Abstract
Inorganic nanocomposites made of an inorganic matrix containing nanoparticle inclusions provide materials of advanced mechanical, magnetic, electrical properties and multifunctionality. The range of compounds that can be implemented in nanocomposites is still narrow and new preparation methods are required to design such advanced materials. Herein, we describe how the combination of nanocrystal synthesis in molten salts with subsequent heat treatment at a pressure in the GPa range gives access to a new family of boron-based nanocomposites. With the case studies of HfB2/β-HfB2O5 and CaB6/CaB2O4(iv), we demonstrate by X-ray diffraction and through (scanning) transmission electron microscopy the crystallization of borate matrices into rare compounds and unique nanostructured solids, while metal boride nanocrystals remain dispersed in the matrix and maintain small sizes below 30 nm, thus demonstrating a new multidisciplinary approach toward nanoscaled heterostructures.
Highlights
Inorganic nanocomposites are made of an inorganic matrix that contains nanoparticle inclusions to target original mechanical, magnetic, electrical properties and multifunctionality.[1,2,3,4,5] The production methods known to yield ceramic matrix nanocomposites consist mostly in two approaches: either embedding preformed nanoparticles into matrices, or precipitating the inclusions directly in the matrix.[2,3] These techniques are restricted to simple oxide and nitride matrices
To ensure evolution under HP-HT of the amorphous matrix toward borates and not boron polymorphs, the powders were exposed to air for 7 days to allow oxidation of the boron matrix
To demonstrate the generalization of the approach, we have explored another boride/borate system, relying on CaB6 nanocrystals (Supplementary Information and Figure 6)
Summary
Inorganic nanocomposites are made of an inorganic matrix that contains nanoparticle inclusions to target original mechanical, magnetic, electrical properties and multifunctionality.[1,2,3,4,5] The production methods known to yield ceramic matrix nanocomposites consist mostly in two approaches: either embedding preformed nanoparticles into matrices, or precipitating the inclusions directly in the matrix.[2,3] These techniques are restricted to simple oxide and nitride matrices. We present a third route based on the crystallization of the matrix around nanoparticles in extreme conditions, in order to combine crystalline matrices and nanoparticles of rare structures and compounds. High pressures and temperatures (HP-HT) ranging from 1 to 20 GPa and from 500 to 1500 °C are a realm of opportunities to synthesize crystalline materials not reachable at pressures close to the atmospheric one.[6,7,8,9] Such HP-HT ranges are compatible with large scale industrial productions.[10] But to yield nanocomposites, nanoscaled inclusions should survive such extreme conditions, questioning the behavior of nanostructured materials under HP-HT treatment
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