Voids entrapped

Weld line(s), flow linefs), melt jetting, air entrapment, voids, warp-ir, stress concentrations, sink mark(s), etc. 

Thermal conductivity is not a static property of a material. It can vary according to the density, operating temperature and type of gas entrapped within the voids. 

Organic acid fluorescence. In a similar manner to trace constituents, such as Mg, Sr and P, concentrations of organic acids present in speleothem calcite are sufficient to observe variation at temporal scales of less than annual in some cases (e.g.. Baker et al. 1993, Shopov et al. 1994). Organic acids (humic and fulvic) are formed in the soil by humification, and transported to the cave void by percolating waters where they are entrapped in precipitating carbonates. Under certain circumstances, where precipitation patterns are strongly seasonal and the nature of vadose percolation is such that seasonal mixing is incomplete, bands with different luminescent intensities can be differentiated after excitation with UV radiation. In other cases, bands are not observable but secular... 

The porosity of refractory bricks has a direct bearing on the thermal conductivity. The densest and the least porous bricks have the highest thermal conductivity owing to the absence of air voids. On the other hand, in porous bricks the entrapped air in the pores acts as a nonconducting material. 

Common defects that are observed in this coating technique are voids, pin holes, thickness variations, and wavy surfaces. Voids and pinholes are caused by air entrapment in the film, surface contamination, or dust particles. 

The occurrence of voids has been thoroughly documented in thick laminates [2], In almost all cases, they are apparently associated with the prepreg surface. The exact mechanism of void formation depends on the system, but in the most general case it can include mechanical entrapment as well as nucleation of stable voids in the resin phase. 

Voids can be formed by either entrapment of air mechanically or by one of two nucleation processes. Mechanical entrapment could include (1) entrained gas bubbles from the resinmixing operation, (2) bridging voids from large particles or particle clusters (quenched DDS curing agent, airborne particles, or paper release agent), (3) voids from wandering tows, fuzz... 

For pure water voids, void nucleation is instantaneous. For air-water voids, air void nucleation or entrapment has already occurred during lay-up or at the beginning of the cure cycle. 

Void formation and growth in addition curing composite laminates is primarily due to entrapped volatiles. Higher temperatures result in higher volatile pressures. Void growth will potentially occur if the void pressure (i.e., the volatile vapor pressure) exceeds the actual pressure on the resin (i.e., the hydrostatic resin pressure) while the resin is a liquid (Fig. 10.9). The prevailing relationship, therefore, is ... 

Of the two hot precompacted laminates (Laminates 3 and 4), the wet exposed laminate was of significantly poorer quality. The attenuation ranges for Laminate 3 was from 30 to 62 dB, whereas that for Laminate 4 was 10 to 42 dB. Because the hot precompaction temperature was only 66°C (150°F), the additional porosity should not be due to voids caused by the vaporization of absorbed moisture, but is more likely due to voids caused by air trapped during collation. The wet exposed prepreg exhibited a noticeable increase in tack that could contribute to increased air entrapment during collation. 

SAS mixes are optimized using a slightly modified Marshall Method (ASTM Method D 1559) and represent a series of trade-offs between stability and flow, air voids content and permeability and fatigue life [15]. The major departure from the Marshall Method is that only 2 hammer blows rather than the conventional 75 blows per face are used. The light compaction is used only to expel entrapped air and to achieve more uniform sample configurations without disturbing sulfur soldification within the aggregate voids. 

Quantitative simulation of spectra as outlined above is complicated for particle films. The material within the volume probed by the evanescent field is heterogeneous, composed of solvent entrapped in the void space, support material, and active catalyst, for example a metal. If the particles involved are considerably smaller than the penetration depth of the IR radiation, the radiation probes an effective medium. Still, in such a situation the formalism outlined above can be applied. The challenge is associated with the determination of the effective optical constants of the composite layer. Effective medium theories have been developed, such as Maxwell-Garnett 61, Bruggeman 62, and other effective medium theories 63, which predict the optical constants of a composite layer. Such theories were applied to metal-particle thin films on IREs to predict enhanced IR absorption within such films. The results were in qualitative agreement with experiment 30. However, quantitative results of these theories depend not only on the bulk optical constants of the materials (which in most cases are known precisely), but also critically on the size and shape (aspect ratio) of the metal particles and the distance between them. Accurate information of this kind is seldom available for powder catalysts. 

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