CN108069720B - Silicon nitride gradient porous capillary core for loop heat pipe and preparation method thereof - Google Patents
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Abstract
The invention relates to a silicon nitride gradient porous capillary wick for a loop heat pipe and a preparation method thereof, wherein the preparation method comprises the following steps: and sequentially injecting at least two silicon nitride slurries with different proportions into a mold for gel curing, drying, sintering and processing to obtain the silicon nitride gradient porous capillary core for the loop heat pipe, wherein the silicon nitride slurries comprise silicon nitride powder, a sintering aid, a gelling agent, a pore-forming agent and water, and the content of the pore-forming agent in the silicon nitride slurry injected into the mold is lower than the content of the pore-forming agent in the silicon nitride slurry injected into the mold.
Description
Technical Field
The invention relates to a silicon nitride capillary core for a loop heat pipe, in particular to design and preparation of a gradient porous structure capillary core, and belongs to the field of temperature control.
Background
The loop heat pipe is used as an efficient phase change heat transfer device and is a key part for stable and long-life operation of electronic elements and aerospace devices at constant temperature, and the capillary pump main core is one of the most key parts in the loop heat pipe. Along with the increasing power consumption of related instruments and equipment, the requirements on system heat dissipation and constant temperature are higher and higher, and the existing metal capillary core main core cannot meet the requirements on heat dissipation, corrosion resistance and the like for stability and long service life. The porous silicon nitride is selected as a preparation material of the capillary core, so that the excellent performance of the silicon nitride ceramic can be fully exerted, the prepared capillary core has the advantages of corrosion resistance, large capillary suction force, low thermal conductivity and the like, and the stability and reliability of the loop heat pipe can be obviously improved.
The loop heat pipe uses the capillary core to need powerful capillary drive power to circulate the working medium on the one hand, on the other hand, the capillary core needs to transfer the steam that produces to the steam pipeline in time. Therefore, for the capillary wick product, under the condition of ensuring higher porosity, the capillary suction force needs to be improved by reducing the capillary pore diameter, and meanwhile, the efficient transfer of steam is also expected to be realized by having larger pores. However, the traditional capillary pumps have single pore size and porosity, and the efficient transfer of vapor while improving the capillary suction force is difficult to achieve. And the traditional pressing process can only prepare some capillary core products with simple shapes and has complex processing.
Disclosure of Invention
In view of the above problems, the present invention aims to provide a silicon nitride gradient porous wick for a functional loop heat pipe, which has good mechanical properties, and has both large capillary suction force and high permeability, and a preparation method thereof.
In one aspect, the invention provides a method for preparing a silicon nitride gradient porous capillary wick for a loop heat pipe, which comprises the following steps: and sequentially injecting at least two silicon nitride slurries with different proportions into a mold for gel curing, drying, sintering and processing to obtain the silicon nitride gradient porous capillary core for the loop heat pipe, wherein the silicon nitride slurries comprise silicon nitride powder, a sintering aid, a gelling agent, a pore-forming agent and water, and the content of the pore-forming agent in the silicon nitride slurry injected into the mold is lower than the content of the pore-forming agent in the silicon nitride slurry injected into the mold.
Preferably, the at least two silicon nitride slurries include a first slurry and a second slurry, and the mass ratio of the silicon nitride powder, the sintering aid, the gelling agent, the pore-forming agent and water in the first slurry is 100: (0.5-10): (0.1-5): (0-10): (20-300);
the mass ratio of silicon nitride powder, sintering aid, gelling agent, pore-forming agent and water in the second slurry is 100: (0.5-10): (0.1-5): (10-50): (20-300);
injecting the obtained first slurry into a mold, curing for 1-10 minutes at 20-30 ℃, pouring out the ungelled first slurry, and injecting the second slurry into the mold to continue curing the gel to obtain a blank;
and (3) demolding the blank, drying, sintering and processing to obtain the silicon nitride gradient porous capillary core for the loop heat pipe.
The invention obtains the required shape by one-step casting and one-step molding, and can obtain the silicon nitride gradient porous capillary core with a column shape, a plate shape or other complex shapes by subsequent sintering. Specifically, two different slurries are first disposed. And injecting the obtained first slurry into a mold, pouring out the ungelled first slurry after the first slurry is cured for 1-10 minutes at 20-30 ℃ (because of the selection of curing temperature and time, only the part of the first slurry close to the inner surface of the mold begins to gel, and the first slurry close to the central part of the mold does not gel), then pouring the second slurry into the hollow inner cavity for continuous curing, and because the surface of the hollow inner cavity is not completely dried, tightly combining the second slurry with the colloid formed by the coagulation of the first slurry during curing to obtain a blank with an interconnected open pore structure. And then demolding, drying, sintering and processing to obtain the silicon nitride gradient porous capillary core for the loop heat pipe. The invention can obtain a sample with better precision through process control, and reduces the processing difficulty. In addition, the invention realizes the layered structure of the inner layer large-aperture layer and the outer layer small-aperture layer through the gradient pore structure design, the interlayer combination is good, and the functionality of the capillary core is expanded through the gradient structure under the condition of good mechanical property.
Preferably, the first slurry is cured at 20-30 ℃ for 1-10 minutes, the second slurry is cured at 20-30 ℃, and the total gel curing time of the first slurry and the second slurry is 1-4 hours.
Preferably, the sintering aid is selected from Y2O3、Al2O3、Yb2O3、Lu2O3、Sm2O3、SiO2、Nd2O3、Eu2O3At least one of (1).
Preferably, the pore-forming agent is a substance that can be removed during sintering, and is preferably at least one of starch, cyclohexane (O/W emulsion), polymethyl methacrylate PMMA, polyvinyl alcohol PVA, and paraffin.
Preferably, the gelling agent is selected from gelatin or/and isobutylene-maleic anhydride copolymer.
Preferably, the drying is performed at 20-150 ℃ for 3-100 hours.
Preferably, the sintering is carried out in an inert atmosphere at 1500-1950 ℃ for 1-12 hours.
Further, the temperature rise rate of the sintering is preferably 1 to 15 ℃/min.
Also, preferably, the inert atmosphere is selected from nitrogen, such as high purity nitrogen.
In one aspect, the invention provides a silicon nitride gradient porous wick for a loop heat pipe, which comprises a primary silicon nitride wick and at least one silicon nitride pore layer coated on the outer surface of the primary silicon nitride wick.
According to the invention, through the design of the gradient pore structure, the gradient structure of the inner silicon nitride large-pore-diameter layer (primary silicon nitride capillary core) and the outer silicon nitride small-pore-diameter layer is realized, and the pore diameter of the silicon nitride large-pore-diameter layer is larger than that of the silicon nitride small-pore-diameter layer, so that the silicon nitride gradient porous capillary core has both large capillary suction force and high permeability. And the silicon nitride macroporous layer and the silicon nitride microporous layer are combined well, namely the silicon nitride macroporous layer and the silicon nitride macroporous layer have communicated open pore structures, so that the functionality of the capillary core can be expanded through a gradient structure under the condition of good mechanical property.
Preferably, the pore diameter of the primary silicon nitride capillary core is 0.1-1 micron, and the porosity is 50-60%. Preferably, the pore size of the silicon nitride pore layer is 0.5 to 30 microns, and the porosity is 55 to 70%.
Preferably, the thickness of the silicon nitride pore layer is 0.5 to 1 mm.
Preferably, the outer shape of the silicon nitride gradient porous capillary wick for the loop heat pipe is a cylinder shape, a disc shape, a plate shape or a flat oval shape.
The invention has the special benefits that:
(1) the gradient porous structure has large capillary suction force and high permeability, and simultaneously, due to the characteristics of the silicon nitride ceramic, the capillary core has the advantages of small density, high strength, corrosion resistance, low thermal conductivity and the like, and the stability and reliability of the loop heat pipe can be obviously improved;
(2) the preparation process is simple, and capillary core products with various shapes and sizes can be obtained through multiple casting and one-step forming;
(3) the application range is wide: the capillary core with single pore size distribution can be prepared by controlling the types and the contents of pore-forming agents in different casting slurries and slurries, and the porous capillary core with different pore size distributions and a multi-layer pore combination can also be obtained.
Drawings
FIG. 1 is a schematic cross-sectional view of a ceramic sample prepared in the present invention;
fig. 2 shows the micro-topography of the inner layer large pores b and the outer layer small pores a of the porous capillary wick obtained in example 1;
fig. 3 is a microscopic morphology of the interlayer bonding sites of the porous wick obtained in example 2.
Detailed Description
The present invention is further illustrated by the following examples, which are to be understood as merely illustrative and not restrictive.
The silicon nitride gradient porous capillary wick for the loop heat pipe comprises a primary silicon nitride capillary wick and one or more layers of silicon nitride pore layers wrapping the outer surface of the primary silicon nitride capillary wick. The pore diameter of the primary silicon nitride capillary core can be 0.5-30 microns, the porosity can be 55-70%, and the primary silicon nitride capillary core has high permeability when the pore diameter is within the range. The pore size of the silicon nitride pore layer can be 0.1-1 micron, the porosity can be 50-60%, and the pore size has higher capillary suction force within the range. The thickness of the silicon nitride micropore layer is 0.5-1 mm. When the silicon nitride pore layer is a multilayer, the pore diameter and the porosity of the multilayer silicon nitride pore layer can form a gradient structure with the porosity and the pore diameter being reduced in sequence from inside to outside.
The silicon nitride gradient porous capillary wick for the loop heat pipe can be cylindrical, disc-shaped, plate-shaped or flat oval in shape. And the outer wall surface of the silicon nitride gradient porous capillary wick for the loop heat pipe is provided with a plurality of steam channels along the axial direction and the radial direction or is a smooth surface without the steam channels, and the sections of the steam channels can be triangular, rectangular, trapezoidal or omega-shaped. As an example, as shown in fig. 1, the silicon nitride gradient porous wick for a loop heat pipe in the present invention includes: the porous structure comprises a silicon nitride macroporous layer, a capillary core inner cavity positioned in the silicon nitride macroporous layer (inner macroporous layer), and a silicon nitride small porous layer (outer small porous layer) coated on the outer surface of the silicon nitride macroporous layer. The silicon nitride macroporous layer and the silicon nitride small pore layer are provided with communicated open pore structures, and the aperture of the silicon nitride macroporous layer is larger than that of the silicon nitride small pore layer.
The following exemplarily illustrates a method for preparing a silicon nitride gradient porous wick for a loop heat pipe according to the present invention.
Preparing a corresponding die according to the shape and the size of the capillary core product; the inner wall of the die cavity is smooth as much as possible, so that the demolding is convenient. The mould can be made of various materials such as metal moulds, plastic moulds, plaster moulds, rubber moulds, glass moulds and the like.
And (4) preparing slurry. Specifically, silicon nitride ceramic slurry is prepared, silicon nitride powder, a sintering aid, a gelling agent and a pore-forming agent are sequentially added into deionized water, and the mixture is mixed in a ball milling mode to obtain uniformly mixed slurry. Wherein, the mass ratio of the silicon nitride powder (generally 0.3-0.8 micron), the sintering aid, the gel, the pore-forming agent and the water can be 100: (0.5-10): (0.1-5): (1-50): (20-300). According to the invention, the addition amount or variety of the pore-forming agent in the slurry is controlled, so that the colloidal porosity of the solidified slurry gel is regulated and controlled, and finally the obtained nitriding for the loop heat pipe is controlledThe pore diameter and porosity of the silicon nitride macroporous layer and the silicon nitride microporous layer in the silicon gradient porous capillary core. Controlling the mass ratio of the silicon nitride powder, the sintering aid, the gel, the pore-forming agent and the water to be 100: (0.5-10): (0.1-5): (0-10): (20-300), adding silicon nitride powder, a sintering aid, a gelling agent and a pore-forming agent into water, and performing ball milling and mixing to obtain a first slurry. Controlling the mass ratio of the silicon nitride powder, the sintering aid, the gel, the pore-forming agent and the water to be 100: (0.5-10): (0.1-5): (10-50): (20-300), adding silicon nitride powder, a sintering aid, a gelling agent and a pore-forming agent into water, and performing ball-milling mixing to obtain a second slurry. The sintering aid can be Y2O3、Al2O3、Yb2O3、Lu2O3、Sm2O3、SiO2、Nd2O3、Eu2O3Any one or a mixture of two or more of them in any proportion. The pore-forming agent can be starch, PMMA, PVA, O/W emulsion, paraffin and the like which can be removed in the sintering process. The gelling agent can be at least one selected from gelatin and isobutylene-maleic anhydride copolymer.
And injecting the obtained first slurry into a mold, pouring out the ungelled first slurry after partial gelation, and injecting the second slurry into the mold to continue gel curing to obtain a blank. Specifically, the obtained first slurry is poured into a mold, and after a part of the first slurry is gelled, the ungelled first slurry is poured out, so that the geocolloid is obtained. And injecting the second slurry into the first colloid, and continuing to carry out gel curing to obtain a blank. Or after the first slurry is completely solidified, the first slurry is processed to form a hollow structure, and then the second slurry is injected into the hollow structure. Therefore, the partial gelation in the present invention is not limited to only a part of the gel. The first slurry is cured for 1-10 minutes at 20-30 ℃. The curing temperature of the second slurry can be 20-30 ℃, and the total gel curing time of the first slurry and the second slurry is 1-4 hours. The above is merely an example of the first slurry and the second slurry, and it is understood that a third slurry, a fourth slurry, etc. may be included in sequence to form a gradient porous structure.
And (3) demolding the blank, drying, sintering and processing to obtain the silicon nitride gradient porous capillary core for the loop heat pipe. The drying condition is drying for 3-100 hours at 20-150 ℃. The sintering is carried out in an inert atmosphere at 1500-1950 ℃ for 1-12 hours. The temperature rise rate of the sintering can be 1-15 ℃/min. Wherein the inert atmosphere is selected from nitrogen (high purity nitrogen). As an example, the sintering can be carried out under the condition of nitrogen, the temperature is raised to 1500-1950 ℃ at 1-15 ℃/min, and the temperature is kept for 1-12 hours.
Wherein the machining is mechanical machining. And machining the sintered capillary core to reach required precision. The mechanical finish machining comprises technological methods such as turning, grinding and drilling, the turning and grinding can realize the shape of the outer wall of the porous silicon nitride capillary core and the dimensional accuracy of the steam channel, and the drilling can realize the shape and the position size of the blind hole in the cylindrical porous silicon nitride capillary core.
Compared with the traditional composite capillary core, the invention optimizes the material and the preparation process, adopts ceramic to replace metal material, improves the stability of the capillary core product, and can achieve higher strength due to the characteristics of silicon nitride ceramic. Compared with the traditional pressing process which can only prepare a plurality of capillary core products with simple shapes and has complex processing, the multi-pouring one-step forming process adopted by the invention can simply and effectively prepare products with various sizes and shapes, thereby increasing the uniformity of the product structure and simultaneously reducing the difficulty of later processing.
The present invention will be described in detail by way of examples. It is also to be understood that the following examples are illustrative of the present invention and are not to be construed as limiting the scope of the invention, and that certain insubstantial modifications and adaptations of the invention by those skilled in the art may be made in light of the above teachings. The specific process parameters and the like of the following examples are also only one example of suitable ranges, i.e., those skilled in the art can select the appropriate ranges through the description herein, and are not limited to the specific values exemplified below.
Example 1:
(1) respectively adding 100g of silicon nitride balls, 40g of deionized water, 2.5g of sintering aid yttrium oxide, 47.5g of silicon nitride powder and 1g of gel gelatin into the two ball milling tanks, wherein the rotating speed of the ball mill is 350rpm, and ball milling is carried out for 2 hours to obtain water-based slurry;
(2) adding 2g and 20g of cyclohexane into the two tanks of slurry respectively, and continuing ball milling for 1 hour to obtain two slurries with different pore-forming agent contents, wherein the slurries 1 and 2 are numbered respectively;
(3) injecting the No. 1 slurry into a gypsum mold, curing for 3 minutes at 25 ℃, pouring out the redundant slurry, continuously pouring the No. 2 slurry, curing for 1 hour at 25 ℃, and obtaining a ceramic wet blank with a double-layer pore structure with a large pore at the inner layer and a small pore at the outer layer after the slurry is gelled;
(4) drying the wet blank for 48 hours at room temperature after demoulding to obtain a ceramic blank;
(5) and (3) heating the ceramic blank to 800 ℃ at a speed of 10 ℃/min in a nitrogen atmosphere, heating to 1680 ℃ at a speed of 3 ℃/min, preserving heat for 2 hours, and cooling along with the furnace after sintering to obtain the porous silicon nitride ceramic. And (4) obtaining the silicon nitride gradient porous capillary wick for the loop heat pipe through mechanical processing. Fig. 2 is a microscopic morphology of inner layer large pores and outer layer small pores of the porous capillary wick obtained in example 1, and it can be seen that both porous layers present uniform pore distribution; the thickness of the silicon nitride small-hole layer is 1 mm.
Example 2:
(1) respectively adding 100g of silicon nitride balls, 40g of deionized water, 2.5g of sintering aid yttrium oxide, 47.5g of silicon nitride powder and 0.2g of gel isobutylene-maleic anhydride copolymer into two ball milling tanks, wherein the rotating speed of the ball mill is 350rpm, and ball milling is carried out for 2 hours to obtain water-based slurry;
(2) adding 0g and 20g of starch into the two tanks of slurry respectively, and continuing ball milling for 1 hour to obtain two slurries with different pore-forming agent contents, wherein the slurries 1 and 2 are numbered respectively;
(3) injecting the No. 1 slurry into a gypsum mold, curing for 1 minute at 25 ℃, pouring out the redundant slurry, continuously pouring the No. 2 slurry, curing for 2 hours at 25 ℃, and obtaining a ceramic wet blank with a double-layer pore structure with a large pore at the inner layer and a small pore at the outer layer after the slurry is gelled;
(4) drying the wet blank for 48 hours at room temperature after demoulding to obtain a ceramic blank;
(5) and (3) heating the ceramic blank to 800 ℃ at a speed of 10 ℃/min in a nitrogen atmosphere, heating to 1680 ℃ at a speed of 3 ℃/min, preserving heat for 2 hours, and cooling along with the furnace after sintering to obtain the porous silicon nitride ceramic. And (4) obtaining the silicon nitride gradient porous capillary wick for the loop heat pipe through mechanical processing. Fig. 3 is a microscopic morphology of the interlayer bonding site of the porous wick obtained in example 2, and it can be seen that the interlayer bonding is continuous and tight. The thickness of the silicon nitride small-hole layer is 0.5 mm.
Table 1 shows the performance parameters of the silicon nitride gradient porous wick for the loop heat pipe prepared by the present invention:
Claims (6)
1. a preparation method of a silicon nitride gradient porous capillary wick for a loop heat pipe is characterized by comprising the following steps:
preparing at least two silicon nitride slurries, wherein the two silicon nitride slurries comprise a first slurry and a second slurry, and the content of a pore-forming agent in the first slurry is controlled to be lower than the content of a pore-forming agent in the second slurry;
the mass ratio of silicon nitride powder, sintering aid, gelling agent, pore-forming agent and water in the first slurry is 100: (0.5-10): (0.1-5): (0-10): (20-300), the pore-forming agent is a substance which can be removed in the sintering process, and the gelling agent is selected from gelatin or/and isobutylene-maleic anhydride copolymer;
the mass ratio of silicon nitride powder, sintering aid, gelling agent, pore-forming agent and water in the second slurry is 100: (0.5-10): (0.1-5): (10-50): (20-300), the pore-forming agent is a substance which can be removed in the sintering process, and the gelling agent is selected from gelatin or/and isobutylene-maleic anhydride copolymer;
injecting the obtained first slurry into a mold, curing for 1-10 minutes at 20-30 ℃, pouring out the ungelled first slurry, and injecting the second slurry into the mold to continue curing the gel to obtain a blank;
demolding the blank, drying, sintering and processing to obtain the silicon nitride gradient porous capillary core for the loop heat pipe, wherein the sintering is carried out in an inert atmosphere at the temperature of 1500-1950 ℃ for 1-12 hours;
the silicon nitride gradient porous capillary core for the loop heat pipe comprises a primary silicon nitride capillary core and at least one silicon nitride pore layer coated on the outer surface of the primary silicon nitride capillary core, wherein the pore diameter of the silicon nitride pore layer is 0.1-1 micron, the porosity is 50-60%, and the pore diameter of the primary silicon nitride capillary core is 0.5-30 microns, and the porosity is 55-70%.
2. The method of claim 1, wherein the sintering aid is selected from Y2O3、Al2O3、Yb2O3、Lu2O3、Sm2O3、SiO2、Nd2O3、Eu2O3At least one of (1).
3. The preparation method according to claim 1, wherein the pore-forming agent is at least one of starch, cyclohexane, PMMA, PVA, and paraffin.
4. The method according to any one of claims 1 to 3, wherein the first slurry is cured at 20 to 30 ℃ for 1 to 10 minutes, the second slurry is cured at 20 to 30 ℃, and the total gel curing time of the first slurry and the second slurry is 1 to 4 hours.
5. The silicon nitride gradient porous capillary wick for the loop heat pipe, which is prepared according to any one of claims 1 to 4, comprises a primary silicon nitride capillary wick and at least one silicon nitride pore layer coated on the outer surface of the primary silicon nitride capillary wick, wherein the silicon nitride pore layer has a pore diameter of 0.1-1 micron and a porosity of 50-60%, and the primary silicon nitride capillary wick has a pore diameter of 0.5-30 micron and a porosity of 55-70%.
6. The silicon nitride gradient porous capillary wick for the loop heat pipe as claimed in claim 5, wherein the thickness of the silicon nitride pore layer is 0.5-1 mm.
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| CN108516806A (en) * | 2018-06-26 | 2018-09-11 | 青岛科技大学 | A kind of preparation method and applications of aluminium oxide ceramics |
| CN110303153B (en) * | 2019-06-28 | 2021-04-27 | 安泰环境工程技术有限公司 | Method for processing capillary core and method for assembling capillary core and tube shell |
| CN115340402A (en) * | 2021-05-13 | 2022-11-15 | 中国科学院上海硅酸盐研究所 | Preparation method of gradient silicon nitride capillary core |
| CN114804842B (en) * | 2022-05-11 | 2022-12-09 | 西安交通大学 | Preparation method of ceramic core with controllable pore distribution and atmosphere |
| CN115385699B (en) * | 2022-08-31 | 2023-11-24 | 山东工业陶瓷研究设计院有限公司 | High-temperature wave-transparent and active cooling integrated material and preparation method thereof |
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