Electrical Properties of Nanocrystalline Tungsten Trioxide
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of gas evaporation was introduced in 1976 [5] and has become a leading method for the production of high-quality nanoparticles [6-8]. In the classical case, the evaporation of the material takes place in the presence of an inert gas. The vapor is then cooled so that nucleation and growth of ultrafine particles take place in the gas. The primary particles can have a narrow size distribution [9]. The gas evaporated particles are normally single crystals. Figure 1 illustrates the experimental equipment for particle fabrication. In this particular case, a pellet of 99.99% pure tungsten is placed on a pedestal in the lower chamber and is surrounded by synthetic air that works as reactive gas and as cooling gas. The pellet is inductively heated by a surrounding copper coil. The synthetic air is introduced below the pedestal. The surface of the pellet is oxidized and the tungsten oxide is sublimated. The transfer pipe is positioned centrally in the vapor zone where particles are formed. Particles in the outer part of the zone are removed by an exhaust pipe connected to a pump. The particles are thereby collected from a small part of the super 15 Mat. Res. Soc. Symp. Proc. Vol. 581 @2000 Materials Research Society
saturated vapor zone and the size distribution is kept narrow. A stream of gas transports the particles through the transfer pipe into the evacuated upper chamber where the particles are collected. Strong precautions were taken to avoid turbulence and eddy formation.
Collection chamber
x, y, z - table
Nozzle for the
particle stream 4
Transfer pipe
Exhaust pipe
Evaporation chamber
Pedestal with source material and heating
device
4-Synthetic air inlet
Fig. 1: Schematicpicture of the gas-evaporationequipment Particles were deposited onto an indium tin oxide (ITO) coated glass substrate in the deposition chamber. The substrate was mounted on a computer controlled x,y,z- table which allowed various patterns to be made. Two samples, referred to as I and II below, were fabricated at a heating power of 1.1 kW. The substrate was kept at room temperature during deposition. The difference in the production between the two samples was the linear speed of the xy,z- table. Sample I was deposited with a speed of 0.38 mm/s and sample II with a speed of 0.19 mm/s. This resulted in a factor of two more particles per unit length in the latter case. Particle size distributions were determined by transmission electron microscopy (TEM), using a Jeol 2000 FX II instrument, and by scanning electron microscopy (SEM), using a LEO 1550 Field Emission Gun with a Gemini column. The film thickness was determined using a FEI XL 30 Environmental SEM with a field emission gun. Dielectric measurements were carried out with a Solartron Instruments SI- 1260 impedance/gain-phase analyzer, a Novocontrol broadband converter and an additional probe station. In order to characterize the dielectric properties of the samples, contacts of aluminium were evaporated onto the surface of the W0 3. The diameter of the evaporated contacts were 1mm.
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