Numerical Simulation And Experimental Study Of The Growth Of Pure And Doped P-Terphenyl Crystals By The Bridgman Method

Organic crystal materials,such as p-terphenyl,tetracene and pentacene,have extensive and considerable applications in the fields of scintillation counter,polymer light emitting diodes,organic field effect transistor and maser.Crystal growth using the Bridgman technology has the advantages of low cost,wide application,strong adaptability and simple equipment.In this thesis,according to the characteristics of the Bridgman technology and the difficulty factors of organic crystal growth,numerical simulations and experiments of p-terphenyl crystal growth by the Bridgman method have been carried out.The influences of the shape and structure of growth vessel(ampoule)on crystal defect,lattice structure and crystal properties have been researched.At the same time,the effects of different doping concentrations on lattice structure and crystal properties have been studied by the crystal growth experiments of p-terphenyl doped tetracene and p-terphenyl doped pentacene.The details are as follows.(1)The transient simulations of the thermal field and fluid field coupling in the process of p-terphenyl single crystal growth by the Bridgman method have been conducted.The effects of the tip shape and cone angle of the ampoule on temperature distribution,melt flow and crystallization interface in the process of crystal growth have been analysed.The simulation results show that compared with the flat bottom ampoule and round bottom ampoule,the tapered tip of the cone bottom ampoule can effectively change the thermal field distribution,and form a flat crystalline interface in the crystallization process and contribute to the growth of crystals.It also can form the upper and lower stable vortices in the flow field,which is beneficial to drive impurities to the edge.Moreover,the smaller the cone angle is,the more favorable the crystallization interface and flow field distribution are to the growth of high quality crystals.(2)According to the Bridgman technics of crystal growth and the properties of organic materials,the design of ampoule has been optimized for the technical difficulties in the preparation process.A series of improvement measures have been introduced,such as funnel loading,small hole pumping,filler sealing and interlayer control.Experiments show that the improved double-wall ampoule can effectively prevent impurities produced in the preparation process from entering into the inner tube,and reduce the crystal defects and improve the crystal quality and performance.(3)Experiments of p-terphenyl single crystals growth by the Bridgman method have been carried out in the single-wall ampoule,original double-wall ampoule and improved double-wall ampoule,respectively.Comparative tests have been conducted to analyze the properties of p-terphenyl single crystals grown in different ampoules.Powder X ray diffraction(XRD),Fourier transform infrared spectroscopy(FTIR)and nuclear magnetic resonance spectroscopy(NMR)show that the spectra of three samples have no additional characteristic peaks except p-terphenyl,and the characteristic peak positions have no obvious shift.Hyperchromic effect of impurities in ultraviolet-visible absorption spectra and the strong fluorescence background in Raman spectra show that the single-wall and original double-wall ampoules samples have more impurities and defects.The XRD analysis indicates that the crystallization of the improved double-wall sample is greatly improved.The fluorescence spectra study shows that the fluorescent properties of the improved double-wall sample are better.(4)Growth experiments of tetracene-doped p-terphenyl crystals(tetracene,0.01 wt% and 0.1 wt%)have been carried out in the improved double-wall ampoule,respectively.The performances of the samples of different doping concentrations and their relations to p-terphenyl single crystals have been tested and analysed.The XRD patterns indicates that two crystals both are of the monoclinic system and space group P21/a.As the doping concentration increases,the average crystallite size decreases,and the crystalline perfection declines.FTIR and NMR show that the spectra of two samples have no additional characteristic peaks except p-terphenyl.Ultraviolet-visible absorption spectra and fluorescence spectra shows that with the addition of tetracene molecules,the intensities of the characteristic peaks due to p-terphenyl molecular transition in two spectra decrease,and the characteristic peaks owing to tetracene molecule transition appears in the two spectra.Moreover,with the increase of doping concentration,the intensities of the characteristic peaks of host molecules decrease continuously,and the intensities of the characteristic peaks of guest molecules increase accordingly.(5)Growth experiments of pentacene-doped p-terphenyl crystals(pentacene,0.01 wt% and 0.1 wt%)have been carried out in the improved double-wall ampoule,respectively.The performances of the samples of different doping concentrations and their relations to p-terphenyl single crystals have been tested and analysed.The XRD,FTIR and NMR studies indicate that no additional peaks are observed in the spectra of two doped crystals.As the doping concentration increases,the average crystallite size decreases,and the crystallinity declines.Ultraviolet-visible absorption spectra and fluorescence spectra shows that with the addition of pentacene molecules,the intensities of the characteristic peaks owing to p-terphenyl molecular transition in two spectra decrease,and the characteristic peaks due to pentacene molecule transition appears in the two spectra.Moreover,with the increase of doping concentration,the intensities of the characteristic peaks of host molecules decrease continuously,and the intensities of the characteristic peaks of guest molecules increase accordingly.