tember 2000 Thermochemical Reactions and equilibria between Fluoromicas and Silicate Matrices two-phase mixture of anorthite and alumina(CaAl2Si2O8 analysis of the Mgo-buffered substrates revealed the presence of c-Al,O3, which is a well-studied, refractory, glass-ceramic matrix fine (s5 um)spinel particles comprising 2 vol% of the total phase) and fluorokinoshitalite(which is capable of withstanding anorthite matrix. temperatures up to -1460.C), to elucidate the specific thermo- chemical reactions occurring at the interface. These results will be ompared with the interfacial response observed between a MgO. ( Feldspar Preparation for Interdiffusion Study buffered, polyphase, anorthite-based matrix and fluorokinoshitalite a different glass-forming feldspar composition was used as the substrates. Thus, effective tactics for stability via Mgo buffering matrix substrate for the cationic interdiffusion studies. The specific will be established. Second. the cationic interdiffusion exhibited composition, based on the feldspar mineral celsian( BaAl2Si2O between(K, Ba)solid-solution feldspars and(K, Ba)solid-solution consists of a monoclinic(Ba, Sr, K) solid-solution phase, hereafter referenced as stabilized celsian (KoaBao sSroslo6AlL6Si24Os) fluoromicas will be examined in detail, to elucidate both the Sr+ cations are added to BaAl, Si,O% because glass compositions thermodynamic and kinetic responses at the interface. Character ization of the local equilibrium conditions that exist at these of 1Bao-1Al2O3 2SiO,(molar basis) crystallize into the high- interfaces may target specific matrix and interphase compositions emperature sian (hexane from which global thermodynamic equilibrium may be established BaAl2SiOs) 3, Hexacelsian is avoided in many glass-ceramic high-temperature, polyphase ceramic composites. In addition, a applications ily because of its large anisotropy in thermal thermodynamic model will be applied to characterize the under expansion coefficient, which causes substantial thermal-shock lying kinetic mechanisms problems, but also because of the existence of a low-temperature (300C) phase transformation from B-hexacelsian to metastable orthorhombic a-celsian. This B-c transformation involves a large Il. Experimental Approach volume decrease(0.3 vol%), which would pose serious difficul (1 Finoromica Preparation Methods ties for a hexacelsian matrix during the temperature cycling experienced in both fiber-composite fabrication and applicati The two fluoromica materials examined in this reaction study However the substitution of Sr+ cations for Ba+ cations in the were the end-member compositions of the trioctahedral fluoromica original glass composition facilitates the crystallization and stabil- solid-solution series [ Ba KL-yMg3[Al+Si3-JJOJoF2, where 0s ity of the desired monoclinic feldspar structure S x s l. fluorokinoshitalite, which is the barium-rich (x= 1) Reagent-grade oxide powders were mixed in the appropriate end-member, and fluorophlogopite, which is the potassium-rich proportions to achieve an overall stabilized-celsian composition (x=O)end-member, were prepared using analytical-reagent-grade such as that previously noted. Then, the mixed-oxide batch was xide, carbonate, and fluoride powders, mixed to produce stoichi- melted, quenched, and pulverized, following the same procedures ometric fluoromica compositions, plus 13.5 wt% additional fluo- as the above-described anorthite-based compositions. The final rine. The excess fluorine(added via the substitution of MgF, for article size of the stabilized-celsian glass powder was <10 um Mgo)compensates for the predictable halogen loss that occurs The glass powder precursor then was cold-pressed into pellets and during melting and homogenization. Consequently, the batch heat-treated at temperatures of 1240-1300C for 3-6h in ambient omposition is oxygen-poor; however, reaction with the melting environment supplies the oxygen required for mica formation air, to promote both particle sintering and glass crystallization. Powder XRD analysis of the dense substrates confirmed a fully Both batches were melted individually in a covered platinum crystalline, single-phase, stabilized-celsian matrix that possessed crucible at 1450C for 5 h in air. The resultant melt was poured clinic structure into a coherent patty, which crystallized to coarse-grained mica Then, the large mica crystals were pulverized, nergy mill, into a fine crystalline powder that had a (4) Reaction-Couple Experiments ze of 5 um. Reaction-couple specimens were produced for both the MgO- buffering and the (2) Feldspar Preparation for Magnesia-Buffering Study old-pressed, crystalline fluoromica pellet between the polished The starting polyphase, feldspar-based matrix composition used surfaces(I-um diamond grit) of two fully crystalline feldspa for the MgO-buffering study consisted of a two-phase mixture of pellets of interest. Then, the oxide sandwich was hot-pressed anorthite(CaAl2Si2O)and alumina(Al2O3) Oxide and carbonate uniaxially(l MPa) at 1200-1400C for 10-24 h in a flowing powders were mixed in the appropriate proportions to produce a starting bulk composition of anorthite that contained -3 wt% mens were heated at a rate of 200% C/h and furnace-cooled at a rate alumina. The mixed-oxide batch was melted in a platinum crucible of~400°Ch at 1650C for 10-12 h in air and then quenched by pouring The fully bonded specimens were carefully cross-sectioned directly into water. The resultant glass cullet was pulverized to a perpendicular to the fluoromica/feldspar interface with a low final particle size of <10 um. To produce the buffered anorthite speed diamond saw. Then, the surfaces of interest were ground and apositions, the previously described alumina-rich anorthite polished to a 1-um finish. After a thin(20 nm)coating of carbon der was combined with -5 wt% reagent-grade, crystal- was deposited onto the polished surface, the thermochemical O powder in a mortar and pestle. The two powders were response of the interface was characterized morphologically(using oined several times with acetone to ensure adequate mixing. microscopy(SEM)and chemically(using EDS and wavelength. powder precursors then were cold-pressed into pellet form and dispersive X-ray spectroscopy (WDS) rocessed at 1350@C for 6 h in air to sinter and fully crystallize the pellets into the desired polyphase, feldspar-based matrix substrate Although powder X-ray diffraction(XRD)analysis of the sintered Ill. Experimental Results: Description and Comment pellets confirmed the triclinic structure of fully crystalline anor- 1) Buffering with Magnesia: An Approach toward a thite, it was unable to resolve the presence of Al,O, in the Stable Fluoromica Interphase unbuffered substrates or spinel in the MgO-buffered substrates Figure I(a) is a BEl micrograph of the interfacial reaction because of their occurrence in minute concentrations. when the between the anorthite-2-vol%alumina aggregate and fluoroki- ombined techniques of backscattered electron (BED)and noshitalite processed at 1200 contrast in this imaging energy-dispersive X-ray spectroscopy(EDs) however technique is due to differences of the small(s5 um), finely dispersed particles of metric alu- atoms comprising the solid; bri ari hter cot directl were observed in the unbuffered represent holes or pores in the specimen. Thug es Micro- to the presence of heavier elements ural point-counting techniques ind final bulk compo- 98 vol% anorthite and fluorokinoshitalite appears brighter than the antwo-phase mixture of anorthite and alumina (CaAl2Si2O8 1 a-Al2O3, which is a well-studied, refractory, glass-ceramic matrix phase) and fluorokinoshitalite (which is capable of withstanding temperatures up to ;1460°C2 ), to elucidate the specific thermo￾chemical reactions occurring at the interface. These results will be compared with the interfacial response observed between a MgO￾buffered, polyphase, anorthite-based matrix and fluorokinoshitalite substrates. Thus, effective tactics for stability via MgO buffering will be established. Second, the cationic interdiffusion exhibited between (K,Ba) solid-solution feldspars and (K,Ba) solid-solution fluoromicas will be examined in detail, to elucidate both the thermodynamic and kinetic responses at the interface. Character￾ization of the local equilibrium conditions that exist at these interfaces may target specific matrix and interphase compositions from which global thermodynamic equilibrium may be established in high-temperature, polyphase ceramic composites. In addition, a thermodynamic model will be applied to characterize the under￾lying kinetic mechanisms. II. Experimental Approach (1) Fluoromica Preparation Methods The two fluoromica materials examined in this reaction study were the end-member compositions of the trioctahedral fluoromica solid-solution series [BaxK12x]Mg3[Al11xSi32x]O10F2, where 0 # x # 1. Fluorokinoshitalite, which is the barium-rich (x 5 1) end-member, and fluorophlogopite, which is the potassium-rich (x 5 0) end-member, were prepared using analytical-reagent-grade oxide, carbonate, and fluoride powders, mixed to produce stoichi￾ometric fluoromica compositions, plus 13.5 wt% additional fluo￾rine. The excess fluorine (added via the substitution of MgF2 for MgO) compensates for the predictable halogen loss that occurs during melting and homogenization.12 Consequently, the batch composition is oxygen-poor; however, reaction with the melting environment supplies the oxygen required for mica formation. Both batches were melted individually in a covered platinum crucible at 1450°C for 5 h in air. The resultant melt was poured into a coherent patty, which crystallized to coarse-grained mica during cooling. Then, the large mica crystals were pulverized, using a fluid-energy mill, into a fine crystalline powder that had a mean particle size of ;5 mm. (2) Feldspar Preparation for Magnesia-Buffering Study The starting polyphase, feldspar-based matrix composition used for the MgO-buffering study consisted of a two-phase mixture of anorthite (CaAl2Si2O8) and alumina (Al2O3). Oxide and carbonate powders were mixed in the appropriate proportions to produce a starting bulk composition of anorthite that contained ;3 wt% alumina. The mixed-oxide batch was melted in a platinum crucible at 1650°C for 10–12 h in air and then quenched by pouring directly into water. The resultant glass cullet was pulverized to a final particle size of ,10 mm. To produce the buffered anorthite compositions, the previously described alumina-rich anorthite glass powder was combined with ;5 wt% reagent-grade, crystal￾line MgO powder in a mortar and pestle. The two powders were recombined several times with acetone to ensure adequate mixing. Both the unbuffered glass and MgO-buffered glass/crystalline powder precursors then were cold-pressed into pellet form and processed at 1350°C for 6 h in air to sinter and fully crystallize the pellets into the desired polyphase, feldspar-based matrix substrate. Although powder X-ray diffraction (XRD) analysis of the sintered pellets confirmed the triclinic structure of fully crystalline anor￾thite, it was unable to resolve the presence of Al2O3 in the unbuffered substrates or spinel in the MgO-buffered substrates, because of their occurrence in minute concentrations. When the combined techniques of backscattered electron imaging (BEI) and energy-dispersive X-ray spectroscopy (EDS) were used, however, small (#5 mm), finely dispersed particles of stoichiometric alu￾mina were observed in the unbuffered anorthite substrates. Micro￾structural point-counting techniques indicated a final bulk compo￾sition of ;98 vol% anorthite and ;2 vol% alumina. Similar analysis of the MgO-buffered substrates revealed the presence of fine (#5 mm) spinel particles comprising ;2 vol% of the total anorthite matrix. (3) Feldspar Preparation for Interdiffusion Study A different glass-forming feldspar composition was used as the matrix substrate for the cationic interdiffusion studies. The specific composition, based on the feldspar mineral celsian (BaAl2Si2O8), consists of a monoclinic (Ba,Sr,K) solid-solution phase, hereafter referenced as stabilized celsian (K0.4[Ba0.5Sr0.5]0.6Al1.6Si2.4O8). Sr21 cations are added to BaAl2Si2O8 because glass compositions of 1BaOz1Al2O3z2SiO2 (molar basis) crystallize into the high￾temperature, hexagonal polymorph of celsian (hexacelsian, b-BaAl2Si2O8).13,14 Hexacelsian is avoided in many glass-ceramic applications, primarily because of its large anisotropy in thermal expansion coefficient, which causes substantial thermal-shock problems, but also because of the existence of a low-temperature (;300°C) phase transformation from b-hexacelsian to metastable, orthorhombic a-celsian. This b–a transformation involves a large volume decrease (;0.3 vol%), which would pose serious difficul￾ties for a hexacelsian matrix during the temperature cycling experienced in both fiber–composite fabrication and application.13 However, the substitution of Sr21 cations for Ba21 cations in the original glass composition facilitates the crystallization and stabil￾ity of the desired monoclinic feldspar structure.15 Reagent-grade oxide powders were mixed in the appropriate proportions to achieve an overall stabilized-celsian composition, such as that previously noted. Then, the mixed-oxide batch was melted, quenched, and pulverized, following the same procedures as the above-described anorthite-based compositions. The final particle size of the stabilized-celsian glass powder was ,10 mm. The glass powder precursor then was cold-pressed into pellets and heat-treated at temperatures of 1240°–1300°C for 3–6 h in ambient air, to promote both particle sintering and glass crystallization. Powder XRD analysis of the dense substrates confirmed a fully crystalline, single-phase, stabilized-celsian matrix that possessed the monoclinic structure. (4) Reaction-Couple Experiments Reaction-couple specimens were produced for both the MgO￾buffering and the interdiffusion studies by sandwiching one cold-pressed, crystalline fluoromica pellet between the polished surfaces (1-mm diamond grit) of two fully crystalline feldspar pellets of interest. Then, the oxide sandwich was hot-pressed uniaxially (;1 MPa) at 1200°–1400°C for 10–24 h in a flowing dry-argon environment (flow rate of ;10 cm3 /min). The speci￾mens were heated at a rate of 200°C/h and furnace-cooled at a rate of ;400°C/h. The fully bonded specimens were carefully cross-sectioned perpendicular to the fluoromica/feldspar interface with a low￾speed diamond saw. Then, the surfaces of interest were ground and polished to a 1-mm finish. After a thin (;20 nm) coating of carbon was deposited onto the polished surface, the thermochemical response of the interface was characterized morphologically (using secondary electron imaging (SEI) and BEI in scanning electron microscopy (SEM)) and chemically (using EDS and wavelength￾dispersive X-ray spectroscopy (WDS)). III. Experimental Results: Description and Comment (1) Buffering with Magnesia: An Approach toward a Stable Fluoromica Interphase Figure 1(a) is a BEI micrograph of the interfacial reaction between the anorthite–2-vol%-alumina aggregate and fluoroki￾noshitalite processed at 1200°C. The contrast in this imaging technique is due to differences in the atomic number (Z) of the atoms comprising the solid; brighter contrast corresponds directly to the presence of heavier elements. Features that appear black represent holes or pores in the specimen. Thus, in this image, fluorokinoshitalite appears brighter than the anorthite–2 vol% September 2000 Thermochemical Reactions and Equilibria between Fluoromicas and Silicate Matrices 2289