2008

Bonding by Hydroxide-Catalyzed Hydration and Dehydration

Room-temperature process can be varied to suit optical and non-optical applications.

A simple, inexpensive method for bonding solid objects exploits hydroxidecatalyzed hydration and dehydration to form silicatelike networks in thin surface and interfacial layers between the objects. (Silicatelike networks are chemical-bond networks similar to, but looser than, those of bulk silica). The method can be practiced at room temperature or over a wide range of temperatures.

The method was developed especially to enable the formation of precise, reliable bonds between precise optical components. The bonds thus formed exhibit the precision and transparency of bonds formed by the conventional optical-contact method and the strength and reliability of high-temperature frit bonds. The method also lends itself to numerous non-optical applications in which there are requirements for precise bonds and/or requirements for bonds, whether precise or imprecise, that can reliably withstand severe environmental conditions. Categories of such non-optical applications include forming composite materials, coating substrates, forming laminate structures, and preparing objects of defined geometry and composition.

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In a Simple Example of Bonding according to the present method, a silicon-oxygen-silicon bridge is formed by hydroxide-catalyzed dehydration betweentwo fused-silica substrates, the mating surfaces of which have been hydrated. When a large number of such bridges form, the substrates are bondedtogether with great strength.
The method is applicable to materials that either (1) can form silicatelike networks in the sense that they have silicatelike molecular structures that are extensible into silicatelike networks or (2) can be chemically linked to silicatelike networks by means of hydroxide- catalyzed hydration and dehydration. When hydrated, a material of either type features surface hydroxyl ( — OH) groups. Examples of materials capable of forming silicatelike networks by means of hydroxide- catalyzed hydration and dehydration include several forms of silica (fused silica, fused quartz, and natural quartz), silica-based glasses, silicon having a thermally-grown surface oxide layer, and some other silica-based or silica- containing materials, including some laser crystals. Examples of materials that cannot form silicatelike networks but can be linked to them by means of hydroxide-catalyzed hydration and dehydration include some metals, oxides of some metals, and some non-silica-based, non-silica-containing laser crystals.

In this method, a silicatelike network that bonds two substrates (see figure) can be formed either by a bonding material alone or by the bonding material together with material from either or both of the substrates. In preparation for bonding, the mating surfaces of the substrates should be cleaned to render them maximally hydrophilic or at least minimally hydrophobic. Typically, an aqueous hydroxide bonding solution is dispensed and allowed to flow between the mating surfaces by capillary action. If at least one of the substrates can form a silicatelike network and if the surface figures of the substrates match with sufficient precision, then a suitable bonding solution would be one that contains a suitable concentration of hydroxide ions but substantially or completely lacks silicate material. If neither substrate material can form a silicatelike network through hydroxide catalysis or if the degree of mismatch between the surface figures of the substrates is such that silicatelike network cannot be formed at a sufficient rate, then a silicate material should be included in the bonding solution.