Effects of calcination temperature and time on the Ca3Co4O9 purity when synthesized using starch-assisted sol-gel combustion method

Ca3Co4O9 is a p-type semiconducting material that is well-known for its thermoelectric (TE), magnetic, electronic, and electro-optic properties. In this study, sol-gel autoignition was used to prepare Ca3Co4O9 at different calcination temperatures (773, 873, 973, and 1073 K) and time (4, 6, 8, 10, 12, and 14 h) using starch as a fuel. The phase and microstructure of the prepared Ca3Co4O9 powder were investigated. Thermogravimetry.differential thermal analysis (TGA) confirms that the final weight loss occurred at 1073 K to form Ca3Co4O9 stable powder. The variable-pressure scanning electron microscopy (VP-SEM) images show that the size of powder particles increases from 1.15 to 1.47 μm as calcination time increases from 4 to 12 h, and the size remains almost constant thereafter. A similar pattern is also observed on the increment of the crystallite size and percentage of crystallinity with X-ray diffraction (XRD) analysis. The highest crystallinity is found about 92.9% when the powder was calcinated at 1073 K for 12 and 14 h with 458 and 460 Å crystallite size, respectively. Energy dispersive X-ray spectroscopy (EDS) analysis demonstrates that the calcinated powder has a high intensity of Ca, Co, and O with uniform distribution. High-resolution transmission electron microscopy (HRTEM) images prove that there is no distinct lattice distortion defect on the crystal structure.


Introduction 
In recent years, utilizing of fossil fuels has shown severe impacts on atmospheric and environmental problems, causing international warming, climate change, greenhouse gas emission, ozone layer depletion, and acid chemical stability. Therefore, they have been recognized as promising thermoelectric materials [4]. However, despite of these advantages, the disadvantage of thermoelectric oxides is their low efficiency (figure of merit). Therefore, these oxides need to improve their thermoelectric properties to enhance the thermoelectric conversion efficiency.
The misfit of calcium cobalt oxide (Ca 3 Co 4 O 9 ) ceramics is eliciting considerable interest for their practical applications in refrigeration devices, recycling of waste heat to electricity, and solar thermoelectric generators [5]. Calcium cobaltite is also extensively studied as a possible thermoelectric oxide material due to its low electrical resistivity and high Seebeck coefficient with low thermal conductivity [6,7]. Therefore, in most thermoelectric applications, doping of Ca 3 Co 4 O 9 ceramics to increase electric conductivity is indispensable. Many kinds of research have been conducted to evaluate the effects of various doping and co-doping elements on the changes of crystal structure and thermoelectric properties of the material. For instance, in the recent developments, significant enhancement in the thermoelectric performance of Ca 3 Co 4 O 9 thermoelectric material was reported through combined strontium substitution and hot-pressing process [8]. While in Ref. [9], Na and W dual doping in Ca 3 Co 4 O 9 system was claimed able to increase figure of merit value more than two times of the undoped sample at 1000 K. The crystal structure of Ca 3 Co 4 O 9 misfit-layered as shown in Fig. 1 consists of two layers (Ca 2 CoO 3 )(CoO 2 ) b 1 /b 2 . The Ca 2 CoO 3 layer is an insulating distorted NaCl type, and the CoO 2 layer is an electrically conductive CdI 2 type. They stack along the c-axis alternately, with b 1 and b 2 representing the length of the (Ca 2 CoO 3 ) and (CoO 2 ) sublattice, respectively, along the b-axis [10,11]. The Ca 2 CoO 3 layer decreases the thermal conductivity due to work as phonon scattering centers, whereas the CoO 2 layer serves as electron transport sites due to having an effectively correlated electron system [7].
Several synthesis methods have been used to produce Ca 3 Co 4 O 9 powder, including thermal hydro-decomposition [15], pechini [16], polymer solution synthesis [17], and solid-state reaction [18][19][20][21][22]. These processes are operating at high reaction temperatures, long process, and low chemical homogeneity [23]. Apart from powder synthesis, only solid-state reaction technique has reported the effect of calcination procedure, temperature, and time on thermoelectric properties. For instance, Smaczyński et al. [24] studied the influence of the solid-state reaction and calcination at different temperatures and time using the thermogravimetry-differential thermal analysis (TGA) test. The results showed that a stable composition of Ca 3 Co 4 O 9 powder was obtained at calcination at 1073 K for a higher soaking time between 20 and 30 h. However, some traces of cobalt oxide were still observed in the calcinated powder because of the Ca 3 Co 4 O 9 nonstoichiometry at room temperature. Bresch et al. [25] investigated the effect of the calcination procedure on the thermoelectric properties of Ca 3 Co 4 O 9 powder using solid-state reaction technique. According to this study, no systematic study had been done on the thermoelectric properties affected by the calcination procedure. Conversely, the advantages of sol-gel combustion method are simple, easy control of homogeneity and stoichiometry, and  [12][13][14]. Reproduced with permission from Ref. [12], © Elsevier Ltd and Techna Group S.r.l. 2018; Ref. [13], © The Royal Society of Chemistry. 2016; and Ref. [14], © American Chemical Society. 2016. economical preparation [26,27]. Previous studies showed that synthesized Ca 3 Co 4 O 9 powder produced by the sol-gel method results in fine-sized particles, as well as regular size distribution, which is significant in improving magnetic, electrical, and optical properties [12,[28][29][30][31].
In the sol-gel preparation method, ethylene glycol, polyethylene glycol, and nitric acid were used to polymerize the solution, induce nitrate salt decomposition, and facilitate new compound formation [32,33]. On the other hand, citric acid and polyethylene glycol were used to polymerize the solution which produces carbonaceous xerogel and needs to be crushed [3,31,[34][35][36]. Some researchers focused on the preparation of Ca 3 Co 4 O 9 powder with different calcination conditions by using different preparation techniques. For example, Chen et al. [7] investigated the influence of the precursor calcination temperature on the microstructure properties of Ca 3 Co 4 O 9 powder. They prepared the calcium cobaltite powder using the sol-gel technique and calcined at various temperatures between 923 and 1073 K. The results showed that the calcination temperature has a significant effect on the microstructure properties. Crystal size increases with the increase in the calcination temperature. However, they did not study the effect of calcination time on the crystallite size.
Most of polymerizing agents used in powder preparation are acid-based which require special care in handling them for preparing ceramic powder. Recent developments show that starch has been increasingly used as a fuel in the sol-gel combustion method [37][38][39][40][41][42][43] due to the ease of preparation and environmental friendliness. Apart from that, it only generates CO 2 , N 2 , and H 2 O after ignition [44,45]. Moreover, the use of starch as a fuel is becoming more favorable because its ability to maintain the pH of the gel [46,47] and it generates non-toxic gases during the calcination process. The method is also capable of producing nanoparticle powder with pure phase [37][38][39]44,48] and homogenous doped particles [37]. It has been reported that the thermoelectric properties improve with smaller powder grain size, which exhibits low thermal conductivity [49,50]. Previous investigations on the properties of Ca 3 Co 4 O 9 powder using starch as fuel were only limited to the preparation of powder at different temperatures [45]. However, there is no report published on the effect of calcination time on the particle size, crystallite size, and crystallinity as yet. The aim of the present study is to evaluate the microstructure of Ca 3 Co 4 O 9 powder when it is prepared via the sol-gel combustion method and uses starch as a fuel during the calcination process.

Materials and experiments
Polycrystalline Ca 3 Co 4 O 9 powder was synthesized using the sol-gel combustion method. Starch (C 6 H 10 O 5 ) n was used as a combustion fuel and gelling agent. Calcium nitrate tetrahydrate Ca(NO 3 ) 2 ·4H 2 O (99%) and cobalt nitrate hexahydrate Co(NO 3 ) 2 ·6H 2 O (99%) were dissolved in distilled water and stirred using a magnetic hot-plate stirrer (IKA-C-MAG HS4, Germany) to obtain a stoichiometric mixture. The starch was mixed with distilled water, and the solution was added gradually into the mixed metal-nitrate solution. The produced transparent pink solution was heated under constant stirring within 353-373 K to obtain a pink gel. Subsequently, the resultant gel was decomposed using a hot-plate at 673 K for 1 h, and stirred until the gel burned and became a black precursor. TGA of the precursor was carried out using TA instruments Q500 TGA (Leatherhead, UK) from room temperature to 1383 K in the air at a heating rate of 10 ℃/min. The precursor was then calcined at different temperatures (773, 873, 973, and 1073 K) and time (4, 6, 8, 10, 12, and 14 h) at a heating rate of 10 ℃/min under ambient using Carbolite CWF 12/23 electric furnace. The morphology of Ca 3 Co 4 O 9 particles was examined using a variable-pressure scanning electron microscope (Model-JEOL JSM-IT300LV) and energy dispersive X-ray spectroscope (JEOL JSM-IT300LV, Akishima, Japan) for elemental mapping. The crystal structures of calcined Ca 3 Co 4 O 9 particles powdered at different dwelling times were characterized using a Rigaku (Smartlab) X-ray diffractometer with Cu Kα radiation (λ = 0.154, 18 nm) and 2θ value was between 5° and 60°. The lattice structure of Ca 3 Co 4 O 9 powder was examined under scanning transmission electron microscopy (STEM) with a JEOL JEM-ARM200F (FEG-STEM/TEM-USA model) at 200 kV with a 0.08 nm resolution, equipped with a Cs-corrector (CEOS GmbH) for the electron probe.

Results and discussion
The black precursor was obtained after the pink gel heated at 673 K for 1 h. The conversion process of the gel is represented by Eq. (1). The synthesized black precursor was then studied using TGA, and the results are shown in Fig. 2. It illustrates the weight loss percentage as a function of temperature for the Ca 3 Co 4 O 9 sample. The TGA curves show that the Ca 3 Co 4 O 9 powder is stable up to 860 K, and no weight loss is observed below this temperature. The maximum weight loss peak occurred at 973 K as a result of Ca 3 Co 4 O 9 compound formation, partially (CaO and Co 3 O 4 ) from the decomposition of calcium carbonate into calcium oxide by the release of carbon dioxide (CO 2 ). The weight loss process is represented by Eq.
(2). The final weight loss occurred at 1073 K, where the decomposition residuals of cobalt oxide and calcium oxide formed a stable Ca 3 Co 4 O 9 product. Equation (3)   illustrate the morphology of black powder after being calcinated at 773, 873, 973, and 1073 K for 4 h, respectively. As the calcination temperature increases, the size of black powder particles also increases, and the shape of particles becomes more defined at 1073 K. Figure 4 demonstrates the X-ray diffraction (XRD) results of each calcinated powder. At low calcinated temperatures (< 973 K), the CaCO 3    calcination time has an effect on the particle size distribution and morphology of these particles. The calcined Ca 3 Co 4 O 9 powder demonstrates a fine particle size with plate-like as well as irregular shape morphology. The distribution of particles size is quite homogenous and normally distributed as shown in Figs. 5(g)-5(l) with a mean of 1.15, 1.19, 1.38, 1.39, 1.47, and 1.47 μm for 4, 6, 8, 10, 12, and 14 h calcination time, respectively. The morphology of the powder has changed slightly with increasing the average particle size, and some agglomerations were observed between fine particles. The number of plate-like shaped particles increased with the increase of calcination time, which is due to the grain growth over the soaking time. The effect of calcination time on lattice-parameters was analyzed using XRD patterns on the powder calcined at 1073 K. These XRD patterns are shown in Fig. 6(a). All peaks were indexed according to ICDD PDF card number 00-021-0139. Structural parameters were refined with the superspace group X2/m(0b0)s0 using JANA 2006 software [53]. The refined XRD pattern of 12 h calcination time sample is shown in Fig.  6(b), as an example. Miller indices are also illustrated in Fig. 6(a) and all the peaks determine the transformation of Ca 3 Co 4 O 9 powder to the monoclinic misfit crystal structure. The lattice parameters were estimated based on the XRD patterns of each powder, as summarized in Table 1. The crystal parameters a = 4.835 , b 1 = 4.556, b 2 = 2.824, c = 10.854, and b 1 /b 2 ratio of 1.613 for the 12 h-calcined powder were close to the crystal parameters as reported by Masset et al. [10,[54][55][56]. From XRD data, the analysis of the where D is the crystallite size (nm), K is a shape factor, which usually takes a value of about 0.9, λ is the wavelength of X-ray source for Cu Kα radiation (λ = 0.15418 nm), β is the full width at half-maximum (FWHM) of the observed peak, and θ is the Bragg diffraction angle (°) [38,[57][58][59].

K
The crystallinities of Ca 3 Co 4 O 9 powder were calculated from the XRD intensity data by assuming a two-phase structure (crystalline-amorphous) with a line through the minimum intensity to get an arbitrary background to diffraction trail, thus an arbitrary crystalline phase segregating from an amorphous phase [43,44]. The crystallinity X cr was calculated by the following equation: where A cr and A am are the integrated areas of the crystalline and amorphous phases, respectively. Figure  7 illustrates the relationship of the calcination time with the degree of crystallinity and crystallite size of the synthesized Ca 3 Co 4 O 9 . It can be seen that the degree of crystallinity and crystallite size increase with the increasing of calcination time. As the calcination time increases, the crystallinity of Ca 3 Co 4 O 9 powder improves near to 92.9%, and it becomes less amorphous. The application of TE materials requires pure phase and thermally stable bulk materials [60]. It is noticed that the crystallite size and crystallinity (%) of powder become insensitive after 12 h calcination time.  Based on the XRD results, it can be deduced that the highest purity of Ca 3 Co 4 O 9 powder is obtained at 1073 K for 12 and 14 h. Considering the energy consumption under these two conditions, 12 h is more favorable since their crystallite size and crystallinity are almost the same. Subsequently, the distribution of the elements on the microstructure of Ca 3 Co 4 O 9 powder at 1073 K for 12 h was characterized using EDS elemental mapping, and the results are shown in an overlay color image in Fig. 8(a). The measured EDX spectrum of Ca 3 Co 4 O 9 elements is illustrated in Fig. 8(b) with peaks of Ca, Co, and O, which verify the existence of Ca (24.9 wt%), Co (54.1 wt%), and O (20.7 wt%). Cobalt is homogeneously distributed in the investigated area, as shown in Fig. 8(c) with rose color. Identical distribution of calcium mapping element is represented in a light blue color, as shown in Fig. 8(d). The oxygen content is high and homogeneously distributed too, which referred by green color, as shown in Fig. 8(e). It can be said that from the SEM-EDS mapping, there were formations of an aggregate consisting of Co, Ca, and O. The quantitative of SEM-EDS analysis confirmed that the chemical compositions of the Ca 3 Co 4 O 9 sample corresponded to the nominal compositions. Furthermore, neither N nor C signals were detected in the EDS spectrum, which means the product is pure and free of any surfactant or impurity.
Figures 9(a)-9(e) show the structure observed for the as-calcined Ca 3 Co 4 O 9 powder at 1073 K for 12 h. It is noticed that the Ca 3 Co 4 O 9 particles appear as non-uniform size and shape, as shown in Fig. 9(a). Enlargement view of Ca 3 Co 4 O 9 particles is illustrated in Fig. 9(b). Darker contrast detected on the particles is attributable to metal-rich cobalt ion aggregates, bound with each other by the calcination process. Highresolution TEM images were obtained from the marked red square area Y in Fig. 9(b). The crystal layered structures of the material with distinct CoO 2 and Ca 2 CoO 3 layers are clearly seen without lattice distortion defect. The atomic arrangement of Ca, Co, and O atoms are schematically presented next to high-resolution transmission electron microscopy (HRTEM) image in Fig. 9(d). This image is identical to the description of Ca 3 Co 4 O 9 crystal structure as described in Fig. 1. The d spacing of planes of Ca 3 Co 4 O 9 along the c direction is determined using Gatan software which is about 1.08 nm. This result is in agreement with the d spacing counted from the 00l peaks of the θ-2θ scan [6,14,18]. The selected area electron diffraction (SAED) patterns of the samples are www.springer.com/journal/40145  shown in Fig. 9(e). The samples had particle sizes in the order of several micrometers; therefore, the SAED covers the area, including small grains. The SAED shows diffraction spots of single crystals mixed with a speckled-ring pattern.

Conclusions
Ca 3 Co 4 O 9 is successfully synthesized using starchassisted sol-gel auto-combustion method. The final powder is obtained through calcination at 1073 K by varying soaking time for 4, 6, 8, 10, 12, and 14 h. All the calcinated powder transforms into a single-phase calcium cobalt oxide with the particle size ranging from 1.15 to 1.47 µm. Similarly, the purity of calcium cobalt oxide particles increases with the increase of calcination time and becomes saturated after 12 h. TEM images also show no distinct lattice distortion defect observed on the crystal structure. The maximum attainable purity of Ca 3 Co 4 O 9 is 92.9%, which is a very encouraging result for the thermoelectric material applications. Besides the purity, the performance of thermoelectric material could be further enhanced via doping elements for improving their thermoelectric properties.