Metal filaments

Values of the average work function (<(), electronvolts) for the commonly used filament metals. The melting points of the metals are also shown to give some guidance as to the maximum temperature at which they can be used. Normally, the practical maximum would lie a few hundred degrees below the melting point to prevent sagging of the filament. 

Schematic diagram showing how placing a thin layer of highly dispersed carbon onto the surface of a metal filament leads to an induced dipolar field having positive and negative image charges. The positive side is always on the metal, which is much less electronegative than carbon. This positive charge makes it much more difficult to remove electrons from the metal surface. The higher the value of a work function, the more difficult it is to remove an electron. Effectively, the layer of carbon increases the work function of the filament metal. Very finely divided silicon dioxide can be used in place of carbon.

The previous discussion demonstrates that measurement of precise isotope ratios requires a substantial amount of operator experience, particularly with samples that have not been examined previously. A choice of filament metal must be made, the preparation of the sample on the filament surface is important (particularly when activators are used), and the rate of evaporation (and therefore temperature control) may be crucial. Despite these challenges, this method of surface ionization is a useful technique for measuring precise isotope ratios for multiple isotopes. Other chapters in this book discuss practical details and applications. 

There are some disadvantages for HFCVD method. Filament is relatively easy to break, and its life time is around 100 h. Second, due to the high working temperature, the filament metal element can evaporate into the gas phase and deposit in the diamond film, which is a kind of contamination (Venter and Neethling 1994). Mehta Menon s group reported that tungsten filament yielded the lowest impurity level (few ppm by mass), whereas rhenium yielded the highest (parts per thousand) (Mehta Menon et al. 1999). Metal contamination in the diamond would affect the electronic application of diamond, even in ppm level, but this is not a big problem for the electrochemical application. 

MehtaMenon, P., Edwards, A., Feigerle, C. S., Shaw, R. W., Coffey, D. W., Heatherly, L., Clausing, R. E., Robinson, L. and Glasgow, D. C. (1999), Filament metal contamination and Raman spectra of hot filament chemical vapor deposited diamond films. Diam. Relat. Mater., 8(1) 101-109. 

C. Horny, Developpement d un reacteur microstructure base sur des filaments metal-liques catalytiques. Production autotherme d hydrogene par steam-reforming oxydatif du methanol, Ecole polytechnique federate de Lausanne, Lausanne, 2005. 

No = emission rate of neutral species W = work function of the filament metal (eV)... 

In general, rollers that are 1 m wide should be about 150 mm in diameter and if 2 m wide, 350 mm in diameter, in order to ensure sufficient rigidity to restrict bending, if any, to a permissible level. Excessive bow would allow the fiber band to move towards the center line, causing fiber-bunching and a build-up of heat in the thicker tow band, which could eventually result in a fire. Rollers can be fitted with wiper blades to remove stray filaments. Metal doctor blades, though efficient, tend to be noisy and therefore, felt is preferred. 

An important milestone was a patent from 1936 describing a method of producing multi-filament metal yam (James et al., 1936). Another patent from 1936 describes heated textiles incorporating iron, nickel, chromium or stainless metal wires (Grisley, 1936). A further patent describes the use of nickel-chrome strips to electrically heat socks (Costanzo, 1936). 

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