Preparation of magnesiumsilicon nitride powderby the carbothermal reductiontechnique

HIROSHIUCHIDA,KIYOSHIITATANI*,MAMORUAIZAWA,F. S. HOWELLand AKIRA KISHIOKADepartmentof Chemistry, Faculty of Science andEngineering, Sophia University,7-1Kioi-cho,Chiyoda-ku,Tokyo102-8554,JapanReceived 18 May1998;accepted17September1998

Abstract-Magnesiumsilicon nitride (MgSiN2)powderwasprepared by carbothermally reducing amagnesiummetasilicate whose chemical composition corresponded toMgO SiO2 orMgSiO3.About0.2gof the powdermixture of magnesiummetasilicate and carbon (C)with the molar ratio of C to MgOSiO2 equal to 6.0 was heated at 1250°C for 7 h in nitrogen atmosphere. Thecrystallinephaseofthecarbothermallyreducedpowderwasonly MgSiN2. The residual carbon could be removed byheatingthepowderat 600°C for 1 h in air. The yieldofMgSiN2 powder was ~ 70%. The resultingpowdercontained 3.22 mol%oxygen.Theprimary particle sizes were rangingfrom 0.1 to 0.5 µm.Keywords: Magnesium siliconnitride;preparation; magnesium metasilicate; nitridation; carbothermalreductiontechnique;chemicalcomposition; powder properties; sinterability.

1.INTRODUCTIONMagnesiumsilicon nitride (MgSiN2)isexpectedto have highthermalconductivity[1],because the crystalstructure is similar to that of aluminium nitride(AIN)whichhasahighthermalconductivity(theoreticalvalue: 319 W/m/K)[2].Since the mechanicalpropertiesofMgSiN2ceramics are superiorto those of AIN ceramics[2],MgSiN2is a potentialcandidate for the electronicsubstrate/packageand heat radiator.Aimingat these applications,Groen et al. [3]and Hintzen et al. [4]measured the thermalconductivityoftheMgSiN2ceramics.However,the thermal conductivitywas as low as 15-17 W/m/K. ThepoorthermalconductivityofMgSiN2ceramicsappearsto be due to the presenceofoxygenatomsinto the crystallattice. The thermalconductivityofMgSiN2ceramics would be improved

*To whom correspondenceshould be addressed.

134by reducing theoxygencontent.Thusit is importanttomakeclear the conditions forpreparing pure MgSiN2 powder.

MgSiN2 powder has been prepared by (i)the solid-state reaction between magne-sium nitride (Mg3 Nz) and silicon nitride (Si3N4) [3, 5], and(ii)the direct nitridationofmagnesiumsilicide(Mg2Si)[5, 6]. The solid-state reaction technique appears tobeasimpleand useful techniquetoprepare large amounts of MgSiN2 powder. However,MgSiN2 powder prepared by thistechniquecontains a largeramount of oxygenthan that prepared by directnitridation,because the starting Mg3N2 powder iseasily hydrated to form hydroxide[7].Inthe case of direct nitridation,the nitrida- tion of Mg2Si proceeds rapidly with the exothermic effects. Since such exothermic effectscause the agglomerationofparticles, grinding oftheresulting powder isneeded before the compactionandsubsequent sintering operations. Inadditionto the above two techniques,carbothermal reduction of metal oxides

is often appliedto the preparationof metal nitrides. For example,AINpowder pre- pared by thistechniqueiscomposedof submicrometer-scaled and controlled-sized particleswithoutthegrinding operation [8, 9]. However,no information on the preparationofMgSiN2 powder by thistechniqueis available yet. Systematic infor-mation on the preparationconditionsof MgSiN2 powder isnecessaryfor the fabrica-tion of dense MgSiN2ceramics. This paperdeals with (i)thepreparationconditionsofMgSiN2 powder by the carbothermal reduction of magnesiummetasilicate and (ii)somepropertiesof the resulting powder.

2.EXPERIMENTALPROCEDURE2. l. Powder preparationMagnesiummetasilicatepowderwaspreparedon the basis of the techniquereportedby Hayashi et al.[10].Thepreparation procedure can be summarized as follows: (i)100 cm3 of concentrated NH3solution was pouredinto100cm3of 0.2 mol /dm 3Mg(N03)2andSi(OCZHS)4ethanol solution (Mg/Siratio= 1.0)to form theprecipitates;and(ii)theresulting precipitates were dried at room temperatureandwere then heated at 700°C for 1 h in air.

Themagnesiummetasilicatepowderwas mixed with the active carbon powderinthepresenceof the hexane,usinganagatemortar and pestle.The molar ratio of carbon to magnesiummetasilicate(nominalcomposition: MgO . Si02orMgSi03)wasadjustedto 6.0. The powdermixture was dried at 150°C for 30 min under reducedpressure(below100Pa),usingarotarypump.About 0.2 gof the powdermixture was placedina graphitecrucible and was heated at a temperatureof1200-1500°C for 1-7 h in a nitrogen atmosphere. 2.2. Evaluation of the powder propertiesThecrystalline phases of the resultingpowderswere identified usinganX-raypowderdiffractometer(XRD;ModelRAD-IIA,Rigaku, Tokyo, Japan)with nickel-

135filteredCuK,,operatingat 40 kV and 25 mA. The crystalline phases were identified with reference to the JCPDS cards. The reflection intensityused for the semi- quantitativeanalysiswasexpressedin terms of the productofheightand half- heightwidth of a reflection. The characteristic reflections used for this examination were:(002)inMgSiN2,(610)inMgSi03(enstatite), (211) inMg2Si04,(201)ina-Si3N4and(200)in,B-ShN4.Thequantitative analysis ofmagnesiumandsilicon content was conducted usinganenergy-dispersiveX-ray microanalyzer (EDX;ModelEMAX-5770, Horiba, Kyoto, Japan). Theoxygenandnitrogencontentswere determined usingan N/O determinator (ModelTC-136, Leco, StJoseph,MI).Theparticle shapes of the powderswere observed usinga transmission electronmicroscope(TEM;ModelEM906,CarlZeiss,Germany; accelerating voltage,120kV).Thespecificsurface area was measured byamultiple-pointBETtechnique, using nitrogen gas as an adsorption gas. Theaverage primary particle size(GBET)was calculated by assuming theprimaryparticlesto be spherical:

wherep is the true densityand s is the specificsurface area of the powder.Thetruedensityof the powderwas measured picnometricallyat 25.0°C. Differential thermalanalysis(DTA)andthermogravimetry(TG)were conducted in nitrogenatmosphereusingDTA-TGequipment(Model8076D1,Rigaku).The thermal expansion/shrinkageof the powder compact witha diameter of 5 mm and a thickness of about 2 mm was measured innitrogen atmosphere using a thermo- mechanicalanalyzer(ModelTAS-100,Rigaku).

3. RESULTS AND DISCUSSION 3.1. Determination ofthepreparationconditions

3.1.1.Someproperties of magnesiummetasilicate. Prior to performingthecarbothermalreduction,we examined the propertiesof the magnesiummetasilicatepowder.The XRD patternrevealed that no crystalline phase was detected from the starting powder. TheMgOandSi02contents of the powderdeterminedbyEDXwere 50.09 and 49.91 mol%,respectively.

Thecompositionoftheresulting powder agrees wellwith the stoichiometric compositionofMgO . Si02orMgSi03.Although MgSi03 was not detected byXRD,itmaybepresenteitherasamorphousmaterial or micro-crystallinematerialwhose size is too small to be detected byXRD.

The TEM observation of the magnesiummetasilicatepowderis shown in Fig.1.Thepowdercontainedagglomerates;theseagglomerateswerecomposedofparti-cles with the sizes below 0.1 1 /,tm.

Thespecificsurface area of the powderwas 392.3 m2/g.Theaverage primary particlesize calculated from the specificsurface area is estimated to be 0.0323 ¡Lm

136

Figure1.TypicalTEMmicrographof the magnesiummetasilicatepowder.(32.3 nm). Thus the particlesobservedbyTEM are regardedas the primaryparticles.

3.1.2.Carbothermal reduction of magnesium metasilicate. We examined the conditions of carbothermal reduction of magnesiummetasilicate for the preparationofpure MgSiN2 powder. First,thepowdermixture of magnesiummetasilicateand carbon was heated at a temperaturebetween 1200 and 1500°C for 3 h in a nitrogen atmosphere. Typical XRDpatternsof the carbothermallyreducedpowdersare shown in Fig.2. The crystalline phase at 1200°C was only Mg2Si04 [11],whereas the crystalline phases at 1300°C were MgSiN2[12],Mg2Si04and,B-Si3N4 (trace) [13].Thecrystallinephasesat 1400°C were a-Si3N4 [14], ?-Si3N4andMgSiN2,whereas those at 1500°C were a-Si3N4and,B-Si3N4.

Figure3 shows the changesinX-rayintensities of the compoundswithcarbother-mal reduction temperatureofmagnesiummetasilicate. The MgSiN2formed above 1250°C and its amount increased upto1350°C.However,it decreased with a fur- ther increase in temperatureanddisappearedat 1450°C. The amount of Mg2Si04decreased with temperatureanddisappearedat 1350°C. Moreover, a-Si3N4 and?-Si3N4which formed above 1350 and 1300°C,respectively,increased with tem- perature up to1450°C,but decreased slightlywith a further increaseintemperatureupto 1500°C.

TheMgZSi04 may be formed bythefollowingroute:

AlthoughSi02is not detected byXRD,it should be presentasamorphousmaterial.TheMgSiN2forms above 1250°C but disappearsat1450°C;it is decomposedto

137

Figure2. TypicalXRDpatternsafterthecarbothermal reduction ofmagnesiummetasilicate at varioustemperaturesfor 3 h. (0)MgSiN2;(A)M92SiO4;1 (m) a-Si3Nq;i (c) ,B-Si3N4'

Figure3.ChangesinX-rayintensities of the compoundswith carbothermal reduction temperature.Carbothermal reduction time: 3 h. (0)MgSiN2(002);(A)Mg2Si04(211); (EI) a-Si3N4(201); (B) #-S13N4(200).

forma-Si3N4and,B-Si3N4:1

Oncea-Si3N4and form,itbecomesdifficult to preparethepure MgSiN2. Onthebasis of these results,we examined more carefullythe conditions for

preparing pure MgSiN2 powder. Since the reflection intensities of MgSiN2becamemaxima at 1350°C,we shortened the carbothermal reduction time from 3 to 1 h at1350°C.AlthoughXRDpatternsofthesepowdersare omitted inthispaper,thepowders carbothermally reduced for 3 and 2 h contained MgSiN2,a-Si3N4and,B-S13N4.Thepowder carbothermally reduced for 1 h included MgSiN2, Mg2Si04 and

138

Figure4. TypicalXRDpatternsafter the carbothermal reduction of magnesiummetasilicate at 1250°C for various times. (0)MgSiNz;(o)MgSi03;(A)MgZSi04.

Figure5. ChangesinX-rayintensities of the compoundswith carbothermal reduction time at 1250°C.(0)MgSiN2(002); (A) MgSi03(610); (A) Mg2Si04(211). ). Regardlessof the decrease in carbothermal reduction time,nopure MgSiN22 could

be obtained when magnesiummetasilicate was carbothermallyreduced at 1350°C. We then examined the optimumtime for preparingpure MgSiN2 by lowering the carbothermal temperaturedownto1250°C,becausenoa-Si3N4andfl-S13N4formed at this temperature(seeFig.2). TypicalXRDpatternsof the resultingpowdersare shown inFig.4.Thecrystalline phases ofthepowder carbothermally reduced for 1 h wereMg2Si04, MgSi03 [15]andMgSiN2(trace).Thecrystallinephasesof the powders carbothermally reducedfor3 and 5 hwereMg2Si04andMgSiN2. Only MgSiN2 was detected from the powder carbothermally reduced for 7 h.

Figure5 shows the changesinX-rayintensities of the compoundswithcarbother-malreductiontime at 1250°C. MgSi03andMg2Si04decreasedwith time and dis- appearedafterthecarbothermalreduction for 2 and 7 h,respectively.The amount ofMgSiN2increasedgraduallywith time upto 4 h and then increased rapidlywithtime from 4 to 7 h.

139

Figure6. EDX spectraof the MgSiN2 powder (a)beforeand(b)after heat treatment of the carbothermallyreducedpowder(1250'Cfor7h)at 600°C for 1h.

SinceMgSi03is decomposedto form Mg2Si04andamorphousSi02 [see equation(2)],MgSiN?is formed bythe carbothermal reduction of Mg2Si04andamorphousSiO,:

It is, therefore, concluded that MgSiN2 powder should be prepared by thecarbothermal reduction of magnesiummetasilicate at 1250°C for 7 h.

Although MgSiN2 powder could be prepared by thecarbothermal reduction of magnesiummetasilicate at 1250°C for 7 h, it still contains unreacted carbon,because the amount of carbon is two times largerthan the stoichiometric amount [C/MgO . Si02(MgSi03)= 3.0;seeequations(2)and(4)].We examined the heat treatment conditions forthe elimination of theresidual carbon inairusingDTA-TG.Althoughthe data were omitted in this paper,we found an exothermic effect (500-700°C)intheDTA curve and a rapidmass decrease (45.5%;450-680°C)in the TG curve,both due to the burn-out of the residual carbon. On the basis of the DTA-TG results,the heat treatment was conducted at 600 ° C for 1 h. The XRD results showed that the powderheat-treated at 600°Cfor 1 h contained onlyMgSiN2.The elements in the powdersbefore and after the heat treatment were checkedusingEDX.Results are shown inFig.6. Although magnesium, silicon,nitrogenand carbon were detected from the as-carbothermally-reduced powder, thecarbondisappearedafter the powderwas heat-treated at 600°C for 1 h.

3.2. Evaluation ofsomeproperties of theMgSiN2 powder As shown in Section3.1,MgSiN2 powder could be prepared by the carbothermal reduction of magnesiummetasilicate at 1250°C for 7 h. We examine somepropertiesof the MgSiN2powderin this section.

140Table 1. Quantitativeanalysesof the powders prepared by varioustechniques

aCarbothermallyreducedmagnesiummetasilicateat1250°Cfor 7 h.3.2.1. Chemical composition.Table 1 shows the chemical compositionof the

resulting MgSiN2 powder examinedusingEDX and a N/0 determinator,togetherwith the theoretical valueandtheanalyticalresultsreportedelsewhere[3, 6]. Themagnesiumand silicon contents were 24.63 and 26.08 mol%,respectively.Thenitrogenandoxygencontents were 46.07 and 3.22 mol%,respectively.

Thesilicon content (26.08 mol%) is higherthan the theoretical value (25.00mol%).Theoxygencontent(3.22 mol%) is lower than that (4.76 mol%) ofthepowder prepared by the solid-state reaction between Mg3N2andSi3N4,but is higherthan that (0.76mol%)of the powderpreparedby nitriding Mg2Si. Although theMg/Siratio of the starting magnesium metasilicatepowderis1.00,that of the carbothermallyreducedpowderis lowered down to 0.94. This fact suggeststhatpartof the magnesium may be eliminated from the system during the carbothermal reduction as follows:

TheyieldofMgSiN2 powder is about 70%. The loss of the yield may be ascribed notonlyto the evaporationofmagnesiumbut also to that of silicon duringthecarbothermalreduction. The excessive silicon content over the stoichiometryandthepresenceofoxygen suggests the formation of amorphousmaterial in the Si-O system.ThepurityofMgSiN2 powder would be improvedif the evaporationofmagnesiumweresuppressed by oneormoretechniques, e.g. carbothermal reduction underhigh nitrogen pressure. 3.2.2. Powder properties.The color of the resulting MgSiN2 powder was white.

Thetruedensityof the powdermeasuredby picnometry was 3.098 g/ cm3. Thisvalueis almost in accordwith the theoretical density(3.148g/cm3).

Thespecificsurface area was 3.6m2 I g. Theaverage primary particle sizecalculated on the basis of the specificsurface area and true densitywas0.53p.m.

We furthermore observed the particle shapes by TEM.AtypicalTEMmicrographof the resulting powder is shown in Fig.7. The TEM micrographshowedthat the powderwascomposedof fine particleswith sizes rangingfrom 0. I to 0.5 Am.

141

Figure7. TEMmicrographof the MgSiN2 powder prepared by the carbothermal reduction ofmagnesiummetasilicate at 1250°C for 7 h.

Figure8. Thermal expansion-shrinkagecurves of theMgSiN2 compacts at a heatingrate of 10°C/min.(a)MgSiN2 compact, originating from the powder prepared by nitriding Mg2Si powder at 1400°C for 1 h [6].(b)PresentMgSiN2 compact.

Theaverage primary particle size(0.53Ecm)calculated from the specificsurfacearea almost correspondsto the sizes of the particlesobservedbyTEM.TheindividualparticlesobservedbyTEM are thus regardedasprimary particles. Theaverage primary particle size is slightly larger than the actual particlesizes,whichindicates that the primary particles partly sinter to one another.

On the basis of the above results,we found that the present MgSiN2 powderhasthefollowingcharacteristics:(i)submicrometer-scaledprimary particles, and(ii)narrowprimary particle sizesrangingfrom 0.1 to 0.5 um.Theoxygencontent(3.22mol%)ofthepresentMgSiN2 powder isslightly higher than that (0.76 mol%) of the MgSiN2powder prepared by nitriding Mg2Si. Thus a furtherinvestigationis needed to make clear whether such oxygenis detrimental to the sinteringornot. Then we examined the shrinkagebehavior of the present MgSiN2 compact

142at the initial stageof the sintering.Results are shown in Fig.8,togetherwith the case of MgSiN2powder prepared by nitriding Mg2Si. Note that little difference inaverage primary particle size(about0.5Mm)was observed between these two MgSiN2 powders. TheMgSiN2 compact, originating fromthepowder prepared bynitridingtheMg2Si,showed little shrinkagewithtemperatureupto 1400°C. On the otherhand,thepresent MgSiN2 compact showedrapidshrinkagewhen the temperatureexceeded 1250°C.

Thesinterabilityofthepowderat the initial stageof the sintering may be affected by physical factors(particleshapeandsize,particle-sizedistribution,defects,etc.)and chemical factors(impurities,adsorbedmaterials,etc.) [16].Since little difference in the average primary particle sizewas observed between these two MgSiN2 powders, we consider that the small amount of oxygenmaycontributetopromotingthe mass transfer at the initial stageofsintering.

We are now conductinga systematicevaluationon the sinterabilities of theseMgSiN2 powders. Detailswill be reportedelsewhere.

4. CONCLUSION Magnesiumsilicon nitride (MgSiN2) powder wasprepared by carbothermally reducing magnesium metasilicate whose chemical composition corresponded toMgO . Si02orMgSi03.The results are summarized as follows: (1)Thepowdermixture of magnesiummetasilicate and carbon with a molar ratio

of C to MgO . Si02orMgSi03 equal to 6.0 was carbothermallyreduced at a temperaturebetween 1200 and 1500°C in nitrogen atmosphere. AsingleMgSiN2 phase was obtained when the powderwascarbothermallyreduced at 1250°Cfor 7 h. The residual carbon in the powdercouldbe removed byheattreatingthepowderat 600°C for 1 h in air.

(2)The chemical compositionof the resulting MgSiN2 powder was:Mg,24.63mol%; Si, 26.08mol%; N, 46.07mol%; 0, 3.22 mol%. The silicon excess over thestoichiometrywould be presentasamorphousSi02.

(3)Theresulting MgSiN2 powder had the followingcharacteristics:(i)submicro-meter-scaledprimary particles, (ii)narrowprimary particle sizesrangingfrom0.1to 0.5 ttm,and(iii)lowersintering temperature thanthe case of the direct nitridationof Mg2Si.

AcknowledgementsThe authors wish to expresstheir thanks to Professor Dr. H. Hirokawa of SophiaUniversityfortakingthe TEM micrographs,and to Dr. A. Tsugeof the National Industrial Research Institute ofNagoyafor the oxygenandnitrogendetermination.

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