Bunn and Alcock were also among the first to propose a mechanism for cold drawing of PE filaments that involved the cleaving of PE crystals. Much later work by a number of researchers has shown that when constrained ultradrawn PE fibers are heated, they do in fact pass through a hexagonal phase prior to becoming an isotropic melt. The authors speculated that this change in the a-axis represented a shift toward a pseudohexagonal packing caused by twisting of the PE molecule away from its all-transplanar conformation. Elevated temperature experiments showed that expansion of the unit cell occurred primarily along the a-axis, and that evidence of ordered structure disappeared around 126☌. They reported an orthorhombic unit cell with the parameters, a-7.42 Å, b-4.93 Å, and c-2.534 Å obtained by wide angle X-ray diffraction (WAXD) experiments with a PE film at room temperature. The first report of crystallographic unit cell parameters for PE was by Bunn and Alcock in 1944.
Gillespie Jr., in Structure and Properties of High-Performance Fibers, 2017 7.3.1 Phase composition of PE fibers The pore diameter is virtually not affected by introduction of aluminum, copper and zinc. In Table 1, the surface area A BET, the pore volume V p and the pore diameter d p, calculated from nitrogen adsorption-desorption isotherms using (Brunauer, Emmett and Teller) BET and (Barrett, Joyner and Halenda) BJH analysis, are summarized. Variation of the n Si/n Al-ratio from 30 to 15 has no significant influence on the adsorption capacity. In agreement with the XRD powder patterns, the nitrogen adsorption is reduced in the zinc-containing samples, while copper introduction does not affect the adsorption capacity. While the addition of copper acetate to the synthesis gel up to a ratio n Si/n Cu = 15 has no influence on the quality of the XRD pattern, the zinc-containing materials show XRD patterns with somewhat lower intensity. XRD and nitrogen adsorption studies reveal that AlMCM-41 can be synthesized in the presence of copper and zinc salts. How could you determine the pattern of the atoms on the surface of a metal experimentally?ģ.A) The pore volumes and diameters were calculated from the desorption branch of the nitrogen isotherms using the BJH-model. Draw the arrangement of atoms on the crystal surface. Label the two directions in the (111) plane.ģ.20 A bcc crystal is cut so that the (011) plane is exposed on the surface. Draw the arrangement of atoms you would see if you looked at this surface. The mass of an atom is the atomic weight times the atomic mass constant $u$ = 1.6605402 × 10 -27 kg.ī) What is the distance between neighboring atoms?Ĭ) The metal is cut so that the (111) is exposed on the surface. The lattice constant is the length of the conventional (cubic) unit cell. The lattice constant is 0.38 nm and the atomic weight of the atoms is 85. What is the Bravais lattice? What shape does the Wigner-Seitz cell have? What are the positions of the atoms of the basis given in fractional coordinates of the conventional (cubic) unit cell? Draw a (111) plane.ģ.18 Calculate the angle between the direction and the direction for a monoclinic lattice with a = 0.3 nm, b = 0.4 nm, c = 0.5 nm, and β = 107°.ģ.19 A metal has an fcc crystal structure. Determine the normal distance between the two planes.ģ.17 Draw the NaCl crystal structure. What is the length of the translation vector with $h = 4$, $k = 3$, and $l = 1$ ?ģ.16 Draw the (111) and (222) planes in a simple cubic unit cell, with lattice constant $a$. From this information determine the primitive lattice vectors. One way is to repeat the primitive unit cell at each translation vector, In a crystal, atoms are arranged in straight rows in a three-dimensional periodic pattern.Ī small part of the crystal that can be repeated to form the entire crystal is called a unit cell.Ī crystal can be specified in several ways.