Chemistry and Morphology of Hydrogarnets Formed in Cement-Based CASH Hydroceramics Cured at 200° to 350°C

We have studied the chemistry and the morphology of hydrogarnet crystals produced in cement-based hydroceramic materials at elevated temperatures (200°–350°C) with silica and alumina additions. Such materials lie within the hydrothermal CaO–Al₂O₃–SiO₂–H₂O (CASH) system. Hydrogarnet Ca₃Al₂(SiO₄)_{3− y} (OH)_{4 y} is the dominant aluminum bearing phase formed and its composition is influenced mainly by the curing temperature and to a lesser degree by the addition of silica. The composition parameter y was estimated by Rietveld refinement of X-ray diffraction (XRD) data. Electron probe microanalysis (EPMA) shows that the hydrogarnets incorporate minor elements such as Fe, Mg, and S. EPMA data confirmed the hydrogarnet composition estimated from XRD. Both octahedral and icositetrahedral forms are observed. The icositetrahedral form is associated with higher minor element content.     

Compound Formation and Phase Equilibria in the System PbO-SiO2

Compound formation in the system PbO-SiO₂ was studied; the results are contrasted with those previously reported. Fifteen binary phases, 4 of which had not been reported, were prepared in the present study. The new phases include Pb₅Si₈O₂₁, an orthorhombic polymorph of PbSiO₃, Pb₃SiO₅, and a high-SiO₂ phase containing ∼ 60 mol% SiO₂. It was shown that Pb₃SiO₅ is thermodynamically stable relative to Pb₂SiO₄ and 4PbO.SiO₂ below 640±5°C. A revised phase equilibrium diagram is presented.     

Phase Relations in the System Lead—Oxygen

Most of the known oxides of lead including the intermediate oxides Pb₁₂O₁₉ and Pb₂O₃ have been prepared under equilibrium conditions. Precise X-ray powder data have been obtained for the intermediate oxide phases. Solid-vapor equilibrium data have been obtained for reactions in the system lead-oxygen over the range 200° to 900° C and 3 to 1400 bars Po₂. Pressure-temperature univariant equilibrium curves are presented for the reactions PbO₂⇋ Pb₁₂O₁₉+ oxygen, Pb₁₂O₁₉⇋ Pb₃O₄+ oxygen, and Pb₃O₄⇋ PbO + oxygen. The enthalpies of reaction are, respectively, 26.8, 34.6, and 33.7 kcal per mole. Eutectic melting between PbO and Pb₃O₄ occurs at 875° C and 124 bars. A stability field for Pb₂O₃ exists above 1000 bars PO₂ at 600° C. Approximate resistivity data are presented.     

SiO2–PbO–CaO–ZrO2–TiO2–(B2O3–K2O), A New Zirconolite Glass–Ceramic System: Crystallization Behavior and Microstructure Evaluation

Zirconolite (CaZrTi₂O₇) is a mineral that has a high containment capacity for actinides and lanthanides and is considered to be a good candidate for the immobilization of radioactive wastes. The glass–ceramic technique seems to be a very suitable and convenient method to produce zirconolite crystals by precipitating them in a specific glass matrix. In this study, development of a new zirconolite-based glass–ceramic belonging to SiO₂–PbO–CaO–ZrO₂–TiO₂–(B₂O₃–K₂O) system was investigated. The presence of PbO, together with B₂O₃ and K₂O, allowed the preparation of a X-ray diffraction (XRD) amorphous glass with a relatively high concentration of ZrO₂ and TiO₂, which was successfully converted to a glass–ceramic containing 34 wt% of zirconolite after heating at 770°C for 4 h. Differential thermal analysis, XRD, scanning electron microscope, and energy dispersive X-ray spectroscopy were used to determine the crystallization conditions, identify the crystallized phases, determine their compositions and quantities and observe and analyze the microstructures. The zirconolite crystals showed a platelet morphology with a monoclinic structure characterized by a =1.246 nm, b =0.7193 nm, c =1.128 nm, and β=100.508°.     

Dielectric and Magnetic Properties of Low-Temperature-Fired Ferrite–Dielectric Composites

The composition effects on the sintering behavior, microstructure evolution, dielectric, and magnetic properties of BaO·(Nd_0.8Bi_0.2)₂O₃·4TiO₂ (BNBT)+Bi₂O₃–B₂O₃–SiO₂–ZnO (BBSZ) glass–(Ni_0.28Cu_0.12Zn_0.6O)–(Fe₂O₃)_0.99 (NiCuZn ferrite) composites were investigated in developing low-temperature-fired composites for high-frequency electromagnetic interference devices. An X-ray diffractometer, a scanning electron microscope, and a dilatometer were used to examine the BNBT+BBSZ glass powder to NiCuZn ferrite ratio effect on the composites densification and chemical reaction between BNBT and NiCuZn ferrite. The results indicate that these composites can be densified at 950°C with no significant chemical reactions occurring between BNBT and NiCuZn ferrites during sintering. The BNBT+BBSZ glass–NiCuZn ferrite composites sintered at 950°C exhibit excellent dielectric and magnetic properties over a wide frequency range.     

Activity–Composition Relations of Chromium Oxide in Silicate Melts at 1500°C under Strongly Reducing Conditions

The solubility and activity–composition relations of chromium oxide in melts of the systems CaO–CrO _x –SiO₂ and CaO–Al₂O₃–CrO _x –SiO₂ have been determined at 1500°C by equilibrating melts with Pt–Cr alloys at known oxygen pressures. It is shown that the increase in the concentration of divalent chromium ions, as the oxygen pressure and the basicity of melt decrease, results in a dramatic increase in the solubility of chromium oxide in the liquid phase. An increase in the Al₂O₃ content of the melt leads to a decrease in the solubility of chromium oxide over the whole composition range studied. The activity coefficient of CrO has been found to increase with increasing melt basicity and decreasing oxygen pressure whereas the activity coefficient of CrO_1.5 decreases sharply with increasing melt basicity for siliceous melts but levels off at a basicity ratio (wt% CaO/wt% SiO₂) of about 0.7. An increase in the Al₂O₃ content of the melts results in an increase in the activity coefficient of CrO.     

Melt Differentiation Induced by Zonal Structure Formation of Calcium Aluminoferrite in a CaO–SiO2–Al2O3–Fe2O3 Pseudoquaternary System

The phase stability in part of the P₂O₅-bearing pseudoquaternary system CaO–SiO₂–Al₂O₃–Fe₂O₃ has been studied by electron probe microanalysis, optical microscopy, and powder X-ray diffractometry. At 1973–1653 K, the α-Ca₂SiO₄ solid solution [α-C₂S(ss)] and melt coexisted in equilibrium, both chemical variations of which were determined as a function of temperature. The three phases of melt, calcium aluminoferrite solid solution (ferrite), and C₂S(ss) coexisted at 1673–1598 K. On the basis of the chemical compositions of these phases, a melt-differentiation mechanism has been, for the first time, suggested to account for the crystallization behavior of Ca₃Al₂O₆ solid solution [C₃A(ss)]. When the α-C₂S(ss) and melt were cooled from high temperatures, the melt would be induced to differentiate by the crystallization of ferrite. Because the local equilibrium would be continually attained between the rims of the precipitating ferrite and coexisting melt during further cooling, the melt would progressively become enriched in Al₂O₃ with respect to Fe₂O₃. The resulting ferrite crystals would show the zonal structure, with the Al/(Al+Fe) value steadily increasing up to 0.7 from the cores toward the rims. The C₃A(ss) would eventually crystallize out of the differentiated melt between the zoned ferrite crystals in contact with their rims.     

High-Pressure-High-Temperature Polymorphism of the Oxides of Lead

The common oxides of lead, PbO, PbO₂, and Pb₃O₄, were examined at pressures of 0 to 60,000 atmospheres and temperatures of 100° to 600° C. The pressure-temperature curve for the litharge-massicot transition was measured, giving a ΔH of transition of 57 cal. per mole. Unusual meta-stability and inversion characteristics of the massicot phase were examined in detail. PbO₂ (rutile) transformed at pressures in excess of 13,000 atmospheres at 300°C. to an ortho-rhombic form. The univariant equilibrium curve for the transition gave a Δ H of 11 cal. per mole. Pb₃O₄ underwent a disproportionation reaction yielding PbO and "Pb₂O₃." The equilibrium curve for this reaction was measured, and Δ H was determined to be 4500 cal. per mole.     

Sintering and Microwave Dielectric Properties of the LiNb0.63Ti0.4625O3 Ceramics with the B2O3–SiO2 Liquid-Phase Additives

The effect of B₂O₃–SiO₂ liquid-phase additives on the sintering, microstructure, and microwave dielectric properties of LiNb_0.63Ti_0.4625O₃ ceramics was investigated. It was found that the sintering temperature could be lowered easily, and the densification and dielectric properties of LiNb_0.63Ti_0.4625O₃ ceramics could be greatly improved by adding a small amount of B₂O₃–SiO₂ solution additives. No secondary phase was observed for the ceramics with B₂O₃–SiO₂ additives. With the addition of 0.10 wt% B₂O₃–SiO₂, the ceramics sintered at 900°C showed favorable microwave dielectric properties with ɛ_r=71.7, Q × f =4950 GHz, and τ_f=−2.1 ppm/°C. The energy dispersive spectra analysis showed an excellent co-firing interfacial behavior between the LiNb_0.63Ti_0.4625O₃ ceramic and the Ag electrode. It indicated that LiNb_0.63Ti_0.4625O₃ ceramics with B₂O₃–SiO₂ solution additives have a number of potential applications on passive integrated devices based on the low-temperature co-fired ceramics technology.     

Electrical Resistance of Some Refractory Oxides and Their Mixtures in the Temperature Range 600° to 1500°C.

Measurements of electrical resistance in the composition systems Al₂O₃–SiO₂, SiO₂–TiO₂, Al₂O₃–Cr₂O₃, and MgO–NiO were made using, in general, dry-pressed disks about 3 cm. in diameter and 0.4 cm. thick and fired to 1500°C. In the Al₂O₃–SiO₂ series minimum resistance was shown by the samples containing 50% SiO₂, 50% Al₂O₃. The resistance of Al₂O₃ was increased by the addition of small amounts of Cr₂O₃. The same effect was observed in the higher temperature range with small additions of NiO to MgO. In other instances the addition of the relatively inert SiO₂, Al₂O₃, and MgO to the semiconductors TiO₂, Cr₂O₃, and NiO resulted in a dilution effect. The resistance of Cr₂O₃ was decreased by the addition of a slight amount of MgO.     

Reaction Hot Pressing of α'- and β'- SiAlON Ceramics

Hot pressing kinetics of α-Si₃N₄, AIN, Al₂O₃, and Y₂O₃ powder mixtures forming α'- and β'-SiAlONs have been studied. Densification proceeds in two steps, first by a small shrinkage upon ternary eutectic oxide melting (SiO₂–Al₂O₃–Y₂O₃) at 1340°C, followed by a massive particle rearrangement and further shrinkage at higher temperature when nitride dissolution begins. With better wettability, AIN initially traps the oxide melt and delays densification. In addition, the preferential dissolution of AIN at 1450°C enriches the melt composition in AI, triggering transient precipitation of supersaturated β'-SiAlON. Full densification is readily achieved at 1550°C without complete α-Si₃N₄ conversion.     

Activity of Nickel(II) Oxide in Silicate Melts

Activity–composition relations of NiO have been determined at 1435°C in melts of the system CaO–NiO–SiO₂ and at 1400°C in melts of the systems CaO–MgO–NiO–SiO₂, CaO–MgO–NiO–Al₂O₃–SiO₂, and CaO–MgO–NiO–K₂O–SiO₂. In the CaO–NiO–SiO₂ and CaO–MgO–NiO–SiO₂ systems the activity coefficient of NiO (γ_NiO) decreases as the polymerization of the melt increases (basicity decrease). γ_NiO stays contant up to several weight percent NiO for melts with similar basicity, an indication of Henry's law behavior. Minima in the NiO activity coefficient are observed in melts of the CaO–MgO–NiO–Al₂O₃–SiO₂ and CaO–MgO–NiO–K₂O–SiO₂ systems at NBO/Si ratios between 1.0 and 1.5; i.e., γ_NiO decreases with decreasing basicity for NBO/Si ratios >1.5 and increases with decreasing basicity for melts with NBO/Si ratios <1.0 (NBO/Si; nonbridging oxygens per silicon). The addition of Al₂O₃ and K₂O to the CaO–MgO–NiO–SiO₂ system results in an increase in the activity coefficient of NiO.     

Suitable Glass-Ceramic Sealant for Planar Solid-Oxide Fuel Cells

Various glass-ceramics were prepared based on the BaO–Al₂O₃–B₂O₃–SiO₂ system with the addition of La₂O₃, ZrO₂, or NiO in an attempt to develop a suitable sealant for planar solid-oxide fuel cells operating at 800°C. To estimate the applicability of these glasses as suitable sealants, their thermal and chemical stabilities as well as the crystallization behavior and bonding characteristics of their parent glasses were investigated. The thermal properties and crystallization behavior of the parent glasses were dependent on the type of additive used, and the bonding characteristics were strongly influenced by the B₂O₃/SiO₂ value in the glass composition. Observation of microstructural change and chemical reaction at the glass-ceramic/electrolyte interface confirmed that the prepared glass-ceramics possessed long-term stability during heat treatment at the operation temperature for up to 1000 h.     

Synthesis of Pb(Mg1/3Nb2/3)O3 in Excess Lead Oxide by Mechanical Activation

Twenty hours of mechanical activation of mixed oxides at room temperature led to the formation of Pb(Mg_1/3Nb_2/3)O₃ (PMN) in excess PbO. The crystallinity of the activation-derived perovskite PMN phase was further established when the activated PMN–PbO phase mixture was subjected to calcination at 800°C. Pyrochlores, such as Pb₃Nb₄O₁₃ and Pb₂Nb₂O₇, were not observed as transitional phases on mechanical activation and subsequent calcination, although 50% excess PbO was deliberately added. The perovskite PMN phase was recovered by washing off excess PbO using acetic acid solution at room temperature. It was sintered to a relative density of 98.9% of theoretical at 1200°C for 1 h and the sintered PMN exhibited a dielectric constant of ∼14 000 at 100 Hz and a Curie temperature of −11°C.     

Surface Decomposition of Strontium-Doped Soft PbZrO3–PbTiO3

Sintering temperature has a pronounced effect on perovskite phase stability at the surface of Pb_0.88Sr_0.12Zr_0.54Ti_0.44Sb_0.02O₃ (PSZT) soft piezoelectric ceramics ( d ₃₃≈ 600 pC/N). After sintering 4 h at 1070°C, XRD reveals only perovskite PSZT peaks in the bulk and at the surface. As sintering temperature increases, XRD from the ceramic surface reveals a second-phase peak at ∼27° (2θ), 0.316 nm ( d -spacing). After 4 h at 1280°C, further second-phase peaks are observed, confirming it to be monoclinic ZrO₂, accompanied by a strong increase in the degree of tetragonality of the perovskite phase. These observations are consistent with decomposition of the PSZT to ZrO₂ and tetragonal PZT (PbZrO₃–PbTiO₃) associated with PbO loss. SEM and cross-sectional TEM indicated that surface decomposition had progressed ∼0.5 mm into the sample after 4 h at 1280°C.     

Sintering and Crystallization of CaO–Al2O3–ZrO2–SiO2 Glasses Containing Different Amount of Al2O3

In this work several complementary techniques have been employed to carefully characterize the sintering and crystallization behavior of CaO–Al₂O₃–ZrO₂–SiO₂ glass powder compacts after different heat treatments. The research started from a new base glass 33.69 CaO–1.00 Al₂O₃–7.68 ZrO₂–55.43SiO₂ (mol%) to which 5 and 10 mol% Al₂O₃ were added. The glasses with higher amounts of alumina sintered at higher temperatures (953°C [lower amount] vs. 987°C [higher amount]). A combination of the linear shrinkage and viscosity data allowed to easily find the viscosity values corresponding to the beginning and the end of the sintering process. Anorthite and wollastonite crystals formed in the sintered samples, especially at lower temperatures. At higher temperatures, a new crystalline phase containing ZrO₂ (2CaO·4SiO₂·ZrO₂) appeared in all studied specimens.     

Use of Mixed-Rare-Earth Oxide in the Preparation of Reaction-Bonded Mullite at ≤1300°C

A process for production of near-net-shape mullite-matrix ceramic composites at ≤1300°C has been achieved by reaction-bonding Al₂O₃, silicon, mullite seeds, and eutectics of Al₂O₃–SiO₂–mixed-rare-earth oxide. The fusion temperature of the eutectic composition utilized is 1175°C. This liquid phase facilitates silicon oxidation, mullitization, and sintering. Mullite phase develops with low residual Al₂O₃ when 7.5 wt% mixed-rare-earth oxide and 5 wt% mullite seeds are used. The final sinter is >90% of theoretical density, >90% mullite (by quantitative XRD), and suffers 2.2% sintering shrinkage.     

Long-Lasting Phosphorescence of Transparent Surface-Crystallized Glass-Ceramics

Transparent surface-crystallized glass-ceramics with molar compositions of 43SiO₂–2B₂O₃–30SrO–25MgO–0.05Eu₂O₃–0.8Dy₂O₃ (sample GC-A) and 43SiO₂–2B₂O₃–24SrO–6CaO–25MgO–0.05Eu₂O₃–0.8Dy₂O₃ (sample GC-B) were prepared by heat-treating the mother glasses at 850°C for 10 h. The precipitated phases in both glass-ceramics were (Sr_{1− x} Ca _x )₂MgSi₂O₇, in which Eu²⁺ and Dy³⁺ ions were incorporated. Phosphorescence with emission peaks at 470 and 480 nm that is due to the 4 f ⁶5 d –4 f ⁷ transition of Eu²⁺ ions was observed from samples GC-A and GC-B, respectively. The phosphorescence lasted for >5 h at room temperature.     

Structural Study on PbO–SiO2 Glasses by X-Ray and Neutron Diffraction and 29Si MAS NMR Measurements

Structure of x PbO–(100− x )SiO₂ ( x =25–89) glasses has been investigated by means of the X-ray and neutron diffraction and ²⁹Si MAS NMR measurements. In the radial distribution functions of all the glasses, the Pb–O correlation was observed at 0.23 nm, indicating that the PbO₃ trigonal pyramids units do exist in the whole glass forming composition range. Furthermore the existence of the first Pb–Pb correlation at ∼0.385 nm in the whole composition range suggests that the basic structural unit is considered to be a Pb₂O₄ unit, which consists of the edge-shared PbO₃ trigonal pyramids. These results strongly imply that the Pb₂O₄ units participate in the glass network constructed by SiO₄ tetrahedra even at low PbO content. Differing from other lead-containing glass systems, these structural characteristics of Pb ions in the PbO–SiO₂ glass system are responsible for the extremely wide glass-forming region.     

Infiltration-Inhibiting Reaction of Gadolinium Zirconate Thermal Barrier Coatings with CMAS Melts

The thermochemical interaction between a Gd₂Zr₂O₇ thermal barrier coating synthesized by electron-beam physical vapor deposition and a model 33CaO–9MgO–13AlO_3/2–45SiO₂ (CMAS) melt with a melting point of ∼1240°C was investigated. A dense, fine-grained, ∼6-μm thick reaction layer formed after 4 h of isothermal exposure to 1300°C. It consisted primarily of an apatite phase based on Gd₈Ca₂(SiO₄)₆O₂ and fluorite ZrO₂ with Gd and Ca in a solid solution. Remarkably, melt infiltration into the intercolumnar gaps was largely suppressed, with penetration rarely exceeding ∼30 μm below the original surface. The microstructural evidence suggests a mechanism in which CMAS infiltration is arrested by rapid filling of the gaps with crystalline reaction products, followed by slow attack of the column tips.