Aligned a-axis films

In the aligned a-axis films several SF structures and APBs with a shift of both c/3 and c/6 are observed as seen in Figs. 12.15 and 12.16. The central APB in Fig. 12.15 has c/3 displacement. It terminates on an unidentified SF with similar displacement which changes smoothly along its length to a 248 

with c/6 displacement, which then connects with a c/6 APB. A continuous change in the nature of SFs along a c-axis layer has also been observed in laser ablated YBCO films [12.32]. In Fig. 12.16, rotated domains and APBs with c/6 displacement are imaged. The c/6 APBs are connected in two places to a SF with the stacking disruption occurring within the YBCO unit cell. The APBs also terminate in two places on the rotated domains. It is seen from these images that displacements due to the nucleation misregistry accommodate a 

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Fig. 12.14. Low-magnification plan-view image of an aligned a-axis film, showing a granular structure of interconnected SF and APBs.

Jc(J K) greater than 10 A/cm. Jc(H) shows weak field dependence indicating an absence of weak-link behavior. For the films of different orientation discussed here the Tc values are of the order of 84-85 K for the two-domain a-axis films, 86-87 K for the c-axis film and aligned a-axis films, and 88 K for the (103) films. 

Fig. 12.18. Resistivity (a) and (b) vs. temperature in the [010] and [001] directions of the aligned a-axis film. The Jc of a two-domain a-axis film and a c-axis film are shown for comparison. The inset of (b) shows the magnetic field dependence of the normalized in the two directions at 77 K compared with a high-/c c-axis film which is known to not be weak-link limited. The scaled field is in kG for the c-axis film and in kG/(mass ratio) for the aligned a-axis film.

Fig. 12.20. Fabrication of single grain boundaries. The lines indicate the CuOg planes of the aligned a-axis region on a PBCO buffer layer and the shaded areas show the c-axis region on the bare substrate. The black pattern is the photolithographic mask used for defining the four measurement lines the BPF and the twist grain boundaries, and the [010] and [001] leads in the a-axis film.

Fig. 12.16. Some c/6 APBs (A), misaligned grains (R), and an SF, which disrupt the YBCO unit cell in the Y—Cu02 layer region (two left arrows), as well as in the chain layer region (right arrow) are imaged in an aligned -axis film.

Consider a nematic film of negative dielectric anisotropy (e < 0) aligned homeotropically between glass plates. If an electric field is applied along the director axis (z axis) a distortion will set in when the field exceeds the critical Freedericksz value given by... 

Strain, usually refers to the direct strain (aligned with the major axis) of a deformable film. The axial extensional strain resulting from bonding two layers. 

The molecular chains in a particular sample of polymer film are almost completely aligned in the simplest type of uniaxial orientation. The fractional transmissions of this sample for radiation of a particular infrared wavelength incident normally are found to be 0.20 and 0.95 for radiation polarised parallel and perpendicular to the alignment axis, respectively. For a second film of the same polymer of lower orientation the eorresponding fractional transmissions are 0.30 and 0.80. Making suitable assumptions, what can be deduced about the degree of orientation of the chains in the second film ... 

When a crystalline polymer is oriented, the random circular film pattern (random orientation) transforms to a collection of defined reflection arcs that are correlated with particular (hkl) planes that can be identified based on the crystal structure and Bragg relationship (see Fig. 12a-b). It follows that the magnitude of the azimuthal spread (x/2) of these reflections is indicative of the degree of orientation. (The breadth, k, of the reflection is related to crystal size and imperfection—see Ref. 32.) Also, the location of the reflection with respect to the sample axes indicates the orientation of the crystallographic planes. For example Fig. 5(a) and (b) show two X-ray photographs of polyethylene that had been cold rolled. From the (200) reflection in sample (a) one sees that the a-axis is aligned preferentially normal to Z whereas in (b) there are two distinct orientations of the a-axis—one along Z and one normal to this. 

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