Showing papers by "Jožef Stefan Institute published in 1987"

Oxygen Partial Pressure and Grain Growth in Donor-Doped BaTiO3

Abstract: The grain growth of donor-doped BaTiO3 at different oxygen partial pressures was studied. Results showed that the oxygen pressure had a pronounced influence on the grain growth and related effects. A model for the grain size anomaly during sintering of donor-doped BaTiO3 in the presence of a TiO2-rich liquid phase is proposed.

Results of a co-ordinated research programme to improve the certification of IAEA milk powder A-11 and animal muscle H-4 for eleven “difficult” trace elements

Abstract: In the original intercomparisons two of the IAEA's reference materials, milk powder A-11 and animal muscle H-4, gave unsatisfactory results, with a wide spread of values for many trace elements of interest, and subsequently the recommended values for Cu and Mn in A-11 have been questioned. In the present work, efforts to improve the status of these two materials through a co-ordinated research programme involving a small group of experienced laboratories is described. The elements chosen were As, Cd, Co, Cr, Cu, Hg, I, Mn, Mo, Ni and Pb, and the techniques used were neutron activation analysis (NAA), atomic absorption spectroscopy (AAS) in various modes, inductively coupled plasma emission spectroscopy (ICP), electrochemical methods, and also for iodine, a catalytic method. The results show a satisfactory measure of concordance for most of the elements (in marked contrast to the original results), and allow proposals to be made for revision of the recommended values. Only in the case of Ni and Pb (where NAA data are not available) is the position not fully resolved.

The role of abstractions in learning qualitative models

Abstract: We present a knowledge acquisition system for the construction of deep knowledge bases, represented by qualitative models. We assume that a model is defined by its structure (a set of components and their connections) and functions of the individual components. The system consists of three subsystems: a learner that hypothesizes functions of components from instances of their behavior, a debugger that locates faulty functions of components and proposes how to correct them, and an interpreter that can use the hypothesized model to derive its behavior. We propose a top-down development of a model from very abstract to more and more specific. In the paper we concentrate on the representation of a model on different levels of abstraction, and on the role that an abstract model plays when refining a more specific model. The system was successfully tested on a difficult problem of constructing a qualitative model of the heart.

The soft mode concept and the history of ferroelectricity

Abstract: I. THE SOFT MODE CONCEPT: In the late fifties ferroelectric research underwent a serious transition. Until that time the theory of ferroelectricity has been either phenomenological or consisted of attempts to develop microscopic theories for specific atomic structures. In 1959 Cochran1 pointed out that at second order ferroelectric transitions where the dielectric constant diverges there must be a transverse optical lattice vibration mode whose frequency goes to zero as the transition is approached. This suggestion turned out to be a tremendous stimulus to ferroelectric theory and research. Many first order transitions which are close to being second order ones also exhibit mode softening as the transition is approached. Whereas the soft mode idea was at first applied only to displacive type ferroelectrics, de Gennes2 extended it to order-disorder type crystals. In these systems one finds instead of soft phonons soft (usually overdamped) pseudo-spin wave type excitations3 or anomalously long relaxation ti...

Low-level simultaneous determination of As and Sb in standard reference materials using radiochemical neutron activation analysis with isotopic 77As and 125Sb tracers

Abstract: The aim of the present work was to develop an older method used in our laboratory based on selective solvent extraction of As and Sb as their iodides with toluene, and by the use of the radioisotopic tracers 77As and 125Sb, to improve the accuracy by better control of the radiochemical yields. 77As was produced for each sample run by concurrent irradiation of a few mg of GeO2 followed by a rapid separation of 77As from 77Ge. The radiochemically purified sample fraction containing 76+77As and 122+125Sb was counted on a Ge detector in good geometry. The γ-lines of the four nuclides do not mutually interfere so that a combined measurement of As and Sb may be made. The method was applied to IAEA Milk Powder A-11, Animal Muscle H-4, Bowen's Kale and some other SRMs. The results obtained are discussed in the light of literature measurements. From present and previous results, together with data by Heydorn, the presently accepted value for As in Bowen's Kale of 140 ng · g−1 may be 20% too high.

Organic geochemistry of two 40-m sediment cores from the Gulf of Trieste (Northern Adriatic)

Abstract: Sediment samples from two 40-m cores drilled in two different sedimentological environments in the Gulf of Trieste (Northern Adriatic), i.e. in the salt marsh of Secovlje and in the inner part of the Bay of Koper, were taken for geochemical analysis of organic matter. The organic geochemical analysis comprised the analyses of organic carbon, nitrogen and phosphorus contents and the analyses of amino acid and hexosamine nitrogen and total carbohydrate contents in the samples. Parallel analyses of carbon and oxygen isotopic composition in calcite, and of the granulometric and mineralogical composition of the samples were performed. Depth variations of the distribution pattern of organic carbon, nitrogen, phosphorus, amino acids, hexosamines and carbohydrates in the cores indicate temporal variability of organic matter deposition and transformations in a changing fluvial-brackish-marine environment in the past. The results also provide information on paleoecological changes in the Gulf during Quaternary sedimentation processes. It appears that the marine environmental conditions in the past were not markedly different from those of the present day.

A Phase Transition and Domain Boundaries in the Modulation Structure of TaTe4

Abstract: TaTe4 is a quasi-one-dimensional crystal with a tetragonal subcell based on axes (a × aa × c). At room-temperature the structure is modulated with supercell axes (aR × aR × cR), where aR = 2a and cR = 3c. Using dark-field electron microscopy, domain boundaries in the modulation structure are analysed in relation to the possible boundaries that could exist in a proposed model. The observed boundary image contrast and the interactions of the boundaries with one another and with subcell defects are discussed. It is shown that a new modulated phase with axes (√2a × √2a × 3c) occurs at about 450 K. Satellite dark-field electron microscopy is used to study the disappearance and reformation of the domain boundaries on cycling through the transition. It is concluded that the domain boundaries in TaTe4 result from this phase change which occurs in a temperature range where the atomic mobility is severely limited. TaTe4 stellt einen quasi-eindimensionalen Kristall mit einer tetragonalen, auf den Achsen (a × a × c) basierenden Unterzelle dar. Bei Zimmertemperatur ist die Struktur mit den Superzellenachsen (aR × aR × cR) moduliert, wobei aR = 2a und CR = 3c ist. Mittels Dunkelfeldelektronen-mikroskopie werden die Domanengrenzen in der Modulationsstruktur in Beziehung zu den moglichen Grenzen analysiert, die in einem vorgeschlagenen Modell existieren konnten. Der beobachtete Grenzen-Bildkontrast und die Wechselwirkungen der Grenzen untereinander und mit Sub-zellendefekten werden diskutiert. Es wird gezeigt, das eine neue modulierte Phase mit den Achsen (√2a × √2a × 3c) bei etwa 450 K auftritt. Satellitendunkelfeldelektronenmikroskopie wird benutzt, um das Verschwinden und die Neubildung der Domanengrenzen beim zyklischen Durchlaufen des Ubergangs zu untersuchen. Es wird angenommen, das die Domanengrenzen in TaTe4 aus dieser Phasenanderung herruhren, die in einem Temperaturbereich auftritt, wo die atomare Beweglichkeit betrachtlich begrenzt ist.

Vibrational study of RbD2PO4 single crystals

Abstract: The Raman (4000–10 cm−1) and infrared (4000–32 cm−1) spectra of RbD2PO4 single crystals were investigated at various temperatures. In the low-temperature phase, the TOLO splitting expected for a P21 polar space group is not observed in the Raman spectra and the vibrational spectra are consistent with a C2h factor group. The II–III phase transition gives rise to a splitting of some skeletal bending modes and the frequency shift of certain vibrations indicates some displacive be haviour, in contrast to the analogous pseudo-unidimensional crystal of CsD2PO4. Hydrogen bonding in RbD2PO4 is discussed.

Distribution of trace metals in human nasal cavity bones

Abstract: X-ray emission spectroscopv was applied to the multi-element analysis of bones of human nasal cavity. Both the 1.5MeV protons and the X-ray photons (Mo tube, Mo secondary target) were used for sample excitation. Results of the analysis of 44 nasal cavity samples taken from one individual are presented.

Quasiparticle excitations in the superconducting state observed in light scattering.

Abstract: In light scattering experiments on superconducting Y-Ba-Cu-O ceramic oxides we observe an unexpectedly intense Stokes-shifted spectrum from about 100 cm/sup -1/ (--13 meV) to 4000 cm/sup -1/ (--0.5 eV). We propose that this is due to light scattered from energy-density fluctuations of quasiparticle states excited above the superconducting ground state. The temperature dependence of the scattering intensity shows strong hysteresis, not observed in other experiments on the oxide superconductors.

Thermal Decomposition Study of Hydrazinium(2+) Heptafluorotantalate Monohydrate and Identification of Intermediate Compounds.

Abstract: The results of TG, DTG, and DTA measurements show that monoclinic N2H6TaF7·H2O decomposes in four steps.

Thermal Decomposition of Hydrazinium(2+) Hexafluorozirconate(IV) and Hexafluorohafnate(IV).

Abstract: A study of the thermal decomposition of N 2 H 6 ZrF 6 and N 2 H 6 HfF 6 showed that in the first step N 2 H 5 ZrF 5 and N 2 H 5 HfF 5 are formed. In the second step the former gives NH 4 ZrF 5 as an intermediate, but an analogous hafnium complex cannot be obtained in a pure state. The end products of the decompositions are the corresponding tetrafluorides. The intermediates were isolated and characterized by chemical analysis X-ray diffraction patterns and vibrational spectroscopy.

On the Synthesis of Xenon(II) Fluorostannates(IV).

Abstract: The xenon(II) fluorostannates(IV) (I) and (II) are prepared by treating xenon difluoride (IV) with tin difluoride (III) or hydrazinium tetrafluorostannate(II) (V).