February 2006 AlPO, Coating on Alumina/ Mullite Fibers 1600A 1500°c 1300°C 1100°c 1000°C 1000110012001300140015001600 Sintering Temp.(°C) Fig. 7. Sintering behavior of aluminium orthophosphate 10 obility of pH. The isoelectric point (IEP) is at pH4.7. Nano AlPO4 Grey vertical line: tridymite AlPO4 forms a stable sol with a negative surface charge in the basic (JCPDF 50-54) region Dark vertical line: cristobalite Figure 5 tracks the phase evolution of AlPO4 at high tem- AlPO4 ( JCPDF 11-500) peratures. As-prepared AlPO4 is amorphous, and crystallization commences at >1000C. A mixture of"tridymite, -and cristobalite-type AlPO4 results at 1100C. The tridymite phase Fig. 5. Phase evolution of aluminium orthophosphate transforms to the cristobalite phase on increasing temperature A trace of Al2O3 appears at 1500.C DTA/TGA data(Fig. 6) exhibit an exothermic peak at 1080.C because of the trans- NH3+H2O→NH4+OH formation from amorphous to tridymitic and cristobalitic AlPO4. No weight loss occurs between 800 and 1400C, but 0. 25% weight loss occurs between 1400 and 1500C,sug- u- gesting minor AlPO4 decomposition at >1400"C. The latter ex so plains the trace of AlO3 at 1500C(Fig. 5). Both the dtA AlPOa nucleates and grows Further increase of pH endothermic peak and the 20% weight loss at <300C suggest dehydration of AlPO4. EDs defines the Al/P molar ratio be for different initial [AF+] and [HPOZ]. The mean particle size tween 1.00 and 1.05, i.e. close to stoichiometric AlPO4. Figure 7 increases from 30 to 100 nm as [AP+] increases from 0.05 to tracks the sintering behavior of APOa bulk ceramics. The green 0.5M. The size distribution is narrow for [AP*]s01M and wide density is -43%, and even though sintered at 1550C, the den- for [Ar+]=0.5M. Figure 3 shows a TEM image of the AlPO4 sity is still low(60%) precipitate for [AP=0.10M. Most particles are 20 nm AlPO4 particles have a negative surface charge at pH>5.0 ongly polyelectrolyte area of aluminum phosphate, and clusters (<50 nm)of several adsor bed on the fiber surfaces to attract the AlPO4 particles electrostatically. Figure 8 shows the fiber weight gain versus bution was obtained for [Ar+=0.1M when the ph was ad- APOA coating cycle following PDADMA burnout. The former justed by a NH4OH solution. The pH increases as the urea is linear with the number of coating cycles, i.e. 1. I wt% gain/ decomposes in situ, so AlPOA nano particles nucleate and grow cycle PDADMA. Assuming the APOa coating is uniform on uniformly. When the NH,OH solution controls the ph, AlPO a the fiber surface(diameter 12 um, 50% relative density), the nucleates and grows to a large size around the Nh4OH drops coating thickness in because of locally induced high pH values. Small particles form hean AlPO4 particle size is 0.05 um(from 0. 10M solution) in the low-pH region. Thus, a wide particle size distribution re- AlPOA multilayer coating forms on the adsorbed sults when NHOH is used as the neutralizing agen PDADMa because of the opposite surface charge. No fiber ght gain was detected on dip coating in AlPOa thout PDADMA treatment 1.2 1.0 12.0% 8.0% 70 02004006008001000120014001600 0.0% Fig. 6. Differential thermal analysis and thermogravimetric analysis curves of aluminium orthophosphate. Fig 8. Fiber weight gain versus coating cycleNH3 þ H2O ! NHþ 4 þ OH
(2) Thus, the pH shifts from acid to alkali and AlPO4 particles nu￾cleate uniformly, i.e. the pH gradually increases with time so AlPO4 nucleates and grows. Further increase of pH produces a white precipitate. Figure 2 tracks the particle size distribution for different initial [Al31] and [HPO4 2
]. The mean particle size increases from 30 to 100 nm as [Al31] increases from 0.05 to 0.5M. The size distribution is narrow for [Al31]r0.1M and wide for [Al31] 5 0.5M. Figure 3 shows a TEM image of the AlPO4 precipitate for [Al31] 5 0.10M. Most particles are B20 nm. They agglomerate after drying because of the ultra-high surface area of aluminum phosphate, and clusters (B50 nm) of several particles (B20 nm) form in the sol. A wider particle size distri￾bution was obtained for [Al31] 5 0.1M when the pH was ad￾justed by a NH4OH solution. The pH increases as the urea decomposes in situ, so AlPO4 nano particles nucleate and grow uniformly. When the NH4OH solution controls the pH, AlPO4 nucleates and grows to a large size around the NH4OH drops because of locally induced high pH values. Small particles form in the low-pH region. Thus, a wide particle size distribution re￾sults when NH4OH is used as the neutralizing agent. Figure 4 shows the electrophoretic mobility of AlPO4 versus pH. The isoelectric point (IEP) is at pHB4.7. Nano AlPO4 forms a stable sol with a negative surface charge in the basic region. Figure 5 tracks the phase evolution of AlPO4 at high tem￾peratures. As-prepared AlPO4 is amorphous, and crystallization commences at 410001C. A mixture of ‘‘tridymite,’’- and ‘‘cristobalite’’-type AlPO4 results at 11001C. The tridymite phase transforms to the cristobalite phase on increasing temperature. A trace of Al2O3 appears at 15001C. DTA/TGA data (Fig. 6) exhibit an exothermic peak at B10801C because of the trans￾formation from amorphous to tridymitic and cristobalitic AlPO4. No weight loss occurs between 8001 and 14001C, but B0.25% weight loss occurs between 14001 and 15001C, sug￾gesting minor AlPO4 decomposition at 414001C. The latter ex￾plains the trace of Al2O3 at 15001C (Fig. 5). Both the DTA endothermic peak and the 20% weight loss at o3001C suggest dehydration of AlPO4. EDS defines the Al/P molar ratio be￾tween 1.00 and 1.05, i.e. close to stoichiometric AlPO4. Figure 7 tracks the sintering behavior of AlPO4 bulk ceramics. The green density is B43%, and even though sintered at 15501C, the den￾sity is still low (B60%). AlPO4 particles have a negative surface charge at pH45.0. PDADMA is a strongly cationic polyelectrolyte. It is pre￾adsorbed on the fiber surfaces to attract the AlPO4 particles electrostatically. Figure 8 shows the fiber weight gain versus AlPO4 coating cycle following PDADMA burnout. The former is linear with the number of coating cycles, i.e. 1.1 wt% gain/ cycle PDADMA. Assuming the AlPO4 coating is uniform on the fiber surface (diameter 12 mm, 50% relative density), the coating thickness increases 0.08 mm after pre-treatment. The mean AlPO4 particle size is B0.05 mm (from 0.10M solution); thus an AlPO4 multilayer coating forms on the adsorbed PDADMA because of the opposite surface charge. No fiber weight gain was detected on dip coating in AlPO4 sol without PDADMA treatment. Grey vertical line: tridymite AlPO4 (JCPDF 50-54) Dark vertical line: cristobalite AlPO4 (JCPDF 11-500) A: alumina 10 20 30 40 50 60 2θ (°C) Intensity (arb.) 300°C 1000°C 1100°C 1300°C 1500°C 1600°C A A A A A Fig. 5. Phase evolution of aluminium orthophosphate. 60 70 80 90 100 0 200 400 600 800 1000 1200 1400 1600 Temp. (°C) DTA (uV/mg) −0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 TGA (wt%) Fig. 6. Differential thermal analysis and thermogravimetric analysis curves of aluminium orthophosphate. 40% 60% 80% 100% 1000 1100 1200 1300 1400 1500 1600 Sintering Temp. (°C) Relative Denstiy Fig. 7. Sintering behavior of aluminium orthophosphate. 0.0% 2.0% 4.0% 6.0% 8.0% 10.0% 12.0% 0 2 4 6 8 10 12 Coating Cycle Weight Gain (wt%) Fig. 8. Fiber weight gain versus coating cycle. February 2006 AlPO4 Coating on Alumina/Mullite Fibers 467