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Enhanced melt-spraying process for tritium breeder pebbles

Enhanced melt-spraying process for tritium breeder pebbles . Outline. Introduction to pebble fabrication by melt spraying Process parameters and microstructure Pebble quality. Standard melt spraying process. Lithium orthosilicate with 2.5 wt.% excess of silica

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Enhanced melt-spraying process for tritium breeder pebbles

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  1. Enhanced melt-spraying process for tritium breeder pebbles

  2. Outline Introduction to pebble fabrication by melt spraying Process parameters and microstructure Pebble quality M.H.H. Kolb, R. Knitter - BA Workshop Meeting on Tritium and Blanket Technologies, Sept. 2011

  3. Standard melt spraying process • Lithium orthosilicate with 2.5 wt.% excess of silica • LiOH & SiO2 as raw materials • Platinum crucible @ 1450 °C • Spraying with an air jet in horizontal direction • Initial composition: Li4SiO4 + Li6Si2O7 • After heat treatment: Li4SiO4 + Li2SiO3 (MSi) • Established at Schott AG, Mainz • Reasonable pebble quality • Long time experience • Moderate yield • Easy and quick reprocessing M.H.H. Kolb, R. Knitter – Workshop on Ceramic Breeder Blanket Interactions, Sept. 2011

  4. Development of Process Modifications • Lithium orthosilicate with 2.5 wt.% excess of silica or other compositions • LiOH & SiO2 as raw materials • Platinum crucible @ 1300 - 1450 °C • Self-decay of thin melt jet • Gathering of the pebbles in liquid nitrogen • Dropping distance can be adjusted • Gas pressure to control melt flow velocity • Modified melt-based process • controlled droplet generation • higher yield • lower porosity • improved process control M.H.H. Kolb, R. Knitter - BA Workshop Meeting on Tritium and Blanket Technologies, Sept. 2011

  5. Element Li (wt.%) Si (wt.%) Pt (µg/g) Reference material 22.8 24.1 46 KALOS 22.5  0.2 24.6  0.1 50  6 Relative changes -1 % +2 % +9 % Development of Process Modifications • Nozzle design improvements • Coping with impurities • Enhancing the melt jet stability • Increasing the output • Verification of pebble purity • Negligible increase of the platinum content • Increase of alkali and alkaline elements due to handling M.H.H. Kolb, R. Knitter – Workshop on Ceramic Breeder Blanket Interactions, Sept. 2011

  6. Pebble size distribution The achievable pebbles size distribution is very narrow ► Improvement of the yield M.H.H. Kolb, R. Knitter – Workshop on Ceramic Breeder Blanket Interactions, Sept. 2011

  7. Effect of the dropping distance 140 cm dropping distance 1400 cm dropping distance Short dropping distances favor large dendritic grains M.H.H. Kolb, R. Knitter – Workshop on Ceramic Breeder Blanket Interactions, Sept. 2011

  8. Effect of dropping distance 140 cm dropping distance 1400 cm dropping distance Mean crush load of shiny pebbles: 3.7 N Mean crush load of opaque pebbles: 13.5 N The dropping distance changes the fraction “shiny”/”opaque” M.H.H. Kolb, R. Knitter – Workshop on Ceramic Breeder Blanket Interactions, Sept. 2011

  9. Variations in the Chemical Composition • Change in the chemical composition • Strengthening by additions of TiO2 • Formation of lithium meta-titanate (MTi) as a second or third phase • Unknown phase equilibria • Decrease of melting temperature • Change in viscosity • Fine grained dendritic microstructure ? F. A. Hummel, T. Y. Tien, K. H. Kim, Journal of The American Ceramic Society Vol. 43, No. 4, pp. 192, 1960 M.H.H. Kolb, R. Knitter – Workshop on Ceramic Breeder Blanket Interactions, Sept. 2011

  10. Effect of titania on pebble microstructure 10 mol% Li2TiO3 15 mol% Li2TiO3 20 mol% Li2TiO3 The grain size is reduced with increasing lithium titanate fraction M.H.H. Kolb, R. Knitter – Workshop on Ceramic Breeder Blanket Interactions, Sept. 2011

  11. Effect of heat treatment on the microstructure 20 mol% MTi As-received 1 week at 950 °C 3 weeks at 950 °C Grain growth of lithium titanate in lithium orthosilicate is comparably slow M.H.H. Kolb, R. Knitter – Workshop on Ceramic Breeder Blanket Interactions, Sept. 2011

  12. Element mapping of as-received material 15 mol% MTi SEM EDX-Mapping SE image Ti mapping Si mapping Lithium titanate is located at the grain boundaries of lithium orthosilicate M.H.H. Kolb, R. Knitter - BA Workshop Meeting on Tritium and Blanket Technologies, Sept. 2011

  13. Element mapping of heat treated material 15 mol% MTi SEM EDX-Mapping SE image Ti mapping Si mapping Lithium titanate is located at the grain boundaries of lithium orthosilicate M.H.H. Kolb, R. Knitter - BA Workshop Meeting on Tritium and Blanket Technologies, Sept. 2011

  14. Composition of phases Initially LiTiO2 (Ti+III) is formed rather than Li2TiO3(Ti+IV) M.H.H. Kolb, R. Knitter – Workshop on Ceramic Breeder Blanket Interactions, Sept. 2011

  15. Mechanical strength of 500 µm pebbles 300 cm (15 mol% MTi) Increase of crush load due to increase of dropping height Increase of crush load due to additions of TiO2 M.H.H. Kolb, R. Knitter – Workshop on Ceramic Breeder Blanket Interactions, Sept. 2011

  16. Summary and Conclusion • Modification of Schott process to control droplet generation • Low porosity • Control of the microstructure • Narrow pebble size distribution • Addition of titania for strengthening • Very little information in the interesting composition range • Fine grained dendritic microstructure • Very limited grain coarsening of titanate phase • Heat treatment necessary to achieve monoclinic Li2TiO3 M.H.H. Kolb, R. Knitter - BA Workshop Meeting on Tritium and Blanket Technologies, Sept. 2011

  17. Thank you for listening M.H.H. Kolb, R. Knitter – Workshop on Ceramic Breeder Blanket Interactions, Sept. 2011

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