Joumal of the American Ceramic Sociery-Boakye et al. Vol 87. No. 10 Table L. Coated Nextel M 720 Fiber Strength for Various Coating Precursors, Heat Treatment Temperatures, Filament strength(GPa). Weibull modulus Argon, as coated Air. I h Coating temperature at-treat temperature Heat-treat temperature Precursor O0°C 1100° 1200°C 600°C 00°C ZNS-C 195,631.88,6.7 69,4.7 1.83.6 1.12.5.3 ZNS-C 93,5.4 2.35*,6.0 ZNS-C 0.19*,1.3 ZP. 1.50,4.8 STrength values are an average from 50 tests. Precursor weight loss(percent: see Fig. 4)above 1000C also is shown. indicates single-filament strengths computed from tow tests(Eq.(I); the Weibull modulii accompanying these values are those measured thanol: TEOS molar ratio of 5.8 and water: TEOS molar ratio of (5) ZP Precursor harge of hydrous Sioz and hydrolyzed ZN are -2 and-10-ll, ethanol and refluxed at 60%C for 4 h. PA (15 g) was added as the mixture was -1. At a pH of 1, Pa electrosterically interacted with 125 g/L and the ZrO PA weight ratio was 40: 60. Precursor the hydrated Sioz and the zirconyl nitrate hydrate to form a stable without Pa is referred to as ZP, and precursor with PA is referred to as ZP-C The ZrO2: SiO2: PA weight ratio was 27 ZrO,-SiO,carbon concentration of 40 iscosities of 1.26, 2.44, and 5.32 cP(1 cP= 1 X 10 N-s/m2) (6) Precursor Characterization respectively. Concentration refers to the ZrO2-SiO2-carbon yield after heat treatment at 140 C for 72 h. Precursor without Pa is viscosities were measured with a programmable referred to as ZNS, and precursor with PA is referred to as ZNS-C rheometer (Model DV-lll, Brookfield Engineering Laboratories Stoughton, MA)at a shear rate of 1/300s. Differential thermal analysis (DTA)and thermogravimetric analysis (TGA)were con- (3) ZES Precursor ducted (Model STA-409, Netzsch, Bayern, Germany). Powder The double alkoxide method was used to test the effects of samples for DTA and TGa were heat-treated at 140C for 18 h and molecular-scale mixing on ZrSiO formation. Zirconium ethoxide X-ray powder diffractometry (XRD)analyses were conducted (ZEOS, 10 g)and TEOS(8.6 g) were added to 1100 cm of Powder samples were heat-treated at 9000-1400C for I h (Model absolute ethanol in a dry box. Ethanol was distilled before use. The Rotaflex, Rigaku Co., Tokyo, Japan). The carbon concentration mixture was refluxed at 78 C for 18 h. For a concentration of 18 was evaluated from TGA curves for each preheat temperature. The g/L, 200 cm of the mixture was extracted, and ammonium ZrSiOa concentration was calculated from the ratio of the peak vanadate and dilute HNO, were added and hydrolyzed at 50 C for intensities for 20= 26.98(ZrSiO4) and the sum of the peak 24 h at pH 2. The water: (TEOS +ZEOS)molar ratio was 20. The intensities of 20=30.17(1-ZrO2), 20=2819%(m-ZrO2), and ZrO2-SiO carbon concentration was limited to 15 g/L by the ZrSiO4 Crystallite sizes were estimated for 1-ZrO2, m-ZrO2, and solubility of ZEOS in ethanol. The ZrO,: SiO,: PA weight ratio was ZrSiO4 using the Scherer formula 27: 13: 60. PA was added as the carbon precursor. Precursor without PA is referred to as ZES, and precursor with PA is referred to as ZES-C (7) Fiber Coating Nextel 720(AlOx-mullite)and Hi-Nicalon-STM(SiC)fiber (4) ZN Precursor tows were coated using the ZNS-C precursor Characteristics of Zirconyl nitrate hydrate(ZN, 25 g) was dissolved in 200 mL of these fibers have been reviewed elsewhere 33,36,58-60 ZNS-C water and refluxed at 60oC for 4 h. PA(15 g) was added. The precursor concentrations of 40-160 g/L were used. The ZES ZrO2-C sol concentration was 125 g/L and the Zro, PA weight precursor was not used, because it had relatively low concentration ratio was 40: 60. Precursor without Pa is referred to as zN. and and high viscosity. Nextel 720 also was coated using the ZP-C and precursor with Pa is referred to as ZN-C ZN-C precursors, using precursor concentrations of 125 g/L. A continuous vertical coater that used hexadecane for immiscible luid displacement was used for fiber coating 6.62 For Nextel 720, the hexadecane layer thickness was varied from 0 to 20 cm to Table Il. Al,O3 Grain Size (Major Axis) study the effect on coating thickness and uniformity. Nextel 720 after Heat Treatment at various coatings were heat-treated in-line at 1000-1300%C and Hi- Te peratures and Times for Nextel M 720 Nicalon-S coatings were heat-treated in-line at 1000%-1600C. al Coated with ZNS-C and zP-C Precursors" at a speed of 1. 4 cm/s in argon. Coated Nextel 720 and Hi- Heat treatment log grain size(nm) Nicalon-S were given further heat treatments in air and argon for 1-100 h at temperatures from 600 to 1200C (Tables I and D) 1.80±0.16 The fiber coatings were characterized using scanning electron microscopy(SEM: Model FEG, Leica, Buffalo, NY) and trans- 000/100 1.80±0.12 mission electron microscopy (TEM: Model CM 200 FEG, Philli 200/100 2.05±0.1 Eindhoven, The Netherlands). Coating phases were determined ZP-O 1000/1 195±0.16 using analytical TEM from energy-dispersive spectroscopy(EDS) in spot mode(5 nm) and from ring patterns obtained by selected area diffraction spectroscopy (SADS)of the fine-grained coatingsethanol:TEOS molar ratio of 5.8 and water:TEOS molar ratio of 10. PA was added as the carbon precursor. The point of zero charge of hydrous SiO2 and hydrolyzed ZN are
2 and
10 –11, respectively.54 –56 The pH of the zirconyl nitrate hydrate–TEOS mixture was
1. At a pH of 1, PA electrosterically interacted with the hydrated SiO2 and the zirconyl nitrate hydrate to form a stable colloidal dispersion. The ZrO2:SiO2:PA weight ratio was 27:13:60. Precursors with ZrO2–SiO2– carbon concentration of 40, 80, and 160 g/L had viscosities of 1.26, 2.44, and 5.32 cP (1 cP  1 10–3 N
s/m2 ), respectively. Concentration refers to the ZrO2–SiO2– carbon yield after heat treatment at 140°C for 72 h. Precursor without PA is referred to as ZNS, and precursor with PA is referred to as ZNS-C. (3) ZES Precursor The double alkoxide method was used to test the effects of molecular-scale mixing on ZrSiO4 formation. Zirconium ethoxide (ZEOS, 10 g) and TEOS (8.6 g) were added to 1100 cm3 of absolute ethanol in a dry box. Ethanol was distilled before use. The mixture was refluxed at 78°C for 18 h. For a concentration of 18 g/L, 200 cm3 of the mixture was extracted, and ammonium vanadate and dilute HNO3 were added and hydrolyzed at 50°C for 24 h at pH 2. The water:(TEOS  ZEOS) molar ratio was 20. The ZrO2–SiO2– carbon concentration was limited to 15 g/L by the solubility of ZEOS in ethanol. The ZrO2:SiO2:PA weight ratio was 27:13:60. PA was added as the carbon precursor. Precursor without PA is referred to as ZES, and precursor with PA is referred to as ZES-C. (4) ZN Precursor Zirconyl nitrate hydrate (ZN, 25 g) was dissolved in 200 mL of water and refluxed at 60°C for 4 h. PA (15 g) was added. The ZrO2–C sol concentration was 125 g/L and the ZrO2:PA weight ratio was 40:60. Precursor without PA is referred to as ZN, and precursor with PA is referred to as ZN-C. (5) ZP Precursor Zirconium propoxide (ZP, 26 g) was dissolved in 200 mL of ethanol and refluxed at 60°C for 4 h. PA (15 g) was added as the carbon precursor. The concentration of the ZrO2–C precursor was 125 g/L and the ZrO2:PA weight ratio was 40:60. Precursor without PA is referred to as ZP, and precursor with PA is referred to as ZP-C. (6) Precursor Characterization Precursor viscosities were measured with a programmable rheometer (Model DV-III, Brookfield Engineering Laboratories, Stoughton, MA) at a shear rate of 1/300 s1 . Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were con￾ducted (Model STA-409, Netzsch, Bayern, Germany). Powder samples for DTA and TGA were heat-treated at 140°C for 18 h and X-ray powder diffractometry (XRD) analyses were conducted. Powder samples were heat-treated at 900°–1400°C for 1 h (Model Rotaflex, Rigaku Co., Tokyo, Japan). The carbon concentration was evaluated from TGA curves for each preheat temperature. The ZrSiO4 concentration was calculated from the ratio of the peak intensities for 2  26.98° (ZrSiO4) and the sum of the peak intensities of 2  30.17° (t-ZrO2), 2  28.19° (m-ZrO2), and ZrSiO4. Crystallite sizes were estimated for t-ZrO2, m-ZrO2, and ZrSiO4 using the Scherer formula.57 (7) Fiber Coatings Nextel 720 (Al2O3–mullite) and Hi-Nicalon-STM (SiC) fiber tows were coated using the ZNS-C precursor. Characteristics of these fibers have been reviewed elsewhere.33,36,58 – 60 ZNS-C precursor concentrations of 40 –160 g/L were used. The ZES precursor was not used, because it had relatively low concentration and high viscosity. Nextel 720 also was coated using the ZP-C and ZN-C precursors, using precursor concentrations of 125 g/L. A continuous vertical coater that used hexadecane for immiscible liquid displacement was used for fiber coating.61,62 For Nextel 720, the hexadecane layer thickness was varied from 0 to 20 cm to study the effect on coating thickness and uniformity. Nextel 720 coatings were heat-treated in-line at 1000°–1300°C and Hi￾Nicalon-S coatings were heat-treated in-line at 1000°–1600°C, all at a speed of 1.4 cm/s in argon. Coated Nextel 720 and Hi￾Nicalon-S were given further heat treatments in air and argon for 1–100 h at temperatures from 600° to 1200°C (Tables I and II). The fiber coatings were characterized using scanning electron microscopy (SEM; Model FEG, Leica, Buffalo, NY) and trans￾mission electron microscopy (TEM; Model CM 200 FEG, Phillips, Eindhoven, The Netherlands). Coating phases were determined using analytical TEM from energy-dispersive spectroscopy (EDS) in spot mode (5 nm) and from ring patterns obtained by selected area diffraction spectroscopy (SADS) of the fine-grained coatings. Table I. Coated Nextel™ 720 Fiber Strength for Various Coating Precursors, Heat Treatment Temperatures, and Atmospheres† Precursor Precursor weight loss (%) Filament strength (GPa), Weibull modulus Argon, as coated Argon, 1 h temperature Air, 1 h Coating temperature Heat-treat temperature Heat-treat temperature 1000°C 1100°C 1200°C 1000°C 1200°C 600°C 1000°C ZNS-C 1.95, 6.3 1.88, 6.7 1.69, 4.7 ZP-C 1.83, 6.2 ZN-C 1.12, 5.3 ZNS-C 1.93, 5.4 2.05*, 5.0 2.35*, 6.0 ZNS-C 0.13 0.19*, 1.3 ZP-C 1.34 1.55*, 5.4 ZN-C 0.90 0.93*, 5.2 No coating 1.50, 4.8 † Strength values are an average from 50 tests. Precursor weight loss (percent; see Fig. 4) above 1000°C also is shown. * indicates single-filament strengths computed from tow tests (Eq. (1)); the Weibull modulii accompanying these values are those measured for tows. Table II. Al2O3 Grain Size (Major Axis) after Heat Treatment at Various Temperatures and Times for Nextel™ 720 Coated with ZNS-C and ZP-C Precursors† Heat treatment log grain size(nm) As received 1.80 0.16 ZNS-C 1000/1 1.80 0.13 1000/100 1.80 0.12 1200/100 2.05 0.12 ZP-C 1000/1 1.95 0.16 † All grain-size distributions were lognormal, and the standard deviations of the distributions are shown. 1968 Journal of the American Ceramic Society—Boakye et al. Vol. 87, No. 10