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Abstract  

Excess molar enthalpies, measured at 298.15 K in a Calvet microcalorimeter, are reported for {x 1 tert-butyl methyl ether (MTBE)+x 2ethanol (EtOH)+(1−x 1x 2)heptane}. Smooth representations of the results are presented and used to construct contant excess molar enthalpy contours on Roozeboom diagrams.

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Abstract  

In the present paper, a new improved expression for -ray detection efficiency of Ge(Li) detectors, 0 , is given. It is represented as a continuous function of x (viz. E –1) with a maximum and decreases very rapidly to a small positive value as -ray energy, E, drops to 40 keV or lower, but slowly as E rises to 1.7 MeV or higher. Since it can well represent the whole physical process of the -ray detection, this expression may be one of the simplest and most precise representations, for 0 at the present time.

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Abstract  

A methodology of comparison is presented as a way to determine the origin of ceramic fragments proceeding from clays and archaeological sites of the 9th Region of Chile, analyzed through X-ray fluorescence (XRF) spectroscopy. Net areas of peak of sample spectra, are analysed using the graphic polygonal method, developed previously.1,2 Criterions of comparison have been defined as a way to determine similarity degrees and proceeding places. Polygonal representations used are triangles, squares, pentagons and hexagons; the similarity grading among samples is defined. Results are compared with clay samples and sand removing effects on fragments are also defined.

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Abstract  

This brief description of cyclic activation analysis includes both a theoretical interpretation, by way of graphical representations and experimental considerations, for the determination of the concentration of elements by measurement of short-lived isotopes. Particular attention is given to the effect of sample transfer and the half-life of the background matrix. The use of a Ge(Li) detector and a low energy photon detector of intrinsic Ge, are also summarily discussed based on the experience of the authors' work in biological and environmental samples and the establishment of rapid, routine, multielemental and instrumental techniques.

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Journal of Radioanalytical and Nuclear Chemistry
Authors: E. L. Dorval, M. A. Arribére, S. Ribeiro Guevara, I. M. Cohen, A. J. Kestelman, R. A. Ohaco, M. S. Segovia, A. N. Yunes and M. Arrondo

Summary  

We have measured the cross sections, averaged over a 235U fission neutron spectrum, for the two high threshold reactions: 75As(n,p)75mGe and 75As(n,2n)74As. The measured averaged cross sections are 0.292±0.022 mb, referred to the 3.95±0.20 mb standard for the 27Al(n,p)27Mg averaged cross section, and 0.371±0.032 mb referred to the 111±3 mb standard for the 58Ni(n,p)58m+gCo averaged cross section, respectively. The measured averaged cross sections were also evaluated semi-empirically by numerically integrating experimental differential cross section data extracted for both reactions from the current literature. The calculations were performed for four different representations of the thermal-neutron-induced 235U fission neutron spectrum. The calculated cross sections, though depending on analytical representation of the flux, agree with the measured values within the estimated uncertainties.

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Journal of Thermal Analysis and Calorimetry
Authors: Visweswararao Badipatla, Dean Pohlman, Manik Pavan Maheswaram, Dhruthiman Mantheni, Naullage Indika Perera, Martin Mittleman, Kenneth Alexander and Alan Riga

Abstract

The current United States Pharmacopeia (USP) test for the disintegration of drug tablets does not measure initial disintegration times and does not adequately describe tablet disintegration mechanisms. An Isothermal Mechanical Analysis (IsoTMA) method meeting USP specifications has been developed to measure the initial time and rate of drug disintegration. TMA monitors the physical dimension of the formulated drug tablet as a function of time, temperature, applied stress, and pH. TMA can be used to measure the swelling, shrinkage, or disintegration of a formulated tablet in a specified fluid. The focus of this study is to validate an efficient and precise IsoTMA method to measure dimensional stability of solid dose tablets. The precision of the method along with the effect of pH (1–10) and temperature (25–37 °C) on the rate of delivery was determined for nine drugs. Graphical representations of dimensional changes over time were created and compared. Drug delivery in a specific liquid medium was measured by UV analysis for the active pharmaceutical ingredient. An increase in temperature decreased the disintegration time and increased the disintegration rate (mm/min). For the drugs that are studied in this article, pH did not have an appreciable effect on the rate of disintegration.

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Melting and thermal decompositions of solids

An appraisal of mechanistic interpretations of thermal processes in crystals

Journal of Thermal Analysis and Calorimetry
Author: Andrew K. Galwey

Abstract

This analysis of interface phenomena considers the alternative processes that may result from heating a crystal, particularly including thermal decomposition, involving chemical reactions, and melting, involving loss of long-range structural order. Such comparisons are expected to provide insights into the factors that determine and control the different types of thermal changes of solids. The survey also critically reviews some theoretical concepts that are currently used to describe solid-state thermal reactions and which provides relevant background information to models used in a recently proposed theory of melting. Probable reasons for the current lack of progress in characterizing the factors that control chemical changes and mechanisms of thermal reactions in solids are also discussed.

It is concluded that some aspects of the macro properties of reaction interfaces in crystal reactions have been adequately described, including geometric representations of interface advance during nucleation and growth processes. In contrast, relatively very little is known about the detailed (micro) processes occurring within these active, advancing interfacial zones: reactant/product contacts during chemical reactions and crystal/melt contacts during fusion. From the patterns of behaviour distinguished, a correlation scheme, based on relative stabilities of crystal structures and components therein, is proposed, which accounts for the four principal types of thermal changes that occur on heating solids: sublimation, decomposition, crystallographic transformation or melting. Identifications of the reasons for these different consequences of heating are expected to contribute towards increasing our understanding of each of the individual processes mentioned and to advance theory of the thermal chemistry of solids, currently enjoying a prolonged quiescent phase.

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The exponential integral in non-isothermal kinetic equations for tempering with linear heating can be represented in the following analytical form

\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{upgreek} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} $$\int\limits_0^T {e^{ - E/kT'} dT'} = \frac{{kT^2 /E}}{{\sqrt {1 + 4kT/E} }}e^{ - E/kT} ,$$ \end{document}
which is one order inkT/E≪1 more accurate than two other representations recently proposed in this journal [1, 2]. A few variants of approximated forms for the exponential integral are compared with regard to the error due to the kind of approximation, which appears when activation energies are evaluated from experimental non-isothermal kinetic curves.

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Rheology of nanocomposites

Modelling and interpretation of nanofiller influence

Journal of Thermal Analysis and Calorimetry
Authors: Christophe Block, Nick Watzeels, Hubert Rahier, Bruno Van Mele and Guy Van Assche

different representations, such as Cole–Cole plots [ 1 , 7 , 8 , 12 ], where the imaginary part of the complex viscosity is plotted against the real part, and Van Gurp–Palmen plots [ 12 , 13 ], where the loss angle is plotted as a function of the modulus

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representations. But, with the multiplication by T to provide the units of energy required for all of the terms in the Gibbs free-energy equation, a value is acquired that, for solid substances, is very close to twice that of the actual thermal energy

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