Field emission-driven microdischarges

For over half a century, it has been known that the onset of field emission can lead to anomalous breakdown for electrode gaps spaced \u3c10 μm apart. In adddition, field emission can serve as a substantial source of primary electrons in a microdischarge operating well below the breakdown threshold at atmospheric pressure. To better understand the role of field emission in a microdischarge, we have developed kinetic simulations and fluid models. Theoretical results from both our simulations and models have shown that the anomalous breakdown at the microscale is due to ion-enhanced field emission. In addition, they predict an entirely new prebreakdown regime – the field emission-driven microdischarge – in which field-emitted electrons interact with the surrounding gas to produce ions and excited metastables. Experiments conducted in our lab agree well with the predictions of our simulations and fluid model for parallel plate electrode spacings ranging from 4.8 to 14 μm

2014 Workshop on Thermionic Energy Conversion for Space and Earth

No abstract availabl

Thermionic Properties of Carbon Based Nanomaterials Produced by Microhollow Cathode PECVD

Thermionic emission is the process in which materials at sufficiently high temperature spontaneously emit electrons. This process occurs when electrons in a material gain sufficient thermal energy from heating to overcome the material's potential barrier, referred to as the work function. For most bulk materials very high temperatures (greater than 1500 K) are needed to produce appreciable emission. Carbonbased nanomaterials have shown significant promise as emission materials because of their low work functions, nanoscale geometry, and negative electron affinity. One method of producing these materials is through the process known as microhollow cathode PECVD. In a microhollow cathode plasma, high energy electrons oscillate at very high energies through the Pendel effect. These high energy electrons create numerous radical species and the technique has been shown to be an effective method of growing carbon based nanomaterials. In this work, we explore the thermionic emission properties of carbon based nanomaterials produced by microhollow cathode PECVD under a variety of synthesis conditions. Initial studies demonstrate measureable current at low temperatures (approximately 800 K) and work functions (approximately 3.3 eV) for these materials

Thermionic Energy Conversion in the Twenty-First Century: Advances and Opportunities for Space and Terrestrial Applications

Thermionic energy conversion (TEC) is the direct conversion of heat into electricity by the mechanism of thermionic emission, the spontaneous ejection of hot electrons from a surface. Although the physical mechanism has been known for over a century, it has yet to be consistently realized in a manner practical for large-scale deployment. This perspective article provides an assessment of the potential of TEC systems for space and terrestrial applications in the twenty-first century, overviewing recent advances in the field and identifying key research challenges. Recent developments as well as persisting research needs in materials, device design, fundamental understanding, and testing and validation are discussed

Part-Scale Model for Fast Prediction of Thermal Distortion in DMLS Additive Manufacturing Part 2: A Quasi-Static Thermomechanical Model

The direct metal laser sintering (DMLS) additive manufacturing process can quickly produce complex parts with excellent mechanical properties. However, thermal stress accumulated in the layer-by-layer build cycles of DMLS may induce part distortion and even cause the failure of the whole build process. This paper is the second part of two companion papers that present a part-scale model for fast prediction of temperature history and part distortion in DMLS. In this paper, a quasi-static thermomechanical (QTM) model is built to estimate the thermal distortion of entire parts in DMLS. Firstly, the thermal contraction in each build cycle is modeled as a quasi-static loading process; the final thermal stress accumulated in the parts is the superposition of thermal stress generated in each build cycle. Secondly, the stress relaxation process after the parts are cut off from the substrate is modeled, and final distortion of the parts is predicted with thermal stress calculated from the thermal contraction processes. In comparison to existing transient thermomechanical models, the QTM can predict thermal distortion in DMLS with much faster computational speed, and a comparison against experiment shows less than 10% error.Mechanical Engineerin

Fast Prediction Of Thermal Distortion In Metal Powder Bed Fusion Additive Manufacturing: Part 2, A Quasi-static Thermo-mechanical Model

The additive manufacturing (AM) process metal powder bed fusion (PBF) can quickly produce complex parts with mechanical properties comparable to that of wrought materials. However, thermal stress accumulated during Metal PBF may induce part distortion and even cause failure of the entire process. This manuscript is the second part of two companion manuscripts that collectively present a part-scale simulation method for fast prediction of thermal distortion in Metal PBF. The first part provides a fast prediction of the temperature history in the part via a thermal circuit network (TCN) model. This second part uses the temperature history from the TCN to inform a model of thermal distortion using a quasi-static thermo-mechanical model (QTM). The QTM model distinguished two periods of Metal PBF, the thermal loading period and the stress relaxation period. In the thermal loading period, the layer-by-layer build cycles of Metal PBF are simulated, and the thermal stress accumulated in the build process is predicted. In the stress relaxation period, the removal of parts from the substrate is simulated, and the off-substrate part distortion and residual stress are predicted. Validation of part distortion predicted by the QTM model against both experiment and data in literature showed a relative error less than 20%. This QTM, together with the TCN, offers a framework for rapid, part-scale simulations of Metal PBF that can be used to optimize the build process and parameters

Fast Prediction Of Thermal Distortion In Metal Powder Bed Fusion Additive Manufacturing: Part 1, A Thermal Circuit Network Model

The additive manufacturing (AM) process metal powder bed fusion (PBF) can quickly produce complex parts with mechanical properties comparable to wrought materials. However, thermal stress accumulated during PBF induces part distortion, potentially yielding parts out of specification and frequently process failure. This manuscript is the first of two companion manuscripts that introduce a computationally efficient distortion and stress prediction algorithm that is designed to drastically reduce compute time when integrated in to a process design optimization routine. In this first manuscript, we introduce a thermal circuit network (TCN) model to estimate the part temperature history during PBF, a major computational bottleneck in PBF simulation. In the TCN model, we are modeling conductive heat transfer through both the part and support structure by dividing the part into thermal circuit elements (TCEs), which consists of thermal nodes represented by thermal capacitances that are connected by resistors, and then building the TCN in a layer-by-layer manner to replicate the PBF process. In comparison to conventional finite element method (FEM) thermal modeling, the TCN model predicts the temperature history of metal PBF AM parts with more than two orders of magnitude faster computational speed, while sacrificing less than 15% accuracy. The companion manuscript illustrates how the temperature history is integrated into a thermomechanical model to predict thermal stress and distortion

Foundations of plasma standards

The field of low-temperature plasmas (LTPs) excels by virtue of its broad intellectual diversity, interdisciplinarity and range of applications. This great diversity also challenges researchers in communicating the outcomes of their investigations, as common practices and expectations for reporting vary widely in the many disciplines that either fall under the LTP umbrella or interact closely with LTP topics. These challenges encompass comparing measurements made in different laboratories, exchanging and sharing computer models, enabling reproducibility in experiments and computations using traceable and transparent methods and data, establishing metrics for reliability, and in translating fundamental findings to practice. In this paper, we address these challenges from the perspective of LTP standards for measurements, diagnostics, computations, reporting and plasma sources. This discussion on standards, or recommended best practices, and in some cases suggestions for standards or best practices, has the goal of improving communication, reproducibility and transparency within the LTP field and fields allied with LTPs. This discussion also acknowledges that standards and best practices, either recommended or at some point enforced, are ultimately a matter of judgment. These standards and recommended practices should not limit innovation nor prevent research breakthroughs from having real-time impact. Ultimately, the goal of our research community is to advance the entire LTP field and the many applications it touches through a shared set of expectations

On the stability of renormalizable expansions in three-dimensional gravity

Preliminary investigations are made for the stability of the $1/N$ expansion in three-dimensional gravity coupled to various matter fields, which are power-counting renormalizable. For unitary matters, a tachyonic pole appears in the spin-2 part of the leading graviton propagator, which implies the unstable flat space-time, unless the higher-derivative terms are introduced. As another possibility to avoid this spin-2 tachyon, we propose Einstein gravity coupled to non-unitary matters. It turns out that a tachyon appears in the spin-0 or -1 part for any linear gauges in this case, but it can be removed if non-minimally coupled scalars are included. We suggest an interesting model which may be stable and possess an ultraviolet fixed point.Comment: 32 pages. (A further discussion to avoid tachyons is included. To be Published in Physical Review D.