The Calcia–Zirconia Phase Diagram Revisited: Stability of the Ordered Phases φ1 and φ2

Single crystals of CaO-stabilized ZrO₂ containing between 15.3 and 18.9 mol% CaO were heat-treated for 5000 h at 1200°C to study the stability of the ordered defect-fluorite phase, CaZr₄O₉ (φ₁). Subsequent TEM analysis of the equiaxed φ₁ domains in samples richer than approximately 18 mol% in CaO showed a random distribution of φ₁ variants, with no preferred interfacial habit planes. A critical review of the literature, combined with the new data, supports the Stubican-Hellmann-Hannon version of the phase diagram in the region 15 to 26 mol% CaO and 1000° to 1400°C and strongly suggests that φ₁ is a stable phase in the ZrO₂-CaO systems.     

Subsolidus Phase Equilibria and Ordering in the System ZrO2-Y2O3

The subsolidus phase relations in the entire system ZrO₂-Y₂O₃ were established using DTA, expansion measurements, and room- and high-temperature X-ray diffraction. Three eutectoid reactions were found in the system: ( a ) tetragonal zirconia solid solution→monoclinic zirconia solid solution+cubic zirconia solid solution at 4.5 mol% Y₂O₃ and ∼490°C, ( b ) cubic zirconia solid solutiow→δ-phase Y₄Zr₃O₁₂+hexagonalphase Y₆ZrO₁₁ at 45 mol% Y₂O₃ and ∼1325°±25°C, and ( c ) yttria C -type solid solution→wcubic zirconia solid solution+ hexagonal phase Y₆ZrO₁₁ at ∼72 mol% Y₂O₃ and 1650°±50°C. Two ordered phases were also found in the system, one at 40 mol% Y₂O₃ with ideal formula Y₄Zr₃O₁₂, and another, a new hexagonal phase, at 75 mol% Y₂O₃ with formula Y₆ZrO₁₁. They decompose at 1375° and >1750°C into cubic zirconia solid solution and yttria C -type solid solution, respectively. The extent of the cubic zirconia and yttria C -type solid solution fields was also redetermined. By incorporating the known tetragonal-cubic zirconia transition temperature and the liquidus temperatures in the system, a new tentative phase diagram is given for the system ZrO₂-Y₂O₃.     

Development of Nano-Composite Microstructures in ZrO2-Al2O3 via the Solution Precursor Method

Aqueous mixtures of zirconium acetate and aluminum nitrate were pyrolyzed and crystallized to form a metastable solid solution, Zr_{1- x} Al _x O_{2− x /2} ( x < 0.57). The initial, metastable phase partitions at higher temperatures to form two metastable phases, viz., t −ZrO₂+γ-Al₂O₃ with a nano-scale microstructure. The microstructural observations associated with the γ- →α-Al₂O₃ phase transformation in the t -ZrO₂ matrix are reported for compositions containing 10, 20, and 40 mol% A1₂O₃. During this phase transformation, the α-Al₂O₃ grains take the form of a colony of irregular, platelike grains, all with a common crystallographic orientation. The plates contain ZrO₂ inclusions and are separated by ZrO₂ grains. The volume fraction of A1₂O₃ and the heat treatment conditions influence the final microstructure. At lower volume fractions of A1₂O₃, the colonies coarsen to single, irregular plates, surrounded by polycrystalline ZrO₂. Interpenetrating microstructures produced at high volume fractions of A1₂O₃ exhibit very little grain growth for periods up to 24 h at 1400°C.     

Phase Relations and Ordering in the System ZrO2-MgO

The Phase relationships in the system ZrO₂-MgO were reinvestigated over a wide range of temperatures and compositions. The extent of the cubic solid solution field was determined with precise lattice parameter measurements and a high-temperature X-ray furnace using analyzed samples. DTA results show that the addition of MgO to ZrO₂ decreases the transition temperature for monoclinic ⇌ tetragonal ZrO₂ and 1 mol% of MgO is soluble in the monoclinic zirconia at ∼1070°C.The invariant eutectiod point is at 13.5 ± 0.3 mol% MgO at 1406°± 7°C, which is in fair agreement with previous results by Grain. The ordered phase Mg₂Zr₅O₁₂ (δ-phase) can form metastably in cubic solid solutions at temperatures as low as 800°C after prolonged annealing. Evidence for the existence of the ordered phase MgZr₆O₁₃(γ-phase) was obtained by electron diffraction technique. Conditions for the formation of this phase are described. The ordered phases in this system are metastable and their formation is an intermediate step in the eutectoid decomposition of the cubic phase.     

Phase Equilibria and Ionic Conductivity in the System ZrO2−Yb2O3−Y2O3

The phase equilibria in the zirconia-rich part of the system ZrO₂−Yb₂O₃−Y₂O₃ were determined at 1200°, 1400°, and 1650°C. The stabilizing effects of Yb₂O₃ and Y₂O₃ were found to be quite similar with <10 mol% of either being necessary to fully stabilize the cubic fluorite-structure phase at 1200°C. The two binary ordered phases, Zr₃Yb₄O₁₂ and Zr₃Y₄O₁₂, are completely miscible at 1200°C. These were the only binary or ternary phases detected. The ionic conductivities of ternary specimens in this system were measured using the complex impedance analysis technique. For a given level of total dopant, the substitution of Yb₂O₃ for Y₂O₃ gives only minor increases in specimen conductivity.     

Densification of Cr2O3-ZrO2 Ceramics by Sintering

Cr₂O₃ and ZrO₂ were mixed in various ratios and pressed to form compacts, which were then sintered in carbon powder. Compacts with >30 wt% Cr₂O₃ were sintered to densities >98% of true density at 1500°C. This method of sintering in carbon powder can be used to prepare very dense Cr₂O₃-ZrO₂ ceramics at a relatively low temperature, (∼1500°C) without additives.     

Crystallization of MgO-Al2O3-SiO2-ZrO2 Glasses

The crystallization of MgO-Al₂O₃-SiO₂-ZrO₂ glasses at 1000°C was studied. Isothermal heat treatments of a cordierite-based glass (2MgO.2Al₂O₃.5SiO₂= Mg₂Al₄Si₅O₁₈) with 7 wt% ZrO₂ produced surface crystallization of α-cordierite and tetragonal ZrO₂ ( t -ZrO₂). These phases advanced into the glass by cocrystallization of t -ZrO₂ rods in an α-cordierite matrix with a well-defined orientation relation. The t -ZrO₂ rods were unstable with respect to diffusional breakup (a Rayleigh instability) and decomposed into rows of aligned ellipsoidal and spheroidal particles. The t -ZrO₂ was very resistant to transformation to monoclinic symmetry. With a similar glass containing 15 wt% ZrO₂, surface crystallization of α-cordierite and t -ZrO₂ was accompanied by internal crystallization of t -ZrO₂ dendrites. Transformation of the dendrites to mono-clinic symmetry was observed under some conditions.     

REVISION OF CRYSTALLINE PHASES IN THE SYSTEM ZRO2-P2O5

Crystalline phases in the system ZrO₂-P₂O_s were reexamined. Samples of varied composition were prepared by the sol-gel route and calcining, as well as by reacting mixtures ofZrO₂ and NH4H2PO4, at temperatures from 200° to 1400°C. Confusion regarding compositions and structures was clarified, and the phase Zr₃(PO₄)₄ was confirmed. The latter can be written Zro.₂sZr₂(PO₄)₃ and is iso-structural withNaZr₂P₃O₁₂. The role of mineralizers and the influence of zirconium salts can now be understood.     

Stability of ZrO2 Phases in Ultrafine ZrO2-Al2O3 Mixtures

Mixtures of ultrafine monoclinic zirconia and aluminum hydroxide were prepared by adding NH₄OH to hydrolyzed zirconia sols containing varied amounts of aluminum sulfate. The mixtures were heat-treated at 500° to 1300°C. The relative stability of monoclinic and tetragonal ZrO₂ in these ultrafine particles was studied by X-ray diffractometry. Growth of ZrO₂ crystallites at elevated temperatures was strongly inhibited by Al₂O₃ derived from aluminum hydroxide. The monoclinic-to-tetragonal phase transformation temperature was lowered to ∼500°C in the mixture containing 10 vol% Al₂O₃, and the tetragonal phase was retained on cooling to room temperature. This behavior may be explained on the basis of Garvie's hypothesis that the surface free energy of tetragonal ZrO₂ is lower than that of the monoclinic form. With increasing A1₂O₃ content, however, the transformation temperature gradually increased, although the growth of ZrO₂ particles was inhibited; this was found to be affected by water vapor formed from aluminum hydroxide on heating. The presence of atmospheric water vapor elevates the transformation temperature for ultrafine ZrO₂. The reverse tetragonal-to-monoclinic transformation is promoted by water vapor at lower temperatures. Accordingly, it was concluded that the monoclinic phase in fine ZrO₂ particles was stabilized by the presence of water vapor, which probably decreases the surface energy.     

Subsolidus Phase Relations in the Pseudoternary System ZrO2-YO1.5-CrO1.5 in Air

The pseudoternary system ZrO₂-YO_1.5CrO_1.5 was studied between 1300° and 1600C in air by °a quenching method. No ordered phase of the type ZrY₆O₁₁ was detected, but an ordered Zr₃Y₄O₁₂ phase at 1300°C and YCrO₃ were observed as intermediate compounds. Solid solutions ofZrO₂ and YO_1.5 coexisting with CrO_1.5 and/or YCrO₃ formed; the apex occurred between 26.5 and 27.5 wt% YO_1.5 for the cubic ZrO₂+CrO_1.5+YCrO₃, three-phase region; CrO_1.5 is slightly soluble in ZrO₂(ss).     

High-Temperature Phase Relations in the System Y2O3-Ta2O5

The phase relations for the system y₂o₃–Ta₂o₅ in the composition range 50 to 100 mol% Y₂O₃ have been studied by solid-state reactions at 1350°, 1500°, or 17000C and by thermal analyses up to the melting temperatures. Weberite-type orthorhombic phases (W2 phase, space group C222₁), fluorite-type cubic phases (F phase, space group Fm3m )and another orthorhombic phase (O phase, space group Cmmm )are found in the system. The W2 phase forms in 75 mol% Y₂O₃ under 17000C and O phase in 70 mol% Y₂O₃ up to 1700°C These phases seem to melt incongruently. The F phase forms in about 80 mol% Y₂O₃ and melts congruently at 2454° 3°C. Two eutectic points seem to exist at about 2220°C 90 mol% Y²O₃, and at about 1990°C, 62 mol% Y₂O₃. A Phase diagram including the above three phases were not identified with each other.     

Pyrochlore Phases in the System ZnO–Bi2O3–Sb2O5: I. Stoichiometries and Phase Equilibria

Two cubic pyrochlore phases exist in the system ZnO–Bi₂O₃–Sb₂O₅. Neither has the supposed "ideal" stoichiometry, Zn₂Bi₃Sb₃O₁₄. One, P ₁, is a solid solution phase, Zn_{2+ x} Bi_{2.96−( x − y )}Sb_{3.04− y} O_14.04+δ where 0< x <0.13(1), 0< y <0.017(2) and a =10.4285(9)−10.451(1) Å. The other, P ₂, is a line phase, Zn₂Bi_3.08Sb_2.92O_13.92 with a =10.462(2) Å. Subsolidus phase relations at 950°C involving phases P ₁ and P ₂ in the ZnO–Bi₂O₃–Sb₂O₅ phase diagram have been determined.     

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%      

Subsolidus Equilibria in the System ZrO2-WO3-P2O5

The existence of stable and metastable forms of 2ZrO₂·P₂O₅ and the subsolidus phase relations in the system ZrO₂-ZrP₂O₇ were confirmed before investigation of the ternary system. The synthesis and thermal behavior of ZrW₂O₈ were reinvestigated, and the system WO₃-P₂O₅ was examined cursorily. A ternary compound, 2ZrO₂·WO₃·P₂O₅, was found, and compatibility triangles for the system between 1105° and 1150°C were established. The ternary compound is compatible with ZrO₂, WO₃, and three binary compounds, giving rise to five composition triangles. In addition, ZrP₂O₇, WO₃, and "W₂O₃(PO₄)₂" were compatible.     

In Situ Martensitic Transformation in a Ternary MgO-Y2O3-ZrO2 Alloy: I, Transformation in Tetragonal ZrO2 Grains

The martensitic transformation in tetragonal ZrO₂ grains in a ternary MgO-Y₂O₃-ZrO₂ alloy has been studied using in situ observations in the transmission electron microscope. Transformation occurred by the nucleation and growth of monoclinic laths; thermoelastic equilibrium can be maintained at different extents of transformation by continuously varying the applied stress. The product phase was always twinned, but two twinning mechanisms were found-sequential formation of twin-related variants and posttransformation deformation twinning. In one example, a (401)/(410) pair of martensite habit planes led to a (100) conjunction twinning plane.     

Phase Relations in the System Bi2O3-WO3

Phase relations in the system Bi₂O₃-WO₃ were studied from 500° to 1100°C. Four intermediate phases, 7Bi₂O₃· WO₃, 7Bi₂O₃· 2WO₃, Bi₂O₃· WO₃, and Bi₂O₃· 2WO₃, were found. The 7B₂O · WO₃ phase is tetragonal with a ₀= 5.52 Å and c ₀= 17.39 Å and transforms to the fcc structure at 784°C; 7Bi₂O₃· 2WO₃ has the fcc structure and forms an extensive range of solid solutions in the system. Both Bi₂O₃· WO₃ and Bi₂O₃· 2WO₃ are orthorhombic with (in Å) a ₀= 5.45, b ₀=5.46, c ₀= 16.42 and a ₀= 5.42, b ₀= 5.41, c ₀= 23.7, respectively. Two eutectic points and one peritectic exist in the system at, respectively, 905°± 3°C and 64 mol% WO₃, 907°± 3°C and 70 mol% WO₃, and 965°± 5°C and 10 mol% WO₃.     

The Al2O3-Nd2O3 Phase Diagram

A tentative phase diagram for the system Al₂0₃-Nd₂O₃ is presented. Three compounds were obtained: a β -A1₂O₃-type compound, the perovskite NdAlO₃, and Nd₄Al₂O₉. The perovskite melts congruently (mp 2090°C), and the two other compounds exhibit incongruent melting behavior: β -Nd/Al₂O₃, mp 1900°C; Nd₄Al₂O₉, mp 1905°C. Two eutectics exist with the following compositions and melting points: 80 mol% Al₂O₃, 1750°C; 23 mol% Al₂O₃,1800°C. Nd₄Al₂O9 decomposes in the solid state at 1780°C.     

Synthesis of ZrO2 and Y2O3-Doped ZrO2 Thin Films Using Self-Assembled Monolayers

Undoped or Y₂O₃-doped ZrO₂ thin films were deposited on self-assembled monolayers (SAMs) with either sulfonate or methyl terminal functionalities on single-crystal silicon substrates. The undoped films were formed by enhanced hydrolysis of zirconium sulfate (Zr(SO₄)·4H₄O) solutions in the presence of HCl at 70°C. Typically, these films were a mixture of two phases: nanocrystalline tetragonal- ( t -) ZrO₂ and an amorphous basic zirconium sulfate. However, films with little or no amorphous material could be produced. The mechanism of film formation and the growth kinetics have been explained through a coagulation model involving homogeneous nucleation, particle adhesion, and aggregation onto the substrate. Annealing of these films at 500°C led to complete crystallization to t -ZrO₂. Amorphous Y₂O₃-containing ZrO₂ films were prepared from a precursor solution containing zirconium sulfate, yttrium sulfate (Y₂(SO₄)₃8·H₂O), and urea (NH₂CONH₂) at pH 2.2–3.0 at 80°C. These films also were fully crystalline after annealing at 500°C.     

Formation and Transformation of Cubic ZrO2 Solid Solutions in the System ZrO2—Al2O3

In the system ZrO₂-Al₂O₃, cubic ZrO₂ solid solutions containing up to 40 mol% Al₂O₃ crystallize at low temperatures from amorphous materials prepared by the simultaneous hydrolysis of zirconium and aluminum alkoxides. The values of the lattice parameter, a, increase linearly from 0.5095 to 0.5129 nm with increasing Al₂O₃ content. At higher temperatures, the solid solutions transform into tetragonal ZrO₂ and α-Al₂O₃. Pure ZrO₂ crystallizes in the tetragonal form at 415° to 440°C.     

Microstructural Characterization and Fracture Toughness of Cordierite-ZrO2 Glass-Ceramics

Crystallization of an MgO-Al₂O₃-SiO₂-ZrO₂ sintered glass frit was studied. Heat treatment at 850° or 900°C caused initial crystallization of μ-cordierite and tetragonal ( t ) ZrO₂. The t -ZrO₂ crystallized with an irregular dendritic morphology and could be transformed to monoclinic ( m ) symmetry under certain conditions; the cordierite underwent the μ→α a transformation with extended annealing. Heat treatments at 1000°C caused crystallization of t -ZrO₂ rods and spheroids in an α-cordierite matrix; these ZrO₂ crystals, however, are resistant to transformation to m -ZrO₂. The beneficial effects of ZrO₂ on the fracture toughness of cordierite-based glass-ceramics are described.