Abstract: Ferroelectric materials have been applied in the photocatalytic field due to the internal electric field generated by the spontaneous polarisation. In this study, it is found that the ferroelectric BaTiO3 catalysts exhibit the excellent photocatalytic property and cycle stability under ultrasonic vibration. In addition, the degraded photocatalytic activity after several cycling tests can resurrect if the BaTiO3 sample is treated by ultrasonic before photocatalytic reaction. The enhanced photocatalytic activity and cycle stability are ascribed to the variational spontaneous polarisation under external strain for ferroelectric materials. Under ultrasonic vibration, the high-frequency alternating strain applied on the BaTiO3 grain changes the internal polarisation electric field. Due to the fast changing polarisation electric field, there is no enough time to form the depolarisation electric field in the BaTiO3 grain. Therefore, the effective electric field existed in BaTiO3 grain is large and can effectively promote the separation the photo-generated electrons/holes; thus improving the photocatalytic activity. It is suggested that ultrasonic vibration is an efficient method to improve the photocatalytic properties and cycle stability of the ferroelectric photocatalysts. PubDate: 2019-01-03T00:00:00Z
Abstract: BaTiO3 (BT) nanoparticles treated in H2 and then coated with a layer of SiO2 were used as the starting material to synthesise BT–SiO2 nanocomposites through the conventional ceramic process. A denser morphology was obtained in the BT–SiO2 composites. The H2 treatment induced not only the oxygen vacancies, but also other changes (i.e. ionic disorders) in the BT. These ionic disorders were more permanent and enhanced the reaction between the BT and SiO2 during the sintering process. Comparing with the nanocomposites sintered from the untreated BT nanoparticles, the nanocomposites sintered from the H2 treated BT nanoparticles exhibit an enhanced dielectric performance (i.e. a higher dielectric constant and a high electric breakdown field). Among the temperatures (i.e. 850, 900, 950 and 1000°C) used to treat the BT nanoparticles in H2, it is experimentally found that 950°C is the best. For example, for the composites with 20 wt% SiO2, the dielectric constant is enhanced more than four times and the breakdown field is enhanced by about 30%. It is believed that the enhancement in the dielectric response is due to the dipoles formed by the ionic disorders, especially the acceptor defects. PubDate: 2018-10-29T00:00:00Z
Abstract: Complex perovskite microwave dielectric ceramics (MWDCs) of A ( B 1 / 3 ′ B 2 / 3 ′ ′ ) O 3 -type (A = Ba, Sr,…; B′ = Mg, Zn,…; B″ = Nb, Ta), which exhibit excellent dielectric properties, have currently been widely used in microwave and millimetre wave devices. Vibrational spectra, including both Raman and far-infrared (FIR) spectra, are powerful tools to investigate the atomic thermal vibrational properties of MWDCs and reveal the intrinsic origin of dielectric properties. In this review, lattice dynamics and phonon characteristics of the A ( B 1 / 3 ′ B 2 / 3 ′ ′ ) O 3 -type MWDCs are summarised and presented in detail to introduce remarkable progress in this field and make a guide for the design of novel advanced MWDCs. The atomic sites and the corresponding modes in Raman and FIR spectra are identified and illuminated. The effects of the processing conditions and the ordered superstructures in the nanoscale region on vibrational modes are summarised systemically. Intrinsic properties can be extrapolated from the fitting results of FIR spectroscopy, which were also discussed based on the Kramers–Krönig relations, Lorentz three-parameter classical model and four-parameter semi-quantum model. The correlations between vibrational modes (phonons), crystal structures, and dielectric properties are created, which can help to build the mathematical models so as to understand the structure–property relationship of MWDCs better. PubDate: 2018-10-18T00:00:00Z
Abstract: The increasing electromagnetic (EM) radiation arise from the vast usage of EM techniques in civilian and military fields has spawned extensive concerns on scientific research with respect to EM wave absorbers. Nanomaterials, as a new type of nano-absorbing agent, have been investigated in detail over the recent years. This review article aims at elaborating the influence of component regulation and structural design on microwave absorption (MA) performance. Combined with the experimental efforts towards the development of nano-absorbers, the relevant absorption mechanisms are also discussed. In addition, from a prospective point, only focus on the exploration of absorbent itself is not enough, but also need matrix material to support. Based on this circumstance, the study summarises various kinds of polymer-based nanocomposites used as excellent absorbers for practical MA applications. PubDate: 2018-09-21T00:00:00Z
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