| KDP(KH2PO4)crystal is widely used in laser frequency conversion,electro-optic modulation,fast light switching,etc.due to its excellent characteristics such as large non-linear photoelectric coefficient,high laser damage threshold and wide refractive index.The high-tech field is one of the key materials in laser confinement nuclear fusion devices.However,KDP crystals also have characteristics that are not conducive to optical processing,such as anisotropy,softness and brittleness,low hardness,easy deliquescent,and high thermal expansion coefficient.Therefore,the research on the mechanical properties of KDP crystals is helpful to solve the technical problems that restrict the ultra-precision processing of KDP crystals.This paper adopts the first-principle calculation method of density functional theory.First,establish the ideal KDP crystal atomic structure with tetragonal structure and analyze its structural characteristics;because the crystal will be subjected to polishing pressure during processing or polishing,it is then studied under certain pressure conditions.Under different pressure conditions,the anisotropy of the tetragonal structure of KDP crystals was studied and analyzed.By calculating the elastic constant,the study found that when the pressure reaches 4.5 GPa,the criterion of elastic stability of the tetragonal crystal under pressure is not met,and the tetragonal phase will undergo a phase change.By studying the charge density maps of0 GPa and 4GPa,the bonding characteristics of(110)crystal plane and(001)crystal plane of KDP crystal are analyzed.Subsequently,KDP crystals with K vacancies were constructed,four KDP crystal structures doped with Al,Fe,and Cr in a substitution manner,the elastic constants of the four KDP crystal structures with defects were calculated,and the elastic modulus,Basic mechanical parameters such as Poisson’s ratio and hardness.By comparing with the ideal KDP crystal and analyzing the changes of the mechanical properties of the defective KDP crystal,it is found that the brittleness of the defective KDP crystal increases in the order of doping Al,Fe,and Cr.In order to analyze the reasons for the changes,the electronic structures of four kinds of defect-containing KDP crystals were studied.Finally,the ideal KDP crystals,KDP crystals containing K vacancies and Al,Fe,Cr doped KDP crystals along the [001] direction of tension,compression and(001)plane [100],[010] and [110] three directions Shearing is studied.When calculating the ideal strength,this paper adopts the method of gradually increasing the equivalent strain,in which the crystal stretching and compression are applied at 0.01 for each step,and the shear is applied at 0.02.It is found that the ideal strength of the defect-containing KDP crystal is lower than that of the ideal KDP crystal in terms of tension,compression and shear.In order to understand the changes in the crystal structure during stretching and compression,the crystal structure diagrams at the starting point,the highest point and the end point in the corresponding stress-strain curve are taken,and the bond length and bond angle of the crystal structure are analyzed in detail.In addition,the relationship between the bond position between atoms and the shearing or stretching is analyzed,which provides a theoretical basis for the ultra-precision processing of KDP crystals. |
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