Sample Holder Considerations

One important experimental parameter is also the furnace which is used in the thermogravimetric apparatus. There always exist pronounced radial and vertical temperature gradients which can be found by calibration runs. As an example, the determination of the vertical temperature distribution by Wiedemann41 may be referred to. Also the amount and shape of the sample can contribute to a temperature gradient. This fact is of special importance for kinetic studies. Considerable temperature differences - up to several °C - can exist at different locations of the sample holder. 

The considerations used to calculate the RCF exerted on a sample spinning in a centrifuge rotor require that the sample be located at a fixed distance, r, from the center of rotation. Owing to rotor design, r varies from top to bottom of the sample holder. The problem is illustrated in the following example. 

Capillaries are unpopular for several reasons firstly, diffuse X-ray scattering from the glass walls, which are approximately 10 pm thick, adds significantly to the background count. Secondly, they take considerably longer to All than the equivalent flat-plate sample holder. Thirdly, capillaries may require careful alignment on the diffractometer to ensure that the axis of the capillary is... 

Another way to increase water vapor pressure in the reactor is to increase, without any exhaust valve, the amount of specimen. Table 1 shows that if all the six sample holders are filled, is considerably longer. The use of an exhaust valve appears necessary to get a constant independent of the amount of the specimen [2]. 

Using the sample holder shown in Figure 9, Schwartz et ai. [18] obtained the dependence of the detonation velocity of lead azide on density (Figure 14). The data are in general agreement with those obtained by others [28-30]. Scott [19] found a value of 4.3 km/sec at a density of 2.86 g/ml and a semicircular cross-section of radius 0.075 in. which is in fair agreement with the Schwartz data. However, both Schwartz and Avrami et at. [18,29] found a considerable variation in /J even in samples of the same (low) density. 

Aside from their field of application, the essential difference between these diffractometers is their geometric layout [GUI 05], In particular, the relative positions of the source, of the sample-holder and of the detector are different. Therefore, the description of these devices will rely essentially on geometric considerations. One of the most significant consequences of choosing a particular geometric configuration is the angular resolution of the device. Therefore, the description of these devices will be illustrated by measurements which make it possible to evaluate this essential feature. 

The slightly improved repeatability of the He- and Hf2-values in comparison with that of the Hfl-value might be caused by the improved thermal contact between sample and sample holder after the fusion process. Nakamura [5] reports a reproducibility of 3°C for the Tm/Tc determination. The difference between the repeatability and the reproducibility values of the Tm/Tc determinations is thus considerably higher than those found for the Tg(onset)-value determination. 

One of the major considerations in DTA is obtaining valid readings of the actual temperature of the sample and reference materials conveniently and repro-ducibly. As in TG, thermal equilibrium is of utmost importance. There is always a definite temperature difference between the outer and inner portions of the sample indeed, reactions often occur at the surface of the sample while the interior is still unreacted. This effect is minimized by using as small a sample as possible with uniform particle-size and packing. Depending upon the instrument used, the thermocouple may be imbedded in the sample, or at the other extreme, may simply be in direct contact with the sample holder. In any case, the thermocouple must be precisely positioned for every experiment. To obtain the best results, the reference and sample thermocouples should be matched in temperature response and the geometric arrangement of the sample and reference thermocouple should be perfectly symmetrical within the oven. 

Samples analyzed in the dilatometer up to temperatures above ISOO C lost considerable amounts of silicon through evaporation that was deposited on the graphite sample holder in the dilatometer. These samples consisted mostly of TiC. Since carbon was present in the furnaces through the graphite heating elements and sample holder/crucible, the decomposition of Ti3SiC2 is likely to follow the reaction initially proposed by Racault et al. ... 

XRF is used for the analysis of solid and liquid samples, and similar sample holders and autosamplers are used for both EDXRF and WDXRF. Sample preparation and other considerations will be discussed in the applications section. For quantitative analysis, the surface of the sample must be as flat as possible, as will be discussed in the applications section. There are two classes of sample holders cassettes for bulk solid samples and cells for loose powders, small drillings, and liquids. A typical cassette for a flat bulk solid such as a polished metal disk, a pressed powder disk, and a glass or polymer flat is shown in Figure 8.20a. The cassette is a metal cylinder, with a... 

A linear instrument baseline is easily obtained because the relatively large furnace heats the atmosphere surrounding the sample holder unit. However, the time required to stabilize the instrument for an isothermal measurement is considerable for both heating and cooling experiments. 

Absorption, emission, fluorescence, and diffraction of X-rays are all applied in analytical chemistry. Instruments for these applications contain components that are analogous in function to the five components of instruments for optical spectroscopic measurement these components include a source, a device for restricting the wavelength range of incident radiation, a sample holder, a radiation detector or transducer, and a signal processor and readout. These components differ considerably in detail from the corresponding optical components. Their functions, however, are the same, and the ways they combine to form instruments are often similar to those shown in Figure 7-1. 

Extrapolation of the baselines from the left and right sides into the reaction range yields a step at the phase transition because of the difference in the specific heats of the two phases and also as a result of the fact that heat transfer between the substance and the container (sample holder) may change considerably at the phase transition. Because this brings about a change of the temperature profile in the steady-state situation (see Figure 6.15), there appears a difference in the Tmi function and consequently in AT and A[Pg.111]

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