标题：HOT ALONE WON‘T DO THE TRICK! 内容：
HEATING ELEMENTS
About 5000 to 7000 furnaces for sintering zirconia are produced every year at the moment. The numbers are rising. There are two different heating element types that are installed in the dental furnaces on the market to achieve the extremely high temperatures, usually in excess of 1400°C (2552°F). By far the largest share of the furnaces on the market is built with molybdenum disilicide (MoSi2) heating elements. In some few cases, silicon carbide (SiC) heating elements will be used. These two element types differ in their properties in addition to the entirely different basic materials.
SiC heating elements are usually used up to a temperature of 1530°C (2786°F) as recommended by the furnace manufacturer. A firing cycle will take several hours. MoSi₂ heating elements can be used up to a temperature of 1750°C (3182°F), depending on the quality and furnace type. The maximum application temperature of these heating elements therefore is clearly above that of SiC heating elements. Additionally, the surface load, which means the power (W) that can be emitted per square centimeter (cm2) is more than twice that of SiC, at above 25 W/cm2. This permits much faster heating-up rates, to much higher application temperatures. Heating to 1550°C (2822°F) in less than 15 minutes is possible.
MolyCom®-Hyper 1800 (MoSi2) heating elements that have been used on the international dental market since 2005 have turned out to be much longer-lived than SiC elements in continuous tests and according to the experience of various furnace manufacturers. SiC elements usually need to be replaced after just a few hundred cycles due to the strong change of resistance, which corresponds to a service life of approx. 1-2 years at common use. MolyCom®-Hyper 1800 elements in contrast have been operated for more than 2500 cycles without any failure or need for exchange. Exchange of the defective element is usually sufficient in MoSi2 heating elements. In contrast to this, all parts of the SiC elements must be exchanged in one due to the strongly impaired resistance.
If we additionally compare the chemical analysis of the heating elements, the standard heating elements show a much higher degree of contamination, in particular by iron oxide. No contamination could be measured at use of the MolyCom®-Hyper 1800 elements up to a temperature of 1630°C (2966°F). No contamination was measured for MolyCom®-Hyper 1800 Super Clean elements up to 1680°C (3056°F).
Additionally, a contaminated zirconia sample on the surface was analyzed with a secondary ion mass spectrometry (SIMS). As the analysis shows, increased contents of iron oxide could be documented in the sample up to a depth of 0.15 μm. While the degree of contamination is very low, it has a considerable visual effect. The vapor pressures of many different oxides at the surface temperatures of the MoSi2 heating elements in the furnace were additionally calculated with Fact Sage.
Among the ceramic materials, silicon nitride ceramics possess good overall performance (e.g., high hardness, high strength, low density, low thermal conductivity, low chemical activity, and high wear resistance), which promotes these applied as rolling bearing, machine tools, sliding bearing in water pumps.
Aluminum titanate has outstanding properties that make it ideally suited for applications involving molten aluminum, including complete resistance to wetting by aluminum alloys, total gas impermeability, and a very low thermal expansion coefficient. This material offers a longer service life and higher reliability when compared to cast iron, calcium silicate or fused silica components. All riser/stalk tubes are 100% leak-down tested, and parts are available with gasketing and ceramic foam filters to provide complete assemblies.
