Electrical Ceramics

• Preparation and Characterization of Ta₂O₅ -TiO₂ Ceramics
• Pb-free Morphotropic Phase Boundary-based Ceramics
• Dielectric and Piezoelectric Analysis of Pb-free Single Crystals
• Pb(Mg_1/3 Nb_2/3 )O₃-PbTiO₃ Phase Formation and Properties
• Characterization of PMN-PT System
• Electro-thermal Imaging of Ferroelectric Materials
• Phase Transformation Studies on PMN-PT Single Crystals
• Densification, Stress Development and Phase Evolution in Sol-Gel-derived PZT Coatings

Materials Chemistry

• Chemical Processing and Characterization of Ta₂O₅ -TiO₂ Films
• Ultrathin Oxide Films on Semiconductors

Preparation and Characterization of Ta₂O₅ -TiO₂ Ceramics
D. A. Payne,* G. L. Brennecka
U.S. Department of Energy, DE-FG02-91-ER45439 (In cooperation with the Materials Research Laboratory)

Ta₂O₅ is widely studied for possible application in VLSI technology as the next generation of higher dielectric constant (K) gate oxide material. Previous work has shown that TiO₂ additions can significantly improve the dielectric properties of Ta₂O₅. Current work on compositions across the Ta₂O₅ -TiO₂ system show evidence of phase separation and the presence of previously unreported phases. A unique oriented microstructure is observed for low-TiO₂ materials, while materials with higher amounts of TiO₂ show distinctly different phases with plate-like morphologies. Work in progress includes sintering and phase transformation studies as well as dielectric measurements.

Pb-free Morphotropic Phase Boundary-based Ceramics
D.A. Payne,* and J. F. Carroll III
U.S. Department of Energy, DE-FG02-91-ER45439 (In cooperation with the Materials Research Laboratory)

Identification of rhombohedral-tetragonal morphotropic phase boundaries in Pb-free material systems is highly sought after. With future global restrictions limiting lead ion content, manufacturers are forced to seek alternative options for piezoelectric based devices. Current investigations underway are focused on the bismuth-based perovskite systems, more specifically at the morphotropic phase boundary in the sodium bismuth titanate and potassium bismuth titanate (NBT-KBT) family of materials. Current research is focused on the dielectric, piezoelectric, and structural behavior as a function of temperature in dense polycrystalline specimens

Dielectric and Piezoelectric Analysis of Pb-free Single Crystals
D.A. Payne,* and J. F. Carroll III
U.S. Department of Energy, DE-FG02-91-ER45439 (In cooperation with the Materials Research Laboratory)

High-quality single crystals of (Na_1/2Bi_1/2)TiO₃ (NBT) have been grown from the melt by a Bridgman method. Current investigations are focused on the temperature and frequency dependence of the relaxor-like system. Supplementary techniques of modulated differential thermal analysis and optical microscopy reveal the structural dependence of dielectric properties. Initial investigations reveal a piezoelectric constant d33, 240 pC/N, a factor of two better than any other previously published data. Future investigations are focused on the determination of the full dielectric and piezoelectric tensor coefficients for NBT.

Pb(Mg_1/3 Nb_2/3 )O₃-PbTiO₃ Phase Formation and Properties
D. A. Payne,* M. A. Kurata
U.S. Department of Energy under grant DEFG02-91-ER45439 (In cooperation with the Materials Research Laboratory)

The piezoelectric material, PMN-PT, has an extraordinarily high electromechanical coupling factor (>0.9), with relatively low dispersion, which enables higher sensitivity, and wider bandwidth devices, compared with PZT and LiNbO₃. Research is in progress on PMN-PT phase formation synthesized via columbite and mixed-oxide techniques. Thermal analysis and in-situ hot-stage microscopy studies are underway to characterize phase formation. After obtaining a better understanding of the system, investigations are planned for various thick film deposition methods for ultrasonic applications. Densification at lower temperatures is a goal for successful integration of this sensor material with current micro-fabrication technology.

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Characterization of PMN-PT System
D. A. Payne,* A. Sehirlioglu, P.D. Han
U.S. Department of Energy, DE-FG02-91-ER45439 (In cooperation with the Materials Research Laboratory)

Crystals in the PMN-PT system are under investigation. The thermal, thermoelastic, dielectric and electromechanical properties were measured as a function of composition, orientation and poling state. Compositions in the morphotropic phase boundary region were chosen. Differential scanning calorimetry, dilatometry dielectric/electromechanical measurements were used to determine the properties for the rhombohedral, tetragonal and cubic phases as a function of temperature.

Electro-thermal Imaging of Ferroelectric Materials
D. A. Payne,* A. Sehirlioglu, P.D. Han , T.J. Mackin, C.E. Deiter
U.S. Department of Energy, DE-FG02-91-ER45439 (In cooperation with the Materials Research Laboratory)

A new method, electro-thermal imaging, was developed to observe polarization reversal in ferroelectric materials. The method is based on a temperature change induced by an applied electric field. An infrared camera was used to detect the temperature change with respect to background. Using software, thermal maps of the ferroelectric material are created. For the first time, polarization reversal was observed remotely by electro-thermal imaging. The poling direction was also determined due to a 180º phase shift in the response between domain states with opposite polarization directions.

Phase Transformation Studies on PMN-PT Single Crystals
D. A. Payne,* A. Sehirlioglu, P.D. Han
U.S. Department of Energy, DE-FG02-91-ER45439 (In cooperation with the Materials Research Laboratory)

Temperature- and field-induced phase transformations for PMN-PT single crystals were studied as a function of composition and orientation with an emphasis on the morphotropic phase boundary region. Dielectric, ferroelectric, thermal expansion and heat flow measurements were used to examine the phase transformation behavior and optical microscopy and transmission electron microscopy was used to study the change in the structure during transformations. Both macroscopic (domain related) and microscopic (unit cell related) effects on phase transformation behavior were studied.

Densification, Stress Development and Phase Evolution in Sol-Gel-derived PZT Coatings
D. A. Payne,* R. J. Ong; T. A. Berfield, N. R. Sottos (TAM)
NSF grant, CMS 00-88206, XYZ-on-a-chip U.S. Department of Energy, DE-FG02-91-ER45439 (In cooperation with the Materials Research Laboratory)

Sol-gel-derived PZT layers deposited on silicon and platinized silicon substrates were examined during heat-treatment for shrinkage behavior using in-situ ellipsometry. Thermal analysis (DTA, TGA) data were correlated to densification data. The resulting stresses in the coating were measured as a function of heat treatment by a laser reflectance technique and related to associated densification phenomena and substrate/layer thermal expansion mismatch. The dielectric, ferroelectric, and piezoelectric properties of crystallized perovskite thin films were then measured and related to the residual stress (resulting from processing) and any applied stress (additional post-processing bending moment) in the film.

Chemical Processing and Characterization of Ta₂O₅ -TiO₂ Films
D. A. Payne,* G. L. Brennecka
U.S. Department of Energy, DE-FG02-91-ER45439 (In cooperation with Materials Research Laboratory)

Thin films of Ta₂O₅ and TiO₂-modified Ta₂O₅ were prepared by a hybrid sol-gel chelate method. Solutions were obtained by chelating a mixture of Ta(OEt)₅ and Ti(OiPr)₄ with acetic acid, then further stabilized with additions of MeOH. Crystallization of Ta₂O₅ -TiO₂ films took place around 700°C. A low- temperature orthorhombic structure was identified in films crystallized below 1000ºC; while films crystallized at higher temperatures (1100-1400ºC) exhibited a high- temperature monoclinic structure, with an oriented microstructure similar to bulk ceramics. Work in progress includes studies of microstructure and texture development, phase transformations, and dielectric properties for integrated films.

Ultrathin Oxide Films on Semiconductors
D. A. Payne,* E. A. Mikalsen; W. G. Klemperer
U.S. Department of Energy, DE-FG02-91-ER45439 (In cooperation with the Materials Research Laboratory)

Research in progress is directed at the development of a method for the controlled deposition of ultrathin oxide films on semiconductor substrates. Applicable as alternative gate oxide materials, zirconia films are under investigation as candidates that provide desirable high capacitances and low leakage current densities (e.g., C/A > 2 F/cm² , J ~ 0.00001 A/cm² ). Using a novel zirconium chemical precursor (Zr4(OPrn)16), the process is capable of near-monolayer deposition cycles using liquid treatment solutions at normal atmospheric pressure in a clean chemical environment. Preliminary results show great promise for a low-leakage, high-k dielectric gate oxide.

 

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