Showing papers on "Ceramic published in 1994"

Ceramic thin-film formation on functionalized interfaces through biomimetic processing.

Abstract: Processing routes have been developed for the production of thin ceramic films through precipitation from aqueous solutions. The techniques are based on crystal nucleation and growth onto functionalized interfaces. Surface functionalization routes have been developed by the mimicking of schemes used by organisms to produce complex ceramic composites such as teeth, bones, and shells. High-quality, dense polycrystalline films of oxides, hydroxides, and sulfides have now been prepared from "biomimetic" synthesis techniques. Ceramic films can be synthesized on plastics and other materials at temperatures below 100 degrees C. As a low-temperature process in which water rather than organic solvents is used, this synthesis is environmentally benign. Nanocrystalline ceramics can be produced, sometimes with preferred crystallite orientation. The direct deposition of high-resolution patterned films has also been demonstrated. The process is well suited to the production of organic-inorganic composites.

Sol-Gel-Derived Ceramic-Carbon Composite Electrodes: Introduction and Scope of Applications

Abstract: A new class of composite electrodes made of sol-gel derived carbon-silica materials is introduced. Modifed porous composite carbon-silica electrodes can exhibit hydrophobic or hydrophilic surface characteristics and can serve as an indicator (inert) electrode, as a potentiometric (selective or reference) electrode, and in amperometric sensing and biosensing. The composite (carbon) ceramic electrodes (CCP) are rigid, porous, easily modified chemically and have a renewable external surface. The electrodes otter higher stability than carbon paste electrodes, and they are moreamenable to chemical modification than monolithic and (organic) composite carbon electrodes

Ceramic microstructures : property control by processing

Abstract: Ceramic processing. General influence of microstructure on ceramic properties. Techniques for characterizing ceramic microstructures. Preparation of ceramic samples for microscopy. Structural oxides I: Al2O3 and mullite. Structural oxides II: ZrO2. Synthetic structural non-oxides. Refractories. Ceramic matrix composites. Index.

Effect of Glass Additions on BaO–TiO2–WO3 Microwave Ceramics

Abstract: The effect of glass addition on the properties of BaO–TiO2-WO3 microwave dielectric material N-35, which has Q= 5900 and K= 35 at 7.2 GHz for samples sintered at 1360°C, was investigated. Several glasses including B2O3, SiO2, 5ZnO–2B2O3, and nine other commercial glasses were selected for this study. Among these glasses, one with a 5 wt% addition of B2O3 to N-35, when sintered at 1200°C, had the best dielectric properties: Q= 8300 and K= 34 at 8.5 GHz. Both Q and K increased with firing temperature as well as with density. The Q of N-35, when sintered with a ZnO–B2O3 glass system, showed a sudden drop in the sintering temperature to about 1000°C. The results of XRD, thermal analysis, and scanning electron microscopy indicated that the chemical reaction between the dielectric ceramics and glass had a greater effect on Q than on the density. The effects of the glass content and the mixing process on the densification and microwave dielectric properties are also presented. Ball milling improved the densification and dielectric properties of the N-35 sintered with ZnO–B2O3.

Sandblasting and silica coating of a glass-infiltrated alumina ceramic: Volume loss, morphology, and changes in the surface composition☆

Abstract: Silica coating can improve bonding of resin to glass-infiltrated aluminum oxide ceramic (In-Ceram), and sandblasting is a pretreatment to thermal silica coating (Silicoater MD system) or a tribochemical coating process (Rocatec system). This study evaluated the effects of sandblasting and coating techniques on volume loss, surface morphology, and surface composition of In-Ceram ceramic. Volume loss through sandblasting was 36 times less for In-Ceram ceramic compared with a feldspathic glass ceramic (IPS-Empress), and sandblasting of In-Ceram ceramic did not change its surface composition. After tribochemical coating with the Rocatec system, a layer of small silica particles remained that elevated the silica content to 19.7 weight percentage (energy-dispersive spectroscopy). Ultrasonic cleaning removed loose silica particles from the surface and decreased the silica content to 15.8 weight percentage, which suggested firm attachment of most of the silica layer to the surface. After treatment with the Silicoater MD system, the silica content increased only slightly from that of the sandblasted specimen. The silica layer created by these systems differs greatly in both morphology and thickness, which could result in different bond strengths. Sandblasting of all ceramic clinical restorations with feldspathic glass materials should be avoided, but for In-Ceram ceramic the volume loss was within an acceptable range and similar to that of noble metals.

Ceramic ferroelectric composite material - BSTO-MgO

Abstract: A novel ceramic ferroelectric material having a low dielectric constant, extremely low loss and high tunability. The material is a composite comprising Barium Strontium Titanate (BSTO) and a ceramic material having a low dielectric constant. The preferred composite is represented by Ba1-x Srx TiO3 --MgO, wherein x is greater than 0.00, but less than or equal to 0.75, and wherein the percent weight ratio between Ba1-x Srx TiO3 and MgO ranges from approximately 99%-40% and 1%-60%, respectively. The novel materials possess superior electronic properties; and they may be employed in various antenna systems at both microwave and millimeter wave range frequencies.

Effects of Glass Additions on (Zr,Sn)TiO4 for Microwave Applications

Abstract: The effects of glass additions on the properties of (Zr,Sn)TiO4 as a microwave dielectric material were investigated. The (Zr,Sn)TiO4 ceramics with no glass addition sintered at 1360°C gave Q= 4900 and K= 37 at 7.9 GHz. Several glasses, including SiO2, B2O3, 5ZnO–2B2O3, and nine commercial glasses, were tested during this study. Among these glasses, (Zr,Sn)TiO4 sintered with ZnO-B2O3–SiO2 (Corning 7574) showed more than 20% higher density than that of pure (Zr,Sn)TiO4 sintered at the same temperature. A 5-wt% addition of SiO2, to (Zr,Sn)TiO4, when sintered at 1200°C, gave the best Q: Q= 2700 at 9 GHz. Results of XRD analysis and scanning electron microscopy and the effect of glass content are also presented.

High Li+ Conducting Ceramics

Abstract: In this Account, the authors describe one of the most suitable ways for preparing a high Li+ conducting polycrystalline material An excellent conductivity was realized by detailed studies of both the lattice size in the Li+ migrating bulk (intragrain) and the condition of the grain boundaries (integrain) 55 refs, 8 figs, 1 tab

Structure and properties of nanocrystalline materials

Abstract: The present article reviews the current status of research and development on the structure and properties of nanocrystalline materials. Nanocrystalline materials are polycrystalline materials with grain sizes of up to about 100 nm. Because of the extremely small dimensions, a large fraction of the atoms in these materials is located at the grain boundaries, and this confers special attributes. Nanocrystalline materials can be prepared by inert gas-condensation, mechanical alloying, plasma deposition, spray conversion processing, and many other methods. These have been briefly reviewed. A clear picture of the structure of nanocrystalline materials is emerging only now. Whereas the earlier studies reasoned out that the structure of grain boundaries in nanocrystalline materials was quite different from that in coarse-grained materials, recent studies using spectroscopy, high-resolution electron microscopy, and computer simulation techniques showed unambiguously that the structure of the grain boundaries is the same in both nanocrystalline and coarse-grained materials. A critical analysis of this aspect and grain growth is presented. The properties of nanocrystalline materials are very often superior to those of conventional polycrystalline coarse-grained materials. Nanocrystalline materials exhibit increased strength/hardness, enhanced diffusivity, improved ductility/toughness, reduced density, reduced elastic modulus, higher electrical resistivity, increased specific heat, higher thermal expansion coefficient, lower thermal conductivity, and superior soft magnetic properties in comparison to conventional coarse-grained materials. Recent results on these properties, with special emphasis on mechanical properties, have been discussed. New concepts of nanocomposites and nanoglasses are also being investigated with special emphasis on ceramic composites to increase their strength and toughness. Even though no components made of nanocrystalline materials are in use in any application now, there appears to be a great potential for applications in the near future. The extensive investigations in recent years on structure-property correlations in nanocrystalline materials have begun to unravel the complexities of these materials, and paved the way for successful exploitation of the alloy design principles to synthesize better materials than hitherto available.

New ultraviolet‐transport electroconductive oxide, ZnGa2O4 spinel

Abstract: ZnGa2O4 having the spinel structure was prepared, and optical and electrical properties of the material were measured. By the measurements of diffuse reflectance spectra, ZnGa2O4 was found to have a wider band gap (∼5 eV) than ITO (indium tin oxide). Electrical conductivity of the H2‐annealed ceramic of ZnGa2O4 was 3×101 S cm−1. Thus the ZnGa2O4 spinel was found to be a new UV‐transparent electronic conductor.

Method of manufacturing ceramic tooth restorations

Abstract: The present invention relates to a method of manufacturing artificial tooth restorations for natural teeth or implants comprising a ceramic densely sintered high strength individual core (B) with dental porcelain (A) by powder metallurgical manufacturing methods. The inner surface (I) of the core (B), which will fit against one or more prepared tooth surfaces (P) or artificial abutments is manufactured by forming a ceramic powder mixture against a surface of a body at which said surface is manufactured by registering the surfaces of the prepared teeth or artificial abutments and their mutual relationship with a three dimensional optical or mechanical reading method directly in the mouth or on a model in e.g. plaster after which the registered surfaces are reproduced in an enlarged size e.g. with a computer controlled milling machine at which the enlargement is calculated from the shrinkage of the ceramic material during sintering to full density and considering the gap for cement.

Electrically powered ceramic composite heater

Abstract: An electrically powered ceramic composite heater useful for devices such as a cigarette lighter. The electrical resistance heater includes a discrete heating segment configuration wherein each individual segment of the heater can be activated using an electric control module, and is capable of heating to a temperature in the range of 600° C. to 900° C. using portable energy devices. The ceramic heater can be made by extrusion of a ceramic precursor material followed by secondary processing steps to obtain discrete heating segments. The heater design is such that a hub on one end of the heater provides structural integrity, and functions as a common for the electrical terminals. The ceramic heater can include one or more insulating or semiconductive metal compounds and one or more electrically conductive metal compounds, the compounds being present in amounts which provide a resistance which does not change by more than 20% throughout a heating cycle between ambient temperatures and 900° C.

Formation of carbon films on carbides under hydrothermal conditions

Abstract: CARBON films find applications in a wide range of fields, ranging from microelectronics to materials science1. In ceramic matrix composites they confer the high strength and toughness needed for applications in aerospace, nuclear and automotive engineering2. Chemical vapour deposition is traditionally used to prepare carbon films, but it is relatively expensive, and not easily adapted to coating samples in the form of whiskers, platelets or powders. Here we report that silicon carbide, the most common component of composite ceramics, can be coated with carbon films of nanometre to micrometre thickness by hydrothermal treatment at 300–800 °C. We have applied the technique to SiC fibres, powders, platelets and single crystals, as well as to other carbides. Our method should provide a general and inexpensive route to high-toughness composites and lubricating coatings.

Effect of composition and temperature on electric fatigue of La-doped lead zirconate titanate ceramics

Abstract: Composition and temperature of ferroelectric La‐doped lead zirconate titanate ceramics influence its electric fatigue behavior, defined as the degradation of the electrical properties under the action of an ac field applied for a long time. Compositions of rhombohedral symmetry exhibit little or no fatigue compared with those of tetragonal and orthorhombic symmetry. At temperatures higher than the dielectric maximum, no fatigue effect was detected. Compositions close to phase boundaries (FE‐AFE, FE‐FE, or FE‐PE) display significant fatigue behavior. Electric fatigue arises from the pinning of domains by space charges or injected carriers or from microcracking. The former (which are charge related) is accompanied by smaller strains and is recoverable by thermal and electrical treatment, while the latter (arising from microcracking) arises from large incompatible stresses between grains and is a permanent damage. The understanding of the mechanism of electric fatigue gained in the present study provides gui...

In vitro marginal adaptation of alumina porcelain ceramic crowns

Abstract: This study evaluated the dimensional stability during firing of In-Ceram alumina porcelain ceramic and examined the marginal fit for three different configurations of tooth preparation. A stereomicroscope was used to measure the space between the margin of restorations and tooth preparations. The three methods of tooth preparation were statistically compared and revealed suitable dimensional stability during the firing and glazing process. A better marginal fit was recorded for artificial crowns fabricated on a chamfer or 50-degree shoulder tooth preparation.

Electric Fatigue in Lead Zirconate Titanate Ceramics

Abstract: Electric fatigue is a major obstacle for some potential applications of ferroelectric materials based on reversals of spontaneous polarization, such as memory devices and high strain actuators. The authors` studies of fine-grained hot-pressed lead zirconate titanate with lanthanum dopant (PLZT 7/68/32) show that fast fatigue is actually caused by contaminated surfaces instead of intrinsic structure deterioration or the change of domain states. All of the specimens with conventionally cleaned surfaces showed significant fatigue after 10{sup 5} switching cycles, but specimens cleaned with a few cleaning procedure did not fatigue even after more than 10{sub 8} switching cycles. This type of fatigue is found to be due to generated microcracking at the ceramic-electrode interface.

Cracking in ceramic actuators caused by electrostriction

Abstract: Many perosvskite-type ceramics deform appreciably under electric fields; they make good actuators which deliver motions upon receiving electrical signals. High electric fields are usually applied to induce large strains. Fracture has been observed in the actuators under electrical loading. In this theoretical study, the phenomenon is examined on the basis of electrostriction and fracture mechanics. Attention is focused on a crack emanating from an internal electrode or a conducting damage path. At the edge of the conducting path, the electric field is intense and nonuniform, inducing incompatible electrostrictive strains. Consequently, a stress field is set up in the ceramic, localized around the edge of the conducting path. The condition for the stress to extend a crack is estimated by two models, using either quadratic or step-like electrostriction law. It is found that, under a given electric field, cracking is suppressed in a multilayer actuator if the ceramic layers are sufficiently thin.

Rare-earth chalcogenides — an emerging class of optical materials

Abstract: Sulphide compounds belong to the family of chalcogenides and are well known for their optical and electronic properties. They possess good optical properties because of their ability to transmit into the infrared (IR) region. Several sulphide glasses are known to exist which exhibit far infrared transmission and are also useful semiconductors. In recent years, there has been an increasing interest in IR materials to be used on surveillance equipment. This led to the identification of several new crystalline sulphide materials which can transmit very far into the IR region (up to a wavelength of 14 Μm). Crystalline and amorphous rare-earth sulphides emerged as a new class of materials, which possess several unique optical and electronic properties. This paper reviews the status of these rare-earth sulphide amorphous and polycrystalline materials, the techniques used to process these materials and discusses their structure, thermal, mechanical and optical properties. Conventional and emergent novel chemical processing techniques that are used for synthesizing these materials are reviewed in detail. The use of metallorganic precursors and the modification of their chemistry to tailor the composition of the final ceramic are illustrated. The potential of these chemical techniques and their advantages over the conventional solid state techniques used for processing sulphide ceramics is discussed, particularly in light of their successful applications in processing novel electronic and optical oxide ceramics.

Fabrication of Al matrix in situ composites via self-propagating synthesis

Abstract: Al matrix composite materials with 30 vol.% TiC, TiB2 and TiC + TiB2 ceramic reinforcements were processed in situ via self-propagating high temperature synthesis (SHS) followed by high pressure consolidation to full density. Non-steady-state oscillatory motion of the combustion wave was observed during the SHS processing, resulting in a typical layered structure of the reaction products. The microstructure and phase composition of the materials obtained were studied using X-ray diffraction, optical microscopy and scanning (SEM) and transmission (TEM) electron microscopy. Very-fine-scale ceramic particles ranging from tens of nanometers up to 1–2 μm were obtained in the Al matrix. Microstructural analysis of the reaction products showed that the TiB2/Al and (TiB2 + TiC)/Al composites contained the Al3Ti phase, indicating that full conversion of Ti had not been achieved. In the TiC/Al composite a certain amount of Al4C3 was detected. High room and elevated temperature mechanical properties (yield stress, microhardness) were obtained in the high-pressure-consolidated SHS-processed TiC/Al and TiB2/Al composites, comparable with the best rapidly solidified Al-base alloys. These high properties were attributed to the high density of the nanoscale ceramic particles and matrix grain refinement.

Particulate silicon nitride-based composites

Abstract: In an attempt to optimize the structure and properties of silicon nitride ceramics, a variety of novel processing techniques and materials compositions have evolved over the last 15 years. Among the most important, was the development of various silicon nitride-based composites. A review of particulate, silicon nitride-based composites other than whisker- or platelets-reinforced, is presented. Materials based on silicon nitride and SiAlONs, with additions of carbides, nitrides and borides of transition metals are described. Special emphasis is placed on TiN- and TiC-containing ceramics. The manufacture of composites by hot pressing, reaction sintering, pressureless and gas-pressure sintering is discussed. The data on properties, including conductivity, density, Young's modulus, strength, fracture toughness, hardness, thermal expansion, wear, creep and oxidation resistance are presented. Analysis of actual and potential uses of the selected composites demonstrates that the particulate composites are very promising as tool, structural and electronic materials.

Large Electromechanical Coupling Factors in Perovskite Binary Material System

Abstract: Similarities of large electromechanical coupling factors (k s) in perovskite binary material systems were studied. Five large-k s material systems, PZT 53/47, PZNT 91/9, PMNT 67/33, PSTT 55/45 and PSNT 57/43, were compared. These five large-ks material systems were found to show four similarities: rhombohedral and tetragonal morphotropic phase boundary (MPB), large polarization (P r>25 µ C/cm2), proper B1 ionic size (70–72 pm) and large B2 ion (64 pm) in Pb(B1, B2)O3 relaxors and proper B-site average ionic sizes of MPB composition (64.7–66.8 pm). For example, in this study, the maximum coupling factors (k p=72 %, k31=45 %, and k33=77 %) were obtained in Pb[(Sc1/2Nb1/2)0.57Ti0.43]O3+0.005Nb2O5(PSNTN 57/43) piezoelectric ceramic materials.

Fabrication and mechanical behaviour of Al2O3/Mo nanocomposites

Abstract: Two types of Al2O3/Mo composites were fabricated by hot-pressing a mixture of γ- or α-Al2O3 powder and a fine molybdenum powder. For Al2O3/5 vol% Mo composite using γ-Al2O3 as a starting powder, the elongated molybdenum layers were observed to surround a part of the Al2O3 grains, which resulted in an apparent high value of fracture toughness (7.1 Mpa m1/2). In the system using α-Al2O3 as a starting powder, nanometre sized molybdenum particles were dispersed within the Al2O3 grains and at the grain boundaries. Thus, it was confirmed that ceramic/metal nanocomposite was successfully fabricated in the Al2O3/Mo composite system. With increasing molybdenum content, the elongated molybdenum particles were formed at Al2O3 grain boundaries. Considerable improvements of mechanical properties were observed, such as hardness of 19.2 GPa, fracture strength of 884 MPa and toughness of 7.6 MPa m1/2 in the composites containing 5, 7.5, 20 vol% Mo, respectively; however, they were not enhanced simultaneously. The relationships between microstructure and mechanical properties are also discussed.

The influence of interface impurities on fracture energy of UHV diffusion bonded metal-ceramic bicrystals

Abstract: In both the practical and theoretical aspects of joining metals to ceramics the influence of impurities on the interfacial bond strength is not fully understood. In the present paper the authors describe part of a study in which the interface of niobium-sapphire bicrystals was contaminated under defined conditions and the interfacial fracture energy of these UHV diffusion bonded bicrystals was determined as a measure of their bond strength. The model combination niobium-sapphire shows at high temperatures a simple reaction in which alumina dissolves in niobium without forming an interlayer. Furthermore, niobium and sapphire are characterized by nearly the same expansion behavior which minimizes the development of thermal stresses during cooling down from the bonding temperature. The authors report on experiments in which silver or titanium atoms have been chosen as interfacial impurity elements. Silver is practically insoluble in niobium and sapphire whereas titanium possesses a highly negative free enthalpy of oxide formation. It is assumed that both these properties are associated with interfacially active elements in the system niobium-sapphire.

Structure and Properties of Ceramics

Abstract: Crystal structures of principal ceramic material oxide ceramics nitride ceramics boride and carbide ceramics glass ceramics diffusion in ceramics mechanical properties of ceramics toughening mechanisms mechanical behaviour of cellular ceramics high temperature engineering ceramics ceramic superionic conductors ferroelectric ceramics ferrimagnetic ceramics semiconducting poly-crystalline ceramics high temperature superconductors.