Fabrication of NiAl Intermetallic Micro Heat Exchangers and Microreactors - B. Paul (Industrial & Manufacturing Engineering)

Micro-scale heat exchangers and chemical microreactors offer opportunities for portable powder generation, on-site waste remediation, point-of-use chemical synthesis, and portable HVAC.

The material requirements for some of these applications include high temperature resistance, chemical inertness and the ability to be fabricated into structures that contain internal features of complex geometries and small (<100 ?m) dimensions. It is recognized that materials with limited formability, like ceramics and intermetallics, will be required for high temperature applications. Microlamination methods for forming arrays of internal microchannels in a NiAl and Ni3Al devices have been developed. These methods have the advantage of ceramic microchannels of better microchannel resolution and dimensional stability. The intermetallic is synthesized from thin sheets of elemental foils. Microchannels are precision machined (via laser ablation) into elemental Ni and Al foils. During bonding, the foils are converted into NiAl. Result show that this is a viable method for producing aluminide-based structures containing complex, internal features.

Development of a Passive Micro-Ball Valve (MBV) for Biotechnology Applications -

B. Paul (Industrial & Manufacturing Engineering), J. Liburdy (Mechanical Engineering), T. Plant (Electrical & Computer Engineering), G. Jovanovic (Chemical Engineering)

Microvalves are critical components for the operation of micro-scale biotechnology systems. In this research, a micro-ball valve (MBV) is developed useful for one-way flow control within many micro-scale biotechnology applications.

Advantages of the MBV over other microvalves include: 1) little dead volume for flow stagnation, biogrowth and fouling; 2) simple, economical construction making it disposable; 3) very low operating pressure in forward flow; 4) very low leakage in reverse flow; and 5) compatibility with a microlamination architecture making it economical to integrate within microfluidic system designs. Details of device design, fabrication and testing are given along with the results of operational conditions for a range of flow rates. Three different MBVs have been developed with nominal major diameters of 900, 700 and 150 µm. Pressure drop diodicities (forward to reverse ratio) in excess of 2,800 have been found.

Pre-Assembly of Micro-Float Valves - B. Paul (Industrial & Manufacturing Engineering)

A major impediment to realizing the potential of MECS devices is the lack of economical assembly methods. The ideal MECS fabrication technology would eliminate assembly altogether by fabricating the device in a "pre-assembled" state. A capacitive dissociation process has been developed to separate a small floating disk from within a float valve body.

Pressure drop tests were performed across the float valve as a function of mass flow rate. Results indicate that the microfloat valve performed with a theoretical orifice size of 0.629 mm for a 1.5 mm valve opening and an average diodicity ratio of 11.2.

Laser Microwelding for Electrical Interconnects - B. Paul (Industrial & Manufacturing Engineering)

The advancement of implantable biomedical devices is pushing for every increasing densities of electrical interconnects. Specific weldment requirements include low electrical resistance, high mechanical strength, biocompatible, and high interconnect density.

Laser microwelding is being considered as an alternative to resistance spot welding for these applications. A quasi-pulse ESI Nd:YAG laser welding system was used to test the feasibility of bonding 25 micron PtIr wires to SS pads. Results show that the process is feasible. Further study is required to determine specific processing parameters and conditions necessary for process control.

Shape Variation in Microlamination - B. Paul (Industrial & Manufacturing Engineering)

Current efforts to fabricate MECS devices include microlamination methods involving the patterning, registration, and bonding of thin material laminae to produce monolithic microfluidic devices. The objective of this project has been to study the source and effects of shape variation within microlamination processes. It is well known that under laminar flow conditions, the shape of a fluid conduit directly influences not only the flow through the channel but also the heat and mass transfer rates within the conduit. Therefore, this project seeks to understand the source of shape variation phenomena in metal microlamination and the effect of shape variation on MECS device performance. Specifically, the focus of the study has been in copper and stainless steel microchannel arrays. Major microchannel shape variation identified within this study include endwall surface roughness, misregistration, and fin warpage. It has been determined that the greatest effect of shape variation is microchannel fin warpage.

Sources of this fin warpage include coefficient of thermal expansion mismatch between the laminae and the tooling fixture, warpage in the sheet stock, residual stress in the sheet stock as well as others. Finite element models are being used to determine the impact and effect of various device geometries on the thermomechanical behavior of the microfluidic device.

Limits on Aspect Ratio in Two-Fluid Micro-Scale Heat Exchangers - B. Paul (Industrial & Manufacturing Engineering)

This research investigates the theoretical limit on aspect ratio within two-fluid counter-flow microchannel heat exchangers. The counter-flow device is comprised of alternating layers of microchannels, which allow the two fluids to flow in opposite directions separated, by fins. A theoretical model for interpreting the span of the fin as a function of the fin thickness is established. The model is verified experimentally by fabricating two kinds of test specimens to simulate the counter-flow device. Results from metallography and leakage testing reveal that for a given set of bonding conditions there exists a maximum permissible value of elastic fin deflection during bonding beyond which leakage is bound to occur.

Following are the conclusions of this study:

i. The fins in two-fluid, microchannel heat exchangers have a certain aspect ratio limit, beyond which poor bonding will occur within the device resulting in leakage and mixing of the two fluids in the device.

ii. For a given set of bonding conditions, the width of the channel can be interpreted as a function of the thickness of the lamina with the key bonding characteristic being a maximum in-process deflection of the fin mid-plane, ycomp, found to be on the order of 1.95 mm for the nominal diffusion bonding condition used in this study. It is expected that this in-process deflection will change across bonding methods and conditions. A more generalizable characteristic may be the minimum pressure allowed for bonding which would be found at the center of the fin. A general analytical solution for this value was not found in this study.

iii. Higher aspect ratios can be accommodated at smaller scales with high aspect ratio (greater than 30) fins possible at lamina thicknesses less than 1.36 mm for this set of bonding conditions.

For more information on Dr. Paul's research see: http://www.ie.orst.edu/people/faculty/paul/index.htm

Capacitor Discharge Microwelding - R.D. Wilson (Industrial & Manufacturing Engineering)

Ordinarily, electrical connections are made using brazing, soldering, or resistance welding. Soldering or brazing requires the use of low melting metallic joint materials and toxic, fluoride/chloride containing fluxes. Alternately, resistance welding is a high heat input process that is difficult to implement for very fine wire diameters.

 

Capacitor discharge welding (CDW) uses no filler metals or fluxes and very low heat input resulting in strong metallurgical joints with very little environmental side effects. Copper and steel wires as small as 0.127 mm in diameter have been autogenously joined using CDW. Characterization of the process shows welding times less than 20 µs and heat affected zones less than 10 µm wide.

As the size of polycrystalline metals decreases to scales smaller than the characteristic lengths of physical processes such as electrical conductivity and magnetism one expects interesting changes in the properties of the metal. In particular, the construction of nanometer scale multilayer metals produces large anisotropies in the electrical and magnetic properties of materials that are normally isotropic in bulk specimens. Because of the large scattering interaction at the interfaces of multilayer metals, the electronic and lattice properties of the solid are different in directions parallel and perpendicular to the interface.

We have been studying the effects of the multilayer structure on the electrical and magnetic properties of superconducting metals using thin-film physical vapor deposition. The sputtering process has been used to allow us to deposit controlled nanometer-scale structures that mimic the two-phase microstructure developed through conventional metallurgical processing. Using the thin-film deposition technique it is possible to build macroscopically thick structures (5-10 micrometers) composed of large numbers of nanometer scaled multilayers (see Figure One). As well, the sputtering target can be manipulated to allow either pure metals or alloys to be deposited, thus providing a large range of electrical and magnetic properties in the thin films. Microscopic properties can be combined in a variety of ways to produce a wide range of bulk behavior in the macroscopic material. An example of a doubly periodic microstructure is seen in Figure Two, in which pure Ti and Nb have been layered in a (five) period 1.1 nm (Nb) / 3.2 nm (Ti) structure, which is alternated with a 27 nm layer of Ti. The small period multilayer approaches the behavior of a uniformly mixed Nb-Ti alloy, while the larger period multilayer exhibits the anisotropies of electrical and magnetic properties associated with larger scale structures.

The potential of this work for microscale devices includes microstructures for controllably anisotropic thermal conductivity, electrical conductivity, and magnetism. Control of heat flow will be important in heat engines and chemical reactors, while electrical and magnetic anisotropies will play a role in actuation of microscale devices such as pumps, valves, and impellers.

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