INVESTIGATIONS OF Na2O INFLUENCE ON STRUCTURE OF Al2O3-CaF2
IN MODEL EXPERIMENT
Bukhtoyarov O.I., Vorontsov B.S., Komogorova S.G.
Kurgan State University Russia
For study was chosen the system having practical application - its components
are part some metallurgical slags [1] - and representing scientific interest. On a
terminology of the polymeric theory its composition includes network-forming oxide,
the oxide-modifier and fluoride that enables studying process of structurization.
Models of objects with contents Al2O3 and CaF2 in the ratio  ( is relation
mol. %) from 0,5 up to 3 and Na2O from 5 up to 20 mol % were studied.
To describe the composition and a relative positioning of ions of object in
study it is used lattice model. In 504 nodes kations sublattice ions Al3+, Ca2+ and Na+
and in 756 nodes anions sublattice ions of oxygen and fluorine were placed. The
fragment of a flat lattice representing system Al2O3-CaF2-Na2O is shown in figure 1.
Fig.1. The fragment of a flat lattice for system Al2O3-CaF2-Na2O. Numbers of
nodes are underlined.
Structural
fragments
are
allocated:
623a) Al4O9 , b) Al3F4O3 , c) Al2F2O3 , d) AlO3 and e) AlF3.
The continuous lattice of bounds Al-O-Al is broken off by ions of sodium and
calcium on kations sublattice and ions of fluorine on anions. In result fragments of a
lattice are formed, which are considered as molecular models for the description of
investigated melts structure. Examples of structural fragments are given in figure 2.
About 100 similar fragments with the contents up to 30 atoms were analysed
by quantum-chemical method MNDO (modified neglect of diatomic overlap) [3].
Calculations have shown, that to within 0,01 % full energy of these structural
622
fragments can be submitted as the sum of two-body and three-body contributions. The
technique of definition of these contributions values is more in detail described in
works [4].
a)
b)
c)
Fig.2. Structural fragments: a - Al4O9Na6, b - Al3O2F5Na, c - Al2CaO2(ONa)4.
Experience of parametrization of quantum-chemical methods has shown, that
consecutive addition of model by new chemical elements, and also integration of
researched fragments results in accumulation of errors. For elimination of this error
simultaneous optimization of contributions in full energy by method Hooke R. and
Jeeves T. [5] on all base of the investigated fragments of structure was carried out.
The problem consist in minimization of a difference between energy, received in
623
independent quantum-chemical calculation and energy, submitted as two-body and
three-body includes.
Values of these contributions received under joint optimization are shown in
Table № 1.
Table 1
Values of contributions for calculation of full energy of model
Type of bonds
Al-O-Al
Al-O-Ca
Al-O-Na
Ca-O-Ca
Ca-O-Na
Na-O-Na
Al-F
Ca-F
Na-F
Energy, received under joint
optimization, 10-18 J/bond
56,916
56,248
55,210
52,845
54,301
53,898
79,747
77,749
78,184
For studying dependence of structure from composition of researched objects
the molecular-statistical method Monte-Carlo [6, 7] is used. The initial configuration
of system was set by arbitrary distribution of ions in nodes of appropriate sublattices.
Change of a configuration was carried out by paired rearrangement of casually chosen
ions in both sublatties.
For calculation of the average structure data sample of configurations on the
importance (in ascending order full energy of system) formed Markov chains. Markov
chains contained  103 configurations for each composition. The temperature of
system was taken into account by use at change of configurations exponent’s
criterion: exp (-)  RAND.
The following characteristics of structure (the terminology also corresponds to
the polymer theory) were analyzed: relative number of ions of oxygen and fluorine of
various type: bridging, non- bridging and free oxygen, F- and F0. The concrete
formula and a charge of structure fragments further was determined, and distribution
fn () by fragments of the various size paid off.
Model experiment shows that at increase  from 0,5 up to 3,0 fraction of
bridging oxygen monotonously grows (fig 3 a) at all additives Na2O, and the quantity
of non-bridging oxygen accordingly decreases. At addition Na2O in melts the quantity
of free oxygen is increased (fig 3 b). Influence Al2O3/CaF2 on the contents of free
oxygen appreciable only at small values ( = 0,5 and 1,0). At  = 1,5 and 3,0 quantity
of free oxygen practically remains without changes.
At transition from  = 0,5 to  = 1,0 at all values Na2O the contents of the
connected fluorine is increased up to value close to unit and the quantity of free
fluorine F- (fig. 3 c) sharply decreases. Then these sizes do not vary at increase
Al2O3/CaF2.
624
Fig.3. Fraction of bridging and free oxygen and free fluorine in dependence
on ratio Al2O3/CaF2 at various additives Na2O in system
Al2O3-CaF2-Na2O.
Concerning distribution by structural fragments, it is possible to note the
following: up to  = 0,5 connected fragments of structure are not observed for all
625
Na2O contents. The greatest variety of fragments of structure corresponds  =11,5.
Here all dependences fn () have obviously expressed extreme character. Exception
make dependence fn>50 (). The size of a maximum consistently grows with increase
of contents Na2O for n10. For values n < 10 obvious dependences of height of a
maximum on contents Na2O it is not revealed.
Fig.4. Influence of additives Na2O on relative number of the structural
fragments containing from 10 up to 19 atoms of aluminium. On axis X –
, in a legend – mol fraction Na2O.
Conducted analysis shows that Na2O influence on the process of structureformation in the system Al2O3-CaF2 is sufficiently packs and is distinguish from its
influences upon the similar process in purely оxyde systems, where Na2O is
"classical" modifier. This possible to give a following explanation. In models for
Al2O3-CaF2 slags a sodium is carry in the lattice direct by Al-O-Al bounds, which are
partly ruined by fluorine ions on аnion sublattice. Herewith occurs an additional
ruining of covalent bonds on kation sublattice and simultaneous its partial recovering
on аnion sublattice.
For this reason in the system Al2O3-CaF2 with a=1,5 (ratio Al2O3/CaF2)
additive Na2O bring about shaping "uniform structure". The whole kation sublattice
is split by sodium and fluorine ions on fragments of approximately alike size, n = 1019 (fig.4), so as most number of Na ions was involved in Al-O-Na bonds. Ions of
fluorine are herewith displace from peripheries of aluminium-oxide formation and
occurs an increasing a share of free fluorine (fig.3 с)
Under a = 3, this compensation mechanism brings about relative stabilization
(in contrast with binary systems) completely "polimerised" conditions.
References:
1. The atlas of slags: the Directory: Translation from German. – Moscow:
Metallurgy. – 1985. – 208 p.
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Metals. - 1976. - №5. - p.45-48.
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3. Dewar M.J.S., Thiel W. Ground states of molecules. 38. The MNDO method.
Approximations and parameters // J. Am.Chem. Soc.- 1977.- V.99.- N15. p.4899-4907.
4. Vorontsov B.S., Revzina L.A. MNDO calculation of two and three-body
energies in glass-forming oxides. – Proc. 16 Int. Congress on Glass. – Madrid.
– Spain. – 1992. – V.3. – p.163-166.
5. Hooke R., Jeeves T.A., J.Assoc. Computer Mach., 8, 212 (1962).
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a method of Monte Carlo for the analysis of structure polimerised melts. //
Magazine Physical Chemistry. – 1985. - v.59. – № 3. - p.753-754.
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