| Potassium dihydrogen phosphate (KH2PO4, or KDP) and its analogs deuterated potassium dihydrogen phosphate ((K(H1-xDx)2PO4, or DKDP), which is an unfailing soluble crystal because of its multifunction properties, has been researched for about80years. It has been widely used in laser frequency conversion, parametric oscillation, electro-optical modulation, high speed Q-switch and piezoelectric transducers and other fields, because of its excellent piezoelectric, electro-optic, ferroelectric and non-linear effects. In recent years, inertial confinement fusion (ICF) project is to flourish. Large size KDP (DKDP) crystal is so far the only non-linear optical crystal materials can be used in ICF project, so the research about this crystal is brought to a new level.Currently, the study about KDP crystal is focused on the following two aspects: one is to accelerate the growth rate of crystals; another is to improve the optical properties of crystals. Very slow growth rate, a few mm a day, of conventional techniques resulting in high cost of final crystals is particularly annoying. For this reason,"point seed" rapid growth technique, by which the growth rate can be one order of magnitude or more large than that of conventional techniques, has been developed during recent years.However, there are two major problems for such rapid growth technique limiting its use. Firstly, this process of rapid growth can be very unstable because of spontaneous nucleation from solutions at the high supersaturation needed to obtain high growth rate. Another, optical properties especially of prismatic sector of rapidly grown crystals, such as UV transmittance, resistance to damage by laser radiation, are deteriorated greatly. Therefore how to enhance the solution stability and improve the optical quality of rapidly grown KDP crystals are both extremely important.Therefore, in this paper we studied the effects of various parameters involving supersaturation, hydrodynamic conditions, impurity metal ions, organic additives and other factors on rapid crystal growth habit and optical quality of KDP crystal. Moreover, we also explored the issue about "optical heterogeneity of larger KDP crystal". The main contents of this paper are as follows:1. The dependence of growth rate of (100) face of KDP crystal on supersaturation was measured by using laser polarization interference technique. KDP crystals were grown from different supersaturation solutions by conventional cooling method and the "point seed" rapid growth method, and optical properties of grown crystals were characterized. The effects of different supersaturation on the growth habit and optical properties of KDP crystals were systematic studied. Experimental results showed that:the greater the degree of supersaturation, the faster the growth rate of the crystal, and the growth rate of X direction is growth faster than that of Z direction, which also led to the decrease of crystal aspect ratio with the increase of the saturation. With the increase of the solution saturation, the solution stability becomes worse, and crystals prone to appear inclusions, cracking, and other macro defects. Moreover, the ultraviolet transmittance of the grown crystal decrease, the optical uniformity become worse, light scattering becomes serious and the laser damage threshold decrease. Additionally, optical quality of pyramidal sector of crystals is superior than that of prismatic sector. Impurity of metal ions is an important factor to cause growth difficulties and performance degradation of crystals grown in high supersaturation solutions.2. The metastable zone widths of growth solutions in different treatment conditions were measured. The effects of stirring, seed crystal, filtering, overheating, and different additives on the solution stability were investigated and the impact mechanisms were analyzed respectively. KDP crystal was grown successfully by "point seed" rapid growth method at growth rate of about10mm/day. Experimental results showed that:The metastable zone width of growth solutions increase with the decrease of solution saturation temperature. The metastable zone width decreases without stirring, and30rpm/min stirring make the solution maintain uniform. The introduction of the rapid growth shelf has little effects on metastable zone width of growth solutions. In the presence of seed crystal, the metastable zone width of growth solutions decreases. The metastable zone width of growth solutions increases with filtration and overheating of solutions. Effect of additives on the metastable zone width is complex, the metastable zone width of growth solutions increases in the presence of100ppm EDTA or DCTA, while it decrease in the presence of100ppm Fe3+orSO42-.3. Some organic compounds, including ethylene diamine tetraacetic acid (EDTA), diethylene triamine pentaacetic acid (DTPA), cyclohexane diamine tetraacetic acid (CDTA) and citric acid (CA) were selected as new additives and were added into the growth solution. A series of KDP crystals were obtained by the "point seed" rapid growth technique from these solutions. The effects of these organic additives on growth habit and optical properties of KDP crystals were described and compared with each other. The impact reason was also analyzed combined with the structure of KDP crystals and the chemical characteristics of these compounds. The results showed that, the addition of an appropriate amount of EDTA, DTPA, CDTA or CA has positive effects on growth habit and optical quality of KDP crystals. It can enhance the solution stability, reduce the "dead zone" and promote the growth rate of (100) faces of the crystal. Furthermore, it can increase the UV transmittance and laser damage threshold, improve the optical homogeneity and mitigated the light scattering of these as grown KDP crystals. However, excess addition had adverse effects. The appropriate addition concentration of EDTA, DTPA and CDTA is all about100ppm. The beneficial influence of EDTA, DTPA and CDTA is due to the chelating action of these organic additives with metal ion impurities in growth solutions. The appropriate addition concentration of CA is about1000ppm. The beneficial influence of CA is considered to be connected with the synergistic bactericidal effect and reducing effect of surface energy.4. Several key optical parameters of large scale KDP and70%DKDP crystal self-grown by our laboratory including the refractive index, the nonlinear coefficient and the linear absorption coefficient were measured. These dates were compared with those of foreign reports. These dates of different sector of large scale crystals were also compared and the reasons for the differences between these dates were analyzed combined with the crystal growth conditions and test conditions. The measured results showed that the optical parameters of KDP crystal has litter difference with those of foreign reports, but the dates of DKDP crystal has large difference with those of foreign reports. The different deuterium contents of DKDP samples is lead to the larger differences dates. Furthermore, there is inhomogeneity in different parts of the large scale crystal. For example, the refractive indices of sample selected from part close to the recovery area of crystal are less than that of selected from part away from the recovery area. These deviations are in the order of10-5-10-4. The inhomogeneity of refractive indices and the nonlinear coefficient is related to the different crystalline quality of different parts in large scale crystal, while the inhomogeneity of linear absorption coefficient is related to the different impurity metal ions contents of different parts in large scale crystal. |
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