Development of high energy storage density glass ceramics and investigation of the properties

High energy density capacitors should have a discharge time of microseconds to milliseconds, an operating voltage of several hundred volts to kilovolts, an energy density of more than 1 J/cm3, and an operating temperature of up to 250 °C for specific applications. High energy storage density dielectric capacitors are important energy storage systems that provide high energy density for power electronics and pulsed power applications by giving high electrical energy in a short time. Dielectric constant and dielectric breakdown strength are two important material properties that determine the energy density of a capacitor. At the same time, electrical conductivity and dielectric loss have a significant effect on the energy storage ability of dielectrics. In order to develop high energy storage capacitors, dielectric materials with high dielectric constant, high breakdown strength, low loss and fast response are needed. Traditional ferroelectric ceramics are widely used in capacitors due to their high dielectric constants, but their low breakdown strength and high permanent polarization due to the inherent defects and porosities limit their applications in high energy storage capacitors. Glass-ceramics (GCs) are produced by the melt-quenching process followed by heat treatment to precipitate the ceramic crystalline phase from the glass matrix. The composition of the parent glass and the crystallization conditions controls the final properties of GCs. One of the most important advantages of glass-ceramics is that the microstructure can be perfectly controlled by the crystallization conditions. Thus, desired material properties can be obtained by the homogeneous distribution of high dielectric permittivity phases in glassy matrix. GCs provide enhanced breakdown strength by combining the unique advantages of controlled crystalline size and porosity-free structure. In addition, the crystalline phase can have a higher dielectric constant than the glass matrix in the glass-ceramic system. Today, dielectric glass-ceramics meet the requirements expected from dielectric capacitors with high energy storage density due to their ability to combine the high dielectric constant of the ferroelectric phase and the high breaking strength of the glass matrix (breakdown strength up to 10 MV/cm due to its porosity-free glass microstructure) and low permanent polarization. In this study, strontium/barium niobate-based glass ceramics with the composition of 25Nb2O5·12.5BaO·12.5SrO·35SiO2·5B2O3·5K2O·5Na2O (%mol) were synthesized and the effect of the 0.5%, 1.0%, 1.5% and 2.0% (mol %) La2O3 and 0.5%, 1.0% and 1.5% (mol %) Gd2O3 dopants on the energy density and electrical properties were evaluated. Firstly, amorphous glass materials were produced from stoichiometric chemical composition by melt-quenching technique. Amorphous structure of the materials was confirmed by X-ray diffraction analysis. Thermal analysis studies were carried out to determine the crystallization heat treatment conditions and the crystallization kinetics of the amorphous materials. Peak temperatures obtained from thermal analysis curves were used to determine the crystallization heat treatment parameters and the heat treatments were carried out accordingly to obtain glass-ceramics. Phase analyses, microstructural investigations and density measurements of undoped and La2O3 and Gd2O3 doped GCs were fulfilled. Frequency-dependent dielectric constant, temperature-dependent dielectric constant and polarization-electric field hysteresis loop measurements were carried out to evaluate electrical properties of the GCs. Phase analyses revealed (Sr4-(m-n)Ba(m-n))Ban(K,Na)(2-2n)Nb10O30 as major, NaNbO3 as the minor and (KxBa1-x)(AlySi1-y)4O8 as trace phases were crystallized in the controlled crystallization heat treatments. As a result of the controlled crystallization heat treatment applied to the undoped compositions at 775 °C, amorphous phase was detected along with homogeneously distributed crystalline niobate phases. Upon increased heat treatment time, morphology of the crystalline structures transform into spherical form and abnormal niobate phase growth (block shaped) was observed. As the temperature rises, it was determined that the abnormal grain growth occurs in a short period when compared to the heat treated at lower temperatures and transforms towards to spherical shape with the increase of the treatment time, and the grain size of the formed spheres increases with the temperature. In La2O3 doped samples, 0.5 mol% La2O3 dopant led to the crystallization of block shaped and coarse-grained niobate phases with sharp corner along with the increase of the amount NaNbO3 phase and it was figured out that the geometric form adapted to the sharp cornered structure. As the La2O3 dopant increases, it has been determined that coarse grained structures lose their geometrical forms and transforms into small grained structures, and thus, microstructure exhibits a structure consisting mixture of niobate phases different from homogeneous/mixed sharp-edged structure. As a result of the controlled crystallization heat treatment applied to the Gd2O3 doped compositions at 750 °C, it was figured out that the microstructure consists of block shaped phases and abnormally grown niobate phases. Furthermore, it was determined that the dark gray colored structures surrounding the niobate phases were distributed as matrix. It was determined that Gd2O3 dopant more than 0.5 mol% in the composition let to higher amount and increased size of the block phases. As the crystallization temperature increased, the amount of light-colored niobate phases increased, and the shape of the dark gray colored structures transformed into block shapes. Microstructural analysis revealed that the amount of the NaNbO3 phase increased as the crystallization heat treatment temperature increased. And also, increasing the amount of the dopants promotes the formation of block phase. La2O3 dopant ratios promotes the formation of the block shaped phase structure up to 1.5 mol%. It leads to the formation of the homogeneously distributed shapeless crystalline phases instead of block shaped phases due to its nucleating effect as the dopant amount increased to 1.5 mol% and above. Gd2O3 dopants also promotes the formation of the block shaped phases. Similar behavior has been observed for La2O3 dopant, but compared to Gd2O3 it had less effect on the formation block shaped phase formation, and it increased abnormal phase growth even at low doping ratios. It was determined that the NaNbO3 phase grown as block structure was in a rod-like structure up to 1.5 mol% Gd2O3 doping and the structure became thin and formless when the Gd2O3 dopant amount was 1.5 mol%. It has also been determined that the NaNbO3 phase is usually in the form of cubic shaped blocks in low percent La2O3 doped compositions. It has been determined that the density values of the glass-ceramic materials are in the range of 3.34–3.82 g/cm3 . An increase in the densities and a decrease in apparent porosity of glass-ceramic materials were observed with the addition of La2O3 and Gd2O3. The highest bulk density and the lowest porosity were obtained in 1.5 mol% La2O3 doped compositions. It was observed that the La2O3 doping is more effective on the densification and the density of the materials with these compositions were higher than the Gd2O3 doped compositions. It has been determined that the polarization mechanism of the undoped, La2O3 and Gd2O3 doped materials is space charge polarization due to the decreasing trend in dielectric constant with the increase in frequency. When the variation of dielectric constant value with frequency is analyzed, higher dielectric constant values were obtained in the La2O3 doped compositions compared to the undoped composition due to the increasing amount of dielectric phase crystallized in the structure. Similarly, although high dielectric constant values obtained in Gd2O3 doped compositions compared to the undoped composition, the values are lower than the La2O3 doped compositions. The highest dielectric constant value at 100 Hz frequency was obtained in 2.0 mol% La2O3 doped sample which is the crystallized at 900°C for 3 hours. The dielectric loss for this sample was measured as 0.46. 1.0 and 1.5 mol% La2O3 doped samples that were crystallized at 900 °C for 1 hour and 3 hours have low dielectric loss and high dielectric constant. These materials' dielectric constants are 2.3-2.9 times higher than the undoped samples. The Curie temperature of the undoped composition was determined as approximately 190±3 °C from the variation of the dielectric constant values depending on the temperature at different frequencies. For the samples doped with 0.5, 1.0 and 1.5 mol% La2O3, the Curie temperature was determined as approximately 95±15 °C, 40±20 °C and 20±5 °C, respectively. For the samples doped with 0.5, 1.0 and 1.5 mol% Gd2O3, the Curie temperature was determined as approximately 100±20 °C, 110±5 °C and 125±20 °C, respectively. Polarization vs. electric field (P-E) hysteresis loops revealed that the lowest spontaneous polarization values of the undoped, 1.5 mol% La2O3 doped and 1.0 mol% Gd2O3 doped samples are 0.243, 0.019 and 0.17 µC/cm2 , respectively. Furthermore, the maximum polarization values obtained from P-E hysteresis loops are 1.06 µC/cm2 for undoped, 1.42 µC/cm2 for 1.0 mol% La2O3 doped and 0.82 µC/cm2 for 1.0 mol% Gd2O3 doped samples. It has been determined that La2O3 doped compositions exhibit relaxor ferroelectric behavior with high energy conversion efficiency.