Temperature Estimation during Pulsed Laser Sintering of Silver Nanoparticles

Comment

In Figure 3 Resistivity values can be added with error bars.

Response

We were limited with our funding and experimental sessions. The Acousto Optic Modulator allowed us to test multiple pulse durations and repetition rates against different laser powers. For example, we tried multiple laser parameters as presented in the table below in search of obtaining the minimum Ag NPs resistivity. We observed that with shorter pulse durations we needed higher laser power to obtain low resistivities but at the same time we were limited with the output power of the AOM that we were using. All the other tested laser parameters produced very high resistivities because of the output power limitations. The minimum resistivity at 135 μm/s and 100 ms was due to longer laser exposure duration that ensured complete sintering of the Ag NPs. The presented data was obtained with the single experiment at 135 μm/s. However, similar resistance values were produced but were not included due to lack of area measurement values.  Our current work is more focused on showing out novel temperature estimation method. Future works will be carried out to better understand the effect of laser parameters on the quality of sintering such as electrical resistivity, surface morphology, and adhesion strength etc.

Comment #1

Authors shows that their model can produce accurate results of the temperature estimation during laser sintering, however, we could expect that they also suggest optimized parameters for their experiments or guidelines of improvements based on their model. To be more specific, authors line 319 to line 326 discuss the impact of high residual temperature versus better sintering induced by longer thermal diffusion length with a longer pulse. Modeling could help here to find a compromise between high residual temperatures and better sintering conditions by deducing the best experimental parameters (pulse duration, power, speed, beam size…).

Response

A detailed discussion has been added under Figure 9 and Figure S4 (supplementary information) in the manuscript to study the effect of pulse duration, repetition rates, scanning speeds and power on the residual temperature and thermal diffusion length through the substrate. Given the available computing power and memory we limited our study to a few seconds of laser scanning for some cases while complete laser scans (for 7000 μm-long Ag NPs line) were performed for others.

Comment #2

Authors remind the two approaches using pulsed and CW lasers for NP sintering. If they discuss in detail the case of pulsed lasers, they did not discuss pros/cons of CW lasers.

Response

Lines 54-56: We have added information in this regards in the introduction section. “CW lasers are known to produce lower resistivities [21], however, they induce large thermal diffusion length and are not compatible with the sintering of sub-micrometer sized patterns due to large heat affected zones [10].”

Comment #3

Line 72-74: Authors could give more details about how NP size affects sintering behavior, the coalescence temperature and the required energy density.

Response

Line 91-93: Additional information has been added. “Due to the thermodynamic size effect in metal nanoparticles exhibit large melting point depression [29], such as 2 nm-sized nanoparticles are reported to melt at around 130-140°C [31].”

Comment #4

The context is well defined by authors, but the requirement of refined temperature modeling, that has motivated authors, is not clear enough. Was a good model missing or the sintering/material parameters were unknown, requiring the use of modeling?

Response

The need for the refined temperature modeling is further elaborated in the introduction section in the manuscript.

Previous studies that are cited in the manuscript either used constant thermal conductivity or temperature dependent thermal conductivity without considering the pulse excitation and relaxation period. One of the group did consider this, however, their model was limited by the melting temperature of the Ag NPs as they had considered phase change when considering the pulse excitation and relaxation periods.

Our model considers temperature dependent thermal conductivity and at the same time considers the pulse relaxation and excitation periods without the limitation of the Ag NPs melting temperature.

Comment #5

At the end of introduction, authors motivate this work by saying their model could help to select the most suitable laser parameters, but it does not appear clear that it is done in the paper discussion or conclusion. Can the authors discuss this point concerning the present experimental work or open perspectives to suggest better laser parameters or design of printed lines?

Response

For the current study we have suggested 100 ms, 1 Hz, and 276 mW and 135 μm/s laser parameters for optimum sintering output.

Comment #6

Line 98-100: Theses information are not used in the article. Are they necessary? Have NP size and density have an impact on viscosity and does the viscosity impacts sintering behavior?

Response

We included all the necessary information regarding the materials used in the experiment to help the other researchers to design their own experiment. The density and viscosity are actually not needed to sinter the nanoparticles as the reviewer commented. However, the density and viscosity can be important parameters for the inkjet printing since our samples are fabricated by the inkjet printing.

Comment #7

Line 104: Can this thermal treatment induces sintering prior laser irradiation?

Response

The pads were printed and sintered before the printing and sintering of the Ag NPs line. The purpose of the pads sintering was to minimize the effect of the high resistivity on the resistivity measurements of the lines after the pulsed laser sintering of the Ag NPs line

Comment #8

Just a remark : Based on your parameters : 135 µm/s (1Hz rep. rate), with a beam of 270 µm in diameter, I indeed find that the next pulse is shifted by 50% of his diameter, but overlapping is rather close to 39% than 50%.

Response

We calculated the pulse overlapping based on the method presented by N. Bellini, R. Geremia & D. Karnakis, Increasing laser pulse overlap restricts picosecond laser ablation, of thin metal films near ablation threshold, Applied Physics A, 123, 2017, https://doi.org/10.1007/s00339-017-0971-9.

Comment #9

Can authors briefly explain why normal reflectivity increase about 200°C? Does sintering impacts reflectivity? If yes, is it a change of surface roughness or the evaporation of organic additives?

Response

The presented figure shows the TGA plot of the Ag NPs ink used in our study (Park Hyeong Jin, Physical characteristics of Ag nanoparticle inks with different size during thermal sintering, Master Thesis (2016).) corresponding to the resistivity drop of the Ag NPs. We observe a sharp weight loss at around 100-170°C and correspondingly a sharp resistivity drop from 100-150°C. The graph indicates that during the evaporation of the additives the sintering of the Ag NPs also occurs causing a sharp drop in the resistivity at 150°C. The almost complete evaporation of the additives combined with the resistivity drop after the 170°C results in the increase of the Ag NPs reflectivity.

Comment #10

Please correct “We Assumed”

Response

The capitalization mistake is rectified.

Comment #11

Line 173-176: A short remind of the method used to deduce the thermal conductivity change with temperature would be helpful. Is the thermal conductivity just temperature dependent or it is a consequence of NP sintering?

Response

The thermal conductivity variation as a function of temperature is as a result of NPs morphology after sintering.

Choi et al. (http://dx.doi.org/10.1016/j.ijheatmasstransfer.2015.01.056) suggested the method of estimating the thermal conductivity of sintered Ag NPs based on Wiedemann-Franz Law. Based on this study Ik-sang Lee et al. (http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.11.095) formulated the temperature dependent thermal conductivity relation which was used in our study.

Comment #12

Line 177-179: I think this sentence is not necessary

Response

The sentence has been removed.

Comment #13

Table 1: The thermal conductivity for Ag NP ink should be a value rather than an equation

Response

The equation shows that the thermal conductivity acts as a function of temperature that is the Ag NPs thermal conductivity is temperature dependent. The relation was first reported by Ik-sang Lee et al.  “http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.11.095.” Putting a single value for the thermal conductivity would mean considering the thermal conductivity as constant during the temperature calculation.

Comment #14

Line 213-217: A remind of the exact origin of such discrepancy might be useful for the reader

Response

To our knowledge, one such study does not exist that tested and reported the effects of all the laser parameters, additives used in the ink, NPs size, and the sintered NPs surface morphology, therefore, we have cited specific references for each. In fact, for better review of the topics, we have added 4 more references to better understand the discrepancy.

Comment #15

Figure 3: How to explain the minimum at 135 µm/s? What is the uncertainty of the measured resistivity ?

Response

We were limited with our funding and experimental sessions. The Acousto Optic Modulator allowed us to test multiple pulse durations and repetition rates against different laser powers. For example, we tried multiple laser parameters as presented in the table below in search of obtaining the minimum Ag NPs resistivity. We observed that with shorter pulse durations we needed higher laser power to obtain low resistivities but at the same time we were limited with the output power of the AOM that we were using. All the other tested laser parameters produced very high resistivities because of the output power limitations. The minimum resistivity at 135 μm/s and 100 ms was due to longer laser exposure duration that ensured complete sintering of the Ag NPs. The presented data was obtained with the single experiment at 135 μm/s. However, similar resistance values were produced but were not included due to lack of area measurement values. 

Our current work is more focused on showing out novel temperature estimation method. Future works will be carried out to better understand the effect of laser parameters on the quality of sintering such as electrical resistivity, surface morphology, and adhesion strength etc.

Comment #16

Line 223: Please correct “positon”

Response

The spelling mistake has been rectified

Comment #17

Figure 8: I am wondering whether an increase of less than 10°C of the residual temperature can cause damage to the underlying substrate?

The statement regarding the damage to the underlying statement is a general statement therefore we have specifically used the word “can”. Such that while looking at the higher residual temperatures with 1 ms and 100 Hz laser scans, we realize that with shorter pulse durations, higher residual temperatures are expected especially with longer periods of continuous laser scans. Therefore, the purpose here was to direct the readers to consider the residual temperatures in choosing the correct laser parameters such as the length of the continuous laser scan and the pulse duration.

Comment #1

The title seems good, the abstract also seems to be fine. Please add one more introductory line of your objective in beginning of abstract.

Response

The introductory line has been precicely added.

Comment #2

Research gap should be delivered on more clear way with directed necessity for the future research work.

Response

We have made sure that the research gap is clearly understood in the introductory section. Extensive changes have been made in this regard in the introduction section.

Comment #3

The novelty of the work must be clearly addressed and discussed, compare previous research with existing research findings and highlight novelty.

Response

The novelty of the current study has been clearly addressed and clearly highlighted in the introductory and discussion section as compared to previous studies.

Comment #4

What is the main challenge?

Response

Previous studies did not consider the pulse excitation and relaxation periods in their studies during a scanning pulsed laser sintering operation. The problem without considering the pulse relaxation and excitation periods is that during the pulse excitation period, the maximum temperature sinters the NPs and changes its thermophysical properties accordingly. However, if the pulse relaxation period is not considered in the model, that means that the attained thermophysical properties will follow the drop in the temperature going back to the properties of the unsintered NPs which is not the case in reality. That is once the NPs get sintered they do not go back to the unsintered state. Our novel numerical model addresses this challenge.

Comment #5

Page 1 Line 32 (Since the current….. processes are not applicable) need this reference to cite-Umar, K.; Yaqoob, A.A.; Ibrahim, M.N.M.; Parveen, T.; Safian, M.T. Chapter Thirteen-Environmental Applications of Smart Polymer Composites. In Smart Polymer Nanocomposites: Biomedical and Environmental Applications; Woodhead Publishing Series in Composites Science and Engineering; Elsevier Inc.: Cambridge, MA, USA, 2021; pp. 295–312.

Response

The recommended reference has been added.

Comment #6

Page 2 Line 60. Please cite this reference -silver nanoparticles: various methods of synthesis, size affecting factors and their potential applications–a review. Similar,

Response

The recommended reference has been added.

Comment #7

The main objective of the work must be written on the more clear and more concise way at the end of introduction section.

Response

The clarity of our objective has been significantly improved at the end of introduction section.

Comment #8

Please add material and chemical reagent section and write chemical specification as well.

Response

Previously we have requested the suppliers for such information, however, they were not willing to disclose the detailed information such as all the chemicals specifications of the ink. The available information such as solid content, viscosity, surface tension, solvent, curing temperature were obtained from the website.

Comment #9

Please check the abbreviations of words throughout the article. All should be consistent.

Response

We have made sure all the abbreviations are consistent throughout the manuscript.

Comment #10

Please include all chemical/instrumentation brand name and other important specification

Response

Previously upon our request via email, the suppliers were not willing to disclose the chemical specifications of the Ag NPs ink. Moreoever, all the experimental instruments brand name pertinent specifications was added in the Experiment section and some information regarding the glass substrate was included in Table 1.

Comment #11

Please provide space between number and units. Please revise your paper accordingly since some issue occurs on several spots in the paper.

Response

The issues were checked and resolved.

Comment #12

Please add chemical reagents section and stated all chemical with brand specifications.

Response

Upon our request from the Ink supplier, such specification were not provided to us, we were only limited to the available data as provided in the website.

Comment #13

Regarding the replications, authors confirmed that replications of experiment were carried out. However, these results are not shown in the manuscript, how many replicated were carried out by experiment? Results seem to be related to a unique experiment. Please, clarify whether the results of this document are from a single experiment or from an average resulting from replications. If replicated were carried out, the use of average data is required as well as the standard deviation in the results and figures shown throughout the manuscript. In case of showing only one replicate explain why only one is shown and include the standard deviations.

Response

We were limited with our funding and experimental sessions. The Acousto Optic Modulator allowed us to test multiple pulse durations and repetition rates against different laser powers. For example, we tried multiple laser parameters as presented in the table below in search of obtaining the minimum Ag NPs resistivity. We observed that with shorter pulse durations we needed higher laser power to obtain low resistivities but at the same time we were limited with the output power of the AOM that we were using. All the other tested laser parameters produced very high resistivities because of the output power limitations. The minimum resistivity at 135 μm/s and 100 ms was due to longer laser exposure duration that ensured complete sintering of the Ag NPs. The presented data was obtained with the single experiment at 135 μm/s. However, similar resistance values were produced but were not included due to lack of area measurement values. 

Our current work is more focused on showing out novel temperature estimation method. Future works will be carried out to better understand the effect of laser parameters on the quality of sintering such as electrical resistivity, surface morphology, and adhesion strength etc.

Comment #14

Section 5 should be renamed by Conclusion and Future perspectives. Conclusion section is missing some perspective related to the future research work, quantify main research findings, highlight relevance of the work with respect to the field aspect

Response

We have made the required additions to the conclusion section marked by the track changes.

Comment #15

To avoid grammar and linguistic mistakes, moderate level English language should be thoroughly checked. Please revise your paper accordingly since several language issue occurs on several spots in the paper.

Response

English language has been significantly improve throughout the manuscript by changing the sentence structures, rectifying the spelling mistakes and typos, and improving the sentence clarity where necessary.

Comment #16

Reference formatting need carefully revision. All must be consistent in one formate. Please follow the journal guidelines.

Response

Page numbers for Ref 32 and 33 was added and we made sure that the formatting of all the references were according to the journal guidelines.