Showing papers on "Ceramic published in 1998"

Piezoelectric Sensors and Sensor Materials

Abstract: This paper reviews the current trends and historical development of piezoelectric sensors and sensor materials technology. It begins with a discussion of the bases of piezo- and ferroelectric activity, followed by an overview of the most commonly used piezoelectric ceramic: lead zirconate titanate (PZT). A discussion of the properties and applications of piezoelectric crystals and additional piezoelectric ceramics is followed by a description of several sensor configurations prepared from bulk ceramics. An extensive review and comparison of piezoelectric ceramic—polymer composite sensors based on the connectivity of the constituent phases is also presented. We conclude our discussion of sensor configurations with recent examples of piezoelectric ceramic—metal composite sensors, and expected future developments in the area of piezoelectric sensors.

Biomorphic Cellular Silicon Carbide Ceramics from Wood: I. Processing and Microstructure

Abstract: Processing of cellular ceramics with anisotropic pore structures by infiltration of liquid Silicon into carbonized wood and subsequent reaction to SiC was investigated. Natural wood of different pore size distribution and composition was carbonized at 800–1800°C in inert atmosphere resulting in a one-to-one reproduction of the original wood structure. The carbon template was converted to SiC by a rapid liquid infiltration-reaction process at 1600°C. Spontaneous infiltration was achieved by using the continous tracheidal cells in wood as a transportation path for liquid silicon. β-SiC formed by solid–liquid reaction at the pore surface exhibits a crystalline texture which may be related to the initial microfibril orientation in the cell walls of wood. Depending on the initial cellular microstructure of the various kinds of wood (ebony, beech, oak, maple, pine, balsa) ceramic materials of different anisotropic pore structures in the form of pseudomorphs of the original wood were obtained.

The luminescence and structure of novel transparent oxyfluoride glass-ceramics

Abstract: New transparent oxyfluoride glass-ceramics can provide a low phonon energy fluoride environment for active rare earth ions while maintaining the formability and physical properties of an oxide glass. Transparent glass-ceramics were made by casting crucible melted glass onto a steel plate and subsequently heating the resultant glass patty above the glass transition temperature to nucleate and grow LaF3 crystals. Eu3+ doped glasses emit only red luminescence from the 5D0 level but after heat treatment emit blue, green, and red luminescence indicative of a low phonon energy rare earth environment. TEM micrographs and XRD show no features in the base glass, but do show extensive LaF3 crystallites after heat treatment that are responsible for the novel optical properties of these hybrid materials.

Reinforcement coatings and interfaces in aluminium metal matrix composites

Abstract: The interface between the matrix and reinforcement plays a crucial role in determining the properties of metal matrix composites (MMC). Surface treatments and coating of the reinforcement are some of the important techniques by which the interfacial properties can be improved. This review reports the state of art knowledge available on the surface treatments and coating work carried out on reinforcements such as carbon/graphite, silicon carbide (SiC) and alumina (Al2O3) and their effects on the interface, structure and properties of aluminium alloy matrix composites. The metallic coatings improved the wettability of reinforcement but at the same time changed the matrix alloy composition by alloying with the matrix. Ceramic coatings reduce the interfacial reaction by acting as a diffusion barrier between the reinforcement and the matrix. Multilayer coatings have multifunctions, such as wetting agent, diffusion barrier and releaser of thermal residual stress. The roles of reinforcement coating as a means of “in situ hybridising” and “in situ alloying” are described.

High-strength alkali-resistant sintered SiC fibre stable to 2,200 °C

Abstract: The high-temperature stability of SiC-based ceramics has led to their use in high-temperature structural materials and composites1,2,3 In particular, silicon carbide fibres are used in tough fibre-reinforced composites Here we describe a type of silicon carbide fibre obtained by sintering an amorphous Si–Al–C–O fibre precursor at 1,800 °C The fibres, which have a very small aluminium content, have a high tensile strength and modulus, and show no degradation in strength or change in composition on heating to 1,900 °C in an inert atmosphere and 1,000 °C in air — a performance markedly superior to that of existing commercial SiC-based fibres such as Hi-Nicalon Moreover, our fibres show better high-temperature creep resistance than commercial counterparts We also find that the mechanical properties of the fibres are retained on heating in air after exposure to a salt solution, whereas both a representative commercial SiC fibre and a SiC-based fibre containing a small amount of boron were severely degraded under these conditions4 This suggests that our material is well suited to use in environments exposed to salts — for example, in structures in a marine setting or in the presence of combustion gases containing alkali elements

Processing and properties of carbon nanotubes-nano-SiC ceramic

Abstract: Carbon nanotubes–nano-SiC ceramic has been fabricated by the hot-press method. The preparation steps involved the use of dispersing nano-SiC powders and carbon nanotubes in butylalcohol using an ultrasonic shaker. The reasonable relative density of about 95% has been achieved by hot-pressing at 2273 K (at 25 MPa in Ar for 1 h). The three-point bending strength and fracture toughness of the composite has about 10% increment over monolithic SiC ceramic which was fabricated under the same process. The reasons for the increment are the strengthening and toughening role of carbon nanotubes occuring in the matrix.

Hydrogen gas-extraction module and method of fabrication

Abstract: A hydrogen gas-extraction module includes an intermediate layer bonded between a porous metal substrate and a membrane layer that is selectively permeable to hydrogen. The metal substrate includes a substantial concentration of a first metal at a surface of the metal substrate, and the intermediate layer includes an oxide of this first metal. In one embodiment, where the module is designed to selectively extract hydrogen at high temperatures, the porous metal substrate comprises stainless steel, and the membrane layer includes palladium or a palladium/silver alloy. A method for fabricating a hydrogen gas-extraction membrane includes reacting the porous metal substrate with an oxidizing agent to form a ceramic intermediate layer on a surface of the porous metal substrate and covering the ceramic coating with the membrane layer that is selectively permeable to hydrogen.

The role of interfaces on an apparent grain size effect on the dielectric properties for ferroelectric barium titanate ceramics

Abstract: We report the effect of interfaces (and thus internal surface area effects) on the value of dielectric constant (K′) calculated from capacitance and geometry data for sub-micron barium titanate (BaTiO3) ceramics prepared with decreasing grain size (and grain volumes). A series model is proposed to explain the decreasing values of apparent K′ obtained for grain sizes below 0.5 μm. A distinction is made between the true dielectric constant (K′) and the apparent dielectric constant (K′) calculated from experimental data. The progressive suppression in K′ is explained in terms of ferroelectric grains of constant dielectric constant (K′1) separated by a lower-K 2 boundary region (i.e., grain boundary) of constant thickness (d 2). The problem is one of an increasing interfacial surface area to grain volume ratio in fine-grain dielectrics. We begin by reporting original dielectric data for high pressure-densified ultrafine-grain BaTiO3 ceramics. Chemically prepared BaTiO3 powder was consolidated at high...

Electrical properties of new type high oxide ionic conductor RE10Si6O27 (RE = La, Pr, Nd, Sm, Gd, Dy)

Abstract: Electrical properties of the high oxide ionic conductive ceramic RE10Si6O27 (RE=La, Pr, Nd, Sm, Gd, Dy) sintered at 1700–1800 °C by the use of MgO-stabilized zirconia setter during a sintering have been investigated. The lowest activation energy and the highest conductivity at 200 °C were achieved for Pr10Si6O27 (55.4 kJmol−1) and La10Si6O27 (1.32 × 10−5 S cm−1), respectively. The conductivity of La10Si6O27 at 200 °C was higher by order of 1.5 × 101 times than that (8.82 × 10−7 S cm−1 at 200 °C) of (Bi2O3)0.75(Y2O3)0.25. The three-point bending strength of La10Si6O27 was 100 MPa.

The design of the fibre-matrix interfacial zone in ceramic matrix composites

Abstract: Ceramic matrix composites are tough when the fibre-matrix bonding is properly controlled during processing, via the use of an interphase. The interphase is either formed in situ as the result of fibre-matrix interactions or deposited on the fibre surface prior to composite fabrication. It has several key functions, including crack deflection, load transfer, diffusion barrier and residual stress relaxation. Four types of interphase are depicted involving weak interfaces, materials with a layered crystal structure (pyrocarbon, BN, micas and phyllosiloxides, or materials with the β-alumina/magnetoplumbite structures), multilayers such as (PyC-SiC)n or (BN-SiC)n or, finally, porous materials. Achieving high mechanical properties and long lifetimes in severe environments require a subtle design of the fibre-matrix interfacial zone, which is depicted for Nicalon/glass–ceramic and Nicalon/SiC-matrix composites.

Ceramic suspensions suitable for stereolithography

Abstract: Ceramic three dimensional parts have been fabricated by a Stereolithography (SL) process using a ceramic slurry containing alumina powder, UV curable monomer, diluent, photoinitiator and dispersant, subsequent removal of organic components and sintering. The SL process consists of fabricating parts with complex shapes layer by layer by laser polymerization of a ceramic/resin mixture. The effects of each component on the rheology of the ceramic suspension were investigated. Both, the addition of dispersant and diluent to the curable monomer and the increase in temperature decrease the viscosity down to suitable values for tape casting of the layers and for SL. The homogeneous and stable high ceramic concentration suspensions (53 vol%) exhibited a shear thinning behavior, which is favorable for casting the layers. Adequate cured depth (above 200 μm) and width were obtained even at high scanning speeds with an argon ionized laser.

Electrodes based on Magnéli phase titanium oxides: the properties and applications of Ebonex® materials

Abstract: Magneli phases are a range of substoichiometric oxides of titanium of the general formula TinO2n−1, (where n is between 4 and 10) produced from high temperature reduction of titania in a hydrogen atmosphere These blue/black ceramic materials exhibit a conductivity comparable to that of graphite and can be produced in a number of forms, such as tiles, rods, fibres, foams and powders While these materials have been studied for many years, they have only recently received interest for use as ceramic electrode materials, commercially termed ‘Ebonex®’, and are beginning to challenge precious metal coated anodes for some applications in aggressive electrolytes Other uses for these materials include electrowinning, electroplating, battery materials, impressed current cathodic protection anodes, electrochemical soil remediation, oxidation of organic wastes, flexible cable materials and electrophoresis The scope of this review considers the structure and properties of Magneli phase titanium oxide materials, together with their electrochemical behaviour and applications

The effect of grain and particle size on the microwave properties of barium titanate (BaTiO3)

Abstract: The use of ferroelectric ceramics and thin films in microwave devices requires that they possess frequency-stable, low-loss dielectric properties. At microwave frequencies, ferroelectric polycrystalline ceramic materials typically exhibit a large dielectric relaxation, characterized by a decrease in the relative permittivity (er) and a peak in the dielectric loss (tan δ). Mechanisms attributed to the relaxation phenomenon include piezoelectric resonance of grains and domains, inertia to domain wall movement, and the emission of gigahertz shear waves from ferroelastic domain walls. As a result, the relaxation phenomenon appears to be intimately linked to the domain state of the ferroelectric. The domain state of a ferroelectric is, in part, dependent upon its microstructure. In this study, the microwave dielectric properties of ferroelectric barium titanate were measured as a function of grain and particle size. Polycrystalline ceramic ferroelectric BaTiO3 (having average grain sizes of 14.4, 2.14, and 0.2...

Processing and properties of particulate reinforced steel matrix composites

Abstract: Metal matrix composites are an attractive choice for aerospace and automotive applications because of their high stiffness-to-weight ratio. Composites with aluminum and magnesium matrices have been investigated extensively, while less work has been carried out on steel matrix composites. In the present study the hot isostatic pressing (HIP) process of steel matrix composites is described, and the factors influencing the reinforcement distribution, interface processes, as well as the mechanical and corrosion properties, are revealed. Both stainless steels and tool steels were used as the matrix material, and the particulate reinforcements were Al 2 O 3 , TiC, Cr 3 C 2 , or TiN. The results are compared with those of the corresponding unreinforced alloys and also with those of aluminum and magnesium matrix composites. It was found that the incorporation of a relatively low volume fraction of ceramic particulate reinforcements significantly increases the wear resistance of the steel matrices, without deteriorating the corrosion properties. On the other hand, reductions in the tensile strength, ductility and toughness were observed. The superaustenitic stainless steel–TiN and hot work tool steel–Cr 3 C 2 composites may offer the best combination of properties.

Ceramic foams by powder processing

Abstract: Ceramic foams show a significant potential of development and application, essentially due to the emergence of environmental preoccupations. A brief overview of the state of the art in cellular ceramic application, preparation and characterization is presented in order to introduce some new data concerning the elaboration of mullite and PZT foams by a replication and a bubble generation method, respectively. Some discrepancies between the theory, developed for describing the properties of open-cell foams, and the experimental mechanical behaviour of these semi-closed cell materials were also shown.

Applications of the FIB lift-out technique for TEM specimen preparation.

Abstract: A site-specific technique for cross-section transmission electron microscopy specimen preparation of difficult materials is presented. A focused ion beam was used to slice an electron transparent membrane from a specific area of interest within a bulk sample. Micromanipulation lift-out procedures were then used to transport the electron-transparent specimen to a carbon-coated copper grid for subsequent TEM analysis. The FIB (focused ion beam) lift-out technique is a fast method for the preparation of site-specific TEM specimens. The versatility of this technique is demonstrated by presenting cross-sectioned TEM specimens from several types of materials systems, including a multi-layered integrated circuit on a Si substrate, a galvanized steel, a polycrystalline SiC ceramic fiber, and a ZnSe optical ceramic. These specimens have both complex surface geometry and interfaces with complex chemistry. FIB milling was performed sequentially through different layers of cross-sectioned materials so that preferential sputtering was not a factor in preparing TEM specimens. The FIB lift-out method for TEM analysis is a useful technique for the study of complex materials systems for TEM analysis.

Biomorphic cellular silicon carbide ceramics from wood : II. Mechanical properties

Abstract: Silicon carbide ceramics with anisotropic pore microstructures pseudomorphous to wood were obtained by liquid Si infiltration of porous carbonized wood templates. Depending on the initial cellular microstructure of the various kinds of wood (ebony, beech, oak, maple, pine, balsa) ceramic materials of different density, pore structure and degree of anisotropy were obtained. Strength and elastic modulus of the pyrolyzed carbon preform and of the final silicon carbide ceramic were measured in different loading directions with respect to the initial cell orientation, e.g. axial, radial and tangential. Generally, the mechanical properties increase with fractional density. Strength and strain-to-failure in axial direction exhibit significantly higher values compared to loading in radial and tangential directions. The orientation dependence of microstructure-property relations may become important for the development of advanced anisotropic light weight structural materials.

Applications for grain boundary engineered materials

Abstract: Advances in automated electron diffraction techniques, microstructural modeling, and the understanding of structure-property relationships for grain boundaries have resulted in the emergence of grain boundary engineering as a formidable tool for cost-effectively achieving enhanced performance in commercial polycrystalline materials (i.e., metals, alloys, and ceramics). In this article, some applications for grain boundary engineering technology that have been developed during the past several years are presented.

Tunable magnetocaloric effect in ceramic perovskites

Abstract: A large entropy variation (magnetocaloric effect) has been discovered in ceramic perovskites with the formulas La0.65Ca0.35Ti1−xMnxO3−z and La0.5+x+yLi0.5−3yTi1−3xMn3xO3−z. Both Curie temperature and entropy change were studied from 4.2 to 400 K for different stoichiometric compositions and applied magnetic fields. Our conclusion is that these materials are excellent candidates for working materials in magnetic refrigeration and liquefaction devices in a wide temperature range.

The simulation of switching in polycrystalline ferroelectric ceramics

Abstract: A polarization switching model for polycrystalline ferroelectric ceramics has been developed. It is assumed that a single ferroelectric crystallite in a ceramic, which is subjected to an electric field and/or a stress, undergoes a complete polarization change and a corresponding strain change if the resulting reduction in potential energy exceeds a critical value per unit volume of switching material. The crystallite’s switch causes a change in the interaction of its field and stress with the surrounding crystallites, which is modeled by the Eshelby inclusion method to provide a mean field estimate of the effect. Thus the model accounts for the effects of the mean electric and stress fields arising from the constraints presented by surrounding crystallites as well as the externally applied mechanical and electrical loads. The switching response of the ceramic polycrystal is obtained by averaging over the behavior of a large number of randomly oriented crystallites. The model, along with the linear dielect...

Ceramic hollow spheres and their applications

Abstract: Ceramic hollow spheres are being developed for electronic uses in piezoelectric transducers and low dielectric constant substrates, for structural applications in syntactic and metal foams, for fluid transport uses in filters and radiant burners, and for acoustic insulation. Fabrication of coarse hollow spheres (1–10 mm) is by nozzle or sacrificial core processes and fine sizes (1–100 microns) are fabricated using sol-gel techniques.

Sintering and Creep Behavior of Plasma-Sprayed Zirconia and Hafnia Based Thermal Barrier Coatings

Abstract: The sintering and creep of plasma-sprayed ceramic thermal barrier coatings under high temperature conditions are complex phenomena. Changes in thermomechanical and thermophysical properties and in the stress response of these coating systems as a result of the sintering and creep processes are detrimental to coating thermal fatigue resistance and performance. In this paper, the sintering characteristics of ZrO2-8wt%y2O3, ZrO2-25wt%CeO2-2.5wt%Y2O3, ZrO2-6w%NiO- 9wt%Y2O3, ZrO2-6wt%Sc2O3-2wt%y2O3 and HfO2-27wt%y2O3 coating materials were investigated using dilatometry. It was found that the HfO2-Y2O3 and baseline ZrO2-Y2O3 exhibited the best sintering resistance, while the NiO-doped ZrO2-Y2O3 showed the highest shrinkage strain rates during the tests. Higher shrinkage strain rates of the coating materials were also observed when the specimens were tested in Ar+5%H2 as compared to in air. This phenomenon was attributed to an enhanced metal cation interstitial diffusion mechanism under the reducing conditions. It is proposed that increased chemical stability of coating materials will improve the material sintering resistance.

Properties of PZT-Based Piezoelectric Ceramics Between -150 and 250 C

Abstract: The properties of three PZT-based piezoelectric ceramics and one PLZT electrostrictive ceramic were measured as a function of temperature. In this work, the dielectric, ferroelectric polarization versus electric field, and piezoelectric properties of PZT-4, PZT-5A, PZT-5H, and PLZT-9/65/35 were measured over a temperature range of -150 to 250 C. In addition to these measurements, the relative thermal expansion of each composition was measured from 25 to 600 C and the modulus of rupture of each material was measured at room temperature. This report describes the experimental results and compares and contrasts the properties of these materials with respect to their applicability to intelligent aerospace systems.

Biodegradable implant material comprising bioactive ceramic

Abstract: Biodegradable polymeric therapeutic implant materials incorporating bioactive ceramics such as Bioglass® are provided. These implants provide increased mechanical properties and pH control, enabling the use of these materials to design porous and nonporous therapeutic implants used as cell scaffolds for healing of tissue defects or fixation devices, having desired degradation times, mechanical properties, elasticity and biocompatibility.