Nelson Sepúlveda

Bending in VO2 -coated microcantilevers suitable for thermally activated actuators

The curvature of VO2-coated silicon microcantilevers was measured as the temperature was cycled through the coating’s insulator-to-metal transition (IMT), which drives the curvature change mainly through the strain generated during this reversible structural transformation. The films were grown by pulsed laser deposition (PLD) on heated substrates. Cantilever tip displacement was measured for a 130 μm long cantilever as the temperature was changed by recording the deflection of a laser beam, and the curvature change and estimated film stress were calculated from this data. A change in curvature of over 2000 m−1 was observed through the narrow temperature range of the IMT, with a maximum rate of ∼485 m−1 per degree. Estimated recoverable stress was ∼1 GPa through the transition region. These results suggest applications in actuator devices with reduced dimensions, including submicron lengths, multifunctional capabilities, and possibly with higher operational frequencies than other thermally actuated devices.

Optoelectronic and all-optical multiple memory states in vanadium dioxide

Vanadium dioxide exhibits a well-known insulator-to-metal transition during which several of its physical properties change significantly. A hysteresis loop develops for each of them as the material is heated and then cooled through the transition. In this work VO2/SiO2 samples were maintained—by heat sinking—at a selected temperature within the heating branch of the hysteresis loops for resistance and near-infrared transmittance, while brief thermal excursions of the VO2 film were caused by either voltage pulses applied to the film or laser light pulses irradiating the film. These pulses had durations from milliseconds to a few seconds and the resulting drops in resistance or transmittance were easily and repeatably measurable without appreciably affecting their new values. A sequence of equal-duration pulses (for either equal-voltage or equal-irradiation pulses) caused the resistance and infrared transmittance to continue to drop, each time by a smaller amount, and larger energy pulses were required in ...

Phase transition behavior in microcantilevers coated with M1-phase VO2 and M2-phase VO2:Cr thin films

Silicon microcantilevers were coated by pulsed laser deposition with vanadium dioxide (VO2) (monoclinic M1 phase) and V1−xCrxO2 with x near 0.024 (monoclinic M2 phase), and their mechanical characteristics were studied as a function of temperature through the films’ insulator-to-metal transition (IMT). The undoped VO2 films grew with (011)M1 planes parallel to the substrate, while Cr-doped VO2 films grew oriented with (201)M2 and (2¯01)M2 planes parallel to the substrate. In both cases, the films transformed reversibly through the IMT to the tetragonal (rutile, R) phase, with film (110)R planes oriented parallel to the substrate. The fundamental resonant frequencies of the cantilevers were measured as the temperature was cycled from ambient temperature, through the IMT, and up to 100  °C. Very high resonant frequency changes were observed through the transition for both types of samples, with increases during heating of over 11% and over 15% for the cantilevers coated with pure and Cr-doped VO2, respectiv...

Reconfigurable UWB Antenna With RF-MEMS for On-Demand WLAN Rejection

A MEMS reconfigurable ultra-wideband (UWB) antenna that rejects on-demand all WLAN signals in the entire 5.15 to 5.825 GHz range (675 MHz bandwidth) is presented. The antenna design, miniaturization procedure, and monolithic integration with the MEMS and biasing network on SiO2 Quartz substrate are described. The integration challenges are addressed and the work is presented in a way that is useful for antenna engineers. A method to vary the rejection bandwidth is also provided. The fabricated prototype is conformal and single-sided. The antenna is measured using a custom-built platform at a university laboratory. Results indicate a successful integration and minimal interference of the MEMS and biasing circuitry with the antenna, paving the road for more integrated reconfigurable antennas on SiO2 using MEMS technology. Such antennas can improve UWB, WLAN and cognitive radio communication links.

High-Performance Polycrystalline Diamond Micro- and Nanoresonators

Cantilever type MEMS resonators were fabricated using boron-doped (~5 times 1019 cm-3) and undoped polycrystalline diamond (poly-C) films that were grown at 600degC or 780degC. The resonator dimensions ranged from 500 mum long, 10 mum wide, and 0.7 mum thick to 40 mum long, 100 nm wide, and 0.6 mum thick. Resonance frequencies and quality factors Qs were measured in vacuum, 10 5 torr, over the temperature range of 23degC-400degC. The measured values of the temperature coefficient of the resonance frequency were in the range of -7.2- -25.6 ppm. degC-1 and seemed to be related to changes in the Youngs modulus with temperature. Undoped poly-C cantilevers exhibit Qs as high as 116 000, the highest value reported for a cantilever resonator fabricated from a polycrystalline film. A thermally activated relaxation process seems to limit the measured Q -values for the highly doped poly-C samples.

Increasing efficiency, speed, and responsivity of vanadium dioxide based photothermally driven actuators using single-wall carbon nanotube thin-films.

Vanadium dioxide (VO2)-based actuators have demonstrated great performance in terms of strain energy density, speed, reversible actuation, programming capabilities, and large deflection. The relative low phase transition temperature of VO2 (∼68 °C) gives this technology an additional advantage over typical thermal actuators in terms of power consumption. However, this advantage can be further improved if light absorption is enhanced. Here we report a VO2-based actuator technology that incorporates single-wall carbon nanotubes (SWNTs) as an effective light absorber to reduce the amount of photothermal energy required for actuation. It is demonstrated that the chemistry involved in the process of integrating the SWNT film with the VO2-based actuators does not alter the quality of the VO2 film, and that the addition of such film enhances the actuator performance in terms of speed and responsivity. More importantly, the results show that the combination of VO2 and SWNT thin films is an effective approach to increase the photothermal efficiency of VO2-based actuators. The integration of SWNT films in VO2 devices can be easily applied to other VO2-based phototransducers as well as to similar devices based on other phase-change materials. While adding a sufficiently thick layer of some arbitrary material with high absorption for the light used for actuation (λ = 650 nm wavelength in this case) could have improved conversion of light to heat in the device, it could also have impeded actuation by increasing its stiffness. It is noted, however, that the low effective Youngs modulus of SWNT film coating used in this work does not impair the actuation range.

Young's modulus of VO2 thin films as a function of temperature including insulator-to-metal transition regime

Young’s modulus of VO2 thin films has been measured for the first time through the material’s insulator-to-metal transition. The resonant frequency of silicon VO2 coated cantilevers was measured in the temperature range 30–90°C. It has been found that during the semiconductor to metallic transition of VO2 thin films, which occurs at a temperature of 68°C, Young’s modulus changes most dramatically with temperature, abruptly reversing its declining trend with increasing temperature. The film is stiffened through the transition and, as the temperature is further raised, the declining trend is reasserted at a similar rate.

Performance of Electro-Thermally Driven

The integration of VO2 thin films in a MEMS actuator device is presented. The structural phase transition of VO2 was induced electro-thermally by resistive heaters monolithically integrated in the MEMS actuator. The drastic mechanical displacements generated by the large stress induced during the VO2 thin film phase transition have been characterized for static and time-dependent current pulses to the resistive heater, for air and vacuum environments. A comprehensive and simplified finite element model is developed and validated with experimental data. It was found that the cut-off frequency of the 300 μm-long VO2-based MEMS actuator operated in vacuum (f3dB=29 Hz) was mostly limited by conductive heat loss through the anchor, whereas convection losses were more dominant in air (f3dB=541 Hz). The cut-off frequency is found to be strongly dependent on the dimensions of the cantilever when operated in air but far less dependent when operated in vacuum. Total deflections of 68.7 and 28.5 μm were observed for 300 and 200 μm-long MEMS cantilevers, respectively. Full actuation in air required ~ 16 times more power than in vacuum.

{\rm VO}_{2}

Vanadium dioxide ( VO2) undergoes a thermally induced solid-to-solid phase transition, which can be exploited for actuation purposes. VO2-coated silicon cantilevers demonstrate abrupt curvature changes when their temperature is varied across the phase transition. Unlike the monotonic hysteresis phenomena observed in many other smart materials, the curvature-temperature hysteresis of VO2 actuators is nonmonotonic due to competing mechanisms associated with the materials phase transition and the different thermal expansion coefficients of the materials that form the bilayered cantilever. Motivated by the underlying physics, a novel model for the nonmonotonic hysteresis that combines a monotonic Preisach hysteresis operator and a quadratic operator is presented. A constrained least-squares scheme is proposed for model identification, and an effective inverse control scheme is presented for hysteresis compensation. For comparison purposes, a Preisach operator with a signed density function and a single-valued polynomial model are considered. Experimental results show that, for a 300- μm -long actuator, the largest modeling errors with the proposed model, the signed Preisach operator, and the polynomial approximation are 46.8, 80.3, and 483 m-1, respectively, over the actuated curvature range of [ -104, 1846] m-1. In addition, both the largest tracking error and root-mean-square error under the proposed inversion scheme are only around 10% of those under the polynomial-based inversion scheme.

-Based MEMS Actuators

The dynamic response of VO2-coated silicon microcantilevers thermally driven over the film’s insulator-to-metal transition was studied using laser light pulses directly incident on the cantilevers. The measured photothermal response revealed very high curvature changes of approximately 2500 m−1 up to pulse frequencies greater than 100 Hz and readily observable vibrations up to frequencies of a few kHz with no amplitude degradation after tens of thousands of pulses. Maximum tip amplitudes for 300-μm-long, 1-μm-thick cantilevers used in these experiments were nearly 120 μm and correspondingly less for 2-μm-thick cantilevers. The main mechanism limiting oscillation amplitude was found to be heat transport response during heating and cooling, which depends mainly on thermal conduction through the cantilever itself to the massive anchor and chip body, which acted as a heat sink at room temperature. For the laser-driven oscillations studied, damping by the surrounding air is unimportant in the range of frequencies probed. Large-curvature response is expected to extend to higher pulse frequencies for cantilevers with smaller dimensions.