Effect of Y2O3-Al2O3 Additives on the Microstructure and Electrical Properties Evolution of Si3N4 Ceramics
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsAlthough this manuscript reports on the electrical properties of Si3N4 ceramics, which could be interesting to some researchers, the employed methods and the material is not novel. Moreover, the temperature window is significantly lower then what Si3N4 is typically used. Therefore, the authors are encouraged to improve the introduction to show the aim and benefits of this study.
Also, there are a few points which are not clear:
lines 118-112: there is no evidence in this study for the role of Yttrium ions in orientation and growth of grains ( Authors can check and use the following paper: https://doi.org/10.1111/j.1551-2916.2008.02448.x or similar ones)
Although mentioned ( a little) in lines 173-177, the effect of grain boundary and its amorphus nature is not discussed. Authors could use the literature on oxynitride glasses.
Comments on the Quality of English Language
The language is fine. However, there are a few minor errors (mostly typos) in the text.
Author Response
- Although this manuscript reports on the electrical properties of Si3N4ceramics, which could be interesting to some researchers, the employed methods and the material is not novel. Moreover, the temperature window is significantly lower then what Si3N4is typically used. Therefore, the authors are encouraged to improve the introduction to show the aim and benefits of this study.
Response 1：The introduction has been revised to add that the operating temperature of ultra-high DC transmission equipment is 80℃, and the resistivity stability of silicon nitride ceramics at this temperature is mainly considered in this paper. The specific content of the revision is marked in red in the introduction
- Also, there are a few points which are not clear:lines 118-112: there is no evidence in this study for the role of Yttrium ions in orientation and growth of grains ( Authors can check and use the following paper: https://doi.org/10.1111/j.1551-2916.2008.02448.x or similar ones)
Response 2：The reference “β-Si3N4 grain growth, part I: Effect of metal oxide sintering” was cited to explain the effect of yttrium oxide/alumina on grain size.
- Although mentioned ( a little) in lines 173-177, the effect of grain boundary and its amorphus nature is not discussed. Authors could use the literature on oxynitride glasses.
Response 3：During sintering, the added sintering additives react with SiO2 on the surface of silicon nitride powder at high temperature to form nitride liquid phase to promote densification, and the liquid phase remains in the form of amorphous or partially crystalline after cooling. After liquid phase cooling, the glass phase is uniformly distributed among the grains in the form of grain boundary film, and due to the long rod-like morphology of β-Si3N4, the excess glass phase is enriched at the polycrystalline junction.
Reviewer 2 Report
Comments and Suggestions for AuthorsThe authors investigated the effect of Y2O3-Al2O3 addition on the microstructure and electrical properties of Si3N4 ceramics. They used two-step pressureless liquid-phase sintering to prepare silicon nitride ceramics with improved electrical properties. The results obtained by changing the content of Y2O3-Al2O3 and the ratio of Y2O3 to Al2O3 are interesting and valuable for researchers who want to modify silicon nitride using some oxide additives. However, several issues listed below should be addressed prior to publication.
1. The authors used the concept of “pressureless liquid phase sintering” in the preparation of silicon nitride with Y2O3-Al2O3 aids. According to the experimental section, the sintering was carried out at 1650 oC for 2h and 1825 oC for 2 hours. Which components meld and form a liquid phase during the sintering process at this temperature?
2. In Figure 1, at the aid content of 12%, there is the formation of Y2SiAlO4N. Then, in what state does the added Y2O3-Al2O3 exist in silicon nitride matrix? Does it exist as an amorphous phase?
3. According to the result of microstructure of the surface of silicon nitride as shown in Figure 2, the authors described that the main phase is black and the grain boundary phase is white in the figure. Then, is the Y2O3-Al2O3 additive located at the grain boundary? If so, the authors should show the distribution of the additives in the matrix using EDS analysis. Also, the grain size of silicon nitride ceramics increases as the sintering aid content increases. For this, the authors explained that the increase of grain size is mainly due to the higher content of liquid phase during the sintering process. What is the rationale for increasing the liquid content as the aid content increases? If relevant literature is reported, it is required to cite it.
4. When comparing the microscopic morphology of samples prepared with changing the aid content, it is helpful to show together with that for pure Si3N4 in Figure 3 and Figure 4. If possible, it is also recommended to add the electrical properties of pure Si3N4 in Figure 4, 6 and 7.
5. There are previous results showing the influence of Y2O3-Al2O3 or MgO-Al2O3 on the electrical properties of silicon nitride (e.g. Ceramics International, volume 45, issue 7 (2017) 9497-9501). It is recommended that the results obtained in this paper be compared with previously reported literature.
6. The main difference of this work with others is thought to be the sintering process, which is called “pressureless liquid phase sintering”. If possible, to help readers understand this process, it would be helpful to add a schematic diagram showing microstructure formation with changing the aid content based on the obtained microscopic morphology shown in Figure 2 and 3.
7. In the text, the ratios of yttrium oxide and aluminum oxide are expressed differently from the values shown in Table 1. For example, lines 128 through 129 show the ratio of yttrium oxide to aluminum oxide as 5:2.35. In order for readers to directly check the ratio, it is necessary to directly use the same ratio value as in Table 1.
8. In the line 133-136, the authors state that “The use of sintering aids with different rare earth/alkaline earth ratios can achieve phase transformation and densification regulation, thereby controlling the microstructure of silicon nitride ceramics, which is particularly important for improving the mechanical and electrical properties of silicon nitride ceramics.” Please cite relevant references.
Author Response
The authors investigated the effect of Y2O3-Al2O3 addition on the microstructure and electrical properties of Si3N4 ceramics. They used two-step pressureless liquid-phase sintering to prepare silicon nitride ceramics with improved electrical properties. The results obtained by changing the content of Y2O3-Al2O3 and the ratio of Y2O3 to Al2O3 are interesting and valuable for researchers who want to modify silicon nitride using some oxide additives. However, several issues listed below should be addressed prior to publication.
- The authors used the concept of “pressureless liquid phase sintering” in the preparation of silicon nitride with Y2O3-Al2O3aids. According to the experimental section, the sintering was carried out at 1650 oC for 2h and 1825 oC for 2 hours. Which components meld and form a liquid phase during the sintering process at this temperature?
Response 1：The added Y2O3-Al2O3 sintering additive reacts with SiO2 on the surface of the silicon nitride powder at high temperature to form a nitride oxide liquid phase, which promotes the particle distribution of the silicon nitride powder.
- In Figure 1, at the aid content of 12%, there is the formation of Y2SiAlO4N. Then, in what state does the added Y2O3-Al2O3exist in silicon nitride matrix? Does it exist as an amorphous phase?
Response 2：The sintering additives Y2O3-Al2O3 react with SiO2 on the surface of silicon nitride powder at high temperature to form nitride liquid phase. It is distributed in the form of amorphous or partial crystallization between grains or polycrystalline junctions.
- According to the result of microstructure of the surface of silicon nitride as shown in Figure 2, the authors described that the main phase is black and the grain boundary phase is white in the figure. Then, is the Y2O3-Al2O3additive located at the grain boundary? If so, the authors should show the distribution of the additives in the matrix using EDS analysis. Also, the grain size of silicon nitride ceramics increases as the sintering aid content increases. For this, the authors explained that the increase of grain size is mainly due to the higher content of liquid phase during the sintering process. What is the rationale for increasing the liquid content as the aid content increases? If relevant literature is reported, it is required to cite it.
Response 3：EDS scan of yttrium aluminum for sample 1825-6.25Y-90SN has been added（Figure 2） in the revised draft. The liquid phase is formed by the reaction of sintering additives and silica on the surface of silicon nitride powder at high temperature, and the liquid phase content is proportional to the auxiliary dose at high temperature.
- When comparing the microscopic morphology of samples prepared with changing the aid content, it is helpful to show together with that for pure Si3N4in Figure 3 and Figure 4. If possible, it is also recommended to add the electrical properties of pure Si3N4in Figure 4, 6 and 7.
Response 4：Figure 3 and Figure 4 are merged. Silicon nitride is a strong covalent bond compound, and pure silicon nitride is difficult to be sintered and densified, so no relevant sample preparation was conducted.
- There are previous results showing the influence of Y2O3-Al2O3or MgO-Al2O3on the electrical properties of silicon nitride (e.g. Ceramics International, volume 45, issue 7 (2017) 9497-9501). It is recommended that the results obtained in this paper be compared with previously reported literature.
Response 4：Reference ( Ceramics International, volume 45, issue 7 (2017) 9497-9501) was added for comparison
- The main difference of this work with others is thought to be the sintering process, which is called “pressureless liquid phase sintering”. If possible, to help readers understand this process, it would be helpful to add a schematic diagram showing microstructure formation with changing the aid content based on the obtained microscopic morphology shown in Figure 2 and 3.
Response 6：Reference (J. Eur. Ceram. Soc, volume41, (2021), 1735–1738) was added to explain）
- In the text, the ratios of yttrium oxide and aluminum oxide are expressed differently from the values shown in Table 1. For example, lines 128 through 129 show the ratio of yttrium oxide to aluminum oxide as 5:2.35. In order for readers to directly check the ratio, it is necessary to directly use the same ratio value as in Table 1.
Response 7：Revised to 6.8:3.2
- In the line 133-136, the authors state that “The use of sintering aids with different rare earth/alkaline earth ratios can achieve phase transformation and densification regulation, thereby controlling the microstructure of silicon nitride ceramics, which is particularly important for improving the mechanical and electrical properties of silicon nitride ceramics.” Please cite relevant references.
Response 8：Reference (Silicon, volume11, (2019), 2689–2699) was added.
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsThe authors revised the paper well to reflect the reviewers' requests. Accordingly, it is judged that the revised paper has been sufficiently improved to be published.
