Metastable Coatings

Thermodynamic Prediction of the Formation and Composition Ranges of Metastable Coating Structures in PVD Processes... 

Table 3 Calculated composition ranges for the cubic and hexagonal metastable coating phases in various MeN-AlN systems at 500 K...

The microstmcture and imperfection content of coatings produced by atomistic deposition processes can be varied over a very wide range to produce stmctures and properties similar to or totally different from bulk processed materials. In the latter case, the deposited materials may have high intrinsic stress, high point-defect concentration, extremely fine grain size, oriented microstmcture, metastable phases, incorporated impurities, and macro-and microporosity. AH of these may affect the physical, chemical, and mechanical properties of the coating. 

A new form of carbon coating is now available which is neither diamond nor graphite and is known as diamond-like carbon (DLC).bl DLC can be considered as a metastable carbon produced as a thin coating with a broad range of structure and composition.l b[42]... 

On discs coated with Zdol films 62 A thick or more, droplets with lateral dimensions of tens of micrometers were observed in some areas of the surface, as shown in Figure 16. Apart from that, however, most of the surface was uniform unless disturbed by contact with the SPFM tip. After such contact, a droplet was usually formed due to capillary effects. Once formed, the droplet slowly grows in volume, indicating that the film was in a metastable state. However, no detectable layering in the surrounding region was observed. 

Adsorbed layers, thin films of oxides, or other compounds present on the metal surface aggravate the pattern of deactivation of metastable atoms. The adsorption changes the surface energy structure. Besides, dense layers of adsorbate may hamper the approach of metastable atom sufficiently close to the metal to suppress thus the process of resonance ionization. An example can be work [130], in which a transition from a two- to one-electron mechanism during deactivation of He atoms is exemplified by the Co - Pd system (111). The experimental material on the interaction of metastable atoms with an adsorption-coated surface of... 

The results of work [ 135] are of specific interest. The work surveyed the influence of the nature and structure of adsorbed layers upon the mechanism of deactivation of He(2 S) atoms. It has been shown that on a surface of pure Ni(lll) coated with absorbed bridge-positioned molecules of CO or NO, the deactivation of metastable atoms proceeds by the mechanism of resonance ionization with subsequent Auger-neutralization. With large adsorbent coverages, when the adsorbed molecules are in a position normal to the surface, deactivation proceeds by the one-electron Auger-mechanism. The adsorbed layers of C2H4 and H2O on Ni(lll) de-excite atoms of He(2 S) by the two-electron mechanism solely. In case of NH3 adsorption, both mechanisms of deactivation are simultaneously realized. Based on the given data, the authors infer that the nature of metastable atoms deactivation on an adsorbate coated metal surface is determined by the distance the electron density of adsorbate valance electrons is removed from the metal lattice. 

Palladium acetate, [PdO —02CCH3)2l3, possesses a unique quality that makes it attractive for solid state decomposition studies as well as technological applications. It can be spin-coated from solution to form a homogeneous, apparently amorphous solid film. This provides large uniform areas over which we can study the effects of various irradiation sources on the chemical nature of the film. The bulky structure of palladium acetate, shown in Figure 1 (8), may offer a partial explanation of the molecule s ability to achieve an amorphous metastable phase upon rapid evaporation of solvent. 

It is worth noting that a monotropic polymorphic system offers the potential of annealing the substance to achieve the preferred form of the thermodynamically stable phase. The use of the most stable form is ordinarily preferred to avoid the inexorable tendency of a metastable system to move toward the thermodynamic form. This is especially important especially if someone elects to use a metastable phase of an excipient as part of a tablet coating, since physical changes in the properties of the coating can take place after it has been made. Use of the most stable form avoids any solid-solid transition that could... 

Infrared absorption spectroscopy is also a powerful tool for matrix isolation studies, which have been carried out extensively for alcohol clusters [34, 88, 103]. Recently, the gap between vacuum and matrix isolation techniques for direct absorption spectroscopy has been closed by the study of nano matrices that is, Ar-coated clusters of alcohols [80]. Furthermore, alcohol clusters can be isolated in liquid He nanodroplets, where metastable conformations may be trapped [160]. 

Reactive sputter deposition is a useful technique not only for coating but also for obtaining metastable phases, especially nitrides. Nitrogen gas is activated in a plasma to enhance nitride formation and heating at high temperature is not required. It is possible to obtain metastable nitrides by sputter deposition in which species in the plasma are quenched on substrates. Compounds with different crystal structures normally do not form solid solutions, but by co-sputter deposition metastable solid solutions may form. 

In many applications, films on solid surfaces are only metastable or they are stable only from a certain thickness. Example are metal films prepared by evaporation [291] and many polymer films. Since most paints and coatings contain a substantial amount of polymer, this is a highly relevant case. 

Modern coating technologies require increasingly thinner polymer films. This requirement is opposed by the surface pressure and the chain elasticity. Below a certain equilibrium thickness, the film is either metastable or even unstable and tends to break into droplets regardless of the chemical structure of the substrate [321, 322]. Anomalous wetting behaviour was observed for amphiphilic polymer films whose stability is controlled by the orientation of the surface active moieties [323,324]. All these phenomena belong to the dewetting problem. 

In addition to the constraints mentioned above, active areas must be sufficiently long-lived and nearly fixed in position for detection by SVET. This is not normally a problem in the study of coatings where the location of active areas are often fixed by existing or emergent coating defects. However, some pitting phenomena, especially metastable pitting, may not be detected well by this technique. 

An answer to the first question was suggested by Lancet and Anders (1970). The principal meteoritic phases stable above 350-400 K (olivine, pyroxene, Fe, FeS) are not effective catalysts for the Fischer-Tropsch reaction, whereas the phases forming below this temperature (hydrated silicates, magnetite) are. P hough metallic iron is often regarded as a catalyst for this synthesis, the catalytically active phase actually is a thin coating of FCjO formed on the surface of the metal (Anderson, 1956)]. Thus CO may have survived metastably until catalysts became available by reactions such as ... 

Ray and Coombe achieved lasing of the NCI (a)/I system by scaling-up their quenching experiment. The apparatus used for this demonstration is shown in Fig. 10. An excimer laser capable of providing 700 mJ pulses at 193 nm was used to generate high densities of NCI (a) metastables. The I laser cavity consisted of mirrors coated for > 99.5% reflectivity at 1.315 m. Representative I emission signals from this device are shown in Fig. 11. 

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