GREEN BODY

FIELD OF THE INVENTION

The present invention is generally directed to the manufacture of advanced ceramics, and is in particular directed to a ceramic casting method for shape forming of a ceramic green body.

BACKGROUND TO THE INVENTION

Advanced ceramics have found many applications such as in solid oxide fuel cells (SOFCs), solar cells, gas sensors, biomedical devices, and gas separation membranes. The high reliability and low cost of manufacturing is the key to commercial production of advanced ceramics. It is well known that fine powders (e.g., <1 μm in size) are usually used to fabricate advance ceramics with desired homogenous microstructures; fine particles are prone to aggregation because their surface force is 3-10 orders of magnitude greater than the corresponding gravitational force. Like starting materials, the shape-forming method has a significant influence on the microstructure of ceramic green body and thus product reliability. Conventional shape-forming methods such as dry powder pressing and paste extrusion do not always meet the requirements of ceramic fabrication because non-uniformly aggregated particles existing in such green bodies tend to cause defects such as cracks in final products. The ceramic shaping reliability is improved by using a colloidal process because ceramic particles can be well dispersed in water or other solvents via repulsive forces between particles; this is especially important for multi-component ceramics because all ingredients need to be fully mixed.

A number of colloidal shape-forming methods including slip casting, tape casting, dip-coating, gelcasting, injection molding, direct coagulation casting, vibraforming, and colloidal isopressing, have been developed so far. Water and/or other solvents are required to prepare a slurry or colloidal suspension, and they need to be removed through drying prior to sintering. During the drying process, the water or solvent is evaporated from the surfaces of the ceramic green body, and capillary tension in liquid often generates non-uniform stresses in the green body, leading to defects (such as cracks). Therefore, the drying process needs to be carefully controlled, and it usually takes a long time, typically from several hours to several days to dry fully. In the case of colloidal isopressing process, the slurry undergoes a plastic-to-elastic transition under high pressure (100-300 MPa), and the green body can be heated directly without drying. However, the high pressure required is not feasible for forming large-dimension ceramic bodies and thin films.

Various methods have been proposed to address the problems associated with the above described methods.

US Patent No. 4,978,643 discloses a method of forming green bodies using a solvent based colloidal suspension. The solvent is evaporated after molding the colloidal suspension by heating. Release of the solvent, however, can lead to cracking and deformation in the green body.

US Patent No. 5,456,877 discloses another water-based colloidal suspension. No mention is made of how the water is removed without distorting the molded article.

US Patent No. 6,228,299 B1 discloses additional water and other solvent based colloidal suspensions, which require an additional heating step to evaporate the solvent.

US Patent No. 5,660,877 discloses a method of forming a liquid based colloidal suspension, which requires an additional step of freeze-drying the molded colloidal suspension to remove the liquid. The freeze-drying step is performed under vacuum for extended time periods to remove all the liquid.

US Patent Application No. 20050203231 A1 discloses a method of manufacture which seeks to address the problem by the development of a polymer colloidal suspension for producing a ceramic green body, comprising polymer, surfactant, dispersant and about 50-70 volume % ceramic powder, wherein the colloidal suspension can be set in a mold. It claims that the colloidal suspension that can be set without distortion or large amounts of shrinkage, has high solids loading and can be formed to have microsized features. The patent application does not however obviate the need for drying.

None of the abovenoted patent documents addresses the problem associated with the need for drying that can result in cracks being formed in the ceramic green body or final ceramic part. It would therefore be advantageous to be able to have a ceramic casting method that eliminates or minimises the need for drying following casting

These are also requirements for ceramic parts having a porous microstructure. These porous ceramic parts should ideally have relatively uniform pore sizes therein. One method used to try to achieve uniform pore size is to add polymer or polymer spheres into the ceramic suspension. These spheres are mixed and dispersed within the suspension. During the final sintering stage, the polymer spheres are burnt off leaving a porous structure behind. There are however a number of problems with this method. Firstly, the addition of the polymer spheres increases the viscosity of the suspension making it more difficult to cast. Secondly, it is difficult to produce spheres of uniform diameter and to disperse these spheres uniformly within the suspension. This results in non- uniformity in the porous microstructure of the final ceramic part. It would therefore also be advantageous to have a ceramic casting method that ensures uniform pore size and distribution within the final ceramic part.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome at least one of the disadvantages of prior art ceramic casting methods.

With this in mind, according to one aspect of the present invention there is provided a colloidal casting method for shape forming of a ceramic green body including the steps of:

a) dispersing ceramic particles in a polymerizable solvent to form a colloidal ceramic suspension,

b) casting the ceramic suspension into the required shape, and c) polymerizing the solvent to solidify the ceramic suspension to form the ceramic green body. The introduction of the polymerizable solvent disperses the fine ceramic particles to form a colloidal ceramic suspension. This ceramic suspension is cast into the required shape, and then solidified by polymerising the solvent to form the ceramic green body. The present invention is therefore virtually drying-free compared with known ceramic casting methods (a small amount of water, typically less than 3.6-7.4 weight percentage (wt%), is produced during the polymerisation process and so the casting method is virtually drying-free).

The polymer networks derived from the polymerizable solvent act as a binder to hold all particles together firmly. In addition, the microstructures, particularly porous structures of the final ceramic products can be tuned by adjusting the amount of the polymerizable solvent given that the solvent-derived polymer can affect sintering process and function as a porogen.

The method according to the present invention may therefore include selecting the weight percentage (wt%) of the polymerizable solvent to thereby control the final porosity of a ceramic part.

The viscosity of the ceramic suspension may also vary depending on the wt% of the polymerizable solvent. Higher viscosity suspensions will need to be cast with the aid of pressure while lower viscosity suspensions can be cast without the need of additional pressure. Therefore, the method according to the present invention may further include selecting the wt% of the polymerizable solvent in dependence on the pressure used during the casting of the ceramic suspension.

The polymerizable solvent may be polymerized at elevated temperatures. It may be preferable to treat the solidified ceramic body at elevated temperatures in the presence of an inert gas, for example nitrogen or argon, to carbonise the polymerized solvent at a high carbon yield. This helps to maintain the porous microstructure during high temperature sintering and can readily be removed at temperatures above 500 ⁰ C when air or oxygen is passed though during the sintering process. The polymerizable solvent may also be polymerized under acidic conditions. An initiator (catalyst) may alternatively be used to polymerize the polymerizable solvent.

According to the present invention furfuryl alcohol (FA) may be used as the polymerizable solvent. FA is a low-viscosity solvent, and polymerizes under an acidic condition and/or at elevated temperature, producing poly (furfuryl alcohol) (PFA). Other liquid monomers can be used as the polymerizable solvent, such as ε-caprolactone, methyl vinyl ketone, methyl methacrylate (MMA), and styrene acrylonitrile. An initiator (catalyst) will be needed to polymerize these monomers. A dispersant, for example poly (vinyl pyrrolidone) (PVP) may also be mixed with the FA to help to form the colloidal suspension. Other surfactants may also be used.

Many different ceramic powders or particles may be used for the present invention, including for example NiO/YSZ (60 wt% NiO: 40 wt% YSZ), AI ₂ O ₃ and YSZ.

According to another aspect of the present invention, there is provided a method of producing a ceramic part including the step of sintering a ceramic green body formed according to the ceramic casting method as described above.

According to a further aspect of the present invention, there is provided a ceramic green body formed using the ceramic casting method as described above.

According to yet another aspect of the present invention, there is provided a ceramic part formed by sintering the ceramic green body described above.

The method therefore has the advantage of allowing for virtually drying- free casting minimising the possibility of cracking in the ceramic green body or in the final ceramic product. Furthermore, the present invention facilitates the manufacture of porous ceramic parts having a microstructure with uniform pore size and distribution.

BRIEF DESCRIPTION OF THE DRAWINGS

It will be convenient to further describe the invention with reference to the accompanying drawings which illustrate embodiments of the present invention.

Other embodiments are possible, and consequently the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention.

In the drawings:

Figure 1 is a graph showing the rheological behaviour of NiO/YSZ suspensions with different amounts of FA;

Figure 2 is a graph showing the TGA curve of NiO-YSZ sample containing

21 .7 wt% PFA;

Figure 3 is a photograph showing (a) a plastic tube used as a mould for forming ceramic tubes (b) and (c), (b) a NiO/YSZ-PFA tube with one closed end, (c) a NiO/YSZ ceramic tube with one closed end, (d) an AI ₂ O ₃ ceramic part with a hexagonal screw nut shape, and (e) a plastic mould for forming the ceramic part

(d);

Figure 4 are two graphs showing the effects of the amount of FA on the sintering shrinkage and porosity (top), and pore size distribution (bottom) of porous Ni/YSZ ceramic determined by mercury porosimetry;

Figure 5 are SEM images of porous Ni/YSZ ceramic prepared from the NiO/YSZ suspension with 21 .7 wt% FA. (a) low magnification, (b) high magnification; and

Figure 6 are SEM images of free-standing dense YSZ film sintered at

1450 ⁰ C for 5 h (a, b, c), and porous YSZ film sintered at 1350 ⁰ C br 3h (d). (a) cross sectional view at low magnification, (b) cross sectional view at high magnification, (c) surface view, and (d) cross sectional view. DETAILED DESCRIPTION OF THE PRESENT INVENTION

The following experiments were conducted by the applicants to demonstrate the ceramic casting method for shape forming of ceramic parts according to the present invention.

Preparation of colloidal suspensions

2 - 4 wt% of poly (vinyl pyrrolidone) (PVP, M _w ~ 40000, Sigma-Aldrich) was dissolved in FA (Sigma-Aldrich), resulting in a PVP-FA solution. Three kinds of powders, NiO/YSZ (a mixture of 60 wt% NiO (10 μm, Jinchuan, China) and 40 wt% YSZ (1 μm, Fanmeiya, China)), AI ₂ O ₃ (10 μm, Sigma-Aldrich), and YSZ (0.2-0.5 μm, Sigma-Aldrich) were used in this study. A given amount of ceramic powder was dispersed in the PVP-FA solution by ball milling for 24h to obtain a uniform colloidal suspension with a FA amount ranging from 19.5 wt% to 39.3 wt%. 60wt% p-toluenesulfonic acid (>98.5 %, Sigma-Aldrich) ethanol (>99.9 %, Sigma- Aldrich) solution was added into the colloidal suspension. The resulting colloidal ceramic suspension with pH of 1 ~ 2 was used for viscosity measurement and casting. Preparation of ceramics

Prior to casting, the colloidal ceramic suspension was degassed under vacuum for 5 min. To form an AI ₂ O ₃ ceramic part with a hexagonal screw nut shape, the suspension was poured into a plastic mould. A NiO/YSZ ceramic tube and a YSZ film were prepared by dip coating of ceramic suspensions on the outer side of plastic tube. The free-standing YSZ film was obtained by removing the plastic tube after polymerization. The cast suspensions were heated in an oven set at 70°C for 2-10 h for FA polymerization and suspension solidification. The resulting ceramic green bodies were sintered at 1350-1450° C for 2-5 h. The porous NiO/ceramic was then reduced into porous Ni/YSZ ceramic by hydrogen gas at 750°Cfor 5 h.

Results

The viscosity of NiO/YSZ suspensions in the shear rate range of 0.06-13 s ^"1 at room temperature is shown in Fig. 1. The viscosity of the NiO/YSZ suspensions initially drops significantly, and then tends to be steady as the shear rate is increased. All the suspensions exhibit a typical shear thinning behaviour. As expected, the viscosity of the suspensions decreased with increasing the amount of FA. It is noted that as a rough indicator the viscosity of suspension should be less than 5 Pa-s for pressureless casting. Therefore, the suspensions with less than around 24.4 wt% FA need to be cast with the aid of pressure such as pressure-assisted gelcasting, and those with around 21.7 wt% or more FA are suitable for pressureless casting such as dip-coating and conventional gelcasting. Further viscosity measurements indicate that AI ₂ O ₃ and YSZ suspensions exhibit rheological behaviors similar to the NiO/YSZ suspension. In this study, 21 .7 wt%, 24.2 wt%, and 34.3 wt% FA were chosen for the preparation of NiO/YSZ, AI ₂ O ₃ , and YSZ ceramics.

The TGA of NiO-YSZ sample containing 21 .7wt% FA is shown in Fig. 2. There is a 2.2% mass loss from room temperature to 250°C. This may be attributed to the loss of water adsorbed and generated via PFA crosslinking reaction in the NiO-YSZ-PFA green body. In the temperature range of 250 ~ 550°C, there is a further 20.4% mass loss owing to PFA and PVP burn-off. The total mass loss is 22.6%, which is close to the total amount of FA and PVP added in the casting process. It is noted that a very small amount of water is produced during FA polymerisation. Given that a maximum of two water molecules are produced in the polymerization reaction between two FA molecules, and the amount of water after complete FA polymerization is 18.4 wt% of FA. Considering the amount of FA in the ceramic suspension varies from 19.5 to 39.3 wt%, the amount of water in the ceramic green body is only 3.6-7.4 wt%. In fact, the formation of oligomers and dimethylene ether linkage may also occur; the actual amount of water produced during FA polymerization is less than 3.6 - 7.4 wt%. As a result, the casting process is virtually drying-free.

The sintering profile was devised according to the TGA result. Since the

TGA result showed that PFA-PVP polymers were completely burnt off at temperatures below 550 °C, all green bodies were heated at a low heating rate of 0.5°Gmin ^~1 from room temperature to 550 ⁰ C and kept at 550 ⁰ C for 3 h to avoid possible cracking during combustion of PFA and PVP. Slow heating is especially important for polymer removal from thick-walled products. Finally, ceramic green bodies were heated to high temperatures of 1350-1450 ⁹ C at a heating rate of 3°Gmin ^~1 , and kept at the temperatures for 2-5 h.

Fig. 3 shows the photos of moulds, ceramic green body and ceramic samples. The NiO/YSZ ceramic tube with one closed end was prepared by dip- coating of ceramic suspension on a plastic tube (Fig.3a), followed by FA polymerization. The dark brown NiO/YSZ-PFA tube (Figure 3b) was readily removed from the plastic tube because almost no drying shrinkage occurred in such a drying-free casting. The green NiO/YSZ tube (Fig. 3c) was obtained after PFA and PVP were burnt off during sintering at 1400 ⁰ C for 2h. The NiO/YSZ tube was porous, and had a nitrogen permeance of 9.94 mol nϊ ² s ^"1 Pa ^"1 at a transmembrane pressure drop of 1 atm. Figure 3d shows an AI ₂ O ₃ ceramic part with a hexagonal screw nut shape, which was cast with a plastic mould (Fig. 3e). It is noted that during casting FA polymerizes slowly from linear unconjugated oligomers to black crosslinked final PFA; meanwhile, its colour changes from light yellow to black, and its viscosity increases gradually. FA polymerization rate is usually very slow at room temperature, and this allows enough time for the colloidal suspension to be cast. Depending on the dimension of intending ceramic part, the moulded suspension is usually solidified by heating at 70° C for minutes to hours to ensure the green body is mechanically strong enough for demoulding.

In the ceramic casting method according to the present invention, FA derived PFA not only acts as a binder to hold ceramic particles together, but also can serve as a porogen for producing porous ceramics with the desired microstructure. The effect of the amount of FA (PFA) on the porous structure of Ni/YSZ ceramic was examined. Ni/YSZ ceramic is commonly used as solid oxide fuel cell anode material. The anode performance heavily depends on its microstructure, which needs to be optimized to produce desirable three-phase- boundaries (TPBs). The porous Ni/YSZ ceramic was obtained from the NiO/YSZ ceramic sintered at 1400°C (Fig. 3c) by reducing NiO into Ni in hydrogen at 750°C for 5 h. Fig. 4 displays the sintering shrinkage, porosity and pore size distribution of porous Ni/YSZ ceramic prepared with different amounts of FA. As the amount of FA increases from 19.3 wt% to 39.3 wt%, the linear sintering shrinkage increases from 20.9% to 29.5% whereas the porosity increases substantially from 36.2% to 65.0%, and the mean pore size increases from 0.68 μm to 1.8 μm. The sintering shrinkage of PFA-Ni/YSZ is comparable to that of the Ni/YSZ ceramics prepared by gel-casting.

Porogens are usually required to create porous structures in the fabrication of porous ceramics, and common porogens such as starch, carbon black, and graphite having particle sizes of 10-100 μm tend to form non-uniform pore structures and undesired large pore sizes; these porogens often increase the viscosity of ceramic suspension drastically, making the casting difficult. In contrast, PFA is produced from solvent FA, and is more uniformly distributed throughout the ceramic green body, resulting in a much more uniform microstructure after PFA removal. Fig. 5 shows the SEM images of porous Ni/YSZ ceramic prepared from the NiO/YSZ suspension with 21.7 wt% FA. The porous Ni/YSZ ceramic consists of uniformly distributed Ni grains (bigger size) and YSZ grain (smaller size), which is more uniform in microstructure than that prepared by gelcasting and using starch as the porogen. Furthermore, PFA is carbonized at a high carbon yield when heated in an inert gas such as nitrogen and argon. PFA derived carbon helps to maintain porous structures during high temperature sintering, and it can be readily removed at a temperature above 500°Cwhen air or oxygen flows through. Therefore, the ceramic casting method according to the present invention offers an effective way to tune product microstructure.

Additionally, the ceramic casting method according to the present invention is also well-suited for preparing dense ceramics. For example, YSZ is commonly used as solid oxide fuel cell electrolyte and oxygen permeation membrane, and high density is required for these applications. A free-standing YSZ film was prepared by dip-coating of YSZ-FA suspension and sintering at 1450° C for 5h. The density of the YSZ film was determined to be 5. 898 g-cm ^"3 with a standard deviation of 0.009 g-cm ^"3 from nine tests. The relative density of the YSZ film is calculated to be 99.13 % based on its theoretical density of 5.950 g-cm ^"3 . SEM images shown in Fig. 6a-c indicate that the YSZ film with a thickness of around 40 μm is well sintered. The nitrogen permeation experiment shows that no nitrogen permeates through the YSZ film, which is consistent with the density measurement result. This further implies the uniformity of ceramic green body prepared by drying-free casting, which is essential for subsequent densification. It is noted that the key factors in affecting ceramic densification include (1 ) uniformity of ceramic body, (2) sinterability of ceramic particles, and (3) sintering conditions (temperature and time). During sintering process, PFA was burnt off at low temperatures (e.g., 550°C), the resulting porous YSZ film was very uniform in terms of microstructure, and the YSZ particles with 0.2-0.5 μm in size exhibited good sinterability because of small sizes. The porous YSZ film was fully densified as the temperature was raised to 1450° C. When the YSZ film was sintered at 1350°C for 3 h, porous YSZ film with uniform submicron-sized pores was produced (Fig.θd). Therefore, it is clear that the porosity of the final YSZ film could be tuned by changing sintering conditions (e.g., lower temperature and shorter time).

The ceramic casting method according top the present invention with a polymerizable solvent exhibits many potential advantages in ceramic forming; in particular, defects due to drying process such as deformation and crack can be completely eliminated, and the microstructure of ceramics can be readily tuned by adjusting the amount of PFA. The ceramic casting method according to the present invention also has high potential for meso- and microscale fabrication for micro-electromechanical systems via direct writing methods. As a result, the ceramic casting method according to the present invention is simple, and cost- effective for fabrication of ceramics with designed shape and controllable microstructure.