Mechanism Investigation Of Neutron Absorption Materials?Tb₂TiO₅,Dy₂TiO₅ And Fe-Dy₂O₃? Prepared By Ball Milling And Sintering

Neutron-absorption materials are unsed to regulate the reactivity of nuclear reactor by absorbing neutrons.Gray control rods are designed to achieve load following capability without substantial use of soluble boron.Tb2TiO5-Dy2TiO5 and Fe-Tb2O3-Dy2O3 are considered as excellent candidate materials used in gray control rods.Microstructures of ball-milling powders,annealed powders and Tb2TiO5 bulks synthesised by ball-milling and sintering were characterized using XRD,SEM,TEM and DSC.Meanwhile,the evolutionary mechanisms of powder mixtures during ball milling and subsequent annealing were analyzed.The microhardness,coefficient of thermal expansion and thermal conductivity of Tb2TiO5 sintered bulks were measured.After 96 h of ball milling at 500 rpm,TiO2 was completely solved in Th4O7 and the nanocrystalline solid solution was obtained in Tb4O7-17.605%TiO2 powder mixtures.At 500 rpm,the Dy2O3-17.636%TiO2 powder mixtures were refined,homogenized,nanocrystallized and later completely amorphized,and the transformation of Dy2O3 from the cubic to the monoclinic crystal structure was observed.The amorphous transformation resulted from monoclinic Dy2O3,not from cubic Dy2O3.However,at 200 rpm,the Dy2O3-17.636%Ti02 powder mixtures were only refined and homogenized.At the same ball-milling conditions,the crystalline structure of TiO2 has less effect on final product during ball milling.Ball milling induced the formation of supersaturated nanocrystalline solid solution of Fe(Dy,O)and Dy2O3 amorphization in Fe-25.68%Dy2O3 powder mixtures.During the whole ball milling process,a rapid decrease in Fe grain size was both observed in the initial stage,approaching a constant value was presented in the later stage.However,an opposite trend was found in the lattice constant of Fe at the initial stage of ball milling.Kinetics analysis indicated that the pressure and many defects generated during ball milling induced a dissolution of a few Dy atoms into the Fe lattice.The transformation of Dy2O3 from cubic to monoclinic crystal structure and then to the amorphization were also observed during ball milling.An intermediate phase with a similar crystal structure to that of cubic Dy2TiO5 was detected in the amorphous mixtures annealed from 800 to 1000 ?,which was a metastable phase that transformed to orthorhombic Dy2TiO5 when the annealing temperature was above 1050 ?.However,at the same annealing temperatures,pyrochlore Dy2Ti2O7 initially formed and subsequently reacted with the remaining Dy2O3 to form orthorhombic Dy2TiO5 in the homogenous mixtures.The microhardness and coefficient of thermal conductivity of sintered Tb2TiO5 bulks increased firstly with increasing ball milling time,and then decreased.However,an opposite trend was found in the coefficient of thermal expansion.For the sintered bulk with powder mixtures milled for 48 h,the highest values of both microhardness and thermal conductivity were observed,whereas the lowest coefficient of thermal expansion was found.With increasing testing temperature,the thermal conductivity of Tb2TiO5 bulks initially fell and then rebounded while an opposite trend was found in the coefficient of thermal expansion.