High Resistivity Materials and their Applications
PROPERTIES
AND APPLICATIONS OF HIGH RESISTIVITY MATERIALS:
MERCURY :
It's made of
silvery-white metal and is a good electrical conductor. At normal temperature,
it is a liquid that dissolves most metals and creates product abeld amalgams.
When it's liquid, it's quite heavy. When heated above 300°C and in the presence
of oxygen or air, it oxidises.
The following
are some of the most important properties:
(a) At room
temperature, it is the sole metal in a liquid condition.
(b)
It's poisonous.
(c) Its
expansion and contraction are consistent across a large temperature range.
(d) If heated
over 350°C in the presence of oxygen, it oxidises.
(e) It has a
boiling point of 357°C.
It is
utilised in mercury vapour lamps, which have a high luminous efficiency of
roughly 40 lumens per watt, and as arc rectifiers, which convert AC to DC. It's
utilised in Buchholz relays and thermometers to make and break contact.
TUNGSTEN :
Following are
some of Tungsten's most important properties:
(a) It is a
grey-coloured resistant substance.
(b) Among all
the metals, it has the greatest melting point (3300°C). As a result, it has
excellent refractory properties.
(c) It is an
extremely hard metal that does not become brittle when exposed to extreme heat.
(d) Filaments
can be made by drawing them into very thin wires.
(e) It has
roughly double the resistivity of aluminium.
(f) In its
thinnest form, it possesses a very high tensile strength.
(g) Even at
temperatures of a few hundred degrees Celsius, it oxidises rapidly in the
presence of oxygen.
(h) It does
not have magnetic qualities when pure, but it may easily be alloyed with
Tungsten Steel, a high-quality magnetic material (i) It has an atomic weight of
184 and a resistance of 5.46 micro ohm/cm2.
(j) It may
easily work up to 2000°C in an atmosphere of inert gases or vacuum.
Because of
its high melting point, it is utilised as a filament in incandescent lamps. In
electron tubes, as a heater coil.
MOLYBDENUM
:
It melts at
2620°C and boils at 3700°C. The resistivity ρ = 0.048 mm2/mm and the
thermal coefficient of expansion are both 5.3 x 10-6 / degrees. It
has a normal temperature resistance coefficient α = 0.047/degree. Because of its
ability to establish a tight seal with glass, it is commonly utilised as an
X-ray tube target and a structural member in high vacuum electron tubes.
TANTALUM
It is a
material with a resistivity (ρ) of 1.24 µΩ - cm and a temperature coefficient of resistance(α) is 0.0036 / degree. 2900°C is its melting temperature,
while 16.6 is its specific gravity. The lamp filament efficiency is around 1.6
watts per candle power. It is rarely used due to its poor efficiency.
MANGANIN
It's a
copper-based alloy with 12% manganese and 2% nickel. The thermal e.m.f. is
limited to roughly 1 micro V/°C due to nickel. The following are the physical
characteristics:
Specific
gravity - 8.4
Resistivity
at 20°C - 48 x 10-8 ohm-m
Temperature
coefficient of resistance - 1 x 10-8 /°C
Working
temperature - 60 to 70°C
Melting point
- 102°C
It's simple
to make thin wires out of it. Although it has a high electrical resistance, it
has a low thermal coefficient of resistance. Wire-wound precision resistances
for measuring instruments shunts for electrical measuring instruments,
resistance boxes, standard resistance coils, and coils for precision electrical
measuring instruments are all made with it.
The second
set of materials can have a high thermal e.m.f. and temperature coefficient of
resistance, but they must also have a high working temperature and be
inexpensive because they are used in large quantities. This group's main alloy
is Constantan (Eureka).
NICHROME :
It's a
nickel-chromium alloy made up of 80% nickel and 20% chromium, or 60% nickel and
20% chromium. Chromium (15%) and iron (25%) are both present. Nichrome has the
following properties:
(a) The
colour is silvery white.
(b) It's
ductile, which means it can be shaped into tiny wires.
(c) The
maximum temperature it can withstand is 1100 degrees Celsius.
(d) At 20°C,
its resistivity is extremely high (100 x 10-8 ohm-m).
(e) It has a
temperature coefficient resistance of 0.0001.
It is
utilised in electric heaters, electric ovens, electric irons, room warmers, and
electric furnaces, among other things.
CONSTANTAN
(EUREKA) :
It's a
copper-nickel alloy with 60 to 65 % Copper and 35 to 40 % Nickel. Hard constantan
wire has a resistivity of 46 to 55 x 10-8 ohm-m and soft constantan
wire has a resistivity of 45 to 48 x 10-8 ohm-m. The temperature
coefficient is very close to zero. When compared to copper, the thermo e.m.f.
is 39 micro V per degree centigrade, making it appropriate for sensing
temperatures up to 700°C. It operates at a temperature of roughly 500°C. When
bare constantan wire is heated in air for 3 seconds at 900°C, it forms a thin
film of electrical insulating oxide that can tolerate up to 1 volt of turn to
turn voltage. In many cases, cheaper alloys are used instead of constantan. Due
to the addition of zinc, Niclin has less nickel than constantan. It has a
resistivity of 40 micro ohm-cm and can work at a maximum temperature of 300
degrees Celsius. A nickel silver alloy with a higher zinc content is another
option. It has a 30 to 32 micro ohm-cm resistivity and a maximum working
temperature of 200 to 300°C. The following are some of Constantan's
characteristics:
(a) It can be
pulled into thin wires.
(b) The
maximum temperature it can tolerate is around 500 degrees Celsius.
(c) It has a
1300°C melting point.
(d) It has an
SG (Specific Gravity) of 8.9.
(e) It has a
temperature coefficient resistance of 0.00002 to 0.00005.
(f) It is
corrosion resistant and does not corrode when exposed to air or moisture.
It's used to
make resistance elements for things like electric motor starters, loading
rheostats, and resistance boxes, as well as thermocouples. It's also utilised
in field regulators to control a generator's generated voltage.
LEAD :
It's a blue-grey
metal with a delicate sheen. It has a specific gravity of 11.36 and a melting
temperature of 326°C. It's ductile and malleable. When alloyed with tin, it is
primarily utilised as a fusing material and also as a soldering material.
Solder is a low-melting-point alloy of two or more metals that are used to
unite two or more metal components. The most common solder is a tin-lead alloy.
The most common compositions are 50% lead and 40% tin or 40% lead and 60% tin.
Lead solder is used to connect copper, bronze, brass, tinned iron, zinc, and
other metals. It has a melting point of about 185°C and conductivity of roughly
10% that of copper. Soft solder and hard solder are the two most common types
of solders.
Lead and tin
are mixed in varying quantities in soft solders. Electronic gadgets, coating of
iron or steel sheets for roofing, and filling of hollow castings, among other
applications, are the most common. The material has a tensile strength of 5.7
kgf/sq. mm and a melting point of 400°C. Due to its low mechanical strength,
this type of solder should not be subjected to mechanical loads.
Copper and
zinc are combined in a hard solder. Its melting point ranges from 790 to 860°C.
Brass, copper, iron, and steel can all be joined with it. Zinc silder solders,
as well as aluminium solders, are used in a 3:2 proportion for brass work.
FUSING
MATERIAL :
A fuse is a
safety device made up of a thin wire or strip that melts when the current
flowing through it surpasses a certain threshold. Lead, tinned copper, zinc,
tin, silver, lead-tin alloys, silver alloys, copper alloys, and other metal
alloys are among the most often used materials for fuse wire. Because of its
high conductivity, lack of oxidation, low specific heat, and non-deteriorating
qualities, silver is regarded as the best material for fuse wire.
PLATINUM
:
It melts at 1770°C
and has a resistivity of 9.27 x 10-8 ohm-m. It is non-corrosive and
silver-white in appearance. It is malleable and ductile and may be easily
moulded. Many compounds have little effect on it. Some of the characteristics
of platinum are:
(a) Its price
is even higher than gold.
(b) It's
chemically inert and rust-proof.
c) It can be
manipulated into thin wires and strips.
(d) It has a
thermal resistance coefficient of 0.00307 / °C.
(e) It is
thermally stable and does not oxidise even when exposed to high temperatures.
(f) It can
make useful alloys with a variety of metals.
In laboratory
ovens and furnaces, it is employed as a heating element. It is utilised in
special-purpose vacuum tubes as an electrical contact material and as a
material for with. Thermo-couples made of platinum are used to measure
temperatures up to 1600 degrees Celsius.
TIN :
Tin is a
glossy white metal that can be hammered into thin foils and is malleable. It's
brittle and soft, but it's corrosion-resistant. It is weak. Nonferrous alloys,
such as bronze and solders, benefit from it. It's a common component in
soldering.
ZINC :
Zinc is a
bluish-white metal that is ductile and malleable to a moderate degree. At room
temperature, it is brittle and has a high rate of creep. Under normal
conditions, it is almost corrosion-proof. Zinc is mostly used to preserve iron
and steel from corrosion.