Pore wall silicon carbide coating of porous ceramics and preparation method thereof
Technical field
The present invention relates to pore wall silicon carbide coating of a kind of porous ceramics and preparation method thereof, belong to the surface modification field of porous ceramics.
Background technology
Porous ceramics is often referred to the block ceramic that the aperture obtains more than 50nm and through oversintering.Advantage such as that porous ceramics has is high temperature resistant, resistance to chemical attack, density are low is widely used in fields such as heat insulation, catalysis, filtrations.Especially as carriers such as melted metal filtering, high-temperature tail gas purification and high-temperature catalytics the time, porous ceramics shows the advantage that other metal polyporous material hardly matches.Along with development of modern industry, the application of porous ceramics is also constantly being widened, and its performance demands is also constantly promoted.In the reality, use porous ceramics more widely and mostly be by oxide compound as principal phase or in conjunction with the composite ceramics of phase, as the carborundum porous ceramics of alumina porous ceramic, mullite porous ceramic, oxide-bonded and the porous SiN ceramic of oxide-bonded etc.Hole wall surface and particle junction at these porous ceramicss all are oxide compounds, and its mechanical property, high-temperature behavior and corrosion resistance nature are mainly determined mutually by these oxide-bonded.Because preparation technology's limitation, there are a lot of tiny crack defectives in the hole wall surface of the porous ceramics of oxide-bonded, thereby has weakened the mechanical property of porous ceramics; Simultaneously, because these oxide-bonded are isolator mutually, thereby they do not have electroconductibility usually.
SiC is the very strong compound of covalence key, has the fusing point height, and hardness height, hot strength height, good characteristic such as Young's modulus is big, wearability good, thermal expansivity is little, thermostability is good are applicable under the extreme environments such as high temperature, high pressure, high corrosion.Simultaneously, silicon carbide has semiconduction again, is a kind of conducting ceramic material.Therefore, silicon carbide can strengthen the high temperature of body material, anti-oxidant, corrosion-resistant and conductivity when being used as coated material.The preparation method of coat of silicon carbide mainly contains chemical Vapor deposition process, Si-C in-situ reaction, recrystallization method etc.CN200510031775 is precursor with the trichloromethyl silane, adopts chemical gaseous phase depositing process to prepare dense silicon carbide coating layer for optical reflection mirrow.CN200510022726 at first makes the rice husk carbonization, is mixed and made into the embedding powder with halide catalyst then, and carbon-carbon composite is buried powder sintering in 1400~1450 ℃ under argon gas atmosphere, obtains coat of silicon carbide at composite material surface.CN97116499 is made into slurry with additives such as silica flour, carborundum powders, and graphite matrix is carried out coating, and sintering utilizes the reaction in of silica flour and graphite and the recrystallize of carborundum powder to form flawless anti-oxidant carbonization silicon coating at graphite surface then.
Because the good characteristic that has of carbofrax material itself, the hole wall of porous ceramic bodies is carried out the crackle that coat of silicon carbide will effectively heal and occur on the hole wall, improve the intensity of material; Silicon carbide can bear the high temperature more than 2000 ℃, and the use temperature of carrying out the porous ceramics behind the coat of silicon carbide also can improve greatly; Silicon carbide has certain electroconductibility simultaneously, thereby can realize the conduction of porous ceramics, widens its Application Areas.Yet the document that porous ceramic bodies is carried out the coat of silicon carbide processing rarely has report, chief reason is, the powder that carries out adopting usually when coating is handled is difficult in the porous ceramics duct in micron order aperture and coats uniformly, simultaneously when slurry floods, because the pressure effect of gas in the duct, slurry is difficult to go deep into porous ceramic bodies inside, also just can not form uniform coat of silicon carbide.
Polycarbosilane is a kind of good presoma of preparation silicon carbide, and the productive rate that generates silicon carbide under inert atmosphere during Pintsch process reaches more than 50%.Polycarbosilane easily is dissolved in organic solvents such as gasoline, dimethylbenzene and ether, but therefore wiring solution-forming floods porous ceramics, makes the cracking at high temperature of dipping body then, thereby generates uniform coat of silicon carbide at the hole wall surface of porous ceramics.
Can the inventor imagines thus, use Polycarbosilane as the silicon carbide precursor body, utilizes appropriate process to produce the pore wall silicon carbide coating of porous ceramics, with the purpose of the resistivity of improving porous ceramics and physical strength, reduction hole.
Summary of the invention
Present situation based on prior art, the pore wall silicon carbide coating and its preparation method that the purpose of this invention is to provide a kind of porous ceramics, dipping by Polycarbosilane solution, on the porous ceramics hole wall, form the silicon carbide precursor body coating that contains Polycarbosilane uniformly, utilize the cracking under the Polycarbosilane high temperature to form coat of silicon carbide then, thereby improve mechanical property, the high-temperature behavior of porous ceramic bodies, make porous ceramics have certain electric conductivity again simultaneously.
In order to reach purpose of the present invention, there are two key issues to solve, the one, guarantee that Polycarbosilane solution is full of the duct of porous ceramics fully, thereby on all hole walls, form uniform silicon carbide precursor body coating, the 2nd, select suitable sintering temperature, make the complete cracking of Polycarbosilane, simultaneously avoid again peeling off, defective such as tiny crack.
The object of the invention implements to be to utilize vacuum-high-pressure impregnation technology, overcome the defective that liquid under the normal pressure is difficult to enter fully hole, make Polycarbosilane solution completely filled in the duct of the porous ceramics that has the interconnected pores structure, drying is after pyroprocessing generates β-SiC coating at the hole wall of porous ceramics.At first, porous ceramics is placed in the container of high vacuum, inject Polycarbosilane solution then, because the vacuum state in the duct, Polycarbosilane solution is easy to rely on capillary force and liquid pressure to enter the duct, and then feed high pressure gas to container, thereby the pressure when increasing solution and entering the porous ceramics duct makes Polycarbosilane solution be full of the duct of porous ceramics fully.The concentration of the Polycarbosilane solution that uses in when dipping should be 10~80wt%, and too small being difficult to of concentration finally forms even successive coat of silicon carbide at the porous ceramics hole wall, is 50~70% because generate the productive rate of silicon carbide during the Polycarbosilane cracking; When concentration was excessive, the viscosity of solution increased, and brings difficulty to dipping, and the impregnation pressure that needs increases.The porous ceramics low for porosity, that the aperture is little should adopt corresponding high vacuum tightness and high pressure, the Polycarbosilane solution of use lower concentration to flood, to guarantee the efficient of dipping.In addition, must control suitable temperature in dipping body sintering process, temperature is low excessively, is difficult to generate silicon carbide, and can forms the Si-C-O vitreum, weakens the performance of coating.
The matrix that the present invention preferentially selects for use is the porous ceramics with interconnection open-celled structure, the aperture should be more than 0.1 micron, open porosity is greater than 30%, and body material is insoluble to organic solvents such as gasoline, dimethylbenzene, ether, and the porous ceramic matrices suitable that the present invention is suitable for mainly contains the silicon carbide of alumina porous ceramic, mullite porous ceramic, porous cordierite ceramics, oxide-bonded or porous SiN ceramic etc.
The present invention implements by following technological process:
(1) raw material is selected for use: with the persursor material of Polycarbosilane as the preparation coat of silicon carbide, gasoline, dimethylbenzene or ether etc. are as the solvent of Polycarbosilane, and porous ceramics is as body material.
(2) concrete preparation process: prepare proper concn (behind 10~80wt%) the Polycarbosilane solution; the porous ceramics of desire dipping is put into suitable containers; then container is placed in the impregnating autoclave of the pressure that can vacuumize and increase; be evacuated to vacuum tightness≤100Pa; inject Polycarbosilane solution by external conduit to the container that holds porous ceramics then; make the complete submergence porous ceramic bodies of solution; slowly in impregnating autoclave, feed high pressure gas (0.2MPa~10MPa) then; thereby make pressure pan inside be in high pressure conditions; and kept 5-20 minute; Polycarbosilane solution enters the duct of porous ceramics fully under the high pressure effect; slow release pressure recovery normal pressure then; after the taking-up; in air under 50-120 ℃ of condition 5-12 hour slowly dry; obtain the dipping body of porous ceramics; dried dipping body under protective atmospheres such as argon gas or nitrogen with less than the temperature rise rate of 10 ℃/min in 900~1500 ℃ of pyroprocessing; make the Polycarbosilane cracking and combine with porous ceramics, the pyroprocessing time is 1-12 hour.Obtain having the porous ceramics of pore wall silicon carbide coating.Repeat the thickness that above-mentioned dipping-cracking high-temperature processing technology can control coating, with performances such as the intensity that changes dipping back porous ceramics, porosity, resistivity.
(3) material characterizes: Fig. 1 is the XRD figure spectrum of split product under the Polycarbosilane high temperature, and the primary product that obtains is β-SiC, and the residual of small amount of carbon (graphite) arranged simultaneously.Fig. 2 is the section microstructure appearance of porous ceramics before and after the Dipping, can see, through dipping, the structure that porous ceramics is loose obtains very big change, and the combination between the particle strengthens.Fig. 3 is the influence of dipping-cracking number of times to the bending strength and the microstructure of porous ceramics, and along with increasing of dipping number of times, the bending strength of porous ceramics improves a lot, but open porosity has certain reduction.The resistivity of the porous ceramics of oxide-bonded is 10
9Ω .m is isolator, and resistivity has substantial degradation after carrying out coat of silicon carbide, and at 0.1~100 Ω .m, Fig. 4 shows that along with the increase of dipping number of times, the resistivity of porous ceramics significantly reduces, and the resistivity after four dipping-cracking only is 0.7 Ω .m.
In sum, the pore wall silicon carbide coating that utilizes the present invention to prepare porous ceramics has the following advantages:
(1) pore wall silicon carbide coating of porous ceramics provided by the invention has the microstructure of even compact, thereby has guaranteed high rigidity, the high strength of coating, good characteristic such as wearability is good, thermal expansivity is little, thermostability is good.
(2) pore wall silicon carbide coating of porous ceramics provided by the invention can change the electrical insulating property of the porous ceramics of oxide porous ceramic or oxide-bonded, reduces their resistivity greatly, makes it possess the conductive characteristic of semiconductor material.
(3) preparation method of the pore wall silicon carbide coating of porous ceramics provided by the invention is simply effective, can the hole wall in all ducts of porous ceramics inside and outside be applied, and especially can carry out coat of silicon carbide to the porous ceramics in micron order aperture.
Description of drawings
The XRD figure spectrum of Fig. 1 Polycarbosilane high-temperature split product, X-coordinate is 2 times of diffraction angle, and unit is degree, and ordinate zou is the relative value of diffracted intensity.
(Fig. 2 is the section microstructure appearance of (Fig. 2 b) mullite bonded carborundum porous ceramics a) and after the coating before Fig. 2 coating.
Fig. 3 floods the influence of number of times to the bending strength and the open porosity of mullite bonded carborundum porous ceramics.
Fig. 4 floods the influence of number of times to mullite bonded carborundum porous ceramics resistivity.
Embodiment is closed
Further specify substantive distinguishing features of the present invention and obvious improvement below by specific embodiment, but the present invention absolutely not only is confined to embodiment.
Embodiment 1
As coated substrate, the combination of porous ceramics is mullite mutually with the carborundum porous ceramics of mullite bonded, and resistivity is 10
9Ω .m is an isolator, and bending strength is 17.8MPa, open porosity 47.7%, volume density 1.57g/cm
3, mean pore size is 5 μ m.With gasoline is that solvent preparation mass percentage concentration is the Polycarbosilane solution of 30wt%, porous ceramic bodies is put into container, place then in vacuum-high-pressure impregnation jar, screw impregnating autoclave, and be evacuated to 5Pa, in the funnel on impregnating autoclave top, pour Polycarbosilane solution then into, the knob of outwarding winding flows in the beaker solution, and porous ceramic bodies is dipped in the Polycarbosilane solution fully, closes knob, kept vacuum state 5 minutes, stop to vacuumize, slowly in impregnating autoclave, feed high pressure nitrogen, make jar interior pressure keep 5MPa, kept then 10 minutes, stop nitrogen and feed, the degassing button of outwarding winding makes jar interior gas slowly be discharged in the atmosphere, recover normal pressure, take out the porous insert after flooding, kept 50 ℃ of dryings 12 hours in baking oven, dried porous ceramics places the interior speed with 1 ℃/min of high temperature argon stove to rise to 2 hours pyroprocessing of 900-1500 ℃ of insulation, naturally cooling obtains the porous ceramics that hole wall has coat of silicon carbide.As shown in Figure 3 and Figure 4, the resistivity of porous ceramics is 8 Ω .m behind the primary coating, presents semi-conductive conductive characteristic, and bending strength increases to 23.2MPa, and open porosity is decreased to 42.2, and it is 1.69g/cm that volume density increases
3
Embodiment 2
The porous ceramics that obtains among the embodiment 1 is taked identical dipping and sintering process, carries out the dipping second time, pyroprocessing, obtains having the mullite bonded carborundum porous ceramics that double-steeping forms dipping-cracked coat of silicon carbide.The resistivity of porous ceramics is 6.93 Ω .m behind the secondary coating, bending strength 38.8MPa, and open porosity 38.3%, volume density are 1.79g/cm
3
As can be seen, repeatedly dipping-cracking can improve the bending strength and the volume density of porous ceramics after the coating, reduces resistivity and open porosity, the mechanical property of strongthener and electrical property.
Embodiment 3
As coated substrate, is that solvent preparation mass percentage concentration be the Polycarbosilane solution of 70wt% with gasoline with aluminum oxide foam pottery, and the aperture of foamed alumina is 5mm, the preparation process of coat of silicon carbide such as embodiment 1.Behind dipping, drying, cracking sintering, obtain the aluminum oxide foam pottery that hole wall has coat of silicon carbide.
Embodiment 4
As coated substrate, is the Polycarbosilane solution of 50wt% with gasoline for the solvent compound concentration with cordierite honeycomb ceramic, is 0.1mm through the aperture of the cordierite honeycomb ceramic of extrusion moulding preparation, the preparation process of coat of silicon carbide such as embodiment 1.Behind dipping, dry, cracking sintering, just the pyroprocessing temperature is low slightly is generally 900-1200 ℃ and makes cracked SiC and matrix bond firmly obtain the cordierite ceramic that hole wall has coat of silicon carbide.