Phase Equilibria in the System ZrO2─InO1.5

Phase equilibria in the system ZrO₂─InO_1.5 have been investigated in the temperature range from 800° to 1700°C Up to 4 mol%, InO_1.5 is soluble in t -ZrO₂ at 1500°C. The martensitic transformation temperature m → t of ZrO₂ containing InO_1.5 is compared with that of ZrO₂ solid solutions with various other trivalent ions with different ionic radii. The diffusionless c → t ' A phase transformation is discussed. Extended solid solubility from 12.4 ± 0.8 to 56.5 ± 3 mol% InO_1.5 is found at 1700°C in the cubic ZrO₂ phase. The eutectoid composition and temperature for the decomposition of c -ZrO₂ solid solution into t -ZrO₂+InO_1.5 solid solutions were determined. A maximum of about 1 mol% ZrO₂ is soluble in bcc InO_1.5 phase. Metastable supersaturation of ZrO₂ in bcc InO _1.5 and conditions for phase separation are discussed.     

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.     

Low-Temperature Equilibria Among ZrO2, Tho2, and UO2

Studies made on low-hafnium-content ZrO₂, show that the monoclinic-tetragonal inversion temperature is 1170°C., and it is raised to approximately 1190°C. in the "natural" ZrO², which contains approximately 2% HfO₂. No explanation could be found for the knownmarked hysteresis during cooling, when the reverse polymorphic transformation takes dace at 1040°C. In the system ZrO₂-ThO₂ the monoclinic-tetragonal ZrO₂, inversion temperature is lowered to 1000°C., although the maximum solid solution extent of ZrO₂, in Thon and vice versa is approximately only 2% at this temperature. Below about 400°C. under hydrothermal conditions it was possible to prepare a continuous, although metastable series of solid solutions with the fluorite structurewith compositions varying from ThO₂, to nearly pure ZrO₂. Contrary to earlier work only 8 mole ZrO₂, dissolves in UO₂ and less than 4 mole of UO, in ZrO₂ at temperatures up to 13OO0C. A continuous series of solid solutions could be made between Th₂ and UO₂ at 13OO°C., and extensive defect fluorite solid solutions could be prepared between Tho₂ and U₃O₈; there is some evidence for exsolution into uranium-rich and thorium-rich members at low temperatures.     

Microstructure and Microwave Dielectric Properties of Ba(Zn1/3Ta2/3)O3 Ceramics with ZrO2 Addition

The effect of ZrO₂ on crystallographic order, microstructure, and microwave dielectric properties of Ba(Zn_1/3Ta_2/3)O₃ (BZT) ceramics was investigated. A small amount of ZrO₂ disturbed the 1:2 cation ordering. The average grain size of the BZT significantly increased with the addition of ZrO₂, which was attributed to liquid-phase formation. The relative density increased with the addition of a small amount of ZrO₂, but it decreased when the ZrO₂ content was increased. Variation of the dielectric constant with ZrO₂ addition ranged between 27 and 30, and the temperature coefficient of resonant frequency increased abruptly as the ZrO₂ amount exceeded 2.0 mol%. The Q value of the BZT significantly improved with the addition of ZrO₂, which could be explained by the increased relative density and grain size. The maximum Q × f value achieved in this investigation was ∼164 000 GHz for the BZT with 2.0 mol% ZrO₂ sintered at 1550°C for 10 h.     

The Zirconia–Hafnia System: DTA Measurements and Thermodynamic Calculations

The equilibrium temperature ( T ₀) at which the tetragonal and monoclinic phases of either ZrO₂ or HfO₂ coexist is generally defined by the middle temperature of A _s (the onset transformation temperature on heating) and M _s (the onset transformation temperature on cooling). It cannot be directly determined due to the athermal nature of the martensitic transformation. Practically, the determination of T ₀ is important for the prediction of A _s and M _s in ZrO₂ or HfO₂-based materials. In this work, the ZrO₂–HfO₂ system was studied experimentally by differential thermal analysis (DTA) to obtain the martensitic tetragonal ⇔ monoclinic transformation temperatures in the temperature range of 1273–1973 K. The T ₀ temperatures obtained for ZrO₂ and HfO₂ are 1367±5 and 2052±5 K, respectively. They are adopted for the assessments of the Gibbs energy parameters of these two oxides. A reasonably calculated ZrO₂−HfO₂ phase diagram is presented.     

Crystallization and Transformation of Zirconia Under Hydrothermal Conditions

During constant-rate heating to 350°C in concentrated NaOH solutions, cubic ZrO₂ crystallized at ∼120°C from hydrated amorphous ZrO₂; these One cubic ZrO₂ particles abruptly changed into needlelike monoclinic ZrO₂ single crystals at 300°C. Crystallization and phase transformation were studied by XRD, TEM, and EPMA. Cubic ZrO₂ appears to crystallize via collapse of the ZrO₂−nH₂O structure and subsequent slight rearrangement of the lattice. The abrupt formation of mono-clinic ZrO₂ was considered to result when the very fine cubic ZrO₂ particles coagulated in a highly oriented fashion.     

Effects of Oxygen Partial Pressure on the Nucleation Behavior and Morphology of Chemically-Vapor-Deposited Zirconia on Hi-Nicalon Fiber and Si

A ZrO₂ coating was prepared on Hi-Nicalon fiber and single-crystal Si by chemical vapor deposition (CVD) using ZrCl₄, CO₂, and H₂ as precursors at 1050°C. The effects of oxygen partial pressure on the nucleation behavior of the CVD-ZrO₂ coating were systematically studied by intentionally varying the controlled amount of O₂ into the CVD chamber. Characterization results suggested that the number density of tetragonal ZrO₂ nuclei apparently decreased with increasing the oxygen partial pressure from 4 × 10⁻³ to 1.6 Pa. Also, the coating layer became more columnar and contained larger monoclinic ZrO₂ grains. The observed relationships between the oxygen partial pressure and the nucleation and morphologic characteristics of the ZrO₂ coating were attributed to the grain size and oxygen deficiency effects, which have been previously reported to cause the stabilization of the tetragonal ZrO₂ phase in bulk ZrO₂ specimens.     

The Pseudobinary Ti-ZrO2

The vertical section Ti-ZrO₂ within the Ti-Zr-O system was investigated by metallographic, X-ray diffraction, electron probe, and melting point studies. Analyses were conducted using arcmelted specimens which had been equilibrated and quenched from temperatures of 600° to 1600°C. The Ti-ZrO₂ section is similar to the Zr-ZrO₂ system. At high temperatures, considerable amounts of Zr and O go into solid solution in Ti, stabilizing α-Ti to 30 wt% ZrO₂. From 30 to 98 wt% ZrO₂ an α-Ti+ZrO₂ region is defined, and at compositions above 98 wt% ZrO₂, single-phase ZrO₂( ss ) exists. At low temperatures an α-Ti+(Ti,Zr)₃O field exists from 22 to 32 wt% ZrO₂; this region decreases in size with increasing temperature until it disappears at 1200°C. Above 32 wt% ZrO₂, a three phase α-Ti+ (Ti,Zr)₃O+ZrO₂ field exists; its stability extends from 1200°C at 30 wt%      

Effect of Zirconia Content on the Oxidation Behavior of Silicon Carbide/Zirconia/Mullite Composites

The oxidation of hot-pressed SiC-particle (SiC_p)/zirconia (ZrO₂)/mullite composites with various ZrO₂ contents, exposed in air isothermally at 1000° and 1200°C for up to 500 h, was investigated; an emphasis was placed on the effects of the ZrO₂ content on the oxidation behavior. A clear critical volume fraction of ZrO₂ existed for exposures at either 1000° or 1200°C: the oxidation rate increased dramatically at ZrO₂ contents of >20 vol%. The sharp transition in the oxidation rate due to the variation of ZrO₂ content could be explained by the percolation theory, when applied to the oxygen diffusivity in a randomly distributed two-phase medium. Morphologically, the composites with ZrO₂ contents greater than the critical value showed a large oxidation zone, whereas the composites with ZrO₂ contents less than the critical value revealed a much-thinner oxidation zone. The results also indicated that the formation of zircon (ZrSiO₄) at 1200°C, through the reaction between ZrO₂ and the oxide product, could reduce the oxidation rate of the composite.     

Phase Diagram of Wollastonite-Zirconia

A reexamination of the system CaO·SiO₂–ZrO₂ has been conducted in order to use this system to obtain ZrO₂-toughened wollastonite materials. The results have shown that CaO·SiO₂–ZrO₂ is a pseudobinary system with an invariant peritectic point at 1467°± 2°C, which is in disagreement with previously reported results.     

Tape Casting of Al2O3/ZrO2 Laminated Composites

Fracture toughness of ZrO₂-toughened alumina could he increased by macroscopic interfaces, such as those existing in laminated composites. In this work, tape casting was used to produce A/A or A/B laminates, where A and B can be Al₂O₃, Al₂O₃/5 vol% ZrO₂, and Al₂O₃/l0 vol% ZrO₂. An increase of toughness is observed, even in the Al₂O₃/Al₂O₃ laminates.     

Chemical Interactions Promoting the ZrO2 Tetragonal Stabilization in ZrO2–SiO2 Binary Oxides

Metastable tetragonal ZrO₂ phase has been observed in ZrO₂–SiO₂ binary oxides prepared by the sol–gel method. There are many studies concerning the causes of ZrO₂ tetragonal stabilization in binary oxides such as Y₃O₂–ZrO₂, MgO–ZrO₂, or CaO–ZrO₂. In these binary oxides, oxygen vacancies cause changes or defects in the ZrO₂ lattice parameters, which are responsible for tetragonal stabilization. Since oxygen vacancies are not expected in ZrO₂–SiO₂ binary oxides, tetragonal stabilization should just be due to the difficulty of zirconia particles growing in the silica matrix. Furthermore, changes in the tetragonal ZrO₂ crystalline lattice parameters of these binary oxides have recently been reported in a previous paper. The changes of the zirconia crystalline lattice parameters must result from the chemical interactions at the silica–zirconia interface (e.g., formation of Si–O–Zr bonds or Si–O⁻ groups). In this paper, FT-IR and ²⁹Si NMR spectroscopy have been used to elucidate whether the presence of Si–O–Zr or Si–O⁻ is responsible for tetragonal phase stabilization. Moreover, X-ray diffraction, Raman spectroscopy, and transmission electron microscopy have also been used to study the crystalline characteristics of the samples.     

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.     

Particle Interactions in Zirconia-Toughened Alumina

In order to determine the effect of the coarse tail of the ZrO₂ size distribution of the monoclinic ZrO₂ content of zirconia-toughened alumina, quantitative hot-stage X-ray measurements were made on (1) A1₂O₃-10% Zr0₂ (submicrometer with a coarse tail >1 μm), (2) A1₂O₃-10% ZrO₂ with the coarse tail of the ZrO₂ carefully removed by centrifugation, and (3) Al₂O₃-10% ZrO₂ in which 10% of Zr0₂(1) and 90% of Zr0₂(2) were used. The monoclinic content of (3) was compared with the weighted average of(1) and (2). A disproportionate amount of monoclinic ZrO₂ was found in (3) and it was concluded that transformation of the coarse particles promotes transformation in finer particles. Microstructural examination supports this conclusion. Results were interpreted as being due both to autocatalytic transformation and to constraint relief from microcracking.     

Stability of Ultrafine Tetragonal ZrO2 Coprecipitated with Al2O3 by the Spray-ICP Technique

The transformation of ultrafine powders (particle size, 0.01 to 0.04 μm) of the system ZrO_2–Al₂O₃, prepared by spraying their corresponding nitrate solutions into an inductively coupled plasma (ICP) of ultrahigh temperature, was investigated. The powders were composed of metastable tetragonal ZrO₂ ( mt- ZrO₂) and γ-Al₂O₃. On heating, the mt- ZrO₂ (or tetragonal ZrO₂, t -ZrO₂) was retained up to 1200°C. At 1380°C the transformation to monoclinic ZrO₂ ( m -ZrO₂) occurred and the amount of the m -ZrO₂ decreased with the increase in Al₂O₃ content, thus indicating the stabilization of the t -ZrO₂ by the Al₂O₃, which seems to be explained in terms of the retardation of grain growth.     

Formation Reaction of ZrO2 Scatterers in ZrF4-Based Fluoride Fibers

This paper clarifies the formation reaction of ZrO₂ crystals which appear as extrinsic scatterers in fluoride fibers. EPMA analysis indicates that BaO exists at grain boundaries of BaF₂ purified by sublimation. BaO reacts with ZrF₄ to form ZrO₂ at 600°C during a glass-melting process. The ZrO₂ formation reaction is influenced by H₂O. Ba(OH)₂, which is formed by the reaction between BaO and water vapor, melts at 370° to 420°C and reacts with ZrF₄ to form ZrO₂ at 450° to 520°C. When low-oxide-content BaF₂ is used for fiber preparation, scatterers significantly decrease.     

Microstructural Studies in Alkoxide-Derived Mullite/Zirconia/Silicon Carbide-Whisker Composites

Mullite composites toughened with ZrO₂ (with or without a MgO or Y₂O₃ stabilizer) and/or SiC whiskers (SiC( w )) were fabricated by hot-pressing powders prepared from Al, Si, Zr, and Mg(Y) alkoxide precursors by a sol–gel process. Micro-structures were studied by using XRD. SEM, and analytical STEM. Pure mullite samples contained prismatic, preferentially oriented mullite grains. However, the addition of ZrO₂, as well as the hot-pressing temperature, affected the morphology and grain size in the composites; a fine, uniform, equiaxed microstructure was obtained. The effect of SiC( W ) was less pronounced than that of ZrO₂. Glassy phases were present in mullite and mullite/SiC( W ) composites, but were rarely observed in Al₂O₃-rich or ZrO₂-containing samples. The formation of zircon due to the reaction between ZrO₂ and SiO₂ and the considerable solid solution of SiO₂ in ZrO₂ prevented the formation of the glassy phase, whereas the reaction between Al₂O₃ and MgO in MgO-containing samples formed a spinel phase and also deprived the ZrO₂ phase of the stabilizer. Intergranular ZrO₂ particles were either monoclinic or tetragonal, depending on size and stabilizer content; small intragranular ZrO₂ inclusions were usually tetragonal in structure.     

Infrared Absorption Spectroscopy in Zirconia Research

Infrared absorption spectra are shown for monoclinic ZrO₂ and for cubic stabilized ZrO₂. Nine bands are reported in monoclinic ZrO₂ in the region 800 to 200 cm^−l, whereas only one broad band is observed in cubic ZrO₂ over the same frequency range.     

Control of Grain-Boundary Pinning in Al2O3/ZrO2 Composites with Ce3+/Ce4+ Doping

The control of the microstructure of Ce-doped Al₂O₃/ZrO₂ componsites by the valence change of cerium ion has been demonstrated. Two distinctively different types of microstructure, large Al₂O₃ grains with intragranular ZrO₂ particles and small Al₂O₃ grains with intergranular ZrO₂ particles, can be obtained under identical presintering processing conditions. At doping levels greater than ∼ 3 mol% with respect to ZrO₂, Ce³⁺ raises the alumina grain-boundary to zirconia particle mobility ratio. This causes the breakaway of grain boundary from particles and the first type of microstructure. On the other hand, Ce⁴⁺ causes no breakaway and produces a normal intergranular ZrO₂ distribution. The dramatic effect of Ce³⁺ on the relative mobility ratio is found to be associated with fluxing of the glassy boundary phase and is likewise observed for other large trivalent cation dopants. The ZrO₂ second phase acts as a scavenger for these trivalent cations, provided their solubility limit in ZrO₂ is not exceeded.     

Hindrance of Grain Growth in Al2O3 by ZrO2 Inclusions.

Alumina and Al₂O₃/ZrO₂ (1 to 10 vol%) composite powders were mixed and consolidated by a colloidal method, sintered to >98% theoretical density at 1550°C, and subsequently heat-treated at temperatures up to 1700°C for grain-size measurements. Within the temperature range studied, the ZrO₂ inclusions exhibited sufficient self-diffusion to move with the Al₂O₃ 4-grain junctions during grain growth. Growth of the ZrO₂, inclusions occurred by coalescence. The inclusions exerted a dragging force at the 4-grain junctions to limit grain growth. Abnormal grain growth occurred when the inclusion distribution was not sufficiently uniform to hinder the growth of all Al₂O₃ grains. This condition was observed for compositions containing ≤2.5 vol% ZrO₂, where the inclusions did not fill all 4-grain junctions. Exaggerated grains consumed both neighboring grains and ZrO₂, inclusions. Grain-growth control (no abnormal grain growth) was achieved when a majority (or all) 4-grain junctions contained a ZrO₂ inclusion, viz., for compositions containing ≥5 vol% ZrO₂. For this condition, the grain size was inversely proportional to the volume fraction of the inclusions. Since the ZrO₂ inclusions mimic voids in all ways except that they do not disappear, it is hypothesized that abnormal grain growth in single-phase materials is a result of a nonuniform distribution of voids during the last stage of sintering.