Engineering surface and electrophoretic deposition of ...

August 2001

Materials Letters 50 Ž2001. 115–119 www.elsevier.comrlocatermatlet

Engineering surface and electrophoretic deposition of SiC powder Marcos A.L. Nobre a,) , Ricardo H.R. Castro b, Douglas Gouvea b a

Instituto de Fısica de Sao 400, P.O. Box 369, CEP 13560-590, Sao ´ ˜ Carlos, USP, AÕ. Trabalhador Saocarlense, ˜ ˜ Carlos-SP, Brazil b Departamento de Engenharia Metalurgica e de Materiais, EPUSP, USP,AÕ. Prof. Mello Moraes 2463, CEP 05508-900, ´ Sao ˜ Paulo-SP, Brazil Received 26 October 2000; accepted 14 November 2000

Abstract Shape SiC tubes were prepared by electrophoresis process. The anodic deposits with thickness from 0.35 to 1.10 mm were attained using low electric field from alcoholic slurry. The SiC dispersion in ethanol with solid loading of 10 vol.% was used. Slurry dispersion degree is a function of SiC surface oxidation degree. High surface oxidation degree give rises a more stable suspension in alcoholic medium. The engineering of the surface allowed creating appreciable amount of free silanol groups ŽSi–OH.. Otherwise, absence of free silanol groups on the surface leads to non-significant or unstable deposit. Very high dispersion degree was attained by addition of a recent commercial deflocculant based on acrylic acid-acrylate copolymer. The enhanced characteristics of the dispersion and particle charge with addition of deflocculating agent are discussed. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Electrophoresis; Alcoholic slurry; Dispersion; Silanol groups; Copolymer; Acrylic acid–acrylate; SiC

1. Introduction Electrophoresis is a physical-chemistry process, in which particle with surface charged move in a liquid medium under effect of an applied potential. Electrophoresis Deposition Process ŽEPD. can be considered an advanced technique of ceramic preparation, whether complex geometry shape and flat surface are required. Actually, this technique exhibits a very interesting cost effectiveness. Complex shaped ceramics w1–3x, ceramic hollow-fibers w4,5x and coatings on the metallic substrate w6x can be easily attained. Recently, EPD has been successfully em)

Corresponding author. E-mail address: [email protected] ŽM.A.L. Nobre..

ployed as inorganic glue for joining of ceramic– ceramic materials w7,8x. A fundamental requisite for application of this technique is that the particles in suspension present high electrophoretic mobility. Comparison between water and polar solvents shows that liquid with polar character exhibits an extra advantage based on the detrimental of the hydrolyze process, a typical phenomenon of water. Otherwise, the polar solvents present relatively high dielectric constant, which allows use of high electric fields. Therefore, a low electrophoretic mobility can be compensated either by high electric field or by surface particle modification via adsorption of deflocculant molecules on the surface particle. Many studies on the dispersion and EPD of silicon carbide have been published. A small number of

00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 4 2 6 - 2

M.A.L. Nobre et al.r Materials Letters 50 (2001) 115–119

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papers have been addressed to alcoholic slurry dispersing by nonionic deflocculant. The main objective of this paper was to investigate the electrophoresis deposition of SiC from alcoholic medium with deposit thickness in the range of millimeters. The dispersion stabilization and charge aids by copolymers developed recently were investigated. New evidences of importance of the reengineering of the particle surface and its correlation with slurry stabilization are provided. In the same way, correlation between surface silanol groups and copolymer absorption was provided. Fig. 1. FTIR spectrum of SiC powder as received SiC-800F.

2. Experimental procedure 2.1. Material The silicon carbide powder used in this study was SiC-800F Alcoa ŽBrazil.. Grain size distribution analysis of SiC powder, as received, was carried out in laser Granulometer Malver model 118. Sodium hexametaphosphate ŽSynth. was used as deflocculant in water medium for this measurement. Table 1 shows the parameters width of distribution of particles Žspan., mean diameter of particle Ž Dw4,3x., median diameter where 50% of the distribution is above and 50% below this value Ž D50 ., diameter where 10% of distribution is below this value Ž D10 . and diameter where 90% of distribution is below this value Ž D 90 .. The parameter span is derived by the equation that follows span s Ž D 90 y D 10 . rD50 .

Ž 1.

The surface of the SiC powder was investigated by Fast Fourie Transform Infrared ŽFTIR. being used equipment Nicolet model Magna 560. FTIR spectrum of the SiC-800F as received is shown in Fig. 1. The spectrum analysis shows that in the surface, one

Table 1 SiC powder characteristics, as received Span

Dw4,3x Žmm.

D 90 Žmm.

D50 Žmm.

D10 Žmm.

1.38

10.74

18.64

10.18

4.60

has been very slight hydrated. Powder exhibits small intensity and broad absorption band in the range from 3550 to 3300 cmy1 . In this range of wavenumber, the absorption bands are commonly assigned to hydroxyl groups vibrations. The band, which centered at around 3380 cmy1 , has been assigned to O–H stretching for Si 3 N4 according to Wang et al. w10x. Thus, slight band shifting can be expected for SiC due structural modifications. Additionally, the band broadening has been assigned to hydrogen bonding between groups distinct of hydroxyls. Thus, the principal specie on the SiC as received in the surface were silanol groups Si–OH w9x. This specie can be considered majority at SiC surface that exhibits some degree of oxidation. 2.2. Electrophoretic deposition Suspension with 10 vol.% of solid loading was prepared. The suspension medium was ethanol alcohol ŽMerck-Analytical Grade.. Powder deflocculation and stabilization were investigated using two new commercial copolymers. The first copolymer was the acrylate–acrylamide . termed LP 10466r9B. copolymer ŽRohm-Germany ¨ The second one was acrylic acid–acrylate copolymer ŽRohm-Germany . termed LP 10466r12B. ¨ Fig. 2 shows schematic representation of electrophoretic cell. Stainless steel becker was the counter electrode Žcathode. and graphite Žanode with 0.7 mm in diameter. was the work electrode. This type of electrode was chosen to receive ceramic deposit,


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Fig. 2. Schematic of electrophoretic deposition apparatus of SiC particles from alcoholic slurry.

since under thermal treatment, as sintering, it suffers total decomposition. The work voltage was a 25-V, dc tension. A gap of 3.5 cm was used between electrodes meaning maximum field strength of 7.1 Vrcm. This data suggest a possibility of using lowtension power supply for electrodeposition. Prior to complete surface recovering by deflocculant, the suspension was mechanically stirred in high-energy disrupter during 15 s. The brief time of the mechanical treatment seems to be adequate, since more long treatment times might introduce deleterious characteristics on the surface particle. In specific, heating generated during mechanical friction might annihilate a fraction of silanol groups w10x according to the equation that follows Si`OH q HO`CH Ž CH 3 . 2 ™ Si`O`CH Ž CH 3 . 2 q H 2 O.

Ž 2.

3. Results and discussion The capability of dispersion of the acrylate– acrylamide and acrylic acid–acrylate copolymers was investigated. Both copolymers exhibit some effectiveness at promoting enhancement of dispersion stabilization of the SiC powder as received, whether

compared with ethanol–SiC dispersion without deflocculant addition. Preliminary electrodeposition shows that the acrylate–acrylamide copolymer does not promote deposit formation. This suggests that surface particle charging was not provided. This behavior can be assigned to pH of suspension of acid typically. Otherwise, the copolymer exhibits pH basic due ammonium sites deprotoned. In this way, the non-polar molecule has great probability of to be surrounded by solvent molecules, instead of going to SiC surface. However, acrylic acid–acrylate copolymer promotes slight deposit formation. The absence of the significant ceramic deposit can be explained by both insignificant charge development on the particle and low deposit strength. In this case, the deposit is disrupted by mechanical vibration during electrode-deposit removal. Recently, Pattanaik and Bhaumik w11x have emphasized that adsorption of polymers on materials surface occurs via hydrogen bond between hydroxyl groups and functional polymer groups, at least for aqueous medium. By hypothesis, the suspension stabilization and charging of SiC surface can be expected considering hydrogen bond between silanol groups and functional polymer groups in alcoholic medium. This hypothesis was investigated by two ways. In the first way, SiC as received was washed in hydrogen perox-

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of surface oxidation can be changed during calcination according to the equation that follows SiC Ž s . q O 2 Ž g . ™ SiO Ž s . q CO Ž g . .

Fig. 3. FTIR spectra of SiC washed in hydrogen peroxide and calcined at 6508C during 24 h in box type furnace and air atmosphere.

ide ŽH 2 O 2 . and dried at 708C. In the second one, SiC as received was calcined at 6508C in furnace type box during 24 h under air atmosphere. Fig. 3 shows FTIR spectra of SiC washed in hydrogen peroxide at 708C and calcined at 6508C during 24 h in air atmosphere. The spectrum of the SiC washed in hydrogen peroxide shows that the vibrations centered at around 3380 cmy1 silanol groups are not detected. Then, the silanol groups on the SiC surface as received ŽFig. 1. were removed. This is further evidence of SiC surface deprotonation of the silanol groups by hydrogen peroxide. According to Section 2.1, the influence of the deflocculant acrylic acid–acrylate copolymer on the dispersion and electrophoretic deposition of washed SiC powder with peroxide was investigated. Removal of silanol groups of the surface leads to unstable suspension, since the polymer does not attained specific site to bond on the surface. Fast settling of suspension was observed at around 5 min. Also, the electrodeposition of the SiC was not detected, indicating absence of superficial charge. The spectrum of SiC calcined at 6508C shows that silanol group vibrations are intensified when compared to those on the SiC surface as received Žsee Fig. 1.. Comparison between the integrated areas of the bands, from 3440 to 3380 cmy1 , shows that calcination leads to increasing of the integrated area at around 17%. Then, the concentration of silanol groups on the SiC surface was increased. The degree

Ž 3.

Therefore, calcination is an effective procedure to the reengineering of the SiC surface and enhancement of SiOŽs. in the surface. The effect of the deflocculant acrylic acid–acrylate copolymer on the dispersion and electrophoretic deposition of calcined SiC powder was also investigated. A great improvement of the suspension stability was observed. This is further evidence of the importance of silanol groups on the dispersion properties. In addition, slight electrodeposition was detected indicating some superficial particle charging. Electrodeposition of calcined SiC from alcoholic slurry with 3 wt.% of deflocculant acrylic acid– acrylate copolymer was successfully carried out. Fig. 4 shows deposit thickness as a function of time of the deposition. The deposits were carried out by applying electric field of 7.1 Vrcm during 15, 30, 60 and 120 s. High thickness deposits were rapidly formed depending on time. The intensity of the field presents similar influence on the deposits’ thickness, data not showed here. Therefore, the surface particle is highly charged. Since, deposition occurs on the anode Žpositive electrode. mean that the surface particle is negatively charged after polymer adsorption. As additional commentary, we highlight that superior deposition times lead to more thick deposits. Unfortunately, the adhesion of deposit to electrode is very

Fig. 4. Thickness deposit as a function of time of deposition for SiC calcined at 6508C during 24 h.


low. It seems that during the removal of electrode and deposit, its comes off under action of the own weight. Otherwise, preliminary investigation shows that giant deposit can be attained using stainless steel electrode and high field. The above discussion indicates clearly a correlation between silanol groups and adsorption of deflocculant. The mechanisms of adsorption of polymers on the oxide surface can be based on interaction of type electrostatic, covalent, hydrophobic and bonding mechanisms, as cited recently w11x. Considering aqueous slurry, stable suspension can be prepared using electrostatic andror steric stabilization. According to Harbard and Nienburg w1x, the hydrogen bond between solvent and surface particle plays an important role on the suspension stabilization. By consequence, hydrogen bond is fundamental, at least indirectly, to the enhancement of the zeta potential magnitude, which is directly correlated with particle mobility w12x. Similar behavior has been suggested for Si 3 N4 powder w10x. Therefore, in alcoholic medium, the adsorption of acrylic acid–acrylate copolymer on SiC occurs via hydrogen bonding. Naturally, this copolymer can be used by dispersing a wide class of materials since hydroxyl groups ŽOH. are present on the particle surface.

4. Conclusion The stabilization of SiC suspension in alcohol medium is possible by surface modifications. This process is performed via physical-chemistry surface changing and surface-active substance containing nitrogen, which adsorb chemically on SiC surface. SiC surface oxidation leads indirectly to the development of free silanol groups, which are the adsorption centers for acrylic acid–acrylate copolymer. After stabilization, the electrophoretic deposition of SiC powder from alcoholic slurry is successfully carried out. New evidences are provided for the adsorption of acrylic acid–acrylate copolymer on the SiC surface occurs via hydrogen bond with free silanol groups.

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Acknowledgements The Brazilian research funding institutions CNPq, CAPES and FAPESP supported this work. M.A.L. Nobre is grateful for the financial support of FAPESP under contract No. 99r03749-3. Also, the authors are grateful to Alcoa-Brazil and Rohm-Germany for ¨ the supply of silicon carbide and deflocculants, respectively.

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