Microstructure, mechanical, thermal, EPR, and optical properties of MgAl2O4:Cr3+ spinel glass-ceramic nanocomposites

The mechanical, thermal, and optical properties, along with the microstructure and electron paramagnetic resonance (EPR) spectra, have been studied for MgAl2O4:Cr3+ spinel glass and glass-ceramics. The activation energy of the crystallization has been estimated from the differential scanning calorimetry (DSC) study using different models and is found to vary within 255-270 kJ/mol for the un-doped precursor glass. The microstructure of the glass-ceramics has been characterized using field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The TEM images demonstrate the presence of cubic crystals in the glass-ceramics of uniform size 10-15 nm. X-ray diffraction (XRD) and Fourier-transform infrared (FT-IR) spectroscopy reveal the presence of MgAl2O4 spinel as the only crystalline phase, formed in the heat-treated glass-ceramics. The EPR spectrum of Cr3+ doped glass sample exhibits a broad resonance signal with effective g = 1.97 whereas in glass-ceramic sample an additional weak resonance signal is observed at g = 3.83. The excitation spectrum exhibits two bands in the visible region. The emission spectrum exhibits an intense red emission at 690 nm which is characteristic of Cr3+ ions caused by the spin-forbidden 2Eg → 4A2g transition. All the mechanical properties are found to have improved in the glass-ceramics when compared to glasses. A good combination of micro-hardness (∼6.0 GPa), high fracture toughness (∼5.0 MPa m1/2), 3 point flexural strength (∼100 MPa) and elastic modulus (∼55 GPa) has been obtained for the glass-ceramic samples.