Thermoelectric Performance of Ca₂Si Synthesized by High-Temperature Melting

Round 1

Reviewer 1 Report

The so called calcium half silicide (Ca2Si) is considered as candidate thermoelectric material for a middle temperature range. However, experimentally obtained thermoelectric figure of merrit is steel far below its theoretically predicted value. That is why, each new research devoted to Ca2Si growth and characterization could potentially enriched ynderstanding of how to improve its thermoelectric performance. In this view, the munuscript under consideration could be interested to materials scientists. For my personal regrets, the manuscript in the present form could not be published and must be extensively improved in accordance with the remarks below.

-=Introduction Section=-

1. First of all, I'm strongly desagree with the statement "However, there are  not many researches on the thermoelectric power of the Si-based compounds, which are mainly concentrated in Mg2Si and its doped compounds". Currently many semiconducting and even semimetal and half Heusler ternary silicides are on the forefront of the thermoelectric materials. Please check, for example, https://doi.org/10.1063/5.0008198; https://doi.org/10.1002/pssa.201800105 and https://doi.org/10.7567/JJAP.56.05DA04. These articles could be very informative and helpful in re-writting introduction parts in term of state-of-the-art silicide thermoelectrics review. 

2. Indeed there are very few reported results on Ca2Si thermoelectric properties, however I did not found any mentions of existing and high quality research on Ca2Si, for example: https://iopscience.iop.org/article/10.1088/1757-899X/18/14/142014;

https://www.scientific.net/DDF.386.3;

https://www.degruyter.com/document/doi/10.3139/146.110492/html;

https://www.springerprofessional.de/en/effects-of-ag-doping-on-thermoelectric-properties-of-ca-2-x-agxs/11022096;

https://www.sciencedirect.com/science/article/pii/S1875389211000393

These articles are deserved to be mentioned in the Introduction Part. In addition, authors should to highlight the differences and benefits of the proposed Ca2Si sample preparation and clearly demonstrate what problem they try to solve in dealing with Ca2Si and its thermoelectric performance.

-=Materials and Methods=-

1. It is not clear why authors chose such a complicated and multistep method of Ca2Si sample growth. Please, provide some explanation and comparisson with other Ca2Si growth techniques.

2. I suggest that comparisson with Mg2Si properties is out of scope. In addition, I doubt that Ca2Si is isomeric with Mg2Si. The former has orthorhombic crystal lattice, while the latter does cubic. This means that Ca2Si unit cell could not be obtain by a simple replacing Mg ions with Ca. Finally, authors did not cite any any articles on Mg2Si properties, which are shown in Figures 4-6.

3. It is a gold standard to provide a picture of the grown bulk sample for a visual control of the resulted crystal quality and growth method used.

4. In dealing with thermoelectric properties, a conductivity type and carrier concentration of the Ca2Si sample must be measured and provided through the text and figures in addition to the existing resistivity measurements. Any suggestion and explanation of the obtained Ca2Si figure of merrit or future strategy of how it can be improved are rather questionable and useless withot these knowledges. That is why, my recomendation is to conduct full Hall resistivity measurements.

5. Please, provide some information about the needs in conducting conductivity and Seebeck measurements under vacuum conditions (Page 2, row 91)

 -=Results and Discussion=-

1. I found two different reason of applying such a complicated growth technique in terms of supressing calcium monosolicide phase formation. The first explanation (Page 2 row 67) is dealing with stochiometry, while second one considers the insufficient reaction temperature (Page 3, row 98). The trick is that the first one is fully incorrect. Ca2Si phase contains higher Ca both Weight and Atomic Percents that shoul results in Ca2Si formation under high vapor pressure of the Ca used. In addition, CaSi should be considered as inclusions of the other silicide phase in Ca2Si rather than impuruites (Page 2, row 66).

2. It is better to additionally provide the simulated XRD pattern for CaSi phase in order to check the real crystal quality and phase composition of the obtained Ca2Si samples.

3. Paragraph 3.2. should be deleted be cause it makes no sense for understanding of the Ca2Si thermoelectric properties. 

4. Measured and calculated Seebeck coefficint and ZT are somewhat below previously obtained ones (see the articles mentioned above). Pleasem illustrate and compare your results with other research groups and make some conclusions concerning the lower Seebeck coefficient instead of comparisson with Mg2Si.

5. The resistivity plot for Ca2Si sample is confusing. It seems to demonstrate semimetal behavior instead of semiconducting. It could be explained by the presence of CaSi phase (which is semimetal) . 

-=Incorrect prases and terminology=-

Page 1, row 7: Ca2Si was synthesized and thermoelectric properties were investigated. - That is the correct order.

Page 1, row 10: "showing strong gold properties" - I did not understand it at al.

Page 1, row 14: the keyword "carrier" is unappropriate.

Page 1, row 20: "convert heat energy and electric energy into each other" should be changed.

Page 1, row 21: "thermoelectric value" should be replaced with "thermoelectric figure of merrit".

Page 1, row 35: "high-temperature section" should be replaced with "high temperature range".

Page 1, row 42: "different sheets of thermoelectric materials" - do the authors mean "thermoelectric legs"?

Page 2, row 55: Text is meaningless and differs in size.

Page 5: row 140: "state density" should be replased with "density of states (DOS)"

Page 2, row 141: "the conductivity of Ca2Si is worse..." should be replaced with "is lower..."

Page 2, row 141: "is not ideal..." should be replaced with "is not so high..."

Page 7, row 189: "Since Si was abundant..." should be replaced with "Since Si is..."

I strictly reccomend to use some Proofreading service before resubmission.

Author Response

Responses to the Reviewer' Comments

Comments and Suggestions for Authors

The so called calcium half silicide (Ca₂Si) is considered as candidate thermoelectric material for a middle temperature range. However, experimentally obtained thermoelectric figure of merrit is still far below its theoretically predicted value. That is why, each new research devoted to Ca₂Si growth and characterization could potentially enriched understanding of how to improve its thermoelectric performance. In this view, the manuscript under consideration could be interested to materials scientists. For my personal regrets, the manuscript in the present form could not be published and must be extensively improved in accordance with the remarks below.

-=Introduction Section=-

1. First of all, I'm strongly desagree with the statement "However, there are not many researches on the thermoelectric power of the Si-based compounds, which are mainly concentrated in Mg₂Si and its doped compounds". Currently many semiconducting and even semimetal and half Heusler ternary silicides are on the forefront of the thermoelectric materials. Please check, for example, https://doi.org/10.1063/5.0008198; https://doi.org/10.1002/pssa.201800105 and https://doi.org/10.7567/JJAP.56.05DA04. These articles could be very informative and helpful in re-writting introduction parts in term of state-of-the-art silicide thermoelectrics review. 

Response: Thanks for providing useful references, and these papers are cited in this revised manuscript. We have made modifications to the description of this section in the manuscript, according to your valuable suggestion: “In recent decades, Si-based thermoelectric materials have gained popularity due to their benefits of minimal toxicity, affordable manufacture, and excellent stability[19-21]. First-principles atomistic calculations have demonstrated the potential for heavy doping to achieve a sufficiently high charge carrier concentration, as well as alloying with isoelectronic elements to attain suitably low phonon thermal conductivity[22]. Significant efforts have been devoted, particularly in the past decade, to developing efficient thermoelectric materials based on silicides. The main focus of these activities has been directed towards reducing the lattice thermal conductivity. This has been achieved through various techniques, including alloying, nanostructuring, and defect engineering[23]. Currently, the most effective silicide-based thermoelectrics are composed of Mg₂(Si-Sn) alloys, MnSi_1.75 and ReSi_1.75[24]. Among which, Mg₂Si and its doped compounds are well-known as one of the best materials for thermoelectric. In previous reports, ZT of P-type doped-Mg₂Si materials can reach above 0.7, and it shows certain potential of thermoelectric application[25-33].”(Please see line 41-55)

2. Indeed there are very few reported results on Ca₂Si thermoelectric properties, however I did not found any mentions of existing and high quality research on Ca₂Si, for example: https://iopscience.iop.org/article/10.1088/1757-899X/18/14/142014; https://www.scientific.net/DDF.386.3;https://www.degruyter.com/document/doi/10.3139/146.110492/html;https://www.springerprofessional.de/en/effects-of-ag-doping-on-thermoelectric-properties-of-ca-2-x-agxs/11022096;https://www.sciencedirect.com/science/article/pii/S1875389211000393These articles are deserved to be mentioned in the Introduction Part. In addition, authors should to highlight the differences and benefits of the proposed Ca₂Si sample preparation and clearly demonstrate what problem they try to solve in dealing with Ca₂Si and its thermoelectric performance.

Response: We gratefully appreciate for your valuable suggestion. As you mentioned, almost no reports on high-quality Ca₂Si thermoelectric materials are currently available, despite that there are currently some reports on Ca₂Si. Therefore, the preparation of Ca₂Si and analysis of its electrical and thermal transport properties that we have done utilizing high-temperature fast synthesis method are quite essential and valuable: “Although Ca₂Si is isomeric with Mg₂Si, thermoelectric properties are still limited by its low conductivity. Therefore, effective synthesis methods and effective conductivity enhancement are crucial for Ca₂Si-based thermoelectric material. Mg₂Si powder was heat treated in Ca gas phase to synthesize polycrystalline Ca₂Si powder. Then Ca₂Si sintered body was prepared by SPS. Subsequently, Ca₂Si sintered compacts were obtained using the Ca₂Si powders through the SPS method. Prolonged SPS treatment time at higher pressure was found to be favorable for achieving Ca₂Si sintered compacts with a higher relative density. At temperatures between 300K and 360K, the Ca₂Si film exhibited a Seebeck coefficient of 300-320 μV·K^-1 and a power factor of 1.37×10^-7 Wm^-1·K^-2[37]. Introducing Ag partial substitution for Ca in p-type Ca₂Si alloys can significantly enhance their electrical and thermal properties. The highest observed ZT value of 0.16 at 837 K corresponds to the Ca_(2-x)Ag_xSi composition[33]. The material exhibited a Seebeck coefficient ranging from 250 to 300 μV·K^-1 within the temperature range of 373 K to 573 K[31, 32, 38]. Based on the ab initio evolutionary algorithm structure searching and density functional theory calculations, scholars predicted the optimized ZT value achieved 0.52 for the p-type doping at 1000 K[32]. The band gap of cubic Ca₂Si decreases with increasing pressure. The Seebeck coefficient decreases with increasing pressure, while electronic conductivity increases with increasing pressure[39].”(Please see line 64-82)   

-=Materials and Methods=-

1. It is not clear why authors chose such a complicated and multistep method of Ca₂Si sample growth. Please, provide some explanation and comparisson with other Ca₂Si growth techniques.

Response: Thanks for your question that inspire us to further improve our manuscript. As suggested, we have added the explanation and comparisson with other Ca₂Si growth techniques : “This synthesis method using melting furnace can quickly reach the reaction temperature. This method has two advantages for obtaining high purity polycrystalline materials: on the one hand, the sample preparation time is greatly reduced and the work efficiency is improved. On the other hand, the raw material can be heated to very high temperatures (up to 1600 °C).”(Please see line 103-107)

2. I suggest that comparisson with Mg₂Si properties is out of scope. In addition, I doubt that Ca₂Si is isomeric with Mg₂Si. The former has orthorhombic crystal lattice, while the latter does cubic. This means that Ca₂Si unit cell could not be obtain by a simple replacing Mg ions with Ca. Finally, authors did not cite any articles on Mg₂Si properties, which are shown in Figures 4-6.

Response: Thanks for your valuable suggestions that help us to improve the quality of our manuscript. As suggested, we has added an explanation that compares the thermoelectric properties of Ca₂Si and Mg₂Si together: “As we known, the Mg₂Si alloy is promising candidate for thermoelectric energy conversion for the middle-high range of temperature. Calcium and magnesium are the same main group elements, and all have the characteristics of low cost, non-toxic and abundant reserves. The metal properties of the same main group elements are gradually enhanced from top to bottom, so calcium is more active than magnesium metals. Although Ca₂Si is isomeric with Mg₂Si, their thermoelectric properties differ significantly. In this paper, Mg₂Si serves as a reference object, helping us to better grasp their differences and giving us crucial knowledge to further enhance the thermoelectric properties of Ca₂Si in further work.”(Please see line 161-169)

In addition, we has cited l article on the thermoelectric properties of Mg₂Si. (see ref. 22-33)

3. 3. It is a gold standard to provide a picture of the grown bulk sample for a visual control of the resulted crystal quality and growth method used.

Response: We gratefully acknowledge your helpful comments. According to your suggestion, we have added pictures of bulk samples the XRD pattern.

4. In dealing with thermoelectric properties, a conductivity type and carrier concentration of the Ca₂Si sample must be measured and provided through the text and figures in addition to the existing resistivity measurements. Any suggestion and explanation of the obtained Ca₂Si figure of merrit or future strategy of how it can be improved are rather questionable and useless withot these knowledges. That is why, my recomendation is to conduct full Hall resistivity measurements.

Response: We gratefully appreciate for your valuable suggestion. Hall measurements have been performed to further research on the electrical properties.

The test method is as follows: “The Hall coefficient, carrier concentration and carrier mobility were measured by Van der Pauw method on the MMR K2500 Hall Effect Measurement System. In order to meet the requirements of the test, the sample was first cut into 4×4 mm square pieces, and then the thickness was polished to less than 0.5mm, and four symmetrical electrodes were made on the side.” (Please see line 136-140)

The hole concentration and the mobility at room temperature are represented in Table 1. The carrier concentration and Hall mobility  are 4.22×10¹⁹ cm^-1 and 15.95 cm²·V^-1·s^-1，respectively. (Please see line 189-190)     

Table 1 Hall coefficient, carrier concentration and mobility of the Ca₂Si at 323 K

5. Please, provide some information about the needs in conducting conductivity and Seebeck measurements under vacuum conditions (Page 2, row 91)

Response: Thanks for your question that inspire us to further improve our manuscript. As suggested, we have revised the error description in the manuscript. The Seebeck test was conducted in an Ar atmosphere, rather than under vacuum conditions: “Under atmosphere of Ar gas, the Seebeck coefficient and conductivity were measured synchronously through a Linseis LSR-3 instrument.”(Please see line 133-135)

 -=Results and Discussion=-

1. I found two different reason of applying such a complicated growth technique in terms of supressing calcium monosolicide phase formation. The first explanation (Page 2 row 67) is dealing with stochiometry, while second one considers the insufficient reaction temperature (Page 3, row 98). The trick is that the first one is fully incorrect. Ca₂Si phase contains higher Ca both Weight and Atomic Percents that shoul results in Ca₂Si formation under high vapor pressure of the Ca used. In addition, CaSi should be considered as inclusions of the other silicide phase in Ca₂Si rather than impurities (Page 2, row 66).

Response: Thank you very much for your constructive comment. As suggested, we have removed the incorrect description that synthetic methods can inhibit the formation of CaSi(Page 2 row 67 in manuscripts). In addition, we have revised the description of CaSi:” which should be considered as inclusions of the other silicide phase in Ca₂Si: “the products often contained a small amount of CaSi, which should be considered as inclusions of the other silicide phase in Ca₂Si.”(Please see line 110-111) 

2. It is better to additionally provide the simulated XRD pattern for CaSi phase in order to check the real crystal quality and phase composition of the obtained Ca₂Si samples.

Response: Thanks for your suggestions. We have added the simulated XRD pattern for CaSi phase in Figure 1.

3. Paragraph 3.2. should be deleted because it makes no sense for understanding of the Ca₂Si thermoelectric properties. 

Response: We gratefully appreciate for your valuable suggestion. Paragraph 3.2 has been deleted in the revision.

4. Measured and calculated Seebeck coefficint and ZT are somewhat below previously obtained ones (see the articles mentioned above). Please illustrate and compare your results with other research groups and make some conclusions concerning the lower Seebeck coefficient instead of comparisson with Mg₂Si.

Response: We gratefully appreciate for your valuable suggestion. We have made a brief comparison and explanation: “Unfortunately, the resistivity of the sample we synthesized was higher than that of ~1.2 mΩ·cm at room temperature reported by C. Wen et al. This may be due to different preparation methods and different densities of Ca₂Si bulks.” (Please see line 185-187)  

5. The resistivity plot for Ca₂Si sample is confusing. It seems to demonstrate semimetal behavior instead of semiconducting. It could be explained by the presence of CaSi phase (which is semimetal) . 

Response: We gratefully acknowledge your helpful comments. According to your suggestion, we have modified resistivity plot for Ca₂Si: “the resistivity of the Ca₂Si sample increases slowly with temperature, indicating semimetallic behavior.” (Please see line 180-182)

This trend is consistent with what has been reported in the literature(DOI: 10.1007/s11664-016-5088-y).

Comments on the Quality of English Language

-=Incorrect prases and terminology=-

Page 1, row 7: Ca₂Si was synthesized and thermoelectric properties were investigated. - That is the correct order.

Response: Thank you! We have revised the summary and this correction. (Please see line 8-18)  

Page 1, row 10: "showing strong gold properties" - I did not understand it at al.

Response: Thank you! We have rephrased the sentence: “demonstrating semimetallic characteristic”(Please see p.1 line 14) 

Page 1, row 14: the keyword "carrier" is unappropriate.

Response: Thank you! This correction was made! "carrier" is replaced by “Seebeck coefficient”. (Please see p.1 line 19)

Page 1, row 20: "convert heat energy and electric energy into each other" should be changed.

Response: Thank you! This correction was made: “Thermoelectric materials are a kind of materials which can convert thermal energy directly to electric energy and vice versa.”(Please see p.1 line 22-23)

Page 1, row 21: "thermoelectric value" should be replaced with "thermoelectric figure of merit".

Response: Thank you! This correction was made: “thermoelectric figure of merit (ZT)”(Please see p.1 line 24)

Page 1, row 35: "high-temperature section" should be replaced with "high temperature range".

Response: Thank you! This correction was made:” Zintl phase compounds and so on in high temperature range” (Please see p.2 line 38-39)

Page 1, row 42: "different sheets of thermoelectric materials" - do the authors mean "thermoelectric legs"?

Response: Thank you! The original intention is to describe the advantages of layered materials. In order to avoid ambiguity, the original expression has been deleted. The new description is: “The anisotropic electronic and phononic transport behaviors in layer-structured materials have become crucial driving forces, leading to a growing interest in this particular family of materials.”(Please see p.2 line 56-58)

Page 2, row 55: Text is meaningless and differs in size.

Response: Thank you! This correction was made.

Page 5: row 140: "state density" should be replased with "density of states (DOS)"

Response: Thank you! This correction was made.

Page 2, row 141: "the conductivity of Ca₂Si is worse..." should be replaced with "is lower..."

Response: Thank you! This correction was made.

Page 2, row 141: "is not ideal..." should be replaced with "is not so high..."

Response: Thank you! This correction was made.

Page 7, row 189: "Since Si was abundant..." should be replaced with "Since Si is..."

Response: Thank you! This correction was made. (Please see p.11 line 246)

I strictly reccomend to use some Proofreading service before resubmission.

Response: This correction have been made and thank you! We did our best to modify the grammar and expression of the paper.

Author Response File: Author Response.pdf

Reviewer 2 Report

This manuscript deals with the thermoelectric properties of a Ca-Si binary ceramic reporting a moderate thermoelectric figure of merit. This paper can be interesting because both Ca and Si are earth-abundant and could constitute an eco-friendly thermoelectric material. However, the manuscript suffers from poor shortcomings that need addressing before being considered for publication:

1) The use of the spark plasma sintering method should be reflected in the abstract.

2) What do the authors mean by “gold properties” in the abstract? Is the resistivity of gold as a metal? In this case, I recommend using the chemical notation. Also, this temperature unit is missing in the same sentence.

3) Occasionally, the manuscript has poor wordings that need thorough polish. For instance, in the methods, the authors confuse words samples with medicines.

4) It is not clear to me from the method section if the authors conducted a phase equilibria investigation that is presented in Fig. 1. However, this figure looks like the study below:

Sigmund, H., Journal of the Electrochemical Society, 1982. 129(12): p. 2809-2812.

If this is the case, the authors have not provided an adequate attribution or citation. I suggest removing this graph altogether in the resubmission.

5) It is not clear how the authors went from the XRD results to the crystal structure. I can see a simulated XRD pattern that might have helped. But I recommend a full Reviled refinement.

6) The manuscript is generally poorly cited. I recommend the following paper to be included in the introduction and discussions to aid the readers in better understanding the context:

Liu et al. Recent Advances of Layered Thermoelectric Materials. Adv. Sustainable Syst. 2018, 2, 1800046. https://doi.org/10.1002/adsu.201800046

Chuang et al. Processing dependence of structural and physical properties of Mg₂Ge thin films prepared by pulsed laser deposition. Thin Solid Films 2012, 520 ,226-6229. https://doi.org/10.1016/j.tsf.2012.05.071

The manuscript requires significant language polish.

Author Response

Responses to the Reviewer' Comments

Comments and Suggestions for Authors

This manuscript deals with the thermoelectric properties of a Ca-Si binary ceramic reporting a moderate thermoelectric figure of merit. This paper can be interesting because both Ca and Si are earth-abundant and could constitute an eco-friendly thermoelectric material. However, the manuscript suffers from poor shortcomings that need addressing before being considered for publication:

1) The use of the spark plasma sintering method should be reflected in the abstract.

Response: We gratefully appreciate for your valuable suggestion. The spark plasma sintering method has been mentioned in the abstract in the revised version: “The synthesis of Ca₂Si was successfully achieved through high-temperature melting furnace and spark plasma sintering process.”(Please see p.1 line 8-9)

2) What do the authors mean by “gold properties” in the abstract? Is the resistivity of gold as a metal? In this case, I recommend using the chemical notation. Also, this temperature unit is missing in the same sentence.

Response: Thank you very much for your valuable comment. According to your advice, we changed the description and added temperature unit: “the resistivity of Mg₂Si decreases gradually with the increase of temperature, from 7.5 mΩ·cm at 323 K to 4.5 mΩ·cm at 873 K. The resistivity of the Ca₂Si sample increases slowly with temperature rising, indicating semimetallic behavior.” (Please see p.7 line 179-180)

3) Occasionally, the manuscript has poor wordings that need thorough polish. For instance, in the methods, the authors confuse words samples with medicines.

Response: We did our best to modify the grammar and expression of the paper.

4) It is not clear to me from the method section if the authors conducted a phase equilibria investigation that is presented in Fig. 1. However, this figure looks like the study below: Sigmund, H., Journal of the Electrochemical Society, 1982. 129(12): p. 2809-2812.If this is the case, the authors have not provided an adequate attribution or citation. I suggest removing this graph altogether in the resubmission.

Response: We gratefully acknowledge your helpful comments. As suggested, the reference has been cited in the revised manuscript. (Please see Ref. 40). And the phase diagram has been removed in the resubmission.

5) It is not clear how the authors went from the XRD results to the crystal structure. I can see a that might have helped. But I recommend a full Reviled refinement.

Response: Thank you very much for your valuable suggestions that help us to improve the quality of our manuscript. As suggested, we have supplemented the simulated XRD patterns (Figure 1).”The simulated pattern of Ca₂Si comes from the standard card (ICSD 191034), which crystallized in the space group of Pnma. Cell parameters a, b and c are 7.609 Å, 4.769 Å and 8.980 Å, respectively.”(Please see p.6 line151-153 )

6) The manuscript is generally poorly cited. I recommend the following paper to be included in the introduction and discussions to aid the readers in better understanding the context:Liu et al. Recent Advances of Layered Thermoelectric Materials. Adv. Sustainable Syst. 2018, 2, 1800046. https://doi.org/10.1002/adsu.201800046. Chuang et al. Processing dependence of structural and physical properties of Mg₂Ge thin films prepared by pulsed laser deposition. Thin Solid Films 2012, 520 ,226-6229. https://doi.org/10.1016/j.tsf.2012.05.071

Response: Thanks for your question that inspire us to further improve our manuscript. As suggested, we have added the literature and description of thin film materials: “The anisotropic electronic and phononic transport behaviors in layer-structured materials have become crucial driving forces, leading to a growing interest in this particular family of materials[34]. The highest electrical conductivity, charge carrier mobility and concentration of Mg₂Ge thin films were 141.86 Ω⁻¹·m⁻¹, 2.62 cm²·V^-1·s^-1 and 8.66 × 10¹⁸ cm⁻³ at room temperature[35].”(Please see p.3 line 56-60)

Comments on the Quality of English Language

The manuscript requires significant language polish.

Response: Thanks for your valuable suggestions. We have did our best to modify the grammar and expression of the paper.

Author Response File: Author Response.pdf

Reviewer 3 Report

The scientific content of this work is also very poor.

Units are missing in the abstract, as well as the definition of abbreviations. In the introduction, line 37, more appropriate references should be given from earlier Ca2Si works (https://www.webofscience.com/wos/woscc/summary/e53d725c-7937-459a-ba24-93a15027a666-98c53104/relevance/1).

In lines 42-43, please explain why stacking layers improves the thermoelectric properties. 

Add references to line 46.

In line 48 the units of the literature value are wrong.

Is fig1 from the literature? case yes, add an appropriate reference. In Fig 2, explain how the simulated pattern was obtained and present fitted and theoretical lattice parameters.

I am not sure if the authors switched the labels of Ca2Si and Mg2Si in Fig4a or if they switched the materials in the discussion in lines 149-158. This trend is for Ca2Si, as seen in Fig 4a and b, unless the labels in the figure are wrong for both materials. The Figure caption in Fig 4 is incomplete.

Regarding Fig4, what are the exact experimental differences in the determination of the Seebeck coefficient and electrical conductivity for Mg2Si in the literature and Ca2Si in this work?

Similarly, in Fig6, what are the exact experimental differences in the determination of K for Mg2Si in the literature and Ca2Si in this work?

The paper is presented in very poor English, which is very hard to read. It is written in a way that sometimes it is difficult to understand what is from the presented work and what is from the literature. The attached PDF has some corrections and highlighted text that needs attention. The abstract must be rewritten using proper grammar. 

Author Response

Responses to the Reviewer' Comments

Comments and Suggestions for Authors

The scientific content of this work is also very poor.

Units are missing in the abstract, as well as the definition of abbreviations. In the introduction, line 37, more appropriate references should be given from earlier Ca₂Si works(https://www.webofscience.com/wos/woscc/summary/e53d725c-7937-459a-ba24-93a15027a666-98c53104/relevance/1).

Response: Thanks for providing useful references, and these papers are cited in this revised manuscript: “Although Ca₂Si is isomeric with Mg₂Si, thermoelectric properties are still limited by its low conductivity. Therefore, effective synthesis methods and effective conductivity enhancement are crucial for Ca₂Si-based thermoelectric material. Mg₂Si powder was heat treated in Ca gas phase to synthesize polycrystalline Ca₂Si powder. Then Ca₂Si sintered body was prepared by SPS. Subsequently, Ca₂Si sintered compacts were obtained using the Ca₂Si powders through the SPS method. Prolonged SPS treatment time at higher pressure was found to be favorable for achieving Ca₂Si sintered compacts with a higher relative density. At temperatures between 300K and 360K, the Ca₂Si film exhibited a Seebeck coefficient of 300-320 μV·K^-1 and a power factor of 1.37×10^-7 Wm^-1·K^-2[37]. Introducing Ag partial substitution for Ca in p-type Ca₂Si alloys can significantly enhance their electrical and thermal properties. The highest observed ZT value of 0.16 at 837 K corresponds to the Ca_(2-x)Ag_xSi composition[33]. The material exhibited a Seebeck coefficient ranging from 250 to 300 μV·K^-1 within the temperature range of 373 K to 573 K[31, 32, 38]. Based on the ab initio evolutionary algorithm structure searching and density functional theory calculations, scholars predicted the optimized ZT value achieved 0.52 for the p-type doping at 1000 K[32]. The band gap of cubic Ca₂Si decreases with pressure increasing. The Seebeck coefficient decreases with increasing pressure, while electronic conductivity increases with increasing pressure[39].”(Please see p.3 line 64-82)

In lines 42-43, please explain why stacking layers improves the thermoelectric properties. 

Response: Thanks for your valuable suggestions that help us to improve the quality of our manuscript. As suggested, we have revised some of the descriptions and explained why layer structure improves the thermoelectric properties: “The anisotropic electronic and phononic transport behaviors in layer-structured materials have become crucial driving forces, leading to a growing interest in this particular family of materials[34].”(Please see p.2 line 56-58)

Add references to line 46.

Response: Thank you very much for your valuable comment. According to your advice, we added the refernces: “Mg₂Si powder was heat treated in Ca gas phase to synthesize polycrystalline Ca₂Si powder.[31]“(Please see p.3 line 66-68)

In line 48 the units of the literature value are wrong.

Response: Thank you very much for your valuable suggestions that help us to improve the quality of our manuscript. As suggested, we have revised the unit from “μV/K” to “μV·K^-1”: “The material exhibited a Seebeck coefficient ranging from 250 to 300 μV·K^-1 within the temperature range of 373 K to 573 K”(Please see p.3 line 76-77)

Is fig1 from the literature? case yes, add an appropriate reference. In Fig 2, explain how the simulated pattern was obtained and present fitted and theoretical lattice parameters.

Response：Thank you very much for your constructive comment. As suggested, We have removed the phase diagram (Figure 1) in the revised version. In addition, we have explained how to obtain simulated patterns: “The simulated pattern of Ca₂Si comes from the standard card (ICSD 191034), which crystallized in the space group of Pnma. Cell parameters a, b and c are 7.609 Å, 4.769 Å and 8.980 Å, respectively.”(Please see p.6 line 151-153)

I am not sure if the authors switched the labels of Ca₂Si and Mg₂Si in Fig4a or if they switched the materials in the discussion in lines 149-158. This trend is for Ca₂Si, as seen in Fig 4a and b, unless the labels in the figure are wrong for both materials. The Figure caption in Fig 4 is incomplete.

Regarding Fig4, what are the exact experimental differences in the determination of the Seebeck coefficient and electrical conductivity for Mg₂Si in the literature and Ca₂Si in this work?

Response：Thank you very much for your valuable comment.

Based on your suggestion, we have improved the figure caption: “Figure 2. (a) Resistivity and (b) Seebeck coefficient of Ca₂Si and Mg₂Si.”

Due to the deletion of two figure, the figure numbers has changed.

Perhaps our description was not clear enough, which caused your confusion. As the temperature increases, the resistivity of Mg₂Si gradually decreases, while the resistance of Ca₂Si gradually increases. Within the same temperature range, the Seebeck coefficient of Mg₂Si increases firstly and then decreases, while the Seebeck coefficient of Ca₂Si gradually increases.  As shown in p.7 line 179- p8. line 200:

As shown in Figure 2(a), the resistivity of Mg₂Si decreases gradually with the increase of temperature, from 7.5 mΩ·cm at 323 K to 4.5 mΩ·cm at 873 K. The resistivity of the Ca₂Si sample increases slowly with temperature rising, indicating semimetallic behavior. At 323 K, the resistivity of Ca₂Si is about 6.5 mΩ·cm, which is slightly lower than that of Mg₂Si. However, the resistivity of Ca₂Si increases to about 12 mΩ·cm at 873 K, which is basically 2.6 times that of Mg₂Si at the same temperature. Unfortunately, the resistivity of the sample we synthesized was higher than that of ~ 1.2 mΩ·cm at room temperature reported by C. Wen et al[33]. This may be due to different preparation methods and different densities of Ca₂Si bulks. However, high resistivity is unfavorable to the thermoelectric properties of high temperature section. As shown in Table 1, the carrier concentration and Hall mobility are 4.22 × 10¹⁹ cm^-1 and 15.95 cm²·V^-1·s^-1，respectively.

The Seebeck coefficient of Mg₂Si is 220 μV·K^-1 at 323 K, and then increases slowly with the increase of temperature, reaching the highest 310 μV·K^-1 at about 623 K. Although the Seebeck coefficient of Mg₂Si slowly declines with increasing temperature after 623 K, it is still around 250 μV·K^-1 at 873 K, which is comparable to many excellent thermoelectric materials.

Similarly, in Fig6, what are the exact experimental differences in the determination of K for Mg₂Si in the literature and Ca₂Si in this work?

Response: We gratefully acknowledge your helpful comments. As suggested, we we have modified the relevant description to make the article easy to understand. As the temperature increases, the thermal conductivity of both Ca₂Si and Mg₂Si undergoes small changes. However, the total thermal conductivity of Mg₂Si is 2.6 times that of Ca₂Si. As shown in p.10 line 229-236:

The total thermal conductivity was shown in Figure 4(a). The total thermal conductivity of Ca₂Si material is only about 1.4 W·m^-1·K^-1 at 323 K. The thermal conductivity of Ca₂Si reaches its lowest point (1.0 W·m^-1·K^-1) at 673 K. As the temperature increases, its thermal conductivity also increases, but the trend is not obvious. Even at 873 K, the thermal conductivity of Ca₂Si is only 1.2 W·m^-1·K^-1, which is comparable to some thermoelectric materials with low thermal conductivity. In contrast, the thermal conductivity of Mg₂Si (5.4 and 4.5 W·m^-1·K^-1 at 323 and 673 K) is 2.6 times that of Ca₂Si.

Comments on the Quality of English Language

The paper is presented in very poor English, which is very hard to read. It is written in a way that sometimes it is difficult to understand what is from the presented work and what is from the literature. The attached PDF has some corrections and highlighted text that needs attention. The abstract must be rewritten using proper grammar. 

Response: Thanks for your valuable suggestions. We have done our best to modify the grammar and expression of the paper.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Manuscript scientific content was greatly improved. Authors did a great job in responding my advices, comments and suggestions. The only crucial thing that still should be improved is English. 

line 14: inadequate electricaL property => not typical of semiconductors behavior electrical properties

line 22: "high temperature range" should be deleted

line 24: low resistivity and thermal conductivity => low both resistivity and thermal conductivity

line 27: which is not conductive => which can hardly result in

line 30: property => properties

line 34: at low temperature => for a low temperature range / near the room temperature

line 35: and so on => etc.

line 38: popularity => attention

line 73: scholars => it was predicted

line 101: through => XRD measurement revealed

lines 112-113: lattice => crystal lattice

line 113: peak => peaks

line 129: it is better to say that "sample was thinned down to 0.5 mm"

line 131: on the side => on the surface / on its surface

line 134: At the beginning => As a first step

line 149: As we know => It is known that

line 151: and all => and both

line 175: section => region

line 186: a big => a contrast 

line 188: growth rate => slope

line 201: it is better to say "While it is not a case for Ca2Si..."

line 214: lowest point => minimal value

And many other mistakes and technical misunderstandings.

Author Response

Response: Thank you very much for your constructive comment. As suggested, we have carefully checked and corrected these errors in the revised manuscript. Simultaneously, we have revised the whole manuscript very carefully and tried our best to avoid both the grammar and spelling errors, and the changes were highlighted in red font in the Revised Manuscript. In addition, the description and analysis in “Materials and Methods” and “Results and Discussion” have been carefully revised to understand better the relevant experiments.

Reviewer 2 Report

Given the revisions made, this paper can be accepted in the current form.

Moderate typesetting and polishing is required.

Author Response

Response: Thank you very much for your valuable suggestion. According to your constructive advice, we have carefully checked and corrected grammatical errors in the revised manuscript. Simultaneously, we have revised and polished the whole manuscript very carefully and tried our best to avoid both the grammar and spelling errors, and the changes were highlighted in red font in the Revised Manuscript.

Reviewer 3 Report

The manuscript is now acceptable for publication.

The manuscript is now acceptable for publication; however, I recommend someone fluent in English to read the paper before.

Author Response

Response: Thank you very much for your valuable suggestion. According to your constructive advice, we have carefully checked and corrected grammatical errors in the revised manuscript. Simultaneously, we have revised and polished the whole manuscript very carefully and tried our best to avoid both the grammar and spelling errors, and the changes were highlighted in red font in the Revised Manuscript.