Ultrafast electron sources, which enable high spatiotemporal resolution in time-resolved electron microscopy and scanning probe microscopy, are receiving increased attention. The most widely used method for achieving ...Ultrafast electron sources, which enable high spatiotemporal resolution in time-resolved electron microscopy and scanning probe microscopy, are receiving increased attention. The most widely used method for achieving ultrafast electron sources involves irradiating metal tips by ultrashort laser pulses, causing electron beam emission via the photoelectric effect [including photon-driven(quantum) or field-driven(classical) emission]. However, the thermionic electrons emission process due to the heating effect of ultrashort lasers, particularly its dynamic aspects, has rarely been addressed in previous studies. In this paper, we improved the signal-to-noise ratio of a two-pulse correlation measurement on the tip electron emission by nearly two orders of magnitude using a delay time modulation method. This allowed us to obtain information on the temperature evolution of hot electrons and phonons in a non-equilibrium state, and to extract characteristic time scales for electron-phonon and phonon-phonon scattering. Our findings indicate that the thermionic electrons emission, unlike the instantaneous photoelectric effect, causes electron emission to lag behind the laser pulse by tens of picoseconds, thus significantly affecting the detection of ultrafast dynamics of samples. Furthermore, such a lagging effect was found to be sensitive to the local structure of the metal tip, offering new insights into the improved design of ultrafast electron sources.展开更多
A microcomputerized measurement system is developed.This system automatically acquiresan I-V characteristic curve in 5 seconds with the accuracy of 0.05％ FSR(full scale range).The Schottkyextrapolation field-free emi...A microcomputerized measurement system is developed.This system automatically acquiresan I-V characteristic curve in 5 seconds with the accuracy of 0.05％ FSR(full scale range).The Schottkyextrapolation field-free emission,the inflection-point emission,the flection-point emission,the average effec-tive work function.the Richardson work function,the work function distribution at the operation tempera-ture and other important information that reflects properties of cathodes are obtained by this system.The re-sults of analysing four types of cathodes demonstrate that this system is very suitable for measuring the activi-ty changes of the cathode in the processes of activation,ageing,poisoning and life test.It is proved that thissystem can also be used to monitor the actlvity of the cathode in the assembled tube by measuring a gridcontrolled travelling-wave tube.展开更多
Magnesium and magnesium thin alloy films were deposited using a thermionic vacuum arc(TVA),which has multiple applications in the field of metallic electrodes for diodes and batteries or active corrosion protection.An...Magnesium and magnesium thin alloy films were deposited using a thermionic vacuum arc(TVA),which has multiple applications in the field of metallic electrodes for diodes and batteries or active corrosion protection.An improved laser-induced TVA(LTVA)method favors the crystallization processes of the deposited magnesium-based films because the interaction between laser and plasma discharge changes the thermal energy during photonic processes due to the local temperature variation.Plasma diagnosis based on current discharge measurements suggests an inelastic collision between the laser beam and the atoms from the plasma discharge.The morphology and surface properties of the obtained thin films differ between these two methods.While the amorphous character is dominant for TVA thin films,enabling a smooth surface,the LTVA method produces rough surfaces with prominent crystallinity,less hydrophobic character and lower surface energy.The smooth surfaces obtained by the TVA methods produce metallic electrodes with good electrical contact,ensuring better diodes and battery charge transport.Both methods allow uniform magnesium alloys to be obtained,but the laser used in the LTVA on the discharge plasma controls the added metal or element ratio.展开更多
By using correlation-detection technique and improving structure of the test tube,the background noise of thermionic-electrons and space charge effect are restrained.The sec-ondary emission coefficient δ of thermioni...By using correlation-detection technique and improving structure of the test tube,the background noise of thermionic-electrons and space charge effect are restrained.The sec-ondary emission coefficient δ of thermionic cathode at high temperature has been studied.Theδ of impregnated scandate cathodes increases exponentially with increasing temperature at lowenergy and current of the bombardment electrons;at high energy or current of the bombardmentelectrons the temperature has little effect on δ.The research shows that an enhanced thermionicemission occurred when the cathode works at high temperature and under electron bombardment.These phenomena are discussed in terms of “internal field model”.展开更多
Thermionic emission is a tunneling phenomenon,which depicts that electrons on the surface of a conductor can be pulled out into the vacuum when they are subjected to high electrical tensions while being heated hot eno...Thermionic emission is a tunneling phenomenon,which depicts that electrons on the surface of a conductor can be pulled out into the vacuum when they are subjected to high electrical tensions while being heated hot enough to overtake their work functions.This principle has led to the great success of the so-called vacuum tubes in the early 20 th century.To date,major challenges still remain in the miniaturization of a vacuum channel transistor for on-chip integration in modern solid-state integrated circuits.Here,by introducing nano-sized vacuum gaps(~200 nm)in a van der Waals heterostructure,we successfully fabricated a one-dimensional(1 D)edge-to-edge thermionic emission vacuum tube using graphene as the filament.With the increasing collector voltage,the emitted current exhibits a typical rectifying behavior,with the maximum emission current reaching 200 p A and an ON-OFF ratio of 10;.In addition,it is found that the maximum emission current is proportional to the number of the layers of graphene.Our results expand the research of nano-sized vacuum tubes to an unexplored physical limit of 1 D edge-to-edge emission,and hold great promise for future nano-electronic systems based on it.展开更多
A combined system model is proposed including a molten carbonate fuel cell(MCFC),a graphene thermionic converter(GTIC)and thermally regenerative electrochemical cycles(TRECs).The expressions for power output,energy ef...A combined system model is proposed including a molten carbonate fuel cell(MCFC),a graphene thermionic converter(GTIC)and thermally regenerative electrochemical cycles(TRECs).The expressions for power output,energy efficiency of the subsystems and the couple system are formulated by considering several irreversible losses.Energy conservation equations between the subsystems are achieved leaned on the first law of thermodynamics.The optimum operating ranges for the combined system are determined compared with the MCFC system.Results reveal that the peak power output density(POD)and the corresponding energy efficiency are 28.22%and 10.76%higher than that of the single MCFC system,respectively.The effects of five designing parameters on the power density and energy efficiency of the MCFC/GTIC/TRECs model are also investigated and discussed.展开更多
A transport equation of momentum for relativistic electrons scattered isotropically was previously reported. Here, a momentum-transport equation for relativistic electrons “scattered anisotropically” by the Coulomb ...A transport equation of momentum for relativistic electrons scattered isotropically was previously reported. Here, a momentum-transport equation for relativistic electrons “scattered anisotropically” by the Coulomb force is inquired into. An ideal plasma consisting of electrons and deuterons is treated again. Also, to raise a generation-ability of a thermionic energy converter, a means of introducing external electric and magnetic fields within “a converter in which an emitter plate and a collector plate face simply each other” is proposed.展开更多
A 2MeV thermionic cathode test stand was established to meet the requirement of the large area thermionic cathode system.A 100ram in diameter type'B'thermionic dispenser cathode was developed.A 1000A emission ...A 2MeV thermionic cathode test stand was established to meet the requirement of the large area thermionic cathode system.A 100ram in diameter type'B'thermionic dispenser cathode was developed.A 1000A emission current was produced at the voltage of the diode about 1.8MV,the pulse width about 90ns(FWHM),and the cathode temperature about 1350℃.The emission current density is 12A/cm^2.The results indicate that a large area thermionic cathode which produces high quality and high current electron beams is visible.The results also indicate that the ability of cathode emission relies on the diode-vacuum and cathode-temperature.展开更多
In this paper,an irreversible thermionic refrigerator model based on van der Waals heterostructure with various irreversibilities is established by utilizing combination of non-equilibrium thermodynamics and finite ti...In this paper,an irreversible thermionic refrigerator model based on van der Waals heterostructure with various irreversibilities is established by utilizing combination of non-equilibrium thermodynamics and finite time thermodynamics.The basic performance characteristics of the refrigerator are obtained.The effects of key factors,such as bias voltages,Schottky barrier heights and heat leakages,on the performance are studied.Results show that cooling rates and coefficients of performances(COPs)can attain the double maximum with proper modulation of barrier heights and bias voltages.Increasing cross-plane thermal resistance as well as decreasing electrode-reservoir thermal resistance and reservoir-reservoir thermal resistance can enhance the performance of the device.The optimal performance region is the interval between the maximum cooling rate point and the maximum COP point.By modulating the bias voltage,the working state of the device can fall into the optimal performance region.The optimal performance of the refrigerator when using single layer graphene and a few layers graphene as electrode material is also compared.展开更多
The work function(WF)of graphene is an essential parameter in graphene electronics.We have derived the WF of graphene by the thermionic emission method.Chemical vapor deposition(CVD)-grown single-layered polycrystalli...The work function(WF)of graphene is an essential parameter in graphene electronics.We have derived the WF of graphene by the thermionic emission method.Chemical vapor deposition(CVD)-grown single-layered polycrystalline graphene on copper foil is transferred to a cross-stacked carbon nanotube(CNT)film drawn from a super-aligned multiwalled CNT array.By decreasing the pore size of the CNT film,the as-prepared CNT-graphene film(CGF)can be Joule heated to a temperature as high as 1,800 K in vacuum without obvious destruction in the graphene structure.By studying the thermionic emission,we derive the WF of graphene,ranging from 4.7 to 4.8 eV with the average value being 4.74 eV.Because the substrate influence can be minimized by virtue of the porous nature of the CNT film and the influence of adsorbents can be excluded due to the high temperature during the thermionic emission,the measured WF of graphene can be regarded as intrinsic.展开更多
A new photovoltaic-thermochemical(PVTC) conceptual system integrating photon-enhanced thermionic emission(PETE) and methane steam reforming is proposed. Major novelty of the system lies in its potential adaptivity to ...A new photovoltaic-thermochemical(PVTC) conceptual system integrating photon-enhanced thermionic emission(PETE) and methane steam reforming is proposed. Major novelty of the system lies in its potential adaptivity to primary fuels(e.g. methane) and high efficiencies of photovoltaic and thermochemical power generation, both of which result from its operation at much elevated temperatures(700–1000 °C)compared with conventional photovoltaic-thermal(PVT) systems. Analysis shows that an overall power generation efficiency of 45.3% and a net solar-to-electric efficiency of 39.1% could be reached at an operating temperature of 750 °C, after considering major losses during solar energy capture and conversion processes. The system is also featured by high solar share(37%) in the total power output, as well as high energy storage capability and very low CO_2 emissions, both enabled by the integration of methane reforming with photovoltaic generation at high temperatures.展开更多
The development of high-entropy borides with combined structural and functional performance holds untold scientific and technological potential,yet relevant studies have been rarely reported.In this work,we report nan...The development of high-entropy borides with combined structural and functional performance holds untold scientific and technological potential,yet relevant studies have been rarely reported.In this work,we report nanocrystalline(La_(0.25)Ce_(0.25)Nd_(0.25)Eu_(0.25))B6 high-entropy rare-earth hexaboride(HEReB6-1)ceramics fabricated through the high-pressure sintering of self-synthesized nanopowders for the first time.The as-fabricated samples exhibited a highly dense(96.3%)nanocrystalline(94 nm)microstructure with major(001)fiber textures and good grain boundaries without any impurities,resulting in a remarkable mechanical,electrical,and thermionic emission performance.The results showed that the samples possessed outstanding comprehensive mechanical properties and a high electrical resistivity from room temperature to high temperatures;these were greater than the average values of corresponding binary rare-earth hexaborides,such as a Vickers hardness of 23.4±0.6 GPa and a fracture toughness of 3.0±0.4 MPa•m^(1/2)at room temperature.More importantly,they showed high emission current densities at elevated temperatures,which were higher than the average values of the corresponding binary rare-earth hexaborides.For instance,the maximum emission current density reached 48.3 A•cm^(−2)at 1873 K.Such superior performance makes the nanocrystalline HEReB6-1 ceramics highly suitable for potential applications in thermionic emission cathodes.展开更多
As the performance of vacuum electron devices is essentially governed by the properties of their cathodes,developing efficient and durable thermionic cathode is necessary and highly desired to meet the boosting requir...As the performance of vacuum electron devices is essentially governed by the properties of their cathodes,developing efficient and durable thermionic cathode is necessary and highly desired to meet the boosting requirements of vacuum electron devices.This review summarized the progress made in the past decades with a detailed discussion on the occurred various thermionic cathodes and their features,and the understandings of the correlation between the emission properties and the composition,where structure and synthesis method are well illustrated.Furthermore,dispenser cathodes with novel structures and emission mechanism are highlighted to indicate the recent achievement in this area of research,and Sc-cathode is considered as a promising candidate for the next-generation vacuum electron devices due to the greatly improved efficiency.However,challenges still exist to meet the ever-growing demands of thermionic cathode with collaborative requirement of high performance,easy fabrication and inadequate reproducibility.展开更多
Purpose The research focuses on the simulation of influence of cathode positions on beam current characteristics in the thermionic electron gun.Methods The electricfield distribution,bias voltage range and electron beam...Purpose The research focuses on the simulation of influence of cathode positions on beam current characteristics in the thermionic electron gun.Methods The electricfield distribution,bias voltage range and electron beam quality under different cathode positions were simulated by CST software.Results Changes in the cathode position can cause changes in the electricfield distributions under the cathode emission surface change,thereby changing the size of the effective emitting area and affecting the relationship between the bias voltage and beam current.Conclusions Only when the cathode is installed in the proper positions,the beam current and the bias voltage have an approximately linear proportional relationship.When the cathode is installed improperly,the electron beam emission is disordered,resulting in decreased electron beam quality.展开更多
Charge transfer mechanisms of contact electrification(CE)are essential for widening applications of the triboelectric nanogenerator,and thus are widely studied by scientists around the world.However,the quantitative m...Charge transfer mechanisms of contact electrification(CE)are essential for widening applications of the triboelectric nanogenerator,and thus are widely studied by scientists around the world.However,the quantitative modeling of CE,especially that between polymers,is still lacking.Herein,a model was proposed to describe the contributions from different mechanisms,including electron transfer and mass transfer in polymer/polymer CE through the fieldassisted thermionic emission,where three groups of charge transfer mechanisms were distinguished by the polarity of the charge transfer and the corresponding electric field.The results indicated that the total generated charge in CE is actually much larger than the measured net surface charge,confirming the bidirectional material-dependent charge transfer mechanisms between two surfaces,which is meaningful for understanding the millennium puzzle in triboelectrification and provides a new perspective for promoting the applications to tailor surface charge generation.展开更多
Peri-implant lesion is a grave condition afflicting numerous individuals with dental implants.It results from persistent periodontal bacteria accumulation causing inflammation around the implant site,which can primari...Peri-implant lesion is a grave condition afflicting numerous individuals with dental implants.It results from persistent periodontal bacteria accumulation causing inflammation around the implant site,which can primarily lead to implant loosening and ultimately the implant loss.Early-stage peri-implant lesions exhibit symptoms akin to gum disease,including swelling,redness and bleeding of the gums surrounding the implant.These signs indicate infection and inflammation of the peri-implant tissues,which may result in bone loss and implant failure.To address this problem,a thermionic strategy was applied by designing a cuprorivaite–hardystonite bioceramic/alginate composite hydrogel with photothermal and Cu/Zn/Si multiple ions releasing property.This innovative approach creates a thermionic effect by the release of bioactive ions(Cu^(2+)and Zn^(2+)and SiO_(3)^(2−))from the composite hydrogel and the mild heat environment though the photothermal effect of the composite hydrogel induced by near-infrared light irradiation.The most distinctive advantage of this thermionic effect is to substantially eliminate periodontal pathogenic bacteria and inhibit inflammation,while simultaneously enhance peri-implant osseointegration.This unique attribute renders the use of this composite hydrogel highly effective in significantly improving the survival rate of implants after intervention in peri-implant lesions,which is a clinical challenge in periodontics.This study reveals application potential of a new biomaterial-based approach for peri-implant lesion,as it not only eliminates the infection and inflammation,but also enhances the osteointegration of the dental implant,which provides theoretical insights and practical guidance to prevent and manage early-stage peri-implant lesion using bioactive functional materials.展开更多
Graphene/semiconductor heterojunction anodes can significantly enhance the output voltage by the photovoltaic effect.However,a significant challenge arises from the high intrinsic work function of heterojunction surfa...Graphene/semiconductor heterojunction anodes can significantly enhance the output voltage by the photovoltaic effect.However,a significant challenge arises from the high intrinsic work function of heterojunction surfaces,which limits efficient electron emission.In this study,we explored the potential of low work function materials modified by Cs/Cs-O adsorption as anodes for thermionic(TI)converters through first principles calculations.The results demonstrate that the work functions of the graphene/MoS_(2) and the graphene/n-type Si surfaces with only Cs coating can decrease to 1.48 eV and 2.46 eV,respectively.The multiple Cs-O atoms co-adsorption enhances the dipole moment,resulting in a further reduction of the work function of the graphene/MoS_(2) surface to 1.25 eV.In addition,the impact of work function on the performance of TI converters is revealed by using concentrated solar energy as heat source.The highest conversion efficiency achieves 15.25%for the Cs-4O:Gr/MoS_(2) anode.This study establishes a robust foundation for further advancement of the TI converters with graphene/semiconductor heterojunction anodes.展开更多
Titanium dioxide （TiO2） is being extensively applied in such fields as photocatalysis, sensing, photoelectrolysis, and pho- tovoltaics [I]. The work function （WF） of a material is one of the vital factors that det...Titanium dioxide （TiO2） is being extensively applied in such fields as photocatalysis, sensing, photoelectrolysis, and pho- tovoltaics [I]. The work function （WF） of a material is one of the vital factors that determine the performance of this ma- terial in electron emission, chemical reaction, surface atomic reconstruction, trapping, and recombination of photogener- ated electrons and holes, interface interaction, and corrosion resistance [2,3]. The WF of metallic Ti is an old topic and abundant data are available [4-7]. In contrast, reports on the WF of TiO2 are relatively rare [2,3,8-10].展开更多
In this work, a comprehensive model that takes the phenomenon of carrier thermionic-emission within the frame work of a diffusion theory of current conducting into account in the type Ⅱ hetero-junction Cd Te solar ce...In this work, a comprehensive model that takes the phenomenon of carrier thermionic-emission within the frame work of a diffusion theory of current conducting into account in the type Ⅱ hetero-junction Cd Te solar cell is developed. According to this model, it is found that the total current flowing through the Cd Te solar cell is limited by the thermionic process for very thin quasi-neutral regions and limited by the diffusion process for the reverse case. In future research of the Cd Te solar cell, such an approach may enable the determination of the boundary conditions for all doping profiles and computation of the conversion efficiency, etc.展开更多
基金supported by the National Key R&D Program under Grant No.2021YFA1400500the National Natural Science Foundation of China under Grant No.22273029+1 种基金the New Cornerstone Science Foundation through the New Cornerstone Investigator Program under Grant No.NCI202303 and the XPLORER PRIZEthe Beijing Outstanding Young Scientist Program under Grant No.JWZQ20240101002。
文摘Ultrafast electron sources, which enable high spatiotemporal resolution in time-resolved electron microscopy and scanning probe microscopy, are receiving increased attention. The most widely used method for achieving ultrafast electron sources involves irradiating metal tips by ultrashort laser pulses, causing electron beam emission via the photoelectric effect [including photon-driven(quantum) or field-driven(classical) emission]. However, the thermionic electrons emission process due to the heating effect of ultrashort lasers, particularly its dynamic aspects, has rarely been addressed in previous studies. In this paper, we improved the signal-to-noise ratio of a two-pulse correlation measurement on the tip electron emission by nearly two orders of magnitude using a delay time modulation method. This allowed us to obtain information on the temperature evolution of hot electrons and phonons in a non-equilibrium state, and to extract characteristic time scales for electron-phonon and phonon-phonon scattering. Our findings indicate that the thermionic electrons emission, unlike the instantaneous photoelectric effect, causes electron emission to lag behind the laser pulse by tens of picoseconds, thus significantly affecting the detection of ultrafast dynamics of samples. Furthermore, such a lagging effect was found to be sensitive to the local structure of the metal tip, offering new insights into the improved design of ultrafast electron sources.
文摘A microcomputerized measurement system is developed.This system automatically acquiresan I-V characteristic curve in 5 seconds with the accuracy of 0.05％ FSR(full scale range).The Schottkyextrapolation field-free emission,the inflection-point emission,the flection-point emission,the average effec-tive work function.the Richardson work function,the work function distribution at the operation tempera-ture and other important information that reflects properties of cathodes are obtained by this system.The re-sults of analysing four types of cathodes demonstrate that this system is very suitable for measuring the activi-ty changes of the cathode in the processes of activation,ageing,poisoning and life test.It is proved that thissystem can also be used to monitor the actlvity of the cathode in the assembled tube by measuring a gridcontrolled travelling-wave tube.
基金funded by CCDI-UEFISCDI project number PN-III-P2-2.1-PED-2021-0828,within PNCDI III and by the program Johannes Amos Comenius(cofunded by the European Union)QM4ST,project number CZ.02.01.01/00/22_008/0004572.
文摘Magnesium and magnesium thin alloy films were deposited using a thermionic vacuum arc(TVA),which has multiple applications in the field of metallic electrodes for diodes and batteries or active corrosion protection.An improved laser-induced TVA(LTVA)method favors the crystallization processes of the deposited magnesium-based films because the interaction between laser and plasma discharge changes the thermal energy during photonic processes due to the local temperature variation.Plasma diagnosis based on current discharge measurements suggests an inelastic collision between the laser beam and the atoms from the plasma discharge.The morphology and surface properties of the obtained thin films differ between these two methods.While the amorphous character is dominant for TVA thin films,enabling a smooth surface,the LTVA method produces rough surfaces with prominent crystallinity,less hydrophobic character and lower surface energy.The smooth surfaces obtained by the TVA methods produce metallic electrodes with good electrical contact,ensuring better diodes and battery charge transport.Both methods allow uniform magnesium alloys to be obtained,but the laser used in the LTVA on the discharge plasma controls the added metal or element ratio.
文摘By using correlation-detection technique and improving structure of the test tube,the background noise of thermionic-electrons and space charge effect are restrained.The sec-ondary emission coefficient δ of thermionic cathode at high temperature has been studied.Theδ of impregnated scandate cathodes increases exponentially with increasing temperature at lowenergy and current of the bombardment electrons;at high energy or current of the bombardmentelectrons the temperature has little effect on δ.The research shows that an enhanced thermionicemission occurred when the cathode works at high temperature and under electron bombardment.These phenomena are discussed in terms of “internal field model”.
基金supported by the National Natural Science Foundation of China(Grant Nos.12004389,12004288,and 12104462)the China Postdoctoral Science Foundation(Grant Nos.2020M68036 and 2021T140430)+1 种基金the support from the Joint Research Fund of Liaoning-Shenyang National Laboratory for Materials Science(Grant No.2019JH3/30100031)the support from the IMR Innovation Fund(Grant No.2021-PY17)。
文摘Thermionic emission is a tunneling phenomenon,which depicts that electrons on the surface of a conductor can be pulled out into the vacuum when they are subjected to high electrical tensions while being heated hot enough to overtake their work functions.This principle has led to the great success of the so-called vacuum tubes in the early 20 th century.To date,major challenges still remain in the miniaturization of a vacuum channel transistor for on-chip integration in modern solid-state integrated circuits.Here,by introducing nano-sized vacuum gaps(~200 nm)in a van der Waals heterostructure,we successfully fabricated a one-dimensional(1 D)edge-to-edge thermionic emission vacuum tube using graphene as the filament.With the increasing collector voltage,the emitted current exhibits a typical rectifying behavior,with the maximum emission current reaching 200 p A and an ON-OFF ratio of 10;.In addition,it is found that the maximum emission current is proportional to the number of the layers of graphene.Our results expand the research of nano-sized vacuum tubes to an unexplored physical limit of 1 D edge-to-edge emission,and hold great promise for future nano-electronic systems based on it.
文摘A combined system model is proposed including a molten carbonate fuel cell(MCFC),a graphene thermionic converter(GTIC)and thermally regenerative electrochemical cycles(TRECs).The expressions for power output,energy efficiency of the subsystems and the couple system are formulated by considering several irreversible losses.Energy conservation equations between the subsystems are achieved leaned on the first law of thermodynamics.The optimum operating ranges for the combined system are determined compared with the MCFC system.Results reveal that the peak power output density(POD)and the corresponding energy efficiency are 28.22%and 10.76%higher than that of the single MCFC system,respectively.The effects of five designing parameters on the power density and energy efficiency of the MCFC/GTIC/TRECs model are also investigated and discussed.
文摘A transport equation of momentum for relativistic electrons scattered isotropically was previously reported. Here, a momentum-transport equation for relativistic electrons “scattered anisotropically” by the Coulomb force is inquired into. An ideal plasma consisting of electrons and deuterons is treated again. Also, to raise a generation-ability of a thermionic energy converter, a means of introducing external electric and magnetic fields within “a converter in which an emitter plate and a collector plate face simply each other” is proposed.
文摘A 2MeV thermionic cathode test stand was established to meet the requirement of the large area thermionic cathode system.A 100ram in diameter type'B'thermionic dispenser cathode was developed.A 1000A emission current was produced at the voltage of the diode about 1.8MV,the pulse width about 90ns(FWHM),and the cathode temperature about 1350℃.The emission current density is 12A/cm^2.The results indicate that a large area thermionic cathode which produces high quality and high current electron beams is visible.The results also indicate that the ability of cathode emission relies on the diode-vacuum and cathode-temperature.
基金supported by the National Natural Science Foundation of China(Grant Nos.51779262,51576207,51306206)the Hubei Province Natural Science Foundation of China(Grant No.2017CFB498)。
文摘In this paper,an irreversible thermionic refrigerator model based on van der Waals heterostructure with various irreversibilities is established by utilizing combination of non-equilibrium thermodynamics and finite time thermodynamics.The basic performance characteristics of the refrigerator are obtained.The effects of key factors,such as bias voltages,Schottky barrier heights and heat leakages,on the performance are studied.Results show that cooling rates and coefficients of performances(COPs)can attain the double maximum with proper modulation of barrier heights and bias voltages.Increasing cross-plane thermal resistance as well as decreasing electrode-reservoir thermal resistance and reservoir-reservoir thermal resistance can enhance the performance of the device.The optimal performance region is the interval between the maximum cooling rate point and the maximum COP point.By modulating the bias voltage,the working state of the device can fall into the optimal performance region.The optimal performance of the refrigerator when using single layer graphene and a few layers graphene as electrode material is also compared.
基金The authors thank Prof.Shuyun Zhou for valuable discussions.The work is financially supported by the National Basic Research Program of China(No.2012CB932301)the National Natural Science Foundation of China(Nos.11274190,51102144,51102147,and 90921012).
文摘The work function(WF)of graphene is an essential parameter in graphene electronics.We have derived the WF of graphene by the thermionic emission method.Chemical vapor deposition(CVD)-grown single-layered polycrystalline graphene on copper foil is transferred to a cross-stacked carbon nanotube(CNT)film drawn from a super-aligned multiwalled CNT array.By decreasing the pore size of the CNT film,the as-prepared CNT-graphene film(CGF)can be Joule heated to a temperature as high as 1,800 K in vacuum without obvious destruction in the graphene structure.By studying the thermionic emission,we derive the WF of graphene,ranging from 4.7 to 4.8 eV with the average value being 4.74 eV.Because the substrate influence can be minimized by virtue of the porous nature of the CNT film and the influence of adsorbents can be excluded due to the high temperature during the thermionic emission,the measured WF of graphene can be regarded as intrinsic.
基金supported by the National Key Research and Development Program of China (2016YFB0901401)the National Natural Science Foundation of China (51676189)the Chinese Academy of Sciences Frontier Science Key Research Project (QYZDY-SSW-JSC036)
文摘A new photovoltaic-thermochemical(PVTC) conceptual system integrating photon-enhanced thermionic emission(PETE) and methane steam reforming is proposed. Major novelty of the system lies in its potential adaptivity to primary fuels(e.g. methane) and high efficiencies of photovoltaic and thermochemical power generation, both of which result from its operation at much elevated temperatures(700–1000 °C)compared with conventional photovoltaic-thermal(PVT) systems. Analysis shows that an overall power generation efficiency of 45.3% and a net solar-to-electric efficiency of 39.1% could be reached at an operating temperature of 750 °C, after considering major losses during solar energy capture and conversion processes. The system is also featured by high solar share(37%) in the total power output, as well as high energy storage capability and very low CO_2 emissions, both enabled by the integration of methane reforming with photovoltaic generation at high temperatures.
基金the National Key Research and Development Program of China(2021YFA0715801)the National Natural Science Foundation of China(51972116 and 52122204)the China Postdoctoral Science Foundation(2021M691051).
文摘The development of high-entropy borides with combined structural and functional performance holds untold scientific and technological potential,yet relevant studies have been rarely reported.In this work,we report nanocrystalline(La_(0.25)Ce_(0.25)Nd_(0.25)Eu_(0.25))B6 high-entropy rare-earth hexaboride(HEReB6-1)ceramics fabricated through the high-pressure sintering of self-synthesized nanopowders for the first time.The as-fabricated samples exhibited a highly dense(96.3%)nanocrystalline(94 nm)microstructure with major(001)fiber textures and good grain boundaries without any impurities,resulting in a remarkable mechanical,electrical,and thermionic emission performance.The results showed that the samples possessed outstanding comprehensive mechanical properties and a high electrical resistivity from room temperature to high temperatures;these were greater than the average values of corresponding binary rare-earth hexaborides,such as a Vickers hardness of 23.4±0.6 GPa and a fracture toughness of 3.0±0.4 MPa•m^(1/2)at room temperature.More importantly,they showed high emission current densities at elevated temperatures,which were higher than the average values of the corresponding binary rare-earth hexaborides.For instance,the maximum emission current density reached 48.3 A•cm^(−2)at 1873 K.Such superior performance makes the nanocrystalline HEReB6-1 ceramics highly suitable for potential applications in thermionic emission cathodes.
基金This work was financially supported by the National Natural Science Foundation of China(Grant No.51534009)the Beijing Municipal High Level Innovative Team Building Program(Grant No.IDHT20170502).
文摘As the performance of vacuum electron devices is essentially governed by the properties of their cathodes,developing efficient and durable thermionic cathode is necessary and highly desired to meet the boosting requirements of vacuum electron devices.This review summarized the progress made in the past decades with a detailed discussion on the occurred various thermionic cathodes and their features,and the understandings of the correlation between the emission properties and the composition,where structure and synthesis method are well illustrated.Furthermore,dispenser cathodes with novel structures and emission mechanism are highlighted to indicate the recent achievement in this area of research,and Sc-cathode is considered as a promising candidate for the next-generation vacuum electron devices due to the greatly improved efficiency.However,challenges still exist to meet the ever-growing demands of thermionic cathode with collaborative requirement of high performance,easy fabrication and inadequate reproducibility.
基金funded by National Natural Science Foundation of China(Nos.51805265,51775527)Natural Science Foundation of Jiangsu Province(No.BK20180472).
文摘Purpose The research focuses on the simulation of influence of cathode positions on beam current characteristics in the thermionic electron gun.Methods The electricfield distribution,bias voltage range and electron beam quality under different cathode positions were simulated by CST software.Results Changes in the cathode position can cause changes in the electricfield distributions under the cathode emission surface change,thereby changing the size of the effective emitting area and affecting the relationship between the bias voltage and beam current.Conclusions Only when the cathode is installed in the proper positions,the beam current and the bias voltage have an approximately linear proportional relationship.When the cathode is installed improperly,the electron beam emission is disordered,resulting in decreased electron beam quality.
文摘Charge transfer mechanisms of contact electrification(CE)are essential for widening applications of the triboelectric nanogenerator,and thus are widely studied by scientists around the world.However,the quantitative modeling of CE,especially that between polymers,is still lacking.Herein,a model was proposed to describe the contributions from different mechanisms,including electron transfer and mass transfer in polymer/polymer CE through the fieldassisted thermionic emission,where three groups of charge transfer mechanisms were distinguished by the polarity of the charge transfer and the corresponding electric field.The results indicated that the total generated charge in CE is actually much larger than the measured net surface charge,confirming the bidirectional material-dependent charge transfer mechanisms between two surfaces,which is meaningful for understanding the millennium puzzle in triboelectrification and provides a new perspective for promoting the applications to tailor surface charge generation.
基金supported by National Natural Science Foundation of China(81991500,81991503)Cross-disciplinary Research Fund of Shanghai Ninth People's Hospital,Shanghai JiaoTong University School of Medicine(JYJC201904)+3 种基金Science and Technology Commission of Shanghai Municipality(18ZR1422400)Innovative Research Team of High-level Local Universities in Shanghai(SSMU-ZDCX20180900)Open Project of the Key Laboratory of Rehabilitation Medicine in Sichuan Province of China(KFYXSZDSYS-01)the Natural Science Foundation of Sichuan Province of China(2022NSFSC1469).
文摘Peri-implant lesion is a grave condition afflicting numerous individuals with dental implants.It results from persistent periodontal bacteria accumulation causing inflammation around the implant site,which can primarily lead to implant loosening and ultimately the implant loss.Early-stage peri-implant lesions exhibit symptoms akin to gum disease,including swelling,redness and bleeding of the gums surrounding the implant.These signs indicate infection and inflammation of the peri-implant tissues,which may result in bone loss and implant failure.To address this problem,a thermionic strategy was applied by designing a cuprorivaite–hardystonite bioceramic/alginate composite hydrogel with photothermal and Cu/Zn/Si multiple ions releasing property.This innovative approach creates a thermionic effect by the release of bioactive ions(Cu^(2+)and Zn^(2+)and SiO_(3)^(2−))from the composite hydrogel and the mild heat environment though the photothermal effect of the composite hydrogel induced by near-infrared light irradiation.The most distinctive advantage of this thermionic effect is to substantially eliminate periodontal pathogenic bacteria and inhibit inflammation,while simultaneously enhance peri-implant osseointegration.This unique attribute renders the use of this composite hydrogel highly effective in significantly improving the survival rate of implants after intervention in peri-implant lesions,which is a clinical challenge in periodontics.This study reveals application potential of a new biomaterial-based approach for peri-implant lesion,as it not only eliminates the infection and inflammation,but also enhances the osteointegration of the dental implant,which provides theoretical insights and practical guidance to prevent and manage early-stage peri-implant lesion using bioactive functional materials.
基金supported by the National Natural Science Foundation of China(No.52325605)the Zhejiang Provincial Natural Science Foundation(No.LR20E060001)the Fundamental Research Funds for the Central Universities(No.2022ZFJH004).
文摘Graphene/semiconductor heterojunction anodes can significantly enhance the output voltage by the photovoltaic effect.However,a significant challenge arises from the high intrinsic work function of heterojunction surfaces,which limits efficient electron emission.In this study,we explored the potential of low work function materials modified by Cs/Cs-O adsorption as anodes for thermionic(TI)converters through first principles calculations.The results demonstrate that the work functions of the graphene/MoS_(2) and the graphene/n-type Si surfaces with only Cs coating can decrease to 1.48 eV and 2.46 eV,respectively.The multiple Cs-O atoms co-adsorption enhances the dipole moment,resulting in a further reduction of the work function of the graphene/MoS_(2) surface to 1.25 eV.In addition,the impact of work function on the performance of TI converters is revealed by using concentrated solar energy as heat source.The highest conversion efficiency achieves 15.25%for the Cs-4O:Gr/MoS_(2) anode.This study establishes a robust foundation for further advancement of the TI converters with graphene/semiconductor heterojunction anodes.
基金supported by the National Natural Science Foundation of China(Grant No.61671022)the Natural Science Foundation of Jiangsu Province,China(Grant No.BK20161423)
文摘Titanium dioxide （TiO2） is being extensively applied in such fields as photocatalysis, sensing, photoelectrolysis, and pho- tovoltaics [I]. The work function （WF） of a material is one of the vital factors that determine the performance of this ma- terial in electron emission, chemical reaction, surface atomic reconstruction, trapping, and recombination of photogener- ated electrons and holes, interface interaction, and corrosion resistance [2,3]. The WF of metallic Ti is an old topic and abundant data are available [4-7]. In contrast, reports on the WF of TiO2 are relatively rare [2,3,8-10].
文摘In this work, a comprehensive model that takes the phenomenon of carrier thermionic-emission within the frame work of a diffusion theory of current conducting into account in the type Ⅱ hetero-junction Cd Te solar cell is developed. According to this model, it is found that the total current flowing through the Cd Te solar cell is limited by the thermionic process for very thin quasi-neutral regions and limited by the diffusion process for the reverse case. In future research of the Cd Te solar cell, such an approach may enable the determination of the boundary conditions for all doping profiles and computation of the conversion efficiency, etc.
