Publications

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The impacts of small niobium additions to processing, microstructure, and electrical properties in the Zr-rich lead zirconate titanate ceramics (PZT 95/5) were investigated. The influence of niobium content on dielectric responses and the characteristics of ferroelectric behaviors, as well as the relative phase stability and the hydrostatic pressure induced ferroelectric-to- antiferroelectric phase transformation are reported. Results indicate that increasing the niobium concentration in the solid solutions enhances densification, refines the microstructure, decreases dielectric constant and spontaneous polarization, and stabilizes the ferroelectric phase. The stabilization of ferroelectric phase with respect to the antiferroelectric phase near PZT 95/5 composition dramatically increases the pressure required for the ferroelectric-to-antiferroelectric phase transformation. These observations were correlated to the creation of A-site vacancies and a slight modification of the crystal structure. The importance of these composition-property relationships on device application will be presented.

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At Sandia National Laboratories, miniaturization dominates future hardware designs, and technologies that address the manufacture of micro-scale to nano-scale features are in demand. Currently, Sandia is developing technologies such as photolithography/etching (e.g. silicon MEMS), LIGA, micro-electro-discharge machining (micro-EDM), and focused ion beam (FIB) machining to fulfill some of the component design requirements. Some processes are more encompassing than others, but each process has its niche, where all performance characteristics cannot be met by one technology. For example, micro-EDM creates highly accurate micro-scale features but the choice of materials is limited to conductive materials. With silicon-based MEMS technology, highly accurate nano-scale integrated devices are fabricated but the mechanical performance may not meet the requirements. Femtosecond laser processing has the potential to fulfill a broad range of design demands, both in terms of feature resolution and material choices, thereby improving fabrication of micro-components. One of the unique features of femtosecond lasers is the ability to ablate nearly all materials with little heat transfer, and therefore melting or damage, to the surrounding material, resulting in highly accurate micro-scale features. Another unique aspect to femtosecond radiation is the ability to create localized structural changes thought nonlinear absorption processes. By scanning the focal point within transparent material, we can create three-dimensional waveguides for biological sensors and optical components. In this report, we utilized the special characteristics of femtosecond laser processing for microfabrication. Special emphasis was placed on the laser-material interactions to gain a science-based understanding of the process and to determine the process parameter space for laser processing of metals and glasses. Two areas were investigated, including laser ablation of ferrous alloys and direct-write optical waveguides and integrated optics in bulk glass. The effects of laser and environmental parameters on such aspects as removal rate, feature size, feature definition, and ablation angle during the ablation process of metals were studied. In addition, the manufacturing requirements for component fabrication including precision and reproducibility were investigated. The effect of laser processing conditions on the optical properties of direct-written waveguides and an unusual laser-induced birefringence in an optically isotropic glass are reported. Several integrated optical devices, including a Y coupler, directional coupler, and Mach-Zehnder interferometer, were made to demonstrate the simplicity and flexibility of this technique in comparison to the conventional waveguide fabrication processes.

The goal of this project was to identify a viable, non-destructive methodology for the detection of cracks in electrically poled piezoelectric ceramics used in neutron generator power supply units. The following methods were investigated: Impedance Spectroscopy, Scanning Acoustic Microscopy, Lock-in Thermography, Photo-acoustic Microscopy, and Scanned Vicinal Light. In addition to the exploration of these techniques for crack detection, special consideration was given to the feasibility of integrating these approaches to the Automatic Visual Inspection System (AVIS) that was developed for mapping defects such as chips, pits and voids in piezoelectric ceramic components. Scanned Vicinal Light was shown to be the most effective method of automatically detecting and quantifying cracks in ceramic components. This method is also very effective for crack detection in other translucent ceramics.

The optical birefringence in an isotropic glass medium was created by using a regeneratively amplified Ti:sapphire femtosecond laser. The regions between two crossed polarizers modified by the femtosecond laser shows bright transmission with respect to the dark background of the isotropic glass. It was found that the angular dependence of transmission through the laser-modified region was consistent with that of an optically birefringent material. It was also observed that the optical axes of laser-induced birefringence can be controlled by the polarization direction of the femtosecond laser.

The electrical properties of lead zirconate titanate ceramics near the 95/5 composition are extremely sensitive to the chemical composition and processing conditions. To precisely control the lead stoichiometry in a solid solution has been a challenge because of lead volatility during high temperature sintering. In this study, we investigated the effect of the amount of lead in the solid solution on crystal structure, dielectric behavior, and phase transformation characteristics for chemically prepared niobium modified PZT 95/5 ceramics. Implications are important for process control and assurance of material performance.

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The polarization reversal process in a rhombohedral ferroelectric ceramic material was investigated using field-induced strain measurements and texture development. Special attention was focused on the difference in the field-induced strains between the first quarter cycle and subsequent loading conditions. Results show that the initial field-induced strain is about twelve times greater than the subsequent strain, which immediately suggests that mechanisms involved in these conditions during the polarization reversal process are different. The difference in the magnitude of field-induced strain is discussed in terms of 180 degree and non-180 degree domain reorientation processes.