X-ray microstructure characterization of ball-milled nanocrystalline microwave dielectric CaZrO₃by Rietveld method

Abstract

The microwave dielectric CaZrO3 has been prepared in nanocrystalline form by high‐energy ball milling an equimolar powder mixture of CaO and monoclinic (m) ZrO2. The formation of CaZrO3 is noticed in the X‐ray diffraction pattern of 2 h ball‐milled powder. At the early stage of milling, up to 4 h, the weight fraction of CaZrO3 increases rapidly and then slowly reaches almost the saturation value of ∼73 wt% within 18 h of milling. It is observed that in the course of milling, the weight fraction of c‐ZrO2 increases continuously from its starting value, ∼2.5 wt%, to ∼43 wt% within 2 h of milling, and then reduces slowly to its starting value. It seems that the c‐ZrO2 phase has been stabilized by CaO and takes part in the formation of CaZrO3 by mechanochemical reaction of nanocrystalline CaO and c‐ZrO2 crystallites. However, post‐annealing of 2, 6 and 18 h ball‐milled samples results in the formation of ∼95 wt% CaZrO3 phase. The formation mechanism, phase transition kinetics, structure and microstructure changes in terms of lattice imperfections and relative phase abundances of individual phases have been estimated from the analysis of X‐ray powder diffraction data of unmilled, ball‐milled and post‐annealed samples by Rietveld powder structure refinement.

Effect of Zirconia Doping on the Structure and Stability of CaO-Based Sorbents for CO2 Capture during Extended Operating Cycles

“…The flame process favors formation of CaZrO 3 , which creates a barrier between CaO grains, thus preventing the sintering-agglomeration leading to improved sorbent performance. 31 When the total molar conversion for free CaO particles in Zr-doped CaO sorbents is calculated, the Zr/Ca (5/10) sorbent exhibited much more capture capacity than all others. Therefore, the formation of CaZrO 3 not only stabilizes the CaO particles but also increases their CO 2 capture capacity.…”

Low‐Temperature Synthesis of CaZrO3 Powder from Molten Salts

“…To avoid using expensive precursors and high temperatures, a mechanochemical alloying (MA) technique has been developed. Nanocrystalline CaZrO 3 powder was obtained via milling CaO and ZrO 2 in a high‐energy ball mill and subsequently heat treating the activated powder at 1000°C for 2 h 15 . In addition to the requirements of expensive high‐energy ball milling and a post‐milling heat treatment, the MA process suffers from a very limited production capacity (only a few grams).…”

Oxalate co-precipitation synthesis of calcium zirconate and calcium titanate powders.

“…This phase has been commonly referred in the literature as calcium stabilized zirconia (Ca 0.15 Zr 0.85 O 1.85 , calcium zirconium oxide, PDF # 00-26-0341) which has a fluorite related structure and is formed in a variety of solution and solid state synthetic routes. 2,5,12,14,15 Our initial assignment of this phase was based on Ca 0.15 Zr 0.85 O 1.85 ; however refinement on our XRD pattern ( Figure 9) and comparison of the weight losses observed during thermal analysis indicates that this phase is stoichiometric in Ca:Zr (Ca 0.50 Zr 0.50 O 1.5 ) based on the refined site occupancies. Data used was from CZ2 and the unit cell parameters for our fluorite phase are listed in Figure 9.…”

“…Solution routes to bulk CZT and CZ materials include metal alkoxide sol-gel 2, 5 , combustion 7,10 , citrate 3,6 , peroxide 11 and oxalate 4,5,8 methods. Some work towards reducing temperatures and particle size for the solid state synthesis of CZ from oxides and carbonates has also been performed using molten salt synthesis 12,13 , mechanochemical activation with vibro milling 14 , and high energy ball milling 15 . The mechanochemical routes, however required long milling times (~20 h) or additional high temperature (900-1100 C) heat treatments to produce phase pure CZ, while the molten salt synthesis produced powders that were found to include salt impurities.…”