T.J. Rudzik, R.A. Gerhardt
Georgia Institute of Technology,
Keywords: nanopowders, SANS, porosity, dc resistivity, impedance spectroscopy, in-situ
Summary:Sintering is one of the primary methods of fabrication of ceramic materials. Therefore, a detailed understanding of the processing-property relationships for materials undergoing sintering is of great potential value. A new testing method combining small angle neutron scattering (SANS) and ac impedance spectroscopy (IS) measured simultaneously during the compaction of ceramic nanopowders has been developed to track the evolution of the powder compact microstructure. SANS and IS are non-destructive tests which can be run in-situ and have proven to be valuable tools for the characterization of porosity and particle-particle interfaces, respectively. For this series of tests, a variety of conducting and semiconducting ceramic nanopowders were compacted in a sapphire die using a computer controlled load frame mounted in the path of the GP-SANS beam at the HFIR facility at Oak Ridge National Lab. Electrical leads attached to steel punches, which applied pressure to the powders, were used to attach the cables for the IS tests, allowing the current to travel through the punches and the powder during compaction. Pressures ranging from 0-300 MPa in 50 MPa increments were applied to the powders. Upon reaching each pressure, the IS and SANS tests were conducted. IS testing was carried out using a looping frequency sweep procedure from 1 MHz-0.1 Hz at a constant voltage of 500 mV. At each pressure, a series of three individual SANS measurements were carried out, with one test using a 2 m sample to detector distance (S-D), one at S-D=18.5 m, and one at S-D=18.5 m with the beam trap moved away from the center. The 2D SANS data for the three tests at each pressure were reduced to two 1-D curves using the SPICE software package for Igor Pro. A consistent issue with the S-D=2 m data necessitated some data manipulation to get a continuous 1-D curve for the entire data range. This continuous 1-D data fit with the size distribution tool in the Irena tool suite for Igor Pro, utilizing the maximum entropy method and assuming spherical scatterers. The output of this fitting was the volume and number distributions of pores, the cumulative surface area, and the cumulative size distribution of pores. Both the IS and SANS data show the expected trends, with resistivity and porosity decreasing as applied pressure increases. SANS-derived porosity and pore concentration values are far too low, likely due to some pores being so large that they were beyond the detection range of our SANS tests. However, the similarities between the SANS and IS data trends show that the tests were successful in tracking the changing microstructure of the powders during the compaction experiment. Therefore, these tests were a successful proof of concept for this powerful new characterization technique. In the future, design and data fitting improvements will be implemented and heating will be incorporated to apply this technique to actual sintering processes.