Anisotropic Thermal Expansion of Tetragonal Zirconia Polycrystals

Thermal expansion coefficients (α _a and α _c ) in two crystallographic axes ( a and c ) of the tetragonal phase are measured at 25°–1200°C in ZrO₂–M₂O₃ (M = Sc, In, Yb) and in ZrO₂–YTaO₄. The difference between these two thermal expansion coefficients, α _c –α _a , decreases with M₂O₃ or YTaO₄ composition even though the tetragonality ( c/a ) behaves differently in these two systems. The locus of α _c =α _a represents a maximum tetragonality for the tetragonal phase, but not the phase boundary for the cubic phase. The relationships among thermal expansion, temperature, and composition are discussed.     

Ferroelectric and Structural Properties of the Pb(Sc1/2Nb1/2)1-x,Tix,O3 System

Extensive solid solution was observed in the system Pb(Sc_1/2/,Nb_1/2,)_1-x,Ti_x,O₃. In the range 0 ≤ x ≤ 0.425 a rhombohedral ferroelectric phase was stable at 25° C. In the range 0.45 ≤ x ≤ 1.00 a tetragonal ferroelectric phase was stable at this temperature. The phase diagram of the system below 500° C strongly resembles that of PbZrO₃−PbTiO₃. The compound Pb(Sc_1/2Nb_1/2)O₃ exhibited rhombohedral perovskite cell symmetry below the ferroelectric ↔ paraelectric transition temperature, and the angle a was acute. The radial coupling coefficient was 0.46 for the composition Sc_1/2Nb_1/2)_0.575Ti_0.4250O₃. At 25°C this composition consisted primarily of the rhombohedral phase with a small amount of the tetragonal phase present. The ferroelectric ↔ paraelectric transition occurred over a temperature range in the rhombohedral phase field. The spontaneous polarization was finite at temperatures considerably above the temperature of the permittivity maximum for a given rhombohedral solid solution.     

Transformation-Induced Twinning: The 90° and 180° Ferroelectric Domains in Tetragonal Barium Titanate

Ferroelectric twin domains resulting from the cubic ( c ) to tetragonal ( t ) phase transformation at the Curie point T _C≈130°C in pressureless-sintered BaTiO₃ ceramics, using TiO₂-excess powder, have been investigated using scanning and transmission electron microscopy. Both 90° and 180° domains were identified by spot splitting along characteristic crystallographic directions in the selected-area diffraction patterns and/or from the shape of domain boundaries. Lamellar domains were found predominantly with the 90° types. The 180° domain boundaries mostly appeared wavy in shape, while the 90° ones, having sharpened ends, attained a dagger shape. Failure of Friedel's law in the non-centrosymmetric t -BaTiO₃ was adopted to validate the existence of the 180° domains. The 90° domains with boundaries lying in     are reflection–inversion twins, and the 180° domains lying in {100) _t and }220) _t are inversion twins. Convergent-beam electron diffraction was performed to ensure that changing of the polar direction [001] _t across the 90° and the 180° domain boundaries was consistent with the domain type. It was also used to confirm whether the 180°-type walls are inversion domain boundaries produced by the loss of an inversion center when the cubic phase transforms into tetragonal symmetry. The formation of such ferroelectric domains is discussed with reference to the crystal symmetry reduction from     ( c -phase) to P 4 mm ( t -phase) with a loss of mirror plane (m) and roto-inversion axis     upon c → t phase transformation.     

Raman Scattering Study of Cubic–Tetragonal Phase Transition in Zr1−xCexO2 Solid Solution

A structural phase transition between the cubic (space group, Fm 3 m) and tetragonal (space group, P 4₂ /nmc) phases in a zirconia–ceria solid solution (Zr_1−xCe_xO₂) has been observed by Raman spectroscopy. The cubic–tetragonal ( c–t" ) phase boundary in compositionally homogeneous samples exists at a composition X₀ (0.8 < X₀ < 0.9) at room temperature, where t " is defined as a tetragonal phase whose axial ratio c/a equals unity. The axial ratio c/a decreases with an increase of ceria concentration and becomes 1 at a composition X'₀ (0.65 < X'₀ < 0.7) at room temperature. The sample with a composition between X₀ and X'₀ is t " ZrO₂. By Raman scattering measurements at high temperatures, the tetragonal ( t" ) → cubic and cubic → tetragonal phase transitions occur above 400°C in Zr_0.2 Ce_0.8O₂ solid solution.     

Influence of Sintering Temperature on Grain Growth and Phase Structure of Compositionally Optimized High-Performance Li/Ta-Modified (Na,K)NbO3 Ceramics

Microstructure characteristics, phase transition, and electrical properties of (Na_0.535K_0.485)_0.926Li_0.074(Nb_0.942Ta_0.058)O₃ (NKN-LT) lead-free piezoelectric ceramics prepared by normal sintering are investigated with an emphasis on the influence of sintering temperature. Some abnormal coarse grains of 20–30 μm in diameter are formed in a matrix consisting of about 2 μm fine grains when the sintering temperature was relatively low (980°C). However, only normally grown grains were observed when the sintering temperature was increased to 1020°C. On the other hand, orthorhombic and tetragonal phases coexisted in the ceramics sintered at 980°–1000°C, whereas the tetragonal phase becomes dominant when sintered above 1020°C. For the ceramics sintered at 1000°C, the piezoelectric constant d ₃₃ is enhanced to 276 pC/N, which is a high value for the Li- and Ta-modified (Na,K)NbO₃ ceramics system. The other piezoelectric and ferroelectric properties are as follows: planar electromechanical coupling factor k _p=46.2%, thickness electromechanical coupling factor k _t=36%, mechanical quality factor Q _m=18, remnant polarization P _r=21.1 μC/cm², and coercive field E _c=1.85 kV/mm.     

Barium Titanate Perovskite Sintered with Lithium Fluoride

The sintering of barium titanate with, respectively, 1 and 2 wt% LiF for two stoichiometries, Ti/Ba=1 and 0.975, was studied using two calcined powders. One was pure barium titanate; the other contained BaTi0₃ plus BaC0₃ and TiO₂ that did not react when burning. The sintering chronology—intermediate phases, appearance, and disappearance of a liquid phase that has been pointed out for the first time—is directly dependent on the used calcined powder, on the LiF amount, and on the firing schedule. In the same way, the obtained perovskite symmetry varies during sintering from tetragonal to cubic and then to a second tetragonal form, whereas most of the Li and F disappear from the ceramic with two different kinetics.     

Phase Transitional Behavior and Piezoelectric Properties of Lead-Free (Na0.5K0.5)NbO3–(Bi0.5K0.5)TiO3 Ceramics

(1− x )(Na_0.5K_0.5)NbO₃–(Bi_0.5K_0.5)TiO₃ solid solution ceramics were successfully fabricated, exhibiting a continuous phase transition with changing x at room temperature from orthorhombic, to tetragonal, to cubic, and finally to tetragonal symmetries. A morphotropic phase boundary (MPB) between orthorhombic and tetragonal ferroelectric phases was found at 2–3 mol% (Bi_0.5K_0.5)TiO₃ (BKT), which brings about enhanced piezoelectric and electromechanical properties of piezoelectric constant d ₃₃=192 pC/N and planar electromechanical coupling coefficient k _p=45%. The MPB composition has a Curie temperature of 370°–380°C, comparable with that of the widely used PZT materials. These results demonstrate that this system is a promising lead-free piezoelectric candidate material.     

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.     

Coexistence of Relaxor and Normal Ferroelectric Phases in Morphotropic Phase Boundary Compositions of Lanthanum-Modified Lead Zirconate Titanate

Hot-stage transmission electron microscopy studies of lanthanum-modified lead zirconate titanate (Pb_{1- x} La _x -(Zr_0.53Ti_0.47)_{1- x /4}O₃ (PLZT 100 x /53/47) have been performed for compositions close to the morphotropic phase boundary (MPB). These studies have revealed the coexistence of relaxor and normal ferroelectric phases. Lanthanum substitution disrupts long-range ferroelectric order, resulting in short-range polar ordering. Investigations also have been performed using dielectric spectroscopy, electrically induced polarization, and strain methods. Three phases have been found for 10/53/47: a low-temperature tetragonal phase, a high-temperature cubic phase, and an intermediate-temperature pseudocubic phase that exhibits relaxor ferroelectric behavior. Induced strain studies of 10/53/47 show a gradual change from butterfly-shaped to slim-quadratic loops with increasing temperature. In addition, the remanent strain decreases with increasing temperature, and reversible strain shows a peak at 80°C that corresponds to the appearance of intermediate phase.     

Preparation of Ultra-Fine Nickel Manganite Powders and Ceramics by a Solid-State Coordination Reaction

A solid-state coordination reaction was adopted to prepare negative temperature coefficient ceramics. A mixed oxalate NiMn₂(C₂O₄)₃·6H₂O, a coordination compound, was synthesized by milling a mixture of nickel acetate, manganese acetate, and oxalic acid for 5 h at room temperature. An ultrafine NiMn₂O₄ powder was obtained by calcining the mixed oxalate in air at 850°C for 2 h. Ceramics with a relative density of more than 97% were achieved by sintering powder compacts at a temperature as low as 1050°C for 5 h. The specific electrical resistivity ρ_25°C and the thermal constant B _25°/85°C were 2174 Ω·cm and 3884 K, respectively. The drift of the resistivity after aging at 150°C for 1000 h was 3.0%.     

Synthesis of High Magnetoelectric Coefficient Composites Using Annealing and Aging Route

In this paper, aging temperature and time dependence of ferroelectric, ferromagnetic, and magnetoelectric properties of lead zirconate titanate and nickel ferrite composite (1– x ) Pb(Zr_0.52Ti_0.48)O₃– x NiFe_1.9Mn_0.1O₄ (PZT–NFM, x =0.03, 0.05, 0.1) are reported. The magnetoelectric composites of different compositions were fabricated by using a process based on the controlled precipitation route. The processing is a combination of conventional mixed oxide sintering and thermal treatment. The thermal treatment consists of annealing at a high temperature (800°C for 10 h), followed by aging in the range of 300–400°C for 3–15 h. X-ray diffractometry patterns show variation in the amount of spinel phase with different aging temperature and time. Scanning electron microscopy investigation showed that the annealing and aging treatment increases the homogeneity of NFM in the PZT matrix. The magnitude of piezoelectric constant ( d ₃₃), piezoelectric voltage constant ( g ₃₃), and the dielectric constant exhibited significant variation in magnitude with aging time and temperature. Aging at 400°C for 5 h exhibited a maximum magnitude of d ₃₃ and g ₃₃ and minimum magnitude of dielectric constant. The magnitude of the magnetoelectric (ME) voltage coefficient (d E /d H ) for 0.9PZT–0.1NFM samples sintered at 1125 °C was found to be of the order of 78 mV/cm Oe. This magnitude increased to 140 mV/cm Oe after annealing and aging at 300°C for 5 h. This significant enhancement in the magnetoelectric coefficient is probably due to the homogenization and precipitation of the spinel (NFM) phase in the perovskite matrix.     

Reaction During Sintering of Barium Titanate with Lithium Fluoride

The reactions occurring during sintering of stoichiometric BaTiO₃ with small additions of LiF were studied at temperatures between 700° and 900°C. BaTiO₃ reacts with LiF to form a cubic solid solution and Li₂TiO₃ During sintering, the cubic solid solution coexists with Li₂Ti0₃ and forms a liquid phase at 740°± 5°C. The occurrence of a liquid phase at this temperature results in an enhancement of the sintering process and leads to the development of a highly dense microstructure.     

Revised Phase Diagram of the System ZrO2-CeO2 Below 1400°C

The phase diagram of the system ZrO₂-CeO₂ was rein-vestigated using hydrothermal techniques. Cubic, tetragonal, and monoclinic solid solutions are present in this system. The tetragonal solid solution decomposes to monoclinic and cubic solid solutions by a eutectoid reaction at 1050°50°C. The solubility limits of the tetragonal and cubic solid solutions are about 18 and 70 mol% CeO₂, respectively, at 1400°C, and about 16 and 80 mol% CeO₂, respectively, at 1200°C. Solubility limits of the monoclinic and cubic solid solutions are about 1.5 and 88 mol% CeO₂ at 1000°C, and 1.5 and 98 mol% CeO₂ at 800°C, respectively. The compound Ce₂Zr₃O₁₀ is not found in this system.     

Polymorphic Behavior of Thin Evaporated Films of Zirconium and Hafnium Oxides

Polymorphism in thin evaporated films of zirconium and hafnium oxides was investigated from 100° to 1500°C by electron diffraction and transmission electron microscopy. The films have metastable cubic structures at room temperature and at moderate temperatures. Zirconium oxide, depending on temperature, exists in cubic, tetragonal, and monoclinic forms, whereas hafnium oxide transforms directly from the cubic to the monoclinic structure. The transformation temperatures depend on the oxygen partial pressure. Air annealing of thin films of ZrO₂ and HfO₂ lowered the temperature of transformation of the tetragonal and the cubic structure into the monoclinic structure by about 150° and 100°C, respectively. The cubic/tetragonal transformation of ZrO₂ is monotropic, whereas the tetragonal monoclinic transformation occurs by the typical nucleation and growth mechanism. Determination of grain size in ZrO₂ at the tetragonal/monoclinic transformation temperature showed that the transformation occurs when a constant grain size of about 800 Å is reached. The oxygen partial pressure, grain size, and temperatures at which the metastable phases exist were correlated. The rate of grain growth is enhanced by increase in oxygen partial pressure. The accelerated transformation in air is attributed to rapid attainment of the critical size; grain boundary energy is an important controlling factor in transformation.     

Enhancing Electrical Properties in NBT–KBT Lead-Free Piezoelectric Ceramics by Optimizing Sintering Temperature

Conventional sintering of (Na_{1− x} K _x )_0.5Bi_0.5TiO₃ (abbreviated as NKBT x , x =18–22 mol%) lead-free piezoelectric ceramics was investigated to clarify the optimal sintering temperature for densification and electrical properties. Both sintered density and electrical properties were sensitive to sintering temperature; particularly, the piezoelectric properties deteriorated when the ceramics were sintered above the optimum temperature. The NKBT20 and NKBT22 ceramics synthesized at 1110°–1170°C showed a phase transition from tetragonal to rhombohedral symmetry, which was similar to the morphotropic phase boundary (MPB). Because of such MPB-like behavior, the highest piezoelectric constant ( d ₃₃) of about 192 pC/N with a high electromechanical coupling factor ( k _p) of about 32% were obtained in the NKBT22 ceramics sintered at 1150°C.     

High-Pressure Phase Transitions in Zirconia and Yttria-Doped Zirconia

Raman spectroscopy has been utilized to characterize the phase transformations and transition pressures in pure and doped zirconia containing 3, 4, and 5 wt% Y₂O₃. The pressure-induced transformations were investigated to over 6 GPa (at room temperature) using a diamond anvil pressure cell. Pure zirconia single-crystal samples transformed to a "new" tetragonal phase (different from the one obtained at high temperatures at atmospheric pressure) at about 4 GPa. The pressure transformation, like the temperature transition, was reversible and exhibited an approximately 0.45-GPa hysteresis at room temperature. The 3 and 4 wt% Y₂O₃ crystals underwent a monoclinic ( P 2_1/b) to tetragonal ( P 4_{2 nmc}) phase transition similar to that observed at high temperatures. This phase change was found to be irreversible on releasing the pressure. The 5 wt% Y₂O₃ at atmospheric pressure consists of a tetragonal modification in a disordered cubic matrix; a gradual, but reversible, disordering transformation of the tetragonal precipitate takes place with pressure.     

The System Zirconia-Scandia

The system zirconia-scandia was investigated using X-ray diffraction analysis, differential thermal analysis, metallographic analysis, and melting point studies. Results reveal the monoclinic α₁ phase (0 to 2 mol% Sc₂O₃), the tetragonal α₂'phase (5 to 8% Sc₂O₃), the rhombohedral β phase (9 to 13% Sc₂O₃), the rhombohedral γ phase (15 to 23% Sc₂O₃), the rhombohedral δ phase (24 to 40% Sc₂O₃), and the cubic % phase (77.5 to 100% Sc₂O₃). The monoclinic α₁ phase and the tetragonal α₂'phase were found to transform to the tetragonal α₂ phase over a wide temperature range depending on composition. The β, γ, and α phases transformed to a cubic phase at temperatures of %600^%, 1100^%, and 1300^%C, respectively. A maximum melting point of %2870^%C was found at %10% Sc₂O₃ and a eutectic at %2400^%C at 55% Sc₂O₃.     

Influences of Sintering Temperature on Piezoelectric, Dielectric and Ferroelectric Properties of Li/Ta-Codoped Lead-Free (Na,K)NbO3 Ceramics

Li/Ta-codoped lead-free (Na,K)NbO₃ ceramics with a nominal composition of [(Na_0.535K_0.480)_0.942Li_0.058](Nb_0.90Ta_0.10)O₃ were synthesized normally at 1070°–1100°C. The XRD patterns of all samples show a single pervoskite structure with tetragonal symmetry. Although MPB separating the orthorhombic and tetragonal symmetries was absent, the maximum piezoelectric coefficient ( d ₃₃), electromechanical coupling coefficient ( k _p), Curie temperature ( T _c), and remanent polarization ( P _r) were optimized as 216 pC/N, 38.1%, 445°C, and 8.73 μC/cm², respectively.     

Phase Transformation and Processing of Poly crystalline Gadolinium Selenide, GdSei.1.49

Polymorphic phase transformation in a thermoelectric material, GdSe_1.49, from cubic to orthorhombic symmetry at temperatures from 800° to 1000°C causes a 1% volume expansion, which generates microcracks. Sintered polycrystalline cubic GdSe_1.49 preforms with at least 97% densities were vacuum annealed at 900°C for 300 h to fully convert to the orthorhombic lattice, encapsulated in nickel containers, and then isostatically hot-pressed. Hot-pressing at or below 1000°C resulted in 96%-dense orthorhombic GdSe_1.49 polycrystals containing large pores, whereas hot-pressing above 1200°C produced fully dense cubic polycrystals with a duplex microstructure. The electrical resistivity of the orthorhombic GdSe_1.49 (hot-pressed below 1000°C) was about 5 times that of the as-sintered cubic GdSe_1.49; the increase appears to be related to the presence of the orthorhombic phase, micro-cracks, large pores, and a Se-rich grain-boundary phase in the hot-pressed orthorhombic GdSei_1.49.     

Phase Transition and Physical Characteristics of Nanograined BaTiO3 Ceramics Synthesized from Surface-Coated Nanopowders

Nanograined BaTiO₃ ceramics prepared from 40-nm-size BaTiO₃ nanopowders exhibited the cubic as well as the tetragonal phase, while nanograined BaTiO₃ ceramics prepared from BaTiO₃ nanopowders coated with Mn had only the tetragonal phase. The dielectric constant of the latter was 10 times larger than that of the former; the latter exhibited PTCR behavior with a resistivity jump ratio of about 5.0 × 10⁴. These physical properties of the BaTiO₃ ceramics appeared to be significantly affected by the strain near grain boundaries; such strain resulted in a phase transition from the cubic to the tetragonal phase in the nanograined BaTiO₃ ceramics, even though the grain size was about 40 nm.