Structure Design and Properties Investigation on Transition Metal-Containing Polymer-derived Ceramics

Author:Zhang Xiao Fei

Supervisor:chen li xin kong jie

Database:Doctor

Degree Year:2018

Download:2

Pages:151

Size:11755K

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Due to the good designability of precursor polymers based on the molecular level,low curing temperature,favarable processability and moldability,moderate heat treatment temperature,controlled ceramic composition and excellent performance,polymer-derived ceramics(PDCs)have been widely used in the aviation,aerospace and nuclear industries using as ceramic fibers,coatings and powders.The introduction of metal elements into the ceramic precursor,especially transition metal elements,can endow ceramic materials more excellent physical and chemical properties,thus greatly expanding the application field of ceramic materials.Although a large number of studies have focused on the synthesis and application of transition metal-containing PDCs,only few types of metallocene-modified precursors were reported,and their cost is high,the preparation conditions are harsh and the preparation process is cumbersome.Therefore,the present study aims to develop new metallocene precursor systems with simple and low-cost preparation method,and obtain the transition metal-containing PDCs nanomaterials with excellent properties by controlling the structural composition of the precursor system and pyrolysis conditions.The present work includes:(1)Using polysilazane(PSZ)as precursor,the influence of NiCp2 on the pyrolysis process and products of polymer precursors was investigated.The pyrolysis behavior of the precursors was investigated by TGA/DSC-Mass Spectrometry.Compared with the pure PSZ,the NiCp2-containing precursor systems have lower thermal crosslinking temperature,decreased deamination reaction and high ceramic yields.The NiCp2 in polymeric precursors induced the formation ofα-Si3N4,SiC,graphitic carbon,and Ni2Si at 1100oC.Nickel silicide nanocrystals containing ceramics exhibited soft magnetism with ultralow hysteresis loss.Owing to the experiment using inexpensive commercial polymeric precursor and NiCp2,we provide a low cost and effective way to prepare tunable transition metal-containing ceramics.(2)A novel and versatile method,phase-separation assisted pyrolysis(PSAP),for preparing PDCs microspheres is reported.Various Si–C–N–(Ni)magnetic microspheres with various dimensions from micro-to nanoscale can be conveniently prepared by using polymeric precursor with an assistance of polymer template of linear polystyrene.The morphology of PDCs microspheres could be controlled through changing the feed ratio of precursor and template,pyrolysis temperature,and centrifugal fractionation.The prepared PDCs microspheres were proved with dense structure,tunable crystals,and ferromagnetic property with an ultralow hysteresis loss.In addition,PDCs microspheres with different nanocrystallites were prepared by selecting different polymers or low molecules as ceramic precursors and other polymers as templates.Therefore,it is indeed a universal and convenient method for synthesizing functional PDCs microspheres with micro-scale dimensions and well-controlled morphology.(3)Ferrocene-containing organosilicon polymers have found wide applications in ceramic nanomaterials,redox sensors,catalysts and nanolithography fields.However,most synthesis methods for ferrocene-containing organosilicon polymers require either strict water-free and oxygen-free conditions or the presence of platinum complex catalysts.In this part,the free radical copolymerization of 1,1?-bis(dimethylvinylsilyl)ferrocene(DVFc)and trimethylol-propane trimethacrylate(TMPTMA)was achieved to construct either soluble branched ferrocene-containing organosilicon polymers or uniform cross-linked microspheres only through tuning the feed ratio of the two monomers.The structures of copolymer and cross-linked microspheres have been characterized by two-dimensional(2D)NMR measurements.Furthermore,the novel organosilicon polymers could be employed as preceramic precursors to prepare magnetic Si-C-Fe-(O)ceramics with favorable ceramic yield at high temperature.The nanocrystals-containing ceramics magnetism could be tuned through changing copolymer structure or pyrolysis condition.(4)We present,for the first time,the synthesis and characterization of magnetic Si-C-Fe hybrid microspheres and their catalytic performance in room temperature reduction of4-nitrophenol as a representative sustainable process for converting environmental pollutants to fine chemicals.The ferrocene-modified polydivinylbenzene(Fc-PDVB)precursor was synthesized by Pt-catalyzed hydrosilylation between the residual vinyl groups on the PDVB surface and 1,1?-bis(dimethylsilyl)ferrocene,where further pyrolysis led to the formation of Fe nanocrystal-containing Si-C-Fe hybrid microspheres.A series of analysis results showed that as-prepared microspherical catalyst exhibited nano-and meso-pores,a high specific surface area(i.e.,347.9 m2 g-1)and good ferromagnetic properties.Efficient catalytic activity(TOF:0.163 s-1),100%selectivity(to 4-aminophenol)and excellent reusability(with easy separation)have been delivered.The achieved microspheres outperform a number of nanomaterials such as supported noble metal particles,composites,monoliths and sheets.We have confirmed by DFT calculations that the activation of 4-nitrophenol via its weak non-covalent interaction with the sp2 carbon domain of Si-C-Fe hybrid microspheres contributed to the superior performance which can be extended to a range of nitrobenzenes.