Characteristic Activation Energy Research Articles

Study of the oxygen reduction reaction on pure and Zr-doped YMnO3+δ SOFC electrode

The effect of Zr-doping on the novel SOFC cathode YMnO3 and its O2-Reduction Reaction (ORR) was studied using electrochemical impedance spectroscopy (EIS) as a function of temperature (500 < T < 800 °C) and the oxygen partial pressure (10−4 < pO2 < 1 atm). EIS technique allows identifying three processes controlling the ORR for each electrode, differentiated by their characteristic frequency, activation energies and pO2 dependence. High frequency contribution, only observed at 600°C for both electrodes, was attributed to oxygen ion transfer trough the electrode/electrolyte interface. In both electrodes, the Intermediate frequency contribution was described by a Gerischer impedance, related to a co-limiting process between oxygen ion diffusion and a surface process. However, whereas the EIS characteristics for YMnO3 can be explained by a surface process controlled by the electron transfer from oxide to O-adsorbed, it seems related for Y0.9Zr0.1MnO3 to the probability of finding a second free-surface site near to a diatomic intermediated for the O2 dissociation. These differences are in agreement with the improvement of the electrical properties when YMnO3 is doped with Zr. The low frequency contribution became important as pO2 decreased and temperature increased, and its characteristics would indicate a secondary surface process, co-limited with O2 gas diffusion for Y0.9Zr0.1MnO3 porous electrode.

Exploring the role of lignin structure in molecular dynamics of lignin/bio-derived thermoplastic elastomer polyurethane blends

The relaxation behavior of two lignins (Alcell organosolv, OSL, and hydroxypropyl modified Kraft, ML) and bio-based thermoplastic polyurethane (TPU) blends have been studied by Differential scanning calorimetry (DSC), Dynamic-mechanical analysis (DMA) and Dielectric relaxation spectroscopy (DRS). The effect of blending on the glass and local relaxation processes as a function of composition, frequency, and temperature has been assessed. The dielectric spectra were resolved into dipolar relaxations as well as conductive processes for differing blend compositions. Characteristic relaxation times, activation energies and dielectric relaxation strengths of lignin/xTPU blends were also investigated. It was found that blending suppresses the α-relaxation process of the blends compared to virgin TPU. On the other hand, while the position of the β-relaxation was not influenced by blending, a reduction of the activation energies, Ea, of this process was observed in the lignin/xTPU blends. Finally, changes are observed in the conductivity behavior of both blends, with conductivity processes more favorable for the OSL/xTPU blends.

Dielectric Relaxation Dynamics of Clay‐Containing Low‐Density polyethylene Blends and Nanocomposites

Series of clay‐containing nanocomposites have been prepared and investigated using frequency‐domain dielectric spectroscopy at different temperatures. Different matrix materials have been used: neat low‐density polyethylene (LDPE) with and without compatibilizer and co‐continuous blends of LDPE with two grades of polystyrene‐b‐poly(ethylene‐co‐butylene)‐b‐polystyrene (SEBS) copolymers. Two major relaxation modes were detected in the dielectric losses of all the nanocomposites and associated with Maxwell–Wagner–Sillars interfacial polarization and dipolar relaxation, respectively. Characteristic relaxation rates, activation energies, dielectric strength, and shape parameters of these relaxation mechanismes were calculated and discussed for the LDPE/clay nanocomposites. The addition of compatibilizer was found to slightly increase the dielectric loss of the nanocomposites while slowing the dynamics due to an improved dispersion. When combined with a high loading of nanofiller (15%), the compatibilizer addition led to low‐frequency dispersion. A new relaxation process was then observed for the nanocomposites with the blend matrix. Several speculations were made as to the origin of this phenomenon, all of which were related to the SEBS phase. POLYM. ENG. SCI., 60:968–978, 2020. © 2020 Society of Plastics Engineers

Non-isothermal crystallization kinetics investigation of different zones at the joining area of a bulk metallic glass welded by friction stir spot welding (FSSW)

The connection cross section of a Ti-based bulk metallic glass welded with friction stir spot welding method, was investigated using Vickers micro-hardness and SEM. Regarding the results, four zones were detected at the connection site. The crystallization kinetics of the four zones identified at the joining area, was studied using the DSC at heating rates of 10–50 K min−1. Apparent activation energy (Ec), frequency factor (A) at characteristic temperatures (Tg, Tx1, Tp1) and local activation energy (Ec(α)) variations of different zones during the first crystallization transformation were calculated by isokinetics and isoconversional models. The crystallization conversion function for each zone was determined using the Sestak-Berggren equation. Sub-tool tip stir zone (TSZ) showed the highest thermal stability. The thermo-mechanically affected zone (TMAZ) with the highest onset crystallization temperature (Tx1) has a higher thermal stability in comparison to sub-tool shoulder stir zone (SSZ) with the lowest thermal stability and base metal (BM).

Effects of deposition temperature on microstructure, cation distribution and electrical properties of Mn1.56Co0.96Ni0.48O4 thin films grown by RF magnetron sputtering

In this work, Mn1.56Co0.96Ni0.48O4 (MCN) thin films have been deposited at 200 °C, 400 °C and 750 °C by radio frequency magnetron sputtering. The crystallinity properties and surface morphologies of the films are greatly influenced by the deposition temperature according to the XRD and FESEM results. The ionic valence states of Mn, Co and Ni were observed by X-ray photoelectron spectroscopy (XPS), the cations distribution and formulas of the MCN films are determined. The Mn3+/Mn4+ ratio increases with the higher deposition temperature, and the films deposited at 400 °C shows maximum concentration of Mn3+-Mn4+ pairs. The results of electrical properties reveal that the resistivity of the MCN films falls down with increasing deposition temperature, while all films possess similar characteristic temperature and activation energy. The deposition temperature dependent resistivity mechanism of the MCN films was further analyzed and discussed based on the crystallinity and XPS results. This study will improve the understanding of the deposition temperature effects on MCN films, which is valuable to its better application in uncooled IR detection.

Quasiparticle Screening near a Bosonic Superconductor-Insulator Transition Revealed by Magnetic Impurity Doping.

Experiments show that the Cooper pair transport in the insulator phase that forms at thin film superconductor to insulator transitions (SIT) is simply activated. The activation energy T_{0} depends on the microscopic factors that drive Cooper pair localization. To test proposed models, we investigated how a perturbation that weakens Cooper pair binding, magnetic impurity doping, and phase frustration affects T_{0}. The data show that T_{0} decreases monotonically with doping in films tuned farther from the SIT and increases and peaks in films that are closer to the SIT critical point. The observations provide strong evidence that the bosonic SIT in thin films is a Mott transition driven by Coulomb interactions that are screened by virtual quasiparticle excitations. This dependence on underlying fermionic degrees of freedom distinguishes these SITs from those in microfabricated Josephson Junction arrays, cold atom systems, and likely in high temperature superconductors with nodes in their quasiparticle density of states.

Use of Passive Age Sensors for Projecting Remaining Thermal Life of Materials

This paper proposes use of passive thermal age sensors and empirical correlation models to project remaining useful life of thermally degradable products and materials. Thermal age sensors, comprising a selected polymeric matrix and conductive fillers, change resistance as the matrix thermally degrades in the same thermal environment as the monitored product or material. Thermal age sensor resistance represents the integrated time-temperature condition of the sensor at its characteristic activation energy. Empirical models correlate sensor resistance to a selected property of the material utilizing multi-temperature thermal aging data of the monitored material. These correlation models project the current condition of the selected product property, or, if end-of-life properties are specified, these models project the percentage of remaining design life of the material. Several applications of this approach are discussed utilizing thermal age sensors attached to monitored materials. An approach utilizing two thermal age sensors is introduced that allows a single tag to predict selected properties of many different materials. PHM tags utilizing passive thermal age sensors do not require an internal source of electrical power or internal memory, eliminating the need for batteries and significantly reducing data management issues. This approach can be expanded to a wide range of products and materials when sufficient thermal age data is available.

Plasma Enhanced CVD Growth of Graphene on Cu and Ge

Thermal chemical vapor deposition (T-CVD) is currently the process of choice to grow large area graphene with good quality on metal surfaces like copper (Cu) or nickel (Ni) due to their catalytic properties. However the typical growth temperatures are still around 1000°C, and the process cannot be easily adapted to alternative, i.e. non-metallic, substrates. Regarding these issues we used a commercially available 4” Aixtron Black Magic system for establishing a plasma enhanced CVD process (PE-CVD) for graphene growth. A pulsed DC plasma was applied to dissociate the precursor material –methane- already in the gas phase and hence reduce the growth temperature. This process facilitates graphene fabrication with good quality down to a growth temperature of 600°C on Cu substrates. By studying the growth rate for different temperatures we have been able to extract a characteristic activation energy of 1.8 eV in PE-CVD, which is reduced by 2.2 eV as compared to T-CVD owing to the dissociation of methane already in the gas phase. In order to understand the plasma enhanced growth process, we analyzed the graphene formed for different growth times at a growth temperature of 700°C. After 45 min (Fig. 1a, top) we observe amorphous carbon around high quality graphene flakes. This is supported by Raman measurements (Fig. 1b), where Raman signatures of both, amorphous carbon and crystalline graphene are found. By increasing the growth time to 3 h, a continuous graphene film is obtained (Fig. 1a, bottom). In Raman spectroscopy, the amorphous signal disappears and Raman signatures of monolayer graphene with strongly reduced defect peak are detected. This indicates that through a recrystallization process the amorphous carbon turns into a graphene film around the crystalline graphene nuclei. The fabricated graphene film has a sheet resistance down to < 0.5 kΩ/sq. when transferred onto a Si/SiO2 substrate. Establishing a PE-CVD process at growth temperatures down to 600°C indicates that the need for catalytical substrates become superfluous. We thus adapted our approach to non-metallic substrates. As an example, we chose crystalline Ge(100), a CMOS compatible material that is used for example in broadband photodetectors in combination with graphene [1]. Graphene growth on Ge is typically performed by T-CVD at temperatures of 900°C or above. Such high temperatures might be critical for practical applications because of the temperature dependent dopant diffusion. [2] We demonstrate that our PE-CVD process can be applied to reduce the growth temperature of graphene on Ge(100) down to 800°C. We observed that by increasing the growth time from minutes to hours a 2D peak appears and the D and G bands isolate indicating growth is initiated from amorphous carbon and proceed to nanocrystalline graphene. In contrast to PE-CVD growth on Cu, a significant defect peak is observed for graphene grown on Ge(100), which we attribute to the numerous nucleation sites arising from the 100 nm large terraces as reported by Luskosius et al. [3] Fig. 1: Graphene growth in PE-CVD at 700°C on Cu foil at different growth times. a) Exemplary SEM images of the graphene flakes at growth times of 45 min (top) and 3 h (bottom). b) Average Raman spectra of the samples presented in a), extracted from the corresponding Raman maps.

Properties of the surface generation-recombination noise in 1.94 µm GaSb-based laser diodes

A detail investigation of generation-recombination (g-r) noise in 1.94 µm GaSb-based type-I ridge waveguide laser diodes (LDs) has been performed in a temperature range (230–295) K. Lorentzian-type noise spectra have been observed in the current range below the threshold at the forward and reverse biases of the LDs with the same characteristic time (3.7 μs) and activation energy (≈0.37 eV) of charge carriers transitions associated with the g-r processes. An equivalent electrical circuit possessing the voltage noise source is presented, which allows the description of both the current-voltage characteristic and the voltage fluctuation spectral density of the laser diode. Results indicate that the origin of the g-r noise in the investigated samples is the surface recombination caused by the surface leakage current channel between n+GaSb and p+GaSb contacts, which is practically independent from the applied bias polarity.

Determination of charge transport activation energy and injection barrier in organic semiconductor devices

Charge carrier transport in organic semiconductor devices is thermally activated with characteristic activation energies in the range of 0.2–0.6 eV, leading to strongly temperature-dependent behaviour. For designing efficient organic semiconductor materials and devices, it is therefore indispensable to understand the origin of these activation energies. We propose that in bilayer organic light-emitting diodes (OLEDs) employing a polar electron transport layer, as well as in metal-insulator-semiconductor (MIS) devices, the hole injection barrier Einj and the hole mobility activation energy Eμ can be decoupled from each other if temperature-dependent capacitance-frequency (C-f-T) and MIS-CELIV (charge extraction by linearly increasing voltage) experiments are combined. While the C-f-T signal contains information of both injection and transport, the CELIV current is expected to be insensitive to the electrode injection properties. We employ numerical drift-diffusion simulations to investigate the accuracy of this analytical parameter extraction approach and to develop criteria for its validity. We show that the implicit assumption of constant charge density and field profiles leads to systematic errors in determining the activation energies. Thus, one should be aware of the intrinsic limitations of the analytical Arrhenius fit, and for more accurate parameter determination a full drift-diffusion modelling is advised. Applying the analytical method to a standard bilayer OLED, we find that the total activation energy of 0.5 eV for the hole current can be split into contributions of ≈0.25 eV each for injection barrier and mobility. Finally, we also discuss the broader applicability of this method for other device stacks and material combinations.

The crossover between tunnel and hopping conductivity in granulated films of noble metals

The conductivity of thin films composed by clusters of gold and silver nanoparticles has been studies in a wide range of temperatures. The switch from a temperature independence to an exponential thermal dependence of the conductivity manifests the crossover between the tunnel and thermally activated hopping regimes of the electronic transport at the temperature of 60 °C. The characteristic thermal activation energy that governs hopping of electrons between nanoparticles is estimated as 1.3 eV. We have achieved a good control of the composition and thicknesses of nano-cluster films by use of the laser ablation method in colloidal solutions.

Anomalous heat transfer in two polymorphs of para-bromobenzophenone

The thermal conductivity of a polycrystalline sample of monoclinic polymorph of para-bromobenzophenone in the T = 3–320 K temperature range was measured using steady-state linear heat flow. The temperature dependences of thermal conductivity are presented as the sum of two independent contributions: a contribution that corresponds to the thermal conductivity of an orientationally ordered crystal structure, and a new additional thermally activated contribution that manifests itself above 130 K. A comparison is made with the data on the thermal conductivity of a single crystal triclinic polymorph of para-bromobenzophenone. It is established that the contribution corresponding to the thermal conductivity of the orientationally ordered crystal structure depends on the molecular crystal packing, and the characteristic activation energy of the thermal activation contribution, which is caused by the intramolecular vibrations of the C-Br bond, does not depend on the grain size or on the structure of the sample.

Impact of tellurium on glass transition and crystallization in the Ge-Se-Te-Bi system

Samples with compositions of Ge20Se67−xTexBi13 (x=0, 5, 10, 15 and 20mol%) were prepared and characterized by XRD, Raman spectroscopy and DSC. XRD patterns and Raman spectra present consistent evidences of glass transition and crystallization in association with the variation of the structure network due to the substitution of Te for Se. Precipitation of thermoelectric (TE) crystals experiences an evolution from Bi2Se2Te (BST) to Bi2SeTe2 (BTS) phases with the increasing Te content. Based on DSC data performed at five heating rates, crystallization kinetics is analyzed by using Kissinger's relation, Ozawa model and Augis-Bennett approximation. Different methods show the identical evaluations of characteristic activation energies (Ec), while additional parameters of frequency factor (K0) and Avrami exponent (n) are available with AB method. In particular, calculated Ec and K0 demonstrate that with the substitution of Se by Te the crystallization process of the target TE BST crystal phase is much enhanced while that of the undesired GeSe2 phase is slightly hindered.

Diffused phase transition of Ba1−x Eu x TiO3 ceramics

By applying the sol-gel method, we fabricated Ba1−x Eu x TiO3 (BET) ceramics as a single peroveskite phase in the composition range of x = 0 ~ 0.20. The BET ceramics displayed a ferroelectric phase transition temperature that changed from 120 ◦C to 80 ◦C, and exhibited the coexistance of the tetragonal, and cubic structures as the Eu composition was increased. They also displayed anomalous dielectric behaviors related to structural relaxation in the temperature range from 200 ◦C to 600 ◦C. We considered the Arrhenius temperature dependence of the dielectric relaxation time by using the electric modulus formalism. The characteristic activation energy was thought to be related with the substitution of Eu (Eu2+, Eu3+) ions for Ba2+ or Ti4+ ions in the perovskite structure.

Intrinsic correlation between β-relaxation and spatial heterogeneity in a metallic glass.

β-relaxation has long been attributed to localized motion of constituent molecules or atoms confined to isolated regions in glasses. However, direct experimental evidence to support this spatially heterogeneous scenario is still missing. Here we report the evolution of nanoscale structural heterogeneity in a metallic glass during β-relaxation by utilizing amplitude-modulation dynamic atomic force microscopy. The successive degeneration of heterogeneity during β-relaxation can be well described by the Kohlrausch–Williams–Watts equation. The characteristic relaxation time and activation energy of the heterogeneity evolution are in accord with those of excess enthalpy release by β-relaxation. Our study correlates β-relaxation with nanoscale spatial heterogeneity and provides direct evidence on the structural origins of β-relaxation in metallic glasses.

Extraction of activation energies from temperature dependence of dark currents of SiPM

Despite several advantages of Silicon Photomultipliers (SiPM) over Photomultiplier Tubes (PMT) like the increased photon detection efficiency (PDE), the compact design and the insensitivity to magnetic fields, the dark count rate (DCR) of SiPM is still a large drawback. Decreasing of the SiPM dark count rate has become a modern task, which could lead to an enormous enhancement of the application range of this promising photo-detector. The main goal of this work is to gain initial information on the dark generation and identify the dominating contributions to dark currents. The chosen approach to fulfill this task is to extract characteristic activation energies of the contributing mechanisms from temperature dependent investigations of dark currents and DCR. Since conventional methods are not suited for a precise analysis of activation energies, a new method has to be developed. In this paper, first steps towards the development of a reliable method for the analysis of dark currents and dark events are presented.

Effects of configurational changes on molecular dynamics in polyvinylidene fluoride and poly(vinylidene fluoride-trifluoroethylene) ferroelectric polymers

We present a comparative study of proton dynamics in unpoled non-ferroelectric polymer polyvinylidene fluoride (PVDF) and in its trifluoroethylene containing ferroelectric copolymer (with 70/30 molar proportion), using quasi-elastic neutron scattering. The neutron data reveal the existence of two distinct types of molecular motions in the temperature range investigated. The slower motion, which is characterized in details here, is ascribed to protons jump diffusion along the polymeric carbon chains, while the faster motion could be attributed to localized rotational motion of methylene groups. At temperatures below the Curie point (Tc ∼ 385 K) of the composite polymer, the slower diffusive mode experiences longer relaxation times in the ferroelectric blend than in the bare PVDF, although the net corresponding diffusion coefficient remains comparatively the same in both polymers with characteristic activation energy of EA ≈ 27–33 kJ/mol. This arises because of a temperature dependent jump length r0, which we observe to be effectively longer in the copolymer, possibly due to the formation of ordered ferroelectric domains below Tc. Above Tc, there is no appreciable difference in r0 between the two systems. This observation directly relates the known dependence of Tc on molar ratio to changes in r0, providing fundamental insight into the ferroelectric properties of PVDF-based copolymers.

Experimental study on the coalescence process of SiO2 supported colloidal Au nanoparticles

We report on an experimental study of the coalescence-driven grow process of colloidal Au nanoparticles on SiO2 surface. Nanoparticles with 30, 50, 80, 100nm nominal diameters on a SiO2 substrate were deposited, from solutions, by the drop-casting method. Then, annealing processes, in the 573–1173K temperature range and 900–3600s time range, were performed. Using scanning electron microscopy analyses, the temporal evolution of the nanoparticles sizes has been studied. In particular, for all classes of nanoparticles, the experimental-obtained diameters distributions evidenced double-peak shapes (i. e. bimodal distributions): a first peak centered (and unchanged changing the annealing temperature and/or time) at the nominal diameter of the as-deposited nanoparticles, 〈D0〉, and a second peak shifting at higher mean diameters, 〈DC〉, increasing the annealing temperature and/or time. This observation suggested us a coalescence-driven growth process of a nanoparticles sub-population. As a consequence, the temporal evolution of 〈DC〉 (for each class of nanoparticles and each annealing temperature), within the well-established particles coalescence theoretical framework, has been analyzed. In particular, by the analyses of the experimental data using relations as prescribed by the theoretical model, a characteristic size-dependent activation energy for the Au nanoparticles coalescence process has been evaluated.

Investigating temperature degradation in THz quantum cascade lasers by examination of temperature dependence of output power

In this paper, we demonstrate a method to investigate the temperature degradation of THz quantum cascade lasers (QCLs) based on analyzing the dependence of lasing output power on temperature. The output power is suggested to decrease exponentially with some characteristic activation energy indicative of the degradation mechanism. As a proof of concept, Arrhenius plots of power versus temperature are used to extract the activation energy in vertical transition THz QCLs. The extracted energies are consistent with thermally activated longitudinal optical-phonon scattering being the dominant degradation mechanism, as is generally accepted. The extracted activation energy values are shown to be in good agreement with the values predicted from laser spectra.

Chiral power change upon photoisomerization in twisted nematic liquid crystals

In this work, we use the photoisomerization of azobenzenes, a phenanthrospirooxazine, and a fulgide in a twisted nematic liquid crystalline phase to change the chiral twisting power of the system. The changes are probed by the rotatory power of linearly polarized light. Time resolved and steady state experiments are carried out. The chiral change and the photoisomerization process have similar characteristic recovery times and activation energy, thus probing that the change is induced by the modification in the chemical composition of the photochromic dopant system. The amplitude of the light twisting power change correlates with the order change in the liquid crystal (LC) but not with the modification in the absorption characteristics of the system. This indicates that the driving force of the chiral change is the microscopic order modification in the LC phase that affects the helical pitch of the phase.