Cluster nozzle

Cluster nozzles are used either without a boom or at the end of booms to extend the effective swath width. One type is simply a large flooding deflector nozzle which will spread spray droplets over a swath up to 70 feet wide from a single nozzle tip. Cluster nozzles are a combination of a center-discharge and two or more off-center-discharge fan nozzles. The spray droplets vary in size from very small to very large, so drifting is a problem. 

For very low cavity distances multi-nozzles called cluster nozzles (Figure 1.53) are favorable. At the same time, however, it should be noted that the branching of the mass flow should happen in a well heated area and not where the melt is already cooling. 

The particle beam — after linear passage from the evacuation chamber nozzle, through the first and second skimmers, and into the end of the ion source — finally passes through a heated grid immediately before ionization. The heated grid has the effect of breaking up most of the residual small clusters, so residual solvent evaporates and a beam of solute molecules enters the ionization chamber. 

The beam of tiny drops passes from the exit nozzle across an evacuated space and into another small orifice (skimmer 1). In this evacuated region, about 90% of the originally injected helium and solvent is removed by vacuum pumps to leave a stream of droplets so small that they are called clusters. 

A stream of a liquid solution can be broken up into a spray of fine drops from which, under the action of aligned nozzles (skimmers) and vacuum regions, the solvent is removed to leave a beam of solute molecules, ready for ionization. The collimation of the initial spray into a linearly directed assembly of droplets, which become clusters and then single molecules, gives rise to the term particle beam interface. 

After condensation, the clusters are transported by the He-flow through a nozzle and a differential pumping stage into a high vacuum chamber. For ionization of the clusters, we used excimer and dye laser pulses at various wavelengths. The ions were then mass analyzed by a time-of-flight mass spectrometer, having... 

Figure 1. Schematic illustration of the laser-vaporization supersonic cluster source. Just before the peak of an intense He pulse from the nozzle (at left), a weakly focused laser pulse strikes from the rotating metal rod. The hot metal vapor sputtered from the surface is swept down the condensation channel in dense He, where cluster formation occurs through nucleation. The gas pulse expands into vacuum, with a skinned portion to serve as a collimated cluster bean. The deflection magnet is used to measure magnetic properties, while the final chaiber at right is for measurement of the cluster distribution by laser photoionization time-of-flight mass spectroscopy.

The cluster reactor is attached to the pulsed cluster source s condensation channel, as shown in Figure 6. (16) To it is attached a high-pressure nozzle from which a helium/hydrocarbon mixture is pulsed into the reactor at a time selected with respect to the production and arrival of the clusters. The effect of turbulent mixing with the reactant pulse perturbs the beam, but clusters and reaction products which survive the travel from the source to the photoionization regime ( 600y sec) and the photoionization process are easily detected. 

Figure 3.9. Transient C02 formation rates on Pd30 (a) and Pd8 (b) mass-selected clusters deposited on a MgO(lOO) film at different reaction temperatures [74]. In these experiments CO was dosed from the gas background while NO was dosed via a pulsed nozzle molecular beam source. The turnover frequencies (TOFs) calculated from the experiments displayed in (a) and (b) are displayed in the last panel (c). C02 formation starts at lower temperatures but reaches lower maximum rates on the larger cluster. (Figure provided by Professor Heiz and reproduced with permission from Elsevier, Copyright 2005).

Figure. 1. Schematic of essential components of the Exxon group cluster laser vaporization source and fast flow tube chemical reactor. On the far left is a 1 mm diameter pulsed nozzle that emits an -200 ysec long pulse of helium which achieves an average pressure of approximately one atmosphere above the sample rod. Immediately before the sample rod position the tube is expanded to 2 mm diameter. The length of this extender section can be varied form 6 mm to 50 mm depending upon the desired integration time for cluster growth. The reactor flow tube is 10 mm in diameter and typically 50 mm long. The reactants diluted in helium are added and mixed with the flow stream via the second pulsed valve.

A variation on this method, passing the vapors emitted from a heated source or sources through a nozzle, may cause clustering. The gas-phase species, which could be ions or neutral molecules, pass from a region of higher pressure to a region of lower pressure. This process has many collisions and then adiabatic expansion often produces cold clusters. If the clusters have not been ionized, they may be ionized in the low-vacuum region. 

The laboratory layout consists of a molecular beam apparatus and a laser system. NaK clusters are created in an adiabatic coexpansion of mixed alkali vapour and argon carrier gas through a nozzle of 70 pm diameter into the vacuum. Directly after the nozzle the cluster beam passes a skimmer. Next, the laser beam coming from perpendicular direction irradiates the dimers and eventually excites and ionizes them. The emerging ions are extracted by ion optics, mass selected by QMS and recorded by a computer. 

In 1984. scientists (Rohlling. Cox. and Caldor at Exxon Research and Engineering) created clusters of carbon (soot) by the laser vaporization of a carbon target rod in connection with a supersonic nozzle. By means of mass spectroscopy, the researchers determined the relative abundance of the carbon clusters produced. Small, 20- to 40-atom clusters of carbon were expected inasmuch as these had been produced a number of times by earlier investigators working on the soot problem. In such experiments, an interesting but unexplained question always arose—Why were only even-numbered carbon clusters produced in the complete absence of odd-numbered clusters See Fig. 3. 

In principle, with cobalt catalysts similar pathways for deactivation exist. Because of the low price of cobalt compared to rhodium, this is less important in unmodified cobalt catalysis, but the deposition of cobalt clusters and metallic cobalt can cause nozzles and valves to plug up, resulting in the shutdown of the plant. In case of a ligand-modified cobalt catalysis, the same problems of ligand deterioration e.g., by oxygen and peroxides, arise, necessitating a meticulous purification of the starting materials. 

The method of choice for the generation of vdW clusters utilizes supersonic expansions. In this technique, the species to be clustered are allowed to expand from a high pressure to a low pressure region through a molecular beam nozzle. The basic principles of adiabatic expansion have been the focus of a number of reviews (Hagena 1974, 1987 Scoles 1988) and only the pertinent aspects will be described here. 

Figure 7-1. Schematic side view of differentially pumped cluster beam apparatus and quadrupole mass spectrometer. The temperature of the nozzle in the stagnation region is regulated by a circulating chiller. Reprinted with permission from Vaidyanathan et al. 1991b. Copyright 1991 American Institute of Physics.

The skimmed and collimated cluster beam passes into the quadrupole mass spectrometer (Extrel, C-50), which has a mass range of 0-1200 amu with unit mass resolution. The mass spectrometer chamber is pumped by a turbomolecular pump (360Is-1). The pressure in the mass spectrometer chamber (P3), when the beam is in operation, is always less than 1 x 10 6 torr. This is necessary to ensure that the contributions from reactions of the cluster ions with the background gas are not significant. The distance of the nozzle from the ion source varies in the range 20.5-22.5 cm, depending on the nozzle to skimmer distance. 

Fig. 12.1. a Schematic diagram of the experimental setup (1) the off-axis //3 parabolic mirror, (2) the laser beam, (3) the specially designed pulsed conical nozzle, (4) the cluster gas jet, (5) the focusing spectrometer with the spherically bent mica crystal, (6) the vacuum-compatible X-ray CCD camera, (7) the ion detector for TOF measurements, b Typical X-ray CCD image measured at an intensity of... 

Fig. 12.2. 2-D calculations of axial radial profiles for the parameters of a two-phase Ar jet at the cross-section positioned 1.5-mm downstream from the nozzle outlet. The solid, dotted, and broken lines represent argon gas pressures of 60, 40, and 20 bar, respectively a the mean cluster concentration, nciust b the gas-phase concentration nat c the mean cluster radius (r) d the mean distance between clusters (d)...

Figure 12.5 shows the cluster size dependence of X-ray emission spectra. The top, middle, and bottom curves represent the spectra measured with a laser contrast of C = 4 x 10-4 at Ar gas pressures of 60, 50, and 40 bar, respectively. Note that no X-rays were observed at an Ar gas pressure of 40 bar. According to hydrodynamic calculations (see Fig. 12.2), at 40 bar, a cluster with an average diameter of 200 nm is one order of magnitude smaller than that with an average diameter of 1.5 pm at 60 bar. Thus, in the case of the 40-bar experiment, the clusters were almost completely destroyed by the prepulse. This result demonstrates the important role of big clusters, and the validity of the nozzle design. 

Systematic investigations of the laser-cluster interaction were carried out by simultaneously measuring high-resolution X-ray emission spectra and ion energy spectra produced by the laser irradiation of micron-sized Ar clusters at laser intensities of 1018 to 1019 W/cm2. To suppress the creation of preplasma, we designed a special conical nozzle and eliminated the laser prepulse. The results indicate that the explosion time scale for micron-sized clusters is much longer than that for nanometer-sized clusters. It is found that hot electrons produced by a higher contrast pulse (a smaller prepulse) allow the isochoric... 

CoO-coated Co cluster and oxide-coated Fe cluster assemblies were prepared by a plasma-gas-aggregation cluster-beam-deposition technique [37-39]. For preparation of CoO-coated Co cluster assembly, oxygen gas was introduced through a nozzle near the skimmer into the deposition chamber. The Co clusters with CoO shells were formed before deposition onto the substrate [37], Figure 8 shows a TEM image of the clusters produced at oxygen gas flow rate R(02) = 1 seem. Clusters are almost monodispersed, with the mean diameter of about 13 nm. Electron diffraction pattern indicated the coexistence of Co and CoO phases. The cluster assemblies were formed on a polyimide film with a thickness of about 100 nm. 

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