Superparamagnetic platelets composed of nanomagnetite-silica composite needles, with optical color effects in dispersion

摘要

The present invention relates to superparamagnetic anisotropic particles composed of a composite containing or consisting of superparamagnetic nanocrystalline iron oxide nanoparticles with diameters of 3 to 50 nm embedded in a siliceous matrix. These particles are obtainable by a process comprising the steps of: a. Providing an acidic ferrofluid, b. Adding, in particular dropwise, a silica or an organically modified silicic acid polycondensate forming agent to the ferrofluid to form a sol, c. Introducing the sol into a magnetic field and adding a water-miscible precipitant or precipitant mixture consisting of or containing a ketone to the sol in an amount such that particle precipitation occurs without removing the magnetic field. The particles of the invention can be used as a component of counterfeited articles and documents and for many other purposes.