Thin foils, measurement

The evaluation of the various XRF measurements will be discussed for different effects in EDXRS the spectra evaluation is perfonned by different programs with varying assumptions, partially different mass attenuation coefficients are used, the calibration procedures are principally different (e.g., thin foils with given thickness, or, infinitely thick samples), measurement under atmospheric pressure or in vacuum, secondary excitation (enhancement) mainly of Al by Si radiation. 

In TOP systems, particle energies are usually determined by SBDs in addition to particle velocities being obtained with a TOP set-up which primarily measures the time needed by a particle to pass the distance between two thin foils 0.5-1 m apart [3.170, 3.171]. The first foil delivers a start signal, the second a stop signal. The stop signal can also be obtained from the SBD, but usually foils provide better timing signals. 

Strain Gages. Strain gages are used to measure the strain or elongation caused by the stress on a material. They are usually made of a thin foil grid laid on a plastic support as shown in Figure 4-259. They are the size of a small postal stamp and are glued to the structure to be stressed. 

The vapor pressure of a molten metal can be measured with a device called a Knudsen cell. This is a container closed across the top by a thin foil pierced by a small, measured hole. The cell is heated in a vacuum, until the vapor above the melt streams from the small hole (it effuses). The weight of the material escaping per second tells the rate at which gaseous atoms leave. 

Cliff and Lorimer (1975) used this equation to form the basis for X-ray microanalysis of thin foils, where the constant kAB contains all the factors needed to correct for atomic number differences. kAB varies with operating voltage, but is independent of sample thickness and composition if the two intensities are measured simultaneously. Its value can be determined experimentally with accuracy, using specimens of known composition. The value of kAB can be determined by calculation more rapidly, but with less accuracy. 

Composition Profile Measurement. Results of Zieba et al. (1997) will be given as an example of the measurement of solute distribution in an alloy undergoing a phase transformation. They studied discontinuous precipitation in cobalt-tungsten alloys, in which a Co-32 wt% W alloy was aged in the temperature range 875 K to 1025 K, and high spatial resolution X-ray microanalysis of thin foils by STEM was used to measure the solute distribution near the reaction front. 

Crystal defects, crystal orientations, accurate lattice parameter measurements, local strain, thin foil thickness can be identified from CBED and LACBED [7],... 

LA-ICP-MS is a very suitable analytical method for direct trace element analysis on a small area of thin pure foil, because no sample preparation is required. The results of the determination of noble metals in a thin difficult to dissolve rhodium foil measured by LA-ICP-MS are... 

We, as well as Chesneau and Fouassier, find that the photospeed increases linearly with light intensity. From this observation one can conclude that chain termination reaction is not the usual interaction between two macroradicals. We have measured the initial rate of photopolymerization using thin foil calorimetry and find a linear relationship between the rate of photopolymerization at low conversions (less than 15%) and the absorbed light intensity. Using the same monomer but with a different photoinitiator (to be discussed in detail later) we observe an order of one half with respect to light intensity both by thin foil calorimetry and by measuring the polymer spike. Therefore we conclude that the linear dependence observed for the Eosin-triethanolamine system is real and not an artifact of the technique employed to determine the photospeed. 

Photopolymerization in thin films was carried out at 514 nm, the rate of heat evolution being measured by thin-foil photocalorimetry. The monomer formulation consisted of 85% TMPTA and 15% HDDA, giving a concen-... 

When performing irradiations with neutrons or high-energy protons, it is common to measure the beam intensity using a monitor reaction. A thin foil of a... 

Spatial Variation of Organic Sulfur. The excellent spatial resolution of focused electron beams offers the possibility of examining variation of organic sulfur within macerals. The electron microprobe and scanning electron microscope allow resolution of a few microns (14-16). The transmission electron microscope allows even better resolution (less than 1 pm) because the thin foils and powders produce less electron scattering. We have used this capability to measure the distribution of S in a number of coals. 

JABLONSKIET AL. Measuring Suifoce Tension of Thin Foils... 

Techniques to measure the surface tension of solids are notoriously difficult and known for their inaccuracies. Reliable surface tension data requires not only a reliable measurement technique but careful control over parameters such as sample purity and the gaseous atmosphere in which the experiments are conducted. TTie zero creep technique is considered one of the most accurate and reliable of these techniques since it requires only a simple length measurement(8). Samples can be either wires or thin foils. Hondros(9) has postulated that the use of thin foils increases the sensitivity of the technique and thus allows more accurate measurements. The thinner the foil, the more it approximates a surface. Wire gauges are limited due to the loads required to strain the sample. Table I lists some of the results obtained using the zero creep foil technique. It should be pointed out that the terms surface tension and surface free energy are often used interchangeably, though they are not equivalent(9,10). 

The Zero Creep Technique. The zero creep technique was developed by Udin, Shaler, and Wulff(8) to measure the surface energy of Cu wires. The technique was later extended for use with thin foils by Hondros(16). Very thin foils, approximating a surface, are readily available. When shaped into a cylinder, the sample will tolerate large loads without necking. Since necking does not occur, the stress can be considered constant throughout the experiment. Figure 1 shows a schematic of a foil and the associated stress under an applied load... 

The surface free energy of thin foils can be determined at temperatures between the melting point and approaching the Tammann temperature of the metal using the zero creep/laser interferometer technique. The use of the laser interferometer allows smaller sample strains to be measured with a higher level of confidence. 

Three experimental units each with different experimental focus were designed, fabricated and delivered to NASA Langley. The first unit contains four passive and four active films samples to be exposed to combined atomic oxygen and vacuum UV. The second unit contains 11 passive and four active samples to be exposed only to vacuum UV (see Fig. 2). The third unit has two passive samples with different thicknesses of protective aluminum layers to be exposed to atomic oxygen and vacuum UV. All passive samples will be characterized, both in terms of chemical and physical properties, after the mission and compared with pre-exposure characterization to determine the effects of the space environmental exposure. Thin foil-type thermocouples are attached and logged underneath the samples to measure actual temperatures experienced. By recording the temperature it will be possible to correlate the deflection properties with temperature conditions of the active samples. 

Ham (1961) and Ham and Sharpe (1961) have discussed the two main methods for determining dislocation densities from electron micrographs of thin foils. In the first method, a set of random lines with a total length L is marked on a given area A of the micrograph, and the number of intersections N that dislocations make with these lines is measured. The dislocation density is then given by... 

Reaction-zone lengths cannot conveniently be measured directly. Their dimensions are deduced from their effects on other parameters, such as initial free-surface velocity of very thin foil flyers. Lengths vary from as little as a hundredth of a millimeter for some high-density high explosives and some liquids, up to several centimeters for some blasting agents. Table 21.1 lists some reaction-zone length data. 

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