Thermal probe method

Let us examine both measurement procedures. The rate of temperature increase dT/dt for a given volume heat output from the probe q(t) is determined by the heat flow dq/dt. A solution of 

The results of dT/dt and T measurements made during curing of two systems - macro(di-isocyanate) based on poly(tetraethylene glycol) and 2,4-toluylene diisocyanate cured by diamine (MDA) and epoxy diene oligomer cured by triethanol(amine titanate) are shown in Figs. 3.9 and 3.10, respectively. The monotonic increase in the parameters, monitored in the experiment, 

The steepest part of the T (t) curve, shown in Fig. 3.10, corresponds to the transition of epoxy resin into the rubbery state. In MDI curing and methyl methacrylate polymerization the amplitude of the response signal (T ) decreases monotononically and then becomes constant when the rigidity of the system reaches a certain level. The completeness of the reaction can be determined by saturation of the dT/dt(tf) curve. The product pCp is relatively insensitive to the transition of an oligomer to a polymer. The shape of the dT/dt(tf) curve is determined primarily by changes in the 

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The simplest experimental arrangement (two-probe method) uses two thermometers one on the thermal bath at Ts, the second on the warm end of the sample together with the heater (see Fig. 11.1). Such configuration can be used when one is sure that contact resistances are negligible compared with the sample thermal resistance. This is seldom the case at very low temperature. A sample bath (and sample support) temperature drop ... 

Fig. 11.1. Scheme of the two-probe method for the measurement of the thermal conductivity. 

For samples with low values of K, < 500 ppm, the laser method, because of its small sample size, becomes more uncertain than a thermal release study with a larger sample size. It is not possible to compensate in the laser probe method by multiple pulses above MOO individual pulses which corresponds to evaporation of approximately 20 pg. It is conceivable that by completely redesigning a mass spectrometer with smaller volumes which is bakeable to higher temperatures to alleviate this problem. We have been able on occasion to work with samples as low as 100 ppm K but certainly not on a routine basis. 

Schematic energy level diagrams for the most widely used probe methods are shown in Fig. 1. In each case, light of a characteristic frequency is scattered, emitted, and/or absorbed by the molecule, so that a measurement of that frequency serves to identify the molecule probed. The intensity of scattered or emitted radiation can be related to the concentration of the molecule responsible. From measurements on different internal quantum states (vibrational and/or rotational) of the system, a population distribution can be obtained. If that degree of freedom is in thermal equilibrium within the flame, a temperature can be deduced if not, the population distribution itself is then of direct interest.

Ideally, the probe molecules used in the chemical characterization experiments should simulate as closely as possible the behaviour of the actual reactants. This is rarely possible and so simple probe molecules (H2, O2, CO, NO) are used instead. Some additional information about the nature of the surface may be obtained using thermal analysis methods, e.g., calorimetry, temperature-programmed desorption, temperature-programmed reaction. 

We have developed a thermal ionization method for use in a standard quadrupole mass spectrometer (1) The method uses a modified solids inlet probe Figure in conjunction with replacable filament assemblies The rhenium filaments are coated with calcium salts precipitated from biological materials in a basic ammonium oxalate solution Calcium is precipitated directly from urine and serum first made basic with ammonium hydroxide fecal samples and aliquots of diet are homogenized with a 9 1 water/nitric acid mixture, centrifuged, made basic and... 

Methods giving absolute energy values thermal measurements (calorimeter, thermal probes). 

When the method used depends upon an estimation of the slope of the tangent at zero time, it is essential to provide good stirring of the liquid. In the absence of stirring the response of the thermal probe is far from simple. 

Thermal probe systems are inexpensive, easy to handle in almost all ultrasonic devices and particularly those used in sonochemistry. Field distributions and optimization of the geometry of the system can be rapidly obtained and the accuracy of the method is high enough to ensure reproducibility. Chemists who make use of ultrasonic equipment should, as a very minimum, consider this method to calibrate and optimize sonication conditions prior to carrying out sonochemical reactions. 

The intensity of sonoluminescence can easily be measured with photocells [159,163] or fiber optics [169] connected to a photomultiplier in darkened surroundings. This measurement is not invasive, and has been suggested as a standard [19]. In principle the sonoluminescence intensity could be correlated to the ultrasonic power, but at this time no direct theoretical correlation has been established. It has been used to determine the areas of maximum cavitational activity in a reactor. Any empirical correlation with power would necessitate preliminary calibration with another method, e.g. with thermal probes. Some care should be exercised when using a sonoluminescence probe for the following reasons ... 

From a practical point of view, the most generally applicable and the easiest dosimeters to use are those based on thermal methods, especially those using thermal probes. These probes have almost no limitations since they can be used (a) in any reaction vessel below or beyond the cavitation threshold, (b) in free or... 

The two main drawbacks to chemical dosimetry are that they have low sensitivity at low power and are often strongly frequency-dependent. For accurate measurements they should also be calibrated with another method, e.g. with a thermal probe. 

The phase composition of the resulted specimens was identified by X-ray diffraction (XRD). Rod-like pieces (3x3xl5mm) and disk-shaped pieces (2mm thickness and 10mm diameter) were cut out for the electrical conductivity measurement and the thermal conductivity measurement, respectively. Microstmcture and phase distribution were observed by a scanning electron microscopy equipped with EPMA (JEOL JXA-8621MX). Electrical conductivity was measured using a D.C. four-probe method. Thermal conductivity was measured using a laser-flash technique. All the measurements were performed in the temperature range of 300 to 1200 K. 

X-ray residual stress determination was performed on the surface of the samples prepared by HIP sintering. The measured residual stress was compared with the results calculated by the finite element method (FEM). The electrical resistivity was measured by the four probes method on the slices cut from the cylinder samples. In order to inspect the thermal stability, the samples were annealed at 900 °C for 24 hour in vacuum. The microstructure on the section was observed by scanning electron microscope. 

Electrical resistivity measurement adopted conventional four probes method. Seebeck coefficient was measured by the standard DC method. Thermal conductivity k was calculated from density, specific heat, and thermal diffiisivity. Specific heat measurement was carried out by use of a differential scanning calorimeter (DSC model 8230, Rigaku, Japan) compared with a standard material of a -AI2O3. The values of thermal diffiisivity obtained from a differential phase analysis of photo-pyroelectric signal (AL- A 0 analysis) [9]. All measiu ements were done at room temperature. 

Figure 7. Experimental set-up for 3 method consisting of thermal probe (ThP), Wheatstone bridge and lock-in amplifier.

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