Structural materials creep resistance

Even though the above work is providing a stable, non-sintering, creep-resistant anode, electrodes made with Ni are relatively high in cost. Work is in progress to determine whether a cheaper material, particularly Cu, can be substituted for Ni to lower the cost while retaining stability. A complete substitution of Cu for Ni is not feasible because Cu would exhibit more creep than Ni. It has been found that anodes made of a Cu - 50% Ni - 5% A1 alloy will provide long-term creep resistance (36). Another approach tested at IGT showed that an "IGT" stabilized Cu anode had a lower percent creep than a 10% Cr - Ni anode. Its performance was about 40 to 50 mV lower than the standard cell at 160 mA/cm. An analysis hypothesized that the polarization difference could be reduced to 32 mV at most by pore structure optimization (37). 

Use of ceramic materials in high temperature structural applications is often limited by creep resistance. However, several recent studies have shown that composite reinforcement can drastically reduce the creep rates compared to the unreinforced ceramic matrix.12,20,27-31 Most of these studies have been conducted in air, which is a strong oxidizing environment. In a few cases, investigators have attempted to isolate the effects of oxidation from creep by conducting parallel experiments in both air and inert atmospheres.27,28 The following is a description of the salient points made in these investigations. 

The anode is stabilized by using mixed powders of Ni-Cr, Ni-Al, or refractory oxides and sintered at 900-1100°C under a reducing atmosphere to provide a creep resistant anode structure. The function of the additives is to reduce the loss of porosity during sintering and develop creep resistant materials. The creep is referred to as the shrinkage in thickness and change in shape. The sintering resistance is increased by the additives, which are usually metals or oxides of metals. 

Highly cross-linked epoxy resins combine high strength stiffness thermal, chemical, and environmental stability adhesion low weight processability excellent creep resistance and favorable economics. These resins are widely applied as coatings, casting resins, structural adhesives, and matrix resins of advanced composite materials. The broad spectrum of applications ranges from the automotive and aerospace industries to corrosion protection and microelectronics. 

Silicon nitride (see Nitrides) is a key material for structural ceramic applications in environments of high mechanical and thermal stress such as in vehicular propulsion engines. Properties which make this material uniquely suitable are high mechanical strength at room and elevated temperatures, good oxidation and creep resistance at high temperatures, high thermal shock resistance, excellent abrasion and corrosion resistance, low density, and, consequendy, alow moment of inertia. Additionally, silicon nitride is made from abundant raw materials. 

Resistance of fiber-reinforced RubCon to long-term loading is the principal criterion of its application as a structural material. We investigated creep of plain RubCon at compression earlier. Experiments were continued for the purpose of studying fibrous RubCon creep. 

Polyarylsulfones are a class of high-use temperature thermoplastics that characteristically exhibit excellent thermal-oxidative resistance, good solvent resistance, hydrolytic stability, and creep resistance (10). In 1965, Union Carbide announced a thermoplastic polysulfone based on dichlorodiphenylsulfone and bisphenol A (11). This polysulfone became commercially available in 1966 and was designated as Udel polysulfone. Since 1966, Imperial Chemical Industry (ICI), Minnesota Mining and Manufacturing (3-M), and Union Carbide have commercialized polyarylsulfones that contain only aromatic moieties in the polymer structure. These materials have been designated Vlctrex polyethersulfone (ICI), Astrel 360 (3-M), and Radel polyphenylsulfone (Union Carbide). 

The polysulfones form another large group, and some of the commercially important structures are shown in Table 15.10. Poly(phenylene sulfone) tends to be too intractable for easy processing, and the copolymer structures are more useful. These are usually amorphous materials with high values, typically in the range of 465 to 560 K. They are thermally stable, show good mechanical properties — particularly creep resistance — and are resistant to attack by dilute acids and alkahs. They can, however, dissolve in polar solvents, and solvent attack may also cause environmental stress cracking. 

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