Surface Treatment of the Templates for the Fabrication of Reticulated Porous Ceramics
Surface Treatment of Templates for Fabrication of Reticulated Porous
Ceramics
Xi-peng Pu,w Li-ping Jia,Da-feng Zhang,and Chang-hua Su
College of Materials Science and Engineering,Liaocheng University,Liaocheng 252059,China
Xue-jian Liu
Shanghai Institute of Ceramics,Chinese Academy of Sciences,Shanghai 200050,China
Before impregnation of the organic templates with an aqueous ceramic slurry,the templates were treated using silica sol.The surface morphology,elasticity of the templates,and the solid loading were studied.After the treatment,the coherence be-tween the template and the slurry was signi?cantly optimized,and the surfaces of the templates were uniformly covered by slurry.The elasticity of the templates was also increased.Con-sequently,the loading content increased considerably.
I.Introduction
R
ETICULATED
porous ceramics (RPCs)are highly porous ce-ramics (70–95vol%)with an open,three-dimensional web structure.Having a rather unique combination of properties,such as low density,low thermal conductivity,low dielectric constant,low thermal mass,high speci?c strength,high perme-ability,etc.,due to their structure,RPCs are considered to be as candidates for a range of applications at room or high temper-atures as molten metal ?lters,catalyst supports,and radiant burners.1–6
The most common process for producing RPCs is the replica technique,7which involves the treatment of polymer templates,the impregnation of a ?exible polymeric sponge with a ceramic slurry,removal of excess slip by squeezing or centrifuging,fol-lowed by drying,a burnout step to eliminate the polymer tem-plate and high-temperature sintering.In this processing,the coating of the templates with ceramic slurries is crucial because it affects the macrostructure,including the thickness of the struts,porosity,mechanical properties,etc.However,the sur-face of the polymer template is always hydrophobic and cannot be uniformly coated by an aqueous slurry.After ?ring,there always remain many ?aws in the structure of RPCs,such as discontinuous ceramic structures,3,8which are deleterious to the structural and mechanical properties of RPCs and limit their applications.9–11Therefore,the surface treatment of the tem-plate is required to optimize the coherence between the template surface and the aqueous slurry.
Other than patent literature,little information about the sur-face treatment of the templates is available.Hargus et al .12found that adhesion between the ?exible template and the aque-ous slurry could be promoted by applying an adhesive coating to the templates before slurry impregnation.The elasticity of the foam tends to be increased by applying an adhesive,which makes impregnation easier.Ravault 13reported that the treat-ment of sponge before slurry impregnation promoted the adhe-sion between the ?exible foam material and the slurry by gen-erating a local ?occulation of slurry or a local formation of a gel or gelatinous precipitate of a type that does not redisperse into the slurry.
This work is thus carried out to optimize the coherence be-tween the template and the slurry via a surface treatment of the templates using silica gel.The effects of the treatment on the surface morphology,elasticity of the templates,and the solid loading were investigated.
II.Experimental Procedure
Commercial polyurethane (PU)sponges,with a cell size of 10–13pores/in.,were chosen in this work and cut into approxi-mately 10mm ?20mm ?120mm samples to test the elasticity.A hydrophilic silica sol was used as the chemical reagent for surface treatment.
Commercially available b -Si 3N 4powder (d 5053m m)was used as the matrix material,and a -Al 2O 3powder (d 5050.62m m)was simultaneously introduced as sintering additives.In addition,silica sol (25.7%,pH 59.2–10),carboxymethylcellu-lose,and Nopco 267-A (Cognis HK Ltd.,Hong Kong,China)were solely added as a binder,thickening agent,and defoamer agent,respectively.The slurry for impregnating PU sponges was prepared by ball milling the admixture of ceramic powder and deionized water containing the above-mentioned additives in a planetary ball miller.The solution pH was adjusted to the op-timum pH around 11using NaOH or HCl.The viscosity mea-surement was performed on a viscometer (Model NDJ-1,Shanghai Balance Instrument Factory,Shanghai,China).
The treatment of the organic templates involved several steps as described below.Firstly,the templates were cleaned using water,dried,and then marinated in 1M NaOH for 24h.After cleaning with deionized water and drying,the templates were surface treated using silica sol to improve the adhesion and co-herence between the templates and the aqueous slurry.Then,the templates were ready to be used for the preparation of RPCs.The changes in the surface morphologies of the templates were characterized using an optical microscope.
The RPCs were fabricated by a two-step centrifuging tech-nique as described in our previous work.14In the ?rst centri-fuging step,the polymer sponges were dipped into slurry and compressed while submerged,to ?ll all of the cells in the sponge.Then,the samples were centrifuged at a high rotational speed to remove the surplus slurry.Thus,a uniform structure coated with a thin slurry layer was obtained.After drying,the preform was dipped into slurry again and centrifuged at an appropriate speed to remove the excessive slurry.
During the fabrication of RPCs using the replica technique,the excess slip is removed by a squeezing or centrifuging
P.Colombo—contributing editor
w
Author to whom correspondence should be addressed.e-mail:puxipeng@https://www.wendangku.net/doc/3f8161861.html,
Manuscript No.22838.Received February 22,2007;approved April 4,2007.J ournal
J.Am.Ceram.Soc.,90[9]2998–3000(2007)
DOI:10.1111/j.1551-2916.2007.01830.x r 2007The American Ceramic Society
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technique.Enough elasticity is necessary for the templates to overcome the resistance of the viscous slurry and recover its original shape,or to withstand a high centrifugal force and re-tain the original shape.Therefore,the compressive stress–strains curves of the treated templates were measured to characterize the changes of the elasticity of the templates,using an INST-RON 5566universal testing machine (Instron Ltd.,High Wycombe,U.K.)at a crosshead speed of 3mm/min.
III.Results and Discussion
According to the theory of solid/liquid wetting,15there are two ways to improve the coherence between the organic templates and the aqueous slurry.One is to modify the template surface from hydrophobic to hydrophilic through surface treatment,and the other is to enhance the surface coarseness to improve the spreading of slurry on the template surface.
Figure 1shows the optical photos of the templates surface during the treatment.Before treatment,the template surface was smooth,as shown in Fig.1(a).After the treatment,using 1M NaOH,as shown in Fig.1(b),the strut of the template was cor-roded and the surface became coarser.After the treatment using
silica sol,the surface was covered by a wrinkle-like hydrophilic silica gel ?lm,as shown in Fig.1(c).Compared with that in Fig.1(b),the surface further treated using silica sol became coarser,which enhanced the spreading of slurry on the templates surface.
Figure 2shows the stress–strain curves of the templates dur-ing the treatment.After the treatment with 1M NaOH,the stress was lower than that before the treatment,which resulted from the corrosion of the strut,as discussed above.In Fig.2,it can clearly be seen that the elasticity increased signi?cantly after the silica sol treatment.Thus,a more viscous slurry with a high solid content can be used for the coating of the templates,which may reasonably improve the solid loading of the slurry on the tem-plate and the mechanical properties of RPCs.
Figure 3presents optical photos of the coated templates after the ?rst centrifugation processing at a high rotational speed of 1000rpm for 60s.The slurry coating of the untreated template was discontinuous,as shown and indicated by arrows
in
Fig.1.Optical photos of the template surface.(a)Before treatment;(b)after 1M NaOH treatment;(c)after silica sol
treatment.
Fig.2.Stress–strain curves of the templates during the
treatment.
Fig.3.Optical photos of the coated templates after the ?rst centrifu-gation processing.(a)Before surface treatment;(b)after surface treatment.
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Fig.3(a),suggesting that the coherence between the organic
templates and the aqueous slurry is weak.The reticulated PU templates are usually fabricated from close-cell PU sponges by an explosion technique,16and small number of the membranes cannot be removed after the explosion technique.The remaining PU membranes produced during the fabrication of the sponges in the structure can be seen in Fig.3(a),which led to formation of ?lled cells and cracks.Figure 3(b)shows that the slurry coating of the treated templates is uniform and continuous.No PU mem-branes could be seen in the coated structure,because the remain-ing PU membranes in the template structure were corroded and washed off after the treatment.It can be concluded that the sur-face treatment is effective in improving the coherence and adhe-sion between the organic templates and the aqueous slurry.The changes in the template weights during the surface treat-ment are shown in Fig.4(a).After the surface modi?cation using silica sol,the template weight increased by about 0.56g (50wt%).Ceramic loadings on the template after the ?rst and sec-ond centrifugation process are presented in Figs.4(b)and (c).After the ?rst and second centrifugation process,the ceramic loadings on the modi?ed template increased about 28and 20wt%than that on the template without surface treatment.This means the coherence between the templates and the slurry can be improved considerably by the surface treatment technique.
IV.Conclusions
The surface of the template was modi?ed using a silica sol as a surface treatment reagent.After the surface treatment,the hydrophobic template surface became hydrophilic and the surface coarseness increased signi?cantly.Consequently,the co-herence and adhesion between the template and the slurry were improved,and the surface of the templates could be uniformly covered by slurry.The elasticity of the templates was also improved after the surface modi?cation.The slurry loading content on the modi?ed template increased signi?cantly in comparison with that on the untreated template,con?rming that the coherence between the templates and the slurry can be improved considerably by this surface treat-ment technique.
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Fig.4.(a)Weight of template after and before surface treatment;slurry loading (after subtraction of the contribution from the silica sol layer)on templates with and without surface treatment after (b)the ?rst and (c)second centrifugation processes using a slurry with a viscosity of 3.4Pa ás,in the weight of the sample (40cm 3in volume).
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