STUDIA UNIVERSITATIS BABES-BOLYAI, PHYSICA, SPECIAL ISSUE, 2003
MICROSTRUCTURAL INVESTIGATIONS OF NEW VITREOUS MATERIAL
FOR CERAMIC INDUSTRY
Cezara Voica1, Liana Gagea3, E. Indrea2, Simina Dreve2 0and I. Bratu2
1
S.C.CEROC S.A., 1 Treboniu Laurean st., 3400 Cluj-Napoca,
Romania
2
National R&D Institute of Isotopic and Molecular
Technologies, P.O. Box 700, R-3400 Cluj-Napoca 5, Romania
3
'Babes-Bolyai' University, 1 Kogalniceanu st., Faculty of
Chemistry, Cluj-Napoca, Romania
Abstract
Colorants and glazes in ceramic industry is using vitreous materials
named “frits”, containing usually rather large amounts of lead. The necessity of
echological and safe products imposed lately the synthesis of new frits lead-free,
having as much as possible, the same structure and properties.
We report here a new frit-type vitreous lead-free material for fabrication
of colorants and glazes in ceramic industry. The chemical composition of the
oxidic mixtures for the classical frit F1 and for the new frit and the technological
procedures for fabrication are presented.
The physical properties as melting point and dilatation coefficient, the
specific surface and the optical properties were determined in correlation with the
microstructural characteristics.
Introduction
In traditional ceramics the presence of lead in frits have a very important
role: to give the hardness and shining properties and to realise the dispersion of the
decoratingcolours in thin layers. Despite this important properties the problems of
the us of lead in frits composition (low chemical resistance, pollution, toxicity)
imposed the elaboration of new frits, replacing the lead oxyde with other
compounds, but keeping the same physical properties. The present work present
the preparation of a new frit-type vitreous material with bismuth oxyde, having the
capacity to replace the frit with lead and having more convenient physico-chemical
properties for use in industrial ceramic.
Experimental
In order to prepare the new frit-type material usual raw materials from the
production process were taken, adjusting the quantities upon the chemical
composition and following the usual production process . The important
modification of the recipe was the replacement of the lead oxide, PbO, with
bismuth oxyde, Bi2O3 , the molar ratios of the chemical compounds in the frit
material being presented in table 1.
CEZARA VOICA, LIANA GAGEA, E. INDREA, SIMINA DREVE AND I. BRATU
Table 1.
Molar ratios of the chemical compounds in the frit.
Oxydes
K2O
Na2O
CaO
MgO
ZnO
PbO
B2O3
Al2O3
Fe2O3
Bi2O3
SiO2
TiO2
Molar ratio
leaded frit (1)
bismuth frit (2)
0.053
0,433
0,034
0,014
0,165
0.301
1,422
0,201
0,002
3,051
0,002
0,076
0,619
0,049
0,019
0,237
2,033
0,288
0,003
0,206
4,363
0,003
Thermal derivatography
To establish the technological firing conditions the thermal analysis of the
new material was performed, with a thermal analyser type MOM Q 1500. The two
frits have an almost similar thermal behaviour, and results. Between 1600 and 1750
C the same endothermal effects are observed, due to the loss of water from the
mixed raw materials. Over 6000 C the effects are very weak, and the loss of weight
is low. A calcinations over 8000 C assur ethe melting and the stability of the
fondant. The new prepared frit have the loss of wight at calcinations decreased with
1.5% than the leaded frit, at the correspondent calcinations temperatures. The
termal analysis curves are represented in fig. 1.
Fig. 1. The curves of thermal analysis for the leaded frit (1) and for the new frit
with bismuth (2).
MICROSTRUCTURAL INVESTIGATIONS OF NEW VITREOUS MATERIAL
Thermal microscopy
The thermal microscopy analysis was performed on the two frits, using a
microscope of high temperatures LEITZ, between 9000 C and 13000 C. The results
are presented in table 2.
Table 2.
Thermal microscopy results for the leaded (1) and bismuth (2) frits.
Nr.
Temperature
900
1100
1300
(0C)
1
Sample
1
2
1
2
1
2
2
Low-melting690
690
675
690
685
710
point
3
Low-softening
780
790
750
815
780
835
point
4
Melting
870
870
850
880
855
920
5
Softening
90
100
75
125
95
125
interval
6
Melting interval
90
80
100
65
75
85
As it can be seen after 11000 C the thermal behaviour of the two kind of
vitreous material is different, and the frit with bismuth is more suited for practical
applications because is has larger melting intervals so it assure a better aderence to
the ceramic support at a better shining .
Structural characterisation
The long-range order characteristic for the crystalline state is not occurring in the
vitreous state, where the short-range order is typical [1]. For the crystalline
structures there are correlations between two atoms disposed at arbitrary large
distances one to other. In the non-crystalline solids like glasses there is a local
order, a short-range order, characterised only for the first coordination spheres of
the component atoms, i.e. at the distances at which the interatomic forces are
acting. The local order in glass and crystalline compounds of the same composition
has both similarities and differences. The differences are assigned to the loss of
long range order, to the deviation from a perfect crystalline structure. The order
extension degree can be investigated using the analysis technique of atom electron
distribution functions obtained form X-ray scattering measurements. There are also
glass systems wherin the local structure is extended at more than some
coordination spheres imposing a middle or intermediate range order [2]. The noncrystalline state of a compound can occur in a large structural variety depending on
the preparation conditions. This is due to the fact that during the vitreous sample
preparation the “freezing” process of the atoms arrangement has an important
effect on the atom bonds and atom relaxation energy barriers. Along with the
CEZARA VOICA, LIANA GAGEA, E. INDREA, SIMINA DREVE AND I. BRATU
neutron scattering the X-Ray scattering from disordered materials is largely used to
investigate their local structure [3-7].
The X-ray scattering patterns were obtained by means of standard DRON-3M
powder diffractometer, working at 40 kV and 30 mA, and equipped with
scintillation counter with single channel pulse height discriminator associated
counting circuitry. The Cu K radiation, Ni filtered, was collimated with Soller
slits. X- ray scattering patterns were recorded in a step-scanning mode with 2 =
0.100 steps. The analysed samples, disposed as a powered disk with diameter of 12
mm and thickness about 2 mm, were investigated by means of a X ray scattering
experiment, using a goniometer in Bragg-Brentano reflection alignment.
The structure of the lead oxide, PbO, and bismuth oxyde, Bi2O3 vitrous systems
was investigated by analysing the atomic radial distribution function obtained from
X ray scattering data using a PEDX program [8]. The computation steps consist
first in the determination of radial distribution function 4πr 2∙ρ(r). The preliminary
processing of scattering data implies the determination of the total experimental
coherent scattering function IT(sk) : IT(sk) = [Iexp(sk) – IF(sk)]·A(sk)/P(sk)
correlation function
g(r)
25
Si - O
1.69
20
B
O-O
Si - Si
15
2.93
A
3.21
10
5
0
-5
1,5
2,0
2,5
3,0
3,5
4,0
4,5
5,0
interatomic distance r [A]
Fig.2. The atomic pair correlation function g(r) for samples of the lead oxide, A ,
and bismuth oxyde, B , vitrous systems.
The atom pairs correlation function g(r) shows maxima and their position, width
and area are determined by the distribution of atom pairs in the structural
disordered sample (Fig. 2). The real space distance corresponding to the maxima
determined from the data obtained in this study may be compared with results
reported for other similar systems [9,10] and allow to identify the atom pairs
MICROSTRUCTURAL INVESTIGATIONS OF NEW VITREOUS MATERIAL
orderly disposed in the the SiO2 matrix vitrous systems. They are summarised in
Table 3.
Table 3
Coordination
Atom pairs
Distance (Å)
sphere
I
Si – O
1.69
II
O–O
2.93
III
Si - Si
3.21
Inspecting the data obtained for the samples with lead oxide, PbO, and bismuth
oxyde, Bi2O3 ,from the analysis of atom pairs correlation function g(r) one remarks
the occurrence of Si – O pair as a first coordination formation, well composed at a
distance of 1.69 Å. In SiO2 vitrous systems , the basic structural building block is
the same as the crystalline silicates, the SiO4 tetrahedron [10]. The distances r
between O - O and Si – Si pairs are ranging from 2.6 to 3.5 Å . The bismuth
addition to the SiO2 matrix frtits vitreous system preserves the vitreous structure,
that evidences the high ability of the precursor glass to accept relatively high Bi 2O3
content without structural changes.
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