The silica coating is typically thin enough (thickness of the order of 1 -¼m or less) to be flexible, so that it can be used on either a rigid or a flexible ceramic substrate without risk of cracking and consequent exposure of the ceramic to moisture.
A silica precursor can be applied to a ceramic substrate by any of a number of techniques for example, solution coating, vapor deposition, low-pressure chemical deposition, immersion, or injection. Some of the traditional waterproofing agents mentioned above are also useful as silica precursors other silica precursors include silicones and siloxanes. The silica coating is formed by first coating with a silica precursor and converting the precursor to silica either in situ during the coating process or subsequently by oxidative pyrolysis. "Coated" as used here does not mean covered on the exterior macroscopic surface it means covered on all microscopic exterior and interior pore surfaces and interstices. The present invention is based on the discovery that even a difficult-to-coat ceramic can easily be coated with a traditional silicon-containing waterproofing agent, provided that it is first coated with a thin layer of silica. These agents can be applied easily to silica ceramics, but cannot be applied easily to other (e.g., alumina) ceramics. The traditional waterproofing agents preferred for this purpose are silanes, substituted silanes, silazanes, and mixtures of these. Thus, what is needed is a means to reduce the intrusion of water to minimize these deleterious effects. These materials can also be damaged by explosive vaporization of trapped water during rapid heating, and by the anomalous expansion and contraction of trapped water during freezing and thawing. Water can thus add undesirably to the weight of these materials. They are also hygroscopic - typically capable of absorbing three to five times their own weight of water. In situations in which ceramics are exposed to water, the need for waterproofing arises as follows: These materials are very porous (void volumes exceed 90 percent in some cases). In this process, traditional waterproofing agents are used, but before the waterproofing agents are applied, the ceramic substrates are first coated with thin layers of silica, as explained below.Īs used here, "waterproofing" of ceramics means rendering them water-repellent, not making them impervious. Ames Research Center, Moffett Field, CaliforniaĪn improved process for waterproofing ceramic materials - especially lightweight ceramic insulation materials in both rigid (tile) and flexible (blanket) form - has been invented.
0 kommentar(er)
