Niobium turning

The hiding of the pyroelectric coefficients seems to be correlated to the maximum c parameter, which in turn corresponds to the transition temperature. The shift along the Oz direction, Az, of the niobium atoms, which are located within the octahedrons, is responsible for the compound s polar properties. When c is at its maximum, this shift is enhanced and leads apparently to maximum spontaneous polarization P The value of Ps increases in the temperature range of 300 to 490K and then decreases at temperatures above 490°K. 

The resulting product depends on the precipitation conditions, and in particular, on the over-saturation level of the solution. Formation of ammonium oxyfluorometalate crystalline compounds occurs at a relatively low pH of the solution. From the standpoint of the interactions described in Equation (143), this means that the interaction between NH4F and Me205 (denoted as interaction 1) is stronger than the interaction denoted as interaction 2. In this case, subsequent processes of the hydroxide treatment lead to some defluorination of the product, but the performance of such processes is usually very problematic. Precipitation at high pH values leads to a strong oversaturation of niobium- or tantalum-containing compounds, which in turn... 

Ammonium hydrofluoride is relatively stable, even in the molten state. In addition to being in contact with tantalum or niobium oxide, the compound will initiate the fluorination process yielding complex tantalum or niobium fluoride compounds. There is no doubt that thermal treatment of the hydroxides at high temperatures and/or at a high temperature rate leads to the enhancement of the defluorination processes, which in turn results in an increase in fluorine content of the final oxides. 

For example, if we consider the list of what Brock (1992) calls Mendeleev s Tater predictions , then alongside the five successes—eka-manganese (technetium, discovered in 1939), tri-manganese (rhenium, 1925), dvi-tellurium (polonium, 1898), dvi-caesium (francium, 1939), eka-tantalum (protactinium, 1917)—there were four failures— coronium (which turned out to be ionised iron), ether, eka-niobium and eka-caesium. (Since many of these predictions were made in the 1871 paper, Brock s reason for calling them the later predictions was presumably that they were the ones whose empirical fate was settled only later.)... 

The first alkali metal-niobium-arsenic compounds were synthesized by accident while attempting the synthesis of alkali-metal main-group arsenides at relatively high temperature. It turns out that niobium and tantalum containers react readily... 

Materials. Niobium metal of low oxygen content was prepared by carbon reduction of high purity niobium(V) oxide obtained from Fansteel Metallurgical Corp. The metal then was arc-melted, cut into fine turnings, and outgassed at a pressure of 8 X 10 5 mm. of Hg and a temperature of 2050° for 2 hours. 

Soon after, Bernd Matthias, then with Bell Laboratories, concocted something called niobium-3 tin, which handled superconductivity at 18° K, a milestone of sorts but still enormously cold when one considers the temperature is equal to -427° F. It s difficult not to compare these early efforts—indeed, the current-day ones as well—to those of medieval alchemists searching for the lapis philosopho-rum, the philosopher s stone, that ill-defined soluble substance endowed with the power to change base metals into gold and silver, a stone capable of purging a metal of its impurities so that it could be turned into some precious substance, or one with some useful purpose. It was all a matter of try it and hope. 

Still, no matter how researchers varied the ingredients and their amounts, or how many they laboriously mixed, pulverized, calcified, crystallized, and precipitated, many of the materials they came up with were not, despite their superconductivity, all that serviceable. Niobium-3 tin, for example, could support large electric currents and remain superconducting, even in intense magnetic fields. But although promising, it proved to be too brittle and, as it turned out, was never used as extensively as scientists had hoped. 

Figure 12.4. Block diagram of a modem NMR spectrometer. These systems use superconducting magnets that are based on a solenoid of a suitable alloy (e.g., niobium/titanium or niobium/tin) immersed in a dewar of liquid helium. The extremely low temperature of the magnet itself (4.2 K) is well insulated from the sample chamber in the center of the magnet bore. The probe in which the sample is housed usually incorporates accurate temperature control over the range typically of 4 to 40°C for biological samples. The rf coil in the probe is connected in turn to a preamplifier, receiver circuitry, analog-to-digital converter (ADC), and a computer for data collection.

Niobium and cobalt clusters exhibit size-sensitive reactions with nitrogen with a reactivity pattern similar to that observed for hydrogen. The reactivity of rhodium clusters (n = 1-12) toward N2 has also been studied. In this case the atoms through the tetramer appear to be inert, with reactivity turning on at Rhj. Maximum reactivity occurs at Rh7, and subsequently drops off by roughly a factor of 2 in going from Rh, to Rh,. Iron clusters appear to be nearly unreactive toward N2. Attempts to induce low-pressure ammonia synthesis on gas-phase iron clusters indicate that hydrogenated iron clusters Fe H are also unreactive toward N2. ... 

Niobium (Colombium) (V) chloride [10026-12-7] M 270.2, m 204.7-209.5 , b 250 (begins to sublime at 125 ), d 2.75. It forms yellow, very deliquescent crystals which decompose in moist air to liberate HCl. It should be kept in a dry box flushed with N2 in the presence of P2O5. Wash it with CCl4 and dry it over P2O5. The yellow crystals usually contain a few small, dirty white pellets among the yellow needles. These should be easily picked out. Upon grinding in a dry box, however, they turn yellow. NbCls has been sublimed and fractionated in an electric furnace. [Epperson Inorg Synth VII 163 1963, Alexander Fairbrother J Chem Soc suppl 233 7949.]... 

---