Low temperature aerosol synthesis (LTAS) of nanostructured alumina particlesM.E. Rabanal1*, M.I. Martín1, L.S. Gómez1, J.M. Torralba1, O. Milosevic2 1 2 University Carlos III of Madrid, Avda. de la Universidad, 30, 28911 Leganés, Madrid, Spain Institute of Technical Sciences of Serbian Academy of Sciences and Arts, K. Mihajlova 35/IV, 11000 Belgrade, Serbia and chemical reaction occur in a dispersed phase and in a single step, there is a possibility to control the important particle properties (size, morphology, chemical composition, etc.) by simple controlling the process parameters (residence time, decomposition temperature)[7-10]. This paper describes the low temperature aerosol synthesis (LTAS) of nanostructured alumina particles for the applications in nanoreinforcements of metalmatrix composites. For that purpose, the conditions for the production of high-purity nano-particles, spherical and free of aggregates will be optimized by assessing the influence of processing parameters (ultrasound frequency, decomposition temperature, residence time, solution properties) on the morphology and structural properties of the nanopowders. Abstract Metal matrix composites (MMCs) having fine-scale and uniformly dispersed phases, are of great technological interest because of improved mechanical properties, particularly the hardness, wear resistance, elastic modulus and yield strength. The addition of ceramic alumina nanoparticles into metal matrix composites might have a huge effect for their implementation into automotive, defense and aerospace application. This paper will present the preparation of nanostructured spherical alumina particles (< 500 nm sized) by low temperature aerosol synthesis (LTAS) for the application in MMCs reinforcement. Synthesis procedure includes aerosol formation ultrasonically from alumina nitrate water solution and its decomposition into a tubular flow reactor at 400ºC. Consequently, as-obtained particles are spherical, smooth, amorphous and in non-agglomerated state. The phase crystallization, either to γ or α-Al2O3 is promoted by additional thermal treatment in the range from 900 to 1300ºC. Detailed phase and structural analysis were proceeded in accordance to X-ray powder diffraction, and electron microscopy (SEM/EDS and TEM). Keywords: Alumina; Nanoparticles; Spray Pyrolysis; Ceramic Materials. Experimental Particle preparation The spray pyrolysis experimental set-up consists of an ultrasonic nebulizer, a quartz tube located inside a cylindrical furnace and a particle collector (Fig. 1). The fine drops of precursor solutions were carried out by the airflow regulated with a flow controller. Al2O3 fine particles were synthesized by pyrolysis of an aerosol generated by ultrahigh frequency of aluminium nitrate aqueous solution: Al(NO3)3·9H2O (0.1 M) according to the following experimental conditions: furnace temperatures: 400 ºC, gas flow rate: 1.5 l/min, frequency of the ultrasonic atomizer (RBI, France): 2.1 MHz, carrier gas: pure air. Introduction Small aluminium oxide nanoparticles have important applications in the ceramic industry[1,2] and can be used as an abrasive material, in heterogeneous catalysis, as an absorbent, a biomaterial and as reinforcements of metalmatrix composites (MMCs)[3-5]. In order to be used for effective discontinuous reinforcements in a continuous metal matrix, several structural and morphological aspects for Al2O3 particles have to be fulfilled: small particle size and narrow size distribution, large surface area, spherical morphology and the absence of agglomerates. When the hot wall aerosol synthesis method (spray pyrolysis), as a basically chemical route for obtaining a versatility of advanced materials is considered, several advantages over conventional synthesis have been reported for the preparation of well defined oxide powders[6]. The technique is based on the generation of micrometric-sized aerosol droplets by ultrasonic waves and their decomposition at intermediate temperatures (400-800ºC). Due to the precipitation, decomposition Fig. 1. Schematic representation of the processing equipment. 1 3a shows SEM micrographs of as prepared Al2O3 particles.758 Å. corresponding to the (113).99Å). Data were collected in the 2θ ranges from 10 to 70° in stepscanning mode with a step size of 0. 3b) prove high compositional uniformity and the single presence of the constitutive elements. although the shape of the diffractograms is still broad. only well defined peaks of α-Al2O3 phase are presented. (116) and (104) reflections. after thermal treatment at 900ºC/12h begins to appear the change in the amorphous behaviour coming out with the low intensities reflections corresponding to the γ-Al2O3 phase (JCPDS 10-0425. However. In both as prepared and thermally treated powder samples (≤800ºC) the amorphous character is typically reflected in the shape of the diffractograms. After this temperature the peaks begin to broaden probably caused by the appearance of the second phase that is obviously coexistent with the γ phase. Results and discussion Fig.u. are also detected.4º and a peak over 35º. the powders were annealed isothermally at 700-1300ºC for 12 h in a chamber furnace in air (CHESA). It is obvious that particle morphology does not change significantly with annealing.04° and a counting time of 2. a = 4. Two phases are formed as a result of the annealing process. (a) (b) Fig. 3: SEM micrograph of as prepared Al2O3 particles (a) and the corresponding EDS analysis (b).924Å). S. the small peaks corresponding to the SiO2 residues of the reactor quartz tube.) (222) (222) (311) 400 300 200 100 0 20 30 * (400) γ γ 800 ºC * 700 ºC * as prepared 40 50 60 2θ Fig. S.G. can be easily identified.8º and 67º. Powders persist in their un-agglomerated form although high temperature regime provokes further crystallisation and growth of the primary particles[6]. Based on the low intensity peaks at approximately 2θ 43º. where the (400) and the (440) planes maximums at 2θ 45. 2. At 1100ºC the main peaks of γ phase are clearly defined. 4 corresponds to SEM images taken with secondary electron mode detector for the thermally treated powder samples at 700-1200º C during 12 h. 2 shows X-ray diffraction patterns for as-prepared and thermally treatment powder samples. non-aggregated and relatively uniform in size (below 500 nm). 400kV. Fig. 2 (440) (440) 500 * γ γ γ γ (440) γ (400) (400) γ (311) (311) 600 * (220) (220) γ γ γ (440) (110) 700 (222) (113) (400) α γα γ (311) Particle characterization γ γ γ (300) α α (116) 900 (012) (113) After synthesis. respectively. and using a CuKα source.) α 1200 ºC 1100 ºC 1000 ºC 900 ºC γ γ γ (111) The crystal structure of the as-prepared and thermally treated powders was analysed by X-ray diffraction (XRD) collected in an automatic X’ Pert Philips diffractometer.70 s per step. Compositional homogeneity and particle morphology were analysed by scanning electron microscopy (SEM/EDS) on a Philips XL Series XL 30 and transmission electron microscopy (TEM. c = 12. Increasing the temperature begins to resolve other maximums related to the γ phase. Experimental X-ray diffraction patterns for asprepared and thermally treatment powder samples. After 1300 º/12 hours annealing. Besides the main phases encountered in the sample. JEOL-JEM. the maximums identified correspond to the α-Al2O 3 phase (JCPDS-42-1468.JCPDS-ICDD[12]. 227 a = 7. 1000 α: α-Al2O3 γ: γ-Al2O3 α α α α 1300 ºC α 70 . Fig. 57. respectively. G. EDS analysis (Fig. Crystalline phases were identified and indexed using the software X-Ray Diffraction Philips Analytical[11] and the Pcpdfwin database . 167. smooth. It can be seen that as-prepared particles derived through aerosol decomposition are highly spherical.(104) (110) (024) *: SiO2 (214) 800 * (220) (104) α (116) α Intensity (a. the latter prevailing at 1300ºC/12 h. (c) (d) Fig. 6 shows a low magnification in bright field mode of the sample annealed at 1100ºC/ 12 h confirming the spherical character in the secondary nanoparticle with a diameter of 331 nm. MAT2006-02458 project. Conclusions The low temperature aerosol synthesis (LTAS) of fine Al2O3 particles (<500 nm) followed by XRPD and SEM characterization is presented.822 ±6. S. Milosevic) as well as The Ministry of Science and Environment Protection.A.I. 5: SEM micrograph of sample annealed at 1300ºC/12 h: (a) secondary electrons (SE). unagglomerated. Juan de la Cierva Program JCI-2005-1892-13 (M.5 nm. XRPD shows the presence of two polycrystalline phases after high temperature treating: γ and α. Fig. Republic of Serbia (Project No 142010). Also the collaboration of the Electron Microscopy Center “Luis Bru” for the attendance in TEM .: Structural transformations of alumina by hight energy ball milling. Hench. Zielinski. 1487-1510. smooth. P. (b) 800ºC/12 h. (d) 1200ºC/12 h. R. 5 showing spherical and non-aggregated particles. Journal of Materials Research 8(11) (1993).(a) (c) Fig. 3 .L. Fig. Kaliaguine et al. (a) Acknowledgements (b) The authors gratefully appreciate the financial support of the Ministry for Education and Science of Spain. Individual primary particles can be resolved with a diameter mean of 17. Microscopic observations indicate that obtained Al2O3 particles are spherical. High temperature (700-1300ºC) annealing did not influence significantly on the particle morphology. Bioceramics: from concept to clinic: Journal of American Ceramic Society 74 (1991). 4: Scanning electron micrographs of the thermally treated powder samples: (a) 700ºC/12 h. Martín). 2985-2992. Particle morphology does not change significantly with thermal treatment at 1300ºC/12 h as being implied at Fig. Schulz. having narrow size distribution. Such well controlled and defined particle’s properties are presumably applicable to be used as a discontinuous reinforcement for MMCs. 6: Low magnification TEM micrograph in bright field mode of the sample annealed at 1100ºC/12 h. The as-prepared particles and samples treated until 800ºC are amorphous. and Sabatic Grant SAB 2004-0035 (O. L. 2. References 1. (b) backscattered electrons (BSE). 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