Preparation And Characterization Of Porous Si₃N₄ Bonded SiC Ceramic Supports

In this article,a series of porous Si₃N₄ bonded SiC ceramic supports have been prepared via reactive sintering.Under nitride atmosphere,the main raw materials including commercially available silicon carbon powder and silicon powder was reacting with appropriate sintering additives and pore-forming agent.Then,the crystal phase,micromorphology,porosity,flexural strength,pore size distribution and acid and alkali resistance have been characterized.The influence of sintering temperature,holding time,heating rate,addition of sintering additives and different content of pore-forming agent on the physicochemical properties of porous ceramic supports were respectively studied,on this basis,the optimal sintering schedule and experiment formula were determined.The growth mechanism of Si₃N₄ and the influence of morphology of silicon nitride on the properties of porous ceramic supports have also been researched.First of all,the effects of sintering temperature,holding time and heating rate on the properties of porous Si₃N₄ bonded SiC ceramic supports were studied,respectively.And as the results shown,the main crystal phases of sintered samples were silicon carbon and silicon nitride.And with the temperature rose from 1400? to 1500?,the needle-like?-Si₃N₄ transformed into rod-like ?-Si₃N₄ which resulted in the decrease of flexural strength from 15.9MPa to 11.8MPa,on the contrary,the porosity gradually rose to 43.8% and the water flux rose to 8.3m3/?m2·h?.With the holding time prolonging from 1h to 4h,the flexural strength rose to 15.7MPa,the porosity decrease to 40.9%simultaneously and the water flux dropped from 6.5m3/?m2·h?to 4.6m3/?m2·h?.With the heating rate increased from 1?/min to 4?/min,the flexural strength declined from 15.7MPato 13.5MPawith the increase of porosity from 40.1% to 43.6%,and water flux decreased to 5.3m3/?m2·h?after rose from 4.6m3/?m2·h?to 5.8m3/?m2·h?.Furthermore,a L16?43?orthogonal experiment has been taken to determine the primary and secondary relationship of the above three influential factors and the optimal sintering processes.The results showed that the sintering temperature influenced the porosity,the holding time influenced the pure water flux and flexural strength most obviously,respectively.And under the condition of 1h of holding time,when the sintering temperatureis 1520? andheating rate is 4?/min,the samples had the biggest porosity,when the sintering temperatureis 1400?heating rate is 2?/min,the samples obtained highest water flux.Meanwhile,under the condition of 1440? of sintering temperature,3h of holding time and 3?/min of heating rate,the samples had the best flexural strength.Hence,the best scheme of preparing excellent porous ceramic membrane supports are required to have high porosity,pure water flux and flexural strength,taking this factors into consideration,the optimal sintering parameter is 1440? of sintering temperature,3h of holding time and 2?/min of heating rate.On the base of above experimental results,the addition of sintering additives were considered to enhance the flexural strength,and the effect of single additive and compound additive on the properties of the ceramic supports were studied.As the results shown,the main phases of the samples were silicon carbon and silicon nitride,there still existed a little impurity phases.And the rod-like silicon nitride mainly came from the single addition of CeO₂ and Y₂O₃,while the needle-like silicon nitride generated from the single addition of Al₂O₃ and complex addition of Al₂O₃ and Y₂O₃.Specifically,when the content of Al₂O₃ was 2wt%,the samples had highest porosity 44.3% and the water flux maximum value 7.9m3/?m2·h?,when the additives was 3wt%Al₂O₃+4wt%Y₂O₃,the samples had highest flexural strength 22.3MPa.To serve as a supports,the flexural strength of 22.3MPa is high enough.Based on this,starch,as the pore-forming agent,adding to the system for further enhancing the porosity has been studied.The results showed that along with the increase of the content of starch the porosity rose to 45.6% while the flexural strength decreased to 12.3MPa,and the water flux declined after rose.When the starch content was 3wt%,the water flux was up to 9.2m3/?m2·h?with 41.8% of porosity,17.3MPa of flexural strength,1.98% of acid resistant and 4.13% of alkali resistant.The other important influential factor,the growth mechanism of silicon nitride and the influence of morphology change of Si₃N₄ on the strength,pore size and pure water flux has been analyzed.At the beginning of the growth of silicon nitride whisker,the prime whisker was ?-Si₃N₄ and the growth mechanism was VS mechanism.However,with the increase of sintering temperature,the solid-state sintering aids gradually transformed into liquid-state,and the ?-Si₃N₄ dissolved into the liquid and turned into ?-Si₃N₄,which was in agreement with VLS growth mechanism.Compared to the rod-like silicon nitride whiskers,the needle-like silicon nitride whiskers were conducive to enhance the mechanical strength.Meanwhile,the mean pore size would decreased,as a result,the water flux declined after rose.Sintering temperature increasing and sintering additives adding would promote the growth of needle-like silicon nitride,and with the holding time prolonging and the heating rate slow down,the content of needle-like silicon nitride whisker gradually increased.Compared to rod-like Si₃N₄,the needle-like Si₃N₄ intertwined between silicon carbon particles as a bridge would be more conducive to improve the strength of the samples,but the whisker would divided the big pore into many small holes,which reduced the average pore size of the sample,and a large number of nanoscale whisker gathered with certain hydrophobic property would result in a loss of the pure water flux.