【作者】 郑斐斐；

【导师】 刘铎； 刘宏；

【作者基本信息】 山东大学 ， 材料物理与化学， 2010， 硕士

【摘要】 钽酸锂(LiTaO3,LT)晶体具有优异的铁电、热电、光电等性能,是一种非常重要的多功能晶体材料。周期极化钽酸锂(Periodically Poled LiTaO3, PPLT),作为一种光学超晶格材料,因其较高的抗光伤阈值,较弱的光弹效应,较宽的通光范围,在准相位匹配技术中具有重要的应用价值。同成分钽酸锂(Congruent LiTaO3, CLT)具有组分均匀的优点,传统的粉料合成采用固相法,在固液同成分配比([Li]／[Ta]=48.6／51.4)熔体中用提拉法生长,但是Li离子的缺失造成晶体中存在大量的空位缺陷,使晶体的许多物理性能受到很大影响。与CLT相比,近化学计量比钽酸锂(Stoichiometric LiTaO3, SLT)晶格完整,缺陷大大减少,电光系数和非线性系数提高,畴反转电压下降,发光强度提高,光折变性能大幅提高。与SLT相比,掺镁化学剂量比钽酸锂(Mg dopedStoichiometric LiTaO3, Mg:SLT)抗光损伤阈值更高,可以实现非临界相位匹配,可透过波段范围更宽,电光和非线性特性更好。制备优质的粉体对后期的晶体生长至关重要,传统生长晶体用的粉料采用固相合成法,虽然简单易行,但在掺杂MgO时掺杂量少,熔点高,反应受扩散控制,容易掺杂不均匀而导致出现包裹体、散射颗粒等缺陷,严重影响晶体的光学质量。因此合成出掺杂均匀的优质Mg：LiTa03粉体对生长高质量光学级晶体是非常重要的。本文以氢氧化钽(Ta(OH)5)、碳酸锂(Li2CO3)、柠檬酸(CA)、盐酸(HCl)、氨水(NH3H2O)作为原料采用溶液-煅烧法合成出化学计量比LT及掺杂均匀的Mg：LT粉体。合成工艺包括：水热环境下Ta(OH)5在浓盐酸中的活化；Ta-CA稳定络合物溶液的制备；过滤掉未溶解的杂质；通过热重分析精确确定溶液中的Ta含量；根据热重得到的Ta含量加入化学剂量比的含Li溶液及需要掺杂的含Mg溶液；对前驱体溶液进行过滤除去不溶解的杂质；对前驱体溶液进行喷雾干燥得到尺寸均匀形貌规则的球形颗粒；最后对前驱体粉料进行煅烧,得到不含杂质的单相多晶粉料。对合成的具体过程、中间产物及最终产物进行了分析,主要内容包括以下几个方面：(1)对合成反应机理进行了探索。(2)对络合反应前后的干燥粉体进行红外分析,证实了络合反应的发生及发生络合反应的原子。(3)对前驱体粉体进行热重分析,得知了粉体在煅烧过程中的具体热行为。(4)对不同煅烧温度下得到的产物进行红外分析对比,得出了确保产物中不含有机杂质的最低煅烧温度。(5)通过对产物进行XRD表征,确定了产物的结晶相。(6)通过对不同温度煅烧得到的粉体进行SEM表征,观察了产物的具体形貌。(7)通过对纯相LT及不同掺杂量的Mg：LT进行XRD表征,确定了Mg元素确实掺进了LT的晶格中,而且随着掺杂量增加,晶胞参数减小。(8)为了确定产物中Li的含量,测量了合成粉体的居里温度,从而证明了合成产物的化学剂量比符合摩尔比[Li]:[Ha]=49.9：50.1。实验合成过程中所用原料价格便宜,无毒,对环境不会造成污染；采用溶液法合成粉体,各种元素在溶液中在原子级别混合均匀,化学计量比容易控制,有利于后期晶体生长长出掺杂均匀的近化学计量比晶体；对前驱体溶液进行喷雾干燥,可以使生产批量化进行,得到尺寸均匀并且可调的球形颗粒,有利于后期生长晶体时的自动连续加料；煅烧温度与固相合成法相比大大降低,避免了锂组分在高温下的挥发,而且煅烧温度低,得到的产物尺寸小,熔点降低,有利于后期多晶粉料化料温度的降低。钽酸钠(NaTaO3)可以用作介电材料和铁电材料,具有优良的光催化性能；钽酸钾(KTaO3)作为铁电材料,具有电、磁和光学性质,庞大的磁阻效应,还有光致发光和催化活性。无机纳米颗粒的可控合成是非常重要的,因为材料的性质会强烈地依赖于颗粒的尺寸和形状,理解晶粒的生长机理对实现材料的可控合成起着非常重要的作用。本文采用水热法合成了NaTaO3及KTaO3粉体,通过XRD,SEM等测试方法对产物进行表征,对其相变特征、生长机理及光化学特征进行了探索,发现：(1)以Ta(OH)5和NaOH,KOH为反应原料,当碱液浓度低时,产物为烧绿石相,当碱液浓度高时,产物为钙钛矿相；(2)当反应原料中同时加入NaOH和KOH时,生成NaTaO3的趋势要远远大于生成KTaO3的趋势；(3)烧绿石相的产物比表面积大,形貌多为八面体,对甲基橙溶液在紫外灯光照下的催化降解能力更强：在反应体系中加入乙醇或柠檬酸可以有效控制产物的形貌,减小产物尺寸,提高光催化降解能力；(4)通过形貌分析发现,碱液浓度低时合成的烧绿石相产物粒度分布较为均匀,而浓度高时合成的产物粒度分布范围广；水热条件下纳米晶粒的生长会采取奥斯瓦尔德熟化、孪晶生长机制和定向聚集的方式。更多还原

【Abstract】 As an important multi-functional material, lithium tantalate (LiTaO3, LT) crystal possesses excellent ferroelectric, piezoelectric and photoelectric properties. Periodically Poled LiTaO3, PPLT, as a kind of optical superlattice material, has important value for quasi phase match technology due to its higher damage threshold, weak photoelastic effect, wide light range.Congruent LiTaO3 (CLT) crystal is grown by Czochralski method with a molar ratio of [Li]/[Ta]=48.6/51.4. However, the deficiency of lithium ions creates a large number of oxygen vacancies and will degrade the performance of CLT crystals. Compared with CLT, stoichiometric LiTaO3 (SLT), with a perfection of lattice and low density of defects, higher electro-optic coefficient and nonlinear coefficient, reduced domain inversion voltage, enhanced luminous intensity and photorefractive performance. Compared with SLT, Mg doped Stoichiometric LiTaO3 (Mg:SLT) can remarkably increase the damage threshold, and has better electro-optical and nonlinear properties.The synthesis of powders of high quality is of importance to grow crystals. Traditionally, solid state method is used to synthesize powders, however, the amount of the doping MgO, with high melting point, is a little, and the reaction is controlled by diffusion, defects such as solid inclusion and scattering particles resulted from un-uniform doping will greatly affect the optical properties. Hence, the successful synthesis of homogenious Mg:LiTaO3 powders is critical to grow optical crystals of high quality.In this thesis, a solution-calcination method was applied to synthesize LiTaO3 and Mg:LiTaO3 powders. The synthesis process is as follows:Ta(OH)5 firstly reacted with HCl under hydrothermal environment, then was cooled down at room temperature; the upper liquid was poured out and deionized water was added to dissolve the white deposite, then a-hydroxy acid citric acid was chosen as coordination agent to form stable Ta-CA, which can further react with Li2CO3 and MgO; after pH value was adjusted to 7 with ammonia, liquid precursor was gained, filter the liquid precursor and dry it with spray drier; the dry powders were calcined at 720℃for 2 h. Analysis about the synthesis mechanism, characterization on the intermediate product and the final product is performed, the main contents are as follows:(1) The reaction mechanism is investigated.(2) IR analysis is used to characterize the dried powders before and after the coordination, and confirm the coordination atoms.(3) TG-DSC analysis is performed on the precursor to study the thermal behaviors during calcination.(4) Comparison on the IR spectra of products obtained through different calcination temperatures is made, and the lowest calcination temperature is fixed to ensure there is no organic impurity.(5) The crystal phase of the product is confirmed through XRD characterization.(6) Through SEM images about the products obtained under different calcination temperature, the morphology is observed.(7) Through XRD analysis on pure LT and Mg:LT, we conclude that Mg element is doped in LT lattice, and the unit-cell parameters decrease with the enhanced amount of doping.(8) To make sure the content of Li, curie temperature is measured to confirm that the molar ratio of [Li]:[Ta]= 49.9:50.1.The raw materials used in the synthesis process are non-poisonous and environmental benign. All elements exist in the solution in atomic level, and it is easy to control the stoichiometry and the homogeneity of the final product. A spray drier was used to dry the precursor solution, and powders with uniform size and composition were obtained, and this is favorable for continued automatic feeding. The low calcination temperature can avoid Li volatilization compared with solid state method. Beside, the melting point of the product decreases due to its small size.Sodium tantalate (NaTaO3) is a useful dielectric and ferroelectric material, possessing excellent photocatalytic performance. Potassium tantalate (KTaO3), as an incipient ferroelectric material, exhibits electric, magnetic, optical properties and bulk magnetoresistive (MR) effects. Besides, it has photoluminescence and photocatalytic properties. Controlled synthesis of inorganic nanomaterials is important because their properties depend intensively on the morphology and size of the particles. An in-depth understanding of crystal growth mechanism is of importance to realize controlled synthesis.We synthesized NaTaO3 and KTaO3 powders via hydrothermal method. XRD and SEM were performed to characterize the products, the photocatalysis performance and growth mechanism were investigated. We find that:(1) when at low alkali concentration, the product was attributed to pyrochlore phase, when at high alkali concentration, the product was attributed to perovskite phase; (2) when NaOH and KOH exist at the same time, there is a stronger trend to create NaTaO3 than KTaO3; (3) the specific surface area for pyrochlore product is large, with octahedron morphology, showed a strong ability to catalyze the decomposition of methyl orange exposed at UV light; the morphology of the product can be effectively controlled when surfactant ethanol or citric acid was added to the synthesis system, and the photocatalysis ability was enhanced. (4) Through SEM analysis, we find that pyrochlore product synthesized at low alkali concentration possesses uniform particle sizes; product synthesized at high alkali concentration showed a broad size distribution; the growth of nanoparticles in hydrothermal environment can take Oswald riping, oriented attachment and twin-assisted growth mechanisms.更多还原