Niobium physical properties

Preparation of Metallic Niobium—Physical Properties of the Metal, Optical Properties, Arc Spectrum—Chemical Properties—Electromotive Behaviour —Atomio Weight—Alloys. 

Low Expansion Alloys. Binary Fe—Ni alloys as well as several alloys of the type Fe—Ni—X, where X = Cr or Co, are utilized for their low thermal expansion coefficients over a limited temperature range. Other elements also may be added to provide altered mechanical or physical properties. Common trade names include Invar (64%Fe—36%Ni), F.linvar (52%Fe—36%Ni—12%Cr) and super Invar (63%Fe—32%Ni—5%Co). These alloys, which have many commercial appHcations, are typically used at low (25—500°C) temperatures. Exceptions are automotive pistons and components of gas turbines. These alloys are useful to about 650°C while retaining low coefficients of thermal expansion. Alloys 903, 907, and 909, based on 42%Fe—38%Ni—13%Co and having varying amounts of niobium, titanium, and aluminum, are examples of such alloys (2). 

Physical Properties. Molybdenum has many unique properties, leading to its importance as a refractory metal (see Refractories). Molybdenum, atomic no. 42, is in Group 6 (VIB) of the Periodic Table between chromium and tungsten vertically and niobium and technetium horizontally. It has a silvery gray appearance. The most stable valence states are +6, +4, and 0 lower, less stable valence states are +5, +3, and +2. 

Occurrence. Niobium and tantalum usually occur together. Niobium never occurs as the metal, ie, ia the free state. Sometimes it occurs as a hydroxide, siUcate, or borate most often it is combiaed with oxygen and another metal, forming a niobate or tantalate ia which the niobium and tantalum isomorphously replace one another with Htde change ia physical properties except density. Ore concentrations of niobium usually occur as carbonatites and are associated with tantalum ia pegmatites and alluvial deposits. Principal niobium-beariag minerals can be divided iato two groups, the titano- and tantalo-niobates. 

The basic corrosion behaviour of stainless steels is dependent upon the type and quantity of alloying. Chromium is the universally present element but nickel, molybdenum, copper, nitrogen, vanadium, tungsten, titanium and niobium are also used for a variety of reasons. However, all elements can affect metallurgy, and thus mechanical and physical properties, so sometimes desirable corrosion resisting aspects may involve acceptance of less than ideal mechanical properties and vice versa. 

Loop Tests Loop test installations vary widely in size and complexity, but they may be divided into two major categories (c) thermal-convection loops and (b) forced-convection loops. In both types, the liquid medium flows through a continuous loop or harp mounted vertically, one leg being heated whilst the other is cooled to maintain a constant temperature across the system. In the former type, flow is induced by thermal convection, and the flow rate is dependent on the relative heights of the heated and cooled sections, on the temperature gradient and on the physical properties of the liquid. The principle of the thermal convective loop is illustrated in Fig. 19.26. This method was used by De Van and Sessions to study mass transfer of niobium-based alloys in flowing lithium, and by De Van and Jansen to determine the transport rates of nitrogen and carbon between vanadium alloys and stainless steels in liquid sodium. 

The 5th group metals a summary of their atomic and physical properties Vanadium, niobium and tantalum have only the bcc, W-type, structure no high-temperature or high-pressure polymorphs are known. 

Other physical properties of molybdenum are given under Chemical Elements. See also summary of properties of refractory metals under Niobium. 

Other important physical properties of niobium are given in the Table 1 and under Chemical Elements. 

The usefulness of tantalum (and niobium) as container material derives from two physical properties of these elements. First, their ductility remains high even in the presence of significant nonmetallic impurities, as noted above. Sec-... 

Note that we have a structure with a "built-in crystal defect, a vacancy. Both the lithium and niobium cations are in an octahedral coordination. In fact, the two ions, Li and Nb , have nearly the same radius and occupy octahedral sites with the same Cgy S5mimetry. The lithium deficiency in congruent crystals is accommodated by means of Nb anti-sites and Nb vacancies in a relative concentration that guaranties overall electrical neutrality. Note that many physical properties depend upon stoichiometry, e.g.- Curie temperature, absorption spectra, lattice parameters and photorefractive yield. 

Lithium niobate [niobium + -ate[ (1966) n. LiNb03. A crystaUine material whose physical properties change in response to pressure or the presence of an electric field and which is used in fiber optics and as a synthetic gemstone. 

Chlorination of ferroniobium has been carried out by the U.K. Atomic Energy Authority first at the Culcheth Laboratories and later at Spring-fields, in a development plant with a capacity of about 1 tonne/annum. As with some other chlorination processes, the design and operation of plant for the reaction with chlorine itself is relatively easy, and a major proportion of the effort must be devoted to the production of the niobium pentachloride or trichloride in a pure state. In this case the problem is made particularly difficult by the presence of a high proportion of tantalum in the ferroniobium, an element which beairs a very close resemblance to niobium in its chemical properties. The physical properties of its compounds are also similar to the corresponding niobium compounds. 

---