Deposition cold surface

With a typical ablated particle size of about 1 -pm diameter, the efficiency of transport of the ablated material is normally about 50% most of the lost material is deposited on contact with cold surfaces or by gravitational deposition. From a practical viewpoint, this deposition may require frequent cleaning of the ablation cell, transfer lines, and plasma torch. 

SiO exists only as a vapor and reforms Si02 particles when it deposits on cold surfaces (97). 

Thermo-diffusion calculations analyze the migration of hazardous material from compartment to compartment to release in containment. These calculations use physico-chemical parameters to predict the retention of hazardous materials by filtration, deposition on cold surfaces and other retention processes in the operation. Containment event trees aid in determining the amount, duration and types of hazardous material that leaves the containment. 

Thermal-Gradient Infiltration. The principle of thermal-gradient infiltration is illustrated in Fig. 5.15b. The porous structure is heated on one side only. The gaseous reactants diffuse from the cold side and deposition occurs only in the hot zone. Infiltration then proceeds from the hot surface toward the cold surface. There is no need to machine any skin and densification can be almost complete. Although the process is slow since diffusion is the controlling factor, it has been used extensively for the fabrication of carbon-carbon composites, including large reentry nose cones. 

The matrix IR spectrum of deposited SiO shows absorptions arising from monomeric (1226 cm1), dimeric (Si202 803, 767 cm 1, D2h symmetry), and trimeric SiO (81303 973 cm"1, symmetry) [11]. The oligomers are formed from monomeric SiO mainly during the deposition process on the cold surface. - After deposition of SiS only the absorptions of monomeric (739 cm"1) and dimeric SiS (504, 469 cm 1) are observed [12],... 

An obvious correlation between polar and alpine environments is the decrease in temperature with increasing latitude or elevation. This temperature change leads to a shift in environmental phase distribution equilibria - i.e. a chemical moves from the atmosphere to terrestrial surfaces, including direct deposition to surface waters, but also to snowpack and soils from which movement into surface and groundwater is possible. This process has been termed cold condensation but should more correctly be called cold-trapping because the contaminants are not actually condensing. 

Finely divided metal samples can also be prepared in the form of evaporated films in high vacuum, usually deposited on IR-transparent alkali halide plates (76-78). Such spectra are of interest in themselves, but tend to be much weaker than those obtained from the metal-particles-in-depth, oxide-supported catalysts. The rough surfaces of films of Cu, Ag, and Au, prepared by deposition on cold surfaces, can lead to very high-quality surface-enhanced Raman spectra (27, 28, 79, 80). The results from such experiments will be discussed in the later sections devoted to particular adsorbed hydrocarbons and metals, alongside the majority of spectra that are obtained on oxide-supported samples. 

Deposition also occurs when frost forms on chilly winter mornings. The water vapor in the air comes in contact with a super-cold surface, such as the windshield of a car, and freezes immediately into tiny ice crystals. Because of the cold temperatures a liquid never forms, and the water vapor changes directly into a solid. 

Normal Raman laser excitation in the visible and NIR region (52) can be used to obtain the SERS effect. The substrate surface is extremely important in providing the necessary enhancement to make the technique as valuable as it has become. A number of substrates have been used (53). These include evaporated silver films deposited on a cold surface at elevated temperature ( 390 K) on a glass substrate, photochemically roughened surfaces (e.g., silver single crystals subjected to iodine vapor, which roughens the surface), grating surfaces, and mechanically abraded and ion-bombarded silver surfaces. 

In the technique of matrix isolation (hereafter denoted "MI"), samples which are liquid or solid at room temperature are vaporized under vacuum, and then mixed with a large excess of a diluent gas (termed the "matrix gas") which in effect, is the "solvent" in the spectroscopic analysis. This gaseous mixture is then deposited on a cold surface for spectroscopic analysis as a solid For most purposes, temperatures of 15 K (which can be obtained by use of commercial closed-cycle refrigerators) are satisfactory for some specialized fluorescence experiments,... 

Single molecules (and not surface artifacts or specs of dust) are easier to see when the surface can be cleaned in ultra-high vacuum at low temperature, imaged to make sure no artifacts are present, and then volatile adsorbates are injected onto the cold surface without breaking vacuum. A nice tour de force was when at 4 K under ultra-high vacuum, Xe atoms were picked up by the STM tip and deposited onto a cold Ni substrate to "write" "IBM" on Ni [28]. It is not too clear exactly how far above the surface the atomically tip floats a good guess is 0.1 nm. 

The original Marsh test involved burning and deposition of the arsenic upon a cold surface. Nowadays the mirror test is usually applied. The silver nitrate reaction (sometimes known as Hofmann s test) is very useful as a confirmatory test. 

In many matrix isolation studies, including those which employ high temperature evaporative ovens, two reactants are allowed to mix, in a large excess of inert gas, immediately in front of the cold deposition surface. The gas phase mixture is rapidly quenched and condensed onto the cold surface, usually maintained between 10 and 20 K, so that very few collisions can occur between reactive partners, and the initial reaction products of the system are isolated. When photolysis is employed in the formation of intermediate species, irradiation can be carried out either during deposition of the reaction mixture, or after the inert gas matrix is rigidly frozen in place. 

Chemical vapor deposition of polynapthalene differs from the parylene and polyimide systems in terms of the deposition process. As mentioned in earlier sections deposition of parylenes and polyimides occurs on cold surfaces, and the deposition rate decreases with increasing substrate temperature. In other words, deposition is done in a "hot wall" reactor. In contrast, CVD of polynapthalenes is performed in a "cold wall" reactor, meaning that the substrate is maintained at a high temperature ( 350°C) while the surrounding wall temperature is kept at near room temperature. A schematic of the CVD reactor setup employed by Lang et al. can be found in Ref. 28. 

In the panicle synthesis, metal atoms produced by the heating collide with the inert gas atoms to decrease the diffusion rate of the atoms from the source region. The collisions also cool the atoms to induce the formation of small clusters of fairly homogeneous size. The clusters grow mainly by cluster-cluster condensation to give nanopaiticles with a broader size distribution. A convective flow of the inen gas between the warm region near the vapour source and the cold surface carries the nanopanicles to the cooled finger, where they are let to deposit. The inen gas pressure, the evaporation rate, and the gas composition can control the characteristics... 

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