Auger studies comparing the surface concentration of barium on tungsten impregnated and M-cathodes

A study comparing the surface concentration of barium on both an impregnated tungsten cathode surface and M-type cathode surface was made to determine whether the higher electron emission capability of the latter can be attributed to a higher average surface barium concentration. Relative quantitative data was obtained by the use of a computerized Auger microprobe spectrometer on the surface of the cathode, half of whose area was M-type, the other half being the tungsten impregnated type. The results indicate that both cathode surfaces have equivalent barium concentrations and that they are approximately equal to a barium monolayer after 3000 h of cathode life.     

Development of a barium-free high-temperature cesium Tacitron

A barium-free, high-temperature, cesium Tacitron has been developed using a platinum, hollow cathode, emitter. The hollow cathode emitter used in our investigation is designed to enhance the current emission of a Tacitron cathode without the use of barium in the switch. In a barium-cesium Tacitron, the barium is known to cover the surface of a molybdenum emitter lowering its work function. The barium however limits the lifetime of the Tacitron, unless sophisticated seal technology is used in the manufacture of the Tacitron. The hollow cathode emitter was operated at current densities from 2.5 A/cm² up to 7.0 A/cm². Continuous operation of the Tacitron was demonstrated at 100-150 V and repetition rates of up to 5.8 kHz with measured voltage drops of 3.5-9.0 V. The results of the experimental characterization are compared to the computer model, and the applicability of this unique Tacitron design is discussed     

A model for the impregnated cathode emission behaviour under ion bombardment

The electron current emitted by a cathode in the saturation region decrease if the cathode is exposed to ion bombardment. The change in surface coverage with barium (oxide) is used in the model to describe the current behaviour. The steady state current reached (equilibrium current) depends on the gas pressure. The experimental results can be explained in terms of the model and the parameters are determined. The temperature dependence of the barium supply rate for an S-and M-type impregnated cathode are determined.     

关于热电子发射理论的评述(Ⅰ)——对氧化物阴极的半导体模型的批判

二十多年来,我们从实践中发现,氧化物阴极的理论与实际之间存在着很多矛盾。本文从逸出功测量、吸收光谱、超额钡浓度、晶格结构、表面能级、涂层导电率、涂层电位分布、薄膜和厚涂层发射的比较、中间层电阻、发射的不均性、闪变噪声、脉冲电流不稳定性、电火花等方面的实验和论证,说明氧化物阴极并不是以超额钡为施主的电子型半导体。提出了表面发射中心的观点,并用它解释了氧化物阴极中的许多现象。     

深紫外激光光发射与热发射电子显微镜在热扩散阴极研究中的应用

对热扩散阴极表面微区发射状态进行原位观察和分析一直是热阴极研究的重要课题.本文着重介绍深紫外激光光发射电子/热发射电子显微镜的基本原理及其在热扩散阴极研究中的典型实例.系统配备了高温激活所用的加热装置,样品可被加热至1400℃.系统具有光发射电子、阴极热发射电子、光发射电子和阴极热发射电子联合三种电子成像模式.应用表明,对于热扩散阴极而言,深紫外激光光发射电子像适于呈现阴极表面的微观结构形貌;热发射电子像适于反映阴极表面的本征热电子发射及均匀性;光电子和热电子联合成像适于对阴极表面的有效发射点做出精确定位.     

Diffusion and desorption of barium and oxygen on tungsten dispenser cathode surfaces

Auger measurements have been made on the concentration of barium and oxygen diffusing out of a pore and slot onto the surface of a simulated tungsten dispenser cathode. Profiles of concentration vs distance from the source were obtained at different temperatures. It is found that at cathode operating temperatures very little gradient of oxygen concentration exists on the surface, and under steady state conditions Ba is the main diffusing species. Ba diffusion distances derived are consistent with previous measurements. However it is found that this distance increases with concentration in spite of the fact that the Ba surface lifetime decreases with concentration. Time dependent measurements made on a clean surface show that the rate of Ba and O build up is limited by the supply rate of oxygen to the surface. This supply rate is not diffusion limited but seems to be limited by the mechanism generating free oxygen. A comparison of emission microscope measurements on a tungsten matrix dispenser cathode with the simulated cathode data indicates that similar oxygen generation processes may be controlling the activation of these cathodes.     

Short Thermionic-Cathode Discharge in Alkaline-Earth Metal Vapor

A short (without a positive column) Knudsen thermionic discharge initiated in barium and strontium vapor has been investigated. It has been shown that the behavior of these discharges and their formation mechanism, effective heating of the plasma electrons by a cathode electron beam, are, in general, similar to the discharge in cesium.     

等离子喷涂含钪氧化物阴极制备及发射特性研究

在三元碳酸盐中加入微量钪,利用大气等离子喷涂方法制备出含钪等离子喷涂氧化物阴极. 制备过程中,在三元碳酸盐原来配比基础上增加12%—22%(摩尔百分比)的碳酸钡进行混合, 并将该混合物进行造粒,在造粒过程中加入微量的钪.利用扫描电子显微镜分析了等离子喷涂材料的形态 和分布,表明符合等离子喷涂对粉末大小和形状的要求,并解决了等离子喷涂过程中钡的损失问题. 对这种新型氧化物阴极的分解排气过程进行详细分析,结果表明,排气时这种阴极比普通喷涂阴极出气少 且分解时间短.对发射性能及寿命进行测试,结果显示这种新型氧化物阴极的性能显著提高,阴极寿命延长.     

Ion scattering spectroscopy studies of barium and oxygen on tungsten and tungsten-based dispenser cathodes

In order to use Ion Scattering Spectroscopy (ISS) for studies of tungsten dispenser cathodes, the relevant ISS sensitivities must be measured. Calibrations have been made using a polycrystalline tungsten ribbon with controlled coverages of oxygen, barium and combinations thereof. Auger Electron Spectroscopy (AES) was used to monitor these controlled surfaces and the escape depths of the tungsten Auger electrons in barium and oxygen have been measured. The absolute ISS sensitivities of all three elements were found to be strongly dependent on the barium coverage of the tungsten surface. This effect has been attributed to the lowering of the work function of the tungsten surface caused by the barium adsorption. However, the relative ISS sensitivities of the three elements are not affected in this way when both barium and oxygen (or oxygen alone) are present on the tungsten surface. ISS spectra of such surfaces have been analyzed quantitatively and found to be in reasonable agreement with AES measurements. The analysis has also been applied to ISS spectra of uncoated tungsten matrix dispenser cathodes in an active state and following exposure to oxygen. Compared to AES, these spectra indicate less oxygen on the active cathode surfaces as a result of the oxygen (associated with barium) not contributing to the oxygen ISS signal. Comparisons of the spectra from the active and oxygen poisoned cathodes suggest that oxygen adsorbed during the oxygen exposure sits on the topmost barium layer whereas the oxygen on the active cathode surface does not.     

新型的覆纳米粒子薄膜阴极的研究

采用直流磁控溅射的方法制备出Ir金属纳米粒子薄膜.利用扫描电子显微镜分析了纳米粒子的形态和分布以及不同工艺条件对粒子粒径及形貌的影响,表明纳米粒子的大小可通过调节溅射气体压强来控制.在25%孔度的W海绵基体内浸入6∶1∶2铝酸盐发射物质,然后在其表面沉积上厚度为200—500 nm的纳米粒子薄膜层,最后在H₂气中1200℃烧结,即制成了新型纳米粒子薄膜阴极.利用阴极发射微观均匀性测试仪对纳米粒子薄膜阴极和传统覆膜阴极的热电子发射的均匀性进行了对比研究.采用飞行时间质谱仪测试了真空本底、纳米粒子薄膜阴极、传统覆膜阴极等各种阴极蒸发物的成分,研究了阴极蒸发速率与阴极温度的关系,比较了不同阴极蒸发速率的大小.研究了Ba-W阴极覆上纳米粒子薄膜后的发射特性. 关键词： 纳米粒子薄膜 热阴极 发射均匀性 蒸发     

“Alternative” auger analysis reveals important properties of M-type and scandate cathodes

It is shown that in the evaluation of Auger spectra, as measured on cathode surfaces, the elemental sensitivities have to be corrected for differences in elemental number density. The substrate and the surface covering material have to be treated differently in the analysis. The result for normal M-type cathodes is that — besides the Ba---O cover — there is an excess oxygen concentration on the surface of about two times the Ba---O concentration. This in spite of the fact that the oxygen to barium peak-to-peak height ratio in the measured spectra is about equal to 2. For a degraded M-cathode the excess oxygen level is found to be much higher than for the normal cathodes. A new type of scandate cathode is described and discussed. Its analysis shows more Ba and, relatively, much less excess oxygen than for the M-type cathodes. This may account for the substantially higher emission.     

Surface composition and barium evaporation rate of “pedigreed” impregnated tungsten dispenser cathodes during accelerated life testing

A study has been made of the surface composition and barium evaporation rate of “pedigreed” impregnated tungsten dispenser cathodes. The effect of air exposure on coated cathodes was examined and was found to have no significant effect on barium evaporation rate although in some cases longer reactivation times were required. No changes in surface topography were apparent following air exposure and reactivation. Life testing was done at 100°C above the typical operating temperature for the cathode, where the typical operating temperature was taken to be 950°C for coated cathodes and 1050°C for uncoated cathodes. The cathodes were examined at different stages of life testing, up to 1200 h. Significant decreases in barium evaporation rates were found after as few as 500 h of life testing. After 1000 h the evaporation rate had decreased more than an order of magnitude. Changes in surface composition were also found. The effects of tungsten particle size, used in manufacture of the billet, on barium evaporation rate were also studied but no correlation was found.     

Dependence of surface coverage on pore geometry in dispenser cathodes

This paper shows that the surface coverage of Ba/BaO on tungsten is more complete when the activator is supplied via slotted pores rather than circular pores. Both theoretical and experimental evidence is given to support this contention. The effect is primarily a geometrical one, since the surface diffusion in the case of circular pores is two-dimensional, whereas the surface diffusion for slotted pores is linear. The contrast becomes less pronounced as the circular pore size decreases. For dimensions of the order of those found on cathode surfaces (e.g., 10 μ m diameter pores), a hexagonal array of circular pores can be optimized to produce an emitting area of 88% of the total, with a pore open area of about 11%. For slotted pores, the slot widths can be made arbitrarily narrow, consequently, the emitting area approaches 100% while pore evaporation losses are minimized. A slotted-pore cathode should, therefore, be capable of higher and more uniform current density with less barium dispensation. When the pore geometry is controlled, either for round holes or slots, the cathode should be less prone to space-charge-limited slump than those based on random sintered pores     

Physical operating principles of scandate cathodes for microwave devices

The electronic structure of barium oxide crystallites determining the emission properties of both dispenser and scandate cathodes has been studied using electron spectroscopy for chemical analysis, electron energy loss spectroscopy, and optical spectroscopy. It has been established that the other elements (calcium, aluminum, scandium, and tungsten) contained in cathode materials are diluted in barium oxide and significantly affect its electronic structure and, consequently, emission properties. The obtained results give an idea about the physical and physicochemical mechanisms of the effect of scandium on the reduction of the work function of scandate cathodes relative to that of the cathodes of other types.     

Investigation of local work function distribution of field emission cathode from carbon materials doped by Ba with the help of AFM/SEM/X-ray

The joint application of atomic force microscopy (AFM) and SEM/X-ray micro-analyzer for investigation of electron work function (EWF) distribution over the surface of field emission cathode (FEC) is described in the paper. This method was used to investigate the graphite cathodes surface after barium doping. The possibility of defining the average value of EWF over the surface of FEC is shown. The results of AFM measurements are compared with results of field emission tests. The reduction of work function down to 3–3.2 eV was found after barium doping.     

绝缘层LiF/Al电极对提高P-PPV发光器件发光性能的研究

为了提高聚合物电致发光器件中Ba（Ca）／Al阴极的稳定性,在该阴极与聚合物发光材料poly（2-（4-ethylexyl）phenyl-1,4-phenylene vinylene）（P-PPV）层之间插入一层7nm的LiF绝缘层,发光器件的发光性能在多项参数（发光器件的电压．电流特性、发光强度及外量子效率,以及电流效率等发光性能指标）上能够与工作性能优良但稳定性较差的Ba（Ca）／Al电极结构PLEDs器件的发光特性具有可比性。这对于研制高效率、高稳定的聚合物电致发光器件并最终将其用于商业目的,具有重大现实意义。     

Structure and Polarization Relaxation of Ba<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript>/(001)Si Films

The structure and dielectric characteristics of strontium barium niobate thin films deposited on single-crystalline silicon substrates without buffer layers are studied. It is found that the c axis in these heterostructures runs largely normally to the substrate surface and the a and b axes are randomly oriented in the plane of the substrate. The polarization relaxation in such heterostructures is investigated. It is shown that the film–substrate interface in the heterostructures grown by rf cathode sputtering may contain a low amount of long-lived charged defects.     

Characterization of scandia doped pressed cathode fabricated by spray drying method

Scandia doped pressed cathode was prepared by a new method of spray drying combined with two-step hydrogen reduction process. The Sc₂O₃ and barium-calcium aluminate co-doped powders have sub-micrometer size in the range of 0.1-1 μm and scandium oxide and barium-calcium aluminate are distributed evenly in the powders. The cathodes sintered by powder metallurgy at 1600 °C_b have a smooth surface and sub-micrometer grain structure with homogeneous distribution of scandium, barium, calcium and aluminum which are dispersed over and among the tungsten grains. This cathode has good emission, e.g., the current density of this cathode reaches 31.50 A/cm² at 850 °C_b. After proper activation, the cathode surface is covered by a Ba-Sc-O active substances layer with a preferable atomic ratio, leading to its good emission property. The evaporation activation energy of SDP cathode with 4.58 eV is the highest among the Ba-W, M-type and SDP cathodes, and the average evaporation velocity v_t of SDP cathode with 1.28 × 10⁻⁸ g cm⁻² s⁻¹ at 1150 °C_b is the lowest one.     

Fast turn-on characteristics of tungsten-based dispenser cathodes. II

In the Proceedings of the 1982 Tri-Service Cathode Workshop, the authors described studies of the reactivations of tungsten-based dispenser cathodes following poisonings of the kind expected during shelf storage of a microwave tube [Appl. Surface Sci. 16 (1983) 73]. Further work on the problems of reactivation following such poisoning is described here. In addition to coated (“M”) and uncoated tungsten matrix cathodes, the tungsten-iridium mixed metal matrix (“MM”) cathode has been studied. In general reproducible results have been obtained from different examples of the same type of “M” and uncoated cathodes. However, although some “MM” cathodes have exhibited good reactivation characteristics, a large variation has been observed between different examples of “MM” cathodes. The composition of the impregnant in the dispenser cathodes has been found to be an important factor in determining the reactivation rate of a cathode. As the barium oxide concentration in the impregnant increases, the cathode will recover faster from a poisoning exposure. Studies of the poisoning caused by combinations of different gases suggest that poisoning will occur if there is a sufficient exposure of a poisoning agent, regardless of the total exposure. The exposures necessary to poison a cathode are so small, that poisoning of the cathode appears probable during shelf storage of a microwave tube. The reactivation results have been summarised in terms of the times and temperatures required to achieve both a given current density and a given degree of reactivation from a poisoned cathode. The studies also indicate that the limiting step during the reactivation process involves the dispensing of fresh material to the cathode surface rather than the desorption or conversion of a poisoned surface layer.     

CBP有机薄膜对MEH-PPV聚合物发光二极管性能的影响

以聚合物发光材料MEH-PPV为发光层的聚合物发光二极管的ITO阳极与发光层之间,加入一层溶解于氯仿中的有机小分子CBP,能显著改善发光器件的电流特性。在电压较低的时候能提高电流,在电压较高的时候能抑制电流,从而增加工作电压范围。此外,器件的电流效率也能得到显著的提高。实验结果表明,加入CBP层后,在低电压时,CBP层能够减缓空穴注入到发光层中,将其限制在CBP层,从而在器件中形成一个内电场,有助于电子的传输,降低开启电压,提高发光亮度。在电压较高时,CBP作为电子阻挡层,能阻挡电子漏泄到阳极,从而使在复合区的空穴与少数载流子电子的复合效率提高,改善器件的性能。