Outgassing diffusive

Secondly, the use of UHV/EHV systems is obviously desirable because of the extreme sensitivity of many materials and processes to residual gas. Work in this pressure range has allowed significant advances to be made in, for example, surface science and the exploitation in many areas of research of synchrotron radiation sources based on electron or positron storage rings. Relevant factors in the operation of these systems, such as adsorption and desorption (including diffusive outgassing), are examined in detail. 

To treat the diffusive outgassing process quantitatively but relatively simply, outgassing from a thin (thickness (= 2d) small compared to length and breadth) sheet of material, e.g. metal, can be considered. This is shown in Figure 6.3. 

This value indicates that diffusive outgassing would be too high even after 3 months of use. 

Outgassing, but that due predominantly to gas adsorbed on the surface rather than incorporated within the structure, was discussed in Chapter 4. (Diffusive outgassing was discussed in Chapter 6.) Surface desorption can be described by a simple relationship containing adjustable parameters which fit measurements. The use of this formula in typical calculations was shown (Examples 4.13-4.18, 4.24). 

The second application concerned aspects of UHV technology. In UHV systems at equilibrium, the predominant gas load arises from the outgassing of internal surfaces. The factors influencing outgassing, including adsorption/desorption, were discussed (Examples 6.10-6.12). Outgassing from the interior of materials (diffusive outgassing), which can arise with both metallic- and non-metallic materials exposed to vacuum, was quantified in Examples 6.13-6.15. 

Differential heats of NH adsorption were measured for the samples outgassed at different temperatures ranging from 400 to 800°C. Ammonia was chosen as a basic probe because its size is small, which may limitate diffusion effects in small pore zeolite materials. The variations of the differential heats of adsorption are plotted in fig. 3 as a function of the successive pulses of... 

The simulations show that steady-state diffusion conditions throughout the wall thickness are attained in 2 hr for the case when hydrogen outgases through the OD surface and 10 hr when the OD surface is impermeable. We defined an effective time to steady state / as the time at which the hydrogen concentration at NILS at the hydrostatic-stress peak location reaches 98% of the final steady state value. We found the corresponding / values equal to 6.4 min for an outgassing pipeline and 34.25 min for an impermeable pipeline. 

Determinations of active hydrogen with methyllithium and with methylmagnesium iodide were undertaken by Uytterhoeven and Fripiat (366). Only after outgassing at 600° or higher, did the results of both methods agree with each other and with the weight loss on calcination. The reactions proceeded more slowly than with Aerosil silica (195). Apparently, diffusion into the pores is hindered, especially with the Grignard compound. 

Figure 1L20 Theoretical age spectra based on Turner s (1968) single-site diffusion model. True age of sample is 4.5 Ga, and outgassing episode took place 0.5 Ga ago. Reprinted from Turner (1968).

The metal ion-implanted titanium oxide catalysts were calcined in O2 at around 725-823 K for 5 hr. Prior to various spectroscopic measurements such as UV-vis diffuse reflectance, SIMS, XRD, EXAFS, ESR, and ESCA, as well as investigations on the photocatalytic reactions, both the metal ion-implanted and unimplanted original pure titanium oxide photocatalysts were heated in O2 at 750 K and then degassed in cells at 725 K for 2 h, heated in O2 at the same temperature for 2 h, and, finally, outgassed at 473 K to 10 lorr [12-15]. 

For metals, ceramics and glasses, this formula is applicable for times up to 100 h or more. With polymers (e.g. PTFE, Nylon, etc.), a single set of parameters for Equation (4.9) may be insufficient to fit the observed behaviour over a given time interval and the further use of Equation (4.9) with other parameters may be necessary. Values of a cover a range from 0.2 to 1.2 but a = 1 or 0.5 are frequently found. The former value is shown for metals, glasses and ceramics and indicates desorption predominantly from the surface of the material. The value of 0.5 is associated with plastics and elastomers and indicates diffusion-controlled outgassing from the bulk. 

A cylindrical vacuum chamber has an internal diameter of 70 cm and a length of 2.5 m. It is made of cleaned stainless steel and evacuated with a combination of diffusion and rotary vacuum pumps. Measurements of the outgassing rate of the chamber material, cleaned according to the same method, give the following data ... 

Figure 2.5 Diffuse reflectance spectra of polycrystalline AEO outgassed at 1073 K. Spectra of MgO, CaO and SrO were recorded in the presence of -lO Pa of non-reactive gas (e g. O2) to quench possible fluorescence emission. The R scale is...

Figure 2.6 The effect of H2O vapor adsorption at 293 K on the diffuse reflectance spectrum of polycrystalline MgO initially outgassed at 1073 K (a) original spectrum, (b) after adsorption of a first sub-monolayer dose of H2O, (c) after adsorption of a second dose. Reprinted from ref [46] with permission from Francis, Taylor Ltd.

Flo. 37. NMR intracrystalline self-diffusion coefficient Dm, (a) and effective self-diffusivity Dcir ( ) of methane in HZSM-5 crystals that were coked for different times by n-hexane cracking (131-133). Before loading with methane (9.2 CHa per u.c.), the coked ZSM-5 crystals were carefully outgassed at 623 K and 10 Pa. The remaining carbonaceous residues were defined as coke. Amounts of coke after different times on stream 1 h, 0.8 wt% C 2 h, 1.3 wt% C 6 h, 3.2 wt% C 16 h, 4.8 wt% C. The starting self-diffusion coefficient is 8.1 x 10" m s . ... 

Fig. 10, Diffuse reflectance spectra of MgO. CaO, SrO, and BaO showing exciton absorption (I, II, and III) due to the surface ions. Solids were outgassed at 1073 K, and the spectra were measured under conditions of quenched fluorescence. The reflectance scale is displaced vertically to avoid overlap of spectra. The dotted portions to the left of the vertical dashed line at n > 52,000 cm (the vacuum UV) are extrapolations (redrawn figure showing the Otc- attached to each band) [reproduced with permission from Ganone et al. (79)].

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