Microstructure, Chemical and Phase Composition of Chromium Silicide Diffusion Coatings on Carbon Steels

The microstructure, chemical and phase composition of chromium silicide diffusion coatings on steels 20, 45, U8, and U10 are studied. It is established that the phases Cr²³C⁶ and Cr⁷C³ with a silicon content in them of up to 0.25 at.% form at the surface of chromium silicide coatings. The maximum silicon content at 7% in steels 20 and 45 is observed beneath the carbide layer at a depth of 40–50 µm, but for steel U10 it is 1.9% at the carbide layer-matrix boundary. It is shown that the corrosion resistance of chromium silicide diffusion coatings increases markedly compared with uncoated specimens: by a factor of 15–30 depending on the acid used (H₂SO₄, HCl, H₃PO₄, HNO₃, CH₃COOH). There is also an increase in the cavitation resistance of articles with chromium silicide coatings in water and 3% NaCl solution.__________Translated from Poroshkovaya Metallurgiya, Nos. 1–2(441), pp. 23–30, January–February, 2005.     

Diatom Biosilica Doped with Palladium(II) Chloride Nanoparticles as New Efficient Photocatalysts for Methyl Orange Degradation

A new catalyst based on biosilica doped with palladium(II) chloride nanoparticles was prepared and tested for efficient degradation of methyl orange (MO) in water solution under UV light excitation. The obtained photocatalyst was characterized by X-ray diffraction, TEM and N₂ adsorption/desorption isotherms. The photocatalytic degradation process was studied as a function of pH of the solution, temperature, UV irradiation time, and MO initial concentration. The possibilities of recycling and durability of the prepared photocatalysts were also tested. Products of photocatalytic degradation were identified by liquid chromatography–mass spectrometry analyses. The photocatalyst exhibited excellent photodegradation activity toward MO degradation under UV light irradiation. Rapid photocatalytic degradation was found to take place within one minute with an efficiency of 85% reaching over 98% after 75 min. The proposed mechanism of photodegradation is based on the assumption that both HO^• and O₂^•− radicals, as strongly oxidizing species that can participate in the dye degradation reaction, are generated by the attacks of photons emitted from diatom biosilica (photonic scattering effect) under the influence of UV light excitation. The degradation efficiency significantly increases as the intensity of photons emitted from biosilica is enhanced by palladium(II) chloride nanoparticles immobilized on biosilica (synergetic photonic scattering effect).     

Influence of clinoptilolite rock on chemical speciation of selected heavy metals in sewage sludge

The chemical speciation of Cd, Cu, Pb, Cr and Ni in Torun municipal sewage sludge is investigated with addition of a natural sorbent (clinoptilolite rock). The total contents of the heavy metals in the sludge are substantially lower than the corresponding limits established by European or Polish legislation excepting nickel only. But the metals concentrations excepting lead exceed significantly the natural background (average contents in soils and in the Earth's crust) in dozens. Application of the sequential chemical extraction indicated that the metals in the sewage sludge are bound mainly (over 50%) in the residual fraction. The metals form the following order by parts of the mobile form: Ni> Cd> Cr> Cu> Pb. Addition of the clinoptilolite to the sludge leads to the metals contents fall in all four fractions of the sequential procedure. Concentrations of mobile forms of cadmium, chromium, copper and nickel decrease by 87, 64, 35 and 24%, respectively, as a result of addition of 9.09% of the clinoptilolite. The total decreases of the metals amount after 9.09% clinoptilolite addition to the sludge are around 11, 15, 25, 41 and 51% for copper, nickel, chromium, cadmium and lead, respectively.     

Clinoptilolite in study of lindane and aldrin sorption processes from water solution

The scope of this study was adsorption of lindane and aldrin from water solution onto clinoptilolite rock with the following desorption using n-hexane. Both kinetic and equilibrium tests were conducted. During kinetic experiment the most part of aldrin and a half part of lindane were sorbed during the first hours. The sorption equilibria with removal of 95% of aldrin amount and about 68% of lindane amount have been set in for 48 h. The pseudo-second-order kinetics model gives somewhat better fit to the both pesticides' sorption data that may testify to the complicated and heterogeneous nature of interaction between active zeolite surface and the pesticides polar-dipole centers. OH(-)-groups and the coordinated exchangeable cations are probably the main active positions for the pesticide sorption on the clinoptilolite surface. Equilibrium experiment results show that adsorption isotherms for the low concentrations of lindane and aldrin (10-200 and 3-100 microg/L, respectively) fit well to the Freundlich and the Liner models. Only 10% of lindane sorbed and about 60% of aldrin sorbed were desorbed from the clinoptilolite using n-hexane under static conditions.     

A novel route to superhard nanocrystalline cubic boron nitride: Emulsion detonation and high-pressure high-temperature transformation-assisted consolidation

Synthesizing bulk nanocrystalline materials is challenging since grain growth should be suppressed whereas densification promoted. Here, we demonstrate a novel route to synthesize superhard bulk nanocrystalline cubic boron nitride (cBN), which combines the use of emulsion detonation and high-pressure high-temperature transformation-assisted consolidation. The emulsion detonation process activates BN to possess unique structure and chemistry, i.e. wurtzitic BN nanograins in hexagonal BN matrix with enhanced structural disordering and oxygen impurity, a combination that enhances the nucleation rate of cBN and its densification leading to the formation of bulk nanocrystalline cBN at reduced conditions. The cBN, synthesized at 7.5 GPa and 1800 °C, displayed Vickers hardness values of 50?62 GPa for 5?20 N loads. The findings in the study suggest a feasible solution to synthesize bulk nanocrystalline cBN in a more scalable way, while also providing design insights on how to refine grain growth while enhancing densification to synthesize bulk nanocrystalline materials.     

Solid-liquid-solid extraction of heavy metals (Cr, Cu, Cd, Ni and Pb) in aqueous systems of zeolite-sewage sludge

This paper presents results of kinetic and equilibrium studies of the removal of heavy metals (Cr, Cu, Cd, Ni and Pb) from the sewage sludge using a new technique of solid-liquid-solid extraction with the adsorption-diffusion column filled by the zeolite. The metal extraction onto the zeolite from aqueous solution of the clinoptilolite and the sludge composition is characterized by three stages: intensive extraction, inversion and stabilization with the moderate extraction increase. Addition of 25% of the zeolite provides extraction efficiency of cadmium and lead of about 84%, chromium, copper and nickel of 66%, 61% and 50%, respectively. The estimated values of Gibbs free energy change DeltaG show that the metals extraction by the clinoptilolite may be considered as a physical adsorption. The negative values of DeltaG testify to an exothermic nature of the process. The difference between energetic potentials of the components is a driving force of the metal redistribution in the system "clinoptilolite-water-sludge".