The development of active, stable, earth -abundant, and cheap catalysts is crucial for renewable energy conversion devices. Catalysts for electrochemical water splitting are prepared using cold uniaxial pressing and consist of a superficial ferrite coating on a nickel substrate. The activity and mechanism of the oxygen (OER) and hydrogen (HER) evolution reactions on powder compacts are investigated. It is found that powder compacts are efficient catalysts for oxygen and hydrogen evolution in a 1 M KOH solution. For Ni/NiFe2O4, the current density of 10 mAcm- 2 in OER and -10 mAcm- 2 in HER shows overpotentials of 272 mV and -43 mV, respectively. In turn, for Ni/CoFe2O4, those overpotentials are 279 mV and -37 mV. Furthermore, using the same material for the anode and cathode in a two -electrode cell configuration, one can achieve a 10 mAcm- 2 water splitting current at only 1.55 V for over 48 h without coating degradation. It is stated that the high activity towards OER and HER results from the large electrochemically active surface area and high electrical conductivity of powder compacts. The results also indicate that the most probable rate -determining steps for OER and HER are the formation of the adsorbed oxide atom and H2 molecule, respectively.

This research is focused on designing new ceramic oxides with the spinel structure for a potential application as sensitive temperature and magnetic field sensors. In this respect, the mixtures of ferrimagnetic NiFe2O4 with low-resistive Ni0.66Cu0.41Mn1.93O4 semiconductor (molar coefficient alpha = 1/5 and 1/2) were prepared using the chemical co-precipitation technique followed by a low-temperature sintering of the cold-pressed powders. For the prepared materials, a variation of the electrical conductivity sigma dc with temperature T (in the range 50 K-400 K) is quantitatively analyzed using different theoretical concepts of the charge carrier hopping transport occurring in disordered materials with strong electron-phonon interaction. It has been shown that an increase in alpha from 0 to 1/2 causes a step-like change of the dc electrical conductivity and, simultaneously, a gradual change in the magnetization M determined in the saturation state. The temperature coefficient of resistance TCR of the prepared ceramics takes the values from -0.6%/K (at 400 K) to-19.6%/K (below 100 K). The highest absolute value of the magneto-resistance coefficient MR determined at 150 K (and at magnetic field 7 T) registered for ceramic oxide with alpha = 1/2 was found to be 3.4%.

In this paper, high-entropy ceramic oxide with the spinel structure and chemical composition Mn0.62(2)Fe0.62 (2)Ni0.60(3)Co0.58(2)Zn0.58(3)O4 was prepared using the co-precipitation technique combined with a lowtemperature sintering process of the cold pressed nanoparticles. Variation of the electrical conductivity sigma in the temperature range from 120 K to 400 K is presented and discussed in detail. The results have been quantitatively analysed using theoretical models describing the mechanisms of the electrical conductivity in disordered ionic materials with a strong electron-phonon interaction. It has also been shown that the material examined exhibits a relatively high value of the temperature coefficient of resistance TCR, which in the temperature range of 150 K - 300 K varies from -10.6(2) %/K to -4.1(2) %/K. The magnetoresistance coefficient determined at T = 150 K and magnetic field 7 T was found to be -4.45(5) %.

In this paper, two novel procedures based on powder sedimentation, thermal treatment, and galvanostatic deposition were proposed for the preparation of porous cobalt ferrite (CoFe2O4) coatings with a metallic and organic binder for use as catalysts in the oxygen evolution reaction (OER). The electrochemical properties of the obtained electrode materials were determined as well, using both dc and ac methods. It was found that cobalt ferrite coatings show excellent electrocatalytic properties towards the oxygen evolution reaction (OER) with overpotential measured at a current density of 10 mAcm−2 from 287 to 295 mV and a Tafel slope of 35–45 mVdec−1. It was shown that the increase in the apparent activity of the CoFe2O4 coatings with an organic binder results mainly from a large electrochemically active area. Incorporation of the nickel binder between the CoFe2O4 particles causes an increase in both the conductivity and the electrochemically active area. The Tafel slopes indicate that the same rate-determining step controls the OER for all obtained coatings. Furthermore, it was shown that the CoFe2O4 electrodes exhibit no significant activity decrease after 28 h of oxygen evolution. The proposed coating preparation procedures open a new path to develop high-performance OER electrocatalysts.

This paper presents synthesis and analysis of critical behavior of (Ni,Zn,Fe,Cu,Mn)3O4 spinel-oxide ceramics near magnetic phase transition. In order to investigate the critical point, a modification of the standard analysis of Arrott-Noakes diagrams was proposed. This new approach takes into account the quality of straightness of isotherms and the fact that the critical straight line (at T=Tc) must pass through the origin of the coordinate system. The determined values of critical parameters β= 0.446, γ= 1.179, and δ= 3.637 are close to those of the mean-field model (β= 0.5, γ= 1, and δ= 3) signifying mainly the isotropic, long-range exchange interactions in the studied ceramic oxide. The self-consistency of the obtained results was confirmed taking into account the scaling theory, and the asymptotic relations for the behaviour of magnetization M in the limits as T→Tc and H→0.

This paper presents synthesis and analysis of critical behavior of (Ni,Zn,Fe,Cu,Mn)(3)O-4 spinel-oxide ceramics near magnetic phase transition. In order to investigate the critical point, a modification of the standard analysis of Arrott-Noakes diagrams was proposed. This new approach takes into account the quality of straightness of isotherms and the fact that the critical straight line (at T = Tc) must pass through the origin of the coordinate system. The determined values of critical parameters beta = 0.446, gamma =1.179, and delta = 3.637 are close to those of the mean-field model (beta = 0.5, gamma = 1, and delta = 3) signifying mainly the isotropic, long-range exchange interactions in the studied ceramic oxide. The self-consistency of the obtained results was confirmed taking into account the scaling theory, and the asymptotic relations for the behaviour of magnetization M in the limits as T-*T-c and H-*0.

Detailed examination of corrosion-induced changes of the 316L steel surface (immersed in 5 wt% NaCl solution) is presented and discussed. The evolution of the stable pit depth (h(av)) with the immersion time (t) was established using 3D maps and statistic techniques. It was found thathav proportional to tnwithn approximate to 0.5. Moreover, determination of the pit area allows estimating the curve current density (j) versus the immersion time and it was found thatj proportional to t-mwithm approximate to 1. A novel technique for surface corrosion degree determination is based on analysis of 2D grayscale images instead of black and white images showing that corrosion morphology was elaborated. For this purpose a three-layered, feed-forward neural network with the Levenberg-Marquardt backpropagation training algorithm was used. It was shown that a dependence corrosion degree versus immersion time (S-type curve) can be fully described by the proposed procedure.

ZnCr2Se4 single crystals doped with holmium ions having concentrations of 0.032, 0.055 and 0.084 (occupying octahedral positions) were successfully synthesised using chemical vapour transport. The crystals were characterised using various method: scanning electron microscopy (SEM), X-ray diffraction (XRD), SQUID and QD-PPMS measurements, and thermal analysis (DSC/TG). The ac and dc magnetic measurements as well as the specific heat studies with an increasing holmium content showed: 1) antiferromagnetic order with the Neel temperature of T-N = 22 K, 2) positive value of the paramagnetic Curie-Weiss temperature increasing from theta = 53 to theta = 100 K, 3) a slight decrease in both the critical field Hc1 characteristic for a metamagnetic transition and the critical field Hc2 corresponding to the breakdown of the helical spin arrangement, 4) the magnetic contribution to the specific heat visible on the sharp peak at T-N, which is strongly shifted to much lower temperatures as the magnetic field increases, and 5) the quite significant Sommerfeld gamma coefficient suggesting the properties of the heavy fermion of the single crystal under study. These effects are interpreted in terms of the exchange integrals including non-stoichiometry, spin and structural defects. The DSC/TG analysis confirmed thermal stability of obtained single crystals.

Application of the power spectral density (PSD) function and the auto-correlation (AC) function to the quantitative determination of surface anisotropy is presented and discussed in detail. A new scaling procedure of the anisotropy ratio is proposed and discussed as well. Our approach is based on the obvious fact that a random system by definition is free from any texture pattern and can be considered as a reference system. The proposed procedure was successfully tested using numerically generated surface maps with a systematic variation of the anisotropy intensity. It was shown that the elaborated method allows determining the surface anisotropy over 80% and even below 1%. Finally, the procedure was used to analyze the grain anisotropy forced by plastic deformation (up to 30%) of AISI 316 L stainless steel. It was shown that the anisotropy intensity is directly proportional to the strain determined from the tensile tests.

In this article, magnetic and structural characteristics of CoFe2O4 nanopowders with mean diameter 17.8(4) and 63.7(2.4) nm [denoted as NM(18) and CP(60), respectively] were studied by applying X-ray diffraction method (XRD), transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM). It was established that for NM(18), the nanoparticle size corresponds to one crystallite while CP(60) nanoparticle forms a multi-crystallite system which means certain kind of averaging of magnetic moments. Magnetic analysis was performed according to the ZFC-FC protocol in the temperature range of 10-380 K, and magnetization curves M(H) were recorded at the magnetic field up to 7 T. It was shown that, independently on nanoparticle size distribution, the magnetic response of both nanopowders is mainly determined by superspin glass type interaction of magnetic moments corresponding to a single crystallite. In addition, basic magnetic characteristics such as anisotropy constant K-1 = (3.9-12.5) x 10(5) J/m(3), anisotropy field mu H-0(A) = 1.80-5.80 T, or magnetic moment per formula unit mu = 3.45(5)mu(B) [for CP(60)] and mu = 3.50(5) mu(B) [for NM(18)] were also determined and discussed.

The paper refers to examination of corrosion resistance improvement of aluminium based material subjected to a sealing procedure. The tests were carried out for non-sealed and sealed anodic coatings using both scanning microscopy and electrochemical techniques. The correlation between electrochemical parameters (i.e. corrosion potential (E-corr), corrosion current density (j(corr))) and contact potential difference (CPD) was examined. It was found that in the first approximation j(corr) as well as E-corr change linearly with CPDav (average CPD). It was shown also, that a decrease of j(corr) and an increase of CPDav caused by anodic oxidation can be explained by an increase in oxide thickness. Further increase in CPDav observed for sealed coatings is related to a decrease of root mean square roughness and/or disappearance of surface anisotropy.

The evolution of frozen magnetic state of Zn delta Co1 - delta Fe2O4 (0 <= delta <= 1) ferrite nanoparticles was studied by applying vibrating sample magnetometer measurements in temperature range 5-350 K and magnetic fields up to 7 T. It was shown that gradual conversion from the inverse spinel (delta = 0) to the normal one (delta = 1.0) is correlated with a drop of freezing temperature T-f (corresponding to blocking of mean magnetic moment of the system) from 238 K (delta = 0) to 9 K (delta = 1.0) and with a decrease of magnetic anisotropy constant K-1 from about 8 . 10(5) J/m(3) to about 3 . 10(5) J/m(3). The percolation threshold predicted for bulk ferrites at 1 - delta approximate to 0.33 was observed as a significant weakness of ferrimagnetic coupling. In this case magnetization curves, determined according to the zero field cooling protocol, reveal two distinct maxima indicating that the system splits into two assemblies with specific ions distribution between A and B sites. (C) 2018 Elsevier B.V. All rights reserved.

Series of Ni45.5-xCo4.5Mn36.6In13.4Bx (at.%, x = 0, 0.05, 0.1, 0.5, 1.0) polycrystalline magnetic shape memory alloys were examined in terms of the magnetic properties, structure and transition temperatures. Depending on the boron concentration single or two phase alloys microstructures were observed. Additionally, the martensitic transformation temperatures decreases with the boron addition. Magnetic-field induced transformation occurs for the alloys with the boron addition up to 0.1 at.%. For alloys with 0.5 and 1.0 at.% of B transformation is hindered.

In the present paper the Co1−δZnδFe2O4 (0 ≤ δ ≤ 1) ferrite nanopowders with a spinel type structure were synthesized using a chemical co-precipitation technique with constant flow rate νFR = 120 cm/min at three different reaction temperatures i.e. Tr = 50 ◦C, 70 ◦C and 90 ◦C. Magnetic and structural characteristics of the obtained materials were investigated by means of X-ray diffraction method, transmission electron microscopy and vibrating sample magnetometer. In the course of studies hysteresis loops M(μ0H) and the relations of magnetization M7T (determined at μ0H = 7 T), squareness ratio S and the Néel temperature TN versus Zn content were determined and discussed in detail. It was shown that for δ < 0.6 the increase in reaction temperature Tr results in a significant increase of the measured magnetic characteristics. In particular, in the case of Co0.8Zn0.2Fe2O4 ferrite nanopowder magnetization M7T reaches maximal value of about 80 emu/g.

In the present paper the Co1-delta Zn delta Fe2O4 (0 <= delta <= 1) ferrite nanopowders with a spinel type structure were synthesized using a chemical co-precipitation technique with constant flow rate nu(FR) = 120 cm(3)/min at three different reaction temperatures i.e. T-r = 50 degrees C, 70 degrees C and 90 degrees C. Magnetic and structural characteristics of the obtained materials were investigated by means of X-ray diffraction method, transmission electron microscopy and vibrating sample magnetometer. In the course of studies hysteresis loops M(mu H-0) and the relations of magnetization M7T (determined at mu H-0 = 7 T), squareness ratio S and the Neel temperature T-N versus Zn content were determined and discussed in detail. It was shown that for delta < 0.6 the increase in reaction temperature T-r results in a significant increase of the measured magnetic characteristics. In particular, in the case of Co0.8Zn0.2Fe2O4 ferrite nanopowder magnetization M7T reaches maximal value of about 80 emu/g.

Series of Ni45.5-xCo4.5Mn36.6In13.4Bx (at.%, x = 0, 0.05, 0.1, 0.5, 1.0) polycrystalline magnetic shape memory alloys produced by the induction melting were examined in terms of the structure and transition temperatures. The structure of the alloys was determined by the X-ray diffraction and transmission electron microscopy. Scanning electron microscopy and electron backscattering diffraction techniques were applied to obtain the microstructure and texture of alloys. Boron addition promotes nucleation of the second Co-rich and In-poor phase as well as causes decrease of the martensitic transformation temperatures.

A new approach to numerical analysis of maps of material surface has been proposed and discussed in detail. It was concluded that the roughness factor RF and the root mean square roughness Sq show a saturation effect with increasing size of the analysed maps what allows determining the optimal map dimension representative of the examined material. A quantitative method of determining predominant direction of the surface texture based on the power spectral density function is also proposed and discussed. The elaborated method was applied in surface analysis of Ni+Mo composite coatings. It was shown that co-deposition of molybdenum particles in nickel matrix leads to an increase in surface roughness. In addition, a decrease in size of the embedded Mo particles in Ni matrix causes an increase of both the surface roughness and the surface texture. It was also stated that the relation between the roughness factor and the double layer capacitance Cdl of the studied coatings is linear and allows determining the double layer capacitance of the smooth nickel electrode.

The crystal structure, magnetic isotherm, magnetic susceptibility, electrical conductivity and specific heat measurements for single-crystalline ZnxDyyCrzSe4 (where x+y+z≈3) spinels are presented. A semiconducting behavior with the activation energy of 0.53eV, an antiferromagnetic order with a Néel temperature TN=22K and a strong ferromagnetic exchange evidenced by a positive Curie–Weiss temperature θ=79, 71 and 70K with increasing Dy-content in the sequence 0.05, 0.13 and 0.19 were established. Below TN the magnetic field dependence of magnetization, M(H), shows two peaks at critical fields Hc1 and Hc2. The values of Hc1 decrease slightly with temperature, especially for the larger Dy-content, while the values of Hc2 drop rapidly with temperature. The magnetic contribution to the specific heat displays a sharp peak at TN, which is strongly shifted to much lower temperatures in the applied magnetic fields. Similar behavior was found for the temperature dependence of the specific heat C(T) plotted as C(T)/T vs. T. The value of the magnetic and phonon contribution to the entropy at TN and at H=0 is only ∼4.8, ∼4.4 and ∼4.2Jmol−1K−1/Cr3+ for y=0.05, 0.13 and 0.19, respectively, much lower than the average magnetic contribution Sm=(z/2)Rln(2S+1)=12.33Jmol−1K−1/Cr3+ calculated for Cr3+ ion with S=3/2, as the dysprosium one is paramagnetic.

CuCr2Te4 can be obtain by mechanical alloying followed by heat treatment. The obtained phase crystallizes in the spinel-type structure of the space group Fd3m. The calculated crystallite size equals to 100 nm. Magnetic susceptibility measurements showed ferrimagnetic order below 21 K.