ALUMINUM-BASED 1D AND 2D MICRO FIN ARRAY STRUCTURES: HIGH THROUGHPUT FABRICATION AND HEAT TRANSFER TESTING LSU: Philip A. Primeaux, Bin Zhang, Xiaoman Zhang, Jacob Miller, W.J. Meng La Tech: Pratik KC, Arden L. Moore

CIMM Research collaboration between – Louisiana State University (LSU) – Louisiana Tech University (La Tech) Manuscript submitted for publication in – Journal of Micromechanics and Micro-engineering Future collaboration between microfabrication and heat transfer performance testing underway

Past Research Microscale compression molding was used successfully to replicate 1D and 2D microscale features directly onto surfaces of high thermal conductivity and high ductility metals, such as aluminum and copper Prototypes of both microchannel heat exchangers and gas chromatographs have been produced in aluminum using compression molding and transient liquid phase bonding

Goals Analyze high throughput roll molding process of 1D and 2D aluminum based micro fin array structures Produce 1D and 2D prototypes for heat transfer testing

Procedure 1D rolling – 2 Aluminum sheet metal strips fed in parallel at room temperature 2D rolling – 1 Aluminum sheet was passed twice, offset by 90 degrees

Procedure Aluminum 1100 and 6061 was heat treated at different temperatures/times to produce test samples with various bulk hardness values The total input torque and normal loading force were measured and recorded during rolling molding Dimensions of micro fin array structures fabricated by roll molding were measured by optical microscope and SEM

Results - 1D Rolling Replication duration was less than 1 minute Successful formation of fin array structures – Fin base and top widths are ~700  m and ~530  m, respectively – Fin height is ~640  m – Fin-to-fin spacing is ~960  m – Taper angle of the fin sidewalls is ~7.6 

Results - 1D Rolling Fin Height vs Normal Load Force Fin Height vs Hardness at a Given Normal Load

Results - 2D Rolling Total rolling time less than 2 minutes Successful formation of protruding micro fin array structures – Protrusion height is ~600  m – Difference in channel depth is less than 20  m

Results - 2D Rolling (a) (b) Difference in fin height in 90  cross rolling: ratio of normal loads vs. fin height difference at the initial normal load of (a) 20000lbf; (b) 23000lbf

Results - 2D Rolling It is reasonable that a ratio of secondary normal load to initial normal load close to 1 is needed to achieve consistent fin heights in the initial and secondary roll passes

Results - 2D Rolling Plastic deformation in the feed direction due to the 90  cross roll is largely symmetric At the ratio of roll molded feature height to roller radius of ≤ 0.01 (1000  m/100mm), effects of the roller curvature appear to be secondary

Heat Transfer Application Use of 1D/2D micro fin array structures for heat transfer applications Enhancing two-phase heat transfer efficiency using metals with microscale 1D/2D surface patterns which have broad industrial processing applications including power plants, refrigeration systems, and food production Two-phase direct immersion cooling has become increasingly attractive as a means of achieving efficient thermal management of high density electronics systems and IT hardware

Heat Transfer Application Ideal two-phase immersion cooling surface enhancer should also be low profile so as not to require large printed circuit board spacing and negatively affect computing density Types of low cost, high throughput micro fin array structures produced in this work promise enhanced heat transfer and widespread industrial adoption Fig:(a) DIC experiment of an array of simulated chips with porous stud boiling enhancers [ W. Nakayama]. (b) Modern DIC using 3M’s Novec fluid [Z. Yao, Y.-W. Lu, and S. G. Kandlikar].

Experimental Setup

Results Heat flux versus excess temperature Nucleate Boiling Film Boiling Natural Convection

Summary One-pass and sequential 90 degree cross roll molding are used successfully for fabrication of 1D/2D aluminum micro fin array structures Room temperature roll molding is a high throughput micromanufacturing technique Aluminum micro fin array structures can have enhanced pool boiling heat transfer performance Full realization of heat transfer benefits requires future considerations of – surface morphology – surface chemistry – surface pattern design for optimal fluid dynamics

Future Heat Transfer Work Comparing heat transfer enhancement offered by increased surface area of micro fin array structures to that of un- patterned surfaces of comparable surface conditions Considerations should also be given to microscale fluid dynamics to facilitate efficient rewetting at fin tips

Future Manufacturing Work Investigate surface treatment processes to achieve – Consistent surface conditions for as-received aluminum sheet metals and roll molded aluminum micro fin array structures – Appropriate surface chemistry for optimal vapor bubble nucleation Experiment with additional geometries and dimensions for the primary roller sleeve design

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