Butrus Khuri-Yakub
Stanford University, Electrical Engineering, Faculty Member
Research Interests: Materials Science, Engineering Mechanics, Composite Materials and Structures, Nondestructive Evaluation, Materials Science and Engineering, and 11 moreMechanics of Materials, High Frequency, Nondestructive testing, Transducers, Ceramic, Ceramic Materials, Scattering, Gain, Attenuation, Ultrasonic Sensor, and Transducer
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We report on capacitive micromachined ultrasonic transducers (CMUTs) with vented cavities for varying their bandwidth and sensitivity in airborne applications. The devices are simulated using a finite element model which incorporates... more
We report on capacitive micromachined ultrasonic transducers (CMUTs) with vented cavities for varying their bandwidth and sensitivity in airborne applications. The devices are simulated using a finite element model which incorporates viscous and thermal fluid losses in the squeeze film and the vent holes, as well as acoustic radiation and acoustic resonance in the backing. Our model accurately predicts the behavior of such CMUTs. The model is also validated with measurements at elevated pressure.
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Advances in vat photopolymerization (VP) 3D printing (3DP) technology enable the production of highly precise 3D objects. However, it is a major challenge to create dynamic functionalities and to manipulate the physical properties of the... more
Advances in vat photopolymerization (VP) 3D printing (3DP) technology enable the production of highly precise 3D objects. However, it is a major challenge to create dynamic functionalities and to manipulate the physical properties of the inherently insoluble and infusible cross‐linked material generated from VP‐3DP without reproduction. The fabrication of light‐ and high‐intensity focused ultrasound (HIFU)‐responsive cross‐linked polymeric materials linked with hexaarylbiimidazole (HABI) in polymer chains based on VP‐3DP is reported here. Although the photochemistry of HABI produces triphenylimidazolyl radicals (TPIRs) during the process of VP‐3DP, the orthogonality of the photochemistry of HABI and photopolymerization enables the introduction of reversible cross‐links derived from HABIs in the resulting 3D‐printed objects. While photostimulation cleaves a covalent bond between two imidazoles in HABI to generate TPIRs only near the surface of the 3D‐printed objects, HIFU triggers cleavage in the interior of materials. In addition, HIFU travels beyond an obstacle to induce a response of HABI‐embedded cross‐linked polymers, which cannot be attainable with photostimulation. The present system would be beneficial for tuning the physical properties and recycling of various polymeric materials, but it will also open the door for pinpoint modification, healing, and reshaping of materials when coupled to various dynamic covalent materials.
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Longitudinal wave piezoelectric transducers of finite diameter which are placed in contact with a buffer rod or a material under test excite weak spurious shear wave modes. We have used a spatial frequency analysis to predict the level of... more
Longitudinal wave piezoelectric transducers of finite diameter which are placed in contact with a buffer rod or a material under test excite weak spurious shear wave modes. We have used a spatial frequency analysis to predict the level of the shear-longitudinal and shear-shear mode as compared to the level of the main longitudinal mode of interest. We find good agreement between theory and experiments for 50-MHz transducers made with PZT, PSN, or LiNbO3 on a quartz buffer rod.
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We report the use of focused acoustic beams to eject discrete droplets of controlled diameter and velocity and from a free liquid surface. No nozzles are involved. Droplet formation has been experimentally demonstrated over the frequency... more
We report the use of focused acoustic beams to eject discrete droplets of controlled diameter and velocity and from a free liquid surface. No nozzles are involved. Droplet formation has been experimentally demonstrated over the frequency range of 5 to 300 MHz, with corresponding droplet diameters from 300 to 5 microns. The physics of droplet formation is essentially unchanged over
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Models of electrostatic transducers that assume a constant shape function for the plate’s deflection facilitate derivation of closed-form design expressions. This work analyses the static and frequency-response characteristics of... more
Models of electrostatic transducers that assume a constant shape function for the plate’s deflection facilitate derivation of closed-form design expressions. This work analyses the static and frequency-response characteristics of capacitive micromachined ultrasonic transducers (CMUTs) using a shape-function based model. This analysis shows that across all CMUT designs of a given plate shape, the CMUT’s static parameters vary with bias voltage in the same manner. Normalizing bias voltage by pull-in voltage and plate deflection by gap height enables this result. The static parameters examined are plate deflection, capacitance, transformer ratio, spring softening, and coupling coefficient. As shown here, assuming a basic second-order model of the CMUT, these static parameters along with plate resonance frequency and quality factor describe a CMUT’s frequency response. This work specifically examines ideal parallel-plate and clamped circular plate CMUTs, but it extends to other plate shapes with substitution of different expressions for capacitance as a function of plate deflection.
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Research Interests: Engineering, Computer Science, Algorithms, Medicine, Image Quality, and 15 moreBeamforming, Transducers, Computer Simulation, Image Reconstruction, Three Dimensional Imaging, Spatial Frequency, Phased Array, Real Time, Front end, Point Spread Function, Ultrasonic Sensor, Transducer, Ultrasonic Imaging, Linear Array, and imaging system
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A microwave-induced thermoacoustic detection system for embedded targets in lossy media is presented. The system achieves reliable detection of 5 cm × 5 cm × 2 cm targets embedded in a large Agarose sample at a 20 cm acoustic standoff.... more
A microwave-induced thermoacoustic detection system for embedded targets in lossy media is presented. The system achieves reliable detection of 5 cm × 5 cm × 2 cm targets embedded in a large Agarose sample at a 20 cm acoustic standoff. Repeated measurements across different target and sample configurations confirm the system’s ability to distinguish between a target signal and a baseline control signal generated by the package without embedded targets. Post-processing techniques including filtering and baseline signal characterization further improve detection performance.
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A capacitive micromachined ultrasonic transducer (CMUT) with substrate-embedded springs has been demonstrated high transmit efficiency with non-flexural parallel-plate piston movement. In this paper, we introduce whole wafer-scale uniform... more
A capacitive micromachined ultrasonic transducer (CMUT) with substrate-embedded springs has been demonstrated high transmit efficiency with non-flexural parallel-plate piston movement. In this paper, we introduce whole wafer-scale uniform nanometer silicon posts with a fabrication technique of combination between DRIE and RIE. In order to maintain our fabrication cost-effective, a photomask with a minimum feature size of $2.0\ \mu m$ was used for whole wafer-scale photolithography, and a size-reduced RIE process followed by DRIE was conducted for achieving the sub-micron or nanometer post area. Furthermore, In-situ nanomechanical tests of the fabricated silicon posts were conducted using a pico-indenter (PI 85L Pico-Indenter, Bruker) monitored under a scanning electron microscopy. The length and the diameter of the silicon post after size-reduction by RIE are measured as $5.6\ \mu m$ and .74\ \mu m$, respectively. The uniformity across the whole 4-inch wafer is less than 5%. The loading-unloading graph by In-situ nanomechanical experiments confirmed that the silicon posts could consistently achieve above 6% elastic strain. We are currently applying this nanometer springs to the CMUTs with substrate-embedded springs.
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Capacitive micromachined ultrasonic transducers (cMUT) can match and even outperform piezoelectric transducers in terms of efficiency and bandwidth. With the advent of silicon micromachining, it is now possible to make capacitors with... more
Capacitive micromachined ultrasonic transducers (cMUT) can match and even outperform piezoelectric transducers in terms of efficiency and bandwidth. With the advent of silicon micromachining, it is now possible to make capacitors with very thin gaps that sustain electric fields of the order of 109 V/m or more. At these levels of electric field, the transformer coupling between the electrical and mechanical parts of the capacitor transducer, and thus its performance, are comparable to that of piezoelectric transducers. We will review the design and performance of cMUTs for airborne, immersion, Lamb and Rayleigh waves’ ultrasound applications. We will show examples of immersion transducers (single element, 1-D and 2-D arrays of elements), air transducers and Lamb wave transducers. We will also present transducer systems (single element, 1-D and 2-D arrays of elements) with dynamic ranges of over 150 dB/Volt/vHz, which are in excellent agreement with the theoretical predictions.
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Focused ultrasound treats tissue deep in the body without damaging surrounding structures. Driven by large-amplitude RF electrical signals, ultrasonic transducers can generate large pressure acoustic wave. Wide-bandgap semiconductors,... more
Focused ultrasound treats tissue deep in the body without damaging surrounding structures. Driven by large-amplitude RF electrical signals, ultrasonic transducers can generate large pressure acoustic wave. Wide-bandgap semiconductors, e.g., Gallium Nitride (GaN), greatly simplify the design and reduce the requisite size and weight of RF amplifiers, and so improves the portability of focused ultrasound device. Here, we demonstrate a compact 8-phase 5 MHz Class-D resonant amplifier for a focused ultrasound cancer therapy application.
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