Preparation And Properties Of Carbon-encapsulated Nano Metal And Alloy Particles

Carbon-encapsulated nano metal particle is a new type ofnano-composite material.The role of the carbon coating in this material is to isolate the particles from eachother, thus avoiding the drawbacks caused by interactions between closely compactedunits, such as the oxidation of the bare nano metal particles. Moreover, the carboncoating can endow these particles with other properties, and then to be able to bewidely used as electronic, magnetic recording, electromagnetic shield/absorbing materials,catalysts, antibacterial materials and biomedicine etc. Since the first finding ofCarbon-encapsulated nano metal particle, it has attracted a great deal of researchinterest. In this thesis, a novel method for the preparation of carbon-encapsulated nanometal particle was proposed, in which D113 resin, a kind of cation exchange resin, wasused as carbon source and metal salt as metal source in order to preparecarbon-encapsulated nano metal (Ni, Co, Fe) and alloy (Fe-Ni, Fe-Co) particles viapyrolysis in the range of 400-700℃. Simplicity, low cost, and controllability ofparticle size are the favorable features of this preparation method.The structural morphologies of the carbon-encapsulated nano metal and alloyparticles are investigated by XRD, IR, SEM, TEM, HRTEM etc and the effects ofpreparation conditions on morphologies of Carbon-encapsulated nano metal and alloyparticle are studied. The formation mechanism is also discussed. Catalytic, magnetic andelectromagnetic absorbing properties are studied as well.Carbon-encapsulated Ni and Co nano-particles are prepared by pyrolysis of Ni/D113and Co/D113 precursors from 400℃to 700℃. It reveales that the nickel and cobaltparticles existe mainly in the form of fcc Ni and Co phase, and no evidence of Ni and Cooxides or carbides are observed, and it is found that the particles sizes of Ni and Coincrease with the increasing of pyrolytic temperature. But when Fe/D113 precursor ispyrolysed in the range of 400-700℃, the crystal structure of the iron particle is different.The products are mainly iron oxides andα-Fe as Fe/D113 precursor pyrolysed at 400℃and 500℃respectively, and as the pyrolytic temperature increases to the range of600-700℃,α-Fe and Fe₃C are observed. Carbon-encapsulated fcc iron-nickel and bcciron-cobalt alloy nano-particles are prepared by pyrolysis of Fe-Ni/D113 and Fe-Co/D113precursors from 400℃to 700℃. The lattice constant of the nano alloy particle increaseswhen increasing the Content of Fe in the nano alloy particles, and the nano alloy particle sizes increase when increasing the pyrolytic temperature.It is found that the structure of the carbon coating is amorphous carbon in thecarbon-encapsulated nano metal and alloy particle when pyrolytic temperature is in therange of 400-500℃, and the amorphous carbon coating changes to graphite whenpyrolytic temperature is in the range of 600-700℃. And the size of encapsulatednanoparticle increases as well. A mechanism reflecting these changes is proposed.The catalysis of these carbon-encapsulated nano metal and alloy particles on thethermal decomposition of NH₄ClO₄ (AP) is investigated by thermal analysis. Resultsindicate that the carbon-encapsulated metal nano and alloy particles lower the higher peaktemperature of AP thermal decomposition. Thermal decomposition dynamics test showsthat the carbon-encapsulated nano metal and alloy particles can lower the activation energyof AP higher temperature decomposition effectively, and increase reaction rate constant ofAP decomposition as well. In addition, the catalytic mechanism of AP thermaldecomposition is discussed.The experimental results show that magnetism of carbon-encapsulated nano metaland alloy particles depends on the nano metal and alloy particle size, which is largelyinfluenced by pyrolysis temperatures. The coercivities H_c of the samples at roomtemperature are much higher than those of the corresponding bulk material, and H_cincreases with the decrease of the particle size firstly, and H_c reaches maximum valuewhen the particle size is about single-domain size (D_s). As the particle size is less than D_s,H_c decreases with the decrease of the particle size. The saturation magnetization M_s ofcarbon-encapsulated nano metal and alloy particles increase with pyrolytic temperatureincreasing, and are less than those of bulk materials. Moreover magnetism ofcarbon-encapsulated nano alloy particles is influenced by the constituent of alloy particles.Electromagnetic absorbing property tests of carbon-encapsulated iron, nickel, FeNiand FeCo alloy nano particles pyrolysed at 500℃indicate that carbon-encapsulatediron nano-particles has more strongly electromagnetic absorbing efficiency than theothers. For 50% mass fraction in wax and 2 mm-thickness of carbon-encapsulated ironnano-particles, at f=11.7 GHz, R_min=-10dB andâ–³dB=6.0 GHz.