Y^3＋-doped （Bi 1/2 Na 1/2） TiO 3-CaTiO 3-BaTiO 3 （BNCBT） positive temperature coefficient of resistivity （PTCR） ceramics sintered in air atmosphere were investigated in this study. （Bi 1/2 Na 1/2） TiO 3 （BNT...Y^3＋-doped （Bi 1/2 Na 1/2） TiO 3-CaTiO 3-BaTiO 3 （BNCBT） positive temperature coefficient of resistivity （PTCR） ceramics sintered in air atmosphere were investigated in this study. （Bi 1/2 Na 1/2） TiO 3 （BNT） component can remarkably increase the onset temperature T c of PTCR ceramics with the expense of the resistivity R 25 increase. CaTiO 3 （9–27 mol%） component can decrease the resistivity, and adjust the effects of BNT phase on the T c point. For the sample containing 3 mol% CaTiO 3 , T c raises from 122 ℃ to 153 ℃ when only 0.6 mol% BNT added, while for the ones with higher CaTiO 3 content （9–27 mol%）, T c is only increased by a rate of 8–9℃/1.0 mol% BNT. The effects of BNT and CaTiO 3 components on R25/Rmin （negative temperature coefficient effect） are also discussed.展开更多
Epoxy resin/Ni@C nanoparticle composites with aligned microstructure were prepared by using a procedure of magnetic field assisted curing. The results show that the resistivity of composites exhibits negative temperat...Epoxy resin/Ni@C nanoparticle composites with aligned microstructure were prepared by using a procedure of magnetic field assisted curing. The results show that the resistivity of composites exhibits negative temperature coefficient （NTC） effect above room temperature, and can be adjusted by varying the content filler and the magnitude of magnetic field applied. Hill＇s quantum tunneling model was modified to understand the electrical conduction mechanism in the composites. It shows that the NTC effect ascribes to the dominant thermal activated tunneling transport of electron across adjacent nanoparticles, as well as the low thermal expansivity of epoxy resin matrix.展开更多
Two-stage ignition exists in the low-temperature combustion process of n-heptane and the first-stage ignition also shows a negative temperature coefficient(NTC) phenomenon. To study key reactions and understand chemic...Two-stage ignition exists in the low-temperature combustion process of n-heptane and the first-stage ignition also shows a negative temperature coefficient(NTC) phenomenon. To study key reactions and understand chemical principles affecting the first-stage ignition of n-heptane, a lumped skeletal mechanism with 62 species is obtained based on the detailed NUIGMech1.0 mechanism using the directed relation graph method assisted by sensitivity analysis and isomer lumping. The lumped mechanism shows good performance on ignition delay time under wide conditions. The study revealed that the temperature after the first-stage ignition is higher and a larger amount of fuel is consumed at lower initial temperatures. The temperature at the first-stage ignition is relatively insensitive to the initial temperature. Further sensitivity analysis and reaction path analysis carried out based on the lumped mechanism show that the decomposition of RO_(2) to produce alkene and HO_(2) is the most important reaction to inhibit the first-stage ignitions. The chain branching explosion closely related to the first-stage ignition will be terminated when the rate constant for the RO_(2) decomposition is larger than that of the isomerization of RO_(2) to produce QOOH. The NTC behavior as well as other characteristics of the first-stage ignition can be rationalized from the competition between these two reactions.展开更多
The Mn_(1.95-x)Co_(0.21)Ni_(0.84)Sr_(x)O_(4)(MCNS)(0≤x≤0.15)based negative temperature coefficient(NTC)materials are prepared by co-precipitation method.The replacement of Mn by Sr plays a critical role in controlli...The Mn_(1.95-x)Co_(0.21)Ni_(0.84)Sr_(x)O_(4)(MCNS)(0≤x≤0.15)based negative temperature coefficient(NTC)materials are prepared by co-precipitation method.The replacement of Mn by Sr plays a critical role in controlling the lattice parameter,relative density,microstructure,and electrical properties.The lattice parameter and relative density increase with the increase of Sr content.A small amount of Sr restrains the grain growth and increases the bulk density.Moreover,the room resistivityρ25,material constant B25/50,activation energy Ea,and temperature coefficientαvalues of MCNS ceramics are influenced by the Sr content and ranged in 1535.0–2053.6Ω·cm,3654–3709 K,0.3149–0.3197 eV,and(–4.173%)–(–4.111%),respectively.The X-ray photoelectron spectroscopy(XPS)results explain the transformation of MCNS ceramics from n-to p-type semiconductors.The conduction could arise from the hopping polaron between Mn3+/Mn4+and Co^(2+)/Co^(3+) in the octahedral sites.The impedance data analysis also discusses the conduction mechanism of the MCNS ceramic,whereas grain resistance dominates the whole resistance of the samples.Furthermore,the aging coefficient(△R/R)of MCNS ceramics is found to be<0.2%,which indicates the stable distribution of cations in the spinel.Finally,the MCNS ceramics demonstrate excellent thermal durability with<1.3%of resistance shift after 100 thermal shock cycles.展开更多
Semiconductor materials with heterogeneous interfaces and twin structures generally demonstrate a higher concentration of carriers and better electrical stability.A variety of Cu-doped Co_(0.98)Cu_(x)Mn_(2.02−x)O_(4)(...Semiconductor materials with heterogeneous interfaces and twin structures generally demonstrate a higher concentration of carriers and better electrical stability.A variety of Cu-doped Co_(0.98)Cu_(x)Mn_(2.02−x)O_(4)(0≤x≤0.5)negative temperature coefficient(NTC)ceramics with dual phases and twin structures were successfully prepared in this study.Rietveld refinement indicates that the content of a cubic spinel phase increases with increasing Cu content.The addition of Cu can promote grain growth and densification.Atomic-level structural characterization reveals the evolution of twin morphology from large lamellae with internal fine lamellae(LIT lamellae)to large lamellae without internal fine lamellae(L lamellae)and the distribution of twin boundary defects.First-principles calculations reveal that the dual phases and twin structures have lower oxygen-vacancy formation energy than those in the case of the pure tetragonal and cubic spinel,thereby enhancing the transmission of carriers.Additionally,the three-dimensional charge-density difference shows that metal ions at the interface lose electrons and dwell in high valence states,thereby enhancing electrical stability of the NTC ceramics.Furthermore,the additional Cu ions engage in electron-exchange interactions with Mn and Co ions,thereby reducing resistivity.In comparison to previous Cu-containing systems,the Co_(0.98)Cu_(x)Mn_(2.02−x)O_(4)series exhibit superior stability(aging value≤2.84%),tunable room-temperature resistivity(ρ),and material constant(B)value(17.5Ω·cm≤ρ≤7325Ω·cm,2836 K≤B≤4315 K).These discoveries lay a foundation for designing and developing new NTC ceramics with ultra-high performance.展开更多
In this paper, a quaternary system of Mn0.43Ni0.9CuFe0.67O4 negative temperature coefficient （NTC） thermistor ceramic prepared by solid/solid reaction was sintered by microwave and conventional method, respectively....In this paper, a quaternary system of Mn0.43Ni0.9CuFe0.67O4 negative temperature coefficient （NTC） thermistor ceramic prepared by solid/solid reaction was sintered by microwave and conventional method, respectively. To characterize the sinterability of the samples, the densification parameter, porosity and grain size distribution of the bulk were determined. The crystal structure, phase compositions, morphology and impedance of the samples were analyzed by X-ray diffraction （XRD）, scanning electron microscopy （SEM）, energy-dispersive spectroscopy （EDS） and impedance analysis. The experimental results confirmed that the sinterability and electrical properties of ceramics were homogenously improved by microwave sintering.展开更多
基金Project supported by the SPAT of Shanghai Committee of Chinese People's Political Consultative Conference and Shanghai Education Development Foundation (Grant No.2008012)
文摘Y^3＋-doped （Bi 1/2 Na 1/2） TiO 3-CaTiO 3-BaTiO 3 （BNCBT） positive temperature coefficient of resistivity （PTCR） ceramics sintered in air atmosphere were investigated in this study. （Bi 1/2 Na 1/2） TiO 3 （BNT） component can remarkably increase the onset temperature T c of PTCR ceramics with the expense of the resistivity R 25 increase. CaTiO 3 （9–27 mol%） component can decrease the resistivity, and adjust the effects of BNT phase on the T c point. For the sample containing 3 mol% CaTiO 3 , T c raises from 122 ℃ to 153 ℃ when only 0.6 mol% BNT added, while for the ones with higher CaTiO 3 content （9–27 mol%）, T c is only increased by a rate of 8–9℃/1.0 mol% BNT. The effects of BNT and CaTiO 3 components on R25/Rmin （negative temperature coefficient effect） are also discussed.
基金supported by the National Natural Sci-ence Foundation of China under grant No. 50704021.
文摘Epoxy resin/Ni@C nanoparticle composites with aligned microstructure were prepared by using a procedure of magnetic field assisted curing. The results show that the resistivity of composites exhibits negative temperature coefficient （NTC） effect above room temperature, and can be adjusted by varying the content filler and the magnitude of magnetic field applied. Hill＇s quantum tunneling model was modified to understand the electrical conduction mechanism in the composites. It shows that the NTC effect ascribes to the dominant thermal activated tunneling transport of electron across adjacent nanoparticles, as well as the low thermal expansivity of epoxy resin matrix.
基金supported by the National Science and Technology Major Project of China (No.2017-I-0004-0004).
文摘Two-stage ignition exists in the low-temperature combustion process of n-heptane and the first-stage ignition also shows a negative temperature coefficient(NTC) phenomenon. To study key reactions and understand chemical principles affecting the first-stage ignition of n-heptane, a lumped skeletal mechanism with 62 species is obtained based on the detailed NUIGMech1.0 mechanism using the directed relation graph method assisted by sensitivity analysis and isomer lumping. The lumped mechanism shows good performance on ignition delay time under wide conditions. The study revealed that the temperature after the first-stage ignition is higher and a larger amount of fuel is consumed at lower initial temperatures. The temperature at the first-stage ignition is relatively insensitive to the initial temperature. Further sensitivity analysis and reaction path analysis carried out based on the lumped mechanism show that the decomposition of RO_(2) to produce alkene and HO_(2) is the most important reaction to inhibit the first-stage ignitions. The chain branching explosion closely related to the first-stage ignition will be terminated when the rate constant for the RO_(2) decomposition is larger than that of the isomerization of RO_(2) to produce QOOH. The NTC behavior as well as other characteristics of the first-stage ignition can be rationalized from the competition between these two reactions.
基金supported by Xinjiang Key Laboratory of Electronic Information Materials and Devices Foundation(Grant No.2018D04006)Tianshan Cedar Project of Xinjiang Uygur Autonomous Region(Grant No.2018XS09)the National Natural Science Foundation of China(Grant No.51872326)。
文摘The Mn_(1.95-x)Co_(0.21)Ni_(0.84)Sr_(x)O_(4)(MCNS)(0≤x≤0.15)based negative temperature coefficient(NTC)materials are prepared by co-precipitation method.The replacement of Mn by Sr plays a critical role in controlling the lattice parameter,relative density,microstructure,and electrical properties.The lattice parameter and relative density increase with the increase of Sr content.A small amount of Sr restrains the grain growth and increases the bulk density.Moreover,the room resistivityρ25,material constant B25/50,activation energy Ea,and temperature coefficientαvalues of MCNS ceramics are influenced by the Sr content and ranged in 1535.0–2053.6Ω·cm,3654–3709 K,0.3149–0.3197 eV,and(–4.173%)–(–4.111%),respectively.The X-ray photoelectron spectroscopy(XPS)results explain the transformation of MCNS ceramics from n-to p-type semiconductors.The conduction could arise from the hopping polaron between Mn3+/Mn4+and Co^(2+)/Co^(3+) in the octahedral sites.The impedance data analysis also discusses the conduction mechanism of the MCNS ceramic,whereas grain resistance dominates the whole resistance of the samples.Furthermore,the aging coefficient(△R/R)of MCNS ceramics is found to be<0.2%,which indicates the stable distribution of cations in the spinel.Finally,the MCNS ceramics demonstrate excellent thermal durability with<1.3%of resistance shift after 100 thermal shock cycles.
基金supported by the National Natural Science Foundation of China(Grant No.52002347)Natural Science Foundation of the Jiangsu Higher Education Institutions of China(Grant No.19KJB430039).
文摘Semiconductor materials with heterogeneous interfaces and twin structures generally demonstrate a higher concentration of carriers and better electrical stability.A variety of Cu-doped Co_(0.98)Cu_(x)Mn_(2.02−x)O_(4)(0≤x≤0.5)negative temperature coefficient(NTC)ceramics with dual phases and twin structures were successfully prepared in this study.Rietveld refinement indicates that the content of a cubic spinel phase increases with increasing Cu content.The addition of Cu can promote grain growth and densification.Atomic-level structural characterization reveals the evolution of twin morphology from large lamellae with internal fine lamellae(LIT lamellae)to large lamellae without internal fine lamellae(L lamellae)and the distribution of twin boundary defects.First-principles calculations reveal that the dual phases and twin structures have lower oxygen-vacancy formation energy than those in the case of the pure tetragonal and cubic spinel,thereby enhancing the transmission of carriers.Additionally,the three-dimensional charge-density difference shows that metal ions at the interface lose electrons and dwell in high valence states,thereby enhancing electrical stability of the NTC ceramics.Furthermore,the additional Cu ions engage in electron-exchange interactions with Mn and Co ions,thereby reducing resistivity.In comparison to previous Cu-containing systems,the Co_(0.98)Cu_(x)Mn_(2.02−x)O_(4)series exhibit superior stability(aging value≤2.84%),tunable room-temperature resistivity(ρ),and material constant(B)value(17.5Ω·cm≤ρ≤7325Ω·cm,2836 K≤B≤4315 K).These discoveries lay a foundation for designing and developing new NTC ceramics with ultra-high performance.
基金financial support from the programs of seed money (No. K08141001)the sci-entific problem tackling (No. G06211002) foundations of Urumqi in China
文摘In this paper, a quaternary system of Mn0.43Ni0.9CuFe0.67O4 negative temperature coefficient （NTC） thermistor ceramic prepared by solid/solid reaction was sintered by microwave and conventional method, respectively. To characterize the sinterability of the samples, the densification parameter, porosity and grain size distribution of the bulk were determined. The crystal structure, phase compositions, morphology and impedance of the samples were analyzed by X-ray diffraction （XRD）, scanning electron microscopy （SEM）, energy-dispersive spectroscopy （EDS） and impedance analysis. The experimental results confirmed that the sinterability and electrical properties of ceramics were homogenously improved by microwave sintering.
