【作者】 黄云霞；

【导师】 曹全喜；

【作者基本信息】 西安电子科技大学 ， 电子科学与技术， 2009， 博士

【摘要】 吸波材料由于能够吸收衰减入射的电磁波,减少电磁波辐射,在军事和民用方面的应用日益重要。本论文以制备性能优良的微波吸收材料为研究方向,系统地研究了金属微粉Fe屑、M型钡铁氧体、改性空心微球和掺杂ZnO等粉体的制备、结构和性能,讨论了产生电磁损耗和微波吸收的机理。采用热分解法对机械加工废料Fe屑进行除油,得到厚度约0.5μm的不规则片状Fe粉。微波电磁性能测试结果表明:在2-18GHz,Fe屑具有一定的磁损耗性能,其磁导率实部μ′、虚部μ″和磁损耗因子tanδM分别为0.770-1.283、0.029-0.471和0.031-0.517。反射率测试结果表明:随样品中Fe屑含量和样品厚度的增加,样品对电磁波的衰减逐渐增大,并且产生最大衰减的频率向低频方向移动。Fe屑含量为33.3%时,有最大衰减-14.371dB,小于-10dB的频宽达2.56GHz。当样品厚度为5.4mm时,最大衰减为-15.719dB,小于-10dB的频宽约为1GHz。分别采用共沉淀-熔盐法和化学沉淀-局部规整法制备了六角晶系BaFe12O19前驱体和胶状BaCO3包覆在铁黄粒子表面的前驱体。将这两种前驱体分别在850℃和1000℃进行煅烧,得到一维纳米片状和针状BaFe12O19粉体。磁损耗性能表明:在2-18GHz,片状与针状BaFe12O19的μ’、μ"和tanδM具有相近的数值,整体上片状BaFe12O19磁损耗稍大,其μ′、μ″和tanδM分别为0.847-1.110、0.031-0.224和0.028-0.231。反射率结果表明:在2-18GHz,片状和针状BaFe12O19的反射率均在-5dB以下,最小反射率分别为-18.197dB和-18.282dB,小于-10dB的频宽达到10.24GHz。采用Sn-Pd胶体溶液作为活化剂,在空心微球表面分别活化-化学镀金属Co、Ni、Ni-Fe和Co-Fe镀层,得到了致密均匀且由纳米颗粒组成的镀层。磁损耗性能表明:化学镀后空心微球μ’、μ"和tanδM在2.6-12GHz的低频范围增加不显著,而在12-18GHz的高频范围则显著增加。反射率结果表明:镀Co和Co-Fe微球对电磁波的衰减明显优于镀Ni和Ni-Fe的空心微球。其中,镀Co微球在6.48GHz产生最大衰减-24dB,低于-10dB的频宽最宽,为8.88GHz。采用固相合成法制备ZnO前驱体,研究了煅烧温度、保温时间和煅烧气氛对ZnO粉体微波介电性能的影响。结果表明,前驱体分别在400-800℃不同温度下保温1.5h煅烧、在600℃不同保温时间煅烧以及在800℃空气、H2气和N2气气氛下进行煅烧,均得到纤锌矿结构的ZnO粉体。介电性能测试结果表明:在8.2-12.4GHz,随煅烧温度升高,样品的介电常数实部ε′与虚部ε″先增大后减小,在700℃达到最大,平均值分别为2.80和0.52。介电损耗因子tanδE在800℃达到最大,平均值为0.19。不同保温时间下所得ZnO粉体的ε′与ε″,随保温时间的增加略有增加,其ε′在2.52-2.62、ε″在0.4-0.49,tanδE的变化不明显。在H2气气氛下煅烧所得ZnO粉体的ε′、ε″和tanδE比在空气和N2气下煅烧所得ZnO粉体的明显增大,分别约为7.5、2.5和0.3。采用第一性原理赝势法,计算了Al掺杂前后ZnO的能带结构、态密度和复介电函数。计算结果表明:掺杂后ZnO的晶胞体积基本不变,费米能级移动进入导带。在能量较低时,Al3+掺杂ZnO的ε′和ε″均比未掺杂时增大。采用固相反应法制备不同Al掺杂的ZnO前驱体,将Al掺杂量为10at%的前驱体分别在400-800℃保温1.5h进行煅烧,将不同Al掺杂量的前驱体在600℃保温1.5h进行煅烧,均得到纤锌矿结构的ZnO粉体。XPS结果表明,在样品中Al是以Al3+的形式存在,形成了替位式掺杂。介电性能测试结果表明:在8.2-12.4GHz,不同煅烧温度下所得Al掺杂ZnO粉体,随温度升高,ε’、ε"和tanδE都是先增加后减小,在600℃达到最大,分别约为4.32、2.34和0.56。随Al掺杂量的增加,ε’、ε"和tanδE明显增加。ε’和ε"在Al掺杂量为20at%时达到最大,分别约为4.73和2.51,tanδE在Al掺杂量为10at%时达到最大,约为0.56。实验结果与第一性原理计算结果相比,都表现出Al3+掺杂后ZnO的介电函数ε′和ε″均比未掺杂时增大的趋势。采用固相反应法制备不同Co掺杂的ZnO前驱体,将Co掺杂量为5at%的前驱体分别在400-800℃保温1.5h进行煅烧,将不同Co掺杂量的前驱体在600℃保温1.5h进行煅烧,以及将Co掺杂量为5at%的前驱体在800℃不同气氛条件下煅烧,均得到纤锌矿结构的ZnO粉体。XPS结果表明,600℃煅烧所得Co掺杂ZnO粉体中,Co是以Co2+或Co2+和Co3+的形式替代Zn2+,形成替位式掺杂;800℃煅烧所得Co掺杂ZnO粉体中,Co仅以Co2+的形式存在。但在H2气气氛中煅烧时,Co是以Co2+和Co3+的形式存在。Co掺杂ZnO粉体电磁性能结果表明:在8.2-12.4GHz,不同煅烧温度下所得Co掺杂ZnO粉体,随温度升高,μ’、μ"以及tanδM是先增大后减小,600℃达到最大,分别约为0.98、0.46和0.52。随Co掺杂量的增加,μ’和μ"逐渐增大,在15at%时达到最大,分别约为1.03和0.54,而tanδM则是先增大后减小,在Co掺杂量为10at%时达到最大,为0.54。在H2气下煅烧所得ZnO粉体的ε′、ε″和tanδE比在空气和N2气下煅烧所得ZnO粉体的明显增大,分别约为5.27、1.21和0.239。同样,在H2气下煅烧所得Co掺杂ZnO粉体的μ′、μ″和tanδM比在空气和N2气下煅烧所得ZnO粉体的明显增大,分别约为9.68、6.37和0.94。更多还原

【Abstract】 The microwave absorbing materials play an important role in the military and civil fields by which the incidence electromagnetic wave can be absorbed and electromagnetic radiation is attenuated. The aim of this paper is to prepare the high quality microwave absorbers. The preparation, structures and properties of metallic Fe chips, M-type barium ferrite, modified cenospheres and ZnO doped by Al and Co were investigated systematically. Electromagnetic loss and microwave absorbing mechanism of these materials in microwave range were discussed.Flaky Fe powders with thickness about 0.5μm were obtained by pyrolyzing the existing oil on the surface of the industrial waste Fe chips. The results of microwave electromagnetic tests show that Fe powders possess magnetic loss performance in the frequency range of 2-18 GHz. The real partμ′and the imaginary partμ″of permeability and magnetic loss factor tanδM of the samples are 0.770-1.283、0.029-0.471 and 0.031-0.517, respectively. The results of reflection show that attenuation of electromagnetic wave in the sample increases with the increasing content of Fe powders in the rubber, and the frequency of maximum attenuation shifts to lower frequency. When the Fe powder content is 33.3%, there is the maximum attenuation of -14.371dB. The frequency width of the attenuation less than -10dB is 2.56GHz. The attenuation of electromagnetic wave of the sample increases as the thickness of sample increases, and the frequency of maximum attenuation also shifts to lower frequency. When the thickness of sample is 5.4mm, there is the maximum attenuation of -15.719dB. The frequency width of the attenuation less than -10dB is 1GHz.The precursors of hexagonal barium ferrite and ferrite yellow coated by colloid BaCO3 were prepared via co-precipitation molten-salt and precipitation-topotactic reaction methods, respectively. The flaky and needle-like hexagonal barium ferrite powders were obtained by calcining the precursors at 850℃and 1000℃, respectively. The magnetic loss properties show the values ofμ’,μ" and tanδM for the flaky and needle-like samples are similar in the frequency range of 2-18 GHz. The magnetic loss of the flaky sample is greater than that of needle-like one on the whole. The values ofμ’,μ" and tanδM for the flaky sample are 0.847-1.110、0.031-0.224 and 0.028-0.231, respectively. The results of reflection test show that both the reflection of the flaky and needle-like samples are less than -5dB in the frequency range of 2-18 GHz. The minimum reflections are -18.197dB and -18.282dB, respectively. The frequency width less than -10dB is up to 10.24GHz.The surfaces of cenosphere particles were activated by colloid solution of Sn-Pd, and the metallic Co, Ni, Co-Fe and Ni-Fe thin films were plated on these surfaces by electroless method, respectively. The continuous and uniform coatings deposited on the cenospheres are composed of nano-particles. The properties of magnetic loss show that the values ofμ’,μ" and tanδM for the modified cenospheres have no obvious increases in the frequency range of 2.6-12GHz, but they increase obviously in the frequency range of 12-18GHz. The results of reflection show the attenuation of electromagnetic wave in the cenospheres deposited by Co and Co-Fe thin films are obviously greater than that of the cenospheres deposited by Ni and Ni-Fe thin films, where the maximum attenuation of the cenosphere with Co film reaches to -24dB at 6.48GHz and the frequency width less than -10dB is maximal, with the value of 8.88GHz.The precursor of ZnO was prepared by the solid state reaction. The effect of the calcined temperature, holding time and synthesis atmosphere on the microwave dielectric properties of ZnO powders were investigated. Results show that the wurtzite structure ZnO powders were produced by calcining the precursors at 400-800℃with holding time of 1.5h, at 600℃with different holding times and at 800℃in air, hydrogen and nitrogen atmospheres. The results of dielectric properties show that both the real partε′and the imaginary partε″of permittivity of the samples increase first and then decrease as the calcined temperature increases in the frequency range of 8.2-12.4 GHz, reaching a maximum values at 700℃, which are 2.80 and 0.52 in average, respectively. The maximum value of dielectric loss factor tanδE is about 0.19 at 800℃. The valuesε′andε″of the ZnO powders prepared with different holding times increase slightly with the increasing time, which are 2.52-2.62 and 0.4-0.49, respectively. The value of tanδE has no obvious change with increasing time. The ZnO powder prepared in hydrogen atmosphere has the greatest values inε′、ε″and tanδE than the samples obtained in air and nitrogen atmospheres, which are about 7.5, 2.5 and 0.3, respectively.The band structure, state density and complex permittivity of the pure ZnO and the Al-doped ZnO were studied by using the first-principle ultrasoft pseudopotential approach of the plane wave based on the density function theory. Results show that the volume of super cell has no obvious change and Fermi energy level introduces into conduction band through introducing Al ions. The values ofε′andε″for the ZnO with doping Al ions increase at low energy.The precursors of ZnO with doping different Al content were prepared by the solid state reaction. The wurtzite structure ZnO powders were prepared by calcining the 10at% Al-doped precursors under 400-800℃for 1.5h and the precursors with different Al content at 800℃for 1.5h, respectively. The results of XPS (X-ray photoelectric spectrum) show that the partial Zn2+ ions are substituted by Al3+ ions in the Al-doped ZnO. The results of dielectric properties for Al-doped ZnO powders show that all theε’,ε" and tanδE of the 10at% Al-doped ZnO powder prepared at different calcined temperatures increase firstly and then decrease with the increasing temperatures in the frequency range of 8.2-12.4 GHz, reaching maximum values at 600℃which are about 4.32, 2.34 and 0.56, respectively. The value ofε’,ε" and tanδE increases obviously as Al-doped content increases. Bothε’ andε" reveal the greatest values when the Al-doped content is 20at% which are about 4.73 and 2.51, respectively. The value of tanδE reaches maximum value of 0.56 at 10at% content. The experimental results show that bothε′andε" of the samples are improved by Al doping, in agreement with the result calculated by first-principle.The precursors of ZnO with doping different Co content were also prepared by the solid state reaction. The wurtzite structure ZnO powders were prepared by calcining the 5at% Co-doped precursors at 400-800℃for 1.5h , the precursors with different Co content at 600℃for 1.5h and the 5at% Co-doped precursor at 800℃in different atmospheres, respectively. The results of XPS show that the partial Zn2+ ions are substituted by Co2+ or Co2+ and Co3+ ions in the Co-doped ZnO prepared at 600oC, and that the partial Zn2+ ions are substituted by Co2+ ions in the Co-doped ZnO prepared at 800oC in air and nitrogen atmospheres. But the partial Zn2+ ions are substituted by Co2+ and Co3+ ions in the Co-doped ZnO prepared at 800oC in hydrogen atmosphere.The results of electromagnetic loss for Co-doped ZnO powders show that all theμ’,μ" and tanδM of the 5at% Co-doped ZnO powder prepared at different calcined temperature increase firstly and then decrease with the increasing temperature in the frequency range of 8.2-12.4 GHz, reaching maximum values at 600℃, which are about 0.98, 0.46 and 0.52, respectively. The values ofμ’ andμ" of the samples increase gradually with the increasing Co content, reaching maximum values of 1.03 and 0.54 at 15at% content, respectively. But the value of tanδM increases firstly and then decrease, reaching maximum values of 0.54 at 10at% content. The sample prepared in hydrogen atmosphere has the greater values inμ′、μ″and tanδM than the samples prepared in air and nitrogen atmospheres, which are 9.68, 6.37 and 0.94, respectively. The sample prepared in hydrogen atmosphere has also the greater values inε′、ε″and tanδE than the samples prepared in air and nitrogen atmospheres, which are 5.27, 1.21 and 0.239, respectively.更多还原