Study On The Mechanical Propenies And Cracking Phenomenon Of KDP Crystal

Potassium dihydrogen phosphate (KH2PO4, or KDP) is one the most famous nonlinear optical crystal, it has been more than80years for their growth and research, and it could be described as a kind of time-honored crystal grown from aqueous solution. KDP was found with large Nonlinear-optical Coefficient, high laser induced damage threshold (LIDT) and high transmittance from near infrared to ultraviolet, as well as frequency doublers of dye laser. Thus, it can be used to make various laser frequency doublers. KDP crystal was also an excellent electro-optical crystal material. In addition, with the application of high power laser system on the controlled nuclear fusion, large KDP crystal is the only nonlinear optical crystal which can be used in the Inertial Confinement Nuclear Fusion (ICF) for its excellent nonlinear optical property.In recent years, with the development of ICF, requirements of the KDP crystal size, quality and quantity was further enhanced. 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, the size, the quantity, and the quality of KDP (DKDP) crystals will be in higher requirements.KDP crystal presents the cracking phenomenon during growth, getting out from crystallizers, transporting, machining and anneal process. The cracking phenomenon restricts the development of technology on large-size and high-quality crystal growth. At present, research actuality of KDP crystal concentrates in the growth technology, growth mechanism and optical quality, while the mechanical characteristic of cracking is reported little. Therefore it's necessary to discover the mechanical parameters, cracking mechanism and impurity influence.The acquisition of KDP crystal is mainly through growing from solution. When grown KDP crystal from solution, the influence of impurities on growth habit, optical quality and mechanical properties of KDP crystal should be taken into consideration. Nowadays, the research about impurity ions effect on mechanical properties of KDP crystal is relative few. Accordingly, we chose Na+, Cr3+and SO42- as additives to study their influence on growth habits, crystal quality and mechanical properties of KDP crystal. The primary coverage is as follows:1. The automatic and high precision system of RMT-150C was carried to test the mechanical parameters of KDP crystal. Elastic modulus, Poission ratio, compressive strength and tensile strength in [001] and [100] were obtained by the uniaxial compressive testing and the cleavage crack testing from the macroscopical point of view. The results indicate that the elastic modulus in [001] and [100] are39.25MPa and16.82MPa while the Poission ratio are0.24and0.16and KDP crystal is transversely isotropic material. In the uniaxial compressive testing, it's found that the stress-strain curves of crystal specimens are linear before the specimens were failure. When the compressive force attach a certain value the curves reduce immediately, the specimen bursts at the end of the test. The failure mode shows KDP crystal is brittle materials.2. Under a certain stress condition, the lattice strain of crystal material is consistent with the macroscopic strain. The lattice strain can be determined through X-ray diffraction technology, the macroscopic strain could be obtained according to the elasticity theory. Therefore the macroscopic stress could be inferred from the obtained lattice strain. Lattice strain of KDP crystal was characterized by High resolution X-ray diffraction (HRXRD) method and lattice stress was analysed quantitatively. It is concluded that KDP crystal's lattice strain maintains at10-3—10-2magnitude, receives the tensile stress along the [001] direction,[100] and [010] direction receives the compressed stress. According to the mechanics character of KDP crystal—resistant to compression but unresistant to tensile, therefore KDP crystal may cleaves easily along (001) direction,which correspond to the crack phenomenon in the practical work.. The major factors causing crack in the crystal growth and the machining process were summarized.3. The appropriate samples for determining the dielectric/piezoelectric/elastic properties have been designed by using coordinate rotation mehod from the sight of KDP crystal structure. The dielectric, piezoelectric and elastic properties have been studied by resonance method in detail at room temperature and compare these parameters with that of the literature values. The results about the elastic constant s11=3.11×10-11m2/N, c,,=4.6×1010N/m2; The piezoelectric strain constant d14=9.51×10-12C/N, corrected the unprecise values reported in the literatures. The other values of electro-elastic constants determined in this test are consistent with the literatures.4. Potassium dihydrogen phosphate (KDP) crystal was rapidly grown by "point-seed" technique. Indentation experiments on different faces of KDP crystal were carried out at a serial of loads by HXS—1000A digital intelligent micro-hardness apparatus. The anisotropy of microhardness has been studied. The results show that the microhardnesses of KDP crystal on (001),(101) and (100) faces were187kg/mm2,156.7kg/mm2,151.3kg/mm2, respectively. The hardness indentation size effect (ISE) phenomenon was also found at loads varying from5to200g. The results show that the microhardnesses decreased with increasing of load for all crystal faces. The cracks appeared and expanded in the radiation form when the loads were higher than25g. The edges of indentation appeared cataclasms with the disintegration shape. In this paper, it was believed that25g was the optimal load for KDP crystal.5. In this paper, we use Na+,Cr3+as the univalent and trivalent metal ion dopants, respectively. The KDP crystals were grown by traditional lowering temperature method and "point seed" technique. The effects of these impurities on the crystal growth solution, thermal expansion coefficient, microhardness, structural integrity, optical uniformity and extinction radio were systematically investigated. The result reveal that the thermal expansion coefficient of [001] direction increased with the increase of Na ion concentration, while the thermal expansion coefficient of [100] direction changed a little. The microhardness of (001) face was bigger than the (100) face. The microhardness of crystal different faces reduced heavily with the Na ion concentration increasing. The full width at half maximum of dopant and undopant KDP crystals were:104.4",122.4" and147.6", respectively. It is obvious that the full width at half maximum was broadened with the increase of Na ion concentration, which suggested that the structure integrity was destroyed with the entering of Na ion into the crystal lattice. When the dopant concentration was high, the black cross of interference figure had a slight distortion, which suggested that the internal structure stress was relatively great, the crystal optical uniformity dropped.Cr3+is selectively to be absorbed onto prismatic sectors. With the increase of Cr3+concentration, the tapering because more and more serious and structural stress increase. Many "hair" mother liquor inclusions which parallel to{101} growth layer are filled in the KDP crystal when the Cr3+concentration in the solution increases to80ppm. The thermal expansion coefficient of Cr3+doped KDP crystals were lower than that pure KDP crystal both in [100] and [001] direction.The microhardness of KDP (001) face decrease with the increase of Cr3+concentration. The diffraction peaks of undoped and dopant crystal with20ppm Cr3+were narrow and incisive, without any additional peak. The full width at half maximum of undoped and dopant crystal with20ppm Cr3+were32.74" and28.8", respectively. But diffraction intensity decreased significantly. When the dopant concentration was40ppm, the diffraction peak appeared split, no longer maintained complete. The full width at half maximum was75.6". When the dopant concentration was60ppm, the rocking curve of (101) face contained two diffraction peaks. The full width at half maximum of the main peak and the low angle boundary were12.69" and11.99", respectively.6. KDP crystals doped with different concentrations of SO42-ions were grown by the traditional temperature lowing method. The macro defects and crack models have been analysis in detail. The main reasons that causing KDP crystal crack were analysis from the view point of crystal growth. The experiment indicate that, the main crack model of KDP crystal doped with SO42-ions was vertical to the growth layer of{101} face. The crystal sectors which had many cracks also had many mother liquor inclusions. With the doping concentration increase, the quality of KDP crystal decreased heavily. The crystal quality and mechanical properties, such as thermal expansion, microhardness, high-resolution X-ray diffractomery were characterized in detail. The results reveal that the thermal expansion coefficient (TEC) of the samples increased slightly at low SO42-dopant concentration (150ppm,500ppm) and increased obviously at high SO42-dopant concentration (1000ppm). The microhardness decreased with the increase of SO42-concentration, when the dopant concentration in solution reach to1000ppm, the microhardness reduced nearly20%compared with undoped crystal. The full width at half maximum (FWHM) of rocking curve increase with the increase of SO42-concentration, the scattered intensity was much more in the positive direction in comparison to that of the negative direction. This feature clearly indicated that the crystal lattice around these defects undergo compressed stress.7. KDP crystals were grown by the traditional temperature lowing method with Z-plate seed and cap-seed, respectively. The crystal properties, such as high-resolution X-ray diffractometry, optical homogeneity, extinction radio were determined in detail. The experiment shows that KDP crystals grown with cap-seed had better crystal quality.