Measuring the effect of demagnetization in stacks of gadolinium plates using the magnetocaloric effect

The effect of demagnetization in a stack of gadolinium plates is determined experimentally by using spatially resolved measurements of the adiabatic temperature change due to the magnetocaloric effect. The number of plates in the stack, the spacing between them and the position of the plate on which the temperature is measured are varied. The orientation of the magnetic field is also varied. The measurements are compared to a magnetostatic model previously described. The results show that the magnetocaloric effect, due to the change in the internal field, is sensitive to the stack configuration and the orientation of the applied field. This may have significant implications for the construction of a magnetic cooling device.     

Ultra-short pulse laser ablation of copper, silver and tungsten: experimental data and two-temperature model simulations

Experimental results of femtosecond laser ablation of the metals copper, silver and tungsten are compared to simulations based on the two-temperature model. The comparison provides new information about the laser-heating process: For the noble metals (Cu, Ag), the energy transport via ballistic electrons must be included, while this effect is negligible for a transition metal (W). The comparison provides values for the range of ballistic electrons in the noble metals. The model calculation is also employed to investigate the dependence of the threshold fluence and melting depth on pulse duration. It is observed that for pulses shorter than approximately 1 ps the threshold fluence and melting depth are independent of the pulse duration, while they increase as τ ^0.47 and τ ^0.51, respectively, for pulses longer than ∼40 ps, in good agreement with approximate analytical expressions predicting a ?{t}\sqrt{\tau} dependence.     

Local Rings of Embedding Codepth 3. Examples

A complete local ring of embedding codepth 3 has a minimal free resolution of length 3 over a regular local ring. Such resolutions carry a differential graded algebra structure, based on which one can classify local rings of embedding codepth 3. We give examples of algebra structures that have been conjectured not to occur.     

Non-intrusive measurements of convective heat transfer in smooth- and rough-wall microchannels: laminar flow

The convective heat transfer behavior of laminar flow through a smooth- and two rough-wall microchannels is investigated by performing non-intrusive and spatially resolved measurements of fluid temperature via two-color fluorescent thermometry under constant heat flux conditions at three of the four microchannel walls. Pressure-drop measurements reveal that the apparent friction factors for all surfaces agree well with established macroscale predictions for laminar flow through rectangular ducts with the onset of transition at Re > Re_cr = 1,800 for smooth-wall flow and deviation from laminar behavior at progressively lower Re with increasing surface roughness. The local Nu for smooth-wall flow agrees well with macroscale predictions in both the thermally developing and developed regimes. With increasing roughness, while an enhancement in local Nu is noted for flow in the thermally developing regime, no measurable influence is noted upon attainment of a thermally developed state. These observations are supported by the examination of temperature profiles across the microchannel at various axial positions and Re, which suggest that the thermal boundary layer may be regenerated locally by roughness in the thermal entrance region of the flow resulting in an increased axial distance (compared to smooth-wall behavior) at which thermally developed flow is attained in the presence of roughness. Finally, estimates of the bulk Nu indicate enhancement in convective heat transfer over the smooth-wall case for laminar flow at higher Re while the smooth-wall bulk Nu data are found to agree well with macroscale predictions.     

A Convenient Synthesis of N-Methoxy-N-Methylamides from Carboxylic Acids

Carboxylic acids can be converted to their corresponding N-methoxy-N-methylamides in high yields using 2-chloro-1-methylpyridinium iodide as the coupling agent. The reaction proceeds without racemization when chiral carboxylic acids are used as the starting material.     

Ammonia synthesis and decomposition on a Ru-based catalyst modeled by first-principles

A recently published first-principles model for the ammonia synthesis on an unpromoted Ru-based catalyst is extended to also describe ammonia decomposition. In addition, further analysis concerning trends in ammonia productivity, surface conditions during the reaction, and macro-properties, such as apparent activation energies and reaction orders are provided. All observed trends in activity are captured by the model and the absolute value of ammonia synthesis/decomposition productivity is predicted to within a factor of 1–100 depending on the experimental conditions. Moreover it is shown: (i) that small changes in the relative adsorption potential energies are sufficient to get a quantitative agreement between theory and experiment (Appendix A) and (ii) that it is possible to reproduce results from the first-principles model by a simple micro-kinetic model (Appendix B).     

A long-pulse millimeter-wave free electron maser

An experimental free electron maser (FEM) facility is described which operates in the millimeter-wave regime. The experiment demonstrates that high gain and beam extraction efficiency can be obtained with low-current but high-quality electron beams. In the oscillator configuration, microwave powers of 110 kW were achieved with a 300-kV, 4-A beam, corresponding to a beam extraction efficiency of 9.2%. At low beam currents, with the system operating in the linear regime, the FEM gain curve was measured. The experiment uses a Pierce-type gun with a calculated beam emittance of 10.7-mm mrad at 300 kV and 4 A. The wiggler is of the iron permanent magnet hybrid design. The experiment is intended as a first step towards the development of continuous-wave (CW) devices that use beam energy recovery to make use of low-current, high-voltage power supplies     

An improved scheme for determining heat transfer correlations from heat exchanger regression models with three unknowns

A nonlinear regression scheme for determining heat transfer correlations from overall heat exchanger measurements is presented. The proposed scheme is valid for any two-fluid heat exchanger data where only one fluid's thermal resistance varies. The resulting regression model has three unknowns; two are associated with the variable resistance, and the third is the sum of all other resistances. The model can be analyzed only by using nonlinear regression because it cannot be transformed into linear form. Unlike existing approximate methods of analysis (known as modified Wilson plots), the proposed scheme is guaranteed to converge if a solution exists. The scheme was successfully tested by analyzing data collected on a tube-fin heat exchanger with continuous plain fins.     

Explicit constructions and properties of generalized shift-invariant systems in $L^{2}(\mathbb {R})$

Generalized shift-invariant (GSI) systems, originally introduced by Hernández et al. and Ron and Shen, provide a common frame work for analysis of Gabor systems, wavelet systems, wave packet systems, and other types of structured function systems. In this paper we analyze three important aspects of such systems. First, in contrast to the known cases of Gabor frames and wavelet frames, we show that for a GSI system forming a frame, the Calderón sum is not necessarily bounded by the lower frame bound. We identify a technical condition implying that the Calderón sum is bounded by the lower frame bound and show that under a weak assumption the condition is equivalent with the local integrability condition introduced by Hernández et al. Second, we provide explicit and general constructions of frames and dual pairs of frames having the GSI-structure. In particular, the setup applies to wave packet systems and in contrast to the constructions in the literature, these constructions are not based on characteristic functions in the Fourier domain. Third, our results provide insight into the local integrability condition (LIC).