Synthesis of Nano-Crystalline Zr0.8Sn0.2TiO4 Powder by Choline-Chloride-Malonate Deep Eutectic Solvents

Chloride-malonate deep eutectic solvent (Cm-DES) was applied to synthesize nano-crystalline Zr 0.8 Sn 0.2 TiO 4 ceramic powder by using Zr(CH 3 COO) 4 , SnCl 2 ∙ 2H 2 O and Ti(OCH 3 CH 3 ) 4 as raw materials. The nano-crystalline Zr 0.8 Sn 0.2 TiO 4 powder was obtained at 450 °C, dramatically lower than that by solid state reaction. Meantime, the grain size of as-synthesized Zr 0.8 Sn 0.2 TiO 4 powder at 450 °C is 10.7 nm. In the synthesized process, Cm-DES, as a solvent and template, affects the synthesis process and the time required for synthesis is greatly reduced. These results show that the Cm-DES is suitable and prospective for the synthesis of nano-crystalline Zr 0.8 Sn 0.2 TiO 4 ceramic powder, which could contribute to reduce energy consumption and time cost.


Introduction
Deep eutectic solvents (DESs) is a new type of solvents like ionic liquids [1,2], which is cheap, ecofriendly and prepared easily, just from the concurrent melting of two simple species. Based on the advantages such as easy synthesis, cost-saving, and high solubility of metal salts, DESs has been widely used in various elds, such as gas adsorption [3], ferroelectrics [4,5] and so on. However, as far as the literature is concerned, there is no report on the synthesis of microwave dielectric ceramic powders by DESs. In this work, Zr 0.8 Sn 0.2 TiO 4 powder was synthesized by the Chloride-malonate deep eutectic solvent (Cm-DES) and the effect of calcination temperature on the phase composition, grain size and microstructure of as-synthesized powder was investigated.

Experimental
Zr 0.8 Sn 0.2 TiO 4 powder was prepared by the Cm-DES using high-purity materials including Zr(CH 3 COO) 4 (Zr,15.0-16.0%), SnCl 2 ·2H 2 O (≥98.0%), and Titanium isopropoxide (TiO 2 , 27.5-28.2%) which were purchased from Sinopharm Chemical Reagent Co., Ltd, China. The chloride-malonate DES (Cm-DES) were prepared using choline chloride (98.0-101.0%) and malonate (≥98.0%) with molar ratio of 1:1, then the mixture was mechanically stirred in a bath (80 °C) until clear solution formed. Stoichiometric SnCl 2 ·2H 2 O and Zr(CH 3 COO) 4 solutions were added to Cm-DES (20 ml) successively. The mixture was then heated to 90°C and stirred until clear solution formed. And then stoichiometric titanium isopropoxide was added to the mixture solution and stirred vigorously. The obtained gel was dried, ground, and calcined at 250-650°C for 2h. X-ray diffraction (D8 Bruker Advance) patterns of the as-synthesized powders were obtained with Cu Ka radiation (λ=0.1541 nm) from 20 ° to 80 ° with a step of 0.02 ° at room temperature. The morphology was characterized using TEM (JEM-2100). The mean grain sizes were characterized by Nano Measurer 1.2 program. Fig. 1 shows the XRD patterns of Zr 0.8 Sn 0.2 TiO 4 powders calcined at various temperatures. Some glass phases were detected for the powder calcined at 250 °C, but the diffraction peaks corresponding to Zr 0.8 Sn 0.2 TiO 4 (PDF No.81-2214) appear from 350 °C. The intensity of diffraction peaks for the powder calcined at 450 °C is much higher and increases with elevating calcination temperature. This means that well-crystallized Zr 0.8 Sn 0.2 TiO 4 powders can be achieved by the Cm-DES at 450 °C, a temperature much lower than that in other preparation methods [6][7][8][9][10]. Generally speaking, high temperature (>1050 °C) is needed to prepare Zr 0.8 Sn 0.2 TiO 4 ceramics by traditional solid state reaction, which limits its application in new elds such as LTCC. The high calcination temperature and coarse grain size of Zr 0.8 Sn 0.2 TiO 4 ceramic powders prepared by solid state reaction may be detrimental to the Qf value of the obtained Zr 0.8 Sn 0.2 TiO 4 ceramics. Furthermore, no second phase was detected in all the powder samples synthesized by this Cm-DES method with higher calcination temperature (≥ 450 °C). The mean grain size of the Zr 0.8 Sn 0.2 TiO 4 powder prepared by the Cm-DES at 450 °C is around 6.6 nm (calculated by Scherer formula), which is signi cantly smaller than that by solid state reaction. Fig. 2 shows HRTEM images and mean grain sizes of as-synthesized powders at different calcination temperatures. The powder calcined at 350 °C is spherical, and the grain boundary between grains is obvious, as shown in Fig. 2A; The powder calcined at 450 °C is also spherical, however its grain size is increased signi cantly. With the further increase of calcination temperature, the grain size of the powder sample increases monotonously, agglomeration becomes more serious, and the particle shape changes from spherical to irregular polyhedron (600 °C, Fig. 2D). In addition, the mean grain sizes of the powder samples obtained at different calcination temperatures were calculated by Nano Measurer 1.2 program, as shown in Fig. 2E. Although the grain size of the as-synthesized powders increases with the increment of calcination temperature and remains unchanged when the calcination temperature is above 550 °C, the maximum grain size within the experimental temperature range is only 13.7 nm, the mean grain size is far smaller than that obtained by solid state reaction method.

Results And Discussion
The formation mechanism of Zr 0.8 Sn 0.2 TiO 4 phase in Cm-DES was also discussed. For the synthesis of BaTiO 3 , Boston et al. [4] presented that the key to the formation of BaTiO 3 phase was the reaction with metal chlorides. We tried to synthesize Zr 0.8 Sn 0.2 TiO 4 and BaTiO 3 directly from metal chlorides.
Unfortunately, no Zr 0.8 Sn 0.2 TiO 4 and BaTiO 3 phase were not detected in the calcined powders, as shown in Fig. 3. On the contrary, Zr 0.8 Sn 0.2 TiO 4 and BaTiO 3 powders were obtained by adding the same metal chlorides into Cm-DES, mixing and stirring evenly, and calcining at the same temperatures. These results indicate that Cm-DES can not only be used as solvent to dissolve the metal chlorides mentioned above, but also promote the formation of Zr 0.8 Sn 0.2 TiO 4 and BaTiO 3 . Generally, there are some interactions, including hydrogen bonding and electrostatic interactions in the DESs. The -C=O groups of malonic acids are mostly free in the eutectic mixture and the -OH groups are strongly associated with the Cl − of choline chloride through hydrogen-bonding interactions [11], which is key because hydrogen bonds lower the lattice energy of choline chloride and the melting point of the eutectic mixture. Herein, Cm-DES exists as a transparent, colorless liquid at room temperature. After the dissolution of SnCl 2 ·2H 2 O, Zr(CH 3 COO) 4 , and titanium isopropoxide in the Cm-DES at 90 °C, the color of the liquid mixture changes from colorless to yellow, corresponding to the formation of the x -. That is to say, the metal ions are primarily absorbed in the Cm-DES cluster at the expense of the partial destruction of the hydrogen-bonded framework of Cm-DES and the hydrogen-bond between -OH (choline) and ···chloride (metal salt) forms. Similar phenomena were detected by Thorat et al. [11] and Abbott et al. [12], respectively. During the calcination, these MCl x metal ions could present at higher temperature and accelerate the decomposition of Cm-DES to produce some light substances such as CO, methane, and ethane. [11], which lower the Gibbs free energy (ΔG) of the formation of Zr 0.8 Sn 0.2 TiO 4 . The oxidation of these substances also promote the formation of Zr 0.8 Sn 0.2 TiO 4 at lower temperature. Meantime, the grain size of Zr 0.8 Sn 0.2 TiO 4 particles is very small due to the template con nement effect of hydrogen bonding network in Cm-DES. While it needs solid phase diffusion with a long diffusion distance before the reaction with each other, herein high heating temperatures and long dwelling time were needed, resulting in some disadvantages such as high energy consuming and undedirable metaphases. Conventional solution chemical methods such as hydrothermal, co-precipitation, sol-gel, and ultrasonic spray pyrolysis method are also developed to prepare Zr 0.8 Sn 0.2 TiO 4 powder/ceramics. Aqueous/liquid states precursor materials were employeed in these solution chemical methods to solve the problems like hard diffussion and long diffussion distances. Unfortunately, special and expensive instruments like high pressure vessels and sonic device limit the application in batch production. In contrast, the synthesis of Zr 0.8 Sn 0.2 TiO 4 powder by Cm-DES reported in this work is low-cost, short process, time-consuming and energy-saving.

Conclusions
The nano-crystalline Zr 0.8 Sn 0.2 TiO 4 ceramic powder was synthesized by Cm-DES at such low temperature as 450 °C. The synthesis temperature is much lower than that for the traditional solid state reaction, reducing time and energy consumption. The corresponding Zr 0.8 Sn 0.2 TiO 4 ceramic powders exhibit well crystallized and the mean grain size is 10.7 nm at 450 °C. In the synthesis process, Cm-DES, as dissolvent materials, react media and template, can promote the Zr 0.8 Sn 0.2 TiO 4 synthesis at lower temperature. Cm-DES process is low-cost, short process, time-consuming and energy-saving, which make it a prospective, cost-saving, and sustainable process for the preparation of microwave dielectric ceramics.