Flextensional Barium Strontium Titanate Actuators

This paper describes the performance of the cymbal flextensional transducer using Dy-doped barium strontium titanate (BST) as the driver material. BST was first characterized for its dielectric and loss behavior as a function of temperature and electrical bias field. With no electrical bias, the transition temperature was measured to be near 20°C and have a dielectric constant >20 000. The strain of a BST disk was then measured and compared with other ceramics. At room temperature the strain and average effective piezoelectric d ₃₃ of this non-lead composition was slightly larger than Navy type I lead zirconate titanate (PZT-4) ceramic. The strain/field behavior was also measured as a function of temperature. Cymbal capped BST ceramic was found to have an amplified displacement of 28×, also very similar to type I ceramic. The stiffness of BST was found to be tunable by dc voltage and 2 to 3 times larger than that of PZT. This material has promise for applications in actuators and transducers with large generative force.     

Cation Interdiffusion in Polycrystalline Cubic C-Type Yttria–Zirconia–Hafnia Solid Solutions

Cation inter diffusion in cubic C-type solid solutions was investigated for the polycrystalline systems Y₂O₃—Zr_xHf₁-_x-1O₂ and Y₂O₃–ZrO₂ in the small-grain, deep-penetration condition at 1615° to 1822°C. Lattice and grain-boundary diffusion parameters were calculated from cation concentration distributions by using Oishi and Ichimura's grain-boundary diffusion equation. The results indicated that Zr–Y and Zr–Hf interdiffusion coefficients decreased with an increase in Y content. The cation diffusivities were lower than anion diffusivities, and the interdiffusion parameters were lower in the C-type cubic than in fluorite-type cubic solid solutions. The results were compared with diffusivities in other C-type cubic oxides.     

Cation Interdiffusion and Phase Stability in Polycrystalline Tetragonal Ceria–Zirconia–Hafnia Solid Solution

Zr–Hf interdiffusions were carried out at 1350° to 1520°C for polycrystalline tetragonal solid solutions of 14CeO₂·86(Zr_{1- x} Hf _x )O₂ with X = 0.02 and 0.10. Lattice and grain-boundary interdiffusion parameters were calculated from the concentration distributions by using Oishi and Ichimura's equation. Lattice interdiffusion coefficients were described by D = 3.0 × 10³ exp[-623 (kJ/mol)/ RT ] cm²/s and grain-boundary interdiffusion parameters by δ D ' = 0.29 exp[-506 (kJ/mol)/ RT ] cm³/s. The cation diffusivity was lower than the anion diffusivity. The results were compared with diffusivities in the fluorite-cubic solid solution. The critical grain radii for stabilization of the tetragonal phase in CeO₂-doped ZrO₂ were 11 and 6 μm for the solutions with 2 and 10 mol% HfO₂ substitution, respectively, both of which are much greater than in the Y₂O₃-doped ZrO₂ solid solution.     

Point Defect Concentrations in Barium Titanate Revisited

An analytic link between the oxygen partial pressure and the concentrations of the point defects for given temperatures and A/B conditions is presented for perovskites. A clear distinction between three different conditions is made. These are the sintering conditions, an intermediate metastable state, and a low-temperature metastable state. The analytical solution for a metastable state resulting from nonequilibrated metal vacancies permits a more accurate and self-consistent approach to calculating the equilibrium constants from conductivity– P (O₂) data. One of the reasons for the higher accuracy is that there is no need to divide the existence regime into subregimes with different approximations to the electroneutrality equation (Brouwer approximation). An excellent fit of the experimental conductivity data to a single function with only two adjustable parameters over all conductivity– P (O₂) space is obtained. The relative importance of frozen-in metal vacancies and foreign acceptors is discussed for BaTiO₃.     

Direct-Current Field Dependence of Dielectric Properties in Alumina-Doped Barium Strontium Titanate

The dielectric properties of (Ba_0.6Sr_0.4)TiO₃ and Al₂O₃-doped (Ba_0.6Sr_0.4)TiO₃ have been characterized. The grain size of the specimen is maximum for (Ba_0.6Sr_0.4)TiO₃ that has been doped with 1 wt% Al₂O₃. The density and the real part of the relative dielectric constant each decrease as the Al₂O₃ content increases. The loss factor is minimum for (Ba_0.6Sr_0.4)TiO₃ that has been doped with 2 wt% Al₂O₃. The dielectric constant of the specimens decreases as the applied dc field increases. The influence of the dc field on the loss factor is much less than that on the dielectric constant. The tunability is ∼24% for (Ba_0.6Sr_0.4)TiO₃ that has been doped with 1 wt% Al₂O₃.     

Crystallization of Sol–Gel-Derived Barium Strontium Titanate Thin Films

Microstructural development of thin-film barium strontium titanate (Ba _x Sr_{1– x} TiO₃) as a function of strontium concentration and thermal treatment were studied, using transmission electron microscopy (TEM) and X-ray diffractometry (XRD). Thin films, ∼250 nm thick, were spin-coated onto Pt/Ti/SiO₂/Si substrates, using methoxypropoxide alkoxide precursors, and crystallized by heat-treating at 700°C. All films had the cubic perovskite structure, and their lattice parameters varied linearly with strontium content. Films with higher strontium concentrations had a larger average grain size. In situ TEM heating experiments, combined with differential thermal analysis/thermogravimetric analysis results, suggest that the gel films crystallize as an intermediate carbonate phase, Ba _x Sr_{1– x} TiO₂CO₃ (with a solid solution range from x = 1 to x = 0). Before decomposition at 600°C, this carbonate phase inhibits the formation of the desired perovskite phase.     

Positive Temperature Coefficient of Electrical Resistivity below 150 K in Barium Strontium Titanate

The resistivity of Ba_{(1– y )(1– x )}Sr _y (1– x )La _x TiO₃ceramics with x = 0.0025 and y = 0.25, 0.5, 0.75, and 0.9 was measured between 50 and 400 K. A resistivity anomaly corresponding to the positive temperature coefficient of electrical resistivity (PTCR) effect was observed for all compositions. The onset temperature decreased from 320 K ( y = 0.25) to 70 K ( y = 0.9). The extent of the PTCR effect was significantly enhanced for the strontium-rich composition and reached ∼8 orders of magnitude for y = 0.9. These results strongly suggested the possibility to fabricate PTCR devices based on (Sr,Ba)TiO₃ ceramics for application at cryogenic temperatures.     

Thermodynamics of Nanoscale Calcium and Strontium Titanate Perovskites

The surface enthalpies of nanocrystalline CaTiO₃ and SrTiO₃ perovskites were determined using high‐temperature oxide melt solution calorimetry in conjunction with water adsorption calorimetry. The nanocrystalline samples were synthesized by a hydrothermal method and characterized using powder X‐ray diffraction, FTIR spectroscopy, and Brunauer–Emmett–Teller surface area measurements. The integral heats of water vapor adsorption on the surfaces of nanocrystalline CaTiO₃ and SrTiO₃ are ?78.63 ± 4.71 kJ/mol and ?69.97 ± 4.43 kJ/mol, respectively. The energies of the hydrous and anhydrous surfaces are 2.49 ± 0.12 J/m² and 2.79 ± 0.13 J/m² for CaTiO₃ and 2.55 ± 0.15 J/m² and 2.85 ± 0.15 J/m² for SrTiO₃, respectively. The stability of the perovskite compounds in this study is discussed according to the lattice energy and tolerance factor approach. The energetics of different perovskites suggest that the formation enthalpy becomes more exothermic and surface energy increases with an increase in ionic radius of the “A” site cation (Ca, Sr, and Ba), or with the tolerance factor. PbTiO₃ shows a lower surface energy, weaker water binding, and a less exothermic enthalpy of formation than the alkaline‐earth perovskites.     

Effect of Acceptor and Donor Dopants on the Dielectric and Tunable Properties of Barium Strontium Titanate

The effects of different concentrations of Mn²⁺, Mg²⁺, Al³⁺, Fe³⁺, La³⁺, and Nb⁵⁺ on the dielectric and tunable properties of Ba_0.6Sr_0.4TiO₃ ceramics were investigated. It was found that doping in small amounts with acceptor ions such as Mg²⁺, Fe³⁺, and Al³⁺ could meliorate the dielectric properties clearly. Decrease of dielectric loss was attributed to the formation of compensating defects originating from acceptor substitution. It was concluded that the tunability was linked to both the dielectric constant and the grain size. A higher figure of merit was obtained by doping the ceramics with smaller ions of Al and Fe, compared to Ti.     

Grain-Boundary Chemistry of Barium Titanate and Strontium Titanate: I, High-Temperature Equilibrium Space Charge

Direct observations using scanning transmission electron microscopy (STEM) of the grain-boundary chemistry of selectively doped SrTiO₃ and BaTiO₃ show the predominant solute segregation in both systems to be that of acceptors (negative effective charge). Appreciable donor segregation is not observed even at lattice concentrations as high as 10 mol%. Donor and acceptor codoped materials show segregation of the acceptor only. The results are consistent with a grain-boundary space-charge distribution consisting of a positive boundary and negative space charge. All grain boundaries examined also show an excess of Ti relative to the A-site cations, suggesting that the positive boundary charge is at least partially accommodated by an excess of Ti ions. The sign and magnitude of the electrostatic potential appear to be remarkably insensitive to changes in lattice defect structure with solute doping. Grain-boundary chemistry appears dominated by space-charge segregation, in contrast with the predictions of recent atomistic simulations which neglect the space-charge potential.     

Diffusion of 110mAg Tracer in Polycrystalline and Single-Crystal Lead-Containing Piezoelectric Ceramics

We have conducted diffusion measurements of radioactive ^110mAg tracer in single-crystal PbMgNbO₃–PbTiO₃ (PMN-PT) and in polycrystalline 50Pb(Ni_1/3Nb_2/3)O₃·35PbTiO₃·15PbZrO₃ (PNN-PT-PZ) piezoelectric ceramics. Both materials measured belong to the perovskite family. Diffusion in PMN-PT is characterized by an activation energy of 277 kJ/mol and pre-exponential factor of 0.0034 m²/s and compares well with cation diffusion in high-temperature superconducting YBa₂Cu₃O_7–δ. Diffusion in polycrystalline PNN-PT-PZ, on the other hand, is many orders of magnitude faster and is attributed to grain boundaries. PNN-PT-PZ has a lower activation energy, 168 kJ/mol, and a combined pre-exponential factor ( s δ( D _b)_o, where s is the segregation factor of silver, δ the thickness, and ( D _b)_o the pre-exponential factor for grain boundaries) of 3.7 × 10⁻⁹ m³/s. The unusually large combined pre-exponential factor infers large segregation of silver at the grain boundaries and small solid solubility within the grains. It is possible, using a semiempirical model, to compute metal– (silver–) ceramic interface energies as a function of temperature, from which values of 90 kJ/mol and 0.9 R are obtained for enthalpy and entropy, respectively, for grain-boundary segregation.     

Influence of Microstructure on the Electrical Properties of Iron-Substituted Calcium Titanate Ceramics

CaTi_0.8Fe_0.2O_3–δ ceramics with grain sizes that varied from 2 to 10 μm were obtained and studied using SEM, TEM, Mössbauer spectroscopy, impedance spectroscopy, and electrochemical oxygen permeability measurements. Smaller grains developed a core–shell microstructure that consisted of a pure CaTiO₃ core and an iron-rich microdomain structure at the shell. The effect of grain size on electronic conductivity was negligible. The ionic conductivity was higher for the ceramics with core–shell grains, which suggested that fast oxygen transport along microdomain walls may have occurred. For the homogeneous ceramics, the ionic conductivity decreased with decreased grain size, in which case the grain boundary represented an additional resistance, probably because of the depletion of oxygen vacancies.     

Bulk Conductivity and Defect Chemistry of Acceptor-Doped Strontium Titanate in the Quenched State

The grain bulk conductivity of acceptor-doped SrTiO₃ ceramics was investigated by the impedance analysis method after quenching from high-temperature equilibria. The influence of the oxygen partial pressure during equilibration, P ^Eq_O2, is described in terms of a defect model for titanates. From a comparison between the experimental results and the predictions of this model, a predominant ionic conductivity is concluded for a wide P ^Eq_O2 range (approximately 10⁻¹¹ to 10⁵ Pa). The influence of the ionization energy of the acceptors and of possible defect association is discussed.     

Preferred Grain Orientation Relationships in Sintered Perovskite Ceramics

Sintered BaTiO₃, SrTiO₃, and Pb(Zr _x Ti_{1− x} )O₃ ceramics were examined using orientation imaging microscopy (OIM^TM), which is a system for the automated measurement of grain orientations by evaluating electron backscatter diffraction patterns in scanning electron microscopy. As expected for sintered materials, none of the three materials exhibited a pronounced texture. However, analysis of the orientation relationships (ORs) between adjacent grains revealed a statistically significant preference of Σ= 3 ORs. Accordingly, the boundaries between grains with a Σ= 3 OR were particularly important for the macroscopic properties of the materials in question.     

Fabrication and Characterization of Colossal Electroresistance Chip Devices Composed of Polycrystalline Lanthanum-Doped Strontium Titanate and Palladium Electrodes

A colossal electroresistance (CER) multilayered chip device composed of polycrystalline 0.8-at% La-doped SrTiO₃ and Pd electrodes has been successfully fabricated. Polycrystalline SrTiO₃ devices exhibit large hysteresis in their current–voltage ( I – V ) characteristics after the forming process. Further, their resistance states can be switched by applying voltage pulses above ±50 V, and the resistance changes by approximately two orders of magnitude (from ∼600 Ω to ∼80 kΩ). These resistance-switching behaviors demonstrate that even ceramics can exhibit resistance changes as large as thin-film devices and provide the possibility of new switching devices with the memory effect composed of ceramics.     

Nonepitaxial Orientation in Sol–Gel Bismuth Titanate Films

Control over crystallographic orientation in ceramic thin films is important for highly anisotropic structures. Layered perovskites, like Bi₄Ti₃O₁₂, have interesting properties associated with their ferroelectric nature, which may be fully exploited only when films are highly textured. Textured films of this titanate were fabricated via a sol–gel technique without using epitaxial growth. Orientation in the film is confirmed by XRD and SEM, and supported by refractive index and dielectric measurements. In an attempt to explain the orienting effect, light scattering experiments were conducted to yield information about the molecular size, shape, and conformation of macromolecules as the sol–gel solution ages and condensation reactions proceed. These experiments clearly show an increase in the size of molecular clusters with time. We believe that it is the organization of these large clusters during spin coating, and the relationship of the backbone chemistry to the crystal structure of Bi₄Ti₃O₁₂, that are responsible for the observed orientation.     

Cation Diffusion in Single-Crystal and Polycrystalline BeO

The diffusion of Be was measured in single crystals of BeO in directions normal and parallel to the hexagonal c axis. The diffusion coefficients, D , in the two directions were nearly equal to each other. The values of D , measured between 1500° and 2000°C were quite similar to values reported previously for high-density, high-purity, polycrystalline BeO. The single-crystal data, taken in conjunction with polycrystalline data, show that the observed diffusion in polycrystalline BeO proceeds entirely by volume diffusion with no significant diffusion along grain boundaries, although there is some evidence of rapid surface diffusion along microcracks in the surface. The data are interpreted in terms of extrinsic impurity-dependent diffusion via mobile vacancies.     

Cation Effects in the Conversion of Methanol on Calcium, Strontium, Barium and Lead Hydroxyapatites

The effect of the nature of the cation on the surface and catalytic properties of calcium, strontium, barium and lead hydroxyapatites and dicationic analogs containing lead and strontium or barium has been studied with methanol and deuterated methanol. XRD, XPS and ³¹P MAS NMR have been used to characterize the samples, and temperature programmed desorption of deuterated methanol has been employed to provide ancillary information. The dehydration/dehydrogenation of methanol is found to depend strongly on the nature of the divalent cation and is related to the binding strengths of methanol on the hydroxyapatites.     

Interdiffusion and marker movements in concentrated polymer-polymer diffusion couples

Intrinsic and interdiffusion coefficients of binary polymer-polymer diffusion couples with initially large concentration gradients are markedly dependent on concentration, even when the polymers differ only in molecular weight. Assuming local thermal equilibrium of vacancies (or free volume) within the couple leads to different intrinsic diffusion coefficients for the two polymer species and to net vacancy fluxes. These fluxes should produce movement of inert markers in the couple relative to a point far from the initial interface. That such marker movements and vacancy fluxes actually occur is demonstrated by experiments in which the motion of 20 nm diameter Au islands is monitored by Rutherford backscattering spectrometry. The markers which are initially at the interface between thin films of monodisperse polystyrene (one 2 × 10⁷M_w and the other 1.1 × 10⁵M_w) are progressively displaced toward the lower molecular weight side of the couple. As expected the marker displacement is proportional to the square root of time at the diffusion temperature.