Microwave vacuum devices are used in a wide variety of areas, such as radar, space technology, electron accelerators and high-power microwave weapons at the future military frontiers. The cathodes, i.e., the electron sources, are the cores of the high-power microwave sources. Nanoparticles have many physical and chemical properties that their corresponding bulk materials do not have, such as surface effect and small-size effect, etc. Based on our previous work on the M-type cathode, we have proposed a strategy to develop a new type cathode. We deposited a layer of thin film of metal nanoparticles on the traditional Ba-W dispenser cathode. Thus the N-type cathode demonstrated some features different from the traditional M-type cathode. The study of the electron emission properties of this N-type cathode would promote the understanding on the electron emission and expand the applications of the nanomaterials. Nanoparticle thin films were grown by magnetron sputtering at room temperature. The results showed that the particle size of the films depended on the deposition rate in the nucleation stage of the films. The effect of thin film characteristics on thermionic emission of dispenser cathodes has been investigated. The chemical components of the vacuum background, the evaporants from an N-type cathode and an M-type cathode were respectively analyzed using a time of flight mass spectrometer (ToFMS). The results have proved that the ToFMS is one of the best tools for studying the evaporation phenomena of impregnated cathodes. Finally, the electron emission performance of the N-type cathodes was studied. I-V characteristics showed that the DC emission current density was 30 A/cm2 and its lifetime was 600 h. The pulse emission current density was 108 A/cm2 and its lifetime was 1500 h.

This paper analyses the degradation factors of a new long life coated impregnated cathode after accelerated life test. The surface state of the cathode is investigated with scanning electron microscope (SEM) as well as the content and variation of the various elements on the surface and the longitudinal section of the cathode are analyzed with Auger electron spectroscopy (AES) before and after the life test. The analyzing results with SEM show that the cathode coating shrinks at the life end and leads to a rise in its work function. The analyzing results with AES show that the percent of the W increases and the active materials Ba decreases on the cathode surface at the life end. Furthermore, there is less Ba underneath the cathode surface but still a lot of Ba in the tungsten matrix at the life end.

The Ir nano particle thin film was grown by dcrect current magnetron sputtering at room temperature. Microstructure of thin films grown at different sputtering pressures were measured by scanning electron microscopy. The results show that the particle size of the films depends on the deposition rate in the nucleation stage of Ir films and the deposition rate can be well controlled by sputtering pressure. A new kind of cathodes are fabricated from 25% porous tungsten impregnated with 6∶1∶2-type barium calcium aluminate and coated with the nano particulate Ir thin film. The coating processes produces a film thickness of 200—500 nm and the cathodes are fired in hydrogen atmosphere for ten minutes at 1200℃ to further improve the coating microstructure. The cathodes have been studied with thermal electron microscope by which the electron-optical picture of a thermal emission cathode could be obtained. Time of flight mass spectrometer （ToFMS） has been used in a study of evaporation from impregnated cathodes. The chemical composition of evaporation of various impregnated cathodes have been measured by ToFMS. The variation of ion current of evaporants from S-type cathode and M-type cathode （coated with iridium） and n-type cathode （coated with iridium nano-particle thin film） with temperature and time have been studied and discussed. Emission current characteristics have been measured as a function of voltage and temperature.

The life of impregnated Ba-W cathodes with a new construction have been evaluated using an accelerated life test at three different temperatures (1170°C,1130°C,1090°C) and constant current density (2A/cm2). According to the relationship of life with operating temperatures, an accelerated equation has been set up. The cathode life at normal operating temperature is deducted based on the accelerated equation. The results show that life of the novel cathode exceeds 190,000 hour at a current density of 2A/cm2.

A novel technique has been developed to dynamically measure the weak cathode evaporation with a quartz oscillator. We found that when high quality quartz oscillator, irradiated by high temperature cathode nearby, is under forced cooling, the frequency of the quartz oscillator decreases monotonically with the increase of cathode-evaporated substance. Dynamic evaporation rate of the cathode can be evaluated by substituting specific frequency decrease in a unit time for Δf of the formula we derived. Compared with the conventional static measurement, the dynamic technique can easily acquire the crucial information at the initial stage of evaporation, which is of much technological interest in fabricating low evaporation, M impregnated dispenser cathode.

A new type dispenser cathode with Os-W/Re dual-layer has been developed. Emission performance and surface microstructure of the cathode with the dual-layer before and after ageing were studied. The performance of the dispenser cathode is improved by introducing a Re intermediate leyer. DC current density of the dispenser cathodes coated with Os-W alloy and Os-W/Re were compared. It is found that the cathode with Os-W/Re shows better emission performance than the cathode with Os-W alloy. Both cathodes were investigated using X-ray photoelectron spectroscopy after full activation. Ternary alloy coating being formed for the cathode with Os-W/Re is the major reason for its better emission performance. Scanning electron microscopy was used for investigating surface microstructures of both kinds of cathodes and the results show that the emitting surface of the cathode with Os-W alloy after ageing appeared non-adherent (flaking) in localized areas. This is one of the reasons for its nonuniform emission. However, surface of the cathode with Os-W/Re does not suffer from peeling under the same conditions, thus ensuring better emission uniformity and functional reliability of the dispenser cathode.

This paper describes the factors which have influence on the heating power of cathode-heater assembly.For satisfying the requirements of X-band traveling wave tubes with high reliability,high efficiency and long life,several improvement measures have been taken,including increasing the effective heated area,determining the material of cathode holder,and optimizing techniques of sintering alumina and assembly.

An optical method for measuring the curvature radius of the cathode surface is presented.The optical principle of the method is analyzed and many measuring factors which affect the absolute error are analyzed.Then the curvature radius of cathode are measured by using Nikon MM—40 microscope.At last,a simple method with high measuring accuracy is given.

A new type of dispenser cathode with dual-layer (Os-W/Re) is developed. The cathode coated with Os-W/Re shows better emission performance than the cathode coated with Os-W alloy. X-ray Photoelectron Spectroscopy (XPS) spectra demonstrate that ternary alloy coating (Os-W-Re) formed on the surface of the cathode with dual-layer (Os-W/Re) after full activation is the major reason why it has better emission than the cathode with Os-W alloy. The surface of each variety of the cathode is characterized with Scanning Electron Microscope (SEM) before and after activation: the emitting surface of the cathode with Os-W alloy after ageing appeared non-adherence (flaking) in localized areas, which is one of the reasons for non-uniform emission. However, the surface of the cathode with dual-layer (Os-W/Re) does not present film peeling under the same conditions. Thus it ensures better emission uniformity and functional reliability for the dispenser cathode.

In order to improve the activation characteristics & emission ability of the conventional Ir-Coated impregnated tungsten cathodes, a new type of dispenser cathode with ternary alloy Ir/Re/W coating was developed. The improved cathodes show higher emission current density and faster activation characteristics than that of the conventional pure Ir-coated impregnated tungsten cathodes. XPS was used to analyze the element compositions on the surface of the cathodes coated with pure Ir and Ir/Re/W alloy. The results will be given in this paper.

An optical method for measuring the curvature radius of the cathode surface was presented. The optical principle of the method was analysed, and many measuring factors which affect the absolute error were analysed. Then the curvature radius of cathode were measured by using Nikon MM-40 microscope. A simple method is given which is of high measuring accuracy.

Hafnium (Hf) films with different preferred crystalline orientation have been prepared on Si(111) by ion beam assisted deposition (IBAD) at room temperature. The effects of the Ar+ ion incidence angle and the ion/atom arrival ratio on the texture were investigated. Hf films deposited by vacuum electron-beam evaporation have polycrystalline orientations. Hf films deposited by sputtering method, with the deposition rate about 0.15 nm/s, have weak (110) orientations. Hf films have preferred (110) orientation when the growing films were bombarded by 500 eV Ar+ ions with the incidence angle of 75° and current density of 90 μA/cm2 with the ion/atom arrival ratio about 0.21. However, as the current density was beyond 120 μA/cm2 with the ion/atom arrival ratio about 0.43, the Hf films exhibited mixed (002) and (100) orientations and which were independent of the ion incident angle. The preferred orientation of Hf films is determined by the competition between the surface energy and the strain energy.

The emission and surface characteristics of the dispenser cathode coated with Os-W alloy and that coated with Os-W/Re are studied and compared. The dispenser cathode coated with Os-W/Re has applied in electron gun measurement system for making measurement of higher emission current and life test. It is found that the dispenser cathode coated with Os-W/Re has higher current density than that coated only with Os-W alloy. In electron gun the dispenser cathode coated with Os-W/Re has pulse current density of 30 A/cm(2) and life of more than 800 hours.

The scandate cathode has excellent electron emission capability. However, applications in vacuum electron devices are limited because of nonuniform emission and weak resistance to ion bombardment. A new method has been adopted for making scandate cathode. This type of scandate cathode showed more uniform emission and fast recovery characteristics after ion bombardment comparing with conventional impregnated scandate cathode. The results of XPS analysis before and after ion bombardment are presented.

Hf films were synthesized by ion beam assisted deposition(IBAD). The influence of ion bombardment during deposition on the preferred orientation of films was studied. Hf film grains exhibit preferred (110) orientation when the growing film is bombarded by 500 eV Ar~+ions at an incident angle of 75° and a current density of 0.9 A/m~2. When the current density is beyond 1.2 A/m~2, the Hf films exhibit mixed (002) and (100) orientation and which is not concerned with the ion incident angle. The reason for the preferred orientation of Hf film was discussed. It is considered that the preferred orientation of Hf films does not simply depend on the channeling effects of ions, or the grain surface energy, but it is the result of mutual competition and actions of several factors, which influences the crystallographic orientation of thin films in the non-equilibrium growth conditions.