Method for producing a highly porous sacrificial template, and highly porous template, and diffusor and / or at least one hollow tube having defined density gradients

Abstract

The invention relates to a method for producing a highly porous sacrificial template for producing a highly porous sacrificial template from tetrapodal zinc oxide having a defined density gradient, the method comprising the following steps: determining the desired density gradients in the highly porous sacrificial template and a desired sacrificial template geometry; producing individual successive sacrificial template layers having the desired density gradient in a mould having the desired geometry, wherein the sacrificial template layers are obtained by defined compression of different amounts of tetrapodal zinc oxide particles in the mould in the temporally successive production of the individual layers, and the process of producing the sacrificial template layers is carried out until the desired density gradient in the highly porous sacrificial template is achieved; heat treating the formed highly porous sacrificial template in order to form a stable tetrapodal zinc oxide network in the highly porous sacrificial template. The invention also relates to a highly porous sacrificial template formed by the production method, and to a diffusor having a defined density gradient, and / or to at least one hollow tube having a submicron wall thickness having a defined density gradient.

Description

[0001] HIGHLY POROUS SACRIFICIAL TEMPLATE MANUFACTURING PROCESS AND HIGHLY POROUS SACRIFICIAL TEMPLATE AS WELL AS DIFFUSER AND / OR AT LEAST ONE HOLLOW TUBE WITH DEFINED DENSITY GRADIENT

[0002] The invention relates to a highly porous sacrificial template manufacturing method for producing a highly porous sacrificial template made of tetrapodal zinc oxide with a defined density gradient.

[0003] Furthermore, the invention relates to a highly porous sacrificial template formed by the manufacturing method as well as a diffuser with a defined density gradient and / or at least one hollow tube with a submicrometer wall thickness with a defined density gradient.

[0004] Laser light diffusers are used to broaden and disperse a directed laser beam and to reduce speckle contrast. This allows the laser light to be used for practical lighting applications. In addition, a diffuser should have low absorption to avoid compromising the efficiency of the lighting system. Almost all standard diffusers currently available on the market have a plate-shaped geometry and, according to the state of the art, are divided into two subclasses: volume diffusers and surface diffusers.

[0005] Volume diffusers are usually made of porous polymers, for example by introducing small air pockets into the polymer during manufacturing.

[0006] Surface diffusers typically consist of a glass substrate coated with a very rough and therefore highly scattering thin film. The surface of the glass itself can be microstructured, for example, by pulsed laser structuring.

[0007] Surface diffusers offer a higher laser damage threshold and also have a higher transmission efficiency (~90%) compared to volume diffusers.

[0008] In "Conversionless efficient and broadband laser light diffusers for high brightness illumination applications," Nature Communications 11, 1437 (2020), https: / / doi.org / 10.1038 / s41467-020-14875-z, Schütt et al. disclose novel high-performance diffusers based on hexagonal boron nitride aeromaterials. These diffusers are characterized by a high laser damage threshold and enable a significant reduction (< 4%) of speckle contrast through multiple scattering. The diffusers are fabricated using zinc oxide ceramic templates.

[0009] US Pat. No. 5,330,465 A discloses a diffuser tip for an optical fiber or an array of fibers, having a proximal end adjacent to the tip of the optical fiber or array of fibers. The diffuser tip has a cylindrical central core made of transparent elastomer containing embedded scattering centers. The scattering centers are distributed within the core such that the concentration of scattering centers increases continuously in a direction from the proximal end of the diffuser tip to the distal end of the diffuser tip.

[0010] Document CN 1 03 518269 A discloses a light-emitting device that improves the light output of an organic light-emitting diode, comprising at least one porous metal or metalloid oxide light-outcoupling layer positioned between the substrate and the transparent conductive material layer in the OLED. The refractive index of the light-outcoupling layer and the light scattering can be adjusted, for example, by changing the pore size, pore density, doping the metal oxide, adding an insulating, conductive, or semiconducting component, or filling the pores.A method for manufacturing the light-emitting device comprises forming at least one light-outcoupling layer comprising a porous metal or metalloid oxide on a substrate, for example using atmospheric pressure chemical vapor deposition (APCVD), and then forming a transparent conductive material on the light-outcoupling layer.

[0011] US Pat. No. 10,090,529 B2 discloses a method for producing and manufacturing highly porous open-cell structures using templates formed by mechanical pressing, injection molding, sintering, or any combination thereof. The processing scheme involves coating, filling, or depositing a material onto or within the porous template. The highly porous structure is formed after selective removal of the template and can be used for various applications such as electrochemical energy storage, including high-performance and high-energy lithium-ion batteries.

[0012] Furthermore, it is known from the publication KR 2016 0 052 990 A to form a porous thin film with a three-dimensional network structure, which improves the reaction properties towards an external substance and the adhesion properties of a substrate, by applying a density gradient in a porous thin film with a three-dimensional network structure in the direction of the thin film thickness, and which is produced in a dry process, as well as a production method therefor.

[0013] Furthermore, the document DE 102018 133 338 A1 discloses a lighting system for a medical-technical therapy and / or diagnostic system, wherein a diffuser element and a diffuser base body comprise an inorganic material, and wherein the surface of the at least one diffuser base body is pore-free and smooth.

[0014] The problems with the state of the art are essentially that currently commercially available volume diffusers are severely limited in terms of their laser damage threshold and efficiency due to light absorption. Currently available surface diffusers, on the other hand, are highly wavelength-dependent. The major disadvantage of surface diffusers, however, is their low scattering power, which leads to high spot contrasts (> 10%) and makes them unsuitable for laser-based illumination.

[0015] The present invention is based on the object of creating a possibility to improve the already known volume diffusers based on aeromaterial with regard to the volumetric scattering properties and their light coupling and to offer a possibility for producing such diffusers based on aeromaterial having a density gradient.

[0016] This object is achieved with a highly porous sacrificial template manufacturing method according to the main claim, a highly porous sacrificial template and a diffuser with a defined density gradient and / or at least one hollow tube with a submicrometer wall thickness with a defined density gradient according to the independent claims.

[0017] The highly porous sacrificial template manufacturing process for producing a highly porous sacrificial template made of tetrapodal zinc oxide with a defined density gradient comprises the following steps:

[0018] - Determination of the desired density gradient in the highly porous sacrificial template and a desired sacrificial template geometry;

[0019] - producing individual successive sacrificial template layers with the respective desired density gradient in a mold with the desired geometry, wherein the sacrificial template layers are obtained by defined compression of different amounts of tetrapodal zinc oxide particles in the mold in chronologically successive production of the individual layers and the process of producing the sacrificial template layers is carried out until the desired density gradient in the highly porous sacrificial template is achieved;

[0020] - Heat treatment of the formed highly porous sacrificial template to form a stable tetrapodal zinc oxide network in the highly porous sacrificial template.

[0021] The heat treatment can be carried out for a period of 3.5 to 6.5 hours and / or 5 hours at a temperature of 1000 to 1200 °C and / or 1150 °C in a sintering furnace.

[0022] In particular, tetrapodal zinc oxide particles with an arm length of 20 to 30 pm and / or an arm diameter of 1 to 3 pm can be used.

[0023] Typically, tetrapodal zinc oxide has a porosity > 90%.

[0024] The individual sacrificial template layers can be produced with a thickness of > 100 pm. The highly porous sacrificial template made of tetrapodal zinc oxide, produced using the highly porous sacrificial template manufacturing process, is characterized by a defined density gradient.

[0025] A preferred use of the highly porous sacrificial template is for the production of a diffuser with a defined density gradient and / or at least one hollow tube with a submicrometer wall thickness with a defined density gradient.

[0026] The diffuser with a defined density gradient and / or the at least one hollow tube with submicrometer wall thickness with a defined density gradient are manufactured using a highly porous sacrificial template.

[0027] The production is preferably carried out in such a way that following the heat treatment of the highly porous sacrificial template, a wet-chemical and / or dry-chemical coating of the template takes place and the highly porous sacrificial template is removed by etching.

[0028] The coating can be carried out in particular with ceramic and / or glass and / or a diffuser material and / or silicon dioxide and / or boron nitride.

[0029] The at least one hollow tube can preferably be partially transparent and / or have a wall thickness of < 0.25 pm.

[0030] Functionalization can be achieved through coating. For example, the absorber material graphene can be used for this purpose.

[0031] A diffuser produced with a highly porous sacrificial template according to the invention can in particular have the following features:

[0032] - Translucent scattering body with a size of at least 100x100x100 pm constructed from a gradient preferably consisting of glass;

[0033] - Free-standing, continuous, connected phase of the scattering body has a density of less than 100 mg / cm 3 ;

[0034] - At least partially transparent ceramic;

[0035] - Internal structural composition mainly consisting of hollow tubes with a wall thickness on the nanoscale (<250 nm) and a diameter between 0.5 and 10 pm;

[0036] - A tube length of 1 to 50 pm;

[0037] - The material may contain plate, triangular and other structures, but always with the characteristic of a hollow body, in particular with a wall thickness < 0.25 pm;

[0038] - The density gradient has the function of efficiently dispersing light in all spatial directions;

[0039] - The density gradient enables efficient light coupling, for example from a laser;

[0040] - The density gradient is used for controlled forward and backward scattering;

[0041] - The density gradient is determined macroscopically by the shape of the scattering body, microscopically by the density of the hollow tubes, and nanoscopically by the wall thickness of the hollow tubes.

[0042] The advantages of the diffuser according to the invention with a defined density gradient and / or the at least one hollow tube with a submicrometer wall thickness with a defined density gradient are shown in that the density gradient enables targeted control over local scattering properties as well as improved light coupling and thus a higher light yield (less backscattering).

[0043] The diffuser according to the invention is a new type of volume diffuser, preferably for scattering laser light, based on highly porous aeromaterials constructed from, in particular, at least partially transparent hollow tubes with a submicrometer wall thickness made of preferably ceramic, more preferably silicon dioxide, characterized by a defined density gradient for efficient light coupling and light scattering.

[0044] Further example:

[0045] The following is a description of the production of a highly porous sacrificial template according to the invention with subsequent production of a diffuser with density gradients using a specific embodiment, which is intended to explain the invention and is not to be considered limiting:

[0046] To produce a porous sacrificial template with a density gradient, tetrapodal zinc oxide microparticles are used, which are produced by flame transport synthesis. In the example, these tetrapods have an arm length of 20-30 pm and an arm diameter of 1-3 pm.

[0047] As an example, the preparation of a sacrificial template with a cylindrical geometry (d = 8 mm, h = 5 mm) and a density gradient of 0.6 g cm -3 to 0.2 g cm -3 described:

[0048] A metal mold with a hole diameter of 8 mm and a precisely fitting stamp (diameter ~8 mm) at the height of the metal mold are used. For the first layer with a density of 0.6 g cm -30.0302 g of tetrapodal zinc oxide is filled into the metal mold and compressed to 1 mm using the stamp. To do this, the stamp is pressed into the metal mold and a 1 mm spacer is used. For the second layer (density 0.5 g cm -3 ), the stamp is withdrawn from the metal mold. Then, 0.0251 g of tetrapodal zinc oxide is applied to the first layer, and the stamp is used again to compress the second layer to 1 mm. A 2 mm spacer is used for this purpose. For the third and fourth layers (density 0.4 g cm -3 and density 0.3 g cm -3 ) the procedure is repeated as described above with a tetrapodal zinc oxide amount of 0.0201 g and 0.0151 g, respectively, and spacers of 3 mm and 4 mm. For the fifth layer (density 0.2 g cm' 3 ) the procedure is also repeated (amount of tetrapodal zinc oxide 0.0101 g, spacer

[0049] 5 mm), but the stamper remains in the metal mold. After the final layer is compressed, the entire metal mold is lifted and held at a distance of at least 6 mm above a ceramic plate. The spacer is removed, and the stamper is pressed completely into the metal mold, forcing the sacrificial template out of the mold.

[0050] The ceramic plate with the template is placed in the sintering furnace and sintered for 5 hours at 1150 °C. After the porous sacrificial template has cooled to a defined density gradient, a wet or dry chemical coating is applied, for example, with silicon dioxide or hexagonal boron nitride, before the template is removed. As an example of a wet chemical coating, the silicon dioxide coating of the tetrapodal zinc oxide surface of a cylindrical sacrificial template with a diameter of 8 mm and a height of 5 mm is described below:

[0051] To coat the tetrapodal zinc oxide surface with silicon dioxide, first mix 10 ml of ethanol with 100 μl of tetraethyl orthosilicate in a beaker and stir for 5 minutes using a magnetic stirrer. Then, add 3 ml of a 25% ammonia solution. After stirring, the solution is immediately poured into another beaker containing the sacrificial template. Ensure that the template is completely covered with the solution. After 45 minutes, the sacrificial template is removed from the solution and rinsed with ethanol and water.

[0052] The highly porous sacrificial template is then etched out with a 10% hydrochloric acid solution, creating a network of interconnected hollow silicon dioxide tubes. To dry the network structure, all traces of the hydrochloric acid solution are removed by repeated exchange with water before the sample is transferred to ethanol and dried in a critical point dryer.

[0053] After the drying process, the diffuser can be used with a defined density gradient.

Claims

CLAIMS 1. Highly porous sacrificial template manufacturing process for producing a highly porous sacrificial template made of tetrapodal zinc oxide with a defined density gradient, comprising the following steps: - Determination of the desired density gradient in the highly porous sacrificial template and a desired sacrificial template geometry; - producing individual successive sacrificial template layers with the respective desired density gradient in a mold with the desired geometry, wherein the sacrificial template layers are obtained by defined compression of different amounts of tetrapodal zinc oxide particles in the mold in chronologically successive production of the individual layers and the process of producing the sacrificial template layers is carried out until the desired density gradient in the highly porous sacrificial template is achieved; - Heat treatment of the formed highly porous sacrificial template to form a stable tetrapodal zinc oxide network in the highly porous sacrificial template.

2. Manufacturing method according to claim 1, characterized in that the heat treatment - for a period of 3.5 to 6.5 hours and / or 5 hours - at a temperature of 1000 to 1200 °C and / or 1150 °C in a sintering furnace.

3. Manufacturing process according to claim 1 or 2, characterized by the use of tetrapodal zinc oxide particles with an arm length of 20 to 30 pm and / or an arm diameter of 1 to 3 pm.

4. Manufacturing method according to one of the preceding claims, characterized by producing the individual sacrificial template layers with a thickness > 100 pm.

5. Highly porous sacrificial template made of tetrapodal zinc oxide produced by the highly porous sacrificial template production method according to one of the preceding claims, characterized in that the highly porous sacrificial template has a defined density gradient.

6. Diffuser with a defined density gradient and / or at least one hollow tube with submicrometer wall thickness with a defined density gradient produced with a highly porous sacrificial template according to the preceding claim.

7. Diffuser with a defined density gradient and / or at least one hollow tube with a submicrometer wall thickness with a defined density gradient according to the preceding claim, wherein following the heat treatment of the highly porous sacrificial template, a wet-chemical and / or dry-chemical coating of the template takes place and the highly porous sacrificial template is removed by etching.

8. Diffuser with a defined density gradient and / or at least one hollow tube with a submicrometer wall thickness with a defined density gradient according to the preceding claim, wherein the coating is carried out with ceramic and / or glass and / or a diffuser material and / or silicon dioxide and / or boron nitride.

9. Diffuser with a defined density gradient and / or at least one hollow tube with a submicrometer wall thickness with a defined density gradient according to the three preceding claims, wherein the at least one hollow tube is partially transparent and / or has a wall thickness of < 0.25 pm.

10. Diffuser with a defined density gradient and / or at least one hollow tube with a submicrometer wall thickness with a defined density gradient according to the three preceding claims, wherein functionalization is carried out by coating.