Point Defects in Metals I

4. Experimental Techniques for Studying Point Defects
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All of the bands in an insulator are either filled or empty. Furthermore, the gap between the highest energy filled band and the lowest energy empty band in an insulator is so large that it is difficult to excite electrons from one of these bands to the other. As a result, it is difficult to move electrons through an insulator. Semiconductors also have a band structure that consists of filled and empty bands.

The gap between the highest energy filled band and the lowest energy empty band is small enough, however, that electrons can be excited into the empty band by the thermal energy the electrons carry at room temperature. Semiconductors therefore fall between the extremes of metals and insulators in their ability to conduct an electric current.

Defects in Crystals - Chemistry LibreTexts

To understand why metals become better conductors at low temperature it is important to remember that temperature is a macroscopic reflection of the kinetic energy of the individual particles. Much of the resistance of a metal to an electric current at room temperature is the result of scattering of the electrons by the thermal motion of the metal atoms as they vibrate back and forth around their lattice points. As the metal is cooled, and this thermal motion slows down, there is less scattering, and the metal becomes a better conductor.

Semiconductors become better conductors at high temperatures because the number of electrons with enough thermal energy to be excited from the filled band to the empty band increases. To understand why semiconductors are sensitive to impurities, let's look at what happens when we add a small amount of a Group VA element, such as arsenic, to one of the Group IVA semiconductors. Arsenic atoms have one more valence electron than germanium and silicon atoms. If the amount of arsenic is kept very small, the distance between these atoms is so large that they don't interact. As a result, the extra electrons from the arsenic atoms occupy orbitals in a very narrow band of energies that lie between the filled and empty bands of the semiconductor, as shown in the figure below.

This decreases the amount of energy required to excite an electron into the lowest energy empty band in the semiconductor and therefore increases the number of electrons that have enough energy to cross this gap. As a result, this "doped" semiconductor becomes a very much better conductor of electricity than the pure semiconductor.

Because the electric charge is carried by a flow of negative particles, these semiconductors are called n-type. These atoms have one less valence electron than silicon or germanium atoms, and they can capture electrons from the highest energy filled band to form holes in this band. The electric charge is now carried by a flow of positive particles, or holes, so these semiconductors are called p-type. Bringing n -type and p -type semiconductors together produces a device that has a natural one-directional flow of electrons, which can be turned off by applying a small voltage in the opposite direction.

This junction between n -type and p -type semiconductors was the basis of the revolution in industrial technology that followed the discovery of the transistor by William Shockley, John Bardeen, and Walter Brattain at Bell Laboratories in You may have noticed that metal ice-cube trays feel significantly colder then plastic ice-cube trays when you remove them from the freezer. The metal trays feel colder because metals are much better conductors of heat than plastic. The ease with which metals conduct heat is related to their ability to conduct an electric current.

Most of the energy absorbed by a metal when it is heated is used to increase the rate at which the atoms vibrate around their lattice sites. But some of this energy is absorbed by electrons in the metal, which move from orbital to orbital through the conduction band. The net result is a transport of kinetic energy from one portion of the metal surface to another.

Metals feel cold to the touch because the electrons in the conduction band carry heat away from our bodies and distribute this energy through the metal object.

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Plastics, on the other hand, are thermal insulators. They are poor conductors of heat because orbitals in which electrons are held tend to be localized on an individual atom or between pairs of atoms. The only way for electrons to carry energy through a plastic is to use this energy to excite an electron from a filled orbital to an empty orbital.

But the difference between the energies of the filled and empty orbitals is so large that this rarely happens. The difference between thermal conductors and thermal insulators can be quantified by defining the thermal conductivity of a substance as the quantity of heat transmitted per second through a plate of the material one centimeter thick and one square centimeter in area when the temperature differential between the two sides of the plate is one degree Celsius or one Kelvin. The copper used to for pots and pans has a thermal conductivity that is more than times the value for the styrofoam used for coffee cups, as shown by the data in the table below.

This table is consistent with experience, which suggests that the air that gets trapped in the fibers of a down-filled jacket is a better insulator than cotton, which is a much better insulator than nylon. It is tempting to think about solids as if the particles were locked into position, the way bricks are used to build a wall.

4.1 Point Defects in Equilibrium

This motion depends on two factors, the temperature of the system and the strength of the interactions that hold the particles together. The higher the temperature, the faster the particles are moving. The stronger the force of attraction between particles, the smaller the distances the particles move apart. Because the van der Waals forces that hold molecules together are much weaker than the bonds between atoms in a metal or between positive and negative ions in an ionic compound, molecular crystal expand more when heated than metals or ionic compounds.

The difference between the coefficients of thermal expansion of iron and copper was the source of a major problem for the Statue of Liberty, which consists of copper plates supported by an iron skeleton. The insulating material used to keep these two metals from coming into contact was inevitably rubbed away because of differences in the rate at which these two metals expand when heated and contract when cooled. When this happened, the two metals came into contact, forming an electric cell that greatly increased the rate at which the iron skeleton corroded.

The same phenomenon, however, is used to form the thermostats that turn electrical appliances on and off. Seller Rating:. Condition: Good. First Edition. Former Library book. Shows some signs of wear, and may have some markings on the inside. Seller Inventory GRP More information about this seller Contact this seller 1. Condition: Very Good. Ships from Reno, NV. Great condition for a used book! Minimal wear.

10. Point Defects

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Defects in Metals

A copy that has been read, but remains in excellent condition. Pages are intact and are not marred by notes or highlighting, but may contain a neat previous owner name.

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Point defects in metals

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Impurities

This book has hardback covers. In good all round condition. No dust jacket. Please note the Image in this listing is a stock photo and may not match the covers of the actual item,grams, ISBN: Seller Inventory More information about this seller Contact this seller 7. Published by The Institute of Metals, London Condition: Acceptable.

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Vacancies and other Point Defects in Metals and Alloys

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Spine ends and leading corners are worn. Tanned and grubby page block. Binding is loose at beginning of book. Contents are clear. More information about this seller Contact this seller 8. More information about this seller Contact this seller 9. Hard Cover. Condition: Fair. No Jacket. The Institute of Metals monograph and report series no A self interstitial atom is an extra atom that has crowded its way into an interstitial void in the crystal structure.

Self interstitial atoms occur only in low concentrations in metals because they distort and highly stress the tightly packed lattice structure. A substitutional impurity atom is an atom of a different type than the bulk atoms, which has replaced one of the bulk atoms in the lattice. An example of substitutional impurity atoms is the zinc atoms in brass. In brass, zinc atoms with a radius of 0. Interstitial impurity atoms are much smaller than the atoms in the bulk matrix. Interstitial impurity atoms fit into the open space between the bulk atoms of the lattice structure.

An example of interstitial impurity atoms is the carbon atoms that are added to iron to make steel. Carbon atoms, with a radius of 0. Vacancies are empty spaces where an atom should be, but is missing. They are common, especially at high temperatures when atoms are frequently and randomly change their positions leaving behind empty lattice sites.