Chemical synthesis of SmAlO3 by stearic acid route: structure, morphology and microwave dielectric properties

A rapid and facile approach was developed for the synthesis of ultrafine SmAlO3 powders through spontaneously combusting stearic acid precursors. The obtained products were characterized by typical techniques including X-ray diffraction (XRD), Fourier Transform Infrared (FT-IR), thermogravimetric and differential thermal analysis (TG-DTA), scanning electron microscopy (SEM) and electron microscopy transmission (TEM) to analyze the phase composition and microstructure. The dielectric characteristics of SmAlO3 microwave ceramics using the as-obtained products as original materials were also studied. Comparing with conventional solid-state reaction method, the synthesis temperature was dramatically reduced to 750 °C. Large size sheet structure was composed of a number of micro/nano-scale crystallites that were almost irregular in shape due to the mutual growth and overlapped shape of adjacent grains. The SmAlO3 ceramics with high density and uniform microstructure were obtained after sintering at 1500 °C for 4 h due to good sintering activity of the as-synthesized powders. In addition, desired dielectric properties at microwave frequencies (dielectric constant εr = 20.22, quality factor Q·f = 74110 GHz, and temperature coefficient of resonant frequency TCf = -74.6 ppm/°C) were achieved.

years. For example, Suvorov et al. reported a new ceramics system with satisfactory microwave dielectric properties (ε r = 41, Q·f = 42,000 GHz, TCf = − 18 ppm/°C) by mixing 0.65CaTiO 3 and 0.35SmAlO 3 at 1450 °C [6]. Soon afterwards, a similar competitive composition of 0.66 CaTiO 3 -0.34SmAlO 3 in this system was found by Xu et al., displaying the best ε r and Q·f of 42.49 and 46069.86 GHz when sintered it at 1500 °C for 4 h [7]. The comprehensive dielectric properties with ε r = 58.3, Q·f = 18800 GHz and TCf = 2.3 ppm/•C could be obtained in the ceramic 0.75(Sr 0.3 Ca 0.427 Nd 0.182 )TiO 3 -0.25SmAlO 3 after being sintered at 1500 •C for 4 h [8]. Solomon revealed that the 0.1SmAlO 3 -0.9Ba(Zn 1/2 Nb 2/3 )O 3 composite sintered at 1400 •C for 4 h had ε r = 37, TCf = 25 ppm/•C and high Q·f [9]. Moreover, the composite could find a potential application in optoelectronics communication. In addition, improved electrical properties were achieved by doping a small amount of SmAlO 3 to (K 0.44 Na 0.52 Li 0.04 )(Nb 0.91 Ta 0.05 Sb 0.04 )O 3 lead-free ceramics [10]. Based on these attempts, it is concluded that SmAlO 3 is one of the most competitive temperature compensation materials due to its rather large and negative TCf.
Nevertheless, an outstanding drawback namely ultrahigh sintering temperature still prevents wide scale industrial application in microwave components for SmAlO 3 -based ceramics owing to the poor reactive nature of the powders obtained from the conventional solid state reaction method. Moreover, the conventional solid state reaction method generally requires a prolonged preparation cycle and thus results in compositional deviation and particle coarsening of final products as well as excessive energy consumption. Especially, SmAlO 3 ceramics prepared by solid state reaction route require very high calcination and sintering temperatures of about 1400 o C and 1650 o C to be densified, respectively [11]. Recently, tremendous efforts have been devoted to enhancing the sintering performance and a variety of synthesis approaches have been carried out, such as co-precipitation method, molten salt synthesis, hydrothermal synthesis, sol-gel route and polymer complex method [12][13][14][15][16][17]. These wet-chemistry methods show several advantages like lower synthesis temperature, shorter reaction time and finer grain size, which influence the sintering and dielectric properties positively.
In our previous work, we investigated the synthesis of SmAlO 3 powders by citrate precursor and EDTA precursor methods [18,19], exhibiting specific advantages in the reduced sintering temperature and enhanced microwave dielectric properties. Providing an important supplement of wet-chemistry methods for SmAlO 3 ceramics, we synthesized pure phase SmAlO 3 nanosized powders with high reaction yield and synthetic reproducibility by stearic acid method, in which stearic acid was used as not only solvent but also complexing agent. The method can also show a cost advantage due to the utilization of inexpensive metal salts and stearic acid as starting materials. The phase composition, microstructure and morphology of the obtained nanoparticles were analyzed and the microwave dielectric properties of the ceramics made by the nanoparticles were also discussed.

Experimental Procedures 2.1 Powder synthesis
SmAlO 3 powders were synthesized according to a synthetic procedure as shown in Fig. 1 solution was added dropwise to the molten stearic acid in the same way. By increasing the heating temperature to 100 o C for a sufficient period of time, the unnecessary water in the mixture was removed. After that, the mixture was continuously magnetic stirred and maintained at 130 o C until a uniform sol-like solution was produced, accompanied by the release of a large volume of reddish brown. By naturally cooling down to room temperature and drying for 12 h in an oven, a dried gel was formed. Subsequently, the dried gel was reheated to 230 o C for 2 h until it transformed into a black resinous precursor. Finally, the SmAlO 3 powder was obtained after calcining the precursor in air at various temperatures.
The suggested chemical reaction is as follows:

Samples preparation
The as-synthesized SmAlO 3 powders along with a small amount of PVA binder were granulated and then pressed cylindrical green samples with a size of about Φ10 mm × 5 mm. Finally, these aspressed cylinders were pressureless sintered for densification at 1350-1550 °C for 4 h.

Characterization and measurements
XRD patterns were collected using a powder X-ray diffraction analyzer (Bruker D8, Germany) with CuKα radiation at 2θ = 20-80 °. Thermal analysis of dried gel was employed on a Shimadzu DTG-60H instrument in a temperature range of 30-900 °C in air with a heating rate 10°/min. FT-IR spectra were obtained on a Nicolet 6700 FT-IR device by the KBr pellet method ranging from 400 to 4000 cm − 1 . 3.2 X-ray diffraction patterns and FT-IR spectra analysis  [19], sol-gel route using malic acid as complexing agent [22]. However, the synthesis temperature of stearic acid method is about 750 o C, which lowers that of the other wet-chemistry routes (800 o C for polymeric precursor method and 950 o C sol-gel technique route). However, this synthesis temperature for the solid-state reaction to obtain pure phase SmAlO 3 is as high as 1400 o C [5]. These evidences indicate that the stearic acid approach is a speedy and energy-saving route for the synthesis of ultrafine SmAlO 3 powders with high sintering activity.
In addition, the refined lattice parameters, lattice strain and crystallite size for the products derived from different synthesis approaches are calculated using the FULLPROF software package and Debye-Scherer's formula [23,24]. Table 1 summarized these calculated results. Where D, λ, β and θ are the average crystallite size, X-ray wavelength, full width at half maximum (FWHM) of the diffraction peak and Bragg's angle, respectively.
It is seen that the cell volume of SmAlO 3 phase obtained by wet-chemistry methods is relatively smaller than that of conventional solid state reaction method, indicating that the powder synthesized by wet-chemistry methods has more defects and thus possesses high reaction activity. The calculated crystallite size is 29 nm for the SmAlO 3 powder calcined at 750 o C while taking the diffraction angle 2θ = 33.9 °.
In order to confirm the structural changes between the precursor and the powders underwent calcination process, FT-IR analysis was done and the results are shown in Fig. 4. Small absorption bands at about 2920 cm − 1 , 1630 cm − 1 and 1067 cm − 1 in the precursor after heat treatment at 230 o C reveal that little amount of organic substances still exists. With increasing the temperature to 750 •C, no obvious absorption bands of organic substances are observed, as shown in Fig. 4(b).
Meanwhile, the absorption bands in the range of 400-700 cm − 1 get stronger, which are attributed to the formation of MO 6 octahedra (M = Sm/Al). In addition, a broad band centered at 3427 cm − 1 is assigned to the adsorbed water of these powders. Figure 5 shows the scanning electron image of SmAlO 3 powder by calcined the precursor at 750 •C for 2 h. As displayed in Fig. 5, a large size sheet structure is composed of a number of micro/nanoscale crystallites. To draw a reliable conclusion based on the above result, the transmission electron analysis was carried out and the corresponding TEM image is present in Fig. 6. It is observed that nearly all crystallites are almost irregular in shape due to the mutual growth and overlapped shape of adjacent grains. The possible reason is high reactive activity and good sintering activity of the assynthesized SmAlO 3 nanopowders. As a result, the particle size estimated from the TEM image is about 70 nm, which is significantly larger than that calculated from the Debye-Scherer's formula.

Sintering of SmAlO 3
Using the powder synthesized by stearic acid method as starting materials to prepare SmAlO 3 ceramics for purpose of application as microwave dielectric materials. The synthesized powders were uniformly mixed with 5 wt% PVA solution, pressed some bulk specimens and were sintered at 1350-1550 •C for 4 h. suggested that a reasonable calcination temperature is critical for obtaining dense ceramics with good microwave dielectric properties [25]. Low or high calcination temperature is disadvantageous to achieve high density ceramics in sintering process. A systematic investigation is worthy to explore for better optimization of density and microwave dielectric properties.
The SEM image of SmAlO 3 ceramics at 1500 o C for 4 h is shown in Fig. 8. A dense, uniform microstructure with the average grain size of about 1.8 µm can be observed, which is advantageous to enhance the microwave dielectric properties. In addition, a layered growth steps morphology can also found according to the SEM image. Similar results have been reported in the ceramic systems of 0.75CaTiO 3 -0.35SmAlO 3 and 0.95(Ca 0.88 Sr 0.12 )TiO 3 -0.05(Bi 0.5 Na 0.5 )TiO 3 [25,26]. These phenomena have a close relationship with the grain growth and mass transfer in sintering process.

Microwave dielectric properties of as-prepared SmAlO 3 ceramics
The microwave dielectric properties of SmAlO 3 ceramics are demonstrated in Fig. 9. As can be seen, the relationship between the ε r value and sintering temperature is in firm agreement with that between the density and sintering temperature. The ε r firstly increases to the saturated value at 1500 °C and then decreases slightly in the investigated sintering temperature. Also, the Q·f value  The sintering properties and microwave dielectric characteristics of SmAlO 3 ceramics derived from various methods are summarized in Table 2. Although the ε r and Q·f values obtained in our present work are slightly low than those achieved by polymeric precursor method, the stearic acid method still exhibits some other advantages such as low cost of raw materials, simple and facile synthesis process and low energy consumption. On the one hand, the stearic acid approach significantly reduces the calcination temperature and sintering temperature of SmAlO 3 compared with the conventional solid-state reaction method. On the other hand, the microwave dielectric properties especially the Q·f value are greatly improved. As a result, the stearic acid method is one of the most facile, energy-saving and promising approaches to prepare SmAlO 3 ceramics with good microwave dielectric properties.

Conclusions
In the current study, SmAlO 3 powder was synthesized successfully by stearic acid method. The characterization results strongly demonstrated that the stearic acid method is one of the facile, low cost and energy-saving approaches to synthesize well-crystallized SmAlO 3 phase after heat treatment at 750 o C for 2 h. The average crystalline size of SmAlO 3 powder synthesized at 750 o C was about 70 nm and in an almost irregular shape. The stearic acid method exhibited many advantages over the other wet-chemistry routes such as low cost of raw materials, simple and facile synthesis process and low energy consumption. The stearic acid approach significantly reduced the calcination temperature and sintering temperature of SmAlO 3 compared with the conventional solid-state reaction method.
Also, the microwave dielectric properties especially the Q·f value were greatly improved.
Declarations Figure 1 Flow diagram of SmAlO3 synthesized by stearic acid method Variation in microwave dielectric properties containing εr, Q×f and τf