The Effect of the Addition of Aluminum Nitride to the Composition of NiAl₂O₄ Ceramics on Hydrogenation Processes and the Increase in Resistance to Swelling and Degradation

Review Report Form 1

1)    The abstract should be more concise. The author should change it.

The author thanks the reviewer for this comment, correction is made to the text of the article, the abstract is reduced.

2)    Line 132: The irradiation temperature was 1000 K- Here, the author used Kelvin for temperature. Please, change it. Units for temperature should be uniform.

The author thanks the reviewer for this comment, correction is made to the text of the article, units of measurement are given in the same format.

3)    Line 479: “…we can conclude”..  We? The author should change this sentence to be more appropriate for one scientific paper. 

The author thanks the reviewer for this comment, correction is made to this text, the sentence is paraphrased.

Analysis of the data obtained revealed that the formation of two-phase ceramics with the main phase of NiAl₂O₄ and impurity inclusions in the form of Al₇O₃N₈ grains located along the grain boundaries has increased resistance to hydrogenation while maintaining stability of strength and thermophysical characteristics at high irradiation doses (20 – 50 dpa), simulating hydrogenation processes.

Review Report Form 2

Please explain why you used the NiO and Al2O3 ratio of 1:3 M. In the NiAl2O4 the ratio of NiO and Al2O3 is 1:1. Please explain the role of the excess alumina in the structure of the spinel.

The author thanks the reviewer for this comment. In response, the following information is provided. 

During mechanochemical synthesis and subsequent thermal sintering, according to previous studies, it was established [32] that the use of a molar ratio of components NiO:Al₂O₃ →1:1 results in the formation of a low content of impurity inclusions in the form of nickel oxide, which makes it impossible to obtain a highly ordered spinel structure.

Line 192. “…the altered shape of diffraction reflections suggests the existence of factors within the structure that introduce distortions. These distortions manifest as reduced structural order and deviations in crystal lattice parameters from their reference values.” Please provide the reference values that you mentioned.

The author thanks the reviewer for this comment. In response, the following information is provided. 

Data from card values PDF# 00-010-0339, a = 8.048 Å, were taken as reference parameters from the PDF-2 NiAl₂O₄ database.

Please explain in detail how you calculate the structural ordering factor.

The author thanks the reviewer for this comment. In response, the following information is provided. 

To calculate the structure factor of ordering, the data on changes in the crystal lattice parameters of the samples obtained by varying the ratio of the components, as well as data on the reference values of the crystal lattice parameters taken from the PDF-00-010-0339 database were used. In the case of a spinel crystal lattice type, the parameters are refined using parameter a (for a cubic crystal lattice type).

Please explain the reason why the spinel lattice parameter decreases with increasing the concentration of AlN.

The author thanks the reviewer for this comment. In response, the following information is provided. 

The reduction in the crystal lattice parameters of the NiAl₂O₄ phase with growing concentration of the AlN dopant is due to the effects of structural ordering associated with the ordering of the crystal lattice and its compaction.

Please explain why reduction of the crystal lattice parameters and volume “signifies an enhancement in the structural order of the ceramics and a decrease in deformation-related irregularities within the structure”.

The author thanks the reviewer for this comment. In response, the following information is provided. 

A decline in the crystal lattice parameters results in a decrease in its volume, which in turn directly affects the number of porous inclusions (density changes). At the same time, a rise in the structural ordering degree (a decrease in the parameters of the crystal lattice, as well as its volume) is due to a decrease in deformation distortions caused by the processes of mechanochemical grinding of samples.

Line 244. I did not notice “diamond-shaped grains” in Fig. 3. Please make correction. Please add the crystal size distribution data to your micrographs.

The author thanks the reviewer for this comment. In response, the following information is provided. 

In this range, an increase in AlN content leads to the development of a densely packed structure consisting of grains of irregular cubic or pentagonal shape.

Also in Figure 3, grain size distribution diagrams have been added.

Please explain in detail how you calculated values presented in Fig. 4.

The author thanks the reviewer for this comment. In response, the following information is provided. 

The swelling value was determined using the following calculation method. The swelling value was determined by calculating changes in the crystal lattice volumes of the studied samples in the initial state (depending on the concentration of the AlN dopant) and their changes as a result of hydrogenation processes under high-dose irradiation.

The structural disorder value was calculated based on changes in the structural order values (calculated for the initial values of the crystal lattice parameters) as a result of external influences (in this case, hydrogenation processes under high-dose irradiation).

Review Report Form 3

In the introduction, the author deals with the ceramic fuel cells and their degradation due to hydrogenation. But, he does not detail which ceramics are commonly used as a ceramic fuel cell. Can the author add few details about this point ? Why do the author study NiAl2O4 in this study ? 

The author thanks the reviewer for this comment. In response, the following information was provided. 

The choice of NiAl₂O₄ ceramics as objects of research is due to their great potential for use as materials for ceramic fuel cells capable of operating in aggressive environments, as well as high temperatures, as reported in several scientific papers [26-28]. Simultaneously, a range of studies [29-31] has emphasized the necessity of modifying these ceramics to enhance their resistance against external factors related to operational processes, such as hydrogenation or mechanical stresses at elevated temperatures.

Section 2.1 : No information are given about the raw powders used in the study (reference of the suppliers, purity).

The author thanks the reviewer for this comment. In response, the following information was provided. 

NiO, Al₂O₃, AlN powders purchased from Sigma Aldrich (St. Louis, Missouri, USA) were used as initial components for the production of ceramics. The sizes of the powders in the initial state were about 10 microns, the chemical purity was 99.95 %.

Section 2.1 : The author claims that a sintering was carried out on the powders. The type of thermal treatment carried out is not clear. Is there a thermal treatment on the powders to obtain the spinel phase and a sintering treatment to densify the tablets ? Can the author clarify this point ?

The author thanks the reviewer for this comment. In response, the following information was provided. 

Annealing of the samples was carried out on samples pressed into tablets with a thickness of about 15 microns and a diameter of 10 mm. The samples were pressed into a special mold under a pressure of 300 MPa. The samples were placed in corundum crucibles, which were placed in the muffle furnace chamber, after which the samples were heated to a temperature of 1500°C at a heating rate of 20°C/min until the set annealing temperature was reached, after which the temperature in the chamber was stabilized and maintained at for 10 hours. After 10 hours, the samples were cooled together with the chamber for 24 hours until they reached room temperature.

Page 7 : The author claims that densification was improved by the addition of AlN. But, no values of final densities for the different samples are given.

The author thanks the reviewer for this comment. In response, the following information was provided. 

Sample density data is presented in Table 1.