EnglishAlmost all conductors generate heat when current passes through them. However, not all conductors are suitable as heating elements. The correct combination of electrical, mechanical and chemical properties is required. Listed below are the characteristics that are important to the design of a heating element.
Resistivity: To generate heat, a heating element must have sufficient resistance. However, the resistance cannot be high enough to be an insulator. Resistance equals resistivity times conductor length divided by conductor cross section. For a given cross-section, to obtain shorter conductors, materials with high resistivity are used.
Oxidation Resistance: Heat generally accelerates the oxidation of metals and ceramics. Oxidation wears down the heating element, reducing its capacity or damaging its structure. This limits the life of the heating element. For metal heating elements, alloying with oxides helps to resist oxidation by forming a passivating layer. For ceramic heating elements, protective anti-oxidative scaling of SiO2 or Al2O3 is most common. Heating element types that are not suitable for use in oxidizing environments, such as graphite, are most commonly used in vacuum furnaces, or furnaces containing non-oxidizing atmosphere gases such as H2, N2, Ar or He, where there is no air in the heating chamber.
Temperature Coefficient of Resistance: Note that the resistivity of a material changes with temperature. In most conductors, resistance increases as temperature increases. This phenomenon affects some materials more significantly than others. The higher temperature coefficient of resistance is mainly used in heat sensitive applications. For heat generation, it is usually better to use lower values. While changes in resistance can be accurately predicted in some cases, a dramatic increase in resistance is required to deliver more power. To adapt the system to changing resistivity, a control or feedback system is employed.
Mechanical Properties: Rigid heating elements deform when used at high temperatures. As a material approaches its melting or recrystallization stage, it weakens and deforms more easily than it does at room temperature. A good heating element will retain its shape even at high temperatures. On the other hand, ductility is also a desirable mechanical property, especially for metallic heating elements. The ductility enables the material to be drawn into strands and shaped without compromising its tensile strength.
Melting point: In addition to the temperature at which oxidation increases significantly, the melting point of a material also limits its operating temperature. Ceramics generally have a higher melting point than metal heaters.
