Exploring the fundamental contributions to the dielectric properties of LiNbO3 and NaNbO3 glass-ceramics with microwave frequency measurements
Restricted (Penn State Only)
- Author:
- Gerace, Katy
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 05, 2023
- Committee Members:
- John Mauro, Program Head/Chair
Susan Trolier-McKinstry, Major Field Member
Michael Lanagan, Outside Unit & Field Member
John Mauro, Chair & Dissertation Advisor
Clive Randall, Major Field Member - Keywords:
- glass
glass-ceramics
dielectric properties
microwave frequency - Abstract:
- With the advent of 5G cellular communications and 6G on the horizon, wireless communication networks are shifting to higher frequencies in an effort to accommodate faster data transfer speeds and higher connection densities. The transition to mm-wave frequencies (30-300 GHz) requires fundamental research into dielectric properties of materials as temperature stable permittivities and low loss become critical properties for high frequency applications. One class of materials that show promise for mm-wave frequency applications is glass-ceramics. Glass-ceramics exploit the processing benefits of the liquid-state formability of glass with the symmetry dependent properties of the ceramic. Glasses offer a wide compositional range, and crystallization temperatures are significantly lower than traditional ceramic sintering temperatures, making glass-ceramics advantageous options for low temperature co-fired ceramics. While glass-ceramics offer an advantageous processing route for dielectrics, much work is still needed to characterize and optimize their dielectric properties at mm-wave frequencies. Because of their composite nature, the bulk properties of glass-ceramics depend on the properties of the glass phase and the ceramic phase, requiring fundamental characterization of both phases and how their interrelationship impacts bulk dielectric properties. In this work, crystallization of LiNbO3 in a 35SiO2-30Nb2O5-35Li2O mol% composition, and crystallization of LiNbO3 and NaNbO3 in a 35SiO2-30Nb2O5-25Li2O-10Na2O mol% composition are examined. Crystallization kinetics are analyzed using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory where the Avrami exponent, n, was calculated to be 1.0-1.5. Microscopical analysis shows dendritic morphology, which when combined with the JMAK analysis, suggests diffusion-controlled one-dimensional growth. Adding Na2O to the glass composition increases the inter-diffusivity of ions which causes LiNbO3 to crystallize at faster rates compared to the single alkali composition. Time-temperature-transformation diagrams are presented which show that the temperature for maximum rate of transformation for LiNbO3 is ~650 ˚C and for NaNbO3 is ~715 ˚C. This work demonstrates that LiNbO3 transformation in niobiosilicate glass-ceramics can be controlled through heat treatments as well as initial composition of the glass as alkali modifiers such as Na2O enhance inter-diffusivity in the glass network and increase rate of crystallization. In addition to understanding the crystallization process in glass-ceramics, another important consideration is the properties of the parent glass. Glass dielectric properties are examined for a set of niobiosilicate glass compositions (100-2x)SiO2-xNb2O5-xLi2O where x = 32.5, 30, 25, and 15 mol% and a 0 mol% Nb2O5 glass with a 70SiO2-30Li2O mol% composition. Permittivity measurements at 10 GHz are used to calculate the polarizability of each constituent oxide in the glass network using the Clausius-Mossotti equation. The SiO2 polarizability in niobiosilicates was found to be 6.16 ± 0.15 Å3, which is much higher than the SiO2 polarizability in fused silica glass (5.25 Å3), alkali modified silicates (5.37 ± 0.04 Å3) and aluminosilicates (5.89 ± 0.18 Å3). The increasing trend in SiO2 polarizability is attributed to the disruption in the connectivity of the SiO4 tetrahedral network as it accommodates different network formers. The high SiO2 polarizability of 6.16 Å3 accurately predicts measured dielectric permittivity when Nb2O5 = 25, 30, and 32.5 mol%, but overpredicts the measured permittivity when Nb2O5 = 15 mol% and 0 mol%. These results are coupled with infrared spectroscopy from 3-20 THz which show a decrease in resonance frequency of the Si-O-Si bending mode with increasing Nb2O5 content in the glass. This suggests that at high Nb2O5 amounts, there is more corner sharing between SiO4 tetrahedra and NbO6 octahedra which causes a high SiO2 polarizability of 6.16 Å3, but as Nb2O5 decreases, SiO4 tetrahedra corner share with other SiO4 tetrahedra and more closely resemble an alkali modified silicate network with a lower SiO2 polarizability of 5.4 Å3. Combining crystallization kinetics with glass dielectric properties, the properties of LiNbO3 and LiNbO3-NaNbO3 glass-ceramics are modeled using the Maxwell-Garnett mixing rule. Permittivity of the glass-ceramics was measured at low frequencies (1kH-1MHz) where space-charge polarization dominates the dielectric properties. But at GHz frequencies, dielectric permittivity is reduced to ionic and electronic polarization contributions. In the LiNbO3 glass-ceramic, measured permittivity decreases as volume fraction of LiNbO3 increases due to the decreasing permittivity of the glass phase as Li2O and Nb2O5 are removed, creating a glass matrix that is more “SiO2 rich.” This is also seen in the LiNbO3-NaNbO3 glass-ceramic when only LiNbO3 is crystallized. However, once NaNbO3 begins to crystallize, the permittivity increases, showing a nonmonotonic behavior that is crystal phase dependent. In order to capture the bulk properties of glass-ceramics, a Modified Maxwell-Garnett (MMG) mixing formula is proposed which takes into account the dynamic properties of the glass phase as crystallization occurs. The results of this work show that crystallization impacts the chemistry of the residual glass matrix which influences many properties such as the dielectric permittivity. Future work on glass-ceramics must account for the dynamic nature of the glass phase to design systems with the desired bulk properties.