Ceramics and joining technology
– Firing processes
CERAMICS AND JOINING TECHNOLOGY // RAW MATERIALS PREPERATION // GRANULATION // SHAPING // FIRING PROCESSES // HARD MACHINING // CONTROLS // LABORATORY // CERAMIC-METAL BRAZING // CERAMIC-CERAMIC AND CERAMIC-METAL SOLDERING
Control of the sinter-firing of oxide and non-oxide ceramic products has with regard to the type of kiln atmosphere, the preheating and cooling ramp as well as the type of kiln setting a crucial influence on the geometric and material-related quality of the ceramics. Natural-gas-heated kilns can be operated to the range of 1800 °C providing they are equipped with suitable burners and a temperature-resistant kiln lining. During operation of such kilns, for reliable regulation of the fuel/air ratio, l-sensors with doped ZrO2 as electrolyte are frequently used. They enable precise adjustment of the gas mix.
Listed in the following are selected technical data of the ZrO2 material, mostly of the type Y-PSZ, frequently used for this purpose:
- Purity >99,7 %
- Density: >95 % of the theoretical value
- Mean bending strength: >350 MPa
- Mean modulus of elasticity: 200 GPa
- Thermal resistance: >1750 °C
- High geometric stability
- He leakage rate: <10-9 mbar*L/s
- Specific electric resistance at 1000 °C: 15 Ω*cm
- Realization of force, form-fit and substance-to-substance joints with metals
The curve of the temperature-time program of a natural-gas-heated ceramic sinter firing is recorded with the help of thermocouples and controlled. In the high-temperature range, thermocouples of the type “platinum/platinum alloy” are used. For reliable electrical insulation of the thermocouples in all operating states of the kiln, they are housed in a capillary tube made of high-purity, dense-sintered Al2O3 ceramic, which for its part is introduced into a protective tube consisting of this ceramic for protection of the thermobead.
In the following, selected typical properties of such a ceramic type relevant for this application are summarized as general technical quality features:
- Purity: >99.5 %
- Density: >98 % of the theoretical value
- Mean bending strength: 300 MPa
- Modulus of elasticity: 380 MPa
- Usable to the region of 2000 °C
- High resistance to thermal shock
- Thermal conductivity: 30 W/m/K (20 °C), 5 W/m/K (1500 °C)
- Specific electrical resistance at 1500 °C: 106 Ω*cm
The weight of the kiln furniture often exceeds the weight of the products fired, sometimes considerably. With regard to the costs of sintering firing, they are therefore not a negligible factor. They are generally thick-walled porous materials classified as refractories.
Recent developments show that thin-walled kiln furniture can be fabricated by means of special laminating techniques as oxide- and non-oxide ceramic hollow and support structures. On account of their comparatively low mass and their microstructure, they enable shorter temperature-time programs and therefore significantly lower energy input.