Showing papers on "Ceramic published in 1991"

(Bi1/2Na1/2)TiO3-BaTiO3 System for Lead-Free Piezoelectric Ceramics

Abstract: One of the (Bi1/2Na1/2)TiO3 (BNT)-based solid solutions, Ba-modified bismuth sodium titanate, (Bi1/2Na1/2)1-xBaxTiO3 (BNBT), is studied for its dielectric and piezoelectric properties as a new group of lead-free piezoelectric ceramics. A rhombohedral (Fα)-tetragonal (Fβ) morphotropic phase boundary (MPB) is shown to exist at x=0.06~0.07 by X-ray data, and dielectric and piezoelectric properties. BNBT ceramics with the MPB composition are superior as piezoelectric ceramics in high-frequency ultrasonic applications or as piezoelectric actuator materials because of a lower free permittivity, e33T/e0, and a high electromechanical coupling factor, kt or k33, along with high mechanical strength.

New design concept of structural ceramics―ceramic nanocomposites

Abstract: Ceramic nanocomposites can be divided into three categories: intergranular nanocomposite, intergranular nanocomposite and nano/nano composite. The intra- and intergranular nanocomposites were found to show the two to five times higher toughness and strength at room temperature than those of monolithic materials. The hardness, toughness, strength and fracture resistance for creep and fatigue at high temperatures as well as the thermal shock fracture resistance were also strongly improved for these composites. On the other hand, the new function such as machinability and superplasticity was observed for the nano/nano composites. The fabrication processes of these nanocomposites by sintering methods, micro and nanostructure observations, improvements of mechanical properties were reviewed and the roles of the nano-size dispersoids were discussed. Finally the new approach on structural materials design will be given.

Modeling 1-3 composite piezoelectrics: thickness-mode oscillations

Abstract: A simple physical model of 1-3 composite piezoelectrics is advanced for the material properties that are relevant to thickness-mode oscillations. This model is valid when the lateral spatial scale of the composite is sufficiently fine that the composite can be treated as an effective homogeneous medium. Expressions for the composite's material parameters in terms of the volume fraction of piezoelectric ceramic and the properties of the constituent piezoelectric ceramic and passive polymer are derived. A number of examples illustrate the implications of using piezocomposites in medical ultrasonic imaging transducers. While most material properties of the composite roughly interpolate between their values for pure polymer and pure ceramic, the composite's thickness-mode electromechanical coupling can exceed that of the component ceramic. This enhanced electromechanical coupling stems from partially freeing the lateral clamping of the ceramic in the composite structure. Their higher coupling and lower acoustic impedance recommend composites for medical ultrasonic imaging transducers. The model also reveals that the composite's material properties cannot be optimized simultaneously; tradeoffs must be made. Of most significance is the tradeoff between the desired lower acoustic impedance and the undesired smaller electromechanical coupling that occurs as the volume fraction of piezoceramic is reduced. >

Ceramic and Glass‐Ceramic Packaging in the 1990s

Abstract: A broad overview of packaging involving interconnecting, powering, protecting, and cooling semiconductor chips to meet a variety of computer system needs is presented. The general requirements for ceramics in terms of their thermal, mechanical, electrical, and dimensional control requirements are presented, both for high-performance and low-performance applications. Glass-ceramics are identified as the best candidates for high-performance systems, and aluminum nitride, alumina, or mullite are identified for low-performance systems. Glass-ceramic/copper substrate technology is discussed as an example of high-performance ceramic packaging for use in 1990s. Lower-dielectric-constant ceramics such as composites of silica, borosilicate, and cordierite, with or without polymers and porosity, are projected as potential ceramic substrate materials by the year 2000.

Composite Polymer Electrolytes

Abstract: The characteristics of composite polymer electrolytes obtained by adding powders of a stable ceramic material having a regular spherical shape with a mean diameter of less than 1 μm, to a poly(ethylene oxide)‐lithium perchlorate complex, are described in terms recrystallization, conductivity, and ion transport number. The addition of the finely dispersed ceramic powder greatly improves the morphological and the electrochemical properties of the polymer electrolytes.

Fixation of hip prostheses by hydroxyapatite ceramic coatings

Abstract: We report the histological findings in post-mortem specimens obtained ten days, 17 days and seven weeks after implantation of hydroxyapatite-coated femoral components of hip arthroplasties. There was early deposition of woven bone on the hydroxyapatite ceramic, identical to that deposited on surviving cancellous trabeculae. The space between these deposits became bridged from both sides by new trabeculae, and there was no evidence of an inflammatory reaction or of fibrous tissue formation. The use of an hydroxyapatite coating seems to allow early, sound, secondary fixation of implants.

Vapor-phase fabrication and properties of continuous-filament ceramic composites.

Abstract: The continuous-filament ceramic composite is becoming recognized as necessary for new, high-temperature structural applications. Yet because of the susceptibility of the filaments to damage from traditional methods for the preparation of ceramics, vapor-phase infiltration has become the fabrication method of choice. The chemical vapor infiltration methods for producing these composites are now being studied in earnest, with the complexity of filament weaves and deposition chemistry being merged with standard heat and mass-transport relationships. Two of the most influential effects on the mechanical properties of these materials are the adhesion and frictional force between the fibers and the matrix, which can be controlled by a tailored interface coating. A variety of materials are available for producing these composites including carbide, nitride, boride, and oxide filaments and matrices. Silicon carbide-based materials are by far the most advanced and are already being used in aerospace applications.

Conversion mechanisms of a polycarbosilane precursor into an SiC-based ceramic material

Abstract: The pyrolysis of a PCS precursor has been studied up to 1600 °C through the analysis of the gas phase and the characterization of the solid residue by thermogravimetric analysis, extended X-ray absorption fine structure, electron spectrocopy for chemical analysis, transmission electron microscopy, X-ray diffraction, Raman and Auger electron spectroscopy microanalyses, as well as electrical conductivity measurements. The pyrolysis mechanism involves three main steps: (1) an organometallic mineral transition (550 < T p < 800 °C) leading to an amorphous hydrogenated solid built on tetrahedral SiC, Si02 and silicon oxycarbide entities, (2) a nucleation of SiC (1000 < T p < 1200 °C) resulting in SiC nuclei (less than 3 nm in size) surrounded with aromatic carbon layers, and (3) a SiC grain-size coarsening (T p > 1400 °C) consuming the residual amorphous phases and giving rise simultaneously to a probable evolution of SiO and CO. The formation of free carbon results in a sharp insulator-quasimetal transition with a percolation effect.

Wafer heating apparatus and with ceramic substrate and dielectric layer having electrostatic chucking means

Abstract: A wafer heating apparatus can be obtained which prevents formation of a local gap caused by deflection or distortion, and the like, of a wafer at the time of heating the wafer so as to improve production yield of the heat treatment of the wafers. The apparatus includes a ceramic substrate, a heat generating resistive element embedded in the ceramic substrate, a film electrode formed on a front surface of the ceramic substrate, and a ceramic dielectric layer formed on the front surface of the ceramic substrate to coat the film electrode. A direct current power source is provided to generate Coulomb's force between the wafer and the film electrode via the dielectric layer to attract the wafer to a wafer-attracting surface of the dielectric layer, while heating the wafer attracted to the wafer-attracting surface by energizing the heat generating element through application of an electric current therethrough. A method of producing the wafer heating apparatus is also disclosed.

Bone bonding mechanism of β‐tricalcium phosphate

Abstract: It has been proposed that the formation of a surface apatite layer in vivo on surface active ceramics is an essential condition for chemical bonding between ceramics and bone tissue. To clarify the difference in bone-bonding mechanisms between surface active ceramics and bioresorbable ceramics, two experiments were performed using plates of dense β-tricalcium phosphate (β-TCP). First, plates of β-TCP were implanted subcutaneously in rats for 8 weeks. Surface change due to bioresorption was observed with scanning electron microscopy. Formation of the apatite layer on the surface was investigated using thin-film x-ray diffraction and Fourier transform infrared reflection spectroscopy. Second, plates of β-TCP were implanted in tibiae of rabbits for 8 and 25 weeks and subjected to the detaching test to measure bone-bonding strength. β-TCP bonded strongly to bone. Undecalcified sections of the interface of bone and β-TCP were examined with SEM-EPMA. However, by physicochemical methods, no formation of surface apatite layer was observed. These results suggest that β-TCP bonds to bone through microanchoring between bone and rough surface of resorbed β-TCP.

Low wear rate of UHMWPE against zirconia ceramic (Y-PSZ) in comparison to alumina ceramic and SUS 316L alloy.

Abstract: Partially stabilized zirconia ceramic is being recognized among ceramics for its high strength and toughness. With this ceramic, is possible to manufacture a 22-mm-size femoral head for low friction arthroplasty of the hip joint in association with an ultra-high-molecular-weight polyethylene socket. Wear-resistant properties of zirconia ceramic were screened on two principally different wear devices. Sterile calf bovine serum, physiological saline, and distilled water were chosen as the lubricant fluid media. Depending on the lubricant medium, the wear factor of polyethylene against zirconia ceramic counterfaces was 40 to 60% less than that against alumina ceramic counterfaces, and 5 to 10 times lower than with the SUS316L metal counterfaces. Polyethylene wear against metal was more susceptible in saline in which it had 2 to 3 times higher wear rate than with serum. On the other hand, different fluid media had little effect on polyethylene wear against ceramic counterfaces. In each set of tests, the wear factor obtained on an unidirectional wear device showed 10 to 15 times higher values, in comparison to the wear factor estimated on a reciprocating wear device.

Method for reducing shrinkage during firing of ceramic bodies

Abstract: A method for reducing X-Y shrinkage during firing of ceramic bodies in which a flexible constraining layer, which becomes porous during firing, is applied to the ceramic body such that the flexible constraining layer conforms closely to the surface of the unfired ceramic body as the assemblage is fired.

Bismuth Ferrites: New Materials for Semiconductor Gas Sensors

Abstract: The gas-sensing characteristics of bismuth ferrites with the compositions BiFeO 3 , Bi 4 Fe 2 O 9 , Bi 2 Fe 4 O 9 as new materials for semiconductor gas sensors are presented. Ceramic samples of composition BiFeO 3 show a high sensitivity to ethanol and acetone vapour. The response and recovery times, long-term stability and reproducibility of gas-sensitive elements, as well as the effect of humidity are studied. A comparison with the characteristics of gas-sensitive elements based on other materials shows the possibility of using bismuth ferrites as sensitive materials for semiconductor gas sensors.

Finite-element analysis of 1-3 composite transducers

Abstract: The vibrational and electromechanical characteristics of a wide range of 1-3 composite structures, comprising ceramic pillars aligned within a polymer phase, are considered using finite-element analysis. The influence of pillar geometry, ceramic volume fraction, and pillar orientation is described in terms of overall transduction efficiency. It is shown that the finite-element method provides a versatile means of analysis and the results obtained permit a set of useful design guidelines to be developed. In general, a small pillar aspect ratio and a relatively high volume fraction provides the most satisfactory performance, in terms of electromechanical efficiency and uniformity of thickness dilation. >

Relative permittivity and dielectric loss tangent of substrate materials for high-Tc superconducting film

Abstract: We measure the relative permittivitye r and dielectric loss tangent tanδ of substrate materials for high-T c superconducting films at 18–300 K and at 5–10 GHz using the cavity-resonator method. The materials measured are single crystals of MgO, LaAlO3, YAlO3, LaSrGaO4, NdGaO3, sapphire, and ZrO2 ceramic. Thee r values are 10–30 and become almost constant below about 50 K. The tanδ values decrease with decreasing temperature and are below 1×10−5 at 77 K except for those of NdGaO3 and ZrO2 ceramic. This suggests that the tanδ values of MgO, LaAlO3, YAlO3, LaSrGaO4, and sapphire do not limit the quality factors of microwave passive components fabricated using high-T c superconducting films. It is also demonstrated that the tanδ of the substrate material is strongly affected by impurities.

A tribological investigation of the graphite-to-diamond-like behavior of amorphous carbon films ion beam deposited on ceramic substrates☆

Abstract: A tribological investigation was conducted on the graphite-to-diamond-like behavior of hard carbon films produced on SiC, Si 3 N 4 and ZrO 2 substrates by ion beam deposition. Friction tests were performed on a ball-on-disk machine with pairs of various ceramic balls and disks coated with hard carbon films in dry and humid air, argon and N 2 . The friction coefficients of carbon films sliding against Si 3 N 4 and sapphire balls were in the range 0.02–0.04 in N 2 and argon, but were significantly higher (about 0.15) in humid air. The wear rates of ceramic disks coated with carbon films were unmeasurable, and, depending on the test environment, the wear rates of counterface ceramic balls were reduced by two to four orders of magnitude below those of balls slid against uncoated ceramic disks. Graphite disks were also tested, to obtain friction data that can help us to understand the graphite-to-diamond-like tribological behavior of carbon films. Micro laser Raman spectroscopy and scanning electron microscopy were used to analyze the structure and chemistry of worn surfaces and to elucidate the graphite- and diamond-like tribological behavior of amorphous carbon films.

Silicon Nitride—From Powder Synthesis to Ceramic Materials

Abstract: Silicon nitride is a ceramic material of great interest to advanced engine construction and mechanical engineering owing to an outstanding combination of favorable properties like high mechanical strength at high temperatures, corrosion and wear resistance, great hardness, and low density. The material is based on high-quality Si3N4 powders, which are shaped and sintered to the ceramic component. This overview outlines the properties required for Si3N4 powders suitable for advanced ceramics. Processes in commerical use and those under development for the production of high quality Si3N4 powders are discussed as well as material manufacturing processes and material properties. By steadily improving powder quality, material properties, and the economy of powder and component production, chemistry and chemical technology play a major role in recent efforts to create a solid fundament for broad applications of silicon nitride ceramics. “If one would surrender to geological phantasies, one could imagine that during the formation of our planet, when elements combined to the compounds making up its crust and mountain ranges, silicon reacted with nitrogen, and the still red-hot nitrogen-silicon, on contact with water, may have decomposed to silicic acid and ammonia. Thus, ammonia may have been formed originally and nitrogen thereby introduced into the forming organic compounds when living nature first started to appear.” H. Sainte-Claire Deville and F. Wohler

Reaction-formed silicon carbide

Abstract: The reaction bonding of silicon carbide (SiC) typifies liquid-solid reaction processes for the synthesis of refractory ceramic composites. These processes have particular advantages over conventional sintering and hot-pressing techniques in their lower processing temperatures, shorter times and near-net shape fabrication capabilities. Two particular modifications that we have employed in order to improve the mechanical properties and the ultimate use temperature of reaction-bonded SiC are the use of microporous carbon pre-forms derived from polyfurfural alcohol for refinement of microstructure, and the use of alloyed melts in order to replace detrimental residual silicon with a refractory silicide. The control of reaction rate is always a key issue in reaction processing. We have studied the kinetics and mechanisms of the liquid SiC reaction. Experiments on carbon fibers and plates show that the principle mechanism is one of solution-reprecipitation. There is an increased solubility of carbon at very fine SiC particles formed by the spallation of the misfitting carbide from the carbon interface, leading to reprecipitation of SiC at defective seed crystals. Molybdenum and boron at low concentrations (3.2 mol.%) have little effect on reaction kinetics, whereas aluminum is able to impede the reaction through the formation of an interfacial carbide layer.

Method of producing parts and molds using composite ceramic powders

Abstract: A method and apparatus for selectively sintering a layer of powder to produce a part comprising a plurality of sintered layers. The apparatus includes a computer controlling a laser to direct the laser energy onto the powder to produce a sintered mass. The computer either determines or is programmed with the boundaries of the desired cross-sectional regions of the part. For each cross-section, the aim of the laser beam is scanned over a layer of powder and the beam is switched on to sinter only the powder within the boundaries of the cross-section. Powder is applied and successive layers sintered until a completed part is formed. Preferably, the powder comprises a plurality of materials having different dissociation or bonding temperatures. The powder preferably comprises blended or coated materials, including precursor materials which are formed into a mass at the irradiated locations, and which either react due to the laser thermal energy or in a later heat treatment to form a compound with properties different than the precursors. Examples are disclosed wherein a compound is formed which has a significantly higher melting point than that of one or more of the precursor powders. A ceramic part, such as a mold for investment casting, may be formed according to the method, where a first material (e.g., ammonium dihydrogen phosphate) has a melting point sufficiently low that it is melted so as to bind the second material (e.g., alumina). Subsequent heat treatment enables chemical reaction of the two materials to form a ceramic (e.g., aluminum phosphate) capable of withstanding high temperatures.

Estimation of Interfacial Shear in Ceramic Composites from Frictional Heating Measurements

Abstract: A new approach for estimating the interfacial frictional shear stress in fiber-reinforced ceramics is presented. The approach is based upon measurement of the temperature rise which occurs during the cyclic loading of ceramic composites. This temperature rise, which is caused by the frictional slip of fibers within the composite, is related to the level of frictional shear stress which exists along debonded interfaces. To illustrate the technique, the interfacial shear

Chain silicate glass-ceramics

Abstract: Glass-ceramics based on chain silicates: enstatite (MgSiO 3 ), potassium fluorrichterite (KNaCaMg 5 Si 8 O 22 F 2 ), and fluorcanasite (K 2 Na 4 Ca 5 Si 12 O 30 F 4 ), display a combination of high flexural strength ( > 200 MPa) and high fracture toughness (> 3 MPa rmm 1 2 ). The relationships between composition, microstrcuture, and properties are discussed. Recent and potential applications are described.

Dielectric ceramic composition

Abstract: PURPOSE:To obtain a dielectric ceramic composition allowing reduced sorts of additives, achieving a high dielectric constant and a low dielectric loss, each of which has an extremely small voltage dependence and deterioration with time by adding a specified wt.% of Mn-based compound to a main composition consisting of SrTiO3, Bi2O3, and TiO2 as an additive. CONSTITUTION:A dielectric ceramic composition consists of 100wt.% a main composition consisting of 50.0-90.5wt.% SrTiO3, 2.0-29.5wt.% Bi2O3, and 5.6-20.5wt.% TiO2 to which a sort of Mn-based compound is added by 0.05-0.20wt.%. A dielectric ceramic composition obtained by molding and baking this composition achieve a high dielectric constant and a low loss, and the voltage dependence and time change of these properties are extremely reduced.

Autogeneic bone marrow and porous biphasic calcium phosphate ceramic for segmental bone defects in the canine ulna.

Abstract: The purpose of this study was to evaluate a porous biphasic hydroxyapatite-calcium phosphate ceramic as a modifier and extender of an autogeneic marrow graft for filling a 2.5-cm segmental bony defect. Twenty adult mongrel dogs were surgically treated to create diaphyseal defects in the left ulnae. The defects were (1) filled with autogeneic bone marrow mixed with granular hydroxyapatite-tricalcium phosphate ceramic (granular ceramic); (2) grafted with a solid block of ceramic soaked in autogeneic bone marrow (block ceramic); (3) received no graft (no implant); or (4) were grafted with autogeneic bone marrow alone (bone marrow). All animals were followed clinically and roentgenographically for 24 weeks and then killed. Repair of diaphyseal defects with the block ceramic led to three solid unions and three fibrous unions; with the granular ceramic implants and marrow, the defects of five dogs formed solid unions, and one progressed to a fibrous union. Defects in all five dogs grafted with autogeneic bone marrow united. The three dogs with no implant formed nonunions. Histology showed normal marrow and only a light immune reaction. Complete bridging of the defect in the dogs treated with the granular ceramic occurred significantly earlier than bridging in the dogs grafted with bone marrow alone. Histomorphometry, performed on the block ceramic implants indicated active resorption of ceramic. Clinically, addition of ceramic to a marrow graft improved the handling characteristics of the graft material and accelerated healing according to roentgenographic evaluation.

Open pore description of mechanical properties of ceramics

Abstract: A dependence of Young's modulus of elasticity on open porosity in ceramics is derived from an open-porosity model, which in the literature, is applied to salinity conductivity and fluid permeability in rocks. A random distribution of grain and pore size is assumed. The relation developed,E(p)=E o(1−"p)m, whereE is the modulus of elasticity of the porous ceramic,E o is the theoretical elastic modulus,p is the porosity andm is an exponent dependent on the tortuosity of the structure of the ceramic, adequately describes the dependence of the modulus of elasticity on porosity. The model is applied to the experimental data from several ceramics such as alumina, silicon nitride, silicon carbide, uranium oxide, rare-earth oxides, and YBa2Cu3O7−δ superconductor, and the value ofm is obtained for each case. We have shown thatm has a value of nearly 2 for sintered ceramics, unless sintering aids or hot pressing have been used during fabrication of the ceramic. Such additional procedures approximately double the magnitude ofm.