Electric Resistance Heating Element Materials
Electric Resistance heating materials are used in furnaces, to
temperatures in excess of 2000°C. In choosing the right material,
several factors have to be looked at:
Environment (Oxidizing, Reducing, or Neutral)
Temperature the heating element expects to see.
The element temperature will be the furnace temperature plus some
increase based on how much energy is transferred through the surface
of the heating element (watt loading). The higher the watt loading
on the surface, the greater the spread between the furnace temperature
and the element temperature.
Physical constraints, can the element be properly supported?
Introduction and Definitions
Three properties must be reviewed. Each will have a role in designing
the proper heating element configuration.
1) Resistivity: This characterizes the ability of a material to
inhibit the flow of electrical current in the presence of an applied
voltage.
2) Temperature coefficient of resistance: This is a correction factor
for changes in the resistivity of a material relative to the temperature.
3) Maximum Temperature: The maximum temperature is the temperature
that the heating element should not exceed during normal use. Lifetime
is relative to how close to this maximum temperature the element
is operated. The maximum temperature is set by the material manufactures
based on their experience and how well the element material performs
in standardized tests. This maximum temperature can also be affected
by the operating environment and any chemical reactions that may
occur between the element material and the environment the element
is in contact with.
Environmental Considerations
Most materials used for heating elements fall into two general categories,
Oxidation Resistant, or Non-Oxidation Resistant. Ignoring the basic
differences between these two categories can be disastrous.
Oxidation Resistant Materials
From the description, Oxidation resistant materials are capable
of operating in environments containing oxygen. The survival of
the heating element is based on its ability to produce a stable
oxide coating that stops further oxidation. There are three groups
of materials that fall into the oxidation resistant category; Metals,
Ceramic-Metals (Cermets), and Ceramics.
Metals: The metallic heating element materials fall into three broad
categories, Nickel Chrome, Iron-Chrome-Aluminum, and a small group
of “Others”.
Nickel-Chrome:
The Nickel-Chrome alloys were first developed in the United States
in the early 1900’s. When heated in an oxidizing environment,
the Nickel-Chrome alloys produce a chromium oxide on their surface.
The Chromium Oxide is relatively stable and protects the underling
material from further oxidation. The maximum element temperature
for the highest grade of Nickel-Chrome Alloys is 1200ºC (2200ºC).
Iron-Chrome-Aluminum:
In the 1930’s the second group, the Iron-Chrome Aluminum alloys,
was developed in Europe. This group when heated produces an Aluminum
Oxide (Alumina). As the Alumina is more stable it can operate up
to 1450ºC (2550ºF).
Others:
The “others” category contains a wide variety of lesser-used
materials. On the high end there are platinum and platinum alloys.
These materials are essentially inert and non-reactive with air
and thus remain stable. They can be used up to temperatures of 1600-1700ºC
(2900-3100ºF). Their obvious drawback is cost. In most cases
failed elements are returned for a material content credit. On the
low end are low temperature heaters. They are seldom if ever used
in Industrial furnaces, but can be used in very low temperature
dryers or space heaters. Examples are aluminum and brass. While
they are low temperature materials, they can be made extremely thin.
This allows them to be put into “Flexible” heating elements
used where fitting to a surface is required. Another “Other”
that is used in some high temperature applications is stainless
steel. Stainless steel behaves very similar to the nickel-chrome
alloys listed above.
Metallic materials are produced in the shape of round wires or rectangular
strips.
Ceramic-Metals (Cermets): Cermets are a class of
materials that contain ceramic components as well as a metallic
component. One such cermet that is used as a heating element material
is Molybdenum Disilicide. This is a combination of Molybdenum and
Silicon that when formed into a heating element and heated in air
produces a silicon oxide (silica). This is extremely stable at elevated
temperature and can be used up to element temperatures of 1900ºC
/ 3400ºF.
Molybdenum-Disilicide:
Molybdenum Dislicide is produced by reacting Molybdenum and Silicon.
The resulting molybdenum disilicide mass is reduced to a powder,
mixed with a binder and extruded into rod form. The rods are then
sintered to achieve near final form. The resulting rods are brittle
and must be heated in order to shape them. The shaping required
special techniques and thus the elements as delivered to the users
are in final form. When heated the element softens and must be properly
supported if used in any position other than a vertical "U"
shape. There are different grades with slightly different resistivity
that allow maximum element temperatures of 1700-1900°C.
Most ceramics are not conductive and many are used as electrical
insulators, but there are three types of ceramic materials that
are used for electric resistance heating, Silicon Carbide (SiC),
Lanthanum Chromite, and Zirconia (ZrO2). All of these materials
are used for high temperatures (above 900ºC/1500ºF), and
each has unique properties that have to be addressed in the design
of the heating system.
Silicon Carbide:
Silicon Carbide (SiC) heating elements are used at elevated temperatures
(above 1400°F / 760°C). The resistivity of SiC has some
unique characteristics that must be accounted for in the design
of the power supply for the elements. First, the resistance at room
temperature is not an accurate reading. The resistance of the material
is widely scattered below 800°C (1475°F) and drops as the
temperature is increased. Above 800°C / 1475°F, the resistance
gradually increases with increasing temperature.
For design purposes, the resistance is measured at 1800°F /
1000°C. The second fact that must be accounted for is that during
operation, the resistance if the heating elements will gradually
increase (Aging).
SiC heating elements are made by sintering Silicon Carbide grains
together at very high temperatures (2200ºC / 4000ºF).
The bonds formed by the sintering process provide the electrical
path to make the element conductive. As the control over the bonding
is not precise, elements produced are each tested at 1000ºC
to check the resistance. Similar values then can be grouped to produce
a matched set.
The increase of resistance over time for SiC elements is caused
by the breaking of the bonds between the grains of Silicon Carbide.
The bonds are generally broken by the formation of oxides between
those bonds. Eventually there are enough bonds broken that either
the resistance is so high that adequate power cannot be generated,
or that the bar becomes so weak from the loss of bonds that it breaks.
Being a ceramic, SiC heating elements are brittle, and cannot tolerate
mechanical shock, but are also strong at elevated temperatures and
can be mounted vertically or horizontally.
Lanthanum Chromite:
Zirconia:
Zirconia has a very high resistivity at room temperature, so high
that it is an insulator. As the temperature of the material is increased,
the resistivity decreases. At a temperature of about 800C / , the
material becomes conductive and will act as a resistive heating
element.
Non-Oxidation Resistant Materials:
These elements do not produce stable oxides, and if exposed to air
at elevated temperature will oxidize. The rate of oxidation can
be relatively slow, or a very rapid disintegration of the material.
Molybdenum, Tungsten, Tantalum, and Graphite are used as heating
elements where they can be protected from contact with oxygen.
Metals:
Molybdenum, Tungsten and Tantalum
Carbon:
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