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; (OC2H5)4Si + H2O (OC2H5)3SiOH + C2H5OH …. 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 MCln ; 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 CharacteristicsChe5700 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 (> 80oC) 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(C2H3O2)2 . 2H2O for synthesis of BaTiO3 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

Polarity index of cosolvent

4.00 4.25 4.50 4.75 5.00 5.25 5.50 5.75

Dia

met

er (

nm)

0

100

200

300

400

500

600

7001,2-Dichloroethane & Ethanol (NH3=1M)

Ethanol & Ethylene glycol (NH3=1M)

Methanol & Ethanol (NH3=1M)

Methanol & Ethylene glycol (NH3=1M)

Methanol & Ethanol (NH3=0.5M)

TEOS=0.28M

Effect of different solvents on particle size (silica) PA’, PB’ = polarity of solvents A, BA, B volume fraction

BBAAAB PPP '''

A Proposal

Si(O

C2H

5)4-

X(O

H)X

Cmax*

Cmin*’

Cs’

Time

Cmax*’

high polaritycosolvent

low polaritycosolvent

Cmin*

Cs

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

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 reactionIII zone – little change in weight, viscous sintering cause shrinkage

Decomposition and Sintering

Che5700 陶瓷粉末處理

Up to 150oC, continuous dehydration, some ligand may desorb and leave, micropores will limit its movement

Over 250oC, 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