Phase and structural evolution of zirconolite ceramics prepared by solid-state reaction sintering

The evolution of phase assemblage and microstructure of stoichiometric zirconolite (CaZrTi₂O₇) ceramics, prepared by a solid-state reaction sintering route, was systematically investigated as a function of sintering temperature. Using powder XRD and quantitative phase analysis data, it was determined that the formation of zirconolite was a one-step reaction, without formation of intermediate phases. The accompanying fractions of secondary CaTiO₃ and ZrO₂ phases were reduced to approximately 2 wt % each after sintering at 1200 °C, with zirconolite formed as the major phase (> 99 wt%) after reaction at 1300 °C. Notable product densification only occurred at T ≥ 1400 °C, at which it was possible to achieve a relative density of 96.97% which is highly desirable for applications as a nuclear wasteform. The zirconolite-2M polytype structure (space group: C2/c) was formed in all products as expected, confirmed by combined high resolution TEM-ED analyses.     

A general method to determine optimal thermal cycles based on solid-state sintering fundamentals

Most technical ceramics require processing up to and including final-stage sintering to obtain a high-density bulk while inhibiting grain growth as dominant sintering process as far as possible. The literature typically highlights the qualitative interdependence of the sintering variables and microstructural parameters, focusing on very simple particulate systems. However, a quantitative method to achieve optimum sintering of actual polycrystalline solids is still lacking.This paper puts forward such a method, which has been satisfactorily tested by the authors. The method consists of a mathematical model, based on the physical phenomena that take place during solid-state sintering. The method leads to two differential equations: a densification rate and a pore-dragged normal grain growth rate equation during final-stage sintering, which mainly depend on sintering temperature and shaping conditions. Simultaneous numerical integration of these two rate equations allows design of an optimal thermal cycle (enhancing densification and controlling grain growth) to obtain the targeted sintered polycrystalline microstructure. Application of this method yields staggered thermal cycles, in addition to the number of steps, as well as the sintering temperature and dwell time in each step.     

Gd2Zr2O7 ceramics synthesized by solid-state reactive sintering: Effects of starting powders with different-scale particle sizes

In this work, the effects of starting oxide powders with different-scale particle sizes on the synthesis of gadolinium zirconate pyrochlore (Gd₂Zr₂O₇, GZO) and its physical properties were studied. Micron Gd₂O₃ (μG), micron ZrO₂ (μZ), nano Gd₂O₃ (nG), and nano ZrO₂ (nZ) powders were used. GZO ceramics were prepared by employing solid-state reactive sintering at 1300 °C, 1400 °C, 1500 °C and 1600 °C with mixed powders of different sizes (μGμZ, μGnZ, nGμZ and nGnZ). X-ray diffraction and Raman analyses of the ceramics revealed that nG has a more significant impact on the crystallization process than nZ. All ceramics synthesized with different sized oxide powders crystallized into pyrochlore phases except for those synthesized with μGnZ mixed powders, which resulted in a fluorite phase. The results indicated that decreasing the particle size of only ZrO₂ to synthesize pyrochlore-phase Gd₂Zr₂O₇ with high crystallinity may not be effective. Samples obtained at 1500 °C were further analyzed. Scanning electron microscopy results revealed that all four ceramics have a non-homogeneous grain size and that the average grain size ranges from 5.40 to 8.30 μm. In addition, the density and Vickers hardness measurements showed that the use of nanopowders significantly improves the mechanical properties.     

The dynamic evolution of aggregated lithium dendrites in lithium metal batteries

Lithium (Li) metal anodes promise an ultrahigh theoretical energy density and low redox potential,thus being the critical energy material for next-generation batteries.Unfortunately,the formation of Li den-drites in Li metal anodes remarkably hinders the practical applications of Li metal anodes.Herein,the dynamic evolution of discrete Li dendrites and aggregated Li dendrites with increasing current densities is visualized by in-situ optical microscopy in conjunction with ex-situ scanning electron microscopy.As revealed by the phase field simulations,the formation of aggregated Li dendrites under high current den-sity is attributed to the locally concentrated electric field rather than the depletion of Li ions.More specif-ically,the locally concentrated electric field stems from the spatial inhomogeneity on the Li metal surface and will be further enhanced with increasing current densities.Adjusting the above two factors with the help of the constructed phase field model is able to regulate the electrodeposited morphology from aggregated Li dendrites to discrete Li dendrites,and ultimately columnar Li morphology.The methodol-ogy and mechanistic understanding established herein give a significant step toward the practical appli-cations of Li metal anodes.     

Dynamical coupling of ion adsorption with fluid flow in nanopores

To describe the commonly existing coupling between adsorption dynamics and fluid flow, a nonequilibrium molecular model is developed, upon which we systematically investigate the dynamical adsorption of ionic components from confined flows onto the charged surfaces of nanoscale pores, and find that a competition relation exists between the adsorption and flow. Promoting flow speed suppresses the adsorption amount, while enhancing adsorption strength reduces the flow speed. With the increase of flow speed, the contact density of co-ion is enhanced while that of counterion is suppressed, leading to overall enhanced accumulation charge densities at pore surfaces. Besides, the accumulation charge density increases monotonically with the applied voltage in large pores, while displays a nontrivial relation with the applied voltage in small pores of several ion sizes. This work not only extends the theoretical framework of nonequilibrium molecular theories, but also provides novel insights into the regulation of interfacial dynamic processes.     

Three-dimensional phase-field study of grain coarsening and grain shape accommodation in the final stage of liquid-phase sintering

A 3-dimensional phase-field model is implemented to simulate the grain evolution in the final stage of liquid-phase sintering. The model considers a liquid phase and a polycrystalline solid phase. Results for varying ratios of the solid–solid interface energy to solid–liquid interface energy and varying solid volume fractions are presented. A variety of microstructures, from fully connected grain structures with liquid pockets at the grain junctions to individual grains fully wetted by the liquid matrix, is seen. The 3 main mechanisms for particle shape accommodation, namely, contact flattening, Ostwald ripening and particle bonding, are reproduced in the simulations. The solid volume fraction, particle size distribution, contiguity, connectivity, particle–particle contact areas and the number of particle contacts per particle are measured as a function of time. The exponent in the power growth law varies between 2.4, for the fully connected grain structures, and 3, for the completely wetted grains.     

Effects of different force fields and temperatures on the structural character of abeta (12-28) Peptide in aqueous solution

The aim of this work is to investigate the effects of different force fields and temperatures on the structural character of Aβ (12-28) peptide in aqueous solution. Moreover, the structural character of Aβ (12-28) peptide is compared with other amyloid peptides (such as H1 and α-syn12 peptide). The two independent temperature replica exchange molecular dynamics (T-REMD) simulations were completed by using two different models (OPLS-AA/TIP4P and GROMOS 43A1/SPC). We compared the models by analyzing the distributions of backbone dihedral angles, the secondary structure propensity, the free energy surface and the formation of β-hairpin. The results show that the mostly populated conformation state is random coil for both models. The population of β-hairpin is below 8 percent for both models. However, the peptide modeled by GROMOS 43A1 form β-hairpin with turn located at residues F19-E22, while the peptide modeled by OPLS-AA form β-hairpin with turn located at residues L17-F20.     

Linear driving force approximation in cyclic adsorption processes: Simple results from system dynamics based on frequency response analysis

The linear driving force (LDF) approximation for cyclic adsorptive processes is discussed on the basis of model equivalence with the homogeneous diffusion model (HDM), the pore diffusion model (PDM) and the intraparticle diffusion and convection model (IDCM). Model equivalence is based on the frequency response of the adsorbent particle, namely on the equality of the amplitude ratio and the phase-lag functions. The analysis of the continuous stirred tank adsorber (CSTA) and of the plug flow adsorber (PFA) is addressed.     

Block copolymers in electric fields

The structural versatility of block copolymers on the nanometer scale make them highly promising candidates for many applications in soft matter nanotechnology, including optics, electronics, and acoustics. In order to harvest the full potential of nanostructured block copolymer materials and achieve widespread use outside of academia, adaptable strategies are required to control and manipulate their spatial orientation, periodicity, connectivity, and long-range order. Over the past two decades the use of an external electric field has been well established as a viable tool to control a wide variety of structural parameters of nanostructured block copolymers on both mesoscopic and nanoscopic length scales. Covering a wide range of experimental and theoretical work, this review aims to illustrate major scientific advances of the past years, focusing in particular on the underlying physics that governs the fundamental interactions between an external electric field and block copolymer mesophases and its impact on phase behaviour and orientational order in bulk, solution, and thin films.