Reaction Between K2O and Al2O3-SiO2 Refractories as Related to Blast-Furnace Linings

An examination was conducted to determine the mechanism of peeling of fire-clay brick in the low-temperature region of a blast furnace where 3 to 10% K₂O is the principal contaminant. In laboratory tests, as-received high-duty and superduty fire-clay brick and 70% alumina brick treated with KCl-K₂CO₃ mixtures showed no peeling at a temperature of 1600°F. Cracks were found in high-duty brick that were treated with KCN at 1500°F. under partially reducing conditions. X-ray diffraction studies of mixtures of crushed brick and K₂CO₃ indicated the formation of leucite (K₂O.Al₂O₃.4SiO₂) and kaliophilite (K₂O.-Al₂O₃.2SiO₂) at temperatures below 1700°F. These latter data, confirmed by specimens from used blast-furnace linings, showed that silica is the first constituent attacked by alkali. Since the formation of leucite and kaliophilite in fire-clay brick is the probable cause of peeling, the increased reaction of silica, in a dense Al₂O₃.SiO₂ refractory of higher silica content than fire-clay brick, should confine the alkali attack to the surface of the brick in low-temperature applications.     

Acicular Mullite Crystals in Vitrified Kaolin

The microstructure of vitrified kaolin ceramic tapes has been studied via scanning and transmission electron microscopy (SEM and TEM). The sintered samples contained crystalline phase of predominantly stoichiometric mullite (3Al₂O₃·2SiO₂), which consisted of high aspect ratio, acicular crystals that are often referred to as secondary mullite. These crystals were interlocked and embedded in an aluminosilicate glass matrix of inhomogeneous composition. The glass matrix contained an average of ∼3.63 wt% K as determined by energy-dispersive X-ray analysis (EDS), whose composition could be approximated to 5Al₂O₃·16SiO₂·0.1MgO·0.3K₂O·0.15TiO₂·0.12Fe₂O₃. The acicular crystals have approximately the stoichiometric composition of Al₂O₃:SiO₂= 3:2. They have grown along a specific crystallographic orientation along the [001] axis. The crystal growth front exhibited facetting on the {110) planes with microfacetting on both the {100) and {010) planes.     

Cassiterite Solubility in High-Silica K2O-Al2O3-SiO2 Liquids

The saturation surface of cassiterite, SnO₂, was determined for liquids in the system K₂O–Al₂O₃–SiO₂ as a function of bulk composition and temperature. At fixed K₂O/Al₂O₃ cassiterite solubility varies weakly with SiO₂ concentration (76 to 84 mol%), temperature (1350° to 1550°C), and log ( f _O2) (−0.7 to −5.3). Cassiterite solubility is also approximately independent of composition in liquids with molar ratios of K₂O/Al₂O₃ lessthan equal to 1 (peraluminous liquids). As K₂O/Al₂O₃ increases from 1 (peralkaline liquids), however, cassiterite solubility increases steeply and approximately linearly with K₂O in excess of Al₂O₃. It is proposed that potassium in excess of aluminum combines with Sn⁴⁺ to form quasi-molecular complexes with an effective stoichiometry of K₄SnO₄.     

Effect of Oxygen Vacancies on the Hot Hardness of Mullite

The structure of mullite, which has a composition ranging from 3Al₂O₃·2SiO₂ to Al₂O₃·2SiO₂, contains ordered oxygen vacancies. Sillimanite, Al₂O₃·SiO₂, has a similar structure but with no vacancies. The indentation hardness of polycrystalline mullite (3Al₂O₃·2SiO₂) was measured from room temperature up to 1400°C and compared with that of single-crystal sillimanite (Al₂O₃·SiO₂) up to 1300°C. It was found that both materials show the same variation in hardness with temperature, suggesting that the structures have a similar resistance to plastic deformation, and therefore that the oxygen vacancies in the mullite structure are not the primary cause of mullite's resistance to high-temperature deformation.     

Thermal Evolution and Sintering Behavior of a 2:1 Mullite Gel

The thermal evolution of a mullite gel of composition 2Al₂O₃·SiO₂ has been investigated. The gel crystallized at 1300°C into an alumina-rich mullite and corundum, instead of single-phase 2Al₂O₃·SiO₂ mullite. The amount of Al₂O₃ that dissolved in the mullite structure has been determined in the 1300–1780°C temperature range by measuring the mullite lattice parameters. A maximum limit for the amount of Al₂O₃ in solid solution has been observed. Densification of the gel powders has been analyzed up to temperatures of 1780°C. The microstructure of dense materials always showed the presence of residual Al₂O₃ particles.     

Characterization and Sintering Behavior of Alkoxide-Derived Aluminosilicate Powders

Submicrometer SiO₂-Al₂O₃ powders with compositions of 46.5 to 76.6 wt% Al₂O₃ were prepared by hydrolysis of mixed alkoxides. Phase change, mullite composition, and particle size of powders with heating were analyzed by DTA, XRD, IR, BET, and TEM. As-produced amorphous powders partially transformed to mullite and Al-Si spinel at around 980°C. The compositions of mullite produced at 1400° and 1550°C were richer in Al₂O₃ than the compositions of stable mullite solid solutions predicted from the phase diagram of the SiO₂-Al₂O₃ system. Particle size decreased with increasing Al₂O₃ content. The sintered densities depended upon the amount of SiO₂-rich glassy phase formed during sintering and the green density expressed as a function of particle size.     

Purification of Mullite by Reduction and Volatilization of Impurities

Mullite materials usually contain a residual glassy phase rich in SiO₂, which concentrates impurities as Na₂O, K₂O, Fe₂O₃, and other minority compounds. A suitable way to minimize this glassy phase is the reduction and volatilization of its components by calcination at high temperatures (1300–1450°C) in atmospheres with a very low partial pressure of O₂. Over 95% of the Na₂O, K₂O, and Fe₂O₃ in mullite can be removed in this way, leaving concentrations lower than 0.02% by weight. To avoid the degradation of mullite that occurs when the partial pressure of O₂ is too low, the material to be purified is covered with TiO₂ plates.     

Mechanical Activation of the Decomposition and Sintering of Kyanite

The influence of attrition milling on the thermal decomposition of kyanite (Al₂O₃·SiO₂) to mullite (3Al₂O₃·2SiO₂) and SiO₂, and its subsequent sintering, was studied. A commercial kyanite was attrition-milled for times up to 12 h. Dilatometry confirmed that as-received unmilled kyanite decomposes between 1300° and 1435°C. The decomposition reaction is slow initially and accelerates during the later stages until about one-half of the decomposition occurs in the last 35°C. For the attrition-milled kyanite, the onset decomposition temperature decreases, the transformation temperature interval is reduced, and both the decomposition reaction and subsequent sintering are accelerated. A dense microstructure of fine equiaxed mullite grains in the 1 μm size range, evenly dispersed in a glassy matrix, is obtained by sintering the attrition-milled kyanites. These results are explained in terms of the energy accumulated during attrition milling, a reduction of the milled kyanite particle size, and the presence of a liquid phase during sintering.     

Fabrication of Mullite Ceramics With Ultrahigh Porosity by Gel Freeze Drying

Porous mullite (3Al₂O₃·2SiO₂) ceramics with an open porosity up to 92.9% were fabricated by a gel freeze-drying process. An alumina (Al₂O₃) gel mixed with ultrafine silica (SiO₂) was frozen and sublimation of ice crystals was carried out by drying the frozen body under a low pressure. Porous mullite ceramics were prepared in air at 1400°–1600°C due to the mullitization between Al₂O₃ and SiO₂. A complex and porous microstructure was formed, where large dentritic pores with a pore size of ∼100 μm contained small cellular pores of 1–10 μm on their internal walls. Owing to the complete mullitization, a relatively high-compressive strength of 1.52 MPa was obtained at an open porosity of 88.6%.     

Hydration of 3CaO·Al2O3 and 3CaO·Al2O3+] Gypsum With and Without CaCl2

Paste samples of tricalcium aluminate alone, with CaCl₂, with gypsum, and with gypsum and CaCl₂ were hydrated for up to 6 months and the hydration products characterized by SEM, XRD, and DTA. Tricalcium aluminate hydrated initially to a hexagonal hydroaluminate phase which then changed to the cubic form; the transformation rate depended on the size and shape of the sample and on temperature. The addition of CaCl₂ to tricalcium aluminate resulted in the formation of 3CaO · Al₂O₃· CaCl₂·10H₂O and 4CaO · Al₂O₃· 13H₂O, or a solid solution of the two. The chloride retarded the formation of the cubic phase 3CaO · Al₂O₃· 6H₂O; the addition of gypsum resulted in the formation of monosulfoaluminate with a minor amount of ettringite. When chloride was added to tricalcium aluminate and gypsum, more ettringite was formed, although 3CaO · Al₂O₃· CaSO₄· 12H₂O and 3CaO · Al₂O₃· CaCl₂· 10H₂O were the main hydration products.     

Thermal Behavior of Alumina—Silica Xerogels During Calcination

Xerogels of 3Al₂O₃·2SiO₂ mullite were prepared by hydrolyzing Al(NO₃)₃·9H₂O and Si(OC₂H₅)₄ solutions with pH values of 8.3, 9.4, 10.1, and 10.4; the xerogels were composed of a combination of singlephase and diphasic materials. A strong alkaline solution enhanced bayerite formation in the gels. Mullite from the diphasic xerogels was produced by reacting θ-Al₂O₃ with amorphous SiO₂, whereas mullite from the single-phase xerogels was transformed from Al-Si spinel. For the single-phase xerogel, the DTA curve closely resembled the kaolinite-to- mullite reaction. For the diphasic xerogels, the Al³⁺ -containing solution gelled to pseudoboehmite, which transformed to bayerite in solution. The bayerite then decomposed to η-Al₂O₃ and to θ-Al₂O₃ sequentially on heating.     

Synthesis of Titanate Derivatives Using Ion-Exchange Reaction

Two types of titanate derivatives, layered hydrous titanium dioxide (H₂Ti₄O₉· n H₂O) and potassium octatitanate (K₂Ti₈O₁₇) with a tunnellike structure, were synthesized using an ion-exchange reaction. Fibrous potassium tetratitanate (K₂Ti₄O₉· n H₂O) was prepared by calcination of a mixture of K₂CO₃ and TiO₂ with a molar ratio of 2.8 at 1050°C for 3 h, followed by boiling-water treatment of the calcined products for 10 h. The material then was transformed to layered H₂Ti₄O₉· n H₂O through an exchange of K⁺ ions with H⁺ ions using HCl. K₂Ti₈O₁₇ was formed by a thermal treatment of KHTi₄O₉· n H₂O. Pure KHTi₄O₉· n H₂O phase was effectively produced by a treatment of K₂Ti₄O₉ with 0.005 M HCl solution for 30 min. Thermal treatment at 250°–500°C for 3 h resulted in formation of only K₂Ti₈O₁₇.     

Reaction-Forming of Mullite Ceramics Using an Aqueous Milling Medium

Mullite and mullite/ZrO₂ ceramics were fabricated starting from Si/Al₂O₃ and Si/Al₂O₃/ZrO₂ powder mixtures, which were mixed and attrition milled with TZP balls in water. Isopressed powder compacts were subjected to a heat treatment in air, during which the Si was oxidized to SiO₂. At } 1410°C, reaction between Al₂O₃ and SiO₂ occurred, resulting in mullite (3Al₂O₃·2SiO₂). Depending on the composition of the starting powders, the end product was either single-phase mullite or a mullite composite. The reaction process was monitored by thermogravimetry and dilatometry. It was found that the microstructure and mechanical properties of the reaction-formed mullite ceramics were significantly improved by ZrO₂ additions.     

Glassy Phase in Alumina–Silica Ceramic Shell Molds

Variation of the amount and the composition of the glassy phase in high-purity Al₂O₃-SiO₂ shell molds and the crystallization of the glassy phase have been studied. The results indicate that the two main modes of transformation from a glassy phase to a crystalline phase during firing are (1) formation of mullite through a chemical reaction and (2) devitrification of the glassy phase that comes from the original materials. The transformation is mainly influenced by the S/ ( A + S ) value ( A and S are the weight percentages of Al₂O₃ and SiO₂, respectively), the Na₂O content, and the firing temperature. Low S /( A + S ), low Na₂O, and high firing temperature are favorable to the transformation.     

Synthetic Mica Investigations: II, Role of Fluorides in Mica Batch Reactions

Various fluorides were studied to determine which might be suitable for the synthesis of fluor-phlogopite mica (K₂Mg₆Al₂Si₆O₂₀F₄). K₂SiF₆, MgF₂, K₂AlF₅, and K₃AlF₆ are preferred in that order. KF, KMgF₃, AlF₃, KAlF₄, and MgSiF₆· 6H₂O are less suitable for the purpose. Solid-state reaction studies were made on the following binary partial systems of the mica batch: MgF₂+ MgO, 3MgF₂+ Al₂O₃, 2MgF₂+ SiO₂, 3MgF₂+ KAlSi₃O₈, K₂SiF₆+ 3MgO, K₂SiF₆+ Al₂O₃, K₃AlF₆+ 3MgO, and K₃AlF₆+ 3SiO₂. Solid-state reaction studies on various types of fluor-phlogopite batches showed that this mica compound can be synthesized at temperatures as low as 750°C. For best results, however, it should be done at 1000° to 1300°C. in closed containers, using anhydrous batch materials to minimize hydrolysis of the fluorides and loss of HF. Many of the mica batches expand as they react in the solid state, and there is danger in large-scale experiments that the container may break unless heating is rapid between 750° and 1200°C.     

Solid Solution Range and Microstructures of Melt-Grown Mullite

The solid solution range of melt-grown mullite was examined by crystal-chemical methods. The maximum Al₂O₃ content as determined by EDX was ∼83.6 wt%, 75 mol%, or the nominal composition 3Al₂O₃.SiO₂. For samples of overall composition 81 to 83 wt% Al₂O₃, extra lines indicating crystallographic superstructure appeared in Guinier X-ray patterns. The corresponding TEM microstructure consisted of a mullite matrix finely twinned on (001), the twins being 0.02 to 0.10 μm wide, with oriented exsolution of α-A1₂O₃, often twinned, also being present. The analogy between mullite superstructure and that of plagioclase feldspars, as well as the relevance of these findings to the SiO₂-Al₂O₃ metastable phase equilibria are discussed.     

Oxygen Grain-Boundary Diffusion in Polycrystalline Mullite Ceramics

Oxygen tracer diffusivities of low- and high-alumina mullite ceramics (72 wt% Al₂O₃, 28 wt% SiO₂ and 78 wt% Al₂O₃, 22 wt% SiO₂, respectively) were determined. Gas/solid exchange experiments were conducted in an atmosphere enriched in the rare stable isotope ¹⁸O, and the resulting ¹⁸O isotope depth distributions were analyzed using SIMS depth profiling. The investigation showed that grain-boundary diffusivities for both mullite ceramics were several orders of magnitude higher than mullite volume diffusivity. Activation enthalpies of oxygen diffusion were 363 ± 25 kJ/mol for the low-alumina and 548 ± 46 kJ/mol for the high-alumina materials. Because the glassy grain-boundary films were not identified, the differences between the low- and high-alumina materials might be explained by different impurity concentrations in the grain boundaries of the two materials.     

Reactions in the System Li2O–MgO–Al2O3–SiO2: I, The Cordierite-Spodumene Join

Phase equilibria along the nonbinary join between cordierite (2MgO · 2Al₂O₃· 5SiO₂) and spodumene (Li₂O · Al₂O₃· 4SiO₂) were investigated in the temperature range 800° to 1550°C. using the quench technique on fourteen compositions. The phase diagram at high temperatures is characterized by a very small region of solid solution on the cordierite side, appreciable solid solution on the spodumene side, and regions of three and four phases toward the center of the system, including liquid, α-cordierite, mullite, spinel, corundum, and β-spodumene and its solid solutions. The liquidus has a flat minimum between 40 and 50% cordierite at 1347°, and rises on one side to the congruent melting point of β-spodumene (1421°) and on the other side to the temperature of complete melting of cordierite (1530°). The lowest temperature at which liquid appears is 1325°. At low temperatures a complete series of metastable solid solutions exists between μ-cordierite and β-spodumene. The significance of the data in the preparation of thermal-shock-resisting bodies is discussed.     

Impurity-Dependent Morphology and Grain Growth in Liquid-Phase-Sintered Alumina

The alumina grains in liquid-phase-sintered (LPS) materials prepared from different commercial sources have a predominantly platelet morphology. Generally, the MgO:(CaO + BaO + Na₂O + K₂O) ratio in the chemical composition controls the morphology in LPS alumina that is 91–94 wt% pure. Within a given range of SiO₂ content (i.e., 4.3–5.2 wt% in the chemical composition), a low MgO:(CaO + BaO + Na₂O + K₂O) ratio (i.e., <1.0) in the LPS compositions favors the formation of elongated grains, whereas ratios of >1.0 result in equiaxed grains. SiO₂ contents outside the 4.3–5.2 wt% range favor the formation of elongated grains. A tendency to form platelike grains is observed for LPS alumina with a purity of 91–94 wt% when both the MgO:(CaO + BaO + Na₂O + K₂O) ratio and the SiO₂ content are relatively low. The sintered density generally increases as the SiO₂ content in the chemical composition decreases.     

Spectroscopic Properties of Er3+-Doped Na2O–La2O3–Al2O3–SiO2 Glasses

Er³⁺-doped sodium lanthanum aluminosilicate glasses with compositions of (90− x )(0.7SiO₂·0.3Al₂O₃)· x Na₂O·8.2La₂O₃· 0.6Er₂O₃·0.2Yb₂O₃·1Sb₂O₃ (in mol%) ( x = 12, 20, 24, 40, 60 mol%) were prepared and their spectroscopic properties were investigated. Judd–Ofelt analysis was used to calculate spectroscopic properties of all glasses. The Judd–Ofelt intensity parameter Ω _t ( t = 2, 4, 6) decreases with increasing Na₂O. Ω₂ decreases rapidly with increasing Na₂O while Ω₄ and Ω₆ decrease slowly. Both the fluorescent lifetime and the radiative transition rate increase with increasing Na₂O. Fluorescence spectra of the ⁴ I _13/2 to ⁴ I _15/2 transition have been measured and the change with Na₂O content is discussed. It is found that the full width at half-maximum decreases with increasing Na₂O.