Kansas City National Security Campus

Laser powder bed fusion (LPBF) is a 3D printing technology that can print metal parts with complex geometries without the design constraints of traditional manufacturing routes. However, the parts printed by LPBF normally contain many more pores than those made by conventional methods, which severely deteriorates their properties. Here, by combining in-situ high-speed high-resolution synchrotron x-ray imaging experiments and multi-physics modeling, we unveil the dynamics and mechanisms of pore motion and elimination in the LPBF process. We find that the high thermocapillary force, induced by the high temperature gradient in the laser interaction region, can rapidly eliminate pores from the melt pool during the LPBF process. The thermocapillary force driven pore elimination mechanism revealed here may guide the development of 3D printing approaches to achieve pore-free 3D printing of metals.

Size and shape of a melt pool play a critical role in determining the microstructure in additively manufactured metals. However, it is very challenging to directly characterize the size and shape of the melt pool beneath the surface of the melt pool during the additive manufacturing process. Here, we report the direct observation and quantification of melt pool variation during the laser powder bed fusion (LPBF) additive manufacturing process under constant input energy density by in-situ high-speed high-energy x-ray imaging. We show that the melt pool can undergo different melting regimes and both the melt pool dimension and melt pool volume can have orders-of-magnitude change under a constant input energy density. Our analysis shows that the significant melt pool variation cannot be solely explained by the energy dissipation rate. We found that energy absorption changes significantly under a constant input energy density, which is another important cause of melt pool variation. Our further analysis reveals that the significant change in energy absorption originates from the separate roles of laser power and scan speed in depression zone development. The results reported here are important for understanding the laser powder bed fusion additive manufacturing process and guiding the development of better metrics for processing parameter design.

Powder spreading is a key step in the powder-bed-based additive manufacturing process, which determines the quality of the powder bed and, consequently, affects the quality of the manufactured part. However, powder spreading behavior under additive manufacturing condition is still not clear, largely because of the lack of particle-scale experimental study. Here, we studied particle-scale powder dynamics during the powder spreading process by using in-situ high-speed high-energy x-ray imaging. Evolution of the repose angle, slope surface speed, slope surface roughness, and the dynamics of powder clusters at the powder front were revealed and quantified. Interactions of the individual metal powders, with boundaries (substrate and container wall), were characterized, and coefficients of friction between the powders and boundaries were calculated. The effects of particle size on powder flow dynamics were revealed. The particle-scale powder spreading dynamics, reported here, are important for a thorough understanding of powder spreading behavior in the powder-bed-based additive manufacturing process, and are critical to the development and validation of models that can more accurately predict powder spreading behavior.

Heusler alloys have been a significant topic of research due to their unique electronic structure, which exhibits half-metallicity, and a wide variety of properties such as magneto-calorics, thermoelectrics, and magnetic shape memory effects. As the maturity of these materials grows and commercial applications become more near-term, the mechanical properties of these materials become an important factor to both their processing as well as their final use. Very few studies have experimentally investigated mechanical properties, but those that exist are reviewed within the context of their magnetic performance and application space with specific focus on elastic properties, hardness and strength, and fracture toughness and ductility. A significant portion of research in Heusler alloys are theoretical in nature and many attempt to provide a basic view of elastic properties and distinguish between expectations of ductile or brittle behavior. While the ease of generating data through atomistic methods provides an opportunity for wide reaching comparison of various conceptual alloys, the lack of experimental validation may be leading to incorrect conclusions regarding their mechanical behavior. The observed disconnect between the few available experimental results and the numerous modeling results highlights the need for more experimental work in this area.

Visual inspection research has a long history spanning the 20 th century and continuing to the present day. Current efforts in multiple venues demonstrate that visual inspection continues to have a vital role for many different types of tasks in the 21 st century. The nature of this role spans the range from traditional human visual inspection to fully automated detection of defects. Consequently, today’s practitioners must not only successfully identify and apply lessons learned from the past, but also explore new areas of research in order to derive solutions for modern day issues such as those presented by introducing automation during inspection. A key lesson from past research indicates that the factors that can degrade performance will persist today, unless care is taken to design the inspection process appropriately.

An ammonium metatungstate precursor (WO3Pr) ink was printed for tungsten oxide (WO3) UV detectors on SiO2/Si wafers with prefabricated Au electrodes. A systematic study was carried out on the printing parameters including substrate temperatures in the range of 22–80 °C, WO3Pr molar concentrations of 0.01, 0.02, and 0.03 M, and printing scan numbers up to 7 to understand their effects on the resulted WO3 film morphology and optoelectronic properties. It has been found that the printing parameters can sensitively affect the WO3 film morphology, which in turn impacts the WO3 photodetector performance. In particular, the printed films experienced a systematic change from discontinuous droplets at below 40 °C to continuous films at 40–60 °C of the substrate temperature. At higher temperatures, the excessive heat from the substrate not only caused drastic evaporation of the printed ink, resulting in highly nonuniform films, but also detrimental heating of the ink in the printer nozzle in proximity of the substrate, preventing continuous printing operation. An optimal printing window of the substrate temperature of 45–55 °C at a molar concentration of 0.02 M of ammonium metatungstate and three printing scans was obtained for the best UV detector performance. A large on/off ratio of 3538 and a high responsivity up to 2.70 A/W at 5 V bias (0.54 A/W·V) represent a significant improvement over the best report of ∼0.28 μA/W·V on WOX photodetectors, which indicates that the printed WO3 films are promising for various applications of optoelectronics and sensors.

Abstract With recent advances of additive manufacturing technology, direct ink write (DIW) printing has allowed to incorporate multi-material printing of various materials with freedom of design and complex geometric shapes to complete functional sensors in a one-step fabrication. This paper introduces the use of DIW 3D printing of polydimethylsiloxane (PDMS) with barium titanate (BTO) filler as stretchable composites with tunable piezoelectric properties that can be used for force sensors applications. To improve the bonding between stretchable piezoelectric composites and electrodes, multi-walled carbon nanotubes was included in the fabrication of electrodes at a fixed ratio of 11 wt. %. The alignment of the BTO dipoles was achieved through corona poling method, which applies an electric charge on the surface layer of the functional material, aligning the dipoles in the desired direction and thus gaining the piezoelectricity. Different BTO mixing ratios (10–50 wt. %) were evaluated in order to obtain tunable piezoelectric properties and compare the sensitivity with respect their elastic properties. Tensile testing and piezoelectric testing were carried out to characterize mechanical and piezoelectric properties. Results showed that fabricated PDMS with 50 wt. % BTO gave the highest piezoelectric coefficient ( d 33 ) of 11.5 pC N −1 and with an output voltage of 385 mV under compression loading of >200 lbF. This demonstrates feasibility of using multi-material DIW printing to fabricate piezoelectric force sensors with integrated electrodes in one-step without compromising the flexibility of the material.

We have developed a pulsed optically pumped magnetometer (OPM) array for detecting magnetic field maps originated from an arbitrary current distribution. The presented magnetic source imaging (MSI) system features 24-OPM channels has a data rate of 500 S/s, a sensitivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.8~\mathrm {pT/}\sqrt {\mathrm {Hz}} $ </tex-math></inline-formula> , and a dynamic range of 72 dB. We have employed our pulsed-OPM MSI system for measuring the magnetic field map of a test coil structure. The coils are moved across the array in an indexed fashion to measure the magnetic field over an area larger than the array. The captured magnetic field maps show excellent agreement with the simulation results. Assuming a 2-D current distribution, we have solved the inverse problem using the measured magnetic field maps, and the reconstructed current distribution image is compared with that of the simulation.

Malicious modifications to printed circuit boards (PCBs) are known as hardware Trojans. These may arise when malafide third parties alter PCBs premanufacturing or postmanufacturing and are a concern in safety-critical applications, such as industrial control systems. In this research, we examine how data-driven detection can be utilized to detect such Trojans at run-time. We develop a flexible and reconfigurable PCB test bed derived from the popular open-source programmable logic controller (PLC) platform “OpenPLC.” We then develop a Trojan detection framework, which utilizes and analyzes multimodal side channels (e.g., timing, magnetic signals, power, and hardware performance counters). We consider defender-configurable input/output (I/O) loopback test, comparison with design-document baselines, and magnetometer-aided monitoring of system behavior under defender-chosen excitations. Our approach can extend to golden-free environments. Golden (known-good) versions of the PCBs are assumed not available, but design information, datasheets, and component-level data are available. We demonstrate the efficacy of our approach on a range of Trojans instantiated in the test bed.

A nanohybrid piezoelectric strain sensor was fabricated by growing vertically aligned (0001)-oriented crystalline zinc oxide nanowires directly on graphene (ZnO-VANWs/Gr) using a facile seedless hydrothermal process. Under mechanical strains, the induced piezoelectric effect on the ZnO-VANWs transduces to a piezoelectric gating effect at the ZnO-VANWs/Gr interface, resulting in a modulation of the conductivity of the Gr channel through electrostatic doping. The vertical alignment of the (0001)-oriented ZnO-VANWs on Gr is ideal to achieving high strain sensitivity, and a low-defect ZnO-VANWs/Gr interface obtained in the seedless hydrothermal process is key to realizing high sensitivity and fast response. Indeed, a high sensitivity up to 3.15 × 10–2 kPa–1 was obtained on the ZnO-VANWs/Gr strain sensors at lower pressures of 1.1 × 10–6–11 Torr, together with a fast response time of ∼0.10 s. In particular, these results represent enhancement factors of ∼7 and 8, respectively, as compared to strain sensors of a similar structure, except having a polycrystalline ZnO seed layer on Gr for the growth of ZnO-VANWs. Therefore, our result illustrates the critical importance of the low-defect interface of the ZnO-VANWs with Gr formed in the seedless ZnO-VANW growth for realizing an optimal electrostatic gating of Gr. In addition, the ZnO-VANWs/Gr nanohybrids can be readily scaled up using the seedless hydrothermal process for commercial applications in optoelectronics and sensors.

Abstract A dual‐phase high‐entropy boride (HEB)/carbide (HEC) ceramic with a fine grain size was synthesized by a sequential boro/carbothermal process. In the first step, an Hf–Nb–Ta–Ti–Zr‐containing carbide was synthesized by a carbothermal reduction of oxides followed by the reaction of the carbide with B 4 C and ZrH 2 to convert part of the carbide to boride. The resulting composition was ∼29 vol% HEB with an average grain size of ∼1.1 μm. Solid solution formation occurred at the densification temperature of 1900°C resulting in a relative density higher than 99%. The Vickers hardness was 26.5 ± 1.4 GPa. This is the first report of synthesizing dual‐phase boride–carbide high‐entropy ceramics from carbothermally synthesized, HEC powders.

Cyanate esters (CEs) are an important class of materials among high-temperature-performance thermosets. They are used in aerospace launch vehicles, heat sinks, booms, trusses of satellites, etc., due to their high glass transition temperatures (>220 °C), excellent thermal stability, and low flammability. Current approaches to improve the thermal stability of CEs include incorporation of siloxanes or phosphorus-based flame retardants. In this work, we have explored boron-based hydroxy (PD)- and epoxy (EP)-functionalized carborane additives to improve the thermal properties of CEs. Carborane fillers were solvent-blended at various mass loadings in the resin and cured to study their effect on thermal properties. PD and EP carboranes react with CEs to form iminocarbonates and oxazolidinone linkages, respectively. Cure kinetic studies at different wt % loadings explained that carboranes catalyze the curing reaction by reducing the curing activation energy by about 54 and 26% for 10 wt % loadings of PD and EP carboranes, respectively. In addition, carborane-filled CE nanocomposites demonstrate an exceptionally high thermal stability as compared to the pristine resin in air and inert environments. Our thermogravimetric analysis (TGA) experiments show that the ultimate char yield of the resin can be increased from 0% to as high as 76 and 82% with 30 wt % PD and EP carborane loadings, respectively, at 1000 °C in air. The initial degradation temperature Td,5 of the composites decreased with increasing carborane loadings in both air and argon. For instance, Td,5 values for CE were 465 and 471.6 °C in argon and air, while those for P20 were 437.4 and 452.1 °C, respectively. Modulated TGA studies gave evidence of the effect of carboranes on degradation mechanism and kinetics in air and inert environments. The effect of bonding between carboranes and CEs at various loadings on the thermal expansion of the matrix was also studied using a thermomechanical analyzer. PD carborane reduced the Tg for P20 to about 225 °C, while CE had Tg > 350 °C.

Sodium ion batteries with Na–Mn–O compounds as cathodes have been widely studied as substitutes for lithium ion batteries due to their abundant resources. However, the relatively poor cycling stability and low capacity of Na–Mn–O compounds significantly limit their applications. Different approaches, including element substitution and surface modification, have been applied to improve the electrochemical performance of those cathode materials. Herein, element doping and coating of ZrO2 on Na0.7MnO2 particles have been achieved by atomic layer deposition followed by post-annealing. The rate capability and cycling stability of the modified materials were significantly improved, and the mechanism of performance enhancement was revealed. The ZrO2 coatings acted as a stable interfacial layer to enhance the cycling stability of Na0.7MnO2 by suppressing side reactions between the electrode and electrolyte. The doping of transition metal ions reduced energy barriers for sodium ion insertion and deintercalation during charge/discharge cycling, further improving the charge/discharge capacity and rate performance of Na0.7MnO2.

Abstract Lithium‐ion batteries (LIBs) are widely used in consumer electronics due to their rechargeability and high energy density. Commercial LIBs are fabricated in fixed geometries such as cylinder, coin, and pouch. However, for specialized applications such as wearable electronics and on‐device power systems, customizable LIBs with arbitrary geometry on three‐dimensional (3D) structures need to be developed. For this purpose, aerosol printing is uniquely suitable due to its flexible working distance, allowing deposition on nonplanar substrates with multiscale surface topologies. Aerosol printing of LiFePO 4 cathodes and Li 4 Ti 5 O 12 anodes for LIBs is presented. Electrodes with an arbitrary geometry, tailorable thickness and on nonplanar substrates can be realized. The highest areal capacity achieved is ≈7.1 mAh cm −2 , which is at least twice that of conventional electrodes. Furthermore, to package the printed electrodes, 3D enclosures are fabricated via fused deposition modeling of polyvinylidene fluoride. The printed electrodes packaged in 3D enclosures exhibit 78.4% capacity retention after 30 cycles. With the two additive manufacturing processes, customizable LIBs on targeted objects can be realized. A nonplanar LIB conformably covering the edge of a block with specific capacity of 135 mAh g −1 is demonstrated.

We present an ultrawideband (UWB) frequency-modulated (FM) microwave radar instrument capable of preserving cm-scale vertical resolution from a very long range (verified up to ~6 km). The system can operate either as a conventional continuous-wave FM radar (2-18 GHz) or in time-delayed stretch processing mode (2-14 GHz), providing high sensitivity, fine range resolution, and very low-range sidelobes. We present an overview of the system and laboratory test results to validate its performance. As a practical demonstration of its capabilities, we tested the system's ability to map snow cover thickness on the Arctic marine ice. We present sample results from airborne measurements carried out at altitudes between ~500 m and ~6 km above the ground level onboard a long-range fixed-wing platform.

Reducing the surface roughness of an additively manufactured (AM) component is one of the most critical factors in determining the suitability of an AM component. As produced surface roughness of an AM component is very high. This prohibits the direct utilization of AM components for the intended applications. For most of the engineering applications, surface roughness must be reduced significantly. Reducing surface roughness is exponentially more challenging for the internal surfaces of a component. This paper reports research in the area of post processing interior surfaces of an AM component. Electropolishing and chemical polishing (chempolishing) methods were applied to reduce the surface roughness of the internal surface. It was found that chempolishing was very effective in simultaneously reducing the internal and external surface roughness of steel AM components for any complicated AM shape and geometry. The electropolishing methodology employed was very effective in reducing the surface roughness of the internal or external surfaces as long as a counter electrode could be positioned in the proximity of the surface to be polished. However, electropolishing produced better performance on the outer surfaces as compared to chempolishing. This paper summarizes research efforts to tackle the critical issue of reducing the surface roughness of complex AM components.

Laser powder bed fusion (LPBF) is an additive manufacturing technology with the capability of printing complex metal parts directly from digital models. Between two available emission modes employed in LPBF printing systems, pulsed wave (PW) emission provides more control over the heat input compared to continuous wave (CW) emission, which is highly beneficial for printing parts with intricate features. However, parts printed with pulsed wave LPBF (PW-LPBF) commonly contain pores, which degrade their mechanical properties. In this study, we reveal pore formation mechanisms during PW-LPBF in real time by using an in-situ high-speed synchrotron x-ray imaging technique. We found that vapor depression collapse proceeds when the laser irradiation stops within one pulse, resulting in occasional pore formation during PW-LPBF. We also revealed that the melt ejection and rapid melt pool solidification during pulsed-wave laser melting resulted in cavity formation and subsequent formation of a pore pattern in the melted track. The pore formation dynamics revealed here may provide guidance on developing pore elimination approaches.

Recoil pressure is a critical factor affecting the melt pool dynamics during Laser Powder Bed Fusion (LPBF) processes. Recoil pressure depresses the melt pool. When the recoil pressure is low, thermal conduction and capillary forces may be inadequate to provide proper fusion between layers. However, excessive recoil pressure can produce a keyhole inside the melt pool, which is associated with gas porosity. Direct recoil pressure measurements are challenging because it is localized over an area proportionate to the laser spot size producing a force in the mN range. This paper reports a vibration-based approach to quantify the recoil force exerted on a part in a commercial LPBF machine. The measured recoil force is consistent with estimates from high speed synchrotron imaging of entrained particles, and the results show that the recoil force scales with applied laser power and is inversely related to the laser scan speed. These results facilitate further studies of melt pool dynamics and have the potential to aid process development for new materials.

The dissolution of nanoparticles, particularly those containing boron, is an important area of interest for polymer nanocomposite formation and material development. In this work, the solubility of boron nitride nanotubes (BNNT), functionalized boron nitride nanotubes (FBNNT), and boron nitride sheets (BN-ZG) is quantified in toluene and THF with static light scattering, refractometry, UV–vis spectroscopy, and physical observations. UV–vis spectroscopy provides a method to determine the concentration and solubility limits of the solutions tested. Using light scattering, the second virial coefficient, A2, is determined and used to calculate χ, the solute–solvent interaction parameter. The Hildebrand solubility parameter, δ, is then extracted from this data using the Hildebrand–Scatchard Solution Theory. A list of potential good solvents based on the estimated δ value is provided for each nanoparticle. Single-walled carbon nanotubes (SWNTs) and prepolymers (EN4 and EN8) used to synthesize polyurethanes were also tested, because the published δ and molar attraction constants of these materials provided a self-consistent check. The dn/dc of SWNTs and boron-containing particles was measured for the first time in this work. A solvent screen for BN-ZG provides additional information that supports the obtained δ and χ. Three systems were found to have χ values below 0.5 and were thermodynamically soluble: BNNT in THF, EN8 in THF, and EN8 in toluene.

, high sensitivity of the gauge factors up to ∼248 and response times of 0.20 s/0.20 s (rise/fall) were achieved on the ZnO-VANWs/Gr/PET strain sensors. Moreover, the response changes polarity when the directions of bending alters between up and down, corresponding to the polarity change of the space charge on the ZnO-VANWs/Gr interface as a consequence of the compressive and tensile strains along the ZnO-VANWs. This result shows that the low-cost and scalable ZnO-VANWs/Gr/PET strain sensors are promising for applications in stress/strain monitoring, wearable electronics, and touch screens.