Friedrich Prinz

Box-skeletons of discrete solids

The Medial Axis Transform (MAT) was defined by Blum in the 1960s as an alternate description of the shape of an object. Since then, its potential applicability in a wide range of engineering domains has been acknowledged. However, this potential has never quite been realized, except recently in two dimensions. One reason is the difficulty in defining algorithms for finding the MAT, especially in three dimensions. Another reason is the lack of incentive for modelling designs directly in MATs. Given this impasse, some lateral thinking appears to be in order. Perhaps the MAT per se is not the only skeleton which can be used. Are there other, more easily derived skeletons, which share those properties of the MAT which are of interest in engineering design? In this work, we identify a set of properties of the MAT which, we argue, are of primary interest. Briefly, these properties are dimensional reduction (in the sense of having no interior), homotopic equivalence, and invertibility. For the restricted class of discrete objects, we define an algorithm for identifying a point set, called a skeleton, which shares these properties with the MAT. Furthermore, this skeleton is to the box-norm (L∞ norm) what the MAT is to the Euclidean norm, and hence the deviation of this skeleton from the MAT is bounded. The algorithm will be developed for both 2D and 3D cases. Proofs of correctness of the algorithm shall be indicated. The use of this skeleton in automated numerical analysis of injection moulded parts shall be demonstrated on industrialsized parts. The use of the 3D skeleton in aiding automatic mesh generation for finite element analysis is also of interest, and shall be discussed.

Finding Narrow Passages with Probabilistic Roadmaps: The Small-Step Retraction Method

Probabilistic Roadmaps (PRM) have been successfully used to plan complex robot motions in configuration spaces of small and large dimensionalities. However, their efficiency decreases dramatically in spaces with narrow passages. This paper presents a new method—small-step retraction—that helps PRM planners find paths through such passages. This method consists of slightly “fattening” robots free space, constructing a roadmap in fattened free space, and finally repairing portions of this roadmap by retracting them out of collision into actual free space. Fattened free space is not explicitly computed. Instead, the geometric models of workspace objects (robot links and/or obstacles) are “thinned” around their medial axis. A robot configuration lies in fattened free space if the thinned objects do not collide at this configuration. Two repair strategies are proposed. The “optimist” strategy waits until a complete path has been found in fattened free space before repairing it. Instead, the “pessimist” strategy repairs the roadmap as it is being built. The former is usually very fast, but may fail in some pathological cases. The latter is more reliable, but not as fast. A simple combination of the two strategies yields an integrated planner that is both fast and reliable. This planner was implemented as an extension of a pre-existing single-query PRM planner. Comparative tests show that it is significantly faster (sometimes by several orders of magnitude) than the pre-existing planner.

Part strength improvement in polymer shape deposition manufacturing

Shape deposition manufacturing (SDM) is a layered manufacturing process which iteratively combines material addition and removal to create artifacts in a variety of materials. Castable thermoset resins have been used to build a variety of parts via polymer SDM. The strength of these parts is determined by the bulk material properties of the part materials and by their interlayer adhesion. Early polyurethane materials had high bulk strength but poor interlayer adhesion, resulting in weak multilayer parts. Interlayer strength improvements were achieved through additional processing steps or the use of different polyurethane and epoxy part materials. These improvements allowed the fabrication of aerodynamic flap mechanisms used in wind‐tunnel testing. These parts are examples of the intricate, functional mechanisms to which the polymer SDM process is ideally suited.

Fabrication and Characterization of Lead Sulfide Thin Films by Atomic Layer Deposition

We present a study of the deposition of lead sulfide (PbS) thin films by atomic layer deposition (ALD). PbS films were deposited from Pb(tmhd)2 and H2S precursors at precursor sublimation temperatures of 165-175 C. The film growth rate was 1.3-1.8 Aa/cycle, which is higher than previously published values. A linear growth rate characteristic of ALD was observed, with no chemical contamination. Film properties are studied using XPS, XRD, SEM and AFM measurements. AFM images show that the films are polycrystalline with grain size increasing with film thickness. Calculations have been performed using Density Functional Theory (DFT) to model the reaction mechanism.

µ-Mold Shape Deposition Manufacturing of Ceramic Parts

Rapid Prototyping is a versatile method to build complex-shaped ceramic or metallic parts on a mesoscopic scale. In this work the fabrication of ceramic gas turbine engine parts with Mold Shape Deposition Manufacturing (Mold SDM) is described. Mold SDM exists of two steps: The pattern generation in wax using deposition and machining of wax layers and the pattern transfer process with gel-casting. For parts with feature sizes that are too small for machining, microfabrication methods are used to fabricate the mold. By combining microand macrofabrication, complex parts in a wide variety of sizes can be manufactured. The surface quality of the mold significantly influences the mechanical properties of the final ceramic part. The contribution of the mold quality to the final mechanical properties is studied in this work.

Oxidative removal of self-assembled monolayers for selective atomic layer deposition

Atomic layer deposition (ALD) is a thin film growth technique that employs a sequence of selflimiting surface reaction steps to afford subnanometer control of the growth process [1-2]. The self-limiting adsorption reactions ensure the precise control of film thickness and uniformity over large areas. While ALD provides perhaps the best available control of material thickness in the zdirection, fabricating precise patterns within the film can be challenging. Several groups have used chemical resists for area selective ALD [3-5], mostly employing selfassembled monolayers (SAMs). SAMs are thin organic films which form spontaneously on solid surfaces. They are well known for modifying the physical, chemical, and electrical properties of surfaces. Moreover, it is known that SAMs can inhibit surface reaction of ALD precursors. A variety of SAMs are stable at temperatures up a few hundred degrees centigrade, unlike the resist layers for photolithography and electron beam lithography. In this presentation, we will demonstrate a new nano-scale fabrication process combining atomic force microscopy (AFM) lithography of SAM for ALD patterning: AFM lithography as a nano-scale patterning method, SAM as a chemical resist for following hydrofluoric acid (HF) etching and ALD process. Due to the unique capability of the precise positioning and imaging, AFM lithography is able to create site-specific and localized patterning. In addition, the topological and electrochemical properties of patterns can be immediately characterized with AFM, and thus in situ pattern fabrication and characterization is possible with AFM lithography. Figure 1 shows the sequence process steps. When an electric field is applied through the AFM tip, an anodic bias can induce local oxidation of SAM-modified silicon substrate. Moving the AFM tip in a predefined fashion enables the creation of oxide patterns on the silicon surface. Subsequent HF etching locally removes oxide together with SAM, exposing silicon layer underneath. This patterned substrate can now be used as template for further ALD processing. To verify the complete removal of SAM at AFM oxidation step, Auger electron spectroscopy (AES) together with scanning electron microscopy (SEM) gas has been used for elemental mapping. (Figure 2) Zirconia ALD patterns with ~100nm line width and ~5nm height were fabricated, as shown in Figure 3. AFM scanning at each step is performed for imaging height/depth profiles. Considering that the negative pattern has a depth of ~3nm, and that the height of the ALD patterns is at least as thick as the SAM mold, a growth rate of ~0.8A per cycle is estimated, which is comparable to the typical ALD growth rates of zirconia.

AC Impedance Investigation of Flooding in Micro Flow Channels for Fuel Cells

The paper presents a study on the transport phenomena related to gas flow through fuel cell micro-channels, specifically the impact of dimensional scale on the order of 100 microns and below. The use of structural photopolymer (SU-8) enabled the direct fabrication of functional fuel cell micro-channels. Previous experimental observation has revealed that if flow channels are too small, they may reduce the performance of fuel due to flooding (Cha et al., 2003). For further investigation, AC Impedance technique has been employed to measure the mass transfer resistance. The result confirmed that in smaller channels, mass transportation resistance increases due to the flooding.Copyright

A microfabricated intravascular ultrasound scanner for intravascular interventions

Minimally invasive medical therapy can reduce both healthcare costs and patient suffering. The development of submillimeter scale instruments falls in a gap of manufacturing technologies between traditional machining and microfabrication techniques. To address this need we have developed a fabrication technique based upon laser machining of tubular structures combined with shaped-memory alloy actuators to create compliant devices for minimally invasive interventions. The initial application of this approach has been to develop a forward viewing intravascular ultrasound scanner for use in guiding intravascular interventions in situations where traditional angiography and intravascular ultrasound are unable to provide adequate guidance. The ultrasound device is less than 1.5 mm in diameter and provides imaging at 20 frames per second. Imaging currently is performed with a 20 MHz 800 micron diameter transducer producing axial resolutions of approximately 150 microns. Device optimization has resulted in peak strains of less than 1% within the compliant structure resulting in device life greater than 200,000 cycles providing usable times greater than twice the anticipated procedure length. The design concepts embodied in this initial implementation will serve as a platform for a variety of self actuated minimally invasive tools.

Direct Water Removal in Gas Diffusion Layer of Proton Exchange Membrane Fuel Cells by a Flexible Electroosmotic Pump

An electroosmotic pump is a non-mechanical pump that can remove water by applied electric field. A flexible electroosmotic (EO) pump was directly attached to a gas diffusion layer (GDL) on the cathode side of a miniature proton exchange membrane fuel cell (PEMFC). The operation of EO pump improved the voltage-current response of a PEMFC. Especially, the increase of limiting current was observed. We assume the reduced flooding in GDL was achieved by the operation of EO pump. Further investigation with AC impedance spectroscopy support the assumption as the reduced mass transport resistance was observed.Copyright