Magnesium silicon nitride (MgSiN 2) was synthesized without any additives under a nitrogen gas flow (200 mL/min) using a nitriding method. The effects of temperature and holding time on its purity and morphology were investigated. A single-phase MgSiN 2 powder was obtained at 1350℃ for 1 h and 1250℃ for 11 h. However, the decomposition of MgSiN 2 occurred at 1450℃, suggesting that the optimum temperature for the preparation of MgSiN 2 from Mg-Si system was 1350℃. The phase purity, morphology, size of the product and elemental composition of the samples were detected by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy spectrometer (EDS), respectively. The evaporation of Mg and Si resulted in the formation of many voids in the blocky product. The temperature gradient promotes the growth of MgSiN 2 on the surface of massive products along the tip. The concentration gradient of Mg and Si vapors in the void resulted in the columnar growth of MgSiN 2.
In recent years, ternary nitrides have been widely investigated due to their higher functionality than binary nitrides. β-SiAlON, Si3N4, and AlN all exhibit excellent thermal performances [
In the past few decades, the preparation of MgSiN2 using different methods and raw materials has been widely studied. Uchlda et al. [
In this study, single-phase MgSiN2 powders were successfully prepared by nitridation of the Mg-Si system, and the effects of temperature and holding time on the purity and morphology of the products were also investigated. The purpose of this study was to obtain the desired products at low temperature, as well as to shorten the required time of nitridation. We believe that this discovery can pave the way for preparing MgSiN2 with low energy consumption.
Mg (>99 wt% purity, Aldrich Reagent Co. Ltd.) and Si (99.99 wt% purity, 300 mesh, Adamas Reagent Co. Ltd.) were used as starting materials to synthesize MgSiN2. The raw Mg and Si materials were mixed and grinded in an agate mortar with a mole ratio of 2:1. Due to the evaporation of Mg, the Mg/Si value deviated from the stoichiometric ratio, a large amount of Mg was consumed. Subsequently, the mixed powders were placed in an alumina crucible, which was covered with a carbon cloth; the mixtures were also covered with a carbon cloth to prevent Mg from evaporating. Then, the crucible containing the mixed powders was sealed and placed in the middle part of a high temperature resistance furnace. After vacuum was pumped, the furnace was filled with nitrogen at a flow rate of 200 mL×min−1, and heated at temperatures between 500˚C and 1450˚C for different holding time. The heating rate was 5˚C∙min−1 for all samples.
After thermal treatment, the products were ground using a mortar and pestle before testing. The phase composition of the samples was examined by using an X-ray powder diffraction (XRD) analyzer (D8 ADVANCE A25, Bruker Corporation, Germany) with Cu Kα radiation, operating at 40 kV and 40 mA. The particle sizes and morphologies of the synthesized powders were determined using scanning electron microscopy (SEM) (Nova Nano SEM 450, FEI Corporation, America).
The XRD patterns of the products synthesized within the temperature range of 500˚C - 1450˚C starting from Mg and Si are shown in
2Mg + Si → Mg 2 Si (1)
The reaction between Mg2Si and N2 may take place as follows:
3Mg 2 Si + 4N 2 → 3MgSiN 2 + Mg 3 N 2 (2)
A large amount of Si was present at 1100˚C; given that the melting point of Mg2Si is 1102˚C, it can be speculated that the decomposition of Mg2Si occurred at this temperature. As the temperature continued to rise, a single-phase MgSiN2 appeared at 1350˚C. The high temperature allowed the starting materials to react completely to generate the nitride, causing the evaporation of the MgO present in the reaction mixture as well as the decomposition of the Mg3N2 product into N2 and Mg (g) as follows:
Mg 3 N 2 → 3Mg ( g ) + N 2 ( g ) (3)
The formation of MgO may be due to the presence of oxygen impurities in the raw material, oxygen pickup during mixing, and oxygen in the N2 atmosphere; thus, the oxygen reacts with Mg or Mg3N2 to form MgO. At 1450˚C, MgSiN2 decomposed to give rise to Si3N4. When the experiments were conducted at 1450˚C for 3 h, the content of Si3N4 increased.
a long time, resulting in a large number of evaporation of magnesium, so the product in addition to MgSiN2 also appeared in Si, MgO and not identified phases [
3MgSiN 2 ( s ) → 3Mg ( g ) + 3xSi ( l ) + ( 1 − x ) Si 3 N 4 ( s ) + ( 1 + 2x ) N 2 ( g ) (4)
Furthermore, a small amount of white fibrous powder was observed around the crucible upon holding for a long time; although the amount was too small to be tested, it most likely consisted of MgO. The oxygen in the gas atmosphere reacted with Mg or Mg3N2 to form MgO.
We also attempted to obtain MgSiN2 at low temperature (1250˚C). Therefore, different amounts of urea were added to the raw materials to promote nitridation and reduce the impurities, but this did not lead to major improvements.
Although the urea could reduce the Si content and produce smaller particles, a small amount of Si and MgO were found to be still present. Thus, we decided to increase the holding time in order to obtain single-phase MgSiN2 and influence the morphology of the products.
From a large number of SEM photographs, it was evident that upon increasing of the holding time, the products with a columnar morphology gradually decreased at 1350˚C. At 1250˚C the products with a columnar morphology mostly appeared in a hollow, which may be caused by the evaporation of Mg. The formation of these voids also provides new space for the production of MgSiN2. Within a void, Mg and Si vapors may have a certain concentration gradient leading to the growth of many columns. Another form of growth is also shown in
Mg and Si resulted in the formation of many voids in the blocky product. The higher is the temperature, the greater is the gas activity, and the smaller is the gas adsorption on the surface of MgSiN2 [
A single-phase of MgSiN2 was obtained either at 1350˚C for 1 h or at 1250˚C for 11 h using Mg/Si as starting materials with a mole ratio of 2:1 under a N2 atmosphere. Although this product could be obtained at low temperature (1250˚C), the holding time required was too long and the process involved great energy consumption. Thus, the most economical temperature was 1350˚C. With the increase of the holding time, the grain size of lumpy shaped crystals became larger, and the size of grains with a columnar morphology also increased becoming more uniform. As the temperature increased, the products with a columnar morphology gradually decreased. Moreover, when the temperature reached 1450˚C, the decomposition of MgSiN2 occurred, and Si3N4 particles could be clearly seen in the SEM images. This simple and energy-efficient method for the preparation of MgSiN2 further promotes its use as a fundamental material for electronic equipment to achieve an enhanced thermal conductivity.
We thank the Life Science Research Core Services of Northwest A & F University for providing scanning electron microscope. This work was supported by the special funds for basic research projects of Northwest Agriculture and Forestry University (NO. Z109021534) and International Science and Technology Cooperative Seed Fund Project of Northwest Agriculture and Forestry University (NO. A213021607).
Guo, R., Yi, X.M., Liu, X.Z., Li, Q.D. and Nomura, T. (2018) Synthesis of MgSiN2 Powders from the Mg- Si System. Journal of Materials Science and Chemical Engineering, 6, 68-79. https://doi.org/10.4236/msce.2018.61008