Ceramic Green Bodies Research Articles

Ultrasonic transducers with functionally graded piezoelectric ceramics

Ultrasonic transducers used for measurement systems are required having a capability of generating a short time waveform ultrasonic pulse, i.e., a broadband frequency spectrum. In order to obtain the broadband transducer, functionally graded piezoelectric ceramics with a ceramic backing are fabricated and their ultrasonic performances are examined. Gradient of the piezoelectric parameter is realized by sintering a layer-structured ceramic green body composed of piezoelectric ceramic calcined powder layers with various compositions or composed of green sheets containing calcined powder mixed with some organic materials. It has been found that the above transducer generates an ultrasonic wave with a broader frequency spectrum compared with that of the conventional one.

Gelcasting of alumina ceramics in the mixed acrylamide and polyacrylamide systems

Ceramic gelcasting without inhibition of gelation due to the presence of oxygen must be performed in a nitrogen atmosphere to avoid surface spallation of the green body, which is a complex and expensive process. This study shows that the surface spallation of ceramic green bodies gelcast in air could be suppressed by introducing a proper amount of water-soluble polymer, polyacrylamide (PAM), to the acrylamide monomer solution. The dispersion of Al2O3 powder in a water solution containing PAM was investigated. The rheological behavior and the gelling behavior of the alumina suspension were analyzed. Flexural strength and microstructure of the gelcast green and sintered bodies were studied.

Processing of Dense MgO Substrates for High-Temperature Superconductors

Abstract Sintering of magnesium oxide ceramics to high density requires high temperatures (1500–1800°C), resulting in coarse-grained microstructures, or additives that promote sintering. An alternative is to use a fine-grained MgO starting powder with an impurity level < 1% that allows appropriate processing conditions. Here, we report about processing by slip casting, resulting in ceramic green bodies that can be sintered to closed porosity at 1600°C. X-ray diffraction and scanning electron microscopy combined with energy-dispersive X-ray spectroscopy showed that the sintered bodies are single-phase. The process described here is suitable to sinter large substrates up to 200 mm × 200 mm and 96% of the theoretical density with closed porosity. The MgO substrates are suitable for partial melt processing of high-quality Bi-2212 thick films.

Pressureless sintering of β-sialon with improved green strength by using metallic Al powder

The “reaction bonding of aluminum oxide” (RBAO) technology is a novel process that has been well developed for manufacturing low-cost alumina or ZTA components. The principle of this technology is using metallic Al as one of the starting components so that the green strength of such Al/ceramic compact is much higher than that of conventional ceramic green bodies. This allows easy machining on green bodies to give a desired shape and dimension instead of the hard work on fired product. In the present study, this technology was used to prepare β-sialon (β′) from starting powders of Si 3N 4, AlN and Al. Precursor powder mixture after attrition milling possessed a much higher green strength than that without Al additive. Al in the mixture was converted to Al 2O 3 via an oxidation firing at 900 °C for 1 h in air. Dense β′ can be achieved by pressureless sintering (PLS) in 0.1 MPa nitrogen at 1700 °C for 2 h. Even lower temperature sintering at 1650 °C yielded the product with same density but a small amount of O-sialon (O′) phase remained in the composition. Results from this research show a great potential of fabrication low-cost sialon ceramics through reaction-sintering process.

Method for the measurement of powder distribution in green ceramic bodies

J. E. ZORZI, C. A. PEROTTONI, J. A. H. da JORNADA Universidade Federal do Rio Grande do Sul, Instituto de Fisica, 91501-970, Porto Alegre, RS, Brazil Universidade de Caxias do Sul, CCET/DEFQ, R. Francisco Getulio Vargas, 1130, Cidade Universitaria, 95070-560, Caxias do Sul, RS, Brazil. InmetroInstituto Nacional de Metrologia, Normalizacao e Qualidade Industrial, Campus de Xerem, Rodovia Washington Luiz, Km 120.5, BR 040, Duque de Caxias, RJ, Brazil

Joining of ceramic green bodies and sintering characteristics

The joining of green bodies prior to sintering is useful in obtaining complexly shaped ceramic parts. Three types of edge shape: I-type, V-type, and X-type, are prepared. Although the amount of scatter in flexural strength of the X-type edge is smallest, the effect of edge shape on the flexural strength of the joining specimen is slight. A joining specimen can be obtained with more than 60% of the base material strength. CIP operation is effective to obtain the high joining specimen strength. Mean flexural strength of the CIPed specimen after joining is about 317 MPa.

Functionally Gradient Piezoelectric Ceramics for Ultrasonic Transducers

Ultrasonic transducers used for measurement systems are required to have the capability of generating a short-time waveform ultrasonic pulse, i.e., a broadband frequency spectrum. In order to obtain the broadband transducer, functionally gradient piezoelectric ceramics with a ceramic backing are fabricated and their ultrasonic performances are examined. The gradient of the piezoelectric parameter is realized by sintering a layer-structured ceramic green body composed of piezoelectric ceramic calcined powder layers with various compositions or composed of green sheets containing calcined powder mixed with some organic materials. It has been found that the above transducer generates an ultrasonic wave with a broader frequency spectrum than that of the conventional one.

Extraction of binders from green ceramic bodies by supercritical fluid: influence of the porosity

Organic additives can be extracted from extruded or injection-moulded ceramic parts by using supercritical fluids leading to defect free green parts in a short time. A model previously defined, that predicts kinetics of extraction of solid binder molecules by solubilization and diffusion of solubilized species has been modified with a corrective term taking into account the capillary migration of a liquid organic phase according to the microstructure (pore size distribution, tortuosity) of the green sample and to the characteristics of the liquid phase (viscosity, surface tension). An analytical expression of this corrective term is proposed, for a paraffin binder and for three alkanes which are representative of its molecular distribution. The corrective term is related to the length scale over which the liquid binder flow occurs and to its linear velocity. The modified model allows the prediction of the kinetics of extraction of a given paraffin binder, by using the effective diffusion coefficient of the binder, determined with large pore size green samples and a capillary factor, calculated from binder and microstructure characteristics of the green part.

Determination of Binder Decomposition Kinetics for Specifying Heating Parameters in Binder Burnout Cycles

The decomposition kinetics of poly(vinyl butyral) binder from barium titanate multilayer ceramic capacitors with platinum metal electrodes were analyzed by thermogravimetric analysis as a function of the heating rate. The activation energy and pre‐exponential factor for the decomposition kinetics were determined from two types of integral equations, from the Redhead method, and from the variation in heating rate method. The accuracy of the kinetic parameters determined from these methods was then evaluated for describing the observed rate of binder decomposition. Although the individual models yielded very different kinetic parameters, all were capable of describing the experimental data within ±15% accuracy. The kinetic parameters were then used in a coupled transport and kinetic model for describing the buildup of pressure within the ceramic green body as a function of the heating cycle. A methodology based on calculus of variations was also developed to predict the minimum duration for the binder burnout cycle.

The use of co-solvents in supercritical debinding of ceramics

Abstract Extraction of organic binders (paraffin waxes) from injected green ceramic parts, by using supercritical fluids, has proven to be very efficient (short time of extraction, no defects in the green part). The extraction is more rapid when the binder is in the liquid state due to capillary migration but the capillary pressure could damage the ceramic green body because of development of internal stresses. In turn, extraction of the binder in solid state avoid formation of defects but the kinetics of extraction by solubilisation in supercritical carbon dioxide is reduced. An organic co-solvent can be added to supercritical CO 2 in order to increase the solid paraffin molecules solubility. Polar and non polar organic co-solvents were tested and their efficiency on the solubility of the paraffin molecules were compared. The influence of the length of the chains and of the number of side groups of co-solvent molecules were examined. The extraction was also performed by adding mixed co-solvent molecules. Mixtures of polar and non polar co-solvents were introduced in supercritical CO 2 . Improved extraction by a factor 5 was obtained with n-hexadecane molecule as co-solvent, which represents a good compromise between destruction of the paraffin binder molecular cohesion, which requires, on one hand, large molecules, and mobility, which requires, on the other hand, small molecules.

Processing of ceramics and composites by rapid prototyping

Injection moulding is accepted as one of the most important methods for shaping complex ceramic cores, which are used to form intricate internal cooling passages of gas turbine blades. But the relatively long lead time and high costs involved in the fabrication of hard tooling render it uneconomical for new products development and low-volume production. In the study, a rapid prototyping process is developed to fabricate complex-shaped alumina-based ceramic core by combining stereolithography (SL) with gelcasting. SL is utilized to fabricate an integral sacrificial resin mold, and gelcasting is utilized to form a wet ceramic core green body through polymerization of aqueous ceramic slurry. The freeze-drying process is adopted to treat the wet green body surrounded by the resin mold, the drying shrinkage is decreased, and the generation of crack can be prevented. The sintering shrinkage of ceramic core is controlled by adding magnesium oxide power and developing a novel sintering process. After the resin mold is burnt out, the complex-shaped alumina-based ceramic core is obtained.

Gelcasting of a rutile mixture applied to extrusion forming

To improve sintering and forming behaviors, many additives are normally included in the rutile powder during extrusion forming. Hence, this complex rutile mixture is difficult to be dispersed into an aqueous solution of acrylamide monomers for gelcasting. In this work, with the aid of a dispersant, the mixture is easy to be dispersed by calcining at high temperature and then ball-milling, and a concentrated rutile suspension with low viscosity was successfully obtained. The suspension can be used to form in-situ a ceramic green body. After sintering the green body, the microstructure and performance of the product are both improved. For example, breakdown strength improved from 12.6 kV/mm to more than 23.6 kV/mm.

The effect of silane hydrolysis on aluminum oxide dispersion stability in ceramics processing

The controlled hydrolysis of organosilanes has been shown to greatly influence both the surface adsorption of silanes on alumina powder, and the resultant dispersion stability of alumina in organic solvent slurries. Plateau adsorption concentrations from settling experiments show that complete surface coverage for hydrolyzed forms of both n-octyltrimethoxysilane (NOS) and N-2-aminoethyl-3-aminopropyltrimethoxysilane (AAPS) is typically achieved at concentrations of approximately 6 μmol/m2. Moreover, the settling densities of dispersions prepared with hydrolyzed silanes are consistently higher than densities achieved with monomeric silanes alone (as seen in case studies involving NOS in toluene, and AAPS in isopropyl alcohol). Similarly, ceramic slips prepared with polystyrene and alumina, or with poly(isobutyl methacrylate) and alumina also lead to ceramic green bodies with increased densities when the slips are prepared with hydrolyzed silanes. In addition, solid state NMR and dynamic mechanical analyses of resultant green bodies reveal that the molecular motional behavior of these polymers is strongly influenced by the presence of hydrolyzed silanes. These results collectively add to a growing body of evidence which supports the idea that not only is hydrolysis required for silanes to produce controllable and predictable effects in many industrial processes, but hydrolysis must be made to occur at the 'right time' within any sequence of steps that define an industrial process. In the case of ceramic slurries, the 'right time' is the period just prior to the mixing of the ceramic slip ingredients.

Hard-skin development during binder removal from Al2O3-based green ceramic bodies

A paraffin-based binder system was developed for low-pressure injection molding of very fine (0.4 μm) average particle size alumina ceramic bodies. Complex chemical reactions take place, during the process of binder removal, when the ceramic parts are fired in air. The effects of these chemical reactions become more pronounced at about 250°C, giving rise to a hard-skin and promoting cracks in the bodies with large cross-sections. The surface hardening occurs only when the bodies are fired in an oxygen-rich atmosphere. The hardness at the surface of the ceramic bodies fired in air at 250°C varies from about 0.4 to 1.2 GPa, depending on binder composition. Upon increasing temperature, the surface hardness decreases monotonically to more conventional values. Infrared absorption spectroscopy was used to study the debinding process and the chemical mechanism behind the hard-skin development in ceramic bodies fired in air. The experimental evidence suggests that cross-linkage between long carbon chains present in the binder and its interaction with the alumina powder surface is responsible for the high surface hardness observed in parts fired in air at 250°C.

New gel-casting process for alumina ceramics based on gelation of alginate

Sodium alginate—a natural innoxious polymer, which can be dissolved in cold water by mechanical stirring and in which irreversible gelation takes place when enough calcium ions are added—is applied for in situ forming process of ceramics by using chelator to control its gelling behavior. In the present work, the viscosity of sodium alginate solution and factors influencing on it were analyzed. How to control the gelation behavior was discussed. The rheology property of the slurry was examined. An appropriate method to apply the gelation of sodium alginate in order to fabricate alumina ceramic green body was systematically investigated. The viscosity of alumina slurry with 50 vol.% Al2O3 and 1.8 vol.% Ca3(PO4)2 dispersed by 1 wt.% sodium alginate solution is less than 400 MPa s when the shear rate is higher than 50 s−1. The green body has uniform structure and smooth surface, and bending strength is about 8 MPa. After being sintered at 1550 °C for 2 h the linear shrinkage of ceramics composed of 95% alumina is about 15.7% during firing and the relative density of the final product is about 98.7%. Bending strength reaches 320 MPa. SEM images show compact and uniform microstructure.

Analysis of Pyrolysis Products during Thermal Decomposition of Organic Components in Ceramic Green Bodies

For improved quality and machinability of ceramic green bodies the use of organic additives is necessary. During the debinding process the polymers are thermally decomposed and partially emitted as ecotoxicologically relevant components. The aim of this study was to determine and to quantify these compounds during pyrolysis with the ultimate goal of reducing the emission of undesirable pyrolysis products. Four organic polymers frequently used in ceramics industry were investigated: polyacrylic acid (PAA), polyvinyl alcohol (PVA), polyethylene glycol (PEG) and latex. The decomposition experiments were carried out using a tube furnace. Cylindrical green bodies, mainly Al 2 O 3 but also SiC, containing a common amount of 5 mass % of organic additives were pressed and heated. To gather the different volatile decomposition products, a branched sampling system with different adsorbents was developed. After elution of these adsorbents separation and quantification of the pyrolysis products were performed using GC/MS and HPLC. The main pyrolysis products of the three aliphatic organic additives are low molecular mass aldehydes in particular the potentially carcinogenic form-, acet-, and crotonaldehyde in the range of mg/g. According to the used polymer, the concentration of aromatic compounds varies over a wide range from μg/g to mg/g. While the pyrolysis of PEG results in nearly undetectable concentrations of aromatics, mono-, di-, and trimers of ethylene glycol, 2-methyl-1,3-dioxolan and 1,4-dioxan are formed in concentrations in the range of mg/g. The formation of the potentially carcinogenic 1,4-dioxan could be of toxicological relevance.

Direct Evidence for Low‐Density Regions in Compacted Spray‐Dried Powders

Pore structure in a ceramic green body was directly observed with a novel characterization tool to examine the detrimental effect of a binder on the microstructure of ceramics for powder compaction processing. The binder segregated on the surface of granules and formed a low‐density region at the boundaries of granules in the compact after binder removal. A network of pores with round shapes was produced in the sintering process from the low‐density region. This paper also explains a new characterization technique, confocal laser scanning fluorescent microscopy.

Decomposition of binder from a ceramic injection molding sample

Ceramic injection molding (CIM) has been recognized as attractive technique for the fabrication of ceramic parts. Binder systems comprise between approximately 5 and 40 vol.% of a given ceramic green body depending on the chosen forming method. In this work, the thermal decomposition behaviors of four binder systems used in CIM were investigated by thermal analysis. The decomposition kinetic parameters were also evaluated using non-isothermal DSC/TG techniques.

Novel Characterization Method for Internal Pore Structure of a Ceramic Green Body.

A new direct method for characterizing porosity in ceramic green bodies was developed. The high suitability of this technique was demonstrated by examining the structure of alumina granule compacts. The compacts were prepared by uniaxial pressing in atmosphere with different humidities. Clear images could locate minor differences in pore structure in the examined specimens.

Infrared Microscopy for Examination of Structure in Spray‐Dried Granules and Compacts

Novel infrared microscopy has been developed to improve the liquid‐immersion method, which is a technique that has been developed by the authors to study the packing structures of powder particles in ceramic green bodies. This paper demonstrates the high potential of the novel technique by presenting clear structures of Si3N4 powder granules and their compacts.