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Introduction to Materials and Atomic Bonding


Out of everything, anything is made out of, there are three main groups of materials being metallics, polymerics and ceramics. As well as this, though, different materials can also be combined to make a composite too. This article will be looking at the properties of metals, ceramics, polymers and composites while also looking at ionic, covalent, metallic and Van der Waals bonds.


Metals

With metals, there are mainly two types. Ferrous metals which have the inclusion of iron (such as irons, carbon steels, stainless steels etc.) and non-ferrous metals (such as aluminium, titanium, copper etc.). Metals account for around two thirds of all the materials found in the periodic table and consequently makes up 24% of the mass of the planet. Metals have some useful properties that some of them are:
  • Tough
  • Ductile
  • Strong
  • Have high melting points
  • Have good thermal conductivity
  • Have good electrical conductivity

Here are a few common metals with the properties that are most useful that are associated with them:
  • Iron/Steel - very strong
  • Aluminium - easy to form, cheap, readily available and recyclable
  • Copper - high ducitly, good electrical and thermal conductivity and does not corrode easily at all (this is why many pipes are made from copper).
  • Titanium - this metal is used when weight is an issue (Titanium is light), when you need high strength at high temperatures (1,000 degrees Fahrenheit) or high corrosion resistance.
  • Nickel - Nickel is used at even higher temperatures (1,500-2,000 degrees Fahrenheit) and when corrosion resistance is required.
  • Refractory materials - Past 2,000 degrees Fahrenheit is when refractory metals are used.
On an atomic level, metals have metallic bonds where the outer electrons of the metal's shell are freed to roam in a sea of electrons around all the other atoms (which is why they are good electrical conductors). 


Polymers

The word 'polymer' means 'many parts' which describes polymers quite well. Polymers have many chemically bonded parts that eventually form a solid. Two important polymers are plastics and elastomers. Plastics are a very large group of synthetic materials which are processed and moulded into shape. Just as there are with metals, there are many different types of plastics. Elastomers, as it kind of says in the name, are a type of polymer which will deform in shape when a large force is applied to it but will form back to its original shape when the force has removed (e.g. a stretched elastic band).

There are many properties of polymers that make them more useful than metals or ceramics. There are many polymers that are:
  • Less dense than metals or ceramics.
  • Corrosive resistance.
  • Are of good use when combined with human tissue.
  • Are great at being resistant to electricity or heat.
Polymer plastics can be divided into two groups: thermoplastics and thermosetting plastics.

Thermoplastics basically mean that the plastic melts on being heated. There are four important thermoplastics which are polyethylene, polypropylene, polystyrene and polyvinyl chloride.

Thermosetting plastics, on the other hand, do not melt on being heated. A few examples of some thermosetting plastics include alkyds, amino and phenolic resins, epoxies, polyurethanes, and unsaturated polyesters.

Although there are naturally occurring polymers, most polymers are man-made by creating and engineering carbon and hydrogen atoms together differently to make different polymers.

A polymer molecule consists of a long chain of covalently bonded atoms with secondary bonds holding the long bonds together (cross linked polymers). Without these secondary bonds, the long chains can freely move around each other changing the state of the polymer. The secondary bonds are broken when melted which is why many polymers move to liquid form when melted and then solidify when cooled and the secondary bonds re-bond.

Man has made some 'super' polymers such as Kevlar that have been used in bulletproof vests and is 20 times stronger than steel. Most polymers are made from petroleum and natural raw gas products.

Ceramics

The word 'ceramic' come from the Greek word 'keramikos' which means 'pottery'. This kind of describes what ceramics are. They are non-metallics that come in powdered form and are heated to produce properties such as hardness, strength, low electrical conductivity and brittleness. They are crystalline in structure and are made from a bond between a metallic and non-metallic element such as Calcium and Oxygen to produce Calcium Oxide (CaO).

Ceramics can be both light or heavy and are typically very strong. The only downside is that they tend to be very brittle (crack easy). Cement, glass and abrasives are all types of ceramics. 

Ceramics, on an atomic level, are kept together by covalent and ionic bonding. Covalent and ionic bonds are generally much stronger than metallic bonds which is why you will find ceramics are brittle and metals are ductile.


Composites

The definition of a composite is a material that is made by two or more different materials that retain their own properties to make a new material (composite) that has a combination of both properties of the two different materials. Examples of composites are fibreglass or carbon fibre.

Here are some classification for composites:
  • Metal-matrix composites
  • Sandwich structures
  • Reinforced plastic
  • Ceramic-matrix composites
  • Concrete
Based on a composite's strengthening mechanism, they can be grouped into three categories being either fibre reinforced, particle reinforced or dispersion strengthened. 
  • Dispersion strengthened composites have a fine distribution of secondary particles in the matrix of the material which enables the composite to deform. You will find that many metal-matrix composites are in fact dispersion strengthened.
  • Particle reinforced composites have a large volume of particles dispersed into the matrix with the load being shared by the particles and the matrix.
  • Fibre reinforced composites have fibres that are the primary load bearing component.
If a composite has been manufactured correctly, the strength of the reinforcement will combine with the toughness of the matrix to achieve something that would not have been achievable with a single material.


There are three main classifications when it comes to the structure of a material:
  • Atomic structure - This includes the type of bonds between atoms and how they are arranged next to each other. The atomic structure affects most properties of the material such as chemical, physical, thermal, electrical, optical and magnetic properties.
  • Micro structure - These are the features of a structure that can be seen at a microscopic level.
  • Macro structure - These are the features of a structure that can be seen with the naked eye.
You will find that the properties of a material can give you a clue into how the material is structured. 


Atomic Bonding

Electrons are the basis of most bonds because the atom's main objective is to have a full outer shell of electrons. The first shell around a nucleus will have a maximum of two electrons, next shell will have 8 and so will the third shell etc. 

If an atom has a few electrons in it's outer shell, it will try to get rid of the electrons (as this does not take much energy).
If an atom has around 5-7 electrons in the outer shell, it will take much more energy to get rid of them. For this reason, you will find the atom tries to take or share electrons from another atom to complete the outer shell.


Ionic Bonding

Ionic bonds occur between a non-metal and metal (making it a ceramic). You will find that metals tend to have 1-3 electrons in their outer shell and non-metals have 5-7 electrons in their outer shell. For this reason, to make both atoms stable, the metal atom gets rid of its electrons and gives them to the non-metal. Now, both atoms have a full outer shell of electrons and are stable. 
The thing about this is that by removing and gaining electrons changes the whole atom's charge. Before, the atoms charge was 0. After, the metal will have a positive charge making it a cation (think that it is 'pussy'tive). The non-metal has gained electrons and therefore will become negative and an anion. Due to the two atoms now have opposite charges, they are attracted to each other. This attraction of a cation and anion is an ionic bond.

Solid materials with ionic bonds have some properties. They are generally: 
  • Hard because it is difficult for the particles to slide over each other.
  • Have high melting points because ionic bonds are very strong and take a lot of energy to break.
  • Brittle because the material tends to crack/cleave than deform because of the strong bonding.
  • Can be transparent because it is the free electrons that interact with photons (but there are no free electrons in ionic bonds).


Covalent Bonding

Covalent bonds occur only between non-metals because non-metals have around 4 or more electrons in the outer shell. This means that it will take more energy for the atom to remove the electrons than to gain a new bond with another atom. Therefore, two non-metals make a new bond by sharing atoms in both their outer shells. This enables both atoms to think they have a full outer shell. This is a covalent bond.
You will find that the bonds between atoms are strong. But, the bonds between molecules are weak. For this reason, covalent bonded materials are usually brittle.
Compounds with covalent bonds can be solid, liquid or gas at room temperature: it all depends on the number of atoms in the compound.  The more atoms in each molecule, the higher the compound's melting and boiling point because it will take more energy to break all the bonds.

Here are some more common features of covalent bonded materials:
  • Hard
  • Good at insulating
  • Brittle (cleave rather than deforms)
  • They can be transparent as there are no free electrons to interact with photons.

Metallic Bonding

With most metals having 1-3 electrons in the outer shell, the force that keeps the electrons attracted to the nucleus of the atom is weak (because the valence electrons are far away from the nucleus and feel less attraction). As well as this, the metal wants to gain a full outer shell. For this reason, the electrons in the outer shell move away from the nucleus and into, what can be described as, a 'sea of electrons' that roam around metal atoms. With electrons leaving each metal atom, the 'sea of electrons' has a negative charge while the atoms will have a positive charge (cations). This is what keeps the atoms together. At first, you may think that the positively packed cations would repel against each other. They do. But, the force of attraction from the sea of electrons is much larger and keeps the atoms close together.

The free flow of electrons makes metals good at conducting electricity and heat while the delocalised nature of metallic bonds makes it possible for metals to deform without breaking unlike the case with ceramics and covalent/ionic bonds.

Due to the free electrons, the cations arrange themselves in a crystalline structure and are very close to each other to maximise the strength of the bonds. Here are some more common properties of metals:
  • They are opaque as the free electrons interact with photons.
  • Relatively ductile.
  • Good at conducting electricity and heat.

Van der Waals Bond

Although van de Waals bonds occur in all materials, they are most important in plastics and polymers.

In essence, electrons are constantly moving around a nucleus of an atom. At one point in time, all the electrons may be on one side of the nucleus making that side very negative and the opposite side positive. This imbalance of charge occurs with every atom and as a molecule gets larger, more of these imbalanced charges will form a much larger overall charge.

Plastics and polymers are made up of long chains of molecules consisting of carbons atoms covalently bonded to the likes of hydrogen or nitrogen etc. The bonds between atoms are very strong. The bonds between the long chains of molecules that allow sliding or ruptures to occur are called Van der Waals forces.

As the molecules become larger, the Van der Waals forces increases. An example of this is polyethylene and ethylene gas. They are both composed of hydrogen and carbon atoms bonded in the same ratios. As the molecules increase in size, the Van de Waals forces increase which is why the matter goes from gas to liquid to eventually solid as the molecules increase in size.


For thermoplastics, heat can be used to break the Van der Waals bonds which means they can be remelted and recycled.  On the other hand, thermosetting materials have three dimensional bonds which makes it difficult to break the bonds through heating. For this reason, they cannot be remelted or formed as easily as thermoplastics.

About Will Green

A student in England studying Automotive Engineering with Motorsport, Will created Ask Will Online back in 2010 to help students revise and bloggers make money. You can follow AskWillOnline via @AskWillOnline.

1 comments so far:

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