Preparation method of high-purity silicon carbide ceramic powder with controllable particle size

Abstract

The application belongs to the field of ceramic materials, and particularly relates to a preparation method of high-purity and particle-size-controllable silicon carbide ceramic powder. The preparation method of the high-purity and particle-size-controllable silicon carbide ceramic powder comprises the following steps: (1) mixing carbon black powder, a binder, a dispersing agent, a pore-forming agent and a solvent by ball milling to obtain carbon powder slurry, and obtaining carbon black granulation powder by spray granulation; (2) mixing the carbon black granulation powder obtained by spray granulation with silicon powder and then performing high-temperature sintering. The carbon black powder is used as raw material, the binder, the dispersing agent and the pore-forming agent are combined, the particle-size-controllable carbon black granulation powder is obtained by spray granulation, the particle-size-controllable carbon black granulation powder is mixed with the silicon powder and then high-temperature sintering is performed, the carbon black granulation powder serves as a reaction template, and finally the high-purity and particle-size-controllable silicon carbide ceramic powder is obtained. The obtained silicon carbide ceramic powder is applied to selective laser sintering technology.

Description

Technical Field

[0001] This invention belongs to the field of ceramic materials, specifically relating to a method for preparing high-purity silicon carbide ceramic powder with controllable particle size. Background Technology

[0002] Silicon carbide ceramics, with their high hardness, high strength, excellent wear resistance, and thermal properties, are widely used in aerospace, electronics, semiconductors, energy, and environmental protection fields. Their high thermal conductivity and low coefficient of thermal expansion make them suitable for high-temperature structural components and high-frequency electronic devices, while their superior corrosion resistance and lightweight properties meet the needs of chemical equipment and protective armor. Furthermore, due to their tunable electrical properties and high-temperature stability, silicon carbide ceramics are used in high-end technology fields such as filtering, sealing, and optical substrates, making them a highly promising advanced ceramic material.

[0003] Silicon carbide ceramics prepared by selective laser sintering (SLS) offer advantages such as complex structures, high material utilization, and moldless production. They can achieve geometries difficult to process using traditional methods, while reducing material waste and production costs. This technology maintains the high hardness, wear resistance, corrosion resistance, and high-temperature stability of silicon carbide while meeting the demands for small-batch, high-precision, and customized production. It is widely used in aerospace, energy, and semiconductor electronics, representing a significant innovative technology for high-performance ceramic manufacturing. SLS preparation of silicon carbide ceramics places extremely stringent requirements on the silicon carbide powder, primarily including high purity, narrow particle size distribution, and good flowability. This is necessary to adapt to the layered powder preparation process, ensuring uniformity during printing and high density and stable performance of the sintered ceramic, thereby meeting the requirements of complex structure manufacturing and high-performance applications. Therefore, preparing a high-purity silicon carbide powder with controllable large particle size (balancing powder flowability and sintering activity) is crucial for the development of SLS-prepared silicon carbide ceramics. Summary of the Invention

[0004] The purpose of this invention is to address the above-mentioned technical problems by providing a method for preparing high-purity silicon carbide ceramic powder with controllable particle size. The method uses carbon black powder as raw material, combined with binder, dispersant and pore-forming agent, and obtains carbon black granulated powder with controllable particle size by spray granulation, which is then mixed and reacted with silicon powder to finally obtain high-purity silicon carbide ceramic powder with controllable particle size.

[0005] The method for preparing high-purity silicon carbide ceramic powder with controllable particle size in the technical solution of this invention includes the following steps:

[0006] (1) Carbon black powder, binder, dispersant, pore-forming agent and solvent are ball-milled and mixed to obtain carbon powder slurry, and carbon black granulated powder is obtained by spray granulation;

[0007] (2) The carbon black granulated powder obtained by spray granulation is mixed with silicon powder and then sintered at high temperature.

[0008] In the reaction process between carbon black granules and silicon powder, the carbon black granules with controllable particle size act as a template, and the silicon carbide ceramic powder generated after the reaction is complete does not change significantly in size, thus obtaining high-purity silicon carbide ceramic powder with controllable particle size.

[0009] Furthermore, in step (1), the particle size of the carbon black powder is less than 1.5 μm, preferably 0.1 to 1.0 μm; excessively large particle size is not conducive to the control of powder morphology and particle size by subsequent spray granulation.

[0010] Furthermore, the binder in step (1) is a thermosetting resin, including but not limited to any one of phenolic resin, furan resin, and epoxy resin. Thermosetting resin can act as a binder in the spray granulation process, binding the carbon black powder for shaping, and can obtain a carbon skeleton with high hardness during the high temperature process, so that the resulting carbon black granulated powder can continue to maintain its morphology, thereby acting as a template when reacting with molten silicon at high temperature.

[0011] Furthermore, the viscosity of the thermosetting resin is 3000–8000 mPa·s.

[0012] Furthermore, the dispersant in step (1) is an organic dispersant, including but not limited to one or more of polyethylene glycol, castor oil, polyacrylamide, fish oil, and cellulose derivatives.

[0013] Furthermore, the pore-forming agent in step (1) is a compound that can generate gas through high-temperature decomposition, including but not limited to one or more of ammonium carbonate, ammonium bicarbonate, ammonium chloride, and oxalic acid; the gas generated by the pore-forming agent during high-temperature sintering can generate continuous pores inside the carbon black granulated powder, and the molten silicon droplets can effectively penetrate into the carbon black granulated powder through capillary action using the continuous pores inside, and fully react with the carbon black granulated powder to generate uniform pure phase silicon carbide ceramic powder.

[0014] Preferably, the solvent in step (1) is any one of ethanol, acetone, dimethyl sulfoxide, and N,N-dimethylformamide.

[0015] Further, in step (1), the mass of the binder, dispersant, and pore-forming agent is 1-30 wt%, 0.5-3.0 wt%, and 0.1-1.0 wt%, respectively, of the carbon black powder.

[0016] Furthermore, in step (1), the mass of the solvent is 1 to 10 times the sum of the masses of carbon black powder, binder, dispersant and pore-forming agent.

[0017] Furthermore, in step (1), the ball milling speed is 100-500 rpm and the time is 1-12 h.

[0018] Furthermore, in step (1), the slurry feed rate during spray granulation is 5-120 rpm, the centrifugal disc speed is 1000-30000 rpm, the air inlet temperature is 80-200℃, and the air outlet temperature is 50-100℃.

[0019] Preferably, spray granulation is carried out in a nitrogen atmosphere using a spray granulation drying device.

[0020] Furthermore, in step (2), the molar ratio of carbon black granulated powder to silicon powder is 1.0 to 1.5:1.0, preferably 1.0 to 1.1:1.0.

[0021] Furthermore, the median particle size of the silicon powder in step (2) is 1–5 μm.

[0022] Furthermore, in step (2), the high-temperature sintering is first held at 1100-1350℃ for 1-12 hours, and then heated to 1500-1900℃ and held for 2-20 hours.

[0023] Preferably, high-temperature sintering is carried out in a high-purity argon atmosphere with a vacuum degree of less than 10. -3 Pa.

[0024] Furthermore, the silicon carbide ceramic powder obtained after high-temperature sintering is subjected to decarburization treatment and shaping treatment in sequence; the decarburization treatment is carried out by sintering at 500-900℃ for 0.5-3.0h in an air environment; the shaping treatment is achieved by mechanical grinding for 0.5-12h.

[0025] The present invention also provides a high-purity silicon carbide ceramic powder with controllable particle size, which is prepared by the above-mentioned method for preparing high-purity silicon carbide ceramic powder with controllable particle size.

[0026] Furthermore, the aforementioned silicon carbide ceramic powder is applied to selective laser sintering technology.

[0027] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0028] (1) This invention uses carbon black powder as raw material, combined with binder, dispersant and pore-forming agent, to obtain carbon black granulated powder with controllable particle size by spray granulation, and then mixes it with silicon powder for high-temperature sintering, which acts as a reaction template, and finally obtains high-purity silicon carbide ceramic powder with controllable particle size.

[0029] (2) Thermosetting resin is used as a binder to bind carbon black powder and shape it. It can also obtain a carbon skeleton with high hardness during high temperature process, so that the carbon black granulated powder obtained by spray granulation can continue to maintain its morphology and act as a template when it reacts with molten silicon at high temperature.

[0030] (3) The gas generated by the decomposition of the pore-forming agent during the high-temperature sintering process can generate continuous pores inside the carbon black granulated powder, so that the molten silicon droplets can effectively penetrate into the carbon black granulated powder through the continuous pores and react fully with the carbon black granulated powder, thereby generating uniform pure phase silicon carbide ceramic powder.

[0031] (4) After high-temperature sintering, decarburization and shaping are carried out to make the obtained silicon carbide ceramic powder more pure and with a more complete appearance structure.

[0032] (5) The high-purity silicon carbide ceramic powder with controllable particle size obtained can be applied to selective laser sintering technology to ensure the uniformity of the printing process and the high density and stable performance of the sintered ceramic, meeting the requirements of complex structure manufacturing and high-performance applications. Attached Figure Description

[0033] Figure 1 Here is a SEM image of the silicon carbide ceramic powder obtained in Example 1;

[0034] Figure 2 The image shows the XRD pattern of the silicon carbide ceramic powder obtained in Example 1. Detailed Implementation

[0035] The technical solution of the present invention will be further described and illustrated below with reference to specific embodiments and accompanying drawings. It should be understood that the specific embodiments described herein are only for the purpose of helping to understand the present invention and are not intended to limit the specific scope of the present invention. Furthermore, the accompanying drawings used herein are merely for better illustrating the content disclosed in the present invention and do not limit the scope of protection. Unless otherwise specified, the raw materials used in the embodiments of the present invention are all commonly used in the art, and the methods used in the embodiments are all conventional methods in the art.

[0036] The silicon powder used in the following examples and comparative examples was purchased from Xiamen Jinwu New Materials Co., Ltd., and the carbon black powder was purchased from Tianjin Tianyi Century Chemical Products Technology Development Co., Ltd.

[0037] Example 1

[0038] The method for preparing silicon carbide ceramic powder in this embodiment includes the following steps:

[0039] (1) 10 parts of carbon black powder (particle size of 0.1 μm), 1.0 part of phenolic resin (viscosity of 5000 mPa·s), 0.05 parts of polyethylene glycol and 0.01 parts of ammonium carbonate were put into a ball mill jar, 35 parts of ethanol were added, and the mixture was ball milled at 300 rpm for 2 hours to obtain carbon powder slurry. Spray granulation was carried out in a nitrogen atmosphere through a spray granulation drying device. The slurry feed rate was 50 rpm, the centrifugal disc speed was 10000 rpm, the air inlet temperature was 130℃, and the air outlet temperature was 70℃.

[0040] (2) The carbon black granulated powder obtained by spray granulation is mixed with silicon powder at a molar ratio of 1.03:1.0. The resulting powder is placed in a crucible and then placed in a sintering furnace. The vacuum degree of the sintering furnace is 5×10⁻⁶. -4 Pa, then high-purity argon gas is introduced to atmospheric pressure, and the cycle is repeated three times to remove the air in the sintering furnace. The temperature is raised to 1350℃ and held for 3 hours, then raised to 1650℃ and held for 3 hours, and then cooled to room temperature to obtain silicon carbide ceramic powder.

[0041] (3) The obtained silicon carbide ceramic powder was heat-treated in air at 600℃ for 1.0 h to remove excess carbon, and then ground for 3 h to obtain silicon carbide ceramic powder with a median particle size of 10.6 μm, such as Figure 1 As shown; the obtained product is a pure-phase silicon carbide ceramic powder, containing no other phases, such as... Figure 2 As shown.

[0042] Example 2

[0043] The method for preparing silicon carbide ceramic powder in this embodiment includes the following steps:

[0044] (1) 10 parts of carbon black powder (particle size of 0.5 μm), 1.0 part of phenolic resin (viscosity of 6000 mPa·s), 0.1 part of polyethylene glycol and 0.01 part of ammonium chloride were put into a ball mill jar, 35 parts of ethanol were added, and the mixture was ball milled at 300 rpm for 2 h to obtain carbon powder slurry. Spray granulation was carried out in a nitrogen atmosphere through a spray granulation drying device. The slurry feed rate was 60 rpm, the centrifugal disc speed was 8000 rpm, the air inlet temperature was 140℃, and the air outlet temperature was 75℃.

[0045] (2) The carbon black granulated powder obtained by spray granulation is mixed with silicon powder at a molar ratio of 1.05:1.0. The resulting powder is placed in a crucible and then placed in a sintering furnace. The vacuum degree of the sintering furnace is 5×10⁻⁶. -4 Pa, then high-purity argon gas is introduced to atmospheric pressure, and the cycle is repeated three times to remove the air in the sintering furnace. The temperature is raised to 1300℃ and held for 4 hours, then raised to 1700℃ and held for 5 hours, and then cooled to room temperature to obtain silicon carbide ceramic powder.

[0046] (3) The obtained silicon carbide ceramic powder was heat-treated in air at 700℃ for 1.5h to remove excess carbon, and then ground for 4h to obtain silicon carbide ceramic powder with a median particle size of 72.1μm.

[0047] Example 3

[0048] The method for preparing silicon carbide ceramic powder in this embodiment includes the following steps:

[0049] (1) 10 parts carbon black powder (particle size of 1.0 μm), 2.0 parts epoxy resin (viscosity of 5000 mPa·s), 0.05 parts polyethylene glycol and 0.01 parts ammonium carbonate were put into a ball mill jar, 35 parts ethanol were added, and the mixture was ball milled at 300 rpm for 2 hours to obtain carbon powder slurry. Spray granulation was carried out in a nitrogen atmosphere through a spray granulation drying device. The slurry feed rate was 80 rpm, the centrifugal disc speed was 8000 rpm, the air inlet temperature was 140℃, and the air outlet temperature was 80℃.

[0050] (2) The carbon black granulated powder obtained by spray granulation is mixed with silicon powder at a molar ratio of 1.01:1.0. The resulting powder is placed in a crucible and then placed in a sintering furnace. The vacuum degree of the sintering furnace is 5×10⁻⁶. -4 Pa, then high-purity argon gas is introduced to atmospheric pressure, and the cycle is repeated three times to remove the air in the sintering furnace. The temperature is raised to 1250℃ and held for 3 hours, then raised to 1700℃ and held for 6 hours. The temperature is then cooled to room temperature to obtain silicon carbide ceramic powder.

[0051] (3) The obtained silicon carbide ceramic powder was heat-treated in air at 800℃ for 1.5h to remove excess carbon, and then ground for 3h to obtain silicon carbide ceramic powder with a median particle size of 107.4μm.

[0052] Example 4

[0053] The difference between this embodiment and Example 1 is only in step (1): 10 parts of carbon black powder (particle size of 0.1 μm), 1.0 part of phenolic resin (viscosity of 5000 mPa·s), 0.05 parts of polyethylene glycol and 0.04 parts of ammonium carbonate are put into a ball mill jar, 35 parts of ethanol are added, and the mixture is ball-milled at 300 rpm for 2 hours to obtain carbon powder slurry. Spray granulation is carried out in a nitrogen atmosphere using a spray granulation drying device. The slurry feed rate is 50 rpm, the centrifugal disc speed is 10000 rpm, the inlet temperature is 130℃, and the outlet temperature is 70℃. Finally, silicon carbide ceramic powder with a median particle size of 9.9 μm is obtained.

[0054] Example 5

[0055] The difference between this embodiment and Example 1 is only in step (1): 10 parts of carbon black powder (particle size of 0.1 μm), 1.0 part of phenolic resin (viscosity of 5000 mPa·s), 0.05 parts of polyethylene glycol and 0.08 parts of ammonium carbonate are put into a ball mill jar, 35 parts of ethanol are added, and the mixture is ball-milled at 300 rpm for 2 hours to obtain carbon powder slurry. Spray granulation is carried out in a nitrogen atmosphere using a spray granulation drying device. The slurry feed rate is 50 rpm, the centrifugal disc speed is 10000 rpm, the inlet temperature is 130℃, and the outlet temperature is 70℃. Finally, silicon carbide ceramic powder with a median particle size of 8.7 μm is obtained.

[0056] Example 6

[0057] The difference between this embodiment and Example 1 is only in step (1): 10 parts of carbon black powder (particle size of 0.1 μm), 1.0 part of phenolic resin (viscosity of 5000 mPa·s), 0.05 parts of polyethylene glycol and 0.12 parts of ammonium carbonate are put into a ball mill jar, 35 parts of ethanol are added, and the mixture is ball-milled at 300 rpm for 2 hours to obtain carbon powder slurry. Spray granulation is carried out in a nitrogen atmosphere using a spray granulation drying device. The slurry feed rate is 50 rpm, the centrifugal disc speed is 10000 rpm, the inlet temperature is 130℃, and the outlet temperature is 70℃. Finally, silicon carbide ceramic powder with a median particle size of 7.2 μm is obtained.

[0058] Example 7

[0059] The method for preparing silicon carbide ceramic powder in this embodiment includes the following steps:

[0060] (1) 10 parts of carbon black powder (particle size of 0.1 μm), 1.0 part of phenolic resin (viscosity of 5000 mPa·s), 0.05 parts of polyethylene glycol and 0.01 parts of ammonium carbonate were put into a ball mill jar, 35 parts of ethanol were added, and the mixture was ball milled at 300 rpm for 2 hours to obtain carbon powder slurry. Spray granulation was carried out in a nitrogen atmosphere through a spray granulation drying device. The slurry feed rate was 50 rpm, the centrifugal disc speed was 10000 rpm, the air inlet temperature was 130℃, and the air outlet temperature was 70℃.

[0061] (2) The carbon black granulated powder obtained by spray granulation is mixed with silicon powder at a molar ratio of 1.03:1.0. The resulting powder is placed in a crucible and then placed in a sintering furnace. The vacuum degree of the sintering furnace is 5×10⁻⁶. -4 Pa, then high-purity argon gas is introduced to atmospheric pressure, and the cycle is repeated three times to remove air from the sintering furnace. The temperature is raised to 1350℃ and held for 3 hours, then raised to 1650℃ and held for 3 hours. After cooling to room temperature, silicon carbide ceramic powder is obtained, and then ground for 3 hours.

[0062] Example 8

[0063] The difference between this embodiment and Embodiment 1 is only in step (2): the carbon black granulated powder obtained by spray granulation is mixed with silicon powder at a molar ratio of 1.0:1.1, and the resulting powder is placed in a crucible and then placed in a sintering furnace. The vacuum degree of the sintering furnace is 5×10⁻⁶. -4 Pa, then high-purity argon gas is introduced to atmospheric pressure, and the cycle is repeated three times to remove air from the sintering furnace. The temperature is raised to 1350℃ and held for 3 hours, then raised to 1650℃ and held for 3 hours, and finally cooled to room temperature to obtain silicon carbide ceramic powder.

[0064] Comparative Example 1

[0065] The only difference between this comparative example and Example 1 is that in step (1), 10 parts of carbon black powder (particle size of 0.1 μm), 1.0 part of phenolic resin (viscosity of 5000 mPa·s) and 0.05 parts of polyethylene glycol are put into a ball mill jar, 35 parts of ethanol are added, and the mixture is ball milled at 300 rpm for 2 hours to obtain carbon powder slurry. The slurry is then spray granulated under a nitrogen atmosphere using a spray granulation drying device. The slurry feed rate is 50 rpm, the centrifugal disc speed is 10000 rpm, the inlet temperature is 130°C, and the outlet temperature is 70°C.

[0066] Comparative Example 2

[0067] The only difference between this comparative example and Example 1 is that in step (1), 10 parts of carbon black powder (particle size of 0.1 μm), 0.05 parts of polyethylene glycol and 0.01 parts of ammonium carbonate are put into a ball mill jar, 35 parts of ethanol are added, and the mixture is ball-milled at 300 rpm for 2 hours to obtain carbon powder slurry. The slurry is then spray-granulated under a nitrogen atmosphere using a spray granulation and drying equipment. The slurry feed rate is 50 rpm, the centrifugal disc speed is 10000 rpm, the inlet temperature is 130°C, and the outlet temperature is 70°C.

[0068] Comparative Example 3

[0069] The only difference between this comparative example and Example 1 is that in step (1), 10 parts of carbon black powder (particle size of 0.1 μm), 1.0 part of polyvinyl butyral, 0.05 parts of polyethylene glycol and 0.01 parts of ammonium carbonate are put into a ball mill jar, 35 parts of ethanol are added, and the mixture is ball-milled at 300 rpm for 2 hours to obtain carbon powder slurry. The slurry is then spray-granulated under a nitrogen atmosphere using a spray granulation and drying equipment. The slurry feed rate is 50 rpm, the centrifugal disc speed is 10000 rpm, the inlet temperature is 130°C, and the outlet temperature is 70°C.

[0070] Examples 1-3 used carbon black powder with a particle size of 0.1-1.0 μm as raw material, combined with appropriate amounts of binder, dispersant and pore-forming agent, to obtain carbon black granulated powder with controllable particle size through spray granulation, which was then mixed with silicon powder and sintered at high temperature to finally obtain silicon carbide ceramic powder with a median particle size of 10.6-107.4 μm; Examples 4-5 added a high content of pore-forming agent, which caused the carbon black granulated powder to generate more pores during high-temperature sintering, resulting in structural instability and a smaller particle size of the final silicon carbide ceramic powder; Example 6 used an excessively high content of pore-forming agent, resulting in more pores in the carbon powder and the obtained silicon carbide ceramic powder, unstable structure, and a more significant reduction in the particle size of the obtained silicon carbide ceramic powder; Example 7 did not perform decarburization treatment, resulting in a decrease in the purity of the obtained silicon carbide ceramic powder, containing some residual carbon. In Example 8, the use of less carbon powder and more silicon powder in the preparation of silicon carbide ceramic powder resulted in a decrease in the purity of the obtained silicon carbide ceramic powder, containing some residual silicon. In Comparative Example 1, no pore-forming agent was added, and no continuous pores were formed inside the carbon black granulated powder during the high-temperature reaction. The molten silicon liquid could not effectively penetrate into the carbon black granulated powder for sufficient reaction, resulting in the presence of unreacted carbon inside the final silicon carbide powder. In Comparative Example 2, no phenolic resin was added, which could not bind and shape the carbon black powder, nor could a carbon skeleton maintain the morphological characteristics of the carbon black powder, making the structure and particle size of the obtained silicon carbide ceramic powder uncontrollable. In Comparative Example 3, thermoplastic resin was used, which caused the carbon black granulated powder obtained by spray granulation to collapse and stick together during the high-temperature sintering process, making it difficult to control the particle size and morphology of the silicon carbide powder generated after the high-temperature reaction.

[0071] Finally, it should be noted that the specific embodiments described herein are merely illustrative of the spirit of the invention and are not intended to limit the implementation of the invention. Those skilled in the art can make various modifications or additions to the described embodiments or use similar methods to replace them; it is neither necessary nor possible to exemplify all embodiments here. However, these obvious variations or modifications derived from the essential spirit of the invention still fall within the scope of protection of the invention, and interpreting them as any additional limitation would contradict the spirit of the invention.

Claims

1. A method for preparing high-purity silicon carbide ceramic powder with controllable particle size, characterized in that, Includes the following steps: (1) Carbon black powder, binder, dispersant, pore-forming agent and solvent are ball-milled to obtain carbon powder slurry, and carbon black granulated powder is obtained by spray granulation; the mass of the binder, dispersant and pore-forming agent is 1~30wt%, 0.5~3.0wt% and 0.1~1.0wt% of the carbon black powder, respectively; (2) The carbon black granulated powder obtained by spray granulation is mixed with silicon powder and then sintered at high temperature to obtain silicon carbide ceramic powder; (3) The silicon carbide ceramic powder is subjected to decarburization and shaping treatment in sequence.

2. The preparation method according to claim 1, characterized in that, In step (1), the particle size of the carbon black powder is less than 1.5 μm.

3. The preparation method according to claim 1, characterized in that, The adhesive in step (1) is a thermosetting resin; and / or the pore-forming agent is a compound that can generate gas when decomposed at high temperatures.

4. The preparation method according to claim 3, characterized in that, The viscosity of thermosetting resins is 3000~8000 mPa·s.

5. The preparation method according to claim 1, characterized in that, In step (1), the mass of the solvent is 1 to 10 times the sum of the masses of carbon black powder, binder, dispersant and pore-forming agent.

6. The preparation method according to claim 1, characterized in that, In step (2), the molar ratio of carbon black granulated powder to silicon powder is 1.0~1.5:1.

0.

7. The preparation method according to claim 1, characterized in that, In step (2), the high-temperature sintering is first held at 1100~1350℃ for 1~12h, and then the temperature is raised to 1500~1900℃ and held for 2~20h.

8. A high-purity silicon carbide ceramic powder with controllable particle size, characterized in that, It is prepared by the preparation method described in claim 1.

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