Dispersion rheology of carbon nanotubes in a polymer matrix

We report on rheological properties of a dispersion of multiwalled carbon nanotubes in a viscous polymer matrix. Particular attention is paid to the process of nanotubes mixing and dispersion, which we monitor by the rheological signature of the composite. The response of the composite as a function of the dispersion mixing time and conditions indicates that a critical mixing time t* needs to be exceeded to achieve satisfactory dispersion of aggregates, this time being a function of nanotube concentration and the mixing shear stress. At shorter times of shear mixing t< t*, we find a number of nonequilibrium features characteristic of colloidal glass and jamming of clusters. A thoroughly dispersed nanocomposite, at t> t*, has several universal rheological features; at nanotube concentration above a characteristic value nc ∼2-3 wt. % the effective elastic gel network is formed, while the low-concentration composite remains a viscous liquid. We use this rheological approach to determine the effects of aging and reaggregation. © 2006 The American Physical Society.

Effects of mechanical and electrical coupling on the parametric sensitivity of mode localized sensors

We compare and contrast the effects of two distinctly different mechanisms of coupling (mechanical and electrical) on the parametric sensitivity of micromechanical sensors utilizing mode localization for sensor applications. For the first time, the strong correlation between mode localization and the phenomenon of 'eigenvalue loci-veering' is exploited for accurate quantification of the strength of internal coupling in mode localized sensors. The effects of capacitive coupling-spring tuning on the parametric sensitivity of electrically coupled resonators utilizing this sensing paradigm is also investigated and a mass sensor with sensitivity tunable by over 400% is realized. ©2009 IEEE.

Experiments and Lagrangian simulations on the formation of droplets in continuous mode

Experimental and computational studies on the dynamics of millimeter-scale cylindrical liquid jets are presented. The influences of the modulation amplitude and the nozzle geometry on jet behavior have been considered. Laser Doppler anemometry (LDA) was used in order to extract the velocity field of a jet along its length, and to determine the velocity modulation amplitude. Jet shapes and breakup dynamics were observed via shadowgraph imaging. Aqueous solutions of glycerol were used for these experiments. Results were compared with Lagrangian finite-element simulations with good quantitative agreement. © 2011 The American Physical Society.

Experiments and Lagrangian simulations on the formation of droplets in drop-on-demand mode

The creation and evolution of millimeter-sized droplets of a Newtonian liquid generated on demand by the action of pressure pulses were studied experimentally and simulated numerically. The velocity response within a model, large-scale printhead was recorded by laser Doppler anemometry, and the waveform was used in Lagrangian finite-element simulations as an input. Droplet shapes and positions were observed by shadowgraphy and compared with their numerically obtained analogues. © 2011 American Physical Society.

A bulk acoustic mode single-crystal silicon microresonator with a high-quality factor

This paper details a bulk acoustic mode resonator fabricated in single-crystal silicon with a quality factor of 15 000 in air, and over a million below 10 mTorr at a resonant frequency of 2.18 MHz. The resonator is a square plate that is excited in the square-extensional mode and has been fabricated in a commercial foundry silicon-on-insulator (SOI) MEMS process through MEMSCAP. This paper also presents a simple method of extracting resonator parameters from raw measurements heavily buried in electrical feedthrough. Its accuracy has been demonstrated through a comparison between extracted motional resistance values measured at different voltage biases and those predicted from an analytical model. Finally, a method of substantially cancelling electrical feedthrough through system-level electronic implementation is also introduced. © 2008 IOP Publishing Ltd.

Ultrasensitive mass balance based on a bulk acoustic mode single-crystal silicon resonator

A single-crystal silicon resonant bulk acoustic mass sensor with a measured resolution of 125 pg cm2 is presented. The mass sensor comprises a micromachined silicon plate that is excited in the square-extensional bulk acoustic resonant mode at a frequency of 2.182 MHz, with a quality factor exceeding 106. The mass sensor has a measured mass to frequency shift sensitivity of 132 Hz cm2 μg. The resonator element is embedded in a feedback loop of an electronic amplifier to implement an oscillator with a short term frequency stability of better than 7 ppb at an operating pressure of 3.8 mTorr. © 2007 American Institute of Physics.

Enhancing parametric sensitivity using mode localization in electrically coupled mems resonators

We use vibration localization as a sensitive means of detecting small perturbations in stiffness in a pair of weakly coupled micromechanical resonators. For the first time, the variation in the eigenstates is studied by electrostatically coupling nearly identical resonators to allow for stronger localization of vibrational energy due to perturbations in stiffness. Eigenstate variations that are orders of magnitude greater than corresponding shifts in resonant frequency for an induced stiffness perturbation are experimentally demonstrated. Such high, voltagetunable parametric sensitivities together with the added advantage of intrinsic common mode rejection pave the way to a new paradigm of mechanical sensing. ©2009 IEEE.

5.4-MHz single-crystal silicon wine glass mode disk resonator with quality factor of 2 million

This paper reports the design and electrical characterization of a micromechanical disk resonator fabricated in single crystal silicon using a foundry SOI micromachining process. The microresonator has been selectively excited in the radial extensional and the wine glass modes by reversing the polarity of the DC bias voltage applied on selected drive electrodes around the resonant structure. The quality factor of the resonator vibrating in the radial contour mode was 8000 at a resonant frequency of 6.34 MHz at pressure below 10 mTorr vacuum. The highest measured quality factor of the resonator in the wine glass resonant mode was 1.9 × 106 using a DC bias voltage of 20 V at about the same pressure in vacuum; the resonant frequency was 5.43 MHz and the lowest motional resistance measured was approximately 17 kΩ using a DC bias voltage of 60 V applied across 2.7 μm actuation gaps. This corresponds to a resonant frequency-quality factor (f-Q) product of 1.02 × 1013, among the highest reported for single crystal silicon microresonators, and on par with the best quartz crystal resonators. The quality factor for the wine glass mode in air was approximately 10,000. © 2009 Elsevier B.V. All rights reserved.

Low loss HF band SOI wine glass bulk mode capacitive square-plate resonator

This paper reports on the design and electrical characterization of a single crystal silicon micromechanical square-plate resonator. The microresonator has been excited in the anti-symmetrical wine glass mode at a resonant frequency of 5.166 MHz and exhibits an impressive quality factor (Q) of 3.7 × 106 at a pressure of 33 mtorr. The device has been fabricated in a commercial foundry process. An associated motional resistance of approximately 50 kΩ using a dc bias voltage of 60 V is measured for a transduction gap of 2 νm due to the ultra-high Q of the resonator. This result corresponds to a frequency-Q product of 1.9 × 1013, the highest reported for a fundamental mode single-crystal silicon resonator and on par with some of the best quartz crystal resonators. The results are indicative of the superior performance of silicon as a mechanical material, and show that the wine glass resonant mode is beneficial for achieving high quality factors allowed by the material limit. © 2009 IOP Publishing Ltd.

Square wine glass mode resonator with quality factor of 4 million

We report on the experimental characterization of a single crystal silicon square-plate microresonator. The resonator is excited in the square wine glass (SWG) mode at a mechanical resonance frequency of 2.065 MHz. The resonator displays quality factor of 9660 in air and an ultra-high quality factor of Q = 4.05 × 106 in 12 mtorr vacuum. The SWG mode may be described as a square plate that contracts along one axis in the fabrication plane, while simultaneously extending along an orthogonal axis in the same plane. The resonant structure is addressed in a 2-terminal configuration by utilizing equal and opposite drive polarities on surrounding capacitor electrodes, thereby decreasing the motional resistance of the resonator. The resonant micromechanical device has been fabricated in a commercial silicon-on-insulator process through the MEMSCAP foundry utilising a minimum electrostatic gap of 2 μm. © 2008 IEEE.

Anchor limited q in flexural mode resonators

This paper reports a preliminary examination of the effect of anchor geometry design on the quality factor of flexural mode resonators operating in vacuum using both FE simulation and measurements of resonator frequency response. Three types of structures have been considered in this study: an elliptical mode ring, a double ended tuning fork, and a doubly-clamped beam. We consider the relative distribution of strain energies in both the resonant structure and the connecting stem, which is indicative of the measured quality factor. The measured quality factors of the different structures are compared against each other, based on which suggestions are proposed for optimizing the anchor limited quality factor (Q) in flexural mode micromechanical resonators. ©2008 IEEE.

Characteristics of multiwalled carbon nanotube nanobridges fabricated by poly(methylmethacrylate) suspended dispersion

Transport measurements were performed on individual PECVD grown MWCNT nanobridge structures. Temperature dependent conductance measurements show that as the temperature is decreased, the conductance also decreases. The nanotubes were able to carry high current densities with the observed maximum at ∼108 A/cm2. High volatile measurements reveal that the PECVD grown MWCNTs break down in segments of nanotube shells.

Fabrication of multiwalled carbon nanotube bridges by poly-methylmethacrylate suspended dispersion

Poly-methylmethacrylate suspended dispersion was used to fabricate multiwalled carbon nanotube (MWCNT) bridges. Using this technique, nanotubes could be suspended between metal electrodes without any chemical etching of the substrate. The electrical measurement on suspended MWCNT bridges shows that the room temperature resistance ranges from under a kω to a few Mω.

Study of multi-walled carbon nanotube structures fabricated by PMMA suspended dispersion

We report on the electrical characteristics of plasma enhanced chemical vapour deposition (PECVD)-grown, multi-walled carbon nanotube (MWCNT) devices made by a new fabrication method, PMMA suspended dispersion. This method makes it possible to suspend nanotubes between metal electrodes and to remove unwanted nanotubes from the substrate. The measurements show that the MWCNTs are metallic and able to maintain a current density ∼2×106 A/cm2 for more than 15 days with a maximum current density of ∼1.8×107 A/cm2. This high current density and reliability will make PECVD-grown MWCNTs applicable to field emission cathodes. © 2002 Elsevier Science B.V. All rights reserved.

Study of lateral mode SOI-MEMS resonators for reduced anchor loss

MEMS resonators fabricated in silicon-on-insulator (SOI) technology must be clamped to the substrate via anchoring stems connected either from within the resonator or through the sides, with the side-clamped solution often employed due to manufacturing constraints. This paper examines the effect of two types of commonly used side-clamped, anchoring-stem geometries on the quality factor of three different laterally-driven resonator topologies. This study employs an analytical framework which considers the relative distribution of strain energies between the resonating body and clamping stems. The ratios of the strain energies are computed using ANSYS FEA and used to provide an indicator of the expected anchor-limited quality factors. Three MEMS resonator topologies have been fabricated and characterized in moderate vacuum. The associated measured quality factors are compared against the computed strain energy ratios, and the trends are shown to agree well with the experimental data. © 2011 IOP Publishing Ltd.