Numerical Simulition And Experimental Study On The Growth Of P-terphenyl Crystals

Organic scintillators have a great application in nuclear experiments for the detection of gamma radiation.In recent years,p-terphenyl doped with pentacene crystals are used as the gain medium of the solid-state maser and show a good application prospect.In this dissertation,vertical Bridgman growth technique has been adopted to grow organic molecular crystals of high quality such as p-terphenyl.The finite volume method is adopted to account for the solidification process of p-terphenyl with different furnace parameters and shapes of the ampoule design by Ansys Fluent.According to the result of simulation,p-terphenyl single crystals and p-terphenyl doped with pentacene crystals are grown and the crystal structure and physical properties of crystals are tested and analyzed.During the growth of the p-terphenyl,the flow field is changed from double-cell structure to one unit cell,with the solid-liquid interface varies from concave to convex.The flow is mainly driven by the radial temperature difference.In the widen furnace,the intensity of convection decreases,and the curvature of m/c interface for different solidification fractions decreases.Keeping the location of the interface unchanged,with the increase of the temperature gradient of the gradient region,the superheating temperature of the melt and the temperature gradient near the interface both increase,and the curvature of the interface decreases.For the growth of p-terphenyl single crystals,it is more suitable to ensure that the temperature gradient is between 5 K/cm and 7 K/cm,and the superheating temperature does not exceed 10 K.The double-wall ampoule introduces a layer of thermal insulation,reducing the radial heat exchange capacity and increasing the stability of the temperature field and flow field.In the shouldering process,the bottom taper angle of the ampoule increases the axial temperature gradient near the bottom of the ampoule.The smaller the cone angle,the smaller the probability of heterogeneous nucleation due to the contact with the ampoule wall,so it is easier to obtain good single crystals at the beginning of crystallization.In the transition process,when the half cone angle is reduced to 30°,the solid-liquid interface begins to protrude toward the melt,which facilitates the stable growth of the p-terphenyl crystals.High quality transparent p-terphenyl single crystals and pentacene-doped p-terphenyl crystals are successfully grown by the vertical Bridgman technique with double-wall ampoule.The p-terphenyl single crystals are proved as the monoclinic crystals with P2₁/a space group and have the strongest diffraction peak in the?201?plane.The single crystals have little impurity and perform good crystallinity and fluorescent efficiency.The guest pentacene moleculars in the p-terphenyl crystals increase the density of the crystal defects,and reduce the crystal crystallinity,grain size and fluorescence efficiency.But the crystal doping in such small amounts does not disorder the monoclinic structure.