Precisely controlled carbothermal synthesis of spherical β-Si₃N₄ granules

Abstract

High purity spherical β-Si₃N₄ particles were synthesized via an efficient carbothermal reduction–nitridation strategy by precise regulation. The results showed that nano-sized reactants with an appropriate ratio was significant for acquiring Si₃N₄ granules with pure β phase. c/a,b-axis length ratio of the β-Si₃N₄ particles could be adjusted by using various additives. Fine spherical β-Si₃N₄ particles can only be obtained with the help of CaO additive by L-S mechanism. Moreover, variations of underlying reaction mechanisms when changing the raw materials were comprehensively revealed. Owing to the high purity and fine spherical morphology, as-obtained β-Si₃N₄ powders are promising fillers for preparing resin-based composites with high thermal conductivities.

Introduction

In recent years, Si₃N₄ granules has received considerable interests as thermally conductive fillers for plastic packages in thermal management engineering. This benefits from its remarkable properties, such as high intrinsic thermal conductivity with a theoretical value of 200–300 W/m/K, high electrical resistivity, low dielectric loss, good chemical stability etc. [[1], [2], [3]] To prepare composites with higher thermal conductivity, it is of great importance to reduce the interface thermal resistance while maintain the good fluidity of the powders/polymers mixtures at the same time [4]. Generally, particles with columnar, equiaxed or irregular shape give rise to the difficulty for the polymers to wet the particles [5]. Consequently, agglomerations appear and the interface thermal resistance grow, leading to lower thermal conductivity. Thereby, Si₃N₄ particles with spherical morphology are regarded as the best candidates for thermally conductive fillers.

Carbothermal reduction-nitridation (CRN) method is a facile yet efficient synthesis strategy to acquire high-pure Si₃N₄ powders [6,7]. The overall reaction can be described as equation (1):

3SiO₂ (s) + 6C (s) + 2N₂ (g) → Si₃N₄ (s) + 6CO (g)

As generally established, this reaction is believed to proceed via a two-step process as follows [7,8]:

SiO₂ (s) + C (s) → SiO (g) + CO (g)

and

3SiO (g) + 3C (s) + 2N₂ (g) → Si₃N₄ (s) + 3CO (g)

SiO gas can be produced by Reaction (2), and reacts with N₂ on the surface of the solid by Reactions (3) by vapor-solid (V–S) route.

In general, Si₃N₄ particles synthesized by CRN strategy prefer to grow along some specific directions to fibers or floccules through V–S route or whiskers and columnar crystallites by V-L-S (vapor-liquid-solid) route [9,10]. Si₃N₄ particles with spherical morphology is difficult to be synthesized because controlling the concentration of SiO vapor is really hard. Meanwhile, SiC, Si and SiO₂ may be formed concurrently during the reaction, reducing the quality of the powders [8,11].

In this work, we concentrated on the influences of reactants and different additives on the properties of the Si₃N₄ particles. Fine spherical β-Si₃N₄ particles can be synthesized by using 10 mol% CaO additive, nano-sized reactants with an appropriate C/SiO₂ weight ratio of 0.5. Furthermore, the underlying relationship between the properties and growth process of the Si₃N₄ particles when utilizing different reactants and additives was tentatively put forward.