Substrate cooling

The substrate may be heated to unacceptable levels by the high temperature of the gases and cooling is usually necessary. Temperature control and substrate cooling remain a problem in arc-plasma systems. However, deposition is rapid and efficient, high... 

There have been few studies of chemisorption on alloys, particularly where the surface has been prepared under very clean conditions, although such studies should help us to understand the catalytic process. It has been shown that homogeneous Pd-Ag alloy films can be prepared without extensive sintering 54) by simultaneous evaporation in UHV with the substrate cooled to 0°C, and so it is now possible to study chemisorption on Pd-Ag alloy films. Surface potentials accompanying CO chemisorption were measured 126) by the diode method. Comparisons of surface potentials were made after exposure to CO at 0°C at a pressure of 2 X 10-< Torr for 10 min. 

Catapult technique Molten metal is catapulted against a stationary substrate (cooling rates is about 5 104oC/s to 105-106oC/s). 

Plasma-jet spraying Alloy powders are molten in a plasma jet and spray-quenched against a cold heat-conducting substrate (cooling rate about 109oC/s). 

Takeuchi et al. prepared films by the evaporation of copper and nickel metals or their alloys on a substrate cooled by liquid oxygen. Prior to use, the film was treated in vacuum at 30 or 250°C. The catalytic activity was tested by the hydrogenation reaction of ethylene. Their result is illustrated in Fig. 8. Essentially similar results have been reported by Volter and Alsdorf... 

Often it is not the deposited material that dictates the use of cooling but the substrate itself. Substrate degradation due to annealing, oxidation, or melting must be eliminated or minimized. Because many materials cannot be deposited onto a substrate cooled sufficiently to avoid substrate degradation, other means must be developed to protect the substrate. In some instances of oxidation or thermally induced film/substrate interaction it has proven beneficial to first apply a protective base thin film layer. The desired material is then deposited on top of this layer perhaps without or minimal substrate cooling. In other cases of substrate degradation, the film must be deposited while the uncooled substrate is moved quickly... 

Coating of soft substrates, cooling after coating sufficient for martensite formation. 

Amorphous layers of light and heavy water 50-500 pm thick were obtained by condensation of vapor in vacuum on a copper substrate cooled by liquid nitrogen. The condensation rate was 50-500 pm/hour. Crystallization was detected by the method of differential-thermal analysis. In experiments with amorphous layer of light and heavy water for the same heating rate the position of the abrupt temperature jump pointing to the sample crystallization was independent within 0.5 K of both the thickness of the sample and the condensation rate in the process of its preparation. In heating amorphous layers of water at a rate of 0.25 K/s crystallization took place at T 166 K. ... 

The second type of films used in UHV studies of SERS are the films deposited onto cold substrates. Wood and Klein evaporated silver (unknown thickness) on a copper substrate cooled to 78 K and detected a surface Raman signal at 2134 cm (presumably, associated with adsorbed CO or carbonate) which was independent of CO pressure. That band was seen together with... 

Care must be present when involing a photo process in CVD. In particular, the absorption induced reaction of a vapor phase precursor molecule must be distinguished from temperature effects. Otherwise, the irreversible result is a combination of a photo-event and a pyro-event. Although substrate cooling will rule out surface mobility of atoms due to light-induced heating, care must be utilized when attributing all gas phase chemistries to purely photo events. 

The allotropic form of oxygen, ozone, can also be employed for the oxidation of saturated hydrocarbons. The reactivity of ozone without additional reagents is not sufficient for the preparative functionalization of alkanes in solution however, its reactivity is increased substantially by the addition of iron(III) chloride [6] or antimony pentafluo-ride. [7] The dry ozonation variant [8] of Mazur et al. [9] by which alkanes are hydroxy-lated at tertiary C atoms with high selectivities and yields, was shown to be especially useful. According to this method, silica gel is coated with roughly 1 wt% of the substrate, cooled to -78 °C, saturated with ozone, and subsequently allowed to warm to room temperature within 0.5-2 h. Adamantane (1) is converted almost quantitatively into 1-adamantanol (4) in this way (Table 1), and this method of oxy-functionalization has been applied successfully even on certain steroids. [10]... 

Substrate cooling is often a problem since cooling by convection is not operational in a vacuum. Substrates can be cooled by being in contact with a cooled substrate fixture. Circulating chilled water or oil, cooled water/ethylene glycol mixture (-25°C), dry ice/ acetone (—78°C), refrigerants (= — 150°C), or LN2 (—196°C) may be used as coolants in the substrate fixturing. 

Amorphous ferromagnetic Co-Au films were first produced by vapor deposition on substrates cooled to liquid nitrogen temperature, but stimulus for expanded effort in this direction resulted from the development of techniques for rapidly quenching alloys from the liquid state to produce metastable phases. These techniques are referred to as splat cooling. Magnetic and Mossbauer studies of (Pd, - tMj8oSi2o alloys, where M = Fe, Co, or Ni, and of Fe-rich Fe-P-C alloys, and magnetic and... 

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