Low Temperature Thermal Properties of Epoxy Resin Matrix Composites as Insulation Materials for Large Scale Superconducting Magnet

Author:Dan Xin Ran

Supervisor:li lai feng


Degree Year:2017





Cryogenic-grade epoxy resins are widely used as impregnated insulating material of superconducting magnets due to their high aspect strength,high dielectric breakdown voltage,excellent thermal stability and chemical stability.However,both the cryogenic thermal contraction and the cryogenic thermal conductivity are urgently required to be precisely tailored for high-field(>10 T),large-scale superconducting magnets of magnetic confined fusion(mass of single coil>350 Ton and electromagnetic force>150 Ton·m-1).This is because the mismatch of coefficients of thermal expansion(CTE)of insulating material and support of superconducting components may cause to formation of micro-cracks in the epoxy resin.In this way,the thermal contraction of the epoxy resin is expected to be significantly reduced to the level of support(usually 316L/316LN austenitic stainless steels).On the other hand,the heat generation during formation of micro-cracks are hard to be rapidly dissipated due to its relatively low cryogenic thermal conductivity,thus the cryogenic thermal conductivity of the resin are expected to be significantly improved.Properties of thermosetting epoxy resins can be modified through incorporation of various functional fillers.This work aimed to tailor thermal properties especially the thermal expansion and thermal conductivity of cryogenic epoxy resin by means of incorporation of ZrW2O8 and LaFe10.5Co1.0si1.5 wtih negative thermal expansion behavior and graphene oxide with high thermal conductivity and high dielectric breakdown voltage.Three types of epoxy resin based composites have been developed and the mechanical properties as well as thermal properties were comprehensively investigated at low temperatures.Ressults indicated that the incorporation of ZrW2O8 tuned both the thermal expansion behavior and mechanical properties of the epoxy resin based composites.It was observed that the CTE of the ZrW2O8/epoxy resin composite monotonically decreases with the increase of the functional additive.It is observed that a reduction of 7.1%of the CTE compared with that of pure resin was obtained in the 6-300 K range when 11.2 wt.%of ZrW2O8 was incorporated.Moreover,the tensile strength of the ZrW2O8/epoxy resion composite increased by 22.8%and 10.7%compared with that of pure resin at room temperature and liquid nitrogen temperature respectively.The thermal expansion behavior of the epoxy resin can also be effectively tailored through incorporation of LaFe10.5Co1.0Si1.5.Similar to ZrW2O8/epoxy resin composites,the decrease in CTE of the LaFel0.5Co1.0Si1.5/epoxy resin composites monotonically depended on the increase of the amount of the LaFel0.5Co1.0Si1.5.A reduction of 25%of the CTE was obtained in the 200-400 K range when 52.3wt.%of LaFe10.5Co1.0Si1.5 was incorporated.Both the cryogenic thermal conductivity and the mechanical properties of the epoxy resin can be significantly improved through the incorporation of high thermal conductivity graphene oxide.In general,it is observed that the increase in the thermal conductivity of the graphene oxide/epoxy resin composites monotonically depended on the increase of the graphene oxide.An increase by 190%in cryogenic themal conductivity compared to the pure resin was achieved at around 6 K when only 0.5wt.%of graphene oxide was incorporated,whereas no deterioration in tensile strength at both room and cryogenic temperatures was observed compared with the pure resin.The present work paves the way to cryogenic applications of epoxy resins with respect to both thermal expansion and thermal conductivity.Moreover,tbe results provide solid material database for design and manufacture of large-scale superconducting magnets.