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ME 411 MATERIALS ENGINEERING

Materials Engineering

- Based largely on the pure sciences of chemistry and physics hence materials obey the laws of physics and chemistry in their formation, reactions and combinations.

Atom

- The smallest part of an element that retains the properties of that element; - The building blocks for engineering materials by bonding together in different patters and with

different types of bonds- Composed of protons (positively charged particles), neutrons (neutral particles) and electrons

(negatively charged particles) which orbit the nucleus (core) of an atom

(a) (b)

Figure 1(a) and (b) shows the configuration of an atom and (c) of Carbon

Element

- Is a pure substance that cannot be broken down by chemical means to a simpler substance

Atomic number

- The number of protons in the nucleus of an atom

EX: The Carbon nucleus has 6 protons and 6 neutrons. Atomic Number (A) is number of protons (6), Atomic Mass (Z) is number of protons+neutrons (plus a little bit = 12.011), which form the nucleus. 

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Figure 2. Elements as the building blocks of all materials

Planet Earth

Matter

Elements

Organic substances Inorganic substances

LiquidsFuelsChemicalsOilsPaintsFood

GasesCO2

CO

LiquidsAcidsWaterBasesChemicals

GasesChlorineArgonHeliumSolids

Living organismsNatural resinsSoils Foods

SolidsMetalsCeramicsCompositesGlassesClaysCementsStone

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Figure 3. Periodic Table of the Elements

In the periodic tables shown above, the atomic number of the elements increases horizontally in the table, and the vertical groupings are based on similarities in valence electronic configurations and similarities in chemical and physical properties of the elements.

Elements in the periodic table with an atomic number greater than 92 donot exist in nature but are produced by nuclear reactions.

Significance of the periodic table in engineering materials

1. Dictionary for the names and chemical symbols for the elements that are the building blocks for all engineering materials.

2. The family groupings indicate which elements behave similarly that aid in the selection problems.

3. The atomic weight is an indication of the density of an element4. The number at the bottom of the vertical column indicates the number of electrons in a valence

shell together with the element grouping provide indicators of how a particular element might combine with other elements.

FORMING ENGINEERING MATERIALS FROM THE ELEMENTS

Alloys – metals combined with one or more elements

Compounds – chemically combined elements with definite proportions of the component elements wherein the smallest component is the molecule

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EX: H2O - water molecule formed by 2 hydrogen atoms and one oxygen atom

H2 – molecule of hydrogen gas composed of 2 hydrogen atoms

(a) (b)

Figure 4. (a) water molecule and (b) hydrogen molecule configurations

Mixtures – a physical blend of two or more substances

Quantum Number

- describes the amount of energy that is given off when an electron moves from one orbit to a lower orbitQuantum numbers and electron configuration of atoms are used in engineering materials to determine the molecular bonding characteristics

EX: Carbon – molecular bonding in polymers

Organic Chemistry – electron configuration related to crystal structure

Electronics – electron configurations and availability of energy levels

Rules in Electron configuration

1. Electrons associated with an atom occupy orbitals and subshells within orbitals.

2. The exact location of electrons in orbitals is defined by four quantum numbers that refer to the energy of the electron (principal quantum number), the shape of an orbital (angular momentum quantum number), the orientation of an orbital (magnetic quantum number and the spin of an electron (spin quantum number).

3. No two (2) electrons can have the same quantum numbers called the Pauli Exclusion Principle.

4. When two electrons reside in the same orbitals, their spins must be paired.

5. The number of electrons in a given orbital is 2n2 where n is the principal quantum number.

6. When atoms interact to form compounds, electrons go into unoccupied orbitals rather than into a partially occupied orbitals.

7. The outermost or valence electrons largely determine the chemical behavior of elements.

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8. In chemical reactions, most elements attempt to attain an electron structure of eight electronic in the outermost energy level – the most stable configuration.

EX: Electron Configuration Notations

A hydrogen atom has 1 electron. That electron will occupy the lowest principal energy level, n = 1, and the only sublevel, s; denoting the electron configuration of hydrogen as

Figure 5. electron configuration

Electron ConfigurationThe electron configuration of an atom describes the specific dispersal of electrons among available subshells. The capacity of an energy level can be found in the following formula:

2 n s

where n is called the principal quantum number and indicates the energy level. In the first energy level, n = 1; the second energy level n = 2; the third energy level n = 3; and so on. On the figure shown below the electrons in these different orbital subshell energy levels.

Electron subshells or orbitals are written with s, p, d, and f as terms for each known energy level. To write the electron configuration of calcium (Ca) as shown above, with an atomic number of 20, it would look like the following: 1s 2 2s 2 p 6 3s 2 p 6 4s 2 . It has 2 electrons in the 1s subshell, 8 electrons in the 2sp, 8 electrons in the 3sp, and 2 electrons in the 4s subshell. Another way of writing this is that there

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are 2 electrons in the n = 1 level, 8 electrons in the n = 2 level, 8 electrons in the n = 3 level, and 2 electrons in the n = 4 level.

Orbitals of the s-type are always singular, p-types form orbital sets of 3, d-type orbitals come in sets of 5, and f-type orbitals are written in sets of 7.

The 4s electrons of calcium are found in the outermost orbit to be filled and from this position react with other elements. Valence electrons affect the reactivity of atoms with other elements.

Molecules that share electrons are generally smaller, have lower melting and boiling points, are insoluble in water, and do not conduct electricity. The s and p orbitals of nearby atoms overlap to form a mixed orbital.

ENGINEERING MATERIALS

Metals

- Is an element with a valence of 1, 2 or 3- Solids composed of atoms held together by a matrix of electrons

Figure 6. Electron matrix of an alkali metal

Ceramics

- A combination of one or more metals with a non-metallic element- Usually have very rigid covalent or ionic bonds between adjacent atoms- usually have a combination of stronger bonds called ionic (occurs between a metal and

nonmetal and involves the attraction of opposite charges when electrons are transferred from the metal to the nonmetal); and covalent (occurs between two nonmetals and involves sharing of atoms). The strength of an ionic bond depends on the size of the charge on each ion and on the radius of each ion. The greater the number of electrons being shared, is the greater the force of attraction, or the stronger the covalent bond. An ion is an atom that has lost or gained an electron.

Figure 7. Covalent bond

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Figure 8. (a) ionic bond of ceramic material and (b) crystalline structure of Al2O3

In ionic bonding, valence electrons from one atom are transferred to another atom and the atoms involved are then held together by the electrostatic attraction between the two oppositely charged ions.

Polymers

- Engineering material known as plastics- Are substances composed of long-chain repeating molecules called mers with the element

carbon forming the backbone of the chain

EX: Polysthylene

Figure 9. Physical structure of polyethylene

Composite

- Combination of two or more materials that has properties that the component materials donot have.

- As a class of engineering materials provide most unlimited potential for higher strength, stiffness and corrosion resistance over the pure material systems of metals, ceramics and polymers.

EX: WOOD – is a composite of cellulose fibers held together with a glue or matrix of soft lignin

a. Plywoodb. Fiber composite – comprise of a mixture of wood mass and cement material

Mer The repeating unit in a polymer chainMonomer A single mer unit (n=1)Polymer Many mer-units along a chain (n=103 or more) Degree of Polymerization The average number of mer-units in a chain.

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Figure 10. Wood- from tree trunk with physical and microscopic structure to final product

COMPOSITION OF WOOD BY ELEMENTS COMPOSITION OF WOOD BY TYPES OF POLYMERSELEMENT % BY WEIGHT POLYMER % BY DRY WEIGHT

CarbonOxygen

HydrogenOthers

494461

CelluloseHemicellulose

Lignin

40-5020-3515-35

Figure 11. Fiberglass process and fiberglass product