Equilibrium of oxide forms of copper in colored optical glasses

Abstract

The formation of the equilibrium Cu(I) « Cu(II) in colored optical glasses under the combined action of several factors is examined. It is shown that the oxidation of copper intensifies with increasing total copper content by weight, increasing content of carbonates in the mix, and increasing basicity of the matrix and depends primarily on the ratio of chromium, arsenic, and copper oxides. A factor is proposed for making a quantitative assessment of the combined effect of the chromium and arsenic oxides on the copper equilibrium.Colored optical glass is used in instrument building to produce color filters that regulate light transmission in the visible and nearby IR and UV regions of the spectrum. To obtain the require spectral characteristics copper oxide is introduced into the mix for blue (BG), blue-green (BGG), and violet (VG) glasses and copper oxide together with chromium oxide is introduced into green (GG) and yellow-green (YGG) glasses (OST 3-4375-79). Up to 0.1% As 2 O 3 (molar content) is introduced into individual compositions in order to correct the optical and operational properties.The following expression is used to evaluate the specific index of absorption c l of a coloring element in glass in order to make a quantitative evaluation of the intensity of absorption by the element:, where t l is the light-transmission of the glass sample, fractions of unity; D rm is a correction for reflection, calculated according to the relation [1] D rm = -2log(1 -r ); l is the thickness of the sample, cm; and, m Me(total) is the content by weight of the coloring element in the glass in terms of metal, %. The scattering index r is related with the index of refraction n of the glass by the expression [1] r = ( ) ( ) n n -+ 1 1 2 2 .Copper and chromium are d elements. In silicate glasses each element coexists in two valence forms: Cu(I), Cu(II) and Cr(III), Cr(VI). Their spectral curves of the specific absorption index c l are presented in Fig. 1 [2]. Univalent copper has no absorption bands in the visible and closest adjoining regions of the spectrum, and Cu(II) forms two complexes with oxygen -a quadruply coordinated "yellow" complex absorbs in the short-wavelength region and a "blue" complex [Cu(II)O 6 ] possesses an intense band at 750 -800 nm.Hexavalent chromium intercepts radiation in the blue part of the spectrum, while tetravalent chromium has two strong bands in the visible region near 460 -480 and 660 -680 nm.It is evident that the spectral curve and the light transmission of glass colored with copper or with a combination of copper and chromium will depend considerably on the ratio of the oxide forms of these elements, i.e., on the equilibrium Cr(III) Cr(VI), Cu(I) Cu(II).The equilibrium between the oxidized and reduced forms of the d elements has been studied in greatest detail for the iron impurity most often encountered in glass and depends on the set of parameters of the technological process: amount of d element, acid -base properties of the glass matrix, oxidation -reduction and temperature -time co...