Structure and Properties of Iron Borophosphate Glasses

1
Structure and Properties of Iron Borophosphate
Glasses
P. A. Bingham (1,), R. J. Hand (1), S. D. Forder
(2), A. Lavaysierre (2), F. Deloffre (2), S. H.
Kilcoyne (3), I. Yasin (3)
(1) Immobilisation Science Laboratory / Ceramics
Composites Laboratory, Dept. of Engineering
Materials, University of Sheffield, Sir Robert
Hadfield Building, Mappin Street, Sheffield S1
3JD, UK. (2) Materials and Engineering Research
Institute, Sheffield Hallam University, Howard
Street, Sheffield, S1 1WB, UK. (3) School of
Physics and Astronomy, University of Leeds,
Leeds, LS2 9JT, UK.
4. Mössbauer Spectroscopy
1. Introduction to Iron Phosphate (IP) Glasses
B0.57Fe0.43PO4
PFe-1M

• P2O5-based glasses generally have lower
  chemical durability than those based on SiO2. The
  structure of phosphate glass comprises P-O-P
  bonds, which are easily hydrated. Unlike silicate
  glasses, addition of modifiers leads to
  replacement of P-O-P bonds with P-O-M bonds, and
  cross- linking of phosphate chains gives rise to
  increased durability and Tg.
• Iron phosphate (IP) glasses exhibit
  superior chemical durability to most phosphate
  glasses and many silicate glasses. Few P-O-P
  bonds exist P-O-Fe bonds predominate and
  phosphate chains are fewer and shorter, with
  monomer (PO4)3- (Q0) and dimer (PO3)2- (Q1) units
  prevalent.
• In addition to high durability, IP
  glasses have low melting temperatures (? 900-1200
  ?C) and the ability to incorporate substantial
  levels of other elements. This combination of
  properties has led to their consideration as
  hosts for the immobilisation of certain toxic 1,
  2 and nuclear 3, 4 wastes.
• One drawback of IP glasses, and
  particularly those based around the 60 P2O5 40
  Fe2O3 (mole ) composition, is poor thermal
  stability compared with that of borosilicate
  glasses used to vitrify high level nuclear waste
  (HLW). IP glasses can crystallise at temperatures
  as low as 600?C. This can cause problems during
  processing and in some situations decreases the
  wasteform durability 4.

• CS, QS and LW were consistent with the
  occupation of distorted octahedral sites by Fe2
  and Fe3. Some tetrahedral sites may occur.
• B2O3 additions had no measurable effects
  on Fe environments.
• Redox ratio, Fe2 / SFe, was increased by
  B2O3 addition. This may be related to changes in
  glass basicity.

B10-P
5. X-Ray Diffraction (XRD)

• All samples were XRD amorphous except
  B10-P.
• On cooling from the melt, B10-P
  developed a phase identified as B0.57Fe0.43PO4
  or a similar boron-iron phosphate phase.

B10
Figure 2. XRD patterns for B10-P B15-Fe
(representative of all other samples)
6. FT-IR Reflectance Spectroscopy

• B2O3 additions introduced new band at 860
  cm1, which has been attributed to tetrahedral
  BO4 groups 5.
• Absorption at 1070 cm1 and 1280 cm1
  grew with B2O3 addition attributed to trigonal
  BO3 groups and / or Q2 Q3 phosphate groups.
• Spectral changes at 750 cm1, 950 cm1,
  1010 cm1, 1070 cm1 and 1140 cm1 indicated
  changes in phosphate Q-group distribution.

2. Benefits of Adding B2O3 to Phosphate Glasses

• B2O3 can suppress crystallisation,
  increase Tg and improve chemical durability in
  various phosphate glass systems including
  P2O5-Al2O3-Na2O 5.
• B2O3 has a thermal neutron cross-section
  which is ? 2 orders of magnitude greater than any
  other glass former, and which is only surpassed
  by a few rare earth oxides, particularly Gd2O3,
  and by CdO. This behaviour may prove very useful
  in immobilisation of certain radioactive wastes.

B20-Fe
Fe3
7. PCT-B Chemical Durability
Fe2

• B2O3 additions of up to 10 had little
  effect on durability, regardless of substituent.
• Additions of 15-20 B2O3 caused
  durability to decrease, but it generally
  remained superior to borosilicate MW and a
  soda-lime- silica (SLS) container glass.

3. Experimental Procedures
Figure 1. Selected Mössbauer spectra fitted with
8 Lorentzian doublets

• Batches to produce 200g glass from gt99
  NH4H2PO4, Fe2O3 and H3BO3 were melted at
  1150-1200?C in mullite crucibles for 3h (stirred
  for final 2h). They were annealed at 450?C for 1h
  then cooled to RT.
• Mössbauer spectroscopy provided CS, QS,
  LW and Fe2/SFe. Measured vs. ?-Fe at room
  temperature. Recoil free fraction f (Fe3) / f
  (Fe2) ? 1.3 in these glasses must be considered
  when assessing redox.
• FT-IR reflectance spectroscopy in the
  range 400 1400 cm-1. Averages of 20
  measurements / sample.
• Chemical durability measured by Product
  Consistency Test B (PCT-B) at 85?C for 7 days.
• DTA from 20-1200?C, 10?C / minute XRD
  using Cu K? radiation, 10-60 ?2q, 0.4 ?/minute.
• SEM-EDS and ICP-OES used for elemental
  analysis of glasses ICP-OES used for leachate
  analyses.

8. DTA Thermal Stability
Figure 3. FT-IR reflectance spectra for samples
with replacement of Fe2O3 by B2O3

• B2O3 additions suppressed crystallisation
  exotherms at ?650?C and ?850?C thermal
  stability of the glass (Tc1 Tg) increased and
  exothermic peak areas decreased. Tg and Tm both
  increased.
• Replacement of Fe2O3 by B2O3 was
  particularly successful in suppressing
  crystallisation thermal stability was comparable
  with UK borosilicate glass MW used in HLW
  immobilisation.
• Stability was high vs. P-4, a complex
  P2O5-Fe2O3-ZnO waste glass

9. Conclusions

• Addition of B2O3 to 60 P2O5 40 Fe2O3
  glasses had little effect on chemical durability
  at levels of up to 10 mole B2O3.
• Thermal stability and crystallisation
  resistance was dramatically improved by addition
  of even small amounts (5-10 ) of B2O3
  replacement of Fe2O3 by B2O3 was particularly
  effective.
• 4-coordinated B3 was present
  3-coordinated B3 was likely but could not be
  confirmed. Replacement of Fe2O3 by B2O3
  increased the average phosphate Q-value.
• B2O3 additions did not measurably affect
  Fe environments, but they increased Fe2 / SFe,
  Tg, and Tm, although all samples but B20-Fe
  could be successfully melted at 1150?C.

10. References and Acknowledgements

• P. A. Bingham R. J. Hand, J. Haz. Mat. B119
  (2003), 125-133.
• P. A. Bingham, R. J. Hand, S. D. Forder, A.
  Lavaysierre, J. Haz. Mat. B122 (2005) 129-138.
• M. G. Mesko, D. E. Day, J. Nucl. Mat. 273 (1999)
  27-36.
• D. E. Day, Z. Wu, C. S. Ray, P. Hrma, J.
  Non-Cryst. Solids 241 (1998) 1-12.
• A. M. Efimov, J. Non-Cryst. Solids 253 (1999)
  95-118.

Figure 4. PCT-B chemical durability results
The authors wish to thank the UK Engineering and
Physical Sciences Research Council (EPSRC) for
funding this work.
Figure 5. DTA traces of sample glasses
comparators phosphate P-4 borosilicate MW
n/m not measured
Table 1. Glass compositions and selected property
measurements