Method for manufacturing transparent ceramic three-dimensional structure using 3D printing

Abstract

The present invention relates to a method for manufacturing a transparent ceramic three-dimensional structure using 3D printing and, specifically, to a method for manufacturing a transparent ceramic three-dimensional structure using 3D printing, the method comprising the steps of: preparing transparent ceramic particles; supplying a 3D printing slurry containing the transparent ceramic particles and a photocurable resin onto a 3D device plate and then irradiating same with light to form a printing layer, and forming a three-dimensional printing structure by repeating the process of forming the printing layer; sintering the three-dimensional printing structure; and polishing the surface of the sintered structure, wherein the maximum diameter of the transparent ceramic particles is less than or equal to the thickness of the printing layer, and the surface polishing step is a chemical-mechanical composite polishing step.

Description

Method for manufacturing transparent ceramic three-dimensional structures using 3D printing

[0001] The present invention relates to a method for manufacturing a transparent ceramic three-dimensional structure using 3D printing.

[0002] Transparent ceramics have attracted considerable attention in recent years due to their unique combination of properties, such as high mechanical strength, thermal and chemical stability, and a wide optical transparency wavelength range, which offer significant advantages over conventional optical materials.

[0003] Among transparent ceramics, yttria (Y2O3) offers the advantages of low phonon energy, high transparency over a wide wavelength range, and excellent chemical stability. Furthermore, its thermal conductivity is higher than that of yttrium aluminum garnet (YAG) single crystals, and its coefficient of thermal expansion is similar, making it suitable for a wide range of applications. Transparent yttria (Y2O3) ceramics have been manufactured through dry pressing, slip casting, and molding, followed by heat treatments such as hot pressing, cold isostatic pressing (CIP), and vacuum sintering. However, these conventional methods can only produce simple geometric structures such as flat or curved shapes, which limits structural control. On the other hand, as research on transparent yttria (Y2O3) increases, there is a growing demand for its application to complex structures such as laser lenses and sensor protection (Effect of powder milling routes on the sinterability and optical properties of transparent Y2O3ceramics, J. Eur. Ceram. Soc. 41 (2021) 775-780., Effects of ZrO2-La2O3 co-addition on the microstructural and optical properties of transparent Y2O3ceramics. Ceramics International, 43(11), 8525-8530.).

[0004] Meanwhile, 3D printing technologies using additive manufacturing (AM) methods such as digital light processing (DLP) are being proposed as promising alternatives for manufacturing complex and diverse 3D shapes with high resolution, as they have a high degree of molding freedom through a computer-based design process.

[0005] However, the transparency of transparent ceramics is governed by the microstructure and surface roughness, and when attempting to manufacture transparent ceramics using a 3D printing method, there is a problem that light scattering and absorption occur due to defects such as pores generated during the printing process, which seriously reduces transparency. In addition, in the process of forming a 3D shape by stacking printed layers, a rough surface is formed (due to the staircase phenomenon), making it difficult to achieve both high transparency and excellent surface quality.

[0006] In this regard, optimization of printing process parameters such as light intensity, exposure time and resin formulation has been explored to improve properties such as green body density and cure depth of ceramic systems in the past (Fabrication of highly transparent yttria by DLP-based additive manufacturing, Journal of the European Ceramic Society 44 (2024) 6037-6046.).

[0007] Meanwhile, in the additive manufacturing 3D printing process, the thickness of a single printed layer directly affects the amount of light that penetrates the photocurable resin, which in turn affects the depth of curing, the presence of uncured areas, and the adhesion between adjacent layers (see Figure 1). That is, the thinner the single printed layer, the higher the printing resolution, but the more layers are required to manufacture the finished product, potentially causing more interface defects and a higher probability of porosity. On the other hand, the thicker the single printed layer, the more light energy is required to cure one layer, which may cause serious overcuring during this process, which may worsen the stair-step phenomenon or increase the probability of uneven crystal growth or porosity due to the occurrence of uncured areas.

[0008] The inventor of the present invention, while conducting research to manufacture a transparent ceramic three-dimensional structure using 3D printing of an additive manufacturing method, discovered that by controlling the ceramic particles and / or the thickness of one layer so that the maximum diameter of the ceramic particles included in the slurry is less than the thickness of one printed layer, a balance is achieved between the conflicting factors described above, thereby forming a denser microstructure of the manufactured three-dimensional structure and reducing surface roughness, thereby ultimately manufacturing a transparent ceramic having high transparency and excellent surface quality, and completed the present invention.

[0009] The purpose of the present invention is

[0010] The present invention provides a method for manufacturing a transparent ceramic three-dimensional structure using 3D printing.

[0011] To achieve the above purpose,

[0012] On one hand,

[0013] Step of preparing transparent ceramic particles;

[0014] A step of forming a 3D printing structure by supplying a 3D printing slurry containing the above transparent ceramic particles and a photocurable resin onto a 3D device plate, irradiating light to form a printed layer, and repeating the process of forming the printed layer;

[0015] A step of sintering the above three-dimensional printing structure; and

[0016] A step of polishing the surface of the sintered structure; including:

[0017] The present invention provides a method for manufacturing a transparent ceramic three-dimensional structure using 3D printing, characterized in that the maximum diameter of the transparent ceramic particles is less than or equal to the thickness of the printed layer.

[0018] On another note,

[0019] Step of preparing transparent ceramic particles;

[0020] A step of forming a 3D printing structure by supplying a 3D printing slurry containing the above transparent ceramic particles and a photocurable resin onto a 3D device plate, irradiating light to form a printed layer, and repeating the process of forming the printed layer;

[0021] A step of sintering the above three-dimensional printing structure; and

[0022] A step of polishing the surface of the sintered structure; including:

[0023] The maximum diameter of the above transparent ceramic particles is less than or equal to the thickness of the printed layer,

[0024] A method for manufacturing a transparent ceramic three-dimensional structure using 3D printing is provided, characterized in that the step of polishing the surface is a chemical-mechanical composite polishing step including a step of simultaneously performing chemical polishing and mechanical polishing using a polishing medium and a chemical polishing solution.

[0025] Here, the step of performing the mechanical polishing and chemical polishing simultaneously may be polishing using at least one of vibration polishing and centrifugal polishing.

[0026] The above polishing solution may be at least one of CeO2, SiO2, and colloidal silica.

[0027] The above polishing medium may include one or more of alumina, SiC, and diamond.

[0028] The above chemical-mechanical composite polishing step

[0029] The step of performing mechanical polishing using a polishing medium may further be included.

[0030]

[0031] The above chemical-mechanical composite polishing step

[0032] A first step of performing mechanical polishing using a first polishing medium; and

[0033] It may include a second step of performing mechanical polishing and chemical polishing simultaneously using a polishing medium smaller than the first polishing medium.

[0034] Additionally, the chemical-mechanical composite polishing step can be performed using a vibrating polisher.

[0035] The thickness of the single printed layer may be 10 to 100 μm.

[0036] The maximum diameter of the above transparent ceramic particles may be 0.9 times or less of the thickness of the single printed layer.

[0037] The method for manufacturing a transparent ceramic three-dimensional structure using the above 3D printing may further include a step of post-heat treating the sintered structure.

[0038] The present invention has the advantage of being able to manufacture a transparent ceramic three-dimensional structure with high transparency and excellent surface quality.

[0039] The present invention has the advantage of being able to manufacture a complex and diverse three-dimensional structure, particularly including a curved surface, with high transparency and excellent surface quality.

[0040] The effects of the present invention are not limited to the effects mentioned above, and other effects not mentioned will be clearly understood by those skilled in the art from the description below.

[0041] Figure 1 is a schematic diagram comparing a three-dimensional structure manufactured by a conventional molding method such as uniaxial pressing, CIP, or HIP with a three-dimensional structure manufactured by a 3D printing method in which printed layers are laminated.

[0042] Figures 2a and 2b are schematic diagrams comparing differences in surface defects such as pores and voids according to the thickness of the printed layer.

[0043] Figures 3a and 3b are drawings to explain the difference in porosity that occurs depending on the thickness of the printed layer during the 3D printing process.

[0044] FIG. 4 is a process diagram schematically illustrating a process for manufacturing a transparent ceramic three-dimensional structure according to one embodiment.

[0045] Figure 5 is a size distribution diagram of ceramic particles manufactured according to Manufacturing Example 1.

[0046] FIGS. 6a to 6e are scanning electron microscope photographs of the microstructure of 3D printed structures (before sintering) manufactured by varying the thickness of the printed layer according to an embodiment, and FIG. 6f is a density graph thereof.

[0047] Figures 7a to 7e are photographs of cross-sections of transparent ceramic structures manufactured with different printing layer thicknesses according to embodiments, observed using a scanning electron microscope.

[0048] Fig. 8a is a graph comparing linear shrinkage rates of transparent ceramic structures manufactured according to an embodiment, and Fig. 8b is a density graph of transparent ceramic structures manufactured by varying the thickness of the printed layer according to an embodiment.

[0049] Figure 9 is a graph showing the results of measuring changes in surface roughness according to the thickness of the printing layer.

[0050] FIG. 10a is an inline transmittance graph measured using a UV-vis-NIR spectrophotometer for a transparent ceramic structure manufactured according to an embodiment, FIG. 10b is a photograph for comparing its transparency, and FIG. 10c is a drawing for explaining a staircase phenomenon that occurs during 3D printing.

[0051] Figure 11 is a graph showing the measurement of the bending strength of a transparent ceramic structure manufactured according to an embodiment.

[0052] Figure 12 is a graph comparing surface roughness after surface polishing.

[0053] Fig. 13a is a graph comparing the transmittance before and after polishing of Fig. 12, and Fig. 13b is a photograph for comparing the transparency of a transparent ceramic structure.

[0054] Fig. 14 is a schematic diagram showing a polishing method performed according to an embodiment.

[0055] Figures 15a to 15e are photographs of the Z-axis surface before polishing and after polishing according to an embodiment, observed using a scanning electron microscope (SEM).

[0056] Figures 16a to 16e are photographs of the Z-axis surface before polishing and after polishing according to an embodiment, observed using an atomic force microscope (AFM).

[0057] Figures 17a to 17f are photographs of the XY plane before polishing and after polishing according to an embodiment, observed using an atomic force microscope (AFM).

[0058] Figure 18 is a graph comparing material removal rates (MMR) according to polishing methods according to embodiments.

[0059] Figures 19a to 19d are surface roughness measurements using an atomic force microscope (AFM) for structures manufactured using different polishing methods according to embodiments.

[0060] Figure 20a is an inline transmittance measured by a UV-vis-NIR spectrophotometer for structures manufactured by different polishing methods according to an embodiment, and Figure 20b is a photograph for comparing the transparency thereof.

[0061] FIGS. 21A to 21D are photographs comparing the transparency of a gradient angle dome, a Fresnel lens, and a porous three-dimensional structure and lens array having a three-dimensional shape including a curved surface manufactured by 3D printing before and after chemical-mechanical composite polishing modification.

[0062] Figures 22a to 22f are photographs of a lens array before the chemical-mechanical composite polishing using CT.

[0063] Figures 23a to 23f are photographs of the lens array after the chemical-mechanical composite polishing using CT.

[0064] Figures 24a and 24b respectively show microstructure photographs and particle size distributions of commercial Y2O3 powder (pure Y2O3) and doped Y2O3 powder (doped Y2O3), and Figure 24c shows an XRD graph thereof.

[0065] Figures 25a and 25b are graphs showing the results of rheological behavior analysis of slurries prepared by varying the dispersant content of slurries containing Y2O3 powder (pure Y2O3) or doped Y2O3 powder (doped Y2O3).

[0066] Figures 26a and 26b are graphs showing the results of rheological behavior analysis of slurries manufactured by varying the ceramic particle content of slurries containing Y2O3 powder (pure Y2O3) or doped Y2O3 powder (doped Y2O3).

[0067] Figures 27a and 27b are graphs of the curing depth and absorption according to UV exposure time of a slurry containing Y2O3 powder (pure Y2O3) or doped Y2O3 powder (doped Y2O3).

[0068] Figures 28a and 28b show the results of thermogravimetric analysis (TGA) and differential gravimetric analysis (DTG) of printed structures manufactured using Y2O3 powder (pure Y2O3) or doped Y2O3 powder (doped Y2O3).

[0069] Figures 29a and 29b are temperature profiles of the degreasing process of a printing structure manufactured using Y2O3 powder (pure Y2O3) or doped Y2O3 powder (doped Y2O3).

[0070] FIGS. 30a and 30b are microstructural photographs of printed structures (green bodies) manufactured with slurries of Manufacturing Example 2 (Doped Y2O3) and Manufacturing Example 3 (Pure Y2O3), respectively, FIGS. 30c and 30d are microstructural photographs of sintered structures, FIG. 30e is an inline transmittance of the two sintered structures, and FIG. 30f is a photograph for comparing the transparency of the sintered structures.

[0071] Figures 31a to 31d show the microstructure of the pre-sintered sintered body, and Figure 31e shows the density of the pre-sintered structure.

[0072] Figures 32a to 32c are photographs showing linear transmittance, density, and transparency evaluations of transparent ceramic structures manufactured according to an embodiment.

[0073] Figures 33a to 33d show the microstructure of a transparent ceramic structure manufactured according to an embodiment.

[0074] Hereinafter, preferred embodiments of the present invention will be described with reference to the attached drawings. However, the embodiments of the present invention may be modified in various different forms, and the scope of the present invention is not limited to the embodiments described below. In addition, the following embodiments are provided to more completely explain the present invention to those with average knowledge in the relevant technical field. Therefore, the shape and size of elements in the drawings may be exaggerated for clearer explanation, and elements indicated by the same reference numerals in the drawings are the same elements. In addition, throughout the specification, the term "including" a certain element does not exclude other elements, but rather means that other elements may be included, unless specifically stated otherwise.

[0075] On one hand,

[0076] Step of preparing transparent ceramic particles;

[0077] A step of forming a 3D printing structure by supplying a 3D printing slurry containing the above transparent ceramic particles and a photocurable resin onto a 3D device plate, irradiating light to form a printed layer, and repeating the process of forming the printed layer;

[0078] A step of sintering the above three-dimensional printing structure; and

[0079] A step of polishing the surface of the sintered structure; including:

[0080] A method for manufacturing a transparent ceramic three-dimensional structure using 3D printing is provided, characterized in that the maximum diameter of the transparent ceramic particles is less than or equal to the thickness of the printed layer.

[0081] Hereinafter, a method for manufacturing a transparent ceramic 3D structure using 3D printing according to one embodiment is described in detail step by step.

[0082] A method for manufacturing a transparent ceramic three-dimensional structure according to one embodiment includes a step of preparing transparent ceramic particles.

[0083] At this time, the transparent ceramic particles are yttria (Y2O3), YAG (Y3Al5O 12 ), magnesia (MgO), spinel (MgAl2O4), zirconia (ZrO2), alumina (Al2O3), and ALON (Aluminum Oxynitride) may be one or more of them.

[0084] The step of preparing the above transparent ceramic particles is

[0085] A step of mixing transparent ceramic powder and sintering additives; and

[0086] It may include a step of calcining the above mixed transparent ceramic powder and sintering additive.

[0087] Through this, the transparent ceramic particles can further include a metal element included in the sintering additive.

[0088] At this time, the sintering additive can be mixed in an amount of 5 to 20 mol per 100 mol of the transparent ceramic powder, and the calcination can be performed at 1000 to 1400°C, but is not limited thereto, and can be performed under various mixing ratios and temperature conditions for including (doping) a metal element of the sintering additive in the conventional ceramic powder.

[0089] By using ceramic particles containing a metal element included in a sintering additive as the above transparent ceramic particles, particle growth can be suppressed during sintering, thereby reducing the occurrence of pores within the sintered body, and ultimately improving the density and light transmittance of the transparent ceramic produced.

[0090] At this time, the sintering additive may be a compound containing at least one element among La, Zr, Yb, Nd, Ca, and Al, and preferably, may be an oxide containing at least one element among La and Zr or a precursor material thereof.

[0091] For example, the transparent ceramic particles may be yttria (Y2O3) doped with one or more elements of La and Zr.

[0092] A method for manufacturing a transparent ceramic three-dimensional structure according to one embodiment includes a step of forming a three-dimensional printed structure by supplying a slurry for 3D printing containing the transparent ceramic particles and a photocurable resin onto a 3D device plate, irradiating light to form a printed layer, and repeating the process of forming the printed layer.

[0093] The above step is a step for manufacturing a 3D printing structure.

[0094] A method for manufacturing a transparent ceramic three-dimensional structure according to one embodiment is characterized by using a molding method, a 3D printing method, specifically, an additive manufacturing (AM) method of 3D printing, and more specifically, a digital light processing (DLP)-based additive manufacturing (AM) method of 3D printing.

[0095] Accordingly, the method for manufacturing a transparent ceramic three-dimensional structure according to one embodiment has the advantage of being able to manufacture not only a three-dimensional structure including a simple plane or curve, but also a three-dimensional shape having a complex shape.

[0096] In the above step, a slurry for 3D printing containing the transparent ceramic particles and a photocurable resin is supplied onto a 3D device plate, and then light is irradiated to form a printed layer, and the process of forming the printed layer is repeated to laminate the printed layer, thereby manufacturing a 3D printed structure.

[0097] The above photocurable resin is intended to cure the slurry by light such as UV.

[0098] The above photocurable resin may include an acrylate series monomer or oligomer having two or more functional groups, and may be, for example, at least one selected from the group consisting of 1,6 hexanediol diacrylate (HDDA), triethylene glycol dimethacrylate (TMPTA), trimethylolpropane triacrylate (TMPTA), urethane dimethacrylate (UDMA), and polypropylene glycol diacrylate (PPGDA), but is not limited thereto.

[0099] The slurry for 3D printing may contain the ceramic particles preferably at least 30% by volume of the total volume of the slurry, and more preferably at 30% by volume to 70% by volume, 35% by volume to 60% by volume, or 40% by volume to 55% by volume.

[0100] This may be to ensure that the photocurable slurry has a viscosity value for an appropriate shear rate so that it can form a normal film shape when applied in a film form. If the photocurable slurry is not applied in a normal film shape, a single printed layer (1 layer) for 3D printing may not be formed normally.

[0101] For example, the slurry for 3D printing may contain yttria (Y2O3) doped with La and Zr in an amount of 44% by volume or less, preferably 40 to 44% by volume.

[0102] If the ceramic particles contained in the photocurable slurry are less than the minimum range, it may be difficult to form a high-density three-dimensional structure to be manufactured, and ultimately, the transparency of the transparent ceramic to be manufactured may be reduced. In addition, if the particles contained in the slurry exceed the maximum range, the viscosity value of the slurry with respect to the shear rate may be too high, making it difficult to apply the slurry in a film form, which may cause a problem in that 3D printing is difficult.

[0103] The above slurry for 3D printing may further include a photoinitiator.

[0104] The above photoinitiator may be a conventional photocurable initiator used in 3D printing. For example, bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide (Irgacure) (BASF, Germany) may be used.

[0105] The above 3D printing slurry may contain 0.05% to 1% by weight of the photoinitiator based on the total weight of the slurry, but the content of the photoinitiator may vary depending on the wavelength or intensity of light.

[0106] Additionally, the slurry for 3D printing may further include a dispersant.

[0107] The above dispersant may be used to control the viscosity of the slurry with respect to the shear rate and also to increase the degree of dispersion of the powder.

[0108] The above dispersant may be at least one selected from the group consisting of CC-9, KD-4, BYK-111, BYK-163, BYK-180, BYK-2001, BYK-2013 and Anti-Terra-U.

[0109] The above 3D printing slurry may contain the dispersant in an amount of 0.5 wt% to 10 wt% based on the total weight of the slurry, but the amount of the dispersant may vary depending on the type and amount of inorganic particles contained in the slurry.

[0110] For example, the slurry containing yttria (Y2O3) doped with La and Zr may contain 1 to 3 wt% of a dispersant, and preferably 1.5 to 2.5 wt% of a dispersant.

[0111] The above 3D printing slurry is supplied onto a 3D device plate, then exposed to light to form a printed layer, and the process of forming the printed layer is repeated to form a 3D printed structure.

[0112] At this time, the 3D device may preferably be a DLP (Digital Light Processing) device.

[0113] The above 3D printing slurry may be supplied to be applied on a 3D device plate (or printing bed) in the form of a film, and at this time, a thin layer may be formed using a doctor blade, but is not limited thereto, and other components that apply the slurry in the form of a film may be used.

[0114] The light in the above step may be ultraviolet (UV) light of 300 nm to 465 nm.

[0115] In the above step, a 3D printed structure can be formed by stacking printed layers by stacking a first printed layer based on a predetermined 3D model, then supplying slurry and performing photocuring on the first printed layer to stack a second printed layer, and then supplying slurry and performing photocuring on the second printed layer to stack a third printed layer.

[0116] At this time, the above “printed layer” means one layer of printed layer cured by light irradiation.

[0117] A method for manufacturing a transparent ceramic three-dimensional structure according to one embodiment can significantly improve the surface quality and transparency of the manufactured transparent ceramic three-dimensional structure by controlling the particle size of the ceramic and / or the thickness of one printed layer.

[0118] Specifically, in a method for manufacturing a transparent ceramic three-dimensional structure according to one embodiment, the maximum diameter of the transparent ceramic particles is less than or equal to the thickness of the printed layer.

[0119] If the maximum diameter of the transparent ceramic particles is larger than the thickness of the printing layer, numerous pores and defects are generated in the three-dimensional printing structure manufactured by the transparent ceramic particles, which causes the surface roughness to increase and the transparency to decrease. Accordingly, the maximum diameter of the transparent ceramic particles is preferably equal to or less than the thickness of the printing layer, more preferably less than the thickness of the printing layer, and preferably 0.9 times or less, 0.8 times or less, 0.7 times or less, 0.6 times or less, or 0.5 times or less of the thickness of the printing layer.

[0120] That is, it is preferable that the thickness of the printing layer is equal to or greater than the maximum diameter of the transparent ceramic particles, preferably exceeds the maximum diameter, and is preferably 1.1 times or more, 1.2 times or more, 1.3 times or more, 1.4 times or more, 1.5 times or more, 1.6 times or more, 1.7 times or more, 1.8 times or more, 1.9 times or more, or 2 times or more of the maximum diameter.

[0121] In addition, if the thickness of the printing layer is too thick, a rough surface is formed due to the step phenomenon that occurs when manufacturing a 3D structure using a 3D printing method of laminated manufacturing when manufacturing a 3D printed structure, which ultimately reduces transparency. Therefore, the thickness of the printing layer may be 100 μm or less, and preferably 10 to 100 μm.

[0122] For example, when forming a three-dimensional structure using a slurry containing ceramic particles having a maximum diameter of about 25 μm, the thickness of the printed layer may preferably be 35 to 55 μm, 40 to 55 μm, 40 to 50 μm, or 42 to 53 μm.

[0123] A method for manufacturing a transparent ceramic three-dimensional structure according to one embodiment includes a step of degreasing the three-dimensional printed structure.

[0124] Additionally, a method for manufacturing a transparent ceramic three-dimensional structure according to one embodiment includes a step of sintering the degreased three-dimensional printed structure.

[0125] At this time, the degreasing is a step for removing organic components such as photocurable resins and dispersants contained within the 3D printed structure, and can be performed at an appropriate temperature range where such organic components can be removed. The degreasing can preferably be performed at a temperature range of 200°C to 700°C in a nitrogen atmosphere.

[0126] Additionally, the sintering step is a step of densifying and forming a final ceramic structure.

[0127] The above sintering step can preferably be performed by a sintering method in a vacuum atmosphere.

[0128] The above vacuum sintering may be performed at a temperature of 1700°C to 1900°C and may be performed at a temperature of 1750°C to 1850°C, but is not limited thereto, and may vary depending on the type of ceramic particles.

[0129] In addition, the above sintering step

[0130] A step of pre-sintering the above 3D printed structure in air; and

[0131] It may include a step of vacuum sintering the above three-dimensional printing structure.

[0132] According to one embodiment, a method for manufacturing a transparent ceramic three-dimensional structure can further improve the density of the sintered body by further performing a pre-sintering step in air before vacuum sintering, thereby ultimately significantly improving the transparency of the transparent ceramic three-dimensional structure.

[0133] At this time, the preliminary sintering can be performed at a temperature of 1100°C to 1500°C, can be performed at a temperature of 1100°C to 1400°C, or can be performed at a temperature of 1300°C to 1400°C.

[0134] For example, when the transparent ceramic particles are yttria (Y2O3), the pre-sintering is performed at 1310 to 1390°C, and the vacuum sintering is performed at 1700°C to 1900°C, thereby significantly improving the transparency of the yttria produced.

[0135] A method for manufacturing a transparent ceramic three-dimensional structure according to one embodiment may further include a step of post-heat treating the sintered structure.

[0136] The above post-heat treatment step is a step for removing oxygen defects that may be contained within the sintered body, and can be performed at 1200°C to 1500°C in air.

[0137] A method for manufacturing a transparent ceramic three-dimensional structure according to one embodiment includes a step of polishing a surface of a sintered structure.

[0138] The above step is a step for reducing surface roughness. Since the method for manufacturing a transparent ceramic three-dimensional structure according to one embodiment manufactures a molded body using a 3D printing method, the surface roughness may be higher than that of a method for manufacturing a sintered body due to the staircase phenomenon. Therefore, it is necessary to reduce the surface roughness through the above polishing step.

[0139] At this time, the polishing can be performed by a method referred to as lap polishing, but since the lap polishing is a method of polishing a flat surface, it may be difficult to apply the lap polishing when the three-dimensional structure being manufactured has a complex structure including various curved surfaces, etc.

[0140] Accordingly, in order to polish the surface of a three-dimensional structure (sintered body) having a complex structure including various curved surfaces, etc., it is preferable that the step of polishing the surface is performed by a method of polishing a three-dimensional structure, and further, in order to further improve the transparency of the structure, it is preferable that the step of polishing the surface includes a chemical-mechanical composite polishing step.

[0141] The above chemical-mechanical composite polishing step includes a step of simultaneously performing chemical polishing and mechanical polishing using an abrasive media and a chemical polishing solution.

[0142] Here, the chemical polishing solution may preferably be at least one of CeO2, SiO2, and colloidal silica, and the polishing medium may include at least one of alumina, SiC, and diamond.

[0143] Additionally, the chemical-mechanical composite polishing step may further include a step of performing mechanical polishing using a polishing medium.

[0144] Here, the mechanical polishing step may preferably be performed before the step of simultaneously performing the chemical polishing and mechanical polishing.

[0145] In addition, the above mechanical polishing step and the step of performing chemical polishing and mechanical polishing simultaneously can all be performed multiple times using a plurality of polishing media of different sizes, by sequentially polishing from a large polishing media to a small polishing media.

[0146] Accordingly, the chemical-mechanical composite polishing step

[0147] A first step of performing mechanical polishing using a first polishing medium; and

[0148] It may include a second step of performing mechanical polishing and chemical polishing simultaneously using a polishing medium smaller than the first polishing medium.

[0149] Specifically, in the first step, mechanical polishing is performed using a first polishing medium, and mechanical polishing is performed multiple times sequentially from a large polishing medium to a small polishing medium, and then, in the second step, mechanical polishing and chemical polishing are performed simultaneously using a second polishing medium smaller than the first polishing medium, and chemical polishing and mechanical polishing can be performed simultaneously multiple times sequentially from a large polishing medium to a small polishing medium.

[0150] At this time, the first polishing medium may have a mesh size of 100 to 3,000, and the second polishing medium may have a mesh size of 4,000 to 15,000.

[0151] In addition, the step of performing the mechanical polishing and the step of simultaneously performing chemical polishing and mechanical polishing can be performed using at least one of a vibrating polisher or a centrifugal polisher, and can preferably be performed using a vibrating polisher. That is, the chemical-mechanical composite polishing step can be performed using a vibrating polisher.

[0152] A method for manufacturing a transparent ceramic three-dimensional structure according to one embodiment can significantly reduce the surface roughness of a three-dimensional structure having a complex structure including various curved surfaces, etc., through chemical-mechanical composite polishing as described above.

[0153] A method for manufacturing a transparent ceramic three-dimensional structure according to one embodiment may further include a washing step.

[0154] At this time, the washing step may be performed using an aqueous solution containing a surfactant. At this time, the surfactant may be Tween 20, sodium dodecyl sulfate, Triton X-100, Pluronic F-127, etc. at a concentration of 0.1% to 2%. Additionally, an ultrasonic washing method may be used for the washing step.

[0155] For example, the washing step can be performed by washing the chemical-mechanical composite polished structure with a 1% Tween 20 aqueous solution using a vibrating polisher, and then ultrasonically washing it in deionized water.

[0156] A method for manufacturing a transparent ceramic three-dimensional structure according to one embodiment can further improve the transparency of the structure manufactured by removing residual impurities through the washing step.

[0157] Hereinafter, the present invention will be described in detail through examples and experimental examples.

[0158] However, the following examples and experimental examples are only illustrative of the present invention, and the content of the present invention is not limited by the following examples.

[0159] <Manufacturing Example 1> Manufacturing of ceramic particles_Manufacturing of doped Y2O3

[0160] Commercial Y2O3 powder (99.99%; YeeYoung Cerachem, Korea) was mixed with La2O3 as a sintering additive and ZrO(CH3COO)2 as a precursor material of ZrO2, and mixed with 2 mm ZrO2 balls in anhydrous alcohol (99.9%; Samchun Pure Chemical, Korea) and milled at 290 rpm for 24 hours. At this time, the amount of Y2O3 powder, La2O3, and ZrO2 was used so that 10 mol% La2O3 and 3 mol% ZrO2 were used per 100 mol% of Y2O3 powder.

[0161] After milling, the suspension was dried in a vacuum oven, then ground and sieved through a 25-μm mesh, and calcined at 1200°C for 4 hours to obtain doped Y2O3. Ceramic particles (doped Y2O3) were manufactured (see Fig. 14).

[0162] <Manufacturing Example 2> Manufacturing slurry for 3D printing_Containing 44% by volume of doped Y2O3

[0163] Doped Y2O3 manufactured in Manufacturing Example 1 A slurry for photocurable 3D printing was prepared by mixing a mixture of TMPTA, HDDA, PPG, and photoinitiator in a rotating centrifugal mixer (ARE-310, THINKY, USA) at 2000 rpm for 10 minutes using 44 vol% of ceramic particles (doped Y2O3), 2 wt% of DISPERBYK111 (Altana AG, Germany) as a dispersant, a mixture of trimethylolpropane triacrylate (TMPTA; Sigma-Aldrich, USA), 1,6-hexanediol diacrylate (HDDA; Sigma-Aldrich, USA), and polypropylene glycol (PPG; Sigma-Aldrich, USA) as a photopolymerizable resin, and Irgacure (BASF, Ludwigshafen, Germany) as a photoinitiator. At this time, the weight ratio of HDDA and TMPTA was set to 8.5:1.5, and the amount of added PPG was set to 30 wt% of the photocurable resin (see Fig. 14).

[0164] <Manufacturing Example 3> Manufacturing slurry for 3D printing_Containing 50% by volume of pure Y2O3

[0165] A slurry for 3D printing was prepared by the same method as in Manufacturing Example 2, except that it contained 50 vol% of commercial Y2O3 powder (99.99%; YeeYoung Cerachem, Korea).

[0166] <Example 1>

[0167] The 3D model was built using SolidWorks software (SolidWorks Corp., USA). The 3D printed structure (green body) was manufactured using the 3D printing slurry of Manufacturing Example 2 at a wavelength of 405 nm and an intensity of 11 mW / cm. 2By applying it to a 3D ceramic printing system based on DLP (Digital Light Processing) utilizing a UV light source, the thickness of one layer of the printing layer was set to 20, 25, 35, 45, and 55 μm as shown in Table 1 below, and a rectangular printed structure having a thickness of 2 mm was manufactured (Examples (1-1 to 1-5)). After 3D printing, the uncured slurry was removed by rinsing with HDDA. After that, degreasing was performed at 600°C in a nitrogen atmosphere to remove organic substances. After that, in order to remove residual carbon and densify, heating was performed at a rate of 2.5°C / min in air, and pre-sintering was performed at a temperature of 1350°C for about 4 hours, and the pre-sintered structure was 1.0×10 3 It was sintered in a vacuum atmosphere of Pa and a tungsten furnace at 1800°C for 10 hours. Afterwards, it was heat-treated in air at 1400°C for 4 hours to remove oxygen defects.

[0168] Afterwards, the surface was polished using a lap polishing method in which the heat-treated sintered body was polished sequentially using 120, 500, 1200, 2000 mesh, 9, 3, and 1 micron circular polishing pads using an automatic polisher, and ultrasonic cleaning was performed using ethanol to complete the manufacturing of a transparent ceramic three-dimensional structure (see Fig. 14).

[0169] Maximum particle size (μm) Printed layer (1-layer) thickness (μm) Example 1-12520 Example 1-22525 Example 1-32535 ​​Example 1-42545 Example 1-52555

[0170] <Example 2 and Comparative Example> In Example 1-4 (printed layer thickness = 45 μm), the surface polishing was performed differently as shown in Table 2 below, and then washing was performed with an aqueous solution containing 1% Tween 20 as a surfactant, followed by ultrasonic cleaning in deionized water. However, the same method as Example 1-4 was performed to complete the manufacture of a transparent ceramic 3D structure (see Fig. 14).

[0171] Comparative example of polishing method 1 ① Not performing surface polishing Example 2-1 ② Centrifugal polishing using SiC polishing powder Example 2-2 ③ Centrifugal polishing using Al2O3 polishing powder Example 2-3 ④ Vibration polishing using SiC polishing powder Example 2-4 ⑤ Vibration polishing using Al2O3 polishing powder Example 1 ⑥ Lap polishing Example 2-5 Chemical-mechanical composite polishing

[0172] Vibratory polishing (SP-B10, Xiamen Aobo Machinery Equipment Co., Ltd., China) was used for vibratory polishing. The vibratory polisher includes a barrel bowl with a vertical vibration motor in the center. The movement of the vibration motor induces three-dimensional vibration, which causes the polishing medium to move rapidly within the bowl and polish the surface of the object to be polished.

[0173] Centrifugal polishing

[0174] A centrifugal grinder (PVC-4, POONG WON INDUSTRIES, INC., Korea) was used for centrifugal grinding. When the machine is activated, the turret rotates around a central axis while simultaneously rotating the barrel in the opposite direction. This simultaneous rotation generates significant centrifugal force that compresses the grinding target and grinding media. As the barrel moves, the grinding media and the grinding target collide, grinding the surface of the target.

[0175] For the above vibration polishing and centrifugal polishing, SiC or Al2O3 polishing powder (polishing powder, Xiamen Aobo Machinery Equipment Co., Ltd., China) was used together with 3 mm × 3 mm × 3 mm SiC triangular polishing abrasive as a polishing medium. The polishing powder was polished multiple times in the order of #300, #800, #1000, #2000, #4000, #12500 from the largest to the smallest, and each polishing was performed at the same speed for 12 hours. At this time, the mass ratio of the SiC abrasive, (SiC, Al2O3) polishing powder, and water were all 10:1:1.

[0176] Chemical mechanical polishing

[0177] Step 1: Mechanical polishing was performed using a vibrating polisher. At this time, 3 mm × 3 mm × 3 mm SiC triangular abrasives and Al2O3 polishing powders were used as polishing media. The polishing powders were polished multiple times in the order of #300, #800, #1000, and #2000 from the largest to the smallest, and each polishing was performed at the same speed for 12 hours. At this time, the mass ratio of the SiC abrasive, Al2O3 polishing powder, and water was all 10:1:1.

[0178] Step 2: The polishing target was washed with acetone and ethanol in sequence, then placed in an oven and dried at 60°C for 30 minutes, and vibrated using the vibration polisher, and chemical polishing and mechanical polishing were performed simultaneously using a 0.05-μm colloidal SiO2 suspension (pH 9.8; ALLIED HIGH TECH PRODUCTS, INC., CA) as a polishing solution.

[0179] At this time, 3mm × 3mm × 3mm SiC triangular abrasive and Al2O3 abrasive powder were used as abrasive media, and the abrasive powder was polished multiple times in the order of #4000, #12500 from the largest abrasive powder to the smallest abrasive powder, and each was polished at the same speed for 12 hours. At this time, SiC abrasive, Al2O3 The mass ratio of the polishing powder and colloidal SiO2 suspension was both 10:1:1.

[0180] <Characteristic Evaluation Method>

[0181] The microstructure of the ceramic particles, printed structures (green bodies), and sintered bodies was observed using a field-emission scanning electron microscope (FE-SEM; JSM-7500 F, JEOL, Tokyo, Japan). The size distribution of the ceramic particles was determined using a laser particle size analyzer (LS13 320, Beckman Coulter, USA). The density was determined using the Archimedes method (ASTM C373-88), and the in-line transmittance was evaluated using a UV-vis-NIR spectrophotometer (Cary 5000, Varian, USA) in the wavelength range of 200–2000 nm.

[0182] Surface roughness and 3D surface morphology were measured using a roughness tester (Surfcorder SEF 3500 K, Kosaka, Japan) and a confocal laser 3D microscope (NS3500, Nanoscope Systems, Inc., Korea). Bending strength was measured using a universal testing system (Instron 6982, Massachusetts, USA). Computed tomography (CT, V tomex m300, GE, USA) was used to detect surface morphology and microstructure. X-ray photoelectron spectroscopy (XPS) was performed using an X-ray photoelectron spectrometer (AXIS SUPRA+, Kratos Analytical Ltd., UK).

[0183] <Experimental Example 1> Transparency Evaluation According to Printing Thickness

[0184] Additive manufacturing (AM), particularly Digital Light Processing (DLP), is a manufacturing process that forms 3D structures by stacking solidified layers one after another. Accordingly, the thickness of each printed layer, along with its microstructure and surface roughness, can play a crucial role in determining the transmittance of transparent ceramics. Therefore, the properties of Y2O3, manufactured with varying layer thicknesses in Example 1, were evaluated using the following method.

[0185] (1) Ceramic particle size

[0186] Doped Y2O3 manufactured in Manufacturing Example 1 The size distribution of ceramic particles (doped Y2O3) was confirmed and the results are shown in Fig. 5.

[0187] Referring to Fig. 5, doped Y2O3 manufactured in Manufacturing Example 1 Ceramic particles (doped Y2O3) are D 50 It was found to be 1914 ± 0065 μm, but it was found to contain particles with a diameter of up to about 25 μm.

[0188] (2) 3D printed structure (Green body)

[0189] Figures 6a to 6e are scanning electron microscope photographs of the microstructure of 3D printed structures (before sintering) manufactured in Examples 1-1 to 1-5 with different printing layer thicknesses, and Figure 6f is a density graph thereof.

[0190] Referring to FIGS. 6a to 6e, it can be seen that multiple pores are formed on the surface of the 3D printed structure (Green body) when the layer thickness is 20 or 25 μm, particularly 20 μm, whereas the number of pores significantly decreases as the layer thickness increases to 35, 45, or 55 μm.

[0191] This can be seen as a result of the fact that when the layer thickness is thinner or closer to the maximum diameter of the ceramic particles (approximately 25 μm), not all powder particles were completely encapsulated within the applied slurry, resulting in a large number of pores and gaps between the layers. In addition, when the thickness of the cured layer is thinner than the particle size, the mechanical interlocking between the layers was insufficient, which increased the possibility of layer delamination. In other words, particles larger than the layer thickness may not properly adhere to the printed layer and the next layer, weakening the adhesive strength and forming pores between the layers. On the other hand, when the layer thickness was 35, 45, or 55 μm, all ceramic particles were completely encapsulated within the slurry, resulting in a dense microstructure of the 3D printed structure without pore formation.

[0192] From the above results, it can be seen that when selecting the thickness of one printing layer, the layer thickness should be formed to be at least equal to or greater than the maximum diameter of the ceramic particles contained in the slurry, and preferably greater.

[0193] Meanwhile, referring to FIG. 6f, it can be confirmed that the density of the printed structure increases as the thickness of the printed layer increases.

[0194] During the photopolymerization process of ceramic slurry, the ceramic powder may undergo surface over-curing due to scattering of incident UV light. This over-curing typically results in weak interlayer adhesion, which can lead to gaps between layers. Referring to the SEM images of Figures 6a to 6e, it can be confirmed that the 3D printed structure has gaps formed between the printed layers. Increasing the thickness of one printed layer while manufacturing a structure with the same overall height can reduce the number of interlayer boundaries, resulting in a denser structure. Furthermore, thicker layers can better support their own weight and maintain their shape, improving structural integrity and stability during printing and minimizing the risk of deformation that could result from gaps or voids between layers.

[0195] (3) Sintered body

[0196] Figures 7a to 7e are photographs of cross-sections of transparent ceramic structures manufactured in Examples 1-1 to 1-5 with different printing layer thicknesses, observed using a scanning electron microscope.

[0197] Referring to FIGS. 7a to 7e, in the case of Examples 1-1 to 1-2 manufactured by setting the printing layer thickness to 20 and 25 μm, micro-sized pores were still visible. This can be seen as because the incomplete 3D printing structure hindered the density increase process of the Y2O3 ceramic, leading to incomplete density increase. On the other hand, in the case of Examples 1-3 to 1-5 manufactured by setting the printing layer thickness to 35, 45, and 55 μm, no clear pores were found.

[0198] Fig. 8a is a graph comparing the linear shrinkage rates of transparent ceramic structures manufactured in Examples 1-1 to 1-5, and Fig. 8b is a density graph of transparent ceramic structures manufactured in Examples 1-1 to 1-5 with different printing layer thicknesses.

[0199] Referring to Fig. 8a, the shrinkage along the Z-axis was significantly higher than the shrinkage along the X- and Y-axes, but there was no noticeable difference between the X- and Y-axes. The Z-axis direction refers to the printing direction, which is the layer deposition direction. Therefore, the larger shrinkage along the Z-axis can be attributed to the fact that the Z-axis direction was formed less densely than the XY-axis direction in the 3D printed structure due to the interlayer voids created by layer deposition in the printing direction. In addition, the shrinkage along the X- and Y-axes was similar at approximately 20% regardless of the layer thickness, whereas the shrinkage along the Z-axis increased with decreasing layer thickness, with significant shrinkage observed for layer thicknesses of 20, 25, and 35 μm. This further confirmed that the layer thickness significantly affects the microstructure of the printed sample, especially in the Z-axis direction.

[0200] Also, referring to Fig. 8b, the relative density of the structure after vacuum sintering and post-heat treatment increased with increasing layer thickness following the same trend as the density of the 3D printed structure, but the difference was minimal when the layer thickness exceeded 25 μm. In particular, when the layer thickness was 45 and 55 μm, the relative density was almost equally high at 99.93% and 99.95%.

[0201] Figure 9 is a graph showing the results of measuring changes in surface roughness according to the thickness of the printing layer.

[0202] The layer-by-layer nature of 3D printing creates a stair-step effect on the surface, resulting in a more irregular surface than when manufactured using conventional methods such as press or injection molding. If the ceramic surface is rough, incident light will not directly penetrate the material but will be scattered in various directions, potentially causing a major loss of transparency.

[0203] As shown in Fig. 9, the roughness of the Z-axis surface, i.e., the printing direction, was significantly higher than that of the XY-axis surface. This indicates that the surface became rougher due to the layer-by-layer lamination process. Furthermore, it can be confirmed that the roughness of the XY-axis surface was not affected by layer thickness, whereas the roughness of the Z-axis surface decreased as layer thickness increased.

[0204] This trend contradicts previous reports that surface roughness in 3D printing increases with increasing layer thickness. Previous reports have shown that thicker layers require more UV light energy for complete curing, and that increased light energy expands the curing width, enhancing the step-like appearance of the surface and increasing roughness. This phenomenon is more evident in slurries with larger curing widths and more severe over-curing. The decrease in surface roughness with increasing layer thickness in Figure 9 can be attributed to the incomplete particle encapsulation, resulting in the formation of surface defects such as pores or voids, when the layer thickness is similar to or smaller than the ceramic particle size.

[0205] FIG. 10a is an inline transmittance graph measured using a UV-vis-NIR spectrophotometer for transparent ceramic structures manufactured in Examples 1-1 to 1-5, and FIG. 10b is a photograph for comparing the transmittance of transparent ceramic structures.

[0206] Referring to FIGS. 10a and 10b, it can be confirmed that the line transmittance increases as the thickness of the printed layer increases from 20 to 45 μm, and has a similar value at layer thicknesses of 45 and 55 μm, which is very close to the theoretical transmittance limit of 84.7%. Specifically, when the layer thickness is 45 μm, the transmittance at a wavelength of 1100 nm was 82.78%, which is 97.73% of the theoretical limit (84.7%), and the transmittance at a wavelength of 400 nm was 71.01%, which is a very high value.

[0207] From the above results, it can be confirmed that the thickness of the printing layer has a significant effect on the microstructure and density of the transparent ceramic 3D structure manufactured using the LP-based 3D printing method, and ultimately has a significant effect on the transparency.

[0208] Specifically, when manufacturing a transparent ceramic 3D structure using a DLP-based 3D printing method, it can be confirmed that by forming the thickness of the printed layer to be equal to or greater than the particle size, more preferably greater than the particle size, and more than twice the particle size, the occurrence of pores within the printed layer can be reduced, thereby reducing the occurrence of light scattering due to the pores and forming a high density, and also reducing surface roughness, ultimately improving the transmittance.

[0209] However, when forming a 3D structure using a DLP-based 3D printing method, if the thickness of the printed layer is too thick, the 3D printing accuracy is expected to deteriorate due to the stair-step phenomenon (see Figure 10c), as reported previously. Therefore, it is preferable that the thickness of each printed layer be formed within the range of 55 μm or less.

[0210] Figure 11 is a graph showing the measurement of the bending strength of transparent ceramic structures manufactured in Examples 1-1 to 1-5.

[0211] Referring to Fig. 11, it can be confirmed that the flexural strength increases with increasing layer thickness from 20 to 45 μm, similar to the trends in density and transmittance, and has a similar value of approximately 199 MPa at layer thicknesses of 45 and 55 μm. In general, higher density leads to higher strength and fracture toughness in transparent ceramics. This is because higher density results in fewer pores, voids, or defects within the material, which act as stress concentrators and crack initiation points, thereby reducing the overall mechanical integrity. In general, high optical transmittance improves mechanical properties due to fewer internal defects, a more uniform microstructure, and lower residual stresses.

[0212] From the above results, it can be confirmed that the transparent ceramic structure manufactured using the DLP-based 3D printing method according to the embodiment can manufacture transparent ceramics having the same or higher density, mechanical properties, and transparency as transparent ceramics manufactured using conventional processes such as conventional uniaxial pressing, CIP, HIP, SPS, and vacuum sintering. In addition, the DLP-based 3D printing method has a superior advantage over conventional processes such as CIP, HIP, SPS, and vacuum sintering in that it can manufacture complex and sophisticated 3D structures.

[0213] <Experimental Example 2> Characteristic Evaluation According to Polishing Method for Polishing a 3D Structure

[0214] (1) Characteristic evaluation according to physical polishing method and polishing powder difference

[0215] Surface roughness is a critical factor in determining the transparency of transparent ceramics. Therefore, to achieve high transmittance, surface roughness must be minimized. Meanwhile, when a ceramic has a complex three-dimensional structure, including curved surfaces as well as flat surfaces, lap polishing, which polishes flat surfaces, cannot achieve this. Therefore, a polishing method capable of polishing three-dimensional structures is required.

[0216] Accordingly, according to Example 2, the microstructure, surface roughness, and transparency of the polished structure using vibration polishing and centrifugal polishing, which are physical methods capable of polishing a three-dimensional structure, were compared with those polished using conventional lap polishing according to Example 1, and the results are shown in FIGS. 12, 13a, and 13b:

[0217] Figure 12 is a graph comparing surface roughness after surface polishing.

[0218] As shown in Fig. 12, the surface roughness varied depending on the surface polishing method, and when comparing the differences in physical polishing methods, when the same polishing powder was used, the surface roughness was lower when vibrating polishing was used than when centrifugal polishing was used. In addition, when the same polishing technique was used, the surface roughness was lower when Al2O3 polishing powder was used than when SiC powder was used.

[0219] Fig. 13a is a graph comparing the transmittance before and after polishing of Fig. 12, and Fig. 13b is a photograph for comparing the transmittance of transparent ceramic structures.

[0220] As shown in Figs. 13a and 13b, it can be confirmed that the transmittance increased as the surface roughness decreased after surface polishing. Among the physical polishing methods ② to ⑤ in Table 2, the most effective method was vibration polishing using Al2O3 polishing powder, which is method ⑤. Compared to the case where surface polishing was not performed, the surface roughness decreased by 92.13% and the in-line transmittance at 1100 nm increased from 7.33% to 63.07%.

[0221] Figures 15a to 15e are photographs of the Z-axis surface before polishing and after polishing ② to ⑤, observed using a scanning electron microscope (SEM).

[0222] Referring to FIGS. 15a to 15e, it can be seen that before polishing, an irregular and rough surface texture was observed due to the presence of a distinct layered arrangement in the Z-axis direction, whereas after polishing, the layered structure of the surface was effectively removed so that it became inconspicuous. However, when examining the surface texture according to the difference in polishing method and polishing powder, it can be seen that a large number of holes were observed when centrifugal polishing and SiC polishing powder were combined, whereas fewer holes were observed when Al2O3 polishing powder was used, and the flattest surface was created when vibration polishing and Al2O3 polishing powder were combined.

[0223] Figures 16a to 16e are photographs of the Z-axis surface before polishing and after polishing ② to ⑤ observed using an AFM, and Figures 17a to 17f are photographs of the XY plane observed using an AFM.

[0224] Referring to FIGS. 16a to 16e and 17a to 17f, both the Z-axis and XY-axis surfaces exhibited very rough textures before polishing, and in particular, a distinct layered pattern appeared on the Z-axis surface, while the roughness was significantly reduced after polishing. Among the physical polishing methods ② to ⑤, the most effective method was vibration polishing using Al2O3 polishing powder, which significantly reduced the surface roughness in the XY direction to 0.091 μm.

[0225] Meanwhile, referring to FIG. 17d, when performing the lap polishing method according to Example 1 of ⑥, the surface roughness in the XY direction was the lowest at 0.060 μm, but this lap polishing method can be used on a flat planar structure, but cannot be applied to a complex three-dimensional structure including a curved surface.

[0226] From the above results, it can be seen that in order to polish a complex three-dimensional structure including not only a flat surface but also a curved surface, a polishing method such as centrifugal polishing or vibration polishing can be used, and among them, when vibration polishing and Al2O3 polishing powder are combined, a transparent ceramic structure having the best surface quality and high transmittance can be manufactured.

[0227] (2) Characteristic evaluation according to chemical-mechanical composite polishing

[0228] As described above, when vibration polishing and Al2O3 polishing powder are combined, the surface quality can be significantly improved by polishing a complex three-dimensional structure including not only a flat surface but also a curved surface, but further improvement in transparency is required.

[0229] Therefore, the final surface roughness was still greater than that achieved through polishing, resulting in a loss of transmittance.

[0230] Accordingly, according to one embodiment, vibration polishing was performed using a chemical-mechanical composite polishing method using #4000 and #12500 polishing powders and a colloidal silica suspension instead of water, and the material removal rate (MMR, MRR = Material removal rate (μμm / h)) was shown in FIG. 18, the surface roughness measured by scanning probe microscopy (AFM) was shown in FIGS. 19a to 19d, the inline transmittance was measured and shown in FIG. 20a, and a photograph comparing the transmittance was shown in FIG. 20b.

[0231] Referring to Fig. 18, it can be seen that when a colloidal silica suspension was used, the MRR was slightly lower than when water was used, but the difference was not significant. However, referring to Figs. 19a to 19d, it can be seen that when a silica suspension was used, the surface roughness was significantly reduced.

[0232] After introducing chemical-mechanical composite polishing, the surface roughness was reduced by up to 95.42%, and the transmittance increased by 66.12%. As a result of evaluating the transmittance after polishing, as shown in Figs. 20a and 20b, it was confirmed that the in-line transmittance at a wavelength of 1100 nm was 74.85%, reaching 88.27% of the theoretical limit.

[0233] The reaction according to the colloidal silica suspension is expected to be as follows.

[0234]

[0235] The hardness of Y2SiO5 (Mohs hardness 5) is lower than that of Y2O3 ceramic and Al2O3 polishing powder, so it is expected to be removed more easily.

[0236] The above method has the advantage of being applicable to three-dimensional structures with complex structures including curved surfaces as well as planes.

[0237] FIGS. 21A to 21D are photographs comparing the transparency of a gradient angle dome, a Fresnel lens, a porous three-dimensional structure, and a lens array having a three-dimensional shape including a curved surface manufactured by 3D printing before and after the chemical-mechanical composite polishing modification.

[0238] As shown in FIGS. 21a to 21d, it can be confirmed that the transparency of the three-dimensional structures manufactured by 3D printing is significantly improved after the chemical-mechanical composite polishing.

[0239] FIGS. 22a to 22f and 23a to 22f are photographs using CT to evaluate the structural and morphological changes of the lens array before and after the chemical-mechanical composite polishing. FIG. 22a is a photograph before polishing, and as shown in FIGS. 22a to 22f, especially in the red box of FIG. 22f, it can be confirmed that a distinct step structure is formed on the surface of the lens array manufactured by 3D printing before polishing. On the other hand, as shown in FIGS. 23a to 23f, especially in the red box of FIG. 23f, it can be confirmed that after polishing, such step structure is removed and a very smooth surface is formed.

[0240] From the above results, it can be confirmed that when manufacturing a transparent ceramic structure having a three-dimensional shape including a curved surface, transparency can be significantly improved through the chemical-mechanical composite polishing.

[0241] <Experimental Example 3>

[0242] In order to improve the transparency of transparent yttria (Y2O3) manufactured by a 3D printing method, the slurries manufactured in Manufacturing Example 2 (Doped Y2O3) and Manufacturing Example 3 (Pure Y2O3) were applied to a LP (Digital Light Processing)-based 3D ceramic printing system in the same manner as in Example 1, but the thickness of one printed layer (1 layer) was set to 35 μm to manufacture a 1 mm thick disk-shaped printed structure, and after a debinding process, vacuum sintering was performed at 1700°C for 10 hours, and thereafter, heat treatment was performed at 1400°C in air for 4 hours to remove oxygen defects. The results are shown in FIGS. 30a to 30f and Table 5.

[0243] At this time, the ceramic powder content in the slurry, the spraying agent content, and the degreasing process temperature profile were determined through the following experiments.

[0244] (1) Evaluation of ceramic particle characteristics

[0245] The morphology and particle size distribution of commercial Y2O3 powder (pure Y2O3) and Y2O3 powder doped with La2O3 and ZrO2 as sintering additives (Doped Y2O3) manufactured in Manufacturing Example 1 are compared in Figs. 24a, 24b, and Table 3 below, the results of elemental analysis using XRF are shown in Table 4, and the results of phase analysis using XRD are shown in Fig. 24c.

[0246] D 10 , μmD 50 , μmD 90 , μmPure Y2O30.306 ± 0.0070.540 ± 0.0171.333 ± 0.021Doped Y2O30.237 ± 0.0161.914 ± 0.06510.491 ± 0.127

[0247] Elemental name Pure Y2O3 / wt.% Doped Y2O3 / wt.% Y8 0.65 187 0.92 3 7 O1 9.3 14 11 7.67 8 3 La-9.2 3 0 1 Zr-2.0 8 5 1 Fe 0.0 3 4 1 0.0 4 1 1 Al-0.0 4 1 6

[0248] As shown in Fig. 24a, Fig. 24b and Table 3 below, Doped Y2O3 showed a wider particle size distribution than Pure Y2O3, and D 50 and D 90 The values ​​also appeared large. This may be due to powder agglomeration. In addition, the purity of the two powders was found to be 99.9% or higher through Table 4 above, and through Table 4 and Figure 24c, although Doped Y2O3 contains La and Zr, the characteristic peaks of the two powders were identical to the peaks of standard cubic Y2O3 (ICDD: 01-071-0049), indicating that La and Zr, which are sintering additives, were doped into Y2O3, and the lattice constant increased through the slight leftward shift of the XRD peak after doping. (2) Particle content and dispersant content in the slurry

[0249] Transparent ceramics require high structural density because existing defects or pores can cause light scattering and hinder transparency. Therefore, high solids loading is necessary to achieve high-density printed structures. However, excessively high solids loading can alter the rheological behavior of the slurry and increase its viscosity, making printing difficult. Therefore, maximizing solids loading by controlling rheological properties is desirable.

[0250] Meanwhile, utilizing an appropriate dispersant concentration can effectively reduce the interparticle attractive forces and prevent agglomeration, thereby reducing the viscosity of the slurry.

[0251] Here, Pure Y2O3 as ceramic particles Alternatively, the rheological behavior as a function of shear rate was analyzed for slurries containing 42 vol% of Doped Y2O3 manufactured in Manufacturing Example 1 but with different concentrations of dispersant, and the results are shown in Figs. 25a and 25b. As a result, as shown in Figs. 25a and 25b, it can be confirmed that the lowest viscosity was achieved at 2% of dispersant in both cases.

[0252] The rheological behavior as a function of shear rate was analyzed for slurries with a dispersant concentration of 2 wt% for higher solids loading, but with the ceramic particle content increased from 44 to 50 vol%, and is shown in Figs. 26a and 26b.

[0253] As shown in FIGS. 26a and 26b, Pure Y2O3 exhibits a low viscosity that allows printing up to 50 vol%, whereas Doped Y2O3 exhibits low viscosity up to 44 vol%, but printing becomes difficult from 46 vol%. Accordingly, it can be seen that when using Doped Y2O3 of Manufacturing Example 1, it is preferable to set the content to 44 vol% or less.

[0254] In general, the smaller the particle size of the powder, the larger the specific surface area and interaction between the particles, which increases the viscosity of the slurry, but the Pure Y2O3 It is expected that the Doped Y2O3 has a higher viscosity than the larger particles due to the formation of irregularly shaped particles through particle agglomeration.

[0255] Reflecting the above rheological behavior evaluation, the curability of the slurry containing 50 vol% of Pure Y2O3 and 44 vol% of Doped Y2O3 was investigated by varying the UV (405 nm wavelength) exposure time, and the results are shown in Fig. 27a. Pure Y2O3 and Doped Y2O3 The results of measuring the absorbance of ceramic particles are shown in Fig. 27b.

[0256] Doped Y2O3 as shown in Fig. 27a The curing depth of pure Y2O3 was very thick. Meanwhile, as shown in Fig. 27b, the absorbance of doped Y2O3 under UV at a wavelength of 405 nm was significantly higher than that of pure Y2O3.

[0257] The above results generally show that the lower the solid content and the larger the particle size, the higher the hardening property of the slurry. However, in the case of the slurry containing Doped Y2O3, despite the low solid content and the large particle size, the ceramic particles have a very high absorbance, which significantly reduces the penetration of UV radiation into the slurry, resulting in a lower hardening depth compared to the slurry containing Pure Y2O3. On the other hand, although the hardening property of Doped Y2O3 was lower than that of Pure Y2O3, it was sufficient to ensure a stable hardened layer and interlayer adhesion.

[0258] (3) Setting the temperature profile for the degreasing process

[0259] The slurries manufactured in Manufacturing Example 2 (Doped Y2O3) and Manufacturing Example 3 (Pure Y2O3) were applied to a 3D ceramic printing system based on LP (Digital Light Processing) in the same manner as in Example 1, but the thickness of one layer of the printing layer was set to 35 μm to manufacture a printed structure in the form of a disk, and a debinding process was performed to remove organic substances such as dispersants, resins, and photoinitiators before sintering.

[0260] At this time, the degreasing process profiles of Figs. 29a and 29b were designed based on the results of thermogravimetric analysis (TGA 55, TA Instruments, USA) and differential gravimetric analysis (DTG) at the rates in the N2 atmosphere of Figs. 28a and 28b.

[0261] As shown in Figs. 28a and 28b, the peaks around 360°C and 415°C were mainly formed by the destruction and volatilization of the cross-linked resin, which is also the largest and fastest mass loss in the debinding process, so the heating rate during this period was set to 0.2°C / min. On the other hand, it was set to 0.5°C / min in the remaining steps. However, since cracks occurred in the pure Y2O3 profile results, the initial heating rate was reduced from 0.2°C / min to 0.1°C / min to produce a crack-free debinding sample.

[0262] Meanwhile, the relative densities of the degreased structures were 57.04 ± 0.68% (Pure Y2O3) and 50.16 ± 0.08% (Doped Y2O3), respectively, which were higher than those of the degreased structures manufactured from the slurry of Manufacturing Example 2, which had a high solid loading.

[0263] (4) Transparency evaluation

[0264] FIGS. 30a and 30b are microstructural photographs of printed structures (green bodies) manufactured with slurries of Manufacturing Example 2 (Doped Y2O3) and Manufacturing Example 3 (Pure Y2O3), respectively, FIGS. 30c and 30d are microstructural photographs of sintered structures, FIG. 30e is an inline transmittance of the two sintered structures, and FIG. 30f is a photograph for comparing the transparency of the sintered structures.

[0265] As shown in FIGS. 30c and 30d, the average grain size of the sintered structure manufactured with the slurry of Manufacturing Example 2 (Doped Y2O3) is approximately 9.97 ± 0.20 μm, which is significantly smaller than the average grain size of 29.49 ± 0.35 μm of the sintered structure manufactured with the slurry of Manufacturing Example 3 (Pure Y2O3). This means that the doped sintering additive successfully suppressed the abnormally fast grain growth. In addition, it can be confirmed that relatively fewer intragranular pores are formed in the sintered structure manufactured with the slurry of Manufacturing Example 2 (Doped Y2O3).

[0266] In addition, as shown in Fig. 30e, it can be confirmed that the transmittance of the sintered structure manufactured with the slurry of Manufacturing Example 3 (Pure Y2O3) at 1100 nm is 15.91%, while the transmittance of the sintered structure manufactured with the slurry of Manufacturing Example 2 (Doped Y2O3) is significantly higher at 51.79%.

[0267] The above results indicate that although the addition of sintering additives negatively affected the flow properties of the ceramic slurry and reduced the maximum solids loading, it ultimately had a significant positive effect on increasing the density of the structure and improving the optical properties.

[0268] Experimental Example 4

[0269] In Experimental Example 3, despite using the slurry of Manufacturing Example 2 (Doped Y2O3) with added sintering additives, the transparency was low at 51.79%, so instead of vacuum sintering at 1700°C for 10 hours, a transparent ceramic structure was manufactured by pre-sintering in air and then vacuum sintering at 1800°C instead of vacuum sintering at 1700°C for 10 hours to make the structure denser.

[0270] At this time, the pre-sintering temperature was changed to 1150, 1250, 1350, and 1450℃, and the microstructure, density, and transparency according to the pre-sintering temperature were evaluated, and the results are shown in Figs. 31a to 31e, Figs. 32a to 32c, and Figs. 33a to 33d.

[0271] Figures 31a to 31d show the microstructure of the structure sintered at the above pre-sintering temperature, and Figure 31e shows the density of the pre-sintered structure.

[0272] As shown in Figures 31a to 31d, no significant difference is observed between the microstructures at 1150 and 1250°C, but upon reaching 1350°C, it can be seen that the grains began to grow and the relative density increased significantly. On the other hand, as the temperature increased to 1450°C, it can be seen that the grains grew rapidly and numerous intragranular pores and closed pores appeared.

[0273] Figures 32a to 32c show photographs of linear transmittance, density, and transparency evaluations of a transparent ceramic structure that was vacuum-sintered at 1800°C, heat-treated in air in the same manner as Example 1, and then polished to complete the structure, and Figures 33a to 33d show its microstructure.

[0274] As shown in Fig. 32a and Fig. 32c, the transmittance increases significantly as the pre-sintering temperature increases from 1150 to 1350°C, and the maximum value of 80.35% at a wavelength of 1100 nm is achieved at 1350°C, resulting in transparent Y2O3. It can be confirmed that 94.86% of the theoretical limit (84.7%) of ceramics was reached. On the other hand, it can be confirmed that it became opaque when the pre-sintering temperature was 1450℃.

[0275] In addition, the relative density of each sample was consistent with the transmittance, as can be seen in Fig. 32b. That is, when the pre-sintering temperature was 1350°C, the relative density showed the maximum value of 99.89%. Referring to the microstructure photographs of Figs. 33a to 33d, it can be confirmed that pores were observed in the microstructure at 1150 and 1250°C, whereas a dense structure without pores was formed when the temperature reached 1350°C. Based on the above results, by performing the pre-sintering at 1350°C and the vacuum sintering at 1800°C, Y2O3 It can be confirmed that the transparency of ceramics can be further improved.

Claims

1. Step of preparing transparent ceramic particles; A step of forming a 3D printing structure by supplying a 3D printing slurry containing the above transparent ceramic particles and a photocurable resin onto a 3D device plate, irradiating light to form a printed layer, and repeating the process of forming the printed layer; A step of sintering the above three-dimensional printing structure; and A step of polishing the surface of the sintered structure; including: A method for manufacturing a transparent ceramic three-dimensional structure using 3D printing, characterized in that the maximum diameter of the transparent ceramic particles is less than or equal to the thickness of the printed layer.

2. Step of preparing transparent ceramic particles; A step of forming a 3D printing structure by supplying a 3D printing slurry containing the above transparent ceramic particles and a photocurable resin onto a 3D device plate, irradiating light to form a printed layer, and repeating the process of forming the printed layer; A step of sintering the above three-dimensional printing structure; and A step of polishing the surface of the sintered structure; including: The maximum diameter of the above transparent ceramic particles is less than or equal to the thickness of the printed layer, A method for manufacturing a transparent ceramic three-dimensional structure using 3D printing, characterized in that the step of polishing the surface is a chemical-mechanical composite polishing step including a step of simultaneously performing chemical polishing and mechanical polishing using a polishing medium and a chemical polishing solution.

3. In paragraph 1 or 2, A method for manufacturing a transparent ceramic three-dimensional structure using 3D printing, wherein the thickness of the above printing layer is 10 to 100 μm.

4. In paragraph 1 or 2, A method for manufacturing a transparent ceramic three-dimensional structure using 3D printing, wherein the maximum diameter of the transparent ceramic particles is 0.9 times or less of the thickness of the single printed layer.

5. In paragraph 1 or 2, A method for manufacturing a transparent ceramic three-dimensional structure using 3D printing, further comprising a step of post-heat treating the sintered structure.

6. In paragraph 2, A method for manufacturing a transparent ceramic three-dimensional structure using 3D printing, wherein the above polishing solution is at least one polishing solution selected from the group consisting of CeO2, SiO2, and colloidal silica.

7. In paragraph 6, The above polishing medium is A method for manufacturing a transparent ceramic three-dimensional structure using 3D printing, the three-dimensional structure comprising at least one of alumina, SiC, and diamond.

8. In paragraph 2, The above chemical-mechanical composite polishing step A method for manufacturing a transparent ceramic three-dimensional structure using 3D printing, further comprising a step of performing mechanical polishing using a polishing medium.

9. In paragraph 2, The above chemical-mechanical composite polishing step A first step of performing mechanical polishing using a first polishing medium; and A method for manufacturing a transparent ceramic three-dimensional structure using 3D printing, comprising a second step of simultaneously performing mechanical polishing and chemical polishing using a polishing medium smaller than the first polishing medium.

10. In paragraph 9, A method for manufacturing a transparent ceramic three-dimensional structure using 3D printing, wherein the above chemical-mechanical composite polishing step is performed using a vibration polisher.

11. In paragraph 2, A method for manufacturing a transparent ceramic three-dimensional structure using 3D printing, further comprising a washing step.

12. In paragraph 1 or 2, The above transparent ceramic particles are yttria (Y2O3), YAG (Y3Al5O 12 ), magnesia (MgO), spinel (MgAl2O4), zirconia (ZrO2), alumina (Al2O3) and ALON (Aluminum Oxynitride), a method for manufacturing a transparent ceramic three-dimensional structure using 3D printing.

13. In paragraph 1 or 2, The step of preparing the above transparent ceramic particles is A step of mixing transparent ceramic powder and sintering additives; and A method for manufacturing a transparent ceramic three-dimensional structure using 3D printing, comprising a step of calcining the above-mentioned mixed transparent ceramic powder and sintering additive.

14. In paragraph 13, The above transparent ceramic particles are yttria (Y2O3), A method for manufacturing a transparent ceramic three-dimensional structure using 3D printing, wherein the above sintering additive is a compound containing one or more elements selected from La, Zr, Yb, Nd, Ca, and Al.

15. In paragraph 1 or 2, A method for manufacturing a transparent ceramic three-dimensional structure using 3D printing, wherein the slurry contains 40 to 70 volume% of the ceramic particles.

16. In paragraph 1 or 2, A method for manufacturing a transparent ceramic three-dimensional structure using 3D printing, wherein the above slurry further contains a dispersant.

17. In paragraph 1 or 2, The step of sintering the above 3D printed structure is: A step of pre-sintering the above 3D printed structure in air; and A method for manufacturing a transparent ceramic 3D structure using 3D printing, comprising a step of vacuum sintering the above 3D printed structure.

18. In paragraph 17, The above transparent ceramic particles are yttria (Y2O3), A method for manufacturing a transparent ceramic three-dimensional structure using 3D printing, wherein the above preliminary sintering is performed at 1310 to 1390°C.

19. In paragraph 1 or 2, A method for manufacturing a transparent ceramic 3D structure using 3D printing, wherein the 3D printed structure has a structure including a curved surface.

20. In paragraph 1 or 2, The above 3D device is a DLP (Digital Light Processing) device, and is a method for manufacturing a transparent ceramic 3D structure using 3D printing.

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