The Electrodegradation Process in PZT Ceramics under Exposure to Cosmic Environmental Conditions
Abstract
:1. Introduction
2. Results
2.1. Electrically-Stressed PZT Ceramics under Vacuum Conditions at an Elevated Temperature (T > TC)
- Stage I—A steep step-like increase of the current flow (green curve in Figure 1c) due to displacement and Ohmic currents after a high voltage is switched on;
- Stage II—Decreasing the electric current due to discharging of the capacitor; only the Ohmic contribution of the so-called leakage current contributing to the total current remains;
- Stage IV—A dramatic increase of the kinetics of the electrodegradation; positive feedback from a Joule heating (J-H peak in Figure 1c) is presented. In this way, in a short time, the resistance of the sample is strongly reduced. The sample temperature can increase in this phase by about 10–20 °C [42];
- Stage V—In this final stage, the electrical decomposition process stagnates, and the resistance change is minimal because, in the current flow, the transport of electrons dominates, and the ionic transport, which is responsible for reducing the oxygen stoichiometry in the ceramic, does not play an important role.
2.2. Oxidation of Electro-Degraded Ceramics. Reversibility of I/M Transition
2.3. Inhomogeneous Electrodegradation. Raman Scattering Analysis of the Electro-Degraded Ceramics
2.4. Resistance and Potential Distribution during Electrodegradation: The Deoxidation and Reoxidation Processes
2.5. Electrical Conductivity in the Nano-Scale of PZT Polycrystalline Thin Films
2.6. DFT Calculations and Source of the Charge Carriers in Electro-Degraded Ceramic
3. Discussion
- Electrodegradation progression increases under reduced oxygen activity, as is the case under cosmic conditions;
- Resistance change during electrodegradation under DC action is comparable to that observed in ternary oxides in the paraelectric phase, as it is in SrTiO3 or KTaO3 crystals [20]. A difference between them centres on the type of ions moving towards the cathode in the PIC ceramics, which creates a virtual anode. Based on the EDS map** and XPS data (Figure S1e, we found that the concentration of Pb in the grain boundary did not increase. However, a reduction in PbO already occurs at 200 °C, and hence the lead in a metallic state was observed at 400 °C on the surface. This kind of electrodegradation is at a relatively low electric field of 100 V/cm. Hence, we state that these are the Pb cations that move into the cathode, creating the aforementioned virtual anode;
- Through in operando measurements of the electric potential distribution, we showed that in a high electric field, the oxygen ions’ movement towards the anode predominates. In this process, the part of the sample lying between the cathode’s geometric and virtual positions is more conductive than the rest of the ceramic, and the maximal potential drop is localised at the virtual cathode. In contrast to the single crystal, the cathode front’s position is not sharp in the ceramic, and when the virtual cathode is close to the anode, maximum oxygen effusion occurs. The oxygen escape allows the electrodegradation process to be classified as electro-induced deoxidation or solid-state electrolysis [20];
- The determination of the transfer number showed that the electric charge transport is a mixture of electrons and ions, but in all phases of the electrodegradation, the electrons predominate. Ionic transport disappears when metallic conductivity, in the last stage IV of electrodegradation, occurs;
- The electrodegradation is an activated process described by the following current power law [5] I(t) = I0 × t−n, where t is the time and the n value is the function of the temperature. With increasing temperature, the exponent n becomes smaller, and so the idea of Sidebottom [56]—stating that the exponent of the power law decreases with decreasing dimensionality of the ionic conducting paths—can be applied to a percolation network of conducting dislocations in electro-degraded PZT (see Supplemental Materials S1). The LCAFM measurement and etching studies have shown the existence of the network of dislocations (conducting filaments) in the grain boundary of thin films. In particular, the dendrite-like fractal structure was proposed by Scott, et al. [15] to analyse the origin of the fatigue effect. An additional argument for the involvement of the mentioned network derives from the discolouration of the ceramics sample with the bar-like geometry. Due to a higher concentration of dislocations in the surface region generated during polishing, this region was mostly electro-degraded;
- The extremely low oxygen outflow, leading to the insulator-metal transition, suggests that the regions (sources) of esca** oxygen are small and galvanically connected. Despite this, these regions are inhomogeneously distributed in the ceramic sample, which is reflected by non-uniform conductivity at the nano-scale;
- The network of conducting filaments is created at the grain boundaries. The core of dislocations constitutes semiconducting nanowires with an additional d1 state of Ti and Zr, which follow the invariance of Burger’s vector and constitute a galvanic short circuit of the insulating grains. During the electrodegradation, the current flows through such a filament network. Hence, the deoxidation process is selective and preferentially reduced to the core of dislocations. Incorporating additional oxygen vacancies in such filaments can generate Ti and Zr states close to the bottom of the conduction band. Then, the filaments can switch into metallic nanowires, and this behaviour is responsible for the current power law;
- Although we did not perform measurements of the local electric field near the virtual anode (e.g., through nano-potentiometric studies), we submit that these fields are extremely high. This agrees with the electric field-driven cold emission of electrons reported for PLZT ceramics above TC, which may even lead to PZT ceramics damage, as was observed [57]. The strength of the electric field locally reaches a value higher than 106 V/cm and transforms the dislocation network into metallic filaments through destruction by hot electrons of the TiO2 or ZrO2 bonding in the vicinity of the virtual anode. The valence of Ti or Zr atoms is thereby reduced, and a new metallic state is created on the core of the electro-degraded network. Therefore, the metallic component of the network is extended.
4. Materials and Methods
5. Conclusions
Supplementary Materials
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
Sample Availability
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Lazar, I.; Rodenbücher, C.; Bihlmayer, G.; Randall, C.A.; Koperski, J.; Nielen, L.; Roleder, K.; Szot, K. The Electrodegradation Process in PZT Ceramics under Exposure to Cosmic Environmental Conditions. Molecules 2023, 28, 3652. https://doi.org/10.3390/molecules28093652
Lazar I, Rodenbücher C, Bihlmayer G, Randall CA, Koperski J, Nielen L, Roleder K, Szot K. The Electrodegradation Process in PZT Ceramics under Exposure to Cosmic Environmental Conditions. Molecules. 2023; 28(9):3652. https://doi.org/10.3390/molecules28093652
Chicago/Turabian StyleLazar, Iwona, Christian Rodenbücher, Gustav Bihlmayer, Clive A. Randall, Janusz Koperski, Lutz Nielen, Krystian Roleder, and Krzysztof Szot. 2023. "The Electrodegradation Process in PZT Ceramics under Exposure to Cosmic Environmental Conditions" Molecules 28, no. 9: 3652. https://doi.org/10.3390/molecules28093652