Search

Magnetic refrigeration is a potentially environmentally-friendly alternative to vapour compression technology that is presented in this paper. The magnetocaloric effect in two magnetocaloric compounds in the La(Fe,Co,Si)13 series is presented in terms of their adiabatic temperature change and the specific heat as a function of temperature at constant magnetic field. A 2.5-dimensional numerical model of an active magnetic regenerative (AMR) refrigerator device is presented. The experimental AMR located at Risø DTU has been equipped with a parallel-plate based regenerator made of the two materials. Experimental zero heat-load temperature spans are presented for different operating conditions and the results are compared to predictions of the numerical model. It is concluded that the model reproduces the experimental tendencies and when including thermal parasitic losses to ambient and the predictions from the model are within 1.5 K of the experimental results.

We report the thermoelectric properties of Nd-, Hf-or Bi-doped SnO2-based ceramics prepared by solid-state sintering. Polycrystalline SnO2-based samples (Sn0.97Sb0.01Zn0.01M0.01O2, M = Nd, Hf or Bi) were prepared by solid-state reactions. We confirmed that Bi-doping increased the power factor due to both the enhanced electrical conductivity and Seebeck coefficient compared to the matrix material. The maximum power factor of 4.8 × 10−4 Wm−1K−2 was attained for the Bi-doped sample at 1060 K.

This article describes in detail the effect of cooling rate on the microstructure of a low-carbon Fe-12 pct Cr alloy. The alloy was prepared using a relatively simple technique, i.e., rapid cooling of the melt in a copper wedge mold. The dependence of microstructure on the cooling rate (∼40 to 105 K/s) has been determined by X-ray diffraction (XRD), microhardness measurement, optical microscopy (OM), and transmission electron microscopy (TEM). It has been found that the matrix structure over a large cooling rate range is composed of columnar ferrite grains, the size of which decreases with increasing cooling rate. Precipitation of second phases has been observed at either the ferrite grain boundaries or within the ferrite grains. The former takes place along the entire wedge sample, whereas the latter characterizes a region 12 mm away from the tip of the wedge sample. The essential structure of the grain boundary precipitates was identified as martensite, which is a transformation product of austenite precipitated at high temperatures. Retained austenite was identified at the tip region as isolated particles (

A mathematical model for accelerating powder particles by a gas and for their thermal behaviour during flight has been developed. Usually, dealing with the solidification of metal droplets, the interaction between an array of droplets and the surrounding gas is not integrated into the modelling of such a process, for example, in the literature the gas temperature is often modelled by an empirical expression. In this model, however, the interaction between the enveloping gas and an array of droplets has been coupled and calculated numerically. The applicability of the empirical relation of the gas temperature proposed in the literature has been discussed in relation to the model. One of the major advantages of this modelling is that it provides a tool to predict the thermal behaviour of droplets during flight without the need for experimental parameters, i.e. gas temperature. Furthermore, the model predicts the effect of process parameters on the size distribution, temperature, velocity histories, fraction-solid and cooling rate for all droplet sizes characterizing the complete droplet size distribution.

The performance of Active Magnetic Regenerators (AMR) does not depend solely on the magnetocaloric effect of their constituents. Rather, it depends on several additional parameters, including, magnetic field, geometry (hydraulic diameter, cross-sectional area, regenerator length etc.), thermal properties (conductivity, specific heat and mass density) and operating parameters (utilization, frequency, number of transfer units etc.). In this paper we focus on the influence of three parameters on regenerator performance: 1) Solid thermal conductivity, 2) magnetostatic demagnetization and 3) flow maldistribution due to geometrically non-uniform regenerators.It is shown that the AMR performance is optimal at an intermediate value of the solid thermal conductivity for many operating conditions. The magnetostatic demagnetization is shown to have a significant influence on the AMR performance, giving a strong dependence on the orientation of the applied field and the regenerator geometry. Finally, the flow maldistribution of non-uniform regenerator geometries is found to degrade the AMR performance even at minor deviations from perfectly homogeneous regenerator matrices.

An improved and reliable method for making sharp scanning tunneling microscopy (STM) tips is described. It is based on the widely used drop-off electrochemical etching procedure, here modified to improve the control of the tip shape. A second etching is applied not only to remove the oxide layer from the tip surface, but also to sharpen the STM tips further. A tip radius less than 20 nm can be obtained reproducibly. The quality of the produced tips was inspected with transmission electron microscopy, and micrographs of tips produced with different times of second etching are shown. To produce tips which are all without an oxide layer an electronic phase control unit is necessary. Even without this etch controller more than half the tips are oxide free, and then only standard laboratory equipment is used for the tip production. Review of Scientific Instruments is copyrighted by The American Institute of Physics.

Magnetic refrigeration is an emerging technology that has the potential to significantly reduce the energy consumption in the refrigeration sector. The technology relies on the heating and cooling of magnetic materials upon the application and removal of a magnetic field, respectively. This magnetocaloric effect is inherent to all magnetic materials, but manifests itself stronger in some materials. The thermodynamically reversible nature of the magnetocaloric effect holds out the promise of a more energy efficient method of refrigeration compared to conventional compressor technology. Coupling this with an absence of ozone depleting and greenhouse contributing gasses gives magnetic refrigeration the potential to become an environmentally sustainable technology. The magnetic refrigeration group at Risø DTU aims to demonstrate the technology in a prototype magnetic refrigeration device. Our work spans a wide range of scientific and technological areas. At the pure science end there is the development and understanding of new magnetocaloric materials, while the design and implementation of a prototype device along with the processing of materials is at the technological engineering end. Tying the work together are advanced numerical computer models of the individual parts of the prototype. A simple yet versatile test machine located at Risø DTU is used to test and characterise new materials and to test the design, configuration and operating conditions relevant for a prototype device, while ensuring understanding through consistency with the numerical models.

Various aspects of numerical modeling of Active Magnetic Regeneration (AMR) are presented. Using a 2-dimensional numerical model for solving the unsteady heat transfer equations for the AMR system, a range of physical effects on both idealized and non-idealized AMR are investigated. The modeled system represents a linear, parallel-plate based AMR. The idealized version of the model is able to predict the theoretical performance of AMR in terms of cooling power and temperature span. This is useful to a certain extent, but a model reproducing experiments to a higher degree is desirable. Therefore physical effects such as thermal parasitic losses have been included. Furthermore, experimentally found magnetocaloric properties are used when available, since the commonly used mean field model can be too idealized and is not always able to determine the magnetocaloric effect accurately. In the present paper preliminary conclusions on which non-ideal physical effects are thought to be dominating considering the performance of experimental AMR are given. The modeling results are compared to experimental results from the AMR test device situated at Risø DTU, Technical University of Denmark. The experimental validation shows that using the measured magnetocaloric properties significantly improves the modeling results compared to using the mean field model.