Specificity in Materials

Finally, though it is usually not presented in this way, we note that one of the most robust results of all for considering materials is the time tested Arrhenius relation 

This generic kinetic relation is at the very heart of our understanding of materials and serves as an ever present reminder of the need for a more fundamental understanding of processes. 

In addition to the type of broad understanding that accompanies a description of phenomena in materials according to certain key scaling relations, much has been learned on the basis of the particular. Analytic and numerical solutions to boundary value problems as well as the advent of numerical simulation have all contributed to our understanding of material- or geometry- or mechanism-specific properties of material systems. 

One of the most intriguing and beautiful aspects of material specificity is revealed by the phase diagrams used to describe the different phases observed in solids. Figs. 1.4 and 6.2 revealed the complexity present even in elemental solids with the case of sulfur exhibiting the wide variety of different structures seen as the temperature and pressure are changed. The alloy case can be even more complex as evidenced by figs. 1.4 and 6.4. From the standpoint of material specificity we only wish to note that even with a single material system the atomic-level structures are as diverse as are the elements themselves. 

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Since 1970 a variety of reaction classification schemes have been developed to allow a more systematic processing of the huge variety of chemical reaction instances (see Chapter III, Section 1 in the Handbook). Reaction classification serves to combine several reaction instances into one reaction type. In this way, the vast number of observed chemical reactions is reduced to a manageable number of reaction types. Apphcation to specific starting materials of the bond and electron changes inherent in such a reaction type then generates a specific reaction instance. 

Manufacture Various methods for the manufacture of acrylates are summarized in Figure 1, showing thek dependence on specific raw materials. For a route to be commercially attractive, the raw material costs and utilization must be low, plant investment and operating costs not excessive, and waste disposal charges minimal. 

As noted, the oxidation resistance of silicon nitride ceramics depends on the type and concentration of the sintering aids. In materials designed for high temperature appHcations the specific weight gain resulting from oxidation upon a 500-h air exposure at 1200°C and 1350°C is about 1—2 g/m and 2—4 g/m, respectively. The kinetics of the oxidation process have been iavestigated (63,64) as has the corrosion resistance (65). Corrosion resistance is also dependent on material formulation and density. 

The properties of commercial rigid foamed plastics are presented in Table 2. The properties of commercial flexible foamed plastics are presented in Table 4. The definition of a flexible foamed plastic is that recommended by the ASTM Committee D 11. The data shown demonstrate the broad ranges of properties of commercial products rather than an accurate set of properties on a specific few materials. Specific producers of foamed plastics should be consulted for properties on a particular product (137,138,142). 

Cables are available in a variety of constmctions and materials, in order to meet the requirements of industry specifications and the physical environment. For indoor usage, such as for Local Area Networks (LAN), the codes require that the cables should pass very strict fire and smoke release specifications. In these cases, highly dame retardant and low smoke materials are used, based on halogenated polymers such as duorinated ethylene—propylene polymers (like PTFE or FEP) or poly(vinyl chloride) (PVC). Eor outdoor usage, where fire retardancy is not an issue, polyethylene can be used at a lower cost. 

Seminars, workshops, and short courses sponsored by professional societies and trade associations provide the needed training in materials standards and specifications. Familiarisation with sources of information in the field, how to prepare specifications and standards, how to tailor requirements for cost effectiveness, and the cross-referencing and correlation of specifications and standards are covered. 

Industrial screening is used essentially for separations over 0.2 mm and in conjunction with cmshers because the efficiency decreases rapidly as particle size decreases. The main objective is to remove undersize material that should not be circulated back to the cmshers, or to remove (scalp) oversize material or trash that should not report to the subsequent processing step. Other appHcations of screening include production of a specification size material (as in quarrys), dewatering, and trash removal from processed material. 

The quantity of boric acid maintained in the reactor coolant is usually plant specific. In general, it ranges from ca 2000 ppm boron or less at the start of a fuel cycle to ca 0 ppm boron at the end. Most plants initially used 12-month fuel cycles, but have been extended to 18- and 24-month fuel cycles, exposing the materials of constmction of the fuel elements to longer operating times. Consequendy concern over corrosion problems has increased. 

Coordinate Indexing and Boolean Logic. Three methods of indexing have been prominent in the chemical Hterature in recent times. The first, articulated indexing, has been used in printed Chemicaly hstracts subject indexes from their earliest days until well into the 1990s. A number of important concepts are identified as permissible index entries, including specific compounds, material types, reactions, and processes. One or more modifying statements foUow each basic index entry. Thus, eg. 

Sepa.ra.tedRecyclables. Even when initial separation of recyclables takes place at the household level, the separated materials stiU requite some preparation before being sent to market. By visually inspecting materials, facihty processors are able to remove any remaining contaminants. This ensures that recyclables will be of sufficient quahty to meet buyer specifications. In addition, materials are often shredded, cmshed, or baled to facihtate cost-effective shipping. 

Sheet form, 914 mm (36 in.) in width, or widths agreed upon by purchaser and suppHer, composed of asphalt-saturated organic felt with approximately half the width of weather side coated with asphalt and surfaced with mineral granules, for use as cap sheet in constmction of BUR. Materials covered by this specification, in minimum mass per unit area, are Type I, 1806 g/m (37.0 lb/100 fU) Type II, 2260 g/m (46.3 lb/100 fU) Type III, 1733 g/m (35.5 lb/100 fU) and Type IV, 2090 g/m (42.8 lb/100 fU). 

Neady every significant class of dyes and pigments has some members that function as sensitizers. Toxicological data are often included in surveys of dyes (84), reviews of toxic substance identification programs (85), and in material safety data sheets provided by manufacturers of dyes. More specific data about toxicological properties of sensitizing dyes are contained in the Engchpedia under the specific dye classes (see Cyanine dyes Polymethine dyes Xanthene dyes). 

Companies shipping dangerous materials sometimes build tank cars with metal thicker than required by the specifications in order to reduce the possibihty of leakage during a wreiB or fire. The punching of couplers or rail ends into heads of tanks is a hazard. 

Dense-media cyclones are generally operated in the (0.7-1.0) X lO -Pa (10-15-lbf in ) range. It is not advisable to go below (0.4-0.56) X 10 Pa because the recoveiy of low-specific-gravity material and the rejection of impurity are improved at higher pressures. 

Selec tion of the correct conveyor for a specific bulk material in a specific situation is complicated by the large number of interrelated factors that must be considered. First, the alternatives among basic types must be weighed, and then the correc t model and size must be chosen. Workabihty is the first criterion, but the degree of performance perfection that can be afforded must be estabhshed. 

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