Higher Thermal Activation Energy Research Articles

High efficient and ultrahigh thermal stability in a rigid rare-earth hybrid molecular crystal: [(CH3CH2)4N]Tb[CH2(SO3)2

Organic-inorganic hybrid molecular fluorescent materials have attracted much attention due to their application potential in white LEDs and high-definition displays. However, achieving high thermal stability for the fields of lighting/displays remains greatly challenging based on the hybrid molecular materials. Herein, we reported a crystalline state 1D rigid rare-earth hybrid coordinate polymer molecular crystal [(CH3CH2)4N]Tb[CH2(SO3)2] (1) along with high quantum yield (QY) 75.2%. Interestingly, this hybrid molecular crystal remains stable up to 728 K (∼455 °C). Remarkably, it exhibits excellent fluorescence thermal stability at high temperatures (150 °C, >95% and 150–200 °C, >82%) and a high thermal activation energy of 568.7 meV. Besides, the fabricated n-UV chips based white-LED by 1, commercial red Y2O3:Eu3+ and blue BaMgAl10O17:Eu2+ phosphor emits white light with color coordinates (CIE, 0.33, 0.32), high color rendering index (CRI) of 91.2, and correlated color temperature (CCT) 5623 K. These results reveal that this ultrahigh thermal stability hybrid molecular crystal as a green phosphor has great potential applications for displays or WLEDs. Therefore, this study opens a new avenue to develop novel rare-earth phosphors and gives new visions for hybrid molecular fluorescent materials.

Insight of the high switching window and data retention in lead-free 2D layered double perovskite resistive memory device

Lead-free robust halides double perovskites (DPs) are evolving as the key materials for the multi-functional resistive memory application. Herein, we aimed to enhance the switching window, i.e., current On/Off ratio by the cutting edge dimensional reduction of the three dimensional Cs2AgBiBr6 DP into two dimensional (2D) BA4AgBiBr8 (BA = butylammonium) DP and studied the insight of their resistive switching anomaly. We affirmed that the improved On/Off ratio (∼103 to ∼106) is attributed to the restricted charge transport in the high resistance state (HRS) of the BA4AgBiBr8 based switching device. The low HRS current can be ascribed to the synergies of higher Schottky barrier at the Au/BA4AgBiBr8 junction and higher thermal activation energy in the layered DP. Owing to the higher switching window in the 2D DP device, reproducible endurance (tested up to 500 cycles) and retentivity (tested up to 104 s) of the resistance states establish that the high On/Off ratio could be retained without significant deviation. The resistive memory behavior could be hypothesized by the charge trapping phenomenon in the BA4AgBiBr8 DP, as the charge retention was persisted over 60 min as evident from surface potential images of Kelvin probe force microscopy.

Synthesis and photoluminescence of novel blue‐emitting phosphor of Eu2+‐activated fluoroborate BaGaBO3F2

AbstractPure and Eu2+‐activated fluoroborate BaGaBO3F2 was prepared using high‐temperature solid‐state reaction. BaGaBO3F2 is a wide band semiconductor with the indirect transition characteristic. The excitation and luminescence spectra of the phosphor were measured, and it was found that Eu2+‐activated BaGaBO3F2 exhibits a bright blue color under ultraviolet (UV) light. The narrow emission band peaked at 425 nm is attributed to the transitions of 4f65d→4f7(8S7/2), and the Stokes shift estimated for this phosphor sample is 3140 cm−1. The lifetime of the luminescence is also reported. The absolute quantum efficiency (QE) of the phosphor was evaluated, and it was found that the absolute QE decreases with increasing Eu2+ concentration. The phosphor shows an excellent quantum efficiency of 72.5% and a high thermal activation energy of 0.342 eV. The study concludes that Eu2+‐doped BaGaBO3F2 phosphor has promising luminescence application abilities and can be used as a blue‐emitting phosphor in a variety of applications.

Synthesis and photoluminescence properties of a novel double perovskite NaGdMgTeO6:Sm3+ red-emitting phosphor for plant growth LEDs and w-LEDs

This research used solid-state reaction methods to synthesize a series of double perovskite structure NaGdMgTeO6:xSm3+ (x = 0.5−30 mol%) phosphors. A systematic study was conducted on the crystal structure, phase purity, photoluminescence (PL) properties, concentration quenching mechanism, color purity, thermal stability, and inner quantum efficiency (IQE) of the samples. Under 405 nm excitation, NaGdMgTeO6:Sm3+ phosphors showed strong red emission at 648 nm, attributed to the 4G5/2 → 6H9/2 transition of Sm3+ ions. The optimal luminescence intensity was achieved at the dopant concentration of 1 mol%, and the dipole-dipole interaction mechanism contributed to the concentration quenching process. High color purity of more than 99.8% and appropriate Commission International de L'Eclairage (CIE) chromaticity coordinates were obtained for the NaGdMgTeO6:Sm3+ phosphors. High thermal stability and activation energy (Ea = 0.293 eV) were found to be excellent characteristics of the phosphors. Ultimately, the fabricated red LED in this study revealed superior spectral matching with the phytochrome Pr. The white light-emitting diode (w-LED) prepared had a high color rendering index (Ra) of 90 and a suitable correlated color temperature (CCT) of 5337 K. As a result, these phosphors are potentially suitable for use in plant growth LEDs and w-LEDs.

Interior crystallographic plane induced cracking behavior of Ni-based superalloy in high-temperature and vacuum environment

Tension-tension fatigue tests at 750 °C were performed to state on fatigue structure-properties of Ni-based superalloy in long-life regime under high temperature by means of ultra-depth-of-field overlay three-dimensional, scanning electron microscopy, energy dispersive spectrometer, electron back scattered diffraction, focused ion beam and transmission electron microscope. Results show that interior crystallographic plane-fisheye failures are divided into the single plane & multiple planes cracking. The formation of plane with micro-crack initiation and growth is mainly under Mode Ⅱ cracking from the Copper grain with large size and the highest Schmid factor or the Goss grain with small size and high Schmid factor. Moreover, owing to high thermal activation energy, grain-related interior cracking is mainly induced by climbing and bypassing mechanisms with bowing of dislocations and zig-zag dislocations, and assisted by shearing mechanism with tangled dislocations. Finally, the interior plane-fisheye cracking behavior with microstructure in high-temperature and vacuum environment is elucidated.

NMR probe of suppressed bulk conductivity in the topological insulator Bi0.5Sb1.5Te3.

The relaxation behavior in the topological insulator (TI) Bi0.5Sb1.5Te3 has been investigated using 125Te nuclear magnetic resonance spectroscopy. We systematically investigate the spin–lattice relaxation rate (1/T1) in bulk electronic states with varying particle sizes. By analyzing the 1/T1 relaxation behavior, we find that with decreasing particle sizes the electronic states in the bulk exhibit more topological insulating behavior, indicative of an increasing energy gap supported by higher thermal activation energy. Besides, the decreasing density of states at the Fermi level was observed in the massive Dirac electrons with decreasing particle size by analyzing the spin–lattice relaxation according to a theoretical model in this spin–orbit coupled system.

Synthesis, characterization and catalytic behavior of MFe2O4 (M = Ni, Zn and Co) nanoparticles on the thermal decomposition of TKX-50

Bimetallic iron oxides (NiFe2O4, ZnFe2O4 and CoFe2O4) were prepared via the facile solvothermal method and characterized using SEM, TEM, XRD, FTIR, XPS and BET instruments. The catalytic behavior of the bimetallic iron oxides on the thermal decomposition of 5, 5′-bistetrazole-1, 1′-diolate (TKX-50) was studied using DSC and TG-DTG methods. Besides, the corresponding kinetic parameters of TKX-50 thermal decomposition were calculated by multi-kinetics methods including traditional, iso-conversional and iteration methods. The thermal decomposition peak temperature (Tp) and the activation energy (Ea) reduced obviously after the addition of bimetallic iron oxides. Among different bimetallic iron oxides, the ZnFe2O4 has the best catalytic activity for TKX-50 thermal decomposition. The high thermal decomposition peak temperature (THDP) and average apparent activation energy (Ea) of TKX-50 were decreased by 47.0 °C and 34.34 kJ mol−1 in the presence of ZnFe2O4 sample. The excellent catalytic activity of ZnFe2O4 for thermal decomposition of TKX-50 makes it a promising combustion catalyst of insensitive solid propellants containing TKX-50.

Photoluminescence properties as a function of growth mechanism for GaSb/GaAs quantum dots grown on Ge substrates

In this work, we use photoluminescence (PL) spectroscopy to investigate how self-assembled GaSb/GaAs quantum dots (QDs) depend on their growth mechanism. Carrier transfer (i.e., carrier recombination in QDs and escape through the barrier layer) is investigated as a function of excitation-power- and temperature-dependent PL measurements. A drastic blueshift of the QD peak energy from 1.23 to 1.30 eV and a further shift to 1.33 eV reveal the influence of the GaSb growth rate and the growth temperature on the optical properties of these QDs. The thermal activation energy is extracted from the temperature-dependent PL by fitting the integrated PL intensity of the QD peaks to the Arrhenius relation. The QDs grown at the growth rate of 0.1 monolayers/s at 450 °C have higher thermal activation energy (109 meV) than those grown at a lower growth rate and higher QD growth temperature. The observed PL characteristics are discussed in terms of QD size, uniformity of QDs, and material intermixing occurring during QD growth on the buffer layer and capping layer.

Sol-Gel Synthesis and Optical Temperature Sensing Behavior of Double-Perovskite Gd2ZnTiO6:Yb3+/Er3+ Phosphors

The Gd2ZnTiO6:Yb3+/Er3+ phosphors (abbreviated as GZT:Yb3+/Er3+) have been successfully prepared by means of the sol-gel method. The GZT:Yb3+/Er3+ phosphor exhibits intense green upconversion (UC) emission bands near 525/547 nm (2H11/2/4S3/2→4I15/2) and weak red UC emission band near 663 nm (4F9/2→4I15/2) upon the 980 nm laser excitation. Furthermore, the sample simultaneously emits near-infrared centered at 1527 nm associated with 4I13/2→4I15/2 transition of Er3+ ions. The high thermal activation energy (0.34 eV) of the GZT:Yb3+/Er3+ phosphor is determined via the temperature dependent luminescence spectra upon the 980 nm laser excitation (low power density: 0.8W/cm2). Two thermally coupled levels (2H11/2/4S3/2) for Er3+ using the fluorescence intensity ratio (FIR) technology were studied in detail. The maximum absolute sensitivity (90.11 × 10−4 K−1 at 539 K) of the targeted phosphor is determined from the experimental results. All results demonstrate a potential of application of as-prepared GZT:Yb3+/Er3+ phosphors in optical thermometry.

Hot tensile deformation behavior and globularization mechanism of bimodal microstructured Ti−6Al−2Zr−1Mo−1V alloy

Abstract The hot deformation behavior, microstructure evolution and fracture characteristics of bimodal microstructured Ti−6Al−2Zr−1Mo−1V alloy were investigated by isothermal tensile tests. Results reveal that flow softening is caused by dynamic globularization of the bimodal microstructure, which also results in a relatively high stress exponent and thermal activation energy. The corresponding SEM, EBSD and TEM observations indicate that the dynamic globularization at 750 and 800 °C is accomplished by the formation of α/α sub-grain boundary and penetration of the β phase. However, dynamic recrystallization (DRX) is the main globularization mechanism at 850 °C, which was proved by the generation of fine grains with a necklace-like character due to the transformation of low-angle boundaries (LABs) into high-angle boundaries (HABs). With an increase in the deformation temperature or a decrease in the strain rate, the fracture mechanism changes from microvoid coalescence to intergranular fracture.

Transport mechanisms in a puckered graphene-on-lattice.

Understanding the fundamental properties of graphene when its topography is patterned by the use of a compliant substrate is essential to improve the performances of graphene sensors. Here we suspend a graphene monolayer on SiO2 nanopillar arrays to form a puckered graphene-on-lattice and investigate the strain and electrical transport at the nanoscale. Despite a nonuniform strain in the graphene-on-lattice, the resistivity is governed by thermally activated transport and not the strain. We show that the high thermal activation energy results from a low charge carrier density and a periodic change of the chemical potential induced by the interaction of the graphene monolayer with the nanopillars, making the use of graphene-on-lattice attractive to further increase the electrical response of graphene sensors.

Enhanced electrical characteristics of FinFET by rapid-thermal-and-laser annealing with suitable power

In order to improve electrical characteristics of FinFETs, various annealing treatments for dopant activation were studied in this work. The treatments including rapid thermal annealing (RTA), microwave annealing, and RTA-and-laser annealing with different powers were investigated. The on-current and carrier mobility of FinFET are significantly improved by a RTA-and-laser annealing with suitable power; meanwhile, the leakage current, interface characteristics and device reliability can be simultaneously maintained. The improvement can be attributed to the high thermal activation energy, less dopant diffusion, and enhanced quality of interfacial layer during the gate first processes. Therefore, a RTA-and-laser annealing with suitable power is promising for manufacturing high performance FinFET.

Light‐induced electrons suppressed by Eu3+ ions doped in Ca11.94−xSrxAl14O33 caged phosphors for LED and FEDs

AbstractControl of light‐induced electron generation is of vital importance for the application of caged phosphors. For Eu‐doped Ca11.94−xSrxAl14O33 caged phosphors, the suppressed effect of strontium doping on the light‐induced electrons is observed compared to the europium‐free Ca11.94−xSrxAl14O33 phosphors. In the presence of europium ions, Sr doping will promote the reduction of Eu3+ to Eu2+. The Rietveld refinement suggests that unit cell volumes of the Ca11.94−xSrxAl14O33:Eu0.06 samples are expanded when Ca2+ ions are replaced by Sr2+ ions. The absorption and FTIR transmittance spectra confirm that the competitive reaction of encaged O2− anions with H2 is suppressed. For the sample (x=0.48), the higher thermal activation energy (~0.40 eV) for luminescence quenching can be attributed to the more rigid framework structure after Sr doping. For Ca11.94−xSrxAl14O33:Eu0.06 phosphors, their emission colours are tuned from red to purple upon 254 nm excitation and from pink to blue under electron beam excitation through Sr substitution. The insight gained from this work may have a significant guiding to design new phosphors for LED and FEDs and novel nanocaged mutifunctional materials.

A novel DOPO-containing flame retardant for epoxy resin

A novel halogen-free flame retardant (6,6′-(((methylenebis(4,1-phenylene))bis(azanediyl))bis((4-hydroxy-3-methoxyphenyl)methylene))bis(6H-dibenzo[c,e][1,2]oxaphosphinine 6-oxide)(DP-DDM))) was synthesized via a one-pot procedure based on the Pudovik reaction between 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide and imine directly resulting from 3-methoxy-4-hydroxybenzaldehyde with 4,4′-methylenedianiline (DDM). The thermal curing behaviors of epoxy resins with different contents of DP-DDM were investigated by differential scanning calorimetry under a nonisothermal condition, aimed to suggest an optimized curing procedure in accordance with the property of DP-DDM. These DP-DDM-modified epoxy thermosets under optimized curing procedure showed high glass transition temperature and thermal degradation activation energy. Moreover, their high flame-retarding performance has been found: the epoxy thermosets with a relatively low addition amount of DP-DDM (on account of the phosphorus content of 0.75 wt%) can reach UL 94 V-0 (Instrumental analysis and measurements) rating with limiting oxygen index value of 34.5.

Achieving excellent thermal stability and very high activation energy in an ultrafine-grained magnesium silver rare earth alloy prepared by friction stir processing

Ultrafine-grained microstructure of a QE22 alloy prepared by Friction Stir processing (FSP) is isochronally annealed to study the thermal stability and grain growth kinetics. The FSPed microstructure of QE22 alloy is thermally stable under ultrafine-grained regime up to 300°C and the activation energy required for grain growth is found to be exceptionally high as compared to conventional ultrafine-grained magnesium alloys. The high thermal stability and activation energy of the FSPed QE22 alloy is due to Zener pinning effect from thermally stable eutectic Mg12Nd and fine precipitates Mg12Nd2Ag and solute drag effect from segregation of Neodymium (Nd) solute atoms at grain boundaries.

Temperature and Oxygen Sensing Properties of Ru(II) Covalently-Grafted Sol-Gel Derived Ormosil Hybrid Materials.

In this article, oxygen and temperature-sensing hybrid materials consisting of [Ru(Phen)3]2+ portions covalently-grafted onto the sol-gel derived ormosil network were prepared by co-condensation of tetraethoxysilane (TEOS) using n-octyltriethoxysilane as the network modifier. For comparison purposes, the hybrid materials in which [Ru(Phen)3]2+ were conventionally physically-incorporated into the matrix were also prepared. The obtained hybrid materials were characterized by Fourier transform infrared (FT-IR), luminescence intensity oxygen quenching Stern-Volmer plots, temperature quenching plots and excited-state lifetime. The near linear Stern-Volmer plots can be attributed to the approximate heterogeneous environment of the luminophore within the ormosil materials. The results reveal that the. covalently-grafted sample is more sensitive to 02, and has a higher sensing sensitivity and a higher thermal activation energy compared to the physically-incorporated one, since these Ru(II) molecules are strongly covalently-grafted onto the Si-O network via the CH2-Si bonds and less -OH group.

Comparison of semi-insulating InAlAs and InP:Fe for InP-based buried-heterostructure QCLs

In a previous work [Flores et al., J. Cryst. Growth 398 (2014) 40] [3] we demonstrated the advantages of using a thin InAlAs spacer layer in the fabrication of buried-heterostructure quantum-cascade lasers (QCLs), as it improves the morphology of the interface between the laser core and the InP:Fe lateral cladding. In this paper we investigate aspects of InAlAs, which are relevant for its role as insulating lateral cladding of the laser sidewalls: carrier traps, electrical resistivity, and functionality as a sole lateral cladding. We find that a thin InAlAs spacer layer not only improves the regrowth interface morphology, but also eliminates interface-related shallow electronic states, thus improving the electrical resistivity of the interface. We further find that bulk InAlAs grown by gas-source molecular-beam epitaxy as well as InP:Fe are semi-insulating at room temperature, with specific resistivities of 3×107Ωcm and 2×108Ωcm, respectively. Both materials have also a high thermal activation energy for electrical conductivity (0.79eV and 0.68eV, respectively). In order to compare the performance of InP:Fe and InAlAs as a lateral cladding, lasers were fabricated from the same QCL wafer with differing stripe insulation materials. The resulting lasers differ mainly by the lateral insulation material: SiO2, InP:Fe (with InAlAs spacer), and pure InAlAs. All devices show a similar performance and similar temperature dependence, indicating insulating properties of InAlAs adequate for application in lateral regrowth of buried-heterostructure QCLs.

Proton Conduction in a Nonporous One Dimensional Coordination Polymer

Complex impedance measurements on single crystals and pellets of a CuCo-orotate coordination polymer, as a function of relative humidity and temperature, reveal proton conduction with a relatively high thermal activation energy, a negligible isotope effect, a large pre-exponential factor in the conductivity model, and a strong dependence on humidity, all of which are consistent with a vehicle mechanism. The proton conductivity found for this polymer compares favorably with that of MOFs and related porous materials even though it has no framework with pores and channels to provide proton-conduction pathways.

Strong infrared photoluminescence from black silicon made with femtosecond laser irradiation

Strong infrared photoluminescence (PL) was observed for the first time from black Si (b-Si) formed by femtosecond laser irradiation and treated by rapid thermal annealing. The PL peak energy changes from 0.78 to 0.84 eV (1.59 to 1.48 μm) when increasing the annealing temperature from 500°C to 1200°C, showing a tunable property. Compared to those after annealing at 1000°C, both the peak intensity and the peak energy change slightly after annealing at higher temperatures of 1100°C and 1200°C, showing a high thermal stability of their structures and PL properties. With the x-ray photoelectron spectroscopy (XPS) results from the b-Si before and after etching in KOH solution, the PL is attributed to dislocation-related (D1) luminescence rather than sulfur-related impurity center luminescence. The integrated intensity of D1 PL as a function of excitation power can be fitted by I [proportionality] P(m) with m of 0.58, suggesting that the PL of the b-Si sample originates from the recombination of bound-to-bound states. The high thermal activation energy of 75.2 meV suggests a high quenching temperature of D1 PL, showing a potential of application at room temperature. This work may promote the progress of research on b-Si in the area of Si-based optoelectronics.

Ferrimagnetism and possible double perovskite structure in half Cr-doped YMn0.5Cr0.5O3

Half Cr-doped YMn0.5Cr0.5O3 polycrystalline has been synthesized by means of a standard solid-state reaction method. Powder X-ray diffraction gives evidence that the sample is a single phase of orthorhombic (Pbnm) or monoclinic (P21/c) structure as observed for other doped manganites, rather than the hexagonal structure as YMnO3. Upon cooling, the magnetic susceptibility increases sharply at TC∼75K, indicating a magnetic ordering occurs. The detailed analysis on the paramagnetic behavior at higher temperatures reveals that the low-temperature order is a ferrimagnetic state. The electrical transport properties show an insulator behavior dominated by the adiabatic small polaron hopping mechanism with high thermal activation energy. The results indicate that the ferromagnetic double exchange (DE) interaction is unreasonable in Cr-doped manganites.