Surface studies on the low work function surface complex of barium on an osmium-ruthenium substrate

Barium and oxygen on an osmium-ruthenium (Os/Ru) surface is the surface complex responsible for the excellent electron emission properties of commercial M-type thermionic cathodes. A computerized Auger technique was used to study the surface properties of this surface complex and compare it with results obtained earlier for Ba-O-W surfaces, which characterize the properties of commercial barium impregnated tungsten thermionic cathodes. Barium desorption, electron emission and barium, barium oxide evaporation were measured at elevated temperatures for this experimental surface study. Desorption measurements, at 1100°C, on the barium on osmium-ruthenium substrate show that the energy for desorption was higher than that for the lowest work function Ba-O-W surface (5.4 to 4.8 eV). Oxygen was always present on the sputtered Os/Ru substrate and contributed to the strong bonding of barium to the Os/Ru surface. The barium and barium oxide evaporative products from such a surface at 1100°C were found to be mainly barium with the Ba/BaO ratio equal to approximately 100.     

Experimental investigation and ab initio calculation of the properties of Sc-, in-doped bismuth titanates with the pyrochlore type structure

Using ab initio calculations, the data have been obtained on the structural, electronic, and optical properties of bismuth titanates with the pyrochlore type structure and compounds with the substitution of scandium or indium atoms for bismuth and titanium atoms. The results of the theoretical calculations agree with the experimentally obtained structural and optical characteristics of the synthesized compounds doped with scandium or indium. It has been shown that the substitution of scandium or indium atoms for bismuth atoms in the pyrochlore structure is energetically favorable. The energies corresponding to the direct and indirect electronic transitions in scandium- and indium-doped bismuth titanates have been determined based on the optical spectroscopy data obtained for the studied samples. These energies are in agreement with the theoretically calculated values.     

亚微米电子发射材料的合成及发射性能

介绍一种气-液相合成技术合成多元碳酸盐及其制备阴极的发射性能.实验表明：采用该方法合成碳酸盐结晶小,形貌呈细丝状,丝径小于1μm,这种亚微米丝状碳酸盐能明显提高氧化物阴极的发射性能.通过对阴极激活后阴极表面的SEM分析,初步讨论了亚微米碳酸盐的发射机理. 关键词： 亚微米碳酸盐 气-液相合成技术 氧化物阴极 发射性能     

Properties and manufacture of top-layer scandate cathodes

The life and/or the staying power against ion bombardment of scandate cathodes can be improved by using a top layer of W + Sc₂O₃ or W + W/ScH₂ on a tungsten matrix. The latter is impregnated with the usual 4-1-1 impregnant. Even at high voltage pulses the current densities are so high that the deviation from space charge limitation is small. The manufacture is discussed and the cathode life at the operating temperature of 1220 K is shown to be very long. Moreover, the relationship between processing parameters and emission recovery after ion bombardment is examined with the aid of combined sputter- and scanning-Auger measurements. It is shown that thin-layer coverage of tungsten by scandia is important to the high emission. This coverage is related to the impregnation process. After extended sputtering it cannot be completely recovered by reactivation. Consequently, the top layer cathodes cannot withstand sufficiently the usual processing and operation of television display tubes. On the other hand, they can improve the life and performance of electron devices with good vacuum and/or relatively low accelerating potentials. Moreover, activated top-layer scandate cathodes are relatively insensitive to exposure to (moist) air.     

“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.     

用于真空电子太赫兹器件的微型热阴极电子束源研究

太赫兹波辐射源是太赫兹(THz)波技术的关键.真空电子太赫兹器件在高频、大功率太赫兹源发展中较其他技术有明显的优势,微米尺度高电流密度微型电子束源则是研制真空电子太赫兹器件的核心之一.本文在研制低温、大电流纳米粒子氧化钪掺杂含钪扩散阴极(nanosized-scandia doped dispenser cathode)的基础上,采用发射抑制膜沉积与聚焦离子束(FIB)刻蚀技术,研制无需压缩直接提供高电流密度的微型电子束的电子源.所研究的电子束源直径400μm,在工作温度950fiC,提供空间电荷限制电流密度50 A/cm2时,已稳定工作1000 h以上,并且层流性良好.本文阐述了阴极制备工艺、电子发射特性、微米尺度电子束源的获得和特性,介绍了发射抑制膜的结构和抑制特性的评估.并探讨了镀膜和刻蚀对发射的影响机理.这一电子束源在常规毫米尺度电子源的基础上产生微米尺度的微区高电流密度的电子束,为真空电子太赫兹辐射源的研制提供了新的途径.     

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.     

LaB6与TaC混合涂层热阴极

研究了一种LaB6与TaC混合涂层热阴极,其发射层通过均匀混合LaB6与TaC粉末得到。比较了该阴极在LaB6与TaC混合比例为1∶1, 2∶1和3∶1下的发射特性,发现混合型LaB6阴极的功函数与多晶LaB6阴极的非常接近,在3∶1的混合比例下,电极的发射电流稳定,得到的电流密度为30 A/cm2。该电极可被应用于需要大电流密度和超大面积阴极的真空电子器件及动态真空电子束设备中。     

XPS on impregnated cathodes: Surface concentrations and thermal stability

Photoelectron spectra are presented for S, M and scandate cathodes before and after the activation procedure. Using the calculatory method and the Ba deposition method the Ba coverages for the S, M and Sc cathodes have been established as 4.0×10¹⁴, 5.8×10¹⁴ and 5.8×10¹⁴ Ba atoms cm^-2 respectively. On the Os-Ru cathode an enhancement of the Ru concentration is found which may be due to segregation. Further a short investigation was made into the thermal stability of the Ba overlayer and its regeneration. It appears that after a heating cycle to 1200 °C the S and M cathodes recover very well whereas the Sc cathode suffers from the loss of Sc which is inherently connected with the fine emission properties of this cathode.     

Structure and hard magnetic properties of barium hexaferrite with and without La2O3 prepared by ball milling

The barium hexaferrites have been prepared by ball milling of a BaO₂ and Fe₂O₃ mixture followed by thermal heat treatments. The structure and magnetic properties were investigated using X-ray diffraction, scanning electron microscopy and vibrating sample magnetometer techniques. The effect of grain refiner was also studied and it was found that the hard magnetic properties were improved significantly. The sintered product of barium hexaferrite powders prepared from ball milling has higher coercive force than that of other barium hexaferrite made from oxide/carbonate.     

The tungsten powder study of the dispenser cathode

The intercorrelation of tungsten powder properties, such as grain size, distribution and morphology, and porous matrix parameters with electron emission capability and longevity of Ba dispenser cathodes has been investigated for the different grain morphologies. It is shown that a fully cleaning step of the tungsten powder is so necessary that the tungsten powder will be reduction of oxide in hydrogen atmosphere above 700 °C. The porosity of the tungsten matrix distributes more even and the closed pore is fewer, the average granule size of the tungsten powder distributes more convergent. The porosity of the tungsten matrix and the evaporation of the activator are bigger and the pulse of the cathode is smaller when the granularity is bigger by the analysis of the electronic microscope and diode experiment.     

Field emission microscope investigation of oxide formation on Mo field emitters

The initial stages of oxide formation on Mo field emitter tips have been examined with the field emission microscopy and field electron spectroscopy. The oxide formed in situ on Mo tips by thermal treatment in pure oxygen atmosphere was found to be non-uniform with little evidence for coherent structure. The electron spectroscopy results are discussed in terms of the hypothesis that the emission process is a hot-electron emission.     

天鹅绒与碳纳米管阴极强流脉冲发射特性

在2 MeV直线感应加速器注入器平台上开展了天鹅绒阴极与碳纳米管阴极的强流脉冲发射特性综合实验。研究结果表明:天鹅绒阴极与碳纳米管阴极均具有强流脉冲发射性能,在1.61 MV的二极管电压下,天鹅绒阴极与碳纳米管的发射电流密度分别为84,108 A/cm2;启动时间分别为21,40 ns;放气量分别为0.29,0.91 PaL;放出气体分子数目与发射电子数目之比分别为64,225;两种冷阴极强流脉冲发射时的放气质谱相似。     

金属基底对石墨烯薄膜阴极气体击穿稳定性影响

利用石墨烯二维材料极好的场发射能力和发射稳定性,提出了石墨烯阴极提高气体开关击穿稳定性的技术路线。采用化学气相沉积法和基底腐蚀转移法两种方法制备金属基底石墨烯薄膜阴极。利用扫描电子显微镜和拉曼光谱表征了石墨烯薄膜阴极质量,确认了石墨烯层数和均匀性。实验研究了两种石墨烯薄膜阴极气体开关,在微秒脉冲均匀电场作用下的击穿特性,获得了击穿电压幅值和分散性的变化规律。结果表明:当气体为0.6 MPa N2、电极间距为5 mm时,铜基底石墨烯薄膜阴极平均击穿电压为85.9 kV,相对标准差为3.2%;不锈钢基底石墨烯薄膜阴极平均击穿电压仅为59.8 kV,相对标准差为2.4%。当两种阴极击穿电压均为80 kV时,相对标准差比较,不锈钢基底仅为铜基底的44%。分析认为,不锈钢基底石墨烯薄膜质量优于铜基底,石墨烯薄膜导致阴极表面微观场增强因子更高,表面分布更均匀,在电场作用下场致发射产生均匀稳定的大量初始电子流,降低了气体开关击穿电压,有效提高了击穿稳定性。     

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     

Effect of barium doping on the physical properties of zinc oxide nanoparticles elaborated via sonochemical synthesis method

The aim of this work is to study the effect of barium (Ba) doping on the optical, morphological and structural properties of ZnO nanoparticles. Undoped and Ba-doped ZnO have been successfully synthesized via sonochemical method using zinc nitrate, hexamethylenetetramine (HMT) and barium chloride as starting materials. The structural characterization by XRD and FTIR shows that ZnO nanoparticles are polycrystalline with a standard hexagonal ZnO wurtzite crystal structure. Decrease in lattice parameters from diffraction data shows the presence of Ba²⁺ in the ZnO crystal lattice. The morphology of the ZnO nanoparticles has been determined by scanning electron microscopy (SEM). Incorporation of Ba was confirmed from the elemental analysis using EDX. Optical analysis depicted that all samples exhibit an average optical transparency over 80%, in the visible range. Room-temperature photoluminescence (PL) spectra detected a strong ultraviolet emission at 330 nm and two weak emission bands were observed near 417 and 560 nm. Raman spectroscopy analysis of Ba-doped samples reveals the successful doping of Ba ions in the host ZnO.     

氧化锌纳米结构的制备及发光性质研究

采用化学气相沉积方法,在氩气和氧气混合气氛下制备了两种四角结构的纳米氧化锌。初始反应物为纯锌粉,反应过程中没有采用任何触媒。采用X射线衍射、扫描电子显微镜、透射电子显微镜、光致发光光谱研究了纳米产物的结构和光学性质。获得的纳米产物为高纯的纤锌矿结构氧化锌。两种氧化锌纳米产物具有三维立体的四角结构,分别为四角锥-片状结构和四角锥-线状结构,具有较大的长径比,呈典型的微/纳结构。通过对两种氧化锌纳米结构的紫外发射峰和可见发射带的对比研究,探讨了氧化锌纳米产物可见发射带的起源,以及影响其发光性质的主要因素。     

Fabrication of barium/strontium carbonate coated amorphous carbon nanotubes as an improved field emitter

Amorphous carbon nanotubes (aCNTs) were synthesized by a chemical reaction between ferrocene and ammonium chloride at a temperature ～250 ^°C in an air furnace. As-synthesized aCNTs were coated with the barium/strontium carbonate through a simple chemical process. The coating of barium/strontium carbonate was confirmed by a high resolution transmission electron microscopy, X-ray diffraction, and Fourier transformed infrared spectroscopy. Morphology of the as-prepared samples was studied by field emission scanning electron microscopy. Thermal gravimetric analysis showed that barium/strontium carbonate coated aCNTs are more stable than the pristine aCNTs. As-prepared barium/strontium carbonate coated aCNTs showed significantly improved field emission properties with a turn-on field as low as 2.5 V/μm. The variation of field emission characteristics of the barium/strontium carbonate coated aCNTs with interelectrode distances was also studied.     

碳纳米管阴极的强流脉冲发射性能研究

采用丝网印刷和涂敷方法制备了两种碳纳米管阴极，并研究了两种阴极的强流脉冲发射特性，表征了阴极表面碳纳米管的形貌及分布.研究结果表明在脉冲宽度为100 ns、电压为1.64×10⁶ V的脉冲电场下，涂敷法制备阴极的场发射电流最高达5.11 kA，最高发射电流密度达260 A/cm².丝网印刷法制备阴极的场发射稳定性优于涂敷法制备阴极，但其发射电流低.阴极表面发射体的形貌与分布影响了阴极的脉冲发射性能.碳纳米管阴极的脉冲发射机理为爆炸电子发射.碳纳米管阴极可以作为强 关键词： 碳纳米管 阴极 脉冲发射 强电流     

氧化物分子束外延技术及其在关联材料电子态研究中的应用

氧化物分子束外延薄膜和异质结生长技术近年来迅速发展,人们已实现以单原子层的精度来精确生长多种复杂量子材料,有力地推动了铜氧化物高温超导电性、二维电子气、氧化物电子学和自旋电子学器件等领域的研究.文章介绍了氧化物分子束外延的技术关键,并以La1-xSrxMnO3薄膜为例,介绍了钙钛矿结构的氧化物薄膜生长和刻画.特别是文章作者通过建立超高真空下的原位样品传送系统,可把薄膜样品直接传送到角分辨光电子能谱仪中,实现了薄膜的原位电子结构测量.所测得的La1-xSrxMnO3的电子结构与能带计算结果较为相符.而此类立方结构的、不可解理材料的电子结构,过去往往是无法直接测量的.