Thin film criteria

The primary consideration for all AEM analysis is that the specimen be thin (generally carbon coated electron microscope grid either dry or in a suitable liquid. If a liquid suspension is used in preparing the specimen, it is important that all elements of interest are insoluble in that liquid. Only particles thin enough to meet AEM thin-film criteria (15) should be analyzed quantitatively. Scraping surface particles from a catalyst pellet for specimen preparation may be more useful than grinding the entire pellet. 

The major disadvantages in application of the AEM method relative to AAS is the time required for specimen preparation and analysis. Care must be taken that the particles analyzed are characteristic of the bulk material, that they are thin enough to meet the "thin-film criteria" of Equations (2) and (3), and that enough particles are analyzed to reduce the random error to acceptable levels. The AEM analyses shown in Figure 6 took several days to collect and involved considerble operator attention. 

A complementary way of considering these attenuation phenomena is to identify the limiting thickness of a sample below which absorption/enhancement effects are essentially negligible and so relevant matrix corrections may be ignored. Analysis of such thin films may then be undertaken without applying any matrix corrections. By convention, a 1% attenuation is normally taken as the limit for thin film criteria, and some representative values are also listed in Table 3. Satisfying thin film criteria is particularly important in applications such as direct analysis of dust particles collected on a filter membrane, which then serves directly as the sample substrate for XRF spectrometry. 

Table 3 Critical penetration depth and thin film criteria for selected lines in silicate and steel matrices...

For an electron-transparent specimen the absorption and fluorescence correction parts can often be neglected, this is the so-called thin-film criterion introduced by Cliff and Lorimer [4.118]. Thus, for a thin specimen containing two elements A and B yielding the net X-ray intensities I a and 1b, the concentration ratio reduces to ... 

For a given detector and a given pair of elements the last two factors give a single constant (Icab) that can be treated as a relative sensitivity factor. Both that factor and the method obtained their names after the two people who introduced them, Cliff and Lorimer (1975). The simplicity originates from the fact that the Uab factor does not depend either on the rest of elements also present in the sample or on the other parameters of the sample (thickness, density), as far as the thin film criterion is fulfilled. The Cliff-Lorimer factors can either be calculated using the known parameters of the detector or can be measured if a well-characterized thin film sample (standard) is available. In the first case the method is standardless. In the second case the known weight fractions and the measured intensity ratio provides the Cliff-Lorimer factor for the pair of elements. 

It can be seen that calculation of the absorption correction also needs the values of the sample density and the linear value of the absorption path length beside the mass absorption coefficients. It is also important to note that it is the difference of the absorption coefficients, what coimts in determining if the thin film criterion is fulfilled for a given sample (geometry). It frequently happens that the sample is thin for one pair of its components and thick if another pair of elements is considered from its components. 

Screen-Printable Polyimide. Bending beam results for a screen-printable polyimide used for chip protection are presented in Figure 8. The EPO-TEK 600 polyimide paste was applied to a quartz beam, then the beam was spun at 5000 rpm to achieve high uniformity. Results are presented as inverse radius of curvature, 1/R, because the 50 m coating thickness on the 84 im substrate violates the thin film criterion of Equation 2. We have no knowledge of the polyimide s viscoelastic modulus, which is needed in order to convert 1/R into interfacial stress using Equation 1. 

Electroluminescent materials are used in the form of powders or thin films Owing to their high brightness and stability, ac thin film devices are the most appropriate electroluminescent systems for display. The main criterion for an electroluminescent material is its ability to have its electrons accelerated loss-freely in electric... 

Obviously, NiO(l 11) is terminated and stabilised by p(2x2) reconstructions with different internal configurations. Importantly the electrostatic criterion is always fulfilled showing that it is very important when polar surfaces are considered. Interestingly, the octopolar reconstruction prediction and the observation of the p(2x2) surface cell were far not enough to understand the polar NiO(lll) surface. Note also that oxide thin films may not exhibit the same surface structure as their bulk counterpart. The studies carried out on thin oxide films may thus only be carefully extrapolated to the bulk surface. 

Current-voltage (I-V) characteristics were also measured at different temperatures in magnetic fields applied perpendicularly to the surface of the substrate, i.e. parallel to the c-axis of the film. The critical current densities at T=2.1 K and in zero field are above 10 A/m. They have been defined by the electric field criterion E=Ec=10 V/cm. As an example, I-V characteristics for different values of the external magnetic field at T = 4.2 K are reported in Fig. 2. The values of the measured critical current density are slightly lower with respect to some others reported in the literature which, however, refer to epitaxial thin films in which Ce doping level was equal to its optimum value x = 0.15 [9]. We believe this is related to the reduced value of the critical temperature T in this system with respect to the optimally Ce-doped samples. 

In trying to answer these questions it is certainly instructive to examine previous fundamental experimental works in which such thin films have been employed. In particular, we have to mention studies on polymer interdiffusion in which thin polymer films were used for testing theoretical concepts of polymer physics [102-106]. Spin-coating was typically used to prepare the films because this process presents an easy way of obtaining smooth film of precisely controllable thickness, even in the nanometer range. This is an essential criterion for investigating, for example, polymer diffusion across an interface between two films. While these experiments successfully supported the model of chain reptation [102-106], they also indicated some deviations that hinted, for example, at the enrichment of chain ends at surfaces [104, 106]. 

To act as UV stabilizers, UV absorbers must be present at a finite concentration in order to fully absorb harmful UV light. At practically used concentrations, this means that a finite absorbing thickness is required. In the case of thin films [poly(ethylene) for agricultural use] or fibers (polypropylene fibers) this thickness criterion is not met and UV absorbers offer inadequate UV light protection. 

Direct transport measurements are generally employed on thin films, tapes and coated conductors. They quickly reach their natural limits as the currents go up and/or the cross-section of the conductor increases. They are fast, i.e., can be started immediately after the magnetic field is set, and trace the I-V characteristics, which is of course most valuable if flux creep or thermally assisted flux flow (Kes et al. 1989) prevails, because the electric field is directly determined. On the other hand, must be defined by a criterion e.g., 1 [xV/cm. It will be affected by the presence of stabilising materials, the silver tube or conducting substrates, and thus generally refers to the overall Jc, wfrich can be converted to the critical current density of the superconductor if the shape and the volume fraction of the superconducting material are known. 

Let us notice that a criterion has been often used to distinguish between interfacial and aqueous bulk reactions. It is based [27] on the dependence of extraction rates upon the specific area of the system, defined by eq 3.19 for an aqueous bulk reaction the extraction rate should be independent of this parameter, because the rate is governed by homogeneous chemical reactions if the transport of the extractant to the aqueous phase, and of the chelated species back to the organic, are not rate determining on the other hand, for an interfacial reaction the extraction rate should vary in proportion to a as shown by eq 3.18. However, for reactions that would occur in a thin film close to the interface (MTWCR hypothesis) application of this criterion would wrongly point to a true interfacial reaction. 

In many particular cases, there is a direct correspondence between the Irwin and Griffith criteria, as was noted above an connection with the result in (4.27). However, the latter criterion has the distinct advantage that the energy release rate can often be determined, or at least estimated, without the need for a complete solution of the boundary value problem for the stress field in the body. For this reason, it is selected as the basis for the present discussion. Many of its special features and numerous extensions of the basic concept will become evident in the sections that follow, in the course of discussing various issues concerned with delamination and fracture in thin film configurations. 

This criterion can be interpreted in the following way. For thin film... 

---