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Precursor synthesis and microwave processing of nickel ferrite nanoparticles
Current Applied Physics 3 (2003) 205–208 www.elsevier.com/locate/cap
Precursor synthesis and microwave processing of nickel ferrite nanoparticles V.K. Sankaranarayanan
a,*
, C. Sreekumar
b
a
b
Microstructure Devices Group, Electronic Materials Division, National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India Time and Frequency Section, Electrical and Electronic Standards Division, National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India
Abstract Nickel based ferrites have several applications in microwave components as dense polycrystalline compacts. In this study we have prepared nickel ferrite nanoparticles by a self-ignition reaction directly from a citrate precursor. XRD patterns and FTIR spectra confirm the formation of single phase nickel ferrite. The sample couples readily with microwaves at ambient temperatures and the residual carbon present in the self-ignited sample could be removed in a few minutes by microwave treatment. The samples, however, could not be sintered by microwaves on account of the high sintering temperature of nickel ferrite at which arc formation takes place. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 75.50 Keywords: Microwave processing; Nickel ferrite; Nanoparticles; Citrate method
1. Introduction Polycrystalline ferrites such as nickel zinc ferrite have variety of applications in microwave control components such as circulators, isolators, and phase shifters [1]. A high sintered density and uniform microstructure are required to minimize the magnetic loss in the high frequency regime. Conventional ceramic methods of synthesis which require high temperature processing at temperatures in excess of 1300 °C, lead to a microstructure consisting of large micrometer sized particles because of the initiation of grain boundary migration at these temperatures. In chemical methods of synthesis such as citrate precursor method and sol–gel method, however, nanoparticles of ternary oxides are obtained on account of the much lower reaction temperatures employed [2,3]. The highly active nanoparticles could in turn be sintered at relatively low temperatures to near theoretical density with uniform microstructures. In this paper we report the formation of nickel ferrite nano-
*
Corresponding author. Fax: +91-11-572-6938. E-mail address: [email protected] (V.K. Sankaranarayanan).
particles directly from a citrate precursor by a selfignition reaction and its microwave processing studies. Use of microwave energy for synthesis and processing of materials is an exciting new field in material science with enormous potential for synthesizing new materials and novel microstructures [4,5]. The growing interest during the past decade is essentially due to the possibility of reduction in manufacturing cost on account of energy savings, shorter processing times and improved product uniformity and yields. The fundamental difference in microwave heating is that the heat is generated internally within the material instead of originating from external heating sources and is responsible for the unique microstructure and uniformity. As a result of this internal and volumetric heating, it is possible to heat materials rapidly and uniformly and to efficiently remove volatile constituents. In the citrate precursor synthesis of different garnets and ferrites, we have observed that removal of residual carbon often necessitates heat-treatment at temperatures up to 600 °C for a few hours. In this study we have utilized microwave processing to efficiently remove the residual carbon in the ferrite samples in a very short time period of 10 min.
1567-1739/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S1567-1739(02)00202-X
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V.K. Sankaranarayanan, C. Sreekumar / Current Applied Physics 3 (2003) 205–208
2. Experimental Aqueous solutions of nickel and ferric nitrates were reacted with citric acid in 1:1 molar ratio. pH of the solution was increased to 7 by addition of ammonium hydroxide and the solution was warmed to complete the reaction to form the nickel iron citrate precursor. The solution was evaporated very slowly over a period of a few hours to dryness. As soon as the solvent removal is completed dried precursor undergoes a self-ignition reaction to form a very fine powder of nickel ferrite consisting of nanoparticles. The fine powder thus obtained was heat-treated in a furnace at 250 °C for 6 h followed by a further heattreatment at 350 °C for 6 h to remove the residual carbon. The self-ignited powder was pressed into pellets of 100 diameter and a thickness of 2 mm for microwave processing. The microwave processing was carried out for a period of 5, 10 15 and 30 min each. X-ray diffraction patterns of the samples were recorded on a Bruker D-500 Diffractometer. FTIR spectra were recorded on a Perkin Elmer Nicolet Spectrometer. Magnetic measurements were carried out on a DMS model 880 Vibrating Sample Magnetometer. Microwave processing of the samples was carried out in a BPL 1200 W home microwave oven at the standard microwave frequency of 2.45 GHz.
3. Results and discussion 3.1. X-ray diffraction XRD pattern of the self-ignited sample is shown in Fig. 1. The characteristic reflections of the spinel phase are clearly evident in the XRD pattern. The rela-
tive intensities of the peaks are typical of the inverse spinel phase which indicate that the nickel ferrite phase has crystallized as the inverse spinel phase with Ni2þ ion occupying half of the octahedral sites. i.e., Fe3þ ðNi2þ Fe3þ Þ2 O4 . It is interesting to see that single phase nickel ferrite is formed directly after self-ignition from the citrate precursor. There are no additional peaks corresponding to extra phases such as a-Fe2 O3 which often appear at low temperatures when the reaction is not complete. It may be remembered that synthesis of single phase nickel ferrite by conventional ceramic methods necessitate repeated grinding and calcining at temperatures in excess of 1000 °C. The very fact that single phase ferrite could be obtained directly by self-ignition from the citrate precursor without any additional heat-treatments using the present synthetic method, is a significant achievement considering the variety of applications of the nickel based ferrites. The highly active powders could be sintered at relatively low temperatures to obtain highly dense and homogeneous polycrystalline ferrites for high frequency applications. In the citrate precursor method the formation of the ferrite phase becomes possible at very low temperatures directly from the precursor because of the atomic level blending of the Ni and Fe ions as a complex in the nickel iron citrate precursor. Being a hydroxy tricarboxylic acid, citric acid has the necessary functional groups for co-ordinating the metal cations. Once the right coordination complex (the citrate precursor) is formed in the reaction under controlled pH conditions, the precursor can be separated by drying and decomposed directly to obtain the ternary spinel oxide phase. In conventional ceramic synthetic methods, physical diffusion of the ions and their reaction involves much higher activation energies and hence ferrite phase formation requires high temperatures. It may be emphasized that
Fig. 1. XRD pattern of the nickel ferrite sample obtained after self-ignition reaction.
V.K. Sankaranarayanan, C. Sreekumar / Current Applied Physics 3 (2003) 205–208
the formation of the citrate precursor complex in solution under controlled pH conditions is critical for the formation of ferrite phase at low temperatures. The selfignition reaction is apparently made possible by the presence of ammonium nitrate in the dried precursor. The self-ignited samples were annealed further at 250 and 350 °C for 6 h each. The samples undergo weight loss during these annealing treatments. However, heattreatment at these temperatures does not make any notable changes in the XRD pattern, even in the relative intensities of the peaks, which shows that the small amount of residual carbon removed during the heattreatments are not present inside the nickel ferrite crystallites. 3.2. FTIR spectra The FTIR spectra of the self-ignited and annealed samples were recorded to confirm the formation of nickel ferrite phase and to understand the nature of the residual carbon in the samples. The self-ignited sample shows characteristic absorptions of nickel ferrite phase, the strong absorption around 600 cm1 and the sharp absorptions at 450 and 410 cm1 , which confirm the crystallization of the nickel ferrite inverse spinel phase in these samples. There are two weak and broad absorptions around 1400 and 1600 cm1 corresponding to the presence of small amount of residual carbon in the samples as observed usually in the case of samples prepared by citrate precursor method [2]. These absorptions in the present case are, however, very weak indicating that the residual carbon has mostly been burnt away during the self-ignition process. The wave numbers of these absorptions indicate that this carbon is in the form of complex carbonates. Heat-treatment at 250 °C for 6 h leads to weakening of these absorptions. A further annealing at 350 °C for 6 h leads to disappearance of these absorptions indicating complete removal of residual carbon from the samples. It may, however, be noticed that prolonged annealing up to 12 h in the furnace is necessary to completely remove the residual carbon. 3.3. Microwave processing Microwave processing has been shown to be a fast and efficient technique for the removal of solvents and volatile species and also for sintering of many materials. In the present synthesis of nickel ferrite we have done microwave processing of the samples obtained after selfignition. The self-ignited samples, as mentioned above, show single phase nickel ferrite phase in the XRD patterns but contain small amount of residual carbon, the removal of which necessitates prolonged heat-treatments at high temperatures. Interestingly, microwave processing for a very short period of 10 min removes all the residual carbon in the samples as indicated in the IR
207
spectra whereas this process, as mentioned earlier, takes up to 12 h heat-treatment in a furnace. The microwave processed samples do not show any notable difference in the XRD patterns indicating that the fast removal of residual carbon does not in any way affect the crystal structure of the nickel ferrite crystallites formed during the self-ignition process. Microwaves readily couple with the sample resulting in a powerful glow and radiations and therefore the sample had to be kept in a thick ceramic enclosure. It may be remembered that many of the ceramic dielectric materials (e.g., Al2 O3 , MgO, SiO2 , and most glasses) are transparent to microwaves at ambient temperature and it becomes necessary to add conductive phases in order to enhance the microwave absorption [5]. How does microwaves couple readily with the present sample? The heating of the material with microwave radiation becomes possible on account of its interaction with electric charges and magnetic dipoles present in the material in the following way. Microwaves are electromagnetic waves that have a frequency in the range of 0.3–300 GHz and the corresponding wavelengths are 1 m–1 mm. When microwaves penetrate and propagate through a magnetic dielectric material such as nickel ferrite the internal electric fields generated induce translational motions of free or bound charges, e.g., electrons or ions, and rotate charge complexes, e.g., electric and magnetic dipoles. Since the induced motions are resisted by inertial, elastic and frictional forces the volumetric electromagnetic heating results from frictional damping of induced motions. The electromagnetic response of a magnetic spinel dielectric material such as nickel ferrite could therefore be described by a complex permittivity e which describes the electrical part and a complex permeability l that describes the magnetic part. In addition to the well known frequency dependence, these properties are a function of temperature, the magnitude of electrical and magnetic fields, the orientation and magnetic state of magnetic domains, and the manner of material preparation. Therefore, at lower temperatures, below the Curie temperature of nickel ferrite, the initial coupling to microwaves is facilitated by the magnetic dipoles and at higher temperatures most of the ceramic materials begins to absorb and couple with microwaves readily anyway. This magnetic loss part could be included in the relative permeability term by writing pffiffiffiffiffiffiffi lr as a complex number, lr ¼ l0 jl00 where j ¼ 1. l00 term describes the magnetic loss arising from damping forces caused by internal friction during domain rotation and Bloch wall propagation and l0 is the permeability. The heat generated by magnetic losses is given by Pm ¼ xl0 l00 H 2 , where l0 is the permeability of free space and x ¼ 2p frequency. In addition to that, in our samples, the residual carbon present in the self-ignited sample could
208
V.K. Sankaranarayanan, C. Sreekumar / Current Applied Physics 3 (2003) 205–208
in the furnace. It shows saturation at a relatively low field of 1 kOe in comparison to 2 kOe for the furnace annealed sample. The magnetization values obtained are very good considering the low temperature at which they are obtained.
4. Conclusions
Fig. 2. Magnetization curves of the nickel ferrite samples: (a) heattreated at 350 °C and (b) the microwave processed sample.
also be helping to enhance the microwave absorption at low temperatures. Since the microwave radiation couple readily with our sample and helps in removing the residual carbon in a very short time, we attempted sintering of the selfignited samples pressed in the form of 100 diameter pellets. The samples, however, could not be sintered after 30 min heating in the microwave. At higher temperatures above the Curie temperature the magnetic loss mechanisms described above would not help in facilitating coupling with the microwave radiation. Also the residual carbon which helps as a dopant is also not present at these temperatures. A detailed investigation of microwave sintering of different spinels has shown that sintering is possible in spinels provided the sintering temperature is not too high to cause arc formation [6]. The sintering temperature of the present nickel ferrite samples are possibly too high to facilitate microwave sintering and densification. 3.4. Magnetization studies The magnetization curves of the nickel ferrite samples obtained after heat-treatment at 350 °C for 6 h and the sample microwave processed for 10 min are shown in Fig. 2(a) and (b) respectively. The microwave processed sample shows a higher saturation magnetization value of 53 emu/g in comparison with the sample heat-treated
Single phase nickel ferrite nanoparticles could be prepared directly from a citrate precursor at relatively low temperatures by a self-ignition reaction. The selfignited samples show the inverse spinel phase of nickel ferrite in the XRD pattern. The characteristic FTIR spectra confirm the formation of the spinel nickel ferrite phase and indicate presence of small amount of residual carbon in the self-ignited sample. Microwaves readily couple with the sample at ambient temperatures presumably due to the magnetic nature and due to the presence of residual carbon. Microwave treatment removes the residual carbon in a very short time of 10 min in comparison with a few hours in a furnace. The samples could not be sintered by microwaves on account of the high sintering temperature of nickel ferrite at which arc formation takes place.
Acknowledgements We thank the Director NPL for the permission to publish this work. Thanks are also due to X-ray and IR section for recording the XRD patterns and FTIR spectra.
References [1] J.D. Adam, S.V. Krishnaswamy, S.H. Talisa, K.C. Yoo, J. Magn. Magn. Mater. 83 (1990) 419. [2] V.K. Sankaranarayanan, N.S. Gajbhiye, J. Am. Ceram. Soc. 73 (1990) 1301. [3] V.K. Sankaranarayanan, Q.A. Pankhurst, D.P.E. Dickson, C.E. Johnson, J. Magn. Magn. Mater. 130 (1994) 288. [4] B. Vaidhyanathan, A.P. Singh, D.K. Aggarwal, T.R. Shrout, R. Roy, J. Am. Ceram. Soc. 84 (2001) 1197. [5] W.H. Sutton, Ceram. Bull. 68 (1989) 376. [6] M. Aliouat, L. Mazo, G. Desgardin, B. Raveau, J. Am. Ceram. Soc. 73 (1990) 2515.
Precursor synthesis and microwave processing of nickel ferrite nanoparticles V.K. Sankaranarayanan
a,*
, C. Sreekumar
b
a
b
Microstructure Devices Group, Electronic Materials Division, National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India Time and Frequency Section, Electrical and Electronic Standards Division, National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India
Abstract Nickel based ferrites have several applications in microwave components as dense polycrystalline compacts. In this study we have prepared nickel ferrite nanoparticles by a self-ignition reaction directly from a citrate precursor. XRD patterns and FTIR spectra confirm the formation of single phase nickel ferrite. The sample couples readily with microwaves at ambient temperatures and the residual carbon present in the self-ignited sample could be removed in a few minutes by microwave treatment. The samples, however, could not be sintered by microwaves on account of the high sintering temperature of nickel ferrite at which arc formation takes place. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 75.50 Keywords: Microwave processing; Nickel ferrite; Nanoparticles; Citrate method
1. Introduction Polycrystalline ferrites such as nickel zinc ferrite have variety of applications in microwave control components such as circulators, isolators, and phase shifters [1]. A high sintered density and uniform microstructure are required to minimize the magnetic loss in the high frequency regime. Conventional ceramic methods of synthesis which require high temperature processing at temperatures in excess of 1300 °C, lead to a microstructure consisting of large micrometer sized particles because of the initiation of grain boundary migration at these temperatures. In chemical methods of synthesis such as citrate precursor method and sol–gel method, however, nanoparticles of ternary oxides are obtained on account of the much lower reaction temperatures employed [2,3]. The highly active nanoparticles could in turn be sintered at relatively low temperatures to near theoretical density with uniform microstructures. In this paper we report the formation of nickel ferrite nano-
*
Corresponding author. Fax: +91-11-572-6938. E-mail address: [email protected] (V.K. Sankaranarayanan).
particles directly from a citrate precursor by a selfignition reaction and its microwave processing studies. Use of microwave energy for synthesis and processing of materials is an exciting new field in material science with enormous potential for synthesizing new materials and novel microstructures [4,5]. The growing interest during the past decade is essentially due to the possibility of reduction in manufacturing cost on account of energy savings, shorter processing times and improved product uniformity and yields. The fundamental difference in microwave heating is that the heat is generated internally within the material instead of originating from external heating sources and is responsible for the unique microstructure and uniformity. As a result of this internal and volumetric heating, it is possible to heat materials rapidly and uniformly and to efficiently remove volatile constituents. In the citrate precursor synthesis of different garnets and ferrites, we have observed that removal of residual carbon often necessitates heat-treatment at temperatures up to 600 °C for a few hours. In this study we have utilized microwave processing to efficiently remove the residual carbon in the ferrite samples in a very short time period of 10 min.
1567-1739/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S1567-1739(02)00202-X
206
V.K. Sankaranarayanan, C. Sreekumar / Current Applied Physics 3 (2003) 205–208
2. Experimental Aqueous solutions of nickel and ferric nitrates were reacted with citric acid in 1:1 molar ratio. pH of the solution was increased to 7 by addition of ammonium hydroxide and the solution was warmed to complete the reaction to form the nickel iron citrate precursor. The solution was evaporated very slowly over a period of a few hours to dryness. As soon as the solvent removal is completed dried precursor undergoes a self-ignition reaction to form a very fine powder of nickel ferrite consisting of nanoparticles. The fine powder thus obtained was heat-treated in a furnace at 250 °C for 6 h followed by a further heattreatment at 350 °C for 6 h to remove the residual carbon. The self-ignited powder was pressed into pellets of 100 diameter and a thickness of 2 mm for microwave processing. The microwave processing was carried out for a period of 5, 10 15 and 30 min each. X-ray diffraction patterns of the samples were recorded on a Bruker D-500 Diffractometer. FTIR spectra were recorded on a Perkin Elmer Nicolet Spectrometer. Magnetic measurements were carried out on a DMS model 880 Vibrating Sample Magnetometer. Microwave processing of the samples was carried out in a BPL 1200 W home microwave oven at the standard microwave frequency of 2.45 GHz.
3. Results and discussion 3.1. X-ray diffraction XRD pattern of the self-ignited sample is shown in Fig. 1. The characteristic reflections of the spinel phase are clearly evident in the XRD pattern. The rela-
tive intensities of the peaks are typical of the inverse spinel phase which indicate that the nickel ferrite phase has crystallized as the inverse spinel phase with Ni2þ ion occupying half of the octahedral sites. i.e., Fe3þ ðNi2þ Fe3þ Þ2 O4 . It is interesting to see that single phase nickel ferrite is formed directly after self-ignition from the citrate precursor. There are no additional peaks corresponding to extra phases such as a-Fe2 O3 which often appear at low temperatures when the reaction is not complete. It may be remembered that synthesis of single phase nickel ferrite by conventional ceramic methods necessitate repeated grinding and calcining at temperatures in excess of 1000 °C. The very fact that single phase ferrite could be obtained directly by self-ignition from the citrate precursor without any additional heat-treatments using the present synthetic method, is a significant achievement considering the variety of applications of the nickel based ferrites. The highly active powders could be sintered at relatively low temperatures to obtain highly dense and homogeneous polycrystalline ferrites for high frequency applications. In the citrate precursor method the formation of the ferrite phase becomes possible at very low temperatures directly from the precursor because of the atomic level blending of the Ni and Fe ions as a complex in the nickel iron citrate precursor. Being a hydroxy tricarboxylic acid, citric acid has the necessary functional groups for co-ordinating the metal cations. Once the right coordination complex (the citrate precursor) is formed in the reaction under controlled pH conditions, the precursor can be separated by drying and decomposed directly to obtain the ternary spinel oxide phase. In conventional ceramic synthetic methods, physical diffusion of the ions and their reaction involves much higher activation energies and hence ferrite phase formation requires high temperatures. It may be emphasized that
Fig. 1. XRD pattern of the nickel ferrite sample obtained after self-ignition reaction.
V.K. Sankaranarayanan, C. Sreekumar / Current Applied Physics 3 (2003) 205–208
the formation of the citrate precursor complex in solution under controlled pH conditions is critical for the formation of ferrite phase at low temperatures. The selfignition reaction is apparently made possible by the presence of ammonium nitrate in the dried precursor. The self-ignited samples were annealed further at 250 and 350 °C for 6 h each. The samples undergo weight loss during these annealing treatments. However, heattreatment at these temperatures does not make any notable changes in the XRD pattern, even in the relative intensities of the peaks, which shows that the small amount of residual carbon removed during the heattreatments are not present inside the nickel ferrite crystallites. 3.2. FTIR spectra The FTIR spectra of the self-ignited and annealed samples were recorded to confirm the formation of nickel ferrite phase and to understand the nature of the residual carbon in the samples. The self-ignited sample shows characteristic absorptions of nickel ferrite phase, the strong absorption around 600 cm1 and the sharp absorptions at 450 and 410 cm1 , which confirm the crystallization of the nickel ferrite inverse spinel phase in these samples. There are two weak and broad absorptions around 1400 and 1600 cm1 corresponding to the presence of small amount of residual carbon in the samples as observed usually in the case of samples prepared by citrate precursor method [2]. These absorptions in the present case are, however, very weak indicating that the residual carbon has mostly been burnt away during the self-ignition process. The wave numbers of these absorptions indicate that this carbon is in the form of complex carbonates. Heat-treatment at 250 °C for 6 h leads to weakening of these absorptions. A further annealing at 350 °C for 6 h leads to disappearance of these absorptions indicating complete removal of residual carbon from the samples. It may, however, be noticed that prolonged annealing up to 12 h in the furnace is necessary to completely remove the residual carbon. 3.3. Microwave processing Microwave processing has been shown to be a fast and efficient technique for the removal of solvents and volatile species and also for sintering of many materials. In the present synthesis of nickel ferrite we have done microwave processing of the samples obtained after selfignition. The self-ignited samples, as mentioned above, show single phase nickel ferrite phase in the XRD patterns but contain small amount of residual carbon, the removal of which necessitates prolonged heat-treatments at high temperatures. Interestingly, microwave processing for a very short period of 10 min removes all the residual carbon in the samples as indicated in the IR
207
spectra whereas this process, as mentioned earlier, takes up to 12 h heat-treatment in a furnace. The microwave processed samples do not show any notable difference in the XRD patterns indicating that the fast removal of residual carbon does not in any way affect the crystal structure of the nickel ferrite crystallites formed during the self-ignition process. Microwaves readily couple with the sample resulting in a powerful glow and radiations and therefore the sample had to be kept in a thick ceramic enclosure. It may be remembered that many of the ceramic dielectric materials (e.g., Al2 O3 , MgO, SiO2 , and most glasses) are transparent to microwaves at ambient temperature and it becomes necessary to add conductive phases in order to enhance the microwave absorption [5]. How does microwaves couple readily with the present sample? The heating of the material with microwave radiation becomes possible on account of its interaction with electric charges and magnetic dipoles present in the material in the following way. Microwaves are electromagnetic waves that have a frequency in the range of 0.3–300 GHz and the corresponding wavelengths are 1 m–1 mm. When microwaves penetrate and propagate through a magnetic dielectric material such as nickel ferrite the internal electric fields generated induce translational motions of free or bound charges, e.g., electrons or ions, and rotate charge complexes, e.g., electric and magnetic dipoles. Since the induced motions are resisted by inertial, elastic and frictional forces the volumetric electromagnetic heating results from frictional damping of induced motions. The electromagnetic response of a magnetic spinel dielectric material such as nickel ferrite could therefore be described by a complex permittivity e which describes the electrical part and a complex permeability l that describes the magnetic part. In addition to the well known frequency dependence, these properties are a function of temperature, the magnitude of electrical and magnetic fields, the orientation and magnetic state of magnetic domains, and the manner of material preparation. Therefore, at lower temperatures, below the Curie temperature of nickel ferrite, the initial coupling to microwaves is facilitated by the magnetic dipoles and at higher temperatures most of the ceramic materials begins to absorb and couple with microwaves readily anyway. This magnetic loss part could be included in the relative permeability term by writing pffiffiffiffiffiffiffi lr as a complex number, lr ¼ l0 jl00 where j ¼ 1. l00 term describes the magnetic loss arising from damping forces caused by internal friction during domain rotation and Bloch wall propagation and l0 is the permeability. The heat generated by magnetic losses is given by Pm ¼ xl0 l00 H 2 , where l0 is the permeability of free space and x ¼ 2p frequency. In addition to that, in our samples, the residual carbon present in the self-ignited sample could
208
V.K. Sankaranarayanan, C. Sreekumar / Current Applied Physics 3 (2003) 205–208
in the furnace. It shows saturation at a relatively low field of 1 kOe in comparison to 2 kOe for the furnace annealed sample. The magnetization values obtained are very good considering the low temperature at which they are obtained.
4. Conclusions
Fig. 2. Magnetization curves of the nickel ferrite samples: (a) heattreated at 350 °C and (b) the microwave processed sample.
also be helping to enhance the microwave absorption at low temperatures. Since the microwave radiation couple readily with our sample and helps in removing the residual carbon in a very short time, we attempted sintering of the selfignited samples pressed in the form of 100 diameter pellets. The samples, however, could not be sintered after 30 min heating in the microwave. At higher temperatures above the Curie temperature the magnetic loss mechanisms described above would not help in facilitating coupling with the microwave radiation. Also the residual carbon which helps as a dopant is also not present at these temperatures. A detailed investigation of microwave sintering of different spinels has shown that sintering is possible in spinels provided the sintering temperature is not too high to cause arc formation [6]. The sintering temperature of the present nickel ferrite samples are possibly too high to facilitate microwave sintering and densification. 3.4. Magnetization studies The magnetization curves of the nickel ferrite samples obtained after heat-treatment at 350 °C for 6 h and the sample microwave processed for 10 min are shown in Fig. 2(a) and (b) respectively. The microwave processed sample shows a higher saturation magnetization value of 53 emu/g in comparison with the sample heat-treated
Single phase nickel ferrite nanoparticles could be prepared directly from a citrate precursor at relatively low temperatures by a self-ignition reaction. The selfignited samples show the inverse spinel phase of nickel ferrite in the XRD pattern. The characteristic FTIR spectra confirm the formation of the spinel nickel ferrite phase and indicate presence of small amount of residual carbon in the self-ignited sample. Microwaves readily couple with the sample at ambient temperatures presumably due to the magnetic nature and due to the presence of residual carbon. Microwave treatment removes the residual carbon in a very short time of 10 min in comparison with a few hours in a furnace. The samples could not be sintered by microwaves on account of the high sintering temperature of nickel ferrite at which arc formation takes place.
Acknowledgements We thank the Director NPL for the permission to publish this work. Thanks are also due to X-ray and IR section for recording the XRD patterns and FTIR spectra.
References [1] J.D. Adam, S.V. Krishnaswamy, S.H. Talisa, K.C. Yoo, J. Magn. Magn. Mater. 83 (1990) 419. [2] V.K. Sankaranarayanan, N.S. Gajbhiye, J. Am. Ceram. Soc. 73 (1990) 1301. [3] V.K. Sankaranarayanan, Q.A. Pankhurst, D.P.E. Dickson, C.E. Johnson, J. Magn. Magn. Mater. 130 (1994) 288. [4] B. Vaidhyanathan, A.P. Singh, D.K. Aggarwal, T.R. Shrout, R. Roy, J. Am. Ceram. Soc. 84 (2001) 1197. [5] W.H. Sutton, Ceram. Bull. 68 (1989) 376. [6] M. Aliouat, L. Mazo, G. Desgardin, B. Raveau, J. Am. Ceram. Soc. 73 (1990) 2515.