Atmospheric exposure, film structures

Rider and Amott were able to produce notable improvements in bond durability in comparison with simple abrasion pre-treatments. In some cases, the pretreatment improved joint durability to the level observed with the phosphoric acid anodizing process. The development of aluminum platelet structure in the outer film region combined with the hydrolytic stability of adhesive bonds made to the epoxy silane appear to be critical in developing the bond durability observed. XPS was particularly useful in determining the composition of fracture surfaces after failure as a function of boiling-water treatment time. A key feature of the treatment is that the adherend surface prepared in the boiling water be treated by the silane solution directly afterwards. Given the adherend is still wet before immersion in silane solution, the potential for atmospheric contamination is avoided. Rider and Amott have previously shown that such exposure is detrimental to bond durability. 

Our own data obtained from energy dispersive X-ray (EDX) analysis on epitaxial siloxene films confirm this general result. Figure 1 shows the typical EDX spectrum of a 1 pm-thick siloxene film on Si(l 11), obtained by exposure to concentrated HCl at 0 °C for one hour and washed in acetone. Whereas Ca is obviously completely removed from the sample, the film still contains some Cl not entirely washed out. From the spectrum, the Si 0 ratio is determined as 2.1 1. Thus, the formation of (Si2HOH)n is confirmed to be favored compared to that of related compounds with a lower or higher oxygen content. Indeed, siloxene is relatively stable in ambient atmosphere, in contrast to polysilyne (SiH) , which is reported to combust spontaneously in air [10]. Since in the ideal siloxene structure the single Si layers are exclusively terminated by H on one side and by OH on the... 

A similar procedure was used to obtain spin-labelled TEE-HEP [49]. The presence of hexafluoropropylene (HFP) groups in this polymer leads to disturbance of the structural ordering typical of PTFE to more complex dynamics of their motion. After y-irradiation of powders and films of TFE-HFP copolymer in air, there are three types of stable peroxy macroradicals in the samples end radicals CF -CF O, secondary middle-chain radicals CF -CF(00 )-CF2, and tertiary middle-chain radicals CF2-C(CF3)(00 )-CF2. In contrast to PTFE, prolonged exposure (>100 hours) of these samples in a NO atmosphere at room temperature does not lead to the formation of aminoxyl macroradicals. However, two types of macroradicals are formed if TFE-HFP is heated with evacuation after the decay of radicals in a NO atmosphere. At 90 °C, the ESR spectrum demonstrates the presence of tertiary alkyl macroradicals CF2-C (CF3)-CF2 formed upon decay of the tertiary nitroso compounds [57]. On further increasing of the temperature up to 180 "C, the tertiary alkyl macroradicals... 

The atmospheric corrosion of zinc starts with the instant formation of a thin film of zinc hydroxide, which seems to occur in different crystal structures, and the subsequent formation of a protective layer of basic zinc carbonate, Zn5(C03)2(0H)g. The pH of the aqueous layer controls the stability of initial corrosion products and results in the dissolution of Zn +. From the HSAB principle one expects Zn, classified as an intermediate acid, to coordinate with a number of different bases. In accordance with this, the prolonged exposure of zinc can proceed along a variety of different paths of reaction sequences depending on the actual deposition rates of atmospheric constituents. Among these Cl and SO2 seem to be the most important. 

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