Since BaTiO3 is known to possess a direct bandgap [19], it is assumed that all the samples are direct band gap materials, although Tauc plots for indirect bandgap data are also reported. To determine if these samples are direct band gap or indirect band gap some additional optical absorption experiments are required. The Tauc plot, raw direct band gap data, is shown in Figure 17. Direct bandgap materials have electrons that can emit a photon because the crystal momentum of electrons and holes is the same in the conduction and valence band [7]. The band gap energy indicates the excitation energy required for an electron to jump from the valence band to the conduction band. Using the Tauc method, the band gap energy was calculated for each sample, and the electron transition value is ½ for the direct bandgap data, and 1 for the indirect bandgap data [19].
The linear portion of the spectra of each sample is extrapolated, and the interception of the x-axis indicates the Eg, which varies by sample.
30
Figure 17: UV-Vis-NIR spectra of samples with ZnW dipole concentration as the identifier.
The activation energy was determined from the Arrhenius plot in Figure 16, and the bandgap energy from Figure 17, which are listed in Table 7. Theory states that twice the Ea should be equivalent to the Eg of that material [7]. The data are not consistent with this relation and may be due to the nature of the measurements. Resistivity is assumably measured through the bulk of the material, although it desires to travel the path of least resistance. From this behavior, the resistivity was most likely measured through the grain boundaries of the material. The activation energy was calculated from the high
temperature resistivity data. As expected for the same crystal structure, the activation energy increases as a function of dipole concentration. UV-Vis measures the bulk of the material, does not see the grain boundary, which is much smaller than the wavelength of light, and is a more accurate representation of the band gap energy.
0 50 100 150 200 250
1 2 3 4 5 6
Reflectance (cm-1eV)1/2
Energy (eV)
0.0000 0.00025 0.0013750 0.002500
0.013750 0.02500 1.0000
31
Table 7: Band gap energies from UV-Vis and resistivity plots.
ZnW Tauc Direct Eg
(eV) Tauc Indirect Eg
(eV) Arrhenius Eg (eV)
0.000000 2.92 2.16 1.89
0.000250 2.99 2.46 1.57
0.001375 2.56 2.28 1.71
0.002500 2.34 1.64 2.35
0.013750 2.73 1.85 2.51
0.025000 2.96 2.04 2.49
1.000000 3.40 2.16 1.59
From Table 7, the double perovskite has the highest band gap energy, therefore is the best insulating material among the lot, so it is more difficult to excite electrons from the valence band to the induction band. For the direct band gap values, there is a trending decrease in energy until the BTZW0.01375 sample, and the band gap energy begins to increase. This may be due to the samples mimicking the properties of the parent materials which they most resemble. Shown in the XRD data as well, the samples with the lower dipole dopant concentrations have similar properties to the pure BaTiO3 and the higher dopant concentrations tend to behave like the double perovskite.
32
Conclusion and Future Work
From these experiments, it has been identified that adding dipoles in small
concentrations can have an impact on the structural, electrical, and dielectric properties of BaTiO3. The dipoles insert a surplus of energy into the lattice of the material by
substitution of Ti4+ pairs, causing the volume to increase and structure to change to lower symmetry. The dipoles are associated with an increase in relative permittivity at low concentrations, which may allow these materials to be made at a smaller volume and possess a higher energy density as pure BaTiO3 assuming the breakdown strength of the material is unchanged from that of BaTiO3. Although, adding dopants in the form of dipoles increases loss factor by about two magnitudes higher than the pure BaTiO3 for the BTZW0.025 and BTZW0.01375 samples, and both have a higher relative permittivity.
The Zn2+-W6+ dipoles also changed the bandgap of these samples, to first decrease and then increase to become more insulating as the concentration of dipoles increased.
This work is promising, and there are many ways to improve the fundamental understanding of how dipole substitutions can affect semiconductors. Future work with these specific materials includes high temperature XRD to differentiate Tm from Tc in the relative permittivity data, sintering at various temperatures to prevent the crystallization of the high dipole concentration samples, and increasing the density and purity of the materials through better lab processing. Also, performing optical absorbance experiments to determine if these materials are indirect or direct band gap would enable improved data interpretation and analysis. It would be intriguing to perform this study at different dipole concentrations as well, for comparison and better understanding of the material behavior.
Overall, this thesis was helpful in determining the effect that substituting incrementally small amounts of dipoles have on BaTiO3 material properties.
33 References
1. Lokhande, Prasad Eknath, et al. “Materials and Fabrication Methods for
Electrochemical Supercapacitors: Overview.” Electrochemical Energy Reviews, vol. 3, no. 1, 2019, pp. 155–186., doi:10.1007/s41918-019-00057-z.
2. Borah, R., et al. “On Battery Materials and Methods.” Materials Today Advances, vol. 6, 2020, p. 100046., doi:10.1016/j.mtadv.2019.100046.
3. Integer Battery Chemistry Comparison Chart, Integer. 2018.
4. Zhu, W.z, and S.c Deevi. “A Review on the Status of Anode Materials for Solid Oxide Fuel Cells.” Materials Science and Engineering: A, vol. 362, no. 1-2, 2003, pp. 228–239., doi:10.1016/s0921-5093(03)00620-8.
5. Bove, Roberto. “Solid Oxide Fuel Cells: Principles, Designs and State-of-the-Art in Industries.” Recent Trends in Fuel Cell Science and Technology, 2007, pp.
267–285., doi:10.1007/978-0-387-68815-2_11.
6. Poonam, Sharma, K., Arora, A., & Tripathi, S. K. “Review of supercapacitors:
Materials and devices.” Journal of Energy Storage, vol 21, 2019, pp. 801–825.
doi:10.1016/j.est.2019.01.010
7. Fulay, Pradeep P., and Jung-Kun Lee. Electronic, Magnetic, and Optical Materials. Taylor & Francis, CRC Press, 2017.
8. R. D. Shannon, "Revised Effective Ionic Radii and Systematic Studies of Interatomie Distances in Halides and Chaleogenides ",Acta. Cryst., 1976.
9. V. L. Miller & S. C. Tidrow (2015) Perovskites: “’Effective’ Temperature and Coordination Dependence of 38 Ionic Radii”, Integrated Ferroelectrics, 2015, pp.
30-47. doi:10.1080/10584587.2015.1092196
10. Veerapandiyan, V., "Inducing Diffuse Phase Transitions in Barium Titanate Using Ga3+-Ta5+ Dipole Pair Substituents"; Master of Science Thesis. Alfred University, New York State College of Ceramics, 2017.
11. “What Is the Mechanism of the Changing of the Capacitance of Ceramic Capacitors over Time?” Murata Manufacturing Co., Ltd., www.murata.com/en- global/support/faqs/capacitor/ceramiccapacitor/char/0013.