Borosilicate glasses and second surface mirrors thereof

Details Borosilicate glasses and second surface mirrors thereof Borosilicate glasses and second surface mirrors thereof

1999-01-26

2001-01-30

Speer, Timothy M. (Department: 1775)

Stock material or miscellaneous articles

Structurally defined web or sheet

Physical dimension specified

C428S426000, C428S432000, C428S433000, C428S434000, C501S055000, C501S065000, C501S066000, C501S072000

Reexamination Certificate

active

06180218

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to glasses having good stability to radiation useful as cladding glasses in space and terrestrial applications, and to cladding panes composed of such glasses.
On irradiation with high energy radiation typically encountered in space, glass tends to discolour, reducing the transmission of the glass and increasing its solar absorptance. Thus, radiation stability is a particular requirement of glasses used as cladding glasses in space applications, for example, as solar cell cover slips or as the glass substrates of second surface mirrors used as cladding to protect spacecraft from overheating.
2. Description of the Prior Art
It is known, for example from EP 0261 885A1 and EP 0 505 061A2, to use borosilicate glasses, stabilised against the effects of irradiation by the incorporation of cerium (typically in amounts of 2% to 5% by weight), for production of solar cell cover slips having a high transmission in the visible and infra-red regions of the spectrum. Cerium has very broad absorption bands in the ultra-violet region of the spectrum at 240 nm and 315 nm. This absorption in the ultra-violet may be beneficial when the glasses are to be used in solar cell cover slips, for example, in protecting the adhesive used to bond the cover slips to the cells from ultra-violet radiation which would otherwise tend to degrade the adhesive. However, when the same base glasses are used, with a reflective coating on the back surface, as second surface mirrors to clad the exterior surface of a space craft and reflect unwanted solar radiation incident upon it, the absorption in the ultra-violet leads to an undesirable build-up of heat in the glass.
Thus there is a need for a method of stabilising a high transmission borosilicate glass to radiation especially radiation encountered in space, which does not rely on the use of cerium (or any other element which absorbs significantly in the spectral region from 250 nm wavelength to 2500 nm wavelength). It has now been found, and the discovery forms the basis of the present invention, that borosilicate glasses may be stabilised against radiation by inclusion of barium.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a radiation stable borosilicate glass in sheet form having a thickness less than 1 mm characterised in that the glass contains more than 5% by weight of barium (calculated as barium oxide) whereby its radiation stability is enhanced.
Preferably, the glass sheet has a cut-on of less than 340 nm (i.e. the transmission of the glass sheet increases above 50% at a wavelength less than 340 nm).
The radiation stability of a glass in space may be estimated by subjecting a thin polished sample of glass to an electronic bombardment in vacuum and measuring the change in the optical characteristics of the glass. Radiation stable glasses typically have a radiation stability such that, if a polished sample pane of the glass 150 microns thick is exposed to 5.7×10
15
1 MeV electrons per square centimeter of glass in vacuum (<1×10
−3
torr), its solar absorptance changes by less than 0.05. Solar absorptance is the ratio of radiant energy absorbed by a body to that incident upon it in the region 250 nm to 2500 nm integrated over the air mass zero solar spectrum. Preferred glasses show a change in solar absorptance of less than 0.04, and especially preferred glasses changes of less than 0.03. For use in second surface mirrors for the cladding of space craft, it is preferred to select glasses having a solar absorptance, after testing as described above, of less than 0.06, and preferably less than 0.04.


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