SCIENCE FOR GLASS PRODUCTION
UDC 666.221:541.53
EQUILIBRIUM OF OXIDE FORMS OF CHROMIUM
IN COLORED OPTICAL SILICATE GLASSES
A. B. Atkarskaya
1
Translated from Steklo i Keramika,No.2,pp.3–7,February, 2011.
The formation of the equilibrium Cr(III) « Cr(VI) in colored optical glasses under the combined effect of
acid-base properties of the matrix, the total concentration of chromium, and the amount of other variable-va
-
lence elements which are in the oxidation – reduction series to the right of chromium is examined. It is shown
that this equilibrium is related with the change of the spectral characteristics of the glass.
Key words: colored optical glass, oxidation-reduction equilibrium of chromium, basicity of glass, color trans
-
mission of glass, absorption coefficient.
Colored optical glass is used for developing light filters
in instrument building instrument that select or cut-off a defi-
nite range in the visible and near IR and UV ranges of the
spectrum. In the group of glasses colored yellow (YG), yel-
low-green (YGG), green (GG), and infrared (IRG), the re-
quired spectral characteristics are obtained by introducing
into the mix copper oxides or chromium oxide in combina-
tion with copper and manganese oxides. These colored
glasses are obtained in the system R
2
O–RO–SiO
2
, where
R
2
O denotes potassium and sodium oxides, RO — CaO,
BaO, ZnO, and MgO [1]. The molar content of lead oxide
does not exceed 5%; the PbO content is 35% only in
YGG-19. Up to 4% B
2
O
3
and 0.1% As
2
O
3
(molar content)
are introduced into individual compositions in order to adjust
the technological and service properties.
To obtain a quantitative evaluation of the absorption in
-
tensity of any colored element in glass the specific absorp
-
tion coefficient (SAC) c
l
of the element is calculated as
c
l,Fe
=
--log t
lr
2D
lm
m
Fe(total)
, (1)
where t
l
is the light transmission of the glass samples in
units starting with 1; D
rm
is the correction for reflection; l is
the thickness of the sample, cm; and, m
Me(total)
is the mass
content of the coloring element in the glass in terms of the
metal, %.
The correction to D
rm
is calculated from the relation [2]
D
rm
= – 2log (1 – r ). (2)
The scattering index r is related with the refractive index
n of glass by the following relation [2]:
r =
()
()
n
n
-
+
1
1
2
2
. (3)
Chromium, copper, and manganese belong to the family
of d elements, and in silicate glass they coexist in two va
-
lence forms: Cr(III) and Cr(VI), Cu(I) and Cu(II), Mn(II) and
Mn(III). The spectral curves of their specific absorption co
-
efficient c
l
are presented in Fig. 1 [3]. We note that univalent
copper and bivalent manganese do not absorb in the visible
and near regions of the spectrum. The Cu(II) ion forms two
complexes with oxygen: quadruply coordinated “yellow”
[Cu(II)O
4
] absorbs in the short-wavelength region and
“blue” [Cu(II)O
6
] possesses a strong band in the 750 –
800 nm region.
Figure 1 shows that the course of the spectral curve and
the light transmission of the glass colored by chromium
alone or in combination with copper and manganese should
depend strongly on the relation between the oxide forms of
these elements, i.e., on the equilibria Cr(III) Cr(VI),
Cu(I) Cu(II), and Mn(II) Mn(III).
The equilibrium between the oxidized and reduced forms
of the d elements in glass has been studied in greatest detail
for iron and depends on many parameters of the technologi
-
cal process: the amount of the d elements, the acid-base
Glass and Ceramics, Vol. 68, Nos. 1 – 2, May, 2011 (Russian Original, Nos. 1 – 2, January – February, 2011)
39
0361-7610/11/0102-0039 © 2011 Springer Science+Business Media, Inc.
1
Affiliate of the V. G. Shukhov Belgorod State Technological Uni
-
versity, Novorossiisk, Russia (e-mail: atkarsk06@mail.ru).