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Friday, 26 July 2019

Properties and Uses of High Resistivity Materials


Properties and Uses of Mercury:


It is silvery white metal. It is in the liquid state at room temperature and dissolves most metals and forms product abeld amalgums. it is a good conductor of heat and electricity. It is heavy in the liquid state. It gets oxidised if heated beyond 300°C and in the presence of oxygen or air.

Some important properties are listed below :

(a) It is the only metal in liquid state at room temperature.
(b) It is poisonous.
(c) Its expansion and contraction is uniform over a wide range of temperature changes.
(d) Its gets oxidised in the presence of oxygen if heated beyond 350°C.
(e) Its boiling point is 357°C.

It is used in mercury vapour lamps with a high luminous efficiency of about 40 lumens per watt and as arc rectifiers to convert AC to DC. It is used for making and breaking contact in buchholz relay and in thermometers.

Properties and Uses of Tungsten:


Some of the important properties of Tungsten are given below

(a) It is grey in colour and is one of the standard resistance material.
(b) It has the highest melting point of 3300°C amongst all the metals. Therefore, its refractory qualities are very much favourable.
(c) It is very hard metal and does not become brittle at high temperatures. (d) It can be drawn into xery thin wires for making filaments.
(e) Its resistivity is about twice that of aluminium.
(f) It has very high tensile strength in its thinnest form.
(g) It oxidises very quickly in the presence of oxygen even at temperatures of a few hundred degrees centigrade.
(h) It does not exhibit magnetic properties when pure but can easily alloy with steel called Tungsten Steel which is a magnetic material of top quality
(i) Its Atomic weight is 184 and resistivity 5.46 micro ohm / cm2.
(j) In the atmosphere of inert gases or in vacuum it can easily work upto 2000°C.

It is used in incandescent lamps as filaments due to its high melting points. As heater coil in electron tubes.



Properties and Uses of Molybdenum :


It has a high melting point of 2620°C and its boiling point is 3700°C. Its thermal co-efficient of expansion is 5.3 X 10-6 / degree and the resistivity ρ= 0.048 0 mm2/mm. Its normal temperature resistance co-efficient α = 0.047 / degree. It is mostly used as a target in X-ray tube and structure member in high vacuum electron tubes because of its ability to form a tight seal with glass.

Properties and Uses of Tantalum :


It is a material having resistivity (ρ) of 1.24 μΩ - cm and its temperature co-efficient of resistance (α) is 0.0036 / degree. Its melting point is 2900°C and its specific gravity is 16.6. Its efficiency for lamp filament is about 1.6 watts per candle power. Due to its low efficiency it is seldom used.

Properties and Uses of Manganin :


It is an alloy of 86% copper, 12% manganese and 2% nickel. Nickel serves to limit the thermo e.m.f to a low value of about 1 micro V/°C. The physical properties are given below :
Specific gravity - 8.4
Resistivity at 20°C - 48 x 10-8 ohm-m
Temperature co-efficient of resistance - 1 x 10-8/°C
Working temperature - 60 to 70°C
Melting point - 102°C

It can easily be drawn into thin wires. It has high electrical resistance but posses a low value of temperature co-efficient of resistance. It is used in making wire-wound precision resistances for measuring instruments, shunts for electrical measuring instruments, resistance boxes, standard resistance coils and coils for precision electrical measuring instruments. Table 3.1 gives the resistivity, temp. co-efficient and thermo e.m.f. of some compositions of Mn, Ni, Cu.

Composition and constant of Manganin

Composition %
Resistivity
Temp. co-efficient of resistivity per °C
Thermo e.m.f. with respect to copper micro V/ °C
Mn
Ni
Cu



67
1
.5
2.03
12 x 10-5
- 0.5
67
-
33
18.8
Almost zero
- 1.0
67
28
5
22
-29 x 10-5
0.12
67
30
10
20.5
-10 x 10-5
Almost zero

In the second group of materials, it may have large thermo e.m.f. and temperature co-efficient of resistance but must have high working temperature and low cost as these materials are required in large quantities. The principle alloy in this group is Constantan (Eureka).

Properties and Uses of Nichrome :


It is an alloy of 80% of nickel and 20% Chromium or 60% nickel 15% Chromium and 25% iron. Some of the properties of Nichrome is given below :

(a) It is silver white in colour.
(b) It is ductile and can be drawn into thin wires.
(c) Its maximum permissible temperature is 1100°C.
(d) It has a high value of resistivity (100 x 10-8 ohm-m) at 20°C.
(e) Its temperature co-efficient of resistance is 0.0001.

It is used in making heating elements for electric heaters, electric ovens, electric iron, room heaters, electrical furnaces etc.

Properties and Uses of Constantan (Eureka):


It is an alloy of 60 to 65% of copper and 35 to 40% Nickel. Soft constantan wire has resistivity of 45 to 48 x 10-8 ohm-m and hard constantan wire 46 to 55 x 10-8 ohm-m. Temperature co-efficient is nearly zero. Thermo e.m.f. is 39 micro V per degree centigrade with respect to copper and therefore it is suitable for measuring temperature upto 700°C. Its working temperature is about 500°C. When bare constantan wire is heated in air for 3 seconds at a temperature of about 900°C, it acquires a thin film of electrical insulating oxide which withstands turn to turn voltage upto 1 volt. In many instance the constantan is replaced by cheaper alloys Niclin, containing less nickel than constantan due to addition of zinc, is one of such alloys. Its resistivity is 40 micro ohm-cm and maximum working temperature is 300°C. Another alloy is nickel silver alloy with still greater zinc. Its resistivity is 30 to 32 micro ohm-cm and maximum working temperature is between 200 to 300°C. Some of the properties of Constantan is given below :

(a) It can be drawn into thin wires.
(b) Its maximum permissible temperature is about 500°C.
(c) Its melting point is 1300°C.
(d) Its specific gravity is 8.9.
(e) Its temperature co-efficient of resistance is 0.00002 to 0.00005.
(f) It is rust proof and does not corrode in the presence of air or moisture.

It is used in making resistance elements for items like loading rheostat and starters for electric motors, resistance boxes and thermo couples. It is also used for resistance elements in the field regulators used for regulating the generated voltage of a generator.

Properties and Uses of Lead :


It is a soft bluish grey metal. Its specific gravity is 11.36 and melting point is 326°C. It is malleable and ductile. It is mostly used as a fusing material and also for soldering material when alloyed with tin. Solder is an alloy of two or more metals of low melting point which is used to join two or more pieces of metals. The most common solder is an alloy of tin and lead. The most popular composition is 50% lead and 40% tin or 40% lead and 60% tin. The lead solder serves to join copper, bronze, brass, tinned iron, zinc etc. Its melting point is about 185°C and its electricity conductivity is about 10% of copper. There are mainly two types of solders viz., soft solder and hard solder.

Properties and uses of Soft Lead

Soft solders are composed of lead and tin in various proportions.  The most important application  is in the field of electronic devices, coating of iron or steel sheets for roofing and filling of hollow castings etc. The tensile strength is 5.7 kgf/sq. mm and melting point is upto 400°C. The joint made of this type of solder should not be subjected to mechanical stresses due to its poor mechanical strength. Its important properties for various percentage of tin content are given in Table.

Properties of Soft Lead


                              Percentage of tin
0
10
20
33
50
62
Specific gravity
11.4
11.0
10.2
9.6
8.9
8.4
Melting point °C
327
60
180
180
180
180
Tensile strength
1.3
2.8
3.5
4.2
5
3
Elongation %
60
30
30
30
40
30
Co-efficient of linear expansion (kgf / sq. mm)
29
28
26
25
23
21
Electrical resistivity (micro ohm-m)
21
19
18
17
15
14

Properties and uses of Hard Solders


A hard solder is an alloy of copper and zinc. It has a high melting point (790 to 860°C). It is used for joining brass, copper, iron and steel. For brass work the proportion is 3 : 2 zinc silder solders, aluminium solders also belong to this type. The important properties of hard solder are given in Table.

Properties of Hard Solders


Solder
Density kg / sq.mm
Tensile strength kg/sq.mm
Melting point °C
Metals joined
Copper-zinc,  54% copper
8.3
22
860
Copper and its alloys
Silver solder, 70% silver
9.8
30 - 35
730
Copper, brass
Silver solder, 25% silver
8.9
28
765
Copper and its alloys
Silver Cadmium solder
9.7
-
790
Copper and its alloys



Properties and Uses of Fusing Material :


A fuse is a protective device which consists of a thin wire or strip which melts when a particular value of current flowing through it exceeds. Most commonly used materials for fuse wire are lead, tinned copper, zinc, tin, silver, lead-tin alloy, silver alloy, copper alloys etc. Silver is considered to be the best material for fuse wire as it has high conductivity, free from oxidation, low specific heat and non deteriorating properties. The chemical composition and melting point of fusible alloys are given in Table. Chemical Composition and Melting Point of Fusible Alloys

Chemical Composition
Melting Point °C
Bi
Pb
Sn
Cd
Hg
20
20
-
-
60
20
50
27
13
10
-
72
52
40
-
8
-
92
53
32
15
-
-
96
54
26
-
20
-
103
29
43
28
-
-
132
50
50
-
-
-
160
33
-
67
-
-
166
20
-
80
-
-
200

Properties and Uses of Platinum : 


It has a high melting point of 1770°C. Its resistivity is 9.27 x 10-8 ohm-m. It is silver white in appearance and non-corrosive. It can be shaped easily and it is malleable and ductile. It is not affected by many chemicals. Some of the properties of Platinum are given below :

(a) It is even more costly than gold.
(b) It is rust proof and chemically inert.
(c) It can be drawn into thin wires and strips.
(d) Its resistance temperature co-efficient is 0.00307 / °C
(e) It possess thermal stability and does not get oxidised even at high temperatures.
(f) It combines with many metals to form useful alloys.

It is used as heating elements in laboratory ovens and furnaces It is used as electrical contact material and as material for grids in special purpose vacuum tubes. Platinum thermo-couples are used for measurement of temperature upto 1600 degrees centigrade.

Properties and Uses of Tin :


Tin is a white lustrous metal, which is malleable and can be hammered into thin foils. It is soft and weak and is highly resistant to corrosion. It possesses little strength. It is useful constituent for non-ferrous alloys such as bronze, solders etc. It is widely used in the manufacture of solders.

Properties and Uses of Zinc : 


Zinc is a bluish white metal which is moderately ductile and malleable. It is brittle at room temperature and has high creep rate. It is practically in corrodible under ordinary atmosphere. Zinc is mostly used as a protective material for iron and steel.

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