Material artificially structured

The term artificially structured materials implies a construction similar to nanostructured materials on a somewhat larger, on the whole unspecified, scale. The terms biomaterials and genetically engineered materials are self explanatory. 

Superlattices, and other kinds of artificially structured materials in which composition varies periodically on a quantum-mechanically significant spatial scale, have excited much interest recently in the materials sciences. Superlattices often have unusual optical and electronic properties, and they may also display extraordinary chemical properties. There are interesting possibilities for synthesis of such materials by various electrodeposition methods. These are worthy of exploration. 

Material characteristics, both chemical and physical, should be considered, especially flowabihty. Abrasiveness, friability, and lump size are also important. Chemical effects (e.g., the effect of oil on rubber or of acids on metal) may dictate the structural materials out of which conveyor components are fabricated. Moisture or oxidation effects from exposure to the atmosphere may be harmful to the material being conveyed and require total enclosure of the conveyor or even an artificial atmosphere. Obviously, certain types of conveyors lend themselves to such special requirements better than others. 

The structures of some natural protein-based materials, such as silk and wool, result in strong, tough fibers. Spiders and silkworms use proteins as a structural material of remarkable strength (Fig. 19.22). Chemists are duplicating nature by making artificial spider silk (Fig. 19.23), which is one of the strongest fibers known. 

The challenge is to form compounds with structures that we design, not Mother Nature. Superlattices are examples of nano-structured materials [1-3], where the unit cell is artificially manipulated in one dimension. By alternately depositing thin-films of two compounds, a material is created with a new unit cell, defined by the superlattice period. 

Composites provide an atPactive alternative to the various metal-, polymer- and ceramic-based biomaterials, which all have some mismatch with natural bone properties. A comparison of modulus and fracture toughness values for natural bone provide a basis for the approximate mechanical compatibility required for arUficial bone in an exact structural replacement, or to stabilize a bone-implant interface. A precise matching requires a comparison of all the elastic stiffness coefficients (see the generalized Hooke s Law in Section 5.4.3.1). From Table 5.15 it can be seen that a possible approach to the development of a mechanically compatible artificial bone material... 

Biooiedieal Applications. In the area of biomedical polymers and materials, two types of applications have been envisioned and explored. The first is the use of polyphosphnzenes as hioinert materials for implantaiion in the body eilher as housing for medical devices or as structural materials for heart valves, artificial blond vessels, and catheters. 

The choice of materials for metallic systems is still expanding and at present various examples of combinations with different atomic radii are being prepared. Here multilayered techniques also show possibilities for new material syntheses. In contrast to materials prepared by chemical procedures, supedattices are made far from equilibrium. The various possibilities for layering the artificial supedattice materials are given in Figure 16c—f. Most of the stacked layers (c, d, f) have more or less sharply defined boundaries and some have a noncrystalline structure in the individual layers (c) or one of the layers is noncrystalline (d). In such situations the structural information is not transmitted between adjacent layers and therefore, stricdy speaking, no supedattice is formed. In the case of an unsharp boundary (e), compositionally modulated alloy-layered structures have been made. The amplitude of composition modulation in the center of a layer can be in the range of 0 to 100%. Supedattices can also be formed with sharp boundaries (<5% of the thinnest layer) between the two components. 

At the higher metal level (2.0-4.5% Ni with up to 2% Sb) used to study artificially contaminated materials, XRD results have shown the formation of Ni-Sb alloys (NiSb x<0.08) whereas XPS data have indicated that a non-reducible antimony oxide, a well dispersed reducible Sb phase together with reducible Sb (that form an alloy with reducible Ni), were present. Selective chemisorption data for unsupported Ni-powders showed that one surface structure can effectively passivate 2-3 Ni atoms with respect to H2 chemisorption. XPS examination confirmed that Sb segregates at the surface of Ni particles where it can drastically affect the electron properties of neighboring Ni atoms thus reducing their activity. 

Dispersions at micron scale are usually made by merging gas and liquid streams in a mixing element and subsequent decay of the gas stream to a dispersion [251-262]. Mixing elements often have simple shapes such as a mixing tee (dual-feed gas-liquid) or triple-feed (liquid-gas-liquid) arrangements. The dispersion is passed either in a microchannel (or many of these) or in a larger environment such as a chamber, which, for example, provides volume to fill in porous materials such as catalyst particle beds, foams or artificial structures (microcolumn array). The mechanisms for bubble formation have not been investigated for all of the devices... 

The last five years have witnessed a tremendous effort to better produce, characterise and study insulating oxide surfaces. Several reasons stem for the rapid development of this field. They are related to experimental considerations — a better control of the fabrication of surfaces, a more thorough use of advanced spectroscopic or structural tools — but also to a more accurate recognition of the technological importance of high quality oxide surfaces in catalysis, magnetic recording, as sensors or as constituents of artificial nano-materials. 

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