Applications of analytical electron microscopy to guide the design of boron carbide

Compositional analysis of boron carbide on nanometer length scales to examine or interpret atomic mechanisms, for example, solid-state amorphization or grain-boundary segregation, is challenging. This work reviews advancements in high-resolution microanalysis to characterize multiple generations of boron carbide. First, ζ-factor microanalysis will be introduced as a powerful (scanning) transmission electron microscopy ((S)TEM) analytical framework to accurately characterize boron carbide. Three case studies involving the application of ζ-factor microanalysis will then be presented: (1) accurate stoichiometry determination of B-doped boron carbide using ζ-factor microanalysis and electron energy loss spectroscopy, (2) normalized quantification of silicon grain-boundary segregation in Si-doped boron carbide, and (3) calibration of a scanning electron microscope X-ray energy-dispersive spectroscopy (XEDS) system to measure compositional homogeneity differences of B/Si-doped arc-melted boron carbides in the as-melted and annealed conditions. Overall, the improvement and application of advanced analytical tools have helped better understand processing–microstructure–property relationships and successfully manufacture high-performance ceramics.     

Manufacturing catalyst-coated membranes by ultrasonic spray deposition for PEMFC: Identification of key parameters and their impact on PEMFC performance

This study deals with the manufacturing of catalyst-coated membranes (CCMs) for newcomers in the field of coating. Although there are many studies on electrode ink composition for improving the performance of proton-exchange membrane fuel cells (PEMFCs), there are few papers dealing with electrode coating itself. Usually, it is a know-how that often remains secret and constitutes the added value of scientific teams or the business of industrialists. In this paper, we identify and clarify the role of key parameters to improve coating quality and also to correlate coating quality with fuel cell performance via polarization curves and electrochemical active surface area measurements. We found that the coating configurations can affect the performance of lab-made CCMs in PEMFCs. After the repeatability of the performance obtained by our coating method has been proved, we show that: (i) edge effects, due to mask shadowing - cannot be neglected when the active surface area is low, (ii) a heterogeneous thickness electrode produces performance lower than a homogeneous thickness electrode, and (iii) the origin and storage of platinum on carbon powders are a very important source of variability in the obtained results.     

Thermal properties of hierarchical YSZ and LZO ceramic microspheres with multi-scaled voids investigated by using theoretical,experimental and simulation methods

Heat transfer within ceramic feedstock powders is still unclear, which impedes optimization of the thermal and mechanical properties of the thermal sprayed coatings. The microspheres (yttria-stabilized zirconia YSZ and lanthanum zirconate LZO) were prepared via the electro-spraying assisted phase inversion method (ESP). The thermal properties of the two ESP microspheres and a commercial hollow spherical powder (HOSP) were investigated by using theoretical, experimental, and simulation methods. Thermal conductivity of the single microsphere was estimated via a novel nest model that was derived from the Maxwell-Eucken 1 and the EMT model. Thermal conductivity of a single YSZ/LZO-ESP microsphere prepared at 1100–1200 °C was within 0.36–0.75 W/m K, which was ～ 20 % lower than that of a single YSZ-HOSP microsphere with a similar porosity. Heat flux simulation showed that high tortuosity around the multi-scaled voids of the ESP microsphere led to a more efficient decrease in thermal conductivity compared with total porosity.     

红外辐射联合回火与冷激杀菌技术对胡萝卜粉品质的影响

首先研究不同红外辐射温度(100,110,120℃)及辐射时间(2.5,5,10 min)对胡萝卜粉微生物及品质的影响,然后根据栅栏效应原理研究红外辐射-回火、红外辐射-冷激联合杀菌对胡萝卜粉微生物、色调值、类胡萝卜素含量等品质的影响。结果表明:100℃、10 min的红外辐射处理使细菌和真菌分别降低了1.9 lg(CFU/g)和2.32 lg(CFU/g),110℃、5 min的红外辐射处使细菌和真菌分别降低了1.58 lg(CFU/g)和2.57 lg(CFU/g)。在上述两种处理条件下胡萝卜粉的水分活度从0.238分别降至0.123和0.147,胡萝卜粉中总类胡萝卜素含量从308.8μg/g降至227.8μg/g和238.8μg/g,色差值(ΔE)为9.11和7.89。与红外辐射单独作用相比,联合回火后的处理没有显著影响细菌数目,处理后保持在5.40~5.80 CFU/g,霉菌、酵母数却在处理过程中显著减少,然而减少量较低,总数仍不低于4.5 lg(CFU/g)。红外辐射-冷激联合处理相比红外辐射单独处理,100℃、10 min联合冷激7 d处理可将细菌数量降低0.25 lg(CFU/g),可将霉菌与酵母数量降低0.28 lg(CFU/g),110℃、5 min联合冷激7 d处可将细菌数量降低0.26 lg(CFU/g),可将霉菌、酵母数量降低0.40 lg(CFU/g),且这些处理下胡萝卜粉的色调值、水分活度、类胡萝卜素含量变化不显著。本试验结果表明,红外辐射-冷激处理具有协同效应,且处理过程中胡萝卜粉的色调值及总类胡萝卜素含量不受影响,这为低水分粉体食品红外辐射联合杀菌提供了参考。     

Synthesis,characterization and adsorption studies of h-BN crystal for efficient removal of Cd2+ from aqueous solution

In the present study, hexagonal boron nitride (h-BN) was synthesized from boric acid and melamine by thermal annealing method in a nitrogen atmosphere. The pure h-BN was used as an efficient sorbent for the uptake of Cd²⁺ ions from the solution phase. The kinetics and sorption studies of metal ions onto the h-BN were carried out in batch adsorption experiments at different temperature, time, pH, sorbent dosage, and concentration of metal ions. The optimum pH for the removal of the Cd²⁺ ions was found to be pH 7. The effect of temperature showed that the process of Cd²⁺ sorption remained endothermic in the range of 298 K–328 K. The Lagergren's first and Ho's second kinetic models were tested to interpret the adsorption kinetic data, however the present data was explained well by Ho's model for kinetics. The thermodynamic perameters ΔG, ΔS and ΔH were determined using the available adsorption data at different temperatures. The physicochemical properties of the synthesized product were also characterized before and after adsorption by different analytical techniques like FT-IR, TGA, XRD and Point of Zero Charge (PZC). The morphology of the surface was analyzed with the help of Scanning Electron Microscopy. The h-BN proved to be an efficient adsorbent for the uptake of the Cd²⁺ ions from aqueous media.     

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.     

Research on the effects of surface modification of ceramic powder on cure performance during digital light processing (DLP)

Digital light processing (DLP) is one of the most important additive manufacture technologies to fabricate ceramic parts with complex geometries. Compared with pure photosensitive resin, the cure performance of ceramic suspensions is obviously different due to the optical property change after the addition of ceramic powders. In this paper, a unique oxidation process was used to modify the optical properties of nitride powders including AlN and Si₃N₄. The properties of oxidized ceramics were investigated and the cure performance of ceramic suspensions was then characterized. The effect of oxidation time on cure performance was evaluated. The results showed that for AlN, oxidation process leads to the smaller cure depth and smaller excess cure width as compared with non-oxidized AlN and for Si₃N₄, oxidation process leads to the larger cure depth and larger excess cure width as compared with non-oxidized Si₃N₄, indicating that both refractive index and light absorbance of ceramic powders have obvious effects on cure behaviors. Additionally, the cure behavior of oxidized ceramic suspension in this study shows that the relationship of cure depth vs. incident energy agrees well with Beer- Lambert model, but the excess cure width vs. incident energy is not consistent with quasi Beer-Lambert model due to the nature of digital micromirror device (DMD).     

Merger of Energetic Affinity and Optimal Geometry Provides New Class of Boron Nitride Based Sorbents with Unprecedented Hydrogen Storage Capacity

Hydrogen is an ideal synthetic fuel because it is lightweight, abundant and its oxidation product (water) is environmentally benign. However, its utilization is impeded by the lack of an efficient storage device. A new building block approach is proposed for an exhaustive search of optimal hydrogen uptakes in a series of low density boron nitride (BN) nanoarchitectures via extensive 3868 ab initio‐based multiscale simulations. By probing various geometries, temperatures, pressures, and doping ratios, these results demonstrate a maximum uptake of 8.65 wt% at 300 K, the highest hydrogen uptake on sorbents at room temperature without doping. Li⁺ doping of the nanoarchitectures offers a set of optimal combinations of gravimetric and volumetric uptakes, surpassing the US Department of Energy targets. These findings suggest that the merger of energetic affinity and optimal geometry in BN building blocks overcomes the intrinsic limitations of sorbent materials, putting hybrid BN nanoarchitectures on equal footing with hydrides while demonstrating a superior capacity‐kinetics–thermodynamics relationship.     

Development of the method of production of the ultrafine macrohomogeneous composite powder

ABSTRACT

A method of ultrafine macro-homogeneous composite powder – B₄C–ZrO₂ production using a planetary mill was developed. From the macro-homogeneous composite high-density ceramics, B₄C–ZrB₂ was produced by the method of reactive sintering (in situ) at 2000°C under the pressure of 41–42?MPa. The effect of ZrO₂ grain size and of its distribution in the matrix on the consolidation parameters, and the microstructure of the obtained ceramics was studied.     

Gel Polymer Electrolyte with Anion‐Trapping Boron Moieties via One‐Step Synthesis for Symmetrical Supercapacitors

A novel gel polymer electrolyte (GPE) which is based on new synthesized boron‐containing monomer, benzyl methacrylate, 1 m LiClO₄/N,N‐dimethylformamidel liquid electrolyte solution is prepared through a one‐step synthesis method. The boron‐containing GPE (B‐GPE) not only displays excellent mechanical behavior, favorable thermal stability, but also exhibits an outstanding ionic conductivity of 2.33 mS cm^?1 at room temperature owing to the presence of anion‐trapping boron sites. The lithium ion transference in this gel polymer film at ambient temperature is 0.60. Furthermore, the symmetrical supercapacitor which is fabricated with B‐GPE as electrolyte and reduced graphene oxide as electrode demonstrates a broad potential window of 2.3 V. The specific capacitance of symmetrical B‐GPE supercapacitors retains 90% after 3000 charge–discharge cycles at current density of 1 A g^?1.