Tungsten hairpin

In principle, energy-analyzer systems can be designed such that their electron-optical properties do not limit the energy resolution attainable, i. e. their intrinsic energy resolution is much better than the energy width of the primary electron beam, which is of the order of approximately 1.5-2.5 eV for a tungsten hairpin cathode, approximately 1 eV for a LaBg cathode, approximately 0.7 eV for a Schottky field emitter, and 0.3-0.5 eV for a pure cold-field emitter. 

Figure 5.3. The tip of a tungsten hairpin filament. (Reproduced by permission of Wiliams and...

The decomposition of formic acid over evaporated Pd-Au alloy films has been studied by Clarke and Rafter (69) the same reaction on Pd-Au alloy wires was studied by Eley and Luetic (128). The alloy films were prepared in a conventional high vacuum system by simultaneous evaporation of the component metals from tungsten hairpins. The alloy films were characterized by X-ray diffraction and electron microscopy. The X-ray diffractometer peaks were analyzed by a method first used by Moss and Thomas (SO). It was found that alloys deposited at a substrate temperature of 450°C followed by annealing for one hour at the same temperature were substantially homogeneous. Electron microscopy revealed that all compositions were subject to preferred orientation (Section III). 

Thermionic cathodes consist of a directly heated tungsten hairpin cathode at = 2500 -3000 K, or an indirectly heated pointed rod of lanthanum or cerium hexaboride (LaB, CeB(,) at 1400 - 2000 K. The electrons must overcome the work function of 4.5 eV (W) or 2.7 eV (LaBfe) by thermal activation (Fig. 78, curve a). Between the cathode at the potential -V and the grounded anode, a negatively biased Wehnelt electrode forms a crossover of diameter 20-50 pm (W) or 10-20 pm (LaBe) as an effective electron source. The emitted electrons show an energy spread A = 1 - 2 eV (W) or 0.5- 1 eV (LaBft). A measure of the quality of an electron gun is the axial gun brightness [i ... 

The SEM uses a beam of electrons to scan the surface of a sample and build a three-dimensional image of the specimen. Electrons are generated in the electron gun. The most commonly used is a tungsten-hairpin gun in which a tungsten... 

Source Tungsten hairpin LaBf Schottky field emission Coldfield emission... 

A simple spectrometer that we have used successfully is shown in Figure 2. Electrons from an electron microscope hairpin tungsten filament are focused with an Einzel lens onto the monochromator entrance slit, pass through the monochromator and exit slit, and are focused on the sample s surface by additional electrostatic... 

A tungsten wire filament may be bent into a V shape (a hairpin filament, figure 5.3) and its typical lifetime at 100 kV operating voltage is 100 h. LaB6 crystals are grown with a (110) orientation in order to enhance their electron emission, and their corresponding lifetime is of the order 500 h. 

A needle source consists of a hairpin filament (M80 pm diameter), usuafly of a refractory metal such as tungsten, with a short length of smaller diameter (M25 pm) wire spot-welded to it, Figure lb. The tip of the latter wire, the emitter, is electrochemically etched to a point with a radius of curvature at the apex of 2-5 pm the etching technique for tungsten has been described in detail by others (7,29). As quickly as possible after the assembly is thermally cleaned under vacuum (n<10 " Pa), the emitter is dipped into a molten pool of liquid metal and then withdrawn. If done correctly, the junction formed by the bend in the filament and the emitter wire will hold a small bead of metal, and the emitter will appear shiny from the thin film of metal on its surface. 

Figure 14.28 Field emitter of single crystal tungsten is shown (left) on its supporting hairpin and (right) in a close up view of the tip. (From Stinger.)...

Glass reactors described by CampbelP each contain a hairpin-shaped filament, 24 in. long, attached to tungsten electrodes which are sealed into the glass. The filament is of pure titanium, not more than about 0 003 in. diameter, suspended vertically, and a second one is sealed into the reactor in a similar manner in case of premature failure in use. A small tungsten weight is attached to the lowest point of the filament to maintain it in a vertical position. 

A critical feature that controls the performance of the SEM, and is a decision made on purchase, is the type of electron gun used as a source for the electron beam. Originally, all electron microscopes used a hot tungsten filament in the form of a simple bent wire—a hairpin. This is a... 

For the FIM experiments, one of the composite specimens was fixed on a hairpin shaped tungsten filament (diameter 0.3 mm) with carbon binder, and it was introduced into an ultra high vacuum (UHV) chamber of 3 x 10 Pa. The detector composed of microchannel plates and a screen for the FIM was placed at a distance of 50 mm from an electron emitter. In order to clean the surface of the CNT tips, the composite specimen was heated to 1000 K in the UHV ehamber. The experiments were carried out at the specimen temperature of 30 K. 

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