JP2006313648A - Porous heat dissipation material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a porous heat dissipation material which has a strong absorptive power by capillary attraction of liquid and also can evaporate liquid efficiently, eliminating an evaporation heat and has an excellent heat dissipation property. <P>SOLUTION: This material is made of a porous sintered material having a frame with porosities around which metal powders are sintered. Preferably, the frame shall have an average capillary diameter of 200 μm or less and an average pore diameter 3000 μm or less and a percentage of voids of the whole porous material shall be 60% or more and 95% or less in volume rate. This material can be used, for instance, for cooling a membrane electrode junction of a direct methanol type fuel cell. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention is a porous heat radiating member that has an absorptive power with respect to a liquid such as water, and that can efficiently transfer heat from the object to be cooled by an endothermic reaction when the liquid evaporates, thereby removing heat. About.

  In the semiconductor industry, metal or ceramic plates with high thermal conductivity are used as heat dissipation members. In addition, there is a problem of cooling in a direct methanol fuel cell (hereinafter abbreviated as DMFC), and moisture discharge and air permeability such as an air electrode of a membrane electrode assembly (hereinafter abbreviated as MEA) are simultaneously performed. In a required part, a metal plate having slits or holes with a size or interval as required may be used also as a current collector plate. However, when used at a high output, if a large amount of water is generated in the air electrode, there is a problem that the air slits or holes are blocked, and the supply of air to the air electrode is obstructed to reduce the output. And if it is heated due to insufficient cooling and burned when applied to a mobile device, or if MEA is placed on the back of the liquid crystal display of a notebook computer, the liquid crystal display will be heated and problems such as discoloration of images will occur. Occurrence is expected.

As for the recovery of the generated water, a proposal has been made to use a porous body such as a sponge or a fiber base material (for example, see Patent Document 1).
JP 2003-368866 A

  When a porous body such as a sponge or a fiber base material is brought into contact with a liquid, the generated water on the air electrode can be recovered by capillary action. However, in order to evaporate the water inside the porous body after absorption, the heat generated by the MEA is used. However, the heat conduction characteristics of the porous body itself for transmitting the heat are insufficient, Accumulation of large water in the porous body may deteriorate the cooling capacity. Moreover, since the strength itself is low, it swells and impairs air permeability, resulting in a decrease in power generation efficiency.

  As described above, in the case of DMFC, for example, cooling by only heat diffusion using a conventional metal plate does not sufficiently cool the MEA, and there is a problem of carrying around and obstacles to peripheral devices. Furthermore, there is a problem that air permeability is hindered by the generation of water on the air electrode.

  It is an object of the present invention to have a strong absorption capacity for liquid by capillary action, and at the same time, by efficiently evaporating the absorbed liquid and utilizing heat removal from the surroundings at the time of evaporation, more powerful cooling ability. It is providing the porous heat radiating member which has this.

  As a result of studying the porous body, the present inventor has solved the above problem with a metal porous sintered body having a skeleton in which metal powder is sintered around the pores, not a simple sintered structure. It came.

  That is, the present invention is a porous heat dissipating member comprising a porous sintered body having a skeleton obtained by sintering metal powder around the pores. Preferably, the skeleton has pores having an average pore diameter of 200 μm or less, the average pore diameter is 3000 μm or less, and the porosity of the entire porous body is 60% by volume or more and 95% by volume or less. To do.

  According to the present invention, the liquid in the porous body can be efficiently evaporated by the heat transferred from the object to be cooled to the skeleton of the porous body at the same time as it has the ability to absorb liquid by capillary action. It is possible to provide a porous heat radiating member having cooling ability. As a result, the surrounding liquid can be removed by volatilization.

  An important feature of the present invention is that a sintered porous body having a framework in which metal powder is sintered around pores is applied for absorption and evaporation of liquid. In other words, by forming a structure in which the sintered portion surrounded by the metal powder surrounds the pores, the sintered portion having the pores absorbs the nearby liquid and also bears heat transfer. To achieve high cooling capacity.

  That is, by forming a skeleton obtained by sintering metal powder around the pores, the liquid is first absorbed by the capillary phenomenon due to the pores of the skeleton and spread in the porous body. Thereby, in addition to ensuring a large contact area between the skeleton and the liquid, the space secured by the pores in the porous body can also increase the interface with the outside air that greatly affects the evaporation efficiency of the liquid. Since the skeleton is made of metal, the heat conductivity is high, and heat can be efficiently transferred from the object to be cooled (for example, MEA) to the liquid spreading in the skeleton. Since the interface with the outside air, that is, the area of the evaporation site is large, it is possible to efficiently evaporate the liquid. When the liquid vaporizes, the porous body is cooled in order to take heat of vaporization from the surroundings, and heat is taken away from the object to be cooled.

  The porous heat dissipating member of the present invention is assumed to be a heat dissipating member for the DMFC air electrode, particularly for mobile use and in-vehicle use. Therefore, the porous heat dissipating member has a metal skeleton for improving vibration resistance and impact resistance. Adopt metal powder as raw material. In addition, metal materials generally have good wettability with liquids and high thermal conductivity, so it is considered suitable as a heat dissipation member using heat of vaporization, but the metal species are affected by the corresponding environment. It is effective to select difficult materials. In addition, it is possible to have the functions of a current collector plate and electrodes at the same time by utilizing the conductivity of metal.

  In addition, when the porous heat dissipation member of the present invention is used for DMFC, it can be used as a fuel transport / supply material to the fuel electrode other than the air electrode described above from a sponge-like porous body or fiber made of resin. Since heat conductivity is high, it can be expected that the heat of the MEA is efficiently released from the fuel electrode side to the housing. Therefore, higher cooling ability can be obtained by disposing the porous member of the present invention on both the air electrode and the fuel electrode of the MEA. In addition, by using excellent conductivity, it is possible to simultaneously function as a current collector for both electrodes.

  And it is preferable that the porous heat radiating member of this invention is 200 micrometers or less in the average pore diameter of a frame | skeleton part. This is to ensure a sufficient liquid suction force due to the capillary action of the skeleton. Moreover, it is preferable that the average diameter of the void | holes which the said frame | skeleton part surrounds and forms is 3000 micrometers or less. These vacancies act as the ventilation and evaporation sites described above. When the diameter becomes too large, large vacancies that do not contribute to cooling (that is, liquid suction and vaporization) are brought into contact with the object to be cooled. This is because there is a problem that uniform cooling is difficult because of the sparseness in the part.

  The porosity of the entire porous body is preferably 60% or more and 95% or less by volume. This is to secure a gas release path after the liquid is vaporized. Increasing the porosity and increasing the porosity improves air permeability in the porous body and promotes evaporation of the liquid. Because. For example, when used on the air electrode side of a DMFC, even in a state where the skeleton part absorbs generated water, in order to supply a sufficient amount of air necessary for the reaction, the connectivity of the holes is increased to some extent, It is necessary to secure a path for guiding air to the air electrode of the MEA. For this purpose, it is preferable that the porosity of the entire porous body is 60% or more. However, in order to ensure the strength of the porous body itself and sufficient liquid absorption and thermal conductivity, it is also necessary to ensure the volume ratio of the skeleton, and the porosity of the entire porous body is 95. % Or less is preferable.

  In a more preferable form of the porous heat radiating member of the present invention, the sintered body forming the skeleton is a sintered skeleton of powder having an average particle size of 100 μm or less, the pore diameter of the skeleton is 5 to 50 μm, and the skeleton surrounds. It is preferable that the diameter of the pore part to be formed is 100 to 2000 μm, and the porosity of the whole porous body is 70 to 90% by volume.

As a method for producing a porous body applied to the present invention, for example, the following method can be applied.
First, metal powder is prepared. The metal powder preferably has a heat conductivity as high as possible, but stainless steel, titanium, a titanium alloy, and the like that are excellent in corrosion resistance are effective depending on the liquid that contacts in use. The particle size of the metal powder is preferably an average particle size of 200 μm or less, more preferably 100 μm or less.

  Next, resin particles and a binder are mixed with the metal powder and kneaded to prepare a kneaded body. The resin particles preferably have an average particle size of 100 to 3000 μm in order to secure pores. Resin can be used as the binder, but when applying an effective method of removing resin particles with a solvent, which is the next step, the binder does not dissolve in the solvent, for example, a binder mainly composed of methylcellulose and water. It is effective to use

  Next, the kneaded body is molded to produce a molded body, which is heated and degreased and sintered. When water is put into the binder, it is preferable to put a drying step after molding, and when removing resin particles with a solvent, a step of extracting the resin particles with a solvent and drying step is given before heat degreasing. Is preferred. In addition, in order to ensure high air permeability to the porous body even after liquid absorption, the volume ratio of the resin particles in the molded body is set to be equal to or higher than the tap density of the resin grains so that the resin grains are not crushed during molding. It is preferable to press within the range to ensure the contact frequency and contact area between the resin particles.

(Preparation of porous heat dissipation member)
SUS316L water atomized powder with an average particle size of 60 μm, commercially available methylcellulose, and resin particles, spherical paraffin wax particles with an average particle size of 1000 μm and 180 μm are mixed, and water and a plasticizer are added and mixed and kneaded. A kneaded body was prepared. The mixing amount of the resin particles is 80% and 10% respectively for the volume ratios of the two types of resin particles having an average particle size of 1000 μm and 180 μm when the total volume of the metal powder and the resin particles is 100%. Was set to be.

  Thereafter, the kneaded body was formed into a disk shape of φ180 mm × 4 mm thickness by a press molding machine (load 0.7 MPa), and then the molded body was dried at 50 ° C. Next, the paraffin wax particles in the molded body were extracted with a solvent and dried at 70 ° C. Subsequently, in a degreasing furnace, the temperature was raised at 40 ° C./h in a nitrogen atmosphere and held at 600 ° C. for 2 hours to disperse the remaining paraffin wax and binder. Then, in a sintering furnace, it was held at 1200 ° C. for 2 hours in a hydrogen atmosphere to perform sintering, thereby obtaining a disk of a porous sintered body having a thickness of 3 mm.

  A cross-sectional microphotograph of the obtained porous sintered body is shown in FIG. The white portion is a metal portion forming a skeleton, and the dark portion is a void that forms pores and pores of the skeleton portion. When the average pore diameter of the skeleton was measured by mercury porosimetry, it was 65.7 μm. In addition, the average pore diameter of the pores introduced by the paraffin wax particles having an average particle diameter of 1000 μm was 650 μm from the cross-sectional microphotograph. Moreover, the porosity of the whole porous body was 86.7%. Furthermore, from the cross-sectional microphotograph, it can be confirmed that these pores communicate with each other, and even when the porous body is seen through, fine pores that transmit light are confirmed. It can also be seen that the sex is secured.

(Evaluation of porous heat dissipation member)
(1) Liquid absorption capacity A porous body of 105 × 20 × 3 (mm) was prepared from the prepared disk, ultrasonically washed with ethanol for 10 minutes, and then dried. Next, it was hung on the electronic balance shown in FIG. 2, and the lower end 10 mm of the porous body was immersed in pure water. And the increase in the mass of the porous body obtained from the electronic balance after immersion was defined as the amount of pure water absorbed by the porous body. The suspended porous body is covered with a case in order to eliminate the influence of water evaporation from the suspended porous body.

FIG. 3 shows the transition of the amount of pure water absorbed per unit cross-sectional area of the porous body with respect to the elapsed time from the immersion. It can be seen that pure water is absorbed by the porous body as time passes from the time of immersion, and in this example, pure water absorption ability of about 3.3 (g / cm ² ) was shown.

(2) Heat dissipation (cooling) capacity As shown in FIG. 4, two 40 × 40 × 1 (mm) SUS304 plates with spot-welded thermocouples at the end are prepared and placed on the electric heater. placed. One of them was used as a standard body and nothing was placed thereon, and the other was used as a test body, on which substances of the respective conditions shown in Table 1 (prepared porous bodies) were placed. In the case of condition 4, water droplets were directly placed on the specimen (FIG. 5). And this standard body and a test body were heated simultaneously with the electric heater, and the cooling capacity in each condition was evaluated from the temperature drop of the test body with respect to the temperature of a standard body.

  In this test, a preliminary test is performed with nothing placed on the specimen, and the specimen on the electric heater and the specimen are placed so that the temperature difference between the specimen and the specimen is within 1 ° C. The positional relationship and the heating rate have been adjusted. The heating rate is the average heating rate of the standard obtained under each condition, and is about 9 ° C./min. And until the temperature of the test body reaches 95 ° C., which is lower than the boiling point of pure water, the test body and the standard body are measured, and as soon as the temperature of the test body reaches 96 ° C., In conditions 1-3, the amount of water remaining in the porous body was measured, and in condition 4, the mass of water droplets was measured. And the evaporation amount of the water | moisture content was calculated | required from the mass difference of the water | moisture content before and behind a test.

  FIG. 6 shows changes in the temperature drop of the test specimen with respect to the temperature of the standard specimen under each condition, and Table 2 shows the amount of water and the amount of evaporation before and after the test.

  In the case of the condition 1 with the dried porous body, the temperature drop with respect to the standard body is about 1 ° C. or less, and the cooling effect is hardly seen. However, by impregnating the porous body with pure water, the larger the amount of pure water soaked in the order of condition 2 and condition 3, the greater the temperature drop with respect to the standard body, and the improvement of the cooling capacity is recognized. And this improvement in cooling capacity shows that heat removal when water evaporates has a large influence because the amount of water evaporation is greater than that of condition 3 when compared with the amount of water evaporation. .

  In addition, when the cooling capacity of Condition 3 placed on the test specimen in the state in which the same amount of pure water as that in Condition 4 was infiltrated into the porous body was evaluated, the temperature drop relative to the standard body was larger than that in Condition 4, It can be seen that the cooling capacity is high. Furthermore, when the amount of water evaporation is compared with 0.37 g of condition 4, that of condition 3 of the present invention is as large as 0.62 g, and the evaporation of water is promoted through the porous body, It can be seen that the cooling capacity is improved. This is because, compared with the case where water droplets are in direct contact with the object to be cooled, moisture is spread on the surface of the skeleton in the porous body, and the area of the evaporation site that is the interface between the pores or the atmosphere of the porous body is expanded. Therefore, it is considered that the heat transmitted through the skeleton is efficiently transferred to moisture and evaporation occurs.

It is an example of the cross-sectional microphotograph of the porous heat radiating member of this invention. It is a figure of the liquid absorptive evaluation test. It is the graph which showed the liquid absorptivity evaluation result. It is a figure of the evaluation test of cooling ability. It is a figure of the evaluation test of cooling ability. It is the graph which showed the evaluation result of cooling capacity.

Claims (2)

A porous heat radiating member comprising a porous sintered body having a skeleton obtained by sintering metal powder around pores. The skeleton has pores with an average pore diameter of 200 μm or less, an average pore diameter of 3000 μm or less, and a porosity of the entire porous body of 60% by volume or more and 95% by volume or less. The porous heat radiating member according to claim 1.