Many heat resistant ferritic steels have superior thermal expansion and conductivity properties compared to Ni-based alloys but are somewhat limited in their applications above ~600 o C by their mechanical properties. Efforts are being made to use precipitation strengthening to improve these properties and thus the efficiency of such alloys [1]. One such effort uses coherent (Ni,Fe)Al precipitates in a ferritic matrix, Fig. 1, and has shown good initial results. Transmission electron microscopy (TEM) has been used to understand globally the precipitate microstructure as it evolves with heat treatment. In order to understand the mechanisms by which such precipitation occurs, it is important to know how the composition variations in the structure on the scale of the precipitates evolve with heat treatment. Atom probe tomography (APT) [2] excels at such atomicscale characterization and is a strong complement to the imaging capabilities of TEM. The local electrode atom probe (LEAP) [3] has been used to characterize the nanoscale microstructure in this alloy in order to determine the chemistry of the third phase (yellow arrow) precipitated inside the (Ni,Fe)Al B2 phase, which is shown as the darkly-imaging phase in Fig. 1b. The Fe-18.1at.% Ni-22.9at.% Al-9.5at.% Cr alloy was produced by vacuum induction melting followed by solution annealing at 1200°C with air cooling. For atom probe analysis, rods of this material were sharpened by electropolishing to have a radius of curvature of less than 100 nm at the apex. Three phases in the microstructure were identified consistent with prior work [1]. A primary Fe-rich phase (A2), a secondary NiAl phase (B2), and a third Fe-rich precipitate. Precipitates of the primary phase within the secondary phase were not seen to form within ~15 nm of large scale primary regions, i.e., there is a denuded zone or precipitate free zone near the primary phase A2 phase. Fig. 2 shows a LEAP analysis with a volume of (80x80x500) nm made up of 110 million atoms. On the right side of Fig. 2, two composition profiles are shown. The top one is across a narrow B2 region between two A2 regions showing the ~50/50 composition of the NiAl phase in this region. The lower profile reveals that the very small (~5nm) precipitates (yellow arrow in Fig. 1) inside the NiAl phase appear to be secondary Fe-rich phase. Through this correlative microscopy effort with TEM and APT, a greater understanding of the precipitation sequence and evolution of the microstructure is obtained.
Controlled field evaporation of organic materials has been a long-term challenge for 3-dimensional (3D) atom probe tomography (APT) [1]. Complications arising from sample electrical/thermal requirements, sample preparation methodologies, and hardware configurations have all conspired to limit APT application for this class of materials. Although polymer laser APT studies have been reported on various polymers including polystyrene [2], polypyrrole [2], and polythiophene [3], these reports have focused on field ionized mass spectroscopy (FIMS) of the polymer and have not even attempted 3D reconstructions.We present data on laser local electrode atom probe (LEAP®) [4] investigations of alkanethiol self-assembled monolayers (SAMs) and poly-3-alkylthiophenes (P3ATs). Alkanethiols represent an opportunity to understand APT field ionization and reconstruction issues, as applied to polymers, at their most basic level. Alkanethiols are monodisperse short-chain molecules which are covalently bonded to a pre-sharpened metal specimen to form a dense, well-ordered SAM on the tip surface. The well understood microstructure [5] and limited potential for molecular fragment permutations of alkanethiols provides a desirable set of well-defined boundaries for subsequent analyses.P3ATs are another family of conducting polymers that have a chemical architecture similar to polypyrrole and polythiophene and thus invite comparison to previously reported results. At their heart, P3ATs are polythiophene with alkane side-chains attached to the conducting polymer main-chain so that P3ATs also bear some similarity to alkanethiols. This investigation of two families of polymers promises to provide a more complete understanding of laser APT than studies of each polymer family individually. 10000 4 CH3+ t o lo 20 30 4SCH2040Hi 7 'CHji. mv;}~~~~~~~~~~~~~mlq (ani) -RoJ u~, |Figure 1: Laser LEAP analIysis of dodecanethiol on Pd tip. 3D reconstruction [left] of mass-to-charge spectrum [top] can be interpreted as low-field alkane fragments (small 7 dark circles) originating from the tip surface. A lack of events at intermediate field results 3 e in a gap artifact in the reconstruction. Peaks at 12, 32, and 33 amu (large gray squares) 0 o |~~~~reside exclusively within the Pd bulk. A number of molecules oniginate from the interface ><. U between alkane and Pd corresponding to peaks at 63, 64, and 65 amu (large light shaded 7 6 5 4 3 2 1 circles). Their location is highly suggestive of fragments corresponding to a missing thiol Depth (nm) component of the dodecanethiol molecule (see chemical diagram top-right insert). Figure 1 shows laser LEAP data for a dodecanethiol coated Pd specimen. Pd wire (99.995 %/ purity) was electropolished to form an APT compatible specimen. A SAM was formed on the tip by submersion in a~-5 mM solution of dodecanethiol in ethanol for~3 hrs. Field evaporation evolution was consistent with fragments of dodecanethiol molecules evaporating at very low field. Once the Pd surface was uncovered, a significantly higher field was n...
The local electrode atom probe (LEAP ® ) [1] geometry enables analyses of multiple microtip specimens fabricated from a planar surface, Fig. 1. However, the preparation of polymeric, biological and particulate specimens for such analyses remains a challenging task. One general approach to solving this problem is the use of mold-replication techniques. We report here the first successful efforts at forming microtip specimens from polymers.In addition to shape, a significant issue with analyzing organic material in atom probe tomography is electrical conductivity. A potential solution is to embed the organic material in a matrix of conductive material. Although most polymers are not electrically conductive, many intrinsically conductive polymers (ICP) are used in various industries for applications such as antistatic surfaces on television screens, flexible electrode materials and polymer light-emitting diodes. In this study, a conductive polyurethane dispersion (CPUD2 [2]) was chosen for its low viscosity and ability to form a thin, uniform coating on a surface. To form the specific specimen shapes required for LEAP, polydimethylsiloxane, a silicone rubber, was used to prepare a mold due to its ability to replicate fine features and to withstand the 50˚C temperature needed to cure CPUD2. A micro-centrifuge tube was used to contain the silicone mold. Initial experiment molds were made from several sizes and types of needles. The polymer filled these molds well, but the needles of polymer were undesirably long (~1-2cm) and flexible, Fig. 2. The tip radii were relatively large (~3-10µm), which necessitated long milling times in a focused-ion-beam (FIB) tool. In addition, this process was time intensive and only created a few good specimens.In the interest of making many identical specimens in parallel, a silicon substrate with a nine-by-nine array of atom-probe-sharp tips [3] was chosen to create a new mold. The microtip coupons, attached to copper stubs were slowly dipped into the silicone and held in place with alligator clips, Fig. 3. These were set aside to cure for twenty-four hours before removing the silicon substrate, Fig. 4. The CPUD2 was poured into the mold, capped and microcentrifuged for 30 minutes at room temperature and 3 RCF (relative centrifugal force). After curing, the CPUD2 replica was removed, trimmed and epoxied to a copper stub, Fig. 5. The extraction of the specimen from the mold must be done slowly and in a linear fashion. The resulting microtips' radii were ~ 2-3µm. With slight FIB milling using annular mill patterns [4], they were ready for attempted LEAP analysis, Fig. 6. This specimen molding technique has the potential to be a relatively quick, simple and repeatable process for creating multiple specimens, enable the encapsulation of particulate specimens within a conductive matrix [5] and enable atom probe analysis of these soft materials.
Although the electropolishing of metals is based in science, there has always been a hint of art involved [1]. The development of an automated polisher has been pursued in order to make specimen preparation for atom probe and other metrologies more accessible and reproducible. The goals have been to develop a computer controlled process to repeat successful polishing routines and to develop an automated, all-in-one tool that performs both rough and fine polishing operations, saving users' time.The basics of metal electropolishing are well known: a metal specimen is placed in an electrolytic solution while another piece of relatively-inert metal -usually platinum -is placed in proximity to the specimen and a voltage is applied to the circuit [2]. Material is dissolved from the specimen, functioning as an anode, and pulled toward inert metal, which serves as the cathode, to create a desired shape. The basic electropolishing process is straight-forward and a variety of self-engineered setups exist in laboratories throughout the world.Existing instrumentation is not able to produce a finely-polished, ready-to-run atom probe specimen automatically, so a multiple-step process is undertaken, starting with rough polishing. To begin rough polishing, a several-millimeter-long specimen is polished until a necking region is formed (Fig. 1). This necking region needs to be long enough to be seen by the operator. Rough polishing is stopped immediately when the necking region breaks off, otherwise the speed of the rough polishing process very quickly dulls the apex and forces a repeat of this step.To automate this procedure, a current-monitoring device replaces visual anticipation and confirmation of the breaking of the necking region. Micro-polishing the specimen to its final shape begins after rough polishing.A specimen that is sharp enough for atom probe analysis -< 200 nm diameter at apexwill be too sharp to resolve with a light microscope. However, a typical procedure is to polish until the apex can no longer be focused by such an instrument, indicating that the tip is sharper than the resolution of the available optics. This highly hands-on process, coupled with limited ability to judge adequate final tip dimension, becomes the "Art of Electropolishing" which intimidates the uninitiated and haunts the experienced.The Simplex Scientific ElectroPointer™ offers an answer to the vagaries of electropolishing and includes automated control of polishing parameters and a hands-off approach that minimizes the need for subjective decision-making. The computer interface allows input of basic parameters to achieve a repeatable tip shape with minimal previous experience. For basic operations, the user can set voltage level, polishing pass width and speed. More advanced operations allow the user to define the number of passes
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