Study On A Novel Method For Preparation Of Fibrous Porous Ultrafine Special Nickel-cobalt Alloy And Nickel-cobalt Complex Oxides Powders

The distinctive properties of special ultrafine powders are closely related to their chemical compositions, structures and granularity as well as morphology. The preparation techniques and processing methods of ultrafine powders which are the necessary means for changing the product properties are able to not only create new materials, but also change and accurately control their physical and chemical properties such as chemical compositions, structures, granularity and morphology. Therefore, it is of great importance for practical application and fundamental academic study to explore novel preparation methods of ultrafine powders.A novel preparation method of coordination coprecipitation-pyrolytic decomposition was proposed to synthesize fibrous porous special ultrafine Ni-Co alloy and Ni-Co complex oxide powders in the mixed solvent of solvent A and water (V_{solvent A} :V_water≥1:3). The key properties of the two types of powders, including the uniformity of the chemical composition, phase composition, purity, specified ratios of Ni²⁺ to Co²⁺, and the control of granularity and morphology, were systematically investigated. The major research accomplishments in this paper are as follows:A novel method of coordination coprecipitation-pyrolytic decomposition in Ni²⁺-Co²⁺-NH₃-NH₄⁺-C₂O₄^2--H₂O-solvent A system was proposed to prepare fibrous porous special Ni-Co alloy and Ni-Co complex oxide powders. The process involves compounding uniformed mixture solution of nickel and cobalt ions, synthesis of fibrous nickel-cobalt oxalate complex salt, pyrolytic decomposition under the controlled atmospheric condition, and the surface oxidation resistance treatment of the powders. The process is carried out under ambient temperature and pressure. It is friendly to environment, easily operated and controlled, and therefore can be easily scaled up for commercial production. In addition, two different types of products can be obtained from the same process by adjusting pyrolytic decomposition atmosphere.The following conclusions were drawn from the theoretical analysis and experimental test. In the solvent of water, the coprecipitated solid solution with fixed ratio of Ni²⁺ to Co²⁺ can be obtained at pH＜5.0, as pH＞5.0, the precipitation process is fractiona and the precipitant is a mixture. In the mixed solvent of solvent A and water, when the precursor powders of Ni-Co solid solution alloy or Ni-Co complex oxide powders are synthesized by coordination coprecipitation, Ni²⁺ and Co²⁺ ratios can be controlled and maintained accurately within a wide range of pH values (2.0～8.6). For the first time, the coprecipitates synthesized in the mixed solvent with wide range of pH values (2.0～8.6) of solution have been validated as homogeneous Ni-Co oxalate complex salt compound by analytical methods of TGA/DTA and XRD. Similarly, the decomposition product obtained in weak reduce gas atmosphere with nitrogen and little hydrogen gas was confirmed by TGA/DTA, XRD as well as thermomagnetometry.It is the first time to synthesize three different morphologies of Ni-Co oxalate complex salt including granular aggregation morphology, rod morphology and fibrous morphology by using coordination coprecipitation method in the mixed precipitating medium. The experimental conditions were investigated on the chemical composition, granularity and morphology of the coprecipitates. It can be concluded that the chemical composition, granularity and morphology of the coprecipitates are dependent on temperature, solution concentration, pH value, the nature of anions, dispersant, etc. The fibrous Ni-Co oxalate complex salt was obtained under the conditions of using ammonia as a complex and pH adjustor, oxalate as coprecipitated agent, reverse-feed method, temperature of 50～65℃, total concentration of Ni²⁺ and Co²⁺ of 0.5mol/L～0.8mol/L, PVP as dispersant, Cl^- as anion and pH value in the range of 8.0～8.4. The type of powders with rod morphology were obtained when pH is 6.0～8.0 and other conditions the same. The coprecipitate powders having granular aggregation morphology can be obtained when pH≤5.0.For the first time, the mechanism of Ni-Co oxalate coprecipitation and differentce of the chemical compositions under different conditions were systematically studied. When pH≤5.0 by adjusting ammonia concentration, composition of the precursor is Ni_xCo_1-xC₂O₄·2H₂O and the morphology is granular aggregation; when pH=6.0～8.0 or pH≥8.0, the morphology of the precursor powders is rod-like or fibre-like respectively due to the involvement of ammonia in the coprecipitation process. The formula of the precursor were identified as Ni_xCo_1-x (NH₃)_yC₂O₄·mH₂O·nNH₃, where x represents molar fraction, while m, n and y represent mole numbers of the components which depend on the solution pH value, the concentration of ammonia and the drying temperature. The changing ranges of y, m, and n are as follows: 0＜y＜0.5, 1.0＜m＜2.0 and 1.0＜n＜2.0. The x value can be controlled accurately by manipulating the ratio of Ni²⁺ to Co²⁺ from the added feed metal salts. Additionally, the formation mechanism of fibrous or rod-like precursor powders (Ni-Co oxalate complex salt) was also investigated. For the Ni-Co oxalate complex salt, the central metal atom was bonded by two C₂O₄^2- ions forming a planar molecule. Perpendicular to the molecular plane are two coordinated H₂O molecules by which crystal grain can grow in an elongate way along axial direction. The surface polar of Ni-Co oxalate molecules is different when the complex switches from H₂O to NH₃ at different pH values in the solution by adding NH₃. Therefore, rod-like or fibre-like powders may be formed through [(NH₃)M-OX-M(NH₃)]²⁺ growth units along the axial direction.For the first time, pyrolytic decomposition mechanism of Ni-Co oxalate complex salt obtained at different solution pH values with different morphologies were thoroughly explored in nitrogen atmosphere:(a) When pH≤5.0, the decomposition mechanism of Ni-Co oxalate complex salt with granular aggregation morphology is described as follows:Ni_xCo_1-xC₂O₄·2H₂O(?)Ni_xCo_1-xC₂O₄(?)Ni_xCo_1-x alloy(b) when 6.0≤pH＜8.8, the decomposition mechanism of Ni-Co oxalate complex salt with rod-like or fibre-like morphology is described as follows:Ni_xCo_1-x(NH₃)_yC₂O₄·mH₂O·nNH₃(?)Ni_xCo_1-x(NH₃)_yC₂O₄·mH₂O (?)Ni_xCo_1-x(NH₃)_yC₂O₄(?)Ni_xCo_1-xC₂O₄(?) Ni_xCo_1-x alloyThe effects of pyrolytic decomposition conditions of Ni-Co oxalate complex salt powders on the product performance and surface treatment were investigated. The pyrolytic decomposition process and surface treatment were conducted in the same furnace. The results showed that porous fibrous Ni-Co alloy powders with large aspect ratio, large specific surface area, low content of carbon and oxygen, well- dispensability and face-centered cubic structure can be obtained through the decomposition of Ni-Co oxalate complex salt under the condition of the proper decomposition: fibrous Ni-Co oxalate complex salt with good dispersion for precursor, weak reductive atmosphere, 360～450℃for decomposition temperature, 30 min for decomposition time, 1.2 L/min～1.8L/min for gas speed and heating up rates at 5～10℃/min.It was also found that fibrous and porous special nickel-cobalt complex oxides powders can be produced by adjusting and controlling decomposition temperature and atmosphere if fibrous nickel-cobalt oxalate complex salt powders were taken as precursor of nickel-cobalt complex oxides. The pyrolytic decomposition mechanism of fibrous Ni-Co oxalate complex salt and the effects of pyrolytic decomposition conditions on the formation of Ni-Co complex oxides phase were systematically investigated for the first time. The results showed that porous fibrous NiCo₂O₄ complex oxide powders with Co_1-x²⁺Co_x³⁺[Co³⁺Ni_y²⁺Ni_1-y³⁺]O₄ structure can be produced under the following conditions: fibrous Ni-Co oxalate complex salt powders with Ni²⁺/Co²⁺=1:2 (mole ratio) as decomposition material, flowing air or oxygen atmosphere, temperature 500℃and retention time 3 hours. Pyrolytic decomposition mechanism of Ni-Co oxalate complex salt in flowing air or oxygen atmosphere is as follow: First, physical absorbed water and ammonia substitution for hydrated water are deprived; and then dehydration process; last, deamination, decomposition of Ni_xCo_1-xCO₄ and oxidation of metal atom take place simultaneously.