US20070089860A1

US20070089860A1 - Heat pipe with sintered powder wick

Info

Publication number: US20070089860A1
Application number: US11/478,392
Authority: US
Inventors: Chuen-Shu Hou; Tay-Jian Liu; Chao-Nien Tung
Original assignee: Foxconn Technology Co Ltd
Current assignee: Foxconn Technology Co Ltd
Priority date: 2005-10-21
Filing date: 2006-06-28
Publication date: 2007-04-26
Also published as: TW200716932A; TWI317414B

Abstract

A heat pipe includes a casing and a sintered powder wick arranged at an inner surface of the casing. The sintered powder wick is made of spherical metal powder (300), the spherical metal powder having an apparent density ranging from 3.3 g/cm³to 4.8 g/cm³to reduce micro pores of the sintered powder wick and improve ability of heat transfer of the heat pipe.

Description

BACKGROUND

1. Field
The present invention relates generally to heat pipes as heat transfer/dissipating device, and more particularly to a heat pipe with a sintered powder wick.
2. Description of related art
Heat pipes have excellent heat transfer properties and therefore are an effective means for transfer or dissipation of heat from heat sources. Heat pipes are widely used for removing heat from heat-generating components such as computer central processing units (CPUs). A heat pipe is usually a vacuum tube containing a working fluid therein, which is employed to carry thermal energy from one section of the heat pipe (typically referred to as the “evaporating section”) to another section thereof (typically referred to as the “condensing section”) via phase transitions between a liquid state and a vapor state of the working fluid. The tube is made of a highly thermally conductive material such as copper or aluminum. Preferably, a wick structure is provided inside the heat pipe, lining an inner wall of the tube, for drawing the working fluid back to the evaporating section after it has condensed in the condensing section. Specifically, as the evaporating section of the heat pipe is maintained in thermal contact with a heat-generating component, the working fluid contained in the evaporating section can absorb heat generated by the heat-generating component and turn into vapor. Due to the difference in vapor pressure between the two sections of the heat pipe, the vapor moves towards the condensing section where the vapor is condensed into liquid, releasing the heat into ambient environment in the process; the heat is then dispersed by, for example, fins thermally contacting the condensing section. Due to the difference of capillary pressure in the wick structure between the two sections, the condensed liquid is then drawn back by the wick structure to the evaporating section where it is again available for evaporation.
FIG. 3 illustrates a wick structure in accordance with related art, provided for a heat pipe and mainly composed of water atomized irregular copper powder 100 with low apparent density. The powder 100 has irregular shape particle and the apparent density ranges from 2.5 g/cm³to 3.3 g/cm³, thereby forming a plurality of micro pores 110 in the powder. Capillary pressure is related to interfacial tension, pore size and contact angle by the following equation:
(Pc)=(2×σ×cos θ)/r
where

Pc=capillary pressure;
s=interfacial tension;
r=radius of pore;
θ=contact angle.

It can be seen from the above equation that the smaller the effective pores 110 sizes the higher the capillary pressure. A flow resistance to working liquid also increases due to a decrease in pores 110 sizes of the wick structure. The working fluid and non-condensation gas in the heat pipe is prone to adhere into the pores 110 having higher capillary pressure and is prevented flowing out of the pores 110 of the wick structure, thereby causing a lack of the working fluid available for heat transfer and reducing the heat transfer performance of the heat pipe. Therefore, the powder 100 is often sintered at a predetermined temperature to eliminate the micro pores 110 of the wick structure. FIG. 4 shows a microstructure of a wick structure formed by sintering the powder particles 100 having a relatively high green density and sintered at a relatively low temperature. FIG. 5 shows a microstructure of a wick structure formed by sintering the powder particles 100 having a relatively low green density and sintered at a relatively high temperature. Compared with the FIG. 5, most of the micro pores 110 of the FIG. 4 is not eliminated. The micro pores 110 can be eliminated only at the relatively high temperature (see FIG. 5). However, when the conventional water atomized irregular copper powder 100 is sintered at the high temperature, not only the number of the micro pores 110 is reduced but also a total porosity of the sintered wick structure is reduced. The reduction of the total porosity of the sintered structure adversely affects the performance of the heat pipe.
Therefore, there is a need for a heat pipe with a sintered powder wick which can solve the above problems.

SUMMARY OF THE INVENTION

A heat pipe in accordance with an embodiment of the present invention includes a casing and a sintered powder wick arranged at an inner surface of the casing. The sintered powder wick is made of spherical metal powder, the spherical metal powder having an apparent density ranging from 3.3 g/cm³to 4.8 g/cm³to reduce micro pores of the sintered powder wick and improve ability of heat transfer of the heat pipe.
The present invention in another aspect, relates to a method for manufacturing a sintered heat pipe. The preferred method includes steps of (1) providing a hollow casing; (2) inserting a mandrel into the casing; (3) filling spherical metal powder into a space between the mandrel and the inner surface of the casing, wherein an apparent density of the spherical metal powder ranges from 3.3 g/cm³to 4.8 g/cm³; and (4) conducting sintering process to the casing and the filled spherical metal powder, whereby a sintered wick structure made of the spherical metal powder is formed inside the casing.
Other advantages and novel features of the present invention will become more apparent from the following detailed description of the embodiment when taken in conjunction with the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present device can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present device. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.
FIG. 1 is a microstructure of powder particles of a heat pipe in accordance with a preferred embodiment of the present invention;
FIG. 2 is a graph of a permeation rate distribution of different types of powder, wherein the permeation rate is changed with time;
FIG. 3 is a scanning electron micrograph of a wick structure formed by sintering a water atomized irregular copper powder;
FIG. 4 is a scanning electron micrograph of a wick structure formed by sintering the water atomized irregular copper powder having a relatively high green density and sintered at a relatively low temperature; and
FIG. 5 a scanning electron micrograph of a wick structure formed by sintering the water atomized irregular copper powder having a relatively low green density and sintered at a relatively high temperature.

DETAILED DESCRIPTION OF THE INVENTION

A heat pipe in accordance with an embodiment of the present invention includes a casing, a sintered powder wick structure arranged at an inner surface of the casing and working fluid filled with the casing. A method of manufacturing the heat pipe comprises following steps: (1) inserting a mandrel into the casing; (2) filling spherical metal powder 300 such as copper or aluminum powder into a space between the mandrel and the inner surface of the casing, wherein an apparent density of the spherical metal powder 300 ranges from 3.3 g/cm³to 4.8 g/cm³; (3) heating the filled spherical metal powder 300 together with the casing at a high temperature, whereby the filled spherical metal powder 300 is sintered and diffusion bonded to the inner surface of the casing and particles of the powder after the sintering have approximately the same diameters to enhance the performance of the heat pipe.
In this embodiment, the spherical metal powder 300 having high apparent density is manufactured by mechanical method, such as cutting, crushing, chipping, etc. Particle diameter of the spherical metal powder 300 ranges from 80 μm to 300 μm, wherein 100 μm˜250 μm is preferable. Fine powder having a diameter smaller than that of the spherical metal powder and acting as an additive is added to the spherical metal powder 300 to improve the sintering strength of the wick structure. The fine powder has a weight which is 1˜10% of a weight of the metal powder 300.
FIG. 1 shows, in an enlarged scale, a structure of particles of the spherical metal powder 300 formed in accordance with an alternative method of the present invention, rather than the afore-mentioned mechanical method such as cutting, crushing, chipping, etc. According to this alternative method, each particle of the spherical metal powder 300 consists of superfine powder 200 having a particle diameter below 10 μm. The superfine powder 200 is processed by controlling parameters of a pressure granulator or centrifugal granulator, to thereby obtain the spherical metal powder 300 with high consistency. A particle diameter of the spherical metal powder 300 formed according to this method ranges from 30 μm to 300 μm. The super fine powder 200 is pre-sintered at a suitable temperature, for example, about 600˜800 degrees Celsius, whereby densification temperature of the spherical metal powder 300 is dropped significantly. At the same time, number of micro pores in the spherical metal powder 300 is reduced and flowability of the spherical metal powder 300 is increased to prevent a bridging effect of the spherical metal powder 300 during the filling of the spherical metal powder 300 into the casing.
FIG. 2 shows a permeation rate distribution of three different types of powder with a porosity of 47%. The three different types of powder comprises the spherical metal powder 300, water atomized irregular copper powder and penniform electrolyte power. It is obviousness that the spherical metal powder 300 has a superior permeation rate to another two types of powder.
The present invention provides the spherical metal powder 300 for forming the sintered wick structure of the heat pipe having various advantages over the heat pipe having water atomized irregular copper powder in accordance with the related art. For example, one advantage is capable of improving ability of heat transfer by using of the spherical metal powder 300 having high apparent density. Another advantage is to reduce micro pores of the spherical metal powder 300 by pre-sintered processing. In addition, the flowability of the spherical metal powder 300 is better than the water atomized irregular copper powder 100.
It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.

Claims

1. A heat pipe comprising:

a casing;

working fluid received in the casing; and

a wick structure sintered at an inner surface of the casing, the wick structure comprising spherical metal powder, the spherical metal powder having an apparent density ranging from 3.3 g/cm³to 4.8 g/cm³.

2. The heat pipe of claim 1, wherein fine powder acting as an additive is added to the spherical metal powder to improve the strength of the wick structure, wherein the fine powder have a particle size smaller than that of the spherical metal powder.

3. The heat pipe of claim 2, wherein a weight of the fine powder is 1˜10% of a weight of the metal powder.

4. The heat pipe of claim 1, wherein a particle diameter of the spherical metal powder ranges from 80 μm to 300 μm.

5. The heat pipe of claim 4, wherein the particle diameter of the spherical metal powder ranges from 100 μm to 250 μm.

6. The heat pipe of claim 1, wherein a particle diameter of the spherical metal powder ranges from 30 μm to 300 μm.

7. The heat pipe of claim 1, wherein the spherical metal powder is made from superfine powder having a particle diameter below 10 μm.

8. A method for manufacturing a heat pipe comprising steps of:

providing a hollow casing;

inserting a mandrel into the casing;

filling spherical metal powder into a space between the mandrel and the inner surface of the casing, wherein an apparent density of the spherical metal powder ranges from 3.3 g/cm³to 4.8 g/cm³; and

conducting sintering process to the casing and the filled spherical metal powder, whereby a sintered wick structure made of the spherical metal powder is formed inside the casing.

9. The method for manufacturing a heat pipe as in claim 8, wherein each of the spherical metal powder consists of superfine powder having a particle diameter below 10 μm and sintered at 600˜800 degrees Celsius.

10. The method for manufacturing a heat pipe as in claim 8, wherein fine powder acting as an addictive supplement is added to the spherical metal powder to improve the strength of the wick structure, the fin powder having a particle diameter smaller than that of the spherical metal powder.

11. The method for manufacturing a heat pipe as in claim 10, wherein a weight of the fine powder is 1˜10% of a weight of the metal powder.

12. The method for manufacturing a heat pipe as in claim 8, wherein a particle diameter of the spherical metal powder ranges from 80 μm to 300 μm.

13. The method for manufacturing a heat pipe as in claim 8, wherein the particle diameter of the spherical metal powder ranges from 100 μm to 250 μm.

14. The method for manufacturing a heat pipe as in claim 8, wherein a particle diameter of the spherical metal powder ranges from 30 μm to 300 μm.

15. The method for manufacturing a heat pipe as claim 8, wherein the spherical metal powder is made from superfine powder having a particle diameter below 10 μm.