Mezmerize Micro Fibers Increase MEMS Durability Over Billions of Cycles

Fibrous micro-composite materials can be formed from micro fibers and be used to build micro-electro mechanical systems (MEMS) that do not show fatigue after the billions of cycles. The fibrous micro-composite materials are utilized as the basis for a new class of MEMS by Mesmerize. Magnetic, piezoelectric, electrostatic, electrothermal, and electrostrictive responsive fibers are used to form a variety of micro-electro-mechanical devices.   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. These materials will enable simultaneous high operating frequencies, large amplitude displacements and or rotations, high reliability under cyclical stresses, according to inventor Shahyaan Desai.

Desai is  a founder of Mesmerize (Ithaca, NY)  which is commercializing  MEMS devices from fabricated nanofibers without the use of a matrix material. 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. Devices can be built where fibers are attached only at a substrate edge (e.g. cantilevers, bridges). Motions can be controlled by adjusting the linkage between multiple fibers with weak coupling (e.g. base, tip, in-between). Driving mechanisms include base-forcing (magnetics, piezo, electrostatics) or tip forcing (magnetics). Mirrors may be formed on free ends of cantilevers to form optical scanners.

Mezmerize is a designer and manufacturer of micro-electromechanical System (MEMS) mirrors and mirror modules based on a novel carbon fiber materials platform. The unique material properties of carbon fibers permit, for the first time, the development of micro-mirrors that can simultaneously deflect through large angles and reach high scanning speeds. Mezmeriz is currently working with partners to integrate their carbon fiber micro-mirror modules in pico-projectors and laser scanning units for laser printers enabling novel functionality in these devices.

Although MEMS manufacturers have pushed to develop silicon (both polycrystalline and single crystal) and other material-based structures, the resulting systems still lack the needed mechanical properties, according to U.S. Patent 7,616,367 held by Cornell Research Foundation, Inc. (Ithaca, NY). A specific example is the case of MEMS based optical scanners 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.

Monolithic materials, such as silicon, metal and ceramic thin films currently used to produce MEMS lack the required combination of high elastic stiffness, high strength, high fatigue lifetime and low density (mass per unit volume) i.e., the basic mechanical flexibility and flaw tolerance necessary for many potential MEMS applications. Polymers are not adequate since they are too flexible and have low strength which limits them to low frequency operation in devices where low forces and/or displacements are required, such as valves and fluidic pumps.

Consequently, moving component MEMS, such as optical scanners, are nearly non-existent commercially today. Most successful applications of MEMS remain based on quasi-static devices such as pressure and acceleration sensors. One moving component MEMS is a digital light processor that is based on bistable positioning of aluminum MEMS mirrors.

The need for advanced capability MEMS devices can be illustrated through a particular application--the MEMS based optical scanner (an OMEMS). Such scanners are envisioned for large area display applications using three-color scanning. Early MEMS optical scanners utilized a torsional silicon micro-mirror produced using wet etching. It was capable of deflecting a beam through a 0.8.degree. angle at a resonance frequency of 16.3 kHz. The majority of OMEMS scanners in development today are still designed using similar thin beams of silicon acting either as torsion bars (around which a silicon mirror element rotates) or as cantilevers (which vibrate to provide the scanning motion). Both of these structure types are efficient, with no moving parts to wear.

General applications are dependent on the resonance frequency, the maximum deflection, and the maximum restoring force--with higher values of each normally desired. These properties are dependent on the size, shape, and mechanical properties of the underlying materials. However, materials used in traditional IC-based MEMS fabrication lack the mechanical characteristics required to allow specific tailoring and optimization for many applications. There is no current way to design simultaneously for high frequency operation, large amplitude deflection, low operating power, robustness, and long-term reliability under cyclic stresses with existing material systems. The basic problem with silicon, and monolithic materials in general, is that while sufficient elastic stiffness, their strength and fatigue lifetime is too low and density too high. This combination limits the ultimate deflection amplitude and frequency, and increases power requirements to sustain oscillation. having sufficient elastic stiffness, their strength and fatigue lifetime is too low and density too high. This combination limits the ultimate deflection amplitude and frequency, and increases power requirements to sustain oscillation.

Fundamental limitations exist in the performance of materials currently used for MEMS and micro-mechanical devices. These materials such as Si, SiO2, SiC, metals, Si3N4 cannot provide large deflections (>100 um) at high speeds (>kHz) necessary for many MEMS actuator applications hampering their widespread commercialization. Most of all, the existing materials do not have the fatigue life necessary to undergo repeated large deformations over the billions of cycles that most actuator MEMS applications require.  Materials developed by Desai overcome may of the problems associated with silicon MEMS devices.

Copper Enigma and Temperature Selection Critical to Nanocarbon Shape Formation

Catalytic Materials LLC (Pittsboro, NC) researchers Terry K. Baker and Nelly M. Rodriguez discovered how to make multi-faceted multiwall carbon nanotubes with crystallinities ranging from greater than 95% to substantially 100% as well as other nanocarbon forms by controlling temperature in the presence of specific catalysts. Carbon nanostructures are generally prepared by reacting the powdered catalyst in a heating zone with the vapor of a suitable carbon-containing compound. While the art teaches a wide variety of carbon-containing compounds as being suitable for the preparation of carbon nanostructures, the inventors found that only a mixture of CO and H2 will yield carbon nanofibers with unexpectedly high crystallinities and in the unique structures of nanofibers of their invention in the temperature range of about 550 degree C to about 725 degree C.

Catalysts used to prepare the carbon nanofibers are bulk metals in powder form wherein the metal is selected from the group consisting of iron, iron-copper bimetallics, iron-nickel bimetallics and also cobalt-magnesium oxide mixtures. It is well established that the ferromagnetic metals, iron, cobalt, and nickel, are active catalysts for the growth of carbon nanofibers during decomposition of certain hydrocarbons or carbon monoxide. Efforts are now being directed at modifying the catalytic behavior of these metals, with respect to nanofiber growth, by introducing other metals and non-metals into the system. In this respect, copper is an enigma, appearing to be relatively inert towards carbon deposition during the CO/H2 reaction. Thus, it is unexpected that Fe or the combination of Cu or Ni with Fe would have such a dramatic effect on carbon nanofiber growth in the CO/H2 system in the temperature range of about 550 degree C to about 725.degree C. For the cobalt-magnesium oxide system the preferred temperature range is 580 degree C. to about 600 degree C. Iron-nickel catalysts and cobalt-magnesium oxide are preferred for preparing the carbon nanostructures.

The following figures illustrates the various shapes formed by Catalytic Materials, LLC’s patented process as well as the temperatures at which certain shapes are formed as revealed in U.S. Patent 7,592,389.

















(click on images to enlarge and for greater clarity)

FIG. 1a is a representation of a platelet carbon nanofiber, which is comprised of substantially graphite sheets that are substantially perpendicular to the longitudinal axis, or growth axis, of the nanofiber.

FIG. 1b is a representation of a cylindrical carbon nanostructure that is comprised of continuous carbon sheets and is in the form of tube within a tube within a tube with a substantially hollow center.

FIG. 1c is a representation of a ribbon carbon nanofiber comprised of non-connected graphitic sheets that are aligned face-to-face and substantially parallel to the longitudinal axis of the nanofiber.

FIG. 1d is a representation of a faceted tubular carbon nanofiber and is comprised of continuous sheets of graphic carbon but having multifaceted flat faces. The graphitic sheets are also substantially parallel to the longitudinal axis of the nanofiber.

FIG. 1e is a representation of a herringbone carbon nanofiber wherein the graphitic platelets or sheets are at an angle to the longitudinal axis of the nanofiber.

FIG. 1f is a representation of multiwall faceted tubular carbon nanofibers

The carbon nanofibers can be dispersed in a polymer by various well-known techniques such as melting and kneading to form an admixture that can be subsequently shaped to form an electrically conductive article. Multi-faceted carbon nanotubes are more conductive electrically than cylindrical carbon nanotubes. The use of conductive nanofibers is highly desirable since a given weight of such a material generates a large number of contact points within a polymer matrix. The widespread interest in electrically conductive polymers is stimulated by the possibility that such materials can have utility in such things as semiconductor chips, integrated circuits, lightweight battery components, sensors, electro-chromic displays, anti-static coatings, static dissipation, electromagnetic and radio-frequency interference shielding, fuel hoses, connectors and packaging items. It is well established that the incorporation of certain types of carbon nanofibers into polymeric materials can impart electrical conductivity to such materials that are generally regarded as insulators.