Kevin W. Kelly

Design and fabrication of a cross flow micro heat exchanger

A cross flow micro heat exchanger was designed to maximize heat transfer from a liquid (water-glycol) to a gas (air) for a given frontal area while holding pressure drop across the heat exchanger of each fluid to values characteristic of conventional scale heat exchangers. The predicted performance for these plastic, ceramic, and aluminum micro heat exchangers are compared with each other and to current innovative car radiators. The cross flow micro heat exchanger can transfer more heat/volume or mass than existing heat exchangers within the context of the design constraints specified. This can be important in a wide range of applications (automotive, home heating, and aerospace). The heat exchanger was fabricated by aligning and then bonding together two identical plastic parts that had been molded using the LIGA process. After the heat exchanger was assembled, liquid was pumped through the heat exchanger, and minimal leakage was observed.

Fabrication and Performance of a Pin Fin Micro Heat Exchanger

Nickel micro pin fin heat exchangers can be electroplated directly onto planar or nonplanar metal surfaces using a derivative of the LIGA micromachining process. These heat exchangers offer the potential to more effectively control the temperature of surfaces in high heat flux applications. Of particular interest is the temperature control of gas turbine engine components. The components in the gas turbine engine that require efficient, improved cooling schemes include the gas turbine blades, the stator vanes, the turbine disk, and the combustor liner. Efficient heating of component surfaces may also be required (i.e., surfaces near the compressor inlet to prevent deicing). In all cases, correlations providing the Nusselt number and the friction factor are needed for such micro pin fin heat exchangers. Heat transfer and pressure loss experimental results are reported for a flat parallel plate pin fin micro heat exchanger with a staggered pin fin array, with height-to-diameter ratios of 1.0, with spacing-to-diameter ratios of 2.5 and for Reynolds numbers (based on the hydraulic diameter of the channel) from 4000 to 20,000

Fabrication of high-aspect-ratio microstructures on planar and nonplanar surfaces using a modified LIGA process

Large surface areas (tens of square centimeters to square meters) covered with high-aspect-ratio microstructures (HARMs) have potential applications in a wide range of fields including heat transfer, adaptive aerodynamics, acoustics, catalysts, seal and bearing design, and composite materials. HARMs are typically hundreds of micrometers in height, with widths ranging from a few micrometers to tens of micrometers, and they can be manufactured from a variety of materials such as metals, polymers, and ceramics. Three of the barriers to extensive use of large HARM-covered surfaces are cost, nonplanarity of typical surfaces, and adhesion of the microstructures to the surface. A starting point for inexpensive reproduction of large arrays of HARMs is the plastic molding step of the LIGA micromanufacturing process. In order to address the latter two problems, the standard LIGA process was modified/extended. Free-standing polymer sheets, perforated with a pattern of high-aspect-ratio throughholes, were clamped to conductive substrates. The sheets provide a template for electrodeposition of nickel microstructures onto the target surface. This process makes it economically feasible to electroform metal microstructures directly onto large planar and nonplanar metal surfaces (cylinders).

Mechanical property evaluation and failure analysis of cantilevered LIGA nickel microposts

An experimental apparatus has been built to measure the elastic modulus and bending strength (modulus of rupture) of LIGA nickel posts. The apparatus uses the static cantilever beam bending approach to measure mechanical properties in a direction parallel to the growth direction. Experimental results are presented for two sets of largely identical posts constructed using an overplating method. One set was electroplated using a Watts bath, and the other set was electroplated using a sulfamate bath. For the Watts bath, the measured modulus of elasticity was slightly lower than that of bulk nickel (182 GPa), while, for the sulfamate bath, it was approximately half (93 GPa). The strength properties of the two sets of posts also differ dramatically. Microhardness measurements, Focused Ion Beam (FIB) images of grain structure, and scanning electron microscopy (SEM) micrographs of failure regions are used to further characterize and explain the differences in the results. This integrated testing approach yields a consistent set of data regarding material properties, grain size/structure and failure mechanisms. Potential sources of experimental error are also identified and improvements in experiment design are suggested to reduce these errors.

Fabrication, modeling, and testing of micro-cross-flow heat exchangers

Planar micro-cross-flow heat exchangers, similar in concept to most automobile radiators, have been fabricated using two different processes. A process that was previously reported (Harris et al., 2000) to fabricate a polymer heat exchanger involved embossing two identical polymer parts using the LIGA process. Then the two parts were aligned and bonded together. In this paper, a process is described to fabricate a nickel micro-cross-flow heat exchanger by embossing a sacrificial polymer mandrel using a LIGA-fabricated mold insert. The mandrel is coated with nickel (using either electroplating or electroless plating), then the sacrificial mandrel is dissolved. Experimental results are reported for both the polymer and nickel heat exchangers to determine the rates of heat transfer between the in-plane liquid (water) and the through-plane gas (air). Pressure drops of both fluid streams were also measured. The experimental results compare favorably with a modified version of the analytical model that was described previously. The fabricated heat exchangers have values of heat transfer/volume that are more than five times higher than conventional scale counterparts (with characteristic dimensions at least one order of magnitude larger than those reported here) and values of heat transfer/mass that are 50% greater than their conventional scale counterparts.

Amorphous hydrocarbon based thin films for high-aspect-ratio MEMS applications

Abstract Methods to engineer the surfaces of micro-electro-mechanical systems (MEMS) are just beginning to be explored. Coating deposition is one such method. In cases where microdevices involve moving parts in contact, such as gear sets, combustion chambers, and mechanical seals, it may be necessary to impart the complex, highly non-planar surfaces with desired mechanical properties and tribological characteristics. In other cases, such as micro-heat exchangers or micro-catalytic converters, the chemical properties of the microdevice surfaces are important. Amorphous hydrocarbon (a-C:H) and metal-containing amorphous hydrocarbon (Me-C:H) thin films possess moderately high hardness, chemical inertness, low coefficient of friction and low wear rate, and are potentially useful in a wide range of microdevice applications. We report on the mechanical properties and tribological characteristics of TiC:H films as a function of the Ti composition. We further demonstrate the conformal deposition of TiC:H thin films over electrodeposited Ni high-aspect-ratio micro-scale structures (HARMs) fabricated by the deep X-ray lithography based microfabrication technique LIGA (Lithographie, Galvanoformung, Abformung). Conformal deposition of nanostructured ceramic thin films over HARMs offers possibilities for improving the tribological characteristics of HARMs based microdevices with parts in relative motion, and for using nanostructured ceramic thin films as structural materials for microdevices. As an example, we demonstrate the fabrication of freestanding, high-aspect-ratio TiC:H microtubes based on conformal deposition over electrodeposited Ni HARMs.

Tapered LIGA HARMs

The standard LIGA process takes advantage of the use of x-ray lithography to produce mold inserts with nearly vertical sidewall; the typical slope of patterns produced by x-ray lithography of polymethylmethacrylate is 0.1%. This lack of significant taper (draft angle) greatly increases the difficulty associated with ejecting parts during demolding. In this paper, a procedure is described to fabricate a mold insert with tapered features having a height of approximately 1 mm and lateral dimensions of approximately 300 μm. A set of six oblique exposures of a thick layer of SU-8 (an EPON epoxy based negative tone resist) is used to create hexagonal posts with a 3° draft angle. An electroforming process is used to fabricate a nickel mold insert with the tapered features. This mold insert is used to injection mold tapered polymer high aspect ratio microstructures. The dimensions of the SU-8 tapered structures (as well as the mold insert) are within 4 μm of desired/predicted values.

Conformal deposition of Ti-C:H coatings over high-aspect-ratio micro-scale structures and tribological characteristics

Results on conformal deposition of Ti-containing hydrocarbon (Ti-C:H) coatings over Ni high-aspect-ratio micro-scale structures (HARMs) fabricated by the lithography, electroplating and molding (LIGA) technique are reported. Using a high-density plasma assisted hybrid chemical vapor deposition/physical vapor deposition technique, it is demonstrated that conformal Ti-C:H deposition can be achieved over HARMs with ∼1000 μm in height and ∼220 μm in mutual spacing. Surface roughness and tribological characteristics of Ti-C:H coatings during unlubricated sliding contact with 52100 steel balls are studied with atomic force microscopy, micro-Raman scattering, and optical microscopy on macro-scale samples. It is shown that unlubricated sliding between Ti-C:H and 52100 steel is significantly influenced by the composition of the Ti-C:H coating.

A pin fin microheat sink for cooling macroscale conformal surfaces under the influence of thrust and frictional forces

Conventional microheat sink design primarily focuses on the use of continuous fin arrays to optimally dissipate thermal energy from electronic components. By contrast, this paper experimentally measures the thermal and structural performance of two micro pin fin heat sinks designed for use in load bearing applications such as mechanical seals and thrust bearings. One pin fin array is of low porosity, which is more optimal for load bearing capacity, and the other is of high porosity, which is more optimal for heat dissipation. By using these two extreme cases, the thermal-structural tradeoff found in load bearing microheat sinks is demonstrated. The heat sinks are constructed of nickel, electrodeposited onto a stainless steel thrust ring using a modified LIGA technique. Under forced air cooling, the thermal performance of each is compared to a simple model based on a combination of macroscale pin fin heat sink results and classical correlations for fins in cross flow. The low porosity design is also tested under the application of a 44.5 N thrust load at 2500 rpm and found to be structurally sound. Experimental temperature profiles demonstrate a substantial benefit of the microheat sink in cooling the load bearing surface.

Ultra-deep x-ray lithography of densely packed SU-8 features: II. Process performance as a function of dose, feature height and post exposure bake temperature

Ultra-tall (2–4 mm), densely packed arrays of high aspect ratio micro structures (HARMs) are required for a variety of heat transfer and mass transfer devices currently under development. Great success has been reported lithographically defining relatively tall features using SU-8 as an x-ray resist, but excellent results are scarce with respect to patterning ultra-tall features with tight packing and small gaps between features. Diffusion of a cross-linking species from exposed regions to unexposed regions is believed to be the primary mechanism that prevents such features from being defined. In a companion paper, solvent content is shown to be an extremely important parameter that controls the quality of ultra-tall, tightly packed micro structure arrays. In this paper, a simple quasi-two-dimensional model is developed to qualitatively show that the adverse effects of diffusion of a cross-linking species can be significant. Also, experiments were performed to isolate the effects of parameters other than solvent content on performance (post exposure bake temperature, dose and top-to-bottom dose ratio, and feature height). These effects are placed in the context of the general hypothesis that preventing diffusion of cross-linking species is a key to successfully defining ultra-tall, tightly packed SU-8 HARMs. Experimental results are provided that demonstrate the ability to define SU-8 features with heights of 3 mm, and gaps between adjacent features of 125 µm (the gap therefore having an aspect ratio of 24).