Hydrogen desorption from crystalline quartz and some related differential-scanning calorimetry and conductivity phenomena

Abstract

Temperature-programmed desorption (TPD) and differential-scanning calorimetry (DSC) analyses were performed on synthetic crystalline quartz, both “as received” and “hydrogen swept”, from 200 to 600°C. The TPD measurements allowed us to establish that interstitial hydrogen ions, present as charge compensators near trivalent aluminium ions substitutional for silicon, are removed from the crystal in the considered temperature range, mostly in the form of H 2 molecules. In the hydrogen-swept samples, release of small amounts of other molecular groups, namely SiH 4 and NaOH, was also observed. The DSC studies revealed the existence of a weak exothermic peak (enthalpy change ⋍ 0.90 J g −1) in the temperature range from 350 to 400°C. The exothermic process, only observable in hydrogen-swept samples, may tentatively be attributed to the total-energy variation involved in processes leading to H 2 formation and to SiO 4 tetrahedra relaxation after H 2 release. The above results have been compared with the intensity variations, following annealing treatments, of near-infrared OH stretching vibrations and with time-transient effects observed in ionic conductivity measurements. The analysis contributes to a deeper understanding of the hydrogen-impurity dynamics in crystalline quartz.