1 Electrons

GUIDE TO SPACE

What are Electrons?

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If you have heard of electrons you know that they have something to do with electricity and atoms. If so you are mostly right in describing what are electrons. Electrons are the subatomic particles that orbit the nucleus of an atom. They are generally negative in charge and are much smaller than the nucleus of the atom. If you wanted a proper size comparison the size of the earth in comparison to the sun would be a pretty close visualization.

Electrons are known to fall into orbits or energy levels. These orbits are not visible paths like the orbit of a planet or celestial body. The reason is that atoms are notoriously small and the best microscopes can only view so much of atoms at that scale. Even if we could view electrons they would move too fast for the human eye. As a matter of fact scientists still can't calculate the exact position of electrons. They can only estimate their locations. That is why the modern model of the atoms has an electron cloud surrounding the nucleus of an atom instead of a defined system of electrons in concentric orbits.

Electrons are also important for the bonding of individual atoms together. With out this bonding force between atoms matter would not be able to interact in the many reactions and forms we see every day. This interaction between the outer electron layers of an atom is call atomic bonding. It can occur in two forms. One is covalent bonding where atoms share electrons in their outer orbits. The other is ionic bonding where an atom gives up electrons to another atom. In either case bonding must meet specific rules. We won't go into great detail, but each electron orbit or electron energy level can only hold so many electrons. Atoms can only bond if there is room to share or receive extra electrons on the outermost orbit of the atom.

Electrons are also important to electricity. Electricity is basically the exchange of electrons in a stream called a current through a conducting medium. In most cases the medium is an acid, metal, or similar conductor. In the case of static electricity, a stream of electrons travels through the medium of air.

The understanding of the electron has allowed for a better understanding of some of the most important forces in our universe such as the electromagnetic force. Understanding its workings has allowed scientist to work out concepts such potential difference and the relationship between electrical and magnetic fields.

We have written many articles about electrons for Universe Today. Here's an article about the atom diagram, and here's an article about the electron cloud model.

If you'd like more info on Electrons, check out the Discussion about Electrons, and here's a link to the History of the Electron Article.

We've also recorded an entire episode of Astronomy Cast all about the Atom. Listen here, Episode 164: Inside the Atom.

Sources:

Wikipedia

Windows to Universe

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Applications of Graphene

Jamie H. Warner, ... Mark H. Rümmeli, in Graphene, 2013

6.5.2.1 Templated Graphene Growth with an Electron Beam

Exposing amorphous carbon to an electron beam of over 100 kV is well known to crystallise freestanding amorphous carbon into carbon onions (concentric graphitic spheres). Carbon is sensitive to a variety of irradiation effects such as knock-on displacements, electronic excitations and radiolysis, and radiation induced diffusion. These effects break chemical bonds in the amorphous carbon, enabling it to rearrange to a more stable form, namely, sp2 carbon. In the case of freestanding amorphous carbon it graphitises in spheres as this results in a structure with no dangling bonds. Moreover, a spherical structure allows for a uniform distribution of the strain because of the out of plane geometry (Ugarte, 1992).

Börrnert (2012) first showed one can also obtain carbon onions from freestanding amorphous carbons with electrons acceleration voltages of 80 kV in low voltage HRTEM which had not previously been demonstrated. They then deposited amorphous carbon on electron cleaned mono-layer graphene, after which they exposed the supported amorphous carbon to the electron beam (in LV HRTEM) for 40 min. The electron dose was 2 × 103 C m−2. After exposure the surface of the supported amorphous carbon is clearly observed to have changed, appearing crystalline with facetted steps indicating various graphene layers have formed. Closer examination shows Moiré patterns typical for graphene layers with rotational stacking faults. In addition, the hexagonal reflexes in the Fourier domain clearly showed the presence of new reflexes indicating not only the formation of new graphene but that this new graphene formed with rotated stacking relative to the underlying monolayer graphene support (see Fig. 6.5.3). They also showed graphene could be manufactured on the same manner on free-standing hexagonal boron nitride membranes. The reason why the amorphous carbon rearranges to planar graphene rather than carbon onions is attributed to the presence of van der Waals forces from the underlying support membrane.

FIGURE 6.5.3. Catalyst-free fabrication of graphene from amorphous carbon supported on graphene. (a) Pristine sample of amorphous carbon residing on a single graphene layer. The dashed ring indicates the area to be irradiated. (b) The Fourier transform from micrograph shown in panel (a) – six spots from the underlying single layer graphene support are visible. (c) The sample shown in panel (a) after 12 mm irradiation. (d) The Fourier transform from micrograph (c) – An additional set of spots as compared to before irradiation (see panel (b)) have now appeared confirming new graphene has formed on the graphene support. (e) magnified section from (b), showing terrace steps of the grown planar few-layer graphene. (f) further magnified section from (e), showing Moiré patterns of the newly formed few-layer graphene. Inset: corresponding Fourier transform, indicating three different rotations from rotational sticking faults between the graphene layers

(Börrnert, 2012).

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Chemical Vapor Deposition

Milton Ohring, in Materials Science of Thin Films (Second Edition), 2002

6.8.5.2.2 Diamondlike Carbon

Amorphous carbons containing hydrogen, are identified as a-C:H materials and possess diamondlike properties. Films are formed when hydrocarbons impact relatively low-temperature substrates with energies in the range of a few hundred eV. Plasma CVD techniques employing RF and DC glow discharges in assorted hydrocarbon gas mixtures commonly produce a-C:H deposits (Ref. 62). Substrate temperatures below 300°C are required to prevent graphitization and film softening. The energetic molecular ions disintegrate upon hitting the surface and this explains why the resulting film properties are insensitive to the particular hydrocarbon employed. It is thought that the incident ions undergo rapid neutralization and the carbon atoms are inserted into C–H bonds to form acetylenic and olefinic polymerlike structures, e.g., C + R–CH3 → R–CH = CH2, where R is the remainder of the hydrocarbon chain. The resultant diamondlike films, therefore, contain variable amounts of hydrogen with H/C ratios ranging anywhere from∼0.2 to∼0.8 or more. Containing an admixture of sp3, sp2, and sp1 bonding, they may be thought of as glassy hydrocarbon ceramics and can be even harder than SiC.

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Special Features of the Electrochemistry of Undoped Tetrahedral Amorphous Carbon (ta-C) Thin Films

T. Laurila, M.A. Caro, in Encyclopedia of Interfacial Chemistry, 2018

Abstract

Amorphous carbon-based electrodes are very promising for electrochemical sensing applications, especially in biological environments. However, there are several specific properties related to the sp3 to sp2 bonded carbon ratio and the spatial distribution of sp2 carbon that must be understood before the observed electrical and electrochemical properties can be rationalized. For this purpose, we present a concise overview of the current state-of-the-art knowledge on the structural, electrical, and electrochemical features of ta-C thin films. Throughout this chapter we will utilize a close combination of computational and experimental methods, as this is the only approach that can provide in-depth understanding of these interesting but complex materials.

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SECONDARY BATTERIES – LITHIUM RECHARGEABLE SYSTEMS | Overview

P. Kurzweil, K. Brandt, in Encyclopedia of Electrochemical Power Sources, 2009

Amorphous carbons

Amorphous carbons show a sloping potential–capacity profile, with no evidence of staging. In the worst case, electrolyte decomposition and gas evolution can occur with no lithium intercalation.

In the first charge and discharge half-cycle, electrolyte is decomposed; the irreversible capacity loss between lithium storage capacity (intercalation) and discharge (ex=deintercalation) is associated with electrolyte decomposition, Qirr=Qin – Qex (in Ah kg−1). The capacity loss Qir increases linearly with rising BET surface area and depends on the electrolyte chosen. In graphitized carbon, the oxidation rate and oxygen chemisorption at the edge sites are much higher than that in the basal plane; the few edge sites are the much more active sites for electrolyte decomposition.

In subsequent cycles, the reversible lithium storage charge capacity approaches Qrev=Qin, which is strongly dependent on the carbon material, electrolyte composition, current density, and potential range.

In the manufacture of lithium-ion batteries, an excess amount of positive electrode material must be used to compensate for the additional capacity needed during the initial formation cycles (irreversible capacity loss).

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Design of ceramic materials for orthopedic devices

Frank Kern, ... Andreas Killinger, in Advances in Ceramic Biomaterials, 2017

10.2.2.1 Diamond-like carbon films

Amorphous carbon films, also known as diamond-like carbon (DLC) coatings are manufactured using PVD or combined PVD/CVD processes and appear as a promising class of coatings in order to improve the surface degradation resistance of metal implants (i.e., wear and corrosion). DLC coatings represent a whole family of carbon- and hydrocarbon-containing amorphous films, exhibiting either a more diamond like character (mainly sp3 hybridization) or a more graphite like character (sp2 hybridization), thus covering a wide range of mechanical properties like microhardness and elastic modulus. By adding hydrogen during deposition, the films can even shift to polymer-like structures showing an increasing grade of amorphous phase with increasing hydrogen concentration. Doping with metals like Cr, Ti, or W can further influence chemical behavior of the coatings like surface wetting, friction behavior, and much more. Basically, most DLC coatings are regarded as biocompatible, and that makes them attractive as they combine many of the unique properties of natural diamond like low friction, high hardness, and high corrosion resistance. Poor adhesion of the DLC coatings on the substrate was reported in vivo, and this is still a major concern for its application at least in load-bearing applications to date (Antilla et al., 1999).

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Molecular Dynamics Simulations of Tribology

J. David Schall, ... Judith A. Harrison, in Superlubricity, 2007

5.4.1 Tribochemistry at the Sliding Interface

Amorphous carbon films have a rich variety of structures depending upon how they are deposited [60]. Experimentally the ratio of sp3 (four-fold) to sp2 (three-fold) coordination and hydrogen content determine the kind of structures obtained. These can range from diamond-like (high sp3-to-sp2 ratio) to graphitic (low sp3-to-sp2 ratio) structures. These films exhibit a wide range of often contradictory tribological properties. For instance, the friction of most DLC films increases with time in inert environments. Yet some DLC films exhibit superlow friction, with friction coefficients less than 0.1 when tested under similar conditions [60–62]. It has been proposed that the random structure of DLC films leads to low lattice commensurability between surface and counterface at atomic scale contacts. In some systems, lattice commensurability has been shown to have a significant effect on the measured friction coefficient. For example, experiments of carbon nanotube rolling and sliding on graphite conducted by Falvo and Superfine [63,64] have shown that nanotubes have preferred orientations on graphite substrates in which the structure of the nanotube is in registry with the structure of the underlying substrate. Falvo and Superfine's experiments also showed that nanotubes preferred to roll when in-registry and slide while out-of-registry. Resisting forces measured while the nanotubes were sliding were significantly lower than those measured during rolling, a somewhat counter-intuitive result. Simulations of carbon nanotube/graphite interactions by Schall and Brenner [65] showed that the difference in observed friction arises from the corrugation of the potential energy surface that the nanotube 'sees' as it moves across the graphite. Carbon nanotubes are best envisioned as rolled up sheets of graphite. When in-registry, the lattices of the graphite and nanotube line up in such away that the potential energy surface has its deepest corrugation. For the nanotube to move, it must either climb out of this deep potential well or roll. When out-of-registry, the potential corrugation is minimized and the nanotube easily slides along the substrate with very little resistance. This same effect has been used to explain the lubricating properties of graphite [66,67].

In early MD simulations of a-C films using Brenner's second-generation REBO potential [51], it was determined that under significant load and shear, film coatings would undergo tribochemical reactions with a hydrogen-terminated counterface [68]. Film composition, (i.e. sp3-to-sp2 ratio and hydrogen content) was also shown to play an important role in the tribo-"reactivity" of the film. Early work on hydrogen-free a-C films showed that tribochemical reactions occurring between films and the counterface give rise to large friction coefficients. The reactions were shown to be dependent on both the structure of the films and the degree of hydrogen-termination on the counterface. Films with similar sp3-to-sp2 hybridization ratios were loaded and sheared against a hydrogen-terminated diamond counterface. At average loads of 300 nN, the results were dramatic as bond rearrangements and adhesion between substrate and counterface occurred. At this load, hydrogen atoms are worn away from the counterface as sliding occurs, creating reaction sites with which unsaturated carbon in the amorphous films can react. This led to the hypothesis that the degree of hydrogen termination of diamond would affect the frictional response due to the greater number of potential reaction sites. Indeed, complete hydrogen saturation of the diamond surface resulted in a lower frictional response, compared to ninety and eighty percent passivation.

In another study, three films were compared: a hydrogen-free film (P00), films containing 20% (P20), and 38% (P38) hydrogen. These films are illustrated in Figure 5.2. Table 5.1 gives a list of their compositions and properties. The films have relatively low sp3-to-sp2 ratios but show no indication of graphite-like layering. The density of the films decreases with increasing hydrogen content. As illustrated in Figure 5.3, films with higher densities (lower hydrogen content) are less compliant than lower density (higher hydrogen content) films. To study the reactivity of the hydrogen containing films three different counterfaces were applied with load and shear. The plot in Figure 5.4 shows that when infinite and curved diamond tips were applied no reactivity was detected. The hydrogen-free film experienced the lowest friction (likely because it is the most ordered film). However, when an amorphous carbon tip that possesses some degree of roughness was applied, thereby introducing potential reaction sites for unsaturated carbons in the films, the hydrogen-free tip was the most reactive. This is reasonable because this film contains the greatest percentage of sp2 hybridized carbon.

Figure 5.2. Films P00 (top), P20 (middle) and P38 (bottom). Carbons colored in red (sp), yellow (sp2), and blue (sp3). Hydrogens colored in green.

Table 5.1. Film composition and properties

FilmTotalCsp(%)sp2(%)sp3(%)Total to HH–H(%)H–C(%)%H in filmczz(GPa)P0030002.285.312.50000358.7P2030002.183.414.57505.694.420295.4P3830006.571.122.418109.590.538193.0

Figure 5.3. Plot of load versus strain for the P00 (squares), P20 (circles) and P38 films (triangles).

Figure 5.4. Friction force versus applied load for three different counterfaces.

The ramifications of these results are two-fold. First, while lattice incommensurability undoubtedly plays an important role in the low friction observed in some DLC films, surface passivation is also very important. As these simulations have shown, high friction in DLC films arises from chemical-bond formation between the surface and counterface due to a lack of passivation. The degree of surface passivation of a film may explain why similar experiments yield widely varying results with high friction in some films and superlow friction in others. Second, the presence of water and other contaminates may act to enhance surface passivation. This may explain discrepancies found in experiments studying the effect of relative humidity on friction in DLC films. Humidity has been shown to reduce friction in some films [69] and increase it in others [70].

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Sodium battery nanomaterials

Lizhuang Chen, Xueying Li, in Advanced Nanomaterials for Electrochemical-Based Energy Conversion and Storage, 2020

4.2.2.3 Amorphous carbon

Amorphous carbon can be classified into soft carbon and hard carbon according to the degree of difficulty in graphitization. Soft carbon, also known as graphitizable carbon, is a transitional carbon that can be converted to graphitized carbon by heat treatment at temperatures above 2000°C. Soft carbon is mainly derived from pyrolysis of organic polymers and petroleum asphalt. Compared with graphitized carbon, the degree of graphitization of soft carbon is low, the grain size is small, and the interplanar spacing is large, which also facilitates the insertion and removal of sodium ions during charge and discharge, and is favorable for compatibility with the electrolyte. Despite this, some of the shortcomings of soft carbon itself limit its use in batteries, mainly due to lower specific capacity and severe voltage hysteresis. Hard carbon is difficult to graphitize even at temperatures above 3000°C. The precursor is the thermal decomposition of hot melt resins, such as some phenolic resins and cellulose present in plants, mainly pyrolytic carbon, resin carbon, and carbon black. Hard carbon has a single carbon atom layer, which is more spaced than the soft carbon layer, which is more conducive to the diffusion of sodium ions. In addition, abundance of lattice defects in the atomic layer provides more active sites for sodium ions, so hard carbon has a larger specific capacity. However, lattice defects also bring some disadvantages while increasing the capacity. Sodium ions are difficult to escape after being embedded in the lattice defects of the atomic layer, which brings about a problem that the first charge and discharge reversible specific capacity loss is large, and the first coulomb efficiency is low. Heteroatoms in hard carbon also cause more severe voltage hysteresis than soft carbon. At the same time, hard carbon has no obvious charging and discharging platform, which also makes the output voltage of the battery unstable.

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Infra-Red Spectra, Electron Paramagnetic Resonance, and Proton Magnetic Thermal Analysis

Osamu Ito, ... Richard Sakurovs, in Carbon Alloys, 2003

2.1 CW-EPR

Amorphous carbons usually give single symmetric signals when using continuous wave (CW)-EPR. Spin concentrations are evaluated by comparing the signal intensity with that of a standard sample. The magnetic field position of the signal corresponds to the 'g-value' of the paramagnetic species which derives from the free electron value of 2.00023 as a result of increases in spin–orbit interactions. Because heteroatoms induce strong spin–orbit interactions, g-values measure the presence of heteroatoms included in the radical content of carbon. The line-width (ΔHpp) of an EPR signal is related to the spin relaxation times (T1 and T2), which are strongly influenced by the presence of adsorbed oxygen. The relative relaxation time (T2) is evaluated, quantitatively, from the dependence of signal intensity upon incident microwave power (P). Measurements of these EPR parameters for a coal-tar pitch are given in Fig. 8 [6]. The plot of the peak–peak signal height (hpp) against log P is normally a parabolic curve. The microwave power at the maximum signal intensity (Pmax) and that at half the maximum signal intensity (P1/2) are known. The Pmax and P1/2 parameters closely relate to spin relaxation times.

Fig. 8. (a) Dependence of EPR signal intensity on microwave power (P) for a coal-tar pitch at room temperature; EPR signals are depicted with changing central magnetic field. (b) Plot of signal intensity (hpp) against log P; Pmax and P1/2 are shown by arrow [7].

EPR parameters obtained by in-situ measurements of coals and a coal-tar pitch change with heat-treatment temperature [7]. On cooling immediately after heating to 500°C, the log (1/Pmax) values return to almost the same values as observed before heating the sample, indicating that the mobility of the molecules in the coal-tar pitch does not change appreciably after heat-treatment at 500°C for a short time. On the other hand, when the coal-tar pitch is kept at 500°C for 2 h, 1/Pmax changes drastically, indicating a significant change in the mobilities of the constituent molecules. For ΔHpp, irreversible changes were observed for a sample maintained at 500°C for 2 h, indicating that chemical changes took during the heat-treatment. The observed increase in the signal intensity with increasing temperature suggests that chemical reactions, producing free radicals, took place at relatively low temperatures. Pmax and P1/2 differ for coals of different rank and maceral contents [6].

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Overview of Rechargeable Lithium Battery Systems

Peter Kurzweil, Klaus Brandt, in Electrochemical Power Sources: Fundamentals, Systems, and Applications, 2019

3.3.1.3 Amorphous carbons

Artificial amorphous carbons (hard carbons, glassy carbon, nongraphitizable) have a high degree of disorder and cannot be graphitized. At about 1000°C, they are obtained by carbonizing thermosetting polymers (phenol-formaldehyde resins, furfuryl alcohol, and divinylbenzene–styrene copolymers), cellulose, charcoal, petroleum pitch, saccharides, and fruit shells. Some hard carbons are known for their good cycling stability; however, they are hygroscopic, and their capacity, which is below that of graphite, varies widely, depending on the degree and type of disorder.

Amorphous carbons exhibit 100–700 mV versus Li/Li+ and ~200 Ah kg−1 at good power and safety, but moderate price and stability. Their potential is more positive than that of graphite and allows higher charge rates, as lithium is not easily plated on the negative electrode. The sloping potential–capacity profile does not show any staging, but a potential plateau at ~0.05 V. In the worst case, electrolyte decomposition and gas evolution can occur with no lithium intercalation.

To be continued

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