Bulk material

Rather different approaches are taken for metallic and nonmetallic materials. Metallic alloys and intermetallic compounds are usually melted first in an arc furnace, where a DC argon arc is struck between a tungsten or graphite electrode and the sample, which lies in a hollow in a water-cooled conducting hearth, or in a radio-frequency induction furnace, where the sample is placed in a few-turns, water-cooled coil and heated by the eddy currents producd by some kilowatts of 150 kHz power. Subsequent heat treatments to produce atomically ordered or phase-segregated microstructures are usually carried out in a resistance furnace, where the temperature and atmosphere can be precisely controlled. When single-crystal samples are required, they may be grown from a seed crystallite in the melt, using the Bridgeman or Czochralzki methods. Crystallization from the melt is a two-step process: initially one or more tiny crystalline nuclei formas a result of a random fluctuation of the atomic positions in the supercooled liquid, then the nuclei grow at a rate dependent on the degree of undercooling and quickly consume the melt.

Amorphous metals demand a different approach. A multicomponent melt is rapidly quenched on a spinning copper wheel, for example, which leaves little time for the nuclei to grow. The surface velocity is of order 50 m s⁻¹. Melt spinning works well at a deep eutectic in the compositional phase diagram, where the melt can be quenched almost instantaneously to a temperature below the glass transition, which is the point where long-range diffusive motion of the atoms is frozen out. An amorphous metal produced in this way is known as a metallic glass. Mechanical alloying of constituent elements in a high-energy ball mill is an alternative means of producing a highly disordered bulk metal. Nonmetals, especially oxides, are freqently prepared by ceramic methods. Mixtures of powders of appropriate precursors with the correct cation ratio, for example Fe₂O₃ and CoO to make CoFe₂O₄, are repeatedly ground and sintered to achieve dense, uniform material by solid-state diffusion. Precursors like carbonates or acetates which have a low decomposition temperature can be used to produce finely divided oxide as a first stage, or else solid solutions can be formed directly as precipitates (gels) from ionic solution.
Ceramics are usually refractory, with high melting points. Crystals may be grown from the melt in an image furnace where infrared radiation is focussed onto a small section of a sintered polycrystalline rod by two parabolic mirrors and the molten zone moves along the rod. Other crystal growth methods include chemical vapour transport, and the flux method, where a mixture of oxides is melted with a flux such as PbF₂ which has no solid solubility in the required crystal. On slow cooling, oxide crystals nucleate and grow throughout the melt. They are extracted by dissolving the flux. Single crystals are indispensable for complete characterization of the anisotropic magnetic properties. Certain techniques like neutron diffraction or measurements of elastic constants require quite large crystals,(1 mm)³–(10 mm)³. Crystal growth is something of an art; its practitioners are star supporting actors in the author lists of numerous publications.

مواضيع ذات صلة

عجائب المجالات المغناطيسية

المجال المغناطيسي الارضي

Planetary magnetism

Therapy and treatment

Cellular biology

Magnetotaxis

Magnetic field effects

Electrodeposition

Magnetic memory

Materials for spin electronics

Actuators

Sensors