Ferromagnetic ‘Ti/Fe- and/or Co-containing TiO2-layer’ composites formed by plasma electrolytic oxidation (PEO) in electrolytes with dispersed particles of iron and/or cobalt hydroxides exhibited photocatalytic activity in methyl orange (MO) degradation under UV and visible light in the presence of hydrogen peroxide. The influence of Fe/Co ratio and additional annealing was studied on their chemical composition, optical, photocatalytic, and adhesion properties. With the proportion of cobalt in the coatings, their adhesion to titanium increased. After annealing, coatings with a higher proportion of iron had higher adhesion. The optical band gap energy Eg determined for direct allowed transitions was 1.37 eV and 1.61 eV for individual Fe-containing and Co-containing composites, respectively. In mixed Fe- and Co-containing PEO-composites, two direct allowed transitions were detected with Eg1=1.68 eV and Eg2=2.21-2.54 eV. Among initial PEO-coated samples, the most active and stable are Fe- and Co-containing PEO layers with a predominance of cobalt: MO degradation under 3-h irradiation with UV and visible light was 64 and 21%, respectively. According to X-ray photoelectron spectroscopy, their surface layers contain a higher percentage of Fe2+ and Co2+ compared to Fe3+ and Co3+. Annealing the samples in air led to an increase in Eg values and a leveling off of photocatalytic activity as a result of the formation of crystalline NaTi2(PO4)3, FePO4, or CoPO4 phases and an increase in the proportion of oxidized iron. The main role in the degradation of MO was found to be played by hydroxyl radicals formed as a result of the interaction of hydrogen peroxide with photogenerated electrons.

The possibility of forming the La0.5Ca0.5MnO3/Al2O3/Al composite by modifying the aluminum oxide matrix, previously formed by plasma electrolytic oxidation (PEO), with calcium lanthanum manganite powder was shown. The aluminum oxide matrix represented a regular system of parallel cylindrical pores with a predominant diameter d similar to 250-300 nm. According to scanning electron microscopy images of the surface and cross-section of the composite, the use of rubbing followed by annealing at 400 degrees C made it possible to firmly fix La0.5Ca0.5MnO3 particles on the surface and in the depth of the porous PEO layer. After the "rubbing + annealing" operation, the orthorhombic modification of the La0.5Ca0.5MnO3 powder was retained with a slight change in the crystal lattice parameters. The surface of the resulting composite contained 1.8 at% Ca, 2.3 at% Mn and 1.5 at% La. The formed composite exhibited weak ferromagnetism at 300 K; lowering the temperature to 3 K led to pronounced ferromagnetic ordering with a H-c value of similar to-855/649 Oe.

“La+Mn-containing TiO2-layer/Ti” composites were formed by plasma electrolytic oxidation in silicate electrolytes with dispersed particles of lanthanum manganite of different composition. The influence of solid-state synthesis temperature on phase composition and magnetic characteristics of lanthanum manganite powder has been studied. An increase in temperature from 870 to 1300 ºС leads to an increase in the coercivity of the powders, which is associated with a change in particle stoichiometry and size. It has been established that the properties of the powder introduced into the electrolyte affect the features of plasma electrolytic formation (voltage-time responses, final voltage), thickness, morphology, and composition of coatings. The addition of powder synthesized at a higher temperature leads to a drop in the formation voltage and a halving of the thickness of the coatings, a greater incorporation of manganese and lanthanum into the composition of the coatings and pores, as well as pronounced ferromagnetism at 3 K. The preservation of magnetic properties over time (1 year) has been tested using the example of a PEO composite obtained in a silicate electrolyte with lanthanum manganite powder synthesized at 870 °C.

A new approach was proposed to the formation of Ti-supported film oxide structures containing lanthanum manganite. To form layered composites, we used plasma-electrolytic oxidation in a silicate electrolyte containing lanthanum manganite powder preliminarily synthesized by the solid-phase method. The formed coating with a thickness of 55 +/- 6 mu m consisted of TiO2 (rutile), LaMnO3 and La7.58(Si1.048O4)6O2 and contained 3.3 at% La and 3.7 at% Mn in the surface layer. Particles enriched in electrolyte metals (La and Mn) were found in all com-ponents of the coating, and most of them were localized in pores and cavities, including in the form of ag-glomerates with a diameter d <= 300 nm. The resulting composite exhibited ferromagnetic ordering at room temperature. When the temperature was lowered to 3 K, the material did not undergo any phase transitions leading to a change in the magnetic ordering.

Oxide layers on titanium have been formed via the method of plasma electrolytic oxidation (PEO) in electrolyte–suspensions containing colloidal particles of iron and nickel hydroxides with a ratio of Fe3+/Ni2+ = 3 : 1 for 5–15 min. The average concentrations of iron, nickel, and titanium in the composition of the coatings were 6.1, 2.2, and 3.5 at %, respectively. For all the composites, the values of coercive force Hс at 300 K did not exceed 59 Oe, which can be attributed to soft magnetic materials. At 2 K, an increase of the magnetization values and a significant increase of Hc up to 496–679 Oe have been observed for all the samples. It has been hypothesized that the contribution to the magnetic behavior of the samples at room temperature is made by the bulk of coatings, whereas at helium temperature it was the contribution of microsized formations with an increased content of iron and nickel found in the pores. Increasing the duration of the PEO process up to 15 min leads to a decrease of Hс values by almost 100 Oe at 2 K, which can result from a decrease of the proportion of iron in the composition of crystallites and the appearance of spherical particles with an increased concentration of phosphorus, titanium, and oxygen in the pores. It has been established that, after long-term storage of the samples in air, the coercive force measured at 2 K decreased by almost twice, which could have been the result of the oxidation of metallic Fe+Ni-containing particles localized in open pores on the coating surface.

Fe-and/or Co-containing coatings have been formed on titanium by plasma electrolytic oxidation (PEO) in electrolytes-sols with colloidal particles of Fe(III) and/or Co(II) compounds. The influence of the electrolyte formula has been studied on the coatings' composition, surface morphology, and magnetic properties. It has been established that the total concentration of iron, cobalt, and titanium in the coating composition is 13.1-22.0 at. %, while their thickness varies from 22 to 37 mu m, and the porosity ranges from 10 to 26 %. Micro-and nanosized crystal-like particles have been found both on the surface (in valleys, around pore mouths, and in caps covering pores) and inside pores (at the bottom and walls). In the particles and the layer lining the bottom of the pores, the concentration of iron, cobalt, and titanium is higher than over the surface (49.7-59.1 at. %). Depending on electrolyte formula, the crystallites located in the pores contain (at. %) 28.4-39.3 O; 3.5-9.5 P; 20.3-29.4 Ti; 2.1-7.5 W; up to 22 Fe and up to 38.8 Co. The values of the coercive force measured at 300 K vary from 6366 to 27,056 A/m, and those at 3 K range from 3581 to 17,428 A/m. The ferromagnetic properties of the coated samples correlate with the elemental composition of the particles found in the pores and on the surface.

Fe-, Ni-containing coatings have been formed on aluminum and titanium by the plasma electrolytic oxidation (PEO) technique in slurry electrolytes containing colloidal particles of Fe(III) and Ni(II) compounds including hydroxides. Their composition, surface morphology, and magnetic properties were studied and compared. In both cases, the concentration of iron, nickel, and substrate metal is higher, and the oxygen concentration is lower in the pores of the coatings than their average concentration over the surface. In the case of titanium, the electrolyte and substrate metals are concentrated in the pores (at. %: 38.6 Fe, 4.7 Ni, 19.5 Ti) in the form of micro- and nanosized crystallites. In the pores of the coating on aluminum, metals are concentrated in the composition of the layers lining the pores (at. %: 15.3 Fe, 50.6 Ni, 18.8 Al). The values of the coercive force and the saturation magnetization of the ferromagnetic component at 300 K for the aluminum sample (H-c = 38 Oe, M-s* = 2.8 x 10(-4) emu/g) are significantly lower than those for the titanium sample (H-c = 73 Oe, M-s*= 7.8 x 10(-3) emu/g). The discrepancy between the ferromagnetic properties of the aluminum and titanium samples correlates with the difference in the elemental composition of the pores and the presence/absence of crystallites in the pores. More pronounced ferromagnetic properties of titanium samples can be associated with the presence of crystallites in the coating pores.

The influence of the nature of valve metals on the ferromagnetic characteristics of oxide coatings formed by plasma electrolytic oxidation (PEO) was studied for the first time. The coatings were obtained under the same current and time conditions in an alkaline electrolyte with colloidal particles of Fe(III) and Co(II) hydroxides. The magnetization dependences of the samples on the external magnetic field strength at room temperature were studied, data on the phase and elemental compositions of the coatings and the morphology of their surfaces were obtained, and the compositions of pores and crystallites in pores were studied. It is shown that the behavior of the magnetization curves for Al-samples differs noticeable from that of the Ti-, Zr-, and Nb-samples. The value of the coercive force decreases in the series Zr > Nb > Ti > Al, and the value of the saturation magnetization of the ferromagnetic component decreases in the series Al > Zr > Nb > Ti. Differences in the ferromagnetic properties of Al-samples and Ti-, Zr- and Nb-samples correlate with differences in the elemental composition of the pores of the coatings.

A comprehensive study of the magnetic behavior, morphology, and composition of Fe-containing oxide coatings on aluminum and titanium has been carried out to investigate the origin of their ferromagnetism. The coatings have been formed by the plasma electrolytic oxidation (PEO) technique in slurry electrolytes containing colloidal particles of iron(III) hydroxides. On the surface of coatings on Al, iron is distributed unevenly concentrating in defective areas with a large number of small pores, and near large pores. On the surface of coatings on Ti, iron and titanium are distributed in antiphase in areas of comparable size. Within the pores, iron concentration appears about 5-10 times higher and oxygen concentration 3-4 times lower than their average concentration over the surface. In both cases, localization of the areas with ferromagnetic properties follows the peculiarities of iron distribution on the surface. The magnetic fraction in the coatings on aluminum appears to be represented by iron-aluminum spinel Fe3-xAlxO4 with x > 0.06, likely cation-deficient. Elemental iron and traces of iron hydroxides are also possibly present. In the coatings on titanium, titanomagnetite (Fe3-xTixO4, where x similar to 0.2-0.3) or its oxidized analogue, titanomaghemite, appear to be present, and possibly also some Fe-Ti alloy particles. (C) 2019 Elsevier B.V. All rights reserved.

Fe-containing oxide coatings fabricated by the plasma-electrolytic oxidation (PEO) method have been studied by means of scanning electron and magnetic force microscopy. The data comparison has demonstrated that ferromagnetic properties of the coatings are mainly associated with concentrating of iron in specific pores. A scheme has been suggested that would explain iron distribution in the course of the coating growth.

The effect replacing Na3PO4 with Na2HPO4 in aqueous phosphate–borate–tungstate electrolyte that additionally contains Fe2(C2O4)3 on the magnetic and magnetoresistive characteristics of oxide coating/ titanium composites formed by means of plasma electrolytic oxidation (PEO) is investigated. It is established that PEO coatings with ferromagnetic characteristics form on titanium in an electrolyte containing Na3PO4 (pH ~ 11) upon adding iron(III) oxalate, while replacing Na3PO4 with Na2HPO4 and the respective drop in the pH of the base electrolyte down to 9.8 results in the formation of coatings with different magnetic characteristics. The correlation between changes in the values of the charge carriers’ activation energy and the magnetic susceptibility is demonstrated for the latter. An increase in the electric resistance of coatings in a magnetic field is observed, and the type of the magnetic resistance temperature dependence is established.

The effect of iron sulfate and citrate addition into the base alkaline phosphate-borate-tungstate electrolyte on the peculiarities of plasma-electrolytic formation of coatings on titanium, their thickness, surface morphology, composition, and magnetic characteristics has been investigated. In the first electrolyte, dispersed particles of iron hydroxides and hydroxo salts are formed, whereas the second one comprises a true solution. Numerous Fe-containing crystallites of a size of ~50 nm united into agglomerates have been found in the suspension electrolyte with FeSO4. Such coatings manifest ferromagnetic properties: coercive force Hc of the samples is 62 and 148 Oe at 300 and 2 K, respectively. In pores of the coatings obtained in the electrolyte with FeC6H5O7 (true solution), the presence of crystallites is less clearly expressed, while crystallites themselves are larger and molten to a higher degree. At room temperature, such coatings are paramagnetic; at 2 K, they manifest ferromagnetic behavior with the Н с value of up to 200 Oe. The available data enable one to associate ferromagnetic properties of the formed coatings with metals concentrated in pores.

The composition, structure, and magnetic characteristics of oxide layers on titanium formed in electrolytes containing colloid particles of iron hydroxo-compounds and their filtrates have been investigated. The obtained results corroborate that formation of Fe-containing crystallites in coating pores occurs due to ingress of negatively charged particles of hydroxo-compounds of transition metals from the electrolyte into breakdown channels and their transformation in local spaces of electric breakdowns. The presence of crystallites in pores is responsible for coatings ferromagnetic properties. Fe-containing crystallites were not found in pores of coatings formed in the electrolyte after filtering of iron hydroxides and hydroxo-salts, whereas coatings contained small concentrations of iron homogeneously distributed over the surface and manifested paramagnetic properties at room temperature.

The paper presents some of the directions of developing the plasma electrolytic oxidation (PEO) technique to form the coatings with magnetic, catalytic, biocompatible or biocidal properties on the valve metals and alloys. It reflects the relationships between the structure, composition and functional properties of PEO coatings. The data presented suggest that PEO is an effective method of physicochemical synthesis on metals and alloys of the surface layers with different chemical composition and certain characteristics.

Layered oxide coatings containing europium ferrite multiferroic have been synthesized on titanium plates through plasma electrolytic oxidation and extraction pyrolysis. The composites have weak ferromagnetic properties: the coercive force attains 45–78 Oe in the temperature range 3–340 K. Their luminescence properties are typical of inorganic materials with europium ions.