Placid M. Ferreira

Ultrathin silicon solar microcells for semitransparent, mechanically flexible and microconcentrator module designs

The high natural abundance of silicon, together with its excellent reliability and good efficiency in solar cells, suggest its continued use in production of solar energy, on massive scales, for the foreseeable future. Although organics, nanocrystals, nanowires and other new materials hold significant promise, many opportunities continue to exist for research into unconventional means of exploiting silicon in advanced photovoltaic systems. Here, we describe modules that use large-scale arrays of silicon solar microcells created from bulk wafers and integrated in diverse spatial layouts on foreign substrates by transfer printing. The resulting devices can offer useful features, including high degrees of mechanical flexibility, user-definable transparency and ultrathin-form-factor microconcentrator designs. Detailed studies of the processes for creating and manipulating such microcells, together with theoretical and experimental investigations of the electrical, mechanical and optical characteristics of several types of module that incorporate them, illuminate the key aspects.

Printed Assemblies of Inorganic Light-Emitting Diodes for Deformable and Semitransparent Displays

Bend Me, Stretch Me In the push toward flexible electronics, much research has focused on using organic conducting materials, including light-emitting diodes (LEDs), because they are more readily processed using scalable techniques. Park et al. (p. 977) have developed a series of techniques for depositing and assembling inorganic LEDs onto glass, plastic, or rubber. Conventional processing techniques are used to connect the LEDs in order to create flexible, stretchable displays, which, because the active diode material only covers a small part of the substrate, are mostly transparent. Methods to fabricate and assemble inorganic light-emitting diodes provide a route toward transparent, flexible, or stretchable display devices. We have developed methods for creating microscale inorganic light-emitting diodes (LEDs) and for assembling and interconnecting them into unusual display and lighting systems. The LEDs use specialized epitaxial semiconductor layers that allow delineation and release of large collections of ultrathin devices. Diverse shapes are possible, with dimensions from micrometers to millimeters, in either flat or “wavy” configurations. Printing-based assembly methods can deposit these devices on substrates of glass, plastic, or rubber, in arbitrary spatial layouts and over areas that can be much larger than those of the growth wafer. The thin geometries of these LEDs enable them to be interconnected by conventional planar processing techniques. Displays, lighting elements, and related systems formed in this manner can offer interesting mechanical and optical properties.

Microstructured elastomeric surfaces with reversible adhesion and examples of their use in deterministic assembly by transfer printing

Reversible control of adhesion is an important feature of many desired, existing, and potential systems, including climbing robots, medical tapes, and stamps for transfer printing. We present experimental and theoretical studies of pressure modulated adhesion between flat, stiff objects and elastomeric surfaces with sharp features of surface relief in optimized geometries. Here, the strength of nonspecific adhesion can be switched by more than three orders of magnitude, from strong to weak, in a reversible fashion. Implementing these concepts in advanced stamps for transfer printing enables versatile modes for deterministic assembly of solid materials in micro/nanostructured forms. Demonstrations in printed two- and three-dimensional collections of silicon platelets and membranes illustrate some capabilities. An unusual type of transistor that incorporates a printed gate electrode, an air gap dielectric, and an aligned array of single walled carbon nanotubes provides a device example.

Computation of stiffness and stiffness bounds for parallel link manipulators

Parallel link spatial mechanisms in general, and Stewart Platforms in particular, are increasingly being studied for possible use in multi-axis machine-tools. An important consideration in the design of such machines is their stiffness. For a given design, stiffness varies with the direction in which it is computed, the posture (or configuration) of the mechanism and the direction of the actuation or disturbing force. This paper addresses the problem of finding the minimum and maximum stiffnesses and the directions in which they occur for a manipulator in a given posture. In addition, the computation of stiffness in an arbitrary direction is also discussed. Since engineers are often interested in the response of the mechanism in the direction of perturbation (called single-dimensional or engineering stiffness in this paper), this paper proves the computed bounds (maximum and minimum) are tight for such a definition. Stiffness computed using the algebraic formulae derived are compared to those obtained from a Finite Element Analysis model to demonstrate correctness of formulation. Finally, minimum, maximum, arbitrary direction and single-dimensional stiffness maps, are produced for a Stewart Platform and their use in machine tool design is discussed.

Polynomial-complexity deadlock avoidance policies for sequential resource allocation systems

The development of efficient deadlock avoidance policies (DAPs) for sequential resource allocation systems (RASs) is a problem of increasing interest in the scientific community, largely because of its relevance to the design of large-scale flexibly automated manufacturing systems. Much of the work on this problem existing in the literature is focused on the so-called single-unit RAS model, which is the simplest model in the considered class of RASs. Furthermore, due to a well-established result stating that, even for single-unit RASs, the computation of the maximally permissive DAP is intractable (NP-hard), many researchers (including our group) have focused on obtaining good suboptimal policies which are computationally tractable (scalable) and provably correct. In the first part of the paper, it is shown, however, that for a large subset (in fact, a majority) of single-unit RASs, the optimal DAP can be obtained in real-time with a computational cost which is a polynomial function of the system size (i.e., the number of resource types and the distinct route stages of the processes running through the system). The implications of this result for the entire class of single-unit RASs are also explored. With a result on the design of optimal DAPs for single-unit RASs, the second part of the paper concentrates on the development of scalable and provably correct DAPs for the more general case of conjunctive RASs.

Deadlock avoidance policies for automated manufacturing cells

This paper suggests a framework for developing provably correct and scalable avoidance policies for the automated manufacturing cell (AMC) model. We provide a formal description of the AMC operation and establish its correspondence to a finite state automaton (FSA). The AMC models deadlock characteristics are studied with the aid of the FSA state transition diagram. We provide a new characterization of AMC state safety which serves as the basis of a new framework for systematic development of avoidance policies. We provide two examples of the framework functionality. First, the framework is used to formally prove the correctness of the resource upstream neighborhood (RUN) avoidance policy (Gaarder, 1993). Then, it is used to provide an alternative proof for the correctness of B-K deadlock avoidance algorithm (BKDAA) (Banaszak et al., 1990). Furthermore, an interesting relationship existing between RUN and BKDAA policies is brought out. Finally, the results are summarized, conclusions drawn, and possible extensions and generalizations of the underlying ideas discussed.

Non-lithographic patterning and metal-assisted chemical etching for manufacturing of tunable light-emitting silicon nanowire arrays

We report a top-down fabrication method that involves the combination of superionic-solid-state-stamping (S4) patterning with metal-assisted-chemical-etching (MacEtch), to produce silicon nanowire arrays with defined geometry and optical properties in a manufacturable fashion.

A correct and scalable deadlock avoidance policy for flexible manufacturing systems

Configuration flexibility and deadlock-free operation are two essential properties of control systems for highly automated flexible manufacturing systems. Configuration flexibility, the ability to quickly modify manufacturing system components and their logical relationships, requires automatic generation of control executables from high level system specifications. These control executables must guarantee deadlock-free operation. The resource order policy is a configurable controller that provides the deadlock-free guarantee for buffer space allocation. It uses a total ordering of system machines and routing information to generate a set of configuration specific linear constraints. These constraints encode the system state along with a buffer capacity function and define a deadlock-free region of operation. Constraint generation and execution are of polynomial complexity.

High-speed and drop-on-demand printing with a pulsed electrohydrodynamic jet

We present a pulsed dc voltage printing regime for high-speed, high-resolution and high-precision electrohydrodynamic jet (E-jet) printing. The voltage pulse peak induces a very fast E-jetting mode from the nozzle for a short duration, while a baseline dc voltage is picked to ensure that the meniscus is always deformed to nearly a conical shape but not in a jetting mode. The duration of the pulse determines the volume of the droplet and therefore the feature size on the substrate. The droplet deposition rate is controlled by the time interval between two successive pulses. Through a suitable choice of the pulse width and frequency, a jet-printing regime with a specified droplet size and droplet spacing can be created. Further, by properly coordinating the pulsing with positioning commands, high spatial resolution can also be achieved. We demonstrate high-speed printing capabilities at 1 kHz with drop-on-demand and registration capabilities with 3–5 µm droplet size for an aqueous ink and 1–2 µm for a photocurable polymer ink.

Verification of form tolerances part II: Cylindricity and straightness of a median line

Abstract Most inspectors measure form tolerances as the minimum zone solution, which minimizes the maximum error between the datapoints and a reference feature. Current coordinate measuring machines verification algorithms are based on the least-squares solution, which minimizes the sum of the squared errors, resulting in a possible overestimation of the form tolerance. Therefore, although coordinate measuring machines algorithms successfully reject bad parts, they may also reject some good parts. The verification algorithms developed in this set of papers compute the minimum zone solution of a set of datapoints sampled from a part. Computing the minimum zone solution is inherently a nonlinear optimization problem. This paper develops a single verification methodology that can be applied to the cylindricity and straightness of a median line problems. The final implementable formulation solves a sequence of linear programs that converge to a local optimal solution. Given adequate initial conditions, this solution will be the minimum zone solution. This methodology is also applied to the problems of computing the minimum circumscribed cylinder and the maximum inscribed cylinder. Experimental evidence that the formulations are both robust and efficient is provided.