Publications

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The National Rotor Testbed (NRT) is a wind turbine blade research program in the Sandia National Laboratories (SNL) Wind Department that has developed a new blade design. Each blade includes bonded-in, threaded metal root inserts that enable the blades to be bolted onto the wind turbine hub. Prior to installing the flight blades on the turbine, root insert strength verification tests exhibited a subset of failures below the design load on one (NRT-02) of four blades. As part of a root cause analysis for the failures, this work analyzes "scraps" of the epoxy adhesive used to bond the metal inserts into the blade and uses surface topography and x-ray fluorescence (XRF) measurements to characterize the exterior surface of the root insert. Samples were taken from inserts that exhibited both high and low loads at failure, as well as some "control inserts" to monitor the state of the surface throughout the manufacturing process. Differences in the calorimetric response of the adhesive from the separate root inserts are apparent but none of them appear to relate to the pull load required to dislodge the inserts. Two takeaways of note include: In the way that the adhesive is processed, it does not reach full cure; and, Something occurred to sample#10 such that the fully-cured adhesive has a significantly lower T_g.

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Contact probing of gaging surfaces is used throughout dimensional metrology. Probe tips such as ruby, sapphire, or diamond are commonly employed as styli for universal length measuring machines (ULMs) and coordinate measuring machines (CMMs) due to the hardness, durability, and wear resistance. Gaging surfaces of gage blocks are precision ground or lapped, with very low surface roughness to enable wringing. Damage or contamination of these surfaces can prevent wringing and lead to measurement error. Experimental investigations using a horizontal ULM and CMM have revealed that even at low force settings (≤0.16 N), probe materials such as ruby and sapphire can cause plastic deformation to hardened carbon chrome steel (such as AISI 52,100) gage block surfaces at the microscale, likely attributed to fretting-associated wear. Under some conditions, permanent transfer of material from the probe stylus to the gaging surface is possible. Results demonstrate irreversible changes and damage to gaging surfaces with repeated probe contact on a ULM and CMM. Optical microscopy, optical profilometry, and scanning electron microscopy (SEM) provide a semi-quantitative assessment of microscale plastic deformation and material transfer. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and Raman techniques confirm chemical constituency of reference materials used (gage blocks and probes) and also identify makeup of deposits on gaging surfaces following probe contact.

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The goal of this project was to qualitatively evaluate the efficacy of using plasma cleaning to remove PDMS from vacuum systems. Silicon containing compounds are notorious for interfering with vacuum system techniques such as x-ray photoelectron spectroscopy (XPS), Auger Electron Spectroscopy (AES) and Secondary lon Mass Spectrometry (SIMS). Finding a way to remotely and rapidly remove contaminants from a system saves time and money for analysts using vacuum analytical techniques.

Tin-germanium alloys are increasingly of interest as optoelectronic and thermoelectric materials as well as materials for Li/Na ion battery electrodes. However, the lattice incompatibility of bulk Sn and Ge makes creating such alloys challenging. By exploiting the unique strain tolerance of nanosized crystals, we have developed a facile synthetic method for homogeneous SnxGe1-x alloy nanocrystals with composition varying from essentially pure Ge to 95% Sn while still maintaining the cubic structure.

In this study, the scaling of polarization and pyroelectric response across a thickness series (5–20 nm) of Hf_0.58Zr_0.42O₂ films with TaN electrodes was characterized. Reduction in thickness from 20 nm to 5 nm resulted in a decreased remanent polarization from 17 to 2.8 μC cm^-2. Accompanying the decreased remanent polarization was an increased absolute pyroelectric coefficient, from 30 to 58 μC m^-2 K^-1. The pyroelectric response of the 5 nm film was unstable and decreased logarithmically with time, while that of 10 nm and thicker films was stable over a time scale of >300 h at room temperature. Finally, the sign of the pyroelectric response was irreversible with differing polarity of poling bias for the 5 nm thick film, indicating that the enhanced pyroelectric response was of electret origins, whereas the pyroelectric response in thicker films was consistent with a crystallographic origin.

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Materials aging is a high-consequence failure mode in electronic systems. Such mechanisms can degrade the electrical properties of connectors, relays, wire bonds, and other interconnections. Lost performance will impact, not only that of the device, but also the function and reliability of next-level assemblies and the weapons system as a whole. The detections of changes to materials surfaces at the nanometer-scale resolution, provides a means to identify aging processes at their early stages before they manifest into latent failures that affect system-level performance and reliability. Diffusion will be studied on thin films that undergo accelerated aging using the nanometer scale characterization technique of Frequency Modulated Kelvin Probe Force Microscopy (FM-KPFM). The KPFM provides a relatively easy, non-destructive methodology that does not require high-vacuum facilities to obtain nanometer spatial resolution of surface chemistry changes. The KPFM method can provide the means to measure surface, and near-surface, elemental concentrations that allow the determination of diffusion rate kinetics. These attributes will be illustrated by assessing diffusion in a thin film couple. Validation data will obtained from traditional techniques: (a) Auger electron spectroscopy (AES), x-ray fluorescence (XRF), and xray photoelectron spectroscopy (XPS).

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Materials aging is a high-consequence failure mode in electronic systems. Such mechanisms can degrade the electrical properties of connectors, relays, wire bonds, and other interconnections. Lost performance will impact, not only that of the device, but also the function and reliability of next-level assemblies and the weapons system as a whole. The detections of changes to materials surfaces at the nanometer-scale resolution, provides a means to identify aging processes at their early stages before they manifest into latent failures that affect system-level performance and reliability. Diffusion will be studied on thin films that undergo accelerated aging using the nanometer scale characterization technique of Frequency Modulated Kelvin Probe Force Microscopy (FM-KPFM). The KPFM provides a relatively easy, non-destructive methodology that does not require high-vacuum facilities to obtain nanometer spatial resolution of surface chemistry changes. The KPFM method can provide the means to measure surface, and near-surface, elemental concentrations that allow the determination of diffusion rate kinetics. These attributes will be illustrated by assessing diffusion in a thin film couple. Validation data will obtained from traditional techniques: (a) Auger electron spectroscopy (AES), x-ray fluorescence (XRF), and xray photoelectron spectroscopy (XPS).

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