Journal of the American Ceramic Society--Bao and Nicholson Vol. 90. No. 4 Cathode Anode 0.8 E0.6 Beaker 0.4 Unidirectional fiber preform for back-filling 0.2 0.0 Insulator 0.1 Fig. 1. Schematic of the electrophoretic infiltration deposition cell for PEI concentration(wt%) fabrication of fiber-reinforced ceramic matrix composites. Fig. 2. Electrophoretic mobility of alumina with polyethyleneimine (PED addition. (PE dispersant(M.w. 10000, Polysciences, Warrington, PA) protonated with glacial acetic acid. The PEl concentration was(Fig. 2). The PEI also acts as a binder for ceramic processing. 26 determined by optimizing the electrophoretic mobility of the 0.5 wt% PEI was used to prepare the alumina suspensions alumina via a Pals Zeta potential analyzer(Brookhaven Instru for EPID ments, Holtsville, NY). Thirty volume percent alumina in eth anol with 0.5 wt% pei was ball milled for 24 h and then diluted to I vol% ension for EPD/EPID. A I vol% alumina sus- (2) Electrophoretic Deposition on Conductive Polypyrrole- pension at pH 4 adjusted with 0. 1M HCl was also used Coated Mullite/Alumina Fibers a submicron layer of conductive polypyrrole was coated onto When the fiber bundle was small. i.e. less than one tow well desized mullite/alumina(Nextel720)fibers(3M, St Paul, MN infiltrated fiber/alumina green opposites were formed. 17 via an in situ polymerization method. With polypyrrole-coated However, particles clogged the bundle surface when more fibers as the cathode and a circular stainless-steel screen as the than eight tows were involved. Figure 3 shows a cross section anode(diameter 2 cm), EPD was conducted at 10-30 V(con- of such a composite synthesized by EPd using a 0.1 mA stant voltage)or 0.05-0.2 mA(constant current) for several mi- constant current. Most alumina particles deposit around the nutes. The deposited Nextel 720 fibers were dried for 12 h in outer layer of the fiber bundle and no particles locate in the saturated ethanol atmosphere and then in air. After drying, th enter of the bundle composite was heated at 900C for 2 h and then vacuum infil trated with epoxy resin to polish for SEM characterization. Figure I shows a schematic of the EPID cell. Desized Nextel (3) Constant-Current Electrophoretic infiltration deposition 20 fibers were aligned in one direction and mounted in a plastic into Non-Conductive Mullite/Alumina Fibers Figure 4 shows a cross section of an epoxy-infiltrated fiber/alu- 25 mmx5 mmx3 mm. The opposite surface of the cathode mina, green composite fabricated by EPID(0.07 mA/cm- for 10 was covered with a plastic plate. The outer surface of the fiber- h)using a I vol% alumina suspension dispersed with 0.5 wt% bundle preform contacted the alumina suspension and alumina PEL. Submicron alumina particles particles were infiltrated there under a constant current (0.01-0.3 A/ cm). The inter-electrode distance was 2 cm. The process was continued until the fiber preform was fully backfilled with the alumina powder Central cavity a pre-weighed porous polyethylene(PE) board(Porex, Fair- with burn GA, 25 mm x 25 mm x 6 mm)with a pore size of 15-45 deposition um and 44% porosity was used as a model porous preform. This replaced the fiber preform in the EPId cell(Fig. 1). Voltage values during deposition were collected by a computer with an A/D acquisition board. A large volume of alumina suspension was used to maintain a constant concentration. The suspension conductivity was measured with a conductivity meter(Model 4100, Man-Tech Associates, Toronto, on) before and after EPID. After deposition, the surface deposit was peeled away and dried in air. The pe board was reweighed after drying. The weight gain is that of deposited alumina within. The green com- posite was vacuum infiltrated with epoxy resin to polish for SEM characterization Il. Results (1) Electrophoretic Mobility of Alumina with a PEl Dispersant Fig 3. Microstructure of conductive polypyrrole-coated Nextel 720(PEI) dispersant (M.W. 10 000, Polysciences, Warrington, PA) protonated with glacial acetic acid. The PEI concentration was determined by optimizing the electrophoretic mobility of the alumina via a Pals Zeta potential analyzer (Brookhaven Instru￾ments, Holtsville, NY). Thirty volume percent alumina in eth￾anol with 0.5 wt% PEI was ball milled for 24 h and then diluted to 1 vol% suspension for EPD/EPID. A 1 vol% alumina sus￾pension at pH 4 adjusted with 0.1M HCl was also used. A submicron layer of conductive polypyrrole was coated onto desized mullite/alumina (Nextelt 720) fibers (3M, St. Paul, MN) via an in situ polymerization method.17 With polypyrrole-coated fibers as the cathode and a circular stainless-steel screen as the anode (diameter 2 cm), EPD was conducted at 10–30 V (con￾stant voltage) or 0.05–0.2 mA (constant current) for several mi￾nutes. The deposited Nextel 720 fibers were dried for 12 h in a saturated ethanol atmosphere and then in air. After drying, the composite was heated at 9001C for 2 h and then vacuum infil￾trated with epoxy resin to polish for SEM characterization. Figure 1 shows a schematic of the EPID cell. Desized Nextel 720 fibers were aligned in one direction and mounted in a plastic mold attached to a metal plate (cathode). The fiber preform was 25 mm
5 mm
3 mm. The opposite surface of the cathode was covered with a plastic plate. The outer surface of the fiber￾bundle preform contacted the alumina suspension and alumina particles were infiltrated there under a constant current (0.01–0.3 mA/cm2 ). The inter-electrode distance was 2 cm. The process was continued until the fiber preform was fully backfilled with the alumina powder. A pre-weighed porous polyethylene (PE) board (Porex, Fair￾burn GA, 25 mm
25 mm
6 mm) with a pore size of 15–45 mm and 44% porosity was used as a model porous preform. This replaced the fiber preform in the EPID cell (Fig. 1). Voltage values during deposition were collected by a computer with an A/D acquisition board. A large volume of alumina suspension was used to maintain a constant concentration. The suspension conductivity was measured with a conductivity meter (Model 4100, Man-Tech Associates, Toronto, ON) before and after EPID. After deposition, the surface deposit was peeled away and dried in air. The PE board was reweighed after drying. The weight gain is that of deposited alumina within. The green com￾posite was vacuum infiltrated with epoxy resin to polish for SEM characterization. III. Results (1) Electrophoretic Mobility of Alumina with a PEI Dispersant Alumina particles are positively charged in ethanol. The elec￾trophoretic mobility increased on PEI dispersant addition (Fig. 2). The PEI also acts as a binder for ceramic processing.26 0.5 wt% PEI was used to prepare the alumina suspensions for EPID. (2) Electrophoretic Deposition on Conductive Polypyrrole￾Coated Mullite/Alumina Fibers When the fiber bundle was small, i.e., less than one tow, well￾infiltrated fiber/alumina green mini-composites were formed.17 However, particles clogged the bundle surface when more than eight tows were involved. Figure 3 shows a cross section of such a composite synthesized by EPD using a 0.1 mA constant current. Most alumina particles deposit around the outer layer of the fiber bundle and no particles locate in the center of the bundle. (3) Constant-Current Electrophoretic Infiltration Deposition into Non-Conductive Mullite/Alumina Fibers Figure 4 shows a cross section of an epoxy-infiltrated fiber/alu￾mina, green composite fabricated by EPID (0.07 mA/cm2 for 10 h) using a 1 vol% alumina suspension dispersed with 0.5 wt% PEI. Submicron alumina particles are uniformly infiltrated + Beaker Anode Cathode Unidirectional fiber preform for back-filling Stirrer Suspension Polymer Insulator _ Fig. 1. Schematic of the electrophoretic infiltration deposition cell for fabrication of fiber-reinforced ceramic matrix composites. 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0 0.1 0.2 0.3 0.4 0.5 0.6 PEI concentration (wt%) Mobility (µm/s)/(V/cm) Fig. 2. Electrophoretic mobility of alumina with polyethyleneimine (PEI) addition. 1mm Central cavity with no deposition Fig. 3. Microstructure of conductive polypyrrole-coated Nextel 720 fiber/Al2O3 composite by electrophoretic deposition with an 8-tow fiber bundle. 1064 Journal of the American Ceramic Society—Bao and Nicholson Vol. 90, No. 4