Atom probe tomography (APT) of carbonate minerals

Atom probe tomography (APT) combines the highest spatial resolution with chemical data at atomic scale for the analysis of materials. For geological specimens, the process of field evaporation and molecular ion formation and interpretation is not yet entirely understood. The objective of this study is to determine the best conditions for the preparation and analysis by APT of carbonate minerals, of great importance in the interpretation of geological processes, focusing on the bulk chemical composition. Results show that the complexity of the mass spectrum is different for calcite and dolomite and relates to dissimilarities in crystalochemical parameters. In addition, APT bulk chemistry of calcite closely matches the expected stoichiometry but fails to provide accurate atomic percentages for elements in dolomite under the experimental conditions evaluated in this work. For both calcite and dolomite, APT underestimates the amount of oxygen based on their chemical formula, whereas it is able to detect small percentages of elemental substitutions in crystal lattices. Overall, our results demonstrate that APT of carbonate minerals is possible, but further optimization of the experimental parameters are required to improve the use of atom probe tomography for the correct interpretation of mineral geochemistry.     

Physical aspects of the pulsed laser deposition technique: The stoichiometric transfer of material from target to film

The physical processes of pulsed laser deposition (PLD) change strongly from the initial light absorption in a target to the final deposition and growth of a film. One of the primary advantages of PLD is the stoichiometric transfer of material from target to a film on a substrate. Even for a stoichiometric flow of material from a multicomponent target, the simultaneous arrival of the target atoms is not sufficient to ensure a stoichiometric film growth. The laser fluence has to be sufficiently high to induce ablation rather than pure evaporation from target, but a high fluence may lead to preferential (self)sputtering and possibly implantation of the light atoms in the film. A background gas of a sufficiently high pressure may reduce sputtering of the film, but may lead the preferential diffusion of the light component to the substrate. The importance of these processes during the entire PLD process will be discussed.     

Determination of the Li/Nb ratio in LiNbO3 crystals grown by Czochralski method with K2O added to the melt

Off-congruent lithium niobate single crystals were grown by the Czochralski method with K₂O added to the congruent melts in a concentration varying between 2 and 10.5 wt%. The Li/Nb ratio was determined by means of indirect methods such as the fundamental absorption edge positon, the IR vibrational spectrum, lattice parameters and the variation of the refractive indices. The data presented in this work show that the Li/Nb ratio can be controlled by the amount of K₂O added to the congruent melt. Moreover, the techniques employed for characterization are good tools for the determination of the Li/Nb ratio.     

Synthesis of tailored WO3 and WOx (2.9 < x < 3) nanoparticles by adjusting the combustion conditions in a H2/O2/Ar premixed flame reactor

Flame synthesis of WO₃ and WO_x (2.9 < x < 3) nanoparticles is carried out by adding a dilute concentration of WF₆ as precursor in a low-pressure H₂/O₂/Ar premixed flame reactor. The reactor is equipped with molecular-beam sampling and particle mass spectroscopy (PMS) to determine particle composition and sizes as a function of height above burner. Varying the H₂/O₂ ratio allowed us to tune the stoichiometry of the product. With a H₂/O₂ ratio of 0.67 white colored stoichiometric WO₃ is formed, whereas the H₂/O₂ ratio >0.8 yields blue colored non-stoichiometric WO_x (2.9 < x < 3) nanoparticles. The size of nanoparticles can be controlled by varying the residence time in the high-temperature zone of the reactor as observed by molecular-beam sampling with subsequent analysis using PMS. Transmission electron microscopy (TEM) images of as-synthesized nanoparticles show that particles are non-agglomerated and have an almost spherical morphology. The X-ray diffraction (XRD) pattern of the as-synthesized material indicates that the powders exhibit poor crystallinity, however, subsequent thermal annealing of the sample in air changes its structure from amorphous to crystalline phase. It is observed that particles with sub-stoichiometric composition (WO_x) show higher conductivity compared to the stoichiometric WO₃ sample.     

Determination of aqueous inclusion complexation constants and stoichiometry of alkyl(methyl)-methylimidazolium-based ionic liquid cations and neutral cyclodextrins by affinity capillary electrophoresis

Affinity CE (ACE) method was developed to characterize the complex formation between seven alkyl(methyl)methylimidazolium-based ionic liquid (IL) cations and eight neutral cyclodextrins (CD). The effective mobility data of the IL cations were processed according to classical nonlinear and linear treatments to obtain the complex stoichiometry and formation constant K. The majority of systems followed a 1:1 complexation stoichiometry model but in four cases a 1:2 stoichiometry was better satisfied. The K values obtained for each IL were compared to elucidate the main influences of IL and CD nature. The availability of these data should lend support to various application areas, including the screening and tailoring of new interactions in the solution for CE.     

Nuclear Magnetic Resonance Study of the Stoichiometry and Stability of Several Li+‐Crown Ether Complexes in Various Acetonitrile‐Nitrobenzene Mixtures

Lithium‐7 NMR spectrometry was used to study the complexation reaction between lithium ions and several 12‐, 15‐ and 18‐membered crown ethers in a number of binary acetonitrile‐nitrobenzene mixtures. Formation constants of the resulting 1:1 complexes in different solvent mixtures were determined by computer fitting of the chemical shift‐mole ratio data. There is an inverse relationship between the complex stability and the amount of acetonitrile in the mixed solvent. Among different sized crown ethers used, 15‐crowns were found to form the most stable Li⁺ complexes in the series. The influence of substitution on the macrocyclic rings on the stability of the resulting complexes is discussed.     

Characterization and cathode performance of Li1 − xNi1 + xO2 prepared with the excess lithium method

Samples of Li_{1 − z}Ni_{1 + x}O₂ with various x values were synthesized and their electrochemical properties, phase transitions, and ordering phenomena were investigated comparatively. In order to synthesize samples with a small x value, an excess lithium was used as a starting material to compensate for lithium loss during the calcination process. A stoichiometric sample with a large reversible capacity of more than 200 mAh g⁻¹ is also described.     

化学计量比和放氢背压对LiBH4+xMg2NiH4体系放氢性能的影响

研究了不同化学计量比(x=0.25, 0.5, 0.75, 1.0, 1.25)和放氢背压(1×10-4和0.4 MPa)对LiBH4+xMg2NiH4复合体系吸放氢性能的影响. 结果表明, 随着化学计量比(x)的增加, 复合体系的放氢温度逐渐降低, 放氢动力学性能得到提高, 但放氢容量逐渐降低; 其中, 在1×10-4和0.4 MPa初始放氢背压下, LiBH4+0.75Mg2NiH4体系具有最佳放氢动力学性能和较高的储氢容量. 结果表明, 放氢背压和化学计量比均会对高温下液相LiBH4 与固态Mg2NiH4 的润湿性产生影响, 进而影响复合体系的放氢路径和放氢动力学性能.     

Structure, intramolecular flexibility, and complexation of aza crown ethers to anions H2PO4 and HSO4 in nonprotic solvents

Both the structure and intramolecular flexibility of a series of aza crown ethers were studied by experimental NMR and theoretical molecular modeling. The stoichiometries of complexation to the anions H₂PO₄⁻ and HSO₄⁻ and resulting complex stabilities were determined by experimental NMR (¹H, ³¹P) titration and, in addition, the structure and mobility changes of the aza crown ethers upon complexation were also examined.