Glass-ceramic compositions designed for radomes

Claim 1

A glass-ceramic suitable for the fabrication of radomes with porous surface layers having a depth of about 0.005"-0.015", said glass-ceramic exhibiting a loss tangent at 25° C. and 8.6×10 9 Hz no greater than about 0.00035, a dielectric constant at 25° C. and 8.6×10 9 Hz no more than about 6, a modulus of rupture after fortification in excess of 30,000 psi, a coefficient of thermal expansion (25°-300° C.) of less than about 45×10 -7 /° C., and wherein cordierite constitutes the predominant crystal phase along with minor amounts of cristobalite, magnesium-aluminum titanate, and rutile, said glass-ceramic having an overall composition consisting essentially, expressed in weight percent on the oxide basis, of about 15-18% MgO, 21-25% Al 2 O 3 , 48-53% SiO 2 , 9.5-11.5% TiO 2 , and 0-1% As 2 O 3 .

Claim 4

A method for making a glass-ceramic suitable for the fabrication of radomes exhibiting a loss tangent at 25° C. and 8.6×10 9 Hz no greater than about 0.00035, a dielectric constant at 25° C. and 8.6×10 9 Hz no more than about 6, a modulus of rupture after fortification in excess of 30,000 psi, and a coefficient of thermal expansion (25°-300° C.) of less than about 45×10 -7 /° C. which comprises the steps of: (a) melting a batch for a glass consisting essentially, expressed in weight percent on the oxide basis, of about 15-18% MgO, 21-25% Al 2 O 3 , 48-53% SiO 2 , 9.5-11.5% TiO 2 , and 0-1% As 2 O 3 ; (b) simultaneously cooling the melt at least below the transformation range thereof and forming a glass shape of a desired configuration therefrom; (c) heat treating said glass shape at a temperature between about 1000°-1300° C. for a period of time sufficient to cause the in situ crystallization of cordierite as the predominant crystal phase along with minor amounts of cristobalite, magnesium-aluminum titanate, and rutile; (d) contacting the surface of said crystallized shape with an alkaline solution to leach out cristobalite therefrom; (e) removing said alkaline solution from the surface of said crystallized shape; (f) contacting the surface of said crystallized shape with an acid solution to leach out residual glass therefrom; (g) removing said acid solution from the surface of said crystallized shape; and then (h) sequentially repeating steps (d)-(g) a sufficient number of times to produce a porous surface layer having a depth of about 0.005"-0.015".

Claim 5

A method for making a glass-ceramic suitable for the fabrication of radomes exhibiting a loss tangent at 25° C. and 8.6×10 9 Hz no greater than about 0.00035, a dielectric constant at 25° C. and 8.6×10 9 Hz no more than about 6, a modulus of rupture after fortification in excess of 30,000 psi, and a coefficient of thermal expansion (25°-300° C.) of less than about 45×10 -7 /° C. which comprises the steps of: (a) melting a batch for a glass consisting essentially, expressed in weight percent on the oxide basis, of about 15-18% MgO, 21-25% Al 2 O 3 , 48-53% SiO 2 , 9.5-11.5% TiO 2 , and 0-1% As 2 O 3 ; (b) simultaneously cooling the melt at least below the transformation range thereof and forming a glass shape of a desired configuration therefrom; (c) heat treating said glass shape at a temperature between about 1000°-1300° C. for a period of time sufficient to cause the in situ crystallization of cordierite as the predominant crystal phase along with minor amounts of cristobalite, magnesium-aluminum titanate, and rutile; (d) contacting the surface of said crystallized shape with an alkaline solution to leach out cristobalite therefrom; (e) removing said alkaline solution from the surface of said crystallized shape; (f) contacting the surface of said crystallized shape with an acid solution to leach out residual glass therefrom; (g) removing said acid solution from the surface of said crystallized shape; and then (h) sequentially repeating steps (d)-(g) a sufficient number of times to produce a porous surface layer having a depth of about 0.005"-0.015".