Thermal Decay Time Research Articles

Rapidly constructed water-resistant composite shell of fluoride red phosphors K2GeF6:Mn4+ by in-situ passivation

White light emitting diodes assembled with Mn4+-doped fluoride red light phosphors are perfect photoconversion devices, but their low water stability prevents them from being used in commercial lighting displays. In this work, in order to increase the moisture resistance of the K2GeF6:Mn4+ (KGFM) phosphor, the Mn4+-free composite shell was prepared by in-situ passivation method with reducing agent SnCl2, and K2GeF6:Mn4+@ K2GeF6, K2SnF6 (PKGFM) with moisture resistance was achieved, which the shell also prevents the energy transfer to the surface defects of the phosphor. The thermal stability and luminescence decay time of the PKGFM are enhanced after surface modification, and IQE was raised from 94.37 % to 96.08 %. After immersion in water for 480 min, the luminescence intensity of the modified phosphors was 91.65 % of its initial, and IQE could still maintain 94.58 %. The WLED prepared with PKGFM was aged at high temperature (85 °C) and high humidity (85 %), after aging for 288 h, the luminous efficiency of the WLED could maintain 83 % of the initial value, and its CCT and CRI were 2979 K and 89.5, indicating that it has excellent color stability. The aging test results further demonstrate that the PKGFM phosphor contained with the Mn4+-free composite shell has good moisture resistance.

A new method for characterizing the heat transfer capacity of the divertor and high heat flux components

The divertor is usually exposed to extreme incident heat flux conditions in nuclear fusion engineering and its ability to remove the heat is extremely crucial for the safe operation of magnetic confinement nuclear fusion device. The evaluation of the heat removal capacity of the divertor has been always based on high heat flux (HHF) tests. Yet such methods are only available for the small size of experimental components, it is more difficult to compare under different loading and cooling conditions. In order to assess the heat removal capacity in real time and accurately, a new method was developed to characterize the heat removal capacity of the divertor and other high heat load components based on the cooling water thermal decay time constant, the time constants model has also been established. The thermal decay time constant was obtained by fitting the characteristic curves of the water temperature differences with time during the cooling phase of different components under HHF tests. This index is used to quantitatively analyze, characterize and evaluate the heat transfer capacity of the system of the divertor.

Efficient continuous-wave Tm,Ho:CaF2 laser at 2.1µm.

We report on the first, to the best of our knowledge, continuous-wave laser operation of a Tm3+,Ho3+-codoped calcium fluoride crystal at ∼2.1 µm. Tm,Ho:CaF2 crystals were grown by the Bridgman method, and their spectroscopic properties were studied. The stimulated-emission cross section for the 5I7 → 5I8 Ho3+ transition is 0.72 × 10-20 cm2 at 2025 nm, and the thermal equilibrium decay time is 11.0 ms. A 3 at. % Tm, 0.3 at. % Ho:CaF2 laser generated 737 mW at 2062-2088 nm with a slope efficiency of 28.0% and a laser threshold of 133 mW. Continuous wavelength tuning between 1985 and 2114 nm (tuning range: 129 nm) was demonstrated. The Tm,Ho:CaF2 crystals are promising for ultrashort pulse generation at ∼2 µm.

High heat flux testing for W/Cu monoblock PFUs with varying defects

The full tungsten divertor is the key component of Experimental Advanced Superconducting Tokamak (EAST) to exhaust excessive heat loads. After EAST discharge operation, defects may appear in the tungsten/copper (W/Cu) joints of W/Cu monoblock plasma-facing units (PFUs). In order to evaluate the heat removal capability and thermal fatigue stability of W/Cu PFUs with defects, the specific heat treatments are used for PFUs to introduce defects of varying degrees. Then, high heat flux (HHF) tests for PFUs with defects are performed on a 100 kW electron-beam (e-beam) device at ASIPP. The W/Cu joints defects and W surface conditions have no significant differences after HHF tests. And the test results show that the fewer defects or the larger water mass flow is, the better heat removal capability of the W/Cu monoblock has. The surface temperature of W/Cu PFUs with major defects (> 2 × 3 mm) is 12% - 17% higher than that of PFUs with minor defects (≈ 2 × 3 mm), and the minor defects PFUs is 6% - 8% higher than the non-defects one (< 2 × 3 mm). Besides, the temperature curves of three W/Cu PFUs are exponentially fitted in cooling down phase, and the obtained average thermal decay time constants are 11.1 s, 8.5 s and 7.4 s, respectively. The time constants quantitatively explain the cooling performance of PFUs with various defects, and provides a feasible method for non-destructive inspection of PFUs defects during EAST experiment. These results are conducive to understanding the relationship between damage degree and heat transfer performance, and can provide relevant basis for improving the acceptance criterion of qualified W/Cu PFUs.

A continuum–atomistic multi-scale analysis of temperature field problems and its application in phononic nano-structures

In this paper, a novel coupling technique is developed in continuum–atomistic multi-scale analysis of temperature field problems. In this manner, a new thermostat is introduced based on the single-atom sub-system, where its capability to control the temperature and produce the canonical ensemble is investigated. Moreover, the performance of proposed thermostat is verified by comparing the distribution of velocities to the Maxwell-Boltzmann distribution. The single-atom sub-system thermostat is then incorporated into the concurrent multi-scale model to relate the temperature field between the continuum and atomistic domains with complex lattice thermal fields. In order to illustrate the capability of proposed coupling continuum–atomistic model, the multi-scale analysis is performed through several numerical examples at various temperature fields and the results are compared with those obtained from the full atomistic model and finite element simulation. Finally, the continuum–atomistic multi-scale technique is applied in numerical simulation of the lattice heat conduction in two-dimensional phononic nano-structures. • A single-atom sub-system thermostat is developed that presents the canonical ensemble. • The performance of proposed thermostat is verified by comparing the velocities distribution to the Maxwell-Boltzmann distribution. • A concurrent multi-scale technique is presented based on the proposed single-atom thermostat. • The concurrent multi-scale technique is applied to model the lattice heat conduction in phononic nano-structures. • The behavior of thermal decay time is investigated for various model parameters in a phononic crystal.

A Robust Infrared Transducer of an Ultra-Large-Scale Array.

A robust micro-electro-mechanical systems (MEMS) infrared thin film transducer of an ultra-large-scale array was proposed and fabricated on a 4-inch silicon wafer. The silicon substrate and micro cavities were introduced. This novel transducer had excellent mechanical stability, time response, and state-of-the-art pixel scale. It could bear a load of 1700 g and its load pressure was improved by more than 5.24 times and time constant decreased by 50.7% compared to the traditional soft infrared thin film transducer. The array scale of its pixels exceeded 2k × 2k. The simulation and measured results of the transient temperature and radiation intensity were well consistent. Illuminated by a 532 nm laser with a frequency of 50 Hz and 50% duty cycle, the thermal decay time of the proposed transducer was 6.0 ms. A knife-edge image was utilized for spatial resolution test and the full width at half maximum (FWHM) of the proposed transducer was 24% smaller than the traditional soft one. High-resolution infrared images were generated using the proposed robust transducer. These results proved that the robust transducer was promising in infrared image generation.

Flow-Driven Resonant Energy Systems

Here we provide a physical and mathematical framework for the description of flow-driven oscillators. These oscillators, differently from frequency-driven harmonic systems, are based on countercurrent mass flows and thermal-energy exchange. We describe this class of oscillators through two countercurrent fluids separated by a heated conductive medium. We show how this configuration embeds the essential elements of harmonic oscillators, such as resonance condition, periodic orbits, and quality factor or decay time. The key advantage of recognizing flow-driven systems as oscillators lies in the possibility to engineer them according to the properties of resonant systems and utilize them to control their temperature, maximize the stored energy, or coupling them in networks. We report examples of simple configurations at their resonant condition, enhancing both thermal energy and decay time by factors larger than 10. We finally show two flow-driven oscillators coupled in series, featuring a reconfigurable internal temperature distribution, depending on the selected resonant condition.

Disordered Tm3+,Ho3+-codoped CNGG garnet crystal: Towards efficient laser materials for ultrashort pulse generation at ∼2 μm

We report on the growth, structure refinement, optical spectroscopy, continuous wave and femtosecond mode-locked laser operation of a Tm3+,Ho3+-codoped disordered calcium niobium gallium garnet (CNGG) crystal. The 2.64 at.% Tm, 0.55 at.% Ho:CNGG is grown by the Czochralski method. Its cubic structure, sp. gr. Ia3¯d - O10h, a = 12.4952(1) Å, is refined by the Rietveld method revealing a random distribution of Ga3+ and Nb5+ cations over octahedral and tetrahedral sites. The Ho3+ transition probabilities are determined within the Judd-Ofelt theory accounting for an intermediate configuration interaction (ICI). For the 5I7 → 5I8 Ho3+ transition, the maximum stimulated-emission cross-section σSE is 0.47 × 10−20 cm2 at 2080.7 nm. The gain bandwidth of Tm,Ho:CNGG at ∼2 μm is > 150 nm and the thermal equilibrium decay time - 6.80 ms. The Tm3+ ↔ Ho3+ energy transfer parameters are determined. A diode-pumped Tm,Ho:CNGG microchip laser generated 413 mW at 2088.4 nm with a slope efficiency of 15.9%. A continuous wavelength tuning between 1940.3 and 2144.6 nm is demonstrated. Ultrashort pulses as short as 73 fs are achieved at 2061 nm from a Tm,Ho:CNGG laser mode-locked by a GaSb semiconductor saturable absorber mirror at a repetition rate of 89.3 MHz.

Performance improvement of an infrared scene generation chip by in-plane microstructures.

An infrared (IR) scene generation chip based on multi-layer thin film was demonstrated. Infrared radiation was efficiently converted from visible light energy through chip absorption. We proposed that the contradictive material parameter requirements aroused by high-spatial resolution and high-frame rate of the dynamic IR scene generator could be successfully solved by a periodical microstructure fabrication. The theoretical simulation results based on a semi-one-dimensional heat transfer model illustrated that the spatial resolution of the generated IR scene was improved using a microstructure with a low contact area ratio and high fill factor, while the thermal decay time of the chip was kept unaffected. Two infrared scene generation chips with different microstructures were fabricated by lithography and site-selective deposition. Measured by non-contact thermography, the chip with low microstructure contact area ratio (0.17) showed a higher spatial resolution (13.2 lp/mm) than that with high microstructure contact area ratio of 0.46 (8.0 lp/mm). Moreover, both chips had the same thermal decay time of 20 ms. The experimental results indicated that the proposed method is an effective and economic way to improve the performance of infrared scene generation chips.

A self-suspended MEMS film convertor for dual-band infrared scene projection

A self-suspended MEMS film convertor used for infrared scene projection was proposed. The film convertor was based on a thin composite film with submicron thickness and high absorption of 95% in visible wavelength range. The microstructure arrays were fabricated by multiple lithography and dry/wet-etching processes to form 1300 × 1300 pixels. By optimizing the geometric parameters of the microstructure, the in-plane thermal diffusion distance was limited to 125 μm. The spatial resolution was therefore improved. Based on this MEMS film convertor, an infrared scene projector was built and its properties were tested. Illuminated by a 532 nm laser, an infrared image with the maximum apparent temperature of 489 K was obtained under the laser power density of 6.4 mW/mm2. By cooling to 278 K, the thermal rising and decay time constants were 5.02 and 2.76 ms, respectively. It indicated that infrared scenes would have a possible frame rate of higher than 100 Hz. The radiation spectrum was similar to a blackbody spectrum, which covered both 3–5 and 8–12 μm wavelength range. These unique properties of the MEMS film convertor made it an ideal candidate to be used in a hardware-in-the-loop simulation system for infrared imaging device performance evaluation.

Enhanced thermal sensitivity of MEMS bolometers integrated with nanometer-scale hole array structures

We have fabricated two-dimensional nanometer-scale hole array structures on GaAs doubly-clamped microelectromechanical system (MEMS) beam resonators to modulate their thermal properties. Owing to the reduction in the thermal conductance of the MEMS beams by introducing the hole array structures, the nano-porous MEMS bolometers show 2-3 times larger thermal sensitivities than the unpatterned reference sample. Furthermore, since the heat capacitance of the MEMS beams is also reduced by introducing the hole array, the thermal decay time of the patterned MEMS beams is increased only by about 30-50%, demonstrating the effectiveness of the hole array structures for enhancing the thermal sensitivities of bolometers without significantly deteriorating their operation bandwidths.

Effect of the replacement of Zn2+ with Mg2+ in Ca14Zn6Ga10O35:Mn4+

To obtain applicable red phosphor for plant lighting, a series of Ca14Zn6-xMgxGa9.86O35:0.14Mn4+ phosphors were synthesized by a conventional high-temperature solid-state method. The XRD, SEM, excitation and emission spectra, temperature-dependent emission spectra, decay time, internal quantum efficiency and luminous efficiency are investigated with the change of the doping concentration of Mg2+. The experiment results suggest that the luminescence properties and quantum efficiency of the phosphor can by improved by replacing Zn2+ with Mg2+ at appropriate doses, while the thermal resistance performance and decay time declined, and the internal quantum efficiency and luminous efficiency reached its maximum, when the Mg2+ concentration is 0.40. Ultimately, the appropriate doping concentration of Mg2+ was determined at x = 0.40, in which case, the phosphor has a promising application in future plant lighting.

Growth and scintillation properties of Ce:Li6Y(BO3)3 crystal enriched with 10B isotopes

In the present work, single crystal of Ce:Li6Y(BO3)3 enriched with 10B isotopes (Ce:LY10BO) has been grown by Czochralski technique. The X-ray powder diffraction patterns showed that Ce:LY10BO crystal belongs to monoclinic system. The optical and scintillation properties of the crystal, such as optical transmittance, photoluminescence, X-ray induced luminescence, multi-channel energy spectrum under the excitation of thermal neutrons and decay time under the excitation of UV light, were measured. The optical transmission from 400 nm to 800 nm of the crystal with 1 mm thickness was above 75%, which revealed that Ce:LY10BO crystal had good optical quality. The energy resolution under the excitation of thermal neutron was estimated to be 33%. And the decay time under the excitation of UV light was fitted to be 24 ns. So Ce:LY10BO crystal was found with good energy resolution and fast decay time, which meant that it was a promising fast scintillator for thermal neutron detection.

Frequency and duty cycle modulation optimization in minimizing thermal accumulation effect in Z-scan measurement with high-repetition-rate laser

In this study, we demonstrate the optimization of the chopper frequency and duty cycle in a Z-scan measurement with a 250 MHz high-repetition-rate (HRR) femtosecond laser to minimize the thermal lensing effect due to cumulative heating of the sample. The result shows that such minimization can be achieved by keeping the modulated exposure time on the sample shorter than the thermal diffusivity decay time tc. The minimum chopper frequency fmin is predicted by relating the duty cycle factor F with tc, while maintaining stable peak and valley transmittances, i.e., ΔTp and ΔTv, respectively. Furthermore, a lower fmin is obtained by taking a stable range of the peak–valley difference ΔTpv into consideration. The optimization allows for the low operational modulation frequency of Z-scan measurement with reduced thermal influence, thus enabling simple management of the thermal lensing effect.

Remote thermal neutron die-away measurements to improve Differential Die-Away Analysis

Differential Die-away Analysis (DDAA) is a very effective detection technique of special nuclear material (SNM). It is based on the detection of fast fission neutrons that decay with a time constant characteristic of the thermal neutron that created them. The presence of fast neutrons with this slow die-away time is a positive, sensitive indication of the presence of SNM in the inspected cargo. The thermal decay time of an inspected medium is not known before a DDAA measurement is taken. The choice of when, from the end of a neutron pulse, data are to be acquired must be taken conservatively. If the thermal decay time of an inspected medium can be determined, a priori, then a better estimate of the expected time of separation between the intrinsic detector signal and the fission signal can be achieved. With a better estimate of this time, the choice of times after the end of the pulse to perform signal integration can be optimized. A more optimal choice for this time will result in higher detection sensitivity. This paper describes a method of remotely assessing the thermal die-away properties of an interrogated cargo, and grossly characterizes the class of material being inspected in terms of neutron attenuation and moderation properties.

Current response and linearity of transition edge sensors in the large signal analysis

A new explicit form for the current response of transition edge sensor microcalorimeters operating with large signals is presented. In the framework of the large signal analysis, it has also been recently shown how the thermal decay time is actually time dependent in presence of electrothermal feedback. As a consequence the combination of such current response with the time dependent thermal response is able to explain the improvement in linearity of the integrated in time signal if compared to the pulse height signal. Moreover, it provides the expected energy linearity, when the extreme electrothermal feedback is applied.

Relaxation of vibrational energy in fullerene suspensions

Picosecond transient absorption is used to measure the thermal decay time of a mixture of higher-order fullerenes suspended in chloroform, toluene, and carbon disulfide. The relaxation time is ≈40 ps, which translates into a small effective interface conductance, G ≈ 14 MW m −2 K −1. Molecular dynamics simulations of a fullerene molecule suspended in octane agree with experiment ( G ≈ 10 MW m −2 K −1) and support the conclusion that the 40 ps decay time results from the relaxation of vibrational energy and not the relaxation of electronic excitations.

Pressure cell for time-resolved calorimetric measurements of photoinitiated reactions on the fractional millisecond and longer time scale

Photoacoustics is an excellent method to obtain enthalpy and volume changes of photochemical reactions. It is particularly useful for photobiological or other reactions in water where the thermal and inherent volume changes can be separated by measurement at the temperature of maximum density. However, its time range is limited to less than ∼10 μs because the amplitude of the observed pressure wave is proportional to the rate of volume change. We describe a closed pressure cell which perfectly complements the usual photoacoustic methodology. It has a rise time of 20 μs and a thermal decay time of several seconds. Its single pulse sensitivity [signal/noise (S/N)=1] is 10 pL or 15 μJ absorbed energy in water. The assembly and calibration of the instrument is described along with its verification by measurement of the enthalpy and volume change of the photodissociation of CO-myoglobin. The apparatus is eminently suited for the study of proton pumping in bacteriorhodopsin and oxygen formation in photosynthesis.

Dissipative mixed quantum-classical simulation of the aqueous solvated electron system

A direct integration of quantum decoherence into the mixed quantum-classical (MQC) molecular dynamics (MD) method mean field with surface hopping (MF/SH) is explored from the context of the aqueous solvated electron system. Within this framework, the time evolution of the reduced density matrix via the MQC Liouville–von Neumann equation includes dissipation of the off-diagonal elements according to some prescribed decoherence time scale. This fixed parameter corresponds, for example, to the characteristic thermal average decay time of nuclear overlaps of the bath. The MF/SH implementation includes decoherence only within the evolution of the primary subsystem that is responsible for transition probabilities but not within the auxiliary equations governing environmental molecular dynamics. Within this implementation, adiabatic MQC propagation is independent of decoherence rate, and only transition times are affected. Simulations with an average decoherence parameter of 6 fs extend the excited-state lifetime of the solvated electron by three and a half times compared to coherent evolution. Since condensed phase environments typically destroy the electronic coherence on such femtosecond time scales, standard MQC methods based on coherent propagation will, in general, overestimate the true transition rate.

Formation of a hydrogen plasma from an incandescently heated hydrogen-catalyst gas mixture with an anomalous afterglow duration

We report the generation of a plasma of hydrogen and certain alkali ions as recorded via EUV spectroscopy and the hydrogen Balmer and alkali line emissions in the visible range. Typically, a hydrogen plasma is generated and the emission of extreme ultraviolet light from hydrogen gas is achieved via a discharge at high voltage, a high power inductively-coupled plasma, or a plasma created and heated to extreme temperatures by RF coupling (e.g. >10 6 K) with confinement provided by a toroidal magnetic field. The observed plasma formed at low temperatures (e.g. ≈10 3 K) from atomic hydrogen generated at a tungsten filament that heated a titanium dissociator and a catalyst comprising one of potassium, rubidium, cesium, and their carbonates and nitrates. These atoms and ions ionize to provide a catalyst with a net enthalpy of reaction of an integer multiple of the potential energy of atomic hydrogen (m×27.2 eV , m=integer) to within 0.17 eV and comprise only a single ionization in the case of a potassium or rubidium ion. Whereas, the chemically similar atoms of sodium and sodium and lithium carbonates and nitrates which do not ionize with these constraints caused no emission. To test the electric dependence of the emission, the weak electric field of about 1 V/ cm was set and measured to be zero in <0.5×10 −6 s . An anomalous afterglow duration of about 1–2 s was recorded in the case of potassium, rubidium, cesium, K 2 CO 3, RbNO 3 , and CsNO 3. Hydrogen line or alkali line emission was occurring even though the voltage between the heater wires was set to and measured to be zero. These atoms and ions ionize to provide a catalyst with a net enthalpy of reaction of an integer multiple of the potential energy of atomic hydrogen to within less than the thermal energies at ≈10 3 K and comprise only a single ionization in the case of a potassium or rubidium ion. Since the thermal decay time of the filament for dissociation of molecular hydrogen to atomic hydrogen was similar to the anomalous plasma afterglow duration, the emission was determined to be due to a reaction of atomic hydrogen with a catalyst that did not require the presence of an electric field to be functional.