Materials and methods for the manufacture and shaping of transparent ceramics

Abstract

The invention relates to a moldable nanocomposite for producing a transparent article made of a ceramic material, the moldable nanocomposite according to the invention comprising: an organic binder; a powder of a ceramic material dispersed in the organic binder, the powder comprising particles having a diameter in the range of 5 nm to 700 nm, and / or a precursor of a ceramic material dispersed in the organic binder, the precursor being at least one metal-containing compound; and a phase former dispersed in the organic binder, the phase former being solid or viscous at room temperature and forming an internal phase in the organic binder, wherein the combined content of the powder and the precursor of the ceramic material in the moldable nanocomposite is at least 5 parts by volume based on 100 parts by volume of the organic binder, and wherein the moldable nanocomposite does not contain any low-viscosity solvent. Furthermore, the invention relates to a method for producing a transparent article made of a ceramic material, the method according to the invention making use of a moldable nanocomposite according to the invention.

Description

[0001] This invention relates to a moldable nanocomposite for producing transparent articles made of ceramic materials. Furthermore, this invention relates to a method for producing transparent articles made of ceramic materials.

[0002] Transparent ceramics such as magnesium aluminate (also known as spinel), alumina, aluminum oxynitride, calcium fluoride, barium titanate, zirconium oxide, and yttrium aluminum garnet (YAG) are promising materials for the production of high-performance optical devices. Various processes for the manufacture and shaping of transparent ceramics are known in the art, with the hot pressing of particles and casting of aqueous dispersions being among the most important.

[0003] In the hot pressing of particles, as described in G. Gilde et al., J. Am. Ceram. Soc. 2005, 88(10), 2747-2751, for example, the particles are densified under high temperature and high pressure. Typically, the hot pressing of particles requires a flat surface to which pressure can be applied. In the casting of aqueous dispersions, as described in A. Krell et al., J. Am. Ceram. Soc. 2003, 86(1), 12-18, for example, the aqueous dispersion is pre-densified into a powder in water, the water is evaporated, and the powder is sintered into a transparent ceramic. Typically, the casting of aqueous dispersions requires that water be removed through the pre-densified structure. As a result of the porosity caused by the removal of water, inherent shrinkage and warping occur.

[0004] Wang Haomin et al., Adv. Mater. 2021, 33(15), 2007072, describe the 3D printing of transparent spinel ceramics with transmittance close to the theoretical limit.

[0005] WO 2020 / 120458 A1 describes a method for producing thin, transparent ceramic parts and thin, transparent ceramic parts.

[0006] CN 109 650 853A describes a method for preparing a transparent ceramic self-locking bracket.

[0007] CN 111 454 067A describes a transparent ceramic orthodontic bracket and its preparation method.

[0008] CN 112 299 828A describes a method for preparing a transparent ceramic backplate for use in 5G solar-powered mobile phones.

[0009] CN 111 499 371A describes a method for preparing magnesium aluminum spinel transparent ceramics.

[0010] CN 108 516 818A describes a method for preparing YAG transparent ceramics based on an improved Isobam gel system.

[0011] KR 2017 0058048A describes a method for forming MgAl2O4 spinel.

[0012] WO 2020 / 200424 A1 describes sinterable raw materials for use in 3D printing equipment.

[0013] US 7,927,538 B2 describes a stereolithography method for preparing light-cured thermoplastic gutta-percha for dental ceramics.

[0014] US2016 / 002117 A1 describes transparent spinel articles and the tape casting method used for their manufacture.

[0015] US2009 / 321971 A1 describes a method for manufacturing dental restorations using nanocrystalline materials.

[0016] Liu Guanwei et al., J.Eur.Ceram.Soc.2012,32(4),711-715, describe the fabrication of translucent alumina ceramics from pre-sintered bodies soaked in a sintering additive precursor solution.

[0017] Therefore, the above processes impose strict limitations on the shape of transparent ceramics. In other words, none of the above processes can produce transparent products made of ceramic materials that can have any desired, i.e., any freely chosen geometry.

[0018] In view of the above, the object of the present invention is to overcome the disadvantages associated with the manufacturing and forming processes of transparent ceramics known in the art. In particular, the technical problem behind the present invention is to provide a method for producing transparent articles made of ceramic materials, which should allow for the acquisition of transparent articles with freely selectable geometries.

[0019] The aforementioned technical problems behind this invention have been solved by providing embodiments having the features of the appended claims.

[0020] Specifically, in one aspect, the above-mentioned technical problems have been solved by providing a moldable nanocomposite material for producing transparent articles made of ceramic materials. The moldable nanocomposite material according to the present invention comprises:

[0021] Organic adhesives;

[0022] A powder of ceramic material dispersed in an organic binder, the powder comprising particles with a diameter in the range of 5 nm to 700 nm, and / or a precursor of ceramic material dispersed in an organic binder, the precursor being at least one metal-containing compound; and

[0023] A phase-forming agent dispersed in an organic binder, which is solid or viscous at room temperature and forms an internal phase within the organic binder.

[0024] The content of the ceramic powder and precursor in the moldable nanocomposite material is at least 5 parts by volume of organic binder per 100 parts by volume, and

[0025] The moldable nanocomposite does not contain any low-viscosity solvents.

[0026] Specifically, on the other hand, the above-mentioned technical problems have been solved by providing a method for producing a transparent article made of ceramic material. The method according to the invention includes the following steps (a) to (d):

[0027] (a) The moldable nanocomposite material according to the invention is shaped into a predetermined geometry before, during and / or after the organic binder has hardened, thereby obtaining a primary structure;

[0028] (b) By removing the organic binder, the primary structure obtained in step (a) is debonded, thereby obtaining a secondary structure having cavities formed therein;

[0029] (c) Optionally, fill the cavity of the secondary structure obtained in step (b) with at least one additive; and

[0030] (d) Sintering the secondary structure obtained in step (b), optionally filled with at least one additive in step (c), to obtain a transparent article.

[0031] As discovered by the inventors, due to the aforementioned features, the moldable nanocomposite material according to the invention used in the method according to the invention can yield transparent articles made of ceramic materials with freely selectable geometries. This is because the organic binder contains powdered and / or precursoral ceramic materials dispersed therein. By hardening the organic binder, the moldable nanocomposite material containing the organic binder and the powdered and / or precursoral ceramic materials dispersed therein can be shaped into a predetermined geometry corresponding to the geometry of the transparent article to be obtained.

[0032] Advantageously, as described in more detail below, since the moldable nanocomposite material can be shaped into a predetermined geometry by any suitable means known in the art, including subtractive manufacturing processes, additive manufacturing processes, replication processes, or combinations thereof, the predetermined geometry is not further limited. Therefore, the geometry of the transparent article ultimately obtained through debinding and sintering is also not further limited. In other words, the geometry of the transparent article made from ceramic materials derived from powders and / or precursors dispersed in an organic binder can be freely chosen. Due to the presence of the phase-forming agent dispersed in the organic binder, the occurrence of cracks during the production of the transparent article can be effectively suppressed. As a result, transparent articles with considerable thickness can be achieved by utilizing the nanocomposite material according to the invention and the method according to the invention. Therefore, according to the invention, the "transparent article made of ceramic material with a freely chosen geometry" to be produced should be understood as an article of any shape, provided that its thickness is generally greater than 250 μm, preferably greater than 600 μm, and even more preferably at least 1.0 mm. It should be noted that the thickness of the transparent article refers to the shortest distance within the transparent article.

[0033] In the following text, refer to Figure 1 The moldable nanocomposite material according to the present invention and the method according to the present invention are described in detail; however, Figure 1 The content is not to be construed as restricting in any way.

[0034] According to the present invention, the nanocomposite material used to produce transparent articles made of ceramic materials is moldable, meaning that its organic binder is in a moldable state or can be transformed into a moldable state. Using an organic binder in a moldable state or transformable into a moldable state, the moldable nanocomposite material can be shaped into a predetermined geometry by hardening the organic binder, as described below in conjunction with the method according to the present invention.

[0035] According to the present invention, the organic binder is not further limited as long as it is in a moldable state or can be transformed into a moldable state, i.e., can be cured. As a result of curing the organic binder, the primary structure obtained in step (a) retains the predetermined geometry of the moldable nanocomposite material that has been shaped.

[0036] In one embodiment of the invention, the organic binder is a thermoplastic that hardens upon cooling. Therefore, cooling causes the softened thermoplastic to solidify, thus removing the organic binder from its moldable state.

[0037] When the organic binder is a thermoplastic, it may be selected from polyesters based on aromatic or aliphatic dicarboxylic acids and diols and / or hydroxycarboxylic acids, polycarbonates based on aliphatic or aromatic diols, such as polyethylene, polypropylene, polybutene, polymethylpentene, polyisobutylene and other polyolefins, poly(ethylene-vinyl acetate), ethylene propylene rubber (EPR), poly(ethylene propylene diene), poly(vinyl butyral) (PVB), polyacrylates and polymethacrylates, cyclic olefin polymers, and polyamides such as polyoxymethylene and other polyacetals, polyethers such as polyethylene glycol (PEG), including bisphenol-based aromatic polyethers, or polyurethanes, or combinations thereof, but not limited thereto.

[0038] In another embodiment of the invention, the organic binder is a resin capable of curing upon initiation of curing or polymerization by an external stimulus. In this context, heat or radiation, particularly UV radiation, can be mentioned as the external stimulus. In some cases, even mixing as an external stimulus may be sufficient, for example in two-part resins, where the liquid components of the resin exhibit sufficient reactivity with each other. Furthermore, as needed, the external stimulus may include an initiator added to the organic binder to promote curing or polymerization of the organic binder. Suitable initiators are known to those skilled in the art, such as acetophenones like 2,2-dimethoxy-2-phenylacetophenone (DMPAP), azo compounds like azobisisobutyronitrile (AIBN), benzophenone derivatives, fluorescein and its derivatives such as Bengal red, quinones such as camphorquinone, and phosphine derivatives such as diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, but are not limited thereto. Thus, curing or polymerization initiated by an external stimulus transforms the liquid components of the resin into solids, causing the organic binder to no longer be in a moldable state. When exposed to external stimuli, depending on the resin used, the resin either cures, resulting in a cross-linked structure, or polymerizes, resulting in a non-cross-linked structure. In other words, the term "resin" as used herein includes not only thermosetting resins but also thermoplastic resins. That is, as a resin, this document may refer to any monomer and / or oligomer and / or polymer composition without limitation.

[0039] When the organic binder is a resin, it may be selected from acrylate resins and methacrylate resins, unsaturated polyester resins, vinyl ester resins, epoxy resins, thiol-olefin resins, or polyurethane resins, but is not limited thereto. In particular, when the organic binder is a resin, 2-hydroxyethyl methacrylate (HEMA) or a mixture of 2-hydroxyethyl methacrylate and tetraethylene glycol diacrylate (TEGDA) may be mentioned as organic binders.

[0040] In addition to the organic binder, the moldable nanocomposite material also includes ceramic material powder and / or precursors as an essential component. The ceramic material powder and / or precursors are dispersed in the organic binder. Depending on the organic binder used, the dispersion of the ceramic material powder and / or precursors can be achieved by any suitable means known in the art. When a thermoplastic is used as the organic binder, the thermoplastic can be softened or dissolved in a suitable organic solvent or gas phase before adding the ceramic material powder and / or precursors to the thermoplastic. When a resin is used as the organic binder, the ceramic material powder and / or precursors can be added directly to the liquid component of the resin. The inclusion of both ceramic material powder and ceramic material precursors can result in higher filler levels, which in turn can lead to less shrinkage during sintering in step (d).

[0041] In the case of moldable nanocomposites comprising ceramic material powder, the powder contains particles with diameters ranging from 5 nm to 700 nm, preferably from 7 nm to 400 nm, more preferably from 20 nm to 350 nm, even more preferably from 30 nm to 300 nm, and most preferably from 50 nm to 250 nm. These particles are also referred to herein as first-type particles. Furthermore, the powder may contain particles with diameters ranging from 1 μm to 50 μm, preferably from 2 μm to 40 μm. These particles are also referred to herein as second-type particles. When the powder contains both first-type and second-type particles, i.e., a bimodal mixture of particles, the smaller-diameter particles can fill the gaps between the larger-diameter particles. Therefore, a denser packing of particles in the moldable nanocomposites is achieved, which in turn leads to less shrinkage during sintering in step (d). In principle, the powder may also contain any other type of particles with diameters different from both the first-type and second-type particles. Such multimodal mixtures of particles are also within the scope of this invention.

[0042] In this document, the diameter of the first type of particles, the second type of particles, and any other type of particles should be understood as the average diameter measured according to ISO 9276-2. According to the invention, the particles need not be (perfectly) spherical. That is, they can also be spherically similar, i.e., they can be near-spherical. For example, for the first type of particles with diameters in the range of 5 nm to 700 nm, preferably in the range of 7 nm to 400 nm, more preferably in the range of 20 nm to 350 nm, even more preferably in the range of 30 nm to 300 nm, and most preferably in the range of 50 nm to 250 nm, this means that these particles can substantially have no size with a diameter less than 5 nm, preferably no size with a diameter less than 7 nm, more preferably no size with a diameter less than 20 nm, even more preferably no size with a diameter less than 30 nm, most preferably no size with a diameter less than 50 nm, and substantially no size with a diameter greater than 700 nm, preferably no size with a diameter greater than 400 nm, more preferably no size with a diameter greater than 350 nm, even more preferably no size with a diameter greater than 300 nm, and most preferably no size with a diameter greater than 250 nm.

[0043] In the case where a precursor of a ceramic material is included in a moldable nanocomposite material, the precursor is at least one metal-containing compound. There are no limitations; the at least one metal-containing compound can be selected from the group consisting of organometallic compounds, metal complexes, and metal salts, or a combination of two or more of these. That is, the precursor of the ceramic material serves as the metal source for the ceramic material. When the ceramic material contains only one metal, the precursor is typically a single metal-containing compound. When the ceramic material contains two or more metals, the precursor is typically a mixture of two or more metal-containing compounds, where each metal-containing compound provides one metal. Depending on the organic binder used, the precursor of the ceramic material can be dispersed within finely divided regions ranging from isolated or complexed ions to ionic clusters and organometallic phases.

[0044] As described above, the transparent articles obtained by the method according to the invention from the moldable nanocomposite material according to the invention are made of ceramic materials derived from powders and / or precursors dispersed in an organic binder. According to the invention, the ceramic materials are not further limited. In particular, the ceramic materials can be selected according to the intended use of the transparent articles to be obtained. There are no limitations; the ceramic materials can be selected from the group consisting of magnesium aluminate, alumina, aluminum oxynitride, calcium fluoride, barium titanate, zirconium oxide, and yttrium aluminum garnet. In this context, the term "powder of ceramic material," also referred to herein as ceramic material powder, means that the particles contained in the powder are made of ceramic material. Furthermore, the term "precursor of ceramic material," also referred to herein as ceramic material precursor, means that the ceramic material is formed from at least one metal-containing compound during the sintering process in step (d).

[0045] According to the present invention, the combined content of ceramic material powder and precursor in the moldable nanocomposite material is at least 5 parts by volume, preferably at least 30 parts by volume, and more preferably at least 35 parts by volume, based on 100 parts by volume of organic binder. The higher the combined content of ceramic material powder and precursor relative to the organic binder, the denser the ceramic material deposition in the transparent article obtained in step (d). Surprisingly, even when the combined content of ceramic material powder and precursor is quite high relative to the organic binder, for example, 55 parts by volume or even more based on 100 parts by volume of organic binder, the moldable nanocomposite material can still be shaped into a predetermined geometry in step (a).

[0046] In addition to an organic binder containing powdered and / or precursor ceramic materials dispersed therein, the moldable nanocomposite material includes a phase-forming agent dispersed in the organic binder as an additional agent. The phase-forming agent promotes the debinding of the primary structure in step (b), thereby effectively suppressing crack initiation, as described in more detail below. The phase-forming agent, which is solid or viscous at room temperature (understood herein as 25°C), forms an internal phase in the organic binder. Hereinafter, the term "viscous" should be understood to mean a viscosity at room temperature of at least 1 mPa·s, preferably at least 5 mPa·s, as measured according to DIN 53019. The phase-forming agent can be removed from the organic binder before or during the debinding of the primary structure in step (b), for example, by means of heat treatment that causes the phase-forming agent to evaporate or sublimate, or to decompose. Furthermore, the phase-forming agent can be removed by means of solvent or gas-phase extraction.

[0047] Examples of phase forming agents include alcohols, ethers, and silicone oils, as well as combinations thereof, which have sufficiently high molecular weights and / or are appropriately functionalized to be solid or viscous at room temperature. As a specific example, phenoxyethanol (POE) can be mentioned as a phase forming agent. POE has a viscosity of about 30 mPa·s at room temperature, and is therefore a viscous substance. It can evaporate at atmospheric pressure at a temperature of 242°C. However, due to its high vapor pressure, a considerable amount of it has already been removed at lower temperatures. Furthermore, PEG, as described above, can also be used as a phase forming agent.

[0048] Typically, the content of the phase forming agent in the moldable nanocomposite material is at least 5 parts by volume, preferably at least 10 parts by volume, and more preferably at least 15 parts by volume, based on 100 parts by volume of organic binder. This ensures sufficient formation of the internal phase within the organic binder to facilitate the debonding of the primary structure in step (b) as further described below.

[0049] In addition to phase forming agents, moldable nanocomposites may contain one or more other additional reagents as needed to facilitate the production of the desired transparent articles. According to the invention, preferably, when the total mass of the moldable nanocomposite is 100% by mass, the content of any additional reagents considered as a whole in the moldable nanocomposite shall not exceed 30% by mass, more preferably not more than 15% by mass, even more preferably not more than 10% by mass, and even more preferably not more than 5% by mass. That is, the moldable nanocomposite according to the invention is substantially composed of an organic binder and powders and / or precursors of ceramic material dispersed therein, including any initiators added to the organic binder. Hereinafter, the term "substantially composed of" means that when the total mass of the moldable nanocomposite is 100% by mass (the remainder being phase forming agents and optional other additional reagents), the content of the organic binder and powders and / or precursors of ceramic material dispersed therein, including any initiators added to the organic binder, preferably reaches at least 70% by mass, more preferably at least 85% by mass, even more preferably at least 90% by mass, and even more preferably at least 95% by mass.

[0050] For example, a dispersant may be present to promote the dispersion of ceramic material powders and / or precursors in an organic binder. Nonionic surfactants, such as polyoxyethylene alkyl ethers or polyoxymethylene, and anionic surfactants, such as fatty acids and their salts or aliphatic carboxylic acids and their salts, such as stearic acid and its salts or oleic acid and its salts, can be mentioned as dispersants without limitation. Another example of a dispersant suitable for use in this invention is 2-[2-(2-methoxyethoxy)ethoxy]acetic acid, which can also be used as a phase forming agent. According to the invention, the presence of a dispersant is not essential. That is, the invention also includes embodiments in which the moldable nanocomposite material does not contain any dispersant.

[0051] It is worth noting that once the primary structure is formed, the organic binder is in a solid state (not a liquid, gel, or paste state). As those skilled in the art will know, a solid is characterized by the inability to determine its viscosity. Typically, the moldable nanocomposite does not contain any thickeners. In particular, the moldable nanocomposite does not contain any low-viscosity solvents, such as water. Low viscosity is defined as a viscosity of less than 5 mPa·s at room temperature, as measured according to DIN 53019. According to specific embodiments of the invention, the moldable nanocomposite contains neither triglycerides, waxes and paraffins, nor any plasticizers, such as phthalates and any derivatives thereof.

[0052] In step (a) of the method according to the invention, the moldable nanocomposite material according to the invention, as described in detail above, is shaped into a predetermined geometry before, during, and / or after the curing of the organic binder. This yields a primary structure (also known as a green body). Depending on the organic binder used, curing is either completed upon cooling or during curing or polymerization induced by an external stimulus. The shape of the primary structure already reflects the shape of the transparent article obtained in step (d).

[0053] The moldingable nanocomposite material can be shaped into a predetermined geometry by any suitable means known in the art. In particular, the moldingable nanocomposite material can be shaped in step (a) by a subtractive manufacturing process, an additive manufacturing process, a replication process, or a combination thereof. Depending on the process applied, an organic binder having powders and / or precursors of ceramic material dispersed therein is hardened before, during, and / or after the shaping of the moldingable nanocomposite material.

[0054] In subtractive manufacturing processes, an organic binder containing powdered ceramic material and / or precursors dispersed therein is hardened before the moldingable nanocomposite material is shaped. In other words, after the organic binder has hardened, the moldingable nanocomposite material is shaped into a predetermined geometry. Suitable subtractive manufacturing processes include, but are not limited to, laser-based structuring techniques and CNC machining techniques such as milling, drilling, grinding, sawing, lathing, and polishing.

[0055] In additive manufacturing processes, an organic binder containing powdered ceramic material and / or precursors dispersed therein hardens during the shaping of the moldable nanocomposite. In other words, during the hardening of the organic binder, the moldable nanocomposite is shaped into a predetermined geometry. Suitable additive manufacturing processes include, but are not limited to: selective laser sintering and selective laser melting, fused filament fabrication (also known as fused deposition modeling), stereolithography, two-photon polymerization, inkjet printing, and volumetric printing techniques.

[0056] In the case of replication processes, an organic binder having a powder and / or precursor of ceramic material dispersed therein hardens after shaping the moldable nanocomposite material. In other words, the moldable nanocomposite material is shaped into a predetermined geometry before the organic binder hardens. Suitable replication processes include, but are not limited to, casting, injection molding, (injection) compression molding, extrusion, thermoforming, cold drawing or hot drawing, hot embossing, nanoimprinting, and blow molding.

[0057] As described above, in order to shape moldable nanocomposites into a predetermined geometry, one or more subtractive manufacturing processes, additive manufacturing processes, and replication processes can be combined. For example, when the moldable nanocomposites are shaped by means of a replication process and the primary structure has a visible artifact produced by the replication process, the subtractive manufacturing process can be applied as a post-processing to the primary structure. Alternatively, a coating technique can be applied as a post-processing to smooth the surface of the primary structure obtained in step (a).

[0058] In step (b) of the method according to the invention, the primary structure obtained in step (a) is debonded by removing the organic binder. Thus, a secondary structure, also known as a brown body, is obtained. As a result of debonding the primary structure, i.e., removing the organic binder, the obtained secondary structure has cavities formed therein.

[0059] Depending on the organic binder used, the primary structure obtained in step (a) can be debonded in step (b) by means of heat treatment, chemical reaction, reduced pressure, solvent or gas-phase extraction, or a combination thereof. For example, prior to heat treatment, the primary structure can first be immersed in a solvent for solvent extraction. In principle, any means that can remove the organic binder can be applied without adversely affecting the powder and / or precursor of the ceramic material forming the secondary structure. In this context, those skilled in the art routinely select appropriate conditions for removing the organic binder in step (b).

[0060] For example, in the case of debonding by heat treatment, depending on the size of the transparent article to be obtained, the temperature applied during debonding is typically in the range of 100°C to 600°C, for example, in the range of 150°C to 550°C; the heating rate is typically in the range of 0.1°C / min to 5°C / min, for example, in the range of 0.5°C / min to 1°C / min; and the holding time is typically in the range of 2 minutes to 12 hours. In cases where the size is relatively small, a few seconds are sufficient for debonding the primary structure in step (b). Heat treatment can also be carried out in a stepwise manner. Based on the above considerations, by means of reduced pressure, i.e., pressure below atmospheric pressure, which makes the organic adhesive more volatile, debonding by heat treatment can be further promoted.

[0061] In the case of moldable nanocomposites containing ceramic material powder, after the removal of the organic binder, the particles contained in the powder adhere together due to hydrogen bonding. This imparts mechanical stability to the secondary structure. Considering the particle size, with diameters in the nanometer range, the particles have a high specific surface area, which allows sufficient interaction to maintain the mechanical stability of the secondary structure. In the case of moldable nanocomposites containing a ceramic material precursor, the secondary structure is stabilized by the inherent cohesive force of at least one metal-containing compound and by its interaction with the particles contained in the ceramic material powder (if present). As an example, organometallic compounds can be stabilized by secondary forces such as hydrophobic interactions, van der Waals forces, and hydrogen bonding, while metal salts form electrostatic interactions that stabilize the secondary structure. Typically, the secondary structure obtained in step (b) has an amorphous or semi-crystalline morphology.

[0062] Before or during the removal of the organic binder in step (b), the phase-forming agent may be removed from the primary structure, for example by evaporation or sublimation, or by decomposition. The removal of the phase-forming agent may also be accomplished by solvent or gas-phase extraction. In principle, the same methods described above for the removal of the organic binder may be applied.

[0063] As a result of removing the phase-forming agent, debonding of the primary structure in step (b) is facilitated. This is because the internal phase formed by the phase-forming agent in the organic binder creates pores in the primary structure when it is removed. Through these pores, the residual organic binder can then be removed in a more controlled manner. Therefore, it is easier to prevent damage to the secondary structure, especially when it is a thick structure. The same applies when the organic binder is removed in several steps. For example, in the case where the organic binder is a combination of two or more binder components exhibiting different thermal decomposition behaviors, debonding can be performed sequentially. In this case, after removing the first binder component, i.e., the binder component with the lowest decomposition temperature, further removal of the binder component is facilitated due to the pores created in the primary structure after the removal of the first binder component.

[0064] In step (b), the debonding of the primary structure also allows for the removal of the immersed template structure, which serves as a lost foam within the primary structure. Once the immersed template structure, composed solely of materials identical or chemically similar to the organic binder, is removed during the debonding of the primary structure, its counterpart remains as a hollow structure within the resulting secondary structure. This alteration is also known as sacrificial template replication.

[0065] In step (c) of the method according to the invention, the cavity of the secondary structure obtained in step (b) may be filled with at least one additive. According to the invention, step (c) is optional. The at least one additive, also called a filler, must have suitable dimensions so that it can be introduced into the cavity formed in the secondary structure. In this document, there are no further limitations on the at least one additive, and it may be appropriately selected.

[0066] For example, to impart a specific color to the desired transparent article, at least one additive may be selected from pigments, such as gold(III) chloride (AuCl3), vanadium(III) chloride (VCl3), and chromium(III) nitrate (Cr(NO3)3). As further described below, the secondary structure is exposed to considerably high temperatures during sintering in step (d), so the pigment needs to withstand these temperatures. Therefore, inorganic pigments like those mentioned above are preferred compared to organic pigments that tend to decompose when heated to the considerably high temperatures applied during sintering.

[0067] For example, typically, to impart specific optical properties to the desired transparent article, at least one additive may also be selected from dopant agents that, upon decomposition during sintering in step (d), produce dopants, such as Ag, Al, Au, B, Ti, F, Fe, Na, Ni, K, Ca, Cr, Ce, Co, Cu, Dy, Er, Eu, Gd, Ho, La, Lu, Nd, P, Pt, Pr, Pm, Rh, Sm, Sc, Tb, Tm, V, Yb, Y, Ge, Pb, Ba, Zr, Zn, and Mg. Suitable dopant agents are known to those skilled in the art.

[0068] Furthermore, to increase the density of the desired transparent product, at least one additive can be selected from the ceramic material powder and the ceramic material precursor. By filling the secondary structure with the ceramic material powder and / or precursor, the shrinkage of the secondary structure during sintering can be reduced. Therefore, after sintering in step (d), the ceramic material originates not only from the powder and / or precursor dispersed in the organic binder, but also from the powder and / or precursor used as at least one additive. In this context, the considerations outlined above regarding the ceramic material powder and / or precursor dispersed in the organic binder are equally applicable to the ceramic material powder and / or precursor used as at least one additive. As will be apparent to those skilled in the art, in addition to what has been outlined above, when the ceramic material powder is used as at least one additive, the particles contained in the powder need to have a suitable diameter, i.e., a suitable size.

[0069] In step (c), the cavity of the secondary structure can be filled with at least one additive by immersing the secondary structure in a solution containing at least one additive, exposing the secondary structure to physical or chemical vapor deposition in an atmosphere containing or generating at least one additive, or a combination thereof. However, in principle, any other filling process can also be applied in this regard. For example, a sol-gel process can also be used. Depending on the at least one additive used to fill the cavity of the secondary structure in step (c), the secondary structure can first be immersed in a solution containing one additive, and then exposed to physical or chemical vapor deposition containing or generating another additive. Depending on the situation, the cavity of the secondary structure can be filled with at least one additive even before the primary structure has completely debonded. In this case, it is a partially debonded primary structure filled with at least one additive.

[0070] In step (d) of the method according to the invention, the secondary structure obtained in step (b) is sintered (optionally filled with at least one additive in step (c)). Thus, a transparent article is obtained.

[0071] Suitable sintering conditions are known to those skilled in the art and are routinely selected as needed. There are no limitations, depending on the size of the transparent article to be obtained. The temperature applied during sintering is typically in the range of 700°C to 2000°C, the heating rate is typically in the range of 1°C / min to 10°C / min, for example 5°C / min, and the holding time is typically in the range of 0.5 hours to 48 hours, for example 4 hours.

[0072] In the case where the moldable nanocomposite material includes a precursor of ceramic material dispersed in an organic binder, and / or where the precursor of ceramic material is used as at least one additive, the secondary structure can be pre-sintered at an intermediate temperature prior to step (d) to transform the precursor of ceramic material into ceramic material. For example, pre-sintering can be carried out at a temperature in the range of 400°C to 700°C.

[0073] Following step (d), the resulting transparent article, typically with a density of at least 90% of the theoretical density, can be cooled to room temperature and used as obtained. In this document, an article is considered transparent when it exhibits a transmittance of greater than 10%, preferably greater than 30%, and even more preferably greater than 50% at wavelengths in the range of 400 nm to 1000 nm for a thickness of 1.0 mm. To achieve complete densification, step (d) may include subjecting the sintered secondary structure to hot isostatic pressing (HIP). That is, when the sintered secondary structure is subjected to hot isostatic pressing, the resulting transparent article has a density of approximately 100% of the theoretical density.

[0074] Hot isostatic pressing (HIP) involves applying high temperature and high pressure in an inert gas atmosphere, such as argon. There are no limitations, depending on the size of the resulting transparent article. The temperature applied during HIP is typically in the range of 1000°C to 1800°C, the heating rate is typically in the range of 1°C / min to 5°C / min, for example, 3°C / min, and the holding time is typically in the range of 0.5 hours to 4 hours, for example, 2 hours. The pressure applied during HIP is typically in the range of 100 MPa to 300 MPa, for example, 150 MPa, but is not limited thereto. As a result of HIP, almost no pores remain within the bulk of the resulting transparent article.

[0075] As described above, the secondary structure obtained in step (b) has an amorphous or semi-crystalline morphology. This also applies to the transparent article obtained in step (d). Therefore, the secondary structure obtained in step (b) or the transparent article obtained in step (d) can be transformed into a single crystal, as needed. For this purpose, the secondary structure obtained in step (b) or the transparent article obtained in step (d) is exposed to external stimuli, such as heat or radiation. The transformation can be induced by anomalous grain growth (e.g., using seed crystals). There are no limitations; to achieve a single-crystal transformation through anomalous grain growth, after seeding the secondary structure obtained in step (b) or the transparent article obtained in step (d), the following approach can be applied in a hydrogen atmosphere:

[0076] Heating rate: 15℃ / minute; 25℃→1880℃; Holding time: 3 hours

[0077] Cooling rate: 15℃ / min: 1880℃→1750℃; Holding time: 100 hours

[0078] Cooling rate: 15℃ / minute: 1750℃ → 25℃ (End)

[0079] If the above scheme is performed after step (b), for example, an additional heat preservation step at 1300°C can be used in advance.

[0080] Single-crystal transformations through anomalous grain growth are well known in the art. For example, details can be found in US 5,549,746, J.Am.Ceram.Soc. 2002, 85(5), 1275-1280 by C. Scott et al., and J.Am.Ceram.Soc. 2007, 90(3), 993-995 by S. Dillon et al.

[0081] In this document, the transformation of the secondary structure obtained in step (b) or the transparent article obtained in step (d) into a single crystal is also referred to as step (e). According to the present invention, step (e) is optional.

[0082] Given the freedom to choose its geometry, transparent articles obtained from the moldable nanocomposite material according to the invention via the method are suitable for a wide variety of applications, including applications in the optical field, such as lenses. Advantageously, transparent articles with considerable thickness can even be achieved using the invention. Since any suitable means known in the art can be applied here, including subtractive manufacturing processes, additive manufacturing processes, replication processes, or combinations thereof, the method according to the invention using the nanocomposite material allows for the high-throughput and high-resolution production of transparent articles made of ceramic materials.

[0083] The attached figure shows :

[0084] Figure 1 An exemplary production of a transparent article made of ceramic material according to the present invention is shown by referring to steps (a) to (e): (a) obtaining a primary structure from a moldable nanocomposite material comprising an organic binder and a ceramic material powder dispersed therein; (b) obtaining a secondary structure from the primary structure; and (c) filling with at least one additive; (d) obtaining a transparent article from the secondary structure; and (e) obtaining a single crystal from the transparent article. Steps (c) and (e) are optional. Ceramic material powder may be used instead, or alternatively, a ceramic material precursor may be used. Furthermore, step (e) may be performed directly after step (b).

[0085] Figure 2 The UV / Vis spectrum of the transparent article made of spinel obtained by hot isostatic pressing in Example 1, which is described further below, is shown.

[0086] Figure 3 A photograph is shown of a transparent article made of spinel obtained by hot isostatic pressing in Example 2, which is described further below.

[0087] Figure 4 A photograph is shown of a transparent article made of spinel obtained by hot isostatic pressing in Example 3, which is described further below.

[0088] Figure 5 A photograph of a broken article made of spinel obtained after sintering, as described further below in the comparative example, is shown. Example

[0089] The present invention is further illustrated by the following embodiments, but the present invention is not limited thereto.

[0090] Example 1

[0091] Transparent products made of spinel are produced by casting.

[0092] A moldable nanocomposite material was obtained by dispersing 38 parts by volume of spinel nanoparticles with an average diameter of 200 nm in 100 parts by volume of a mixture of 2-hydroxyethyl methacrylate (68 parts by volume) and tetraethylene glycol diacrylate (7 parts by volume) as organic binders, and 25 parts by volume of 2-[2-(2-methoxyethoxy)ethoxy]acetic acid (25 parts by volume) as a dispersant and phase forming agent. 2,2-Dimethoxy-2-phenylacetophenone was added as an initiator to promote the photocuring of the organic binder.

[0093] A moldable nanocomposite material is cast into a polyethylene mold, and the organic binder is cured by light curing in the wavelength range of 300 nm to 400 nm to obtain a green body. The organic binder is removed from the obtained green body by thermal debinding using the following method to obtain a semi-green body:

[0094]

[0095] The semi-green body obtained by sintering using the following method achieves a density of approximately 96% to 100% of the theoretical density:

[0096] Heating rate: 5℃ / minute; 25℃→1530℃; Holding time: 4 hours

[0097] Cooling rate: 5℃ / minute: 1530℃ → 25℃ (End)

[0098] Complete densification was achieved by hot isostatic pressing in an argon atmosphere at a pressure of 150 MPa using the following method:

[0099] Heating rate: 3℃ / minute; 25℃→1700℃; Holding time: 2 hours

[0100] Cooling rate: 3℃ / minute: 1700℃ → 25℃ (End)

[0101] After hot isostatic pressing, a transparent product made of spinel is obtained. This product has a polycrystalline morphology and exhibits a transmittance of greater than 50% in the wavelength range of 400nm to 1000nm for a thickness of 1.0mm. Figure 2 What I saw in the video.

[0102] Example 2

[0103] Transparent products made of spinel are produced using stereolithography.

[0104] A moldable nanocomposite material was obtained by dispersing 38 parts by volume of spinel nanoparticles with an average diameter of 200 nm in 100 parts by volume of a mixture of 2-hydroxyethyl methacrylate (68 parts by volume) and tetraethylene glycol diacrylate (7 parts by volume) as organic binders, and 25 parts by volume of 2-[2-(2-methoxyethoxy)ethoxy]acetic acid (25 parts by volume) as a dispersant and phase forming agent. 2,2-Dimethoxy-2-phenylacetophenone was added as an initiator to promote the photocuring of the organic binder.

[0105] Moldable nanocomposite materials are printed using a conventional 3D printer (Asiga Pico 2 stereolithography system). During printing, the organic binder hardens by photocuring with wavelengths in the 300nm to 400nm range provided by a light source integrated into the printer, thus obtaining a green body.

[0106] The same hot debonding, sintering, and hot isostatic pressing were performed as described above with respect to Example 1 to obtain a transparent article made of spinel. A transparent article made of spinel with a thickness of 1.0 mm is shown below. Figure 3 As shown.

[0107] Example 3

[0108] Transparent products made of spinel are produced by injection molding.

[0109] A moldable nanocomposite material was obtained by dispersing 38 parts by volume of spinel nanoparticles with an average diameter of 200 nm in 100 parts by volume of a mixture of polyvinyl butyral (42.5 parts by volume) as an organic binder, stearic acid (15 parts by volume) as a dispersant, and polyethylene glycol (42.5 parts by volume) as a phase forming agent. The organic binder was preheated to disperse the spinel nanoparticles.

[0110] The moldable nanocomposite material was injected for 4 seconds at 130°C and 8 bar using a micro injection molding machine (DSM X-Plore). The pressure was maintained at 8 bar, and the mold temperature was set to 55°C. This yielded a green compact.

[0111] The same hot debonding, sintering, and hot isostatic pressing were performed as described above with respect to Example 1 to obtain a transparent article made of spinel. A transparent article made of spinel with a thickness of 4.5 mm is shown below. Figure 4 As shown.

[0112] Comparative example

[0113] Products made of spinel are produced by casting without the use of phase forming agents.

[0114] A moldable nanocomposite material was obtained by dispersing 38 parts by volume of spinel nanoparticles with an average diameter of 200 nm in 100 parts by volume of a mixture of 2-hydroxyethyl methacrylate (93 parts by volume) and tetraethylene glycol diacrylate (7 parts by volume) as an organic binder. The spinel nanoparticles were dispersed using a cationic polymer dispersant (Hypermer KD1). 2,2-Dimethoxy-2-phenylacetophenone was added as an initiator to promote the photocuring of the organic binder.

[0115] A moldable nanocomposite material is cast into a polyethylene mold, and the organic binder is cured by light curing in the wavelength range of 300 nm to 400 nm to obtain a green body. The organic binder is then removed from the green body by thermal debinding using the following method to obtain a semi-green body:

[0116]

[0117] The semi-green body obtained by sintering using the following method achieves a density of approximately 96% to 100% of the theoretical density:

[0118] Heating rate: 5℃ / minute; 25℃→1530℃; Holding time: 4 hours

[0119] Cooling rate: 5℃ / minute: 1530℃ → 25℃ (End)

[0120] After sintering, a product made of spinel with a polycrystalline morphology is obtained. Because there is no phase-forming agent in the moldable nanocomposite material, cracking occurs during thermal debonding, resulting in product breakage. A broken product with a thickness of 1.5 mm is shown below. Figure 5 As shown.

Claims

1. A moldable nanocomposite for producing a transparent article made of a ceramic material, the moldable nanocomposite comprising: an organic binder; a powder of a ceramic material dispersed in the organic binder, the powder comprising particles having a diameter in the range of 5 nm to 700 nm, and in addition thereto, the powder comprising particles having a diameter in the range of 1 pm to 50 pm, and / or a precursor of a ceramic material dispersed in the organic binder, the precursor being at least one metal-containing compound; and a phase-forming agent dispersed in the organic binder, the phase-forming agent being solid at 25 °C or having a viscosity of at least 5 mPa-s at 25 °C measured according to DIN 53019, and forming an internal phase in the organic binder, wherein the combined content of the powder and the precursor of the ceramic material in the moldable nanocomposite is at least 5 parts by volume based on 100 parts by volume of the organic binder, wherein the content of the phase-forming agent in the moldable nanocomposite is more than 5 parts by volume and less than 100 parts by volume based on 100 parts by volume of the organic binder, wherein the moldable nanocomposite does not contain any solvent having a viscosity of less than 5 mPa-s at 25 °C measured according to DIN 53019, wherein the moldable nanocomposite does not contain any wax, nor phthalate esters and derivatives thereof, wherein the organic binder is a thermoplastic plastic capable of hardening upon cooling, or the organic binder is a resin capable of hardening upon initiation of a solidification or polymerization by an external stimulus, and wherein the content of the organic binder with the powder and / or the precursor of the ceramic material dispersed therein, including any initiator added to the organic binder, is at least 70 mass-%, in case the total mass of the moldable nanocomposite is 100 mass-%.

2. The moldable nanocomposite according to claim 1, wherein the moldable nanocomposite comprises a precursor of the ceramic material dispersed in the organic binder, the precursor being at least one metal-containing compound selected from the group consisting of organometallic compounds, metal complexes, and metal salts, or a combination of two or more thereof.

3. The moldable nanocomposite according to claim 1 or 2, wherein the ceramic material is selected from the group consisting of magnesium aluminate, aluminum oxide, aluminum oxynitride, calcium fluoride, barium titanate, zirconium oxide, and yttrium aluminum garnet.

4. The moldable nanocomposite according to claim 1 or 2, wherein the combined content of the powder and the precursor of the ceramic material in the moldable nanocomposite is at least 30 parts by volume based on 100 parts by volume of the organic binder.

5. The moldable nanocomposite according to claim 1 or 2, wherein the content of the phase-forming agent in the moldable nanocomposite is at least 10 parts by volume based on 100 parts by volume of the organic binder.

6. A method of producing a transparent article made of a ceramic material, the method comprising the following steps (a) to (d): (a) providing a moldable nanocomposite according to any one of claims 1 to 5, (b) filling the moldable nanocomposite into a mold, (c) removing the mold from the moldable nanocomposite, and (d) sintering the moldable nanocomposite in the mold. (a) before, during and / or after hardening of the organic binder, shaping the moldable nanocomposite according to any one of claims 1 to 5 into a predetermined geometry, thereby obtaining a primary structure; (b) debinding the primary structure obtained in step (a) by removing the organic binder, thereby obtaining a secondary structure having cavities formed therein; (c) optionally filling the cavities of the secondary structure obtained in step (b) with at least one additive; and (d) sintering the secondary structure obtained in step (b), optionally filled with at least one additive in step (c), thereby obtaining the transparent article, wherein the phase-forming agent is removed from the organic binder before or during debinding of the primary structure in step (b).

7. The method according to claim 6, wherein the moldable nanocomposite is shaped in step (a) by means of a subtractive manufacturing process, an additive manufacturing process, a replication process, or a combination thereof.

8. The method according to claim 6 or 7, wherein the primary structure obtained in step (a) is debound in step (b) by means of a thermal treatment, a chemical reaction, a reduced pressure, a solvent or gas-phase extraction, or a combination thereof.

9. The method according to claim 6 or 7, wherein the cavities of the secondary structure obtained in step (b) are filled in step (c) with the at least one additive selected from the group consisting of pigments, doping agents, powders of the ceramic material, and precursors of the ceramic material.

10. The method according to claim 6 or 7, wherein the cavities of the secondary structure obtained in step (b) are filled in step (c) with the at least one additive by immersing the secondary structure in a solution comprising the at least one additive, exposing the secondary structure to a physical or chemical vapor deposition in an atmosphere comprising or generating the at least one additive, or a combination thereof.

11. The method according to claim 6 or 7, wherein step (d) comprises subjecting the sintered secondary structure to hot isostatic pressing.

12. The method according to claim 6 or 7, further comprising the following step (e): (e) transforming the secondary structure obtained in step (b) or the transparent article obtained in step (d) into a single crystal by exposing the secondary structure obtained in step (b) or the transparent article obtained in step (d) to an external stimulus, wherein the transformation is induced by abnormal grain growth with a seed crystal.

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