Surfaces, thermal probing

The extent to which the molecules formed by recombination are in thermal equilibrium with the catalyst is of fundamental interest for the light it sheds on the nature of the interaction with the surface at the instant of reaction. It is also of practical interest, particularly in the use of thermal probes for the determination of atom concentrations, where the need to take account of factors influencing energy transfer processes has not always been recognised. Fresh interest in the phenomenon has been stimulated by the demands of space technology for information on surface heating due to recombination during re-entry into the earth s atmosphere. 

Figure 15. Thermal probe response vs. distance from surface for vessels of different shape.

The effect of film thickness on Tg can also be determined by measuring the surface softening temperature of corresponding polymer films using a heated AFM-like tip in scanning thermal probe microscopy. Scanning thermal probe microscopy probes the change in film modulus via the cantilever deflection induced by the thermal probe. As shown in Fig. 4.31, the cantilever deflects markedly when the PS film softens and hence, the onset of the Tg can be assessed. Systematic studies showed that PS on SiC>2 exhibits an altered Tg for film thicknesses below 50 nm. 

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... 

Wherever it is possible moulds should be provided with pockets for temperature measuring and recording instrumentation. This is essential if the moulding conditions are to be properly optimised and also provides a thermal finger-print for audit of the process during the production cycle. Surface temperature probes are better than nothing, but give results which may well differ between operators and are affected by the condition of the mould surface [5]. 

An AFM probe equipped with a thermal probe heats up sample resulting topographical imaging and thermal analysis of specific areas of the sample surface, sudden change in deflection of the heated cantilever indicates a thermeil transition Hme-of-flight secondary ion mass spectrometry (ToF-SIMS) Mass spectrometry of sample surface maps spatial compositional information the distribution of compounds within 1-2 nm depth... 

A number of other specialized adhesion testing methods have been tried, but most apply only to a very limited class of materials. These other methods use many other techniques, such as surface indentation probes or acoustic or thermal shock of the coating to get an idea of the film adhesion. In addition to being applicable only to a small class of materials, all of these techniques are plagued by the lack of a direct relation to adhesive strength and limited use and experience with the technique. Reference 16 reviews a few of the more common methods of adhesion testing. 

In addition to DESI and AP-MALDI, a large variety of other, sometimes closely related, atmospheric-pressure desorption ionization techniques have been introduced in the past decade, connected to a huge number of acronyms. Van Beikel et al. [76] tried to classify these emerging techniques into four categories, i.e., (1) thermal desorption ionization, (2) laser desorption/ablation ionization, (3) liquid-jet and gas-jet desorption ionization, and (4) hquid extraction surface sampling probe ionization. 

Figure 7.7. Thermal signal versus distance between the thermal probe and silicon and polystyrene surfaces (solid lines) together with deflection data (dashed line) [from Gorbunov (2000a), reprinted with permission of Tayior Francis Group],...

In principle, it can be envisaged that a thermal probe could be used in conjunction with an optical microscope that would image the sample surface to facilitate selection of the locations for analysis in other words, an atomic force microscope is not strictly necessary. In practice, all micro/nano-TA to date has been performed using an AFM, so we will consider this option only in this section. In its modern form micro/nano-TA can be interfaced with a wide range of AFMs, and so exact details of the procedure will vary from instrument to instrument, here we will discuss generic requirements. 

Microthermal analysers afford images based on thermal properties such as surface thermal conductivity (10 txm deep) and thermal diffusivity (with modulated frequencies for depth probing) and permit thermal analysis on samples of /rm area by combining the imaging ability of AFM and the thermal characterisation ability of temperature modulated DSC (/rMTDSC). As only measurements made on small samples can follow the temperature modulation, modulated temperature experiments are particularly indicated for small samples. 

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