Journal of the American Ceramic Society-Bao and Nicholson Vol. 89. No. 2 DEsizing fibers A Treating fiber surfaces with PDADMA Electrostatic attraction to coat fibers with AlPO4 particles Nashing coated fibers Drying coated fibers Heat treatment of coated fibers if necessary smission electron spectroscopy image of aluminium ortho- (4) Characterization of the AlPO, Sol, Coating, and the Is coating of desired Fracture Surface of AlPOrCoated Fiber-Reinforced Al2O3 The electrophoretic mobility of AlPO4(Pals Zeta Potential anal zer, BIC, Holtsville, NY) versus pH was measured, adjusting Fig. 1. Flow chart of the electrostatic attraction coating. the latter with 0.01M HNO3 or 0.01M NH4OH solution. The AlPO4 particle size( Capa-700 particle analyzer, Horiba, Kyoto, Japan)was measured at pH 7.5 and also observed by con- St Louis, MO) for 30 min to induce a positive surface charge on ventional tranmission electron spectroscopy (TEM) imaging the fibers. After washing with distilled water to remove excess AlPO4 powder phase evolution(Miniflex X-ray diffractome- PDADMA, the bundle was soaked in 0.05M AlPO4 sol for ter, Rigaku, Tokyo, Japan) was determined after heating to dif- 30 min and then washed with distilled water to remove excess ferent temperatures for 2 h. Differential thermal analysis/ AlPO4 and finally dried at 110C. The coated fiber bundle was thermogravimetric analysis DTA-TGA)(STA 409PC, Netzsch heated at 1100.C for I h to develop bonding between the AlPO GmbH, Hamburg, Germany) was conducted with a tempera and the fibers. The coating cycle was repeated until the desired ure ramp of 10.C/min. Energy-dispersive spectroscopy(EDS) thickness was obtained. Figure 1 illustrates the procedure of was used to analyze the composition of the washed powder com- electrostatic attraction coating pared with an AlPO4 standard prepared by heating Al(NO3)3 NHa]HPO4 at 1000.C. AlPO4 pellets were uniaxially pressed (3) Alumina/Mullite Fiber-Reinforced Al,O, Composites at 120 MPa with AlPO4 nano powder preheated at 300C TM-DAR ALO, powder (Taimei Chemicals, Tokyo, Japan) and sintered at 1100% C for 2 h to determine sintering with a low sintering temperature(1300.C)was used to prepare behavior. The relative green and fired density was calculated fiber/Al2O3 composites. Several AlPO4-coated fiber tows were via weight and dimension( theoretical density 2.6 g/cm) The AlPO4 coating morphology and fracture surfaces of the infiltrated with a 10 vol% Al2 O3 aqueous slurry at pH-40 un- fiber composites were examined by scanning electron micros der vacuum and then dried, embedded in alumina in a graph die and hot-pressed at 1250C/20 MPa for I h. a pure TM- copy(SEM). EDS was conducted on the coating. DAR alumina pellet was also hot-pressed for comparison. The density of the hot-pressed composites was measured by the II. Results and discuss Urea in aqueous solution decomposes at elevated temperatures and shifts the pH of the AlPO4 sol from acid to alkali, i.e. CO(NH)2+H20-CO2+ 2NH3 90 05MA3 60 0.25 10 00.050.10.150.20250.3 Particle Size (um) 125 Fig. 2. Aluminium orthophosphate particle size distribution via urea decomposition Fig 4. Electrophoretic mobility of aluminium orthophosphate particles.St. Louis, MO) for 30 min to induce a positive surface charge on the fibers. After washing with distilled water to remove excess PDADMA, the bundle was soaked in 0.05M AlPO4 sol for 30 min and then washed with distilled water to remove excess AlPO4 and finally dried at 1101C. The coated fiber bundle was heated at 11001C for 1 h to develop bonding between the AlPO4 and the fibers. The coating cycle was repeated until the desired thickness was obtained. Figure 1 illustrates the procedure of electrostatic attraction coating. (3) Alumina/Mullite Fiber-Reinforced Al2O3 Composites TM-DAR Al2O3 powder (Taimei Chemicals, Tokyo, Japan) with a low sintering temperature (13001C) was used to prepare fiber/Al2O3 composites. Several AlPO4-coated fiber tows were infiltrated with a 10 vol% Al2O3 aqueous slurry at pHB4.0 un￾der vacuum and then dried, embedded in alumina in a graphite die and hot-pressed at 12501C/20 MPa for 1 h. A pure TM￾DAR alumina pellet was also hot-pressed for comparison. The density of the hot-pressed composites was measured by the Archimedes method. (4) Characterization of the AlPO4 Sol, Coating, and the Fracture Surface of AlPO4-Coated Fiber-Reinforced Al2O3 Composites The electrophoretic mobility of AlPO4 (Pals Zeta Potential anal￾yzer, BIC, Holtsville, NY) versus pH was measured, adjusting the latter with 0.01M HNO3 or 0.01M NH4OH solution. The AlPO4 particle size (Capa-700 particle analyzer, Horiba, Kyoto, Japan) was measured at pH B7.5 and also observed by con￾ventional tranmission electron spectroscopy (TEM) imaging. AlPO4 powder phase evolution (Miniflex X-ray diffractome￾ter, Rigaku, Tokyo, Japan) was determined after heating to dif￾ferent temperatures for 2 h. Differential thermal analysis/ thermogravimetric analysis (DTA–TGA) (STA 409PC, Netzsch GmbH, Hamburg, Germany) was conducted with a tempera￾ture ramp of 101C/min. Energy-dispersive spectroscopy (EDS) was used to analyze the composition of the washed powder com￾pared with an AlPO4 standard prepared by heating Al(NO3)3 (NH4)2HPO4 at 10001C. AlPO4 pellets were uniaxially pressed at 120 MPa with AlPO4 nano powder preheated at 3001C and sintered at 11001–15501C for 2 h to determine sintering behavior. The relative green and fired density was calculated via weight and dimension (theoretical density 2.6 g/cm3 ). The AlPO4 coating morphology and fracture surfaces of the fiber composites were examined by scanning electron micros￾copy (SEM). EDS was conducted on the coating. III. Results and Discussion Urea in aqueous solution decomposes at elevated temperatures and shifts the pH of the AlPO4 sol from acid to alkali, i.e., COðNH2Þ2 þ H2O ! CO2 þ 2NH3 (1) Desizing fibers Treating fiber surfaces with PDADMA Electrostatic attraction to coat fibers with AlPO4 particles Washing coated fibers Drying coated fibers Heat treatment of coated fibers if necessary Is coating of desired thickness? No Fig. 1. Flow chart of the electrostatic attraction coating. 0 10 20 30 40 50 60 70 80 90 100 0 0.05 0.1 0.15 0.2 0.25 0.3 Particle Size (µm) Cumulative #% 0.1M Al3+ 0.5M Al3+ 0.05M Al3+ Fig. 2. Aluminium orthophosphate particle size distribution via urea decomposition. Fig. 3. Transmission electron spectroscopy image of aluminium ortho￾phosphate from [Al31] 5 0.1M solution. −1.25 −1 −0.75 −0.5 −0.25 0 0.25 0.5 2 3 4 5 6 7 8 9 10 pH Mobility (u/s)/(V/cm) Fig. 4. Electrophoretic mobility of aluminium orthophosphate particles. 466 Journal of the American Ceramic Society—Bao and Nicholson Vol. 89, No. 2