Structural and electrical properties of the glassy semiconductor system Cu-As-Te

Journal of Materials Science - A method has been developed for measuring electrical conductivity in bulk samples of glassy semiconductors, based on the procedure described by Valdes, which is used...     

Kinematic Viscosities for Ether + Alkane Mixtures: Experimental Results and UNIFAC-VISCO Parameters

Kinematic viscosities for the binary mixtures of diisopropylether, dibutylether or methyl ter-butyl ether with 3-methylpentane, hexane or heptane have been measured at 283.15 K, 298.15 K, and 313.15 K. The experimental values have been correlated by the McAllister equation. Using these results, new UNIFAC-VISCO parameters, O_ether−CH₂ and O_ether−CH₃, have been calculated.     

Use of an environmental specimen bank for evaluating the impact of the Prestige oil spill on the levels of trace elements in two species of Fucus on the coast of Galicia (NW Spain)

In the present study we investigated possible contamination by trace elements due to the oil slick caused by the shipwreck of the Prestige oil tanker, in two species of brown macroalgae (Fucus vesiculosus and Fucus ceranoides) collected from the coastal area most severely affected by the spill (Galicia, NW Spain). The oil slick apparently did not cause significant increases in the concentrations of the trace elements studied, except vanadium. It appears that the magnitude of terrestrial inputs to coastal waters is sufficiently high to mask the inputs of trace elements from the fuel. The observed exception of V suggests that bioaccumulation of this element by the two species of Fucus may be used to indicate exposure to petrochemical products similar to the Prestige fuel. The findings of the study are another example of the importance of the existence of Environmental Specimen Banks for studies of this type.     

Influence of temperature and interelectrode distance on the negative differential resistance in metal-chalcogenide glassy semiconductors

An investigation has been carried out, to elucidate some aspects of the current-controlled negative differential resistance (CCNDR) effect in bulk metal-chalcogenide glassy semiconductors. Because this phenomenon has been shown to be mainly of a thermal nature, a model, from the thermodynamic point of view, was developed, including some aspects related to the thermistors theory. The main conclusion from this model is the appearance of a current filament, which showed up when the material switched from the high electrical resistance to the low electrical resistance state, forming a crystalline filament between both electrodes. The variation of the CCNDR parameters with temperature and interelectrodic distance was studied, using both coplanar point electrodes and coplanar disc electrodes. The experimental results show a good agreement with the expected behaviour from the proposed thermal model (especially when natural convection was considered as the heat-exchanging process between the material and the ambient surrounding). In addition an algorithm was found to simulate the phenomenon computationally, using the experimentally determined physical parameters for the samples under study.     

A theoretical method for determining the crystallized fraction and kinetic parameters by DSC, using non-isothermal techniques

A procedure has been developed for determining expressions for the volume fraction crystallized and for the kinetic parameters in non-isothermal reactions in solid systems involving the formation and growth of nuclei. This method makes use of an equation for the evolution with time of the volume fraction crystallized. This equation has been integrated under non-isothermal conditions and assuming an Arrhenian temperature dependence of the nucleation frequency and of the crystal growth rate, thus obtaining a general expression for the volume fraction crystallized for each value of the related parameter with the dimensionality of the crystal. The kinetic parameters have been deduced, obtaining the maximum crystallization rate, bearing in mind the fact that, in the non-isothermal processes, the reaction rate constant is a time function through its Arrhenian temperature dependence. Finally, the theoretical expressions of the kinetic parameters have been applied to the experimental data corresponding to a set of glassy alloys, quoted in the literature, thus obtaining mean values that agree very satisfactorily with the published data. This fact shows the reliability of the theoretical method developed.     

Design of an aquatic biomonitoring network for an environmental specimen bank

The aim of the present study was to establish an aquatic biomonitoring network for the Galician Environmental Specimen Bank (BEAG) (NW Spain). For this, a sampling system was designed that comprised of 121 points distributed throughout Galician rivers, from which samples of water and of three species of bryophytes were collected. The results obtained allowed selection of 74 sampling points and 2 species (Fontinalis antipyretica Hedw. and F. squamosa Hedw.) as the most suitable for use in future BEAG sampling surveys. The two species selected showed a strong similarity in their capacity to accumulate the 17 elements determined (Al, As, Ca, Cd, Co, Cr, Cu, Fe, Hg, K, Mg, Mn, Na, Ni, Pb, Se and Zn), as well as in their ecological preferences. Furthermore, the levels of contamination of epicontinental waters were lower than those observed in previous surveys.     

Speeds of Sound and Isentropic Compressibilities of Binary Mixtures Containing Cyclic Ethers and Haloalkanes at 298.15 and 313.15 K

Speeds of sound of binary mixtures formed by either 1,3-dioxolane or 1,4-dioxane and isomeric chlorobutanes at 298.15 and 313.15 K are reported in this paper. Isentropic compressibilities and isentropic compressibility deviations have been calculated from experimental measurements. Isentropic compressibility deviations have been fitted to a Redlich–Kister equation, and the results have been discussed in terms of molecular interactions and structural effects. Isentropic compressibilities have been estimated at 298.15 K using the Prigogine–Flory–Patterson theory.     

Corn starch in the treatment of patients with glycogenosis type I and III

RATIONALE: Administering raw corn starch can maintain normoglycemia for long periods after being ingested, thus facilitating control in patients with type I and III glycogenosis. METHODS: The metabolic effects and the effects on the nutritional status of a treatment with fractionated administrations of raw starch are assessed in two patients with type I glycogenosis (ages 18 and 12 years) and one patient with type III glycogenosis (aged 13 years). In the first two cases the response was previously studied after administering a load of raw corn starch in a water suspension, in an amount similar to the estimated rate of endogenous glucose production during the fasting period (5 mg/kg/minute). RESULTS: The results of the overload of starch showed a normoglycemia and an absence of lactoacidosis between 4 and 6 hours after its ingestion. The three patients were given two doses of raw corn starch (2 g/kg/dose) at 1.00 and 5.00 hours during the night. After one year of treatment, all patients showed glycemia levels at 9.00 AM that were greater than 90 mg/dl and lactic acid levels that were lower than 2.4 mmol/l. Moreover, in two of the cases there was an increase in the growth rate. In all cases the amount of the hepatomegaly decreased as did the size of the hepatic adenomas that were present in two of the cases. CONCLUSIONS: In patients with type I and III glycogenosis, raw corn starch can balance the results of the nightly gastric glucose infusion, both with regard to the metabolic control and with regard to the growth.     

An integral method to analyze the glass-crystal transformation kinetics by differential scanning calorimetry under non-isothermal regime. Application to the crystallization of the Ge0.08Sb0.15Se0.77 chalcogenide glass

In this article, a theoretical procedure has been developed for the kinetic study of the glass-crystal transformations under continuous heating regime in materials involving formation and growth of nuclei, obtaining the corresponding parameters: kinetic exponent, activation energy, and pre-exponential frequency. It is an integral procedure, which is based on a transformation rate independent of the thermal history of the material and assumes that the quoted rate depends only on the transformed fraction, x, across the f(x) function, and temperature, considering that these variables are independent ones. Therefore, the transformation rate is expressed as the product of two separable functions of absolute temperature and the transformed fraction. The quoted f(x) function corresponds to a theoretical method that we have developed and recently published, whose details are given in the Sect. “Basic theory” of this study. The above-mentioned integral procedure considers the same pair of temperatures for the different heating rates and obtains a constant value for temperature integral and, therefore, a plot of a function of the transformed fraction versus the reciprocal of the heating rate leads to a straight line with an intercept of zero, if the value of kinetic exponent is correctly chosen. The process may be repeated for other pairs of temperatures and, consequently, other straight lines will be obtained. By using different values of kinetic exponent for each of the quoted lines, it is taken the best correlation coefficient to choose the quoted exponent more suitable. On the other hand, by using the first mean value theorem to approach the temperature integral, one obtains a relationship between a function of the temperature and other function of the transformed fraction. The logarithmic form of the quoted relationship leads to a straight line, whose slope and intercept allow the obtaining of the activation energy and of the pre-exponential frequency. In addition, this study applies the quoted procedure to the analysis of the crystallization kinetics of the Ge_0.08Sb_0.15Se_0.77 glassy alloy resulting in ranges of variation both of the kinetic exponent, n, with the temperature and of the activation energy, E, with the crystallized fraction, which contain the values of n and E, obtained according to the already quoted theoretical method, that we have developed and recently published. This fact shows the reliability of the theoretical procedure described in this article to analyze the glass-crystal transformation kinetics of glassy alloy.