Energy-efficient green synthesis of Nd:Y₂O₃ nanopowder by microwave gel combustion

A novel method for the synthesis of nanocrystalline neodymium-doped yttria (Nd:Y₂O₃) by gel combustion in microwave without long period of calcination at high temperature is described. The method leads to rapid formation of phase pure product with saving of time and energy. Gels were prepared by selecting citrate to nitrate (C/N) ratios of 0.5:1 & 1:1 followed by combustion in microwave for drying of gel leading to precursor formation. Thermogravimetric-differential thermal analysis (TG-DTA) of precursor with (C/N) ratios of 1:1 exhibited a total loss in weight of about 31.7% up to 1300 °C and did not show M–O bonds characteristic of yttria by FTIR indicating requirement of calcination at high temperature. Crystallization to phase pure yttria took place only on thermal treatment at 1000 °C for 4 h in oxygen atmosphere confirmed by XRD with particle size 40 nm. While the microwave combusted precursor with C/N ratio of 0.5:1 exhibited a weight loss of only 4.7% up to 1300 °C and showed M–O bonds characteristic of yttria in FTIR spectrum without calcinations at higher temperature. This microwave combustion precursor with C/N ratio of 0.5:1 was found to be crystalline phase pure yttria by X-ray diffraction (XRD) with primary particle size 28 nm by Scherrer’s equation and 30–50 nm of uniform morphology by transmission electron microscopy (TEM). In the present work the ratio of citric acid to nitrates is playing a crucial role in terms of saving time and energy involved in calcinations of microwave combusted precursor for the composition with higher citrate content.