Naval Postgraduate School

About: Naval Postgraduate School is a education organization based out in Monterey, California, United States. It is known for research contribution in the topics: Tropical cyclone & Boundary layer. The organization has 5246 authors who have published 11614 publications receiving 298300 citations. The organization is also known as: NPS & U.S. Naval Postgraduate School.

Roberts: Public Entrepreneurship and Innovation

Abstract: This article presents a conceptualframeworkofpublic entrepreneurship Public entrepreneurship is defined as the generation ofa novel or innuuative idea and the design and implementation of the innovative idea into publicsector practice The conceptualfiamework is used to distinguish between public entrepreneurs and other actors in the policy process, and to clarify the differences between policy, political, executive, and bureaucratic entrepreneurs Taking a functionalist perspective, the article difmentintes between individual and coZkctiue entrepreneurship and ginezates pmpaitims to move us cher to a theory of public entrepreneurship

Impression creep characterization of rapidly cooled Sn–3.5Ag solders

Abstract: The creep behavior of solders crucially influences solder joint reliability in microelectronic packages. This, in conjunction with the ongoing industry-wide transition to lead-free solders, has resulted in significant current emphasis on developing approaches for expeditiously creep testing miniaturized samples of a wide range of solder compositions to determine their suitability for packaging applications. Here we characterize the creep response of Sn–3.5Ag solders via impression creep testing. The tested microstructures represent a rapidly cooled state, resembling the highly refined microstructures often found in tiny microelectronic solder joints. It is shown that in the as-reflowed state, the solder creeps by a viscous glide-controlled mechanism at low stresses due to the presence of a non-equilibrium amount of dissolved Ag in Sn, and a particle-limited climb mechanism at high stresses. Following slight aging, which allows the dissolved solute concentration to drop to near-equilibrium levels, the low stress mechanism changes to dislocation climb-control. Based on these observations, a unified view of creep of Sn–3.5Ag solders is presented, which is consistent with observations of a transition from low to high stress sensitivity with increasing stress, which has been reported in several studies.

Modelling and Residual Analysis of Nonlinear Autoregressive Time Series in Exponential Variables

Abstract: : An approach to modelling and residual analysis of nonlinear autoregressive time series in exponential variables is presented; the approach is illustrated by analysis of a long series of wind velocity data which has first been detrended and then transformed into a stationary series with an exponential marginal distribution The stationary series is modelled with a newly developed type of second order autoregressive process with random coefficients, called the NEAR(2) model; it has a second order autoregressive correlation structure but is nonlinear because its coefficients are random The exponential distributional assumptions involved in this model highlight a very broad four parameter structure which combines five exponential random variables into a sixth exponential random variable; other applications of this structure are briefly considered Dependency in the NEAR(2) process not accounted for by standard autocorrelations is explored by developing a residual analysis for time series having autoregressive correlation structure; this involves defining linear uncorrelated residuals which are dependent, and then assessing this higher order dependence by standard time series computations Application of this residual analysis to the wind velocity data illustrates both the utility and difficulty of nonlinear time series modelling

The Use of Organic Coatings to Promote Dropwise Condensation of Steam

Abstract: Fourteen polymer coatings were evaluated for their ability to promote and sustain dropwise condensation of steam. Nine of the coatings employed a fluoropolymer as a major constituent; four employed hydrocarbons and one a silicone. Each coating was applied to 25-mm-square by approximately 1-mm-thick metal substrates of brass, copper, copper-nickel, and titanium. While exposed to steam at atmospheric pressure, each coating was visually evaluated for its ability to promote dropwise condensation. Observations were also conduction over a period of 22,000 hr. Hardness and adhesion tests were performed on selected specimens, On the basis of sustained performance, six coatings were selected for application to the outside of 19-mm-dia copper tubes in order to perform a heat transfer evaluation. These tubes were mounted horizontally in a separate apparatus through which steam flowed vertically downward. Steam-side heat transfer coefficients were inferred from overall measurements. Test results indicate that the steam-side heat transfer coefficient can be increased by a factor of five to eight through the use of polymer coatings to promote dropwise condensation.

Influence of dislocation density and distribution on the aging behavior of 6061 AlSiCw composites

Abstract: Precipitation hardening metal-matrix composites (MMCs) are known to age more rapidly than the unreinforced matrix alloys. It has been proposed that the accelerated aging is due to some dislocation defect mechanism which enhances nucleation, growth or both. Theoretical Avrami-type precipitation curves were generated assuming dislocation density dependent nucleation only and dislocation density dependent nucleation and growth. Curves were generated for both uniform dislocation density (to model cold work) and a dislocation density gradient (to model MMCs). These theoretical results were compared to precipitation curves generated by differential scanning calorimetry of unstrained and plastically strained unreinforced 6061 Al and 10 vol.% SiC whisker reinforced 6061 Al MMC. It was found on aging that β′ precipitation in the MMC initiates earlier and is completed later than in the unreinforced alloy with the same amount of plastic work as the composite. The reaction rate order for β′ precipitation was also determined from the calorimetric studies. These experimental results were interpreted in terms of the theoretical calculations. It is concluded that the matrix dislocation density distributions of metal-matrix composites can strongly influence macroscopically observed nucleation and growth rates.