Tungsten wires

The most connnonly used detector in EPR is a semiconducting silicon crystal in contact with a tungsten wire, which acts as an MW rectifier. At microwatt powers, crystal detectors are typically non-linear and render a... 

Boron filaments are formed by the chemical vapor deposition of boron trichloride on tungsten wire. High performance reinforcing boron fibers are available from 10—20 mm in diameter. These are used mainly in epoxy resins and aluminum and titanium. Commercial uses include golf club shafts, tennis and squash racquets, and fishing rods. The primary use is in the aerospace industry. 

The experiment may also be repeated using a platinum (indicator) electrode and a tungsten wire reference electrode. If the tungsten electrode has been left idle for more than a few days, the surface must be cleaned by dipping into just molten sodium nitrate (CARE ). The salt should be only just at the melting point or the tungsten will be rapidly attacked it should remain in the melt for a few seconds only and is then thoroughly washed with distilled water. 

The term plastic is not a definitive one. Metals, for instance, are also permanently deformable and are therefore plastic. How else could roll aluminum be made into foil for kitchen use, or tungsten wire be drawn into a filament for an incandescent, light bulb, or a 100 ton ingot of steel be forged into a rotor for a generator. Likewise the different glasses, which contain compounds of metals and nonmetals, can be permanently shaped at high temperatures. These cousins to polymers and plastics are not considered plastics within the plastic industry or context of this book. 

Figure 5.7. A diamond-coated tungsten wire that is about the same diameter as a human hair.

A further improvement was introduced by Weiss and co-workers,35 who connected the circuit between the lamellae and the lower portion of the wire through an electrolyte in a conducting beaker. This avoids introducing any forces on the tungsten wire whilst retaining the automatic etching current... 

Figure 3.13 Three methods of chemically etching metal tips for STM. In (a) the current cut-off is manually or electronically triggered when the end of the etched wire falls the finite time delay inherent in this approach results in a blunting of the final tip as etching continues after separation, (b) This shows an adaptation in which the etching current is automatically cut off when the lower portion of the wire drops - it is the lower portion that is used as an STM tip. (c) This shows an improved design in which the etching current is fed to the lower portion of the tungsten wire through an electrolyte held in a conductive beaker. In this case the upper portion of the etched wire is kept.

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. 

Specimens for field emission sources are of a very fine needle shape, usually in the form of tungsten wire with a tip radius of <0.1 pm (Figure 5.4). Application of a potential of lkV thus generates a field of 106V/m which lowers the work function barrier sufficiently for electrons to tunnel out of the tungsten. FEG electron microscopes usually employ a gun potential of 3-4 keV. 

Fig. 1. Schematic diagram of the multimass ion imaging detection system. (1) Pulsed nozzle (2) skimmers (3) molecular beam (4) photolysis laser beam (5) VUV laser beam, which is perpendicular to the plane of this figure (6) ion extraction plate floated on V0 with pulsed voltage variable from 3000 to 4600 V (7) ion extraction plate with voltage Va (8) outer concentric cylindrical electrode (9) inner concentric cylindrical electrode (10) simulation ion trajectory of m/e = 16 (11) simulation ion trajectory of rri/e = 14 (12) simulation ion trajectory of m/e = 12 (13) 30 (im diameter tungsten wire (14) 8 x 10cm metal mesh with voltage V0] (15) sstack multichannel plates and phosphor screen. In the two-dimensional detector, the V-axis is the mass axis, and V-axis (perpendicular to the plane of this figure) is the velocity axis (16) CCD camera.

Fig. 3. Exit port of the energy analyzer. (1) Stainless steel plates to support the ceramic plates (2) grooved ceramic plates to support the concentric cylindrical electrodes (3) inner cylindrical electrode (4) 30 pm diameter tungsten wire (5) 15 concentric cylindrical electrodes separated equally between the inner and outer cylindrical electrodes (6) outer cylindrical electrode.

Figure5.5 Setupforthetip production byelectrochemicaletching of a tungsten wire. (Reproduced with permission from Ref. [21].)...

All the STM results from our group presented in this chapter employed the variable temperature STM, with tips made by electrochemical etching of tungsten wire. For noncontact AFM (NC-AFM), we employ commercial conducting silicon cantilevers with force constants of approximately 2-14 rn 1 and resonant frequencies of approximately 60-350kHz (Nanosensors and Mikromasch). The NC-AFM images we present here were recorded in collaboration with Professor Onishi at Kobe University and employed a UHV JEOL (JSPM-4500A) microscope. 

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