Heat Spreader With Thermal Conductivity Inversely Proportional To Increasing Heat

Abstract

A heat spreading apparatus has a body defining a void. A fluid positioned within the void distributes heat via a complete thermodynamic cycle. A disruption of the complete thermodynamic cycle is inversely proportional to the magnitude of dynamic body forces, thereby diminishing heat spreading activity by the heat spreading apparatus.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. Ser. No. 13/649,044, filed Oct. 10, 2012.

FIELD OF THE INVENTION

This invention relates generally to a heat distribution device used in connection with a heat generating surface. More particularly, this invention relates to a heat spreader that has a thermal conductivity that is inversely proportional to increasing heat applied to it.

BACKGROUND OF THE INVENTION

U.S. Pat. Nos. 6,167,948 and 6,158,502 disclose thin, planar heat spreaders in various configurations. These heat spreaders endeavor to have improved thermal conductivity with increased exposure to heat. In some engineering applications it is desirable to have decreased thermal conductivity with increased exposure to heat. Accordingly, it would be desirable to provide a heat spreader that achieves this counterintuitive result.

SUMMARY OF THE INVENTION

A heat spreading apparatus has a body defining a void. A fluid positioned within the void distributes heat via a complete thermodynamic cycle. A disruption of the complete thermodynamic cycle is inversely proportional to the magnitude of dynamic body forces, thereby diminishing heat spreading activity by the heat spreading apparatus.

BRIEF DESCRIPTION OF THE FIGURES

The invention is more fully appreciated in connection with the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a section view of a planar heat spreader.

FIG. 2 illustrates a segment of the heat spreader of FIG. 1 with incipient bubble formation

FIG. 3 illustrates a segment of the heat spreader of FIG. 1 with increased bubble formation.

FIG. 4 illustrates a segment of the heat spreader of FIG. 1 with further increased bubble formation.

FIG. 5 illustrates a segment of the heat spreader of FIG. 1 with indentations to promote bubble formation.

FIG. 6 illustrates a segment of the heat spreader of FIG. 1 with hydrophilic properties to promote formation of a bubble with a first characteristic.

FIG. 7 illustrates a segment of the heat spreader of FIG. 1 with hydrophobic properties to promote formation of a bubble with a second characteristic.

FIG. 8 illustrates a heat spreader in accordance with an embodiment of the invention.

FIG. 9 illustrates a heat spreader in accordance with another embodiment of the invention.

FIG. 10 illustrates a heat spreader in accordance with another embodiment of the invention.

FIG. 11 illustrates a heat spreader with a wick structure of varying fluid transport capacity utilized in accordance with an embodiment of the invention.

FIG. 12 illustrates a heat spreader with a fluid return path that operates against gravity in accordance with an embodiment of the invention.

FIG. 13 illustrates a heat spreader subject to a body force with a disrupted thermodynamic cycle.

FIG. 14 illustrates a heat spreader subject to a body force to complete a thermodynamic cycle.

FIG. 15 illustrates a heat spreader with counter-current flow forces utilized in accordance with an embodiment of the invention.

FIG. 16 illustrates a state diagram for a device utilized in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a section view of a planar heat spreader 100 configured in accordance with an embodiment of the invention. The planar heat spreader 100 includes a first body 102 and a second body 104 which define a void 106. At least one surface of one body has a heat accumulation surface geometry 108.

The heat accumulation surface geometry disrupts the thermodynamic cycle of vaporizing fluid. Consequently, heat spreading activity by the heat spreader 100 is diminished with increasing heat. The heat accumulation surface geometry may be in the form of indentations to promote bubble growth or surface treatments, such as hydrophilic surface treatments and hydrophobic surface treatments. The heat accumulation surface geometry may also be in the form of capillary wick structures, such as screens, sintered metals, grooves, arteries, planar capillaries and combinations thereof.

FIG. 2 illustrates second body 104 of the heat spreader 100 of FIG. 1. Fluid 202 has fluid flow paths 204 adjacent to a heat accumulation surface geometry to promote the formation of an incipient bubble 206. An embodiment of the invention promotes the formation of such bubbles to disrupt efficient thermal performance of the planar heat spreader 100.

FIG. 3 illustrates increased bubble formation. In particular, bubble 206 of FIG. 3 is larger than bubble 206 of FIG. 2. The bubble 206 grows larger with increased exposure to heat from the heat generating surface. This bubble of increased size dislocates fluid path 204, as shown with fluid dislocation segment 208.

FIG. 4 illustrates further increased bubble formation. In particular, bubble 206 of FIG. 4 is even larger than bubble 206 of FIG. 3. The bubble 206 grows larger with increased exposure to heat from the heat generating surface. This bubble of further increased size further dislocates fluid path 204, as shown with fluid dislocation segment 208.

Bubble 206 effectively has a liquid perimeter and a vapor interior. As shown in FIG. 4, the bubble 206 displaces fluid 202 from much of the surface of body 104. Consequently, the fluid 202 is exposed to less surface area of body 104, which is attached to a heat generating surface. The reduced surface exposure reduces vaporization and its concomitant heat transfer action. Thus, with increased temperature and increased bubble formation, the thermal conductivity of the device 100 is reduced. This stands in contrast to typical designs that endeavor to increase thermal conductivity in the presence of increased exposure to heat.

FIG. 5 illustrates body 104 of the heat spreader 100 of FIG. 1. Fluid 502 is adjacent to a heat accumulation surface geometry in the shape of indentations 504 to promote bubble growth. FIG. 5 illustrates a bubble 506 formed in one such indentation. Although the indentations are shown as spherical, they may be any shape, such as cylindrical, conical, or trapezoidal.

FIG. 6 illustrates body 104 of the heat spreader 100 of FIG. 1. Fluid 602 is adjacent to a heat accumulation surface geometry with hydrophilic properties to promote bubble formation. For example, a hydrophilic material, a hydrophilic film or hydrophilic surface features may be used to promote hydrophilic properties. A hydrophilic surface minimizes surface exposure to a liquid. Thus, bubble 604 forms with a relatively small footprint 606 on the surface of segment 104.

FIG. 7 illustrates body 104 of the heat spreader 100 of FIG. 1. Fluid 702 is adjacent to a heat accumulation surface geometry with hydrophobic properties to promote bubble formation. For example, a hydrophobic material, a hydrophobic film or hydrophobic surface features may be used to promote hydrophobic properties. A hydrophobic surface maximizes surface exposure to a liquid. Thus, bubble 704 forms with a relatively large footprint 706 on the surface 104.

The selection of a hydrophilic surface or hydrophobic surface is contingent upon the application and the desired configuration of the bubble. A single surface may include both hydrophilic and hydrophobic regions.

The foregoing examples illustrate the formation of a single or few bubbles. Alternate embodiments of the invention facilitate the formation of increased number of bubbles with increased exposure to heat.

Table I illustrates performance results achieved in accordance with an embodiment of the invention.

TABLE I
		Thermal
Power	Temperature	Conductivity
(W)	(° C.)	(W/m * K)
0.0	60.0	2410.5
9.9	60.9	2025.3
15.3	61.5	1820.1
25.0	62.8	1532.9
50.0	67.5	1064.9
75.0	75.0	760.0
100.4	85.6	577.1
125.0	99.4	323.0

Observe that this embodiment experiences thermal conductivity changes from 2410 to 323 (650% thermal conductivity change) over approximately 40° C. (from 99.4° C. to 60° C.). Thus, unlike typical devices, thermal conductivity decreases with increasing heat exposure.

The techniques of the invention may be used to form heat transfer devices of various configurations. FIG. 8 illustrates a section view of one such device 800. Device 800 includes a first body 804 a second body 806 and vertical sidewalls 808 and 812 which define a void 802 for vapor flow. At least a portion of the bodies and sidewalls interior surfaces have a heat accumulation surface geometry 814. A fluid (not shown) is positioned adjacent to the heat accumulation surface geometry and vertical support 810. The heat accumulation surface geometry is configured for bubble formation, as previously described.

The sidewalls 808, 812 and vertical support 810 facilitate efficient heat transfer. This efficient heat transfer is countered by the heat accumulation surface geometry, which has a thermal conductivity that is inversely proportional to increasing applied heat.

FIG. 9 illustrates a section view of device 900 generally corresponding to device 800 of FIG. 8, but with additional thermal resistance promoting features. The device 900 includes a first body 904 a second body 906 and vertical sidewalls 908 and 912, which define a void 902 for vapor flow. At least a portion of the bodies and sidewalls interior surfaces have a heat accumulation surface geometry 914. A fluid (not shown) is positioned adjacent to the heat accumulation surface geometry and vertical support 910. The heat accumulation surface geometry is configured for bubble formation, as previously described.

The sidewalls 908, 912 and vertical support 910 have corresponding cut-outs 916, 918, 920, 922, 924, and 926 to reduce heat transfer efficiency. Specifically, these cut-outs reduce the heat flow cross-sectional area, and increase the heat flow length, reducing the heat transfer efficiency, which supplements the heat accumulation surface geometry design goal of thermal conductivity that is inversely proportional to increasing applied heat.

Embodiments of the invention rely upon a heat accumulation surface geometry that promotes dry out. Dry out is the absence of a fluid. The absence of a fluid in the heat spreading apparatus disrupts the thermodynamic cycle and thereby diminishes heat spreading activity. For example, dry out occurs when the fluid pressure from the condenser region is insufficient to provide enough fluid to the evaporator region. This leads to dry out in the evaporator. Dry out prevents the thermodynamic cycle from continuing and therefore heat spreading activity is diminished, thus satisfying the heat accumulation surface geometry design goal of thermal conductivity that is inversely proportional to increasing applied heat.

Techniques of the invention may be realized in a variety of configurations. For example, various capillary configurations are disclosed in the previously referenced U.S. Pat. Nos. 6,167,948 and 6,158,502, which are incorporated herein by reference.

Alternate embodiments of the invention are optimized for an environment in which the device is attached to a heat source. The device is operative to alternately engage and disengage as a heat path in response to body forces. The body forces may be gravitational, acceleration, electromagnetic, centrifugal, Euler and/or Coriolis forces. The body forces may have any magnitude and direction, including harmonic, periodic, linear, reciprocating, random, circular, rotary, curvilinear, rotational, oscillating, spiral, helical, static, dynamic, multiaxial, compound, and combinations thereof.

A body force or the absence of a body force may cause a transient disruption of the thermodynamic cycle. For example, a heat spreader subject to a vertical body force may provide a poor heat spreading path. The same heat spreader subject to a body force that is 45 degrees from vertical may provide a high heat spreading path. Finally, the same heat spreader subject to a horizontal body force may provide a low heat spreading path. Another example where the absence of transient body forces disrupt the thermodynamic cycle, involves a heat spreader where the disruption of the complete thermodynamic cycle is inversely proportional to the magnitude of dynamic body forces applied to the heat spreader. In other words, when the heat spreader has adequate dynamic body forces to complete the thermodynamic cycle, less than adequate body forces will disrupt the complete thermodynamic cycle, thereby diminishing heat spreading activity by the heat spreading apparatus. A further example where body forces or the absence of body forces disrupt the complete thermodynamic cycle, involves a heat spreader where a component of these body forces provides a threshold to the disruption of the complete thermodynamic cycle. In other words, while body forces or the absence of body forces are disrupting the complete thermodynamic cycle, other body forces may limit the disruption and provide minimum heat spreading activity.

FIG. 10 illustrates a section view of device 1000. The device 1000 includes a first body 1004 a second body 1006 and curved sidewalls 1008 and 1010, which define a void 1002 for vapor flow. At least a portion of the bodies and sidewalls interior surfaces have a heat accumulation surface geometry 1012. A fluid (not shown) is positioned adjacent to the heat accumulation surface geometry. The heat accumulation surface geometry is configured for bubble formation, as previously described.

The sidewalls 1008, 1010 reduce heat transfer efficiency. Specifically, these sidewalls reduce the heat flow cross-sectional area, and increase the heat flow length, reducing the heat transfer efficiency, which supplements the heat accumulation surface geometry design goal of thermal conductivity that is inversely proportional to increasing applied heat.

Embodiments of the invention may also rely upon the promotion of diminishing fluid transport capacity. A dry out region is a region with zero fluid transport capacity. Diminishing fluid transport capacity regions in the heat spreading apparatus impede the thermodynamic cycle and thereby reduce heat spreading activity. A fluid transport capacity retarding surface geometry may be used to accomplish a diminishing fluid transport capacity. For example, the heat spreading apparatus may have a fluid wicking structure with greater cross-sectional area (thicker), and greater fluid transport capacity, and a fluid wicking structure with less cross-sectional area (thinner) and less fluid transport capacity. The fluid wicking structure may include screens, sintered metals, grooves, arteries, planar capillaries and combinations thereof. The fluid transport capacity retarding surface geometry may be in the form of a surface treatment.

FIG. 11 illustrates one such device 1100, having a fluid transport capacity retarding surface geometry that may be used in accordance with an embodiment of the invention. Device 1100 includes a first body 1104 a second body 1106 and curved sidewalls 1108 and 1110, which define a void 1102 for vapor flow. A portion of the bodies and sidewalls interior 1112 have a fluid wicking structure with greater cross-sectional area (thicker) and greater fluid transport capacity 1114, and a fluid wicking structure with progressively less cross-sectional area (thinner) and less fluid transport capacity 1116, thus satisfying the promotion of diminishing fluid transport capacity.

The thermodynamic cycle may be disrupted by applying body forces to the heat spreading apparatus. For example, body forces opposing the fluid return to the evaporator region will assist in diminishing fluid transport capacity, thereby reducing heat spreading activity.

In one embodiment, the body force is gravity. The heat distribution device is oriented such that the fluid return path must work against gravity. FIG. 12 illustrates a heat spreading apparatus 1200 oriented against gravity, such that the fluid return path must work against gravity. Device 1200 includes a first body 1204 a second body 1206 and curved sidewalls 1208 and 1210, which define a void 1202 for vapor flow. Fluid 1212 and its fluid return path 1214 must work against gravitational force 1216 in order to complete the thermodynamic cycle. Body forces (gravity for this embodiment) have interrupted the thermodynamic cycle by preventing complete fluid return, and thereby causing a dry out region 1218.

In another embodiment, where the thermodynamic cycle may be disrupted by applying body forces to the heat spreading apparatus, involves heterogeneous fluid transport capacity surface regions, or surface regions with varying fluid transport capacity, where body forces govern fluid connection between adjacent or remote surface regions.

FIG. 13 illustrates a heat spreading apparatus 1300, where body force 1302 obstructs fluid 1304 from connecting surface region 1306 to either surface region 1308 or 1310. Fluid from surface region 1306 needing to return to surface region 1310 in order to complete the fluid flow cycle has been disrupted; therefore, the thermodynamic cycle has been disrupted. Dynamic body forces may promote a complete thermodynamic cycle. If a body force component, such as 1302 (e.g., gravity), provides a disruption threshold, minimum heat spreading activity occurs within the heat spreading apparatus. In this case, fluid 1304 is limited to one region and therefore a complete thermodynamic cycle does not exist. If an additional body force component (not shown) was promoting a complete thermodynamic cycle, body force component 1302 may provide a heat distribution threshold by allowing fluid 1304 to pool and partially block heat transfer surface region 1306, thereby blocking and limiting heat spreading activity by the heat spreading apparatus.

FIG. 14 illustrates a heat spreading apparatus 1400, where body force 1402 allows fluid 1404 to connect surface region 1406 to surface regions 1408 and 1410. Fluid from surface region 1406 needing to return to surface region 1410 in order to complete the fluid flow cycle has been closed; therefore the thermodynamic cycle has been closed. Accordingly, an evaporation and condensing thermodynamic cycle is once again operative. If an additional body force component (not shown) was disrupting the complete thermodynamic cycle, body force 1402 may provide a disruption threshold by allowing fluid 1404 to connect surface region 1406 to surface regions 1408 and 1410, thereby providing minimum heat spreading activity by the heat spreading apparatus.

Increasing the thermal resistance may also be supported by the type of fluid used in the heat spreading apparatus. Certain fluids, including composite and heterogeneous fluids have a lower figure of merit, which results in a lower heat transport capacity, thereby supporting diminishing heat spreading activity, for a specified applied heat. The fluid figure of merit for a phase change (evaporating and condensing) apparatus is equal to the quantity of the fluids' latent heat of vaporization times the liquid density times the liquid surface tension, divided by the liquid dynamic viscosity. Example fluids that may be used in accordance with embodiments of the invention and their room temperature figure of merit are shown in Table 2.

TABLE 2
		Latent Heat of	Liquid	Liquid Dynamic	Liquid Surface
Working	Figure of	Vaporization	Density	Viscosity	Tension
Fluid	Merit	(J/kg)	(kg/m{circumflex over ( )}3)	(kg/m * s)	(N/m)
acetone	3.229E+10	5.470E+05	7.860E+02	3.056E−04	2.295E−02
methanol	3.832E+10	1.185E+06	7.872E+02	5.405E−04	2.221E−02
water	1.971E+11	2.443E+06	9.978E+02	8.902E−04	7.198E−02

The thermodynamic cycle may also be disrupted by having a heat accumulation surface geometry that promotes counter-current fluid flow. The heat accumulation surface geometry may be in the form of a surface treatment. Counter-current fluid flow occurs when fluid vapor flowing from the evaporator to the condenser is opposed by condensed fluid returning from the condenser to the evaporator. Increasing the applied heat to the heat spreading apparatus increases these opposing fluid velocities, resulting in diminished fluid mass flow, and thereby reducing the heat spreading activity. An example of a heat accumulation surface geometry that promotes counter-current flow, thereby disrupting the thermodynamic cycle, is an open longitudinal channel. Example of a heat accumulation surface geometries that minimize counter-current flow, thereby promoting the thermodynamic cycle are closed or semi-closed tunnels or arteries.

FIG. 15 illustrates a heat accumulation surface geometry 1500 including vapor flow direction 1502, fluid 1504 and fluid flow direction 1506. Counter-current flow forces have interrupted the thermodynamic cycle by preventing complete fluid return, and thereby causing a dry out region 1508.

Additional embodiments of the heat spreading apparatus may periodically disrupt the thermodynamic cycle. For example, transient body forces may cycle between opposing and supporting fluid return to the evaporator. Further embodiments of the heat spreading apparatus having transient disruption to the thermodynamic cycle, where transient body forces are applied, may involve over-charging. Over-charging is the introduction of more fluid than may be necessary for normal activity. When over-charging and transient body forces are combined, body forces may cause excess fluid to accumulate in either the evaporator region, the condenser region, or any other regions, depending on the direction and magnitude of the body forces. FIGS. 12-14 illustrate such excess fluid accumulation. When fluid accumulates in the evaporator region, evaporation and heat spreading are improved, and when fluid accumulates in the condenser region, evaporation and heat spreading are reduced.

Embodiments of the heat spreading apparatus where dry out occurs, may need to recover from this dry out activity. For example, dry out recovery may be improved by the reduction of applied heat, or by body forces that support the complete thermodynamic fluid flow cycle, or by the use of fluids having dry out recovery properties, or by the degree of over-charging, or any combination thereof.

FIG. 16 illustrates a state diagram associated with device utilized in accordance with an embodiment of the invention. In an initial mode, a complete thermodynamic cycle exists 1600 and therefore the device has its best heat spreading capacity. If no force change exists (1602—No), then the device remains in this state. So, for example, in the case where the device is attached to a heat source, the device will convey heat at its best heat spreading capacity.

If there is a force change (1602—Yes), then an incomplete thermodynamic cycle exists 1604. This results in reduced heat spreading performance. Thus, in the case where the device is connected to a heat source, the device will limit heat spreading activity and effectively isolate the heat source.

If no restoring force exists (1606—No), then the incomplete thermodynamic cycle 1604 remains. If a restoring force exists (1606—Yes), then the complete thermodynamic cycle 1600 state returns, resulting in improved heat spreading.

The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that specific details are not required in order to practice the invention. Thus, the foregoing descriptions of specific embodiments of the invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed; obviously, many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, they thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the following claims and their equivalents define the scope of the invention.

Claims

1. A heat spreading apparatus, comprising:

a body defining a void; and

a fluid positioned within the void for distributing heat via a complete thermodynamic cycle;

wherein a disruption of the complete thermodynamic cycle is inversely proportional to the magnitude of dynamic body forces, thereby diminishing heat spreading activity by the heat spreading apparatus.

2. The heat spreading apparatus of claim 1 wherein the body defines a void with a fluid transport retarding surface geometry to disrupt the thermodynamic cycle, thereby diminishing heat spreading activity by the heat spreading apparatus.

3. The heat spreading apparatus of claim 1 wherein the fluid has a figure of merit that diminishes heat transport capacity, thereby diminishing heat spreading activity by the heat spreading apparatus.

4. The heat spreading apparatus of claim 1 wherein the body defines a void with a counter-current fluid flow surface geometry to disrupt the thermodynamic cycle of returning condensed fluid, thereby diminishing heat spreading activity by the heat spreading apparatus.

5. The heat spreading apparatus of claim 1 wherein the amount of fluid is in excess of the amount of fluid necessary for optimal heat transfer activity, such that excess fluid in an evaporator region of the void improves heat spreading activity and access fluid in a condenser region of the void reduces heat spreading activity.

6. A heat spreading apparatus, comprising:

a body defining a void; and

a fluid positioned within the void for distributing heat via a complete thermodynamic cycle;

wherein body forces disrupt the complete thermodynamic cycle, while a component of these body forces provides a disruption threshold, thereby providing minimum heat spreading activity by the heat spreading apparatus.

7. The heat spreading apparatus of claim 6 wherein the body defines a void with a fluid transport retarding surface geometry to disrupt the thermodynamic cycle, thereby diminishing heat spreading activity by the heat spreading apparatus.

8. The heat spreading apparatus of claim 6 wherein the fluid has a figure of merit that diminishes heat transport capacity, thereby diminishing heat spreading activity by the heat spreading apparatus.

9. The heat spreading apparatus of claim 6 wherein the body defines a void with a counter-current fluid flow surface geometry to disrupt the thermodynamic cycle of returning condensed fluid, thereby diminishing heat spreading activity by the heat spreading apparatus.

10. The heat spreading apparatus of claim 6 wherein the amount of fluid is in excess of the amount of fluid necessary for optimal heat transfer activity, such that excess fluid in an evaporator region of the void improves heat spreading activity and access fluid in a condenser region of the void reduces heat spreading activity.