A silicon nitride flat capillary core for a loop heat pipe with an extremely small pore channel curvature and a preparation method thereof

Abstract

The present invention relates to a silicon nitride flat capillary core for a loop heat pipe with an extremely small pore channel tortuosity and a preparation method thereof. The silicon nitride flat capillary core for the loop heat pipe has a unidirectional pore channel inside, and small pores are distributed on the pore walls of large pores; the porosity of the silicon nitride flat capillary core for the loop heat pipe is 65.45% - 85.37%, the size of the small pores is 0.8 - 1.3 μm, the size of the large pores is 2.5 - 100.7 μm, and the pore channel tortuosity is 2.0 - 6.0.

Description

Technical Field

[0001] The present invention relates to a silicon nitride flat capillary wick for a loop heat pipe with an extremely small pore channel tortuosity and a preparation method thereof, belonging to the field of thermal control. Background Art

[0002] With the rapid development of electronic technology, the high heat flux density of electronic components has increased the demand for their heat dissipation. As a phase change heat transfer device, a loop heat pipe can complete efficient heat transfer and temperature control by using the evaporation and condensation of a liquid working medium. Therefore, it has received extensive attention in the fields of aerospace and electronic equipment heat dissipation. The capillary wick in the evaporator, as the core component of the loop heat pipe, provides the driving force for the two-phase working medium flow inside the heat pipe. Therefore, the selection of the wick material and the pore parameters are crucial for the heat transfer efficiency of the loop heat pipe. Since polymer materials have poor working medium compatibility and high-temperature stability, and metal cores represented by nickel, copper, and stainless steel have too high a thermal conductivity, which is easy to cause backward heat leakage. At the same time, during the processing, plastic deformation is easy to cause the closure of the micropores on the surface. Therefore, ceramic capillary wicks that can overcome the above disadvantages have become a research hotspot.

[0003] Porous silicon nitride ceramics have excellent thermal stability and chemical stability. The microstructure of the rod-shaped grains cross-linked and overlapped ensures that the ceramic can maintain sufficient strength at a relatively high porosity, which is beneficial to the assembly of the capillary wick and its application in a complex heat flux coupling environment. In addition, traditional capillary wicks with a single pore size distribution are difficult to meet the requirements of the capillary pumping performance and permeation performance of the capillary wick for the normal operation of the loop heat pipe under high heat flux density. The homogeneous capillary wick with a double pore distribution can, to a certain extent, achieve the goal of "small pores sucking liquid and large pores exhausting gas", but it still cannot fully meet the requirements of the loop heat pipe for the functionality of different parts of the capillary wick. For example, the new capillary wick requires that the capillary wick part near the evaporation surface has better thermal conductivity, smaller pore size, and a relatively uniform pore size distribution, while the capillary wick part near the compensation chamber or the liquid reservoir should have relatively poor thermal conductivity to reduce backward heat leakage and larger pore size to reduce the resistance of the working medium flow. In addition, the new capillary wick should also have a small pore channel tortuosity. The pore channel tortuosity refers to the ratio between the path of the fluid flowing through the inside of the porous medium and the macroscopic thickness of the porous medium in the flow direction. A smaller pore channel tortuosity will reduce the flow resistance of the fluid in the porous medium. Therefore, the design of a multi-scale pore structure with a small pore channel tortuosity has become the key to maximizing the function of the capillary wick. Summary of the Invention

[0004] Aiming at the above problems, the present invention aims to provide a silicon nitride flat capillary wick for a loop heat pipe with an extremely small pore channel tortuosity and a preparation method thereof. The capillary wick has excellent capillary pumping performance and permeation performance, and its unique structural characteristics are very consistent with the design concept of a high-performance flat capillary wick.

[0005] In the first aspect, the present invention provides a silicon nitride flat capillary core for a loop heat pipe with extremely small pore curvature, wherein the silicon nitride flat capillary core for the loop heat pipe has a unidirectional pore inside, and small pores are distributed on the pore walls of large pores; the porosity of the silicon nitride flat capillary core for the loop heat pipe is 65.45% to 85.37%, the small pore size is 0.8 to 1.3 μm, the large pore size is 2.5 to 100.7 μm, and the pore curvature is 2.0 to 6.0.

[0006] Preferably, the silicon nitride flat capillary wick for the loop heat pipe has a compression strength of 2.0 to 15.7 MPa along the freezing temperature gradient direction, a capillary suction coefficient of 18 to 69 seconds, and a Darcy permeability of 1.6*10 -12 ~5.9*10 -11 m 2 .

[0007] In a second aspect, the present invention provides a method for preparing the above-mentioned silicon nitride flat capillary core for the loop heat pipe, comprising: mixing Si3N4 powder, a sintering aid, a binder aqueous solution and a dispersant to obtain a uniform ceramic slurry; using a single cold source for freeze-solidification and freeze-drying to obtain a ceramic blank; and then pre-firing and sintering to obtain the silicon nitride flat capillary core for the loop heat pipe.

[0008] The present invention utilizes an environmentally friendly freeze-drying method, combined with a water-based ceramic slurry, to prepare a silicon nitride flat capillary wick with a continuously changing pore structure in the direction of the temperature gradient. This capillary wick with a special pore structure meets the requirements of the new flat capillary wick for the pore structure. In particular, compared with traditional capillary wicks, the capillary wick has a smaller pore curvature, which greatly reduces the flow resistance of the two-phase working fluid inside the capillary wick during the operation of the loop heat pipe. The capillary wick prepared by this method has the characteristics of high porosity, small pore curvature, excellent capillary suction performance and permeability, and high compressive strength along the freezing temperature gradient direction.

[0009] Among them, under a single cold source, ice crystals grow along the temperature gradient direction and gradually increase in size. After freeze drying, a unidirectional channel with continuous size change is left in the ceramic blank, with a bimodal pore size distribution. The large pores come from the pores left by the sublimation of large ice crystals growing in one direction, and the small pores mainly come from the pores formed by the sublimation of small-sized ice crystals and the overlap of grains. If an all-round cold source is used, the ice crystals grow disorderly, and at the same time, the growth of ice crystals is inhibited, the ice crystal size is relatively small, and the channel curvature is relatively large.

[0010] Preferably, the sintering aid is at least one of Y2O3, Al2O3, SiO2, Nd2O3, Yb2O3, Lu2O3, and MgO, and the added amount of the sintering aid is preferably 0.5% to 12% of the mass of the Si3N4 ceramic powder, and more preferably 1% to 8%.

[0011] Preferably, the binder is at least one of sodium carboxymethyl cellulose, polyvinyl alcohol, and polyethylene glycol, and the addition amount of the binder is preferably 0.05% to 10% of the mass of the Si3N4 ceramic powder, more preferably 1% to 8%.

[0012] Preferably, the dispersant is at least one of ammonium polyacrylate, sodium polyacrylate, and sodium tripolyphosphate, and the addition amount of the dispersant is preferably 0.01% to 5% of the mass of the Si3N4 ceramic powder, more preferably 0.1% to 3%.

[0013] Preferably, the water content is preferably 100% to 320% of the mass of the Si3N4 ceramic powder, more preferably 125% to 280%.

[0014] Preferably, the temperature of the freeze-curing is -196 to -5 °C, and the time is 0.5 to 8 h.

[0015] Preferably, the parameters of the freeze-drying include: the freeze-drying vacuum degree is 1 to 10 Pa, the temperature of the condensation pipeline is -50 °C to -40 °C, the temperature of the heating plate is -45 °C to 40 °C, and the drying time is 5 to 120 h.

[0016] Preferably, the pre-sintering is completed in an air atmosphere, the heating rate is 1 to 5 °C / min, the holding temperature is 500 to 700 °C, and the holding time is 0.5 to 3 h.

[0017] Preferably, the sintering is completed in a nitrogen atmosphere, the heating rate is 1 to 10 °C / min, the holding temperature is 1500 to 1800 °C, the holding time is 1 to 5 h, and the nitrogen atmosphere pressure is preferably 0.01 to 0.5 MPa.

[0018] Advantageous Effects:

[0019] The advantageous effects of the present invention are to prepare a silicon nitride flat capillary core with a continuously varying pore structure by an environmentally friendly method with relatively low cost; under a single cold source, ice crystals grow along the temperature gradient direction and the size gradually increases. After freeze-drying, unidirectional pores with continuously varying sizes are left in the ceramic green body. This greatly reduces the pore channel curvature of the capillary core and is beneficial for the capillary core to obtain excellent permeability. At the same time, the difference in the pore structures at both ends of the capillary core near the evaporation surface and the compensation cavity meets the requirements of the new capillary core for the functional structures of different parts. Description of the Drawings

[0020] Figure 1 It is a macroscopic morphology diagram of the silicon nitride capillary core in Example 1.

[0021] Figure 2 It is a microscopic morphology diagram of the silicon nitride capillary core in Example 2.

[0022] Figure 3 It is the microscopic morphology diagram of the silicon nitride capillary core in Example 3.

[0023] Figure 4 It is the microscopic morphology diagram of the silicon nitride capillary core in Comparative Example 1. Detailed implementation manners

[0024] The present invention will be further described below in conjunction with the accompanying drawings and the following implementation manners. It should be understood that the following implementation manners are only used to illustrate the present invention and do not limit the present invention.

[0025] In the present disclosure, by the method of freezing and solidifying the ceramic slurry with a single cold source, ice crystals grow along the temperature gradient direction. Utilizing the law that the size of the ice crystals gradually increases, after processes such as freeze-drying and sintering, a silicon nitride flat capillary core with continuously changing pore structure sizes is obtained. At the same time, the pore parameters of the capillary core can be regulated by changing the types and contents of sintering aids, the solid content of the slurry, the cold source temperature, and the sintering regime, etc.

[0026] The preparation method of the silicon nitride flat capillary core for the loop heat pipe described in the present invention is exemplarily described below.

[0027] Prepare the ceramic slurry. Select Si3N4 powder as the raw material, and uniformly mix it with sintering aids, binders, dispersants, and water in proportion to obtain the ceramic slurry. Among them, the sintering aid can be at least one of Y2O3, Al2O3, SiO2, Nd2O3, Yb2O3, Lu2O3, and MgO. The addition amount of the sintering aid can be 0.5% - 12% of the mass of the Si3N4 ceramic powder, preferably 1% - 8%. The binder can be at least one of sodium carboxymethyl cellulose, polyvinyl alcohol, and polyethylene glycol. The addition amount of the binder can be 0.05% - 10% of the mass of the Si3N4 ceramic powder, preferably 1% - 8%. The dispersant is at least one of ammonium polyacrylate, sodium polyacrylate, and sodium tripolyphosphate. The addition amount of the dispersant is preferably 0.01% - 5% of the mass of the Si3N4 ceramic powder, more preferably 0.1% - 3%. The water content can be 100% - 320% of the mass of the Si3N4 ceramic powder, preferably 150% - 280%. The mixing method can be ball milling. The rotation speed of the ball milling can be 150 - 350 r / min, and the mixing time can be 1 - 20 h. The ball-to-material ratio can be (1 - 4):1, preferably (2 - 3):1.

[0028] After subjecting the obtained ceramic slurry to vacuum degassing and defoaming treatment, it is poured into a mold with a cold source connected to the bottom for freeze-curing and freeze-drying to obtain a ceramic green body. Among them, the time of the vacuum defoaming is less than or equal to 45 min. The temperature of the cold source can be -196 to -5 °C, and the freeze-curing time can be 0.5 to 8 h. Subsequently, the frozen sample is transferred to a freeze-dryer for freeze-drying to obtain a ceramic capillary core green body. Among them, the vacuum degree of the freeze-dryer is 1 to 10 Pa, the temperature of the condensation pipeline is -50 °C to -40 °C, the temperature of the heating plate is -45 °C to 40 °C, and the drying time is 5 to 120 h.

[0029] Pre-sintering for debinding treatment. The above-mentioned ceramic green body is subjected to debinding treatment in a muffle furnace to remove organic substances. Among them, the specific process may include: in an air atmosphere, the heating rate is 1 to 5 °C / min, the holding temperature is 500 to 700 °C, and the holding time is 0.5 to 3 h.

[0030] Sintering. The capillary core body from which the organic substances have been removed is sintered in a sintering furnace. Among them, the specific process may include: in a nitrogen atmosphere, the heating rate is 1 to 10 °C / min, the holding temperature is 1500 to 1800 °C, the holding time is 1 to 5 h, and the nitrogen atmosphere pressure is preferably 0.1 to 3 MPa.

[0031] In the present invention, the porosity of the capillary core is measured by the Archimedes drainage method, the pore size distribution and pore channel tortuosity are measured by a mercury porosimeter, the compressive strength is measured by a universal mechanical testing machine, the Darcy permeability is calculated by fitting with the Darcy formula, and the capillary suction coefficient test refers to the paper Int.J.Heat Mass Tran.169(2021)120985. The smaller the suction coefficient, the better the capillary core suction performance.

[0032] As a preferred embodiment, when the sintering aid is Y2O3, the porosity of the porous silicon nitride capillary core is 65.45% to 85.37%, the small pore size is 0.8 to 1.3 μm, the large pore size is 2.5 to 100.7 μm, the pore channel tortuosity is 2.0 to 6.0, the compressive strength along the freezing temperature gradient direction is 2.0 to 15.7 MPa, the capillary suction coefficient is 18 to 69 s, and the Darcy permeability is 1.6*10 -12 ~5.9*10 -11 m 2 .

[0033] The shape of the silicon nitride flat capillary core can be circular, rectangular, disc-shaped, oval, etc., which are shapes suitable for the assembly of flat capillary cores. On the outer wall surface of the silicon nitride flat capillary core, a plurality of steam channels can be arranged along the axial and radial directions or it can be a smooth surface without steam channels. The cross-section of the steam channels can be triangular, rectangular, trapezoidal, etc., which are shapes that can be designed.

[0034] The following are further examples to illustrate the present invention in detail. It should also be understood that the following examples are only used to further illustrate the present invention and cannot be construed as limiting the protection scope of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the above content of the present invention all fall within the protection scope of the present invention.

[0035] Example 1

[0036] 100 g of Si3N4 powder, 4 g of Y2O3, 2 g of ammonium polyacrylate, 2 g of polyvinyl alcohol, 275 g of water and 766 g of silicon nitride balls were ball-milled and mixed at 300 r / min for 4 h to obtain a uniform ceramic slurry. After vacuum degassing treatment, the slurry was transferred to a metal mold with a single cold source at the bottom for freeze-curing. The temperature of the cold source was -18 °C. The cured ceramic green body was placed in a freeze dryer for freeze-drying. Among them, the vacuum degree was 1 Pa, the temperature of the condenser was -50 °C, the temperature of the heating plate was 30 °C, and the drying time was 48 h. Subsequently, the ceramic green body was heated to 600 °C at 3 °C / min under air conditions and held for 1 h. After pre-sintering, it was cooled with the furnace. Finally, the pre-sintered ceramic green body was transferred to a sintering furnace and heated to 1700 °C at 5 °C / min under a nitrogen atmosphere of 0.3 MPa and held for 2 h. After sintering, it was cooled with the furnace to obtain a silicon nitride capillary core.

[0037] The porosity of the silicon nitride capillary core prepared in this Example 1 was 81.33%, the median of the large pore diameter was 68.5 μm, the median of the small pore size was 1.1 μm, the pore channel tortuosity was 2.2, the compressive strength was 3.7 MPa, the capillary suction coefficient was 32 s, and the Darcy permeability was 2.4×10 -11 m 2 . The macroscopic morphology of the obtained capillary core was as shown in Figure 1 and had a unidirectional pore structure reflecting a smaller pore channel tortuosity.

[0038] Example 2

[0039] In this Example 2, the preparation process of the capillary core was referred to Example 1, the difference being that the water content was 135 g and the mass of the silicon nitride balls was 486 g.

[0040] The porosity of the silicon nitride capillary core prepared in this Example 2 was 69.33%, the median of the large pore diameter was 50.4 μm, the median of the small pore size was 0.8 μm, the pore channel tortuosity was 3.4, the compressive strength was 5.9 MPa, the capillary suction coefficient was 54 s, and the Darcy permeability was 1.2×10 -11 m 2 . The microscopic morphology of the obtained capillary core was as shown in Figure 2 and the unidirectional pore size was larger.

[0041] Example 3

[0042] In Example 3, the preparation process of the capillary wick refers to Example 1, with the difference that the cold source temperature is -196°C.

[0043] The silicon nitride capillary wick prepared in Example 3 has a porosity of 80.42%, a median large pore diameter of 7.3 μm, a median small pore size of 1.1 μm, a pore channel tortuosity of 3.8, a compressive strength of 5.1 MPa, a capillary suction coefficient of 44 s, and a Darcy permeability of 9.7*10 -12 m 2 。

[0044] Example 4

[0045] In Example 4, the preparation process of the capillary wick refers to Example 3, with the difference that the water content is 135 g and the mass of silicon nitride spheres is 486 g.

[0046] The silicon nitride capillary wick prepared in Example 4 has a porosity of 68.72%, a median large pore diameter of 2.9 μm, a median small pore size of 0.8 μm, a pore channel tortuosity of 5.7, a compressive strength of 11.2 MPa, a capillary suction coefficient of 56 s, and a Darcy permeability of 3.8*10 -12 m 2 。The microscopic morphology of the obtained capillary wick is as Figure 3 shown, with relatively small unidirectional pore sizes.

[0047] Comparative Example 1

[0048] In Comparative Example 1, the preparation process of the capillary wick refers to Example 2, with the difference that the cold sources are different. Although the cold source temperatures of both are -18°C, in Example 2, the cold source only exists at the bottom of the metal mold, while in Comparative Example 1, the cold source exists on each surface of the mold. Therefore, the ice crystal growth forms and rules are different.

[0049] The silicon nitride capillary wick prepared in Comparative Example 1 has a porosity of 70.49%, a median large pore diameter of 13.9 μm, a median small pore size of 0.8 μm, a pore channel tortuosity of 6.9, a compressive strength of 4.8 MPa. The capillary suction coefficient is 58 s, and the Darcy permeability is 9.8*10 -13 m 2 。The microscopic morphology of the obtained capillary wick is as Figure 4 shown, with an orientation-disordered pore structure.

[0050] Table 1: Performance parameters of the capillary wicks prepared in the examples and comparative examples.

[0051]

Claims

1. A preparation method of a silicon nitride flat capillary core for a loop heat pipe with an extremely small pore channel curvature, characterized in that, The loop heat pipe uses a silicon nitride flat capillary wick with unidirectional pores of continuously varying sizes inside. The small pores are distributed on the walls of the large pores. The porosity of the silicon nitride flat capillary wick for the loop heat pipe is 65.45% - 85.37%. The size of the small pores is 0.8 - 1.3 μm, the size of the large pores is 2.5 - 100.7 μm, and the pore channel tortuosity is 2.0 - 6.

0. The preparation method includes: mixing Si3N4 powder, sintering aids, an aqueous binder solution, and a dispersant to obtain a uniform ceramic slurry; using a single cold source at the bottom for freeze-curing and freeze-drying to obtain a green ceramic body; and then through pre-sintering and sintering to obtain the silicon nitride flat capillary wick for the loop heat pipe. The temperature of the freeze-curing is -18 to -5 °C, and the time is 0.5 to 8 h. The parameters of the freeze-drying include: the vacuum degree of the freeze-drying is 1 to 10 Pa, the temperature of the condensation pipeline is -50 °C to -40 °C, the temperature of the heating plate is -45 °C to 40 °C, and the drying time is 5 to 120 h.

2. The preparation method according to claim 1, characterized in that, The silicon nitride flat capillary wick for the loop heat pipe has a compression strength of 2.0 to 15.7 MPa along the freezing temperature gradient direction, a capillary suction coefficient of 18 to 69 seconds, and a Darcy permeability of 1.6*10 -12 ~5.9*10 -11 m 2 .

3. The preparation method according to claim 1, characterized in that, The sintering aids are at least one of Y2O3, Al2O3, SiO2, Nd2O3, Yb2O3, Lu2O3, and MgO. The addition amount of the sintering aids is 0.5% - 12% of the mass of the Si3N4 ceramic powder.

4. The preparation method according to claim 3, characterized in that, The addition amount of the sintering aids is 1% - 8% of the mass of the Si3N4 ceramic powder.

5. The preparation method according to claim 1, wherein The binder is at least one of sodium carboxymethyl cellulose, polyvinyl alcohol, and polyethylene glycol. The addition amount of the binder is 0.05% - 10% of the mass of the Si3N4 ceramic powder.

6. The preparation method according to claim 5, characterized in that, The addition amount of the binder is 1% - 8% of the mass of the Si3N4 ceramic powder.

7. The preparation method according to claim 1, characterized in that, The dispersant is at least one of ammonium polyacrylate, sodium polyacrylate, and sodium tripolyphosphate. The addition amount of the dispersant is 0.01% - 5% of the mass of the Si3N4 ceramic powder.

8. The preparation method according to claim 7, characterized in that, The addition amount of the dispersant is 0.1% - 3% of the mass of the Si3N4 ceramic powder.

9. The preparation method according to claim 1, characterized in that, The water content is 100% - 320% of the mass of the Si3N4 ceramic powder.

10. The preparation method according to claim 9, characterized in that, The water content is 125% - 280% of the mass of the Si3N4 ceramic powder.

11. The preparation method according to any one of claims 1-10, characterized in that, The pre-sintering is completed in an air atmosphere. The heating rate is 1 - 5 °C / min, the holding temperature is 500 - 700 °C, and the holding time is 0.5 - 3 h. The sintering is completed in a nitrogen atmosphere. The heating rate is 1 - 10 °C / min, the holding temperature is 1500 - 1800 °C, the holding time is 1 - 5 h, and the nitrogen atmosphere pressure is 0.01 - 0.5 MPa.

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