Zirconium oxide thermal conductivity

Fig. 7.16.2 shows a cross section of the sensing element. The sensing element mainly consists of six layers of yttrium-doped zirconium oxide, which has a high capability against thermal stress and promotes O2- conductivity. The basis for such Oz conductivity is the temperature of more than 600 °C. The pumping electrodes mainly consist of platinum. 

The major use for zirconium is in the nuclear industry. Zirconium alloys (zircaloys) are used extensively as a cladding for nuclear (uranium oxide) fuel rods in water cooled reactors. Zircaloys were favoured over stainless steel cladding because they had a considerably lower neutron cross-section, appropriate thermal conductivity and both corrosion and mechanical resistance. As indicated, hafnium is an impurity in nearly all zirconium ores. Hafnium, however, has a much higher neutron cross-section than zirconium and, as such, the two elements must be separated prior to using zirconium in fuel rod cladding. For many years the separation was very difficult due to the chemical similarity of the two elements. Zirconium hydride is used as a moderator in nuclear reactors. 

Refractory Fibers Recently, zirconia-based insulating material with a low density and a low thermal conductivity has been developed in the form of fibers, paper, felt, board and shaped articles. The material is a cubic zirconia soHd solution stabilized with yttria, and has a maximum usable temperature of >2100 C. The innovative fabrication technique involves the use of an organic precursor fiber as a structural template, impregnated with an aqueous solution of zirconium chloride and yttrium chloride. The metallic salts are deposited within the organic fiber, which can subsequently be burned off by a controlled oxidation. The hollow remainder is then fired at a sufficiently high temperature (800-1300 °C) so as to induce crystallization, after which the oxide particles are sintered to develop a ceramic bond. Other techniques to produce refractory fibers involve phase inver-... 

The mineral zircon ZrSiO is used for moulds in foundries. Its high thermal conductivity improves the coohng rate compared to other mould materials. Zirconium oxide is used in firebricks. It has very good temperature resistance and is, after sintering, also used for crucibles and other components for work at high temperatures. When zirconia is heated, a phase transformation occurs at a temperature of about 1200°C. 

This additional component of heat conduction in metals results in metals having higher thermal conductivities than most ceramics, which have virtually no electrical conductivity. Exceptions to this statement are beryllium oxide, aluminum nitride, diamond, and cubic zirconium oxide. These materials will be discussed more fuUy in later sections. 

BeO is a metallic oxide with a very high thermal conductivity. BeO is chemically compatible with UO2, most sheath materials including zirconium alloys, and water. In addition to its chemical compatibility, BeO is insoluble with UO2 at temperatures up to 2160°C. As a result, BeO remains as a continuous second solid phase in the UO2 fuel matrix while being in good contact with UO2 molecules at the grain boundaries. BeO has desirable thermochemical and neutronic properties, which have resulted in the use of BeO in aerospace, electrical, and nuclear applications. For example, BeO has been used as the moderator and the reflector in some nuclear reactors. However, the major concern with beryllium is its toxicity. But the requirements for safe handling of BeO are similar to those of UO2. Therefore, the toxicity of BeO is not a limiting factor in the use of this material with UO2 (Solomon et al., 2005). 

The sol-gel chemistry of Zr02 is similar to other tetravalent metal compounds such as Si02 and Ti02 Precursors such as zirconium halides and alkoxides are largely available, and they all hydrolyze rapidly in the presence of water. Zirconium oxide (and especially yttrium-stabilized Zr02 (YSZ)) is widely used as a thermal barrier but also as an ionic conductor (electroceramic). Even if it is not widely used for gas detection, its ionic conductivity makes it attractive as a sensor to control the oxygen level and thus the air/luel ratio in internal combustion engines. 

Polydimethylsiloxane (PDMS)/zirconia/HPW hybrid membranes 80 and 500 pm thick were cast by modified sol-gel technique [160], where HPW (1.6-14 wt.%) was incorporated with Zr/PDMS hybrid matrix by Coulomb force and/or ionic bonding. Flexible, homogeneous, and transparent hybrid membranes were formed in the molar ratio of zirconium to PDMS from 2 to 8, while at higher zirconium concentration, the membrane became brittle and soft, and the hybrid membranes synthesized with HPW were heterogeneous. IR studies confirmed the formation of bond between Zirconium oxide moiety and PDMS via Zr-O-Si bonds. Zr/PDMS hybrid membranes, therefore, possess a good thermal stability at least up to 300°C and shows small proton conductivity of the order of 10 S/cm, and with increase in temperature, the conductivity increased from 4 x 10 S/cm at 30°C to 5 X 10 S/cm at 150°C. The conductivity of the membrane was improved on the... 

Ceramic borides, carbides and nitrides are characterized by high melting points, chemical inertness and relatively good oxidation resistance in extreme environments, such as conditions experienced during reentry. This family of ceramic materials has come to be known as Ultra High Temperature Ceramics (UHTCs). Some of the earliest work on UHTCs was conducted by the Air Force in the 1960 s and 1970 s. Since then, work has continued sporadically and has primarily been funded by NASA, the Navy and the Air Force. This article summarizes some of the early works, with a focus on hafnium diboride and zirconium diboride-based compositions. These works focused on identifying additives, such as SiC, to improve mechanical or thermal properties, and/or to improve oxidation resistance in extreme environments at temperatures greater than 2000°C. 

The pyrotechnic paper or heat paper supplies the thermal energy to elevate the thermal battery cell to operating temperatures. The pad consists of a ceramic fibre paper, which acts as a binder/carrier for a slurry of zirconium metal fuel and barium chromate oxidant. The paper is extremely fast burning, making it ideal for a fast activation cell. Because the remaining ash of the heat pad is electrically non-conductive, it is necessary to provide a means for series intercell connection. 

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