In recent years, magnesium silicon nitride (MgSiN2) has been attracted great interest due to the crystal structure is similar to aluminium nitride (AlN). Compared with AlN, MgSiN2 is a simple covalent insulator, and shows more excellent mechanical properties than AlN ceramics   . MgSiN2 has many attractive properties such as high thermal conductivity, low dielectric constant, high hardness, high thermal stability, good oxidation resistance (up to 920˚C) and high electrical resistance at room temperature  . All of the above features make MgSiN2 very suitable for the electronic substrate/package and heat radiator. MgSiN2 has been successfully used as effective sintering additive of nitrogen ceramics or growth promoter of β-Si3N4 rod crystal    . MgSiN2 is also considered to be a promising luminescent material for light emitting diode (LED) applications  .
MgSiN2 can be prepared by various methods, such as carbothermal reduction , direct nitridation   , hot-pressing , solvothermal method by the reaction of SiCl4, N2H4·HCl and Mg , the solid-state metathesis route using SiO2 and Mg3N2 as reactants . However, most of these methods usually require high-energy consumption, high-temperature, long-time treatment.
Combustion synthesis (CS), also called self-propagating high-temperature synthesis (SHS) is well-know to prepare a series of advanced materials, due to its energy-efficient, time-saving, low processing cost, mass production, high production rate  . Thus, preparation of MgSiN2 by combustion synthesis in nitrogen gas was also widely studied using different starting materials (Mg/Si3N4, Mg/Si, Mg2Si) as reactant . However, the reaction mechanism is still unclear during combustion synthesis process for preparation of MgSiN2.
Sintering methods such as hot press sintering and reaction sintering have been previously used to produce MgSiN2 ceramic. As a promising rapid and effective densification technique, spark plasma sintering (SPS) have been previously used to produce some ceramics as well as other hard materials. However, to the author’s knowledge, so far, there is no information on the use of this technique to synthesize MgSiN2.
In this paper, MgSiN2 powder was prepared by combustion synthesis between Mg and Si3N4 without additive, using a combustion synthesis apparatus in different N2 pressures. Then the CSed powders were sintered using spark plasma sintering (SPS) system for obtaining bulk MgSiN2 product. We hope this research can pave the way for prepare high thermal conductivity of MgSiN2.
2. Experimental Procedure
2.1. Synthesis of MgSiN2 Powder
Mg (purity, 180 μm in size) and α-Si3N4 (purity, 0.5 μm in size) powders were used as raw materials. The chemical reaction for the synthesis of MgSiN2 from the above mentioned starting materials can be shown as follows:
3Mg +Si3N4 + N2 ® 3MgSiN2 + ΔH (1)
when the raw mixtures are ignited, the heat released (ΔH) will keep the reaction going to the end. The schemata for the step-wise synthesis were shown in Figure 1. MgSiN2 powders were prepared by the raw materials of Mg and Si3N4 with the mole ratio of 3:1. Then, the reactant mixtures with the compositions shown in Table 1 were mechanically milled by a planetary ball mill for 15 min in an alumina container of 250 ml. Silicon nitride balls were Æ 5 mm in diameter as medium, and the weight ratio of ball to powder was 10:1. The ball milling was processed at 200 rpm. After milling, the mixture was charged into a cylindrical
Figure 1. Flow chart of the experimental procedure for synthesizing MgSiN2.
Table 1. Compositions of the raw reactants and experimental conditions.
graphite crucible (diameter: 40 mm; length: 65 mm). And after the procedure of evacuation, nitrogen gas (99.99% in purity) was finally introduced to raise the pressure to preset condition into the chamber. The combustion reaction was triggered by igniting the sample by passing an electric current through a carbon foil placed on the top of sample. One W-Re thermocouple was inserted into the center of the sample to record the combustion temperature profile. The apparatus can be found other else .
The CSed powder was ball milled for 30 min for SPS using the planetary ball milling. After milling, the powders were compacted into a carbon die of 10 mm in inner diameter and sintered by a SPS system under 50 MPa of compressive stress. The resulting compacts heated from room temperature to 600˚C in 5 min, and then were heated to 1500˚C at a rate of 30˚C/min, and maintained at this temperature for 10 min before turning off the power.
The phases of the combustion products were identified by an X-ray diffraction (XRD) analyzer (Mini Flex, Rigaku Corporation, Tokyo, Japan). The morphologies of the reaction products were investigated by scanning electron microscopy (SEM) (FE-SEM JSM-7400F, JEOL, Tokyo, Japan). The bulk density of the SPSed specimens was measured according to the Archimedean principle, using distilled water as the medium. The Vickers hardness was measured using a Vickers microhardness tester with a diamond indenter of regular pyramid with an opposite angle of 136˚. The thermal diffusivity was measured by the laser-flash method (TC-7000, ULVAC Sinku RikoCo., Yokohama, Japan) at room temperature.
3. Results and Discussion
Figure 2 shows the temperature history of sample M-2 at CS under 0.7 MPa N2 pressure, and the thermal couples were set at the center of the sample. It can be seen, only in several seconds, the temperature sharply reached its apex of 1840˚C, and then began to decrease. It nearly held about 100 s above 1091˚C, which is the boiling point of Mg.
The product picture of sample M-2 at CS under 0.7 MPa N2 pressure is shown in Figure 3. All of the products for M-1, M-2, and M-3 showed similar. The cross section of the product can be clearly divided into three parts according to the color. The outside was dark and the center part was grey-brown, while the intermediate layer was white.
The XRD patterns of the central parts for samples M-1, M-2, and M-3 are shown in Figure 4. It can be seen that all of the central parts for the three samples are pure MgSiN2 as the N2 pressure increasing from 0.3 MPa to 1.0 MPa. The XRD patterns of different parts for sample M-2 are shown in Figure 5. For
Figure 2. Temperature history of sample M-2 at CS under 0.7 MPa N2 pressure, the thermal couples were set at the center of the sample.
Figure 3. The portrait of sample M-2 for CS at 0.7 MPa N2 pressure.
Figure 4. XRD patterns of combustion product synthesized at different N2 pressures.
Figure 5. XRD patterns of the combustion product of sample M-2.
all of the three parts, the major phase was MgSiN2. The XRD peaks of the central part are very sharp, while those of the outer part are relatively flat, which indicates the degree of crystallization of MgSiN2 gradually decreasing from inside to outside. A small amount of MgO peaks appeared in both intermediate and outside parts. Combined with the temperature history shown in Figure 2, due to the high temperature at central part, Mg evaporated over 1091˚C. The Mg vapor diffused from center to outside of the sample. In addition, a heavy odor was smelled when the CS equipment was opened. Combining the above phenomena, when the combustion synthesis reaction is ignited, the target product MgSiN2 is obtained according to above mentioned Equation (1). However, a small amount of Mg vapor diffuses into the outer layer and reacts with nitrogen to obtain Mg3N2 due to the high temperature in the core of the sample. When the furnace is opened, Mg3N2 reacts with water in the air to obtain MgO and ammonia. The reaction can be expressed as follows:
3Mg + N2 → Mg3N2 (2)
Mg3N2 + 6H2O → 3MgO + 2NH3(g) (3)
Furthermore, the oxygen may also come from the oxygen impurity of starting materials or nitrogen gas. Although the color of the outer layer was greenish yellow, no Mg3N2 phase peaks was detected by XRD, possibly due to too little content or too low crystallinity. Another possible reason for this phenomenon may be the decomposition of magnesium silicon nitride .
The SEM images of the products synthesized at different N2 pressure are shown in Figure 6. All of the three samples showed that some small grains clustered together to form larger particles, and there were a large number of pores among the clusters. The average diameter of the small grains looked smaller than 1mm. However, it seemed that the grain size tends to increase with the increase of nitrogen pressure based on the SEM pictures. Figure 7 shows the SEM images of different parts for sample M-3. It can be seen that the grain size and crystallinity decrease gradually from the center to the outer layer due to temperature gradient between different parts.
Table 2 summarizes the characteristics of the CS-SPSed MgSiN2 products. bulk density was 3.11 g/cm3, Vickers hardness was 1673.1 kgf/mm2, and thermal diffusivity was 8.718E−2 cm2/s.
Table 2. Properties of the bulk MgSiN2 sintered at 1500˚C by spark plasma sintering.
Figure 6. SEM images of combustion products synthesized at different N2 pressures.
Figure 7. SEM images of sample M-3: (a) Center; (b) Intermediate; (c) Outside; (d) Magnification of (c).
Single-phase MgSiN2 was synthesized by combustion synthesis method under N2 pressures of 0.3 - 1.0 MPa using Mg/Si3N4 as reactants. The CSed product can be divided into three distinct layers according to its color. SEM observation showed the grain size decreased gradually from the center to the outer layer. Some small grains clustered together and formed larger particles, and there were a large number of pores among the clusters. The bulk density of CS-SPSed MgSiN2 was 3.11 g/cm3, Vickers hardness was 1673.1 kgf/mm2, and thermal diffusivity was 8.718E−2 cm2/s.
This work was financially supported by Shaanxi Key R&D Program (No. 2018GY-116) and by Yangling Demonstration Zone Science and Technology Plan Project, Shaanxi, China (No. 2018GY-05).
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