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The manufacture, characterisation and microwave properties of aligned M ferrite fibres
Journal of Magnetism and Magnetic Materials 186 (1998) 326—332
The manufacture, characterisation and microwave properties of aligned M ferrite fibres R.C. Pullar!, S.G. Appleton", A.K. Bhattacharya!,* ! Centre for Catalytic Systems and Materials Engineering, Department of Engineering, University of Warwick, Coventry CV4 7AL, UK " Structural Materials Centre, Defence Evaluation and Research Agency, Farnborough, Hants GU14 6TD, UK Received 5 September 1997
Abstract Gel fibres of strontium and barium M ferrite were blow spun from an aqueous inorganic sol and collected as aligned tow blankets. Both were then calcined to 1000°C and characterised using a variety of techniques. The ceramic fibres were shown to be the respective single phase crystalline M ferrites at 1000°C by X-ray diffraction, and compared to standard commercially available M ferrites at this temperature they demonstrated a favourable grain structure of less than 1 lm. Measurement of the microwave permeability spectra showed both materials exhibiting ferromagnetic resonance frequencies consistent with those reported in the literature. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 75.50.Gg; 82.70.Gg; 76.50.#g Keywords: Hexagonal ferrites; Fibres; Sol—gel; Ferromagnetic resonance
1. Introduction The M-type hexaferrites are among a group of useful magnetic compounds discovered by Philips between 1952 [1] and 1956 [2]. Strontium and barium M ferrites have the formulae Sr- or BaFe O , and the hexagonal magnetoplumbite 12 19 structure [3]. They are uniaxial with the direction of magnetisation parallel to the c-axis [2] and are magnetically hard materials, barium M ferrite being one of the most commercially important
* Corresponding author. Tel.: 0044 1203 523523; fax: 0044 1203 524027; e-mail: [email protected].
permanent magnetic materials [4]. The M ferrites also have high resistivities [5], magnetocrystalline anisotropies [6] and saturisation magnetisations [7], low dielectric losses [8] and are thermally stable well above their Curie temperatures [9]. These properties make M ferrites potentially ideal for use in non-reciprocal microwave devices provided the resonance line-width can be reduced to single crystal values, by exploiting the effect of gyromagnetic resonance that appears when the material is subjected to a low power microwave field perpendicular to the axis of magnetisation. This investigation into the formation, characterisation and microwave properties of aligned M ferrite fibres is part of an ongoing programme into the
0304-8853/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 1 0 7 - 3
R.C. Pullar et al. / Journal of Magnetism and Magnetic Materials 186 (1998) 326—332
development of refractory fibres, manufactured from aqueous sol—gel routes. The many advantages of sol—gel processes, fibrous versus bulk materials and fibre composites have been discussed in previous publications [10,15].
2. Experimental 2.1. Sample preparation An acid-peptised, halogen-stabilised iron(III) hydroxide sol (Fe : anion"3 : 2) was doped with a stoichiometric amount of a strontium salt, which had been previously dissolved into a solution with an organic liganding agent. Spinnability was bestowed by the addition of a small amount of polyethylene oxide as a spinning aid, and the fibres produced on a proprietary blow spinning process [11]. The resulting gel fibres were collected as an aligned tow blanket and stored in a circulating air oven at 110°C. The gel fibres were heat treated in a muffle furnace, firstly being pre-fired to 400°C at 100°C/h to remove water and organic compounds, and then further heat-treated at 200°C/h to 1000°C in a re-crystallised alumina vessel for three hours to form strontium M ferrite. Barium M ferrite was prepared by a similar method as previously reported [10], but was this time also collected as an aligned tow blanket and calcined under the same firing regime, and the characteristics of the two ferrite fibres were compared. 2.2. Characterisation 2.2.1. Photon correlation spectroscopy (PCS) Particle size measurement of the sol above the 3 nm diameter range was measured on a Malvern Instruments Lo-C autosizer and series 7032 multi-8 correlator, using a 4 mW diode laser, 670 nm wavelength. 2.2.2. Scanning electron microscopy (SEM) Scanning electron micrographs and analysis of the morphology of the samples was carried out on a Cambridge Instruments Stereoscan 90 SEM operating at 15 kV. Conducting samples were prepared by gold sputtering fibre specimens.
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2.2.3. Surface area and porosity measurements Surface areas and pore size distributions of the fibres were performed on a Micrometrics ASAP 2000 using N as the adsorption gas. Samples were 2 degassed at 300°C for 6 h prior to analysis. 2.2.4. X-ray photoelectron spectroscopy (XPS) The XPS analysis was performed using a Kratos XSAM 800 spectrometer fitted with a dual anode (Mg/Al) X-ray source and a multichannel detector. The spectrometer was calibrated using the Ag3d5@2 line at 397.9 eV and the AgMVV line at 901.5 eV. Al K radiation (1486.6 eV) was the exciting source a (120 W) and spectra were collected in the high resolution mode (1.2 eV) and Fixed Analyser Transmission (FAT). The Kratos DS800 software was used for data acquisition and analysis. 2.2.5. X-ray fluorescence spectrometry (XRF) The elemental composition of the samples was measured on a Philips PW2400 sequential X-ray spectrometer fitted with a rhodium target end window X-ray tube and Philips X-40 analytical software. The samples were analysed in the form of a fused bead, where 1 g of sample was fused with 10 g of lithium tetraborate flux at 1250°C for 12 min and then cast to form a glass bead. 2.2.6. X-ray powder diffraction (XRD) measurement X-ray powder diffraction patterns of the samples treated at various temperatures were recorded in the region of 2h"10—80° with a scanning speed of 0.25°/min on a Philips PW1710 diffractometer using Cu K radiation with a nickel filter. Cell a parameters were calculated and further refined using linear regression procedures applied to the measured peak positions of all major reflections up to 2h"90° with the Philips APD 1700 software. This software was also used to calculate the average size of the crystallites in a sample using the wellknown Scherrer equation: D"Kj/h cos h 1@2 where D is the average size of the crystallites, K the Scherrer constant (0.9]57.3), j the wavelength of radiation (1.5405), h the peak width at half height 1@2 and h corresponds to the peak position.
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2.2.7. Assessment of fibre alignment The fibres were collected as a blanket on a high speed rotor, in a manner similar to that used for ‘Safimax’ alumina fibres [12]. There were differences compared to “Safimax”, which was an aligned blanket with 90% of the fibres within $10° and all within $20°. A small proportion of fibres crossing the general alignment was estimated using an optical microscope at 40]magnification. The number of aligned fibres in a field was counted, together with the few crossing the alignment in the same field. Counts were made in up to 5 separate fields summing to several hundred generally aligned fibres and up to about 50 crossing the alignment. The direction of the generally aligned fibres was analysed by traversing the electron micrographs with a protractor normal to the axis of alignment and measuring the deviations of at least 100 individual fibres. Two sets of data were taken from opposite sides of each micrograph. Both the protractor measurement on micrographs or direct measurement at 40]magnification are viewing deviations set into the fibre on a 1—2 mm scale. These cannot be removed by subsequent tensioning and can affect the packing into composites. 2.2.8. Microwave measurements The complex permeability (k*) spectrum of the ceramic ferrite fibres were determined using established techniques. The fibres were milled to a fine powder and dispersed in paraffin wax to a volume loading of 30% ferrite. The wax/ferrite mixture was die-pressed to form a rectangular specimen which was inserted in a waveguide cell. Measurements of transmission and reflection were made between 40 and 60 GHz using a Hewlett Packard HP8510C vector network analyser coupled to a millimetre wave S-parameter test-set. The values of k* for the ferrite/wax composite were calculated from the measured S-parameters using the transmission line method of Nicholson and Ross [13]. The intrinsic ferrite properties were isolated using the Lichtenecker effective medium expressions: DkD "» DkD #(1!» )DkD , %&& & &%33*5% & 8!9 tan d "tand #tand , %&& &%33*5% 8!9
where DkD , DkD and DkD are the modulus of %&& &%33*5% 8!9 permeability of the composite, the ferrite and wax, respectively. » is the volume fraction of ferrite and & tan d is the loss tangent, defined by the relation tan d"kA/k@, where k@ is the real permeability and kA is the imaginary permeability.
3. Results and Discussion 3.1. Sol characterisation and stability The stability and the resulting size of the sol particles is sensitive to the preparative techniques and conditions employed, and PCS enabled us to measure and control the properties of the strontium doped iron (III) sol to a certain extent. The PCS data indicates that the average particle size of the strontium doped iron (III) sol was 6.4 nm, with a polydispersity of 0.82 and an average particle weight of 4.2]104 amu. By volume distribution, the mean size was found to be 7.0 nm, with an upper limit of 30 nm, and these results are similar to the volume distribution of the barium doped sol (Fig. 1). The characteristics of the barium doped iron (III) sol have been reported in a previous publication [10], and are reproduced in Table 1 for comparison to those of the strontium doped sol. It must be considered that the technique is unable to detect particles below the 3 nm threshold, and therefore these measurements may be greater than the actual true figures. 3.2. M ferrite fibre characterisation and morphology The SrM ferrite fibres had smooth, parallel sides and had retained their fibrous nature with a diameter of 3—6 lm. The grain size was generally found to be at the submicron level, between a range of 0.1—1 lm, and there were no pores visible on the surface of the fibres (Fig. 2). By comparison, commercial M ferrite specimens normally demonstrate a grain size of 1—5 lm at 1000°C [14]. Surface area data on the SrM ferrite fibre gave a low surface area of 0.88 m2 g~1 and an average pore diameter of 48 nm using N adsorption. 2
R.C. Pullar et al. / Journal of Magnetism and Magnetic Materials 186 (1998) 326—332
329
Fig. 1. PCS volume distribution curves for (1) Ba doped and (2) Sr doped iron (III) sols.
Table 1 Comparison of Sr and Ba doped iron (III) sols
Sol Z average (nm) Polydispersity Molecular weight (amu]103) Volume average (nm) Volume upper limit (nm)
Sr doped sol
Ba doped sol
6.4 0.82 42 7.0 30
7.4 0.63 56 6.6 40
A deviation in either the stoichiometry [16] or oxidation state of the iron (III) [5] in the ferrite can have an adverse affect on its magnetic and microwave properties, and therefore the composition of the fibres was established. The XPS analysis of the SrM fibres showed the oxidation state of the iron to be Fe (III) with a binding energy of 710.5 eV for the main Fe 2p peak. The XRF elemental analysis for the oxides SrO and Fe O confirmed the composi2 3 tion to be SrFe O , and the XRD pattern of the 12 19 powdered fibres fired to 1000°C proved them to be single phase SrM ferrite (Fig. 3). The average crystallite size was estimated to be 59 nm from the width-at-half-height of the 100% peak at 2h" 34.2° using the Scherrer equation.
Fig. 2. SEM micrograph of SrM ferrite fibres fired to 1000°C for 3 h.
The composition of the aligned BaM ferrite fibres was also confirmed by XRF and XRD techniques (Fig. 3), and they were shown to be similar in both quality and microstructure to the random BaM fibres reported previously [10]. Various characteristics of the two aligned M ferrite fibres are compared in Table 2, and the similarities between the two different fibres can be clearly seen.
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Fig. 4. SEM micrograph of an aligned BaM fibre blanket fired to 1000°C for 3 h.
Fig. 3. XRD patterns of (a) BaM and (b) SrM ferrite fibres fired to 1000°C.
Table 2 Comparison of SrM and BaM aligned ferrite fibres
Crystallite size (nm) Fibre diameter (lm) Surface area (m2 g~1) Average pore diameter (nm) Alignment (within $20° of axis)
SrM fibres
BaM fibres
59 3—6 0.88 48 80%
57 3—5 0.85 53 88%
3.3. Alignment of the M ferrite fibre blankets The BaM ferrite fibres were well aligned with 88% of the fibres being within $20° of the axis of alignment and 69% within $10° (Fig. 4). However, the SrM ferrite fibres were not as well aligned. The fibres were undulating, with some individual fibres being looped almost perpendicularly to the axis of alignment. Although only some 80% of the
Fig. 5. SEM micrograph of an aligned SrM fibre blanket fired to 1000°C for 3 h.
SrM fibres were aligned within $20° of this axis and 62% within $10° (Fig. 5), it is thought that the differences between the two fibres are due to minor variations during the actual spinning and collection, rather than any intrinsic variance in the properties of the two sols. Both of these fibres compare less well with commercially available fibres, such as “Safimax” alumina [12], a development product, where all the fibres are within $20 and 90% within $10°, than those previously manufactured at Warwick [15,17], which have been reported with an alignment of up to 98%
R.C. Pullar et al. / Journal of Magnetism and Magnetic Materials 186 (1998) 326—332
331
Fig. 6. FMR permeability components k@ (the real permeability) and kA (the imaginary permeability) for SrM ferrite fibres fired to 1000°C.
Fig. 7. FMR permeability components k@ (the real permeability) and kA (the imaginary permeability) for BaM ferrite fibres fired to 1000°C.
within $20° and 85% within $10° of the axis of alignment. The proprietary “Safimax” fibre process was modified to allow the collection of more friable gel fibres. The collection rotor was operated well below, rather than slightly above the fibre generating velocity, and an open diverging rather than a converging air duct was used. A known result of this differential between the velocity of the fibre draw and the speed of the collection drum is looping, and this problem should be removed with further optimisation of the spinning process, thus improving the alignment of both fibres.
ceramic methods are 51.5 and 45 GHz, respectively, which is in good agreement with our measurements.
3.4. Microwave measurements There is a wealth of information on the M-type hexagonal ferrites, some of which deals with their high frequency properties [18,19] and their expected natural ferromagnetic resonance (FMR) frequencies. The measured spectra of k@ and kA are shown in Figs. 6 and 7, where k@ is the real permeability and kA is the imaginary permeability. The FMR frequency is defined as the point at which kA is a maximum, and which also coincides with the half-peak value of k@. The FMR frequency recorded for the SrM ferrite fibres was 50 GHz (Fig. 6) and that of the BaM fibres occurred at 43.5 GHz (Fig. 7). Literature values for the FMR frequencies of SrM and BaM manufactured by conventional
4. Conclusions A stable, strontium doped iron (III) sol was produced, and from it gel-fibres were successfully spun and collected in the form of an aligned tow blanket. The gel fibres were calcined to 1000°C and then characterised by various techniques. They were found to be pure phase SrM ferrite at this temperature whilst maintaining their fibrous nature, and with an improved microstructure compared to conventionally prepared specimens. BaM fibres were also spun from a doped sol, and collected as an aligned tow for the first time and compared with SrM fibres. These were also characterised, and found to be identical to the previously reported random BaM ferrite fibres [10]. Both the SrM and BaM fibres were found to be less well aligned than other aligned hexagonal ferrite fibres previously reported by the Warwick group, and generally the SrM blankets were undulating, rather than linear in appearance. To maintain a useful packing density in composite materials fibres are required to be both straight and well aligned, but we feel certain that the alignment and linearity of both fibres can be improved upon with further refinement of the spinning process.
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The measured FMR frequencies of both materials were in excellent accord with the literature data, and further investigations into their magnetic characteristics are currently underway. Acknowledgements R.C. Pullar wishes to thank the Centre for Catalytic Systems and Materials Engineering and the DERA for providing funding for his research associateship. Our thanks to D. Croci for surface area measurements and R.C. Reynolds for the XPS and XRD characterisation (both at the Centre for Catalytic Systems and Materials Engineering, Department of Engineering, University of Warwick) and R. Burton for the XRF analysis (Materials Research Institute, Sheffield Hallam University). The microwave data contained herein is published with the permission of the controller of Her Brittanic Majesty’s Stationery Office. References [1] J.J. Went, G.W. Rathenau, E.W. Gorter, G.W. Van Oosterhaut, Phil. Tech. Rev. 13 (1952) 194. [2] H. Jonker, H.P. Wijn, P.B. Braun, Phil. Tech. Rev. 18 (1956) 145. [3] V. Adelskold, Arkiv. Kemi. Min. Geol. A 12 (1938) 1.
[4] E.A.M. Van Der Broek, A.L. Stuijts, Phil. Tech. Redsch. 37 (1977—8) 169. [5] J. Nicolas, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials, vol. 2, North-Holland, Amsterdam, 1980, p. 291. [6] E.E. Riches, Ferrites, Mills, Boon Technical Library, London, 1972, p. 59. [7] J. Smit, H.P.J. Wijn, Ferrites, Philips Technical Library, Eindhoven, 1959, p. 195. [8] W.H. Von Aulock, C.E. Fay, in: L. Marton (Ed.), Linear Ferrite Devices for Microwave Applications, Academic Press, New York, 1968, p. 14. [9] H. Stablein, in: E.P Wohlfarth (Ed.), Ferromagnetic Materials, vol. 3, North-Holland, Amsterdam, 1982, p. 448. [10] R.C. Pullar, M.D. Taylor, A.K. Bhattacharya, J. Mater. Sci. 32 (1997) 349. [11] M.J. Morton, J.D. Birchall, J.E. Cassidy (ICI), UK Patent 1360200 (1974). [12] M.H. Stacey, M.D. Taylor (ICI), Eur. Patent 318203 (1987). [13] A.M. Nicholson, G. Ross, IEEE Trans. Instr. Meas. 19 (1970) 377. [14] H. Stablein, Tech. Mitt. Krupp., Forsch.-Ber. 26 (1968) 81. [15] R.C. Pullar, M.D. Taylor, A.K. Bhattacharya, J. Mater Sci. 32 (1997) 873. [16] J. Smit, H.P.J. Wijn, Ferrites, Philips Technical Library, Eindhoven, 1959, p. 221. [17] R.C. Pullar, S.G. Appleton, M.D. Taylor, M.H. Stacey, A.K. Bhattacharya, J. Magn. Magn. Mater. 186 (1998) (this issue). [18] H. Kojima, in: E.P Wohlfarth (Ed.), Ferromagnetic Materials, vol. 3, North-Holland, Amsterdam, 1982, pp. 345—347. [19] J. Smit, H.P.J. Wijn, Ferrites Philips Technical Library, Eindhoven, 1959, pp. 78—84.
The manufacture, characterisation and microwave properties of aligned M ferrite fibres R.C. Pullar!, S.G. Appleton", A.K. Bhattacharya!,* ! Centre for Catalytic Systems and Materials Engineering, Department of Engineering, University of Warwick, Coventry CV4 7AL, UK " Structural Materials Centre, Defence Evaluation and Research Agency, Farnborough, Hants GU14 6TD, UK Received 5 September 1997
Abstract Gel fibres of strontium and barium M ferrite were blow spun from an aqueous inorganic sol and collected as aligned tow blankets. Both were then calcined to 1000°C and characterised using a variety of techniques. The ceramic fibres were shown to be the respective single phase crystalline M ferrites at 1000°C by X-ray diffraction, and compared to standard commercially available M ferrites at this temperature they demonstrated a favourable grain structure of less than 1 lm. Measurement of the microwave permeability spectra showed both materials exhibiting ferromagnetic resonance frequencies consistent with those reported in the literature. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 75.50.Gg; 82.70.Gg; 76.50.#g Keywords: Hexagonal ferrites; Fibres; Sol—gel; Ferromagnetic resonance
1. Introduction The M-type hexaferrites are among a group of useful magnetic compounds discovered by Philips between 1952 [1] and 1956 [2]. Strontium and barium M ferrites have the formulae Sr- or BaFe O , and the hexagonal magnetoplumbite 12 19 structure [3]. They are uniaxial with the direction of magnetisation parallel to the c-axis [2] and are magnetically hard materials, barium M ferrite being one of the most commercially important
* Corresponding author. Tel.: 0044 1203 523523; fax: 0044 1203 524027; e-mail: [email protected].
permanent magnetic materials [4]. The M ferrites also have high resistivities [5], magnetocrystalline anisotropies [6] and saturisation magnetisations [7], low dielectric losses [8] and are thermally stable well above their Curie temperatures [9]. These properties make M ferrites potentially ideal for use in non-reciprocal microwave devices provided the resonance line-width can be reduced to single crystal values, by exploiting the effect of gyromagnetic resonance that appears when the material is subjected to a low power microwave field perpendicular to the axis of magnetisation. This investigation into the formation, characterisation and microwave properties of aligned M ferrite fibres is part of an ongoing programme into the
0304-8853/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 1 0 7 - 3
R.C. Pullar et al. / Journal of Magnetism and Magnetic Materials 186 (1998) 326—332
development of refractory fibres, manufactured from aqueous sol—gel routes. The many advantages of sol—gel processes, fibrous versus bulk materials and fibre composites have been discussed in previous publications [10,15].
2. Experimental 2.1. Sample preparation An acid-peptised, halogen-stabilised iron(III) hydroxide sol (Fe : anion"3 : 2) was doped with a stoichiometric amount of a strontium salt, which had been previously dissolved into a solution with an organic liganding agent. Spinnability was bestowed by the addition of a small amount of polyethylene oxide as a spinning aid, and the fibres produced on a proprietary blow spinning process [11]. The resulting gel fibres were collected as an aligned tow blanket and stored in a circulating air oven at 110°C. The gel fibres were heat treated in a muffle furnace, firstly being pre-fired to 400°C at 100°C/h to remove water and organic compounds, and then further heat-treated at 200°C/h to 1000°C in a re-crystallised alumina vessel for three hours to form strontium M ferrite. Barium M ferrite was prepared by a similar method as previously reported [10], but was this time also collected as an aligned tow blanket and calcined under the same firing regime, and the characteristics of the two ferrite fibres were compared. 2.2. Characterisation 2.2.1. Photon correlation spectroscopy (PCS) Particle size measurement of the sol above the 3 nm diameter range was measured on a Malvern Instruments Lo-C autosizer and series 7032 multi-8 correlator, using a 4 mW diode laser, 670 nm wavelength. 2.2.2. Scanning electron microscopy (SEM) Scanning electron micrographs and analysis of the morphology of the samples was carried out on a Cambridge Instruments Stereoscan 90 SEM operating at 15 kV. Conducting samples were prepared by gold sputtering fibre specimens.
327
2.2.3. Surface area and porosity measurements Surface areas and pore size distributions of the fibres were performed on a Micrometrics ASAP 2000 using N as the adsorption gas. Samples were 2 degassed at 300°C for 6 h prior to analysis. 2.2.4. X-ray photoelectron spectroscopy (XPS) The XPS analysis was performed using a Kratos XSAM 800 spectrometer fitted with a dual anode (Mg/Al) X-ray source and a multichannel detector. The spectrometer was calibrated using the Ag3d5@2 line at 397.9 eV and the AgMVV line at 901.5 eV. Al K radiation (1486.6 eV) was the exciting source a (120 W) and spectra were collected in the high resolution mode (1.2 eV) and Fixed Analyser Transmission (FAT). The Kratos DS800 software was used for data acquisition and analysis. 2.2.5. X-ray fluorescence spectrometry (XRF) The elemental composition of the samples was measured on a Philips PW2400 sequential X-ray spectrometer fitted with a rhodium target end window X-ray tube and Philips X-40 analytical software. The samples were analysed in the form of a fused bead, where 1 g of sample was fused with 10 g of lithium tetraborate flux at 1250°C for 12 min and then cast to form a glass bead. 2.2.6. X-ray powder diffraction (XRD) measurement X-ray powder diffraction patterns of the samples treated at various temperatures were recorded in the region of 2h"10—80° with a scanning speed of 0.25°/min on a Philips PW1710 diffractometer using Cu K radiation with a nickel filter. Cell a parameters were calculated and further refined using linear regression procedures applied to the measured peak positions of all major reflections up to 2h"90° with the Philips APD 1700 software. This software was also used to calculate the average size of the crystallites in a sample using the wellknown Scherrer equation: D"Kj/h cos h 1@2 where D is the average size of the crystallites, K the Scherrer constant (0.9]57.3), j the wavelength of radiation (1.5405), h the peak width at half height 1@2 and h corresponds to the peak position.
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2.2.7. Assessment of fibre alignment The fibres were collected as a blanket on a high speed rotor, in a manner similar to that used for ‘Safimax’ alumina fibres [12]. There were differences compared to “Safimax”, which was an aligned blanket with 90% of the fibres within $10° and all within $20°. A small proportion of fibres crossing the general alignment was estimated using an optical microscope at 40]magnification. The number of aligned fibres in a field was counted, together with the few crossing the alignment in the same field. Counts were made in up to 5 separate fields summing to several hundred generally aligned fibres and up to about 50 crossing the alignment. The direction of the generally aligned fibres was analysed by traversing the electron micrographs with a protractor normal to the axis of alignment and measuring the deviations of at least 100 individual fibres. Two sets of data were taken from opposite sides of each micrograph. Both the protractor measurement on micrographs or direct measurement at 40]magnification are viewing deviations set into the fibre on a 1—2 mm scale. These cannot be removed by subsequent tensioning and can affect the packing into composites. 2.2.8. Microwave measurements The complex permeability (k*) spectrum of the ceramic ferrite fibres were determined using established techniques. The fibres were milled to a fine powder and dispersed in paraffin wax to a volume loading of 30% ferrite. The wax/ferrite mixture was die-pressed to form a rectangular specimen which was inserted in a waveguide cell. Measurements of transmission and reflection were made between 40 and 60 GHz using a Hewlett Packard HP8510C vector network analyser coupled to a millimetre wave S-parameter test-set. The values of k* for the ferrite/wax composite were calculated from the measured S-parameters using the transmission line method of Nicholson and Ross [13]. The intrinsic ferrite properties were isolated using the Lichtenecker effective medium expressions: DkD "» DkD #(1!» )DkD , %&& & &%33*5% & 8!9 tan d "tand #tand , %&& &%33*5% 8!9
where DkD , DkD and DkD are the modulus of %&& &%33*5% 8!9 permeability of the composite, the ferrite and wax, respectively. » is the volume fraction of ferrite and & tan d is the loss tangent, defined by the relation tan d"kA/k@, where k@ is the real permeability and kA is the imaginary permeability.
3. Results and Discussion 3.1. Sol characterisation and stability The stability and the resulting size of the sol particles is sensitive to the preparative techniques and conditions employed, and PCS enabled us to measure and control the properties of the strontium doped iron (III) sol to a certain extent. The PCS data indicates that the average particle size of the strontium doped iron (III) sol was 6.4 nm, with a polydispersity of 0.82 and an average particle weight of 4.2]104 amu. By volume distribution, the mean size was found to be 7.0 nm, with an upper limit of 30 nm, and these results are similar to the volume distribution of the barium doped sol (Fig. 1). The characteristics of the barium doped iron (III) sol have been reported in a previous publication [10], and are reproduced in Table 1 for comparison to those of the strontium doped sol. It must be considered that the technique is unable to detect particles below the 3 nm threshold, and therefore these measurements may be greater than the actual true figures. 3.2. M ferrite fibre characterisation and morphology The SrM ferrite fibres had smooth, parallel sides and had retained their fibrous nature with a diameter of 3—6 lm. The grain size was generally found to be at the submicron level, between a range of 0.1—1 lm, and there were no pores visible on the surface of the fibres (Fig. 2). By comparison, commercial M ferrite specimens normally demonstrate a grain size of 1—5 lm at 1000°C [14]. Surface area data on the SrM ferrite fibre gave a low surface area of 0.88 m2 g~1 and an average pore diameter of 48 nm using N adsorption. 2
R.C. Pullar et al. / Journal of Magnetism and Magnetic Materials 186 (1998) 326—332
329
Fig. 1. PCS volume distribution curves for (1) Ba doped and (2) Sr doped iron (III) sols.
Table 1 Comparison of Sr and Ba doped iron (III) sols
Sol Z average (nm) Polydispersity Molecular weight (amu]103) Volume average (nm) Volume upper limit (nm)
Sr doped sol
Ba doped sol
6.4 0.82 42 7.0 30
7.4 0.63 56 6.6 40
A deviation in either the stoichiometry [16] or oxidation state of the iron (III) [5] in the ferrite can have an adverse affect on its magnetic and microwave properties, and therefore the composition of the fibres was established. The XPS analysis of the SrM fibres showed the oxidation state of the iron to be Fe (III) with a binding energy of 710.5 eV for the main Fe 2p peak. The XRF elemental analysis for the oxides SrO and Fe O confirmed the composi2 3 tion to be SrFe O , and the XRD pattern of the 12 19 powdered fibres fired to 1000°C proved them to be single phase SrM ferrite (Fig. 3). The average crystallite size was estimated to be 59 nm from the width-at-half-height of the 100% peak at 2h" 34.2° using the Scherrer equation.
Fig. 2. SEM micrograph of SrM ferrite fibres fired to 1000°C for 3 h.
The composition of the aligned BaM ferrite fibres was also confirmed by XRF and XRD techniques (Fig. 3), and they were shown to be similar in both quality and microstructure to the random BaM fibres reported previously [10]. Various characteristics of the two aligned M ferrite fibres are compared in Table 2, and the similarities between the two different fibres can be clearly seen.
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Fig. 4. SEM micrograph of an aligned BaM fibre blanket fired to 1000°C for 3 h.
Fig. 3. XRD patterns of (a) BaM and (b) SrM ferrite fibres fired to 1000°C.
Table 2 Comparison of SrM and BaM aligned ferrite fibres
Crystallite size (nm) Fibre diameter (lm) Surface area (m2 g~1) Average pore diameter (nm) Alignment (within $20° of axis)
SrM fibres
BaM fibres
59 3—6 0.88 48 80%
57 3—5 0.85 53 88%
3.3. Alignment of the M ferrite fibre blankets The BaM ferrite fibres were well aligned with 88% of the fibres being within $20° of the axis of alignment and 69% within $10° (Fig. 4). However, the SrM ferrite fibres were not as well aligned. The fibres were undulating, with some individual fibres being looped almost perpendicularly to the axis of alignment. Although only some 80% of the
Fig. 5. SEM micrograph of an aligned SrM fibre blanket fired to 1000°C for 3 h.
SrM fibres were aligned within $20° of this axis and 62% within $10° (Fig. 5), it is thought that the differences between the two fibres are due to minor variations during the actual spinning and collection, rather than any intrinsic variance in the properties of the two sols. Both of these fibres compare less well with commercially available fibres, such as “Safimax” alumina [12], a development product, where all the fibres are within $20 and 90% within $10°, than those previously manufactured at Warwick [15,17], which have been reported with an alignment of up to 98%
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Fig. 6. FMR permeability components k@ (the real permeability) and kA (the imaginary permeability) for SrM ferrite fibres fired to 1000°C.
Fig. 7. FMR permeability components k@ (the real permeability) and kA (the imaginary permeability) for BaM ferrite fibres fired to 1000°C.
within $20° and 85% within $10° of the axis of alignment. The proprietary “Safimax” fibre process was modified to allow the collection of more friable gel fibres. The collection rotor was operated well below, rather than slightly above the fibre generating velocity, and an open diverging rather than a converging air duct was used. A known result of this differential between the velocity of the fibre draw and the speed of the collection drum is looping, and this problem should be removed with further optimisation of the spinning process, thus improving the alignment of both fibres.
ceramic methods are 51.5 and 45 GHz, respectively, which is in good agreement with our measurements.
3.4. Microwave measurements There is a wealth of information on the M-type hexagonal ferrites, some of which deals with their high frequency properties [18,19] and their expected natural ferromagnetic resonance (FMR) frequencies. The measured spectra of k@ and kA are shown in Figs. 6 and 7, where k@ is the real permeability and kA is the imaginary permeability. The FMR frequency is defined as the point at which kA is a maximum, and which also coincides with the half-peak value of k@. The FMR frequency recorded for the SrM ferrite fibres was 50 GHz (Fig. 6) and that of the BaM fibres occurred at 43.5 GHz (Fig. 7). Literature values for the FMR frequencies of SrM and BaM manufactured by conventional
4. Conclusions A stable, strontium doped iron (III) sol was produced, and from it gel-fibres were successfully spun and collected in the form of an aligned tow blanket. The gel fibres were calcined to 1000°C and then characterised by various techniques. They were found to be pure phase SrM ferrite at this temperature whilst maintaining their fibrous nature, and with an improved microstructure compared to conventionally prepared specimens. BaM fibres were also spun from a doped sol, and collected as an aligned tow for the first time and compared with SrM fibres. These were also characterised, and found to be identical to the previously reported random BaM ferrite fibres [10]. Both the SrM and BaM fibres were found to be less well aligned than other aligned hexagonal ferrite fibres previously reported by the Warwick group, and generally the SrM blankets were undulating, rather than linear in appearance. To maintain a useful packing density in composite materials fibres are required to be both straight and well aligned, but we feel certain that the alignment and linearity of both fibres can be improved upon with further refinement of the spinning process.
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The measured FMR frequencies of both materials were in excellent accord with the literature data, and further investigations into their magnetic characteristics are currently underway. Acknowledgements R.C. Pullar wishes to thank the Centre for Catalytic Systems and Materials Engineering and the DERA for providing funding for his research associateship. Our thanks to D. Croci for surface area measurements and R.C. Reynolds for the XPS and XRD characterisation (both at the Centre for Catalytic Systems and Materials Engineering, Department of Engineering, University of Warwick) and R. Burton for the XRF analysis (Materials Research Institute, Sheffield Hallam University). The microwave data contained herein is published with the permission of the controller of Her Brittanic Majesty’s Stationery Office. References [1] J.J. Went, G.W. Rathenau, E.W. Gorter, G.W. Van Oosterhaut, Phil. Tech. Rev. 13 (1952) 194. [2] H. Jonker, H.P. Wijn, P.B. Braun, Phil. Tech. Rev. 18 (1956) 145. [3] V. Adelskold, Arkiv. Kemi. Min. Geol. A 12 (1938) 1.
[4] E.A.M. Van Der Broek, A.L. Stuijts, Phil. Tech. Redsch. 37 (1977—8) 169. [5] J. Nicolas, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials, vol. 2, North-Holland, Amsterdam, 1980, p. 291. [6] E.E. Riches, Ferrites, Mills, Boon Technical Library, London, 1972, p. 59. [7] J. Smit, H.P.J. Wijn, Ferrites, Philips Technical Library, Eindhoven, 1959, p. 195. [8] W.H. Von Aulock, C.E. Fay, in: L. Marton (Ed.), Linear Ferrite Devices for Microwave Applications, Academic Press, New York, 1968, p. 14. [9] H. Stablein, in: E.P Wohlfarth (Ed.), Ferromagnetic Materials, vol. 3, North-Holland, Amsterdam, 1982, p. 448. [10] R.C. Pullar, M.D. Taylor, A.K. Bhattacharya, J. Mater. Sci. 32 (1997) 349. [11] M.J. Morton, J.D. Birchall, J.E. Cassidy (ICI), UK Patent 1360200 (1974). [12] M.H. Stacey, M.D. Taylor (ICI), Eur. Patent 318203 (1987). [13] A.M. Nicholson, G. Ross, IEEE Trans. Instr. Meas. 19 (1970) 377. [14] H. Stablein, Tech. Mitt. Krupp., Forsch.-Ber. 26 (1968) 81. [15] R.C. Pullar, M.D. Taylor, A.K. Bhattacharya, J. Mater Sci. 32 (1997) 873. [16] J. Smit, H.P.J. Wijn, Ferrites, Philips Technical Library, Eindhoven, 1959, p. 221. [17] R.C. Pullar, S.G. Appleton, M.D. Taylor, M.H. Stacey, A.K. Bhattacharya, J. Magn. Magn. Mater. 186 (1998) (this issue). [18] H. Kojima, in: E.P Wohlfarth (Ed.), Ferromagnetic Materials, vol. 3, North-Holland, Amsterdam, 1982, pp. 345—347. [19] J. Smit, H.P.J. Wijn, Ferrites Philips Technical Library, Eindhoven, 1959, pp. 78—84.