Mechanical Properties of Tape-Cast Alumina-Glass Dental Composites

Alumina-glass dental composites were prepared by tape casting and sintering at 1120°C, followed by glass infiltration at 1100°C. Flexural strength and fracture toughness of the composites were investigated in terms of the influence of tape constituents, namely, alumina powder, binder, and plasticizer, on the mechanical properties. Both strength and toughness increased with increasing alumina fraction in tapes and decreased with increasing binder content in binder/plasticizer mixtures. These observations were consistent with the influence of the constituents on mean alumina particle distance in tapes, suggesting that the high strength of glass-infiltrated alumina composites is related to toughening by crack bowing. The strength and fracture toughness of the tape-cast composites, optimized for forming dental crowns, were 508 MPa and 3.1 MPa·m^1/2, respectively, obtained from biaxial tests. Shrinkage of the composites decreased with increasing thermocompression pressure, applied to the tapes prior to sintering, and heating rate to the sintering temperature.     

Aqueous tape casting of super-hard B4C laminates with rGO-enriched reinforcing interlayers

Superhard B₄C parts with microarchitectures constituted by ceramic layers and evenly-spaced rGO-enriched reinforcing interlayers were fabricated for the first time. To this end, a concentrated slurry of B₄C with its Ti-Al sintering additive was first prepared by aqueous colloidal processing, optimizing its total solids loading and content of both binder and plasticizer to obtain, by tape casting, handleable and flexible green tapes. A semi-dilute aqueous suspension of B₄C with Ti-Al and abundant GO was also prepared to dip-coat those green tapes with a GO-enriched layer, optimizing the withdrawal rate and the dipping time. The bare and GO-coated B₄C+Ti-Al tapes were then sequentially laminated, thus yielding green multilayered laminates that finally were appropriately debinded and densified by spark plasma sintering. Vickers indentation tests demonstrated that these multilayered laminates are superhard (～31 GPa), and that their rGO-enriched reinforcing interlayers are effective in arresting crack propagation.     

Aqueous Tape Casting of Zirconium Diboride

Aqueous tape casting of ZrB₂ powder with sintering additives was investigated. The dispersion of ZrB₂ suspensions in aqueous media was studied and characterized in terms of zeta potential, sedimentation, and rheological measurements. A well-stabilized suspension with a high solid content (up to 45 vol%) was prepared in the alkaline pH region with 0.4 wt% Lopon 885 as the dispersant. Several suspensions with different compositions of binder and plasticizer were prepared for comparison. Crack-free green tapes with a maximum thickness of approximately 250 μm were obtained with a binder content of 18–23 wt%. The green tapes had high qualities, such as homogeneity, good flexibility, and a smooth surface. Results showed that the slurries at selected formulations met the needs of the tape-casting process.     

Effect of pore orientation on anisotropic shrinkage in tape-cast products

During tape casting, an anisotropic shrinkage can be observed, which is attributed to particle alignment during the casting process. The understanding of the relationship between green body microstructure and shrinkage anisotropy is of great importance for further miniaturization of multilayer ceramics. In the current study, four alumina powders with different particle shape (spherical, standard, plate-like and extreme plate-like) were used to cast green tapes. The sintering shrinkage behavior and the microstructure were analyzed. In particular, the pore orientation was determined quantitatively by using a modified linear intercept method. The relationship between pore alignment and anisotropic sintering shrinkage of cast green tapes is discussed in all three spatial directions. The shrinkage anisotropy could be correlated quantitatively with the pore anisotropy. Furthermore, this correlation was verified by mathematical modeling based on elongated particles and pores.     

Functionally Graded Ceramics Fabricated with Side‐by‐Side Tape Casting for Use in Magnetic Refrigeration

Functionally graded ceramic tapes have been fabricated by a side‐by‐side tape casting technique. This study shows the possibility and describes the main principles of adjacent coflow of slurries resulting in formation of thin plates of graded ceramic material. Results showed that the small variations of solvent and binder system concentrations have a substantial effect on slurry viscosity. Varying these parameters showed that side‐by‐side tape casting with a well‐defined interface area is possible for slurries with viscosities above 3500 mPa s at a casting shear rate of 3.3 s^?1. As it was expected, the choice of de‐bindering and sintering regimes significantly influences crack formation, and a three‐step heating programme was found to result in tapes of the highest quality. The interface regions of green graded tapes were investigated structurally by scanning electron microscopy; for a distinct identification of the interface region and analysing the degree of cross‐interface diffusion, the isothermal entropy change was measured by a vibrating sample magnetometer as the magnetic transition temperature (Curie temperature) is very sensitive to the dopant level in ceramics. Also the purpose of developing this graded ceramic tape casting was applications of these specific magnetocaloric properties within the magnetic refrigeration technology.     

Shrinkage of Tape Cast Products During Binder Burnout

During sintering of tape cast products, anisotropic shrinkage occurs, which can be attributed to an anisotropic green tape structure concerning particle and pore orientation. Little is known about the shrinkage during binder burnout (BBO) and its relation to the microstructure of green tapes including the binder–plasticizer phase. Therefore, the article determines the shrinkage behavior of green tapes derived from alumina powders with different particle shape during binder burnout and prefiring in all spatial directions. The shrinkage after prefiring relative to the green and the debindered states is also discussed. The interrelation between shrinkage behavior and microstructure is investigated in dependence on different process parameters and specifically on the thermal behavior of the binder–plasticizer phase in the green tapes. It is shown that the subtraction of the BBO shrinkage from the total shrinkage results in completely different data for the sintering shrinkage anisotropy in z direction.     

Pyrolysis of polyvinyl butyral (PVB) binder in thermoelectric green tapes

The pyrolysis of polyvinyl butyral (PVB) binder and other organic additives in thermoelectric green tapes, are analysed through differential thermal analysis (DTA), thermogravimetric analysis (TGA) and published results of fourier transformer infrared spectroscopy (FTIR). Based on these analyses the optimum balance of binder degradation mechanism, heating rate, burnout temperature and burnout atmosphere were determined. The maximum upper temperature at which pyrolysis can take place in an oxidising atmosphere, was imposed at 450°C, to avoid the risk of oxidising the thermoelectric material above this temperature, which could degrade its thermoelectric properties. Thermoelectric cast green tapes made with PVB formulation were found to leave char residue after pyrolysis at 450°C, estimated to be almost 20% of the total PVB content in the tape. Different pyrolysis atmospheres of air, argon, CO₂ and Ar/H₂O were used to minimise the char content. The best pyrolysis for the PVB was obtained with the use of CO₂ atmosphere at 450°C with a hold-out time of 5 h, which reduced the char residue to only 1%. Even with this small percentage, the char residue was in the form of a very fine black powder (soot) which covered the thermoelectric material powder in the tape, preventing its densification at the later stages of the sintering process. It was therefore concluded that the PVB system was not a suitable binder candidate to be used in the fabrication of thermoelectric generator by the tape casting method.     

Thickness control and interface quality as functions of slurry formulation and casting speed in side-by-side tape casting

A novel method of co-casting called side-by-side tape casting was developed aiming to form thin functionally graded films with varying properties within a single plane. The standard organic-based recipe was optimized to co-cast slurries into thick graded tapes. Performed numerical simulations identified the stable flow beneath the blade with a shear rate profile independent of slurry viscosity as long as the slurry load in the casting tank was low. Thickness and interface shape could be well predicted if the rheological behaviour of slurries is known and the processing parameters are well-controlled. A well-defined steep interface was obtained by co-casting slurries with similar viscosities above 4000 mPas at a speed of 40 cm/min. The elastic properties of green tapes were proven to be defined by the binder concentration in the recipe formulation. The interfaces in graded tapes were shown to withstand high stresses identifying a good adhesion between side-by-side cast materials.     

Tape casting of non-aqueous silicon nitride slips

Tape casting is a powerful method for the manufacturing of flat, large area ceramic components. Silicon nitride is a reference material for high temperature structural applications. Between them, thick film/coating technologies and ceramic–ceramic joining are receiving an increased attention. In this work, the rheological behavior on non-aqueous silicon nitride slips for tape casting was investigated considering different solvent and binding systems, by controlling the total binder and plasticizer content and the binder to plasticizer ratio. A phosphate ester was used as dispersant. The characteristics of the green tapes obtained in different conditions were studied in terms of density, thickness and microstructure and related with the rheological properties of the slips. Once the slip properties were adjusted, manufacturing parameters, such as the casting speed and the gap between the blades and the carrier substrate, were also analyzed in order to improve the green tape properties and the process reliability. In order to obtain sinterable compacts, Al₂O₃ and Y₂O₃ were used as sintering aids. The effect of the sintering aids in both the rheological behavior and the green characteristics was also studied.     

Manufacturing porous ceramic materials by tape casting—A review

Tape casting is a well-established technique to fabricate ceramic tapes. This technique has been usually applied to produce dense substrates for electronic applications, but recently there are increasing efforts regarding the production of porous cast tapes. The aim of this paper is to present the latest strategies and achievements to manufacture porous ceramic materials by tape casting. The pores morphology can be manipulated by adjusting particle size, sacrificial pore formers, sintering conditions, and combined techniques (phase inversion and freeze casting). Moreover, tape casting enables adjusting the thickness of the product, which is a key property in separation applications using membranes and/or support materials with tailorable structure.     

Processing of porous yttria-stabilized zirconia tapes: Influence of starch content and sintering temperature

The tape-casting process was used to produce porous yttria-stabilized zirconia (YSZ) substrates with volume fractions of porosity ranging from 28.9 to 53 vol.% by using starch as a fugitive additive. Concentrated aqueous YSZ slips with different amounts of starch and an acrylic latex binder were prepared. The influence of the volume fraction of starch and sintering temperature on the sintering behavior and final microstructure were investigated. The microstructure consisted of large pores created by the starch particles with lengths between 15 and 80 μm and smaller pores in the matrix with lengths between 0.6 and 3.8 μm. The pores in the matrix reduced the sinterability of the YSZ leading to the retention of closed porosity in the sintered tapes. The porosities were above those predicted for each of the starch contents. However, larger deviations from the predicted porosity were found as more starch was added. The open to total porosity ratio in the sintered tapes could be controlled by the volume fraction of added starch as well as by the sintering temperature. As the volume fraction of starch increased from 17.6 to 37.8 vol.% there was a gradual increase in the interconnectivity of the pore structure. The sintering shrinkage of the tapes at a given temperature could be directly related to the YSZ packing density in the matrix.     

Particle Shape and Size Effects on Anisotropic Shrinkage in Tape-Cast Ceramic Layers

The particle shape of a commercial low-temperature cofired ceramic (LTCC) composite powder was determined quantitatively in the as-received and milled state using a new particle image analyzer. All grades of the milled powder with average particle sizes of 3.0, 2.4, and 1.8 μm, respectively, exhibit a considerable stretched particle shape, because 40% of their particles have circularity values below 0.95. On the basis of the fast particle image analyzer, the influence of the raw materials on particle alignment during tape casting was investigated using "design of experiments" (DOE). In the cast LTCC green tapes, the degree of particle orientation was measured and correlated with the information from the particle shape analyses and with other material and process factors from the DOE. The results showed that the degree of particle alignment correlates significantly with the measured particle shape and size; more than 80% of the particles were oriented in the casting direction if their shape factor was below 0.5. The particle orientation causes shrinkage anisotropy. The use of a coarser LTCC powder with an average particle size d ₅₀ of 3.0 μm instead of 1.8 μm increased the sintering anisotropy factor of LTCC tapes and laminates significantly from 1.0% to 1.85% and from 3.6% to 7.6%, respectively. The use of more binder or less solvent led to higher shrinkage anisotropies too. The casting velocity showed only a minor effect on the degree of particle orientation and sintering anisotropy, which is due to the shorter shearing period in which particle rotation can take place.     

Mechanical and lamination properties of alumina green tapes obtained by aqueous tape-casting

Mechanical characterisation and lamination were carried out on alumina green tapes prepared by aqueous tape casting using two acrylic emulsions having different glass transition temperatures (T_g) as binders. The tensile strength and strain were strongly dependent on the binder nature and content. Namely, the mechanical properties of the green tapes reflected those of the binders at room temperature: the green tapes obtained with the higher T_g binder showed a brittle behaviour, whereas those obtained with the lower T_g binder showed an elastoplastic behaviour. The mechanical properties of the green tapes prepared by mixing the two acrylic binders lies in between, giving the possibility of tailoring the flexibility and strength in the range of the values obtained for pure binders. Lamination gave rise to an increase of both green and sintered densities, compared with monolayer specimens, whatever the composition of the binder system. Such improvements significantly depended on lamination pressure, but were insensitive to lamination temperature for the two temperatures tested higher than the T_g of the two binders. ©     

The preparation of SiCP/Ag–Cu–Ti hybrid tapes and application in the joining of silicon carbide ceramics

Abstract

A novel method of tape casting to fabricate ceramic-particulate-reinforced composite filler alloy tapes with low organics (no more than 6?wt.%) was developed, with which SiC_P/Ag–Cu–Ti hybrid tapes were successfully prepared and used in joining of sintered silicon carbide ceramics. The stress rheometer, scanning electron microscopy and energy dispersive spectrometer were used to characterize the rheological properties of slurry and microstructure of green tapes and joints. The slurry for tape casting consists of dispersant, binder, solvent, a mixture of ceramics particulates and metal powders, and no plasticizer was added. Castor oil phosphate was proven the suitable dispersant for the slurry, and the content of dispersant, binder, solid loading was optimized as 3?wt.%, 2.6?wt.% and 26?vol.%, respectively. The fabricated hybrid tapes possess good ductility and uniform thickness. The SiC particulates were homogeneously distributed in the metal powders matrix in both sides of the green tapes, and the distribution was retained in the SiC/SiC joints.     

Tape casting of UV-curable aluminium nitride-based slurries

Aluminium nitride (AlN) is one of the materials used for the preparation of substrates for electronic circuits. Such substrates can be prepared via tape casting method. Until now, only non-aqueous and aqueous tape casting was developed and studied for AlN. In this work, the development of UV-curable tape casting for AlN, including the use of photopolymerisable binder, is shown. Two different dispersing agents, BYK-W 9010 and glycerol trioleate, were used to stabilize and homogenize AlN dispersions. In order to modify the powder surface, a pre-treatment step was used, where the powder was first mixed with the dispersing agent in an azeotropic solvent mixture, followed by the evaporation of the solvents and the redispersing of the pre-conditioned powder into the reactive binder. The effective concentration of the dispersants, the impact of the solid loading on the viscosity and slurry behaviour, as well as the effect of the powder pre-conditioning, were studied by means of rheological measurements. Green tapes were optimised by evaluating the effect of the casting gap and the photo- and co-initiator concentrations. Finally, FTIR measurements were used to estimate the polymer conversion degree as a function of exposure time for the green tapes.     

Processing of large and complex-shaped fine-grained alumina bodies with high transparency

The paper presents the development of unique processing methods for large alumina windows, domes, and thin sheets with a fine-grained structure and high optical transmission. Gelcasting combined with post-hot isostatic pressing proved to be a suitable processing method for windows and domes. This method provided alumina ceramics with a real in-line transmission of up to 72 % at a wavelength of 633 nm and with a theoretical in-line transmission in the infrared region between 1500 and 4000 nm for 0.8 mm thick bodies. Complex-shaped bodies, domes, were prepared with minimum warping during sintering. Thin transparent alumina sheets were prepared by gel-tape casting. This novel tape casting method utilized the advantages of the gelcasting method. The real in-line transmission of the tapes was lower compared with the windows. Nevertheless, the thin tapes with a thickness below 350 μm exhibited reasonable transparency even after pressureless sintering and without a polishing of sintered samples.     

Aqueous tape casting processing of low dielectric constant cordierite-based glass-ceramics—selection of binder

In this paper, four different binders were investigated in the process of aqueous tape casting of cordierite-based glass-ceramics and their effects on the rheological behaviour of the suspensions and on the microstructures of the green tapes were compared. Meanwhile, a good compatibility between the dispersant and binder was found to be a predominant factor to obtain an optimised cordierite glass-ceramic tape. The microstructure of the green tape was observed by SEM and the weight loss during binder burn out process was determined by DTA/TG. The dielectric constant and dielectric loss of the sintered tapes (at 1150 °C for 2 h) was also measured.     

Dispersion and Rheological Characterization of TiO2 Tape Casting Slurry

Dispersion behavior of TiO₂ in different solvent systems in combination with two different dispersants was studied and optimized for the dispersion of TiO₂. Based on sedimentation, viscosity, and rheological characteristics, zeotropic ethanol: xylene with a ratio of 50:50 along with 1 wt% menhaden fish oil is found to be the best solvent–dispersant combination for TiO₂. Tape casting slurry was optimized using polyethylene glycol 400 and benzyl butyl phthalate as plasticizers and polyvinyl buyral as the binder. Cyclohexanone was used as homogenizer. TiO₂ tapes were obtained by double doctor blade tape casting process. As cast tapes were dried in air at room temperature. The results show that it is possible to obtain homogenous defect-free green tapes of 58.7% solid loading and green density of 55% having thickness of ∼90 μm.     

Low toxicity binder systems for tape cast Ce0.9Gd0.1O1.95 laminates

Conventional binder systems for tape casting contain toxic phthalate plasticizers and butanone (MEK) as part of the solvent. The effects of exchanging the phthalate with a non-toxic alternative, and butanone with ethanol, were studied on laminates of high-green density CGO (Ce_0.9Gd_0.1O_1.95) tapes. Samples were prepared with a binder system containing DBP (dibutyl phthalate) plasticizer and MEK solvent, and with a binder system based on a non-toxic non-phthalate plasticizer and ethanol. In both systems, the weight ratio of plasticizer to the PVB (polyvinyl butyral) binder was varied between 0.4 and 0.7. Substitution to the less toxic binder system had no adverse impacts on the microstructure. In fact, denser packing and improved homogeneity were observed with the non-phthalate-based system at ratio 0.5 indicating improved dispersion in this system. The denser packing also coincided with a maximum in z-shrinkage and molecular weight of the binder system, which could be related to the distribution of the binder system.     

Critical analysis of aqueous tape casting,sintering, and characterization of planar Yttria‐Stabilized Zirconia electrolytes for SOFC

Tape casting is an established forming technique for several industries, however, researches focus more on slurry composition. In this work, the combined use of design of experiment and materials characterization techniques showed tape casting process parameters have great influence on the microstructure and mechanical properties of green tapes. Formulation and processing optimization allowed obtaining YSZ green tapes with good mechanical characteristics and homogeneous microstructure without laminating step. The optimized sintering schedule and sintering load allowed obtaining planar electrolytes with high density, tensile strength, and electrical conductivity. This work provides an environmental friendly procedure for large‐scale production of SOFCs planar electrolytes.