Electrical resistivity yttrium

Working with the yttrium mixture, the researchers placed it in a vacuum and fired several thousand laser shots at it, ten pulses per second. The process, called pulsed excimer laser evaporation, produces a vapor from the superconducting compound with each shot. It is the vapor that is deposited on a sample material to form the ultrathin layers. After the film has been built up, it is baked at high temperature, and when cooled, it shows a large reduction in electrical resistivity beginning at about 90° K (the so-called onset temperature) and full superconductivity (zero resistivity) at 83° K. These temperatures are in the same range as in the original bulk material. 

The most frequently used source of infrared light for infrared spectrometers is so called the Nemst stick. This stick is about two to four centimeters long and one to three millimeters thick, and is made from zirconium oxide with additions of yttrium oxide and oxides of other metals. This mixture of oxides has a negative temperature coefficient of electrical resistance. This means that its electrical conductivity increases with an increase in temperature. At room temperature, the Nemst stick is a non-conductor. Thus, an auxiliary heating is necessary for ignition of the Nernst stick. Even if the Nernst stick is red-hot, it can be heated further by electricity. The normal operating temperature of this infrared light source is approximately 1900 K. 

Figure 5-3. Temperature dependence of the electrical resistivity of yttrium oxide. (Reproduced with permission from ref 6. Copyright 2000 Indian Association for the Cultivation of Science.)...

Markova et al. (1967) investigated the terbium-yttrium system by means of microscopy, X-ray diffraction, thermal analysis, hardness and electrical resistance measurements. Their starting materials were distilled yttrium of 99.6 to 99.7 (wt )% purity and terbium of 98.5 to 99% purity. Impurities in their terbium included yttrium, gadolinium, dysprosium, calcium, copper, iron and tantalum. Both metals contained gaseous impurities. Alloys were melted in an arc furnace under a helium atmosphere and annealed at 850°C for 70hr. 

Investigations of phase equilibria in the erbium-yttrium system have been conducted by Markova et al. (1964) and by Spedding et al. (1973). The first group used distilled yttrium metal of 99.6(wt )% purity and erbium metal of about 98(wt )% purity. The principal impurities included Ca, Fe, Cu, Ta and other rare earth metals. The alloys were arc-melted under purified helium and studied in the annealed state. Microscopy, differential thermal analyses and X-ray methods were utilized and measurements of hardness and electrical resistivity were performed on the alloy specimens. The main difficulty experienced in their thermal analysis was the narrow temperature interval between the melting of their alloys and the polymorphic transformations. 

Fig. 1. The variation in electrical resistivity (dashed line) and optical transmittance (solid line) as a function of hydrogen exposure time for a 300 nm yttrium film capped with 20 nm of Pd over layer on exposure to H2 gas at 10 Pa pressure at room temperature (Huiberts et al., 1996a Griessen, 2001).

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. 

Chu kept it up, trying one recipe after another like a chef in pursuit of the perfect sauce. Finally, he and his team, along with Maw-Kuen Wu at the team s University of Alabama unit (Wu was a former graduate student of Chu s), replaced the lanthanum with the rare earth yttrium. They heated the mixture for hours at 1,652° F, ground the solid mass produced, and sintered it at 2,192° F. Wu drenched it with coolant, but this time he used liquid nitrogen. When Wu passed an electric current through the new ceramic, its resistance dropped sharply—at what physicists would later call a balmy 93° K (-292° F). We were so excited and so nervous, recalled Wu, that our hands were shaking. At first we were suspicious that it was an error. ... 

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. 

One of the important uses for yttrium is in superconductors. Superconductors were first discovered by Dutch physicist Heike Kamerlingh-Onnes (1853-1926) in 1911. Kamerlingh-Onnes found that certain metals cooled to nearly absolute zero lose all resistance to an electric current. Absolute zero is the coldest temperature possible, about 59°F (-273°Q. Once an electric current gets started in these very cold metals, it keeps going forever. These metals are called superconductors. 

Superconductors are materials that have the ability to conduct electricity without resistance below a critical temperature above absolute zero. The phenomenon of superconductivity was first seen in mercury at liquid helium temperatures. Great interest developed in this area in the late 1980s, when Muller and Bednorz discovered that even ceramic-like materials can exhibit superconductivity. C. W. Chu subsequently found yttrium barium copper oxide (YBCO) to be superconducting above liquid nitrogen temperatures. Indeed, various books are devoted to this subject. > In the following subsections we highlight representative force field applications that have aided the understanding of static and dynamic properties of superconductors. 

F is the Faraday, Mq2 ehemical potential of molecular oxygen 2pq), f, the ionie transport number (see section 11.1.6) function of M02 T, the average transport in the membrane thickness (not very different from 1 for a solid electrolyte). The measure of this electric potential is a direct measure of the differenee in oxygen ehemieal potential. If one of the two is a reference, then the cell acts as an oxygen probe. Tlie solid electrolyte is zirconia stabilized with lime or yttrium. The addition of magnesium reinforces the resistance to thermal shocks. The reference eould be either air (P02 the combination of a metal and its... 

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