Stoichiometric Solution Research Articles

YAG thermal barrier coatings deposited by suspension and solution precursor thermal spray

Yttrium aluminium garnet (YAG) is a promising topcoat material for thermal barrier coatings due to its high temperature stability and better CMAS (calcium-magnesium-alumino-silicate) resistance. YAG topcoats were deposited by suspension and solution precursor high-velocity oxy-fuel (HVOF) thermal spray. The relationships between processing, microstructure and final properties were studied through a range of characterization techniques and thermal cycling tests. The microstructure of the as-sprayed YAG topcoat from stoichiometric solution precursor (SP-YAG) had distributed pores and inter-splat boundaries, while the as-sprayed topcoat produced from suspension (S-YAG) had vertical and branched micro cracks, pores, and inter-splat boundaries. Both as-sprayed coatings were composed of amorphous phase, hexagonal yttrium aluminium perovskite (YAP) and cubic YAG. In thermal cycling tests, 20% of SP-YAG failure was reached after the 10th cycle; whereas, S-YAG reached the failure criteria between the 60th and 70th cycle. The failure of both the SP-YAG and the S-YAG topcoats occurred due to thermal stresses during the thermal cycling.

High-performance methylammonium-free ideal-band-gap perovskite solar cells

Summary The development of mixed tin-lead (Sn-Pb)-based perovskite solar cells (PSCs) with low band gap (1.2–1.4 eV) has become critical not only for pushing single-junction devices toward the maximum efficiency given by the Shockley-Queisser limit, but also for enabling all-perovskite tandem devices beyond this limit. However, achieving high power-conversion efficiency (PCE) and long-term device operation stability simultaneously remains a significant challenge for Sn-Pb-based PSCs. Here, we demonstrate near ideal-band-gap (∼1.34 eV) methylammonium-free Sn-Pb-based PSCs with high efficiency (∼20%) and promising operational stability of maintaining >80% of initial PCE over 750 h under maximum-power-point tracking. The key to this success is the use of a SnCl2⋅3FACl complex additive that improves the microstructure and reduces the development of residual stress in the Sn-Pb perovskite thin films, which in turn enhances the efficiency and stability of the Sn-Pb-based ideal-band-gap PSCs.

A humidity-controlled precipitation technique enabling discovery of Rb3(H1.5PO4)2

The previously unknown compound Rb3(H1.5PO4)2 is successfully synthesized here using a newly developed variant of aqueous precipitation crystal growth. The approach exploits the phenomenon of boiling point elevation in concentrated solutions. Crystals of the title compound were obtained upon heating a stoichiometric solution from 110 to 150 ​°C under a high steam partial pressure of 0.83 ​atm. Single crystal X-ray diffraction studies revealed Rb3(H1.5PO4)2 crystallizes in space group C2/m and is isostructural to Cs3(H1.5PO4)2. As evidenced by thermal analysis, Rb3(H1.5PO4)2 does not undergo a phase transition to a trigonal superprotonic phase upon heating. Even under a steam partial pressure of 0.82 ​atm, under which dehydration is suppressed to a temperature of 263 ​°C, no polymorphic transition is detected. The behavior parallels that of Cs3(H1.5PO4)2 and contrasts that of several structurally and chemically similar selenate compounds. The crystal growth approach developed here may prove particularly useful for obtaining water soluble compounds which are thermodynamically or kinetically disfavored at temperatures close to ambient.

Effect of cation ratio on microstructure and optical absorbance of magnesium aluminate spinel

This work is pertaining to the synthesis of fine magnesium aluminate spinel (MgAl2O4) powders of varied trivalent:bivalent cation ratio along line of homogeneity of the solid solution (MgO. xAl2O3, x = 1, 1.25, 1.50, 1.75, and 2) via gel combustion method. Magnesium- and aluminum- nitrate were used as the oxidants in combustion reaction fuelled by urea in combination with stoichiometric formaldehyde solution acting as reductant. Synthesized powders were characterized in terms of microscopic analysis and optical absorbance measurements. The cation ratio, through a change in gel structure influences the nature of crystallization of the product, while on the other hand does not affect grain shapes and sizes. Distinct enhancement in both absorption intensity and the corresponding estimated energy band gap has been observed against increasing excess than stiochiometric alumina concentrations. Evaluated optical band gaps were widened in proportion to the Al: Mg ratio which may be attributed to Burstein-Moss effect in consequence of substitutional insertion of introduced Al3+ ions in spinel lattices.

Synthesis and characterization of thiazole based selective chemosensor for copper ions

A new Schiff base probe was designed and synthesized using thiazole based ligand (L). Chemical structure of the probe was confirmed by IR, 1H &13C –NMR spectroscopy. The probe was investigated for its recognition abilities against various cations. A stoichiometric solution of probe was prepared using acetonitrile: water (1:9) mixture and added with 5 equivalents of various metal ion perchlorate salts (Ca2+, Mg2+,Mn2+, Cd2+, Fe3+, Co2+, Ni2+, Cu2+, Hg2+, Zn2+, Al3+,Na+, K+) for recognition studies. Naked eye investigation of the solution produce visible color change for Cu and the UV–Visible spectroscopic analysis show red shift due to complex formation. Similarly the probe solution was mixed with 5 equivalents of quaternary ammonium salts of various anions (F−, Cl−, Br−, I−, CN−, SCN−, N3−, H2PO4−) and was investigated for its recognition ability but did not show any visible colour change. Ratiometric titration and Job’s plot shows 1:1 binding stoichiometry. This confirms the ion recognition of ligand (L) towards Cu for sensing properties.

The Cesium doping using the nonstoichiometric precursor for improved CH3NH3PbI3 perovskite films and solar cells in ambient air

Organic-inorganic hybrid perovskite solar cells have shown great prospect as a low-cost and high efficiency photovoltaic technology. The quality of the perovskite absorber layer is most critical to the performance of the device, and the stability remains one of the challenging issues. In this paper, the Cesium (Cs) doping perovskite film was prepared from nonstoichiometric precursor solution in ambient and humidity-controlled conditions. The results showed that the crystallinity, uniformity, absorption, Photo-luminescence intensity and the thermal stability of these films can be effectively improved compared with the films fabricated from stoichiometric one, which is attributed to the combined effect of the Cs doping and excess methylammonium cations passivation. Finally, the highest efficiency of the perovskite solar cells fabricated from nonstoichiometric solution reached to 14.1%, which is 12.8% higher than that of the control device from stoichiometric solution (12.5%). Furthermore, the device displayed the high stability and efficiency degradation of only 2% occurring over a period of 5 weeks in ambient air without encapsulation. This reported work provides a pathway for further improving the performance of perovskite solar cells with higher stability.

Near Infrared Spectrometric Investigations on the behaviour of Lactate.

In patients with life-threatening illnesses, the metabolic production and disposal of lactate are impaired, which leads to a build-up of blood lactate. In critical care units, the changes in lactate levels are measured through intermittent, invasive, blood sampling and in vitro assay. Continuous monitoring is lacking, yet such monitoring could allow early assessment of severity and prognosis to guide therapy. Currently, there is no routine means to measure lactate levels continuously, particularly non-invasively. The motivation of this study was to understand the interaction of lactate with light in the Near Infra Red (NIR) region of the electromagnetic spectrum. This was to create an opportunity to explore the possibility of a non-invasive sensing technology to monitor lactate continuously.In vitro studies were performed using solution samples with varying concentration levels of sodium lactate in isotonic Phosphate Buffer Solution (PBS) at constant pH (7.4). These samples were prepared using stoichiometric solution compositions and spectra for each sample were taken using a state-of-the-art spectrometer in the NIR region. The spectra were then analysed qualitatively by 2D correlation analysis, which identified the regions of interest. Further analysis of these regions using linear regression at four randomly selected wavelengths showed bathochromic shifts, which, moreover, showed systematic variation correlating with lactate concentration.

Phosphorous acid promoted isomerization of propargyl alcohols to α,β-unsaturated carbonyl compounds

A metal-free and two-phase protocol for the Meyer-Schuster isomerization of propargyl alcohols to the corresponding α,β-unsaturated carbonyl compounds has been achieved in the presence of stoichiometric phosphorous acid aqueous solution, which produces the desired products in high yields with excellent stereoselectivity. Compared with the traditional methods, the procedure features broad scope of the substrates, mild conditions, and easy separation, providing an appealing alternative to the Meyer-Schuster reaction.

A new strategy for the synthesis of monomethylhydrazine using the Raschig process. 1. Study of the stability of monochloramine

In this paper, we describe a new strategy developed for the synthesis of MMH using the Raschig process. For the first time, it consists in synthesizing chloramine under stoichiometric conditions which leads to a more economical process because it avoids the additional steps of extraction due to the presence of excess reagents. However, monochloramine is not very stable and, therefore, it is necessary to study its stability. The various batch tests show that the reaction must be strictly controlled. As shown from DSC analyses on stoichiometric chloramine solutions, a risk of decomposition is present from 36 °C. Thus, controlling the temperature rise requires efficient cooling in the synthesis reactor or synthesis at low temperature, in order to avoid any sudden decomposition of the synthesis solution.

Superconductor YBa2Cu3−xNixO7−δ compounds prepared by electrospinning

This work describes the synthesis and characterization of (0.00 ≤ x ≤ 0.04) compounds prepared by an electrospinning technique. Stoichiometric solutions of Y-Ba-Cu-Ni were used to electrospin nanowires of these compounds, which were dried and heat-treated at temperatures ranging from 350 °C to 925 °C in an oxygen atmosphere. All samples were characterized by x-ray diffraction (XRD), scanning electron microscopy, and temperature magnetization curves. As prepared and dried at 100 °C, these samples present a nanowire shape with a smooth surface, with external diameters of ∼600 nm and a large length (>10 μm). After the heat treatment at 780 °C, all of them maintain their nanowire morphology with a rough surface with external diameters of ∼600 nm and length values ranging from 5 to 10 μm. All samples crystallize into the perovskite structure with orthorhombic symmetry (Pmmm space group). The most intense Bragg reflections arise from the phase and low-intensity peaks of additional phases. The temperature dependence of magnetization of these nanowires reveals a diamagnetic transition at temperatures close to the critical temperature, Tc, which is related to the superconductor transition in these compounds. The Tc values range between 70 and 93.2 K for samples with x = 0.04 to x = 0.00, respectively, where the substitution of Cu with Ni results in a lower Tc.

Tetrahydrofuran Hydrate in Clayey Sediments—Laboratory Formation, Morphology, and Wave Characterization

AbstractFine‐grained sediments host more than 90% of gas hydrates on Earth. However, the fundamental properties of hydrate‐bearing silty and clayey sediments are much less understood than those of hydrate‐bearing sands, mainly due to the experimental challenges in synthesizing gas hydrate in fine‐grained sediments in the laboratory as the way they form in nature. This study forms tetrahydrofuran (THF) hydrate in kaolinite, visualizes the hydrate distribution and morphology using X‐ray computed tomography, and uses P and S waves to characterize the formed hydrate‐bearing clayey sediments. The results show that THF hydrate formed in clay is innately segregated and heterogeneous, no longer as a pore constituent as in sandy sediments. Hydrate nucleation, growth, and distribution in clays are dominated by the thermal condition and constrained by water activity and mass transport, which can result in low stoichiometric‐solution‐to‐hydrate conversion ratios of approximately 0.4–0.7. Thus, the estimation of hydrate volume in clays based on the mass of stoichiometric solution can be erroneous. The heterogeneity in hydrate‐bearing clays imposes challenges in wave velocity based characterization. The bulk elastic properties of hydrate‐bearing clays can be well predicted using the self‐consistent model. Specimens with higher hydrate volume fraction VFh show higher wave attenuations Q−1, highlighting the dominant role of THF hydrate in the wave attenuation of hydrate‐bearing clays. The results suggest that Qp−1 = 0.08 + 0.4VFh = 2Qs−1, which underlines the potential of using wave attenuation based methods to quantify the hydrate volume fraction in clayey sediments.

Engineering Perovskite Nanocrystal Surface Termination for Light‐Emitting Diodes with External Quantum Efficiency Exceeding 15%

AbstractHybrid organic–inorganic metal halide perovskites are particularly promising for light‐emitting diodes (LEDs) due to their attractive optoelectronic properties such as wavelength tunability, narrow emission linewidth, defect tolerance, and high charge carrier mobility. However, the undercoordinated Pb and halide at the perovskite nanocrystal (NC) surface causes traps and nonradiative recombination. In this work, the external quantum efficiency of iodide‐based perovskite LEDs is boosted to greater than 15%, with an emission wavelength at 750 nm, by engineering the perovskite NC surface stoichiometry and chemical structure of bulky organoammonium ligands. To the stoichiometric precursor solution for the 3D bulk perovskite, 20% molar ratio of methylammonium iodide is added in addition to 20% excess bulky organoammonium iodide to ensure that the NC surface is organoammonium terminated as the crystal size is decreased to 5–10 nm. This combination ensures minimal undercoordinated Pb and halide on the surface, avoids 2D phases, and acts to provide nanosized perovskite grains which allow for smooth and pinhole‐free films. As a result of time‐resolved photoluminescence (PL) and PL quantum yield measurements, it is possible to demonstrate that this surface modification increases the radiative recombination rate while reducing the nonradiative rate.

CATALYTIC ACTIVITY AND COKING RESISTANCE ON HYDROXYAPATITE FOR THE OXIDATIVE DEHYDROGENATION OF ISOBUTANE

Hydroxyapatite (HAp) samples were prepared from precursor solutions with different Ca/P molar ratios. The HAp samples were utilized as catalysts for the oxidative dehydrogenation (ODH) of isobutane. Also, the HAp samples were characterized by N2 and H2O adsorption-desorption measurements, X-ray diffraction, inductively coupled plasma optical emission spectrometry, thermogravimetry-differential thermal analyses, and temperature-programmed oxidation (TPO). The Ca/P molar ratio of the precursor solution had a major influence on the catalytic activity and coking resistance ability. The HAp catalyst prepared from the stoichiometric precursor solution [HAp (1.67)] showed the highest isobutene yield. Furthermore, the white color of HAp (1.67) remained unchanged during the ODH of isobutene, indicating the absence of coke on the surface of HAp (1.67) after the reaction and its higher coking resistance ability. The results of the TPO measurements showed that the HAp (1.67) catalyst possesses the strongest oxidation ability which is likely related to its higher coking resistance ability.

Removal of Microcystin-LR from spiked natural and synthetic waters by anion exchange

Cyanobacterial blooms are becoming a serious challenge across the globe due to changing climate and rainfall patterns as a consequence of human activities. In the present study, the fundamental interactions involved during the removal of Microcystin-LR (MCLR), one of the most commonly occurring cyanobacterial toxins, were investigated by employing strongly basic anion exchange (IX) resins. Several factors including the stoichiometric coefficients, competitive fractions and solute affinities were determined under various concentrations of inorganic ions and natural organic matter. The results indicated that suphates were the most competitive fractions with high affinity (α (affinity coefficient) values ~ 9) followed by nitrates (α ~ 4.7) and NOM fractions (α ~ 4.5, p < 0.05). The Equivalent Background Concentration Mode (EBC), that arises from the Ideal Adsorption Solution Theory (IAST), indicated a competitive fraction of ~2 μeq/L NOM, which approximates to <10% of the initial NOM concentrations, indicating a small fraction of the NOM resulting in the competitive effect. Further, studies with natural surface waters indicated that the MCLR uptake could be modeled using the IAST-EBC model and the IX resin could simultaneously removal of >90% of NOM, inorganic ions and MCLR at resin dosages of 3.6 meq/L or higher.

Dye adsorption in ZIF-8: The importance of external surface area

Two kinds of ZIF-8 crystals have been prepared from the ultrasound-assisted hydrothermal reaction of a stoichiometric metal-ligand precursor solution. The addition of surfactant promoted the formation of nanocrystals having a much larger external surface area compared to the non-promoted one. The two samples were then employed to examine the effects of particle size on dye adsorption. The adsorption capacity of Rhodamine B (RB) and methyl orange (MO) was found directly proportional to the external surface area of the ZIF-8 particles, while that of methylene blue (MB) did not vanish with decreasing external area. The residue adsorption is the most clear-cut evidence confirming the penetration of the MB molecule into the ZIF-8 framework, but not the similarly sized MO molecule. The larger RB molecule was also excluded from the internal pores and cover only the external surface of the crystals. Our calculations suggested that each RB and MO molecule occupied 4.7 and 7.9 nm2 of the external surface, respectively. In contrast, MB adsorption occurs both inside the ZIF-8 structure, at a loading of 0.11 molecule per unit cell, and on the external surface with 2.3 nm2/molecule.

An unexpected lanthanum chloride carbonate hydrate: synthesis, crystal structure, vibrational spectra and thermal degradation of LaCl[CO3]·3H2O

Abstract Microcrystalline powders of LaCl[CO3]·3H2O were precipitated and isolated from stoichiometric aqueous solutions of LaCl3·7H2O and Na2[CO3]·10H2O. This confirmed the surprising emergence of corresponding single crystals during a reaction of equimolar aqueous solutions of LaCl3·7H2O and Na[N3] under ambient conditions by the uptake of atmospheric CO2. According to the X-ray structure analysis of colorless, transparent and rectangular single-crystals, LaCl[CO3]·3H2O adopts an orthorhombic structure, space group Pbca with the unit-cell parameters a=856.82(5), b=1598.57(9) and c=967.68(6) pm for Z=8. The tenfold coordination polyhedron around La3+ consists of two monodentate as well as two bidentate [CO3]2− anions together with three oxygen atoms from coordinating water molecules and one chloride anion. According to DSC/TG studies and X-ray powder diffraction, LaCl[CO3]·3H2O loses first its water and then carbon dioxide under thermal treatment up to 900°C to leave LaOCl behind. Vibrational infrared and Raman spectra confirmed the presence of H2O and [CO3]2− in LaCl[CO3]·3H2O and its stepwise degradation at elevated temperatures.

Low-temperature heat capacity VO2±δ and solid solutions V1-XFeXO2

The article reports the study results of the heat capacity of vanadium dioxide compounds (VO2) (within the homogeneity range) and stoichiometric solid solutions V1-xFexO2 at temperature range from 5 κ to 100 κ. Separation of various constituents of heat capacity with its additivity assumption has been conducted: crystal lattice heat capacity and defects heat capacity. It is shown that a considerable contribution to the heat capacity is produced by the crystal lattice defects (till ~ 25 κ) at low temperatures, and at higher temperatures the heat capacity is determined mainly with the crystal lattice. Using the Debye function table for the heat capacity for all compounds, the characteristic Debye temperatures θD were determined.

Mechanochemical Synthesis of Methylammonium Lead Mixed–Halide Perovskites: Unraveling the Solid-Solution Behavior Using Solid-State NMR

Mixed-halide lead perovskite (MHP) materials are rapidly advancing as next-generation high-efficiency perovskite solar cells due to enhanced stability and bandgap tunability. In this work, we demonstrate the ability to readily and stoichiometrically tune the halide composition in methylammonium-based MHPs using a mechanochemical synthesis approach. Using this solvent-free protocol we are able to prepare domain-free MHP solid solutions with randomly distributed halide ions about the Pb center. Up to seven distinct [PbClxBr6–x]4– environments are identified, based on the 207Pb NMR chemical shifts, which are also sensitive to the changes in the unit cell dimensions resulting from the substitution of Br by Cl, obeying Vegard’s law. We demonstrate a straightforward and rapid synthetic approach to forming highly tunable stoichiometric MHP solid solutions while avoiding the traditional solution synthesis method by redirecting the thermodynamically driven compositions. Moreover, we illustrate the importance of co...

Relative impacts of methylammonium lead triiodide perovskite solar cells based on life cycle assessment

The environmental performance of four different device assembly procedures based on hybrid halide perovskite solar cell (PSC) were assessed from cradle to grave using life cycle assessment (LCA) methodology. In addition, a new environmental indicator was defined to measure the time evolution of an impact category, specifically in this case, human toxicity cancer payback time. PSCs procedures accounted for the probably three more used basic recipes for laboratory perovskite deposition: 1) spin coating of stoichiometric precursor solution, 2) spin coating of precursor solution using lead chloride precursor and 3) the two step deposition method. Also, the two most widely used substrate configurations (planar and mesoporous substrate)were considered. LCA included three realistic scenarios for the end of life: 1) residual landfill, 2) reuse and residual landfill and 3) reuse and recycling. The remaining variable parameters to assemble the device were fixed in common for all four devices, which were the major responsible of the whole PSC impact. Lead of PSCs had no significant contribution in environmental impacts. Beyond shared procedure steps, impacts generated by the two-step method and the use of mesostructured type substrate were higher. End of life scenario with reuse and recycling improved the toxicity impact categories.

Ferromagnetic and insulating behavior of LaCoO3 films grown on a (001) SrTiO3 substrate: A simple ionic picture explained ab initio

This paper shows that the oxygen vacancies observed experimentally in thin films of LaCoO3 subject to tensile strain are thermodynamically stable according to ab initio calculations. By using DFT calculations, we show that oxygen vacancies on the order of 6 % forming chains perpendicular to the (001) direction are more stable than the stoichiometric solution. These lead to magnetic Co2+ ions surrounding the vacancies that couple ferromagnetically. The remaining Co3+ cations in an octahedral environment are non magnetic. The gap leading to a ferromagnetic insulating phase occurs naturally and we provide a simple ionic picture to explain the resulting electronic structure.