This invention relates to a method of producing either practically pure silica glass or a modified silica glass containing at least one metal other than silicon by using a silicon alkoxide solution, which may contain additional alkoxide(s) of the desired metal(s) to produce a modified silica glass.
Recently, it has attracted increasing attention to produce silica glass or a "modified silica glass," which contains in the glass matrix at least one metal other than silicon as exemplified by titanium-containing silica glass, without melting or fusing raw materials by using a silicon alkoxide solution or a mixed alkoxide solution containing a silicon alkoxide and at least one additional metal alkoxide. (In the following description, the term "metal alkoxide solution" means either a solution of silicon alkoxide or the aforementioned mixed alkoxide solution.)
In principle, a glass producing method of this type consists of the steps of first preparing a metal alkoxide solution which contains an organic solvent in addition to water, then allowing the metal alkoxide(s) in the solution to hydrolyze to increase the viscosity of the solution until gelation of the solution, and finally heating the gelled material to convert it into glass. In practice, there is the need of shaping the metal alkoxide solution into a desired form, i.e. intended form of the glass to be produced, such as a coating film on a solid body, independent film, fiber or lump at an intermediate stage of the aforementioned hydrolysis step while the solution exhibits a sufficiently high viscosity but has not yet gelled. As will be understood, a suitable viscosity of the metal alkoxide solution to carry out the solution-shaping operation is variable depending on the type of the shaping method, and in every case the shaping operation can successfully be carried out only in a relatively narrow range of the viscosity values of the solution. The viscosity of a metal alkoxide solution and the rate of its increase considerably vary depending on various factors such as the kind of the metal alkoxide, amount of the organic solvent and the rate of progress of the hydrolysis of the alkoxide. In general the viscosity of each metal alkoxide solution becomes higher as the amount of the organic solvent is decreased and as the hydrolysis of the alkoxide progresses. However, it is difficult to freely vary the amount of the organic solvent which is employed for the purpose of assisting the dissolution of the metal alkoxide and enhancing the miscibility of the metal alkoxide with water, and the rate of hydrolysis is determined fundamentally by the kind of the metal alkoxide. Since silicon alkoxides are generally low in the rate of hydrolysis, it takes a very long time such as several days for a metal alkoxide solution to reach a viscosity suited to the shaping operation.
In the described glass producing method, it is also a matter for consideration that the degree of hydrolysis of the alkoxide in the solution at the time of the shaping operation affects the manner of gelation of the shaped viscous solution and the properties of the glass obtained through a subsequent heating process. Until now, however, no technique has yet been proposed to control the viscosity of a metal alkoxide solution independently of the rate or degree of hydrolysis of the metal alkoxide in the solution.
A more detailed description will be given with respect to the case of forming a glass coating film on a glass or ceramic substrate, or a surface of a solid body or any other form, by using a metal alkoxide solution. Coating of various substrates with glass film by this method is expected to have much application for the purpose of enhancing the chemical resistance and scratch resistance of the substrates and/or modifying the electrical characteristic or optical property of the substrate.
In this case the aforementioned shaping of a metal alkoxide solution is embodied in coating of a substrate with a film of the alkoxide solution which has become adequately viscous. More particularly, the coating is achieved by immersing the substrate in the metal alkoxide solution not yet gelled and withdrawing therefrom or by spraying the alkoxide solution onto a major surface of the substrate, or still alternatively by dropping the solution onto the surface of the substrate and swiftly rotating the substrate in a horizontal plane. In every case the thickness of the liquid film and, hence, the thickness of the glass coating film formed by this method depend primarily on the viscosity of the metal alkoxide solution.
To fully achieve the object of this coating technique, it is important to form a glass coating film which is uniform and high in the strength of its adhesion to the substrate surface and has a sufficient thickness, though this film is a very thin film usually less than one micron in thickness. For example, in the case of a colored glass coating film with the desire of a deep color, it becomes necessary to form the coating film to a considerably large thickness because it is impossible to freely increase the content of the coloring element in the metal alkoxide solution without failing to form a uniform glass coating film. As a different example, where the glass film coating technique is applied to a substrate that is rather inferior in its surface smoothness it is desirable that the glass coating film has sufficient thickness to ensure effectiveness of the coating over the entire surface area and ensure durability of the coating film. Therefore, it becomes necessary to perform the aforementioned step of forming a liquid film on the substrate when the metal alkoxide solution exhibits sufficiently high viscosity. Since the composition of the metal alkoxide solution must be determined principally in view of the chemical composition of the intended glass coating film rather than the viscosity of the solution, a sufficient increase in the viscosity of the solution should necessarily be achieved by allowing the hydrolysis of the metal alkoxide in the solution to proceed to considerable extent before coating of the substrate with this solution. In other words, it will be necessary to carry out the coating operation when the metal alkoxide solution is in a highly viscous state shortly before gelation of the solution. By using a metal alkoxide solution after the progress of the hydrolysis to such extent, certainly it is possible to coat the substrate with a relatively thick liquid film which can be expected to give a glass coating film of a sufficiently large thickness. Actually, however, heating of the liquid film formed in this way results in significant peeling and/or cracking of the resultant glass coating film and therefore fails to give a uniform coating film of glass on the surface of the substrate.
Thus, it is a matter of great inconvenience in the known methods of producing glass from a metal alkoxide solution that the viscosity of the solution can hardly be controlled independently of the progress of the hydrolysis of the metal alkoxide in the solution.