James W. Palko

Energy breakdown in capacitive deionization

We explored the energy loss mechanisms in capacitive deionization (CDI). We hypothesize that resistive and parasitic losses are two main sources of energy losses. We measured contribution from each loss mechanism in water desalination with constant current (CC) charge/discharge cycling. Resistive energy loss is expected to dominate in high current charging cases, as it increases approximately linearly with current for fixed charge transfer (resistive power loss scales as square of current and charging time scales as inverse of current). On the other hand, parasitic loss is dominant in low current cases, as the electrodes spend more time at higher voltages. We built a CDI cell with five electrode pairs and standard flow between architecture. We performed a series of experiments with various cycling currents and cut-off voltages (voltage at which current is reversed) and studied these energy losses. To this end, we measured series resistance of the cell (contact resistances, resistance of wires, and resistance of solution in spacers) during charging and discharging from voltage response of a small amplitude AC current signal added to the underlying cycling current. We performed a separate set of experiments to quantify parasitic (or leakage) current of the cell versus cell voltage. We then used these data to estimate parasitic losses under the assumption that leakage current is primarily voltage (and not current) dependent. Our results confirmed that resistive and parasitic losses respectively dominate in the limit of high and low currents. We also measured salt adsorption and report energy-normalized adsorbed salt (ENAS, energy loss per ion removed) and average salt adsorption rate (ASAR). We show a clear tradeoff between ASAR and ENAS and show that balancing these losses leads to optimal energy efficiency.

Approaching the limits of two-phase boiling heat transfer: High heat flux and low superheat

We demonstrate capillary fed porous copper structures capable of dissipating over 1200 W cm−2 in boiling with water as the working fluid. Demonstrated superheats for this structure are dramatically lower than those previously reported at these high heat fluxes and are extremely insensitive to heat input. We show superheats of less than 10 K at maximum dissipation and varying less than 5 K over input heat flux ranges of 1000 W cm−2. Fabrication of the porous copper layers using electrodeposition around a sacrificial template allows fine control of both microstructure and bulk geometry, producing structures less than 40 μm thick with active region lateral dimensions of 2 mm × 0.3 mm. The active region is volumetrically Joule heated by passing an electric current through the porous copper bulk material. We analyze the heat transfer performance of the structures and suggest a strong influence of pore size on superheat. We compare performance of the current structure to existing wick structures.

Rapid Slow Off-Rate Modified Aptamer (SOMAmer)-Based Detection of C‑Reactive Protein Using Isotachophoresis and an Ionic Spacer

We present an on-chip electrophoretic assay for rapid protein detection with a SOMAmer (Slow Off-Rate Modified Aptamer) reagent. We used isotachophoresis (ITP) coupled with an ionic spacer to both react and separate SOMAmer-protein complex from free SOMAmer reagent. ITP accelerates the reaction kinetics as the ionic spacer concurrently separates the reaction products. We developed a numerical and analytical model to describe ITP spacer assays, which involve low-mobility, nonfocusing targets that are recruited into the ITP zone by higher-mobility, ITP-focused probes. We demonstrated a proof-of-concept of this assay using C-reactive protein (CRP) in buffer, and achieved a 2 nM limit of detection (LOD) with a combined 20 min assay time (10 min off-chip preparation of reagents and 10 min on-chip run). Our findings suggest that this approach has potential as a simple and rapid alternative to other homogeneous immunoassays. We also explore the extension of this assay to a diluted serum sample spiked with CRP, where we observe decreased sensitivity (an LOD of 25 nM in 20-fold diluted serum). We describe the challenges in extending this assay to complex samples and achieving higher sensitivity and specificity for clinical applications.

Increasing Hybridization Rate and Sensitivity of Bead‐Based Assays Using Isotachophoresis

We present an electrokinetic technique to increase the reaction rate and sensitivity of bead-based assays. We use isotachophoresis (ITP) to preconcentrate and co-focus target molecules and beads into a single ITP zone. The process achieves rapid mixing, stirring, and strongly increases the binding reaction rate. We demonstrate our assay with quantitative detection of 24 nt single-stranded DNA over a dynamic range of three orders of magnitude and multiplexed detection of ten target species per sample. We show that ITP can achieve approximately the same sensitivity as a well-stirred standard reaction in 60-fold reduced reaction time (20 min versus 20 h). Alternately, compared to standard reaction times of 30 min, we show that 20 min ITP hybridization can achieve 5.3-fold higher sensitivity.

High heat flux two-phase cooling of electronics with integrated diamond/porous copper heat sinks and microfluidic coolant supply

We here present an approach to cooling of electronics requiring dissipation of extreme heat fluxes exceeding 1 kW/cm2 over ~1 cm2 areas. The approach applies a combination of heat spreading using laser micromachined diamond heat sinks; evaporation/boiling in fine featured (5 μm) conformal porous copper coatings; microfluidic liquid routing for uniform coolant supply over the surface of the heat sink; and phase separation to control distribution of liquid and vapor phases. We characterize the performance of these technologies independently and integrated into functional devices. We report two-phase heat transfer performance of diamond/porous copper heat sinks with microfluidic manifolding at full device scales (0.7 cm2) with heat fluxes exceeding 1300 W/cm2 using water working fluid. We further show application of hydrophobic phase separation membranes for phase management with heat dissipation exceeding 450 W/cm2 at the scale of a single extended surface (~300 μm).

Tailoring of Permeability in Copper Inverse Opal for Electronic Cooling Applications

Microporous metals are extensively applied in convective cooling of high heat flux systems such as electronics. Traditional fabrication approaches, such as sintering of metallic particles, however, produce materials with limited fluid transport capability. Here, we demonstrate control and enhancement of the permeability of porous copper inverse opals produced via electrodeposition around a sacrificial polymer template. Sintering of the template is used to control the fluid transport network microstructure, with permeability increasing for increasing sintering times. These electrodeposited structures achieve permeabilities greater than 1×10−12 m2 with 5 μm pores, roughly 5 times larger than those of porous sintered copper with comparable feature sizes. The high permeability and small feature sizes, with attendant high specific surface area and strong capillarity, offered by the sintered template electrodeposited copper are attractive for two phase cooling applications.Copyright

Burst behavior at a capillary tip: Effect of low and high surface tension.

Liquid retention in micron and millimeter scale devices is important for maintaining stable interfaces in various processes including bimolecular separation, phase change heat transfer, and water desalination. There have been several studies of re-entrant geometries, and very few studies on retaining low surface tension liquids such as fluorocarbon-based dielectric liquids. Here, we study retention of a liquid with very low contact angles using borosilicate glass capillary tips. We analyzed capillary tips with outer diameters ranging from 250 to 840 μm and measured Laplace pressures up to 2.9 kPa. Experimental results agree well with a numerical model that predicts burst pressure (the maximum Laplace pressure for liquid retention), which is a function of the outer diameter (D) and capillary exit edge radius of curvature (r).

Microchannel cooling strategies for high heat flux (1 kW/cm 2 ) power electronic applications

The wide band-gap (WBG) semiconductor electronics such as silicon carbide (SiC) and gallium nitride (GaN) are becoming more popular in power electronics applications due to their excellent functionality at higher operating temperatures, powers, frequencies and in high radiation environments compared to Si devices. However, the continued drive for higher device and packaging densities has led to extreme heat fluxes on the order of 1 kW/cm2 that requires aggressive microchannel cooling strategies in order to maintain the device junction temperature below acceptable limits. A reduced order single/two phase thermal-fluidic model is developed to investigate the effect of micro-channel geometry parameters, packaging materials and fluid flow conditions on the cooling performance of various cooling strategies. Water and R245fa refrigerant are used as single- and two-phase working fluids, respectively. We consider three cooling strategies: • Design A: copper cold-plate micro-channel module bonded to the device substrate • Design B: embedded micro-channels directly etched into the device substrate and • Design C: embedded micro-channels with a 3D manifold with inlet and outlet module. The proposed embedded micro-channels with 3D-manifold with R245fa working fluid has the potential to achieve the lowest thermal resistance ∼0.07 K/W and pressure drop ∼10 kPa for flow rate Q ∼ 0.21 l/min (Tin = 90 °C) and exit quality x = 0.44.

Characterization of the capillary performance of copper inverse opals

In this work, we characterize the capillary performance of copper inverse opals (CIOs) fabricated using template-assisted electrodeposition. A bed of sintered 5 μm-diameter polystyrene spheres is used as a template. The extent of sintering tailors the window diameter between adjacent pores of the CIOs; these windows largely determine the permeability and capillary characteristics of the CIO structures. Capillarity is characterized through rate-of-rise measurements and compared with corresponding changes in permeability determined previously. Trends in these measured values are studied as a function of sintering parameters for the template and resulting porosities of the CIO structures. Findings from this study provide insights into the flow characteristics of copper inverse opal structures, paving the way for designing optimal thermal solutions to electronic cooling applications with microporous metals.

Enhanced Heat Transfer Using Microporous Copper Inverse Opals

Enhanced boiling is one of the popular cooling schemes in thermal management due to its superior heat transfer characteristics. This study demonstrates the ability of copper inverse opal (CIO) porous structures to enhance pool boiling performance using a thin CIO film with a thickness of 10 lm and pore diameter of 5 lm. The microfabricated CIO film increases microscale surface roughness that in turn leads to more active nucleation sites thus improved boiling performance parameters such as heat transfer coefficient (HTC) and critical heat flux (CHF) compared to those of smooth Si surfaces. The experimental results for CIO film show a maximum CHF of 225 W/cm (at 16.2 C superheat) or about three times higher than that of smooth Si surface (80 W/cm at 21.6 C superheat). Optical images showing bubble formation on the microporous copper surface are captured to provide detailed information of bubble departure diameter and frequency. [DOI: 10.1115/1.4040088]