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.     

Making reusable reaction-bonded silicon nitride crucibles for silicon casting from kerf-loss silicon waste

Silicon kerf loss during wafer slicing and the broken quartz crucibles after silicon casting are two major solid wastes from photovoltaic (PV) industry. Especially, the recycle of kerf-loss silicon has become an urgent issue because near 100 000 t of solid wastes are generated every year. One of the most meaningful recycle routes of the kerf-loss silicon is to make silicon nitride crucibles to replace the quartz crucibles. In this study, we demonstrated how this is feasible through acid leaching refining, slip casting, and nitridation. The reaction-bonded silicon nitride (RBSN) crucibles after oxidation were found pure enough for silicon ingot growth. More importantly, they could be reused after ingot growth. With the present examples, the potential of using the kerf-loss silicon for fine ceramics is prominent.     

Ultrasonic monitoring of slip casting kinetics of silicon nitride ceramic

An ultrasonic technique has been used to perform insitu monitoring of ceramics formed by the slip casting of silicon nitride. The technique can monitor the cast thickness as a function of time. Experiments have been performed to study the casting kinetics as a function of slip pH and viscosity, and the results correlated with the resulting cast green density.     

Optimization of non-aqueous tape casting of high solid loading slurry for aluminum nitride ceramic substrates

Aluminum nitride (AlN) ceramic substrates have been fabricated using non-aqueous tape casting and pressureless densification under flowing N₂ atmosphere. Considering the economic and environmental impact, a new strategy of solvent and dispersant system was adopted to prepare AlN slurries with high solid loading. According to the viscosity characteristics of AlN slurries, dispersant content was adjusted to be 0.5 wt% of AlN powder in order to optimize the rheological behavior of AlN slurries. The addition contents of binder and plasticizer were both optimized as 5 wt% of AlN powders by combining the viscosity of slurries and tensile strength of green tapes. Green AlN tapes were fabricated with an optimized tape casting process such as dry temperature. The exclusion process of organic additives was investigated by employing thermogravimetric analysis. Flat and dense AlN ceramic substrates with a relative bulk density over 99.75 % were achieved after being sintered under 1800°C for 6 hours, which had a maximum size of 110 × 110 mm. The thermal conductivity of the AlN substrate could reach 145 Wm⁻¹K⁻¹.     

Application of polypropylene carbonate for the tape casting of silicon nitride ceramics

QPAC40 (polypropylene carbonate), with a little decomposition residue, is commonly used as a binder in aluminum nitride (AlN) tape casting. In this paper, we tried to explore its application in silicon nitride (Si₃N₄) tape casting. By studying the influence of dispersant, binder, plasticizer/binder ratio, and solid loading on slurry and green tape properties, the optimum formulation of the tape casting of Si₃N₄ slurry was determined, and the green tape with a uniform structure and relative density up to 63.16% was prepared. Si₃N₄ ceramics were obtained by debinding at 600°C for 1 h in vacuum and gas-pressure sintering at 1830°C for 2 h in N₂. The thermal conductivity and flexural strength of Si₃N₄ ceramics were 56.28 ± 1.21 W/(m·K) and 1130.67 ± 23.58 MPa, respectively. These results indicated that QPAC40 can be used to prepare Si₃N₄ sheets through tape casting.     

Thermal and corrosion properties of silicon nitride for copper die casting components

Due to the high melting temperature of copper and copper alloys, conventional die-steel components used in pressure die casting these materials exhibit short service lifetimes and undergo thermal fatigue. Thermal and corrosion properties of silicon nitride were studied to assess the material's applicability in substituting conventional die-steels in casting copper and copper alloys. In this study, experiments were conducted to test the thermal shock resistance and corrosion behaviour of a commercial silicon nitride in contact with molten pure copper. The results did not indicate any corrosive reaction between silicon nitride and pure copper. However, the presence of copper oxides at high temperatures accelerated the oxidation of the ceramic resulting in considerable loss of material. The thermal shock behaviour of silicon nitride proved to be adequate for the application.     

Direct coagulation casting of silicon nitride suspension via a dispersant reaction method

A direct coagulation casting method for silicon nitride suspension via dispersant reaction was reported. Tetramethylammonium hydroxide (TMAOH) was used as dispersant to prepare silicon nitride suspension with high solid loading and low viscosity. Influences of TMAOH and pH value on the dispersion of silicon nitride powder were investigated. Glycerol diacetate (GDA) was used to coagulate the silicon nitride suspension. Influences of the concentration of glycerol diacetate on the viscosity and pH value of the suspension were investigated. It was indicated that high viscosity sufficient to coagulate the suspension was achieved by adding 1.0–2.0 vol% glycerol diacetate at 40–70 °C. The coagulation mechanism was proposed that the silicon nitride suspension was destabilized by dispersant reacting with acetic acid which was hydrolyzed from glycerol diacetate at elevated temperature. Coagulated samples could be demolded without deformation by treating 50 vol% silicon nitride suspensions with 0.2 wt% tetramethylammonium hydroxide and 1.0–2.0 vol% glycerol diacetate at different temperatures. Dense silicon nitride ceramics with relative density above 98.8% had been prepared by this method using glycerol diacetate as coagulating agent sintered by different methods.     

Rheological and casting properties of ceramic slips

Translated from Steklo i Keramika, No. 3, pp. 22–25, March, 1991.     

The characterisation of aqueous silicon slips

The optimum conditions of aqueous silicon slips for slip casting was achieved by employing various experimental techniques like zeta potential, particle/floc size distribution, sedimentation, viscosity and rheological measurements as a function of pH of the slips in the pH range of 2–11. Silicon suspensions display a maximum in zeta potential values and a minimum in sedimentation height and viscosity in two pH regions of 4–5 and 8. The slips behaved as near-newtonian at these pH values up to a solid loading of 52 wt% and as non-newtonian with thixotropic behaviour above this solid loading. Samples with highest green density (68%) were produced from 72 wt% solid loaded slips which were conditioned at a pH of 8 and milled for 26 h. The cast bodies of these slips were nitrided to obtain reaction bonded silicon nitride products which were also characterised.     

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.     

Tape casting of proton conducting ceramic material

This work explores experimental procedures for tape-cast proton conducting ceramic fuel cells (PCFC) based on Yttrium-doped Barium Cerate (BCY10). The work is based on several years experience on aqueous tape-casting applied to the shaping of YSZ-based SOFC: however, water-based tape casting of BCY10 appeared to be impracticable for reasons associated with the high basicity of this material that results in rapid hydrolysis when in contact with water. Organic tape casting was therefore developed for BCY10, but only on Electrolyte (BCY10)/Anode (BCY10 + NiO) half cells since up to now no cathode material is available. Planar 20 mm diameter circular half-cells were obtained with the aid of a small load on top of the bi-layer to counterbalance the inevitable warping of the samples. Back-scattered SEM and X-Ray computer-controlled microtomography showed sedimentation of some large grains in the green tapes which are believed to have formed by a mechanism associated with a porosity gradient. The deformation occurring during sintering was modelled taking into account the elastic, thermal, viscoplastic and sintering components of the total deformation. 2D and 3D Finite Element numerical simulations showed that the driving force for deformation is associated with this porosity gradient.     

Tape casting and characterizations of MgO ceramics

We report a high density MgO ceramic substrate produced by the tape casting technology. The tape casting formulation and process produced a uniform tape free of cracking. Y₂O₃ and SiO₂ were used as the sintering aid for the pressureless sintering of the green tape. X-ray diffraction phase identification indicates that MgO is the main phase, while both Y₂O₃ and SiO₂ sintering aids react with MgO to form MgY₄Si₃O₁₃ as the second phase. Liquid phase sintering occurs in the temperature range from 1030°C to ~1500°C, which is confirmed by the simultaneous Thermal Gravitation Analysis/Differential Scanning Calorimeter (TGA/DSC) and the percent linear shrinkage and densification. A 96.5% theoretical density was achieved by presureless sintering at 1650°C for 2 hours, which was further increased to a fully dense structure using hot-isostatic-pressing(HIP) at 1650°C and 207 MPa in argon. Scanning electron microscopy (SEM) and energy dispersive(EDS) spectroscopic analysis on the HIP’ed sample show that MgY₄Si₃O₁₃is located at the MgO grain boundary and the sample has a fully dense structure. The refractive indices and extinction coefficient were measured on the HIP’ed sample along with thermal properties and dielectric properties. Thermal diffusivity and heat capacity were measured to calculate the thermal conductivity.     

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.     

Tape casting of nanocrystalline ceria gadolinia powder

A ceramic ceria gadolinia solid solution membrane for solid oxide fuel cells was fabricated by tape casting using a nanopowder of 37 nm average particle size. A novel combination of solvent and dispersant was used to disperse the nanoparticles. The polymer was added in a dilute stage to guarantee a homogeneous distribution. After casting a remarkable densification of the cast tape suspension from a solid loading of 20 up to 42 vol.% was observed during drying. The green tape was sintered to >92% theoretical density and was dense towards perfusion. The resulting grain size in the sintered specimen still was <200 nm.     

Structure evolution in thermoplastic slips based on aluminum nitride

Conclusions The content of the free and the fixed dispersing medium and the limiting concentration of the solid phase in the thermoplastic slips used for hot pressure casting of aluminum nitride preforms are close to the values of the corresponding parameters of the aqueous dispersions of the ceramic oxide materials. When selecting the composition of the thermoplastic binders, the maximum value of the critical content of the solid phase forms the controlling factor. Other things being equal, the plastic viscosity depends on the degree of filling-up of the system and the volume fraction of the kinetically free dispersing medium; the density of the preforms and their shrinkage during the firing process are determined by the volume fraction of the solid phase in the slip and the volume fractions of the kinetically free dispersing medium.Translated from Ogneupory, No. 3, pp. 11–14, March, 1993.     

Tape casting of PLZST tapes via aqueous slurries

A well-dispersed aqueous slurry for tape casting of PLZST (Pb_0.97La_0.02Zr_0.66Sn_0.23Ti_0.11) was prepared. Properties of PLZST slurries with and without polyelectrolyte dispersant were characterized by zeta potential, sedimentation etc. The experimental results show that the pH value of the slurries is changing with ball-milling time, and that the polyelectrolyte dispersant has a significant effect on the isoelectric point of PLZST. TGA analysis indicates that the organic additives in the green tapes can be completely removed by heat treatment at 600 °C. The density of PLZST ceramics is tightly related to the plasticizer and the sintering temperature. However, the results demonstrated that it is possible to prepare a dense PLZST thick film through aqueous slurry tape casting.