Fabrication and Characterization of Ferroelectric Thin Films by the MOCVD Method
H. H. Chen,* C. H. Lin, P. Friddle
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

This work encompasses the fabrication and characterization of ferroelectric Pb(Sc0.5Ta0.5)O3(PST) thin films by metal-organic chemical vapor deposition (MOCVD) techniques. For infrared detector applications, we perform noise, blackbody responsivity, and spectral response and frequency response measurements. The PST detectors' performance will be compared with commercial state-of-the-art pyroelectric detectors. The ultimate goal of this work is to develop a room-temperature infrared radiation detector. We base our work on the ferroelectric PST system and a novel detection approach which utilizes both the ferroelectric and the pyroelectric properties of PST. We also fabricate other perovskite thin films such as PT, PZT, and BST on various types of substrates and investigate the structure-property-processing relationships.

Oxide Thin Films for Electronic Applications
H. H. Chen,* S. W. Lee, J. Finch
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

The primary objective of this research is to produce oxide thin films for electronic applications. One example is a flammable gas sensor using SnO2 and the other is Ta2Os as dielectric material or thermal barrier. These oxide thin films are grown by the MOCVD technique. Microstructural analysis is also carried out on the grown films using XRD, SEM, TEM, AES, and XPS. Sensing tests are done for SnO2 using resistivity measurements with reducing gases such as H2, alcohol, etc. Dielectric constant, C-V, and leakage current density measurements are made. Models are suggested to explain the measured electrical properties.

Ionic Transport in Composite Electrolytes
J. Kieffer,* R. L. Houston, E. Guilbert
National Science Foundation, DMR 93-15779 REU

Composites consisting of organic polymers and inorganic glassy phases are developed for use as electrolyte materials. The organic polymer contains the charge-carrying species and acts as the conducting phase, whereas the inorganic glass provides mechanical stability. High ionic conductivities can be achieved with this constellation because the conducting phase can be maintained above the glass transition temperature. The electrolytes are synthesized via a sol-gel process. This project is concerned with controlling the structure at the nanoscale to optimize component performance. The electrolytes synthesized in the laboratory are characterized using impedance spectroscopy, small-angle x-ray scattering, and thermal analysis.

Transmission Electron Microscopy Studies of Incommensuration in Modified High Zr Content Lead Zirconate Titanate Perovskites
D. A. Payne,* Z. Xu
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

Lead zirconate titanate (PZT) perovskite materials are of considerable technological importance, particularly with regard to physical properties such as pyro- and piezoelectricity and electric field-induced antiferroelectric-to-ferroelectric phase switching. A systematic study of the effects of temperature, Sn- and La-substituents, and Zr/Ti ratio on the evolution of incommensurate phases in high Zr-content PZT is carried out by hot- and cold-stage TEM. Particular attention is paid to the possible development of 1/x[110] incommensurate modulations and to the resul-tant influence on the macroscopic properties. The pur-pose of this investigation is to obtain a fundamental understanding of microstructure-property relationships for electroceramics.

Densification and Stress Development in Sol-Gel-derived PZT Coatings
D. A. Payne,* R. J. Ong, in collaboration with L. H. Allen
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

Sol-gel-derived PZT thin layers deposited on silicon were examined for their shrinkage behavior using in situ ellipsometry. Correlating densification data with thermal analysis (DTA, TGA), the effect of pyrolysis and crystallization on layer shrinkage is observed as experimental parameters such as water of hydrolysis, heating rate, and layer thickness are systematically varied. The resulting stresses in the coating are measured as a function of heat treatment by a laser reflectance technique and related to associated densification phenomena and substrate/layer thermal expansion mismatch.

Temperature-dependent Dielectric Relaxation Phenomena-an Enhanced Brick-Wall Model for Ferroelectric Materials
D. A. Payne*, E. A. Mikalsen
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

This study addresses the temperature and frequency dependencies of dielectric anomalies in heterogeneous media by modeling equivalent circuits for the appropriate boundary conditions. Permittivity and resistivity distributions are considered for representatives microstructures, with temperature dependencies in the Curie-Weiss behavior of the permittivites and in the activation energies for the resistivities. Unusual dielectric relaxation phenomena are generated where the temperature for maximum dielectric response in heterogeneous media is further considered from the macrostructure to the nanostructure. New applications are under investigation for the unusual properties, which can be designed, in appropriate heterogeneous media. The prediction of "relaxor-type" behavior in heterogeneous media is verified by experimental measurement.

Synthesis and Characterization of Perovskite Pb(Zn1/3Nb2/3)O3-PbTiO3 Ceramics
D. A. Payne,* P. Han, Z. Xu
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

Composition near the morphotropic phase boundary in the solid solution of Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN)-PbTiO3 (PT) system display ultrahigh piezoelectric coefficients, strain levels, and electromechanical coupling. This superior piezoelectric property has only been realized in single crystals. In this research, ultrahigh pressure forming will be used to synthesize PZN-PT ceramics with 100% perovskite structure. Transformation from the pychlore to preovskite phase as a function of synthesis conditions will be studied by XRD. Analytical TEM will be used to study microstructure development for different synthesis conditions. Particular attention will be directed to investigation of synthesis conditions, microstructure development, and piezoelectric property relationships.

Studies on Crystal Growth, Characterization, and Properties of SrBi2Ta2O9
D. A. Payne,* X. Lu, P. Han, Z. Xu
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

Recently, strontium bismuth tantalate has been recognized as a leading candidate material for practical nonvolatile ferroelectric memory applications. However, its exact structure and intrinsic properties are not well understood. This research is directed at the growth of sizable single crystals and the study of crystallographic structure and phase transitions, as well as properties such as dielectric, piezoelectric, pyroelectric, ferrolectric, elastic, and thermal properties. Crystals as thick as 1.5 nm have been grown. Structure is being studied by TEM and XRD methods.

Size Effects in Ferroelectric Ceramics
D. A. Payne,* R. J. Ong, A. P. Predith
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

Chemically derived nanocrystalline ferroelectric ceramics are under investigation to determine if any finite size effects exist for properties. Materials under investigation include PbTiO3 and Pb(Zr0.53Ti0.47)O3. Nanocrystalline powders are derived from a 2-MOE based metalloorganic decomposition process. A high-pressure consolidation method is used to produce nearly dense ceramics of average grain size <50 nm. The effects of crystallite size and defect structure on ferroelectric phase stability, dielectric behavior, and piezoelectric properties are of primary interest.

Single Crystals and Thin Films of Doped Rare Earth Manganese Perovskites
D. A. Payne,* B. A. Clothier
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

Rare earth manganese perovskites doped with divalent cations, such as Ca, Sr, Ba, and Pb, have been shown to exhibit greatly enhanced magnetoresistive properties when compared with metallic heterolayers. Improved magnetoresistive behavior is greatly desired for sensor applications, such as magnetic read heads, or advanced computer memories, such as MRAM cells. Current research is concerned with the electronic behavior and morphological characterization of doped rare earth manganese perovskite single crystals. Resulting candidate materials are deposited by low-temperature solution processing and self-assembled patterning to form integrated structures for devices.

Conformal Patterning of an Infrared Sensory Array on Planar and Nonplanar Surfaces
D. A. Payne,* E. A. Mikalsen, in collaboration with R. G. Nuzzo and R. Brandman in the Department of Chemistry
Defense Advanced Research Projects Agency (In cooperation with the Materials Research Laboratory)

This study is involved with the development of thin-film materials for high-resolution infrared detectors. Soft lithographic methods, including micromolding in capillaries (MIMIC) and mediated monolayer patterning by microcontact printing ((CP), are under investigation for the selective deposition of solution-derived oxide thin films. Our approach is to develop soft lithographic methods and sol-gel deposition processes to integrate infrared sensory arrays on both planar and nonplanar surfaces. Materials under investigation include pyroelectric lead zirconium titanate and barium titanate and semiconducting vanadium pentoxide. The radiation sensors and temperature bolometers operate on the thermally stimulated pyroelectric current or temperature coefficient of resistivity.

A New Approach to Doped Barium Titanate Ceramics
D. Viehland,* R. T. Zhang, Z. Xu
U.S. Office of Naval Research; Naval Undersea Warfare Center, 15-20621

Barium titanate is one of the most important ferroelectric materials. The interrelationships between the structure and properties in this material are being studied by changing the La and Sn contents. These investigations are being performed by dielectric spectroscopy and transmission electron microscopy, with particular attention being paid to the influence of impurities on the phase transformational characteristics and domain structure. The purpose of this study is to develop new lead-free relaxor-like ferroelectric materials for electrostrictive applications.

Electrical Properties of Grain Boundaries in Manganese Ferrite
G. P. Wirtz*
University of Illinois

Minor amounts of CaO and SiO2 are being added to MnFe2O4 to study their effects on the electrical transport properties of the ferrite. The additives are concentrated in the grain boundaries, which is believed to produce high-resistance grain boundaries, surrounding more conductive grains, thus reducing eddy current losses at high frequencies in the ferrite. It is also found that the concentration of dopant in the grain boundary prevents decomposition of the ferrite in oxidizing atmospheres, thus maintaining the stoichiometry of the high-resistance ferrite. Impedance spectroscopy, thermogravimetry, and electrochemical cell measurements are being used in the study.