Microhardness of fused periclase

Abstract

Fused periclase is widely used in the manufacture of several types of refractory articles and also as an electrical-insulation material in tubular electric heaters (TEH). During the manufacture of the electric heaters, the metal tubes filled with fused periclase are compressed on roller machinery. As a result, there is a significant compaction of the powdered periclase, for example, from an apparent density of 2.30-2.35 to 2.90 g/cm 3. Therefore, it seemed important to study the microhardness of periclase and the possibility of lowering this parameter with the aim of improving the compression process and the pressing of the refractory articles.The mierohardness of minerals is associated with the strength of the crystal. As Lomonosov stated; "the harder the solid, the more strongly associated are its particles" [i]. In the classical crystallochemical works by GolVdshmidt [2] and Fersman [3], the hardness is associated with the strength of the crystal lattice of the mineral which depends on the cation and anion charges and the interatomic distances.A further development of crystallochemical theorywas made by A. S. Povarennykh [4,5] who suggested that as well as the valence (charge) of the ions and the interatomic distances, the coordination number, the packing density of the atoms, the strength of the ionic and covalent bonds, and the structure of the electron shell must also be taken into account.However, the dependences obtained on the basis of crysta!lochemical hypotheses only give approximate values for the hardness, for example, for periclase by comparison with other minerals.In fact, the hardness is a strength characteristic and can change significantly even for the same crystal depending on the plane chosen for the measurement, the presence of dislocations, and other factors [6]. One of the important factors affecting the strength of crystals and, consequently, their hardness is the interaction between dislocations [7]. The interaction between dislocations leads to an increase in the resistance to movement in the plane where the dislocations lie in proportion to the accumulation of defects and to the strengthening (increase in hardness) of the crystal.An important effect on the hardness of crystals is provided by defects and impurities. Their action is twofold: on the one hand, the defects in the structure help to lower the hardness and, on the other, they strengthen the crystal by preventing the free movements of the dislocations.The strengthening of the structure can be explained by the fact that when the dislocation passes a hindrance (structural defects, inclusion, impurity of atom of a different size) it is forced to bend round the hindrance (to form a loop) and then straighten out again (Fig. i). Such a process is accompanied by the elongation of the dislocation and the local distortion of the structure for which an additional expenditure of energy is required. As a result, the defective part of the crystal is characterized by a high resistance to deformation, i.e., by an increase in hardness.As in ot...

High-quality fused electrotechnical periclase from chemically purified raw material

“…In the USSR electrotechnical periclase powder is prepared mainly from natural raw material: brucites from the Kul'dursk, and magnesites from the Kirgiteisk deposits [2].…”