| Carbon coated metals core/shell nanoparticles are a new type of nanomaterials composed of functional cores and modified shells.They have promising applications in fields such as magnetism,biomedicine,high-efficiency catalysts and electromagnetic wave absorbers.CoNi alloy nanoparticles have attracted considerable attention due to their characteristics of high curie temperature,high saturation magnetization,low coercivity,low magnetocrystalline anisotropy,and obvious Snock's limit.At present,various methods such as IR-laser heating method,high-temperature arc plasma evaporation method,solvothermal method,hydrothermal method and modified arc discharge method have been developed to fabricate CoNi/C nanoparticles and their corresponding derived architectures.However,the low consumption,high yield and controlled synthesis of CoNi/C nanoparticles remains a major challenge.In this paper,CoNi/C core/shell nanoparticles were synthesized in a horizontal tube furnace with hydrogen as carrier gas by one-step MOCVD,which used acetylacetone cobalt?Co?acac?3?and acetylacetone nickel?Ni?acac?2?as precursors.Then the phase,morphology and microstructure of the synthesized samples were systematically characterized by various analytical means,such as X-ray diffraction?XRD?,Raman spectrometer?Raman?,transmission electron microscope?TEM?,plasma emission spectrometer?ICP-OES?and elemental analyzer.The microwave absorption characteristics of samples were studied by vector network analyzer and transmission line theory.The main results of this paper are summarized as follo?w1s?:The effects of mass ratio of Co?acac?3 to Ni?acac?2 and deposition temperature?region?on the phase,morphology and structure of CoNi/C core/shell nanoparticles were studied:?a?When the mass ratio of Co?acac?3 to Ni?acac?2 is 1:2,1:1 and 2:1,the average grain sizes of the CoNi cores are 27.3,25.4 and 21.4 nm,respectively,and the content of Co in the alloy increased with the increase of the content of Co?acac?3 in the precursors.?b?The products collected by the quartz plate in a reaction zone with a temperature of 700 oC in a horizontal tube furnace were CoNi/C core/shell particles with an average core diameter of 24 nm and an average shell thickness of 7.9 nm.And the products collected by the steel wire ball at the end of the tube furnace with a temperature of about 300 oC were CoNi/C core/shell particle with an average core diameter of 3.7 nm and an average shell thickness of 1 nm.?2?The formation mechanism of CoNi/C core/shell nanoparticles was analyzed and divided into the following four steps:?a?Co?acac?3 and Ni?acac?2form vapor phase precursors in the evaporation zone of 220 oC,and then transported to the reaction zone with hydrogen;?b?the vapor phase precursors Co?acac?3 and Ni?acac?2 were thermally decomposed in the reaction zone.CoNi atomic clusters and gaseous carbonaceous compounds?CO2 and CH4,etc.?were formed at the same time;?c?gaseous carbonaceous compounds were catalytically decomposed on the surface of CoNi clusters,and formed C atoms were adsorbed on the surface to form CoNi-C nanoclusters;?d?CoNi-C nanoclusters agglomerated due to high surface adsorption when colliding with each other,and formed CoNi/C core/shell nanoparticles through fusion and precipitation processes.?3?According to the transmission line theory and the vector network analyzer,the microwave absorption performance of the synthesized CoNi/C core/shell nanoparticles was studied.It was found that the complex permittivity,complex permeability and reflection loss of CoNi/C core/shell nanoparticles with very small size exhibited enhanced effects,as well as expanded the effective absorption bandwidth. |
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