NMR crystallography of amino acids.

The development of NMR crystallography methods requires a reliable database of chemical shifts measured for systems with known crystal structure. We measured and assigned carbon and hydrogen chemical shifts of twenty solid natural amino acids of known polymorphic structure, meticulously determined using powder X-ray diffraction. We then correlated the experimental data with DFT-calculated isotropic shieldings. The small size of the unit cell of most amino acids allowed for advanced computations using various families of DFT functionals, including generalized gradient approximation (GGA), meta-GGA and hybrid DFT functionals. We tested several combinations of functionals for geometry optimizations and NMR calculations. For carbon shieldings, the widely used GGA functional PBE performed very well, although an improvement could be achieved by adding shielding corrections calculated for isolated molecules using a hybrid functional. For hydrogen nuclei, we observed the best performance for NMR calculations carried out with structures optimized at the hybrid DFT level. The high fidelity of the calculations made it possible to assign additional signals that could not be assigned based on experiments alone, for example signals of two non-equivalent molecules in the unit cell of some of the amino acids.

A copper-containing analog of the biomineral whitlockite: dissolution-precipitation synthesis, structural and biological properties.

In the present work, copper whitlockite (Cu-WH, Ca18Cu2(HPO4)2(PO4)12) was successfully synthesized and comprehensively characterized, founding the base knowledge for its future studies in medicine, particularly for bone regeneration. This material is a copper-containing analog of the well-known biomineral magnesium whitlockite (Mg-WH, Ca18Mg2(HPO4)2(PO4)12). The synthesis of powders was performed by a dissolution-precipitation method in an aqueous medium under hydrothermal conditions. Phase conversion from brushite (CaHPO4·2H2O) to Cu-WH took place in an acidic medium in the presence of Cu2+ ions. Optimization of the synthesis conditions in terms of medium pH, temperature, time, Ca/Cu molar ratio and concentration of starting materials was performed. The crystal structure of the synthesized products was confirmed by XRD, FTIR and Raman spectroscopy, 1H and 31P solid-state NMR, and EPR. Morphological features and elemental distribution of the synthesized powders were studied by means of SEM/EDX analysis. The ion release in SBF solution was estimated using ICP-OES. Cytotoxicity experiments were performed with MC3T3-E1 cells. The study on thermal stability revealed that the synthesized material is thermally unstable and gradually decomposes upon annealing to Cu-substituted ß-Ca3(PO4)2 and Ca2P2O7.

Homogeneous Precipitation of Lanthanide Oxalates.

Oxalic acid is an important separation agent in the technology of lanthanides, actinides, and transition metals. Thanks to the low solubility of the oxalate salts, the metal ions can be easily precipitated into crystalline material, which is a convenient precursor for oxide preparation. However, it is difficult to obtain oxalate monocrystals due to the fast precipitation. We have developed a synthetic route for homogeneous precipitation of oxalates based on the thermal decomposition of oxamic acid. This work primarily concerns lanthanide oxalates; however, since no information was found about oxamic acid, a brief characterization was included. The precipitation method was tested on selected elements (Ce, Pr, Gd, Er, and Yb), for which the kinetics was determined at 100 °C. Several scoping tests at 90 °C or using different starting concentrations were performed on Ce and Gd. The reaction products were studied by means of solid-state analysis with focus on the structure and morphology. Well-developed microcrystals were successfully synthesized with the largest size for gadolinium oxalate.

Ultrahigh Temperature Flash Sintering of Binder-Less Tungsten Carbide within 6 s.

We report on an ultrarapid (6 s) consolidation of binder-less WC using a novel Ultrahigh temperature Flash Sintering (UFS) approach. The UFS technique bridges the gap between electric resistance sintering (≪1 s) and flash spark plasma sintering (20-60 s). Compared to the well-established spark plasma sintering, the proposed approach results in improved energy efficiency with massive energy and time savings while maintaining a comparable relative density (94.6%) and Vickers hardness of 2124 HV. The novelty of this work relies on (i) multiple steps current discharge profile to suit the rapid change of electrical conductivity experienced by the sintering powder, (ii) upgraded low thermal inertia CFC dies and (iii) ultra-high consolidation temperature approaching 2750 °C. Compared to SPS process, the UFS process is highly energy efficient (≈200 times faster and it consumes ≈95% less energy) and it holds the promise of energy efficient and ultrafast consolidation of several conductive refractory compounds.

Speedy bioceramics: Rapid densification of tricalcium phosphate by ultrafast high-temperature sintering.

Due to unique osteogenic properties, tricalcium phosphate (TCP) has gained relevance in the field of bone repair. The development of novel and rapid sintering routes is of particular interest since TCP undergoes to high-temperature phase transitions and is widely employed in osteoconductive coatings on thermally-sensitive metal substrates. In the present work, TCP bioceramics was innovatively obtained by Ultrafast High-temperature Sintering (UHS). Ca-deficient hydroxyapatite nano-powder produced by mechanochemical synthesis of mussel shell-derived calcium carbonate was used to prepare the green samples by uniaxial pressing. These were introduced within a graphite felt which was rapidly heated by an electrical current flow, reaching heating rates exceeding 1200 °C min-1. Dense (> 93%) ceramics were manufactured in less than 3 min using currents between 25 and 30 A. Both ß and α-TCP were detected in the sintered components with proportions depending on the applied current. Preliminary tests confirmed that the artifacts do not possess cytotoxic effects and possess mechanical properties similar to conventionally sintered materials. The overall results prove the applicability of UHS to bioceramics paving the way to new rapid processing routes for biomedical components.

Preparation of High-Entropy (Ti, Zr, Hf, Ta, Nb) Carbide Powder via Solution Chemistry.

High-entropy ceramics is a new class of materials having a great potential and wide application. The carbide of Ti, Zr, Hf, Ta, Nb is a typical member of this group. It has been synthesized mostly through blending, milling, and high-temperature solid-state reaction of metal carbide precursors for each metal. This route needs extremely high temperature (2300 °C), which makes it energy and technology demanding. We have developed a chemical route for high-entropy carbide powder that needs a synthetic temperature that is several hundred degrees Celsius lower. A solution of desired metal citrates with an excess of citric acid was converted into a metal oxide/active carbon nanocomposite. Starting from a solution enabled ideal mixing of precursors on a molecular level, allowing us to skip any milling and blending steps. The nanocomposite was treated in vacuum at 1600 °C, giving a phase-pure high-entropy carbide. The intermediate compounds and products were characterized by means of solid-state analysis.

Poly[oligo(ethylene glycol) methacrylate]-b-poly[(vinyl benzyl trimethylammonium chloride)] Based Multifunctional Hybrid Nanostructures Encapsulating Magnetic Nanoparticles and DNA.

We report on the preparation of novel and multifunctional hybrid spherical-shaped nanostructures involving a double-hydrophilic block copolymer, namely the neutral cationic poly[oligo(ethylene glycol) methacrylate]-b-poly[(vinyl benzyl trimethylammonium chloride)] (POEGMA-b-PVBTMAC) diblock copolymer, initially complexed with hydrophilic anionic magnetic nanoparticles (MNPs), and subsequently, with short deoxyribonucleic acid (113 bases DNA). The POEGMA-b-PVBTMAC copolymer, the copolymer/MNPs and the copolymer/MNPs/DNA tricomponent hybrid electrostatic complexes were studied by dynamic/electrophoretic light scattering (DLS/ELS) and cryogenic transmission electron microscopy (cryo-TEM) techniques for the determination of their structure and solution properties. The MNPs were complexed efficiently with the oppositely charged diblock chains, leading to well-defined hybrid organic-inorganic spherical-shaped nanostructures. A significant aggregation tendency of the MNPs is noticed in cryo-TEM measurements after the electrostatic complexation of DNA, implying an accumulation of the DNA macromolecules on the surface of the hybrid tricomponent complexes. Magnetophoretic experiments verified that the MNPs maintain their magnetic properties after the complexation initially with the copolymer, and subsequently, within the block polyelectrolyte/MNPs/DNA nanostructures.

Cerium Oxalate Morphotypes: Synthesis and Conversion into Nanocrystalline Oxide.

Cerium dioxide is a scientifically and technologically important material with a wide range of potential applications, particularly in solid oxide fuel cells and catalysis. Herein, we report a study focusing on the synthesis of nanocrystalline cerium dioxide via thermal decomposition of the oxalate salt. Simply by changing reaction conditions (temperature, concentration, acidity, strike) during the precipitation of the cerium solution with oxalic acid, we were able to obtain different morphologies. The main reaction parameters were mapped and linked to the morphology of the final products. Additionally, it was proved that oxalate precipitation is a robust reaction proceeding at relatively extreme contitions. Moreover, the conversion of cerium oxalate to nanocrystalline oxide was followed to monitor the progress of the reaction, the texture evolution, and the grain growth. The results showed that, for unvaried heating cycle, the grain size of converted material is linked to morphology. The thinner was the original microcrystal the smaller were the CeO2 nanocrystals after calcination. In addition, the grains were found smaller near the edges of the ex-oxalate microcrystals. In both cases, this behavior results from asymmetrical limitations of diffusion during grain growth.

On the Role of the Electrical Field in Spark Plasma Sintering of UO2+x.

The electric field has a large effect on the stoichiometry and grain growth of UO2+x during Spark Plasma Sintering. UO2+x is gradually reduced to UO2.00 as a function of sintering temperature and time. A gradient in the oxidation state within the pellets is observed in intermediate conditions. The shape of the gradient depends unequivocally on the direction of the electrical field. The positive surface of the pellet shows a higher oxidation state compared to the negative one. An area with larger grain size is found close to the positive electrode, but not in contact with it. We interpret these findings with the redistribution of defects under an electric field, which affect the stoichiometry of UO2+x and thus the cation diffusivity. The results bear implications for understanding the electric field assisted sintering of UO2 and non-stoichiometric oxides in general.