Developments in Ceramic Materials Research

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136 José M. Rojo, José L. Mesa and Teófilo Rojo Figure 31. Schematic views of the exchange pathways for M(PO3) 3 (M= Mo and Fe). Exchange through one (PO 4) group only are represented for clarity. Projection of the spin orientation up (+) and down (-) in a layer for Fe(PO3) 3 is shown. In the case of the Mo phase the orientations are equal in each layer exhibiting ferromagnetic arrangements. These angles are characteristic of antiferromagnetic couplings, being in good agreement with those observed in other phosphates and oxo acids (sulphuric, molydic…) exhibiting these kinds of magnetic interactions [51,54,57]. These types of interactions are observed in other chromium salts such as α-and β-Cr(XO4) (X= P, As) in which Cr-O-Cr exchange pathways are also present increasing the force of the magnetic interactions [59]. Finally, considering the similar exchange magnetic angles in both B and C type Cr(PO3)3 metaphosphates, the differences observed in the magnetic behavior of these phases can be explained considering the different packing of both (CrO6) octahedra and (PO4) tetrahedra. In this way, the stronger packing (ρexp= 3.11 gcm -3 ) of Cr(PO3)3 [(PO4) groups disposed in chains] than that of Cr2(P6O18) in which the (PO4) tetrahedra are ordered in cycles of six (ρexp= 2.51 gcm -3 ) could be responsible for the weaker antiferromagnetic interactions observed in the latter compound as described from the magnetic measurements. On the other
Synthesis, Spectroscopic and Magnetic Studies… 137 hand, for iron and molybdenum metaphosphates, the higher spin number in Fe(III) (S= 5/2) than in Mo(III) (S= 3/2) and its magnetically isotropic behavior [41] can gives rise to new competing interactions. So, the differences observed in the magnetic behavior of the molybdenum and iron metaphosphates can be attributed to both the slight geometrical differences and the small magnetic frustration in the (MO6) octahedra together with the electronic configurations in M(III) (d 3 ) ions with respect to the Fe(III) (d 5 ) ones. The complexity of the three-dimensional arrangements, the presence of distinct exchange pathways and the existence of nine distinct phosphorous ions make it impossible to find a simple magnetic model to explain the differences in the magnetic behavior observed in these systems. 9. MAGNETOCALORIC EFFECT IN M(PO3)3 (M= FE, CR AND MO) METAPHOSPHATES The magnetocaloric effect (MCE) is the heating or the cooling of magnetic materials due to the varying magnetic field. Actually there is a renewed interest in MCE due to its possibilities of application in magnetic refrigeration [60]. In this way, the research in the area of magnetic refrigeration has been concentred in found materials with large magnetocaloric response capable of operating at different temperatures range, depending on the intended application. Hear, we presented the magnetocaloric properties of the M(PO3)3 metaphosphates (M= Fe, Cr and Mo) studied by means of heat capacity under fields up to 90 KOe. Previous studies of the magnetic properties show that this family orders antiferromagnetically at temperatures closer to the liquefaction of helium one. At zero field, the heat capacity shows a well-defined λ-type anomaly centered at 7.8, 4.2 and 4.1 K for M= Fe, Cr and Mo respectively, which is associated to the presence of long-range magnetic order. With increasing magnetic fields, initially the peak becomes broader and shifts to lower temperatures; confirming the antiferromagnetic character of the transition. However, in the Cr and Mo compounds, at higher field a broad maximum emerges above the Néel temperature indicating a transformation to ferromagnetic behavior. We have found than both the isothermal magnetic entropy (ΔS) and the adiabatic temperature change (ΔT), display a peak around the Néel temperature in the three studied materials. At low fields the MCE is negative (ΔS>0 and ΔT< 0) proving the antiferromagnetic order, however by increasing the field the MCE become positive. In particular, the stronger MCE has been found in Cr(PO3)3 which exhibits a maximum value of ΔS= -5.4 J/Kmol and ΔT= 11.5 K at μ0H= 90 KOe. These values are larger considering the small magnetic moment of the Cr(III) (S= 3/2) and show that Cr(PO3)3 is a good candidate to be used in magnetic refrigeration at low temperatures. 10. CONLUDING REMARKS The M(PO3)3 (M= Ti, V, Cr, Mo and Fe) metaphosphates have been synthesized by the ceramic method at 800 °C in air, except the titanium phase which synthesis required a inert nitrogen atmosphere. The IR spectra show bands characteristic of metaphosphates groups with chain or ring structure, this later observed for the chromium(III) hexametaphosphate.
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Copyright © 2007 by Nova Science P
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PREFACE Ceramics are refractory, in
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Preface ix crystals is discussed. A
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Preface xi spectra and the directio
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2 Leslie G. Cecil comprehensive dat
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4 Leslie G. Cecil Ringle et al. (19
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6 Leslie G. Cecil The main architec
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8 Leslie G. Cecil can study “the
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10 Leslie G. Cecil Middleton et al.
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12 Leslie G. Cecil The laser ablate
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14 Leslie G. Cecil There are two ge
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16 Leslie G. Cecil distinct recipes
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18 Leslie G. Cecil second group (Fi
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20 Leslie G. Cecil The Vitzil-Orang
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22 Leslie G. Cecil Table 3. Mahalan
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24 Leslie G. Cecil Ixlú and Ch’i
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26 Leslie G. Cecil structures (D. R
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28 Leslie G. Cecil paste and Macanc
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30 Leslie G. Cecil Bieber, A. M. Jr
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32 Leslie G. Cecil Kepecs, S. M., a
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34 Leslie G. Cecil Schele, L., Grub
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36 Z. C. Li, Z. J. Pei and C. Tread
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62 I, % 100 80 60 40 20 0 T. T. Bas
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68 Fluorescence, a.u. 1 T. T. Basie
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78 k, cm -1 20 18 16 14 12 10 8 6 4
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82 Photon counts 300 250 200 150 10
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84 Fluorescence, a.u. I transfer ,
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Modeling of Thermal Transport in Ce
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q x = Modeling of Thermal Transport
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212 R. Ramesh, H. Kara, Ron Stevens
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220 ε S 33 R. Ramesh, H. Kara, Ron
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242 Development of Display Technolo
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244 Li Chen technology moved to the
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246 Li Chen The continuing evolutio
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248 Li Chen the back contact cathod
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250 Li Chen Figure 3. Vertical side
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252 Li Chen Figure 6. Molybdenum mi
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254 Li Chen Figure 8(a). I-V curve
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256 Li Chen Figure 9. Life time tes
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258 Figure 11(a). Plasma generated
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260 Li Chen [13] C.A. Spindt, K.R.
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262 S. Ardizzone, C. L. Bianchi, G.
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A absorption spectra, 66, 67, 77, 7
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correlation function, 156 corrosion
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group membership, 13, 20, 21, 22, 2
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microstructure(s), x, xi, 73, 74, 2
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