Temperature–Time Texture Transition of Pb(Zr1−xTix)O3 Thin Films: II, Heat Treatment and Compositional Effects

Texture transition of Pb(Zr_1−xTi_x)O₃ thin films grown from a metallo–organic solution onto Pt/Ti/SiO₂/Si substrates has been studied. The orientation obtained depends on the crystallization temperature and time and can be drastically altered by modifying the heat treatment schedule. A [100] texture requires an initial seeding treatment in the intermediate temperature range. This is attributed to a nucieation advantage associated with the formation of an intermediate PbO[001] texture that tends to form at intermediate temperature over time. A [111] texture develops at higher temperature during rapid heating and can be rationalized by the formation of an epitaxial intermetallic phase Pt_5–7Pb, at the PZT/Pt interface that provides lattice matching between PZT(lll), Pt_5–7Pb(lll), and Pt(lll). Temperature–time transformation texture (TTT) diagrams of PZT texture have been constructed for seeding various orientations. The effect of composition (Pb excess and Zr/Ti ratio) and the possible manipulation of these texture selection relations by combining compositional tailoring and heat treatment schedules are illustrated.     

Methane Oxidation on Perovskite-Type Ca(Mn1−xTix)O3−δ

X-ray photoelectron spectroscopy was performed to elucidate the catalytic activity of CH₄ oxidation on perovskite-type Ca(Mn_{1− x} Ti _x )O_3−δ synthesized at 1173 K in a flow of oxygen from a gel with citric acid and ethylene glycol. The Mn ion content decreases and the ratio of the Mn³⁺ ion in the Mn ion increases with increases in x . Ca(Mn_{1− x} Ti _x )O_3−δ has a high catalytic activity of CH₄ oxidation at x =0.4. These results indicate that the catalytic activity strongly depends on the Mn³⁺ ion content of the surface.     

A Study of Conductivity in the Sr(Fe1−xTix)O3−δ System

The conductivity and conductivity-temperature characteristics of the Sr(Fe_1−xTi_xO_3−δ system fired in air are studied by means of Mössbauer spectroscopy at room temperature and at 78 K. On the basis of quantitative analysis of the Fe⁴⁺ content in the solid solution, the concentrations of oxygen vacancies are calculated and the relationship between Fe⁴⁺ content and conductivity is found. It is also found that the turning points of conductivity and material constant B, as well as Fe⁴⁺ and the percentage of oxygen vacancies, occur at x=0.7 on the curve. When x=0.7, Fe³⁺ (II), which has never been reported before, disappears and the lattice parameters no longer vary with x. Besides Fe⁴⁺, Fe³⁺(II) is also a factor affecting the conductivity of this system.     

Sol—Gel Fabrication of Pb(Zr0.52 Ti0.48)O3 Thin Films Using Lead Acetylacetonate as the Lead Source

Lead zirconium titanate (PZT) thin films of the morphotropic phase boundary composition [Pb(Zr_0.52Ti_0.43)O₃] were deposited on platinum-coated silicon by a modified sol-gel process using lead acetylacetonate as the lead source. The precursor solution for spin coating was prepared from lead acetylacetonate, zirconium n -butoxide, and titanium isopropoxide. The use of lead acetylacetonate instead of the widely used lead acetate trihydrate provided more stability to the PZT precursor solution. Films annealed at 700°C for 12 min formed well-crystallized perovskite phase of Pb(Zr_0.52Ti_0.48)O₃. Microstructures of these films indicated the presence of submicrometer grains (0.1 to 0.2 μm). The dielectric constant and loss values of these films measured at 10 kHz were approximately 1200 and 0.04, respectively, while the remanent polarization and coercive field were ∼ 13 μC/cm² and ∼ 35 kV/cm. Aging of the solution had almost no effect on the dielectric and ferroelectric properties of these films.     

A Phase Diagram for Epitaxial PbZr1−xTixO3 Thin Films at the Bulk Morphotropic Boundary Composition

A phase diagram of temperature versus strain was constructed for a (001)-oriented PbZr_{1− x} Ti _x O₃ epitaxial single crystal thin film near the bulk morphotropic boundary composition ( x =0.47) on an (001)-oriented cubic substrate. The phase-field approach is employed. It is shown that a mixture of distorted rhombohedral, orthorhombic, and tetragonal phases exists under small values of strain, i.e., close to the in-plane clamped boundary condition. This result contradicts thermodynamic calculations assuming a single-domain state that predicted a single distorted rhombohedral phase under similar strains. Furthermore, it is demonstrated that under a large tensile strain current phase-field simulations showed a tetragonal phase with a ₁ /a ₂ twin structures as the stable state whereas thermodynamic calculations predicted an orthorhombic phase.     

Modifying Structures and Ferroelectric Properties of Pb(Zr0.6Ti0.4)O3 Thin Films by Constraint-Annealing of Precrystallized Film

A tensile or compressive mechanical constraint was applied, during annealing, on the Pb(Zr_0.6Ti_0.4)O₃ (PZT) ferroelectric films to investigate the effects of stress on its crystal structure and electric properties. The external stress was applied by bending the substrate into a circular section. By using both precrystallized film structure and high constraint strain (0.08%), the stress states of PZT during the crystallization process became controllable. Structural change of polycrystalline PZT was observed when crystallized under a compression constraint. Moreover, these films with compression constraint annealing exhibited enhanced remnant polarization by ∼70% and increased dielectric constant by ∼68%. The variations in ferroelectric behaviors were correlated to domain configuration, texture, amount of pyrochlore phase, grain size and residual stress, which are dependent on the stress state during annealing process.     

Texture Control of Sol-Gel Derived Pb(Mg1/3Nb2/3)O3–PbTiO3 Thin Films Using Seeding Layer

Lead magnesium niobium titanate (PMNT) thin films with a composition near the morphotropic phase boundary were prepared on conventional Pt(111)/Ti/SiO₂/Si substrates using a modified sol-gel process. A PbO seeding layer was introduced to the interface between the PMNT layer and the substrate to enhance the [001]-preferential orientation of the PMNT film. Single-phase perovskite PMNT films with highly [001]-preferential orientation were obtained at reduced annealing temperatures compared with the PMNT films directly deposited on the same substrates. The dielectric and ferroelectric properties of the prepared PMNT films were evaluated as a function of annealing temperature.     

Dielectric Properties of Pb0.97La0.02(Zr0.87−xSnxTi0.13)O3 Thin Films with Compositions near the Morphotropic Phase Boundary

Pb_0.97La_0.02(Zr_{0.87− x} Sn _x Ti_0.13)O₃ (PLZST, x =0.27, 0.17, 0.07)) thin films with the compositions in ferroelectric rhombohedral (FE_R) region, near the morphotropic phase boundary (MPB), were deposited on the Pt-electroded silicon (PtSi) substrates by the sol–gel process. The phase structure and surface morphology of PLZST thin films were analyzed by XRD and SEM, respectively. The dc electric field and temperature-dependent dielectric properties of the PLZST thin films were investigated in detail. The results indicated that the dielectric constant, remnant polarization, and the Curie temperature ( T _c) of PLZST films were elevated with the decrease of Sn content. Hence, the larger dielectric tunability (τ) was obtained for PLZST thin films with x =0.07, and the maximum τ value was 78.1%.     

Transmission Electron Microscopy of Pb(Zr0.52Ti0.48)O3

Transmission electron microscopy of Pb(Zr_0.52Ti_0.48)O₃ sintered with 3 wt% excess PbO indicates that there is an amorphous grain-boundary phase 10 nm thick. The grain-boundary phase has a higher Pb content than the bulk material. Influence of such a phase on sintering is discussed.     

High Piezoelectric and Dielectric Properties of La-Doped 0.3Pb(Zn1/3Nb2/3)O3–0.7Pb(ZrxTi1−x)O3 Ceramics Near Morphotropic Phase Boundary

La-doped 0.3Pb(Zn_1/3Nb_2/3)O₃–0.7Pb(Zr _x Ti_{1− x} )O₃ ( x =0.5–0.53) piezoelectric ceramics with pure perovskite phase were synthesized by a two-step hot-pressing route. The piezoelectric properties of various compositions near the morphotropic phase boundary (MPB) were systematically investigated. Not only was the exact MPB of this system determined via X-ray diffractometry analysis, but also the peak of piezoelectric properties was found near the MPB. The optimum piezoelectric properties of this series were observed in the specimen with Zr/Ti=51/49. The piezoelectric coefficient ( d ₃₃) and electromechanical coupling factor ( k _p) were 845 pC/N and 0.70, respectively, which have not been reported in this system so far. Large permittivity (ɛ_r=4088) and permittivity maximum (ɛ_m=29 500) were also obtained for the poled specimens. The temperatures ( T _max) of the permittivity maxima ranged from 206° to 213°C with various Zr/Ti ratios.     

Ferroelectric and Piezoelectric Properties of Na1−xBaxNb1−xTixO3 Ceramics

Piezoelectric ceramics Na_{1− x} Ba _x Nb_{1− x} Ti _x O₃ with low BaTiO₃ concentrations x have been prepared by the solid-state reaction method, and their ferroelectric and piezoelectric properties have been studied. The ceramics are classic ferroelectrics when x ≤0.10, and the ferroelectric–paraelectric phase transition becomes diffusive when x ≥0.15. A low doping level of BaTiO₃ changes the NaNbO₃ ceramics from antiferroelectric to ferroelectric. With the increase in BaTiO₃ doping level, the Curie temperature of ceramics decreases linearly and the remnant polarization and coercive field also decrease, while their dielectric constant increases. Na_0.9Ba_0.1Nb_0.9Ti_0.1O₃ ceramics show the largest piezoelectric constant d ₃₃ (147 pC/N) and good sinterability, suggesting that it is a good candidate for lead-free piezoelectric ceramics.     

Sequence of Phase Formation in Chemically Derived Ferroelectric Lead Zirconate Titanate Pb(Zr0.4Ti0.6)O3 Thin Films

A qualitative model for the sequence of phase transforma-tions leading to perovskite phase formation in sol–gel-derived PZT thin films is presented. It is suggested that for sol–gel-derived Pb(Zr_xTi_1–x)O₃ (x = 0.4) thin films, prepared using the molecularly modified alkoxide precursors (MMAP) approach, complete (or partial) conversion of the originally amorphous film into a "pyrochlore" phase, although possible, may not be necessary. Differences between the sequence of phase formation encountered in the tradi-tional (oxide-mix) synthesis and that in chemically derived ceramics are discussed. The sequence of phase formation in these thin films also appears to be intertwined with the texture development in that a (111) PZT texture develop-ment appears to be controlled by nucleation of the perovs-kite phase, assisted by a solid-phase epitaxial effect, and the (100)/(001) texture appears to be growth controlled.x     

Nucleation- or Growth-Controlled Orientation Development in Chemically Derived Ferroelectric Lead Zirconate Titanate (Pb(ZrxTi1−x)O3, x= 0.4) Thin Films

The profound influence of low-temperature processing steps, hitherto assumed benign from the viewpoint of texture development, is reported. On inclusion of a 400°C step and further heat treatment at 700°C, PZT (40/60) sol-gel thin films with a strong (111) texture were obtained. When the 400°C step was excluded, the films exhibited a strong (100) texture. The (111) PZT texture development is nucleation-controlled and is attributed to the solid-phase epitaxial effect. The (100) texture is considered growth-controlled in that (100) oriented grains grow preferentially so as to minimize surface energy.     

Cation Ordering and Domain Boundaries in Ca[(Mg1/3Ta2/3)1−xTix]O3 Microwave Dielectric Ceramics

Cation ordering and domain boundaries in perovskite Ca[(Mg_1/3Ta_2/3)_{1− x} Ti _x ]O₃ ( x =0.1, 0.2, 0.3) microwave dielectric ceramics were investigated by high-resolution transmission electron microscopy (HRTEM) and Rietveld analysis. The variation of ordering structure with Ti substitution was revealed together with the formation mechanism of ordering domains. When x =0.1, the ceramics were composed of 1:2 and 1:1 ordered domains and a disordered matrix. The 1:2 cation ordering could still exist until x =0.2 but the 1:1 ordering disappeared. Neither 1:2 nor 1:1 cation ordering could exist at x =0.3. The space charge model was used to explain the cation ordering change from 1:2 to 1:1 and then to disorder. A comparison between the space charge model and random layer model was also conducted. HRTEM observations showed an antiphase boundary inclined to the (111) _c plane with a projected displacement vector in the 〈001〉 _c direction and ferroelastic domain boundaries parallel to the 〈100〉 _c direction.     

Structure and Microwave Dielectric Characteristics of Ca1+xNd1−xAl1−xTixO4 Ceramics

CaNdAlO₄ microwave dielectric ceramics were modified by Ca/Ti co-substitution, and their dielectric characteristics were evaluated along with their structure and microstructures. Ca_{1+ x} Nd_{1− x} Al_{1− x} Ti _x O₄ ( x =0, 0.025, 0.05, 0.10, 0.15, 0.20) ceramics with the relative density of over 95% theoretical density were obtained by sintering at 1400°–1450°C in air for 3 h, where the K₂NiF₄-type solid solution single phase was determined from the compositions of x <0.20, while a small amount of CaTiO₃ secondary phase was detected for x =0.20. With Ca/Ti co-substitution in CaNdAlO₄ ceramics, the dielectric constant (ɛ_r) increased with increasing x , and the temperature coefficient of resonant frequency (τ_f) was adjusted from negative to positive, while the Q × f ₀ value increased significantly at first and reached an extreme value at x =0.025 and the maximum at x =0.15. The best combination of microwave dielectric characteristics were achieved at x =0.15 (ɛ_r=19.5, Q × f ₀=93 400 GHz, τ_f=−2 ppm/°C). The improvement of the Q × f ₀ value primarily originated from the reduced interlayer polarization with Ca/Ti co-substitution, while the decreased tolerance factor, the subsequent increased interlayer stress, and the appearance of CaTiO₃ secondary phase brought negative effects upon the Q × f ₀ value.     

Phase Transition and Ferroelectric Characteristics of Pb[(Mg1/3Nb2/3)1-xTix]O3 Ceramics Modified with La(Mg2/3Nb1/3)O3

The effects of 0–5 mol% addition of La(Mg_2/3Nb_1/3)O₃ (LMN) on the phase transition and ferroelectric behaviors of Pb[(Mg_1/3Nb_2/3)_1-xTi_x]O₃ (PMNT) ceramics with compositions near the morphotropic phase boundary (MPB) were studied. An evolution of structure from rhombohedral to tetragonal was found with increasing PbTiO₃ (PT) content across the MPB (at ∼32.5 mol% PT), and a coexistence of both rhombohedral and tetragonal phases was also found at the MPB. The dual-phase field extended toward the lower PT content side of the MPB, and, moreover, the rhombohedrality or tetragonality was reduced, especially for the compositions near the MPB, by the addition of La in PMNT. The ferroelectric transition was found to change from normal to diffuse as the La content increased and the compositions became more rhombohedral. In accordance with the structural evolution, the change of remanent polarization ( P _r) and coercive field ( E _c) also became gradually indistinct, and both P _r and E _c were reduced. For compositions near the MPB, both PMNT and La-modified PMNT had a similar electromechanical factor ( k _p) in a range around 0.55–0.60, but the mechanical quality factor ( Q _m) was significantly reduced for the La-modified PMNT. The piezoelectric coefficient ( d ₃₃), however, was largely improved with increasing La content in PMNT of compositions at MPB. A high value of d ₃₃∼ 815 pC/N was obtained for the 5-mol%-La-modified ceramics, but it was associated with a low value of Q _m.     

Intrinsic Nonstoichiometry in Single-Phase Pb(Zr0.5Ti0.5)O3

Two-phase crucibles made of Pb(Zr,Ti)O₃ with small additions of either PbO or (Zr _x Ti _1-x )O₂, which provide atmospheres of constant PbO activity, permit study of intrinsic nonstoichiometry by thermogravimetry. The width of the single-phase region can be determined by alternate equilibration of a sample with crucible atmospheres of high and low PbO activity. The width of this region for Pb(Zr_0.5Ti_0.5)O₃ is 2.48 mol% PbO at 1100°C.     

Metal Propionate Synthesis of Magnetoresistive La1−x(Ca,Sr)xMnO3 Thin Films

A flexible chemical solution deposition (CSD) method for the preparation of magnetoresistive La_1−x (Ca,Sr)x MnO₃thin films based completely on metal propionates is pre-sented.A number of polycrystalline thin films with varying stoichiometries were deposited on different substrate ma-terials at temperatures between 550° and 850°C. The crys-tallization behavior on selected substrates was found to de-pend on the thin film stoichiometry. Magnetoresistivity and magnetization were measured as a function of temperature. For the selected samples, a magnetic Curie temperature TC, a metal–semiconductor transition, and magnetoresistive behavior were observed. These measurements demon-strated that La_1−x(Ca,Sr)x MnO₃ thin films with properties well known from films deposited by PLD or sputtering can be prepared by a simple, propionate-based CSD method.     

Microstructure of KTaxNb1−xO3 Thin Films on MgO (100) Single Crystals

Thin KTa_xNb_1−xO₃ (KTN) films were prepared by deposition of sol–gel precursor solutions on MgO (100) single crystals. Crystal structure and microstructure of the films as a function of processing parameters, such as rate, duration, and temperature of postdeposition heat treatment, were studied. Several techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were employed to analyze the films. It was observed that slow heating of KTN films promotes pyrochlore formation while fast-firing of the films results in predominant formation of the perovskioe phase. In slow-heated samples, TEM showed randomly oriented pyrochlore crystallites with a vermicular nanostructure of 10–30 nm with an interpenetrating porosity of the same range. In fast-fired samples, large perovskite pockets with pyrochlore crystallites scattered among them were seen. The large perovskite grains were on the order of 0.1–0.5 μm, irregular in shape and porous. Transmission electron diffraction indicated these were single crystals, and ferroelectric domains were observed in them. Films of up to 1 μm thick were obtained by multiple deposition of the sol–gel KTN. Dense films were achieved when each layer was densified at 750°C for 2 h before the next layer was deposited.     

In-Plane Polarized 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 Thin Films

Ferroelectric 0.7Pb(Mg_1/3Nb_2/3)O₃–0.3PbTiO₃ (PMN-PT) thin films were deposited on ZrO₂/SiO₂/silicon substrates using a chemical-solution-deposition method. Using a thin PZT film as a seed layer for the PMN-PT films, phase-pure perovskite PMN-PT could be obtained via rapid thermal annealing at 750°C for 60 s. The electrical properties of in-plane polarized thin films were characterized using interdigitated electrode arrays on the film surface. Ferroelectric hysteresis loops are observed with much larger remanent polarizations (∼24 μC/cm²) than for through-the-thickness polarized PMN-PT thin films (10–12 μC/cm²) deposited on Pt/Ti/Si substrates. For a finger spacing of 20 μm, the piezoelectric voltage sensitivity of in–plane polarized PMN-PT thin films was ∼20 times higher than that of through-the-thickness polarized PMN-PT thin films.