A High Pressure Cell for Spark Plasma Sintering

Many nanostructured materials have been shown to have performance gains strongly dependent on the grain size in the material. Nanostructured thermoelectric materials for instance have found great performance increases through reduction of the grain sizes, due mostly to the scattering of phonons while retaining a good electrical conductivity. Other such examples abound where the grain size plays an important role in the performance of the material, including magnetic materials, proton fuel cell membranes, or simply improving the mechanical properties of a system through the Hall-Petch relationship. A considerable amount of effort has been applied into reducing the grain size of an existing powder as well as retaining the small grain size during a sintering operation to create a bulk specimen. Frequently conventional sintering methods use temperatures and time scales that lead to deleterious grain growth. Spark Plasma Sintering (SPS) has been found to reduce the temperatures and times required to densify a sintered material by rapidly heating the sample compared to conventional “diffusive” sintering techniques. More recently the addition of high pressure to the SPS process has been shown to reduce the temperatures and sintering times even further, allowing for the retention of grain sizes as small as 10 nm. In this work we design and test a new kind of pressure cell for SPS to deviate from the conventional graphite die arrangements common to the literature. The new swaged alumina core cell was designed for pressures of 1 GPa and temperatures in excess of 1,000 °C, with a sample diameter of 12.7 mm. It is hoped that this design will lead to improved cells that are fully reusable allowing for the economical production of sintered samples with grain sizes smaller than 50 nm. The cell also tests the use of the NiCrAl alloy as an electrode material. This alloy, having been fabricated for neutron scattering studies, has properties that may make it useful for high temperature, high strength applications. In addition to the work with the SPS cell the fabrication and characterization of this alloy is discussed