Magnetic Field Effects On Chemical Reduction Of Ni²⁺ And Magnetic Domain Structures Of Nickel

The morphology and properties of materials can be controlled by use of magnetic field in chemical reactions.Magnetism mainly comes from electron spin magnetic moments,so studying magnetic domains contributes to understand magnetic properties of magnetic materials in the view of microcosmic.Therefore,the objective of this dissertation is to explore magnetic field effects on chemical reduction of Ni²⁺, magnetic domain structures and magnetic properties of nickel.More details are summarized as follows:1.In order to study the influence of magnetic field on chemical reactions,the reaction system imposed by the magnetic field should meet the following conditions. Firstly,the research objects should be paramagnetic ions,such as Ni²⁺ or Cu²⁺; Secondly,the reaction temperature can not be too high,for the Curie temperature of nickel is 358℃.Also,if the temperature is too high,the thermal motion would be dominant in the system,so the influence of magnetic field would become weak.Lastly, the reaction should be carried out as slow as possible,and then the slow nucleation and growth of crystal would be helpful to observe the induction of magnetic field on the particles.Therefore,low-temperature solvothermal system is an optimal reaction system,and facilitates the imposition of external magnetic field.So,in a moderate low-temperature solvothermal system,Ni-PVP complex was selected.The reaction conditions involved were studied systematically to select the optimal reaction parameters to reveal the effects of magnetic fields on chemical reactions.Uniform polycrystalline nickel nanospheres were prepared in a water-ethanol mixed solvent solution using Nickel Chloride(NiCl₂),hydrazine(N₂H₄),and polyvinylpyrrolidone (PVP) as starting materials.The product exhibited good ferromagnetism,and its saturation magnetization(Ms) and coercivity(Hc) are 53 emu/g and 82 Oe, respectively.It can be deduced that 1D nickel wires with controlled shapes can be obtained by introducing a magnetic field in the same system.However,research shows that the influence of magnetic field on the reduction reaction of copper ions was not obvious,as copper is not a ferromagnetic material. 2.Based on the study of several control experiments performed without a magnetic field,1D nickel wires with controlled shapes were obtained by use of a magnetic field in the optimal reaction systems.The involved chemical reaction was taken as an example to study the effects of magnetic fields on the chemical reduction of Ni²⁺,and the nucleation and growth of nickel nanowires.The formation of our 1D nickel wires can be ascribed to the cooperative effect of the reaction rate and magnetic field.The reaction rate in the system is controlled mainly by adjusting the reactant concentration when other reaction conditions are fixed.When the concentration of Ni²⁺ is low,the reaction rate is correspondingly slow,so the complex[Ni(N₂H₄)_x]²⁺ preferentially migrates to the magnetic line of force since paramagnetic metal ions are attracted toward the maximum field.Then the chemical reduction of[Ni(N₂H₄)_x]²⁺ may occur along the magnetic line of force due to the enhanced electrode potential,leading to the formation of uniform 1 D wires.While with the increase of the concentration of Ni²⁺, the reaction rate increases too,so it can be deduced that the chemical reduction of [Ni(N₂H₄)_x]²⁺ may be taken place quickly in the whole system,leading to the formation of isolated particles coexisted with the wires composed of microspheres which formed along the magnetic line of force.Additionally,to explore well the effect of magnetic fields on the nucleation and growth of magnetic particles,a postsynthesis magnetic alignment experiment was carried out on the sample prepared without a field.However,one-dimensional wires were not found,which interprets that magnetic fields applied during the chemical reaction have influenced the nucleation and growth of nickel,instead of inducing a simple assembly of particles.Magnetic measurement results show that the nickel wires prepared with a magnetic field had remarkably improved magnetic properties,compared to that of nickel nanospheres prepared without a field.Moreover,the enhanced microwave absorption property of the nickel wires/PMMA composite at 8.5-12.5 GHz in comparison with that of nickel microspheres/PMMA composite was observed,which indicated that the anisotropic wires were not the simple assembly of particles,and the magnetic domain structures in wires had been changed by the applied field in chemical reaction.3.Magnetic force microscopy(MFM) investigations were carried out to study the domain structures of the nickel samples,and magnetic field effects on magnetic domain structures and magnetic properties of nickel were explored.Nickel wires with a diameter of approximately 250 nm had single-domain structure,while unique core-shell cylindrical domain structures were found in wires with a diameter of 2μm. It is suggested that the unique multidomain may be aroused by the applied field in the chemical reaction.Then a schematic domain structure was used to tentatively explain the unique multidomain structure.To further support the core-shell cylindrical model, nickel film AF was prepared with the same condition as the wires,while film ZF was obtained by the same synthesis procedure as AF except without a field applied.MFM images of the AF sample show many dark circular domains,corresponding to the wires in a diameter of about 250 nm.Additionally,there are also some typical concentric annular zone domains with dark inner core and bright outer shell.While the dark spot domains of ZF are corresponding to the nanoparticles in the sample.The different magnetic properties of AF and ZF resulted from their different domains that controlled by the magnetic field in the chemical reactions.The magnetic measurements show a good agreement with our MFM results,further confirming the magnetic domain model proposed.So magnetic field in chemical reactions can be used to control magnetic domains of materials.