Tungsten Bronze-Type Ceramics for Temperature-Stable Energy Storage Properties: A Feasibility Study

Round 1

Reviewer 1 Report

In this work, The four compositions of tungsten bronze phase ceramics, i.e., Ba6Ti2Nb8O30, Ba6Zr2Nb8O30, Sr3TiNb4O15, Sr3ZrNb4O15 ceramics are systematically studied with a focus on their microstructure, dielectric response and its correlation with their temperature dependence of energy storage properties. This manuscript is well prepared with reliable experimental data. I basically recommend publication of this work in Crystals. In order to make it more understandable and innovative, I want to suggest several revisions on general points. Please see below.

1) This manuscript includes only figures with experimental data. Therefore, outline and targets materials are not well understandable for non-specialist readers. I recommend the authors to add the initial figure to explain outline of the target materials (structures of the compounds).

2) I recommend the authors to include a new conceptual word to the title for increasing innovative impression. This work is based on nanostructure investigation. Therefore, I may recommend use of an emerging conceptual term, nanoarchitectonics, in the title. For example, the title like ... Nanoarchitectonics of Tungsten Bronze Type Compounds for Temperature-Stable Energy Storage Properties ... may sound more innovative.

3) Conclusive description is rather short. Most of them are short summary. It may not give strong impression. Impact of this research and meaning of this research had better be more intensively described.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The title of the paper must be changed. The energy storage ability of the materials appeared to be quite low as found by the authors. The present title appeal to high energy storage ability of the materials which is not the case. Moreover, the title touches tungsten bronze-type compounds, but no tungsten compounds were synthesised. This can also result in some misunderstandings.

Please use the correct notation for watts.

Milling for 24 h can result in a severe contamination of the ceramics with the milling materials. Was such a contamination checked? The exact conditions for the first and the second milling must be disclosed. The pressure, additives etc. for the pressing stage must be disclosed.

Was the density of the samples measured? How the density changes with the temperature? If one choose the temperature higher than 1300C, will the materials can exhibit higher energy storage property?

Some references to the figures are missing. Please check!

The experimental diffraction patterns must be processed with a refinement software and presented in ESI along with the difference curves. The estimated amount of admixture phases (e.g. Ba5Nb4O15) must be presented in the R&D section.

SEM of the pellet surface does not provide the information on the inner structure of the ceramics. Please polish the samples and conduct SEM from the polished surfaces. Also, the porosity of the materials must be estimated.

EDX or WDX measurements must be provided to show the chemical homogeneity and the composition of the materials.

Fig. 3 must be presented in a different way. In the present form, it is not clear which curve corresponds to the Y-axes.

In the Conclusions, the claim “Doping would be an ideal method to achieve a dense structure with smaller grains, enhanced relaxor nature, and promising energy storage properties” had nothing in common with the findings of the paper and must be omitted.

Overall, the paper needs further methodological polishing and, in the current form, can not be accepted. The major issues touch the title, chemical purity of the ceramics and its porosity. Nevertheless, the paper contains some valuable results which can be interesting for the audience of Ceramics. Thus, I recommend a major.

Some mistakes and misprints are present, but the English is acceptable.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

I thank the authors for they tried to address my comments. Unfortunately, I am not satisfied with some answers, some comments were ignored (I do not understand why).

Again, the title refers to TUNGSTEN bronzes. I understand what the authors mean, but such a title allows misunderstanding. In this regard, the claim “With different cations at A and B sites, those four ceramics exhibit different crystal structures” is confusing. Additionally, “structured” refers to a specific structure (microstructure) of the ceramics, which is not what the authors mean. Here, they mean exactly atomic structure.

Concerning the chemical purity of the ceramics. The authors did not introduce any discussion of this item in the manuscript. I think that it’s a very important question and must be even mentioned. To demonstrate the purity, the raw EDS spectra of the final ceramic materials must be provided, this will be very informative. The references given in the authors’ answer can also be used. But the spectra must be provided as they have already been collected.

Note that the density of the ceramics was quite different (70–93%), why? There is a confusion between the density values of the samples, “BTN, BZN, STN and SZN samples is calculated to be 93.5 %, 70 %, 89.9 %, and 82.9 %” and the porosity, “the porosity is estimated to be 2.44 %, 2.27 %, 1.61 %, and 141 6.35 % for BTN, BZN, STN, and SZN”. These values are not in line with each other though both sets of data were calculated from the Archimedes principle.

It is claimed that “It is suggested the intensively distributed pores and unclear grain boundaries in those samples (especially in BZN and SZN) are due to the release of CO2 gas during the sintering process, resulting in low sample density” is very strange as the sintering process followed the annealing at 950°C. How can CO2 eliminate from the thermally treated, re-milled and pressed reaction mixture? Here, there is an obvious mistake!

I wonder why the authors did not processed the XRDs. They claim that “the XRD data is not sufficient to provide in depth structural information of TTB sturcutre because the XRD diffraction patterns are mostly dominated by the heavy elements”. This is true. But the admixture phases are like Ba5Nb4O15 and do contain very heavy elements. I understand that the exact refinement of oxygen positions is complicated in the case (and not needed at all). I do not ask to refine oxygen, I just ask to show the correctness of the XRD interpretation and the estimations of the phase composition of the ceramics.

Generally, I am sure (and I insist on) that, for any functional material, the information on the chemical and phase composition of is obligatory.

Again, I halted between two options, a major or a reject. Again, I choose a major and I hope the authors will clear my concerns of the chemical purity, porosity and the composition of the ceramics. The title must be also taken into account.

Thorough English polishing can be required.

Author Response

Please see the attachment

Author Response File: Author Response.pdf