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Staged microstructural study of flash sintered titania
Materialia 8 (2019) 100451
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Staged microstructural study of flash sintered titania Han Wang a,1, Xin Li Phuah a,1, Harry Charalambous b, Shikhar Krishn Jha b, Jin Li a, Thomas Tsakalakos b, Xinghang Zhang a, Haiyan Wang a,c,∗ a
School of Materials Engineering, Purdue University, West Lafayette, IN 47907, United States Department of Materials Science and Engineering, Rutgers University, New Brunswick, NJ 08901, United States c School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47907, United States b
a r t i c l e Keywords: Titania Flash sintering TEM Microstructure Defects
i n f o
a b s t r a c t Flash sintering offers more control of microstructure owing to its additional sintering conditions compared to conventional sintering, including electric field, current density limit and holding time. A systematic study of flash sintered TiO2 is presented to investigate the effects of flash sintering conditions on the asymmetrical microstructure and defect structure across the two electrodes of the samples. Grain growth has been found to be more prominent near the positive electrode and significantly influenced by the current density limit. Extended defects, including dislocations and stacking faults, have been observed near the positive electrode and the defect density is highly dependent on the strength of electric field. Lowering the current density and holding time could maximize the defect density in TiO2 for improved properties.
1. Introduction Flash sintering of ceramics has gained significant attention from the ceramics community ever since the first publication in 2010 [1]. Flash sintering results in rapid densification of ceramics by applying an electric field across the sample while heating in a furnace. At a certain furnace temperature and electric field combination, current will begin to flow through the sample and result in rapid densification. The flash sintering process is often described in the following three stages: Stage I (Voltage control): Electric field is applied across the sample while the furnace temperature could either be heating up or held isothermally during this stage. This stage has been known as the incubation period, where the sample transitions from being insulating to conductive [2,3]. The incubation time depends on the magnitude of electric field and furnace temperature and can last from a fraction of a second to several hours [4]. Stage II (Switching from voltage to current control): As the sample becomes more conductive, current begins to rapidly flow through the sample and undergoes Joule heating [3]. To avoid an indefinite increase in current, the power supply is switched from voltage control to current control to limit the maximum current density. The electric field drops to maintain the constant current and this creates a large power spike. This stage is where most of the sample densification occurs and only lasts for several seconds.
∗
1
Stage III (Current control): During current control, the current flows through the sample and resulting in an increase in sample temperature from Joule heating. This stage has approximately constant power dissipation and can be held indefinitely even when the furnace is turned off, as long as the sample remains conductive. Since most of the densification occurred during Stage II, holding the steady state power dissipation will lead to final densification and grain growth [4,5]. Flash sintering of various ceramics, including yttria-stabilized zirconia (YSZ) [1,6–8], TiO2 [5,9–11], ZnO [12–15], CeO2 [16], SrTiO3 [17–19], MnCo2 O4 [20], SnO2 [21] and BiFeO3 [22,23] has been demonstrated. The investigation of the flash sintering mechanism(s) is still ongoing and most of mechanical studies and models developed are based on the ionic conductor YSZ [2,24–27]. YSZ is a complex system which exhibits phase changes at high temperatures [28] and Ysegregation at grain boundaries due to space charge [8,29]. These characteristics may influence the flash sintering process and complicate the understanding of the mechanisms for flash sintering. Currently, majority of the detailed flash sintering studies have been performed on YSZ systems. The studies performed on other flash sintered ceramics are scarce and there is a compelling need for detailed studies on other ceramic systems with simple composition which may behave differently from YSZ. Rutile TiO2 , a mixed conductor, is an ideal system to study because of its good phase stability at high temperatures [30,31]. Different
Corresponding author at: School of Materials Engineering, Purdue University, West Lafayette, IN 47907, United States. E-mail address: [email protected] (H. Wang). These authors contributed equally.
https://doi.org/10.1016/j.mtla.2019.100451 Received 9 August 2019; Accepted 19 August 2019 Available online 22 August 2019 2589-1529/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
H. Wang, X.L. Phuah and H. Charalambous et al.
Materialia 8 (2019) 100451
diffusion mechanisms are suggested for flash sintering of TiO2 based on the strength of the electric field [9], where ionic conductivity is dominant at low electric field while electronic conductivity overtakes at higher electric fields. In previous flash sintered TiO2 studies, inhomogeneous characteristics were reported across the sample [5,11]. Prior in situ X-ray diffraction studies during flash sintering of TiO2 revealed a temperature distribution throughout the sample [11], while post analysis using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy revealed the reduction of TiO2 which occurred near the positive electrode [5]. In this work, a detailed microstructure and defect analysis has been conducted on flash sintered TiO2 with the key flash sintering parameters varied for the three different stages discussed above. More specifically, the key flash sintering parameters include the electric field applied in Stage I, the current density limit in Stage II and III, and the holding time in Stage III. With this staged study, the impacts of electric field, current and holding time on the overall grain morphology, pore structure and defect type and density can be effectively explored. Since inhomogeneity exists across the two electrodes of the samples, the microstructure characteristics are compared for TEM samples taken near the positive and negative electrodes for each of the staged samples. This study provides a detailed understanding of how each key sintering parameter affects the resulting microstructure and allow for optimized flash sintering conditions. The defect structures of flash sintered TiO2 , including high density dislocations and stacking faults, observed for the first time in TiO2 , could also play a critical role in future materials designs with enhanced mechanical and physical properties. 2. Experimental Rutile TiO2 powder (Inframat Advanced Materials, Product # 22N-0814R, 50 ± 20 nm particle size) was pressed uniaxially to make cylindrical green bodies. The dimensions of the green bodies are 6 mm diameter by 6 ± 0.5 mm height with 40–45% density. Fig. 1 shows the experimental setup to perform flash sintering. A small pressure of 10 kPa was used to maintain electrical contact between the green body and Pt electrodes during the flash sintering experiment. Specimens were heated at a rate of 10 °C/min in air to the pre-flash temperature of 900 °C. Once the pre-flash temperature was reached, an electric field was applied across the sample thickness. The rise in conductivity of the sample led to a non-linear rise in the current until it has reached the limit set in the feedback loop. The power supply was switched from voltage control to current control and held constant for a preset time. The power was switched off and the samples were cooled down to room temperature. A series of experiments were carried out with varying applied electric field, current density limit and holding time, summarized in Table 1. The flash sintered TiO2 samples were sectioned near the positive and negative electrodes. The sectioned samples were polished and thermally etched at 900 °C for 30 min to reveal the grain boundaries. Scanning electron microscopy was performed on the samples on a Nova NanoSEM. TEM samples were prepared through the conventional approach, which includes cutting, manual grinding, polishing, dimpling and final ion polishing in a precision ion milling system. An FEI TALOS F200X TEM/STEM operated at 200 kV was used for microstructure characterization. The average grain sizes were averaged over 100 grains from multiple images. The dislocation and stacking fault densities were
Fig. 1. A schematic of the experimental setup to perform flash sintering.
Fig. 2. (a) Plots of electric field, current density, and power density during flash sintering. The applied field was 120 V/cm with a current density limit of 1.5 A/cm2 , and holding time of 60 s. The microstructure near the (b) positive and (c) negative electrodes was compared.
measured by line-intercept method and averaged over three images. The phase of the sintered TiO2 samples was identified with X-ray diffraction (XRD, PANalytical Empyrean). 3. Results and discussion 3.1. Microstructure and defect structure of flash sintered TiO2 Fig. 2(a) shows the plots of electric field, current density, and power density as a function of the time with an applied field of 120 V/cm, current density limit of 1.5 A/cm2 and holding time of 60 s during the current control stage. The current increased rapidly around 60 s, resulting in a sudden increase in power density. When the power supply was switched to current control, the power density drops back down to a steady state since the electric field drops to maintain constant current. Due to the rapid densification occurring during Stage II, there may be differences in the power density curve during Stage III between samples, as it is difficult to maintain perfect electrical contact at all times. The corresponding TEM micrographs of the samples from the positive and negative electrodes are shown in Fig. 2(b) and (c), respectively. The grain size was found to be larger near the positive electrode (∼0.9 μm) than that near the negative one (∼0.6 μm). Additionally, the porosity near the positive electrode (1.29%) was less than that near the negative electrode (3.53%). It is noted that, despite that the TEM has limited fields of view compared to SEM, the microstructure analysis of TEM has shown more details about the characteristics of flash sintered samples compared to SEM. For example, a previous study on flash sintered 3YSZ study suggested much smaller grain size by TEM than that
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Materialia 8 (2019) 100451
Table 1 Summary of experimental conditions, grain size and defects density. Furnace temperature (°C)
Electric field (V/cm)
Current density (A/cm2 )
Holding time (s)
Grain size (𝜇m)
Defects density (𝜇m−1 )
Positive
Negative
Dislocations (DL) Stacking faults (SF)
900 900 900 900 1100
120 60 120 120 –
1.5 1.5 4.5 1.5 –
60 60 60 600 –
0.9 ± 0.3 0.7 ± 0.2 8.8 ± 1.5 1.9 ± 0.5 1.0 ± 0.4
0.6 ± 0.2 0.4 ± 0.1 0.7 ± 0.1 1.5 ± 0.4
26.4 ± 4.4 – 14.4 ± 2.0 13.2 ± 0.8 –
101.6 ± 3.3 – 54.9 ± 29.2 30.0 ± 3.8 –
Fig. 3. High density (a) dislocations (DL) and (b) stacking faults (SF) observed near the positive electrode of the flash sintered TiO2 with an applied field of 120 V/cm, current density limit of 1.5 A/cm2 , and holding time of 60 s. (c) Defects were not observed near the negative electrode.
in SEM, due to the texturing of adjacent grains [8]. To have a complete grain size distribution analysis, we have combined both TEM and SEM analysis for this work. Fig. S1 shows the SEM micrographs at the positive and negative electrodes. The average grain size from the positive side was estimated to be 0.9 μm, while the negative side was measured to be 0.7 μm. The measured grain sizes from SEM are comparable with that of the TEM results which is different from the previous report on flash sintered YSZ. The porosity was also estimated from the SEM images and they were lower than the TEM results (0.76% for the positive side and 2.96% for the negative side). Another advantage of using the TEM is to allow for observation of any defects present, and thus detailed defect analysis was conducted on flash sintered TiO2 . Fig. 3(a) and (b) shows high density dislocations and stacking faults near the positive electrode. The dislocation density was estimated to be 26.4 μm−1 and stacking fault density was 101.6 μm−1 , using the linear density calculation method. In the region near the negative electrode, which was not as well-sintered, there were no obvious defects observed and most of the grains had minimal contrast caused by strain or defects (Fig. 3(c)). For comparison with a field-free sample, a sample was sintered at 1100 °C with no applied electric field. The microstructure is shown in Fig. S2 for reference, where the average grain size (1 μm) is similar to the positive electrode of the flash sintered sample and there were no obvious extended defects observed. Ceramics are known to have limited plasticity due to the difficulty for slip to occur. In the case of flash sintered ceramics, high density dislocation arrays were previously observed in the YSZ system [8] and has shown to enhance the plasticity [32]. Other reports of defects include stacking faults in ZnO [15] and dislocations in CeO2 [16], all of which are related to the accumulation of oxygen vacancies. Thus, it is necessary to investigate the types of defects present in the various flash sintered ceramic systems and how they can be controlled. The positive and negative electrodes have shown contrasting microstructures, which strongly suggest the role of point defects during flash sintering. In the negative electrode region, smaller grain sizes and higher porosity suggest slower densification and grain growth kinetics. Gradients of grain sizes and porosities have been a common observation for several other flash sintered ceramic systems [15,16,19,33]. Numerous studies have been performed for the YSZ system and the asymmetric grain sizes were mostly explained by the lowered migration energy for grain growth near the cathode for the YSZ system due to accumulation
of oxygen vacancies [33,34]. This explanation could work similarly for TiO2 , as Ti4+ ions could be reduced to maintain neutrality from the oxygen vacancies. TiO2 has shown enhanced kinetics in a reduced atmosphere [35] and would explain the asymmetric microstructure. It is noted that the reduction occurred near the positive electrode in flash sintered TiO2 , which is consisted with the previous report [5]. In other systems, such as YSZ and SrTiO3 , the negative electrodes tend to have higher concentrations of positively charged oxygen vacancies. Defect thermodynamics revealed that reduced TiO2 is a defect structure formed when the equilibrium concentration of electronic defects exceeds the ionic defects [36]. If the formation of electrons is preferential, it could occur near the positive electrode which also creates oxygen vacancies. This confirms that electronic conduction is the dominant conductivity during flash sintering of TiO2 and have a contrasting mechanistic behavior compared to the ionic conductor systems. This agrees with the enhanced electronic conductivity in nanocrystalline TiO2 and the conductivity study reported for flash sintered TiO2 [9,37]. The higher concentration of oxygen vacancies near the positive electrode is further supported by the observation of high density extended defects in this work. Stacking fault is a planar fault caused by an error in the stacking sequence and can be created by non-stoichiometry in the TiO2 system [38,39]. A previous in situ bias study in TEM demonstrated the formation of planar faults for TiO2 under an applied electric field due to the coalescence of oxygen vacancies [40]. On the other hand, dislocations are line defects created by imperfections in the crystal. Both types of defects are likely to form due to the high concentration of oxygen vacancies. Similar planar faults have also been observed in flash sintered ZnO [15] but with much lower stacking fault densities than that of TiO2 . This is because TiO2 can accommodate large deviations from stoichiometry by forming crystallographic shear planes [41]. Magnéli phases are commonly identified in heavily reduced TiOx (1.67 < x < 1.999) after the accumulation and ordering of shear planes [42,43]. X-ray diffraction (XRD) was performed, shown in Fig. S3. It is clear that the flash sintered sample remained rutile and did not show any formation of the Magnéli phase. Since the reduced phase was not identified, it is more likely that the oxygen vacancies did not reach to the ordered phase during the reduction process and resulted in high densities of stacking faults and dislocations to accommodate the change in stoichiometry.
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Materialia 8 (2019) 100451
The flash sintered TiO2 sample, with an electric field of 120 V/cm, current density limit of 1.5 A/cm2 and the holding time of 60 s, is used as a baseline for microstructure comparison of different flash sintering conditions. Table 1 summarizes all of the sintering conditions, along with the average grain sizes near the positive and negative electrodes and the defect densities at the positive electrode. 3.2. Effect of flash sintering conditions on the microstructure and defect structure 3.2.1. Stage I: electric field During Stage I, the sintering conditions which can be controlled are the electric field and furnace temperature. For this study, the effect of decreasing the applied field while maintaining the other flash conditions, was compared for TiO2 . Fig. 4(a) compares the power density for the samples with different initial electric fields of 120 V/cm and 60 V/cm. The sample with lower electric field experienced a lower maximum power density and prolonged incubation time of 200 s compared to that of samples under higher electric field. A similar trend has been previously observed in YSZ, where the incubation time increases from a few seconds up to 60 s after decreasing the electric field from 125 V/cm to 75 V/cm [7]. Fig. 4(b) and (c) shows the microstructures near the positive and negative electrodes under the lower electric field (60 V/cm), with average grain sizes were 0.7 μm and 0.4 μm, respectively. The reduced electric field did decrease the grain sizes but the asymmetrical structure remained. The lower field sample is also less dense, since the porosity of the sample is estimated to be around 3.28%, compared to the higher electric field (1.29%). There were also no obvious extended defects observed with a very low density of stacking faults observed, as shown in Fig. S4, near the positive electrode. Based on the observations in Fig. 4, the strength of the electric field appears to play a critical role in controlling the concentration of defects. The weaker electric field resulted in slower formation and an overall lower concentration of defects, thus required a prolonged time for an increase in conductivity for Stage II. This agrees with the observation of smaller grain sizes, higher porosity and minimal extended defects, since less oxygen vacancies are formed. This would suggest that for
Fig. 4. (a) Power density plots for the flash sintered TiO2 with electric fields of 60 V/cm and 120 V/cm. Microstructures were compared for the (b) positive and (c) negative electrode regions from the sample with lower electric field, 60 V/cm.
Fig. 5. (a) Power density plots for the flash sintered TiO2 with current density limits of 1.5 A/cm2 and 4.5 A/cm2 . Microstructures were compared for the (b) positive and (c) negative electrode regions from the sample with higher current density, 4.5 A/cm2 .
better densification and higher defects density, a higher electric field may be preferred. Additional comparisons will be needed at higher electric fields and will help to further verify this correlation. 3.2.2. Stage II: current density limit The non-linear rise in conductivity during Stage II leads to a rapid rise in the current up to the limit set by the power supply. With the same initial electric field and holding time, the power density plot for the current density limits of 1.5 A/cm2 and 4.5 A/cm2 were compared in Fig. 5(a). As evidenced in Fig. 5(b) for the 4.5 A/cm2 sample, the increase in the current density limit led to a significant grain growth, i.e., the average grain size for the sample near the positive electrode region increases from ∼0.9 μm (Fig. 2(b), the 1.5 A/cm2 sample) to ∼8.8 μm (Fig. 5(b), the 4.5 A/cm2 sample). The negative electrode region of the 4.5 A/cm2 sample (Fig. 5(c)) did not experience the same rapid grain growth as the positive region. It shows fine grains and higher porosity, similar to the negative electrode with the lower current density limit (Fig. 2(c)). Furthermore, the pore morphology for the 1.5 A/cm2 and 4.5 A/cm2 samples are distinctive due to the different grain growth rates. To achieve a clearer observation of the pore morphology, scanning transmission electron microscope (STEM) images for the positive electrode regions of the two samples were compared in Fig. 6. The 1.5 A/cm2 sample demonstrates primarily intergranular pores (i.e., pores between grains) with only a few intragranular pores (i.e., pores within grains), as shown in Fig. 6(a). On the contrary, the pores for the 4.5 A/cm2 sample are predominantly intragranular (Fig. 6(b)). Due to the rapid grain growth, the grain boundary velocity significantly exceeds the pore velocity, causing pore breakaways from the grain boundaries and become trapped inside the grains. These intragranular pores shown in Fig. 6(b) are well faceted and mostly square-shaped with various pore sizes. Other morphologies of these intragranular pores were also found, including rectangular and hexagonal, as shown in Fig. S5. The overall pore density is estimated to be around 4.37% for the 4.5 A/cm2 sample, which is much higher than the lower current density. These well-faceted intragranular pores are made of low surface energy crystal facet (i.e. {110} for rutile), which are typically obtained after a long, high temperature annealing process due to pore coalescence [31]. It is more common to observe intragranular pores which are spherical as they have the lowest free energy. Because of the high current density, the power dissipation and specimen temperature are considerably higher especially at the positive side, which led to significant grain growth and prompted the formation of these faceted, intragranular pores.
H. Wang, X.L. Phuah and H. Charalambous et al.
Fig. 6. STEM images comparing the pore morphology for the flash sintered TiO2 with current density limits of (a) 1.5 A/cm2 and (b) 4.5 A/cm2 .
In addition, the 4.5 A/cm2 current–density limit sample has reasonably high density of dislocation arrays and stacking faults near the positive electrode region, as shown in Fig. 7(a) and (b), respectively. The dislocation density was estimated to be 14.4 μm−1 and the stacking fault density was 54.9 μm−1 , which are approximately half of the defect density of the 1.5 A/cm2 sample (Fig. 3(a) and (b)). Significant grain growth typically causes the removal of defects as grain boundaries move past them, which leads to the reduction of dislocation and stacking fault densities. There is also significant variation in stacking fault density among the grains, which leads to the large deviation. Hence, it is critical to adjust the current density limit to control the uniformity of microstructure and defects density. 3.2.3. Stage III: holding time The flash state can be held indefinitely during Stage III as long as the sample remains conductive. In this stage, the holding time was increased from 60 s to 600 s (Fig. 8(a)). The micrographs near the positive and negative electrodes are shown in Fig. 8(b) and (c) for this 600 s case. First, obvious grain growth has been observed in the microstructure near both positive and negative electrodes for the sample with longer holding time. The average grain size was measured to be 1.9 μm and 1.5 μm for the positive and negative electrode, respectively. These are more than twice of the average grain sizes for the case of 60 s sample on both electrodes (Fig. 2(b) and (c)). The grain growth at both electrodes were comparable, unlike the case of high current density limit where a very significant grain growth was observed in the positive electrode region. Since less grain growth was observed for a long holding time compared with a high current density limit, one could expect a minor decrease in defects density. Fig. 9(a) and (b) shows the defect structure
Materialia 8 (2019) 100451
Fig. 7. (a) Dislocations and (b) stacking faults observed near the positive electrode of the flash sintered TiO2 with the increased current density limit of 4.5 A/cm2 .
of the flash sintered sample held for 600 s where the dislocation and stacking fault densities were estimated to be 13.2 μm−1 and 30.0 μm−1 , respectively. It is clear that the defect density decreases significantly with a longer holding time. This would suggest that the holding time during Stage III leads to the removal of defects through grain growth and annihilation of defects from annealing.
3.3. Suggestions for optimizing flash sintering conditions for TiO2 Flash sintering has demonstrated the ability to significantly reduce furnace temperatures and sintering time compared to conventional sintering. Moreover, with the rapid heating rates caused by Joule heating, grain growth could be minimized from the initial coarsening. The final microstructure and defects density can be further controlled by the electrical parameters, as demonstrated and discussed in the previous sections, for flash sintered TiO2 . This opens new possibilities for controlled and enhanced properties in TiO2 with various applications, such as photocatalysis [44] and sensing devices [45]. Smaller grain sizes are usually desired for enhanced properties, such as fracture strength and dielectric constant [46]. Based on the results from this study, the grain size can be influenced by setting the current density limit. Using high current densities will lead to significant grain growth and are also known to cause large thermal gradients in the sample and hot spots within the samples [12,47]. However, the current density is known to be a critical parameter to control the final density and setting a low current density limit may consequently reduce the ability to reach high density [48]. Thus, using an intermediate current density
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Materialia 8 (2019) 100451
sintered TiO2 at room temperature due to the formation of nanoscale stacking faults [49]. Based on the results from this study, the flash sintering parameters can be further tuned to achieve the desired defects density. High density of dislocations and stacking faults can be achieved by applying a stronger electric field (e.g., ∼120 V/cm for TiO2 ), as well as limiting the current density (e.g., ∼1.5 A/cm2 ) and holding time (e.g., ∼60 s) to prevent removal by grain growth and annihilation. With both fine grained microstructure and high density defects, ceramics with enhanced mechanical behavior can be achieved through flash sintering. 4. Conclusions
Fig. 8. (a) Power density plots for the flash sintered TiO2 with holding time of 60 s and 600 s. Microstructures were compared for the (b) positive and (c) negative electrode regions from the sample with longer holding time, 600 s.
The microstructural characteristics of flash sintered TiO2 have been effectively studied in this work. The positive electrode region presents larger grains and higher porosity compared to the negative electrode region. The enhanced kinetics near the positive electrode is correlated with the accumulation of oxygen vacancies, since high density of stacking faults and dislocations were observed. Different from the well-known ionic conductor YSZ, the positively charged oxygen vacancies accumulate at the positive electrode in TiO2 . Due to the preferential electronic conductivity of TiO2 at high electric fields, the oxygen vacancies are created due to the production of electrons. A systematic microstructure study comparing the grain size and defect density at the two electrodes for the three stages has been conducted and the major findings are listed as follows: 1. In Stage I, the weaker electric field is less effective in the formation of charge carriers and thus required a longer time before the sample was conductive for current to flow. Additionally, less oxygen vacancies were formed in the lower electric field, which resulted in a slower rate of densification and minimal formation of any extended defects. In order to achieve higher density of defects, a higher electric field should be utilized. 2. In Stage II, increasing the current density limit results in a significant grain growth near the positive electrode. Consequently, a large number of faceted intragranular pores are present and the defect density is reduced. This makes the current density limit a critical controlling condition for achieving more symmetric microstructure while maintaining fine grain structure. 3. In Stage III, a longer holding time leads to additional grain growth throughout the specimen. Increasing the holding time also leads to defect removal by grain growth and annihilation, and therefore the holding time should be minimized to prevent further grain growth and defect removal. Overall, flash sintering offers a large degree of flexibility in controlling the microstructure of sintered TiO2 , such as grain size, porosity, point defects, stacking faults, dislocations, etc. The findings suggest the flash sintering conditions could be tuned effectively to obtain desired microstructures and enhanced properties. Declaration of Competing Interest The authors declare that they have no competing interests. Acknowledgments
Fig. 9. (a)-(b) Dislocations and stacking faults observed near the positive electrode of the flash sintered TiO2 with increased holding time of 600 s.
(e.g., ∼1.5 A/cm2 for TiO2 ) will be necessary to achieve high densification while maintaining a uniform and fine microstructure. The introduction of extended defects through flash sintering can definitely make an impact on the mechanical behavior of ceramics. In another study, we have investigated the mechanical behavior of flash sintered TiO2 and have demonstrated the enhanced plasticity in flash
We would like to acknowledge the support from the U.S. Office of Naval Research (Contract number: N00014-17-1-2087 for sintering effort and N00014-16-1-2778 for TEM). The effort at Rutgers University was supported by the U.S. Office of Naval Research (Contract number: N00014-15-1-2492). Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.mtla.2019.100451.
H. Wang, X.L. Phuah and H. Charalambous et al.
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Full Length Article
Staged microstructural study of flash sintered titania Han Wang a,1, Xin Li Phuah a,1, Harry Charalambous b, Shikhar Krishn Jha b, Jin Li a, Thomas Tsakalakos b, Xinghang Zhang a, Haiyan Wang a,c,∗ a
School of Materials Engineering, Purdue University, West Lafayette, IN 47907, United States Department of Materials Science and Engineering, Rutgers University, New Brunswick, NJ 08901, United States c School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47907, United States b
a r t i c l e Keywords: Titania Flash sintering TEM Microstructure Defects
i n f o
a b s t r a c t Flash sintering offers more control of microstructure owing to its additional sintering conditions compared to conventional sintering, including electric field, current density limit and holding time. A systematic study of flash sintered TiO2 is presented to investigate the effects of flash sintering conditions on the asymmetrical microstructure and defect structure across the two electrodes of the samples. Grain growth has been found to be more prominent near the positive electrode and significantly influenced by the current density limit. Extended defects, including dislocations and stacking faults, have been observed near the positive electrode and the defect density is highly dependent on the strength of electric field. Lowering the current density and holding time could maximize the defect density in TiO2 for improved properties.
1. Introduction Flash sintering of ceramics has gained significant attention from the ceramics community ever since the first publication in 2010 [1]. Flash sintering results in rapid densification of ceramics by applying an electric field across the sample while heating in a furnace. At a certain furnace temperature and electric field combination, current will begin to flow through the sample and result in rapid densification. The flash sintering process is often described in the following three stages: Stage I (Voltage control): Electric field is applied across the sample while the furnace temperature could either be heating up or held isothermally during this stage. This stage has been known as the incubation period, where the sample transitions from being insulating to conductive [2,3]. The incubation time depends on the magnitude of electric field and furnace temperature and can last from a fraction of a second to several hours [4]. Stage II (Switching from voltage to current control): As the sample becomes more conductive, current begins to rapidly flow through the sample and undergoes Joule heating [3]. To avoid an indefinite increase in current, the power supply is switched from voltage control to current control to limit the maximum current density. The electric field drops to maintain the constant current and this creates a large power spike. This stage is where most of the sample densification occurs and only lasts for several seconds.
∗
1
Stage III (Current control): During current control, the current flows through the sample and resulting in an increase in sample temperature from Joule heating. This stage has approximately constant power dissipation and can be held indefinitely even when the furnace is turned off, as long as the sample remains conductive. Since most of the densification occurred during Stage II, holding the steady state power dissipation will lead to final densification and grain growth [4,5]. Flash sintering of various ceramics, including yttria-stabilized zirconia (YSZ) [1,6–8], TiO2 [5,9–11], ZnO [12–15], CeO2 [16], SrTiO3 [17–19], MnCo2 O4 [20], SnO2 [21] and BiFeO3 [22,23] has been demonstrated. The investigation of the flash sintering mechanism(s) is still ongoing and most of mechanical studies and models developed are based on the ionic conductor YSZ [2,24–27]. YSZ is a complex system which exhibits phase changes at high temperatures [28] and Ysegregation at grain boundaries due to space charge [8,29]. These characteristics may influence the flash sintering process and complicate the understanding of the mechanisms for flash sintering. Currently, majority of the detailed flash sintering studies have been performed on YSZ systems. The studies performed on other flash sintered ceramics are scarce and there is a compelling need for detailed studies on other ceramic systems with simple composition which may behave differently from YSZ. Rutile TiO2 , a mixed conductor, is an ideal system to study because of its good phase stability at high temperatures [30,31]. Different
Corresponding author at: School of Materials Engineering, Purdue University, West Lafayette, IN 47907, United States. E-mail address: [email protected] (H. Wang). These authors contributed equally.
https://doi.org/10.1016/j.mtla.2019.100451 Received 9 August 2019; Accepted 19 August 2019 Available online 22 August 2019 2589-1529/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
H. Wang, X.L. Phuah and H. Charalambous et al.
Materialia 8 (2019) 100451
diffusion mechanisms are suggested for flash sintering of TiO2 based on the strength of the electric field [9], where ionic conductivity is dominant at low electric field while electronic conductivity overtakes at higher electric fields. In previous flash sintered TiO2 studies, inhomogeneous characteristics were reported across the sample [5,11]. Prior in situ X-ray diffraction studies during flash sintering of TiO2 revealed a temperature distribution throughout the sample [11], while post analysis using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy revealed the reduction of TiO2 which occurred near the positive electrode [5]. In this work, a detailed microstructure and defect analysis has been conducted on flash sintered TiO2 with the key flash sintering parameters varied for the three different stages discussed above. More specifically, the key flash sintering parameters include the electric field applied in Stage I, the current density limit in Stage II and III, and the holding time in Stage III. With this staged study, the impacts of electric field, current and holding time on the overall grain morphology, pore structure and defect type and density can be effectively explored. Since inhomogeneity exists across the two electrodes of the samples, the microstructure characteristics are compared for TEM samples taken near the positive and negative electrodes for each of the staged samples. This study provides a detailed understanding of how each key sintering parameter affects the resulting microstructure and allow for optimized flash sintering conditions. The defect structures of flash sintered TiO2 , including high density dislocations and stacking faults, observed for the first time in TiO2 , could also play a critical role in future materials designs with enhanced mechanical and physical properties. 2. Experimental Rutile TiO2 powder (Inframat Advanced Materials, Product # 22N-0814R, 50 ± 20 nm particle size) was pressed uniaxially to make cylindrical green bodies. The dimensions of the green bodies are 6 mm diameter by 6 ± 0.5 mm height with 40–45% density. Fig. 1 shows the experimental setup to perform flash sintering. A small pressure of 10 kPa was used to maintain electrical contact between the green body and Pt electrodes during the flash sintering experiment. Specimens were heated at a rate of 10 °C/min in air to the pre-flash temperature of 900 °C. Once the pre-flash temperature was reached, an electric field was applied across the sample thickness. The rise in conductivity of the sample led to a non-linear rise in the current until it has reached the limit set in the feedback loop. The power supply was switched from voltage control to current control and held constant for a preset time. The power was switched off and the samples were cooled down to room temperature. A series of experiments were carried out with varying applied electric field, current density limit and holding time, summarized in Table 1. The flash sintered TiO2 samples were sectioned near the positive and negative electrodes. The sectioned samples were polished and thermally etched at 900 °C for 30 min to reveal the grain boundaries. Scanning electron microscopy was performed on the samples on a Nova NanoSEM. TEM samples were prepared through the conventional approach, which includes cutting, manual grinding, polishing, dimpling and final ion polishing in a precision ion milling system. An FEI TALOS F200X TEM/STEM operated at 200 kV was used for microstructure characterization. The average grain sizes were averaged over 100 grains from multiple images. The dislocation and stacking fault densities were
Fig. 1. A schematic of the experimental setup to perform flash sintering.
Fig. 2. (a) Plots of electric field, current density, and power density during flash sintering. The applied field was 120 V/cm with a current density limit of 1.5 A/cm2 , and holding time of 60 s. The microstructure near the (b) positive and (c) negative electrodes was compared.
measured by line-intercept method and averaged over three images. The phase of the sintered TiO2 samples was identified with X-ray diffraction (XRD, PANalytical Empyrean). 3. Results and discussion 3.1. Microstructure and defect structure of flash sintered TiO2 Fig. 2(a) shows the plots of electric field, current density, and power density as a function of the time with an applied field of 120 V/cm, current density limit of 1.5 A/cm2 and holding time of 60 s during the current control stage. The current increased rapidly around 60 s, resulting in a sudden increase in power density. When the power supply was switched to current control, the power density drops back down to a steady state since the electric field drops to maintain constant current. Due to the rapid densification occurring during Stage II, there may be differences in the power density curve during Stage III between samples, as it is difficult to maintain perfect electrical contact at all times. The corresponding TEM micrographs of the samples from the positive and negative electrodes are shown in Fig. 2(b) and (c), respectively. The grain size was found to be larger near the positive electrode (∼0.9 μm) than that near the negative one (∼0.6 μm). Additionally, the porosity near the positive electrode (1.29%) was less than that near the negative electrode (3.53%). It is noted that, despite that the TEM has limited fields of view compared to SEM, the microstructure analysis of TEM has shown more details about the characteristics of flash sintered samples compared to SEM. For example, a previous study on flash sintered 3YSZ study suggested much smaller grain size by TEM than that
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Table 1 Summary of experimental conditions, grain size and defects density. Furnace temperature (°C)
Electric field (V/cm)
Current density (A/cm2 )
Holding time (s)
Grain size (𝜇m)
Defects density (𝜇m−1 )
Positive
Negative
Dislocations (DL) Stacking faults (SF)
900 900 900 900 1100
120 60 120 120 –
1.5 1.5 4.5 1.5 –
60 60 60 600 –
0.9 ± 0.3 0.7 ± 0.2 8.8 ± 1.5 1.9 ± 0.5 1.0 ± 0.4
0.6 ± 0.2 0.4 ± 0.1 0.7 ± 0.1 1.5 ± 0.4
26.4 ± 4.4 – 14.4 ± 2.0 13.2 ± 0.8 –
101.6 ± 3.3 – 54.9 ± 29.2 30.0 ± 3.8 –
Fig. 3. High density (a) dislocations (DL) and (b) stacking faults (SF) observed near the positive electrode of the flash sintered TiO2 with an applied field of 120 V/cm, current density limit of 1.5 A/cm2 , and holding time of 60 s. (c) Defects were not observed near the negative electrode.
in SEM, due to the texturing of adjacent grains [8]. To have a complete grain size distribution analysis, we have combined both TEM and SEM analysis for this work. Fig. S1 shows the SEM micrographs at the positive and negative electrodes. The average grain size from the positive side was estimated to be 0.9 μm, while the negative side was measured to be 0.7 μm. The measured grain sizes from SEM are comparable with that of the TEM results which is different from the previous report on flash sintered YSZ. The porosity was also estimated from the SEM images and they were lower than the TEM results (0.76% for the positive side and 2.96% for the negative side). Another advantage of using the TEM is to allow for observation of any defects present, and thus detailed defect analysis was conducted on flash sintered TiO2 . Fig. 3(a) and (b) shows high density dislocations and stacking faults near the positive electrode. The dislocation density was estimated to be 26.4 μm−1 and stacking fault density was 101.6 μm−1 , using the linear density calculation method. In the region near the negative electrode, which was not as well-sintered, there were no obvious defects observed and most of the grains had minimal contrast caused by strain or defects (Fig. 3(c)). For comparison with a field-free sample, a sample was sintered at 1100 °C with no applied electric field. The microstructure is shown in Fig. S2 for reference, where the average grain size (1 μm) is similar to the positive electrode of the flash sintered sample and there were no obvious extended defects observed. Ceramics are known to have limited plasticity due to the difficulty for slip to occur. In the case of flash sintered ceramics, high density dislocation arrays were previously observed in the YSZ system [8] and has shown to enhance the plasticity [32]. Other reports of defects include stacking faults in ZnO [15] and dislocations in CeO2 [16], all of which are related to the accumulation of oxygen vacancies. Thus, it is necessary to investigate the types of defects present in the various flash sintered ceramic systems and how they can be controlled. The positive and negative electrodes have shown contrasting microstructures, which strongly suggest the role of point defects during flash sintering. In the negative electrode region, smaller grain sizes and higher porosity suggest slower densification and grain growth kinetics. Gradients of grain sizes and porosities have been a common observation for several other flash sintered ceramic systems [15,16,19,33]. Numerous studies have been performed for the YSZ system and the asymmetric grain sizes were mostly explained by the lowered migration energy for grain growth near the cathode for the YSZ system due to accumulation
of oxygen vacancies [33,34]. This explanation could work similarly for TiO2 , as Ti4+ ions could be reduced to maintain neutrality from the oxygen vacancies. TiO2 has shown enhanced kinetics in a reduced atmosphere [35] and would explain the asymmetric microstructure. It is noted that the reduction occurred near the positive electrode in flash sintered TiO2 , which is consisted with the previous report [5]. In other systems, such as YSZ and SrTiO3 , the negative electrodes tend to have higher concentrations of positively charged oxygen vacancies. Defect thermodynamics revealed that reduced TiO2 is a defect structure formed when the equilibrium concentration of electronic defects exceeds the ionic defects [36]. If the formation of electrons is preferential, it could occur near the positive electrode which also creates oxygen vacancies. This confirms that electronic conduction is the dominant conductivity during flash sintering of TiO2 and have a contrasting mechanistic behavior compared to the ionic conductor systems. This agrees with the enhanced electronic conductivity in nanocrystalline TiO2 and the conductivity study reported for flash sintered TiO2 [9,37]. The higher concentration of oxygen vacancies near the positive electrode is further supported by the observation of high density extended defects in this work. Stacking fault is a planar fault caused by an error in the stacking sequence and can be created by non-stoichiometry in the TiO2 system [38,39]. A previous in situ bias study in TEM demonstrated the formation of planar faults for TiO2 under an applied electric field due to the coalescence of oxygen vacancies [40]. On the other hand, dislocations are line defects created by imperfections in the crystal. Both types of defects are likely to form due to the high concentration of oxygen vacancies. Similar planar faults have also been observed in flash sintered ZnO [15] but with much lower stacking fault densities than that of TiO2 . This is because TiO2 can accommodate large deviations from stoichiometry by forming crystallographic shear planes [41]. Magnéli phases are commonly identified in heavily reduced TiOx (1.67 < x < 1.999) after the accumulation and ordering of shear planes [42,43]. X-ray diffraction (XRD) was performed, shown in Fig. S3. It is clear that the flash sintered sample remained rutile and did not show any formation of the Magnéli phase. Since the reduced phase was not identified, it is more likely that the oxygen vacancies did not reach to the ordered phase during the reduction process and resulted in high densities of stacking faults and dislocations to accommodate the change in stoichiometry.
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The flash sintered TiO2 sample, with an electric field of 120 V/cm, current density limit of 1.5 A/cm2 and the holding time of 60 s, is used as a baseline for microstructure comparison of different flash sintering conditions. Table 1 summarizes all of the sintering conditions, along with the average grain sizes near the positive and negative electrodes and the defect densities at the positive electrode. 3.2. Effect of flash sintering conditions on the microstructure and defect structure 3.2.1. Stage I: electric field During Stage I, the sintering conditions which can be controlled are the electric field and furnace temperature. For this study, the effect of decreasing the applied field while maintaining the other flash conditions, was compared for TiO2 . Fig. 4(a) compares the power density for the samples with different initial electric fields of 120 V/cm and 60 V/cm. The sample with lower electric field experienced a lower maximum power density and prolonged incubation time of 200 s compared to that of samples under higher electric field. A similar trend has been previously observed in YSZ, where the incubation time increases from a few seconds up to 60 s after decreasing the electric field from 125 V/cm to 75 V/cm [7]. Fig. 4(b) and (c) shows the microstructures near the positive and negative electrodes under the lower electric field (60 V/cm), with average grain sizes were 0.7 μm and 0.4 μm, respectively. The reduced electric field did decrease the grain sizes but the asymmetrical structure remained. The lower field sample is also less dense, since the porosity of the sample is estimated to be around 3.28%, compared to the higher electric field (1.29%). There were also no obvious extended defects observed with a very low density of stacking faults observed, as shown in Fig. S4, near the positive electrode. Based on the observations in Fig. 4, the strength of the electric field appears to play a critical role in controlling the concentration of defects. The weaker electric field resulted in slower formation and an overall lower concentration of defects, thus required a prolonged time for an increase in conductivity for Stage II. This agrees with the observation of smaller grain sizes, higher porosity and minimal extended defects, since less oxygen vacancies are formed. This would suggest that for
Fig. 4. (a) Power density plots for the flash sintered TiO2 with electric fields of 60 V/cm and 120 V/cm. Microstructures were compared for the (b) positive and (c) negative electrode regions from the sample with lower electric field, 60 V/cm.
Fig. 5. (a) Power density plots for the flash sintered TiO2 with current density limits of 1.5 A/cm2 and 4.5 A/cm2 . Microstructures were compared for the (b) positive and (c) negative electrode regions from the sample with higher current density, 4.5 A/cm2 .
better densification and higher defects density, a higher electric field may be preferred. Additional comparisons will be needed at higher electric fields and will help to further verify this correlation. 3.2.2. Stage II: current density limit The non-linear rise in conductivity during Stage II leads to a rapid rise in the current up to the limit set by the power supply. With the same initial electric field and holding time, the power density plot for the current density limits of 1.5 A/cm2 and 4.5 A/cm2 were compared in Fig. 5(a). As evidenced in Fig. 5(b) for the 4.5 A/cm2 sample, the increase in the current density limit led to a significant grain growth, i.e., the average grain size for the sample near the positive electrode region increases from ∼0.9 μm (Fig. 2(b), the 1.5 A/cm2 sample) to ∼8.8 μm (Fig. 5(b), the 4.5 A/cm2 sample). The negative electrode region of the 4.5 A/cm2 sample (Fig. 5(c)) did not experience the same rapid grain growth as the positive region. It shows fine grains and higher porosity, similar to the negative electrode with the lower current density limit (Fig. 2(c)). Furthermore, the pore morphology for the 1.5 A/cm2 and 4.5 A/cm2 samples are distinctive due to the different grain growth rates. To achieve a clearer observation of the pore morphology, scanning transmission electron microscope (STEM) images for the positive electrode regions of the two samples were compared in Fig. 6. The 1.5 A/cm2 sample demonstrates primarily intergranular pores (i.e., pores between grains) with only a few intragranular pores (i.e., pores within grains), as shown in Fig. 6(a). On the contrary, the pores for the 4.5 A/cm2 sample are predominantly intragranular (Fig. 6(b)). Due to the rapid grain growth, the grain boundary velocity significantly exceeds the pore velocity, causing pore breakaways from the grain boundaries and become trapped inside the grains. These intragranular pores shown in Fig. 6(b) are well faceted and mostly square-shaped with various pore sizes. Other morphologies of these intragranular pores were also found, including rectangular and hexagonal, as shown in Fig. S5. The overall pore density is estimated to be around 4.37% for the 4.5 A/cm2 sample, which is much higher than the lower current density. These well-faceted intragranular pores are made of low surface energy crystal facet (i.e. {110} for rutile), which are typically obtained after a long, high temperature annealing process due to pore coalescence [31]. It is more common to observe intragranular pores which are spherical as they have the lowest free energy. Because of the high current density, the power dissipation and specimen temperature are considerably higher especially at the positive side, which led to significant grain growth and prompted the formation of these faceted, intragranular pores.
H. Wang, X.L. Phuah and H. Charalambous et al.
Fig. 6. STEM images comparing the pore morphology for the flash sintered TiO2 with current density limits of (a) 1.5 A/cm2 and (b) 4.5 A/cm2 .
In addition, the 4.5 A/cm2 current–density limit sample has reasonably high density of dislocation arrays and stacking faults near the positive electrode region, as shown in Fig. 7(a) and (b), respectively. The dislocation density was estimated to be 14.4 μm−1 and the stacking fault density was 54.9 μm−1 , which are approximately half of the defect density of the 1.5 A/cm2 sample (Fig. 3(a) and (b)). Significant grain growth typically causes the removal of defects as grain boundaries move past them, which leads to the reduction of dislocation and stacking fault densities. There is also significant variation in stacking fault density among the grains, which leads to the large deviation. Hence, it is critical to adjust the current density limit to control the uniformity of microstructure and defects density. 3.2.3. Stage III: holding time The flash state can be held indefinitely during Stage III as long as the sample remains conductive. In this stage, the holding time was increased from 60 s to 600 s (Fig. 8(a)). The micrographs near the positive and negative electrodes are shown in Fig. 8(b) and (c) for this 600 s case. First, obvious grain growth has been observed in the microstructure near both positive and negative electrodes for the sample with longer holding time. The average grain size was measured to be 1.9 μm and 1.5 μm for the positive and negative electrode, respectively. These are more than twice of the average grain sizes for the case of 60 s sample on both electrodes (Fig. 2(b) and (c)). The grain growth at both electrodes were comparable, unlike the case of high current density limit where a very significant grain growth was observed in the positive electrode region. Since less grain growth was observed for a long holding time compared with a high current density limit, one could expect a minor decrease in defects density. Fig. 9(a) and (b) shows the defect structure
Materialia 8 (2019) 100451
Fig. 7. (a) Dislocations and (b) stacking faults observed near the positive electrode of the flash sintered TiO2 with the increased current density limit of 4.5 A/cm2 .
of the flash sintered sample held for 600 s where the dislocation and stacking fault densities were estimated to be 13.2 μm−1 and 30.0 μm−1 , respectively. It is clear that the defect density decreases significantly with a longer holding time. This would suggest that the holding time during Stage III leads to the removal of defects through grain growth and annihilation of defects from annealing.
3.3. Suggestions for optimizing flash sintering conditions for TiO2 Flash sintering has demonstrated the ability to significantly reduce furnace temperatures and sintering time compared to conventional sintering. Moreover, with the rapid heating rates caused by Joule heating, grain growth could be minimized from the initial coarsening. The final microstructure and defects density can be further controlled by the electrical parameters, as demonstrated and discussed in the previous sections, for flash sintered TiO2 . This opens new possibilities for controlled and enhanced properties in TiO2 with various applications, such as photocatalysis [44] and sensing devices [45]. Smaller grain sizes are usually desired for enhanced properties, such as fracture strength and dielectric constant [46]. Based on the results from this study, the grain size can be influenced by setting the current density limit. Using high current densities will lead to significant grain growth and are also known to cause large thermal gradients in the sample and hot spots within the samples [12,47]. However, the current density is known to be a critical parameter to control the final density and setting a low current density limit may consequently reduce the ability to reach high density [48]. Thus, using an intermediate current density
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Materialia 8 (2019) 100451
sintered TiO2 at room temperature due to the formation of nanoscale stacking faults [49]. Based on the results from this study, the flash sintering parameters can be further tuned to achieve the desired defects density. High density of dislocations and stacking faults can be achieved by applying a stronger electric field (e.g., ∼120 V/cm for TiO2 ), as well as limiting the current density (e.g., ∼1.5 A/cm2 ) and holding time (e.g., ∼60 s) to prevent removal by grain growth and annihilation. With both fine grained microstructure and high density defects, ceramics with enhanced mechanical behavior can be achieved through flash sintering. 4. Conclusions
Fig. 8. (a) Power density plots for the flash sintered TiO2 with holding time of 60 s and 600 s. Microstructures were compared for the (b) positive and (c) negative electrode regions from the sample with longer holding time, 600 s.
The microstructural characteristics of flash sintered TiO2 have been effectively studied in this work. The positive electrode region presents larger grains and higher porosity compared to the negative electrode region. The enhanced kinetics near the positive electrode is correlated with the accumulation of oxygen vacancies, since high density of stacking faults and dislocations were observed. Different from the well-known ionic conductor YSZ, the positively charged oxygen vacancies accumulate at the positive electrode in TiO2 . Due to the preferential electronic conductivity of TiO2 at high electric fields, the oxygen vacancies are created due to the production of electrons. A systematic microstructure study comparing the grain size and defect density at the two electrodes for the three stages has been conducted and the major findings are listed as follows: 1. In Stage I, the weaker electric field is less effective in the formation of charge carriers and thus required a longer time before the sample was conductive for current to flow. Additionally, less oxygen vacancies were formed in the lower electric field, which resulted in a slower rate of densification and minimal formation of any extended defects. In order to achieve higher density of defects, a higher electric field should be utilized. 2. In Stage II, increasing the current density limit results in a significant grain growth near the positive electrode. Consequently, a large number of faceted intragranular pores are present and the defect density is reduced. This makes the current density limit a critical controlling condition for achieving more symmetric microstructure while maintaining fine grain structure. 3. In Stage III, a longer holding time leads to additional grain growth throughout the specimen. Increasing the holding time also leads to defect removal by grain growth and annihilation, and therefore the holding time should be minimized to prevent further grain growth and defect removal. Overall, flash sintering offers a large degree of flexibility in controlling the microstructure of sintered TiO2 , such as grain size, porosity, point defects, stacking faults, dislocations, etc. The findings suggest the flash sintering conditions could be tuned effectively to obtain desired microstructures and enhanced properties. Declaration of Competing Interest The authors declare that they have no competing interests. Acknowledgments
Fig. 9. (a)-(b) Dislocations and stacking faults observed near the positive electrode of the flash sintered TiO2 with increased holding time of 600 s.
(e.g., ∼1.5 A/cm2 for TiO2 ) will be necessary to achieve high densification while maintaining a uniform and fine microstructure. The introduction of extended defects through flash sintering can definitely make an impact on the mechanical behavior of ceramics. In another study, we have investigated the mechanical behavior of flash sintered TiO2 and have demonstrated the enhanced plasticity in flash
We would like to acknowledge the support from the U.S. Office of Naval Research (Contract number: N00014-17-1-2087 for sintering effort and N00014-16-1-2778 for TEM). The effort at Rutgers University was supported by the U.S. Office of Naval Research (Contract number: N00014-15-1-2492). Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.mtla.2019.100451.
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