【作者】 杜雪莲；

【导师】 李建功；

【作者基本信息】 兰州大学 ， 材料物理与化学， 2008， 博士

【摘要】 本研究本着两个主要的研究思想:一是从制取α-Al₂O₃纳米颗粒的方法上探求出一种低成本、高产出的新的制取方法;二是从制备α-Al₂O₃纳米颗粒的过程中来对其形貌和尺寸实现人为地控制,探索由简单的液相沉淀法批量制备单分散的α-Al₂O₃纳米颗粒的制备工艺。以廉价的硝酸铝、氨水为原料采用化学沉淀法制备出α-Al₂O₃的前驱物-氢氧化铝,通过分析不同晶体结构的氢氧化铝在相转变过程中的热行为、晶粒长大现象、以及得到最终产物α-Al₂O₃的产率;再藉由不同的处理方式,研究处理过的氧化铝前驱体在煅烧过程的晶型转变及α-Al₂O₃晶粒的成核与长大,观察在低温下所制得α-Al₂O₃纳米颗粒的颗粒尺寸、团聚程度;自主研发了一种新型的制备方法,隔离相辅助煅烧方法。通过在氧化铝前驱体里混入隔离相,研究隔离相在煅烧过程中的隔离机制与所制得α-Al₂O₃纳米颗粒的粒径、分散性的关系。研究结果表明,在氧化铝前驱体中加入的α-Al₂O₃晶种提供异质成核点,使得α-Al₂O₃晶粒在过渡态Al₂O₃基质中的异质成核的活化能△G_hu极大地降低,促使α-Al₂O₃晶相在相对较低的温度形成,获得低团聚的α-Al₂O₃纳米颗粒粉体。前驱体的软研磨处理,在微细化粉体颗粒的同时,会造成Al原子亚晶格的紊乱,促使由紊乱的Al原子亚晶格组成的变形结构向更稳定的α-Al₂O₃晶相转变。同时,在研磨处理过程,软研磨的提供的能量可能会促使某些活性羟基的脱去,但没有剩余或足够大的能量使颗粒表面吸附的羟基键合,因而不会使得氧化铝前驱体颗粒间因形成氢键而团聚厉害。在研磨活化和添加α-Al₂O₃晶种的共同作用下,α-Al₂O₃前驱体在煅烧过程通过α-Al（OH）₃→γ-AlOOH→γ-Al₂O₃→α-Al₂O₃晶相转变途径转化为α-Al₂O₃晶相,α-Al₂O₃晶相的起始形成、相变完成温度分别降低了800℃与150℃,获得的α-Al₂O₃粉也由平均粒径100 nm的严重烧结的颗粒粉体变为平均粒径40 nm分散的颗粒粉体。在研磨活化和添加α-Al₂O₃晶种的共同作用下,较细结构的γ-Al（OH）₃前驱体在煅烧过程,小部分通过γ-Al（OH）₃→χ-Al₂O₃→α-Al₂O₃晶型转变途径在300℃直接转变部分α-Al₂O₃晶粒,大部分通过γ-Al（OH）₃→χ-Al₂O₃→κ-Al₂O₃→α-Al₂O₃晶型转化途径转化为α-Al₂O₃晶粒;基于χ-Al₂O₃、κ-Al₂O₃以及α-Al₂O₃三者结构上的相似性,α-Al₂O₃晶粒更容易的异质成核,在750℃煅烧时保温10天几乎完全转变为α-Al₂O₃晶相,制得分散的20 nm近球型α-Al₂O₃纳米颗粒。采用的隔离相辅助煅烧方法,即在制备α-Al（OH）₃前驱体过程中引入化学稳定的无机盐充当隔离相辅助煅烧。隔离相的含量,对制备单分散的α-Al₂O₃纳米颗粒的尺寸、形貌以及团聚程度有直接的影响。隔离相含量太低时,不能完全隔离开Al₂O₃纳米颗粒,煅烧时发生纳米颗粒间烧结、团聚以及长大。随隔离相含量增加,α-Al₂O₃纳米颗粒间烧结、团聚明显减弱,但氧化铝前驱体转变为α-Al₂O₃晶相所需的煅烧温度升高;同时,在煅烧时,在液态NaCl隔离相和固态Al₂O₃纳米颗粒相的混合体系中发生固液两相相分离,使得制得的α-Al₂O₃纳米颗粒呈现α-Al₂O₃多晶大颗粒与α-Al₂O₃单晶小颗粒两种形态。对制备单分散的α-Al₂O₃纳米颗粒,隔离相的含量存在临界值;当隔离相与硝酸铝的摩尔比为20:1时,隔离相在煅烧时有效地隔离Al₂O₃纳米颗粒,避免Al₂O₃纳米颗粒在高温煅烧时粗化、烧结以及长大,在1000℃煅烧就可以制得颗粒尺寸分布窄、平均粒径10nm的单分散球形α-Al₂O₃纳米颗粒。更多还原

【Abstract】 Two strategies forα-Al₂O₃ nanopowder preparation are presented in this dissertation. One is to develop a new method with low cost and high yield to prepare nanosizedα-Al₂O₃ powders. And the other one is to realize artificial control ofα-Al₂O₃ nanoparticle size and shape during the preparation process. A simple chemical precipitation method was explored to prepare monodiperseα-Al₂O₃ nanoparticles. The cheap aluminum nitrate and ammonia were used as raw materials to prepared alumina precursors, aluminum hydroxides, by chemical precipitation. Thermal behavior, grain growth, andα-Al₂O₃ productivity of aluminum hydroxides with different crystal structures during phase transformation process were analyzed. During calcinations, the phase transformation of precursors withα-Al₂O₃ seeding and grinding treatment and the nucleation and grain growth ofα-Al₂O₃ were investigated. The particle size and agglomeration degree of calcinatedα-Al₂O₃ nanopowder were analyzed. Then, a novel calcinations method, namely the isolating phase assistant calcination, was developed to prepare monodisperseα-Al₂O₃ nanoparticles. That is, by introducing a salt as isolating phase to isolate alumina precursor particles to avoid the contacts between the alumina precursor particles, between the transition alumina particles as well as between theα-Al₂O₃ particles to eliminate the particle growth and agglomeration of theα-Al₂O₃ particles during calcinations.It was found that addition ofα-Al₂O₃ seeds in alumina precursors could supply the heterogeneous nucleation sites, reduce the activation energy of heterogeneous nucleation ofα-Al₂O₃ grains in transition Al₂O₃ matrix, and accelerate the formation of theα-Al₂O₃ crystal phase at relative low temperatures to obtainα-Al₂O₃ nanopowder with low-degree agglomeration.Soft grinding treatment of precursors refines the precursor particles, results in the lattice distortion of the initial particles, and leads to a strained framework of disordered Al-atom sublattice to easily rearrange into the stable framework ofα-Al₂O₃ phase. Meanwhile, the energy introduced by soft grinding might lead to the removal of a part of active hydroxyl but is not enough to bond the hydroxyl adsorbed on the surface of particles; thus no serious agglomeration due to the formation of hydrogen bond between alumina precursor particles was formed. Due toα-Al₂O₃ seeding and soft-grinding treatment, bayerite undergoes a series of phase transformations via theα-Al（OH）₃→γ-AlOOH→γ-Al₂O₃→α-Al₂O₃ path without the formation of theθ-Al₂O₃ transition phase and transforms intoα-Al₂O₃ crystal phase. The onset and completion temperatures of the transformation toα-Al₂O₃ in the ground bayerite are about 800℃and 150℃lower than that in the unground bayerite, respectively. The obtainedα-Al₂O₃ nanoparticles with an average diameter of 40 nm are relatively disperse.Due toα-Al₂O₃ seed addition and soft-grinding treatment, during calcinations of fine gibbsiteγ-Al（OH）₃, a small part ofγ-Al（OH）₃ transformed intoα-Al₂O₃ at 300℃viaγ-Al（OH）₃→χ-Al₂O₃→α-Al₂O₃ phase transition sequence, and mostγ-Al（OH）₃ transformed intoα-Al₂O₃ viaγ-Al（OH）₃→χ-Al₂O₃→κ-Al₂O₃→α-Al₂O₃ phase transition path. The similar structures ofα-Al₂O₃,κ-Al₂O₃, andχ-Al₂O₃ （hexagonal closed-packing of oxygen atoms） lead to more easy heterogeneous nucleation; disperseα-Al₂O₃ nanoparticles with an average diameter of 20 nm were achieved by calcining ground gibbsite at 750℃for 10 days.The isolating phase assistant calcination method was employed to preapre monodieperseα-Al₂O₃ nanoparticles. In this novel method, the inorganic salt （NaCl） serving as isolating phase was introduced in preparation process of precursor,α-Al（OH）₃. The content of isolating phase influences directly the particle size, shape, and agglomeration of theα-Al₂O₃ nanoparticles. When a small quantity of isolating phase was used, the Al₂O₃ nanoparticles could not be isolated completely; and the sintering, agglomeration, and grain growth ofα-Al₂O₃ nanoparticles occurred. With increasing content of the isolating phase, the sintering, agglomeration, and grain growth ofα-Al₂O₃ nanoparticles obviously reduce, but the completation temperature of phase transformation ofα-Al₂O₃ increases. Meanwhile, during calcinations at high temperatures, the solid phase-liquid phase segregation occurred in the system composed of a liquid isolating phase （NaCl） and a solid phase of the Al₂O₃ nanoparticles. The solid phase-liquid phase segregation occurring during high temperature calcinations results in two sorts ofα-Al₂O₃ nanoparticles. One sort ofα-Al₂O₃ nanoparticles formed in the region of relatively higher isolating phase content and were small single-crystal nanoparticles of about 10 nm in size. The other sort ofα-Al₂O₃ nanoparticles formed in the region of relatively lower isolating phase content and were large polycrystal nanoparticles of 20-30 nm in size. For preparation of monodisperseα-Al₂O₃ nanoparticles, the isolating phase content has a critial value. When the molar ratio of salt to aluminum nitrate is about 20:1, the isolating phase can isolate the Al₂O₃ nanoparticles well to avoid the sintering, agglomeration, and grain growth of Al₂O₃ nanoparticles during calcinations. Monodisperse and sphericalα-Al₂O₃ nanoparticles with a narrow size distribution and an average diameter of 10 nm were obtained by a 1000℃calcination.更多还原