skip to main content

Title: 3D Printed Stress Sensors for Non-Destructive Evaluation of Space Structures
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 more » 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. « less
Authors:
Award ID(s):
1701983
Publication Date:
NSF-PAR ID:
10314516
Journal Name:
AIAA 2021 Regional Student Conference, April 2021.
Sponsoring Org:
National Science Foundation
More Like this
  1. The future of aerospace structures is highly dependent on the advancement of reliable and high-performance materials, such as composite materials and metals. Innovation in high resolution non-invasive evaluation of these materials is needed for their qualification and monitoring for structural integrity. Aluminum oxide (or α-alumina) nanoparticles present photoluminescent properties that allow stress and damage sensing via photoluminescence piezospectroscopy. This work describes how these nanoparticles are added into a polymer matrix to create functional coatings that monitor the damage of the underlying composite or metallic substrates. Different volume fractions of α-alumina nanoparticles in the piezospectroscopic coatings were studied for determining the sensitivity of the coatings and successful damage detection was demonstrated for an open-hole tension composite substrate as well as 2024 aluminum tensile substrates with a subsurface notch.
  2. Abstract

    Printing functional devices on flexible substrates requires printing of high conductivity metallic patterns. To prevent deformation and damage of the polymeric substrate, the processing (printing) and post-processing (annealing) temperature of the metal patterns must be lower than the glass transition temperature of the substrate. Here, a hybrid process including deposition of a sacrificial blanket thin film, followed by room environment nozzle-based electrodeposition, and subsequent etching of the blanket film is demonstrated to print pure and nanocrystalline metallic (Ni and Cu) patterns on flexible substrates (PI and PET). Microscopy and spectroscopy showed that the printed metal is nanocrystalline, solid with no porosity and with low impurities. Electrical resistivity close to the bulk (~2-time) was obtained without any thermal annealing. Mechanical characterization confirmed excellent cyclic strength of the deposited metal, with limited degradation under high cyclic flexure. Several devices including radio frequency identification (RFID) tag, heater, strain gauge, and temperature sensor are demonstrated.

  3. Recent advancements in additive manufacturing such as Direct Write Inkjet printing introduced novel tools that allow controlled and precise deposition of fluid in nano-liter volumes, enabling fabrication of multiscale structures with submillimeter dimensions. Applications include fabrication of flexible electronics, sensors, and assembly of Micro-Electro-Mechanical Systems (MEMS). Critical challenges remain in the control of fluid deposition parameters during Inkjet printing to meet specific dimensional footprints at the microscale necessary for the assembly process of microscale structures. In this paper we characterize an adhesive deposition printing process with a piezo-electric dispenser of nano-liter volumes. Applications include the controlled delivery of high viscosity Ultraviolet (UV) and thermal curable adhesives for the assembly of the MEMS structures. We applied the Taguchi Design of Experiment (DOE) method to determine an optimal set of process parameters required to minimize the size of adhesive printed features on a silicon substrate with good reliability and repeatability of the deposition process. Experimental results demonstrate repeatable deposition of UV adhesive features with 150 μm diameter on the silicon substrate. Based on the observed wettability effect of adhesive printed onto different substrates we propose a solution for further reduction of the deposit-substrate contact area for microassembly optimization.
  4. Direct write Inkjet Printing is a versatile additive manufacturing technology that allows for the fabrication of multiscale structures with dimensions spanning from nano to cm scale. This is made possible due to the development of novel dispensing tools, enabling controlled and precise deposition of fluid with a wide range of viscosities (1 – 50 000 mPas) in nano-liter volumes. As a result, Inkjet printing has been recognized as a potential low-cost alternative for several established manufacturing methods, including cleanroom fabrication. In this paper, we present a characterization study of PEDOT: PSS polymer ink deposition printing process realized with the help of an automated, custom Direct Write Inkjet system. PEDOT: PSS is a highly conductive ink that possesses good film forming capabilities. Applications thus include printing thin films on flexible substrates for tactile (touch) sensors. We applied the Taguchi Design of Experiment (DOE) method to produce the optimal set of PEDOT:PSS ink dispensing parameters, to study their influence on the resulting ink droplet diameter. We experimentally determined that the desired outcome of a printed thin film with minimum thickness is directly related to 1) the minimum volume of dispensed fluid and 2) the presence of a preprocessing step, namely air plasmamore »treatment of the Kapton substrate. Results show that an ink deposit with a minimum diameter of 482 μm, and a thin film with approximately 300 nm thickness were produced with good repeatability.« less
  5. Tunneling field effect transistors (TFETs) have gained much interest in the previous decade for use in low power CMOS electronics due to their sub-thermal switching [1]. To date, all TFETs are fabricated as vertical nanowires or fins with long, difficult processes resulting in long learning cycle and incompatibility with modern CMOS processing. Because most TFETs are heterojunction TFETs (HJ-TFETs), the geometry of the device is inherently vertically because dictated by the orientation of the tunneling HJ, achieved by typical epitaxy. Template assisted selective epitaxy was demonstrated for vertical nanowires [2] and horizontally arranged nanorods [3] for III-V on Si integration. In this work, we report results on the area selective and template assisted epitaxial growth of InP, utilizing SiO2 based confined structures on InP substrates, which enables horizontal HJs, that can find application in the next generation of TFET devices. The geometries of the confined structures used are so that only a small area of the InP substrate, dubbed seed, is visible to the growth atmosphere. Growth is initiated selectively only at the seed and then proceeds in the hollow channel towards the source hole. As a result, growth resembles epitaxial lateral overgrowth from a single nucleation point [4], reapingmore »the benefits of defect confinement and, contrary to spontaneous nanowire growth, allows orientation in an arbitrary, template defined direction. Indium phosphide 2-inch (110) wafers are used as the starting substrate. The process flow (Fig.1) consists of two plasma enhanced chemical vapor deposition (PECVD) steps of SiO2, appropriately patterned with electron beam lithography (EBL), around a PECVD amorphous silicon sacrificial layer. The sacrificial layer is ultimately wet etched with XeF2 to form the final, channel like template. Not shown in the schematic in Fig.1 is an additional, ALD deposited, 3 nm thick, alumina layer which prevents plasma damage to the starting substrate and is removed via a final tetramethylammonium hydroxide (TMAH) based wet etch. As-processed wafers were then diced and loaded in a Thomas Swan Horizontal reactor. Successful growth conditions found were 600°C with 4E6 mol/min of group III precursor, a V/III ratio of 400 and 8 lpm of hydrogen as carrier gas. Trimethylindium (TMIn) and tertiarybutylphosphine (TBP) were used as In and P precursors respectively. Top view SEM (Fig.2) confirms growth in the template thanks to sufficient Z-contrast despite the top oxide layer, not removed before imaging. TEM imaging shows a cross section of the confined structure taken at the seed hole (Fig.3). The initial growth interface suggests growth was initiated at the seed hole and atomic order of the InP conforms to the SiO2 template both at the seed and at the growth front. A sharp vertical facet is an encouraging result for the future development of vertical HJ based III-V semiconductor devices.« less