Sol-gel modified gel casting of ceramic powders

Abstract: The present invention comprises a method for manufacturing a moulded ceramic artefact, comprising the following subsequent steps: preparation of a dispersion of a ceramic powder in an acid aqueous solution, addition of a sufficient amount of a sol-gel component to said dispersion until said dispersion is coagulating, a gelling step, and a drying step.

Description: Field of the invention The invention relates to a method for moulding ceramic powders. State of the art Ceramics in complex shapes are regularly required in several areas of technology. Several methods for manufacturing such complex shapes are known in the art. Moulding ceramic powders under pressure to the desired shape, followed by a sintering step is one of them. The disadvantages of these classic technologies are well known and include long manufacturing time, low precision, form defects, expensive moulds, the lack of flexibility of the method and usually the need for aftertreatments (after sintering) that are necessary to obtain an acceptable end product. U.S. Patents 4,894,194 and 5,028,362 describe a method for moulding ceramic powders using a water based alkaline suspension of ceramic powders, comprising a dispersant such as ammonium polyacrylate, an acrylate monomer and a free radical initiator compound. A catalyst is added just before the slurry is cast in a mould. Polymerisation by heating or catalysis results in a solid mass that can be dried, followed by removing the polymer by heating, and sintering the moulded ceramic powder. Important disadvantages of this technology are the use of poisonous compounds (acryl monomers) and the need for burning out the polymers. Other polymerising agents can be used such as gelatine, proteins, cellulose ethers, polymerising under external influences such as pH change, temperature or (UV) light (Bergström et al, Key Engin. Mat., 132-136, 1997). Another possibility is to coagulate an aqueous suspension of ceramic powder by Direct Coagulation Casting (DCC), using pH change or salt formation (Baader et al, Ceramic Processing Science and Technology, eds Hausner et al, Ceramic Transactions, vol. 51 (1994) 463, The American Ceramic Society; Graule et al, Scientific Forum, cfi/Ber.DKG, 71 (1994) 317.). US patent 5,701,577 describes a method utilising liquid absorbing substances to solidify the ceramic powder suspension in a mould. Sol-gel technology has been used to manufacture high quality optical components. Starting from a TEOS (Tetraethoxysilane) sol comprising colloidal SiO 2 particles, a moulded gel can be formed through hydrolysis/condensation reactions. Heat treatment transforms said gel into a quartz component, dimensioned about 3 times smaller than the mould dimensions (Beghi et al, Eurogel '91, eds Vilminot et al, Elsevier (1992)213; Hench, Ceramics International, 17 (1991)209; Shoup, Ceram. Bull., 70 (1991) 1505.). Ceramic particles can be bound together via sol-gel bonds if the ceramic articles have hydroxyl groups at the surface (I.M. Thomas, SPIE proc., vol 895 (1988) 278). Suspensions of ceramic particles in a sol-gel solution can be used to manufacture a thick ceramic layer by applying the suspension to a substrate surface and sintering the layer (U.S. Patent 5,585,136). Aims of the invention A primary aim of the present invention is to provide a new and improved method for moulding ceramic powders to provide ceramic products in complex shapes. Said new method should solve the problems present in the prior art. General description of the invention The present invention is a method for manufacturing a moulded ceramic artefact, comprising the following subsequent steps: prepare a dispersion of a ceramic powder in an acid aqueous solution, addition of a sufficient amount of a sol-gel component to said dispersion until said dispersion is coagulating, a gelling step, and a drying step. Said ceramic powder is preferably selected from the group consisting of (1)oxides such as Al 2 O 3 , ZrO 2 , TiO 2 , Ta 2 O 5 , CeO 2 ..., (2) mixed oxides such as Y-doped ZrO 2 , (3) oxidic compounds such as PZT, BaTiO 3 , LaMnO 3 ..., (4) cermets such as Ni-ZrO 2 . Said sol-gel component is preferably selected from the group consisting of alkoxides such as TEOS and chemically modified alkoxides such as acetylacetonate-modified aluminiumisopropylate. In a specific embodiment of the present invention, the gelling step comprises raising of the pH, preferably by exposing the coagulated dispersion to ammonia vapour. The drying step in the method according to the present invention comprises preferably the removal of molecular water by exposing the gelled coagulated dispersion to a temperature comprised between room temperature and 150 °C, removal of reaction water and hydrolysed groups by exposing the gelled coagulated dispersion to a temperature comprised between 150 °C and 450°C, and burnout of the remaining hydrocarbon compounds exposing the gelled coagulates dispersion to a temperature higher than 450 °C. The method of the present invention can further comprise a sintering step, comprising exposing the dried gelled coagulated dispersion to a temperature comprised between 500°C and 2000°C for a time comprised between 15 and 180 minutes. The method of the present invention can be further characterised in that the coagulated dispersion and optionally the gelled coagulated dispersion are prepared in a mould. Detailed description of the invention A sol-gel solution of compounds such as alkoxides combined with ceramic particles is used in a new method for ceramic powder moulding. Hydrolysis of alkoxides leads to the replacement of an alkoxide group by a hydroxyl group: -Me-OC x H y + H 2 O →-Me-OH + C x H y OH The hydrolysed compound can be chemically coupled to a ceramic particle comprising hydroxyl groups at its surface: -Me-OH + HO-Particle → -Me-O-Particle + H 2 O Sol-gel compounds form three-dimensional networks through hydrolysis-condensation reactions and can be used to immobilise ceramic particles to a solid, green mass. Performing these reactions in a mould, followed by drying and sintering yields a ceramic product. Hydrolysis of alkoxides is catalysed in acid media while condensation is inhibited. On the other hand, condensation is catalysed in alkaline solutions. The new method of the invention combines sol-gel casting and ceramic gel casting. In a preferred embodiment of the present invention, metal alkoxides are used. These compounds hydrolyse in acid medium with the replacement of alkoxide groups by hydroxyl groups. In moderate alkaline medium, a condensation reaction between hydroxyl groups takes place to form water and an oxide bond. A gel is formed which is transformed into an oxide layer through thermal heating, light irradiation or plasma treatment. The use of the metal alkoxides as in the present invention has the major advantage that no new components have to be introduced to obtain the gel. Indeed, the alkoxides can be transformed, after gelling, to the corresponding oxides, using only moderate temperatures (< 200°C) to remove the alkoxy groups. Further, the three-dimensional gel network is reinforced and inclusion of ceramic particles promoted since chemical bonds (oxide bonds) are formed between the ceramic particles and the sol-gel compounds, both resulting in oxide bonds between ceramic compounds. Drying of the obtained gel in the desired form is performed using the known techniques for sol-gel drying (Van de Leest, R.E. et al.,Appl.Surf.Sci., 106 (1996) 412 and Keddy, J.L. et al., J.Amer.Ceram.Soc., 77 (1994) 1592). Usually, three stages are required: 1. Temperature (T°) between room temperature and 150°C: removal of molecular water 2. T° between 150 and 400°C: removal of reaction water and hydrolysed groups such as alcohol, acetic acid, acetaldehyde, ... 3. T° above 400°C: burnout of the remaining hydrocarbon compounds. Gel casting needs to be performed with a high ceramic particle dispersion degree (preferably higher than 50 vol%) to limit shrinking effects. The particles have to be dispersed in an acid solution. To obtain such high dispersion, the solution needs to be sufficiently acidified, depending on the type of acid used. E.g. a high degree dispersion of Al 2 O 3 is feasible in an HNO 3 or HCl solution, but not in H 2 SO 4 or acetic acid. Dispersion agents can be added. The present invention will be further clarified using non-limiting examples and figures. Short description of the figures: Figure 1 describes a flow chart of a method according to a preferred embodiment of the present invention. Example 1: This example describes how a ceramic product can be formed by casting a sol-gel dispersion into a mould. The dispersion degree of 50 vol% Al 2 O 3 in 0.3N HNO 3 with 0.1N TEOS recedes to about 37.5 vol%. The addition of TEOS until a concentration of about 1N is reached results in a coagulation. In the case that the TEOS is not hydrolysed, coagulation can take a few hours, resulting in a green gel. Exposure of the green gel to ammonia vapour raises alkalinity of the gel and provokes condensation reactions, leading to a stronger gel structure. This gel is mechanically workable and can easily be taken out of the mould. Drying is performed by using the following temperature profile: Temperature (°C) Duration (hours) 150 2 450 2 700 2 The weight loss after drying is less than 1%. The dried mass is further sintered at a temperature of 1575 °C during 15 minutes. The result is a heterogeneous ceramic mass comprising aluminiumoxide particles sizing between 0.5 and 3 µm and silicon dioxide particles measuring up to 10 µ m. The density is about 89% of the theoretical density. Example 2: The same procedure is followed as in example 1, but now acetylacetonate modified aluminiumisopropylate is added up to a concentration of 0.1N instead of adding TEOS. Coagulation with aluminiumisopropoxide can be controlled through the incorporation of non hydrolysable groups such as acetylacetone groups. The aluminiumoxide-aluminiumisopropoxide system results in a homogeneous ceramic mass comprising particles sizing up to about 5 µ m and having a density of about 97% of the theoretical density.

Claims: Method for manufacturing a moulded ceramic artefact, comprising the following subsequent steps: preparation of a dispersion of a ceramic powder in an acid aqueous solution, addition of a sufficient amount of a sol-gel component to said dispersion until said dispersion is coagulating, a gelling step, and a drying step. The method as in claim 1, characterised in that said ceramic powder is selected from the group consisting of oxides such as Al 2 O 3 , mixed oxides such as Y-doped ZrO 2 , oxidic compounds such as BaTiO 3 and cermets such as Ni-ZrO 2 . The method as in claim 1 or 2, characterised in that said sol-gel component is selected from the group consisting of alkoxides such as TEOS and chemically modified alkoxides such as acetylacetonate - modified aluminiumisopropylate. The method as in any of the claims 1 to 3, characterised in that the gelling step comprises raising of the pH. The method as in claim 4, characterised in that the raising of the pH comprises exposing the coagulated dispersion to ammonia vapour. The method as in any of the claims 1 to 5, characterised in that said drying step comprises removal of molecular water by exposing the gelled coagulated dispersion to a temperature comprised between room temperature and 150 °C, removal of reaction water and hydrolysed groups by exposing the gelled coagulated dispersion to a temperature comprised between 150 °C and 450°C, and burnout of the remaining hydrocarbon compounds exposing the gelled coagulates dispersion to a temperature higher than 450 °C. The method as in any of the claims 1 to 6, further comprising a sintering step, comprising exposing the dried gelled coagulated dispersion to a temperature comprised between 500°C and 2000 °C for a time comprised between 15 and 180 minutes. The method as in any of the claims 1 to 7, characterised in that the coagulated dispersion and optionally the gelled coagulated dispersion are prepared in a mould.

Inventors: LUYTEN JAN, VAN DE LEEST RENE, LUYTEN, JAN, VAN DE LEEST, RENE, van de Leest, René
Applicants: VITO, "VLAAMSE INSTELLING VOOR TECHNOLOGISCH ONDERZOEK"
Application year: 2000
Main classification: C04B35
IPC codes: C04B35