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).

The Advanced Fuel Cycle Initiative (AFCI) is a Department of Energy (DOE) program, that has been investigating technologies to improve fuel cycle sustainability and proliferation resistance. One of the program's goals is to reduce the amount of radioactive waste requiring repository disposal. Cesium and strontium are two primary heat sources during the first 300 years of spent nuclear fuel's decay, specifically isotopes Cs-137 and Sr-90. Removal of these isotopes from spent nuclear fuel will reduce the activity of the bulk spent fuel, reducing the heat given off by the waste. Once the cesium and strontium are separated from the bulk of the spent nuclear fuel, the isotopes must be immobilized. This study is focused on a method to immobilize a cesium- and strontium-bearing radioactive liquid waste stream. While there are various schemes to remove these isotopes from spent fuel, this study has focused on a nitric acid based liquid waste. The waste liquid was mixed with the bentonite, dried then sintered. To be effective sintering temperatures from 1100 to 1200 degrees C were required, and waste concentrations must be at least 25 wt%. The product is a leach resistant ceramic solid with the waste elements embedded within alumino-silicates and a silicon rich phase. The cesium is primarily incorporated into pollucite and the strontium into a monoclinic feldspar. The simulated waste was prepared from nitrate salts of stable ions. These ions were limited to cesium, strontium, barium and rubidium. Barium and rubidium will be co-extracted during separation due to similar chemical properties to cesium and strontium. The waste liquid was added to the bentonite clay incrementally with drying steps between each addition. The dry powder was pressed and then sintered at various temperatures. The maximum loading tested is 32 wt. percent waste, which refers to 13.9 wt. percent cesium, 12.2 wt. percent barium, 4.1 wt. percent strontium, and 2.0 wt. percent rubidium. Lower loadings of waste were also tested. The final solid product was a hard dense ceramic with a density that varied from 2.12 g/cm3 for a 19% waste loading with a 1200 degrees C sintering temperature to 3.03 g/cm3 with a 29% waste loading and sintered at 1100 degrees C. Differential Scanning Calorimetry and Thermal Gravimetric Analysis (DSC-TGA) of the loaded bentonite displayed mass loss steps which were consistent with water losses in pure bentonite. Water losses were complete after dehydroxylation at ~650 degrees C. No mass losses were evident beyond the dehydroxylation. The ceramic melts at temperatures greater than 1300 degrees C. Light flash analysis found heat capacities of the ceramic to be comparable to those of strontium and barium feldspars as well as pollucite. Thermal conductivity improved with higher sintering temperatures, attributed to lower porosity. Porosity was minimized in 1200 degrees C sinterings. Ceramics with waste loadings less than 25 wt% displayed slump, the lowest waste loading, 15 wt% bloated at a 1200 degrees C sintering. Waste loading above 25 wt% produced smooth uniform ceramics when sintered >1100 degrees C. Sintered bentonite may provide a simple alternative to vitrification and other engineered radioactive waste-forms.

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.

The geochemical analysis of bioapatite in vertebrate skeletal tissues is an important tool used to obtain ecological information from fossil animals. An important consideration when conducting stable isotope and trace element analyses is obtaining biogenic information that has been unaffected by diagenetic processes. A two-step pretreatment procedure is commonly used remove diagenetically altered material by removing organic material, via an oxidation reaction with H2O2 or NaOCl, and secondary carbonate, via dissolution in dilute acetic acid, from bioapatite. While much work has been done to determine the efficacy of the pretreatment process, little research has been conducted to determine the potential effects of this process on the oxygen isotope composition of enamel bioapatite. A comparison between δ18O values of fossil enamel treated with 18O-depleted (δ18O = -10.0 / V-SMOW) and 18O-enriched solutions (δ18O = +16.4 / V-SMOW). On average, samples treated with 18O-enriched solutions had δ18O values at least 0.4 / V-PDB more positive than samples treated with 18O-depleted solutions. These results suggest that the isotopic composition of solutions used in the pretreatment process can significantly affect the δ18O values of fossil enamel prior to isotopic analysis. Diagenetic alteration can potentially be assessed using the linear relationship between δ18O values of the carbonate and phosphate components of bioapatite, as any deviation from a slope of 1 suggests alteration. Comparing the relationship between δ18Op and δ18Oc for fossil mammals from the Hadar Formation suggests that this method is successful at identifying samples that are significantly different from the remaining samples due to diagenetic alteration. The relationship between δ18Oc and δ18Op for the majority of sampled fossils from the Hadar Formation has a slope very close to 1 with an average offset, and apparent fractionation factor, between δ18Oc and δ18Op consistent with modern mammals. Therefore, it is likely that many of the fossil mammals sampled from the Hadar Formation retain in vivo δ18O values relating to the δ18O value of the water they consumed. The reliability of paleodietary reconstructions using trace element ratios (notably Sr/Ca and Ba/Ca) is strongly dependent on the preservation of biogenic trace element concentrations. Although most trace element ratio research relies on bone bioapatite, enamel has a better preservation potential and may successfully preserve biogenic trace element relationships. An analysis of Sr/Ca, Ba/Ca, and Zn/Ca ratios indicates that, despite rare earth element concentrations greater than the 1ppm observed in modern enamel, mammals from the Hadar and Busidima formations potentially reflect feeding strategies observed in modern counterparts, i.e. grazers have higher Sr/Ca and Ba/Ca ratios than browsers and omnivores

Previous studies have shown that municipal water can provide a substantial surface water and groundwater recharge source for the Edwards aquifer in central Texas. Knowledge of how water sources to urban watersheds change with urbanization is essential for sustainable water resource management. The range for 87Sr/86Sr values for Austin municipal water (0.7086–0.7094) is distinct from that of naturally occurring phreatic groundwater (0.7076-0.7079) and stream discharge in many rural watersheds (0.7077– 0.7084). Many streams in urbanized Austin watersheds have elevated 87Sr/86Sr values (0.7085–0.7088) relative to these rural streams. These differences demonstrate the potential for Sr isotopes to serve as a tracer of municipal water inputs to urban streamflow. A few urban streams and springs, however, have 87Sr/86Sr values higher than those of municipal water. Soil is the likely source of these elevated values. Spatial variability in the distribution of high 87Sr/86Sr soil and temporal variability in soil-exchangeable Sr contributions to groundwater may result in naturally high streamflow 87Sr/86Sr values, making the identification and quantification of municipal water as a streamflow source using Sr isotopes unreliable in some instances. Temporal variability in climatic conditions and resulting changes in effective moisture can result in distinct natural groundwater 87Sr/86Sr and Mg/Ca ratio variations, due to differences in overall groundwater residence times and water-rock interaction. Unlike natural water sources, municipal water inputs to urban watersheds peak during the summer (and periods of drought) when natural recharge inputs (precipitation) are minimal or nonexistent. Thus, proportions of natural vs. municipal water sources in the streamflow of some highly urbanized streams vary seasonally, resulting in distinct 87Sr/86Sr and Mg/Ca temporal trends, based on the recharge source. In some urban watersheds, municipal water appears to be a significant streamflow component during dry periods. However, temporal variation in natural Sr inputs to vadose and phreatic groundwater may result in the overestimation of municipal water contributions to streamflow and groundwater recharge during relatively wet periods. / text

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

No description available.

No description available.

Thesis (M.S.)--University of Wyoming, 2007. / Title from PDF title page (viewed on Feb. 17, 2009). Includes bibliographical references (p. 69-74).

Thèse--Sc. nat.--Strasbourg I, 1976. / Bibliogr. p. 229-244. Résumés en anglais et en allemand.