the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Jan 2026
Nascent Titanium/Silicon-Containing Particle Formation in Corona Discharge Assisted Combustion
Abstract. Plasma-assisted combustion (PAC) is a technology that introduces high concentrations of charges, ions, and radicals, making the flame more stable and efficient. At the same time, PAC has also been shown to alter particle formation during combustion. Here, we investigate the effect of a high-frequency (~ 21 kHz) alternating current (AC) corona discharge on particle formation and growth in a premixed flame, especially at the initial stages (with particle sizes below 10 nm). We first examined the mobility size distribution of ions generated from non-plasma combustion and corona discharge-assisted combustion. The mobility size for positive ions does not change with the introduction of plasma. However, the negative ions change towards a larger size, likely due to different ion chemistry from plasma. We then introduced corona discharge with varying powers into the flame that contains titanium isopropoxide (TTIP) or tetraethyl orthosilicate (TEOS) and obtained the size distribution of the synthesized nanoparticles. We found that particle growth is suppressed by the corona discharge under relatively higher precursor feed rates (above ~ 29 mg h-1 for TTIP and above ~ 60 mg h-1 for TEOS). The mobility diameter is suppressed by up to 12 % for TTIP and by up to 20 % for TEOS. We further used different charging models to examine the impact of plasma on particle formation. In the case of higher precursor feed rates, the incipient particle concentration is high within the flame region. As higher number of charges accumulated on particles from negative charge carriers (including electrons and negative ions) than positive ions, the particles are preferentially charged negative. Such preferential charging results in particle-particle repulsion which suppresses coagulation particle growth. The findings of this study can guide nanoparticle synthesis and particulate matter control using PAC.
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Status: open (until 13 Feb 2026)
- RC1: 'Comment on ar-2025-41', Anonymous Referee #1, 13 Jan 2026 reply
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RC2: 'Comment on ar-2025-41', Anonymous Referee #2, 23 Jan 2026
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The work is an experimental study on the effect of corona discharge plasma parameters on metal oxide nanoparticle production in flames.
The introduction section is quite long (nearly 3 pages). This discussion provides relevant and thorough background but the long discussion makes the thrust of the current work more difficult to follow. Perhaps the goals and objectives of the current work can be made more clear throughout this intro section.
pg 5 line 142, Why does the stoichiometric flame condition reduce the flame temperature effect of adding precursor?
Is the HRDMA also a TSI instrument?
Why is air used for dilution of the metal oxide aerosol? Is the probe inlet temperature low enough to prevent further oxidation by air in the dilution probe flow?
What are details of the thermocouple probe? Type? Shape? Size? Could there be energy losses to the temperature probe affecting the interpretation of the temperature readings?
Is the thermocouple grounded or protected from plasma interference?
pg 8, How is the flame height of 6.35 mm defined?
In industrial applications, the precursor concentrations are in the heavily loaded regime, would the currently observed plasma affects apply by simply increasing the ion concentration to match the heavy precursor concentration?
Citation: https://doi.org/10.5194/ar-2025-41-RC2 -
RC3: 'Comment on ar-2025-41', Anonymous Referee #3, 01 Feb 2026
reply
This manuscript presents measurements of the (mobility) size distribution functions (SDFs), performed via dilution sampling followed by high-resolution differential mobility analysis (HRDMA), of charged nanoparticles and ions formed in Bunsen-type premixed flames of near-stoichiometric methane/air mixtures and trace amounts of either titanium isopropoxide (TTIP) or tethraethyl orthosilicate (TEOS). The measurements are performed at a fixed but unspecified height above the burner (HAB) of about 9mm (according to the inset of Fig.1), with and without the perturbation of an AC corona discharge generated by two coaxial tungsten needles facing each other radially at HAB =3mm, and two discharge powers (i.e., 56 W and 125 W), both at about 21kHz. The measurements are processed upon performing estimates of the fraction of nanopraticles getting charged in the flame via diffusion and field charging. The objective is to investigate the effects of the plasma discharge on the mechanisms of nanoparticle formation in flames, with some indications of the corona enhancing nucleation but suppressing coagulation. The manuscript includes a good survey of the relevant literature, and the results are interesting and worthy of presentation in the archival literature. On the other hand, I think that the writing style and organization quality of the manuscript need some significant improvements before publication, and I would also ask the authors to address my more specific comments below.
- Some details of the experimental method are missing. What is the diameter of the sampling orifice and sampling underpressure? What type and size of (coated or uncoated) thermocouple was used to measure the temperature?
- The manuscript mentions the height of the (I assume conically shaped) flame being about 6.35mm, and an unquantified elongation of the flame caused by the corona discharge. Can you acquire digital images of the flames with and without the corona discharge during sampling (and possibly also without the dilution probe)?
- The evaluation of the values of the parameter in equations 4 and 5 is not properly described. This is particularly the case for N0 and E0. Also, it is not clear which one of the two equations (or what type of their combination) is used to correct the number concentration of the measured charged particles to report the SDFs.
- The evaluation of the exposure time of particles to the ions based on the flame height is not correct and should, instead, be based on the distance from the blue layer of the flame and the sampling position. This is the case because nanoparticles cannot be formed in the cold, unreacted region upstream of the main reaction zone (i.e., the blue layer) of a premixed flame, unless their formation is caused by the plasma rather than by the flame.
- Related to point D above, it is more than possible that the observed suppression of coagulation is caused by the reduction in the residence time of the particle in the hot post-flame region (where they form) between the flame blue layer and the sampling position, as a result of the flame elongation ensuing the corona discharge.
- What is the transport time of the aerosol from the sampling orifice to the HRDMA? As reported in the literature you cite, diffusion charging is still active in the diluted flow.
- Have all the SDFs in Figs 2-4 been corrected for the estimated charged fraction? How? If not, which ones have been?
- It’s too easy to lose track of the important message while reading many confusing percentage changes reported in Paragraphs 3.1 and 3.2. This could be substantially improved by changing the current organization of the figures, which highlight the effect of the precursor concentration rather than that of the corona discharge power. Grouping datasets at different powers of the corona discharge in the same panels of the figures ( a panel for each precursor type and flow rate) would greatly improve the presentation of the effect of the corona discharge.
- With the same token, it would be nice to compare each SDF of the TTIP and TEOS nanoparticles (without correction for charged fraction) with that of the ions generated in the flame. How distinguishable is the raw signal of the nanoparticle from that of ions in the flames with the smallest nanoparticle precursor flow rates?
- Please describe Figure 5 more properly and include it in the manuscript where it is discussed first. What is the main message of this figure? How is the modal diameter determined? Can you include an error bar bracketing the standard deviation of the mobility diameter of the mode?
- The same argument applies also to fig. 6, which is plotted with a log-scale ordinate spanning 9 orders of magnitude, making it impossible to visualize any effect. Would it be wiser to have separate panels for the different considered charging mechanisms?
- The manuscript discusses the expectation that the charging in negative polarity should be more efficient due to free electrons (e.g., in the last paragraph of page 13). Yet, if I did not get too lost in the wording, the experimental results do not support this expectation, especially those with TEOS in Fig. 4. Can you please clarify?
Other observations:
- Page 1, lines 10 and 18. Wording is not clear: “more stable” and “higher” compared to what?
- Page 2, line 34. I think the authors are referring to flame spray pyrolysis in turbulent conditions. Regardless, this statement should be supported by at least one citation. Also, the ensuing discussion may also benefit from distinguishing studies in laminar and turbulent (as well as premixed and non-premixed)
- Page 3, line 84. Do you mean 1E23 rather than 1023?
- Page 3, line 93. Ion wind (and undiscussed space charge) effects may still be relevant under AC conditions. Regardless, this statement should be supported by at least one citation.
- Page 9, line242. This is not surprising since electrons are more likely to affect the flame compared to much larger cations.
- Page 11, lines 280-282 and 293-24. To what suppression and promotion effects is the prose referring to?
Citation: https://doi.org/10.5194/ar-2025-41-RC3
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This work clearly illustrates the transformation process of precursors in a plasma flame. It investigates the effects of feed rate on the formation of nanoparticles. The study provides guidance for the synthesis of nanoparticles in flames and for controlling the morphology of particulate matter. The research content and conclusions are consistent with the standards of AR journals, and it is recommended for publication after appropriate revisions.
1. The authors could consider increasing the testing and evaluation of the product nanoparticles to enhance the contribution and impact in the field of nanomaterials synthesis.
2. The evaluation methods in this work all use organic sources as raw materials. It could be worthwhile to explore whether similar conclusions hold when using volatile inorganic precursors, which could appropriately expand the application scope of this work.