Graphene Sheets Make Fibrous Micro-Composite Materials Used for New Class of MEMS

Cornell Research Foundation, Inc. (Ithaca, NY) earned U.S. Patent 7,675,698 for fibrous micro-composite materials formed from micro fibers. The fibrous micro-composite materials are utilized as the basis for a new and more durable class of MEMS. 

In addition to simple fiber composites and microlaminates, fibrous hollow and/or solid braids, can be used in structures where motion and restoring forces result from deflections involving torsion, plate bending and tensioned string or membrane motion. The fibrous elements are formed using high strength, micron and smaller scale fibers, such as carbon/graphite fibers, carbon nanotubes, fibrous single or multi-ply graphene sheets, or other materials having similar structural configurations. Cantilever beams and torsional elements can ber formed from the micro-composite materials according to inventors Shahyaan Desai, Michael O. Thompson, Anil N. Netrvali  and S. Leigh Phoenix

           

Present day micro-electro-mechanical systems (MEMS) based actuator devices have fundamental performance issues that severely limit their widespread commercialization. Although MEMS manufacturers have pushed to develop silicon and other material-based structures, the resulting systems still lack the needed mechanical properties. A specific example is the case of MEMS based optical scanner and switches (OMEMS). Such devices need to produce large angular deflections (several tens of degrees) and resonant frequencies exceeding tens of kilohertz with lifetime reliability over billions of cycles. 

The Cornell fibrous micro-composite materials are utilized as the basis for a new class of MEMS. In addition to simple fiber composites and microlaminates, fibrous hollow and/or solid braids, can be used in structures where motion and restoring forces result from deflections involving torsion, plate bending and tensioned string or membrane motion. In some embodiments, these materials will enable simultaneous high operating frequencies, large amplitude displacements and or rotations, high reliability under cyclical stresses. 

The fibrous elements are formed using high strength, micron and smaller scale fibers, such as carbon/graphite fibers, carbon nanotubes, fibrous single or multi-ply graphene sheets, or other materials having similar structural configurations. 

Cantilever beams can be fabricated from single fibers, single/multilayer aligned arrays of fibers, or single/multilayer fabrics. Such fabrics exploit the special strong anisotropic mechanical properties and high strength along the fiber axis of the fibers yielding structures with high bending stiffness, and low mass, yet large bending curvatures. Single fiber cantilevers provide the highest possible operating frequencies for potential applications such as RF sensors, at the expense of lateral stiffness and strength. Multifiber cantilevers benefit from statistical improvements and stability based on averaging properties and load sharing in the event of fiber damage or intrinsic faults. The natural extension is to more complex fabrics with optimized properties in multiple directions or multiple modes of deflection. Such cantilevers can also be produced from braided torsion elements, producing both lateral and angular displacements. 

Plates (two dimensional minimally deformable objects) are fabricated from single/multilayer aligned arrays of fibers, or single/multilayer fabrics. This configuration optimizes the stiffness to mass ratio together with the strength required for high frequency motion, such as required for the mirror element in a scanner MEMS. Relative stiffness in the two axes may be tailored to balance driving forces through fiber density, type, orientation, positioning and/or weave characteristics. 

 Hollow or tubular micro-braids made from micron-scale fibers are used as torsional deflecting elements in devices to provide high performance MEMS actuators. Braids permit the transformation of stresses within the torsion bar from shear (resulting from twisting motion) to tensile/compressive stresses (with some bending) along the orthogonal fiber axes at plus/minus 45 degrees. In effect, the braid allows the fibers to act in a mode in which their behavior is exceptional. Torsion elements at the sub 100 micron scale (comparable to MEMS device dimensions) can be fabricated from fibers 5 microns in diameter; smaller fibers produce commensurately smaller braids.

Additionally by manipulating the dimensions of the braid, the braiding angle, the types of fibers used to construct the braid, and the size and number of fibers in the braid, torsional elements with tailored strength, elastic stiffness, density, and other mechanical properties can be produced. This enhancement and tailorability of the strength and elastic stiffness of the torsional element results in MEMS devices capable of producing large angular deflections and forces at high frequencies and speeds without failure.