1.2 MICROSTRUCTURES

 

• To consider materials properly we must start with the basic atomic structure and then look at the more macroscopic aspects, and how they are related to the microscopic components.

 

 

1.2.1 Atomic Structures

 

• In an atom there are some fundamental ratios,

• Each atom is understood to have a basic structure with a nucleus and orbiting electrons.

• The nucleus is a combination of neutrons and protons.

• The number of protons and neutrons in an atom are equivalent and these determine the atomic number. If there are additional neutrons in the nucleus this is called an isotope.

• The mass of the atom is determined by the sum of the neutrons and protons (the electron mass is much smaller).

• In a mole of material there are 6.023*10**23 atoms.

 

• How these components fit together is described in models,

Bohr model

- electrons have quantized energy levels

- electrons are discrete and orbit the nucleus

- a free electron has a negative energy level

Wave-mechanic model

- electron waves can behave like particles or waves

- an electron is described as an electron cloud

- electrons have energy levels including ground levels

- valence electrons are the outermost and most likely to be removed first

 

• The valences of electrons are determined with the ’spdf’ numbers.

 

• The basic atomic elements are listed in the periodic table. This is in sequence of the atomic masses, as well as proton counts. It can also be used to determine similarities in properties by proximity in the table.

 

 

• In the periodic table the metals are in the left hand side. They have 1 to 3 valence electrons. They tend to give up electrons when bonding.

 

• In the upper right hand of the periodic table are the non-metals. They typically are 1 to 3 valence electrons short of a full valence level. As a result they tend to consume electrons when bonding. These are,

He, N, O, F, Ne, P, S, Cl, Ar, Br, Kr, I, Xe, At, Rn

 

• There is a band of semimetals - including semiconductors. These often consume and give up electrons when bonding. These are

B, O, Si, Ge, As, Se, Te.

 

 

1.2.1.1 - Crystal Structures

 

• Understanding crystal structures can help understanding of crystalline materials such as metals.

 

• Think of dropping balls into a box. it can fall randomly, but often it will fall into patterns. This is like atoms in a solid.

 

• If all of the balls fall into a single organized pattern then we can say there is a single crystal.

 

• Three of the basic structure types to consider are,

bcc - body centered cubic

fcc - face centered cubic

hcp - hexagonal close packed

 

 

• In a common solid there will be many regions in the crystal, but there will also be boundaries where the crystal properties change. These are known as boundaries.

 

• A common effect that can occur is slippage along one of the planes of the crystal. An example is pictured below,

 

 

• Different crystal structures will result in different possible slip planes.

bcc has 48 possible slip planes

fcc has 12 possible

hcp has 3 possible

 

• Other slip structures are also possible

 

 

 

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