PSI - Issue 50

S.A. Filin et al. / Procedia Structural Integrity 50 (2023) 91–99 S. A. Filin at al. / Structural Integrity Procedia 00 (2022) 000 – 000

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(Makshantsev et al. (1985)). Therefore, insufficient cleaning of the surface from impurities can at a certain stage cause a decrease in the optical damage as a result of a local increase in absorption. Therefore, probably, other azeotropes did not allow reaching the maximum value of optical damage, due to the increase in surface roughness during cleaning. While cleaning with azeotropes (№ 9 -11, 17) probably made it possible to reveal a process close to optimal - the removal of the bulk of absorbing impurities. In this case, it is ensured that the roughness is smoothed by the remaining impurities. They cause a decrease in optical damage. Cleaning with azeotropes (№№ 9 -11, 17) made it possible to reveal a composition close to the optimal variant, which ensures the removal of the bulk of impurities while achieving an acceptable roughness. An abnormally high threshold was reached after purification with the azeotrope "freon-114B2 - acetone" (№ 17) because it contains the maximum number of fluorine atoms compared to other azeotropes - 4 (for comparison, freon 113 has 3, freon- 112 - 2 fluorine atoms). At the same time, there is little other component in the azeotrope (97.6 weight % freon- 114B2 (№ 17)) compared to other azeotropes and the polar selectivity of the components is the same as in the components of the model contaminant. Fluorine compounds are adsorbed on the surface (Filin et al. (2019); Ilyin et al. (1995)) during cleaning due to extremely high values of the ionization potential and ionization energy (Ageev and Volkov (2015)). They provide an increase in the optical damage of the metal (Filin et al. (2020)) and prevent, under the action of a laser, the growth of the electron concentration in the space above the surface to values corresponding to the ionized gas (Lidin et al. (1987); Gurvich et al. (1974)). If there is a significant emission of electrons into the space near the mirror, these electrons will be absorbed by freon-114B2 for some time. Freon vapor is present in front of the mirror as a result of laser evaporation. This is due to the significant electron affinity of the fluorine and bromine atoms that make up freon-114B2 (Rambidi and Berezkin (2008)). Work function data for some chemical elements in terms of electron affinity are presented in (Gurvich et al. (1974); Tables of physical quantities: Handbook (1976). After cleaning the mirrors with an azeotrope of "freon-114B2 - acetone", the probability of optical damage is significantly reduced. This is due to the removal of a significant part of organic impurities from the surface. The results of cleaning from model contaminants showed that it is most effective, at a content of 45 weight % pekkanifol re sin in contaminent. They are removed with azeotrope № 12, and when the resin content in the contamination is 30 weight % - with azeotropes № 13 and № 14. With a resin content in the contamination of 15 weight percentage, aze otrope № 17 is most effective. The maximum optical damage, close to the calculated one for copper (1.0), was obtained for model contaminant with a peccanifol resin content of 45-weight percentage , when treated with azeotropes № 13 and № 14. For model contaminant with a resin content of 30 weight, percentage the maximum optical damage was obtained, when treated with azeotropes № 15 and № 16, and for contaminants with a content of pekkanifol resin 15-weight percentage - when treated with azeotrope № 16. Optical damage values close to those calculated for copper were obtained for similar model contaminants on all mirrors, when cleaning with azeotrope № 17 (Fig. 1, curves 3, 4). It was found that the maximum value of optical damage is provided by azeotropes, as a rule, with the polar selectivity of the components corresponding to the polar selectivity of the contaminant components, but with a solubility parameter, that differs from the experimentally determined solubility parameter of the model contaminant. In absolute value, the difference bet ween the solubility parameters δ of the azeotrope to be cleaned and contamination, at which the most effective cleaning of the mirror is achieved, should be at least 0.3-1.0 J- 1/2∙cm 3/2. It also requires the presence in the azeotrope of a fluorine-containing component with the highest possible relative content of fluorine. The asymmetry of the curve sections (Fig. 1) with respect to the optimal value of the solubility parameter, at which the maximum optical damage is reached, is associated with a higher ionization energy and electron affinity of fluorine-containing molecules and radicals compared to chlorine-containing ones. Their properties affect the optical damage both on the surface and near it. Let us note the expediency of repeated use of the most effective azeotrope "freon-114B2 - acetone" (№ 17 in Table 1), which provides the most maximum and fastest removal of technological impurities in a wide range of composition of technological impurities when cleaning mirrors. The use of compounds containing hydroxyl groups (phenol, tert-butanol) as stabilizing additives can lead to a decrease in the destruction of freons. In (Bronin and Chernov (1978)) tert-butanol was used as a Freon-114B2