Wavelet Estimation of Time Series Regression with Long Memory Processes

This paper studies the estimation of time series regression when both regressors and disturbances have long memory. In contrast with the frequency domain estimation as in Robinson and Hidalgo (1997), we propose to estimate the same regression model with discrete wavelet transform (DWT) of the original series. Due to the approximate de-correlation property of DWT, the transformed series can be estimated using the traditional least squares techniques. We consider both the ordinary least squares and feasible generalized least squares estimator. Finite sample Monte Carlo simulation study is performed to examine the relative efficiency of the wavelet estimation.Discrete Wavelet Transform

Synthesis of Palladium and Palladium-Copper Nanostructures as Electrocatalysts

Pd and its alloys are alternatives of Pt as promising catalysts and electrocatalysts for many reactions. Size controlled synthesis of nanoparticles remains a major research subject, since smaller size particles show better catalytic performance. In this work, we developed a modified chemical wet method to prepare Pd and Pd-Cu nanostructures with uniform small size. Different sizes and shapes of Pd nanostructures were successfully synthesized by using the two reducing agents (i.e., L-ascorbyl-6-palmitate or phenylphosphinic acid). The reducing agents play a role to control the final morphologies and sizes of particles. The use of L-ascorbyl-6-palmitate favors to form irregular branch shapes or rods; in contrast, the use of phenylphosphinic acid tends to form spherical nanoparticles. Furthermore, phenylphosphinic acid can assist with size control of Pd particles. Co-reducing Pd and Cu precursors can obtain Pd-Cu nanostructures with different sizes and shapes. The growth mechanism is followed the deposition of Cu on Pd seeds which are reduced prior to Cu. Similar to pure Pd synthesis, phenylphosphinic acid reduced the precursors to form small uniform spherical particles compared to L-ascorbyl-6-palmitate. It was also found that the composition could also be tuned by using different reducing agents. The catalytic activity of Pd and Pd-Cu nanostructures for ethanol oxidation reaction (EOR) has been tested in basic solution for alkaline fuel cell applications. The specific areas of these Pd and Pd-Cu are much higher than those reported previously. It was found that both Pd and Pd-Cu nanostructures exhibited enhanced catalytic activities and to some extent resisted CO-like intermediates poisoning. Most catalysts had enhanced current densities after 500 cycle scan, indicating enhanced stability of those catalysts

Temperature Separation in a Vortex Tube and convection driven by vorticity

Why the temperature gradient in the vortex tube deviates significantly from the adiabatic gradient is a very important but unresolved issue in vortex tube research. In compressible fluids, the vorticity of a fluid parcel that is different from its environment can affect its own pressure or density, just like temperature. Gases near the wall have very high vorticity, and when they detach from the wall, their higher vorticity compared to the environment makes their density lower than the environment. Driven by vorticity, some fluid parcels undergo multiple collisions near the axis, achieving temperature separation.Comment: 6pages, 1figr