Properties: structure insensitive

List four different structure-insensitive properties. 

There are, of course, many more ceramics available than those listed here alumina is available in many densities, silicon carbide in many qualities. As before, the structure-insensitive properties (density, modulus and melting point) depend little on quality -they do not vary by more than 10%. But the structure-sensitive properties (fracture toughness, modulus of rupture and some thermal properties including expansion) are much more variable. For these, it is essential to consult manufacturers data sheets or conduct your own tests. 

When the catalytic properties of metals are examined, the importance of the non-uniformity of sites depends on the reaction under study. For some reactions, the activity of the metal catalyst depends only on the total number of sites available and these are termed structure-insensitive reactions. For other reactions, classified as structure-sensitive reactions, activity may be much greater on sites associated with a particular crystal face or even with some type of defect structure. The alternative names of facile or demanding have been used to describe structure-insensitive or structure-sensitive reactions, respectively. 

Because of the chemical structure insensitive (nonspecific) nature of plasma polymerization illustrated above, the structure of the monomer appears to have relatively little influence on the polymerization characteristics as well as on the characteristics of plasma polymers. This is largely true in the context of selective reactivity due to the chemical structure of monomers in conventional polymerization (i.e., monomer vs. nonmonomer). However, the influence of monomer structure, as classified by five types in Table 7.2, is actually accentuated when the operational conditions are varied. In this context, therefore, the chemical structure of a monomer is a key factor in its deposition characteristics and also in determining the properties of the deposition. 

Ethylene (C H ) hydrogenation to ethane (C H ) was nsed as a first test reaction (conditions 50 mbar C H, 215 mbar H, 770 mbar He) [43, 51, 55], As expected for a structure-insensitive reaction the steady-state turnover frequency (TOE) at 300 K ( 6 s ) was independent of particle size (1.3-6.1 nm) (and similar to the TOE for Pd(lll)). The reaction orders (ethylene -0.3 hydrogen 1) and the activation energy (about 50-60 kJ mol ) were also very similar to values reported for technical catalysts, demonstrating that Pd-AljO3/NiAl(110) model catalysts closely mimic the properties of impregnated catalysts. 

Catalytic properties of supported clusters identified as primarily Ira or Ir6 were reported by Xu et al [15], who investigated a structure-insensitive reaction, toluene hydrogenation. The support was NaY zeolite or, for comparison, MgO. EXAFS spectroscopy (Table 3) showed that the first-shell Ir-Ir coordination numbers characterizing both the fresh and used MgO-supported catalysts made by decarbonylation of supported [Ir4(CO)i2] or [HIr4(CO)ii] are indistinguishable fi om 3, the value for a tetrahedron, as in [Ir4(CO)i2] and [HIr4(CO)n]. The decarbonylated clusters retained or nearly retained this metal frame. EXAFS data show that the decarbonylated Ire clusters had metal fi ames indistinguishable from the octahedra of the precursor hexairidium carbonyls, indicated by the coordination number of approximately 4. 

Although the Ira and Ir clusters catalyze the same reactions as metallic iridium particles, their catalytic character is different, even for structure-insensitive hydrogenation reactions. It is inferred [15] that the clusters are metal-like but not metallic consistent with the structural inferences stated above, we refer to them as quasi molecular. Thus these data show the limit of the concept of structure insensitivity it pertains to catalysis by surfaces of structures that might be described as metallic, i.e., present in three-dimensional particles about 1 nm in diameter or larger. This conclusion suggests that supported metal clusters may be found to have catalytic properties superior to those of conventional supported metals for some reactions. The suggestion finds some support in the results observed for platinum clusters in zeolite LTL, as summarized below. 

Zeolite-supported metal clusters are a new class of catalyst made possible by syntheses involving organometallic chemistry and by precisely controlled treatment of metal complexes in zeolite cages. Elucidation of the preparation chemistry would not have been possible without the guidance of EXAFS spectroscopy. Clusters such as Ir4, Ir, and Pt (where n is about 6) are small enough to be considered quasi molecular rather than metallic. Their catalytic properties are distinct from those of metallic particles, even for structure-insensitive reactions. The zeolite pores seem to confer some properties on the clusters that are not yet well understood. 

A comparative review on the evaluation of the catalysts Al, A2, B1 and B2 reveals that activity and dispersity are two closely related parameters which is influenced by the mode of preparation especially with respect to removal of chloride or to the final calcination temperature. Earlier works had identified benzene hydrogenation on supported platinum catalysts as a facile or structure insensitive reaction(5). A facile reaction may be defined as one for which the specific activity is practically independent of its mode of preparation(6). In other words all surface atoms are believed to be the active sites in a facile reaction without any dependence on the coordination number of site or on the collective properties of the crystallite. 

Jim Goodwin, Soo Kim, and William Rhodes (Clemson Univeristy, Clemson, SC) review the concept of turnover frequency, a widely used measure of catalytic reaction rates. They review various methods of measuring this property chemisorption and isotopic tracing, for example. Their analysis also compares TOF values for structure-insensitive reactions like methanation and structure-... 

To understand the importance of defects in polymer crystals, one must distinguish structure-insensitive properties from structure-sensitive properties. For crystals of small molecules and rigid macromolecules (see Fig. 1.6), the structure-insensitive properties often can be derived directly from the ideal crystal structure as summarized in Fig. 5.80. The density, for example, can be calculated from the unit cell dimensions (see Sect. 5.1). The polymeric materials in form of flexible macromolecules are, in... 

To determine the crystallinity of a sample, many structure-insensitive properties can be used, since they can be separated into contributions from the amorphous and the crystalline phase. Three examples are given in this section X-ray, infrared, and... 

For bulk materials, all techniques based on structure-insensitive properties, as described in this section and elsewhere, yield closely similar data. The crystallinity model is thus a valid defect concept to describe structure-insensitive properties of semicrystalline polymers. It breaks down for three-phase systems, consisting, for example, of a crystalline phase, a mobile amorphous phase, and a rigid-amorphous fraction (see Chap. 6). In addition, one does not expect valid answers for structure-sensitive properties. 

It is used to measure the deflection or stiffness of materials under load, and is an important design parameter. It is the structure-insensitive property of a material [30]. The magnitude of the elastic modulus varies widely for different metals for example, E for low-carbon steel is approximately 30 X 10 psi and E for aluminum is 10 X 10 psi. This is only marginally affected by a small variation... 

Polymer crystals are always semicrystalline, and thus, they are far from perfect. Limited crystal sizes and complicated aggregates make the basic understanding of the structure-property relationships very difficult. To simplify, two types of properties in general exist for crystals of small molecules structure-insensitive and structure-sensitive properties. The structure-insensitive properties include... 

The dimensions of the RVE are not constant for a material structure, i.e. they may vary with the loading and with development of the crack pattern if the RVE is selected to represent the cracked material in its actual state. Also the RVE may be different for different material characteristics if the focus of material representation is reduced to only one group of its characteristics, then the RVE does not take into account the others. In that sense two different groups of material properties are distinguished structure-sensitive and structure-insensitive ones. 

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