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The sol-gel technique offers a low-temperature method for synthesizing materials that are either totally inorganic in nature or both inorganic and organic. The process, which is based on the hydrolysis and condensation reaction of organometallic compounds in alcoholic solutions, offers many advantages for the fabrication of coatings, including excellent control of the stoichiometry of precursor solutions, ease of compositional modifications, customizable microstructure, ease of introducing various functional groups or encapsulating sensing elements, relatively low annealing temperatures, the possibility of coating deposition on large-area substrates, and simple and inexpensive equipment. Within the past several years, a number of developments in precursor solutions, coating processes and equipment have made the sol-gel technique even more widespread.
Chemical PrecursorsThe sol-gel process uses inorganic or metal organic precursors. The most commonly used organic precursors for sol-gel film formation are metal alkoxides (M(OR)z), where R stands for an alkyl group (CxH2x+1). Normally, the alkoxide is dissolved in alcohol and hydrolyzed by the addition of water under acidic, neutral or basic conditions, although film formation is also possible by the deposition of alkoxides followed by exposure to moisture. Hydrolysis replaces an alkoxide ligand with a hydroxyl liquid, as shown in the equation:
M(OR)z + H2O®M(OR)z-1(OH) + ROH
Condensation reactions involving the hydroxyl ligands produce polymers composed of M-O-M bonds. In most cases, these reactions also produce the byproducts water or alcohol, as shown in the following equations, for silicon condensation:
Si(OR)3OH + Si(OR)4®(RO)3Si-O-Si(OR)3 + ROH
2Si(OR)3OH®(RO)3Si-O-Si(OR)3 + H2O
Further reactions lead to the formation of silicon oxides. The chemical reactivity of metal alkoxides is related to the R; the larger the R, the slower the hydrolysis of metal alkoxides.
Based on the success of these new precursors, other precursors have also been developed for use in sol-gel processes, including metal carboxylate, metal dialkylamides, amorphous and crystalline colloidal sol solutions, and organic/inorganic hybrids.
Sol-Gel Coating Processes and Coating EquipmentSeveral methods can be used to make sol-gel coatings with the sol-gel process. Spin coating and dip coating are two basic techniques used to deposit sol-gel coatings. Spin coating produces a one-sided coating, while dip coating yields a double-sided coating. Both techniques are used in manufacturing to make different coatings and thin films.
Roll coating is another coating technique and is widely used for industrial coatings, especially for flexible substrates. It can make coatings at a speed of up to 200 ft per minute.
- Stage 1: The deposition of the coating fluid onto the substrate
- Stage 2: Aggressive fluid expulsion from the substrate surface by the rotational motion
- Stage 3: Gradual fluid thinning
- Stage 4: Coating thinning by solvent evaporation
Dip coating processes are used for plate glass by Schott, based on developments made by Schr?der2 and Dislich3,4; for solar energy control systems (Calorex®) and for anti-reflective coatings (Amiran®) on windows. The dip coating technique is also used for optical coatings, such as on bulbs, and for optical filters or dielectric mirrors by various small and medium sized enterprises (SMEs) and other companies that must fabricate multilayer systems with up to 30 or 40 coatings with very high precision. A large-scale fused silica dip coating system (FSDCS) has been delivered to Lawrence Livermore National Laboratory (LLNL) to make uniform anti-reflective sol-gel coatings over large precision optics measuring 44 cm x 44 cm x 41 cm and weighing up to 150 lbs. The FSDCS produces a speed ripple of less than ±1% at speeds of 2-20 cm/s, with strict control of vibration for both the optic and the tank.
The most important feature of a gravure coater is the patterned chrome roll. The pattern of cells or grooves is engraved onto the surface of the roll by mechanical engraving (knurling), chemical etching or electromechanical engraving. An excess of coating solution is applied to the gravure roll, which is then doctored by a flexible blade. The blade meters the cells partially full of coating liquid. The cells then pass into a nip where a fraction of the coating in the cells is transferred either to the web (in direct gravure) or to the offset roll (in offset gravure). In an offset gravure coater, the final transfer of coating to the web is at a second nip.
The main strength of gravure coating is the ability to apply thin coatings at high speed, with the coating thickness and uniformity controlled by the cell volume and cell uniformity. However, this method also has several drawbacks. Changing the coating thickness more than a small amount requires the gravure roll to be changed because the coating thickness is primarily determined by the volume of the cells on the gravure roll. Each cell must act like all the others, and the stability of the removal of the film from each cell is difficult to achieve at higher speeds and lower coating thickness. Additionally, wear of the gravure roll can be a problem when using abrasive coating formulations.
Developing a Turnkey ProcessA number of oxides and organic/inorganic hybrid coatings can be deposited on substrates of metal, plastic, semiconductor materials and ceramics by the sol-gel technology. The key to successful applications of sol-gel coating technology is developing a turnkey coating process. The entire production process should be very well controlled by specifically designed equipment and integrated process procedures. The equipment should facilitate the coating process and avoid ambient environmental effects to a significant degree, if not completely. Automation can also be used to reduce human error. The combination of improved solution stability, proper equipment and automation can be expected to greatly improve the reproducibility of the sol-gel coating process and accelerate commercial applications.
For More InformationFor more information about sol-gel coating technology, contact Chemat Technology, Inc., 9036 Winnetka Ave., Northridge, CA 91324; (818) 727-9786, ext 109; fax (818) 727-9477; e-mail email@example.com; or visit http://www.chemat.com. Or contact Shanghai Chemat Advanced Ceramics Technology Co., Ltd., 1288 Ye-Cheng Rd., Jiading, Shanghai, China 201800; (86) 21-59167109; fax (86) 21-59166967.
*These precursors were developed by Chemat Technology Inc.
**The Chemalux SR Coating System, supplied by Chemat Technology Inc.
†The DipMaster Dip Coating Systems, supplied by Chemat Technology Inc.