The effects of ball milling parameters, namely, the ball-to-powder mass ratio and milling speed, on the synthesis of (K0.5Na0.5)NbO3 nanopowders by high-energy ball milling method from a stoichiometric mixture containing Na2CO3, K2CO3, and Nb2O5 were investigated in this paper. The results indicated that the single crystalline phase of (K0.5Na0.5)NbO3 was received in as-milled samples synthesized using optimized ball-to-powder mass ratio of 35 : 1 and at a milling speed of 600 rpm for 5 h. In the optimized as-milled samples, no remaining alkali carbonates that can provide the volatilizable potassium-containing species were found and (K0.5Na0.5)NbO3 nanopowders were readily obtained via the formation of an intermediate carbonato complex. This complex was mostly transformed into (K0.5Na0.5)NbO3 at temperature as low as 350°C and its existence was no longer detected at spectroscopic level when calcination temperature crossed over 700°C.
Nowadays, with the tendency towards the sustainable and eco-friendly development, lead-containing compounds in general and commonly-used Pb (Zr,Ti)O3-based piezoelectric materials with about 60 wt.% of lead in particular are restricted to use and are going to be expelled due to their high toxicity [
The effects of ball-to-powder mass ratio and milling speed on the synthesis of (K0.5Na0.5)NbO3 nanopowders by high-energy ball milling method using Na2CO3, K2CO3, and Nb2O5 as starting materials were described in detail in this paper. In addition, the thermal decomposition of the carbonato complex as an intermediate species during the reactive high-energy ball milling process was also investigated.
For high-energy ball milling synthesis of (K0.5Na0.5)NbO3, all analytical grade K2CO3, Na2CO3, and Nb2O5, which were purchased from Aldrich (Germany), were dried at 200°C for 2 h prior to use in order to remove moisture. Stoichiometric mixtures of these raw materials were placed in a 125 mL stainless-steel vial of a Fritsch Pulverisette 6 planetary mill. The rotational speed was changed from 300 to 600 rpm. Different numbers of stainless-steel milling balls with diameters of 10 and 20 mm were used depending on the ball-to-powder weight ratio ranging from 25/1 to 40/1. For investigation of thermal decomposition of intermediate carbonate complex, as-milled samples synthesized at optimized conditions were calcined at temperatures ranging from 300 to 1000°C for 3 h. X-ray diffraction diagrams of obtained samples were recorded using Siemens D-5000 diffractometer (Siemens, Germany) with CuK
X-ray diffraction (XRD) diagrams of an unmilled mixture containing K2CO3, Na2CO3, Nb2O5, and as-milled (K0.5Na0.5)NbO3 samples synthesized at rotational speed of 600 rpm for 5 h with different ball-to-powder weight ratios (BPRs) of 25/1, 35/1, 37/1, and 40/1 were shown in Figure
X-ray diffraction diagrams of unmilled mixture and as-milled (K0.5Na0.5)NbO3 prepared mechanochemically at 600 rpm for 5 h with different ball-to-powder mass ratios of 25/1, 35/1, 37/1, and 40/1.
FTIR spectra of as-milled (K0.5Na0.5)NbO3 prepared mechanochemically at 600 rpm for 5 h with different ball-to-powder mass ratios of 25/1, 35/1, 37/1, and 40/1.
(a) DTA and (b) TG curves of as-milled (K0.5Na0.5)NbO3 prepared mechanochemically at 600 rpm for 5 h with different ball-to-powder mass ratios of 25/1, 35/1, 37/1, and 40/1.
Figure
X-ray diffraction diagrams of as-milled (K0.5Na0.5)NbO3 prepared mechanochemically for (a) 5 h and (b) 10 h with the ball-to-powder mass ratio of 35/1 at 300, 400, 500, and 600 rpm.
In order to study the thermal decomposition of intermediate carbonato complex, as-milled samples synthesized at optimized conditions were calcined at 300, 400, 500, 700, and 1000°C for 3 h. For all studied samples, only crystalline phase of (K0.5Na0.5)NbO3 was observed in XRD diagrams as shown in Figure
X-ray diffraction diagrams of as-milled (K0.5Na0.5)NbO3 and samples calcined at 300, 400, 500, 700, and 1000°C for 3 h.
FTIR spectra of as-milled (K0.5Na0.5)NbO3 and samples calcined at 300, 400, 500, 700, and 1000°C for 3 h.
FE-SEM images of (a) as-milled (K0.5Na0.5)NbO3 and samples calcined at (b) 700 and (c) 1000°C for 3 h.
The effects of the ball-to-powder mass ratio and milling speed on the synthesis of (K0.5Na0.5)NbO3 nanopowders by reactive high-energy ball milling method using Na2CO3, K2CO3, and Nb2O5 as starting materials were systematically investigated. By selecting the rotational speed of 600 rpm with the ball-to-powder weight ratio of 35/1 after 5 h of milling time as optimized synthesis parameters, the volatilization of potassium-containing species can be avoided and (K0.5Na0.5)NbO3 nanopowders were readily obtained at a large scale via the formation of an intermediate carbonato complex. This complex was mostly transformed into (K0.5Na0.5)NbO3 at temperature as low as 350°C and the carbonato complex-free (K0.5Na0.5)NbO3 nanopowders were obtained with the average grain size of 40 nm for samples calcined at 700°C.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was financially supported by Vietnam’s National Foundation for Science and Technology Development (NAFOSTED) with Project code 103.02-2011.06.