Multiphasic composite glass

Figure 10.7 A simplified schematic representation of the multiphasic composite glass (a) [23] and examples of a dye-doped multiphasic composite glass monolith (b).

Composite glasses prepared via the sol-gel technique are of high optical quality and large monolithic bulk forms can be made for various photonic functions [22,23,79,88,101-103] such as lasing, optical power limitation, nonlinear optical response, and so on. Figure 10.9 presents an example of a solid dye laser based on multiphasic composite monolith doped with BASF LFO-240 laser dye placed in optical resonator. 

In multiphase compositions, we have to consider the relationships among amount, distribution, and orientation of separate phases. The most common structure is one or more phases dispersed in a continuous matrix. These can be prismatic crystals in a glass, such as forsterite glass ceramic (Figure 11.6), or there can be crystals precipitated in a crystalline matrix. 

In practice ceramics are usually multiphase, consisting of crystalline phases, glasses and porosity. The overall behaviour depends on the distribution as well as the properties of these constituents. A minor phase that forms a layer round each crystallite of the major phases, and therefore results in a 3-0 connectivity system (see Section 2.7.4), can have a major effect. If the minor phase is conductive it can greatly reduce the resistivity of the composite or, if insulating, it can reduce its conductivity. Also, an abrupt change in the mode of conduction at the main phase-intercrystalline phase boundary may introduce barriers to conduction that dominate the overall electrical behaviour. In contrast, minor phases present as small discrete particles, or porosity present as empty cavities, can only modify properties to a minor extent as indicated by one of the mixture relations such as Lichtenecker s rule (see Section 2.7.4). 

In three-phase reactors, one of the main problems is often the mass transport limitations, which may reflect internal as well as external mass transfer resistances. The use of filamentous catalytic materials for multiphase reactions may help reduce or even avoid mass transfer limitations [63,132,133]. Filamentous woven cloths made of glass, composite mixed oxides, metallic alloys, or activated carbon (Figure 18) can be used as supports for active components such as platinum, palladium, or transition metal oxides. The diameters of the filaments are of the order of several micrometers and correspond to the typical diameters of catalysts that are suspended in the reaction medium. By using such small diameters, internal mass transfer limitations can be avoided. 

From the viewpoint of macrostructure, homo- and copolymers are generally defined as single-phase systems. Polyblends and composites are always classified as multiphase systems consisting of a polymeric continuous phase or matrix and of a dispersed phase. The latter can be either another polymer or any other foreign material such as glass fibers, fillers, or minerals. 

K. M. Prewo, The Development of Fibre Reinforced Glasses and Glass-Ceramics, in Mat. Sci. Research Vol. 20 Tailoring Multiphase and Composite Ceramics, R. E. Tressler, G. Messing, C. Pantano and R. Newnham eds.. Plenum Press, New York (1986) 529-547. 

A multilayered glass composite concept has been considered for the safe recycling of hazardous waste containing sihcate phases [60]. in this concept, the use of layered and graded microstructures in multiphase glass-containing composites has two major objectives [60] ... 

A composite material is a two-phase or multiphase compact material with its components (phases) separated by interfaces which can be formed naturally or be manmade. One of the composite material phases is the matrix (phase I). It exists in the solid (crystalline or amorphous) state of aggregation. Within the matrix, particles are distributed discretely. This is phase II or disperse phase [23]. Biocomposites are composite materials made from natural fiber and petroleum-derived nonbiodegradable polymers like PP, PE, and epoxies or biopolymers like poly lactic acid (PLA), cellulose esters. Composite materials derived from biopolymer and synthetic fibers such as glass and carbon come under biocomposites. Biocomposites derived from plant-derived fiber (natural/biofi-ber) and crop/bioderived plastics (biopolymer/bioplastic) are likely more ecofriendly, and such biocomposites are sometimes termed green composites [24]. 

The sintered microstructure of thick film ceramic devices can be quite complex. This is caused by two factors (1) the ceramic itself may be multiphase, such as low-firing glass—ceramic packaging material and (2) dissimilar materials, such as ceramics and metals, are in contact during the high-temperature sintering process. Therefore, a variety of characterization techniques must be used in concert to determine adequately the phase and composition distributions in these materials. The following examples illustrate this point. 

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