Resistance above room temperature

Figure 1. The resistance above room temperature for nominal compositions of Er-Ba Cu OQ (triangles) and Y Ba Cu OQ (squares) upon heating. The inset enhances the region between 200 C to 400 C for the Er BaoCuo0Q. sample.

Polyimide. Polyimide is a biaxiaHy oriented high performance film that is tough, flexible, and temperature- and combustion-resistant. Its room temperature properties compare to poly(ethylene terephthalate), but it retains these good characteristics at temperatures above 400°C. Its electrical resistance is good and it is dimensionally stable. The principal detriment is fairly high moisture absorbance. The main uses are for electrical insulation, particularly where high temperatures are prevalent or ionizing radiation is a problem. The films may be coated to reduce water absorption and enhance... 

Most metals, concrete, and other constmction materials are corroded by hydrobromic acid. Suitable materials of constmction include some fiber glass-reinforced plastics, some chemically resistant mbbers, PVC, Teflon, polypropylene, and ceramic-, mbber-, and glass-lined steel. Metals that are used include HasteUoy B, HasteUoy C, tantalum, and titanium. The HasteUoys can only be used at ambient temperatures. Liquid hydrogen bromide under pressure in glass at or above room temperature can attack the glass resulting in unexpected shattering. 

Resistivity measurements of doped, alpha-siUcon carbide single crystals from —195 to 725°C showed a negative coefficient of resistivity below room temperature, which gradually changed to positive above room temperature (45). The temperature at which the changeover occurred increased as the ionization of the donor impurity increased. This is beUeved to be caused by a change in conduction mechanism. 

In Chapters 3 and 11 reference was made to thermoplastic elastomers of the triblock type. The most well known consist of a block of butadiene units joined at each end to a block of styrene units. At room temperature the styrene blocks congregate into glassy domains which act effectively to link the butadiene segments into a rubbery network. Above the Tg of the polystyrene these domains disappear and the polymer begins to flow like a thermoplastic. Because of the relatively low Tg of the short polystyrene blocks such rubbers have very limited heat resistance. Whilst in principle it may be possible to use end-blocks with a higher Tg an alternative approach is to use a block copolymer in which one of the blocks is capable of crystallisation and with a well above room temperature. Using what may be considered to be an extension of the chemical technology of poly(ethylene terephthalate) this approach has led to the availability of thermoplastic polyester elastomers (Hytrel—Du Pont Amitel—Akzo). 

Spreading resistance—the resistance between the deep interior of a semiconductor and a very sharp metal point pressed on the surface— measures the local resistivity on a scale of the order of the contact radius (Ehrstein, 1974). It thus measures the amount of hydrogen taking part in donor or acceptor passivation, whether this occurs by complex formation or by compensation. However, some methods of preparing samples for a spreading resistance measurement may involve heating above room temperature, and this may cause redistribution even of hydrogen bound in some types of complexes (Mu et al., 1986). 

The flexural modulus and heat deflection temperature of these aryl polyesters are increased by the incorporation of reinforcing fillers. PET and related aryl polyesters are resistant to nonoxidizing acids, alkalis, and salts, as well as to polar and nonpolar solvents at room temperature. (Above room temperature some alkalis and acids begin to degrade polyesters.)... 

The properties of the wide band-gap semiconductor SiC have been extensively studied by HFEPR because knowledge of the defect states is needed for its application in high power and radiation resistant devices. (The main method of doping SiC is by ion implantation that inevitably also introduces defects into the lattice.) The primary defects that can be produced are vacancies, interstitials and anti-sites. In contrast to silicon the primary defects in SiC seem to be stable at and even far above room temperature. 

In the procedure (ASTM D-2639), the sample is air dried prior to preparation and the temperature should not exceed 15°C (59°F) above room temperature, and drying should not be continued to the extent that oxidation of the coal occurs and the plastic properties of the coal are not altered by oxidation. The apparatus is then immersed in the heating bath and a known torque applied to the stirrer. During the initial heating no movement of the stirrer occurs, but as the temperature is raised, the stirrer begins to rotate. With increasing temperature, the stirrer speed increases until at some point the coal resolidifies and the stirrer is halted (Figure 7.4). The plastic properties of the sample are then measured by the resistance to motion of the fluid mass in the plastometer. 

LaFe4Sbn is a poor metal or heavily doped semiconductor with good thermoelectric properties above room temperature (700-1000 K) (Sales et al., 1996, 1997). Only polycrystalline samples have been investigated. The room temperature resistivity is about 0.5 m 2cm de-... 

Some selected values near and above room temperature are listed in Table 1.16. The resistivity of liquid tungsten close to the melting point and at 5000 K was given as 131 pfJ cm and 160 cm, respectively [1.84]. 

It was observed that THF did not adsorb on Ag at room temperature. However, THF-like TPD features were observed above room temperature if THF was condensed on a bare Ag substrate and then the TPD was initiated at 113 K. The adsorption of the THF was probably induced by the electrons from the filament used for heating che sample. In the last chapter, a suggestion will be made of replacing electron bombardment heating by resistive heating for future work. 

In this paper we have measured the resistance dependence and oxygen content of Er Ba2Cu 0Q and Y.Ba Cu OQ - samples above room temperature under the conditions or a simulated sintering and annealing process. The results provide insight into the importance of specific sample preparation procedures. We will start with a detailed discussion of the two most common preparation procedures for copper oxide superconductors at this moment the solid state reaction method and the coprecipitation method. A discussion of the resistance and thermogravimetric analysis (TGA) data and how it correlates to the sample formation will follow. 

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