| 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 C2O42- 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)n2-(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-C2O42--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-C2O42--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-C2O42--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-C2O42--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.
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