Localised thermal analysis

Price DM, Reading M, Hammiche A, et al. Micro-thermal analysis scanning thermal microscopy and localised thermal analysis. Int J Pharm 1999 192(l) 85-96. 

The introduction and development of Micro-Thermal Analysis are described and discussed by Duncan Price in Chapter 3. The atomic force microscope (AFM) forms the basis of both scanning thermal microscopy (SThM) and instruments for performing localised thermal analysis. The principles and operation of these techniques, which exploit the abilities of a thermal probe to act both as a very small heater and as a thermometer, in the surface characterisation of materials are described in detail. The... 

A good reference material has a number of desirable properties including a well-documented value, avmlability in a suitable form for analysis, homogeneity, stability, low toxicity, and traceability to a national reference laboratory (NRL). In traditional DSC and TMA, metals like indium, tin, and zinc meet these criteria. These metals are not suitable for temperature calibration for localised thermal analysis, however, as they may contaminate the probe tip thus changing its resistance and defeating the object of calibration.. 

A final effect often observed during localised thermal analysis arises from the high heating-rates that can be employed. Many thermal transitions are governed by kinetic laws which define the time dependence of such processes as devitrification and thermal degradation. As a consequence of this, some... 

Figure 7. Localised thermal analysis of semi-crystalline poly(ethylene terephthalate) showing two consecutive measurements at the same location (solid line is the probe displacement, broken line is the probe power, filled symbols denote first upwards temperature scan, open symbols denote second upwards temperature scan, heating rate 10 s ).

Since its development and subsequent commercialisation, localised thermal analysis has been employed in a number of different disciplines. A review by Pollock and Hammiche [85], summarises the wide range of materials which have been studied by these techniques. An earlier paper by Price et al. [102] describes measurements on polymers, electronic components and biological specimens in the same publication as an illustration of Ihe wide applicability of such measurements. Articles in popular technical publications have also served to illustrate this point [72,90, 103-105],... 

Figure 8. Localised thermal analysis of multi-layer polyolefin film shown in Figure 4 [105].

Two examples of the use of localised thermal analysis are provided in order to illustrate the generic applications of this approach. Figure 8 shows localised thermomechanical analysis of the surface of the multi-layer film in Figure 4. Measurements were made at points within this image describing the bulk polymer, the central gas-barrier layer and the thin tie-layer between this and the bulk film. The melting transition temperatures are consistent with high density polyethylene, poly(ethylene-co-vinyl alcohol) and medium density polyethylene for the bulk, gas barrier and tie layers, respectively [101],... 

Figure 10. Results fiom localised thermal analysis of the specimen shown in Figure 9. The solid symbols denote measurements on the normal film whereas the open symbols denote measurements made in the gel particle [103].

Figure 11. Localised thermal analysis of the analgesic tablet shown in Figure 5(a) with measurements made in the low thermal conductivity (dark) region (solid line) and the high thermal conductivity (bright) region (broken line) of the image [70],...

Micro-thermal analysis has been used to examine a number of specimens of biological origin. Gorbunov et al. have reported the study of the infrared receptors of snakes using SThM and localised thermal analysis, whereby the... 

Figure 13. Localised thermal analysis of the LED shown in Figure 12. The upper set of curve show measurements made. within the high thermal conductivity central region and the lower set of curves show measurements made outside this area [102]. (With the permission of Akademiai Kiado.)...

The results of this method, in addition to the work described above, have also been combined with measurements by SThM and localised thermal analysis. Polymers [106] and pharmaceuticals [23] represent the largest classifications of materials Aat have been investigated, although this technique has been used in cellular biology to monitor the life-cycles of cells [168]. 

The recent technique of micro-thermal analysis ()U-TA), which now has a variety of measurement modes, is included here because usually two or more measurements are made simultaneously. Micro-TA combines the imaging capabilities of atomic force microscopy (AFM) with a form of localised thermal analysis, and is able to measure thermal transitions on an area of a few microns. A good introduction to the whole family of these methods is available on the internet, from which application studies can... 

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