Piling Form of Packing

Conversely, the least dense packing can be seen if the filler is stacked by particles with the same size or shape. It contains more space and therefore the whole stack system occupies a larger space. For example, it is impossible for spheres with the same diameter to be placed in the gaps among spheres with the same diameter and filler particles with the same needle shape are often mixed and piled in a disorderly manner into a loose group because of different particle orientations in a static state. 

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Organic ion-radicals exist as salts with counterions. As seen in the preceding chapters, neutral molecules with strong acceptor/donor properties form rather stable ion-radical salts. Under certain conditions (see later), components of an ion-radical salt pack up in a crystal lattice in a special manner. Different ion-radical parts (cations and anions separately) line up one over the other in the form of endlessly long, practically linear one-dimensional (1-D), piles-chains. These piles-chains form a crystal. 

In the photooligomerizable crystals of m-PDA Me and CVCC Me48, nearly planar molecules are piled up and overlap completely along the shortest crystal axis of about 4 A. In the photostable crystalline DSP (y)47, molecules form a characteristic layer-type packing without any overlap of adjacent molecules. The y-form of DSP has been shown to undergo an unusual, thermally stimulated, topotactic phase transformation into the a-form53. ... 

The mean-field assumption of random contacts breaks down, however, for 2D piles formed under gravitational collapse, as most interactions between particles bring about alignment and there is not sufficient entanglement to prevent this from occurring. This was demonstrated in experiments by Stokely et al. [43], who formed 2D piles under gravity and compared both the packing fraction and orientational correlations with the mean-field predictions. 

Monomers decorated with chiral side-chains can induce a transfer of chirality at the supramolecular level. Indeed, several examples show that disklike molecules, designed to pile up into long reversible columns, form in fact helical columns, driven by the favorable packing of the chiral lateral substituents [30,32,93,94,99]. This chiral packing effect is strong enough to induce a chiral amplification a low amount of chiral monomer mixed with a non-chiral monomer can still drive the formation of helical columns. 

As is obvious in Fig. 10, there are common features of molecular packing in photo-reactive crystals. In all the photopolymerizable crystals in Table 4, nearly planar molecules are piled up and displaced in the direction of the molecular longitudinal axis by about half a molecule to form a parallel plane-to-plane stack. The periodicity in the stack is about 7 A. The shortest intermolecular distance between the double bonds in photopolymerizable crystals is about 3.9 A (Table 4) and it is found between molecules related to the center of symmetry in the stack. The second shortest distance between molecules in different stacks is more than 5 A. Therefore, the double bonds in the stack react to form a cyclobutane ring consequently, polymer chains should grow in the direction of the stack. The crystal axis along the stack in each photopolymerizable crystal, i.e. the presumed chain-growth direction, is indicated by c) in Table 4. 

Shape and uniformity of the particles also affect the behaviour of the product under storage. If the crystals are packed in bags and stacked on top of one another, the pressure in the bags near the bottom of the pile tends to force the crystals into closer contact with non-xmiform crystals this compaction may be quite severe and, in extreme cases, many of the crystals may be crushed. If the solubility of the salt in water increases with an increase in pressure, traces of solution may be formed under the high local pressure at the points of contact. The solution will then tend to flow into the voids, where the pressure is lower, and crystallize. Storage of crystalline materials xmder pressure should always be avoided if possible. 

Clay or firmly packed earth, upon which fine coal is rolled, should form the base of the storage pile and the coal should be spread over the entire area in thicknesses of approximately 1-2 ft and compacted. The formation of conical piles should be avoided and the top and sides of the pile should be compacted or rolled to form a seal and exclude air. An effective seal for a coal pile is afforded by a continuous layer of fine coal followed by a covering of lump coal to prevent the loss of the seal through the action of wind and rain. 

The channel type structures are constructed of a-cyclodextrin molecules piled on top of each other like coins in a roll, with the cavities forming an endless linear channel (Figure 6). The a-cyclodextrin stacks are packed in patterns belonging either to (pseudohexagonal) triclinic, to truly hexagonal or to tetragonal space group symmetries. 

The spherical fuel particle measuring about 1 mm in diameter consists of an inner nuclear kernel coated in successive layers of carbon and ceramics. Thousands of the particles are packed in graphite matrix into a spherical pebble of roughly tennis ball size or a cylindrical compact about the size of man s thumb. A pebble bed core contains a large number of fuel pebbles (for example, 27,000 in the HTR-10 core), and the helium coolant flows in the void volume formed in the pile of the pebbles. On the other hand, a prismatic core contains many hexagonal graphite blocks (150 in the HTTR core) in which the fuel compacts are embedded and the hehum coolant flows in the channels provided in the block. Both cores are surrounded by graphite reflector and enclosed in steel pressure vessel. Reactivity control rods (RCRs) are inserted from above the reactor pressure vessel (RPV). 

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