Barium electrical resistivity

Positive temperature coefficient (PCT) thermistors are solids, usually consisting of barium titanate, BaTiOi, in which the electrical resistivity increases dramatically with temperature over a narrow range of temperatures (Fig. 3.38). These devices are used for protection against power, current, and thermal overloads. When turned on, the thermistor has a low resitivity that allows a high current to flow. This in turn heats the thermistor, and if the temperature rise is sufficiently high, the device switches abruptly to the high resisitvity state, which effectively switches off the current flow. 

An exceptionally pure form of reduced iron has been obtained by Lambert and Thomson 4 by reduction of pure, colourless crystals of ferric nitrate.5 The crystals were first converted into oxide or basic nitrate by ignition in an iridium boat.6 The whole was then introduced into a silica tube and heated in an electric resistance furnace to just above 1000° C. in a current of pure hydrogen gas, obtained by the electrolysis of barium hydroxide solution. 

The electrical resistivity and thermopower of metallic calcium, strontium, and barium have been measured from room temperature to near their melting points. From discontinuities observed in these parameters as functions of temperature, the f.c.c.-b.c.c. phase transition was determined in calcium at 428 2 °C and in strontium at 542 2 C, both at ambient pressure. Four compounds have been identified in the Ca-Ni system by means of X-ray methods. The intermetallic... 

Titanates are double oxides of the form MeTiOa or Me2Ti04. Barium titanate BaTiOa and its solid solution crystals with other titanates are especially well-known. BaTiOs crystallizes in the perovskite structure. Its technical importance results from its ferroelectric and associated piezoelectric properties, its high dielectric constant at room temperature, and the interesting semiconducting properties which it exhibits when doped [13]. The remarkable temperature dependence of the electrical resistance of such doped material (the temperature coefficient can be metal-like) is used to advantage in control and circuit devices. 

C.700 HIGH ALUMINA ceramics C.800 OXIDE CERAMICS The standard also defines six classes of glass insulating materials, based on composition G.lOO alkali-lime-silica G.200 and G.300, borosilicates (chemically and electrically resistant respectively) G.400 alumina-lime-silicates G.500 lead oxide alkali silica G.600 barium oxide alkali silica. 

The discovery of high-temperature superconductivity in mixed oxides, such as the lanthanum-barium-copper oxide complexes, has created a great deal of interest in these materials. Superconductivity, that is, the absence of any resistance to the flow of electric current, is now possible at temperatures above the temperature of liquid nitrogen (77K). Many problems remain in the development of practical processes for these materials and commercialization is not likely to occur until these problems are solved. Among the several processing techniques now used, CVD appears one of the most successful. 

One of the most exciting developments in materials science in recent years involves mixed oxides containing rare earth metals. Some of these compounds are superconductors, as described in our Chemistry and Technology Box. Below a certain temperature, a superconductor can carry an immense electrical current without losses from resistance. Before 1986, it was thought that this property was limited to a few metals at temperatures below 25 K. Then it was found that a mixed oxide of lanthanum, barium, and copper showed superconductivity at around 30 K, and since then the temperature threshold for superconductivity has been advanced to 135 K. 

The compound consisting of yttrium, copper, and barium oxide, commonly called compound 1-2-3, was formed in 1987 by research scientists at the universities of Alabama and Houston. It had limited superconducting capabilities. It has been known for some time that conductors of electricity such as copper resist, to some extent, the flow of electrons at normal temperatures, but at temperatures near absolute zero (zero Kelvin = -273°C), this resistance to the flow of electrons in some materials is reduced or eliminated. The 1-2-3 compound proved to be superconducting at just 93°K, which is still much too cold to be used for everyday transmission of electricity at normal temperatures. Research continues to explore compounds that may achieve the goal of high-temperature superconductivity. 

Some polysiloxanes are curable with lead monoxide, with a consequent reduction in both curing time and temperature. High-frequency electrical energy vulcanizes in one case at least. Zirconium naphthenate imparts improved resistance to high temperatures. Barium salts are said to prevent blooming. Sulfur dichloride is also used. Some resins are solidified by pressure vulcanization, using di-f-butyl peroxide. Improvements are to be found in lower condensation temperatures and shorter times of treatment... 

One fateful day in 1980, as the people down at the Institute of Electrical and Electronic Engineers like to tell the story, Rustum Roy, a physical chemist at Penn State, became disenchanted with his experiments in superconductivity, which is the ability of some substances, when cooled to very low temperatures, to conduct electricity without resistance and without loss. He had been experimenting for five years with ceramics—notably with a barium-lead-bismuth oxide mixture—but despite the long hours and the hard work, he could not get his concoction to superconduct at temperatures any higher than a few degrees above what one might encounter in outer space. 

K later they determined that the drop was a fluke, that subtle shifts in resistance in the contacts between the electrical leads and the sample, and not in the sample itself, were responsible. Sumitomo Electric Industries of Japan came in with 300° K (no confirmation]. In Michigan, researchers at Energy Conversion Devices announced that part of a synthetic material made of fluorine (a highly dangerous yellow gas), yttrium, barium, and copper oxide had superconducted at 45° to 90° F. (The part that super-conducted, it turned out, represented less than 1 percent of the material tested, and the samples were far too small to lose all resistance. It is incredibly difficult to identify the exact portion of any material that shows superconductivity and then produce a pure sample of it.) In New Delhi, at the National Physical Laboratory, scientists saw evidence of superconductivity in material heated to 80° F, but the electrical signals were misleading, an artifact of the measurement process. 

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