Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences; and, (B.S.)--Massachusetts Institute of Technology, Dept. of Biology, 1978. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND LINDGREN. / Includes bibliographical references. / B.S. / M.S.

Ferroelectric and piezoelectric ceramic reinforced metal matrix composites are new materials being explored for vibration damping purposes. The high damping ability of ferroelectric and piezoelectric ceramics such as barium titanate (BaTiO3) and lead zirconate titanate (PZT) is due to the anelastic response of ferroelastic domain walls to applied external stress. In piezoelectric ceramics, vibration energy can also be dissipated through the direct piezoelectric effect if the appropriate electric circuit is connected across the ceramic. In this work we have examined the vibration damping behavior of BaTiO3, nickel-barium titanate (Ni-BaTiO3) composites and nickel-lead zirconate titanate (Ni-PZT) composites. BaTiO3 ceramics were fabricated by a combination of uniaxial pressing and cold isostatic pressing followed by sintering in air. Low frequency (0.1Hz-10Hz) damping capacity of BaTiO3, tanδ has been measured in three-point bend configuration on a dynamic mechanical analyzer. Tanδ has been found to increase with temperature up to the Curie temperature (Tc) of BaTiO3, after which there was a drop in damping capacity values due to the disappearance of ferroelectric domains above Tc. Furthermore within the frequency range tested, tanδ has been found to decrease with increasing vibration frequency. We also observed that tanδ decays with the number of vibration cycles (N). The decrease in tanδ with N, however, is fully recovered if BaTiO3 is heated above the Tc. Ni-BaTiO3 composite composed of a layer of BaTiO3 ceramic sandwiched between two layers of Ni were fabricated using a combination of electroless plating and electroforming. The damping behavior of the composite was analyzed in terms of the damping mechanisms below Tc and the damping mechanisms above Tc of BaTiO3. Below Tc, vibration damping ability of the composite was highly influenced by ferroelastic damping in the BaTiO3 component. Above the Curie temperature, the damping capacity was influence more by the inherent damping mechanisms in the nickel matrix. The damping mechanisms in Ni-PZT composites were evaluated at a low vibration frequency of 1Hz. In these composites we identified ferroelastic domain wall motion as the main damping mechanism active below the Tc of PZT. Using a poled PZT ceramic enhanced the damping capacity of the composite because of favorable ferroelastic domain orientation in the direction of applied stress. Based on our experimental results, we found no evidence of a direct piezoelectric damping mechanism in the Ni-PZT composites. / Ph. D.

Metal matrix composites offer unique opportunities for achieving multi-functionality in materials. In an attempt to investigate the possibility of enhancing damping characteristics of structural metals, copper was reinforced with tetragonal ferroelectric BaTiO3 particulates (Cu-BaTiO3 composites) using powder metallurgy techniques. The effect of particulate size and three processing conditions, sintering atmosphere, cooling rate and, uniaxial compaction pressure on the tetragonality and hence the ferroelectric properties of barium titanate powder were investigated using differential scanning calorimetry (DSC) and x-ray diffraction (XRD). The results show that sintering atmosphere and cooling rates have little effect on the tetragonality of barium titanate powder. Tetragonality of barium titanate powder decreased gradually with decreasing particle size. The decrease in tetragonality with decreasing particle size, however, was only severe in the very fine powders. Although no direct relationship was found between uniaxial compaction pressure and tetragonality, uniaxial pressure may also decrease the tetragonality of barium titanate. Three Cu-BaTiO3 composites, D1, D2 and D3 reinforced with 40vol% barium titanate particles of average sizes 209μm, 66μm and 2μm were respectively fabricated. The retention of the ferroelectric tetragonal phase of barium titanate after composite processing was confirmed by DSC. Composite microstructures observed using optical and scanning electron microscopy revealed uniform dispersions of barium titanate particles in D1 and D2. In D3, the barium titanate formed a chain-like structure because of extensive agglomeration of the fine reinforcement particles. Damping characteristics of the composites were evaluated between 25oC and 165oC at a frequency of 1Hz using dynamic mechanical analysis (DMA). The relative damping capacities (tanδ) in the composites were higher than the unreinforced metal. The damping capacity of composites D1 and D2 was also found to be dependent on temperature. Damping capacity was high from room temperature up to the Curie point of barium titanate, after which there was a slight drop in damping values probably due to a loss in ferroelectric properties. The small drop in damping values recorded in excess of the Curie temperature is an indication that ferroelectricity contributes little to the overall damping capacity of the Cu-BaTiO3 composites. This results from either a reduced ferroelectric damping in barium titanate particles or, poor stress transfer from matrix to reinforcement because of the weak and porous copper-barium titanate interface. / Master of Science

<div>Understanding grain growth in dielectric ceramics is essential to controlling the electrical and mechanical properties necessary to produce ceramic capacitors and sensors. The effect of alloying barium titanate with strontium titanate on the equilibrium crystal shape was investigated in order to determine possible impacts on grain growth. The equilibrium crystal shape was studied through three experimental methods to identify possible changes in grain boundary energy or anisotropy with changing composition.</div><div>The first method was by imaging intergranular pores to observe faceting behavior and relative interfacial energies. Intergranular pores were reconstructed to determine the relative surface energies of the identified facets. The second method was to perform atomic force microscopy on surface facets to collect topography data. The topography data was combined with orientation data obtained by EBSD analysis from the same region, and used to calculate the normal vector of the surface facets. These datasets were plotted in a stereographic projection to study the faceting anisotropy. The third method involved collecting EBSD orientation data and images of surface faceting behavior. The surface faceting behavior of each grain was categorized by type of facet and plotted on a stereographic projection at the corresponding orientation. This allowed for the analysis of faceting transitions and the differentiation of faceted and continuous regions of the equilibrium crystal shape. The analysis of faceting behavior across compositions has implications on grain growth of the barium titanate/strontium titanate system.</div>

Reservoir-type dispenser hollow cathodes are currently being developed for use on NASAs Prometheus 1 mission. In these cathodes, the reaction between a barium source material and tungsten powder contained in a cavity surrounding a porous tungsten emitter produces barium vapor which is crucial to operation of the cathode. The primary objective of this research was to investigate the reactions between tungsten and a commercial barium source material in a simulated reservoir hollow cath-ode environment. Mixtures of tungsten and a barium calcium aluminate material were sealed inside molybdenum capsules with porous tungsten closures and heated to 1000?1200?and 1300?or 100, 200, and 400 hours. Based on the reaction products, which were identified to be BaAl2O4 and Ba2CaWO6, a reaction was proposed for the barium calcium aluminate material with tungsten. The bottom pellets in the capsules were found to have reacted to a much further extent than the top pellets in all of the samples, possibly due to a temperature gradient or excessive moisture in the base of the capsules. Quantita-tive and semi-quantitative x-ray analysis results did not show a clear trend as to how the concentrations of BaAl2O4 and Ba2CaWO6 vary with time. Most of the barium source materials are hygroscopic, and hydration of the materi-als would substantially reduce the performance of the cathode. Therefore, the environ-mental stability of several barium compounds, 3BaO??2O3 (B3A), 6BaO????2O3 (612), 4BaO????O3 (411), Ba2.9Ca1.1Al2O7 (B4ASSL), and Ba3Sc4O9, were investi-gated in order to evaluate their suitability for use as barium source materials. A micro-balance was used to measure weight gain of the materials as they were exposed to dew points of ??C and 11?t room temperature. The results showed that B3A hydrated more extensively than any of the other materials tested in the low- and intermediate-humidity environments, while the 612, 411, and B4ASSL materials were all reasonably stable in the low-humidity environment. The Ba3Sc4O9 was extremely stable compared to the barium aluminates in the intermediate-humidity conditions.

Ion traps are an incredibly versatile tool which have many applications throughout the physical sciences, including such diverse topics as mass spectrometry, precision frequency metrology, tests of fundamental physics, and quantum computing. In this thesis, experiments are presented which involve trapping and measuring properties of Th³⁺. Th³⁺ ions are of unique interest in part because they are a promising platform for studying an unusually low-lying nuclear transition in the 229Th nucleus which could eventually be used as an exceptional optical clock. Here, experiments to measure electronic lifetimes of Th³⁺ are described. A second experimental topic explores the production of sympathetically cooled molecular ions. The study of cold molecular ions has a number of applications, some of which include spectroscopy to aid the study of astrophysical objects, precision tests of quantum electrodynamics predictions, and the study of chemical reactions in the quantum regime. The experiments presented here involve the production of barium monohalide ions, BaX⁺ (X = F, Cl, Br). This type of molecular ion proves to be particularly promising for cooling to the rovibrational ground state. The method used for producing BaX⁺ ions involves reactions between cold, trapped Ba⁺ ions and neutral gas phase reactants at room temperature. The Ba⁺ ion reaction experiments presented in this thesis characterize these reactions for producing Coulomb crystals composed of laser cooled Ba⁺ ions and sympathetically cooled BaX⁺ ions.

This thesis deals with the investigations on the atomic composition, chemical surface states and microstructural features of barium titanate and other electronic ceramics namely barium polytitantes, calcium manganites and magnesium calcium titanate by surface analytical techniques. After presenting a brief introduction on the ceramic materials studied in terms of their crystal structures, electrical properties, nonstoichiometry and interfacial characteristics, the thesis describes the synthesis of the ceramics and the methodology of the different surface analytical techniques utilized such as backscattering spectrometry (BS), an ion beam analysis (IBA) technique, X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDS). The XPS investigations on the chemical surface states of polycrystalline barium titanate having well-defined electrical characteristics reveal the prevalence of Ba in two distinct chemical environments : the one corresponding to the lower binding energy is related to the dielectric while the other having higher binding energy is correlated to semiconducting properties of the ceramics. Processes such as abrasion or polishing make the surfaces more reactive and susceptible to atmospheric contamination. Sputter cleaning causes surface modification leading to changes in the Ba (3d) and Ti (2p) spectra. Studies on the surface atomic composition by BS and microstructural features of doped barium titanate ceramics reveal their interfacial characteristics in terms segregation of dopants or metal ion constituents. Surfaces of these ceramics exhibit cationic as well as anionic nonstoichiometry depending on the processing steps involved. Ceramics synthesized by oxalate precursor route are Ti-rich while those prepared by gel-to-crystallite method are Ba-rich. These are correlated to the chemical processes and background impurities which in turn control the microstructures. Barium titanate substitued with > 1 at. % Mn are deficient in oxygen and exist as the hexagonal polymorph. Acceptors segregate at the grain boundaries accompanied by the enrichment of Ti leading to PTCR or GBLC characteristics. The oxygen nonstoichiometry prevailing in the surface regions of differently processed calcium manganites is investigated by way of depth profile measurements involving 16O(a,a) 16O resonant scattering. These studies reveal extensive compositional heterogeneity across the surface layers particularly in the manganite specimens annealed in lower po2 leading to the stabilization of brownmillerite phase. Two of the microwave dielectric ceramics namely dibarium nona-titanate and barium tetra-titanate with suitable variations in Ba:Ti ratios have been synthesized by the carbonate-gel precipitation. The corresponding dense ceramics have high permittivity (~ 52) and low temperature coefficient of permittivity (TCK ~ 5 ppm /0C). Extensive miscibility between the ilmenite-type MgTiO3 and perovskite-type CaTiO3 over a wide compositional range is brought about by the simultaneous equivalent substitution of Al3+ + La3+. The resulting (Mg1-(x+y)CaxLay)(Ti1-yAly)O3 ceramics exhibit improved microwave dielectric properties by way of high permittivity, low TCK and high quality factor. The microarea elemental distribution and chemical surface state studies reveal the complexity in the Mg/Ca distribution and its correlation with the solid state miscibility as well as dielectric properties. The discontinuous changes in the local site symmetry of the cationic substituents in these ceramics have been investigated by the photoluminescence spectra using Pr3+ as the emission probe.

Numerous organisms produce ornately detailed inorganic structures (often known as shells) with features on length scales from the nanoscale to the microscale. One organism, commonly referred to as a diatom, originates from algae and is found throughout the oceans on Earth. These diatoms possess skeletal structures, frustules, made from silicon dioxide. This chemical makeup limits the number of possible applications for which these structures can be used. Using a series of gas displacement reactions, these frustules can be converted to other useful materials, such as magnesium oxide and titanium dioxide, while maintaining the features of the frustule template. In the current research, silicon dioxide frustules were converted to titanium dioxide replicas using method previously devised by our group. The titanium dioxide replicas were subjected to a hydrothermal reaction by exposing the replicas to an aqueous basic solution containing barium hydroxide to form barium titanate and barium strontium titanate replicas. The effects of reaction temperature, time, and solution composition on extent of conversion were examined. The conventional method of converting titanium dioxide to barium titanate, using a convection heating oven, was compared with a microwave assisted heating method to study the advantages of using microwave heating over convection heating.

With the digital systems moving towards higher frequencies, lower operating voltages and higher power, supplying the required current at the right voltage and at the right time to facilitate timely switching of the CMOS circuits becomes increasingly challenging. The board level power supply cannot meet these requirements directly due to the high inductance of the package interconnections. To overcome this problem, several thin film decoupling capacitors have to be placed on the IC or close to the IC in the package. Two approaches were pursued for high-k thin film decoupling capacitors. 1) Low cost sol-gel based thin film capacitors on organic board compatible Cu-foils 2) RF-sputtered thin film capacitors on silicon substrate for silicon compatible processes While sol-gel provides cost effective technology, sputtered ferroelectric devices are more compatible from manufacturing stand point with the existing technology. Nano-crystalline barium titanate and barium strontium titanate film capacitor devices were fabricated and characterized for organic and silicon substrates respectively. Sol-gel barium titanate films were fabricated first on a bare Cu-foil and then transferred to organic board through a standard lamination process. With process optimization and film doping, a capacitance density of 3 µF/cm2 was demonstrated with breakdown voltage greater than 12V. Leakage current characteristics, breakdown voltages, and electrical reliability of the devices were significantly improved through doping of the barium titanate films and modified film chemistry. Films and interfaces were characterized with high resolution electron microscopy, SEM, XRD, and DC leakage measurements. RF sputtering was selected for ferroelectric thin film integration on silicon substrate. Barium strontium titanate (BST) films were deposited on various electrodes sputtered on silicon substrates. The main focus was to improve interface stabilities for high-k thin films on Si to yield large-area defect-free devices. Effect of bottom electrode selection and barrier layers on device yield and performance were investigated carefully. High yield and high device performance was observed for certain electrode and barrier layer combination. A capacitance density up to 1 µF/cm2 was demonstrated with a breakdown voltage above 15 V on large area, 7 mm2, devices. These two techniques can potentially meet mid-high frequency future decoupling requirements.

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