The principle properties of materials which are of importance to the engineer in selecting materials. These can be broadly divided into:
Physical properties of materials
These properties concerned with such properties as melting, temperature, electrical conductivity, thermal conductivity, density, corrosion resistance, magnetic properties, etc. and the more important of these properties will be considered as follows :
1. Density
Density is defined as mass per unit volume for a material. The derived unit usually used by engineers is the kg/m3 . Relative density is the density of the material compared with the density of the water at 4?C. The formulae of density and relative density are:
2. Electrical conductivity
Figure 1 shows a piece of electrical cable. In this example copper wire has been chosen for the conductor or core of the cable because copper has the property of very good electrical conductivity. That is, it offers very little resistance to the flow of electrons (electric current) through the wire. A plastic materials such as polymerized has been chosen for the insulating sheathing surrounding the wire conductor. This material has been chosen because it is such a bad conductor, where very few electrons can bass through it. Because they are very bad conductors they are called as insulators.
MAC.BARAA.H.HADI Notes Engineering Materials 2016
2 | P a g e
There is no such thing as a perfect insulator, only very bad conductors
For example, metallic conductors of electricity all increase in resistance as their
temperatures rise. Pure metal shows this effect more strongly than alloys. However, pure metals generally have a better conductivity than alloys at room temperature. The
conductivity of metals and metal alloys improves as the temperature falls. Conversely, non-metallic materials used for insulators tend to offer a lower resistance to the passage of electrons, and so become poorer insulators, as their temperatures rise. Glass, for example, is an excellent insulator at room temperature, but becomes a conductor if
raised to red heat.
3. Melting temperature of material
The melting temperatures and the recrystallization temperatures have a grate effect on the materials and the alloys of the materials properties and as a result on its applications.
4. Semiconductors
So far we have examined the conductivity of the metals and the insulating properties of the non-metals (exception : carbon). In between conductors and isolators lies a range of materials known as semiconductors. These can be good or bad conductors depending
upon their temperatures. The conductivity of semiconductor materials increases rapidly for relatively small temperature increases.
This enable them to be used as temperature sensors in electronic thermometers.
Semiconductor materials are capable of having their conductors properties changed during manufacture. Examples
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
|