【作者】 刘志宏；

【导师】 张多默；

【作者基本信息】 中南大学 ， 有色金属冶金， 2007， 博士

【摘要】 研究了镍、钴草酸盐制备中的形貌与粒度控制，包括三方面内容：凝胶—溶胶法制备单分散草酸镍粉末；纤维状草酸镍的形成机理及其形貌与粒度控制；纤维状草酸钴的形成机理及其形貌与粒度控制。研究中，采用SEM、XRD、FTIR、TGA—DTA、GCMS等方法，对粉末样品进行表征。1)凝胶—溶胶法制备单分散草酸镍粉末提出了一种凝胶—溶胶法制备单分散草酸镍的方法，由Ni（OH）₂凝胶生成—NiC₂O₄·2H₂O颗粒形核—凝胶（Ni（OH）₂）向溶胶（NiC₂O₄·2H₂O）转化3个步骤组成。制备的草酸镍为单斜晶型α-NiC₂O₄·2H₂O，具有菱柱体形貌，底面菱形的夹角分别为106°和74°；随条件变化，粉末D₅₀值可在0.3～3.0μm间调节，单分散性σ值可达1.10。实验证实，粉末颗粒的形成机理为溶解—再结晶。过程中，Ni（OH）₂凝胶包覆在NiC₂O₄·2H₂O颗粒表面，既作为Ni²⁺的“缓释源”；又起到了阻隔颗粒碰撞团聚的作用。实验研究了温度、起始NiCl₂浓度、表面活性剂、Na₂C₂O₄加入量及NaOH浓度的影响，揭示了Gel-sol法制备草酸镍中，形貌与粒度控制的主要规律：（1）在实验研究的条件范围内，制备的颗粒形貌均为菱柱状，粒度在0.6μm以上时，轴向比在0.5左右波动；（2）颗粒生长中，在偏高的过饱和度下，二次异相晶核与母晶的融合生长以及颗粒的孪晶生长，使颗粒呈现类似多颗粒团聚体形貌且尺度超大；（3）粉末粒度主要取决于颗粒形核及生长过饱和度，形核过饱和度高，晶核数多，粉末粒度细小；生长过程中，若过饱和度控制偏高，在颗粒长大的同时，伴随着异相形核大量发生，形成的二次晶核或与母晶融合生长，或脱离母晶独立生长，使得粉末平均粒度增大或减小，粒度分布变宽。（4）生长过程的过饱和度及其稳定性，主要决定于NiC₂O₄·2H₂O结晶反应速率，因此，往往与晶核数的多少密切相关，形核过饱和度大，形成的晶核数多，颗粒表面积大，生长反应（即NiC₂O₄·2H₂O的结晶）速度快，过程中Ni（OH）₂与H₂C₂O₄离解产生的Ni²⁺与C₂O₄^2-在系统中无累积，颗粒在低的过饱和度下生长，有利于得到单分散性好的粉末。2)纤维状草酸镍的形成机理及其形貌与粒度控制采用Na₂C₂O₄溶液与XiCl₂和NH₃·H₂O混合溶液并流加料合成的方法，研究了纤维状草酸镍的制备。研究证实，在Ni²⁺-NH₃-H₂O体系碱性条件下，Ni（Ⅱ）离子以Ni（NH₃）_n^2-（n=0，1，2，...，6）形态存在，改变了草酸镍的沉淀形态及其形核与生长过饱和度，是草酸镍形成长纤维状形貌的内在原因。研究发现，在Ni²⁺-NH₃-C₂O₄^2--H₂O系中，依条件不同，可形成系列Ni（NH₃）_nC₂O₄·2H₂O（n=0，0.5，1.0，1.5，2.0，...），其中，n值为0时，即为α-NiC₂O₄·2H₂O，其余均为JCPDS尚无卡片的含NH₃草酸镍新化合物，对n值为0.5，1.0，1.5，2.0的4种，已制备出较纯的样品，确定其分子式及XRD图谱与数据。研究发现，在Ni²⁺-NH₃-C₂O₄^2--H₂O体系碱性条件下，初生沉淀一般是数种Ni（NH₃）_nC₂O₄·2H₂O（n=0.5，1.0，1.5，2.0，...）的混合物，形貌为细小晶体团聚体，陈化中，不同化合物间因溶解度差别使其发生溶解—再结晶，溶解度大的化合物溶解，源源不断提供结晶物质，使溶解度较小的化合物在低过饱和度下生长，形成大轴向比纤维状形貌颗粒。研究发现，在Ni²⁺-NH₃-C₂O₄^2--H₂O体系碱性条件下，实验研究的范围内，Ni（NH₃）_1.5C₂O₄·2H₂O是稳定性最高的物相，充分陈化后，往往是体系中唯一存在的草酸镍化合物。研究发现，生长与颗粒间沿长轴方向的取向聚并融合，是颗粒长大的方式。研究了温度、pH值、加料速度、浓度、[NH₃]_T/[Ni]_T摩尔比、表面活性剂及其添加量、搅拌等对颗粒形貌与粒度的影响，主要结论为：（1）草酸镍化合物种类是决定颗粒形貌与粒度的最主要因素；（2）提高陈化中溶解—再结晶速度，有利于得到物相单一，形貌规整，轴向比大的粉末；（3）不同条件下，颗粒形核与生长过饱和度改变，也会对粉末形貌与粒度产生一定影响；（4）适量添加PVP25有利于大轴向比纤维状草酸镍的制备。3)纤维状草酸钴的形成机理及其形貌与粒度控制采用CoCl₂溶液与（NH₄）₂C₂O₄和NH₃·H₂O混合溶液并流加料合成的方法，研究了纤维状草酸钴的制备。研究证实，在Co²⁺-NH₃-H₂O体系碱性条件下，Co（Ⅱ）离子以Co（NH₃）_n^2-（n=0，1，2，...，6）形态存在，改变了草酸钴的沉淀形态及其形核与生长过饱和度，是草酸钴形成长纤维状形貌的内在原因。研究发现，在Co²⁺-NH₃-C₂O₄^2--H₂O体系中，依条件不同，可形成两种草酸钴，其一为β-CoC₂O₄·2H₂O；第二种为Co（NH₃）_1.5C₂O₄·2H₂O，是一种JCPDS尚无卡片的含NH₃草酸钴新化合物，已确定其分子式及XRD图谱与数据。研究发现，生长与颗粒间沿长轴方向的取向聚并融合，是颗粒长大的方式。研究了温度、pH值、加料速度、浓度、PVP25添加等对颗粒形貌与粒度的影响，主要结论为：（1）草酸钴化合物种类是决定颗粒形貌与粒度的最主要因素；（2）不同条件下，颗粒形核与生长过饱和度改变，会对粉末形貌与尺度产生一定影响；（3）陈化中草酸钴化合物种类并无改变，尺度不同的颗粒问按Ostwald陈化机理发生溶解与再结晶，对颗粒形貌与粒度产生影响。（4）PVP25的添加对颗粒形貌与粒度影响并不显著。更多还原

【Abstract】 The formation mechanisms, as well as the morphology and size control of nickel and cobalt oxalate particles with rhombohedron and fibre morphologies were investigated systematically. The samples synthesized in the experiments were characterized by SEM, XRD, FTIR, TGA-DTA and GCMS.1) The preparation of monodisperse nickel oxalate particles with rhombohedron form by a novel gel-sol processA novel process for the preparation of monodisperse nickel oxalate particles was proposed and investigated systematically. The process is composed of three steps: the formation of Ni（OH）₂ gel; the nucleation of nickel oxalate; the transition from Ni（OH）₂ gel into nickel oxalate sol.The nickel oxalate synthesized is identified by XRD to beα-NiC₂O₄·2H₂O with rhombohedron morphology, while two angles of the rhombus at underside are 106 and 74 degrees separately. The D₅₀ of the particles can be controlled between 0.3 and 3.0μm by regulating synthesizing conditions, while the index number of particle monodispersity,σreaches 1.10.It was verified experimentally that formation mechanism of the particles is dissolution-recrystallization from Ni（OH）₂ gel into nickel oxalate sol. During the transition process from gel to sol, the surfaces of nickel oxalate particles are coated by Ni（OH）₂ gel, which not only works as the slow-release source of nickel ions, but also protects nickel oxalate particles from aggregation.The effects of temperature, initial NiCl₂ concentration, surfactants, amount of sodium oxalate added and NaOH concentration on morphology and size of the particles in gel-sol process were investigated experimentally. The conclusions obtained are as follows: （1） The morphology of the particles is independent of preparation conditions. In case of the size of particles is above 0.6μm, the ratio of breadth and length of rhombohedron （aspect ratio） is about 0.5. （2） During particle growing under a higher supersaturation, the amalgamation growth of secondary nuclei formed heterogeneously in situ with mother crystals as well as the formation of twin crystals take place, resulting in the formation of larger particles with morphology similar to the agglomerates of particles. （3） The size of particles is mainly governed by the supersaturation during nucleation and growth of the particles, the higher the supersaturation of nucleation, the more the number of nuclei formed, therefore, the smaller the size of particles synthesized; if the supersaturation is controlled too high during the process of particle growth, the heterogeneous formation of secondary nuclei takes place largely in company with the growth of particles, the secondary nuclei formed either grow in situ, amalgamating with mother crystals, or leave their mother crystals and grow independently, resulting in the increasing or decreasing of the average particle sizes and broadening particle size distribution. （4） The growth supersaturation and its stability are mainly decided by the rate of crystallization reaction of nickel oxalate, therefore, dependent on the number of nuclei formed; the higher the supersaturation in nucleation, the more the number of nuclei formed, then, the larger the surface of particles, therefore, a higher growth reaction rate can be obtained. Due to no accumulation of Ni²⁺ and C₂O4^2- ions generated by the dissociation of Ni（OH）₂ and H₂C₂O₄ respectively, particles grow in a lower supersaturation, which is benefit to the preparation of monodisperse particles.2) The formation mechanism as well as the morphology and size control of fibrous nickel oxalate particlesThe nickel oxalate particles with fibre morphology were prepared using Na₂C₂O₄ solution and mixed NiCl₂, NH₃·H₂O solution by a double-jet process.It was verified by thermodynamic calculations that the internal cause of nickel oxalate being fibre morphology, is owing to the formation of Ni（NH₃）_n^2-（n=1,2,...,6）, in alkali Ni²⁺-NH₃-H₂O system, resulting in variations of the form and supersaturation of nickel oxalate precipitates.It was confirmed that with the change of synthesizing conditions, a series of Ni（NH₃）_nC₂O₄·2H₂O （n=0, 0.5,1.0,1.5,2.0,...） can be precipitated in Ni^<sup>2+)-NH₃-C₂O₄^2--H₂O system, among them, nickel oxalate with n equal to zero isα-NiC₂O₄·2H₂O, while the others being new species of nickel oxalate containing NH₃, which are no cards in JCPDS at present. Rather pure samples of nickel oxalates, with n value equal to 0.5, 1.0 1.5, 2.0 individually, were synthesized and furthermore, their molecular formula, XRD pattern and data were determined.It was found out that under alkali condition in the Ni²⁺-NH₃-C₂O₄^2--H₂O system, the firstborn precipitates are admixtures of several kinds of Ni（NH₃）_nC₂O₄·2H₂O （n=0.5,1.0,1.5,2.0,...）, with the morphology of agglomerate of small crystals. During aging, dissolution- recrystallization processes take place, due to the difference of their solubilities, that is, the species with higher solubilities dissolve, releasing the crystallization substances constantly, while the species with lower solubilities grow under a lower supersaturation, forming fibrous nickel oxalate particles with high aspect ratio.It was found out that under alkali condition in the Ni²⁺-NH₃-C₂O₄^2--H₂O system, Ni（NH₃）_1.5C₂O₄·2H₂O is the most stable phase of nickel oxalates in the experimental range of conditions,generally existing as the only species of nickel oxalates in the systems aged fully.It was found out that both crystal growth and orientational aggregation along long axis of the particles are the ways of particle growing up.The effects of such parameters as temperature, pH, feeding rate,concentration, [NH₃]_T/[Ni]_T molar ratio, surfactants and agitation, on the morphology and size of the particles, were studied and conclusions obtained are as follows: （1） The morphology and size of the particles are mainly decided by the species of nickel oxalates present. （2） Speeding up dissolution-recrystallization process benefits the preparation of the particles of being composed of a single species with uniform morphology and high aspect ratio. （3） The variations of nucleation and growth supersaturation under different conditions have definite effects on the morphology and size of the particles. （4） Adding PVP25 at a proper amount is beneficial to the preparation of fibrous nickel oxalate particles with high aspect ratio. 3) The formation mechanism as well as the morphology and size control of fibrous cobalt oxalate particlesThe cobalt oxalate particles with fibre morphology were prepared using CoCl₂ solution and mixed（NH₄）₂C₂O₄, NH₃·H₂O solution by a double jet process.Thermodynamic calculations revealed that immanent cause of cobalt oxalate presenting fibre form, is owing to the formation of Co（NH₃）_n²⁺ （n=1,2,...,6） in alkali Co²⁺-NH₃-H₂O system, resulting in variations of the species and supersaturation of cobalt oxalate precipitated.It was found out that in Co²⁺-NH₃-C₂O₄^2--H₂O system, two species of cobalt oxalates can be precipitated with the changes of synthesizing conditions, one of which isβ-CoC₂O₄·2H₂O, while the other is Co（NH₃）_1.5C₂O₄·2H₂O, a new species of cobalt oxalate, without record in JCPDS files at present.It was found out that both crystal growth and orientational aggregation along long axis of the particles are the ways of particle growing up.The effects of such factors as temperature, pH, feeding rate, concentration and PVP25 addition, on the morphology and size of the particles, were investigated and following conclusions can be obtained: （1）The morphology and size of the particles is governed mainly by the kind of cobalt oxalate species. （2） The changes in nucleation and growth supersaturation of the particles have a certain effect on their morphology and size at varying conditions. （3） In aging process, the species of cobalt oxalate do not change, but dissolution-recrystallization process takes place among the particles with different sizes, following Ostwald ripening mechanism, resulting in variations of morphology and size of the particles. （4） The addition of PVP25 has no obvious effects on the morphology and size of fibrous cobalt oxalate particles.更多还原