Fabrication Of Ordered Porous Non-Oxide Ceramics Using Preceramic Polymer/Template Techniques

As an important field of material science, ordered porous materials have been widely applied in the adsorption, separation and catalyst. In the process of tens years' development, the interest on this kind of material stepped from micropore to mesopore, and to macropore. Nowadays, macroporous materials with high stability, high BET surface area, 3D ordered structure, tailored composition and pore morphology have been paid more attention due to their many potential applications.In this study, a series of ordered macroporous non-oxide ceramics were successfully fabricated for the first time by combining template and preceramic polymer techniques. The fabrication process includes 4 basic steps: (1) selection and preparation of preceramic polymer and template; (2) infiltrating the polymer solution or melt into the template; (3) pyrolysis of the polymer; (4) removal of the template.Six kinds of porous structures were studied in this work: sphere shape pore (S), sphere void shape pore (SV), cylinder shape pore (C), cylinder void shape pore (CV), cubic shape pore (Q) and filter-template pore (FT). The structures of the pore shapes, except the FT pore, are well ordered. As for the composition of the porous materials, 6 kinds of non-oxide ceramics were investigated: SiC, SiC/Si, SiC/C, SiC/MoSi₂, SiCN and BN.Template technique is the key point in the fabrication of ordered macroporous materials. In this study, three kinds of templates were prepared. For S-type porous materials, homogenous and mono-dispersed silica spheres were prepared by Sol-Gel method, where the diameter of spheres can be tailored from 70 to 1000nm. The spheres were packed into a 3D ordered hcp lattice by natural sedimentation, and turned into the template after drying. For SV-type porous materials, the template is actually 3D S-type porous carbon, which is prepared by the following process: infiltrating phenolic resin into the above-mentioned silica sphere template, followed by carbonization and etching out the silica spheres. In the preparation of the template of Q-type porous materials, the above-mentioned ordered silica spheres were transformed into nearly cubic shape by pressure.Carbon fibers or carbon nanotubes were used directly as the templates of C-type porous material. Commercial porous alumina membrane templates were applied to make CV-type pores. While for FT materials, the templates are different filters, such as G6, Nylon-66, Whatman40 and MFS filters.Six kinds of preceramic polymers were synthesized to prepare non-oxide ceramics: polymethylsilane (PMS), polycarbosilane (PCS), polysilazine (PSZ), polyborazine (PBN), Molybdenum modified PMS (MoPMS) and BN-PMS-PCS. Among these, PSZ was synthesized by the ammonification of dichloromethylsilane and dichloromethyl-vinylsilane; PBN was polymerized from borazine at room temperature using M0Cl₅ as crosslinking agent; MoPMS was obtained from the condensation of PMS and M0Cl₅ in situ with the elimination of HC1 at room temperature.S-type porous SiC/Si, SiC/C, stoichiometric SiC, SiC/MoSi₂, SiCN and BN ceramics were prepared by infiltrating into 3-D ordered silica sphere templates with PMS, PCS, BN-PMS-PCS, MoPMS , PSZ and PBN, respectively, followed by pyrolysis in Ar or N₂ and removal of template by acid etching.The BET surface areas and the pore volumes of S-type porous ceramics can be controlled with diameters of silica spheres of the templates. There are three kinds of pores in the system: sphere pores originated from silica spheres, the small windows caused by the contacting parts of the spheres and the micropores of l⁵nm from the pyrolysis of the polymer. It is the small windows that connected the sphere pores. The large amount of micropores contributes to the surprisingly high BET surface areas.The S-type porous materials were modified on the walls with Pt-Ru catalyst alloy deposited, where the particle size is 7-10nm, by borohydride reduction. The catalyst supported on the porous structures might greatly enhance the catalyst efficiency.3D ordered SV-type porous structures were fabricated for the first time by using 3D S-type porous carbon as templates. Low viscosity preceramic polymer melt such as PSZ resulted into connected solid spheres. Alternately, when solution of PMS was applied, connected hollow spheres (h-SV structure) were obtained due to the evaporation of the solvent in pyrolysis. Therefore two kinds of pores in h-SV structure: sphere void andhollow cores of the spheres. The sphere diameters, wall thicknesses of hollow spheres, and therefore the BET surface areas and pore volumes can be tailored by pore size of the porous carbon templates and the concentration of the polymer.The S-type porous carbon template technique can also be expanded into other materials, such as porous metals. When RuCb/FkPtC^ and AgNO3 solution were infiltrated into the templates, 3D ordered h-SY porous Pt-Ru and Ag were achieved after borohyride reduction reaction, with very high BET surface areas and pore volumes. The porous metals, with unique conducting behavior, will be expected potential applications in photonic, eletronic and catalyst fieldsOrdered CV-type porous SiC were prepared for the first time using cylinder porous alumina membranes (cylinder diameter is of 100, 200, 400nm) as templates and PMS solutions in THF. The resulted h-CY structures contain two kinds of pores such as cylinder void pores and the hollow tubular pores. The wall thickness of the tubes can be tailored with the concentration of PMS, for which, higher concentration leads thicker wall or even solid rods. The achieved h-CY porous SiC were modified with deposited Pt-Ru catalyst on the inner walls of the tubes with high surface area.Other kinds of porous ceramics, such as C, Q, FT, S/FT and FT/FT types porous SiC and FT type porous SiCN were prepared. It was found the porous materials from silica based template exhibit very high BET surface areas due to the micropores in the material, which are originated from the reaction between the silica spheres and the preceramic polymer during pyrolysis.In addition, a series of ceramic microapparatus and porous ceramic microapparatus were fabricated by sphere assistant template technique and patterned PDMSO elastic template technique. Owing the unique properties of the non-oxide ceramics, the above microapparatus are expected to replace the polymer products and will be widely applied in micro-reactors, micro-fiuidic system and purification apparatus of the tail gas from autos.