Fundamental Structure-Property Relationships towards Engineering of an Integrated NP0 Capacitor for Bismuth Pyrochlore Systems

Open Access
Author:
Nino, Juan Claudio
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 14, 2002
Committee Members:
  • Xiaoxing Xi, Committee Member
  • William Blaine White, Committee Member
  • Susan E Trolier Mckinstry, Committee Member
  • Michael T Lanagan, Committee Chair
  • Clive A Randall, Committee Chair
Keywords:
  • dispersion analysis
  • crystal structure
  • NP0 capacitors
  • dielectric relaxation
  • low fire ceramics
  • phase refinement
  • Bismuth pyrochlores
Abstract:
A comprehensive investigation of the processing-structure-property-performance interrelationship in the Bi2O3-ZnO-Nb2O5 (BZN) pyrochlore system towards engineering of integrated capacitors with silver electrodes for LTCC (low temperature cofire ceramics) microwave applications is presented. The first area of investigation covers the phase formation for the cubic pyrochlore with nominal composition Bi3/2ZnNb3/2O7 and monoclinic zirconolite Bi2Zn2/3Nb4/3O7 compounds through X-ray diffraction phase analysis. Formation reactions sequences are identified. In both phases, BiNbO4 is found to be a precursor that reacts primarily with ZnO to form the main phases. It is also found that in order to kinetically limit the residual BiNbO4, the calcination and sintering profiles should spend limited time at temperatures between 650°C and 750°C. It is shown that high ZnO activity is essential for the phase formation process and therefore the process is extremely dependent on milling and mixing. In addition, batching with excess ZnO and low pO2 processing atmospheres are demonstrated as alternative processing routes to reduce residual BiNbO4. The solubility of the main BZN phases to ZnO is investigated revealing wide compositional windows. These results serve as a guide for the appropriate modifications in batching and processing required to obtain a BZN composite ceramic exhibiting desirable characteristics for integrated capacitors (temperature coefficient of capacitance (TCC) ~ 15 ppm/°C, mean dielectric constant of 96 and dielectric loss ~ 0.001 at 1 MHz), which can be densified at 950°C with no sintering additives. The second area covers the phase refinement of the main phases in the Bi2O3-ZnO-Nb2O5 pyrochlore system performed via detailed X-ray and neutron diffraction, and transmission electron microscopy (TEM). Crystal symmetry and atomic positions are presented. Bi2Zn2/3Nb4/3O7 is characterized as a monoclinic zirconolite-like structure with space group C2/c and lattice parameters a = 13.1037(9) Å, b = 7.6735(3) Å, c = 12.1584(6) Å, and &#61538; = 101.318(5)°. Essential features in this phase are revealed by neutron diffraction analysis like the sheet like structure based on HTB- (hexagonal tungsten bronze) like layers formed by [Nb(Zn)O6] octahedra and stacked along the c-axis. Analysis of TEM dark field images demonstrate the presence of stacking faults in the Bi2Zn2/3Nb4/3O7 phase parallel to the (00l) planes which is consistent with the HTB sheet-like structure characterized. It is also shown that Zn occupies the central position of the rings in the HTB layers and is displaced from the center, leading to the odd (5+1) coordination with oxygen. This displacement results in two equivalent sites that are half-occupied and introduces disorder in the system. Through studying the closely related Bi2O3-MgO-Nb2O5 pyrochlore system, it is shown that the ability to establish the (5+1) coordination is essential for the thermodynamic stability of the phase. The phase refinement of the cubic pyrochlore phase reveals the presence of free ZnO in the composition Bi3/2ZnNb3/2O7. The correct stoichiometry for the cubic pyrochlore phase is determined as Bi1.5Zn0.92Nb1.5O6.92, space group Fd3m and lattice parameter a = 13.1037(9) Å. Neutron diffraction analysis confirms the multiple and random occupancy of Bi, Zn and vacancy in the A-site, putting an end to the debate regarding the unlikely occupancy of Zn in the A-site. More importantly, it is shown that both the A- and O’-sites are displaced from the ideal pyrochlore positions. There are six equivalent positions for the A-site along the <112> directions, and twelve equivalent positions for the O’-site along the <110> directions. The displacements introduce disorder in the system that leads to a break in local symmetry. Combination of the multiple occupancy and displacement enables the presence of random fields and nonergodicity in the system that ultimately explains the anomalous properties observed in Bi-cubic pyrochlores. Finally, a crystallographic comparison between the cubic pyrochlore and monoclinic zirconolite is presented. The third area investigated corresponds to the dielectric properties (e’ and e’’) of the main phases in the BZN pyrochlore system. A series of different measuring techniques to cover from Hz to THz in the frequency space, and from 12K to 400 K in the temperature space are utilized. Measurements on the Bi2Zn2/3Nb4/3O7 phase reveal a very frequency stable dielectric constant. e’ is found to be approximately 80 throughout most of the measured range, with a TCC value +170 ppm/°C from -55 to 125 °C at ~ 1 GHz, and quality factor Q ~ 500 at ~ 5 GHz. These results confirm the BZN monoclinic zirconolite phase as an extremely appealing candidate for microwave applications. A low temperature (~37 K) anomaly in the dielectric properties is reported and its nature discussed on the basis of the (5+1) coordination by Zn characterized in the phase refinement experiments. Experimental data of the dielectric properties of the BZN cubic pyrochlore phase reveal a temperature and frequency dependent dielectric relaxation with great variation across the frequency-temperature space. For example, it is found that at room temperature and 1 MHz, &#61541;’ ~ 150 and tand = 0.003, but at 10 GHz and room temperature, e’ ~130 and tand = 0.115. TCC at 1MHz = -390 ppm/°C. The main characteristics of the dielectric relaxation are qualitatively discussed. It is shown that despite the dielectric relaxation influencing the dielectric constant and primarily the dielectric loss present in the cubic pyrochlore, the opposing TCC of the monoclinic zirconolite allows the synthesis of a bi-phase composite with a compensated TCC and low loss. The fourth research area presents the collection and analysis of Raman and infrared spectra of the main BZN phases. Utilizing a factor group analysis, the Raman and infrared active phonon modes are identified and assigned. Based on the crystallographic information, a comparison between the spectra of the cubic pyrochlore and the monoclinic zirconolite is presented. Unique features are revealed in both phases with temperature dependence analysis of the spectra. In Bi3/2ZnNb3/2O7, the lowest frequency infrared active phonon mode, corresponding to the O’-A-O’ is found to be main contributor to the dielectric permittivity in the cubic pyrochlore in the microwave regime. Absorption parameters show that this lattice mode corresponds to an overdamped vibration with a temperature dependent damping coefficient. This behavior is explained by random occupancy and multiple equivalent positions in the A- and O’- sites presented in the phase refinement studies. Additional proof for the structural disorder is given by the temperature independence of the mode intensities in the Raman spectra. The absorption parameters are utilized to predict the dielectric properties at microwave frequencies. The base limit for the dielectric loss in BZN cubic pyrochlore is determined and a protocol that can be extended to any material in order to calculate the intrinsic loss arising from phonon losses is demonstrated. A peak narrowing and intensity increase of the active modes Raman spectra of BZN monoclinic zirconolite phase reveals ordering of the structure with decreasing temperature. Dispersion analysis of the Bi2Zn2/3Nb4/3O7 infrared spectra reveal phonon hardening with increasing temperatures. These results are consistent with the crystallographic nature of the phase (HTB layers and (5+1) coordination of Zn) characterized in the phase refinement studies and can account for the temperature dependence of the dielectric permittivity observed at low frequencies. Analysis of the trends in the infrared absorption bands (phonon modes) with compositional changes is performed based on the spectra of four Bi-based cubic pyrochlore systems. All measured trends are found to be in fair agreement with those observed in regular cubic pyrochlores. The fifth area discusses in detail and extensively analyzes the dielectric relaxation phenomenon observed in Bi-based cubic pyrochlores. Characteristics of the 3D nature (frequency-temperature space) of the dielectric loss are outlined. Quantitative analysis of Tm (the temperature at which the maxima in the dielectric loss curve occurs at a given frequency) is reported, and the behavior is successfully modeled with Arrhenius equations. The pre-exponential coefficient in the Arrhenius equation representing the attempt jump frequency governing the phenomenon is found to closely correspond to the resonant frequency for the O’-A-O’ bending mode reported from the vibrational spectroscopy studies, thus relating the crystal structure with the relaxation observed in the dielectric properties. The shape of the dielectric loss (tand) curve is analyzed and a function (J) capable of modeling tan&#61540; throughout the entire frequency-temperature space where the relaxation phenomenon is active is introduced. The J function is modified to include the base dielectric loss L0 as an input primarily based on the dispersion analysis calculations (fourth area) rather than a meaningless parameter of the model. In addition, the influence of excess Bi+3 on the dielectric relaxation is investigated. Limited solubility to Bi2O3 is found in the BZN cubic pyrochlore, as little as 5% mol Bi-excess results in a multiple phase ceramic. Changes in the dielectric properties are explained by mixing rules. To extend this analysis, the Bi+3 solubility in the Bi3/2MgNb3/2O7 cubic pyrochlore is studied. Enhanced dielectric constant and shift in Tm to higher temperatures are found with increasing Bi-excess. A strong correlation between lattice strain and Tm shift is demonstrated, and solid solution behavior of the A-site in the cubic pyrochlore structure is confirmed. Finally, bringing together all the evidence collected from the different characterization techniques employed in the areas investigated, it is concluded that the relaxation is associated with the following sources: nonergodicity, random fields, lone-pair electrons, site disorder, break of local symmetry, site solubility, and overdamped phonon modes. As a result a self consistent schematic of the mechanism responsible for the dielectric relaxation in Bi-pyrochlores is proposed. In the sixth and final area, several issues regarding the manufacturing of BZN based NP0 (negative, positive, but almost zero temperature coefficient of capacitance) capacitors are covered: The appropriate composition leading to composite exhibiting NP0 characteristics is presented. Densification curves for BZN ceramics are determined and analyzed on the basis of the phase formation reactions. The compatibility of BZN ceramics with silver electrodes is investigated. A reaction between BiNbO4 and Ag/AgO forming a silver based pyrochlore is identified. It is demonstrated that if there is any residual BiNbO4 in the BZN composite, the cofirability with silver electrodes is hindered. More importantly however, it is confirmed that if phase purity is ensured in BZN pyrochlore ceramics, then successful cofiring with silver electrodes is attainable. This is demonstrated by building and testing prototype multilayer capacitors. Several electrode pastes are tested. It is shown that a pure non-glass Ag conductor provides the best results in terms of density, electrode continuity, no delamination and no ceramic-electrode interaction. In addition, the compatibility of BZN ceramics with LTCC technology and silver electrodes is further exemplified by the successful cofiring of an embedded BZN NP0 capacitor with a commercial LTCC system utilizing silver as internal electrodes. These prototypes serve to the electronics industry as encouragement to incorporate Bi-based pyrochlores and in particular BZN dielectrics to their commercial applications.