| Alkali halide single crystals(Na Cl,KCl and KBr)have been widely used in the fields of infrared windows,tunable lasers,thermoluminescence dosimeters,food inspection and X-ray monochromators with the advantages of wide infrared transmission band,low intrinsic absorption coefficient,negative refractive index temperature coefficient and high optical distortion quality factor.However,the alkali halide crystals have shortcomings such as low hardness,easy deliquescence and difficult processing,which limit their further application.Researchers have found that forming mixed crystals or ion doping can solve the above problems and improve the mechanical,thermal,electrical and optical properties of these alkali halide crystals.Therefore,in the present research work,we have carried out investigations on the growth and performance of pure,Ce-doped and mixed single crystals based on Na Cl,KCl and KBr.The growth(and processing technology),and structural,mechanical,thermal,electrical and optical properties of the grown crystals were systematically studied.The key scientific and technical difficulties such as raw material purification,defect control,crystal processing and performance control were solved.The following are considered as the important aspects of this research work.High-quality pure,Ce-doped and mixed single crystals of Na Cl,KCl and KBr were grown by an improved resistance heating pulling method using the composition design and purification,temperature field design and control,crystal growth and annealing technologies.In the process of crystal growth,the pulling speed,rotating speed,circulating water temperature,cooling rate and other parameters were tested and analyzed,and the best process parameters were found(Rotating speed:6-8 r/min,pulling speed:1-2 mm/h,circulating water temperature:28±1?,cooling rate:8-10?/h,axial temperature gradient:1-2?/mm).The growth of Ce-doped single crystals have been carried out in a protective atmosphere to prevent Ce ions from being oxidized.The cracking factors in the process of crystal growth were discussed,which were mainly caused by low raw material purity,unreasonable process parameters and poor temperature field design.Also,the melt structure in the crystal growth process was analyzed.A quasi-lattice model was proposed based on density,energy,coordination number and ion distance.The heat conduction equation was used to analyze the boundary conditions during the crystal growth process.Based on the calculation results,the influence of the melt flow on the solid-liquid interface was discussed.There are available mainly natural convection,forced convection and Marangoni convection due to temperature and concentration differences in the melt,which interact during crystal growth to adjust the shape of the solid-liquid interface.The XRD,EDX and AFM techniques were used to carry out theoretical research on the deliquescent mechanism of alkali halide crystals.It was found that no new substance was produced on the surface of alkali halide crystals after deliquescence.Deliquescence is just a physical process,which is closely related to temperature and humidity.When the humidity exceeds the critical humidity,deliquescence will occur,and the rate of deliquescence will become faster and faster.Based on the deliquescent properties of alkali halide crystals,a polishing solution with water as the solvent,ethanol as buffer and surfactant X as active agent was designed.The best ratio between them is 9.5:87.5:3.The effects of polishing pressure,polishing fluid flow rate,rotating speed of polishing disc and polishing time on the removal rate and roughness of the crystal surface were studied,and the best polishing process parameters were found:the best pressure is 0.1042 MPa,the polishing fluid flow rate is 15 ml/min,the rotating speed of polishing disc is 30 r/min and the polishing time is 20 min.The crystal structure,thermal,optical,mechanical and electrical properties were analyzed by XRD,hardness,stress,absorption spectrum,transmittance,electrical conductivity and dielectric constant of alkali halide single crystals and mixed crystals.The study found that the XRD of alkali halide single crystals,(KCl)x(KBr)1-xand(Na Cl)0.1(KCl)y-x(KBr)1-ycrystals consist of a single phase,while other binary and ternary mixed crystals are composed of two phases.From the XRD data,the lattice constant was calculated,the volume composition of the crystal was estimated,the Debye-Waller factor of all crystals was determined by the Wilson diagram method,and the mean square amplitude,Debye temperature,Debye frequency,compressibility and average speed of sound of the crystal were calculated.It is found that the density and refractive index of alkali halide crystals have a linear relationship with the composition,while the Debye-Waller factor and Debye temperature have a nonlinear relationship with the composition.The vibrational frequencies of all the crystals studied are in the infrared range.Na Cl,KCl,KBr,and(KCl)x(KBr)1-xcrystals have no absorption in the test band,and the transmittance is around 90%,but KCl and(KCl)x(KBr)1-xcrystals have a small amount of absorption in the transmission band,which is caused by impurities in the crystal raw materials.The highest transmittance of other binary and ternary mixed crystals is less than 40%,and a large amount of impurity absorption occurs in the band.The hardness and thermal stress of the binary and ternary mixed crystals are higher than that of the pure end member crystals.The DC and AC electrical conductivities,dielectric constant,and dielectric loss factor of all the crystals studied are found to increase with the increase in temperature,and their changes with composition are nonlinear.The mechanical and optical properties of the studied crystals have been found to be improved by Ce doping.XRD,XPS and ICP analyses have shown that Ce ions have been successfully incorporated into the single crystal and they predominantly exist as Ce3+ions.The concentration of Ce ions in the crystal is found to increase with the vertical length of the crystal.The hardness of the crystal is found to increase with the increase of Ce ion concentration and the crystal composition.When y=0.8 and x=0.7,the hardness of(Na Cl)x(KCl)y-x(KBr)1-y:Ce crystal reaches the maximum value,which is 71.7 kg/mm2.The stress of Ce doped crystals is found to increase with the increase in composition.After Ce doping,the crystal shows an absorption peak in the absorption spectrum,and with the increase of Ce concentration,the absorption peak blue-shift can be observed.The Ce doped crystal has an excitation peak near 262 nm,which corresponds to the 4f?5d transition of Ce ions,and there are emission peaks of different intensities in the range of 342-394 nm,corresponding to the 5d?4f transition of Ce ions.The thermoluminescence intensity of Ce doped crystals are found to be higher than that of the pure(un-doped)crystals.Except for (Na Cl)0.5(KCl)0.5:Ce crystals,Ce doped alkali halide crystals have only one luminescence peak.It shows that there is only one trap energy level in the crystal.Both the photoluminescence(PL)and thermoluminescence(TL)emission peak intensities of the crystals are found to increase with the increase in Ce concentration.When the concentration reaches 0.8%,the emission peak intensity reaches the maximum.Therefore,the optimal doping concentration of Ce ions is considered as 0.8%.The thermoluminescence parameters such as activation energy(E),luminescence dynamics order and frequency factor(s)of the Ce doped alkali halide crystals were determined.It has been found that the decay time of the crystal gradually decreases with the increase of system components and Ce concentration. |
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