Effect of Electric Current on SPS Densification of Spherical Copper Powder
and Frédéric Bernard 1,*Round 1
Reviewer 1 Report
Page 1 Line 21
Typo (i) h the current flows
Page 2
2. Materials and Methods
How many specimens did you use for each experimental condition? What about the reproducibility of the experimental data (density and conductivity)?
Page 3 Line 113-114
The measurement method of density should be more detailed. The same explanation of how to calculate the temporal density increase (Fig. 3~Fig. 7) needs to be described in Section 2 Materials and Methods by using the equation with d_0, d_f, and L_f.
Page 3 Line 114-115
The measurement method of electrical conductivity should be more detailed. It is not clear how you measured it during sintering as shown in Fig. 9. If you merely measured it after sintering the data should be plotted (not as line drawing) by showing the relationship between the electrical conductivity and sintering temperature.
Page 4 Line 140-141
What is the cause of the fluctuation of density at the low-temperature range of StB? What do you mean by “temperature regulation problem”? The decrease in density cannot be explained by such a simple cause. I suspect your data does not reflect the net density of copper itself.
Page 12 Line 346
“Densification is sensitive to thermocouple position.”
This explanation may mislead readers. Which data or figure support this conclusion? The “Densification” process itself must be independent of the measurement; wherever you put a thermocouple, if the heating method and heating profile are identical. Rather, you must mean that the temperature distribution (and/or measurement error) existed as shown in your simulation. Please conclude your results with consideration of the uncertainty of your experiment and simulation.
Fig.3~Fig.7 These figures show the continuous change of the density of sintered copper. The question is how did you measure the temporal increase of copper density?
Fig. 10 A temperature contour is shown on the right side of the center-bottom subfigure. Is it common for all bottom subfigures at t=600s?
Table 2 The configuration (evaluated fraction ratio) in the first row of the table is hard to figure out exactly. It is strongly recommended to illustrate (i.e. by drawing arrows) each flow of “punches to sample heat flow” and “die to sample heat flow”, etc in another figure.
References
Page13 Line398-399
The reference number is missing.
Author Response
Dear Reviewer 1,
See in attachment our answer point by point
Best regards
Author Response File: 
Author Response.pdf
Reviewer 2 Report
- In Figure 2, the thermocouple actually pierced the upper punch in Position A, so in can’t be ensured that the upper and lower temperatures are the same. Besides, how to ensure that the thermocouple of Position A is always in the sample during sintering and will not be broken with the longitudinal displacement of the punch?
- In the NC experiment, alumina was added to the upper and lower punches, but no alumina was added to the contact surface between the sample and the cavity mold. Therefore, in the NC experiment, it cannot be proved that the heat source of the sample is only from the die and punch, Joule heat may generate in the powders.
- In the FC experiment, why not conduct a sintering test with a pressure of 28 MPa? In the actual SPS process, experiment like FC ensures that the current passes through the sample completely, instead of passing through the cavity mold, ensuring that most of the Joule heat is generated in the sample, which can better explain the effect of the electrical current on the sample in SPS.
- In Lines 216 and 217, the author believes that no significant difference could be highlighted between FC and St configurations, but a small systematic difference between St and NC configurations was observed. However, according to Figs. 6 and 7, the difference between St and NC is also not obvious.
- Where does the 20 MPa in Line 226 come from?
- According to the discussion on the change of electrical conductivity with relative density from Line 222 to Line 233, when the relative density is low, the electrical conductivity of the sample is not large, then in the St experiment of Figure 2, during most sintering process, the density of the sample is not very high, and the electrical conductivity of the sample is low because of high porosity. In that case, almost no current passes through the sample in the most process of St experiment, but passes through the cavity mold instead. So, there is almost no difference between the St and NC experiments in Figure 2.
- How to ensure that the electrical current mainly passes through the powders during simulation with Comsol? If graphite is selected as the material of the mold, the conductivity of graphite should be greater than that of porous powders, so most of the electrical current should pass through graphite, and the electrical density in the powders should be lower than that in the cavity mold. In addition, how to set the electrical insulation parameters and connection parameters in Comsol to ensure that the electrical current flows into the powders from the punch?
- In Line 305~ Line 309, it is said that the electrical conductivity of the powders is higher than that of graphite, then the current passing through the powders will definitely produce a strong Joule heating effect. In addition, in Line 346 to Line 347, Only the absence of current in the specimen leads to a decrease in densification. However, in Line 315 to Line 316, it is said that Joule heating has little effect. These statements are contradictory.
- The authors stated that the densification is sensitive to thermocouple position. However, the thermocouples are only used to measure the temperature, and their positions don’t affect the density.
Author Response
Dear colleague,
See in attachment our answer point by point
Best regards
F. Bernard
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
I feel that the authors revised their manuscript with enough quality and scientific soundness by responding to reviewers' comments.
Reviewer 2 Report
The comments had been well addressed by the authors, and the paper is recommended to accept.
