Improved Hot-Pressed Electrooptic Ceramics in the (Pb,La)(Zr,Ti)O3 System

Physical, electrical, and optical properties of selected compositions within the lead lanthanum zirconate-titanate (PLZT) system are presented, including chemically and thermally etched microstructures, examples of residual porosity defects, temperature dependence of remanent polarization and coercive field, lateral switching strains, and thermal expansion and optical transmission data. Optical-quality ceramics were produced by stringent raw material selection and improved ceramic processing and hot-pressing. Optimum results were obtained from starting oxides with higher purity (99.9%) and smaller particle size (<2 μm) than those used in previous investigations. These oxides were processed by methods developed to minimize segregation and promote chemical homogeneity and hot-pressed at temperatures (1050° to 1250°C) higher than those used previously to remove residual porosity.     

Controlled Oxygen Partial Pressure Sintering of (Pb,La)(Zr,Ti)O3 Ceramics

Transparent PLZT(7/60/40) ceramics with large piezoelectric coefficients were obtained using a two-step sintering process with controlled oxygen partial pressure. Specifically, low-oxygen-pressure and low-temperature sintering were used in the first step, followed by a high-oxygen-pressure, high-temperature sintering cycle. High-density ceramics with small grain sizes of about 3 µm were prepared. As a result, k _p= 0.71, k ₃₃= 0.78, d ₃₃= 850 × 10^-12 C/N, and a transparency of 15% (λ= 610 nm, thickness of 1 mm) have been achieved; 20% improvement of d ₃₃ was gained compared to conventional processed PLZT ceramics ( d ₃₃= 710 × 10^-12 C/N).     

Distribution of A-Site and B-Site Vacancies in (Pb,La)(Ti,Zr)O3 Ceramics

It was shown experimentally that A -site vacancies predominate in La-substituted lead titanate-zirconate (PLZT) solid solutions. The formation of B -site vacancies is favored under increased PbO vapor pressure, but preparation of PLZT-perovskite phases with B -site vacancies only is impossible since the increase in the partial pressure of PbO is naturally limited by the condensation of liquid PbO at the grain boundaries of the ceramic.     

Excimer Laser Crystallized (Pb,La)(Zr,Ti)O3 Thin Films

A KrF pulsed excimer laser (248 nm) was utilized to crystallize sputtered La-modified Pb(Zr,Ti)O₃ (3:30:70) (PLZT) films on LaNiO₃-coated silicon substrates. The film surface was irradiated with defocused laser pulses in an oxygen ambient at various substrate temperatures. Polycrystalline, phase pure perovskite PLZT thin films were produced for substrate temperatures of 250°C and higher. The dielectric constant and loss tangent values of laser-assisted crystallized (10 min exposure at 10 Hz using a substrate temperature of 400°C) PLZT thin films at 10 kHz were 406 and 0.027; in comparison, rapid thermal annealed films (annealed at 700°C for 1 min) showed values of 400 and 0.021, respectively. Laser crystallized films exhibited a remanent polarization value of 14 μC/cm² with a coercive field |( E _+c+ E _−c)|/2 of 95 kV/cm.     

Synthesis of (Pb,La)(Zr,Ti)O3 Inverse Opal Photonic Crystals

(Pb,La)(Zr,Ti)O₃ (PLZT) inverse opal photonic crystals were synthesized by a sol-gel process with synthetic opal templates of monodisperse submicrometer polystyrene spheres. This process involves infiltration of precursors into the interstices of the opal template, followed by hydrolytic condensation of the precursors and removal of the polystyrene opal combined with crystallization of PLZT perovskite by calcination at a final temperature of 750°C. By this method, PLZT inverse opal photonic crystals with a periodical structure of 280 nm center-to-center distance between the air spheres are prepared from opal templates of polystyrene microspheres with a mean diameter of 400 nm. The PLZT inverse opals reflect yellow-green light strongly.     

Low-Field Depoling Characteristics of Pb(Zr, Ti)O3 Ceramics

Low-electric-field depoling measurements were made on Pb(Zr, Ti)O₃ ceramics. The apparent activation energy for the depoling process was calculated to be 33 kcal/mol. The data indicate that the materials tested should be very stable when subjected to depoling fields on the order of one-fourth of the coercive field.     

Preparation of Full-Dense Pb(Zr,Ti)O3 Ceramics by Aerosol Deposition

Full-dense fine-grained bulk Pb(Zr,Ti)O₃ (PZT) ceramics with a thicknesses of 300 μm were successfully obtained by aerosol deposition (AD) employing the starting powder subjected to an adequate pretreatment. It was found that formation of internal pores during the postdeposition firing treatment takes place in two different mechanisms on different scales. Residual carbons contained in the as-deposited PZT ceramics are responsible for the formation of extra-large pores, inducing severe distortion to the shape of the sample. On the other hand, trapped agglomerates in the as-deposited PZT ceramics are responsible for the formation of intermediate pores. The revealed mechanisms are discussed with a focus on the positive/negative correlation between the pretreatment procedure of starting powders and the formation of internal pores.     

Metalorganic Chemical Vapor Deposition of Ferroelectric Pb(Zr,Ti)O3 Thin Films

Ferroelectric Pb(Zr_xT_1–x)O₃, films were successfully and reproducibly deposited by both hot–wall metalorganic chemical vapor deposition (MOCVD) and cold-wall MOCVD. One of the important problems associated with the MOCVD techniques is the selection of ideal precursors. After an intensive investigation for the most suitable precursors for MOCVD PZT films, the safe and stable precursors, namely lead tetramethylheptadione [Pb(thd)₂], zirconium tetramethylheptadione [Zr(thd)₄], and titanium ethoxide [Ti(OEt)₄], were chosen. The films were deposited at temperatures as low as 550°C and were single-phase perovskite in the as-deposited state. Also, the films were smooth, specular, crack-free, and uniform, and adhered well to the substrates. The stoichiometry of the films can be easily controlled by varying the individual precursor temperature and/or the flow rate of the carrier gas. Auger electron spectroscopic (AES) depth profile showed good compositional uniformity through the thickness of the films. The AES spectra also showed no carbon contamination in the bulk of the films. As-deposited films were dense and showed uniform and fine grains (≅0.1 μm).The optical properties of the films on the sapphire disks showed high refractive index ( n = 2.413) and low extinction coeflicient ( k = 0.0008) at a wavelength of 632.8 nm. The PZT (82/18) film annealed at 600°C showed a spontaneous polarization of 23.3 μC/cm² and a coercive field of 64.5 kV/cm.     

Bipolar Fatigue Caused by Field Screening in Pb(Zr,Ti)O3 Ceramics

Bipolar cycling of lead zirconate titanate ceramics can lead to massive material damage in regions close to the electrode. The damaged region can be identified by color changes, and the microstructure in this region shows signs of interface melting. This damaged region can screen the sample volume from the applied voltages and reduced fields are applied to the undamaged part of the sample. This has two effects. The first one is that the bulk is effectively subjected to smaller fields, but the measured parameters are assigned to the applied field, yielding apparent fatigue curves. The second effect is that with further cycling, field screening protects the bulk of the sample from fatigue due to the reduced effective fields. If the damaged region is mechanically removed and the ferroelectric hystereses are measured again, nearly unfatigued parameters are obtained.     

Fabrication of Optically Transparent Lead Lanthanum Zirconate Titanate ((Pb,La)(Zr,Ti)O3) Ceramics by a Three-Stage-Atmosphere-Sintering Technique

An easy technique has been developed to fabricate optically transparent lanthanum-modified lead zirconate titanate (PLZT) ceramics. This technique consists of three stages: (1) sintering in an oxygen atmosphere, (2) elimination of pores in a carbon dioxide atmosphere, and (3) elimination of oxygen vacancies in an oxygen atmosphere. The carbon dioxide atmosphere enhances the diffusion of oxygen from the pores to outside the sintered body. The experimental results reveal that use of a carbon dioxide atmosphere effectively decreases residual pores and improves optical transmittance. From commercially available raw powders, an optical transmittance of 51% (wavelength of 550 nm) can be achieved for 0.7 mm thick polished PLZT9/65/35 ceramics using a carbon dioxide atmosphere, whereas the value is only 34% without a carbon dioxide atmosphere. The advantage of this technique is that PLZT ceramics having high optical quality can be obtained using conventional sintering tools.     

Electron Paramagnetic Resonance Investigation of Acceptor Centers in Pb(Zr,Ti)O3 Ceramics

The nature of extrinsic point defects in lead zirconate titanate (PZT) ceramics of various compositions prepared by solution chemistry has been explored. Using electron paramagnetic resonance (EPR), several impurity sites have been identified in the as-received materials, which include Fe³⁺oxygen vacancy (V_O^‥) complex and isolated Cu²⁺ ions; both of these ions are incorporated into the lattice by replacing the Ti(Zr) ion. A Fe₃₊V_O^‥ complex serves as a sensitive probe of the local crystalline field environment of the ceramic; the symmetry of this defect is roughly correlated with its phase diagram as the composition is varied from PbTiO₃ to PbZrO₃. The Fe³⁺V_O^‥ complex experiences tetragonal, rhombic, or orthorhombic symmetry as the composition is varied from PbTiO₃ to PbZrO₃. As the composition of the Cu²⁺ ion is varied, it appears as though the addition of Zr, and not necessarily a change in phase, is largely responsible in determining the local environment of this acceptor impurity. Also, the Cu²⁺ resonance parameters weakly reflect the relative Ti–O(Zr–O) bond covalency in the perovskite lattice.     

Effects of Crystal-Lattice Distortion on Optical Transmittance of the (Pb, La)(Zr, Ti)O3, System

Optical transmittance and crystal-lattice distortion and their relation were studied in hot-pressed La-doped Pb zirconate-titanate (PLZT) ferroelectric ceramics at the ferroelectric-antiferroelectric morphotropic phase boundary. In the thermally annealed condition, the crystal system of PLZT at the FE-AFE morphotropic phase boundary was pseudotetragonal and similar to the antiferroelectric phase (AFE_β) appearing in Pb(Zr, Ti)O₃ and (Pb, Sr)ZrO₃. Optical transmittance and crystal-lattice distortion change monotonically with respect to the Zr/Ti ratio, and the optical transmittance increases approximately linearly with decreasing lattice distortion. In view of these relations and the temperature dependence of the optical transmittance, it is suggested that light scattering in PLZT ceramics is caused mainly by the variation in refractive index encountered as the light travels from one domain or grain into another.     

Effect of Additives on the Electromechanical Properties of Pb(Zr,Ti)O3–Pb(Y2/3W1/3)O3 Ceramics

Dielectric and piezoelectric properties of 0.02Pb(Y_2/3W_1/3)O₃ 0.98Pb(Zr0.52Ti0.48)O3 ceramics doped with additives (Nb₂O₅, La₂O₃, MnO₂, and Fe₂O₃) were investigated. The grain sizes of these ceramics decreased with increasing amounts of additives. For additions of MnO₂ and Fe₂O₃, dielectric losses decreased, while for Nb₂O₅ and La₂O₃, these values increased. The maximum values of the mechanical quality factor Q_m were found to be 956 and 975 for additions of 0.9 wt% Fe₂O₃ and 0.7 wt% MnO₂, respectively, but donor dopants (Nb₂O₅ and La₂O₃) did not change the values of Q_m . On the other hand, the piezoelectric constant d₃₃ and the electromechanical coupling factor k_p decreased with additions of MnO₂ and Fe₂O₃, but improved with additions of Nb₂O₅ and La₂O₃.     

Preparation of Lead Lanthanum Zirconate Titanate (PLZT, (Pb,La)(Zr,Ti)O3) Fibers by Sol-Gel Method

Lead lanthanum zirconate titanate (PLZT, (Pb,La)(Zr,-Ti)O₃) ceramic fibers were prepared by the sol-gel method from a solution of lead acetate trihydrate, lanthanum isopropoxide, zirconium n -propoxide, and titanium isopropoxide that contained 2-methoxyethanol as the solvent. The sols obtained from the solution were concentrated at 156°-174°C for 2 h. Concentration at higher temperatures resulted in more-viscous sols of higher specific gravities. The concentration resulted in the formation of spinnable sols, which had viscosities >10⁵ mPas and exhibited Newtonian flow properties. These spinnable sols were formed to be so stable that no change in viscosity and spinnability was observed for more than three months when stored in a sealed container at room temperature. Gel-to-ceramic fiber conversion was investigated by means of X-ray diffractometry, infrared spectroscopy, and thermal analysis. Single-phase perovskite PLZT ceramic fibers 5-200 μm in diameter and >20 cm in length were obtained. Scanning electron microscopy (SEM) observation and Brunauer-Emmett-Teller (BET) measurement showed that heat treatment of the fibers at a lower rate resulted in the formation of fibers of denser microstructure. Although the SEM image of the cross section of the fibers revealed a relatively dense microstructure and a laser beam could be transmitted through a fiber 6 mm in length, BET measurement of the fibers indicated that the fibers had more than a few percent of open porosities, and scattering of light was observed in the laser-beam guiding test.     

Highly Oriented, Chemically Prepared Pb(Zr, Ti)O3 Thin Films

We have fabricated highly oriented, chemically prepared thin films of Pb(Zr_0.04Tio._0.60)O₃ (PZT 40/60) on both insulating and conducting substrates. While (100) MgO single crystals were used as the insulating substrates, the conducting substrates. were fabricated by RF magnetron sputter deposition of 100-nm-thick (100) Pt films onto (100) MgO substrates. For comparison, we also fabricated PZT 40/60 films that had no significant preferential orientation on platinized MgO substrates. Sputter deposition of an underlying amorphous Pt film was used to fabricate randomly oriented PZT 40/60 films. Highly (001) oriented PZT 40/60 films had higher remanent polarization (61 μC/cm² compared to 41 μC/cm²) and lower relative dielectric constant (368 compared to 466) than PZT 40/60 films that were randomly oriented.     

Shape Evolution of Pb (Zr,Ti)O3 Nanocrystals Under Hydrothermal Conditions

Pb (Zr,Ti)O₃ nanocrystals with different morphologies have been prepared hydrothermally with poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) as surfactants. The morphology of Pb (Zr,Ti)O₃ nanocrystals develops from a particle to a rod and then a wire by adjusting the ratio of PVA/PAA and reaction time in a hydrothermal system. The mixed surfactant system has been shown to promote anisotropic growth effectively and thus the formation of nanorods and single crystalline nanowires of Pb (Zr,Ti)O₃. A possible mechanism for such a shape evolution of nanocrystals is simply discussed.     

Structure and Dielectric Properties of Pb(Sc2/3W1/3)O3–Pb(Zr/Ti)O3 Relaxors

The structure and dielectric properties of (1− x )Pb(Sc_2/3W_1/3)O₃–( x )Pb(Zr/Ti)O₃ ceramics have been investigated over a full substitution range. All compositions with x < 0.5 adopt a cubic perovskite structure; however, for x ≤ 0.25 a doubled cell results from a 1:1 ordered distribution of the B-site cations. The structural order in Pb(Sc_2/3W_1/3)O₃ (PSW) can be described by a random-site model with one cation site occupied by Sc³⁺ and the other by a random distribution of (Sc_1/3³⁺W_2/3⁶⁺). The ordering is destabilized in solid solutions of PSW with PbZrO₃ (PSW–PZ), but stabilized by PbTiO₃ in the (1− x )PSW–( x )PT system. The changes in order are accompanied by alterations in the dielectric response of the two systems. For PSW–PZ the temperature of the permittivity maximum ( T _ɛ,max) increases linearly with x ; however, for PSW–PT T _ɛ,max decreases in the ordered region (up to x = 0.25) and then increases rapidly as the order is lost. Similar effects were produced by modifying the degree of order of (0.75)PSW–(0.25)PT; when the order parameter was reduced from ∼1.0 to ∼0.65, T _ɛ,max increased by more than 60°C.