Design And Properties Of The Reactive Single Source Precursors For Silicoboron Carbonitride Ceramics

Polymer-derived ceramics（PDCs）is a novel of multifunctional ceramics synthesized by thermal pyrolysis of preceramic polymers.Compared with traditional ceramics,PDCs have many outstanding properties.For example,PDCs exhibit excellent thermal stability,high temperature mechanical properties and oxidation/corrosion resistance.PDCs also exhibit tunable semiconductor properties as well as optoelectronic properties.Among various PDCs,siliconboron carbonitride（SiBCN）ceramics have received special attention due to their excellent high-temperature stability,creep resistance and electrical properties,granting them potential applications in coatings,microelectronics,and fibers used in high temperature stability composites.Although there are many kinds of single-source precursors for SiBCN ceramics,most of them contain highly active and air-sensitive Si-Cl or B-Cl groups,which cause difficulties in storage and usage.In addition,their curing method is thermal curing requiring ammonia or amines as the curing agent.In response to these problems,we designed and synthesized a series of single-source precursors containing reactive reaction groups for SiBCN ceramics,including the study of UV-curable single-source precursors.The main research contents and conclusions are as follows:1.UV-curable single-source ceramic precursors for SiBCN ceramic prepared via transmetallation.The single-source precursors a-TSMB and e-TSMB with Si and B connected by methylene are synthesized via transmetallation reaction.They also contain borazine ring as the backbone,and reactive groups as the terminal groups.The reactive groups of a-TSMB are acrylate groups,and the reactive groups of e-TSMB are vinyl ether.The molecular structures of a-TSMB and e-TSMB are characterized by Fourier transform infrared（FTIR）and NMR.The results of DPC and RT-FTIR indicate that a-TSMB and e-TSMB have excellent UV-curing properties.When a-TSMB is irradiated by ultraviolet light,the exothermic peak appeared at 5s,the curing exothermic peak end within 60 s,and the final conversion was about 90%.When e-TSMB is irradiated by ultraviolet light,the exothermic peak appeared within 40 s,the curing exothermic peak end within 84 s,and the final conversion was about 75%.The pyrolytic process of cured a-TSMB（P_a）and e-TSMB（P_e）are investigated by TG.The results show that the pyrolytic residual of P_a and P_e are 30%and 29.6%respectively at 1400℃.For pyrolysis product C_a and C_e,XPS and elemental analysis show that C_a and C_e contain Si,B,C,N and O elements.The composition of C_a can be approximated to that of Si₄B₃N₃C₂₈O₁₃.And the composition of C_e can be approximated to that of Si₄B₃N₃C₂₄O₁₁.The results of XRD pattern show that C_a and C_e remain amorphous at 1400℃.XPS,elemental analysis and XRD show that C_a and C_e are amorphous ceramics containing Si,B,C,N and O.2.UV-curable single-source ceramic precursors for SiBCN ceramic prepared via hydrobration.Single-source precursors a-CSEB,a-DSEB and a-TSEB with Si and B connected by ethyl are synthesized by hydroboration reaction.a-CSEB,a-DSEB and a-TSEB have similar structure but different functionalities.They also contain borazine ring as the backbone,and acrylate groups as the terminal groups.The molecular structures of a-CSEB,a-DSEB and a-TSEB are characterized by FTIR and NMR.By calculation of ¹H NMR spectra,the structure of a-CSEB has introduced 2.7 acrylate units,the structure of a-DSEB has introduced 4.5 acrylate units,the structure of a-TSEB has introduced 6.3 acrylate units.The results of DPC and RT-FTIR indicate that a-CSEB,a-DSEB and a-TSEB have excellent UV-curing properties,they can be UV-cured within 60 s.Under the same conditions（light intensity:110mW/cm²;initiator content:1.5 wt%;reaction temperature:room temperature）,the final conversions of a-CSEB,a-DSEB,and a-TSEB are 98%,92%,and 83%,respectively.With the increase of functionality,the final conversion decreases.In addition,DPC is used to study the effect of photoinitiator content,temperature and light intensity on a-TSEB photo-curing behavior.The results show that the effect of irradiation temperature on the photo-curing is relatively small.With the increase of photoinitiator content,the fastest polymerization rate and the final conversion rate first increase and then decrease.With addition of 1.0 wt%photoinitiator,the polymerization rate and final conversion reach the extreme values(0.072 s^-1,84.54%).The effect of UV light on the photo-curing is similar to that of photoinitiator content,first increase and then decrease.At 66mW/cm²,the polymerization rate and final conversion show the extreme values(0.125s^-1,87.53%).FTIR and TG-MS results show that their pyrolysis mechanism is depolymerization reaction and rearrangement reaction.The pyrolytic residue of the cured a-CSEB,cured a-DSEB and cured a-TSEB are 21.7%,26.1%and 33.7%at 800℃,respectively.The results reveal that the pyrolytic residual of the single-source precursors increases with the increase of functionality.XPS results for C₁,C₂ and C₃ derived from a-CSEB,a-DSEB and a-TSEB indicate that C₁,C₂ and C₃ contain Si,B,C,N and O elements.The results of SEM images show that there are holes and a large number of microspheres on the surface of C₁ and C₂.The surface of C₃ doesn’t appear microspheres,but only with tiny debris.XRD results show that C₁,C₂ and C₃ remain amorphous at 1400℃.XPS,SEM and XRD show that C₁,C₂ and C₃ are amorphous ceramics containing Si,B,C,N and O.a-TSEB is used as the initial material to prepare ceramic coating.Via spin coating,UV-polymerization and pyrolysis,the ceramic coating is prepared successfully.SEM micrographs show that the compacted coatings formed without any penetrating cracks or holes on samples and the interface bonding between the coating and substrate is good.3.Single-source ceramic precursor for SiBCN ceramic prepared via silazane cleavage reaction.Single-source precursor TSAB is prepared by the method of silazane cleavage.The molecular structure of TSAB contains borazine ring as the backbone and the vinyl as the terminal group.Its structure is characterized by FTIR and NMR.FTIR and DSC results show that its thermal curing is achieved by addition polymerization of double bonds.The optimum content of initiator in curing was 2.25 wt%.The favorable polymerization process is 130℃/1hr+160℃/2hr+190℃/1hr.The curing reaction kinetic are studied by KAS method and Crane equation,the activation energy Ea and reaction order n are 114.90 kJ/mol and 0.86,respectively.The TG-MS results show that the pyrolytic residual of cured TSAB is 60.7%at 800℃.The results of FTIR and XPS indicate that the pyrolytic product C_A derived from TSAB contains Si,B,C and N.The boron atoms and silicon atoms mainly bonded to nitrogen atoms,the carbon atoms mostly exist as C-C bond.XRD powder pattern and Raman spectra show that C_A maintain amorphous until 1500℃,the graphite and h-BN appeared at 1500℃.The room temperature conductivity increases with the pyrolysis temperature,the highest is about10^-4（Ω?cm）^-11 for C_A pyrolyzed at 1500℃.The PL emissions are in the visible range,the peaks appears at 418,466,498,593 nm（2.97,2.66,2.49,2.09 eV）.