Enhanced meltspraying process for tritium breeder pebbles M
Enhanced melt-spraying process for tritium breeder pebbles M. H. H. Kolb, R. Knitter INSTITUTE FOR APPLIED MATERIALS - MATERIAL PROCESS TECHNOLOGY / DEPARTMENT CERAMICS KIT – University of the State of Baden-Wuerttemberg and National Research Center of the Helmholtz Association www. kit. edu
Outline Introduction to pebble fabrication by melt spraying Process parameters and microstructure Pebble quality 2 M. H. H. Kolb, R. Knitter - BA Workshop Meeting on Tritium and Blanket Technologies, Sept. 2011 Institute for Applied Materials (IAM-WPT)
Standard melt spraying process Established at Schott AG, Mainz Reasonable pebble quality Long time experience Moderate yield Easy and quick reprocessing Lithium orthosilicate with 2. 5 wt. % excess of silica Li. OH & Si. O 2 as raw materials Platinum crucible @ 1450 °C Spraying with an air jet in horizontal direction Initial composition: Li 4 Si. O 4 + Li 6 Si 2 O 7 After heat treatment: Li 4 Si. O 4 + Li 2 Si. O 3 (MSi) 3 M. H. H. Kolb, R. Knitter – Workshop on Ceramic Breeder Blanket Interactions, Sept. 2011 Institute for Applied Materials (IAM-WPT)
Development of Process Modifications Modified melt-based process ► ► controlled droplet generation higher yield lower porosity improved process control Lithium orthosilicate with 2. 5 wt. % excess of silica or other compositions Li. OH & Si. O 2 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 4 M. H. H. Kolb, R. Knitter - BA Workshop Meeting on Tritium and Blanket Technologies, Sept. 2011 Institute for Applied Materials (IAM-WPT)
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 5 Element Reference material KALOS Relative changes Li (wt. %) Si (wt. %) 22. 8 24. 1 22. 5 0. 2 24. 6 0. 1 -1 % +2 % Pt (µg/g) 46 50 6 +9 % M. H. H. Kolb, R. Knitter – Workshop on Ceramic Breeder Blanket Interactions, Sept. 2011 Institute for Applied Materials (IAM-WPT)
Pebble size distribution The achievable pebbles size distribution is very narrow ► Improvement of the yield 6 M. H. H. Kolb, R. Knitter – Workshop on Ceramic Breeder Blanket Interactions, Sept. 2011 Institute for Applied Materials (IAM-WPT)
Effect of the dropping distance 140 cm dropping distance 1400 cm dropping distance Short dropping distances favor large dendritic grains 7 M. H. H. Kolb, R. Knitter – Workshop on Ceramic Breeder Blanket Interactions, Sept. 2011 Institute for Applied Materials (IAM-WPT)
Effect of dropping distance 140 cm dropping distance 1400 cm dropping distance Mean crush load of shiny pebbles: Mean crush load of opaque pebbles: 3. 7 N 13. 5 N The dropping distance changes the fraction “shiny”/”opaque” 8 M. H. H. Kolb, R. Knitter – Workshop on Ceramic Breeder Blanket Interactions, Sept. 2011 Institute for Applied Materials (IAM-WPT)
Variations in the Chemical Composition Change in the chemical composition Strengthening by additions of Ti. O 2 Formation of lithium metatitanate (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 9 M. H. H. Kolb, R. Knitter – Workshop on Ceramic Breeder Blanket Interactions, Sept. 2011 Institute for Applied Materials (IAM-WPT)
Effect of titania on pebble microstructure 10 mol% Li 2 Ti. O 3 15 mol% Li 2 Ti. O 3 20 mol% Li 2 Ti. O 3 The grain size is reduced with increasing lithium titanate fraction 10 M. H. H. Kolb, R. Knitter – Workshop on Ceramic Breeder Blanket Interactions, Sept. 2011 Institute for Applied Materials (IAM-WPT)
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 11 M. H. H. Kolb, R. Knitter – Workshop on Ceramic Breeder Blanket Interactions, Sept. 2011 Institute for Applied Materials (IAM-WPT)
Element mapping of as-received material 15 mol% MTi SEM SE image EDX-Mapping Ti mapping Si mapping Lithium titanate is located at the grain boundaries of lithium orthosilicate 12 M. H. H. Kolb, R. Knitter - BA Workshop Meeting on Tritium and Blanket Technologies, Sept. 2011 Institute for Applied Materials (IAM-WPT)
Element mapping of heat treated material 15 mol% MTi SEM SE image EDX-Mapping Ti mapping Si mapping Lithium titanate is located at the grain boundaries of lithium orthosilicate 13 M. H. H. Kolb, R. Knitter - BA Workshop Meeting on Tritium and Blanket Technologies, Sept. 2011 Institute for Applied Materials (IAM-WPT)
Composition of phases Initially Li. Ti. O 2 (Ti+III) is formed rather than Li 2 Ti. O 3 (Ti+IV) 14 M. H. H. Kolb, R. Knitter – Workshop on Ceramic Breeder Blanket Interactions, Sept. 2011 Institute for Applied Materials (IAM-WPT)
Mechanical strength of 500 µm pebbles 300 cm (15 mol% MTi) 15 Increase of crush load due to increase of dropping height Increase of crush load due to additions of Ti. O 2 M. H. H. Kolb, R. Knitter – Workshop on Ceramic Breeder Blanket Interactions, Sept. 2011 Institute for Applied Materials (IAM-WPT)
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 Li 2 Ti. O 3 16 M. H. H. Kolb, R. Knitter - BA Workshop Meeting on Tritium and Blanket Technologies, Sept. 2011 Institute for Applied Materials (IAM-WPT)
Thank you for listening 17 M. H. H. Kolb, R. Knitter – Workshop on Ceramic Breeder Blanket Interactions, Sept. 2011 Institute for Applied Materials (IAM-WPT)
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