Microwave radiometric study of ferro–paraelectric phase transitions in KNO3

A recently developed technique of microwave passive radiometry is adopted for the study of the para–ferroelectric phase transitions in KNO3. Informations are obtained about the temperature dependence of the optical constants which are of twofold relevance: superimposed to the basic features of the thermal spectra, which reproduce those already reported for the dielectric constants, double peaks of emissivity appear during each of the phase transitions, whose explanation is attempted. Accurate calorimetric measurements are carried out in order to serve as guidelines for the determination of the transition temperatures in the radiometric experiment. The transition temperature dependence on the rate of cooling is also confirmed for this material. On a adopte une technique recemment developpee de radiometrie passive a micro-ondes pour etudier les transitions para-ferroelectriques du KNO3. On obtient ainsi des informations sur la dependance des constants optiques de la temperature doublement de relief: les spectres thermiques reproduissent les variations deja reportees des constantes dielectriques a microondes; superposes a ces-la, on releve des doublets d'emission en correspondance de chaque changement de phase. Une explication de ce dernier phenomene est tentativement donnee. On a effectue aussi des tres accurees measures calorimetriques pour avoir une guide pour la determination exacte des temperatures de transition dans les experiences radiometriques. On a eu aussi confirme de la dependence des temperatures de transition de la vitesse de refroidissement.