Effect of V\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e5\u3c/sub\u3e Doping on the Sintering and Dielectric Properties of \u3ci\u3eM\u3c/i\u3e-Phase Li\u3csub\u3e1+x-y\u3c/sub\u3eNb\u3csub\u3e1-x-3y\u3c/sub\u3eTi\u3csub\u3ex+4y\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e Ceramics

The effect of the addition of V2O5 on the structure, sintering and dielectric properties of M-phase (Li1+x-yNb1-x-3yTix+4y)O3 ceramics has been investigated. Homogeneous substitution of V5+ for Nb5+ was obtained in LiNb0.6(1-x) V0.6xTi0.5O3 for x ≤ 0.02. The addition of V2O5 led to a large reduction in the sintering temperature and samples with x = 0.02 could be fully densified at 900oC. The substitution of vanadia had a relatively minor adverse effect on the microwave dielectric properties of the M-phase system and the x = 0.02 ceramics had εr = 66, Q x f = 3800 at 5.6 GHz, and τf = 11 ppm/oC. Preliminary investigations suggest that silver metallization does not diffuse into the V2O5-doped M-phase ceramics at 900oC, making these materials potential candidates for low-temperature cofired ceramic (LTCC) applications

Synthesis and Dielectric Properties of Li\u3csub\u3e1-\u3ci\u3ex+y\u3c/i\u3e\u3c/sub\u3eTa\u3csub\u3e1-\u3ci\u3ex\u3c/i\u3e-3\u3ci\u3ey\u3c/i\u3e\u3c/sub\u3eTi\u3csub\u3e\u3ci\u3ex\u3c/i\u3e+4\u3ci\u3ey\u3c/i\u3e\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e \u3ci\u3eM\u3c/i\u3e-Phase Solid Solutions

The synthesis, structure, and dielectric properties of the so-called M-phase solid solutions in the Li2O-Ta2O5-TiO2 system were investigated. Although the range of stability of the tantalate phases is more limited compared with their niobate counterparts, they have identical structures based on intergrowths of LiTaO3-type blocks separated by corundum type layers. The dielectric constants of the tantalate M-phases range from 68 to 52 and they all exhibit a negative temperature coefficient of capacitance. The temperature coefficient of the resonant frequency measured in the microwave region can be tuned to zero, and the system shows quite good quality factors with the highest value reaching a Q x f = 10500 at 6.7 GHz

Analysis of phase distributions in the Li\u3csub\u3e2\u3c/sub\u3eO–Nb\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e5\u3c/sub\u3e–TiO\u3csub\u3e2\u3c/sub\u3e system by piezoresponse imaging

The M-phase solid solutions Li1+x-yNb1-x-3yTix+4yO3) (0.1 ≤ x ≤ 0.3, 0 ≤ y ≤ 0.175) in the Li2O–Nb2O5–TiO2 system have promising microwave dielectric properties. However, these compounds can contain small quantities of ferroelectric impurities that affect the polarization response of the material. Due to their low concentration and their chemical similarity to the host material, the impurities cannot be detected by x-ray diffraction or local elemental analysis. Scanning surface potential microscopy and piezoresponse imaging were used to analyze phase compositions in this system. Piezoresponse imaging demonstrated the presence of thin (\u3c200–300 nm) ferroelectric layers on the grain boundaries oriented along the c-axis of the M-phase. Differences between the surface potential and the piezoresponse of ferroelectric multicomponent systems are discussed

Dopants adsorbed as single atoms prevent degradation of catalysts

The design of catalysts with desired chemical and thermal properties is viewed as a grand challenge for scientists and engineers. For operation at high temperatures, stability against structural transformations is a key requirement. Although doping has been found to impede degradation, the lack of atomistic understanding of the pertinent mechanism has hindered optimization. For example, porous gamma-Al2O3, a widely used catalyst and catalytic support, transforms to non-porous alpha-Al2O3 at ~1,100C. Doping with La raises the transformation temperature to ~1,250C, but it has not been possible to establish if La atoms enter the bulk, adsorb on surfaces as single atoms or clusters, or form surface compounds. Here, we use direct imaging by aberration-corrected Z-contrast scanning transmission electron microscopy coupled with extended X-ray absorption fine structure and first-principles calculations to demonstrate that, contrary to expectations, stabilization is achieved by isolated La atoms adsorbed on the surface. Strong binding and mutual repulsion of La atoms effectively pin the surface and inhibit both sintering and the transformation to alpha-Al2O3. The results provide the first guidelines for the choice of dopants to prevent thermal degradation of catalysts and other porous materials.Comment: RevTex4, 4 pages, 4 JPEG figures, published in Nature Material

Theory-assisted determination of nano-rippling and impurities in atomic resolution images of angle-mismatched bilayer graphene

Ripples and impurity atoms are universally present in 2D materials, limiting carrier mobility, creating pseudo–magnetic fields, or affecting the electronic and magnetic properties. Scanning transmission electron microscopy (STEM) generally provides picometer-level precision in the determination of the location of atoms or atomic 'columns' in the in-image plane (xy plane). However, precise atomic positions in the z-direction as well as the presence of certain impurities are difficult to detect. Furthermore, images containing moiré patterns such as those in angle-mismatched bilayer graphene compound the problem by limiting the determination of atomic positions in the xy plane. Here, we introduce a reconstructive approach for the analysis of STEM images of twisted bilayers that combines the accessible xy coordinates of atomic positions in a STEM image with density-functional-theory calculations. The approach allows us to determine all three coordinates of all atomic positions in the bilayer and establishes the presence and identity of impurities. The deduced strain-induced rippling in a twisted bilayer graphene sample is consistent with the continuum model of elasticity. We also find that the moiré pattern induces undulations in the z direction that are approximately an order of magnitude smaller than the strain-induced rippling. A single substitutional impurity, identified as nitrogen, is detected. The present reconstructive approach can, therefore, distinguish between moiré and strain-induced effects and allows for the full reconstruction of 3D positions and atomic identities

A sacrificial coating strategy toward enhancement of metal-support interaction for ultrastable Au nanocatalysts

Supported gold (Au) nanocatalysts hold great promise for heterogeneous catalysis; however, their practical application is greatly hampered by poor thermodynamic stability. Herein, a general synthetic strategy is reported where discrete metal nanoparticles are made resistant to sintering, preserving their catalytic activities in high-temperature oxidation processes. Taking advantage of the unique coating chemistry of dopamine, sacrificial carbon layers are constructed on the material surface, stabilizing the supported catalyst. Upon annealing at high temperature under an inert atmosphere, the interactions between support and metal nanoparticle are dramatically enhanced, while the sacrificial carbon layers can be subsequently removed through oxidative calcination in air. Owing to the improved metal–support contact and strengthened electronic interactions, the resulting Au nanocatalysts are resistant to sintering and exhibit excellent durability for catalytic combustion of propylene at elevated temperatures. Moreover, the facile synthetic strategy can be extended to the stabilization of other supported catalysts on a broad range of supports, providing a general approach to enhancing the thermal stability and sintering resistance of supported nanocatalysts

Population and hierarchy of active species in gold iron oxide catalysts for carbon monoxide oxidation

The identity of active species in supported gold catalysts for low temperature carbon monoxide oxidation remains an unsettled debate. With large amounts of experimental evidence supporting theories of either gold nanoparticles or sub-nm gold species being active, it was recently proposed that a size-dependent activity hierarchy should exist. Here we study the diverging catalytic behaviours after heat treatment of Au/FeOx materials prepared via co-precipitation and deposition precipitation methods. After ruling out any support effects, the gold particle size distributions in different catalysts are quantitatively studied using aberration corrected scanning transmission electron microscopy (STEM). A counting protocol is developed to reveal the true particle size distribution from HAADF-STEM images, which reliably includes all the gold species present. Correlation of the populations of the various gold species present with catalysis results demonstrate that a size-dependent activity hierarchy must exist in the Au/FeOx catalyst

Towards spin-polarized two-dimensional electron gas at a surface of an antiferromagnetic insulating oxide

The surfaces of transition-metal oxides with the perovskite structure are fertile grounds for the discovery of novel electronic and magnetic phenomena. In this article, we combine scanning transmission electron microscopy (STEM) with density functional theory (DFT) calculations to obtain the electronic and magnetic properties of the (001) surface of a ( LaFe O 3 ) 8 / ( SrFe O 3 ) 1 superlattice film capped with four layers of LaFe O 3 . Simultaneously acquired STEM images and electron-energy-loss spectra reveal the surface structure and a reduction in the oxidation state of iron from F e 3 + in the bulk to F e 2 + at the surface, extending over several atomic layers, which signals the presence of oxygen vacancies. The DFT calculations confirm the reduction in terms of oxygen vacancies and further demonstrate the stabilization of an exotic phase in which the surface layer is half metallic and ferromagnetic, while the bulk remains antiferromagnetic and insulating. Based on the calculations, we predict that the surface magnetism and conductivity can be controlled by tuning the partial pressure of oxygen

Towards spin-polarized two-dimensional electron gas at a surface of an antiferromagnetic insulating oxide

The surfaces of transition-metal oxides with the perovskite structure are fertile grounds for the discovery of novel electronic and magnetic phenomena. In this article, we combine scanning transmission electron microscopy (STEM) with density functional theory (DFT) calculations to obtain the electronic and magnetic properties of the (001) surface of a ( LaFe O 3 ) 8 / ( SrFe O 3 ) 1 superlattice film capped with four layers of LaFe O 3 . Simultaneously acquired STEM images and electron-energy-loss spectra reveal the surface structure and a reduction in the oxidation state of iron from F e 3 + in the bulk to F e 2 + at the surface, extending over several atomic layers, which signals the presence of oxygen vacancies. The DFT calculations confirm the reduction in terms of oxygen vacancies and further demonstrate the stabilization of an exotic phase in which the surface layer is half metallic and ferromagnetic, while the bulk remains antiferromagnetic and insulating. Based on the calculations, we predict that the surface magnetism and conductivity can be controlled by tuning the partial pressure of oxygen