W. Wang

Influence of composite oxidation on electron emission characteristics of thick porous silicon

A new composite oxidation method combining rapid thermal oxidation (RTO) and electrochemical oxidation (ECO) is proposed to improve electron emission characteristics of electron emitters based on thick porous silicon (PS) layers. Four groups of electron emitters with 8 μm PS layers were oxidized by RTO, ECO, composite RTO–ECO, and composite ECO–RTO, respectively. The experimental results indicated greater emission current density and efficiency of electron emitters oxidized by composite oxidation versus ECO or RTO alone. The most favorable results occurred for an electron emitter oxidized by ECO–RTO, with an emission current density of 2.2u2009A/m2 and efficiency of 2.9%. The higher and more uniform the oxygen content in the PS layer after oxidation, the better the electron emission characteristics that the authors noted. Thick PS layers can be oxidized completely and uniformly by composite oxidation to achieve adequate oxygen content with uniform distribution. This subsequently leads to formation of a longer...

Riveting parameter design that satisfies requirements for driven rivet head dimensions

Riveted joints are extensively adopted in designing aircraft structures. Riveting implies a squeezing process of the rivet with large plastic deformations to form the driven rivet head. The driven rivet head dimensions (height H, diameter D) depend on riveting force (X1), rivet length and diameter tolerance (X2 and X3), as well as rivet hole tolerance (X4). Incorrect selection in these parameters could induce the excessive stress concentration that results in initial crack and also results in improper rivet head deformation leading to loose rivet. The present research is conducted on a MS2047AD6-6 rivet and 2.286u2009mm thick aluminum alloy sheets and mainly focuses on the design of riveting parameters X1, X2, X3, and X4 using the proposed three-step statistical experiment designs including fractional factorial design, steepest ascent design, and central composite design while satisfying the quality requirements for driven rivet head dimensions (H, D) mentioned in Standard Aircraft Handbook. Fractional factorial design is used to evaluate the impact of riveting parameters X1, X2, X3, and X4 on H and D. Based on the effective ranges of the significant riveting parameters obtained from steepest ascent design, a five-level central composite design is proposed to derive the statistical relations between H, D and the significant riveting parameters, and the statistical models are used to find the feasible region resulting from the combination of the significant riveting parameters while satisfying the quality requirements for H and D. Finally, the feasible ranges of X1, X2, X3, and X4, namely [16,470u2009N 22,730u2009N], [−0.1491u2009mm 0.3891u2009mm], [−0.0466u2009mm 0.1216 mm], and [−0.0375u2009mm 0.2125 mm], are determined from the perspective of adjustable accuracy of X1 and that of the manufacturability for X2, X3, and X4. It implies that any combination of X1, X2, X3, and X4 that falls within this feasible region can result in a good quality riveted joins, namely that the quality requirements for the driven riveting head dimension (H, D) can be satisfied.