David R. Clarke

A series of bottlebrush silicone elastomers have been synthesized by using a solvent-free process based on hydrosilylation and thiol–ene reactions. The use of thiol-functionalized PDMS effectively suppresses the side reactions of Si–H bonds during the hydrosilylation process. The dynamic covalent C–S bonds in these materials can undergo reversible cleavage under UV illumination, imparting excellent forming and self-healing properties under UV. Remarkably, the elastomer not only retains a low elastic modulus but can heal within seconds, exhibiting toughness of up to 86% of its original value. The synthesis approach also allows small silane molecules to be efficiently grafted as spacers onto the polymer backbone, and the effects of both spacers with different steric hindrances and functional groups are reported. Spacers with different steric hindrances significantly altered the stiffness, while those with functional groups, such as fluorinated spacers, effectively modified the surface property. Potential applications are illustrated by using the material in 3D printing by digital light processing, healing under UV and recycling.

An optically addressable and configurable electrode architecture for dielectric elastomer actuators and arrays is described. It is based on embedding photoconductive, zinc oxide (ZnO) nanowires in the DEA to create electrodes. Normally, a network of ZnO nanowires is electrically insulating but it becomes conductive in the presence of UV light with a photon energy above the optical bandgap. Taking advantage of this characteristic optical induced switching behavior, we create an optically addressable electrode design, and create new, localized capacitor structures. As the ZnO nanowires are only conductive where, and when, illuminated the effective electrode structure is not fixed, as is the case with CNT and carbon-black electrodes currently used in DEAs. This provides greater, previously unattainable, freedom in the design of dielectric elastomer actuators for soft robotics and devices.

Multicomponent oxides, such as many minerals and high entropy oxides, show promise as materials for protection in extreme environments. Similar to other phononically dominated materials, the spectrum of vibrational carriers and phonon scattering heavily influences thermal transport in multi-cation oxides. In this work, we experimentally and computationally investigate the nature of phonon scattering and thermal transport in a series of single and multi-cation rare earth sesquioxides and zirconates. A reduction in thermal conductivity was observed from the single to multi-cation oxides, which is directly correlated to measured optical mode lifetimes. Via spectroscopic ellipsometry, we observe red shifting of the optical modes from local bonding distortion. Density functional theory calculation was used to evaluate how bonding distortions influence the phononic scattering rate observed through modal broadening and reduced thermal conductivity. Compared to single-cation oxides, the multi-cation oxides, especially those with larger cation size variance, exhibited lower effective coordination number and greater bond distortion.

<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" class="first" dir="auto" id="d13750e107">Thermoelectric layered superlattices of (ZnO) _mIn ₂O ₃ show tremendous promise for converting heat energy lost in power plants back into electricity [ <a class="xref-link" href="#r1">1</a>]. Understanding how these materials function at the most fundamental level requires exploring the nature of the chemical bonds within them. An accurate knowledge of the atomic structure of these compounds is a vital pre-requisite to bonding electron density measurements. Studies of bonding electron density have always been limited to materials with small and equiaxed unit cells. In the case of (ZnO) _mIn ₂O ₃, the length of <i>c</i> is at least an order of magnitude greater than the <i>a</i> and <i>b</i> lattice parameters. Therefore, even though electron scattering in these materials can be modelled by the Bloch wave method [ <a class="xref-link" href="#r2">2</a>], the layered compositional variation within these materials makes analysis by the multislice method [ <a class="xref-link" href="#r3">3</a>] far more tractable. <p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" dir="auto" id="d13750e147">In this study, a multislice approach [ <a class="xref-link" href="#r3">3</a>, <a class="xref-link" href="#r4">4</a>] to quantitative convergent-beam electron diffraction (QCBED) was applied to elucidate the atomic structure of (ZnO) ₄In ₂O ₃, starting with what was guessed to be an isomorphous reference structure, namely (ZnO) ₄LuFeO ₃ [ <a class="xref-link" href="#r5">5</a>]. The quality of the pattern matching of numerous CBED patterns by our multislice-based QCBED program, <i>QCBEDMS,</i> lends confidence to the atomic structure of (ZnO) ₄In ₂O ₃ that we have thus elucidated, leading the way to future bonding electron density measurements in a layer-by-layer fashion.

Abstract The high melting temperatures, large optical bandgaps, and large refractive indices of oxides favor their use as fibrous insulation at extreme temperatures where radiative heat transfer is significant. In this work, the reflectance of mats of fibers is modeled with the Monte Carlo (MC) method using the fiber optical properties (such as the scattering efficiency and scattering coefficient), the fiber radius, refractive index, volume fraction ( f v ), as well as the thickness of insulation ( L ). Two key metrics are identified for the design of highly reflective insulation. One is a novel metric, the Kuhn scattering length, which is based on an analogy between the anisotropic random walks in MC radiative transfer and polymer physics. It is used to determine approximate the effective thickness of insulation ( f v L ). The second is a size metric, which indicates that fiber mats are most reflective when the fiber radius is . The model is validated on experimental measurements of commercially available insulation consisting of fibers with 3–5 µm radius.

Yttria-stabilized zirconia (YSZ) coatings have been developed for high temperature energy applications including gas turbines. The objective of this work is to understand how aliovalent Fe substitution affects the optical absorption spectrum of the host YSZ and ${\mathrm{ZrO}}_{2}$ systems in the ultraviolet--visible--near infrared wavelength range (from 245 to 2500 nm) using both experimental and computational techniques. In the Fe-substituted ${\mathrm{ZrO}}_{2}$ system, phase-pure ($&gt;99$% purity) samples were synthesized in the monoclinic crystal structure, whereas Fe substitution in YSZ resulted in a two-phase mixture of coexisting tetragonal and monoclinic phases. Optical property characterization performed at room temperature revealed two broad absorption bands in both systems: one centered around 1000 nm and the other centered around 500 nm. Tauc plot analysis of the optical absorption data showed that as the Fe concentration increases, the optical band gaps of both materials systems decrease. Many-body perturbation theory methods, based on ${G}_{0}{W}_{0}$ and the Bethe-Salpeter equation, were used to computationally model the optical absorption spectrum as a function of Fe substitution in the tetragonal and monoclinic crystal structures of YSZ and ${\mathrm{ZrO}}_{2}$. Supercells were constructed and several Fe- and/or Y-atom configurations were explored in Zr sites. Charge compensating O vacancies were introduced to maintain electrical neutrality. The computations reveal that the observed optical excitations centered around 1000 nm likely have an excitonic character due to defect states, whose origin is traced to the electronic transitions between $\mathrm{Fe}\text{\ensuremath{-}}3d$ and $\mathrm{Fe}\text{\ensuremath{-}}3d$ orbitals. Intriguingly, both the tetragonal and monoclinic crystal structures appear to support local polyhedral distortions that promote excitations in the 1000 nm wavelength region. The excitation centered around 500 nm is attributed to the optical band gap of these materials. The outcomes of this work shed light on the radiative properties of Fe-substituted YSZ with implications in thermal barrier coating composition design.

Abstract Continuous sucker rod is used in progressing cavity (PC) pump applications to minimize the tubing wear and connections failures that can occur with conventional jointed sucker rods. While PC pumps are often assumed to have relatively steady state loading conditions, rod string rotation combined with local curvature results in high frequency low magnitude cyclic bending stresses. Variations in produced fluids and certain modes of pump friction also give rise to lower frequency moderate to higher magnitude cyclic torsional and axial loads. These cyclic stresses, particularly when combined with corrosive downhole environments, can result in rod fatigue failures at loads well below their specified load ratings often with short runtimes. Alloyed rod materials can slow the impact of corrosion but usually the failures persist. Corrosion inhibitors can reduce downhole equipment corrosion but their effectiveness on continuous rod can be diminished due to the rotational contact impairing the build-up of a protective layer on the rod surface. Rod coatings can provide barrier protection to the underlying bare rod material from production fluid exposure eliminating corrosion and its impact on accelerating fatigue failures. Certain coatings can also protect the rod body against mechanical damage that contributes to fatigue as well as reduce rod and tubing wear and its associated contact friction. Summarized in this paper is ten plus years of experience with a first-generation coated continuous rod product that employs a thick durable polyethylene coating. Deployment of several thousand rod strings in an area with highly directional heavy oil wells reduced rod fatigue failures from multiple breaks a year to multi-year runtimes. Broader application deployment also demonstrated reductions in fatigue failures but identified well depth and produced fluid limitations that significantly limited the products application range leading to a second-generation development. This paper provides an overview of the second-generation coated rod development including design requirements, materials development and laboratory testing, product prototyping and validation, new coated rod manufacturing processes and service equipment testing. In support of this development were evaluations using custom rotary tribology equipment and multiaxial fatigue equipment select findings of which are included. Lastly it details field testing and initial commercial deployment that confirms benefits like those with the first-generation design but over an expanded application range.

Future gas turbine engines will operate at higher gas temperatures and consequentially hot-section components such as blades, vanes and combustors, will be subject to higher thermal radiation fluxes than today. Current thermal barrier coating materials are translucent over the spectral region of the heat flux so future coatings will also have to provide a barrier to thermal radiation. The effects of optical absorption and scattering properties of coating materials on the temperatures and heat fluxes through coatings are explored using a two-flux heat transfer model, and promising combinations are identified that reduce the coating-alloy interface temperatures. Lower interface temperatures occur for thickness normalized absorptions of $\overline{\kappa} L$ $>$1. The effect of both a narrow and a broad band spectrally selective absorbing Gd${_2}$Zr${_2}$O$_{7}$ based coating materials are then studied. These show that large values of the product of the normalized absorption length and the spectral width of the absorption are required to significantly decrease the radiative heat transport through a coating. The results emphasize the importance of enhancing the optical absorption of the next generation barrier materials as a strategy to increase gas turbine engine efficiency by decreasing compressor bleed air cooling requirements.

Abstract The demand for more efficient gas turbines relies heavily on the development of new environmental‐thermal barrier coatings (ETBCs). However, there is still uncertainty about which alloys and composites will be used for the next generation of turbine blades, as well as the most promising coating materials. Herein, an ETBCs development strategy is presented by integrating the coating design, synthesis, and screening using an ultrafast high temperature sintering (UHS) technique to accelerate coating improvements. The initial basis for composition selection is their thermal expansion mismatch with the substrate alloys; for which a temperature‐dependent coefficient of thermal expansion database is created. By combining tape casting method with the UHS technique a high‐throughput synthesis of single and multi‐layer coatings are realized with different compositions, layer stacking sequences, and layer thicknesses. To evaluate the coatings, thermal cycling tests from room temperature to 1300 °C are conducted. The approach enabled coatings on objects with complex geometries, multi‐layer ETBCs, and porosity tailoring by using staged UHS that runs with different temperatures and durations. The fast iteration strategy is more cost‐effective for the screening of ETBCs compared to conventional methods and greater throughput which can be further extended for rapid optimization of other materials systems.

PhotoCAN silicone elastomers, based on the thiol–ene reaction, exhibit rapid and reversible changes in dynamic modulus at room temperature when illuminated by UV. By combining results from magic angle spinning solid-state NMR as well as EPR and rheometry measurements, both under UV, it is concluded that the mechanical response can be attributed to a combination of dissociative, associative, and oxidation reactions. The cleavage of the C–S bonds under UV in the presence of an excess of thiyl radicals is identified as the reversible dissociative reaction responsible for abrupt drops in the storage modulus. A slower but concurrent reaction is a termination process involving thiyl radicals to form disulfide bonds. A kinetic model is developed that successfully relates the rates of the underlying reaction mechanisms to changes in the storage modulus. The results provide a basis for designing new, ambient temperature photoresponsive covalently adaptive network materials.

Our ARPA-E funded project, New Environmental-Thermal Barrier Coatings for Ultrahigh Temperature Alloys, has made significant strides in advancing the understanding of design and fabrication of novel thermal barriers coatings. Through rigorous research and innovative approaches, our team has explored new frontiers, pushing the boundaries of knowledge in this critical field.

The timely discovery and monitoring of fatigue cracks on steel bridges is critical in ensuring structural safety and continuous operations. Existing sensing solutions, for example foil gauges, can be used to monitor known cracks, yet cannot be used for crack discovery because of their highly localized nature. A solution is the development and deployment of large-area electronics capable of detecting local states over large surfaces. Polymer-based nano-structured materials that respond to external impacts, such as mechanical strain, with color changing properties have attracted significant attention in structural health monitoring (SHM) community, because their passive optical/visual properties can be used to assist inspectors at quickly localizing new fatigue cracks. Here, the authors proposed a multifunction skin sensor that combines optical and electrical sensing properties. The optical function is passive and engineered to visually assist in localizing fatigue cracks, and the electrical function is added to send timely warnings to infrastructure operators. This is achieved by modifying the nanoscale structures within a photo-elastomer to obtain a soft stretchable optically-active film that is sandwiched between transparent carbon nanotube electrodes (CNT) to form a capacitor structure for adding electrical functionality. The developed sensor has a stiffness of 460 kPa and withstands reversible strain levels of up to 40%. Additionally, it exhibits a reversible and repeatable color change from light blue to deep blue, and changes the reflected color in the Vis spectra from approximately 500 nm to 600 nm and insensitive to viewing angle. The performance of the sensor is characterized through a free-standing dynamic test and further extended to a free-vibration test conducted on a steel cantilever plate. A correlation- based image processing algorithm was developed to discriminate color change and further quantify strain. The measured shift in the material’s reflection center wavelength was merged with an RGB correlations matrix and an optical gauge factor of 0.52 was obtained. An electrical gauge factor of 0.48 was obtained by subjecting the sensor to a triangular load. It was found that the parallel capacitance measurements exhibited better performance by yielding higher accuracy for free vibration strain measurements.

Source data for: Natalie M. Larson, Jochen Mueller, Alex Chortos, Zoey S. Davidson, David R. Clarke, Jennifer A. Lewis. Rotational multimaterial printing of filaments with subvoxel control. Nature 613, 682–688 (2023). https://doi.org/10.1038/s41586-022-05490-7

Abstract Fatigue-induced cracking in steel components and other brittle materials of civil structures is one of the primary mechanisms of degrading structural integrity and can lead to sudden failures. However, these cracks are often difficult to detect during visual inspections, and off-the-shelf sensing technologies can generally only be used to monitor already identified cracks because of their spatial localization. A solution is to leverage advances in large area electronics to cover large surfaces with skin-type sensors. Here, the authors propose an elastic and stretchable multifunctional skin sensor that combines optical and capacitive sensing properties. The multifunctional sensor consists of a soft stretchable structural color film sandwiched between transparent carbon nanotube electrodes to form a parallel plate capacitor. The resulting device exhibits a reversible and repeatable structural color change from light blue to deep blue with an angle-independent property, as well as a measurable change in capacitance, under external mechanical strain. The optical function is passive and engineered to visually assist in localizing fatigue cracks, and the electrical function is added to send timely warnings to infrastructure operators. The performance of the device is characterized in a free-standing configuration and further extended to a fatigue crack monitoring application. A correlation coefficient-based image processing method is developed to quantify the strain measured by the optical color response. Results show that the sensor performs well in detecting and quantifying fatigue cracks using both the color and capacitive signals. In particular, the color signal can be measured with inexpensive cameras, and the electrical signal yields good linearity, resolution, and accuracy. Tests conducted on two steel specimens demonstrate a minimum detectable crack length of 0.84 mm.