Comment on ar-2023-14

Abstract

Abstract. Particles generated by spray flame synthesis (SFS) can exhibit impurities, with surface adsorbates and soot being the best-known examples. Unfortunately, the extent to which the formation of these impurities can be characterized, and how their formation can be avoided is not fully understood yet. In order to contribute to the understanding of the formation of carbonaceous impurities, we investigated their formation using a standardized burner type (SpraySyn burner) while synthesizing maghemite by the gas-to-particle conversion. Two approaches were followed to characterize the formation of soot and surface adsorbates in this paper. Firstly, fabricated powders were analyzed by complementary powder analysis. In this term, the detection of soot was negative, but powders contained carboxylates and carbonates, which were bound to particle surfaces. The methodological procedure used allowed a reliable quantification of those adsorbates, and it was possible to normalize their relative mass fractions (up to ~16 %) to the powders' respective specific surface areas. The normalization approach yielded a surface loading values of 0.84–0.85 mg m-2. The methodical procedure and the data provided can be used in the future to denote/compare SFS materials in sensitivity studies. Secondly, particles were thermophoretically extracted from the process in situ and subsequently characterized by transmission electron microscopy (TS-TEM). Interestingly, particles extracted at 5 cm height above the burner (HAB) (representing the center of the visible flame) showed amorphous core-shell structures and amorphous aggregates, indicating a coexistence of soot alongside maghemite particles. Via reducing the sampling time of the TS system to 1.5 ms samples were taken either from flame pulses (a state of high flame activity) or from flame flickering (lower flame activity). A comparison of these samples showed that heterogeneous particle structures formed predominantly in flame pulses. As samples extracted at 15 cm HAB neither showed amorphous coating nor amorphous aggregates, it was indicated that soot fractions oxidize between 5 and 15 cm HAB.

Authors 7

1. Ricardo Tischendorf corresponding

   Paderborn University

   Affiliation as printed

   Particle Technology Group, University of Paderborn, 33102 Paderborn, Germany

2. Kristina Duschik

   Paderborn University

   Affiliation as printed

   Department of Material Science, University of Paderborn, 33100 Paderborn, Germany

3. Sophie Dupont Aachen Institute of Heat and Mass Transfer

   RWTH Aachen University

   Affiliation as printed

   Institute of Heat and Mass Transfer, RWTH Aachen, 52056 Aachen, Germany

4. Heinz G. Pitsch Aachen Institute for Combustion Technology

   RWTH Aachen University

   Affiliation as printed

   Institute for Combustion Technology, RWTH Aachen, 52056 Aachen, Germany

5. Reinhold Kneer Aachen Institute of Heat and Mass Transfer

   RWTH Aachen University

   Affiliation as printed

   Institute of Heat and Mass Transfer, RWTH Aachen, 52056 Aachen, Germany

6. Mirko Schaper

   Paderborn University

   Affiliation as printed

   Department of Material Science, University of Paderborn, 33100 Paderborn, Germany

7. Hans‐Joachim Schmid

   Paderborn University

   Affiliation as printed

   Particle Technology Group, University of Paderborn, 33102 Paderborn, Germany