Vacuum conditions


If the adsorption energy is large enough to be comparable to chemical bond energies, we now speak of chemisorption. The adsorbate tends to be localized at particular sites (although some surface diffusion or mobility may still be present), and the equilibrium gas pressure may be so low that the adsorbent-adsorbate system can be studied under high-vacuum conditions. This allows the many diffraction and spectroscopic techniques described in Chapter VIII to be used to determine what actual species are present on the surface and their packing and chemical state.) This is also tme for physisorption systems if the surface is well defined and the temperature low enough that the equilibrium pressure is very low, see Fig. XVII-17 for example.  [c.571]

The course of a surface reaction can in principle be followed directly with the use of various surface spectroscopic techniques plus equipment allowing the rapid transfer of the surface from reaction to high-vacuum conditions see Campbell [232]. More often, however, the experimental observables are the changes with time of the concentrations of reactants and products in the gas phase. The rate law in terms of surface concentrations might be called the true rate law and the one analogous to that for a homogeneous system. What is observed, however, is an apparent rate law giving the dependence of the rate on the various gas pressures. The true and the apparent rate laws can be related if one assumes that adsorption equilibrium is rapid compared to the surface reaction.  [c.724]

Surface science techniques, including the ability to transfer a system rapidly from reaction to high-vacuum conditions, have established that Fe(III) surfaces are by far the most reactive [262,263]. The contemporary wisdom is that the slow step is the dissociative chemisorption of N2 on a Fe site having C7 coordination (seven nearest neighbors) (see Refs. 13, 264). The general mechanism is of the Langmuir-Hinshelwood type. Hydrogen also adsorbs dissociatively, and the surface reactions N(ads) + H(ads) = NH(ads), NH(ads) + H(ads) = NH2(ads), and NH2(ads) + H(ads) = NH3(ads) then occur in sequence followed by desorption of product NH3. The energetic scheme is shown in Fig. XVIII-23 [256].  [c.729]

For example, energy transfer in molecule-surface collisions is best studied in nom-eactive systems, such as the scattering and trapping of rare-gas atoms or simple molecules at metal surfaces. We follow a similar approach below, discussing the dynamics of the different elementary processes separately. The surface must also be simplified compared to technologically relevant systems. To develop a detailed understanding, we must know exactly what the surface looks like and of what it is composed. This requires the use of surface science tools (section B 1.19-26) to prepare very well-characterized, atomically clean and ordered substrates on which reactions can be studied under ultrahigh vacuum conditions. The most accurate and specific experiments also employ molecular beam teclmiques, discussed in section B2.3.  [c.899]

Electron microscopy (see section B1.18) is very valuable in characterizing particles (see, for instance, figure C2.6.1). The suspension stmcture is, of course, not represented well because of tire vacuum conditions in tire microscope. This can be overcome using environmental SEM [241.  [c.2671]

Until recently, lasers were not much used in mass spectrometry. It has been known for many years that light can ionize substances if the light energy (wavelength) is sufficient. Generally, this wavelength is in the far-ultraviolet end of the UV/visible spectrum, in which region all substances absorb the radiation. The mass analyzers in mass spectrometers operate under vacuum, so any light sources to be used for ionization must also operate under similar vacuum conditions if the irradiation is to reach and ionize the sample molecules to be analyzed. For this reason, ion sources based on ionization of sample molecules by light were mostly research curiosities. When lasers first became commercially available, the useful laser sources emitted mostly visible light of an energy that is insufficient for ionization. As the laser beams became more intense (more photons per unit area per unit time), multiphoton absorption events could be achieved easily. Whereas one photon can be absorbed by a molecule but with insufficient energy for ionization, two photons absorbed within a short space of time can cause ionization (Figure 18.17).  [c.134]

The poor yield of molecular ions during El is partly due to the inherent instability of. some cation radicals formed upon ionization. Since Cl gives quasi-molecular ions that have been largely thermally equilibrated, there is little fragmentation. However, both Cl and El require the sample to be in the gas phase, which means that thermal instability of the sample can be a serious problem. Thus a substance such as toluene can be easily vaporized under the vacuum conditions inside a mass spectrometer, but sugars, proteins, or oligonucleotides are nonvolatile and degrade extensively when heated to modest temperatures. Obtaining an El or Cl spectrum of toluene is easy, but polar, high-molecular-mass compounds will give only spectra characteristic of breakdown products. (There are some instruments that make use of this thermal effect to investigate complex mixtures, as in pyrolysis/MS or even pyrolysis/GC/MS.)  [c.283]

Under high-vacuum conditions, an ion can travel quite long distances before it meets a neutral molecule. A resulting collision causes both ion and neutral to be deflected from their original paths. However, unlike the higher pressure conditions, because collisions are so infrequent, an ion can be deflected right away from its initial velocity direction. In the absence of any forces to bring it back onto its original trajectory, the ion will eventually strike the walls of the mass spectrometer and be lost entirely (Figure 49.4c).  [c.376]

The principal concern in designing a tubeside reflux condenser is that an excessive velocity of the entering gas will prevent downflow of the condensed solvent. This is usually not a problem in atmospheric condensation. However, because of the higher gas velocities at low pressures, it may cause flooding under vacuum conditions unless an adequate number of tubes is provided to lower the velocity. This can produce a heat-exchanger configuration that uses many more tubes (albeit shorter ones) than might ordinarily be required were the design based strictly on heat-transfer considerations. A horizontal shell side reflux condenser is less prone to flooding, but is usually not as effective in lowering the exiting gas temperature. A demonstrated procedure for designing in-tube reflux condensers that will not flood was developed by Holmes and has been pubUshed in Perm s Chemical Engineers Handbook (1).  [c.254]

Instmmentation for tern is somewhat similar to that for sem however, because of the need to keep the sample surface as clean as possible throughout the analysis to avoid imaging surface contamination as opposed to the sample surface itself, ultrahigh vacuum conditions (ca 10 -10 Pa) are needed in the sample area of the microscope. Electron sources in tern are similar to those used in sem, although primary electron beam energies needed for effective tern are higher, typically on the order of ca 100 keV.  [c.272]

Paschen s Rule and Breakdown Voltage. As pressure decreases to vacuum conditions, the breakdown voltage (BDV) first decreases, then increases, resulting in a minimum as shown in Figure 1. Table 3 gives BDV data for SF and other dielectrics. For optimum utiUty of a dielectric, a  [c.241]

Distillation equipment for soap—lye and esterification cmde requires salt-resistant metallurgy. The soHd salt which results when glycerol is vaporized is removed by filtration or as bottoms from a wiped film evaporator. The Luwa scraped wall evaporator is capable of vaporizing glycerol very rapidly and almost completely, such that a dry, powdery residue is discharged from the base of the unit (8). Distillation of glycerol under atmospheric pressure is not practicable since it polymerizes and decomposes glycerol to some extent at the normal boiling point of 204°C. A combination of vacuum and steam distillation is used in which the vapors are passed from the stiH through a series of condensers or a packed fractionation section in the upper section of the stUl. Relatively pure glycerol is condensed. High vacuum conditions in modem stills minimize glycerol losses due to polymerization and decomposition (see Distillation).  [c.348]

Suitable catalysts include the hydroxides of sodium (119), potassium (76,120), calcium (121—125), and barium (126—130). Many of these catalysts are susceptible to alkali dissolution by both acetone and DAA and yield a cmde product that contains acetone, DAA, and traces of catalyst. To stabilize DAA the solution is first neutralized with phosphoric acid (131) or dibasic acid (132). Recycled acetone can then be stripped overhead under vacuum conditions, and DAA further purified by vacuum topping and tailing. Commercial catalysts generally have a life of about one year and can be reactivated by washing with hot water and acetone (133). It is reported (134) that the addition of 0.2—2 wt % methanol, ethanol, or 2-propanol to a calcium hydroxide catalyst helps prevent catalyst aging. Research has reported the use of more mechanically stable anion-exchange resins as catalysts (135—137). The addition of trace methanol to the acetone feed is beneficial for the reaction over anion-exchange resins (138).  [c.493]

The process for the defatting of whole peanuts can be found in References 109 and 138. Whole kernels are roasted at 305°C for eight minutes, or partly roasted at 215°C, and then extracted with hexane at room temperature for various amounts of time, after which the rate of oil removal is deterrnined. Fully roasted nuts lose 81% of their oil content and have the best appearance after extraction for 120 hours. Solvent removal requires forced draught or vacuum conditions for 9—10 hours of drying. Salting is achieved by dipping into saturated brine or, preferably, by dipping into water followed by sprinkling with salt.  [c.278]

Initially, the product to be made using lyophilization is prepared as an aqueous solution or suspension, which is then cooled rapidly to a predeterrnined temperature. Such temperature is below the eutectic point and generally approaches — 50°C. The freezing chamber is sealed and the frozen material subjected to heat under high vacuum conditions. The Hquid portion sublimes, leaving the desired soHd dmg or biological. The process continues until less than 1% moisture remains in the dried components. Reabsorption of moisture can occur, necessitating quick removal from the freezer chamber into appropriate containers in a low humidity environment. When the lyophilized product is to be prepared for parenteral use, sterile conditions are maintained throughout the process. The dried dmg or biological residue is porous upon sublimation of the ice crystals. Such surface character increases its rate of dissolution.  [c.234]

Vacuum Flow When gas flows under high vacuum conditions or through very small openings, the continuum hypothesis is no longer appropriate if the channel dimension is not very large compared to the mean free path of the gas. When the mean free path is comparable to the channel dimension, flow is dominated by collisions of molecules with the wall, rather than by colhsions between molecules. An approximate expression based on Brown, et al. J. Appl. Phys., 17, 802-813 [1946]) for the mean free path is  [c.640]

Comparison Data—Plate Dryers Comparative studies have been done on products under both atmospheric and vacuum drying conditions. See Fig. 12-79. These curves demonstrate (1) the improvement in drying achieved with elevated temperature and (2) the impact to the drying process obtained with vacuum operation. Note that cui ve 4 at 90°C, pressure at 6.7 kPa absolute, is comparable to the atmospheric cui ve at 150°C. Also, the comparative atmospheric cui ve at 90°C requires 90 percent more diying time than the vacuum condition. The dramatic improvement with the use of vacuum is important to note for heat-sensitive materials.  [c.1217]

Capable of operation under high pressure (to 150 Ibf/in ) or vacuum conditions  [c.2180]

Opening a manual valve. Manual valves which are normally closed to isolate two or more pieces of equipment or process streams can be inadvertently opened, causing the release of a high-pressure stream or resulting in vacuum conditions. Other effects may include the development of critical flows, flashing of liquids, or the generation of a runaway chemical reaction.  [c.2289]

Failure of compo- Ensure all system components, including flexible nents in connectors are rated for maximum feasible subatmospheric vacuum conditions pressure convey-, Ensure adequate pressure control system and ing operations. back-up (e.g., vacuum relief devices) API 2000 CCPS G-3 CCPS G-11 CCPS G-22 CCPS G-29 CCPS G-3 9  [c.96]

Values listed are guides, and final line sizes and flow velocities must be determined by appropriate calculations to suit circumstances. Vacuum lines are not included in the table, but usually tolerate higher velocities. High vacuum conditions require careful pressure drop evaluation.  [c.7]

Elevated temperature, pressure and oxidant concentration relative to air may significantly reduce MIE (5-9.6 and 5-9.7). Conversely, reduced pressure elevates MIE. Eigure 5-1.4b, developed from data in [142], shows how propane s MIE varies with both absolute pressure and oxygen concentration in an oxygen-nitrogen oxidant mixture. The propane concentration is optimized in the range 5-15 vol% depending on oxygen concentration. In air (—21% oxygen) a pressure reduction from 1 atm to 0.2 atm increases the MIE from 0.25 mJ to nearly 6 mJ, a factor of about 25. The significance of this is that under many vacuum conditions such as in vacuum truck cargo tanks the MIE of typical hydrocarbons exceeds the maximum effective energy of brush discharges. As oxygen in the oxidant mixture is increased from 21% to 100% the MIE falls by a factor of about 100 at all pressures in the range examined.  [c.91]

Under vacuum conditions the flash point of liquid in the cargo tank is reduced (5-1.1.3) although vapor accumulation in the truck tank is offset by air throughput. Single component volatile liquids having a vapor pressure exceeding about 80 mmHg at 20°C should not be picked up using a vacuum truck because they might boil, creating downwind air pollution and possibly a vapor cloud hazard. Boiling is an obvious hazard for Class lA liquids but has also been found to be a problem for Class 1B liquids, the more volatile of which (such as acetone) may be substantially lost by evaporation, in the case of mixtures, the listed vapor pressure may be due to components representing only a small fraction of the bulk liquid and a small amount of boiling may be acceptable provided the vapors are discharged to a safe location also, the more volatile fractions may have weathered off by the time pick-up is scheduled.  [c.137]

One objective is to verify oil flow, friction horsepower, and heat rejection to the lube oil. Very often these tests are carried out under vacuum conditions resulting in very low aerodynamic thrust loads. It should be noted that the thrust friction horsepower constitutes the greatest loss and, therefore, the results will have to be adjusted by calculation from the test thrust to full design thrust. The error found from the test compared to design is of little significance to compressor efficiency, but is serious to oil cooling capacity. It should be noted that no-load to full-load friction loss is a linear relationship for thrust bearings and for gears. Errors in predicted loss are due to variations in running clearance and oil flow iliat affect the churning loss. It is important that tests are carried out at ite design temperatures with the correct viscosity of oil.  [c.409]

The reader is urged to review the XPS and AES articles in this Encyclopedia to obtain an adequate introduction to these techniques, since XPD and AED are actually their by-products. In principle two additions to XPS and AES are needed to perform diffraction studies, an automated two-axis sample goniometer and an angle-resolved analyzer. Ultrahigh-vacuum conditions are necessary to maintain surface cleanliness. Standard surface cleaning capabilities such as specimen heating and Ar sputtering, usually followed by sample annealing, are often needed. Sample size is rarely an issue, especially in AED where the analysis area may be as small as 300 A, using electron field emitter sources.  [c.241]

The advantages of LA are now well-known - no sample preparation is needed, conducting and non-conducting samples of arbitrary structure can be analyzed directly, spatial resolution up to a few microns can be obtained, high vacuum conditions are not required, rapid simultaneous multi-element analysis is possible, and it is possible to obtain complete analytical information with a single laser pulse. A brief overview of the potential and limitations of LA will be given in this chapter.  [c.231]

RAIRS is a non-destructive infrared technique with special versatility - it does not require the vacuum conditions essential for electron spectroscopic methods and is, therefore, in principle, applicable to the study of growth processes [4.270]. By use of a polarization modulation technique surfaces in a gas phase can be investigated. Higher surface sensitivity is achieved by modulation of the polarization between s and p. This method can also be used to discriminate between anisotropic near-sur-face absorption and isotropic absorption in the gas phase [4.271].  [c.250]

Another exception to the rule of contaminated surfaces involves very small particles, generally referred to as nanoclusters. These are generally formed and the adhesion of the.se particles to substrates studied in situ, under ultrahigh vacuum conditions. Owing to the vacuum and the short existence of these particles prior to deposition, it is possible for chemistry to occur.  [c.161]

Dry wire-pipe ESPs and other ESPs in general, because they act only on the particulate to be removed, and only minimally hinder flue gas flow, have very low pressure drops (typically less than 13 millimeters (mm) (0.5 in.) water column). As a result, energy requirements and operating costs tend to be low. They are capable of very high efficiencies, even for very small particles. They can be designed for a wide range of gas temperatures, and can handle high temperatures, up to 700°C (1300°F). Dry collection and disposal allows for easier handling. Operating costs are relatively low. ESPs are capable of operating under high pressure (to 1,030 kPa (150 psi)) or vacuum conditions. Relatively large gas flow rates can be effectively handled, though are uncommon in wire-pipe ESPs.  [c.425]

The advantages of this equipment are as follows. Wet wire-pipe ESPs and other ESPs in general, because they act only on the particulate to be removed, and only minimally hinder flue gas flow, have very low pressure drops (typically less than 13 millimeters (mm) (0.5 in.) water column). As a result, energy requirements and operating costs tend to be low. They are capable of very high efficiencies, even for very small particles. Operating costs are relatively low. ESPs are capable of operating under high pressure (to 1,030 kPa (150 psi)) or vacuum conditions, and relatively large gas flow rates can be effectively handled (AWMA, 1992).  [c.432]

Low-density steam under vacuum conditions can cause a linear velocity to be higher than is allowable with steam lines.  [c.59]

Gaseous-phase carbon adsorption systems can be classed in several ways. The first category is between regenerable and nonregenerable processes. The majority of industrial systems are regenerable operations that allow the user to recover the adsorbate and continue to reuse the activated carbon adsorbent. Regeneration relies on the continuity of gaseous adsorption achieved through equipment cycling to a desorption or regeneration phase of operation in which the temporarily exhausted beds of carbon are generated by removing the adsorbate. Regeneration operations are categorized in the following mechanisms thermal swing regeneration, pressure swing regeneration, inert gas purge stripping, and displacement desorption. Thermal swing is widely used for regeneration in purification adsorption operations. The spent bed is heated to a level at which the adsorptive capacity is reduced so that the adsorbate leaves the activated carbon surface and is removed in a stream of purge gas. Pressure swing relies on the reduction of pressure at constant temperature to reduce the adsorptive capacity for an adsorbate. Pressures can drop from elevated to atmospheric or from atmospheric to vacuum conditions. Inert purge stripping relies on the passage of a liquid or gas, without adsorbable molecules and in which the adsorbate is soluble, through the spent carbon bed at constant temperature and pressure. Displacement desorption relies on the passage of a fluid containing a high concentration of an adsorbable molecule or a more strongly adsorbable molecule than the adsorbate presently on the carbon. Gaseous-phase adsorption systems are also categorized as either fixed-bed adsorbers or movable-bed adsorbers.  [c.280]

For species chemisorbed on well-defined surfaces, especially metals, high-vacuum spectroscopy gives better resolution and the method of choice is electron-eneigy-loss spectroscopy (EELS) or the high-resolution version, HREELS. As illustrated in Fig. XVlII-4, intermediate species can be identified. Figure XVIII-5 illustrates how HREELS can distinguish between various bonding geometries for adsorbed NO2. Actual catalyzed reactions may be followed since it is possible to go from ambient to high vacuum conditions in less than a minute [32]. Additional, representative references are an overall review [33], CHj and CH radicals on Ni(m) [34], CICH2I on Pt(in) [35], and H on Ni(UI) [36].  [c.689]

Recently, in situ studies of catalytic surface chemical reactions at high pressures have been undertaken [46, 47]. These studies employed sum frequency generation (SFG) and STM in order to probe the surfaces as the reactions are occurring under conditions similar to those employed for industrial catalysis (SFG is a laser-based teclmique that is described in section A 1.7.5.5 and section BT22). These studies have shown that the highly stable adsorbate sites that are probed under vacuum conditions are not necessarily tlie same sites that are active in high-pressure catalysis. Instead, less stable sites that are only occupied at high pressures are often responsible for catalysis. Because the active  [c.302]

The distinctive chemical and physical properties of surfaces and interfaces typically are dominated by the nature of one or two atomic or molecular layers [2, 3], Consequently, usefiil surface probes require a very high degree of sensitivity. How can this sensitivity be achieved For many of the valuable traditional probes of surfaces, the answer lies in the use of particles that have a short penetration depth through matter. These particles include electrons, atoms and ions, of appropriate energies. Some of the most familiar probes of solid surfaces, such as Auger electron spectroscopy (AES), low-energy electron diffraction (FEED), electron energy loss spectroscopy (EELS) and secondary ion mass spectroscopy (SIMS), exploit massive particles both approaching and leaving the surface. Other teclmiques, such as photoemission spectroscopy and inverse photoemission spectroscopy, rely on electrons for only half of the probing process, with photons serving for the other half These approaches are complemented by those that directly involve the adsorbate of interest, such as molecular beam techniques and temperature progranuned desorption (TPD). While these methods are extremely powerfiil, they are generally restricted to—or perfonn best for—probing materials under high vacuum conditions. This is a significant limitation, since many important systems are intrinsically incompatible with high vacuum (such as the surfaces of most liquids) or involve interfaces between two dense media. Scaiming tuimellmg microscopy (STM) is perhaps the electron-based probe best suited for investigations of a broader class of interfaces. In this approach, the physical proximity of the tip and the probe penuits the method to be applied at certain interfaces between dense media.  [c.1264]

A related teclmique that also relies on the interference of x-rays for solid characterization is extended x-ray absorption fine structure (EXAFS) [65, 66]. Because the basis for EXAFS is the interference of outgoing photoelectrons with their scattered waves from nearby atoms, it does not require long-range order to work (as opposed to diffraction techniques), and provides infonnation about the local geometry around specific atomic centres. Unfortunately, EXAFS requires tlie high-mtensity and tunable photon sources typically available only at synclirotron facilities. Further limitations to the development of surface-sensitive EXAFS (SEXAFS) have come from the fact that it requires teclmology entirely different from that of regular EXAFS, involving in many cases ultrahigh-vacuum enviromnents and/or photoelectron detection. One interesting advance in SEXAFS came with the design by Stohr et al of fluorescence detectors for the x-rays absorbed by the surface species of small samples that allows for the characterization of well defined systems such as single crystals under non-vacuum conditions [67]. Figure Bl.22.9 shows the S K-edge x-ray absorption data obtained for a c (2 X 2)S-Ni(100) overlayer using their original experimental set-up. This approach has since been extended to the analysis of lighter atoms (C, O, F) on many different substrates and under atmospheric pressures [68].  [c.1791]

In order to use any material for commercial purjDoses, it is important to understand its phase behaviour. Bulk gold, for example, melts at 1065°C and thus would be an unwise choice as an electrical interconnect in a high-temperature environment many ceramics can crack during thennal processing due to solid-solid phase transfonnations that lower the volume of the crystal [178]. The extrapolation of such bulk phase behaviour to the properties of nanocrystals is not a straightforward problem. Nanocrystals are intrinsically metastable materials which, given the right circumstances, would fuse to create bulk crystals. Indeed, metal nanocrystals prepared on surfaces under high-vacuum conditions do spontaneously fuse into larger grains [179, 180]. On the other hand, solutions of nanocrystals stabilized with organic agents can exist for months or even years with unchanging sizes. Evidently, the metastability of nanocrystals is a sensitive function of their surface bonding, but the nanocrystal surface affects the crystallite in two distinct ways. First, surface atoms can make up 5-40% of the mass of a nanocrystal, and thus contribute significantly to the overall thennodynamic properties of the material. Second, the nanocrystal surface chemistry can raise the activation barriers for many thennodynamically favoured processes. Thus, it is important to consider the surface in both the kinetic and thennodynamic treatments of phase behaviour.  [c.2912]

Liquids examined by FAB or LSIMS are moved on the end of a probe until the liquid becomes situated in the atom or ion beam. Because of the high-vacuum conditions existing in a mass spectrometer ion source, there would be little point in trying to examine a solution of a sample substance dissolved in one of the common solvents used in chemistry (water, ethanol, chloroform, etc.). Such solvents would evaporate extremely quickly, probably as a burst, upon introduction into the ion source. Instead, it is necessary to use a high-boiling liquid as solvent (matrix). A low-temperature probe has been described, which does utilize low-boiling solvents. Finally, upon bombardment, the solvent itself forms ions that appear as background in a mass spectrum. Very often, protonated clusters of solvent ions can be observed (Figure 4.5).  [c.20]

The physical properties of sulfonic acids vary gready depending on the nature of the R-group. Sulfonic acids are found in both the soHd and Hquid forms at room temperature. No examples of gaseous sulfonic acids are known as of the mid-1990s. Sulfonic acids can be described as having similar acidity characteristics to sulfuric acid. Sulfonic acids are prone to thermal decomposition, ie, desulfonation, at elevated temperatures. However, several of the alkane-derived sulfonic acids show excellent thermal stabiUty, as shown in Table 1. Arene-based sulfonic acids are thermally unstable. These must be distilled under extreme vacuum conditions using a minimal amount of heating to avoid thermal decomposition. Polyaromatic compounds, such as 1-naphthalenesulfonic acid and 2-naphthalenesulfonic acids [83-47-2] and [120-18-3] respectively, readily decompose upon attempted distillation even at very high vacuum.  [c.95]

The successful appHcation of carbon—graphite as a sliding contact depends on the proper use of additives and impregnants ia the carbon—graphite materials. Carbon—graphite, long considered to be self-lubricating, depends on the presence of adsorbed films of water vapor and/or oxygen for its low friction and low wear properties. This adsorbed boundary layer is soon lost when the operation is conducted at high altitude, high temperature, or ia cold, dry air. A substitute boundary layer can be formed by iacorporating additives such as metallic sulfides, oxides, and haHdes, and impregnants such as thermoplastic and thermosetting resias. Additives and impregnants also serve to improve oxidation resistance, provide impermeability to high pressure gases and Hquids, and even permit operation under high vaccum conditions (25), a primary requirement of equipment used for exploring outer space.  [c.516]

Processing. Traditional ceramic dental restorations are custom fabricated by skilled technicians in specially equipped laboratories. Processing generally involves formation of powder-water slurries that are incrementally built up in layers of varying color and translucency using bmshes or spatulas. Consohdation of the greenware is achieved by a combination of vibration and wieking away excess water. Sintering is generally performed at 900—1000°C under vacuum conditions of ca 3.3 kPa (25 Torr) using heating rates in the range of 40—60°C/min and hold times of 0.5—10 min at the maximum temperature.  [c.471]

Vacuum. Vacuum conditions in process eqmpmeut can develop due to a wide variety of situations including  [c.2289]

Water addition to vessels that have been steam-purged If vacuum conditions can develop, then either the equipment must be designed for vacuum conditions or a vacuum relief system must be instiled.  [c.2289]

The value of the change in partial pressure of the transporting gas, at one atmosphere where pressure, with temperature indicates the balance between the standard heat of the reaction and temperature times the standard enU opy change. In the case of chloride U ansport this balance sometimes occurs only at an unacceptably high temperature due to tire large heats of formation of most chlorides. The optimum temperature for vapour transport is significantly reduced by the use of bromine, by approximately lOOkJmol and even more by the use of iodine, by roughly double tlrat amount, as the transporting halogen. This is because the heats of formation of conesponding gaseous halides reflect tire decrease in dre electron affinities of the halogens in the sequence Cl > Br > I, and the fact that the metal-halogen bond length increases in the opposite sequence, M-Cl < M-Br < M-I. The electron affinities determine the irragnimde of the ionic contribution to the metal-halogen bond, and the bond lengtlr determines the covalent as well as the ionic contributions. Furthermore, iodine is the most practical species when the reaction is caiTied out on a small scale in a sealed system, or under vacuum conditions, because it may be introduced into the container in solid form, and the container can be sealed at room teirrperamre before tire experiment or application is begun.  [c.91]

In the (Cd, Zn) (S, Se, Te) family of compounds the principal crystal structure is sphalerite, a face-centred cubic structure of the metal atoms with the Group VI elements along the cube diagonals in four coordination with the metal atoms. The exceptions are CdS and CdSe which have the related wurtzite (h.c.p) structure. The lattice parameters of the sphalerite phases range between 0.54 nm for ZnS to 0.64 nm for InSb. There are therefore a number of solid solutions which can be formed between drese compounds, where the lattice parameters differ by less than about 15%, such as GaAs-InAs. A practical example is tire solid solution between CdTe (band gap = 1.44 eV, a — 0.6488 nm) and HgTe (band gap = 0.15eV, a — 0.6459 nm). This solution has a variable band gap as a function of the Cd/Hg ratio, and a wide range of miscibility. Another example is the solid solution between InP (a — 0.5969 nm) and GaAs (a — 0.5654 nm) in which the band gap varies direcdy as the mole fraction of each component. These solutions are best prepared by vapour phase deposition under ulU a-high vacuum conditions, using separate Knudsen cells for each element because of the widely differing vapour pressures of the constituent elements.  [c.158]

Commercial spectrometers are usually bakeable, can reach ultrahigh-vacuum pressures of better than 10" Torr, and have fast-entry load-lock systems for inserting samples. The reason for the ultrahigh-vacuum design, which increases cost considerably, is that reactive surfaces, e.g., clean metals, contaminate rapidly in poor vacuum (1 atomic layer in 1 s at 10 Torr). If the purpose of the spectrometer is to always look at as-inserted samples, which are already contaminated, or to examine rather unreactive surfaces (e.g., polymers) vacuum conditions can be relaxed considerably.  [c.294]

Both IR and Raman have the great practical advantage of working in ambient atmosphere, and one can even study interfaces through liquids. The third vibrational technique discussed here, HREELS, requires ultrahigh vacuum conditions. A monochromatic, low-energy electron beam (a few eV) is reflected from a sample surftice, losing energy by exciting vibrations (cf., Raman scattering) as it does so. Since the reflected part of the beam does not penetrate the surface, the vibrational information obtained relates only to the outermost layers. Actually two separate scattering mechanisms occur. Scattering in the specular direction is a long-range dipole process that has the same selection rules as for IR. Impact scattering is short range and nonspecular. It is an order of magnitude weaker than dipole scattering and has relaxed selection rules. Taking data in both the specular and off-specular  [c.414]

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]

An additional advantage to neutron reflectivity is that high-vacuum conditions are not required. Thus, while studies on solid films can easily be pursued by several techniques, studies involving solvents or other volatile fluids are amenable only to reflectivity techniques. Neutrons penetrate deeply into a medium without substantial losses due to absorption. For example, a hydrocarbon film with a density of Ig cm havii a thickness of 2 mm attenuates the neutron beam by only 50%. Consequently, films several pm in thickness can be studied by neutron reflecdvity. Thus, one has the ability to probe concentration gradients at interfaces that are buried deep within a specimen while maintaining the high spatial resolution. Materials like quartz, sapphire, or aluminum are transparent to neutrons. Thus, concentration profiles at solid interfaces can be studied with neutrons, which simply is not possible with other techniques.  [c.661]


See pages that mention the term Vacuum conditions : [c.8]    [c.214]   
Applied Process Design for Chemical and Petrochemical Plants, Volume 1 (1999) -- [ c.128 ]