Scalable and repeatable determinations of continuous wave (CW) laser-induced damage thresholds are required to develop materials for applications ranging from deformable mirrors to momentum transfer. Current standards assume sample geometries and beam conditions where CW damage thresholds are constant in linear power density, depend strongly on substrate thermal conductivity, and are insensitive to environmental conditions. In this work, the CW laser response of thin PET films with a reflective Al/MgF2 coating are experimentally assessed over a range of beam diameters and irradiances. The laser-induced damage threshold decreases with increased exposure time down to a temporally-independent irradiance, decreases with increased beam diameter to an irradiance that is independent of spot size, and depends on radiative and convective cooling. Models are used to define the minimum spot size and exposure time required to achieve such constant damage threshold irradiances for thin reflectors.

Low-cost unmanned aircraft that use affordable manufacturing and have limited service life can enable mission concepts in which there is a higher tolerance for aircraft loss, or attrition. Because of their higher risk tolerance, these low-cost attritable aircraft could also integrate emerging technology which may have previously been considered too risky for integration into expensive and long life aircraft. Light-weight multifunctional structural composites have the potential to integrate additional functions and enable mission agility without significantly adding weight or reducing payload capacity. However, design and development of these material systems are often difficult because of the traditional “building block” development approach used for traditional composites, the large option space available for structural and functional properties, and the potential complexity of the multiscale and multiphysics coupling. To realize integrated functionality, new multi-scales and multi-physical experimental mechanical characterization techniques should be merged with maturing integrated materials models. We discuss this need using examples of a reconfigurable liquid metal Structurally Embedded Vascular Antenna (SEVA), a plasmonic nanoparticle based method for measuring internal temperature gradients, and embedded micro-cantilever carbon-nanotube based sensors. The latter of these is also used to discuss the potential to accelerate development of multifunctional structural concepts by provide air flow measurement and structural feedback during testing of complex structures. This could, in turn, eliminate some testing of intermediate elements in the slow and expensive traditional “building block” approach.

Artificial hair flow sensors were fabricated using piezoresistive, radially grown carbon nanotube arrays on glass fibers and investigated for their dynamic aerodynamic response as measured within an instrumented plane-wave tube. The sensors were experimentally observed to provide both a large bandwidth of operation below first resonance and a strong resonance response at selected frequencies above first resonance. The frequency of first resonance was easily tunable by adjusting the length of the exposed hair and could be made to vary from a few hundred hertz to over 13 kHz. Higher frequency bands were accessible for a given hair length using higher-order resonance modes, up to five of which were observed. All of the responses were understood and modeled using a vibrating Euler-Bernoulli beam analysis.

Understanding the shear strain, viscoelastic response, and onset of damage within bonded composites is critical to their design, processing, and reliability. This presentation will discuss the multidisciplinary research conducted which led to the conception, development, and demonstration of two methods for measuring the shear within a bonded joint - dual-plane digital image correlation (DIC) and a micro-cantilever shear sensor. The dual plane DIC method was developed to measure the strain field on opposing sides of a transparent single-lap joint in order to spatially quantify the joint shear strain. The sensor consists of a single glass fiber cantilever beam with a radially-grown forest of carbon nanotubes (CNTs) within a capillary pore. When the fiber is deflected, the internal radial CNT array is compressed against an electrode within the pore and the corresponding decrease in electrical resistance is correlated with the external loading. When this small, simple, and low-cost sensor was integrated within a composite bonded joint and cycled in tension, the onset of damage prior to joint failure was observed. In a second sample configuration, both the dual plane DIC and the hair sensor detected viscoplastic changes in the strain of the sample in response to continued loading.

Distributed arrays of artificial hair sensors have bio-like sensing capabilities to obtain spatial and temporal surface flow information which is an important aspect of an effective fly-by-feel system. The spatiotemporal surface flow measurement enables further exploration of additional flow features such as flow stagnation, separation, and reattachment points. Due to their inherent robustness and fault tolerant capability, distributed arrays of hair sensors are well equipped to assess the aerodynamic and flow states in adverse conditions. In this paper, a local flow measurement from an array of artificial hair sensors in a wind tunnel experiment is used with a feedforward artificial neural network to predict aerodynamic parameters such as lift coefficient, moment coefficient, free-stream velocity, and angle of attack on an airfoil. We find the prediction error within 6% and 10% for lift and moment coefficients. The error for free-stream velocity and angle of attack were within 0.12 mph and 0.37 degrees. Knowledge of these parameters are key to finding the real time forces and moments which paves the way for effective control design to increase flight agility, stability, and maneuverability.

: This report summarizes research activity performed under this task order. The overall objective was to develop novel material interfaces with the potential to respond to changes in mechanical, thermal, or electromagnetic stimulus. The primary thrust was on the further development of artificial hair sensors (AHS) featuring a responsive carbon nanotube (CNT) array to serve as a piezoresistive element. This report outlines the fabrication and modeling of these systems, along with investigation of their potential for several applications of AFRL interest. Additionally, the task order included a preliminary investigation of polymer matrix composites under laser irradiation, which is reported under separate cover, as AFRL Interim Report AFRL-RX-WP-TR-2016-0071 dated 30 October 2015.

While numerous flow sensor architectures mimic the natural cilia of crickets, locusts, bats, and fish, the prediction of sensor output for given flow conditions based on the sensor properties has not been achieved. Challenges include difficulty in determining the electromechanical properties of the sensors, limited working knowledge of the boundary layer, low sensitivity to small hair deflections, and lack of models for large deflections. Within this work, hair sensors are fabricated using piezoresistive arrays of carbon nanotubes (CNTs) without traditional microelectromechanical processing. While correlating the CNT array electromechanical properties to synthesis conditions remains a challenge, a consistent, proportional, and predictable response to steady, boundary‐determined air flow is obtained using theory and measurement for various lengths of hairs. The moment sensitivity is shown to scale inversely with the CNT length and stiffness to a typical maximum of 1.3 ± 0.4% resistance change nN−1 m−1. The normalized CNT piezoresistivity is constant (1.1 ± 0.2) for a majority of the more than two dozen sensors examined despite the orders‐of‐magnitude variability in both sensitivity and CNT compressive modulus. The sensor sensitivity and noise both distinctly change as the flow transitions from steady and laminar to turbulent, suggesting the sensor may be capable of detecting flow transitions.

Crickets, locusts, bats, and many other animals detect changes in their environment with distributed arrays of flow-sensitive hairs. Here we discuss the fabrication and characterization of a relatively new class of pore-based, artificial hair sensors that take advantage of the mechanical properties of structural microfibers and the electromechanical properties of self-aligned carbon nanotube arrays to rapidly transduce changes in low speed air flow. The radially aligned nanotubes are able to be synthesized along the length of the fibers inside the high aspect ratio cavity between the fiber surface and the wall of a microcapillary pore. The growth self-positions the fibers within the capillary and forms a conductive path between detection electrodes. As the hair is deflected, nanotubes are compressed to produce a typical resistance change of 1-5% per m/s of air speed which we believe are the highest sensitivities reported for air velocities less than 10 m/s. The quasi-static response of the sensors to point loads is compared to that from the distributed loads of air flow. A plane wave tube is used to measure their dynamic response when perturbed at acoustic frequencies. Correlation of the nanotube height profile inside the capillary to a diffusion transport model suggests that the nanotube arrays can be controllably tapered along the fiber. Like their biological counterparts, many applications can be envisioned for artificial hair sensors by tailoring their individual response and incorporating them into arrays for detecting spatio-temporal flow patterns over rigid surfaces such as aircraft.

We demonstrate that vertically aligned carbon nanotubes (CNTs) can be precisely machined in a low pressure water vapor ambient using the electron beam of an environmental scanning electron microscope. The electron beam locally damages the irradiated regions of the CNT forest and also dissociates the water vapor molecules into reactive species including hydroxyl radicals. These species then locally oxidize the damaged region of the CNTs. The technique offers material removal capabilities ranging from selected CNTs to hundreds of cubic microns. We study how the material removal rate is influenced by the acceleration voltage, beam current, dwell time, operating pressure, and CNT orientation. Milled cuts with depths between 0–100 microns are generated, corresponding to a material removal rate of up to 20.1 μm3/min. The technique produces little carbon residue and does not disturb the native morphology of the CNT network. Finally, we demonstrate direct machining of pyramidal surfaces and re-entrant cuts to create freestanding geometries.

A technique is reported for measuring and mapping the maximum internal temperature of a structural epoxy resin with high spatial resolution via the optically detected shape transformation of embedded gold nanorods (AuNRs). Spatially resolved absorption spectra of the nanocomposites are used to determine the frequencies of surface plasmon resonances. From these frequencies the AuNR aspect ratio is calculated using a new analytical approximation for the Mie-Gans scattering theory, which takes into account coincident changes in the local dielectric. Despite changes in the chemical environment, the calculated aspect ratio of the embedded nanorods is found to decrease over time to a steady-state value that depends linearly on the temperature over the range of 100-200 °C. Thus, the optical absorption can be used to determine the maximum temperature experienced at a particular location when exposure times exceed the temperature-dependent relaxation time. The usefulness of this approach is demonstrated by mapping the temperature of an internally heated structural epoxy resin with 10 μm lateral spatial resolution.

A new method is developed for the determination of internal temperatures and spatially resolved thermal gradients using the change in aspect ratio of dispersed gold nanorods and corresponding changes in their measured plasmon resonance spectrum. Previously, changes in the plasmon resonance spectral peaks of gold nanorods as a function of temperature were observed by several researchers in solution. These spectral shifts were related to the change in aspect ratio of the nanorods as they transitioned continuously from an elongated ellipsoid towards a sphere. Aspect ratios are confirmed with X-ray scattering and TEM microscopy. When the gold nanorods are dispersed in a structural epoxy resin, their spectral changes are used to infer the steady state temperature over a range of temperatures and thermal gradients. Using spatially resolved spectroscopy, an extension of current theory, and isothermal calibration curves, we demonstrate that the maximum temperature experienced within a composite can be spatially mapped over a useful temperature and temporal range. This provides an important means of understanding a composite's thermal history and to validate thermal models without disrupting the composite structures by either embedded thermocouples or inferring the internal temperature from surface thermal measurements. The technique requires very low loading of materials (<0.05vol%) and has a spatial limitation limited by the optical detection system being employed. We measure thermal gradients of >1 degrees C/mu m within an epoxy resin in the proximity of a single carbon fiber that is resistively heated. We compare the measurements with continuum models and obtain good agreement. We also demonstrate the applicability of the approach to macroscale thermal gradient measurements. A gradient of 4 degrees C/mm was measured with comparable capability to a commercial thermal camera. The general applicability of this approach to other thermally sensitive nanomaterials and for validation of composite processing models will be discussed.

Here we discuss the characterization and modeling of a relatively new class of embedded artificial hair sensors. The sensors take advantage of the mechanical properties of structural S2 glass microfibers as the hair element and the electromechanical properties of self-aligned carbon nanotube arrays to rapidly transduce small changes in force or displacement into changes in resistance. While traditionally envisioned for air flow sensing, this approach can also provide fundamental understanding of the electrical and mechanical properties of fuzzy fibers and may have additional applications to structural monitoring. The materials of the entire sensor are chosen to survive CNT synthesis conditions (700 degrees C, inert atmosphere) as well as the typical processing and operating conditions for aerospace composite skin materials. Each sensor is individually contained within a small footprint. While other hair sensor designs suffer from reduction in bandwidth due to mechanical coupling of the hair by the transducer, our models indicate that the stiffness of the distributed CNT array supporting the S2 fiber is high relative to the low mass of the fiber. As a result the resonance frequency of the hair is maximized as if rigidly fixed at its base (e.g. the opening of the pore), yet the base of the hair deflects enough to induce a resistance change. In comparison to other published results, we believe these sensors display the highest sensitivity for air velocities less than 10 m/s. The electromechanical responses of the sensors to both point loads and to the distributed loads from airflow are compared, and their responses under both quasi-static and dynamic conditions are correlated to the mechanical properties of the hair and nanotubes. Employing the sensors to measure the mechanical strain in composites is also investigated.

Performance demands of future unmanned air vehicles will require rapid autonomous responses to changes in environment. Towards this goal, we expect that the next generation flight control systems will include advanced sensors beyond the contemporary array. One promising scenario correlates measurements of flow footprints over aircraft surfaces with aerodynamic data to aid navigation and feedback control algorithms. As a sensor for this concept, we construct artificial hair sensors (AHSs) based on glass microfibers enveloped in an annular, radially-aligned piezoresistive carbon nanotube (CNT) forest to measure air flow in boundary layers. This study includes an analysis of the sensitivity based on laboratory scale electromechanical testing. The sensors in this work utilize nine micron diameter S2 glass fibers as the sensing mechanism for coupling to boundary layer air flows. The annular CNT forest resides in a fused silica microcapillary with electrodes at the entrance. The sensor electrical transduction mechanism relies on the resistance change of the CNT forest due to changes in both the bulk and contact resistance as a function of mechanical loading on the fiber. For the electromechanical analysis, the sensors are controllably loaded to measure both the force and moment acting at the base of the hair and the resulting deflection of the CNT forest inside of the microcapillary is measured to estimate the stress on the forest and the pressure between the forest and the electrode. The electrical responses of the sensors are compared to the mechanical state of the CNT forest. This work represents the development of a characterization tool to better understand and control the response of CNT based AHSs.

Novel freestanding foils of vertically aligned carbon nanotube arrays (VANTAs) fused with metals are fabricated in a scalable four-step process of (1) VANTA synthesis, (2) deposition of partially infused metal film, (3) removal from growth substrate, and (4) optional deposition of a second metal film on the back side. Mechanical and electrical testing demonstrates excellent contact between the VANTA and the interface-infused metal layers.

We analyze transport data from a quantum point contact (QPC), fabricated on a modulation doped Si/SiGe heterostructure, to extract experimental estimates for the valley splitting. The experimental data are fit to a form derived from a valley coupling theory that takes into account the fact that the quantum well is grown on a miscut substrate. The results of the fitting analysis are compared to the results obtained by fitting to a simple phenomenological form; both methods indicate that electrostatic confinement and magnetic confinement enhance the valley splitting by reducing the lateral spatial extent of the electronic wavefunction. Consequently, the valley splitting can be much larger than the spin splitting for small magnetic fields. We observe different valley splittings for the two lowest orbital modes of the QPC, supporting the notion that when steps are present at the quantum well interface the spatial extent of the wavefunction plays a dominant role in determining the valley splitting.

Silicon quantum devices have progressed rapidly over the past decade, driven by recent interest in spintronics and quantum computing. Spin coherence has emerged as a leading indicator of suitable devices for quantum applications. In particular, the technique of electron-spin resonance (ESR) has proven powerful and flexible for probing both the magnitude and the nature of spin scattering, when compared to theoretical predictions. Here, we provide a short review of silicon quantum devices, focusing on silicon/silicon-germanium quantum wells. Our review touches on the fabrication and lithography of devices including quantum dots, and the development of Schottky top gates, which have recently enabled the formation of few-electron quantum dots with integrated charge sensors. We discuss recent proposals for quantum-dot quantum computing, as well as spin- and valley-scattering effects, which may limit device performance. Recent ESR, studies suggest that spin scattering in high-mobility Si/SiGe two-dimensional electron gases may be dominated by the D'yakonov and Perel' mechanism arising from Bychkov-Rashba spin-orbit coupling. These results rely on theoretical predictions for the dependence of the coherence time T-2* on the orientation of an external applied magnetic field. Here, we perform ESR experiments on a series of samples fabricated by different methods, including samples recently used to obtain few-electron quantum dots. While we observe some similarities with recent experiments, we find that for five out of six samples, the angular dependence of T-2* was far larger than the theoretical predictions. We discuss possible causes for this discrepancy, but conclude that the theoretical understanding of these samples is not yet complete.