Direct ink writing of ceramic matrix composite structures

We present the development of an ink containing chopped fibers that is suitable for direct ink writing (DIW), enabling to obtain ceramic matrix composite (CMC) structures with complex shape. We take advantage of the unique formability opportunities provided by the use of a preceramic polymer as both polymeric binder and ceramic source. Inks suitable for the extrusion of fine filaments (<1 mm diameter) and containing a relatively high amount of fibers (>30 vol% for a nozzle diameter of 840 μm) were formulated. Despite some optimization of ink rheology still being needed, complex CMC structures with porosity of ~75% and compressive strength of ~4 MPa were successfully printed. The process is of particular interest for its ability to orient the fibers in the extrusion direction due to the shear stresses generated at the nozzle tip. This phenomenon was observed in the production of polymer matrix composites, but it is here employed for the first time for the production of ceramic matrix ones. The possibility to align high aspect ratio fillers using DIW opens the path to layer‐by‐layer design for optimizing the mechanical and microstructural properties within a printed object, and could potentially be extended to other types of fillers.     

Liquid chromatographic determination of caffeine and adrenergic stimulants in food supplements sold in Brazilian e-commerce for weight loss and physical fitness

Methyl-xanthines and adrenergic stimulants, such as caffeine and synephrine, are commonly added to food supplements due to their stimulating and thermogenic effects. In addition, the abusive consumption of food supplements with ergogenic and aesthetic purposes has been observed worldwide. This work describes the study of caffeine, p-synephrine, hordenine, octopamine, tyramine, ephedrine and salicin as stimulants in dietary supplements marketed in Brazil for weight loss and physical fitness claims. A total of 94 different products were acquired from 30 Brazilian websites. Thus, the sampling of marketed supplements was performed in virtual commerce (e-commerce) with claims of weight loss, appetite reduction, fat burning and metabolism acceleration. The developed analytical method involved the separation of the stimulants by HPLC with diode array detection (HPLC-DAD) by using a gradient elution of flow rate (0.7–2.5 ml min^?1) and mobile phase composition (0.1% H₃PO₄/methanol). The validated method was applied to the study of 46 dietary supplements. Caffeine, p-synephrine and ephedrine were found to be present as stimulants in 52% of the studied samples marketed as encapsulated or bulk forms. Caffeine was found to be present in concentrations that represent doses from 25.0 to 1476.7 mg day^–1. Synephrine was found in concentrations that represent doses from 59.1 to 127.0 mg day^–1. Ephedrine was found to be associated with caffeine in one formulation at a concentration representing a 26.1 mg day^–1 dosage.     

Social organization,constraints and opportunities for kitchen garden implementation: ScalA and ScalA-FS assessment tools in Morogoro and Dodoma,Tanzania

Tanzania is the second largest country in East Africa with about 50 million inhabitants in 2014, and it is also considered as one of the poorest countries in the world. The country strongly depends on agriculture production. Like many other poor countries in sub-Saharan Africa, Tanzania sees food security as a central part of development and poverty reduction efforts. This work aims to investigate the opportunities and constraints of implementing a policy of “Kitchen Gardens” as a practice for two regions of Tanzania. The research was carried out with a qualitative approach through a first round of semi-structured interviews using a Scaling up Assessment Tool (ScalA), and a second round with a questionnaire survey, using a Scaling up Assessment Tool for Food Security (ScalA-FS) by Tanzanian and German experts from the Trans-SEC project. The experts assessed implementation suitability and the institutional requirements of Kitchen Garden across the food value chains in two Tanzanian regions with different climate regions, namely Dodoma (semi-arid) and Morogoro (sub-humid). Kitchen Garden assessments did not differ significantly between these regions. The ScalA tools provided a range of statements that allowed an overview of the structural situation to be obtained, which could enable Kitchen Garden activity to be incentivized and scaled up. However, a number of specific aspects, potentials, challenges, and likely bottlenecks of implementation related to their feasibility and institutional requirements, were indicated, which should be carefully monitored during implementation. Adopting the recommended strategies could help to close gaps in implementation, enhance community empowerment and social network development, reduce food insecurity and improve the health of the communities.     

Quantum junction solar cells

Colloidal quantum dot solids combine convenient solution-processing with quantum size effect tuning, offering avenues to high-efficiency multijunction cells based on a single materials synthesis and processing platform. The highest-performing colloidal quantum dot rectifying devices reported to date have relied on a junction between a quantum-tuned absorber and a bulk material (e.g., TiO(2)); however, quantum tuning of the absorber then requires complete redesign of the bulk acceptor, compromising the benefits of facile quantum tuning. Here we report rectifying junctions constructed entirely using inherently band-aligned quantum-tuned materials. Realizing these quantum junction diodes relied upon the creation of an n-type quantum dot solid having a clean bandgap. We combine stable, chemically compatible, high-performance n-type and p-type materials to create the first quantum junction solar cells. We present a family of photovoltaic devices having widely tuned bandgaps of 0.6-1.6 eV that excel where conventional quantum-to-bulk devices fail to perform. Devices having optimal single-junction bandgaps exhibit certified AM1.5 solar power conversion efficiencies of 5.4%. Control over doping in quantum solids, and the successful integration of these materials to form stable quantum junctions, offers a powerful new degree of freedom to colloidal quantum dot optoelectronics.     

Hybrid passivated colloidal quantum dot solids

Colloidal quantum dot (CQD) films allow large-area solution processing and bandgap tuning through the quantum size effect. However, the high ratio of surface area to volume makes CQD films prone to high trap state densities if surfaces are imperfectly passivated, promoting recombination of charge carriers that is detrimental to device performance. Recent advances have replaced the long insulating ligands that enable colloidal stability following synthesis with shorter organic linkers or halide anions, leading to improved passivation and higher packing densities. Although this substitution has been performed using solid-state ligand exchange, a solution-based approach is preferable because it enables increased control over the balance of charges on the surface of the quantum dot, which is essential for eliminating midgap trap states. Furthermore, the solution-based approach leverages recent progress in metal:chalcogen chemistry in the liquid phase. Here, we quantify the density of midgap trap states in CQD solids and show that the performance of CQD-based photovoltaics is now limited by electron-hole recombination due to these states. Next, using density functional theory and optoelectronic device modelling, we show that to improve this performance it is essential to bind a suitable ligand to each potential trap site on the surface of the quantum dot. We then develop a robust hybrid passivation scheme that involves introducing halide anions during the end stages of the synthesis process, which can passivate trap sites that are inaccessible to much larger organic ligands. An organic crosslinking strategy is then used to form the film. Finally, we use our hybrid passivated CQD solid to fabricate a solar cell with a certified efficiency of 7.0%, which is a record for a CQD photovoltaic device.