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.     

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.     

Aqueous tape casting of silicon nitride

Slurries consisting of a low cost silicon nitride powder, sintering aids yttria and alumina, dispersants, binders, defoamers and water as a solvent were optimised for tape casting by electroacoustic and viscosity measurements and by casting experiments. The slurries exhibit shear-thinning behaviour due to the highly shear-thinning binder emulsion. Crack free tapes with a maximum thickness of approx. 250 μm and a binder content of 13–15 wt.% could be obtained. The green tapes with a high flexibility and green strength could be laminated easily by compression at room temperature. After sintering a dense microstructure developed. A significant shrinkage anisotropy parallel and perpendicular to the cast direction was observed.     

Tape Casting of Lanthanum Chromite

The effects of process additives, ball milling, and solids loading were evaluated for tape casting suspensions of glycinenitrate-synthesized La_0.7Ca_0.31CrO₃ powder. An optimized formulation was obtained based on rheological characterization, electrokinetic sonic amplitude measurements, qualitative examination of green tapes, and the sintered microstructure. The tape casting formulation incorporated 66:34 methyl ethyl ketone/ethyl alcohol solvent, an aliphatic phosphate ester dispersant, and 80 wt% (35 vol%) solids. The best binder/plasticizer system was 12 wt% (15 vol%) poly(isobutyl methacrylate) and 5 wt% (6.3 vol%) benzyl butyl phthalate plasticizer (binder:plasticizer = 2.3). Cast tapes were sintered at 1300°C for 2 h, producing a bulk density of 96.2% theoretical, with linear shrinkage of 22% and an approximate grain size of 1.3 μm.     

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.     

Effect of Particle Shape on Anisotropic Packing and Shrinkage Behavior of Tape-Cast Glass–Ceramic Composites

In this study, the influence of particle shape anisometry and particle alignment in tape-cast green sheets on the shrinkage behavior of low-temperature co-fired ceramics (LTCCs) was investigated quantitatively. A new method for the characterization of particle shape with the use of a particle image analyzer is presented, and its application to real material systems demonstrated. A commercial LTCC system and three developed composite powders with different average particle sizes were analyzed. After tape casting, particle alignment in the green sheets was analyzed using image analysis of SEM micrographs of cross sections. The investigations showed that the degree of particle alignment correlates significantly with the particle shape and size of the materials. A further increase in particle orientation was seen after the lamination process. Additionally, the powder packing of both single layers and laminates was analyzed by mercury porosity. The anisotropic shrinkage behavior during the sintering process was determined by means of optical dilatometry. The data obtained on the particle morphology, particle orientation in the tapes, and their effects on the shrinkage anisotropy will be discussed.     

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.     

Factors Influencing the Green Body Properties and Shrinkage Tolerance of LTCC Green Tapes

Low-temperature co-fired ceramics (LTCC) is a powerful technology for the manufacture of integrated electronic devices. To fulfill the needs of miniaturization, the applied materials must be of constant quality and the manufacturing processes must be well controlled. The reproducibility in shrinkage is one of the most important quality issues of LTCC tapes. To guarantee the customer a narrow shrinkage tolerance, the tape producer must know the important influencing factors of tape manufacturing on shrinkage. In this work, the effects of slurry composition, tape-casting parameters like casting speed and opening of the blades, and tape handling on the sintering shrinkage behavior were investigated by using design of experiments. The investigation showed that the green density and therefore the shrinkage values of tapes depend considerably on the variation of the solvent mixture. Furthermore, significant interactions were quantified between the tape-casting velocity as well as the particle size and the shrinkage behavior. The shrinkage values were also influenced by mechanical deformations of the tapes before firing.     

Effect of powder,binder and process parameters on anisotropic shrinkage in tape cast ceramic products

The effect of powder, binder and process parameters on the properties of cast alumina tapes and their anisotropic shrinkage were investigated. Three alumina powders with different particle shapes (platelets, spherical, standard) and three PVB binders with different chain lengths were used. In addition, casting velocity and blade gap height were varied. The orientation of the particles in the tape was detected quantitatively by image analysis of micrographs. The shrinkage anisotropy is more than 12% for the platelet shaped powder and 8% for the standard powder, whereas the spherical particles lead to almost isotropic shrinkage. The influence of the organic binder chain length proved to be minor compared to the influence of the particle morphology. The variation of casting speed and blade gap height has no effect on anisotropic shrinkage in the investigated parameter range. This is explained by theoretical considerations of particle rotation in a sheared fluid.     

Influence of Green Formulation and Pyrolyzable Particulates on the Porous Microstructure and Sintering Characteristics of Tape Cast Ceramics

Porous morphology and total porosity produced in sintered ceramic tapes was controlled by the amount and distribution of pyrolyzable graphite particles added to a colloidal suspension during a tape casting operation. A conceptual model of the green tape microstructure was used to explain the influence of graphite and tape formulation on sintering characteristics. The creation of a connected, open porous network in the sintered body was the result of graphite particle percolation within the green body. Additional voidage introduced by particle bridging was the source of excess porosity and also resulted in a bimodal pore size distribution. Sintering shrinkage was determined by the ceramic packing density, which was primarily determined by the tape formulation.     

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.     

Laser-Sintered Barium Titanate

Laser sintering of alkoxy-derived ultrafine BaTiO₃ powders was investigated. The temperature increases of the sample with laser irradiation were measured with a thermocouple. It was found that laser irradiation could generate enough heat to sinter ceramics. A slurry was prepared by mixing an alkoxy-derived BaTiO₃ powder, binder additives, solvent, and plasticizer. The slurry was tape cast and dried to give a green sheet. The green sheet was laser sintered and was then characterized by SEM, XRD, and density measurements. The effect of burnout before laser irradiation and the characteristic microstructure of laser-sintered BaTiO₃ are described.     

Prediction and Explanation of Aging Shrinkage in Tape-Cast Ceramic Green Sheets

A simple test to predict linear shrinkage of tape-cast green sheets as a function of time was developed. Shrinkage was found to correlate inversely with the amount of organic phase bound to ceramic particle surfaces. The test was developed for alumina green tapes with acrylic binder. The correlation was found to be independent of the medium used for the two slurry systems evaluated, water and toluene/ethanol. Treatments developed to dimensionally stabilize green sheets reduced shrinkage after 35 days of aging to less than 0.15%. Stabilization treatments involved heating and/or polymer swelling which facilitated the binding of organic phase to alumina particles, causing the tape to be in a more relaxed state.     

Optimization of the tape casting process for the development of high performance silicon nitride substrate

In this paper, tape casting of Si₃N₄ substrate were investigated and optimized. The effects of dispersant content, binder, plasticizer/binder ratio, and solid loading on the green sheet properties were studied. An optimal formulation for the tape casting slurries was proposed, green tape with homogeneous microstructure and higher relative density of 56.08% was developed. After gas‐pressure sintering and annealing, Si₃N₄ substrate with a relative density of above 99% and thermal conductivity as 58 W/m/K was obtained. Results showed that the combination of tape casting and gas‐pressure sintering is feasible for the development of Si₃N₄ circuit substrates for power electronic devices.     

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.     

Anisotropic Shrinkage in Tape-Cast Alumina: Role of Processing Parameters and Particle Shape

Aqueous suspensions of alumina and other powders containing no visible morphological anisotropy were tape cast under different conditions to study the role of processing parameters and powder characteristics in the origin of in-plane sintering shrinkage anisotropy. High anisotropic shrinkage, with the typical shrinkage in the transverse direction exceeding that in the casting direction by more than 10%, was observed for tape-cast alumina cast at high solids loading and high shear rates. Tape casting without any binder also resulted in similar anisotropy. Lower solids loading with increased agglomeration resulted in a drastic reduction of shrinkage anisotropy. Under similar casting conditions, BaTiO₃ did not show any shrinkage anisotropy unlike alumina. Cubic zinc ferrite and titania powders also showed some consistent shrinkage anisotropy. A correlation was found between in-plane particle orientation and anisotropic shrinkage proving that powder characteristics, particularly particle shape, control anisotropic shrinkage even in apparently equiaxed powders.     

Mo-(Fe-B)-Fe混合粉末流延成型薄层坯体技术研究

以无水乙醇和丁酮的共沸体系为溶剂 ,三油酸甘油酯为分散剂 ,聚乙烯醇缩丁醛为粘结剂 ,丙三醇和邻苯二甲酸二正辛酯为增塑剂 ,环己酮为均化剂 ,将Mo -(Fe -B) -Fe混合粉末制备成均匀分散、稳定悬浮的流延料浆。利用此料浆 ,采用玻璃模具浇制法流延成型出颗粒分布均匀、结构致密、厚度可控、具有适宜柔韧和拉伸强度的薄层坯体。对流延成型薄层坯体技术中的主要工艺参数和影响因素进行了研究     

Development of a tape casting process for making thin layers of Si3N4 and Si3N4 + TiN

A process for the tape casting of silicon nitride ceramics has been developed and is described in detail. A solvent (ethanol) based recipe was developed using polyvinyl butyral and polyethylene glycol as the binder and plasticizer, respectively. The effect, of milling times, dispersant, solvent, plasticizer and binder contents were all investigated as well as that of the binder to plasticizer ratio. In addition the beneficial effect of multi-stage milling of the slip was evaluated. The removal of the ceramic tape from the carrier film is described. In addition the recipe and process used for producing silicon nitride tape was successfully adapted for the production of two different silicon nitride + titanium nitride composite based tapes. From the tapes produced it was possible to hot press dense multi-layer laminate structures with the thinnest layers being 45 μm thick.     

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.     

Role of organic additives on non-aqueous tape casting of SiAlON ceramics

Suspensions consisting of precursor α/β SiAlON forming powders, azeotropic solvent mixture of 60 MEK/40E, dispersant, binder, and plasticizer were optimized for tape casting by rheological measurements and tape properties. Sodium tripolyphosphate (STPP) was introduced as a dispersant for low temperature applications of α/β-SiAlONs. Optimum STPP amount was determined as 0.012 g/m² (of the particle surface) for stable α/β-SiAlON suspensions. Different amounts of binder/plasticizer mixtures were added to the slurries and the effects on rheological and green tape properties were investigated. Green tapes with dibutyl phthalate (DBP), and plasticizer mixture, poly(ethylene glycol) (PEG) and DBP, exhibited centered cracks with high plasticity, on the other hand, polyvinyl butral (PVB) and PEG showed no crack but low plasticity. Therefore, many different parameters were found to be effective on final tape properties. In addition, tapes were prepared with 6 vol% PVB + PEG, sintered at 1800 °C for 2 h and exhibited almost 97%TD in room temperature applications of α/β SiAlONs.