A method for integrated production of tubular porous metal microfiltration membrane elements

Abstract

The application discloses a kind of integrated preparation methods of tubular porous metal microfiltration membrane element, the method comprises: one, in the hard outer mould both ends are equipped with upper end positioner and lower end positioner, obtain shell mould;Two, configuration membrane preparation slurry, prepare microfiltration membrane element blank in the hard outer mould inner wall;Three, in the inner wall of microfiltration membrane element blank forming support body, obtain tubular porous metal microfiltration membrane element roughcast;Four, after one-step high-temperature sintering, remove hard outer mould and obtain tubular porous metal microfiltration membrane element.The application changes the stress mode of traditional tubular filter element in cold isostatic pressing process, combines twice sintering of traditional metal microfiltration membrane element into one sintering, greatly reduces production energy consumption and manufacturing cost, effectively improves the production efficiency of high-precision tubular porous metal microfiltration membrane material, and the surface of membrane material is smooth, easy to blowback cleaning regeneration.

Description

Technical Field

[0001] This invention belongs to the field of porous metal materials technology, specifically relating to an integrated preparation method for a tubular porous metal microfiltration membrane element. Background Technology

[0002] Powder-sintered porous materials, due to their highly interconnected three-dimensional pore structure, high specific surface area, and low density, play an irreplaceable role in filtration, fluidization distribution control, and thermal management processes in modern industries such as nuclear industry, chemical machinery, and aerospace. However, due to factors such as powder particle size and formability, traditional powder-sintered porous elements with uniform pore structures struggle to achieve efficient separation of particles smaller than 2μm. Invention patents with application numbers 201310435548.4 and 201910151019.9, etc., employ a wet powder spraying process to prepare a precision control layer composed of fine powder particles on the surface of traditional powder-sintered porous elements. The resulting porous metal microfiltration membrane element effectively improves the filtration precision of the material while ensuring permeation flux. Furthermore, methods such as sol-gel or dip-coating can also be used to prepare a precision transition layer, resulting in high-precision metal microfiltration membrane elements.

[0003] However, in the above-mentioned manufacturing process, the metal filter elements all need to undergo two high-temperature sintering processes. The first sintering is used to prepare the support body, and the second sintering is used to prepare the precision control membrane layer on the surface of the support body, which significantly increases the energy consumption and production cost of enterprises in mass production.

[0004] Therefore, there is a need for an integrated fabrication method for tubular porous metal microfiltration membrane elements. Summary of the Invention

[0005] The technical problem this invention aims to solve is to address the shortcomings of the prior art by providing an integrated fabrication method for tubular porous metal microfiltration membrane elements. This method overturns the limitation of traditional techniques that first prepare the support and then the microfiltration membrane layer. Through process innovation, it combines the two sintering processes in the microfiltration membrane element fabrication process into a single sintering, achieving integrated forming of the tubular porous metal microfiltration membrane element. This effectively improves the production efficiency of high-precision tubular porous metal microfiltration membrane materials and significantly reduces energy consumption and costs in the production process. Furthermore, this invention changes the stress distribution of traditional tubular filter elements during cold isostatic pressing, resulting in a significantly smoother surface roughness compared to traditional filter elements. Under the same filtration precision conditions, this facilitates system backflushing and cleaning, and significantly extends the element's service life.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: an integrated preparation method for a tubular porous metal microfiltration membrane element, characterized in that the method includes the following steps:

[0007] Step 1: Install the upper and lower cylindrical locators inside the two ends of the cylindrical rigid outer mold to obtain the outer shell mold;

[0008] Step 2: Mix the fine metal powder, solvent and binder, then pour it into a horizontally placed outer shell mold, and centrifuge the outer shell mold around the axis of the rigid outer mold for 30 min to 60 min to obtain a microfiltration membrane element blank that adheres to the inner wall of the rigid outer mold in the outer shell mold.

[0009] Step 3: Remove the upper locator from the outer shell mold obtained in Step 2, and insert the inner steel sleeve and the soft rubber sleeve that fits tightly with the inner steel sleeve into the rigid outer mold. Then fill the cavity formed by the rigid outer mold and the soft rubber sleeve with coarse metal powder particles. After that, assemble the upper locator to obtain an integrated molding mold containing the sample. Then put the integrated molding mold containing the sample into an isostatic press and hold it under a pressure of 120MPa to 220MPa for 30s to 120s. A tubular porous metal microfiltration membrane element blank is obtained in the integrated molding mold.

[0010] Step 4: Sequentially remove the upper locator, inner steel sleeve, soft rubber sleeve, and lower locator of the integrated molding die obtained in Step 3. Then, sinter the rigid outer mold together with the tubular porous metal microfiltration membrane element blank adhered inside it at a high temperature of 1100℃~1400℃ and hold it at that temperature for 1h~4h. After cooling, remove the rigid outer mold to obtain the tubular porous metal microfiltration membrane element.

[0011] In this invention, cylindrical upper and lower locators are respectively installed inside the two ends of a cylindrical rigid outer mold, which is equivalent to setting plugs at both ends of the rigid outer mold. This allows the slurry formed by the added fine metal powder, solvent and binder to be uniformly coated inside the rigid outer mold under the condition of rotating along the axis, forming a microfiltration membrane element blank. The wall thickness of the upper and lower locators is greater than the thickness of the microfiltration membrane element blank.

[0012] In this invention, the inner steel sleeve has several through holes, and the area with through holes is wrapped with a soft rubber sleeve. The length of the soft rubber sleeve is greater than the length of the rigid outer mold. When it is subjected to isostatic pressing, the soft rubber sleeve applies pressure evenly to the coarse metal powder particles, forming a layer of coarse metal powder particle support inside the microfiltration membrane element blank, thus obtaining a tubular porous metal microfiltration membrane element blank.

[0013] In this invention, the rigid outer mold and the tubular porous metal microfiltration membrane element blank adhered inside it are sintered together, and the sintering process of the support and the membrane layer is combined. This reduces the energy consumption of the sintering process by more than 50%, and significantly shortens the manufacturing cycle, effectively reducing the production cost of the tubular metal microfiltration membrane element.

[0014] The forming pressure in this invention is 120MPa to 220MPa, which effectively ensures the bonding between the membrane layer and the support without damaging the membrane pore structure, and is crucial for obtaining the aforementioned structure. If the forming pressure is less than 120MPa, the support and membrane layer cannot achieve precise bonding, resulting in defects such as separation of the membrane layer and support after sintering, affecting the lifespan of the microfiltration membrane element. If the forming pressure exceeds 220MPa, large particles of powder in the support will damage the membrane layer embedded in the inner wall of the cylindrical mold, causing damage to the membrane structure. Similarly, a holding time that is too short, less than 30s, has the same effect as too low pressure, while a holding time that is too long will also cause damage to the membrane pore structure.

[0015] In this invention, when the sintering temperature is between 1100℃ and 1400℃, optimal metallurgical bonding occurs between the coarse-particle powder of the support, the fine-particle powder of the microfiltration membrane, and the interface between the two. The pore structure is fully developed, and the material strength meets the requirements for use. If the sintering temperature is below 1100℃, the sintering necks between the support powders cannot form an effective metallurgical bond, resulting in insufficient overall strength of the microfiltration membrane material. If the sintering temperature exceeds 1400℃, the densification degree of the membrane layer and the support powder will exceed the predetermined value, causing severe pore shrinkage and near-complete pore closure, resulting in the so-called over-burning phenomenon.

[0016] In this invention, the particle size of the fine metal powder in step two is smaller than the particle size of the coarse metal powder in step three.

[0017] The above-described integrated fabrication method for a tubular porous metal microfiltration membrane element is characterized in that, in step one, the rigid outer mold remains unchanged at 1100℃~1400℃, the surface roughness Ra of its inner wall is ≤1.6μm, and it does not react with fine or coarse metal powder particles. The rigid outer mold is made of alumina, zirconium oxide, silicon oxide, heat-resistant steel, molybdenum, or a molybdenum alloy. This invention ensures structural stability by controlling the parameters of the rigid outer mold, preventing it from reacting with fine or coarse metal powder particles, thus improving the quality of the tubular porous metal microfiltration membrane element.

[0018] The above-described integrated fabrication method for a tubular porous metal microfiltration membrane element is characterized in that the thickness of the microfiltration membrane element blank in step two is no greater than 30 μm, and the surface roughness Ra ≤ 10 μm. The thickness of the membrane blank in this invention, not exceeding 30 μm, is crucial for obtaining a defect-free microfiltration membrane material with high permeation flux. This is because, for microfiltration membrane materials, most of the permeation resistance originates from the membrane layer formed by the sintering of fine powder particles. If the blank thickness is ≥ 30 μm, it will not only lead to a significant decrease in the membrane material flux, but also cause defects such as delamination and cracking during subsequent sintering.

[0019] The above-mentioned integrated preparation method of a tubular porous metal microfiltration membrane element is characterized in that the tubular porous metal microfiltration membrane element in step four is made of one or two of stainless steel, nickel-based alloy, titanium, titanium alloy, and copper alloy, with an outer diameter of 10mm to 120mm and a wall thickness of 2mm to 4mm. In this invention, the fine metal powder and coarse metal powder particles can be of the same or different materials, selected according to actual needs, suitable for different working conditions. The microfiltration membrane element diameter of 10mm to 120mm in this invention can effectively utilize the advantages of high material precision and high flux, making it particularly suitable for the purification and separation of dust-laden gases in industries such as polysilicon and coal chemical engineering. If the microfiltration membrane element diameter is less than 10mm, the internal space of the tube is limited, making it difficult to perform the relevant operations in the preparation process; when the diameter is greater than 120mm, the filtration area per unit volume of the membrane material decreases, limiting the selectivity of engineering applications. The microfiltration membrane element wall thickness of 2mm to 4mm in this invention can maximize material savings and control costs while ensuring strength, further improving the market competitiveness of the microfiltration membrane material. If the wall thickness of the microfiltration membrane element is less than 2mm, the overall strength is low and it cannot guarantee the high-frequency backflushing vibration during filtration. If the wall thickness is greater than 4mm, although the overall strength of the material is improved, the resulting excessive weight will increase production costs and cause serious material waste.

[0020] Compared with the prior art, the present invention has the following advantages:

[0021] 1. This invention combines the sintering processes of the support and the membrane layer, reducing the energy consumption of the sintering process by more than 50%, significantly shortening the manufacturing cycle, and effectively reducing the production cost of tubular metal microfiltration membrane elements.

[0022] 2. The isostatic pressing of the present invention is from the inside out, and the surface roughness of the tubular metal microfiltration membrane element prepared is ≤8μm, which is significantly better than the membrane material elements prepared by the existing process (above 15μm). This makes it easier for the element to backflushed during service and effectively improves the service life of the membrane material.

[0023] 3. In the tubular metal microfiltration membrane element prepared by this invention, the precision control layer composed of fine metal powder particles and the support composed of coarse metal powder particles are tightly bonded under the dual action of pressure and temperature, resulting in higher membrane strength.

[0024] 4. This invention has a wide range of applications. It is applicable not only to the preparation of microfiltration membranes made of the same material, but also to the preparation of components with membrane layers and supports made of different materials.

[0025] 5. The preparation process of this invention is simple, the production cost is low, and the requirements for production equipment are low. It is suitable for the preparation of tubular porous materials of various specifications and is suitable for large-scale industrial production.

[0026] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the integrated molding die used in this invention.

[0028] Figure 2 This is a cross-sectional microstructure diagram of the tubular porous metal microfiltration membrane element prepared in Example 1 of the present invention.

[0029] Explanation of reference numerals in the attached figures:

[0030] 1—Hard outer mold; 2—Inner steel sleeve; 3—Soft rubber sleeve;

[0031] 4—Upper positioner; 5—Lower positioner; 6—Cavity. Detailed Implementation

[0032] Figure 1 This is a schematic diagram of the integrated molding die used in this invention. Figure 1 As can be seen from the figure, the integrated forming mold of the present invention includes a hard outer mold 1. The inner sides of the left and right ends of the hard outer mold 1 are provided with an upper locator 4 and a lower locator 5. An inner steel sleeve 2 and a soft rubber sleeve 3 that fits tightly with the inner steel sleeve 2 are also inserted into the inner side of the hard outer mold 1. The soft rubber sleeve 3, the hard outer mold 1, the upper locator 4 and the lower locator 5 form a cavity 6.

[0033] Example 1

[0034] This embodiment includes the following steps:

[0035] Step 1: Install the upper cylindrical locator 4 and the lower cylindrical locator 5 inside the two ends of the cylindrical hard outer mold 1 to obtain the outer shell mold; the material of the hard outer mold 1 is 310S heat-resistant steel and the inner wall roughness is 1.2μm;

[0036] Step 2: D 50 =12.8μm 316L stainless steel powder, ethanol and polyvinyl butyral are mixed evenly in a mass ratio of 3:8:1, poured into a horizontally placed outer shell mold, and centrifuged around the axis of the rigid outer mold 1 for 30 minutes to obtain a microfiltration membrane element blank that adheres to the inner wall of the rigid outer mold 1 in the outer shell mold.

[0037] Step 3: Remove the upper locator 4 from the outer shell mold obtained in Step 2, and insert the inner steel sleeve 2 and the soft rubber sleeve 3 that fits tightly with the inner steel sleeve 2 into the rigid outer mold 1. Fill the cavity 6 formed by the rigid outer mold 1 and the soft rubber sleeve 3 with 316L stainless steel powder particles of -100 to +160 mesh. Then assemble the upper locator 4 to obtain an integrated molding mold containing the sample. Then place the integrated molding mold containing the sample into an isostatic press and hold it under a pressure of 180MPa for 40s to obtain a tubular porous metal microfiltration membrane element blank in the integrated molding mold.

[0038] Step 4: Sequentially remove the upper locator 4, inner steel sleeve 2, soft rubber sleeve 3, and lower locator 5 of the integrated forming mold obtained in Step 3. Then, sinter the 310S heat-resistant steel hard outer mold 1 together with the tubular porous metal microfiltration membrane element blank adhered inside it at a high temperature of 1250℃ for 3 hours. After cooling, remove the hard outer mold 1 to obtain a tubular stainless steel microfiltration membrane element with an outer diameter of 60mm and a wall thickness of 3.0mm.

[0039] Figure 2 The cross-sectional morphology of the tubular porous metal microfiltration membrane element prepared in this embodiment is shown. Figure 2 As can be seen, the thickness of the precision control layer is approximately 16 μm, i.e., D 50 The layer formed by 12.8 μm 316L stainless steel powder exhibits well-developed sintering necks between both the coarse powder of the support and the fine powder of the precision control layer, indicating that the microfiltration membrane material prepared by this invention is fully sintered, achieving integrated preparation of the membrane material. The tubular stainless steel microfiltration membrane element prepared in this embodiment was tested according to GB / T 5249-2013 "Determination of Pore Size by Bubble Test of Permeable Sintered Metal Materials". The results show that its average pore size is 2.8 μm, and the surface roughness Ra of the tubular stainless steel microfiltration membrane element is only 4.76 μm, possessing the ability to achieve efficient filtration and separation of ultrafine dust particles, while effectively improving the backflushing efficiency and service life of the material.

[0040] Example 2

[0041] This embodiment includes the following steps:

[0042] Step 1: Install cylindrical upper end locator 4 and lower end locator 5 inside both ends of the cylindrical hard outer mold 1 to obtain the outer shell mold; the material of the hard outer mold 1 is alumina and the inner wall roughness is 1.6μm.

[0043] Step 2: Mix 316L stainless steel powder with D50=20μm, ethanol and polyvinyl butyral in a mass ratio of 4:6:1, pour into a horizontally placed forming mold, and centrifuge it around the axis of the rigid outer mold 1 for 50 minutes to obtain a microfiltration membrane element blank that adheres to the inner wall of the rigid outer mold 1 in the outer shell mold.

[0044] Step 3: Remove the upper locator 4 from the outer shell mold obtained in Step 2, and insert the inner steel sleeve 2 and the soft rubber sleeve 3 that fits tightly with the inner steel sleeve 2 into the rigid outer mold 1. Fill the cavity 6 formed by the rigid outer mold 1 and the soft rubber sleeve 3 with 316L stainless steel powder particles of -100 to +200 mesh. Then assemble the upper locator 4 to obtain an integrated molding mold containing the sample. Then place the integrated molding mold containing the sample into an isostatic press and hold it under a pressure of 120MPa for 30s to obtain a tubular porous metal microfiltration membrane element blank in the integrated molding mold.

[0045] Step 4: Sequentially remove the upper locator 4, inner steel sleeve 2, soft rubber sleeve 3, and lower locator 5 of the integrated forming mold obtained in Step 3. Sinter the alumina hard outer mold 1 together with the tubular porous metal microfiltration membrane element blank adhered inside it at a high temperature of 1250℃ and hold for 3 hours. After cooling, remove the hard outer mold 1 to obtain a tubular stainless steel microfiltration membrane element with an outer diameter of 120mm and a wall thickness of 4.0mm.

[0046] The cross-sectional morphology test results of the tubular porous metal microfiltration membrane element prepared in this embodiment show that the thickness of the precision control layer is approximately 29 μm. Furthermore, the sintering necks between the coarse powder of the support and the fine powder particles of the precision control layer are well-developed, indicating that the tubular stainless steel microfiltration membrane element prepared by this invention is fully sintered, achieving integrated preparation of the membrane material. The tubular stainless steel microfiltration membrane element prepared in this embodiment was tested according to GB / T 5249-2013 "Determination of Pore Size in Bubble Test of Permeable Sintered Metal Materials". The results show that its average pore size is 2.038 μm, and the surface roughness Ra of the tubular stainless steel microfiltration membrane element is only 3.86 μm, demonstrating its ability to achieve efficient filtration and separation of ultrafine dust particles, while effectively improving the backflushing efficiency and service life of the material.

[0047] Example 3

[0048] This embodiment includes the following steps:

[0049] Step 1: Install the upper cylindrical locator 4 and the lower cylindrical locator 5 inside the two ends of the cylindrical hard outer mold 1 to obtain the outer shell mold; the material of the hard outer mold 1 is silicon oxide, and the inner wall roughness is 1.2-0.6.

[0050] Step 2: Mix TA2 powder with D50 = 12.8 μm, ethanol and polyvinyl butyral in a mass ratio of 3:10:2, pour into a horizontally placed forming mold, and centrifuge and rotate it around the axis of the rigid outer mold 1 for 60 minutes to obtain a microfiltration membrane element blank that adheres to the inner wall of the rigid outer mold 1 in the outer shell mold.

[0051] Step 3: Remove the upper locator 4 from the outer shell mold obtained in Step 2, and insert the inner steel sleeve 2 and the soft rubber sleeve 3 that fits tightly with the inner steel sleeve 2 into the rigid outer mold 1. Fill the cavity 6 formed by the rigid outer mold 1 and the soft rubber sleeve 3 with TA2 powder particles of -300 to +400 mesh. Then assemble the upper locator 4 to obtain an integrated molding mold containing the sample. Then place the integrated molding mold containing the sample into an isostatic press and hold it under a pressure of 220 MPa for 120 seconds to obtain a tubular porous metal microfiltration membrane element blank in the integrated molding mold.

[0052] Step 4: Sequentially remove the upper locator 4, inner steel sleeve 2, soft rubber sleeve 3, and lower locator 5 of the integrated molding die obtained in Step 3. Sinter the silica hard outer mold 1 together with the tubular porous metal microfiltration membrane element blank adhered inside it at 1100℃ for 3 hours. After cooling, remove the hard outer mold 1 to obtain a tubular stainless steel microfiltration membrane element with an outer diameter of 10mm and a wall thickness of 2.0mm.

[0053] The cross-sectional morphology test results of the tubular porous metal microfiltration membrane element prepared in this embodiment show that the thickness of the precision control layer is approximately 12 μm. Furthermore, the sintering necks between the coarse powder of the support and the fine powder particles of the precision control layer are well-developed, indicating that the tubular TA2 microfiltration membrane element prepared by this invention is fully sintered, achieving integrated preparation of the membrane material. The tubular microfiltration membrane element prepared in this embodiment was tested according to GB / T 5249-2013 "Determination of Pore Size in Bubble Test of Permeable Sintered Metal Materials". The results show that its average pore size is 1.757 μm, and the surface roughness Ra of the tubular microfiltration membrane element is only 3.14 μm, demonstrating its ability to achieve efficient filtration and separation of ultrafine dust particles.

[0054] Example 4

[0055] The difference between this embodiment and Embodiment 1 is that: the material of the rigid outer mold 1 in step one is zirconium oxide; in step three, the cavity 6 formed by the rigid outer mold 1 and the soft rubber sleeve 3 is filled with -300 to +400 mesh Monel 400 powder; and in step four, the rigid outer mold 1 together with the tubular porous metal microfiltration membrane element blank adhered inside it is sintered at 1400°C and held at that temperature for 1 hour.

[0056] Example 5

[0057] The difference between this embodiment and Embodiment 1 is that the material of the rigid outer mold 1 in step one is molybdenum, and in step four, the rigid outer mold 1 together with the tubular porous metal microfiltration membrane element blank adhered inside it is subjected to high-temperature sintering at 1200°C and held at that temperature for 4 hours.

[0058] Example 6

[0059] The difference between this embodiment and embodiment 1 is that the material of the hard outer mold 1 in step one is a molybdenum alloy, and in step three, TC4 powder of -300 to +400 mesh is used to fill the cavity 6 formed by the hard outer mold 1 and the soft rubber sleeve 3.

[0060] Example 7

[0061] The difference between this embodiment and embodiment 1 is that in step three, copper alloy powder of -300 to +400 mesh is used to fill the cavity 6 formed by the hard outer mold 1 and the soft rubber sleeve 3.

[0062] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any way. Any simple modifications, alterations, and equivalent changes made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. An integrated fabrication method for a tubular porous metal microfiltration membrane element, characterized in that, The method includes the following steps: Step 1: Install cylindrical upper end locator (4) and lower end locator (5) inside both ends of the cylindrical rigid outer mold (1) to obtain the outer shell mold; the surface roughness Ra of the inner wall of the rigid outer mold (1) is ≤1.6μm; Step 2: Mix the fine metal powder, solvent and binder evenly, then pour it into the shell mold obtained in Step 1, which is placed horizontally, and centrifugally rotate the shell mold around the axis of the rigid outer mold (1) for 30 min to 60 min to obtain a microfiltration membrane element blank that adheres to the inner wall of the rigid outer mold (1) in the shell mold; the thickness of the microfiltration membrane element blank is not greater than 30 μm; Step 3: Remove the upper locator (4) from the outer shell mold obtained in Step 2, and insert the inner steel sleeve (2) and the soft rubber sleeve (3) that fits tightly with the inner steel sleeve (2) into the rigid outer mold (1). Then fill the cavity (6) formed by the rigid outer mold (1) and the soft rubber sleeve (3) with coarse metal powder particles. Then assemble the upper locator (4) to obtain an integrated molding mold containing the sample. Then put the integrated molding mold containing the sample into an isostatic press and hold it under a pressure of 120MPa~220MPa for 30s~120s. Obtain the tubular porous metal microfiltration membrane element blank in the integrated molding mold. Step 4: Sequentially remove the upper locator (4), inner steel sleeve (2), soft rubber sleeve (3) and lower locator (5) of the integrated forming mold obtained in Step 3. Then, the rigid outer mold (1) together with the tubular porous metal microfiltration membrane element blank adhered inside it is subjected to high-temperature sintering at 1100℃~1400℃ and kept at that temperature for 1h~4h. After cooling, the rigid outer mold (1) is removed to obtain the tubular porous metal microfiltration membrane element.

2. The integrated fabrication method of a tubular porous metal microfiltration membrane element according to claim 1, characterized in that, The hard outer mold (1) mentioned in step one does not deform at 1100℃~1400℃ and does not react with fine metal powder and coarse metal powder particles. The material of the hard outer mold (1) is alumina, zirconium oxide, silicon oxide, heat-resistant steel, molybdenum or molybdenum alloy.

3. The integrated fabrication method of a tubular porous metal microfiltration membrane element according to claim 1, characterized in that, The surface roughness Ra of the microfiltration membrane element blank in step two is ≤10μm.

4. The integrated fabrication method of a tubular porous metal microfiltration membrane element according to claim 1, characterized in that, The tubular porous metal microfiltration membrane element described in step four is made of one or two of stainless steel, nickel-based alloy, titanium, titanium alloy and copper alloy, with an outer diameter of 10mm to 120mm and a wall thickness of 2mm to 4mm.

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