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Final Report: Preparation of Superferromagnetic Lanthanide Nanoparticulate Magnetic RefrigerantsEPA Grant Number: R828132
Title: Preparation of Superferromagnetic Lanthanide Nanoparticulate Magnetic Refrigerants
Investigators: Wagner, Michael J. , Bennett, Lawrence H.
Institution: George Washington University
EPA Project Officer: Karn, Barbara
Project Period: June 1, 2000 through May 31, 2003
Project Amount: $254,557
RFA: Technology for a Sustainable Environment (1999) RFA Text | Recipients Lists
Research Category: Nanotechnology , Sustainability , Pollution Prevention/Sustainable Development
The objective of this research project was to develop a magnetic nanocomposite, important to the construction of a magnetic refrigeration system, that can operate in the range of ambient atmospheric temperatures and be applied to practical consumer refrigeration systems. Magnetic nanocomposites offer important advantages over paramagnetic and ferromagnetic materials for magnetic cooling, producing large entropy changes for smaller magnetic fields, having a more uniform distribution of ΔS(T) as a function of temperature, and allowing high cycling frequency to be utilized while minimizing any eddy current losses. The magnetic properties of these materials and their ambient temperature cooling capability in a magnetic refrigerator are being tested.
The major advantages of magnetic refrigeration technology, with application to automobile air conditioners, household refrigerators, and heat pumps, are: (1) the prevention of environmental damage caused by harmful working fluids often involved in existing refrigeration systems; (2) the reduction of energy consumption resulting from the significantly improved energy efficiency inherent in the utilization of the magneto-caloric effect; and (3) potentially dramatic reductions in the complexity, size, and mass of the cooling unit. The ability of the magnetic refrigerant to transfer heat during the isothermal magnetization stage is an important advantage over conventional gas refrigerators. Nonisothermal compression is the most serious cause of inefficiency in conventional refrigerators.
Each of these advantages has potentially significant ecological benefits. The elimination of ozone-depleting gases currently employed in cooling applications is of direct benefit, whereas both improved energy efficiency and reduced mass will result in the abatement of pollution associated with fuels used to operate cooling systems and the vehicles on which they are mounted. Success of this research project will provide materials with promise for implementation of ambient temperature magnetic cooling and the consequent realization of the inherent ecological benefits.
A relatively new and unexplored method, the reduction of metal salts by alkalides and electrides, was employed to synthesize supported Gadollinium (Gd) and Dysprosium (Dy) nanocomposites and a number of alloys.
The rare earth elements Gd, Dy, Terbium, Holium, and their alloys, are considered to be among the leading candidates for use as magnetic refrigerants. Gd is the refrigerant in most magnetic refrigerators operating at room temperature today. Gd is a ferromagnetic metal with a large moment and whose Curie temperature, 293.2 ± 0.4 K, is close to ambient. Gd and its alloys and compounds exhibit the largest magnetocaloric effects known near room temperature. The theoretical magnetic entropy of Dy (23.05 J/gK) is actually larger than that of Gd (17.29 J/gK), but its magnetocaloric effect is somewhat less, although still excellent (ΔTad approximately 9K versus 11.5 K) because of its complex magnetic behavior. In fact, Dy has been suggested to be the refrigerant of choice in the temperature range of approximately 100 to 220 K for a proposed eight-stage active magnetic refrigerator.
To the principal investigator's knowledge, there have been no previous reports of the chemical synthesis of rare earth nanoparticles. Only three previous studies of Gd and Dy nanoparticles have appeared, all by gas phase methods. Chemical synthesis has a number of advantages that make it especially attractive, including simplicity, control of stoichiometry and size, narrow size distribution, and the ease of scale-up to industrial quantities. Producing rare earth nanoparticles by chemical methods, however, is technically challenging because they have very high reduction potentials, making it necessary to use a powerful reductant and carefully protect the reaction and products from oxidation.
Dr. Wagner's research team employed alkalide solutions to reduce Gd and Dy salts to nanoscale metal particles. This novel technique utilizes the alkali metal anion, Na-, as the reductant. Solvated Na- is capable of simultaneous two electron transfers and has the ability to affect fast homogeneous reductions of even the most active metal cations while rigorously protecting oxyphyllic products. To date, alkalide reduction is the only chemical method shown to be capable of making and protecting rare earth nanoscale particles.
The Dy and Gd nanoparticles synthesized by alkalide reduction exhibit paramagnetism behavior at room temperature rather than superparamagnetism or superferromagnetism. Thus, the magnitude of the magnetocaloric effect at high temperatures are low; however, the large moments of Dy and Gd yield excellent ΔSm values at low temperature. Although the materials we report here are not in large part superferromagnetic, we do see some evidence of superferromagnetic coupling in some samples of Gd. This indicates that optimization/alteration of material processing parameters may be effective in increasing the intra- and interparticle ferromagnetic interactions. The weakness of these interactions could be because of the surface composition, crystallinity, or morphology, all of which can all have dramatic effects on the magnetic behavior of nanomaterials. For instance, an oxide surface layer might mediate antiferromagnetic interactions or atomic disorder might inhibit cooperative behavior. It should be possible to increase the intra- and interparticle ferromagnetic coupling though control of these and other factors by post-synthesis processing and tune the behavior to achieve a highly effective, high temperature magnetic refrigerant.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
|Other project views:||All 6 publications||2 publications in selected types||All 2 journal articles|
||Nelson JA, Bennett LH, Wagner MJ. Dysprosium nanoparticles synthesized by alkalide reduction. Journal of Materials Chemistry 2003;13(4):857-860||
||Nelson JA, Bennett LH, Wagner MJ. Solution synthesis of gadolinium nanoparticles. Journal of the American Chemical Society 2002;124(12):2979-2983||