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Applied Surface Science 257 (2011) 5375–5379

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

ynthesis of nanocrystalline magnesium nitride (Mg3N2) powder using thermallasma

ong-Wook Kim, Tae-Hee Kim, Hyun-Woo Park, Dong-Wha Park ∗

epartment of Chemical Engineering and Regional Innovation Center for Environmental Technology of Thermal Plasma (RIC-ETTP),NHA University, 253 Yonghyun-dong, Nam-gu, Incheon 402-751, Republic of Korea

r t i c l e i n f o

rticle history:vailable online 13 December 2010

ACS:

a b s t r a c t

Nanocrystalline magnesium nitride (Mg3N2) powder was synthesized from bulk magnesium by thermalplasma at atmospheric pressure. Magnesium vapor was generated through heating the bulk magnesiumby DC plasma jet and reacted with ammonia gas. Injecting position and flow rates of ammonia gas were

2.77.−j1.07.Wx

eywords:agnesium nitride

lasma

controlled to investigate an ideal condition for Mg3N2 synthesis. The synthesized Mg3N2 was cooledand collected on the chamber wall. Characteristics of the synthesized powders for each experimentalcondition were analyzed by X-ray diffractometer (XRD), scanning electron microscopy (SEM) and ther-mogravity analysis (TGA). In absence of NH3, magnesium metal powder was formed. The synthesis withNH3 injection in low temperature region resulted in a formation of crystalline magnesium nitride withtrigonal morphology, whereas the mixture of magnesium metal and amorphous Mg3N2 was formed when

temp

NH3 was injected in high

. Introduction

Magnesium nitride (Mg3N2) is widely applied as catalystso prepare some metal nitrides or non-metal nitrides. Espe-ially Mg3N2 is used to prepare cubic boron nitride (cBN) [1–4].lso Mg3N2 has been used to form high thermal conductiveaterials [5–8]. In recent years, it has attracted considerable inter-

st as an alternative material to aluminum nitride (AlN) andallium aluminum nitride compositions (GaxAlyN) in the opto-lectrical field. Indirect-bandgap semiconductors like AlN andaxAlyN have a lower light-emitting efficiency than direct-bandgapemiconductors. Therefore researches on the application of theirect-bandgap metal nitride semiconductors are being widelyerformed [9]. Especially the nitride of alkali earth metals suchs beryllium and magnesium is treated as promising alternativeaterials [10,11].The traditional synthesis of Mg3N2 is performed using direct

itridation of magnesium and low-pressure chemical vapor deposi-ion [12–14]. However these methods require high-cost equipmentnd have low yield and low reaction stability. An alternative

ethod is a combustion synthesis under low pressure or an inert

as atmosphere [15]. However it has difficulties in atmosphere con-rol and getting nanosized-powder.

∗ Corresponding author. Tel.: +82 32 874 3785, fax: +82 32 876 8970.E-mail address: dwpark@inha.ac.kr (D.-W. Park).

169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2010.12.029

erature region. Also, vaporization process of magnesium was discussed.© 2010 Elsevier B.V. All rights reserved.

The thermal plasma has numerous activated species suchas electrons, protons and radicals with high temperature over10,000 K. Thermal plasma process for preparation of nanopowdervaporizes the raw materials that react with the activated species invapor phase and decides the reaction path by controlling the atmo-sphere in the process [16,17]. Finally, the rapid cooling system isused to limit the growth of the products and form the nano-sizedproducts.

In this paper, Mg3N2 nanopowder was synthesized from thebulk magnesium using the thermal plasma. This process couldobtain the crystalline magnesium nitride with high yield. Also anypre- or post-treatment was not required, simplifying the entireprocess.

2. Experimental

2.1. Comparison of Ar plasma and Ar–N2 plasma

To confirm the effect of the nitrogen in the plasma gas, vaporiza-tion rate of magnesium was measured as a presence of the nitrogenin the plasma gas. Two types of the plasma gas (Ar 15 l/min, Ar15 l/min + N2 1.5 l/min) were introduced and the plasma was

operated to vaporize magnesium for 10 min. After turning off theplasma, the weight loss of the magnesium was measured. Then, thespecific energies were calculated by plasma power/vaporizationrate. The detailed calculation results are summarizedin Table 1.

5376 D.-W. Kim et al. / Applied Surface Sc

Table 1The calculation results of the vaporization efficiency of Ar–N2 plasma.

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Plasma power 8.25 kWVaporization rate 1.5 g/minSpecific energy 91 Wh/g

.2. Synthesis of Mg3N2

Fig. 1 shows an experimental set-up for the synthesis of Mg3N2.magnesium ingot in a tungsten crucible was placed on a water-

ooled copper holder. A plasma torch was fixed on the top of thehamber. Operating conditions were given in Table 2. Argon wassed as the main plasma gas and a small amount of nitrogen wasixed with the argon to improve a plasma power and a vapor-

zation efficiency of magnesium. The flow rates and the injectingosition of ammonia gas were controlled as operating variables. Inun 1, nitrogen gas was introduced as nitrogen source. In Run 2,and 4, ammonia gas was used as nitrogen source. Ammonia gasas injected through the inlet nozzle positioned at radial distance

f 5 mm from the plasma gas inlet nozzle on the plasma torch inun 2 while injected at the chamber wall in Run 3 and 4. Also, thehamber was purged with the nitrogen prior to examination. Thenhe plasma arc was discharged using the argon as the plasma gasnd the crucible and the magnesium ingot were pre-heated at theow temperature region. After the pre-heating process the crucible

as approached to the plasma arc and the magnesium ingot startedo melt. Then, 1 l/min of nitrogen gas was added to the plasma gasnd it vaporized the magnesium. The generated magnesium fumeeacted with the ammonia to form magnesium nitride. The synthe-ized magnesium nitride was cooled and collected on the chamberall.

The phase formation of the synthesized powders was con-rmed by X-ray diffractometer (XRD, DMAX 2500, Rigaku Co.).he morphologies and size distribution of the obtained powdersere analyzed using scanning electron microscopy (FE-SEM, S-

300, Hitachi Co.). Additionally, thermogravimetric analysis and

ig. 1. Experimental set-up for the synthesis of magnesium nitride. (1: Plasma torch withater cooled copper holder.)

ience 257 (2011) 5375–5379

differential thermal analysis (TGA-DTA, TGA/SDTA851e, METTLERTOLEDO) were performed.

3. Results and discussion

The argon plasma and the argon–nitrogen mixture plasmawere compared to vaporize magnesium. When only argon wasused to generate the plasma, the magnesium ingot was meltbut barely vaporized. Although Ar plasma has the high temper-ature to vaporize magnesium in the center of the plasma flame,it is hard to make bulk magnesium to approach to the centerof the plasma flame. Moreover heat loss caused by quenchingsystem at sample holder disturbed the vaporization of magne-sium. On the other hand, Ar–N2 plasma efficiently vaporized themagnesium and the calculation results about that are shown inTable 1. That is why the dissociated nitrogen in thermal plasma pro-motes the vaporization of metal. Some of the dissociated nitrogenatoms were adsorbed on the surface of metal and then recombineeach other instantly. This recombination is exothermic reaction(N + N → N2 �H = −958.383 kJ/mol). The formation enthalpy of N2promotes vaporization of metal [18]. The results showed althoughthe small amount of nitrogen was added in the argon gas it helpsthe vaporization of the magnesium.

Fig. 2(a)–(c) and (e) is the X-ray diffraction (XRD) patterns ofthe products which are collected at chamber wall in Run 1, 2, 3and 4 respectively. Fig. 2(d) displays the XRD patterns of the syn-thesized powders collected at the plasma torch in Run 3. Most ofpeaks in Fig. 2(a) indicate magnesium, which implies that nitro-gen did not react with both the solid magnesium and the vaporizedmagnesium. Magnesium oxide peaks at 42.9◦ and 62.32◦ are dueto the contact with air when collected. The main phase of Fig. 2(b)

and (c) is the amorphous Mg3N2 and minor phase is the crystallineMg3N2. To compare the products of Run 2 and 3 the products col-lected at the plasma torch were analyzed. Although the product ofRun 2 collected at the plasma torch had the same XRD patterns withFig. 2(b), the product of Run 3 showed the crystalline Mg3N2 peaks

tungsten cathode and copper anode; 2: DC power supply; 3: bulk magnesium; 4:

D.-W. Kim et al. / Applied Surface Science 257 (2011) 5375–5379 5377

Table 2Operating conditions for the synthesis of magnesium nitride.

Run 1 Run 2 Run 3 Run 4

Plasma gas The mixture of Ar and N2 (Ar: 15 l/min, N2: 1.5 l/min)Plasma power (kW) 8.25 kW (300 A, 27.5 V)

zzle in

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particles over its crystallite size because of high concentration ofmagnesium vapor at the vaporizing step.

Fig. 4 shows the result of TGA/DTA analysis in air aboutthe product of Run 4. The weight increased from 100% (origi-nal weight) to 116% at near 500 ◦C. When Mg3N2 (100.9 g/mol)

Pressure (atm) 1Flow rates of NH3 (l/min) – 20Flow rates of N2 (l/min) 10 –NH3 injecting position – Gas inlet no

s the main phase and this result was shown in Fig. 2(d). These XRDesults demonstrate that N2 injected into the plasma flame did noteact with magnesium, but NH3 have an important role to synthe-ize Mg3N2. When NH3 was injected to the plasma flame, most ofH3 was pyrolized and released hydrogen and nitrogen. However

he atomized nitrogen is very unstable and recombines each otherithout the reaction with magnesium. It is almost same in both Runand Run 2. In case of Run 3, the amount of the decomposed NH3

s far smaller than that of Run 2. It seems that the presence of therystalline Mg3N2 in Fig. 2(d) was due to the high concentration ofhe vaporized magnesium near the plasma torch. To obtain furtherrystalline Mg3N2, the more amount of NH3 was injected to thehamber wall. Amorphous magnesium nitride was formed undernsufficient ammonia supply. The growth of magnesium nitrideccurs rapidly because of rapid evaporation rate of magnesium andapid quenching rate. Therefore excess supply of NH3 is requiredo obtain crystalline magnesium nitride. XRD patterns of Run 4 in

ig. 2(e) show the crystalline Mg3N2. Therefore it is concluded thathe injecting position and the flow rate of NH3 affect the conver-ion of the crystalline Mg3N2 in the vapor phase method using thehermal plasma process.

ig. 2. X-ray diffraction patterns of the nanopowders obtained at the chamber walln (a) Run 1, (b) Run 2, (c) Run 3, (e) Run 4 and at the plasma torch in (d) Run 3.

20 40– –

plasma torch Chamber wall Chamber wall

Fig. 3 shows SEM and TEM images of the products of Run 4. Thesynthesized Mg3N2 have polygonal morphologies and the particlessize ranged from 50 nm to 400 nm. The SEM and TEM images showthe broad size distribution of the synthesized Mg3N2. Furthermorethe crystallite size was calculated from Scherrer’s equation. Thecrystallite size of the sample 4 is 28.54 nm which is different fromthe particle size observed from SEM and TEM images. It is due toagglomeration of primary single crystal. Although crystallite sizeis about 30 nm, the particles were agglomerated and formed large

Fig. 3. (a) SEM and (b) TEM images of the synthesized Mg3N2.

5378 D.-W. Kim et al. / Applied Surface Science 257 (2011) 5375–5379

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3Mg(g) + 2N(g) → Mg3N2 (2)

Overall reaction: 3Mg(g) + 2NH3(g) → Mg3N2 + 3H2

Fig. 4. TGA/DTA curve of the synthesized Mg3N2.

onverts to 3MgO (3 × 40.3 g/mol = 120.9 g/mol), the theoreticaleight-increase ratio is 120% which is similar with 116% of

he experimental result. Additionally, DTA analysis exhibits thendothermic peak when the reaction (Mg3N2 → 3MgO) occurs.hen Mg (24.3 g/mol) converts to MgO (40.3 g/mol), the theoreticaleight-increase ratio is 166%. If the product contains metal mag-esium, the weight-increase ratio will be higher than 120%. Hence

t can be concluded that Mg3N2 convert to MgO at near 500 ◦C andhis reaction is exothermic.

Thermodynamic equilibriums based on the Gibbs’ free energyere calculated using the thermodynamic software ‘FACTSAGE’

nd shown in Fig. 5. These calculations show the most thermo-ynamically stable phase composition which is composed of theonsidered chemical species, but not chemical reaction. Accordingo the calculations, magnesium nitride is stable at a low tempera-ure region in both Mg–N2 system and Mg–N2–NH3 system. Hence,t was supposed that magnesium nitride would be formed throughooling after vaporization in these systems. Fig. 5(a) and (b) ispplied to Run 1 and Run 2, respectively. In Fig. 5(a), nitrogenN2) starts to be decomposed to nitrogen atom (N) at 4100 K andeacts with magnesium gas to form magnesium nitride at 1700 K.owever Mg3N2 was not actually formed in Mg–N2 system since2 is inactive. It is similar in Fig. 5(b). Ammonia (NH3) is per-

ectly decomposed to nitrogen (N2) and hydrogen (H2) at 700 K.he nitrogen formed by the decomposition of NH3 is used to formg3N2 in the calculation, but not in the experiment. In Run 3 and

un 4, the temperature of NH3 injection nozzle on the chamberall was the room temperature and it was expected that NH3

emained undecomposed. Although NH3 is gradually decomposedt room temperature, its decomposition is severe near 500 K. HenceH3 could react with the magnesium fume in vapor phase as notecomposed, which led the formation of Mg3N2. While the sampleynthesized in Run 3 includes some amorphous products becausef deficiency of NH3, the composition of the sample synthesizedn Run 4 almost consists of crystalline Mg3N2. This phenomenonan be explained as following. When vaporized magnesium con-acts with ammonia gas in gas phase, ammonia is decomposed athe interface with magnesium vapor because of high temperaturef magnesium vapor. The magnesium vapor has temperature over373 K (it is boiling point of magnesium) which is over decomposi-ion temperature of ammonia (about 600 K). So dissociated N and

were instantly formed and penetrate into magnesium vapor.

There are two possible reactions: (1) 3Mg(g) + 2N(g) → Mg3N2

nd (2) Mg(g) + 2H(g) → MgH2. Fig. 6 indicates that Gibbs’ freenergy of formation of Mg3N2 is lower than that of MgH2 in all tem-erature region. Therefore the formation of Mg3N2 is preferred. The

Fig. 5. Thermodynamic equilibrium calculations in (a) Mg–N2 system and (b)Mg–N2–NH3 system.

reaction mechanism can be written as:

2NH3 → 2N + 6H (at interface of Mg gas and NH3)

Fig. 6. Gibbs’ free energy of formation of Mg3N2 and MgH2.

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D.-W. Kim et al. / Applied Sur

Therefore it is concluded that the nitrogen molecule hardlyeacts with the magnesium gas, but the nitrogen produced fromhe decomposition of NH3 at interface of magnesium gas andmmonia reacts with the magnesium gas. From these results,e could form two conclusions: (1) the path of the reaction is

Mg(g) + 2NH3(g) → Mg3N2(s) + 3H2(g) and (2) the conversion ofg3N2 depends on the flow rate of NH3.

. Conclusions

Magnesium nitride (Mg3N2) was successfully synthesized usingC thermal plasma. Bulk magnesium was vaporized by Ar–N2ixture plasma and then N2 or NH3 was introduced as a nitro-

en source. It is confirmed that the small amount of the nitrogen inhe plasma gas has an important role to vaporize the bulk magne-ium. The synthesized nanopowders were analyzed by XRD, TGAnd SEM. In nitrogen atmosphere the vaporized magnesium wasot nitridized because the most of ionized nitrogen recombinedith each other before reacting with the magnesium vapor. In case

f NH3 atmosphere, NH3 injection position was varied; one was theorch nozzle (near plasma flame) and another was the chamber wallfar from plasma flame). When NH3 was injected near plasma flame,itrogen formed by the decomposition of NH3 barely reacted withhe magnesium vapor because of the recombination of the nitro-en. When NH3 was injected to the chamber wall, NH3 reacted with

he vaporized magnesium to form magnesium nitride. In this case,he flow rate of NH3 had an important role on the crystallizationf Mg3N2. TGA/DTA results exhibited Mg3N2 convert to MgO atear 500 ◦C and this reaction is exothermic. SEM and TEM imagesisplayed the polygonal particles with the size of 50–400 nm.

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ience 257 (2011) 5375–5379 5379

In combination with the experimental results and the ther-modynamic calculations, it can be concluded that the vaporizedmagnesium reacted with the ammonia gas to form Mg3N2, that is, the reaction path is 3Mg(g) + 2NH3(g) → Mg3N2 + 6H2.

Acknowledgement

This work was supported by INHA University Research Grant.

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