Abstract
This paper reports the electrochemical, optical and thermal effects occurring during flash sintering of 8 mol % yttria-stabilized zirconia (8YSZ). In-situ observations of polycrystalline and single crystal specimens revealed electrochemical blackening/darkening during an incubation period prior to flash sintering. The phenomenon is induced by cathodic partial reduction under DC fields. When using a low frequency AC field (0.1–10 Hz) the blackening is reversible, following the imposed polarity switching. Thermal imaging combined with sample colour changes and electrical conductivity mapping give a complete picture of the multi-physical phenomena occurring during each stage of the flash sintering event. The partial reduction at the cathode causes a modification of the electrical properties in the sample and the blackened regions, which are close to the cathode, are more conductive than the remainder of the sample. The asymmetrical nature of the electrochemical reactions follows the field polarity and causes an asymmetry in the temperature between the anode and cathode, with the positive electrode tending to overheat. It is also observed that the phenomena are influenced by the quality of the electrical contacts and by the atmosphere used.
Highlights
Flash Sintering (FS) is a field-assisted sintering technology where an electric field is directly applied to a green sample and the current that flows causes Joule heating [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17]
The results presented in the difference following paragraphs mainlyfor refer experiments carried outthe on sample reached the current limit: asymmetrical in the case
Electrochemical blackening develops during flash incubation in 8 mol % yttria-stabilized zirconia (8YSZ) when the experiments are carried out at DC or low-frequency AC (
Summary
Flash Sintering (FS) is a field-assisted sintering technology where an electric field is directly applied to a green sample and the current that flows causes Joule heating [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17]. The sample is heated in a furnace until it reaches a suitable temperature where the conductivity suddenly rises, producing rapid heating and allowing full densification in just a few seconds. This phenomenon, called Flash Event (FE), takes place at the onset of the electric field and furnace temperature combination. The driver for the FE is the thermal runaway by Joule heating [22,23,24], in materials with a negative temperature coefficient for electrical resistivity, it is still matter of debate whether When using generators operating under current and voltage limits, the flash process can be divided into three stages: Stage I is the incubation period where the material is resistive and the power source works in voltage control; Stage II is the flash event, the system switches from voltage to current control and a peak power is achieved
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