Wave-absorbing material powder with oxidation resistance and salt fog resistance and preparation method thereof

Abstract

Wave-absorbing material powder of the present invention has oxidation resistance and salt fog resistance, which includes an iron-containing wave-absorbing material powder, and a metal oxide ceramic layer and a metal phosphate layer sequentially coated on an outside of the iron-containing wave-absorbing material powder from the inside to the outside. A method for preparing the wave-absorbing material powder includes using atomic layer deposition to coat the iron-containing wave absorbing material powder with a metal oxide ceramic coating, and then adopting the atomic layer deposition to coat the metal oxide ceramic coating with a metal phosphate layer; repeating the above steps to form an alternating nano-stack of the metal oxide ceramic coating and the metal phosphate layer outside the iron-containing absorbing material powder; and finally performing a high-temperature annealing treatment. The present invention improves temperature resistance, corrosion resistance and oxidation resistance of wave-absorbing materials.

Claims

1. A method for preparing wave-absorbing material powder with oxidation resistance and salt fog resistance, comprising steps of: step 1: putting iron-containing wave-absorbing material powder into a porous container; wherein a wave-absorbing material comprises carbonyl iron, iron-silicon-aluminum, and iron-cobalt; and the iron-containing wave-absorbing material powder is particles, flakes or chopped fibers; step 2: putting the porous container into an ALD (atomic layer deposition) reaction chamber; then repeatedly vacuumizing, and filling in nitrogen gas at least three times; step 3: using atomic layer deposition to coat the iron-containing wave-absorbing material powder with a metal oxide ceramic coating, wherein a non-oxygen element in the metal oxide ceramic coating is aluminum, hafnium, yttrium, zirconium, titanium, zinc or silicon; step 4: repeating the step 3 until a predetermined coating thickness is deposited; step 5: fluidizing powder obtained from the step 4 under nitrogen or argon atmosphere, wherein a fluidization pressure is 1-1000 torr; or rotating the porous container to disperse the powder; step 6: selecting a precursor according to a type of a deposited ceramic coating, and setting parameters of the ALD reaction chamber as: a deposition temperature is 100° C.-400° C., and a deposition pressure is 0.01 torr-500 torr; wherein the precursor is volatile trimethylaluminum, Ti(OEt).sub.4, Zr[N(CH.sub.3).sub.2].sub.4, or Hf[N(CH.sub.3)(C.sub.2H.sub.5)].sub.4; step 7: introducing precursor vapor into the ALD reaction chamber with nitrogen or argon, and holding for 10-300 seconds; step 8: purging the ALD reaction chamber with nitrogen or argon to remove residual precursor; step 9: introducing dimethyl phosphate vapor into the ALD reaction chamber with nitrogen or argon, and holding for 10-300 seconds; step 10: purging the ALD reaction chamber with nitrogen or argon to remove an excess dimethyl phosphate vapor oxygen source and by-products; step 11: repeating the steps 5-10 until a predetermined thickness of a metal phosphate coating is deposited, wherein the metal phosphate coating is AlPO.sub.4, Ti.sub.3(PO.sub.4).sub.4, Zr(HPO.sub.4).sub.2, or Hf.sub.3(PO.sub.4).sub.4; and step 12: sintering obtained powder at 600-900° C. under argon atmosphere.

2. The method, as recited in claim 1, further comprising a step between the step 11 and the step 12: repeating the steps 3-10 to form alternating nano-stack layers composed of the metal oxide ceramic coating and AlPO.sub.4, Ti.sub.3(PO.sub.4).sub.4, Zr(HPO.sub.4).sub.2, or Hf3(PO.sub.4).sub.4.

3. The method, as recited in claim 1, wherein the step 3 comprises specific steps of: Step 31: fluidizing the iron-containing wave-absorbing material powder in nitrogen or argon atmosphere, wherein a fluidization pressure is 1-1000 torr; or rotating the porous container to disperse the iron-containing wave-absorbing material powder; step 32: selecting a precursor according to a type of a deposited oxide coating, and setting the parameters of the ALD reaction chamber as: the deposition temperature is 25° C.-400° C., and the deposition pressure is 0.01 torr-500 torr; step 33: introducing precursor vapor into the ALD reaction chamber with nitrogen or argon, and holding for 10-300 seconds; step 34: purging the ALD reaction chamber with nitrogen or argon to remove residual precursor; step 35: introducing oxygen source vapor into the ALD reaction chamber with nitrogen or argon, and holding for 10-300 seconds; and step 36: purging the ALD reaction chamber with nitrogen or argon to remove an excess oxygen source and by-products.

4. The method, as recited in claim 1, wherein the precursor in the step 3 is selected from a group consisting of volatile silicon ions, metal alkylamino salts, metal organic compounds, halides, alkoxides, and metal β-diketone complexes; and metal ions in the metal alkylamino salts, the metal organic compounds, the halides, the alkoxides, and the metal β-diketone complexes are aluminum, hafnium, yttrium, zirconium, titanium, and zinc ions.

5. The method, as recited in claim 2, wherein the precursor in the step 3 is selected from a group consisting of volatile silicon ions, metal alkylamino salts, metal organic compounds, halides, alkoxides, and metal β-diketone complexes; and metal ions in the metal alkylamino salts, the metal organic compounds, the halides, the alkoxides, and the metal β-diketone complexes are aluminum, hafnium, yttrium, zirconium, titanium, and zinc ions.

6. The method, as recited in claim 3, wherein the precursor in the step 3 is selected from a group consisting of volatile silicon ions, metal alkylamino salts, metal organic compounds, halides, alkoxides, and metal β-diketone complexes; and metal ions in the metal alkylamino salts, the metal organic compounds, the halides, the alkoxides, and the metal β-diketone complexes are aluminum, hafnium, yttrium, zirconium, titanium, and zinc ions.

7. The method, as recited in claim 4, wherein the oxygen source is water, hydrogen peroxide, oxygen, ozone, or atomic oxygen.

8. The method, as recited in claim 5, wherein the oxygen source is water, hydrogen peroxide, oxygen, ozone, or atomic oxygen.

9. The method, as recited in claim 6, wherein the oxygen source is water, hydrogen peroxide, oxygen, ozone, or atomic oxygen.

10. The method, as recited in claim 1, wherein a carrying gas flow rate in the step 7 and the step 9 is 5-8000 sccm; and a purging gas flow rate in the step 8 and the step 9 is 10-5000 sccm.

11. The method, as recited in claim 3, wherein a carrying gas flow rate in the step 33 and the step 35 is 5-8000 sccm; and a purging gas flow rate in the step 34 and the step 36 is 10-5000 sccm.

12. The method, as recited in claim 3, wherein the fluidization pressure in the step 3 and the step 5 is 1-1000 torr.

13. Wave-absorbing material powder with oxidation resistance and salt fog resistance prepared by the method as recited in claim 1, comprising: iron-containing wave absorbing material powder, and a coating on the iron-containing wave absorbing material powder, wherein the coating is composed of multiple alternating metal oxide ceramic coatings and metal phosphate coatings; a non-oxygen element in the metal oxide ceramic coating is aluminum, hafnium, yttrium, zirconium, titanium, zinc or silicon, and the metal phosphate coating is AlPO.sub.4, Ti.sub.3(PO.sub.4).sub.4, Zr(HPO.sub.4).sub.2, or Hf3(PO.sub.4).sub.4; the coating is a film layer with a crystalline structure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] FIG. 1 illustrates resistivity of wave-absorbing powder material powder before and after using an anti-corrosion coating of the present invention;

[0046] FIG. 2 is a cross-sectional electron microscope photo of a nano-laminate composed of ZrO.sub.2 and AlPO.sub.4 of the wave-absorbing material powder with oxidation resistance and salt fog resistance prepared in an embodiment;

[0047] FIG. 3 is results of a corrosion test after soaking carbonyl iron powder of embodiments 2-4 in hydrochloric acid for three days;

[0048] FIG. 4 is results of a neutral corrosion resistance test of original powder, an absorbent with a single-layer coating and an absorbent with a lamination coating;

[0049] FIG. 5 is a TGA diagram of the original powder (an oxidation temperature is 665.7° C.);

[0050] FIG. 6 is a TGA diagram of the absorbent with the single-layer coating (an oxidation temperature is 694.9° C.);

[0051] FIG. 7 is a TGA diagram of the absorbent with the lamination coating (an oxidation temperature is 781.9° C.);

[0052] FIG. 8 is a curve of real parts of electromagnetic parameter: complex permittivity of the original powder, the absorbent with the single-layer coating, an uncalcined absorbent with the laminating coating, and an absorbent prepared in the embodiment 1;

[0053] FIG. 9 is curve of imaginary parts of the electromagnetic parameter: the complex permittivity of the original powder, the absorbent with the single-layer coating, the uncalcined absorbent with the laminating coating, and the absorbent prepared in the embodiment 1;

[0054] FIG. 10 is curve of real parts of the electromagnetic parameter: complex magnetic permeability of the original powder, the absorbent with the single-layer coating, the uncalcined absorbent with the laminating coating, and the absorbent prepared in the embodiment 1;

[0055] FIG. 11 is curve of imaginary parts of the electromagnetic parameter: the complex magnetic permeability of the original powder, the absorbent with the single-layer coating, the uncalcined absorbent with the laminating coating, and the absorbent prepared in the embodiment 1;

[0056] FIG. 12 illustrates a simulated reflectivity of the original powder;

[0057] FIG. 13 illustrates a simulated reflectivity of ZrO.sub.2 with a single-layer coating formed by atomic layer deposition;

[0058] FIG. 14 illustrates a simulated reflectivity of the uncalcined absorbent with the laminating coating; and

[0059] FIG. 15 illustrates a simulated reflectivity of the absorbent prepared in the embodiment 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0060] Referring to the drawings, the present invention will be further illustrated. Embodiments of the present invention are implemented based on the technical scheme of the present invention, which are described in detail. However, the protection scope of the present invention is not limited to the following embodiments.

[0061] A method for preparing wave-absorbing material powder with oxidation resistance and salt fog resistance, comprises steps of:

[0062] step 1: putting wave-absorbing material powder into a porous container; wherein a wave-absorbing material comprises carbonyl iron, nickel carbonyl, cobalt carbonyl, metal powder, silicon carbide, iron silicon aluminum, iron cobalt, or metal powder that can be used as a wave-absorbing material; and the iron-containing wave-absorbing material powder is particles, flakes or chopped fibers;

[0063] step 2: putting the porous container into an ALD (atomic layer deposition) reaction chamber; then repeatedly vacuumizing, and filling in nitrogen gas at least three times;

[0064] Step 3: fluidizing the iron-containing wave-absorbing material powder in nitrogen or argon atmosphere, wherein a fluidization pressure is 1-1000 torr; or rotating the porous container to disperse the iron-containing wave-absorbing material powder; wherein the fluidization pressure is preferably 10-100 torr;

[0065] step 4: selecting a precursor according to a type of a deposited ceramic coating, and setting the parameters of the ALD reaction chamber as: the deposition temperature is 100° C.-400° C., and the deposition pressure is 0.01 torr-500 torr; the precursor is selected from a group consisting of volatile silicon ions, metal alkylamino salts, metal organic compounds, halides, alkoxides, and metal β-diketone complexes; and metal ions in the metal alkylamino salts, the metal organic compounds, the halides, the alkoxides, and the metal β-diketone complexes are aluminum, hafnium, yttrium, zirconium, titanium, and zinc ions;

[0066] step 5: introducing precursor vapor into the ALD reaction chamber with nitrogen or argon, and holding for 10-300 seconds;

[0067] step 6: purging the ALD reaction chamber with nitrogen or argon to remove residual precursor;

[0068] step 7: introducing oxygen source vapor into the ALD reaction chamber with nitrogen or argon, and holding for 10-300 seconds; wherein the oxygen source is water, hydrogen peroxide, oxygen, ozone, or atomic oxygen;

[0069] step 8: purging the ALD reaction chamber with nitrogen or argon to remove an excess oxygen source and by-products;

[0070] step 9: repeating the steps 5-8 until a predetermined coating thickness is deposited, so as to obtain wave-absorbing powder with an atomic layer deposition oxide coating; and

[0071] step 10: choosing another deposit and repeating the steps 3-8 until a predetermined coating thickness is deposited, wherein the other deposit is metal phosphate, and a precursor thereof is volatile trimethylaluminum and dimethyl phosphate; the obtained powder is subjected to high-temperature annealing treatment in argon atmosphere at 600-900° C. to obtain powder with an atomic layer deposition metal phosphate coating;

[0072] wherein the step 10 comprises specific steps of:

[0073] step 101: fluidizing the iron-containing wave-absorbing material powder in nitrogen or argon atmosphere, wherein a fluidization pressure is 1-1000 torr; or rotating the porous container to disperse the iron-containing wave-absorbing material powder; wherein the fluidization pressure is preferably 10-100 torr;

[0074] step 102: selecting a precursor according to a type of a deposited ceramic coating, and setting parameters of the ALD reaction chamber as: a deposition temperature is 100° C.-400° C., and a deposition pressure is 0.01 torr-500 torr; wherein the precursor is volatile Ti(OEt).sub.4, Zr[N(CH.sub.3).sub.2].sub.4, or Hf[N(CH.sub.3)(C2H.sub.5)].sub.4;

[0075] step 103: introducing precursor vapor into the ALD reaction chamber with nitrogen or argon, and holding for 10-300 seconds;

[0076] step 104: purging the ALD reaction chamber with nitrogen or argon to remove residual precursor;

[0077] step 105: introducing dimethyl phosphate vapor into the ALD reaction chamber with nitrogen or argon, and holding for 10-300 seconds;

[0078] step 106: purging the ALD reaction chamber with nitrogen or argon to remove an excess dimethyl phosphate vapor oxygen source and by-products; and

[0079] step 107: repeating the step 10 until a predetermined thickness of the AlPO.sub.4, Ti.sub.3(PO.sub.4).sub.4, Zr(HPO.sub.4).sub.2, or Hf3(PO.sub.4).sub.4 coating is deposited.

[0080] The powder with the atomic layer deposition metal phosphate coating obtained in the step 10 comprises iron-containing wave absorbing material powder, and a coating on the iron-containing wave absorbing material powder, wherein the coating is composed of multiple alternating metal oxide ceramic coatings and metal phosphate coatings; a non-oxygen element in the metal oxide ceramic coating is aluminum, hafnium, yttrium, zirconium, titanium, zinc or silicon, and the metal phosphate coating is AlPO.sub.4, Ti.sub.3(PO.sub.4).sub.4, Zr(HPO.sub.4).sub.2, or Hf.sub.3(PO.sub.4).sub.4; the coating is a film layer with a crystalline structure.

[0081] Embodiment 1

[0082] The embodiment 1 comprises the following steps of:

[0083] (1) putting iron-cobalt powder into a porous container with micropore size;

[0084] (2) putting the porous container into an ALD reaction chamber, then repeatedly vacuumizing, and filling in nitrogen gas three times; heating the reaction chamber to 200° C., and maintaining a pressure of the reaction chamber at 5 torr;

[0085] (3) rotating the porous container so that the powder is fully mixed in the porous container;

[0086] (4) pulsing a precursor Zr[N(CH.sub.3).sub.2].sub.4 into the reaction chamber with N.sub.2 at a flow rate of 50 sccm, adsorbing on the iron-cobalt powder until 6 torr, and keeping for 60 seconds; purging with 50 sccm N.sub.2 for 30 s and removing residual Zr[N(CH.sub.3).sub.2].sub.4; then purging H.sub.2O into the reaction chamber with 50 sccm N.sub.2 until the gas pressure reaches 6 torr, and keeping for 60 seconds to chemically reacts with the Zr[N(CH.sub.3).sub.2].sub.4 adsorbed on the iron-cobalt powder, thereby generating ZrO.sub.2 for 60 s; and then purging with 50 sccm N.sub.2 to remove excess water and by-products for 30 s, thus completing an ALD deposition cycle;

[0087] (5) repeating the step (4) 10 times to obtain iron-cobalt powder with a 1 nm-thick ZrO.sub.2 coating layer;

[0088] (6) rotating the porous container again, so that the iron-cobalt powder with the ZrO.sub.2 coating is fully mixed in the porous cavity;

[0089] (7) pulsing a precursor trimethylaluminum vapor into the reaction chamber with N.sub.2 at a flow rate of 50 sccm, adsorbing on the ZrO.sub.2 coating layer of the powder obtained in the step (5) until 6 torr, and keeping for 60 seconds; purging with 50 sccm N.sub.2 for 30 s and removing residual trimethylaluminum; then purging dimethyl phosphate into the reaction chamber with 50 sccm N.sub.2 until the gas pressure reaches 6 torr, and keeping for 60 seconds to chemically reacts with the trimethylaluminum adsorbed on the ZrO.sub.2 coating layer, thereby generating AlPO.sub.4 for 60 s; and then purging with 50 sccm N.sub.2 to remove excess dimethyl phosphate and by-products for 30 s, thus completing an ALD deposition cycle;

[0090] (8) repeating the steps (6)-(7) 10 times, so that a coating thickness is lnm, thereby complete a nano-stack, wherein phosphates are more resistant to water and acid corrosion; when different coatings of ZrO.sub.2 and AlPO.sub.4 are formed, discontinuous and mismatched characteristics of their grain boundaries can be used to more efficiently block oxygen and ions.

[0091] (9) repeating the steps (3)-(8) 5 times to form a nano-stack composed of ZrO.sub.2 and AlPO.sub.4 with a total thickness of 10 nm; wherein after multiple repetitions, the coating thickness increases to provide better oxidation and corrosion resistance; and (10) processing the powder obtained in the step (9) in argon atmosphere at 600° C. for 4 hours, so as to obtain a final product; and sintering at a high temperature to improve wave-absorbing performance.

[0092] The more the step (9) is repeated, the more the thickness will be, which means better oxidation and corrosion resistance, but the thicker it is, the worse the magnetic reflection will be. However, after the high-temperature annealing treatment in the step (10), iron elements are precipitated to the surface layer, so that the magnetic reflection is improved.

[0093] The scanning electron microscope photo of the final product prepared in the embodiment 1 is shown in FIG. 2. ZrO.sub.2 and AlPO.sub.4 constitute a nano-stack, and each nano-coating layer is dense and uniform. FIG. 1 shows conductivities of the iron-cobalt powder and the powder obtained in the step (10). As the pressure during conductivity test increases, the conductivity of the coated iron-cobalt powder decreases when the nano ceramic layer coating the powder is dense, indicating the denser the coating layer, the lower the conductivity.

[0094] Embodiment 2

[0095] The embodiment 2 comprises the following steps of:

[0096] (1) putting carbonyl-iron powder into a porous container with micropore size;

[0097] (2) putting the porous container into an ALD reaction chamber, then repeatedly vacuumizing, and filling in nitrogen gas three times; heating the reaction chamber to 200° C., and maintaining a pressure of the reaction chamber at 5 torr;

[0098] (3) rotating the porous container so that the powder is fully mixed in the porous container;

[0099] (4) pulsing a precursor Zr[N(CH.sub.3).sub.2].sub.4 into the reaction chamber with N.sub.2 at a flow rate of 50 sccm, adsorbing on the carbonyl-iron powder until 6 torr, and keeping for 60 seconds; purging with 50 sccm N.sub.2 for 30 s and removing residual Zr[N(CH.sub.3).sub.2].sub.4; then purging H.sub.2O into the reaction chamber with 50 sccm N.sub.2 until the gas pressure reaches 6 torr, and keeping for 60 seconds to chemically reacts with the Zr[N(CH.sub.3).sub.2].sub.4 adsorbed on the carbonyl-iron powder, thereby generating ZrO.sub.2 for 60 s; and then purging with 50 sccm N.sub.2 to remove excess water and by-products for 30 s, thus completing an ALD deposition cycle;

[0100] (5) repeating the step (4) 10 times to obtain carbonyl-iron powder with a 1 nm-thick ZrO.sub.2 coating layer;

[0101] executing the steps (6)-(8) of the embodiment 1, using trimethylaluminum and dimethyl phosphate as precursors for AlPO.sub.4 coating, thereby forming a nano-stack with a coating thickness of 1 nanometer; and

[0102] repeat the above steps to form 5 nano-stacks consisting of ZrO.sub.2 and AlPO.sub.4 with a total thickness of 10 nm; processing the powder in argon atmosphere at 300° C. for 4 hours to obtain the final product.

[0103] Embodiment 3

[0104] The embodiment 3 comprises the following steps of:

[0105] (1) putting carbonyl-iron powder into a porous container with micropore size;

[0106] (2) putting the porous container into an ALD reaction chamber, then repeatedly vacuumizing, and filling in nitrogen gas three times; heating the reaction chamber to 200° C., and maintaining a pressure of the reaction chamber at 5 torr;

[0107] (3) rotating the porous container so that the powder is fully mixed in the porous container;

[0108] (4) pulsing a precursor Zr[N(CH.sub.3).sub.2].sub.4 into the reaction chamber with N.sub.2 at a flow rate of 50 sccm, adsorbing on the carbonyl-iron powder until 6 torr, and keeping for 60 seconds; purging with 50 sccm N.sub.2 for 30 s and removing residual Zr[N(CH.sub.3).sub.2].sub.4; then purging H.sub.2O into the reaction chamber with 50 sccm N.sub.2 until the gas pressure reaches 6 torr, and keeping for 60 seconds to chemically reacts with the Zr[N(CH.sub.3).sub.2].sub.4 adsorbed on the carbonyl-iron powder, thereby generating ZrO.sub.2 for 60s; and then purging with 50 sccm N.sub.2 to remove excess water and by-products for 30 s, thus completing an ALD deposition cycle;

[0109] (5) repeating the step (4) 10 times to obtain carbonyl-iron powder with a 1 nm-thick ZrO.sub.2 coating layer;

[0110] executing the steps (6)-(8) of the embodiment 1, using trimethylaluminum and dimethyl phosphate as precursors for AlPO.sub.4 coating, thereby forming a nano-stack with a coating thickness of 1 nanometer; and repeat the above steps to form 5 nano-stacks consisting of ZrO.sub.2 and AlPO.sub.4 with a total thickness of 10 nm.

[0111] Embodiment 4

[0112] The embodiment 4 comprises the following steps of:

[0113] (1) putting carbonyl-iron powder into a porous container with micropore size;

[0114] (2) putting the porous container into an ALD reaction chamber, then repeatedly vacuumizing, and filling in nitrogen gas three times; heating the reaction chamber to 200° C., and maintaining a pressure of the reaction chamber at 5 torr;

[0115] (3) rotating the porous container so that the powder is fully mixed in the porous container;

[0116] (4) pulsing a precursor Zr[N(CH.sub.3).sub.2].sub.4 into the reaction chamber with N.sub.2 at a flow rate of 50 sccm, adsorbing on the carbonyl-iron powder until 6 torr, and keeping for 60 seconds; purging with 50 sccm N.sub.2 for 30 s and removing residual Zr[N(CH.sub.3).sub.2].sub.4; then purging H.sub.2O into the reaction chamber with 50 sccm N.sub.2 until the gas pressure reaches 6 torr, and keeping for 60 seconds to chemically reacts with the Zr[N(CH.sub.3).sub.2].sub.4 adsorbed on the carbonyl-iron powder, thereby generating ZrO.sub.2 for 60 s; and then purging with 50 sccm N.sub.2 to remove excess water and by-products for 30 s, thus completing an ALD deposition cycle;

[0117] (5) repeating the step (4) 10 times to obtain carbonyl-iron powder with a 1 nm-thick ZrO.sub.2 coating layer;

[0118] (6) rotating the porous container again, so that the carbonyl-iron powder with the ZrO.sub.2 coating is fully mixed in the porous cavity;

[0119] (7) pulsing a precursor trimethylaluminum vapor into the reaction chamber with N.sub.2 at a flow rate of 50 sccm, adsorbing on the ZrO.sub.2 coating layer of the powder obtained in the step (5) until 6 torr, and keeping for 60 seconds; purging with 50 sccm N.sub.2 for 30 s and removing residual trimethylaluminum; then purging dimethyl phosphate into the reaction chamber with 50 sccm N.sub.2 until the gas pressure reaches 6 torr, and keeping for 60 seconds to chemically reacts with the trimethylaluminum adsorbed on the ZrO.sub.2 coating layer, thereby generating AlPO.sub.4 for 60 s; and then purging with 50 sccm N.sub.2 to remove excess dimethyl phosphate and by-products for 30 s, thus completing an ALD deposition cycle;

[0120] (8) repeating the steps (6)-(7) 10 times, so that a coating thickness is lnm, thereby complete a nano-stack, wherein phosphates are more resistant to water and acid corrosion; when different coatings of ZrO.sub.2 and AlPO.sub.4 are formed, discontinuous and mismatched characteristics of their grain boundaries can be used to more efficiently block oxygen and ions.

[0121] (9) repeating the steps (3)-(8) 5 times to form a nano-stack composed of ZrO.sub.2 and AlPO.sub.4 with a total thickness of 10 nm; wherein after multiple repetitions, the coating thickness increases to provide better oxidation and corrosion resistance; and

[0122] (10) processing the powder obtained in the step (9) in argon atmosphere at 600° C. for 4 hours, so as to obtain a final product; and sintering at a high temperature to improve wave-absorbing performance.

[0123] The more repetitions of steps (3)˜(8) in step (9), the greater the thickness of the nano layer, which means better oxidation and corrosion resistance, but the thicker it is, the worse the magnetic reflection will be. However, after the high-temperature annealing treatment in the step (10), iron elements are precipitated to the surface layer, so that the magnetic reflection is improved.

[0124] In order to test the acidic corrosion resistance of carbonyl-iron powder t before and after coating, 0.8 g carbonyl-iron powder with no coating and carbonyl-iron with ALD cyclic coating obtained in the embodiments 4 and 2 were respectively weighed at room temperature. Powder samples were placed in 0.2 mol/L HCl solution and stirred quickly with a glass rod. As shown in FIG. 3, corrosion test was performed by immersing in salt water for three days. Severe discoloration occurred to the embodiment 3, while no discoloration occurred to the embodiments 2 and 4. Therefore, the anti-HCl corrosion resistance of the wave-absorbing materials with the lamination coating in the embodiments 4 and 2 is better than that of the wave-absorbing materials with a single-layer ZrO.sub.2 coating in the prior art.

[0125] At the same time, neutral corrosion resistance test was carried out on the uncoated original wave-absorbing material, the wave-absorbing material with the single-layer ZrO.sub.2 coating and the wave-absorbing material with the laminated coating. The results are shown in FIG. 4. After soaking in 5% salt water for 24 hours, the original iron powder turned yellow obviously, while the coated powder showed no obvious corrosion.

[0126] In addition, TGA test was carried out on the original wave-absorbing material, the wave-absorbing material with the single-layer coating and the wave-absorbing material with the laminated coating. Referring to FIG. 5, the results show that the uncoated wave-absorbing material suffered thermal oxidation with a temperature higher than 665.7° C. due high-temperature oxidation, and the carbonyl-iron powder was oxidized to Fe.sub.2O.sub.3 or Fe.sub.3O.sub.4. The thermal gravity of the wave-absorbing material increased significantly.

[0127] As shown in FIG. 5, after the single-layer coating of the present invention, the increment of thermal gravity of the wave-absorbing material was reduced, and the thermal oxidation temperature increased to 694.9° C.

[0128] As shown in FIG. 7, after the lamination coating of the present invention, the thermal oxidation temperature of the absorbing material increased to 781.9° C., and the anti-oxidation performance was significantly improved.

[0129] A coaxial method was used to test the electromagnetic parameters of the original wave-absorbing material, the wave-absorbing material with the single-layer coating, the uncalcined wave-absorbing material with the lamination coating, and the wave-absorbing material prepared in the embodiment 1. The results are shown in FIGS. 8-11, wherein the magnetic permeability of the wave-absorbing material without high-temperature annealing treatment is the lowest, indicating that the magnetic loss of the magnetic wave-absorbing material is directly affected by the lamination coating. The wave-absorbing material with only the metal oxide layer (the single-layer coating) has the highest magnetic permeability, but FIG. 6 shows that the anti-oxidation performance of the wave-absorbing material with the single-layer coating is weak, while the magnetic permeability of the wave-absorbing material prepared in the embodiment 1 is larger, indicating that such coating has little effect on the magnetic loss of the magnetic wave-absorbing material, while the dielectric constant increases significantly. That is to say, the electrical loss is enhanced, which is beneficial to the absorption of electromagnetic waves.

[0130] By simulating and calculating the electromagnetic absorption properties of the original wave-absorbing material, the wave-absorbing material with the single-layer coating, the uncalcined wave-absorbing material with the lamination coating, and the wave-absorbing material prepared in the embodiment 1, it is found that with the same wave-absorbing material content and the same thickness, the electromagnetic wave absorption peak of the coated and high-temperature-annealed wave-absorbing material moves to the low frequency, while the peak and valley are deeper, reflecting the radar absorption performance of the wave-absorbing material after stacking and coating of the embodiment 1 is improved. The reflectivity simulation results are shown in FIGS. 12-15.

[0131] In general, when the electromagnetic parameters of the material are fixed, and the reflectivity of the material can be adjusted by changing the thickness of the nano layer. When the thickness is fixed, the material should be redesigned to change the electromagnetic properties thereof, so as to adjust the reflectivity of the material.

[0132] In general, when the thickness of the nano layer is large, the absorption peak of the wave-absorbing material is located at the low frequency, and the absorption bandwidth becomes narrower. In addition, the wave-absorbing materials should not be prepared without purpose, but should combine specific project indicators to understand the required reflectivity of each frequency band, so as to design electromagnetic parameters and matching thickness.

[0133] In short, the wave-absorbing material is relatively complex, and the above parameters should be considered comprehensively during research. The reflectivity should be calculated based on the transmission line theory to find the optimal electromagnetic parameters and matching thickness.