Exploration Growth Of Several Kinds Of Bismuth-based Photo-electric Functional Crystals And Their Basic Properties Characterization

Photo-electric functional crystals have always been the research hotspot in the field of material science.Among the photo-electric functional crystal materials developed at present,some crystals with excellent comprehensive performance have been developed into various optoelectronic devices(sensors and lasers,etc.)and pushed to practical applications.With the rapid development of science and technology,many kinds of application scenarios showing the increasingly demands to promote the performance of the devices,thus urgently needing to explore new photo-electric functional crystals.Therefore,bismuth-based crystals were studied in this thesis.Single crystal growth for bismuth octaborate(α-Bi2B8O15),bismuth calcium titanate(CaBi4Ti4O15,CBT)and bismuth titanate(Bi2Ti2O7,BTO)crystals were investigated,using the Kyropoulos,flux and Bridgman-Stockbarger methods.Crystal structure,mechanical,thermal,electrical and optical properties of these crystals were also studied.Main contents and conclusions were as following:(I)Growth and quality characterization of bismuth-based crystals(1)Crystal growth for α-Bi2B8O15 CBT and BTO crystals were tried by using the Kyropoulos,flux and Bridgman-Stockbarger methods.It was found that the α-Bi2B8O15 and CBT crystals showed strong layered growth habitus,which made it difficult to grow into large size with high quality.In addition,through the optimization of crystal growth parameters,high quality BTO crystal was obtained by flux method with dimensions up to 16×13×5 mm3.(2)The crystalline quality of αa-Bi2B8O15 and BTO crystals was characterized.The rocking curve tests showed that the full width at half maximum(FWHM)values for theα-Bi2B8O15 and BTO crystals were 0.04° and 72",respectively.In addition,Laue diffraction tests exhibited that the diffraction spots of the grown crystals were symmetrical and maintained good consistency.The above results indicated that the grown α-Bi2B8O15 and BTO crystals had good crystallinity.(3)The growth defects for α-Bi2B8O15 and BTO crystals were studied.It was found that there were several kinds of common macroscopic defects,e.g.,inclusions,negative grains and addition grains in the grown crystals.The(010)facet and(111)facet for α-Bi2B8O15 and BTO crystals were observed by scanning probe microscope,respectively,and there was no growth hill defect was observed.The dislocations of the crystals were studied by chemical etching method.By observing the etched samples under microscope,it was found that the morphology of the dislocations etched pits was consistent with the symmetry characteristics of the crystal structure.(Ⅱ)Crystal stucture and basic physical properties(1)The structures of α-Bi2B8O15,CBT and BTO crystals were refined by Rietveld method.The reasons of the layered growth behavior for α-Bi2B8O15 and CBT crystals were explained from the perspective of microstructure.Besides,based on the refined structure data,the ideal morphology of α-Bi2B8O15,CBT and BTO crystals were calculated by using the Bravais-Friedel Donnay-Harker method,and the calculated results were in good agreement with the crystal morphology obtained by experiments.(2)The basic physical and chemical properties of α-Bi2B8O15 and BTO crystals,such as X-ray photoelectron spectroscopy(XPS),density and mechanical properties(hardness,yield strength,fracture toughness and brittleness index),have been studied.The results showed that the α-Bi2B8O15 and BTO crystals had good chemical stability.In addition,the relatively weak mechanical properties of α-Bi2B8O15 crystal were related to the layered structure.Besides,the BTO crystal had good mechanical strength,but the brittleness was relatively high,so more attention should be paid to the subsequent processing and applications.(3)The thermal properties of α-Bi2B8O15,CBT and BTO crystals were measured and studied.It was found that the α-Bi2B8O15 crystal had good thermal stability with melting point of 722℃,while CBT and BTO crystals were incongruent melting compounds with decomposition points of 1240℃ and 1254℃,respectively.The specific heats of α-Bi2B8O15 and BTO crystals increased with increasing temperature,and the specific heat values of α-Bi2B8O15 and BTO crystals were found to be 0.61 J·g-1·K-1 and 0.45 J·g-1·K-1 at room temperature,respectively,which were comparable to that of BIBO crystal at room temperature(0.48 J·g-1·K-1).In addition,the large thermal expansion coefficient(36.64×10-6 K-1)and relatively small thermal diffusion coefficient(0.32 mm2/s@25℃)and thermal conductivity(0.80 W·m-1·K-1@25℃)of the α-Bi2B8O15 crystal were related to the layered structure in the Y direction.The thermal expansion coefficient of the BTO crystal was small(5.80×10-6K-1),which was smaller than that of BIBO crystal(α11=7.3×10-6 K-1).In addition,the thermal conductivity of BTO crystal was positively correlated with temperature,and the thermal conductivity was found to be 1.55 W·m-1 K-1 at room temperature,which was comparable to that of YCa4O(BO3)3 crystal(κ22=1.72 W·m-1·K-1).(Ⅲ)Basic photo-electric properties(1)The high temperature resistivity,dielectric permittivity and dielectric loss of α-Bi2B8O15 and BTO crystals were studied.The results showed that the α-Bi2B8O15 crystal had high resistivity,low dielectric loss(tanδ22:2.5%@550℃)and good temperature stability(Δε22/ε0:15%@550℃).Furthermore,the electrical resistivity along the Y-axis was found to be 1.5× 1010 Ω·cm at 300℃,which was nearly two orders of magnitude higher than that of the La3Ga5.5Nb0.5014 crystal(～8×108 Ω·cm).The BTO crystal had relatively small resistivity,and the resistivity decreased rapidly with increasing temperature(<104Ω·cm@600℃).The small resistivity of the BTO crystal was related to the corresponding band gap value(2.83 eV)and the disorder structure.The relative dielectric permittivity of BTO crystal was relatively high(270@600℃),and the dielectric relaxation phenomenon was found in this crystal.The dielectric relaxation of BTO crystal was related to the disordered displacement of the high symmetry position in the crystal structure.In addition,the piezoelectric activities of theα-Bi2B8O15 and CBT crystals were analyzed from the perspective of polyhedral distortion and dipole moments.It was found that the net dipole moment of CBT crystal was 37.35 D,which was slightly higher than that of BIBO crystal(35 D)with excellent piezoelectric properties,indicating that the CBT crystal may have excellent piezoelectric properties.(2)The optical properties of α-Bi2B8O15,CBT and BTO crystals were measured and studied.UV-Vis diffuse reflectance spectroscopy measurements showed that the band gap values of α-Bi2B8O15,CBT and BTO crystals were found to be 4.41 eV,3.11eV and 2.83 eV,respectively.Thus the electrical resistivity of α-Bi2B8O15 crystal was higher than the other two kinds of crystals in theory,which was consistent with the experimental resistivity values of α-Bi2B8O15(1.5×1010Ω·cm@300 ℃)and BTO(1.6×107Ω·cm@300℃)crystals.The transmittance spectra of α-Bi2B8O15 and BTO crystals showed wide transmittance range(0.26～4.4 μm;0.44-7.3 μm)and high transmittance(>60%@0.78～3.3 μm;>60%@0.69～5.4 μm).The infrared cutoff edge of α-Bi2B8O15 crystal(4.4 μm)was larger than thoses of borate crystals such as β-BaB2O4(3.5 μm),LiB3O5(3.2 μm)and CsB3O5(3.0 μm),besides,the infrared transmittance range of BTO crystal covered an important band of atmospheric window(3～5 μm).The above analysis showed that the α-Bi2B8O15 and BTO crystals have potential research value in the mid-infrared band.The Raman spectroscopy showed that the BTO crystal had low phonon vibration frequency(<800 cm-1),which was also one of the reasons for the BTO crystal had a large infrared cutoff edge.In addition,the electronic structures of α-Bi2B8O15 and BTO crystals have been studied combined with previous reports.It was found that the BiO6 octahedron and BOx(x=3,4)polyhedron played a key role in the optical properties of α-Bi2B8O15 crystal,and the TiO6 octahedron was the main contributing group of BTO crystal.