When alumina tubes are heated consistently, they tend to expand uniformly. However, during material processing in furnaces, temperature gradients often develop due to factors such as the presence of the material inside the tube.
While professional furnace design and careful control of tube diameter can help manage these gradients, they can't be entirely eliminated.
These temperature variations also affect the tube's tensile strength. In particular, 998 alumina experiences a more significant loss of tensile strength at temperatures above 1000 °C.
Recommended Ways to Limit Tube Loss
- Limiting furnace ramp speeds to 60 degrees Celsius per hour reduces the danger of thermal shock caused by gradients.
- Tubes are susceptible to mechanical stress. To keep tubes from being restrained during heat cycling, secure them tightly at one end only. Alternatively, Deltech's water-cooled end caps have O-rings that allow for tube expansion and contraction while also providing excellent sealing to protect the integrity of inert atmospheres within the tube.
- Whenever feasible, avoid using large-diameter, thick-walled tubes. The elastic strain caused by heat gradients grows with the size of the tube. When tubes exceeding 100 mm in diameter are required for the process, Deltech offers furnace design solutions to help mitigate the consequences.
- Thermal gradients are more noticeable where the process tube passes through the furnace wall(s). Deltech reduces their incidence by employing insulation designs that accommodate fluctuations in thermal expansion without causing considerable heat loss from the furnace.
- When placing parts within a horizontal furnace, use a fixed bed of small-diameter ceramic rods along the bottom. Crucibles, trays, or boats can be placed on refractory supports or D-tubes that are already fitted inside the furnace tube. Deltech's technical team can assist you in selecting the optimal solution for your process.
- When using end seals, utilize internal refractory tubes or discs to shield them from radiant heat. Deltech's technical staff may make recommendations on the best design and placement.
Deltech vertical tube furnace and control system. Image Credit: Deltech, Inc
This information has been sourced, reviewed and adapted from materials provided by Deltech, Inc.
For more information on this source, please visit Deltech, Inc.