Properties of Pyrophoric Metals

Physical Properties. An overview of the metallurgy (qv) and soUd-state physics of the rare earths is available (6). The rare earths form aUoys with most metals. They can be present interstitiaUy, in soUd solutions, or as intermetaUic compounds in a second phase. Alloying with other elements can make the rare earths either pyrophoric or corrosion resistant. It is extremely important, when determining physical constants, that the materials are very pure and weU characteri2ed. AU impurity levels in the sample should be known. Some properties of the lanthanides are Usted in Table 3. 

Zirconium. While almost any metal in the finely divided state exhibits pyrophoric properties, a few metals when abraded emit a shower of sparks of sufficient temp to ignite hydrocarbon vapors. Cerium is the best known metal of this kind for commercial purposes, such as gas lighter flints. For military purposes Zr is the most used. It has found applications in HE and armor-piercing incendiary ammo, the lining of shaped-charge rounds, and in incendiary cluster bombs (Ref 7, pp 100-01)... 

Tin finds widespread use because of its resistance to corrosion, or as foil or to provide protective coats/plates for other metals. Properties of lead which make industrial application attractive surround its soft, plastic nature permitting it to be rolled into sheets or extruded through dies. In the finely-divided state lead powder is pyrophoric in bulk form the rapidly-formed protective oxide layer inhibits further reaction. It dissolves slowly in mineral acids. Industrial uses include roofing material, piping, and vessel linings, e.g. for acid storage. 

Finely divided aluminium powder or dust forms highly explosive dispersions in air [1], and all aspects of prevention of aluminium dust explosions are covered in 2 recent US National Fire Codes [2], The effects on ignition properties of impurities introduced by recycled metal used to prepare dust were studied [3], Pyrophoricity is eliminated by surface coating aluminium powder with polystyrene [4], Explosion hazards involved in arc and flame spraying of the powder are analysed and discussed [5], and the effect of surface oxide layers on flammability was studied [6], The causes of a severe explosion in 1983 in a plant producing fine aluminium powder are analysed, and improvements in safety practices discussed... 

Considering the right-hand side of this equation as a simple mathematical function, it can be plotted versus (Sir). The left-hand side is known for a given metal it contains known thermochemical and thermophysical properties, thus (Sir) is determined. The mass of the oxide formed is greater than the mass of the metal consumed, consequently the original size of the metal that would be pyrophoric (rm) can be calculated from S, r and the physical properties of the oxide and metal and their molecular weights. These results have been presented in the... 

Another difficulty arises from the chemical properties of the actinide metals. They are chemically reactive, rapidly corroded by moist air, pyrophoric, and, when in the molten state, dissolve common crucible materials. The radioactivity of short-lived isotopes of Am and Cm makes their long-term storage difficult small amounts can be stored successfully under ultrahigh vacuum. Large amounts of isotopes such 238pu with a Ti/2 of only 87.7 years are best stored under a pure inert... 

U is a member of the actinide series of elements which, together with the rare earths and the transition elements, possess a high heat of oxidation, a low oxide density compared with that of the metal, and the presence of an unfilled d shell in its electronic structure. While the reasons for the high pyrophoric potential of U are not clearly understood, they are thought to be related to these aforementioned properties (see under Pyrotechnics in Vol 8, P511 and Pyrophoric Incendiary Agents , P503-L)... 

The combustion of Zn powder in air has been investigated (Ref 18). Owing to the higher vapor pressure of Zn compared with that of Al, ignition temps for Zn are also lower. Complete combustion of Zn powder takes place above 980° whereas the ignition temp for Al powder under the same exptl conditions is 1765° with a combustion temp of 2160—2290°. The process of self-oxidation of Zn dust is stimulated by humidity and the presence of chlorides, whereas the composition of the metallic phase, and the specific surface of the Zn dust were shown to be of minor importance in determining the pyrophoric properties (Ref 19). For a discussion of... 

W.W. Hillstrom (Ref 21) attempted to correlate the pyrophoricity of bulk metals with the kinetic energy at impact on a variety of targets, both projectile and target being characterized by Brinnel hardness. Such a study would be of value in the opinion of this author, if the shock propagation properties of the materials at the moment of impact had been used instead. Hillstrom s findings may be instructive (Table 5) ... 

Pyrophoricity is a property of metals and oxides of lower oxidation states, including radioactive ones, in which they spontaneously ignite during or after stabilization. If the waste also contains other combustible matter, it will burn. For a waste form to be safe from spontaneously igniting, the metals and oxides of lower oxidation states must be fully oxidized. To determine whether a particular waste is pyrophoric or not, one must identify the pyrophoric components within the waste. Such identification is done by analytical methods such as X-ray diffraction. 

Similarly to the lanthanides, actinides in the elemental state are reactive electropositive metals and pyrophoric in finely dispersed form. Strong reducing agents are necessary to prepare the metals from their compounds, for instance reduction of the halides by Ca or Ba at 1200°C (e.g. Pup4 + 2Ca Pu -I- 2CaF2). Some properties of the actinides in the metallic state are fisted in Table 14.5. The number of metallic modifications and the densities are remarkably high for U, Np and Pu. Some modifications of these elements are of low symmetry this is an exception for metals that is explained by the influence of the f electrons. The properties of Am and the following elements correspond to those of the lanthanides. 

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