Radiation effects specimen heating

Figures 1(a) and (b) show relations between the currents measured and the voltages applied for the specimens with and without H, respectively, at several ionizing dose rates for the first cycle. The currents at the applied voltage increased as the ionizing dose rate increased. Moreover, the irradiation temperature was also gone up to 473 K by gamma-heating. The increment of the irradiation temperature has a little influence on the increment of the current. However, it is, for the temperature below 473 K, much lower than the radiation effects.

Thickness. The traditional definition of thermal conductivity as an intrinsic property of a material where conduction is the only mode of heat transmission is not appHcable to low density materials. Although radiation between parallel surfaces is independent of distance, the measurement of X where radiation is significant requires the introduction of an additional variable, thickness. The thickness effect is observed in materials of low density at ambient temperatures and in materials of higher density at elevated temperatures. It depends on the radiation permeance of the materials, which in turn is influenced by the absorption coefficient and the density. For a cellular plastic material having a density on the order of 10 kg/m, the difference between a 25 and 100 mm thick specimen ranges from 12—15%. This reduces to less than 4% for a density of 48 kg/m. References 23—27 discuss the issue of thickness in more detail. 

Radiation damage and sample heating arise from the absorption of X-rays in the sample. The sensitivity of the specimen to these effects depends markedly on the temperature of the specimen. Initially comments will be restricted to room (or near room) temperature where protein crystals maintain their liquid-like nature in the solvent channels going through the crystal. 

In a passive mode, such devices function like thermocouple probes described above. These can be used (for example) to map the temperature distribution in energised electronic devices simultaneously with their topography [38,39]. If the surface is illuminated with infrared radiation, the photo-thermal effect arising from the absorption of energy specific to the infra-red (IR) active modes of the specimen may be used to obtain the sample s IR spectrum [40-47]. In the active mode, the heat flow from the tip can be used to detect surface and subsurface defects of different thermal conductivity than the matrix [48,49]. 

Dinner et al. [17] presented the design and construction of a calorimeter in which the specimen may be heated by microwave radiation, as well as by hot air. The effect of the intensity of microwave radiation was examined by measuring the melting of benzyl and the solid-state phase transition in silver iodide. For the latter, the transition temperature has been foimd to vary significantly with the intensity of microwave irradiation. 

Test methods of flame spread comprise all the techniques where a specimen is subjected to the effect of an igniting source (flame, heat radiation, glowing body) followed by recording of the consequences of ignition. 

Like in any porous insulation material, the total heat losses are the sum of the skeletal conduction (phonons), gas conduction (collisions between gas molecules), and radiation contributions. The particular mesoscopic structure of aerogels leads to a considerable reduction of the gas-phase conduction contribution due to a trapping effect of the pore gas. For most terrestrial applications, the thermal conductivity at ambient temperature and pressure is relevant, which although well studied, is still a challenge to determine accurately [203], primarily due to the lack of large-area homogeneous aerogel specimens. 

---