Insulator-insulator Contact Charging As A Function Of Pressure

Metal - metal and to an extent metal - insulator contact or triboelectric charging are well known phenomena with good theoretical understanding of the charge exchange mechanism. However, insulator insulator charging is not as well understood. Theoretical and experimental research has been performed that shows that the surface charge on an insulator after triboelectric charging with another insulator is rapidly dissipated with lowered atmospheric pressure. This pressure discharge is consistent with surface ions being evaporated off the surface once their vapor pressure falls below the saturation vapor pressure. A two-phase equilibrium model based on an ideal gas of singly charged ions in equilibrium with a submonolayer adsorbed film was developed to describe the pressure dependence of the surface charge on an insulator. The resulting charge density equation is an electrostatic version of the Langmuir isotherm for adsorbed surface particles, which describes well the experimental observations

A content analysis of presentations of electrostatics in South African upper secondary school textbooks

A thesis submitted to the faculty of humanities, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Doctor of philosophy Johannesburg, 26 May 2017The reality of South African education leaves little doubt that the school science textbook is the primary means by which the „what is taught and learnt‟ in science classrooms is determined. Reports from different countries suggest the same trait. The possibility that not all learners‟ „naïve ideas‟ originate in everyday life has also emerged in the literature along with allusions to the quality of textbooks. If school textbooks are to be blamed, even partially, for learners‟ naïve ideas, a systematic analysis of their subject content becomes requisite. The present study is a systematic content analysis of presentations of foundational aspects of Electrostatics, in approved South African physical sciences textbooks in use after the first democratic elections of 1994, thus representing and addressing three curricula school education has gone through since. The study was perceived as a first step to an anticipated analysis of the entire topic Electromagnetism to which Electrostatics is part of, given its difficulty as has been widely reported in the literature and its status in school curricula. Using the conceptual framework of the Classical Electromagnetic Theory, six foundational aspects of Electrostatics were demarcated for the analysis, targeting the concept charge, its origins, transfer and conservation, the distinction between conductors and insulators, the attraction between charged and uncharged objects, as well as global perceptions of Electrostatics and its place within Electromagnetism. Categorisation tables with theoretically grounded indicators were developed as the primary constructs against which texts were analysed, but inductive categorisation tables emerged from the texts as well. An additional construct was necessitated and developed, the “Organisation of the science educator‟s thought”, based on the notion of a scientific explanation and the nature of scientific models, for analysing links between macro and micro. The analysis revealed that the subject matter content of Electrostatics in South African textbooks is of major concern, giving learners no reason to make sense or develop an appreciation for science, physics in particular. In fact it is not science. The analysis suggests that the long lists of problems revealed, have their origin in two main drawbacks: Firstly, inadequate author understanding of the concept charge, disregarded or misused in the texts, and secondly, author unawareness of the inferred nature of science models, affecting purpose of accounts, explanations and reasoning. Furthermore, certain unprofessional author practices are suggested, such as lack of familiarity with curricula and the content of other topics (not a single link was found), lack of research, and general disregard for learners‟ difficulties, while misconceptions identified in the literature are all communicated in the texts, most explicitly so. The findings suggest that science textbook authors are in need of training.MT201

Electrostatic instrumentation and measurements on powders and powder mixtures

In the powder industry, such as the pharmaceutical or chemical industry, powder particles collide frequently with the surrounding surfaces and with other particles and become electrostatically charged in different processes. The charging may cause problems, for example, in powder mixing and conveying. High charge values increase particle adhesion on surfaces. Charged particles may adhere to sensors or production lines of a pharmaceutical plant. In the worst case, even a dust explosion may occur. The aim of this work was to develop instrumentation for these kinds of processes and to study the charging and electrical resistivity of powders and powder mixtures. The charge and resistivity are the fundamental quantities when electrostatic effects are studied. The charge is a direct measure for the intensity of the electrostatic charging, and the resistivity is related to the dissipation of the accumulated charge. In the thesis, a method for studying the powder resistivity as a function of humidity was developed. The new method proved to be significantly faster than the traditional steady-state measurements. For instance, the resistivity of a lactose powder was measured in 24 hours with the new one-step method but took four weeks with the traditional method. The electrical resistivity of powder mixtures was also studied. It was observed that the particle size difference of the component materials played an important role in the effective resistivity. Two induction probes for measuring the charge density of powder in a fluidized bed were developed. The probes were designed using computer simulations, calibrated with frictionally charged spheres, and finally tested in a laboratory-scale fluidized bed. The charging of lactose and salbutamol sulphate was studied at different humidities. The humidity had a special effect on the charging of their mixtures. At low humidities, the mixtures charged negatively. As the humidity was increased, the charge polarity flipped from negative to positive. When the once humidified samples were dried, the polarity did not flip back to negative but remained positive. It was suggested that the small salbutamol sulphate particles adhered to larger lactose particles due to increased capillary forces and remained adhered even if the powder was again dried. A method for studying the charging of powders on-line during surface adhesion was also developed and used in measurements. The powder was slid in a pipe, and the charge and mass of the transferred powder were simultaneously monitored. With powder mixtures, it was observed that the dominant charging mechanism changed from powder–pipe contacts to powder–powder contacts between the two dissimilar powders quickly after the other powder component started to adhere to the pipe surface

The influence of surface functional groups of organic crystals on triboelectrification

Triboelectric charging is a ubiquitous phenomenon with both historical significance in the discovery of electricity and contemporary importance in applications like particulate processing and small-scale power generation through ambient energy harvesting. However, despite its long history, the fundamental mechanisms underlying triboelectric charging remain poorly understood. This complexity arises from the sensitivity of the phenomenon to a multitude of physical and environmental factors, making the interpretation of experimental results challenging. Recent advances, particularly the development of triboelectric nanogenerators, have led to a growing consensus that the electron transfer mechanism dominates in triboelectric charging, although this perspective is not universally accepted among researchers. First principles calculations have demonstrated their ability to probe the underlying mechanisms for charge transfer and this work has been applied to gain insight into various triboelectric charging phenomena observed in experiments. This thesis employs Density Functional Theory (DFT) to gain insights into various aspects of triboelectric charging observed in experimental settings. Firstly, the influence of surface properties, crystal facets, and the presence of surface contaminants on various electronic structure properties relevant to triboelectric charging is investigated. This analysis underscores the substantial impact that surface conditions can have on charge transfer phenomena. Secondly, the work function, a key parameter for modeling triboelectric charging, is predicted for different facets of pharmaceutical crystals using DFT. These calculations predict variations in work function, depending on the crystal facet and the presence of surface water. Finally, the charging characteristics of individual Active Pharmaceutical Ingredient (API) molecules are investigated through a theoretical "slab-molecule" approach. To elucidate the mechanisms of charge transfer, charge density difference, density of states and Hirschfeld charge analysis were employed. This thesis aims to highlight, through the application of first principles calculations, the substantial influence that surface chemistry and contamination can exert on triboelectric charging. Additionally, it seeks to introduce innovative methods for gaining insights into the triboelectric charging properties of individual molecules

Direct-current triboelectricity from miniaturised sliding metal– monolayer–semiconductor contacts

Triboelectric nanogenerators (TENGs) represent an autonomous and sustainable technology for power generation, aiming to convert small vibrations into electricity. Through molecular control of Si crystals and the application of conductive atomic force microscopy, I have addressed crucial unresolved inquiries about the influencing factors of tribocurrent. This understanding has been leveraged to elucidate the operational principles of semiconductor-based direct-current triboelectric nanogenerators (DC-TENGs)

Improving the performances of direct-current triboelectric nanogenerators with surface chemistry

Over the past decade, triboelectric nanogenerators (TENGs) – small and portable devices designed to harvest electricity from mechanical vibrations and friction – have matured from a niche theme of electrical engineering research into multidisciplinary research encompassing materials science, physics, and chemistry. Recent advances in both the fundamental understanding and performances of TENGs have been made possible by surface chemistry, electrochemistry, and theoretical chemistry research entering this active and promising field. This short review focuses on the recent developments of direct-current (DC) TENGs, where sliding friction or repetitive contact–separation cycles between the surface of polymers, metals, chemically modified semiconductors, and more recently even by the simple contact of surfaces with water solutions, can output DC suitable to power electronic devices without the need of additional rectification. We critically analyze the role of surface chemistry toward maximizing DC TENG outputs and device longevity. The major current hypotheses about their working mechanism(s) are also discussed

Transient aspects of plasma luminescence induced by triboelectrification of polymers

Transient electric gas discharges that occur around sliding interfaces during contact electrification of polymers were studied at millisecond timescales and with micrometre resolution. Deduced vibrational temperatures indicate cold plasma resulting from positive corona discharge. At millisecond timescales, previously unseen rapid discharge events are observed, and modelling suggests that these result from streamer development, triggered by electron emission from the polymer surface. Those which occur over a period of several seconds are shown to be caused by competition between charge generation and the formation of polymer films. The findings explain the interplay between charging and plasma generation and their dependence on wear processes.UK Engineering and Physical Sciences Research Council (EP/J002100/1), Additional equipment funding provided by the Taiho Kogyo Tribology Research Foundation (11A01). Spanish Ministry of Economy, Industry and Competitivity through the grant BIA2016-79582-