G. Flor

Electrical properties of the ZrO2-CeO2 system

Abstract The electrical properties of ZrO 2 -CeO 2 solid solutions are investigated in detail as functions of the temperature between 200 and 1000 °C and of the ceria molar fraction between 16 and 100% (pure CeO 2 ). The samples are studied by combining impedance spectroscopy, ionic transport number and thermoelectric power measurements so as to define the role and the contribution of hole and electron carriers to the conductivity mechanism. In particular, the transition from p- to n-type conductivity of undoped CeO 2 at 750 °C is clearly demonstrated. The conductivity dependence on the oxygen partial pressure is also studied in the ZrO 2 -80 mol% CeO 2 composition that exhibits the highest electronic conductivity.

Combustion synthesis of ZrAl intermetallic compounds

Abstract The combustion syntheses of all the intermetallic compounds in the ZrAl binary system have been investigated. Analyses of the reaction products show that for compositions of the reacting mixture richer than 60 at.% in Al the products always contain a mixture of ZrAl4, ZrAl2 and Zr2Al4, in different proportions. Zr-rich samples always produce a mixture of Zr5Al3, Zr3Al2, ZrAl2 and Zr2Al3. DTA investigations have shown that the exothermic process responsible for the front propagation is represented, for all starting compositions, by the reaction of molten aluminum with solid zirconium to form ZrAl3. When fine powdered reactants are used, solid-state interaction also contributes to the reaction ignition.

Nature and amount of carriers in Ce doped Nd2CuO4 I. High-temperature characterization

Abstract DC conductivity, ionic transport number, and thermopower have been measured of Nd2−xCexCuO4±δ materials (x = 0, 0.05, 0.10, 0.15, 0.20) at different temperatures [300 ⩽ T ⩽ 900°C] and under different oxygen partial pressures [1 atm ⩽ P(O2) ⩽ 3 × 10−6 atm]. For the most heavily doped samples, the current carriers are electrons essentially produced by the substitutional defect CeNd., while the crystal-gas equilibria involving Oi″ account for a smaller amount of carriers. The behavior of Nd1.95Ce0.05CuO4±δ is intermediate between more doped and undoped materials. When coupled with oxygen non-stoichiometry data, the results indicate that the mobility of the electronic carriers exhibits a transition from small-polaron to large-polaron behavior with increasing doping level.

Solid-state reaction study on the system Ni-Li2CO3

Abstract Thermogravimetry, differential scanning calorimetry, X-ray diffractometry and scanning electron microscopy have been used to investigate the formation reaction of the solid solution LixNi1−xO in the solid-state system Ni-Li2CO3. Thermogra vimetric results show that carbonate decomposition takes place well below its usual temperature when nickel is being oxidized and that as a consequence of the simultaneous oxidation and decomposition processes, a solid solution forms in a temperature range where no appreciable volume diffusion can occur. X-ray diffraction measurements confirm this conclusion showing that lattice constant values of thermogravimetric samples linearly decrease with increasing mixture lithium content. Notwithstanding all experimental evidences agree in indicating that all lithium oxide participates to solid solution formation, sensible departures from the linear relationship arise for mixtures of high lithium content (XLi>0.2010). It is shown that cation vacancies are responsible of such a behaviour and that their concentration increases with increasing the relative amount of lithium oxide produced by spontaneous carbonate decomposition that enters solid solution.

Oxygen non-stoichiometry and high temperature DC conductivity of SmBa2Cu3O6+x

Abstract The paper reports on coupled DC electrical conductivity measurements and oxygen non-stoichiometry determinations on SmBa2Cu3O6+x at high temperatures (300–750°C) under oxygen partial pressures from 10−1 to 10−5 atm. Under the lowest oxygen pressures (10−4 and 10−5 atm) and at the highest investigated temperatures (T>700°C, for the former pressure) the material shows semiconducting behavior. In the other [T, P(O2)] field the material shows metallic conductivity. The amount of (positive) carriers is controlled by equilibrium with external oxygen coupled to oxygen point defects. A change in the mechanism of hole injection is well apparent at the transition between the tetragonal and the orthorhombic phases. Indications are given about the large polaron nature of the holes at high oxygen content.

A thermal study on moisture absorption by epoxy composites

The water sorption kinetics of an epoxy matrix-carbon fiber composite with different degrees of polymerization was studied. It was observed that the diffusion coefficients, determined perpendicularly to the fiber direction, are increased by increasing the cure degree. Only a small portion of the bonded water is released when the specimen are heated to 200 °C.The effects of moisture on some properties of the composite were also investigated. TheTg values determined on fully polymerized samples, show lowerings of 70 degrees; moreover, the plasticization power of the bonded water increases on decrease of the temperature of conditioning of the specimens.Finally, the water present in the matrix considerably reduces the thermal stability of the composite.ZusammenfassungEs wurde die Wassersorptionskinetik eines Graphitfaserverbundstoffes auf Epoxymatrixbasis mit verschiedenen Polymerisationsgraden untersucht. Es zeigte sich, daß die senkrecht zur Faserrichtung bestimmten Diffusionskonstanten mit anwachsendem Vernetzungsgrad zunehmen. Beim Erhitzen des Materials auf 200 °C wird nur ein geringer Teil des gebundenen Wassers abgegeben. Der Einfluß des Feuchtgehaltes auf einige Eigenschaften des Verbundstoffes wurde ebenfalls untersucht. Die an vollkommen polymerisierten Proben bestimmtenTg Werte zeigen eine Abnahme von 70 Grad; die Plastifizierungswirkung des gebundenen Wassers ist um so höher, je niedriger die Konditionierungstemperatur der Probe war. Das in der Matrix gegenwärtige Wasser senkt beträchtlich die Wärmebeständigkeit des Verbundstoffes.РезюмеИзучена кинетика сор бции воды композитом графитное волокно-эп оксидная матрица с различной степенью полимеризации. Устан овлено, что коэффициенты диффуз ии, определенные в направлении перпен дикулярно волокнам, увеличиваются с увел ичением степени отверждения. Только н ебольшая часть связа нной воды выделяется при нагре вании этого композита до 200 °С. Изуч ено также влияние вла ги на некоторые свойства к омпозитного материала. Значения Tg, о пределенные на полно стью полимеризованных об разцах, показали понижение температу ры на 70 °С. Однако степе нь пластикации связанн ой воду увеличиваетс я с уменьшением темпера туры обработки образ цов. Вода, находящаяся в эпокси дной матрице, значительно понижае т термоустойчивость композитного матери ала.

Performances of a heat flux DSC cell

Abstract Five different metals have been used to determine the calibration constant of a heat flux DSC cell and measurements have been performed under a variety of experimental conditions. Different results have been obtained by different standard meaning that some physical property of the sample may affect the instrument response. An analysis of the details of the instrument response showed that sample density and latent heat of fusion per unit mass can both affect the final result. In addition the heating rate too has been shown to exert an influence on the calibration constant value.

Nature and amount of carriers in Ce-doped Nd2CuO4 II: Low temperature transport and XAS characterisation

Abstract This paper discusses the electrical (conductivity) and spectroscopic (XAS) evidence of Ce-doped Nd 2 CuO 4 samples in which the charge carrier density has been fixed to a known value by annealing in well known and well controlled conditions, and then quenching to room temperature. All the experimental findings presented here are consistent with a band structure where no impurity levels are present in the energy gap. In addition, the XAS results indicate that the unoccupied band of lower energy is originated by a Mott-Hubbard splitting. It has been found that both Ce-doping and reduction of the oxygen excess δ concur in giving rise to the charge carrier density. For low charge carrier density, the conductivity data indicate an Anderson localisation of the charge carriers, which is seemingly responsible of the semiconducting-like behavior. As the charge carrier density increases, conductivity becomes less dependent on temperature, and superconductivity (above 10 K) is found only when the charge carrier density is greater than the value of 0.082 electrons per copper atom.

An analysis of the factors affecting the peak shape and the quantitative reliability of a heat flux DSC cell

Abstract In a previous work it was shown that unexpected problems may arise in the calibration of a heat flux DSC cell. To understand the origin of these problems an analysis has been undertaken of what happens in the cell before, during and after the peak. Here the first part of this analysis is reported dealing with the baseline and with the dependence of the recorded signal on time. Analytical expressions are obtained connecting the measured signal with experimental variables and instrument constants. Completely independent on sample related properties, a maximum measurable ΔT is introduced as limiting factor for quantitative reliability.

On the quantitative reliability of heat flux DSC

In an attempt to explain how the calibration factor of a heat flux DSC cell depends both on the standard utilized and on the experimental variables, a study has been undertaken of the entire DSC trace.This report deals with the second half of the peak and shows that, other experimental conditons being fixed, the peak area increases with increasing heating rate.This accounts for the observed decrease in the calibration factors with increasing heating rate.ZusammenfassungDie gesamte DSC-Kurve wurde untersucht, um zu klären, wie der Eichfaktor einer Wärmefluss-DSC vom verwendeten Eichstandard und von den Versuchsbedingungen abhängt. Der vorliegende Teil behandelt den zweiten Abschnitt eines DSC-peaks (vom Maximum an). Es wird gezeigt, dass bei Konstanthalten aller anderen Versuchsbedingungen die peak-Flächen mit steigender Aufheizgeschwindigkeit zunehmen. Das wird als Ursache für die beobachtete Verringerung des Eichfaktors mit steigender Aufheizgeschwindigkeit angesehen.РЕЗЮМЕДля установления зав исимости калибровоч ного коэффициента теплов ого потока в ячейке ДСК от станда ртных и эксперимента льных переменных параметр ов, изучена полная запись ДСК измерений. Первая часть проведе нного исследования уже был а опубликована. Насто ящее сообщение касается в торой половины пика и показано, что при каком-либо фик сированном экспериментальном у словии, площадь пика увеличивается с увел ичением скорости наг ревания. Этим объясняется наб людаемое уменьшение калибровочного коэф фициента с увеличени ем скорости нагрева.