: ; ; : (compared to liquid, gas phases), /, core, , ,

Che5700

SiC; Acheson process: SiO2 + C (coke) (> 2000oC) SiC , ; , Si, Fe, O; SiO2 + C SiO + CO (, fine dust) : SiH4, SiCl4, chlorosilane + CH4 (or C2H4) (even by plasma, or laser) , , (> 99.8%); (~99.5%), (~99%), (~90%) petroleum coke

Che5700

SiC; HFSiO2

Si3N4: Si + (, , Si): SiCl4 + NH3 Si(NH)2 + NH4Cl NH4Cl, HCl Si(NH)2 1000oCamorphous Si2N3H (remove NH3) 1400-1500oCSi3N4: , NH3 silicon imide Si(NH)2

Che5700

Si3N4(2)SiCl4 (g) + NH3 (g) Si(NH)2 + NH4Cl (s) H = -161.5 Kcal/mol . ,

(4) SiO2 + C powder (may have some hydrogen to minimize oxidation)

Mostly heterogeneous reactions; homogeneous reactions

Che5700

SiCl4 NH3

Si3N4, , , or amorphous form; beta form, ,

, ()

;

Am. Cer. Bull. 70(1), 1991.

AlN Powder Synthesis: AlCl3 + 4 NH3 AlN + 3 NH4Cl; 900-1500oK, >5 hr : R3Al(l) + NH3 R3AlNH3 in sequence to get AlN + 3 RH; 400-1000oK (, ): Al2O3 + N2 + 3C 2 AlN + 3 CO; 1500-2200oK, >5hr; : 2 Al + N2 2 AlN; 1000-1500oK, >5hr, : , Che5700

, , : (), , () Any carrier (solvent, or carrier gas)? , ? () (), ?Che5700

Shrinking core & shrinking sphere models

Examples of Shrinking Core ReactionsFeO + H2 Fe + H2OCaCO3 + heat CaO + CO2

spontaneous reaction; G , spontaneous, or (most likely).: decomposition, oxidation, reduction; : : , , , or even CO2; Che5700

: (a) ; (b) ; (c) ; (d) ; (e) ..,

Che5700

TA Ring, 1996, ,

Shrinking Sphere Modelflakes off the original particle, shrinking sphere model, CaCO3: (a) mass transfer of A to particle; (b) surface reaction; (c) mass transfer of product away from particle; (d) heat transfer

: nucleation and growth model e.g. 7 C + 2 B2O3(l) B4C (s) + 6 CO (g) ; B4C; Avrami kinetics: ln( 1 XB) = - (k t)m (general form); Che5700

Solid-Solid Reactions, , e.g. NiO + Al2O3 NiAl2O4 ZnO + Al2O3 ZnAl2O4 BaCO3 + TiO2 BaTiO3 + CO2 (g) 4 B + C B4C SiO2 + C SiC + CO2 carbothermal reaction, , , e.g. SiO2 + C SiO (g) + CO; SiO + 2C SiC + CO (1900oK),

Che5700

CO + O2 CO2 K = PCO2/[PCO x PO2]

Solid-Solid Inter-diffusionDiffusion flux Ci, ion mobility Bi electrochemical potential gradient , = chemical potential; = electrical potential; Z = valence of species; F = Faraday constant;Carter eq. For solid reaction kinetics

TA Ring, 1996;Charge balance, space charge, (in opposite direction);diffusion couple;

(Fe): formwhisker;

: nucleation of Si3N4 on Si growth + CVD Si3N4 (whisker form) SiN2T