Search

We have studied conditions for the synthesis of intermetallic phases in the Cu–Al–Ni system by selective laser sintering/melting, in particular by heating a powder mixture to 300°C. The effects of laser synthesis and heating on the microstructure of the intermetallic phases in the samples obtained have been studied using electron microscopy, optical metallography, and X-ray diffraction analysis. The results demonstrate high sinterability of stoichiometric mixtures. Resistivity measurements indicate that the samples exhibit a shape memory effect. We discuss the feasibility of producing biomicroelectromechanical systems using layerby- layer synthesis.

MgNi2 intermetallic was synthesized by powder metallurgy and crystallizes with a Laves-type C36 structure (space group P63/mmc (No. 194); a = 4.826; c = 15.832 A). At 300 °C during interaction with hydrogen (deuterium) gas compressed to 2.8–7.4 GPa, a trihydride MgNi2H(D)3.2 was synthesized. The trihydride remained metastable at ambient conditions allowing its structure, stability and magnetic properties to be studied. The formation of MgNi2H3.2 is associated with a complete rebuilding of the initial hexagonal structure into the orthorhombic distorted MoSi2-type sublattice (space group Fmmm (No. 69); a = 4.55; b = 4.69; c = 8.80 A). Neutron diffraction of the MgNi2D3.2 demonstrated that D atoms fill sites having octahedral Mg4Ni2 (D1/4b) and planar Ni2 (D2/8f) coordination. Within the framework of the density functional theory, density of states (DOS) calculations showed the formation of a structure around −10 to −6 eV caused by the chemical bonds of hydrogen and its 1s states mainly via interaction with the 3d states of Ni. Analysis of the electronic structure revealed a charge transfer from Mg to Ni, and to the H atoms. The calculated enthalpy of formation of MgNi2H3 is about −30 kJ/mol-H2, which is consistent with the stability of the hydride at normal conditions. The initial sample contained a small amount of a secondary MgNi3 intermetallic, which has been formed during the equilibrium interaction of magnesium and nickel at 800 °C. Thus this compound should be included in the phase diagram of the Mg–Ni system. MgNi3 decomposes under high-temperature/high-pressure hydrogenation conditions and forms nickel monohydride.

The mechanical activation (MA) of a charge of a hard magnetic 22Kh15KT alloy is studied by wet and dry milling in a planetary mill in a medium of argon and ethyl alcohol with addition of surface-active materials and without them. It is shown that, upon dry MA, powder alloy components are alloyed with formation of two bcc solid solutions and, upon wet MA, charge particles are only intensely dispersed. Dispersion is developed at the highest degree in the first five minutes of MA.

Spherical zinc particles ranging in average size from 275 to 760 nm and covered with platelike zinc oxide particles on the order of 10 nm in size have been prepared by levitation-jet aerosol synthesis through condensation of zinc vapor in an inert-gas flow. The nanoparticles have been characterized by transmission electron microscopy, X-ray diffraction, BET measurements, and vibrating-sample magnetometry. The results indicate that the observed ferromagnetic ordering is due to changes in unit-cell volume on the surface of the nanoparticles. High-temperature magnetization data demonstrate that the ferromagnetic ordering of the nanoparticles persists up to 750 K.

Ferromagnetic Ni nanoparticles were formed by a levitation-jet aerosol synthesis under different gas environments and metal precursor feed rates. At a constant background gas inlet temperature, it was found that a higher Ni loading resulted in enhanced particle growth through coalescence. He partial atmosphere favors surface condensation of evaporated Ni atoms over coalescence as the surface area reduction mechanism in the nanoparticles. A flow of 2.5% air in the background gas mixture was enough to oxidize 75% of the initial Ni load, inducing a drastic destabilization of particle size and shape distribution. Regardless of the background inert gas composition, necked nanoparticles were observed in samples prepared with a 1 g/h Ni feed rate, whereas discrete nanoparticles resulted from a higher feed rate of ca. 4 g/h, confirming the key role of Ni loading on the rate of coalescence. The highest saturation magnetization (51.75 A m 2 kg −1 measured at 300 K) and the lowest coercivity (0.008 T) were obtained under an Ar flow. Zero-field cooled and field-cooled magnetization curves measured under an applied field of 10 −2 T revealed that the blocking processes of nanoparticles are dominated by their particle size distributions, with some features attributable to interparticle interactions.

Nickel oxide nanoparticles (NPs) 6–24 nm size have been prepared by a levitation-jet method based on metal vapor condensation in a mixture of gaseous streams of helium and air (or oxygen). Particles were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) N 2 adsorption and SQUID magnetometry. It is revealed that at room-temperature (RT) the nanoparticles show a small magnetic moment and the magnetic field dependence of their susceptibility a minimum lying below the high-field values. After the appropriate subtraction of both ferromagnetic and paramagnetic components from the initial magnetization curves, the appearance of a diamagnetic contribution is observed. The possible link between the latter contribution and the occurrence of some features related to RT superconductivity in NiO NPs is discussed.

Pseudospherical copper ferrite particles 20 to 90 nm in average size were prepared by an aerosol method through condensation of iron and copper vapors in an inert-gas flow, followed by the oxidation of the resulting two-phase powder under heterogeneous combustion conditions to an almost single-phase product. The nanoparticles were characterized by scanning electron microscopy, X-ray diffraction, BET measurements, and vibrating-sample magnetometry. Analysis of the X-ray diffraction data and the behavior of the magnetization of reaction intermediates and final synthesis products in the range 400–1100 K made it possible to propose models for the nanostructure of the particles and establish the likely sequence of the observed phase transformations.

Pseudospherical nickel ferrite particles 25 to 70 nm in average size were prepared by a crucibleless aerosol method through cocondensation of Fe and Ni vapors in an inert-gas flow containing a small amount of air. The particles were characterized by transmission electron microscopy, X-ray microanalysis, X-ray diffraction, BET measurements, and vibrating-sample and SQUID magnetometry. The results were used to optimize process parameters for the preparation of particles with a tailored size, specific surface area, and saturation magnetization. A dc electric field applied to the condensation zone can serve to improve the phase purity of nickel ferrite nanoparticles, reduce their size, and change their Curie temperature.

Spherical oxidized nickel particles 15 to 200 nm in average size have been produced by a crucibleless aerosol method involving metal vapor condensation in an inert gas flow and oxidation processes. The particles have been characterized by scanning electron microscopy, X-ray microanalysis, X-ray diffraction, BET surface area measurements, and vibrating-sample magnetometry. The process parameters have been optimized for the preparation of particles with tailored size, specific surface area, and saturation magnetization. A dc electric field applied to the condensation zone during the oxidation process reduces the size and increases the extent of oxidation of the particles. We have studied low-temperature oxidation of carbon monoxide and propane on nickel nanopowders differing in particle size and extent of oxidation. The nanoparticles with optimized characteristics have been shown to have a marked catalytic effect on these processes.

We have prepared ferromagnetic barium hexaferrite powders by self-propagating high-temperature synthesis (SHS) in an electric field applied by a contactless method. Our results indicate that the field strength has a significant effect on the phase composition of the synthesis product. This effect is interpreted in terms of the decomposition kinetics of the solid oxidant and reactant diffusion. Microstructural examination demonstrates that an electric field has a marked effect on the morphology of SHS materials. The magnetic properties of the synthesized powders depend significantly on both the strength and direction of the applied field.

The electrical resistivity of TiB2–AlN–BN ceramics with Y2O3 additions was measured between 300 and 1300 K as a function of the Y2O3 content of the starting mixture. The resistivity of the ceramics was found to exhibit anomalous, hysteretic behavior in a wide temperature range. This behavior is tentatively attributed to the metastability of percolation processes in the multicomponent conductive ceramics.

Hydrofullerites C 60 H x synthesised at hydrogen pressures of 0.6 and 3 GPa were found to possess ferromagnetic properties at room temperature. The magnitude of magnetisation varied from sample to sample and reached 0.001–0.16 Bohr magnetons per C 60 molecule at H =10 kOe. The coercivity of all the samples was about 100 Oe. The hydrofullerites had either an fcc or bcc lattice formed of C 60 H x units. The maximum values of magnetisation were observed for the fcc hydrofullerites with x ≈24.

An X-ray study of a single crystal of the NBu4[MnCr(C2O4)3 composition reveals a layered structure with an ordered arrangement of Mn2+ and Cr3+ ions. According to the data of the orientation dependence of magnetization at 4.5K, the magnetic easy axis is aligned along the crystal c-axis. Mossbauer data analysis of isostructural NBu4[FeCr(C2O4)3] powder material gives, on the contrary, the evidence of strong confinement of Fe2+ spins to the crystal basal plane. The ferromagnetic system which arises here is a quasi-2D system in terms of the nature of its exchange interactions.

The steady expansion of the fields of applications of ultrafine powders (UFPs) necessitates studying and developing their methods of manufacture. In particular, there is a need to identify the factors influencing the particle size of UFPs. Of special interest in this connection is the levitation method [i], which enables UFPs to be produced with very narrow particle size ranges and mean particle sizes varying from 0.01 to 0.5 ~m in a continuous process. The present work was therefore undertaken with the aim of examining, using nickel UFPs as an example, the effect of the key parameters of production of particles by the levitation method on their size. Use of a ferromagnetic material made it possible to study at the same time the effect of the dimensional factor on the magnetic properties of such UFPs. The first results of this investigation have been reported in [2]. As can be seen from the diagrammatic representation shown in Fig. I, evaporation of a metal drop i in the apparatus employed is performed in a suspended state with the aid of a countercurrent generator 2 supplied from a VChG4-10/0.44 high-frequency generator 3. Nickel vapor entrained by an inert gas stream condenses, with the formation of spherical particles (Fig. 2), which are trapped by a filter 4. At the end of the process the particles are removed from the filter by means of a rod 5 into a capsule 6, another rod 7 serving to withdraw a sample of the particles for electron microscopy. During evaporation fresh metal is continually supplied to the drop from a wire of the starting material 8, which passes through a feeding device 9 and a stuffing box I0. The velocity of the cooling inert gas stream v is regulated by means of a valve ii and a rotameter 12. In our work all investigations were carried out at an inert gas pressure close to atmospheric. A study was made of the effect of the rate of particle condensation s, rate of flow of the cooling inert gas, and its atomic weight on the size and magnetic properties of the nickel particles being formed. Magnetic properties were studied using a vibration magnetometer. The rate of condensation for levitation under self-regulating conditions is usuallymade equal to the rate of consumption of the metal wire [i], which can easily be regulated by varying the speed of rotation of the feeding device rollers. In the apparatus employed the rollers were driven by an RD-09 reversing motor through a special reductor. The linear wire feed rate in the system was varied steplessly and in steps in the range 0.03-30 m m/sec (for 0.2-mmdiameter nickel wire the corresponding variation of s was from 0.01 to i0 mg/sec). At constant rates of _fl~ of inert gas through the vaporizer the dependence of the mean nickel UFP particle size d, calculated on the basis of analyses of micrographs, on s was as shown in Fig. 3. The power-function character of the ~ vs s relationship was maintained wit h various inert gases and at various gas flow rates. The slopes of the straight-line plots in the figure indicate a typical d ~ s~/~ relationship.