Mass contrast


The im< e mode produces an image of the illuminated sample area, as in Figure 2. The imj e can contain contrast brought about by several mechanisms mass contrast, due to spatial separations between distinct atomic constituents thickness contrast, due to nonuniformity in sample thickness diffraction contrast, which in the case of crystalline materials results from scattering of the incident electron wave by structural defects and phase contrast (see discussion later in this article). Alternating between imj e and diffraction mode on a TEM involves nothing more than the flick of a switch. The reasons for this simplicity are buried in the intricate electron optics technology that makes the practice of TEM possible.  [c.105]

By contrast, in the system propionic acid d) - methyl isobutyl ketone (2), (fi and are very much different when y 1, Propionic acid has a strong tendency to dimerize with itself and only a weak tendency to dimerize with ketone also,the ketone has only a weak tendency to dimerize with itself. At acid-rich compositions, therefore, many acid molecules have dimerized but most ketone molecules are monomers. Acid-acid dimerization lowers the fugacity of acid and thus is well below unity. Because of acid-acid dimerization, the true mole fraction of ketone is signi-  [c.35]

The contractor is paid per foot drilled. Whilst this will provide an incentive to make hole quickly, the same risks are involved as in the turnkey contract. Footage contracts are often used for the section above the prospective reservoir where hole conditions are less crucial from an evaluation or production point of view.  [c.62]

As the name implies the company basically rents the rig and crew on a per day basis. Usually the oil company also manages the drilling operation and has full control over the drilling process. This type of contract actually encourages the contractor to spend as much time as acceptable on location . With increased cost consciousness, day rate contracts have become less favoured by most oil companies.  [c.62]

In contrast to an oil production profile, which typically has a plateau period of 2-5 years, a gas field production profile will typically have a much longer plateau period, producing around 2/3 of the reserves on plateau production in order to satisfy the needs of the distribution company to forecast their supplies. The Figure 8.9 compares typical oil and gas field production profiles.  [c.194]

To protect both parties in a contract arrangement it is good practice to make a contract in which the scope of work, completion time and method of reimbursement are agreed. Contracts are normally awarded though a competitive tendering process or after negotiation if there is only one suitable contractor.  [c.301]

The economic analysis of investment opportunities requires the gathering of much information, such as capital costs, operating costs, anticipated revenues, contract terms, fiscal (tax) structures, forecast oil/gas prices, the timing of the project, and the expectations of the stakeholders in the investment. These data must be collected from a number of different departments and bodies (e.g. petroleum engineering, engineering, taxation and legal, host government) and each data set carries with it a range of uncertainty. Data gathering and establishing realistic ranges of uncertainty can be very time consuming.  [c.304]

Market forces determine the demand for a product, and the demand will be used to forecast the sales of hydrocarbons. This will be one of the factors considered by some governments when setting the production targets for the oil company. For example, much of the gas produced in the South China Sea is liquefied and exported by tanker to Japan for industrial and domestic use the contract agreed with the Japanese purchaser will drive the production levels set by the National Oil Company.  [c.346]

There are two different methods to produce hard surfaces by laser beam dispersing At the single-staged method the powdery particles are transported simultaneously into the melting bath produced by a laser. The double-staged method takes two steps. First the coating powder is placed on the surface (for example by silk screen printing with binding materials). Then it is fused in the second interval. A homogeneous distribution of the particles in the solidification zone is necessary for optimal wear characteristics of surfaces. The particles should not decompose themselves or dissolve [3]. The knowledge of the interaction between laser beam, substrate and the particles during the dispersing is essential for the control of the process. The previous findings are based on simulations [4] and analysis [5] of dispersed samples after the treatment. In-situ examinations of the transport of hard particles on laser beam dispersing are unknown. This paper shows the results of transportation of tungsten carbide particles in the melting bath of a titanium based substrate during laser dispersing. The process was observed by use of a high speed radiographic system. In contrast to conventional methods of observing the process, it is possible now to make visible the movement of the particles and to make statements on the processes inside the melting bath. These processes detennine the distribution of the particles in the solidified structure and so the characteristic of wear.  [c.543]

It should be noted that these results are only preliminary and have to be considered as a proof of concept. As is clear from eq. (2) the phase contrast can be improved drastically by improving the global resolution and sensitivity of the instrument. Currently, a high resolution desktop system is under construction [5] in which the resolution is much better than that of the instrument used in this work, and in which the phase contrast is expected to be stronger by one order of magnitude.  [c.577]

In the case of Langmuir monolayers, film thickness and index of refraction have not been given much attention. While several groups have measured A versus a, [143-145], calculations by Knoll and co-workers [146] call into question the ability of ellipsometry to unambiguously determine thickness and refractive index of a Langmuir monolayer. A small error in the chosen index of refraction produces a large error in thickness. A new microscopic imaging technique described in section IV-3E uses ellipsometric contrast but does not require absolute determination of thickness and refractive index. Ellipsometry is routinely used to successfully characterize thin films on solid supports as described in Sections X-7, XI-2, and XV-7.  [c.126]

Many more complex polymers are receiving attention because of their technological importance. The adsorption of polyelectrolytes combines the interesting features of polymer configurational statistics with the added complexity of electrostatics [87-90], requiring characterization of such features as the charge on the polymer and the surface [91] and the effect of counterions on adsorption [92,93]. The adsorption of diblock copolymers is particularly interesting for colloidal stabilization as they produce polymers anchored to the surface by one end. These tethered chains have received much theoretical and experimental attention in recent years [94-99]. Many of the experiments have focused on the forces between surfaces bearing these polymer layers [91,93,100] using the surface forces apparatus described in Chapter VI. In contrast to adsorbed homopolymers, diblock copolymers show an adsorbed layer thickness increasing linearly with the molecular weight of the soluble block, making them attractive for colloidal stabilization [98,100].  [c.403]

The interest in vesicles as models for cell biomembranes has led to much work on the interactions within and between lipid layers. The primary contributions to vesicle stability and curvature include those familiar to us already, the electrostatic interactions between charged head groups (Chapter V) and the van der Waals interaction between layers (Chapter VI). An additional force due to thermal fluctuations in membranes produces a steric repulsion between membranes known as the Helfrich or undulation interaction. This force has been quantified by Sackmann and co-workers using reflection interference contrast microscopy to monitor vesicles weakly adhering to a solid substrate [78]. Membrane fluctuation forces may influence the interactions between proteins embedded in them [79]. Finally, in balance with these forces, bending elasticity helps determine shape transitions [80], interactions between inclusions [81], aggregation of membrane junctions [82], and unbinding of pinched membranes [83]. Specific interactions between membrane embedded receptors add an additional complication to biomembrane behavior. These have been stud-  [c.549]

Another specialized application of EM image contrast is mass measurement. Using the elastic dark-field image in the STEM or the inelastic image in the EETEM, a direct measurement of the scattering mass can be performed. Eor reviews on this teclmique see [60.61].  [c.1645]

Metal clusters are bonded by strong covalent or metallic bonds. Clusters of the low melting point metallic elements are produced using the thennal evaporation teclmique. With the laser vaporization teclmique, metal clusters from all the metallic elements in the periodic table can be made. Simple metal clusters include those main group elements whose cluster properties are dominated by the delocalized nature of their valence electrons. In contrast to the simple metal clusters, transition-metal clusters are extremely complicated. Because of the unfilled d orbitals, transition-metal clusters possess a high density of electronic states. Transition-metal clusters possess both metallic and covalent bonding characters and exhibit interesting chemical, magnetic and electronic properties. Studies of transition-metal clusters are directly relevant to heterogeneous catalysis, surface science, metal cluster chemistry and metal-metal bonding in inorganic chemistry [33]. Although accurate theoretical descriptions of transition-metal clusters still pose a tremendous challenge, improved experimental and theoretical teclmiques are expected to make significant progress in the investigation of transition-metal clusters.  [c.2391]

In neutron reflectivity, neutrons strike the surface of a specimen at small angles and the percentage of neutrons reflected at the corresponding angle are measured. The an jular dependence of the reflectivity is related to the variation in concentration of a labeled component as a function of distance from the surface. Typically the component of interest is labeled with deuterium to provide mass contrast against hydrogen. Use of polarized neutrons permits the determination of the variation in the magnetic moment as a function of depth. In all cases the optical transform of the concentration profiles is obtained experimentally.  [c.50]

In multiphase, amorphous or glassy materials, regions containing a phase of high aver< e Z will scatter electrons more efficiendy and to higher angles than regions containing a low average Z The objective aperture in bright-field blocks this scattered intensity, making the high-Z material appear darker (less transmitted intensity) than the low-Z material. This is mass contrast, due primarily to incoherent elastic scattering. The scattering is largely incoherent because spadal relationships between scattering centers in these materials are not periodic. A priori there are no well-defined phase relationships between electrons scattered by such materials. Under these circumstances, the transmitted intensity distribution is determined from the principle of the additivity of individual scattered intensities, without consideration for the individual scattered ampUtudes.  [c.110]

In Neutron Reflectivity the neutron beam strikes the sample at grazing incidence. Below the critical angle (around 0.1°), total reflection occurs. Above it, reflection in the specular direction decreases rapidly with increasing angle in a manner depending on the neutron scattering cross sections of the elements present and their concentrations. On reaching a lower interface the transmitted part of the beam will undergo a similar process. H and D have one of the largest mass contrasts in neutron-scattering cross section. Thus, if there is an interface between a H-containing and a D-containing hydrocarbon, the reflection-versus-angle curve will depend strongly on the interface sharpness. Thus interdiffusion across hydrocarbon material interfaces can be studied by D labeling. For polymer interfaces the depth resolution obtained this way can be as good as 10 A at buried interface depths of 100 nm, whereas the alternative techniques available for distinguishing D from H at interfaces, SIMS (Chapter 10) and ERS (Chapter 9), have much worse resolution. Also, neutron reflection is performed under ambient pressures, whereas SIMS and ERS require vacuum conditions. Labeling is not necessarj if there is sufficient neutron mass contract already available—e.g., interfaces between fluorinated hydrocarbons and hydrocarbons. The technique has also been used for biological films and, magnetic thin films, using polarized neutron beam sources, where the magnetic gradient at an interface can be determined.  [c.646]

The action of small quantities of electrolytes in coagulating hydrophobic (i.e. unhydrated) colloids is important. The colloid is coagulated by the adsorption of ions of charge opposite to that which it carries. Thus an arsenic sulphide sol (negative) can be coagulated by unipositive cations, but dipositive cations are more efficient and tripositive cations much more so, the latter producing coagulation even at great dilutions. The addition of a large excess of the coagulant may stabilize a sol of opposite charge to that of the initial colloid, by adsorption of a large excess of (in the above example, positive) ions. Other coagulating influences include ultrasonic vibrations, u.v. light and boiling (as in denaturization of proteins) the two latter effects are essentially chemical. Hydrated or hydrophilic colloids are also coagulated by electrolytes, but only in high concentrations. This effect is termed salting out in contrast to the simple adsorption effect outlined above. See colloid, electrophoresis, peptization, salting out .  [c.103]

Currently a substantial part of NR applications in NDT are still bound to stationary strong neutron sources, which enable the high resolution and high contrast radiographic film based neutron imaging, required by the nature of NDT problems in several fields of modern industry and high technology development, e g. inspection of nuclear fuel, applications in space and aircraft industry. Here the replacement of inherently slow high quality film based neutron imaging with much faster and in image quality comparable IP based imaging could offer a significant reduction in time and cost. Second, the wide spread use of NR as in-the-field NDT tool in the industry is limited by relatively weak neutron intensity of existing mobile or portable neutron sources based on particle accelarators or radioisotope sources. To increase the utilization of NR as in-the-field NDT method the current activities go both in the direction of development of new high intensity mobile neutron generators as well as in the development and introduction of new more sensitive neutron imaging techniques. The introduction of IP into NR with weak mobile neutron sources clearly offers new opportunities for NR in the industry.  [c.507]

This focal spot diameter is very much smaller than the spots of conventional X-ray tubes. The goodness of a X-ray image is influenced by contrast and sharpness. Caused by the almost punctual point of origin of the X-rays, a low geometrical unsharpness according to equation 1 is reached.  [c.544]

A much better way would be to use phase contrast, rather than attenuation contrast, since the phase change, due to changes in index of refraction, can be up to 1000 times larger than the change in amplitude. However, phase contrast techniques require the disposal of monochromatic X-ray sources, such as synchrotrons, combined with special optics, such as double crystal monochromatics and interferometers [2]. Recently [3] it has been shown that one can also obtain phase contrast by using a polychromatic X-ray source provided the source size and detector resolution are small enough to maintain sufficient spatial coherence.  [c.573]

The quantitative description of visibility is based on the contrast threshhold dC which is the smallest contrast necessary to recognize an object. The contrast threshold depends on the viewing conditions and the optical abilities of the object. Quantitative correlations are based on experimental investigations which are recommended by the Commission International d Eclairage CIE [6]. The results are presented as dependant on the adaptation luminance and the object dimension as parameter [3]. The object dimension is described by the viewing angle (in angle minutes ) of a circle disc. As usual [5], the presentation time was set on 0.2 s. and the detection probability on 50 %. These quantitative correlations describe the fact, that visibility increases with increasing adaptation luminance if the contrast remains constant. The contrast threshold was determined by test persons under optimal conditions which included viewing and environmental conditions as well as the visual acuity. In any real visual inspection (e.g. detection of small objects) the contrast between the object and the surroundings should be much higher than the contrast threshold. This factor is defined as Visibility Level VL = C/dC. The factor is a quantitative measure for the visibility of objects Recommended values are dependant on the inspection task and are in the range VL = 3 - 60.  [c.670]

It has been found that the contrast in film density caused by very small local variations in mass density of the concrete is considerable, e g. S D = 0.12 for a 6 mm diameter hole in a 250 mm thick concrete beam. The image quality provided by fine-grained films (Agfa Gaevert D7) was sufficient to distinguish the thin walls of a pre-stressing duct in a 750 mm thick concrete bridge slab.  [c.1002]

Conventional optical microscopy can resolve features down to about the wavelength of visible light or about 500 nm. Surface faceting and dislocations may be seen as in Fig. VII-10. Confocal microscopy adds the ability to make three-dimensional scans in solutions, biological membranes, and polymeric systems. [11]. A new approach, p/ioton tunneling microscopy (PTM), improves resolution and contrast down to 1 nm vertically and one quarter of the wavelength of light in the lateral directions [12]. The striking improvement in resolution of surface features is due to the unique properties of the evanescent wave created when light tunnels from a medium of higher index of refraction into one of  [c.293]

The effect is more than just a matter of pH. As shown in Fig. XV-14, phospholipid monolayers can be expanded at low pH values by the presence of phosphotungstate ions [123], which disrupt the stmctival order in the lipid film [124]. Uranyl ions, by contrast, contract the low-pH expanded phase presumably because of a type of counterion condensation [123]. These effects caution against using these ions as stains in electron microscopy. Clearly the nature of the counterion is very important. It is dramatically so with fatty acids that form an insoluble salt with the ion here quite low concentrations (10 M) of divalent ions lead to the formation of the metal salt unless the pH is quite low. Such films are much more condensed than the fatty-acid monolayers themselves [125-127].  [c.557]

Fig. XVin-3. AFM image of DNA strands on mica. Lower figure image obtained in the contact mode under water. The contrast shown covers height variations in the range of 0-2 nm. Upper figure observed profile along the line A-A of the lower figure. (From S. N. Magnov and M.-H. Whangbo, Surface Analysis with STM and AFM, VCH, New Yoric, 1996.) Fig. XVin-3. AFM image of DNA strands on mica. Lower figure image obtained in the contact mode under water. The contrast shown covers height variations in the range of 0-2 nm. Upper figure observed profile along the line A-A of the lower figure. (From S. N. Magnov and M.-H. Whangbo, Surface Analysis with STM and AFM, VCH, New Yoric, 1996.)
Photodissociation of molecular ions occurs when a photon is absorbed by the ion and the energy is released (at least partly) by the breaking of one of the molecular bonds. The photodissociation of a molecular ion is conceptually similar to that for neutral molecules, but the experimental techniques differ. Photodissociation events are divided into two categories direct dissociation, in which the photoexcitation is from a bound state to a repulsive state and predissociation, in which a quasi-bound state is accessed in the excitation. Direct dissociation takes place rapidly (fs to ps timescale). The shape of the direct dissociation cross section curve against photon energy is governed by the (Franck-Condon) overlap of wavefiinctions of the initial state (usually the ground state) and the final, repulsive state. It will nonnally consist of peaks corresponding to vibrational structure in the initial level of the target ion with shapes skewed by the overlap with the repulsive state. One can model these shapes to obtain the potential curves of both the initial and repulsive states. Predissociation, in contrast, may take place over a much longer timescale the lifetune of a particular predissociating state may be detennined from the width of tire resonance observed. Measurements of the lifetime for a series of predissociating states gives a picture of the predissociation mechanism.  [c.800]

In principle the study of ion-molecule kinetics and dynamics is no different from studies of the same processes in neutral species however, there are additional forces that govern reactivity, often leading to behaviour that is fiindamentally different from neutral processes. An important factor in detenuining ion-molecule rate constants and cross sections is the rate at which the reactants collide, i.e. the collision rate. In contrast to neutral kinetics, the collision rate at low energies or temperatures is detenuined not by the size of the molecule but by electrical forces. The ion-molecule collision rate is detenuined by the classical capture cross section for a point charge interacting with a stmctureless multipole. This was first described analytically for a point charge interacting with a polarizable species with no other multipole. In this case, the collisional value of the rate constant is independent of temperature [Ml- The only other force of any significance is from the ion-penuanent dipole interaction. Other forces, such as those between the ion and tire quadmpole moment of the neutral, and between the neutral dipole and the induced dipole of the ion, have been shown to be of minor importance [, 59, 60, 61, 62 and 63]. If the physical size of tlie reactants is greater than the capture radius, e.g. at translational energies of several tenths of an electron volt and greater, more conventional notions apply. Except for species with very small polarizabilities and systems of large mass, ion-molecule collision rates are above 10 molecules cm s, or about a factor of ten larger than neutral collision rates.  [c.806]

NMR is an important teclnhque for the study of flow and diflfiision, since the measurement may be made highly sensitive to motion without in any way influencing the motion under study. In analogy to many non-NMR-methods, mass transport can be visualized by imaging the distribution of magnetic tracers as a fiinction of time. Tracers may include paramagnetic contrast agents which, in particular, reduce the transverse  [c.1534]

By far the greatest advantage of pulsed EPR [31, 32] lies in its ability to manipulate the spin system nearly at will and, thus, to measure properties that are not readily available from the CW EPR spectra. Nevertheless, EPR has long remained a domain of CW methods. In contrast to the rapid development of pulsed NMR spectroscopy, the utilization of the time domain in EPR took a much longer time, even though the underlying principles are essentially the same. There are several reasons for this slow development of pulsed EPR. (1) The large energies involved in electron spin interactions (see Figure BL15.3) can give rise to spectral widths of the order of 10-25% of the carrier frequency  [c.1572]

The discussion of electron-specimen interactions shows that, for a given incident electron dose, a certain quantity of resulting scattered electrons and secondary electrons or photons is produced. The majority of energy transfer into the specimen leads to beam damage and, finally, to the destruction of the sample structure. Therefore it is desirable to simultaneously collect as much infonnation from the interactions as possible. This concept could lead to an EM mstmment based on the design of a STEM but including many different detectors for the elastic dark field, phase contrast, inelastically scattered electrons, BSE, SE, and EDX. The complexity of such an instmment would be enonnous. Instead, specific instmments developed in the past can coarsely be categorized as TEM for stnictural studies on thin samples, STEM for analytical work on thin samples and SEM for analytical and surface topography studies.  [c.1630]

Thundat T, Warmack R J, Allison D P, Bottomley L A, Lourenco A J and Ferrell T L 1992 Atomic force microscopy of deoxyribonucleic acid strands adsorbed on mica the effect of humidity on apparent width and image contrast J. Vac. Sol. Technol. A 10 630  [c.1727]

It can be seen from figure B2.3.2 that scattering angles (relative to the direction of one of the reagent beams) are different in the laboratory and CM coordinate systems. If the detected product is very heavy compared with its partner, or if its translational energy is very small, then its speed will be small compared with the speed of the centre of mass of the system. In this case, the product is scattered into a small range of scattering angles about c, and detennination of the CM angrilar distribution will be difficult. Moreover, the scattered intensity at one laboratory scattering angle can come from two CM scattering angles, as can be seen in figure B2.3.2. From the intensity estimates presented in section B2.3.2.1. this concentration of the scattered product into a small laboratory angrilar range will facilitate detection of the product molecules. By contrast, if the product CM speed is large, then the product can be scattered into all laboratory angles.  [c.2064]

CFIDF end group, no selective reaction would occur on time scales above 10 s. Figure B2.5.18. In contrast to IVR processes, which can be very fast, the miennolecular energy transfer processes, which may reduce intennolecular selectivity, are generally much slower, since they proceed via bimolecular energy exchange, which is limited by the collision frequency (see chapter A3.13).  [c.2137]

In the MPPT/MBPT method, once the reference CSF is chosen and the SCF orbitals belonging to this CSF are detennined, the wavefiinction T and energy E are detennined in an order-by-order maimer. The perturbation equations determine what CSFs to include and their particular order. This is one of the primary strengdis of this technique it does not require one to make fiirtlier choices, in contrast to the MCSCF and Cl treatments where one needs to choose which CSFs to include.  [c.2177]


See pages that mention the term Mass contrast : [c.162]    [c.978]    [c.560]    [c.779]    [c.848]    [c.1280]    [c.1312]    [c.1368]    [c.1520]    [c.1740]    [c.1828]    [c.1925]    [c.1941]    [c.2062]   
Encyclopedia of materials characterization (1992) -- [ c.110 ]