Ferroelectric glass-ceramics

Ferroelectric glass-ceramics have been investigated from the Bi203-Ge02, BiOl.s-Ge02-B0I.5, Bi0I.5-Ge02-Te02, 5PbO-3Ge02, PbO-Ge02-NbzOs, and PbsGe30u-PbNbz06-Si02+15%Ah03 systems. DTA, XRD and SEM analysis were used to obtain crystallographic and microstructural information. The dielectric properties and ferroelectric hysteresis loop behaviour of selected samples were determined. The stoichiometric Bjz03:Ge02 (BjzGeOs) composition devitrified on cooling, giving rise to the investigation of new systems BiO\.S-Ge02-B0I.5 and Bi0I.5-Ge02-Te0 2. It was found that the glass-forming region in Bi0I.5-Ge02-Te02 is narrow and good parent glasses for precipitating BjzGeOs crystals were not obtained. However, pure BizGeOs based glass-ceramic can be successfully formed from BiOl.s-Ge02-BOI.5. SEM backscatter imaging of these glass-ceramics showed surface crystallisation and XRD analysis confirmed that the preferred orientation is perpendicular to (311) planes. The dielectric behaviour and ferroelectric hysteresis loop study of the Bi2GeOs based glass-ceramic heat treated at 475°C for 12 hours, showed that this material can be ferroelectric at room temperature with Ps = 14 flC/cm2 and has Curie temperature at about 407°C. Glasses of compositions PG(PbSGe30I J)-xPN(PbNhz06) (x = 0.5, I, 2 3) were investigated from the PbO-Ge02-NbzOs system. Most of the samples devitrified on cooling and have poor mechanical strength except the sample PG-O.S PN sample which also contains interesting phases: ferroelectric PbSGe3011 and dielectric pyroniobate PhzNhz07. The surface crystallisation of PbSGe3011 with a-axis orientation and the bulk crystallisation of PhzNbz07 phase in this sample could be observed using SEM and XRD analysis. By applying heat treatment at 667°C for 48 hours to this sample, surface crystallisation along the a-axis can be enhanced. The Curie temperature of this heat treated sample is about 166 °C with Ps = 1 flC/cm2 from dielectric measurement and ferroelectric hysteresis loop behaviour. More samples were also investigated but it was difficult to form glass-ceramics containing both PbSGe3011 and PbNbz06 crystals from this PbO-Ge02-NbzOs system. In order to obtain the multiple ferroelectric PbSGe3011 and PbNbz06 based-glass ceramics, six glasses along the tie line from 62.5 mol%PbO: 25 mol%Ge02: 12.5 mol%Si02 to 40 mol%PbNbz06: 60 mol%Si02 were investigated from the PbSGe3011: PbNhz06: Si02+ 15%Ah03 system. Most of the glasses exhibited glass-in glass phase separation. From DT A analysis and subsequent crystallisation information, the most likely possible parameters, which control the glass-in glass phase separation, may be the NbzOslSi02 ratio for the glasses near the PbSGe3011 rich composition and Ah03 for the glasses near the PhzNhz06 rich composition. This system offered many interesting materials such as cubic pyrocWore PhzNbz07 based glass-ceramics and the orthorhombic PbNbz06 based glass-ceramics, and they are also mechanically robust