The Clay Tradition

The specific properties of clay minerals-water masses have been exploited from the beginning. Clay minerals form agglomerates composed of 2-5 crystallites with thicknesses of 2-8 nm and widths 100 nm. As a result of Coulombic attraction of hydroxide layers in crystallites and water molecules, the agglomerates are wetted by water, which forms thin layers on their surface. This results in the formation of water bridges between crystallites (Fig. 2.1). Since, the wetting angle ( Fig. 4.2 in Chap. 4) is less than 90°, the meniscus (i.e. the water-air interface) of the water bridges is concave and, due to the low thickness of the water layers, has a very small radius of curvature. 

There is a relationship between the chemical potential of atoms and surface curvature  

In equilibrium, the capillary forces in the water bridges are balanced by identical compressive forces acting perpendicularly to them (Fig. 2.2). This causes agglomerates of clay mineral crystallites to be strongly attracted to one another. 

To lower the resulting cohesion of crystallites more water must be added and to initiate deformation of the clay in this state stress has to be applied. On increasing the applied stress above a limit value (plasticity limit) the lubricating action of water facilitates a further increase in deformation. After the load is relieved, the capillary forces cause the clay-water mix to retain the deformation from the load. This is referred to as plastic behaviour. 

Deformation of material by shearing requires, as a mle, the application of a lower stress than for other modes of applying load. Such a deformation can be achieved at many points of the clay-water mix, if tangent stress is applied to its surface. The discovery of this behaviour of the clay-water mix is believed to have been the reason behind an innovation introduced around 5,000 years ago in Sumer, namely, the formation of clay products with a potter s wheel. 

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Several types of continuous ceramic fibres are used here. Fibres made of -type glass s ( 2. Tradition continued. The silica tradition) in composites used for constmction of boats, carbon fibres (Fig. 3.12) in composites used for aviation and sports equipment. Materials other than the continuous fibres can also be applied. Namely, particles where at least one dimension is <100 nm, like clay minerals ( 2. The tradition continued. The clay tradition), carbon nanombes and graphenes (—> 5. Unusual ceramic dielectrics and conductors). Due to a high ratio of length to thickness such fillers are in contact with the polymer matrix over a large area. Therefore, their effect on strength and fracture toughness—can be observed even with a low-volume fractions of fillers (firom 0.5 to 15 %wt). Due to the dimensions of these fillers, these composites are often referred to as nanocomposites. 

The product is baked in a special clay-lined oven. Above the clay there is an arch of 6 mm of Inconel, a special alloy steel, to stand the heat. The oven is intended to give the same taste as a traditional tandoor. Unlike the traditional tandoor the oven is horizontal rather than vertical. There are five burners, three above the Naan and three below. The lower burners provide base heat as in a tandoor while the burners above throw heat across the surface. 

Onium salts, such as tetraethylammonium bromide (TEAB) and tetra-n-butylammonium bromide (TBAB), were also tested as PTCs immobilized on clay. In particular, Montmorillonite KIO modified with TBAB efficiently catalyzed the substitution reaction of a-tosyloxyketones with azide to a-azidoketones, in a biphasic CHCI3/water system (Figure 6.13). ° The transformation is a PTC reaction, where the reagents get transferred from the hquid to the solid phase. The authors dubbed the PTC-modified catalyst system surfactant pillared clay that formed a thin membrane-hke film at the interface of the chloroform in water emulsion, that is, a third liquid phase with a high affinity for the clay. The advantages over traditional nucleophilic substitution conditions were that the product obtained was very pure under these conditions and could be easily recovered without the need for dangerous distillation steps. 

In the case of LDPE-clay nanocomposites, the mechanical behavior does not depend only on the degree of exfoliation or the clay content but also on the presence of substantial amounts of compatibilizer [93, 94]. Besides the chemical modification of the clay, it is then necessary to turn the polymer matrix more polar with the grafting of polar groups. Traditionally, MAH has been used as the polar group to induce compatibility due to the high reactivity of the anhydride group. 

Simple shapes such as traditional vases may be made in one piece, a skilled but fairly simple operation. For complicated shapes like figures, or even just the handles for our vase, the original model will have had to be cut up and the finished object made in a number of parts. It is at this stage, when the clay is basically dry but still in what we call a green or leather state, that the parts are reassembled. In the case of a simple vase it may just be a simple case of dabbing a little of the original slip onto the ends of the handles and sticking them carefully in place. 

Alpha method The alpha procedure was derived by Drewry, Weidler, and Hwong (1977) from the same data set used for the original lambda correlation. For pile-capacity calculations the procedure utilizes a multiplier a on the undrained shear strength (Su) of the clay. The API code has traditionally recommended a value of 1.0 for Gulf of Mexico-type clays or a value that varies with strength for other types of clays. The API (2000) code varies the a value with the ratio of undrained soil strength to effective soil overburden ( / = S /a ) is as given in Equation 10.21 ... 

If the densification of liquid-phase sintering is achieved due to the viscous flow of a liquid that is able to fill up the pore spaces between the solid grains, it is called vitrification [112-114]. The driving force for vitrification is the reduction of solid-vapor interfacial energy, because the flow of the liquid covers the surfaces of the solid. Traditional clay-based ceramics are usually densified through vitrification. However, it is very unlikely to be observed in the processing of transparent ceramics, because the content of liquid phase must be controlled to a limited level. 

Finally, the comparison of both local (EDS) and bulk chemical analysis (AA) gives the opportunity to follow the compositional changes occurring in the clay platelets during the pillaring reaction. Furthermore, the true composition of the clay fraction, which is precluded from traditional chemical analysis, can be obtained by BIDS methods. 

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