Thesis (Ph. D.)--Oregon Graduate Center, 1982.

Thesis (Ph. D.)--Oregon Graduate Center, 1980.

The thermoelectric properties of lanthanum-doped strontium titanate (LSTO) were investigated for high temperature applications. Ceramics with the formulation La(2x/3)Sr(1-x)TiO(3-delta) (x = 0.1, 0.3, 0.5, 0.7, 0.9) were produced using the conventional mixed oxide route. After 18 hours of milling the powders were calcined at 1373 K for 4 hours and sintered at 1733 K (± 180 K/hour) for 4 hours in air. SEM, XRD, and TEM techniques were employed to characterise the microstructure of the ceramics as well as density measurements. Subsequently the electrical conductivity, Seebeck coefficient, and thermal conductivity in order to determine the thermoelectric figure of merit of the ceramics. Key thermal conductivity results were further investigated using a computational approach. This production method resulted in high quality, high density (> 97 %) ceramics that were mostly single phase determined by XRD with a Pm3m space group, with the exception of the x = 0.9 ceramic that had a Cmmm space group. SEM imaging confirmed this finding and revealed a core-shell structure in x = 0.1 and x = 0.3 ceramics whereby the core was La-rich/Sr-deficient. Thermal conductivity of the ceramics decreased with increasing La content. This was investigated further computationally employing the Green-Kubo method. It was established that the decrease in thermal conductivity was due to phonon-scattering from A-site vacancies, and not from the stabilisation of oxygen vacancies as suggested elsewhere. The electrical properties were dramatically improved through sintering the LSTO ceramics in a 5 % H2 95 % Ar atmosphere. This led to a reduction of Ti4+ to Ti3+. The maximum electrical conductivity increased to 789 S.cm-1 resulting in a power factor of 0.0013 W.m-1 K-2 at 477 K for x = 0.3. A zT of 0.27 at 870 K for x = 0.5 was obtained due to a lower thermal conductivity. The ceramic LSTO x50H was subsequently doped with excess lanthanum; A maximum of 3 % excess was able to be fully incorporated into the lattice. This resulted in a further increase of the electrical conductivity to 875 S.cm-1 at 377 K. A decrease in the lattice thermal conductivity of ∼ 1 W.m-1 K-1 was also achieved due to the oxygen vacancies that were introduced as a result of the reducing sintering conditions. Overall a zT 0.27 at 1016 K was obtained for 3 % excess lanthanum. The ceramic LSTO x50H was also doped with niobium and vanadium. Reduction in the lattice parameter from the La substitution inhibited the niobium from fullyincorporating into the matrix. This was not the case for vanadium that resulted in an electrical conductivity of 144 S.cm-1 , Seebeck coefficient of -106 µV.K-1 , and thermal conductivity of 2.08 W.m-1 K-1 at 308 K, resulting in an overall maximum zT of 0.08 at 1070 K.

The area of advanced gate dielectrics has gained considerable attention recently, and there are significant leakage current and reliability concerns for oxy-nitride in this regime. So it¡¦s an important business to use alternate high-k dielectrics instead of oxy-nitride. Titanium dieoxide shows a high dielectric constant for dielectric applications. Besides, strontium can create additional oxygen vacancies that can enhance dielectric constant. In this study, we prepared SrTiO3 film by liquid phase deposition which is a novel material considered to have high dielectric constant. From several characteristic measurements, we found that SrTiO3 with exhibiting higher dielectric constant and well interface state which is very promising candidates to instead of titanium dieoxide. The physical and chemical properties of SrTiO3 films by means of several measuring instruments, including Fourier transform infrared spectrometer (FTIR), secondary ion spectrometer (SIMS), and X-Ray diffractometer (XRD). An Al / SrTiO3 / Si metal-oxide-semiconductor (MOS) capacitor structure was used for the electrical measurements. To improve the electrical properties, we investigated the characteristics of SrTiO3 films after annealing in oxygen, nitrous oxide, and nitrogen ambient. Including the variations of thickness, structure, dielectric constant, and leakage current were discussed in this work.

Recent interests on the plastic deformation of strontium titanate (SrTiO3) are derived from its unusual ductile-to-brittle-to-ductile transition (DBDT). The transition is divided into three regimes (A, B and C) corresponding to the temperature range of 113 K to 1053 K (-160oC to 780oC), 1053 K to ~ 1503 K (780oC to ~ 1230oC) and ~ 1503 K to 1873 K (~ 1230oC to 1600oC), discovered by Sigle and colleagues in the MPI-Stuttgart. We report the dislocation substructures in (001) single crystal SrTiO3 deformed by Vickers indentation at room temperature, studied by scanning and transmission electron microscopy (SEM and TEM). Dislocation dipoles of screw and edge character are observed and confirmed by inside-outside contrast using g-vector by weak-beam dark field imaging. They are formed by edge trapping, jog dragging and cross slip-pinching off. Similar to dipole breaking off in deformed sapphire (£\-Al2O3) at 1200oC and £^-TiAl intermetallic at room temperature, the dipoles pinch off at one end, and emit a string of loops at trail. Two sets of slip systems {110}<-11 0> and {100}<011> are activated under both 100 g and 1 kg load. The suggestion is that plastic deformation has reached the stage II work hardening, which is characterized by multiplication of dislocations through cross slip, interactions between dislocations, and operating of multiple slip systems. In nanoindentation experiments, it is generally believed that the shear stress at the onset of plasticity can approach the theoretical shear strength of an ideal. Here we report direct evidence that plasticity in a single crystal SrTiO3 can begin at very small forces, remarkably. However, the shear stresses associated with these very small forces is excess the theoretical shear strength of SrTiO3 (16.1 GPa). Our observations entail correlating quantitative load¡Vdisplacement measurements with individual stage microstructure during nanoindentation experiments in a transmission electron microscope. We also report direct evidence that with the prevalent notion that the first obvious displacement excursion in a nanoindentation test is indicative of the onset of plastic deformation. The SrTiO3 deforms elastically before the pop-in depth, but exhibits a plastic-elastic behavior after that. TEM observations reveal that the slip band is the predominant deformation mechanism in SrTiO3 during indentation. The cracks usually initiate at the intersection of slip bands to produce the sessile dislocations with Burger vectors [1-10] (or [110]) along the (110) (or (1-10)) crack plane. In addition, theoretical analysis confirms that the pop-in event is associated with the onset plasticity of SrTiO3. The plastic deformation of (001) single crystal SrTiO3 is investigated using compression along [001] at room temperature. A total plastic strain of ~19+2% is consistently obtained. The stress-strain curve exhibiting four work-hardening stages are describable using the stage 0 of axis rotation, the stage I ¡§easy glide¡¨, the stage II multiple slip and the wall-and-cell structure, and the stage III work softening and dynamic recovery before sample fracture takes place. It is revealed by analyzing the microstructure for each work-hardening stage that the plastic deformation of single crystal SrTiO3 closely resembles that of metals. The primary slip systems of [011](0-11) and [01-1](011) predominate in stage I where plastic deformation occurs by the migration of kink pairs in collinear partial dislocations. The activation of multiple slips including [101](-101) and [10-1](101), and [011](0-11) and [0-11](011) in stage II produces the cell-and-wall structure which is also characteristic of plastically deformed metals. In stage III with decreasing work-hardening rate, the bow-out dislocation interaction from opposite walls results in annihilation. The reaction between dislocations from adjacent walls produces the resultant dislocations with b = [-110] parallel to the load axis [001]. These dislocations are sessile, which eventually leads to sample fracture. We have analyzed the microstructure of <001> SrTiO3 single crystal deformed using compression at room temperature using transmission electron microscopy. A representative stress-strain (£m-£`) curve is established, similar to that for metals it consists of three hardening stages before failure occurs at a strain £` = 19+2%. Dislocation analysis suggests that the primary slip systems in [011](0-11) and [0-11](011) are activated in the £m-£` curve stress plateau region usually addressed as easy glide. Three characteristic features are identified from samples deformed to stage I hardening by easy glide: (a) rectangular glide loops, (b) collinear partials, and (c) kink pairs. Dislocations have predominantly pure edge character. Kink pairs are observed only on the edge segments suggesting that screw dislocations have higher mobility. In easy glide, the migration and annihilation of kink pairs occurring on both the trailing and leading partials lends support to a previous report by Castillo-Rodriguez and Sigle (2011) that dislocation glide is controlled by the long-segment limit of a kink-pair model. Pure edge dislocations are dissociated into collinear partials with b = 1/2[011] (or 1/2[0-11]) by glide in (0-11) (or(011)), and kink pairs are formed on both leading and trailing partials. The suggestion is that in the low-stress regime hardening by dislocation pile-up in stage I is compensated for by kink pair nucleation and migration. The overall hardening rate thus remains unchanged at approximately zero, resembling easy glide in the deformation of metals, over an increasing strain of £` ? 4% before reaching stage II hardening. Microcrack nucleation and propagation behavior in the crack tip was investigated by using transmission electron microscopy (TEM) through compressive test and Vickers indenter. Observation results showed that fracture process was completed in this <001> SrTiO3 single crystal material by connecting dislocations. The crack were nucleated and developed in the dislocation free zone (DFZ) or super thinned area ahead of crack tip under local high stress concentration. The cracks were linked with each other by mutual dislocation emission which expedites the propagation of crack tips effectively. We suggested a dislocation based the Hirsch et al. model of plastic-zone evolution in which dislocations emitted from the crack tip glide away to form a crack-tip plastic zone. Each emitted dislocation reduces the crack tip stress intensity via elastic interactions (the ¡¥¡¥shielding¡¨ effect).

We use the spin polarized pseudofunction(PSF) method with the local-spin-density approximation (LSDA) to calculate the electronic structures and the total and partial charge densities (TDOS, PDOS) of bulk Ba1-xSrxTiO3 (BSTO) with x = 0, 0.25, 0.5, 0.75, and 1. We find that the calculated direct energy gap Eg bows up as a function of the Sr concentration, which is related to the bowling downward of the Ti-O bond length inferred from the experimental lattice constants. The calculated partial densities of unoccupied O-p derived states (PDOS) agree well with the features observed in the O K-edge XANES spectra of BaTiO3 and SrTiO3. The calculated total densities of valence-band states (TDOS) and the valence-band widths for BaTiO3 and SrTiO3 agree well with the UPS spectra.

A proposed design of the variable energy positron annihilation spectroscopy (VEPALS) system based on secondary electron (SE) emission from a thin carbon foil has been investigated practically. The SE yield and the positron transmission coefficient were investigated as a function of the positron beam energy, the annular electrode potentials, and the column lengths of the annular electrode. The positron lifetime spectra of single crystal p-type silicon(Si) sample under different annular electrode potentials were analyzed. The result gives a supposed annular electrode potential of 1.5 kV. In view of this, the positron lifetime spectra were measured under different positron beam energy by fixing the annular electrode potential. It can be seen that all the spectra have the main p-type Si bulk lifetime component of 234 ps occupying more than 60% intensities. The intensity of the 234 ps component reaches up to 84.5 ±1.3 % when the positron beam energy is 15 keV. Further, the origin of the satellite peaks in the positron lifetime spectra are also investigated. It has been shown that the satellite peaks is attributed to the overflowing positrons on the MCP detector. The single crystal strontium titanate (STO) substrates after vacuum annealing treatment have been investigated in detail by several experimental techniques. The crystallization changes induced by the vacuum annealing were investigated by X-ray diffraction(XRD). Secondary phases were occurred after annealing treatment. The measured X-ray photoelectron spectroscopy (XPS) at O1s and C1s core levels were analyzed. The additional peaks after annealing are attributed to hydroxyl species, C-OH compounds, and carbonates. The variable energy Doppler broadening spectroscopy (VEDBS) and the traditional coincidence positron annihilation lifetime spectroscopy (PALS) were used to probe defects in STO samples. For long annealing time samples, the S parameters decrease below the reference level. The S-Wplot suggests that almost the same type of vacancy defects were induced during the annealing treatment. The positron lifetime results suggest that the main defects in annealed samples are oxygen monovacancies or divacancies and Sr-O vacancy complexes. The sample with annealing time of 110h has minimum positron effective diffusion length and maximum average lifetime, which is attributed to the increase of the vacancy-type defects during the long annealing treatment. / published_or_final_version / Physics / Master / Master of Philosophy

Doctor of Philosophy / Department of Chemistry / Kenneth J. Klabunde / A new approach to synthesize nanosized strontium titanate (SrTiO3) and barium titanate (BaTiO3) has been developed. Nanocrystals of mixed metal oxide were synthesized by a modified aerogel procedure from alkoxides. The textural and surface characteristic properties were studied by nitrogen BET analysis, transmission electron microscopy, and powder XRD. The crystallite sizes of aerogel prepared powders can vary from 6 to 25 nm by the use of different solvents. A mixture of ethanol and toluene was found to be the best binary solvent for supercritical drying, which produced a SrTiO3 sample with a surface area of 159 m2/g and an average crystallite size of 8 nm, and a BaTiO3 sample with a surface area of 175 m2/g and an average crystallite size of 6 nm. These titanates have been studied for photocatalytic oxidation of volatile organic compounds and acetaldehyde (CH3CHO) in particular. The big band gaps of the bulk (3.2 eV for SrTiO3 and 3.1 eV for BaTiO3) limit their application to a UV light region only. The modification of titanates by doping with transition metal ions (partial substitution of Ti ions with metal ions) creates a valence band or electron donor level inside of the band gap, narrows it, and increases the visible light absorption. The enhanced adsorption of visible light was achieved by the synthesis of nanosized SrTiO3 and BaTiO3 by incorporating Cr ions during the modified aerogel procedure. Gaseous acetaldehyde photooxidation has been studied on pure SrTiO3 and BaTiO3, and on chromium doped Cr-SrTiO3 and Cr-BaTiO3 under UV and visible light irradiation, and compared with the photoactivity of P25 TiO2. SrTiO3 doped with antimony/chromium shows absorption in visible light and show photocatalytic activity for CH3CHO oxidation. The reason for the codoping of SrTiO3 with Sb/Cr was to maintain the charge balance and to suppress oxygen defects in the lattice. This photocatalyst shows high photoactivity under visible light irradiation even after several continuous runs. The photoactivity under visible and UV light irradiation was almost identical for the Sb/Cr-SrTiO3 photocatalyst. Dielectric properties of aerogel prepared barium titanate samples have being studied and the bulk resistance values of AP-BaTiO3 were significantly lower than that of commercial BaTiO3, by several orders of magnitude.

Low temperature sintering has become a very important research area in ceramics processing and sintering as a promising process to obtain grain size below 100nm. For electronic ceramics, low temperature sintering is particularly difficult, because not only the required microstructure but also the desired electronic properties should be obtained. In this dissertation, the effect of liquid sintering aids and particle size (micrometer and nanometer) on sintering temperature and Positive Temperature Coefficient Resistivity (PTCR) property are investigated for Ba1-xSrxTiO3 (BST) doped with 0.2-0.3mol% Sb3+ (x = 0.1,0.2,0.3,0.4 and 0.5). Different sintering aids with low melting point are used as sintering aids to decrease the sintering temperature for micrometer size BST particles. Micrometer size and nanometer size Ba1-xSrxTiO3 (BST) particles are used to demonstrate the particle size effect on the sintering temperature for semiconducting BST. To reduce the sintering temperature, three processes are developed, i.e. 1 using sol-gel nanometer size Sb3+ doped powders with a sintering aid; 2 using micrometer size powders plus a sintering aid; and 3 using nanometer size Sb3+ doped powders with sintering aids. Grain size effect on PTCR characteristics is investigated through comparison between micrometer size powder sintered pellets and nanometer size powder sintered pellets. The former has lower resistivity at temperatures below the Curie temperature (Tc) and high resistivity at temperatures above the Curie temperature (Tc) along with higher ñmax/ñmin ratio (ñmax is the highest resistivity at temperatures above Tc, ñmin is the lowest resistivity at temperatures below Tc), whereas the latter has both higher ñmax and ñmin. Also, ñmax/ñmin is smaller than that of pellets with larger grain size. The reason is that the solid with small grain size has more grain boundaries than the solid with large grain size. The contribution z at room temperature and high temperature and a lower ñmax/ñmin ratio value.

<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>