Crystal Structure, Phase Transitions and Photoferroelectric Properties of KNbO3-Based Lead-Free Ferroelectric Ceramics: A Brief Review

Ferroelectric KNbO3 (KN) ceramics were first fabricated in the 1950s, however, their use in commercial technical applications has been hampered by inherently challenging processing difficulties. In the early 1990s, the interest in KN ceramics was revived by the pursuit of Pb-free piezoceramics. More recently the search for inexpensive photovoltaic materials alternative to Si prompted bandgap engineering studies in KN-based solid solutions. If the ferroelectric and piezoelectric properties of KN-based ceramics are now well established, the understanding of chemical doping on the bandgap of KN-based ceramics is still in its infancy. Here we provide a brief review on the current understanding of the structure-property relationships in this class of materials, which successively covers crystal structures, structural phase transitions, lattice dynamics, polarization, solid solutions and bandgap engineering of KN

Study of the physical properties of a novel lithium aluminosilicate dental glass-ceramic

The aim of this study is to simultaneously establish the processability and physical properties of Lithium aluminosilicate-based (LAS) glass for dental restorations. An eventual outcome is the production of glass-ceramic matching both the aesthetics and mechanical properties of natural tooth. The two LASbased glass compositions, refer to as LAS1 glass and LAS2 glass, are investigated using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), Differential Scanning Calorimetry (DSC), X-ray Diffraction (XRD), Raman Spectroscopy (RS), Ultrasonic Testing (UT), Vickers Hardness and threepoint bending flexural testing. ICP-OES analyses reveal LAS1 glass and LAS2 glass to be compositionally similar, however LAS2 glass contains traces of vanadium. XRD analyses reveal the presence of Li3PO4 and Li2SiO3 crystals in LAS1 glass, which apparently are not detected in LAS2 glass, however RS analyses obviously show vestiges of these phases in LAS2 glass. DSC reveals LAS1 glass and LAS2 glass to exhibit similar thermal behaviour. LAS1 glass shows a glass transition temperature of ~500°C, two major thermal exothermic events at 615°C and 705°C, which are followed by two minor thermal exothermic events at 750°C and 790°C, and finally a major endothermic event at 910°C. Based on In-situ XRD analyses carried out between 540°C to 790°C, the first exothermic event centred at ~615°C can be associated with the successive crystallisation of Li2SiO3, Li0.25Al0.25Si0.75O2 and LiAlSi4O10, whereas the second peak centred at ~705°C can be associated with the crystallisation of LiAlSi2O6 and Li2Si2O5. Similar results are obtained for isothermal treatments of 30 minutes in the temperature range of 610°C and 870°C, as shown by combined ex-situ by XRD and RS analyses. The incorporation of a nucleation step of 300 minutes at 550°C, reduces the crystallisation temperature of LiAlSi4O10 and Li0.25Al0.25Si0.75O2 by ~20°C, but also leads to increase of the crystallite sizes. Following this initial evaluation of the impact of isothermal heat treatments, other heat treatments are strategically carried out at temperatures below and above the exothermic events in order to evaluate again their impact on both phase assemblage and physical properties, such as hardness, elastic modulus, fracture toughness and colour. Hence, based on the DSC data, nucleation is carried out at a temperature of 550°C for 300 minutes, and crystallisations are carried out at 670°C, 780°C, 800°C, 830°C and 850°C, for different time lengths. XRD results reveal LiAlSi2O6 to be the dominant crystalline phase, followed by Li2Si2O5 and Li2SiO3 for both LAS1 glass and LAS2 glass. Both LAS1 and LAS2 glassceramics exhibit high values of mechanical properties when the heat treatment is at 550°C for 300min,780°C for 120min and 830°C-850°C for 120 min. Moreover, LAS1 glass and LAS2 glass heat treated above 770°C are both aesthetically suitable for dental restorations. Regarding the LAS1 glass, the colour is white, whereas LAS2 glass colour is identical to several standard shades including D2, C1 and B2, depending on the heat treatment temperatur