Geoff L. Brennecka

Electrical conductivity in oxygen-deficient phases of tantalum pentoxide from first-principles calculations

We apply first-principles density-functional theory (DFT) calculations, ab-initio molecular dynamics, and the Kubo-Greenwood formula to predict electrical conductivity in Ta2Ox (0 ≤ x ≤ 5) as a function of composition, phase, and temperature, where additional focus is given to various oxidation states of the O monovacancy (VOn; n = 0,1+,2+). In the crystalline phase, our DFT calculations suggest that VO0 prefers equatorial O sites, while VO1+ and VO2+ are energetically preferred in the O cap sites of TaO7 polyhedra. Our calculations of DC conductivity at 300 K agree well with experimental measurements taken on Ta2Ox thin films (0.18 ≤ x ≤ 4.72) and bulk Ta2O5 powder-sintered pellets, although simulation accuracy can be improved for the most insulating, stoichiometric compositions. Our conductivity calculations and further interrogation of the O-deficient Ta2O5 electronic structure provide further theoretical basis to substantiate VO0 as a donor dopant in Ta2O5. Furthermore, this dopant-like behavior is sp...

Neutron irradiation effects on domain wall mobility and reversibility in lead zirconate titanate thin films

The effects of neutron-induced damage on the ferroelectric properties of thin film lead zirconate titanate (PZT) were investigated. Two sets of PbZr0.52Ti0.48O3 films of varying initial quality were irradiated in a research nuclear reactor up to a maximum 1 MeV equivalent neutron fluence of (5.16 ± 0.03) × 1015 cm−2. Changes in domain wall mobility and reversibility were characterized by polarization-electric field measurements, Rayleigh analysis, and analysis of first order reversal curves (FORC). With increasing fluence, extrinsic contributions to the small-signal permittivity diminished. Additionally, redistribution of irreversible hysterons towards higher coercive fields was observed accompanied by the formation of a secondary hysteron peak following exposure to high fluence levels. The changes are attributed to the radiation-induced formation of defect dipoles and other charged defects, which serve as effective domain wall pinning sites. Differences in damage accumulation rates with initial film qual...

Phase and texture evolution in solution deposited lead zirconate titanate thin films: Formation and role of the Pt3Pb intermetallic phase

Solution deposition is widely used for the fabrication of lead zirconate titanate (PZT) thin films on platinized silicon substrates. However, phase and texture evolution during the crystallization process is not well understood, particularly due to the difficulty in tracking changes in the thin films in situ during heating. In this work, we characterized phase and texture evolution in situ during heating and crystallization of PZT thin films using high-energy X-ray diffraction. Films were pyrolyzed at either 300 °C or 400 °C and heated at various rates between 0.5 °C/s and ∼150 °C/s. For films that were pyrolyzed at 300 °C, the most rapid heating rates first induced strong intensities from a transient Pt3Pb phase. The Pt3Pb phase inherited the texture of the pre-existing platinum layer. Combined with other observations, the results suggest the conversion of the platinum to the intermetallic phase near the interface due to the interdiffusion of lead. In all experimental variations, the pyrochlore phase was...

In situ x-ray diffraction of solution-derived ferroelectric thin films for quantitative phase and texture evolution measurement

An in situ measurement technique is developed and presented, which utilizes x-rays from a synchrotron source with a two-dimensional detector to measure thin film microstructural and crystallographic evolution during heating. A demonstration experiment is also shown wherein the measured diffraction patterns are used to describe phase and texture evolution during heating and crystallization of solution-derived thin films. The diffraction images are measured sequentially while heating the thin film with an infrared lamp. Data reduction methodologies and representations are also outlined to extract phase and texture information from the diffraction images as a function of time and temperature. These techniques and data reduction methods are demonstrated during crystallization of solution-derived lead zirconate titanate ferroelectric thin films heated at a rate of 30 °C/min and using an acquisition time of 8 s. During heating and crystallization, a PtxPb type phase was not observed. A pyrochlore phase was obse...

Tuning the piezoelectric and mechanical properties of the AlN system via alloying with YN and BN

Recent advances in microelectromechanical systems often require multifunctional materials, which are designed so as to optimize more than one property. Using density functional theory calculations for alloyed nitride systems, we illustrate how co-alloying a piezoelectric material (AlN) with different nitrides helps tune both its piezoelectric and mechanical properties simultaneously. Wurtzite AlN-YN alloys display increased piezoelectric response with YN concentration, accompanied by mechanical softening along the crystallographic c direction. Both effects increase the electromechanical coupling coefficients relevant for transducers and actuators. Resonator applications, however, require superior stiffness, thus leading to the need to decouple the increased piezoelectric response from the softened lattice. We show that co-alloying of AlN with YN and BN results in improved elastic properties while retaining some of the piezoelectric enhancements from YN alloying. This finding may lead to new avenues for tu...

Crystallographic changes in lead zirconate titanate due to neutron irradiation

Piezoelectric and ferroelectric materials are useful as the active element in non-destructive monitoring devices for high-radiation areas. Here, crystallographic structural refinement (i.e., the Rietveld method) is used to quantify the type and extent of structural changes in PbZr0.5Ti0.5O3 after exposure to a 1 MeV equivalent neutron fluence of 1.7 × 1015 neutrons/cm2. The results show a measurable decrease in the occupancy of Pb and O due to irradiation, with O vacancies in the tetragonal phase being created preferentially on one of the two O sites. The results demonstrate a method by which the effects of radiation on crystallographic structure may be investigated.

Neutron flux characterization techniques for radiation effects studies

In the field of radiation effects in materials, a detailed and precise description of the radiation environment used to damage samples is often required to make sense of subsequent materials analysis. The types of reactions and extent of damage that occur during irradiation strongly depend on the flux spectrum of the particular facility. Different neutron activation techniques for characterizing neutron flux spectra were performed on the University of Texas at Austin TRIGA research reactor’s in-core facilities. The results were compared in terms of spectral detail and precision. Activation of Au foils with multiple correction factors, and multiple foil activation employing different deconvolution techniques comprise the methods tested.

Micropen Printing of Electronic Components

This chapter explains a direct-write approach for fabricating highly integrated, multilayer components using a Micropen to deposit slurries in precise patterns. Micropen deposition enables printing of multilayer material structures on nonplanar substrates, enabling high-density circuitry with integrated passive components. Furthermore, this chapter presents the technologies instrumental to material integration by the Micropen, including equipment operation, slurry preparation, deposition constraints, and cofiring of multimaterial integrated passives. The four example device elements include: high precision resistors, high-capacitance density dielectrics, integrated inductor coils, and chemical sensors. The direct-write approach provides the ability to fabricate multifunctional, multimaterial integrated ceramic components (MMICCs) in an agile way with rapid turnaround. This technique is used to fabricate devices such as integrated RC filters, multilayer voltage transformers, and other passive components. Thus with advent of new levels of pen tip sophistication, vision control systems, agile materials delivery, and ultralow temperature materials, pen-dispense direct write appears to be most promising route towards high-speed, multimaterial electronics fabrication.

Phonon scattering mechanisms dictating the thermal conductivity of lead zirconate titanate (PbZr1−xTixO3) thin films across the compositional phase diagram

This work represents a thorough investigation of the thermal conductivity (κ) in both thin film and bulk PbZr1–xTixO3 (PZT) across the compositional phase diagram. Given the technological importance of PZT as a superb piezoelectric and ferroelectric material in devices and systems impacting a wide array of industries, this research serves to fill the gap in knowledge regarding the thermal properties. The thermal conductivities of both thin film and bulk PZT are found to vary by a considerable margin as a function of composition x. Additionally, we observe a discontinuity in κ in the vicinity of the morphotropic phase boundary (MPB, x = 0.48) where there is a 20%–25% decrease in κ in our thin film data, similar to that found in literature data for bulk PZT. The comparison between bulk and thin film materials highlights the sensitivity of κ to size effects such as film thickness and grain size even in disordered alloy/solid-solution materials. A model for the thermal conductivity of PZT as a function of com...

Thermal Conductivity of Self-Assembling Symmetric Block Copolymer Thin Films of Polystyrene-Block-Poly(methyl methacrylate)

The thermal conductivities of both disordered and self-assembled symmetric polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) copolymer films were measured using time-domain thermoreflectance (TDTR). The variation in out-of-plane thermal conductivity with changing block copolymer thickness is similar to that of PMMA polymer brushes and thick spun-cast films. The results suggest that the interfaces between the PS and PMMA, and reorganization of the PS and PMMA chains around these interfaces, do not significantly affect the thermal transport in these PS-bPMMA films. However, for thin PS-b-PMMA films, the thermal boundary resistances at the sample interfaces limit the thermal transport. [DOI: 10.1115/1.4031701]