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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”.

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).

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).

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 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 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.


Molybdenum, Tungsten and Tantalum






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