Stabilisation of FeCoNiCuPt high-entropy alloy nanoparticles by surface capping.

Influence of surface capping on oxygen reduction catalysis: A case study of 1.7 nm Pt nanoparticles

High throughput synthesis of CoCrFeNiTi high entropy alloys via directed energy deposition

Copper doped ZnS nanoparticles with different capping agents have been synthesized through chemical co-precipitation method. Polyvinylpyrrolidone (PVP), 1-thioglycerol and Trioctylphosphine oxide (TOPO) were used as capping agents. X-ray diffraction (XRD) studies revealed the average particle size of ZnS:Cu2+ with different capping agents to vary from 3 nm to 4 nm. Energy dispersive X-ray spectroscopy (EDAX) measurements confirmed the presence of Cu in the ZnS lattice. UV-Vis spectroscopy revealed blue shift for all the samples with respect to the bulk value. The band gap energy values were in the range of 4.29 eV to 4.86 eV. The blue shift was attributed to the quantum confinement effect. Room temperature photoluminescence (PL) spectrum of the ZnS:Cu2+ capped nanoparticles exhibited emission in the visible region with multiple peaks at an excitation wavelength of 250 nm

This study investigates the influence of various organic modifiers that serve as capping agents on the structure, morphology, electronic properties, colloidal stability, and photocatalytic behavior of ZnO nanoparticles synthesized using the sonochemical method. The capping agents used include citric acid (CA), ethylene glycol (ETG), oleic acid (OA), and poly-(vinylpyrrolidone) (PVP). The synthesized ZnO nanoparticles were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray Spectroscopy (EDS), diffuse reflectance ultraviolet-visible spectroscopy (DRUV-vis), dynamic light scattering (DLS), and electrophoresis (EP). XRD analysis confirmed that all samples exhibited a Wurtzite-type hexagonal structure, with crystal sizes ranging from 17 to 22 nm and band gap energies (E g) between 3.16 and 3.25 eV. Notably, samples synthesized with ETG and OA showed additional absorbance in the visible region. Among the samples, ZnO-ETG demonstrated the highest photodegradation efficiency for reactive black-5 azo-dye (RB5), serving as a model reaction. This work highlights the ability to modulate the properties of ZnO nanoparticles, such as crystal size, morphology, hydrodynamic size, surface charge and Urbach energy, through the choice of organic modifiers that serves as stabilizers, impacting their potential applications in photocatalysis.

In this study, AlCoCrFeMnNiTi based refractory-free high entropy superalloys (HESA) were prepared by powder injection moulding (PIM) for aviation applications. Aging was done in order to enhance mechanical properties. Ni-based superalloys are expensive and heavy due to expensive alloying elements with high density. Strength, oxidation and creep resistance of the high entropy alloys are low for engine applications. HESA would be cheaper and lighter than superalloys and would have higher strength, creep resistance and oxidation resistance than high entropy alloys. In order to obtain solid solution, mixing entropy and mixing enthalpy values were adjusted. Valance electron concentrations was adjusted in order to obtain face centered cubic structure. There is grain-coarseing and segregation in HESA produced by vacuum arc melting. HESA with finer grain sizes and low segregation could be produced by using mechanical alloying (MA)-PIM. Alloy powders were prepared by MA. Alloys with high solid solubility, complex shaped, fine grained superalloy specimens were produced by MA-PIM. Polymer binder consisted polyethylene, paraffin, and stearic acid. Feedstock consisted of 45% of binder and 55% of alloy powder. PIM was carried out at 185°C. After the PIM, binder was removed by chemical debinding and thermal debinding. Sintering was carried out at 1,250°C. Elevated temperature properties of the HESA was enhanced by intermetallic precipitates which were obtained by alloying and heat treatments. Corrosion properties, high temperature oxidation properties, thermal properties, mechanical properties were determined.

To achieve higher engine combustion efficiency while reducing emissions, it is necessary to address the challenges posed by elevated operating temperatures. High Entropy Alloys (HEAs) have emerged as promising materials for this purpose, offering exceptional properties at high temperatures, including synergistic effects and excellent resistance to oxidation and corrosion. In this study, a FeCoNiCrAl HEA was investigated as a bond coat material due to its excellent balance of strength and ductility, coupled with outstanding oxidation resistance. It was deposited using HVAF M3 and i7 guns equipped with different nozzles/powder injectors and pressures. Notably, this research marks the first study of the i7 gun globally for the HEA bond coat, coupled with the optimization of HVAF parameters for both i7 and M3 guns. Characterization of both powder and as-sprayed samples was carried out using X-ray Diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDS), and Field Emission Scanning Electron Microscopy (FESEM) techniques. The results revealed the formation of a dense and homogeneous microstructure. Additionally, isothermal oxidation tests were conducted to analyze the behavior of the thermally grown oxide. After 50 hours at 1000 °C, a dense, uniform, and thin alumina TGO layer was observed to have formed. These tests revealed that FeCoNiCrAl HEA exhibits significant potential to enhance oxidation resistance at high temperatures.

Enhancing the oxidation resistance of nanocrystalline high-entropy AlCuCrFeMn alloys by the addition of tungsten

Design and characterisation of AlCrFeCuNbx alloys for accident-tolerant fuel cladding

We report here the zeta potential study of Bi2S3 nanoparticles (NPs), synthesized by facile polyol method using CTAB and EDTA as surfactants or capping agents. X-ray diffraction (XRD), Rietveld refinement of XRD data and Raman spectroscopy confirms the single phase orthorhombic crystal structure of Bi2S3 with space group Pnma. In Energy-dispersive X-ray spectroscopy (EDAX) data ratio of Bi to S was little deviated from 2:3 due to excess sulfur taken during synthesis. Field Emission Scanning Electron Microscope (FESEM) images show the agglomeration of rod-like Bi2S3. Effect of different capping agents on the surface charge of Bi2S3 NPs was studied using Zeta potential measurement with ethanol as dispersion medium at different pH values. It reveals that these NPs are more stable when it was synthesized in presence of EDTA than that of CTAB or without capping agent. The isoelectronic point was found at pH=4.2 for pure sample and 3.2 and 7.3 for CTAB and EDTA capped Bi2S3 samples.

ZnS nanoparticles were synthesized by co-precipitation method using different capping agents used EDTA (ethylenediaminetetraacetic acid) and PVP (polyvinylpyrrolidone). Field emission Scan Electron Microscope (FE-SEM) with Energy-Dispersive X-ray spectroscopy (EDX), X-Ray Diffraction (XRD), Atomic Force Microscope (AFM), Fourier Transform Tnfrared spectra were used to examine the structural properties and the elemental of the synthesized nanoparticles. UV-optical spectroscopy was used for determination of the optical band gap and the obtained values between 3.4-3.2eV have been found. Average Crystallite Size of nanoparticles measured between 3.7-3.6 nm from XRD pattern. The particle size of nanoparticles were also determined by the capping agent’s quality.

This investigation reports a simple, controllable, chemical reduction method for the formation of copper nanoparticles by reducing copper sulfate with sodium hypophosphite in ethylene glycol using different capping agents: sodium dodecyl sulfate (SDS), polyethylene glycol 2000 (PEG2000), and polyethylene glycol 4000 (PEG4000). The synthetic parameters influencing the purity, size, and morphology of the copper nanoparticles such as reaction time, reaction temperature, reaction solvent, initial copper sulfate concentration, and capping agent were investigated. The products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). The results revealed that pure copper nanoparticles with an average crystallite size of 28 nm were successfully prepared in the presence of PEG4000 capping agent, at 70 °C, and in 45 min. The formation mechanism of the as-prepared Cu nanoparticles was also proposed.

Development of cost-effective high-entropy alloys with superior mechanical properties

Due to the aggressive operation conditions of turbine hot sections, protective coatings are required to provide oxidation and hot corrosion resistance for superalloy components. Thermal barrier coatings (TBCs) are comprised of a ceramic top coat and a metallic bond coat (BC) and are typically used as thermal protection systems against these aggressive environments. Conventional BC materials are MCrAlX, with M being metals or alloys (e.g., Ni, Co or NiCo) and X being reactive elements such as Y, Hf, Ta, Si. Due to their strength, thermal stability, and oxidation resistance, high-entropy alloys (HEAs) have presented promise for use as BC materials in hightemperature applications. Owing to its cocktail effect, optimally chosen HEAs could help to enhance the hot corrosion resistance of BCs by forming a more continuous, dense, and uniform thermally grown oxide (TGO). Furthermore, HEAs could help to control the diffusion between the bonding layer and substrate in elevated temperature environments. This paper will discuss the thermodynamic, mechanical, and microstructural behaviour of HEAs. Furthermore, the selection and usage of HEAs as BCs will be explored and compared to conventional BCs in TBC systems.

High entropy alloys (HEAs) contain five or more alloying elements typically in equimolar concentrations. In contrast, traditional solid solution alloys typically consist of one majority solvent element and several solute elements in comparatively low atomic proportions. HEAs are of interest for development of high performance materials particularly corrosion resistant alloys, because they can be engineered to form a single solid solution phase which reduces compositional and microstructural heterogeneity. Several hypotheses have been proposed to explain why HEAs form solid solutions, including a high entropy of mixing, and a near-zero enthalpy of mixing1 , 2. Many desired properties have been reported to arise in HEAs such as high hardness, superior wear resistance, high-temperature strength, and structural stability1 , 2. Excellent corrosion and oxidation resistance has been proposed but testing is limited1-3. HEAs are expected to perform superior to conventional alloys due to their unique combination of elements as well as an increase in the degrees of freedom in design which can be used to regulate the structure and properties of these alloys through alloying process control, and which cannot be achieved in typical binary and ternary systems3. This enhanced control over process variables in HEA design enables a systematic scientific examination of the effect alloy composition on corrosion properties, particularly passivity, metastable pitting, and repassivation. Characterization of the protective film formed on the HEA surface will provide an understanding of how alloy composition, microstructure, and oxide properties combine to improve passivity-based corrosion resistance and prevent film breakdown4. The focus of this talk will be to investigate alloy passivation as a function of chloride concentrations in an effort to elucidate the mechanism of aqueous corrosion in nickel-based HEAs, with the overarching goal of predictive corrosion-resistant alloy (CRA) design. In the present study a single HEA of composition 33.2Ni-16.85Cr-16.3Fe-8.57Mo-9.56Ru-5.48W, wt%, was designed and synthesized. The HEA demonstrated a stable FCC structure. The HEA was tested and compared to commercially pure Ni as well as the crystalline corrosion-resistant Ni-based alloy, C22 (Ni-21Cr-3.9Fe-13.3Mo-0.72Co-0.0035 C-0.23Mn-2.9W-0.026Si-0.011P-0.013V-0.0039S, wt%). Cyclic potentiodynamic polarization (CPP) scans were performed in solutions of increasing chloride concentration to assess the stability of the passive film. These tests enabled the identification of the passive current density, pitting, and repassivation potentials. A single frequency sinusoidal potential perturbation of 20 mV at 1 Hz was superimposed on the CPP scan to provide electrochemical impedance spectroscopy (EIS) data as a function of applied potential within the passive region. The electronic properties of the oxide film were also assessed using Mott-Schottky analysis of the EIS data. Ex-situ characterization was conducted to determine the corrosion morphology after electrochemical testing. The molecular identity of the oxide was determined using Raman and X-ray photoelectron (XPS) spectroscopies. Preliminary results indicated good corrosion resistance and a broad potential range for the passive region. Results were compared to pure Ni and C22. The passive range was compared to E-pH stability diagrams that were constructed using the CALPHAD approach and provided by Questek®. Acknowledgements This work was supported as part of the Center of Performance and Design of Nuclear Waste Forms and Containers, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0016584

Resistance sintering of CoCrFeNiAlx (x = 0.7, 0.85, 1) high entropy alloys: Microstructural characterization, oxidation and corrosion properties