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Experimental investigation on microexplosion of single isolated burning droplets containing titanium tetraisopropoxide for nanoparticle production
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Experimental investigation on microexplosion of single isolated burning droplets containing titanium tetraisopropoxide for nanoparticle production H. Li, C.D. Rosebrock, N. Riefler, T. Wriedt, L. Mädler∗ Department of Production Engineering, Foundation Institute of Material Science (IWT), University of Bremen, Badgasteiner Str. 3, Bremen 28359, Germany Received 3 December 2015; accepted 12 September 2016 Available online xxx
Abstract The microexplosion and nanoparticle formation of single isolated burning droplets using titanium tetraisopropoxide (TTIP) as nanoparticle precursor are investigated experimentally. Spherical along with fine agglomerated titanium dioxide (TiO2 ) nanoparticles are obtained from the single droplet combustion of TTIP dissolved in pure xylene, pure ethanol and the mixture of xylene and ethanol. Strong, global, and continuous microexplosions are observed during the combustion of TTIP/xylene droplet and TTIP/xylene– ethanol droplet. We proposed a hypothesis of droplet microexplosions as follows: hydrolysis of TTIP at or in the droplet surface leads to formation of TiO2 nanoparticles, which together with the low volatility component near the droplet surface creates an impermeable shell for the high volatility component. The high volatility component in the interior is superheated to induce heterogeneous nucleation of bubbles beneath the shell, which increases the pressure inside the shell until microexplosion occurs. These insights are very important mechanisms for the well-established flame spray pyrolysis (FSP) process. © 2016 by The Combustion Institute. Published by Elsevier Inc. Keywords: Titanium tetraisopropoxide; Isolated droplet combustion; Microexplosion; Titanium dioxide nanoparticles; Flame spray pyrolysis
1. Introduction As a promising and versatile technique for synthesizing nanoparticles, flame spray pyrolysis (FSP) takes advantage of the ability to dissolve the precursors directly in the fuel [1,2]. High tem∗
Corresponding author. Fax: +49 421 218 51211. E-mail address: [email protected] (L. Mädler).
perature pyrolysis and fast quenching rates of the near transonic spray flame result in a one-step production process of highly pure, single-component and multicomponent nanoparticles with a wide variety of elements. Because of their high purity and controlled size distribution [2,3], nanoparticles produced with FSP technique have already applications for ceramics, electronics, optics and catalysis [4–6]. During FSP synthesis, the droplets vaporize and release the nanoparticle precursors into the flame,
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Please cite this article as: H. Li et al., Experimental investigation on microexplosion of single isolated burning droplets containing titanium tetraisopropoxide for nanoparticle production, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.017
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where pyrolysis and particle formation via coagulation and sintering take place [1,2]. Due to strong interactions of the droplets, high temperatures, turbulence and the particle formation process taking only a few milliseconds in the spray flame [7], tracing of the particle formation is experimentally very challenging. To obtain fundamental insights into the mechanisms of droplet combustion in spray flames containing nanoparticle precursor solutions, simplified experiments such as the single burning droplet method have shown to be very helpful [8,9]. Many combustion studies have been carried out on single isolated droplets. The pioneering works of Godsave [10], Spalding [11], Goldsmith and Penner [12], and Wise et al. [13] lead to the classical diffusion-controlled combustion model of single-component droplet burning in an oxidizing atmosphere. This theory, referred as the d2 -law, is commonly used to describe the quasi-steady combustion of pure fuel droplets. Multicomponent droplet combustion is a more complicated process, where volatility differences and liquid-phase diffusional resistance become important, occasionally leading to microexplosions. Lasheras et al. [14] observed the disruptive burning of isolated free droplets of binary n-paraffin mixtures. Interior superheating, induced by volatility differences of fuels and slow liquid-phase mass diffusion, was attributed to the disruptive burning. Combustion of slurry droplets composed of Boron/JP-10 was studied by Takahashi et al. [15,16]. They described the disruptive burning process as three consecutive stages: (1) uniform combustion according to the d2 -law; (2) shell formation at the droplet surface due to volatility differences and pyrolysis; (3) microexplosions due to internal superheating of the high volatility component. Similar observations were made by Rosebrock et al. [8,9], who investigated the disruptive burning of precursor/solvent droplets for nanoparticle production. They showed that droplet microexplosions with continuing combustion are prerequisite for the generation of homogeneous nanoparticles [9]. Considering single isolated droplet combustion for nanoparticle production, the previous investigations mainly focus on the mechanism of droplet microexplosions of moderate reactive precursors [8], and the role of microexplosions on homogeneous nanoparticles production using cheap, low reactive metal precursors [9]. However, single isolated droplet combustion of highly reactive precursors, especially the mechanism of droplet microexplosions, still needs further investigations. Highly reactive precursors are widely used in FSP due to their ability to produce homogeneous nanoparticles with controlled size distribution. Investigations of single isolated droplet combustion of highly reactive precursors are necessary to better understand the FSP process and to further improve the particle quality. The purpose of this paper is to ex-
tend and to generalize the knowledge of microexplosions to highly reactive alkoxide precursors such as titanium tetraisopropoxide (TTIP). It is known that titanium dioxide (TiO2 ) can be formed via hydrolysis of titanium alkoxides compounds [17–19]. Considering that water is a major product during combustion, water vapor could diffuse back to the burning droplet and condense onto the surface if the boiling point of the fuel is lower than that of water and the fuel is miscible with it [20,21]. Thus, hydrolysis of TTIP to form TiO2 would be expected. This phenomenon could explain the observed micro-sized droplets from TTIP during FSP synthesis with high volatility fuels such as ethanol. The effect of TTIP on droplet combustion could provide new insights for synthesis strategies toward cost-effective FSP precursors and further improvement of the homogeneity of TiO2 nanoparticles. 2. Experimental setup and procedure The experiments were conducted under the condition of normal gravity and normal pressure. As shown in Fig. 1, the setup for single isolated droplet combustion mainly includes a piezoelectric dropleton-demand generator, ignition electrodes, a high speed camera, a gas bottle, a mass flow controller, electronics for the droplet generator and a small solvent reservoir. Pure liquids or precursor/solvent mixtures were fed from the solvent reservoir to the generator with plastic tubes. The generator produced stable isolated droplets at a frequency of 4 Hz, and these reproducible droplets moved upward in a square glass cuvette with an initial velocity from 0.5 to 1 m/s. The initial diameter of the stable droplet is dependent on the investigated liquids, the nozzle of the generator, and the parameters that control the generator. Considering the process of the atomized droplet burning initially in a pure oxygen atmosphere during FSP, the concentrated oxygen was used to provide a similar oxidizing atmosphere. Co-flowing dry oxygen (purity 99.95%) from the gas bottle flowed to the glass cuvette at
Fig. 1. Experimental setup for single isolated droplet combustion.
Please cite this article as: H. Li et al., Experimental investigation on microexplosion of single isolated burning droplets containing titanium tetraisopropoxide for nanoparticle production, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.017
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a flow rate of 0.6 L/min, controlled with the mass flow controller. The buoyancy effect on the burning droplets in our experiments can be neglected because the flame is spherical and concentric to the droplet [22,23]. Thus, the droplet combustion can be considered as spherical symmetric, which makes the classical d2 -law available to describe the burning characteristic. The droplet in the glass cuvette was ignited with a spark discharge from adjacent 100 μm diameter electrodes. The spark discharge can change the droplet trajectory and cause droplet surface oscillations at the moment of ignition. This effect was avoided by minimizing the duration and energy of the spark discharge, and adjusting the relative position between the droplet and the spark at the moment of ignition. The burning process of isolated droplet in the glass cuvette was recorded with the high speed camera at a frequency of 40,000 Hz. These images consist of 1024 X 96 pixels corresponding to a monitored area of 6.4 mm × 0.6 mm (L × W), and the merged images of droplets shown in this paper are cropped in the length direction in order to remove useless part. The droplet diameter was obtained by converting image pixels to micrometers with a Matlab-based edge detection program. The average uncertainty of the droplet diameter was ±1 pixel corresponding to ±6.25 μm. Particles produced from the single isolated droplet combustion were collected with carbon-coated copper grids (Plano GmbH-S162), which are located 5 cm above the ignition electrodes. A transmission electron microscope (TEM) Titan 80-300 (FEI) was used to provide the information of chemical composition, particle size and morphology of the particles. TTIP (Sigma Aldrich, >97% purity), xylene (VWR, 98.5% purity), and ethanol (VWR, 99.8% purity) were used to prepare precursor/solvent mixtures. Six groups of droplets with different components were studied: pure ethanol, pure xylene, and xylene–ethanol (volume ratio 1:1) droplets as well as 0.5 mol/L TTIP/ethanol, TTIP/xylene, and TTIP/xylene–ethanol (volume ratio 1:1) droplets. The setup for single isolated droplet combustion can provide stable and reproducible burning process for hours, which makes the evolution of the droplet diameter fit well with each other at the same conditions. The evolution of the normalized droplet diameter of one specific liquid in this paper is obtained by averaging the droplet diameters of at least ten droplets. 3. Results and discussion 3.1. Combustion of pure ethanol and TTIP/ethanol droplets Figure 2 shows two image sequences of a 122 μm ethanol and a 106 μm 0.5 mol/L TTIP/
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Fig. 2. Merged image sequences of isolated burning droplets: (a) an ethanol droplet with an initial diameter of 122 μm; (b) a 0.5 mol/L TTIP/ethanol droplet with an initial diameter of 106 μm. The initial droplet image is obtained at the moment of ignition. From left to right the droplets move in the upward direction in the experiment.
Fig. 3. Normalized droplet diameter for a 122 μm ethanol and a 106 μm 0.5 mol/L TTIP/ethanol droplet. The ignition of the droplet corresponds to a normalized time equal to zero with a droplet diameter to d0 .
ethanol droplet burning in pure oxygen. The time interval between each droplet image is 0.9 ms. After ignition, the ethanol droplet burns continuously with time and stops burning at the end of the image sequences (Fig. 2a). In contrast, the TTIP/ethanol droplet shows moderate microexplosions at 3.6 ms after a period of steadily burning (Fig. 2b). The droplets might slightly deviate from the straight upward direction due to the effect of the spark, drag, and co-flowing of oxygen. Figure 3 shows the evolution of the normalized diameter for the ethanol and the TTIP/ethanol droplet. The normalized droplet diameter for the pure ethanol decreases almost linearly with the time, as suggested by the d2 -law [10–13]. At approximately 0.5 μs/μm2 , the decline of the normalized droplet diameter decreases, indicating that the droplet combustion stops. This deviation from the d2 -law can be attributed to the boiling point difference between water and ethanol and their
Please cite this article as: H. Li et al., Experimental investigation on microexplosion of single isolated burning droplets containing titanium tetraisopropoxide for nanoparticle production, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.017
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Fig. 4. Image sequences of the microexplosion of a 0.5 mol/L TTIP/ethanol droplet. The time interval is measured from the moment of the first visible microexplosion.
mutual solubility. The water vapor produced during the combustion diffuses back to the droplet and condenses onto the droplet surface due to the lower boiling point of ethanol compared to that of water [20,21]. The water subsequently dissolves into ethanol due to the complete solubility between both liquids. The continuing accumulation of water during combustion decreases the amount of ethanol exposed at the droplet surface, resulting in a decrease of the vaporization rate of ethanol and subsequently a lack of heat of reaction. When the water to ethanol ratio increases substantially, the droplet burning stops. Comparing the remaining droplet volume with the initial droplet volume, the ratio is around 2%, showing that only a small amount of water vapor condenses into the ethanol droplet. The evolution of the normalized diameter of the TTIP/ethanol droplet initially coincides well with that of the ethanol droplet up to about 0.2 μs/μm2 . Then, the burning rate decreases to almost zero for a short period of time, and subsequently microexplosions are observed. The corresponding volume ratio of TTIP to ethanol in the initial droplet is 15–85. Before the microexplosion, 56% (by volume) of the droplet is consumed. The calculations of the volume ratio in this paper are under the assumption of ideal mixtures. In order to describe the microexplosion phenomena more clearly, image sequences of the microexplosion process of a 0.5 mol/L TTIP/ethanol droplet are shown in Fig. 4. The first image was captured at 2.875 ms after ignition in real experiment, and the corresponding normalized time is around 0.25 μs/μm2 . After deformation of the droplet due to microexplosion, a large bright emission is observed and the droplet ceases burning. Next, we would like to discuss two possible mechanisms in order to explain the observed microexplosions. The observed microexplosions of the TTIP/ethanol droplet can be triggered either by homogeneous or heterogeneous vapor bubble nucleation [8,14,16]. At the moment of ignition, ethanol and TTIP are exposed at the droplet surface. Due to its high volatility, ethanol is pref-
erentially evaporated from the droplet surface, which can be seen by the evolution of the normalized droplet diameter for the ethanol droplet and the TTIP/ethanol droplet in Fig. 3 (from 0 to 0.2 μs/μm2 ). However, as combustion proceeds, ethanol is continuously withdrawn from the liquid surface and the liquid-phase mass diffusion rate is at least one order of magnitude slower than the liquid-phase thermal diffusion rate [24–26], leading to an accumulation of TTIP at the surface. The increasing amount of TTIP presented at the droplet surface will be heated and vaporized, resulting in a cease of the combustion as seen from the continuously deviation between the normalized droplet diameters (Fig. 3). The surface temperature might attain values close to the boiling point (∼232 °C) of TTIP [27], which is above the boiling point (∼85 °C) and the homogenous limit of superheat temperature (∼190 °C) of ethanol [28]. It can be assumed that the droplet interior consists mostly of the initial TTIP/ethanol concentration due to slow mass diffusion [26,29]. The probability of superheating the ethanol increases, occasionally leading to bubble growth and microexplosions from homogeneous vapor phase nucleation. The other possible mechanism is similar to the microexplosions of slurry droplets [30], where three sequential steps are observed: (1) a period of continuous droplet combustion according to the d2 -law; (2) a period of constant diameter due to the accumulation and partial shell formation of rigid solids and intermediates at the droplet surface; (3) a period of microexplosions resulting from heterogeneous vapor phase nucleation in or at the shell boundary. In light of the observations made in Figs. 2 and 4, and considering that TTIP hydrolyzes in the presences of water, TiO2 particles could be generated at the droplet surface that heterogeneously superheat the high volatility ethanol adjacent to it. Moreover, as the droplet continues to vaporize, there is more TTIP hydrolysis and TiO2 particle formation and the initial porous shell might become impermeable for the high volatility component to diffuse through it. The subsequent
Please cite this article as: H. Li et al., Experimental investigation on microexplosion of single isolated burning droplets containing titanium tetraisopropoxide for nanoparticle production, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.017
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Fig. 6. Merged image sequences of isolated burning droplets: (a) a xylene–ethanol droplet with an initial diameter of 142 μm; (b) a 0.5 mol/L TTIP/xylene–ethanol droplet with an initial diameter of 139 μm. The initial droplet image is obtained at the moment of ignition.
3.2. Combustion of xylene–ethanol and TTIP/xylene–ethanol droplets
Fig. 5. TEM image of TiO2 nanoparticles from the combustion of 0.5 mol/L TTIP/ethanol droplets.
suppressed vaporization might lead to a substantial heat up of the shell due to the high melting/boiling point of the intermediate and the solid phase. Thus, heterogeneous nucleation of ethanol bubbles could be generated near the inner part of the shell, creating a pressure build-up inside the droplet that results in microexplosions. Previous studies support this mechanism [31,32]. Li et al. [31] studied the chemical reaction between an electrodynamically levitated microdroplet of titanium tetraethoxide and water, and found that the initially rapid rate of reaction leads to the formation of a TiO2 shell which inhibits and eventually stops the reaction. Ahonen et al. [32] prepared nanocrystalline titania powders via aerosol pyrolysis of titanium tetrabutoxide precursors, and observed the hydrolysis reaction with the titanium alkoxide. They suggested that water from the combustion diffuses to the droplet surface and hydrolyzes the titanium tetrabutoxide, resulting in a dense and smooth shell [32]. In the present study, spherical along with fine agglomerated TiO2 nanoparticles are observed (Fig. 5). The TiO2 nanoparticles can be formed by two routes. The preceding discussion suggests that the TiO2 particles could be produced by hydrolysis of the TTIP at the droplet surface, forming partially permeable shells that hinder the highly volatile ethanol to evaporate through the shell. Moreover, the TiO2 nanoparticles are also likely to be produced within the gas phase by gas-to-particle conversion after TTIP is released [1,9]. To point out the dominating mechanism for TTIP/ethanol droplets, the combustions of xylene– ethanol droplets as well as TTIP/xylene–ethanol droplets are investigated.
Figure 6 shows two image sequences of a 142 μm xylene–ethanol droplet and a 139 μm 0.5 mol/L TTIP/xylene–ethanol droplet burning in pure oxygen. The time interval between each droplet image is 1.05 ms. The microexplosions seem to occur solely in the TTIP/xylene–ethanol droplet burning process, whereas the xylene/ethanol droplet burns continuously. The evolution of the droplet diameter for ten xylene–ethanol droplets and ten 0.5 mol/L TTIP/xylene–ethanol droplets are shown in Figs. S1 and S2, respectively. It can be seen in Fig. S1 that the ten curves of xylene–ethanol droplets coincide very well. Moreover, the ten curves of TTIP xylene–ethanol droplets coincide well and the occurring times of microexplosions are almost the same for these ten droplets (Fig. S2). Both figures can support the highly reproducible droplet combustion, and nearly the same occurring time of microexplosions confirms that the observed droplet microexplosions are not stochastic compared to that induced by homogeneous nucleation. Quantitative results are shown in Fig. 7, where the evolution of the normalized droplet diameter for the xylene–ethanol droplet and the 0.5 mol/L TTIP/xylene–ethanol droplet are plotted. The xylene–ethanol droplet burns according to the d2 law, which shows that the homogeneous vapor bubble nucleation mechanism for the xylene–ethanol droplet case does not take place. In contrast, several strong, global, and continuous microexplosions are observed during the combustion of the TTIP/xylene–ethanol droplet. After around 56% (by volume) of the droplet is consumed, the first microexplosion occurs, coinciding with that of the TTIP/ethanol droplet case. In comparison to the TTIP/ethanol droplet, the microexplosions of the TTIP/xylene–ethanol droplet are stronger because the high boiling point xylene is taking part in the shell formation, leading to a more rigid shell. Image sequences of the first microexplosion are shown in Fig. 8. The black object with the same size in each image sequence is caused by a contamination on the lens of the high speed
Please cite this article as: H. Li et al., Experimental investigation on microexplosion of single isolated burning droplets containing titanium tetraisopropoxide for nanoparticle production, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.017
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Fig. 9. Merged image sequences of isolated burning droplets: (a) a xylene droplet with an initial diameter of 86 μm; (b) a 0.5 mol/L TTIP/xylene droplet with an initial diameter of 91 μm. The initial droplet image is obtained at the moment of ignition.
3.3. Combustion of pure xylene and TTIP/xylene droplets
Fig. 7. Normalized droplet diameter for a 142 μm xylene–ethanol droplet and a 139 μm 0.5 mol/L TTIP/xylene–ethanol droplet. The ignition of the droplet corresponds to a normalized time equal to zero with a droplet diameter to d0 .
camera that can serve as a reference. In Fig. 8, especially at 0.05 and 0.75 ms, it can be seen that explosive fragments are ejected from the outer layer of the droplet, which supports the existence of the shell and heterogeneous vapor phase nucleation beneath the shell. Ejected fragments are ignited to release a bright light. Figure S3 of supplemental material (SM) shows spherical TiO2 nanoparticles probably formed at the droplet surface and in the gas phase. The residual liquid of the droplet forms a smaller droplet due to surface tension, and the second microexplosion occurs after a short period of time. In average, five microexplosions have been observed in the burning of the 0.5 mol/L TTIP/xylene–ethanol droplet. Image sequences of the second, third, fourth, and fifth microexplosion are shown in Fig. S4.
Figure 9 shows image sequences of an 86 μm xylene droplet and a 91 μm 0.5 mol/L TTIP/xylene droplet burning in pure oxygen. The time interval between each droplet image is 0.75 ms. While the xylene droplet burns continuously until the fuel is completely consumed, the TTIP/xylene droplet undergoes strong microexplosion after a short period of stable burning, as seen from the bright emissions in Fig. 9b. The spherical flame can be seen in the second, third, and fourth images in Fig. 9b. It is necessary to be noted that the visibility of the flame is restricted by soot formation of the investigated liquids and the intensity of the background light. Because of the strong intensity of the background light, it is difficult to observe the flames in this paper. In order to show the spherical flame clearly, images obtained from the high speed camera are colored according to the luminance of the grayscale image (Fig. S6). In Fig. S6c and d, the spherical flames of a 0.5 mol/L TTIP/xylene–ethanol droplet and a 0.5 mol/L TTIP/xylene droplet are marked by white circles. Figure S7 shows the evolution of the normalized droplet diameter for the xylene droplet and the 0.5 mol/L TTIP/xylene droplet. The burning process of the xylene droplet follows the d2 -law. During the period of stable burning, the curve of TTIP/xylene droplet coincides with that of the xylene droplet from 0 to 0.3 μs/μm2 . Subsequently,
Fig. 8. Image sequences of the first microexplosion of a 0.5 mol/L TTIP/xylene–ethanol droplet. The time interval is measured from the occurrence of the first visible microexplosion. Arrow indicates a contamination on the lens which serves as a reference.
Please cite this article as: H. Li et al., Experimental investigation on microexplosion of single isolated burning droplets containing titanium tetraisopropoxide for nanoparticle production, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.017
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the droplet diameter stops decreasing, reaching a short period of constant diameter, which could be attributed to the formation of the shell. It can be seen from Fig. S7 that the first explosion of the TTIP/xylene droplet occurs at 0.33 μs/μm2 , which is later than both of the TTIP/ethanol droplet and the TTIP/xylene– ethanol droplet. Since xylene is not soluble to water and has a higher boiling point than water, TTIP cannot readily react with the water vapor from the combustion. Comparing to TTIP/ethanol droplets, the strong microexplosion of the TTIP/xylene droplet is due to the large accumulated amount of the high boiling point TTIP in the shell, which makes the shell rigid and hard to destroy. Image sequences of two sequential microexplosions of the TTIP/xylene droplet, and the spherical TiO2 nanoparticles produced from these combustion events are shown in Figs. S8 and S9, respectively.
4. Summary and conclusions In order to understand the microexplosion mechanisms of single isolated droplets using TTIP as precursor, six groups of droplets with different components have been investigated. By comparing image sequences, normalized droplet diameter and TEM images of TiO2 nanoparticles from the isolated burning droplets, the following conclusions can be drawn: Spherical along with fine agglomerated TiO2 nanoparticles are obtained during combustion of single isolated droplets containing TTIP as precursor independent of the solvent/fuel used. The TiO2 nanoparticles could be generated from the reaction between TTIP and water vapor at the droplet surface, and gas-to-particle conversion both by the undisturbed vaporization of TTIP from the solution and by TTIP release during the microexplosions. The microexplosion mechanism of single isolated droplets containing TTIP could be described as the three sequential steps: (1) a period of continuous droplet combustion according to the d2 law; (2) a period of constant diameter due to the accumulation and partial shell formation of rigid solids and intermediates at the droplet surface; (3) a period of microexplosions resulting from heterogeneous vapor phase nucleation in or at the shell boundary. Comparing with the decomposition of the metal precursor in our previous study, the solids taking part in the shell formation in this study are from hydrolysis of highly reactive TTIP at the droplet surface. Strong and global microexplosions are observed during the combustion of TTIP/xylene–ethanol droplets and TTIP/xylene droplets. Microexplosions existing in the combustion of TTIP/ethanol droplets are moderate.
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For the first time microexplosions have been observed for the highly reactive FSP precursor. These observations are unexpected, and require the adaption of established FSP process models that assume a distilled evaporation of the precursor and the solvent. In future, these models have to take into account, the high reactivity of TTIP with water that causes reactions and particle formation on the droplet surface leading to a shell formation and microexplosions. Further investigations are needed to distinguish TiO2 nanoparticles formed by hydrolysis of the TTIP at the droplet surface and by gas-to-particle conversion, and to improve the homogeneity of TiO2 nanoparticles. Acknowledgments The authors would like to thank the Deutsche Forschungsgemeinschaft (DFG) for funding this project under grants of MA 3333/4-1. The authors also thank Horst Woyczechowski from the Foundation Institute of Material Science (University of Bremen) for providing technical supports for the droplet-on-demand generators, and Thorsten Mehrtens from the Institute of Solid State Physics (University of Bremen) for the TEM measurements. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi: 10.1016/j.proci.2016.09.017. References [1] W.Y. Teoh, R. Amal, L Mädler, Nanoscale 2 (2010) 1324–1347. [2] L. Mädler, H.K. Kammler, R. Mueller, S.E. Pratsinis, J. Aerosol Sci. 33 (2002) 369–389. [3] S.E. Pratsinis, Prog. Energy Combust. Sci. 24 (1998) 197–219. [4] L. Mädler, Kona 22 (2004) 107–120. [5] M.J. Height, L. Mädler, S.E. Pratsinis, F. Krumeich, Chem. Mater. 18 (2006) 572–578. [6] R. Strobel, S.E. Pratsinis, PCCP 13 (2011) 9246–9252. ¨ , S.E. Pratsinis, A. Sánchez-Ferrer, [7] A.J. Grohn R. Mezzenga, K. Wegner, Ind. Eng. Chem. Res. 53 (2014) 10734–10742. [8] C.D. Rosebrock, N. Riefler, T. Wriedt, L. Mädler, S.D. Tse, AIChE J 59 (2013) 4553–4566. [9] C.D. Rosebrock, T. Wriedt, L. Mädler, K. Wegner, AIChE J 62 (2016) 381–391. [10] G.A.E. Godsave, Symp. (Int.) Combust. 4 (1953) 818–830. [11] D.B. Spalding, Symp. (Int.) Combust. 4 (1953) 847–864. [12] M. Goldsmith, S.S. Penner, Jet Propul 24 (1954) 245–251.
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Please cite this article as: H. Li et al., Experimental investigation on microexplosion of single isolated burning droplets containing titanium tetraisopropoxide for nanoparticle production, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.017
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Proceedings of the Combustion Institute 000 (2016) 1–8 www.elsevier.com/locate/proci
Experimental investigation on microexplosion of single isolated burning droplets containing titanium tetraisopropoxide for nanoparticle production H. Li, C.D. Rosebrock, N. Riefler, T. Wriedt, L. Mädler∗ Department of Production Engineering, Foundation Institute of Material Science (IWT), University of Bremen, Badgasteiner Str. 3, Bremen 28359, Germany Received 3 December 2015; accepted 12 September 2016 Available online xxx
Abstract The microexplosion and nanoparticle formation of single isolated burning droplets using titanium tetraisopropoxide (TTIP) as nanoparticle precursor are investigated experimentally. Spherical along with fine agglomerated titanium dioxide (TiO2 ) nanoparticles are obtained from the single droplet combustion of TTIP dissolved in pure xylene, pure ethanol and the mixture of xylene and ethanol. Strong, global, and continuous microexplosions are observed during the combustion of TTIP/xylene droplet and TTIP/xylene– ethanol droplet. We proposed a hypothesis of droplet microexplosions as follows: hydrolysis of TTIP at or in the droplet surface leads to formation of TiO2 nanoparticles, which together with the low volatility component near the droplet surface creates an impermeable shell for the high volatility component. The high volatility component in the interior is superheated to induce heterogeneous nucleation of bubbles beneath the shell, which increases the pressure inside the shell until microexplosion occurs. These insights are very important mechanisms for the well-established flame spray pyrolysis (FSP) process. © 2016 by The Combustion Institute. Published by Elsevier Inc. Keywords: Titanium tetraisopropoxide; Isolated droplet combustion; Microexplosion; Titanium dioxide nanoparticles; Flame spray pyrolysis
1. Introduction As a promising and versatile technique for synthesizing nanoparticles, flame spray pyrolysis (FSP) takes advantage of the ability to dissolve the precursors directly in the fuel [1,2]. High tem∗
Corresponding author. Fax: +49 421 218 51211. E-mail address: [email protected] (L. Mädler).
perature pyrolysis and fast quenching rates of the near transonic spray flame result in a one-step production process of highly pure, single-component and multicomponent nanoparticles with a wide variety of elements. Because of their high purity and controlled size distribution [2,3], nanoparticles produced with FSP technique have already applications for ceramics, electronics, optics and catalysis [4–6]. During FSP synthesis, the droplets vaporize and release the nanoparticle precursors into the flame,
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Please cite this article as: H. Li et al., Experimental investigation on microexplosion of single isolated burning droplets containing titanium tetraisopropoxide for nanoparticle production, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.017
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where pyrolysis and particle formation via coagulation and sintering take place [1,2]. Due to strong interactions of the droplets, high temperatures, turbulence and the particle formation process taking only a few milliseconds in the spray flame [7], tracing of the particle formation is experimentally very challenging. To obtain fundamental insights into the mechanisms of droplet combustion in spray flames containing nanoparticle precursor solutions, simplified experiments such as the single burning droplet method have shown to be very helpful [8,9]. Many combustion studies have been carried out on single isolated droplets. The pioneering works of Godsave [10], Spalding [11], Goldsmith and Penner [12], and Wise et al. [13] lead to the classical diffusion-controlled combustion model of single-component droplet burning in an oxidizing atmosphere. This theory, referred as the d2 -law, is commonly used to describe the quasi-steady combustion of pure fuel droplets. Multicomponent droplet combustion is a more complicated process, where volatility differences and liquid-phase diffusional resistance become important, occasionally leading to microexplosions. Lasheras et al. [14] observed the disruptive burning of isolated free droplets of binary n-paraffin mixtures. Interior superheating, induced by volatility differences of fuels and slow liquid-phase mass diffusion, was attributed to the disruptive burning. Combustion of slurry droplets composed of Boron/JP-10 was studied by Takahashi et al. [15,16]. They described the disruptive burning process as three consecutive stages: (1) uniform combustion according to the d2 -law; (2) shell formation at the droplet surface due to volatility differences and pyrolysis; (3) microexplosions due to internal superheating of the high volatility component. Similar observations were made by Rosebrock et al. [8,9], who investigated the disruptive burning of precursor/solvent droplets for nanoparticle production. They showed that droplet microexplosions with continuing combustion are prerequisite for the generation of homogeneous nanoparticles [9]. Considering single isolated droplet combustion for nanoparticle production, the previous investigations mainly focus on the mechanism of droplet microexplosions of moderate reactive precursors [8], and the role of microexplosions on homogeneous nanoparticles production using cheap, low reactive metal precursors [9]. However, single isolated droplet combustion of highly reactive precursors, especially the mechanism of droplet microexplosions, still needs further investigations. Highly reactive precursors are widely used in FSP due to their ability to produce homogeneous nanoparticles with controlled size distribution. Investigations of single isolated droplet combustion of highly reactive precursors are necessary to better understand the FSP process and to further improve the particle quality. The purpose of this paper is to ex-
tend and to generalize the knowledge of microexplosions to highly reactive alkoxide precursors such as titanium tetraisopropoxide (TTIP). It is known that titanium dioxide (TiO2 ) can be formed via hydrolysis of titanium alkoxides compounds [17–19]. Considering that water is a major product during combustion, water vapor could diffuse back to the burning droplet and condense onto the surface if the boiling point of the fuel is lower than that of water and the fuel is miscible with it [20,21]. Thus, hydrolysis of TTIP to form TiO2 would be expected. This phenomenon could explain the observed micro-sized droplets from TTIP during FSP synthesis with high volatility fuels such as ethanol. The effect of TTIP on droplet combustion could provide new insights for synthesis strategies toward cost-effective FSP precursors and further improvement of the homogeneity of TiO2 nanoparticles. 2. Experimental setup and procedure The experiments were conducted under the condition of normal gravity and normal pressure. As shown in Fig. 1, the setup for single isolated droplet combustion mainly includes a piezoelectric dropleton-demand generator, ignition electrodes, a high speed camera, a gas bottle, a mass flow controller, electronics for the droplet generator and a small solvent reservoir. Pure liquids or precursor/solvent mixtures were fed from the solvent reservoir to the generator with plastic tubes. The generator produced stable isolated droplets at a frequency of 4 Hz, and these reproducible droplets moved upward in a square glass cuvette with an initial velocity from 0.5 to 1 m/s. The initial diameter of the stable droplet is dependent on the investigated liquids, the nozzle of the generator, and the parameters that control the generator. Considering the process of the atomized droplet burning initially in a pure oxygen atmosphere during FSP, the concentrated oxygen was used to provide a similar oxidizing atmosphere. Co-flowing dry oxygen (purity 99.95%) from the gas bottle flowed to the glass cuvette at
Fig. 1. Experimental setup for single isolated droplet combustion.
Please cite this article as: H. Li et al., Experimental investigation on microexplosion of single isolated burning droplets containing titanium tetraisopropoxide for nanoparticle production, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.017
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a flow rate of 0.6 L/min, controlled with the mass flow controller. The buoyancy effect on the burning droplets in our experiments can be neglected because the flame is spherical and concentric to the droplet [22,23]. Thus, the droplet combustion can be considered as spherical symmetric, which makes the classical d2 -law available to describe the burning characteristic. The droplet in the glass cuvette was ignited with a spark discharge from adjacent 100 μm diameter electrodes. The spark discharge can change the droplet trajectory and cause droplet surface oscillations at the moment of ignition. This effect was avoided by minimizing the duration and energy of the spark discharge, and adjusting the relative position between the droplet and the spark at the moment of ignition. The burning process of isolated droplet in the glass cuvette was recorded with the high speed camera at a frequency of 40,000 Hz. These images consist of 1024 X 96 pixels corresponding to a monitored area of 6.4 mm × 0.6 mm (L × W), and the merged images of droplets shown in this paper are cropped in the length direction in order to remove useless part. The droplet diameter was obtained by converting image pixels to micrometers with a Matlab-based edge detection program. The average uncertainty of the droplet diameter was ±1 pixel corresponding to ±6.25 μm. Particles produced from the single isolated droplet combustion were collected with carbon-coated copper grids (Plano GmbH-S162), which are located 5 cm above the ignition electrodes. A transmission electron microscope (TEM) Titan 80-300 (FEI) was used to provide the information of chemical composition, particle size and morphology of the particles. TTIP (Sigma Aldrich, >97% purity), xylene (VWR, 98.5% purity), and ethanol (VWR, 99.8% purity) were used to prepare precursor/solvent mixtures. Six groups of droplets with different components were studied: pure ethanol, pure xylene, and xylene–ethanol (volume ratio 1:1) droplets as well as 0.5 mol/L TTIP/ethanol, TTIP/xylene, and TTIP/xylene–ethanol (volume ratio 1:1) droplets. The setup for single isolated droplet combustion can provide stable and reproducible burning process for hours, which makes the evolution of the droplet diameter fit well with each other at the same conditions. The evolution of the normalized droplet diameter of one specific liquid in this paper is obtained by averaging the droplet diameters of at least ten droplets. 3. Results and discussion 3.1. Combustion of pure ethanol and TTIP/ethanol droplets Figure 2 shows two image sequences of a 122 μm ethanol and a 106 μm 0.5 mol/L TTIP/
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Fig. 2. Merged image sequences of isolated burning droplets: (a) an ethanol droplet with an initial diameter of 122 μm; (b) a 0.5 mol/L TTIP/ethanol droplet with an initial diameter of 106 μm. The initial droplet image is obtained at the moment of ignition. From left to right the droplets move in the upward direction in the experiment.
Fig. 3. Normalized droplet diameter for a 122 μm ethanol and a 106 μm 0.5 mol/L TTIP/ethanol droplet. The ignition of the droplet corresponds to a normalized time equal to zero with a droplet diameter to d0 .
ethanol droplet burning in pure oxygen. The time interval between each droplet image is 0.9 ms. After ignition, the ethanol droplet burns continuously with time and stops burning at the end of the image sequences (Fig. 2a). In contrast, the TTIP/ethanol droplet shows moderate microexplosions at 3.6 ms after a period of steadily burning (Fig. 2b). The droplets might slightly deviate from the straight upward direction due to the effect of the spark, drag, and co-flowing of oxygen. Figure 3 shows the evolution of the normalized diameter for the ethanol and the TTIP/ethanol droplet. The normalized droplet diameter for the pure ethanol decreases almost linearly with the time, as suggested by the d2 -law [10–13]. At approximately 0.5 μs/μm2 , the decline of the normalized droplet diameter decreases, indicating that the droplet combustion stops. This deviation from the d2 -law can be attributed to the boiling point difference between water and ethanol and their
Please cite this article as: H. Li et al., Experimental investigation on microexplosion of single isolated burning droplets containing titanium tetraisopropoxide for nanoparticle production, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.017
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Fig. 4. Image sequences of the microexplosion of a 0.5 mol/L TTIP/ethanol droplet. The time interval is measured from the moment of the first visible microexplosion.
mutual solubility. The water vapor produced during the combustion diffuses back to the droplet and condenses onto the droplet surface due to the lower boiling point of ethanol compared to that of water [20,21]. The water subsequently dissolves into ethanol due to the complete solubility between both liquids. The continuing accumulation of water during combustion decreases the amount of ethanol exposed at the droplet surface, resulting in a decrease of the vaporization rate of ethanol and subsequently a lack of heat of reaction. When the water to ethanol ratio increases substantially, the droplet burning stops. Comparing the remaining droplet volume with the initial droplet volume, the ratio is around 2%, showing that only a small amount of water vapor condenses into the ethanol droplet. The evolution of the normalized diameter of the TTIP/ethanol droplet initially coincides well with that of the ethanol droplet up to about 0.2 μs/μm2 . Then, the burning rate decreases to almost zero for a short period of time, and subsequently microexplosions are observed. The corresponding volume ratio of TTIP to ethanol in the initial droplet is 15–85. Before the microexplosion, 56% (by volume) of the droplet is consumed. The calculations of the volume ratio in this paper are under the assumption of ideal mixtures. In order to describe the microexplosion phenomena more clearly, image sequences of the microexplosion process of a 0.5 mol/L TTIP/ethanol droplet are shown in Fig. 4. The first image was captured at 2.875 ms after ignition in real experiment, and the corresponding normalized time is around 0.25 μs/μm2 . After deformation of the droplet due to microexplosion, a large bright emission is observed and the droplet ceases burning. Next, we would like to discuss two possible mechanisms in order to explain the observed microexplosions. The observed microexplosions of the TTIP/ethanol droplet can be triggered either by homogeneous or heterogeneous vapor bubble nucleation [8,14,16]. At the moment of ignition, ethanol and TTIP are exposed at the droplet surface. Due to its high volatility, ethanol is pref-
erentially evaporated from the droplet surface, which can be seen by the evolution of the normalized droplet diameter for the ethanol droplet and the TTIP/ethanol droplet in Fig. 3 (from 0 to 0.2 μs/μm2 ). However, as combustion proceeds, ethanol is continuously withdrawn from the liquid surface and the liquid-phase mass diffusion rate is at least one order of magnitude slower than the liquid-phase thermal diffusion rate [24–26], leading to an accumulation of TTIP at the surface. The increasing amount of TTIP presented at the droplet surface will be heated and vaporized, resulting in a cease of the combustion as seen from the continuously deviation between the normalized droplet diameters (Fig. 3). The surface temperature might attain values close to the boiling point (∼232 °C) of TTIP [27], which is above the boiling point (∼85 °C) and the homogenous limit of superheat temperature (∼190 °C) of ethanol [28]. It can be assumed that the droplet interior consists mostly of the initial TTIP/ethanol concentration due to slow mass diffusion [26,29]. The probability of superheating the ethanol increases, occasionally leading to bubble growth and microexplosions from homogeneous vapor phase nucleation. The other possible mechanism is similar to the microexplosions of slurry droplets [30], where three sequential steps are observed: (1) a period of continuous droplet combustion according to the d2 -law; (2) a period of constant diameter due to the accumulation and partial shell formation of rigid solids and intermediates at the droplet surface; (3) a period of microexplosions resulting from heterogeneous vapor phase nucleation in or at the shell boundary. In light of the observations made in Figs. 2 and 4, and considering that TTIP hydrolyzes in the presences of water, TiO2 particles could be generated at the droplet surface that heterogeneously superheat the high volatility ethanol adjacent to it. Moreover, as the droplet continues to vaporize, there is more TTIP hydrolysis and TiO2 particle formation and the initial porous shell might become impermeable for the high volatility component to diffuse through it. The subsequent
Please cite this article as: H. Li et al., Experimental investigation on microexplosion of single isolated burning droplets containing titanium tetraisopropoxide for nanoparticle production, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.017
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Fig. 6. Merged image sequences of isolated burning droplets: (a) a xylene–ethanol droplet with an initial diameter of 142 μm; (b) a 0.5 mol/L TTIP/xylene–ethanol droplet with an initial diameter of 139 μm. The initial droplet image is obtained at the moment of ignition.
3.2. Combustion of xylene–ethanol and TTIP/xylene–ethanol droplets
Fig. 5. TEM image of TiO2 nanoparticles from the combustion of 0.5 mol/L TTIP/ethanol droplets.
suppressed vaporization might lead to a substantial heat up of the shell due to the high melting/boiling point of the intermediate and the solid phase. Thus, heterogeneous nucleation of ethanol bubbles could be generated near the inner part of the shell, creating a pressure build-up inside the droplet that results in microexplosions. Previous studies support this mechanism [31,32]. Li et al. [31] studied the chemical reaction between an electrodynamically levitated microdroplet of titanium tetraethoxide and water, and found that the initially rapid rate of reaction leads to the formation of a TiO2 shell which inhibits and eventually stops the reaction. Ahonen et al. [32] prepared nanocrystalline titania powders via aerosol pyrolysis of titanium tetrabutoxide precursors, and observed the hydrolysis reaction with the titanium alkoxide. They suggested that water from the combustion diffuses to the droplet surface and hydrolyzes the titanium tetrabutoxide, resulting in a dense and smooth shell [32]. In the present study, spherical along with fine agglomerated TiO2 nanoparticles are observed (Fig. 5). The TiO2 nanoparticles can be formed by two routes. The preceding discussion suggests that the TiO2 particles could be produced by hydrolysis of the TTIP at the droplet surface, forming partially permeable shells that hinder the highly volatile ethanol to evaporate through the shell. Moreover, the TiO2 nanoparticles are also likely to be produced within the gas phase by gas-to-particle conversion after TTIP is released [1,9]. To point out the dominating mechanism for TTIP/ethanol droplets, the combustions of xylene– ethanol droplets as well as TTIP/xylene–ethanol droplets are investigated.
Figure 6 shows two image sequences of a 142 μm xylene–ethanol droplet and a 139 μm 0.5 mol/L TTIP/xylene–ethanol droplet burning in pure oxygen. The time interval between each droplet image is 1.05 ms. The microexplosions seem to occur solely in the TTIP/xylene–ethanol droplet burning process, whereas the xylene/ethanol droplet burns continuously. The evolution of the droplet diameter for ten xylene–ethanol droplets and ten 0.5 mol/L TTIP/xylene–ethanol droplets are shown in Figs. S1 and S2, respectively. It can be seen in Fig. S1 that the ten curves of xylene–ethanol droplets coincide very well. Moreover, the ten curves of TTIP xylene–ethanol droplets coincide well and the occurring times of microexplosions are almost the same for these ten droplets (Fig. S2). Both figures can support the highly reproducible droplet combustion, and nearly the same occurring time of microexplosions confirms that the observed droplet microexplosions are not stochastic compared to that induced by homogeneous nucleation. Quantitative results are shown in Fig. 7, where the evolution of the normalized droplet diameter for the xylene–ethanol droplet and the 0.5 mol/L TTIP/xylene–ethanol droplet are plotted. The xylene–ethanol droplet burns according to the d2 law, which shows that the homogeneous vapor bubble nucleation mechanism for the xylene–ethanol droplet case does not take place. In contrast, several strong, global, and continuous microexplosions are observed during the combustion of the TTIP/xylene–ethanol droplet. After around 56% (by volume) of the droplet is consumed, the first microexplosion occurs, coinciding with that of the TTIP/ethanol droplet case. In comparison to the TTIP/ethanol droplet, the microexplosions of the TTIP/xylene–ethanol droplet are stronger because the high boiling point xylene is taking part in the shell formation, leading to a more rigid shell. Image sequences of the first microexplosion are shown in Fig. 8. The black object with the same size in each image sequence is caused by a contamination on the lens of the high speed
Please cite this article as: H. Li et al., Experimental investigation on microexplosion of single isolated burning droplets containing titanium tetraisopropoxide for nanoparticle production, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.017
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Fig. 9. Merged image sequences of isolated burning droplets: (a) a xylene droplet with an initial diameter of 86 μm; (b) a 0.5 mol/L TTIP/xylene droplet with an initial diameter of 91 μm. The initial droplet image is obtained at the moment of ignition.
3.3. Combustion of pure xylene and TTIP/xylene droplets
Fig. 7. Normalized droplet diameter for a 142 μm xylene–ethanol droplet and a 139 μm 0.5 mol/L TTIP/xylene–ethanol droplet. The ignition of the droplet corresponds to a normalized time equal to zero with a droplet diameter to d0 .
camera that can serve as a reference. In Fig. 8, especially at 0.05 and 0.75 ms, it can be seen that explosive fragments are ejected from the outer layer of the droplet, which supports the existence of the shell and heterogeneous vapor phase nucleation beneath the shell. Ejected fragments are ignited to release a bright light. Figure S3 of supplemental material (SM) shows spherical TiO2 nanoparticles probably formed at the droplet surface and in the gas phase. The residual liquid of the droplet forms a smaller droplet due to surface tension, and the second microexplosion occurs after a short period of time. In average, five microexplosions have been observed in the burning of the 0.5 mol/L TTIP/xylene–ethanol droplet. Image sequences of the second, third, fourth, and fifth microexplosion are shown in Fig. S4.
Figure 9 shows image sequences of an 86 μm xylene droplet and a 91 μm 0.5 mol/L TTIP/xylene droplet burning in pure oxygen. The time interval between each droplet image is 0.75 ms. While the xylene droplet burns continuously until the fuel is completely consumed, the TTIP/xylene droplet undergoes strong microexplosion after a short period of stable burning, as seen from the bright emissions in Fig. 9b. The spherical flame can be seen in the second, third, and fourth images in Fig. 9b. It is necessary to be noted that the visibility of the flame is restricted by soot formation of the investigated liquids and the intensity of the background light. Because of the strong intensity of the background light, it is difficult to observe the flames in this paper. In order to show the spherical flame clearly, images obtained from the high speed camera are colored according to the luminance of the grayscale image (Fig. S6). In Fig. S6c and d, the spherical flames of a 0.5 mol/L TTIP/xylene–ethanol droplet and a 0.5 mol/L TTIP/xylene droplet are marked by white circles. Figure S7 shows the evolution of the normalized droplet diameter for the xylene droplet and the 0.5 mol/L TTIP/xylene droplet. The burning process of the xylene droplet follows the d2 -law. During the period of stable burning, the curve of TTIP/xylene droplet coincides with that of the xylene droplet from 0 to 0.3 μs/μm2 . Subsequently,
Fig. 8. Image sequences of the first microexplosion of a 0.5 mol/L TTIP/xylene–ethanol droplet. The time interval is measured from the occurrence of the first visible microexplosion. Arrow indicates a contamination on the lens which serves as a reference.
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the droplet diameter stops decreasing, reaching a short period of constant diameter, which could be attributed to the formation of the shell. It can be seen from Fig. S7 that the first explosion of the TTIP/xylene droplet occurs at 0.33 μs/μm2 , which is later than both of the TTIP/ethanol droplet and the TTIP/xylene– ethanol droplet. Since xylene is not soluble to water and has a higher boiling point than water, TTIP cannot readily react with the water vapor from the combustion. Comparing to TTIP/ethanol droplets, the strong microexplosion of the TTIP/xylene droplet is due to the large accumulated amount of the high boiling point TTIP in the shell, which makes the shell rigid and hard to destroy. Image sequences of two sequential microexplosions of the TTIP/xylene droplet, and the spherical TiO2 nanoparticles produced from these combustion events are shown in Figs. S8 and S9, respectively.
4. Summary and conclusions In order to understand the microexplosion mechanisms of single isolated droplets using TTIP as precursor, six groups of droplets with different components have been investigated. By comparing image sequences, normalized droplet diameter and TEM images of TiO2 nanoparticles from the isolated burning droplets, the following conclusions can be drawn: Spherical along with fine agglomerated TiO2 nanoparticles are obtained during combustion of single isolated droplets containing TTIP as precursor independent of the solvent/fuel used. The TiO2 nanoparticles could be generated from the reaction between TTIP and water vapor at the droplet surface, and gas-to-particle conversion both by the undisturbed vaporization of TTIP from the solution and by TTIP release during the microexplosions. The microexplosion mechanism of single isolated droplets containing TTIP could be described as the three sequential steps: (1) a period of continuous droplet combustion according to the d2 law; (2) a period of constant diameter due to the accumulation and partial shell formation of rigid solids and intermediates at the droplet surface; (3) a period of microexplosions resulting from heterogeneous vapor phase nucleation in or at the shell boundary. Comparing with the decomposition of the metal precursor in our previous study, the solids taking part in the shell formation in this study are from hydrolysis of highly reactive TTIP at the droplet surface. Strong and global microexplosions are observed during the combustion of TTIP/xylene–ethanol droplets and TTIP/xylene droplets. Microexplosions existing in the combustion of TTIP/ethanol droplets are moderate.
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For the first time microexplosions have been observed for the highly reactive FSP precursor. These observations are unexpected, and require the adaption of established FSP process models that assume a distilled evaporation of the precursor and the solvent. In future, these models have to take into account, the high reactivity of TTIP with water that causes reactions and particle formation on the droplet surface leading to a shell formation and microexplosions. Further investigations are needed to distinguish TiO2 nanoparticles formed by hydrolysis of the TTIP at the droplet surface and by gas-to-particle conversion, and to improve the homogeneity of TiO2 nanoparticles. Acknowledgments The authors would like to thank the Deutsche Forschungsgemeinschaft (DFG) for funding this project under grants of MA 3333/4-1. The authors also thank Horst Woyczechowski from the Foundation Institute of Material Science (University of Bremen) for providing technical supports for the droplet-on-demand generators, and Thorsten Mehrtens from the Institute of Solid State Physics (University of Bremen) for the TEM measurements. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi: 10.1016/j.proci.2016.09.017. References [1] W.Y. Teoh, R. Amal, L Mädler, Nanoscale 2 (2010) 1324–1347. [2] L. Mädler, H.K. Kammler, R. Mueller, S.E. Pratsinis, J. Aerosol Sci. 33 (2002) 369–389. [3] S.E. Pratsinis, Prog. Energy Combust. Sci. 24 (1998) 197–219. [4] L. Mädler, Kona 22 (2004) 107–120. [5] M.J. Height, L. Mädler, S.E. Pratsinis, F. Krumeich, Chem. Mater. 18 (2006) 572–578. [6] R. Strobel, S.E. Pratsinis, PCCP 13 (2011) 9246–9252. ¨ , S.E. Pratsinis, A. Sánchez-Ferrer, [7] A.J. Grohn R. Mezzenga, K. Wegner, Ind. Eng. Chem. Res. 53 (2014) 10734–10742. [8] C.D. Rosebrock, N. Riefler, T. Wriedt, L. Mädler, S.D. Tse, AIChE J 59 (2013) 4553–4566. [9] C.D. Rosebrock, T. Wriedt, L. Mädler, K. Wegner, AIChE J 62 (2016) 381–391. [10] G.A.E. Godsave, Symp. (Int.) Combust. 4 (1953) 818–830. [11] D.B. Spalding, Symp. (Int.) Combust. 4 (1953) 847–864. [12] M. Goldsmith, S.S. Penner, Jet Propul 24 (1954) 245–251.
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Please cite this article as: H. Li et al., Experimental investigation on microexplosion of single isolated burning droplets containing titanium tetraisopropoxide for nanoparticle production, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.017