钛酸锶钡基介质材料微波特性及频率选择特性设计

Microwave characteristics and design of frequency selective properties based on barium strontium titanate

选用传统的固相烧结法合成Ba_xSr_(1-x)TiO_3陶瓷,其中x为0.2,0.3,0.4,0.45,分别记为BST20*,BST30*,BST40*,BST45*,对上述不同组分的Ba_xSr_(1-x)TiO_3陶瓷进行介电性能的测试分析,最终选择了烧结温度为1420℃且具有高介电常数低损耗Ba_(0.4)Sr_(0.6)TiO_3陶瓷作为后续的研究对象.为解决纯BST陶瓷的介电常数和介电损耗偏高导致的插损高、性能优化困难等问题,在BST40*的中掺杂钛酸镁(Mg_2TiO_4,MT)和镁钛酸镧(La(Mg_(0.5)Ti_(0.5))O_3,LMT)改性,得出了掺杂不同组分的Ba_(0.4)Sr_(0.6)TiO_3陶瓷在微波频段下具有介电常数可调(ε=48~108)的特性,同时介电损耗降至1×10^(-3)数量级.选取介电常数为ε=65.87高介低损陶瓷,仿真设计了一款在X波段具有三通带的频率选择表面结构.基于准静态条件下的等效媒质理论和介质谐振理论对等效参数提取结果进行了数值分析,对等效磁导率、等效介电常数和归一化阻抗的关系做了一定的说明,确定电磁谐振是产生通带的主要原因,并对动态场矢量分布情况进行了跟踪分析,进一步确定了谐振模式和通阻特性的形成机理.相关研究为制作高介低损的全介质频率选择表面提供了一种方法.

Frequency selective surface（FSS） is a kind of electromagnetic functional devices with a spatial filtering efficiency. It can transmit or reflect electromagnetic waves at certain frequencies and is widely used in microwave, infrared and terahertz electromagnetic frequency. Conventional FSSs usually consist of a 2D periodic array of metal structures, but they are prone to breakdown, oxidization and corrosion, especially in high-power and high-temperature applications. According to Maxwell＇s equations, the conduction current and displacement current have similar effect in the excited time-varying field. Hence, high-permittivity ceramic particles can be employed as the elementary unit cell to fabricate all-dielectric metamaterial FSSs. First, solid sintering method is used to prepare BaxSr（1-x）TiO3 ceramics. Here, the x refers to 0.2, 0.3, 0.4 and 0.45, the BaxSr（1-x）TiO3 are marked as BST20＊, BST30＊, BST40＊, BST45＊ respectively. The induced density and dielectric properties of different BaxSr（1-x）TiO3 ceramics were tested and discussed. The testing results indicate that when sinter temperature is up to 1420℃, the Ba（0.4）Sr（0.6）TiO3 ceramics can obtain a high dielectric constant and a low loss and it is selected for the following research. For the pure BST ceramics, the dielectric constantis high, which results that the resonant occurs many times in a narrow frequency band. So, it＇s difficult for designing of the FSS. In addition, the relative higher dielectric loss leads to a larger insertion loss and a wide cut-off frequency band. In order to improve its performance, the Mg2TiO4（MT） and LA（Mg（0.5）Ti（0.5）O3）（LMT） are mixed into BST40＊. Through testing, the dielectric constant of the improved Ba（0.4）Sr（0.6）TiO3 ceramics with different components can be obtained（ε=48–108） and dielectric loss is reduced to 10^（-3） order of magnitudes. After that, the high-permittivity and low-loss ceramics with ε=67.87 is chosen to design the 3 band-pass all-dielectric FSSs, which implemented by the two-dimensional periodic arrays. The center frequencies are 8.7, 10.4 and 11.3 GHz respectively, and the passband fluctuation is flat. The -3 d B bandwidths of the lower and upper passbands are f1=0.26（8.5–8.8 GHz）, f2=0.42（10.2–10.6 GHz） and f3=0.50 GHz（11.1–11.6 GHz） respectively. Based on the effective medium and dielectric resonance theory, the effective permeability, the effective permittivity and the normalized impedance are discussed. The results show that the major cause of the passing band is electromagnetic coupling. Finally, the distribution of dynamic vector field is investigated. Through which, the mechanism of resonance and band-pass characteristics are further verified. Take the first transmission pass band（8.5–8.8 GHz） as an example, after 8 GHz, the effective permittivity gradually increases to a maximum value（15） at 8.6 GHz, and then decreases rapidly until reaches a negative evalue（-50）, which means a strong electric resonance occurred. In addition, at the right side of the 8 GHz, the effective permeability firstly increases to a maximum value, and then decreases to a minimum valueat the 8.6 GHz, which means strong magnetic resonance occurred. Therefore, the electric and magnetic resonance can be coupled to achieve the passband filtering characteristics. In the first passband, the real part of normalized impedance gradually decreases from more than 1 to 1 on the right side of at 8.6 GHz, and then continues to decrease until less than 1. When the impedance value is 1, it is fine for air matching, while larger or smaller than 1, slightly worse. Although the minimum ｜S（21）｜ value of the first passband is approximately 0.87, the insertion loss is small and it still can achieve a good transmission effect. This proposal provides a method for designing the high-permittivity and low-loss all-dielectric FSS.