The heating element material properties narrow the selection to a few materials. The most common materials are nickel-chromium, iron-chromium-aluminum alloy, molybdenum silicide, and silicon carbide. These materials can work at high temperatures because of their ability to resist high-temperature oxidation. The other group consists of graphite, molybdenum, tungsten and tantalum. These materials are oxidized at high temperatures and are mostly used in vacuum environments or furnaces with oxygen-free atmospheres.
Ni-chromium (Ni-Cr) alloy
Due to its ductility, high resistivity and oxidation resistance even at high temperatures, this type is one of the most widely used heating element materials. The most common composition of nickel-chromium alloys is 80/20 or 80% nickel, 20% chromium. Other compositions depend on the manufacturer. Due to its high ductility, it is often drawn into a wire when used as a heating element. A common application that exhibits this property is hot wire foam cutters. The maximum heating temperature of nickel-chromium wire is about 1,100 to 1,200°C.
Iron-chromium-aluminum (Fe-Cr-Al) alloy
The chemical composition of this type of ferritic iron-chromium-aluminum alloy is typically 20 to 24% chromium, 4-6% aluminum, and iron as margins. Compared to nickel-chromium, iron-chromium-aluminum heaters are flexible and lower in weight. They can also produce higher temperatures than nickel-chromium wire, around 1,300 to 1,400°C. Due to its iron-based metal, the price of this alloy fluctuates less than Ni-Cr, which is mainly composed of nickel. The disadvantage of using iron-chromium-aluminum alloys is that they have reduced strength at higher temperatures.
Iron-chromium-aluminum alloys can be made better by a process called powder metallurgy. In this process, alloy ingots are turned into powder and compressed into molds. It is then sintered or hot pressed (hot isostatic pressing) in a temperature-controlled atmosphere to create a metallurgical bond without completely melting the powdered metal. Dispersions are added to the alloy mixture to enhance the mechanical properties of the material, thereby imparting additional strength and toughness at higher temperatures.
Molybdenum disilicide (MoSi2)
Molybdenum disilicide is a refractory cermet (ceramic-metal composite) mainly used as a heating element material. Due to its high melting point and good corrosion resistance, this is an ideal material for high-temperature furnaces. Molybdenum silicide heating elements are produced through a variety of energy-intensive processes such as mechanical alloying, combustion synthesis, impact synthesis, and hot isostatic pressing.
MoSi₂ heaters can achieve heating temperatures up to 1,900°C. The disadvantages of using molybdenum silicide are its low toughness and high-temperature creep under environmental conditions. It is brittle at room temperature and needs to be handled very carefully. Higher toughness is achieved at a brittle-tough transition temperature of around 1,000°C. On the other hand, a higher creep rate causes the heating element to deform easily at high temperatures. The most common type of MoSi2 element is the 2-handle hairpin design, which is usually suspended from the furnace roof and located around the furnace wall. Other shapes are often used in combination with ceramic insulation molders to provide mechanical support and thermal insulation as an integrated package.
Silicon carbide (SiC)
This is a ceramic produced by the recrystallization or reaction combination of SiC grains at temperatures above 2,100°C. Silicon carbide heating elements are porous (typically 8-25%), in which the atmosphere inside the furnace can react through a cross-section of the material. The entire heating element may gradually oxidize, which causes the resistance characteristics of the element to increase over time (often referred to as "aging") A variable voltage supply is often required to increase the voltage of the element over the life of the element by gradually maintaining the desired power output of the element. This aging ultimately limits the life and performance of the heating element.
Silicon carbide has many properties that make it suitable for the manufacture of heating elements suitable for extremely high operating temperatures. This ceramic has no liquid phase. This means that the elements do not sag or deform due to creep at any temperature, and no support is required inside the furnace. Silicon carbide is directly sublimated at a temperature of about 2,700°C. In addition, it is chemically inert to most process fluids and has high rigidity and low coefficient of thermal expansion. Silicon carbide heaters can reach heating temperatures of approximately 1,600 to 1,700°C.
graphite
Graphite is a mineral composed of carbon in which atoms are arranged in a hexagonal structure. This mineral, also in its synthetic form, is a good conductor of heat and electricity. Graphite can generate heat at temperatures above 2,000°C. At high temperatures, its resistance increases significantly. In addition, it can withstand thermal shock and will not become brittle even after rapid heating and cooling cycles. The main disadvantage of using graphite is that it oxidizes easily at temperatures around 500°C. Continued use within this range will eventually lead to material consumption. Graphite heating elements are often used in vacuum furnaces where oxygen and other gases are discharged from the heating chamber. Lack of oxygen prevents oxidation not only of the molten metal, but also of the heating element itself. Graphite can be used for sealing film, made into carbon crystal electric heating film, graphene electric heating film film and other film heater products.
