E Stoll et al. /Joumal of the European Ceramic Society 26(2006)1567-1576 (adjustable posite electrode deposition electrode Aqueous or non aqueous suspension o8 Ceramic particles (usually 10-500 nm) 8°68。。08 Frame for fixing fibre mats to the elctrodode otion of the particles Fig. 1. Schematic representation of the typical EPD cell suitable for the infiltration of non-conductive fibre mats with ceramic particles(positively charged in this schema), for the fabr (ii) simultaneous infiltration of multiple Nextel M 720 fibre equipment(Sonifier 450 Branson Ultrasonic SA, Carong mats with Al2O3 particles by means of a single EPD step Geneve, Switzerland). Subsequently, the suspension was left in a newly developed EPD cell. to rest for 24 h at room temperature before being used for The electrochemical behaviour of the Nextel TM 720 fib 2. Experimental procedures mmersed in ethanol with addition of 4-HBS was charac terised using ESA signal measurements. For this experiment, 2.. Materials desized NextelTM 720 fibres were ground using a ball mill (Type P6, Fritsch Company, Germany) to a fine powder of The fibres used in this work are NextelTM 720(3M mean particle size 1.35 um, with particle sizes varying be- Corporation, St Paul,MN,USA)woven into eight-harness tween 0.5 and 2.5 um, as determined by laser scattering pa satin fabrics 26,27 The woven fabrics contain 400 individ- ticle analysis (Zetasizer, Malvern Instruments GmbH). The ual filaments with diameters between 10 and 12 um. Be- objective of the ESA signal measurements on the powered fore using the fibres for composite fabrication, they were NextelTM fibres was to characterise the surface charge ac- desized by a heat treatment at 700oC in air for 10 min. quired by the fibres when immersed in ethanol and the The 100% a-alumina powder used for the matrix was the fect of the 4-HBS dispersant on the surface charge of the commercial powder AKP-50 (Sumitomo, Chemicals, Tokyo, fibre Japan), which has a particle size distribution between 100 and 300 nm and a BET surface area of 10.6m/g(manufacturer 2.3. Composite processing 2.3.1. General description of the process 2.2. Preparation of suspensions for EPD Two different techniques were developed for the fabrication of the Al2 O3 matrix/Nextel 720 fibre com- Ethanol was chosen as the suspension medium to avoid posites. The first method is a combination of EPD of any formation of porosity in the deposits during EPD single fibre mats(individual layers) with a subsequent lam- due to gas evolution, which usually occurs in EPD from ination procedure to fabricate the multilayered composite aqueous suspensions. 8 According to previous investiga- The second method involves the simultaneous infiltration tions on the electrochemical behaviour of Al2O3 parti- of several (three or more)Nextel M 720 fibre mats by cles in ethanol, 29 suspensions containing an optimised a single EPD step in an advanced tailor-made EPD cell concentration of 25 wt %o Al2O3 powder were prepared. 4- Fig. 2 shows the flow chart describing the basic steps in- ydroxybenzoic acid (4-HBS)was used as dispersant agent volved. The following main processing parameters were aconcentration of 4 wt. of solid content. This was the op- used after a preliminary trial-and-error optimisation ap timised concentration of dispersant found by electronic sonic proach similar to the one discussed in the literature amplitude(ESA)measurements, which showed that Al2O3 constant electric voltage= 100 V and distance between elec particles in ethanol exhibit a positive charge. The suspension trodes =2 cm. In all cases, the CMCs green bodies after ras dispersed and homogenized in high frequency ultrasound EPD were dried in air at room temperature and subsequently1568 E. Stoll et al. / Journal of the European Ceramic Society 26 (2006) 1567–1576 Fig. 1. Schematic representation of the typical EPD cell suitable for the infiltration of non-conductive fibre mats with ceramic particles (positively charged in this schema), for the fabrication of oxide–oxide ceramic matrix composites. (ii) simultaneous infiltration of multiple NextelTM 720 fibre mats with Al2O3 particles by means of a single EPD step in a newly developed EPD cell. 2. Experimental procedures 2.1. Materials The fibres used in this work are NextelTM 720 (3M Corporation, St. Paul, MN, USA) woven into eight-harness satin fabrics.26,27 The woven fabrics contain ∼400 individ￾ual filaments with diameters between 10 and 12
m. Be￾fore using the fibres for composite fabrication, they were desized by a heat treatment at 700 ◦C in air for 10 min. The 100% -alumina powder used for the matrix was the commercial powder AKP-50 (Sumitomo, Chemicals, Tokyo, Japan), which has a particle size distribution between 100 and 300 nm and a BET surface area of 10.6 m2/g (manufacturer data). 2.2. Preparation of suspensions for EPD Ethanol was chosen as the suspension medium to avoid any formation of porosity in the deposits during EPD due to gas evolution, which usually occurs in EPD from aqueous suspensions.28 According to previous investiga￾tions on the electrochemical behaviour of Al2O3 parti￾cles in ethanol,29 suspensions containing an optimised concentration of 25 wt.% Al2O3 powder were prepared. 4- Hydroxybenzoic acid (4-HBS) was used as dispersant agent in a concentration of 4 wt.% of solid content. This was the op￾timised concentration of dispersant found by electronic sonic amplitude (ESA) measurements, which showed that Al2O3 particles in ethanol exhibit a positive charge. The suspension was dispersed and homogenized in high frequency ultrasound equipment (Sonifier 450 Branson Ultrasonic SA, Caronge￾Geneve, Switzerland). Subsequently, the suspension was left to rest for 24 h at room temperature before being used for EPD. The electrochemical behaviour of the NextelTM 720 fibre immersed in ethanol with addition of 4-HBS was charac￾terised using ESA signal measurements. For this experiment, desized NextelTM 720 fibres were ground using a ball mill (Type P6, Fritsch Company, Germany) to a fine powder of mean particle size 1.35
m, with particle sizes varying be￾tween 0.5 and 2.5
m, as determined by laser scattering par￾ticle analysis (Zetasizer, Malvern Instruments GmbH). The objective of the ESA signal measurements on the powered NextelTM fibres was to characterise the surface charge ac￾quired by the fibres when immersed in ethanol and the ef￾fect of the 4-HBS dispersant on the surface charge of the fibre. 2.3. Composite processing 2.3.1. General description of the process Two different techniques were developed for the fabrication of the Al2O3 matrix/NextelTM 720 fibre com￾posites. The first method is a combination of EPD of single fibre mats (individual layers) with a subsequent lam￾ination procedure to fabricate the multilayered composite. The second method involves the simultaneous infiltration of several (three or more) NextelTM 720 fibre mats by a single EPD step in an advanced tailor-made EPD cell. Fig. 2 shows the flow chart describing the basic steps in￾volved. The following main processing parameters were used after a preliminary trial-and-error optimisation ap￾proach similar to the one discussed in the literature30: constant electric voltage = 100 V and distance between elec￾trodes = 2 cm. In all cases, the CMCs green bodies after EPD were dried in air at room temperature and subsequently