Developments in Ceramic Materials Research

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182 M. A. Sheik Ten sheets of the woven carbon fibre fabric are assembled between forming platens; which are then housed within a chemical vapour infiltration (CVI) chamber. The assemblage is then CVI processed, and the effect of temperature and pressure results in the fibres within each tow being embedded within a carbon matrix, created by diffusion of carbon from the fibres. The tows are, in turn, embedded within the infiltrated silicon carbide matrix. The process involves time-dependent temperature and pressure distributions within the structure of the laminate. Figure 5 schematically represents a plain weave arrangement and Figures 4 and 6 illustrate the geometric configuration and coordinate global system of this material. 3.2. Classes of Porosities Internal damage in the form of cracks and voids can be introduced either during manufacturing or in service. During manufacturing, damage is caused by a mismatch of thermo-mechanical properties of the constituent materials, thus inducing thermal gradients, thermal stresses, localised failure and hence damage when cooled. This manifests itself after cooling as micro-porosity. Additionally, damage is created in service by mechanical overloads, fatigue, time-dependent and environmental effects. As damage evolves, a limiting condition is reached when an engineering component becomes mechanically unserviceable. It then requires either repair or replacement. The dominant effect on material serviceability is that the thermal transport properties are dramatically reduced due to the evolution of damage, which can be highly directional, ultimately rendering the component thermally unserviceable due to impaired thermal efficiency. Despite the existence of a very strong coupling between mechanical behaviour and thermal properties, these relations are currently ill comprehended and are not capable of being accurately predicted at the present time. Paolo et al [16] have achieved the quantification as well as classification of initial porosity through the usage of optical electron microscopy and SEM. These techniques have been selected for their ease of use and availability; and, for their potential to cross check observations and measurements. 4 classes of porosities have been identified and schematically shown in Figure 7. Figure 7. Schematic drawing of a general plane orthogonal to either the X or the Y-direction illustrating the four classes of porosity. [Circled letters denote the porosity type].
Modeling of Thermal Transport in Ceramics Matrix Composites 183 3.2.1. Inter-Fibre Micro-Porosity (Class A) This type of porosity occurs between adjacent fibres contained within a tow. It comprises either of a series of voids or large cracks between the fibres. These can be seen within as black dots A1 or cracks parallel to fibre tows A2 in Figures 8(a)-(c) and schematically represented in Figure 7. These two kinds of porosity are mainly generated by the manufacturing process. The spheroidal pores, shown in Figure 8(b), are the result of incomplete infiltration due to shrinkage from the processing temperature which is around 1550°C for molten silicon impregnation. The shrinkage de-bonding is due to different thermal contractions of the fibre and the matrix during cooling. This generates thermal stresses across the interface, which in turn causes interface de-bonding. a b c Figure 8(a)-(c). Three different images of orthogonal planes obtained using optical microscopy. (a) Plane normal to laminate and the Z axis, (b) plane normal to X axis, and (c) plane normal to Y axis. The locations of four different classes of porosity (A, B, C, D) are indicated. 3.2.2. Trans-Tow Cracks (Class B Porosity) This class of porosity may be observed as cracks B in Figure 8(a)-(c). The porosity is comprised of cracks, which run through the tows in planes parallel to the fibres. They occur in planes which are orthogonal to X, Y and Z directions. Porosity classes A and B are also shown schematically in Figure 7. Cracks of this type are formed during manufacture as a consequence of differential cooling, coupled with the different mechanical/thermal properties in adjacent orthogonal tows.
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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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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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232 R. Ramesh, H. Kara, Ron Stevens
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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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