Effects of surface oxide species and contents on SiC slurry viscosity

Transcription

1 RARE METALS Vol. 24, No. 3, Sep 25, p. 24 Effects of surface oxide species and contents on SiC slurry viscosity NING Shufan 1,2), LI Hongyan 1), CHEN Wei 1), LIU Bin 1), and CHEN Shoutian 1) 1) State Key Laboratory of Electrical Insulation for Power Equipment, Xi an Jiaotong University, Xi an 7149, China 2) School of Technology Physics, Xidian University, Xi an 7171, China (Received ) Abstract: The disadvantageous effects of colloidal SiO 2 layer and micro-content of metal oxide adsorbed on SiC powder surface on SiC slurry stable dispersion were studied, and the novel method to avoid this disadvantage was proposed. By acidwashing, on the one hand, because the maximum Zeta potential of SiC powder increases to mv with the decreasing content of metal oxide adsorbed on the SiC powder surface, the repulsion force between SiC powders that dispersed in slurry is enhanced, thus the SiC powder can be fully dispersed in slurry. On the other hand, after HF acidwashing, with the OH group adsorbed on SiC powder surface destroyed and replaced by the F ion, the hydrogen bond adsorbed on the OH group is also destroyed. Therefore, the surface property of the SiC powder is changed from hydrophilic to hydrophobic; H 2 O that adsorbed on SiC powder surface is released and can flow freely, and it actually increases the content of the effective flow phase in the slurry. These changes of SiC powder surface property can be proved by XPS and FTIR analysis. Finally, the viscosity of SiC slurry is decreased greatly, and when the viscosity of the slurry is lower than 1 Pa s, the solid volume fraction of SiC powder in the slurry is maximized to 61.5 vol.%. Key words: SiC powder water base slurry; acidwashing; surface oxide; Zeta potential; hydrophobic; slurry viscosity [This work was financially supported by the Doctoral Foundation of Xi an Jiaotong University (No. DFXJTU24-4).] 1 Introduction During the past decade, an increasing number of novel colloidal processing techniques have been presented to the ceramic community, such as Direct Coagulation Casting and Gelcasting, by which the reliable ceramic green body with a complex shape can be formed [1-2]. To form the ceramic green body with a good quality, the preparation of low viscosity and high solid volume fraction ceramic slurry is necessary [3]. Most defects of ceramic green body can be avoided and enough intensity of the green body can be obtained by decreasing the viscosity of ceramic slurry to keep it lower than 1 Pa s and increasing the solid volume fraction of slurry to keep it higher than 5 vol.% [4]. Hence, the preparation of low slurry viscosity and high solid volume fraction ceramic slurry becomes a very important issue. There are many factors that affect the viscosity and solid volume fraction of ceramic slurry. Besides the shape of ceramic powder, powder-size distribution, the kind and concentration of soluble metal ions, the surface property of the ceramic powder is also an important factor [5-6]. For all kinds of silicon carbide powder, there are a layer of colloidal SiO 2 and micro-content of metal oxide covering on their surface. Therefore, the surface property of SiC powder essentially is the properties of SiO 2 layer and the metal oxide [7-8]. Hence, it is very important to study the effect of SiC powder surface oxide impurity on the viscosity and solid volume fraction of SiC slurry. Two kinds of SiC powder were used to study the issue mentioned above. 2 Experiment and methods 2.1 Raw materials and powder surface treatment methods Two kinds of green α-sic powder named SiC-1 and SiC-2 respectively were used in this study. The Corresponding author: NING Shufan sic_whiskers@yahoo.com

2 Ning S.F. et al., Effects of surface oxide species and contents on SiC slurry viscosity 241 average diameter of either kind of the powder was.8 µm. The SiO 2 layer and the metal oxide adsorbed on SiC powder surface were removed by acid leaching according to the following chemical equation: SiO 2 + 4HF SiF 4 + 2H 2 O (1) The process detail is as follows: 12 g SiC powder and 3 ml 1% HF solution were placed into a 5 ml plastic beaker, stirred for 24 h, and then leached with distilled water until the ph value of the leaching water reached Experimental methods The Zeta potential of SiC powder was measured with a Zeta potential instrument (Model ZETA-SIZER4, MALVEN, England). All infrared spectra of hydroxyl ( OH) group, hydrogen bond group, and Si OH group absorbed on SiC powder surface were measured with an FTIR spectrophotometer (Model IRPrestige-21 FTIR-84S, SHI- MADZU Corporation, KYOTO Japan). The XPS spectra of Si F and Si O bonds of SiC powder surface were measured with a XPS instrument (Model ESCALABMK-II, VG SCIENTIFIC LTD. England). The contents of SiC powder surface metals were measured by a plane grating spectrograph made in Beijing (model: WPG-1), accorded with atomic emission spectrometry. The slurry viscosity was measured through the following steps: (1) 1 g SiC powder was mixed with distilled water until the slurry viscosity was lower than 5 mpa s, and the ph value of the mixture was adjusted to 1. Then the quantity of distilled water that had been used was recorded. (2) 2 g SiC powder was added to the slurry each time and stirred. Then the viscosity of SiC slurry was measured. Measurement of the slurry viscosity was performed on a viscosimeter (Model NDJ-8, Shanghai Balance Instrument LTD., China). 3 Results and discussion 3.1 Effect of surface oxide content on Zeta potential of SiC powder The surface colloidal SiO 2 contents, before and after acid leaching, of SiC-1 and SiC-2 are shown in table 1. There were only.2735 wt.% of SiO 2 on the surface of SiC-1 powder and.1594 wt.% on SiC-2 after acid leaching. Compared with the samples before acid leaching, the surface colloidal SiO 2 contents of SiC-1 and SiC-2 decreased 85.9% and 96.2%, respectively. The Zeta potentials of SiC-1 and SiC-2 before and after acid leaching are shown in figures 1 and 2. Before acid leaching, the IEP (isoelectric point) of the SiC powder was within the bound of ph = 2-4. After acid leaching, the IEP of Table 1 Contents of SiO 2 on SiC powder surface before and after acid leaching Sample SiO 2 contents / wt.% Before acid leaching After acid leaching SiC SiC Zeta potential / mv SiC-1, before acid leaching SiC-1, after acid leaching ph Figure 1 Zeta potential as a function of ph for SiC-1 before and after acid leaching. Zeta potential / mv SiC-2, before acid leaching SiC-2, after acid leaching ph Figure 2 Zeta potential as a function of ph for SiC-2 before and after acid leaching.

3 242 RARE METALS, Vol. 24, No. 3, Sep 25 the SiC powder was within the bound of ph = 6-7. And after acid leaching, in the alkaline solution, the maximum absolute value of the Zeta potential of SiC-1 was mv and that of SiC-2 was mv (here ph = 1), much greater than 6.71 mv of SiC-1 and mv of SiC-2 before acid leaching. The reasons why the Zeta potential changed after acid leaching are as follows. First, the content of the hydroxyl ( OH) group adsorbed on SiC powder surface was decreased because of acid leaching. The negative electric charge adsorbed on SiC powder surface is obtained in two ways described below [9]: (a) The OH group adsorbs on the solid-liquid interface; (b) The Si OH group decomposes to Si O according to the following chemical equation: Si OH + OH Si O + H 2 O (2) Before acid leaching, the content of SiO 2 was quite high, a large number of O 2 ions existed, the OH group formed easily and the ability of SiC powder surface to form negative electric charge was comparatively good. Hence, the ph value at IEP was comparatively low (ph = 2-4). After acid leaching, with the sharp decrease of the contents of SiO 2 and O 2 ions and the chance of forming the OH group, the ph value at IEP changed from 2-4 to 6-7 and the ability of SiC powder surface to adsorb negative electric charge was weakened. Second, during the acid leaching processing, not only the SiO 2 film but also the fractional metal oxide was removed from SiC powder surface. Meanwhile, the soluble metal ions, for example, Mg 2+, Ca 2+, and Na +, were also removed from SiC powder surface. The difference between the contents of soluble metal ions before and after acid leaching is shown in table 2. Most of soluble metal ions adsorbed on SiC powder surface; especially, high valence ions were removed from SiC powder surface. This situation results in the thickening of double electrode layer and the increase of the Zeta potential of SiC powder. 3.2 Effect of SiO 2 content on chemical bond group adsorbed on SiC powder surface Before acid leaching, the contents of SiO 2 and O 2 ions on SiC powder surface were quite high, as shown in table 1. The O 2 ion existed as non-bridging oxygen: one bond connected with Si 4+, and another bond forming a OH group when reacting with H 2 O. The hydroxyl group adsorbed H 2 O with hydrogen bonding force. When H 2 O was adsorbed by the hydroxyl group, this H 2 O took a directional arrangement around SiC powder, because the hydrogen bonding force was too strong for H 2 O to move freely, as shown in figures 3(a) and 3(b). Table 2 Contents of soluble metal ions adsorbed on SiC powder surface Soluble metal ions SiC-1 / 1 6 As received Acid leached SiC-2 / 1 6 As received Acid leached Al Fe Ca Mg Na Figure 3 Sketch map of SiC surface. After acid leaching, the contents of SiO 2 and O 2 on SiC powder surface decreased sharply. This situation led to the result that, on the one hand, the hydroxyl group could not form; on the other hand, OH was replaced by F because of some of the properties of OH and F, for example, ionic radius, ionic valence, and ionic polarization which are very

4 Ning S.F. et al., Effects of surface oxide species and contents on SiC slurry viscosity 243 similar, as shown in figure 3(c). In this case, the hydrogen bond on SiC powder surface cannot form and H 2 O cannot be adsorbed either. SiC powder surface was changed from hydrophilic to hydrophobic. The situation of SiC powder surface discussed above can be proved by FTIR analysis. The FTIR spectra of SiC powders as received and after acid leaching are shown in figure 4. It is obvious that the infrared absorption band at 1128 cm 1 of SiC powder has been weakened after acid leaching, indicating that the SiO 2 content on SiC powder surface was decreased by acid leaching. With the decrease of SiO 2 content, the stretching vibration band 3499 cm 1 of the hydroxyl group, the bending vibration band 1628 cm 1 of the hydrogen bond, and the stretching vibration band 935 cm 1 of Si OH weakened, illuminating that the hydroxyl group and the hydrogen bond adsorbed on SiC powder surface were decreased. X-ray photoelectron spectroscopy analysis can also illustrate that after acid leaching most of oxygen on SiC powder surface was removed, and with the decrease of surface oxygen content, most of OH was replaced by F, as shown in figures 5-7 and table 3. From figures 5 and 6 and table 3, it can be obtained that there was a great deal of oxygen (22.36 at.%) and a little fluorin (.49 at.%) on SiC powder surface before acid leaching, and after acid leaching, the oxygen content was decreased (15.79 at.%) and the fluorin content was increased (4.63 at.%). Before acid leaching, oxygen has a binding energy band at ev, corresponding to Transmittance / % As recieved Acid leached 3499 Intensity / counts SiC, as recieved SiC, HF treated Wavenumber / cm 1 Figure 4 FTIR spectra of SiC powders as received and by HF leached. Intensity / counts SiC, as recieved SiC, HF treated Binding energy / ev Figure 6 XPS spectra of O 1s of SiC powders. Intensity / counts Binding energy / ev Figure 5 XPS spectra of F 1s of SiC powders SiC, as recieved SiC, HF treated Binding energy / ev Figure 7 XPS spectra of Si 2p of SiC powders.

5 244 RARE METALS, Vol. 24, No. 3, Sep 25 the binding energy band ev of SiO 2 (gel), and after acid leaching, the binding energy band of oxygen moved to ev, indicating that the SiO 2 (gel) on SiC powder surface was dissolved after acid leaching, Si O bonds were broken and replaced by Si F bonds (684.7 ev), as shown in figure 5. The binding energy ev of Si F moved to ev after acid leaching, as shown in figure 5, because there were a few unstable SiF 3 and SiF 2 groups (685.3 ev) adsorbed on the SiC powder as received, and these unstable groups were decomposed during acid leaching and drying processing and transformed into stable SiF groups (684.7 ev). Figure 7 shows that before acid leaching, the binding energy band Si 2p was 1.7 ev, corresponding to Si O bonds, and after acid leaching, this value moved to 1.1 ev, corresponding to Si C bonds, which illustrated in the other way that SiO 2 (gel) on SiC powder surface was dissolved after acid leaching. Table 3 XPS analysis data of SiC powders Peak center / ev Content / at.% Peak As received HF leached As received HF leached F 1s O 1s Si 2p C 1s From table 3, it can be also obtained that the binding energy peak position of C 1s as received was at ev, corresponding to C C bonds, and after acid leaching, the peak position was at ev, corresponding to C Si bonds, and the content of C 1s decreased from at.% to 18.7 at.%. It is indicated that the graphite C adsorbed on SiC surface was removed by acid leaching processing. 3.3 Relationship between surface oxide content and SiC slurry viscosity Two main effects of the SiC powder surface oxide content on the viscosity of SiC slurry are discussed below. With the decrease of SiC powder surface oxide content and the increase of the Zeta potential discussed above, the repulsion force increased, Van der Vaals force was overcome, and the powder was dispersed to individual and kept a colloidal stability. Therefore, the SiC slurry viscosity was decreased and the solid volume fraction was increased. The hydroxyl group adsorbed on SiC powder surface was broken and the surface was changed into hydrophobic because of the decrease of oxide content. Therefore, a great deal of H 2 O was adsorbed tightly on SiC powder surface when the hydrogen bond was released and the flow phase content of slurry was increased. As a result, the SiC slurry viscosity was decreased and the solid volume fraction was greatly increased. Especially when the average diameter of SiC powder was only.8 µm, its specific surface area could be 1-15 m 2 /g. Hence, the effects discussed above become more important. With combination of the two effects, the SiC slurry viscosity was decreased and the solid volume fraction was greatly increased. When the SiC slurry viscosity is 1 Pa s, the solid volume fraction of SiC-1 was increased from 51.5 vol.% as received to 61.5 vol.% after acid leaching, and the solid volume fraction of SiC-2 was increased from 41. vol.% as received to 51.5 vol.% after acid leaching, respectively, as shown in figures 8(a) and 8(b). 4 Conclusions The IEP of SiC powder is changed from ph = 2-4 to ph = 6-7 and the absolute value of the Zeta potential is increased in the alkaline condition by the acid leaching processing. At ph = 1, the maximum absolute value of the Zeta potential of SiC-1 is 72.49mV and that of SiC-2 is mv, much greater than 6.71 mv of SiC-1 and mv of SiC-2 before acid leaching, respectively. With decreasing oxide content, the hydroxyl group adsorbed on SiC powder surface is broken and replaced by F. As a result, SiC powder surface is changed from hydrophilic to hydrophobic. With the increase of the Zeta potential, the repulsion force between SiC powder increases, and the powder is dispersed to individual and keeps a colloidal stability. When SiC surface property changes into hydrophobic, the H 2 O molecule adsorbed

6 Ning S.F. et al., Effects of surface oxide species and contents on SiC slurry viscosity SiC-1, acid washed SiC-1, as recieved 3 SiC-2, acid washed SiC-2, as recieved Viscosity / (Pa s) Viscosity / (Pa s) SiC-1 solid volume fraction / vol.% SiC-2 solide volume fraction / vol.% Figure 8 Slurry viscosity as a function of solid volume fraction for SiC-1 and SiC-2 as received and by acid leached. tightly on SiC powder surface by the hydrogen bond is released and the flow phase content of slurry is increased. With combination of the two effects, the SiC slurry viscosity is decreased and the solid volume fraction is greatly increased. When the SiC slurry viscosity is 1 Pa s, the solid volume fraction of SiC-1 is increased from 51.5 vol.% as received to 61.5 vol.% after acid leaching, and the solid volume fraction of SiC-2 is increased from 41 vol.% as received to 51.5 vol.% after acid leaching, respectively. References [1] Si W.J., Graule T.J., Baader F.H., and Gauckler L.J., Direct coagulation casting of SiC the solidification process, J. Inorg. Mater., 1994, 11 (1): 171. [2] Omatete O.O., Janney M.A., and Mumm S.D., Gelcasting: from laboratory development toword industrial production, J. Eur. Ceram. Soc., 1997, 17 (2): 47. [3] Wang J. and Gao L., Rheology of concentrated Y-TZP aqueous suspensions, J. Inorg. Mater. (in Chinese), 1999, 14 (4): 652. [4] Si W.J., Graule T.J., and Baader F.H., Direct coagulation casting of silicon carbide components, J. Am. Ceram. Soc., 1999, 82 (5): [5] Wolfgang M.S., Novel Powder-Processing Methods for Advanced Ceramics, J. Am. Ceram. Soc., 2, 83 (7): [6] Brynestad J., Bamberger C.E., Heatherly D.E., and Land J.F., Removal of oxide contamination from silicon carbide powers, Commun. Am. Ceram. Soc., 1984, 67 (9): C-184-C-185. [7] Merle-Mejean T., Abdelmounim E., and Quintard P., Oxide layer on silicon carbide powder: a FT-IR investigation, J. Molec. Struct., 1995, 349 (Apr. 1): [8] Rahaman 15. M.N., Boiteux Y., and Jonghe L.C.D., Surface characterization of silicon nitride and silicon carbide powders, Am. Ceram. Soc. Bull., 1986, 65 (8): [9] Huang Q., Gu M., Sun K., and Jin Y., Effect of pretreatment on the properties of silicon carbide aqueous suspension. Ceram. Int., 22, 28 (7): 747.