A. Baszczuk

Tuning of magnetic and electronic states by control of oxygen content in lanthanum strontium cobaltites

We report on the magnetic, resistive, and structural studies of perovskite La1/3Sr2/3CoO3−�. By using the relation of synthesis temperature and oxygen partial pressure to oxygen stoichiometry obtained from thermogravimetric analysis, we have synthesized a series of samples with precisely controlled � = 0.00 0.49. These samples show three structural phases at � = 0.00 0.15, � 0.25, � 0.5, and two-phase behavior for other oxygen contents. The stoichiometric material with � = 0.00 is a cubic ferromagnetic metal with the Curie temperature TC = 274 K. The increase ofto 0.15 is followed by a linear decrease of TC to � 160 K and a metal-insulator transition near the boundary of the cubic structure range. Further increase ofresults in formation of a tetragonal 2ap ×2ap ×4ap phase for � � 0.25 and a brownmillerite phase for � � 0.5. At low temperatures, these are weak ferromagnetic insulators (canted antiferromagnets) with magnetic transitions at Tm � 230 and 120 K, respectively. At higher temperatures, the 2ap × 2ap × 4ap phase is G-type antiferromagnetic between 230 K and �360 K. Low temperature magnetic properties of this system for � 1/3, there appears to be a combination of Co 2+ and Co 3+ ions, both in the high-spin state with dominating antiferromagnetic interactions.

Corrosion resistance of tin coatings deposited by cold spraying

This article presents a study of corrosion resistance of tin and tin-based composite (Sn + Al2O3 and Sn + SiC) coatings deposited onto aluminium alloy substrates using the low-pressure cold spraying method. The samples were subjected to three different corrosion tests at a room temperature: (i) Kesternich test, (ii) a cyclic salt spray test and (iii) anodic polarisation test. The selected tests allowed extreme environment simulation typical for wide range of urban, industrial and marine service conditions. Evaluation of corrosion was carried out by analysing changes on the coating surface as well as in the microstructure. Additionally, the physicochemical tests were carried out using X-ray diffraction. XRD analysis clearly showed that abhurite Sn21O6Cl16(OH)14 is the main corrosion product established after the polarisation measurements. Despite extreme corrosive conditions used in experiments all tin coatings showed minor corrosion changes which implies that abhurite serves as a buffer between the metal and the oxidising natural environment.

Active Sol-Gel Materials

Sol-gel technology has attracted considerable attention due to possibility of obtaining submicron and nano-sized materials. The method of silica and titania nanopowders and thin films obtaining will be presented. Also properties and prospective application of these materials will be express. Additionally, methods of obtaining nanomaterials with different grains shape and specific properties (submicron spherical silica powders, titania nanofibers) will be showed. One of the main advantages of the sol-gel technique is the easiness of doping of the obtained materials with various substances (inorganic, organic, biological). Materials activated this way possess several useful properties. For example, silica spherical matrices with metallic nano-islands on their surface will be presented. Such silver-doped silica powders display anti-microbial capabilities and can be used to obtain doped thin-film coatings e.g. for the production of bacteriostatic textiles. Moreover, materials obtained by the sol-gel method have found wide application in the area of sensors. Examples of optical sensors based on sol-gel derived thin films and optical fiber or planar wave-guide will be also presented.

Aluminium to steel resistance spot welding with cold sprayed interlayer

ABSTRACT Studies on bonding of aluminium alloys to steels are popular because these are structural materials widely used in a variety of industries. However, joining these dissimilar materials is difficult mainly because of the formation of brittle intermetallic compounds. This paper presents a study on welding aluminium to steel by resistance spot welding. Before the welding, steel surface was covered by cold spraying with the layer of aluminium, nickel and nickel–aluminium. This way, instead of the welding of dissimilar materials, the welding of aluminium to aluminium (or nickel) layer pre-deposited on the steel sample was performed. The feasibility of using interlayers for improving the welding of dissimilar materials was tested using SEM, EDX and XRD. Mechanical properties of welds were investigated by microhardness and shear strength tests. The results showed that the coating allowed to decrease hardness in the welding zone and to increase the shear strength of the weld.

Insights into the multistep transformation of titanate nanotubes into nanowires and nanoribbons

Abstract Different types of titanate one-dimensional nanostructured materials were synthesized and characterized using scanning and transmission electron microscopy, X-ray diffraction and Raman spectroscopy. The results presented in this work unquestionably showed dependence of morphology and structure of the titanate nanopowders on parameters of hydrothermal synthesis. It was found that nanotubes, nanowires and nanoribbons are three unavoidable kinetic products of hydrothermal reaction. Moreover, increasing temperature of reaction or hydrothermal treatment duration results in acceleration of nanotube-nanowire-nanoribbon transformation. However, the sequence of titanate morphology transformation is invariable. The detailed studies further revealed that the crystal structure of hydrothermally prepared nanotubes and nanowires are indistinguishable but the determination of the exact structure is practically impossible. Because of higher crystallinity, the structure of nanoribbons can be established. It was shown that it corresponds to the monoclinic layered trititanic acid H2Ti3O7 and is isostructural with sodium derivatives Na2_xHxTi3O7.nH20 (with x near 2).

Crystal structure, magnetic and electrical properties of new indium copper-oxycarbonates Ba2CuInx(CO3)1−xO2+2.5x+δ

Abstract A new indium copper oxycarbonate Ba 2 CuIn x (CO 3 ) 1− x O 2+2.5 x + δ , existing for 0.0625≤ x ≤0.25 in the In 2 O 3 –BaO(CO 2 )–CuO pseudo-ternary system, has been identified and characterised. Rietveld refinement of powder X-ray diffraction data combined with electron microscope studies shows that this phase crystallises in the tetragonal system (space group P 4/ mmm ) with unit cell ( a =4.01333(1) A, c =8.0572(1) A for x =0.125) based on a doubling of the basic perovskite subcell in the c -direction. The refined crystal structure is characterised by a random distribution of copper atoms in the [Cu 1− x In x –O] as well as the [(CO 3 ) 1− x Cu x –O] layers. The oxygen atoms from carbonate groups occupy all 14 possible structural positions around the carbon atoms. The Cu +2 ions with spin S = 1 2 form a disordered sublattice of corner-sharing CuO 4 units and isolated linear CuO 2 dumbbells. All samples with δ =0 indicate non-superconductive properties in the investigated temperature range. The curves of the magnetisation versus magnetic field at 1.9 K, M ( H ), and magnetic susceptibility versus temperature, χ s ( T ), do not indicate long range magnetic order, but the presence of low-dimensional magnetism with suppressed values of magnetic parameters due to zero-point spin fluctuations and covalency effects. The compound has a temperature dependence of the resistivity characteristic for disordered systems, satisfying Mott’s variable-range-hopping (VRH) law ρ=ρ 0 exp [T 0 /T] p with p =1/4 and T 0 ≈3.10 7 K. In Ba 2 CuIn x (CO 3 ) 1− x O 2+2.5 x compound the In sites in disordered [Cu 1− x In x –O] layers may be the hopping centers, although estimated values for the density of localised states at the Fermi level N ( E F ), seem to be independent of the indium content x .

Low-Temperature Transformation of Amorphous Sol–Gel TiO2 Powder to Anatase During Cold Spray Deposition

Nanocrystalline TiO2 coatings were successfully deposited on metallic substrate surfaces through low-pressure cold spray. Sol–gel amorphous TiO2 dry gel was for the first time used as a feedstock powder. Agglomerated morphology of the amorphous powder with high-density packing of particles and reduced pores number was an important factor for successful low-pressure cold spraying. However, in the present study one more factor contributing to anchoring of TiO2 particles by cold gas spraying onto a substrate was indicated. This factor enabling effective powder deposition and coating production is based on an exotic for conventional ceramic materials’ plastic deformation of amorphous powder. Furthermore, the detailed analysis of SEM, Raman and XRD measurements clearly demonstrated that amorphous titanium dioxide is transformed into crystalline anatase form during the process of low-pressure cold spraying.

Corrosion Resistance of Copper Coatings Deposited by Cold Spraying

In the article, a study of corrosion resistance of copper and copper-based cermet (Cu+Al2O3 and Cu+SiC) coatings deposited onto aluminum alloy substrate using the low-pressure cold spraying method is presented. The samples were subjected to two different corrosion tests at room temperature: (1) Kesternich test and (2) a cyclic salt spray test. The selected tests were allowed to simulate service conditions typical for urban, industrial and marine environment. Examination of corroded samples included analysis changes on the coating surface and in the microstructure. The physicochemical tests were carried out using x-ray diffraction to define corrosion products. Moreover, microhardness and electrical conductivity measurements were conducted to estimate mechanical and physical properties of the coatings after corrosion tests. XRD analysis clearly showed that regardless of corrosion conditions, for all samples cuprite (Cu2O) was the main product. However, in the case of Cu+Al2O3 cermet coating, chlorine- and sulfate-containing phases such as Cu2Cl(OH)3 (paracetamite) and Cu3(SO4)(OH)4 (antlerite) were also recorded. This observation gives better understanding of the lowest microstructure changes observed for Cu+Al2O3 coating after the corrosion tests. This is also a justification for the lowest decrease in electrical conductivity registered after the corrosion tests for this coating.

Structural, transport, and magnetic properties of the cation-ordered cobalt perovskite Ho{sub 1/3}Sr{sub 2/3}CoO{sub 3-delta}.

Structural, magnetic, and thermoelectric properties of Ho{sub 1/3}Sr{sub 2/3}CoO{sub 3-{delta}} have been studied within the oxygen content range 2.67<3-{delta}<2.80. The studied samples demonstrate antiferromagnetic order (G type) and semiconductive behavior. No ferromagnetic ordering has been observed in contrary to a recent study [A. Maignan et al., J. Solid State Chem. 178, 868 (2005)] on a closely related Sr{sub 2/3}Y{sub 1/3}CoO{sub 3-{delta}} with almost identical oxygen contents. We conclude that the high degree of both cation and oxygen vacancy ordering determines the observed properties of this material via strong localization of Co{sup 3+} electrons.