Development of microvalves for gas flow control in micronozzles using PVDF piezoelectric polymer

This work describes a fabrication and test sequence of microvalves installed on micronozzles. The technique used to fabricate the micronozzles was powder blasting. The microvalves are actuators made from PVDF (polivinylidene fluoride), that is a piezoelectric polymer. The micronozzles have convergent-divergent shape with external diameter of 1mm and throat around 230μm. The polymer have low piezoelectric coefficient, for this reason a bimorph structure with dimensions of 2mm width and 4mm of length was build (two piezoelectric sheets were glued together with opposite polarization). Both sheets are recovered with a conductor thin film used as electrodes. Applying a voltage between the electrodes one sheet expands while the other contracts and this generate a vertical movement to the entire actuator. Appling +300V DC between the electrodes the volume flux rate, for a pressure ratio of 0.5, was 0.36 sccm. Applying -200V DC between the electrodes (that means it closed) the volume flux rate was 0.32 sccm, defining a possible range of flow between 0.32 and 0.36 sccm. The third measurement was performed using AC voltage (200V AC with frequency of 1Hz), where the actuator was oscillating. For pressure ratio of 0.5, the flow rate was 0.62 sccm. © 2008 IOP Publishing Ltd.

The significance and determination by image analysis of microcrack density in hard chromium plating

It is well known that the microcrack density is a fundamental parameter in hard chromium electroplating. The chemical and mechanical properties of this coating are widely dependent on its microcrack density. In this paper a simple image analysis procedure to determine microcrack density is presented in order to demonstrate it as a fundamental tool to estimate the fatigue, corrosion and wear behavior, as well as the residual stress field of a coated component. For this purpose, the image analysis procedure was carried out on two kinds of hard chromium plating - one called accelerated (high velocity of deposition and fluoride-free) and the other conventional (with fluoride). The coatings were applied on samples of AISI 4340 aeronautical steel, which is widely used in aircraft landing gear components. To characterize the practical significance of this study, the microcrack density results were related to the fatigue, wear and corrosion behavior from previous study and to the residual stress field in the coatings.