Finite element simulation of diffusion into polycrystalline materials

Diffusion in polycrystalline materials is investigated by means of numerical finite element simulations for constant source conditions. The grain boundaries are assumed to provide fast diffusion paths. Main emphasis is put on situations that typically occur for nanocrystals, viz. on situations in which (i) the diffusion length is significant compared with grain size, (ii) the influence of boundaries that are parallel to the surface become important in addition to the perpendicular ones. Furthermore, we treat the influence of blocking space charge layers sandwiching the core pathways and thus channeling grain boundary diffusion.     

Fast grain boundary diffusion and surface exchange reactions at active surface sites of polycrystalline materials

Analytical solutions to the diffusion equations for fast grain boundary diffusion and surface exchange reactions at active surface sites are derived. The microstructure of the polycrystalline sample of finite thickness is modelled by parallel grain boundaries. The ratio between the surface exchange coefficient and the diffusion coefficient for the grain boundaries is assumed to be greater than or equal to that for the bulk. The analytical solutions allow the calculation of diffusion profiles for thin films. Special emphasis is laid on a detailed analysis of the time dependence of the total amount of diffusant exchanged between the constant diffusion source (e.g. gas phase) and the polycrystalline sample (e.g. oxide ceramics) which corresponds to relaxation curves obtained from, e.g., oxygen exchange measurements. The calculated relaxation curves refer to Harrison's type-A kinetics where homogeneous medium solutions are satisfactorily applicable, introducing effective kinetic parameters. Apart from expressions for the effective diffusion coefficient analogous relations for the effective surface exchange coefficient are proposed, depending on the microstructure of the polycrystalline material and the corresponding kinetic parameters of bulk and grain boundary regions, respectively.     

Microstructure and electrical properties of aluminium-substituted La(Sr)Ga(Mg)O3–δ-based solid electrolytes

Abstract  A study of the influence of the substitution of Al for Ga in the ceramic processing and electrical properties of La_0.95Sr_0.05Ga_0.90–x Al _x Mg_0.10O_3–δ (0 ≤ x ≤ 0.3) solid electrolytes is presented. The materials retained orthorhombic symmetry over the entire substitution range, whereas a deviation from Vegard’s law for x > 0.20 suggested a maximum Al solubility of x = 0.20. Scanning electron microscopy analysis of ceramic samples revealed that grain growth was inhibited for x ≥ 0.2. This microstructural change was related to an apparent deterioration of mechanical properties, as suggested by room-temperature Vickers hardness measurements. Impedance spectroscopy revealed a significant degradation of the grain-boundary electrical properties for x ≥ 0.20, whereas the bulk conductivity was enhanced for 0.10 ≤ x ≤ 0.15. Oxygen-permeability measurements confirmed that the studied materials remain essentially pure ionic conductors. An ionic conductivity maximum of 0.047 S/cm at 700 °C was obtained for x = 0.10. The effect of aluminium in the grain-bulk ionic conductivity is discussed in terms of defect cluster models and assuming fast oxygen diffusion along domain walls. Graphical Abstract        

Modeling of surface exchange reactions and diffusion in composites including transport processes at grain and interphase boundaries

Surface exchange reactions and diffusion of oxygen in ceramic composites consisting of a dilute and random distribution of inclusions in a polycrystalline matrix (host phase) are modeled phenomenologically by employing the finite element method. The microstructure of the mixed conducting composite is described by means of a square grain model, including grain boundaries of the matrix and interphase boundaries between the inclusions and grains of the host phase. An instantaneous change of the oxygen partial pressure in the surrounding atmosphere may give rise to an oxygen exchange process, i.e., oxidation or reduction of the ceramic composite. Relaxation curves for the total amount of exchanged oxygen are calculated, emphasizing the role played by fast diffusion along the interfaces. The relaxation curves are interpreted in terms of effective medium diffusion, introducing appropriate equations for the effective diffusion coefficient and the effective surface exchange coefficient. When extremely fast diffusion along the grain and interphase boundaries is assumed, the re-equilibration process shows two different time constants. Analytical approximations for the relaxation process and relations for the separate relaxation times are provided for this limiting case as well as for blocking interphase boundaries. Furthermore, conductivity relaxation curves are calculated by coupling diffusion and dc conduction. In the case of effective medium diffusion, the conductivity relaxation curves do not deviate from those for the total amount of exchanged oxygen. On the contrary, the conductivity relaxation curves differ remarkably from the time dependence of the total amount of exchanged oxygen, when the different phases of the composite re-equilibrate with separate time constants.     

A novel microwave route for the preparation of ZrC-SiC composites

A novel microwave-assisted carbothermal reduction and carburization route has been used to prepare ZrC-SiC composite powders. Both zircon and mixtures of ZrO₂ and SiO₂ were used as starting materials along with amorphous carbon. Carbothermal reduction and carburization were examined in both argon and nitrogen atmospheres. Reaction kinetics in microwave field was found to exhibit notable differences for the two different starting materials. However, a complete oxide to carbide conversion was achieved in less than 30 min in both cases when argon was used as an ambient gas. The possible structural mechanism involved in the reactions has been discussed.     

Study of the surface chemistry and morphology of single walled carbon nanotube-magnetite composites

The study of the morphologies of the single walled carbon nanotube (SWCNT), magnetite nanoparticles (MNP), and the composite based on them was carried with combined X-ray diffraction (XRD), Raman spectroscopy (RS), scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM). These techniques together with thermogravimetric analyses (TGA) and diffuse reflectance infrared transform spectroscopy (DRIFTS) confirmed the production of pure single phases, and that the composite material consisted of MNP attached to the outer surface of the SWCNT. The Mössbauer spectroscopy (MS) research showed the presence of a large quantity of Lewis acid sites in the highly dispersed magnetite particles supported on the SWCNT outer surface. The DRIFTS carbon dioxide adsorption study of the composites revealed significant adsorption of carbon dioxide, fundamentally in the Lewis acid sites. Then, the Lewis acid sites were observed to be catalytically active. Further, the electron exchange between the Lewis acid sites and the basic or amphoteric adsorbed molecules could influence the magnetic properties of the magnetite. Consequently, together with this first ever use of MS in the study of Lewis acid sites, this investigation revealed the potential of the composites for catalytic and sensors applications.     

Electrochemical Investigations of Mn and Al Co‐doped Li2FeSiO4/C Cathodes for Li‐Ion Battery

Few studies on orthosilicate cathodes co‐doped with two cations have been reported until now. Here, we report the synthesis of Mn and Al co‐doped Li₂Fe_0.8?xMn_0.2Al_xSiO₄ (x = 0.05 and 0.1) by a solid‐state reaction route and characterized by X‐ray diffraction (XRD), particle size analysis, scanning electron microscopy (SEM), galvanostatic charge/discharge tests, and capacity intermittent titration technique (CITT), as compared to the single‐doped Li₂Fe_0.8Mn_0.2SiO₄. Though the co‐doping leads to a slight decreased capacity owing to the increased impurity and Al³⁺ inertia, a better cycling performance is obtained as expected. Especially when x is 0.05, the modified sample (Li₂Fe_0.75Mn_0.2Al_0.05SiO₄) shows an initial discharge capacity of 159.3 mAh/g and high capacity retention of 78% after 50 charge/discharge cycles. The present work indicates that a synergistic effect of Mn and Al co‐substitution at the Fe site could partly make up the disadvantage of single Mn doping, and might provide an effective guide for the dopant incorporation to Li₂FeSiO₄ systems.     

Defect-induced anisotropic surface reactivity and ion transfer processes of anatase nanoparticles

Surface reactivity and ion transfer processes of anatase TiO₂ nanocrystals were studied using lithium bis(trifluoromethylsulfone)imide (LiTFSI) as a probing molecule. Analysis of synthesized anatase TiO₂ by electron microscopy reveals aggregated nanoparticles (average size ~8 nm) with significant defects (holes and cracks). With the introduction of LiTFSI salt, the Li⁺-adsorption propensity towards the surface along the anatase (100) step edge plane is evident in both x-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) analysis. Ab initio molecular dynamics (AIMD) analysis corroborates the site-preferential interaction of Li⁺ cations with oxygen vacancies and the thermodynamically favorable transport through the (100) step edge plane. Using ⁷Li nuclear magnetic resonance (NMR) chemical shift and relaxometry measurements, the presence of Li⁺ cations near the interface between TiO₂ and the bulk LiTFSI phase was identified, and subsequent diffusion properties were analyzed. The lower activation energy derived from NMR analysis reveals enhanced mobility of Li⁺ cations along the surface, in good agreement with AIMD calculations. On the other hand, the TFSI^– anion interaction with defect sites leads to CF₃ bond dissociation and subsequent generation of carbonyl fluoride-type species. The multimodal spectroscopic analysis including NMR, electron paramagnetic resonance (EPR), and x-ray photoelectron spectroscopy (XPS) confirms the decomposition of TFSI^– anions near the anatase surface. The reaction mechanism and electronic structure of interfacial constituents were simulated using AIMD calculations. Overall, this work demonstrates the role of defects at the anatase nanoparticle surface on charge transfer and interfacial reaction processes.     

Kinetic study of surface reactions of O2 and H2 on polycrystalline copper

Adsorption and transformation of O₂, H₂, water and methanol on polycrystalline copper in the 300–400 K range have been studied by surface potential variations. In particular, O₂ reacts with H(a) to form OH(a) with a first order kinetic law, whereas OH(a) reacts with H₂ to form water which desorbs and restores H(a) with an Elovich kinetic law.     

Surface silylation and pore structure development of silica aerogel composites from colloid and TEOS-based precursor

Flexible aerogel-fiber composites were prepared by silylation and ambient drying of colloidal silica and tetraethylorthosilicate (TEOS)-based sol. After immersing glass fiber matrices into silica sol with colloid-based, colloid/TEOS-based, and TEOS-based silica sol, it was surface-modified in a trimethylchlorosilane/n-hexane solution and heat-treated at 230 °C in ambient atmosphere. Surface silylation of silica aerogel synthesized from colloid and TEOS-based silica sols showed different behaviors. For colloid silica gel, it was comprised of small sized mesopores because colloid-based silica gel has dense networks through great degrees of hydrolysis and condensation. On the contrary, TEOS-based aerogel was consisted of relatively large-sized pores because of comparatively lesser degree of hydrolysis and condensation. Through this study, we can know that the pore structures of silica aerogel could be controlled by choosing colloid or TEOS-based precursor and surface silylation reaction.     

Rapid and solvent-free solid-state synthesis and characterization of Zn3V2O8 nanostructures and their phenol red aqueous solution photodegradation

Zinc vanadate (Zn₃V₂O₈) nanostructures have been successfully synthesized via simple, rapid and solvent-free solid-state method by using different complex precursors of Zn and NH₄VO₃ as novel starting materials. Effects of various zinc (II) Schiff base complex precursors and calcination temperatures were investigated to reach optimum condition. It was found that particle size and optical property of the as-prepared products could be greatly influenced via these parameters. The products were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectra, energy dispersive X-ray microanalysis (EDX), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Photoluminescence and ultraviolet-visible (UV–Vis) spectroscopy. The photocatalytic activity of zinc vanadate nano and bulk structures were compared by degradation of phenol red aqueous solution.     

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.     

The options of interference microscopy to explore the significance of intracrystalline diffusion and surface permeation for overall mass transfer on nanoporous materials

After a short introduction into interference microscopy and its potentials in monitoring transient concentration profiles in nanoporous materials, we concentrate on the special options of an analysis of these profiles close to the crystal surfaces. We shall in particular introduce a novel route of correlating the overall uptake, at a certain instant of time, with the current boundary concentration. In this way, the significance of surface resistances to overall molecular uptake may be most vividly demonstrated. Considering a large variety of nanoporous host-guest systems, including methanol in zeolites ferrierite, methanol in MOF Manganese(II)-formate and methanol in SAPO STA-7, quite different patterns of surface resistivities may be observed. A generalized analysis is complicated by the fact that both the diffusivities and the surface permeabilities are found to notably depend on the actual concentration. As a consequence, for one and the same system and over identical pressure steps, the relative contributions of diffusion and surface permeation to the overall process may be quite different for desorption and adsorption.     

Solid‐State NMR Investigations on the Amorphous Network of Precursor‐Derived Si2B2N5C4 Ceramics

Employing a multitude of modern solid state NMR techniques including ¹³C{¹⁵N}REDOR NMR, ¹H–¹³C CP NMR, ¹¹B MQMAS NMR spectroscopic experiments, the structural organization of Si₂B₂N₅C₄ ceramic has been studied. The experiments were executed on double isotope enriched (¹³C, ¹⁵N) and natural isotope abundance Si₂B₂N₅C₄ ceramics. The materials were synthesized by aminolysis and subsequent pyrolysis of intermediate pre‐ceramic polymers that were obtained from the single source precursor TSDE, 1‐(trichlorosilyl)‐1‐(dichloroboryl)ethane (Cl₃Si–CH(CH₃)–BCl₂). The result of the ¹³C{¹⁵N} REDOR NMR spectroscopic experiment shows that carbon atoms are incorporated into the network by bridging to nitrogen, which already occurs during the polymerization step. Furthermore, the combined results of ¹¹B NMR and ¹¹B MQMAS NMR indicate that boron atoms may also be connected to carbon in addition to nitrogen.     

The surface characteristics of polycrystalline cobalt electrooxidised in sulfuric acid solutions

The oxidation of cobalt electrodes has been carried out by means of cyclic voltammetry and coulometry under controlled potential in sulfuric acid solutions of different concentrations. The electrochemical scanning tunneling microscope/scanning tunneling microscope (ECSTM/STM) systems constructed by the authors and scanning electron microscopy (SEM) with the SEM-EDX system of surface analysis of the elements have been used. The procedure applied in this work made it possible to observe the fragments of the same surface by means of SEM and ECSTM/STM. The most typical images for a polycrystalline Co electrode with a ±10% accuracy at the scales of 4800 nm × 4800 nm and 100 nm × 100 nm are presented and the results are discussed. In a diluted electrolyte (0.1 M), irregular forms of a stable cobalt oxide with Co:O ratio ∼1:1 appear. Unreproducible results have been obtained in a 1.0 M H₂SO₄ solution. Compact and relatively regular layers of cobalt oxide of the same ratio have been obtained in 0.1 M H₂SO₄, as well as in 10.0 M sulfuric acid solution, under controlled oxidation potential at the passivation range. Received: 6 January 1999 / Accepted: 5 May 1999     

Effects of laser scanning speed on surface roughness and mechanical properties of aluminum/Polylactic Acid (Al/PLA) composites parts fabricated by fused deposition modeling

Although fused deposition modeling (FDM) has gradually become one of the popular additive manufacturing technologies in different industries, high surface roughness has always been an inevitable disadvantage of FDM parts. Laser polishing represents a recent and novel application of laser surface irradiation that can be used for precise, post-process smoothing of the rough surfaces commonly encountered on FDM parts. The influence of laser polishing on surface modification and mechanical properties of aluminum/Polylactic Acid (Al/PLA) composites has been investigated. Laser scanning speed was varied to evaluate its effect on the surface quality and mechanical properties of the experimental samples. The results indicated that laser polishing could enable reductions in surface roughness of over 86.6% (from 23.27 μm to 3.11 μm Sa). The tensile strength of specimen also increased from 41.01 MPa to 51.31 MPa with increasement of 25.1%. The dynamic mechanical analysis (DMA) results showed that there was a remarkable improvement in the storage modulus and glass transition temperature of Al/PLA composite specimens after laser polishing, which suggested that the laser polishing treatment could play a role in decreasing the porosity inside the FDM parts and improve interfacial adhesion between the PLA matrix and Al fibers. Moreover, the fracture morphologies were observed to investigate the possible strengthening mechanism. These results demonstrate how laser polishing can simultaneously smooth and modify the surface characteristics of a FDM part surface.     

Synthesis and characterization of surface modified SBA-15 silica materials and their application in chromatography

Hexagonally ordered SBA-15 mesoporous silica spheres with large uniform pore diameters are obtained using the triblock copolymer, Pluronic P123, as template with a cosurfactant cetyltrimethylammonium bromide (CTAB) and the cosolvent ethanol in acidic media. A series of surface modified SBA-15 silica materials is prepared in the present work using mono- and trifunctional alkyl chains of various lengths which improves the hydrothermal and mechanical stability. Several techniques, such as element analysis, nitrogen sorption analysis, small angle X-ray diffraction, scanning electron microscopy (SEM), FTIR, solid-state (29)Si and (13)C NMR spectroscopy are employed to characterize the SBA-15 materials before and after surface modification with the organic components. Nitrogen sorption analysis is performed to calculate specific surface area, pore volume and pore size distribution. By surface modification with organic groups, the mesoporous SBA-15 silica spheres are potential materials for stationary phases in HPLC separation of small aromatic molecules and biomolecules. The HPLC performance of the present SBA-15 samples is therefore tested by means of a suitable test mixture.     

The surface and materials science of tin oxide

The study of tin oxide is motivated by its applications as a solid state gas sensor material, oxidation catalyst, and transparent conductor. This review describes the physical and chemical properties that make tin oxide a suitable material for these purposes. The emphasis is on surface science studies of single crystal surfaces, but selected studies on powder and polycrystalline films are also incorporated in order to provide connecting points between surface science studies with the broader field of materials science of tin oxide. The key for understanding many aspects of SnO₂ surface properties is the dual valency of Sn. The dual valency facilitates a reversible transformation of the surface composition from stoichiometric surfaces with Sn⁴⁺ surface cations into a reduced surface with Sn²⁺ surface cations depending on the oxygen chemical potential of the system. Reduction of the surface modifies the surface electronic structure by formation of Sn 5s derived surface states that lie deep within the band gap and also cause a lowering of the work function. The gas sensing mechanism appears, however, only to be indirectly influenced by the surface composition of SnO₂. Critical for triggering a gas response are not the lattice oxygen concentration but chemisorbed (or ionosorbed) oxygen and other molecules with a net electric charge. Band bending induced by charged molecules cause the increase or decrease in surface conductivity responsible for the gas response signal. In most applications tin oxide is modified by additives to either increase the charge carrier concentration by donor atoms, or to increase the gas sensitivity or the catalytic activity by metal additives. Some of the basic concepts by which additives modify the gas sensing and catalytic properties of SnO₂ are discussed and the few surface science studies of doped SnO₂ are reviewed. Epitaxial SnO₂ films may facilitate the surface science studies of doped films in the future. To this end film growth on titania, alumina, and Pt(1 1 1) is reviewed. Thin films on alumina also make promising test systems for probing gas sensing behavior. Molecular adsorption and reaction studies on SnO₂ surfaces have been hampered by the challenges of preparing well-characterized surfaces. Nevertheless some experimental and theoretical studies have been performed and are reviewed. Of particular interest in these studies was the influence of the surface composition on its chemical properties. Finally, the variety of recently synthesized tin oxide nanoscopic materials is summarized.     

Solid state electrochemical reactions in systems with miscibility gaps

Cyclic voltammograms of electroactive solid compounds with partial immiscibility between the oxidized and reduced phases can exhibit a splitting of the peaks. If the free energy of transformation between the oxidized and reduced phases is small, the formal potentials of the redox pair will be almost the same in both solid phases. This results in an inert potential range in which no appreciable electrochemical activity is possible. The kinetic implications of this situation have been analysed in relation to the width of the miscibility gap. The diffusion of ions in the particle, which is hindered by the immiscibility, can proceed when a transition zone between the two phases exists in which the crystal structure is changed. If there is no such transition zone the voltammogram will display several spikes, which are caused by the collapse of concentration barriers at the sharp interfaces between the two phases in the mixed crystals. Received: 14 October 1999 / Accepted: 4 November 1999