C. Cãpãþînã,
P. Baltã,
Journal of the University of Chemical
Technology
and Metallurgy, 43, 1, 2008, 81-86
STUDY REGARDING THE NANO-HETEROGENEOUS STRUCTURE
OF A TERNARY GLASS
C. Cãpãþînã1, P. Baltã2
„Constantin Brâncuºi” University,
Engineering Faculty,
3 Geneva Street, Targu-Jiu, Gorj, Romania
E-mail: [email protected]
2
Politehnical University of Bucharest, Faculty of
Applied Chemistry and Materials Science, Department
of Science and Engineering of Oxidic Materials and
Nanomaterials, 1-7 Polizu Street,
011061 Bucharest, Romania
1
Received 05 May 2007
Accepted 12 August 2007
ABSTRACT
The paper presents the study of a silica-calco-sodium glass in the ternary system Na2O-CaO-SiO2 for which there
were defined the binary systems Na2O-SiO2 and CaO-SiO2, so that the calculus of the distribution of the structural nanoaggregated species using the Pretnar method should be possible. Using Excel the properties of the solid glasses based on
the structural nano-aggregated species distribution were calculated. The results have proved that due to different compositions, the properties of the structural nano-aggregated species cover very large ranges of values with differences of
dozens of percents between the maximum and the minimum values.
Keywords: nano-aggregates, ternary glass, nano-heterogenous, Pretnar method.
INTRODUCTION
No matter the definitions proposed for nano
materials or the discussions regarding the characterization of different materials being nano or not, this tendency becomes stronger in the field of glass as well.
Taking into account the fact that in the currently used
technology the structure of glass is formed in the melt
at high temperature, it is concluded that this is the
moment to look for and to identify the ways of influencing the nano-heterogeneous structure [1-8].
In the melt the chemical structural equilibrium
of the glass components is established, which can interact in specific ways [9-12].
As a result the melt represents a mixture of structural groups which have in different works various de-
posits: molecules, polymers, clusters, aggregates, etc.
Their dimensions, shape and, of course, their properties, depend on the present chemical components, on
the equilibrium constant, influenced as well as by the
temperature, the degree of reactions progress – respectively on time, on the modification of the interactions
between components depending on temperature, etc.
For a binary melt the equilibrium reaction may
be written [3]:
SiO4 Na4 + SixO3 x +1 Na2 x + 2 ↔ Six +1O3 x +4 Na2 x + 4 + Na2O
(1)
considering that from left to right a polycondensation
reaction takes place.
The equilibrium constant of reaction (1) is given
by the relation:
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Journal of the University of Chemical Technology and Metallurgy, 43, 1, 2008
Fig. 1. Ternary diagram Na2O-CaO-SiO2
K=
N Six+1 ⋅ N O2
N Six ⋅ N Si
(2)
where:
N Six+1 - the molar fraction of the silica with (x +
1) tetrahedrals SiO4 in macroanion;
N Six - the molar fraction of the silica with x
tetrahedrals SiO4 in macroanion;
NSi - the molar fraction of the insular silica;
N O2 - the molar fraction of the free oxide.
When modifying the temperature the equilibrium
constant also changes in the sense determined by the
exothermal character of the reactions between oxides.
It is deduced that by choosing a melting temperature
where the equilibrium constant has a convenient value
there may be obtained the modification of the polymers’ distribution present in the melt and persistent in
the solid glass. So, the properties of the glass may be
influenced by acting on the structure at nano level.
When increasing the melting temperature and the
period of maintaining at high temperatures other processes occur: high thermal energy of the system determines the breaking – covering of some chemical bonds
82
as well as the networks disintegration and the minimization of structural aggregates: some compounds dissociate thermally, some gases, like oxygen, are lost, etc.
The aim of this paper is to study the influence of
the nano-aggregates from the binary component systems
on the properties of the corresponding ternary glass,
indirectly obtaining proofs of their presence.
The place of the ternary glass in the Na2O-CaOSiO2 system.
The studied glass is silica – calco – sodium of
window type. Its composition in gravimetrical percents
varies within the following limits depending on the forming procedure and on the wanted properties:
SiO2
71 – 74 %
Na2O
13,8 –15,5 %
CaO
6,6 – 10 %
MgO
2,8 – 4 %
Al2O3
0,6 – 2%
Fe2O3
sub 0,2%
Taking into consideration only the first three main
oxides the used composition in molar percents was: SiO2
76.43 %, Na2O 14.65 % and CaO 8.92 %. This composition
was placed in the ternary diagram presented in Fig. 1 [1].
C. Cãpãþînã, P. Baltã,
MODEL DEVELOPMENTS
The calculus of the distribution of structural
aggregates and of glass properties
In the literature there were presented moles and
calculus methods of the structural aggregates distribution inspired more or less from the field of organic
polymers.
In the paper the calculus method of polymers
distribution elaborated by Pretnar was used, which allows the theoretical treatment of the chemical equilibrium in acid silica melts, with the molar fraction of
SiO2 smaller than 0.8 but bigger than 0.5. Over this
value the existence of a disordered network of
Zachariasen type is very likely.
Prentar defined a polymerization degree which
represents the report between the actual number of
bridges from the structure ∑ f x nx and the theoretical
number of possible bridges
Fig. 2. Polymers distribution in the melt in the Na 2O – SiO 2
system.
(2n ) [9, 12]:
SiO2
P=
∑fn
x
x
2nSiO
(3)
2
where: nx
- number of moles in x species; nSiO2 number of SiO2 moles.
Based on a sophisticated calculus program where
there was taken into account the gravimetric fraction of
every polymer species given by the relation [9, 12]:
Gx =
Basicity weight factor
Fig. 3. Polymers distribution in the melt in the CaO – SiO 2
system.
M x ⋅ Nx
n
∑M
x
⋅ Nx
(4)
1
where:
Mx - molecular mass of the polymer species having the formula
M (2 x − 2 f x ) / v SixO (2 x − f x ) ,
v - cation valence;
Nx - molar fraction of every polymer species,
calculated with the relation [9, 12]:
Nx =
nx
where nx – number of moles from the molecule
species having x silica atoms.
RESULTS AND DISCUSSION
n
∑n
x
1
Fig. 4. The variation of basicity gravity of the structural aggregate
species. On the abscissa there was represented the number of x
silica atoms of every species
(5)
The results obtained through the calculus of the
polymers distribution for the compositions of the Na2O
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Journal of the University of Chemical Technology and Metallurgy, 43, 1, 2008
Fig. 5. The variation of basicity gravity of the structural aggregate
species. On the abscissa there was represented the number of x
silica atoms of every species
– SiO2 and CaO – SiO2 systems are presented in Figs. 2
and 3.
It is observed the presence of a number of nanoaggregates higher than 150, 200, respectively, suggesting their high structural complexity.
By using the calculus program, the mass and
molar ratios, there are calculated the properties based
on the glass composition applied to each nano-aggregate specie and then the balanced averages are calculated according to the respective distribution.
In order to calculate the gravity of the basicity of
an oxidic compound, the following relationship is used
[10, 11]:
pB = ∑ pBi ⋅ ci
(6)
where:
pBi – basicity gravity;
ci – the concentration in mass ratios.
By calculating the basicity of each structural aggregate species present according to the calculated distribution, the values represented in the diagram from
Figs. 4 and 5 are obtained.
A first observation that emphasizes the nanoheterogeneous character of the material is the fact
that the property value varies from species to species, in very wide ranges, representing cca. 26 % in
the case of the glass in the Na2O – SiO 2 system and
cca. 22 % in the case of the glass in the CaO – SiO2
system. On this basis, the usual image of glass as a
homogeneous, monolith material must be corrected
admitting that in the structure there are aggregates
and zones with slightly different properties which react differently at exterior stimuli.
84
Fig. 6. Basicity gravity variation of the structural aggregate species.
On the abscissa there was represented the number of x silica
atoms of every species.
Fig. 7. The values of the dilatation coefficient of aggregated
species according to the distribution.
The basicity of the whole may be expressed with
the relation [10]:
pBst = ∑ pBx ⋅ cx
(7)
where pBx is the basicity of every nano-aggregate
species, and cx is the respective concentration in mass
fractions.
It is obtained:
for the composition from the
Na2O – SiO2 system pBNaSi = 63.33;
for the composition from the CaO –
SiO2 system pBCaSi = 56.83.
With these data, based on the same principle, it
is calculated the basicity of the ternary glass:
pBst 3 = pBNaSi cNaSi + pBCaSi ⋅ cCaSi
(8)
C. Cãpãþînã, P. Baltã,
The obtained value is pBst3 = 60.740, comparable with 60.855 obtained by the calculation based on
oxidic composition.
The oxides’ basicity gravity is determined according to the basicity scale, with the formula:
pB = 1,9 ⋅ ( NC )
0,02
− 0,023⋅
Pi
NC
(9)
where:
Pi – ionization potential, in eV corresponding to
the oxidation number of the respective atom;
NC – coordination number.
The values represented in the diagram from Fig.
6 are obtained by the calculation of the basicity of every structural aggregate species present according to the
calculated distribution.
The thermal dilatation coefficient is calculated
using Appen’s method, with the formula [9]:
mi ⋅α i
α=∑
100
(10)
where:
mi – oxides concentration in molar percents
α i – factors representing the contribution of
the respective oxide to the glass characteristic.
In order to take into account the more complex
influence of the SiO2 concentration, Appen proposed
the determination of α SiO2 with the equation [9]:
α SiO ⋅ 107 = 38 − ( mSiO − 67 )
2
In cases where
becomes:
α SiO ⋅107 = 38
2
(11)
2
mSiO
2
< 67 %
, the relation
mol
(12)
The obtained values for the structural aggregate
species according to the distribution are presented in
Fig. 7.
The data from this diagram confirm the structural heterogeneity at nano level. The variation of the
dilatation coefficients of the nano-aggregates species with
46 % in the case of CaO – SiO2 system and even with
64 % in Na2O – SiO2 system allows to understand the
experimentally determined mechanical resistance decrease as a phenomenon, as compared to the theoreti-
cally estimated one, as well as the occurrence and persistence of superficial micro-fissures.
CONCLUSIONS
There were determined some glass properties
(basicity and thermal dilatation coefficient) for the systems Na2O – SiO2 and CaO – SiO2 and for a large number of species.
There was found that due to different compositions the properties of structural nano-aggregate species
cover wide ranges of values with differences of tens of
percents between the maximum and minimum values.
The structural nano-heterogeneity of the binary
systems and of the ternary system which includes them
were presented in the paper, becoming reflection and
interpretation elements of the behaviour of glasses, apparently homogeneous when in contact with the environment.
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