SOFT-MAGNETIC POWDER COMPRISING COATED PARTICLES

Abstract

The invention is related to a soft-magnetic powder comprising coated particles, the coated particles comprising a core and a shell, the core having an average particle size D.sub.50 in a range from 0.1 μm to 100 μm and comprising iron, wherein the shell has a thickness of not more than 20 nm and comprises at least two solid oxides and wherein the shell comprises at least three layers and the shell comprises more than one layers of a first solid oxide and at least one layer of a second solid oxide, wherein the more than one layers of the first solid oxide and the at least one layer of the second solid oxide are arranged in an alternating manner. The invention is further related to a process for the production of the soft-magnetic powder, a use of the soft-magnetic powder and an electronic component comprising the soft-magnetic powder.

Claims

1.-12. (canceled)

13. A soft-magnetic powder comprising coated particles (1), the coated particles (1) comprising a core (3) and a shell (5), the core (3) having an average particle size D.sub.50 in a range from 0.1 μm to 100 μm and comprising iron (7), wherein the shell (5) has a thickness of not more than 20 nm and comprises at least two solid oxides (9, 11) and wherein the shell (5) comprises at least three layers (13, 15) and the shell (5) comprises more than one layers of a first solid oxide (13) and at least one layer of a second solid oxide (15), wherein the more than one layers of the first solid oxide (13) and the at least one layer of the second solid oxide (15) are arranged in an alternating manner and wherein the core (3) comprises carbonyl iron powder (CIP).

14. The soft-magnetic powder according to claim 13, wherein the number of layers of the first solid oxide (13) comprised in the shell (5) is equal to the number of layers of a second solid oxide (15) comprised in the shell (5).

15. The soft-magnetic powder according to claim 13, wherein the shell (5) comprises from 3 to 20 layers (13, 15).

16. The soft-magnetic powder according to claim 13, wherein a thickness of each of the at least three layers (13, 15) is in a range from 0.1 nm to 5 nm.

17. The soft-magnetic powder according to claim 13, wherein each of the at least three layers (13, 15) is amorphous, crystalline or a combination thereof.

18. The soft-magnetic powder according to claim 13, wherein each of the at least two solid oxides (9, 11) is an oxide of a metal, a metalloid or a transition metal.

19. The soft-magnetic powder according to claim 18, wherein the metal is Al, the metalloid is Si and/or the transition metal is selected from the group consisting of Hf, Zn, Zr, Co, Mn, Ni and Ti.

20. The soft-magnetic powder according to claim 13, wherein a first solid oxide (9) of the at least two solid oxides (9, 11) is Al.sub.2O.sub.3 and/or a second solid oxide (11) of the least two solid oxides (9, 11) is ZrO.sub.2 or SiO.sub.2.

21. A process for the production of the soft-magnetic powder according to claim 13, wherein the shell (5) is deposited on the core (3) by Atomic Layer Deposition (ALD).

22. A process according to claim 21, wherein each of the at least three layers (13, 15) is produced by more than one cycle of Atomic Layer Deposition (ALD).

23. Use of the soft-magnetic powder according to claim 13 for coil cores, magnetorheological fluids (MRF), powder injection moulding, radio-frequency identification tags or electromagnetic shielding.

24. An electronic component comprising the soft-magnetic powder according to claim 13.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0072] The present invention is described in more detail at hand of the accompanying schematic drawings and images, respectively, in which:

[0073] FIGS. 1 a)-d) shows cross-sections of a coated particle comprising a core and a shell,

[0074] FIGS. 2 a)-i) shows detailed sections of a coated particle comprising a core and a shell,

[0075] FIG. 3 shows TEM images of a coated particle comprising a core and a shell with layers and

[0076] FIG. 4 shows an EDXS-Linescan of the coated particle according to FIG. 3.

[0077] FIGS. 1 a) to 1 d) show cross-sections of a coated particles 1 comprising a core 3 and a shell 5. Six different coated particles 1 are represented. All coated particles 1 possess a core 3 comprising iron 7 and differ in the composition of the shell 5.

[0078] Coated particle 1, shown in FIG. 1 a), has a shell 5 on its core 3, wherein the shell 5 comprises a layer of a first solid oxide 13 and a layer of a second solid oxide 15. The layer of the first solid oxide 13 comprises a first solid oxide 9 and the layer of the second solid oxide 15 comprises a second solid oxide 11. Further, the shell 5 has an interface 17 between the layer of the first solid oxide 13 and the layer of the second solid oxide 15, where the first solid oxide 9 and the second solid oxide 11 are in contact with each other.

[0079] Coated particle 1, shown in FIG. 1 b), comprises a shell 5 comprising four layers 13, 15, namely two layers of the first solid oxide 13, comprising the first solid oxide 9, and two layers of the second solid oxide 15, comprising the second solid oxide 11. The layers of the first solid oxide 13 are arranged in an alternating manner with the layers of the second solid oxide 15.

[0080] Further, the shell 5 possesses three interfaces 17, where one of the layers of the first solid oxide 13 is in contact with one of the layers of the second solid oxide 15.

[0081] Coated particle 1, shown in FIG. 1 c), comprises a core 3 and a shell 5, comprising four layers of the first solid oxide 13 and four layers of the second solid oxide 15, which are arranged in an alternating manner. The shell 5 possesses seven interfaces 17, each between one of the layers of the first solid oxide 13 and one of the layers of the second solid oxide 15, respectively.

[0082] Coated particle 1, shown in FIG. 1 d), comprises a core 3 and a shell 5, comprising eight layers of the first solid oxide 13 and eight layers of the second solid oxide 15, which are arranged in an alternating manner. The shell 5 possesses 15 interfaces, each between one of the layers of the first solid oxide 13 and one of the layers of the second solid oxide 15, respectively.

[0083] FIGS. 2 a) to 2 i) show detailed sections of nine different coated particles 1, each comprising a core 3, comprising iron 7 and a shell 5. The coated particles 1 differ in the composition of the shell 5, which are represented in more detail.

[0084] The section of coated particle 1, shown in FIG. 2 a), has a shell 5 comprising a layer of a first solid oxide 13, which comprises a first solid oxide 9, and a layer of a second solid oxide 15, which comprises a second solid oxide 11. The layer of the first solid oxide 13 is in contact with the layer of the second solid oxide 15 at an interface 17. Further, the layer of the first solid oxide 13 is located directly on the core 3 and followed by the layer of the second solid oxide 15. A thickness of the layer of the first solid oxide 13 is superior to a thickness of the layer of the second solid oxide 15.

[0085] The section of the coated particle 1, shown in FIG. 2 b), has a shell 5 comprising three layers of the first solid oxide 13 and three layers of the second solid oxide 15, which are arranged in an alternating manner. One layer of the first solid oxide 13 is arranged directly on the core 3. One layer of the second solid oxide 15 is arranged on the outside of the coated particle 1.

[0086] The represented section of coated particle 1, shown in FIG. 2 c), differs from coated particle 1 of FIG. 2 b) in that the order of three layers of the first solid oxide 13 and three layers of the second solid oxide 15 is inverted. The layers of the first solid oxide 13 and the layers of the second solid oxide 15 are also arranged in an alternating manner, but here starting with a layer of the second solid oxide 15, which is arranged directly on the core 3 and a layer of the first solid oxide 13 on the outside of the coated particle 1.

[0087] The represented section of coated particle 1, shown in FIG. 2 d), corresponds to coated particle 1 of FIG. 2 b) with the difference that the coated particle 1 j) comprises eight layers of the first solid oxide 13 and eight layers of the second solid oxide 15.

[0088] The section of coated particle 1, shown in FIG. 2 e), corresponds to coated particle 1 of FIG. 2 c), with the difference that the coated particle 1 comprises eight layers of the first solid oxide 13 and eight layers of the second solid oxide 15.

[0089] The section of coated particle 1, shown in FIG. 2 f), possesses a shell 5 comprising four layers of the first solid oxide 13 and four layers of the second solid oxide 15. One layer of the second solid oxide 15 is arranged directly on the core 3 and one layer of the first solid oxide 13 is arranged on the outside of the coated particle 1. A thickness of the layers of the second solid oxide 15 is superior to a thickness of the layers of the first solid oxide 13.

[0090] The section of coated particle 1, shown in FIG. 2 g), differs from the coated particle 1 of FIG. 2 f) in that the order of the layers of the first solid oxide 13 and the layers of the second solid oxide 15 is inverted as well as in the thickness of the layers. A layer of the first solid oxide 13, which is thicker compared to a layer of the second solid oxide 15, is arranged directly on the core 3 and the layer of the second solid oxide 15, which is thinner compared to the layer of the first solid oxide 13, is arranged on the outside of the coated particle 1.

[0091] The section of coated particle 1, shown in FIG. 2 h), has a core 3 with a shell 5. The shell 5 comprises a first solid oxide 9 and a second solid oxide 11. The first solid oxide 9 and the second solid oxide 11 are each arranged with a concentration gradient in the shell 5. The concentration of the first solid oxide 9 decreases from the core 3 to the outside of the coated particle 1, whereas the concentration of the second oxide 11 increases from the core to the outside of the coated particle 1.

[0092] The section of coated particle 1, shown in FIG. 2 i), corresponds to the coated particle 1 of FIG. 2 h) with the difference that the concentration gradients of the first solid oxide 9 and the second solid oxide 11 are inverted. In this shell 5 the concentration of the first solid oxide increases from the core 3 to the outside of the coated particle 1 and the concentration of the second solid oxide 11 decreases from the core 3 to the outside of the coated particle 1.

[0093] FIG. 3 shows transmission electron microscopy (TEM) images of a section of a coated particle 1 comprising a core 3 and a shell 5. The core 3 comprising iron 7 is represented at the lower left end of the images.

[0094] Image b) shows two illuminated layers of a first solid oxide 9, comprising aluminum, as part of the shell 5 of the coated particle 1. Image c) shows two illuminated layers of a second solid oxide 11, comprising zirconium, as part of the shell 5 of the coated particle 1.

[0095] On image d) the first solid oxide 9, comprising aluminum, as well as the second solid oxide 11, comprising zirconium, are illuminated such that the complete shell 5 of the coated particle 1 is visible.

[0096] FIG. 4 shows an Energy-dispersive X-ray spectroscopy (EDXS)-Linescan of the coated particle according to FIG. 3. On the abscissae 19 a distance in nm, referring to the surface of the iron core, is given, whereas on an ordinate 21 net counts of the Linescan are shown in percent. Graphs are represented for detected iron 7, aluminum 23, zirconium 25 and oxygen 29. Two first layers 13, each comprising aluminum 23, and two second layers 15, each comprising zirconium 25, are visible.

Examples and Comparative Examples

[0097] Cores of carbonyl iron particles were coated by atomic layer deposition with Al.sub.2O.sub.3 or ZrO.sub.2 alone or only comprising one layer of each oxide (compare samples 2 to 8) as comparative example. Further, cores of carbonyl iron particles were coated by atomic layer deposition with a combination of Al.sub.2O.sub.3 and ZrO.sub.2 (compare samples 9 to 15) or with a combination of Al.sub.2O.sub.3 and SiO.sub.2 (compare sample 16), see Table 1. Additionally, carbonyl iron particles without a coating (compare sample 1) were tested.

[0098] The chemical composition of the applied solid oxides, the number of layers in the shell and the number of ALD cycles for the production of one layer was varied as well as the deposition temperature.

[0099] Here, a layer of the first solid oxide was arranged directly on the core followed by a layer of the second solid oxide, where applicable. Layers of the first solid oxide and layers of the second solid oxide were arranged in an alternating manner. The total amount of layers covering the core and forming the shell is the sum of the indicated number of layers of the first solid oxide and the indicated number of layers of the second solid oxide.

[0100] For example 60 g of uncoated carbonyl iron powder was placed into a fluidized bed reactor (FBR) of a Beneq TFS200 ALD. The FBR was evacuated to a pressure of approximately 100 Pa while it was heated to 180° C. During the experiment nitrogen with a purity of 99.999 mol-% was led through the FBR with a flow in a range from 10 sccm to 20 sccm. The FBR was in addition mechanically vibrated to assist the movement of the powder. Off-gasses from the FBR were monitored in real time by utilizing a Vision 2000C quadrupole mass spectrometer/residual gas analyzer from MKS. The powder was dried under these conditions for 3.5 hours to remove physisorbed water.

TABLE-US-00001 TABLE 1 First solid oxide Second solid oxide ALD Nb. of Nb. of Deposition Nb. of cycles/ Nb. of cycles/ Sample temperature Chemistry layers layer Chemistry layers layer 1 — — — — — — 2 180° C. Al.sub.2O.sub.3 1 25 — — — 3 180° C. ZrO.sub.2 1 50 — — — 4 180° C. Al.sub.2O.sub.3 1 64 — — — 5 180° C. Al.sub.2O.sub.3 1 64 — — — 6 180° C. ZrO.sub.2 1 70 — — — 7 180° C. Al.sub.2O.sub.3 1 55 ZrO.sub.2 1 11 8 180° C. Al.sub.2O.sub.3 1 32 ZrO.sub.2 1 35 9 180° C. Al.sub.2O.sub.3 2 16 ZrO.sub.2 2 17 10 180° C. Al.sub.2O.sub.3 4 8 ZrO.sub.2 4 9 11 180° C. Al.sub.2O.sub.3 8 4 ZrO.sub.2 8 4 12 120° C. Al.sub.2O.sub.3 2 16 ZrO.sub.2 2 17 13 240° C. Al.sub.2O.sub.3 2 16 ZrO.sub.2 2 17 14 180° C. ZrO.sub.2 2 17 Al.sub.2O.sub.3 2 16 15 180° C. ZrO.sub.2 4 9 Al.sub.2O.sub.3 4 8 16 240° C. Al.sub.2O.sub.3 2 16 SiO.sub.2 2 27 17 240° C. Al.sub.2O.sub.3 2 24 SiO.sub.2 2 14

[0101] For the powder samples, the oxidation onset temperature was measured using a simultaneous thermal analyzer Q600 from TA instruments. The analyzer comprised a microbalance and a furnace enabling the measurement of weight change versus temperature. The powders were heated in air at a rate of 20° C. per minute and the onset of weight gain (oxidation) was determined.

[0102] Select powders were coated as described in table 1 and pressed into inductor cores to measure the initial magnetic permeability. These cores were placed into an oven at 180° C. and the resulting voltage difference was monitored over time to gage the thermal stability. In such a test, a voltage of 0 V is ideal indicating the existence of electrically insulating layers surrounding the iron powder. Contrastingly, a high voltage measurement indicates that insulating shell has become conductive and compromises the performance of the powder. Additionally, the voltage was monitored for the same sample at the start of the experiment, after 24 hours, after 48 hours, after 72 hours and after 96 hours. The lower the measured voltage was, the higher was the electrical resistivity of the sample

[0103] Other magnetic cores produced using these powders were placed in a chamber maintained at 85° C. with 85% relative humidity (rH) to measure the corrosion resistance of the shell. The cores were visually inspected every 24 hours for appearance of rust stains. Cores prepared using powders 7, 9, 10 and 17 performed extremely well under these aggressive conditions in contrast to cores that were prepared with uncoated powder or using single oxides of comparable thickness (e.g. powders 3, 4, 5 and 6). For example, cores prepared with samples 1, 3, 4, 5 and 6 showed significant surface rust (>10% of exposed area), which was evident after only 24 hours of exposure to 85% relative humidity at 85° C. Contrastingly, powders 7, 9, 10 and 17 showed no evidence of surface rust even after 96 hours of exposure to the same conditions. The results indicate that the moisture induced corrosion resistance does not necessarily correlate to the oxidation onset temperature determined by the thermal analysis.

TABLE-US-00002 TABLE 2 Shell thick- Oxidation onset Relative ness temperature Permeability No. [nm] [° C.] [μ/μ.sub.0] 1 — 394 36.0 2 3.0 499 20.0 3 5.0 365 20.4 4 7.0 601 18.3 5 6.7 642 19.6 6 6.8 388 20.1 7 7.0 580 18.5 8 8.1 527 20.2 9 7.4 546 19.8 10 8.8 572 19.1 11 8.1 584 19.6 12 9.0 557 19.5 13 7.2 568 20.4 14 8.6 626 20.0 15 7.9 610 19.7 16 7.2 642 18.8 17 6.9 643 18.5

[0104] In table 2 the relative permeability refers to the permeability of a vacuum μ.sub.0.

TABLE-US-00003 TABLE 3 85° C./85% rH Voltage Voltage Voltage Voltage Voltage corrosion (0 hr) (24 hr) (48 hr) (72 hr) (96 hr) resistance [visually No. [V] [V] [V] [V] [V] assessed] 1 0 >290 — — — Poor 2 — — — — — 3 0 254 >254 >254 >254 Poor 4 0 52 >210 — — Poor 5 0 226 — — — Poor 6 0 41 178 249 — Poor 7 0 0.45 48 160 202 Excellent 8 0 7 203 242 — — 9 0 0.15 31 170 218 Excellent 10 0 1.50 53 169 223 Excellent 11 0 77 188 248 — 17 0 223 278 — — Excellent

[0105] As illustrated in Tables 2 and 3, the presence of at least two solid oxides in the shell of the coated particles led to a high electrical resistivity (low voltage) and/or to superior corrosion resistance. At the same time, the coated particles still possessed a good magnetic permeability.

[0106] Further, cross-sections of powders were prepared using a focused ion beam (FIB) and examined using HAADF-STEM (High Angle Annular Dark Field—Scanning Transmission Electron Microscopy) with EDXS (Energy Dispersive X-ray Spectroscopy) to enable mapping the distribution of elements in the sample. Sample 7 was analyzed and it was shown that the iron particle, representing the core, was uniformly coated with alternating layers with a thickness of about 2 nm of alumina, zirconia, alumina and zirconia.

REFERENCE NUMERALS

[0107] 1 Coated particle [0108] 3 Core [0109] 5 Shell [0110] 7 Iron [0111] 9 First solid oxide [0112] 11 Second solid oxide [0113] 13 Layer of a first solid oxide [0114] 15 Layer of a second solid oxide [0115] 17 Interface [0116] 19 Abscissae [0117] 21 Ordinate [0118] 23 Aluminum [0119] 25 Zirconium [0120] 29 Oxygen