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Studies On Preparation And Microwave Absorption Properties Of Cobalt-Based Metals Coated Strontium Ferrite Composite Powders

Posted on:2009-02-08Degree:DoctorType:Dissertation
Country:ChinaCandidate:X F PanFull Text:PDF
GTID:1101360242495172Subject:Materials science
Abstract/Summary:PDF Full Text Request
Recently, the research of electromagnetic wave absorbent drew more and more attention. The ideal electromagnetic wave absorbent is expected to possess the following properties: strong absorption, wide bandwidth, thin coating thickness and light mass. However, there is no such kind of absorbent which could meet all the above-mentioned requirements. The composite of different absorbents is considered the best way to develop good microwave absorption materials. The M-type strontium ferrite is thought to be a very important magnetic absorbent for its special properties, such as high resonance frequency, high resistance, low permittivity and so on. But the microwave absorption properties of the strontium ferrite are sensitive to its synthesis technique. It is hard to improve the permeability of the strontium ferrite further. The microwave absorption properties of strontium ferrite are not as good as ferromagnetic metal at high frequency. The magnetic metal is another kind of very important microwave absorbent. It is widely used as microwave absorbent for its high specific saturation magnetization, high magnetic susceptibilty and stead properties to temperature changing. However, the microwave absorption properties of the magnetic metal are confined by the"skin effect", its use must under the conditions of small particle size, low particle concentration and well-proportioned distribution in the electromagnetic wave absorption coating. The electromagnetic properties of the strontium ferrite and magnetic metal are complementary. The combination of them is expected to create good electromagnetic wave absorption composite. In this paper a new"core-shell"type cobalt-based metals coated strontium ferrite composite powder is fabricated by electroless plating with the aid of sonication. The complement of electromagnetic properties of the strontium ferrite and the magnetic cobalt-based metals, the special"core-shell"structure of the composite particle and the interactions of the core and shell are all expected to give rise to the electromagnetic wave absorption properties of the composite powder.First, the strontium ferrite was synthesized by sol-gel low temperature self-propagating method. The effects of solution such as water or glycol, Fe/Sr molar ratio and synthesis temperature were investigated carefully to get the strontium ferrite powder, which is aimed to be the core of the"core-shell"composite powder. The results indicated that the application of glycol, the suitable Fe/Sr molar ratio and synthesis temperature are all very important to fabricate the strontium ferrite powder with excellent microwave absorption properties. When glycol as solution, Fe/Sr molar ratio is 11.2 and synthesis temperature is 850℃, the strontium ferrite possesses the best microwave absorption properties. The minimum reflection loss and matching thickness and 10dB bandwidth of the strontium ferrite are -29.77dB and 2.5mm and 4.32GHz, respectively.Then, the strontium ferrite was plated with cobalt-based soft magnetic metals by electroless plating with the aid of sonication at room temperature. The results showed that the deposited cobalt-based metal on the surface of the strontium ferrite is in the form of very tiny particles. The Co-P coating is crystalline, but the Co-Ni-P is amorphous. The improvement in microwave absorption properties of as-plated composite powers is not obvious. The Co-P-coated strontium ferrite possesses the best microwave absorption properties. The minimum reflection loss decreases to -35.44 dB, compared with -29.77dB of strontium ferrite. The microwave absorption properties of Co-Ni-P-coated strontium ferrite decrease with the increase of plated layer content and nickel content in it, until even lower than that of the strontium ferrite. After heat-treated at 400℃for 1h, the amorphous Co-Ni-P coating crystallized. The microwave absorption properties of the composite powders are also improved very much in comparison with those of as-plated powder. After heat treatment, the sample SM3 possesses the best microwave absorption properties. The minimum reflection loss decreases from -31.03dB to -43.46dB after heat treatment. In comparison with the strontium ferrite, the decrease of minimum reflection loss of the heat treated sample SM3 is about 13.69dB. After heat treatment, the increase of the electromagnetic wave absorption properties of Co-P-coated strontium ferrite is not as abrupt as the Co-Ni-P-coated strontium ferrite. But the minimum of reflection loss of it is also decreased about 31% in comparison with strontium ferrite. The matching thickness and 10dB bandwidth of the heat treated powder are changed very much with the increase of plated layer content and nickel content in it. The matching frequency of the heat treated powder shifts to the lower frequency in comparison with that of before heat treatment. Finally, the Co-coated strontium ferrite composite powder was fabricated. The results indicated that there is an optimism cobalt content for the composite powder. In our study, it is 21.1%. With such a cobalt content, the Co-coated strontium ferrite composite powder possesses very excellent microwave absorption properties. The minimum of reflection loss, matching thickness and 10dB bandwidth of it are -41.24dB, 2.2mm and 4.12GHz, respectively. Its minimum reflection loss is always less than -29dB when the thickness of the microwave absorption coating is changed from 2mm to 3mm. The cobalt powder was fabricated by chemical reduction method. The microwave absorption properties of the Co-coated strontium ferrite composite powder and the mixture of the cobalt powder and strontium ferrite powder were studied carefully to discover the mechanism of microwave absorption of the cobalt-based metals coated strontium ferrite core-shell composite powder. The microwave absorption properties of the mixture of the cobalt powder and strontium ferrite powder is better than that of the strontium ferrite powder, but they are not as good as that of the Co-coated strontium ferrite composite powder. The minimum reflection loss, matching thickness and 10dB bandwidth are–33.07dB, 2.3mm and 3.61GHz, respectively.The electromagnetic wave absorption mechanisms of the core-shell composite powder are supposed to be as follows: (1) The electromagnetic properties of the strontium ferrite and magnetic metal are really complementary. (2) The"core-shell"structure of the composite powder increases the transmission course of the electromagnetic wave, enhancing the attenuation of the electromagnetic wave. (3) The interface polarization and magnetic crystalline exchange-coupling effect increase the microwave absorption properties of the composite powder too.
Keywords/Search Tags:Electromagnetic property, Electroless plating, Magnetic powder, Microwave absorption, Surface modification
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