United States Air Force Office of Scientific Research

Future generations of NASA and U.S. Air Force vehicles will require lighter mass while being subjected to higher loads and more extreme service conditions over longer time periods than the present generation. Current approaches for certification, fleet management and sustainment are largely based on statistical distributions of material properties, heuristic design philosophies, physical testing and assumed similitude between testing and operational conditions and will likely be unable to address these extreme requirements. To address the shortcomings of conventional approaches, a fundamental paradigm shift is needed. This paradigm shift, the Digital Twin, integrates ultra-high fidelity simulation with the vehicle s on-board integrated vehicle health management system, maintenance history and all available historical and fleet data to mirror the life of its flying twin and enable unprecedented levels of safety and reliability.

High-order neural networks have been shown to have impressive computational, storage, and learning capabilities. This performance is because the order or structure of a high-order neural network can be tailored to the order or structure of a problem. Thus, a neural network designed for a particular class of problems becomes specialized but also very efficient in solving those problems. Furthermore, a priori knowledge, such as geometric invariances, can be encoded in high-order networks. Because this knowledge does not have to be learned, these networks are very efficient in solving problems that utilize this knowledge.

A review is given of recent results in developing improved fabrication processes for ZnO devices with the possible application to UV light emitters, spin functional devices, gas sensors, transparent electronics, and surface acoustic wave devices. There is also interest in integrating ZnO with other wide band-gap semiconductors, such as the AlInGaN system. In this article, we summarize recent progress in controlling n- and p-type doping, materials processing methods, such as ion implantation for doping or isolation, Ohmic and Schottky contact formation, plasma etching, the role of hydrogen in the background n-type conductivity of many ZnO films, and finally, the recent achievement of room-temperature ferromagnetism in transition-metal (Mn or Co)-doped ZnO. This may lead to another class of spintronic devices, in which the spin of the carriers is exploited rather than the charge as in more conventional structures.

A review is given here of recent results in developing improved control of growth, doping, and fabrication processes for ZnO devices with possible applications to ultraviolet (UV) light emitters, spin functional devices, gas sensors, transparent electronics, and surface acoustic wave devices. ZnO can be grown on cheap substrates such as glass at relatively low temperatures and may have advantages over the GaN system in some of these applications.

A review of recent results on transition metal doping of electronic oxides such as ZnO, TiO2, SnO2, BaTiO3, Cu2O, SrTiO3 and KTaO3 is presented. There is interest in achieving ferromagnetism with Curie temperatures above room temperature in such materials for applications in the field of spintronic devices, in which the spin of the carriers is exploited. The incorporation of several atomic per cent of the transition metals without creation of second phases appears possible under optimized synthesis conditions, leading to ferromagnetism. Pulsed laser deposition, reactive sputtering, molecular beam epitaxy and ion implantation have all been used to produce the oxide-based dilute magnetic materials. The mechanism is still under debate, with carrier-induced, double-exchange and bound magnetic polaron formation all potentially playing a role depending on the conductivity type and level in the material.

Laser propulsion has the advantage of acting at the speed of light over great distances. The practical concept originated with Arthur Kantrowitz of AVCO Everett Research Laboratories back in 1972. I will review two quite different types of LP in this talk: LAP (laser ablation propulsion) and PPP (photon pressure propulsion). The first one is good for traveling within our solar system, while the second is the only way we (or at least an instrument) will get far out of our region in the universe to gain a clear perspective on where we are. I will give several examples of practical applications for each. The full realization of PPP depends on the development of a fusion energy economy, in order to get the GW or PW in CW laser power required for best performance. LAP is available now using repetitive pulse lasers with 100ps pulses.

Currently, the performances of thin film solar cells are limited by poor light absorption and carrier collection. In this research, large, broadband, and polarization-insensitive light absorption enhancement was realized via integrating with unique metallic nanogratings. Through simulation, three possible mechanisms were identified to be responsible for such an enormous enhancement. A test for totaling the absorption over the solar spectrum shows an up to approximately 30% broadband absorption enhancement when comparing to bare thin film cells.

This paper provides an overview of the National Jet Fuels Combustion Program led by the Federal Aviation Administration, the U.S. Air Force Research Laboratory, and the NASA. The program follows from basic research from the U.S. Air Force Office of Scientific Research and results from the engine-company-led Combustion Rules and Tools program funded by the U.S. Air Force. The overall objective of this fuels program was to develop combustion-related generic test and modeling capabilities that can improve the understanding of the impact of fuel chemical composition and physical properties on combustion, leading to accelerating the approval process of new alternative jet fuels. In this paper, the motivation and objectives for the work, participating universities, gas-turbine-engine companies, other federal agencies, and international partners are described. This paper provides an in-depth discussion on the benefits to the fuels approval process, the rationale in selecting conventional and alternative fuels to study, the referee rig used for fuel testing, and the modeling approaches. High-level results are also briefly discussed, and will be covered in detail in separate university-led papers. Lastly, an Appendix reviewing past programs, events, and workshops that lay the groundwork for this program is also included for reference.

Abstract Organically modified montmorillonite was synthesized with a novel 1,2‐dimethyl‐3‐ N ‐alkyl imidazolium salt or a typical quaternary ammonium salt as a control. Poly(ethylene terephthalate) montmorillonite clay nanocomposites were compounded via melt‐blending in a corotating mini twin‐screw extruder operating at 285 °C. The nanocomposites were characterized with thermal analysis, X‐ray diffraction, and transmission electron microscopy to determine the extent of intercalation and/or exfoliation present in the system. Nanocomposites produced with N , N ‐dimethyl‐ N , N ‐dioctadecylammonium treated montmorillonite (DMDODA‐MMT), which has a decomposition temperature of 250 °C, were black, brittle, and tarlike resulting from DMDODA degradation under the processing conditions. Nanocomposites compounded with 1,2‐dimethyl‐3‐ N ‐hexadecyl imidazolium treated MMT, which has a decomposition temperature of 350 °C, showed high levels of dispersion and delamination. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2661–2666, 2002

Ninety-six patients who had undergone disc excision and midline spinal fusion and 36 patients who had had simple disc excision had spinal radiographs made 10 or more years postoperatively. Claw spurs were found most commonly at the L2-3 and L3-4 levels in fusion patients, particularly male laborers. Traction spurs with segmental hypermobility were found more commonly at the L4-5 level in patients whose spines were not fused, particularly women. Total lumbar flexion-extension was greater in nonfusion than in fusion patients, but the L1-3 mobility was greater in those who had undergone fusion, suggesting a compensatory increase in the range of lumbar motion. Segmental mobility at levels of surgery in nonfusion patients was similar in those with good and those with poor clinical results. Disc space narrowing was common at levels of operation, but did not correspond to the clinical result. Pseudarthrosis was demonstrated in 26% of fusion patients, but was of no clinical significance. Although complex radiographic changes follow lumbar disc surgery, with or without failure, it is concluded that the plane radiograph is of little aid in determining the source of postoperative pain. The sole exception is that of acquired spondylolysis, which was found in 2.5% of this group of fusion patients, and was clearly associated with a poor clinical outcome. Symptomatic degenerative disc disease at levels above lumbar spinal fusions appears to be an uncommon clinical problem.

Room temperature ionic liquids (RTILs) have emerged as tunable and potentially “greener” solvents for a multitude of applications. To investigate the solvent properties and potential use as a thermal fluid, a study was initiated to determine the effects of anion type, C-2 hydrogen substitution, and alkyl chain length on the flammability, thermal stability, and phase change characteristics of 1,2,3-trialkylimidazolium room temperature ionic liquids. A Setaflash flashpoint apparatus was used to determine the flammabilities of the RTILs. No flashpoints were detected for any of the imidazolium based RTILs below 200 °C, the maximum temperature of the instrument. The thermal stabilities of the RTILs were measured using the technique of thermogravimetric analysis. The 1,2,3-trialkylimidazolium compounds exhibit slightly higher thermal stabilities than the comparable 1,3-dialkylimidazolium compounds; RTILs with nucleophilic anions decompose about 150 °C lower than RTILs with bulky fluoride containing anions; the alkyl chain length does not have a large effect on the thermal stability of the RTILs; and the pyrolysis decomposition exhibits higher thermal stabilities via a different mechanism than the oxidative decomposition. In addition, it was found that although the calculated onset temperatures were above 350 °C, significant decomposition does occur 100 °C or more below these temperatures. The phase change behaviors of several imidazolium based RTILs were characterized by differential scanning calorimetry. The melting points of the RTILs increased with increasing alkyl chain length. Most of the salts studied exhibited significant undercooling, which decreased as the length of the alkyl chain was increased. The hexafluorophosphate and bromide RTILs exhibited polymorphic and liquid crystalline behaviors as the alkyl chain length was increased above C10. The clearing point temperatures increased more rapidly with alkyl chain length than the melting point temperatures.

We report a 50% increase in the power conversion efficiency of InAs/GaAs quantum dot solar cells due to n-doping of the interdot space. The n-doped device was compared with GaAs reference cell, undoped, and p-doped devices. We found that the quantum dots with built-in charge (Q-BIC) enhance electron intersubband quantum dot transitions, suppress fast electron capture processes, and preclude deterioration of the open circuit voltage in the n-doped structures. These factors lead to enhanced harvesting and efficient conversion of IR energy in the Q-BIC solar cells.

A new method for producing long, small-diameter, single- and few-walled, boron nitride nanotubes (BNNTs) in macroscopic quantities is reported. The pressurized vapor/condenser (PVC) method produces, without catalysts, highly crystalline, very long, small-diameter, BNNTs. Palm-sized, cotton-like masses of BNNT raw material were grown by this technique and spun directly into centimeters-long yarn. Nanotube lengths were observed to be 100 times that of those grown by the most closely related method. Self-assembly and growth models for these long BNNTs are discussed.

Design procedures are presented for the compensation of linear multivariable-feedback systems. The design objectives which are noninteraction and low-order compensators are obtained by state-variable feedback. A <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</tex> th order plant with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</tex> inputs and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</tex> outputs where <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q&gt;m</tex> is treated. All the state variables are assumed to be available and measurable. It is shown that it is often possible by state-variable feedback to obtain noninteraction without an increase in system order. When the determinant of the plant transfer-function matrix has right half plane zeros the state-variable feedback method cannot be used to obtain noninteraction since the resulting compensated system would be unstable. It is shown that it is not possible to change these right-half plane zeros by either the well-known transfer-function design methods or by the state-variable feedback method. A combination of both transfer-function and state-variable techniques is discussed and shown to lower the order of the compensators required for the compensation of certain systems.

The Child–Langmuir Law (CL), discovered a century ago, gives the maximum current that can be transported across a planar diode in the steady state. As a quintessential example of the impact of space charge shielding near a charged surface, it is central to the studies of high current diodes, such as high power microwave sources, vacuum microelectronics, electron and ion sources, and high current drivers used in high energy density physics experiments. CL remains a touchstone of fundamental sheath physics, including contemporary studies of nanoscale quantum diodes and nano gap based plasmonic devices. Its solid state analog is the Mott–Gurney law, governing the maximum charge injection in solids, such as organic materials and other dielectrics, which is important to energy devices, such as solar cells and light emitting diodes. This paper reviews the important advances in the physics of diodes since the discovery of CL, including virtual cathode formation and extension of CL to multiple dimensions, to the quantum regime, and to ultrafast processes. We review the influence of magnetic fields, multiple species in bipolar flow, electromagnetic and time dependent effects in both short pulse and high frequency THz limits, and single electron regimes. Transitions from various emission mechanisms (thermionic-, field-, and photoemission) to the space charge limited state (CL) will be addressed, especially highlighting the important simulation and experimental developments in selected contemporary areas of study. We stress the fundamental physical links between the physics of beams to limiting currents in other areas, such as low temperature plasmas, laser plasmas, and space propulsion.

Abstract Well‐dispersed multiwalled carbon nanotube (MWNT)/polystyrene nanocomposites have been prepared via melt extrusion, using trialkylimidazolium tetrafluoroborate‐compatibilized MWNTs. Quantification of the improvement is realized via transmission electron microscopy and laser scanning confocal microscopy image analysis. Differential scanning calorimetry and Fourier‐transform infrared and X‐ray diffraction analysis show evidence for a π‐cation, nanotube–imidazolium interaction and the conversion from an interdigitated bilayer, for the imidazolium salt, to an ordered lamellar structure, for the imidazolium on the surface of the MWNTs.

ZnO has gained considerable interest recently as a promising material for a variety of applications. To a large extent, the renewed interest in ZnO is fuelled by its wide direct band gap (3.3 eV at room temperature) and large exciton binding energy (60 meV) making this material, when alloyed with, e.g., Cd and Mg, especially attractive for light emitters in the blue/ultraviolet (UV) spectral region. Unfortunately, as with other wide-gap semiconductors, ZnO suffers from the doping asymmetry problem, in that the n-type conductivity can be obtained rather easily, but p-type doping proved to be a formidable challenge. This doping asymmetry problem (also dubbed as the p-type problem in ZnO) is preventing applications of ZnO in light-emitting diodes and potential laser diodes. In this paper, we provide a critical review of the current experimental efforts focused on achieving p-type ZnO and discuss the proposed approaches which could possibly be used to overcome the p-type problem.

Natural fiber welded (NFW) yarns embedded with porous carbon ­materials are described for applications as electrodes in textile electrochemical capacitors. With this fabrication technique, many kinds of carbons can be embedded into cellulose based yarns and subsequently knitted into full ­fabrics on industrial knitting machines. Yarns welded with carbon and ­stainless steel have device capacitances as high as 37 mF cm ‐1 , one of the highest reported values for carbon‐based yarns. The versatility of this ­technique to weld any commercially available cellulose yarn with any ­micro‐ or nanocarbon means properties can be tuned for specific applications. Most importantly, it is found that despite having full flexibility, increased strength, and good electrochemical performance, not all of the electrode yarns are ­suitable for knitting. Therefore, it is recommended that all works reporting on fiber/yarn capacitors for wearables attempt processing into full fabrics.

Situational awareness (SA) is defined as the perception of entities in the environment, comprehension of their meaning, and projection of their status in near future. From an Air Force perspective, SA refers to the capability to comprehend and project the current and future disposition of red and blue aircraft and surface threats within an airspace. In this article, we propose a model for SA and dynamic decision-making that incorporates artificial intelligence and dynamic data-driven application systems to adapt measurements and resources in accordance with changing situations. We discuss measurement of SA and the challenges associated with quantification of SA. We then elaborate a plethora of techniques and technologies that help improve SA ranging from different modes of intelligence gathering to artificial intelligence to automated vision systems. We then present different application domains of SA including battlefield, gray zone warfare, military- and air-base, homeland security and defense, and critical infrastructure. Finally, we conclude the article with insights into the future of SA.

During Fusion 2019 Conference (https://www.fusion2019.org/program.html), leading experts presented ideas on the historical, contemporary, and future coordination of artificial intelligence/machine learning (AI/ML) with sensor data fusion (SDF). While AI/ML and SDF concepts have had a rich history since the early 1900s—emerging from philosophy and psychology—it was not until the dawn of computers that both AI/ML and SDF researchers initiated discussions on how mathematical techniques could be implemented for real-time analysis. ML, and in particular deep learning, has demonstrated tremendous success in computer vision, natural language understanding, and data analytics. As a result, ML has been proposed as the solution to many problems that inherently include multi-modal data. For example, success in autonomous vehicles has validated the promise of ML with SDF, but additional research is needed to explain, understand, and coordinate heterogeneous data analytics for situation awareness. The panel identified opportunities for merging AI/ML and SDF such as computational efficiency, improved decision making, expanding knowledge, and providing security; while highlighting challenges for multi-domain operations, human-machine teaming, and ethical deployment strategies.