Thermal operating environments

In order to select materials that will maintain acceptable mechanical characteristics and dimensional stability one must be aware of both the normal and extreme thermal operating environments to which a product will be subjected. TS plastics have specific thermal conditions when compared to TPs that have various factors to consider which influence the product s performance and processing capabilities. TPs properties and processes are influenced by their thermal characteristics such as melt temperature (Tm), glass-transition temperature (Tg), dimensional stability, thermal conductivity, specific heat, thermal diffusivity, heat capacity, coefficient of thermal expansion, and decomposition (Td) Table 1.2 also provides some of these data on different plastics. There is a maximum temperature or, to be more precise, a maximum time-to-temperature relationship for all materials preceding loss of performance or decomposition. Data presented for different plastics in Figure 1.5 show 50% retention of mechanical and physical properties obtainable at room temperature, with plastics exposure and testing at elevated temperatures. 

The residual carbon-carbon double bond in nitrile butadiene rubber (NBR) can be catalytically hydrogenated to yield its tougher and more stable derivative, hydrogenated nitrile butadiene rubber (HNBR).2 This class of specialty elastomer was developed to expand the range of operating environments possible for nitrile butadiene rubber NBR in environments that expose the rubber to chemical and thermal attack. 

The physical and chemical properties of both the solidified adhesive and the plastic substrate affect the quality of the bonded joint. Major elements of concern are the thermal expansion coefficient, modulus, and glass transition temperature of the substrate relative to the adhesive. Special consideration is also required of polymeric surfaces that can change during normal aging or on exposure to operating environments. 

Conversely where less stringent thermal demands are appropriate, choices are broadened to encompass materials more commonly referred to as "commodity resins". Ultimate end product operational environments will impact the material selection process. Where in-service temperatures range from room ambient to moderate thermal extremes, lesser thermally tolerant resins may be considered. Conversely, for hostile environments such as those encountered in "under the hood" automotive, down-hole (geothermal), tropical, or corrosive climatic axtremes, enhanced material properties will be required. 

The acrylic foam-coated nonwoven fabrics produced in this manner are capable of continuous operation at temperatures of up to approximately 120 °C. However, they are not normally resistant to hydrolytic conditions, these leading to the collapse of the structure and hence, premature pressurisation. The latter notwithstanding, in view of the success of foam-coated structures operating in relatively safe conditions , the future will undoubtedly see more advanced products of this type, leading to sttuctures that are both more efficient in particle capture and also are capable of operation in more chemically and thermally challenging environments. 

Finally, conductive heat protection is required for textiles that may come into direct contact with a heat source other than a flame. Major threats here include those met by metal industrial workers, who risk contact with hot metal tool handles and molten metal splashes. For this reason, not only must the thermal insulating characteristics of the textile be paramount, but also surfaces which minimise contact, for instance by resisting wetting by molten metals, must be considered. However, in many thermally hazardous environments, a combination of conduction, convection (or flame) and radiation may be operating in concert, and usually the last two are associated with flame sources in particular. 

Table 1.4 summarizes the major application/market segment and the technical attributes that influence the selection of ceramic interconnect technology in that segment. As shown in this table, thermal management — thermal performance, thermal stability, and high-temperature operation as it relates to both operating environment and lead-free assembly — is common to most applications. High-performance electrical property, and the related interconnect density, direct die attach, and embedded passives or embedded functions also span many applications. This comparison demonstrates that many... 

B., Schnick, T.M., and Vuoristo, P. (1998) Thermally sprayed silicon nitride-based coatings on steel for application in severe operation environments preliminary results, in Microstructural Science Analysis of In-Service Failures and Advances in Microstructural Characterization, vol. 26 (eds E. Abramovici, D.O. Northwood,... 

The GDL sheets are then heated to higher temperatures (2500-3300 K) that are required for graphi-tization (i.e., the phase transition from amorphous carbon to crystalline graphite). This transformation is crucial to enhance the electrical, thermal, and mechanical material properties. It also changes the surface characteristics and chemical response to the fuel cell operating environments (e.g., hydrophobicity and corrosion resistance). Differences in the process conditions (temperature, prevailing atmosphere or vacuum, etc.) can result in large differences in material properties. Hence, care must be... 

A very limited number of materials exist that can be used to fabricate the various functional and strucmral components of SOFCs and that are sufficiently stable thermally and chemically to withstand their operating environment. As an example, take the interconnectors between cells for which mechanically weak lanthanum chromite ceramics or thermally stable high-alloy steels must be used. These materials are very expensive, so the interconnectors make a considerable contribution to the fuel cell s total cost. Numerous difficulties also arise in the selection of sealing materials. 

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