- Email: [email protected]
Fine metallic and ceramic fibers by melt extraction
158
Materials" Science and Engineering, A 17~)/A I A'O (1994) 158-162
Fine metallic and ceramic fibers by melt extraction J. O. S t r 6 m - O l s e n , G. R u d k o w s k a a n d P. R u d k o w s k i
Centerfor the Physics of Materials, McGill University, Department of Physics, 3600 UniversityStreet, Montreal, Que. H3A 2T8 (Canada) M. A l l a h v e r d i a n d R. A. L. D r e w
Department of MetallurgicalEngineering, McGill University, 3450 UniversityStreet, Montreal, Que. H3A 2A 7 (Canada)
Abstract Techniques are described for the fabrication of metal and ceramic fibers of diameter below 20/~m by a method of melt extraction. The process is containerless and has been successfully applied to reactive metals and high temperature ceramics. The properties of the fibers, where exceptional, are reviewed.
1. Fabrication The demand for fine metal and ceramic fibers has led to the development of a variety of techniques for their manufacture. For metals, perhaps the simplest method is to draw fibers down sequentially in bundles from coarser fibers, which are first coated to prevent cold welding. The process tends to leave the fiber with a very damaged surface. Drawing them down molten in a Pyrex tube (the Taylor process) [1, 2] yields betterquality material, but with a limited production rate, and with additional problems associated with reactions between the liquid metal and glass tube. Liquid quenching into water [3] improves the production rate but gives wires of 100/~m diameter or more which need further drawing. Ceramic fibers, of course, cannot be made by any of these techniques because of the high melting temperature and the resistance to plastic deformation. Methods used here include chemical vapor deposition, sol-gel processing and slurry spinning. In the present article, we review a method of making either metal or ceramic fibers by modifications of the familiar melt extraction process [4, 5] in which the material is cast directly from the melt by introducing a sharpened wheel into the liquid. The method that we shall describe has several advantages over other techniques and produces material of very high quality. Figure l(a) shows the principle of operation applied to metals. The metal is first formed into a rod of diameter 5-10 mm. It passes through a loosely fitting guide of boron nitride or similar refractory insulating material and is heated at the tip by an almost flat r.f. induction coil. The electromagnetic levitation forces
from the coil combine with surface tension and gravity to allow the molten region to be formed into a fine tip which, unlike the pendant drop method [4], is introduced onto the rod from below. The fine tip is an essential feature of the method and permits continuous casting of high quality fibers below 20 ktm diameter. The molten tip is generally quite stable once accessed by the wheel, and the rod may be steadily fed onto the wheel to replace the material being extracted. The rate of feed and the tangential velocity of the wheel deter-
/
Mo
WHEEL
Mo
WHEEL
LASER
REA--H--M[CZl3 rnrnCERAMICROO
(b/
1~i
Fig. I. Schematic diagram (a) for melt extraction of a metal and (b) for a ceramic. Elsevier Sequoia.
,';5I)1 (t921-5093(93)05799-U
J. O. Strrm-Olsen et al.
/
Fine metallic and ceramic fibers by melt extraction
mine within limits the diameter of the fiber. For ferrous metals a rod of 6 mm diameter may be fed at about 0.25 mm s- 1 onto the wheel with a tangential velc, city of about 30 m s-1 to produce fibers of diameters 15 + 5 am. If the relatively small amount of material lost through oxides and other impurities is ignored, these values imply that the wheel is extracting fibers for about 75% of the time. The process runs automatically without the need for further fine adjustment. At McGill we have left the system operating essentially unattended for up to i h. It should be stressed that, although the fabrication process is continuous, the fiber as a rule is not. Small defects on the wheel, particles of oxide and other impurities, vibration and other effects serve to break the fiber and it is not easy to obtain lengths longer than several meters. The extraction process is generally carried out under an inert gas at a reduced pressure (e.g. 15 kPa) both to control oxidation and to reduce the turbulence associated with gas boundary layers. Some environmental gas is desirable. Casting under nominal vacuum leads to problems with plasma discharge from the r.f. coil and evaporation of the molten metal. Also, the gas provides some useful cooling of the fiber after extraction and of the extraction wheel. Figure l(b) shows how melt extraction is applied to ceramics. The heat source is a 180 W CO2 laser focused down to 2-3 mm. Since electromagnetic levitation can no longer be used to shape the molten tip, other methods must be found. A simple solution is to use a thinner feed rod so that surface tension alone provides a sufficiently fine tip. Once a stable configuration is found, the system again operates automatically. Typical production parameters are fibers of 10 _+2/~m diameter for a tangential wheel speed of 10 m s- ~. The extraction wheel should be made of a refracting material of good thermal conductivity able to take a good edge and to withstand rapid wide fluctuations in temperature. Figure 2 shows the tip of a molybdenum wheel in cross-section that has been successfully used to extract both ceramic and metal fibers. The range of metals and ceramics that may be cast by this process is limited only by the constraints of sufficient wetting of the wheel and reasonably low viscosity, ideally below 1 P. Liquid metals offer few problems and almost all may be extracted into fibers. With ceramics the situation is slightly less favorable; wetting of the metal wheel may be poor and viscosity, especially for ceramics containing silica, may be high. Ceramics that have been successfully cast include mixtures of A1203,ZrO2, TiO2 and CaO, as well as a wide range of ferrites, and yttrium and bismuth-based high Tc superconductivity oxides. Figures 3 and 4 show fibers of a metal and a ceramic respectively. Both show all the principal characteristics
159
Fig. 2. A cast of the tip of a molybdenum wheel successfullyused for high quality fiber production.
Fig. 3. Melt-extracted fiber of Permalloy. Note the track of the extraction wheel.
Fig. 4. A melt-extracted fiber of ZrO2-Al203. As in Fig. 3, note the wheel track.
160
J. O. StrOm-Olsen et al.
/
Fine metallic and ceramic fibers by melt extraction
of the fibers made by this process: a cross-section which is essentially circular, a line of contact with the wheel and an extremely clean and defect-free surface. The last important feature is a direct consequence of the fact that most of the surface is free-formed and does not come into contact with a cooling substrate (cf. melt spinning). The circular cross-section results from the dominance of the force of surface tension. If the liquid has surface tension a and a density p and is being extracted at a tangential velocity V by a wheel of radius a, then the pressure due to surface tension exceeds that of centrifugal force for a fiber radius less than ( o a / p V 2 ) 1/2. Under typical extraction conditions for metals, this is about 50 ,urn and for ceramics about 200/zm. The fact that the cross-section of the fiber is a circular drop hanging from the wheel shows that the liquid solidifies after being drawn from the molten tip. If the wheel is deeply inserted into the melt, then solidification may occur before extraction and a crosssection of shape closer to the isotherms surrounding the tip in the melt results. Under these conditions, many of the desirable properties of the fibers discussed below are lost. This is one reason why it is desirable to have the molten tip small, so that extraction from the melt before solidification is relatively easy. It also suggests that the size of the fibers is determined by the viscous boundary layer (whose thickness is difficult to estimate because of the temperature dependence of the viscosity). The apparent upper limit for circular fibers is between 20 and 30/~m. In comparing the process of making metal fibers with ceramic fibers, it was noted above that metals have good wetting of the molybdenum wheel and have relatively low viscosities (about 10 -2 P). As a result, the extraction of metal fibers is a "forgiving" technology, in contrast with ceramic fibers where poorer wetting, higher viscosity and lower thermal conductivity restrict the acceptable range of parameters. In particular, running the extraction wheel at too high a velocity leads to intermittent contact and hence a reduced quenching rate. Under these conditions, which approximate high speed drawing, the fibers develop Rayleigh waves (Fig. 5). As mentioned above, cooling rates are comparable with those found in the melt-spinning process. If we compare a metal ribbon of thickness h with a metal fiber of radius r making contact with the wheel over a length l of its circumference, the ratio of heat loss to the wheel (fiber to ribbon) is of the order lh/TEr 2, s o that a 10/zm fiber cools by this mechanism at the same rate as a 25 ~m ribbon. However, total quenching of the fiber may be significantly greater because of convective losses at the surface through the environmental gas. For ceramics, heat conduction is much lower and it is difficult to estimate the heat flow into the wheel. However,
Fig. 5. A fiber of amorphous Ba-Sr-Ca-Cu-O twisted after manufacture.
because of the much higher melt temperatures, radiative cooling can play a significant role. For a ceramic fiber of radius r at a temperature J] the instantaneous quenching rate once it leaves the laser beam is QR = r/
2osT a 2 M R
3r p
(1)
where o s is Stefan's constant, R the gas constant, M the molecular weight, p the density and ~1 the emissivity. For typical ceramic fibers Q~ can easily exceed 105 K s ~at the melt temperature. Finally, we should emphasize one great advantage of this technique over most liquid quenching methods, namely that the process is containerless, the melt being supported by the same material in the solid state. This allows fibers to be made from highly reactive metals, such as Zr and Ti, and from high temperature ceramics, such as A120~, for which finding a crucible is challenging.
2. Properties The fibers exhibit several very unusual properties. Mechanically they are almost always very flexible, even when made of highly brittle ceramics. Figure 6 shows B i - C a - S r - C u - O twisted into a small loop after manufacture. This fiber is amorphous (as shown by X-ray diffraction), but even polycrystalline materials, especially metals, retain some flexibility. As a corollary to the flexibility, the fibers have a very high fracture strength, which typically increases as the diameter decreases. Figure 7 shows fracture strengths for amorphous and crystalline fibers of metals and ceramics, together, for
J. O. StrOm-Olsen et al.
/
Fine metallic and ceramic fibers by melt extraction
comparison, with data for fibers produced by melt drawing or water spinning. Not surprisingly, amorphous materials generally have the highest fracture strengths but even crystalline fibers are exceptionally strong. To date the strongest fiber that we have made is
Fig. 6. A ZrO2-A1203 fiber cooled too slowly to prevent Rayleigh wave formation.
10
8 []
o 8
o
O
0 4
I~
=o
•
v
D D [] --~IDOQ []
Q= 2
=o
• •
0~
1'0
2'0
| •go
0
3tO 4'0 d (/.zm)
5'0
60
Fig. 7. Yield strength as a function of diameter: n, amorphous FelsSi10B15 (data from ref. 5);~m-.'-~, amorphous Fe75Si10B15 water spun and drawn (data from ref. 3); o, crystalline Fe67.sCosCr10NisCuzMo0.5B~0 (data from ref. 5); A, Fe67.5CosCrioNisCu2Moo.sB,~ filaments obtained by melt-spinning (data from ref. 2); •, crystalline ZrO2-A120 3 ceramic fibers; ", amorphous ZrO2-A1202-TiO2.
161
a 3/~m fiber of Fe-Si-B whose fracture strength was about 10 GPa. Elastic moduli are comparable with those of the bulk material. Magnetically, too, there is considerable enhancement of performance, at least for soft magnetic alloys. Table 1 shows permeability and coercive fields for some representative amorphous alloys including high performance commercial alloys in ribbon and fiber form. The data were taken with the sample in toroidal form, showing that the enhancement in performance is intrinsic and not a shape effect. Also shown in Table 1 are data for a crystalline Permalloy fiber. This last result is particularly interesting since good performance from Permalloy-type magnetic material can usually only be obtained after long and careful anneali n g - t h e reverse of the melt extraction process! In particular, melt-spun Permalloy has a very poor magnetic performance [6]. In fact, to our knowledge this is the first time that rapid quenching methods have been successfully applied to Permalloys. Although not quite as good as the best amorphous alloys, Permalloy fibers offer the advantage that they may be used at temperatures beyond the devitrification point of amorphous alloys (typically 4 5 0 - 5 0 0 °C). The reason why fibers behave this way is a matter for speculation. The fact that the surface is so clear and free of faults is undoubtedly a major contributor to the high yield strength, and may also play a role in enhancing the magnetic performance. Also, we believe that the fibers are formed with a considerable level of quenched-in stress. When the fiber is extracted, it first cools at the tip and around the circumference, placing the still liquid core under compression. We have some indirect evidence for this from the behavior of the Permalloy fibers. As made, they not only show good performance, but also resist deterioration under mechanical deformation. When annealed (e.g. at 400°C for 1 h), however, the permeability diminishes and the resistance to stress disappears. We interpret this to mean that the fibers as made have so high a level of stress that any additional stress introduced by deformation is negligible. Once this stress is relieved, then the fibers have similar stress sensitivity as conventional Permalloys.
TABLE 1, Properties of some representative soft magnetic alloys in fiber and ribbon form Alloy
Form
D.c. permeability
D.c. coercive field (mOe)
Coercive field at 6 kHz (mOe)
Metglas 2605-$2 Metglas 2605-$2 Vitrovac 6025Z CoxIRe4NbzSi~gB4
Ribbon (3.3 mm × 25/~m) Fiber (16 pm) Ribbon (2.3 mm × 25/~m) Fiber (9/am) Fiber (7 pm) Fiber ( 16 pm)
1.0 × 105 2.3 × 105 1.5 x 105 2.5 x 105 7.5 × 104 2.8 × 104
190 50 50 20 30 490
750 240 200 175 160 750
Ni45Co26Fe6Si19BI3
Permalloy (Ni72Fe1iCUl3MO2Mn2)
162
Y. O. StrfJm-Olsen et al.
/
Hne metallic and ceramic.fibers by melt extraction
The Permalloy fibers have been successfully used to measure small magnetic fields by measuring magnetoresistance. Thin film magnetoresistive sensors have been in use for some time. They exploit the small difference in resistance when the current is aligned perpendicular or parallel to the magnetization. The domain structure of the Permalloy fibers is such that they make excellent devices of this type while having a ruggedness and flexibility denied thin films. The small size of the fibers ensures a large resistance which changes by about 2-3% on saturation. Typically these sensors have a linear response to the field over most of the range with a saturation field of a few Oersteds [7]. Other applications not involving magnetic properties include metal reinforcement by ceramic fibers ~qd rechargeable hydride electrodes. Fig. 8. The fiber in Fig. 3 treated by a grain boundary etch, revealing the crystallites growing away from the line of contact (data from ref. 7).
The microstructures of the fibers is similar to those of melt-spin ribbons, varying from amorphous through nanocrystalline to microcrystalline. However, the different geometry of cooling reflected in the stress pattern and the crystallite pattern. A good example is shown in Fig. 8, where a Permalloy fiber is shown which has crystallites growing away from the line of contact with the wheel in a generally fan-like structure. The favorable properties of the fibers are being developed for several applications including two exploiting the magnetic properties. The high permeability and favorable aspect ratio of the soft magnetic sensors are used in particular for magnetic article surveillance devices. Small tags containing as little as 1 mg of material may be detected in conventional detection gates with 99% certainty. The small size of the fiber makes direct incorporation into paper relatively easy, thus avoiding the additional step of attaching the magnetic material to the tag.
Acknowledgments This research has been supported by Natural Sciences and Engineering Research Council of Canada, Les Fonds pour la Formation des Chercheurs et "hl'Aide ~ Recherche du Qu6bec, Quebec and Pitney Bowes Inc. The authors wish to thank W. B. Muir for his assistance with the measurement of mechanical properties.
References 1 I.W. Donald, J. Mater. Sci., 22 ( 1987 ) 2661-2678. 2 T. Goto and T. Toyama, J. Mater. Sci., 20 (1985) 1883-1888. 3 M. Hagiwara, A. Inoue and T. Masumoto, Mater. Sci. Eng., 54 (1982) 197-207. 4 R. E. Maringer and C. E. Mobley, J. Vac. Sci. Technol., 11 (1974) 1067-1071. 5 E Rudkowski, G. Rudkowska and J. O. Str6m-Olsen, Mater. Sci. Eng., A133 (1991) 158-161. 6 A. Kalkami, J. V. Wood and M. R. J. Gibbs, J. Magn. Magn. Mater. A, 133 ( 1991 ) 395-398. 7 P. Ciureanu, G. Rudkowska, E Rudkowski and J. O. Str6mOlsen, 1EEE Trans. Magn., 29 (1993) 2251-2257.
Materials" Science and Engineering, A 17~)/A I A'O (1994) 158-162
Fine metallic and ceramic fibers by melt extraction J. O. S t r 6 m - O l s e n , G. R u d k o w s k a a n d P. R u d k o w s k i
Centerfor the Physics of Materials, McGill University, Department of Physics, 3600 UniversityStreet, Montreal, Que. H3A 2T8 (Canada) M. A l l a h v e r d i a n d R. A. L. D r e w
Department of MetallurgicalEngineering, McGill University, 3450 UniversityStreet, Montreal, Que. H3A 2A 7 (Canada)
Abstract Techniques are described for the fabrication of metal and ceramic fibers of diameter below 20/~m by a method of melt extraction. The process is containerless and has been successfully applied to reactive metals and high temperature ceramics. The properties of the fibers, where exceptional, are reviewed.
1. Fabrication The demand for fine metal and ceramic fibers has led to the development of a variety of techniques for their manufacture. For metals, perhaps the simplest method is to draw fibers down sequentially in bundles from coarser fibers, which are first coated to prevent cold welding. The process tends to leave the fiber with a very damaged surface. Drawing them down molten in a Pyrex tube (the Taylor process) [1, 2] yields betterquality material, but with a limited production rate, and with additional problems associated with reactions between the liquid metal and glass tube. Liquid quenching into water [3] improves the production rate but gives wires of 100/~m diameter or more which need further drawing. Ceramic fibers, of course, cannot be made by any of these techniques because of the high melting temperature and the resistance to plastic deformation. Methods used here include chemical vapor deposition, sol-gel processing and slurry spinning. In the present article, we review a method of making either metal or ceramic fibers by modifications of the familiar melt extraction process [4, 5] in which the material is cast directly from the melt by introducing a sharpened wheel into the liquid. The method that we shall describe has several advantages over other techniques and produces material of very high quality. Figure l(a) shows the principle of operation applied to metals. The metal is first formed into a rod of diameter 5-10 mm. It passes through a loosely fitting guide of boron nitride or similar refractory insulating material and is heated at the tip by an almost flat r.f. induction coil. The electromagnetic levitation forces
from the coil combine with surface tension and gravity to allow the molten region to be formed into a fine tip which, unlike the pendant drop method [4], is introduced onto the rod from below. The fine tip is an essential feature of the method and permits continuous casting of high quality fibers below 20 ktm diameter. The molten tip is generally quite stable once accessed by the wheel, and the rod may be steadily fed onto the wheel to replace the material being extracted. The rate of feed and the tangential velocity of the wheel deter-
/
Mo
WHEEL
Mo
WHEEL
LASER
REA--H--M[CZl3 rnrnCERAMICROO
(b/
1~i
Fig. I. Schematic diagram (a) for melt extraction of a metal and (b) for a ceramic. Elsevier Sequoia.
,';5I)1 (t921-5093(93)05799-U
J. O. Strrm-Olsen et al.
/
Fine metallic and ceramic fibers by melt extraction
mine within limits the diameter of the fiber. For ferrous metals a rod of 6 mm diameter may be fed at about 0.25 mm s- 1 onto the wheel with a tangential velc, city of about 30 m s-1 to produce fibers of diameters 15 + 5 am. If the relatively small amount of material lost through oxides and other impurities is ignored, these values imply that the wheel is extracting fibers for about 75% of the time. The process runs automatically without the need for further fine adjustment. At McGill we have left the system operating essentially unattended for up to i h. It should be stressed that, although the fabrication process is continuous, the fiber as a rule is not. Small defects on the wheel, particles of oxide and other impurities, vibration and other effects serve to break the fiber and it is not easy to obtain lengths longer than several meters. The extraction process is generally carried out under an inert gas at a reduced pressure (e.g. 15 kPa) both to control oxidation and to reduce the turbulence associated with gas boundary layers. Some environmental gas is desirable. Casting under nominal vacuum leads to problems with plasma discharge from the r.f. coil and evaporation of the molten metal. Also, the gas provides some useful cooling of the fiber after extraction and of the extraction wheel. Figure l(b) shows how melt extraction is applied to ceramics. The heat source is a 180 W CO2 laser focused down to 2-3 mm. Since electromagnetic levitation can no longer be used to shape the molten tip, other methods must be found. A simple solution is to use a thinner feed rod so that surface tension alone provides a sufficiently fine tip. Once a stable configuration is found, the system again operates automatically. Typical production parameters are fibers of 10 _+2/~m diameter for a tangential wheel speed of 10 m s- ~. The extraction wheel should be made of a refracting material of good thermal conductivity able to take a good edge and to withstand rapid wide fluctuations in temperature. Figure 2 shows the tip of a molybdenum wheel in cross-section that has been successfully used to extract both ceramic and metal fibers. The range of metals and ceramics that may be cast by this process is limited only by the constraints of sufficient wetting of the wheel and reasonably low viscosity, ideally below 1 P. Liquid metals offer few problems and almost all may be extracted into fibers. With ceramics the situation is slightly less favorable; wetting of the metal wheel may be poor and viscosity, especially for ceramics containing silica, may be high. Ceramics that have been successfully cast include mixtures of A1203,ZrO2, TiO2 and CaO, as well as a wide range of ferrites, and yttrium and bismuth-based high Tc superconductivity oxides. Figures 3 and 4 show fibers of a metal and a ceramic respectively. Both show all the principal characteristics
159
Fig. 2. A cast of the tip of a molybdenum wheel successfullyused for high quality fiber production.
Fig. 3. Melt-extracted fiber of Permalloy. Note the track of the extraction wheel.
Fig. 4. A melt-extracted fiber of ZrO2-Al203. As in Fig. 3, note the wheel track.
160
J. O. StrOm-Olsen et al.
/
Fine metallic and ceramic fibers by melt extraction
of the fibers made by this process: a cross-section which is essentially circular, a line of contact with the wheel and an extremely clean and defect-free surface. The last important feature is a direct consequence of the fact that most of the surface is free-formed and does not come into contact with a cooling substrate (cf. melt spinning). The circular cross-section results from the dominance of the force of surface tension. If the liquid has surface tension a and a density p and is being extracted at a tangential velocity V by a wheel of radius a, then the pressure due to surface tension exceeds that of centrifugal force for a fiber radius less than ( o a / p V 2 ) 1/2. Under typical extraction conditions for metals, this is about 50 ,urn and for ceramics about 200/zm. The fact that the cross-section of the fiber is a circular drop hanging from the wheel shows that the liquid solidifies after being drawn from the molten tip. If the wheel is deeply inserted into the melt, then solidification may occur before extraction and a crosssection of shape closer to the isotherms surrounding the tip in the melt results. Under these conditions, many of the desirable properties of the fibers discussed below are lost. This is one reason why it is desirable to have the molten tip small, so that extraction from the melt before solidification is relatively easy. It also suggests that the size of the fibers is determined by the viscous boundary layer (whose thickness is difficult to estimate because of the temperature dependence of the viscosity). The apparent upper limit for circular fibers is between 20 and 30/~m. In comparing the process of making metal fibers with ceramic fibers, it was noted above that metals have good wetting of the molybdenum wheel and have relatively low viscosities (about 10 -2 P). As a result, the extraction of metal fibers is a "forgiving" technology, in contrast with ceramic fibers where poorer wetting, higher viscosity and lower thermal conductivity restrict the acceptable range of parameters. In particular, running the extraction wheel at too high a velocity leads to intermittent contact and hence a reduced quenching rate. Under these conditions, which approximate high speed drawing, the fibers develop Rayleigh waves (Fig. 5). As mentioned above, cooling rates are comparable with those found in the melt-spinning process. If we compare a metal ribbon of thickness h with a metal fiber of radius r making contact with the wheel over a length l of its circumference, the ratio of heat loss to the wheel (fiber to ribbon) is of the order lh/TEr 2, s o that a 10/zm fiber cools by this mechanism at the same rate as a 25 ~m ribbon. However, total quenching of the fiber may be significantly greater because of convective losses at the surface through the environmental gas. For ceramics, heat conduction is much lower and it is difficult to estimate the heat flow into the wheel. However,
Fig. 5. A fiber of amorphous Ba-Sr-Ca-Cu-O twisted after manufacture.
because of the much higher melt temperatures, radiative cooling can play a significant role. For a ceramic fiber of radius r at a temperature J] the instantaneous quenching rate once it leaves the laser beam is QR = r/
2osT a 2 M R
3r p
(1)
where o s is Stefan's constant, R the gas constant, M the molecular weight, p the density and ~1 the emissivity. For typical ceramic fibers Q~ can easily exceed 105 K s ~at the melt temperature. Finally, we should emphasize one great advantage of this technique over most liquid quenching methods, namely that the process is containerless, the melt being supported by the same material in the solid state. This allows fibers to be made from highly reactive metals, such as Zr and Ti, and from high temperature ceramics, such as A120~, for which finding a crucible is challenging.
2. Properties The fibers exhibit several very unusual properties. Mechanically they are almost always very flexible, even when made of highly brittle ceramics. Figure 6 shows B i - C a - S r - C u - O twisted into a small loop after manufacture. This fiber is amorphous (as shown by X-ray diffraction), but even polycrystalline materials, especially metals, retain some flexibility. As a corollary to the flexibility, the fibers have a very high fracture strength, which typically increases as the diameter decreases. Figure 7 shows fracture strengths for amorphous and crystalline fibers of metals and ceramics, together, for
J. O. StrOm-Olsen et al.
/
Fine metallic and ceramic fibers by melt extraction
comparison, with data for fibers produced by melt drawing or water spinning. Not surprisingly, amorphous materials generally have the highest fracture strengths but even crystalline fibers are exceptionally strong. To date the strongest fiber that we have made is
Fig. 6. A ZrO2-A1203 fiber cooled too slowly to prevent Rayleigh wave formation.
10
8 []
o 8
o
O
0 4
I~
=o
•
v
D D [] --~IDOQ []
Q= 2
=o
• •
0~
1'0
2'0
| •go
0
3tO 4'0 d (/.zm)
5'0
60
Fig. 7. Yield strength as a function of diameter: n, amorphous FelsSi10B15 (data from ref. 5);~m-.'-~, amorphous Fe75Si10B15 water spun and drawn (data from ref. 3); o, crystalline Fe67.sCosCr10NisCuzMo0.5B~0 (data from ref. 5); A, Fe67.5CosCrioNisCu2Moo.sB,~ filaments obtained by melt-spinning (data from ref. 2); •, crystalline ZrO2-A120 3 ceramic fibers; ", amorphous ZrO2-A1202-TiO2.
161
a 3/~m fiber of Fe-Si-B whose fracture strength was about 10 GPa. Elastic moduli are comparable with those of the bulk material. Magnetically, too, there is considerable enhancement of performance, at least for soft magnetic alloys. Table 1 shows permeability and coercive fields for some representative amorphous alloys including high performance commercial alloys in ribbon and fiber form. The data were taken with the sample in toroidal form, showing that the enhancement in performance is intrinsic and not a shape effect. Also shown in Table 1 are data for a crystalline Permalloy fiber. This last result is particularly interesting since good performance from Permalloy-type magnetic material can usually only be obtained after long and careful anneali n g - t h e reverse of the melt extraction process! In particular, melt-spun Permalloy has a very poor magnetic performance [6]. In fact, to our knowledge this is the first time that rapid quenching methods have been successfully applied to Permalloys. Although not quite as good as the best amorphous alloys, Permalloy fibers offer the advantage that they may be used at temperatures beyond the devitrification point of amorphous alloys (typically 4 5 0 - 5 0 0 °C). The reason why fibers behave this way is a matter for speculation. The fact that the surface is so clear and free of faults is undoubtedly a major contributor to the high yield strength, and may also play a role in enhancing the magnetic performance. Also, we believe that the fibers are formed with a considerable level of quenched-in stress. When the fiber is extracted, it first cools at the tip and around the circumference, placing the still liquid core under compression. We have some indirect evidence for this from the behavior of the Permalloy fibers. As made, they not only show good performance, but also resist deterioration under mechanical deformation. When annealed (e.g. at 400°C for 1 h), however, the permeability diminishes and the resistance to stress disappears. We interpret this to mean that the fibers as made have so high a level of stress that any additional stress introduced by deformation is negligible. Once this stress is relieved, then the fibers have similar stress sensitivity as conventional Permalloys.
TABLE 1, Properties of some representative soft magnetic alloys in fiber and ribbon form Alloy
Form
D.c. permeability
D.c. coercive field (mOe)
Coercive field at 6 kHz (mOe)
Metglas 2605-$2 Metglas 2605-$2 Vitrovac 6025Z CoxIRe4NbzSi~gB4
Ribbon (3.3 mm × 25/~m) Fiber (16 pm) Ribbon (2.3 mm × 25/~m) Fiber (9/am) Fiber (7 pm) Fiber ( 16 pm)
1.0 × 105 2.3 × 105 1.5 x 105 2.5 x 105 7.5 × 104 2.8 × 104
190 50 50 20 30 490
750 240 200 175 160 750
Ni45Co26Fe6Si19BI3
Permalloy (Ni72Fe1iCUl3MO2Mn2)
162
Y. O. StrfJm-Olsen et al.
/
Hne metallic and ceramic.fibers by melt extraction
The Permalloy fibers have been successfully used to measure small magnetic fields by measuring magnetoresistance. Thin film magnetoresistive sensors have been in use for some time. They exploit the small difference in resistance when the current is aligned perpendicular or parallel to the magnetization. The domain structure of the Permalloy fibers is such that they make excellent devices of this type while having a ruggedness and flexibility denied thin films. The small size of the fibers ensures a large resistance which changes by about 2-3% on saturation. Typically these sensors have a linear response to the field over most of the range with a saturation field of a few Oersteds [7]. Other applications not involving magnetic properties include metal reinforcement by ceramic fibers ~qd rechargeable hydride electrodes. Fig. 8. The fiber in Fig. 3 treated by a grain boundary etch, revealing the crystallites growing away from the line of contact (data from ref. 7).
The microstructures of the fibers is similar to those of melt-spin ribbons, varying from amorphous through nanocrystalline to microcrystalline. However, the different geometry of cooling reflected in the stress pattern and the crystallite pattern. A good example is shown in Fig. 8, where a Permalloy fiber is shown which has crystallites growing away from the line of contact with the wheel in a generally fan-like structure. The favorable properties of the fibers are being developed for several applications including two exploiting the magnetic properties. The high permeability and favorable aspect ratio of the soft magnetic sensors are used in particular for magnetic article surveillance devices. Small tags containing as little as 1 mg of material may be detected in conventional detection gates with 99% certainty. The small size of the fiber makes direct incorporation into paper relatively easy, thus avoiding the additional step of attaching the magnetic material to the tag.
Acknowledgments This research has been supported by Natural Sciences and Engineering Research Council of Canada, Les Fonds pour la Formation des Chercheurs et "hl'Aide ~ Recherche du Qu6bec, Quebec and Pitney Bowes Inc. The authors wish to thank W. B. Muir for his assistance with the measurement of mechanical properties.
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