Research On The In-situ Preparation Of Porous Ceramic With A Framework Structure Of Magnesium Borate Whiskers

Generally,whisker reinforced metal matrix composites?MMCs?are generally fabricated by stir casting,vacuum pressure infiltration or powder metallurgy in which whiskers are externally added.However,there are some drawbacks of these methods such as high cost of the whiskers and severe health hazards associated with the processing and handling of reinforcements.To solve these problems,research on the in situ synthesis of whisker-reinforced MMCs have been carried out.The technical routes of the synthesis and growth of whiskers and sintering of composites happened at the same time are benefit to full play the advantages of in situ synthesis method,however,ceramic whisker-reinforced MMCs cannot easily be fabricated by such an in situ synthesis method if the melt points of metal and ceramic whisker are quite different.Alternatively,it's possible to prepare these kinds of composites by a two-step process combining in situ whisker synthesis of porous ceramics with a framework structure and subsequent molding process.In this paper,new porous ceramics with a framework structure of magnesium borate whiskers?MBOw?were in situ synthesized by use of basic magnesium carbonate?BMC?and boric acid with the molar ratio of Mg:B of 1:1 an 1:1.5,in which MBOw bonded together and distributed quite random in three dimensions.For the identification of the reaction process,mixed but uncalcined powders of reactants were examined by TG-DTA analysis.Powder X-ray diffraction?XRD?,scanning electron microscope?SEM?and mechanical properties examination were also employed to characterize the crystal structure and micromorphology of MBOw and the phase constitution,mechanical properties and microstructure of synthesized porous MBOw ceramics influenced by different technical conditions.Under the boron rich reactant system comprised of BMC and H₃BO₃ with Mg to B mole ratio of 1:1.5,study on the preparation of porous ceramics with MBOw influenced by the fluxes?KOH?Fe?NO₃?₃?Ni?NO₃?₂?and their mix proportion?1%-10%?and sintering temperature?750-1000°C?,have been made.And the results are shown below:?1?When heat the mixture of Mg?OH?₂ and H₃BO₃ at 900°C for 4h,the products were magnesium borate rods with a diameter and length of 0.3-0.5?m and2-3?m,as well as a large amount of impurities.When BMC and H₃BO₃ were selected as starting materials,uniform distributed MBOw with an average diameter of0.3-0.5?m and length of 3-6?m and the length to diameter of MBOw was 10-15could be easily synthesized.?2?When heat the mixture of BMC and H₃BO₃ with Mg to B mole ratio of 1:1 at900°C for 4 h,the diameter and length of synthesized MBOw was 0.4-0.5?m and 2-5?m,respectively.If the Mg to B mole ratio changed to 1:1.5,MBOw have a diameter and length of 0.2-0.3?m and 3-6?m separately;and the porosity and compressive strength of synthesized porous ceramic was 65%and 4.72 MPa separately.?3?The optimal technical condition for the in-situ synthesis of porous MBOw ceramics was to heat the mixture of BMC and H₃BO₃ with Mg to B mole ratio of 1:1.5 at 900°C for 4 h.?4?According to the reaction process analysis,identification of crystal structure and observation of micro-morphology of MBOw,we suggest a solution-liquid-solid?SLS?mechanism,which could be the appropriate explanation for the nucleation and growth of MBOw.?5?1wt.%KOH was added into the mixtures,the average diameter and length of MBOw sintered at 900°C was 0.5?m and 4-5?m,respectively,the length to diameter was 10,but the surface of whiskers was not smooth,there existed lots of particles.When 5wt.%KOH was added,the average diameter and length of MBOw sintered at 800°C was 0.2-0.3?m and 3-9?m,respectively,the length to diameter was 10-15.When sintering temperature increased to 900°C,the synthesized porous ceramic had a porosity and compressive strength of 45%and 7.6 MPa,comprised of a number of bulk or granular impurities and large size of MBOw with an average diameter and length of 0.8-1?m and 3-10?m,respectively,and the length to diameter ratio was more than 10.?6?1wt.%,5wt.%and 10wt.%amount of Fe?NO₃?₃ were separately added into the mixtures,all the green pellets were sintered at 900°C,with the increase of the addition amount of Fe?NO₃?₃,MBOw grew more sufficiently,and the bulk impurities in the inner of porous ceramic became much less,the compressive strength of synthesized porous ceramics exhibit a tendency of increase at first,and then decreased,among them,the strength of porous ceramics with 5wt.%amount of Fe?NO₃?₃addition was the highest,which indicated that the addition of Fe?NO₃?₃ not only promote the grain growth of MBOw,but also enhance the densification and compressive strength of porous MBOw ceramics.When the amount of Fe?NO₃?₃ was5wt.%,porous MBOw ceramics with a porosity of 60%and compressive strength of22.32 MPa,and MBOw with a diameter of 0.3-0.8?m and length of 3-5?m could be synthesized.However,there existed a few of bulk impurities in the inner of porous ceramic,and the morphology and length to diameter ratio of MBOw wasn't ideal,which indicated that the growth of MBOw is inadequate.?7?When 1wt.%,5wt.%and 10wt.%amount of Ni?NO₃?₂ were separately added into the mixtures,the growth of MBOw was demonstrated to be accelerated,and the length to diameter ratio of MBOw maintained stably at 10 to 20.The results showed that,When 5wt.%Ni?NO₃?₂ was added,MBOw in synthesized porous ceramic were uniform distributed,and there were no other impurities exist,the diameter and length of MBOw is 0.2-0.3?m and 4-8?m,respectively,and the length to diameter ratio of MBOw reached 20,the porosity and compressive strength of synthesized porous ceramic was 56%and 15.72 MPa separately.According to the experiment results,NiO particles decomposed through Ni?NO₃?₂ play the part of not only promote the growth of MBOw,but also enhance the densification and compressive strength of porous ceramics,which proved that Ni?NO₃?₂ was the optimal additive for the preparation of porous MBOw ceramics under this system.According to the previous analysis,the optimal conditions for the preparation of porous MBOw ceramic by in situ solid-phase method with the presence of additives was:heat the mixture of BMC and H₃BO₃ with Mg to B mole ratio of 1:1.5 and an additive of 5wt.%Ni?NO₃?₂ at 900°C for 4 h.