Resistive heating evaporation

Resistive Heating Combined with Reaction Zone. The simplest way to form nitride UFPs on a laboratory scale is to adopt the reactive gas condensation method with a resistive heating evaporator. UFPs of the Fe-N system were synthesized by evaporating iron from a tungsten basket heated to 1600-2300°C in NH gas pressure of 27 kPa (37). Crystalline phases of a-Fe, y-Fe including about 2.5wt% of nitrogen, and y -Fe4N depend on the NH3 gas pressure and the evaporation temperature. 

Various types of resistance heated evaporation sources. 

This method is most common. The evaporation material is placed in a container made of Mo, , W or C which can be in the form of a boat, crucible, coil or strip. In some cases ceramic crucibles or inserts are also employed, made of AI2O3 and BeO, BN or BN/TiB2. The container is heated by current flow and the material is evaporated or sublimed from this. Various types of resistance-heated evaporation sources are shown in Fig. 53. Undesired but possible chemical reactions can cause film contamination [274]. The formation and evaporation of vapourizable compounds of the boat material upon contact of the hot boat wall, with reactive gases, as well as with some reactive and/or decomposable film material, is not always low [275], but can, however, often be avoided by appropriate choice of the evaporation source material and by special pre-treatment. A further evaporation method uses radiation heating. The radiationheated vapour source consists generally of a resistance-heated spiral radiator of tungsten wire which is mounted above the evaporant surface in an open crucible. It can be used for the evaporation of easy volatilised materials. 

Some simple metal halides can be evaporated practically without any dissociation as shown in Table 9a, the mass spectrum of MgF2 vapour above a resistance heated evaporation source [294]. Complex types decompose into the components and show a fractionation so that the films are no longer stoichiometrically composed, as can be seen for example in Tab. 9b for cryolite Na3(AlF6) —> 3NaF + A1F3, [294,295]. NaF is found in the films and by recombination the desired Na3(AlF6) is formed as well as various amounts of Na(AlF4) [296] as can be seen in Fig. 60. 

Horizontal cylindrical coaling unit with spiral shaped resistance heated evaporators. 

Phtovoltaic cells were fabricated from the higher T films by depositing CdS from a resistance heated evaporator over the rf-sputtered films. The best a device having an area of 0.2 cm ... 

Resistance heating (evaporation) The Joule or I R heating of an electrical current (1) passing through a material having an electrical resistance (R). 

Vacuum Deposition. Vacuum deposition, sometimes called vacuum evaporation, is a PVD process in which the material is thermally vaporized from a source and reaches the substrate without coUision with gas molecules in the space between the source and substrate (1 3). The trajectory of the vaporized material is therefore line-of-sight. Typically, vacuum deposition takes place in the pressure range of 10 10 Pa (10 10 torr), depending on the level of contamination that can be tolerated in the resulting deposited film. Figure 3 depicts a simple vacuum deposition chamber using a resistively heated filament vaporization source. 

Uses. In spite of unique properties, there are few commercial appUcations for monolithic shapes of borides. They are used for resistance-heated boats (with boron nitride), for aluminum evaporation, and for sliding electrical contacts. There are a number of potential uses ia the control and handling of molten metals and slags where corrosion and erosion resistance are important. Titanium diboride and zirconium diboride are potential cathodes for the aluminum Hall cells (see Aluminum and aluminum alloys). Lanthanum hexaboride and cerium hexaboride are particularly useful as cathodes ia electronic devices because of their high thermal emissivities, low work functions, and resistance to poisoning. 

Nonferrous alloys account for only about 2 wt % of the total chromium used ia the United States. Nonetheless, some of these appHcations are unique and constitute a vital role for chromium. Eor example, ia high temperature materials, chromium ia amounts of 15—30 wt % confers corrosion and oxidation resistance on the nickel-base and cobalt-base superaHoys used ia jet engines the familiar electrical resistance heating elements are made of Ni-Cr alloy and a variety of Ee-Ni and Ni-based alloys used ia a diverse array of appHcations, especially for nuclear reactors, depend on chromium for oxidation and corrosion resistance. Evaporated, amorphous, thin-film resistors based on Ni-Cr with A1 additions have the advantageous property of a near-2ero temperature coefficient of resistance (58). 

The size distribution of the clusters produced in the cluster source is quite smooth, containing no information about the clusters except their composition. To obtain information about, for example, the relative stability of clusters, it is often useful to heat the clusters. Hot clusters will evaporate atoms and molecules, preferably until a more stable cluster composition is reached that resists further evaporation. This causes an increase in abundance of the particularly stable species (i.e., enhancing the corresponding peak in the mass spectrum, then commonly termed fragmentation spectrum ). Using sufficiently high laser fluences (=50 /iJ/mm ), the clusters can be heated and ionized simultaneously with one laser pulse. 

Other uses are in thin film technology where coatings are applied by vacuum deposition. Tungsten boats, or coils fabricated from wire are heated by direct resistance heating and used to evaporate Ag, Al, Au, B, Ba, Ce, Cr, Fe, In, Mg, Mn, Ni, Pa, Pt, SiO, Te, V, Zn and Zr. 

Ion Vapour Deposition A variant of the process is ion vapour deposition, in which a high negative potential is applied to the workpiece during chemical vapour deposition. The process has been employed on a commercial scale chiefly for depositing aluminium on to steel and titanium in the aerospace industry as an alternative to cadmium plating, which is liable to cause hydrogen embrittlement, especially of high tensile steel components. The aluminium is evaporated from a wire-fed resistance-heated boat . 

To evaporate the source material, various heating methods are used such as resistance heating, electron beam, pulsed excimer laser, or cathodic arc (where the source is the cathode). 

A rotary apparatus is used for metal evaporation as shown in Fig. 1. Similar equipment is employed for the preparation of organometallics. For Au clusters, Au metal is evaporated from a resistively heated W boat onto an ethanol film, in which the reactants, e.g., AufPAr IX and/or PAr, are dissolved (see Table 1, method B, 8.2.2.2). 

Most of the work initially was with the more volatile transition metals, i.e. the first row metals plus palladium, silver and gold, because these were easy to evaporate in reasonable quantities in simple apparatus. However, efforts to use the less volatile metals of the second and third rows gained momentum. Skell used sublimation of resistively heated wires of molybdenum and tungsten to make the remarkably stable [Mo(rj4-C4H6)3] and [W(1j4-C4H6)3] (42). Green... 

There are different types of DEPs in that some of them rely on conductive heating from the ion source block and/or the heated tip of a modified DIP, [49] and others - now widespread in use - that are capable of rapid resistive heating of a little loop made of chemically inert metal wire (rhenium). Resistively heated probes allow rates of several hundred °C s and temperatures up to about 1500 °C (Fig. 5.16). As a consequence of rapid heating, fast scanning, e.g., 1 s per scan over the m/z range of interest, is required to follow the evaporation of the analyte. 

Cl in conjunction with a direct exposure probe is known as desorption chemical ionization (DCI). [30,89,90] In DCI, the analyte is applied from solution or suspension to the outside of a thin resistively heated wire loop or coil. Then, the analyte is directly exposed to the reagent gas plasma while being rapidly heated at rates of several hundred °C s and to temperatures up to about 1500 °C (Chap. 5.3.2 and Fig. 5.16). The actual shape of the wire, the method how exactly the sample is applied to it, and the heating rate are of importance for the analytical result. [91,92] The rapid heating of the sample plays an important role in promoting molecular species rather than pyrolysis products. [93] A laser can be used to effect extremely fast evaporation from the probe prior to CL [94] In case of nonavailability of a dedicated DCI probe, a field emitter on a field desorption probe (Chap. 8) might serve as a replacement. [30,95] Different from desorption electron ionization (DEI), DCI plays an important role. [92] DCI can be employed to detect arsenic compounds present in the marine and terrestrial environment [96], to determine the sequence distribution of P-hydroxyalkanoate units in bacterial copolyesters [97], to identify additives in polymer extracts [98] and more. [99] Provided appropriate experimental setup, high resolution and accurate mass measurements can also be achieved in DCI mode. [100]... 

Copper is best evaporated from an alumina crucible of 2-5 mL capacity, resistance heated either by an external molybdenum or tungsten wire spiral or, preferably, by imbedded molybdenum or tungsten wire (See Sec. 11). If the crucible is insulated with a 10-mm layer of a refractory wool, for example, Kaowool (see Sec. 11), a power input of 180-250 W will heat the crucible to 1400° and evaporate the copper. The power required to give the desired rate of evaporation has to be determined for a new crucible by a trial evaporation in the absence of BC13. The volt and amp settings then found should be valid for many runs with the same crucible. An ordinary grade of soft copper rod is satisfactory... 

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