autogenous shrinkage strain.

Luminescence characteristics and site-occupancy of Eu2+- and Eu3+-doped MgZn2(PO4)2 phosphors

Eu2+- and Eu3+-doped MgZn2(PO4)2 were prepared using the high temperature solid-state reaction method. The excitation spectra in the UV and VUV region, the emission spectra, and the luminescence decay curves of the Eu2+ and Eu3+ ions were investigated. The crystallographic distributions of the Eu2+ and Eu3+ sites in MgZn2(PO4)2 lattices are different. MgZn2(PO4)2:Eu2+ presents a bluish green luminescence with two distinct emission centers at 450 and 525 nm. MgZn2(PO4)2:Eu3+ presents an reddish orange color with luminescence transitions from the 5D0 level of the Eu3+ ions. There exist two Eu2+-crystallographic sites in MgZn2(PO4)2:Eu2+, which occupy both Mg2+ and Zn2+ sites. However, there is only one Eu3+-site in the MgZn2(PO4)2:Eu3+ lattices, which was confirmed by site-selective excitation and emission spectroscopy, and the luminescence decay in the 5D0 → 7F0 region of the Eu3+ ions under a pulsed, tunable, narrowband dye laser. The Eu3+ ion was suggested to occupy the octahedral Mg2+-site in the MgZn2(PO4)2 lattices. The spectral characteristics, the temperature-dependent luminescence intensity and activation energies of thermal stabilities and the microstructures were discussed on the basis of the crystal structure and the luminescence results.

Defect luminescence at grain boundaries in multicrystalline silicon

AbstractMulticrystalline silicon is characterised by extended defects like dislocations and grain boundaries. Grain boundaries in this material can show radiative recombination in the near infrared (NIR) region even at room temperature (Dreckschmidt et al., Proc. 23rd European Photovoltaic Solar Energy Conference, Valencia, Spain, 2008, p. 407. [1]). Here we present results of further investigations by the electroluminescence method regarding these phenomena. It turned out that carrier recombination at these centres is negligible. From temperature dependent measurements activation energies of the different centres were determined. Defect schemes for the grain boundary luminescence are proposed (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Alignment and Relaxation Dynamics of Dye Molecules in Host−Guest Inclusion Compounds As Probed by Dielectric Spectroscopy

The alignment and relaxation dynamics of a polar dye molecule, N,N-dimethyl-4(4-nitrophenylazo)aniline (DNAA), in zeolite L and perhydrotriphenylene (PHTP) channels were investigated by means of a combination of optical, dielectric, and quantum-chemical methods. Both the zeolite L and PHTP channels enable the dye molecules to align along the channel axis. An amplified net dipole moment of DNAA in PHTP is observed and attributed to enhanced 1D close alignment of dye molecules. In zeolite L channels, a concentration gradient is found with aggregation at the channel entrances. The dynamics of the dye in zeolite L channels reveals localized conical rotational fluctuation modes following Arrhenius-type activation with energy of 0.31 eV, which we assign to small noninteracting fluctuating polar units of the dyes being loosely aligned or isolated. Unlike zeolite L, relaxations in PHTP are characterized by cooperative wobbling motions interpreted as increased intermolecular dipole interaction due to a closely packed one-dimensional array. Temperature-dependent activation energies of 0.25 eV below 0 degrees C and 0.37 eV at ambient temperature reflect the role of the soft channel walls in the activation process.