Calcium Titanate from Food Waste: Combustion Synthesis, Sintering, Characterization, and Properties
Calcium titanate (CaTiO3) was combustion synthesized from a calcium source of waste duck eggshell, anatase titanium dioxide (A-TiO2), and magnesium (Mg). The eggshell and A-TiO2 were milled for 30 min in either a high-energy planetary mill or a conventional ball mill. These powders were then separately mixed with Mg in a ball mill. After synthesis, the combustion products were leached and then sintered to produce CaTiO3 ceramic. Analytical characterization of the as-leached combustion products revealed that the product of the combustion synthesis of duck eggshell + A-TiO2 that had been high-energy-milled for 30 min before synthesis comprised a single perovskite phase of CaTiO3. The high-energy milling of the reactant powder had generated a large reactive surface area and induced structural defects, both of which drove the completion of the combustion reaction and the phase conversion of the reactants into the product. A calcium titanate ceramic, fabricated by sintering as-leached powdered combustion product at 1350°C for 180 min, achieved a maximum density of 3.65 g/cm3 and a minimum porosity of 0.54%. The same fabricated calcium titanate ceramic product also exhibited the highest dielectric constant (∼78) and the lowest dielectric loss (∼0.02), which resulted from the simplified charge polarization process.
The alkaline earth titanate, calcium titanate (CaTiO3), has attracted attention because it is semiconductive, ferroelectric, and photorefractive. In addition, dielectrics based on calcium titanate are widely used in different high-frequency electronic applications such as capacitors, oscillators, filters, and resonators for microwave systems . Several approaches are adopted for the synthesis of calcium titanate either by soft chemistry such as sol-gel or by hydrothermal or solvothermal methods, coprecipitation, or organic-inorganic solution [2–5]. High-temperature solid state synthesis of CaTiO3 has been conducted using mixtures of calcium carbonate (CaCO3), calcium oxide (CaO), and titanium dioxide (TiO2) [6–9] and also mechanoactivated synthesis has been carried out with various mixtures of CaCO3, calcium hydroxide (Ca(OH)2), CaO, and TiO2 [10–14]. However, soft chemistry methods require complex starting materials and processes, while solid state and mechanoactivated processes consume a lot of energy and time. Recently, the advantages of combustion synthesis over other methods have seen it more widely used to synthesize several ceramic materials. The experimental set-up is uncomplicated, the method is energy-efficient and also has a short processing time, and a high-temperature furnace and large energy input are not required . Moreover, except for the necessary elements and metal oxides, waste materials, such as those from mining, agriculture, or food consumption, can be used as reactants because the high-exothermic heat of the reaction eliminates the organic and traces inorganic materials from the product.
The objective of the present work was to synthesize calcium titanate (CaTiO3) by combustion synthesis using powdered duck eggshells and anataste titanium dioxide (A-TiO2) as starting materials and magnesium (Mg) as fuel. The duck eggshell and A-TiO2 were mixed together for 30 minutes in either a high-energy planetary mill or a ball mill, and the synthesis products of the two different reactant powders were compared. Finally, powdered calcium titanate products from the combustion syntheses were fabricated into monolithic ceramic samples by sintering at 1350°C for 60, 120, and 180 min of holding time. In addition, the various products were characterized in terms of phase composition, microstructure, and physical, thermal, and electrical properties.
2. Experimental Procedure
2.1. Combustion Synthesis of Calcium Titanate Powder
Duck eggshells, received from Trang province in Thailand, provided a source of calcium carbonate (CaCO3). XRD analysis in Figure 1 confirmed that the as-received duck eggshell was composed entirely of calcium carbonate (CaCO3: ICDD no. 00-005-0586).
Anatase titanium dioxide (A-TiO2, 99.5%, <45 μm, Degussa AG) was used as a reactant because an anatase phase is an intermediate state of TiO2, which implies that A-TiO2 may be highly reactive . In addition, magnesium (Mg, 99%, ∼45 μm, Riedel-Detlaen) was used as fuel. The composition of the starting reaction used in the synthesis of calcium titanate was calculated from stoichiometric ratios and is expressed as follows:
Duck eggshells were ground into a friable powder and sieved through a 325 mesh to give particles of 45 μm. Powdered duck eggshell and A-TiO2 were high-energy-planetary-milled (Fritsch GMBH, Pulverisette 6, Germany) with an Si3N4 vial and ball for 30 min at 400 rpm. This procedure was adapted from the work of Palaniandy and Jamil . The eggshell and A-TiO2 was also milled in a conventional ball mill to obtain an alternative reactant powder for comparison. The ball- and high-energy-milled powder mixtures were then separately mixed with Mg powder by ball milling for 120 min using a nylon vial and zirconia (ZrO2) ball. With a hydraulic press (Huat Seng, 1939-15T, Thailand), 20 g of a reactant powder mixture were uniaxially pressed at 5 psi into a prehardened tooled steel mold to form a cylindrical pellet 25.4 mm in diameter, with a green body density 50–60% of the theoretical density (Figure 2(a)). The experimental set-up was schematically represented in a previous study  that used an oxy-acetylene flame as the external heat source.
The magnesiothermic reaction that took place within the reactant compact of high-energy-milled duck eggshell + A-TiO2/Mg started after a short period of heating and, preceded by a macroscopic combustion front, progressed quickly in gravitational self-propagating mode along the entire reactant compact to form a final product in an average time of about 20–30 seconds. Images of typical ignition, propagation, relaxation, and cooling down stages of the combustion reaction are shown in Figure 3. The reactant compact of ball-milled duck eggshell + A-TiO2/Mg, however, had to be continually provided with heat from the oxy-acetylene flame because the system could not produce a self-sustained combustion reaction. The inability of this reactant compact to sustain self-propagated combustion was due to the low surface area of the duck eggshell + A-TiO2 mixture.
Being cooled to room temperature, the as-combusted products (Figure 2(b)) were mechanically pestled into friable powders in a ZrO2 mortar. In order to remove the by-products of magnesium oxide (MgO) and magnesium titanate (MgTiO4), the as-combusted powders were leached using the 2 M HCl solution for 120 min under moderate stirring. The ratio of powdered product to leaching agent was 10 g : 100 mL throughout the experiment. After leaching, the leached product was filtered through filter paper coupled with a pump inlet and washed with deionized (DI) water several times in order to adjust pH to 7.0 before final drying at 100°C in an oven.
2.2. Sintering of Calcium Titanate Ceramic
To study the sinterability of the powders obtained from the present combustion method, 1.2 g of the as-prepared calcium titanate powders was mixed with 3% polyvinyl alcohol (PVA) binder and pressed in a hydraulic press (CARVWR, INC, 4128, USA), using a pressure of about 5 psi, to form a cylindrical pellet 15 mm in diameter. Following the research of Luo et al. , the pellet was then sintered at 1350°C for 60, 120, and 180 min of holding time under air atmosphere in a muffle furnace (Lenton, UAF16, USA). The temperature profile of the sintering process is in Figure 4.
The mean particle size of the duck eggshell and A-TiO2 powder obtained by planetary milling and ball milling was determined with a laser diffraction particle size analyzer (LPSA, Beckman Coulter, LS 230, USA). To study the combustion reaction mechanisms in the duck eggshells-TiO2-Mg system, the prepared mixed reactant powders were tested using simultaneous thermal analysis (STA, 449 F3, Jupiter, Netzsch, Germany) in TG and DSC modes. The samples were heated from 30 to 1300°C at a rate of 10°C/min in the N2 atmosphere. The morphologies of the as-combusted powder, as-leached powders, and as-sintered calcium titanate ceramics were characterized using scanning electron microscopy (SEM, Quanta 400, FEI, USA). X-ray diffraction (XRD, X’ Pert, MPD PHILIPS, the Netherlands) phase identification of all materials was carried at 40 kV, and 30 mA using CuKα radiation (0.15406 nm). The average crystallite size was calculated from the Debye–Scherrer equation :where is the crystallite size, is X-ray the wavelength, is the full width at half maximum (FWHM) of the peak in radians, and is Bragg’s angle.
2.4. Density and Porosity Measurement
The sintered density and apparent porosity of the calcium titanate ceramics were measured according to the Archimedes principle and can be calculated using equations (3) and (4), respectively:where is the sintered density, is the apparent density; are the dry weight, weight in water of the water-saturated specimen, and weight in air of the water-saturated specimen, respectively; and is the density of water (1 g/cm3) .
2.5. Dielectric Properties
For room temperature dielectric studies, both flat surfaces of the sintered calcium titanate ceramic pellet were polished and the thickness controlled at ∼1 mm. Silver (Ag) paste electrodes were applied to the pellet, which was then dried at 50°C for 24 h. The pellet was applied as a disc capacitor in which the sintered product was the dielectric medium. Capacitance was measured using an LCR meter (GW INSTEX, LCR-821, USA) within the frequency range of 0 to 200 kHz. The dielectric constant was calculated according to the following equation :where is the dielectric constant, is the equivalent parallel capacitance obtained from the data of measurement, is the thickness of the fabricated ceramic material, A is the combined surface electrode area of the electrode discs, and is the permittivity of vacuum (8.85 × 10−12 F/m). The dielectric loss obtained from the value of the dissipation factor can be calculated from by the following equation :where is the dielectric loss, is the dissipation factor obtained from data of measurement, and is obtained from equation (5).
3. Results and Discussion
Figure 5 shows the XRD patterns of duck eggshell + A-TiO2 mixed by ball- and high-energy milling. The diffraction peaks of the high-energy-milled sample have lower intensity and broader bases than the peaks of the ball-milled sample. The main cause of the difference between the two diffraction patterns was the alteration to the crystal structure of the TiO2 and CaCO3 caused by the more intensive grinding process of the high-energy milling process. The reduction of peak intensities implied the formation of a partially amorphous structure in the high-energy-milled powders , which was related to the alteration of long-range lattice periodicity caused by large numbers of dislocations and their related strain fields . However, no phase conversion occurred during the present high-energy milling process. Also, the particle size of the powder mixture was 13.52 μm when high-energy milling was used, compared with 49.52 when ball milling was used (Table 1).
Before the addition of the Mg powder fuel, the thermal properties of the two duck eggshell + A-TiO2 mixtures were studied by TG and DSC techniques. The TG thermograms show that the decomposition of calcium carbonate in both prepared powders occurred in a single step from 600°C onwards (Figures 6(a) and 6(b)). This step was initiated by the release of carbon monoxide (CO) which gave rise to calcium oxide (CaO) . In addition, the mass loss of the powder obtained from the high-energy milling was higher than that of the ball milling, and the loss started and finished at lower temperature ranges as well. The reactant mixture from 30-minute high-energy milling lost mass swiftly between about 620°C and 770°C, whereas the mixture from 30-minute ball milling, underwent most of its mass loss between about 650 and 825°C. This difference is attributable to the larger reactive surface area of the high-energy-milled mixture, which increased the thermal decomposition rate of calcium carbonate.
Reactant powders milled in both conditions produced large endothermic peaks in the DSC curves between 600 and 825°C (Figures 7(a) and 7(b)). This endothermic reaction was produced by the phase transformation of calcium carbonate to calcium oxide and the emission of carbon monoxide , which is consistent with the TG data. After the endothermic peak, an exothermic peak is present at a higher temperature (about 1100°C), and this peak can be attributed to the reaction of calcium oxide with titanium dioxide to form calcium titanate [8, 10, 13, 24]. Furthermore, the endothermic peak of the 30-minute high-energy ball-milled reactant mixture (Figure 7(b)) occurred at a lower temperature and the peak area was smaller than the peak area of the 30-minute ball-milled reactant mixture (Figure 7(a)). This tendency is regulated by energy accumulation within or at the surface of crystals due to the limit of fragmentation of particles . The TG and DSC analyses indicate that complete combustion was promoted by high-energy ball-milled duck eggshell + A-TiO2 when Mg fuel was included in the reaction, which is further discussed in the next section.
The diagram of a previous phase conversion between CaO and TiO2  indicated that the formation of stoichiometric calcium titanate by solid state reaction typically occurred at a temperature above 1450°C after a long heating time. In this work, however, the incorporation of Mg fuel supported the reaction between the solid powders by releasing heat from an exothermic reaction at the melting point (660°C). Consequently, the reactants were simultaneously combusted to completion in a short time.
The XRD patterns of the as-combusted products (ball-milled) indicate the presence of calcium titanate (CaTiO3: ICDD no. 01-074-8732), a by-product of magnesium oxide (MgO: ICDD no. 01-089-4248), unreacted anatase titanium dioxide (A-TiO2: ICDD no. 03-065-5714), and a minor unstable phase of magnesium titanate (MgTiO4: ICDD no. 00-025-1157) (Figure 8(a)). The XRD pattern of the as-leached product synthesized from ball-milled duck eggshell + A-TiO2/Mg (Figure 8(b)) indicates calcium titanate coupled with a large amount of unreacted titanium dioxide and unstable magnesium titanate, whereas the as-leached product synthesized from high-energy-milled duck eggshell + A-TiO2/Mg (Figure 8(c)) presents calcium titanate as a major phase with a small amount of unreacted titanium dioxide. The reason why more calcium titanate appears in Figure 8(c) than Figure 8(b) is that the mechanical activation produced by high-energy milling developed more surface area as well as various sorts of structural defects in the reactant powders, which increased the chemical reactivity of the solid compact.
Typical SEM images of the combustion product before leaching (Figure 9(a)) reveal agglomerated particles which included distinct phases, but, in the images taken after the leaching out of by-products (Figure 9(b)), the fast growth time and cooling rate of the combustion reaction appear to have produced fine particles in a submicron size range.
XRD patterns of the calcium titanate ceramics sintered at 1350°C with different holding times (Figure 10) show that all the calcium titanate ceramic samples had a perovskite phase structure (CaTiO3: ICDD no. 01-074-8732) with an orthorhombic crystal system (space group: Pbnm). However, low amount of secondary phases of titanium dioxide in an anatase structure (A-TiO2: ICDD no. 03-065-5714) is present. The increments of holding time from 60 to 180 min slightly increased the average crystal size (from 47.40 to 47.94 nm) without changing the crystal system.
The density and porosity of the sintered calcium titanate ceramic were approximately dependent on holding time (Figure 11). The plot of sintered density and apparent porosity against holding time shows that longer holding time resulted in calcium titanate ceramics of greater sintered density. Long holding time facilitated tenacious migration of calcium titanate particles and led to closer packing of the calcium titanate particles. On the other hand, the apparent porosity was dramatically reduced by increasing holding time from 60 to 180 min. This result agrees well with the SEM images in Figures 12(a), 12(c), and 12(e). In addition, the SEM micrographs shown in Figures 12(b), 12(d), and 12(f) indicate that the microstructure was also dependent on the holding time. At a holding time of 60 min, a well-sintered sample could not be obtained. At a holding time of 120 min, a relatively uniform grain size of approximately 2-3 μm could be achieved. However, the grain size increased to about 5‐6 μm and exhibited abnormal grain growth when the holding time was increased to 180 min. This occurred because the longer holding time can enlarge the diffusion coefficient and make grain boundary migration easier .
The dielectric constant and dielectric loss of all samples of sintered CaTiO3 ceramic decreased when the frequency was increased (Figures 13(a) and 13(b)). The reduction in the dielectric constant with increased frequency can be attributed to the lagging of the dipoles in the material, which is a typical Debye-type behavior exhibited by most dielectric materials . The available Ti ions on the octahedral sites in calcium titanate were at maximum polarization at low frequencies. As the applied frequency increased, the polarized Ti ions could not respond to the changing frequency and orientation polarization was arrested. As a result, the dielectric constant and the capacitance were reduced [7, 25]. The highest dielectric constant (∼78) and lowest dielectric loss (∼0.02) at a frequency of 200 kHz occurred in calcium titanate, sintered at 1350°C with a holding time of 180 min. These results were attributed to the denser structure of the material and the fewer defects (pores) present in it. The high density of the sample simplified the process of charge polarization, resulting in a larger dielectric constant [17, 26].
In the present work, calcium titanate (CaTiO3) was successfully synthesized by combustion synthesis utilizing duck eggshell, as a source of calcium, mixed with anatase titanium dioxide (A-TiO2) and using magnesium (Mg) as fuel. Based on the experimental results and analysis, the following conclusions are presented:
The as-leached product from 30-minute high-energy milling of duck eggshell + A-TiO2 showed a single perovskite phase of CaTiO3. The high-energy milling promoted the generation of a large reactive surface area and increased the structural defects in the reactant powder. These physical changes drove the combustion reaction to completion in a self-propagating mode and improved the phase conversion of the reactants into the product.
The highest density (3.65 g/cm3) and lowest porosity (0.54%) of the fabricated calcium titanate ceramics were achieved by sintering the as-leached powdered combustion product at 1350°C for 180 min.
The densified calcium titanate ceramics sintered at 1350°C for 180 min also exhibited a high dielectric constant (∼78) with a low dielectric loss (∼0.02) due to the simplification of the charge polarization process.
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This work was financially supported by Faculty of Science, Prince of Songkla University, under the revenue budget for fiscal year 2017. The authors would like to thank Dr. Saowanee Singsarothai for providing the oxy-acetylene set and Assist. Prof. Dr. Pornsuda bomlai for providing the LCR meter. Sincere thanks and appreciation are extended to Mr. Thomas Duncan Coyne for editing and proofreading English in this article.
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