Concentrated aqueous Si3N4 -Y2O3 -Al2O3 slips stabalized with tetramethylammonium hydroxide

Dispersões aquosas concentradas de Si3N4 - Y2O3 - Al2O3 estabilizadas com hidróxido de tetrametilamônio

Concentrated aqueous Si₃N₄ -Y₂O₃ -Al₂O₃ slips stabalized with tetramethylammonium hydroxide

(Dispersões aquosas concentradas de Si₃N₄ - Y₂O₃ - Al₂O₃ estabilizadas com hidróxido de tetrametilamônio)

M. P. Albano^*, L. B. Garrido^**

Centro de Tecnología de Recursos Minerales y Cerámica (CETMIC),

C.C. 49 (1897) M.B. Gonnet, Provincia de Buenos Aires, Argentina

fax: [phone].

Abstract

In order to obtain well dispersed concentrated aqueous Si₃N₄ slips for slip casting, the influence of pH and sintering aid powders (Y₂O₃ and Al₂O₃) on the viscosity and on the amount of tetramethylammonium ions adsorbed were determined. 35 vol% aqueous Si₃N₄ and Si₃N₄-6wt%Y₂O₃-4wt% Al₂O₃ slips were prepared in an attrition mill. Tetramethylammonium hydroxide was added to adjust the pH values in a range of 9.7 to 12.3. The viscosity of Si₃N₄ slips and the amount of [(CH₃)₄N]⁺ ions in solution increased with increasing pH. These counterions contributed mainly to increase the ionic strength of the solution with increasing the slip viscosity. The sintering aid powders had a positive effect on the dispersion of the Si₃N₄ powder at pH 10.3-12.3 since low viscosity values could be obtained. This was attributed to the lower concentration of counterions in solution.

Keywords: Si₃N₄ , slip casting, tetramethylammonium ion adsorption.

Resumo

A influência do pH e dos aditivos de sinterização Y₂O₃ e Al₂O₃ na viscosidade e na quantidade de íons tetrametilamônio adsorvidos foi determinada a fim de se obter dispersões aquosas concentradas de Si₃N₄ para colagem por barbotina. Foram preparadas em moinho atritor dispersões aquosas (35 vol%) de Si₃N₄ e Si₃N₄ -6 peso% Y₂O₃ -4 peso% Al₂O₃. Hidróxido de tetrametilamônio foi adicionado para ajuste dos valores de pH na faixa 9,7-12,3. A viscosidade das dispersões de Si₃N₄ e a quantidade de íons [(CH₃)₄N]⁺ em solução aumenta com o aumento de pH. Esses contra-íons contribuíram principalmente para aumentar a ionicidade da solução com o aumento da viscosidade da dispersão. Os pós aditivos de sinterização tiveram um efeito positivo na dispersão do pó de Si₃N₄ para valores de pH 10,3-12,3 porque baixos valores de viscosidade puderam ser obtidos. Esse resultado foi atribuído à menor concentração de contra-íons em solução.

Palavras-chave: Si₃N₄, dispersões, adsorção de íons tetrametilamônio.

INTRODUCTION

Colloidal processing of ceramic slurries is widely used in the industry to avoid heterogeneities and to improve the final properties of the product .

Slip casting is a suitable colloidal processing technique to produce high packing density Si₃N₄ compacts with complex shapes and microstructural homogeneity. As the sintering process requires addition of sintering additives powders, the mixture silicon nitride/ sintering aid powders (typically Al₂O₃ and Y₂O₃ ) must be used [1].

It is commonly accepted that to obtain materials with high green densities by slip casting well deflocculated slips with a high content of solids must be prepared. However, concentrated Si₃N₄-Y₂O₃-Al₂O₃ slips are rheologically complex since the three powders have different values of the isoelectric point (IEP). Consequently, at pH ranges between the IEPs heterocoagulation occurs [2].

Thus, the preparation of a well dispersed, concentrated slip of a submicron Si₃N₄ powder with the corresponding sintering aid additives is considered as a critical step in slip casting[3].

It has been shown that well dispersed concentrated slips prepared from the as-received Si₃N₄ powder, without any surface treatment, could be obtained at pH 11-12 with the addition of tetramethylammonium hydroxide (TMAH) [3].

The TMAH is successfully used to obtain concentrated aqueous Si₃N₄ slips [3]; however, the properties of the slip depend on the starting powder and more information is required on the stabilization effect of the TMAH and on the influence of the sintering aid powders on the slip rheology. Therefore, this work was undertaken to study the influence of pH and sintering aid powders on the degree of dispersion, measured by the viscosity, of 35 vol% aqueous Si₃N₄ slips.

MATERIALS AND METHODS

Materials

A commercial a-Si₃N₄ powder (SN-E10, UBE Industries, Japan) was used in this study. The mean particle diameter and the specific surface area were 0.6 µm and 10 m²/g, respectively. The oxygen content of the Si₃N₄ powder taken from the manufacturer specifications was lower than 2 wt%. A mixture of 6 wt% Y₂O₃ and 4 wt% Al₂O₃ was used as sintering aid. The mean particle diameter of the Al₂O₃ and Y₂O₃ powders was 0.4 and 2.8 µm, respectively. The specific surface area was 11 m²/g for the Al₂O₃ powder and 1.7 m²/g for the Y₂O₃ powder.

Zeta Potential determinations

The electrophoretic mobility was measured (Penkem Laser Zee Meter 501, USA) and used to calculate zeta potential according to Smoluchowski's equation. Electrophoretic measurements were used to determine the isoelectric point (IEP) of the Si₃N₄ powder in 10^-2 M NaCl and 10^-2 M (CH₃)₄NCl aqueous solutions. For each determination, 0.05g of sample were dispersed in 100 ml of 10^-2 M NaCl or (CH₃)₄NCl solutions and the slurry stirred magnetically for 10 min before the measurements were carried out. The pH of the slurry was adjusted using dilute HCl and NaOH solutions to generate zeta potential versus pH curves and from them the IEP.

Slip Preparation

Slips with a solid content of 35 vol% were prepared. A commercial tetramethylammonium hydroxide solution (Fluka AG, CH-9470 Bucks) was used to adjust the pH.

Aqueous slips were prepared by desagglomeration of the powder in an attrition mill at different pH values in a range of 9.7 to 12.3. The milling time was half an hour. Then the slip was allowed to equilibrate for a few minutes before the pH was measured.

Viscosity Measurements

Slip rheological properties were determined using a concentric cylinder viscometer (Haake RV3, Germany) at 25 ºC. The slips with high viscosity values showed a slightly pseudoplastic behavior with very small thixotropic hysteresis cycle. In this case the up curve was used to determine the viscosity. The slips with low viscosity values showed a nearly newtonian behavior. The apparent viscosity was calculated as the ratio between the shear stress and the shear rate at 175 s^-1.

Adsorption measurements of tetramethylammonium ion

In order to determine the amount of tetramethylammonium ion adsorbed, slips were centrifuged for 30 min at 2500 rpm and washed twice with distilled water. Afterwards, the solid was dried at 100 ºC and analyzed by thermal gravimetric analysis (TGA) (Model STA 409, Netzsch Inc., Germany) at a heating rate of 10 ºC/min in N₂ atmosphere. The TGA data showed a water weight loss at temperatures near 100 ºC and a weight loss due to the tetramethylammonium ion decomposition at a temperature range from 250 ºC to 500 ºC. This weight loss was used to determine the amount of tetramethylammonium ion adsorbed on each sample.

The amount of tetramethylammonium ion in solution for each sample was taken as the difference between the amount added and the amount adsorbed.

Soluble Silica Measurements

The slips were centrifuged for 1h at 3000 rpm and the supernatant was drawn-off. The clear supernatant of each slip was analyzed colorimetrically with the formation of a silicomolybdate acid complex (Standard Method IRAM 41318).

RESULTS AND DISCUSSION

Zeta Potential versus pH curves

Fig. 1 shows the zeta potential versus pH curves of the Si₃N₄ powder in 10^-2 M NaCl and 10^-2 M (CH₃)₄NCl aqueous solutions.

Figure 1:
Zeta Potencial versus pH curves of the Si₃N₄ powder in different solutions : (a) 10^-2 MNaCl, (b) 10^-2 M (CH₃)₄NCl

The IEP in 10^-2 M NaCl solution was at about pH 6 (Fig. 1a). This was in agreement with the reported electroacoustic behavior of this Si₃N₄ powder [4]. Bergströn also found an IEP at pH 6.2 for this Si₃N₄ powder [5].

The IEP in 10^-2 M (CH₃)₄NCl solution was at about pH 7 (Fig. 1b). The increase in the IEP with respect to that determined in NaCl solution (pH_IEP 6) indicated that the tetramethylammonium ion was adsorbed specifically on the Si₃N₄ particles.

The zeta potential of the powder in (CH₃)₄NCl solution had lower negative values than those in NaCl solution in the pH range from 7 to 10. Thus, the adsorbed [(CH₃)₄N]⁺ ions decreased the negative charge of the Si₃N₄ powder at alkaline pH range. However, at pH range from 10 to 12 the powder had the same zeta potential values in both electrolytes. This adsorption behavior at pH 10-12 will be explained below.

Viscosity, amount of tetramethylammonium ion adsorbed and amount of species in solution versus pH

Figs. 2a and 2b show the viscosity as a function of pH for 35 vol% Si₃N₄ and 35 vol% Si₃N₄/sintering aid slips, respectively. The Si₃N₄ powder could be dispersed only at pH ³ 9.7 in the pH range investigated (Fig. 2a). The slip viscosity increased with an increase in pH. A minor increase in viscosity up to pH 11.2 was found, followed by a greater increase with further increasing of pH up to 12.2.

The zeta potential measurements (Fig. 1) suggested that the most stable dispersions of the powder should occur at pH near 12 where the powder had the maximum negative surface charge; however, well dispersed slips at that pH could not be obtained. Well deflocculated slips of the Si₃N₄ powder could be formed over a pH range of 9.7 to 11.2. Thus, the dispersion of the Si₃N₄ powder in suspension was not determined by the magnitude of the powder surface charge.

Fig. 3 shows the amount of [(CH₃)₄N]⁺ ions adsorbed and the amount in solution versus pH for 35 vol% slips.

Figure 3:
Amount of [(CH₃)₄N]⁺ ions adsorbed and amount of [(CH₃)₄N]⁺ ions in solution versus pH for different 35Vol% slips: (a) and (c) Si₃N₄ slips, (b) and (d) Si₃N₄/sintering aid slips.

The amount of [(CH₃)₄N]⁺ ions adsorbed on the Si₃N₄ powder (Fig. 3a) was negligible at the pH range studied and adsorption was not detected at pH ³11.4.

The Si₃N₄ surface consisted of silanol (Si-OH) and amine (=N-H) functional groups. The silanol groups are the sites on the Si₃N₄ surface at which adsorption occurs. Ionic attraction between [(CH₃)₄N]⁺ ions and Si-O- groups exists[6]. However, at pH>9 the silica dissolves generating soluble silicate (HSiO₃) ions[6].

Figs. 4a and 4b show the amount of soluble SiO₂ as a function of pH for 35 vol% Si₃N₄ and 35 vol% Si₃N₄ /sintering aid slips, respectively.

The amount of soluble SiO₂ in Si₃N₄ slips increased slightly with an increase in pH in the range from 9.7 to 12.2 (Fig. 4a). Therefore, the negligible adsorption of the [(CH₃)₄N]⁺ ions on the Si₃N₄ powder at the pH range studied was due to the dissolution of the SiO₂ layer on the powder surface.

The similarity found between the zeta potential values of the powder in NaCl and (CH₃)₄NCl solutions at pH 10-12 (Fig. 1) was consistent with the low adsorption of the [(CH₃)₄N]⁺ ions at those pH values.

The amount of [(CH₃)₄N]⁺ ions in solution for Si₃N₄ slips was high and increased with increasing pH.

The concentration of [(CH₃)₄N]⁺ ions was about 4 to 16 times higher than the concentration of (HSiO₃)^- ions (Figs. 3c and 4a). Consequently , the counterions contributed mainly to increase the ionic strength of the solution with increasing the slip viscosity (Fig. 2a).

The IEP of the Y₂O₃ and Al₂O₃ powders was found to be around 10.3 [2] and 8 [7] respectively.

The viscosity of Si₃N₄/sintering aid slips at pH<10.3 was high, upon further increase of the pH to 10.5 the slip could be effectively dispersed (Fig. 2b).The viscosity increased at pH higher than 10.5, however, the viscosity values at pH 10.3-12.3 were lower than those observed for Si₃N₄ slips at the same pH range (Figs. 2a and 2b).

A greater adsorption of [(CH₃)₄N]⁺ ions on the Si₃N₄ /sintering aid powders with respect to the Si₃N₄ powder was found at all pH values (Figs. 3a and 3b). Thus, the adsorption of [(CH₃)₄N]⁺ ions on the sintering aid particles was responsible for the greater adsorption.

The amount of [(CH₃)₄N]⁺ ions adsorbed increased with increasing pH from 9.7 to 10.5 and then decreased upon further increasing of pH up to 12.3 (Fig. 3b). At pH<10.3 the Y₂O₃ particles were positively charged and the [(CH₃)₄N]⁺ ions were only adsorbed at the negative (-Al-O-) sites of the Al₂O₃ powder surface . At pH higher than 10.3 the Al₂O₃ and Y₂O₃ powders were both negatively charged, therefore a greater adsorption was expected. However, the amount of [(CH₃)₄N]⁺ ions adsorbed decreased slightly at pH higher than 10.5 (Fig. 3b). This behavior might be produced by an increase of the solubility of Al₂O₃ at pH values >10.5 as it was mentioned by Yariv[8], reducing the amount of counterions adsorbed on its surface.

The amount of [(CH₃)₄N]⁺ ions in solution for Si₃N₄/sintering aid slips shows a minimum at pH 10.5 (Fig. 3d) where the [(CH₃)₄N]⁺ ion adsorption has its maximum (Fig. 3b). The amount of (HSiO₃)^- ions was very low with respect to the amount of counterions in solution (Figs. 3d and 4b) therefore the silicate ions had a minor effect on the ionic strength of the solution.

High viscosity values at pH<10.3 (Fig. 2b) could be expected since the Y₂O₃ powder had the opposite sign zeta potential (positive) with respect to Si₃N₄ and Al₂O₃ powders and hetero-coagulation occurred, decreasing the negative surface charge of the powders and consequently the electrostatic repulsion between particles. At pH values higher than 10.3 the Y₂O₃ powder became negatively charged so mutual repulsion then occurred. The minimum viscosity was observed at pH 10.5 where electrostatic repulsion between particles occurred and the amount of counterions in solution was minimum (Fig. 3d).

The increase in viscosity in the pH range from 10.5 to 12.3 was produced by the increase of the amount of counterions in solution at that pH range (Fig. 3d) which had a detrimental effect on the slip stability as they increased the ionic strength of the solution.

The substitution of 10 wt% Si₃N₄ by 6 wt% Y₂O₃ and 4 wt% Al₂O₃ in Si₃N₄ /sintering aid slips produced a decrease in the negative surface charge of the powders with respect to Si₃N₄ slips (Al₂O₃ and Y₂O₃ powders have lower zeta potential values at 10.3-12.3 pH range compared with the Si₃N₄ powder [2]). However, lower viscosity values at 10.3-12.3 pH range could be obtained with Si₃N₄ /sintering aid slips due to the lower amount of counterions in solution. Thus, the degree of slip dispersion at high pH values was governed by the amount of free [(CH₃)₄N]⁺ ions in solution.

CONCLUSIONS

The Si₃N₄ powder can be dispersed at pH ³ 9.7. The slip viscosity increases with an increase in pH at 9.7-12.2 pH range. The increase in the amount of counterions in solution with increasing pH increases the ionic strength of the solution and consequently the slip viscosity.

A greater adsorption of the Si₃N₄ /sintering aid powders with respect to the Si₃N₄ powder is found at the pH range studied due to the adsorption of tetramethylammonium ions on the sintering aid particles. Therefore, the equilibrium concentration of counterions in solution is lower for Si₃N₄ /sintering aid slips.

The minimum viscosity for Si₃N₄ /sintering aid slips is observed at pH 10.5. At pH lower than 10.3 the viscosity is high since hetero-coagulation occurs. The increase in viscosity at pH higher than 10.5 is attributed to the increasing amount of counterions in solution.

(Rec.07/08/99, Ac. 16/02/99)

** CONICET Research Member.

• [1] T.Hayashi, H.Munakata, H.Suzuki, H. Saito, J. Mater. Sci . 21, 3 (1986) 501.
• [2] J. P. Pollinger, D. D. Newson, J. J. Nick, "The Implementation of Acoustophoresis for the Characterization of Silicon Nitride Based Aqueous Suspensions" in Handbook on Characterization Techniques for the Solid-Solution Interface. Edited by J. H. Adair, J. A. Casey and Sridhar Venigalla. The Am. Ceram. Soc., Inc. Westerville, Ohio (1996) 125.
• [3] S. R. H. M. Castanho, R. Moreno, in Proc. Third Euro-Ceramics, 1 (1993) 513.
• [4] D. Liu and S. G. Malgham,. Colloids Surfaces A, 110 (1996) 37.
• [5] L. Bergströn, R. J. Pugh, J. Am. Ceram. Soc. 72, 1 (1989) 103.
• [6] R. K. Iler, "The Surface Chemistry of Silica" in The Chemistry of Silica, Vol. III, John Wiley & Sons, New York (1978) 654.
• [7] P. A. Smith, R. A. Haber, J. Am. Ceram. Soc. 78, 7 (1995) 1737.
• [8] S. Yariv, H. Cross in Geochemistry of Colloid Systems, Springer-Verlag, Berlin (1979).160.

*

CIC and UNLP Research Member.

Publication Dates

• Publication in this collection
  31 Mar 2000
• Date of issue
  May 1999

History

• Accepted
  16 Feb 1999
• Received
  07 Aug 1999