Research On Growth And Residual Stress Of KDP/DKDP Crystals

Inertial Confinement Fusion(ICF)with safety,cleanliness and controllability,is one of effective methods for human beings to resolve the energy problem in the future,which has attracted attentions of the countries around the world.Since the frequency conversion crystal is an important optical element in high power laser system for ICF instrument,it is required that the crystal have large size,high damage threshold,and excellent nonlinear property.Throughout all the nonlinear single crystal materials,at present,KDP and DKDP crystal is the only nonlinear crystal material used in high power system for ICF instrument.It is usually used as third harmonic generation(THG)crystal in ICF project,due to the fact that DKDP can efectly minimize the transverse stimulated Raman scattering caused by high power laser.Today,the DKDP crystal was mainly grown by using the seed along Z crystallographic direction,however,the Z seed can cause much dislocation in crystal.These dislocations and the substitution of deuterium for hydrogen will make residual stress in DKDP crystal,which limit the fluence quality of output laser.Thus to reduce the dislocation density in DKDP crystal,in this dissertation,we studied the growth conditions and growth mechanism of DKDP crystal grown rapidly with directional seed.And then,neutron diffraction technology was used to investigate crystal structure and the residual stress of DKDP crystals with different deuterium contents.The crystal structure and the three-dimensional stress were successfully measured,respectively.The main research contents of this dissertation included:1.The KDP crystals were grown rapidly with 59° direction seed at different temperature ranges and in solutions with various pH values.Then the optimum growth conditions suitable for KDP crystal with 59° direction seed were ensured through transmission spectrum and conoscopic figure of these crystals,the results of which tell us the optimum growth temperature range and solution's pH value are 58??52? and 3.6,respectively.In order to deeply understand the growth habit of KDP/DKDP crystal using 59°direction seed,we further studied the microscopic growth mechanism on prismatic face of KDP crystal.The research results tell us that the main growth mechanism is dislocation at low supersaturation(??0.06).In addition,the AFM morphology shows that a hollow core occurs in the center of growth hillock,which is the partial reason for occlusion and cracking.Due to the hollow core,the slope of hillock firstly increases with increasing supersaturation,and then tends to become a constant value.Refering to AFM findings,we founded one model of growth hillock to analyze the variation of slope with supersaturation.Meanwhile,with the theoretical calculation,it shows that the hollow core will generate if the Burger's vector is more than 5.9 A.The AFM morphologies grown in solutions with different pH values show that the main growth mechanism of KDP crystal includes dislocation and two-dimension nuclear.In the same supersaturation condition,the lower pH value of growth solution is,the more easily two-dimension nuclear occurs.According to all the AFM results,we draw a line that distinguishes dislocation and two-dimension nuclear at a certain supersaturation and pH value,which can be regarded as a reference for choosing the growth condition of KDP/DKDP crystal.The crystallization process of DKDP crystals were studied by in-situ infrared spectroscopy,the results of which show the growth element of DKD crystal is(H/D)2PO4-.Since the deuterium bond is weaker than hydrogen bond,the D2O in(H/D)2PO4-[(H/D)2O]hydrated ion takes off more easily than H2O.Correspondingly,the deuterium concentration in growth element is lower than hydrogen concentration,which makes the deuterium segregation coefficient of DKDP crystal smaller than 1.2.By adoppting neutron powder diffraction technology,we studied the crystal cell structure of series DKDP crystal grown by traditional method.It shows that the lattice parameter a increase with increasing deuterium content in crystal,whereas,the deuterium content has little impact on the lattice parameter c.The variation of lattice parameter a is caused by the increase of(O-D-O)between adjacent PO4 groups with increasing deuterium content.In addition,the deuterium content in crystal was measured by neutron diffraction technology.And then,we obtained the deuterium segregation coefficient(Dc = 0.64x exp(0.00421Ds)×Ds)of DKDP crystal grown by traditional method depending on the deuterium contents in crystals and the corresponding solution.We also studied the relationship of Raman,Infrared spectroscopy and deuterium content.And we normalized the step of testing deuterium content of DKDP crystal by using Raman and Infrared spectroscopy.As for the rapid growth DKDP crystal with high supersaturation(?= 0.04?0.06),we studied the deuterium segregation using Raman and Infrared spectroscopy,and obtained the deuterium segregation coefficient Dc(%)Average= 0.447 ×exp(0.00785Ds)×Ds.Through the research of the effect of supersaturation on deuterium content in DKDP crystal grown from 80%solutions,it shows the deuterium content in crystal decrease with increasing supersaturation less than 0.03,and then,the average deuterium content tends to become a constant value with increasing supersautration between 0.03 and 0.10,however,the homogeneity of deuterium distribution become worse with increasing supersaturaion.3.The three-dimension stress of DKDP crystal grown rapidly with Z seed were measured by neutron diffraction.In crystallographic coordinate,the macro-strain is at 10-3?10-4 magnitude,and the macro-stress were calculated through the Hooke law.The normal stress is much larger than shear stress,meanwhile the maximum normal stress can be up to 320 MPa.The macro-stress is uncorrelated with the deuterium content in crystal.The stress along Z direction for all the tested DKDP crystals is compressive,however,at least one of normal stresses along X and Y direction is tensile,when the compressive stress along Z direction is up to 115 MPa,both of the stresses along X and Y direction are tensile.The average micro-strain of DKDP crystal,calculated from the full width at half maximum(FWHM)of diffraction peak,increases firstly when deuterium content is less than 50%,and then decrease with deuterium content,in other words,the micro-strain in DKDP crystal with 50%deuterium content reaches the maximum of 0.0023,illustrating the lattice distortion is largest in this deuterium content.The normal stress along X direction in DKDP crystal grown with 59° seed with wedge shape were also studied by neutron diffraction technology,finding that the stress of directional DKDP crystal is smaller than that of DKDP crystal grown with Z seed.The largest tensile stress,the largest compressive stress and average micro-strain in directional DKDP crystal all increase with their growth supersaturation.Compared with the growth conditions,we find that the optimum supersaturation for DKDP crystal growth is 0.06.The normal stress in 70%DKDP crystal at different temperatures was measured by using the in-situ neutron diffraction technology.From the results,the phase-transition temperature along Z direction is different from that of X direction.The phase-transition temperature along Z direction ranges from 165? to 170?,however,the phase-transition temperature along X direction ranges from 190? to 195?.The phase-transition of DKDP crystal along Z direction occurs earlier than cracking,whereas,DKDP crystal along X direction is contrary to Z direction.The DKDP crystal along Z direction can endure the largest thermal stress of 272 MPa,while the X direction can endure the largest thermal stress of 1163 MPa.From the change of the intensity,F WHM,and the shape of neutron diffraction peak,it can be illustrated the dislocation recovery in the annealing process.The atoms with small mismatched degree get enough energy and first return to period lattice at the prior annealing process,while the atoms with large mismatched degree get enough energy and return to period lattice at high temperature.The experiment results show that two temperatures are critical to annealing process,which are 90? and 140?.Firstly,the temperature should be slowly increased to 90?,and then be kept for a long time until partial defects return.Afterwards the temperature continues to be increased up to 140?,and be kept for a long time until other defects return,at last be cooled slowly to room temperature.The measurement of normal stress and strain along X direction of THG-cut 70%DKDP plate shows that the pyramidal region exist tensile residual stress reaches up to 175MPa.Thus the pyramidal region of 70%DKDP crystal should be carefully carried and cutted in case of cracking.The micro-strain in 70%DKDP grown by traditional method is less than 0.009,which is quite less than that in 70%DKDP crystal grown by rapid growth method.