Recent widespread application of colloidal crystals in research has attempted to gain fundamental insights into colloidal forces and self-assembly [1,2] in addition to providing precursors for the next generation of advanced materials. [3,4] One such class of materials that benefits from these systems is dimensionally periodic dielectric structures that exhibit a photonic bandgap, often referred to as photonic crystals. [5,6] Colloidal crystals have been exploited in this field since they may undergo self organization at a nanometer length scales, resulting in spatial periodicities that may range from ca. 10 2 to 10 3 nm.Two forms of manipulating monodisperse spherical colloidal particles for the generation of photonic crystals have emerged. One approach involves the assembly of the particles into close-packed arrays through sedimentation, and typically relies on non-specific particle±particle ªhard sphereº packing to induce order. Particle assembly via this method is attractive in terms of both simplicity and versatility.[7±12] The second approach utilizes the long-range electrostatic repulsive interactions of charged colloidal spheres suspended in a liquid medium to produce order. [2,13,14] These systems will often adopt a minimum energy crystal structure with either body-centered cubic (bcc) or face-centered cubic (fcc) symmetry, [1,15,16] though the ordering in these systems is readily perturbed through minor mechanical disturbances or ionic contamination of the suspending medium. Approaches to stabilize both inorganic [17,18] and organic electrostatically stabilized arrays [19,20] through an in-situ polymerization of a monomer around the ordered arrays have been pioneered. These materials have been observed to exhibit a mechanochromic response, [17,21,22] where the response has been attributed to an affine deformation of the lattice. Recently, poly(methyl methacrylate) (PMMA) inverse opals were presented, which exhibited stop-band shifts when the opal was deformed above the glass transition of PMMA.[23]Though these approaches have increased the likelihood that these systems may find commercial applications exploiting mechanochromism, the underlying slow recovery time after the cessation of stress or the poor mechanical performance of the photonic crystal is a disadvantage. In this communication, we report on a procedure we have developed that allows for the tailoring of the thermomechanical properties of a photonic crystal to suit end-use criteria, with a specific interest in developing mechanochromic photonic crystal composites that exhibit reversible color variations at deformation frequencies up to 200 Hz. The first phase in generating a photonic crystal composite requires the stabilization of a crystalline colloidal array composed of monodisperse crosslinked polystyrene spheres dispersed in water (typical characteristics include a diameter of 109 ± 26 nm (mean and standard deviation) and a particle density of 10 13 ±10 14 cm ±3) through the encapsulation of the arrays with a photoinitiated free-radical p...
Piezoelectric micromachined ultrasound transducer (PMUT) matrix arrays were fabricated containing novel through-silicon interconnects and integrated into intracardiac catheters for in vivo real-time 3-D imaging. PMUT arrays with rectangular apertures containing 256 and 512 active elements were fabricated and operated at 5 MHz. The arrays were bulk micromachined in silicon-on-insulator substrates, and contained flexural unimorph membranes comprising the device silicon, lead zirconate titanate (PZT), and electrode layers. Through-silicon interconnects were fabricated by depositing a thin-film conformal copper layer in the bulk micromachined via under each PMUT membrane and photolithographically patterning this copper layer on the back of the substrate to facilitate contact with the individually addressable matrix array elements. Cable assemblies containing insulated 45-AWG copper wires and a termination silicon substrate were thermocompression bonded to the PMUT substrate for signal wire interconnection to the PMUT array. Side-viewing 14-Fr catheters were fabricated and introduced through the femoral vein in an adult porcine model. Real-time 3-D images were acquired from the right atrium using a prototype ultrasound scanner. Full 60° × 60° volume sectors were obtained with penetration depth of 8 to 10 cm at frame rates of 26 to 31 volumes per second.
Piezoelectric micromachined ultrasound transducers (pMUTs) are a new approach for the construction of 2-D arrays for forward-looking 3-D intravascular (IVUS) and intracardiac (ICE) imaging. Two-dimensional pMUT test arrays containing 25 elements (5 x 5 arrays) were bulk micromachined in silicon substrates. The devices consisted of lead zirconate titanate (PZT) thin film membranes formed by deep reactive ion etching of the silicon substrate. Element widths ranged from 50 to 200 microm with pitch from 100 to 300 mum. Acoustic transmit properties were measured in de-ionized water with a calibrated hydrophone placed at a range of 20 mm. Measured transmit frequencies for the pMUT elements ranged from 4 to 13 MHz, and mode of vibration differed for the various element sizes. Element capacitance varied from 30 to over 400 pF depending on element size and PZT thickness. Smaller element sizes generally produced higher acoustic transmit output as well as higher frequency than larger elements. Thicker PZT layers also produced higher transmit output per unit electric field applied. Due to flexure mode operation above the PZT coercive voltage, transmit output increased nonlinearly with increased drive voltage. The pMUT arrays were attached directly to the Duke University T5 Phased Array Scanner to produce real-time pulse-echo B-mode images with the 2-D pMUT arrays.
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