Sintering behavior of alumina rich cordierite porous ceramics

Materials Chemistry and Physics67(2001)146–150Sintering behavior of alumina rich cordierite porous ceramicsT.D.Senguttuvan∗,H.S.Kalsi,S.K.Sharda,B.K.DasSpecial Ceramics Group,Electronic Materials Division,National Physical Laboratory,New Delhi110012,IndiaAbstractPorous ceramics are increasingly becoming important in particulate gasfiltration.They are an effective alternative to the honeycombed structure in the case of diesel engines.Porous ceramics are made by infiltrating ceramic slurry into a polymeric sponge followed by drying and heating to burn out the polymeric sponge and obtain a ceramic sponge with large open porosity.In this paper we report optimized drying and sintering conditions for alumina cordierite porous ceramics.The microwave drying time ranging from5–30min depending on the power and the amount of loading.The shapes of the object was maintained in most of the structures.The sintering temperature was optimized around1380◦C.These results were analyzed using XRD data and SEM micrographs.©2001Published by Elsevier Science B.V. Keywords:Porous ceramics;Alumina;Cordierite;Sintering;Microwave1.IntroductionPorosity is the main cause for reduction in mechanical properties of ceramic and brittle solids.These class of ceram-ics are essential for many industries where high permeability, high surface area,and insulating characteristics are required. The search for porous ceramics with good mechanical strength has stimulated the development of several technolo-gies.There are many applications of porous ceramics:filters for diesel emission,filters for molten metals,devices where chemical durability and permeability are required at high temperature.[1,2].The properties of porous ceramics greatly depends on pore morphology,size and distribution.This in turn depends on the processing steps involved.Porous ceramics are generally prepared by the polymer-sponge method,foaming method or by the gel casting method.[3,4] With increased environmental awareness particulate emission standards have been tightened to achieve particu-late emission as low as0.13mg/Wh in diesel engines.Some attempts have been made to use the honeycombed structures to reduce the particulate emission in diesel engines.Such honeycombedfilters have cell density of15–30cells cm−2, wall thickness of0.30mm and porosity of50–80%.Ceramic materials tried for suchfilters were cordierite and mullite. Filtration efficiency varies from60to80%for suchfilters. Alternatives to the honeycombed structure are ceramic foam and ceramic spongefilters.Ceramic foams have been made by adding a foamable organic substance to the green ce-ramic body which foams up during sintering.However,ce-ramic foam has high closed porosity and a not so high open ∗Corresponding author.porosity required for particulatefiltration.Therefore,foam ceramicfilters do not have highfiltration efficiency although they have high porosity.On the other hand,ceramic sponge has been made by infiltrating ceramic slurry into a polymeric sponge followed by drying and heating to burn out the poly-meric sponge and obtain a ceramic sponge with large open porosity.Due to the large open porosity present and the sim-ple method of fabrication,these reticulate sponge ceramics have become an attractive alternative to honeycombed ce-ramics as a particulatefilter and catalytic converter.Besides automotive applications,since such porous ceramicfilters have a highfiltration efficiency of>95%for particles as small as0.1␮m,they have been also found to be specially suitable(when made using SiC)forfiltering hot industrial gases coming out of incinerators andfluidized bed combusti-on chamber.One of the main problems facing these particula-tefilters is the clogging of thefilter due to soot deposition and concomitant reduction in thefilter efficiency and increase in back pressure and to withstand thermal shock. In this paper,we examine the process property rela-tionship of alumina rich cordierite ceramics prepared by polymer-sponge method.2.Experimental2.1.Description of slurry infiltration processA typical slurry infiltration method is shown in the sponge ceramic fabrication process sheet(Fig.1)which is used to produce sponge ceramics.It can produce ceramics with both close and open cell porosity.We have mainly used this0254-0584/01/$–see front matter©2001Published by Elsevier Science B.V. PII:S0254-0584(00)00432-6T.D.Senguttuvan et al./Materials Chemistry and Physics67(2001)146–150147Fig.1.Sponge ceramic fabrication process. polymeric sponge infiltration method.It involves unit pro-cesses such as preparation of the slurry,selection of sponge, immersion of the sponge,drying and sintering.2.2.Preparation of ceramic slurryThe basic slurry is made up offinely divided ceramic particles,water and additives.The particle chosen depends on the particular application and the desired properties of thefinal product.Ceramic materials used to make reticulate structures for varieties of applications are zirconia,mullite, talc,silica,alumina,magnesia,cordierite and calcined clays. Typically,the ceramic particles are<45␮m in size.Water is most often used as the carrier in the slurry.The amount of water can range from10to50%of total weight of the slurry.Apart from powder and water,other additives such as binders,rheological agent,anti foaming agents,flocculating agents and anti-setting agents are added.They are ball milled or attrition milled depending on the need and properties of thefinal slurry.2.3.Selection of spongePolymeric sponge is selected on the basis of porosity,pore size,resiliency and volatility.Pore size of the foam decides the pore size of thefinally formed ceramics.Resiliency is required to achieve thefinal shape of the ceramics product. V olatility is required to burn off the base material.Typically, the pore size requirements are2–25pores/linear cm.The best suitable organic material amongst the usually used ones such as polyurethane,cellulose,polyvinyl chloride,polystyrene and latex is polyurethane.This is due to its high softening temperature.2.4.Immersion of the spongeOnce the polymeric sponge has been selected and the ce-ramic slurry prepared,the next step is to impregnate the sponge with the slurry.Typically,the sponge is compressed to remove air and then it is immersed in the slurry and al-lowed to expand.This process is repeated to achieve desired ceramic loading.2.5.DryingAfter the desired loading,the infiltrated sponge is dried. Drying can be done in air,oven or by microwave heating[5]. Air drying typically takes between8–24h.During this time there is a possibility of deshaping of the green body.Oven drying takes around15min to6h and here also the possi-bility of deshaping is difficult to avoid.Whereas microwave drying takes around5–30min depending on the power and the amount of loading,the shapes of the object is maintained in most of the structures.2.6.SinteringSintering is carried out in two steps.Thefirst step is to burn out the organics from the slurry slowly and it is carried out by heating them in air in temperature ranging from350to 800◦C.The second step is densification of ceramics network by sintering.In the sintering stage,controlled heating is required to prevent the collapse of sponge ceramic frame. This is achieved by having different temperature schedules. Sintering in general is carried out in the temperature range 1000–1700◦C depending on the ceramic material.3.Results and discussion3.1.Development of new composition of cordierite ceramic In order to achieve the much needed thermal shock resis-tance,a high alumina cordierite composition was evolved. Starting from high silica composition58mol%SiO2, 23mol%MgO and18mol%Al2O3,high alumina47mol% SiO2,23mol%MgO and30mol%Al2O3cordierite com-position was developed.The new ceramic composition of cordierite was developed using ball clay,alumina and talc as starting raw materials.The IR studies on these samples processed at various temperatures shows IR peaks corre-sponding to SiO2(1110,820,500cm−1),MgO(500cm−1) and Al2O3(870,680,600,500cm−1).We observe,the peaks become well defined beyond1300◦C.Fig.2shows the IR spectra of samples processed at various temperatures viz.1300◦C(A2),1200◦C(A3),800◦C(A4)and air dried (A5).The new composition required sintering to be done at1320–1380◦C for duration up to6h.The X-ray studies (Fig.3)revealed that all peaks corresponding to cordierite148T.D.Senguttuvan et al./Materials Chemistry and Physics 67(2001)146–150Fig. 2.The IR spectra of samples processed at various temperatures namely,1300◦C (A2),1200◦C (A3),800◦C (A4)and air dried(A5).Fig.3.The XRD pattern of new cordierite composition showing various (h k l )peaks due to the cordierite phase.were observed in these compositions but the peaks were found shifted to higher ‘d ’values.This may be due to the formation of alumina rich cordierite phase.Table 1gives (h k l )values of the observed peaks in order of intensity ratios (I :I 0)corresponding to various compositions.To use these materials for fabrication of filter candles,it is essential to study the true density of the sintered ceramicTable 1The (h k l )values of XRD peaks observed for various cordierite ceramic compositions investigated Composition in (mol%)(h k l )values of the peaks observed in XRD in decreasing order of I /I 0SiO 2MgO Al 2O 3562718112202222311582318110311222202463518110311112202472330222112202311Table 2Density and sintering temperature for new cordierite composition Sintering temperature (◦C)True density (g cm −3)1320 2.451340 2.491360 2.4913802.50as it determines the strength of the porous body.The true density was measured by crushing the sintered porous body and measuring the powder density using a specific gravity bottle.Our objective here was to obtain the best possible true density.The true density versus the sintering tempera-ture is given in Table 2.From the table,it can be concluded that a true density of 2.5g cm −3was achieved when the sin-tering temperature was in the range 1340–1380◦C.Beyond 1380◦C,the ceramic body started melting.These studies were also substantiated by the XRD studies which showed the presence of alumina rich cordierite phase at all sintering temperatures.3.2.Optimization of slurry infiltration process3.2.1.Water content of slurryAfter standardizing the composition,our next objective was optimization of the slurry infiltration process.The pro-perties under observations were viscosity of the slurry and penetrability.Low viscosity slurries could easily penetrate the foam stencil but are difficult to be retained in the pores of the foam stencil.These low viscosity slurries gave rise to the deformity in the candle filter.While high viscosity slurries could give a better shape,it was difficult to infiltrate the foam stencil with them.The water to material content in the slurries determined the viscosity of the slurries.Water content in the slurry was decided based on the ease of in-filtration and observed distortion after drying.It was found that,without any rheological agents,the infiltration was dif-ficult for all slurry compositions whose water to solid con-tent was more than 1:1.For water to solid content beyond 1.2:1,distortions in the samples were observed.Observa-tions on the various solid contents are tabulated in Table 3.Table 3Effect of water content of the slurry on distortion Ratio of water to solid content in slurry Remarks0.6:1Slurry too thick for infiltration 0.7:1Slurry too thick for infiltration0.8:1Slurry thick,but infiltration occurred0.9:1Easy infiltration,no distortion after drying 1.0:1Easy infiltration,no distortion after drying 1.1:1Easy infiltration,no distortion after drying 1.2:1Infiltration occurred,but distortion present 1.3:1Infiltration occurred,but distortion presentT.D.Senguttuvan et al./Materials Chemistry and Physics 67(2001)146–150149Table 4Effect of loading of sponge during infiltration a Loading of sponge (g cm −3)Green porosity (%)Remarks 1.7565Oval distortion 1.5070No ovality 1.3575No ovality 1.0080No ovalityaWater to solid ratio in the slurry =1:1.True density of cordierite =2.5g cm −3.When 0.1%of tetra sodium di-phosphate was added to the slurry,we were able to achieve distortion free samples for water to solid ratio in the range 0.8:1.3.2.2.Weight of slurry infiltratedFabricating the components,using the slurry infiltration process,it was found that there was considerable distortion of the component during drying of the infiltrated sponge.This made it difficult to assemble the filter assembly from sintered sponge body.The next parameter examined was the amount of ceramic slurry loading in the sponge.Effect of ce-ramic loading of the slurry,method of pressing of the sponge during and after infiltration and rate of temperature rise dur-ing drying were the various parameters studied.The weight of slurry infiltrated per unit volume of the sponge given (loading of the sponge)in units of g cm −3was an important experimental variable.This was affected by the method of pressing the sponge inside the slurry during infiltration and after infiltration and number of repeated infiltrations carried out.The weight of slurry infiltrated also determined the dis-tortion after drying besides a host of other properties like porosity,permeability and fracture strength.It was found that the ovality of the dried product depended critically on the loading as shown in Table 4.Ovality also depended on the rate of heating of the porous body.It has been found that the ovality of the processed sponge was inevitable in drying conditions in the oven.They were avoided by going in for slow drying to the tune of 3–4h.However,microwave dry-ing has given us the best results.We have used microwave drying using a domestic microwave oven operated at 800W power for a time of 10min to get the best results.Select-ing a sponge with higher strength can also reduce ovality.They were found to have greater pore size leading to low strength in the finally sintered body.It was found that load-ing of the sponge was independent of the size and shape ofTable 5Effect of sintering temperature on density,porosity and gas permeability a Sintering temperature (◦C)Sintered density (g cm −3)Porosity (%)Permeability (m 2Mpa −1s −1)13200.70471.8 6.9313400.69472.8 6.9713600.67572.97.1113800.69772.126.73aSponge loading =1.5g cm −3.Table 6Effect of loading on crushing strengthLoading ofsponge (g cm −3)Sintered density (g cm −3)Porosity (%)Crushingstrength (MPa)1.000.42982.750.641.350.58776.50 2.61.500.68372.98 3.811.750.79068.455.09Table 7Effect of sintering temperature on crushing strength a Sinteringtemperature (◦C)Sintered density (g cm −3)Porosity (%)Crushingstrength (MPa)13200.79568.2 4.9413400.79568.2 5.0013600.79068.4 4.9213800.79068.44.95aSponge loading =1.7g cm −3.the sponge.For a given type of the sponge,we have tried to load 25mm thick polyurethane foam as well as 90mm polyurethane foam and we found that the results were simi-lar to what has been observed in 25mm thick foam.The dif-ference between them was in the duration of heating in the microwave drier.It requires around 30min to dry the 90mm size completely.Rate of heating in the microwave was not varied since the power into the microwave oven could not be varied.3.2.3.Effect of sintering temperatureIt was found that sintering temperature did not have sig-nificant effect on the porosity,density,permeability or frac-ture strength of the samples.This is illustrated for a sponge loading of 1.5g cm −3for density and permeability in Ta-ble 5.The conclusion was equally true for all other sponge loadings.All measurements were averaged over two sam-ples for each set of experimental variables.3.2.4.Crushing strength studiesTable 6gives the variation of crushing strength with load-ing of the sponge.Crushing strength of 3MPa could be ob-tained in loading of more than 1.50g cm −3.The crushing strength was found to be independent of sintering tempera-ture within experimental error in the range of 1320–1380◦C as shown in Table 7.150T.D.Senguttuvan et al./Materials Chemistry and Physics 67(2001)146–150Table 8Effect of loading on density,porosity and gas permeability Loading ofsponge (g cm −3)Sintered density (g cm −3)Porosity (%)Permeability (m 2MPa −1s −1)1.000.42982.7510.171.350.58776.508.691.500.68372.987.211.750.79068.456.33Fig.4.The SEM micrograph of porous cordierite fabricated.Please note the interconnected pores size 20–40␮m.3.2.5.Optimization based on gas permeabilityEffect of loading the sponge during infiltration on the sin-tered density,porosity and gas permeability were studied.The loading greatly affected these properties as shown in Table 8.The results are average over eight samples.All samples satisfied the requirement of gas permeability of 5m 2MPa −1s −1.Porosity of 70–80%could be achieved in the range of loading studied.The analysis of our sample reveled a linear dependance of porosity with permeability.The pore size and the number of pores in the porous boobies were analyzed by SEM micrographs.They are shown in Fig.4.4.ConclusionsIn conclusion,it can be said that one of the material pro-cess parameters for fabricating porous sponge cordierite ce-ramic has been optimized to get the desired combination of porosity,gas permeability and crushing strength.A good quality ceramic sponge was obtained when 46mol%SiO 2,36mol%MgO and 18mol%Al 2O 3was ball milled with 1:1weight proportions of water and the sponge was infiltrated to a loading of 1.5g cm −3,dried in microwave oven with 800W power for 10min/30min and sintered at 1380◦C.References[1]L.M.Sheppard,Porous ceramics:processing and applications,Ceram.Trans.31(1993)3–23.[2]D.-M.Liu (Ed.),Porous ceramics materials,Trans.Tech.Publication,Switzerland,1996.[3]J.Saggio-Woyansky,C.E.Scoot,Am.Ceram.Soc.Bull.71(1992)1674–1682.[4]P.Sepulveda,Am.Ceram.Soc.Bull.76(1997)61–65.[5]D.K.Agarwal,Cu.Opt.Solid-State Mater.Sci.3(1998)480–485.。