Che5700 陶瓷粉末處理 溶膠凝膠法 Sol-Gel Process More and more applications, become an independent category; Use metal alkoxide, to go through hydrolysis reaction, then condensation polymerization, to obtain desired colloids During process, system viscosity increase with time, it can also be used for coating into films, or even directly forming into desired objects  variety; Most common example: TEOS (tetraethyl orthosilicate) hydrolysis to make silica; (OC 2 H 5 ) 4 Si + H 2 O  (OC 2 H 5 ) 3 SiOH + C 2 H 5 OH  …. Gradual hydrolysis and then condensation reactions……

Advantages of Sol-Gel Processes Che5700 陶瓷粉末處理 In general: Able to get uniform & small sized powder Can get at low temperature high density glass, without high temperature re-crystallization Can get new compositions of glass New microstructure and composition Easy to do coating for films Can get objects or films with special porosity For improved adhesion Can get metal (inorganic) – organic composites Can coat onto large area or complex shape objects Can get fibers High uniformity, multicomponent systems

More Process Characteristics Che5700 陶瓷粉末處理 Mixing of reacting system: all alkoxide precursor, or 1 alkoxide + 1 metal salt; Things to consider: (a) cost of precursor; (b) relative hydrolysis rate; (c) compatibility of chemicals (uniform mixing) Colloidal sol & polymerized sol; Peptization: adding proper solvent or adjust pH, to re- disperse weak agglomerates back into sols M(OR) n  mostly derived from MCl n ; often sensitive to light, moisture, temprature etc.; M-O bonding: ionic nature – mostly solids; covalent nature - liquids To decompose alkoxide: (a) add water; (b) heat (to decompose)

Some statistic data on Alkoxide synthesis Che5700 陶瓷粉末處理 Source: Am. Cer. Soc. Bull., 72(10), 73, 1993;

Some real examples

Reaction Characteristics Che5700 陶瓷粉末處理 In simple terms, whole process control is on control of relative rate between (a) hydrolysis ; (b) poly- condensation M-OH + M-OR  M-O-M + ROH; Yet M-OH & M-OH condensation rate, and whether M-OR & M-OR can condense; whether linear condensation or branched form  affect microstructure Effect of catalyst: acid or base Effect of temperature: e.g. Al(OR) 3 low temperature hydrolysis  amorphous form, further aging  hydroxide; at high temperature (> 80 o C)  crystalline boehmite AlO(OH), sintering behavior different Zr(OR) 4 hydrolysis, easy to get oxo bond, instead of hydroxy bond

More Characteristics Che5700 陶瓷粉末處理 Alcohol as solvent, same as alcohol in alkoxide, one can also use different alcohols (or co-solvent), may affect reaction; sometime the steric effect Additive with carbonate groups, will get esterfication ( 酯 化 ), and polyesterification to get gel-like product; (e.g. Pb acetate; Pb(C 2 H 3 O 2 ) 2. 2H 2 O for synthesis of BaTiO 3 powder) Another reason for gelation: solvent evaporation, increase of concentration; mostly due to reaction;  end result 3D network structure, system viscosity increase  continue aging, stronger structure, begin to shrink, expel solvent.

Taken from TA Ring, 1996; Three different network structures

Taken from TA Ring, 1996; sol characteristics also affect film structure and properties

Acid or base (catalyst) and /or salt: will change surface charge  and final structure

Polarity Effect  Effect of different solvents on particle size (silica)  P A ’, P B ’ = polarity of solvents A, B   A,  B volume fraction

A Proposal  Adding solvent of different polarity, change solubility of precursor species, change supersaturation,  nucleation to get more nuclei  smaller final particle size

Gel Characteristics Che5700 陶瓷粉末處理 Gel stage: time to form film, fiber, etc. forming period. Due to proper viscosity to work with. Gel densification: Continuous cross-linking & dehydration to expel solvent Reduce free volume (relaxation of microstructure) Reduce surface area Capillary contraction: due to solvent evaporation All above mechanisms  shrink structure, may cause crack, especially as films (constrained by substrate) Sol  gel can be considered as phase transition

Low coordination number  large pore inside structure; high CN  dense structure

Theoretical composition Che5700 陶瓷粉末處理

Filtration of ordinary small particles, may also form gel layer

During formation of gels, sample may adhere to wall and cause crack

Gel Drying Che5700 陶瓷粉末處理 Gel drying period, can get kinetic data from weight loss Similar to ordinary drying process, classified as (a) constant rate drying period; (b) reach a critical point (prone to cracking); (c) first falling rate period; (d) second falling rate period To prevent cracking during drying, control drying rate (slow during certain period), some proposed to add “drying control chemical additive (DCCA) – objective: to lower capillary pressure, to lower solvent pressure; or to use supercritical evaporation method

I zone – de-hydration, solvent evaporation, slight capillary contraction II zone – continue to dehydrate, molecule cracking; skeletal densification, structural relaxation may be polymerization reaction III zone – little change in weight, viscous sintering cause shrinkage

Decomposition and Sintering Che5700 陶瓷粉末處理 Up to 150 o C, continuous dehydration, some ligand may desorb and leave, micropores will limit its movement Over 250 o C, molecule begin to decompose, continue to loss weight. Decomposition affected by gel structure, extent of cross-linking; atmosphere also important, incomplete decomposition may have residual char; Skeleton collapse, particle sintering and densification, mostly by viscous sintering (faster than conventional diffusion mechanism), I.e. can be achieved at lower temperature (one advantage of sol-gel process); if fast heating, may contain residual pores.

Sol to Gel to Glass Che5700 陶瓷粉末處理 sol- gel- glass: one advantage – no need of high temperature treatment, can obtain special composition, high purity, high uniformity Difficult to make one whole piece glass without crack, often very slow (because of slow drying to avoid cracking) Ref: Am Cer. Soc. Bull., 64(11), 1463, 1985

* TEOS + boric acid + phosphoric acid + acid catalyst + glycerol + formamide  PE or teflon container  room temperature; B+P content may reach 12 wt%

Example of Reverse Micelle Method Micelle = oil in water; reverse micelle = water in oil