Study On Thermal Stability And Mechanical Properties Of High Temperature Gd-MOF/Polymer Shielding Composites

The Generation IV nuclear energy system with strong neutron fluence,high temperature and complex structure has much high requirements on shielding materials.As one of the light neutron shielding materials,the commercial boron-containing polyethylene hardly meets the high service requirements of advanced nuclear energy system owing to its limited serviced temperature,relatively low strength and neutron shielding performance.Gd-contained glycidylamine epoxy resin(AFG-90H)and Gd-contained polyimide(PI)are regarded as candidate shielding materials for advanced nuclear energy systems due to their good neutron shielding properties,thermal stability and mechanical properties.However,thermal stability and mechanical properties of composites will be affected by the harmful Gd-contained second phases.Metal organic frameworks(MOFs)with unique porous structure and organic surface can enhance the interfacial interaction between fillers and polymer matrix,thus improve the thermal stability and mechanical properties of polymer composites.Based on this,Gd(1,4-BDC)1.5 and Gd4(1,2,4,5-BTEC)3 were added into AFG-90H and PI to prepare Gd-MOF/AFG-90H and Gd-MOF/PI composites,respectively.The effect of Gd-MOF content on the neutron shielding properties,thermal stability and mechanical properties of polymer composites was studied.The main results of the study are as follows:1)The effect of interface between MOFs and polymer matrix on the material performance:Zr-MOF with 1 wt%content was added into bisphenol A epoxy resin(EP)to form UiO-66/EP and UiO-66(NH2)/EP composites.Compared with EP and UiO-66/EP,the glass transition temperature of UiO-66(NH2)/EP increased by 10.0%and 7.6%,the tensile strength at room temperature(RT)increased by 14.8%and 0.6%,the elongation at break at RT increased by 34.0%and 30.7%,respectively.Because of the interfacial physical interpenetration and chemical bonding,the binding of UiO-66(NH2)to EP polymer chains,the "bridge" connection of polymer chains and the crosslinking density of UiO-66(NH2)/EP were improved,thus improved the thermal stability and mechanical properties of composites.The effect of interface on the thermal stability and mechanical properties of MOFs/polymer composites was studied for preparing Gd-MOF/polymer composites.2)The design and performance of Gd-MOF/AFG-90H:Gd-MOF/AFG-90H composites with various Gd(1,4-BDC)1.5 contents were designed for the storage and transportation systems of spent fuels.The thermal neutron shielding ability of Gd-MOF/AFG-90H with the same thickness increased with the content of Gd(1,4-BDC)1.5 ranging from 0 wt%to 30 wt%.Compared with pure AFG-90H,the thermal decomposition temperature of Gd-MOF/AFG-90H increased by 29.4%at 50%weight loss in the thermogravimetric analysis,because AFG-90H polymer chains degraded into oligomers,whose thermal movements were hindered by Gd(1,4-BDC)1.5 agglomerates-The tensile properties at RT increased first and then decreased with the increase of Gd(1,4-BDC)1.5 content.Compared to pure AFG-90H,the maximum increase in the tensile strength(?73.5%)and elongation at break(76.8%)at RT were observed for 10 wt%content of Gd(1,4-BDC)1.5,while those at 150? were observed for 20 wt%content of Gd(1,4-BDC)1.5.The reason is that the"bridge" effect of polymer chains and the "pinning" effect of fillers improved tensile properties for low content of Gd(1,4-BDC)1.5,while the stress concentration caused by Gd(1,4-BDC)1.5 agglomerates reduced the tensile properties for high content of Gd(1,4-BDC)1.5.In conclusion,Gd-MOF/AFG-90H had good neutron shielding property,thermal stability and mechanical properties with the Gd(1,4-BDC)1.5 content in the range of 10?20 wt%.In addition,the glass transition temperature and tensile strength at 150? were lower than those of AFG-90H,which needed to be further improved.3)The design and performance of Gd-MOF/PI:Gd-MOF/PI composites with various Gd4(1,2,4,5-BTEC)3 contents were designed for advanced nuclear energy systems.The thermal neutron shielding ability of Gd-MOF/PI with the same thickness increased with the content of Gd4(1,2,4,5-BTEC)3 ranging from 0 wt%to 30 wt%.Compared with pure PI,glass transition temperature of Gd-MOF/PI increased by 7.2%in the thermogravimetric analysis,because the thermal movements of PI polymer chains were hindered by Gd4(1,2,4,5-BTEC)3.The tensile strength at RT increased first and then decreased with the increase of Gd4(1,2,4,5-BTEC)3 content.Compared to pure PI,the maximum increase in the tensile strength(?74.6%)at RT were observed for 3 wt%content of Gd4(1,2,4,5-BTEC)3,while that at 400? were observed for 1 wt%content of Gd4(1,2,4,5-BTEC)3.The reason is that the high stress transfer efficiency improved tensile strength for low content of Gd4(1,2,4,5-BTEC)3,while the defects caused by Gd4(1,2,4,5-BTEC)3 agglomerates reduced the tensile strength for high content of Gd(1,4-BDC)1.5.In conclusion,Gd-MOF/PI had good neutron shielding property,thermal stability and mechanical properties with the Gd4(1,2,4,5-BTEC)3 content in the range of 1-3 wt%.In addition,the elongation at break at RT and 400? were lower than those of PI,which needed to be further improved.In summary,the Gd-MOF/AFG-90H and Gd-MOF/PI composites were designed and fabricated,the effect of Gd-MOF content on the neutron shielding properties,thermal stability and mechanical properties of composites were studied.Based on this,the composite compositions were optimized.The shielding composites have potential to be used in the advanced nuclear energy systems and the storage and transportation systems of spent fuels.