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Amorphization of crystalline orthoboric acid on a vitreous B2O3 substrate

Published online by Cambridge University Press:  31 January 2011

Eric McCalla  and
Ralf Brüning
Eric McCalla
Affiliation:
Physics Department, Mount Allison University, Sackville, NB, Canada E4L 1E6
Ralf Brüning*
Affiliation:
Physics Department, Mount Allison University, Sackville, NB, Canada E4L 1E6
*
a)[email protected]
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Abstract

Time-resolved x-ray diffraction measurements were carried out during and after the exposure of anhydrous boron oxide glass films to humid air. B(OH)3 crystals grew on the glass substrate with planar sheets of B(OH)3 molecules aligned parallel to the surface. After the space above the sample was evacuated, the crystal peaks began to disappear. During the decay, new crystals of orthoboric acid grew with the orientation of the B(OH)3 sheets perpendicular to the substrate. Then crystals with all orientations decayed completely, and an amorphous scattering pattern remained. The water acquired by the sample during the exposure eventually reacted with the anhydrous B2O3 glass to produce amorphous BO(3+y)/2Hy. The decay of the crystals took from one to several days, depending on the length of the exposure to humid air. Measurements of the metastable equilibrium phases in the B2O3–H2O binary system showed that at room temperature the amorphous phase could reach the composition BO1.85H0.7. At higher water concentrations this saturated amorphous phase coexisted with crystalline B(OH)3. This result is consistent with the instability of the crystalline B(OH)3 film in contact with anhydrous glass, and we present a thermodynamic model for this process. In the present case, amorphization is driven by the outdiffusion of water from a crystalline phase rather than interdiffusion in conventional solid-state amorphization.

Type
Articles
Information
Journal of Materials Research , Volume 17 , Issue 12 , December 2002 , pp. 3098 - 3104
Copyright
Copyright © Materials Research Society 2002

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