Ideal Stoichiometric Composition Research Articles

(Invited) Understanding the Impact of the Stoichiometry on Cation-Mixing in Ni-Rich Cathodes

Although LiNiO2 is chemically similar to LiCoO2 and offers a nearly identical theoretical capacity, LiNiO2 and related Co-free Ni-rich cathode materials suffer from degradation during electrochemical cycling. Especially the formation of Li/Ni anti-site defects can lead to a reduced cyclability and poor capacity retention. We report evidence that such cation mixing in LiNiO2 is linked to the difficulty of synthesizing the material in its ideal stoichiometric composition. In practice, off-stoichiometries (Li1-xNi1+xO2) can hardly be avoided. We determined the Ni migration mechanism in ideal and off-stoichiometric LiNiO2 using first-principles calculations and identified the energetically most probable pathways for Ni migration and for the phase transition to the spinel structure. Our results show that off-stoichiometries promote the formation of Li/Ni defects. Furthermore, we find that the formation of the spinel phase is a highly concerted mechanism, explaining why the phase transition is usually not observed during cycling at room temperature. Our study is a first step towards understanding degradation in Co-free cathodes and underlines the importance of considering defects in materials modeling.

Growth and Characterization of Vacuum Annealing AgCuInSe2 Thin Film

The influence of annealing on quaternary compound Ag0.9Cu0.1InSe2 (ACIS) thin film is considered a striking semiconductor for second-generation solar cells. The film deposited by thermal evaporation with a thickness of about 700 nm at R.T and vacuum annealing at temperatures (373,473) K for 1 hour. It was deposited in a vacuum of 4.5*10-5 Torr on a glass substrate. From XRD and AFM analysis, it is evident that Ag0.9Cu0.1InSe2 films are polycrystalline in nature, having ideal stoichiometric composition. Structural analysis indicated that annealing the films following the deposition resulted in the increasing polycrystalline phase with the preferred orientation along (112) direction. , increasing crystallite size and average grain size after annealing . The optical properties of these films are determined by the wavelength between 400 nm – 1000 nm. The band gap of the Ag0.9Cu0.1InSe2 films was evaluated to be (1.68-1.5) eV, and the optical constants calculated, such as the refractive index, the extinction coefficient, and the real and imaginary parts of the dielectric constant.

Mechanism of stress- and thermal-induced fct → hcp → fcc crystal structure change in a TiAl-based alloy compressed at elevated temperature

The mechanism of stress- and thermal-induced fct → hcp → fcc crystal structure change in a TiAl-based alloy was investigated via experiments and molecular dynamics simulations. According to the experimental results, laths occurring in γ-phase grains were identified as γ-phase twins, and increasing strain promoted the γ → α2 (fct → hcp) phase transformation when the alloy was deformed at 1150 °C. A large number of ultrafine γ lamellae precipitated when deformed at 1200 °C, which was attributed to the thermal-mechanical coupling. The atomic mechanisms of the α2(α) → γlamella phase transformations were investigated using molecular dynamics simulations. The results showed that the generalised stacking fault energy along different directions (<1‾010>) exhibited remarkable anisotropy on the basal plane (0001) of the hcp crystal structure, regardless of the ordering or disordering phase. The disordering of the hcp crystal structure significantly affected the formation of the γ lamellae. Regardless of the ideal stoichiometric or off-stoichiometric composition, the stress-induced hcp → fcc crystal structure transformation occurred more easily in the disordered phases. Grain-boundary nucleation dominated the precipitation of the ultrafine γ lamellae. In addition, intragranular nucleation of γ lamellae was observed, which was mainly attributed to the activation and movement of the Shockley partial dislocation loop with a Burgers vector of 1/6<1‾010> whose formation was not directly related to the dissociation of the superpartial dislocation 1/6<112‾0>.

Crystal Chemistry and Structural Variations for Zircon Samples from Various Localities

This study investigates the variations of structural parameters and chemistry of a partially metamict and seven detrital zircon samples from different localities using single-crystal X-ray diffraction, synchrotron high-resolution powder X-ray diffraction, and electron-probe micro-analysis techniques. The unit-cell parameters for the eight zircon samples vary linearly with increasing unit-cell volume, V. A zircon sample from the Canadian Arctic Islands has the smallest unit-cell parameters, bond distances, ideal stoichiometric composition, unaffected by α-radiation damage, and is chemically pure. A zircon sample from Jemaa, Nigeria has the largest unit-cell parameters because of the effect of α-radiation doses received over a long time (2384 Ma). All the samples show good correlations between Zr and Si apfu (atom per formula unit) versus unit-cell volume, V. The α-radiation doses in the samples are lower than ~3.5 × 1015 α-decay events/mg. Substitutions of other cations at the Zr and Si sites control the variations of the structural parameters. Relatively large unit-cell parameters and bond distances occur because the Zr site accommodates other cations that have larger ionic radii than the Zr atom. Geological age increases the radiation doses in zircon and it is related to V.

Plasma resistance of sintered and ion-plated yttrium oxyfluorides with various Y, O, and F composition ratios for use in plasma process chamber

Yttrium oxyfluoride was developed for use in the plasma process chamber using various corrosive gases. In this paper, sintered yttrium oxyfluorides with various Y, O, and F composition ratios (YxOyFz) and ion-plated YOF and Y5O4F7 films were prepared, and the physical etching behavior due to Ar ion bombardment and NF3/Ar plasma resistance was investigated. It was found that the etching rate of the sintered yttrium oxyfluoride due to the bombardment of Ar ions with an energy of 500 eV decreased as the oxygen composition ratio in the samples decreased, i.e., F-rich yttrium oxyfluoride had better resistance against energetic-ion bombardment. It was also found that the surface roughness of sintered YOF and Y5O4F7, both of which had the stable phases, was much smoother after Ar ion bombardment than that of yttrium oxyfluorides without the stable phase. NF3/Ar plasma resistance was also investigated. For the sintered yttrium oxyfluoride, both YOF and Y5O4F7 showed good resistance against the NF3/Ar plasma, where the ideal stoichiometric atomic composition ratio could be kept even after plasma irradiation. For both as-deposited ion-plated YOF and Y5O4F7 films, the F composition ratio was slightly smaller than the ideal stoichiometric ratios, suggesting that some amount of fluorine was escaped from the starting materials of YOF and/or Y5O4F7 during the ion-plated film deposition processes. After NF3/Ar plasma irradiation, F composition ratios were increased for both ion-plated YOF and Y5O4F7 films, and the atomic composition ratio becomes closer to the ideal stoichiometric ratios for both films.

Ab initioexplanation of disorder and off-stoichiometry in Fe-Mn-Al-Cκcarbides

Carbides play a central role for the strength and ductility in many materials. Simulating the impact of these precipitates on the mechanical performance requires the knowledge about their atomic configuration. In particular, the C content is often observed to substantially deviate from the ideal stoichiometric composition. In the present work, we focus on Fe-Mn-Al-C steels, for which we determined the composition of the nano-sized kappa carbides (Fe,Mn)3AlC by atom probe tomography (APT) in comparison to larger precipitates located in grain boundaries. Combining density functional theory with thermodynamic concepts, we first determine the critical temperatures for the presence of chemical and magentic disorder in these carbides. Secondly, the experimentally observed reduction of the C content is explained as a compromise between the gain in chemical energy during partitioning and the elastic strains emerging in coherent microstructures.

Thermomechanical and large deformation behaviors of antiplasticized epoxy resins: Effect of material formulation and network architecture

A series of amine‐hardened epoxies were prepared by systematically varying the stoichiometric ratio and the relative amounts of diepoxide and monoepoxide resin to chemically control the material composition and macromolecular architecture (chain segments flexibility; crosslink density; amounts of dangling groups). Positron Annihilation Lifetime Spectroscopy was used to investigate the free volume associated to the various epoxy formulations, while dynamic‐mechanical analysis was employed to investigate their network density and their primary and secondary mechanical relaxations. The mechanical behavior at small and large strain was studied by means of tensile and compression tests. The results pointed out that deviations from the ideal stoichiometric composition and the addition of monoepoxide resins lead to significant room temperature stiffening, together with a reduction of the crosslink density and glass transition temperature. This behavior, phenomenologically associated to antiplasticization, was interpreted according to the specific macromolecular architecture and ascribed to chain mobility hindrance, as revealed by secondary transitions, whereas no significant contribution from the free volume could be evidenced. Furthermore, it was shown that depending on the strain scale and on the corresponding deformational mechanisms, the mechanical response may be differently influenced either by the presence of dangling groups or by the network density. POLYM. ENG. SCI., 57:553–565, 2017. © 2017 Society of Plastics Engineers

Orthorhombic phase formation in a Nb-rich γ-TiAl based alloy – An in situ synchrotron radiation investigation

Novel low aluminium, high niobium containing γ-TiAl based alloys, such as Ti-42Al-8.5Nb (at.%), can exhibit an excellent combination of high-temperature strength and ductility. Nano scaled modulated microstructures consisting of lamellae with a tweed substructure are believed to contribute to the balanced properties. These tweed-like appearing lamellae are an arrangement of multiple stable or metastable phases with at least one orthorhombic constituent. Due to the crystallographic similarity of these orthorhombic phases to the hexagonal α2-Ti3Al and cubic B2 ordered βo-TiAl phases an unambiguous identification of their crystal structure and formation path is difficult. By using in situ high-energy synchrotron X-ray diffraction (HEXRD) measurements, the nature of the orthorhombic phases involved as well as their formation paths were directly observed and identified in the temperature range between 550 °C and 700 °C. Calculations and Rietveld analysis based on the in situ data showed that the crystal structure of the newly formed orthorhombic phase is comparable to that of the O phase but deviates from the ideal stoichiometric composition (Ti2AlNb). The formation of the orthorhombic phase out of the α2 phase was monitored in situ.

Improving crystalline quality of sputtering-deposited MoS2 thin film by postdeposition sulfurization annealing using (t-C4H9)2S2

A sputtered MoS2 thin film is a candidate for realizing enhancement-mode MoS2 metal–oxide–semiconductor field-effect transistors (MOSFETs). However, there are some sulfur vacancies in the film, which degrade the device performance. In this study, we performed postdeposition sulfurization annealing (PSA) on a sputtered MoS2 thin film in order to complement sulfur vacancies, and we investigated the fundamental properties of the MoS2 film. As a result, a high-quality crystalline 10-layer MoS2 film with an ideal stoichiometric composition was obtained at a relatively low process temperature (500 °C). The MoS2 film had an indirect bandgap of 1.36 eV and a high Hall mobility compared with the as-deposited sputtered MoS2 film.

Uraniferous paleoplacers of the Mesoarchean Mahagiri Quartzite, Singhbhum craton, India: Depositional controls, nature and source of > 3.0 Ga detrital uraninites

The quartz-pebble conglomerate (QPC)-hosted detrital uranium mineralization is unique in character in terms of their restricted distribution before 2.2Ga atmosphere during pre-Great Oxidation Event (pre-GOE). Such QPC paleoplacer deposits over the world are good targets for moderate to high tonnage and low grade uranium deposits and more importantly for their gold content. The Mahagiri Quartzite, dated c. 3.02Ga for their youngest detrital zircon population, is developed unconformably over the Mesoarchean Singhbhum Granite (3.44Ga to 3.1Ga). The Mahagiri Quartzite includes a conglomerate-pebbly sandstone dominated subaerial alluvial fan to coastal braided plain sequence in the lower parts and shallow marine mature quartz arenite in the upper parts. The alluvial fan-braided plain deposits in the lower parts host a number of pyritiferous and uraniferous conglomerate and pebbly sandstone beds. The uraninite grains are rounded to subrounded in outline suggesting mechanical transport and detrital origin. Together with detrital pyrite and uraninite constitute the example of >3.0Ga paleoplacer closely comparable to the Witwatersrand Au–U deposits. EPMA and SEM-EDS studies suggest that the uraninite grains are rich in Th (>4wt.%), S and REE-Y. Chemical formula calculations from EPMA analyses suggest uraninite grains belong to two populations with different oxidation states as revealed from Y/REE and cation U4+: U6+ [apfu] ratios. The U contents of the detrital uraninite grains from Mahagiri are significantly lower than that of the ideal stoichiometric composition of UO2. This is mainly due to higher amount of heterovalent cationic substitution by Th, REE, Y, Pb, and Ca in Mahagiri QPC uraninite structures, and partial alteration and metamictization of uraninites. Alteration due to metamictization resulted in elevated concentration of Si, Al, P, and Ca in more altered and metamict uraninite grains. The REE pattern is typically flat with comparable LREE–HREE concentration. The high Th content flat REE-pattern suggests that the uraninitere presents high temperature phases (>350°C) and are magmatic in origin. The Mahagiri detrital uraninite grains suggest existence of highly felsic and K-rich (richer than TTG) granodiorite–granite–monzogranite suites (GGM) of rocks older than 3.1Ga in the Singhbhum craton.

Studies on optical characteristics of CuInSe2 thin films

Abstract CuInSe2 is considered as a striking semiconductor for second generation solar cells. An investigation of optical properties of CuInSe2 thin films is essential to evaluate its perfectibility as high efficiency solar cells. The films were fabricated by thermal co-evaporation technique. For this experiment, a shimadzu spectrophotometer of model number 1201 is used. The optical properties of these films are determined for the wavelength range 350 nm–1100 nm. From the experiment it is evident that the reflectance and transmittance of the films are negligible in comparison to the absorption of these films. The high absorption coefficient of the order of 104/cm of the film material also supports this. The band gap of the CuInSe2 films was evaluated to be 1.1 eV. From XRD and EDAX analysis it is evident that CuInSe2 films are polycrystalline in nature having ideal stoichiometric composition.

ZnTe electrodeposition from organic solvents

The electrodeposition of zinc telluride was investigated from various organic solvent baths such as methanol, acetonitrile, propylene carbonate. The potential window increases with increasing boiling point of the solvents. In particular, ZnTe with high crystallinity could be obtained in propylene carbonate by the under-potential deposition (UPD) of Zn. The current efficiency for ZnTe deposition in propylene carbonate was enhanced considerably compared to that in acidic aqueous solutions. A single-phase ZnTe film close to the ideal stoichiometric composition of ZnTe could be obtained at 423 K and −0.8 V versus Ag/AgCl in propylene carbonate with a molar ratio of Zn(II)/Te(IV) = 10 and a low Te(IV) concentration; the film had a smooth and dense granular crystal morphology.

Electrochemical oxidation of titanium by pulsed discharge in electrolyte

We propose a high-voltage (>1000 V) pulsed discharge method of preparation of thin anodic films on valve metals under conditions of extremely high rates of the film growth. The peculiarities of this method have been demonstrated on the anodic oxidation of titanium in sulfuric acid solutions. A light flash at the electrode was generated upon the pulsed anodization of titanium. The emission spectra consist of narrow lines assigned to electronically excited O, H and Ti atoms, indicating that a hot plasma is created at the electrode surface. The current efficiency of TiO 2 anodic film formation exceeds markedly 100% for the pulsed Ti anodization. This non-faradaic yield was associated with the contribution of the plasma-generated high-energy radicals to the oxide growth processes. We applied various techniques to compare the composition, morphology, structure and semiconducting properties of the anodic films prepared by the high-voltage pulsed discharge and the conventional galvanostatic anodization. The anodic TiO 2 films prepared by the high-voltage pulsed discharge have nearly ideal stoichiometric composition and are more amorphous as compared to galvanostatically grown ones. Thin TiO 2 films (50–150 nm) formed in the pulsed regime exhibit significantly higher photocurrent quantum yields at λ < 310 nm and a significantly lower donor concentration.

Energy dependence of electron inelastic mean free paths in bulk GaN crystals

Recent advances in fabrication and commercialization of high-brightness blue and green light-emitting devices based on gallium nitride have renewed intense research of its basic properties. Since information on electron transport processes in GaN is scarce, their systematic studies are highly desirable. The electron inelastic mean free path (IMFP) is a crucial parameter for quantitative interpretation of surface electron spectra. The energy dependence of IMFP for bulk GaN crystals with different surface concentrations of their constituents was obtained from elastic peak electron spectroscopy (EPES) with use of the Ni standard in the energy range 200–2000 eV. The measured IMFPs were compared with the values predicted by the TPP-2M and G-1 formulae. A reasonable agreement was found between the measured IMFPs in bulk GaN with an ideal stoichiometric surface composition and the corresponding calculated IMFPs. Compared with the bulk IMFPs, experimental IMFPs valid for the GaN sample with a thin surface layer enriched in ∼70 at.% Ga are only slightly smaller by 5–10%, depending on the electron energy.

A comparison of EDS microanalysis in FIB-prepared and electropolished TEM thin foils

This paper reports the results of a fine-probe EDS microanalytical study of cellular precipitation in a Cu–Ti binary alloy. Compositional profiles across the solute depleted Cu-rich FCC lamellae and the Cu 4Ti lamellae within isothermally formed cellular colonies were measured in a FEG-TEM from thin-foil specimens prepared by conventional electropolishing and by a technique using a Ga + focused ion-beam (FIB). The Cliff–Lorimer ratio method, with an absorption correction, was employed to quantify the compositions. Two FIB samples were prepared with different orientations of the lamellae with respect to the ion-milling direction. The compositional profiles across the Cu-rich FCC lamellae and the Cu 4Ti compound lamellae in both the FIB-prepared samples and the electropolished sample were, within experimental error, numerically equivalent. The composition of the Cu 4Ti compound phase lamellae was very close to the ideal stoichiometric composition of 20 at% Ti. It is concluded that for this system, and for the specimen preparation procedures used in this study, the Ga + ion-milling process has had no detectable effect on the chemistry changes across the interlamellar interface at the scale studied. These results indicate that the possible sources of chemical artifacts which include redeposition, preferential sputtering and ion-induced atomic migration can be minimized if several precautions are taken during milling in the FIB. Consistent with previous investigators, it was also found that the ion-milling process does introduce significant structural artifacts (e.g., dislocations) into the softer FCC Cu-rich phase compared with a specimen produced by conventional electropolishing.

Hydrogen-Isotope Exchange in Halloysite: Insight from Room-Temperature Experiments

Halloysite is a common pedogenic clay mineral, often found in young soils developed on volcanic deposits (Dixon, 1989), It is a member of the kaolin group of clays with the same ideal stoichiometric composition as kaolinite [Al2Si2O5(OH)4]. Halloysite, however, often contains water of hydration (i,e., Al2Si2O5(OH)4·nH2O), and is commonly found with a tubular morphology, This “rolling” of halloysite has received a great deal of study because there is no generally agreed upon mechanism for the process and there is no corresponding phenomenon in natural kaolinite (e.g. , Bates et al., 1950; Bailey, 1989; Singh, 1996; Singh and Mackinnon, 1996). The crystal structure of halloysite often shows stacking disorder. This property, combined with a rolled morphology, makes identification by X-ray diffraction (XRD) difficult. The XRD peaks at 7.5, 4.4, and 3.6 Å are often asymmetric with a large width at half peak height (Bailey, 1989).

Characteristics of ore minerals associated with gold at the Prestea mine, Ghana

AbstractGold in Early Proterozoic Birimian greenstone at Prestea in Ghana is associated with base metal sulphides and sulphosalts including arsenopyrite, pyrite, sphalerite, chalcopyrite, pyrrhotite, galena, tetrahedrite, bournonite, boulangerite and jamesonite. The occurrence of the gold is intimately associated with arsenopyrite and the sulphosalts, and to a lesser extent with the other sulphides. The tetrahedrites at Prestea constitute the major component of sulphosalts associated with gold and occurring in two distinct types. Type I show ideal stoichiometric composition. Type II tetrahedrites deviated from the ideal stoichiometry and are represented approximately by the average formula (Cu,Ag)9.61(Fe,Zn)2.39(Sb,As)4S13. The tetrahedrites co-precipitated with gold exhibit ideal characteristics indicating an equilibruim state of the mineralizing fluid during precipitation. Three types of pyrites were distinguished by electron-microprobe analyses based on their As, Co and Ni composition. The As content in type I vary from 0.15 to 0.37 wt%, and contain up to 2 wt.% Co.Type II pyrites are As-rich and form the most dominant with As content ranging from 0.2 to 2.69 wt.%. Ni content varies from below-detection to 1000 ppm. Type III pyrites are poor in the trace elements and consistent with the stoichiometric composition. The mineralization occurred in three paragenetic stages from at least a two-phase hydrothermal fluid, with stage II forming a prolonged and main stage of the ore and gold mineralization. Redox changes in ore fluid which were triggered by episodic pressure releases during fissuring and fracturing caused fluctuation of the activity of the As/Ni ratio and subsequent oscillatory zoning of Ni in As-rich ores.

Diagenetically altered sabkha‐type Pleistocene dolomite from the Arabian Gulf

ABSTRACTDiagenetically altered Pleistocene dolomite occurs in the shallow subsurface of the Arabian Gulf, offshore of Al Jubayl, Saudi Arabia. This dolomite accumulated in relatively shallow marine to sabkha depositional environments. In contrast with the thin extent of most other Quaternary sabkha and sabkha‐related dolomite deposits, these deposits comprise a thick (&gt;56 m) accumulation. Additionally, this Pleistocene dolomite displays a high degree of ordering and has a more nearly ideal stoichiometric composition than the dolomite from the depositionally and diagenetically analogous Abu Dhabi sabkha complex. The Pleistocene dolomite also has lower δ13 and δ18O values than the modern Abu Dhabi sabkha dolomite, and higher values than those commonly reported for analogous dolomite from the ancient rock record. The low δ18O values, in conjunction with the geological setting, indicate that the diagenetic waters were meteoric or mixed meteoric and marine in composition. Thus, the degree of ordering, stoichiometric and stable isotopic values indicate that this dolomite has undergone diagenetic alteration relative to its presumed Holocene precursor.

Modifications in the microscopic structure of Ru and Pb phthalocyanine films by intermolecular interactions

Ruthenium and lead phthalocyanine (RuPc, PbPc) thin films were characterized by photoelectron spectroscopies (XPS and UPS), optical spectroscopies (UV-Vis and photocurrent) and were prepared by mass-spectrometer-controlled sublimation of ultrapure materials using Ag substrates. Phthalocyanine molecules can be sublimed without fragmentation in the gas phase. However, there are strong intermolecular interactions in Knudsen-deposited films even for substrate temperatures of 300 K which cause the formation of dimers and excess metal atoms. This results in specific changes of the ideal stoichiometric composition, the electronic structure and the interaction with oxygen.

Polymerized complex synthesis and intergranular coupling of Bi-Pb-Sr-Ca-Cu-O superconductors characterized by complex magnetic susceptibility

Superconducting Bi-Pb-Sr-Ca-Cu-O ceramics have been synthesized by the polymerized complex method at the Bi1.8Pb0.2Sr2Ca2Cu3Oy, Bi1.6Pb0.4Sr2Ca2Cu3Oy and Bi1.68Pb0.32Sr1.75Ca1.82Cu2.83Oy compositions. The polymerized complex method includes three important major steps: (1) formation of stable citrate complexes uniformly dispersed into ethyleneglycol and water, (2) fabrication of polymerized complexes which are formed through dehydration (or esterification) between ethyleneglycol and metal-citrate complexes, and (3) pyrolysis of the polymerized complex precursor and the subsequent heat treatments for producing the Bi-Pb-Sr-Ca-Cu-O superconductor. The present technique offers more convenient and easier fabrication of (Bi,Pb)2Sr2Ca2Cu3Oy superconductors compared with other more elaborate preparation procedures based on wet-chemical techniques, as it eliminates many steps such as ageing, filtration, washing, and pH control. ‘‘X-ray-clean’’ (Bi,Pb)-2223 superconductors have been obtained for all the three compositions after sintering at 850 °C in air for 94–235 h, depending upon the nominal composition of the starting batch. Noticeable is that a small deficiency of Sr, Ca, and Cu relative to the ideal stoichiometric composition of the (Bi,Pb)2Sr2Ca2Cu3Oy significantly accelerates the formation of the (Bi,Pb)-2223 phase. Complex magnetic susceptibility measurements show that Tc(onset) of the bulk (Bi,Pb)-2223 phase remains almost constant at around 108 K, independent of the compositions. However, it is stressed that low-level and/or amorphous impurity phases possibly concentrated on grain boundaries can have dramatic effects on the overall magnetic properties even for materials which look like very ‘‘clean’’ by x-ray diffraction. The best superconducting properties were obtained with samples with a nominal composition of Bi1.6Pb0.4Sr2Ca2Cu3Oy after sintering in air at 850 °C for 180–280 h. The onset temperature of superconducting transition obtained by complex susceptibility was 108.0 K and the transition width (10%–90%) of full diamagnetism was 5.2 K for a measuring field of 100 mOe.