Sofoklis S. Makridis

High coercivity in boron substituted Sm-Co melt-spun magnets

The structural and magnetic properties of nanocomposite melt-spun Sm(Co/sub 0.74-x/Fe/sub 0.1/Cu/sub 0.12/Zr/sub 0.04/B/sub x/)/sub 7.5/ magnets have been investigated as a function of boron content (x = 0.005 0.05), wheel speed and annealing conditions. The as-spun ribbons are nanocrystalline with fine microstructure and average grain size of 60-100 nm. X-ray diffraction indicates that the as-spun samples have the metastable hexagonal TbCu/sub 7/-type structure phase and fcc-Co as a secondary soft phase. Magnetization at nonsaturating 5 T field is 45-72 emu/g and the reduced remanence (Mr/Ms) is above 0.8. The loop shape exhibits a characteristic step due to the soft magnetic phase. At room temperature, Hc values of 20-28 kOe are obtained for as spun samples, with a record value of 38.5 kOe for x = 0.04. At 380/spl deg/C Hc values higher than 5 kOe are observed. Coercivity and loop shape are strongly dependent on annealing conditions.

Effects of boron substitution on the structural and magnetic properties of melt-spun Sm(Co,Fe,Zr)7.5 and Sm(Co,Fe,Zr,Cu)7.5 magnets

In this work we examine the effect of small boron substitution (x=0, 0.005, 0.010, 0.015) on the structural and magnetic properties of Sm(Co0.86−xFe0.1Zr0.04Bx)7.5 and Sm(Co0.74−xFe0.1Cu0.12Zr0.04Bx)7.5 melt-spun samples, as a function of wheel speed and annealing conditions. Boron substituted as-spun ribbons are found to have increased coercivity, Hc>5 kOe, and small grain size of 60–100 nm. For copper containing samples, the highest coercivity (Hc=16.3 kOe) was obtained in as-spun ribbons with x=0.015. In samples without copper the coercivity increased after short annealing (Hc=12 kOe for x=0.015). The large coercivities are attributed to a fine microstructure consisting mainly of hexagonal TbCu7-type phase and a small amount of soft-phase grains.

Modeling and Simulation for Absorption- Desorption Cyclic Process on a Three-Stage Metal Hydride Hydrogen Compressor

Abstract In the current work a mathematical and a simulation study on a three – stage metal hydride hydrogen compressor (MHHC) is presented. Multistage MHHC uses a combination of different materials acting as metal hydrides in order to increase the final compression ratio, while maximizing the absorption capacity of every single stage supply pressure. The performance of a MHHC can be predicted by solving simultaneously the heat, mass and momentum differential equations. The materials used for the current study are LaNi 5 , MmNi 4.6 Al 0.4 and Ti 0.99 Zr 0.01 V 0.43 Fe 0.99 Cr 0.05 Mn 1.5 . This three – stage compression system yields a pressure ratio of 25:1, for supply conditions 20 ° C and 5 bar. The delivery pressure achieved is 115 bar for 100 ° C desorption temperature.

Structural and magnetic properties of Sm(Co0.7Fe0.1Ni0.12Zr0.04B0.04)7.5 melt spun isotropic and anisotropic ribbons

Abstract We investigated the structural and magnetic properties of Sm(Co 0.7 Fe 0.1 Ni 0.12 Zr 0.04 B 0.04 ) 7.5 melt spun ribbons. Samples were arc melted then melt spun at 37 m/s up to 55 m/s to obtain ribbon for powdering. Annealing was performed in argon atmosphere for 30–75 min at 600–870°C. In as-spun ribbons the hexagonal SmCo 7 (TbCu 7 -type of structure) of crystal structure was determined from x-ray diffraction patterns, while fcc-Co has been identified as a secondary phase. After annealing, the 1:7 phase of the as-spun ribbons transformed into 2:17 and 1:5 phases. X-ray patterns for as-milled powders exhibited very broad peaks making it difficult to identify a precise structure but represented the 1:7 structure after annealing at low temperature (650°C). TEM analysis showed a homogeneous nanocrystalline microstructure with average grain size of 30–80 nm. Coercivity values of 15–27 kOe were obtained from hysteresis loops traced up to a field of 5 T. The coercivity decreased as temperature increases, but it maintained values higher than 5 kOe at 380°C. The maximum energy product at room temperature increased, as high as 7.2 MGOe, for melt-spun isotropic ribbons produced at higher wheel speeds. Anisotropic ribbons had a maximum energy product close to 12 MGOe.

Integration of Hydrogen Energy Technologies in Autonomous Power Systems

The never-ending stories on an alternative energy supply for a cleaner environment, recently related with efforts to decrease global CO2 emissions, has been revived by the steep increase in oil prices (over 100

Two-Stage Hydrogen Compression Using Zr-Based Metal Hydrides

/barrel) and the parallel controversy about the potential and public acceptance of nuclear energy. Thus, it is now the right time for the scientific community and energy producers to synthesise their knowledge in order to achieve realistic solutions towards a cleaner energy system. Taking into account concerns that are related to environmental protection, security in the energy supply, and the utilisation of energy sources that promote the economic growth of societies, the concept of a “hydrogen economy era” is moving beyond the realm of scientists and engineers into the lexicon of political and business leaders. Interest in hydrogen, the simplest and most abundant element in the universe, is also emerging due to technical advances in fuel cells — the potential successors to batteries in portable electronics, power plants, and the internal combustion engine (Marban et al., 2007).

Effect of V Substitution on the Composite Zr-Ti-Cr-V-Ni Intermetallic Hydrides

Zr-based AB2-Laves phase type alloys containing the same type of A and B metals, have been prepared from pure elements by melting and subsequent re-melting under argon atmosphere by using a HF-induction levitation furnace. Characterization of the alloys has resulted from powder X-Ray Diffraction (XRD) measurements and SEM/EDX analyses. Systematic PCI (Pressure-Composition-Isotherms) measurements have been recorded at 20 and 90 °C, using a high-pressure Sieverts type apparatus. The purpose of this study is to find a series of alloys promptly forming metal hydrides (MH) with suitable properties in order to build a MH-based hydrogen compressor, working in the same way between 20 and ~100 °C.

Effect of wheel speed and boron content on microstructure and crystallographic texture of boron substituted Sm-Co melt spun ribbons

Hydrogen storage in reversible metal hydrides is attractive because it can be stored at relatively low pressures with a high volumetric density. In the present research work have been investigated compounds with nominal compositions Zr0.8Ti0.2Cr1.2V0.4Ni0.4 and Zr0.8Ti0.2Cr0.8V0.8Ni0.4 that have been prepared by arc-melting under argon atmosphere. Structural characteristics have been studied by using X-ray powder diffraction while the patterns have been analyzed by using the Rietveld analysis. A main hexagonal Laves phase MgZn2 (C14) and a secondary MgCu2 (C15)-type of, have been found in the powdered compounds. The bulk samples have composite microstructures but the V-rich is characterized by a dendritic microstructure. The high resolution scanning electron microscopy (HR-SEM) and energy dispersive X-ray analysis (EDAX) have been used for the morphology and quantitative analysis, respectively. Hydrogenations and dehydrogenations have been obtained after crucial activation procedure. The alloys were found to be more active under hydrogen after activation while the desorbed amount of hydrogen has been measured by using a Sievert-type apparatus.

Nanostructured melt-spun Sm(Co-Fe-Zr-B)/sub 7.5/ alloys for high temperature magnets

The Sm(Co0.71Fe0.1Cu0.12Zr0.04B0.03)7.5 (2:17 type magnet) melt spun ribbons have been produced from bulk as cast samples at low (5 m/sec) to medium (40 m/sec) wheel speed by the melt spinning technique. The crystallographic texture on wheel side, the microstructural characteristics and magnetic properties have been investigated. The soft magnetic fcc-Co forms a very high degree of texture especially at low velocities but for the first time a degree of texturing has been remarked on fcc-Co grains. Diffraction patterns have been traced by x-ray scattering using Cu-Kα radiation on the wheel and free side of the ribbons. In the pattern of ribbons which have been produced at 5 m/sec the (002) plane of fcc-Co is almost the dominant peak while at 40 m/sec this peak diminishes in parallel to the appearance of the structure type TbCu7 and (111) plane of fcc-Co structure. Scanning electron microscopy on the wheel side of the ribbons has been used to observe microstructural characteristics and showed that the formation of texture is attributed to the appearance of dendrites, with their long axis parallel to the longitudinal direction of the ribbons. Dendrites’ density depends on the wheel speed of the roller and boron content. It decreases as the velocity increases while for constant velocity of about 40 m/sec, higher boron content stabilizes higher degree of texturing. Magnetic properties are also examined from low to medium wheel speed by also using the magnetooptical Kerr microscopy. Therefore coercive field as high as 3.4 kOe and reduced remanence (mr) of ~0.76 has been detected from the hysteresis curve for as spun Sm Co0.71Fe0.1Cu0.12Zr0.04B0.03)7.5 ribbons at 5 m/sec.

Intermetallic Hydrides Based on (Zr-Ti)(Fe-Cr) 2 Type of Compounds

In this paper, we report high coercivity in as spun and annealed ribbons of Cu free Sm(Co/sub bal/Fe/sub z/Zr/sub y/B/sub x/)/sub 7.5/ samples, with varying B, Zr and Fe content (x-0.005-0.06, y=0.02-0.16, z=0.08-0.3) were studied. Also TEM studies have been used to investigate microstructure and grain size distribution. Average grain size is 30-50 nm is as spun ribbons and 70 to 100 nm for annealed ribbons. The grain distribution is very uniform in the scale of nanometers for the samples with square loop. Very good loop squareness and a maximum energy product of (BH)/sub max/=10.7 MGOe has been obtained at RT for sample with composition Sm(Co/sub bal/Fe/sub 0.3/Zr/sub 0.02/B/sub 0.04/)/sub 7.5/ after annealing at 600/spl deg/C for 75 min.