Influence of interface structure on mass transport in phase boundaries between different ionic materials Experimental studies and formal considerations

Abstract  

Internal and external interfaces in solids exhibit completely different transport properties compared to the bulk. Transport parallel to grain or phase boundaries is usually strongly enhanced. Transport perpendicular to an interface is usually blocked, i.e., transport across an interface is often much slower. Due to the high density of interfaces in modern micro- and nanoscaled devices, a severe influence on the total transport properties can be expected. In contrast to diffusion in metal grain boundaries, transport phenomena in boundaries of ionic materials are still less understood. The specific transport properties along metal grain boundaries are explained by structural factors like packing densities or dislocation densities in the interface region. In most studies dealing with ionic materials, the interfacial transport properties are merely explained by the influence of space charge regions. In this study the influence of the interface structure on the interfacial transport properties of ionic materials is discussed in analogy to metallic materials. A qualitative model based on the density of misfit dislocations and on interfacial strain is introduced for (untilted and untwisted) phase boundaries. For experimental verification, the interfacial ionic conductivity of different multilayer systems consisting of stabilised ZrO₂ and an insulating oxide is investigated as a funtion of structural mismatch. As predicted by the model, the interfacial conductivity increases when the lattice mismatch is increased.     

Analysis and modelling of transport processes in alumina scales on high temperature alloys

Summary The oxidation behaviour of a number of FeCrAl-based oxide dispersion strengthened (ODS) alloys of the type MA 956 was investigated at temperatures between 1100°C and 1200°C. The main emphasis was placed on the effect of alloy yttria content on scale composition and structure. Studies of three model alloys with various alloy yttria contents in the range 0.02–0.7% showed an increase in the oxide growth rate with increasing yttria content. Alumina scales on alloys with 0.17% and 0.7% yttria grow almost exclusively by oxygen diffusion, whereas the scale on the alloy with 0.02% yttria exhibits a significant contribution from cation diffusion. By using SIMS depth-profiling and SIMS-imaging it was found that the oxygen diffuses through the alumina scale via oxide grain boundaries. TEM-studies revealed that the yttria is incorporated in the scale grain boundaries as precipitations and as a segregation layer. This behaviour of the yttrium is believed to be responsible for its influence on scale growth mechanisms. A mathematical model has been developed which describes the alumina scale growth by oxygen diffusion through grain boundaries. The model accounts for the observed increase in aluminium oxide grain size in scale growth direction.     

Microstructural analysis of a carbon fibre reinforced AZ91D magnesium alloy composite

Interfacial regions in metal matrix composites are important in controlling the mechanical and thermal properties of these materials. An ultrahigh modulus fibre‐reinforced magnesium alloy matrix composite has been studied, with particular attention paid to the interfacial and precipitate microstructures. Fibres were surface treated but uncoated prior to composite manufacture. Observations revealed that an interface consisting of polycrystalline magnesium oxide with occasional Mg₁₇Al₁₂ (β) precipitate particles predominates. Discontinuous β particles formed at fibre surfaces, and continuous spherical and lamellar β precipitates nucleated at grain boundaries and fibre surfaces. High dislocation densities exist at the interface indicating matrix‐yielding subsequent to manufacture and that a high mean residual compressive stress acts on fibres. The effect that the observed microstructural features has on composite properties and on interfacial bonding is discussed and compared to examples in the literature. Copyright © 2005 John Wiley & Sons, Ltd.     

Investigation on the influence of sulfur segregation on the adherence of protective oxide layers on high temperature materials

Summary Oxide layers of various thicknesses have been produced on Fe-15Cr and NiAl doped with sulfur. For thin oxide layers, depth profiles demonstrated that no sulfur is enriched at the oxide-metal interface. For thick oxide layers, the interface has been made accessible by bending or scratching the specimen. Sulfur has been found on the spalled area by AES. Micrographs proved that excessive void formation is responsible for the bad adherence of the oxide layers to the substrate. Sulfur segregaton decreases the surface energy, thus reducing the radius of nucleus formation and promoting void formation. Reactive elements form refractive sulfides with sulfur, thereby improving scale adherence.     

Interfacial analyses for elucidation of the intergranular disintegration upon oxidation of intermetallic phases

Summary In a range of intermediate temperatures from 600 to 1000°C and in the presence of oxygen, numerous intermetallic phases show the phenomenon pest, an intergranular disintegration into small pieces. Thermo-gravimetric investigations and annealings in quartz ampoules have been performed, for annealing the oxygen pressures were established in the range 10^–30 to 10^–10 bar O₂ using metal/oxide mixtures. The specimens after annealing were fractured in UHV and the intergranular fracture faces were analyzed by AES. The Auger peaks of Al are markedly different for the intermetallic phase and for Al₂O₃, therefore it can be distinguished if oxygen has diffused into grain boundaries and not yet reacted or if Al₂O₃ was formed. The fracture face of NbAl₃ shows oxide precipitates near the surface and oxygen which had penetrated into the interior. Also in NiAl, Al₂O₃ was detected as well as oxygen penetrated into the grain boundaries. The pest obviously is a complex interplay of the processes: 1) penetration of oxygen through the outer oxide layer on the surface into grain boundaries of the intermetallic phase; 2) inward diffusion of oxygen along the grain boundaries into the interior of the intermetallic phase; 3) precipitation of Al₂O₃, beginning near the surface or (at low oxygen pressure) in the whole cross section, and cracking of the materials by the growth of Al₂O₃ into grain boundaries and cracks. Depending on the range of oxygen pressure different steps can be rate determining.     

Influence of interface structure on mass transport in phase boundaries between different ionic materials

Abstract  Internal and external interfaces in solids exhibit completely different transport properties compared to the bulk. Transport parallel to grain or phase boundaries is usually strongly enhanced. Transport perpendicular to an interface is usually blocked, i.e., transport across an interface is often much slower. Due to the high density of interfaces in modern micro- and nanoscaled devices, a severe influence on the total transport properties can be expected. In contrast to diffusion in metal grain boundaries, transport phenomena in boundaries of ionic materials are still less understood. The specific transport properties along metal grain boundaries are explained by structural factors like packing densities or dislocation densities in the interface region. In most studies dealing with ionic materials, the interfacial transport properties are merely explained by the influence of space charge regions. In this study the influence of the interface structure on the interfacial transport properties of ionic materials is discussed in analogy to metallic materials. A qualitative model based on the density of misfit dislocations and on interfacial strain is introduced for (untilted and untwisted) phase boundaries. For experimental verification, the interfacial ionic conductivity of different multilayer systems consisting of stabilised ZrO₂ and an insulating oxide is investigated as a funtion of structural mismatch. As predicted by the model, the interfacial conductivity increases when the lattice mismatch is increased. Graphical abstract  

AES and LEED study of well‐ordered oxide film grown on Cu–9at.%Al(111)

An alloy of Cu–9at.%Al(111) has been oxidized in a low‐energy electron diffraction (LEED)/AES and a scanning AES instrument at elevated temperatures. Dosing with 1300 L of oxygen at 995 K gives rise to well‐ordered oxide layer formation on the Cu–9at.%Al alloy. The structure of the ordered oxide confirmed by LEED is ( ) R30°. The chemical state of the oxide was Al₂O₃. The morphology of the surface observed with SEM in the scanning AES instrument revealed flat oxide growth with triangular defects of the same orientation. The possible epitaxy between the alloy substrate and alumina layer has been discussed. Copyright © 2003 John Wiley & Sons, Ltd.     

溶胶-凝胶法设计与制备金属及合金纳米材料的研究进展

溶胶-凝胶法是常见的制备金属氧化物的方法之一。在溶胶-凝胶法中,各种反应物能达到分子级的均匀混合,因此能制备成份复杂的氧化物材料。目前,溶胶-凝胶法也应用于设计与制备金属纳米材料,特别是合金纳米颗粒。例如,溶胶-凝胶法能应用于制备CoPt、FePt等磁性纳米合金材料以及CoCrCuNiAl高熵合金纳米材料,以及物相结构为有序相的Cu3Pt合金纳米材料。本文综述溶胶-凝胶法设计制备金属纳米材料的研究进展,包括溶胶-凝胶法实施的基本步骤、该方法在制备金属纳米材料方面的具体应用,并着重论述采用热力学计算设计金属及化合物的基本原理。该基本原理包括计算金属氧化物与还原性气体如氢气的还原反应的吉布斯自由能的变化量、金属氧化物的标准电极电位(不同于金属离子的标准电极电位)。最后探讨溶胶-凝胶法设计制备金属纳米材料存在的问题以及后续可能的发展方向。     

Direct observation and analysis of annealing-induced microstructure at interface and its effect on performance improvement of organic thin film transistors

For the first time direct observation and analysis of microstructural variations of crystalline domains and grain boundaries at atomic scale in the buried interface of an organic semiconductor thin film of poly(2,6-bis(3-alkylthiophen-2-yl)dithieno[3,2- b;2',3'- d]thiophene) (PBTDT), a new synthesized solution-processed polymer is achieved for demonstrating a different network nanostructure of crystalline nanofibers at the interface from the outside surface of the film observed. It is also discovered that structural variations of crystalline domains and grain boundaries at an atomic scale caused by annealing, which include larger domains with enhanced crystallinity, reduced pi-pi stacking distance, reduced disorders in the grain boundaries, and small tilt-angle boundaries well explain the significant performance improvement of the PBTDT based organic thin film transistor (OTFT) after annealing. This work provides a highly resolutioned image on the microstructures at an organic semiconducting interface for deep scientific insights of the OTFT performance improvement through microstructure optimization.     

Influence of atmosphere kind on temperature programmed decomposition of noble metal chlorides

The hydrated chlorides of noble metals located in the 8-10 groups of Periodic System were used in this work: Ru, Rh, Pd, Ir and Pt. The common mechanism of chloride compounds decomposition in oxidative, inert and reductive atmospheres was postulated and mutual similarities and differences were discussed. These compounds play a significant role in heterogeneous catalysis, as starting materials for metal/oxide catalysts, and also in analytical chemistry and industry.     

Plasma SNMS investigations on powder metallurgical Cr and Ti-48Al-2Cr after oxidation in air and 15N2/18O2 atmosphere

Oxidation experiments at 800°C and 900°C, partly in atmospheres enriched in isotope tracers, with subsequent SNMS depth profiling confirmed that two well-known oxidation mechanisms are also valid for the title materials: In powder metallurgical Cr, Y addition shows the reactive element effect, most completely by means of Y implantation and to a lesser extent by the addition of Y₂O₃ dispersion, classical fast Cr outward transport dominated kinetics is replaced by kinetics that is governed by slow inward diffusion of oxygen. In γ-TiAl with 2 at-% Cr, N is identified as the “frontline” oxidizing element being able to penetrate existing oxide and nitride layers inward towards the bulk metal. These results exemplify that the combination of plasma SNMS depth profiling with oxidation experiments, especially in ¹⁵N₂/¹⁸O₂ tracer atmosphere, and with reactive element implantation is an excellent method to obtain basic insights into corrosion mechanisms. Especially advantageous is the use of SNMS because of its much lesser matrix dependence compared to SIMS which has more frequently been used for this kind of experiments.     

金属锌表面膜形成的椭圆偏振技术研究

介绍了近年作者课题组使用椭圆偏振技术研究金属锌表面氧化膜的形成,包括多晶锌表面自然氧化物薄膜的形成及其光学性能和电子结构、不同气氛自然氧化物膜的生长研究以及在碱性碳酸盐介质金属锌的电化学过程等方面的工作. 旨在通过原位和非原位椭圆偏振技术了解金属锌表面氧化物膜层的光、电性能以及膜层结构的改变和生长动力学,这对评估锌氧化层的总体性能有着重要意义.     

Surface chemistry and diffusion of trace and alloying elements during in vacuum thermal deoxidation of stainless steel

Removal of the native surface oxide from steel is an important initial step during vacuum brazing. Trace and alloying elements in steel, such as Mn, Si, and Ni, can diffuse to the surface and influence the deoxidation process. The detailed surface chemical composition and grain morphology of the common stainless-steel grade 316L is imaged and spectroscopically analyzed at several stages of in-vacuum annealing from room temperature up to 850°C. Measurements are performed using synchrotron-based X-ray photoemission and low-energy electron microscopy (XPEEM/LEEM). The initial native Cr surface oxide is amorphous and unaffected by the underlying Fe grain morphology. After annealing to ~700°C, the grain morphology is seen at the surface, persisting also after the complete oxygen removal at 850°C. The surface concentration of first Mn and then Si increases significantly when annealing to 500°C and 700°C, respectively, while Ni and Cr concentrations do not change. Mn and Si are not located only in grain boundaries or clusters but are distributed across over the surface. Both Mn and Si appear as oxides, while Cr oxide becomes metallic Cr. Annealing from 500°C up to 850°C leads to the removal of first the Mn and then Si oxides from the surface, while Cr and Fe are completely reduced to metals. Deoxidation of Cr occurs faster at the grain boundaries, and the final Cr metal surface content varies between the grains. The findings are summarized in a general qualitative model, relevant for austenite steels.     

Atomic‐Scale Insight into Exsolution of CoFe Alloy Nanoparticles in La0.4Sr0.6Co0.2Fe0.7Mo0.1O3−δ with Efficient CO2 Electrolysis

In situ exsolution of metal nanoparticles in perovskite under reducing atmosphere is employed to generate a highly active metal–oxide interface for CO₂ electrolysis in a solid oxide electrolysis cell. Atomic‐scale insight is provided into the exsolution of CoFe alloy nanoparticles in La_0.4Sr_0.6Co_0.2Fe_0.7Mo_0.1O_3?δ (LSCFM) by in situ scanning transmission electron microscopy (STEM) with energy‐dispersive X‐ray spectroscopy and DFT calculations. The doped Mo atoms occupy B sites of LSCFM, which increases the segregation energy of Co and Fe ions at B sites and improves the structural stability of LSCFM under a reducing atmosphere. In situ STEM measurements visualized sequential exsolution of Co and Fe ions, formation of CoFe alloy nanoparticles, and reversible exsolution and dissolution of CoFe alloy nanoparticles in LSCFM. The metal–oxide interface improves CO₂ adsorption and activation, showing a higher CO₂ electrolysis performance than the LSCFM counterparts.     

S8‐Mediated Cyclization of 2‐Aminophenols/thiophenols with Arylmethyl Chloride: Approach to Benzoxazoles and Benzothiazoles

A metal‐free approach to benzazoles from arylmethyl chlorides and 2‐mercaptan/2‐hydroxyanilines using elemental sulfur as a traceless oxidizing agent has been developed. The reactions proceeded in good to excellent yields, exhibiting good functional groups tolerance and gram‐scale ability. A key mechanistic investigation indicated that the key intermediate trisulfide 6 , which was characterized by NMR, HRMS and crystal X‐ray crystallography, was separated in the reaction prior to the formation of the product.     

Electrochemical oxide film formation at noble metals as a surface-chemical process

The mechanisms of electrochemical oxide film formation at noble metals are described and exemplified by the cases of Pt and Au, especially in the light of recent experimentation by means of cyclic voltammetry, ellipsometry and vacuum surface-science studies using LEED and AES.

Unlike the mechanisms of base-metal oxidation, e.g., in corrosion processes, anodic oxide film formation at noble metals proceeds by surface chemical processes involving, initially, sub-monolayer, through monolayer, formation of 2-dimensional OH/O arrays. During such 2-d processes, place-exchange between electrosorbed OH or O species on the surface, and Pt or Au atoms within the surface lattice, takes place leading to a quasi-2-d compact film which then grows ultimately to a multilayer hydrous oxide film, probably by continuing injection of ions of the substrate metal and their migration through the growing film under the influence of the field.

The initial, sub-monolayer stage of electrosorption of OH involves competitive chemisorption by anions, e.g. HSO₄⁻, ClO₄⁻, Cl⁻, which inhibits onset of the first stage of surface oxidation. These processes are demonstrable in experiments on single-crystal surfaces. The combination of such anion effects with place-exchange during the extension of the film, leads to a general mechanism of noble metal oxide film formation.

The formation of the oxide films can be examined in detail by recording the distinguishable stages in the film's electrochemical reduction in linear-sweep voltammetry which is sensitive down to OH/O fractional coverages as low as 0.5% and over time-scales down to 50μs in experiments on time-evolution and transformation of the states of the oxide films.

By means of LEED, AES and STM or AFM experiments, the reconstructions and perturbations (e.g. generation of stepped terraces) which oxide films cause on singlecrystal surfaces can be followed.     

Oxide and nitride protective layers on stainless steel studied by AES,WDS and XPS

Protective surface layers on AISI 321 stainless steel were prepared by thermal treatments at two different temperatures in air and two controlled atmospheres. Different oxide and/or nitride layers were formed. Surface morphology of the layers was investigated by scanning electron microscopy (SEM). Auger electron spectroscopy (AES) depth profiling of the samples was performed. Since depth profiling suggested layer thicknesses of the order of hundreds of nanometres, an attempt was made to obtain some fast, averaged information about the layer compositions using wavelength dispersive spectroscopy (WDS) at two different beam energies to obtain probing depths best suited to the layer thickness. X‐ray photoelectron spectroscopy (XPS) profiling of one layer was also performed to obtain information about the chemical states of the elements inside the layer. The analysed samples showed considerable differences with respect to their surface morphology, oxide/nitride layer thicknesses, compositions and layer–metal interface thickness. Copyright © 2007 John Wiley & Sons, Ltd.     

Intergranular fracture surface analysis of molybdenum

Bend properties of plasma beam and/or electron beam melted molybdenum and several dilute alloys containing C, B, T, V and Re were investigated. Fracture surfaces and microstructure were examined in detail by SEM and AES. Considerable intergranular fracture occurred in almost all of the samples as the test temperature decreased. But proper alloy addition, such as B and Ti, and hot-rolling induced a change in the fracture mode to a transgranular mode. Segregation of nitrogen and/or oxygen to grain boundaries was often recognized in plasma beam or electron beam melted ingots and decreased the strength of grain boundaries. Small precipitates due to comparatively low alloy addition such as C, B, Ti and Re, were effective to enhance the grain boundary cohesion.     

电极界面浓差极化对锂金属沉积的影响

锂金属作为下一代高能量密度电池的理想负极材料受到研究人员广泛关注。然而,锂枝晶生长引起的安全隐患和循环寿命短等问题严重影响了锂金属电池的实用化进程。本文以电化学现象和理论为依据,从浓差极化角度详细分析锂金属电沉积过程中枝晶生长、死锂形成和全电池失效机制,并对目前研究较多的多孔宿主电极中的浓差极化及枝晶抑制进行分析,提出锂金属界面浓差电池现象。本文得到的结论为研究人员更深入地探究锂金属保护策略提供了理论依据。     

Fe-50Cu纳米涂层的制备及氧化行为

为探讨晶粒尺寸变化对多相合金氧化行为的影响,采用磁控溅射技术制备了Ｆｅ－５０Ｃｕ过饱和固溶体纳米涂层,并利用热重实验对比研究了该涂层与同成分的铸态合金在８００℃空气中的氧化行为。结果表明,铸态合金中形成了包括铁和铜的氧化物及其复合氧化物在内的复杂外氧化膜,同时发生了铁的内氧化。涂层中发生了铁的单一外氧化,但涂层退化后,合金基体中仍出现铁的内氧化。这种氧化模式的转变机制在于涂层尺寸的纳米化显著扩展了