沸石和磷酸盐分子筛晶体的空间生长

沸石晶体是应用领域最广的分子筛材料.基于这一原因,长期以来人们几乎把"沸石"一词等同于分子筛.沸石晶体不仅可应用于催化、吸附、分离等过程,还可用于微型激光器、非线性光学材料及纳米器件等新兴领域.因此,分子筛材料的合成和晶体生长引起科学界和工业部门越来越浓厚的研究兴趣.1982年美国联合碳化物公司首次合成了新型磷酸铝分子筛,打破了分子筛由硅氧四面体和铝氧四面体组成的传统观念,此后许多AlPO4-n分子筛不断涌现.由于在地面上难以生长出较大尺度的沸石晶体,因而,自20世纪80年代以来,沸石晶体的合成已成为空间材料科学的研究热点之一.前苏联、欧洲、日本和美国已在空间完成了多次沸石和磷酸铝晶体生长试验.本文将首先介绍微重力环境对于分子筛晶体生长的有利条件.然后,简述国际上已完成的空间实验,包括:前苏联PHOTO返回卫星实验、欧洲和日本CASIMIR飞行实验、欧洲EURECA空间飞行平台实验、美国STS航天飞机实验、国际空间站(ISS)实验.最后综述空间实验的主要研究结果.     

Nd^3＋：L-Lys：thiourea：TGS晶体的生长与性能研究

利用水溶液降温法生长了掺杂稀土离子Nd^3＋、L-赖氨酸和硫脲的TGS晶体。测试了掺杂TGS晶体的晶胞参数和热释电性能。实验表明,三掺杂的TGS晶体的热释电系数、热释电优值比和内偏压场大于纯TGS。     

空间低温氧化物熔体晶体生长的数字模拟研究

地面上 ,由于受浮力对流、分层和沉淀等因素的影响 ,难以阐明晶体生产和凝固现象的本质 ,从而无法获得组份均匀、结构完整和性能优良的材料 ,因此自七十年代以来 ,已进行了大量的空间晶体生长实验 ,但由于对空间环境在晶体生长过程中的流体效应参数缺乏了解 ,实验结果往往与设想的不一致。数字模拟方法可以模拟实际晶体生产过程 ,了解晶体生长参数的变化对晶体生长的影响 ,本文利用数字模拟的方法 ,对本实验室建立的空间三维实时观察装置中 ,低温生产NaNO3熔体晶体中的流体效应及温场进行了数字模拟研究 ,结果表明 ,在地面生长 ,熔体内部存在复杂的双涡流动模式 ,重力对熔体中的温场和速度场的分布产生强烈的作用 ,而在空间 ,当微重力水平达到一定程度时 ,可以使熔体中的流动模式简单化 ,从而降低流动效应对传热、传质造成的不稳定性和不均匀性 ,有利于提高晶体生长的质量。同时通过对流场中的温度分布分析表明 ,降低重力可以明显改变晶体生长固液界面附近的温度梯度 ,并使温场 ,速度场分布朝着稳态生长的方向发展     

Nd∶KGd(WO_4)_2激光晶体生长

以K2 W2 O7为助熔剂 ,采用熔盐顶部籽晶 (TSSG)法生长出尺寸为 2 3mm× 2 0mm× 19mm的Nd∶KGd(WO4) 2 激光晶体 .比较了K2 WO4和K2 W2 O7两种助熔剂的性能及对晶体生长的影响 ,认为K2 W2 O7熔点较低可以有效地降低晶体生长温度 ,有利于控制晶体生长和生长环境 .进行了KGd(WO4) 2 -KNd(WO4) 2 系统二元相图的研究 ,认为两者互溶性好 ,有利于晶体生长 ,并且Nd3+ 易于以化学剂量比取代Gd3+ .采用XRD、偏光显微镜及TG -DTA对晶体性能进行了研究 ,实验表明所生长的晶体为高温相的 β -Nd∶KGW .用光学显微镜对晶体表面裂纹、生长条纹、生长丘、生长台阶和包裹物等缺陷进行了观察 ,认为它们形成的原因是由于晶体生长工艺不稳定 ,温度梯度过大 ,拉速和降温速率过快     

α-LiIO_3晶体包裹体形成机制研究

本文通过氩离子激光束扫描下晶体内包裹体的超显微法观测,总结了α-LiIO_3晶体中包裹体的空间分布,发现了α-LiIO_3晶体中包裹体形成的晶体尺寸效应和溶液流体效应,并确认流体效应在包裹体形成过程中起重要作用。所得实验结果,已在α-LiIO_3晶体生长中得到应用和验证,并为进一步在理论上研究α-LiIO_3晶体包裹体的起因提供了实验依据。     

在粗孔αAl2O3载体上合成NaA沸石膜

采用稀释的水玻璃作为分散介质配成0.5(ω)的NaA沸石悬浮液,对粗孔α-Al2O3(孔径3～5μm)载体管修饰并预涂晶种,进一步采取原位水热晶化法在((Al2O3载体管外表面制备NaA沸石膜. 重复合成5次后,在载体表面形成一层致密、连续的沸石晶体层. 由XRD确定该晶体为A型沸石,由SEM可观察到膜厚约15-20μm,膜表面上的沸石晶体大小约为3-5μm,晶体之间紧密孪生在一起,看不出晶间空隙. 制备的NaA沸石膜的H2渗透率为3.05×10-6 mol/(m2(s(Pa), 对H2/N2和H2/C3H8的理想分离因数分别为6.9和15.6,超过对应的努森扩散值3.74和4.69,说明所制备的NaA沸石膜具有分子筛分性能.     

氘化掺杂TGS(DATGS)晶体生长与完整性

本文着重说明从重水溶液中采用慢降温法生长氘化掺杂TGS(DATGS)晶体的实验。然后,借助于光学显微镜、X射线形貌术对晶体内不同类型的缺陷及其分布进行了观察研究,并与TGS晶体作了对比,发现DATGS晶体的缺陷较为严重,其严重程度依赖于生长参数。最后,为了改善DATGS晶体的完整性,作者意图建立晶体的完整性与其生长参数间的联系。     

透明质酸对脱硫石膏制备高分散性α-半水石膏形貌和强度的影响

在常压,Ca(NO3) 2-KCl溶液中,以脱硫石膏为原料,研究了聚合物大分子透明质酸转晶剂浓度和pH值对α-半水石膏晶体生长的影响.实验结果表明,pH值为5.5,转晶剂透明质酸浓度为3.0g·L-1时,制备的α-半水石膏为规整度高、分散性好的六边短柱状晶体.α-半水石膏水化硬化浆体力学性能测试显示,浆体抗压强度和抗折强度随着α-半水石膏晶体长径比减小和规整度的增加而逐渐变大,其最大值分别为58.8 MPa和28.5 MPa,属于高强石膏.     

稀土和双有机取代基TGS晶体生长和性能研究

选择稀土离子Ce3＋、Nd3＋、Eu3＋作为掺杂阳离子,采用水溶液降温法生长了7种双有机取代基TGS晶体。研究了其晶胞参数、主要的热释电和铁电性能参数。发现这几种掺杂TGS晶体的品质因子有了提高。     

在粗孔a-Al_2O_3载体上合成NaA沸石膜

采用稀释的水玻璃作为分散介质配成0.5%(w)的NaA沸石悬浮液,对粗孔a-Al2O3(孔径3~5 mm)载体管修饰并预涂晶种,进一步采取原位水热晶化法在a-Al2O3载体管外表面制备NaA沸石膜. 重复合成5次后,在载体表面形成一层致密、连续的沸石晶体层. 由XRD确定该晶体为A型沸石,由SEM可观察到膜厚约15~20 mm,膜表面上的沸石晶体大小约为3~5 mm,晶体之间紧密孪生在一起,看不出晶间空隙. 制备的NaA沸石膜的H2渗透率为3.0510-6 mol/(m2sPa), 对H2/N2和H2/C3H8的理想分离因数分别为6.9和15.6,超过对应的努森扩散值3.74和4.69,说明所制备的NaA沸石膜具有分子筛分性能.     

Nanosecond dynamics of phase transitions in chromophoredoped ferroelectric crystals

The microscopic dynamics of phase transitions in ferroelectric crystals are studied using time-resolved fluorescence spectroscopy. Fluorescence decay times of the molecular probe tryptophan, embedded in the ferroelectric crystals potassium dihydrogen phosphate (KDP) and triglycine sulfate (TGS), are measured over a large temperature range. The dependence of the lifetime data on temperature were different between the two crystals. The magnitude of the fluorescence lifetime shows a rapid change around the phase transition temperature in KDP, while the changes in the lifetime data of the TGS crystal have been smooth. The data indicate that the order of the phase transition in solids (first or second) can be recovered using this technique. Since fluorescence lifetime is related to the dynamic interactions between the chromophore and the environment, insight into the emergence of macroscopic phase transition behavior from microscopic fluctuations near the phase transition region may be realized.     

空间微重力晶体生长研究

空间晶体生长是一个有前景的前沿学样，这一研究主要包括：在微重力条件下晶体生长的基本理论、方法和过程。本文是一篇空间晶体生长的综述，包括５个部分：（１）微重力环境；（２）微重力晶体生长研究背景；（３）晶体生长的空间实验；（４）空间晶体生长的地基实验研究和理论模拟；（５）我国微重力晶体生长发展前景。     

水溶液中硫酸钾晶体生长动力学

The single crystal growth rates of potassium sulfate in pure aqueous solution under different conditions were determined by photomicrography in a flow system for crystal growth. The effects of themain controlling factors, such as supersaturation, crystal size, solution velocity and crystal growth temperature, on crystal growth rates of potassium sulfate were discussed in detail by using non-linear regression from the experimental data, and several empirical relationships were given. The results showed that the growth rates of crystals increased with supersaturation, crystal size, solution velocity and temperature. Moreover supersaturation was the most important controlling factor influencing growth rates of crystals, crystal size and solution velocity were the secondary and temperature was the least.Furthermore, It was found that the growth rate of crystals along the [100] crystallographic axis was higher than that along the [001] in the same condition. The effect of every factor on crystal growth rates along the [100] crystallographic axis was stronger than that along the [001].     

Crystal habit modification studies with ammonium and potassium dihydrogen phosphate

Face rates of growth of ammonium and potassium dihydrogen phosphate crystals have been measured for crystallisation from pure solutions and from solutions that contained known amounts of foreign ions. The level of supersaturation and the solution pH both effect the crystal habit. Low supersaturation at normal pH (3.8) tends to give tapered crystals, and at a slightly higher pH (?5), the linear growth rates of both principal axes are considerably increased. The presence of foreign ions such as Cr³⁺, Fe³⁺ and Al³⁺ in low concentrations (? 1 × 10^?3 g ion/1 or ? 50 ppm) also causes a tapering habit. A mechanism of habit modification based on the presence of aquo ions at the crystal face is suggested.     

KDP–ADP Crystal Growth by the Solution-Dropping Method

Potassium dihydrogen phosphate (KDP), ammonium dihydrogen phosphate (ADP), and K _x A_{1− x} DP ( x =0–1 step 0.1) crystals were fabricated on KDP and ADP seed crystals by dropping a KADP solution. The KADP solution, with a temperature of 18°C, was dropped onto a substrate at a temperature of 35°C. The KADP crystals were subsequently deposited onto the substrate because of the different solubilities of the dropped solution and the solution at the substrate. The composition of the KADP crystal could be controlled by changing the precursor solution ratio of KDP and ADP. From this simple process, the K _x A_{1− x} DP ( x =0–1, step 0.1) crystals were obtained.     

Relationship between Surface Roughness,Internal Crystal Perfection,and Crystal Growth Rate

Potential mechanisms affecting growth rate dispersion (GRD) are investigated. Previous studies have identified surface roughness and internal lattice perfection as key mechanisms which are both evaluated with respect to GRD. Crystal growth of potassium dihydrogen phosphate was studied in two solvent mixtures, water and water‐ethanol. The surface roughness was analyzed by atomic force microscopy and the internal crystal perfection by X‐ray diffraction using a synchrotron source. The crystals grown at higher supersaturation have more pronounced and more frequent surface irregularities, supporting previous findings on a feedback mechanism between surface roughness and growth rate. No significant relationship was found between internal crystal perfection and growth rate, however, this is likely due to the size of the crystals analyzed herein and not the absence of any such mechanism in small crystals.