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). )e eggshell and A-TiO2 were milled for 30min in either a high-energy planetary mill or a conventional ball mill. )ese 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 30min before synthesis comprised a single perovskite phase of CaTiO3. )e 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 180min, achieved a maximum density of 3.65 g/cm and a minimum porosity of 0.54%. )e 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.


Introduction
e alkaline earth titanate, calcium titanate (CaTiO 3 ), 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 [1].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 organicinorganic solution [2][3][4][5].High-temperature solid state synthesis of CaTiO 3 has been conducted using mixtures of calcium carbonate (CaCO 3 ), calcium oxide (CaO), and titanium dioxide (TiO 2 ) [6][7][8][9] and also mechanoactivated synthesis has been carried out with various mixtures of CaCO 3 , calcium hydroxide (Ca(OH) 2 ), CaO, and TiO 2 [10][11][12][13][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.e 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 [15].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.e objective of the present work was to synthesize calcium titanate (CaTiO 3 ) by combustion synthesis using powdered duck eggshells and anataste titanium dioxide (A-TiO 2 ) as starting materials and magnesium (Mg) as fuel.e duck eggshell and A-TiO 2 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.

Combustion Synthesis of Calcium Titanate Powder.
Duck eggshells, received from Trang province in ailand, provided a source of calcium carbonate (CaCO 3 ).XRD analysis in Figure 1 confirmed that the as-received duck eggshell was composed entirely of calcium carbonate (CaCO 3 : ICDD no.00-005-0586).
Anatase titanium dioxide (A-TiO 2 , 99.5%, <45 μm, Degussa AG) was used as a reactant because an anatase phase is an intermediate state of TiO 2 , which implies that A-TiO 2 may be highly reactive [14].In addition, magnesium (Mg, 99%, ∼45 μm, Riedel-Detlaen) was used as fuel.e 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-TiO 2 were high-energy-planetarymilled (Fritsch GMBH, Pulverisette 6, Germany) with an Si 3 N 4 vial and ball for 30 min at 400 rpm. is procedure was adapted from the work of Palaniandy and Jamil [12].e eggshell and A-TiO 2 was also milled in a conventional ball mill to obtain an alternative reactant powder for comparison.
e 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 (ZrO 2 ) ball.With a hydraulic press (Huat Seng, 1939-15T, ailand), 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)).e experimental set-up was schematically represented in a previous study [16] that used an oxy-acetylene flame as the external heat source.
e magnesiothermic reaction that took place within the reactant compact of high-energy-milled duck eggshell + A-TiO 2 /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. e reactant compact of ball-milled duck eggshell + A-TiO 2 /Mg, however, had to be continually provided with heat from the oxyacetylene flame because the system could not produce a selfsustained combustion reaction.e inability of this reactant compact to sustain self-propagated combustion was due to the low surface area of the duck eggshell + A-TiO 2 mixture.
Being cooled to room temperature, the as-combusted products (Figure 2(b)) were mechanically pestled into friable powders in a ZrO 2 mortar.In order to remove the by-products of magnesium oxide (MgO) and magnesium titanate (MgTiO 4 ), the as-combusted powders were leached using the 2 M HCl solution for 120 min under moderate stirring.e 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.

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

Characterizations.
e mean particle size of the duck eggshell and A-TiO 2 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-TiO 2 -Mg system, the prepared mixed reactant powders were tested using simultaneous thermal analysis (STA, 449 F3, Jupiter, Netzsch, Germany) in TG and DSC modes.e samples were heated from 30 to 1300 °C at a rate of 10 °C/min in the N 2 atmosphere.e 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 CuK α radiation (0.15406 nm).e average crystallite size was calculated from the Debye-Scherrer equation [18]: where D is the crystallite size, λ is X-ray the wavelength, β is the full width at half maximum (FWHM) of the peak in radians, and θ is Bragg's angle.

Density and Porosity
Measurement.e 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 D sintered is the sintered density, P is the apparent density; W 0 , W 1 , W 2 are the dry weight, weight in water of the water-saturated specimen, and weight in air of the watersaturated specimen, respectively; and ρ water is the density of water (1 g/cm 3 ) [19].

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.e 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.e dielectric constant was calculated according to the following equation [9]: where ε r is the dielectric constant, C is the equivalent parallel capacitance obtained from the data of measurement, d is the thickness of the fabricated ceramic material, A is the combined surface electrode area of the electrode discs, and ε 0 is the permittivity of vacuum (8.85 × 10 −12 F/m).e dielectric loss obtained from the value of the dissipation factor can be calculated from by the following equation [20]: where ε i is the dielectric loss, D is the dissipation factor obtained from data of measurement, and ε r is obtained from equation ( 5).

Results and Discussion
Figure 5 shows the XRD patterns of duck eggshell + A-TiO 2 mixed by ball-and high-energy milling.e diffraction peaks of the high-energy-milled sample have lower intensity and broader bases than the peaks of the ball-milled sample.e main cause of the difference between the two diffraction patterns was the alteration to the crystal structure of the TiO 2 and CaCO 3 caused by the more intensive grinding process of the high-energy milling process.
e reduction of peak intensities implied the formation of a partially amorphous structure in the highenergy-milled powders [12], which was related to the alteration of long-range lattice periodicity caused by large numbers of dislocations and their related strain fields [21].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-TiO 2 mixtures were studied by TG and DSC techniques.e 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)). is step was initiated by the release of carbon monoxide (CO) which gave rise to calcium oxide (CaO) [22].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.e reactant mixture from 30-minute high-energy milling lost mass swiftly between about 620 °C and 770 °C, whereas the mixture from 30minute ball milling, underwent most of its mass loss between about 650 and 825 °C. is 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)). is endothermic reaction was produced by the phase transformation of calcium carbonate to calcium oxide and the emission of carbon monoxide [23], 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 ballmilled 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)). is tendency is regulated by energy accumulation within or at the surface of crystals due to the limit of fragmentation of particles [21].e TG and DSC analyses indicate that complete combustion was promoted by highenergy ball-milled duck eggshell + A-TiO 2 when Mg fuel was included in the reaction, which is further discussed in the next section.e diagram of a previous phase conversion between CaO and TiO 2 [8] 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.Advances in Materials Science and Engineering e XRD patterns of the as-combusted products (ballmilled) indicate the presence of calcium titanate (CaTiO 3 : ICDD no.01-074-8732), a by-product of magnesium oxide (MgO: ICDD no.01-089-4248), unreacted anatase titanium dioxide (A-TiO 2 : ICDD no.03-065-5714), and a minor unstable phase of magnesium titanate (MgTiO 4 : ICDD no.00-025-1157) (Figure 8(a)).
e XRD pattern of the asleached product synthesized from ball-milled duck eggshell + A-TiO 2 /Mg (Figure 8(b)) indicates calcium titanate coupled with a large amount of unreacted titanium dioxide and unstable magnesium titanate, whereas the asleached product synthesized from high-energy-milled duck eggshell + A-TiO 2 /Mg (Figure 8(c)) presents calcium titanate as a major phase with a small amount of unreacted titanium dioxide.e 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 (CaTiO 3 : 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-TiO 2 : ICDD no.03-065-5714) is present.
e 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.e density and porosity of the sintered calcium titanate ceramic were approximately dependent on holding time (Figure 11).
e 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.is 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.is occurred because the longer holding time can enlarge the diffusion coefficient and make grain boundary migration easier [18].e dielectric constant and dielectric loss of all samples of sintered CaTiO 3 ceramic decreased when the frequency was increased (Figures 13(a) and 13(b)).e 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 [9].e 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].
e 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.
ese results were attributed to the denser structure of the   Advances in Materials Science and Engineering material and the fewer defects (pores) present in it.e high density of the sample simplified the process of charge polarization, resulting in a larger dielectric constant [17,26].

Conclusions
In the present work, calcium titanate (CaTiO 3 ) was successfully synthesized by combustion synthesis utilizing duck eggshell, as a source of calcium, mixed with anatase titanium dioxide (A-TiO 2 ) and using magnesium (Mg) as fuel.Based on the experimental results and analysis, the following conclusions are presented: e as-leached product from 30-minute high-energy milling of duck eggshell + A-TiO 2 showed a single perovskite phase of CaTiO 3 .e high-energy milling promoted the generation of a large reactive surface area and increased the structural defects in the reactant powder.ese physical changes drove the combustion reaction to completion in a self-propagating mode and improved the phase conversion of the reactants into the product.e highest density (3.65 g/cm 3 ) 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.
e 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.

Figure 1 :
Figure 1: XRD pattern of as-received duck eggshell powder used in this work.

Figure 2 :
Figure 2: Digital photographs of a typical (a) green pellet of the reactants and (b) as-combusted product.

Figure 3 :Figure 4 :
Figure 3: Snapshot images illustrating the various stages of combustion reaction within reactant compact of high-energy-milled duck eggshell + A-TiO 2 /Mg carried out in air.(a) ignition, (b) self-combustion, (c) relaxation, and (d) final product.

Figure 11 :
Figure 11: Effect of holding time on the sintered density and porosity of calcium titanate ceramics sintered at 1350 °C in air.

Figure 12 :
Figure 12: SEM images of the microstructure of calcium titanate ceramics sintered at 1350 °C in air represent fracture and top surface with different holding times (a, b) 60, (c, d) 120, and (e, f ) 180 min.

Figure 13 :
Figure 13: Variation of dielectric constant ε r (a) and dielectric loss ε i (b) as a function of frequency with different holding times.

Table 1 :
Mean particle size of mixed duck eggshell + A-TiO 2 obtained from different milling processes.