Thermal spiking

A number of mechanisms have been proposed by which this common irradiated state is obtained. The most widely accepted is the thermal spike theory, which considers the heat generated in the wake of a fast particle passing through a soHd as being sufficient to cause severe stmctural disturbances which are then fro2en in by rapid cooling. Many property changes can be explained by this theory (146). 

The origin of the sputtered atoms may result from displacement below the surface of the source, a so-called thermal spike, as well as dhectly from the surface, depending on the incident energy of tire bombarding ions, ,. This must reach a tlrreshold value, before any atoms are dislodged from the target and... 

Thermally spiked 204 Thermal spikes 195 Three-layer models 152, 174, 175 Three-term models 149 Three-term unfolding model 176,185 Tie molecules 124 Tie-taut molecules 141 Time-dependent hardness 121 Tire cord 6... 

Figure 6 shows the sequence of events in a laser desorption FTMS experiment. First, a focused laser beam traverses the analyzer cell and strikes the crystal normal to the surface. Molecules desorbed by the thermal spike rapidly move away from the crystal and are ionized by an electron beam which passes through the cell parallel to the magnetic field and 3 cm in front of the crystal. 

As the thickness of the laminate increases, the strength of this thermal spike and the degree of thermal lag during heat-up increases. Figure 8.8 shows the results for a 62.5-mm (500 ply) laminate of the same material. Now the center-line temperature never reaches the autoclave temperature during the first dwell, and the thermal spike during the second dwell is nearly 135°C. The thermal spike is directly related to the release of internal heat during cure. The thermal lag is a manifestation of the low thermal diffusivity of polymer matrix composites. 

In early theories of the chemical effect of radiation it was supposed that the changes one observes in an irradiated medium are due to formation of local heated regions, which subsequently became known as thermal spikes. Later it was shown that for a particle with low LET the heating is neither considerable nor is sufficiently long to have an essential effect on chemical transformations. For instance, according to estimates made by Mozumder,23 the rise of the temperature inside a spur is only about 30 K, while the time r1/2 by which the temperature in the center of the spur lowers to a half of its initial value is about 6 x 10 12 s. 

For the c-BN formation a stress threshold was observed in the deposited layers. The h-BN intermediate layer shows a preferred orientation, where the c-axis of the h-BN is parallel to the substrate. Both effects are explained by the compressive biaxial stress induced by the ion bombardment. The mechanism for the conversion of h-BN into c-BN is explained by rather high temperatures originated during thermal spikes (direct h-BN —> c-BN transformation). The stress caused by the bombardment with high energetic ions is considered to be a reason for the growth of the c-BN crystals [190, 191]. A stress within the layer of up to 10 GPa has been observed. This biaxial stress causes a hydrostatic pressure up to the values usual in HP-HT synthesis. 

The difficulty of synthesizing C-N films makes it crucial to develop and apply theoretical models principally to guide and optimize the design of experimental methods for this purpose. The most widely applied models can be classified into three - thermodynamic, molecular dynamics, and thermal spike models. These models are described below. 

Although the effect of chemical bonding on energy dissipation in chemical systems is not included [54,55], the cylindrical thermal spike model takes into account the specific features of an ion-beam bombardment process such as energy loss and collision cascade. In particular, the individual ion bombardment will induce an initial energy deposited in a finite volume through collision cascades as illustrated in Figure 16.5. 

What is the primary assumption underlying the application of the thermal spike model to ion-beam assisted deposition of C-N materials ... 

This technique is used in the study of surfaces and in the analysis of the local composition of samples, such as inclusions in minerals or in cell organelles. It normally allows selective ionization by adjusting the laser wavelength. However, in most conventional infrared LD modes, the laser creates a thermal spike, and thus it is not necessary to match the laser wavelength with the sample. 

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