A series of YAG：Ce,Tb phosphors were synthesized by vacuum sintering method.Moreover,their spectral properties,thermal quenching behaviors and color rendering properties were investigated systematically.The photolumi...A series of YAG：Ce,Tb phosphors were synthesized by vacuum sintering method.Moreover,their spectral properties,thermal quenching behaviors and color rendering properties were investigated systematically.The photoluminescence emission spectra of YAG：Ce,Tb show a great red shift compared with that of YAG：Ce.Direct energy transfer from Tb^（3＋） to Ce^（3＋） ions is verified based on the analysis of different photoluminescence spectra.The quenching temperature for Tb^（3＋）-doped YAG：Ce phosphors is about 490 K.The thermal activation energy is estimated to be 0.18 and 0.291 eV for Tb^（3＋）-doped YAG：Ce and YAG：Ce phosphors,respectively.The smaller activation energy for Tb^（3＋）-doped YAG：Ce means a more rapid nonradiative transition from 5d to 4f state,thus resulting in the lower quenching temperature.In addition,white LEDs with improved color rendering properties are achieved by using modified YAG：Ce,Tb phosphors.展开更多
Herein,double-perovskite Ba_(2)LaTaO_(6) Eu-doped orange-red phosphors were successfully synthesized using a high-temperature solid-phase method.The phosphor phase purity was investigated using X-ray diffraction and m...Herein,double-perovskite Ba_(2)LaTaO_(6) Eu-doped orange-red phosphors were successfully synthesized using a high-temperature solid-phase method.The phosphor phase purity was investigated using X-ray diffraction and microscopic morphology analyses.Their luminescence properties were investigated using absorption,emission,excitation,and temperature-dependent spectra.The transition mechanism mainly involves a magnetic-dipole transition with an energy transfer mode featuring multipole-multipole interactions,and concentration quenching is achieved via dipole-dipole interactions.In addition,the intensity of the temperature-dependent spectrum increases abnormally between 298 and 373 K,with the luminous intensity at 373 K increasing to 110%of that observed at room temperature.This phenomenon can be attributed to lattice defects in Ba_(2)LaTaO_(6):Eu^(3+),and the phosphor luminous intensity at473 K remains at 80.62%of that at room temperature.In addition,white-light-emitting diode devices based on this novel Ba_(2)LaTaO_(6):0.35Eu^(3+)phosphor were fabricated to evaluate the potential applications of the as-prepared phosphor.展开更多
Luminescent materials often suffer from thermal quenching(TQ),limiting the continuation of their applications under high temperatures up to 473 K.The formation of defect levels could suppress TQ,but rational synthesis...Luminescent materials often suffer from thermal quenching(TQ),limiting the continuation of their applications under high temperatures up to 473 K.The formation of defect levels could suppress TQ,but rational synthesis and deep understanding of multiple defects-regulated luminescent materials working in such a wide temperature range still remain challenging.Here,we prepare a negative thermal quenching(NTQ)phosphor LiTaO_(3)∶Tb^(3+)by introducing gradient defects V_(Ta)^(5−),Tb_(Li)^(2+),and(V_(Ta)Tb_(Li))^(3−)as identified by advanced experimental and theoretical studies.Its photoluminescence significantly becomes intense with rising temperatures and then slowly increases at 373 to 473 K.The mechanism studies reveal that gradient defects with varied trapping depths could act as energy buffer layers to effectively capture the carriers.Under thermal disturbance,the stored carriers could successively migrate to the activators in consecutive and wide temperature zones,compensating for TQ to enhance luminescence emission.This study initiates the synthesis of multi-defect NTQ phosphors for temperature-dependent applications.展开更多
The yellow SrSi2O2N2：Eu2＋ phosphor has been synthesized by using a simple solid-state reaction method with Sr2SiO4：Eu2＋ as the precursor. It shows a broad excitation band extending from 250 to 520 nm and an asymme...The yellow SrSi2O2N2：Eu2＋ phosphor has been synthesized by using a simple solid-state reaction method with Sr2SiO4：Eu2＋ as the precursor. It shows a broad excitation band extending from 250 to 520 nm and an asymmetric emission band with a main peak at about 550 nm. The emission intensity of the SrSi202N2：Eu2＋ is about 1.2 times higher than the commercial yellow phosphor YAG：Ce3＋ （P46-Y3）. The temperature- dependent luminescence characteristic of SrSi202N2：Eu2＋ has been investigated in this paper. With increasing temperature, the emission band of SrSi202N2：Eu2＋ shows anomalous blue-shift along with decreasing emission intensity and the broadening full width at half maximum （FWHM）. Particularly, compared with YAG：Ce3＋ （P46-Y3）, the yellow SrSi202N2：Eu2＋ phosphors exhibit higher thermal stability due to their weaker electron-phonon coupling strength （1.1）, lower stokes shift （0.0576 eV） and larger activation energy （0.288 eV）. All these results indicate that SrSi202N2：Eu2＋ yellow phosphors have potential application for white light-emitting diodes （LEDs）, What＇s more, an energy level scheme is constructed to explain the anomalous blue-shift phenomenon.展开更多
In this paper,luminescence thermal quenching of M2SiO4:Eu^2+(M=Sr,Ba) orthosilicate phosphors and mechanisms for thermal quenching proposed by different authors are briefly reviewed.Depending on preparation conditions...In this paper,luminescence thermal quenching of M2SiO4:Eu^2+(M=Sr,Ba) orthosilicate phosphors and mechanisms for thermal quenching proposed by different authors are briefly reviewed.Depending on preparation conditions and/or Eu^2+-doping concentrations,the quenching temperature(T1/2) and activation energy for thermal quenching of the same orthosilicate phosphor reported by different authors are inconsistent.The common conclusion is that T1/2 of the intermediate compound(Ba1-xSrx)2 SiO4:Eu^2+(x≈0.5) is higher than that of either Sr2So4;Eu^2+or Ba2SiO4;Eu^2+end-member.Moreover,T1/2 of the best-performing SrBaSiO4:Eu^2+is evidently lower than that of YAG:Ce3+and some Eu^2+-doped nitride phosphors.Rega rding the quenching mechanism,most of the investigators attributed thermal quenching to a thermally assisted 4f-5d cross-over in the configuration coordinate diagram.Only a few authors ascribed thermal quenching to a thermally assisted photoionization of 5 d electron to conduction band of the host.Nonetheless,a close inspection of T1/2 and Stokes shift derived from the vibrational spectra of the intermediate compound and end-member phosphors indicates that the 5d electron photo ionization model instead of the 4f-5d crossing decay model should be the genuine mechanism for the thermal quenching of M2 SiO4:Eu^2+(M=Sr,Ba) phosphors.Since the relationship between T1/2 and Stokes shift of the phosphors does not support the 4 f-5 d crossing decay model.The ionization probability of the 5 d electron depends on the energy gap(EdC) between 5 d1 level of the Eu^2+and conduction band minimum(CBM) of the host at higher temperatures.Lattice thermal expansion would result in an elevating 5 d1 level of the Eu^2+along with a diminishing CBM of the host and as a consequence a reduction in EdC and an enhanced photo ionization probability at elevated temperatures.A less rigid lattice and hence a larger coefficient of thermal expansion of M2SiO4 hosts should be the physical origin of poorer thermal quenching properties of the orthosilicate phosphors.展开更多
CO2 pyrolysis by thermal plasma was investigated,and a high conversion rate of 33% and energy efficiency of 17% were obtained.The high performance benefited from a novel quenching method,which synergizes the convergin...CO2 pyrolysis by thermal plasma was investigated,and a high conversion rate of 33% and energy efficiency of 17% were obtained.The high performance benefited from a novel quenching method,which synergizes the converging nozzle and cooling tube.To understand the synergy effect,a computational fluid dynamics simulation was carried out.A quick quenching rate of 10~7Ks（-1） could be expected when the pyrolysis gas temperature decreased from more than 3000 to 1000 K.According to the simulation results,the quenching mechanism was discussed as follows： first,the compressible fluid was adiabatically expanded in the converging nozzle and accelerated to sonic speed,and parts of the heat energy converted to convective kinetic energy; second,the sonic fluid jet into the cooling tube formed a strong eddy,which greatly enhanced the heat transfer between the inverse-flowing fluid and cooling tube.These two mechanisms ensure a quick quenching to prevent the reverse reaction of CO2 pyrolysis gas when it flows out from the thermal plasma reactor.展开更多
A system was developed to identify characteristics of quenchants. It consists of computerized hardware, temperature acquisition software as well as changeable probes of steels. The feature of the ability to use probes...A system was developed to identify characteristics of quenchants. It consists of computerized hardware, temperature acquisition software as well as changeable probes of steels. The feature of the ability to use probes made of any metallic material enables evaluation of hardening power of quenchants. Three thermal couples located on the same cross-section plane in the middle of probe’s length give cooling curves that present cooling behavior at different depths from the surface of probe. Microstructure and hardness of the quenched probes on the same plane in the middle of probe’s length from surface to center was investigated to estimate hardening effect. A number of quenchants were tested by this characterization system with several thermal probes of typical steels. Experimental data were stored in QuenDB quenchant database, which was designed for quenchants identification and hardness distribution storage.展开更多
Heat treated steel components often suffer distortion and residual stress effects when cooled to room temperature. A numerical analysis of a vacuum gas quenched die block made of H 13 was carried out utilising a nonli...Heat treated steel components often suffer distortion and residual stress effects when cooled to room temperature. A numerical analysis of a vacuum gas quenched die block made of H 13 was carried out utilising a nonlinear thermoelastic-plastic stress model together with the fluid flow and thermal profiles within the furnace. Simulation procedures for stress behaviour of the die were developed for both direct quench and marquench processes. Results of the initial thermal analysis indicated that the temperature difference between the surface and core of the die during direct quenching (gas pressure is 4 bar) was larger than that due to marquenching ( gas pressure is 4 bar at the beginning and 2 bar near isothermal hold). Simulation of the cooling rates at the surface and core of the die during marquenching correlated well with the experimental data. Further stress simulation indicated that the final thermal distortion and residual stresses in the die after direct quenching were larger than those due to marquenching. The findings of the numerical analysis suggested that marquenching is recommended for this die because it could reduce the temperature difference in the die and thus result in less thermal distortion and residual stresses.展开更多
The effect of yttrium on the thermal stability and crystallization behavior of Nd-Fe-Al-Ni amorphous alloys was investigated using X-ray diffraction （XRD）, differential scanning calorimeter （DSC）, and transmission...The effect of yttrium on the thermal stability and crystallization behavior of Nd-Fe-Al-Ni amorphous alloys was investigated using X-ray diffraction （XRD）, differential scanning calorimeter （DSC）, and transmission electron microscopy （TEM）.The results indicated that the as-cast Nd60Fe20Al10Ni10-xYx（X=-0, 2） amorphous alloys were fabricated with some quenched-in crystals, which could be restrained by Y. With the effect of yttrium, both the crystallization temperature and exothermic peak shifted to higher temperatures, illustrating that the thermal stability could be improved. The addition of Y changed the crystallization process and final crystallization results. Moreover, the crystallites in the amorphous matrix became more homogeneous and smaller. Meanwhile, Y was useful for the passivation of oxygen in chemistry and restrained the negative effect of oxygen. The activation energies of the start of crystallization and peaking were 1.21 and 1.16 eV, respectively, according to the Kissinger equation.展开更多
基金supported by the National Natural Science Foundation of China(51272282,51302311)the Beijing Committee of Science and Technology(Z13111000280000)the Education Commission of Beijing(2011010329)
文摘A series of YAG：Ce,Tb phosphors were synthesized by vacuum sintering method.Moreover,their spectral properties,thermal quenching behaviors and color rendering properties were investigated systematically.The photoluminescence emission spectra of YAG：Ce,Tb show a great red shift compared with that of YAG：Ce.Direct energy transfer from Tb^（3＋） to Ce^（3＋） ions is verified based on the analysis of different photoluminescence spectra.The quenching temperature for Tb^（3＋）-doped YAG：Ce phosphors is about 490 K.The thermal activation energy is estimated to be 0.18 and 0.291 eV for Tb^（3＋）-doped YAG：Ce and YAG：Ce phosphors,respectively.The smaller activation energy for Tb^（3＋）-doped YAG：Ce means a more rapid nonradiative transition from 5d to 4f state,thus resulting in the lower quenching temperature.In addition,white LEDs with improved color rendering properties are achieved by using modified YAG：Ce,Tb phosphors.
基金Project supported by the National Natural Science Foundation of China (52262020)the Science and Technology Foundation of Guizhou Province (ZK[2021]yiban 328)。
文摘Herein,double-perovskite Ba_(2)LaTaO_(6) Eu-doped orange-red phosphors were successfully synthesized using a high-temperature solid-phase method.The phosphor phase purity was investigated using X-ray diffraction and microscopic morphology analyses.Their luminescence properties were investigated using absorption,emission,excitation,and temperature-dependent spectra.The transition mechanism mainly involves a magnetic-dipole transition with an energy transfer mode featuring multipole-multipole interactions,and concentration quenching is achieved via dipole-dipole interactions.In addition,the intensity of the temperature-dependent spectrum increases abnormally between 298 and 373 K,with the luminous intensity at 373 K increasing to 110%of that observed at room temperature.This phenomenon can be attributed to lattice defects in Ba_(2)LaTaO_(6):Eu^(3+),and the phosphor luminous intensity at473 K remains at 80.62%of that at room temperature.In addition,white-light-emitting diode devices based on this novel Ba_(2)LaTaO_(6):0.35Eu^(3+)phosphor were fabricated to evaluate the potential applications of the as-prepared phosphor.
基金the National Key Research and Development Program of China(Grant No.2018YFB0704103)the Natural Science Foundation of Shanghai(Grant Nos.22ZR1472100 and 20ZR1465900)+3 种基金the National Natural Science Foundation of China(Grant Nos.92163117 and 62175210)the State Key Laboratory of ASIC&System(Grant No.2020KF002)the Innovation Project of Shanghai Institute of Ceramics(Grant No.E21ZC1770G)L.L.thanks the Taiwan Light Source for technical support.J.W.thanks the Program of Shanghai Academic Research Leader(Grant No.20XD1424300)for financial support.The authors thank Professor Zhijun Zhang for providing valuable assistance in stress luminescence testing.
文摘Luminescent materials often suffer from thermal quenching(TQ),limiting the continuation of their applications under high temperatures up to 473 K.The formation of defect levels could suppress TQ,but rational synthesis and deep understanding of multiple defects-regulated luminescent materials working in such a wide temperature range still remain challenging.Here,we prepare a negative thermal quenching(NTQ)phosphor LiTaO_(3)∶Tb^(3+)by introducing gradient defects V_(Ta)^(5−),Tb_(Li)^(2+),and(V_(Ta)Tb_(Li))^(3−)as identified by advanced experimental and theoretical studies.Its photoluminescence significantly becomes intense with rising temperatures and then slowly increases at 373 to 473 K.The mechanism studies reveal that gradient defects with varied trapping depths could act as energy buffer layers to effectively capture the carriers.Under thermal disturbance,the stored carriers could successively migrate to the activators in consecutive and wide temperature zones,compensating for TQ to enhance luminescence emission.This study initiates the synthesis of multi-defect NTQ phosphors for temperature-dependent applications.
基金the financial supports of the National Natural Science Foundations of China (Nos.50903038, 21171071, 51203053, and 21071063)the project sponsored by SRF for ROCS, SEM (No. [2011] 1139)the project of the combination of Industry and Research by the Ministry of Education and Guangdong Province (No. 2012B091100181)
文摘The yellow SrSi2O2N2：Eu2＋ phosphor has been synthesized by using a simple solid-state reaction method with Sr2SiO4：Eu2＋ as the precursor. It shows a broad excitation band extending from 250 to 520 nm and an asymmetric emission band with a main peak at about 550 nm. The emission intensity of the SrSi202N2：Eu2＋ is about 1.2 times higher than the commercial yellow phosphor YAG：Ce3＋ （P46-Y3）. The temperature- dependent luminescence characteristic of SrSi202N2：Eu2＋ has been investigated in this paper. With increasing temperature, the emission band of SrSi202N2：Eu2＋ shows anomalous blue-shift along with decreasing emission intensity and the broadening full width at half maximum （FWHM）. Particularly, compared with YAG：Ce3＋ （P46-Y3）, the yellow SrSi202N2：Eu2＋ phosphors exhibit higher thermal stability due to their weaker electron-phonon coupling strength （1.1）, lower stokes shift （0.0576 eV） and larger activation energy （0.288 eV）. All these results indicate that SrSi202N2：Eu2＋ yellow phosphors have potential application for white light-emitting diodes （LEDs）, What＇s more, an energy level scheme is constructed to explain the anomalous blue-shift phenomenon.
文摘In this paper,luminescence thermal quenching of M2SiO4:Eu^2+(M=Sr,Ba) orthosilicate phosphors and mechanisms for thermal quenching proposed by different authors are briefly reviewed.Depending on preparation conditions and/or Eu^2+-doping concentrations,the quenching temperature(T1/2) and activation energy for thermal quenching of the same orthosilicate phosphor reported by different authors are inconsistent.The common conclusion is that T1/2 of the intermediate compound(Ba1-xSrx)2 SiO4:Eu^2+(x≈0.5) is higher than that of either Sr2So4;Eu^2+or Ba2SiO4;Eu^2+end-member.Moreover,T1/2 of the best-performing SrBaSiO4:Eu^2+is evidently lower than that of YAG:Ce3+and some Eu^2+-doped nitride phosphors.Rega rding the quenching mechanism,most of the investigators attributed thermal quenching to a thermally assisted 4f-5d cross-over in the configuration coordinate diagram.Only a few authors ascribed thermal quenching to a thermally assisted photoionization of 5 d electron to conduction band of the host.Nonetheless,a close inspection of T1/2 and Stokes shift derived from the vibrational spectra of the intermediate compound and end-member phosphors indicates that the 5d electron photo ionization model instead of the 4f-5d crossing decay model should be the genuine mechanism for the thermal quenching of M2 SiO4:Eu^2+(M=Sr,Ba) phosphors.Since the relationship between T1/2 and Stokes shift of the phosphors does not support the 4 f-5 d crossing decay model.The ionization probability of the 5 d electron depends on the energy gap(EdC) between 5 d1 level of the Eu^2+and conduction band minimum(CBM) of the host at higher temperatures.Lattice thermal expansion would result in an elevating 5 d1 level of the Eu^2+along with a diminishing CBM of the host and as a consequence a reduction in EdC and an enhanced photo ionization probability at elevated temperatures.A less rigid lattice and hence a larger coefficient of thermal expansion of M2SiO4 hosts should be the physical origin of poorer thermal quenching properties of the orthosilicate phosphors.
基金the funding of National Natural Science Foundation of China (Grant No.11775155)
文摘CO2 pyrolysis by thermal plasma was investigated,and a high conversion rate of 33% and energy efficiency of 17% were obtained.The high performance benefited from a novel quenching method,which synergizes the converging nozzle and cooling tube.To understand the synergy effect,a computational fluid dynamics simulation was carried out.A quick quenching rate of 10~7Ks（-1） could be expected when the pyrolysis gas temperature decreased from more than 3000 to 1000 K.According to the simulation results,the quenching mechanism was discussed as follows： first,the compressible fluid was adiabatically expanded in the converging nozzle and accelerated to sonic speed,and parts of the heat energy converted to convective kinetic energy; second,the sonic fluid jet into the cooling tube formed a strong eddy,which greatly enhanced the heat transfer between the inverse-flowing fluid and cooling tube.These two mechanisms ensure a quick quenching to prevent the reverse reaction of CO2 pyrolysis gas when it flows out from the thermal plasma reactor.
文摘在压水堆冷却剂丧失事故(Loss-of Coolant Accident,LOCA)中,处于高温条件下的燃料棒由于棒内压力过高,可能导致包壳发生鼓胀。包壳形变会造成堆芯局部流道堵塞,进而影响失水事故再淹没阶段的堆芯换热。然而,大多数系统分析程序都是基于假设的流道堵塞率来模拟事故进程,导致模拟结果与实际情况不符合。本文将已开发的燃料棒热-力行为分析模块(Fuel Rod Thermal-Mechanical Behavior,FRTMB)集成在自主开发的严重事故分析程序ISAA(Integrated Severe Accident Analysis Code)中,通过改进已有的流道堵塞模型,使其能够模拟由于燃料棒形变导致的冷却剂流量变化。最后,使用ISAA-FRTMB模拟QUENCH-LOCA-0实验,通过对比包壳峰值温度,验证改进的流道堵塞模型的正确性和有效性,并在此基础上研究包壳形变对堆芯换热以及后续事故进程的影响。
文摘A system was developed to identify characteristics of quenchants. It consists of computerized hardware, temperature acquisition software as well as changeable probes of steels. The feature of the ability to use probes made of any metallic material enables evaluation of hardening power of quenchants. Three thermal couples located on the same cross-section plane in the middle of probe’s length give cooling curves that present cooling behavior at different depths from the surface of probe. Microstructure and hardness of the quenched probes on the same plane in the middle of probe’s length from surface to center was investigated to estimate hardening effect. A number of quenchants were tested by this characterization system with several thermal probes of typical steels. Experimental data were stored in QuenDB quenchant database, which was designed for quenchants identification and hardness distribution storage.
文摘Heat treated steel components often suffer distortion and residual stress effects when cooled to room temperature. A numerical analysis of a vacuum gas quenched die block made of H 13 was carried out utilising a nonlinear thermoelastic-plastic stress model together with the fluid flow and thermal profiles within the furnace. Simulation procedures for stress behaviour of the die were developed for both direct quench and marquench processes. Results of the initial thermal analysis indicated that the temperature difference between the surface and core of the die during direct quenching (gas pressure is 4 bar) was larger than that due to marquenching ( gas pressure is 4 bar at the beginning and 2 bar near isothermal hold). Simulation of the cooling rates at the surface and core of the die during marquenching correlated well with the experimental data. Further stress simulation indicated that the final thermal distortion and residual stresses in the die after direct quenching were larger than those due to marquenching. The findings of the numerical analysis suggested that marquenching is recommended for this die because it could reduce the temperature difference in the die and thus result in less thermal distortion and residual stresses.
基金the National Natural Science Foundation of China (50571052)
文摘The effect of yttrium on the thermal stability and crystallization behavior of Nd-Fe-Al-Ni amorphous alloys was investigated using X-ray diffraction （XRD）, differential scanning calorimeter （DSC）, and transmission electron microscopy （TEM）.The results indicated that the as-cast Nd60Fe20Al10Ni10-xYx（X=-0, 2） amorphous alloys were fabricated with some quenched-in crystals, which could be restrained by Y. With the effect of yttrium, both the crystallization temperature and exothermic peak shifted to higher temperatures, illustrating that the thermal stability could be improved. The addition of Y changed the crystallization process and final crystallization results. Moreover, the crystallites in the amorphous matrix became more homogeneous and smaller. Meanwhile, Y was useful for the passivation of oxygen in chemistry and restrained the negative effect of oxygen. The activation energies of the start of crystallization and peaking were 1.21 and 1.16 eV, respectively, according to the Kissinger equation.
