PowerPoint bemutat

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Network structure of multi-component
borosilicate waste glasses from high Q neutron
diffraction study and RMC modelling
--------------------------- Erzsébet
SVÁB Research Institute for Solid State Physics
and Optics, Budapest, Hungary -------------- Co-au
thors Margit Fábián , PhD student - RMC
calculations Gy. Mészáros Th. Proffen LANSCE,
USA E. Veress Cluj, Romania
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Motivation and aim of the work
Multi-component alkali borosilicate glasses are
suitable materials for the storage of high-level
radioactive waste materials (HLW) (i.e. UO3 or
PuO2). Actual compositon, we are interested
in (65-x)SiO2xB2O325Na2O5BaO5ZrO2 (mol)
network former
modifiers
Fábián, Z. Krist.(2006)
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Effect of modifier different mechanism in
silica and diborate glasses
v-SiO2 SiO4 Mv ? SiO4 NBO tetrahedrally
coordinated 4Si basic units are unchanged,
network is breaking
v-B2O3 BO3 Mv ? BO3 BO4 3-fold coordinated
units transform into superunits, which contain
both 3B and 4B coordination
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What is in multi-component borosilicates?
Problem high number of contributing elements
(5-7) and the overlapping atomic distances. Our
concept systematic study, starting from the
simple oxide glasses and, added step-by-step by a
new element up to the 6-component glass.
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Investigated samples systematic series of
glassy specimens

• (65-x)SiO2xB2O325Na2O5BaO5ZrO2 with x5,
  10, 15 mol (SiBxNaBaZrO 6-component)
• 70SiO225Na2O5BaO (SiNaBaO)
• - 70SiO230Na2O (SiNaO) (see poster)
• B2O3 and SiO2 (ISIS database)
• 11B-isotope enriched to 99.6.
• Samples were prepared by melt-quench technique
  (1400-1600 ºC )
• Elemental composition was verified by PGAA

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Neutron diffraction experiments
PSD at the 10 MW research reactor, Budapest,
?01.068 Å, Q0.9-10 Å -1
NPDF at LANSCE pulsed neutron source, USA,
Qmax 50 Å -1
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Structure factor S(Q) and interference function
I(Q)QS(Q)-1
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Total distribution function, G(r) calculated by
sine-Fourier transformation
weighting factors, wij ()
SiO2 SiNaO SiBxNaBaZrO B2O3
x5 10
15 Si-O 38.8 25.7 19.8 18.7
15.0 - B-O - - 6.1
10.3 14.4 49.1 O-O 54.3 43.5
42.2 39.9 40.3 32.1 Si-Si 6.9
3.8 2.3 2.2 1.4 -
B-B - - 0.2 0.7
1.2 18.8 Na-O - 19.2 14.0
12.2 11.2 - Ba-O - -
1.6 1.6 1.6 - Zr-O -
- 3.9 3.7 4.1
-
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Reverse Monte Carlo modelling

• Starting model a disordered atomic configuration
  was built up with a simulation box containing
  4000-6000 atoms, and box length 19-21 Å.
• Constraints density,
• nearest neighbour distances for all pairs,
  connectivity for Si-O and B-O.
• Density of the multi-component samples was
  calculated from G(r)-4??0r
• Nearest neighbour distances as resolved in G(r),
  and data taken from literature were applied.
• step by-step, with increasing number of
  components

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B-O pair correlation functions
B-O expected 1.37 Å (BO3) and 1.47 Å (BO4)
Swenson and Börjesson PRB (1995) Cormier et
al. PRB (2000), etc. 1.60 Å ???
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RMC constraints no Si-O and B-O overlap
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Constraints for SiBxNaBaZrO glasses in the final
RMC run
cut-off distance, r(Å)
x5 10 15 Si-O 1.45 1.5
1.5 B-O 1.0 0.8 1.0 Si-Si 2.86 2.8 2.81
Si-Na 2.48 2.35 2.47 O-O 2.15 2.15 2.15
Na-O 2.07 2.05 2.05 Na-Na 2.53 3.01 3.02
Ba-O 2.42 2.51 2.45 Zr-O 1.95 1.9 1.98
Si-B 2.75 2.44 2.41
Number of neighbours (CNij) in a restricted
interval (Å ) Si-O
B-O x CN 4 3 4
5 1.45-2.0 1.0-1.9 10 1.5-1.9
0.8-1.9 15 1.5-1.9 1.0-1.9
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Result of final RMC simulation
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gtotal(r) for SiBxNaBaZrO glasses - comparison
of FT and RMC result
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Partial pair correlation functions, gij(r) for
SiB15NaBaZrO sample
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Structural characteristics from partial
correlation function Si-O and Si-Si
Si-O coordination number distribution
Conclude the network consisting of tetrahedral
SiO4 units is highly stable even in the
multi-component glasses.
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Zr O correlation function
coordination number distribution
Zr is coordinated by 6 O bonden to 2 O at a
shorter distance due to its strong charge
compansating ability
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O-O partial correlation function
In the multi-component glasses two characteristic
peaks are present at 2.3Å and 2.6Å, and with
increasing boron content the lower peak becomes
more pronounced
Suggests a network configuration consisting of
boron rich and silicon rich regions.
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B-O partial correlation function and coordination
number distribution
1. 60 Å
1.40 Å
From NMR, Raman, XANES 3B-O-Si , 4B-O-Si ,
3B-O- 3B, 3B-O-4B, 4B-O-4B
coordinations e.g.Du and Stebbins JNCS(2003)
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Correlation between the population of the two B-O
distances and 3/4B coordination numbers?
If we suppose that the 1st (sub)peak at 1.40 Å is
formed completely by 3B, even than about 30 of
3B contribute to the 2nd (sub)peak at 1.60 Å
in addition to the number of 4B units.
3/4B form superunits and they are linked to
4Si, forming a mixed network. This
interpretation agrees with Raman and NMR
spectroscopy results.
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Conclusion
Using RMC modelling we have successfully revealed
most of the partial atomic pair correlations for
a 6-component glass
- Si-O network consisting of tetrahedral SiO4
units is highly stable even in the
multi-component matrix glass.
- B-O shows two distinct first neighbour
distances at 1.40 Å and 1.60 Å, and both 3 and
4-fold coordinated boron atoms are present.
- O-O distribution suggests a network
configuration consisting of boron rich and
silicon rich regions.
- We propose a model, where the B- rich network
contains mostly trigonal BO3 units, and in the
Si- rich network the terahedrally coordinated
Si-O network contains different mixed 4B-O-Si
and 3B-O-Si linkages.
Thanks for your attention
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Elemental composition was verified by Prompt
Gamma Activation Analysis
Elemental composition (at)
Si B Na O
Ba Zr B5 Nominal 19.67 3.28 16.39 57.38 1.64
1.64 PGAA 19.1 3.69 15.37
58.0 1.3 2.19 B10 Nominal 17.46 6.35 15.87 57.
14 1.59 1.59 PGAA 19.1 6.59 14.3 58.4 1.3
2.18 B15 Nominal 15.38 9.23 15.38 56.92 1.54 1.
54 PGAA 15.24 9.15 13.08 58.5 1.3
2.42 PGAA data (1.5-2)
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Interatomic distances, rij(Å) SiO2 SiNaO
SiNaBaO x51015 B2O3 Si-O 1.615 1.62
1.60 1.60 - B-O - -
- 1.40/1.60 1.36 O-O 2.63
2.61 2.60 2.3/2.6 2.35 Si-Si 3.10
3.05 3.10 3.0 - B-B - - -
2.4-2.6 2.4 Si-B - - -
2.5-3.1 - Na-O - 2.29 2.30 2.2/2.6
- Si-Na - 2.8-3.5 2.7-3.2 - Zr-O
- - - 2.0 - Ba-O - - 2.70
2.6- 2.7 -
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Coordination
Number, CNij SiO2 SiNaO SiNaBaO x5
x10 x15 B2O3 Si-O 3.9 3.94 3.9 3.9
3.8 3.9 - B-O - - - 3.5 3.1 3.1 2.9
O-O 6.4 6.0 5.7 6.1 6.0 6.1 6.3 Na-O -
3.9 4.0 4.7 4.7 4.8 - Zr-O - - - 1.9
1.6 1.9 -
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Elemental composition was verified by Prompt
Gamma Activation Analysis
Elemental composition (at)
Si/B Si B Na O
Ba Zr B5 Nominal 5.99 19.67 3.28
16.39 57.38 1.64 1.64 PGAA 5.17
19.1 3.69 15.37 58.0 1.3
2.19 B10 Nominal 2.74 17.46 6.35 15.87
57.14 1.59 1.59 PGAA 2.89
19.1 6.59 14.3 58.4 1.3
2.18 B15 Nominal 1.66 15.38 9.23
15.38 56.92 1.54 1.54 PGAA 1.66
15.24 9.15 13.08 58.5 1.3 2.42
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weighting factors, wij() SiO2
SiNaO SiNaBaO x5 x10 x15 B2O3 Si-O
38.8 25.7 26.0 19.8 18.7 15.0 - B-O - - -
6.1 10.3 14.4 49.1 O-O 54.3 43.5 44.2
42.2 39.9 40.3 32.1 Si-Si 6.9 3.8 3.9
2.3 2.2 1.4 - B-B - - - 0.2 0.7 1.2
18.8 Na-O - 19.2 16.2 14.0 12.2 11.2 -
Ba-O - - 2.3 1.6 1.6 1.6 - Zr-O - -
- 3.9 3.7 4.1 - Si-Na - 5.7 4.8 3.3
2.9 2.1 - Si-B - - - 1.4 2.4 2.7 -
B-Na - - - 1.0 1.6 2.0 - Na-Na - 2.5
1.5 1.1 0.9 0.8 -
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Partial structure factors Sij(Q) for
SiB15NaBaZrO sample
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Na - O