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.     

Immiscibility Area in the System TiO2–ZrO2–SiO2

A furnace for use in conjunction with the X-ray spectrometer was developed which was capable of heating small powdered specimens in air to temperatures as high as 1850°C. This furnace was also used for the heating and quenching of specimens in air from temperatures as high as 1850°C. An area of two liquids coexisting between 20 and 93 weight % TiO₂ above 1765°± 10°C. was found to exist in the system TiO₂–SiO₂, which is in substantial agreement with the previous work of other investigators. The area of immiscibility in the system TiO₂–SiO₂ was found to extend well into the system TiO₂–ZrO₂–SiO₂. The two liquids were found to coexist over a major portion of the TiO₂ (rutile) primary-phase area with TiO₂ (rutile) being the primary crystal beneath both liquids. The temperature of two-liquid formation in the ternary was found to fall about 80°C. with the first additions of ZrO₂ up to 3%. With larger amounts of ZrO₂ the change in the temperature of the boundary of the two-liquid area was so slight as to be within the limits of error of the temperature measurement. Primary-phase fields for TiO₂ (rutile), tetragonal ZrO₂, and ZrTiO₄ were found to exist in the system TiO₂–ZrO₂–SiO₂. SiO₂ as high cristobalite is known to exist in the system TiO₂–ZrO₂–SiO₂.     

A Study of the Phase Relations of ZrO2–TiO2 and ZrO2–TiO2–SiO2

The phase relations of the systems ZrO₂–TiO₂ and ZrO₂–TiO₂–SiO₂ were investigated. X-ray diffraction techniques served as the principal means of analysis. The binary system ZrO₂–TiO₂ was found to be one of partial solid solutions with no intermediate compounds. A eutectic point was found to exist at 50 to 55 weight % ZrO₂ and 1600°C. A preliminary investigation of the ternary system ZrO₂–TiO₂–SiO₂, although not extensive, resulted in a better understanding of this system, with a fairly accurate location of some of its boundary lines. A eutectic point was located at 2% ZrO₂, 10% TiO₂, and 88% SiO₂ at approximately 1500°C.     

Synthesis, Densification, and Phase Evolution Studies of Al2O3–Al2TiO5–TiO2 Nanocomposites and Measurement of Their Electrical Properties

Alumina–aluminum titanate–titania (Al₂O₃–Al₂TiO₅–TiO₂) nanocomposites were synthesized using alkoxide precursor solutions. Thermal analysis provided information on phase evolution from the as-synthesized gel with an increase in temperature. Calcination at 700°C led to the formation of an Al₂O₃–TiO₂ nanocomposite, while at a higher temperature (1300°C) an Al₂O₃–Al₂TiO₅–TiO₂ nanocomposite was formed. The nanocomposites were uniaxially compacted and sintered in a pressureless environment in air to study the densification behavior, grain growth, and phase evolution. The effects of nanosize particles on the crystal structure and densification of the nanocomposite have been discussed. The sintered nanocomposite structures were also characterized for dielectric properties.     

Stable and Metastable Phase Relationships in the System ZrO2–ErO1.5

Stable and metastable phase relationships in the system ZrO₂–ErO_1.5 were investigated using homogeneous samples prepared by rapid quenching of melts and by arc melting. The rapidly quenched samples were annealed in air for 48 h at 1690°C or for 8 months at 1315°C. Two tetragonal phases ( t - and t '-phases) were observed after quenching samples heated at 1690°C to a room temperature, whereas one t -phase and cubic ( c -) phase were found in those treated at 1315°C. Since the t '-phase is obtained through a diffusionless transformation during cooling from a high-temperature c -phase, t - and c -phases can coexist at high temperature. The t - and c -phases field spans from 4 to 10 mol% ErO_1.5 at 1690°C and from 3 to 15 mol% ErO_1.5 at 1315°C. The equilibrium temperature T ^t-m ₀ between the t - and monoclinic ( m -) phases estimated from A_s and M_s temperatures decreased with increasing ErO_1.5 contents.     

Influence of the Y2O3 Content and Temperature on the Mechanical Properties of Melt-Grown Al2O3–ZrO2 Eutectics

The effect of Y₂O₃ content on the flexure strength of melt-grown Al₂O₃–ZrO₂ eutectics was studied in a temperature range of 25°–1427°C. The processing conditions were carefully controlled to obtain a constant microstructure independent of Y₂O₃ content. The rod microstructure was made up of alternating bands of fine and coarse dispersions of irregular ZrO₂ platelets oriented along the growth axis and embedded in the continuous Al₂O₃ matrix. The highest flexure strength at ambient temperature was found in the material with 3 mol% Y₂O₃ in relation to ZrO₂(Y₂O₃). Higher Y₂O₃ content did not substantially modify the mechanical response; however, materials with 0.5 mol% presented a significant degradation in the flexure strength because of the presence of large defects. They were nucleated at the Al₂O₃–ZrO₂ interface during the martensitic transformation of ZrO₂ on cooling and propagated into the Al₂O₃ matrix driven by the tensile residual stresses generated by the transformation. The material with 3 mol% Y₂O₃ retained 80% of the flexure strength at 1427°C, whereas the mechanical properties of the eutectic with 0.5 mol% Y₂O₃ dropped rapidly with temperature as a result of extensive microcracking.     

Al2O3–ZrO2–SiO2 Ternary Glasses for Molybdenum Oxidation Barriers

The wettability of binary and ternary glasses belonging to SiO₂–Al₂O₃–ZrO₂ diagram has been studied using the sessile drop technique at 1750° and 1800°C. The ternary SiO₂–Al₂O₃–ZrO₂ (90–5–5 wt%) glass has proved to be well appropriated as a molybdenum oxidation barrier coating. The addition of 5 wt% of MoO₂ slightly improves its wettablity at higher temperatures without affecting its oxidation barrier properties. The Mo comes into the glass network as a mixture of Mo⁵⁺, Mo⁴⁺, and Mo⁶⁺. After oxidation at 1000°C in oxygen atmosphere, the molybdenum remains in the glass network as Mo⁶⁺.     

Zirconium Aluminum Carbides: New Precursors for Synthesizing ZrO2–Al2O3 Composites

ZrO₂–Al₂O₃ nanocrystalline powders have been synthesized by oxidizing ternary Zr₂Al₃C₄ powders. The simultaneous oxidation of Al and Zr in Zr₂Al₃C₄ results in homogeneous mixture of ZrO₂ and Al₂O₃ at nanoscale. Bulk nano- and submicro-composites were prepared by hot-pressing as-oxidized powders at 1100°–1500°C. The composition and microstructure evolution during sintering was investigated by XRD, Raman spectroscopy, SEM, and TEM. The crystallite size of ZrO₂ in the composites increased from 7.5 nm for as-oxidized powders to about 0.5 μm at 1500°C, while the tetragonal polymorph gradually converted to monolithic one with increasing crystallite size. The Al₂O₃ in the composites transformed from an amorphous phase in as oxidized powders to θ phase at 1100°C and α phase at higher temperatures. The hardness of the composite increased from 2.0 GPa at 1100°C to 13.5 GPa at 1400°C due to the increase of density.     

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.     

Homogeneous ZrO2–Al2O3 Composite Prepared by Nano-ZrO2 Particle Multilayer-Coated Al2O3 Particles

ZrO₂–Al₂O₃ nanocomposite particles were synthesized by coating nano-ZrO₂ particles on the surface of Al₂O₃ particles via the layer-by-layer (LBL) method. Polyacrylic acid (PAA) adsorption successfully modified the Al₂O₃ surface charge. Multilayer coating was successfully implemented, which was characterized by ξ potential, particle size. X-ray diffraction patterns showed that the content of ZrO₂ in the final powders could be well controlled by the LBL method. The powders coated with three layers of nano-ZrO₂ particles, which contained about 12 wt% ZrO₂, were compacted by dry press and cold isostatically pressed methods. After sintering the compact at 1450°C for 2 h under atmosphere, a sintered body with a low pore microstructure was obtained. Scanning electron microscopy micrographs of the sintered body indicated that ZrO₂ was well dispersed in the Al₂O₃ matrix.     

Synthesis of Y-Doped BaTiO3–(Bi1/2K1/2)TiO3 Lead-Free Positive Temperature Coefficient of Resistivity Ceramics and Their PTC Effects

As a lead-free positive temperature coefficient of resistivity (PTCR) material, (1– x mol%) BaTiO₃– x mol% (Bi_1/2K_1/2) TiO₃– y mol% Y₂O₃–0.5 mol% TiO₂ (BT– x BKT–2 y Y–0.5TiO₂) systems were prepared by the conventional solid-state reaction method. All samples containing <2 mol% BKT sintered in air possessed relatively low room-temperature resistivity (ρ₂₅) and high positive temperature coefficient (PTC) effect. However, when the BKT content exceeded 2 mol%, the sample was not semiconductive after sintering in air. The effects of sintering schedule on the properties of PTCR ceramics were discussed. The results showed that the optimum composition of BT–1BKT–0.2Y–0.5TiO₂, sintered at 1330°C for not-soaking and then fast quenched in air, achieved rather low ρ₂₅ of 28 Ω·cm and a high jump of resistivity (maximum resistivity [ρ_max]/minimum resistivity [ρ_min]) of 4.0 orders of magnitude with T _c about 155°C. The ρ₂₅ of the as-sintered sample could be further reduced to about 10 Ω·cm by annealing in N₂ at 450°C for 30 min, accompanied decrease on the PTC effect.     

Experimental Investigation and Thermodynamic Modeling of the ZrO2–SmO1.5 System

Phase equilibria of the ZrO₂–SmO_1.5 system have been studied by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The compositions of phases in the tetragonal+fluorite, fluorite+pyrochlore, and fluorite+B-Sm₂O₃ two-phase fields have been determined for samples quenched from temperatures between 1400° and 1700°C. The heat content of the fluorite phase with 30 mol% SmO_1.5 and of the pyrochlore phase with 50 mol% SmO_1.5 has been measured in the temperature range 200°–1400°C using high-temperature drop calorimetry. The transition between pyrochlore and fluorite phases is clearly first order in the SmO_1.5-rich region, while no fluorite+pyrochlore two-phase region has been detected for the samples with ZrO₂ excess. Based on the obtained experimental results and literature data, the phase diagram and thermodynamic properties were optimized using the CALPHAD approach.     

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°.     

Phase Equilibrium in the System PbO-TiO2–ZrO2

Phase equilibrium relations in the system PbO–TiO₂–ZrO₂ were studied by quenching in the range where the PbO content is 50 mole % and more. Isotherms were examined at 1100°, 1200°, and 1300°C and tie lines were determined between the liquid and solid solution in equilibrium. The incongruent melting point of PbZrO₃ was 1570°C and the equilibrium between liquid, PbO-type solid, and PbZrO₃ is peritectic. Pb(Zr,Ti)O₃ solid solutions containing more than 14 mole % PbZrO₃ decomposed to liquid, ZrO₂, and Pb(Zr,Ti)O₃ and the decomposition temperature rises from 1340° to 1570°C with increasing PbZrO₃ content. The system PbTiO₃–PbZrO₃ should not be treated as a binary, but as a section of the ternary system.     

Microwave Dielectric Properties of 5Li2O–0.583Nb2O5–3.248TiO2 Ceramics with V2O5

The effects of V₂O₅ addition on the sintering behavior, microstructure, and the microwave dielectric properties of 5Li₂O–0.583Nb₂O₅–3.248TiO₂ (LNT) ceramics have been investigated. With addition of low-level doping of V₂O₅ (≤2 wt%), the sintering temperature of the LNT ceramics could be lowered down to around 920°C due to the liquid phase effect. A secondary phase was observed at the level of 2 wt% V₂O₅ addition. The addition of V₂O₅ does not induce much degradation in the microwave dielectric properties but lowers the τ_f value to near zero. Typically, the excellent microwave dielectric properties of ɛ_r=21.5, Q × f =32 938 GHz, and τ_f=6.1 ppm/°C could be obtained for the 1 wt% V₂O₅-doped sample sintered at 920°C, which is promising for application of the multilayer microwave devices using Ag as an internal electrode.     

Homogeneous Fabrication of Al2O3–ZrO2–SiC Whisker Composite by Surface-Induced Coating

Al₂O₃–ZrO₂–SiC whisker composites were prepared by surface-induced coating of the precursor for the ZrO₂ phase on the kinetically stable colloid particles of Al₂O₃ and SiC whisker. The fabricated composites were characterized by a uniform spatial distribution of ZrO₂ and SiC whisker phases throughout the Al₂O₃ matrix. The fracture toughness values of the Al₂O₃–15 vol% ZrO₂–20 vol% SiC whisker composites (∼12 MPa.m^1/2) are substantially greater than those of comparable Al₂O₃–SiC whisker composites, indicating that both the toughening resulting from the process zone mechanism and that caused by the reinforced SiC whiskers work simultaneously in hot-pressed composites.     

Densification and Phase Transformation During Pressureless Sintering of Nanocrystalline ZrO2–Y2O3–CuO Ternary System

ZrO₂–Y₂O₃–CuO nanocrystalline powders have been synthesized using a chemical coprecipitation method. Nano-powders were compacted uniaxially and densified in a muffle furnace. Densification studies show that the presence of CuO accelerates the densification process of ZrO₂(3Y). A fully dense (>96%) pellet of ZrO₂(3Y)/5 mol% CuO was obtained after sintering at 900°C, with a very small grain size of 44 nm calculated by X-ray line broadening.     

Dielectric Properties of (1−x)CaTiO3–xCa(Zn1/3Nb2/3)O3 Ceramic System at Microwave Frequency

The microwave dielectric properties of the (1− x )CaTiO₃– x Ca(Zn_1/3Nb_2/3)O₃ ceramic system have been investigated. The ceramic samples sintered at 1300°–1450°C for 4 h in air exhibit orthorhombic pervoskite and form a complete solid solution for different x value. When the x value increased from 0.2 to 0.8, the permittivity ɛ_r decreased from 115 to 42, the unloaded quality factor Q × f increased from 5030 to 13 030 GHz, and the temperature coefficient τ_f decreased from 336 to −28 ppm/°C. When x =0.7, the best combination of dielectric properties, a near zero temperature coefficient of resonant frequency of τ_f∼−6 ppm/°C, Q × f ∼10 860 GHz and ɛ_r∼51 is obtained.     

The Bi2O3–Nb2O5–NiO Phase Diagram

The Bi₂O₃–Nb₂O₅–NiO phase diagram at 1100°C was determined by means of solid-state synthesis, X-ray diffraction, and scanning electron microscopy. A ternary eutectic with a melting point below 1100°C was found to exist in the field between NiO, Bi₂O₃, and the end-member of the δBi₂O₃–Nb₂O₅ solid solution. The existence of the previously reported Bi₃Ni₂NbO₉ phase was disproved. A pyrochlore homogeneity range around Bi_1.5Ni_0.67Nb_1.33O_6.25 was determined together with all the phase relations in this phase diagram.     

Major Influences on the Particle-Size–Transformation-Temperature Relation in Al2O3–ZrO2 Composites

The energetics of martensitic transformation in ZrO₂ is studied using a thermodynamic approach, with particular reference to Al₂O₃–ZrO₂ composites. The different characters of three types of transformation-toughened ceramics are analyzed, and several factors affecting the t → m transformation in Al₂O₃–ZrO₂ composites are discussed. The expression of transformation temperature dependence on particle size is derived and has good agreement with experimental results. The energetics concerned with nucleation of martensitic transformation is also discussed.