- Email: [email protected]
Gel-casting of fused silica based core packing for investment casting using silica sol as a binder
Available online at www.sciencedirect.com
Journal of the European Ceramic Society 33 (2013) 2745–2749
Gel-casting of fused silica based core packing for investment casting using silica sol as a binder Fei Wang, Fei Li ∗ , Bo He, Donghong Wang, Baode Sun The State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China Received 24 November 2012; received in revised form 5 April 2013; accepted 8 April 2013 Available online 23 April 2013
Abstract A novel approach for the fabrication of core packing via silica sol gel-casting is described. Concentrated slurry dispersed in silica sol with high solid loading but low viscosity is successfully prepared at about pH 10.2. In situ consolidation of the slurry is realized through adjustment of NH4 Cl concentration to control the gelation time of the slurry. High compaction and uniform green body is obtained by gel-casting technology without de-airing process. The results from flexural strength tests show that wet gel bodies with 0.5 wt.% calcium aluminate obtained by silica sol gel-casting have exceptionally high strength, which are responsible for the integrity of core packing during autoclaving. © 2013 Elsevier Ltd. All rights reserved. Keywords: Strength; Core packing; Gel-casting; Fused silica; Calcium aluminate
1. Introduction In investment casting, ceramic cores are used to form complex internal designs of castings.1,2 But, it will tremendously increase casting cost, because of the high price of ceramic core mold and its production process. For some castings with extremely large internal structures, the wax patterns cannot withstand the weight of big precast ceramic cores. In these cases, core packing is needed. Different from conventional precast ceramic core, the core packing and the shell surrounding it combine together to form the internal structure of the casting, their role is much like the ceramic core. The production of the investment casting core packing can be summarized as follows. First, concentrated ceramic slurry dispersed in silica sol with high solid loading and good fluidity should be prepared. Then, after the slurry have been poured into the internal space build by first several dips of shell, in situ consolidation of the slurry will be realized through adjustment of the electrolyte concentration or pH value to promote gelation of the silica sol in the slurry. Finally, after the solidification of the
∗
Corresponding author. Tel.: +86 21 34202951; fax: +86 21 34202951. E-mail addresses: [email protected], [email protected] (F. Li).
0955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.04.002
slurry, the shell build process is continued to produce a graded mold. Since the solidified slurry is enclosed by mold, the moisture removal from the wet gel body is very slow. Consequently, the strength of the wet gel body is very low. As most investment foundries use wax as the pattern material, steam autoclaving is the most common method used to remove the wax pattern to leave the hollow shell. The shell and core packing exposed to steam at a high pressure and a relatively high temperature within a sealed vessel will expand during autoclaving. Cracking and fracture occur when the strength of the wet gel body cannot support the expansion stress. This is prone to cause cracking of the surrounding mold especially at tips or in a region of stress concentration. Therefore, it would be of great significance to improve the wet strength of core packing, especially the wet tensile strength of the green gel body of core packing. However, little attention has been focused on research of this. In the present work, calcium aluminate was used to add into the slurry to increase the strength of the wet gel body of the core packing due to its rapid hardening and enhanced durability properties.3,4 The wet gel bodies obtained by silica sol gel-casting using non-toxic silica sol as a binder and a NH4 Cl-coagulating agent showed exceptionally high strength.
2746
F. Wang et al. / Journal of the European Ceramic Society 33 (2013) 2745–2749
Fig. 1. Particle size distribution of the fused silica powder.
2. Experimental procedure Slurries consisted of colloidal silica binder (Nalco, 1130), 99.5 wt.% fused silica (Minco, Min-Sil) filler and 0.5 wt.% calcium aluminate (Liaoning Tianhe Technology Co., Ltd., China) filler were prepared. The particle size distribution of the fused silica powder was analyzed as given in Fig. 1. The average particle size of the fused silica filler and the calcium aluminate filler is 78.6 m and 1.3 m. The fillers were premixed evenly in a V-type mixer. The slurries with different solid loadings were prepared through directly dispersing the fillers in the silica sol with various pH values, which were adjusted in advance using 1 mol/L HCl solution in one-way direction, by mechanically stirring for 20 min. The gelation time of the slurries were controlled through adjustment of the NH4 Cl concentration of silica sol in the slurry by adding finely ground NH4 Cl powder while continuously stirring in this program. It has reported that the de-airing process had little effect on the green body prepared by silica sol gel-casting when the slurry did not contain dispersant or other organic materials such as gelatin, egg-white, and even the aqueous monomer, etc.5 . Consequently, as the slurry added NH4 Cl was stirred uniformly, the slurry was cast into a 160 mm × 40 mm × 40 mm steel mold without deairing. After the slurry gelled, the wet gel body was de-molded. To simulate the relative closed environment of the wet gel body surrounded by shell mold, the wet gel body was aging by wrapping it with plastic film. For the sintered samples, they were fired at 1000 ◦ C for 4 h after the wet gel body was de-molded and dried at 22 ◦ C, 55% relative humidity for 24 h. The bulk density (ρ0 ) and the true density (ρ) of the green and the sintered bodies were determined by Archimedes’method in water and by water displacement method. The pH values of the slurries were measured with a kind of pH meter (KL-009 (I) A, China). A rotational viscometer (Model NXS-11A, Chengdu Instrument Factory, China) was employed to measure the viscosity of the slurries. The slurries have to be stirred uniformly before testing. Once the reading of the viscometer is relatively stable,
Fig. 2. Influence of slurry pH value on viscosity with solid loading of 43 vol.%.
it should be recorded within 10 s. The gelation time was determined by visual observation. The prepared slurries (8 mL) were placed into the glass tube. The gelation time was determined while the test tube was gently inverted. The time when the sample stopped flowing was classically taken as the gel point.5 Flexural strength tests were made in three-point bending over a span of 100 mm using 5 test pieces of 160 mm × 40 mm × 40 mm for each group of samples. Samples were loaded in an Instron 8500 tensile testing machine at a constant load rate of 1 mm/min until failure. Scanning electron microscopy (Sirion 200, FEI) was used to observe the microstructure of the green and sintered bodies. 3. Results and discussion 3.1. Preparation of concentrated suspension The viscosity (at about 2.5 s−1 ) of the slurries with solid loading of 43 vol.% dispersed in silica sol is plotted as a function of pH values in Fig. 2. The measured pH value of the slurry is about 10.2 when the solid fillers are directly dispersed in the silica sol without any pH value adjustment. And the measured viscosity of the slurry is 0.36 Pa s. With decreasing the pH value, the viscosity of the slurry increases slowly when the pH value is still over about 7.0. But the viscosity increases rapidly with further decreasing the pH value. The slurry shows the highest viscosity at around pH 5.7. Then the viscosity decreases rapidly with decreasing the pH value and the slurry shows very low viscosity below pH 2.0. As indicated in Fig. 2, the pH value should be controlled either at very low pH value by adding HCl or at about pH 10.2 without adjustment of the pH value to prepare slurries with low viscosity but high solid loadings. Silica sols can be destabilized for lower salt concentrations as the pH value is increased away from the isoelectric point (iep). This was ascribed to the presence of a thick hydration layer at pH close to the iep, resulting from hydrogen bonding of the water molecules with the protonated silanol ( Si OH) sites, and to its
F. Wang et al. / Journal of the European Ceramic Society 33 (2013) 2745–2749
2747
Fig. 3. Viscosity vs. solid loading of the slurries at pH 10.2.
7.6,7
destruction at pH above At pH above 7 but lower than 11, where many negative sites are present on the colloidal particle surface, it was found that ammonium ions (structure-breaker) was more efficient in destabilizing silica colloidal particles than the sodium ions (structure-maker).8,9 And the addition of salt with various concentrations can cause a time-dependent gelation of the silica sol. Thus, all the slurries were prepared without any adjustment of the pH value for gel-casting, and the gelation time of the slurry was controlled through adjustment of the NH4 Cl concentration of silica sol in the slurry by adding NH4 Cl power in this program. The influence of solid loading on the viscosity near pH 10.2 at about 13 s−1 is shown in Fig. 3. The viscosity keeps increasing with increasing of the solid loading in the slurry. When the solid loading is below 60 vol.%, the viscosity increases slowly with increasing the solids content, but increases rapidly when it is beyond 60 vol.%, and the viscosity reaches around 1.1 Pa s when the solids content is 62 vol.%. Gel-casting system generally requires well dispersed concentrated ceramic slurries having viscosity less than 1 Pa s.10 Thus, the maximum solid loading for gel-casting in this program is about 60 vol.%.
Fig. 4. Effect of addition of various concentrations of NH4 Cl on viscosity variation with time of 60 vol.% solid loading slurries.
Fig. 5 shows variation of pH with time of 60 vol.% solid loading slurries after the addition of various concentrations of NH4 Cl. After the adding of NH4 Cl, hydrogen ions are produced by hydrolyzation of the NH4 Cl, leading to a decrease in the slurry’s pH value. The pH of the slurries decrease rapidly with time in the initial few second after the addition of NH4 Cl and thereafter tend to steady. At NH4 Cl concentrations in the range of 0.3-1.4 wt.%, the slurry pH decreases from 10.2 to 8.9-9.7 after the addition of NH4 Cl. As the addition of salt with various concentrations can cause a time-dependent gelation of the silica sol.9 Thus, the gelation time of the slurry can be controlled through adjustment of the NH4 Cl concentration in the slurry by adding NH4 Cl. The gelation time of the slurry with solid loading of 60 vol.% as a function of the NH4 Cl concentration of the slurry is plotted in Fig. 6. It can be seen that the gelation time decrease with increasing the NH4 Cl concentration. The slurries containing 0.3-1.4 wt.% NH4 Cl cast
3.2. Control of the gelation time Fig. 4 shows effect of addition of various concentrations of NH4 Cl on viscosity variation with time of 60 vol.% solid loading slurries prepared at room temperature. The viscosity measurements were taken at 13 s−1 . At 0.3 wt.% NH4 Cl, the slurry does not show further increase in viscosity even after 60 min aging. However, at NH4 Cl concentrations in the range of 0.6-0.9 wt.%, slurry viscosity increase with time and reach a gel-like consistency (viscosity near 50 Pa s) in 6-40 min, respectively. When NH4 Cl is added to the slurry, NH4 + is preferably adsorbed on the surface of colloidal particle where many negative sites are present. With the concentration of NH4 + increasing, the hydration layer thickness is decreased. Therefore, with increment of NH4 + concentration, the silica gel with a three-dimensional contexture is formed by (≡Si− O − Si ≡)n interconnecting.
Fig. 5. Variation of pH with time of 60 vol.% solid loading slurries after the addition of various concentrations of NH4 Cl.
2748
F. Wang et al. / Journal of the European Ceramic Society 33 (2013) 2745–2749
Fig. 8. Photograph of cross sections of wet gel bodies and ceramic molds after de-waxing. (a) Core packing without CA and (b) core packing with 0.5 wt.% CA. Fig. 6. Gel time vs. NH4 Cl concentration of the slurry.
in steel mold attain gel-like consistency (viscosity ∼50 Pa s) at time in the range of 0.6-540 min. In general, the gelation time of the slurry should be controlled within 20 min to avoid the sedimentation of particles in highly dispersed slurries during the gelling process. In the meantime, the gelation time should be controlled above 6 min to provide sufficient stirring time and good fluidity of the slurry before casting. Thus, the concentration of the NH4 Cl in the slurry should be controlled between 0.7 wt.% and 0.9 wt.% for the silica sol gel-casting process. 3.3. Green strength The average yield strength of the wet gel bodies with 60 vol.% solid loading and various aging time is shown in Fig. 7. In all these, NH4 Cl concentration of the slurry used for gelation is 0.7 wt.%. The wet gel samples with 0.5 wt.% CA (calcium aluminate) and without CA have almost the same yield strength of 0.25 MPa when the aging time is 2 h. The yield strength of the wet gel bodies without CA increased from 0.25 MPa to 0.47 MPa
Fig. 7. Yield strength vs. aging time of the wet gel bodies.
when aging time increased from 2 h to 24 h. Above 24 h, the yield strength of wet gel bodies without CA increased marginally with the aging time. However, the yield strength of the wet gel bodies with 0.5 wt.% CA increased considerably, from 0.25 MPa to 0.75 MPa when aging time increased from 2 h to 24 h. Above 24 h, the yield strength of wet gel bodies with 0.5 wt.% CA increased marginally with the aging time and reached a value of 0.80 MPa at aging time of 48 h. For integrity of core packing during autoclaving, the wet gel bodies prepared by silica sol gel-casting should have a high strength. The strength of the wet gel body prepared from slurry mainly depends on the bond between the colloidal particles in the gel network. CA is the main component of CACs (calcium aluminate cements) which are mainly used in refractory and building chemistry applications, such as floor screeds and rapid hardening mortars.11 On reaction with water the nature of the CA hydrates formed is dependent on the temperature of hydration. At room temperatures, CAH10 with a high early strength is the first hydrate formed within 24 h. The CA, as a cementitious binder, bind the fused silica particles in the wet gel body. Therefore, as the CA power distribute uniformly throughout the wet gel body, an increase in strength of the wet gel bodies with an increase of aging time is obvious. Meanwhile, as the hydration reaction consume some water in the wet gel body, it is also benefit to the strength. In addition to the binding effect, the cross-linked CA hydrates and fused silica particles may have entrapped part of the liquid medium. This leads to low effective volume fraction of water in the gel body and results in a higher van der Waals attractive force due to closer approximation of neighboring particles. This van der Waals attractive force between particles also contributes to an increase in strength of wet gel bodies with an increase of aging time. As the temperature increases, the strength of CA hydrates decrease slowly during the conversion to C2 AH8 , C3 AH6 and AH3 . But as the conversion process is very slow, so the strength of the CA hydrates has little decline during autoclaving. As the CAH10 with high early strength is formed in wet gel bodies, the strength of the wet gel bodies can be greatly improved by incorporation of CA in the slurries. Fig. 8 shows a photograph of cross sections of wet gel bodies and molds after autoclaving.
F. Wang et al. / Journal of the European Ceramic Society 33 (2013) 2745–2749
2749
4. Conclusion
Cracks of the wet gel body without CA are clearly evidenced in the photograph. And this cause cracking of the surrounding mold at tip.
A gel-casting technology using non-toxic silica sol as a binder and a NH4 Cl-coagulating agent has been developed to prepare core packing green body. Through investigation of the effect of pH values on the viscosity of the slurry dispersed in the silica sol, concentrated slurry dispersed in silica sol with high solid loading but low viscosity was successfully prepared at about pH 10.2. Calcium aluminate was used to add into the slurry to increase the strength of the wet gel body of the core packing due to its rapid hardening and enhanced durability properties. In situ consolidation of the slurry was realized through adjustment of the NH4 Cl concentration as electrolyte to promote gelation of the silica sol in the slurry. High compaction and uniform green body without entrapped pores has been obtained. The wet gel bodies with 0.5 wt.% calcium aluminate obtained by silica sol gel-casting showed exceptionally high strength.
3.4. Microstructure of the sintered body
References
Fig. 9. SEM microstructure of core packing sintered at 1000 ◦ C for 4 h.
The gelled bodies did not show any deformation or crack during drying at a temperature of 22 ◦ C, 55% relative humidity. The strip-shape bodies prepared at NH4 Cl concentrations in the range of 0.6-0.9 wt.% showed very close values of green density and sintering linear shrinkage. The green density and sintering linear shrinkage observed are in the ranges of 64.5-65.2% TD (theoretical density) and 0.60-0.64%, respectively. The green bodies prepared by gel-casting process at NH4 Cl concentration in the range of 0.6-0.9 wt.% achieve a density of sintered to ∼73% TD at 1000 ◦ C for 4 h. However, the sintered bodies prepared at NH4 Cl concentration in the range of 0.6-0.9 wt.% showed more or less similar microstructures. Fig. 9 shows SEM microstructure of the fractured surface of the core packing prepared by silica sol gel-casting process sintered at 1000 ◦ C for 4 h. It should be noted that the ceramics by gel-casting with silica sol has a uniform microstructure. The steric exclusion between silica colloidal particles prevented fillers from agglomerating, thereby enhancing the stability of the slurry and led to more difficulty in forming a structural cluster.
1. Shaw C. Silica-based ceramic cores. Foundry Trade J 1946;78(31):1534–9. 2. Robb RR, Yaker C, Burd L. Ceramic cores for manufacturing hollow metal castings. US Patent No. 4190450, 1980. 3. Juenger MCG, Winnefeld F, Provis JL, Ideker JH. Advances in alternative cementitious binders. Cem Concr Res 2011;41:1232–43. 4. Scrivener KL, Cabiron J-L, Letourneux R. High-performance concretes from calcium aluminate cements. Cem Concr Res 1999;29:1215–23. 5. Kong Deyu, Yang Hui, Wei Su, Li Dongyun, Wang Jiabang. Gelcasting without de-airing process using silica sol as a binder. Ceram Int 2007;33:133–9. 6. Colic M, Fisher ML. Effect of the size and affinity for water of excluded ions on interparticle forces. Chem Phys Lett 1998;291:24–30. 7. Lawrence HA, Egon M. Stability of colloidal silica: I. Effect of simple electrolytes. J Colloid Interface Sci 1969;31:287–96. 8. Depasse J. Coagulation of colloidal silica by alkaline cations: surface dehydration or interparticle bridging? J Colloid Interface Sci 1997;194:260–2. 9. Trompette JL, Meireles M. Ion-specific effect on the gelation kinetics of concentrated colloidal silica suspensions. J Colloid Interface Sci 2003;263:522–7. 10. Omatate OO, Janney MA, Strelow RA. Gelcasting, a new ceramic forming process. Am Ceram Soc Bull 1991;70:1641–9. 11. Scrivener KL, Capmas A. Calcium aluminate cements. In: Hewlett PC, editor. Lea’s chemistry of cement and concrete. Oxford, UK: Elsevier Ltd.; 1998. p. 713–82.
Journal of the European Ceramic Society 33 (2013) 2745–2749
Gel-casting of fused silica based core packing for investment casting using silica sol as a binder Fei Wang, Fei Li ∗ , Bo He, Donghong Wang, Baode Sun The State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China Received 24 November 2012; received in revised form 5 April 2013; accepted 8 April 2013 Available online 23 April 2013
Abstract A novel approach for the fabrication of core packing via silica sol gel-casting is described. Concentrated slurry dispersed in silica sol with high solid loading but low viscosity is successfully prepared at about pH 10.2. In situ consolidation of the slurry is realized through adjustment of NH4 Cl concentration to control the gelation time of the slurry. High compaction and uniform green body is obtained by gel-casting technology without de-airing process. The results from flexural strength tests show that wet gel bodies with 0.5 wt.% calcium aluminate obtained by silica sol gel-casting have exceptionally high strength, which are responsible for the integrity of core packing during autoclaving. © 2013 Elsevier Ltd. All rights reserved. Keywords: Strength; Core packing; Gel-casting; Fused silica; Calcium aluminate
1. Introduction In investment casting, ceramic cores are used to form complex internal designs of castings.1,2 But, it will tremendously increase casting cost, because of the high price of ceramic core mold and its production process. For some castings with extremely large internal structures, the wax patterns cannot withstand the weight of big precast ceramic cores. In these cases, core packing is needed. Different from conventional precast ceramic core, the core packing and the shell surrounding it combine together to form the internal structure of the casting, their role is much like the ceramic core. The production of the investment casting core packing can be summarized as follows. First, concentrated ceramic slurry dispersed in silica sol with high solid loading and good fluidity should be prepared. Then, after the slurry have been poured into the internal space build by first several dips of shell, in situ consolidation of the slurry will be realized through adjustment of the electrolyte concentration or pH value to promote gelation of the silica sol in the slurry. Finally, after the solidification of the
∗
Corresponding author. Tel.: +86 21 34202951; fax: +86 21 34202951. E-mail addresses: [email protected], [email protected] (F. Li).
0955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.04.002
slurry, the shell build process is continued to produce a graded mold. Since the solidified slurry is enclosed by mold, the moisture removal from the wet gel body is very slow. Consequently, the strength of the wet gel body is very low. As most investment foundries use wax as the pattern material, steam autoclaving is the most common method used to remove the wax pattern to leave the hollow shell. The shell and core packing exposed to steam at a high pressure and a relatively high temperature within a sealed vessel will expand during autoclaving. Cracking and fracture occur when the strength of the wet gel body cannot support the expansion stress. This is prone to cause cracking of the surrounding mold especially at tips or in a region of stress concentration. Therefore, it would be of great significance to improve the wet strength of core packing, especially the wet tensile strength of the green gel body of core packing. However, little attention has been focused on research of this. In the present work, calcium aluminate was used to add into the slurry to increase the strength of the wet gel body of the core packing due to its rapid hardening and enhanced durability properties.3,4 The wet gel bodies obtained by silica sol gel-casting using non-toxic silica sol as a binder and a NH4 Cl-coagulating agent showed exceptionally high strength.
2746
F. Wang et al. / Journal of the European Ceramic Society 33 (2013) 2745–2749
Fig. 1. Particle size distribution of the fused silica powder.
2. Experimental procedure Slurries consisted of colloidal silica binder (Nalco, 1130), 99.5 wt.% fused silica (Minco, Min-Sil) filler and 0.5 wt.% calcium aluminate (Liaoning Tianhe Technology Co., Ltd., China) filler were prepared. The particle size distribution of the fused silica powder was analyzed as given in Fig. 1. The average particle size of the fused silica filler and the calcium aluminate filler is 78.6 m and 1.3 m. The fillers were premixed evenly in a V-type mixer. The slurries with different solid loadings were prepared through directly dispersing the fillers in the silica sol with various pH values, which were adjusted in advance using 1 mol/L HCl solution in one-way direction, by mechanically stirring for 20 min. The gelation time of the slurries were controlled through adjustment of the NH4 Cl concentration of silica sol in the slurry by adding finely ground NH4 Cl powder while continuously stirring in this program. It has reported that the de-airing process had little effect on the green body prepared by silica sol gel-casting when the slurry did not contain dispersant or other organic materials such as gelatin, egg-white, and even the aqueous monomer, etc.5 . Consequently, as the slurry added NH4 Cl was stirred uniformly, the slurry was cast into a 160 mm × 40 mm × 40 mm steel mold without deairing. After the slurry gelled, the wet gel body was de-molded. To simulate the relative closed environment of the wet gel body surrounded by shell mold, the wet gel body was aging by wrapping it with plastic film. For the sintered samples, they were fired at 1000 ◦ C for 4 h after the wet gel body was de-molded and dried at 22 ◦ C, 55% relative humidity for 24 h. The bulk density (ρ0 ) and the true density (ρ) of the green and the sintered bodies were determined by Archimedes’method in water and by water displacement method. The pH values of the slurries were measured with a kind of pH meter (KL-009 (I) A, China). A rotational viscometer (Model NXS-11A, Chengdu Instrument Factory, China) was employed to measure the viscosity of the slurries. The slurries have to be stirred uniformly before testing. Once the reading of the viscometer is relatively stable,
Fig. 2. Influence of slurry pH value on viscosity with solid loading of 43 vol.%.
it should be recorded within 10 s. The gelation time was determined by visual observation. The prepared slurries (8 mL) were placed into the glass tube. The gelation time was determined while the test tube was gently inverted. The time when the sample stopped flowing was classically taken as the gel point.5 Flexural strength tests were made in three-point bending over a span of 100 mm using 5 test pieces of 160 mm × 40 mm × 40 mm for each group of samples. Samples were loaded in an Instron 8500 tensile testing machine at a constant load rate of 1 mm/min until failure. Scanning electron microscopy (Sirion 200, FEI) was used to observe the microstructure of the green and sintered bodies. 3. Results and discussion 3.1. Preparation of concentrated suspension The viscosity (at about 2.5 s−1 ) of the slurries with solid loading of 43 vol.% dispersed in silica sol is plotted as a function of pH values in Fig. 2. The measured pH value of the slurry is about 10.2 when the solid fillers are directly dispersed in the silica sol without any pH value adjustment. And the measured viscosity of the slurry is 0.36 Pa s. With decreasing the pH value, the viscosity of the slurry increases slowly when the pH value is still over about 7.0. But the viscosity increases rapidly with further decreasing the pH value. The slurry shows the highest viscosity at around pH 5.7. Then the viscosity decreases rapidly with decreasing the pH value and the slurry shows very low viscosity below pH 2.0. As indicated in Fig. 2, the pH value should be controlled either at very low pH value by adding HCl or at about pH 10.2 without adjustment of the pH value to prepare slurries with low viscosity but high solid loadings. Silica sols can be destabilized for lower salt concentrations as the pH value is increased away from the isoelectric point (iep). This was ascribed to the presence of a thick hydration layer at pH close to the iep, resulting from hydrogen bonding of the water molecules with the protonated silanol ( Si OH) sites, and to its
F. Wang et al. / Journal of the European Ceramic Society 33 (2013) 2745–2749
2747
Fig. 3. Viscosity vs. solid loading of the slurries at pH 10.2.
7.6,7
destruction at pH above At pH above 7 but lower than 11, where many negative sites are present on the colloidal particle surface, it was found that ammonium ions (structure-breaker) was more efficient in destabilizing silica colloidal particles than the sodium ions (structure-maker).8,9 And the addition of salt with various concentrations can cause a time-dependent gelation of the silica sol. Thus, all the slurries were prepared without any adjustment of the pH value for gel-casting, and the gelation time of the slurry was controlled through adjustment of the NH4 Cl concentration of silica sol in the slurry by adding NH4 Cl power in this program. The influence of solid loading on the viscosity near pH 10.2 at about 13 s−1 is shown in Fig. 3. The viscosity keeps increasing with increasing of the solid loading in the slurry. When the solid loading is below 60 vol.%, the viscosity increases slowly with increasing the solids content, but increases rapidly when it is beyond 60 vol.%, and the viscosity reaches around 1.1 Pa s when the solids content is 62 vol.%. Gel-casting system generally requires well dispersed concentrated ceramic slurries having viscosity less than 1 Pa s.10 Thus, the maximum solid loading for gel-casting in this program is about 60 vol.%.
Fig. 4. Effect of addition of various concentrations of NH4 Cl on viscosity variation with time of 60 vol.% solid loading slurries.
Fig. 5 shows variation of pH with time of 60 vol.% solid loading slurries after the addition of various concentrations of NH4 Cl. After the adding of NH4 Cl, hydrogen ions are produced by hydrolyzation of the NH4 Cl, leading to a decrease in the slurry’s pH value. The pH of the slurries decrease rapidly with time in the initial few second after the addition of NH4 Cl and thereafter tend to steady. At NH4 Cl concentrations in the range of 0.3-1.4 wt.%, the slurry pH decreases from 10.2 to 8.9-9.7 after the addition of NH4 Cl. As the addition of salt with various concentrations can cause a time-dependent gelation of the silica sol.9 Thus, the gelation time of the slurry can be controlled through adjustment of the NH4 Cl concentration in the slurry by adding NH4 Cl. The gelation time of the slurry with solid loading of 60 vol.% as a function of the NH4 Cl concentration of the slurry is plotted in Fig. 6. It can be seen that the gelation time decrease with increasing the NH4 Cl concentration. The slurries containing 0.3-1.4 wt.% NH4 Cl cast
3.2. Control of the gelation time Fig. 4 shows effect of addition of various concentrations of NH4 Cl on viscosity variation with time of 60 vol.% solid loading slurries prepared at room temperature. The viscosity measurements were taken at 13 s−1 . At 0.3 wt.% NH4 Cl, the slurry does not show further increase in viscosity even after 60 min aging. However, at NH4 Cl concentrations in the range of 0.6-0.9 wt.%, slurry viscosity increase with time and reach a gel-like consistency (viscosity near 50 Pa s) in 6-40 min, respectively. When NH4 Cl is added to the slurry, NH4 + is preferably adsorbed on the surface of colloidal particle where many negative sites are present. With the concentration of NH4 + increasing, the hydration layer thickness is decreased. Therefore, with increment of NH4 + concentration, the silica gel with a three-dimensional contexture is formed by (≡Si− O − Si ≡)n interconnecting.
Fig. 5. Variation of pH with time of 60 vol.% solid loading slurries after the addition of various concentrations of NH4 Cl.
2748
F. Wang et al. / Journal of the European Ceramic Society 33 (2013) 2745–2749
Fig. 8. Photograph of cross sections of wet gel bodies and ceramic molds after de-waxing. (a) Core packing without CA and (b) core packing with 0.5 wt.% CA. Fig. 6. Gel time vs. NH4 Cl concentration of the slurry.
in steel mold attain gel-like consistency (viscosity ∼50 Pa s) at time in the range of 0.6-540 min. In general, the gelation time of the slurry should be controlled within 20 min to avoid the sedimentation of particles in highly dispersed slurries during the gelling process. In the meantime, the gelation time should be controlled above 6 min to provide sufficient stirring time and good fluidity of the slurry before casting. Thus, the concentration of the NH4 Cl in the slurry should be controlled between 0.7 wt.% and 0.9 wt.% for the silica sol gel-casting process. 3.3. Green strength The average yield strength of the wet gel bodies with 60 vol.% solid loading and various aging time is shown in Fig. 7. In all these, NH4 Cl concentration of the slurry used for gelation is 0.7 wt.%. The wet gel samples with 0.5 wt.% CA (calcium aluminate) and without CA have almost the same yield strength of 0.25 MPa when the aging time is 2 h. The yield strength of the wet gel bodies without CA increased from 0.25 MPa to 0.47 MPa
Fig. 7. Yield strength vs. aging time of the wet gel bodies.
when aging time increased from 2 h to 24 h. Above 24 h, the yield strength of wet gel bodies without CA increased marginally with the aging time. However, the yield strength of the wet gel bodies with 0.5 wt.% CA increased considerably, from 0.25 MPa to 0.75 MPa when aging time increased from 2 h to 24 h. Above 24 h, the yield strength of wet gel bodies with 0.5 wt.% CA increased marginally with the aging time and reached a value of 0.80 MPa at aging time of 48 h. For integrity of core packing during autoclaving, the wet gel bodies prepared by silica sol gel-casting should have a high strength. The strength of the wet gel body prepared from slurry mainly depends on the bond between the colloidal particles in the gel network. CA is the main component of CACs (calcium aluminate cements) which are mainly used in refractory and building chemistry applications, such as floor screeds and rapid hardening mortars.11 On reaction with water the nature of the CA hydrates formed is dependent on the temperature of hydration. At room temperatures, CAH10 with a high early strength is the first hydrate formed within 24 h. The CA, as a cementitious binder, bind the fused silica particles in the wet gel body. Therefore, as the CA power distribute uniformly throughout the wet gel body, an increase in strength of the wet gel bodies with an increase of aging time is obvious. Meanwhile, as the hydration reaction consume some water in the wet gel body, it is also benefit to the strength. In addition to the binding effect, the cross-linked CA hydrates and fused silica particles may have entrapped part of the liquid medium. This leads to low effective volume fraction of water in the gel body and results in a higher van der Waals attractive force due to closer approximation of neighboring particles. This van der Waals attractive force between particles also contributes to an increase in strength of wet gel bodies with an increase of aging time. As the temperature increases, the strength of CA hydrates decrease slowly during the conversion to C2 AH8 , C3 AH6 and AH3 . But as the conversion process is very slow, so the strength of the CA hydrates has little decline during autoclaving. As the CAH10 with high early strength is formed in wet gel bodies, the strength of the wet gel bodies can be greatly improved by incorporation of CA in the slurries. Fig. 8 shows a photograph of cross sections of wet gel bodies and molds after autoclaving.
F. Wang et al. / Journal of the European Ceramic Society 33 (2013) 2745–2749
2749
4. Conclusion
Cracks of the wet gel body without CA are clearly evidenced in the photograph. And this cause cracking of the surrounding mold at tip.
A gel-casting technology using non-toxic silica sol as a binder and a NH4 Cl-coagulating agent has been developed to prepare core packing green body. Through investigation of the effect of pH values on the viscosity of the slurry dispersed in the silica sol, concentrated slurry dispersed in silica sol with high solid loading but low viscosity was successfully prepared at about pH 10.2. Calcium aluminate was used to add into the slurry to increase the strength of the wet gel body of the core packing due to its rapid hardening and enhanced durability properties. In situ consolidation of the slurry was realized through adjustment of the NH4 Cl concentration as electrolyte to promote gelation of the silica sol in the slurry. High compaction and uniform green body without entrapped pores has been obtained. The wet gel bodies with 0.5 wt.% calcium aluminate obtained by silica sol gel-casting showed exceptionally high strength.
3.4. Microstructure of the sintered body
References
Fig. 9. SEM microstructure of core packing sintered at 1000 ◦ C for 4 h.
The gelled bodies did not show any deformation or crack during drying at a temperature of 22 ◦ C, 55% relative humidity. The strip-shape bodies prepared at NH4 Cl concentrations in the range of 0.6-0.9 wt.% showed very close values of green density and sintering linear shrinkage. The green density and sintering linear shrinkage observed are in the ranges of 64.5-65.2% TD (theoretical density) and 0.60-0.64%, respectively. The green bodies prepared by gel-casting process at NH4 Cl concentration in the range of 0.6-0.9 wt.% achieve a density of sintered to ∼73% TD at 1000 ◦ C for 4 h. However, the sintered bodies prepared at NH4 Cl concentration in the range of 0.6-0.9 wt.% showed more or less similar microstructures. Fig. 9 shows SEM microstructure of the fractured surface of the core packing prepared by silica sol gel-casting process sintered at 1000 ◦ C for 4 h. It should be noted that the ceramics by gel-casting with silica sol has a uniform microstructure. The steric exclusion between silica colloidal particles prevented fillers from agglomerating, thereby enhancing the stability of the slurry and led to more difficulty in forming a structural cluster.
1. Shaw C. Silica-based ceramic cores. Foundry Trade J 1946;78(31):1534–9. 2. Robb RR, Yaker C, Burd L. Ceramic cores for manufacturing hollow metal castings. US Patent No. 4190450, 1980. 3. Juenger MCG, Winnefeld F, Provis JL, Ideker JH. Advances in alternative cementitious binders. Cem Concr Res 2011;41:1232–43. 4. Scrivener KL, Cabiron J-L, Letourneux R. High-performance concretes from calcium aluminate cements. Cem Concr Res 1999;29:1215–23. 5. Kong Deyu, Yang Hui, Wei Su, Li Dongyun, Wang Jiabang. Gelcasting without de-airing process using silica sol as a binder. Ceram Int 2007;33:133–9. 6. Colic M, Fisher ML. Effect of the size and affinity for water of excluded ions on interparticle forces. Chem Phys Lett 1998;291:24–30. 7. Lawrence HA, Egon M. Stability of colloidal silica: I. Effect of simple electrolytes. J Colloid Interface Sci 1969;31:287–96. 8. Depasse J. Coagulation of colloidal silica by alkaline cations: surface dehydration or interparticle bridging? J Colloid Interface Sci 1997;194:260–2. 9. Trompette JL, Meireles M. Ion-specific effect on the gelation kinetics of concentrated colloidal silica suspensions. J Colloid Interface Sci 2003;263:522–7. 10. Omatate OO, Janney MA, Strelow RA. Gelcasting, a new ceramic forming process. Am Ceram Soc Bull 1991;70:1641–9. 11. Scrivener KL, Capmas A. Calcium aluminate cements. In: Hewlett PC, editor. Lea’s chemistry of cement and concrete. Oxford, UK: Elsevier Ltd.; 1998. p. 713–82.