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
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
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
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 poly...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 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
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
Hollow cathode researches used to focus on the inner cavity or downstream plume,however,rarely on the gap between the throttling orifice plate and the keeper plate(T-K gap),which was found to impact the self-sustainin...Hollow cathode researches used to focus on the inner cavity or downstream plume,however,rarely on the gap between the throttling orifice plate and the keeper plate(T-K gap),which was found to impact the self-sustaining margin of hollow cathode discharge in this paper.Near the lower margin,the main power deposition and electron emission and ionization regions would migrate from inner cavity and downstream plume to the T-K gap,in which case,the source and destination of each m A current therein matter for the self-sustaining capability.Changing the metal surfaces in the T-K gap with emissive materials proved effective in lowering the lower margin by supplementing auxiliary thermionic emission,compensating electron loss on cold absorbing walls and suppressing discharge oscillations.By doing so,the lower margin of a 4 A hollow cathode was lowered from 1 to 0.1-0.2 A,enabling it to couple with low power Hall thruster without extra keeper current.展开更多
The recent advances in Mo La 2O 3 thermionic cathode materials were presented. It is shown that Mo La 2O 3 cathode has better ductility, radioactive pollution free, excellent thermionic electron emission properties an...The recent advances in Mo La 2O 3 thermionic cathode materials were presented. It is shown that Mo La 2O 3 cathode has better ductility, radioactive pollution free, excellent thermionic electron emission properties and lower operating temperature compared with W ThO 2 cathode. At operating temperature 1?350 ～ 1?400?℃, the average saturation current of the Mo La 2O 3 cathode is 118?mA, the corresponding average current density is 367?mA/cm 2, and the average emission efficiency is 11.8?mA/W. The lifetime of diode is more than 2?000?h when the stable emission current is 80?mA. Moreover, the lifetime of practical 6T51 type triode is more than 1?000?h. These advances show that the Mo La 2O 3 cathode electron tube is closer to industry application.展开更多
The emission and surface characteristics of a dispenser cathode coated with Re arestudied.It is found that the dispenser cathode coated with Re has both higher current densityand more uniform distribution of emission ...The emission and surface characteristics of a dispenser cathode coated with Re arestudied.It is found that the dispenser cathode coated with Re has both higher current densityand more uniform distribution of emission than the S-type cathode.The Auger images of Bashow that the Ba distribution on the surface of the cathode coated with Re is more uniform thanthat on the surface of the S-type cathode.The analytical results by XPS and low energy AESshow that the Ba on the surface of the cathode coated with Re has stronger metallic propertythan that on the surface of the S type cathode.展开更多
A scanning electron probe is used to study the secondary emission properties of the im-pregnated barium scandate dispenser cathode, the influence of the activation on the secondary emis-sion property, the secondary em...A scanning electron probe is used to study the secondary emission properties of the im-pregnated barium scandate dispenser cathode, the influence of the activation on the secondary emis-sion property, the secondary emission image and the secondary emission distribution over the surfaceof the cathode. At optimal activation, δ<sub>m</sub>=3.56 (E<sub>pm</sub>=700eV). The activation has a larger effect onδ when the activation temperature is higher than 1100℃. The distribution over the surface of thecathode is non-uniform. The half-peak width of the distribution curve over the surface is 0.4. Theexperimental phenomena are discussed in relation to surface analysis of the cathode. The theoreticallyderived binomial distribution curves over the surface fit much better with the experimental results.展开更多
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.
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The authors thank Prof. Shuyun Zhou for valuable discussions. The work is financially supported by the National Basic Research Program of China （No. 2012CB932301） and 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.
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.
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.
基金National Natural Science Foundation of China(Nos.61571166 and 51736003)for supporting the research。
文摘Hollow cathode researches used to focus on the inner cavity or downstream plume,however,rarely on the gap between the throttling orifice plate and the keeper plate(T-K gap),which was found to impact the self-sustaining margin of hollow cathode discharge in this paper.Near the lower margin,the main power deposition and electron emission and ionization regions would migrate from inner cavity and downstream plume to the T-K gap,in which case,the source and destination of each m A current therein matter for the self-sustaining capability.Changing the metal surfaces in the T-K gap with emissive materials proved effective in lowering the lower margin by supplementing auxiliary thermionic emission,compensating electron loss on cold absorbing walls and suppressing discharge oscillations.By doing so,the lower margin of a 4 A hollow cathode was lowered from 1 to 0.1-0.2 A,enabling it to couple with low power Hall thruster without extra keeper current.
文摘The recent advances in Mo La 2O 3 thermionic cathode materials were presented. It is shown that Mo La 2O 3 cathode has better ductility, radioactive pollution free, excellent thermionic electron emission properties and lower operating temperature compared with W ThO 2 cathode. At operating temperature 1?350 ～ 1?400?℃, the average saturation current of the Mo La 2O 3 cathode is 118?mA, the corresponding average current density is 367?mA/cm 2, and the average emission efficiency is 11.8?mA/W. The lifetime of diode is more than 2?000?h when the stable emission current is 80?mA. Moreover, the lifetime of practical 6T51 type triode is more than 1?000?h. These advances show that the Mo La 2O 3 cathode electron tube is closer to industry application.
文摘The emission and surface characteristics of a dispenser cathode coated with Re arestudied.It is found that the dispenser cathode coated with Re has both higher current densityand more uniform distribution of emission than the S-type cathode.The Auger images of Bashow that the Ba distribution on the surface of the cathode coated with Re is more uniform thanthat on the surface of the S-type cathode.The analytical results by XPS and low energy AESshow that the Ba on the surface of the cathode coated with Re has stronger metallic propertythan that on the surface of the S type cathode.
文摘A scanning electron probe is used to study the secondary emission properties of the im-pregnated barium scandate dispenser cathode, the influence of the activation on the secondary emis-sion property, the secondary emission image and the secondary emission distribution over the surfaceof the cathode. At optimal activation, δ<sub>m</sub>=3.56 (E<sub>pm</sub>=700eV). The activation has a larger effect onδ when the activation temperature is higher than 1100℃. The distribution over the surface of thecathode is non-uniform. The half-peak width of the distribution curve over the surface is 0.4. Theexperimental phenomena are discussed in relation to surface analysis of the cathode. The theoreticallyderived binomial distribution curves over the surface fit much better with the experimental results.
