Magnetic composite article and manufacturing method using Fe-Al-Si powder

Abstract: Soft magnetic powder of Fe-Al-Si system of which magnetostrictive constant λ takes a positive value at the room temperature is employed to produce a magnetic composite article so that a temperature characteristic of core-loss of the article takes a negative value at the room temperature. Excellent magnetic characteristics such as a low core-loss and a high permeability can be obtained at a high frequency band.

Description: Technical Field The present invention relates to a magnetic composite article using soft magnetic powder of Fe-Al-Si system alloy that is employed in transformer-cores of power supplies, choke coils, or magnetic heads, and a manufacturing method of the same. Background Art Electric appliances and electronic devices have been downsized, which entails a demand for small and efficient magnetic articles. Ferrite cores and dust cores are thus employed as choke coils that are used at a high frequency band. The ferrite core has a problem of a low saturation-magnetic-flux-density, while the dust core formed by compacting magnetic powder has a substantially higher saturation-magnetic-flux-density than the ferrite core. The dust core has thus an advantage over the ferrite core in the way of downsizing appliances and devices. On the other hand, the dust core is inferior to the ferrite core in regard to permeability and power loss. Because of these points, when the dust core is used as a choke coil or an inductor, a great amount of core loss raises the core temperature, which is an obstacle to downsizing. The core loss of dust core comprises, in general, hysteresis loss and eddy current loss. The eddy current loss increases in proportion to the square of the frequency and the square of the size of eddy current i.e. the square of path length of the eddy current. Therefore, the magnetic-powder-surface is coated with insulating resin so that the dust core restrains itself from producing eddy current. The dust core is formed generally with the compacting pressure of not less than 5 ton/cm 2 . Its magnetostrictive is increased, while the permeability is lowered through this process. As a result, the hysteresis loss is increased. In order to release the magnetostrictive, a heat treatment has been applied to the dust core after the compacting. Some of the heat treatments are disclosed e.g. in the Japanese Patent Application Non-examined Publication Nos. H06-342714, H08-37107, and H09-125108. However, a conventional dust core using powder of Fe-Al-Si system alloy has a drawback that the core loss increases in step with temperature rising. To be more specific, when a temperature coefficient of the core loss is positive around the room temperature, the transformer or choke coil produces heat due to the core loss during its active use. Its temperature thus rises and the core loss further increases, which induces a greater heat. This vicious circle is repeated to provoke a thermo-run-away. In order to avoid the thermo-run-away, it is therefore crucial that the dust core should have a temperature characteristic such that the core loss is minimized at 80°C - 100°C in an active use. In general, the Fe-Al-Si system alloy shows its maximum permeability steeply around the composition of Si=9.6%, Al=5.5%, and the remainder of Fe (the figures are wt%) where the crystal magnetic anisotropy K ≒ 0 and magnetostricitve constant λ ≒ 0, as shown in Figs. 2 and 3. The composition within this range is generally called "sendust". This maximum permeability was taken into consideration, and the magnetic composite articles employing the powder of Fe-Al-Si system alloy have been proposed, and some of them are disclosed in the patent gazettes of the Japanese Patent Application Non-examined Publication Nos. H06-342714, H08-37107 and H09-125108. US-A-5 651 841, representing the closest prior art, discloses a magnetic composite article prepared by mixing an insulating binder with a soft magnetic "sendust" powder, applying a compacting pressure to the mixed powder and subjecting said mixed powder to a heat treatment. However, no description about the relation between the core loss and the temperature characteristic is found in any of these proposals. The temperature characteristic of the core-loss is determined by behavior of the hysteresis loss, i.e. the temperature characteristic of permeability. Ferrite in the conventional manner has shown its maximum permeability at a given temperature and shown also its minimum loss at the same point. This is because the crystal magnetic anisotropy K shows "0" at the given temperature, where magnetic domain walls can move with ease, and therefore, the hysteresis loss decreases. A conventional "sendust" dust-core employing the soft magnetic powder of Fe-Al-Si system alloy increases its core-loss monotonously as shown in Fig. 1 when the temperature is not lower than the room temperature. Therefore, this dust-core has been evaluated not good for a large-power transformer. Disclosure of the Invention The present invention addresses these problems and aims to provide magnetic composite article having excellent characteristics such as a high permeability and a low-core-loss. A magnetic composite article according to the present invention employs soft magnetic powder of Fe-Al-Si system alloy, of which magnetostrictive constant λ is positive at the room temperature so that a temperature coefficient of the core loss at the room temperature is negative. The soft magnetic powder employed in the article preferably comprises 4.5% ≦ Al ≦ 8.5%, 7.5%≦ Si≦ 9.5%, and the remainder of Fe, (the figures are wt%). This structure realizes a core having a low core-loss even at a high frequency, an excellent temperature characteristic such that the temperature coefficient of the core loss is negative, and an excellent permeability. However, according to the present invention, in a case of the magnetic composite article employing the soft magnetic powder of Fe-Al-Si system alloy, the crystal magnetic anisotropy K does not govern the temperature characteristic contrary to the established theory, but the magnetostrictive constant λ that has not drawn attention hitherto governs it. Further, the following fact is found. That is, when the magnetostrictive constant λ takes positive value at the room temperature (around 20-30°C), the temperature coefficient of the core-loss has a negative inclination. In particular, when employing the soft magnetic powder of Fe-Al-Si system alloy that comprises 4.5% ≦ Al ≦ 8.5%, 7.5%≦ Si≦ 9.5%, and the remainder of Fe (the figures are wt%), the inventors can obtain an excellent temperature characteristic such as a high permeability and a low core-loss. Preferably the soft magnetic powder that comprises 5.0% ≦ Al ≦ 6.5%, 8.2%≦ Si≦ 9.2% and the remainder of Fe is used, whereby the more effective result is obtained. Brief Description of the Drawings Fig. 1 shows a temperature characteristic of the core-loss of the present invention, compared with a prior art. Fig. 2 shows how much the maximum permeability (µ m ) of the Fe-Al-Si system alloy depends on the composition of Fe, Si and Al. Fig. 3 shows how much an initial permeability (µ i ) of the "sendust" at its center composition area depends on the composition of Fe, Si and Al. Detailed Description of the Preferred Embodiments (Exemplary Embodiment 1) The soft magnetic powder of Fe-Al-Si system alloy is produced by the water atomizing method so that its final composition is shown in Table 1. The volume content of oxygen in the powder show 2000 - 3000 ppm. The powder is sifted with a sieve so that an average grain size is 50 µm. The powder is mixed with butyral resin as an insulating binder by a mixer in the weight ratio of 100: 2. A single axis press machine provides the mixed powder with compacting pressure of 981 MPa (10 ton/cm 2 ) to produce a toroidal compacted article of which dimension is outer diameter=25 mm, inner diameter=15 mm, thickness=10 mm. After this, heat treatment is provided to the resultant article at 690°C in nitrogen gas, then silicone resin is impregnated therein. The samples are thus obtained. The permeability of the sampless is measured with an LCR meter at 10 kHz. The core-loss thereof is measured with an ac B-H curve measuring device at 50 kHz and 0.1T magnetic flux density. Both of these values are measured from 20°C to 120°C with an interval of 20°C, and temperature characteristics of both the values are also measured. The values at the minimum loss temperature are shown in Table 1. In the case that the minimum loss temperature exceeds 120°C or keeps under 20°C, the core-loss and permeability at these temperatures are shown. When the article is used for a choke coil of an active filter against harmonic distortion, the samples having the following excellent characteristics are obtained: core-loss≦1000 kW/m 3 , permeability ≧50 and the minimum-loss-temperature≧80°C, at the condition of measured frequency = 50 kHz and measured magnetic flux density=0.1T. As Table 1 shows, when the soft magnetic powder of Fe-Al-Si is used, which comprises 4.5% ≦ Al ≦ 8.5%, 7.5%≦ Si≦ 9.5%, and the remainder of Fe (the figures are wt%), the samples of a high permeability and a low core-loss are obtained. Preferably the soft magnetic powder of Fe-Al-Si is used, which comprises 5.0% ≦ Al ≦ 6.5%, 8.2%≦ Si≦ 9.2% and the remainder of Fe (the figures are wt%), to produce the more effective results. Sample No. Final composition (wt%) Characteristics at minimum loss temperature Al Si Fe Temperature (C°) Core-loss kW/m 3 Permeability 1 4.4 7.5 Remainder ≧120 1100 55 Comparison 2 9.5 Remainder 80 1200 75 Comparison 3 4.5 7.4 Remainder ≧120 1210 80 Comparison 4 7.5 Remainder ≧120 580 84 Embodiment 5 9.5 Remainder 80 770 80 Embodiment 6 9.6 Remainder 40 1100 72 Comparison 7 4.9 8.2 Remainder 100 500 80 Embodiment 8 9.2 Remainder 100 550 78 Embodiment 9 5.0 8.1 Remainder 120 510 95 Embodiment 10 8.2 Remainder 100 270 105 Embodiment 11 9.2 Remainder 100 430 122 Embodiment 12 9.3 Remainder 80 530 113 Embodiment 13 6.5 8.1 Remainder ≧120 520 90 Embodiment 14 8.2 Remainder 100 220 95 Embodiment 15 9.2 Remainder 100 220 118 Embodiment 16 9.3 Remainder 80 580 115 Embodiment 17 6.6 8.2 Remainder 100 330 80 Embodiment 18 9.2 Remainder 100 350 76 Embodiment 19 8.5 7.4 Remainder ≧120 1280 35 Comparison 20 7.5 Remainder ≧120 850 56 Embodiment 21 9.5 Remainder 80 900 52 Embodiment 22 9.6 Remainder 60 1260 32 Comparison 23 8.6 7.5 Remainder ≧120 1350 35 Comparison 24 9.5 Remainder 80 1170 35 Comparison (Exemplary Embodiment 2) Soft magnetic powder is produced by an ingot grinding method so that the final composition thereof is Al = 6.0%, Si = 9.0%, and the remainder of Fe (the figures are wt%). The volume content of oxygen in each sample powder ranges from 1000 ppm to 2000 ppm. The powder is sifted with a sieve or an air classifying method so that an average grain size is as shown in Table 2. The magnetic powder is mixed with organic silicone resin as an insulating binder by a mixer in a weight ratio of 100: 5. A single axis press machine provides the mixed powder with compacting pressure of 687 MPa (7 ton/cm 2 ) to produce a toroidal compacted article of which dimension is outer diameter=25 mm, inner diameter=15 mm, thickness=10 mm. After this, heat treatment is provided to the resultant article at 720°C in nitrogen gas, then epoxy resin is impregnated therein. The samples are thus obtained. The permeability of the samples is measured with an LCR meter at 10 kHz. The core-loss thereof is measured with an ac B-H curve measuring device at 50 kHz and 0.1T magnetic flux density. Both of these values are measured from 20°C to 120°C with an interval of 20°C, and temperature characteristics of both the values are also measured. The values at the minimum loss temperature are shown in Table 2. In the case that the minimum loss temperature exceeds 120°C or keeps under 20°C, the core-loss and permeability at these temperatures are shown. When the article is used for a choke coil of an active filter against harmonic distortion, the samples having the following excellent characteristics are obtained: core-loss≦1000 kW/m 3 , permeability ≧50 and the minimum-loss-temperature≧80°C, at the condition of measured frequency = 50 kHz and measured magnetic flux density=0.1T. As shown in Table 2, the core-loss stays at a low level when the average grain size is between 1µm and 100µm, and preferably, the core-loss is ensured at a low level when the average grain size is between 1 µm and 50 µm. Sample No. Average grain size (µm) Characteristics at minimum loss temperature Temperature (C°) Core-loss kW/m 3 Permeability 25 110 ≧120 1370 125 Comparison 26 100 ≧120 940 121 Embodiment 27 60 ≧120 560 97 Embodiment 28 50 ≧120 400 77 Embodiment 29 20 ≧120 240 64 Embodiment 30 5 ≧120 110 54 Embodiment 31 1 ≧120 100 50 Embodiment 32 0.8 ≧120 340 35 Comparison (Exemplary Embodiment 3) Soft magnetic alloy powder is produced by a water atomizing method so that the final composition thereof is Al= 5.8 wt%, Si=8.6 wt% and the remainder of Fe, and the average grain size thereof is 30µm. The resultant magnetic powder is mixed with butyral resin as an insulating binder and TiO 2 of which average grain size is 1µm as a space control material by a mixer in a weight ratio of 100: 1: 0.5. The resultant mixed powder is deaerated and ground into granulation of which average grain size is not more than 500 µm. A single axis press machine provides the granulation with compacting pressure of 1177 MPa (12 ton/cm 2 ) to produce a toroidal compacted article of which dimension is outer diameter=25 mm, inner diameter=15 mm, thickness=10 mm. Then the compacted article is degreased in the air at 450°C. After this, heat treatment is provided to the resultant article at 730°C in nitrogen gas, then epoxy resin is impregnated therein. The samples are thus obtained. The permeability of the samples is measured with an LCR meter at 10 kHz. The core-loss thereof is measured with an ac B-H curve measuring device at 50 kHz and 0.1T magnetic flux density. Both of these values are measured from 20°C to 120°C with an interval of 20°C, and temperature characteristics of both the values are also measured. The values at the minimum loss temperature are shown in Table 3. In the case that the minimum loss temperature exceeds 120°C or keeps under 20°C, the core-loss and permeability at these temperatures are shown. When the article is used for a choke coil of an active filter against harmonic distortion, the samples having the following excellent characteristics are obtained: core-loss≦1000 kW/m 3 , permeability ≧50 and the minimum-loss-temperature≧80°C, at the condition of measured frequency = 50 kHz and measured magnetic flux density=0.1T. Sample No. Oxygen volume (ppm) Characteristics at minimum loss temperature Temperature (C°) Core-loss kW/m 3 Permeability 33 900 ≧120 1280 95 Comparison 34 1000 ≧120 650 85 Embodiment 35 3000 ≧120 670 82 Embodiment 36 5000 ≧120 720 74 Embodiment 37 8000 ≧120 780 70 Embodiment 38 8100 ≧120 2430 35 Comparison As shown in Table 3, a low core-loss is achieved when the volume content of oxygen stays between 1000 ppm and 8000 ppm. (Exemplary Embodiment 4) Soft magnetic powder of Fe-Al-Si system alloy is produced by a gas atomizing method so that the final composition thereof is as shown in Table 4. The powder is then sifted with a sieve so that an average grain size is 60µm. The sifted powder is mixed with butyral resin as an insulating binder by a mixer in the weight ratio of 100: 5. A single axis press machine provides the granulation with compacting pressure of 687 MPa (7 ton/cm 2 ) to produce a toroidal compacted article of which dimension is outer diameter=25 mm, inner diameter=15 mm, thickness=10 mm. After this, heat treatment is provided to the resultant article at 710°C in nitrogen gas, then silicone resin is impregnated therein. The samples are thus obtained. The permeability of the samples is measured with an LCR meter at 10 kHz. The core-loss thereof is measured with an ac B-H curve measuring device at 50 kHz and 0.1T magnetic flux density. Both of these values are measured from 20°C to 120°C with an interval of 20°C, and temperature characteristics of both the values are also measured. The values at the minimum loss temperature are shown in Table 4. In the case that the minimum loss temperature exceeds 120°C or keeps under 20°C, the core-loss and permeability at these temperatures are shown. When the article is used for a choke coil of an active filter against harmonic distortion, the samples having the following excellent characteristics are obtained: core-loss≦1000 kW/m 3 , permeability ≧50 and the minimum-loss-temperature≧80°C, at the condition of measured frequency = 50 kHz and measured magnetic flux density=0.1T. As Table 4 shows, when the soft magnetic powder of Fe-Al-Si is used, which comprises 4.5% ≦ Al ≦ 8.5%, 7.5%≦ Si≦ 9.5%, and the remainder of Fe (the figures are wt%), the samples of a high permeability and a low core-loss are obtained. Preferably the soft magnetic powder of Fe-Al-Si is used, which comprises 5.0% ≦ Al ≦ 6.5%, 8.2%≦ Si≦ 9.2%, and the remainder of Fe (the figures are wt%), to produce the more effective results. Sample No. Final composition (wt%) Characteristics at minimum loss temperature Al Si Fe Temperature (C°) Core-loss kW/m 3 Permeability 39 4.4 7.5 Remainder ≧120 1200 70 Comparison 40 9.5 Remainder 80 1170 83 Comparison 41 4.5 7.4 Remainder ≧120 1210 87 Comparison 42 7.5 Remainder ≧120 750 90 Embodiment 43 9.5 Remainder 80 920 86 Embodiment 44 9.6 Remainder 40 1070 82 Comparison 45 4.9 8.2 Remainder 100 550 85 Embodiment 46 9.2 Remainder 100 530 84 Embodiment 47 5.0 8.1 Remainder 120 530 95 Embodiment 48 8.2 Remainder 100 350 105 Embodiment 49 9.2 Remainder 100 460 122 Embodiment 50 9.3 Remainder 80 530 113 Embodiment 51 6.5 8.1 Remainder ≧120 510 98 Embodiment 52 8.2 Remainder 100 210 104 Embodiment 53 9.2 Remainder 100 250 110 Embodiment 54 9.3 Remainder 80 600 115 Embodiment 55 6.6 8.2 Remainder 100 330 90 Embodiment 56 9.2 Remainder 100 380 91 Embodiment 57 8.5 7.4 Remainder ≧120 1270 35 Comparison 58 7.5 Remainder ≧120 880 60 Embodiment 59 9.5 Remainder 80 930 57 Embodiment 60 9.6 Remainder 60 1350 30 Comparison 61 8.6 7.5 Remainder ≧120 1370 42 Comparison 62 9.5 Remainder 80 1250 37 Comparison (Exemplary Embodiment 5) Soft magnetic powder is produced by a gas atomizing method so that its final composition is Al=6.0 wt%, Si=9.0 wt% and the remainder of Fe. The powder is sifted with a sieve so that its average grain size is as shown in Table 5. The sifted magnetic powder is mixed with organic silicone resin by a mixer in the weight ratio of 100 : 3. A single axis press machine provides the granulation with compacting pressure of 883 MPa (9 ton/cm 2)to produce a toroidal compacted article of which dimension is outer diameter=25 mm, inner diameter=15 mm, thickness=10 mm. After this, heat treatment is provided to the resultant article at 730°C in nitrogen gas, then epoxy resin is impregnated therein. The samples are thus obtained. The permeability of the samples is measured with an LCR meter at 10 kHz. The core-loss thereof is measured with an ac B-H curve measuring device at 50 kHz and 0.1T magnetic flux density. Both of these values are measured from 20°C to 120°C with an interval of 20°C, and temperature characteristics of both the values are also measured. The values at the minimum loss temperature are shown in Table 5. In the case that the minimum loss temperature exceeds 120°C or keeps under 20°C, the core-loss and permeability at these temperatures are shown. When the article is used for a choke coil of an active filter against harmonic distortion, the samples having the following excellent characteristics are obtained: core-loss≦1000 kW/m 3 , permeability ≧50 and the minimum-loss-temperature≧80°C, at the condition of measured frequency = 50 kHz and measured magnetic flux density=0.1T. As shown in Table 5, the core-loss stays at a low level when the average grain size is not greater than and 100 µm, and preferably, the core-loss is ensured at a low level when the average grain size is not greater than 50 µm. Sample No. Average grain size (µm) Characteristics at minimum loss temperature Temperature (C°) Core-loss kW/m 3 Permeability 63 110 ≧120 1120 145 Comparison 64 100 ≧120 950 125 Embodiment 65 60 ≧120 620 135 Embodiment 66 50 ≧120 460 100 Embodiment 67 20 ≧120 260 85 Embodiment 68 5 ≧120 120 62 Embodiment (Exemplary Embodiment 6) Soft magnetic powder is produced by a gas atomizing method so that the final composition thereof is Al= 5.8 wt%, Si=8.6 wt% and the remainder of Fe, and the average grain size thereof is 40µm. The resultant magnetic powder is mixed with butyral resin as an insulating binder and MgO of which average grain size is 1µm as a space control material by a mixer in a weight ratio of 100: 1: 1. The resultant mixed powder is deaerated and ground into granulation of which average grain size is not more than 500 µm. A single axis press machine provides the granulation with compacting pressure of 981 MPa (10 ton/cm 2 ) to produce a toroidal compacted article of which dimension is outer diameter=25 mm, inner diameter=15 mm, thickness=10 mm. Then the compacted article is degreased in the air at 450°C. After this, heat treatment is provided to the resultant article in nitrogen gas as shown in Table 6, then epoxy resin is impregnated therein. The samples are thus obtained. The permeability of the samples is measured with an LCR meter at 10 kHz. The core-loss thereof is measured with an ac B-H curve measuring device at 50 kHz and 0.1T magnetic flux density. Both of these values are measured from 20°C to 120°C with an interval of 20°C, and temperature characteristics of both the values are also measured. The values at the minimum loss temperature are shown in Table 6. In the case that the minimum loss temperature exceeds 120°C or keeps under 20°C, the core-loss and permeability at these temperatures are shown. When the article is used for a choke coil of an active filter against harmonic distortion, the samples having the following excellent characteristics are obtained: core-loss≦1000 kW/m 3 , permeability ≧50 and the minimum-loss-temperature≧80°C, at the condition of measured frequency = 50 kHz and measured magnetic flux density=0.1T. Sample No. Heat treatment temperature Characteristics at minimum loss temperature Temperature (C°) Core-loss kW/m 3 Permeability 69 480 ≧120 1500 38 Comparison 70 500 ≧120 850 80 Embodiment 71 630 ≧120 590 90 Embodiment 72 650 ≧120 350 114 Embodiment 73 800 ≧120 470 115 Embodiment 74 820 ≧120 660 125 Embodiment 75 900 ≧120 770 135 Embodiment 76 920 ≧120 3520 165 Comparison As Table 6 shows, a low core-loss is realized when the temperature treatment is provided at the temperature ranging from 500°C to 900°C. Preferably, the lower core-loss is expected at the temperature ranging from 650°C to 800°C. (Exemplary Embodiment 7) Soft magnetic powder (inventive article) is produced by a gas atomizing method so that the final composition thereof is Al= 7.5 wt%, Si=8.5 wt% and the remainder of Fe. Another soft magnetic powder (conventional article) is produced by also the gas atomizing method so that its final composition is Al=5.4 wt%, Si=9.6 wt% and the remainder of Fe. Each powder is sifted with a sieve so that the average grain size of each is 40µm. The resultant magnetic powder is mixed with organic silicone resin as an insulating binder by a mixer in a weight ratio of 100: 4. A single axis press machine provides each mixed powder with compacting pressure of 981 MPa (10 ton/cm 2 )to produce a toroidal compacted article of which dimension is outer diameter=25 mm, inner diameter=15 mm, thickness=10 mm. After this, heat treatment is provided at 720 °C in nitrogen gas, then epoxy resin is impregnated therein. The samples are thus obtained. Fig.1 shows a temperature characteristic of core-loss at a measured frequency of 50 kHz and a measure magnetic flux of 0.1T. This characteristic graph tells that the inventive sample has a negative inclination around the room temperature (20°C - 30°C) and a minimum loss temperature is 80°C or more. The conventional article, on the other hand, has a positive inclination around the room temperature and a minimum loss temperature is not higher than 20°C, Therefore, the conventional sample has a possibility of thermo-run-away at a high temperature. (Exemplary Embodiment 8) Soft magnetic powder of Fe-Al-Si system alloy is produced with a water atomizing method so that its final composition is as shown in Table 7. Then the powder is sifted with a sieve so that its average grain size is 50µm. The sifted magnetic powder is mixed with butyral resin as an insulating binder in a weight ratio of 100: 1.5. A single axis press machine provides each mixed powder with compacting pressure of 981 MPa (10 ton/cm 2 )to produce "E" and "I" shaped compacted articles. After this, heat treatment is provided to the resultant articles in nitrogen gas at 700°C, then epoxy resin is impregnated therein. The samples are thus obtained. The "E" shaped and "I" shaped samples are combined into a power-choke-coil of DC/DC converter in a notebook type personal computer. This choke coil mounted in the active personal computer is evaluated at 200 kHz. Temperature-rise of this evaluation is shown in Table 7. Sample No. Final composition (wt%) Temperature rise (C°) Al Si Fe 77 5.0 8.1 Remainder 25 Embodiment 78 7.5 9.0 Remainder 30 Embodiment 79 4.0 7.0 Remainder 52 Comparison 80 8.5 9.6 Remainder 60 Comparison As Table 7 shows, the temperature-rise is not higher than 30°C when the soft magnetic powder is used, which comprises 4.5% ≦ Al ≦ 8.5%, 7.5%≦ Si≦ 9.5%, and the remainder of Fe (the figures are wt%.) As the foregoing exemplary embodiments described, the magnetic composite article is formed by employing soft magnetic powder of Fe-Al-Si system alloy of which magnetostrictive constant λ is positive at the room temperature. Since the temperature coefficient of the core-loss at the room temperature can stay negative, excellent magnetic characteristics such as a low core-loss and a high permeability even at a high frequency range can be obtained. Preferably, the minimum loss temperature of the magnetic composite article is not lower than 80°C. The magnetic composite article according to the present invention comprises mainly the soft magnetic powder of Fe-Al-Si system alloy and an insulating material such as remainders after the heat treatment of the insulating binder, resin for impregnation or hollow holes. In view of magnetic characteristics, a volume content of the soft magnetic powder is preferably between 70 - 99 volume %. The soft magnetic powder preferably comprises 4.5% ≦ Al ≦ 8.5%, 7.5%≦ Si≦ 9.5%, and the remainder of Fe (the figures are wt%.) When a small amount of impurities or additive are included therein, as far as they do not negatively influence the magnetic characteristics, the same effect can be expected. The magnetic composite article can include magnetic powders other than the main component i.e. the soft magnetic powder of Fe-Al-Si system alloy. Further preferably, the magnetic composite article formed with the following methods is employed to produce more stable and excellent magnetic characteristics, i.e. soft magnetic powder of Fe-Al-Si system alloy is turned into powder by a gas-atomizing, water-atomizing method, or is directly ground after being alloyed. The same result can be obtained when the soft magnetic powder can be shaped into anyone of spherical, compressed, or polygonal state. The article is preferably formed by the soft magnetic powder of Fe-Al-Si system alloy of which average grain size ranges from 1 µm to 100 µm. When the average grain size is smaller than 1 µm, the core compact becomes less densely whereby the permeability is lowered. Therefore, the powder of which average grain size is not less than 1µm and preferably ranges from 1 µm to 50 µm is desirably used. The powder is preferably coated with an oxide film of not less than 5 nm thickness, the article of higher insulating and more effective to reduce the eddy current loss can be obtained. The present invention provides the following manufacturing method of the magnetic composite article. 1) Mix the soft magnetic powder of Fe-Al-Si system alloy of which magnetostrictive constant λ is positive at the room temperature with electrical insulating binder, 2) apply compacting pressure, and 3) provide a heat treatment ranging from 500°C to 900°C. The heat treatment after the compacting pressure contributes to reduce the hysteresis loss, whereby a stable and an excellent magnetic characteristics can be obtained. The electrical insulating binder preferably consists mainly of at least one of epoxy resin, phenol resin, polyvinyl chloride resin, butyral resin, and organic silicone resin. Since the heat treatment is provided at the temperature ranges from 500°C to 900°C, the ingredients of the binder preferably less diffuse into the magnetic powder, and a non-oxide atmosphere is preferred for the heat treatment in view of preventing the alloy powder from being oxidized. The heat treatment can be also provided in the air. After the heat treatment, the magnetic composite article is preferably put into insulating impregnant. Because the heat treatment over 500°C dissolves the binder such as resin, mechanical strength of the article is lowered, therefore, the insulating impregnant is impregnated into the article after the heat treatment so that the core strength is improved, magnetic powder is prevented from being oxidizing, and surface resistance is increased. Vacuum impregnation is preferred because the impregnant invades into inside of the core. The soft magnetic powder of Fe-Al-Si system alloy according to the present invention comprises 4.5% ≦ Al ≦ 8.5%, 7.5%≦ Si≦ 9.5%, and the remainder of Fe (the figures are wt%). The volume content of oxygen preferably reanges from 1000 ppm to 8000 ppm, and the magnetostrictive constant λ is positive at the room temperature. When this material is used, the temperature coefficient of the core-loss at the room temperature can stay negative, therefore, excellent magnetic characteristics such as a low core-loss and a high permeability even at a high frequency can be produced. When the volume content of oxygen is 1000 ppm or more, the eddy current loss is decreased. Because the resistance value of the magnetic powder increases in step with the increasing of oxygen-volume-content, the eddy current loss is decreased. When the volume content of oxygen exceeds the upper limit of 8000 ppm, the hysteresis loss increases, the total core-loss thus increases.

Claims: A magnetic composite article comprising soft magnetic powder consisting of 4.5% ≦ Al ≦ 8.5%, 7.5% ≦ Si ≦ 9.5%, and the remainder of Fe and impurities (the figures are wt%), said magnetic composite article having a magnetostrictive constant taking a positive value at room temperatures whereas the temperature coefficient of core-loss is taking a negative value at room temperatures. The magnetic composite article as defined in claim 1, wherein said soft magnetic powder comprises 5.0% ≦ Al ≦ 6.5%, 8.2% ≦ Si ≦ 9.2%, the balance being iron and impurities (the figures are wt%). The magnetic composite article as defined in claim 1 or 2, wherein the core loss is minimized at not lower than 80°C. The magnetic composite article as defined in one of claims 1 to 3, wherein an average grain size of said soft magnetic powder is between 1µm and 100µm. A method of manufacturing a magnetic composite article as defined in claim 1, said method comprising the steps of mixing an insulating binder with the soft magnetic powder , applying a compacting pressure to the mixed powder, and subjecting said mixed powder to a heat treatment between 500°C and 900°C. The method of manufacturing a magnetic composite article as defined in claim 5, wherein the insulating binder comprises at least one of epoxy resin, phenol resin, polyvinyl chloride resin, butyral resin and organic silicone resin. The magnetic composite article as defined in one of claims 1 to 4, wherein the volume content of oxygen in the soft magnetic powder is between 1000 ppm and 8000 ppm and the magnetostrictive constant λ takes a positive value at room temperature.

Inventors: MATSUTANI NOBUYA, MIDO YUJI, ONISHI KAZUAKI, MATSUTANI, NOBUYA, MIDO, YUJI, ONISHI, KAZUAKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD
Application year: 1998
Main classification: H01F1
IPC codes: H01F41, H01F1