John N. Harb

Modeling the thermal behavior of a surface-micromachined linear-displacement thermomechanical microactuator

Thermomechanical microactuators possess a number of desirable attributes including ease of fabrication and large force and displacement capabilities relative to other types of microactuators. These advantages provide motivation for improving thermomechanical microactuator designs that are more energy efficient and thus better suited for low-power applications. To this end, this paper describes the development and experimental validation of a finite-difference thermal model of a thermomechanical in-plane microactuator (TIM). Comparisons between the model and experimental results demonstrate the importance of including the temperature dependence of several parameters in the model. Strategies for reducing the power and energy requirements of the TIM were investigated using model simulations as a guide. Based on design insights gained from the model, the energy efficiency of the TIM has been improved significantly by operating in a vacuum environment and providing short-duration, high-current pulse inputs. These improvements have been validated experimentally.

Metallization of Branched DNA Origami for Nanoelectronic Circuit Fabrication

This work examines the metallization of folded DNA, known as DNA origami, as an enabling step toward the use of such DNA as templates for nanoelectronic circuits. DNA origami, a simple and robust method for creating a wide variety of shapes and patterns, makes possible the increased complexity and flexibility needed for both the design and assembly of useful circuit templates. In addition, selective metallization of the DNA template is essential for circuit fabrication. Metallization of DNA origami presents several challenges over and above those associated with the metallization of other DNA templates such as λ-DNA. These challenges include (1) the stability of the origami in the processes used for metallization, (2) the enhanced selectivity required to metallize small origami structures, (3) the increased difficulty of adhering small structures to the surface so that they will not be removed when subject to multiple metallization steps, and (4) the influence of excess staple strands present with the origami. This paper describes our efforts to understand and address these challenges. Specifically, the influence of experimental conditions on template stability and on the selectivity of metal deposition was investigated for small DNA origami templates. These templates were seeded with Ag and then plated with Au via an electroless deposition process. Both staple strand concentration and the concentration of ions in solution were found to have a significant impact. Selective continuous metal deposition was achieved, with an average metallized height as small as 32 nm. The shape of branched origami was also retained after metallization. These results represent important progress toward the realization of DNA-templated nanocircuits.

Modeling of ash deposition in large-scale combustion facilities burning pulverized coal

Traditional approaches to the prediction of the deposition behavior of a coal usually involve the use of empirical indices and ASTM ash fusion temperatures. These approaches, however, can give misleading results and are often unreliable. In recent years, considerable effort has been made in the development of models which overcome some of the deficiencies of the traditional approaches, as reviewed in the first part of this paper. In spite of advances, these models still fail to describe the effect of deposition on boiler operation. The second part of this paper documents the efforts in the Advanced Combustion Engineering Research Center (ACERC) to integrate an ash deposition model with a comprehensive 3D coal combustion model. An ash deposition submodel, which includes the effects of both ash chemistry and operating conditions on slagging deposits, has been incorporated into the comprehensive combustion code, PCGC-3. The submodel includes a statistically-based particle cloud model for determination of impaction rates of fly ash on boiler walls. The fraction of impacting particles that stick to the surface is determined from the physical characteristics (viscosity) of both the particles and the deposit surface. The model includes a description of deposit growth which approximates both the physical properties and chemistry of the deposit as a function of combustion conditions (operating conditions). A key feature of the model is its ability to account for the effect of deposition on operating conditions in the boiler. Simulations of deposition in both pilot-scale and utility-scale combustion facilities are reported in the paper.

Additive Behavior during Copper Electrodeposition in Solutions Containing Cl − , PEG, and SPS

This paper documents an experimental investigation of additive behavior in acidic copper plating solutions containing chloride ions, polyethylene glycol (PEG), and bis-(3-sodiumsulfopropyl disulfide) (SPS). These solutions represent a simplified model of solutions used industrially for electroplating copper interconnects. Experiments were conducted with use of a rotating disk electrode under both galvanostatic and potentiostatic conditions and the transient behavior was observed. Linear sweep voltammetry was also performed. A transition time for the system to reach steady state was observed under both galvanostatic and potentiostatic conditions and found to be a strong function of SPS concentration. Superfilling is made possible by the existence of a long transition time that permits establishment of a significant rate difference between the top and bottom of the cavity. Experimental results provide evidence for slow adsorption and desorption of the accelerator and SPS incorporation into the deposit. Linear sweep voltammetry indicates that behavior of the accelerator is potential dependent. The same mechanism used to explain superfilling can be applied to explain the leveling/brightening of copper surfaces in these solutions.

The Role of SPS, MPSA, and Chloride in Additive Systems for Copper Electrodeposition

This paper documents an experimental study to compare the behavior of bis-(3-sodiumsulfopropyl) disulfide (SPS)-Cl-polyethylene glycol (PEG) and 3-mercaptopropanesulfonic acid sodium salt (MPSA)-Cl-PEG additive systems during copper electrodeposition. These systems are analogous to those used industrially to plate copper lines onto integrated circuits. Galvanostatic experiments show that in the absence of chloride ion, either SPS or MPSA added to the plating solution inhibited copper deposition. Upon the addition of Cl - , a rapid transition from inhibition to acceleration was observed for both additives, illustrating the critical role played by the chloride ion in these systems. Potentiostatic experiments performed for the Cl-PEG-SPS and Cl-PEG-MPSA additive systems show that the potential dependency of the Cl-PEG-SPS system was much stronger than that of the MPSA-Cl-PEG system. Differences between the SPS and MPSA additive systems suggest that acceleration may occur through an MPSA pathway. Finally, results with chemical compounds similar to MPSA indicate that the thiol group is associated with inhibition and that a synergistic accelerating relationship exists between the sulfonate group and chloride, presumably due to complex formation.

Microbatteries for self-sustained hybrid micropower supplies

This paper describes the characteristics of microbatteries suitable for use in a hybrid micropower supply for powering autonomous MEMS and other microsystems. The hybrid power supply includes an energy conversion device, microscopic batteries for energy storage, and control/interface circuitry. Comparison of the hybrid approach with single power sources (either a battery or energy conversion device alone) shows that it offers several potential advantages including reduced size, increased flexibility, long lifetime and increased reliability. Such an approach is well suited to the expected duty cycles of remote microsensors. Realization of the advantages of a hybrid system depends on the availability of a battery with the required characteristics. Initial experimental results demonstrate the feasibility of fabricating microbatteries with the proper characteristics and the use of these batteries as part of a hybrid micropower supply. It is anticipated that hybrid micropower supplies with suitable microbatteries will play a critical role in the successful implementation of a wide variety of autonomous microsystems.

Modeling of Particle-Particle Interactions in Porous Cathodes for Lithium-Ion Batteries

In this work, we present a mathematical model and associated experiments for describing the performance of porous electrodes under high rates of charge and discharge. By increasing the physical accuracy of porous battery modeling, we hope to enable improved design of cells for high-power applications, such as hybrid and plug-in-hybrid electric vehicles. The model includes an improved accounting of electron transfer between different-size particles or materials, including the conductive carbon additive, as well as a modified Bruggeman relation to handle liquid-phase ion transport through porous electrodes. Both types of resistance, electronic and liquid-phase ionic, are strongly coupled to particle properties, including size and volume-fraction distributions. The model is used to better understand the cause for decreased utilization of active material for relatively highly loaded lithium-ion cathodes at high discharge rates. It was found for Li x CoO 2 cathodes with loading around 1.6 mAh/cm 2 that voltage losses at 1C discharge rate are mostly governed by local interparticle resistances. At 5C discharge rate, diffusional resistance in the liquid electrolyte had the greatest influence on cell performance.

Fireside corrosion in pc-fired boilers

This review has examined fireside corrosion of pc-fired boilers in both the waterwall and superheater regions. The present understanding of corrosion phenomena has resulted in the development of strategies to control tube wastage

DNA origami metallized site specifically to form electrically conductive nanowires.

DNA origami is a promising tool for use as a template in the design and fabrication of nanoscale structures. The ability to engineer selected staple strands on a DNA origami structure provides a high density of addressable locations across the structure. Here we report a method using site-specific attachment of gold nanoparticles to modified staple strands and subsequent metallization to fabricate conductive wires from DNA origami templates. We have modified DNA origami structures by lengthening each staple strand in select regions with a 10-base nucleotide sequence and have attached DNA-modified gold nanoparticles to the lengthened staple strands via complementary base-pairing. The high density of extended staple strands allowed the gold nanoparticles to pack tightly in the modified regions of the DNA origami, where the measured median gap size between neighboring particles was 4.1 nm. Gold metallization processes were optimized so that the attached gold nanoparticles grew until gaps between particles were filled and uniform continuous nanowires were formed. Finally, electron beam lithography was used to pattern electrodes in order to measure the electrical conductivity of metallized DNA origami, which showed an average resistance of 2.4 kΩ per metallized structure.

Microscopic Nickel-Zinc Batteries for Use in Autonomous Microsystems

Microscopic nickel-zinc batteries have been built using microfabrication procedures similar to those used in the microelectronics industry. These batteries were designed for use with autonomous microsystems and are of particular interest for applications that involve remote sensing. It is envisioned that the batteries would he part of a hybrid micropower system that would also include a device for scavenging energy from the environment in order to recharge the batteries and provide for a long operational lifetime. The nickel-zinc batteries that have been developed have a typical footprint of 0.02 cm 2 and a capacity of approximately 0.555 mWh/cm 2 . Over 1000 batteries have been fabricated on a single 4 in, wafer, One of the most important characteristics of these batteries is their ability to discharge at high rates (≥100 mA/cm 2 ), making it possible to extract more than I mW (per cell) from these very small cells Also, initial results show that more than 2000 cycles of a microelectromechanical systems (MEMS) type duty cycle are possible. The high rate performance, small size and long cycle life of these batteries make them suitable for microsystem applications. In addition, the microfabrication procedure that has been developed should enable the fabrication of these batteries on the same substrate used for other system components