MICROSYSTEM FOR ELECTROMECHANICAL MEASUREMENTS OF CARBON NANOFIBER LOADING AND FAILURE J.J. Brown 1 , J.W. Suk 2 , G. Singh 1 , D.A. Dikin 3 , R.S. Ruoff 2 , and V.M. Bright 1 1 Department of Mechanical Engineering, University of Colorado, Boulder, CO, USA. 2 Department of Mechanical Engineering, University of Texas, Austin, TX, USA. 3 Department of Mechanical Engineering, Northwestern University, Evanston, IL, USA. ABSTRACT A thermally actuated uniaxial testing stage for nanofiber materials has been designed and fabricated. Electrical separation of portions of the stage allows two-point electrical measurements simultaneously with mechanical testing. Using this stage, a carbon nanofiber was subjected to mechanical loading and simultaneous electrical impedance characterization, which provides a means to derive fiber resistance measurements when a fiber is mechanically coupled using highly resistive contacts. INTRODUCTION Electromechanical measurements on nanoscale fibers are of interest to enable integration of these materials into sensors and other microdevices. [1,2] Uniaxial testing is desirable in mechanical testing to ensure uniform loading throughout a fiber specimen [3,4], and to this end several devices have been developed to perform uniaxial mechanical testing on a nanofiber. [5-8] Furthermore, electrical coupling to a fiber specimen can allow thermomechanical characterization. The novelty of the device presented here lies in the ability to perform electrical measurement of a fiber specimen under mechanical loading, and mechanical loading of an electrically heated specimen. DESIGN Actuation Stage actuation is realized by thermal expansion through Joule heating of a set of angled beams. Beams symmetrically connected to the stage ensure uniaxial motion and also serve as heat sinks. Using the actuator geometry and a plane strain condition, it was estimated through solid mechanics mathematical analysis that the thermal actuator would be capable of providing up to 400 μN force. In order to avoid the specimen temperature changes caused by the actuator in [5,6], the moving specimen stage was separated from the thermal actuator and mechanically anchored. Mechanical Measurements Tensile measurement is enabled by fixing a material specimen across a gap between the moving and fixed portions of the stage. The experiment reported here uses SEM observation as a direct means of measuring the carbon nanofiber strain. Microscopy requires the interpretation of micrographs in order to derive strain data, which can be a slow process. Sensors could provide a faster approach to acquisition of strain data, and to that end, several indirect strain measurement mechanisms were built into the reported device. These alternative devices can be seen at left in Figure 2, and as built in the center of Figure 1. These measurement approaches include a diffraction grating, piezoresistive beam bending, and electron emission or gas ionization. All these approaches require further characterization. The diffraction grating is designed to measure submicron stage displacement by examination of a reflected laser beam. The electron or ion sensor works by comparing an electron or ion current arriving at a fixed and a moving electrode. For the piezoresistive beam measurements, electrical current is carried in two of the polysilicon anchoring beams. As the stage moves, the beams bend, modifying the electrical resistance of each beam. This concept has been explored in [9], but experimental results thus far have been inconclusive. The successful implementation of one of the displacement sensors above can be used in combination with bending of a beam to derive a measured force as applied to the fiber, as in the approach taken in [8]. Direct force measurement has not yet been implemented in this design. An alternative approach to force measurement can be developed from measurements of the actuator input power and the stage displacement. The simulation shown in Figure 4 indicates that at any given applied voltage, there is a linear relation between force and displacement. This suggests that it may be possible to develop a map that translates electrical and stage displacement data into force measurements. Electrical Characterization Although two gold connections have been provided on the moving stage, they are electrically linked by the underlying polysilicon layer. Two separate electrical contacts were defined on the fixed side of the stage, effectively enabling a 3 point conductivity measurement. In practice, the difficulty of carbon nanofiber placement limited actual connections to only two: one on the moving stage, and one on the closest portion of the fixed stage. The fiber was mechanically clamped to the stage with amorphous carbon deposits created in an SEM, and these Figure 1. Left: Diagram of actuator and stage system. The moving stage at left is linked by bending beams to rectangular anchor points. A thermal actuator pulls the moving stage away from the fixed stage, at right. Center: SEM image of overall system. Right: Close-up of stage showing mounted carbon nanofiber, 120 nm diameter, before loading. 0-9640024-7-7/HH2008/$20©2008TRF Solid-State Sensors, Actuators, and Microsystems Workshop Hilton Head Island, South Carolina, June 1-5, 2008 182