Preparation And Microwave Absorption Properties Of FeCl₃-NiCl₂-GICs And Its Reduction Products

With many excellent characteristic, Graphite intercalation compounds （GICs）have a great potential application on high electrical and thermal conductivities,sealing, energy storage and conversion performances, etc. Howerver, it is difficult instably controlling the material structure and improving the yield of the products byuse of the traditional preparation methods of GICs including the two-zone vaportransport technique, the liquid intercalation method, and the electrochemical method,etc. The molten salts method which has simple process and equipment belongs to theliquid intercalation method, and is usually used to prepare ternary ormulti-component GICs. It has the biggest possibility to produce GICs with a largescale, while the reactor needs to be vacuumized and airproofed, so glass tube is usedas the reactor, of which the volume is small and the heat-resistant temperature is low.In this dissertation, ternary FeCl₃-NiCl₂-GICs were synthesized by molten saltsmethod in air. The natural graphite powders were used as host material and themixture of NiCl₂and FeCl₃were used as intercalants. The technological factors,process and microwave absorption properties of FeCl₃-NiCl₂-GICs and its reductionproducts were investigated. The conclusions are summarized as follows:（1） Systematic investigations were carried out on the effects of technologicalfactors upon the stage index of FeCl₃-NiCl₂-GICs under the new technique, and thestainless steel reactor with big volume was designed and employed. The results showthat the ternary FeCl₃-NiCl₂-GICs with different stage structure can be obtained inthe glass tube with a little air. The influence of particle size of graphite, reactiontemperature, reaction time, pressure, NiCl₂/FeCl₃and C/transition metal chlorideson the stage index of FeCl₃-NiCl₂-GICs, were similar with that of theFeCl₃-NiCl₂-GICs prepared in vacuum. XPS results showed that the oxygen elementmainly distributed on the surface of graphite flakes and have little influence on otherparts of GICs when it reacted with graphite. The weight of the products prepared inthe stainless steel designed and manufactured by ourselves was ca.250g for eachreaction. The process simplification and the improvement in reactor of the moltensalts method pave the way for the preparation of new GICs, the performance andapplication research.（2） In order to evaluate the microstructure and the conditions of ternaryFeCl₃-NiCl₂-GICs, four parameters were put forward, i.e., the volume fraction of theobject stage GICs（V1）, the Ni standard deviation of the products（σ）, Ni/Fe ratio in the products （S）, and the ration of Ni/Fe ratio of the products and Ni/Fe ratio in rawmaterials（S′）. According to these parameters, the optimized preparation conditionsof the FeCl₃-NiCl₂-GICs with pure stage index in the new container were obtained.The four parameters have guiding significance for the microstructre and processoptimization of ternary GICs. And the quantitative characterization of the degree ofdiversification for ternary GICs is realized by them.The microstructure of the FeCl₃-NiCl₂-GICs such as surface morphology,composition, element distribution, crystal structure was investigated by XRD, SEM,EDS, Raman and HRTEM. The results show that the FeCl₃&NiCl₂and the layer ofgraphite are arranged alternately, and the stage domain size is about5-10nm. Due tothe intercalation reaction, the broken lattice fringes of graphite are no longer straight.The flake edge of the corroded zigzag stage1FeCl₃-NiCl₂-GICs was observed. Withthe increase of the stage index of GICs, the corroding degree is weakened and finallylooks like the original graphite.（3） The properties of microwave （2-18GHz）absorption of the GICs werestudied in details. It is found that FeCl₃-NiCl₂-GICs are good microwave absorbingmaterials（MAM） and have an optimum absorption efficiency lying in lowerfrequency region. The main microwave absorption mechanism of GICs is dielectricloss, and the absorbing capability of GICs is related to the electrical conductivity ofGICs. The optimal mass ratio of GICs and paraffin wax is1:1. The maximum TRL ofthe stage1FeCl₃-NiCl₂-GICs which size is100mush is-11dB when the thickness ofMAM is5mm, and the matching frequency is3.2GHz.（4）The intercalation process of FeCl₃-NiCl₂-GICs and the diffusion of NiCl₂were analyzed. The results show that the intercalation process is divided into fivestages. The first is the selective absorption involved the intercalation between FeCl₃and NiCl₂. The second stage is the formation stage of FeCl₃-GICs. The third is theNiCl₂distribution stage including NiCl₂and FeCl₃together. The forth is thedistribution stage of NiCl₂alone. And the last one is the NiCl₂intercalating with theGICs domain stage. The speed of the intercalation process is greatly depended on thediffusion of Ni. The intercalation ways of NiCl₂are either embedding or meltingtogether with FeCl₃to form NiFe2Cl8when the condition is under500℃and lessthan72h, and the method of embedding is predominant. These conclusions about theintercalation process and mechanism show the method and way to improve themicrostructure of ternary GICs to us.（5） The reduce conditions of FeCl₃-NiCl₂-GICs and the properties of microwaveabsorption of the reduction products were investigated in details. The reductioneffects of FeCl₃-NiCl₂-GICs by H2, Na, hydrazine hydrate and sodium borohydride were studied. The results show that the reduction effect by H2is the best, and noimpurity is introduced into reduction products in H2reduction environment. Thereduced conditions of FeCl₃-NiCl₂-GICs by H2, such as temperature, time and theparticle size of graphite, were investigated. The results indicate that the content ofthe Cl in the reduction product is decreased with increasing of reduction temperature,prolonging of reduction time and the minimizing the particle size ofFeCl₃-NiCl₂-GICs. The molar content of Cl in the reduction product, which wasprepared by reducing the stage2under the optimized conditions, is about31%. Thereduction products of FeCl₃-NiCl₂-GICs are composed of FeNi, graphite and FeCl₂.A lot of FeNi particles with size of20²00nm are located in the graphite layers. Thekey reason that the FeCl₃-NiCl₂-GICs cannot be reduced thoroughly is the sterichindrance effect of graphite.The reduction products of FeCl₃-NiCl₂-GICs, a kind of soft magnetic material,have good microwave absorption properties. The absorbed frequency bandwidth ofthe FeNi/C500reaches7GHz, covering the whole Ku band （12.4-18GHz）, when thethickness of MAM is1mm. The maximum TRL of FeNi/C600is-4.5dB at3.5GHzwhen the thickness of the MAM is2mm, which exhibits good absorption efficiencyat lower frequency region.6、According to these research conclusions, FeCl₃-CoCl₂-GICs and its reductionproducts were designed and manufactured, and the the properties of microwaveabsorption of them were investigated. These studies about the reduction method,process and performance of ternary transition metal chlorides-GICs provide theexperimental evidences for the design and preparation of the new transition metal/graphite nanocompositions, and set out a way to design and preparate the new MAMwith the low density and wide-frequency properties.