Structure and microwave dielectric properties of BaAl2−2xLi2xSi2O8-2x ceramics

BaAl_2-2xLi_2xSi₂O_8-2x (x = 0, 0.005, 0.0075, 0.01, 0.02, 0.03) ceramics were synthesized by solid-state sintering method. Based on density functional theory, the first-principle calculations provided by the Cambridge Sequential Total Energy Package (CASTEP) software were introduced to the BaAl₂Si₂O₈ (BAS) system. In an effort to confirm the site occupied by Li⁺, we discussed the formation energy and final energy of different positions of Li⁺ doped BAS. The result demonstrated that Li⁺ should substitute Al³⁺ to promote the hexacelsian-to-celsian transformation with the aid of generated oxygen vacancies. The sintering behavior, crystal structure, surface appearance, and microwave dielectric properties of samples were investigated. Completely transformed celsian could be obtained when x = 0.005–0.03, which lowered the sintering temperature from 1400 °C (x = 0) to 1300 °C (x = 0.03), as well as strikingly improved the compactness, quality factor (Q × f) value and temperature coefficient of resonant frequency (τ_f) of BAS ceramics. When x = 0.1, unveiling the significant effects of Al-position ion substitution, BaAl_1.98Li_0.02Si₂O_7.98 ceramic sintered at 1350 °C for 5 h exhibited a supreme Q × f value of 48,620 GHz, and the ε_r and τ_f values were 6.99 and -23.29 × 10^?6 °C^?1, respectively.     

Resistive switching behavior and improved multiferroic properties of Bi0.9Er0.1Fe0.98Co0.02O3/Co1-xMnxFe2O4 bilayered thin films

Bilayered Bi_0.9Er_0.1Fe_0.98Co_0.02O₃/Co_1-xMn_xFe₂O₄ (BEFCO/CM_xFO) thin films were deposited by the sol-gel method. Structural variations between the triclinic-P1 and trigonal-R3c:H (two-phase coexistence) phases in the BEFCO layer were observed owing to the trigonal-R-3m:H phase existing in the CM_xFO layer. The oxygen vacancy concentrations of the BEFCO/CM_xFO bilayered films are reduced by Mn-doping in the bottom CFO layer. The BEFCO/CFO films showed high oxygen vacancy concentrations with a high leakage current. This induced changes of the significant potential barrier at the interface between the BEFCO and CM_xFO layers in the processes of electron capture and release. Thus, the BEFCO/CFO film exhibited obvious resistive switching (RS) effect. The high leakage current also caused a fake polarization phenomenon with a blow up of the P-E loop in the BEFCO/CFO films. However, the real and outstanding ferroelectric properties, which resulted from the fewer oxygen vacancies and the 38% triclinic-P1 structure, were obtained in the BEFCO/CM_0.3FO films (P_r~156.3?μC?cm^?2). In addition, the typical capacitance-voltage curve further confirmed its superior ferroelectric performance. The RS effect almost disappeared in the BEFCO/CM_0.3FO bilayered films. Moreover, the enhanced ferromagnetic properties (M_s~100.36?emu?cm^?3, M_r~55.38?emu?cm^?3) were obtained for the BEFCO/CM_0.1FO films, which was attributed to the magnetic properties of BEFCO (a more triclinic-P1 phase and numerous Fe²⁺ ions), in addition to the CM_xFO layer. The introduction of the doped magnetic layer into the bilayered films thus represented a highly effective method for enhancing the multiferroic properties of BFO.     

p-Type/n-type behaviour and functional properties of KxNa(1-x)NbO3 (0.49 ≤ x ≤ 0.51) sintered in air and N2

Potassium sodium niobate (KNN) is a potential candidate to replace lead zirconate titanate in sensor and actuator applications but there are many fundamental science and materials processing issues to be understood before it can be used commercially, including the influence of composition and processing atmosphere on the conduction mechanisms and functional properties. Consequently, KNN pellets with different K/Na ratios were sintered to 95% relative density in air and N₂ using a conventional mixed oxide route. Oxygen vacancies (V_O^??) played a major role in the semi-conduction mechanism in low p(O₂) for all compositions. Impedance spectroscopy and thermo-power data confirmed KNN to be n-type in low p(O₂) in contradiction to previous reports of p-type behaviour. The best piezoelectric properties were observed for air- rather than N₂-sintered samples with d₃₃?=?125?pC/N and k_p?=?0.38 obtained for K_0.51Na_0.49NbO₃.     

Improved dielectric,nonlinear and magnetic properties of cobalt-doped CaCu3Ti4O12 ceramics

The dielectric properties and voltage–current nonlinearity of the pure and various cobalt-doped CaCu₃Ti₄O₁₂ (Co-doped CCTO) prepared by solid-state reactions were investigated. The improved dielectric properties in the Co-doped CCTO, with a dielectric constant $\text{ε'}$ ≈7.4?×?10⁴ and dielectric loss $\text{tan}\text{δ}$?≈?0.034, were observed in the sample with a Co doping of 5% (CCTO05) at room temperature and 1?kHz. The related multi-relaxations, RII (?20 to 40?°C) and RIII (100–150?°C), were demonstrated to be a Debye-like relaxation and a Maxwell–Wagner relaxation related to oxygen vacancies. The low dielectric loss of CCTO05 was associated with the high grain boundary resistance and the increase in cation vacancies. The improved nonlinear electrical properties (CCTO05, with nonlinear coefficients $\text{α}$?≈?5.22 and breakdown electric field ${\text{E}}_{\text{b}}$?≈?300.46?V/cm) and the ferromagnetism in Co-doped CCTO were also discussed.     

Metal organic framework derived Ni/CeO2 catalyst with highly dispersed ultra-fine Ni nanoparticles: Impregnation synthesis and the application in CO2 methanation

CO₂ methanation is a promising strategy to convert the greenhouse gas into synthetic natural gas, which is known as a clean fuel with higher energy density. Ni/CeO₂ is one of the active catalysts for CO₂ methanation reaction. However, Ni/CeO₂ with conventional metal/support structure suffers from the problem of nanocrystal coarsening during high-temperature reaction and thus decreasing catalytic activity. In this work, a Ni/CeO₂ catalyst with novel structure was designed and synthesized through impregnation of Ce-based metal organic framework (MOF) with Ni precursor followed by calcination process. Due to the confinement effect of ultra-small pores derived from the MOF, Ni/CeO₂ catalysts with ultrafine Ni nanoparticles, high dispersion and good thermostability were resulted. Among all the samples, Ni/CeO₂ calcined at 600 °C showed the best catalytic performance due to the highest amount of oxygen vacancies. This work demonstrates a facile way to synthesize a broad range of ultra-fine metal/metal oxide nanocomposite catalysts with high catalytic activity and good stability for various applications.     

Anomalous electrical performance of A-site double-bivalent-doped Bi0.49Na0.5TiO3-δ ceramics from nominal oxygen deficiency to excess

Perovskite structural Bi_0.5Na_0.5TiO₃ (BNT) ferroelectrics can exhibit considerable ionic conductivity, giving a new area for the application as oxygen ion conductors. Through acceptor doping and A-site nonstoichiometry, an ionic conductive mechanism associated with well-resolved arcs in the complex impedance spectra is proposed. Different dielectric responses in ceramics can be deduced to probe the electrical inhomogeneities in separative regions, such as bulk, grain boundary, and electrode, regions. Generally, ionic conductivity only arises at the nominal oxygen deficiency composition of BNT based ceramics. Although large current leakage can exist in the BNT ferroelectrics, they overall display an insulative nature and dominate the electronic conductive contribution, and only a large main arc can be identified in the Nyquist plots. In this work, A-site bivalent doped Bi_0.49-x(SrBa)_xNa_0.5TiO_3-δ and Bi_0.49-x(SrCa)_xNa_0.5TiO_3-δ ceramics ranging from oxygen deficiency to excess are investigated. However, anomalous ionic conductive characteristics are achieved at nominal oxygen excess composition. Herein, the polarization and conductive mechanisms are discussed to elucidate the effect of compositions and their phase and microstructure on the AC impedance, dielectric, and ferroelectric performances. The inhomogeneous distribution of oxygen vacancies, resulting from element segregation or distorted phases in the respective electroactive domains, is a crucial issue in the design of BNT based dielectrics or ionic conductors.     

Synthesis,properties and applications of ZnO nanomaterials with oxygen vacancies: A review

ZnO nanomaterials have attracted tremendous interest in the fields of photocatalysis, sensors, solar cells, supercapacitors, etc. However, the performance of intrinsic ZnO is limited by several factors such as insufficient light absorption, poor charge transport and low conductivity. Extensive studies have indicated that the controlled introduction of oxygen vacancies (V_O) into ZnO can manipulate their optical, electronic and surface properties, enabling their enhanced performance in various applications. Here, we present a state-of-the-art review on the various synthetic approaches of ZnO nanomaterials with V_O and their defect-related properties including structural characteristics, band structure, optical, electrical and ferromagnetic properties. In addition, their use in various applications such as photocatalysts, photoelectrodes, antibacterial agents, gas sensors, supercapacitors and other electronic devices related to V_O-rich ZnO are outlined. Furthermore, we offer some perspectives on the challenges and new directions in this field. We hope that this review would provide some useful information to the design, synthesis and applications of metal oxide nanomaterials with defects.     

An investigation on the photocatalytic activity of sub-stoichiometric TiO2-x coatings produced by suspension plasma spray

To enhance the photocatalytic activity, sub-stoichiometric TiO_2-x films were coated on stainless steel substrates by Suspension Plasma Spraying. Because the TiO₂ particles are exposed to high temperature during deposition by plasma spray, TiO_2-x coating are typically produced. To achieve different levels of oxygen vacancies, as-sprayed TiO_2-x coatings were annealed at four different temperatures for 48 h in air. In this work, the degradation of methylene blue was performed to evaluate the photocatalytic activity under visible light. The results indicated that oxygen vacancy positively affects the photocatalytic activity of TiO_2-x by introducing some energy levels into the bandgap of titania. Moreover, these energy levels could act as traps for photo-excited holes and electrons, reducing the recombination rate of charges, thus improving the photocatalytic activity under the visible lamp. Additionally, coatings were analyzed by X-ray diffraction, confocal laser microscopy, scanning electron microscopy, Raman spectroscopy, thermogravimetric analysis, Fourier-transform infrared spectroscopy, and UV–vis spectroscopy.     

Optimizing the stability and electrical transport properties of CeNbO4+δ-based oxide ceramics by regulating oxygen ion content

CeNbO_4+δ ceramics have attracted extensive research interest because of their unique mixed ion-electron transport characteristics and interesting structure-functional characteristics caused by the difference in oxygen ion content. Although the change of oxygen ion content brings rich redox properties, it also causes serious crystal transformation and abnormal electrical transport properties. In order to obtain stable structure and excellent electrical transport properties, the directional regulation of the oxygen ion content has been realized through introducing Al₂O₃ and high temperature aging. After 600 h of aging at 1073 K, the prepared composite ceramics not only obtain a stable structure without crystal transformation, but also show good negative temperature coefficient (NTC) thermistor characteristics in the temperature range of 473 K–1273 K, in which the linear fitting maximum Pearson's r of the relationship between lnρ and 1000/T can reach 99.97%. The proposed method provides a new thought for the design and application of high-temperature electronic ceramics.     

Enhanced photocatalytic activity of La1-xSrxCoO3/Ag3PO4 induced by the synergistic effect of doping and heterojunction

This study sought to design and synthesize a series of perovskite-based La_1-xSr_xCoO₃/Ag₃PO₄ (with x = 0–1) heterojunction photocatalysts with different Strontium (Sr) doping contents by a simple sol-gel method and properties of the material were comprehensively characterized. Moreover, tetracycline (TC) was chosen as the target pollutant to assess the effect of Sr doping on the catalytic performance of LaCoO₃/Ag₃PO₄. Our results demonstrated that the partial replacement of La³⁺ with Sr²⁺ coupled with shifting Co³⁺ to the mixed-valence state of Co³⁺-Co⁴⁺ led to the formation of substantially more oxygen vacancies in the crystal lattice of La_1-xSr_xCoO₃/Ag₃PO₄. Therefore, the doped catalyst La_1-xSr_xCoO₃/Ag₃PO₄ exhibited enhanced photocatalytic performance. When x = 0.9, the obtained La_0·1Sr_0·9CoO₃/Ag₃PO₄ exhibit an optimal performance for TC degradation. Kinetic analyses demonstrated that the degradation rate constant of TC in La_0·1Sr_0·9CoO₃/Ag₃PO₄ system was 0.0098 min^?1, which is 1.78 times that of LaCoO₃/Ag₃PO₄, and 2.45 times that of SrCoO₃/Ag₃PO₄. Additionally, free radical sequestration experiments indicated that OH^?, h⁺, and O₂^?? all participated in the degradation of TC in the following order: h⁺>O₂^??>OH^?. Finally, analyses of photocatalytic mechanisms suggested that the enhanced photocatalytic activity of La_0·1Sr_0·9CoO₃/Ag₃PO₄ was due to its strong electron transfer properties and the formation of substantially more surface oxygen vacancies in Sr-doped La_0·1Sr_0·9CoO₃.