Distribution by Scientific Domains
Distribution within Polymers and Materials Science

Terms modified by Densification

  • densification behavior
  • densification process

  • Selected Abstracts

    Microstructural Bases for the Superior Densification of Gels doped with Alumina Nanoseeds

    J. Tartaj
    The densification behavior of alumina gels has been studied. We have found that in addition to the effect of volume transformed, the thermal history of gels plays an important role in their densification process. Particularly, prior to the ,- to , transformation, the nanoseeded gels preserve a close packed and evenly distributed porous structure that helps to achieve full density at lower temperatures. [source]

    Densification of Oxide Nanoparticle Thin Films by Irradiation with Visible Light

    Massimo F. Bertino
    Abstract A technique is presented that allows for altering of the physical characteristics of films of TiO2 nanoparticles by exposure to visible light. In this technique, dye-sensitized oxide nanoparticles are deposited on a substrate by dip-coating. Photodissociation of the organic ligand layer leads to cross-linking of the nanoparticles. Consequently, irradiated films have a decreased porosity, an increased index of refraction and an increased hydrophobicity. Films irradiated with green light are compared to films irradiated with UV light. Within experimental error, visible- and UV-illumination induces the same changes in the films. The mechanism of surfactant elimination in dye-sensitized oxide particles is discussed, patterning is demonstrated, and prospective applications of the technique are considered. [source]

    Synthesis, Densification, and Phase Evolution Studies of Al2O3,Al2TiO5,TiO2 Nanocomposites and Measurement of Their Electrical Properties

    Vikas Somani
    Alumina,aluminum titanate,titania (Al2O3,Al2TiO5,TiO2) nanocomposites were synthesized using alkoxide precursor solutions. Thermal analysis provided information on phase evolution from the as-synthesized gel with an increase in temperature. Calcination at 700C led to the formation of an Al2O3,TiO2 nanocomposite, while at a higher temperature (1300C) an Al2O3,Al2TiO5,TiO2 nanocomposite was formed. The nanocomposites were uniaxially compacted and sintered in a pressureless environment in air to study the densification behavior, grain growth, and phase evolution. The effects of nanosize particles on the crystal structure and densification of the nanocomposite have been discussed. The sintered nanocomposite structures were also characterized for dielectric properties. [source]

    Densification of the PLZT Films Derived from Polymer-Modified Solution by Tailoring Annealing Conditions

    Z. H. Du
    The Pb0.91La0.09(Zr0.65,Ti0.35)0.9775O3 films were prepared from the solution containing polyvinylpyrrolidone, with Mw of 360 000. A fast heating rate of 40C/min was used to heat our samples to temperatures in the range of 550,750C for different durations. Both a rapid heating rate and isothermal holding at a proper temperature are found to be essential for obtaining dense and crack-free PLZT films. The TG-DTA and Fourier-transform infrared results indicated that the fast heating rate could keep a certain amount of polyvinylpyrrolidone residues undecomposed to a higher temperature (<700C), which was found to be beneficial for the densification of the films during subsequent isothermal hold. The possible mechanisms responsible for the enhanced densification have been discussed. In addition, the effects of annealing temperature on the electrical and optical properties were also investigated. [source]

    Normal Sintering of (K,Na)NbO3 -Based Ceramics: Influence of Sintering Temperature on Densification, Microstructure, and Electrical Properties

    Yuhua Zhen
    Normal sintering of Li-doped and Li/Ta-codoped potassium sodium niobate (KNbO3,NaNbO3, KNN)-based ceramics was investigated to clarify the optimal sintering condition for densification, microstructure, and electrical properties. It was found that density increased greatly within a narrow temperature range but tended to decrease when the sintering temperature slightly exceeded the optimal one, accompanied by the appearance of abnormal grain growth, which was considered to be due to the intensified volatilization of alkali metal oxides. Piezoelectric and dielectric properties also showed a similar relationship between the density and sintering temperature, but the highest piezoelectric strain coefficients were obtained at the temperatures lower than that for the highest density, because both densification and composition affect the electrical properties. The highest d33 value of 206 pC/N was obtained for the Li- and Ta-codoped KNN ceramics prepared at 1090C. [source]

    Effect of Varying Quartz Particle Size and Firing Atmosphere on Densification of Brazilian Clay-Based Stoneware

    G. P. Souza
    Body mixes for stoneware containing a Brazilian red clay, nepheline syenite, and quartz with two different median particle sizes (,2 and 18 ,m) were fired under oxidizing and reducing atmospheres. The densification behavior was followed by dilatometry simulating the firing schedule, as well as by water absorption, linear shrinkage, and bulk density measurements on as-fired specimens. It was revealed that finer quartz led to interconnected pore closure at 1125C when fired under an oxidizing atmosphere. Densification was systematically related to the uniformity of the compacts in the unfired state. Phase and microstructural examination by X-ray diffractometry, scanning electron microscopy/energy-dispersive spectroscopy (EDS), and transmission electron microscopy/EDS showed that mullite developed more extensively in the stonewares fired in an oxidizing atmosphere, along with cristobalite and haematite, whereas metallic iron was found in stonewares fired under a reducing atmosphere. [source]

    Pressureless Densification of Zirconium Diboride with Boron Carbide Additions

    S. C. Zhang
    Zirconium diboride (ZrB2) was densified (>98% relative density) at temperatures as low as 1850C by pressureless sintering. Sintering was activated by removing oxide impurities (B2O3 and ZrO2) from particle surfaces. Boron oxide had a high vapor pressure and was removed during heating under a mild vacuum (,150 mTorr). Zirconia was more persistent and had to be removed by chemical reaction. Both WC and B4C were evaluated as additives to facilitate the removal of ZrO2. Reactions were proposed based on thermodynamic analysis and then confirmed by X-ray diffraction analysis of reacted powder mixtures. After the preliminary powder studies, densification was studied using either as-received ZrB2 (surface area ,1 m2/g) or attrition-milled ZrB2 (surface area ,7.5 m2/g) with WC and/or B4C as a sintering aid. ZrB2 containing only WC could be sintered to ,95% relative density in 4 h at 2050C under vacuum. In contrast, the addition of B4C allowed for sintering to >98% relative density in 1 h at 1850C under vacuum. [source]

    Simultaneous Spark Plasma Synthesis and Densification of TiC,TiB2 Composites

    Antonio M. Locci
    The simultaneous synthesis and densification of dense TiC,TiB2 composite has been investigated starting from Ti, B4C, and C as reactants, and using the spark plasma sintering (SPS) technique. The optimal conditions for complete conversion of the reactants to the composite were determined for different applied DC current levels. A kinetic investigation performed allows us to conclude that solid-state diffusion is the mechanism governing the synthesis process. It is seen that TiC is the first phase formed, while TiB2 formation occurs afterward. Two intermediate boride phases, i.e. TiB and Ti3B4, are also formed but, as the SPS holding time was augmented, they were gradually and completely converted to TiB2. Moreover, it is found that in order to reach relatively high dense products, an electric current needs to be applied for time intervals longer than those required for obtaining complete conversion. A pure dense product (relative density ,98%) was obtained when an electric pulsed current of 1100 A and a mechanical pressure of 20 MPa were applied for about 4 min. [source]

    Densification of Si3N4 with LiYO2 Additive

    Branko 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 1250C, resulting in a significant reduction of sintering temperature. On sintering at 1500C the bulk density increases to more than 90% of the theoretical density with only minor phase transformation from ,-Si3N4 to ,-Si3N4 taking place. At 1600C 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]

    Effect of Yttria and Yttrium-Aluminum Garnet on Densification and Grain Growth of Alumina At 1200,1300C

    Michael K. Cinibulk
    Densification and grain growth of alumina were studied with yttria or yttrium-aluminum garnet (YAG) additives at the relatively low temperatures of 1200,1300C. Yttria doping was found to inhibit densification and grain growth of alumina at 1200C and, depending on dopant level, had a lesser effect at 1300C. At 1200C, yttria inhibits densification more than it hinders grain growth. The rate of grain growth increases faster with temperature than the rate of densification. Alumina-YAG particulate composites were difficult to sinter, yielding relative densities of only 65% and 72% after 100 h at 1200 and 1300C, respectively. Pure YAG compacts exhibited essentially no densification for times up to 100 h at 1300C. [source]

    Large Photoinduced Densification in Organically Modified Germanosilicate Glasses

    Jae Hyeok Jang
    Organically modified germanosilicate (ORMOGSIL) glasses prepared by a sol-gel method showed a large refractive index change on ultraviolet exposure. The large photoinduced refractive index change in the ORMOGSIL glasses is mainly due to the structural densification caused by ultraviolet irradiation. The shifts in frequency of the Raman bands measured at room temperature reveal structural densification by reduction of the average intertetrahedral bonding angle , in the ORMOGSIL glasses. Surface relief patterns by photoinduced densification were directly inscribed on the ORMOGSIL glasses. [source]

    Effect of Rigid Inclusions on the Densification and Constitutive Parameters of Liquid-Phase-Sintered YBa2Cu3O6+x Powder Compacts

    Samuel M. Salamone
    The presence of rigid inclusions in a powder compact leads to a reduction in the densification rate of the compact and may also lead to processing defects. In this paper, the densification rate and the constitutive parameters of both homogeneous YBa2Cu3O6+x and composite powder compacts (YBa2Cu3O6+x powder with 10 vol% dense inclusions of YBa2Cu3O6+x) are reported. A small amount of liquid phase, which formed during sintering, was present in the samples. However, even with the presence of a liquid phase, the addition of inclusions still reduces the densification rate of the composite and increases its viscosity. The results have been compared with a published analysis of the problem using measured values of the constitutive parameters. Both the viscosity and viscous Poisson's ratio of the porous body have been measured. [source]

    Simultaneous Synthesis and Densification of Titanium Nitride/ Titanium Diboride Composites by High Nitrogen Pressure Combustion

    Masachika Shibuya
    Composites of TiN/TiB2 were synthesized by a combustion process of BN, Ti in a nitrogen atmosphere. The effect of the BN/Ti ratio and the nitrogen gas pressure on the synthesis of these composites was investigated. Dense TiN/TiB2 composites with relatively high hardness and toughness were fabricated by combustion synthesis from Ti and BN under a nitrogen pressure of 4.0 MPa. The Vickers microhardness of the products obtained from reactants with a BN/Ti mole ratio of 0.11 increased with an increase in nitrogen pressure and had a maximum value of ,25 GPa. Fracture toughness, KIC, of the products increased from 3.1 to 5.9 MPam1/2 as the BN/Ti ratio increased from 0.11 to 0.20. However, products formed under nitrogen pressures higher than 6.0 MPa exhibited circumferential macrocracks due to thermal shock. [source]

    Effect of Nitrogen Atmosphere on the Densification of a 3-mol%-Yttria-Doped Zirconia

    Yeong-Kyeun Paek
    The densification behavior of a 3-mol%-Y2O3 -doped ZrO2 (3Y-ZrO2) has been investigated under N2 and O2 atmospheres. Powder compacts have been sintered at 1550 and 1400C for various times. The density of the specimen sintered at 1550C is higher in N2 than in O2, while the contrary result is obtained in the case of the specimen sintered at 1400C. Such results can be explained in terms of nitrogen solubility and oxygen vacancy in a ZrO2 matrix. Because nitrogen solubility into the ZrO2 increases with an increase in heat-treatment temperature, leading to the formation of oxygen vacancy, the densification rate becomes higher. The present study thus shows evidence of nitrogen solubility into the ZrO2 and its role on the densification behavior of 3Y-ZrO2. [source]

    Sintering and Microstructure Modification of Mullite/Zirconia Composites Derived from Silica-Coated Alumina Powders

    Viktor Yaroshenko
    This paper addresses the densification and microstructure development during firing of mullite/zirconia composites made from silica-coated-alumina (SCA) microcomposite powders. Densification occurs in two stages: in the presence of a silica,alumina mixture and after conversion to mullite. The first stage of densification occurs through transient viscous phase sintering (TVS). This is best promoted by rapid heating, which delays the crystallization of silica to higher temperatures. A further sintering stage is observed following mullitization. The introduction of seeds promotes solid-state sintering, most probably due to refinement of the mullite matrix. For seed concentrations up to about 1% the sintering kinetics depend on seed concentration. This suggests that nucleation still remains the rate-controlling mullitization step. Above this concentration the reaction becomes growth controlled. Introduction of seeds also promotes direct mullitization without transient zircon formation that was observed in a previous study of the same process without seeding. Seeding also promotes the development of elongated grains by way of a solid-state recrystallization process. [source]

    Microwave Hybrid Post-Heat Treatment of Reaction Sintered Alumina/Lanthanum Hexaaluminate Composite Ceramics,

    Zahra Negahdari
    One of the main problems in development of in situ reaction sintered alumina/lanthanum hexaaluminate composite ceramics is achievement of simultaneous densification and in situ formation of lanthanum hexaaluminate (LHA) platelets inside the matrix. Microwave hybrid post-sintering was investigated as a method to enhance the solid-state reaction of LHA formation and the densification of composite ceramics with 2.8,80 vol% LHA. Comparison of the conventionally and microwave assisted sintered alumina/lanthanum hexaaluminate composite ceramics revealed that utilization of microwave heating in second stage of sintering could enhance the solid-state reaction, the densification, and the anisotropic grain growth of the LHA platelets in ceramics containing more than 20 vol% LHA and for heat treatment at 1500,C. [source]

    Beneficial Effects of AlN as Sintering Aid on Microstructure and Mechanical Properties of Hot-pressed ZrB2,

    F. Monteverde
    Higher density of ZrB2 ceramics than with the pure material is achieved when 4.6,% of aluminum nitride are added before hot-pressing as a sintering aid. AlN supports densification and prevents grain coarsening, mainly by virtue of its ability to remove the boron oxide layer that otherwise covers ZrB2 particles. The new material (see Figure for an SEM image of a polished section) has outstanding mechanical properties, e.g. strength values of 600 and 200 MPa at 25 and 1500,C. [source]

    The effects of water table draw-down (as a surrogate for climate change) on the hydrology of a fen peatland, Canada

    Peter N. Whittington
    Abstract Hydrological response to climate change may alter the biogeochemical role that peatlands play in the global climate system, so an understanding of the nature and magnitude of this response is important. In 2002, the water table in a fen peatland near Quebec City was lowered by ,20 cm (Experimental site), and hydrological response was measured compared to Control (no manipulation) and Drained (previously drained c. 1994) sites. Because of the draw-down, the surface in the Experimental pool decreased 5, 15 and 20 cm in the ridge, lawn and mat, respectively, increasing bulk density by ,60% in the Experimental lawn. Hydraulic conductivity (K) generally decreased with depth and from Control (25,125 cm) 10,1 to 10,5 cm s,1 to Experimental (25,125 cm) 10,2 to 10,7 cm s,1 and to Drained (25,75 cm) 10,2 to 10,6 cm s,1. In similar topographic locations (ridge, lawn, mat), K trended Control > Experimental > Drained, usually by an order of magnitude at similar depths in similar topographic locations. Water table fluctuations in the Drained site averaged twice those of the Control site. The water table in the Control lawn remained at a stable depth relative to the surface (,, 1 cm) because the lawn peat floats with changes in water table position. However, the Drained lawn peat was more rigid because of the denser degraded peat, forcing the water to fluctuate relative to the surface and further enhancing peat decay and densification. This provides a positive feedback loop that could intensify further peat degradation, changing the carbon cycling dynamics. Copyright 2006 John Wiley & Sons, Ltd. [source]

    Habitat influences on urban avian assemblages

    IBIS, Issue 1 2009
    Urbanization is increasing across the globe and there is growing interest in urban ecology and a recognition that developed areas may be important for conservation. We review the factors influencing urban avian assemblages, focusing on habitat type and anthropogenic resource provision, and analyse data from a common bird monitoring scheme to assess some of these issues. The review suggests that (1) local factors are more important than regional ones in determining the species richness of urban avian assemblages, raising the potential for the management of urban sites to deliver conservation; (2) habitat fragmentation frequently influences urban avian assemblages, with the effects of patch size being greater than those of isolation, and (3) urban bird assemblages appear to respond positively to increasing the structural complexity, species richness of woody vegetation and supplementary feeding, and negatively to human disturbance. Data from Britain's Breeding Bird Survey, combined with habitat data obtained from aerial photographs, were used to assess a number of these issues at the resolution of 1-km squares. Green-space constituted 45% of these squares, and domestic gardens contributed 50% of this green-space, though their contribution to large continuous patches of green-space was negligible. There was no significant positive correlation between the densities of individual species in urban areas and surrounding rural areas. Rural species richness declined with increasing latitude, but urban species richness was not correlated with latitude. This contrast contributes to slightly higher avian species richness in rural squares in Southern England than urban ones. Occupancy and abundance were strongly positively correlated in urban avian assemblages, and some indicator species of conservation concern occurred in few urban areas and at low densities. Such species will require conservation action to be precisely targeted within urban areas. Of the urban indicators of conservation concern, only the House Sparrow Passer domesticus and Common Starling Sturnus vulgaris were more abundant in urban than rural areas. Moreover, the densities of these two species were strongly and positively correlated, indicating that they may be limited by shared resources, such as nest-sites or supplementary food. There was little evidence that high densities of nest-predating corvids were associated with reduced densities of their prey species. Species richness and the densities of individual species frequently declined with an increasing number of buildings. Current trends for the densification of many British urban areas are thus likely to be detrimental for many bird species. [source]

    A Screening Design Approach for the Understanding of Spark Plasma Sintering Parameters: A Case of Translucent Polycrystalline Undoped Alumina

    Yann Aman
    An experimental screening design was used to evaluate the effects of spark plasma sintering (SPS) parameters such as heating rate, sintering temperature, dwell duration, and green-shaping processing on the relative density, grain size, and the optical properties of polycrystalline alumina (PCA). It is shown that heating rate and sintering temperature are the most critical factors for the densification of PCA during SPS. Green-shaping processing could prevent grain growth at low SPS sintering temperatures. No predominant SPS parameters are observed on the optical properties. Hence, the optical properties of PCA are controlled by microstructural evolution during the SPS process. [source]

    THIS ARTICLE HAS BEEN RETRACTED Effect of Silica Sol on the Properties of Alumina-Based Duplex Ceramic Cores

    Yexia Qin
    A series of alumina-based ceramic cores sintered at 1300C, 1400C, and 1500C for 5 h were prepared, and the phases and microstructures were characterized by X-ray diffraction and scanning electron microscopy. The effect of colloidal silica sols on the properties of ceramic core was discussed. The properties of these materials were determined. The results indicated that the microstructure of the core is characterized by the presence of substantially unreacted Al2O3 particles having a polycrystalline composition consisting essentially of in situ synthesized 3Al2O32SiO2 on the surface of the Al2O3 particles. The colloidal silica sol contents do not have an appreciable effect on the densification and shrinkage of the alumina ceramic core. The ceramic cores of 5 wt% colloidal silica sol contents sintered at 1500C for 5 h showed the smallest creep deformation in the present research. [source]

    Structural characterization of undoped and Sb-doped SnO2 thin films fired at different temperatures

    Alessandro P. Rizzato
    SnO2 thin films were obtained by the sol-gel method starting from inorganic precursor solutions. In this work, we compare the structure of undoped and Sb-doped SnO2 films prepared by dip-coating. The films were deposited on quartz substrates and then fired at different temperatures ranging from 383 up to 1173 K. The density and the thickness of the films were determined by X-ray reflectivity (XRR) and their porous nanostructure was characterized by grazing-incidence small angle X-ray scattering (GISAXS). XRR results corresponding to undoped and Sb-doped samples indicate a monotonous decrease in film thickness when they are fired at increasing temperatures. At same time, the apparent density of undoped samples exhibits a progressive increase while for Sb-doped films it remains invariant up to 973 K and then increases for T = 1173 K. Anisotropic GISAXS patterns of both films, Sb-doped and undoped, fired above 573 K indicate the presence of elongated pores with their major axis perpendicular to the film surface. For all firing temperatures the nanopores in doped samples are larger than in undoped ones. This suggests that Sb-doping favours the pore growth hindering the film densification. At the highest firing temperature (1173 K) this effect is reversed. [source]

    Compressive response and energy absorption of foam EPDM

    Biqin Wang
    Abstract Ethylene,propylene,diene terpolymer foam was prepared by two different processing routes. The microstructure and mechanical properties of the foams with wide relative density ranging from 0.11 to 0.62 have been studied via scanning electron microscopy and mechanical testing, respectively. Scanning electron microscopy shows that the foam with lower relative density has a unique bimodal cell size structure, which the larger cells inlay among the smaller cells, while the foam articles with higher relative density have thicker cell walls with few small cells. The compressive stress,strain curves show that the foam articles with lower relative density have three regimes: linear elastic, a wide slightly rising plateau, and densification, while the foam articles with higher relative density have only two regimes: the longer linear elastic and densification. The relative modulus increases with the increase in the relative density. The contribution of the gas trapped in the cell to the modulus could be neglected. The energy absorbed per unit volume is relationship with the permitted stress and the relative density. The efficiency and the ideality parameter were evaluated from the compressive stress,strain plots. The parameters were plotted against stress to obtain maximum efficiency and the maximum ideality region, which can be used for optimizing the choice for practical applications in cushioning and packaging. 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007 [source]

    Micro-computed tomography evaluation of vertebral end-plate trabecular bone changes in a porcine asymmetric vertebral tether

    Jean-Michel Laffosse
    Abstract We conducted a micro-CT analysis of subchondral bone of the vertebral end-plates after application of compressive stress. Thoracic and lumbar vertebral units were instrumented by carrying out left asymmetric tether in eleven 4-week-old pigs. After 3 months of growth, instrumented units and control units were harvested. Micro-CT study of subchondral bone was performed on one central and two lateral specimens (fixated side and non-fixated side). In control units, bone volume fraction (BV/TV), number of trabeculae (Tb.N), trabecular thickness (Tb.Th), and degree of anisotropy (DA) were significantly higher, whereas intertrabecular space (Tb.Sp) was significantly lower in center than in periphery. No significant difference between the fixated and non-fixated sides was found. In instrumented units, BV/TV, Tb.N, Tb.Th, and DA were significantly higher in center than in periphery. BV/TV, Tb.N, and Conn.D were significantly higher in fixated than in non-fixated side, while Tb.Sp was significantly lower. We noted BV/TV, Tb.N, and Tb.Th significantly lower, and Tb.Sp significantly higher, in the instrumented levels. This study showed, in instrumented units, two opposing processes generating a reorganization of the trabecular network. First, an osteolytic process (decrease in BV/TV, Tb.N, Tb.Th) by stress-shielding, greater in center and on non-fixated side. Second, an osteogenic process (higher BV/TV, Tb.N, Conn.D, and lower Tb.Sp) due to the compressive loading induced by growth on the fixated side. This study demonstrates the densification of the trabecular bone tissue of the vertebral end-plates after compressive loading, and illustrates the potential risks of excessively rigid spinal instrumentation which may induce premature osteopenia. 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 28:232,240, 2010 [source]

    In situ Raman scattering studies of high-pressure stability and transformations in the matrix of a nanostructured glass,ceramic composite

    Kristina E. Lipinska-Kalita
    Abstract High-pressure Raman scattering studies have been performed on a glass-based composite consisting of nanometer-sized gallium oxide aggregates embedded in a potassium-silicate host glass using the diamond anvil cell technique. The Raman spectra of this heterophase nanocomposite showed a range of pressure-induced structural transformations occurring in the glass matrix. Compression from ambient pressure up to 10.8 GPa indicated a progressive reduction in the width of the intertetrahedral SiOSi angle distribution, which was completely reversible on decompression to ambient pressure. At higher pressures, the Raman spectra demonstrated a breakdown of the intermediate-range order in the glass matrix of the nanocomposite. The enhancement of scattering intensity in the region of the D-defect band at 565 cm,1 together with the blue shift of the main SiOSi symmetric stretching wavenumber are evidence of a permanent reduction in SiO4 ring statistics toward smaller-than-six-ring configurations in the three-dimensional glass network. Starting from 13 GPa, the Raman spectra displayed a remarkable decrease in the scattering intensity of the SiOSi symmetric stretching that has been related to a coordination change of the silicon atom. The Raman spectrum of the composite quenched from 23 GPa to ambient conditions illustrated the pressure-driven, permanent reconstructive modification of the glass matrix in the nanocomposite. The pressure-induced evolution of the Raman peaks assigned to the gallium oxide phase indicated a progressive densification of the nanocrystalline phase, reversible on decompression to ambient pressure. Copyright 2005 John Wiley & Sons, Ltd. [source]

    Structural Changes in Silica Glass by Continuous-Wave Laser Backside Irradiation

    Hirofumi Hidai
    We report on a permanent change in the physical properties inside silica glass by rapid heating and quenching using a continuous-wave laser beam. The absorption of the glass was enhanced by laser heating, and the heated spot moved as a result of thermal radiation and conduction. To trigger heating, an absorbent material was placed on the backside of a glass plate and irradiated through the glass. Laser illumination with a power of 11 W focused on the absorbent material induced a cylindrical modified zone along the laser beam with a length of up to 5.5 mm that was modified at a rate of ,130 mm/s. The characteristics of the modified silica glass were studied. The modified area consists of two layers, and the diameters of the inner and outer zones are ,40 and ,55 ,m, respectively. The inner zone was modified by laser heating. The fictive temperature is estimated to be ,1900 K. The etch rate and hardness of the modified glass increased owing to the increment of the fictive temperature. The outer zone was modified by tensile stress due to the densification of the inner zone. In the outer zone, the etch rate is increased and hardness is decreased. [source]

    Synthesis, Processing, and Properties of Submicrometer-Grained Highly Transparent Yttria Ceramics

    Karn Serivalsatit
    In this article, we report on the fabrication of 0.3-,m average grain-sized transparent yttria ceramics using a modified two-step sintering approach. This process yielded full densification of the yttria ceramics with a drastically reduced grain growth. These transparent yttria ceramics exhibited a transparency equivalent to that of single crystals in the near-infrared spectral region. The microhardness and fracture toughness of the 0.3-,m average grain-sized transparent yttria ceramics fabricated by a modified two-step sintering were found to exceed those of ,300-,m average grain-sized transparent yttria ceramics fabricated by conventional sintering by 25 and 70%, respectively. [source]

    Al3BC3 Powder: Processing and Synthetic Mechanism

    Sea-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 1800C 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 1000C, and became the main synthetic mechanism of Al3BC3 at 1100C. The Al3BC3 particles synthesized at 1100C were porous and were composed of fine hexagonal crystals. The main synthetic mechanism of A3BC3 changed into solid,solid reaction above 1100C, and a gas,solid reaction promoted the densification of the porous Al3BC3 powder above 1340C. [source]

    Sintering Behavior and Conductivity Study of Yttrium-Doped BaCeO3,BaZrO3 Solid Solutions Using ZnO Additives

    He Wang
    The effect of ZnO on the crystal structure, sintering behavior, and electrical conductivity of yttrium-doped BaCeO3,BaZrO3 was investigated by unfixing or fixing the yttrium content noted as BaCe0.5Zr0.3Y0.2,xZnxO2.9,0.5x and BaCe0.5Zr0.3Y0.2O2.9+yZnO, respectively. Studies on the two series revealed that BaOZnO eutectic, rather than ZnO, was responsible for the sintering densification. For BaCe0.5Zr0.3Y0.2,xZnxO2.9,0.5x, the evaporation of ZnOBaO eutectic was observed after sintering at 1300C for 10 h, and few impurities were detected by XRD with x<0.20. For BaCe0.5Zr0.3Y0.2O2.9+yZnO, the concomitant loss of BaO with ZnO caused A-site deficiency and led to impurities of Y2O3 for y=0.08 and 0.14, and Y2BaZnO5 for y=0.20 during the sintering. For both series, ZnO enhanced the relative density, which was above 97% with x or y varying from 0.02 to 0.08. Energy-dispersive X-ray spectroscopy analysis revealed that ZnO hardly entered the perovskite phase. The conductivity study also suggested that ZnO did not serve as a dopant and that yttrium content was essential for sustaining a high ionic conduction. Excessive ZnO was especially detrimental to the grain boundary conduction and thus lowered the total electrical conduction. The optimized composition of BaCe0.5Zr0.3Y0.2O2.9+0.04ZnO has been obtained, with both a high relative density (,98.5%) and a high electrical conductivity (1.35 10,2 S/cm at 600C). [source]

    Sintering and Microwave Dielectric Properties of the LiNb0.63Ti0.4625O3 Ceramics with the B2O3,SiO2 Liquid-Phase Additives

    Yanping 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 900C showed favorable microwave dielectric properties with ,r=71.7, Qf=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]