EP3973240B1

EP3973240B1 - Heat transfer device and method for manufacturing such a heat transfer device

Info

Publication number: EP3973240B1
Application number: EP19745036.4A
Authority: EP
Inventors: Leonid Leonardovich VASILIEV; Yun Tang; Sviataslau Alehavich FILATAU
Original assignee: Huawei Technologies Co Ltd
Current assignee: Huawei Technologies Co Ltd
Priority date: 2019-06-17
Filing date: 2019-06-17
Publication date: 2023-10-04
Anticipated expiration: 2039-06-17
Also published as: WO2020252555A8; EP3973240A1; WO2020252555A1

Description

TECHNICAL FIELD

The present invention relates to a heat transfer device for transferring heat generated by a heat source to a heat sink via a fluid working in evaporating-condensing cycles. The fluid evaporates to a vapor phase near the heat source and then condenses to a liquid phase near the heat sink, the vapor phase flowing toward the heat sink and the liquid phase flowing toward the heat source. Further, the present invention relates to a method for manufacturing such a heat transfer device.

BACKGROUND

A heat source like an electronic device in service, e.g. a chip or a circuit, generates heat that has to be transferred to a heat sink where the heat can be dissipated to the ambient. A heat transfer device, for example a heat pipe, comprises an evaporation region and a condensation region, a vapor channel extending between them, and a porous structure surrounding the vapor channel.
Such a heat transfer device may transfer heat on long distances via a fluid flowing in the vapor phase through the vapor channel, and flowing back to the evaporator in the liquid phase through the porous structure that generates a capillary pressure. In order to allow for mass production the porous structure of a conventional heat pipe has uniform pore size from the evaporation region to the condensation region, and the porous structure is uniformly distributed around the vapor channel.
The liquid pressure drop in the porous structure mostly depends on the permeability of the porous structure: A porous structure having big pores, hence big particles, only causes a low liquid pressure drop and can hence provide a large maximum transferrable heat load. But on the other hand, big particles can only generate low capillary forces, thus limiting the liquid flow rate in the porous structure. Thus, a conventional heat transfer device only provides a limited maximum transferrable heat load, because the pore or particle size of its porous structure is selected as a tradeoff between the pressure drop and the capillary pressure.
In theory a porous structure having a non-uniform distribution of pore or particle sizes could provide both a high capillary pressure around the evaporation region (finer particles) and a low liquid pressure drop around the condensation region (larger particles). Thus, a heat transfer device comprising such a porous structure could theoretically provide a large maximum transferrable heat load. For example, US4170262A describes a heat transfer device in which the porous structure has a continuously varying pore or particle size. However, such a heat transfer device is not suitable for mass production and its manufacturing is in any case very expensive and not reliable.
Besides, US2006162907A1 describes a heat transfer device having a porous structure comprising first and second segments respectively in an evaporation region and in a condensation region. This porous structure further comprises a third segment between the first and second segments. The pore size is smaller in the first, evaporation segment than in the third, intermediate segment and smaller in the third, intermediate segment than in the second, condensation segment. However, such a heat transfer device is not optimal, at least in view of the distribution of the heat through its different regions.
US 2006/219391 discloses a heat pipe having a three layer porous structure, in which segments of higher and lower porosity alternate.

SUMMARY

In view of the above-mentioned problems and disadvantages, embodiments of the present invention aim to improve the current implementations. An objective is to provide a heat transfer device, which may efficiently transfer heat and which can reliably be manufactured in mass production.
The obj ective is achieved by the embodiment of the invention as described in the enclosed independent device claim. Advantageous implementations of the present invention are further defined in the dependent claims.
An aspect of the invention provides a heat transfer device according to claim 1.
In the aspect according to claim 1, the sequence of segments further include at least one intermediate segment extending between the first segment and the second segment, the at least one intermediate segment having a third effective pore size, the third effective pore size being larger than the first effective pore size.
Thus, the at least one intermediate segment enables manufacturing a heat transfer device that can be long and efficient since the third effective pore size enhances the maximum transferrable heat load, as it helps increase the capillary pressure while decreasing the liquid pressure drop.
In the aspect according to claim 1, the second effective pore size of the second segment is smaller than the third effective pore size of the at least one intermediate segment.
Thus, the sequence of segments makes it possible to decrease thermal resistance in the condensation region.
The fluid may form an internal working media with two phases (liquid phase, vapor phase). In a particular implementation form, the fluid may be water or any other evaporative liquid. When the heat transfer device is in service the fluid may evaporate as the heat-receiving outer surface receives heat from the heat source, i.e. from an electronic device. Evaporation of the fluid occurs in an evaporation region. The evaporated fluid may flow through the vapor chamber and toward the heat-emitting outer surface. The evaporated fluid may condense when the heat-emitting outer surface emits heat and hence dissipate it in the ambient. Condensation of the fluid occurs in a condensation region. The condensed fluid may flow through the porous structure back toward the heat-receiving outer surface.
Thanks to its smaller effective pore size the first segment may generate a large capillary pressure, in other words large capillary forces, while the second segment may cause only a low liquid pressure drop thanks to its larger effective pore size, which helps increase the maximum transferrable heat load. Such a heat transfer device may be manufactured in mass production since the porous structure has a relatively simple structure. Such a heat transfer device may have a relatively low thermal resistance.
The maximum possible capillary pressure P_cap can be estimated via the following equation: $P_{cap} = \frac{4 σ \cos θ}{d_{eff}}$
where:

σ is the surface tension of the fluid, in N/m;
θ is the contact angle between the fluid and the material of porous structure, in degree;
d_eff is the effective pore size, in m.

The effective pore size d_eff may be calculated as 0.8d_max , where d_max is the maximum pore diameter according to the method A of the standard "Standard Test Methods for Pore Size Characteristics of Membrane Filters by Bubble Point and Mean Flow Pore Test" ASTM F316 - 03(2011) (available at: https://www.astm.org/Standards/F316.htm); the maximum pore diameter dmax may be measured using the Advanced Capillary Flow Porometer iPORE 1200 provided by company Porous Materials, Inc. (available at: http://www.pmiapp.com/products/advanced-capillarv-flow-porometer).
In a particular implementation form, the heat transfer device may operate in a so-called anti-gravity configuration, in which the liquid phase flows vertically upwards, hence against the gravity, from the condensation region to the evaporation region. In another particular implementation form, the heat transfer device may operate in the opposite configuration, the evaporation region being located below the condensation region. In yet another particular implementation form, the heat transfer device may operate in a horizontal or inclined orientation.
In a particular implementation form, the porous structure in the condensation region may include two or more segments having different effective pore sizes, which effective pore sizes decrease in a direction going from the condensation region to the evaporation region.
In a particular implementation form, the porous structure in the evaporation region may include two or more segments having different effective pore sizes, which effective pore sizes decrease in a direction going from the condensation region to the evaporation region.
In an implementation form of the aspect, the first segment may comprise a first sintered material formed of sintered particles having a first average particle size and the second segment may comprise a second sintered material formed of sintered particles having a second average particle size.
Thus, the first and second sintered materials allow for a simplified mass production of the heat transfer device.
In a particular implementation form, the particles having the first average particle size may be selected within the group consisting of: copper powder, copper meshes, aluminum powder, aluminum meshes, nickel powder, nickel meshes, steel powder, steel meshes, titanium powder, titanium meshes, ceramics, and polymers. In a particular implementation form, the particles having the second average particle size may be selected within the group consisting of: copper powder, copper meshes, aluminum powder, aluminum meshes, nickel powder, nickel meshes, steel powder, steel meshes, titanium powder, titanium meshes, ceramics, and polymers. In a particular implementation form, the copper powder may be of dendritic shape or of irregular shape.
In a particular implementation form, the at least one intermediate segment may define a transport region for transporting the liquid under capillary pressure through the porous structure from the condensation region to the evaporation region.
In an implementation form of the aspect, the number of intermediate segments may range from 1 to 5, each intermediate segment having a respective effective pore size, the intermediate segments being sequentially arranged along a longitudinal direction extending from the condensation region toward the evaporation region such that the effective pore sizes decrease stepwise from one intermediate segment to the next intermediate segment in the sequence of segments from the condensation region toward the evaporation region.
Thus, the or each intermediate segment enables manufacturing a longer yet efficient heat transfer device. For example, the number of intermediate segments may range from 2 to 5.
In an implementation form of the aspect, a third cross-sectional area of the vapor channel in the at least one intermediate segment may be smaller than at least one of: i) a first cross-sectional area of the vapor channel in the first segment, and ii) a second cross-sectional area of the vapor channel in the second segment.
Thus, due to the different cross-sectional areas, the total pressure drop generated in the porous structure may be comparable to the total pressure drop generated in a porous structure having a constant cross-sectional area, while the thermal resistance may be smaller as the temperature difference may be decreased.
In an implementation form of the aspect, the number of spaces having different cross-sectional areas in the vapor channel may differ from the number of segments having different effective pore sizes in the sequence of segments.
In an implementation form of the aspect, the evaporation region may be delimited i) by a heat-receiving side wall configured to be thermally coupled to the heat source and ii) by an opposite side wall located opposite the heat-receiving side wall with respect to a longitudinal direction extending from the condensation region toward the evaporation region, the porous structure being thinner at the heat-receiving side wall than at the opposite side wall.
Thus, the thermal resistance of the porous structure may be reduced near the heat source, which may increase the maximum transferrable heat load.
In an implementation form of the aspect, the vapor channel may extend closer to the heat-receiving side wall than to the opposite side wall.
Thus, the porous structure may be thinner at the heat-receiving side wall than at the opposite side wall.
In an implementation form of the aspect, the condensation region may be delimited i) by a heat-dissipating side wall configured to be thermally coupled to the heat sink and ii) by a facing side wall facing the heat-dissipating side wall with respect to a longitudinal direction extending from the condensation region toward the evaporation region, the porous structure being thinner at the heat-dissipating side wall than at the facing side wall.
Thus, the thermal resistance of the porous structure may be reduced near the heat sink, which may increase the maximum transferrable heat load.
In an implementation form of the aspect, the vapor channel may extend closer to the heat-dissipating side wall than to the facing side wall.
Thus, the porous structure may be thinner at the heat-dissipating side wall than at the facing side wall.
In an implementation form of the aspect, the vapor channel may be inclined with respect to a longitudinal direction extending from the condensation region toward the evaporation region.
Thus, in case the heat source and the heat sink are arranged on opposite sides with respect to a longitudinal direction extending from the condensation region to the evaporation region, the porous structure may be thinner near the heat source and near the heat sink, which may increase the maximum transferrable heat load.
In an implementation form of the aspect, at least one segment of the sequence of segments may have a main portion and a boundary portion, the boundary portion being closer to an interface with the consecutive segment in the sequence of segments than the main portion, the cross-sectional area of the porous structure in the boundary portion being larger than the cross-sectional area of the porous structure in the main portion.
Thus, a larger cross-sectional area of the porous structure in the boundary portion may decrease the local pressure drop in the liquid flow.
In an implementation form of the aspect, the porous structure may have at least one interface that is arranged between two consecutive segments in the sequence of segments and that extends obliquely to a longitudinal direction extending from the condensation region toward the evaporation region.
Thus, such an obliquely extending interface may locally increase the cross-sectional area of the porous structure, which may decrease the local pressure drop in the liquid flow.
In a particular implementation form, the interface or each interface between two consecutive segments may be planar.
In an implementation form of the aspect, the cross-sectional area of the porous structure in the first segment may range from 60% to 450% of the cross-sectional area of the vapor channel, and wherein the cross-sectional area of the porous structure in the second segment may range from 60% to 450% of the cross-sectional area of the vapor channel.
In an implementation form of the aspect, a first length of the first segment may range from 50% to 200% of a length of the evaporation region, and wherein a second length of the second segment may range from 50% to 300% of a length of the condensation region.
In an implementation form of the aspect, an electronic assembly may comprise an heat source, for example an electronic device, a heat sink, and the heat transfer device of any one of the preceding claims, wherein the evaporation region may be in thermal contact with the heat source and the condensation region is in thermal contact with the heat sink.
The electronic assembly may comprise at least one heat source like an electronic component that generates heat when functioning. The electronic assembly may comprise several heat sources. The heat sink may comprise at least one heat dissipating element that dissipates heat in the ambient or surrounding environment. In a particular implementation form, the heat sink may comprise several heat dissipating elements.
In a particular implementation form, the heat transfer device may further comprise walls arranged to enclose the evaporation region, the condensation region, the porous structure and the vapor channel, the porous structure being arranged around the vapor channel.
In a particular implementation form, the heat transfer device may be elongated at least along a longitudinal direction extending between the evaporation region and the condensation region. In a particular implementation form, the heat transfer device may generally have the shape of a cylinder having a circular or an oblong cross-section. Thus, the heat transfer device may form a heat pipe. Alternatively, the heat transfer device may generally have the shape of a plate having, for example, a rectangular outline.
Further, the objective is achieved by the embodiment of the invention as described in the enclosed independent method claim. Advantageous implementations of the present invention are further defined in the dependent claims.
An aspect of the invention provides a method according to claim 16 for manufacturing a heat transfer device.
Thus, such a method make it possible to manufacture the heat transfer device in mass production with a reduced cost and enhanced reliability.
In an implementation form of the aspect, the method further comprises:

placing the particles having the first average particle size in a first place in a housing,
placing the particles having the second average particle size in a second place in the housing, and
subjecting the particles having the first average particle size and the particles having the second average particle size to a sintering process while they are placed in the first place and in the second place in the housing.

In a particular implementation form, the first place corresponds to the first segment and the second place corresponds to the second segment.
In a particular implementation form, the sintering process may comprise: providing an atmosphere containing at least one of nitrogen gas and argon gas and a sintering temperature ranging from 860 degrees Celsius to 890 degrees Celsius. The sintering process may have a duration in the range of 1.2 h to 2.0 h, advantageously in the range of 1.5 h to 2.0 h.
In an implementation form of the aspect, the method further comprises:

a porous structure extending from the condensation region to the evaporation region such that condensed fluid may flow from the condensation region to the evaporation region through the porous structure by action of capillary forces,
- wherein the porous structure comprises a sequence of segments including at least i) a first segment extending in the evaporation region and ii) a second segment extending in the condensation region,
- wherein the method comprises:
  - producing the first segment by sintering particles having a first average particle size, and
  - producing the second segment by sintering particles having a second average particle size, the second average particle size being larger than the first average particle size.

Thus, such a method make it possible to manufacture the heat transfer device in mass production with a reduced cost and enhanced reliability.
In an implementation form of the aspect, the method further comprises:

In a particular implementation form, the first place corresponds to the first segment and the second place corresponds to the second segment.
In a particular implementation form, the sintering process may comprise: providing an atmosphere containing at least one of nitrogen gas and argon as and a sintering temperature ranging from 860 degrees Celsius to 890 degrees Celsius. The sintering process may have a duration in the range of 1.2 h to 2.0 h, advantageously in the range of 1 .5 h to 2.0 h.
In an implementation form of the aspect, the method further comprises:

before the sintering process: placing a filling material in a space corresponding to the vapor channel; and
after the sintering process: removing the filling material.

In a particular implementation form, the filling material is a core pin.
In an implementation form of the aspect, the first average particle size is comprised in the range of 40 urn to 290 µm, advantageously of 40 µm to 75 µm, and wherein the second average particle size is comprised in the range of 50 µm to 300 µm, advantageously of 200 µm to 300 µm.
The average particle sizes (first, second) of filling material, for each one of the first and second segments, may be calculated as d_p,av in the following equation: $d_{p, av} = d_{p, \min} d_{p, \max} \sqrt{\frac{2}{d_{p, \max}^{2} + d_{p, \min}^{2}}}$
where d_p,min - minimum particle size of filling material, in m; d_p,max - maximum particle size of filling material, in m. Minimum and maximum particle sizes of filling material, for each one of the first and second segments, may be measured during sieve analysis according to standard ASTM C136 / C136M - 14 "Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates" (available at: https://www.astm.org/Standards/C136.htm); design, e.g. size selection, of a sieve shaker for measuring particle sizes may follow the specification ASTM E11-17, which is referred to in relation to Method A of standard ASTM C136 / C136M - 14; for example the sieve shaker "Gilson SS-8R" (available at: https://www.globalgilson.com/gilson-tapping-sieve-shakers) may be used for measuring particle sizes.
The first average particle size and the second average particle size arc measured before the particles forming the first and second sintered materials are sintered, hence before the sintering process.
It has to be noted that all devices, elements, units and means described in the present application could be implemented in any technically applicable combination of the implementation forms. All steps which are performed by the various entities described in the present application as well as the functionalities described to be performed by the various entities are intended to mean that the respective entity is adapted to or configured to perform the respective steps and functionalities. Even if, in the following description of specific embodiments, a specific functionality or step to be performed by external entities is not reflected in the description of a specific detailed element of that entity which performs that specific step or functionality, it should be clear for a skilled person that these methods and functionalities can be implemented in any technically applicable combination of the implementation forms.

BRIEF DESCRIPTION OF DRAWINGS

The above described aspects and implementation forms of the present invention will be explained in the following description of specific embodiments and aspects in relation to the enclosed drawings, in which

FIG. 1: is a schematic cross-sectional view, along plane I in FIG. 2, illustrating a heat transfer device according to a first embodiment.
FIG. 2: is a schematic cross-sectional view along plane II in FIG. 1.
FIG. 3: is a schematic cross-sectional view along plane III in FIG. 1.
FIG. 4: is a schematic cross-sectional view along plane IV in FIG. 1.
FIG. 5: is a schematic cross-sectional view, along plane V in FIG. 6, illustrating a heat transfer device according to a second embodiment.
FIG. 6: is a schematic cross-sectional view along plane VI in FIG. 5.
FIG. 7: is a schematic cross-sectional view along plane VII in FIG. 5.
FIG. 8: is a schematic cross-sectional view along plane VIII in FIG. 5.
FIG. 9: is a schematic cross-sectional view along plane IX in FIG. 5.
FIG. 10: is a schematic diagram illustrating changes of effective pore sizes along the heat transfer device of FIG. 5.
FIG. 11: is a schematic diagram similar to FIG. 10 and illustrating the effective pore size along a heat transfer device according to a third embodiment.
FIG. 12: is a schematic cross-sectional view, along plane XII in FIG. 13, illustrating a heat transfer device according to a fourth embodiment.
FIG. 13: is a schematic cross-sectional view along plane XIII in FIG. 12.
FIG. 14: is a schematic cross-sectional view along plane XIV in FIG. 12.
FIG. 15: is a schematic cross-sectional view along plane XV in FIG. 12.
FIG. 16: is a schematic cross-sectional view along plane XVI in FIG. 12.
FIG. 17: is a schematic diagram illustrating thicknesses of porous structure along the heat transfer device of FIG. 12.
FIG. 18: is a view similar to FIG. 12 illustrating a heat transfer device according to a fifth embodiment.
FIG. 19: is a view similar to FIG. 5 illustrating a heat transfer device according to a sixth embodiment.
FIG. 20: is a schematic cross-sectional view, along plane XX in FIG. 21, illustrating a heat transfer device according to a seventh embodiment.
FIG. 21: is a schematic cross-sectional view along plane XXI in FIG. 20.
FIG. 22: is a schematic cross-sectional view along plane XXII in FIG. 20.
FIG. 23: is a schematic cross-sectional view along plane XXIII in FIG. 20.
FIG. 24: is a schematic cross-sectional view along plane XXIV in FIG. 20.
FIG. 25: is a schematic cross-sectional view, along plane XXV in FIG. 26, illustrating a heat transfer device according to an eighth embodiment.
FIG. 26: is a schematic cross-sectional view along plane XXVI in FIG. 25.
FIG. 27: is a schematic cross-sectional view along plane XXVII in FIG. 25.
FIG. 28: is a schematic cross-sectional view along plane XXVIII in FIG. 25.
FIG. 29: is a schematic cross-sectional view along plane XXIX in FIG. 25.
FIG. 30: is a schematic, diagram illustrating thicknesses of porous structure along the heat transfer device of FIG. 25.
FIG. 31 to 38: are cross-sectional views illustrating several steps of a method according to an embodiment for manufacturing a heat transfer device.
FIG. 39: is a cross-sectional view illustrating a step of a method according to another embodiment for manufacturing a heat transfer device.
FIG. 40: are cross-sectional views illustrating several steps of a method according to yet another embodiment for manufacturing a heat transfer device.
FIG. 41: is a schematic diagram illustrating a technical effect of a heat transfer device according to an embodiment as compared to a conventional heat transfer device.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 to 4 illustrate a heat transfer device 1, according to a first embodiment, for transferring heat generated by a heat source 2 to a heat sink 4 via a fluid (not shown).
Heat transfer device 1 comprises an evaporation region 6 where a fluid in the liquid phase may evaporate. Evaporation region 6 corresponds to heat source 2 when heat transfer device 1 is assembled with heat source 2. Heat transfer device 1 comprises a condensation region 8 where the fluid in the vapor phase may condense. Condensation region 8 corresponds to heat sink 4 when the heat transfer device 1 is assembled with heat sink 4. Further, heat transfer device 1 may comprise a transport region 9 between evaporation region 6 and condensation region 8.
Heat transfer device 1 comprises a porous structure 10 extending from condensation region 8 to evaporation region 6 such that condensed fluid may flow from condensation region 8 to evaporation region 6 through porous structure 10.
Heat transfer device 1 comprises a vapor channel 12 extending from evaporation region 6 to the condensation region 8 such that evaporated fluid may flow from evaporation region 6 to condensation region 8 through vapor channel 12. Porous structure 10 may be arranged around vapor channel 12.
Heat transfer device 1 may be elongated along a longitudinal direction Z extending between evaporation region 6 and condensation region 8. Heat transfer device 1 may generally have the shape of a cylinder having an oblong basis across longitudinal direction Z.
Porous structure 10 may have the shape of a tube extending in longitudinal direction Z. Vapor channel 12 may extend along, i.e. parallel to or coincident with, longitudinal direction Z. Vapor channel 12 may have the same shape and the same cross section area all along heat transfer device 1. Vapor channel 12 may generally have the shape of a cylinder having an oblong basis across longitudinal direction Z.
Porous structure 10 comprises a sequence of segments including i) a first segment 20 extending in evaporation region 6 and ii) a second segment 22 extending in condensation region 8. First segment 20 has a first effective pore size and second segment 22 has a second effective pore size. The second effective pore size is larger than the first effective pore size. In the example of FIG. 1 to 4, the first size may be of about 30 µm and the second effective pore size may be of about 70 µm.
First segment 20 may comprise a first sintered material, e.g. copper, formed of sintered particles, which particles have a first average particle size of about 60 to 75 µm before being sintered. Second segment 22 may comprise a second sintered material, e.g. copper, formed of sintered particles, which particles have a second average particle size of about 200 to 300 µm before being sintered.
Heat transfer device 1 may comprise walls 14 arranged to enclose evaporation region 6, condensation region 8, porous structure 10 and vapor channel 12. In particular, heat transfer device 1 may comprise a top wall (not shown) arranged atop first segment 20 and a bottom wall (not shown) arranged beneath second segment 22, so as to hermetically contain the fluid. An inner, cylindrical surface of the walls of heat transfer device 1 may be covered by porous structure 10.
In the example of FIG. 1 to 4 evaporation region 6 may be about 25 mm long, condensation region 8 may be about 80 mm long, and transport region 9 may be about 80 mm long. First segment 20 may be about 40 mm long, second segment 22 may be about 195 mm long.
Heat transfer device 1 may have a length of 300 mm from top of first segment 20 to bottom of second segment 22. Heat transfer device 1 may have a width of 11 mm including the walls enclosing porous structure 10 as measured in a width direction X perpendicular to longitudinal direction Z. Vapor channel 12 may have a width W12 of 6 mm. Heat transfer device 1 may have a thickness of 3 mm including the walls enclosing porous structure 10 as measured in a thickness direction Y perpendicular to longitudinal direction Z and to width direction X. Porous structure 10 may have a thickness of about 0.85 mm.
Further, the sequence of segments may include a third segment 24, which is an intermediate segment located between the first segment 20 and the second segment 22. Third segment 24 has a third effective pore size that is larger than the first effective pore size (of first segment 20). Third segment 24 may be formed from particles having a size of about 100 to 150 µm. Third segment 24 may be about 65 mm long.
Third segment 24 may define a border region 24.20 with first segment 20, and a border region 24.22 with second segment 22. Border region 24.20 may be located in transport region 9. Likewise, border region 24.22 may be located in transport region 9. In other words, first segment 20 may extend partially in evaporation region 6 and partially in transport region 9, and second segment 20 may extend partially in condensation region 8 and partially in transport region 9. As particles or pores of different sizes are in contact at border regions 24.20 and 24.22 the flow conditions change there when heat transfer device 1 is in service.
When heat transfer device 1 is in service the smaller first effective pore size at first segment 20 may generate a relatively large capillary pressure, in other words relatively large capillary forces, for moving the liquid phase back to evaporation section 6. By contrast, the largest second effective pore size at second segment 22 and the large third second effective pore size at third segment 24 may only cause a relatively small liquid pressure drop in the liquid phase moving back to evaporation section 6. So second segment 22 and third segment 24 may have a higher permeability than first segment 20 due to their big pores located between their large particles.
In FIG. 1 to 4 heat transfer device 1 may form an electronic assembly together with an electronic device forming heat source 2, for example a chip, and heat sink 4. In the electronic assembly evaporation region 6 is in thermal contact with heat source 2 and condensation region 8 is in thermal contact with heat sink 4. In service heat source 2 may generate heat and heat sink 4 may be an element that dissipates heat in the ambient or surrounding environment. Heat transfer device 1 may operate in a so-called anti-gravity configuration, in which the liquid phase flows vertically upwards, hence against the gravity, from condensation region 8 to evaporation region 6.
FIG. 5 to 10 illustrate a heat transfer device 1 not within the scope of the invention. The afore-detailed description of FIG. 1 to 4 may be applied to FIG. 5 to 10, except for the hereinafter-mentioned noticeable differences. An element of heat transfer device 1 of FIG. 5 to 10 is given the same reference sign as an element having a similar structure or function in FIG. 1 to 4.
Heat transfer device 1 of FIG. 5 to 10 differs from heat transfer device I of FIG. 1 to 4 in that the sequence of segments of porous structure 10 further comprises a fourth segment 26, which is an intermediate segment located between second segment 22 and third segment 24. Fourth segment 26 may have a fourth effective pore size that is larger than the first effective pore size (of first segment 20).
The intermediate segments, third segment 24 and fourth segment 26, may have respective effective pore sizes. Thus, the third effective pore size is different from the fourth effective pore size. FIG. 10 illustrates the effective pore size EPS on the horizontal axis with respect to the length in longitudinal direction Z on the vertical axis. The intermediate segments, i.e. third segment 24 and fourth segment 26, may be sequentially arranged along longitudinal direction Z from condensation region 8 toward evaporation region 6 such that the effective pore sizes EPS decrease stepwise from one intermediate segment, herein fourth segment 26, to the next intermediate segment 24, herein third segment 24, in the sequence of segments from condensation region 8 toward evaporation region 6.
Like in FIG. 1 to 5 third segment 24 may define a border region 24.20 with first segment 20. By contrast to FIG. 1 to 5 third segment 24 is not in contact with second segment 22. Instead, fourth segment 26 may define a border region 26.22 with second segment 22. Also, fourth segment 26 may define a border region 26.24 with third segment 24.
Further, by contrast to FIG. 5 to 10, border region 24.20 may be located in evaporation region 6 in lieu of transport region 9. Likewise, border region 26.22 may be located in condensation region 8, in lieu of transport region 9. Besides, border region 26.24 may be located in transport region 9.
Besides, the dimensions of heat transfer device 1 of FIG. 5 to 10 may differ from the afore-mentioned dimensions of heat transfer device 1 of FIG. 1 to 4.
FIG. 11 illustrate a heat transfer device 1 according to a second embodiment. The afore-detailed description of FIG. 5 to 10 may be applied to FIG. 11, except for the hereinafter-mentioned noticeable differences. An element of heat transfer device 1 of FIG. 11 is given the same reference sign as an element having a similar structure or function in FIG. 5 to 10.
FIG. 11 illustrates the effective pore size EPS on the horizontal axis with respect to the length in longitudinal direction Z on the vertical axis. Heat transfer device 1 of FIG. 11 differs from heat transfer device 1 of FIG. 5 to 10 in that the fourth effective pore size of fourth segment 26 is larger than the first effective pore size of second segment 22. Likewise, the third effective pore size of third segment 24 is larger than the first effective pore size of second segment 22. Thus, each intermediate segment, 24 or 26, has a larger effective pore size than second segment 22.
The sequence of segments of FIG. 11 makes it possible to decrease thermal resistance in condensation region 8. Since the second average particle size in condensation region 8 is smaller than the third and fourth average particle sizes in transport region 9, the pressure drop in the liquid flow is increased in condensation region 8 with respect to say FIG. 10. But this increase is compensated by a decrease in the pressure drop along transport region 9 where the average particle size is large.
FIG. 12 to 17 illustrate a heat transfer device 1 according to a second embodiment. The afore-detailed description of FIG. 5 to 9 may be applied to FIG. 12 to 17, except for the hereinafter-mentioned noticeable differences. An element of heat transfer device 1 of FIG. 12 to 17 is given the same reference sign as an element having a similar structure or function in FIG. 5 to 9.
Heat transfer device 1 of FIG. 12 to 17 differs from heat transfer device 1 of FIG. 5 to 9 in that vapor channel 12. of FIG. 12 to 17 may include three spaces of different cross-sectional areas along longitudinal direction Z:

a first space 12.20 extending approximately along first segment 20,
a second space 12.22 extending approximately along second segment 20, and
a third space 12.24 extending approximately along third, intermediate segment 24, hence between first space 12,20 and second space 12.22.

As visible on FIG. 12 first space 12.20 extends across border region 24.20 and second space 12.22 extends across border region 26.22.
The cross-sectional area of third space 12.24 may be smaller than the cross-sectional area of first space 12.20 and smaller than the cross-sectional area of second space 12.22. The cross-sectional area of first space 12.20 may be similar or identical to the cross-sectional area of second space 12.22. For example, a ratio between the cross-sectional areas of third space 12.24 and of first space 12.20 or of second space 12.22 may range between 20% and 50%.
FIG. 17 illustrates the thickness T of porous structure 10 on the horizontal axis with respect to the length in longitudinal direction Z on the vertical axis. FIG. 17 illustrates the different thicknesses of porous structure 10 in correspondence to first space 12.20, second space 12.22 and third space 12.24. The different thicknesses of porous structure 10 correspond to different cross-sectional areas of vapor channel 12 in first space 12.20, second space 12.22 and third space 12.24.
Correspondingly porous structure 10 may have different cross-sectional areas along first space 12.20, second space 12.22 and third space 12.24, the cross-sectional area along third space 12.24 being larger than the cross-sectional area along first space 12.20 and larger than the cross-sectional area along second space 12.22. In other words, porous structure 10 may be thicker in transport region 9 than in condensation region 8 and in evaporation region 6.
The number of spaces, i.e. three, having different cross-sectional areas in vapor channel 12 differs from, and herein is smaller than, the number of segments, i.e. four, having different effective pore sizes in the sequence of segments of porous structure 10.
When heat transfer device 1 is in service the smaller cross-sectional areas of porous structure 10 at first space 12.20 and at second space 12.22 cause a reduced thermal resistance as compared to the design of FIG. 5 to 9. Due to the smaller cross-sectional areas a pressure drop in the liquid flow through porous structure 10 at first space 12.20 and at second space 12.22 is increased as compared the design of FIG. 5 to 9, but this increase in the pressure drop is compensated by the decrease of pressure drop in the liquid flow at third space 12.24 because of its larger cross-sectional area.
Thus, the total pressure drop generated in porous structure 10 may be similar to the design of FIG. 5 to 9, while the thermal resistance may be smaller as the temperature difference e.g. along evaporation region 6 may be decreased.
FIG. 18 illustrate a heat transfer device 1 according to a second embodiment. The afore-detailed description of FIG. 12 to 17 may be applied to FIG. 18, except for the hereinafter-mentioned noticeable differences. An element of heat transfer device 1 of FIG. 18 is given the same reference sign as an element having a similar structure or function in FIG. 12 to 17.
Heat transfer device 1 of FIG. 18 differs from heat transfer device 1 of FIG. 12 to 17 in that first space 12.20 does not extend across border region 24.20, and in that second space 12.22 does not extend across border region 26.22. By contrast, in FIG. 18, first space 12.20 extends only within first segment 20 and second space 12.22 extends only within second segment 22.
Heat transfer device 1 of FIG. 18 also differs from heat transfer device 1 of FIG. 12 to 17 in that first segment 20 of the sequence of segments has a main portion and a boundary portion, the boundary portion being closer to an interface with the consecutive segment, herein third segment 24, in the sequence of segments than the main portion. The main portion extends along first space 12.20, while the boundary portion corresponds to a downstream end of third space 12.24.
The cross-sectional area of porous structure 10 in the boundary portion may be larger than the cross-sectional area of porous structure 10 in the main portion. Likewise, second segment 22 is configured with a main portion and a boundary portion of larger cross-sectional area than its main portion, the boundary portion of second segment 22 being closer to the border region 26.22 than the main portion of second segment 22.
As visible when comparing FIG. 12 and 18 the lengths of first segment 20, second segment 22, third segment 24 and fourth segment 26 are similar in FIG. 18 as in FIG. 12. Conversely, first space 12.20 and second space 12.22 of vapor channel 12 in FIG. 18 are shorter, along longitudinal direction Z, than in the design of FIG. 12. So porous structure 10 has a large cross-sectional area at border regions 24.20 and 26.22. Thus, a contact area between first segment 20 and third segment 24 is larger in FIG. 18 than in FIG. 12. Likewise, a contact area between second segment 22 and fourth segment 26 is larger in FIG. 18 than in FIG. 12.
When heat transfer device 1 is in service the average velocity of liquid flow near border regions 24.20 and 26.22 is reduced, thus reducing pressure drop, which is proportional to the square of the velocity. Further, a large contact area between at border regions 24.20 and 26.22 limits the risk of insufficient contact between particles of different sizes in the sequence of segments.
In a non-illustrated alternative embodiment the vapor channel may have only two spaces of different cross-sectional areas. For example, the vapor channel may have a larger cross-sectional area at the first segment and a smaller cross-sectional area at the second segment, or vice-versa.
In a non-illustrated alternative embodiment the vapor channel may generally have a frustoconical shape tapering from condensation region toward evaporation region, or tapering from evaporation region toward condensation region.
FIG. 19 illustrates a heat transfer device 1 according to a second embodiment. The afore-detailed description of FIG. 5 to 9 may be applied to FIG. 19, except for the hereinafter-mentioned noticeable differences. An element of heat transfer device 1 of FIG. 19 is given the same reference sign as an element having a similar structure or function in FIG. 5 to 9.
Heat transfer device 1 of FIG. 19 differs from heat transfer device 1 of FIG. 5 to 9 in that porous structure 10 has border regions 24.20, 26.22 and 26.24 between consecutive segments 20/24, 26/22, 24/26 in the sequence of segments define interfaces which extend obliquely to longitudinal direction Z. In the plane of FIG. 19 the obliquely extending interfaces are planar, parallel and each of them forms an angle of approximately 45 degrees with longitudinal direction Z.
Thus, obliquely extending planar interfaces offer an enlarged contact area between consecutive segments 20/24, 24/26, 26/22 of the sequence of segments. Such an enlarged contact area limits the risk of insufficient contact between particles of different sizes in the sequence of segments.
FIG. 20 to 24 illustrates a heat transfer device 1 according to a second embodiment. The afore-detailed description of FIG. 5 to 9 may be applied to FIG. 20 to 24, except for the hereinafter-mentioned noticeable differences. An element of heat transfer device 1 of FIG. 20 to 24 is given the same reference sign as an element having a similar structure or function in FIG. 5 to 9.
Like in the embodiment of FIG. 5 to 9 vapor channel 12 generally has the shape of a cylinder having an oblong basis across longitudinal direction Z. Unlike in the embodiment of FIG. 5 to 9 vapor channel 12 extends closer to the side of heat transfer device 1 where heat source 2 and heat sink 4 are arranged than to the opposite side of heat transfer device 1.
Evaporation region 6 is delimited i) by a heat-receiving side wall 6.2 configured to be thermally coupled to heat source 2 and ii) by an opposite side wall 6.3 located opposite heat-receiving side wall 6.2 with respect to longitudinal direction Z. Likewise, condensation region 8 is delimited i) by a heat-dissipating side wall 8.4 configured to be thermally coupled to heat sink 4, and ii) by a facing side wall 8.5 facing heat-dissipating side wall 8.4 with respect to longitudinal direction Z.
Heat transfer device 1 of FIG. 20 to 24 differs from heat transfer device 1 of FIG. 5 to 9 in that: i) vapor channel 12 extends closer to heat-receiving side wall 6.2 than to opposite side wall 6.3, and in that ii) vapor channel 12 extends closer to heat-dissipating side wall 8.4 than to facing side wall 8.5.
Thus, heat transfer device 1 of FIG. 20 to 24 differs from heat transfer device 1 of FIG. 5 to 9 in that: i) porous structure 10 is thinner at heat-receiving side wall 6.2 than at opposite side wall 6.3, and in that ii) porous structure 10 is thinner at heat-dissipating side wall 8.4 than at facing side wall 8.5. In other words, the thickness of porous structure 10 on heat-receiving side wall 6.2 is smaller than the thickness of porous structure 10 on opposite side wall 6.3 as particularly visible on FIG. 21. Likewise, the thickness of porous structure 10 on heat-dissipating side wall 8.4 is smaller than the thickness of porous structure 10 on facing side wall 8.5 as particularly visible on FIG. 24.
Thus, thermal resistance of porous structure 10 may be reduced near heat source 2 and near heat sink 4, which may increase the maximum transferrable heat load.
Besides, the respective cross-sectional areas of first segment 20, second segment 22 and of vapor channel 12 may be similar in the configuration of FIG. 20 to 24 as in the configuration of FIG. 5 to 9, thus generating a similar pressure drop in the liquid flow.
FIG. 25 to 30 illustrates a heat transfer device 1 according to a second embodiment. The afore-detailed description of FIG. 20 to 24 may be applied to FIG. 25 to 30, except for the hereinafter-mentioned noticeable differences. An element of heat transfer device 1 of FIG. 25 to 30 is given the same reference sign as an element having a similar structure or function in FIG. 20 to 24.
Heat transfer device 1 of FIG. 25 to 30 differs from heat transfer device 1 of FIG. 20 to 24 in that heat source 2 and heat sink 4 are arranged on opposite sides of heat transfer device 1 with respect to longitudinal direction Z.
Further, heat transfer device 1 of FIG. 25 to 30 differs from heat transfer device 1 of FIG. 20 to 24 in that vapor channel 12 is inclined with respect to longitudinal direction Z extending from condensation region 8 toward the evaporation region 6. In addition, vapor channel 12 extends closer to heat-receiving side wall 6.2 than to opposite side wall 6.3, and in that ii) vapor channel 12 extends closer to heat-dissipating side wall 8.4 than to facing side wall 8.5.
FIG. 30 illustrates the thickness T of porous structure 10 on the horizontal axis with respect to the length in longitudinal direction Z on the vertical axis. As illustrated in FIG. 30 a thickness of porous structure 10 in first segment 20 decreases linearly along longitudinal direction Z. and toward second segment 22. Similarly, a thickness of porous structure 10 in second segment 22 increases linearly along longitudinal direction Z and toward first segment 20.
In the configuration of FIG. 25 to 30 the thermal resistance of porous structure 10 can be reduced near heat source 2 and near heat sink 4, thus enhancing the heat transfer.
Besides, the respective cross-sectional areas of first segment 20, second segment 22 and of vapor channel 12 may be substantially equal in the configuration of FIG. 20 to 24 as in the configuration of FIG. 5 to 9, thus generating a similar pressure drop in the liquid flow.
FIG. 31 to 38 illustrate several steps of a method according to an embodiment for manufacturing a heat transfer device which is akin to heat transfer device 1 of FIG. 5 to 9.
As shown in FIG. 38 a heat transfer device 1 manufactured through this method comprises:

an evaporation region 6 where the fluid (not shown) may evaporate,
a condensation region 8 where the fluid may condense,
a vapor channel 12 extending from evaporation region 6 to the condensation region 8 such that evaporated fluid may flow from evaporation region 6 to condensation region 8 through vapor channel 12, and
a porous structure 10 extending from condensation region 8 to the evaporation region 6 such that condensed fluid may flow from condensation region 8 to evaporation region 6 through porous structure 10.

Porous structure 10 comprises a sequence of segments including i) a first segment 20 extending in evaporation region 6, ii) a second segment 22 extending in condensation region 8, iii) a third segment 24 and a fourth segment 24, which are intermediate segments extending between first segment 20 and second segment 22.
The method of FIG. 31 to 37 for manufacturing heat transfer device 1 of FIG. 38 comprises:

producing first segment 20 by sintering particles having a first average particle size, and
producing second segment 22 by sintering particles having a second average particle size, the second average particle size being larger than the first average particle size.

Thus, a second effective pore size of second segment 22 may be larger than a first effective pore size of first segment 20.
Besides, the method of FIG. 31 to 37 may comprise:

providing a housing 30 (FIG. 31); the housing may be comprised of walls, which may form a metal envelope;
inserting a pin 32, or an equivalent filling material, into housing 30 (FIG. 32); pin 32 may obstruct a bottom port 33 while carrying out the method steps so that vapor channel 12 has an open bottom end; pin 32 may be formed of a core pin;
placing particles P22 having the second average particle size between pin 32 and housing 30; under action of gravity force the particles P22 may be charged on the bottom of housing 30, thus occupying a second place in housing 30;
placing particles P26 having a fourth average particle size between pin 32 and housing 30; under action of gravity force particles P26 may be charged on particles P22, thus occupying a fourth place in housing 30;
placing particles P24 having a third average particle size between pin 32 and housing 30; under action of gravity force particles P24 may be charged on particles P26, thus occupying a third place in housing 30;
placing particles P20 having a first average particle size between pin 32 and housing 30; under action of gravity force particles P20 may be charged on particles P24, thus occupying a first place; and
subjecting the particles P20, P22, P24, P26 to a sintering process while they are placed in housing 30.

The first, second, third and fourth places respectively correspond to first segment 20, second segment 22, third segment 24 and fourth segment 26 of heat transfer device 1. The step of placing particles may be repeated a number of times equal to the number of desired segments in the sequence of segments eventually composing the porous structure.
As an example the first average particle size (powder P20) may be comprised in the range of 40 µm to 290 µm, advantageously of 40 µm to 75 µm, and the second average particle size (powder P22) is comprised in the range of 50 µm to 300 µm, advantageously of 200 µm to 300 µm. The third and fourth particle sizes may be selected according to the details given in relation to FIG, 10 and 11.
When filling of powder particles (P22, P24, P26, (20) in housing 30 is performed step by step and starting from larger particles, the smaller particles of the consecutive segment, say P26, permeate into previous, larger particles, say P22, under the action of gravity force. Such powder permeation helps mix particles of different average particle sizes at the border regions, thus preventing the appearance of gaps between two consecutive segments. Mixing of particles at the border regions between different segments help create smoother transitions in the flow condition at the border regions, thus decreasing local pressure drop in the liquid flow.
The sintering process may comprise: providing an atmosphere containing at least one of nitrogen gas and argon gas and a sintering temperature ranging from 860 degrees Celsius to 890 degrees Celsius. The sintering process may have a duration in the range of 1.2 h to 2.0 11,
As afore-mentioned the method of FIG. 31 to 38 may comprise:

before the sintering process: placing pin 32, or an equivalent filling material, in a space corresponding to the vapor channel 12 to be left in porous structure 10; and
after the sintering process: removing pin 32, or an equivalent filling material.

Further, the method of FIG. 31 to 37 may comprise:

after completion of the sintering process, removing pin 2, thus defining vapor channel 12;
charging heat transfer device 1 with an evaporative fluid like water, thus providing an internal working fluid;
after charging heat transfer device 1, sealing, for example by welding, bottom end 34 and top end 36 of heat transfer device 1.

FIG. 39 illustrates a step of a method according to another embodiment for manufacturing a heat transfer device which is akin to heat transfer device 1 of FIG. 25 to 29. The afore-detailed description of FIG. 31 to 38 may be applied to FIG. 39. except for the hereinafter-mentioned noticeable differences.
The step illustrated on FIG. 39 may replace the step of FIG. 32 in the method of FIG. 31 to 38. The method of FIG. 39 differs from the method of FIG. 25 to 29 in that filling material 32 is placed in an inclined orientation with respect to housing 30. A longitudinal axis Z32 of filling material 32 is inclined with respect to a longitudinal direction Z of housing 30. Such an inclined orientation of filling material 32 enables for example to manufacture heat transfer device 1 of FIG. 25 to 29 with an oblique vapor channel 12 after completion of the previous steps as described in relation to FIG. 31 to 38.
FIG. 40 illustrates a step of a method according to another embodiment for manufacturing a heat transfer device which is akin to heat transfer device 1 of FIG. 39. The afore-detailed description of FIG. 39 may be applied to FIG. 40, except for the hereinafter-mentioned noticeable differences.
The method of FIG. 40 differs from the method of FIG. 39 in that filling material 32 includes a primary pin 32.1 and a secondary pin 32.2. Secondary pin 32.2 may be integral with primary pin 32.1, thus forming a combined pin. Secondary pin 32.2 may be arranged so as to obstruct port 33 of housing 30 when charging powder particles in housing 30.

EXAMPLE

The diagram of FIG. 41 illustrates Temperature difference (in degrees Celsius) as a function of heating power Q (in Watt) that is transferred:

i) along a heat transfer device 1 akin to the one of FIG. 1 (right curve in FIG. 41).

FIG. 41

heat transfer device

FIG. 1

transfer device

FIG. 1

The diagram of FIG. 41 shows that, beyond a heating power Q of 16 W, the conventional heat transfer device stops operating properly, because the Temperature difference increases rapidly (left curve). By contrast, the Temperature difference for heat transfer device 1 of FIG. 1 (right curve) does not change rapidly even when the heating power Q exceeds 35 W. Hence, heat transfer device 1 of FIG. 1 may transfer much more heat as compared to a conventional heat transfer device.
Similar results are obtained when the same heat transfer devices are tested operating in a horizontal orientation.
Therefore, a heat transfer device as described above may efficiently transfer heat, and a method as described above make it possible to manufacture such a heat transfer device in mass production with a reduced cost and an enhanced reliability.
The present invention has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed invention, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word "comprising" does not exclude other elements or steps and the indefinite article "a" or "an'^- does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.

Claims

A heat transfer device (1) for transferring heat generated by a heat source (2) to a heat sink (4) via a fluid, the heat transfer device (1) comprising:
- an evaporation region (6) where the fluid in the liquid phase may evaporate,

- a condensation region (8) where the fluid in the vapor phase may condense,

- a vapor channel (12) extending from the evaporation region (6) to the condensation region (8) such that evaporated fluid may flow from the evaporation region (6) to the condensation region (8) through the vapor channel (12), and

- a porous structure (10) extending from the condensation region (8) to the evaporation region (6) such that condensed fluid may flow from the condensation region (8) to the evaporation region (6) through the porous structure (10) by action of capillary forces,

wherein the porous structure (10) comprises a sequence of segments including at least i) a first segment (20) extending in the evaporation region (6) and ii) a second segment (22) extending in the condensation region (8),

the first segment (20) having a first effective pore size, the second segment (22) having a second effective pore size, the second effective pore size being larger than the first effective pore size, wherein the sequence of segments further includes at least one intermediate segment (24, 26) extending between the first segment (20) and the second segment (22), the at least one intermediate segment (24, 26) having a third effective pore size, the third effective pore size being larger than the first effective pore size;

characterized in that the second effective pore size of the second segment (22) is smaller than the third effective pore size of the at least one intermediate segment (24, 26).
The heat transfer device (1) of claim 1, wherein the first segment (20) comprises a first sintered material formed of sintered particles having a first average particle size and the second segment (22) comprises a second sintered material formed of sintered particles having a second average particle size.
The heat transfer device (1) of any of the preceding claims, wherein the number of intermediate segments (24, 26) ranges from 1 to 5, each intermediate segment (24, 26) having a respective effective pore size, the intermediate segments (24, 26) being sequentially arranged along a longitudinal direction (Z) extending from the condensation region (8) toward the evaporation region (6) such that the effective pore sizes decrease stepwise from one intermediate segment (24, 26) to the next intermediate segment (24, 26) in the sequence of segments from the condensation region (8) toward the evaporation region (6).
The heat transfer device (1) of any one of the preceding claims, wherein a third cross-sectional area of the vapor channel (12) in the at least one intermediate segment (24, 26) is smaller than at least one of: i) a first cross-sectional area of the vapor channel (12) in the first segment (20), and ii) a second cross-sectional area of the vapor channel (12) in the second segment (22).
The heat transfer device (1) of claim 4, wherein the number of spaces (12.20, 12.22, 12.24) having different cross-sectional areas in the vapor channel (12) differs from the number of segments (20, 22, 24, 26) having different effective pore sizes in the sequence of segments.
The heat transfer device (1) of any of the preceding claims, wherein the evaporation region (6) is delimited i) by a heat-receiving side wall (6.2) configured to be thermally coupled to the heat source (2) and ii) by an opposite side wall (6.3) located opposite the heat-receiving side wall (6.2) with respect to a longitudinal direction (Z) extending from the condensation region (8) toward the evaporation region (6), the porous structure (10) being thinner at the heat-receiving side wall (6.2) than at the opposite side wall (6.3).
The heat transfer device (1) of claim 6, wherein the vapor channel (12) extends closer to the heat-receiving side wall (6.2) than to the opposite side wall (6.3).
The heat transfer device (1) of any of the preceding claims, wherein the condensation region (8) is delimited i) by a heat-dissipating side wall (8.4) configured to be thermally coupled to the heat sink (4) and ii) by a facing side wall (8.5) facing the heat-dissipating side wall (8.4) with respect to a longitudinal direction (Z) extending from the condensation region (8) toward the evaporation region (6), the porous structure (10) being thinner at the heat-dissipating side wall (8.4) than at the facing side wall (8.5).
The heat transfer device (1) of claim 8, wherein the vapor channel (12) extends closer to the heat-dissipating side wall (8.4) than to the facing side wall (8.5).
The heat transfer device (1) of any of the preceding claims, wherein the vapor channel (12) is inclined with respect to a longitudinal direction (Z) extending from the condensation region (8) toward the evaporation region (6).
The heat transfer device (1) of any of the preceding claims, wherein at least one segment (20) of the sequence of segments has a main portion and a boundary portion, the boundary portion being closer to an interface with the consecutive segment in the sequence of segments than the main portion, the cross-sectional area of the porous structure (10) in the boundary portion being larger than the cross-sectional area of the porous structure (10) in the main portion.
The heat transfer device (1) of any of the preceding claims, wherein the porous structure (10) has at least one interface that is arranged between two consecutive segments in the sequence of segments and that extends obliquely to a longitudinal direction (Z) extending from the condensation region (8) toward the evaporation region (6).
The heat transfer device (1) of any of the preceding claims, wherein the cross-sectional area of the porous structure (10) in the first segment (20) ranges from 60% to 450% of the cross-sectional area of the vapor channel (12), and wherein the cross-sectional area of the porous structure (10) in the second segment (22) ranges from 60% to 450% of the cross-sectional area of the vapor channel (12).
The heat transfer device (1) of any of the preceding claims, wherein a first length of the first segment (20) ranges from 50% to 200% of a length of the evaporation region (6), and wherein a second length of the second segment (22) ranges from 50% to 300% of a length of the condensation region (8).
An electronic assembly comprising a heat source (2), a heat sink (4), and the heat transfer device (1) of any one of the preceding claims, wherein the evaporation region (6) is in thermal contact with the heat source (2) and the condensation region (8) is in thermal contact with the heat sink (4).
A method for manufacturing a heat transfer device (1) for transferring heat generated by a heat source (2) to a heat sink (4) via a fluid, wherein the heat transfer device (1) comprises:
- an evaporation region (6) where the fluid in the liquid phase may evaporate,

- a condensation region (8) where the fluid in the vapor phase may condense,

- a vapor channel (12) extending from the evaporation region (6) to the condensation region (8) such that evaporated fluid may flow from the evaporation region (6) to the condensation region (8) through the vapor channel (12), and

- a porous structure (10) extending from the condensation region (8) to the evaporation region (6) such that condensed fluid may flow from the condensation region (8) to the evaporation region (6) through the porous structure (10) by action of capillary forces,
wherein the porous structure (10) comprises a sequence of segments including at least i) a first segment (20) extending in the evaporation region (6) and ii) a second segment (22) extending in the condensation region (8),

wherein the sequence of segments further includes at least one intermediate segment (24, 26) extending between the first segment (20) and the second segment (22), the at least one intermediate segment (24, 26) having a third effective pore size, the third effective pore size being larger than the first effective pore size;

wherein the method comprises:
- producing the first segment (20) by sintering particles having a first average particle size, and

- producing the second segment (22) by sintering particles having a second average particle size, the second average particle size being larger than the first average particle size, characterized in that the second effective pore size of the second segment (22) is smaller than the third effective pore size of the at least one intermediate segment (24, 26).
The method of claim 16, further comprising:
- placing the particles having the first average particle size in a first place in a housing,

- placing the particles having the second average particle size in a second place in the housing, and

- subjecting the particles having the first average particle size and the particles having the second average particle size to a sintering process while they are placed in the first place and in the second place in the housing.
The method of claim 17, further comprising:
- before the sintering process: placing a filling material (32) in a space corresponding to the vapor channel (12); and

- after the sintering process: removing the filling material (32).
The method of any one of claims 16 to 18, wherein the first average particle size is comprised in the range of 40 µm to 290 µm, advantageously of 40 µm to 75 µm, and wherein the second average particle size is comprised in the range of 50 µm to 300 µm, advantageously of 200 µm to 300 µm.