Developments in Ceramic Materials Research

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122 José M. Rojo, José L. Mesa and Teófilo Rojo has been also observed in some gadolinium and europium based compounds (S= 7/2), being attributed to the filling-up of the 2S + 1 quantum energy levels [51,52]. Preliminary studies using Mean Field Theory indicate that a shoulder superimposed onto the λ-type anomaly appears for systems with S≥ 5/2 as in the case of Fe(PO3)3. The thermal evolution of the magnetic entropy, Sm, for Cr(PO3)3 is shown in Figure 19, in comparison with that of Mo(PO3)3. The total magnetic entropy for the chromium compound was found to be 1.15 R, which agrees with the 83% of the theoretical value (1.39 R) for a S= 3/2 system. The small difference of the experimental entropy with respect to the theoretical value could originate from the procedure to calculate the lattice contribution which is approximate. Specific heat measurements of Cr2(P6O18) were not carried out due to its low magnetic ordering temperature (at the limit of our experimental temperature range). However, these ordering temperatures are accessible to neutron diffraction measurements which would allow us to determine the magnetic structure of both metaphosphates and to better understand the magnetic behavior of these compounds. The total magnetic entropy, Sm= ∫(Cm/T)dT, was found to be 1.17R and 1.43R for the Mo and Fe metaphosphates, respectively (Figure 20). These results agree with 84% of the theoretical value (1.39 R) for an S= 3/2 system and 80% for an S= 5/2 (1.79R) (see Figure 20). An amount of 0.54R (47%) of the experimental entropy of Mo(PO3)3 was gained below the ordering temperature, TN= 4 K, whereas in the case of Fe(PO3)3 the magnetic entropy reaches 74% (1.06R) of the experimental entropy at the transition point. The value obtained for the Mo phase is similar to that observed in other three-dimensional phosphate systems where the entropy normally obtained near the Néel temperature reaches values of 1/3 of the theoretical value [50]. The high value obtained for Fe(PO3)3 must be attributed to the existence of magnetic anomalies as was previously described. Sm (J/Kmol) 15 10 5 Sm teor = 1.39 R = 11.5 J/Kmol S = 3/2 Cr(PO 3 ) 3 T N Mo(PO 3 ) 3 0 0 5 10 15 20 25 30 35 T(K) Figure 19. Magnetic entropy of Cr(PO 3) 3 as a function of temperature together with that of Mo(PO 3) 3 for comparison. The horizontal solid line represents the theoretical Sm. The vertical solid line indicates the ordering temperature.
Sm (J/Kmol) Sm (J/Kmol) 15 10 5 Synthesis, Spectroscopic and Magnetic Studies… 123 Sm teor = 1.39 R = 11.5 J/Kmol S = 3/2 T N Mo(PO 3 ) 3 0 0 5 10 15 20 25 30 35 T(K) 20 15 10 5 a Sm teor = 1.79 R = 14.9 J/Kmol S = 5/2 T N Fe(PO 3 ) 3 0 0 5 10 15 20 25 30 35 T(K) Figure 20. Magnetic entropy of (a) Mo(PO 3) 3 and (b) Fe(PO 3) 3 as a function of temperature. The horizontal solid line represents the ΔS maximum. The vertical solid line indicates the ordering temperature. b The most important difference between both S= 3/2 systems, Cr and Mo, is observed at the ordering temperature where the experimental entropy gained in Cr(PO3)3 is significantly higher than that obtained in Mo(PO3)3. This is probably due to the different size of the metallic cations (see Table 3). The S(TN)/Smax ratio exhibits similar values for the Cr (S= 3/2) and Fe (S= 5/2) metaphosphates, being higher than that of the Mo (S= 3/2) phase, indicating than in those compounds the relatively value of the entropy in the ordering temperature seems to be more dependent on the cation size than on the magnetic spin.
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DEVELOPMENTS IN CERAMIC MATERIALS R
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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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54 T. T. Basiev, V. A. Demidenko, K
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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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70 Fluorescence, a.u. 1 0.1 0.01 0.
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Page 113 and 114: Synthesis, Spectroscopic and Magnet
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In: Developments in Ceramic Materia
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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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efraction index, 61 refractive inde
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time, vii, ix, 1, 3, 7, 8, 11, 26,
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