Thin fly ash/ ladle furnace slag geopolymer: Effect of elevated temperature exposure on flexural properties and morphological characteristics

The flexural properties and thermal performance of 10 mm-thin geopolymers made from fly ash and ladle furnace slag were evaluated before and after exposure to elevated temperatures (300 °C, 600 °C, 900 °C, 1100 °C and 1150 °C). Class F fly ash was mixed with liquid sodium silicate (Na₂SiO₃) and 12 M sodium hydroxide (NaOH) solution using aluminosilicate/activator ratio of 1:2.5 and Na₂SiO₃/NaOH ratio of 1:4 to synthesise thin fly ash (FA) geopolymers. 40 wt% of ladle furnace slag was partially replacing fly ash to produce fly ash/slag-based (FAS) geopolymers. Thermal treatment enhanced the flexural strength of thin geopolymers. In comparison to the unexposed specimen, the flexural strength of FA geopolymers at 1150 °C and FAS geopolymers 1100 °C was increased by 161.3% to 16.2 MPa and 208.9% to 24.1 MPa, respectively. A more uniform heating was achieved in thin geopolymers which favoured the phase transformation at high temperatures and contributed to the substantial increase in flexural strength. The joint effect of elevated temperature exposure and the incorporation of ladle furnace slag further improved the flexural strength of thin geopolymers. The calcium-rich slag refined the pore structure and increased the crystallinity of thin geopolymers which aided in high strength development.     

Fast microwave syntheses of fly ash based porous geopolymers in the presence of high alkali concentration

Microwave synthesis of porous fly ash geopolymers was achieved using a household microwave oven. Fly ash paste containing SiO₂ and Al₂O₃ component was mixed with sodium silicate (Na₂SiO₃) solutions at different concentrations of sodium hydroxide (NaOH) of 2, 5, 10, and 15 M, which were used as NaOH activators of geopolymerization. The mass ratio of Na₂SiO₃/NaOH was fixed at 2.5 with SiO₂/Al₂O₃ at 2.69. After the fly ash and alkali activators were mixed for 1 min until homogeneous, the geopolymer paste was cured for 1 min using household microwave oven at different output powers of 200, 500, 700, and 850 W. Porous geopolymers were formed immediately. Micro X-ray CT and SEM results showed that the porous structure of the geopolymers was developed at higher NaOH concentrations when using 850 W power of the microwave oven. These results derive from the immediate increase of the temperature in the geopolymer paste at higher NaOH concentrations, meaning that aluminosilicate bonds formed easily in the geopolymers within 1 min.     

Capillary rise properties of porous geopolymers prepared by an extrusion method using polylactic acid (PLA) fibers as the pore formers

The geopolymers were prepared from sodium silicate, metakaolinite, NaOH and H₂O at SiO₂:Al₂O₃:Na₂O:H₂O of 3.66:1:1:x, where x = 8–17, and curing temperatures of 70–110 °C. Since the bending strength of the geopolymers was highest (36 MPa) where H₂O/Al₂O₃ = 9 and the curing temperature = 90 °C, these conditions were adopted. The porous geopolymers were prepared by kneading PLA fibers of 12, 20 and 29 μm diameter into the geopolymer paste, at fiber volumes of 13–28 vol%. The resulting paste was extruded using a domestic extruder, cured at 90 °C for 2 days then dried at the same temperature. The PLA fibers in the composites were removed by alkali treatment and/or heating. The highest capillary rise was achieved in the porous geopolymers containing 28 vol% of 29 μm fibers. The capillary rise of this sample, estimated by the equation of Fries and Dryer¹ was 1125 mm.     

Two-step process for synthesis of a single phase Na–A zeolite from coal fly ash by dialysis

Using the amorphous aluminosilicate in coal fly ash (FA), a single phase Na–A zeolite was synthesized from FA by dialysis. The FA and NaOH solution added into the tube made by semipermeable membrane were pretreated in the same NaOH solution at 85 °C for 24 h. After the pretreatment, the tube was removed and NaOH–NaAlO₂ solution was added into the residual solution to control SiO₂/Al₂O₃ molar ratio of the solutions from 0.9 to 4.3. The precipitates thus formed were aged for 24 h at 85 °C. The amorphous aluminosilicate in FA was dissolved during the pretreatment. When the NaOH–NaAlO₂ solution was added into the solution after the pretreatment and then aged, white precipitates were yielded over the whole SiO₂/Al₂O₃ range. At SiO₂/Al₂O₃=0.9, the material formed was identified as a single phase Na–A zeolite. The Na–X zeolite was slightly produced at SiO₂/Al₂O₃≥1.7.     

Synthesis and thermal properties of inorganic polymers (geopolymers) for structural and refractory applications from volcanic ash

The volcanic ash occurring as an abundant and readily accessible natural resource in the Central African country of Cameroon was used to synthesize aluminosilicate geopolymers using sodium hydroxide as the sole alkaline activator. Both the curing conditions and the Na₂O/SiO₂ molar ratio were found to influence the development of compressive strength of the geopolymer cement paste, which achieved a maximum strength of 55 MPa at Na₂O/SiO₂ = 0.3. The formation of a mortar by the addition of 40 wt% sand to the optimized geopolymer cement composition reduced the compressive strength to 30 MPa, still within the useful range for construction applications. The geopolymers consist largely of X-ray amorphous material with a small content of crystalline phases. Scanning electron microscopy showed a homogenously distributed mixture of lath-shaped and agglomerated morphologies, with a homogeneous distribution of Si, Al and O in the geopolymer matrix. The geopolymers are relatively stable to heat, shrinking only slowly and retaining about 60% of their as synthesized compressive strength on heating to 900 °C. The FTIR spectra of both the as synthesized and heated geopolymers show two broad absorbance bands, between 820-1250 cm⁻¹ and 450-730 cm⁻¹ assigned to the internal vibrations of Si-O-Si, and Si-O-Al respectively. The compressive strengths and the thermal stability of these materials suggest their suitability for building applications and low-grade refractories.     

XRD Analysis of the Role of Cesium in Sodium‐Based Geopolymer

The purpose of this work was to study the role of cesium in sodium‐based geopolymer and its thermal stability for nuclear waste management. A series of mixed sodium and cesium geopolymer samples (Na_1?x Cs _x )₂O·Al₂O₃·SiO₂·12H₂O (referred to as (Na_{1? x} Cs _x )‐GP, where x = 0, 0.08, 0.15, 0.42, 1) have been prepared. All geopolymer samples were heated at 1100°C for 24 h. Pollucite (CsAlSi₂O₆) and feldspathoid (CsAlSiO₄) were crystallized from Cs‐GP. Nepheline (NaAlSiO₄) and a small amount of crystallized silica were obtained from Na‐GP. The other geopolymers (Na_{1? x} Cs _x )‐GP (x = 0.08, 0.15, 0.42) led to pollucite and nepheline main phases. Amorphous silica phase was observed in all the geopolymer samples with various amounts. Phase quantification and scanning electron microscope revealed that higher Cs concentrations in Na‐GP tend to decrease the amorphous phase while improving pollucite and nepheline phase quantification. The amorphous geopolymers have also been studied by pair distribution function analysis. Tetrahedral chains formed by T–O bonding (with T = Si, Al) were shown to be more tighten around Cs⁺ than around Na⁺. It led to shorter Cs–T bond than Na–T bond matching the higher solvation property of Na⁺. Furthermore, thermal study analysis pointed out the fact that geopolymer samples (Na_{1? x} Cs _x )‐GP, can be considered as solid solutions.     

The Coexistence of the Poly(phospho-siloxo) Networks and Calcium Phosphates on the Compressive Strengths of the Acid-Based Geopolymers Obtained at Room Temperature

This work aims to investigate the coexistence of the poly(phospho-siloxo) networks and calcium phosphates on the compressive strengths of the acid-based geopolymers obtained at room temperature. Waste fired brick and phosphoric acid were used as an aluminosilicate and chemical reagent, respectively. Calcium aluminate hydrate was prepared by mixing calcium hydroxide from the calcined eggshell and calcined bauxite. Calcium silicate hydrate was obtained by the mixture of rice husk ash and calcium hydroxide. The molar ratios CaO/Al₂O₃ and CaO/SiO₂ in the calcium aluminate and calcium silicate hydrates are equals to 1.0. The X-ray patterns of the acid-based geopolymers indicate the broad hump structure between 18 and 38°(2θ). In addition to this broad band, those from the mixture of calcium sources show the reflection peaks of monetite and brushite. The compressive strength of the reference is 56.43 MPa. Those obtained with the addition of 10, 20, 40 and 50 g of calcined eggshell are 30.15, 22.85, 21.16 and 13.47 MPa, respectively. The ones from calcium aluminate hydrate are 32.62, 31.58, 17.83 and 16.33 MPa, respectively. Whereas those containing calcium silicate hydrate are 44.02, 42.71, 40.19 and 18.59 MPa, respectively. This work demonstrates that the formation of calcium phosphates in the structure of the acid-based geopolymers decreases the poly(phospho-siloxo) chains and therefore reduces their compressive strengths. The moderate addition of calcium silicate hydrate reduces slightly the compressive strengths of the acid-based geopolymers which can be comparable to the one of CEM II 42.5R.

     

Water retention properties of porous geopolymers for use in cooling applications

A series of geopolymers were prepared with varying ratios of sodium silicate, metakaolinite, NaOH and H₂O and their porous properties, water retention and mechanical properties were determined, to develop materials for counteracting heat island effects. Samples were prepared with the molar ratios SiO₂:Al₂O₃:Na₂O:H₂O of 3.66:1:x:y, where x = 0.92–1.08 and y = 14.2–19.5. The porous and mechanical properties of the geopolymers showed a good correlation with the H₂O/Al₂O₃ ratio (y); an increase in y produced an increase in the pore volume (from 0.26 to 0.46 ml/g), the pore size (from 15 to 390 nm) and the water absorption (from 27.2 to 51.1%). The same increase in y decreased the bulk density (from 1.29 to 0.99 g/cm³), the bending strength (from 14.2 to <5 MPa) and the water retention. Thus, the H₂O/Al₂O₃ ratio is the most important factor for controlling the porous properties of these materials, since geopolymers with higher H₂O/Al₂O₃ ratios are more porous and have higher water absorption rates, making them suitable as materials for surface cooling by water evaporation. Geopolymers with lower H₂O/Al₂O₃ ratios are more suitable for water retention applications, and have the advantage of higher mechanical strength.     

Optimization of new eco-material synthesis obtained by phosphoric acid attack of natural Moroccan pozzolan using Box-Behnken Design

Natural pozzolan is a promising precursor for the synthesis of alkali-activated geopolymers. However, the acid activation of pozzolan is very poorly developed. In this respect, this paper presents an optimization study of the synthesis of geopolymers based on pozzolan and phosphoric acid using the Design Of Experiments (DOE) methodology. Indeed, a Box Behnken Design (BBD) associated with the Response Surface Methodology (RSM) is used. The analysis of variance ANOVA showed that the factors used for the modeling are significant in their sets. Thus, the study of the adjustment confirms the validation of the established models, by very high values of adjusted coefficient of determination (R²_Adj-Cs = 95.63%, R²_Ad-PWV = 95.73%, R²_Adj-D = 97.62%, R²_Adj-P = 98.24%, R²_Adj-WA = 95.52%). Similarly, the RSM tool showed that the optimal conditions to elaborate a geopolymer with good physicomechanical properties (Compressive strength = 21.57 MPa, P-wave velocity = 2110 m/s, Density = 1.95 g/cm³, Porosity = 12.8%, Water absorption = 6.06%) are as follows: Si/P = 1.25, H₂O/H₃PO₄ = 2, and Li/So ratio = 0.9. Microstructural characterization of the optimal geopolymer by XRD and FTIR showed that the pozzolan is actively reacted with phosphoric acid, under the optimal conditions found by the Box Behnken Design model. This is revealed by the formation of a significant amount of amorphous phase, the formation of a new monetite-like mineral phase as well as the formation of Si–O–P bonds, which are the basis of the physicomechanical properties found.     

Effects of artificial aging on the biaxial flexural strength of Ce-TZP/Al2O3 and Y-TZP after various occlusal adjustments

The aim of this study was to determine the effect of aging on the biaxial flexural strength (BFS) of Ce-TZP/Al₂O₃ and Y-TZP after occlusal adjustment. NanoZr block (Ce-TZP/Al₂O₃ nanocomposite) and Katana zirconia block (Y-TZP) were prepared by milling with the aid of CAD/CAM into disk-shaped specimens. For each type of zirconia, 16 specimens were prepared without grinding for the control group (diameter of 16 mm and thickness of 1.20±0.05 mm, mean±SD), while 48 specimens were prepared for 3 experimental groups (n=16 each; 16 mm in diameter and 1.50±0.05 mm thick) with different types of surface grinding: superfine diamond bur (group I), zirconia stone bur (group II), and zirconia stone and fine polishing bur (group III). These specimens underwent an aging process in a steam autoclave for 5 h at 0.2 MPa and 134 °C, and then X-ray diffractometry was applied along with measurements of surface roughness and BFS. After occlusal adjustment, the monoclinic phase percentage increased in 3 experimental groups. Overall the increase was greater for Ce-TZP/Al₂O₃ than for Y-TZP. The Ra value showed similar changes for both types of zirconia. Following the aging process, Y-TZP showed a greater increase in the monoclinic phase percentage, but the change was not statistically significant. The Ra value showed similar changes in both types of zirconia, with no significant differences between before and after the aging process. The results of the BFS test showed that applying the aging process after grinding significantly increased the strength of both types of zirconia, with Ce-TZP/Al₂O₃ being significantly stronger than Y-TZP. The specimens treated by a superfine diamond bur exhibited the highest BFS in the four tested groups. Ce-TZP/Al₂O₃ had a higher BFS and greater resistance to low-temperature degradation than did Y-TZP.     

Preparation and characterization of fully waste-based glass-ceramics from incineration fly ash,waste glass and coal fly ash

Municipal solid waste incineration (MSWI) fly ash (FA) is a typical hazardous waste due to its high contents of toxic heavy metals, and hence its disposal has attracted global concern. In this work, it was recycled into environmental-friendly CaO–Al₂O₃–SiO₂ system glass-ceramics via adding coal fly ash (CFA) and waste glass (WG). The effects of CaO/SiO₂ ratios and sintering temperatures on the crystalline phases, morphologies, mechanical and chemical properties, heavy metals leaching and potential ecological risks of glass-ceramics were investigated. The results showed that wollastonite (CaSiO₃), anorthite (CaAl₂Si₂O₈) and gehlenite (Ca₂Al₂SiO₇) were the dominant crystals in the glass-ceramics, which were not affected by CaO/SiO₂ ratio and sintering temperature. The compressive strength increased, while the Vickers hardness and microhardness decreased as increasing the sintering temperatures from 850 to 1050 °C, which reached their maximum values of 660.69 MPa, 6.14 GPa, and 7.43 GPa, respectively. However, the increase of CaO/SiO₂ ratio resulted into the reduction of the three mechanical parameters. As varying CaO/SiO₂ ratio from 0.48 to 0.86, the maximum compressive strength, Vickers hardness and microhardness were 611.80 MPa, 5.43 GPa, and 6.56 GPa, respectively. Besides, all the glass-ceramics exhibited high alkali resistance of >97%. The extremely low heavy metals leaching concentrations and low potential ecological risk of glass-ceramics further revealed its environmentally friendly property and potential application feasibility.     

Impact of the solidification path of FeOx–SiO2 slags on the resultant inorganic polymers

Aiming to reduce the carbon dioxide emissions associated with cement production, alternative binders such as inorganic polymers currently receive substantial attention and slags from the non-ferrous metallurgy are promising precursors. However, studies  that correlate their chemistry and crystallinity with the newly formed binder remain limited. In this work, the effect of three different solidification methods on glass formation and reactivity of FeO_x–SiO₂ slags, as well as on the molecular structure of the resultant Fe-rich inorganic polymers, was investigated. The inorganic polymers were synthesized by mixing the slags (approximate molar ratio FeO/SiO₂ = 1.6) with an alkali silicate solution (molar ratios SiO₂/Na₂O = 1.6 and H₂O/Na₂O = 25). Results demonstrated that higher cooling rates promoted higher glass formation and faster reaction kinetics when the slags were activated. ⁵⁷Fe Mössbauer spectroscopy indicated that all the slags consisted predominantly of Fe²⁺ ions with a minor amount of Fe³⁺ ions, regardless of the variability in glass content. The binder phase of all inorganic polymers consisted of iron in both Fe²⁺ and Fe³⁺ states, after 28 days of curing. After pulverizing the inorganic polymer pastes and exposing the powder to air for 28 additional days, the Fe²⁺ state in the binder transformed to Fe³⁺. The compressive strength evolution of the three slags showed that the 2-day strength was higher for the samples with a higher amorphous fraction, while after 28 days, this influence was less pronounced.     

Phase evolution and microwave dielectric properties of SrTiO3 added ZnAl2O4–Zn2SiO4–SiO2 ceramics

Phase evolution and microwave dielectric properties of SrTiO₃ added ZnAl₂O₄–3Zn₂SiO₄–2SiO₂ ceramics system were investigated. With the addition of SrTiO₃, the sintering temperature for dense ceramic is reduced from 1320 °C to 1180–1200 °C. According to the nominal composition ZnAl₂O₄–3Zn₂SiO₄–2SiO₂-ySrTiO₃, phase evolution is revealed by XRD patterns and Back Scattering Electron images: Zn₂SiO₄, ZnAl₂O₄ and SiO₂ phases coexist at y = 0; SrTiO₃ reacts with ZnAl₂O₄ and SiO₂ to form SrAl₂Si₂O₈, TiO₂ and Zn₂SiO₄ at y = 0.2 to 0.8, and SiO₂ phase disappears at y = 0.8; new phase of Zn₂TiO₄ is obtained at y = 1. The existence of TiO₂ has important effect on the dielectric properties. The optimized microwave dielectric properties are obtained at y = 0.6 and the ceramics show low dielectric constant (7.16), high-quality factor (57, 837 GHz), and low temperature coefficient of resonant frequency (−30 ppm °C⁻¹).     

Use of glass polishing waste in the development of ecological ceramic roof tiles by the geopolymerization process

The production process of ceramic roof tiles requires a large consumption of natural raw material, such as clay, and energy consumption in the sintering process. Thus, the objective of this research was the development of more ecological tile for civil construction, adopting the process of geopolymerization, which does not require burning, and the use of glass polishing waste, in partial replacement of natural raw material. Prismatic specimens were made with a ratio of alkaline solution/ (metakaolin + waste) = 0.26 and variation of the curing time in 7, 28, and 60 days, and a ratio of precursors with SiO₂/Al₂O₃ varying in 2.5, 3.0, 3.5, and 4.0 for the evaluation of technological properties such as apparent specific mass, linear shrinkage, water absorption, and mechanical resistance to flexion, in addition to microstructural evaluations and the physical, chemical, and mineralogical characterization of the waste. The results showed that the glass waste had potential for use as a precursor in the geopolymerization process, and that the specimens with a 7 days cure and a SiO₂/Al₂O₃ = 4 ratio are the most recommended for the production of roof tiles for civil construction.     

Micro-mesoporous TiO2/SiO2 nanocomposites: Sol-gel synthesis,characterization, and enhanced photodegradation of quinoline

Micro-mesoporous TiO₂/SiO₂ nanocomposite powders have been successfully synthesized by the sol-gel process with different TiO₂/SiO₂ molar ratios and were applied in the UV-photodegradation of quinoline (λ = 254 nm). The structural, morphological, and textural characterization of the powders showed a homogeneous distribution of TiO2 nanoparticles within a porous amorphous SiO2 matrix. Due to the micro-mesoporous character of the materials, their textural characteristics were evaluated by the N2 adsorption method, by comparing BET, DR, Langmuir, and DFT theories. Si₆₀Ti₄₀ powders (60%SiO₂/40%TiO₂) presented the highest specific surface area (SSA) obtained from BET (SSA = 363 m²g^-1), DR (SSA = 482 m²g^-1), and Langmuir (SSA = 492 m²g^-1) due to the adequate particle size of TiO₂ and its high dispersion in the porous matrix. A higher degradation of quinoline in the presence of H₂O₂ (66%) was achieved using Si₈₀Ti₂₀ powders (80%SiO₂/20%TiO₂), as compared to pure sol-gel TiO₂ powders, (51%) under the same reaction conditions (1 UVC lamp - 250W, t = 180 min). The better performance of the Si₈₀Ti₂₀ nanocomposite could be attributed to the small TiO₂ anatase crystallite size (<5.7 nm), high dispersion of these crystallites in the SiO₂ matrix, great specific surface area (DR SSA = 342 m² g^?1), and the formation of Ti–O–Si bond, which is associated with new catalytic sites in TiO₂/SiO₂ composite.     

Conversion of coal fly ashes into faujasite under soft temperature and pressure conditions: Influence of additional silica

The hydrothermal conversion of a French coal fly ash (FA) into NaX zeolite (faujasite) was studied under soft conditions (30 °C, atmospheric pressure). One used molar composition had previously been found during a preliminary study aiming to determine an optimal composition for the synthesis of NaX from this FA, i.e., 1SiO₂:0.36Al₂O₃:3.8NaOH:72.7H₂O, the Al₂O₃/SiO₂ ratio corresponding to the one of the FA, and the zeolitisation procedure consisting only in adding a NaOH solution to the FA. As this composition is much poorer in silica than the one given in the literature for the synthesis from commercial reactants, new experiments were performed with addition of external soluble silica, leading to the following composition: 1SiO₂:0.17Al₂O₃:4.0NaOH:75.5H₂O. The zeolitisation kinetics – followed mainly by XRD and N₂ adsorption – were compared in presence or absence of additional silica. The addition of silica leads to a final higher content in faujasite (30 wt.% instead of 20–25 wt.%), a higher purity of the zeolitic fraction (co-formation of traces of other more stable zeolites avoided). In presence of external silica, the faujasite crystals are also smaller and display a slightly higher Si/Al framework ratio (1.2 instead of 1.1). One significant point is that the addition of soluble silica leads to a better solubilisation of the FA, about 20% more Al being mobilised. Finally, the zeolisation reaction is delayed when silica is added, the maximum faujasite content being reached after about 50 days instead of about 20 days.     

Structural behavior and in vitro bioactivity evaluation of hydroxyapatite-like bioactive glass based on the SiO2-CaO-P2O5 system

Silicate bioglass is of great importance in bone engineering because of its excellent bioactivity and osteogenic effects. In this study, hydroxyapatite-like bioactive glass based on the xSiO₂-CaO-P₂O₅ (x = 30, 45, 60 and 90 mol.%, Ca/P = 1.67) system was synthesized by the sol-gel method, and the corresponding structural evolution, apatite-forming ability and cytotoxicity were systematically investigated. The results suggest that both a higher heat treatment temperature and a lower SiO₂ content increase the crystallinity tendency of the bioglass, and the samples become obviously compact as the SiO₂ amount increases from 30 to 90 mol.%. Compared with the samples with higher SiO₂ content, the 30Si sample shows more remarkable internal connected mesoporous structures, with a higher specific surface area up to 129.12 m²/g, exhibiting excellent hydroxyapatite formation in simulated body fluid. Moreover, no obvious inhibitory effect was presented on human periodontal ligament cells (hPDLCs) for any of the silicate glass samples.     

Effect of fiber factor on the workability and mechanical properties of polyethylene fiber-reinforced high toughness geopolymers

In this work, we studied the influence of fiber factor (F) on the workability, mechanical properties, and internal defects of polyethylene (PE) fiber-reinforced high toughness geopolymers, and two specific fiber parameters, the critical fiber factor (F_c = 150) and dense fiber factor (F_d = 600), were proposed. Subsequently, the workability and hardening properties of the 26 PE fiber-reinforced geopolymer (FRG) groups were divided into three grades. When F was lower than F_c, the fibers had an insignificant effect on the workability and hardening properties of the material. However, when F was higher than F_d, many fibers agglomerated and deteriorated the workability and mechanical properties. Thus, when F was between F_c and F_d, satisfactory workability and excellent mechanical properties were simultaneously obtained. Furthermore, with similar fiber stiffness coefficients, the relative yield stress of FRG was similar to the fiber-reinforced Portland cement-based composites. Moreover, the flexural and compressive strength values of the PE FRG were both higher than the PE fiber-reinforced Portland cement-based composites. As a result, this work provides relevant conclusions for the theoretical reference and design of FRG with satisfactory workability and mechanical behaviors for concrete engineering.     

Synthesis and microstructural characterization of Bi12SiO20 (BSO) thin films produced by the sol–gel process

We have produced Bi₁₂SiO₂₀ (BSO) thin films using the sol–gel process. The stable sol was synthesized using Bi(NO₃)₃·5H₂O and Si(OC₂H₅)₄ (TEOS) as the precursors, acetic acid and 2-ethoxyethanol as the solvents, and ethanolamine as the stabilizer. The stability of the solution, which depends on the concentration and the R_h value (R_h = [H₂O]/[M]), directly affects the microstructure of the BSO thin film. We determined that the optimal concentration for the preparation of BSO thin films is 0.76 M. The influences of the substrates, the annealing temperature, the concentration and the R_h = value of the solution on the microstructure of the Bi₁₂SiO₂₀ thin films were investigated. X-ray diffraction (XRD) showed that the Bi₁₂SiO₂₀ starts to form at 500 °C and that single-phase Bi₁₂SiO₂₀ polycrystalline thin films are formed at 700 °C. The coated films were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM).     

Silica Activity Measurements in the Y2O3–SiO2 System and Applications to Modeling of Coating Volatility

Rare‐earth silicate compounds, such as those in the Y₂O₃–SiO₂ system, are promising candidates for coatings of SiC‐based ceramics and ceramic matrix composites in combustion environments. The predicted lower activity of silica in these silicates will lead to less reactivity with the water vapor combustion products. A procedure for measuring silica activities in this system is discussed. Knudsen effusion mass spectrometry is used and the measured vapor pressure of SiO(g) is correlated to activity. Due to the low vapor pressure of SiO(g) in the temperature range of interest, a reducing agent is utilized to boost this vapor pressure without altering the solid‐state composition. In addition, corrections are made for nonequilibrium vaporization. The measured silica activities are: Y₂O₃ + Y₂O₃·SiO₂ two phase field: log[a(SiO₂)] = ?5200.26(1/T) + 0.0567 (1532 < T(K) < 1670) Y₂O₃·SiO₂ + Y₂O₃·2SiO₂ two phase field: log[a(SiO₂)] = 4.2252(1/T)?0.5531 (1628 < T(K) < 1747) It is shown how these results can be used to predict reduced volatilization rates of the coatings.