Basics of Material Science and Engineering #1
molecular orbital (MO) theory
( linear combinations of atomic orbitals (LCAO).) is a method for determining molecular structure in which *electrons are not assigned to individual bonds* between atoms, but are treated as moving under the influence of the nuclei in the *whole molecule* . Divided into bonding orbitals, anti-bonding orbitals, and non-bonding orbitals. A *bonding orbital* concentrates electron density in the region between a given pair of atoms, so that its electron density will tend to attract each of the two nuclei toward the other and hold the two atoms together. An *anti-bonding orbital* concentrates electron density *"behind" each nucleus* (i.e. on the side of each atom which is farthest from the other atom), and so tends to pull each of the two nuclei away from the other and actually weaken the bond between the two nuclei. Electrons in *non-bonding orbitals* tend to be associated with atomic orbitals that do not interact positively or negatively with one another, and electrons in these orbitals *neither contribute to nor detract from bond strength*
Characteristics of Ionic bond
(I) Crystalline in nature (ii) High strength (iii) High hardness (iv) Having high melting and boiling temperature due to strong electrostatic forces binding atoms. (v) Have brittleness (vi) *Non-conducting of electricity*
Miller indices
(h k l)→ denotes a plane [h k l]→ denotes a direction {h k l}→ family of plane <h k l>→ family of direction
Classification of crystalline imperfections
*"crystalline defect"* is meant a lattice irregularity having one or more of its dimensions on the order of an atomic diameter. Classification of crystalline imperfections 1. point defects (those associated with one or two atomic positions), 2. linear (or one-dimensional) defects, 3. surface or interfacial defects, or boundaries, which are two-dimensional.
Properties due to metallic bond: Lustre, Malleable and ductile, Electrical conductivity, High melting and boiling points and High density
*Lustre* : Light is reflected by the sea of delocalized electron *Malleable and ductile*: electron continue to hold the metal structure together *Electrical conductivity* : electron are free to move through cations lattice *High melting and boiling points*: Attraction b/n cation and electron cloud is strong *High density*: cations are closely packed in a three-dimensional network
Crystallographic methods (three types of radiation interact with the specimen)
*X-rays interact* with the *spatial distribution* of electrons in the sample. *Electrons* are charged particles and therefore interact with the *total charge distribution* of both the atomic nuclei and the electrons of the sample. *Neutrons* are *scattered by the atomic nuclei *through the strong nuclear forces, but in addition, the magnetic moment of neutrons is non-zero. They are therefore also *scattered by magnetic fields*.
Comparison of Crystalline and Noncrystalline Soild
*crystalline Soild* 1. The arrangement of atoms is in a periodically repeating manner 2. It has high density due to its closed packing of atoms in the structure 3. It presents a sharp diffraction pattern. 4. It exhibits a pinpointed melting temperature. 5. It has well-defined crystal structure and geometries. * Noncrystalline Soild* 1. It possesses entangled chain of atoms without any periodicity 2. It has lower density as the packing of atoms take place in a zigzag manner. 3. It does not present any sharp diffraction pattern. 4. it melts over a range of temperatures. 5. It has varying structure and geoetries.
Ionic bonding
*electrostatic attraction* between oppositely charged ions, and is the primary interaction occurring in ionic compounds. The ions are atoms that have gained one or more electrons (known as *anions*, which are negatively charged) and atoms that have lost one or more electrons (known as *cations*, which are positively charged). This transfer of electrons is known as *electrovalence* in contrast to covalence. In simpler words, an ionic bond is the transfer of electrons from a metal to a non-metal in order to obtain a full valence shell for both atoms. bonded atom results in a *lower energy (more negative ) than for the separate atoms*
point lattice (space lattice or lattice) and Lattice Point
*point lattice* A point lattice is a regularly spaced array of points. In the plane, point lattices can be constructed having unit cells in the shape of a square, rectangle, hexagon, etc. Point lattices are frequently simply called "lattices, *Lattice Point* A point at the intersection of two or more grid lines in a point lattice.
single crystal and polycrystalline
*single crystal*: For a crystalline solid, when the periodic and repeated arrangement of atoms is perfect or extends throughout the entirety of the specimen without interruption, the result is a single crystal. *polycrystalline*: Most crystalline solids are composed of a collection of many small crystals or grains;
total number of Bravais lattices in two-dimensional space
, there are 5 Bravais lattices, grouped into four crystal families.
order for filling the "subshell" orbitals
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p
chemical bond
A chemical bond is a lasting attraction between atoms that enables the formation of chemical compounds. The bond may result from the *electrostatic force* of attraction between atoms with opposite charges, or through the *sharing of electrons* as in the covalent bonds. The strength of chemical bonds varies considerably; there are "strong bonds" or *"primary bond"* such as metallic, covalent or ionic bonds and "weak bonds" or *"secondary bond"* such as Dipole-dipole interaction, the London dispersion force and hydrogen bonding.
CRYSTALLOGRAPHIC DIRECTIONS
A crystallographic direction is defined as a line between two points, or a vector. The following steps are utilized in the determination of the three directional indices: 1. A vector of convenient length is positioned such that it passes through the origin of the coordinate system. Any vector may be translated throughout the crystal lattice without alteration, if parallelism is maintained. 2. The length of the vector projection on each of the three axes is determined; these are measured in terms of the unit cell dimensions a, b, and c. 3. These three numbers are multiplied or divided by a common factor to reduce them to the *smallest integer values*. 4. The three indices, not separated by commas, are enclosed in square brackets, thus: *[uvw]*. The u, v, and w integers correspond to the reduced projections along the x, y, and z axes, respectively
Nonpolar molecules
A molecule may be nonpolar either when there is an equal sharing of electrons between the two atoms of a *diatomic molecule* or because of the *symmetrical arrangement of polar bonds* in a more complex molecule. For example, boron trifluoride (BF3) has a trigonal planar arrangement of three polar bonds at 120°. This results in no overall dipole in the molecule. outer edges are negative so there is no asymmetry in charges (polar means one end is +ve other end is -ve) in C02 both ends having same charges
Polar molecules
A polar molecule has a net dipole as a result of the opposing charges (i.e. having partial positive and partial negative charges) from polar bonds arranged asymmetrically. Water (H2O) is an example of a polar molecule since it has a *slight positive charge on one side and a slight negative charge on the other*. The dipoles do not cancel out resulting in a net dipole. Due to the polar nature of the water molecule itself, polar molecules are generally able to dissolve in water. Other examples include sugars (like sucrose), which have many polar oxygen-hydrogen (−OH) groups and are overall highly polar. For example, the water molecule (H2O) contains two polar O−H bonds in a bent (nonlinear) geometry. The bond dipole moments do not cancel, so that the molecule forms a molecular dipole with its negative pole at the oxygen and its positive pole midway between the two hydrogen atoms. In the figure each bond joins the central O atom with a negative charge (red) to an H atom with a positive charge (blue).
Crystallography: Theory
An image of a small object is made using a lens to focus the beam, similar to a lens in a microscope. However, the wavelength of *visible light* (about 4000 to 7000 ångström) is three orders of *magnitude longer than the length of typical atomic bonds and atoms* themselves (about 1 to 2 Å). Therefore, obtaining information about the *spatial arrangement* of atoms requires the use of radiation with *shorter wavelengths*, such as X-ray or neutron beams.
Principal quantum number (n)
As n increases, the *number of electronic shells increases* and the electron spends more time farther from the nucleus. As n increases, the electron is also at a higher potential energy and is therefore less tightly bound to the nucleus. first created for use in the *semiclassical Bohr model* of the atom, distinguishing between different energy levels. With the development of modern quantum mechanics, the simple Bohr model was replaced with a more complex theory of atomic orbitals. *Schrödinger wave equation* describes energy eigenstates having corresponding real numbers En with a definite total energy which the value of En defines. The bound state energies of the electron in the hydrogen atom are given by: *En=-13.6xZ^2 /n^2 ev n=1,2,3,...* In the notation of the periodic table, the main shells of electrons are labeled: K (n = 1), L (n = 2), M (n = 3), etc.
Types of covalent bonds
Atomic orbitals (except for s orbitals) have specific *directional properties* leading to different types of covalent bonds. *Sigma (σ) bonds* are the strongest covalent bonds and are due to *head-on overlapping of orbitals* on two different atoms. A single bond is usually a σ bond. *Pi (π) bonds* are weaker and are due to *lateral overlap* between p (or d) orbitals. *A double bond* between two given atoms consists of one σ and one π bond, and a *triple bond* is one σ and two π bonds.
Biomaterials
Biomaterials are employed in *components implanted into the human body* for replacement of diseased or damaged body parts. These materials must *not produce toxic substances* and must be compatible with body tissues (i.e., must not cause adverse biological reactions). All of the above materials—metals, ceramics, polymers, composites, and semiconductors—may be used as biomaterials. For example, some of the biomaterials that are utilized in artificial hip replacements
Ceramics
Ceramics are compounds between metallic and nonmetallic elements; they are most frequently *oxides, nitrides, and carbides*. For example, some of the common ceramic materials include aluminum oxide (or alumina,Al2O3),silicon dioxide (or silica, SiO2), silicon carbide (SiC), silicon nitride (Si3N4), and, in addition, what some refer to as the traditional ceramics—those composed of *clay minerals (i.e., porcelain), as well as cement, and glass*. With regard to mechanical behavior, ceramic materials are relatively stiff and strong. In addition, ceramics are typically *very hard*. On the other hand, they are extremely *brittle* and are highly *susceptible to fracture*. These materials are typically *insulative* to the passage of heat and electricity , and are more resistant to high temperatures and harsh environments than metals and polymers.With regard to optical characteristics, ceramics may be *transparent, translucent, or opaque*, and some of the oxide ceramics (e.g., Fe3O4) *exhibit magnetic behavior*.
Dipole-dipole interactions
Dipole-dipole interactions are electrostatic interactions between molecules which have permanent dipole(s). These interactions tend to align the molecules to increase attraction (reducing potential energy).
substitutional solid solution and interstitial solid solution (imperfection)
Impurity point defects are found in solid solutions, of which there are two types: substitutional and interstitial. For the substitutional type, solute or impurity atoms replace or *substitute for the host atoms* For interstitial solid solutions, impurity atoms fill the voids or interstices among the host atoms. For metallic materials that have relatively high atomic packing factors, these interstitial positions are relatively small
polarity
In chemistry, polarity is a separation of electric charge leading to a molecule or its chemical groups having an *electric dipole or multipole moment*. Polar molecules must contain *polar bonds* due to a difference in electronegativity between the bonded atoms. A polar molecule with two or more polar bonds must have an *asymmetric geometry*(one end positive other end -ve, denoted as δ+ (delta plus) and δ− (delta minus)) so that the bond dipoles do not cancel each other. Polar molecules interact through *dipole-dipole intermolecular forces and hydrogen bonds*. Polarity underlies a number of physical properties including surface tension, solubility, and melting and boiling points.
Atomic orbital (electron, Quantum mechanics)
In quantum mechanics, an atomic orbital is a mathematical function that describes the *wave-like behavior* of either one electron or a pair of electrons in an atom. This function can be used to calculate the *probability of finding any electron* of an atom in any specific region around the atom's nucleus. The term, atomic orbital, may also refer to the physical region or *space where the electron can be calculated to be present*, as defined by the particular mathematical form of the orbital.
Bravais lattices in 3 dimensions
In three-dimensional space, there are *14 Bravais lattices*. These are obtained by combining one of the crystal families with one of the centering types. The centering types identify the locations of the lattice points in the unit cell as follows: *Primitive (P)*: lattice points on the cell corners only (sometimes called simple) *Base-centered (A, B, or C)*: lattice points on the cell corners with one additional point at the center of each face of one pair of parallel faces of the cell (sometimes called end-centered) *Body-centered (I)*: lattice points on the cell corners with one additional point at the center of the cell *Face-centered (F)*: lattice points on the cell corners with one additional point at the center of each of the faces of the cell *Rhombohedrally-centered (R)*: lattice points on the cell corners with two additional points along the longest body diagonal (only applies for the hexagonal crystal family)
CRYSTALLOGRAPHIC PLANES
Intercepts→Reciprocal→Reduction→(hkl)
Ion-dipole and ion-induced dipole forces
Ion-dipole and ion-induced dipole forces are similar to dipole-dipole and induced-dipole interactions but involve ions, instead of only polar and non-polar molecules. Ion-dipole and ion-induced dipole forces are *stronger than dipole-dipole interactions* because the charge of any ion is much greater than the charge of a dipole moment. Ion-dipole bonding is stronger than hydrogen bonding. An *ion-dipole force* consists of an ion and a polar molecule interacting. They align so that the positive and negative groups are next to one another, allowing maximum attraction. An ion-induced dipole force consists of an *ion and a non-polar molecule* interacting. Like a dipole-induced dipole force, the charge of the ion causes distortion of the electron cloud on the non-polar molecule
Amphiphilic molecules
Large molecules that have *one end with polar groups* attached and *another end with nonpolar groups* are described as amphiphiles or amphiphilic molecules. They are good surfactants and can aid in the formation of stable emulsions, or blends, of water and fats. Surfactants reduce the interfacial tension between oil and water by adsorbing at the liquid-liquid interface.
Metals
Materials in this group are composed of one or more metallic elements (such as iron, aluminum, copper, titanium, gold, and nickel), and often *also nonmetallic elements* (for example, carbon, nitrogen, and oxygen) in relatively small amounts. Atoms in metals and their alloys are arranged in a *very orderly manner*, and in comparison to the ceramics and polymers, are *relatively dense* mechanical characteristics, these materials are relatively *stiff and strong* , yet are *ductile*, and are *resistant to fracture* Metallic materials have large numbers of *nonlocalized electrons*; that is, these electrons are not bound to particular atoms. Many properties of metals are directly attributable to these electrons. For example, metals are extremely *good conductors* of electricity and heat, and are *not transparent* to visible light; a polished metal surface has a *lustrous* appearance. In addition, some of the metals (viz., Fe, Co, and Ni) have *desirable magnetic properties*.
electronegativity
Not all atoms attract electrons with the same force. The *amount of "pull" an atom exerts on its electrons* is called its electronegativity. Atoms with high electronegativities - such as fluorine, oxygen and nitrogen - exert a greater pull on electrons than atoms with lower electronegativities. In a bond, this leads to *unequal sharing of electrons* between the atoms, as electrons will be drawn closer to the atom with the higher electronegativity.
crystal system
On this basis there are *seven* different possible combinations of a, b, and c, and α,β, and γ and each of which represents a distinct crystal system. These seven crystal systems are cubic, tetragonal, hexagonal, orthorhombic, rhombohedral,2 monoclinic, and triclinic. The *cubic system*, for which and has the greatest degree of symmetry. Least symmetry is displayed by the *triclinic* system, since and a ≠b≠c, α≠β≠γ≠90 monoclinic -- a ≠b≠c, α=γ=90, β≠90 cubic -- a=b=c, α=β=γ=90 Tetragonal -- a=b≠c, α=β=γ=90 Orthorhombic -- a≠b≠c, α=β=γ=90
Quantum Numbers
Principal quantum number (n) Azimuthal quantum number (ℓ) Magnetic quantum number (m) Spin quantum number (s)
Semiconductors
Semiconductors have electrical properties that are *intermediate between the electrical conductors (viz. metals and metal alloys) and insulators* (viz. ceramics and polymers). Furthermore, the electrical characteristics of these materials are *extremely sensitive* to the presence of minute concentrations of *impurity atoms*, for which the concentrations may be controlled over very small spatial regions. Semiconductors have made possible the advent of integrated circuitry that has totally revolutionized the electronics and computer industries over the past three decades.
POLYMORPHISM AND ALLOTROPY
Some metals, as well as nonmetals, may have *more than one crystal structure*, a phenomenon known as polymorphism. When found in elemental solids, the condition is often termed *allotropy*. The prevailing crystal structure depends on both the temperature and the external pressure. One familiar example is found in carbon: graphite is the stable polymorph at ambient conditions, whereas diamond is formed at extremely high pressures. Also, pure iron has a BCC crystal structure at room temperature, which changes to FCC iron at C ( F). Most often a modification of the density and other physical properties accompanies a polymorphic transformation
Atomic Packing Factor
The APF is the sum of the sphere volumes of all atoms within a unit cell (as-suming the atomic hard sphere model) divided by the unit cell volume
Unit cell
The atomic order in crystalline solids indicates that *small groups of atoms* form a repetitive pattern.Thus, in describing crystal structures, it is often convenient to sub-divide the structure into small repeat entities called unit cells. Thus, the unit cell is the basic structural unit or building block of the crystal structure and defines the crystal structure by virtue of its geometry and the atom positions within the unit cell having the *highest level of geometrical symmetry*.
Coordination number (crystal structure)
The coordination number is defined as the number of *nearest and equidistant atoms* with respect to any other atom in a unit cell SCC(simple cubic crystal) - - 6 BCC--8 FCC--12 HCP-- 12 DCC--4
VACANCIES AND SELF-INTERSTITIALS
The simplest of the point defects is a vacancy, or *vacant lattice site*, one normally occupied from which an atom is missing A self-interstitial is an atom from the crystal that is *crowded into an interstitial site*, a small void space that under ordinary circumstances is not occupied
Bond polarity
The terms *"polar" and "nonpolar"* are usually applied to covalent bonds, that is, bonds where the polarity is not complete. To determine the polarity of a covalent bond using numerical means, the difference between the electronegativity of the atoms is used. Bond polarity is typically divided into three groups that are loosely based on the difference in electronegativity between the two bonded atoms. According to the Pauling scale: *Nonpolar bonds* generally occur when the difference in electronegativity between the two atoms is *less than 0.5* *Polar bonds* generally occur when the difference in electronegativity between the two atoms is roughly between *0.5 and 2.0* *Ionic bonds* (completely polar bond is more correctly called an ionic bond,) generally occur when the difference in electronegativity between the two atoms is *greater than 2.0*
The Hexagonal Close-Packed Crystal Structure
The top and bottom faces of the unit cell consist of *6* atoms that form regular hexagons and surround a *single* atom in the center. Another plane that provides *3* additional atoms to the unit cell is situated between the top and bottom planes. The atoms in this midplane have as nearest neighbours atoms in both of the adjacent two planes. The equivalent of *six atoms* is contained in each unit cell; 1/6 of each of the 12 top and bottom face corner atoms, one-half of each of the 2 center face atoms, and all 3 midplane interior atoms. c/ a ratio should be *1.633*; The coordination number and the atomic packing factor for the HCP crystal structure are the same as for *FCC: 12 and 0.74*,
ADVANCED MATERIALS
These advanced materials are typically traditional materials whose *properties have been enhanced*, and, also newly developed, high-performance materials. Furthermore, they may be of *all material types* (e.g., metals, ceramics, polymers), and are normally expensive. Advanced materials include semiconductors, biomaterials, and what we may term *"materials of the future"* (that is, smart materials and nanoengineered materials)
planes of HCP and FCC (h k l)
These close-packed planes for HCP are *(0001)*-type planes, FCC→ (111)
hybrid spⁿ orbitals
Under special circumstances, the s and p orbitals combine to form hybrid spⁿ orbitals, where n indicates the number of p orbitals involved, which may have a value of 1, 2, or 3 The driving force for the formation of hybrid orbitals is a *lower energy state for the valence electrons*. For carbon the sp3 hybrid is of primary importance in organic and polymer chemistries. The shape of the sp3 hybrid is what determines the *109° (or tetrahedral)*[105°-for H2O] angle found in polymer chains
covalent bond
also called a *molecular bond*, is a chemical bond that involves the *sharing of electron pairs* between atoms. These electron pairs are known as shared pairs or bonding pairs, and the *stable balance of attractive and repulsive forces* between atoms. For many molecules, the sharing of electrons allows each atom to attain the equivalent of a full outer shell, corresponding to a stable electronic configuration. The prefix *co- means jointly*, associated in action, partnered to a lesser degree, etc.; thus a "co-valent bond", in essence, means that the atoms share "valence" Covalent bonds are also affected by the *electronegativity* of the connected atoms which determines the chemical *polarity of the bond*. Two atoms with equal electronegativity will make *nonpolar covalent bonds such as H-H*. An unequal relationship creates a polar covalent bond such as with H−Cl. However polarity also requires *geometric asymmetry*, or else dipoles may cancel out resulting in a non-polar molecule.
Carbonated Beverage Containers- ability of storage
aluminum and glass containers retain their carbonization (i.e., "fizz") for several years, plastic bottles *"go flat"* within a few months. Glass is impervious to the passage of carbon dioxide, plastic bottle is *not as impervious* to the passage of carbon dioxide as the aluminum and glass.
Frenkel defect
an atom is *displaced* from its lattice position to an interstitial site, creating a vacancy at the original site and an interstitial defect at the new location within the same element without any changes in chemical properties
Tin (Its Allotropic Transformation)
another common metal that experiences an allotropic change is tin. *White (or ) tin*, having a body-centered tetragonal crystal structure at room temperature, transforms, at *13.2 C , to gray (or ) tin*, which has a crystal structure similar to diamond (i.e., the diamond cubic crystal structure)
CLOSE-PACKED CRYSTAL STRUCTURES
both FCC and hexagonal close-packed crystal structures have atomic packing factors of *0.74*, which is the *most efficient packing* of equal-sized spheres or atoms Both crystal structures may be generated by the stacking of these close-packed planes on top of one another; the difference between the two structures *lies in the stacking sequence*. This stacking sequence, *ABABAB* . . . , is repeated over and over. Of course, the ACACAC . . . arrangement would be equivalent For the face-centered crystal structure, the centers of the third plane are situated over the C sites of the first plane. This yields an *ABCABCABC . . .*
Composites
composed of two (or more) individual materials, which come from the categories discussed above—viz., metals, ceramics, and polymers.The design goal of a composite is to achieve a *combination of properties* that is not displayed by any single material, and also to incorporate the best characteristics of each of the component materials. Furthermore, some naturally-occurring materials are also considered to be composites—for example, *wood and bone*. However, most of those we consider in our discussions are synthetic (or man-made) composites. One of the most common and familiar composites is *fiberglass*, in which small glass fibers are embedded within a polymeric material (normally an epoxy or polyester). The glass fibers are relatively strong and stiff (but also brittle), whereas the polymer is ductile (but also weak and flexible). Thus, the resulting fiberglass is relatively *stiff, strong, flexible, and ductile*. In addition, it has a low density . Another of these technologically important materials is the *"carbon fiber-reinforced polymer"(or "CFRP")* composite—carbon fibers that are embedded within a polymer. These materials are stiffer and stronger than the glass fiber-reinforced materials , yet they are more expensive.
Characteristics of covalent bond
covalent bonds are *directional in nature* and covalent compound can be solids, liquids and gaseous. The compounds having covalent bond have following general characteristics (I) high strength. (ii) Have high melting and boiling temperature, (iii) Atom movement within the material (deformation) requires the breaking of distinct bonds, thereby making the material characteristically brittle, iv) Electrical conductivity depends upon the bond strength, ranging from *conductive* tin (weak covalent bond) through semiconductive silicon and germanium to *insulating* diamond. (v) Covalent solids *do not form closed pack* structures as bonding has directional nature,
crystalline
crystalline material is one in which the atoms are situated in a repeating or periodic array over large atomic distances All metals, many ceramic materials, and certain poly-mers form crystalline structures under normal solidification conditions.
cubic (or isometric) crystal system
cubic (or isometric) crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals. There are three main varieties of these crystals: Primitive cubic (abbreviated cP and alternatively called simple cubic) Body-centered cubic (abbreviated cI or bcc), Face-centered cubic (abbreviated cF or fcc, and alternatively called cubic close-packed or ccp)
shielding effect
describes the attraction between an electron and the nucleus in any atom with more than one electron shell. Shielding effect can be defined as a *reduction in the effective nuclear charge on the electron cloud*, due to a difference in the attraction forces of the electrons on the nucleus. It is also referred to as the *screening effect (or) atomic shielding*. because of this Zeff reduces
Azimuthal quantum number (ℓ)
determines its orbital angular momentum and describes the *shape of the orbital*. Quantum Subshells Each of the different angular momentum states can take *2(2ℓ + 1)* electrons maximum number of electrons in the nth *energy level is 2n^2*
magnetic quantum number mℓ
distinguishes the *orbitals* available within a subshell, and is used to calculate the azimuthal component of the orientation of orbital in space. Electrons in a particular subshell (such as s, p, d, or f) are defined by values of ℓ (0, 1, 2, or 3). The value of m can range from *-ℓ to +ℓ*, inclusive of zero. Thus the s, p, d, and f subshells contain 1, 3, 5, and 7 orbitals each, with values of m within the ranges *±0, ±1, ±2, ±3* respectively. Each of these orbitals can accommodate *up to two electrons* (with opposite spins), forming the basis of the periodic table.
Nanoengineered Materials
general procedure utilized by scientists to understand the chemistry and physics of materials has been to begin by studying large and complex structures, and then to investigate the fundamental building blocks of these structures that are smaller and simpler. This approach is sometimes termed *"top-down"* science. design new materials that are built from simple atomic-level constituents. This ability to carefully arrange atoms provides opportunities to develop mechanical, electrical, magnetic, and other properties that are not otherwise possible. We call this the *"bottom-up"* approach, and the study of the properties of these materials is termed *"nanotechnology"*; the "nano" prefix denotes that the dimensions of these structural entities are on the order of a nanometer (10^9 m)—as a rule, less than 100 nanometers (equivalent to approximately 500 atom diameters). example of a material of this type is the *carbon nanotube*.
The point imperfection may be created by
i thermal fluctuations ii quick cooling (quenching) of material from a high temperature iii deformation of material by forging (hammering and rolling) and iv displacement of atoms in a material with the bombardment of high energy oarticles.
Polymers
include the *familiar plastic and rubber materials*. Many of them are *organic compounds* that are chemically based on carbon, hydrogen, and other nonmetallic elements (viz. O, N, and Si). Furthermore, they have very large molecular structures, often *chain-like in nature* that have a backbone of carbon atoms. Some of the com-mon and familiar polymers are polyethylene (PE), nylon, poly(vinyl chloride) (PVC), polycarbonate (PC), polystyrene (PS), and silicone rubber. These materials typically have *low densities*, whereas their mechanical characteristics are generally dissimilar to the metallic and ceramic materials—they are *not as stiff nor as strong*. However, on the basis of their low densities, many times their stiffnesses and strengths on a per mass basis are comparable to the metals and ceramics. In addition, many of the polymers are extremely *ductile and pliable* (i.e., plastic), which means they are easily formed into complex shapes. In general, they are *relatively inert chemically and unreactive* in a large number of environments. One major drawback to the polymers is their *tendency to soften and/or decompose at modest temperatures*, which, in some in-stances, limits their use. Furthermore, they have *low electrical conductivities and are nonmagnetic*.
materials science
involves investigating the relationships that exist *between the structures and properties of materials* . *structure* of a material usually relates to the *arrangement of its internal components*. Subatomic structure involves electrons within the individual atoms and interactions with their nuclei. On an atomic level, structure encompasses the organization of atoms or molecules relative to one another properties of solid materials may be grouped into six dif-ferent categories: mechanical, electrical, thermal, magnetic, optical, and deteriorative.
octet rule
is a chemical rule of thumb that reflects observation that atoms of main-group elements tend to combine in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. The rule is especially applicable to carbon, nitrogen, oxygen, and the halogens, but also to metals such as sodium or magnesium.
diamond cubic crystal structure
is a repeating pattern of 8 atoms that certain materials may adopt as they solidify. While the first known example was diamond, other elements in group 14 also adopt this structure, including α-tin, the semiconductors silicon and germanium, and silicon/germanium alloys in any proportion.
Metallic bonding
is a type of chemical bonding that arises from the *electrostatic attractive force between conduction electrons (in the form of an electron cloud of delocalized electrons) and positively charged metal ions*. It may be described as the sharing of free electrons among a lattice of positively charged ions (cations). Metallic bonding accounts for many physical properties of metals, such as strength, ductility, thermal and electrical resistivity and conductivity, opacity, and luster. Metallic bonding is *not the only type* of chemical bonding a metal can exhibit, *even as a pure substance*.
Schottky defect
is a type of point defect in a crystal lattice In non-ionic crystals it means a lattice vacancy defect. In ionic crystals, the defect forms when *oppositely charged ions leave* their lattice sites, creating vacancies. These vacancies are formed in stoichiometric units, to maintain an overall *neutral charge* in the ionic solid. The vacancies are then free to move about as their own entities. Normally these defects will lead to a *decrease in the density* of the crystal.
Bravais lattice
is an infinite array of discrete points in three dimensional space generated by a set of discrete translation operations When the discrete points are atoms, ions, or polymer strings of solid matter, the Bravais lattice concept is used to formally define a crystalline arrangement and its (finite) frontiers. A crystal is made up of a periodic arrangement of one or more atoms (the basis) repeated at each lattice point. Consequently, the crystal looks the same when viewed from any equivalent lattice point, namely those separated by the translation of one unit cell (the motif)
fermion
is any subatomic particle characterized by Fermi-Dirac statistics. These particles *obey the Pauli exclusion principle*. Fermions (24) include *all quarks(6) and leptons(6) along with the corresponding antiparticle of each of these*, as well as all composite particles made of an odd number of these, such as all baryons and many atoms and nuclei. Fermions differ from bosons, which obey Bose-Einstein statistics. A fermion can be an *elementary particle, such as the electron*, or it can be a composite particle, such as the proton and neutrons.
Polarizability
is the ability to form instantaneous dipoles. It is a property of matter. Polarizabilities determine the dynamical response of a bound system to external fields, and provide insight into a molecule's internal structure
hydrogen bond
is the attraction between the lone pair of an electronegative atom and a hydrogen atom that is bonded to either *nitrogen, oxygen, or fluorine*. The hydrogen bond is often described as a *strong electrostatic dipole-dipole interaction*. However, it also has some features of covalent bonding: it is directional, *stronger than a van der Waals* force interaction, produces interatomic distances shorter than the sum of van der Waals radius, and usually involves a limited number of interaction partners, which can be interpreted as a kind of valence. bond strength (descending order) *Ionic >ion-dipole >dipole-dipole(Hydrogen) > van der Waals*
Crystallography
is the experimental science of *determining the arrangement of atoms* in the crystalline solids Crystallographic methods now depend on analysis of the *diffraction patterns of a sample* targeted by a beam of some type. *X-rays are most commonly used; other beams used include electrons or neutrons*. This is facilitated by the wave properties of the particles.
Pauli exclusion principle
is the quantum mechanical principle which states that *two or more identical fermions (particles with half-integer spin) cannot occupy the same quantum state* within a quantum system simultaneously. In the case of electrons in atoms, it can be stated as follows: it is impossible for two electrons of a poly-electron atom to have the same values of the four quantum numbers: n, the principal quantum number, ℓ, the angular momentum quantum number, mℓ, the magnetic quantum number, and ms, the spin quantum number. For example, if two electrons reside in the same orbital, and if their n, ℓ, and mℓ values are the same, then their ms must be different, and thus the electrons must have *opposite half-integer spins of 1/2 and −1/2*
motif or basis (atom)
motif or basis is an atom or group of atom associated with each lattice point, the generation of cystal structure from a lattice and basis
London dispersion force
part of the van der Waals forces The LDF is a *weak intermolecular force* arising from quantum-induced *instantaneous polarization* multipoles in molecules. They can therefore act between molecules *without permanent multipole moments*. London forces are exhibited by *non-polar molecules*. generally weaker than ionic bonds and hydrogen bonds. London forces become *stronger as the atom becomes larger*, and to a smaller degree for large molecules. This is due to the increased polarizability of molecules with larger, more dispersed electron clouds. This trend is exemplified by the halogens (from smallest to largest: F2, Cl2, Br2, I2). Fluorine and chlorine are gases at room temperature, bromine is a liquid, and iodine is a solid. The London forces also become *stronger with larger amounts of surface contact*. Greater surface area means closer interaction between different molecules.
primitive cell
primitive cell is a minimum volume cell (a unit cell) corresponding to a *single lattice point* of a structure with discrete translational symmetry.
The four components of the discipline of materials science and engineering
processing-structure-properties-performance
s, p, d, f abbreviation
sharp, principal, diffuse, and fundamental
why Volume of Water Expansion Upon Freezing?
solid ice, each water molecule participates in *4 hydrogen bonds* Upon melting, this structure is partially destroyed, such that the water molecules become more closely packed together at room temperature the average number of nearest-neighbor water molecules has increased to approximately *4.5*; this leads to an increase in density
Smart Materials
these materials are able to *sense changes in their environments and then respond to these changes* in predetermined manners— traits that are also found in living organisms. Components of a smart material (or system) include some type of *sensor* (that detects an input signal), and an *actuator* (that performs a responsive and adaptive function). Actuators may be called upon to change shape, position, natural frequency, or mechanical characteristics in response to changes in temperature, electric fields, and/or magnetic fields. Four types of materials are commonly used for *actuators*: shape memory alloys, piezoelectric ceramics, magnetostrictive materials, and electrorheological/magne-torheological fluids. *Piezoelectric ceramics* expand and contract in response to an applied electric field (or voltage); conversely, they also generate an electric field when their dimensions are altered . For example, one type of smart system is used in helicopters to reduce aero-dynamic cockpit noise that is created by the rotating rotor blades. Piezoelectric sensors inserted into the blades monitor blade stresses and deformations; feedback signals from these sensors are fed into a computer-controlled adaptive device, which generates *noise-canceling* antinoise.
CLASSIFICATION OF MATERIALS
three basic classifications: metals, ceramics, and polymers. This scheme is based primarily on *chemical makeup and atomic structure*, and most materials fall into one distinct grouping or another, although there are some intermediates. In addition, there are the *composites*, combinations of two or more of the above three basic material classes Another classification is *advanced materials*—those used in high-technology applications— viz. semiconductors, biomaterials, smart materials, and nano-engineered materials
Atoms bond primarily to
to reduce their potential energy and gain stability
spin quantum number
which completely describe the quantum state of an electron. It is designated by the letter s. It describes the *energy, shape and orientation of orbitals* each possible *state of the electron* to be described by three "quantum numbers". These were identified as, respectively, the electron "shell" number n, the "orbital" number l, and the "orbital angular momentum" number m. The spin angular momentum is characterized by a quantum number; *s = 1/2* specifically for electrons the two different spin orientations are sometimes called *"spin-up" or "spin-down"*.
