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Herein, we proposed a cascade refrigerant system with R1270 and carbon dioxide refrigerants. The natural refrigerant R1270 was adopted in the high-temperature circuit to replace NH3 and R290. A prototype cascade refrigerant system with two semi-hermetic piston compressors was manufactured, and its performance was investigated experimentally. The results establish R1270 as a good alternative in the high-temperature circuit of the cascade refrigerant system. Moreover, with a constant condensation and evaporation temperature of cascade system, as the R1270 evaporation temperature was decreased (from −7 °C to −19 °C), the coefficient of performance (COP) of the cascade system increased. The isentropic efficiency of R1270 compressor decreased from 78.5% to 74.7% when the R1270 evaporation temperature increased from −19 °C to −7 °C.

Compressor in air supply circuit influences flow rate of oxygen in cathode of polymer electrolyte membrane (PEM) fuel cell systems. In this paper, an air-cooled dry oil-free twin-screw compressor for fuel cell systems was developed and adopted successfully in a type of truck with fuel cell system. Extensive experimental investigations including the recording of p-V indicator diagram were carried out to study the performance of the prototype compressor. Rotation speed of the female rotor varied from 5000 rpm to 10,000 rpm, and discharge pressure from 0.14 MPa to 0.22 MPa. It was found that the volume flow rate decreases almost linearly with the reduction of rotation speed and this is beneficial to the fuel cell systems under part-load. Between speed of 7000 rpm and 10,000 rpm at a given discharge pressure, the variations of isentropic efficiency and specific power are very limited. This indicates the twin-screw compressor can operate efficiently in a wide range of power output from the fuel cell systems. Under the design conditions with discharge pressure of 0.2 MPa and female rotor rotation speed of 9000 rpm for a fuel cell output of 50 kW, the volumetric efficiency and isentropic efficiency of the prototype compressor developed are 70% and 55% respectively. And the mechanical efficiency and the polytropic process index regarding to discharge temperature are 80% and 1.58. These values can be used in the design of other air-cooled dry oil-free twin-screw compressors for similar applications.

In this study, a vapor injection technique was applied in a high temperature heat pump (HTHP) for providing hot water at temperatures up to 88 °C. A prototype HTHP system with economizer vapor injection was developed and its performance was experimentally investigated under various operating conditions. Results showed that the vapor injection pressure had a large effect on heating capacity and on refrigerant temperature at the inlet of the electronic expansion valve (EEV). As the injection pressure increased from 0.82 to 0.98 MPa, the vapor injection flow ratio increased from 7.3% to 22.61% and the heating capacity increased by 7%. The system COP did not show significant change although the COP trend showed an optimal value for the injection pressure. The refrigerant temperature at the EEV inlet showed a subcooling of more than 16 °C under all studied conditions, which improved the EEV operating reliability.

In this paper, a high-temperature heat pump (HTHP) is developed and manufactured to replace the traditional oil-fired boiler heater for crude oil heating. It extracts thermal energy from waste hot water separated from the crude oil to provide high-temperature hot water to heat the crude oil. A prototype of the HTHP system is installed in the Jinzhou oil treatment station in Liaoning, China and the field test is conducted for about 6000 h. A typical 144 h of field testing data is analyzed to evaluate the performance of the designed HTHP system. It is observed that the temperature of hot water provided by the HTHP unit varies from 86 ℃ to 95 ℃ throughout the whole operating period and is sufficient for the crude oil heating (80–90 ℃). The heating capacity and power consumption of the HTHP system varies from 1350 to 1785 kW, and 171 to 197 kW, respectively. The overall system coefficient of performance ranges from 3.5 to 4.4 with an average value of 3.8. Based on the experimental results, a primary energy ratio is introduced to evaluate and compare the economics of the studied HTHP system and oil-fired boiler heater. The comparison shows that the energy consumed by the HTHP unit is only 57% of that consumed by the oil-fired boiler heater. If all traditional oil-fired boiler heaters are replaced by the HTHPs in the Jinzhou oil treatment station, the total yearly energy saving is around 1.12 × 104 tons of equivalent coal which equates to 1.76 × 104 tons of CO2 emissions.

Fuel cell system has gained extensive attention due to increasing tendency of environmental protection, and Roots blower has great potential as the recirculation pump in its hydrogen recirculation system while there are rare of research about this ancillary component. In this paper, a three-dimensional, transient model of a three-lobe Root pump was established and solved and the internal flow characteristic was investigated. The results show that pressure ratio can influence the mass flow rate and torque significantly and high pressure ratio will lead to mass flow reflux. The most dramatic variation of pressure occurs in the moment that the displacement chamber connects with the exhaust region. The vortex will arise from both the inlet and outlet pocket. Different methods are needed to reduce the vortexes since their position are quite different in the inlet and outlet pocket.

This study investigated the effect of geometrical parameters of gas coolers on system performance and the optimal discharge pressure with a developed and experimentally validated numerical model of an air-source transcritical CO 2 heat pump water heater system. This model fully considered actual heat transfer process in the gas cooler and characteristics of major components adopted in the prototype system. Simulation results showed a good agreement with experimental results in terms of heat yields, COP and the optimal discharge pressure. Parametric analysis reveals that a larger heat transfer area of the gas cooler increases heat yields but decreases the optimal discharge pressure while other components in the system remain unchanged. In addition, at the same operating condition with the same heat transfer area, heat yields increase with CO 2 mass fluxes by reducing tube diameters and flow passages of gas coolers, and beyond certain range of mass fluxes, the optimal discharge pressure increases sharply. As one result, the optimal range of CO 2 mass flux in heat exchange tubes should be the range of 155–325 kg m −2 s −1 at given condition in this paper.

Sugarcane is one of the most important economic crops in Guangxi. For a long time, the sugarcane cultivated areas were estimated via sampling data statistics, while effective and accurate dynamic monitoring data keep absent. High spatial resolution is one of the advantages of high-resolution remote sensing images, through which the texture of sugarcane fields is found clear and unique; however, effective and accurate methods are lacking extracting them automatically in the past. In this paper, a novel deep learning method for sugarcane field extraction from high-resolution remote sensing images is proposed based on DeepLab V3+. It consists of blocks for multi-temporal remote sensing images fusion, which increases the ability of DCNN temporal factors processing. The experiment shows 94.32% extraction accuracy of sugarcane field. Also, its processing speed is superior to the traditional object-oriented extraction method, which solves the problems of low extraction accuracy and slow processing speed using traditional methods.

ZnO, as a wide-band gap semiconductor, has recently become a new research focus in the field of ultraviolet optoelectronic semiconductors. Laser molecular beam epitaxy (L-MBE) is quite useful for the unit cell layer-by-layer epitaxial growth of zinc oxide thin films from the sintered ceramic target. The ZnO ceramic target with high purity was ablated by KrF laser pulses in an ultra high vacuum to deposit ZnO thin film during the process of L-MBE. It is found that the deposition rate of ZnO thin film by L-MBE is much lower than that by conventional pulsed laser deposition (PLD). Based on the experimental phenomena in the ZnO thin film growth process and the thermal-controlling mechanism of the nanosecond (ns) pulsed laser ablation of ZnO ceramic target, the suggested effective ablating time during the pulse duration can explain the very low deposition rate of the ZnO film by L-MBE. The unique dynamic mechanism for growing ZnO thin film is analyzed. Both the high energy of the deposition species and the low growth rate of the film are really beneficial for the L-MBE growth of the ZnO thin film with high crystallinity at low temperature.

ZnO is a direct wide-band gap semiconductor with Eg=3.3 eV, which has high saturation velocity and is radiation tolerant as well. It can be expected being utilized for the room temperature nuclear radiation detector. In this paper, we fabricate the p+-Si/n-ZnO abrupt heterojunction and study the X-ray detection properties. The I-V characteristics were measured in the temperature range of −40 °C-40 °C. The built-in potential and the interface state density are separately calculated to be 0.25 eV and 2.1×1011 cm−2 from the temperature dependent dark current. The detector exhibits photovoltaic behavior and shows linear photo-current response to the incident X-ray intensity. The open-circuit voltage is 0.21 V and the short-circuit current is 0.15 μΑ under the X-ray dose rate of 1.53 Gy/s. The sensitivities of the detector are 95 nC/Gy and 115 nC/Gy at the bias voltages of 0V and −5V respectively. The transient response presents the fast and stable response of the detector to the X-ray.

This paper studies relationship between phase shifts of carriers and passive intermodulation (PIM) products. Theoretical results have been obtained first through analytical derivation and Fourier transformation. And then a simple measurement method was proposed to conduct the relative phase shift test using easily available RF instruments. In this method, phase controllable RF signal sources were not required for test and only power levels are need to be measured. Microwave isolator with obvious nonlinear property was used as device under test. It's demonstrated that the relative phase shift of carriers lead to phase shift of PIM products. 3rd order PIM product behaviors as the theory predicts while 5th order PIM product shows different tendency with prediction to some degree.

Schottky diode has been widely used in RF fields. Because of its nonlinear conductivity characteristic, it often works as a strong nonlinear device and achieves the goal of spectrum transformation, such as mixer, multiplier. Meanwhile, the intermodulation of Schottky diode can be very considerable due to its strong nonlinear conductivity characteristic. In this paper, a novel proposed passive intermodulation test unit based on micro-strip is designed to verify the intermodulation of a strong nonlinear device (Avago HSMS2820). This circuit unit integrates combiner, intermodulation detector, and works as a mini passive intermodulation (PIM) test sub-system. The reverse and forward intermodulation of the diode in this unit has been tested simultaneously and the measured 3rd PIM level has a good agreement with the harmonic balance (HB) simulation, which proves that the unit is feasible for the verification of intermodulation distortion. In the future, this unit can be used to test other devices with different nonlinear conductivity and give a revelation for the research of the mechanism of PIM generation in frequency domain.

Secondary electron emission (SEE), which is a frequent phenomenon in space high power microwave systems, is one of the basic inducement of multipactor in space microwave components. It is already verified that lowering SEE is an effective method to mitigate the undesirable effect. Metal black nanostructures have ever been reported to suppress SEE remarkably, however, the SEE characteristics of the gold nanostructures are rarely investigated. In this work, we use the thermal evaporation to fabricate the gold nanostructures under various evaporation gas pressures, and further analyze their SEE characteristics as well as energy distribution information. Experimental results reveal that the evaporation gas pressure determines the morphology of gold nanostructure, and the morphology dominates the SEE level of the gold nanostructure. To be specific, as the evaporation gas pressure rises, the porosity of the nanostructure increases and the SEE yield decreases. The energy distribution information indicates that the gold nanostructure just suppresses the true secondary electrons (TSEs) effectively. However, the effect of the nanostructure on the back scattered electrons (BSEs) is heavily dependent on the surface morphology. Specifically, the nanostructure fabricated at 70 Pa suppresses the BSEs weakly while the nanostructures fabricated at 40-60 Pa enhance the BSEs to some degree. To theoretically explain the experimental phenomena, we establish an equivalent model, which is made up of the periodical combination of a hemisphere and a composite groove, to imitate the fabricated gold nanostructure and simulate its SEE characteristics based on the SEE phenomenological probability model. Simulation results indicate that the hemisphere induces more TSEs and BSEs while the composite groove suppresses them, besides, the groove suppresses the TSEs much more remarkably than the BSEs. The SEE level of the nanostructure model is determined by the weighted average effect of both the hemisphere and the groove. The simulations qualitatively explain the experimental phenomena. This work in depth reveals the SEE mechanism for the gold nanostructures, and is of considerable significance for developing the low SEE surface on a nanometer scale in a space high power microwave-system.

The multipactor effect is a resonant vacuum electron discharge that can occur in microwave and millimeter-wave subsystems,such as filters,multiplexers,and radio-frequency satellite payloads.In a high-power microwave device,multipator discharge can cause the device to break down,and thus degrading its performance.Fortunately,the multipactor effect can be mitigated by reducing the secondary electron yield (SEY) of the material which a microwave device is made from.Therefore,how to reduce the SEY of material is an important matter.In view of this problem,a new method to reduce the SEY is presented in this paper.This method is based on the fact that when aluminum sheet is treated with anodizing,many porous structures with high height-to-width ratios can be formed on the surface of sheet.These porous structures are conducive to reducing SEY.However,the alumina film covers these porous structures.Because alumina has poor performance in conductivity,the loss of high-power microwave device will increase if the microwave device is anodized.In consequence,the performances of the microwave device will deteriorate.In order to avoid this problem, silver film is chosen,and is electroplated on the anodized aluminum sheet.Although silver film is electroplated on the aluminum sheet,there are still many porous structures on the surface.In order to validate the method in this paper, some aluminum samples are anodized.And then,the SEYs of these samples are obtained by the SEY measurement system.The results show that this method is efficient for reducing the SEY.Compared with the non-anodized sample, the uncleaned sample on whose surface there exists the adsorption or contamination shows that the value of the first energy crossing point of the measured curve of emission coefficient of secondary electrons,E1,increases from 45 eV to 77 eV,and the maximum value of SEY (SEYmax) decreases from 2.68 to 1.52;when the samples are all cleaned (in order to obtain ideal surface by wiping off adsorption or contamination),the value of E1 increases from 40 eV to 211 eV, and the value of SEYmax decreases from 2.55 to 1.36.Furthermore,the multipactor threshold of an X-band impedance transformer is simulated with using these SEY data to validate this method.And it is concluded that compared with the threshold of the original design,the multipactor threshold of the impedance transformer which is treated with the method increases from 7000 W to 125000 W.Therefore,it can be seen that the method presented in this paper is helpful in solving the problem of the multipactor in high-power microwave device for space.Meanwhile,as a usual method,the method can also be used to push forward the researches of vacuum electron devices and accelerators.

The dependence of the resonance frequency of thermally excited microcantilever resonators on temperature has been reported by several papers. All these papers ignored the film grown or deposited on the surface of a cantilever and regarded the beam as a single layer one. As a result, there was difference between the theoretical model and experimental results. The paper proposes a bilayer beam model to calculate the dependence of resonance frequency on temperature. This novel model regard the film as another mechanical layer and takes the induced mass by the surrounding gas into account. The calculated results are in good agreement with experimental results.

The controllable growth processes of ZnO nanowires by evaporation of metal zinc with high purity and its luminescence properties have been investigated in detail. Firstly, the power of ZnO nanowires with high yield and homogeneous dimension was synthesized using the special quartz boat at 600°C. Then, the oriented ZnO nanowires with about 20nm diameter were synthesized by using a 90nm-thick layer of ZnO nanocrystals on the Si substrate as the seed layer. Both fabrication processes are repeatable and no catalysts are necessary. Finally, photoluminescence (PL) spectroscopy for ZnO nanowires using an He-Cd laser line of 325nm as the excitation source were measured at room temperature and both samples showed a sharp strong ultraviolet (UV) near-band edge emission. However, different UV peak positions (385nm for ZnO nanowire powder, 377nm for ZnO nanowire array) can be observed. The size confinement effect for excitons and carriers is proposed to explain the blue shift of the near-band edge emission with decreasing size and the native defects are responsible for the green emission.

Gas injection technology is often used in cold regions to solve heat pump’s low heating capacity and high discharge temperature at low ambient temperature. Injecting gas into port opened at specific position of compressor could increase mass flow rate of compressor and total heating capacity of heat pump. Gas injection also changes compression ratio of compressor and decreases discharge temperature. An optimal gas injection pressure is got when the coefficient of performance reached to peak value at a certain working condition. It’s a feasible way to increase performance of heat pump at cold regions. High temperature heat pump could provide higher temperature water for industrial usage regions but there still existed some problems on its usage. Total heating capacity decreased and discharge temperature increased with the raise of condensation temperature. Refrigerant temperature before throttling valve was high and may exceed working temperature range of electrical expansion valve. Gas injection technology with an economizer was adopted to solve these problems. A new high temperature heat pump cycle was designed based on gas injection and outlets water temperature of the prototype manufactured was reached to 90?. Temperature before throttling valve was well controlled by the usage of economizer. Heating capacity, discharge temperature, compressor power consumption of the heat pump system at different amount of injected gas was conducted by theoretical and experimental research in this paper. This study showed the function of gas injection technology which used in high temperature heat pump.

Air-source transcritical CO2 heat pump water heater (ATHW) can supply hot water from 60 ? to 90 ? at high efficiency with environment-friendly refrigerant CO2 for commercial, residential and industrial applications. Several optimal discharge pressure correlations for transcritical CO2 heat pump have been proposed in the past few years, most of which are related to the ambient temperature, the evaporation temperature and the gas cooler outlet temperature. In an earlier study, the authors’ research group had presented a study on the dependency of the optimal discharge pressure on the ambient temperature and the hot water outlet temperature. In this study, a revised model for optimal discharge pressure is developed based on experimental results. In order to validate the optimal discharge pressure model developed, field tests are conducted to evaluate the performance of an air-source transcritical CO2 heat pump water heater in practical application. The system is comprised of a semi-hermetic reciprocating compressor, a counter-flow tube-in-tube gas cooler, a counter-flow internal heat exchanger, a fin-and-tube evaporator, and an electronic expansion valve (EEV) driven by electrically operated step motor. A Siemens SIMATIC S7-200 Programmable Logic Controller (PLC) was used to regulate the compressor discharge pressure by adjusting the EEV opening and the water flow rate by changing the frequency of the variable speed water pump. Field tests were conducted under three different operating scenarios: the nominal test condition, high water supply temperature condition and low ambient air temperature condition. The results show that the coefficient of performance (COP) can achieve 3.76 in the nominal test condition with 15? water inlet temperature and 80? hot water supply temperature. Even when the hot water temperature is higher than 90?, the COP remains at 3.21 with 20? dry-bulb temperature and 15? wet-bulb temperature. Under low ambient air temperature condition, the COP was 2.19 with the hot-water supply temperature of 60?. Comparison between the field test results and the model predictions show that the maximum relative error of discharge pressure control was 5.6% in the low temperature condition, while the maximum relative error of system COP was only 4.7%. With the reasonable agreement observed between the field test results and the model prediction. It is reasonable and effective to model the optimal discharge pressure as the function of the ambient temperature and the water outlet temperature.

In industrial applications, 90-150? hot water needed during production processes is traditionally provided by fuel-fired boilers. The usage of fuel-fired boiler has a low efficiency of energy use and potential danger of explosion. 30-60?waste water with large amount of energy is rejected to environment directly, which causes a great energy loss and thermal pollution. Seeking for a much high efficiency and safe heating pattern is essential to replace the boilers used now. High temperature heat pump is an ideal solution to recover the heat in waste water and to produce hot water needed. It has a higher efficiency than boilers through the use of energy contained in waste water. Water temperature provided by high temperature heat pump reached to 90-120?. In this paper, an industrial high temperature heat pump with twin screw compressor was developed. Optimized heat pump cycle was made and heat exchangers were designed for using in specific workplace. For operating at high condensation and evaporation temperature safely, the twin screw compressor used in high temperature heat pump should be modified, like the changes of inner volume ratio and motor capacity. Discharge temperature was regulated by liquid injection and oil cooling system. Moreover, extensive experimental tests were carried out under several working scenarios and the results showed high temperature heat pump can fully meet the requirement in industrial field to supply 90-120? hot water with excellent system performance.

Metallic thin films deposited on non-conductive substrates are widely used in areas like microwave absorbers, photovoltaic, packaging, electromagnetic shielding, and integrated circuits. From scientific and engineering point of view, measuring sheet resistance of metallic thin films is important. In this study, we develop a theory of evaluating sheet resistance by using transmission coefficient of a rectangular waveguide (RG) and verify it with sputtered silver films of various thickness values. According to the field distribution of RG working under the fundamental mode and corresponding electromagnetic boundary conditions, we first analytically derive the transmission coefficient of an RG with the metallic thin film exactly occupying its cross section. Comparing existing theory, we take the effect of the non-conductive substrate supporting the metallic thin film into consideration. According to this derivation, we establish a method to calculate the sheet resistance of metallic thin films from the amplitude of RG transmission coefficient. To verify our derivation, we also conduct full-wave simulations of a standard WR-75 RG used for characterizing the metallic thin film at 13.65 GHz. Both the analytical derivations and full-wave simulations show that the amplitude of the transmission coefficient depends on the logarithm of the sheet resistance in a linear manner. It is also demonstrated that the substrate effect may not be ignored. To facilitate measurement, we propose a sandwiched structure by placing the metallic thin film between two waveguide flanges. This modification removes the stringent requirements for sample preparation. Simulations of this sandwiched structure indicate that it is possible to realize non-contact measurement if the air gap between metallic thin film and waveguide flange is below 0.1 mm. Through full-wave simulations, we also show the feasibility of metallic thin film evaluation by using such transmission lines as dielectric filled RG, circular waveguide, and coaxial line. Finally, we prepare various silver films with sheet resistances ranging from 20 m/square to 1 /square (measured by the four-point probe technique) on the top of high resistance silicon and glass substrates, respectively. We measure the amplitudes of transmission coefficient of these metal films in RG by using vector network analyzer. The obtained experimental results are well consistent with the derivation and simulation results, thereby verifying the proposed method. It is recommended that the proposed method is suitable for conductive films with sheet resistances ranging from 0.05 /square to 0.5 /square. The results of this study are of potential value for characterizing the conductive thin films in micro/nano fabrication and relevant areas.

ZnO nanowires with high quality and high yield can be obtained based on the simple thermal evaporation of metal zinc in situ at low temperature, which are repeatable. It's suggested that we can use the ZnO nanowires to develop nano-optoelectronics devices and sensors

As a wide bandgap semiconductor material, ZnO has huge potential in applications such as light emitting devices and sensors. Compared with GaN and SiC, ZnO has a bandgap of 3.37 eV and exciton binding energy of 60 meV at room temperature, indicating it is a promising candidate of UV detector. ZnO based metal-semiconductor-metal photoconductive ultraviolet detector has the advantages of high optical gain and strong responsivity. However, due to the photoconductive relaxation and surface effect of the ZnO material, a ZnO-based photoconductive UV detector has a slow response which is defective for practical application. The intrinsic defects typically generated during the synthesis of ZnO, e.g. oxygen vacancy, should be responsible for the slow response. Therefore, we have fabricated the high-resistive ZnO thin film based UV detector and studied its UV response characteristic. High resistance ZnO thin film is fabricated on glass by RF magnetron sputtering and followed by lift-off photolithography to form Al interdigital electrodes. SEM and XRD images show that the as-fabricated ZnO thin film grows with preferential orientation along c-axis. A linear I-V curve under UV illumination indicates the ohmic contact between Al and ZnO. From these results, we can calculate the resistivities to be 3.71×109 Ω · cm and 7.20×106 Ω · cm respectively when in the dark and under 365 nm UV light of 303 μW/cm2. The light-to-dark current ratio is up to 516 with bias of 40 V. Besides, the ZnO thin film detector shows a stable, rapid, repeatible and reproducible response with a rise time of 199 ms and a fall time of 217 ms when exposed to periodically switched UV light illumination at a bias voltage of 40 V. Moreover, the detector has a high selectivity for 365 nm UV light and the responsivity is 0.15 mA/W with the intensity of 303 μW/cm2. Furthermore, the transient response process is analyzed using the theory of surface recombination and bulk recombination of ZnO semiconductor. For a high resistance ZnO thin film based UV detector, the surface recombination process is weakened ascribed to the decrease of intrinsic defects and the bulk recombination process plays a leading role, resulting in the fast response. Results show that high resistivity ZnO thin film based UV detectors have outstanding UV photoresponse characteristics for potential applications in UV/radiation detection.

Ag-ZnO-Ag X-ray detectors based on ZnO film and nanowires are both fabricated in this paper. Results of continuous X-ray radiation measurement show that the two detectors have high responsivity: the responsivity of the ZnO film device is about 0.12 μC/Gy under a 100 V bias voltage, and that of the ZnO nanowires device is about 0.15 μC/Gy under a 50 V bias voltage. Surface effect due to the absorption and desorption of oxygen on the ZnO surface, which makes the carrier lifetime increase, is decisive to the high responsivity. ZnO film and nanowires have their potential applications in the X-ray dose rate measurement.

Suppression of secondary electron yield attracted much attention in areas such as accelerator and high power microwave components in recent years. To evaluate the suppression efficiencies of different surface topographies, the secondary electron yields (SEYs) of four kinds of micro-structured surfaces for trapping secondary electrons, i.e., triangular groove, rectangular groove, cuboid, cylindrical, are obtained by the phenomenological probabilistic model of secondary electron emission. The simulation results show that the SEYs of these structures are much dependent on the shape parameters such as aspect ratio or porosity. There are mainly three findings: 1) the SEY decreases with increasing aspect ratio and porosity; 2) the traps with cuboid or cylindrical shape are more efficient than triangular or rectangular traps for the SEY suppression; 3) the SEY dependence of micro-structured surface on incident angle is not as obvious as that of flat surface. Micro-trapping structure surfaces are fabricated by mechanical method, photolithography process and chemical etching respectively. The measured SEYs of these samples validate the theoretical results. All these results show that the proposed micro-structures as secondary electron traps have potential applications in SEY suppression in fields such as multipactor and electron-cloud effects.

The band structure model of commensurate double-walled carbon nanotube (DWNT) was built by using tight-binding Hamiltonian with curvature effect included. The field emission characteristics of commensurate DWNT were quantitatively investigated basing on its dependence on the band structure. The results indicate that interwall coupling interaction (ICI) can form additional electronic energy level in the band structure of DWNT so as to increase the emitting channel of electrons;and at the same time, it can also increase the energy gap of the metallic DWNT and decrease that of the semiconducting DWNT, which will result in a change in the number of electrons emitted from the valence band. At a given applied electric field, the emission current of DWNT has an obvious increase, compared with that of DWNT when ICI is turn off, and the current increment of semiconducting DWNT is larger than that of the metallic one. At an applied electric field of 5V/μm, the field emission currents of commensurate DWNTs (6,6)&(12,12), (10,0)&(20,0) and (8,2)&(16,4) have an increase of about 3%,10% and 4%, respectively, compared with those when ICI is turn off. The results reveal that ICI contributes to the transport and electron emission of DWNT, which is helpful for understanding the field emission mechanism of DWNT ,and hence that of MWNT.