Patent Description:
Various wicking structures have been designed that exploit capillary action to continuously absorb liquid into various solid structures as it condenses at the condenser regions, and to then draw the absorbed liquid back to the evaporator regions. Exemplary wicking structures include channel-type wicking structures that are etched into the inner surfaces of the vapor chamber to induce capillary forces that draw the liquid back to the evaporator regions. Unfortunately, channel-type wicking structures are not designed to decrease the overall thermal resistance across all liquid-vapor interfaces (menisci) at evaporation regions of the device. For example, although efficient at returning liquid to evaporation regions, channel-type wicking structures are not suited for increasing a thin-liquid-film evaporation area within the evaporation regions. This is because the channel-type wicking structures typically include relatively wide channels (e.g., <NUM> microns or more) so as not to unduly increase fluid resistance that restricts liquid flow from the condenser region to the evaporator region. Thus, a majority of the liquid-vapor boundary that is formed within such channel-type wicking structures will be an intrinsic meniscus region - which has a relatively higher thermal resistance than thin-film evaporation regions.

<CIT> relates to a flexible thermal ground plane. A flexible thermal ground plane may include a support member. The flexible thermal ground plane may include an evaporator region or multiple evaporator regions configured to couple with the support member. The flexible thermal ground plane may include a condenser region or multiple condenser regions configured to couple with the support member. The evaporator and condenser region may include a microwicking structure. The evaporator and condenser region may include a nanowicking structure coupled with the micro-wicking structure, where the nanowicking structure includes nanorods. The evaporator and condenser region may include a nanomesh coupled with the nanorods and/or the microwicking structure. It is the object of the present invention to provide an improved thermodynamic system having a porous microstructure sheet. This object is solved by the subject matter of the independent claim.

Technologies described herein provide a two-phase thermodynamic system that includes a porous microstructure sheet to increase an aggregate thin-film evaporation area of a working fluid. Generally described, embodiments disclosed herein include a porous microstructure sheet that is disposed at a liquid-vapor boundary of a working fluid that is contained within the two-phase thermodynamic system. The porous microstructure sheet includes a plurality of pores through which the working fluid flows from a liquid flow path on one side of the porous microstructure sheet to a vapor flow path on the other side of the porous microstructure sheet. Individual pores induce the working fluid to form thin-film evaporation regions. In various embodiments, the porous microstructure sheet has a pore density (e.g., a number of pores per unit area) that is optimized so as to increase an aggregate thin-film evaporation area of the working fluid. It can be appreciated that increasing the pore density may be accomplished by decreasing the size (e.g., dimensions) of the individual pores or decreasing the spacing in between the individual pores, or both.

By appropriately sizing and spacing the individual pores, the overall thermal resistance across all liquid-vapor interfaces (menisci) of the working fluid is substantially decreased over conventional vapor chambers that merely incorporate channel-type wicking structures for wicking the condensed liquid back to the evaporator region. Thus, the technologies described herein enable two-phase thermodynamic systems (e.g., heat pipes, vapor chambers, etc.) to transmit latent heat across the liquid-vapor interface at substantially higher rates than conventional systems. This enables the two-phase thermodynamic systems disclosed herein to exploit phase-change processes for maximizing thermal conductivity at higher throughput heat power rates than conventional systems.

The thermodynamic system of the invention includes walls that form a cavity which contains a working fluid in two phases (i.e., a liquid phase and a vapor phase). During operation, the liquid phase of the working fluid absorbs heat that is generated and/or transferred by a heat source that is external to the device. In some instances, the heat source may be physically touching a portion of the walls of the thermodynamic system. The absorbed heat continually converts mass of the working fluid from the liquid phase into the vapor phase, which then transfers this heat in latent form away from the heat source. Specifically, inside the cavity is an evaporator region or regions for absorbing heat to convert a liquid fraction of the working fluid into a vapor fraction of the working fluid. The working fluid is a bi-phase fluid that evaporates from a liquid state into a gaseous (vapor) state upon absorbing latent heat. The working fluid may then flow through the cavity, in the vapor state, to carry the absorbed latent heat away from the evaporator region. Exemplary working fluids include, but are not limited to, water, refrigerant substances (e.g., R134), ammonia-based liquids, or any other fluid suitable for efficient absorption and release of heat to effect phase changes (evaporation and condensation respectively) change a liquid and a gaseous (vapor) state.

Inside the cavity there is also a condenser region or regions for releasing the latent heat out of the working fluid and expelling this released heat into an external environment. This release of latent heat occurs via spontaneous condensation wherein the vapor fraction is continually converted back into the liquid fraction. It will be appreciated by those skilled in the art that the specific amount (e.g., mass) of the vapor fraction that condenses back into the liquid fraction in any given time interval of operation depends on the specific amount of latent heat that is dissipated from the vapor fraction. It will also be appreciated that in steady state operation, this mass amount is equal to the mass amount of liquid fraction that is evaporated (converted into the vapor phase) during an equal duration of time. It will further be appreciated that in steady-state operation, the total heat power flowing into all evaporator region(s) from the external source(s) will be equal to the total heat power flowing out of all condenser region(s) into the external environment.

The thermodynamic system includes a vapor flow path through which the vapor fraction of the working fluid convectively carries the latent heat absorbed at the evaporator region(s) away from the heat source. The vapor flow path may be any path suitable for the vapor fraction of the working fluid to freely flow from the evaporator region(s) to the condenser region(s). The thermodynamic system may also include a liquid flow path through which the liquid fraction of the working fluid continuously flows back to the evaporator region(s) near the heat source. Thus, the liquid flow bath may extend from the condenser region(s) to the evaporator region(s). The liquid flow path includes a plurality of ribs that form channels extending from the condenser region(s) back to the evaporator region(s). The channels fill up (at least partially) with the liquid fraction as it condenses at the condenser region(s). The channels then draw this liquid fraction back to the evaporator region (e.g. via capillary action). It can be appreciated that the channels form a wicking structure that exploits capillary action to ensure the evaporator region remains sufficiently wetted by continually drawing the liquid fraction back into the evaporator region - ideally at sufficient rates to keep up with evaporation.

The thermodynamic system includes a porous microstructure sheet that forms a boundary between the liquid flow path and the vapor flow path. The liquid flow path is generally defined by the channels, the porous microstructure sheet is disposed over the plurality of ribs that form the channels. The porous microstructure sheet includes a plurality of pores that individually induce the liquid fraction of the working fluid to form thin-film evaporation regions. A pore density of the porous microstructure sheet may be sufficiently high such that an aggregate thin-film evaporation area of the working fluid is increased - as compared to conventional systems. For example, it can be appreciated that near where a liquid-vapor boundary nears a solid material (e.g., metal), a liquid may form into one or more distinctively behaving regions. These distinctively behaving regions may include an adsorbed layer region, an thin-film evaporation region, and an intrinsic meniscus region. It can be appreciated that a relative rate at which the liquid fraction evaporates into the vapor fraction will be greatest at the thin-film evaporation region(s), as compared to the adsorbed layer region and the intrinsic meniscus region of the liquid-vapor boundary. Since the pore density of the porous microstructure sheet is sufficiently high such that an aggregate thin-film evaporating area of the working fluid is increased as compared to conventional systems, the technologies described herein are suitable for increasing the number and robustness of low-thermal-resistance microscopic evaporation sites for uninterrupted optimum performance.

In various embodiments, the porous microstructure sheet may be in the form of a grid mesh in which the individual pores are repeatably arranged into an ordered grid of pores having defined pore sizes and/or pore shapes. The individual pores of the invention include at least one interior corner. As a specific but nonlimiting example, the individual pores may include four sides which come together to form four interior corners. These "microscopic" interior corners may individually induce the formation of thin-film evaporation regions of the liquid fraction. Furthermore, since the liquid-vapor boundary may be located within the individual pores, thin-film evaporation regions may also form away from the corners on the individual sides of each pore. Thus, even away from corners, each side (edge) of each grid block shaped pore will also function as an evaporator site as evaporation is relatively higher via thin-film evaporation regions of a liquid as opposed to other "deeper" regions of the liquid (e.g., where in intrinsic meniscus region forms).

In some embodiments, the porous microstructure sheet may be formed by converting a metallic foil into a grid mesh by etching the plurality of pores into the metallic foil. It can be appreciated that various dry etching techniques facilitate the formation of microstructures having relatively high aspect ratio. For example, various dry etching techniques enable the formation of deep "pin" holes type microstructures.

In some embodiments, the porous microstructure sheet may be formed by weaving a plurality of metallic fibers together so that individual metallic fibers are interconnected with other metallic fibers to form the plurality of pores. As a specific but nonlimiting example, the weave of metallic fibers may include a first array of metallic fibers that extend in a first direction and a second array of metallic fibers that extend in a second direction. The first array of metallic fibers may be weaved together with the second array of metallic fibers.

These and various other features will be apparent from a reading of the following Detailed Description and a review of the associated drawings. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended that this Summary be used to limit the scope of the claimed subject matter.

References made to individual items of a plurality of items can use a reference number with another number included within a parenthetical to refer to each individual item.

The following Detailed Description describes a two-phase thermodynamic system that includes a porous microstructure sheet that is specifically designed to induce a working fluid to form numerous thin-film evaporation sites to increase an aggregate thin-film evaporation area of the working fluid. Generally described, various embodiments disclosed herein include a porous microstructure sheet that is disposed at or near a liquid-vapor boundary of the working fluid. The porous microstructure sheet includes a plurality of pores through which the working fluid flows from a liquid flow path on one side of the porous microstructure sheet to a vapor flow path on the other side of the porous microstructure sheet. Individual pores induce the working fluid to form thin-film evaporation regions and the porous microstructure sheet may have a pore density that is optimized so as to increase an aggregate thin-film evaporation area of the working fluid. In this way, the overall thermal resistance across all liquid-vapor interfaces (menisci) of the working fluid is substantially decreased over conventional vapor chambers (e.g., vapor chambers that merely incorporate channel-type wicking structures for wicking the condensed liquid back to the evaporator region(s)).

Thus, the technologies described herein enable two-phase thermodynamic systems (e.g., heat pipes, vapor chambers, etc.) to transmit latent heat across the liquid-vapor interface at substantially higher rates than conventional systems. Additionally, substantially increasing the number of robust evaporation sites that are present within an evaporator region(s) results in the two-phase thermodynamic systems disclosed herein being substantially more resistant to dry-out. This enables the two-phase thermodynamic systems disclosed herein to exploit phase-change processes for maximizing thermal conductivity at higher throughput heat power rates than conventional systems.

The present invention is believed to be applicable to a variety of two-phase thermodynamic systems and approaches involving the utilization of porous microstructure sheet(s) to increase an aggregate thin-film evaporation area of a working fluid. Aspects of the invention disclosed below are predominantly described in the context of a single porous microstructure sheet being disposed over a channel-type wicking structure so that the porous microstructure sheet is located at and/or induces the formation of a liquid-vapor boundary. While the present invention is not necessarily limited to such embodiments, an appreciation of various aspects of the invention is best gained through a discussion of examples in this context. Accordingly, aspects of the disclosure below that are not expressly recited in the claims are not to be interpreted as limiting of the claims in any way whatsoever.

<FIG> is a perspective cut-away view of a "prior art" two-phase thermodynamic system <NUM> that includes a channel-type wicking structure <NUM> to induce fluid flow from a condenser region <NUM> and an evaporator region <NUM>. As illustrated, the thermodynamic system <NUM> includes walls <NUM> that contain a working fluid (omitted from illustration to expose the channel-type wicking structure <NUM>). During operation, a heat source <NUM> (e.g., a Central Processing Unit, a battery, etc.) emits heat that is absorbed into the working fluid at the evaporator region <NUM> (as shown by the QIN symbol) of the thermodynamic system <NUM>. The absorbed heat causes a liquid fraction of the working fluid to evaporate into a vapor fraction of the working fluid. As additional mass (e.g. as quantified in gram units) of the liquid fraction is continually evaporated into the vapor fraction, some mass of the vapor fraction is displaced towards the condenser region <NUM>. Thus, as additional heat is absorbed into the working fluid converting the liquid fraction into the vapor fraction, latent heat is carried by the vapor fraction through a vapor flow path <NUM> (shown as a white arrow disposed above the channel-type wicking structure <NUM>) to the condenser region <NUM>. Then, when the vapor fraction reaches the condenser region <NUM>, the latent heat is transferred out of the working fluid (as shown by the QOUT symbol) via a combination of condensation phase change (converting the vapor fraction of the working fluid back into its liquid fraction) and conductive heat transfer out of the device and into the external environment adjacent to the outer device wall in the vicinity of the condenser region.

As the latent heat is released from the condenser region <NUM> (e.g., through the walls <NUM> and into the ambient environment), the channel-type wicking structure <NUM> continually wicks or draws the newly condensed portions of the liquid fraction back into the evaporator region <NUM>. Although somewhat useful for drawing the liquid fraction back into the evaporator region <NUM>, the channel-type wicking structure <NUM> Is not useful for inducing the formation of large numbers of robust (e.g., dry-out resistant) thin-film evaporation sites. The undesirable quantity and geometry of the evaporation sites produced by the channel-type wicking structure <NUM> all too often leads to dry-out occurring within the evaporator region <NUM>. Dry-out is a phenomenon wherein the liquid fraction of the working fluid evaporates too fast as heat is absorbed, thereby causing the evaporator region to "dry-out. " Dry-out can lead to high localized temperature rises or spikes and, therefore, extreme temperature non-uniformity across the thermodynamic system <NUM>.

<FIG> is a side view of the "prior art" two-phase thermodynamic system <NUM> that illustrates the formation of thin-film evaporation sites within the channel-type wicking structure <NUM>. As illustrated, the thermodynamic system <NUM> includes eleven ("<NUM>") individual ribs <NUM> (only the first rib <NUM>(<NUM>) and the eleventh rib <NUM>(<NUM>) are labelled) that together with the side walls form twelve ("<NUM>") individual channels through which the liquid fraction of the working fluid is drawn into the evaporator region <NUM>. It can be appreciated that since the thin-film evaporation sites will form where the liquid-vapor boundary nears a solid structure (e.g., the ribs <NUM> and/or the interior side of the walls <NUM>), in the illustrated example the inner geometry of the thermodynamic system <NUM> will induce the formation of twenty-four ("<NUM>") thin-film evaporation region. Specifically, so long as the level of the working fluid (shown with a dotted fill pattern) remains below the top of the ribs <NUM>, then a single thin-film evaporation region will form against each side of each individual channel.

Turning now to <FIG>, illustrated is an exemplary two-phase thermodynamic system <NUM> that includes a porous microstructure sheet <NUM> to induce the formation of numerous thin-film evaporation sites (also referred to herein as "thin-film evaporation regions") to increase an aggregate thin-film evaporation area of a working fluid. <FIG> illustrates a side view of the two-phase thermodynamic system <NUM> of <FIG>. As illustrated, the thermodynamic system <NUM> includes walls <NUM> that form a cavity <NUM> (labelled only in <FIG>). The cavity <NUM> contains the working fluid in two phases (i.e., a liquid phase and a vapor phase). It is worth noting that in order to expose the structural details of the porous microstructure sheet, the working fluid is omitted from both of <FIG>. During operation, the liquid phase of the working fluid absorbs heat that is generated and/or transferred by a heat source <NUM> that is external to walls <NUM> of the two-phase thermodynamic system <NUM>. As described above, the absorbed heat continually converts mass of the working fluid from the liquid phase into the vapor phase. The vapor phase then transfers this heat in latent form away from the heat source <NUM>.

In the illustrated example, the two-phase thermodynamic system <NUM> includes a plurality of ribs <NUM> that together form a plurality of channels <NUM>. The channels <NUM> form a liquid flow path through which the liquid phase of the working fluid flows from a condenser region <NUM> back to an evaporator region <NUM>. The channels <NUM> induce capillary action on the liquid fraction of the working fluid to draw the liquid fraction toward the evaporator region <NUM>.

In the illustrated embodiment, the porous microstructure sheet <NUM> is disposed over the top of the ribs <NUM> to induce the liquid fraction of the working fluid to form thin-film evaporation regions. Specifically, the porous microstructure sheet <NUM> includes a plurality of individual pores <NUM> that may be specifically shaped so as to cause thin-film evaporation regions to be formed therein. In this way, if the cavity <NUM> is filled with an appropriate amount of the working fluid such that a liquid-vapor boundary forms slightly above the ribs <NUM>, then small amounts of the liquid fraction will be drawn into individual ones of the pores <NUM> and caused to form thin-film evaporation regions. It can be appreciated therefore that the porous microstructure sheet <NUM> may be disposed substantially at the liquid-vapor boundary such that the vapor flow path is on one side of the porous microstructure sheet <NUM> (e.g., above the sheet as shown in <FIG>) and the liquid flow path is on the opposite side of the porous microstructure sheet <NUM> (e.g., below the sheet as shown in <FIG>).

The porous microstructure sheet <NUM> may have a pore density that is optimized so as to increase an aggregate thin-film evaporation area of the working fluid - as compared to the amount of thin-film evaporation area that would form due to the channel-type wicking structure alone (e.g., as described in relation to <FIG>). It should be appreciated that inclusion of the porous microstructure sheet <NUM> at (or near) a liquid-vapor boundary within the two-phase thermodynamic system <NUM> may substantially decrease the overall thermal resistance across all liquid-vapor interfaces (menisci) of the working fluid. Thus, the porous microstructure sheet <NUM> may enable the two-phase thermodynamic system <NUM> to transmit latent heat across the liquid-vapor interface at substantially higher rates than conventional systems and, therefore, exploit phase-change processes for maximizing thermal conductivity at higher throughput heat power rates than conventional systems.

In various embodiments, the individual pores <NUM> within the porous microstructure sheet <NUM> may be "microscopic" in size. As some specific but nonlimiting examples, the individual pores <NUM> may have a total pore area of: less than <NUM> square microns (µ) (e.g., as would be formed by a 50µ by 50µ square pore), less than <NUM> square microns (µ) (e.g., as would be formed by a 45µ by 45µ square pore), less than <NUM> square microns (µ) (e.g., as would be formed by a 40µ by 40µ square pore), less than <NUM> square microns (µ) (e.g., as would be formed by a 35µ by 35µ square pore), less than <NUM> square microns (µ) (e.g., as would be formed by a 30µ by 30µ square pore), less than <NUM> square microns (µ) (e.g., as would be formed by a 25µ by 25µ square pore), less than <NUM> square microns (µ) (e.g., as would be formed by a 20µ by 20µ square pore), less than <NUM> square microns (µ) (e.g., as would be formed by a 15µ by 15µ square pore), or less than <NUM> square microns (µ) (e.g., as would be formed by a 10µ by 10µ square pore).

Turning now to <FIG>, illustrated is a detailed view of the two-phase thermodynamic system <NUM> of <FIG>. The detailed view shown in <FIG> specifically corresponds to the circle that is labeled Detail A in <FIG>. In the illustrated example, the porous microstructure sheet <NUM> is in the form a grid mesh in which the individual pores <NUM> are repeatedly arranged into an ordered grid of pores having predefined and/or repeatable pore sizes. For example, as illustrated, the individual pores <NUM> are arranged into ordered columns and rows of pores. In this example, the individual rows of pores are separated from one another by a predefined and repeatable grid row spacing whereas the individual columns of pores are separated from one another by a predefined and repeatable grid column spacing. In some implementations, the grid row spacing may be nominally equal to the grid column spacing. In some implementations, the grid row spacing may be nominally different from the grid column spacing. In some implementations, the "nominal" grid row spacing and/or the grid column spacing may be between <NUM> microns and <NUM> microns, between <NUM> microns and <NUM> microns, between <NUM> microns and <NUM> microns, between <NUM> microns and <NUM> microns, or less than <NUM> microns.

In the illustrated example, the individual pores include four sides which come together to form four interior corners. As described in more detail below in relation to <FIG>, individual thin-film evaporation regions may be formed within these interior corners. Furthermore, since the liquid-vapor boundary may be located within the individual pores, thin-film evaporation regions may also form away from the corners on the individual sides of each pore. Thus, even away from corners, each side (edge) of each grid block will also function as an evaporator site as evaporation is relatively higher via thin-film evaporation regions of a liquid as opposed to other "deeper" regions of the liquid. It can be appreciated from <FIG> that since the individual pores <NUM> and interior corners thereof are much more numerous than the individual sides of the ribs <NUM>, the number of thin-film evaporation regions that are formed within the two-phase thermodynamic system <NUM> will be substantially greater than would be formed within the "prior art" system <NUM> described in relation to <FIG>. It can further be appreciated that although those thin-film evaporation regions that would be formed against the ribs of the system <NUM> individually be larger than those formed within the individual pores <NUM>, the aggregate area of the thin-film evaporation regions formed against the ribs of the system <NUM> will be substantially less than the aggregate are of the thin-film evaporation regions formed within the porous microstructure sheet <NUM>.

It can be appreciated from <FIG> that in embodiments in which the porous microstructure sheet <NUM> is disposed over the top of ribs <NUM> that form channels <NUM>, the width and/or height of the individual pores may be substantially less than the width of the individual channels <NUM>. In some implementations, the individual channels <NUM> span a width that is at least twice as long as the width of the individual pores <NUM>. As a specific but nonlimiting example, the individual pores <NUM> may be substantially square pores that sides that span <NUM> microns (or less) while the individual channels <NUM> may span <NUM> microns (or more). As another specific but nonlimiting example, the individual pores <NUM> may span <NUM> microns (or less) while the individual channels <NUM> may span <NUM> microns (or more). As another specific but nonlimiting example, the individual pores <NUM> may span <NUM> microns (or less) while the individual channels <NUM> may span <NUM> microns (or more). As another specific but nonlimiting example, the individual pores <NUM> may span <NUM> microns (or less) while the individual channels <NUM> may span <NUM> microns (or more).

In some embodiments, the porous microstructure sheet illustrated in <FIG> is formed by etching the individual pores <NUM> into a metallic foil sheet. It can be appreciated that various dry etching techniques facilitate the formation of microstructures having relatively high aspect ratio. For example, various dry etching techniques enable the formation of deep "pin" holes type microstructures. An exemplary such dry-etching technique may include, but is not limited to, reactive-ion etching which is commonly used in microfabrication. In addition to dry etching techniques, any other manufacturing technique that is suitable for forming the plurality of pores <NUM> into a metallic foil may also be deployed to form the porous microstructure sheet <NUM>.

Turning now to <FIG>, illustrated is top view of an exemplary two-phase thermodynamic system <NUM> that includes a porous microstructure sheet <NUM> that is formed by a plurality of metallic fibers <NUM>. More specifically, the porous microstructure sheet <NUM> is formed by weaving the plurality of metallic fibers <NUM> together so that individual metallic fibers <NUM> are interconnected with other metallic fibers <NUM> to form a plurality of pores <NUM>. In various embodiments, the "weave" of metallic fibers <NUM> includes multiple different arrays of metallic fibers <NUM> that extend in multiple different directions. In the specific but nonlimiting example illustrated in <FIG>, the "weave" of metallic fibers <NUM> that makes up the porous microstructure sheet <NUM> includes a first array of metallic fibers <NUM> that extend in a first direction (illustrated vertically) and a second array of metallic fibers that extend in a second direction (illustrated horizontally). In this example, individual pores <NUM> are formed by the weaved combination of four individual metallic fibers <NUM>.

It can be appreciated that in <FIG> a portion of the wall <NUM> is cut-away to reveal an inner portion of the two-phase thermodynamic system <NUM>. It can further be appreciated that a relatively smaller portion of the porous microstructure sheet <NUM> is also cut-away to reveal an unobstructed top view of a channel-type wicking structure that is formed by a plurality of ribs <NUM>. Thus, the two-phase thermodynamic system <NUM> is generally similar to the two-phase thermodynamic system <NUM> with that the porous microstructure sheet <NUM> is formed from a "weave" of interconnected metallic fibers <NUM> whereas the porous microstructure sheet <NUM> is formed by cutting, stamping, etching, or otherwise adding pores to a metallic foil sheet.

<FIG> is a side view of the two-phase thermodynamic system <NUM> of that further illustrates the individual metallic fibers <NUM> being woven together. The side view of <FIG> is taken at a cross section that corresponds to a first metallic fiber <NUM>(<NUM>). In the illustrated example, the first metallic fiber <NUM>(<NUM>) is woven underneath a second metallic fiber <NUM>(<NUM>), and then over the top of a third metallic fiber <NUM>(<NUM>), and then underneath a fourth metallic fibers, and so on. In some embodiments, the individual metallic fibers <NUM> of the porous microfiber sheet <NUM> are interconnected at predetermined and repeatable intervals. As a specific but non-limiting example, the individual metallic fibers <NUM> may be interconnected at predefined and repeatable spacings.

It can be appreciated that in embodiments in which a porous microstructure sheet (e.g., <NUM> and/or <NUM>) is disposed directly over a channel-type wicking structure, some portion of the working fluid may become trapped between the top of the individual ribs <NUM> and the porous microstructure sheet. This results in this "trapped" portion of the working fluid becoming inactive (e.g., "dead") in the sense that it ceases to participate in the heat dissipation action that is desired of the two-phase thermodynamic system. In order to mitigate this unfortunate phenomenon, in various embodiments of the two-phase thermodynamic system disclosed herein the porous microstructure sheet is welded to the top of the individual ribs <NUM>. It can be appreciated that by welding the porous microstructure sheet to the "shoulder" (e.g., top) of the individual ribs, the amount of the working fluid that becomes "dead" (e.g., trapped) may be minimized. In this way, a smaller amount of the bi-phase fluid will be needed to achieve a desired amount of heat dissipation ability.

Turning now to <FIG>, illustrated is an enlarged view of an exemplary porous microstructure sheet <NUM> in which the individual pores <NUM> are rectangular in shape and in which thin-film evaporation regions of the working fluid are shown to be formed within the interior corners of the individual pores. As illustrated, the individual pores <NUM> are arranged according to an ordered grid of pores (e.g., an arrangement of pores having rows or pores and columns of pores). The exemplary microporous microstructure sheet <NUM> is illustrated as being disposed at a liquid-vapor boundary of the working fluid. Specifically, a liquid fraction of the working fluid is shown as being disposed underneath the porous microstructure sheet <NUM> and above an outer wall. In this way, disposed underneath the porous microstructure sheet <NUM> is a liquid flow path through which the illustrated liquid fraction of the working fluid is able to return from the condenser region (not shown in <FIG>) to the evaporator region (also not shown in <FIG>). It should be appreciated that <FIG> omits one or more outer walls which form the cavity and that these outer walls are omitted in order to provide an adequate and uncluttered view of the thin-film evaporation sites being formed within the porous microstructure sheet.

As further illustrated, a small portion of the liquid fraction of the working fluid collects within the individual corners of the individual pores <NUM>. It will be appreciated by those skilled in the art that the liquid fraction will form thin-film evaporation regions within the corners. Thus, under conditions in which the liquid-vapor boundary is disposed within the individual pores of the porous microstructure sheet <NUM>, thin-film evaporation regions may form at each of the interior corners of the pores. For individual pores as illustrated that have four interior corners, at least four thin-film evaporation regions will form within each of the individual pores. Furthermore, the size of the individual pores may be specifically sized such that a substantial fraction of the liquid-vapor boundary is formed into a "thin-film" in the sense that that fraction has a relatively lower thermal resistance than other fractions of the liquid-vapor boundary. In this way, by spacing the individual pores closely together with a relatively small separation between pores (e.g., <NUM> microns, <NUM> microns, etc.), the aggregate area of the liquid-vapor boundary that is induced to forming into a thin-film may be substantially increased over conventional vapor chambers and heat pipes. It can further be appreciated that the relatively small size of the thin-film evaporation regions formed within the porous microstructure sheets described herein will have increased (e.g., smaller radii) meniscus curvatures as compared to conventional vapor chambers. The increased meniscus curvatures will, in turn, will increase the resistance of most evaporation sites to dry-out at higher heat-power through-put levels (especially when transient).

Claim 1:
A thermodynamic system, comprising:
one or more walls (<NUM>) forming a sealed cavity (<NUM>) that contains a bi-phase fluid, the bi-phase fluid having at least a vapor fraction and a liquid fraction;
a plurality of ribs (<NUM>) forming channels (<NUM>) that extend from an evaporator region (<NUM>) of the sealed cavity (<NUM>) to a condenser region (<NUM>) of the sealed cavity (<NUM>), the evaporator region (<NUM>) for absorbing heat into the bi-phase fluid to convert the liquid fraction into the vapor fraction, the condenser region (<NUM>) for dissipating the heat out of the bi-phase fluid to convert the vapor fraction into the liquid fraction; and
a porous microstructure sheet (<NUM>; <NUM>) disposed over the plurality of ribs (<NUM>) to form a boundary between a liquid flow path, that is defined by the channels (<NUM>), and a vapor flow path, wherein the porous microstructure sheet (<NUM>; <NUM>) includes a plurality of pores (<NUM>; <NUM>) characterised in that
the plurality of pores (<NUM>; <NUM>) individually include an interior corner, that is formed by an intersection between two interior walls that intersect, wherein the liquid fraction forms a thin-film evaporation region within the interior corner at a liquid-vapor boundary between the liquid fraction and the vapor fraction, wherein the liquid-vapor boundary resides within individual pores of the porous microstructure sheet (<NUM>; <NUM>).