16 W. Dressler, R. riedel composites, by using high melting sintering AB except that it is rotated by 180% on the c- additives, 4 by reduction of additive content* axis. s Consequently, the channels present in and by devitrification of grain boundary pha- the B-modification are closed off into two inter ses.as reviewed in this article. Additionally, stices and the c-dimension of a-si3N an alternative route for the fabrication of silicon (c=0. 5617 nm) is approximately twice that of nitride, silicon carbide ceramics and Si3N,/SIC- B-Si3N4(c=0. 29107 nm). The lattice parameters composites, the hybrid processing, 4-0, 43,44 in the a directions are similar: a(a-Si3N4) shows exceptional possibilities to meet the =0.7818 nm, a(B-Si3 NA)=0-7595 nm aforementioned requirements. Here, organosili The technical synthesis routes lead mainly to con polymers(hybrids)are converted to silicon- a-Si,N, which converts to B-Si3 Na during liquid based inorganic materials this processing phase sintering. Grun" calculated the free technique not only the phase composition and enthalpy of a-to B-transformation at 10 Pa and microstructure but also the thermo-mechanical 298 K to be -30 kJ mol. Hampshire and properties of the final ceramics can be tailored Jack*/ reported that the activation energy for by the choice of organosilicon system, the the a-to B-transformation during liquid phase design of the intermediate processing steps(to sintering is similar to the dissociation energy of convert the system to an inorganic amorphous the Si-n bond, 435+38 kJ mol Thus the intermediate) and the use of annealing treat- mechanism of the reconstructive transformation ments(to transform the inorganic intermediate seems to be the breaking of Si-N bonds, the into the desired final crystalline ceramics) solution of the less stable a-phase in the formed This article in general reviews the progress in liquid phase and finally the reprecipitation of Si3N4 and Sic based materials achieved in the less soluble, more stable b-modification past decade. In particular, the fabrication and The technical production of Si,n4 powder properties of Si,N,/SiC composites derived predominantly performed on four different from (i) conventional processing and (ii) from routes: (i) the direct nitridation of silicon pow advanced techniques are comparatively dis- der,(ii) the carbothermal reduction and subse quent nitriding of Sio2,(ii the diimide process and, (iv) the gas phase reaction of silanes with ammonia 2 SILICON NITRIDE (Si,N,)-INTRINSIC The direct nitridation of elemental silicon to STRUCTURAL PROPERTIES AND achieve stoichiometric Si, Na was developed SYNTHESIS from Weiss and Engelhardt in 1910 and is still the most common industrial processing Si3 N, is a highly covalent compound(70% cova-route lence)having a density of 3 19g cm-3and 3Si+2N2-100140c occurring in two different crystal structures the a-and B-modification. The a-structure possesses AH=-750 kJ mol the space group P3lc the B-modification is hex The resulting SiN, powder is subsequently agonal(P63/m)as depicted in Figs 1 and 2. The milled and consists mainly of the a-modifica- polymorphs consist of slightly distorted tetra tion. In this connection, the choice of starting hedral SiN,(sp hybridization of Si)and planar silicon powder quality (grain size and purity) NSi,(sp- hybridization of f N units The SiN determines on the one hand the price of the tetrahedrons are joined by sharing nitrogen cor- resulting Si,N, and on the other hand the purity ners so that each nitrogen is common to three of the product. Especially the use of semicon tetrahedra. The unit cells of a-Si3n4 and ductor silicon leads to extremely pure Si, NA- B-Si3NA are represented by Si12N16 and Sions, powders. 4 The carbothermal reduction method respectively, as shown in Figs 1 and 2. The starts from a mixture of fine Sio, and carbon B-structure is composed of puckered rings of powder. This mixture is converted into Si N4 at alternating Si andn atoms having a stacking 1500 C in flowing N2 quence of ABAB+ and forming channels 1500°C ( diameter about 0. 15 nm) along the c-direction 3SiO2+6C+2N2 Si3N4+6Co(2) The a-modification contains the same AB layer In order to convert the initial SiOz completel and an additional layer CD which is similar to into Si3n4 excess carbon is necessary. .>3 By16 IV.. Dressier, R. Riedel composites, 27-39 by using high melting sintering additives, 4°'4~ by reduction of additive content 43 and by devitrification of grain boundary pha￾ses 3"42 as reviewed in this article. Additionally, an alternative route for the fabrication of silicon nitride, silicon carbide ceramics and Si3N4/SiC￾composites, the hybrid processing, 34-36"43"44 shows exceptional possibilities to meet the aforementioned requirements. Here, organosili￾con polymers (hybrids) are converted to silicon￾based inorganic materials. Using this processing technique not only the phase composition and microstructure but also the thermo-mechanical properties of the final ceramics can be tailored by the choice of organosilicon system, the design of the intermediate processing steps (to convert the system to an inorganic amorphous intermediate) and the use of annealing treat￾ments (to transform the inorganic intermediate into the desired final crystalline ceramics). This article in general reviews the progress in Si3N 4 and SiC based materials achieved in the past decade. In particular, the fabrication and properties of Si3N4/SiC composites derived from (i) conventional processing and (ii) from advanced techniques are comparatively dis￾cussed. 2 SILICON NITRIDE (Si3N4) -- INTRINSIC STRUCTURAL PROPERTIES AND SYNTHESIS Si3N4 is a highly covalent compound (70% cova￾lence) having a density of 3.19g cm -3 and occurring in two different crystal structures the a- and fl-modification. The a-structure possesses the space group P31c the fl-modification is hex￾agonal (P63/m) as depicted in Figs 1 and 2. The polymorphs consist of slightly distorted tetra￾hedral SiN4 (sp 3 hybridization of Si) and planar NSi3 (sp 2 hybridization of N) units. The SiN4 tetrahedrons are joined by sharing nitrogen cor￾ners so that each nitrogen is common to three tetrahedra. The unit cells of a-Si3N4 and fl-Si3N 4 are represented by Si12Ni6 and Si6Ns, respectively, as shown in Figs 1 and 2. The fl-structure is composed of puckered rings of alternating Si and N atoms having a stacking sequence of ABAB 45 and forming channels (diameter about 0.15 nm) along the c-direction. The a-modification contains the same AB layer and an additional layer CD which is similar to AB except that it is rotated by 180 ° on the c￾axis. 45 Consequently, the channels present in the fl-modification are closed off into two inter￾stices and the c-dimension of a-Si3N 4 (c=0.5617nm) is approximately twice that of fl-Si3N4 (c=0.29107 nm). The lattice parameters in the a directions are similar: a(a-Si3N4) =0.7818 nm, a(fl-Si3N4)=0.7595 nm. 46 The technical synthesis routes lead mainly to a-Si3N4 which converts to fl-Si3N4 during liquid phase sintering. Grfin 46 calculated the free enthalpy of a- to fl-transformation at 105 Pa and 298 K to be -30kJ mol-'. Hampshire and Jack 47 reported that the activation energy for the a- to fi-transformation during liquid phase sintering is similar to the dissociation energy of the Si-N bond, 435+38kJ mol -~. Thus the mechanism of the reconstructive transformation seems to be the breaking of Si-N bonds, the solution of the less stable a-phase in the formed liquid phase and finally the reprecipitation of less soluble, more stable fi-modification. The technical production of Si3N 4 powder is predominantly performed on four different routes: (i) the direct nitridation of silicon pow￾der, (ii) the carbothermal reduction and subse￾quent nitriding of SiO2, (iii) the diimide process and, (iv) the gas phase reaction of silanes with ammonia. The direct nitridation of elemental silicon to achieve stoichiometric Si3N4 was developed from Weiss and Engelhardt in 191048 and is still the most common industrial processing route 5,.52 3Si + 2N2 1 lOO-14oo°c ~" Si3N4 AH= -750 kJ mol-1. (1) The resulting Si3N4 powder is subsequently milled and consists mainly of the a-modifica￾tion. In this connection, the choice of starting silicon powder quality (grain size and purity) determines on the one hand the price of the resulting Si3N 4 and on the other hand the purity of the product. Especially the use of semicon￾ductor silicon leads to extremely pure Si3N n￾powders? 4 The carbothermal reduction method starts from a mixture of fine SiO2 and carbon powder. This mixture is converted into Si3N 4 at 1500°C in flowing N~. 3Si02+6C+2Nz 15oooc > Si3N4+6C0 (2) In order to convert the initial SiOz completely into Si3N4 excess carbon is necessary. 5~'53 By