Introduction:
Biaolian elements are made from highly pure alpha silicon carbide grits extruded as rods or tubes before firing at about 2500℃ to form recrystallised bonds between the adjacent particles. The grain structure of Biaolian elements is optimized to resist oxidation and corrosion in both industrial and laboratory applications in order to ensure the best possible service life over a wide range of operating temperature and atmospheric conditions. Each element consists of one hot zone and two cold ends. Each cold end is jointed with the hot zone by using special SiC cement. As the resistance of the hot zone is much higher than that of the cold ends Biaolian elements achieve a high efficiency and have an excellent performance. The cold ends, which pass the furnace lining remain relatively cool and save energy. Additionally the ends are sprayed over a short length with aluminum to form a low contact resistance for the electrical connection.

Advantages:
As all silicon carbide elements are rigid at elevated operating temperatures up to 1600℃, it is possible to install them horizontally or vertically without any additional support. This simplifies the furnace construction as the elements can be replaced while the furnace is in operation, thus there is no loss of production. This is a major advantage for all kinds of continuous operating processes and therefore Biaolian elements are preferably used for manufacturing of electronic components, optical glass, fluorescence powder, as well as in aluminum holding, melting, and die-casting furnaces, and for heat treatment of both ferrous and non-ferrous metals. All Biaolian elements have a relatively high load capacity and are ideal for applications where a high load is required within a limited space.

Electrical Characteristics:
Silicon carbide is a semiconductor material and has a much higher resistance than metals. The room temperature resistance of Biaolian elements is rather high compared to that at 1000℃ and is declining at elevated temperatures whereby a minimum value is reached at about 800℃. Above this temperature the resistance will rise up again (see Fig. 1).

Minor impurities have a disproportionate effect on the resistance at temperatures below 600℃ as indicated by the dotted curves. Therefore, resistance measurements carried out at room temperature do not give any indication of the resistance at higher temperatures. Resistance measurements shall always be carried out at a constant temperature of 1000℃ or above and the value can be calculated by dividing the voltage of each element and the Amps value passing the element.

The nominal resistance of all Biaolian elements is based on an element temperature of 1000℃ and can vary within a tolerance of ±15%. All elements are tested with a standard test voltage depending on element size and amps value marked on the element. The nominal resistance can be used for calculating the voltage requirements of the power supply, but it should be taken into the account the resistance tolerance of ±15% when the maximum current is calculated. At surface temperatures between 600℃ and 800℃ the resistance will decline by 20% compared to the value at 1000℃, which shall be considered for the current rating of the power supply. For example, if an element has a nominal resistance of Ω at 1000℃, the minimum will be 0.85Ω only (1Ω×(1-15%)=0.85Ω). Under consideration of the temperature characteristics the lowest resistance at 700℃ is 0.85Ω×(1-20%)=0.68Ω

Element Performance:
All silicon carbide elements are aging over the time which means that the resistance will rise up during use depending on following factors: 1. Element loading (W/cm²); 2. Operating temperature. 3. Process atmosphere; 4. Continuous or intermittent operation; 5. Operating procedures

Biaolian elements may show a lower aging rate and may achieve a longer service life of up to 7000hrs in clean oxidizing atmosphere at temperatures of up to 1400℃. To optimize the service life the lowest specific load shall be taken which is usually between 3 and 8W/cm² (see Fig. 2).

Under specific atmospheric conditions Biaolian elements are available with varies types of coatings which protect the elements from chemical attacks and increase their service life.

Coated heating elements:
SiC heating elements can be used in all types of atmospheres, but under severe atmospheric conditions the element life can be effected and shorten substantially, which makes the firing process ineffective. In order to improve the service life of those elements in presence of aggressive volatiles the elements can be delivered with various kinds of coatings which protect the SiC structure from accelerated aging. Therefore, whenever the use of untreated SiC elements lead to an unsatisfactory result it is recommended to install coated heating elements instead. Depending on the nature of the atmosphere several coatings from type U, H and TS are available which give the elements a perfect performance in certain atmospheres. The coating with the best performance shall be selected under the supervision of our company. The table below shows the application range of the different types of coatings, which shall be used as a guideline only, as in fact several volatiles can be present in the atmosphere at the same time whereby even harmless ones can become hazardous in combination with others. Therefore, it is highly recommended to undertake practically test in order to find the best solution for your individual problem.