High-purity metals, melting points

National Institute of Standards and Technology (NIST). The NIST is the source of many of the standards used in chemical and physical analyses in the United States and throughout the world. The standards prepared and distributed by the NIST are used to caUbrate measurement systems and to provide a central basis for uniformity and accuracy of measurement. At present, over 1200 Standard Reference Materials (SRMs) are available and are described by the NIST (15). Included are many steels, nonferrous alloys, high purity metals, primary standards for use in volumetric analysis, microchemical standards, clinical laboratory standards, biological material certified for trace elements, environmental standards, trace element standards, ion-activity standards (for pH and ion-selective electrodes), freezing and melting point standards, colorimetry standards, optical standards, radioactivity standards, particle-size standards, and density standards. Certificates are issued with the standard reference materials showing values for the parameters that have been determined. 

Soft white, ductile metal high-purity metal is very ductile at ordinary temperatures occurs in three allotropic forms (i) body-centered cubic form, alpha iron stable up to 910°C, (ii) face-centered cubic form, gamma iron occurring between 910 to 1,390°C, and (iii) body-centered delta iron allotrope forming above 1,390°C. Density 7.873 g/cm at 20°C melting point 1,538°C vaporizes at 2,861°C hardness (Brinell) 60 electrical resistivity 4.71 microhm-cm at 0°C tensile strength 30,000 psi Poisson s ratio 0.29 modulus of elasticity 28.5 X 10 psi thermal neutron absorption cross-section 2.62 bams velocity of sound 5,130 m/s at 20°C. 

The recrystallization temperature of a metal is dependent on sample purity and amount of prior cold work as well as the melting point. High purity metals will recrystallize at a lower temperature than low purity metals. Metals with analyses approximately the same as given in tables 2.1, 2.3-2.5 were found by Spedding and Beaudry (1971) and Beaudry and Palmer (1974b) to be completely recrystallized in 20 hours when heated to the temperatures listed in table 2.11 after a 25% reduction in thickness. 

Since the vast majority of calibration experiments are done on heating, we will first describe the calibration for heating runs heating calibration). Traditionally, the true temperature in heating calibration experiments is taken from melting points of high-purity water and very high-purity metal standards. The... 

Wire method This method of temperattue calibration uses wires made from high purity metals of known melting points such as In, Sn, Pb, Zn, Al, Ag, Au, Ni and Pd. These wires are used to attach a mass to the sample pan, which will detach immediately when the wire melts. These substances cover the broad temperature range (430-1800 K) of measurement encountered in the present work. The observed melting points were used in systems data acquisition program for subsequent temperatme corrections. 

Metals which have been used (generally in inert or reducing atmosphere) as container materials are W (melting point 3422°C), Mo (2623°C), Pt (1769°C), Fe (1538°C), Ni (1455°C), Cu (1085°C), Au (1064°C), Ag (962°C). W and Mo do not react with many elements they must be protected however from air oxidation. Pt and Au cannot be used, owing to their reactivity, for melting metallic materials they are useful for other types of synthesis. Fe, of very high purity and with very low carbon content, could possibly be used for melting alkaline and alkaline earth metals and a number of their alloys. 

Levitation melting (Section III, A) of Pa metal in high vacuum results in a considerable increase in purity. Metallic Pa resembles Th metal in that it has a very high melting point and a low vapor pressure. 

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