Exclusive Growth of Low-Aspect Ratio, Polyhedral h-BN Crystals in Molten Li₂CO₃ as the Reactive Flux

Abstract

Hexagonal boron nitride (h-BN) exhibits a wide range of properties, including insulation and high thermal conduction. Despite these advantages, h-BN often forms with a plate-like morphology, with an aspect ratio much greater than 1, which gives ineffective thermal conduction at the particle interfaces. The problem arises because of the plate-like morphology that renders the crystal a high aspect ratio, much greater than 1, in its typical growth form. Herein, we used the flux method to grow and develop the morphology of h-BN crystals with a much smaller aspect ratio. After screening the flux and heating conditions, we found that crystals of h-BN grown in molten Li 2 CO 3 had a new polyhedral structure that was over 3 μm in size. These crystals had low aspect ratio (around 1), which was unusual, and nonequilibrium morphology. Crystallographic evaluation revealed that the polyhedral h-BN crystals were not single crystals but were polycrystals with oriented tablet-type single crystals. This unique morphology was further investigated by in situ thermal analyses of the phase transition, chemical reaction, and morphological change during the growth of h-BN in molten Li 2 CO 3 . Based on these results, we discuss the mechanism of the growth of the crystal and the role of the flux. Our observations could be explained based on the reactivity of the flux and its probable absorbed manner on h-BN, which is dependent on its crystal facets in the flux.

Improved water oxidation activity of a Sillén SrBi3O4Cl3 photocatalyst by flux method with an appropriate binary-component molten salt

“…3a-c, via the ux synthesis, the crystal size of the plate-like particles increased with increasing temperature, which indicates the controllability of the crystal size with calcination temperature as with the case of the liquid phase syntheses of other materials. 11,35 Although the size of the sample prepared via the SSR method was also changed by the calcination temperature, its degree was smaller than that obtained via the ux method, as shown by the surface area (Fig. S9 †).…”

Process-Informatics-Assisted Preparation of Lithium Titanate Crystals with Various Sizes and Morphologies

“…For instance, regarding the BaTaO 2 N photocatalysts, the use of KCl flux-assisted nitridation enables the preparation of symmetric BaTaO 2 N crystals that are co-exposed with anisotropic facets, enabling efficient separation of photogenerated electrons and holes . Another representative example is hexagonal boron nitride grown from Li 2 CO 3 flux . The use of the flux growth method enabled the formation of hexagonal boron nitride crystals of an aspect ratio of ca.…”

“…Crystalline materials have been widely used in various fields such as photocatalysts, − electrodes, biomaterials, heat conductors, and catalysts. , Functionalities of crystalline materials such as (opto)­electronic, intercalation, and catalytic properties strongly depend on the crystal structures as well as the particle size, exposed facets, and particle morphology; , which must be controlled to improve the functionalities. We focus here on the flux growth method of preparing crystalline materials, especially layered titanates.…”

“…3 Another representative example is hexagonal boron nitride grown from Li 2 CO 3 flux. 8 The use of the flux growth method enabled the formation of hexagonal boron nitride crystals of an aspect ratio of ca. 1 that were >3 μm in size.…”

Experimental Investigation of an Integrated Crystallization and Wet-Milling System with Temperature Cycling to Control the Size and Aspect Ratio of Needle-Shaped Pharmaceutical Crystals

“…Several particle engineering strategies have been adopted in industry for the control of size uniformity and AR. These include temperature cycling, using wet milling, and addition of growth rate modifiers. , Novel batch crystallization designs were also developed to control the size distribution . These approaches can also be applied in combination for improved control of size and shape.…”