阴离子掺杂对KDP晶体生长及其性能影响的研究

【作者】 张建芹；

【导师】 王圣来；

【作者基本信息】 山东大学 ， 材料学， 2006， 硕士

【摘要】 KDP／DKDP晶体是一种光学性能非常优良的非线性晶体，其生长研究已经有70多年的历史，是一种经久不衰的水溶性晶体之一；被广泛应用于激光变频、电光调制和快速光开关等领域。近些年来，随着ICF的迅速发展，高激光损伤阈值特大尺寸的KDP晶体与其性能的研究，在国际上又进入了一个新阶段，尽管目前新型的非线性光学材料不断涌现，但观其综合的非线性光学性能，尤其是所要求的特大尺寸晶体，到目前为止，能用于激光核聚变等高功率系统中的晶体，也仅仅只有KDP晶体，正因为如此，生长特大尺寸的高激光损伤阈值的KDP晶体研究，当前在国内外均受到重视。激光核聚变应用中不仅要晶体有较大的尺寸，而且要有求有较高的光学质量（高的光伤阈值，低的光学吸收，高的光学均匀性和良好的透过波段等）。为了给ICF工程提供足够的晶体，同时为了提高晶体的利用率，目前各国正在努力发展快速生长技术。这一技术对原料、生长设备及生长条件要求比较严格，所以大尺寸KDP和DKDP晶体在国内还是以传统降温法为主。KDP晶体的质量好坏与其生长工艺有很大关系，溶液中的杂质、过饱和度的大小、溶液的稳定性、晶体生长的速度和温度，都会对晶体的质量产生影响。明确KDP晶体中各类杂质影响机制，减轻或消除有害杂质影响一直是研究的一个主要方面。从文献报道来看，关于阳离子杂质研究的报道较多，阴离子杂质要少一些。这些杂质对KDP晶体的生长习性和光学质量产生很大影响。本论文采用传统降温法和“点籽晶”快速生长法生长了KDP晶体，进行了硫酸盐（K₂SO₄），硝酸盐（KNO₃）和卤盐（KCl）掺杂，研究了SO₄^2-、NO₃^-和Cl^-离子对KDP晶体生长习性、光散射、透过光谱、光学均匀性以及电导率的影响，通过超显微法和ICP等方法测量和观察了晶体中的光散射和杂质元素的含量，测量了不同阴离子掺杂下KDP晶体的透过光谱和电导，并对其影响机制和原因进行了分析。本论文首先结合晶体的生长，研究了SO₄^2-、NO^3-和Cl^-对晶体溶液稳定性、生长速度、形态及应力缺陷等生长习性的影响。溶液的稳定性是KDP晶体生长的基础，它不仅关系到晶体的生长能否顺利进行，还影响到晶体的质量。通过研究分析发现，三种阴离子杂质掺杂到一定浓度时，生长溶液均会出现杂晶，稳定性受到破坏。硫酸盐掺杂对晶体的生长速度和形态产生较大的影响，高浓度掺杂时，晶体出现开裂、包臧和添晶等缺陷；通过硫酸盐掺杂KDP晶体的X射线衍射图显示，硫酸盐掺杂后晶体的结构没有发生很大的变化。硝酸盐掺杂导致晶体成帽变得困难，柱面出现扩展。卤盐掺杂对KDP晶体的生长影响较小。KDP晶体中的散射颗粒是衡量其质量优劣的一个重要指标，散射颗粒的出现能引起晶体的光损耗，甚至引起在强光下的破坏。有证据已表明晶体中的散射颗粒是造成激光损伤阈值低的潜在因素。散射颗粒对晶体的透过率，均匀性等方面也有影响。SO₄^2-掺杂晶体中的散射颗粒的密度随其掺杂浓度增加有所增大，但散射颗粒的数量增加程度较小。硝酸盐和卤盐掺杂加剧了传统降温法所得晶体的光散射，散射重，散射颗粒密度高；但是在“点籽晶”快速生长法所得晶体中，柱面和锥面生长区的散射颗粒均较少，与纯KDP晶体试样相比，变化不大。KDP晶体的透过波段一般认为0.176-1.5μm，可对波长为1.064μm激光实现二倍频和三倍频。通过分析掺杂晶体的透过性能，我们发现低浓度硫酸盐掺杂时，KDP晶体的透过光谱变化不大；高浓度掺杂时，传统降温法晶体在紫外波段的透过率较纯KDP晶体降低很多。对于“点籽晶”快速生长法所得晶体，锥面生长区的透过光谱的变化规律与传统降温法晶体的相类似，柱面生长区的透过率在紫外波段下降很快，截止波长为300nm。NO₃^-和Cl^-掺杂对KDP晶体的透过率无明显影响。KDP晶体中存在的应力，会引起应力双折射和折射率梯度，降低晶体的光学均匀性。我们发现，硫酸盐掺杂导致晶体出现大量宏观缺陷，一定程度降低了KDP晶体的均匀性。通过测量晶体的锥光干涉图来研究了晶体的光学均匀性，锥光干涉图中黑十字的畸变说明高浓度硫酸盐掺杂时晶体的均匀性降低。电导在国外是衡量DKDP晶体质量的一个关键指标，通过研究晶体材料的电导性质可以获知其内部化学缺陷的信息，对于KDP晶体也是很重要的，它决定了KDP晶体在电光开关中的应用。硫酸盐掺杂后其电阻率随着掺杂浓度增大而降低，电导率随着掺杂浓度的增大而增加，主要是硫酸盐掺杂后晶体内部产生空位等微观缺陷，使晶体更易导电所致。NO₃^-和Cl^-离子掺杂后，KDP晶体的电阻率和电导率变化不大。总之，本论文第一次比较系统地研究了SO₄^2-、NO₃^-和Cl^-三种阴离子掺杂对KDP晶体的生长和光学质量产生的影响，其中SO₄^2-对KDP晶体的影响较大，我们认为，这是由于SO₄^2-具有四面体结构，较强的电负性以及氢键结合能力都使SO₄^2-较易进入晶体并占据生长格位，从而对晶体的生长和性能产生较大影响。此外，发现尺寸较小的NO₃^-和Cl^-加重了晶体中的光散射。因此，在KDP晶体生长中必须严格控制SO₄^2-、NO₃^-和Cl^-的浓度。本论文的工作对大尺寸KDP晶体的生长，解决晶体生长中出现问题，以及晶体生长的基础工作都具有一定意义。更多还原

【Abstract】 KDP/DKDP crystals are non-linear optical crystal materials with excellent optical properties. The investigation of crystal growth has been lasting for more than 70 years. The type of crystals has been one of the lasting water-soluble crystals and has been widely applied in the high technology fields such as laser frequency conversion, electro-optic modulation and optical rapid switch etc.In recent years, with the development of Inertial Confinement Fusion （ICF）, the investigation of large KDP/DKDP crystals with high laser damage threshold and their optical properties has entered a new phase internationally. Although many new non-linear materials have been turning up constantly, considering the excellent optical properties and the requirement of large crystal, so far KDP/DKDP crystals are the only kind of crystals that meet the requirement of high-power laser system such as ICF. So the investigation of the growth of large KDP crystals has been paid attention to at home and aboard at present. The requirements of KDP/DKDP crystals in ICF include not only large size, but also their excellent optical qualities such as high laser damage threshold, low optical absorption, high optical homogeneity and wide transparent wave band.In order to provide enough crystals for ICF and enhance the usage ratio of crystals, now many countries are trying to develop the rapid growth technique, which has higher requirement for raw materials, growth devices and growth conditions. Now large KDP/DKDP crystals are mainly grown by the traditional temperature- lowering method in our country. The growth period is much longer and the growth is more difficult. The optical qualities are affected by many factors such as impurities, supersaturation, stability of the growth solution, the growth rate and so on. So it is an important subject to judge the affection mechanism of different impurities on the KDP crystals and to find the method of decreasing or eliminating the influence of the harmful impurities. From the literature reports, the investigation of the effect of cations on KDP crystals is much more than that of anions. These impurity ions have great effect on the growth habit and optical qualities of crystals.In this paper, the crystals were grown from the aqueous solution by the traditional temperature-lowering method and a "point seed" rapid growth technique. Potassium sulphate （K₂SO₄） potassium nitrate （KNO₃） and potassium chloride （KCl） were chosen as additives. The effect of SO₄^2-, NO₃^- and Cl^- on the growth habit, light scatter, transparent spectra, optical homogeneity and electric conductivity of KDP crystals were studied. Many measurement means such as ultra-microscopy, XRD and ICP were applied to study the light scatter, the element content of crystals. Meanwhile the mechanism and reason of the affection were also analyzed.First, the effect of SO₄^2-, NO₃^- and Cl^- on the stability of growth solution, the growth habit, stress and defects were studied. The stability of the solution is the basis of the growth of KDP crystals, which decides not only the regular growth of the crystals, but also the optical quality. It’s seen from the experiments that the spontaneous crystals appeared in the growth solution and the stability of the solution was destroyed when the dopant concentration of three ions exceeded some value. At high dopant concentration of suphate, the KDP crystals showed many defects such as mother liquid inclusions, parasite crystals and cracks. From the result of XRD, the structure of KDP crystal didn’t change after the doping of sulphate. The high nitrate dopant made the crystals’ capping become difficult and the expansion of the prismatic sectors occurred.Scatter particles is an important parameter to evaluate the properties of the KDP crystals. Appearance of scatter particles can cause the light loss of the crystals and decrease the optical homogeneity of the crystals. Some results have proved that scatter particles is the latent factor of the low laser damage threshold of KDP crystals. And scatters also affect the transmission of the crystals. SO₄^2- ions had little effect on the light scatter of KDP crystals and the scatters have low density. With the increase of the dopant concentration, the density of scatter particles increased a little. For KDP crystals grown by the traditional temperature-lowering method, the existent of NO₃^- and CI ions seriously aggravated the light scatter of KDP crystals and the scatter particles had larger scale with high density. The effect on the light scatter of KDP crystals grown by a "point seed" rapid method was not evident.Transparent wave band of KDP crystals is generally thought from 0.176μm to 1.5μm, which can achieve double frequency and thrice frequency for the laser with wavelength of 1.064μm. Through analyzing the transparency of the dopant KDP crystals, we found that SO₄^2- ions affected the transparency of the crystal evidently. At high dopant concentration, KDP crystals’ transmittance decreased a lot in ultraviolet band compared with pure KDP crystals; at low dopant concentration, the transmittance didn’t have evident change. The transmittance of the crystals by the rapid method in prismatic sectors decreased rapidly in the ultraviolet band and the cutoff wavelength was about 300nm. The dopants of NO₃^- and Cl^- ions didn’t have evident effect on the transparency of KDP crystals.The stress inside KDP crystals can decrease the homogeneity of crystals. The doping of sulphate made the crystals form many defects, which caused the stress of crystals and decreased the homogeneity to some extent. The homogeneity of KDP crystals were studied by measuring the conoscopic figure. The distortion of black cross can prove of the decrease of the homogeneity of crystals.Electric conductivity is a key index to evaluate the KDP/DKDP crystals, which decides the application of electro-optic switches of crystals. For KDP crystals doped with sulphate, its electric conductivity improved with the increase of the dopant concentration, which is subjected to have relation to the defects of crystals caused by SO₄^2-ions. The dopants of NO₃^- and Cl^- ions had no evident effect on electric conductivity of KDP crystals.In this paper, the author systematically studied the effect of SO₄^<sup>2-, NO₃^- and Cl^- on the growth and optical qualities of KDP crystals for the first time. Experiments showed that SOn₄^2-ions had great effect on the growth and properties of KDP crystals. SO₄^2- has tetrahedron configuration and strong H-bond combinability ability, which make SO₄^2-ions easily join into the KDP crystal lattices through H-Bond and electron attraction. Furthermore, the existence of small-sized NO₃^- and Cl^- seriously aggravated the light scatter of KDP crystals. So, we think that it is necessary to control the concentration of SO₄^2-, NO₃^- and Cl^- in the growth solution to obtain KDP crystals of high quality. The work of this paper is important to the growth of large KDP crystals and the theory of crystals growth.更多还原