Superparamagnetic Magnetocaloric Materials
Fabrication and Investigation of Superparamagnetic Nanocomposite and Molecular Cluster Materials Suitable for Using as Working Bodies in Magnetic Cooling Devices
Tech Area / Field
- CHE-SYN/Basic and Synthetic Chemistry/Chemistry
- MAT-SYN/Materials Synthesis and Processing/Materials
- NNE-HCS/Heating and Cooling Systems/Non-Nuclear Energy
- PHY-SSP/Solid State Physics/Physics
3 Approved without Funding
Moscow State University / Department of Physics, Russia, Moscow
- VNIIKhT (Chemical Technology), Russia, Moscow
- Iowa State University / College of Engineering, USA, IA, Ames\nUniversity of Victoria / Department of Mechanical Engineering, Canada, BC, Victoria\nIPHT-Jena / Institut für Physicalishe Hochtechnologie e.v., Germany, Jena
Project summaryMagnetocaloric effect (MCE), displays itself in emitting or absorption of heat by a magnetic material under the action of a magnetic field. Under adiabatic conditions magnetic field can cause cooling or heating of the material as a result of variation of its internal energy. On the basis of MCE it is possible to create magnetic refrigerators or cryocoolers – the machines where magnetic materials are used as working bodies instead of a gas, and magnetization/demagnetization is used instead of compression/expansion in conventional refrigerators. In general case a magnetic refrigerator or cryocooler should include the following main parts: magnetic working body, magnetization system, hot and cold heat exchangers and heat transfer fluid with system providing its flow. The general operational principle of a refrigerator is as follows: the magnetic working material (working body) absorbs heat from low temperature load (cold heat exchanger) and discharges heat at high temperature sink (hot heat exchanger). As a result of cyclic repetition of this process the load is cooled. In magnetic refrigerators the working material is a magnetic material, which changes its temperature and entropy under action of a magnetic field. For magnetic refrigeration a nonregenerative Carnot cycle and also magnetic type regenerative Brayton, Ericsson and AMR (active magnetic regenerator) cycles can be used. A Carnot cycle consists of two adiabatic processes and two isothermal processes (magnetization and demagnetization). The lattice entropy of solids strongly increases above 20 K, which leads to a decrease of the Carnot cycle area and loss of its efficiency. That is why applications of Carnot-type refrigerators are optimal to the cryogenic temperature region (especially below 20 K), where lattice and electronic entropies are small and the main part of the total entropy is the magnetic entropy.
One of the main parts of magnetic cryocoolers is a magnetic working body. The task of searching of the effective working body for the necessary thermal interval is very important and many laboratories and scientific groups in the world are involved in this work. For magnetic cryocoolers working on a Carnot cycle below 20 K oxides with low magnetic ordering temperatures such as rare earth garnets (R3Ga5O12) and orthoaluminates (RAlO3) are used in present time.
In magnetic systems comprising small single domain particles or clusters consisting of a certain number of magnetic atoms (such systems are called superparamagnets) the MCE enhancement is observed, which can be shown theoretically and also was discovered in nanocomposite magnetic materials containing nanosized particles or crystallites [1-3]. One of examples of systems displaying superparamagnetic properties are materials on the basis of molecular clusters. In the last two decades molecular chemistry provided a perse variety of magnetic clusters with nanometer range sizes and high value of total spin (S = 10 – 14) and consequently the magnetic moment of the cluster. The molecular clusters are the objects with internal magnetic ordering, crystal structure and well-defined and uniform size and shape and in principle can be regarded as single domain nanosize (few nm) magnetic particles. There are many molecular clusters containing 3d transition metal atoms with total ground state spin equals 10 and higher. The total spin of the cluster is the result of internal magnetic ordering (usually antiferromagnetic in character with incomplete compensation) due to the superexchange interactions between 3d atoms via ligand ions. Among magnetic molecular cluster it is possible to mention the follows:
“Fe8” cluster, whose formula is [(tacn)6Fe8O2(OH12)]8+, where tacn is the organic ligand triazacyclononane and total spin S = 10 ; dodecanuclear manganese cluster (“Mn12ac”) of formula [Mn12O12(CH3COO)16(H2O)4] 2CH3COOH 4H2O and S = 10 ; hexanuclear Mn(III) cluster NaMn6 ([NaMn6(OMe)12(dbm)6]+, dbm = dibenzoylmethane) with S = 12 ; “Mn10” molecular clusters ([Mn10O4(biphen)4X12]4-, X = Cl-, Br-, biphen = 2,2’- biphenoxide) with total spin S = 14 . Theoretical calculations of electronic structure of Mnx (x = 2 – 8) and (MnO)x (xЈ9) clusters made by Pederson et al.  and Nayak and Jena  showed that (MnO)x clusters can have internal ferromagnetic ordering with total magnetic moments 4 – 5 mB per MnO unit. The ground states of Mn4, Mn5, Mn6, Mn7, and Mn8 clusters are ferromagnetic with moments 20, 23, 26, 29 and 32 mB, correspondingly.
In the systems containing molecular clusters one can expect enhancement of the magnetocaloric properties, as it should be observed in superparamagnets. In the work of Spichkin et al.  it was shown theoretically that the cluster superparamagnetic system should give enhancement of magnetocaloric properties, so that the magnetic entropy change (this is one of characteristics of the MCE) in such systems should essentially exceed this value in the oxide compounds using for magnetic cooling or considered as promising for this application at present time: gadolinium gallium garnet (GGG), gadolinium gallium iron garnet (GGIG) Gd3Ga3.25Fe1.75O12 and erbium orthoaluminate ErAlO3.
Magnetic clusters can be placed in various porous matrices – inorganic (Al2O3, ZrO2, zeolites, etc), polymeric and other matrices. Polymeric matrices have natural pores forming as a result of polymeric chains packing under the material synthesis. Clusters and nanoparticles can be inserted in these pores. Zeolites can also be used to create another promising magnetocaloric superparamagnetic materials, in which various magnetic metals, alloys and compounds are included into the closed zeolite’s porous. The magnetic component in this case is introduced into the zeolite matrix during its preparation. The porous in zeolite are of tens nanometers size and such novel nanocomposite material should also exhibit superparamagnetic properties with corresponding enhancement of magnetocaloric characteristics. So, molecular cluster and nanocomposite superparamagnetic materials should have parameters superior for those of oxide materials using now in this temperature range and that is why are promising for applications as magnetic working bodies in cryocoolers working in low temperature range (below 20 K). Their application can essentially increase characteristics of the magnetic cryocoolers and enhance their commercial potential.
The goal of this project is to develop the technology of fabrication of new molecular cluster and nanocomposite magnetic materials with high magnetocaloric properties, investigate and optimize their magnetic and magnetocaloric characteristics. The results of the investigations made in the frameworks of the project will allow to achieve deeper understanding of mechanisms of the magnetocaloric effect in superparamagnetic systems and will promote development of new effective magnetocaloric materials. The developed materials can be used as effective working bodies in magnetic cryocoolers and refrigerators. Magnetic cryocoolers are more effective than conventional gas cryocoolers and can be used in various fields such as gas liquefiers, high speed computers and SQUID systems cooling, cooling in laboratory investigations, etc. After completion of the project the authors are going to continue the development in order to use the obtained materials in magnetic cryocoolers prototype and create effective commercial models of the cryocoolers. The proposed project will help to: provide weapons scientists and engineers in the CIS the ability to take part in the investigations directed on creation of new materials using in energy saving technology (magnetic cooling); promote integration of scientists of CIS states into the international scientific community by solving of one of the actual scientific tasks on which many laboratories, scientific groups and research centers all over the world are working now; support applied research and development on energy saving technology of magnetic cooling; contribute to the solution of important international scientific problem – development of new effective magnetocaloric materials; reinforce the transition to market-based economies responsive to civil needs by development of highly effective technology of magnetic cooling, which obviously have great potential for commercial application.
The scientific groups involved in the project have essential experience in magnetic, magnetocaloric and structural investigations, chemical synthesis and studies. Leading scientist, doctor of science Tishin A.M. is a head of laboratory in Moscow State University conducting investigations in the field of magnetic properties of nanosize and low dimension systems and also in magnetocaloric effect and its applications. A.M. Tishin is an author of about 150 scientific papers devoted to magnetic and magnetocaloric properties of various magnetic materials and published in leading Russian and foreign scientific journals. In 2003 A.M. Tishin together with Y.I. Spichkin published in IoP Publishing (England) the book entitled “The Magnetocaloric Effect and Its Applications” , where the today state of art in magnetocaloric effect and its application for magnetic cooling is considered. The Moscow State University scientific group has in its disposal the experimental equipment necessary for conduction of magnetic and magnetocaloric effect experimental studies.
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