MXPA97006840A - Source of power and method for heating by induction of articu - Google Patents

Abstract

The present invention relates to a method for heating an assembly by means of electromagnetic radiation, the assembly comprising: (1) a composition comprising: (a) a host material that is not heated by electromagnetic radiation, and (b) ferromagnetic particles that are dispersed in the host material and have a Curie temperature, and (2) a loose component that is composed of a material that can be heated by electromagnetic radiation and that does not have a Curie temperature, said process comprises: (A) exposing the assembly to the electromagnetic radiation of a first energy that heats the ferromagnetic particles and the loose component to a first temperature, the first temperature is at or near a Curie temperature of the ferromagnetic particles, and (B) immediately after the stage ( A), exposing the assembly to the electromagnetic radiation of a second energy that heats the loose component to a smaller scale than the radiation of the first energy, the second energy is such that the heat generated within the loose component is approximately equivalent to the heat lost from the assembly

Description

SOURCE OF POWER AND METHOD FOR HEATING BY INDUCTION OF ITEMS Technical Field of the Invention This invention relates to a power source for heating an article by exposing the article to an electromagnetic field and to a method of heating an article.

Antecedents of the Invention Several technologies require the heating of material to achieve a transition of the material from an initial state to a final state that exhibits the desired characteristics. For example, heat is used to recover heat-recoverable polymeric articles such as heat-recoverable tubing and castings, cure gels, melt or cure adhesives, activate foaming agents, dry inks, cure ceramics, initiate polymerization, initiate or accelerate catalytic reactions or, treat parts with heat, among other applications. The speed at which the material is heated is an important consideration in the efficiency and effectiveness of the overall process. It is often difficult to obtain uniform heat distribution in the material through its center. In cases where the center of the material is not adequately heated, the transition from the initial state to the final state may not occur totally or uniformly. Alternatively, in order to obtain the desired temperature in the center of the article, the application of excessive heat on the surface may be required where the temperature conditions may lead to degradation of the material surface. Due to these disadvantages of external heating, mass or internal heating methods are often preferred to provide rapid, uniform and efficient heating. As described in commonly assigned U.S. Patent No. 5,378,879 issued January 3, 1995 to Monovoukas and entitled "Induction Heating of Charged Materials" which is incorporated herein by reference for all purposes , the induction heating can be used to heat a non-conductive material in situ quickly, uniformly, selectively and in a controlled manner. A non-magnetic and electrically non-conductive material is transparent to the magnetic field and, therefore, can not be coupled with the field to generate heat. However, said material can be heated by magnetic induction heating by uniformly distributing ferromagnetic particles within the material and exposing the article to an alternating high frequency electromagnetic field. Ferromagnetic particles for induction heating are added to the non-magnetic host material electrically non-conductive and exposed to the field? Alternating electromagnetic high frequency such as those produced in an inductor coil. The temperature of the ferromagnetic particles increases until the particles reach their Curie temperature and then, the particles are self-regulated at that temperature. While the induction heating of the ferromagnetic material is fast, effective and self-regulating in temperature, other components of the article can be damaged when subjected to the energy levels used to heat the ferromagnetic material. For example, in a case where copper conductors coated with insulator are present, the copper is inductively heated; however, copper does not have a Curie temperature, it is not self-regulating in temperature and it continues to be heated while supplying energy. Therefore, the insulation surrounding the copper continues to heat up due to the heat generated by the copper and, in this way, it is damaged. The window period, in which adequate heating of the article occurs without damage to the components, can be extremely small, if any.

Brief Description of the Invention It has been found that it is possible to extend the window period and improve the heating results of an article by induction heating by exposing the article to an electromagnetic field at a first energy level for a predetermined time and, subsequently, reduce the energy level. A first aspect of the invention comprises a method of heating an assembly by means of electromagnetic radiation, the assembly comprising: (1) a composition comprising: (a) a material that is not heated by electromagnetic radiation, and (b) ferromagnetic particles that are dispersed in the host material and have a Curie temperature; and (2) a loose component that is composed of a material that can be heated by electromagnetic radiation and that does not have a Curie temperature; the process comprising: (A) exposing the assembly to the electromagnetic radiation of a first energy that heats the ferromagnetic particles and the loose component, and (B) immediately after the stage (A), exposing the material to the electromagnetic radiation of a second energy that heats the loose component at a lower speed than the radiation of the first energy. A second aspect of the invention comprises an apparatus for heating an article, the article comprising (1) a composition comprising: (a) a host material; and (b) ferromagnetic particles dispersed in the host material; and (2) a solid component that does not self-regulate at temperature; the apparatus comprising: (A) a power supply for supplying energy to an induction heating coil; (B) a first fixation for providing energy to a first energy level so that the particles of ferromagnetic material are heated by induction heating and reach a first temperature; and (3) a second fixation to provide power to a second level, wherein the second energy level is reduced from the first energy level, the second fixation being such that the ferromagnetic particles are maintained at or near the first temperature while the heat generated in other parts of the article is reduced. A further aspect of the invention comprises a blocked cable arrangement, including a plurality of metallic conductors, the arrangement comprising an adhesive including a host material in which the ferromagnetic particles are dispersed, the adhesive having been heated by the following method: (1) supplying energy to the induction heating coil to a first energy so that the ferromagnetic particles reach a first temperature; and (2) immediately after step (1), supplying energy to the induction heating coil to a second energy. The second energy being less than the first energy, so that the ferromagnetic particles are kept at or near the first temperature and, the heat generated in the conductors is reduced, so that the heat generated in the arrangement is approximately equal to the heat loss from the disposal. A further aspect of the invention comprises a method of heating an arrangement comprising: (1) providing a plurality of metal conductors; (2) placing an article in close proximity to the conductors, wherein the article comprises: (a) a host material; and (b) ferromagnetic particles dispersed in the host material; (3) provide a cover around said article; (4) heating the arrangement by exposing it to electromagnetic radiation from an inductor coil at a first energy, where the ferromagnetic particles reach a first temperature on the scale of 130 ° C to 220 ° C; and (5) immediately after step (4), heating the arrangement by exposing it to a second energy, the second energy being 15-40% of the first energy, where the ferromagnetic particles are kept at a temperature on the scale of 130 ° C up to 220 ° C, while the heat generated in other parts of the arrangement is reduced; and wherein, the heat generated in other parts of the arrangement is approximately equal to the heat loss from the arrangement. Other features and advantages of the present invention will arise from the following description in which the preferred embodiment has been established in detail together with the accompanying drawings.

Brief Description of the Drawing (s) Fig. 1 illustrates a circuit diagram of the power source according to the present invention. Fig. 2 illustrates a perspective view of an arrangement for forming a fluid block. Fig. 3 is a graph illustrating temperature versus time for an article subjected to the dual energy system of the present invention.

Description of the Preferred Modalities The present invention comprises an apparatus for heating an article by exposing it to an electromagnetic field, such as one produced in an inductor coil. Induction heating is produced internally by exposing the article to electromagnetic fields. The article comprises a host material that includes ferromagnetic particles dispersed therein. The ferromagnetic particles, such as those described by Monovoukas, referred to above, provide an efficient article that is heated, fast, internally, uniformly and selectively and is self-regulating in temperature. In each application, the article is heated to transform it from its initial state to a new condition. The host material is electrically non-conductive and non-magnetic and can be any material that may be desirable to treat with heat. Examples include gels, adhesives, foams, inks, ceramics and heat-recoverable polymeric articles, such as tubing. Heat-recoverable articles are articles, the dimensional configuration of which can be made to change substantially when subjected to heat treatment. Usually, these items are recovered, upon heating, to an original shape from which they had previously been deformed. In the typical prior art heating methods that apply only an individual energy, the energy is maintained at a constant level even after the Curie temperature of the particles has been reached. Any remaining loose components continue to heat up, so that the window period in which effective sealing is achieved without damage to the components is relatively small, if any. By reducing the energy level after a predetermined time to a second energy, the present invention provides a greater window period in which effective sealing can occur. In addition, in many cases that would not otherwise have a window period, the present invention creates a window in which effective sealing occurs. For some applications, the window period seems to extend indefinitely, so that the burning of the material does not occur. guest in the second reduced energy. (See for example Samples 11-22 as described in Table I, below) Using the present invention, an article is heated rapidly, as long as no component is damaged. The article, which in the present invention includes a loose component such as a metallic conductor, is heated by exposure to the electromagnetic field of the inductor coil at a first energy for a first predetermined period. Once the ferromagnetic particles in the host material reach their Curie temperature, the particles maintain their Curie temperature even at reduced energy, although minimal energy is required. The article is heated immediately to a second energy during a second predetermined period wherein the second energy is reduced from the first. The first energy and the first predetermined period are such that the ferromagnetic particles reach a first temperature, preferably their Curie temperature and where the second energy is such that the ferromagnetic particles are kept at or near the first temperature, while the heat generated in other parts of the article, for example, copper from the isolated conductor, is reduced. The heat generated in those other parts of the article is approximately equal to the heat lost through conduction and radiation. The first energy level can be total energy, while the second level of energy is sufficient to maintain the host material at a temperature such that the heat lost due to conduction and the radiation is equal to the heat added to the article. The heat loss through conduction and radiation can be measured with thermocouples or by examination of a cross section of the article With this measurement, it is possible to determine the second desired energy level. The second energy level is preferably between 5-70%, more preferably between 10-50% and more preferably between 15-40% of the total energy. The measurements of the thermocouples can also be used to determine the first and second predetermined periods. Once the desired temperature is reached at full power, the energy level is reduced to the second energy level, as described above. The second predetermined period is sufficient to ensure complete sealing, while still within the window period. The first and second energies and the predetermined periods are set by the first and second fixings, respectively, which can be controlled by an individual synchronizer or, a separate synchronizer for each energy and the corresponding predetermined period. It should be noted that while it is preferred that the ferromagnetic particles reach their Curie temperature upon exposure to the electromagnetic field at a first energy level during the first predetermined period, it is not necessary for the present invention that the particles reach their Curie temperature and, in some cases, it may be preferable that the first temperature is less than the Curie temperature of the ferromagnetic particles. In alternative embodiments, the method may include heating to a third additional energy during a predetermined third period. The third energy may be greater or less than the first and second energies and may include the energy completely stopped for a predetermined period If desired, the first and second energies and the third energy. if applicable, they can be summarized in cycles. As described above, the ferromagnetic particles used in the present invention are preferably those referred to above described by Monovoukas, in which the selection of the particles results in the fastest, most uniform and most controlled heating. These particles advantageously have the configuration of a leaflet, that is, a configuration similar to a thin disk. The efficiency of the heat generation of these particles allows a smaller volume in percentage of particles in the host material so that the desired properties of the host material remain essentially unchanged. The particles preferably employed in the present invention have a configuration including first, second and third orthogonal dimensions, wherein each of the first and second orthogonal dimensions is at least 5 times the third orthogonal dimension. The first and second orthogonal dimensions, which are the largest dimensions, are preferably between about 1 μm and about 300 μ. A composition containing ferromagnetic particles in an amount of between 0.5% and about 10% by volume is also preferred. In some applications, for example, in cases where the highest speeds are desired and certain properties, such as viscosity, elongation at rupture or conductivity may be compromised, bar-like particles or higher concentrations may be employed. However, it should be noted that, the present invention contemplates any composition or configuration of ferromagnetic particles. Fig. 1 illustrates the circuit for an oscillating voltage energy generator 2. The methods of developing the grating signal varies from one oscillator to another. The present method employs a generator of 2.5 kW that includes a Hartley type oscillator. The oscillation circuit includes a tank circuit 4. The tank circuit 4 describes an apparatus consisting of a series of tank capacitors 6 connected in parallel with a tank coil 8 and a work coil 10. The energy stored in the capacitors is CV2 / 2 where V is the voltage charged by an equivalent capacitor C. This energy is transferred to the inductance L of the coil tank and the work coil so that L = inductance of the coil tank + inductance of the work coil and energy returns back to capacitor 6. The speed of this energy swing process, that is, the oscillation frequency, f, depends on the values of L and C so that Z 71 ('-tank + ^ -work / \ "tank) In this way the attenuation oscillations occur because a certain amount of energy is dissipated by the tank coil 8 and the work coil 10. To compensate for those losses, the tank circuit 4 is supplied with additional energy through a plate 14. of the vacuum tube 12. The tank coil 8 induces current in a grid coil 16. The tank coil and grid currents are 180 ° out of phase one of the other. The grid coil 16 couples the energy from the tank coil 8 to the grid 15 of the vacuum tube 12. The grid circuit 18, by varying its voltage with respect to the vacuum tube 12, controls the flow of electrons towards the tank circuit 4. The oscillation effect in the tank circuit 4 produces a large RF current in the tank coil 8 and the work coil 10. The passage of this large RF current through the work coil 10 creates a magnetic field on the which proportionally generates heat. The article is placed inside the work coil 14 to be heated by induction Fig. 1 has been described with reference to a tank circuit generator having automatic frequency coupling. However, it should be noted that a fixed frequency oscillator can also use rse. In a preferred embodiment, the present invention can be employed, for example, in an arrangement for forming a block in a cable against the transmission of fluid along the cable, wherein the cable includes a plurality of cables, as described in Monovoukas. , referred to above, and U.S. Patent No. 4,972, 042 entitled "Blocking Disposition to Suppress Fluid Transmission in Cables" issued November 20, 1990 to Seabourne et al. , which is incorporated herein by reference for all purposes. The cable lock assembly, as shown in Fig. 2, comprises an adhesive that includes a host material in which the ferromagnetic particles are dispersed therein. A cable lock assembly 20 comprises a generally planar body construction 22 having approximately five open end passages 24 extending therethrough. Each passage 24 has a slot 26 associated therewith which allows an electrical cable 28 to be inserted into the passage simply by placing the cable through the slot 26 and pressing the cable 28 into the passage 24. It is possible that any number of cables are inserted through each passage, depending on the relative dimensions of the cables and the passages. In the present embodiment, all slots are located on the same side of the construction. Although the body construction is illustrated as a flat body, any type of body construction that can be placed in the vicinity of the cables, either by encircling the cables of the cable bundle or placed with the cable bundle, or any construction that includes openings for receiving the cables, is within the scope of the present invention. The arrangement is placed inside the work coil 14 and heated by exposure to the electromagnetic radiation having a first energy for a first predetermined period. The temperature reached by the ferromagnetic particles is in the range from 80 ° C to 360 ° C, preferably in the scale from 100 ° C to 250 ° C and, more preferably in the scale from 130 ° to 220 ° C. Immediately thereafter, the arrangement is heated by exposure to electromagnetic radiation having a second energy during a second predetermined period, the second energy being less than the first energy, preferably in the range of 5-70%, more preferably in the scale of 10. -50% and more preferably 15-40% of the first energy. The temperature of the ferromagnetic particles is maintained in the range from 80 ° C to 360 °, preferably in the range of 100 ° C to 250 ° C and more preferably in the range of 130 ° C to 220 ° C. In the preferred mode, a cover is secured around the blocking structure to control the flow of the composition as the viscosity of the article 22 is reduced to heating. The cover can be a recoverable sleeve with heat placed around the blocking structure. A recoverable sleeve with heat would coat as the blocking structure and ,. therefore, the entire arrangement heats up. Alternatively, the cover can be removable. For example, the cover may comprise a polytetrafluoroethylene fastener which holds the blocking structure during heating and which is subsequently removed. Fig. 3 illustrates the temperature (T) against time (t) for an article that is heating. The use of double energy levels of the present invention, it can be seen that once the desired temperature T, is reached in a time ti, the energy is reduced to a level such that the heat generated by the loose component, in this case the cables, is equal to the heat lost through conduction and radiation. In this way, the desired temperature of the arrangement is maintained. The second energy level is sufficient to maintain the temperature of the arrangement on the working temperature scale, which is between the sealing temperature, T ', in this case about 130 ° C and, slightly above the desired temperature, T ", in this case of approximately 160 ° C. At full power, the temperature of the continuous heating arrangement / as also shown in Fig. 3), as the loose component heats up, possibly damaging the arrangement SAMPLES 1-14 Samples 1-9 and Comparative Samples were prepared -14 providing 57 wire bundles 0.304 meters long, each comprised of non-interlaced polyethylene that has a scale of 150 ° C. Each mace consisted of 29 20-gauge cables, 17 18-gauge cables, 4 14-gauge cables, 4 coaxial braided casing cables and 3 twisted pairs. The cables of each hub were inserted in 6 collectors of five channels (such as article 22 as seen in Fig. 2) that were alternating. A recoverable pipe with 40 mm long heat was placed around each hub. Samples prepared according to the procedure for Samples 1-9 were exposed to an electromagnetic field by a U-channel inductor coil at approximately 1500 W of energy, i.e. total energy, for 26 seconds. Subsequently, the energy was reduced to approximately 500 W for additional periods of up to 28 seconds. The samples prepared according to the procedure for Samples 1-9 were calculated to have 28 seconds of sealing after the initial exposure. The cables prepared in accordance with the procedure for Samples 1-9 were damaged after exposure to electromagnetic fields for 54 seconds (26 seconds at full power plus 28 seconds at reduced power). Comparative Samples 10-14 were exposed to an electromagnetic field by a U-channel inducing coil at approximately 15 W of energy, i.e. at full power, for 24, 26, 28, 32 and 34 seconds, respectively. Samples prepared in accordance with the procedure for Samples 1-14 were calculated to have a seal after 28 seconds. The cables prepared in accordance with the procedure for Samples 1-9 were damaged after exposure to electromagnetic fields 34 seconds after exposure to electromagnetic fields at full power. Therefore, the Samples window prepared in accordance with the procedure for Samples 1-9 was 24 seconds (52 seconds of total time minus 28 seconds for sealing). The Samples window prepared according to the procedure for Samples 10-14 was 6 seconds (32 seconds of total time minus 26 seconds for sealing).

SAMPLES 15-22 Samples 15-22 were prepared as Samples 1-14. Samples 15-22 were exposed to an electromagnetic field by a U-channel inductor at approximately 1500 W of energy, i.e., total energy, for 19 seconds. Subsequently, the energy was reduced to approximately 500 W for additional periods of up to 36 seconds. Samples prepared in accordance with the procedures for Samples 15-22 were calculated to have a seal after 22 seconds. The cables prepared in accordance with the procedure for Samples 15-22 were calculated to have a seal after 22 seconds. The wires prepared in accordance with the procedure for Samples 15-22 showed no signs of damage after 58 seconds (19 seconds at full power plus 39 seconds at reduced power), when exposure to the electromagnetic field was stopped The Sample window prepared in accordance with the procedure for Samples 15-22 it was at least 36 seconds (58 seconds of total time minus 22 seconds for sealing).

SAMPLES 23-31 Samples 23-31 were prepared as Samples 1-14. Samples 23-31 differ from previous Samples in that the conductors were 2.73 m in length. Samples 23-31 were exposed to an electromagnetic field by a U-channel inductor coil at approximately 1500 W of energy, i.e., total energy, for 26 seconds. Subsequently, the energy was reduced to approximately 500 W for additional periods of up to 30 seconds. Samples prepared in accordance with the procedure for Samples 23-31 were calculated to have a seal after 30 seconds. Cables prepared in accordance with the procedure for Samples 23-31 showed no signs of damage after 58 seconds (26 seconds at full power plus 32 seconds at reduced power), when the exposure to the electromagnetic field was stopped. The window of the Samples prepared in accordance with the procedure for Samples 23-31 was at least 30 seconds (58 seconds of total time minus 28 seconds for sealing).

Sample # Time (s) to Time (s) Additional Condition to Total Energy Reduced Energy Sealing 1 26 0 not yet sealed 2 26 2 not yet sealed 3 26 4 sealing 4 26 8 sealing 5 26 20 sealing 6 26 22 sealing 7 26 24 sealing 8 26 26 sealing 9 26 28 damaged 10 * 24 0 not yet sealed 11 * 26 0 not yet sealed 12 * 28 0 sealing 13 * 32 0 sealing 14 34 0 damaged 15 19 0 not yet sealed 16 19 3 not yet sealed 17 19 5 sealing 18 19 23 sealing 19 19 29 sealing 20 19 33 sealing 21 19 37 sealing 22 19 39 not yet sealing 23 26 0 not yet sealing 24 26 2 sealing 25 26 4 sealing 26 26 8 sealing 27 26 10 sealing 28 26 24 sealing 29 26 26 sealing 30 26 28 sealing 31 26 30 sealing Comparative samples The Examples set forth above illustrate the invention with respect to a mode including ferromagnetic particles dispersed in an adhesive. As described above, it should be noted that the ferromagnetic particles can be dispersed within the host material of any article to be heated, such as gels, foams, inks, ceramics or heat-recoverable polymeric articles.

Claims (20)

1. A method for heating an assembly by means of electromagnetic radiation, the assembly comprising: (1) a composition consisting of: (a) a host material that is not heated by electromagnetic radiation, and (b) ferromagnetic particles that are dispersed in the host material and have a Curie temperature; and (2) a loose component that is composed of a material that can be heated by electromagnetic radiation and that does not have a Curie temperature; said process comprising: (A) exposing the assembly to the electromagnetic radiation of a first energy that heats the ferromagnetic particles and the loose component, and (B) immediately after the step (A), exposing the assembly to the electromagnetic radiation of a second energy that heats the loose component to a smaller scale than the radiation of the first energy.

The method as defined in claim 1, wherein the article reaches a first temperature in step (A) and wherein the second energy is such that the heat generated within the loose component in step (B) is approximately equal to the heat lost from the article.

3. The method as defined in claim 1, wherein the ferromagnetic particles reach a first temperature in step (A) and are maintained at or near said first temperature in step (B).

4. The method as defined in claim 3, wherein the first temperature is in the range of 130 ° C to 220 ° C.

The method as defined in claim 1, wherein the second energy is from 15-40% of the first energy.

6. The method as defined in claim 1, wherein the loose component is a metallic conductor and is surrounded by a polymeric insulator.

The method as defined in claim 1, wherein the loose component is a solid through steps (A) and (B) and the composition flows during step (B).

The method as defined in claim 7, wherein the assembly further comprises a cover that controls the flow of the composition during step (B).

The method as defined in claim 8, wherein the cover comprises a recoverable sleeve with heat, and the sleeve is recovered in steps (A) and (B).

The method as defined in claim 8, wherein the cover is removable.

11. An apparatus for heating an article, the article comprising: (1) a composition consisting of: (a) a host material; and (b) ferromagnetic particles dispersed in the host material; and (2) a loose component that does not self-regulate at temperature; the apparatus comprising: (A) a power source for supplying power to an induction heating coil; (B) a first fixation for providing energy to a first energy level so that the ferromagnetic particles are heated by induction heating and reach a first temperature; and (3) providing a second fixation to provide energy at a second energy level, wherein the second energy level is reduced from the first energy level, the second fixation being such that the ferromagnetic particles are maintained at or near the first temperature while the heat generated in other parts of the article is reduced.

The apparatus as defined in claim 11, wherein the heat generated in other parts of the apparatus is approximately equal to the heat lost from the article.

The method as defined in claim 11, wherein the first temperature is in the range of 130 ° C to 220 ° C.

14. A blocked wire arrangement, including a plurality of metallic conductors, the arrangement comprising an adhesive that includes a host material in which ferromagnetic particles are dispersed, the adhesive having been heated by the following method: (1) supplying power to the heating coil by induction at a first energy so that the ferromagnetic particles reach a first temperature; and (2) immediately after step (1), supplying energy to the induction heating coil to a second energy, the second energy being less than the first energy, so that the ferromagnetic particles are maintained at or near the first temperature and, the heat generated in the conductors is reduced, so that the heat generated in the arrangement is approximately equal to the heat lost from the arrangement.

15. The arrangement as defined in claim 14, wherein the first temperature is at or near the temperature Curie of ferromagnetic particles.

16. The arrangement as defined in claim 14, wherein the first temperature is in the range of 130 ° C to 220 ° C.

17. A method for heating an arrangement comprising: (1) providing a plurality of metal conductors; (2) placing an article in close proximity to the conductors, wherein the article comprises: (a) a host material; and (b) ferromagnetic particles dispersed in the host material; (3) provide a cover around said article; (4) heating the arrangement by exposing it to electromagnetic radiation from an inductor coil to a first energy, where the ferromagnetic particles reach a first temperature on the scale of 130 ° C to 220 ° C; and (5) immediately after step (4), heating the arrangement by exposing it to an electromagnetic field at a second energy, the second energy being 15-40% of the first energy, where the ferromagnetic particles are kept at a temperature on the scale of 130 ° C to 220 ° C, while the heat generated in other parts of the arrangement is reduced; and wherein the heat generated in other parts of the arrangement is approximately equal to the heat lost from the arrangement. The method as defined in claim 17, further comprising securing the cover around the blocking construction prior to step (4). The method as defined in claim 17, wherein the cover comprises a heat shrinkable sleeve and, the sleeve is recovered in steps (4) and (5). The method as defined in claim 17, wherein the first temperature is at or near the Curie temperature of the ferromagnetic particles.