Bio-dielectric based on superconductors yttrium calcium barium copper oxide (YCaBa₂Cu₃O_7−x) from eggshell as calcium oxide source via sol-gel process

The chemical substances – yttrium oxide (Y₂O₃), barium chloride (BaCl₂), copper oxide (CuO), caustic soda (NaOH), and quicklime (CaO) – were supplied by Ajax Finechem Co., Ltd. Waste eggshell was gathered from the local restaurant at the Kasetsart University, Bangkok, Thailand. Dried eggshell was ground to a fine powder using a rapid porcelain mill for 1 h and calcined at the firing temperature of 900 °C for 2 h.

An X-ray diffractometer (XRD; Bruker, AXS analyzer-D8 Discover) with a VANTEC-1 detector and a double-crystal wide-angle goniometer was used to measure the phase composition and crystallinity at a scan speed of 5°/min with increments in 2θ of 0.05° or 0.03° from 10° to 90°, using CuK_α radiation at a λ value of 0.15406 nm. The standard X-ray diffraction patterns were measured according to the international standards (Joint Committee on Powder Diffraction Standards, JCPDS).

A scanning electron microscope (SEM; JEOL, 5200) was used to characterize the microstructures of YCaBCO samples. The samples were coated with gold at a thickness of 0.1 μm to induce electrical conductivity. The electrical voltage was accelerated to 15 kV and subjected to 1,000x magnification.

Fourier transform infrared (FTIR) spectroscopy (Perkin Elmer, Bruker Alpha E) was used to characterize the chemical-functional groups of superconductors in the range 500–4,000 cm⁻¹. The samples were prepared and mixed using potassium bromide (KBr) as a reference substance in the wave number range of 500–4,000.

A simultaneous thermal analyzer (STA; Netzsch 409) was utilized to measure the thermal reaction, decomposition temperature, and thermal stability. The superconductor specimens were measured at the heating rate of 10 °C/min under air conditioned from room temperature (25 °C) to 1,000 °C.

An impedance analyzer (dielectric test fixture: Hewlett Packard 4248A) was used to measure the electrical properties (capacitance, electrical conductivity, dielectric constant, and dielectric loss) in the frequency range of 500 Hz – 1 MHz at room temperature. The obtained powder sample after firing was pressed into a disk-shaped mold with thickness of 0.5–1.0 mm, as shown in Figure 2.

Fig. 2

The formulas of superconducting samples with/without adding calcium oxide (CaO) were prepared to be five formulas via sol-gel process, as indicated by the data in Table 1. The precursor solution was prepared by mixing four solutions by controlling the stoichiometry. Each precursor was prepared from the chemical substances (Y₂O₃, BaCl₂, CuO, and CaO) by dissolving in the solvent. Four types of solvents were used, including water (H₂O), ethyl alcohol (C₂H₅OH), or hydrochloric acid (HCl), before mixing the precursors together by stirring at room temperature following Eq. (1). The stoichiometric ratio of Y₂O₃:BaCl₂:CuO: CaO was 1:2:3:1 based on the molar mass.

The solubility testing of raw materials was done according to Table 2. To make yttrium oxide solution precursor, barium chloride solution precursor, calcium oxide solution precursor, and copper oxide solution precursor, we had to choose a suitable solvent. Yttrium oxide (Y₂O₃) was easily soluble in ethanol but slightly solubility in hydrochloric acid, while barium chloride had excellent soluble in water. Both calcium oxide (CaO) and copper oxide (CuO) were easily soluble in hydrochloric acid. The calcium oxide was obtained from firing eggshell at a firing temperature of 900 °C for 2 h, as shown in Eq. (2). The fired eggshell was mainly constituted by calcium oxide (CaO), which was in the form of soft, fine, white powder and was odorless. The average particle size of the CaO from eggshell was 3.04 μm. The carbon dioxide (CO₂) gas diffused out of the chemical reaction.

Sodium hydroxide (NaOH) was used to adjust the pH and to accelerate gel formation in the third and the fifth formulas. All powder samples had a light green–blue color when they were dried in an oven at 110 °C for 3 days. After that, the dry samples were fired at 900 °C or 1,000 °C for 30 h at a heating rate of 10 °C/min to obtain superconductor samples. The first formula without adding CaO and specifying firing at 900 °C and 1,000 °C yielded products labeled as YBCO-900-ref and YBCO-1000-ref, respectively. The second and fourth formulas were used for sample preparation by adding CaO from eggshell and commercial grade product, respectively. Further, the third and the fifth formulas were used for preparation with CaO from eggshell and commercial grade CaO, along with NaOH, labeled as YCaBCO-NaOH-firing temperature (900 °C or 1,000 °C)-CaO from eggshell or commercial grade product.

The dried precursors were studied based on the thermal reaction to select the suitable firing temperature by using an STA from room temperature (25 °C) to 1,000 °C with a heating rate of 10 °C/min, as shown in Figure 3. The residual mass values of YCaBCO-eggshell and YCaBCO-com grade were analyzed approximately at 75.76% and 71.08%, respectively. In addition, both dried precursor samples with added CaO from eggshell and commercial grade CaO had four sharp peaks and displayed endothermic peaks at 147.3 °C, 315.0 °C, 393.6 °C, and 459.3 °C, respectively. The obtained STA results of Figures 3A and 3B had the same behaviors. In addition, the displayed endothermic peaks caused by the decomposition of the organic matter and other intermediate components of YCaBCO were in the range of 200–600 °C. From the differential thermal analysis (DTA) curve of both samples with/without the addition of CaO, we obtained the suitable temperature for preparation of the superconducting YBCO and YCaBCO with CaO (from eggshell and commercial grade product) at the same range (850–1,000 °C). The thermogravimetric analysis (TGA) results of YCaBCO with added CaO from eggshell and commercial grade CaO showed that it had the approximately 30–40 wt% weight loss at temperatures ranging from room temperature (27 °C) to 1,100 °C. Furthermore, the study by Harabor et al. [19] presented the DSC curves of samples on heating in air and in the temperature range of 20–1,000 °C. Ochsenkühn-Petropulu et al. [20] characterized the temperature to be in the range of 20–1,080 °C and discovered the endothermic reactions caused by the organic decomposition of Y-Ba-Cu salt, BaCO₃, and YBCO at 180 °C, 450 °C, and 850 °C, respectively.

Fig. 3

The characteristics of obtained samples with/without the addition of CaO into YBCO were fine and black-colored powders that were odorless, as shown in Figure 4. The densities of YBCO (without the addition of CaO) and YCaBCO (with CaO from eggshell fired at 900 °C) were 6.87 ± 0.53 g/cm³ and 6.10 ± 0.06 g/cm³, respectively. The theoretical density of the superconductor (YBCO or YBa₂Cu₃O₇) without the addition of CaO was 6.30 g/cm³, and it was characterized as a solid black powder, insoluble in water, with a melting temperature >1,000 °C, and a perovskite structure, as shown in Figure 5 [15].

Fig. 4

Fig. 5

The FTIR spectra of the superconductors with/without the addition of CaO measured in the wave number range of 500–4,000 cm⁻¹ are shown in Figure 6. The YBCO sample without adding CaO was fired at 900 °C for 30 h, and the functional groups were measured by FTIR. YBCO had many small peaks in the range of 500–750 cm⁻¹. Many peaks appeared at 482 cm⁻¹ belonging to Y-Cu-O and Ba-O, around 500 cm⁻¹ for ν (Cu-O), at 528 cm⁻¹ for ν (Cu-O), at 550 cm⁻¹ and 692 cm⁻¹ for ν (Ba-Cu-O), at 565 cm⁻¹ and 588 cm⁻¹ for ν (Y-O) and ν (Y-O-Y), and at 1,420 cm⁻¹ for strong ν (C=O). The obtained FTIR spectra of the YCaBCO samples fired at 900 °C had a peak at 1,250–1,500 cm⁻¹ for ν (C-O) and were close to the FTIR result of YBCO-900-ref. However, the FTIR spectra of the YCaBCO samples fired at 1,000 °C had no peak for ν (C-O) at 1,250–1,500 cm⁻¹. The YCaBCO samples fired at 900 °C and 1,000 °C showed sharp peaks at 1,480–1,600 cm⁻¹ due to ν(C=O) and ν(C=C), respectively. The other peak of the YCaBCO samples fired at 900°C and 1,000 °C was at 3,450–3,500 cm⁻¹ for ν (O-H) due to the water molecule in their structures. Furthermore, Thuy et al. [21] described the FTIR results of YBCO showing a sharp peak at 565 cm⁻¹ for Y₂O₃ and BaCO₃, assigned to ν (Y-O) and ν (Ba-O), respectively, while Ozabaci [22] suggested peaks of YBCO assigned at 588 cm⁻¹, 1,026 cm⁻¹, 1,085 cm⁻¹, and 1,216 cm⁻¹ for ν (Y-O-Y). The strong absorption bands of YBCO located at 591 cm⁻¹ and 701 cm⁻¹ were assigned to the vibration of Cu-O. In addition, Hamadneh et al. [23] reported bands at 1,660 cm⁻¹, 2,382 cm⁻¹, as well as a broad band peak at 2,800–3,100 cm⁻¹, belonging to ν (C=O), ν (C-O), and ν (O-H), respectively.

The XRD peak patterns of the YBCO superconductors with/without CaO addition before and after firing in the temperature range of 10°–90° were characterized as shown in Figure 7. The dried gel samples of YBCO and YCaBCO before firing had XRD peak patterns corresponding with the JCPDS file numbers 00-035-0690 (copper chloride, CuCl₂, monoclinic form), 00-035-1258 (calcium copper chloride hydrate, Cu₃(OH)₆CaCl₂·H₂O, hexagonal form), 00-046-0946 (barium chloride hydrate, BaCl₂·0.5H₂O), and 00-047-0703 (calcium oxide hydrate, tetragonal). The XRD peak pattern of YBCO-900-ref without the addition of CaO corresponded with the JCPDS file peak pattern no. 01-079-1613 of yttrium barium copper oxide (YBa₂Cu₃O₇) with an orthorhombic structure, as shown in Figure 8 [15]. The highest five peaks of YBCO-900-ref appeared as in the YBCO standard pattern at 32.769° (110), 33.111° (103), 46.561° (020), 47.479° (200), and 58.909° (213). The YCaBCO doped with CaO from eggshell and commercial grade product and fired at 900 °C or 1,000 °C showed XRD peak patterns corresponding with the standard peak patterns of JCPDS file numbers 01-082-0268 (Y_0.9Ca_0.1Ba₂Cu₄O₈), 01-084-1853 (Y₂Cu₂O₅), and 01-079-1613 (YBa₂Cu₃O₇). The crystal structures of (Y_0.9Ca_0.1)Ba₂Cu₄O₈, Y₂Cu₂O₅, and YBa₂Cu₃O₇ in Figure 7 had an orthorhombic phase formation, with densities of 6.14 g/cm³, 5.41 g/cm³, and 6.47 g/cm³, respectively; these were close to the obtained experimental density values (6.10 ± 0.06 g/cm³). Furthermore, the phase formation of the superconductor YCaBCO samples, with added CaO from commercial grade product and eggshell and then fired at 1,200 °C, had the XRD peaks of barium oxide (BaO) and YBa₂Cu₃O₆ caused by decomposition of barium chloride to barium oxide. Therefore, the excess firing temperature changed the phase formation of these superconductors. Furthermore, Amoudeh et al. [18] and Ozabaci [22] suggested that the XRD peak patterns of YBCO had polycrystalline structure with orthorhombic phase formation. The asymmetrical orthorhombic structure affected its electrical properties such as inducing polarization at the electromagnetic frequency and increasing oxygen absorption during the critical current density.

Fig. 6

Fig. 7

Fig. 8

The SEM micrographs of CaO from eggshell, YBCO-900-ref, YCaBCO-900-eggshell, YCaBCO-900-com grade, YCaBCO-NaOH-900-eggshell, and YCaBCO-NaOH-900-com grade at a magnification of 1,000× were analyzed and reported as shown in the Figure 9. SEM microstructure of CaO obtained from the fired eggshell showed agglomeration. The obtained superconductor without the addition of CaO (YBCO-900-ref) and fired at 900 °C was an irregular solid black powder. Further, the SEM microstructures of YCaBCO-900-eggshell, YCaBCO-900-com grade, YCaBCO-NaOH-900-eggshell, and YCaBCO-NaOH-900-com grade showed grain connectivity, grain agglomeration, and grain growth, being characterized as a solid black powder; the obtained powder was hard and brittle with a random orientation. Adding CaO from eggshell supported good orientation and rectangular-like grains, and facilitated the filling-in of pores, as shown in Figures 9C and 9E. Furthermore, CaO from the eggshell affected the agglomeration and grain growth of the superconductor YCaBCO (Figures 9C and 9E) more than for the superconductor with added commercial grade CaO (Figures 9D and 9F).

Fig. 9

Table 3 and Figures 10 and 11 exhibit the electrical properties (capacitance, electrical conductivity, dielectric constant, and dielectric loss) of the specimens (raw materials and superconductors with and without the addition of CaO) at electromagnetic frequencies ranging from 500 Hz to 1 × 10⁶ Hz at room temperature (27°C). The electrical conductivity values of CuO, Y₂O₃, BaCl₂, CaO (commercial grade), and CaO (eggshell) at 500 Hz and 27 °C were 1,960.00 ± 18.70 S/m, 236.00 ± 9.04 S/m, 1,200.00 ± 16.10 S/m, 185.00 ± 2.30 S/m, and 8,700.00 ± 38.70 S/m, respectively. The dielectric constants of CuO, Y₂O₃, BaCl₂, CaO (commercial grade), and CaO (eggshell) at 500 Hz and 27 °C were 8.334 ± 0.080, 71.366 ± 2.706, 13.808 ± 0.184, 94.627 ± 1.189, and 1.966 ± 0.009, respectively. The addition of CaO from a commercial grade product or eggshell powder resulted in greater values for the electrical conductivity, capacitance, and dielectric constant of the superconductors, better than in the starting materials, at magnitudes of approximately 10⁴–10⁵, 10², and 10²–10³, respectively. Comparison of the superconductor YBCO with added CaO from commercial grade product and eggshell with the sample of YCaBCO showed that the capacitance and the dielectric constant of the superconductors with added CaO from eggshell were higher than those of samples with added commercial grade CaO. However, the electrical conductivity of these superconductors with added CaO from the eggshell was close to that of the samples with added commercial grade CaO. When the electrical frequency increased from 500 Hz to 1 × 10⁶ Hz, the values of electrical conductivity and dielectric constant decreased—as shown in Figures 10 and 11—and these results were consistent with the polarization theory of materials versus the electromagnetic frequency, as suggested by Waheed et al. [7] (see Figure 12). Therefore, the superconductors YBCO and YCaBCO with/without the addition of CaO from eggshell had electrical conductivity and dielectric constant values close to the theoretical values. On comparison with the electrical properties (electrical conductivity and dielectric constant versus the electromagnetic frequency) of the superconductor YCaBCO, samples doped with CaO from eggshell and commercial grade product showed the same tendency as in theory. In addition, the superconductor YCaBCO with added CaO had an orthorhombic crystal structure that increased oxygen absorption due to the increase in the surface – volume ratio, un-symmetrical structure (a ≠ b), and the critical current density. The superconductor with CaO from eggshell (YCaBCO) had electrical properties (both conductivity and dielectric constant) better than that of the superconductor (YBCO) without added CaO, as shown in Figure 11. Adding sodium hydroxide (NaOH) to the superconductor YCaBCO, to adjust the pH for gel formation as a basic catalyst – for firing at 900 °C or 1,000 °C, increased the electrical properties of their structures, as shown in Figures 10 and 11. Furthermore, all superconductor samples with and without the addition of CaO from either commercial source or eggshell had low loss in their dielectric values, which offers an important advantage over dielectric materials, making them suitable as superconducting materials in various industries.

Fig. 10

Fig. 11

Fig. 12