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Continuous production of γ-Fe2O3 ultrafine powders by laser pyrolysis
May 1998
Materials Letters 35 Ž1998. 227–231
Continuous production of g-Fe 2 O 3 ultrafine powders by laser pyrolysis S. Veintemillas-Verdaguer ) , M.P. Morales, C.J. Serna Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain Received 18 July 1997; revised 13 October 1997; accepted 14 October 1997
Abstract Pure g-Fe 2 O 3 particles were prepared by a continuous process from cw CO 2 laser induced pyrolysis of a 30% solution of iron pentacarbonyl in isopropanol with a yield of about 50% and an average productivity of 0.05 grh. From TEM the particle size is 5 " 2 nm with a low degree of aggregation, which agrees with the size obtained from the width of the X-ray diffraction peaks. The nanoparticles seem to be well crystallised according to his infrared spectrum. Moreover, superparamagnetic behaviour was observed at room temperature with a saturation magnetisation value of 30.5 emurg. q 1998 Elsevier Science B.V. All rights reserved. PACS: 81.35.q k; 82.30.Lp; 75.60.Jp; 42.55.Em Keywords: Laser pyrolysis; Ultrafine powders; IronŽIII. oxide; g-Fe 2 O 3 ; Maghemite; Superparamagnetism
1. Introduction Laser-induced processes for the continuous production of nanometric powders was initiated by Cannon w1,2x. Since then several workers obtained the Si, SiC, Si 3 N4 and composite SirCrN powders under a variety of conditions, with sizes ranging from 5 to 20 nm with narrow size distribution, no aggregation and high surface area w3,4x. In the field of magnetic nanoparticles the continuous laser synthesis of Fe 3 C, Fe 7 C 3 , a-Fe w5x, g-Fe w6x and g X-Fe 4 N w7x has been reported. Here we report the continuous formation of g-Fe 2 O 3 ultrafine particles by a cw CO 2 laser pyrolysis method. In addition, structural characterisation ) Corresponding author. Tel.: q34-1-3349061; fax: q34-13720623; e-mail: [email protected].
of the obtained particles and the effect on their magnetic properties is also included.
2. Experimental The CO 2 laser pyrolysis technique w1,2x, see Fig. 1, has a small reaction zone defined by the overlap between the vertical reactant gas stream and the horizontal laser beam. The reaction zone is safely away from the chamber walls. This design provides an ideal environment for the nucleation of small particles in the nanometer range, with less contamination and narrower size distribution than those prepared by more conventional thermal methods w8,9x. A dissolution of 30% FeŽCO.5 in isopropanol was nebulized by means of a ultrasound Ž15 W, 8 kHz.
00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 7 - 5 7 7 X Ž 9 7 . 0 0 2 5 1 - 6
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S. Veintemillas-Verdaguer et al.r Materials Letters 35 (1998) 227–231
and the cloud formed partially dragged by the air stream Ž6 sccm.. The mixture then passed vertically out of a stainless steel nozzle Ždiameter 1.5 mm. inside the 6-way stainless steel cross Žchamber. and intersected a horizontal unfocused CO 2 laser beam ŽSYNRAD Duo-Lase model 57-2-208 W., operated at 100 W and with a spot diameter of 4 mm. The reaction zone was protected by a coaxial flow of 30 sccm Ar gas Ž99.999%.. The energy coupling of the laser to the reactant mixture is realized by the over-
lap of the laser wavelength Ž10.60 " 0.05 m m. to the isopropanol band at 10.50 " 0.08 m m, see Fig. 2. A flux of 770 sccm of Ar gas was employed to avoid deposition of the powder on the ZnSe window. The powder was collected by means of a stainless steel filter of 0.05 m m of pore size. The chamber pressure Ž400 mbar. was controlled through a diaphragm valve located between the reaction chamber and a mechanical vacuum pump. The productivity ranged between 0.033 to 0.096 grh and the yield, calculated as
Fig. 1. CO 2 laser pyrolysis system. Ž1. Laser beam, Ž2. ZnSe window, Ž3. water refrigerated aluminium target, Ž4. nozzle, Ž5. pressure gauge, Ž6. ultrasonic bath, Ž7. 30% iron pentacarbonyl solution in isopropanol, Ž8. not return valve, Ž9. ball valve, Ž10. three ways ball valve, Ž11. argon rotameter, Ž12. massic controller of air flux, Ž13. stainless steel filter to collect the produced powders, Ž14. heating resistance, Ž15. pressure controller valve, Ž16. rotary vacuum pump, Ž17. filter to capture oil mist.
S. Veintemillas-Verdaguer et al.r Materials Letters 35 (1998) 227–231
229
Fig. 2. IR spectrum of the dissolution of 30% FeŽCO.5 in isopropanol showing the near coincidence between the absorption band of isopropanol and the emission line of CO 2 laser.
grams of product obtained from 100 g of FeŽCO.5 pyrolysed, between 12 to 84%. The variations of productivity and yield are mainly coming from changes in nebulization conditions which produced reactive clouds with different droplet densities. The colour of the obtained powders was brown being
slightly darker in some samples probably reflecting an incomplete oxidation of the initially formed magnetite to maghemite. The particles were characterized by means of elemental analysis ŽPerkin Elmer 2400CHN., X-ray diffractometer ŽPhilips PW1710. with Cu K a radiation, Infrared spectroscopy ŽIR.
Fig. 3. XRD pattern of ultrafine maghemite powder.
230
S. Veintemillas-Verdaguer et al.r Materials Letters 35 (1998) 227–231
Fig. 4. TEM micrographs of the synthesised g-Fe 2 O 3 particles.
ŽNicolet 20SXC FT-IR., transmission electron microscope ŽTEM. ŽJEOL-2000 FXII, 200 KeV. and Squid magnetometer ŽQuantum Design MPMS-2..
3. Results and discussion
shoulders at 694, 724, 638, 584, 558, 442 and 396 cmy1 which are characteristic of well ordered maghemite w10x. Disordered g-Fe 2 O 3 particles present only two broad bands at about 600 and 400 cmy1 as expected from a cubic structure whose octahedral positions are distributed at random w8,9x.
Carbon and hydrogen were analysed employing standard elemental analysis techniques. The averaged values obtained from five samples were 1.5 " 0.1% for carbon and 0.37 " 0.05% for hydrogen. Fig. 3 shows the X-ray diffraction pattern of a typical sinthesised powder. The diffraction peaks are broad, suggesting that the sample consists of very small particles. The average grain size obtained using the Scherrer’s equation was 5 nm. All the peaks observed can be assigned to the cubic maghemite phase and no extra peaks of iron or other iron oxides were observed. From the positions of the peaks we ˚ in agreement calculated the lattice parameter 8.35 A ˚ .. Ž to that of pure cubic maghemite a s 8.350 A Fig. 4 shows a TEM photograph of the prepared g-Fe 2 O 3 particles. The particles have a rounded morphology and low degree of aggregation. The average size obtained by counting 35 particles was 5 " 2 nm. This size is similar to the grain size determined by X-ray diffraction mentioned above. The infrared powder spectrum of the nanoparticles ŽFig. 5. shows two broad bands along with some
Fig. 5. IR spectrum of ultrafine maghemite powder Žbelow.. For comparison the spectrum of a pure well ordered maghemite is included w10x.
S. Veintemillas-Verdaguer et al.r Materials Letters 35 (1998) 227–231
231
pentacarbonyl and isopropanol. The average productivity obtained was 0.05 grh with a yield of about 50%. The particles obtained were approximately spherical with 5 " 2 nm diameter and low aggregation. The results presented here evidence the important effects of the particle size on the magnetic properties of the sample, in particular when the particles are smaller than 10 nm. The coercivity enhancement in this case may be due to the existence of an additional anisotropy to the magnetocrystalline one coming from the high surface-volume ratio. Acknowledgements
Fig. 6. Magnetisation curve at room temperature and 5 K for ultrafine g-Fe 2 O 3 powder.
In Fig. 6, we show the hysteresis loops obtained at 300 K and 5 K for a representative sample. Its magnetic behaviour is well described in terms of superparamagnetism. At room temperature no hysteresis is observed consistent with the small particle size of the sample Ž5 nm., however, at 5 K, the particles are magnetically blocked and hysteresis appears. The coercivity value found 700 Oe is 100 = higher than the value calculated for spherical single domain particles following Stoner–Wolhfarth theory. The saturation magnetisation is 30.5 emurg which is also far from the value expected for bulk g-Fe 2 O 3 . Similar effects have been previously reported not only for g-Fe 2 O 3 nanoparticles prepared by laser evaporation w11x and spray pyrolysis w12x but also for other Fe-based systems of similar particle size w13x. This magnetic behaviour is attributed to the existence of a spin-glass-like surface around the particle which gives rise to a reduction in the saturation magnetization as well as a large increase in the coercivity at low temperature. In Fig. 6, a feature that should be noted is the nonclosed loop up to 1 T indicating an anisotropy field is greater than this value. 4. Conclusions
g-Fe 2 O 3 nanoparticles were prepared by cw CO 2 laser induced decomposition of a solution of iron
This work was carried out by Project PB95-0002. Thanks are given to Domingo Martinez Foundation for its financial support in the construction of the laser pyrolysis equipment. We are very grateful to Dr. T. Gonzalez Carreno ˜ for her many helpful comments about this work. Valuable help from M. Cauchetier for the construction of the apparatus is gratefully acknowledged. References w1x W.R. Cannon, S.C. Danforth, J.H. Flint, J.S. Haggerty, R.A. Marra, J. Am. Ceram. Soc. 65 Ž1982. 324. w2x W.R. Cannon, S.C. Danforth, J.S. Haggerty, R.A. Marra, J. Am. Ceram. Soc. 65 Ž1982. 330. w3x M. Cauchetier, O. Croix, N. Herlin, M. Luce, J. Am. Ceram. Soc. 77 Ž1994. 993. w4x E. Borsella, S. Botti, R. Fantoni, R. Alexandrescu, I. Morjan, C. Popescu, T. Dikonimos-Makris, R. Giorgi, S. Enzo, J. Mater. Res. 7 Ž1992. 2257. w5x X.-X. Bi, B. Ganguly, G.P. Huffman, F.E. Huggins, M. Endo, P.C. Eklund, J. Mater. Res. 8 Ž1993. 1666. w6x X.Q. Zhao, F. Zheng, Y. Liang, Z.Q. Hu, Y.B. Xu, Mater. Lett. 21 Ž1994. 285. w7x X.Q. Zhao, F. Zheng, Y. Liang, Z.Q. Hu, Y.B. Xu, G.B. Zhang, Mater. Lett. 23 Ž1995. 305. w8x T. Gonzalez-Carreno, ´ ˜ M.P. Morales, M. Gracia, C.J. Serna, Mater. Lett. 18 Ž1993. 151. w9x B. Martınez, A. Roig, E. Molins, T. Gonzalez-Carreno, ´ ´ ˜ C.J. Serna, J. Appl. Phys., submitted. w10x M.P. Morales, C. de Julian, C.J. Serna, J. ´ J.M. Gonzalez, ´ Mater. Res. 9 Ž1994. 135. w11x B.J. Jonsson, T. Turkki, V. Strom, ¨ ¨ M.S. El-Shall, K.V. Rao, J. Appl. Phys. 79 Ž1996. 5063. w12x B. Martinez, A. Roig, E. Molins, T. Gonzalez-Carreno, ´ ˜ C. Serna, J. Appl. Phys, submitted. w13x R.H. Kodama, A.E. Berkowitz, E.J. McNiff Jr., S. Foner, Phys. Rev. Lett. 77 Ž1996. 394.
Materials Letters 35 Ž1998. 227–231
Continuous production of g-Fe 2 O 3 ultrafine powders by laser pyrolysis S. Veintemillas-Verdaguer ) , M.P. Morales, C.J. Serna Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain Received 18 July 1997; revised 13 October 1997; accepted 14 October 1997
Abstract Pure g-Fe 2 O 3 particles were prepared by a continuous process from cw CO 2 laser induced pyrolysis of a 30% solution of iron pentacarbonyl in isopropanol with a yield of about 50% and an average productivity of 0.05 grh. From TEM the particle size is 5 " 2 nm with a low degree of aggregation, which agrees with the size obtained from the width of the X-ray diffraction peaks. The nanoparticles seem to be well crystallised according to his infrared spectrum. Moreover, superparamagnetic behaviour was observed at room temperature with a saturation magnetisation value of 30.5 emurg. q 1998 Elsevier Science B.V. All rights reserved. PACS: 81.35.q k; 82.30.Lp; 75.60.Jp; 42.55.Em Keywords: Laser pyrolysis; Ultrafine powders; IronŽIII. oxide; g-Fe 2 O 3 ; Maghemite; Superparamagnetism
1. Introduction Laser-induced processes for the continuous production of nanometric powders was initiated by Cannon w1,2x. Since then several workers obtained the Si, SiC, Si 3 N4 and composite SirCrN powders under a variety of conditions, with sizes ranging from 5 to 20 nm with narrow size distribution, no aggregation and high surface area w3,4x. In the field of magnetic nanoparticles the continuous laser synthesis of Fe 3 C, Fe 7 C 3 , a-Fe w5x, g-Fe w6x and g X-Fe 4 N w7x has been reported. Here we report the continuous formation of g-Fe 2 O 3 ultrafine particles by a cw CO 2 laser pyrolysis method. In addition, structural characterisation ) Corresponding author. Tel.: q34-1-3349061; fax: q34-13720623; e-mail: [email protected].
of the obtained particles and the effect on their magnetic properties is also included.
2. Experimental The CO 2 laser pyrolysis technique w1,2x, see Fig. 1, has a small reaction zone defined by the overlap between the vertical reactant gas stream and the horizontal laser beam. The reaction zone is safely away from the chamber walls. This design provides an ideal environment for the nucleation of small particles in the nanometer range, with less contamination and narrower size distribution than those prepared by more conventional thermal methods w8,9x. A dissolution of 30% FeŽCO.5 in isopropanol was nebulized by means of a ultrasound Ž15 W, 8 kHz.
00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 7 - 5 7 7 X Ž 9 7 . 0 0 2 5 1 - 6
228
S. Veintemillas-Verdaguer et al.r Materials Letters 35 (1998) 227–231
and the cloud formed partially dragged by the air stream Ž6 sccm.. The mixture then passed vertically out of a stainless steel nozzle Ždiameter 1.5 mm. inside the 6-way stainless steel cross Žchamber. and intersected a horizontal unfocused CO 2 laser beam ŽSYNRAD Duo-Lase model 57-2-208 W., operated at 100 W and with a spot diameter of 4 mm. The reaction zone was protected by a coaxial flow of 30 sccm Ar gas Ž99.999%.. The energy coupling of the laser to the reactant mixture is realized by the over-
lap of the laser wavelength Ž10.60 " 0.05 m m. to the isopropanol band at 10.50 " 0.08 m m, see Fig. 2. A flux of 770 sccm of Ar gas was employed to avoid deposition of the powder on the ZnSe window. The powder was collected by means of a stainless steel filter of 0.05 m m of pore size. The chamber pressure Ž400 mbar. was controlled through a diaphragm valve located between the reaction chamber and a mechanical vacuum pump. The productivity ranged between 0.033 to 0.096 grh and the yield, calculated as
Fig. 1. CO 2 laser pyrolysis system. Ž1. Laser beam, Ž2. ZnSe window, Ž3. water refrigerated aluminium target, Ž4. nozzle, Ž5. pressure gauge, Ž6. ultrasonic bath, Ž7. 30% iron pentacarbonyl solution in isopropanol, Ž8. not return valve, Ž9. ball valve, Ž10. three ways ball valve, Ž11. argon rotameter, Ž12. massic controller of air flux, Ž13. stainless steel filter to collect the produced powders, Ž14. heating resistance, Ž15. pressure controller valve, Ž16. rotary vacuum pump, Ž17. filter to capture oil mist.
S. Veintemillas-Verdaguer et al.r Materials Letters 35 (1998) 227–231
229
Fig. 2. IR spectrum of the dissolution of 30% FeŽCO.5 in isopropanol showing the near coincidence between the absorption band of isopropanol and the emission line of CO 2 laser.
grams of product obtained from 100 g of FeŽCO.5 pyrolysed, between 12 to 84%. The variations of productivity and yield are mainly coming from changes in nebulization conditions which produced reactive clouds with different droplet densities. The colour of the obtained powders was brown being
slightly darker in some samples probably reflecting an incomplete oxidation of the initially formed magnetite to maghemite. The particles were characterized by means of elemental analysis ŽPerkin Elmer 2400CHN., X-ray diffractometer ŽPhilips PW1710. with Cu K a radiation, Infrared spectroscopy ŽIR.
Fig. 3. XRD pattern of ultrafine maghemite powder.
230
S. Veintemillas-Verdaguer et al.r Materials Letters 35 (1998) 227–231
Fig. 4. TEM micrographs of the synthesised g-Fe 2 O 3 particles.
ŽNicolet 20SXC FT-IR., transmission electron microscope ŽTEM. ŽJEOL-2000 FXII, 200 KeV. and Squid magnetometer ŽQuantum Design MPMS-2..
3. Results and discussion
shoulders at 694, 724, 638, 584, 558, 442 and 396 cmy1 which are characteristic of well ordered maghemite w10x. Disordered g-Fe 2 O 3 particles present only two broad bands at about 600 and 400 cmy1 as expected from a cubic structure whose octahedral positions are distributed at random w8,9x.
Carbon and hydrogen were analysed employing standard elemental analysis techniques. The averaged values obtained from five samples were 1.5 " 0.1% for carbon and 0.37 " 0.05% for hydrogen. Fig. 3 shows the X-ray diffraction pattern of a typical sinthesised powder. The diffraction peaks are broad, suggesting that the sample consists of very small particles. The average grain size obtained using the Scherrer’s equation was 5 nm. All the peaks observed can be assigned to the cubic maghemite phase and no extra peaks of iron or other iron oxides were observed. From the positions of the peaks we ˚ in agreement calculated the lattice parameter 8.35 A ˚ .. Ž to that of pure cubic maghemite a s 8.350 A Fig. 4 shows a TEM photograph of the prepared g-Fe 2 O 3 particles. The particles have a rounded morphology and low degree of aggregation. The average size obtained by counting 35 particles was 5 " 2 nm. This size is similar to the grain size determined by X-ray diffraction mentioned above. The infrared powder spectrum of the nanoparticles ŽFig. 5. shows two broad bands along with some
Fig. 5. IR spectrum of ultrafine maghemite powder Žbelow.. For comparison the spectrum of a pure well ordered maghemite is included w10x.
S. Veintemillas-Verdaguer et al.r Materials Letters 35 (1998) 227–231
231
pentacarbonyl and isopropanol. The average productivity obtained was 0.05 grh with a yield of about 50%. The particles obtained were approximately spherical with 5 " 2 nm diameter and low aggregation. The results presented here evidence the important effects of the particle size on the magnetic properties of the sample, in particular when the particles are smaller than 10 nm. The coercivity enhancement in this case may be due to the existence of an additional anisotropy to the magnetocrystalline one coming from the high surface-volume ratio. Acknowledgements
Fig. 6. Magnetisation curve at room temperature and 5 K for ultrafine g-Fe 2 O 3 powder.
In Fig. 6, we show the hysteresis loops obtained at 300 K and 5 K for a representative sample. Its magnetic behaviour is well described in terms of superparamagnetism. At room temperature no hysteresis is observed consistent with the small particle size of the sample Ž5 nm., however, at 5 K, the particles are magnetically blocked and hysteresis appears. The coercivity value found 700 Oe is 100 = higher than the value calculated for spherical single domain particles following Stoner–Wolhfarth theory. The saturation magnetisation is 30.5 emurg which is also far from the value expected for bulk g-Fe 2 O 3 . Similar effects have been previously reported not only for g-Fe 2 O 3 nanoparticles prepared by laser evaporation w11x and spray pyrolysis w12x but also for other Fe-based systems of similar particle size w13x. This magnetic behaviour is attributed to the existence of a spin-glass-like surface around the particle which gives rise to a reduction in the saturation magnetization as well as a large increase in the coercivity at low temperature. In Fig. 6, a feature that should be noted is the nonclosed loop up to 1 T indicating an anisotropy field is greater than this value. 4. Conclusions
g-Fe 2 O 3 nanoparticles were prepared by cw CO 2 laser induced decomposition of a solution of iron
This work was carried out by Project PB95-0002. Thanks are given to Domingo Martinez Foundation for its financial support in the construction of the laser pyrolysis equipment. We are very grateful to Dr. T. Gonzalez Carreno ˜ for her many helpful comments about this work. Valuable help from M. Cauchetier for the construction of the apparatus is gratefully acknowledged. References w1x W.R. Cannon, S.C. Danforth, J.H. Flint, J.S. Haggerty, R.A. Marra, J. Am. Ceram. Soc. 65 Ž1982. 324. w2x W.R. Cannon, S.C. Danforth, J.S. Haggerty, R.A. Marra, J. Am. Ceram. Soc. 65 Ž1982. 330. w3x M. Cauchetier, O. Croix, N. Herlin, M. Luce, J. Am. Ceram. Soc. 77 Ž1994. 993. w4x E. Borsella, S. Botti, R. Fantoni, R. Alexandrescu, I. Morjan, C. Popescu, T. Dikonimos-Makris, R. Giorgi, S. Enzo, J. Mater. Res. 7 Ž1992. 2257. w5x X.-X. Bi, B. Ganguly, G.P. Huffman, F.E. Huggins, M. Endo, P.C. Eklund, J. Mater. Res. 8 Ž1993. 1666. w6x X.Q. Zhao, F. Zheng, Y. Liang, Z.Q. Hu, Y.B. Xu, Mater. Lett. 21 Ž1994. 285. w7x X.Q. Zhao, F. Zheng, Y. Liang, Z.Q. Hu, Y.B. Xu, G.B. Zhang, Mater. Lett. 23 Ž1995. 305. w8x T. Gonzalez-Carreno, ´ ˜ M.P. Morales, M. Gracia, C.J. Serna, Mater. Lett. 18 Ž1993. 151. w9x B. Martınez, A. Roig, E. Molins, T. Gonzalez-Carreno, ´ ´ ˜ C.J. Serna, J. Appl. Phys., submitted. w10x M.P. Morales, C. de Julian, C.J. Serna, J. ´ J.M. Gonzalez, ´ Mater. Res. 9 Ž1994. 135. w11x B.J. Jonsson, T. Turkki, V. Strom, ¨ ¨ M.S. El-Shall, K.V. Rao, J. Appl. Phys. 79 Ž1996. 5063. w12x B. Martinez, A. Roig, E. Molins, T. Gonzalez-Carreno, ´ ˜ C. Serna, J. Appl. Phys, submitted. w13x R.H. Kodama, A.E. Berkowitz, E.J. McNiff Jr., S. Foner, Phys. Rev. Lett. 77 Ž1996. 394.