Thermal probes heating

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... 

Hitze-probe, /. heat teat, -strahlung, /. heat radiation, -wirkung, /. action of heat, heat effect, thermal effect, -wirkungsgrad, m. thermal efficiency. 

In order to provide a steady-state signal, heat must be continuously evolved. An ideal reaction for this is the biocatalytic enzymatic reaction, which combines high substrate specificity with a high amplification factor. Thus, an enzyme-containing layer is deposited over the thermal probe, and the substrate is allowed to diffuse in. 

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. 

Thermal probes can be constructed quite easily and cheaply. Their response is non-directional, and very small devices with diameters between 0.5 and 1 mm can be fashioned. They are very simple to use with almost every kind of ultrasonic equipment and measurements can be made very rapidly. Several kinds of thermal probes have been described which are basically thermocouples or thermistors used bare or embedded in an absorbing medium. Bare probes are used to measure the actual temperature of the medium, just as in a calorimeter. Coated probes will generate internal heat under the influence of the sound wave and are used to determine local power dissipation in the absence of stirring. Coated probes are often used in conjunction with a bare probe, and the temperature difference between the two probes is then proportional to the acoustic power. Great care should be taken since the response of a coated probe strongly depends on its nature and geometry, and on the medium used. 

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. 

This technique based on a hot wire thermal probe with AC excitation and 3 CO lock-in detection. Since the principle and procedures of the technique have been described in details previously [51] only a brief description is given here. We consider a thermal probe (ThP) consisting of a metallic wire of length 21 and radius r immersed in a liquid sample, acting simultaneously as a heater and as a thermometer. The sample and probe thermophysical properties are the volume specific heat pc and the thermal conductivity k, with the respective subscripts (5) and (p). The wire is excited by ac current... 

Fig. 4.3 Automatic system for the determination of humidity in solid samples (Mettler IR LP 16 desiccator. (1) Pair of dual infrared heating bars (2) two thermal probes (3) rising humidity (4) infrared radiation (5) air-cut (6) sample scale (7) air gates (8) outside air (9) input keyboard (10) high-precision balance. (Courtesy of Mettler).

For micro-pyrolysis-td-GC-MS, the sorbent tube is modified to end in a short section of stainless steel hypodermic tubing the open end of which can be placed immediately adjacent to the heated thermal probe using a micro-manipulator. As the tip is heated, a pump is used to draw gas through the tube. After sampling, the tube is placed in a suitable carrier that fits into a standard thermal desorption unit interfaced to a GC-MS system. Blank desorption runs of the... 

Modulated radiation from a standard spectrometer is directed onto a sample and detected by a microprobe (see Fig. 20) [137, 138]. The IR imager s thermal probe is the same as used in micro-thermcd techniques. It detects photo-thermal response of a region on the specimen heated by exposure to the beam from a FTIR... 

Figure 7.2. An illustration of the effect of sample topography on the heat flux from a thermal probe to its surroundings.

It should be noted that the use of the L-TA power signal was previously termed micro-DTA and not micro-DSC. This is because, as discussed above, the volume and hence the mass of the heated sample is unknown and its calculation at present does not seem possible. Hence it is argued that micro/nano-TA cannot be described as a form of calorimetry, because it cannot measure the heat capacity of a material or the enthalpies associated with particular transitions. This is a major limitation of thermal probing. However, the ability of this technique to detect transitions occurring in a sample at the micrometer and submicrometer levels and to measure the temperature range over which those transitions occur is a major advantage. 

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