Influence of surface silanols on the dielectric properties of nitrogen adsorbed on activated silica

• Published in: Surface science Vol. 315; no. 3; pp. 323 - 336
• Main Authors: Guermeur, Robert, Jacolin, Christian
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 20.08.1994; Amsterdam Elsevier Science; New York, NY
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Dielectric properties of solids and liquids; Dielectrics, piezoelectrics, and ferroelectrics and their properties; Exact sciences and technology; Physics; Polarization and depolarization; Adsorption; Polarizability; Silicon oxides; Surface properties; Nitrogen; Aerosil; Experimental study
• ISSN: 0039-6028; 1879-2758
• DOI: 10.1016/0039-6028(94)90136-8

Microwave measurements were used to study the dielectric properties of silica powder (Aerosil) containing surface silanol groups. The same substrate, activated under vacuum at increasing temperatures between 200°C and 800°C, was used in experiments at low temperatures (63–100 K). A change in the surface polarisability was recorded during adsorption of controlled amounts of nitrogen molecules on silica powder in a cell placed in a microwave cavity. The results strongly depend on activation conditions and on the temperature at which adsorption isotherms are recorded. A model taking into account four adsorption sites is used to fit the experimental results. Interaction between adsorbed molecules and surface silanols is clearly demonstrated. Different silanol species related to the different adsorption sites induce characteristic polarisability changes when the sites are filled with adsorbed molecules. The number of adsorption sites is unchanged by activation. Adsorption enthalpies are not much affected: a variation of about 15% is noted after activation at 800°C. Dependence of the surface polarisability on temperature is found to be p ≈ (T 0 − T) T where T 0 is about 100 K in the range 63 ⩽ T⩽ 100 K.