Georg Dr. Gärtner

Emission properties of top-layer scandate cathodes prepared by LAD

Abstract In the first part an overview of scandate cathode types, their historical development and the current status of research is given. Then recent improvements obtained by our group are elucidated: A new type of top-layer scandate cathode has been prepared with the aid of laser ablation deposition. Emission current characteristics have been measured as a function of voltage and temperature. Zero field emission i10% is determined as 10% deviation from space-charge law. At 965°CMo-Br i10% of about 400 A/cm2 and at 760°CMo-Br of 32 A/cm2 were achieved, which is more than a factor of 3 higher than the best values for other scandate type cathodes given in literature. It corresponds to a work-function of about 1.16 eV with a Richardson constant of 7.8 A cm−2 K−2. Other emission properties such as uniformity and cathode life are discussed. A description of I U characteristics via the superposition model is given.

Supply and loss mechanisms of Ba dispenser cathodes

Abstract Ba dispenser cathodes, especially W–I, Re–I, Os/Ru–I and Sc 2 O 3 /Re top-layer Scandate cathodes have been investigated with respect to Ba resupply, Ba evaporation and ion bombardment behaviour. Since a 411 impregnated base was used for all these types, the same Ba evaporation activation energy of 3.15 eV was found despite different surface coatings. These and further results are compared with literature data. From ion bombardment (=IB) investigations different sputter yields are derived. With respect to IB stress tests, dynamic shielding of the surface complex rest by Ba sitting on top is introduced, which allows the extrapolation of resistivity data to real tube conditions. Information about the Scandate surface complex is evaluated.

Quantitative LEIS analysis of thermionic dispenser cathodes

An UHV LEIS setup has been converted into a dedicated apparatus to study the surface composition, structure and dynamics of real dispenser cathodes and cathode model systems based on W single crystals. LEIS, AES and LEED are available to investigate the surface characteristics, and the cathode emission properties are derived in situ from a close-spaced diode configuration. In this paper, the focus is on the quantitative surface composition of B-type and M-type dispenser cathodes by LEIS. A straightforward quantification is hampered by the influence of the cathode workfunction on the neutralisation of the ions. It is shown that the ion fraction decreases as the workfunction of the cathode decreases. The Ba surface density is observed to increase with decreasing workfunction. However, before an accurate quantitative surface analysis can be performed a validation of the model used to correct for the influence of the ion fraction has to be performed.

Dynamic shielding during ion bombardment of Ba dispenser cathodes

Abstract Ion bombardment (IB) resistivity is a key parameter for cathodes to be operated in TV tubes and CMTs over lifetimes of several years. Thus, accelerated stress tests with much higher ion dose rates than those present in real tube environment are normally used. Here, IB resistivity of Os/Ru-I and top layer Scandate cathodes was investigated. It was observed by variation of the dose rates, that Ba resupply modified the survived ion dose and led to a dynamic shielding of the rest of the surface complex/dipole below Ba. Enhanced IB resistivity w.r.t. total ion dose was observed for reduced ion dose rates. Based on a model of shielding an extrapolation from stress tests to real tube conditions has been established, where the correction factor is in the range of 1–8, depending on the test conditions.

Impregnated cathode surface investigations by SFM/STM and SEM/EDX

Abstract The surfaces of 411-impregnated S-cathodes have been investigated by SFM/STM (= scanning force microscopy and scanning tunnelling microscopy) and SEM/EDX. The S-cathodes of different origin consist of a tungsten matrix (20% porosity) and are impregnated with 4BaO · CaO · Al 2 O 3 (abbreviation: WI-cathodes). Surface structure and topography and elemental constituents of the partly contaminated tungsten surface have been identified. A comparison between non-activated and activated WI-cathodes is given. The specific advantages and the resolution limits of both characterization tools are discussed.

Light extraction for a doubly resonant cavity organic LED: the RC2LED

The RC2LED is a substrate emitting OLED which has three additional interference layers between the ITO electrode and the glass substrate. This creates two resonant optical cavities. The RC2LED has 2 resonant optical cavities. The first cavity is also present in regular devices and is formed by metal/organic layers/ITO. The second cavity is formed by 3 additional layers: a high refractive index layer (Nb2O5), a low refractive index layer (SiO2) and a high refractive index layer (Nb2O5). The additional layers introduce a strong wavelength dependent improvement of the extraction efficiency compared to the OLED without the additional layers. Our simulations show an improvement of the extraction efficiency of over 70% over a wavelength range of 75 nm compared to an OLED without the 3 layers. Light extraction is worse compared to the reference OLED for wavelengths outside this wavelength range. the when compared to the OLED. This improvement has been experimentally verified for a green OLED with an emission between 500nm and 650 nm. A numerical study shows a relative improvement of 10% for the luminous power efficiency of a 3 color white OLED with the additional layers. The emitted white corresponds with the light emitted by illuminant A. The WOLED has been composed of a fluorescent blue emitter, green and red phosphorescent emitters.

Dynamic behavior of thermionic dispenser cathodes under ion bombardment

We have investigated the surface coverage and electron emission of thermionic dispenser cathodes during 3 keV Ar+ ion bombardment, thereby simulating the bombardment of the cathodes by residual gases that takes place in cathode-ray tubes as used in television sets. During the ion bombardment at the operating temperature of 1030 °C, a dynamic equilibrium is established between the sputter removal and resupply mechanisms of the Ba and O atoms that form the dipole layer on the cathode substrate. We demonstrated that the performance of the cathodes under ion bombardment is governed by the O removal and resupply rates. It was found that the Ba resupply rate is almost an order of magnitude higher than the O resupply rate, but that the Ba can only be present on the surface bound to O atoms. Therefore, the Ba/O ratio is approximately equal to unity during the ion bombardment. Based on the investigations of the removal and resupply processes, we proposed a model that accurately describes the surface coverage and e...

Cleaning procedure for single-crystal tungsten substrates

Single-crystal tungsten substrates produced by electron-beam melting were cleaned by annealing in an oxygen atmosphere (∼10–3Pa, ≃1500°C) for a few hours to remove adsorbed carbon and by high-temperature ultrahigh-vacuum annealing (≃2 × 10–8Pa, ≥2500°C) for a few minutes to remove residual oxygen. The process was followed by low-energy ion scattering, Auger electron spectroscopy, and low-energy electron diffraction. The carbon and oxygen adsorbed on the surface of single-crystal tungsten were found to form ordered, oriented structures. The resultant high-purity, structurally perfect W substrates were used to study electron emission in the Ba/W and Re/W systems.