Electrical Behavior of Electric Field-Assisted Pressureless Sintered Ceria-20 mol% Samaria
Round 1
Reviewer 1 Report
A few comments were made within the text. Please refer to them in the revised paper.
Comments for author File: 
Comments.pdf
Author Response
We would like to acknowledge the comments that helped to improve the paper. All raised questions are answered below, and the corresponding changes (in red) may be found in the revised manuscript.
1) Too many (irrelevant) references for 3YSZ and 8YSZ, materials which are not involved in the present manuscript.
I suggest to leave only one reference from each research group among the 7-47, except ref. 39 and 47 which were cited for other purposes too.
We reduced the number of references of 40 to just over 15, leaving only few works of some groups that developed flash sintering with 3 mol% and 8 mol% stabilized zirconia.
2) ... while only oxide ions take part... Please see hole conductivity in the recent paper by Induced p‐type semi‐conductivity in yttria‐stabilized zirconia Xavier Vendrell, Anthony R. West J Am Ceram Soc 2019
The introduction section was modified and reference [34] was added:
"That means that at usual flash occurring temperatures, oxide ions and electrons are the charge carriers in ceria compounds, while mainly oxide ions take part in the electrical conductivity of zirconia compounds [34]."
[34] Vendrell, X.; West A.R.; Induced p‐type semi‐conductivity in yttria‐stabilised zirconia, J. Am. Ceram. Soc. 2019. doi.org/10.1111/jace.16492.
3) Comment to:
This is due to the percolation ...pellet which is higher...
The following correction was done in the discussion section:
"This is due to the percolation of the electric current through the pellet, which is expected to be higher in samples with higher pore content, pores being considered the preferential pathway for the electric current. The Joule heating caused by the electric current increases the internal temperature of the sample enhancing the densification rate, achieving near full density."
4) Then, which type of charge carriers were identified from these activation energies?
The discussion section was modified to:
"…depending on the powder synthesis and sintering profile, meaning that O2- ions are the charge carriers [38]."
5) correct to:
same shrinkage level, but higher than the level reached by conventional sintering at 1500 °C
The following sentence was modified in the Conclusion section:
"(CeO2)0.8(Sm2O3)0.2 ceramics sintered by applying an electric field without pressure (flash sintering) at 800°C, 1000°C and 1200°C reached the same shrinkage level, which is higher than the level reached by conventional sintering at 1500oC."
Author Response File: Author Response.pdf
Reviewer 2 Report
1. The manuscript contains too many references. The number of references between references 7 – 47 can be greatly reduced, as many of them are not mentioned again in the manuscript.
2. The authors need to make clear what is novel about this manuscript. I presume it is the use of an AC electric field in the flash-sintering of samaria-doped ceria.
3. In the Materials and Methods section, the authors mention that the Archimedes density of sintered samples was measured, but this data is missing from the manuscript. Please include the density data in the revised manuscript.
4. Likewise, the XRD patterns of sintered samples are missing. Please include them in the revised manuscript.
5. In the revised manuscript, please include details of the equivalent circuit that was used to deconvolute the impedance spectroscopy results.
6. In the Discussion section, the authors state that as the flash sintering temperature increases, the amount of shrinkage induced by the applied electric field decreases. The authors state that this is due to the reduction in porosity limiting the percolation of current through the sample. A simpler explanation would be that degree of sintering the sample experiences before application of the electric field increases as the flash sintering temperature increases. (The dilatometry curves of the flash sintered samples follow that of the conventionally sintered sample up until the flash sintering temperature is reached.) Therefore the samples need to undergo less shrinkage before full densification is attained.
7. Can the authors explain why the flash-sintered samples appear more porous than the conventionally-sintered sample?
8. The authors state that, “The relatively lower bulk resistivity of the flash sintered samples may be due to the diffusion of the samarium ions located at the space charge region back to the bulk due to the intense Joule heating caused by the electric current pulse.” This should be confirmed by comparing the distribution of samarium at the grain boundaries and bulk grains for the flash sintered and conventionally sintered samples e.g. by using element mapping or EDS line scans of the grains in the SEM or TEM.
9. Please include mean grain size data measured from the SEM micrographs.
Author Response
Answer to Comments
We would like to acknowledge the comments that helped to improve the paper. All raised questions are answered below, and the corresponding changes (in red) may be found in the revised manuscript.
1. The manuscript contains too many references. The number of references between references 7 – 47 can be greatly reduced, as many of them are not mentioned again in the manuscript.
The number of references was reduced from 40 to 15, citing few papers from groups that developed flash sintering with 3 mol% and 8 mol% stabilized zirconia.
2. The authors need to make clear what is novel about this manuscript. I presume it is the use of an AC electric field in the flash-sintering of samaria-doped ceria.
The Introduction section was modified:
"This work for the first time reports results on AC electric field-assisted (flash) pressureless sintering CeO2: 20 mol% Sm2O3 green pellets at different temperatures with fixed electric field and current limit with fairly high density."
3. In the Materials and Methods section, the authors mention that the Archimedes density of sintered samples was measured, but this data is missing from the manuscript. Please include the density data in the revised manuscript.
A new sentence was inserted in the Results section:
"The apparent densities (%T.D.) of the specimens flash sintered at 800oC, 1000oC and 1200°C were 82%, 88% and 92%, respectively; the specimen conventionally sintered at 1500°C achieved 96% T.D."
4. Likewise, the XRD patterns of sintered samples are missing. Please include them in the revised manuscript.
The X-ray diffraction patterns were included in the manuscript:
"Figure 2 shows X-ray diffraction results of the SDC20 specimens sintered at 1500°C and with application of 200 V cm-1 during 5 min at 1200°C. Both patterns are identical and correspond to the ICDD 28-792 file."
Figure 2. XRD patterns of (CeO2)0.8(Sm2O3)0.2 sintered at 1500oC/2 h and flash sintered with application of 200 V cm-1 at 1200°C during 5 min.
5. In the revised manuscript, please include details of the equivalent circuit that was used to deconvolute the impedance spectroscopy results.
The equivalent circuit was included in Figure 4 and the following sentence in the Results section:
"Figure 4 shows the [- Z''(w) x Z'(w)] impedance spectroscopy diagrams and the equivalent circuit of ..."
Figure 4. Impedance spectroscopy diagrams of (CeO2)0.8(Sm2O3)0.2 flash sintered at 800oC, 1000oC and 1200oC under 200 V cm-1, 1 kHz, 1.5 A limiting current during 5 min, and conventionally sintered at 1500oC/2 h. Temperature of measurement: 270°C. Inset: equivalent circuits.
6. In the Discussion section, the authors state that as the flash sintering temperature increases, the amount of shrinkage induced by the applied electric field decreases. The authors state that this is due to the reduction in porosity limiting the percolation of current through the sample. A simpler explanation would be that degree of sintering the sample experiences before application of the electric field increases as the flash sintering temperature increases. (The dilatometry curves of the flash sintered samples follow that of the conventionally sintered sample up until the flash sintering temperature is reached.) Therefore the samples need to undergo less shrinkage before full densification is attained.
The following sentence was included in the Discussion section:
This is due to the percolation of the electric current through the pellet, which is expected to be higher in samples with higher pore content, pores being considered the preferential pathway for the electric current. The Joule heating caused by the electric current increases the internal temperature of the sample enhancing the densification rate, achieving near full density.
7. Can the authors explain why the flash-sintered samples appear more porous than the conventionally-sintered sample?
For conventional sintered samples, sintering occurs during furnace heat reaching the external surfaces, allowing the gaseous species from the laboratory to expand and leave the specimen, closing the pores. In flash sintering experiments, heating happens in short times during the electric current pulse, trapping gaseous species, producing porous structures.
8. The authors state that, “The relatively lower bulk resistivity of the flash sintered samples may be due to the diffusion of the samarium ions located at the space charge region back to the bulk due to the intense Joule heating caused by the electric current pulse.” This should be confirmed by comparing the distribution of samarium at the grain boundaries and bulk grains for the flash sintered and conventionally sintered samples e.g. by using element mapping or EDS line scans of the grains in the SEM or TEM.
Similar result was observed by the same authors in yttria stabilized zirconia [25]. It is a proposition to be further experimentally confirmed (or not) by FIB microscopy, under consideration but not available in our laboratories.
9. Please include mean grain size data measured from the SEM micrographs.
The following statement was introduced in the Results section.
"Grain size ranges 0.28 µm, 0.34 µm and 0.69 µm in samples flash sintered at 800oC, 1000oC and 1200°C, respectively. The sample conventionally sintered at 1500 °C, on the other hand, has larger average grain size (approximately 1.5 mm), as expected, and apparently devoid of pores." Pores are visible in the flash sintered sample surfaces.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The manuscript is much improved and is now suitable for publication.
