Voids periodic appearance

Figure 5.12 shows the SAXS intensity curves of PTeOX prepared at 84 °C (similar to Fig. 5.11). The intensity curve of the as-polymerized sample decreases monotonically except of a small shoulder at ca. 5 . This small shoulder becomes clearer by irradiation with 0.1 MGy and forms a peak with 0.5 MGy. The peak position of ca. 7 corresponds to a long spacing of ca. 800 A, indicating the periodical appearance of voids along the c-axis.

Following this initial period of polyimide development, interest reached a steady-state and remained there until the late 1970s. During this time a major impetus to the polyimide area was provided by the aerospace industry. The need for composite matrix resins as well as structural adhesives with excellent oxidative and thermal stability appeared to be at least partially met by polyimide type resins. Ultimately, requirements of high flow and low void content in relatively thick parts directed these efforts into different directions. Another upswing occurred in the early 1980s with the potential application of... 

It appears as if one of the level K electrons of the atom disappears into the nucleus. The void created induces X-ray fluorescence from the nucleus Y. There are several known radionucleides of this type, which have sufficiently long periods that they can be used as different energy sources (see Table I3.l). These are typically used in portable instruments. The activity of these isotopic sources is generally in the order of a few mCi and they can yield a flux of 106 to 108 photons/s/steradian. Because these sources require a permit for use as well as permanent protection because they emit continuously, their use is rapidly diminishing. 

The formation of defects in porous anodic alumina films fabricated in phosphoric acid solutions has been studied. TEM and SEM examinations revealed that defects appear at the triple point junctions of the cell boundaries, where neighbouring celis meet. The defects represent voids in the anodic alumina and develop into spatial structures that comprise a central void at the triple-point junction extending into tubular branches that pass toward the pore wall. The defects are generated continuously during the anodic film growth under appropriate anodising conditions. Additional post-anodising treatment also results in periodic arrays of circular nanoholes in the pore walls. 

It is likely also that biofilms will become more dense with the passage of time since the voids within a biofilm, will gradually fill with cells as a result of reproduction. The effects of the age of a biofilm of Pseudomonas fluorescens on mass transfer through the biofilm were demonstrated by Vieira et al [1994]. Using Li Cl as the diffusing solute there was a fall of around 60% in mass transfer coefficient in a period of about 150 hours. The Reynolds numbers used in the experiments were 8250 and 14700 but there appeared to be little dependence as would be anticipated, of mass transfer through the biofilm due to changes in Reynolds number. 

Luminescence differs from chromic effect in that luminescence is not a colour change but emits light. Opticoluminescence is a typical effect encountered in optical fibres. Opticoluminescent fibres have been widely used in textiles for the function of accurately monitoring body and environmental conditions. Recently photonic crystal fibres (PCFs) have been introduced. The cross-section of the PCF contains either periodically arranged micron-scale air voids or a periodic sequence of micron-scale layers of different materials. " PCFs can appear coloured due to optical interference effects in the microstructured regions. 

In practice, there are problems with powders, and to some extent with fibre blankets, due to thermal contraction and expansion of the inner vessel and pipework. Over a period of time, the insulations settle or move so as to form voids. The presence of such voids are indicated by the appearance of cold spots or frost patches on the outer containing skin. These problems can be avoided wifli the use of PUF and other plastic foams. 

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