Developments in Ceramic Materials Research

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184 M. A. Sheik 3.2.3. Matrix Cracks (Class C Porosity) This class of porosity is composed of cracks, embedded in the matrix, which surround the tow in planes perpendicular to the tow axes. C matrix cracks run circumferentially around the tows. The generation of the matrix cracks occurs during cooling of the composite due to tensile stresses generated in the matrix arising from the difference in the values of the linear coefficients of thermal expansion for the matrix and the fibre parallel to its axis. Class C porosity is schematically given in Figure 7. Figure 8(b) is a section taken in the Y-Z plane which cuts through the tow centres running in the Y direction. This is shown in Figure 10(a). Likewise, Figure 8(c) is a section taken in the X-Z plane which cuts through the silicon carbide matrix encapsulating the tows running in the X-direction. Hence it can be seen that the Class C matrix cracks run circumferentially around the tows as shown schematically in Figure 10(b). In addition, Figure 9 shows a SEM primary electron image of Class C defects in a plane normal to the X direction. Figure 9. Class C porosity (matrix cracks): SEM primary electron image of a plane normal to the X direction. Region (a) has fibres running normal to the plane; region (b) is silicon carbide matrix; and region (c) has fibres running parallel to the plane. The matrix region (b) surrounds the fibre tow (c). Two Class C periodic matrix cracks are shown in region (b) which are circumferential to the in-plane tow (c). Figure 10(a). Two orthogonal tows sectioned normal to the X-axis in a Y-Z plane cutting through the centre of the lower tow.
Modeling of Thermal Transport in Ceramics Matrix Composites 185 Figure 10(b). Two orthogonal tows sectioned normal to the Y axis in a X-Z plane cutting through the silicon carbide matrix encapsulating the lower tow. 3.2.4. Denuded Matrix Regions (Class D Porosity) This type of porosity is due to the large voids, which occur at the intersection of four orthogonal tows. This class of porosity occurs during manufacture due to difficulties in the CVI process, these are largely due to the difficulty and time required to fill the large void which occurs at the intersection of four tows. Porosity of such type is illustrated in Figure 8(a)-(c), Figures 7 and 11. Figure 11. A schematic view of a section through a unit cell showing one unit of Class D porosity (spotted region). 3.3. Quantification of Porosity Various classes of porosities for DLR-XT are quantified by utilising a porosimeter as well as detailed observations of optical micrographs. The porosimeter received its input data in the format of a digital two dimensional surface image. A software was then used to edit black and white images by the selection of a group of varying levels of greyness. Discernment between levels of greyness and porosity area fractions was calibrated with standard reference test pieces. Eventually, the result generated was an area fraction expressed as a percentage. Pertinent area fractions in connection with porosity classes A and D are quantitatively determined with the porosimeter. Optical micrographs were used in the case of porosity classes B and C. For these two classes of porosities, the length scale is that associated with the height of the tow measured in the Z-direction. This is equal to the diameter of the circle
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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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Intensity (a.u.) Intensity (a.u.) 8
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234 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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