Resistivity and temperature coefficient of resistance
- table or chart of the resistivity and temperature coefficient of
resistance of a variety of materials many of which are used in electronics
equipment.
Two important electrical features of any substance are its
resistivity and its temperature coefficient of resistance. These two properties
are particularly important and will often determine whether a substance can be
used in the manufacture of a wide variety of electrical and electronics
components from wire to components such as resistors, potentiometers and many
more.
Resistivity
The electrical resistivity of a material is also known as its specific
electrical resistance. It is a measure of how strongly a material opposes the
flow of electric current. The resistivity is the electrical resistance per unit
length and per unit of cross-sectional area. This is for a particular material
at a specified temperature. It is also possible to define the resistivity of a
substance as the resistance of a cube of that substance having edges of unit
length, with the understanding that the current flows normal to opposite faces
and is distributed uniformly over them. The SI unit for electrical resistivity
is the ohm metre, although it is also sometimes specified in ohm centimetres.
This means that a low resistivity indicates a material that readily allows the
movement of electrons. Conversely a high resistivity material will have a high
electrical resistance and will impede the flow of electrons.
Elements such as copper and aluminium are known for their low levels of
resitivity. Silver and in particular, gold have a very low resistivity, but for
obvious cost reasons their use is restricted.
Temperature coefficient of resistance
The temperature coefficient of resistance, often designated alpha, is defined as
the amount of change of the resistance of a material for a given change in
temperature. A positive value of alpha indicates that the resistance increases
with temperature; a negative value of alpha indicates the resistance decreases;
and a value of zero a indicates that the resistance is constant. For most metals
it is found that the resistance increases with temperature, whereas the opposite
is true for semiconductor where the resistance falls with increasing
temperature.
| Substance |
Resistance at 0C
Ohm metres |
Temperature Coefficient
K-1 |
| Aluminium |
0.25 x 10-6 |
38 x 10-4
(18C - 100C) |
| Antimony |
3.9 x 10-6 |
40 x 10-4 |
| Bismuth |
10.6 x 10-6 |
42 x 10-4 |
| Brass |
~0.6 - 0.9 x 10-6 |
10 x 10-4 |
| Cadmium |
0.60 x 10-6 |
40 x 10-4 |
| Cobalt |
0.56 x 10-6 |
33 x 10-4 |
| Copper |
0.16 x 10-6 |
43 x 10-4 |
| Ebonite |
2 x 1014 |
-- |
| German silver |
1.6 - 4.0 x 10-6 |
Approx 4.5 x 10-4 |
| Glass |
1013 |
-- |
| Gold |
0.20 x 10-6 |
40 x 10-4 |
| Graphite |
300 x 10-6 |
-5.6 x 10-4 |
| Iron |
0.89 x 10-6 |
62 x 10 -4 |
| Lead |
1.9 x 10-6 |
43 x 10-4 |
| Manganin |
4.2 x 10-6 |
~0.1 x 10-4 |
| Mica |
9 x 1014 |
-- |
| Nickel |
0.61 x 10-6 |
27 x 10-4 |
| Palladium |
1.0 x 10-6 |
37 x 10-4 |
| Phosphor-bronze |
0.5 - 1.0 x 10-6 |
-- |
| Platinum |
0.98 x 10-6 |
38 x 10-4 |
| Quartz |
1 x 1013 |
-- |
| Silver |
0.15 x 10-6 |
40 x 10-4 |
| Tantalum |
1.3 x 10-6 |
33 x 10-4 |
| Tin |
1.1 x 10-6 |
45 x 10-4 |
| Tungsten |
0.49 x 10-6 |
51 x 10-4 |
| Zinc |
0.55 x 10-6 |
36 x 10-4 |
Applications
Many of the materials found in the list above are widely used in electronics.
Aluminium and particularly copper are used for their low levels of resistance.
Most wire used these days for interconnections is made from copper as it offers
a low level of resitivity at an acceptable cost. Gold while much better is more
costly and is used in much smaller quantities. Often gold plating is found on
high quality low current connectors where it ensures the lowest contact
resistance. Silver is not so widely used because it tarnishes and this can
result in higher contact resistances. The oxide can also under some
circumstances act as a rectifier which may cause some annoying problems in RF
circuits.
Tantalum is used in capacitors, and nickel and palladium are
used in the end connections for many surface mount components such as
capacitors. Quartz finds its main use as a piezo electric resonant element.
Quartz crystals are sued as frequency determining elements in many oscillators
where its high value of Q enables very frequency stable circuits to be made.
They are similarly used in high performance filters.
|
