【作者】 潘喜峰；

【导师】 顾明元；

【作者基本信息】 上海交通大学 ， 材料学， 2008， 博士

【摘要】 近年来,具有吸收电磁波功能的材料成为研究和应用的热点。理想的吸波材料应该具有强吸收、宽频段、厚度薄、质量轻的特点。但迄今为止,还没有发现任何一种材料能够完全满足这些要求。因此对现有材料进行适当改性和设计就成为了研究的重点。六角晶M型锶铁氧体因其自然共振频率较高,在高频下不易产生趋肤电流,介电常数较小,可与其它吸波材料混合使用来对吸波涂层的电磁参数进行调节等优点而成为一类非常重要的电磁波吸收剂。其缺点是吸波性能对制备工艺十分敏感。要进一步提高单一铁氧体材料的磁导率μr很难。而铁磁性金属粉吸收剂具有良好的温度稳定性,具有最高的饱和磁化强度Ms,其微波磁导率较大,优点非常明显。但是铁磁性金属粉吸收剂在微波频段下因受限于“趋肤效应”而粒度不能太大,并且要分布均匀,体积分数不能过大。由此可见,铁氧体吸波材料和铁磁性金属粉吸波材料的电磁特性具有很强的互补性,把二者有效结合起来有可能创造出高效的吸波剂。本文致力于设计和制备一类新型铁磁性钴基金属包覆硬磁性锶铁氧体的“核壳型”电磁波吸收材料。设计和制备这类“核壳型”复合粒子的出发点就是不但要利用二者电磁性能的互补性,而且还希望能够发挥复合形式也就是“核壳型”结构以及“核”和“壳”的各种特殊界面效应的作用,使它们也能对电磁波吸收做出贡献。首先,利用溶胶-凝胶低温自蔓延燃烧法制备了锶铁氧体超细粉末。并系统地研究了乙二醇溶剂和常规的水溶剂、铁锶比和合成温度等工艺参数对锶铁氧体的制备过程和电磁波吸收性能的影响,制备出具有相对较佳吸波性能的锶铁氧体,并以其作为“核壳型”复合粉末的核心。研究结果表明:以乙二醇为溶剂时,所得锶铁氧体的吸波性能明显好于以水为溶剂时所得锶铁氧体的吸波性能;只有在合适的铁锶比和合成温度下得到的锶铁氧体才会具有较好的吸波性能。在以乙二醇为溶剂、铁锶比为11.2和合成温度为850℃的条件下合成的锶铁氧体超细粉末具有相对较佳的电磁波吸收性能,其反射损耗峰值可达-29.77dB,匹配厚度为2.5mm, 10dB带宽为4.32GHz。然后,以锶铁氧体为核,利用低温超声波化学镀方法制备出一类新型钴磷或钴镍磷软磁合金包覆硬磁锶铁氧体的磁性复合吸波粉末。研究结果表明钴磷合金和钴镍磷合金是以极细小的颗粒为单位沉积在锶铁氧体表面。钴磷合金镀层呈晶态,钴镍磷合金镀层随着镍含量的增加逐渐呈非晶态。镀态复合粉末与单纯锶铁氧体吸波粉末相比,其吸波性能的提高并不十分显著。镀态下,钴磷合金镀覆复合粉末的吸波性能相对较好,复合粉末的反射损耗最小值由单纯锶铁氧体的-29.77dB下降到-35.44dB,下降了5.67dB。钴镍磷合金镀覆复合粉末的吸波性能随镀覆量和镀层中镍含量的增加逐渐降低,直至低于纯锶铁氧体的吸波性能。当镀态复合粉末在400℃热处理后,钴镍磷非晶合金镀层变为晶态,复合粉末的电磁波吸收性能显著提高。热处理后样品SM3(钴镍磷合金镀覆量为20.4%,镀层理论厚度为12nm时的样品)的吸波能力最强,其反射损耗最小值由热处理前-31.03dB降低到热处理后的-43.46dB,降幅最大,达到12.43dB。其反射损耗最小值与单纯锶铁氧体吸波材料相比下降了13.69dB,下降幅度达到46%。钴磷合金镀覆复合粉末即样品SM1热处理前、后的反射损耗最小值的变化最小,为3.61dB,但是和锶铁氧体单组分吸波材料相比还是下降了9.28dB,下降了近31%。热处理后,随着镀覆量和镀层中的镍含量的增加,复合粉末的匹配厚变化不大;10dB带宽变化不规律,并且变化幅度也不大;复合粉末的匹配频率呈降低趋势,与热处理之前相比也略向低频移动。最后,采用水合肼为还原剂,利用低温超声波化学镀方法制备了纯钴金属包覆锶铁氧体复合粉末。研究结果表明,随着金属钴镀覆量的增加,复合粉末的电磁波吸收性能并不一直增加,而是有一最佳镀覆量。在本研究中这一最佳镀覆量为21.1%。在最佳镀覆量,复合粉末的反射损耗峰值可达-41.24dB,匹配厚度为2.2mm, 10dB带宽为4.12GHz。在2.0mm到3.0mm厚度范围内涂层对电磁波的反射损耗最小值都小于-29dB。利用化学还原法制备了钴金属粉末,对钴金属粉末与锶铁氧体粉末的混合粉末和钴金属包覆锶铁氧体复合粉末的电磁波吸收性能作了详细的对比研究。实验结果表明,混合粉末的吸波性能要比锶铁氧体粉末的略好,但是要比钴金属包覆锶铁氧体复合粉末的吸波性差很多。混合粉末在匹配厚度下的反射损耗峰值只有-33.07dB,与复合粉末的反射损耗峰值相差了8.17dB。并且混合粉末的10dB带宽也比复合粉末的窄了0.51GHz。两种粉末的匹配厚度相差不大,混合粉末的匹配频率更高一些。对复合粉末的吸波机理进行了分析研究,结果表明钴基金属包覆锶铁氧体“核壳型”复合粉末之所应具有优异吸波性能主要有下面几个原因:一、铁磁性金属与锶铁氧体的吸波性能确实具有互补性。二、复合方式对复合粉末吸波性能具有重要的影响。核壳型结构增加了电磁波在吸波介质中的波程长,增大了对电磁波的损耗。三、“核壳”之间界面处发生的界面极化和磁晶交换耦合作用均会增强复合粉末对电磁波的吸收。更多还原

【Abstract】 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.更多还原