Showing papers on "Ceramic published in 2013"

Hydrophobicity of rare-earth oxide ceramics

Abstract: Metallic and ceramic surfaces can be rendered hydrophobic through a combination of multiscale surface structures and polymeric modifiers, but the imparted hydrophobicity is not robust to harsh environments. It is now shown that the lanthanide oxide series—a class of ceramics—is intrinsically hydrophobic as a result of their unique electronic structure, even after exposure to high temperatures and abrasive wear.

Salt melt synthesis of ceramics, semiconductors and carbon nanostructures

Abstract: Materials synthesis in the liquid phase, or wet-chemical synthesis, utilizes a solution medium in which the target materials are generated from a series of chemical and physical transformations. Although this route is central in organic chemistry, for materials synthesis the low operational temperature range of the solvent (usually below 200 °C, in extreme 350 °C) is a serious restriction. Here, salt melt synthesis (SMS) which employs a molten inorganic salt as the medium emerges as an important complementary route to conventional liquid phase synthesis. Depending on the nature of the salt, the operational temperature ranges from near 100 °C to over 1000 °C, thus allowing the access to a broad range of inorganic crystalline materials and carbons. The recent progress in SMS of inorganic materials, including oxide ceramic powders, semiconductors and carbon nanostructures, is reviewed here. We will introduce in general the range of accessible materials by SMS from oxides to non-oxides, and discuss in detail based on selected examples the mechanisms of structural evolution and the influence of synthetic conditions for certain materials. In the later sections we also present the recent developments in SMS for the synthesis of organic solids: covalent frameworks and polymeric semiconductors. Throughout this review, special emphasis is placed on materials with nanostructures generated by SMS, and the possible modulation of materials structures at the nanoscale in the salt melt. The review is finalized with the summary of the current achievements and problems, and suggestions for potential future directions in SMS.

Fabrication and deformation of three-dimensional hollow ceramic nanostructures

Abstract: Creating lightweight, mechanically robust materials has long been an engineering pursuit. Many siliceous skeleton species— such as diatoms, sea sponges and radiolarians—have remarkably high strengths when compared with man-made materials of the same composition, yet are able to remain lightweight and porous1–7. It has been suggested that these properties arise from the hierarchical arrangement of different structural elements at their relevant length scales8,9. Here, we report the fabrication of hollow ceramic scaffolds that mimic the length scales and hierarchy of biological materials. The constituent solids attain tensile strengths of 1.75 GPa without failure even after multiple deformation cycles, as revealed by in situ nanomechanical experiments and finite-element analysis. We discuss the high strength and lack of failure in terms of stress concentrators at surface imperfections and of local stresses within the microstructural landscape. Our findings suggest that the hierarchical design principles offered by hard biological organisms can be applied to create damage-tolerant lightweight engineering materials.

Ultra high energy density nanocomposite capacitors with fast discharge using Ba0.2Sr0.8TiO3 nanowires.

Abstract: Nanocomposites combining a high breakdown strength polymer and high dielectric permittivity ceramic filler have shown great potential for pulsed power applications. However, while current nanocomposites improve the dielectric permittivity of the capacitor, the gains come at the expense of the breakdown strength, which limits the ultimate performance of the capacitor. Here, we develop a new synthesis method for the growth of barium strontium titanate nanowires and demonstrate their use in ultra high energy density nanocomposites. This new synthesis process provides a facile approach to the growth of high aspect ratio nanowires with high yield and control over the stoichiometry of the solid solution. The nanowires are grown in the cubic phase with a Ba0.2Sr0.8TiO3 composition and have not been demonstrated prior to this report. The poly(vinylidene fluoride) nanocomposites resulting from this approach have high breakdown strength and high dielectric permittivity which results from the use of high aspect rati...

Influence of B4C on the tribological and mechanical properties of Al 7075–B4C composites

Abstract: In the present investigation, the influence of B 4 C on the mechanical and Tribological behavior of Al 7075 composites is identified. Al 7075 particle reinforced composites were produced through casting, K 2 TiF 6 added as the flux, to overcome the wetting problem between B 4 C and liquid aluminium metal. The aluminium B 4 C composites thus produced were subsequently subjected to T6 heat treatment. The samples of Al 7075 composites were tested for hardness, tensile, compression, flexural strengths and wear behavior. The test results showed increasing hardness of composites compared with the base alloy because of the presence of the increased ceramic phase. The wear resistance of the composites increased with increasing content of B 4 C particles, and the wear rate was significantly less for the composite material compared to the matrix alloy. A mechanically mixed layer containing oxygen and iron was observed on the surface, and this acted as an effective insulation layer preventing metal to metal contact. The coefficient of friction decreased with increased B 4 C content and reached its minimum at 10 vol% B 4 C.

Additive manufacturing of ZrO2-Al2O3 ceramic components by selective laser melting

Abstract: Purpose – The purpose this paper is to develop an additive manufacturing (AM) technique for high‐strength oxide ceramics. The process development aims at directly manufacturing fully dense ceramic freeform‐components with good mechanical properties.Design/methodology/approach – The selective laser melting of the ceramic materials zirconia and alumina has been investigated experimentally. The approach followed up is to completely melt ZrO2/Al2O3 powder mixtures by a focused laser beam. In order to reduce thermally induced stresses, the ceramic is preheated to a temperature of at least 1,600°C during the build up process.Findings – It is possible to manufacture ceramic objects with almost 100 percent density, without any sintering processes or any post‐processing. Crack‐free specimens have been manufactured that have a flexural strength of more than 500 MPa. Manufactured objects have a fine‐grained two‐phase microstructure consisting of tetragonal zirconia and alpha‐alumina.Research limitations/implications...

Optical Properties of (Oxy)Nitride Materials: A Review

Abstract: (Oxy)nitride materials, consisting mainly of transition metal and ionic-covalent (oxy)nitrides, show a vast number of interesting physical and chemical properties due to their substantial structural diversity. The optical properties of these (oxy)nitrides, in combination with their excellent mechanical strength, thermal properties, and chemical stability, enable (oxy)nitrides to be used in a variety of industrial fields, such as photovoltaic, photothermal, photocatalytic, pigment, lighting and display, optoelectronic, and defense industries. The optical properties are extremely related to the electronic band structure of (oxy)nitrides, and can be varied significantly by changing the chemical composition (e.g., the oxygen to nitrogen ratio) and preparation/processing conditions. This article overviews the optical properties (including refractive index, reflectance, absorbance, band gap, photoluminescence, and transmittance) of (oxy)nitride materials that are in the form of thin films, powders, or bulk ceramics, and highlights their applications as antireflection coatings, solar spectral selectivity coatings, visible-light-driven photocatalysts, ecological pigments, phosphors for light-emitting diodes, and transparent window materials.

Highly Conductive Porous Graphene/Ceramic Composites for Heat Transfer and Thermal Energy Storage

Abstract: A novel architecture of 3D graphene growth on porous Al2O3 ceramics is proposed for thermal management using ambient pressure chemical vapor deposition. The formation mechanism of graphene is attributed to the carbothermic reduction occurring at the Al2O3 surface to initialize the nucleation and growth of graphene. The graphene films are coated on insulating anodic aluminum oxide (AAO) templates and porous Al2O3 ceramic substrates. The graphene coated AAO possesses one-dimensional isolated graphene tubes, which can act as the media for directional thermal transport. The graphene/Al2O3 composite (G-Al2O3) contains an interconnected macroporous graphene framework with an extremely low sheet electrical resistance down to 0.11 Ω sq−1 and thermal conductivity with 8.28 W m−1 K−1. The G-Al2O3 provides enormous conductive pathways for electronic and heat transfer, suitable for application as heat sinks. Such a porous composite is also attractive as a highly thermally conductive reservoir to hold phase change materials (stearic acid) for thermal energy storage. This work displays the great potential of CVD direct growth of graphene on dielectric porous substrates for thermal conduction and electronic applications.

Wetting: Intrinsically robust hydrophobicity

Abstract: Ceramic surfaces can be rendered hydrophobic by using polymeric modifiers, but these are not robust to harsh environments. A known family of rare-earth oxide ceramics is now found to exhibit intrinsic hydrophobicity, even after exposure to high temperatures and abrasive wear.

Review of graphene–ceramic matrix composites

Abstract: Graphene has remarkable mechanical properties, which makes it potentially a good reinforcement in ceramic composites. It also has unique electrical and thermal properties, which makes it an attractive filler for producing multifunctional ceramics for a wide range of applications. In the past few years, relatively little attention has been focused on graphene ceramic matrix composites (GCMC) in comparison to polymer composites. This review gives a comprehensive overview on the state of the art of GCMC, including materials synthesis, densification and characterisation. The published literature allows us to define the critical steps for processing GCMC, and identify its influence on the multifunctional and mechanical properties of the composites. Finally, the potential future applications and current research trends in GCMC are presented.

Real-time quantitative imaging of failure events in materials under load at temperatures above 1,600 °C

Abstract: Gathering information on the evolution of small cracks in ceramic matrix composites used in hostile environments such as in gas turbines and hypersonic flights has been a challenge. It is now shown that sequences of microcrack damage in ceramic composites under load at temperatures up to 1,750 °C can be fully resolved with the use of in situ synchrotron X-ray computed microtomography.

An ‘aqueous route’ for the fabrication of low-temperature-processable oxide flexible transparent thin-film transistors on plastic substrates

Abstract: Metal oxide semiconductor structures can be both electrically conductive and optically transparent — an attractive combination for electronic devices. Their fabrication has typically involved the deposition of the precursors under vacuum, a method that is costly and can result in materials lacking uniformity over large surface areas. A team of researchers led by Byeong-Soo Bae at the Korea Advanced Institute of Science and Technology has alleviated these issues by devising a low-temperature fabrication route in aqueous media. Solution-based processes had been investigated, but often involved alcohol solvents that require high processing temperatures or complex and unstable precursors. An indium oxide thin film has now been prepared from an aqueous solution, at temperatures lower than 200 °C, that further served as the active layer of a fully transparent thin-film transistor. In addition, using plastic as a substrate enabled the construction of a transparent, flexible device.

Enhanced dielectric properties of BaTiO3/poly(vinylidene fluoride) nanocomposites for energy storage applications

Abstract: In this work, homogeneous ceramics-polymer nanocomposites consisting of surface treated BaTiO3 (BT) particles as fillers and poly(vinylidene fluoride) polymer as matrix have been prepared using a solution casting process. The nanocomposites exhibit enhanced dielectric permittivity and reduced loss tangent. The frequency and temperature dependencies of the dielectric permittivity and loss tangent of the nanocomposites suggest that the introduced BT phase and interface areas contribute to the improvement of the dielectric responses. Meanwhile, the X-ray diffraction patterns and Differential Scanning Calorimetry (DSC) curves indicate that the incorporation of ceramic particles contributes to the decrease of the crystallite size, the increase of the crystallinity, and the shift of the crystallization temperature of the polymer matrix. Furthermore, the dielectric displacement and energy density of the nanocomposites are significantly enhanced and an energy density of 3.54 J/cm3 was obtained under an electric field of 200 MV/m with the BT concentration of 20 vol. %. The results indicate that the introduced ceramic fillers and interface areas have positive influences on the structure of the polymer matrix and contribute to the enhancement of the dielectric responses and energy storage properties of the nanocomposites.

Halloysite clay nanotubes as a ceramic “skeleton” for functional biopolymer composites with sustained drug release

Abstract: Halloysite, naturally occurring clay nanotubes, is described as an additive for functional polymer composites. Due to the empty tubular lumen capable of being loaded with chemically active agents, halloysite provides additional functions (drug delivery, antibiotic, flame-retardant, anticorrosion, and crack self-healing) to polymeric composites synergistically combined with enhanced tensile, impact and adhesive strength. Doping loaded clay nanotubes into a polymeric matrix provides a kind of ceramic “skeleton”, and these “skeleton bones” are loaded with functional chemicals like real bones loaded with marrow. Tunable controlled release of active agents through synthesis of artificial nano-caps at the tube endings and halloysite lumen enlargement by selective etching allowed for tubular nanocontainers with chemicals release time from 10 to 200 h and a loading capacity of ca. 30 wt%. Halloysite is well mixable with polymers of high and medium polarities without any surface modification.

Preliminary investigation of flash sintering of SiC

Abstract: The feasibility of flash sintering a covalent ceramic, SiC, has been investigated for the first time. Flash sintering involves the application of an electrical potential difference across a powder compact during heating, which leads to sintering at low furnace temperatures in a few seconds and has only been demonstrated with ionic ceramics previously. Near-theoretical density was achieved using Al 2 O 3 + Y 2 O 3 sintering aids at a furnace temperature of only 1170 °C and in a time of 150 s. Specimen temperatures were significantly higher than the furnace temperature owing to Joule heating and consequently heat loss limited densification in the near surface region. It was not possible to reach high densities using “ABC” sintering aids (aluminium–boron–carbon) or pure SiC. The mechanisms involved and potential commercial advantages are briefly discussed.

Main Challenges for High Performance NAS Battery: Materials and Interfaces

Abstract: The progress in the research work and real applications of sodium-sulfur (NAS) battery in large scale energy storage is introduced. The key materials and interfaces of the battery, particularly the role of Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS), are systematically reviewed. As the most important and difficult part, the high-quality beta- alumina ceramic electrolyte tubes are prepared by a low-cost solid state reaction process; their sealing performance and interfacial behavior with molten sodium and sulfur electrodes could be substantially improved by glass ceramic type sealants and surface modification, respectively. Combination of carbon and additives like SiO2 with different wetting behaviors for sulfur and the discharge product sulfides is shown to be significant in improving the electrochemical performances of NAS battery. Conductive ceramic coatings are developed as anti-corrosion media of the current collector of sulfur electrode; this is identified as an effective route to protect the metal parts.

Structure, dielectric, ferroelectric, and energy density properties of (1 − x)BZT–xBCT ceramic capacitors for energy storage applications

Abstract: We investigate the dielectric, ferroelectric, and energy density properties of Pb-free (1 − x)BZT–xBCT ceramic capacitors at higher sintering temperature (1600 °C). A significant increase in the dielectric constant, with relatively low loss was observed for the investigated {Ba(Zr0.2Ti0.8)O3}(1−x){(Ba0.7Ca0.3)TiO3}x (x = 0.10, 0.15, 0.20) ceramics; however, electric breakdown was low (~140, 170, 134 kV/cm), and of which room temperature (300 K) charging curve energy density values are largest ~0.88, 0.94, and 0.87 J/cm3 with maximum high dielectric constant values ~7800, 8400, and 5200, respectively. Bulk ceramic BZT–BCT materials have shown interesting energy densities with good energy storage efficiency (~72 %) at high sintering temperature; they might be one of the strong candidates for high energy density capacitor applications in an environmentally protective atmosphere.

Mechanical properties of ZrB2- and HfB2-based ultra-high temperature ceramics fabricated by spark plasma sintering

Abstract: Flexural strengths at room temperature, at 1400 °C in air and at room temperature after 1 h oxidation at 1400 °C were determined for ZrB 2 - and HfB 2 -based ultra-high temperature ceramics (UHTCs). Defects caused by electrical discharge machining (EDM) lowered measured strengths significantly and were used to calculate fracture toughness via a fracture mechanics approach. ZrB 2 with 20 vol.% SiC had room temperature strength of 700 ± 90 MPa, fracture toughness of 6.4 ± 0.6 MPa, Vickers hardness at 9.8 N load of 21.1 ± 0.6 GPa, 1400 °C strength of 400 ± 30 MPa and room temperature strength after 1 h oxidation at 1400 °C of 678 ± 15 MPa with an oxide layer thickness of 45 ± 5 μm. HfB 2 with 20 vol.% SiC showed room temperature strength of 620 ± 50 MPa, fracture toughness of 5.0 ± 0.4 MPa, Vickers hardness at 9.8 N load of 27.0 ± 0.6 GPa, 1400 °C strength of 590 ± 150 MPa and room temperature strength after 1 h oxidation at 1400 °C of 660 ± 25 MPa with an oxide layer thickness of 12 ± 1 μm. 2 wt.% La 2 O 3 addition to UHTCs slightly reduced mechanical performance while increasing tolerance to property degradation after oxidation and effectively aided internal stress relaxation during spark plasma sintering (SPS) cooling, as quantified by X-ray diffraction (XRD). Slow crack growth was suggested as the failure mechanism at high temperatures as a consequence of sharp cracks formation during oxidation.

Thermal conductivity of high porosity alumina refractory bricks made by a slurry gelation and foaming method

Abstract: Alumina has high heat resistance and corrosion resistance compared to other ceramics such as silica or mullite. However, for its application to refractory bricks, its high thermal conductivity must be reduced. To reduce this thermal conductivity by increasing the porosity, a GS (gelation of slurry) method that can produce high porosity solid foam was applied here to produce the alumina refractory brick. This method was successfully applied to produce alumina foam with high porosity and thermal conductivity of the foam is evaluated. At room temperature, the thermal conductivity was about 0.12 W/mK when the foam density was 0.1 g/cm 3 . At elevated temperature above 783 K, thermal conductivity of the foam was strongly affected by heat radiation and increased with increasing temperature, in contrast to the thermal conductivity of alumina itself, which decreased with increasing temperature. The alumina foams developed here achieved sufficient thermal insulating properties for use in refractory bricks. © 2013 The Authors. Published by Elsevier Ltd. All rights reserved.

Oxygen-vacancy-related dielectric relaxations in SrTiO3 at high temperatures

Abstract: We herein present comparative investigation on the dielectric properties of both ceramic and single crystal SrTiO3 samples in the temperature from room temperature to 1073 K. Two relaxations were observed in both samples. They behave as Debye-like and relaxor-like relaxations in ceramic and single crystal samples, respectively. These relaxations were found to be bulk effect related to oxygen-vacancy. In single crystal sample, the relaxations result from the long-range conduction associated with singly and doubly charged oxygen vacancies. In ceramic sample, the oxygen vacancies are more strongly localized in relation to the crystal. This leads to a new phenomenon of formation and dissociation of oxygen vacancy clusters before the vacancies make contribution to the long-range conduction. The low-temperature relaxation in ceramic sample was determined by the clustering and dissociating processes of the oxygen vacancies. The high-temperature relaxation in ceramic sample was found to share the same mechanism as that in the single crystal sample.