Electrochemical and optical properties of silane layers grown on anodized zinc

P.R. SERÉ; J.O. ZERBINO; C.DEYA; W, EGLI; C.I. ELSNER; A.R. DI SARLI

The Hague

Congreso; 67th Annual Meeting of the International Society of Electrochemistry. Electrochemistry; 2016

ISE

Organo-functional silanes improve the adhesion between organic polymers and inorganic materials and are used for the development of integrated circuits in the transistors industry, in wear-resistant coatings, solid oxide fuel cells and oxygen sensors.Films of mercaptopropyltrimethoxysilane (MPTMS) are prepared by hydrolysis, condensation and curing at 80oC on zinc and steel [1]. The metals were polished with diamond paste up to 0.25 m to achieve a specular surface. Silane coatings were applied by dipping the metallic samples into MPTMS solutions of two concentrations 8% or 4% for 60 s.On bare zinc, the poor wetting of the surface hinder the silane film formation. Different anodic treatments of zinc were used to improve the silane covering.The optical parameters for silane deposition after two or three coating steps are fitted assuming an isotropic and homogeneous film. The global treatment of data (obtained at different wavelengths (, and incident angles (  65o, 69o and 74o) increases the accuracy of the calculated optical parameters leading to a univocal mathematical solution, which becomes independent of the initial parameters used in the fitting procedure.The optical indices, n, k and the thickness d are calculated using ?in-situ? the ellipsometry technique in the case of anodic oxide formation on the zinc surfaces and ?ex-situ? measurements for the silane coverings. A programme is developed to fit a wide set of ellipsometric  and data in the visible optical region 400 nm <  < 600 nm [2].Several researchers have used ellipsometry to estimate the thickness of coats assumed as non-absorbing layers, i.e. optical absorption k ≈ 0. However, the presence of pores can produce an increase of k, which, in turn, can have a significant effect upon the calculated n and d values. An increase in the optical absorption k is detected for the lower concentration of MPTMS related to that obtained for higher silane concentrations and it is attributed to the higher light absorption from the pores of dimensions of the order of the wavelength.For isotropic materials, the n and k dependence on the composite volume fractions, Q, can be tested with effective medium theories (EMT). Several models are proposed including the Maxwell-Garnett theory, the Bruggeman effective medium approximation, and the Volume Averaging Theory (VAT) [3]. The VAT calculations allow modeling the optical absorption for pore size of the order of the wavelength. In these conditions, the effective refraction index can be smaller and the absorption index higher than those of the dispersed phases.The decrease in the precursor concentration produces coats with higher porosity and optical absorption. The higher optical indices and relatively lower absorption probably indicate that a higher solvent amount is retained within the most compact silane layers obtained using higher precursor concentration.