Surface ablation

Heatshield thickness and weight requirements are determined using a thermal prediction model based on measured thermophysical properties. The models typically include transient heat conduction, surface ablation, and charring in a heatshield having multiple sublayers such as bond, insulation, and substmcture. These models can then be employed for any specific heating environment to determine material thickness requirements and to identify the lightest heatshield materials. 

We shall employ a simplified analysis of the ablation problem utilizing the coordinate system and nomenclature shown in Fig. 12-18. The solid wall is exposed to a constant heat flux of (q/A)0 at the surface. This heat flux may result from combined convection- and radiation-energy transfer from the highspeed boundary layer. As a result of the high-heat flux the solid body melts, and a portion of the surface is removed at the ablation velocity V . We assume that a steady-state situation is attained so that the surface ablates at a constant... 

For the experiments, only macroscopic front-surface ablation (and if necessary modification) was considered. For an in situ evaluation and the decision if ablation takes place or not, a HeNe laser was focused onto the interaction region between (femtosecond) laser and sample. In the case of ablation, changing Speckle patterns can be observed. 

Lascr-microprt)be mass speciromelers arc used for the study of solid surfaces. Ablation of the surface is accomplished with a high-power, pulsed laser, usually a Nd-YAG laser. After frequency quadrupling, the Nd-YA(i laser can produce 266-nm radiation focused to a spot as small as 0.5 pm. The power density of the radiation within this spot can be as high as 10 to 10" W/cm On ablation of the surface a small fraction of the atoms arc ionized. The ions produced are accel eraied and then analyzed, usually by limc-of-nighi mass spectrometry. In some cases laser microprobes have been combined with quadrupole ion traps and with Fourier transform mas speciromelers. Laser-microprobe tandem mass speclromeiry is also reeeiv-... 

The authors of this chapter conducted a recent study, which evaluated the efficacy of plasma treatment on a model wood substrate. In order to imderstand the effect of plasma treatment on wood-based fibers, clear pine wood veneers with an identical chemical composition was used as a model substrate. Plasma treatment was conducted using a custom built 13.56MHz inductively coupled plasma chamber in continuous wave mode (CW) and continuous wave mode plus pulse plasma mode [60]. During continuous wave mode the plasma is always on , so the formation of functional groups is accompanied by their destruction due to the continuous ion bombardment or surface ablation. During the pulse mode, the plasma source is on for a specific period and the plasma generated is similar to the continuous wave mode, where the plasma reactive species interact with the surface of the wood substrate. During the period when the... 

This is in concordance with the spectroscopic results where regardless of the carrier gas, surface ablation was observed. 

Figure 11.2 One pass of surface ablation experiment with a 50% beams overlap.

For more profound textile surface modifications, gases such as tetrafluoromethane (CF4) are useful. Specifically, tetrafluoromethane will form a thin hydrophobic layer over textile fibers after use within a plasma discharge. There are a number of studies which indicate that ablation accompanies the deposition of these thin films on fiber surfaces. In reference [30], Yip et al. suggested that shorter CF4 plasma exposure time will lead to more efficient polymerization effects, whereby longer CF4 plasma exposures lead to better surface ablation and lowered surface tension. 

Ablative polymers are used for the purpose of cooling which is due to the fact that work has to be done at the surface. Endothermic processes take place (e.g. melting, vaporization, thermal decomposition, etc.). Some heat is dissipated by radiation from the very hot surfaces. Pyrolysis of an ablative polymer starts when its decomposition temperature is reached at the surface. Ablation may be different from bulk degradation as the surface is preferentially heated. One refers to surface pyrolysis or "linear pyrolysis". The loss of material from the surface is termed "surface regression". 

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