Monte Carlo modeling of low-energy electron-induced secondary electron emission yields in micro-architected boron nitride surfaces

Surface erosion and secondary electron emission (SEE) have been identified as the most critical life-limiting factors in channel walls of Hall-effect thrusters for space propulsion. Recent wall concepts based on micro-architected surfaces have been proposed to mitigate surface erosion and SEE. The idea behind these designs is to take advantage of very-high surface-to-volume ratios to reduce SEE and ion erosion by internal trapping and redeposition. This has resulted in renewed interest to study electron-electron processes in relevant thruster wall materials. In this work, we present calculations of SEE yields in micro-porous hexagonal BN surfaces using stochastic simulations of electron-material interactions in discretized surface geometries. Our model consists of two complementary parts. First we study SEE as a function of primary electron energy and incidence angle in flat surfaces using Monte Carlo simulations of electron multi-scattering processes. The results are then used to represent the response function of discrete surface elements to individual electron rays generated using a ray-tracing Monte Carlo model. We find that micro-porous surfaces result in SEE yield reductions of over 50% in the energy range experienced in Hall thrusters. This points to the suitability of these micro-architected surface concepts to mitigate SEE-related issues in compact electric propulsion devices

El mundo caótico del director(a) de un programa de enfermería

Aim: The environment of the nursing program dean or director within a community college or state university can be politically, fiscally, and emotionally challenging. There are few studies that investigate that environment. The purpose of this study was to describe the major barriers and incentives facing these nursing deans or directors as they implemented their proposed interventions related to the Central Valley Nursing Diversity Project. Additionally, we sought to identify successful strategies used to keep the programs competitive for resources and status within their institutions and within their local communities. Methodology: The study is descriptive; the data collection method was structured interviews and data were analyzed using content analysis. Findings: Findings indicate that among the most difficult barriers faced by the directors and the faculty was the over subscribed status (more applicants than positions) of the programs. The deans or directors described three significant points that acted as barriers. These were 1) limited space in science laboratory pre-requisite courses, 2) limited classroom space in nursing courses, and 3) limited space in clinical (hospital) sites. The largest single external pressure reported was the reduction in funding and all deans or directors indicated they had difficulty hiring qualified or credentialed faculty. Conclusion: Colleges must manage more effectively student demand by modifying admissions criteria to be more selective and admit students with greater likelihood of graduating; encourage innovative partnerships between employers and schools of nursing; and increasing funding for nursing faculty salaries, classrooms, and laboratories. Objetivo: El ambiente de un(a) decano(a) o director(a) o de un Programa de Enfermería en un colegio comunitario o una universidad estatal puede ser un reto político, físico y emocional. Hay pocos estudios que investiguen ese ambiente. El propósito de este estudio fue describir las principales barreras e incentivos que enfrentan las(os) decanas(os) o directoras(es) al implementar las intervenciones del Proyecto de Diversidad en Enfermería en el Valle Central. Adicionalmente, se identificaron estrategias exitosas para mantener competitivos a los programas en cuanto a recursos y estatus en las comunidades locales. Metodología: El estudio es descriptivo; la información se recolectó mediante entrevistas estructuradas y fue analizada mediante análisis de contenido. Hallazgos: La barrera más difícil fue la demanda exagerada de los programas. Las(os) decanas(os) o directoras(es) describieron tres barreras significativas: 1) espacio limitado en laboratorios de ciencias para los cursos pre-requisito, 2) espacio limitado en los salones de clase para los cursos de enfermería, 3) espacio limitado en los sitios de práctica clínica. La principal presión externa fue la reducción de financiación, lo que produce dificultades para conseguir docentes calificados. Conclusión: Las instituciones deben manejar más efectivamente la demanda estudiantil, modificar los criterios de admisión para hacerlos más selectivos y admitir estudiantes con mayor posibilidad de graduarse; promover alianzas innovadoras entre empleadores y escuelas de enfermería e incrementar la financiación para salarios de docentes, aulas de clase y laboratorios

Relativistic analysis of application of Helmholtz theorem to classical electrodynamics

In this work we discuss the relationship between the instantaneous-action-at-a-distance solutions of Maxwells equations obtained using Helmholtz theorem and the Lorentzs invariant solutions of the same equations obtained using Special Relativity postulates. We show that Special Relativity postulates are not consistent with Helmholtzs theorem in the presence of charges and currents, but in the vacuum, without charges and currents, Helmholtzs theorem and Special Relativity agree because the instantaneous-action-at-a-distance solutions can be eliminated using a gauge transformation

High-Pressure Properties of Several Narrow Bandgap Semiconductors from First-Principles Calculations

The electronic, thermodynamic, and structural properties of three semiconducting materials, ZnO, InN, and PbS, at high pressure are investigated utilizing first-principles calculations based on density function theory. The first two systems, ZnO and InN, crystalize as hexagonal structures at ambient conditions and transition to a cubic structure at higher pressures. The last system, PbS, is cubic at ambient conditions, but transitions to an orthorhombic structure at higher pressure. At ambient conditions, these materials are well known semiconductors with vast amount of research and a variety of wide ranging applications in electrical devices. However, there is a lack of understanding of their physical properties at high pressures. In this thesis, an attempt is made to establish an understanding of the fundamental properties of the high-pressure phase of these materials. DFT and Boltzmann transport theory are used to find how pressure- induced phase transitions affect the electronic and heat transport of these materials. From harmonic approximations, a frozen phonon method is used to calculate the phonon frequencies and thermodynamic properties

Beyond hollowing out: Public sector managers and the use of external management consultants

Expenditure on management consultants in public sector organizations is generally seen as contributing to a “hollowing out” of the state through the substitution of internal management capability. However, there is little systematic evidence for this view which also ignores how public sector managers may, themselves, drive consulting use. Looking at 125 English public hospitals over 6 years, we explore the relationship between the development and composition of management functions and spending on consultants. Our findings show the absence of a substitution effect and, therefore, challenge the “hollowing out” thesis. Instead, they point to a more active, occupationally varied and political use of consultancy. We find that larger management functions overall are associated with greater reliance on consultants— a complementary relationship. However, where a higher proportion of managers are engaged in internal consulting functions, this results in the lower use of external management consultants, with implications for theory, research and policy

Establishing the First Student-Run Clinic to Provide Free Health Care to a South Texas Colonia

The University of Texas Rio Grande Valley School of Medicine (UTRGVSoM) opened its student-run clinic (SRC) in March 2018 to provide free health care for the residents of Pueblo de Palmas colonia in Hidalgo County. Located along the United States-Mexico border near some of the country\u27s poorest and most medically underserved communities, UTRGVSoM has the unique opportunity to extend quality primary care to those who otherwise would go without. The physical location of the clinic was determined by a partnership with Proyecto Desarrollo Humano, a nonprofit organization within the Pueblo de Palmas colonia. The free clinic, located in the town’s community center, opens its doors to the public every month on a chosen Saturday. This article hopes to detail the experience of being the first SRC to operate inside a Texas colonia

Tetranucleosome Interactions Drive Chromatin Folding

The multiscale organizational structure of chromatin in eukaryotic cells is instrumental to DNA transcription, replication, and repair. At mesoscopic length scales, nucleosomes pack in a manner that serves to regulate gene expression through condensation and expansion of the genome. The particular structures that arise and their respective thermodynamic stabilities, however, have yet to be fully resolved. In this study, we combine molecular modeling using the 1CPN mesoscale model of chromatin with nonlinear manifold learning to identify and characterize the structure and free energy of metastable states of short chromatin segments comprising between 4- and 16-nucleosomes. Our results reveal the formation of two previously characterized tetranucleosomal conformations, the “α-tetrahedron” and the “β-rhombus”, which have been suggested to play an important role in the accessibility of DNA and, respectively, induce local chromatin compaction or elongation. The spontaneous formation of these motifs is potentially responsible for the slow nucleosome dynamics observed in experimental studies. Increases of the nucleosome repeat length are accompanied by more pronounced structural irregularity and flexibility and, ultimately, a dynamic liquid-like behavior that allows for frequent structural reorganization. Our findings indicate that tetranucleosome motifs are intrinsically stable structural states, driven by local internucleosomal interactions, and support a mechanistic picture of chromatin packing, dynamics, and accessibility that is strongly influenced by emergent local mesoscale structure