Powders layering

Based upon a piezoelectric 1-3-composite material, air-bome ultrasonic probes for frequencies up to 2 MHz were developped. These probes are characterized by a bandwidth larger than 50 % as well as a signal-to-noise ratio higher than 100 dB. Applications are the thickness measurement of thin powder layers, the inspection of sandwich structures, the detection of surface near cracks in metals or ceramics by generation/reception of Rayleigh waves and the inspection of plates by Lamb waves. 

In wide sectors of industry there is a growing need of inspection methods which go without liquid coupling media. The excitation of bulk and surface waves by means of air-coupled ultrasonic probes is therefore an attractive tool for NDE. This is tme e.g. for the rapid scanning of large composite structures in the aerospace industry [1]. In other cases, the use of liquid couplants is prohibitive like the thickness measurement of powder layers. 

Centrifugal granulators 0.3 to 3 Moderate to high Up to 200 kg hatch Powder layering and coating applications pharmaceuticals, agricultural chemicals... 

This first industrial device has been designed by MES company [65] for drying. It could be used for solid state reactions with powder reactants. Consequently, the reactor cannot be a classical chemical vessel or a classical chemical reactor with stirrer and others associated technical devices but a container able to enclose a reactant powder layer. The geometrical shape of the microwave applicator is parallelepiped box and the reactants are supported by a dielectric conveyor belt with edges as described by the Fig. 1.18. 

Powder-layer reactors are not appropriate for full-scale commercial applications, however. Loose catalyst powders are generally not compatible with high-flow-rate situations, as the powder may become compressed or dispersed into the airstream, depending on the direction of the airflow. Powder-layers are also a poor choice in situations where the reactor may be subjected to vibrations or sudden shocks, as might be expected in reactors associated with certain mobile... 

Figure 2 Schematic diagram of a continuous-flow, powder-layer photoreactor. (From Ref. 39.)...

In all four cases, tlie initial reaction rates at the start of illumination in the continuous-feed photoreactor were higher than the pseudo-steady-state reaction rates the reaction rates declined over time until pseudo-steady-state operation was achieved. Tliis apparent deactivation phenomenon, often observed with aromatic contaminants, is discussed in Sec. III.E. In a transient reaction system, the time required to reach pseudo-steady-state operation also appears to increase in the same order as the reaction rates. For example, for the continuous photocatalytic oxidation of aromatic contaminants at 50 mg/m in a powder-layer photoreactor, the time required for pseudo-steady-state operation to be achieved was reported to be approximately 90 min for benzene, 120 min for toluene, and as long as 6 hr for wz-xylene [50,51]. Under such conditions, the difference in reaction rates between the aromatic contaminants is magnified by the fact that the more reactive aromatics retain their higher transient reaction rates for longer periods (Fig. 7). 

The presence of gas-phase water is generally beneficial to the photocatalytic oxidation of aromatic contaminants. In continuous photoreactors, humidity appears to prolong catalyst activity and delay or prevent catalyst deactivation. The effects of humidity on reaction rates, however, appear to vary, depending on the aromatic contaminant concentration and the humidity level. For example. Petal and Ollis [18] examined the continuous photocatalytic oxidation of m-xylene in a powder-layer photoreactor at several different relative humidity levels. The m-xylene photo-oxidation reaction rate was observed to increase for gas-phase water concentrations up to 1000 mg/m (—7% relative humidity). Increasing the humidity level further (up to 5500 mg/m ) produced a gradual decrease in the observed reaction rate, possibly due to increased adsorption-site competition between xylene and water. The reported xylene removal rate for a water concentration of 5500 mg/m was approximately half that seen at 1000 mg/m. ... 

Don t let it overflow the powder layer and run directly through the filter paper and into the funnel. This is the nonpolar extract. 

The SORS concept was first demonstrated (see Fig. 3.3a) on a two-layer sample composed of a 1mm thick PMMA powder layer on top of a trans-stilbene powder sublayer [18]. The Raman spectra obtained at different spatial offsets are illustrated in Fig. 3.4. The conventional Raman spectrum is that obtained with zero spatial offset. The introduction of a non-zero spatial offset led to a more rapid decrease of the surface-generated Raman signal (PMMA)... 

Initial results enabled a simple discrimination between calcification types (buried at 16 mm in chicken breast tissue) using a difference spectrum method of analysis. Furthermore, signal could be obtained from a thin (100-300 pm) powder layer placed with the tissue. This gave a relative volume of calcifications to tissue of between 0.625 and 1.875%. This compares with an approximate physiological level of around 0.05-0.14% [116]. 

Fig. 11 SEM pictures of a y-Al203 powder layer and b screen-printed layer of the y-Al203 powder with colloidal silica binder...

Figure 11 shows the SEM pictures of (a) the y-Al203 powder layer and (b) the screen-printed layer of the y-Al203 powder with the colloidal silica binder. Because the porous structure has space between particles the penetration of gas into the printed layer is allowed. 

To increase thermal conductivity of powder layer metal powders of copper, aluminium are added. Composites are compacted in pellets, which can be sintered in addition. Their main characteristics are coefficient of effective thermal conductivity and coefficient of gas-permeability. The weight fraction of powder in such compacts serves as the controlled parameter, and it has the optimum, when gas-permeability does not worsen sharply at acceptable thermal conductivity. Encapsulation of hydride powder by material with high thermal conductivity followed by compaction of pellets and their sintering is also used. 

By results of our experiments mean coefficient of effective thermal conductivity of powder LaNi5 is Xe l.25 0.05 W/(m-K). The porosity of powder layer was 0.5-0.55. Effective thermal conductivity of ZrCrFei.2 powder was 0.5 W/(m-K). 

The sample, In which uranyl nitrate powder layer and 20-3Q<-sec active charcoal layer are sandwiched between Kleenex paper pads In a polyethylene tubing, is Irradiated for a short time (5-20 sec). 

In this method the slices are produced by first spreading layers of powdered material onto a surface, foUowed by selective deposition of a liquid composition onto the powder layer surface. The hquid composition, which is deposited by Inkjet, binds the powder particles together to form a solid but fragile material. 

FlG. 24. Cross-section of the dipleg with arched powder layers (Li, 1994). 

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