Abstract
In this work, the properties of mechanochemically produced (by using mills made from different materials) barium-titanate (BaTiO3) and zinc-titanate (ZnTiO3) perovskites are compared. Mechanochemistry is a process that can cover the energy demand of some reaction pathways between solid materials. This process is called “high-energy milling”, for which not all types of mills are suitable. In our case, a planetary ball mill provided the necessary energy. Using a model, the required energy is determinable; the energy released during an impact of a milling ball (Eb – ball-impact energy), as well as during the whole milling (Ecum – cumulative milling energy). Thus, a milling-energy map was created, with which the applied Eb and Ecum values were visualized depending on the different grinding parameters. The parameters changed were the material of the grinding vessels, the number of grinding balls, and the rotational speed. The transformation was tracked by X-ray diffraction (XRD) measurements, and electron microscopic images (TEM and SEM) of the perovskites produced were taken. This study aimed to draw conclusions that will help later in the synthesis of materials with other perovskite structures by choosing optimal milling parameters.
Introduction
Mechanochemical reactions and mechanical activation in planetary ball mills have long been known. In the case of a planetary ball mill, there are several parameters that fundamentally influence these processes: the rotational speed; the material of the milling balls and vessel; the number of balls and the filling ratio of the balls and reactants; the milling time; the temperature and atmosphere in the vessel, the physical and chemical properties of the reactants etc. These parameters are not independent of each other and play an important role in achieving the best available yield. The value of ball-impact energy affects the increase in the particle size of materials, and as the temperature increases, compounds of different compositions may be formed [1]. For example, a long milling process can result in inadequate products, while too low milling energy does not allow for proper conversion of starting materials [2].
Due to its excellent properties, the group of chemical compounds with a perovskite structure includes a wide range of electrochemical materials: high-temperature superconductors, superionic conductors, and semiconductor dielectrics [3, 4]. The number of perovskites that can be produced by means of mechanochemistry is expanding. Our study aimed to explore the energy mapping of the process. Thereby, the mechanochemical perovskite synthesis can become a general process by which a large number of products can be prepared under laboratory conditions.
Experimental
Mechanochemical perovskite production was carried out using a Fritsch Pulverisette-6 planetary ball mill. Each experiment was conducted in grinding vessels with a volume of 80 mL. The material of the three different grinding vessels was: silicon-nitride (Si3N4), stainless-steel (FeNi) and tungsten-carbide (TC). The density of these vessels is 3.25 g cm−3, 7.7 g cm−3 and 14.3 g cm−3, respectively. The excess energy generated by the density of the material thus approximately doubles by using harder grinding vessels. The grinding balls (10 mm in diameter) and 2.00 g of BaO with 1.04 g of TiO2 or 1.50 g of ZnO with 1.47 g of TiO2 were added to the vessel in each case. The explanation for this is that the total weight had to be kept at around 3 g. The number of grinding balls was changed between 10 and 25, while the rotational speed was changed between 300 and 500 rpm.
Powder X-ray diffraction patterns were obtained with a Rigaku Miniflex-II instrument operating with Cu-Kα radiation (λ = 1.5406 Å). The 2Θ Bragg angles were scanned over a range of 5–90° at a rate of 1.0° min−1. Transmission Electron Microscope (TEM) analysis was performed by an FEI-Tecnai G2/20/X-TWIN instrument with a point resolution of 0.26 nm. Samples were placed on holey carbon-coated copper grids of 300 mesh.
Results and discussion
Using the milling-energy model, the Eb values were determined with different settings. From these, a milling-energy map was created, which is shown in Fig. 1. It illustrates the relationship between Eb and the material of the applied vessel and between Eb and the rotational speed. In the case of FeNi and TC vessels, samples milled at 200, 300, 400, and 500 rpm are illustrated. Whereas, in the case of the samples milled in the silicon-nitride vessel, the conversion of the starting materials only started to take place at 300 rpm with 25 milling balls (200 rpm data are not shown). Figure 1 shows that in the case of higher density grinding vessel, increasing the rotational speed has a more significant impact on the dynamics of the growth of Eb. With the TC vessel, the minimum rotational speed will determine the applied energy resolution, nevertheless, a much wider energy spectrum can be achieved with it in contrast to the Si3N4.
Eb points are defined by Eq. 1 as a function of the rotational speed (ωp)
Citation: Resolution and Discovery 6, 1; 10.1556/2051.2022.00092
Each grinding was performed for 3 h. Meanwhile, an hourly sample of the powder was taken, which was immediately measured by XRD. Figure 2 shows the XRD results of samples milled in different vessels with the same parameters (ωd = 400 rpm, Nb = 20 balls). As expected, the formation of both perovskites differs significantly in the three grinding vessels.
Results of XRD measurements. The 0-h sample means the BaO/ZnO-TiO2 starting materials mixture. ■ typical reflections of BaTiO3 perovskite. ● typical reflections of ZnTiO3 perovskite
Citation: Resolution and Discovery 6, 1; 10.1556/2051.2022.00092
The typical reflections of BaTiO3 between 2 theta 20–90° are listed below: 2 theta 22.1° (100); 31.5° (110); 38.8° (111); 45.1° (200); 50.7° (210); 56.1° (211); 65.7° (220); 70.2° (300); 74.7° (310); 78.9° (311). [8] BaTiO3 produced at a sintering temperature above 1,000 °C, is typically tetragonal and changed to the hexagonal structure above 1,400 °C [9]. The typical reflections of ZnTiO3 between 2 theta 20–90° are listed below: 2 theta 23.8° (101); 29.7° (012); 32.8° (104); 35.2° (110); 40.3° (113); 48.8° (024); 53.3° (116); 56.5° (018); 61.8° (214); 63.1° (300). The ZnTiO3 exhibits a pure crystalline phase of hexagonal [10].
The Eb can double depending on the material of the grinding vessel. This is well reflected in the XRD results. The production of BaTiO3 is already sufficient at a lower Eb value, resulting in 35.5 mJ/hit. After 60 min typical reflections appear during treatment in the case of the FeNi grinding vessel. The transformation of precursors in the Si3N4 grinding vessel does not take place at this Eb value in the case of ZnTiO3. The characteristic reflections of crystalline ZnTiO3 appear only after 2 h in the FeNi milling vessel.
For the same samples, transmission electron microscopic measurements were also performed
Figures 3 and 4 shows the TEM images of BaTiO3 and ZnTiO3, respectively. It is clear that by increasing the density of the grinding vessels, i.e., the Eb, the morphology of the particles becomes sharper for both perovskites. While only a mixture of starting materials can be seen in the Si3N4 grinding vessel, as confirmed by the XRD results, individual particles can be distinguished in the samples made in the FeNi grinding vessel.
TEM images of BaTiO3 perovskites synthesized in the (a) Si3N4, (b) FeNi and (c) TC grinding vessels
Citation: Resolution and Discovery 6, 1; 10.1556/2051.2022.00092
TEM images of ZnTiO3 perovskites synthesized in the (a) Si3N4, (b) FeNi and (c) TC grinding vessels
Citation: Resolution and Discovery 6, 1; 10.1556/2051.2022.00092
In the case of FeNi grinding vessels, the presence of synthesized perovskite and the mixture of starting materials are present at the same time, especially in the case of ZnTiO3 (Fig. 4b). Separate particles of both BaTiO3 and ZnTiO3 can already be observed in the TC grinding vessel. The size of the particles falls within the nanoscale.
With the microscopic images, size distribution histograms were made of the samples synthesized in the stainless-steel and tungsten-carbide grinding vessels (Fig. 5). Due to the high-energy milling, crystal growth is inhibited during the formation of the perovskite structure, so the size of individual particles falls within the nano range. The conspicuous difference between the material produced in the two grinding vessels is that the increase in particle size was measurable in the TC vessel, which provides more impact energy (Eb). This can be explained by the sintering caused by excess energy, which creates ever-larger particles by merging smaller separated particles. Overall, the average diameter of the perovskite particles is 14.5 and 19.8 nm.
Histograms of the particle size distribution of BaTiO3 (a, b) and ZnTiO3 (c, d) perovskites produced in the FeNi (a, c) and TC (b, d) grinding vessels
Citation: Resolution and Discovery 6, 1; 10.1556/2051.2022.00092
Conclusion
For both perovskites, the formation of the structure can be achieved mechanochemically in a similar energy range. From this, we can conclude that the experience gained during the research can already be used to produce perovskites from the components of metal oxide.
As a result, it can be a general mechanochemical perovskite synthesis model: with adequate Eb, all three grinding vessels are suitable for the production of perovskites, while only the harder stainless steel and tungsten-carbide grinding drums are capable of the formation of nanoscale particles.
Acknowledgements
This study was prepared with the support of the Bolyai János Research Scholarship No. BO/00835/19/7 for G.K. and BO/00384/21/7 for A.R., and with the professional support of the New National Excellence Program of the Ministry of Innovation and Technology No. ÚNKP-21-5-SZTE-547 for G.K and ÚNKP-21-5-SZTE-576 for A.R. Project no. TKP2021-NVA-19 has been implemented with the support provided by the Ministry of Innovation and Technology of Hungary from the National Research, Development and Innovation Fund, financed under the TKP2021-NVA funding scheme.
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