METHOD OF MANUFACTURING WICK STRUCTURE FOR HEAT PIPE

Abstract

A method is disclosed to produce a wick structure for a heat pipe. The wick structure is a sintered powder wick and is produced by sintering process. A group of powders is firstly provided. The group of powders is then classified into many sub-groups in terms of powder size. At least one sub-group of the powders is selected to form the wick structure via the sintering process. Thus, the powders used to construct the wick structure are confined to powders having a relatively narrower range of powder size in relative to the group of powders as originally provided. This has greatly reduced the complexity involved in the sintering process, and as a result, the required sintering temperature and the required time for the sintering process are easier to be determined.

Description

FIELD OF THE INVENTION

The present invention relates generally to an apparatus for transfer or dissipation of heat from heat-generating components such as electronic components, and more particularly to a method of manufacturing a wick structure for a heat pipe.

DESCRIPTION OF RELATED ART

Heat pipes have excellent heat transfer performance due to their low thermal resistance, and therefore are an effective means for transfer or dissipation of heat from heat sources. Currently, heat pipes are widely used for removing heat from heat-generating components such as central processing units (CPUs) of computers. A heat pipe is usually a vacuum casing containing therein a working fluid, which is employed to carry, under phase transitions between liquid state and vapor state, thermal energy from one section of the heat pipe (typically referring to as the “evaporating section”) to another section thereof (typically referring to as the “condensing section”). The casing is made of high thermally conductive material such as copper or aluminum. Preferably, a wick structure is provided inside the heat pipe, lining an inner wall of the casing, for drawing the working fluid back to the evaporating section after it is condensed at 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 at the evaporating section absorbs heat generated by the heat-generating component and then turns into vapor. Due to the difference of vapor pressure between the two sections of the heat pipe, the generated vapor moves towards and carries the heat simultaneously to the condensing section where the vapor is condensed into liquid after releasing the heat into ambient environment by, for example, fins thermally contacting the condensing section. Due to the difference of capillary pressure developed by 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.

The wick structure currently available for heat pipes includes fine grooves integrally formed at the inner wall of the casing, screen mesh or bundles of fiber inserted into the casing and held against the inner wall thereof, or sintered powders combined to the inner wall by sintering process. Among these wicks, the sintered powder wick is preferred to the other wicks with respect to heat transfer ability and ability against gravity of the earth.

Currently, a conventional method for making a sintered powder wick includes filling a group of metal powders necessary to construct the wick into a hollow casing which has a closed end and an open end. A mandrel has been inserted into the casing through the open end of the casing; the mandrel functions to hold the filled powders against an inner wall of the casing. Then, the casing with the powders is sintered at high temperature for a specified time period to cause the powders to diffusion bond together to form the wick. In the method, it requires a sintering temperature (or temperature range) suitable for the sintering process.

The group of powders to be formed as the wick can be obtained by well-known method such as mechanical grinding. Generally, the powders thus obtained are a mixture of powders with different sizes due to a tolerance in producing the powders. That is, the powders generally have a wide range of powder sizes. In addition, the proportion between these different sized powders is unknown before the sintering process. As a result, it is difficult to determine the sintering temperature required by the sintering process and how long the sintering process should take. The general rule is that a lower sintering temperature and a shorter period of time are required to sinter and interconnect the small-sized powders in relative to the large-sized powders. If the sintering temperature is much too lower or the time to carry out the sintering process is much shorter than it should be, the powders as applied to form the wick cannot be effectively diffusion bonded together. To the contrary, if the sintering temperature is much too higher or the time is much longer than what is indeed required, the powders with relatively small sizes are apt to be overheated and melt. When the powders with relatively small sizes have a large proportion in the group of powders, the wick accordingly formed will shrink significantly into a compact, high-density structure, noticeably reducing the pore size of the wick and causing the pores formed in the wick to be disconnected. The disconnected pores in the wick cannot provide a continuous passageway for the condensed liquid to return back along the wick.

Therefore, it is desirable to provide a method of manufacturing a sintered powder wick by a sintering process. In the method, both the required sintering temperature and the required time for the sintering process can be easily determined and controlled.

SUMMARY OF INVENTION

The present invention relates to a method of manufacturing a wick structure applicable in a heat pipe. A preferred method includes the following steps: (1) providing a group of powders; (2) classifying the powders into multiple sub-groups in terms of powder size, the sub-groups having powder sizes different from each other; (3) selecting at least one sub-group of the powders; and (4) sintering the selected powders to form the wick structure.

In the method, if the selected powders are consisted of more than one sub-groups, these sub-groups are preferably mixed in a prescribed proportion by weight. Thus, the powders used to form the wick structure are confined to a relatively narrow range of powder sizes, and before sintering, the proportion between these sub-groups is already known. It is therefore easier to determine both the required sintering temperature and the required time for the sintering process.

Other advantages and novel features of the present invention will become more apparent from the following detailed description of preferred embodiment when taken in conjunction with the accompanying drawings, in which:

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow chart of a preferred method in accordance with the present invention, for manufacturing a wick structure applicable in a heat pipe;

FIG. 2 is a graph of the powder size distribution of a group of powders as provided by the method of FIG. 1;

FIGS. 3-5 are schematic diagrams of one example of the present invention, showing in different stages of two sub-groups of the powders in forming the wick structure by using the method of FIG. 1; and

FIGS. 6-7 are schematic diagrams of another example of the present invention, showing in different stages of one sub-group of the powders in forming the wick structure by using the method of FIG. 1.

DETAILED DESCRIPTION

FIG. 1 shows a preferred method in accordance with the present invention for producing a porous wick structure that can be suitably applied to heat pipes or other heat transfer devices such as vapor chamber-based heat spreaders. The wick structure is constructed from small-sized powders and a sintering process is required to form the wick structure. Firstly, a group of powders is provided, for example, by mechanical grinding a metal stock. Typically, the powders thus obtained are consisted of powders with various different sizes. The sizes of the powders may in fact range widely, for example, from several to hundreds of micrometers, based on the extent of preciseness and tolerance involved in the production of the powders. Thus, the powders as thus originally obtained are not suitable for being immediately used to construct the wick structure since it has a wide range of powder size and will raise uncertainty to the determination of the sintering temperature and time required by the sintering process. Thus, in this method, the powders are then classified into many sub-groups based on their powder sizes with each sub-group has a relatively narrow range powder sizes, as will be discussed in more details later. For easier understanding, in this embodiment, the powders used to construct the wick as originally obtained are presumed to have powder sizes ranging from 20 to 400 mesh. The “mesh” used herein represents the number of openings defined in per unit area of a standard screen. Standard screens are well known apparatus widely used to classify objects (such as powders or the like) based on their sizes. If a standard screen is used to classify powders, the number of openings in per unit area of the standard screen is usually used to indicate the powder size of the powders that pass through the standard screen. In this regard, it can be inferred that the powders having a powder size of 20 mesh are larger than those having a powder size of 400 mesh. For illustrative purpose, the powder size distribution of the powders illustrated in FIG. 2 is generally in the form of a normal distribution.

Since the powders as originally obtained are generally not immediately suitable for the sintering process, the powders subsequently are divided into multiple sub-groups in terms of powder size. The aforementioned standard screens will serve the purpose of the classification job. Specifically, the powders are brought to pass through a series of standard screens that have different meshes (i.e., openings) in per unit area thereof. For example, if the powders are brought to sequentially pass through a series of standard screens that have 200, 140, 100, and 40 meshes respectively in per unit area thereof, the powders will be classified five sub-groups, i.e., the sub-group A having a powder size of 200-220 mesh, the sub-group B having a powder size of 140-200 mesh, the sub-group C having a powder size of 100-140 mesh, the sub-group D having a powder size of 40-100 mesh and the sub-group E having a powder size of 20-40 mesh, as shown in FIG. 2. In this figure, the X-coordinate represents the powder size of the five sub-groups of the powders and the Y-coordinate represents the powder distribution (by weight) of each sub-group of the powders. It can be seen from this figure that a majority of the powders has a powder size of 100-140 mesh.

After the original group of powders is divided into these sub-groups, at least one sub-group of the powders is selected to form the wick structure. Selecting which group or groups of the powder to form the wick, however, is mainly based on what kind of characteristic the wick structure is intended to have. For example, if the wick structure to be formed is intended to have a large capillary force, sub-groups A-C with small-sized powders are generally preferred. To the contrary, if the wick structure to be formed is intended to have a large permeability, then sub-groups C-E with large-sized powders are helpful. If multiple sub-groups of powders are selected, they should be mixed in certain proportions by weight, respectively. Finally, the selected sub-groups of powders are thoroughly mixed and sintered at a required temperature for a required period of time to form the intended wick structure.

An example of forming a wick structure in a heat pipe by selecting the powders of sub-groups B and D is illustrated in FIGS. 3-5. The powders 10 of sub-group D and the powders 20 of sub-group B are preferably mixed in a ratio by weight of 5:1 to 20:1. When the two sub-groups B and D are mixed, the small-sized powders 20 are nested in the spaces (not labeled) formed between the large-sized powders 10, as illustrated in FIG. 3. Although it is not shown in the drawings, it is well known by those skilled in the art that the powders 10, 20 after mixed are then filled into a casing of the heat pipe and a mandrel is typically used to hold the powders 10, 20 against an inner wall of the casing. The casing is then placed into an oven and the powders 10, 20 are subsequently sintered. Before the sintering process, the powder sizes of the selected powders (i.e., sub-groups B and D) and the proportion between them by weight are already known. Furthermore, the powders used to construct the wick structure are limited to the selected sub-groups each having a relatively narrower range of powder sizes. It is therefore easier to determine the sintering temperature required by the sintering process. On this basis, the required time for the sintering process can also be easily determined. As the sintering process is conducted under the determined sintering temperature and time, the small-sized powders 20 of the sub-group B become to melt and gradually turn into a molten state, as illustrated in FIG. 4. At this time, however, the large-sized powders 10 of the sub-group D are almost intact except that their outer surfaces are melted. After the selected powders are sintered under the determined sintering temperature for the determined period of time, the wick structure is formed, wherein the large-sized powders 10 are interconnected together by a plurality of necks 20′ which are formed from the small-sized powders 20, as illustrated in FIG. 5. Meanwhile, a plurality of voids 30 is formed between the large-sized powders 10. These voids 30 are communicated with each other so as to form a continuous, liquid passageway. In this example, the small-sized powders 20 are helpful to form the necks 20′ between the large-sized powders 10. In order for easier illustration and understanding, in the drawings the large-sized powders 10 are presumed unchanged throughout the sintering process. In this example, since the proportion of the powders 20 of the sub-group B in the mixture is controlled, the melting of the powders 20 will not cause the mixture to have an excessive shrinkage which may result in a disconnection between the voids 30. The required temperature and time for the sintering process in this example are selected to melt the powders 20 substantially entirely and the outer surfaces of the powders 10.

Another example of forming a wick structure by selecting only one group of the powders, such as group C, is illustrated in FIGS. 6-7. The powders 40 of group C have a powder size of 100-140 mesh, larger than the powder size of group B (140-200 mesh) but smaller than the powder size of group D (40-100 mesh). In this case, since only one sub-group of the powders is selected to form the wick structure, the sintering temperature and time required in the sintering process is quite easy to be determined. As the sintering process is conducted, the outer surfaces of the powders 40 become to melt, and meantime the powders 40 as a whole become to shrink and the contacting surface between neighboring powders increases, as illustrated in FIG. 4. After the sintering process, the powders 40 are connected together by the melted outer surfaces thereof. Meanwhile, a plurality of voids 50 is defined between the powders 40. The required temperature and time for the sintering process in this example are selected to melt the outer surfaces of the powders 40. The selected group C of the powders has a range of powder size which is located at a middle of the distribution and includes a median of the distribution of FIG. 2.

Following the above-mentioned examples, a wick structure may also be constructed by selecting, for example, the sub-groups B and C, the sub-groups C and E, or the sub-groups B, C and D. A wick structure having a multi-layer structure may also be formed, for example, one layer thereof being formed by selecting the powders of sub-groups B and D, and the other layer thereof being constructed from the group C, thus forming a gradient in capillary force between these layers of the wick structure. Further, the powders used to construct the wick structure may be copper powders, nickel powders, stainless steel powders, ceramic powders or combinations thereof.

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 method of manufacturing a wick structure for a heat pipe comprising steps of:

providing a group of powders;

classifying the powders into multiple sub-groups in terms of powder size, the sub-groups having powder sizes different from each other;

selecting at least one sub-group of the powders; and

sintering the selected powders to form said wick structure.

2. The method of claim 1, wherein the powders as provided are one of copper powders, nickel powders, stainless steel powders and ceramic powders.

3. The method of claim 1, wherein the powders as provided are classified into the multiple sub-groups by passing through a series of screens.

4. The method of claim 1, wherein, when more than one sub-groups of the powders are selected, the selected sub-groups of the powders are mixed in a prescribed ratio by weight.

5. A method of manufacturing a wick structure for a heat pipe comprising steps of:

providing multiple groups of powders with each group having an average powder size different from that of each of the other groups;

selecting at least one group of powders from said multiple groups; and

sintering the selected powders to form said wick structure.

6. The method of claim 5, further comprising a step of mixing the selected powders in a prescribed proportion by weight before sintering.

7. The method of claim 6, wherein the multiple groups of powders are obtained by classification from an original group of powders having the multiple groups of powders.

8. The method of claim 7, wherein the original group of powders is classified into the multiple groups of powders by passing through a series of screens.

9. The method of claim 7, wherein the powders are ceramic powders.

10. The method of claim 7, wherein the powders are one of copper powders, nickel powders and stainless steel powders.

11. A method for forming a wick structure for a heat pipe, comprising:

preparing a group of powders;

separating the group of powders into a plurality of sub-groups of powders according to a size distribution of the powders, the sub-groups occupying different regions of the size distribution, respectively; selecting at least one of the sub-groups of powders and filling the selected powders into a casing of the heat pipe; and heating the casing and the selected powders to sinter the selected powders in the casing to thereby obtain the wick structure in the casing of the heat pipe.

12. The method of claim 11, wherein two sub-groups of powders are selected, size ranges of the selected two sub-groups of powders are at discontinuous regions of the size distribution of the powders.

13. The method of claim 11, wherein the size distribution is a normal distribution, and one sub-group of powders is selected which includes a median of the normal distribution.

14. The method of claim 12, wherein a heating temperature and period of time for the heating step are selected to melt the selected powders of one of the two sub-groups substantially entirely, which have a smaller powder size than the selected powders of the other of the two sub-groups.

15. The method of claim 12, wherein one of the two sub-groups, which has a smaller powder size has a weight less than that of the other of the two sub-groups, which has a larger powder size.

16. The method of claim 15, wherein the other of the two sub-groups, which has the larger powder size has a weight which is five to twenty times of that of the one of the two sub-groups, which has the smaller powder size.