Home About us Contact
|
Home About us Contact | ||
|
|
||
Secondary Phases (secondary + phase)
Selected AbstractsText as design, writers as designersENGLISH IN EDUCATION, Issue 2 2005Ian Maun Abstract Acknowledging the changing nature of writing in the 21st century, particularly the increasing significance of visual characteristics in written texts, this paper explores the implications of multimodality for the pedagogy of writing. It considers the potential disjunction between children's life experiences of written texts and the demands of the writing curriculum, particularly in the secondary phase, and whilst arguing for greater recognition of the role of the visual, the paper also notes the importance of ensuring all children also have access to powerful verbal texts. Drawing on two separate research studies, the paper demonstrates how visual characteristics of written texts influence readers' responses to texts, but also how writers are aware of some of the choices they make in shaping verbal texts. The paper argues for a reconceptualisation of the writing process as a design process, and for a pedagogy of writing which encourages, supports and enables writers to become confident and effective designers of texts. [source] Spontaneous Outcropping of Self-Assembled Insulating Nanodots in Solution-Derived Metallic Ferromagnetic La0.7Sr0.3MnO3 FilmsADVANCED FUNCTIONAL MATERIALS, Issue 13 2009César Moreno Abstract A new mechanism is proposed for the generation of self-assembled nanodots at the surface of a film based on spontaneous outcropping of the secondary phase of a nanocomposite epitaxial film. Epitaxial self-assembled Sr,La oxide insulating nanodots are formed through this mechanism at the surface of an epitaxial metallic ferromagnetic La0.7Sr0.3MnO3 (LSMO) film grown on SrTiO3 from chemical solutions. TEM analysis reveals that, underneath the La,Sr oxide (LSO) nanodots, the film switches from the compressive out-of-plane stress component to a tensile one. It is shown that the size and concentration of the nanodots can be tuned by means of growth kinetics and through modification of the La excess in the precursor chemical solution. The driving force for the nanodot formation can be attributed to a cooperative effect involving the minimization of the elastic strain energy and a thermodynamic instability of the LSMO phase against the formation of a Ruddelsden,Popper phase Sr3Mn4O7 embedded in the film, and LSO surface nanodots. The mechanism can be described as a generalization of the classical Stranski,Krastanov growth mode involving phase separation. LSO islands induce an isotropic strain to the LSMO film underneath the island which decreases the magnetoelastic contribution to the magnetic anisotropy. [source] Microwave Dielectric Properties of Ba3(VO4)2,Mg2SiO4 Composite CeramicsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 2 2010Siqin Meng Novel Ba3(VO4)2,Mg2SiO4 microwave dielectric composite ceramics with compositions of (1,x)Ba3(VO4)2,xMg2SiO4 (x=0.50,0.65) have been prepared by firing mixtures of Ba3(VO4)2 and Mg2SiO4. X-ray diffraction and scanning electron microscopy analysis revealed that they coexist without forming secondary phases. Because their resonant frequency temperature coefficients (,f) have opposite signs, the ,f of the composite ceramic can be adjusted passing through zero by changing the relative content of the two phases. Dielectric property measurements were performed using the resonator method at around 11 GHz. Ceramics with Mg2SiO4 55 wt% sintered at 1175°C exhibited microwave dielectric properties of dielectric constant ,r=9.03, Q×f=52 500 GHz, and ,f=0.6 ppm/°C at 11.3 GHz. In this composite ceramic, we observed that the formation of the MgSiO3 secondary phase was suppressed. [source] Sintering and Microwave Dielectric Properties of the LiNb0.63Ti0.4625O3 Ceramics with the B2O3,SiO2 Liquid-Phase AdditivesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2009Yanping Long The effect of B2O3,SiO2 liquid-phase additives on the sintering, microstructure, and microwave dielectric properties of LiNb0.63Ti0.4625O3 ceramics was investigated. It was found that the sintering temperature could be lowered easily, and the densification and dielectric properties of LiNb0.63Ti0.4625O3 ceramics could be greatly improved by adding a small amount of B2O3,SiO2 solution additives. No secondary phase was observed for the ceramics with B2O3,SiO2 additives. With the addition of 0.10 wt% B2O3,SiO2, the ceramics sintered at 900°C showed favorable microwave dielectric properties with ,r=71.7, Q×f=4950 GHz, and ,f=,2.1 ppm/°C. The energy dispersive spectra analysis showed an excellent co-firing interfacial behavior between the LiNb0.63Ti0.4625O3 ceramic and the Ag electrode. It indicated that LiNb0.63Ti0.4625O3 ceramics with B2O3,SiO2 solution additives have a number of potential applications on passive integrated devices based on the low-temperature co-fired ceramics technology. [source] A New Compound with Ultra Low Dielectric Loss at Microwave FrequenciesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2009Ching-Fang Tseng A new ultra low loss microwave dielectric ceramic, Mg(Sn0.05Ti0.95)O3 (MSnT), was found and investigated. The compounds were prepared by the conventional solid-state route, and sintered at 1360°,1480°C for 2,6 h. The investigations show that the MgTi2O5 secondary phase was observed. Moreover, the dielectric properties were correlated with the formation second phase. The excellent microwave dielectric properties of Q×f=322 000 (GHz), ,r=17.4, and ,f=,54 ppm/°C were obtained from the new MSnT ceramics sintered at 1390°C for 4 h. [source] Effects of Ca/Ti Cosubstitution upon Microwave Dielectric Characteristics of CaSmAlO4 CeramicsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 2 2009Xie Cheng Fan (Ca1+xSm1,x)(Al1,xTix)O4 (0,x,0.4) ceramics were synthesized by solid-state reaction method and their microstructures and microwave dielectric properties were investigated. X-ray diffraction analysis and energy-dispersive X-ray analysis indicated that the matrix phase was a solid solution with a composition represented by the chemical formula (Ca1+xSm1,x) (Al1,xTix)O4 and minor amount of (Ca,Sm)(Al,Ti)O3 secondary phase was detected. Ca/Ti cosubstitution could significantly improve the microwave dielectric characteristics of CaSmAlO4 ceramics, and the excellent microwave dielectric characteristics were obtained in the modified ceramics as ,r=19,23, Q×f=49 100,118 700 GHz, and ,f=,15,15 ppm/°C. [source] Processing and Thermal Conductivity of Sintered Reaction-Bonded Silicon Nitride: (II) Effects of Magnesium Compound and Yttria AdditivesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2007Xinwen Zhu The effects of the magnesium compound and yttria additives on the processing, microstructure, and thermal conductivity of sintered reaction-bonded silicon (Si) nitride (SRBSN) were investigated using two additive compositions of Y2O3,MgO and Y2O3,MgSiN2, and a high-purity coarse Si powder as the starting powder. The replacement of MgO by MgSiN2 leads to the different characteristics in RBSN after complete nitridation at 1400°C for 8 h, such as a higher ,-Si3N4 content but finer ,-Si3N4 grains with a rod-like shape, different crystalline secondary phases, lower nitrided density, and coarser porous structure. The densification, ,,, phase transformation, crystalline secondary phase, and microstructure during the post-sintering were investigated in detail. For both cases, the similar microstructure observed suggests that the ,-Si3N4 nuclei in RBSN may play a dominant role in the microstructural evolution of SRBSN rather than the intergranular glassy chemistry during post-sintering. It is found that the SRBSN materials exhibit an increase in the thermal conductivity from ,110 to ,133 (Wm·K),1 for both cases with the increased time from 6 to 24 h at 1900°C, but there is almost no difference in the thermal conductivity between them, which can be explained by the similar microstructure. The present investigation reveals that as second additives, the MgO is as effective as the MgSiN2 for enhancing the thermal conductivity of SRBSN. [source] Densification of Si3N4 with LiYO2 AdditiveJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2004Branko Matovic This paper deals with the densification and phase transformation during pressureless sintering of Si3N4 with LiYO2 as the sintering additive. The dilatometric shrinkage data show that the first Li2O- rich liquid forms as low as 1250°C, resulting in a significant reduction of sintering temperature. On sintering at 1500°C the bulk density increases to more than 90% of the theoretical density with only minor phase transformation from ,-Si3N4 to ,-Si3N4 taking place. At 1600°C the secondary phase has been completely converted into a glassy phase and total conversion of ,-Si3N4 to ,-Si3N4 takes place. The grain growth is anisotropic, leading to a microstructure which has potential for enhanced fracture toughness. Li2O evaporates during sintering. Thus, the liquid phase is transient and the final material might have promising mechanical properties as well as promising high-temperature properties despite the low sintering temperature. The results show that the Li2O,Y2O3 system can provide very effective low-temperature sintering additives for silicon nitride. [source] Analytical Electron Microscopy Study of Green Ceramics Formed from Aqueous Suspensions Using the Hydrolysis-Assisted Solidification ProcessJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2002a Novak During the hydrolysis-assisted solidification (HAS) of aqueous ceramic suspensions, aluminum hydroxides are formed that bind the ceramic particles into a stiff wet body. Transmission electron microscopy investigations of HAS-processed Al2O3 and ZrO2 green parts after drying revealed that the secondary phase is amorphous and distributed uniformly around the host ceramic particles. The estimated thickness of this layer was 3,5 nm. Moreover, areas of a few tens of nanometers in size were found at three-particle junctions that contained an amorphous phase and individual nanocrystals of boehmite. [source] Reciprocative sliding friction and wear properties of electrical discharge machined ZrO2 -based compositesLUBRICATION SCIENCE, Issue 9 2009Koenraad Bonny Abstract Hot-pressed, laboratory-made, ZrO2 -based composites with 40 vol. % WC, TiCN or TiN were surface finished by electrical discharge machining in order to compare their reciprocating sliding friction and wear response against WC,6wt%Co cemented carbide in unlubricated conditions. Sliding experiments were performed using a Plint TE77 pin-on-flat wear test rig, revealing a strong impact of the secondary phase on the tribological behaviour of the ZrO2 -based composites. The worn surfaces and wear debris were characterised by scanning electron microscopy, energy dispersive X-ray analysis and surface topography scanning, pointing out abrasion, polishing and adhesion as main wear mechanisms. The most favourable friction and wear characteristics were encountered with ZrO2,WC composites compared to the other grades with equal amount of volumetric secondary phase. Copyright © 2009 John Wiley & Sons, Ltd. [source] Investigations of p-type signal for ZnO thin films grown on (100) GaAs substrates by pulsed laser depositionPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 4 2006D. J. Rogers Abstract In this work we investigated ZnO films grown on semi-insulating (100) GaAs substrates by pulsed laser deposition. Samples were studied using techniques including X-ray diffraction (XRD), scanning electron microscopy, atomic force microscopy, Raman spectroscopy, temperature dependent photoluminescence, C-V profiling and temperature dependent Hall measurements. The Hall measurements showed a clear p-type response with a relatively high mobility (,260 cm2/V s) and a carrier concentration of ,1.8 × 1019 cm,3. C-V profiling confirmed a p-type response. XRD and Raman spectroscopy indicated the presence of (0002) oriented wurtzite ZnO plus secondary phase(s) including (101) oriented Zn2As2O7. The results suggest that significant atomic mixing was occurring at the film/substrate interface for films grown at substrate temperatures of 450 ºC (without post-annealing). (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] ALTERNATIVE CURRICULUM: The integration of an alternative curriculum: Skill ForceBRITISH JOURNAL OF SPECIAL EDUCATION, Issue 3 2009Lynne Rogers The introduction of alternative curricula in the UK for students in the secondary phase is one of a number of strategies designed to improve attendance at school, reduce exclusion and improve attainment. Skill Force is a charitable youth initiative that offers 14- to 16-year-old students a key skills based vocational alternative to the traditional curriculum. In this article, Lynne Rogers, Susan Hallam and Jacquelene Shaw of the Institute of Education, University of London, and Jasmine Rhamie of the University of Southampton set out to explore the views of Skill Force instructors and team leaders, school staff and Skill Force Regional Directors. These participants perceived the critical factors in the successful integration of Skill Force to be: effective introduction of the programme to pupils and parents; careful selection of students; clear introduction of the programme to staff; integrated discipline policies; strong support from senior management; good communication; and a willingness to resolve practical difficulties. [source] Weathering and allophane neoformation in soils developed on volcanic ash in the AzoresEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 2 2007M. Gérard Summary On Faial and Pico islands (Azores), we studied two profiles on basaltic pyroclasts that contain buried horizons, and we focussed on petrography, micropedology and mineralogy. Emphasis was given to weathering of the lapilli and ashes, and the neoformation of allophane. A combination of optical studies, in situ chemical analyses, X-ray diffraction and infrared spectroscopy of clay fractions revealed that allophane is present both in the micromass of the groundmass, in alteromorphs after lapilli or pumice, and in clay coatings. Whereas most studies describe allophane as a colloidal fraction formed by the congruent and total dissolution of the ashes, this study shows evidence for the formation of allophane alteromorphs, due to leaching of Si and cations, with preservation of the original shapes of the tephra. The allophane alteromorphs often display optical characteristics that resemble those of palagonite. Increasing alteration is observed through three steps: (i) hydration of the glass associated with strong cation and Si leaching, (ii) allophane hypocoatings, and (iii) allophane alteromorphs with development of intragrain bridges. The chemical signature of the alteromorphs varies from a pure alumino-silicate at one extreme to an Fe(Ti) enriched alumino-silicate at the other. Between those two extremes, the colour grades from yellow to dark orange, with microzonations. An Al-rich allophane composition is associated with gibbsite in the EUR6-Pico profile, whereas at the base of the EUR5-Faial profile, Si-rich allophane is associated with halloysite. Some variations of the palaeo-environment are also suggested by strong iron segregation observed with various secondary phases (ferrihydrite, haematite, iddingsite). [source] Microstructure and Properties of an HfB2 -SiC Composite for Ultra High Temperature Applications,ADVANCED ENGINEERING MATERIALS, Issue 5 2004F. Monteverde An ultra-high-temperature ceramic (UHTC) based on HfB2 was produced. The microstructure consisted of fine and regular diboride grains (2 ,m average size), with SiC particulate distributed intergranularly, not rarely in clustered formation, and low levels of secondary phases were identified. The resulting thermo-mechanical properties proved interesting results for microhardness and fracture toughness. The microstructural alteration experienced within the explored temperature range renders the material unsuitable for service in extreme conditions of temperature and pressure. [source] Raman identification of natural red to yellow pigments: ochre and iron-containing oresJOURNAL OF RAMAN SPECTROSCOPY, Issue 5 2008Françoise Froment Abstract Red and yellow natural (roasted) pigments have been used for millennia. This paper reports on the Raman characterization of about 50 pigments of the Ôkhra ,Mathériauthèque' collection from different origins: violet, red, orange, apricot, yellow ochre, Umbrian and Siena earths from France (Puisaye, Ardennes, Vaucluse), Italy (Siena, Sardinia, Venice, etc.), Germany, Sweden, Cyprus and India (Madras). Comparison is made with ,Bengara' Japanese pigments. Goethite is the chromophore of yellow pigments, and haematite (sometimes with magnetite) that of red ochre and earths. Umbrian pigment (,Terre d'Ombre') contains additional manganese oxides. Color shades are related to the content of secondary phases: iron oxides (haematite, goethite, magnetite), quartz, gypsum and anhydrite, calcite, dolomite, alums (jarosites), and to the Fe/M substitution in the haematite structure. The set of secondary phases can be used to confirm a provenance signature (Supplementary Material available). Copyright © 2008 John Wiley & Sons, Ltd. [source] Nanostructured La1,xSrxGa1,yMgyO3,, Ceramics Processed by Spark Plasma Sintering of Mechanosynthesized PrecursorsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2010Alberto Moure Nanostructured ceramics with La1,xSrxGa1,yMgyO3,, composition have been processed by spark plasma sintering of precursors obtained by mechanosynthesis. The compositions with x=0.10 and y=0.20 are single-phase ceramics even at the lowest processing temperatures of 900°C. For x=0.20, y=0.17, typical secondary phases as SrLaGaO4 and SrLaGa3O7 appear at all the processing temperatures, from 900° to 1100°C. The high reactivity of the precursors due to the prolonged milling allows highly densified ceramics (>98%) to be obtained at moderate applied pressures (100 MPa), and at temperatures for which the nanostructure scale is maintained. Single-phase ceramics present higher conductivity than those with isolating secondary phases. The nanostructure appears to break the vacancies ordering, and the conductivity is produced through random vacancies paths, diminishing the activation energy and increasing the total conductivity of the ceramics. [source] Microwave Dielectric Properties of Ba3(VO4)2,Mg2SiO4 Composite CeramicsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 2 2010Siqin Meng Novel Ba3(VO4)2,Mg2SiO4 microwave dielectric composite ceramics with compositions of (1,x)Ba3(VO4)2,xMg2SiO4 (x=0.50,0.65) have been prepared by firing mixtures of Ba3(VO4)2 and Mg2SiO4. X-ray diffraction and scanning electron microscopy analysis revealed that they coexist without forming secondary phases. Because their resonant frequency temperature coefficients (,f) have opposite signs, the ,f of the composite ceramic can be adjusted passing through zero by changing the relative content of the two phases. Dielectric property measurements were performed using the resonator method at around 11 GHz. Ceramics with Mg2SiO4 55 wt% sintered at 1175°C exhibited microwave dielectric properties of dielectric constant ,r=9.03, Q×f=52 500 GHz, and ,f=0.6 ppm/°C at 11.3 GHz. In this composite ceramic, we observed that the formation of the MgSiO3 secondary phase was suppressed. [source] Al3BC3 Powder: Processing and Synthetic MechanismJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2009Sea-Hoon Lee The processing parameters for the synthesis of Al3BC3 powder were optimized, and the synthetic mechanism was investigated. The mechanical mixing of the raw powders promoted the formation of secondary phases due to mechanical alloying effect and contamination. Nearly X-ray pure Al3BC3 powder was obtained after the calcination of the raw powder mixture at 1800°C for 2 h in Ar by suppressing the vaporization of aluminum. During calcination, Al4C3 layer was formed at the surface of aluminum powder by the reaction with carbon, which maintained the morphology of the aluminum powder above its melting temperature. The nucleation and growth of Al3BC3 within aluminum melt began to occur at 1000°C, and became the main synthetic mechanism of Al3BC3 at 1100°C. The Al3BC3 particles synthesized at 1100°C were porous and were composed of fine hexagonal crystals. The main synthetic mechanism of A3BC3 changed into solid,solid reaction above 1100°C, and a gas,solid reaction promoted the densification of the porous Al3BC3 powder above 1340°C. [source] Effects of La2O3 Addition and PbO Excess on the Transmittance of PbZrO3,PbTiO3,Pb(Zn1/3Nb2/3)O3 Ceramics by Spark Plasma SinteringJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2008Yong Jun Wu The effects of La2O3 addition and PbO excess on the microstructures and optical properties of PbZrO3,PbTiO3,Pb(Zn1/3Nb2/3)O3 (PZ,PT,PZN) ceramics prepared by spark plasma sintering were investigated. When 1 mol% La2O3 was added, the highest transmittance of 35% at 700 nm for PZ,PT,PZN ceramics was obtained. The improved transmittance was attributed to the increased relative density and the decreased optical anisotropy. The samples containing more than 1 mol% La2O3 showed decreased transmittance, due to the appearance of secondary phases. The transmittance of PZ,PT,PZN ceramics increased slightly to 29% at 700 nm with increasing amount of excess PbO up to 10 mol% and thereafter decreased rapidly. [source] Thermodynamic Studies on the AlN Sintering Powders Treated With Phosphate SpeciesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2007Susana Maria Olhero The processing of aluminum nitride (AlN) ceramics in aqueous media requires the use of a surface layer to protect the surface of the particles against hydrolysis. This surface layer might influence the densification, affecting the reactions between AlN and sintering additives. The present paper describes a thermodynamic and experimental approach to evaluate the effects of a phosphate-based protecting surface layer on the densification of AlN in the presence of YF3,CaF2 as sintering aids, and to predict the densification behavior during sintering using thermodynamic assessments. Based on thermodynamic calculations and the measured weight loss of the samples during heating to sintering temperature, the chemical reactions occurring during firing were proposed. The proposed reactions were related to the experimental results as well as the final properties of the AlN samples, namely, thermal conductivity, microstructure, secondary phases, and density. [source] Processing and Thermal Conductivity of Sintered Reaction-Bonded Silicon Nitride: (II) Effects of Magnesium Compound and Yttria AdditivesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2007Xinwen Zhu The effects of the magnesium compound and yttria additives on the processing, microstructure, and thermal conductivity of sintered reaction-bonded silicon (Si) nitride (SRBSN) were investigated using two additive compositions of Y2O3,MgO and Y2O3,MgSiN2, and a high-purity coarse Si powder as the starting powder. The replacement of MgO by MgSiN2 leads to the different characteristics in RBSN after complete nitridation at 1400°C for 8 h, such as a higher ,-Si3N4 content but finer ,-Si3N4 grains with a rod-like shape, different crystalline secondary phases, lower nitrided density, and coarser porous structure. The densification, ,,, phase transformation, crystalline secondary phase, and microstructure during the post-sintering were investigated in detail. For both cases, the similar microstructure observed suggests that the ,-Si3N4 nuclei in RBSN may play a dominant role in the microstructural evolution of SRBSN rather than the intergranular glassy chemistry during post-sintering. It is found that the SRBSN materials exhibit an increase in the thermal conductivity from ,110 to ,133 (Wm·K),1 for both cases with the increased time from 6 to 24 h at 1900°C, but there is almost no difference in the thermal conductivity between them, which can be explained by the similar microstructure. The present investigation reveals that as second additives, the MgO is as effective as the MgSiN2 for enhancing the thermal conductivity of SRBSN. [source] Electronic Structure and Bonding of All Crystalline Phases in the Silica,Yttria,Silicon Nitride Phase Equilibrium DiagramJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2004Wai-Yim Ching This paper reviews the structures and properties of 10 binary, ternary, and quaternary crystals within the equilibrium phase diagram of the SiO2,Y2O3,Si3N4 system. They are binary compounds SiO2, Y2O3, Si3N4; ternary compounds Si2N2O, Y2Si2O7, and YSi2O5; and quaternary crystals Y2Si3N4O3 (M-melilite), Y4Si2O7N2, (N-YAM), YSiO2N (wallastonite), and Y10(SiO4)6N2 (N-apatite, N-APT). Although the binary compounds are well-known and extensively studied, the ternary and the quaternary crystals are not. Most of the ternary and the quaternary crystals simply have been referenced as secondary phases in the processing of nitrogen ceramics. Their crystal structures are complex and not precisely determined. In the quaternary crystals, there exists O/N disorder in that the exact atomic positions of the anions cannot be uniquely determined. It is envisioned that a variety of cation,anion bonding configurations exist in these complex crystals. The electronic structure and bonding in these crystals are, therefore, of great interest and are indispensable for a fundamental understanding of structural ceramics. We have used ab initio methods to study the structure and bonding properties of these 10 crystals. For crystals with unknown or incomplete structural information, we use an accurate total energy relaxation scheme to obtain the most likely atomic positions. Based on the theoretically modeled structures, the electronic structure and bonding in these crystals are investigated and related to various local cation,anion bonding configurations. These results are presented in the form of atom-resolved partial density of states, Mulliken effective charges, and bond order values. It is shown that Y,O and Y,N bonding are not negligible and should be a part of the discussion of the overall bonding schemes in these crystals. Spectroscopic properties in the form of complex, frequency-dependent dielectric functions, X-ray absorption near-edge structure (XANES), and the electron energy-loss near-edge structure (ELNES) spectra in these crystals also are calculated and compared. These results are discussed in the context of specific bonding configurations between cations (silicon and yttrium) and anions (oxygen and nitrogen) and their implications on intergranular thin films in polycrystalline Si3N4 containing rare-earth elements. [source] Sintering of AlN Using CaO-Al2O3 as a Sintering Additive: Chemistry and Microstructural DevelopmentJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2002Eirik Hagen The densification of aluminum nitride using Ca12Al14O33 as a sintering aid has been studied with emphasis on the effect of using coarse or fine powder, the amount of sintering aid, the sintering temperature, and embedding. Both crystalline and amorphous grain boundary phases were observed. Significant weight losses were observed for coarse-grained samples, and if suitable embedding was not used. Porous and coarse-grained ceramics with high contiguity and minor amounts of secondary phases were obtained by enhanced evaporation while dense ceramics were obtained limiting the evaporation. High weight losses in the graphite environment resulted in formation of a dense AlN surface layer. [source] Effect of Microstructure on the Thermal Conductivity of Hot-Pressed Silicon Nitride MaterialsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2002Angel De Pablos Si3N4 materials with distinct microstructures were prepared by hot-pressing, varying the holding time at the maximum temperature, and using different types and amounts of sintering additives. Materials with thermal conductivities of 15,82 W·(m·K),1 were obtained by changing the processing variables. The highest conductivity was measured for the material with the coarsest microstructure. The effect of microstructural parameters, such as percentage of secondary phases, grain size, and texture on thermal properties of Si3N4 ceramics, were studied. Hot-pressed Si3N4 ceramics were modeled as a two-phase composite made of large grains of high conductivity and a small-grained phase of low conductivity. [source] Compressive Creep Performance of SrFeO3JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2001Kjersti Kleveland Compressive creep performance of strontium-deficient and strontium-excess SrFeO3-, materials has been investigated in the temperature range 800°,1000°C and in the stress range 2.5,25 MPa. The absolute densities of the strontium-deficient and strontium-excess materials are 4.99 and 5.25 g/cm3, respectively, which corresponds to porosities of ,2% and 5%, respectively. Both materials contain secondary phases because of the cation nonstoichiometry. The creep rate is faster for the strontium-deficient material than the strontium-excess material. The stress exponent is approximately unity, and the activation energy is 260± 30 kJ/mol for both materials. The results can be explained by a cation diffusion mechanism. The present findings are discussed in relation to previous sintering data and the possible application of these materials as oxygen-permeable membranes. [source] Crystallization and Dielectric Properties of SrO,BaO,Nb2O5,SiO2 Tungsten-Bronze Glass-CeramicsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2000Jiin-Jyh Shyu The crystallization and dielectric properties of SrO,BaO,Nb2O5,SiO2 glass-ceramics have been investigated. Glass-ceramics that contain strontium barium niobate (SBN) as a primary crystalline phase, which has a tungsten bronze structure, are produced. The formation of crystalline secondary phases also has been studied. The SBN phase shows evidence of both surface nucleation and bulk nucleation, and the crystals have an average composition of Sr0.47Ba0.53Nb2O6. The dendritic morphology of the SBN crystals has been examined. The SBN content and composite dielectric constant each has been studied as a function of heating temperature/time. The highest SBN content and dielectric constant obtained in the present study are 42 vol% and 180, respectively. The dielectric constant of the glass-ceramics is determined primarily by the SBN content and the residual glass phase. The dielectric constant of the randomly oriented SBN crystal in the glass-ceramics is calculated, using dielectric mixture rules, to be ,400. [source] 36Cl, 26Al, and O isotopes in an Allende type B2 CAI: Implications for multiple secondary alteration events in the early solar systemMETEORITICS & PLANETARY SCIENCE, Issue 7-8 2007Takayuki Ushikubo CAI#2 has two distinct alteration domains: the anorthite-grossular (An-Grs) domain that mostly consists of anorthite and grossular, and the Na-rich domain that mostly consists of sodalite, anorthite, and Fe-bearing phases. Large 36S excesses (up to ,400,) corresponding to an initial 36Cl/35Cl ratio of (1.4 ± 0.3) × 10,6 were observed in sodalite of the Na-rich domain, but no resolvable 26Mg excesses were observed in anorthite and sodalite of the Na-rich domain (the initial 26Al/27Al ratio < 4.4 × 10,7). If we assume that the 36Cl- 36S and the 26Al- 26Mg systematics were closed simultaneously, the 36Cl/35Cl ratio would have to be on the order of ,10,2 when CAIs were formed. In contrast to sodalite in Na-rich domain, significant 26Mg excesses (up to ,35,) corresponding to an initial 26Al/27Al ratio of (1.2 ± 0.2) × 10,5 were identified in anorthite of the An-Grs domain. The 26Al- 26Mg systematics of secondary phases in CAI#2 suggest that CAIs experienced multiple alteration events. Some of the alteration processes occurred while 36Cl (half-life is 0.3 Myr) and 26Al (half-life is 0.72 Myr) were still alive, whereas others took place much later. Assuming that 26Al was homogeneously distributed in the solar nebula, our study implies that alteration of CAIs occurred as early as within 1.5 Myr of CAI formation and as late as 5.7 Myr after. [source] Planar deformation features and impact glass in inclusions from the Vredefort Granophyre, South AfricaMETEORITICS & PLANETARY SCIENCE, Issue 6 2002P. C. Buchanan This unit contains inclusions of country rock that were derived from different locations within the impact structure and are predominantly composed of quartzite, feldspathic quartzite, arkose, and granitic material with minor proportions of shale and epidiorite. Two of the least recrystallized inclusions contain quartz with single or multiple sets of planar deformation features. Quartz grains in other inclusions display a vermicular texture, which is reminiscent of checkerboard feldspar. Feldspars range from large, twinned crystals in some inclusions to fine-grained aggregates that apparently are the product of decomposition of larger primary crystals. In rare inclusions, a mafic mineral, probably biotite or amphibole, has been transformed to very fine-grained aggregates of secondary phases that include small euhedral crystals of Fe-rich spinel. These data indicate that inclusions within the Vredefort Granophyre were exposed to shock pressures ranging from <5 to 8,30 GPa. Many of these inclusions contain small, rounded melt pockets composed of a groundmass of devitrified or metamorphosed glass containing microlites of a variety of minerals, including K-feldspar, quartz, augite, low-Ca pyroxene, and magnetite. The composition of this devitrified glass varies from inclusion to inclusion, but is generally consistent with a mixture of quartz and feldspar with minor proportions of mafic minerals. In the case of granitoid inclusions, melt pockets commonly occur at the boundaries between feldspar and quartz grains. In metasedimentary inclusions, some of these melt pockets contain remnants of partially melted feldspar grains. These melt pockets may have formed by eutectic melting caused by inclusion of these fragments in the hot (650 to 1610 °C) impact melt that crystallized to form the Vredefort Granophyre. [source] A comparative study of La0.67Ca0.33MnO3 composites with different secondary phasesPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2009Guang Ming Ren Abstract Two kinds of composites with the nominal composition of (1 , x)La0.67Ca0.33MnO3 (LCMO)/x MgO and (1 , x)LCMO/ x SrO were investigated. Different electrical and magnetic transport properties were observed between the two kinds of composites. For the LCMO/MgO composites, the insulator,metal transition temperature (Tp) decreases rapidly with the increase of MgO content and a remarkably enhanced magnetoresistance (MR) peak is observed near Tp. However, for the LCMO/SrO composites, the SrO content has little effect on Tp and the low-field MR is enhanced over almost the whole temperature range. Moreover, a two-step magnetization behavior was observed in the LCMO/MgO composites. By calculating in terms of a ferromagnetic grain coupling model, we attribute this difference to the different effects of the secondary phases on the ferromagnetic coupling between the neighboring LCMO grains, which results from the formation of the different interfacial phases at the grain boundary in the two kinds of composites. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Effect of the substrate temperature on the properties of the RF sputtered TiO2 thin filmsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 9 2010I. Ben Mbarek Abstract Titanium dioxide (TiO2) thin films were deposited by RF magnetron sputtering on glass and silicon substrates at different substrate temperatures (20, 100, 200 and 300 °C). The structural and morphological characteristics of the films were investigated by X ray Diffractometry (XRD) and Atomic Force Microscopy (AFM) while the optical properties of the films were studied by optical spectroscopy. It was shown that at room temperature, TiO2 films grown on glass were amorphous following the substrate structure. At higher temperatures, XRD detected only a nanocrystalline rutile TiO2 structure. This indicated that the transition temperature toward the most stable TiO2 phase was obtained from 100 °C and the crystallinity was enhanced at higher growth temperature. For TiO2 films grown on Si substrates, only a nanocrystalline anatase TiO2 structure was obtained at room temperature. At higher temperatures, we noticed the appearance of other secondary phases related to rutile, anatase and brookite structures. From AFM images, we noticed that at room temperature, the films were porous. With increasing the temperature, the structure of the films became crystallized showing a columnar structure. Film growth and structural properties were discussed in terms of the Thornton model. From optical analysis, the films were transparent with an indirect band gap and a refraction index which reached 3.09 eV and 2.7, respectively. The reflectance and transmittance spectra showed, not only that there was a little translation from UVB to UVA and near-visible range, but also a decrease of reflection with a temperature increase indicating that the films could be used as anti-reflection coatings. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | |||||||||||||||||||