Liquid crystals film detection

Based on the generation of a temperature pattern by dissipation of power, liquid crystals can be employed to detect faults in electronic devices. The liquid crystalline material is painted on the surface of the device and the device is then switched on. The defective areas are clearly indicated by the change in color of the liquid crystal film at the points of excessive thermal stress. This technique is particularly useful for locating faults in solid state devices, especially integrated circuits [15]. 

Fig. 1 Unit for shear force measurements attached to the surface force apparatus. The sectioned piezo tube (Morgan Matroc) bends depending on the amplitude and polarity of the applied input voltage 17in. The symmetrical leaf springs transfer this bending into lateral oscillations of the liquid crystal film which is sandwiched between two mica sheets glued to the crossed quartz lenses the upper lens attached to the piezo tube and the lower lens mounted on the cantilever spring. The magnitude of these oscillations is detected as the output voltage Uom of the capacitance probe (Japan ADE). In this configuration our shear force unit is a modification of the one proposed first by Klein et al. [8]...

There are two possibilities for the detection of the interferences the film detection and the registration of x-ray counts with scintillation counters or position-sensitive detectors. However, the SAXD method does not detect interferences from which the interlayer spacings can be calculated. It rather makes it possible from the sequence of the interferences to decide the type of liquid crystal [13,14]. 

The investigations of solid-liquid crystal interaction by SHG are not only restricted to LG monolayers and thin films. As most of the bulk liquid crys-talhne phases are centrosymmetric, the surface SHG response can be detected also from thick liquid crystal cells , in which the material is sandwiched between two substrates. By this kind of measurements it is possible to study a relation between the bulk and the surface liquid crystalline ordering and consequently probe various pretransitional phenomena and other surface related effects. 

There are several other reasons why LB films are likely to be used in preference to other pyroelectric materials. The figure of merit referred to above, (p/e), is only a measure of the signal strength, and does not take into account the noise sources in a complete infrared detection system. If this is done, one discovers that it is essential to have low dielectric loss in the film over an extended frequency range. All ferroelectric liquid crystals are ruled out of contention on this basis, because they have high tan 5 values in the range IHz to lOkHz. Moreover, it is not possible to prepare very thin films in structures with low thermal mass and low thermal conductivity. 

Another type of thermotropic liquid crystals is called cholesteric liquid crystals. The color of cholesteric liquid crystals changes with temperature and therefore they are suitable for use as sensitive thermometers. In metallurgy, for example, they are used to detect metal stress, heat sources, and conduction paths. Medically, the temperature of the body at specific sites can be determined with the aid of liquid crystals. This technique has become an important diagnostic tool in treating infection and mmor growth (for example, breast tumors). Because locahzed infections and tumors increase metabolic rate and hence temperature in the affected tissues, a thin film of liquid crystal can help a physician see whether an infection or tumor is present by responding to a temperature difference with a change of color. 

Homeotropic liquid crystal cells remain important in liquid crystal deflec-toscopy (flow detection) [155, 167]. In this case, the controlling potential is created by the defect hole and could be approximated by item 3, Table 5.2. A typical homeotropic liquid crystal cell for testing dielectric films is shown in Fig. 5.27. The defect hole creates a nonuniform field, which becomes visible against a dark background made by crossed polaroids. 

Nematic and cholesteric liquid crystals can be used for the nondestructive study of electrical defects in transistors and integrated circuits [81, 82], for the detection defects in film capacitors prepared by vacuum deposition [83], for the visualization of electrically active defects or rapidly diffusing dopants, as well as for quality control at various stages of integrating circuits production [84-86]. The most suitable effect for this purpose would appear to be the B effect [85] and the fiexoelectric effect in spatially nonuniform field [84, 86], which permits the distribution of the electrical potential in operating the integrated circuits to be visualized. 

In addition to thin film models, the stabilizing action of liquid crystals in foams has been well established, especially in the case of nonionic surfactant systems. At low concentrations, no significant structure buildup has been detected. However, in the early 1960s convincing evidence of a stabilizing effect of liquid crystals at high concentrations was reported. It was suggested that the crystals had a twofold function, in that... 

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