This article presents a microwave circuit-level approach to enhance the power-handling capability of an overmoded Ka-band relativistic Cherenkov oscillator (RCO) operating under low guiding magnetic fields. Distributed impedance engineering is implemented through a segmented energy extraction slow wave structure (SEE-SWS), enabling spatially distributed beam energy extraction and suppression of localized resonant surface fields. In addition, a novel axial extraction cavity (AEC), functioning as an integrated microwave extraction circuit, provides efficient beam energy extraction, intrinsic TM01 output mode purification, and effective load sharing with the SWS. The design is optimized using a neural-network-based genetic algorithm (NNGA). Particle-in-cell simulations show that, compared with the prototype, the maximum surface emission field is reduced from 3.38 to 0.94 MV/cm, while the output power is increased by approximately 25%. Experiments demonstrate that the device delivers a 620-MW, 20-ns flat-top microwave pulse at 29.4 GHz under a 0.62-T guiding magnetic field, with no pulse shortening, outperforming the prototype (400 MW, 7 ns). Moreover, stable 50-Hz repetitive operation for 4 s is achieved without active water cooling. These results highlight the effectiveness of distributed impedance engineering and axial extraction circuit design for power-handling enhancement in high-power millimeter-wave sources, providing a general microwave circuit-level methodology for overmoded millimeter-wave oscillator design.

Weak surface insulation performance of the vacuum insulators is a bottleneck problem of the large pulsed power devices. Recent studies have found that molecular group containing delocalized electrons is able to decrease the secondary electron emission yield (SEY) and increase the surface insulation performance of the vacuum insulators. Therefore, in this paper, a kind of copolymer coating consisting of dopamine and aniline is designed and in situ synthesized on the surface of alumina insulators to introduce delocalized electrons so as to improve the surface insulation performance. Influence of the different dopamine-aniline molar ratios of the copolymer coating on the SEY and surface insulation performance is studied. The results show that the copolymer coating with molar ratios of 1:1 can endow alumina insulators the lowest SEY and highest surface insulation performance than other molar ratios steadily. Further synthesizing the copolymer with dopamine-aniline molar ratios of 1:1 on the surface of several polymer insulators can also improve the surface insulation performance, which demonstrate the wide applicability of the electron-delocalized copolymer coating in improving the surface insulation performance.

In this study, an innovative approach was developed to address the critical need for a convenient and environmentally sustainable method for preparing high-performance TiB2/TiC bilayer coatings. This method involves pack boriding TiC coatings using BN powders at temperature ranging from 1100 degrees C to 1500 degrees C. The prepared coatings were characterised using X-ray diffractometry, X-ray photoelectron spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. At 1100 degrees C, a small proportion of TiC transformed into TiB2. With increasing temperature, TiB2 became the predominant phase, forming a denser microstructure. At 1500 degrees C, the coatings exhibited a composite microstructure with distinct TiB2 and TiC layers. These layers, with an adhesion strength of 25.18 N, were strongly bonded through robust mechanical interlocking, demonstrating seamless integration. This innovative methodology highlights significant potential not only for fabricating TiB2 coatings but also for converting other carbide coatings into boride or silicon-doped carbide coatings.

Surface modification of vacuum insulators is a promising way to improve their surface insulation performance but commonly used surface modification technologies in surface insulation area such as fluorination and plasmas treatment are unable to control the surface chemical components at the molecular level, which makes it hard to explore molecular groups with high surface insulation performance. In this paper, molecule self-assembly is introduced to control the surface molecular structure precisely and vacuum surface insulation performance of a special molecular structure electron-delocalized group is studied. Hexagonal phenyl group containing delocalized electrons and cyclohexyl group with the same hexagonal molecular structure but without delocalized electrons are grafted onto the surface of alumina insulators through molecule self-assembly. Secondary electron emission yield test shows that phenyl group endows alumina insulators lower SEY than cyclohexyl group. Correspondingly, phenyl group grafted alumina insulators show higher surface insulation performance than cyclohexyl group. The results indicate that group containing delocalized electrons can endow insulators higher surface insulation characteristics.

Currently, backward current in diodes is a significant factor limiting the application of high-power microwave (HPM) devices packaged with a permanent magnet. This article experimentally observed typical morphological changes in diodes, such as the attachment morphology, micro-damage, and composition of sputtered products on the cathode rod surface caused by the backward current. Combined with particle-in-cell simulation, the connection between the backward current and typical morphological changes is revealed, illustrating the important influence of the anode on diode stability. Finally, an electrostatic shielding groove structure (ESGS) on the anode is developed from the perspective of slowing down the anode plasma movement. Experiments proved that this method can effectively suppress the backward current and improve diode stability.

In this study, nanosized Hf(C,N,O) ceramics were successfully prepared from a novel precursor synthesised by combining HfCl4 with ethylenediamine and dimethylformamide. Subsequently, the carbothermal reduction of these Hf(C,N,O) ceramics into hafnium carbide was investigated. The Hf(C,N,O) ceramics comprised Hf2ON2 and HfO2 nanocrystals and amorphous carbon. Upon carbothermal reduction, conversion began at 1300 degrees C, when HfC first appeared, and continued to completion at 1500 degrees C, resulting in irregularly shaped crystallites measuring 50-150 nm. Upon increasing the dwelling time, the oxides were completely converted into carbides at 1400 degrees C. Furthermore, nitrogen was introduced into the reaction to catalyse the conversion of oxides into carbides considering the beneficial gas-solid reaction between CO and Hf2ON2. We expect that the ceramics prepared in this study will be suitable for the fabrication of high-performance composite ceramics, with properties superior to those of current materials.

In this paper, through the application of 3D printing technology, insulators with different surface microstructures are prepared. Surface flashover test of the printed insulators shows that surface microstructures have a strong influence on the surface flashover voltage. This indicates that through proper surface microstructure design and the application of 3D printing technology insulators with high surface voltage withstand strength can be prepared. The influence of gas release characteristics, with and without thermal treatment, on the surface flashover voltages is studied. The results indicate that insulators with lower gas release rate own higher voltage withstand strength.

The TM 02 mode has advantage in power capacity over the fundamental TM 01 mode in relativistic backward-wave oscillator (RBWO), whereas in our formal experiment, the microwave pulse seemed not as wide as we expected. In this article, the power capacity of the TM 02 mode is investigated theoretically and the mechanisms corresponding to the pulse shortening are analyzed in detail. It is found that the pulse shortening in the RBWO results from the breakdown on the window, the bombardment of the stray electrons traveling in the low guiding magnetic field, and the breakdown induced by the intensive electric field on the surface of the slow wave structure (SWS). The approach of eliminating the pulse shortening is put forward based on the theoretical research and the numerical optimization using our artificial intelligence (AI) code and is validated in the experiment. With larger aperture antenna adopted, post resonator simplified, and emitting electric field greatly decreased, the pulsewidth is lengthened from ~9 to 18 ns with the output power of 360 MW at guiding magnetic field of 0.9 T.

In the development of a Ka-band overmoded Cerenkov oscillator, the issue of asymmetric modes competition should be paid attention to as they can be potentially excited in the overmoded cavity and then affect the performance of device seriously. Based on the 3-D models, the influences of the structural parameter such as “ldr” on the resonance characteristics for the symmetric modes and the asymmetric modes are analyzed. Then according to the start conditions for the modes, the roles of the Ezfield distribution and the guiding magnetic field in spurring the asymmetric modes are discussed and accordingly the ways to alleviate the asymmetric modes competition are suggested. The 3-D particle-in-cell (PIC) simulation results show that, with the proposed conditions, the optimized device is operating stably with a purified output mode of TM 01 . Finally, the optimized structure is experimentally tested. In accordance with the theoretical prediction and the simulation results, the experimental results show that under specific conditions the asymmetric modes are very likely to be excited and they can even dominate the beam-wave interaction process. Nevertheless, using the proposed method, the output mode is pure of TM 01 , demonstrating the feasibility of the proposed method in suppressing the asymmetric modes competition.

Resistance to intense electron beam bombardment of TiC/Graphite was investigated numerically and experimen-tally. The results indicate that the decreasing of initial electron energy and the increasing of TiC coating thickness could both enhance the electron energy deposition inside the surface layer of the TiC/Graphite, and finally cause the increasement of the temperature differences at the interface between TiC coating and graphite substrate. The graphite substrate is well coated with TiC in all the TiC/graphite targets, and the morphology of the TiC coatings could be controlled by adjusting its thickness. Under electron beam with accelerating voltage of 850 keV, current of 11 kA, pulsewidth of 40 ns and pulse number of 30 times, TiC/Graphite target with TiC coating thickness of 1 mu m shows more excellent electron beam bombardment resistance, agreeing with the results of the numerical modeling.

Precursors with different nitrogen contents were synthesised using hafnium (IV) chloride (HfCl4), dicyandiamide (DICY) and triethylamine (Et3N) as raw materials and were used for preparing Hf-based nanocomposites. The synthesis, ceramic conversion and microstructural evolution of HfC-based nanocomposites derived from precursors with different nitrogen contents were investigated. The results indicate that the composition of the synthesised precursors can be controlled by adjusting the molar ratio of HfCl4 and dicyandiamide. Pyrolysis of the precursors at 1200 degrees C led to the formation of Hf2CN- or HfN-based ceramics. As the nitrogen content of the synthesised precursors increased, the Hf2CN crystallites present in the ceramics annealed at 1200 degrees C gradually transformed to HfN. Furthermore, at 1600 degrees C, the HfN and Hf2CN phases gradually converted to HfC(N). The results also indicate that nitrogen favours the sintering of the obtained nanocomposites. The HfC-based ceramics should be promising for application as high-performance composite ceramics. (C) 2018 Elsevier B.V. All rights reserved.

Preparation of TaC-SiC fibers derived from Ta-doped polycarbonsilane precursor were reported. Ta-doped precursor was prepared using nanometer tantalum powders (nano-Ta) and polycarbonsilane (PCS), according to the quality of tantalum powders and PCS at 10 wt%, via a ultrasonic and ball mill mixing method. TaC-SiC fibers were prepared by melt-spinning, electron beam irradiation and heat-treatment. The composition, structure and pyrolysis process of the obtained green fibers were investigated by IR, TG, XRD, SEM and so on. The results show that the evolution of volatile gases is inhibited via introduction of nano-Ta in the precursor and the yield of precursor is increased to 69.32 wt%. TaC ceramics can be prepared via reaction of tantalum powders with free carbon in the fibers. The ceramic fibers are compact and smooth. Thus, TaC-SiC fibers can be prepared using the doping method.

The synthesis and characterization of nano-tantalopolycarbosilanes (TS) and their transformation into ceramic materials were reported. The TS-5, TS-10, TS-25, and TS-55 hybrid precursors were prepared using nanometer tantalum powders (nano-Ta) and polycarbosilane (PCS), according to the quality of tantalum powders 5 wt%, 10 wt%, 25 wt%, 55 wt%, via a ultrasonic and ball mill mixing method. The composition, structure, uniformity and pyrolysis process of the obtained precursors were investigated by infrared (IR), thermogravimetric (TGA), element analysis, SEM characterization, and so on. The results show that nano-Ta is dispersed uniformly in PCS. The ceramic yield of the precursors increases gradually with the increase of nano-Ta proportion. The inorganic conversion is almost completed at 1073 K and TaC crystal appears, and nano-Ta is completely converted into TaC at 1673 K. As the temperature increases, the crystallization of TaC is sharper and sharper, which indicates the growth of TaC grain. The non-oxygen structure, high ceramic yield, and uniform composition enable the as-received hybrid precursor as promising materials to prepare high performance ultrahigh-temperature ceramics.

The growing popularity of ultra-high-temperature ceramics inspired us to prepare Hf-based ceramic fibres by sequential melt hand-drawing, ultraviolet (UV) crosslinking, and pyrolysis of a novel polymeric precursor synthesised by co-polymerisation of HfCl4, ethylenediamine, and allylamine. The above precursor featured a backbone comprising Hf–N and CH=CH groups and exhibited good melt spin ability due to having a relatively linear-chain structure, an optimal molecular weight (Mn = 2569g/mol), and a suitable softening point (135–155°C), being easily melt-drawn into green fibres (diameter = 25µm) and exhibiting a high ceramic yield of 46.38wt% at 1500°C. UV curing of green fibres at room temperature and their subsequent pyrolysis in an inert atmosphere at 1200 and 1600°C afforded Hf-based ceramic fibres (diameter = 13µm), indicating the great potential of the above precursor and the prepared ceramic fibres for ceramic matrix composites used in high-temperature applications.

The high-temperature durability of SiBNC ceramics is significantly influenced by Si/B ratios and the synthetic procedures. Single-source synthetic routes can yield homogeneous ceramics at the atomic level, but the Si/B ratio cannot be efficiently adjusted. In this paper, a simple and efficient method for the synthesis of SiBNC precursor polyborosilazanes (PBSZs) with different Si/B ratios has been established via a one-pot reaction involving boron trichloride, dichloromethylsilane and hexamethyldisilazane in different molar ratios. The Si/B ratios of the derived SiBNC ceramics were consistent with that of the precursor PBSZs. When pyrolysed at 1000°C, PBSZs with 0.52, 0.94 and 2.12 Si/B ratios transformed into SiB2.6N5C2.2, SiB0.9N2.7C1.3 and Si2BN3C1.4 ceramics respectively. The polymer-to-ceramic process was also studied and featured ceramic yields of 43.2wt%, 50.1wt% and 62.2wt%, respectively. The derived ceramic SiB0.9N2.7C1.3 resisted crystallization up until 1700°C, whereas the SiB2.6N5C2.2 and Si2BN3C1.4 could remain amorphous up to 1600°C only. Using the precursor with 0.94 Si/B ratio, the SiBNC ceramic fibres were also obtained.

Curing green fibres infusible is an essential procedure for the preparation of SiBNC ceramic fibres. Previously, green fibres had been fabricated by one-pot synthesis of polyborosilazane (PBSZ) and melt-spinning. In this paper, we attempted to use the method of electron beam irradiation to crosslink green fibres. The variation of molecular structures from green fibres to cured fibres and the properties of sintered SiBNC fibres were investigated. Via electron beam irradiation, the free radicals are formed at the C atoms and Si atoms on the -N-SiH(CH3)- main chain units and terminal -Si(CH3)3 groups. The radicals react with each other to produce cross-linking, coupling and grafting among PBSZ chains, which all contribute to improvement of the cross-linking density of green fibres. The cured fibres performed a high ceramic yield of 80.4wt%. After pyrolysis at 1500°C, SiBNC ceramic fibres were acquired, which exhibited a good flexibility with 12µm in diameter and 1.22GPa in tensile strength. The obtained fibres could remain amorphous up to 1700°C and showed no mass loss at this temperature.

A single-source precursor for the preparation of HfC-SiC ceramics was synthesized via a Grignard reaction using bis(cyclopentadienyl) hafnium(IV) dichloride, trans-1,4-dibromo-2-butene, and (chloromethyl) trimethylsilane as raw materials. The composition, structure, pyrolysis process and high-temperature behavior of the precursor were investigated. The results show that the precursor with a backbone comprising Hf-C, Si-C and CH=CH groups exhibits good solubility in common solvents, such as tetrahydrofuran, dimethylbenzene, and chloroform. Pyrolysis of the precursor at 1000 degrees C yielded a microcrystalline HfC phase with a ceramic yield of 63.86 wt%. The pyrolytic products at 1600 degrees C were HfC-SiC nanocomposite ceramics, which exhibited good thermal stability up to 2400 degrees C. The formation of a (Hf,Si)C solid-solution would be beneficial for densification during the sintering process. The non-oxygen structure, high ceramic yield, homogeneous composition and excellent high-temperature behavior of the pyrolytic products make the as-prepared precursor a promising material for the preparation of high-performance ultra-high-temperature ceramics.

A novel precursor was synthesized by reacting hafnium chloride with dicyandiamide and dimethylformamide. The precursor was characterized via FT-IR and NMR, as well as TG. Subsequently, the precursor was annealed in Ar over a range of temperatures from 1000°C to 2000°C, and the microstructural evolution of the ceramics was investigated by XRD, XPS, and TEM. The results show that the carbothermal reduction of the precursor starts at 1150°C and the ceramic yields at 1500°C reach 44.6wt%. The obtained powders exhibit a uniform distribution and are composed of N-doped HfC and graphite. The N-doped structure postponed the oxidation of the HfC(N) ceramics. The HfC(N) ceramics were first oxidized to yield HfO2, carbon, and nitrogen, and then the carbon was oxidized with the evolution of CO2. The presented synthesis method is believed to be applicable to the preparation of other high-performance ceramics.

In this study, amorphous silicon nitride fibers were prepared through the nitridation of cured polycarbosilane fibers. It was observed that their composition and properties can be controlled by adjusting the flow of NH3 during the nitridation process. Based on their compositional and structural stability, the samples could be divided into two classes: stable fibers and unstable fibers. As indicated by the electron spin resonance spectra, the amount of the residual free radicals in the unstable fibers was significantly higher than that in the stable fibers. Because of the reactions of the radicals with the moisture in the air, the oxygen content of the unstable fibers increased day by day until the residual radicals were exhausted. Thus, to develop silicon nitride fibers with both low oxygen content and low carbon content, the amount of NH3 used should be optimized to eliminate the free radicals. These results suggest that it is possible to tailor the nitridation conditions for preparing high-purity silicon nitride materials so that they exhibit desirable properties, such as compositional and structural stability, good mechanical properties, and high electrical resistivity.