Hydrogen barrier coatings are protective layers consisting of materials with a low intrinsic hydrogen diffusivity and solubility, showing the potential to delay, reduce or hinder hydrogen permeation. Hydrogen barrier coatings are expected to enable steels, which are susceptible to hydrogen embrittlement, specifically cost-effective low alloy-steels or light-weight high-strength steels, for applications in a hydrogen economy. Predominantly, ceramic coating materials have been investigated for this purpose, including oxides, nitrides and carbides. In this review, the state of the art with respect to hydrogen permeation is discussed for a variety of coatings. Al₂O₃, TiAlN and TiC appear to be the most promising candidates from a large pool of ceramic materials. Coating methods are compared with respect to their ability to produce layers with suitable quality and their potential for scaling up for industrial use. Different setups for the characterisation of hydrogen permeability are discussed, using both gaseous hydrogen and hydrogen originating from an electrochemical reaction. Finally, possible pathways for improvement and optimisation of hydrogen barrier coatings are outlined.

This study investigates a thermal synthesis in which iron(II) and iron(III) chlorides are used to form maghemite nanoflowers in the presence of sodium hydroxide and a solvent mixture of N-methyldiethanolamine and diethylene glycol. The agglomeration process leading to nanoflower formation is examined by testing temperatures of synthesis ranging from 180 °C to 220 °C and holding times from 2 h to 12 h. A temperature of 220 °C and a holding time of 2 h are confirmed to be suitable synthesis parameters for nanoflower formation. The process of primary particle ordering during agglomeration leading to cooperative magnetic behaviour is discussed. The stoichiometric ratio of Fe²⁺ and Fe³⁺ ions was varied in the precursor solution and it is shown to have a strong influence on particle and crystallite sizes, and thereby on the magnetic properties. A larger Fe³⁺ ion content during the synthesis leads to larger particles and crystallites, while a higher content in Fe²⁺ ions favours nucleation at the expense of growth. An additional treatment with iron nitrate leads to further growth of crystallites as well as particles. Stable, mostly monodisperse suspensions of multicore particles with diameters ranging from 18.4 nm to 28.7 nm show Brownian relaxation times between 0.2 μs and 9 μs and dynamic susceptibilities at 25 kHz from about 7 × 10⁻³ m³ kg⁻¹ [Fe] to 20 × 10⁻³ m³ kg⁻¹ [Fe], making the particles interesting candidates for magnetic hyperthermia and magnetic particle imaging.