Anomalous Temperature Dependence of the Electric Field Gradient at the Y Site in In₀.₁Y₀.₉Ba₂Cu₃O\u3csub\u3e9- δ\u3c/sub\u3e

Perturbed angular correlation (PAC) spectroscopy has been used to measure a well-defined static electric quadrupole interaction in a superconducting ceramic, In0.1Y0.9Ba2Cu3O9-δ. Perturbation functions, Fourier transforms, and the derived PAC parameters are given for data taken at temperatures ranging from 77 to 1070 K. Indirect evidence is presented for the occupation of the Y site by the 111In-111Cd PAC probe. The derived electric field gradients were found to increase linearly with temperature. This result suggests the presence of soft, anisotropic vibrations in the structure. Additional evidence is presented to indicate that O2--ion or O-vacancy transport may not occur in the Y coordination sphere

Structural Characterization of a High-Temperature, Ionic Conducting Ceramic using Perturbed Angular Correlation Spectroscopy

Perturbed angular correlation (PAC) spectroscopy has been used to characterize several structural aspects of a high-temperature, ionic conducting ceramic, CaZr3.95Hf0.05P6O24. Hafnium was introduced into the material to provide the PAC probe nuclei, 181Hf/181Ta, which were located primarily at Zr sites. PAC measurements were made over a range of temperatures from 77 to 1180 K, and they have been analyzed and interpreted using several simple models. The distorted octahedral crystal field at the Zr site produced a (low-frequency) static electric quadrupole interaction which can be accurately described by the point-charge model. But, the temperature dependence of the associated electric field gradient (EFG) cannot be described accurately by purely static considerations via the point-charge model and high-temperature x-ray diffraction data. Although a high-frequency static interaction was also observed, the measurements were not sufficiently accurate to identify its origin unambiguously. Some of the high-temperature measurements show evidence of a time-varying interaction, which may result from Ca2+-ion jumping. But, jump frequencies derived classically from high-temperature electrical dc conductivity measurements are too low to agree with those indicated by the PAC data. However, the dc conductivity measurements support a simple model of thermally activated Ca2+-ion transport. The temperature dependence of the EFG (corresponding to the low-frequency interaction) was used to determine an effective Debye-Waller factor. As a result of using this approach to analyze this type of PAC data, this factor was shown also to agree qualitatively with the predictions of the Debye crystal model, although significant theoretical limitations were encountered. These particular results suggest that the PAC technique may provide new insights into understanding advanced ceramic materials

The spotted gar genome illuminates vertebrate evolution and facilitates human-teleost comparisons

To connect human biology to fish biomedical models, we sequenced the genome of spotted gar (Lepisosteus oculatus), whose lineage diverged from teleosts before teleost genome duplication (TGD). The slowly evolving gar genome has conserved in content and size many entire chromosomes from bony vertebrate ancestors. Gar bridges teleosts to tetrapods by illuminating the evolution of immunity, mineralization and development (mediated, for example, by Hox, ParaHox and microRNA genes). Numerous conserved noncoding elements (CNEs; often cis regulatory) undetectable in direct human-teleost comparisons become apparent using gar: functional studies uncovered conserved roles for such cryptic CNEs, facilitating annotation of sequences identified in human genome-wide association studies. Transcriptomic analyses showed that the sums of expression domains and expression levels for duplicated teleost genes often approximate the patterns and levels of expression for gar genes, consistent with subfunctionalization. The gar genome provides a resource for understanding evolution after genome duplication, the origin of vertebrate genomes and the function of human regulatory sequences

IMECE2003-43991 DEPENDENCE OF PRESSURE LOSSES ON ANGLE OF ATTACK FOR FLOW THROUGH PERFORATED PLATES

ABSTRACT We have measured the dependence of pressure losses on angle of attack in pipe flow through a set of seven perforated metal plates. Pressure losses were measured in air flow at temperature and pressure about 24 o C and 736 mm Hg. The nominal pipe diameter is 3.5" resulting an internal pipe diameter of 90.17 mm. Specifically, the plates differ in type of metal, thickness, hole size, and hole spacing. Using a pipe-flow apparatus in which the angle of attack for the plate was set at either 0°, or 45°, or 22.5°, we measured the pressure losses over speeds ranging up to approximately 30 m/s. Generally, the pressure loss increases linearly with the square of the air speed; and, in this context, the ratio of pressure loss to air speed squared represents a classical loss coefficient, K, for perforated plates. For a specific air speed, the magnitude of the loss coefficients correlates positively with the ratio of the distance between hole centers to the hole diameter, s/D. The geometric effects associated with changing the angle of attack are complex, and a single dimensionless ratio such as s/D provides insufficient information to describe this effect. However, the measurements do indicate that the loss coefficient K depends on the ratio, s/D, raised to a power of approximately 4.8. Flow visualization may provide some more insight into the specific physical phenomena responsible for these pressure losses

Investigating Phase Transitions in ABO₃ Perovskites Using Perturbed Angular Correlation Spectroscopy

Perturbed Angular Correlation (PAC) measurements were performed at several temperatures on BaTiO3, SrTiO3, BaHfO3 ceramic and PbTiO3 powder samples. In the PbTiO3 and BaTiO3 samples, both the tetragonal (ferroelectric) and the cubic (paraelectric) phases were investigated. For the tetragonal phase temperatures, the perturbation functions for PbTiO3 show strongly decreasing electric field gradients (efgs) and increasing linebroadening with increasing temperature. These two features are evidence for an order-disorder phase transition mechanism. Although the data for tetragonal BaTiO3 provide less information, linebroadening is evident, which is not inconsistent with an order-disorder mechanism. Above the Curie temperatures, both compounds show featureless perturbation functions that decay slowly with increasing time. In addition, this interaction shows little temperature sensitivity. Similarly, PAC measurements on cubic BaHfO3 and cubic SrTiO3 show this behavior. The literature reports similar phenomena for cubic PbZrO3 and cubic PbHfO3, although the explanations differ from report to report. These results suggest that a nuclear spin relaxation mechanism was responsible for the perturbations

Defects and magnetic hyperfine fields in ZrFe/sub 2/ investigated using perturbed-angular-correlation spectroscopy

We have measured the temperature dependence of the electric and magnetic hyperfine interactions at 181Ta nuclei substituted into the Zr site in the Laves-phase compound ZrFe2, using the perturbed angular correlation of g rays emitted after the b2 decays of 181Hf probe nuclei. Although the overall crystal structure is cubic, a weak strongly damped electric-quadrupole interaction is observed, which shows no significant temperature dependence over the investigated temperature range from 290–1300 K. Thus below the magnetic ordering temperature TC of 631~2! K we observe combined magnetic-dipole and electric-quadrupole hyperfine interactions. Two separate magnetic components characterize the magnetic-dipole interactions. For the interaction at the primary site, which is occupied by 70–80% of the probes, the Larmor frequency measured at temperature has a value of vL5407(1) Mrad sec21. The secondary site is populated by the remaining 20–30% of the probes, for which the corresponding Larmor frequency has a room temperature value of vL 5579(3) Mrad sec21. We attribute the primary interaction to the ‘‘perfect-crystal’’ probe environment at the Zr site, whereas we ascribe the secondary interaction to the enhancement of the transferred hyperfine field by the presence of Fe antisite defects near the Zr site. At temperatures below but very close to TC , those frequencies cannot be determined for either interaction, because the magnetic-hyperfine and the electricquadrupole frequencies converge to comparable values

Defects and magnetic hyperfine fields in ZrFe/sub 2/ investigated using perturbed-angular-correlation spectroscopy

We have measured the temperature dependence of the electric and magnetic hyperfine interactions at 181Ta nuclei substituted into the Zr site in the Laves-phase compound ZrFe2, using the perturbed angular correlation of g rays emitted after the b2 decays of 181Hf probe nuclei. Although the overall crystal structure is cubic, a weak strongly damped electric-quadrupole interaction is observed, which shows no significant temperature dependence over the investigated temperature range from 290–1300 K. Thus below the magnetic ordering temperature TC of 631~2! K we observe combined magnetic-dipole and electric-quadrupole hyperfine interactions. Two separate magnetic components characterize the magnetic-dipole interactions. For the interaction at the primary site, which is occupied by 70–80% of the probes, the Larmor frequency measured at temperature has a value of vL5407(1) Mrad sec21. The secondary site is populated by the remaining 20–30% of the probes, for which the corresponding Larmor frequency has a room temperature value of vL 5579(3) Mrad sec21. We attribute the primary interaction to the ‘‘perfect-crystal’’ probe environment at the Zr site, whereas we ascribe the secondary interaction to the enhancement of the transferred hyperfine field by the presence of Fe antisite defects near the Zr site. At temperatures below but very close to TC , those frequencies cannot be determined for either interaction, because the magnetic-hyperfine and the electricquadrupole frequencies converge to comparable values

Electric-field gradients at the Zr sites in Zr/sub 3/Fe : measured using perturbed-angular-correlation spectroscopy and calculated using band theory

We have measured the electric-field-gradient (EFG) parameters Vzz and η and their temperature dependences at the two Zr sites in the intermetallic compound Zr₃Fe using perturbed-angular-correlation spectroscopy and the probe ¹⁸¹Hf→¹⁸¹Ta. At temperatures below the peritectic transformation at ≈1158 K, at each Zr site, we observed well-defined EFG’s, characterized by sharp spectral lines. A high-frequency, very asymmetric nuclear electric-quadrupole interaction characterizes the first Zr site, which represents approximately one-third of the probes. A low-frequency, nearly axially symmetric nuclear electric-quadrupole interaction characterizes the second Zr site, which represents approximately two-thirds of the probes. Near and above the peritectic transformation, the results show the effects of the decomposition of Zr₃Fe into Zr and Zr₂Fe and subsequent melting. We have compared the values of Vzz and h measured at laboratory temperature to those calculated using the first-principles, self-consistent real-space linear muffin-tin atomic sphere approximation (RS-LMTOASA) band-theory method. Overall the magnitudes of Vzz and η calculated using the RS-LMTO-ASA method agree reasonably well with the experimental values