Results Of A Remedial Laboratory Program Based On A Piaget Model For Engineering And Science Freshmen

Engineering and science freshmen who enter our institution with algebra deficiencies have a very low probability of performing satisfactorily in the required college-level calculus course. Two years ago, a remedial one-semester program was conducted for 36 freshmen who were identified by an entrance examination as being weak in algebra. The course was based on the assumption that the students were concrete in the Piagetian sense. The main part of the course required a general physics laboratory that provided selected experiences to bridge the concrete-formal gap. After three semesters, 31% of the participants in the course were receiving grades of C or above in calculus courses, as compared to 16% of those students who did not take the remedial program. © 1977, American Association of Physics Teachers. All copy rights reserved

Ferroelectric Properties of PbZrO₃-BiFeO₃ Solid Solutions

Solid solutions of BiFeO and PbZrO3 were investigated for crystallographic and dielectric properties. A new pseudocubic perovskite phase was found, which has been tentatively indexed as orthorhombic with the same unit cell as PbZrO3 but with dielectric properties characteristic of a ferroelectric substance. The Curie point in this series of solid solutions has a minimum of about 155°C at a composition of about (PbZrO3)0.80 (BiFeO3)0.20, which at room temperature lies on the boundary between the orthorhombic antiferroelectric and the pseudocubic ferroelectric phases. This composition has a relative dielectric constant of 800 at room temperature and shows appreciable dielectric nonlinearity. The Curie point rises to about 400°C for (PbZrO3)0.50 (BiFeO3)0.50 and is extrapolated to about 900°C for pure BiFeO3, in agreement with previous data from similar solid solutions. It is felt that this is additional evidence that the distortion from cubic symmetry in BiFeO3 may be of ferroelectric origin

Increased Optical Damage Resistance In Lithium Niobate

We have confirmed greatly improved resistance to photorefractive damage in compositions of lithium niobate containing 4.5 at. % MgO or more. Holographic diffraction measurements of photorefraction demonstrated that the improved performance is due to a hundredfold increase in the photoconductivity, rather than a decrease in the Glass current. The diffraction efficiency shows an Arrhenius dependence on temperature, with an activation energy of 0.1 eV for the damage-resistant compositions, compared with 0.5 eV for undoped or low-magnesium compositions. The damage-resistant compositions are distinguished by a 2.83-μm absorption line instead of the usual 2.87-μm line due to the OH-stretch vibration

Dielectric Properties of Solid Solutions of BiFeO₃ with Pb(Ti, Zr)O₃ At High Temperature and High Frequency

Solid solutions of BiFeO3 with PbTiO3, PbTi0.5Zr0.5O3, and PbZrO3 were prepared. The crystallographic data on these solutions, which are basically perovskitic, are given. The dielectric constants of the materials were determined at a frequency of 0.53 GHz and at temperatures up to 800°C. Dielectric Curie points were found in solutions containing up to 90 mole % BiFeO3. These results leave little doubt that BiFeO3 is ferroelectric or antiferroelectric. The extrapolated Curie point for BiFeO3 is above 850°C. BiFeO3 appears more likely to be ferroelectric than antiferroelectric, but the distinction between the two classifications may not be sharp

Aspectos de la teoría de nudos

Tesis (Lic. en Matemática)--Universidad Nacional de Córdoba, Facultad de Matemática, Astronomía, Física y Computación, 2019.Los nudos, tal cual aparecen en nuestra vida cotidiana, son un objeto de estudio en la Matemática. La Teoría de Nudos es la rama de la Matemática que se encarga de su estudio. Un problema central es el de poder decir si dos nudos dados son equivalentes o no. Los matemáticos, en la búsqueda de responder esta pregunta, entre otras, han desarrollado diversas técnicas y herramientas en esta área de estudio. En este trabajo se hace un recorrido en el estudio de la Teoría de Nudos, comenzando con las definiciones más elementales, hasta llegar a estudiar herramientas sofisticadas como el polinomio de Alexander, el grupo de un nudo y las matrices de Seifert, entre otros. En los dos últimos capítulos se investigan los dos temas siguientes: nudos virtuales y presentaciones de Wirtinger. En este último se hace un aporte, dando una nueva familia infinita de presentaciones de Wirtinger no geométricas.The knots we usually see in our lifes are studied in mathematics in the branch called Knot Theory. A main problem is to decide whether two knots are equivalent or not. Many tools and techniques have been developed by mathematicians in order to answer this and other related questions. In this work, we study Knot Theory from the beginning, with definitions and elementary notions, until some sophisticated concepts and tools like the Alexander polynomial, the knot group and Seifert matrices, among others. In the last two chapters, we work on the following two particular subjects: virtual knots and Wirtinger presentations. In this last one, we made a small contribution by presenting a new infinite family of Wirtinger presentations which are not geometric.Fil: Gutierrez Quispe. Robert Gerson. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía, Física y Computación; Argentina

Conductive Strontium Titanate Layers Produced By Boron-ion Implantation

The ion implantation of boron has been found to be an effective method for increasing the conductivity of strontium titanate. A highly conductive layer was formed by implantations at doses of 3.6x1016 to 1.0x1017 ions/cm2, using an accelerating voltage of 100 kV, corresponding to a boron range of about 300 nm. The conductivity of the implanted layer was found to be further enhanced by about four orders of magnitude after annealing at 225 °C. The surface resistivity attained was typically 1000 Ω/square at room temperature. The resistance increased with rising temperature between 77 and 500 K. All samples were found to be n type, as determined by Hall-effect and thermoelectric measurements. The measured Hall mobility range was from 100 cm2/V sec at 77 K, decreasing to 5 cm2/V sec at 300 K. The existence of several defect energy levels can be inferred from the electrical data

Photoconductivity Parameters In Lithium Niobate

Measurements on a variety of doped (magnesium and/or iron) and undoped lithium niobate crystals in the oxidized state demonstrate an Arrhenius dependence of dark conductivity on reciprocal temperature between 460 and 590 K. All of the crystals had roughly the same conductivity and activation energy (1.21 eV) over the temperature range, implying that all have about the same free-carrier concentration and mobility. The enhanced photoconductivity of magnesium-doped lithium niobate is attributed to a greatly reduced trapping cross section of Fe3+ for electrons, the smaller cross section being due to a changed substitutional site for Fe3+. The Fe3+ trapping cross section is calculated from photoconductivity data to be of order 10-18 m2 in undoped lithium niobate. This implies a photoelectron lifetime of order 6x10-11 s in a relatively pure (2-ppm Fe) oxidized crystal

Ariel - Volume 5 Number 6

Editors J.D. Kanofsky Mark Dembert Entertainment Robert Breckenridge Joe Conti Gary Kaskey Photographer Scot Kastner Overseas Editor Mike Sinason Circulation Jay Amsterdam Humorist Jim McCann Staff Ken Jaffe Bob Sklaroff Janet Welsh Dave Jacoby Phil Nimoityn Frank Chervane

Threshold Effect In Mg-doped Lithium Niobate

Optical absorption spectra were obtained after reducing (i.e., vacuum annealing) a series of LiNbO3 crystals grown from melts having various Mg concentrations and Li/Nb ratios. A band peaking at 500 nm, and assigned to oxygen vacancies containing two electrons, was the only absorption present in one set of crystals following reduction. In contrast, two overlapping bands peaking near 1200 and 760 nm were present in the other set of crystals immediately after the reduction. The 1200-nm band is assigned to a previously unreported electron trap and the 760-nm band to oxygen vacancies containing only one electron. These data are interpreted in terms of a threshold level for Mg doping; however, the threshold Mg doping level is not a constant but depends on the ratio of Mg ions to Li vacancies