فایل ورد کامل ساختار فرامواد فراکتال درجه دوم مورد استفاده در مواد جاذب مایکرویو فرکانس پایین

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تعداد صفحات این فایل: ۲۲ صفحه

 هدف این مقاله استفاده از فرامواد برای کاهش ضخامت ماده مغناطیسی در صورت استفاده از جاذب فرکانس می باشد و در عین حال باند جاذب  توسعه داده شود. هر دوی این اهداف حاصل شدند. ضخامت MM   به ۱ کاهش یافت که مطلقا در فرکانس پایین بی مصرف است. از سوی دیگر، نتایج آزمایش نشان داد که باند جاذب بین ۳ و ۵۵ گیگاهرتز ، ۱۰۹ گیگاهرتز  عریض تر از ساختار فراماده بود  و باند جاذب تا ۸۸ درصد توسعه یافت. در نهایت،  این مقاله بحث مختصری در خصوص اثر  ضخامت های مختلف MM انجام داده و  نتیجه بر این شد که باند جذبی افزایشی زمانی دارای عمیق ترین پیک یا قله است که ضخامت صحیح باشد و پایداری آن می تواند دلیل مثبتی بر افزایش  باند جاذب در فرکانس پایین تر باشد.

عنوان انگلیسی:A second-order cross fractal meta-material structure used in low-frequency microwave absorbing materials~~en~~

Introduction Microwave absorber is widely used in many different fields. With the development of detective technology, the performance of microwave absorber needs to be improved, especially when it comes to low frequency. We normally use magnetic materials (MM) to achieve low-frequency absorption [1]. But the MM is very heavy and thick, and thus this is unacceptable in many fields. Recently, meta-material has draw a lot of attention due to its unique properties [2]. As we all know, meta-material is a kind of artificial periodic array of metal, which can realize a negative refractive index, negative permittivity and negative permeability [3–۵]. When it is used as microwave absorber, its ability of designable absorption band shows a bright future [6–۸]. We can easily move the absorbing band of meta-material by adjusting the parameters of the metal structures [9]. At the same time, the electromagnetic parameters of substrate material also play a crucial role in changing the absorbing band [10]. However, meta-material has an obvious shortage. Due to its metal resonance absorption, the absorbing band is very narrow [11]. Nowadays, researchers have come up with a lot of methods to broaden the bandwidth of meta-material absorbers. The most common way is to combine a serial of different parameters of meta-materials structure to achieve a combination of different absorbing band [12–۱۵]. This method has made certain achievements, but according to the equivalent medium theory, the number of different meta-materials structures is limited [16], especially in low-frequency band. So it is impossible to realize a wider and perfect absorber band by simply using meta-materials structure combination. In this paper, we combined the advantages of metamaterial and traditional magnetic absorbing material, focusing on using meta-material to decrease the weight and thickness of magnetic material layer and to achieve a wider absorbing band at the same time. First we designed and fabricated a second-order cross fractal meta-material structure. Then we combined it with a very thin magnetic material which is only 1 mm. The experiment results indicate that the absorbing band of 1 mm magnetic material was moving to low frequency and compared with unloaded meta-material structure the bandwidth of the absorbing band expanded by 88 %. 2 Simulation and discussion The fractal structure has self-similar structures which could widen the resonant bandwidth of meta-material [17]. On that case, we came up with the cross fractal structure to create a meta-material structure. The whole absorber consists of a magnetic layer, cross fractal meta-material, fr-4 substrate and metal backing from the top down, as you can see in Fig. 1a. The magnetic material’s electromagnetic parameters is denoted in Fig. 1b. The red layer is fr-4 material which has a permittivity of 4.4 and a loss tangent of 0.021. Then three different cross fractal structures were designed based on their different orders, as you can see in Fig. 1c–e. The simulation work was conducted using the commercial finite-difference time domain (FDTD) solver Microwave Studio by CST. The unit boundary condition in the x–y plane was applied. EM radiation was polarized, the wave vector was perpendicular and went to the front of the slab and the H and E fields ware parallel to the X and Y directions, respectively. The simulation was performed in free space. The absorbance was calculated from the Fig. 1 different structures a the whole absorber structure, b the electromagnetic parameters of the magnetic material, c the first-order structure, d the second-order structure and e the third-order structure Fig. 2 S11 curves of different order fractal structure D. Huang et al. 123 reflectance and expressed by the S11 parameter as A = 1 – S11 2 because of the metal backing. So the S11 curve can describe the absorption ability. Then the S11 parameter of those different order fractal structures was calculated and the results are demonstrated in Fig. 2. As what is illustrated in Fig. 2, there is a main deep absorption band around 6 GHz in each fractal model which we can give credit to the MM. At the same time we found out that each fractal model has an addition absorption band, but the position and depth are different. The addition band that is caused by the first-order cross fractal structure appears around 2 GHz and its depth is only -3 dB. The addition band that is caused by the second-order cross fractal appears around 5 GHz and its depth achieves -18 dB. . The addition band that is caused by the thirdorder cross fractal appears around 4.5 GHz and its depth becomes -10 dB. From the analysis, we can tell that the change of the position and the depth of the addition band Fig. 3 The distribution of surface current a the first-order model, b the second-order model and c the third-order model A second-order cross fractal meta-material 123 are not linear. With the increase of the order of the different structures, the positions of the addition absorption band become close to the main absorption band at first and then go far away from it. And the same thing happens to the depth that go deep at first then become shallow, which means the second-order structure can easily and optimally realize the expansion of the absorption band. In order to support our conclusion, we further analyzed the distribution of the surface current around addition absorption peak, and the results are show in Fig. 3. Figure 3 indicates two phenomena.

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