Alloys, structure

Outlet Superheater (SHI header of Unit 4 (600 MW. supercritical multi-fliel l of an ENEL power station it also consists of 2 twin and independent bodies (23 m length, 215 mm internal diameter, 103 mm thickness material ASTM A335 P22 - 2.25CrlMo - low alloy). Structural integrity problems and monitoring requirements and objectives same as above. 

Another important property is alloy structural stability. This means freedom from formation of new phases or drastic rearrangement of those originally present within the metal structure as a result of thermal experience. Such changes may have a detrimental effect upon strength or corrosion resistance or both. 

Interna] Insulation The practice of insulating within the vessel (as opposed to applying insulating materials on the equipment exterior) is accomplished by the use of fiber blankets and hghtweight aggregates in ceramic cements. Such construction frequently incorporates a thin, high-alloy shroud (with slip joints to allow for thermal expansion) to protect the ceramic from erosion. In many cases this design is more economical than externally insulated equipment because it allows use of less expensive lower-alloy structural materials. 

Nickel and nickel alloys possess a high degree of resistance to corrosion when exposed to the atmosphere, much higher than carbon and low-alloy steels, although not as high as stainless steels. Corrosion by the atmosphere is, therefore, rarely if ever a factor limiting the life of nickel and nickel alloy structures when exposed to that environment. 

The inclusion of both covalent and intermetallic crystals in Chapter 6 is predicated on the close relation between the covalent and metallic bonds, as discussed in Chapter 3. SP 54 and SP 55 are beautiful examples of the complexity of the atomic packing and bonding arrangements in alloy structures, which fascinated Pauling. 

PtRu nanoparticles can be prepared by w/o reverse micro-emulsions of water/Triton X-lOO/propanol-2/cyclo-hexane [105]. The bimetallic nanoparticles were characterized by XPS and other techniques. The XPS analysis revealed the presence of Pt and Ru metal as well as some oxide of ruthenium. Hills et al. [169] studied preparation of Pt/Ru bimetallic nanoparticles via a seeded reductive condensation of one metal precursor onto pre-supported nanoparticles of a second metal. XPS and other analytical data indicated that the preparation method provided fully alloyed bimetallic nanoparticles instead of core/shell structure. AgAu and AuCu bimetallic nanoparticles of various compositions with diameters ca. 3 nm, prepared in chloroform, exhibited characteristic XPS spectra of alloy structures [84]. 

F. C. Frank, J. S. Kasper, Complex alloy structures regarded as sphere packings. I Definitions and basic principles, Acta Crystallogr. 11 (1958) 184. II Analysis and classification of representative structures, Acta Crystallogr. 12 (1959) 483. 

Serious problems may arise if gallium or its liquid alloys contact aluminium alloy structural components in aircraft, when rapid amalgamation and weakening occurs. 

F. C. Frank and J. S. Kasper, Complex Alloy Structures Regarded as Sphere Packings. II. Analysis and Classification of Representative Structures, Acta Cryst., 12, 483 (1959). 

Figure 9.56 The structure of a typical QD nanocrystal includes a semiconductor alloy core surrounded by a shell consisting of a different alloy structure. Early QD compositions involved the use of CdSe core with a ZnS shell, but many different alloy compositions have been used and are possible.

Apart from structures that are built of slabs, modular structures that can be constructed of columns in a jigsawlike assembly are well known. In the complex chemistry of the cuprate superconductors and related inorganic oxides, series of structures that are described as tubular, stairlike, and so on have been characterized. Alloy structures that are built of columns of intersecting structures are also well known. Structures built of linked columns, tunnels, and intersecting slabs are also found in minerals. Only one of these more complex structure types will be described, the niobium oxide block structures, chosen as they played a significant role in the history of nonstoichiometry. 

Alloy Structure (RT-800°C) Resistance Strength ability Cost... 

There are three anions that may loosely claim to be nitrides. Pentazolides (salts of cyclic N ) will all be explosive. Some azides (salts of N3) fall just short of being explosive but all are violently unstable. The true nitrides, nominal derivatives of N3-, are more various. In addition to some ionic structures, there are polymeric covalent examples, and some monomeric covalent ones, while most of those of transition metals are best considered as alloys. Several are endothermic and explosive, almost all are thermodynamically very unstable in air with respect to the oxide. Many are therefore pyrophoric if finely divided and also may react violently with water and, more particularly, acids, especially oxidising acids. A few are of considerable kinetic stability in these circumstances. There is no very clear classification of probable safety by position in the periodic table but polymeric and alloy structures are in general the more stable. Individual nitrides having entries ... 

S. Lpken, J.K. Solberg, J.P. Maehlen, R.V. Denys, M.V. Lototsky, B.P. Tarasov, V.A. Yartys, Nanostructured Mg-Mm-Ni hydrogen storage alloy Structure-properties relationship, J. Alloys Compd. 446-447 (2007) 114-120. 

In alloys, structural ordering transformations can proceed with either a continuous or discontinuous change in entropy at the transformation temperature, while in... 

Mondolfo, L. F. (1976) Aluminium Alloys Structure and Properties (Butterwoiths, London). 

In this way we find that, in oxides for example, many cation arrangements are identical to the arrangements of the atoms in known (or plausible) alloy structures. (In some cases the cations and atoms are identical in the two cases.) This is particularly helpful in those oxide examples where it has not previously been possible to describe the structure in any simple terms, often because the anion array is not regular in any simple way, for instance in many metal sulphates. But it is also revealing in some cases that are describable in conventional terms, e.g. the humites. A long (but still incomplete) list of examples is given in Table 3. Sufficient specific cases are described and discussed in detail in the text in order to expose the principles and some of the advantages of this unfamiliar approach. 

Table 2 records some examples of this phenomenon in which oxygen arrays in oxides are the same as metal atom arrays in alloys. Recognition of this fact has been exploited to simplify the description of complex alloys (see especially Andersson ), which is essentially the reverse of what we propose to do here, namely to simplify the description of oxide structures by giving them in terms of known alloy structures. Nevertheless Tables 1 and 2 provide striking evidence of Nature s parsimony in the use of patterns in crystal structures. 

There are of course very many structures that appear not to be based on known alloy structure types. 

Table 3. Examples of oxide, nitride and fluoride structures in which the cation arrays are alloy structures...

Ru/Cu (22) and Pt/Ir (23) on silica. They analyzed the detailed structure of these samples by an extended x-ray absorption fine structure (EXAFS) technique, showing an alloy structure for the nanoparticles with a diameter of 1-3 nm, associated with their special properties. 

Coreduction of Mixed Ions. Coreduction of mixed ions is the simplest method to synthesize bimetallic nanoparticles. However, this method cannot be always successful. Au/Pt bimetallic nanoparticles were prepared by citrate reduction by Miner et al. from the corresponding two metal salts, such as tetrachloroauric(III) acid and hexachloroplatinic(IV) acid (24). Reduction of the metal ions is completed within 4 h after the addition of citrate. Miner et al. studied the formation of colloidal dispersion by ultraviolet-visible (UV-Vis) spectrum, which is not a simple sum of those of the two monometallic nanoparticles, indicating that the bimetallic nanoparticles have an alloy structure. The average diameter of the bimetallic nanoparticles depends on the metal composition. By a similar method, citrate-stabilized Pd/Pt bimetallic nanoparticles can also be prepared. 

The examples are shown in Figure 9.1.10, which gives x-ray diffractograms of three types of physical mixtures of PVP-stabilized Pd, Pt, and Au monometallic nanoparticles, and the corresponding PVP-stabilized bimetallic nanoparticles (53). The diffraction patterns of the physical mixtures are consistent with the sum of two individual patterns, and are clearly different from those of the bimetallic nanoparticles, which have two broader peaks, indicating that several interatomic lengths exist in a single particle. By XRD one can easily understand if the obtained multi-metallic nanoparticles have an alloy structure or are simple physical mixtures of monometallic particles. 

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