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1. Measurements for stress sensing of composites using tailored piezospectroscopic coatings

   https://doi.org/10.1063/1.5084964  

   Esteves, Remelisa; Hernandez, Johnathan; Vo, Khanh; Hoover, Ryan; Freihofer, Gregory; Raghavan, Seetha (May 2019, AIP Advances)

   Chromium-doped α-alumina is naturally photo-luminescent with spectral properties that are characterized by R-lines with two distinct peaks known as R1 and R2. When the material is subjected to stress, shifts in the R-lines occur, which is known as the piezospectroscopic (PS) effect. Recent work has shown that improved sensitivity of the technique can be achieved through a configuration of nanoparticles within a polymer matrix, which can be applied to a structure as a stress-sensing coating. This study demonstrates the capability of PS coatings in mechanical tests and investigates the effect of nanoparticle volume fraction on sensing performance. Here, measurements of spectral shifts that capture variation in stress of the coating during mechanical testing and in the region of substrate damage showed that stress contours are more noticeable on a soft laminate than hard laminate. It was found that the 20 % volume fraction PS coating showed the most distinct features of all the coatings tested with the highest signal-to-noise ratio and volume fraction of α-alumina. Post failure assessment of the PS coatings verified that the coatings were intact and peak shifts observed during mechanical testing were due to the stress in the substrate. The results suggest the ability to design and tailor the “sensing” capability of these nanoparticles and correlate the measured stress variations with the presence of stress and damage in underlying structures. This study is relevant to nondestructive evaluation in the aerospace industry, where monitoring signs of damage is of significance for testing of new materials, quality control in manufacturing and inspections during maintenance. 

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2. 3D Printed Stress Sensors for Non-Destructive Evaluation of Space Structures

   Perla Latorre-Suraez, Rohan Madathil (April 2021, AIAA 2021 Regional Student Conference, April 2021.)

   Self-sufficient and non-contact sensors play multiple roles in lunar, planetary exploration, and Earth structures. These sensors allow engineers to accurately examine structural integrity and defects on mechanical components for optimal operations. Structural integrity allows the industry to ensure the safety and capacity of key structures. Materials like α-alumina can be employed as sensors due to the photoluminescent properties that they possess. Piezospectroscopy is a non-destructive evaluation (NDE) method capable of capturing in-situ stress using α-alumina due to the chromium ion impurities that it contains. The chromium ion impurities carry spectral characteristics, that when excited with an Nd: YAG laser (532 nm), demonstrate capabilities for structural integrity monitoring. In this work, a 3D printing method is developed to autonomously create sensors that are compatible with use in space environments. The 3D printing method intends to provide the industry flexible and adaptive solutions for structural integrity monitoring. This method includes a modified Fused Deposition Method printer by exchanging its original nozzle with a syringe base nozzle. The printing parameters such as printing speed, printing bed temperature, coating thickness, and syringe volume are determined during the testing process. Challenges include achieving uniform integration and nanoparticle dispersion as well as adhesion between the matrix and the substrates. The parameters to encounter these challenges will depend on the materials used. Experiments with three different volume fractions (VF) of α-alumina within an epoxy were performed to address the printing challenges. The sensors were applied to nine specimens, three of each VF but with varying deposition rates after the mixture process. These experiments considered the mixing and deposition method while testing the dispersion within the α-alumina and the epoxy matrix. The substrates, on which the epoxy matrix was deposited, underwent a surface treatment to ensure adhesion between the substrate and the sensor matrix. During this experiment, the epoxy matrix was deposited with a syringe onto a substrate and cured at room temperature. The specimens were tested with a tensile load using an electromechanical MTS. While the samples are tensile loaded, the sensors were characterized via photoluminescent piezo spectroscopy to determine which VF demonstrates the best stress sensing capabilities, along with the adhesion between the matrix and the substrate. The data collected allows the optimal VF to be established for future applications. 

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3. Effect of Polymer Removal on the Morphology and Phase of the Nanoparticles in All-Inorganic Heterostructures Synthesized via Two-Step Polymer Infiltration

   https://doi.org/10.3390/molecules26030679  

   Berman, Diana; Sha, Yuchen; Shevchenko, Elena V. (February 2021, Molecules)

   Polymer templates play an essential role in the robust infiltration-based synthesis of functional multicomponent heterostructures with controlled structure, porosity, and composition. Such heterostructures are be used as hybrid organic–inorganic composites or as all-inorganic systems once the polymer templates are removed. Using iron oxide/alumina heterostructures formed by two-step infiltration of polystyrene-block-polyvinyl pyridine block copolymer with iron and aluminum precursors from the solution and vapor-phases, respectively, we show that the phase and morphology of iron oxide nanoparticles dramatically depend on the approach used to remove the polymer. We demonstrate that thermal and plasma oxidative treatments result in iron oxide nanoparticles with either solid or hollow morphologies, respectively, that lead to different magnetic properties of the resulting materials. Our study extends the boundaries of structure manipulations in multicomponent heterostructures synthesized using polymer infiltration synthesis, and hence their properties. 

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4. Accessibility and Mechanical Stability of Nanoporous Zinc Oxide and Aluminum Oxide Coatings Synthesized via Infiltration of Polymer Templates

   https://doi.org/10.3390/polym15204088  

   Omotosho, Khalil D.; Lyon, Zachary; Shevchenko, Elena V.; Berman, Diana (October 2023, Polymers)

   The conformal nanoporous inorganic coatings with accessible pores that are stable under applied thermal and mechanical stresses represent an important class of materials used in the design of sensors, optical coatings, and biomedical systems. Here, we synthesize porous AlOx and ZnO coatings by the sequential infiltration synthesis (SIS) of two types of polymers that enable the design of porous conformal coatings—polymers of intrinsic microporosity (PIM) and block co-polymer (BCP) templates. Using quartz crystal microbalance (QCM), we show that alumina precursors infiltrate both polymer templates four times more efficiently than zinc oxide precursors. Using the quartz crystal microbalance (QCM) technique, we provide a comprehensive study on the room temperature accessibility to water and ethanol of pores in block copolymers (BCPs) and porous polymer templates using polystyrene-block-poly-4-vinyl pyridine (PS75-b-P4VP25) and polymers of intrinsic microporosity (PIM-1), polymer templates modified by swelling, and porous inorganic coatings such as AlOx and ZnO synthesized by SIS using such templates. Importantly, we demonstrate that no structural damage occurs in inorganic nanoporous AlOx and ZnO coatings synthesized via infiltration of the polymer templates during the water freezing/melting cycling tests, suggesting excellent mechanical stability of the coatings, even though the hardness of the inorganic nanoporous coating is affected by the polymer and precursor selections. We show that the hardness of the coatings is further improved by their annealing at 900 °C for 1 h, though for all the cases except ZnO obtained using the BCP template, this annealing has a negligible effect on the porosity of the material, as is confirmed by the consistency in the optical characteristics. These findings unravel new potential for the materials being used across various environment and temperature conditions. 

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5. The alkali-silica reaction in belitic calcium sulfoaluminate (BCSA) cement concrete

   https://doi.org/10.1016/j.conbuildmat.2025.140726  

   Adnan, Tayyab; Thomas, Robert J (April 2025, Construction and Building Materials)

   This paper studies the alkali-silica reaction (ASR) in rapid-strength belitic calcium sulfoaluminate (BCSA) cement systems. Theoretically, its low alkalinity and high alumina content should make BCSA less prone to ASR than portland cement (PC), but little experimental evidence has been published, and the theorized mechanisms have not been examined critically. We examine this problem using expansion tests, microstructural analysis, and pore solution analysis. Accelerated expansion tests show increased expansion in BCSA mortars with reactive aggregates, but we argue that the test conditions are unsuitable for the cement. Long-term expansion tests show a significant reduction in expansion in BCSA mortars with reactive aggregates, but later-age measurements still exceed ASTM C1778 limits and microstructural investigations indicate ASR damage. Curiously, BCSA mortars with nonreactive aggregates also expanded significantly, but no ASR damage was observed. BCSA pore solutions had ten times more aluminum than PC and one-tenth as much calcium. While the pH was sufficiently high to initiate ASR, the alkali reserves can be half or less than in PC. Overall, BCSA cement is not immune to ASR, but it is more resistant than PC. This is mostly related to the lower alkalinity of the cement and, to a lesser degree, to the abundance of alumina and shortage of soluble calcium. 

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