We report on the syntheses of magnetoresponsive, superparamagnetic nanostructures with highly anisotropic shapes, i.e., nanochains of controlled length and their bundles (nanobundles). These nanochains and nanobundles were obtained by the simultaneous magnetic assembly of superparamagnetic nanoparticle clusters (SNCs) and the fixation of the assembled SNCs with an additional layer of deposited silica, produced by a sol-gel process. This low-cost approach provides excellent length control of the short nanochains (approximately 6 or 14 SNCs per nanochain) and fine-tuning of the spacing between the neighboring SNCs inside an individual nanochain. Our magnetically responsive superparamagnetic nanostructures have a controlled aspect ratio, a uniform size, and a well-defined shape, and they express good colloidal stability. This general approach should lead to new, advanced applications of the nanochains and nanobundles in the treatment of cancer and in the ability to magnetically manipulate liquid and photonic crystals.

The solid solubility of R ions (R = Ho3+, Dy3+, and Y3+) in the BaTiO3 perovskite structure was studied by quantitative electron‐probe microanalysis (EPMA) using wavelength‐dispersive spectroscopy (WDS), scanning electron microscopy (SEM), and X‐ray diffractometry (XRD). Highly doped BaTiO3 samples were prepared using mixed‐oxide technology including equilibration at 1400° and 1500°C in ambient air. The solubility was found to depend mainly on the starting composition. In the TiO2‐rich samples a relatively low concentration of R incorporated preferentially at the Ba2+ lattice sites (solubility limit ∼Ba0.986R0.014Ti0.9965(V″Ti″)0.0035O3at 1400°C). In BaO‐rich samples a high concentration of R entered the BaTiO3 structure at the Ti4+ lattice sites (solubility limit ∼BaTi0.85R0.15O2.925(VO••)0.075at 1500°C). Ho3+, Dy3+, and Y3+incorporated preferentially at the Ti4+ lattice sites stabilize the hexagonal polymorph of BaTiO3. The phase equilibria of the Ho3+–BaTiO3 solid solutions were presented in a BaO–Ho2O3–TiO2phase diagram.

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The hydrothermal treatment of an appropriate suspension of Ba and Fe hydroxides in the presence of a large excess of OH(-) results in the formation of Ba hexaferrite at temperatures as low as 150 degrees C. This low formation temperature enables the synthesis of uniform, ultrafine Ba hexaferrite nanoparticles. These nanoparticles have a disc-like shape, approximately 10 nm wide, but only approximately 3 nm thick. When the temperature of the hydrothermal treatment is increased, large platelet Ba hexaferrite crystals appear as a consequence of secondary re-crystallization (Ostwald ripening). In this work, this undesired process of secondary re-crystallization has been evaluated. We show that the secondary re-crystallization can be totally suppressed with the use of an oleic acid surfactant. The addition of oleic acid enabled the synthesis of uniform, ultrafine nanoparticles at temperatures up to 240 degrees C. The nanoparticles were hydrophobic and could be suspended in nonpolar liquids to form relatively concentrated ferrofluids. Such stable suspensions of hexaferrite nanoparticles will be technologically important, especially as precursors for the preparation of new nanostructured materials, for example nanocomposites or nanostructured ceramic films.

Stable suspensions of superparamagnetic iron oxide nanoparticles in water (water-based ferrofluids) were prepared using citric acid (CA) as a surfactant. The influences of different factors on the amount of nanoparticles in a stable suspension were systematically studied. These factors, including the temperature, the pH value and the concentration of CA applied during the adsorption of the CA onto the nanoparticles and during their suspension in water, were evaluated. The highest content of nanoparticles in a stable suspension was obtained when the CA was absorbed at pH values of around 5.2, where two carboxyl groups are dissociated, and when the nanoparticles were suspended at a pH of around 10, where all three carboxyl groups of the CA are in a dissociated state.

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