Solid State Phenomena Vols. 124-126 (2007) pp 619-622 online at http://www.scientific.net © (2007) Trans Tech Publications, Switzerland Online available since 2007/Jun/15
Industrial Applications of Sol-Gel Technology Youngjun Hong*, Gi-Ra Yi Advanced Materials R&D, LG Chem Research Park, 104-1 Moonji-Dong, Yuseong-Gu, Daejeon 305-380, Korea Corresponding author:
[email protected]
Keywords: sol-gel processing, nanoparticles, nano-composites, industrial application
Abstract. In present paper, various industrial applications of the sol-gel technology will be briefly introduced. In particular, the current development status and technological issues of sol gel derived functional nanoparticles in electronic and industrial materials will be reviewed. Productions and surface modifications of nanoparticles by sol-gel processing, their incorporations into nanocomposites and final applications are described. Introduction Sol-gel processing has received considerable attentions due to its applications for new materials.[1] Films, fibers, monolith or stable colloidal suspensions of nanoparticles are produced from metal alkoxides. Moreover, the flexibility of compositional change using designed precursors or mixtures enables us to control their physical properties precisely such as hardness, transparency, chemical resistance, porosity, and thermal conductivity.[2] Furthermore, using organically modified metal alkoxides, organic-inorganic hybrid materials with homogeneity at molecular level can be fabricated. Due to unique properties of metal oxide nanoparticles, academic research groups have been trying to development efficient way for producing those nanoparticles in large quantities. Over ten years, we have been working on the implementation of such sol-gel derived inorganic polymers and nanoparticles to nanocomposites for various industrial applications. One of our main applications areas is display industry such as anti-glare and anti-reflection coatings. Currently, the main issue of the display industry is manufacturing cost. Therefore, one of the most important factors is production speed. In order to increase the speed, companies prefer UV cure system rather than thermal cure and they are focusing in developing UV curable acrylate system with functional nanofillers. For this reason, the importance of nanoparticles derived from sol gel technology is becoming more important compared to inorganic binder in the display coating. Sol-gel derived Nanoparticles: Productions and Applications Production of nanoparticles can be achieved through several different methods. Most common approaches include solid-state methods, vapor methods, chemical synthesis, and gas-phase synthesis methods.[3] Solid-state methods including milling process are quite well established and have been used for mass production over tens of years. However, those are not appropriate for All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 130.203.133.34-16/04/08,14:18:56)
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precise control of size distribution and working only on bulk for limited materials and applications. Vapor methods are highly versatile but higher throughput at lower cost is still challenging. Chemical synthesis by sol-gel reaction is the most popular methods. The main problems of this sol-gel process are reproducibility between batches and producing particles in large quantities. Controlled scale-up methods can solve both problems today, provided that there is finding the appropriate surfactant or coupling agent that will eliminate agglomeration and determine functionality of the nanoparticles. Because of this reason, colloidal chemistry will continue to be developed over the next 10 years. Nanoparticles have broad industrial applications as suspension itself or fillers for nanocomposites. Suspensions of dense nanoparticles are mainly used as abrasive materials for CMP(chemical mechanical polishing) process, which is important of microelectronic device fabrication. For planarization of interlayer dielectrics, silica nanoparticles have been used as abrasive from the early stage of the development of CMP process. However, their market volume is fairly mature and prices are getting to drop since production increases. Recently, ceria slurries have been developed for shallow trench isolation (STI) formations, which are challenging since they have to polish one of the two different materials (silicon oxide and silicon nitride) selectively as shown in Figure 1 and the feature size of microelectronic chips is reduced to sub-100nm.[4] Before CMP QP
SiO2
QP
Si3N4
After CMP
QP
Si Figure 1. Structure of STI and planarization by CMP process As shown in Figure 2, nanoparticles under 50nm are used as fillers for flat panel display coatings since they are optically transparent when they are incorporated into nanocomposites. For instance, conductive nanoparticles such as indium tin oxide are using for antistatic function layer. Void SiO2 and TiO2 are also most common nanoparticles to adjust reflective index in the display films.
Figure 2. TEM/SEM images of functionalized transparent nanoparticles. Recently, TiO2 nanoparticles have also taken lots of attraction due to UV protecting function in personal care products. Nanophase Technologies and Oxonica are recently involved in this
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application. Moreover, photo catalytic activation of TiO2 have been applied to self-cleaning applications for removing dirt or other substances to adhere to the surface. However, TiO2 has a problem when it is coated on organic substrate because it makes the organic substrate decompose due to photo catalytic effect. In order to solve this problem, we developed nano sized silica based self cleaning coating. Figure 3 shows PVC vinyl siding coated by the silica based self cleaning and there are obvious difference in hydrophilicity and self cleaning effect.
Uncoated
Coated
Figure 3. Effect of vinyl siding coated silica based self cleaning material. Surface modification (Silane coupling agent) Most nanoparticles by sol-gel processing are stable in specific solvent system. However, their surface should be modified for making homogeneous and well dispersed solution when particles are mixed with binders having different chemical properties and solvent system. For most of metal oxide nanoparticles, silane coupling agents as shown in Figure 4 are used because depending on chemical structure of organic side chain (X in Figure 4) they can control degree of hydrophilicity(or hydrophobicity) and reactivity with the binder. Typical examples for the surface modifications on inorganic surfaces of hydroxyl groups are also shown in Figure 4. Variety of commercially available silane coupling agents could be supplied. Titanium-based coupling agents are also used for the surface modifications of indium tin oxide nanoparticles [5] but diversity of commercially available coupling agent is still limited compare to silane coupling agent.
X
X
X
X
X
H2O Si
HO
Si
O
Si
O
S
OH
OH
OH
OH
OH
OH
OH
RO
OH
S
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OR
OH Hydrolysis
Inorganic material
X
Si
HO H
O O
O H
H
X
X
Si
Si
O
O
O
H
H
O
Inorganic material
Dehydration Condensation
OH H
HO
X
Si O
X
X
O
Si
Si
O
OH
O
O H
O
H
Inorganic material
Figure 4. Surface modification of inorganic materials using silane coupling agent
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Summary In this paper, I briefly reviewed nanocomposites including production and surface modifications of sol-gel derived nanoparticles, their incorporations into polymeric networks and final application. So far, optically transparent films with various functions such as anti-reflection and self cleaning with good mechanical strength are produced using nanoparticle-based composites as we described here. In near future, using advanced nanosized fillers with additional value or higher function, we hope that cost-effective process of more functional films is developed. References [1] C. J. Brinker, G. W. Sherer, Sol-Gel Science, Academic Press, 1989. [2] Alain C. Pierre Introduction to Sol--Gel Processing, Springer, 1998. [3] Juan Pérez, Laszlo Bax, Carles Escolano, Roadmap Report on Nanoparticles, 2005 (www.nanoroadmap.it/roadmaps/NRM_Nanoparticles.pdf) [4] Thomas Abraham, Chemical Mechanical Polishing Equipment and Materials: A Technical and Market Analysis, 2003 [5] Y. S. Cho, G. R. Yi, J. J. Hong, S. H. Jang, and S. M. Yang, Colloidal Indium Tin Oxide Nanoparticles for Transparent and Conductive Films, Thin Solid Films, In Press, 2006.
