Mixed ionic electronic conductivity and flash sintering
March 10-15, 2019
In this contribution, we present and discuss similarities and differences between two phenomena connected to creation and transport of point defects. One is flash sintering (FS) on which this conference is focused on and the other is switching in some non-volatile memory devices. The explanation of flash sintering (FS) is based on the understanding that the solids being sintered contain point defects, both mobile ions and quasi free conducting electrons i.e. that they are mixed-ionic-electronic-conductors (MIECs). We shall refer specifically to oxide MIECs. The explanation also requires understanding how these MIECs respond to temperature changes, to changes in the oxygen chemical potential (or oxygen partial pressure) and how chemical diffusion can take place. The oxygen chemical potential can be fixed by the ambient or by polarization under an electric current applied on polarizing electrode. To present those properties we address a specific memory system which has much in common with FS, show the similarity where it exists and point at the differences.
The memory system is constructed of one single nano grain (crystallite), which is an insulator at room temperature, placed between two metal electrodes, subjected to a high electrical field. The temperature exhibits runaway, very similar to FS, the current increasing rapidly after an incubation period. The increase in the current of a few orders of magnitude is facilitated by both the increase in temperature and the reduction of the oxide. In this single grain system, the I-V relations are not linear (Ohmic) but rather super linear, while in FS they may be ohmic changing only due to the temperature increase.
The key difference between this system and FS is by the absence of grain boundaries within the bulk and no need for densification and grain growth. Yet not only electrical current but also material transport take place in the single grain system under an applied electric field. Heat losses in the single grain system are due to conduction of electrode and in FS mainly by radiation. Due to the small size of the system the incubation starts at room temperature despite the insulating nature of the grains, however the electrical field is of the order of 106 V/cm. The response time is very short and can be as low as 1 ns.
The region of high vacancy concentration in the single grain system and in FS during the transient step (II) can be either near the anode or the cathode. It is only under conditions close to steady state of step III that the vacancies concentrate near the cathode.
When the single crystal is replaced by a polycrystalline solid, mobile ions that are injected into it under an applied voltage form a percolation pattern with one branch reaching the opposite electrode, accompanied by a few shorter branches. This is reminiscent of possible percolation path during FS.
We shall discuss also the interaction of water vapor with oxides and the possible impact on surface as well as bulk conductivity.
Ilan Riess, "Mixed ionic electronic conductivity and flash sintering" in "Electric Field Enhanced Processing of Advanced Materials II: Complexities and Opportunities", Rishi Raj, University of Colorado, USA Olivier Guillon, Forschungzentrum Jülich, Germany Hidehiro Yoshida, National Institute for Materials Science, Japan Eds, ECI Symposium Series, (2019). https://dc.engconfintl.org/efe_advancedmaterials_ii/62