Mo掺杂对高纯Ce1-xNdxO2-x/2(x=0.10, 0.15)体系烧结温度及晶界电性能的影响

用柠檬酸硝酸盐法制备高纯Ce1-xNdxO2-x/2(x=0.10, 0.15)固溶体, 加入摩尔分数为5%的Mo, 研究了Mo掺杂对烧结温度、结构及电性能的影响. 通过X射线衍射、电感偶合等离子体和场发射扫描电镜等手段对氧化物进行了结构表征, 采用交流阻抗谱测试其电性能. 柠檬酸硝酸盐法制备的前驱体经1450 ℃烧结24 h得到致密度大于96%的陶瓷材料; 加入5%Mo, 在1250 ℃下烧结8 h即可达到理想的致密度(>95%). 加入Mo在烧结过程中可加快晶界迁移, 促进晶粒生长, 显著提高了晶界电导率. 在600 ℃时Ce0.85Nd0.15O1.925的晶界电导率为2.56 S/m, 加入Mo后材料的电导率增加到5.62 S/m.     

ReaxFF reactive force field for the Y-doped BaZrO3 proton conductor with applications to diffusion rates for multigranular systems

Proton-conducting perovskites such as Y-doped BaZrO 3 (BYZ) are promising candidates as electrolytes for a proton ceramic fuel cell (PCFC) that might permit much lower temperatures (from 400 to 600 degrees C). However, these materials lead to relatively poor total conductivity ( approximately 10 (-4) S/cm) because of extremely high grain boundary resistance. In order to provide the basis for improving these materials, we developed the ReaxFF reactive force field to enable molecular dynamics (MD) simulations of proton diffusion in the bulk phase and across grain boundaries of BYZ. This allows us to elucidate the atomistic structural details underlying the origin of this poor grain boundary conductivity and how it is related to the orientation of the grains. The parameters in ReaxFF were based entirely on the results of quantum mechanics (QM) calculations for systems related to BYZ. We apply here the ReaxFF to describe the proton diffusion in crystalline BYZ and across grain boundaries in BYZ. The results are in excellent agreement with experiment, validating the use of ReaxFF for studying the transport properties of these membranes. Having atomistic structures for the grain boundaries from simulations that explain the overall effect of the grain boundaries on diffusion opens the door to in silico optimization of these materials. That is, we can now use theory and simulation to examine the effect of alloying on both the interfacial structures and on the overall diffusion. As an example, these calculations suggest that the reduced diffusion of protons across the grain boundary results from the increased average distances between oxygen atoms in the interface, which necessarily leads to larger barriers for proton hopping. Assuming that this is the critical issue in grain boundary diffusion, the performance of BYZ for multigranular systems might be improved using additives that would tend to precipitate to the grain boundary and which would tend to pull the oxygens atoms together. Possibilities might be to use a small amount of larger trivalent ions, such as La or Lu or of tetravalent ions such as Hf or Th. Since ReaxFF can also be used to describe the chemical processes on the anode and cathode and the migration of ions across the electrode-membrane interface, ReaxFF opens the door to the possibility of atomistic first principles predictions on models of a complete fuel cell.     

Exclusion of impurity particles during grain growth in charged colloidal crystals

We examine the spatial distribution of fluorescent-labeled charged polystyrene (PS) particles (particle volume fraction ? = 0.0001 and 0.001, diameter d = 183 and 333 nm) added to colloidal crystals of charged silica particles (? = ?(s) = 0.035-0.05, d = 118 nm). At ?(s) = 0.05, the PS particles were almost randomly distributed in the volume-filling polycrystal structures before the grain growth process. Time-resolved confocal laser scanning microscopy observations reveal that the PS particles are swept to the grain boundaries of the colloidal silica crystals owing to grain boundary migration. PS particles with d = 2420 nm are not excluded from the silica crystals. We also examine influences of the impurities on the grain growth laws, such as the power law growth, size distribution, and existence of a time-independent distribution function of the scaled grain size.     

In-situ characterization of the electrochemistry of grain and grain boundary of an X70 steel in a near-neutral pH solution

An in-situ scanning vibrating electrode technique (SVET) was used to characterize microscopically the local electrochemistry of grain and grain boundary in an X70 steel in a near-neutral pH solution. It was found that the grain was anodic relative to the grain boundary. Localized corrosion, such as pitting, preferentially occurs on the grain, rather than on the grain boundary. Once a stress corrosion crack initiates from the corrosion pit, there is a higher electrochemical driving force for the crack growth within a grain than that at grain boundary. Therefore, the stress corrosion cracking follows a transgranular mode.     

The reactivity of defects at the sidewalls of single-walled carbon nanotubes: the Stone-Wales defect

The reactivity of 5/7/7/5 (Stone-Wales, SW) defects is compared to that of the pristine sidewalls of (5,5) and (10,0) carbon nanotubes (CNTs) using density functional theory (PBE). Infinite tube models (periodic boundary conditions) are used to investigate the reaction energy for CH(2) addition to the ten [5,6], [5,7], [6,7], and [7,7] C-C junctions resulting from SW rotations of the two unique bonds in (5,5) and (10,0) CNTs. In all cases, at least one of the junctions associated with the SW defects is more highly reactive than the pristine tubes. The orientation of these junctions with respect to the tube axis mainly determines the exothermicity. The [7,7] junctions are not the most reactive sites in SW defects of (5,5) and (10,0) CNTs.     

Ion-transfer- and photo-electrochemistry at liquid|liquid|solid electrode triple phase boundary junctions: perspectives

Ion transfer at liquid|liquid junctions is one of the most fundamental processes in nature. It occurs coupled to simultaneous electron transfer at the line junction (or triple phase boundary) formed by the two liquids in contact to an electrode surface. The triple phase boundary can be assembled from a redox active microdroplet deposit of a water-immiscible liquid on a suitable electrode surface immersed into aqueous electrolyte. Ion transfer voltammetry measurements at this type of electrode allow both thermodynamic and kinetic parameters for coupled ion and electron transfer processes to be obtained. This overview summarises some recent advances in understanding and application of triple phase boundary redox processes at organic liquid|aqueous electrolyte|working electrode junctions. The design of novel types of electrodes is considered based on (i) extended triple phase boundaries, (ii) porous membrane processes, (iii) hydrodynamic effects, and (iv) generator-collector triple phase boundary systems. Novel facilitated ion transfer processes and photo-electrochemical processes at triple phase boundary electrodes are proposed. Potential future applications of triple phase boundary redox systems in electrosynthesis, sensing, and light energy harvesting are indicated.     

Tilt grain boundaries in a diblock copolymer ordered nanocomposite lamellar phase

A hybrid self-consistent field theory/density functional theory method is applied to predict tilt (kink) grain boundary structures between lamellar domains of a symmetric diblock copolymer with added spherical nanoparticles. Structures consistent with experimental observations are found and theoretical evidence is provided in support of a hypothesis regarding the positioning of nanoparticles. Some particle distributions are predicted for situations not yet examined by experiment.     

稀土及杂质元素对ZA27合金晶间腐蚀的影响

To understand the influence of impurities and rare earth elements in nature on the intergranular corrosion of the zinc-aluminum alloys, taking efficient methods to retard the inter granular corrosion, the atomic cluster of α phase grain boundary including impurities (Pb, Sn, Cd), rare earth elements (La, Y) and phase grain was constructed by computer programming based on the coincidence-site lattice theory. The recursion method was used to calculate the charge transfer over the grain boundary of α phase, and discuss influence of impurities (Pb, Sn, Cd) and Rare Earth elements (La, Y) on the electrode-potential of Zn and Al. The results shows that impurity elements (Pb, Sn, Cd) can increase the charge transfer between atoms over the grain boundary, leading to the large difference of electrode-potential between Zn and Al. Therefore the eroding is speeding up, but the RE element can reduce the charge transfer between atom sover the grain boundary, and lower the difference of electrode-potential between Zn and Al. Thus RE element is of the role of restraining the inter granular corrosion of zinc-aluminum alloys.     

Ab initio study of chemical bond interactions between covalently functionalized carbon nanotubes via amide,ester and anhydride linkages

In this paper, we have investigated the chemical bond interactions between covalently functionalized zigzag (5,0) and (8,0) SWCNT–SWCNT via various covalent linkages. Side-to-side junctions connected via amide, ester and anhydride linkages were particularly studied. The geometries and energy of the forming reaction were investigated using first-principles density functional theory. Furthermore, the band structures and the total density of states (DOS) of the junctions have also been analyzed. Our results show that several promising structures could be obtained by using chemical connection strategy and particularly the junctions formed by coupling amino functionalized SWCNT and carboxylic acid functionalized SWCNT was more favorable.     

Solid-liquid interfacial energy of neopentylglycol

The grain boundary groove shapes for equilibrated solid neopentylglycol (2,2-dimethyl-1,3-propanediol) (NPG) with its melt were directly observed by using a horizontal temperature gradient stage. From the observed grain boundary groove shapes, the Gibbs-Thomson coefficient (Gamma), solid-liquid interfacial energy (sigma(SL)), and grain boundary energy (sigma(gb)) of NPG have been determined to be (7.4+/-0.7)x10(-8) Km, (7.9+/-1.2)x10(-3) Jm(-2), and (15.4+/-2.5)x10(-3) Jm(-2), respectively. The ratio of thermal conductivity of equilibrated liquid phase to solid phase for the NPG has also been measured to be 1.07 at the melting temperature.     

Unusual decrease in conductivity upon hydration in acceptor doped, microcrystalline ceria

The impact of hydration on the transport properties of microcrystalline Sm(0.15)Ce(0.85)O(1.925) has been examined. Dense, polycrystalline samples were obtained by conventional ceramic processing and the grain boundary regions were found, by high resolution transmission electron microscopy, to be free of impurity phases. Impedance spectroscopy measurements were performed over the temperature range 250 to 650 °C under dry, H(2)O-saturated, and D(2)O-saturated synthetic air; and over the temperature range 575 to 650 °C under H(2)-H(2)O atmospheres. Under oxidizing conditions humidification by either H(2)O or D(2)O caused a substantial increase in the grain boundary resistivity, while leaving the bulk (or grain interior) properties unchanged. This unusual behavior, which was found to be both reversible and reproducible, is interpreted in terms of the space-charge model, which adequately explains all the features of the measured data. It is found that the space-charge potential increases by 5-7 mV under humidification, in turn, exacerbating oxygen vacancy depletion in the space-charge regions and leading to the observed reduction in grain boundary conductivity. It is proposed that the heightened space-charge potential reflects a change in the relative energetics of vacancy creation in the bulk and at the grain boundary interfaces as a result of water uptake into the grain boundary core. Negligible bulk water uptake is detected under both oxidizing and reducing conditions.     

Origin and evolution of exon/intron junctions

In nuclear mRNA genes, exon/intron junctions (both exon/intron and intron/exon junctions in this paper) possess the specific duplex pattern with the corresponding ends (3′ to 3′, 5′ to 5′) of exons and introns more or less identical. In genes with group I or group II introns, overall analyses indicate there are also related patterns in their exon/intron junctions. From the analysis of these specific regions of split genes and the study of the composition of primitive genomes, it is proposed that the sequences of primitive exons and introns are identical at least in their corresponding boundary regions. And more fundamentally, it may be concluded that exon/intron junctions were originally related to tandem repeated sequences in the earliest genomes. Results from a preliminary analysis of specific motifs in modern repeated sequences support such a view on the origin of exon/intron junctions. As for the evolution of exon/intron junctions, there have been multiple rather than single paths.     

Twist grain boundary and frustrated liquid crystal phases

Research into the structures and properties of frustrated liquid crystal phases continues apace. A number of new twist grain boundary phases have been discovered, of particular interest are those with their rod-like molecules inclined to the layer planes, i.e. the TGBC phases. A new anticlinic TGBC phase has also been discovered complimenting the synclinic TGBC phases. In addition a number of smectic blue phases (BP_Sm) have recently been discovered.     

Tight junction development between cultured hepatoma cells: possible stages in assembly and enhancement with dexamethasone.

Freeze-fracture and thin-section methods were used to study tight junction formation between confluent H4-II-E hepatoma cells that were plated in monolayer culture in media with and without dexamethasone, a synthetic glucocorticoid. Three presumptive stages in the genesis of tight junctions were suggested by these studies: 1) "formation zones" (smooth P-fracture face ridges deficient in intramembranous particles), apparently matched across a partially reduced extracellular space, develop between adjacent cells; 2) linear strands and aggregates of 9--11 nm particles collect along the ridges of the formation zones. The extracellular space was always reduced when these structures were found matched with pits in gentle E-face depressions; 3) the linear arrays of particles on the ridges associate within the membranes to form the fibrils characteristic of mature tight junctions. The formation zones resemble tight junctions in terms of size, complexity and the patterns of membrane ridges. Although some of the beaded particle specializations may actually be gap junctions, it is unlikely that all can be interpreted in this way. No other membrane structures were detected that could represent developmental stages of tight junctions. Dexamethasone (at 2 x 10(-6)M) apparently stimulated formation of tight junctions. Treated cultures had a greater number of formation zones and mature tight junctions, although no differences in qualitative features of the junctions were noted.     

Grain boundary resistance of fast lithium ion conductors: Comparison between a lithium-ion conductive Li-Al-Ti-P-O-type glass ceramic and a Li1.5Al0.5Ge1.5P3O12 ceramic

The structure and ionic conductivity of a lithium-ion conductive Li-Al-Ti-P-O-type glass ceramic (LATP) was studied by means of powder X-ray diffraction, transmission electron microscopy, and broadband impedance spectroscopy. The results were compared to a Li_1.5Al_0.5Ge_1.5P₃O₁₂ (LAGP) ceramic. While the grain conductivity of LATP is higher than that of LAGP, the total conductivity of LATP is lower due to a large grain boundary resistance. The grain boundary resistance of LATP is characterized by a slightly higher activation energy and a slightly higher pre-exponential factor than the grain resistance. Our results indicate that the origin of the grain boundary resistance in these fast lithium ion conductors is clearly distinct from oxide ion conductors.     

Nonlinear electrical grain boundary properties in proton conducting Y-BaZrO3 supporting the space charge depletion model

The overall proton conductivity of polycrystalline acceptor-doped BaZrO(3) is limited by the high resistivity of its grain boundaries. To investigate the nature of the electrical response of the grain boundaries as a function of the DC bias, Y-doped BaZrO(3) ceramics with a very large grain size (up to 200 μm) have been prepared in an infrared image furnace. The grains are so large that even individual grain boundaries can be addressed by microelectrodes. DC voltage-dependent resistance and capacitance of the grain boundaries are discussed in terms of the space charge model. The results corroborate carrier depletion (OH(O)˙, h˙, V(O)˙˙) as origin of the pronounced grain boundary resistance. This picture fits well into the space charge scenario found for various related oxide materials, and leads to strategies for improving grain boundary conductivity.     

Electronic transport in Z-junction carbon nanotubes

In this paper, the electronic transport in different Z-shape carbon nanotubes containing double knee junction structures on the same tube is studied. One consists of (5,5)-(9,0)-(5,5) double knee nano-metal-metal-metal junctions and another consists (6,6)-(10,0)-(6,6) double knee nano-metal-semiconductor-metal junctions. With the nearest-neighbor pi-orbital tight-binding model, quantum conductances of these double knee junctions are calculated using the Landauer formula. The interesting conductance curves are provided to exhibit a potential application in the arena of molecular electronics.     

First-principles study of the triwing graphene nanoribbons: junction-dependent electronic structures and electric field modulations

Using first-principles calculations, we investigate the structural, electronic and magnetic properties of triwing zigzag graphene nanoribbons (TW-ZGNRs), as well as the electric field effects on their electronic structures. The TW-ZGNRs have comparable energetic stabilities to the normal ZGNRs and exhibit fascinating junction-dependent electronic properties. With the sp(2) hybridized junctions, the TW-ZGNRs undergo a Peierls distortion and behave as ferromagnetic metals. While the TW-ZGNRs with sp(3) hybridized junctions become semiconductors, which have a ferrimagnetic ground state. An external electric field can further modulate the band structures of semiconducting TW-ZGNRs. The parallel electric field directly moves the flat bands around the Fermi level, while the perpendicular field controls the edge states at the ribbon wings. By these electric field modulations, the band gaps are effectively tuned and half-metallicity can be induced into TW-ZGNRs. Our studies demonstrate that the junctions play an important role in the electronic structures of TW-ZGNRs, which have well-tunable electronic and magnetic properties for potential applications in nanoelectronics and spintronics.     

Impurity and vacancy clustering at the Σ3(1 1 1)[1 −1 0] grain boundary in strontium titanate

The interaction of selected point defects and the Σ3(1 1 1)[1 −1 0] symmetrical twin boundary in SrTiO₃ is investigated by density-functional band-structure calculations. The pristine boundary is SrO₃-terminated and exhibits electronic properties which are comparable with the pure bulk phase. Both the mirror-symmetric twin and some laterally shifted structures have low grain boundary energies, thus they may coexist in a real crystal if external stress is applied. With varying chemical potential of the electrons, the TiO₃ sublattice of the pristine boundary is destabilised by both electron enrichment and electron depletion. These results may explain, why different translation states can be observed by electron microscopy within the same bicrystal.Substitutional doping of the Ti columns next to the boundary plane with formally 3+ cations yields a contraction compared with the pure boundary. The same result, accompanied by more pronounced local geometry changes, is obtained for the translation state in the presence of O vacancies. Neutral O vacancies in the boundary plane are the most stable species followed by positively charged O vacancies in the neighbouring Sr/O plane parallel to the boundary. The Fermi level is shifted either upward to metal-derived states by substitutional doping or downward into the O-2p manifold for O vacancies. These findings may rationalise both the observed low, mainly electronic, grain boundary conductivity of SrTiO₃ boundaries and the changes in local potential found at the grain boundary by tunnelling and force microscopy experiments.     

Determination of lattice and grain boundary diffusion coefficients in protective alumina scales on high temperature alloys using SEM,TEM and SIMS

Grain and grain boundary diffusion coefficients in alumina scales on FeCrAl-based ODS alloys have been determined. The boundary diffusion-coefficients have been derived by combining gravimetrically determined growth rate data with SEM and TEM analyses of the oxide scale microstructure. The diffusion coefficients determined have been used as input parameters for a computer model describing the oxygen isotope exchange between grain and grain boundary in the alumina scale which forms during a two-stage oxidation using (18)O-tracers. This comparison of the calculated tracer profiles with profiles determined experimentally by SIMS allows the estimation of the lattice diffusion coefficient of oxygen in the alumina scale.