Study On The Thermal Property And Uniformity Of Large KDP/DKDP Crystals

Potassium dihydrogen phosphate (KH2PO4, or KDP) crystal is one of the most famous nonlinear optical crystals. Due to the characteristics of large Nonlinear-optical Coefficient, high transmittance from near infrared to ultraviolet, high laser induced damage threshold (LIDT), as well as frequency doublers of dye laser, it can be used to make various laser frequency doublers. In addition, KDP crystal is also a kind of excellent electro-optical crystal material. Large KDP crystal is the unique nonlinear optical material which can be used in the Inertial Confinement Nuclear Fusion (ICF) for its excellent electro-optical and nonlinear optical properties.With the development of ICF, the study of the KDP (DKDP) crystal size, quantity and quality has become a top priority. For example, the National Ignition Facility Project (NIF) need about 600 pieces of KDP/DKDP chips, which cross-sectional area is about 410mm × 410mm, used as frequency converter and Pockels cells, and those back-up chips did not included in the quantity. In China, the current "SG-Ⅲ" host and the ignition project has been one of the national medium and long-term major projects. In the future, there will be more demands for high quality large KDP/DKDP crystals.As a typical elastic and brittle matetial, KDP/DKDP crystal presents the cracking phenomenon as the temperature changes during the process of growth, getting out from crystallizers, transporting, machining, annealing and laser thermal radiation. The cracking phenomenon restricts the crystal growth technology development on large size, high quality and high service performance. The cracking phenomenon happends when the stress exceeds the tensile strength or compressive strength of material. The thermal properties are used to characterize the organizational structure changes of the material affected by the temperature changes. In order to understand the cracking mechanism of the large KDP crystal, we carried out a study of its thermal properties.The growth conditions of KDP/DKDP crystal will be different in the growth process due to the characteristics of slow growth rate, long growth cycle, etc., which will bring the nonuniformity of the thermal properties in different crystal growth stages. KDP/DKDP crystal is mainly grown from aqueous solution, impurity ions in aqueous solution will bring very important influence on the growth and the properties of crystal. Currently, there are numerous studies on the impact of the impurity ions bring to crystal optical properties, while fresh report on its impact on the thermal properties. In this paper, the thermal properties of KDP/DKDP crystal were studied. The primary coverage is as follows:1. Studied the thermal expansion coefficient uniformity of the large KDP/ DKDP crystal positions which grown in different growth intervals. Experimental results indicate:Large KDP/DKDP crystal thermal expansion coefficient has nonuniformity exist in different growth intervals, the thermal expansion coefficient of the crystal grown in the initial stage and the final stage have large deviations to the mid-section of the crystal growth; Got the average linear thermal expansion coefficients from 25 ℃ to 110 ℃ of KDP/DKDP crystals in each tangential.2. Tested the thermal conductivity of large KDP/DKDP which grown in different growth intervals. Experimental results indicate:Large KDP/DKDP crystal thermal conductivity has nonuniformity exist in different growth intervals, the thermal conductivity of the crystal grown in the initial stage and the final stage have large deviations to the mid-section of the crystal growth. In addition, we got the thermal conductivity of large KDP/DKDP crystal in 25,50,75,100,125 and 150 ℃.3. Grown a series of KDP crystals doping with Cs+, Mg2+, Al3+and ClO4-, studied their influence on the thermal expansion coefficients of the crystals. The doping results showed that:As the doping amount of Cs+ increases, the thermal expansion coefficient of x direction of the prismatic growing district first decreases and then increases. It has the same influence pattern to the x direction of pyramidal growing district, but it was less affected than the prismatic growing district. The thermal expansion coefficient of x direction decreases with the Mg2+ doping, the thermal expansion coefficient of z direction decreases when doped Mg2+, but it has no obvious relationship with the doping amount; The thermal expansion coefficient of KDP decreases when doped with Al3+, as the doping amount increased, the thermal expansion coefficient of prismatic growing district slightly reduce, the thermal expansion coefficient of the pyramidal growing district changes irregularly.The thermal expansion coefficient of prismatic growing district x direction first increases and then decreases with the ClO4- doping doping amount increase, the thermal expansion coefficient of x direction decrease to smaller than when it has not doped until the doping amount reached 12800ppm, the thermal expansion coefficient of z direction remained unchanged within 100ppm, after reaching 400ppm, it decreases with the doping amount increase, prismatic growing district has the same influence pattern to the pyramidal growing district; The impact of the four ions doping on x direction are significantly greater than z direction; The impact of Mg2+、Al3+ and ClO4- doping bring to prismatic growing district are significantly greater than pyramidal growing district.