Values Of Thermal Activation Energy Research Articles

Tuning β-Si3Al3O3N5 phosphors by Eu or Ce doping for white light-emitting diodes (wLEDs)

SiAlON-based phosphors have garnered considerable attention in the field of white lighting owing to their excellent thermal stability, and rare earth doped SiAlON-based phosphors are expected to be used in a new generation of lighting sources. In this work, Eu2+ or Ce3+ doped Si3Al3O3N5 phosphors were synthesized by high temperature solid phase method. The microstructure of the phosphors exhibited irregular grain. The two phosphors exhibit green and cyan luminescence under 365 nm excitation, which are derived from the 5d-4f transitions of Eu2+ and Ce3+, respectively. The quenching concentrations are 3 % and 4 % for Eu2+ and Ce3+doped phosphors, respectively. The fluorescence intensity of Eu2+ and Ce3+ activated SiAlON powders decreased to 50 % of the room temperature intensity at 179 °C and 170 °C, respectively. The thermal activation energy values of the two phosphors were 0.35 eV, and the coordinate configuration diagram was constructed to comprehensively analyze the thermal quenching behavior of phosphors. Finally, the two phosphors powders were mixed with red and blue fluorophore to obtain white LEDs, showing low color temperature, high color rendering, high power unsaturation and long half-life. The SiAlON-based phosphors with excellent thermal stability obtained in this work achieve different wavelengths of luminescence in the green region. The results show that the β-Si3Al3O3N5 phosphor can be used as a candidate material for high-quality green light supplement for wLEDs.

Temperature- and size-dependent photoluminescence in colloidal CdTe and Cd x Zn1−x Te quantum dots

Semiconductor colloidal quantum dots (QDs) of CdTe and alloyed Cd x Zn1−x Te QDs with N-acetyl-L-cysteine capping ligands are synthesized by a reflux method in aqueous solution. Alloying provides a new degree of freedom to tune the optical and electronic properties of the nanocrystals. The photoluminescence (PL) of Cd x Zn1−x Te QDs is sharper and displays a highly enhanced quantum yield (QY) of 65% relative to the 16% of CdTe QDs. The fluorescence of Cd x Zn1−x Te QDs is observed to be highly stable for over 12 months without degradation, while that of CdTe QDs begins to mildly flocculate around 8 months of storage. To characterise the material structure and composition, UV-Vis absorption spectroscopy, x-ray powder diffraction, transmission electron microscopy, and inductively coupled plasma mass spectrometry measurements are carried out. To understand the fundamental processes that play in the luminescence behaviour, temperature- and size-dependent PL spectra are investigated in the range 80–300 K. The Varshni and O’Donnell equations fit well on the PL peak emission energies and the Huang–Rhys parameter indicates the strengthening of exciton–phonon coupling in the nanocrystals upon alloying and with decreasing nanocrystal sizes. PL linewidth analysis reveals that the inhomogeneous broadening is considerably reduced in Cd x Zn1−x Te QDs relative to CdTe. Moreover, the quantum confinement effect of the nanocrystals leads to an increase in exciton–acoustic phonon interactions with the coefficients ranging between 26.9 and 95.6 µeV K−1 compared to the bulk CdTe value of 0.72 µeV K−1. Exciton–longitudinal optical phonon interactions are made stronger by the ZnTe alloying with the coefficients lying in the range between 24.8 and 41.7 meV and also with the effect of increasing crystal size. An Arrhenius plot of PL integrated area is used to calculate the thermal activation energy value E a of the non-radiative recombination channel, which is 132 meV for CdTe QDs and a higher value of 185 meV for Cd x Zn1−x Te QDs. This is consistent with the observed QY enhancement in Cd x Zn1−x Te QDs as a higher E a value indicates reduced generation of non-radiative recombination centres and a decrease in defect densities upon alloying. Cd x Zn1−x Te QDs with enhanced fluorescence properties serve both as a medium for studying fundamental effects of alloying and its properties, and for practical applications such as biomedical labelling and optoelectronics.

The effect of Sn doping on the optical properties and thermal stability of rare-earths-doped Na3Gd(VO4)2 double vanadate materials

The Sn single doped and Sn/rare-earth (RE) ions (Eu3+, Dy3+, and Sm3+) co-doped Na3Gd(VO4)2 (NGVO) materials were synthesized using a sol-gel method by mixing stoichiometric amounts of the precursors. All the samples gave intense diffraction peaks without any shift in the 2θ values and did not show any impurity phases. The scanning electron microscope images of Sn and RE ions co-doped samples did not show significant changes with the dopant ions, and the elements present in the samples was analyzed with energy dispersive X-ray spectrometry. Introducing Sn into the V site showed enhanced optical characteristics due to the formation of oxygen vacancies and lattice distortions in the host lattice. Similarly, when the NGVO host was co-doped with Sn and RE ions, the optical properties of the RE were enhanced. Temperature-dependent optical properties of the Sn co-doped NGVO: RE samples were also fortified due to the Sn co-doping and showed an increase in thermal activation energy values when compared with NGVO: RE samples.

Luminescence and temperature sensing abilities of zincate phosphors co-doped bismuth Bi3+ and lanthanide Eu3+/Sm3+

We proposed the FIR method-based optical materials involving dual luminescent centers of Bi3+ and lanthanide (Ln): Eu3+/Sm3+. Resulted from the f-f characteristic transition of Eu3+, the tunable color region changing from cyan to jacinth with the augment of Eu3+/Bi3+ ratio exists . Owing to Bi3+ being susceptive to crystal surroundings, and asymmetric crystal sites of Sm3+ in the host, phenomena respectively involving redshift and energy level splitting occur. In addition, benefiting from the unshielded 3d orbitals, the thermal quenching of Bi3+ ion occurs through energy-level crossing relaxation (ELCR), while the heating quenching of Ln3+ ions takes place due to the shielded 4f orbitals via multi-phonon deexcitation (MPD). Considering the symmetric crystal environment, stronger sensitivity compared with Sm3+ ion to the outer environment due to its smaller value of thermal activation energy and the weaker crystal field than CAZO:Bi3+,Sm3+, a more excellent temperature sensing sensitivity is excepted by applying Eu3+ as the temperature-detecting signal.

Gamma irradiation effects on the structural, thermal and optical properties of samarium doped lanthanum–lead- boro-tellurite glasses

Sm3+- doped Lanthanum Lead Boro Tellurite glasses are synthesized through Melt-quenching method and the effect of gamma irradiation on the properties of the glasses are evaluated before and after exposed to gamma radiation for 30 kGy. Samples exhibit colour change from pale yellow to dark red, indicates the creation of radiation induced defects. Glasses retain amorphous nature even after gamma irradiation. Significant mass loss and the thermal activation energy values are estimated, which shows increase in thermal stability is observed after gamma irradiation due to creation of non-bridging oxygens, results in the increase in the rigidity of the glasses. Optical studies reveal the decrease in the energy band gap after irradiation. Also, the luminescence studies show the emission property is unaltered and sustains the radiation effects up to 2 mol% Sm3+- in the host matrix, that is, sample shows radiation resistant behavior. In addition, glass samples exhibit single exponential behavior before irradiation while bi-exponential behavior after irradiation. The results show that the prepared glasses exhibit radiation resistant behavior for gamma irradiation up to 30 kGy, could be potential candidates for radiation resistant applications.

Impact of Sn4+ Substitution at Cr3+ Sites on Thermoelectric and Electronic Properties of p-Type Delafossite CuCrO2

The effects of Sn4+ substitution at Cr3+ sites on the thermoelectric and electronic properties of p-type delafossite CuCrO2 have been studied. CuCr1−xSnxO2 (x = 0.01, 0.03) samples were prepared via conventional solid-state reaction. The Seebeck coefficient results confirmed that all the samples exhibited p-type conduction. X-ray photoelectron spectroscopy verified the presence of Sn4+ ions and the appearance of mixed-state Cr3+/Cr2+ ions. The experimental results revealed that addition of Sn impacted the electrical conductivity due to the mixed Cu1+/Cu2+ states while the Seebeck coefficient was affected by the mixed Cr3+/Sn4+ states. The electrical conductivity was governed by the polaron hopping mechanism, and the large Seebeck coefficient was controlled by the spin and orbital degeneracy of the mixed Cr3+/Sn4+ ions. The electrical conductivity activation energy values were 0.410 eV and 0.407 eV while the thermal activation energy values were 0.208 eV and 0.160 eV, for x = 0.01 and 0.03, respectively. The results confirm that Sn4+ substitution at Cr3+ sites impacted the Seebeck coefficient and electronic behavior of delafossite CuCrO2 oxide even for low dopant levels of x = 0.01 and 0.03, resulting in p-type conduction and optically conducting materials.

Electrical transport properties of focused ion beam lithography Au contacts on PbTiO3 nanorods

PbTiO3 nanorods with tetragonal phase were synthesized by hydrothermal method and heat treatment, and temperature-dependent electrical transport properties of individual PbTiO3 nanorod were investigated. The results show that the conductivities of PbTiO3 nanorods are gradually enhanced with temperature increasing from 77.4 to 295[Formula: see text]K, and exhibit the typical nonlinear I–V characteristics. The barrier height between Au electrode and nanorod is reduced from 0.137 to 0.088[Formula: see text]eV with increasing bias from 0.2 to 1[Formula: see text]V. The corresponding values of thermal activation energies are 0.172 and 0.06[Formula: see text]eV below the conduction band for 180–295 and 77.4–180[Formula: see text]K, respectively. This semiconductor-like behavior may result from the larger number of surface defects or localized states in the amorphorized PbTiO3.

Spectroscopic, thermal and geometrical structures of Cu(II) azo rhodanine complexes

Copper(II) complexes of azo rhodanine (1–3) were prepared and characterized by elemental analyses, IR spectra, X-ray diffraction, thermogravimetric analysis (TGA), molar conductance and magnetic susceptibilities as well as mass spectra. The IR spectra showed that the ligands act as a monobasic bidentate coordinating via nitrogen atom of −N=N− group and oxygen atom of enol form. The X-ray diffraction pattern of complex (2) is polycrystalline phase. Quantum chemical parameters and geometric structural of the complexes were theoretically computed and the results were studied. The alternating current conductivity (σac) and dielectric properties of Cu(II) complexes (1–3) were investigated in the frequency range of 0.1–100kHz and temperature range of 303–500K. The values of thermal activation energies of electrical conductivity (ΔE1 and ΔE2) for Cu(II) complexes are calculated and discussed. It was found that the electrical conductivity depends on the nature of substituents of the complexes. The correlated barrier hopping (CBH) is the dominant conduction mechanism for all complexes.

Temperature Dependence of Electrical Properties of Organic Thin Film Transistors Based on pn Heterojuction and Their Applications in Temperature Sensors

Organic thin film transistors based on an F16CuPc/α6T pn heterojunction have been fabricated and analyzed to investigate the temperature dependence of electrical properties and apply in temperature sensors. The mobility follows a thermally activated hopping process. At temperatures over 200 K, the value of thermal activation energy (EA) is 40. 1 meV, similar to that of the single-layer device. At temperatures ranging from 100 to 200 K, we have a second regime with a much lower EA of 16.3 meV, where the charge transport is dominated by shallow traps. Similarly, at temperatures above 200 K, threshold voltage (VT) increases linearly with decreasing temperature, and the variations of VT of 0.185 V/K is larger than the variation of VT (~0.020 V/K) in the single layer devices. This result is due to the interface dipolar charges. At temperatures ranging from 100 K to 200 K, we have a second regime with much lower variations of 0.090 V/K. By studying gate voltage (VG)-dependence temperature variation factor (k), the maximum value of k (~0.11 dec/K) could be obtained at VG = 5 V. Furthermore, the pn heterojunction device could be characterized as a temperature sensor well working at low operating voltages.

Effects of Deformation Heating on Constitutive Analysis of a New Al-Mg-Si-Cu Alloy during Hot Compression

A new Al-Mg-Si-Cu aluminum alloy was hot compressed at temperatures between 320°Cand 530°Cand strain rates between 0.001 s1 and 10 s1 using a Gleeble-1500 thermo-simulation machine. The effects of deformation heating on flow stress were analyzed and corrected, and a comparison was made of hyperbolic-sine constitutive equations with and without correction. The results show that the deformation heating apparently increases the flow stress at strain rates of 1 s1, 10 s1. The thermal activation energy values were 325.12 KJ/mol and 304.47 KJ/mol before and after correction, respectively.

Synthesis and photoluminescence properties of the 4H-SiC/SiO2 nanowires

4H-SiC/SiO2 nanowires are synthesized and the temperature-dependent photoluminescence (PL) properties of the nanowires are studied. Their structure and chemical composition are studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectra. At room temperature, an ultraviolet PL peak and a green PL band are observed. From the PL spectrum measured in the temperature range from 80 K to 300 K, the free excition emission, donor bound excition emission and their multiple-phonon replicas have been observed in ultraviolet region, and their origins have been identified. Moreover, it has been found that the temperature dependence of the free exciton peak position can be described by standard expression, and the thermal activation energy values extracted from the temperature dependence of the free exciton and bound exciton peak integral intensity are about 40 meV and 181 meV, respectively. 2014 Tianjin University of Technology and Springer-Verlag Berlin Heidelberg.

Effect of high energy milling on the synthesis temperature, magnetic and electrical properties of barium hexagonal ferrite

Nanoparticles of barium hexagonal ferrite (BaFe12O19) have been synthesized by initial high energy milling of the precursors and sintering subsequently. X-ray Diffraction pattern reveals the hexagonal phase with a crystallite size of 42nm. Scanning Electron Micrograph (SEM) shows the size of nanoparticles to lie in the range of single domain particles. X-ray Photoelectron Spectroscopy (XPS) indicates Fe in +3 state and an associated surface peak indicates its presence at different environments. Vibrating Sample Magnetometer measurements (VSM) illustrate ferrimagnetism with high coercivity and non-saturation of hysteresis behavior up to 8500Oe. The initial high energy milling confines the particle size to the nanoscale which leads to high coercivity. Temperature dependent magnetization shows a sharp peak before transition which is the characteristic of the single domain magnetic behavior. Resistivity plot shows a decreasing trend with temperature. Two thermal activation energy values of 0.81eV and 0.29eV indicate the existence of two different conduction processes. The frequency dependent dielectric constant plot shows a semiconducting behavior. The dielectric loss plot shows a drastic change at high temperature due to the anion vacancies in addition to four types of polarization.

Dielectrical, conduction mechanism and thermal properties of rhodanine azodyes

In this paper, we report on the differential scanning calorimetry analysis (DSC) and thermogravimetric analysis (TGA) performed for 5-(4'-derivatives phenylazo)-2-thioxothiazolidin-4-one (HLn) (n=1, R=OCH3; n=2, R=CH3; n=3, R=H; and n=4, R=NO2) in the temperature range 46–800°C. The values of the thermal activation energies of decomposition of HL1, HL3 and HL4 are found in the range 59.10–299.72kJ/mol. The molecular and electronic structures of the investigated compounds (HLn) were also studied using quantum chemical calculations. The alternating current conductivity (σac) and dielectrical properties of HLn were investigated in the frequency range 0.1–100kHz and temperature range 303–500K. The temperature and frequency dependence of the real and the imaginary dielectrical constants are studied. The values of the thermal activation energy for derivatives under investigation were calculated at different frequencies. The values of thermal activation energies of electrical conductivity ΔE1 and ΔE2 for all ligands decrease with increasing the test frequency. The activation energies, ΔE1 and ΔE2, increase according to the following order p-(NO2>H>CH3>OCH3). This is in accordance with that expected from Hammett's substituent coefficients (σR). The conductivities are found to be dependent on the structure of the compounds. The values of σac are related to the frequency as σacα ωS where the behavior of the exponent S determines the operating conduction mechanism. The correlated barrier hopping (CBH) is the dominant conduction mechanism for HLn. The values of maximum barrier height (Wm) were calculated.

Dielectric relaxation and AC conductivity of modified polysulfones with chelating groups

The frequency-dependent dielectric properties and conductivity of partially quaternized polysulfones, quaternized polysulfones containing chelating groups, and chelated quaternized polysulfones with Cu2+ have been studied. The permittivity has low values and is dependent on the chemical characteristic of samples, in relation with the charge transfer complex and free volume and, consequently, with packing of the polymer chains and of the polarizable groups per volume units. At temperatures below 150 °C, all polysulfone films develop two relaxation processes, i.e., γ and β relaxation, involving different enthalpy and entropy contributions induced by their chemical structures. Frequency–temperature-dependent conductivity showed that conductivity increased with frequency, while the values of thermal activation energy of electrical conduction, lower that 1, suggest that a model based on energy bandgap representation could be suitable for explaining the temperature influence on AC conductivity for all samples. In addition, enhancement of mobility of the charge carrier upon complexation was observed for chelated quaternized polysulfones with Cu2+. All these typical semiconducting properties recommend the studied polymers as potential candidates for use in various applications in electrotechnical industry.

Conductivity and dielectric relaxation in niobium alkali borate glasses

The frequency and temperature dependent conductivity investigations for alkali niobium borate glasses of composition xNb 2O 5·(30− x)M 2O·70B 2O 3 (where M=Li, Na; x=0, 4, 8 mol%) have been carried out using impedance spectroscopy (IS). The complex impedance data have been analyzed by using both the conductivity and the electric modulus formalisms. The conductivity decreases with the decrease in M 2O:Nb 2O 5 ratio. The effect of temperature on the scaling of dielectric modulus indicates that the conductivity relaxation mechanism is temperature independent. The overlapping of the normalized peaks corresponding to impedance and electric modulus and the identical values of thermal activation energy for conduction and relaxation suggest the single mechanism for the dynamic processes occurring in the present glasses. The variation of density, molar volume and glass transition temperature with glass composition has also been reported to supplement the conductivity results.

Charge Carrier Mobility and Ageing of ZnPc/C60 Solar Cells

Cu and Zn Phtalocyanines (CuPc and ZnPc), and C60 are materials frequently used for organic Solar cell engineering. Their energy levels form a donor-acceptor junction, and they have high absorption coefficients and a complementary absorption for the Sun spectrum. We have investigated ageing properties of ZnPc/C60 Solar cells as they are influenced by the charge carrier mobility and variation of the potential barrier height of the ZnPc/C60 interface. The structures ITO/ZnPc/C60/C60:AOB/Al with a reasonable energy conversion efficiency of ∼1.5% were investigated. The samples were aged for 1300 hours upon illumination with blue LED, with peak emission at 475 nm, and incident light power density of 10 mW/cm2. The aged devices showed a strong and fast degradation of the short circuit current and of the fill factor after several hours followed by an almost constant behaviour of these values. The reference samples kept in the dark at the room temperature did show only very small changes in their I-V curves. Carrier mobility dependencies on electric field strength at different temperatures were measured by the Charge Extraction by Linearly Increasing Voltage (CELIV) method. It was demonstrated that mobility values decrease during degradation as compared to the reference samples. Nevertheless only mobility changes cannot explain the observed drop of device current. The increase of the effective barrier height at the interface of ZnPc and C60 by about 0.1 eV from ∼0.55 eV up to ∼0.65 eV was observed in the aged samples. Meanwhile thermal activation energy values of the electrical conductivity grew from about 0.28 eV prior to degradation up to about 0.34 eV after ageing.

Conductivity, charge carrier mobility and ageing of ZnPc/C60 solar cells

We have investigated conductivity and ageing of ZnPc/C60 solar cells as they are influenced by the charge carrier mobility and variation of the potential barrier height. The test structures with a reasonable energy conversion efficiency of ∼1.5% were investigated. The samples were aged for 1300 h upon illumination by the blue LED with peak emission at 475 nm. Upon ageing the devices showed a strong and fast degradation of the efficiency, short circuit current and of the fill factor within several hours followed by a much slower decrease of them. The reference samples kept in the dark at the room temperature did show only very small changes in their I– V curves. Carrier mobility dependencies on electric field strength at different temperatures were measured by the Charge Extraction by Linearly Increasing Voltage (CELIV) method. It was demonstrated that mobility values decrease during degradation as compared to the reference samples. Nevertheless only mobility changes cannot explain the observed drop of device current. The increase of the effective barrier height by about 0.1 eV from ∼0.55 eV up to ∼0.65 eV was observed in the aged samples. Meanwhile thermal activation energy values of the electrical conductivity grew from about 0.28 eV prior to degradation up to about 0.34 eV after ageing.

Characterization of effective charge carrier mobility in ZnPc/C60 solar cells after ageing

AbstractWe have investigated the ageing of ZnPc/C60 solar cells in terms of changes of the charge carrier mobility and potential barrier height. The initial fast degradation of device performance within several hours was followed by a much slower one. The mobility decreases during degradation by about 30 to 50%. Moreover, an increase of the effective barrier height at ZnPC/C60 interface from ∼0.55 eV up to ∼0.65 eV was observed. Meanwhile thermal activation energy values of the electrical conductivity grew from about 0.28 eV to about 0.34 eV after ageing. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Investigation of Photoelectrical and Current Transient Behaviour of TlBr

Photoelectrical and current transient properties of TlBr were investigated in the temperature region from − 20 C up to + 20 C, in which the effect of ionic conductivity changes significantly. The thermal activation energy value of conductivity of about 0.78 eV could stand for several mechanisms, e.g., electronic and ionic conductivity. From the spectral dependencies of photocurrent several defect-related maxima in the region between 1 and 2.5 eV were identified, being dependent on sample prehistory, i.e., its excitation by light and/or voltage. The transient photocurrent kinetics had demonstrated a complex behaviour that could be explained by the combined influence of electron generation/recombination processes superimposed upon the effect of ionic conductivity. The results indicate that ionic conductivity can take place also at − 20 C, though its influence is less because of the thermally activated character.

Carrier transport and capture in GaN single crystals and radiation detectors and effect of the neutron irradiation

Carrier transport and trapping were investigated in GaN single crystals and semi-insulating epitaxial MOCVD layers by thermally stimulated current (TSC) and thermally stimulated depolarization (TSD) spectroscopy. Effect of irradiation by 100 keV reactor neutrons with fluencies of up to 10 16 n/cm 2 was identified. We demonstrate that in the unirradiated samples TSC spectra might be caused not by carrier generation from traps, but it rather originates from the thermal mobility variation. The numerical analysis revealed that mobility limited by ionized impurities varies as ∼ T 2.8 and lattice scattering causes the dependence ∼ T −3.5. The highest mobility values were up to 1550 cm 2/Vs at 148–153 K, indicating relatively high quality of the unirradiated samples. The irradiation, depending on the doses, induced complex changes of the carrier transport. The growth of the sample resistivity took place because of the more intense carrier trapping and scattering accompanied by appearance of the percolation transport phenomena. The whole ensemble of the defect traps was identified by the TSC in multiple heating regime and by measurements at different applied biases. In the samples irradiated by 10 16 n/cm 2 the following activation energy values were found: 0.16–0.2, 0.27–0.32, 0.36–0.45 and 0.73–0.74 eV. Effects associated with the percolation transport in a disordered media were observed in the samples irradiated by 5×10 14–10 16 n/cm 2, e.g., current instabilities and decrease of the effective thermal activation energy values by heating in the temperature region below 150 K. A model of carrier percolation transport is presented, in which effect of potential fluctuations due to crystal inhomogeneities is involved. The TSD measurements had confirmed the intensifying effect of inhomogeneities upon irradiation.