George Robert Burns

Femtosecond laser-pulse-induced birefringence in optically isotropic glass

We used a regeneratively amplified Ti:sapphire femtosecond laser to create optical birefringence in an isotropic glass medium. Between two crossed polarizers, regions modified by the femtosecond laser show bright transmission with respect to the dark background of the isotropic glass. This observation immediately suggests that these regions possess optical birefringence. The angular dependence of transmission through the laser-modified region is consistent with that of an optically birefringent material. Laser-induced birefringence is demonstrated in different glasses, including fused silica and borosilicate glass. Experimental results indicate that the optical axes of laser-induced birefringence can be controlled by the polarization direction of the femtosecond laser. The amount of laser-induced birefringence depends on the pulse energy level and number of accumulated pulses.

Electric field effect on the rhombohedral–rhombohedral phase transformation in tin modified lead zirconate titanate ceramics

The ferroelectric–to–ferroelectic phase transformation between the high temperature (FERH) and the low temperature (FERL) rhomobohedral phases in lead based perovskite under the dc bias conditions was investigated. Dielectric measurements show that an external electric field stabilizes the FERL phase and moves the phase transformation to a higher temperature. The observation has been further verified by an in situ microfocused x-ray study where an external field can effectively induce the oxygen octahedral tilting in the crystalline lattice and extends the thermal stability region of the FERL phase to a higher temperature. An analysis based on the combination of the Clausius–Clapeyron relationship with lattice dynamic principles suggests that the transformation from FERH to FERL is driven by a short-range interaction in the crystalline lattice. The origin of this short-range interaction is proposed, based on the structural evolution during the phase transformation. Experimental evidence suggests that such...

Field-enhanced piezoelectric deformation during the high temperature/low temperature rhombohedral (FERh/FERL) phase transformation for tin modified lead zirconate titanate ceramics

An unusual field-enhanced piezoelectric deformation near the FERH/FERL structural phase transformation was observed in a tin modified lead zirconate titanate solid solution. In addition to the typical field-induced domain reorientation and the piezoelectric strain, this additional field-enhanced deformation only observed near the phase transformation increases linearly with external electric field strength. A 78% increase in field-enhanced strain was observed at a field strength of 32 kV/cm. Comparison of the dielectric susceptibility at low and high field conditions suggests that the observed unusual behavior is created by a field-induced lattice softening during the structural phase transformation. Experimental observations on the field-induced softening phenomena are reported.

In situ dissolution or deposition of Ytterbium (Yb) metal in microhotplate wells for a miniaturized atomic clock

Current atomic clocks are burdened by size, weight, power and portability limitations to satisfy a broad range of potential applications. One critical need in the fabrication of a miniaturized atomic clock is small, low-power metallic sources. Exploiting the relatively high vapor pressure of ytterbium (Yb) and its dissolution in anhydrous ammonia, we report two independent techniques for depositing Yb inside a well micromachined into a microhotplate. Subsequent in situ evaporation of Yb from the microhotplate well serves as a low-power metallic source suitable for atomic clocks. The deposition and evaporation of Yb were confirmed using a variety of physicochemical techniques including quartz crystal microbalance, scanning electron microscopy, energy dispersive X-ray spectroscopy, and laser fluorescence. We also describe the fabrication of the microhotplate device, an integral component of our Yb-based miniature atomic clock. The Yb deposition/evaporation on a microhotplate well is thus useful as a low power Yb source during the fabrication of a miniaturized atomic clock, and this technique could be used for other applications requiring a vapor of a metal that has a moderate vapor pressure.

Direct-write embedded waveguides and integrated optics in bulk glass by femtosecond laser pulses

Embedded waveguides and integrated optical devices are fabricated from a computer-aided design file by scanning the focus spot in amorphous silica, using femtosecond laser pulses (800 nm, ,125 fs, at 1 kHz). The effect of laser processing conditions on the optical prop- erties of direct-written waveguides and an unusual laser-induced birefrin- gence in an optically isotropic glass are reported. Several integrated optical devices, including Y coupler, directional coupler, and Mach- Zehnder interferometer, are made to demonstrate the simplicity and flex- ibility of this technique in comparison to the conventional waveguide fab- rication processes.

Direct-write waveguides and structural modification by femtosecond laser pulses

Embedded waveguides and their optical properties in bulk silicate glasses fabricated by femtosecond (fs) laser pulses (800 nm, <120 fs, at 1 kHz) are reported. Experimental results show that there is a narrow operating window for our system to produce low loss waveguides. An angular dependence of light transmission measured between two crossed polarizers on these laser-modified regions suggests that these regions possess an optical birefrigent property. Furthermore, the optical axes of laser-induced birefringence can be controlled by the polarization direction of the fs laser. Permanent optical birefringence induced by the fs laser pulses can be produced in amorphous silica, and borosilicate glass. Raman spectroscopy of the modified glass shows a densification and reconstruction of silica network in the glass. Results show that the amount of laser-induced birefringence depends on pulse energy level and the number of accumulated pulses. Mechanisms that contribute to the observed laser induced birefringence behavior are discussed.

The effects of process parameters on injection-molded PZT ceramics part fabrication- compounding process rheology.

Solid solutions of lead-based perovskites are the backbone materials of the piezoelectric components for transducer, actuator, and resonator applications. These components, typically small in size, are fabricated from large sintered ceramic slugs using grinding and lapping processes. These operations increase manufacturing costs and produce a large hazardous waste stream, especially when component size decreases. To reduce costs and hazardous wastes associated with the production of these components, an injection molding technique is being investigated to replace the machining processes. The first step in the new technique is to compound an organic carrier with a ceramic powder. The organic carrier is a thermoplastic based system composed of a main carrier, a binder, and a surfactant. Understanding the rheology of the compounded material is necessary to minimize the creation of defects such as voids or cavities during the injection-molding process. An experiment was performed to model the effects of changes in the composition and processing of the material on the rheological behavior. Factors studied included: the surfactant of the organic carrier system, the solid loading of the compounded material, and compounding time. The effects of these factors on the viscosity of the material were investigated.

Ceramic processing of template-induced microstructure textured ceramics PI008

The target of this work is to develop an advanced manufacturing process that results in a bulk polycrystalline electroceramic component through a texture induced forming method. The technique produces a bulk ceramic component that exhibits enhanced macroscopic properties when compared to a traditional electroceramic material that has electrically induced ferroelectric texture or crystallographic texture. Templated texturing can involve the ¿laying down¿ of seed crystals with planar morphology to induce preferential grain growth within a pre-designed orientation of the bulk ceramic. Through well-controlled slurry processing of ceramic powders and the addition of templated crystals it is possible to induce this preferential grain orientation for sintered ceramics. This work will present the results of a comparison between three separate advanced ceramic forming techniques; tape casting, thick film screen printing, and extrusion, examining the degree of microstructure texture developed by these methods.

Textured processing, reactive templated grain growth, and electrical property relationships for sodium bismuth titanate PI015

The investigation of lead-free piezoelectric ceramic compositions has recently gained an increased level of interest due to the efforts to reduce lead based components. The most widely used piezoelectric/ferroelectric ceramic material today, including specialized ceramics for military applications (ex. sonar), consists of PbTiO3-PbZrO3 (i.e. PZT-system). It has become imperative to integrate a processing strategy with a lead-free ferroelectric material capable of competing with or surpassing the properties of lead-based compositions. This work examines the development of optimal processing parameters through texturing and reactive templated grain growth to selectively engineer a polycrystalline ceramic microstructure. It presents how these parameters can affect the electro-mechanical properties for a sodium bismuth titanate based composition. The final properties for all ceramic materials are highly influenced by the processing steps and forming techniques used to construct the bulk ceramic component. Texturally modified ceramic compositions have recently exhibited enhanced properties that, depending on the system, match and even surpass those of an optimum modified lead-based composition. In this work we report on the development and use of a texture induced forming process combined with reactive templated grain growth to produce grain-oriented polycrystalline bulk ceramics. Thermal analysis, x-ray diffraction characterization, microstructure stereology and the dielectric and electromechanical performance will be presented. A processing space has been characterized and mapped in order to drive towards achieving maximized electrical performance for this lead-free system.

Characterization of the electro-mechanical behavior of zirconia-rich PZT ceramics

Lead zirconate titanate (PZT) ceramics near the morphotropic phase boundary have been the backbone materials for piezoelectric applications for more than 50 years. The electro-mechanical responses for these materials have been well studied. In this study, we investigated the electro-mechanical behavior of two zirconia-rich PZT compositions, with and without tin (Sn) modification. These materials, close to the antiferroelectric phase region, have been used for power supply and actuator applications due to their unique ferroelectric(FE)-antiferroelectric(AFE) phase transformation behavior. However, limited information is available characterizing their electromechanical responses, especially outside of room temperature. In this work, we present the electromechanical properties of these compositions as a function of temperature. Special attention has been placed on the electromechanical responses near the FE-FE phase transformation.