The Addition of Urea in the Ultrasonic Spray Pyrolysis Process and its Influence on the Morphology of the Products Obtained

Transcription

1 The Addition of Urea in the Ultrasonic Spray Pyrolysis Process and its Influence on the Morphology of the Products Obtained Maxymme Mendes de Melo Federal Institute of Education, Science and Technology of Rio Grande do Norte Santa Cruz/RN Brazil Mara Tatiane de Souza Tavares Fabiana Villela da Motta Maurício Roberto Bomio Delmonte Melise Carina Duarte de Almeida Materials Testing Laboratory Center for Gas Technology and Renewable Energy Andréa Santos Pinheiro de Melo Materials Testing Laboratory Center for Gas Technology and Renewable Energy Carlos Alberto Paskocimas Abstract The ultrasonic spray pyrolysis technique is a method of only one step for continuous production of powder and easy control of the parameters. Despite its simplicity, there are at least five parameters that can directly affect the final product, as follows: (i) the carrier gas, (ii) flow of the carrier gas, (iii) pyrolysis, (iv) concentrating the solution precursor, and (v) the presence of additives. With so many variables, the results can be quite different. Thus, this study aims to evaluate the influence of the addition of urea on the morphology of the product formed. For the experiments, there was a 0.25 M solution of aluminum nitrate prepared and urea was added to it in the following concentrations: 0.25, 0.50, 1, 2 and 4 M. The carrier gas was air with a flow of 4 L/min. The pyrolysis temperature was 1000 C and the product was collected in an electrostatic precipitator. The morphology of the powders was evaluated by SEM and particle size distribution was made by software based on the images obtained. It was found that the presence of urea causes contraction and generates holes in the particles. The average particle diameter does not change significantly and about 90% of all concentrations were in the submicron range. Keywords: ultrasonic spray pyrolysis, particles, urea, morphology. 1 I. INTRODUCTION The ways to synthesize powders can be divided into five, namely/called: (i) chemical via wet method, such as sol-gel, microemulsion, hydrothermal synthesis, chemical precipitation, etc.; (ii) mechanical preparation such as milling, or friction; (iii) gaseous methods such as chemical or physical vapor deposition; (iv) methods of liquid phase spray, such as molten metal spray, spray pyrolysis, spray drying; and (v) methods of liquid/gas phase, such as spray pyrolysis by combustion [1]. For the powders presented with submicron dimensions or in some cases nanoscale, they can be synthesized in both liquid and gas phase [2]. Thus, many simpler chemical methods such as Pechini, the sol-gel method, emulsion method, and the citric acid method, have been used due to the disadvantages of solid state reactions. However, these methods often involve complicated preparations in order to obtain the correct stoichiometry and still require calcination powder to obtain high crystallinity [3]. The liquid routes are processes which involve a large amount of solvents. Another disadvantage is the impurities, since they

2 are more present in liquids than in gases, and most of the advanced process use surfactants that contaminate the produced particles. On the other hand, the gas phase methods that form aerosols offer high purity. In addition, techniques using aerosol reduce waste generation which makes them more attractive for large scale production [2]. The techniques based on the use of sprays are simple methods of only one step, suitable for the production of a wide range of powders with controlled properties for special applications, especially in the chemical, material and food industries. These methods include: spray drying, spray pyrolysis, spray pyrolysis by combustion and spray of molten metal [1, 4]. Among these, spray pyrolysis and spray drying are the two most common methods. They are techniques that have advantages of excellent reproducibility, being continuous processes, for presenting an excellent potential for large scale production and require a simple apparatus in comparison to other techniques. The spray pyrolysis technique can be used for large production of materials in powder form including metals, metal oxides, non-oxidized ceramics, superconductor materials and nanophase materials [1, 5, 6]. The spray pyrolysis technique consists of five basic steps: (i) generating a mist using a generator of droplets from a precursor liquid; (ii) transporting the spray by a flow of gas during the evaporation of the solvent which occurs concurrently with the precipitation of the solute when the solubility limit is exceeded in the droplets; (iii) thermolysis of precipitated particles at elevated temperatures to form porous micro/nanoparticles; (iv) intra-particle sintering to form dense particles; (v) finally, the particle extraction [4, 7]. The precursor solution may be atomized by a pneumatic or ultrasonic nebulizer, but the latter produces more uniform size droplets [6, 8, 9]. When this is used, the technique is called ultrasonic spray pyrolysis (USP). At this stage of the process, a mist is created and then carried by a carrier gas into the reactor and heated by a horizontal electric furnace. For the mist to be converted into solid oxides, the following phenomena will take place: solvent evaporation, precipitation of the solution, its drying, heating and sintering [10]. Thus, the higher the gas flow, the shorter is the mist passage to the reactor, and consequently, all of these steps, especially the sintering may occur in a different way [11]. Several authors have discussed theoretical models about the stages through which the droplet passes to become a particle [12-15]. Fig. 1 shows a general diagram of what occurs with the droplets in the processes of Spray Drying and Pyrolysis [1]. In general, thermolysis of aerosol solutions lasts 2 to 20 seconds [13]. Figure 1. Basic stages of the Spray Drying and Pyrolysis methods [1] Although spherical, most of the particles produced by USP may present other morphologies, namely: spherical surfaces with holes in the middle, fractured in an ordered or disordered way and also contracted [16]. Given the multiple possibilities of particle morphology generated by the USP process, this study aims to evaluate the influence of adding urea in determining the final morphology of the particles. Thus, particles of amorphous alumina were synthesized by USP from an aluminum nitrate solution with various additions of urea. II. EXPERIMENTAL A. Synthesis of particles To synthesize the particles, a solution of aluminum nitrate (Al(NO 3) 3.9H 2O) was prepared in distilled water at a concentration of 0.25 M, and variations of urea concentration (CH 4N 2O) as shown in Table I. TABLE I. VARIATION OF THE UREA CONCENTRATION IN THE ALUMINUM NITRATE SOLUTIONS. Aluminum nitrate concentration (M) 0.25 Urea concentration (M) Fig. 2 illustrates the USP process followed in this work. The mist was generated by an ultrasonic nebulizer at 2.4 MHz and transported by compressed air flow of 4 L/min until the quartz reactor with 3.7 cm 3.7 cm of internal diameter and is 150 cm long in length of which 30 cm inside the electric oven and heated to 1,000 o C. After leaving the reactor, the product generated was collected in an electrostatic precipitator powered by a 5 kv DC source. The exposure time of the mist in the reactor was around 24 seconds. Figure 2. Schematic diagram of the experimental apparatus for spray pyrolysis ultrasonic used B. Analysis of particles generated The material produced had its morphology analyzed by scanning electron microscopy on a Shimadzu SSX-550 SEM. In order to determine the average particle size, Image Pro Plus 6.0 software was used to evaluate the SEM images generated. From the measurements obtained, we used the software Matlab to generate curves of granulometric distribution for each set of particles produced. III. RESULTS AND DISCUSSION Figs. 3 to 8 show the images obtained by SEM together with the charts of the particle size distribution obtained from counting the particles seen in the images. The curves of the particle size distribution indicate that the produced particles had a modal distribution in all cases. 2

3 Fig. 3, which displays the images of the material that was synthesized without the addition of urea, shows that virtually all the particles have spherical shape, low rugosity and without any holes or irregularities in the topography. Although the distribution is modal, it is possible to observe a significant dispersion in the particle size. Figure 3. SEM micrographs of the sample without urea in two magnifications and the distribution curve of the particle size All products containing urea, regardless of concentrations, showed different morphologies. In the product with 0.25 M (Fig. 4) the presence of holes (indicated by arrows) in some particles was noticed, however the surface appears smooth, spherical and with low rugosity in the majority. These holes occur during its formation when the wall of the particle does not show uniform thickness. Thus, during the evaporation step, and while the solvent remaining in the interior of the particle tends to evaporate, it pushes out the wall [10], as illustrated in Fig. 9, and finding no opened channels, ends up breaking the thinnest part, creating such holes [16]. As can be seen, the mass flow of gases from the pyrolysis acts in pushing the wall of the particle in formation, while the flow of heat follows from the outside to inside the particle [14, 17]. Figure 4. SEM micrographs of the sample with 0.25 M of urea in two magnifications and the distribution curve of the particle size Figure 5. SEM micrographs of the sample with 0.5 M of urea in two magnifications and the distribution curve of the particle size 3

4 Figure 6. SEM micrographs of the sample with 1 M of urea in two magnifications and the distribution curve of the particle size Figure 7. SEM micrographs of the sample with 2 M of urea in two magnifications and the distribution curve of the particle size Figure 8. SEM micrographs of the sample with 4 M of urea in two magnifications and the distribution curve of the particle size As the urea solution was added the precursor to, generated products began to show less spherical and more rugous particles, with holes or contracted. The concentration of 0.5 M (Fig. 5) generated more holes than 0.25 M and caused the moderate contraction of a number of particles, but also kept various particles smooth and spherical. The same happened with the concentration of 1 M urea (Figure 6), in which the presence of contracted particles was greater than the concentration of 0.5 M. In Fig. 7, which displays the images of the particles with the addition of 2 M urea, there was a drastic reduction in the presence of particles with spherical and smooth 4 shape, being largely full of surfaces with holes or contractions. The arrow pointing to a smaller particle that was adhered to a larger particle and is contracted may be explained by the following: during heating of the droplets and the consequent decomposition of the solvent, the presence of urea in the solution generates gas production which may further increase the internal pressure of the particle being formed [18]. From this phenomenon, two situations may occur: (i) swelling and subsequent rupture of the interconnecting structure of the primary particles present in each droplet while still forming the particle, generating even smaller particles [18]; or (ii) swelling of

5 the structure and resisting buildup of internal pressure by the crust formed in the droplet, followed by the contraction of the particle when it later exits from the reactor caused by condensation of the gases inside the particle [19]. The arrows in Fig. 8 (4 M) show various particles that have suffered large shrinkage during pyrolysis, stating that for the variation of urea investigated the more urea that was added, the greater the amount of produced particles contracted. Table II gives the mean particle diameter and the submicrometric percentage of particles for each synthesized material. Despite differing values, the results indicate that varying the concentration of urea in the precursor solutions to the range used in this study barely affects the particle size, although it significantly interferes in the morphology of the particles. Figure 9. Illustration of the flow of heat and mass acting on particle formation during pyrolysis [14] TABLE II. AVARAGE PARTICLE SIZE AND PERCENTAGE LESS THAN 1000 nm DUE TO THE VARIATION OF THE CONCENTRATION OF UREA IN THE SOLUTIONS. Urea concentration (M) Mean diameter (nm) < 1000 nm (%) of contracted particles was increasingly more as more urea was added to the precursor solution. From the dimensional point of view, the average particle diameter did not significantly change, and about 90% of all concentrations were in the submicron range, as the curves of the particle size distribution remained in the same format, simple modal. REFERENCES [1] M. Eslamian, M. Shekarriz. Recent Patents on Nanotechnology. 3, (2009). [2] G. Biskos, V. Vons, C. U. Yurteri, A. Schmidt-Ott. KONA Powder and Particle Journal, 26, (2008). [3] I. Taniguchi. Mat. Chem. and Phys. 92, (2005). [4] N. Reuge, B. Caussat. Comput. and Chem. Eng. 31, (2007). [5] Y. C. Kang, S. B. Park. J. Mat. Sci. 31, (1996). [6] A. Dalmoro, A. A. Barba, M. D amore. The Scientific World Journal, Article ID (2013). [7] C. H. Lu, T. Wu, H. Wu, M. Yang, Z. Guo, I. Taniguchi. Mat. Chem. and Phys. 112, (2008). [8] B. Bittner, T. Kissel. T. Journal of Microencapsulation 16, 3, (1999). [9] K. Park, Y. Yeo. Patent. US (2003). [10] Z. Bakenov, I. Taniguchi. Solid State Ionics. 176, (2005). [11] W. Widiyastuti, S. Y. LEE, F. Iskandar, K. Okuyama. Advan. Powd. Tech. 20, (2009). [12] S. Nesic, J. Vodnik. Chem. Eng. Sci. 46, 2, (1991). [13] G. V. Jayanthi, S. C. Zhang, G. L. Messing. Aerosol Science and Technology 19, 4, (1993). [14] M. Mezhericher, A. Levy, I. Borde. Chem. Eng. and Proc. 47, (2008). [15] M. Mezhericher, A. Levy, I. Borde. Chem. Eng. Sci. 66, (2011). [16] M. Eslamian, N. Ashgriz. J. Eng. Mat. and Tech. 129, Janeiro (2007). [17] M. Eslamian, M. Ahmed, N. Ashgriz. Drying Technology 27, 1, 3-13 (2009). [18] N. Ashgriz, Handbook of atomization and sprays theory and applications, Springer, 849 (2011). [19] P. Seydel, A. Sengespeick, J. Blömer, J. Bertling. Chem. Eng. Tech. 5, 27, (2004). IV. CONCLUSIONS Based on the results obtained, it was found that the presence of urea in the precursor solution generates holes and causes contractions in the synthesized particles. Only a few particles had some holes observed at a concentration of 0.25 M urea. In turn, the concentration of 0.5 M made several particles undergo moderate contraction in addition to generating more holes than 0.25 M, although various smooth and spherical particles still remained. Thus, in all other concentrations tested, the presence of smooth spherical particles was increasingly fewer and the presence 5