Carbon nanotubes (CNTs) have attracted broad attentions for developing innovative nanocomposites
due to their exceptional properties. However, the success of developing nanocomposites is largely
dependent on material processing. Authors' group has developed an innovative electrochemical co-deposition
form synthesis of CNT reinforced metallic (Ni). Results indicate that the yield and
ultimate strength of the as-deposited pure nickel are found to be about 350MPa and 625MPa,
respectively, which are comparable to the reported data. However, it demonstrates that the yield
strength and ultimate strength of Ni/MWCNT composite are about 1,290MPa and 1,700MPa,
respectively. In other word, the ultimate strength of Ni/MWCNT nanocomposite is about 270%
higher than that of pure nickel. Authors believe the largely increased mechanical strength of Ni/CNT
nanocomposite is attributed to the novel fabrication that high temperature associated degradation
problems in metallic composite processing such as diffusion, chemical reaction and mismatch of
coefficient of thermal expansion (CTE) are eliminated.
Authors have demonstrated that by controlling the mixing ratio of polydimethylsiloxane's (PDMS's) two components-base polymer (part A) and a curing agent (part B)-different mechanical properties of PDMS can be achieved. Test results show that the Young's modulus decreases as the increasing of mixing ratios (A:B). However, there is a transitional mixing ratio (part A:part B=10) after which the Young's modulus is almost independent of the mixing ratio. The PDMS's thickness plays an important role in determining the mechanical properties. The results show that the thinner the PDMS, the stiffer it behaves. The bonding strength between two cured PDMS parts with different mixing ratios shows that it depends on the mixing ratio. A maximum bonding strength of 130 kPa occurs on a bonded couple with mixing ratios of 30A:1B and 3A:1B, respectively. The fracture on bonded specimens does not occur at the bonding interfaces. Instead it occurs at the side with a larger portion of part A. The intermediate material property formed at the interface is attributed to the diffusion layer formed.
Patterning thick SU-8 with conventional photolithography facilities is important for fabricating various MEMS structures. However, the fabrication of thick SU-8 MEMS has experienced severe problems such as cracks, distortions, or delaminations during the fabrication process and/or postservices, due to the large internal stress generated during the photolithography process. In this work, an in-depth finite element analysis (FEA) is performed to investigate the causes and effects of the internal stresses. Analytical results show that the post-exposure bake (PEB) temperature is the main factor in developing the resulted internal stress. Under the guidance of analytical results, an optimized UV photolithography process for the fabrication of ultra-thick low-stress SU-8 patterns is developed with conventional (simple) equipment. A low PEB temperature of 55°C reduces the internal stresses by more than 70% compared to those fabricated with the recommended procedure. Experimental results indicate that cracks, distortions, and delaminations are eliminated from the fabricated SU-8 structures using the newly developed procedure. In addition, the patterned SU-8 has a Young's modulus of 2.5 GPa and an ultimate strength of 50 MPa, which is about 50% higher than previous reported values.
Compact robust hydraulic actuators are very important for space related applications because of their capability of producing much larger forces per unite volume/mass than existing technologies. The major components of these actuators are PZT stacks (pusher) and microvalves. The PZT pusher works at high frequencies to produce large flow rates (proportional to displacement traveled) and high pressures. As a component of the hydraulic actuator, the microvalves are challenged in matching the requirements of the PZT in terms of high operational frequencies, large flow rates and high-pressure support capabilities. In order to fulfill these requirements, the authors have developed robust self-assembled solid nickel micro valve arrays consisting of 80 single micro check valves, to achieve the required flow rate (>10 cc/second). A single micro check valve consists of an inlet channel (200 μm in diameter), a specially designed valve flap held by four identical micro beams, and outlet channels. All these structures are made from electroformed nickel and are self-assembled during a novel in situ UV-LIGA fabrication process. Finite element simulation results show that the micro check valve has a 1st resonant frequency of 16 kHz and is able to support pressures greater than 10 MPa. Test results show the flow rate is 19 cc/s at a pressure difference of 100 psi, and is roughly proportional to the pressure applied. Based on Poiseuille's law, it is reasonable to predict larger flow rates if higher-pressure differences are applied.
MEMS based SnO2 gas sensor with sol gel synthesized mesoporous nanocrystalline (<10 nm) semiconductor thin (100~150 nm) film has been recently developed. The SnO2 nano film is fabricated with the combination of polymeric sol gel chemistry with block copolymers used for structure directing agents. The novel hydrogen sensor has a fast response time (1s) and quick recovery time (3s), as well as good sensitivity (about 90%), comparing to other hydrogen sensors developed. The improved capabilities are credited to the large surface to volume ratio of gas sensing thin film with nano sized porous surface topology, which can greatly increase the sensitivity even at relatively low working temperature. The gas sensing film is deposited onto a thin dielectric membrane of low thermal conductivity, which provides good thermal isolation between substrate and the gas-sensitive heated area on the membrane. In this way the power consumption can be kept very low. Since the fabrication process is completely compatible with IC industry, it makes mass production possible and greatly reduces the cost. The working temperature of the new sensor can be reduced as low as 100°C. The low working temperature posse advantages such as lower power consumption, lower thermal induced signal shift as well as safe detection in certain environments where temperature is strictly limited.
SU-8 is an ultra-thick negative photoresist with low optical absorption in the near UV range, which makes it an ideal material
for generating thick molds for electroforming as well as a structural material for MEMS devices. However, the MEMS fabrication of using SU-8 is largely limited by its well-known poor adhesion to metallic layers as well as the high internal stress induced after baking. In this paper, an optimized process for fabricating ultra-thick low stressed SU-8 mold is developed and good adhesion between SU-8 and metals is obtained by applying a newly developed material, Omnicoat from Microchem Inc. A laminated (sandwiched) micro heat exchanger has been fabricated using the developed process in which sandwiched microchannels (one layer of Ni and one layer of SU-8) has been fabricated using the patterned SU-8 and nickel electroforming process. Test results show that the micro structure fabricated can stand at cryo temperature (77K) without damages such as cracks or delamination.
This paper describes the development of a micro-machined passive check valve for an SMA-based compact hybrid actuator device (CHAD). The overall diameter of the valve is 12 mm and the thickness is 1 mm. The structure houses an array of 56 micro check valves. Each micro valve has a 250 μm diameter orifice covered by 10 mm thick nickel flap. Stoppers on each micro valves limit the displacement of the flaps during an opening. This design allows the Ni flaps to withstand high-pressure gradient created by the actuator while achieving high flow rate. A finite element analysis is used to characterize the static and dynamic behaviors of the valve flap for the prediction on flow rate. The prediction is found to be in good agreement with the experiment on static flow rate. The test results indicate that the flaps are able to withstand pressure difference of 0.28 MPa while achieving flow rate of 20 cc/sec. The valve also has low cracking pressure and reverse leakage.
Robust compact hydraulic actuators are extremely needed in space industry where payload is critical. Microvalves are key component for compact hydraulic actuators. Robust microvalves with large load bearing ability, large flow rate, and high operational frequency are objectives of this research. A FEM analytical approach was used to optimize the valve design. The microvalves were fabricated by novel microfabrication process and scaling laws. Electroformed nickel on silicon substrate was used to make the valve flap and deep RIE etching was adopted to make the valve channels while the metallic valve flap as the etching stop. Test results shown that the flow rate is proportional to the pressure applied. The flow rate is larger than 10 cc/sec at pressure or 40 psi. These microvalves can be used to solve engineering problems where both load bearing and flow rate are major concerns.
The development and frequency response of a novel proof-of- concept prototype Mesoscale Actuator Device (MAD) is described in this paper. The MAD is similar to piezoelectric driven inchworm motors with the exception that mechanically interlocking microridges replace the traditional frictional clamping mechanisms. The interlocked microridges, microfabricated from single crystal silicon, are shown to support macroscopic loads. Tests conducted on the current design demonstrate that the interlocked microridges support 16 MPa in shear or that two sets of 3 X 5 mm locked chips support a 50 kgf. Operation of three generations of prototype MAD device containing microridges are accomplished at relatively large frequencies using an open loop control signal. Synchronizing the locking and unlocking of the microridges with the elongating and contracting actuator requires a dedicated waveform in the voltage signal supplied and permitted large operational frequencies. First generation operates at 0.6 Hz and demonstrated 1000s microridges can be engaged without problem, second generation moves like an inchworm up to 32 Hz, and the third generation including an external force was successfully operated from 0.2 Hz to 500 Hz corresponding to speeds from 2 micrometers /s to 5 mm/s. The upper limit (500 Hz) was imposed by software limitations and not related to physical limitations of the current device.
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