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  • 11
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Science Inc
    Journal of the American Ceramic Society 88 (2005), S. 0 
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The high-temperature hexagonal (h) phase has been found to retain metastability at room temperature and co-exist with the room-temperature tetragonal (t) phase in hot-pressed BaTiO3 ceramics. The crystallographic orientation relationships between two polytypes have been analyzed by TEM and selected-area diffraction patterns (SADPs). The relationships are illustrated using transformation matrices for both the forward h→t phase transformation (Mf) and the backward t→h phase transformation (Mb) based on two independent microstructures containing h-particle in t-matrix and t-particle in h-matrix, respectively. Self-consistent transformation matrices are derived from the coincident reflections in corresponding SADPs. The forward and backward matrices are inverse matrix to each other.
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  • 12
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Journal of the American Ceramic Society 87 (2004), S. 0 
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The microstructure of vitrified kaolin ceramic tapes has been studied via scanning and transmission electron microscopy (SEM and TEM). The sintered samples contained crystalline phase of predominantly stoichiometric mullite (3Al2O3·2SiO2), which consisted of high aspect ratio, acicular crystals that are often referred to as secondary mullite. These crystals were interlocked and embedded in an aluminosilicate glass matrix of inhomogeneous composition. The glass matrix contained an average of ∼3.63 wt% K as determined by energy-dispersive X-ray analysis (EDS), whose composition could be approximated to 5Al2O3·16SiO2·0.1MgO·0.3K2O·0.15TiO2·0.12Fe2O3. The acicular crystals have approximately the stoichiometric composition of Al2O3:SiO2= 3:2. They have grown along a specific crystallographic orientation along the [001] axis. The crystal growth front exhibited facetting on the {110) planes with microfacetting on both the {100) and {010) planes.
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  • 13
    Electronic Resource
    Electronic Resource
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 87 (2004), S. 0 
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Planar defects in the metastably retained h-BaTiO3 exhibiting α-fringe pattern have been characterized via transmission electron microscopy (TEM). The eligible fault vectors were determined by adopting the invisibility criteria of 2πg·R = 0 or 2nπ augmented by high-resolution imaging. Three stacking faults, F1, F2, and F3, of the extrinsic nature have been fully analyzed. The eligible fault vectors for faults F1 and F3 contained a basal component respectively of ⅓[0001] and ⅙[0001] and a common prismatic component of ⅓〈10[1-macr]0〉. However, only three of the 〈10[1-macr]0〉 vectors are the eligible prismatic component for the fault vectors RF1=⅓[0[1-macr]11], ⅓[10[1-macr]1], and ⅓[[1-macr]101], and RF3=⅙[02[2-macr]1], ⅙[2[2-macr]01], and ⅙[[2-macr]021] that have fulfilled the invisibility criteria. On the other hand, all fault vectors RF21=⅙〈[4-macr]223〉 for fault F2, containing six vectors of the 〈[2-macr]110〉 family, is eligible. Unlike the faults of πRF=⅙〈[2-macr]203〉 found in the D019 intermetallics of Ni3Sn and Co3W, neither fault F1 nor F3 is the π-rotation type. Fault F2, however, is a π-rotation fault since a 60°-rotation clockwise about [0001] has produced another eligible fault vector.
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  • 14
    Electronic Resource
    Electronic Resource
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 85 (2002), S. 0 
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Two types of superlattice reflections have been observed in pressureless-sintered Pb(Fe2/3W1/3)O3 perovskite ceramics from selected-area electron diffraction patterns (SADPs). The presence of commensurate superlattice reflections of the {1/2 1/2 1/2} type found in as-sintered samples persisted after prolonged postsintering annealing at 800°C for 300 h. These reflections, conventionally termed the F-spots, represent the existence of the nanoscale cation-ordered domains of cubic perovskite Pb1−x(Fe1/2W1/2)O3. However, the satellite-type incommensurate superlattice reflections, appearing as sideband spots to the fundamental reflections and occurring only along 〈111〉, 〈110〉, and 〈100〉, have gradually reduced their intensities and finally disappeared on postsintering annealing at 800°C. Observations via transmission electron microscopy (TEM) suggested that the “black dots” revealed in bright-field (BF) images have given rise to the incommensurate superlattice sideband spots. These dots of ∼5–10 nm are in fact a different type of nanocrystalline domain whose structure (and chemical composition) differs from those of the disordered Pb(Fe2/3W1/3)O3 matrix and the ordered nanoscale Pb1−x(Fe1/2W1/2)O3 domains. The nanocrystalline precipitates containing the paracrystalline phase are pyrochlore (Pb2(Fe,W)O6.5) formed during powder preparation via the solid-state mixed-oxide route. They become dispersed in the Pb(Fe2/3W1/3)O3 perovskite matrix grain and have also undergone phase transformation to perovskite progressively during sintering and postsintering annealing. It is suggested that the nanocrystalline pyrochlore precipitates are metastable and disappear when chemical composition within the perovskite grains becomes homogenized from solid-state diffusion on postsintering annealing.
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  • 15
    Electronic Resource
    Electronic Resource
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 85 (2002), S. 0 
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The dependence of flexural strength on crystalline phase content, residual porosity, and sintering temperature has been investigated for ceramic cores based on a mixture of fused silica and zircon. The rule of binary particle mixture, adopted for three particle sizes of fused silica, predicts a critical weight fraction of fine (zircon) particles of 46.9 wt% for the best packing. However, the optimal composition of a 37.8 wt% zircon–fused silica mixture for the flexural strength of ceramic cores found experimentally is lower in zircon content. The content of α-cristobalite +α-quartz crystallized from infilling silica sols exceeds ∼3 wt% when the strength decreases from ∼10 MPa to ∼6 MPa in samples sintered at 899–927°C. The degradation is ascribed to microcracking and the loss of coherency between the fused silica grains and the infilled silica due to β- →α-phase transformation on cooling and differential densification during sintering.
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  • 16
    Electronic Resource
    Electronic Resource
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 85 (2002), S. 0 
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: A commercial TiO2-excess BaTiO3 powder has been sintered and its microstructure analyzed for crystallographic facetting via both scanning and transmission electron microscopy (SEM and TEM). Facetted grain surfaces are developed initially from {111} at a low temperature of 1215°C, which are then altered to {111} and {100} at 1290°C in the presence of a grain-boundary liquid phase. The grain shape is also modified correspondingly from platelike to polygonal. Facetting of the intragranularly located residual pores in BaTiO3 along the {141} planes further develops on the (quasi-)equilibrium shape after annealing at 1400°C for 100 h from the initially well-characterized {111}, {110}, and {100} in as-sintered samples sintered at the same temperature for 10 h. The Wulff plots derived from the residual pores in as-sintered and annealed samples are constructed for the 〈011〉 zone. Microstructural analysis also suggests that the shape of grains and intragranular residual pores is modified progressively upon annealing. The initial solid–vapor surface energy has become less anisotropic crystallographically. Abnormal grain growth in relation to the surface energy anisotropy is discussed.
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  • 17
    Electronic Resource
    Electronic Resource
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 83 (2000), S. 0 
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Pressureless-sintered TiO2-excess BaTiO3 (BT) ceramic doped with Na2O (as an acceptor oxide) has been investigated for the inhibition of platelike (111) twin grains and the associated microstructure development. Solid-state reaction between the Na2O dopant and the excess TiO2 of the nonstoichiometric BaTiO3 powder during sintering has resulted in the formation of orthorhombic Na4TiO4(N4T) at temperatures of ≤1215°C. Enhanced densification is due to a liquid-phase sintering mechanism, where the liquid eutectic is generated by reacting the excess TiO2 and the trace impurities of SiO2 and Al2O3 with the Na2O dopant. Suppression of the abnormally large, platelike grains occurs at a critical acceptor concentration of ∼0.50 mol% Na2O when sintering is conducted at 1215°C. The inhibition mechanism is associated with the secondary phases, whose formation gradually modifies the initially TiO2-excess powder toward the stoichiometric composition. The corresponding sintered microstructure is analyzed using both scanning and transmission electron microscopy. The second-phase precipitates of Na4TiO4, which are located intragranularly and intergranularly in the tetragonal BaTiO3 matrix grains, exhibit the following crystallographic orientation relationships: [110]BT// [100]N4T, (1〈inlineGraphic alt="inline image" href="urn:x-wiley:00027820:JACE2155:JACE_2155_mu1" location="equation/JACE_2155_mu1.gif" extraInfo="missing"/〉1)BT// (010)N4T, and (112)BT// (001)N4T.
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  • 18
    Electronic Resource
    Electronic Resource
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 82 (1999), S. 0 
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Pressureless sintering studies have been conducted for excess Al2O3, stoichiometric, and excess MgO compositions of MgAl2O4 at 1500-1625°C. Initial powders of various compositions are prepared by solid-state reaction of MgO and Al2O3. A Brouwer defect equilibrium diagram is constructed that assumes intrinsic defects of the Schottky type. The densification rate derived from sintering kinetics is compared with the compositions investigated when the concentration is converted to the activity of the two oxide components in MgAl2O4. The grain-size exponent of p similar/congruent 3 suggests that densification takes place by a lattice-diffusion mechanism in the solid state. Determined activation enthalpies of 489-505 kJmol-1 are close to those obtained from oxygen self-diffusion derived in previous sintering studies. It is, therefore, proposed that oxygen lattice diffusion through vacancies is the rate-controlling mechanism for the sintering of nonstoichiometric MgAl2O4 compositions. The discrepancy between densification-rate ratios in experimental results and oxygen vacancy concentration in the Brouwer diagram is accounted for by the defect associates formed in the nonstoichiometric compositions.
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  • 19
    Electronic Resource
    Electronic Resource
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 83 (2000), S. 0 
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Low-temperature co-fired ceramics (LTCCs) that are composed of a RuO2-based resistor and a cordierite–glass substrate have been sintered at temperatures of 850° and 900°C. The microstructure of the resistor/substrate interface has been investigated using scanning and transmission electron microscopy, and its correlation to the overall resistance has been discussed. X-ray diffractometry has revealed that lead ruthenate pyrochlore (Pb2Ru2O6.5) in peak-fired thick-film resistors (TFRs) disappears and the co-fired samples contain only RuO2 in the resistor film when sintered at 900°C. The overall resistance of the LTCC resistors is increased by a factor of ∼3 when temperature is increased from 850°C to 900°C. The cordierite–glass composition of the initial substrate reacts with glass in the resistor film. The greatly extended layer of the resistor/substrate interface that contains the conductor particles is either broad or diffuse, which contrasts the abrupt interface that often is observed in conventional TFRs. This layer contains predominantly faceted platelike crystals of anorthite, in addition to other phases (such as diopside, sapphirine, and cristobalite) that apparently crystallize during co-firing as vitrification and chemical reactions between glass compositions of the substrate and the resistor occur. The increase in the resistance of the LTCC resistors is attributed to the interruption of the conducting path by platelike anorthite crystals that are produced in the resistor/substrate interface when subjected to co-firing.
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  • 20
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Establishing cellular polarity is critical for tissue organization and function. Initially discovered in the landmark genetic screen for Drosophila developmental mutants, bazooka, crumbs, shotgun and stardust mutants exhibit severe disruption in apicobasal polarity in embryonic epithelia, ...
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