Patent Application
20090211095
This invention is related to the wick structures, and more specifically to microgrooves (<0.2 mm wide) used as wick structures in heat pipes and method for manufacturing the same.
A heat pipe is a highly efficient heat transfer device that typically includes a vacuum vessel. The vacuum vessel has a wick structure on its inner wall and contains a small quantity of working fluid. When a heat source is applied to an evaporator portion, the working fluid evaporates into vapor that spreads quickly in the vessel. The vapor carries latent heat to a condenser portion and condenses to liquid as the latent heat dissipates to outside of the heat pipe by conduction or convection. The working fluid is transported by the capillary force back to the evaporator portion, thereby completing a two phase heat transfer cycle without consuming any power.
Generally, heat pipes are made from highly thermally conductive metals such as stainless steel, copper, and aluminum. Working fluids that are compatible with these heat pipe materials include water, mercury, and other chemicals depending on the working temperature range. Copper and pure water are the most common combination for the heat pipes used in computer and electronic systems. To overcome gravity so that evaporator and condenser can be in any orientation, the wick structure in a heat pipe provides the pumping mechanism that transports the working fluid back to the evaporator portion.
Rather than having a round or oblong tube shape of a typical heat pipe, a flat heat pipe has a plate shape and is usually made of metal sheets or plates. The flat heat pipe has a vapor chamber enclosing a working fluid. The vapor chamber has capillary structures on the inner surfaces of the top and bottom plates. The evaporator portion is one or more small areas on the outer surface of either the top or bottom plate that contact one or more heat sources (e.g., an electronic device). All other areas of the top and bottom plates serve as the condenser portion.
Typical capillary structures in heat pipes include sintered metal powders, fibers, meshes and grooves. Heat pipes with sintered metal powders, such as a sintered copper powder, have great capillary force so that they can be used at any orientation. However, it is complex and expensive to manufacture this type of heat pipes, and the thermal resistance is higher than other type heat pipes because the sintered metal powders are porous. Heat pipes made with fibers and meshes work at small angles. Furthermore, they are also expensive and complicated to be manufactured. When compared with the aforementioned technologies, heat pipes with grooves are inexpensive and easy to manufacture. However, they are only used at horizontal condition or small angles because the conventional grooves do not provide enough capillary force.
Heat pipes with grooves, usually V-shape or other shapes, are generally manufactured by a seamless pipe process such as extrusion. However, the size of the grooves are large (about >0.35 mm wide) relative to heat pipe dimensions due to the limitations on the tooling. The resulting capillary force is not large enough to pump the working fluid back to the upper condenser at large working angles. Therefore, a method for fabricating microgrooves (about <0.2 mm wide) is needed to take advantage of the low cost and ease of manufacturing of heat pipes with grooves, as well as to improve the thermal performance of the heat pipes.
Use of the same reference numbers in different figures indicates similar or identical elements.
In accordance with the invention, one embodiment of a method for fabricating microgrooves on a metal plate or strip includes two sequential steps in a single pass. A first blade with first multi-plowshares is used in the first step to turn up material on the plate or strip to form large grooves, and then a second blade with second multi-plowshares is used in the second step to rebury the large size grooves with the material turned up in the first step to form microgrooves. The microgrooves can have various shapes and are used as wicks in heat pipes. The microgrooves are formed from the relative movement between the blades and the plate or strip into which the plowshares enter. As the microgrooves can be fabricated with very small dimensions, which are controlled by the amount of the reburied material, the heat pipes can perform at large working angles due to increased capillary force.
In one embodiment, microgrooves on plates are manufactured with fluting or slotting machines where the plates are fixed on the worktable and the blades moves along a track on the machine. In one embodiment of the method, the microgrooves are formed along two directions so they intersect and allow a working fluid to travel between the microgrooves. The plates with the microgrooves can be used to make flat heat pipes or vapor chambers.
In one embodiment, microgrooves are manufactured on a metal strip such that the blades are fixed and a reel of the metal strip is unwound forward. Tubular heat pipes with the microgrooves can then be easily manufactured by integrating the above process in a conventional pipe production line using seam-welding such as high frequency induction heating (HFI). In order to have a better flow mechanism, regular V-shape grooves in another direction can be first formed by rolling to allow the working fluid to flow across the microgrooves.
It is well known that narrow grooves provide large capillary force and therefore large working angle for heat pipes. Grooves of various shapes in current heat pipes are typically formed by extrusion and are generally greater than 0.3 mm wide. The microgrooves in accordance with the invention are mini/micro-scaled grooves that are less than 0.2 mm wide. The two sequential steps in accordance with the invention may be the only available approach for mass producing grooves of this scale at present time. The principle is as simple as a farmer plowing a trench in the soil and then reburying the trench after seeds are planted. To accomplish the process, two blades are used. A first blade of first multi-plowshares is used in the first step to turn up material on a metal plate or strip to form large grooves, and then a second blade with second multi-plowshares is used in the second step to rebury the large size grooves with the material turned up in the first step to form microgrooves. The two sequential steps are simultaneously applied in a single pass. As more material is reburied, the groove size becomes smaller. The microgrooves are formed from the relative movement between the blades and the plate or strip into which the plowshares enter. The plate or strip is typically a malleable metal such as copper, copper alloy, aluminum, or aluminum alloy when the method uses cold-pressing steps. Alternatively, the plate or strip can be of harder metal such as stainless steel when the method uses hot-pressed steps.
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The base of bottom cover 606 has a pedestal depression 610 that protrudes downward from the base for contacting a heat source below flat heat pipe 600. The base of bottom cover 606 further has microgrooves 602 formed along two perpendicular directions as shown more clearly in
A spacer 612 is seated in pedestal depression 610 between top cover 604 and bottom cover 606. Spacer 612 adds to the mechanical stiffness of flat heat pipe 600 and provides a heat conductive path from the heat source to top cover 604 to improve heat dissipation.
Spacers 614 are sandwiched between top cover 604 and bottom cover 606 to control the height of the cavity defined between the covers. Holes 616 are defined in top cover 604 and bottom cover 606 for fasteners to mounting flat heat pipe 600. For example, flat heat pipe 600 is mounted to an electronic board to cool a processor in contact with pedestal depression 610.
Strip 804 is optionally fed under a roller 806 to form optional grooves 808 (only one is labeled for clarity) that are diagonal to the travel of strip 804. Diagonal grooves 808 are of typical shape and size like grooves found in a conventional heat pipe. For example, diagonal grooves 808 are V-grooves and have a width greater than 0.3 mm. When included, diagonal grooves 808 interconnect microgrooves 802 so that a working fluid in the resulting heat pipe can travel via diagonal grooves 808 between microgrooves 802. This allows the resulting heat pipe to function not just along the direction of microgrooves 802 but essentially along any direction.
In one embodiment, the process of
Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention. For example, the microgrooves of the present invention are formed from the relative motion between the plate or strip and the blades. Thus, the plate/strip can move against stationary blades, the blades can move against stationary plate/strip, or they can all move relative to each other. Numerous embodiments are encompassed by the following claims.
