MetE 215 Sol-Gel Processing 1. Introduction 1.1 Glass from liquid precursors: What is sol-gel? Glasses are usually prepared by mixing the different oxides precursors (carbonates, nitrates, sulfates, oxides..) at solid state and then this mixture is melted at relatively high temperatures (1300-2000 C) to obtain a liquid. The high temperature structure of the liquid is characterized by an amorphous state (no order at long distance) which is preserved by cooling the melt rapidly at room temperature. The resulted solid is structurally amorphous and presents, when heated, the characteristic glass transition temperature (Tg). Glass researchers in the late 70's start investigating intensively low temperature processing routes for making glass. Network randomness is typical characteristic of glass. During conventional glass making from oxide we consume a lot of energy to destroy the order initially present on the various crystalline precursors. Random network can be also from using liquid precursors, in the same way macromolecular chemists do to elaborate polymers. The glass amorphous state can be created by using a "bottom-up" approach by chemical routes named as sol-gel. In this case, there is no need to go to high melting temperatures since the network structure can already be elaborated at relatively low temperatures (20-80 C). This chemical approach of making a material starting with molecular precursors and building the structural blocks (here Si-O-Si network) is widely used today for tailoring structures at the molecular level to create new materials with enhanced performances. The advantages of this chemical approach over the traditional melting technique are: - Less energy consumption, - Better homogeneity: mixing at the molecular level, - Higher purity: Liquids can by successive distillation be purified at ppb or even lower levels, whereas such purity is much difficult to achieve in the solid state. These features make sol-gel an attractive processing method, if the cost is not prohibitively high. The first attempt to synthesize glass from gels was focused on silica (a simple form that can be made by sol-gel, but extremely difficult to melt oxide). That was the beginning of an great research effort started in the late 70s' known today as sol-gel process. A chemical route for obtaining glassy and ceramic materials at relatively low temperatures starting from liquids. Although the sol-gel technique has drastically evolved since, it is now used for a variety of materials in different forms (thin and thick films, fibers, nanopowders, bulk porous or dense ceramics), composition and structure (inorganic, organic-inorganic hybrids, semiconductors, nanotextured materials, etc.) This teaching material is partially produced from the tutorial publications of The Sol-Gel Gateway entitled Silica Glass from Aerogels by Michel Prassas (Corning European Research Center, France) and Wet Coating Technologies for Glass by H. Schmidt, M. Mennig (INM, Institut für Neue Materialien, Saarbrücken, Germany.) 1
1.2 Chemistry of sol-gel Let's take for the simplicity of the purpose the example of a single component glass SiO2. Quartz the major natural crystalline form of SiO2 is a periodic structure of [SiO4] tetrahedra where Si atoms occupy the center coordinates with four oxygen atoms. A regular network in a plane representation is given in Figure 1a. Silica glass on the other hand is made from the same elemental tetrahedron except that there is no apparent regularity (amorphous) in the construction of this network shown in (Figure 1b). One needs to go up to more than 1800 C in order to melt the quartz and transform it into a new product of desired shape. In addition -due to the high viscosity of silica- successive melting is required before achieving optical quality. Glass chemists overcame the difficulty by building the amorphous silica network at room temperature using liquid silicon precursors known as silicon alkoxides. The general formula of this molecular precursors is Si(OR)4 alkoxy groups (OR where R= CnH 2n+1) are readily hydrolyzed by water and progressively replaced by OH groups forming silanols (Si-OH). This is known as HYDROLYSIS. Then, silanols (Si-OH) can further react between them or /and with non hydrolyzed alloy groups to form a siloxane bond (Si-O-Si), which is the beginning of the silica network formation. This second step is known as CONDENSATION reactions. These reactions are schematically represented in Figure 2. By these chemical reactions, an amorphous 3-dimensional interconnected silica network can be formed in progressive fashion. The viscosity of the solution (sol) is continuously increasing up to the point where the entire solution is gelified. The resulting structure (gel) is constituted by a coherent continuous solid silica network impregnated by a liquid solvent phase and the reaction by-products. So, typical sol-gel system for making silica requires four main ingredients. 1. A liquid precursor, serving as a silicon and oxygen source: silicon alkoxide, Si (OR)4 such as Si(OCH3)4, tetramethylortohosilicate (TMOS), or Si(OC2H5)4, tetraethylortohosilicate (TEOS) etc. 2. A solvent to dissolve the organic precursor: alcohol methanol CH3OH or ethanol C2H5OH etc. 3. Water, acting as hydrolyzing agent 4. A catalyst (an acid or base to initiate and modify the hydrolysis and condensation reactions) Although simple in its illustration the system is rather complex. A number of parameters can change the reactivity scheme and ultimately the properties of the final product. These are; concentration (silicon alkoxide:aqueous media ratio), water content, ph (catalyst type and amount), temperature, drying conditions. Meanwhile, these parameters, give additional degree of freedom to tailor specific properties such as pore volume, pore size, specific surface area of the resultant amorphous silica. 2. Processing of monolithic sol-gel derived products Two challenging processing steps for achieving conditions for forming a monolithic glass product from a gel are i. drying, removal of liquid chemicals (mutual solvent, water and reaction by products) present within the gel network and, ii. sintering to densify the gel by eliminating the porosity left after drying without destroying the amorphous structure. 2
(a) (b) Figure 1: 2-dimensional schematics of (a) ordered SiO 2 structure (crytalline), (b) random network of SiO 2 (amorphous or glassy SiO 2 ). H 2 O, solvent TEOS (Tetraethyl-orthosilicate) HYDROLYSIS Hydrolysis & Condensation Si-O-Si (Glassy Network) CONDENSATION Figure 2: Fundamental chemical reactions of sol-gel derived SiO 2 3
In order to preserve the expected sol-gel advantages, the performs, i.e. gels, should maintain their structural integrity throughout all subsequent processing steps (drying, sintering ) which eventually transform the gel to a glassy product. Freshly prepared silica gels contain an appreciable amount of solvent (usually 70 to 90 wt. %) which must be eliminated. The solid silica network formed by hydrolysis and polycondensation of silicon alkoxide is made up by silica species of a few tens of nanometer size. Capillary stress appears when the liquid moves inside the pores during drying and form a liquid-gas curved interface (Figure 3). The liquid inside the pores (which size is similar to ultimate silica particles) exerts a stress (P) in the "walls" of the capillaries during drying, which is inversely proportional to the pore radius (r) according to 2 Cos P r In the case of an alcoholic solvent (i.e. methanol, surface tension = 0.0022 Nm -1 ) and for a 10 nm pore radius, the capillary stress will be then in the order of more than 10 6 N/m². This capillary stress is high enough to break the gel into useless tiny pieces. However, employing specially controlled drying procedures (sometimes lasting weeks or months) and controlling the reaction chemistry, monolithic products from alkoxides derived gels can be formed. At least under experimental conditions, it will be somewhat demonstrated in the first part of this laboratory session. It is worth mentioning that nowadays there are some special techniques for achieving controlled solvent removal and drying (known as Hypercritical Drying, will not be discussed here) for obtaining monolithic sol-gel derived products by technologically relevant and feasible way. 3. Processing of sol-gel derived coatings Large area/high volume coatings on different substrates have been developed to achieve different functions. Different material properties and functions can be achieved by coatings or thin films including, easy-to-clean, antifogging, scratch resistance, photochromic or electrochromic, antireflective (AR), IR reflecting, color, conductivity. Depending on the effect to be obtained and the material to be used for this, either gas phase, vacuum or wet coating techniques can be used. In the case of vacuum coating the equipment and the technology can be rather costly, wet coating technologies based on sol-gel approaches on the other hand can be advantageous due to simple processing needs. Making coatings by sol-gel simply involves i. Depositing of a liquid sol onto the substrate (coating step) before reaching the gel state. ii. Subsequent maturing (usually by a thermal treatment) of the deposited sol. Variety of methods can used for depositing the sol in the form of a coating or thin film. These include dip coating, flow coating, spray coating, spin coating, roll coating, and screen printing. Of particular interest in our lab work is spin coating technique. We will learn to use spin coating technique for making glassy silica coating on polycarbonate. Such sol-gel derived coating can improve the scratch resistance of the polymer or organic-based engineering materials with intrinsically poor hardness compared to those of glass and ceramics. In the spin coating process, the substrate spins around an axis which should be perpendicular to the coating area. The schematics are shown in Figure 4. The sol can be deposited when the 4
substrate is stationary or during the spinning. A coating thickness of several hundreds of nanometers and up to couple micrometers can be realized by the choice of spinning speed, rheological properties of the coating sol and by performing multiple coating operations. The overall quality of the coating depends on optimization of these processing parameters. 4. Laboratory Work: Low Temperature Glass Synthesis This lab work includes sol-gel processing of glassy silica in two different forms. Initially, a monolithic (bulk) silica product will be produced by sol-gel starting from silicon alkoxide-teos. Preparation of the sol-gel and the effect of different catalysis conditions on gelling behavior of the sols will be demonstrated. The second part involves making silica coatings on a planar polycarbonate substrate by spin-coating. 4.1 Procedure for making monolithic silica and coatings (1) Clean and dry all glassware, ending with an ethanol rinse. (2) The acids, bases, and organics used in this lab are toxic and can cause severe burns. Please wear latex gloves throughout the experiment and report any spills immediately to the instructors. (3) The instructor will give the students the procedure for preparation on an acid, or twostep acid-base catalyzed reaction sequence. (4) Prepare two different sols according to the given protocols, in two separate beakers. (5) Remember to stir for 5 min after each step during mixing the ingredients. (6) After mixing all the ingredients, measure the ph and record for each sol. (7) Seal the sol in the first beaker (acid catalyzed) label it with date and group name. Keep it aside-and observe the changes until the end of lab-session. (Note that this sol, after 1-week of aging, will be used by the following week group) (8) Pour the sol in the second beaker (acid-base catalyzed) in equal amounts into to the test tubes. Close the test tubes with a cap and seal with Parafilm. After a certain time (within 1 hour) the sol should be gelled in clear liquid form-record the gelation time. (8i) After 3 days, remove seals and open to air to dry. (8ii) After 2 days, reseal with parafilm, and perforate with 6 small holes. (8ii) After 5 days, remove seals and place in drying oven at 75 C for 24 hours. (Observe and record any changes during these steps). (9) Take the aged acid-catalyzed gel prepared by the previous weeks group and spin-coat on the polycarbonate substrates according to the given protocol by the lab instructor. 4.2 Report The report should contain the following: 1) Compare your sol formulations with the actual raw materials and masses used during batching. 2) Gelling times for both sols 3) Description of the changes for both sols during one week of time span. 5. Further Reading and References 1. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, C. J. Brinker and G. W. Scherer, Academic Press, 1990 (TP810.5.B75) 5
2. Introduction to Sol-Gel Processing, A. C. Pierre, Kluwer, 1998 (TP810.5.P54) 3. Sol-Gel Science and Technology, Ed. by E. J. A. Poppe, S. Sakka and L. C. Klein, American Ceramic Society, 1995 (TP810.5.S665 1994) Figure 3. Formation of capillary stresses at the pores during solvent removal of a gel drying. Figure 4. Schematic for processing thin film coating by spin coating 6
Following are required to perform the suggested lab module Equipments 1. Laboratory Hood with adequate bench space 2. Spin Coater 3. Hot-plate (2x) 4. ph-meter and thermometer 5. Volumetric (Micropipettors) or weight-based measuring (Balance) units 6. Conventional drying oven Consumables 1. TEOS (Tetra ethylorthosilicate-2x1000 ml) 2. Polycarbonate substrates (10x) 3. Ethanol (200 proof-2.5 lt) and DI-water 4. HClaq and NaOHaq 5. Borosilicate glass beakers (20x-50 ml) 6. Magnetic bars (10x) 7. Polystyrene test tubes and tube holders 8. Parafilm 9. Volumetric measuring cylinders 7