Aerosol techniques for manufacturing piezoelectric materials
J. Akedo, in Advanced Piezoelectric Materials, 2010
14.4.1 Deposition ratio and influence of starting powder properties
High deposition rates can be achieved easily with AD because the source material is already in particle state form. These deposition rates are at least 30 times higher than other conventional thin film formation methods. Therefore, AD is an attractive manufacturing process due to high throughput. The particle velocity and concentration in the aerosol jet at the nozzle increase with increasing carrier gas flow rate, resulting in increased deposition efficiency. Starting particle properties, such as the average particle size and the size distribution, mechanical and surface properties can also strongly affect the deposition efficiency. [41]
The deposition rates for PZT AD layers using powders subjected to various milling procedures are indicated in Fig. 14.8. It can be seen that, by increasing the milling time, the deposition rate of the PZT layer significantly increased and reached a maximum of 73 μm/min for a 5 mm2 deposition area when powder was milled for 5 h. This value is 30 times higher than that for a starting powder without the milling procedure. An interesting fact is that the deposition rate decreased as a result of further milling to 30 h. It is assumed that particle surface properties (for example, surface activation, defects, gas absorption) will change by longtime milling making them less probable to be deposited in the same conditions.
The milling procedure is also strongly influenced by the film density. The cross-sectional SEM images of the deposited layers vs. the milling time are shown in Fig. 14.9. With the increase of powder milling time, density and hardness of the deposited layer decreased. On the other hand, for 5 h and 30 h milling procedures, grain images of starting powders with the diameters from 100 to 300 nm and porous structures were distinct in the deposited layer images. At the same time, color and transmittance of layers were markedly changed from yellow to white due to the increase of optical scattering. [41] Thus the milling procedure is applicable to control the porosity of ceramic layers deposited by the AD process.
The starting powder particle size and shape strongly influence the RTIC phenomenon in the AD process. If spherical α-Al2O3 ultra-fine particles with average particle size around 50 nm were used, after AD deposition the films have a pressed-like structure and the RTIC phenomenon could not be observed even if the ejecting particle velocity from the nozzle was over 400 m/sec and particle size was very small (Fig. 14.10(a)). In contrast, if non-spherical α-Al2O3 fine powder with average particle size around 1 μm was used, the deposited particles on the substrate were consolidated at room temperature and RTIC phenomenon was observed even for particle velocities around 200 m/sec, as shown in Fig. 14.10(b). As a result, high density and transparent ceramic layers were obtained. These results are explained by aerodynamic properties of the particle jet flow near the substrate. If particle size and weight are too small, the particle follows the carrier gas flow as shown schematically in Fig. 14.11. Therefore the particle velocity normal to the substrate is largely decreased and is not high enough to obtain RTIC phenomenon. Detailed further investigation about the particles’ aerodynamic properties in the AD process still needs to be conducted.
An advantage of AD over conventional thin film and thermal spray coating methods is that substrate surface does not need pre-cleaning to achieve good deposition. During the initial deposition stage, the particles impacting the substrate will act as cleaning agents in a similar way as in sand-blasting processes. Surface contaminants such as dirt and oils are removed by the initial particle collisions. The deposition automatically begins when the surface becomes sufficiently clean. The film adhesive strength to glass and metal substrates may be in excess of 30 MPa, because an anchoring layer having a thickness of about 100–200 nm was formed in the interface between the substrate and the deposited layer. To obtain maximum adhesive strength, a substrate with suitable hardness and elasticity is needed to allow the formation of the anchoring layer. A substrate that is very soft will be etched by the particle jet flow and the deposition will not occur. On the other hand, when a substrate with large hardness value is used, the adhesion strength between the deposited layer and substrate is weak and the film may easily peel off.