The present invention relates to a method of preparing a stable, nanometer-sized colloid and to a method of coating a substrate with the colloid to produce continuous coatings.

BACKGROUND OF THE INVENTION

In the past there has been a need for a method of producing a stable metal oxide sol for use in coating substrates. Zirconium alloys in pressurized and boiling water reactors corrode in the high-temperature coolant producing an oxide on the exterior of the alloy and a build-up of hydrogen (or deuterium if the coolant is heavy water) within the alloy itself. If the solubility of hydrogen in the zirconium alloy is exceeded in a localized region, zirconium hydride will precipitate. As well, cooling of reactor components can also cause hydride formation as the hydrogen solubility decreases with temperature. The brittle hydride renders components susceptible to cracking, whether these hydrides form at reactor operating conditions or in a cold shut-down mode. Thus, the lifetime of zirconium alloy reactor components can be seriously degraded by excessive hydrogen absorption. Hydrogen can arise from neighbouring sources or through corrosion (accelerated or otherwise) of the component.

The rate of corrosion and hydrogen absorption of zirconium alloys used in nuclear power plants is controlled by the chemistry of the high temperature coolant as well as by control of the alloy composition, purity and microstructure. In the past, it has been thought that coatings applied to the alloy could reduce the rate of corrosion or hydrogen absorption but such coatings have been porous and non-protective.

Zirconium oxide grows by a reaction at the metal-oxide interface between zirconium cations and oxygen or hydroxide anions. Anions diffuse through the growing oxide to the metal-oxide interface and electrons migrate in the opposite direction towards the water-oxide interface. The transport of the charged species through the oxide depends on the oxidation

potential as well as on the extent of porosity, cracks, crystallite size, and the presence of precipitates, which in turn depend on the alloy composition and microstructure. Thus, there has been a need for providing the zirconium alloy with a protective coating that reduces the rate of transport of anions and electrons to the interfaces in order to reduce the rates of corrosion and hydrogen absorption and for coatings where the levels of corrosion and hydrogen absorption are independent of the composition and microstructure of the zirconium alloy.

A sol (or colloid) is defined as a dispersion of solid particles in a liquid phase where the particles are small enough to remain suspended indefinitely due to Brownian motion. The solid particles are agglomerations of smaller particles which form to lower the surface energy of the smaller particles. Sols can be dispersions in water or an organic solvent. As such, sols are a convenient starting material for the preparation of coatings applied by dipping. The relatively small particle size means that the coating can be sintered to theoretical density at a relatively low temperature. The smaller the particle size, the lower the sintering temperature. As well, the shape of the agglomerations affects the coating process and the sintering temperature where amorphous agglomerations of small particles provide more uniform coatings at relatively lower sintering temperatures.

Zirconium alkoxides are known to produce fme particles of zirconia upon hydrolysis and, hence, are attractive starting materials for the preparation of zirconia ceramics or powders which will sinter at relatively low temperatures. Zirconium alkoxides have been used as starting materials for the preparation of powders composed of spherical particles ranging from approximately 0.05 to l.Oμm in size. Because of the rapid hydrolysis of zirconium alkoxides, complexing agents have been added to slow down the hydrolysis and prevent the precipitation of zirconia when a continuous coating on a substrate is desired.

In that it is desirable to produce a sol with a small particle size in order to lower the sintering temperature and thereby simplify the coating of a substrate, in the past, there has been a need for a zirconium based sol with a particle size less than 50 nm that forms

amorphous agglomerates, that can be formed without the need for complexing agents and that can be sintered on a substrate at relatively low sintering temperatures.

Past work in this area have not addressed the above problems. For example, Shane & Mecartney (J. Mat. Sci 25 (1990)) describe a method for applying zirconium barrier coatings using a sol-gel method with complexing agents. The product formed with a complexing agent was a gel that had to be carefully dried to avoid cracking.

As well, United States Patent 4,543,341 describes the chemical synthesis and the production of fme metal powders of metallic oxides, including zirconia. Chemical synthesis of these powders involve reacting the metallic alkoxide with water in the presence of a dilute alcohol. Alkoxide concentrations used ranged from 0.03 to 0.2 molar. Particle sizes of 0.05 to 0.7 microns were prepared and used to make powder compost which were sintered at 1200°C to achieve maximum theoretical density.

Furthermore, United States Patent 5,047,174 describes the production of stable metal oxide sols used for forming metal oxide coatings on substrates. The method of this patent involves adding water to a metal alkoxide solution to form the corresponding metal oxide solids, removing the water and redispersing the metal oxide solids in an anhydrous alcohol solvent to form the desired sol.

SUMMARY OF THE INVENTION

In accordance with the invention a process for the production of a stable, non-settling zirconia sol is provided comprising the steps of:

a) mixing alcohol diluted water with an alcohol diluted solution of zirconium alkoxide to form a solution having nanometer-sized particles with a colloid concentration of 3- 10 grams of ZrO ₂ per litre and a H ₂ O/Zr molar ratio of between 80/1 to 2/1; b) allowing the solution to form a stable sol by agglomeration of the nanometer-sized particles into amorphous clusters.

In another embodiment the invention further comprises adding up to 1 wt% of a polymer soluble in the stable sol to increase the viscosity of the stable sol.

In another embodiment, step a) comprises mixing an alcohol diluted solution of zirconium alkoxide and an aliquot of a mineral acid to form a solution with a colloid concentration of 3-10 grams of ZrO ₂ per litre and a H ₂ O/Zr molar ratio of between 80/1 to 2/1.

In another embodiment, step a) comprises vigorously mixing alcohol diluted zirconium alkoxide with water to form a solution with a colloid concentration of 3-10 grams of ZrO ₂ per litre and a H ₂ O/Zr molar ratio of between 80/1 to 2/1.

In an alternate embodiment the invention provides a process for coating a substrate with a zirconia sol comprising the steps of: a) dipping a substrate in the zirconia sol and withdrawing the substrate from the zirconia sol at a constant linear pull rate; b) drying the substrate to evaporate the alcohol; c) sintering at a temperature between 350°C and 800°C.

The invention also provides a process where the dipped substrate is placed in a dry inert environment to burn off the polymer and other volatiles prior to sintering and where the substrate is sintered without a bond coat in a dry inert atmosphere.

In another embodiment, the invention provides a sol gel composition comprising a solution of: a) alcohol diluted water and; b) an alcohol diluted solution of zirconium alkoxide the composition having nanometer-sized particles and a colloid concentration of 3-10 grams of ZrO ₂ per litre and a H ₂ O/Zr molar ratio of between 6/1 to 4/1.

In a specific embodiment, the invention provides a process for forming a layer of zirconium oxide on a substrate comprising the steps of:

a) mixing n-propanol diluted water with an n-propanol diluted solution of zirconium n-propoxide to form a solution having nanometer-sized particles with a colloid concentration of 3-10 grams of ZrO ₂ per litre and a H ₂ O/Zr molar ratio of between 6/1 to 4/1; b) allowing the solution to form a stable sol by agglomeration of the nanometer-sized particles into amorphous clusters; c) adding up to 1 wt% of polyvinylbutyral to increase the viscosity of the sol; d) dipping the substrate in the sol and withdrawing the substrate from the sol at a constant linear pull rate of 5 to 15 mm/s; e) drying the substrate in an inert atmosphere at around 250°C to evaporate the alcohol and burn off the polyvinylbutyral; f) sintering the substrate in an inert atmosphere at 350°C - 800°C.

BRIEF DESCRIPTION OF THE DRA INGS

These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings wherein:

FIGURE 1 is a scanning electron micrograph of spherical agglomerates of zirconia produced by the hydrolysis of zirconium alkoxide;

FIGURE 2 is a scanning electron micrograph of amorphous agglomerates of zirconia produced by the hydrolysis of zirconium alkoxide in accordance with the invention.

FIGURES 3a, 3b, 3c, 3d and 3e are scanning electron micrographs of zirconia coatings sintered at 600 °C from sols prepared according to the invention; and,

FIGURES 4a, 4b, and 4c are scanning electron micrographs of zirconia coatings sintered at 350°C, 800°C and 1000°C, respectively, from sols prepared according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENT

The zirconia sol (or colloid) is prepared by hydrolysing a dilute solution of zirconium n-alkoxide in its parent alcohol according to the following reaction:

Zr(OR) ₄ + xH ₂ O → ZrO ₂ + 4ROH + (x-2)H ₂ O where OR is an n-alkoxide.

There are two variations to the procedure. In the first, both the water and alkoxide are diluted with alcohol and in the second, only the alkoxide is diluted in alcohol. In both cases, the concentrations of the reagents should be such that the colloid concentration is about 3 to 10 grams ZrO ₂ per litre. For example, 6 volumes of a 0.3 molar solution of water in n-propanol added to 1 volume of a 0.3 molar solution of zirconium n-propoxide in n-propanol produces a 5 g/L ZrO ₂ colloid. Alternatively, the same colloid can be prepared by adding 1 volume of water to 225 volumes of a 0.04 molar solution of zirconium n- propoxide in n-propanol.

When pure water is added to the dilute solution of zirconium alkoxide in n-propanol, the colloid forms in two distinct stages. The solution immediately turns a very pale blue as a result of the formation of nanometer-sized particles of ZrO ₂ . The pale colour indicates the presence of particles that are small enough to exhibit Raleigh scattering, indicating that the particle size is less than 1/20 the wavelength of light or less than approximately 25 nm. Over the next hour or two the solution turns a milky white as these nanocrystalline particles agglomerate to form larger structures approximately equal to the wavelength of light where they are large enough to reflect light. These agglomerates or clusters remain suspended for months.

The agglomerates prepared in accordance with the invention are amorphous in shape, that is, each is without a distinct shape. Furthermore, it has been determined that the shape of the agglomerates affects coating the sol on a substrate. For example, spherical

agglomerates, prepared by methods of the prior art, have the tendency to shrink during sintering and pull away from one another on the surface of the substrate resulting in a porous coating or a coating that does not cover the surface. On the other hand, amorphous agglomerates, prepared in accordance with the invention, do not shrink away from one another during sintering, thus providing a continuous coating on the substrate.

The amorphous shape of the agglomerates also permits sintering at a lower temperature. Figure 1 is a scanning electron micrograph of spherical agglomerates of zirconia produced by the hydrolysis of zirconium alkoxide prepared by a prior art method which are not suitable for low temperature sintering. Figure 2 is a scanning electron micrograph of amorphous agglomerates of ZrO ₂ produced by the hydrolysis of zirconium alkoxide in accordance with the invention. The amorphous agglomerates make the sol particularly useful for coatings.

The rate of the agglomeration growth step decreases with a decrease in any one of the following parameters: the colloid concentration, the magnitude of x= H ₂ O/Zr, the concentration of water in the water/alcohol solution, the rate of addition of the water, and the temperature. For example, for x— 6 and water added undiluted with alcohol in a single aliquot at room temperature, the colloids turned white within seconds at colloid concentrations greater than 7 g/1 ZrO ₂ . The same process takes 2 hours at a ZrO ₂ concentration of 5 g/1 and is still not complete after several months for a ZrO ₂ concentration of 3.7 g/1. When the water is added as a 1.3M solution in alcohol at the rate of 3 ml/min to make a 7 g/1 colloid, the agglomeration takes approximately 12 hours.

If the initial colloid is prepared at a ZrO ₂ concentration greater than 10 g/L, however, the agglomerates grow large enough to settle, leaving a clear layer on top. If the ZrO ₂ colloid concentration is less than about 3 g/L the agglomeration step does not occur. The colloid remains a very pale blue colour and is unsuitable for the preparation of coatings. When both the water and alkoxide are diluted prior to mixing, the solution turns from clear to milky over a period of about 24 hours.

Other zirconium alkoxides including ethoxide, iso-propoxide, and n-butoxide were examined. Sols prepared with n-propoxide were found to form agglomerates that produced the best continuous, uniform coating. The relatively low sintering temperature of the coatings are a direct result of the nanometer-sized particles that compose the agglomerates. A sintering temperature as low as 350°C has provided uniform coatings.

Zirconium Colloid Synthesis

The initial dilution of zirconium n-propoxide in n-propanol (to approximately 0.3 molar Zr) is done in a dry environment to prevent hydrolysis of the stock solution. Subsequent operations may be performed on the bench top but un-necessary exposure of the diluted alkoxide to the atmosphere should be avoided. The final product is -a homogenous ZrO ₂ colloid with a concentration of 5 to 10 g/L ZrO ₂ . Initial colloid concentrations above 10 g/L ZrO ₂ are not stable and will settle leaving a clear layer on top thus rendering them unsuitable for coatings. However, once a stable colloid has been prepared, it may be concentrated up to 50 g/L without settling. ZrO ₂ colloids have been prepared with the H ₂ 0/Zr ratio ranging from 2 to 80. For the purposes of coating, a colloid of 10 g/L ZrO ₂ provides the best coating on a substrate and, accordingly, it is preferred that a colloid is initially prepared with a ZrO ₂ concentration of approximately 5-7 g/L ZrO ₂ which is subsequently concentrated to 10 g/L ZrO ₂ by evaporation of the alcohol.

Examples

Synthesis # 1

A 10 g/L ZrO ₂ colloid was prepared with a H ₂ O/Zr molar ratio = 6/1 according to the following procedure: a) 20 ml of stock zirconium n-propoxide (approximately 2.2 molar) was diluted with n-propanol to a final volume of 140 ml (approximately 0.3 molar); b) 5 ml of water was diluted with n-propanol to a final volume of 860 mL (approximately 0.3 molar); c) Solution b) was added to solution a) at a rate of 15 - 30 mlJmin with stirring;

d) The solution was let stand for 24 hours for agglomeration to complete at which time the colloid was translucent; e) The colloid was concentrated on a rotary vacuum evaporator at approximately 60 °C to 1/2 its original volume. The final concentration of ZrO ₂ was approximately 10 g/L

A scanning electron micrograph of a substrate dipped in the above colloid and sintered at 600°C is shown in Fig. 3a.

Synthesis # 2

A 10 g/L ZrO ₂ colloid was prepared with a H ₂ O/Zr molar ratio = 4/1 according to the following procedure: a) 20 ml of stock zirconium n-propoxide (approximately 2.2 molar) was diluted with n-propanol to a final volume of 140 ml (approximately 0.3 molar); b) 3 ml of water was diluted with n-propanol to a final volume of 860 mL (approximately 0.2 molar); c) Solution b) was added to solution a) at a rate of 15 - 30 mL min with stirring at which time the colloid was translucent; d) The solution was let stand for 24 hours for agglomeration to complete; e) The colloid was concentrated on a rotary vacuum evaporator at approximately 60°C to 1/2 its original volume. The final concentration of ZrO ₂ was approximately 10 g/L.

A scanning electron micrograph of a substrate dipped in the above colloid and sintered at 600°C is shown in Fig. 3b.

Synthesis 3

A 10 g/L ZrO ₂ colloid stabilized in acid was prepared with a H ₂ O/Zr molar ratio = 6/1 according to the following procedure:

a) 20 ml of stock zirconium n-propoxide (approximately 2.2 molar) was diluted with n-propanol to a final volume of 1 L (approximately 0.044 molar); b) A 4.5 ml aliquot of 0.001 molar HNO ₃ was added to a) with vigorous stirring; c) The solution was let stand for 1 hour at which time the colloid was translucent; d) The colloid was concentrated on a rotary vacuum evaporator at approximately 60 ^β C to 1/2 its original volume. The final concentration of ZrO ₂ was approximately 10 g/L.

A scanning electron micrograph of a substrate dipped in the above colloid and sintered at 600°C is shown in Fig. 3c.

Synthesis U 4

A 10 g/L ZrO ₂ colloid was prepared with a H ₂ O/Zr molar ratio = 6/1 according to the following procedure: a) 20 ml of stock zirconium n-propoxide (approximately 2.2 molar) was diluted with n-propanol to a final volume of 1 L (approximately 0.044 molar); b) A 4.5 ml aliquot of water was added to a) with vigorous stirring; c) The solution was let stand for 1 hour at which time the colloid was translucent. The colloid of this synthesis was less translucent than those of synthesis #'s 1 - 3; d) The colloid was concentrated on a rotary vacuum evaporator at approximately 60 °C to 1/2 its original volume. The final concentration of ZrO ₂ was approximately 10 g/L.

A scanning electron micrograph of a substrate dipped in the above colloid and sintered at 600°C is shown in Fig. 3d.

Po-yro er Addition

A suitable polymer may be added to the colloid to increase the viscosity of the sol. A concentration of around 1 wt% polymer in the zirconia sol has been shown to suitably increase the viscosity. Polyvinylbutyral (PVB) is an example of a suitable polymer.

Coating a Substrate

Continuous, adherent coatings may be made from colloids prepared in accordance with the invention with concentrations up to about 10 g/L ZrO ₂ . At higher concentrations, the coatings are too thick and they spall during the drying process.

A substrate may be coated by withdrawing it from the colloid at a controlled linear pull rate. A pull-rate of 5 - 15 mm/s has been shown to provide continuous coatings.

Drying and Sintering

The coated substrate is dried at room temperature and then heated in a dry argon environment at 250°C for around 15 minutes to bum of the polymer and other volatiles. A scanning electron micrograph of a dipped substrate sintered at 600 °C is shown in Fig. 3e showing the effect of holding at 250°C to bum off PVB.

Sintering is conducted between 350°C and 600°C for 10 minutes to one hour. Sintered coatings up to 150mm were obtained from a single dipping application. A more translucent colloid indicates a lower level of agglomeration of the nanometer-sized particles.

Preparations 1-3 were developed to prepare colloids less agglomerated than those produced by preparation #4 in order to reduce the extent of crack formation in the coating during the sintering process. The colloids prepared by preparations 1 - 3 were more translucent than the colloids of preparation #4 indicating preparations 1-3 were less agglomerated. Figures 3a, 3b and 3c show that coatings made from the colloids prepared in preparations 1-3 had fewer cracks than that of preparation #4, as shown in Figure 3d.

Figures 4a, 4b and 4c show the practical sintering temperatures for sols prepared in accordance with the invention. Figure 4a shows an SEM micrograph of a zirconia sol-gel coating sintered for 10 minutes at 350°C under an argon atmosphere. The quality of the coating is equivalent to a coating sintered for 1 hour at 600 °C. Figure 4b shows an SEM micrograph of a zirconium sol-gel coating sintered for 1 hour at 800 °C. This micrograph also shows that the coating integrity is good. Figure 4c shows a sol-gel coating sintered for 1 hour at 1000°C. It is clear from this micrograph that the morphology of the coating has changed, where the coating has formed into spheres and no longer provides a continuous covering of the surface probably indicating a phase change of the ZrO ₂ from tetragonal to monoclinic.

The terms and expressions which have been employed in this specification are used as terms of description and not of limitations, and there is no intention in the use of such terms and expressions to exclude any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the claims.