Thermal Hydrogen Reduction Research Articles

Synthesis and microwave electromagnetic properties of CoFe alloy nanoflakes prepared with hydrogen-thermal reduction method

CoFe alloy nanoflakes (NFs) with diameter and thickness on nanoscale were prepared by hydrogen-thermal reduction in CoFe2O4 flakes at 400 °C for 60 min. The effective complex permittivity and permeability of CoFe alloy NFs/paraffin composites were measured and compared with that of CoFe alloy nanoparticles (NPs)/paraffin composites. Due to the two-dimensional shape character, the real part of permittivity and permeability of CoFe alloy NFs was rather higher than that of CoFe alloy NPs. Electromagnetic wave absorbing (EMA) performance of both CoFe alloy NFs and NPs was evaluated by using transmission line theory. The effective EMA band position of the coating with CoFe alloy NFs as fillers was found to locate in the range of 2–4 GHz, while the effective EMA band position of the coating containing CoFe alloy NPs as fillers was located in the range 8–18 GHz. A maximum reflection loss (RLmax) of −57.8 dB was achieved in a coating containing CoFe alloy NFs as fillers, which is much higher than the −16.6 dB of coatings with CoFe alloy NPs.

Preparation of CoFe alloy nanoparticles with tunable electromagnetic wave absorption performance

CoFe alloy nanoparticles with different particle sizes were successfully prepared by hydrogen-thermal reduction of CoFe 2O 4 microspheres at different temperatures. It was found that the real part of the permeability of the CoFe alloy nanoparticles increased with the decrease of the particle size, and accordingly the reflection loss ( RL) peak moved towards the low-frequency region. The maximum RL of the coating based on CoFe alloy nanoparticles exceeded −23 dB, and a wide effective absorption band width ( RL≤−10 dB) of about 6 GHz was simultaneously achieved. The EMA performance was confirmed to be tunable by the control of the absorber's particle size, and thus a new clue for the design of the EMA applications was supplied.

Electromagnetic properties of FeNi alloy nanoparticles prepared by hydrogen-thermal reduction method

FeNi alloy nanoparticles were prepared by hydrogen-thermal reduction in nickel ferrite nanoparticles at 400 °C for 1 h. FeNi alloy nanoparticles with average particle size of 150 nm in diameter are composed of Fe, Ni, and FeNi solid solution. The effective complex permittivity and complex permeability of FeNi alloy nanoparticles in the frequency range of 2–18 GHz were measured, and the microwave reflection loss of FeNi/wax with different assumed coating thicknesses was calculated according to the transmission line theory. The hydrogen-thermal reduced FeNi alloy nanoparticles were found to possess high permeability and superior microwave absorption performance in the microwave band.

Dissolution of Au nanoparticles in hydrochloric acid solution as studied by optical absorption

Nanoscale Au particles dispersed in pores in mesoporous silica were prepared by the sol–gel technique, followed by thermal reduction in hydrogen. In contrast to bulk gold metal, Au nanoparticles as so-called microelectrodes can be dissolved in hydrochloric acid (HCl), as measured by optical absorption. We attribute this dissolution phenomenon to the high chemical activity of nano-sized particles.

Stabilization and reactivity of Ni + ions on TiO 2 and TiO 2/SiO 2 systems

Coordinately unsaturated Ni + cations were generated by either mild thermal reduction in hydrogen at 450 K or photoreduction in hydrogen at 77 K of Ni 2+ ions dispersed on TiO 2 and TiO 2/SiO 2 supports. Upon adsorption of carbon monoxide, the Ni + ions lead to the formation of surface Ni + -carbonyl complexes which have been characterized by EPR spectroscopy. Photoreduction is more effective in the formation of definite valence and coordination state of Ni(I). Thermal reduction produced Ni 0 metal particles and Ni + ions. The results showed that the Ni + ions were probably located at the metal-support interfaces. On further thermal reduction, metal particles can be procuded with unusual catalytic properties, possibly because of specific Ni + or Ti 3+ anchoring sites.