| Complex perovskites compounds have shown many important applications, such as wireless communication, satellite broadcasting and intelligent transport systems, because they have moderate dielectric constant(e), very low dielectric loss(tand), and near-zero temperature coefficient of resonant frequency. Basing on these backgrounds, Ba(Mg1/3B21/3)O3(B2=Ta and Nb) materials were studied in this research. Firstly, the electronic structures of BMNT were studied by the first-principle calculations based on DFT theory, and then the Ba(Mg1/3B22/3)O3 thin films were fabricated by an entire aqueous solution-gel route. The effects of fabrication conditions, material compositions, and material interfaces on the dielectric properties of BMNT films were carefully investigated. Lastly, the related dielectric response mechanisms were discussed.The crystalline structure and cell parameters were obtained through refining the XRD patterns of BMT and BMN. Five BMNT supercells with Nb:Ta=0:9, 1:8, 2:7, 3:6, and 9:0 were constructed, and then the first-principle calculations were performed. It is found that there exist obvious Mg-O and Ba-O ionic interactions, and Ta-O and Nb-O covalent interactions. Especially, the Nb-O covalent interactions in [Ta O6] octahedral and the Ta-O covalent interactions in [Nb O6] octahedral play the primary role in dielectric properties of BMNT. The mean cause that the dielectric constant of BMN is higher than that of BMT is that [Nb O6] octahedral is bigger than [Ta O6] octahedral.The BMN precursor solutions are formed through the combination of three single citrate complexes with NH4+ acting as crosslinks. The metal coordinated bonds of the citrate complexes can be easily broken by heat treatment, which can result in the crystallization of BMN films at low annealing temperature. For the first time, the single perovskite BMN thin films were prepared after annealing at 550 °C by this method. The 1:2 long range order(LRO) degree of BMN thin films increases with the increase of annealing temperature, and the sample annealed at 750 °C presents the best dielectric properties. At 100 KHz, e and tan d of this sample are 30.7 and 0.018; at 1MHz they are 30.4 and 0.025, respectively. Combined structure and the dielectric properties analysis, it can be known that the dependence of e and tand on the annealing conditions can be attributed to the change of film densification and LRO degrees. The sample annealed at 750 °C shows steady dielectric and capacitive-bias properties below 1MHz. The leakage density sharply increases under direct current electric field. However, the leakage value is lower than 3.0′10-8 A/cm2 when electric field is smaller than 250 k V/cm, which is a low value for thin films. The BMN films have different dielectric responses machanism at different frequency range. When w is lower than 100 k Hz, e and tan d decrease as the frequency increases because space charges play an important role in the dielectric response. When w range is 100 KHz-1 MHz, e tends to be constant and tan d increases with the increase of frequencies, indicating that the dipolar response starts to take effect. When w range is 1 MHz-1 GHz, e decreases with increasing frequencies because the dipolar polarization obviously lags behind the applied electric field, and tan d is highest at 292 MHz. When w range is 1-4 GHz,e and tan d of BMN thin film tend to be constant(e and tan d are 28.5 and 0.0030±0.0015, respectively), which is due to the contributions of inoic and electronic cloud displacement polarization. BMN thin films are very suitable for high-frequency(> 1GHz) microwave dielectric materials, because the contribution of ionic displacement polarization is the greatest among various polarization in the BMN thin films.The dielectric properties of BMNT thin films were studied. It was found that the dielectric constants of BMNT films increase with the increase of Nb amount. Meanwhile, dielectric loss show first–decrease-then-increase as the Nb amount increases. The dependences of e and tand on Nb amount are attributed to morphology and electronic structure of BMNT thin films. It was seen that low Nb substitution strengthen Ta-O covalent interaction and more Nb atoms weaken Ta-O interaction. Consequently, the BMN1T7 thin film presents the best dielectric properties. The e and tand at 100 KHz are 24.89 and 0.0186, at 1MHz are 24.35 and 0.0264, and at 1-4 GHz are 22.46 and 0.0045±0.0015, respectively.The interface structure and micromave dielectric properties of BMT/BMN films were investigated in detail. Some fluctuant and diffusion on the interface of BMT/BMN layered films can be observed. Well-crystalline interface layer exists, which has no disadvantage on dielectric performances. The e and tand at high frequencies are lower than that at low frequencies because only the ionic and electron cload displacement can contribute to the dielectric reponses. The dielectric properties of BMN films are better than that of BMT films, because the BMN films comprise mainly hexaganol perovskite structure and the BMT films comprise mainly cubic perovskite structure, respectively. Ionic polarizations are divided into many vibration modes with small amounts of 4prj for each in hexaganol perovskite structure. The coupling between each mode is small, so the vibration energy is inhibited to leak from one mode to another, consequently leading to a low tan d of BMN films. Therefore, the dielectric properties of BMT films is much lower than that of BMT ceramics, and we can obtain BMT films with better dielectric properties through introducing Nb and constructing heterogeneous layered structure. |
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