Viscosity of liquid boron

Abstract

Viscosity of liquid boron was measured over the temperature range from 2325 to 2556 K using an electrostatic levitation method combined with an oscillation drop technique. The results obtained revealed that the viscosity increases slowly with decreasing temperature from 2.2 mPa s at 2550 K to 2.6 mPa s at 2370 K, and substantially increases with further decrease in temperature below the melting temperature ͑T m = 2360 K͒, becoming as large as 6.4 mPa s at 2325 K. The increase in the viscosity suggests that clusters with extension may appear in supercooled liquid of boron.Boron ͑B͒ and B-rich compounds combine useful properties such as hardness, low density, and chemical inertness. 1 B-rich solids are characterized by a series of uncommon crystal structures associated with unusual bonding in the solid state. They range from superconducting metals to widegap insulators. The large range of properties originates from the peculiarity of the covalent bonding in pure B and modifications of the bonding when other elements are added to the B network.The crystal structures of B and B-rich compounds are dominated by B 12 units each of which consists of 12 B atoms and possesses the icosahedral structure. The atoms in the B 12 icosahedral unit of B-based compounds are firmly connected by covalent bonds with each other and appear to be remarkably stable. 2 Moreover, the B 12 icosahedral units also seem to survive in the amorphous B. 3 Thus, the question has been raised whether the icosahedral units may survive into liquid B.Until very recently, there have been few experimental measurements on liquid B since it reacts with the walls of containers. The advent of levitation techniques made possible to handle a sample without contamination and to carry out the measurements on the physical properties of liquid B in the temperature region down to the supercooled state. The density and surface tension have been measured with a gas levitation technique. 4 Different density measurements have been performed with a vacuum electrostatic levitator, revealing that B contracts by nearly 3% upon melting. 5 The electrical conductivity of liquid B was measured using a contactless method coupled with gas levitation. It has been shown that the semiconducting property remains even in the liquid state. 6 X-ray diffraction and inelastic x-ray scattering measurements on liquid B were carried out coupled with gas levitation technique in order to address the question whether B 12 icosahedral units may survive into the liquid state. 7,8 No evidence has been obtained on the survival of the icosahedral units into liquid. Ab initio molecular-dynamics simulations of liquid B also suggested that icosahedral arrangements are destroyed upon melting. 8,9 Both of these experimental and simulation studies seem to strongly deny the existence of B 12 icosahedral units in liquid B. Instead, as Price et al. 8 pointed out in their paper, there still remains possibility that small clusters such as pentagonal units could exist in liquid B. In the more deeply supercooled...

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Wetting of carbide ceramics (B4C, SiC, TiC and ZrC) by molten Ni at 1753 K

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