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Introduction to Heterogeneous Catalysis in Organic Transformation
Published in Varun Rawat, Anirban Das, Chandra Mohan Srivastava, Heterogeneous Catalysis in Organic Transformations, 2022
Garima Sachdeva, Gyandshwar Kumar Rao, Varun Rawat, Ved Prakash Verma, Kaur Navjeet
An electron microscope is a type of microscope that uses a beam of accelerated electrons as a source of illumination. Since an electron’s wavelength can be much shorter than that of visible light photons, as a consequence, electron microscopes have a higher resolving power and can depict the structure of small particles. This method is used to create high-resolution pictures of biological and non-biological specimens. Electron microscopy is a recognized standard tool for nanomaterial characterization and can be used for determining the shape and size of solid catalysts [26]. It can be done in two modes: SEM (scanning electron microscopy) and TEM (transmission electron microscopy). The transmission electron microscope (TEM), the first-ever type of electron microscope, uses a large electron beam to visualize specimens and creates an image of the material. The scanning electron microscope (SEM) generates images by probing the sample with a concentrated electron beam that scans through a rectangular section of the sample. SEM is effective for visualizing catalyst particles (composition) and surfaces (topography) with micrometer diameters, but TEM is helpful for obtaining detailed structural information.
Finite-Element Electrostatic Solutions
Published in Stanley Humphries, Field Solutions on Computers, 2020
Electrostatics describes the forces between charged bodies at given positions. We denote the charge on an object, a positive or negative quantity, by the symbol q. Throughout this book, we shall use the International System of Units where the charge is given in units of coulombs. The charge of an electron is –1.60219 × 10-19 coulombs. The empirical relationship for the forces between stationary charges was discovered by Coulomb in 1771. Suppose there are two small objects with charges q1 and q2. As shown in Figure 2.1, the unit vector n^21 points from object 2 to object 1 and the distance between the charges is r21. The electric force on object 1 from object 2 is () F1=n^21q1q24πεo r212.
Orbitals and Bonding
Published in Michael B. Smith, A Q&A Approach to Organic Chemistry, 2020
The electron configuration is the distribution of electrons of an atom or molecule in atomic or molecular orbitals. Electrons are distributed in shells, each of which has different types of electrons: s, p, d, f. Each orbital (energy level) occurs further from the nucleus; the electrons are held less tightly. Each orbital can hold a maximum of two electrons and each energy level will contain different numbers of electrons (one electron for the 1s1 and two electrons for the 1s2 orbital, as shown. There are six electrons for p-orbitals; two each is possible for each of the three-degenerate p-orbitals. There are ten electrons for d-orbitals; two each for the five d-orbitals. Orbitals will fill from lowest energy to highest energy orbital, according to the order shown in the mnemonic for the electronic filling order of orbitals.
Colloidal lead in drinking water: Formation, occurrence, and characterization
Published in Critical Reviews in Environmental Science and Technology, 2023
Javier A. Locsin, Kalli M. Hood, Evelyne Doré, Benjamin F. Trueman, Graham A. Gagnon
In some cases, direct visualization of colloids may also be desired, which is impossible in methods previously discussed. Electron microscopes use electrons to produce an image of an object with magnification controlled by electric fields (Fig. 1). Two common electron microscopy methods are scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Since conventional SEM and TEM must be operated under a high vacuum, samples are dried. Thus, sample preparation is critical: the drying process may make it difficult to distinguish between agglomeration due to drying and aggregates already present in the sample or induce the crystallization of salts and structural alteration of colloids (Domingos et al., 2009). In contrast, environmental SEM (ESEM) and liquid TEM allows the in-situ observation of the sample (Daulton et al., 2001; Hülsey et al., 2019). However, they suffer from lower resolution due to electron interactions with the liquid layer (Peckys & de Jonge, 2011).
The Hall current effect of magnetic-optical-elastic-thermal-diffusive semiconductor model during electrons-holes excitation processes
Published in Waves in Random and Complex Media, 2022
Shreen El-Sapa, Kh. Lotfy, Alaa A. El-Bary
In modern physical studies, moving charge carriers are free of particles, yet they carry electric charges and this is clearly shown during the study of semiconductors. There are many charge carriers such as electrons, ions, and holes. In semiconductor material electrons and holes are charge carriers. At absolute temperatures, the free electrons in the atoms of the semiconductors are present in the lower levels (the valence energy band). In this case, the electrons cannot move or move from one place to another, and the electric current cannot flow. Since the internal resistance of semiconductors decreases with increasing temperature, and with the gradual temperature rise, some electrons can jump from the valence band to the conduction band. In this case, with the movement of electrons, a flow of electric current is created. With each transition of an electron into the conduction band, there will be a hole in the valence band. Therefore, electrons and holes are adjacent in semiconductors. In any case, the electric current created in a semiconductor is caused by the free electrons. In some special cases where the material is exposed to gradient temperatures, the holes also transmit electric current.
Analysis of the current–voltage curves and saturation currents in burner-stabilised premixed flames with detailed ion chemistry and transport models
Published in Combustion Theory and Modelling, 2018
Memdouh Belhi, Jie Han, Tiernan A. Casey, Jyh-Yuan Chen, Hong G. Im, S. Mani Sarathy, Fabrizio Bisetti
Analysis of non-thermal electron transport properties in methane–air flames suggests a breakdown threshold of 140 Td [39]. Since the reduced electric field strength does not exceed this threshold in the configurations considered in our work, electrons are assumed to be thermal. The electron mobility was computed as a function of temperature and mixture composition by [40]: where m = 9.109 × 10−31 kg is the mass of the electron. The effective cross-sections of electron-neutral pairs (σj) were collected from the LXcat repository [41]. The electron diffusivity was estimated from the electron mobility using Einstein's relation: Sensitivity of the electric currents to the transport properties of ions and electrons is discussed in Section 5.2.2.