MDME: MANUFACTURING, DESIGN, MECHANICAL ENGINEERING

# CLASSIFICATION and STRUCTURE

Classification of materials by properties: Mechanical, physical, chemical properties

Notes 2011:  Classification.pdf     Classification.one

 Image Video Lesson Description and Link Duration Date Download Atomic Structure: Part 1 19:28 min 20140728 Atomic Structure: Part 2 Bonds 26:53 min 20140728

Atomic Structure: Part 1

Atomic Structure: Part 2 Bonds

## Atomic Structure

Atoms
Atoms are composed of  a nucleus of protons and neutrons which are orbited by electrons. Diagrams show neat orbits, but they are actually fuzzy. Here is a simplified model of an atom - Carbon.

 Property Electron Proton Neutron Charge -1 e +1 e 0 Relative Mass 0.000545 1.0000 1.0004

Charge of 1e = 1.6 × 10-19 Coulombs, and mass of 1 proton (amu) = 1.66 × 10-27 kg.

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.

To converts atomic mass to grams you need 1 mole (6.023E23) of atoms.  Atoms that might be about 0.25nm apart, are roughly 4000 per micron, or 4 million per mm.  Atoms are mostly empty space, the nucleus is about 10 000 times smaller than the whole atom.  The diameter of an atom is in the order of 10E-10 m, whereas the diameter of the nucleus in the order of 10E-15  m.  See chart comparing the size of atoms and their ions.  periodic_table_diameters.jpg

The negative electrons and positive protons must be balanced, otherwise the atom will be charged (ionized). Generally, the interactions at the electron level between atoms is chemistry and electronics, but playing with the nucleus is nuclear science.
An atom is identified by the number of protons.
A different number of electrons produces an ion
A different number of neutrons produces an isotope

If Carbon gets an extra 2 neutrons it becomes the unstable isotope Carbon 14, which decays by one of the neutrons turning into a proton, now a completely different atom, N. This radioactive decay converts half the C14 in about 5700 years (the halflife), so a lower C14:C12 ratio means an older piece of wood. This assumes we know the original C14:C12 ratio and that it was not disturbed by other C14:C12 ratio carbon in the meantime.
First paper from Science 1949; http://hbar.phys.msu.ru/gorm/fomenko/libby.htm

NUCLEUS DENSITY: What happens if you had a lump of solid nucleus material (taken from a neutron star)?

It's the extreme gravity in the neutron star that keeps the matter compressed to extremely high densities. Once you removed the baseball sized clump of matter from the neutron star, the pressure of gravity would no longer be compressing it, and the matter would expand violently (i.e, explode). There is no way you could get it back to earth to try your experiment. You couldn't put it into some sort of "bottle" to keep it compressed, because no ordinary matter is strong enough to keep it compressed,

...in a hypothetical world in which one could bring back a handful of neutron star matter and place it on the ground here on earth, it wouldn't matter how strong or dense the ground beneath the neutron star material is - it would never be as dense as the neutron star material itself. and so such an object would punch a hole in the ground and would not settle until it reached the center of the earth. A spoonful of neutron star matter could weigh as much as a mountain on earth (hundreds of millions of tons). The reason a mountain doesn't punch a hole in the surface of the earth and sink to the center is b/c its weight is distributed over several, perhaps hundreds, of square miles. an object that weighs as much as a mountain, but whose weight is only distributed over several square inches, would sink to the center of the earth provided it can maintain its density here on earth (which it can't).

## The Periodic Table

The number of electrons in the outermost layer or shell (called the valence band) of an atom determines how it will behave chemically. A full outer shell is the most stable. This is usually 8 electrons, except for small atoms that try to get a shell of 2 electrons.
An Interactive Periodic table is here (off-site); http://www.dayah.com/periodic/
Click to get large version of chart below;  periodic_table_long.jpg

Elements are listed by atomic number, and within a column they have similar properties. The noble gases have a full outer shell so they do not gain or lose electrons easily - very unreactive. Alkalis can easily lose an electron to become a positive ion with a closed shell; halogens can easily gain one to form a negative ion with a closed shell. When atoms have interacting electrons they join together to form a molecule. For example, NaCl (Sodium Chloride or salt). Hydrogen joins in pairs to share each single electron to approximate the complete outer shell that Helium has.

The ability to gain or lose electrons is termed electronegativity or electropositivity, an important factor in ionic bonds.

## Bonding Atoms

Joining Atoms Together

If atoms come too close they repel each other - because the electrons are all negative. At a larger distance and at a temperature below the boiling point, they attract to each other. So the atoms act like there are spring between them. Some attractions between atoms are stronger than others, which is called a high bond energy. These structures are harder to melt (break the atoms loose from the grid), or to evaporate (break the atoms completely apart into a gas).

#### Primary Interatomic Bonds

Ionic Bonding

This is the bond when one of the atoms is negative (has an extra electron) and another is positive (has lost an electron). Then there is a strong, direct Coulomb attraction.  Ionic bonds are the strongest bonds. In real solids, ionic bonding is usually combined with covalent bonding.  An example is NaCl (salt). In this molecule, there are more electrons around Cl, forming Cl- and less around Na, forming Na+. The result the a cubic crystal - salt.

Covalent Bonding

In covalent bonding, electrons are shared between the molecules, to saturate the valency. The simplest example is the H2 molecule, where the electrons spend more time in between the nuclei than outside, thus producing bonding.

Metallic Bonding

In metals, the atoms are ionized, loosing some electrons from the valence band. Those electrons form a electron sea, which binds the charged nuclei in place, in a similar way that the electrons in between the H atoms in the H2 molecule bind the protons.

Image of atoms in a crytal lattice produced by scanning tunneling microscope.

#### Secondary Bonding (Van der Waals)

Fluctuating Induced Dipole Bonds

Since the electrons may be on one side of the atom or the other, a dipole is formed: the + nucleus at the center, and the electron outside. Since the electron moves, the dipole fluctuates. This fluctuation in atom A produces a fluctuating electric field that is felt by the electrons of an adjacent atom, B. Atom B then polarizes so that its outer electrons are on the side of the atom closest to the + side (or opposite to the – side) of the dipole in A. This bond is called van der Waals bonding.  Examples - Wringing of gauge blocks - precision ground hardened flat steel surfaces that can stick together. Gecko's feet are also considered to be an example of van der Waals forces.

Polar Molecule-Induced Dipole Bonds

A polar molecule like H2O (H partially +, O partially – ), will induce a dipole in a nearby atom, leading to bonding. This is why water dissolves lots of things. (Solvent)

Permanent Dipole Bonds

This is the case of the hydrogen bond in ice. The H end of the molecule is positively charged and can bond to the negative side of another dipolar molecule, like the O side of the H2O dipole.

#### Static Charge Bonding

This is a bulk effect (much larger scale than atomic/molecular level), but has some similarities. The famous example is static electricity causing paper pieces to attract stick to a charged plastic rod. The rod got charged by rubbing it on a cloth (a woollen jumper (sweater) works well.

The same applies to plastic foodwrap and some adhesive labels. Clingfilm is either made from PVC or low density polyethylene that's treated to make it stretch. When you unroll the clingfilm, some of the electrons on the surface of one layer get pulled away onto the adjacent layer. This creates patches of positive and negative electrostatic charge. Because clingfilm is a good insulator, this charge persists for quite a while. When you wrap the clingfilm around itself or another insulator (like glass) the electrostatic charge induces an opposite charge in the other surface and the two stick together. If you try this on a conductor, like metal, it won't stick because the charge is dispersed immediately. From BBC Focus Magazine

## Molecules

If molecules formed a closed shell due to covalent bonding (like H2, N2) then the interaction between molecules is weak, of the van der Waals type. Thus, molecular solids usually have very low melting points (as in they might normally be liquid or gas).

## Physical Properties

The most obvious properties of a material are density and melting point.
Density = Mass/Volume. Density is most dependent on the Atomic Mass.
Melting points depends on bond energy - which is more complex. The melting point of alloys is usually somewhere in between the component metals. But it  can also be completely different (esp lower)- like cast iron (1200C) made of pure iron (1536C) and carbon (3652C) for it's graphite form. Graphite is not at all like the arrangement of carbon in steel, the small percentage of carbon only lowers the melting point, rather than raise it.

Density and Melting point of Some Engineering Materials

 Metal or Alloy Density  (kg/m3) Melting Point (0C) Admiralty Brass 8525 900-940 Aluminum 2712 660 Aluminum - melted 2560 - 2640 600 - 655 Aluminum bronze (3-10% Al) 7700 - 8700 600 - 655 Aluminum foil 2700 -2750 600 - 655 Antifriction metal 9130 -10600 - Beryllium 1840 1285 Beryllium copper 8100 - 8250 865 - 955 Brass - casting 8400 - 8700 - Brass - rolled and drawn 8430 - 8730 930 Bronze - lead 7700 - 8700 - Bronze - phosphorous 8780 - 8920 - Bronze (8-14% Sn) 7400 - 8900 - Cadmium 8650 321 Cast iron 6800 - 7800 1175 - 1290 Chemical Lead 11340 - Chromium 7140 1860 Cobolt 8746 1495 Copper 8930 1084 Cupronickel 8940 1170 - 1240 Delta metal 8600 - Electrum 8400 - 8900 - Gold 19320 1063 Hastelloy 9245 1320 - 1350 Inconel 8497 1390 - 1425 Incoloy 8027 1390 - 1425 Iron 7850 1536 Lead 11340 327.5 Light alloy based on Al 2560 - 2800 - Light alloy based on Mg 1760 - 1870 - Magnesium 1738 650 Manganese 7470 1244 Manganese Bronze 8359 865 - 890 Mercury 13593 -38.86 Molybdenum 10188 2620 Monel 8360 - 8840 1300 - 1350 Nickel 8800 1453 Nickel silver 8400 - 8900 - Platinum 21400 1770 Plutonium 19816 640 Red Brass 8746 - Silicon 2330 1411 Silver 10490 961 Sodium 968 97.83 Solder 50/50 Pb Sn 8885 - Stainless Steel 7480 - 8000 1510 Steel 7850 1425 - 1540 Tin 7280 232 Titanium 4500 1670 Tungsten 19600 3400 Uranium 18900 1132 Vanadium 5494 1900 White metal 7100 - Wrought Iron 7750 - Yellow Brass 8470 905 - 932 Zinc 7135 419.5

Thermal Properties of some major materials
Melting and boiling occurs with every material (apart from combustion of course)...

 Substance (1 atm) Melting Point1) - Tm - (K) Heat of Fusion2) - Lf - (cal/g) Boiling Point3) - Tb - (K) Heat of Evaporation4) - Lv - (cal/g) Hydrogen 13.8 14.0 20.3 108 Oxygen 54.4 3.3 90.2 50.9 Nitrogen 63.3 6.1 77.3 48.0 Ethyl Alcohol 156 24.9 351 205 Mercury 234 2.7 630 70.0 Water 273.15 79.7 373.15 539 Lead 600 5.9 2,023 208 Aluminum 932 94.5 2,740 2,500 Gold 1,336 15.4 2,933 377 Copper 1,359 32.0 1,460 1,210 Iron 1,808 69.1 3,023 1,520
1) The melting point of a solid is the temperature at which it changes state from solid to liquid.

2) Heat of fusion is the amount of energy (heat) that is required for a material to undergo a change of phase from solid to fluid. It is also called the latent heat of fusion or the enthalpy of fusion.

3) The boiling point of a substance is the temperature at which it can change state from a liquid to a gas throughout the bulk of the liquid.

4) Heat of evaporation describes the amount of energy in the form of heat that is required for a material to undergo a change of phase from fluid to gas.

## Classification

Engineering materials are often classified like this;

Metals        Polymers        Ceramics        Other
Then each of these can be subdivided another level;
Metals > Ferrous and non-ferrous metals
Polymers > Thermoplastics and thermosets. Another attribute is elastomeric properties
Ceramics > Ceramics, cermets, glasses or also by use; hardness, high temperature (refractory)
Other > Natural materials (wood, fibres), concrete

Exercise
Construct a chart of material classification, using an appropriate computer method to lay out the information. (e.g. Word, Excel, 2D CAD, image editing, charting software, etc). include approx 50 materials and categories of material.

Lesson Whiteboard:

Assignment:

• Review Test #10101.
• Do a practice test 10101cp