| Mullite whisker is used as an excellent kind of reinforcements in ceramic matrix composite,due to its good properties such as high temperature resistance,good oxidation resistance,small coefficient of thermal expansion,good strength in high temperature,good thermal shock resistance and so on.In this thesis,preparation and dispersion of the mullite whiskers has been researchede in order to obtain the mullite whiskers with high purity,high aspect ratio and well dispersion,which is preliminary research for its industrial production in the future.Mullite whiskers were synthesized through solid reaction method using ?-Al2O3 and SiO2 as raw materials,aluminum fluoride as additives.Phase composition of the prepared material was analyzed by XRD and microstructure was observed by SEM.The results show that mullite whisker powders with high purity and relatively high aspect ratio may be prepared when the raw powder materials with 10wt%fluoride are naturally packing in the flat and fat crucible.The results also show that calcining temperature,holding time and the amount of the aluminum fluoride have effects on the aspect ratio and composition of mullite whiskers.Three factors of calcining temperature,holding time and the amount of the aluminum fluoride are considered in an orthogonality test with four levels adopted in every factor.And these three influential factors were analyzed with the experiment by means of range and variance analysis.It is showed that among the three influential factors,the holding time is the most important one,followed by the amount of the aluminum fluoride and calcining temperature(1400-1480 ?).In these three factors,holding time is the significant influencing factor,the amount of the aluminum fluoride is the relatively significant one,while calcining temperature is insignificant influencing factor in the range of 1400 ? to 1480 ?.The optimal solution is AlF3·3H2O with a lOwt%of total amount of powder,calcining of the powders at 1450 ? and holding for 6 hours.Experiment has been done for verification of the optimal solution.Mullite whisker powders with high purity and the maximum aspect ratio of about 26 have been fabricated.Various studies have been down on the dispersibility of prepared mullite whiskers,which is characterized by measuring final settlement volume.The results show that the dispersibility of the whiskers from different height of the crucible is are different and the dispersibility of the powders situated in the upside part of the crucible is the best while the dispersibility of the powder situated in the bottom part is the worst.The dispersion effect obviously promotes,but after 15 minutes' ultrasonic vibration it will not promote any more.The dispersion effect in the solvent with the pH value of 3.5 or 9 is better than that with the pH value of 6.5.Compared with ammonium polyacrylate,calgon do good to better dispersion effect,and the dispersant concentration has a significant influence on the dispersibility of the mullite whiskers.The corrosion effect of hydrofluoric acid with low concentration is obviously better than that of hydrochloric acid,nitric acid and sulphuric acid,and the aspect ratio of prepared mullite whiskers will be shorten obviously over 24 hours' corrosion by hydrofluoric acid with low concentration.The optimal dispersion parameters are:using the middle and upside layer powder of the crucible,etching by hydrofluoric acid with 3 mol/L for 24 hours,adjust the pH value of the polarity solution to 9,using 0.3wt%calgon as dispersant and conducting ultrasonic dispersion for 15 minutes.,After the dispersion experiment with the optimum dispersion process,the average final settlement volume is 0.7ml which is reduced nearly 4 times than that of the as prepared powders.The result shows the dispersibility of mullite whiskers is significantly improved. |
PDF Full Text Request