The Principle of FAST/SPS-Sintering
During FAST/SPS sintering, the current flows directly through the sintering mold or the conductive sintered material. The electrical resistance causes rapid heating, known as Joule heating. The temperature only arises where it is needed.
The sintering cycles are significantly shortened because there is no need to heat up the furnace atmosphere. This leads to less grain growth and an improved microstructure of the sintered material. Nanomaterials remain nano after sintering because grain growth is minimal.
The sintering mold can be made of graphite, steel or TZM. You can work with loose powder or cold pre-pressed green compacts. Compared to conventional furnaces, productivity increases while sintering unit costs decrease.
Fast sintering enables new material combinations because a soft phase can arise between layers. This is how, for example, FGMs (Functionally Graded Materials) and connections with carrier bodies are created.
A vacuum is created during the sintering process and the sintering chamber can be flushed with protective gas. Dr. Fritsch FAST/SPS presses have a standard vacuum of 20mbar, optionally also 0.05mbar.
The temperature measurement is carried out contactlessly with a pyrometer or with thermocouples in the lower temperature range. To avoid excessive temperatures, Dr. Fritsch FAST/SPS sintering presses use the highest of three temperature measurements for control.
The "International Powder Metallurgy Directory" (01/2012) published an article by Prof. Kieback, former director of Fraunhofer IFAM, comparing the pulsed and unpulsed FAST/SPS processes. The result showed that only the short sintering cycles, fast heating rates and lower sintering temperatures are relevant for the results. Dr. Fritsch avoids the unnecessary and expensive pulsing of the current, which confirms this finding.