23CHY109 Engineering Chemistry – B Course Objectives: The objective of the course is to impart knowledge on the concepts of chemistry involved in the application of engineering materials that are used in the industry/day-to-day life. Course Outcomes • CO1: Characterize the solids using X-ray diffraction technique and analyse the materials using computational tools. • CO2: Apply the fundamental principles of electrochemistry to illustrate the functioning of electrochemical energy systems. • CO3: Understand the application of polymers in fabricating integrated electronic devices. Unit 1: Solid state Crystalline and amorphous solids, isotropy and anisotropy, - Miller indices, space lattice and unit cell, Bravais lattices, the seven crystal systems and their Bravais lattices, X-ray diffraction - Bragg’s equation and experimental methods (powder method and rotating crystal technique), types of crystals molecular, covalent, metallic and ionic crystals - close packing of spheres – hexagonal, cubic and body centred cubic packing, elements of symmetry in crystal systems, defects in crystals – stoichiometric, non-stoichiometric, extrinsic and intrinsic defects. Vesta – for visualization of crystal structures. Solar energy - introduction, utilization and conversion, photovoltaic cells design, construction and working, panels and arrays. Advantages and disadvantages of PV cells. DSSC (elementary treatment). 2 Types Amorphous: Pseudo solids/ Supercooled liquid Crystalline: True solids Isotropic materials: • have the same or similar properties throughout the material. This is due to their uniform composition throughout. • They show the same properties in all directions • Ex; Glass, metals, diamond Anisotropic materials: have varying properties in different orientations of the mineral surface. • The differences in properties are related to the compositional differences. • Ex: Wood, Crystals (Except cubic) Anisotropy Arrangement of particles in space results in several structures : Ex: Arrangement of Carbon in space SOLIDS CRYSTALLINE • Regular shape AMORPHOUS • Irregular shape • Sharp M.P (Crystalline Quartz) • Range of Temp (Glass of quartz) • Isotropic in nature • Anisotropic in nature • True solids • Psoudo solids/ Super cooled liqu • Application: Glass , rubber, Plast PV cells, etc Question Q.1. Polyethylene , Naphthalene, Benzoic acid, Teflon, Potassium nitrate, Cellophane, PVC, Fibre glass, Copper Answer: Polyethylene , Naphthalene, Benzoic acid, Teflon, Potassium nitrate, Cellophane, PVC, Fibreglass, Copper Crystalline Q.2. Refractive index of solid is observed same value in all directions. (Crystalline or Amorphous) Answer: Amorphous Crystal lattice & Unit cell • Edge-length of ‘x’ axis = a • Edge-length of ‘y’ axis = b • Edge-length of ‘z’ axis = c • Angle opposite to ‘a’ = α • Angle opposite to ‘b’ = β • Angle opposite to ‘c’ = ϒ Characteristics of crystal lattice • Each point in a lattice is called lattice point or site • Each point represents one particle(atom, molecule) • Lattice points are joined by straight which decides the geometry of compound. Unit cell 7 Crystal systems Cubic a=b=c, α=β=γ=90° Orthorhombic a≠b≠c, α=β=γ=90° Tetragonal, a=b≠c, α=β=γ=90 Rhombohedral a=b=c, α=β=γ≠90° Monoclinic a≠b≠c, α=γ=90°, β≠90 Triclinic a≠b≠c, α≠β≠γ≠90° Hexagonal a=b≠c, α=β=90°, γ=120 Bravais lattices-14 Types Unit cells -14 Bravais lattices Miller Indices • Also known as Miller-Bravais indices – a symbolic notation system used to describe the crystallographic planes and direction in the crystal lattice. • Represented by a set of 3 integers enclosed in parenthesis, written as (h,k,l). • h, k, l are the reciprocals of intercepts made by the plane or crystallographic axes Miller indices Vectors and planes in a crystal lattice are described by the threevalue Miller index notation. This syntax uses the indices ℓ, m, and n as directional parameters. Planes with different Miller indices in cubic crystals 1. Plane intercepts axes : 3a, 2b, 2c 2. Reciprocal of intercepts : 1/3, 1/2 , 1/2 3. Miller Indices of plane [h, k, l] : (2, 3 , 3) C b a 1. Plane intercepts axes (a,b,c) : 1, ∞, ∞, Z 2. Reciprocal of intercepts : 1/1, 1/ ∞, 1/ ∞ 3. Miller Indices of plane [h, k, l] : (1, 0 , 0) c b a X [h, k, l] : (1, 0 , 0) Y 1. Plane intercepts axes (a,b,c) : ∞, 1, ∞, Z 2. Reciprocal of intercepts : 1/ ∞, 1/1, 1/ ∞ 3. Miller Indices of plane [h, k, l] : (0, 1 , 0) c b a X [h, k, l] : (1, 0 , 0) Y X-ray diffraction Interplanar spacing (d-spacing) of a crystal is used for identification and characterization purposes It is possible to detect and quantify elements of interest based on the characteristic X-ray wavelengths produced by each element. The phenomenon by which X-rays are reflected from the atoms in a crystalline solid is called diffraction. The diffracted X-rays generate a pattern that reveals the structural orientation of each atom in a given compound Each crystalline solid has its unique characteristic Xray powder pattern - fingerprint" for its identification. X-ray diffraction is, a phenomenon in which the atoms of a crystal, by virtue of their uniform spacing, cause an interference pattern of the waves present in an incident beam of X-rays. Incident X rays d d Diffracted X rays Ɵ Ɵ Ɵ Ɵ λ Ɵ= Braggs angle λ= angle of incidence d= distance between planes n is an integer When a crystal is bombarded with X-rays of a fixed wavelength (similar to spacing of the atomic-scale crystal lattice planes) and at certain incident angles, intense reflected X-rays are produced when the wavelengths of the scattered X-rays interfere constructively. Bragg's Law, expressed as: n λ = 2d sinΘ where n (an integer) is the "order" of reflection, λ is the wavelength of the incident X-rays, d is the interplanar spacing of the crystal and Θ is the angle of incidence. Bragg's Law. When x-rays are scattered from a crystal lattice, peaks of scattered intensity are observed which correspond to the following conditions: The angle of incidence = angle of scattering. The path length difference is equal to an integer number of wavelengths. XRD – 2 types 1. Powder XRD 2. Single crystal XRD Powder XRD It is used on microcrystalline powder samples. Relatively quick in comparison to single crystal XRD due to the significantly reduced difficulty in the sample preparation step. It can be very challenging to grow high-quality single crystals of sufficient size to perform single-crystal XRD measurements for many materials but powder XRD can be performed on much smaller crystal sizes. One issue with powder XRD is that while the sample preparation is relatively straightforward, it is demanding in terms of the amounts of sample required for a measurement. Single crystal XRD differs to powder diffraction not just in terms of the sample preparation but also in terms of the equipment required. Powder samples tend to give rise to diffraction ‘rings’ that are continuous. This can result in ambiguities in the data interpretation and the need for trialing different fittings to the data to interpret the final structures. In single crystal XRD, single, discrete diffraction peaks are observed. These can then be transformed into a series of coordinates to recover the underlying lattice dimensions of the sample of interest. The interpretation of single crystal XRD is much less ambiguous than powder diffraction methods but the challenge in these experiments is to be able to prepare the single crystal samples, which can often be a highly laborious and timeconsuming process. https://images.app.goo.gl/JNguBLmAFxFVHErM6 Single crystal Diffraction Pattern Powder XRD: Circular Diffraction Pattern. Imperfection in Crystals https://www.youtube.com/watch?v=Z0Ks3wjFnrk Vacancy Defect Interstitial defects Interstitial sites Defects in Ionic Solids Schottky Defect Frankel Defect Impurity Defects Defects in solids Although crystalline solids have short range as well as long range order in the arrangement of their constituent particles, yet crystals are not perfect. Usually a solid consists of an aggregate of large number of small crystals. These small crystals have defects in them. This happens when crystallisation process occurs at fast or moderate rate. Single crystals are formed when the process of crystallisation occurs at extremely slow rate. Even these crystals are not free of defects. The defects are basically irregularities in the arrangement of constituent particles. Broadly speaking, the defects are of two types, namely, point defects and line defects. Point defects are the irregularities or deviations from ideal arrangement around a point or an atom in a crystalline substance, whereas the line defects are the irregularities or deviations from ideal arrangement in entire rows of lattice points. These irregularities are called crystal defects. We shall confine our discussion to point defects only. Point defects can be classified into three types : (i) stoichiometric defects (ii) impurity defects and (iii) non-stoichiometric defects. (a) Stoichiometric Defects These are the point defects that do not disturb the stoichiometry of the solid. They are also called intrinsic or thermodynamic defects. Basically these are of two types, vacancy defects and interstitial defects. (i) Vacancy Defect: When some of the lattice sites are vacant, the crystal is said to have vacancy defect . This results in decrease in density of the substance. This defect can also develop when a substance is heated. (ii) Interstitial Defect: When some constituent particles (atoms or molecules) occupy an interstitial site, the crystal is said to have interstitial defect. This defect increases the density of the substance. Vacancy and interstitial defects as explained above can be shown by non-ionic solids. Ionic solids must always maintain electrical neutrality. Rather than simple vacancy or interstitial defects, they show these defects as Frenkel and Schottky defects. Interstitial sites Na Cl- Na (iii) Frenkel Defect: This defect is shown by ionic solids. The smaller ion (usually cation) is dislocated from its normal site to an interstitial site. It creates a vacancy defect at its original site and an interstitial defect at its new location. Frenkel defect is also called dislocation defect. It does not change the density of the solid. Frenkel defect is shown by ionic substance in which there is a large difference in the size of ions, for example, ZnS, AgCl, AgBr and AgI due to small size of Zn2+ and Ag+ ions. (iv) Schottky Defect: It is basically a vacancy defect in ionic solids. In order to maintain electrical neutrality, the number of missing cations and anions are equal (Like simple vacancy defect, Schottky defect also decreases the density of the substance. Number of such defects in ionic solids is quite significant. For example, in NaCl there are approximately 106 Schottky pairs per cm3 at room temperature. In 1 cm3 there are about 1022 ions. Thus, there is one Schottky defect per 1016 ions. Schottky defect is shown by ionic substances in which the cation and anion are of almost similar sizes. For example, NaCl, KCl, CsCl and AgBr. It may be noted that AgBr shows both, Frenkel as well as Schottky defects. (b) Impurity Defects If molten NaCl containing a little amount of SrCl2 is crystallised, some of the sites of Na+ ions are occupied by Sr2+ . Each Sr2+ replaces two Na+ ions. It occupies the site of one ion and the other site remains vacant. The cationic vacancies thus produced are equal in number to that of Sr2+ ions. Another similar example is the solid solution of CdCl2 and AgCl. Interstitial defects Non-Stoichiometric Defects The defects discussed so far do not disturb the stoichiometry of the crystalline substance. However, a large number of nonstoichiometric inorganic solids are known which contain the constituent elements in non-stoichiometric ratio due to defects in their crystal structures. These defects are of two types: (i) metal excess defect and (ii) metal deficiency defect. (i) Metal Excess Defect Metal excess defect due to anionic vacancies: Alkali halides like NaCl and KCl show this type of defect. When crystals of NaCl are heated in an atmosphere of sodium vapour, the sodium atoms are deposited on the surface of the crystal. The Cl– ions diffuse to the surface of the crystal and combine with Na atoms to give NaCl. This happens by loss of electron by sodium atoms to form Na+ ions. The released electrons diffuse into the crystal and occupy anionic sites (Fig. 1.28). As a result the crystal now has an excess of sodium. The anionic sites occupied by unpaired electrons are called F-centres (from the German word Farbenzenter for colour centre). They impart yellow colour to the crystals of NaCl. The colour results by excitation of these electrons when they absorb energy from the visible light falling on the crystals. Similarly, excess of lithium makes LiCl crystals pink and excess of potassium makes KCl crystals violet (or lilac). Metal excess defect due to the presence of extra cations at interstitial sites: Zinc oxide is white in colour at room temperature. On heating it loses oxygen and turns yellow. Now there is excess of zinc in the crystal and its formula becomes Zn(1+x)O. The excess Zn2+ ions move to interstitial sites and the electrons to neighbouring interstitial sites. (ii) Metal Deficiency Defect There are many solids which are difficult to prepare in the stoichiometric composition and contain less amount of the metal as compared to the stoichiometric proportion. A typical example of this type is FeO which is mostly found with a composition of Fe0.95O. It may actually range from Fe0.93O to Fe0.96O. In crystals of FeO some Fe2+ cations are missing and the loss of positive charge is made up by the presence of required number of Fe3+ ions.
