Gas Flow Parameters

The specification of gas flow parameters should always include a measurement of the total pressure p and the gas flow rate Q. This is easily done with simple instrumentation yet much data appears in the literature with only one of these 

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Since initiation of the decomposition is dependent on the heat flux supplied by the high-temperature gas flow, the ignition process is dependent on the various gas-flow parameters, such as temperature, flow velocity, pressure, and the physicochemical properties of the gas. 

The role of this system is to generate precursor vapour, and then deliver it to the CVD reactor. Gaseous precursors are preferred because they can be readily metered to control the gas flow parameters. Liquid and solid reactants can usually be vaporised at suitable temperatures and transported by a carrier gas to the reaction chamber. A typical gas delivery system with four delivery lines is shown in Figure 3.3. In lines 1, 2 and 3 gas flows and their associated parameters (e g. flow rate, pressure) are regulated by three mass flow controllers and the relevant valves. H2, MTS-H2 and Ar gases are conveyed to the gas mixer, then to the CVD chamber. Line 4 is employed to purge the reactor before and after the CVD process. For a delivery system, three types of precursors should be considered. 

Inductively coupled plasma-mass spectrometry (ICP-MS) is a modern and more sensitive variation of MS detection of bismuth. Bismuthine is generated in a hydride generator and swept by argon directly into the ICP unit. The ions are then introduced into the mass spectrometer. Optimization of the mass spectrometer, reagent, and gas flow parameters leads to a detection limit of 20ngL (IfQand 1993). Phillips etal. (2001) examined the safety aspects of colloidal bismuth subcitrate (CBS) quadruple therapy for Helicobacter pylori. These authors used ICP-MS to determine blood Bi levels in 34 patients receiving CBS quadruple therapy, with whole blood Bi levels being deter-... 

Xie, Z.-P. (2003) A Study on Coal Seam Gas Flow Parameters Inversion. Research and Development of Mine Disaster Prevention and Control Strategy (China). China University of Mining and Technology Press, 12, 236-239. 

The series of diagrams in Fig. 11.28 obtained with the help of the calculated model [51, 52], and supported by conclusions from [48] give an idea about the anticipated gas flow parameters along the blast wave axial trajectory and of the trailing hydrogen jet. Diagrams in Fig. 11.28 have been plotted for Pc = 30 MPa and d = 3 mm and the instant of the container wall rupture. 

The packing parameter ( ) (m) reflects the influence of the Hquid flow rate as shown in Figure 20. reflects the influence of the gas flow rate, staying at unity below 50% of the flooding rate but beginning to decrease above this point. At 75% of the flooding velocity, = 0.6. Sc is the Schmidt number of the Hquid. 

The optoelectronic properties of the i -Si H films depend on many deposition parameters such as the pressure of the gas, flow rate, substrate temperature, power dissipation in the plasma, excitation frequency, anode—cathode distance, gas composition, and electrode configuration. Deposition conditions that are generally employed to produce device-quahty hydrogenated amorphous Si (i -SiH) are as follows gas composition = 100% SiH flow rate is high, --- dO cm pressure is low, 26—80 Pa (200—600 mtorr) deposition temperature = 250° C radio-frequency power is low, <25 mW/cm and the anode—cathode distance is 1-4 cm. 

CVD reactions are most often produced at ambient pressure in a freely flowing system. The gas flow, mixing, and stratification in the reactor chamber can be important to the deposition process. CVD can also be performed at low pressures (LPCVD) and in ultrahigh vacuum (UHVCVD) where the gas flow is molecular. The gas flow in a CVD reactor is very sensitive to reactor design, fixturing, substrate geometry, and the number of substrates in the reactor, ie, reactor loading. Flow uniformity is a particulady important deposition parameter in VPE and MOCVD. 

The expanded version of the van Deem ter equation is used to help understand the relationships between the packing parameters and the gas flow. 

FIG. 6-26 Parameters for pressure drop in liqiiid/gas flow through horizontal pipes. (Based on Lockhatt and Maitinelli, Chem. Engr. Prog., 45, 39 [1.94.9],)... 

ICP-SFMS (Thermo Finnigan, Flement) with cold vapour generation was developed with a guard electrode and a gold amalgamation device using an Au-sorbent for sample pre-concentration to improve the sensitivity. Instrumental parameters of ICP-SFMS such as take-up time, heating temperature of Au-sorbent, additional gas flow, and sample gas flow were optimized. Detection limit calculated as 3 times the standard deviation of 10 blanks was 0,05 ng/1, RSD = 7-9 %. 

Expander effieieney is related to gas flow, enthalpy drop, and shaft rotating speed. The eombination of these parameters defines impeller blade geometries required to maximize thermal effieieney. Expander... 

Detection limits for a particular sample depend on a number of parameters, including observation height in the plasma, applied power, gas flow rates, spectrometer resolution, integration time, the sample introduction system, and sample-induced background or spectral overlaps. ... 

Parameter studies have shown that single-layer nanotubes can be produced by the arc method under a wide range of conditions, with large variations in variables such as the buffer gas pressure (100-500 Torr), gas flow rate, and metal concentration in the... 

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