Developments in Ceramic Materials Research

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262 S. Ardizzone, C. L. Bianchi, G. Cappelletti et al. INTRODUCTION Zircon, ZrSiO4, assumes a garnet-like structure in which guest metal ions can be incorporated to give coloured materials. Their colour is yellow, blue or pink when, respectively, praseodymium, vanadium or iron ions are introduced into the lattice [1-5]. Pigments based on zircon (ZrSiO4) are widely used in ceramic industries because they offer superior stability at high temperatures and in corrosive environments. ZrSiO4 pigments are important from an industrial point of view not only for their structural and chemical properties but also for the interesting tonalities of colour they may develop. The principal structural unit of zircon is a chain of alternating edge-sharing SiO4 tetrahedra and ZrO8 triangular dodecahedra, characterized by the presence of empty octahedric cavities (Figure 1). The metal dopant can be accommodated into the zircon network in interstitial positions, it can substitute either Zr 4+ or Si 4+ in their lattice positions or it can form an encapsulated phase. In several cases controversial conclusions are reached in the literature about the actual location of the metal ion and its valence in the zircon lattice [2- 16]. In the case of Pr-doped materials, the cations are generally assumed to form a solid solution with the zircon lattice but the actual valence of the dopant (either +3 or +4) has not been definitely established yet. The position occupied by Pr ions has not been clarified, either. In general Pr ions are considered to be located at the triangular dodecahedral positions [3,6,14,15] of Zr 4+ , however Shoyama et al. [14] suggest that Pr(IV) may substitute for both Zr 4+ and Si 4+ of the zircon lattice. In the case of Pr-zircon pigments prepared by a sol-gel procedure [17] we have observed that Pr ions, besides being distributed into the host matrix, were present also in separate crystalline phases. Figure 1. Zircon (ZrSiO 4) crystal lattice structure.
Coloured ZrSiO4 Ceramic Pigments 263 In fact separate Pr containing phases (either encapsulated into the host lattice or not) were appreciable in X-ray spectra in the case of the larger Pr amounts and EDX analyses showed, at the same time, a non homogeneous distribution of the metal. For V-ZrSiO4 pigments the actual location of vanadium in the zircon structure is still unclear and has been the subject of many studies, in some cases leading to controversial interpretations. Several authors suggest that V ions should substitute for Zr in the dodecahedral lattice positions [18,19]. Exactly the opposite conclusion was reached by Di Gregorio and de Waal [20,21] that indicated V 4+ preferred the tetrahedral silicon site. A further possibility some workers have preferred is that both sites can be occupied to a significant extent [22,23]: Xiayou [22] and Chandley [23] have, in fact, reported very little difference in the energy of the V 4+ occupying either of the two principal lattice sites. In the case of V-ZrSiO4 prepared in our laboratory by a sol-gel procedure in the absence of mineralizers [24] we have observed, also on the grounds of the elaboration of XANES results, that for vanadium amounts lower than 0.05, the metal is prevailingly substituted to Si in the tetrahedral positions of the lattice, while for loadings larger than 0.05, additional V is only localized in the Zr 4+ dodecahedral positions of the lattice. In the case of iron, Tartaj et al. [4] suggest that the formation of a solid solution between the iron cations and the tetragonal zirconia lattice occurs before the formation of zircon. This process generates vacancies which favour the nucleation of zircon. The authors [4] suggest that after the zircon formation iron (III) segregates with the ensuing formation of hematite and that hematite is the only responsible of the red colour of the iron zircon pigment. The same authors have observed by XPS that the surface Fe/ZrSiO4 ratio is comparable to the average iron content of the sample indicating that hematite is homogeneously distributed in the zircon matrix. In the case of Fe-ZrSiO4 samples prepared in our laboratory by a sol-gel procedure [25] we have instead observed, by quantitative elaboration of diffuse reflectance spectra, the concomitant presence of both interstitial iron and hematite, in partial agreement with what reported by Berry et al. [7,8]. The presence in the literature of divergent results concerning various aspects of these systems is, at least in part, due to the manifold features of these materials, and to the close interplay between different parameters introduced by the preparative steps. In fact the adopted preparative route plays, in the present authors opinion, a key role in defining the final features of the pigments. Consistently with these considerations, Monros et al. [5] have recently shown that, in the case of coral-red zircon pigments, the conditions adopted for the sol-gel process and particularly the sequence of the iron addition to the reacting mixture lead to the formation of samples with different phase composition and colour. In this work Pr-, V-, and Fe-doped ZrSiO4 pigments were obtained by a common sol-gel path. The reaction, starting from Si and Zr alkoxides, in water-ethanol mixtures, was employed to obtain the xerogels which were thermally treated in the temperature range 600- 1200°C. The role played by the addition of different mineralizers (LiF, LiCl, NaF, NaCl, KF, KCl) to the reaction mixture was investigated in the case of V-doped pigments which develop the desired blue colour only in the presence of an added salt. X-ray diffraction data were refined by the Rietveld method and UV-Vis diffuse reflectance spectra were recorded for all samples. XPS analyses were obtained in the case of V-doped pigments to obtain indications on the localization of both mineralizer and metal dopant.
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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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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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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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86 ln(I transfer (t)) -4.2 -4.4 -4.
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88 Fluorescence, a.u. I transfer ,
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In: Developments in Ceramic Materia
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Synthesis, Spectroscopic and Magnet
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a c Synthesis, Spectroscopic and Ma
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Transmission Transmission Transmiss
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Sm (J/Kmol) Sm (J/Kmol) 15 10 5 Syn
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The Use of Ceramic Pots in Old Wors
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3.2. Wall-in Positions The Use of C
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IACC ( τ ) = t2 ∫ The Use of Cer
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D50 0.8 0.7 0.6 0.5 0.4 0.3 The Use
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Definition (D50) 1 0.95 0.9 0.85 0.
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Modeling of Thermal Transport in Ce
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q x = Modeling of Thermal Transport
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Page 226 and 227: 212 R. Ramesh, H. Kara, Ron Stevens
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Page 260 and 261: 246 Li Chen The continuing evolutio
Page 262 and 263: 248 Li Chen the back contact cathod
Page 264 and 265: 250 Li Chen Figure 3. Vertical side
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Page 268 and 269: 254 Li Chen Figure 8(a). I-V curve
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Page 274 and 275: 260 Li Chen [13] C.A. Spindt, K.R.
Page 278 and 279: 264 S. Ardizzone, C. L. Bianchi, G.
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Page 305 and 306: time, vii, ix, 1, 3, 7, 8, 11, 26,
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