Synthesis of MgSiN 2 Powders from the Mg-Si System

Magnesium silicon nitride (MgSiN2) 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 MgSiN2 powder was obtained at 1350 ̊C for 1 h and 1250 ̊C for 11 h. However, the decomposition of MgSiN2 occurred at 1450 ̊C, suggesting that the optimum temperature for the preparation of MgSiN2 from Mg-Si system was 1350 ̊C. 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 MgSiN2 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 MgSiN2.


Introduction
In recent years, ternary nitrides have been widely investigated due to their higher functionality than binary nitrides.β-SiAlON, Si 3 N 4 , and AlN all exhibit excellent thermal performances [1]- [7].The crystal structure of MgSiN 2 belongs to the orthorhombic system similar to AlN; however, the mechanical properties of MgSiN 2 are superior to those of AlN.Thus, MgSiN 2 has attracted extensive attention owing to its high fracture toughness (3 MPa•m 1/2 ), high stress intensity (280 MPa), high hardness (20 GPa), high-temperature electrical insulation, R. Guo  high-temperature oxidation resistance (up to 920˚C), excellent thermal conductivity, etc. [8]- [13].In view of its theoretical thermal conductivity value of up to 75 W/m•K, MgSiN 2 should replace the AlN material as a new generation of ceramic materials with high thermal conductivity [14].It can also be used as substrate material, packaging material, fluorescent material, for sintering aid of non-oxide ceramics with high thermal conductivity, and as growth additive in the combustion synthesis of β-Si 3 N 4 rod crystals.Therefore, it is considered a very promising engineering and functional ceramic material [15]- [25].
In the past few decades, the preparation of MgSiN 2 using different methods and raw materials has been widely studied.Uchlda et al. [26]  In this study, single-phase MgSiN 2 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 MgSiN 2 with low energy consumption.

Materials and Methods
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 MgSiN 2 .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 R. Guo et al.
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).

Results and Discussion
The XRD patterns of the products synthesized within the temperature range of The reaction between Mg 2 Si and N 2 may take place as follows: A large amount of Si was present at 1100˚C; given that the melting point of Mg 2 Si is 1102˚C, it can be speculated that the decomposition of Mg 2 Si occurred at this temperature.As the temperature continued to rise, a single-phase MgSiN 2 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 Mg 3 N 2 product into N 2 and Mg (g) as follows: ( ) ( ) 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 N 2 atmosphere; thus, the oxygen reacts with Mg or Mg 3 N 2 to form MgO. At 1450˚C, MgSiN 2 decomposed to give rise to Si 3 N 4 .When the experiments were conducted at 1450˚C for 3 h, the content of Si 3 N 4 increased.Figure 2  a long time, resulting in a large number of evaporation of magnesium, so the product in addition to MgSiN 2 also appeared in Si, MgO and not identified phases [11].Oxygen impurities will reduce the thermal conductivity of the product, but the MgO can be washed off by acid washing [29].As of now, there has been no literature to report that Mg and Si can be synthesized by combustion synthesis to obtain a single phase of the MgSiN 2 powder.
Figure 3 shows the XRD patterns of the products synthesized at 1350˚C for different holding time.It was found that the powder of the middle part of the sample was measured and the result was single phase MgSiN 2 .Although after 8 h some black powders appeared around the product that were completely separated from it.Figure 4 shows the XRD pattern of these black powders, which suggested the presence of some Si 3 N 4 and Si impurities in addition to MgSiN 2 .This indicates that not only the decomposition of MgSiN 2 into Mg and Si 3 N 4 occurred owing to the long reaction time, but also Si 3 N 4 decomposed according to the following reaction [32]: 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 Mg 3 N 2 to form MgO.
We also attempted to obtain MgSiN 2 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 MgSiN 2 and influence the morphology of the products.Figure 5 shows the XRD patterns of the products synthesized at 1250˚C for different holding time.As the holding time increased, the Si impurity gradually decreased, and single-phase MgSiN 2 was obtained when the holding time was up to 11 h.By increasing the holding time, Si could be removed at low temperatures.Figure 6 shows the SEM images and EDS patterns of the product synthesized at 1250˚C after holding for 5 h.The crystals mainly grew into two types, i.e., lump and columnar.The EDS results of the sample are shown in the lower left corner of Figure 6, confirms that both types of products are MgSiN 2 .With the increase of the holding time, the grain size of the lumpy shaped crystals became larger.Figure 7 shows the SEM images of the products synthesized at 1250˚C for (a) 8 h, (b) 1 h and at 1350˚C for (c) 1 h, while the diagrams on the right are partial enlargements of the left graphs.Figure 7(b) shows that the grains did not grow due to the low temperature and short time.As shown in Figure 7(a), the grains gradually gathered and grew along the original column with the increase of the holding time.Form Figure 7(c), it is clear that when the temperature was high enough for the reaction to go to completion, the grains gradually sintered into blocks.In summary, with the increase of temperature and holding time, the particles gradually became larger; however, the temperature had a greater effect on the particle size than the holding time.Journal of Materials Science and Chemical Engineering   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 MgSiN 2 .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 Figure 8. Figure 8(a) shows that the products with columnar morphologies grew on a solid surface.The columnar formation may be due to a larger temperature gradient on the solid surface.The farther away from the solid, the lower the temperature, the smaller the activity of the gas, and the easier it is to absord the reaction gas; thus, MgSiN 2 easily grows along the tip.As shown in Figure 8(b), it is possible that Mg and Si continued to grow along the inner surface of the cavity because of the relatively strong adsorption of nitrogen on the surface of Mg and Si.The schematic illustration of the mechanism of particle formation on solid surfaces and voids is shown in Figure 9.As the temperature increased, the reactants gradually reacted to form massive amounts of MgSiN 2 .However, the temperature was much higher than the boiling point of Mg, so a large amount of Mg and a small amount of Si evaporated.The evaporation of  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 MgSiN 2 [33].This may be because the surface of block products possesses a certain temperature gradient.Thus, the farther away from the surface of the block product, the lower is the temperature, the more easily adsorbed is the gas; this promotes the growth of MgSiN 2 on the surface of massive products along the tip.Within the voids of the bulk products, it is possible that the evaporation of Mg and Si results in a concentration gradient of Mg and Si vapors in the void, resulting in the columnar growth of MgSiN 2 .However, it is possible that some of the surfaces of Mg and Si still have adsorption properties, which can absord N 2 , Mg and Si atoms, so that some MgSiN 2 grows along the surface of the block products.

Conclusion
A single-phase of MgSiN 2 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 N 2 at-mosphere.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 MgSiN 2 occurred, and Si 3 N 4 particles could be clearly seen in the SEM images.This simple and energy-efficient method for the preparation of MgSiN 2 further promotes its use as a fundamental material for electronic equipment to achieve an enhanced thermal conductivity.

Figure 1 .
Figure 1.XRD patterns of the products synthesized at different temperatures.
Figure 2. SEM images and EDS analysis results of the products synthesized at 1450˚C after holding for (a) and (b) 1 h; (c) and (d) 3 h.

Figure 3 .
Figure 3. XRD patterns of the products synthesized at 1350˚C after holding for different holding time.

Figure 4 .
Figure 4. XRD pattern of the black powders surrounding the synthesized white products at 1350˚C after holding for 8 h.

Figure 5 .
Figure 5. XRD patterns of the products synthesized at 1250˚C for different holding times.

Figure 6 .
Figure 6.SEM images and EDS patterns of the product synthesized at 1250˚C after holding for 5 h.

Figure 7 .
Figure 7. SEM images of the products synthesized at 1250˚C after holding for (a) 8 h; (b) 1 h and at 1350˚C for (c) 1 h.

Figure 8 .
Figure 8. SEM images of the products synthesized at 1250˚C after holding for 8 h.

Figure 9 .
Figure 9. Schematic illustration of the mechanism of particle formation within solid surfaces and voids.
et al.
DOI: 10.4236/msce.2018.6100869 Journal of Materials Science and Chemical Engineering [27]ined single-phase MgSiN 2 by nitridation of Mg 2 Si at 1400˚C for 1 h.Bruls et al.[27]used Mg 3 N 2 /Si 3 N 4 as starting mixture to obtain MgSiN 2 with oxygen content of only 0.1 ± 0.1 wt%.Mg and Si have also been used as raw materials to synthesize MgSiN 2 at 1250˚C for 16 h; however, no single-phase products were obtained.
[30]es et al.[28]synthesized MgSiN 2 by direct nitridation of complex mixtures consisting of Mg/Si/Si 3 N 4 /Mg 2 Si, and reported the thermal analysis, phase composition, and characterization of the resulting MgSiN 2 powders.Khajelakzay et al.[9]prepared MgSiN 2 nanopowders by mechanical alloying and heat treat- ment in two steps, using Mg/Si as starting mixtures and adding a small amount of stearic acid.Yang et al.[29]synthesized single-phase MgSiN 2 powders starting from Mg/Si 3 N 4 by combustion synthesis, followed by acid washing.The preparation of MgSiN 2 by carbothermal reduction was also reported[30].The synthesis of MgSiN 2 by a solvothermal method used SiCl 4 , N 2 H 4 •HCl, and Mg as