Preparation Of A New Type Of MgO Foam Ceramics Materials And Its Application

Foam ceramics (FC), a kind of special materials with the 3D porous structures andexcellent texture properties, have showed superiority in the field of molten metal filtration.The advantage of process of foam ceramics processed by impregnation of an open-cellpolyurethan(ePU)foam by immersion into a ceramic slurry have a promising prospect, dueto its simple process, low cost and prepared easily.Recently, Magnisium have been used widely in the field of transportation, aerospaceand communication for its properties of high specific strength, specific rigidity and specificelastic modulus, good damping resistance properties. However, the magnesium made fromboth electrolytic method and reduction method have a varity of inclusions which havenegative effects on products, such as worse mechanical and anticorrosion properties.Compared with some foam ceramic filter, such as Al₂O₃,ZrO₂,SiC,Si₃N₄, MagnesiumOxide(MgO) foam ceramic filter can absorb flux and clean up fine inclusions efficiently inmolten magnesium spontaneously as well as can not react with molten magnesium itself,so it is a better candidate to purify molten magnesium. But until now there are not reporton MgO foam ceramic filter to filter molten crude magnesium in detail. So our work is toexplore ways of making MgO foam ceramics and of purifying technologies of moltenmagnesium so as to improve the fineness of magnesium. We carry out a series ofexperiments for the purpose above, and draw some conclusions as following:Magnesia hydration is a key concern in refractory castable processing, the reasonabove can result in cracks or even explosion during the first heating-up,so we use differentadditives to inhibit MgO hydration. The hydration degree of MgO powder modified byKH-550 decrease from 54.36% to 17% compared with unmodified MgO powder whenMgO powders react with water in 25℃for 180min, that is to say, coupling agent KH-550have partial effect on inhibiting MgO hydration.We have studied the hydration kinetic equations of MgO powders treated byammonium phosphate(AP) and KH-550, respectively. According to the ArrheniusEquation,we obtained the datum of MgO hydration activation energy was Ea(AP)= 78.95kJ/mol and Ea(KH-550)=81.17kJ/mol,respectively. Hydrated rates of both of them arecontrolled by chemical reaction instead of diffusion. The results showed hydration reactionof MgO powder treated by KH-550 is more difficult to hydrate than that of MgO powdertreated by AP.When the dosage of SN5040, a kind of polyacrylic salt dispersants, is between 0.1%and 0.2%, the thick degree and relative viscosity of MgO slurry are 1.024-1.034 and1.058-1.083, respectively, the values of them are optimum simultaneously. Theexperimental results showed SN5040 is the optimum dispersant for dispersing MgOpowders in water than PAM and CMC-Na.Our experiment results prove that the design ofbicomponent composition is beneficial to improve the sample's compressive strength andrelative density, and the values of compressive strength and relative density of sampleincrease from 3.35MPa and 0.42 to 5.12MPa and 0.55, respectively. Nevertheless, thevalues of compressive strength and relative density of sample come down in the conditionof tricomponent composition.The volume of slurry coating of MgO on PU foams is largerand more uniform than that of those untreated when PU foams immersed in phenolicresin-ethanol solution. The slurry weight on PU foam after modified is 9 times than theweight of PU foam while the slurry weight on PU foam unmodified is only 5 times thanthe weight of PU foam; The powder is adhere to the surface of foam more firmly and thetemperature resistant of PU foam is improved and the shrinkage collapse probability ofgreen foam ceramics decrease.The sintering temperature of MgO foam ceramics candecrease from 1500℃to 1200℃or so on the condition of adding a capful of sinteringadditive MgF₂to MgO powders, and the sintered fused MgO ceramics on the condition oflower temperature have a stable 3D structure and the little defect in them.Compared with different filtration methods of molten magnesium, we can draw aconclusion that MgO foam ceramics can clean MgO inclusions and flux effectively. Theresearch result indicates that filtrating ability are raised, inclusions and flux are less andgrains are more uniform and perfect by decreasing hole dimension of MgO foamceramics.Based on the characteristic of filtration system of molten magnesium, a moltenmagnesium purifying efficiency equation relative to foam ceramics filtration wasestablished.The experiment result indicates that the purifying efficiency of the foamceramics could be reduced with the increase of the inclusions on the network surface offoam ceramics because of smaller 3D sizes of holes and slower flow velocity of molten magnesium in foam ceramics.We analyzed some influencing factors on equation toinclusion thickness on the network surface of foam ceramics, for it is helpful to guaranteethe cast process goes well, so this will be helpful for extending the industrial application ofMgO foam ceramics to filter molten magnesium.