Specimen thickness

Thick or bulky specimens are commonly used in scanning electron microscopy. However, as shown in Fig. 4.4.2, there are two disadvantages in using such thick specimens (1) a low spatial resolution, and (2) interferences from the effects of backscattering, absorption and fluorescence (Russ 1974). The poor spatial resolution can be remedied by using thin specimens (Fig. 4.4.2). The use of cross-sections of 0.5thickness, for example, would permit analysis of the compound middle lamella in cell walls as a separate entity (Saka and Thomas 1982). The second problem can also be minimized or eliminated by using thin specimens (Russ 1974, Saka and Thomas 1982). 

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Dielectric Strength lEC 243-1. This is a measure of the dielectric breakdown resistance of a material under an applied voltage. The applied voltage just before breakdown is divided by the specimen thickness to give the value in kV/mm. Since, however, the result depends on the thickness this should also be specified. 

Polycarbonates with superior notched impact strength, made by reacting bisphenol A, bis-phenol S and phosgene, were introduced in 1980 (Merlon T). These copolymers have a better impact strength at low temperatures than conventional polycarbonate, with little or no sacrifice in transparency. These co-carbonate polymers are also less notch sensitive and, unlike for the standard bis-phenol A polymer, the notched impact strength is almost independent of specimen thickness. Impact resistance increases with increase in the bis-phenol S component in the polymer feed. Whilst tensile and flexural properties are similar to those of the bis-phenol A polycarbonate, the polyco-carbonates have a slightly lower deflection temperature under load of about 126°C at 1.81 MPa loading. 

It should be noted that for TEM at accelerating voltages of 100-400 keV the specimen thickness must be of the order of 10-100 nm which requires dedicated preparation techniques [2.173, 2.176, 2.178]. 

Figure 2.36 A shows a typical low-loss spectrum taken from boron nitride (BN). The structure of BN is similar to that of graphite, i. e. sp -hybridized carbon. For this reason the low-loss features are quite similar and comprise a distinct plasmon peak at approximately 27 eV attributed to collective excitations of both n and a electrons, whereas the small peak at 7 eV comes from n electrons only. Besides the original spectrum the zero-loss peak and the low-loss part derived by deconvolution are also drawn. By calculating the ratio of the signal intensities hot and Iq a relative specimen thickness t/2 pi of approximately unity was found. Owing to this specimen thickness there is slight indication of a second plasmon.

The phenomena of beam broadening as a function of specimen thickness are illustrated in Fig. 4.20 each figure represents 200 electron trajectories in silicon calculated by Monte Carlo simulations [4.91, 4.95-4.97] for 100-keV primary energy, where an infinitesimally small electron probe is assumed to enter the surface. In massive Si the electrons suffer a large number of elastic and inelastic interactions during their paths through the material, until they are finally completely stopped. The resulting penetration depth of the electrons is approximately 50 pm and in the... 

First, consider uniaxial tension loading in the 1-direction on a flat piece of unidirectionally reinforced lamina where only the gage section is shown in Figure 2-20. The specimen thickness is not just one lamina, but several laminae all of which are at the same orientation (a single lamina would be too fragile to handle). The strains and E2 are measured so, by definition,... 

In each of the following equations a = maximum tensile stress, E = modulus of elasticity and t = specimen thickness. 

One of the most stringent and most widely accepted test is UL 94 that concerns electrical devices. This test, which involves burning a specimen, is the one used for most flame-retardant plastics. In this test the best rating is UL 94 V-0, which identifies a flame with a duration of 0 to 5 s, an afterglow of 0 to 25 s, and the presence of no flaming drips to ignite a sample of dry, absorbent cotton located below the specimen. Tlie ratings go from V-0, V-l, V-2, and V-5 to HB, based on specific specimen thicknesses. 

It should be noted that test information would vary with specimen thickness, temperature, atmospheric conditions, and different speed of straining force. This test is made at 73.4°F (23°C) and 50% relative humidity. For brittle materials (those that will break below a 5% strain) the thickness, span, and width of the specimen and the speed of crosshead movement are varied to bring about a rate of strain of 0.01 in./in./min. The appropriate specimen size are provided in the test specification. 

Specimens are thin sheets or plates having parallel plane surfaces and are of a size sufficient to prevent flashing over. Dielectric strength varies with thickness and, therefore, specimen thickness must be reported. The dielectric strength varies inversely with the thickness of the specimen. The dielectric strength of plastics will drop sharply if holes, bubbles, or contaminants are present in the specimen being tested. 

Variable, independent An experimental factor that can be controlled (temperature, pressure, order of test, etc.) or independently measured (hours of sunshine, specimen thickness, etc.). Independent variables may be qualitative (such as a qualitative difference in operating technique) or quantitative (such as temperature, pressure, or duration). Thus, if variable A is a function of variable B, than B is the independent variable. 

At constant conditions, different fluids will diffuse at different rates into a particular elastomer (with their rates raised proportionally by increasing the exposed area), and each will reach the far elastomer-sample surface proportionally more rapidly with decreasing specimen thickness. Small molecules usually diffuse through an elastomer more readily than larger molecules, so that, as viscosity rises, diffusion rate decreases. One fluid is likely to diffuse at different rates through different elastomers. Permeation rates are generally fast for gases and slow for liquids (and fast for elastomers and slow for thermoplastics and thermosets). 

Interpretation of the images is still not straightforward even when there seems to be a simple one-to-one correspondence between black (or white) dots in the image and atom positions. Especially when quantitative data on interatomic distances is to be derived, detailed calculations based on many-beam dynamical theory ( ) must be applied to derive calculated images for comparison with experiment. For this purpose the experimental parameters describing the imaging conditions and the specimen thickness and orientation must be known with high accuracy. 

If the sampling volume is now treated as a truncated cone, and if the specimen thickness is t nm, then the lateral beam spread B (in nm) for thin specimens is given by ... 

The thicker the specimen, the longer it takes for penetration to be complete. A 1-mm thick piece of tissue will take a certain length of time to become penetrated. A 2-mm piece of the same tissue will take four times as long (double squared), and a 3-mm piece will take nine times as long (triple squared). For very thick gross specimens, complete penetration may not occur in any reasonable period of time. If you want very rapid fixation and processing, specimen thickness must be kept as thin as possible. 

Local thickness variations in a thin specimen complicate the quantitative analysis of a single element in the absence of precise knowledge of specimen thickness and without the ability to compare the measured x-ray intensities with those of thin standards. To avoid this difficulty, the x-ray intensity for the element of interest can be divided either by the intensity of a region of background between peaks as in the Hall method[8], or by the intensity from another element as in the Cliff-Lorimer method[9]. The former is largely used for biological analysis while the latter has become the standard thin specimen microanalysis method for materials science applications. The Cliff-Lorimer method is expressed in the following equation ... 

A calibration (reference) standard shall be prepared from a representative sample. Longitudinal (axial) reference notches shall be introduced on the outer and inner surfaces of the standard in accordance with Fig. 2(c) of ASTM E 213 to a depth not greater than the larger of 0.1 mm (0.004 in.) or 4% of specimen thickness and a length not more than 10 times the notch depth. 

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