Materials selection and design of microelectrothermal bimaterial actuators

A common form of MEMS actuator is a thermally actuated bimaterial, which is easy to fabricate by surface micromachining and permits out of plane actuation, which is otherwise difficult to achieve. This paper presents an analytical framework for the design of such microelectrothermal bimaterial actuators. Mechanics relationships for a cantilever bimaterial strip subjected to a uniform temperature were applied to obtain expressions for performance metrics for the actuator, i.e., maximum work/volume, blocked (force) moment, and free-end (displacement) slope. Results from finite-element analysis and closed form relations agree well to within 1%. The optimal performance for a given pair of materials and the corresponding thickness ratio were determined. Contours of equal performance corresponding to commonly used substrates (e.g., Si, SiO2) were plotted in the domain of governing material properties (thermal expansion coefficient and Young's modulus) to identify candidate materials for further development. These results and the accompanying methodology provide a rational basis for comparing the suitability of "standard" materials for microelectrothermal actuators, as well as identifying materials that might be suitable for further research

Low force electrical switching using gold coated vertically aligned multi-walled carbon nanotubes surfaces

Gold coated vertically aligned multi-walled carbon-nanotubes (Au/MWCNT) surfaces are investigated to determine the electrical contact performance under low force conditions with repeated load cycling. The multi-walled CNT's are synthesized on silicon planar and sputter coated with a gold film. These planar surfaces are mounted on the tip of a PZT actuator and mated with a coated Au hemispherical probe. The load is typical of MEMs devices, with a 4V supply, 1 and 10mA current, and applied force of 1mN. The contact resistance (Rc) is monitored with the repeated loading cycles (over 1000 and a million cycle) to determine reliability and durability testing. The surfaces are compared with a reference Au-Au contact under the same experimental conditions. This study shows the potential for the application of CNT surfaces as an interface in low force electrical contact applications

Gold coated carbon nanotube surfaces as low force electrical contacts for MEMS devices: part 1

An experimental investigation of a gold coated vertically aligned carbon nanotube surfaces is undertaken to determine the limits of the electrical contact performance over a large number of switching cycles under low force conditions and with current loading (1mA-50mA at 4V). The multi-walled CNT’s (MWCNT’s) are synthesized on a silicon planar and sputter coated with a gold film. The planar surfaces are mounted on the tip of a PZT actuator and mated with a coated Au hemispherical probe. The electrical load is selected to reflect typical MEMs relay loads with a 4V supply, 1 and 10mA current load with an applied force of 1mN. The surfaces tested maintain a stable contact resistance over 106 switching cycles. To determine the limits, the contact force is increased to 3mN under dry circuit conditions and the current increased at the 1mN load to 20mA-50mA. The surfaces are compared with a reference Au-Au contact under the same experimental conditions. For the surfaces investigated the current loading limit was determined to be 20mA where the contacts failed after 50x106 cycles

The influence of toughening-particles in CFRPs on low velocity impact damage resistance performance

The role of particle-toughening for increasing impact damage resistance in carbon fibre reinforced polymer (CFRP) composites was investigated. Five carbon fibre reinforced systems consisting of four particle-toughened matrices and one system containing no toughening particles were subjected to low velocity impacts ranging from 25 J to 50 J to establish the impact damage resistance of each material system. Synchrotron radiation computed tomography (SRCT) enabled a novel approach for damage assessment and quantification. Toughening mechanisms were detected in the particle-toughened systems consisting of particle–resin debonding, crack-deflection and crack-bridging. Quantification of the bridging behaviour, increase in crack path length and roughness was undertaken. Out of the three toughening mechanisms measured, particle systems exhibited a larger extent of bridging suggesting a significant contribution of this toughening mechanism compared to the system with no particle

Fluidic packaging of microengine and microrocket devices for high pressure and high temperature operation

The fluidic packaging of Power MEMS devices such as the MIT microengine and microrocket requires the fabrication of hermetic seals capable of withstanding temperature in the range 20-600/spl deg/C and pressures in the range 100-300 atm. We describe an approach to such packaging by attaching Kovar metal tubes to a silicon device using glass seal technology. Failure due to fracture of the seals is a significant reliability concern in the baseline process: microscopy revealed a large number of voids in the glass, pre-cracks in the glass and silicon, and poor wetting of the glass to silicon. The effects of various processing and materials parameters on these phenomena were examined. A robust procedure, based on the use of metal-coated silicon substrates, was developed to ensure good wetting. The bending strength of single-tube specimens was determined at several temperatures. The dominant failure mode changed from fracture at room temperature to yielding of the glass and Kovar at 600/spl deg/C. The strength in tension at room temperature was analyzed using Weibull statistics; these results indicate a probability of survival of 0.99 at an operational pressure of 125 atm at room temperature for single tubes and a corresponding probability of 0.9 for a packaged device with 11 joints. The residual stresses were analyzed using the method of finite elements and recommendations for the improvement of packaging reliability are suggested

Partial volume correction for approximating crack opening displacements in CFRP material obtained from micro-focus X-ray CT scans

This paper presents a partial volume correction technique that applies a measurement weighting based on grey scale intensity values, allowing crack opening displacements (CODs) to be better estimated in micro-focus computed tomography (?CT) scans. These were tested on 3D data obtained from two separate ?CT scanners on particle toughened and non-particle toughened carbon fibre material subjected to low velocity impact. Direct comparisons of COD estimations were made with higher resolution measurements obtained using synchrotron radiation computed tomography (SRCT) scans taken at the European Synchrotron Radiation Facility (ESRF). In this study, partial volume correction is reported to improve the accuracy of these measurements to within 20% of SRCT measurements, whereas measurements based on counting interconnected voxels representing a detectable crack are reported to consistently overestimate crack openings by up to 500%. Scatter in estimations was dependent on material type, noise, and artefacts associated with ?CT volumes

Quasi-static indentation and compression after impact damage growth monitoring using microfocus X-ray computed tomography

In this study interrupted quasi-static indentation and post-impacted compression tests were performed at incremental load steps with X-ray computed tomography performed at each step. This enabled non-destructive, three-dimensional damage assessments to be carried out allowing initiation and propagation of different damage modes to be monitored. Preliminary results from these experiments are reported in this paper