Answer to Question #287394 in Material Science Engineering for jay

Question

List the THREE (3) classification ofthe imperfections and THREE (3) defects in solids.

D.                Which properties of material would you need to consider if you required materials which were:

Capable of being bent into a fixed shape, iii. Capable Of not fracturing when small cracks are present, iv. Not easily broken

v. Act as electrical insulator vi. A good conductor of heat vii. Capable of being used as the lining for a tank storing acid.

E.                 List the TWO (2) applications of the following materials:

i. Plain Carbon steel 0.3 % Carbon ii. Medium Carbon Steel 0.7 Carbon iii. High Carbon Steel 0.9 — 1.5% Carbon iv. Stainless steel Cast Iron vi. Wrought iron

Answer 1

Crystalline solids have a definite structure. They have long term arrangement of their molecules. This means that they have repeated arrangement of a unit cell to form a space lattice structure. So the arrangement of molecules, in the long run, will remain the same.

However, these crystals are not perfect. They have defects or imperfections in their arrangement of constituent particles. These defects are a deviation from their uniform arrangement of particles. The defects are of two kinds

• Point Defects: When the deviation occurs around an atom/particle it is a point defect. It could be due to displacement, an extra particle or a missing particle.
• Line Defects: When there is an abnormality in the arrangement of an entire row, then it is a line defect.

Point Defects

Point defects usually occur when the crystallization has occurred very rapidly, not allowing a perfect crystal arrangement to form. Although it should be noted there are imperfections in even during a slow crystallization. There are mainly three different kinds of point defects. Let us study each of them.

Stoichiometric Defects

Stoichiometric compounds are those who maintain their stoichiometry. That means they maintain their ratios of cations and anions as indicated by their chemical formula. The defects in the solid do not affect this ratio. They are of various kinds, such as

Vacancy Defect

Here the lattice site is simply vacant, which means there is a missing particle. In a perfect crystal, there would not be this vacancy. This defect will lead to a reduced density of the solid. Some surrounding particle may move to fill in the gap, but the vacancy will only shift in the opposite direction, Also the solid structure of the crystal will ensure that the particles surrounding the vacant spot do not collapse.

Interstitial Defect

There is an unoccupied space at the very center of the cube structure of the solid. When the eight spheres of a unit cell meet at the center they leave a little space, the interstitial site. Sometimes another particle will occupy this space. This is what we call an interstitial defect.

These are extra atoms or molecules or ions that occupy space which was supposed to be empty, which is why it is a defect. The density of the solid also increases due to such defects.

Schottky Defects

In this defect, more than one particle is missing. But the number of cations missing is equal to the number of anions missing. So the electrical composition remains the same.

Since more than one particle is missing, the mass of the structure will decrease. But since the volume remains the same, the density of the solid will also decrease. If there are too many particles missing, then the lattice structure may be compromised. this could mean the stability of the structure may suffer. An example of this defect is NaCl, KCl, and other such ionic compounds.

Frenkel Defects

When a cation is missing from its normal position and has instead occupied an interstitial site it is a Frenkel Defect. Here the electrical neutrality will be unaffected. Also, it is usually cations that cause this defect since they can easily fit in the interstitial site due to their small and compact size.

Here no particles are actually missing, only the position is wrong. So the density of the solid will not change. It usually occurs in compounds where there is a significant size difference between cations and anions. Some examples are AgCl, AgBr etc.

D. ii. Malleability - being able to bend or shape easily would make a material easily malleable, eg sheet metal such as steel or silver is malleable and can be hammered into shape.

Ductility - materials that can be stretched are ductile, eg pulling copper into wire shows it is ductile.

iii. Metals hold the highest values of fracture toughness. Cracks cannot easily propagate in tough materials, making metals highly resistant to cracking under stress and gives their stress–strain curve a large zone of plastic flow.

v. Electrical insulators are materials with a high resistivity (resistivity is a property of the material) so they can make objects with a high resistance. This allows insulators to prevent electric current from flowing where it's not wanted. Insulators are useful for coating wires, or acting as dielectrics in capacitors.

vi. Considering their use in electrical systems, what makes a good conductor is decided on different criteria. Usually good current carrying conductors should have following properties:

1. Low resistivity/ / high conductivity
2. Low temperature coefficient of resistivity
3. Good thermal conductivity
4. Should be easily available
5. Environmental stability
6. Malleability
7. Should be highly ductile
8. Amenable to manufacturing process
9. Easy to get in shapes, rods and wires.
10. Economic viability

Copper scores over other in most respects, and is universally preferred. Aluminium is also finding good use due to cost factor as also some properties are better— like thermal conductivity, malleability etc.

Having said that, good conductor for different requirements will be different. Brass is most common conductor after copper, and gets preference due to its workable nature and lower cost compared to copper.

In critical applications Silver gets preference despite high costs because of its superior properties as conductor. Even gold finds its place for contact materials in extreme cases.

For switchgear contacts tungsten is common because of its ability to withstand arcing, low wear and relative inert nature / resistance to oxidation.

Even GI wire is considered good material for earthing conductor, as it is cheap, can withstand rough weather and humidity. It is easily available and tough against mechanical stresses. It goes deep underground and remains in contact with salty water.

vii. Epoxy Linings

Epoxy tank coatings are made from epoxy material, which can be developed to be chemical-resistant. They are strong linings that are aggressive and capable of working under high-temperature environments. Epoxy is moisture tolerant and solvent-free, making it an obvious choice for water tanks.

Polyurethane Linings

Polyurethane tank linings fall somewhere in the middle in terms of flexibility, though they’re certainly more flexible than other options. That said, they’re ideal for concrete-made structures.

Vinyl Ester Tank Liners

Vinyl ester liners are made from vinyl material and provides some of the highest resistance to chemical infiltration and contamination. This material has found extensive use in most chemical industries because of its high resistance to temperature and chemical damage.

Polyurea Tank Liners

Polyurea material is rare in the tank lining industry. This newer commercialized lining material is applied in a gel-like form using specialized spraying equipment. Polyurea provides perfect flexibility and strength as a tank liner. It also has elongation properties of up to 900%. There are many variations of polyurethane and they’re designed for varying applications. Polyurea tank linings are also abrasion resistant and solvent-free.

Cementitious Liners

There are two common types of cementitious linings, which include epoxy and polymer modified cementitious linings. These materials find common use in the lining of concrete tanks used in water treatment and to safely contain chemicals. These linings are ideal for waterproofing underwater retaining or collection structures.

Zinc Tank Liners

A zinc-made tank liner has zinc silicate material and is anti-corrosive. Its additives and binders can act as effective waterproofing material. This material has zinc dust levels of a high percentage. These liners give a zinc-to-zinc contact with cathode-like protective layers like some that occur in a galvanizing process. These linings are non-porous, which makes their cleaning a little challenging. However. they can still safely contain chemicals. The zinc coating is effective because it fills up the capillaries and pores. These coatings also resist the power of solvents and they can get used to line solvent tanks. But they’re not resistant to bases and strong acids.

Stainless Steel Liners

Stainless steel linings are made from polished or rough stainless steel. Rough-surfaced steel linings are most commonly used. Tanks with stainless steel linings should get cleaned to an ideal Water White Standard on a regular basis.

You can safely contain chemicals and avert tank leakages from all kinds of tanks by ensuring that you buy the right kind of tank lining material. There are numerous tank lining materials and they have varying effectiveness in serving as tank liners, which can prevent leakages and chemical contaminations. Your choice of a liner should be based on various factors, which include its ability to serve as a perfect water-tight sealant.

E. i. It has a good combination of strength and ductility and may be hardened and carburized. Used for simple structural applications such as cold formed fasteners and bolts.

ii. These steels represent a good balance of strength and ductility, and they have good wear resistance. You will find medium-carbon steels used for forgings, automotive parts and large machine parts. These steels are mainly used for making shafts, axles, gears, crankshafts, couplings, and forgings.

iii. Common applications of higher carbon steels include forging grades, rail steels, spring steels (both flat rolled and round), pre-stressed concrete, wire rope, tire reinforcement, wear resistant steels (plates and forgings), and high strength bars.

iv.

• Culinary uses. Kitchen sinks. Cutlery. Cookware.
• Surgical tools and medical equipment. Hemostats. Surgical implants.
• Architecture (pictured above: Chrysler Building) Bridges. Monuments and sculptures.
• Automotive and aerospace applications. Auto bodies. Rail cars.

v. It is used to make decor items like table base, candle holder, curtain rods, in making pipes, fences and gates, nuts, bolts, rivets, chains, crane hooks, plates.