Effects of surface oxide species and contents on SiC slurry viscosity

RARE METALSVol. 24, No.3, Sep 2005, p.240Effects of surface oxide species and contents on SiC slurry viscosityNING Shufan'2), LI Hongyan", CHEN Wei"), LIU Bin), and CHEN Shoutian)1) State Key Laboratory of Electrical Insulation for Power Equipment, Xi an Jiaotong University, Xi'an 710049, China2) School of Technology Physics, Xidian University, Xi1 an 710071, China(Received 2004-12-14)Abstract: The disadvantageous effects of colloidal SiO2 layer and micro-content of metal oxide adsorbed on SiC powdersurface on SiC slurry stable dispersion were studied, and the novel method to avoid this disadvantage was proposed. Byacidwashing, on the one hand, because the maximum Zeta potential of SiC powder increases to 72.49 mV with the decreas-ing content of metal oxide adsorbed on the SiC powder surface, the repulsion force between SiC powders that dispersed inslurry is enhanced, thus the SiC powder can be fully dispersed in slurry. On the other hand, after HF acidwashing, with theOH group adsorbed on SiC powder surface destroyed and replaced by the F ion, the hydrogen bond adsorbed on the OHgroup is also destroyed. Therefore, the surface property of the SiC powder is changed from hydrophilic to hydrophobic; .H2O that adsorbed on SiC powder surface is released and can flow freely, and it actually increases the content of the effec-tive flow phase in the slurry. These changes of SiC powder surface property can be proved by XPS and FTIR analysis. Fi-nally, the viscosity of SiC slurry is decreased greatly, and when the viscosity of the slurry is lower than 1 Pas, the solidvolume 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; slury viscosity[This work was financially supported by the Doctoral Foundation ofXi 'an Jiaotong University (No. DFXJTU2004-04).]1 Introductionthe shape of ceramic powder, powder-size distribu-tion, the kind and concentration of soluble metalDuring the past decade, an increasing number ofions, the surface property of the ceramic powder isnovel colloidal processing techniques have beenalso an important factor [5-6]. For all kinds of sili-presented to the ceramic community, such as Directcon carbide powder, there are a layer of colloidalCoagulation Casting and Gelcasting, by which theSiO2 and micro-content of metal oxide covering onreliable ceramic green body with a complex shapetheir surface. Therefore, the surface property of SiCcan be formed[1-2]. To form the ceramic green bodypowder essentially is the properties of SiO2 layer andwith a good quality, the preparation of low viscositythe metal oxide [7-8]. Hence, it is very important toand high solid volume fraction ceramic slurry isstudy the effect of SiC powder surface oxide impu-necessary [3]. Most defects of ceramic green bodyrity on the viscosity and solid volume fraction of SiCcan be avoided and enough intensity of the greenslurry. Two kinds of SiC powder were used to studybody can be obtained by decreasing the viscosity ofthe issue mentioned above.ceramic slurry to keep it lower than 1 Pars and in-creasing the solid volume fraction of slurry to keep itExperiment and methodshigher than 50 vol.% [4]. Hence, the preparation oflow slurry viscosity and high solid volume fraction2.1 Raw materials and powder surface treatmentmethod中国煤化工ceramic slurry becomes a very important issue.There are many factors that affect the viscosityTwo|YH.CNMHGernamedSiC-1and solid volume fraction of ceramic slurry. Besidesand SiC-2 respectively were used in this study. TheCorresponding author: NING ShufanE-mail: sic_ whiskers@yahoo.comNing S.E et al, Efects of surface oxide species and contents on SiC slurry viscosity241average diameter of either kind of the powder wastable 1. There were only 0.2735 wt.% of SiO2 on the0.8 um.surface of SiC-1 powder and 0.1594 wt.% on SiC-2The SiO2 layer and the metal oxide adsorbed onafter acid leaching. Compared with the samples be-SiC powder surface were removed by acid leachingfore acid leaching, the surface colloidal SiO2 con-according to the following chemical equation:tents of SiC-1 and SiC-2 decreased 85.9% andSiO2 + 4HF←→SiF4+ 2H2O(1)96.2%, respectively. The Zeta potentials of SiC-1The process detail is as follows: 120 g SiC pow-and SiC-2 before and after acid leaching are shownder and 300 mL 10% HF solution were placed into ain figures 1 and 2. Before acid leaching, the IEP500 mL plastic beaker, strred for 24 h, and then(isoelectric point) of the SiC powder was within theleached with ditilledl water until the pH value of thebound ofpH= 2-4. After acid leaching, the IEP ofleaching water reached 7-8.Table 1 Contents of SiO2 on SiC powder surface be-fore and after acid leaching2.2 Experimental methodsSiO2 contents / wt.%The Zeta potential of SiC powder was measuredSampleBefore acid leachingAfter acid leachingwith a Zeta potential instrument (ModelSiC-11.94040.2735ZETA-SIZER4, MALVEN, England). All infraredSiC-24. 18050.1594spectra of hydroxyl (-OH) group, hydrogen bondgroup, and Si- OH group absorbed on SiC powder■SiC-1, before acid leachingsurface were measured with an F TIR spectropho-40-SiC-1, after acid leachingtometer (Model IRPrestige-21 FTIR-8400S, SHI-MADZU Corporation, KYOTO Japan). The XPSspectra ofSi- F and Si- O bonds of SiC powder sur-face were measured with a XPS instrument (Model-20ESCALABMK-II, VG SCIENTIFIC LTD. England).-40-The contents of SiC powder surface metals were-60measured by a plane grating spectrograph made inBeijing (model: WPG 100), accorded with atomic-80emission spectrometry. The slury viscosity was261012measured through the following steps: (1) 100 g SiCpHpowder was mixed with distilled water until theFigure 1 Zeta potential as a function of pH for SiC-1slurry viscosity was lower than 500 mPas, and thebefore and after acid leaching.pH value of the mixture was adjusted to 10. Then thequantity of distilled water that had been used was ,■siC-2, before acid leaching●- SiC-2, after acid leachingrecorded. (2) 2 g SiC powder was added to the slurryeach time and stirred. Then the viscosity of SiC20slurry was measured. Measurement of the sluryviscosity was performed on a viscosimeter (Model-20-NDJ-8, Shanghai Balance Instrument LTD., China).3 Results and discussion3.1 Effect of surface oxide content on Zeta poten-0tial of SiC powder中国煤化工The surface colloidal SiO2 contents, before andFigureMYHCNMHGofpHforSiC-2after acid leaching, of SiC-1 and SiC-2 are shown in242RARE METALS, Vol. 24, No. 3, Sep 2005the SiC powder was within the bound of pH = 6-7.O- ions on SiC powder surface were quite high, asAnd after acid leaching, in the alkaline solution, theshown in table 1. The 0- ion existed asmaximum absolute value of the Zeta potential ofnon-bridging oxygen: one bond connected with Sit+SiC-1 was 72.49 mV and that of SiC-2 was 63.546and another bond forming a -OH group when re-mV (here pH = 10), much greater than 60.71 mV ofacting with H2O. The hydroxyl group adsorbed H2OSiC-1 and 55.728 mV of SiC-2 before acid leaching.with hydrogen bonding force. When H2O was ad-The reasons why the Zeta potential changed aftersorbed by the hydroxyl group, this H2O took a direc-acid leaching are as follows.tional arrangement around SiC powder, because theFirst, the content of the hydroxyl (-0H) grouphydrogen bonding force was too strong for H2O toadsorbed on SiC powder surface was decreased be-move freely, as shown in figures 3(a) and 3(b).cause of acid leaching. The negative electric chargeadsorbed on SiC powder surface is obtained in twoTable 2 Contents of soluble metal ions adsorbed onSiC powder surfaceways described below [9]:(a) The OH group adsorbs on the solid-liquidSolubleSiC-1/ 10-SiC-2/ 10-6interface;metalAs AcidAAcid(b) The Si-OH group decomposes to Si-O" ac-ionsreceived leachedAl330120cording to the following chemical equation:Fe31002013015=Si-OH+OH←→=Si-O~ + H2O(2)Ca2+3Before acid leaching, the content of SiO2 wasquite high, a large number of 0- ions existed, theMg23(2OH group formed easily and the ability of SiCNat1powder surface to form negative electric charge was/ OHcomparatively good. Hence, the pH value at IEP wasjScomparatively low (pH = 2-4). After acid leaching,OHwith the sharp decrease of the contents of SiO2 and(a) Dry SiC surface0- ions and the chance of forming the OH group,the pH value at IEP changed from 2-4 to 6-7 and theHability of SiC powder surface to adsorb negative,0-H一 Oelectric charge was weakened.; Si.Second, during the acid leaching processing, notO一H一Qonly the SiO2 film but also the fractional metal oxidewas removed from SiC powder surface. Meanwhile,(b) Interfacc of SiC and H20the soluble metal ions, for example, Mg-+, Ca2, andNa, 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(C) SiC surface leached by HF2. Most of soluble metal ions adsorbed on SiC pow-Figure 3 Sketch map of SiC surface.der surface; especially, high valence ions were re-moved from SiC powder surface. This situation re-After acid leaching, the contents of SiO2 and 02sults in the thickening of double electrode layer andon SiC powder surface decreased sharply. Thisthe increase of the Zeta potential of SiC powder.situation led to the result that, on the one hand, thehydroxyL group could not form: on the other hand,3.2 Effect of SiO2 content on chemical bond-OH中国煤化工of some of thegroup adsorbed on SiC powder surfacepropert|YHC N M H Gple, ionic radius,Before acid leaching, the contents of SiO2 andionic valence, and 1onic polarnzation which are veryNing S.E et al, Efects of surface oxide species and contents on SiC slurry viscosity243similar, as shown in figure 3(c). In this case, the hy-ened, iluminating that the hydroxyl group and thedrogen bond on SiC powder suface cannot formhydrogen bond adsorbed on SiC powder surfacend H2O cannot be adsorbed either. SiC powderwere decreased.surface was changed from hydrophilic to hydropho-X-ray photolectron spectroscopy analysis canbic.also ilustrate that after acid leaching most of oxygenThe situation of SiC powder surface discussedon SiC powder surface was removed, and with theabove can be proved by FTIR analysis. The FTIRdecrease of surface oxygen content, most of OHspectra of SiC powders as received and after acidwas replaced by F，as shown in figures 5-7 andleaching are shown in figure 4. It is obvious that thetable 3.infrared absorption band at 1128 cm 1 of SiC pow-From figures 5 and 6 and table 3, it can be ob-der has been weakened after acid leaching, indicat-tained that there was a great deal of oxygening that the SiO2 content on SiC powder surface was(22.36 at%) and a lttle fuorin (0.49 at.%) on SiCdecreased by acid leaching. With the decrease ofpowder surface before acid leaching, and after acidSiO2 content, the stretching vibration band 3499leaching, the oxygen content was decreasedof the hydroxyl group, the bending vibration(15.79 at.%) and the fluorin content was increasedband 1628 cm 1 of the hydrogen bond, and the(4.63 at.%). Before acid leaching, oxygen has atretching vibration band 935 cm-' of Si- OH weak-binding energy band at 532.7 eV, corresponding to0-2300 t684.7. SiC, as recieved- SiC, HF treatedAs recieved只30-35685.3210020-3499---目20001628349190001000200030004000680685 690 695 700705Wavenumber/ cmBinding energy/eVFigure4 FTIR spectra of SiC powders as receivedFigure 5 XPS spectra ofF 1s of SiC powders.and by HF leached.2300 ,3400 |532.7SiC, as recieved100.1. SiC, HF treated2000 t100.7,532.21400 t直22001100 t.... SiC, HF treated1800 t800525530535540545中国煤化工5110Binding energy/ eVHCNMHGeVFigure 6 XPS spectra of 0 1s of SiC powders.Figure 7 XPS spectra of Si 2p of SiC powders.244RARE METALS, Vol. 24, No. 3, Sep 2005the binding energy band 532.8 eV of SiO2 (gel), andVaals force was overcome, and the powder was dis-after acid leaching, the binding energy band of oxy-persed to individual and kept a colloidal stability.gen moved to 532.15 eV, indicating that the SiO2Therefore, the SiC slurry viscosity was decreased(gel) on SiC powder surface was dissolved after acidand the solid volume fraction was increased.leaching, Si- O bonds were broken and replaced byThe hydroxyl group adsorbed on SiC powderSi- -F bonds (684.7 eV), as shown in figure 5. Thesurface was broken and the surface was changed intobinding energy 685.3 eV of Si- F moved to 684.7 eVhydrophobic because of the decrease of oxide con-after acid leaching, as shown in figure 5, becausetent. Therefore, a great deal of H2O was adsorbedthere were a few unstable- -SiF3 and -SiF2 groupstightly on SiC powder surface when the hydrogen(685.3 eV) adsorbed on the SiC powder as received,bond was released and the flow phase content ofand these unstable groups were decomposed duringslury was increased. As a result, the SiC slury vis-acid leaching and drying processing and transformedcosity was decreased and the solid volume fractioninto stable - -SiF groups (684.7 eV). Figure 7 showswas greatly increased. Especially when the averagethat before acid leaching, the binding energy banddiameter of SiC powder was only 0.8 um, its spe-Si 2p was 100.7 eV, corresponding to Si- 0 bonds,cific suface area could be 10-15 m*/g. Hence, theand after acid leaching, this value moved to 100.1 eV,effects discussed above become more important.corresponding to Si- C bonds, which ilustrated inWith combination of the two effects, the SiCthe other way that SiO2 (gel) on SiC powder surfaceslurry viscosity was decreased and the solid volumewas dissolved after acid leaching.fraction was greatly increased. When the SiC slurryviscosity is 1 Pas, the solid volume fraction ofTable 3 XPS analysis data of SiC powdersSiC-1 was increased from 51.5 vol.% as received toPeak center/ eVContent / at.%61.5 vol.% after acid leaching, and the solid volumePeaks received HF leached As received HF leachedfraction of SiC-2 was increased from 41.0 vol.% asF 1s684.15683.550.494.63received to 51.5 vol.% after acid leaching, respec-Ols532.75532.1522.3615.79tively, as shown in figures 8(a) and 8(b).Si2p100.60 .100.3042. 1761.51C ls284.65282.8534.9818.07ConclusionsFrom table 3, it can be also obtained that theThe IEP of SiC powder is changed from pH = 2-4binding energy peak position of C ls as receivedto pH = 6-7 and the absolute value of the Zeta po-was at 284.65 eV, corresponding to C- C bonds, andtential is increased in the alkaline condition by theafter acid leaching, the peak position was at 282.85acid leaching processing. At pH = 10, the maximumeV, corresponding to C- Si bonds, and the content ofabsolute value of the Zeta potential of SiC-1 isC ls decreased from 34.98 at.% to 18.07 at.%. It is72.49mV and that of SiC-2 is 63.546 mV, muchindicated that the graphite C adsorbed on SiC sur-greater than 60.71 mV of SiC-1 and 55.728 mV offace was removed by acid leaching processing.SiC-2 before acid leaching, respectively.With decreasing oxide content, the hydroxyl3.3 Relationship between surface oxide contentgroup adsorbed on SiC powder surface is brokenand SiC slurry viscosityand replaced by F . As a result, SiC powder surfaceTwo main effects of the SiC powder surface 0x-is changed from hydrophilic to hydrophobic.ide content on the viscosity of SiC slurry are dis-With the increase of the Zeta potential, the repul-cussed below.sion force between SiC powder increases, and theWith the decrease of SiC powder surface oxidepowder中国煤化工nd keeps a col-content and the increase of the Zeta potential dis-loidal SYHc N M H Groperty changescussed above, the repulsion force increased, Van derinto hyu upvuic，uc 1I2U iuecule adsorbedNing S.E et al, Efects of surface oxide species and contents on SiC slurry viscosity2452.0■SiC-2, acid washed一t - SiC-1, acid washed+ SiC-2, as recieved1.6-SiC-1, as recievede 1.2-基2g 0.8- .号“0.4-6240424446485052.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 acidleached.tightly on SiC powder surface by the hydrogen bondY-TZP aqueous suspensions, J. lnorg. 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