WO2017089960A1 - Surfaces microstructurées pour un meilleur transfert de chaleur par changement de phase - Google Patents

Surfaces microstructurées pour un meilleur transfert de chaleur par changement de phase Download PDF

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Publication number
WO2017089960A1
WO2017089960A1 PCT/IB2016/057042 IB2016057042W WO2017089960A1 WO 2017089960 A1 WO2017089960 A1 WO 2017089960A1 IB 2016057042 W IB2016057042 W IB 2016057042W WO 2017089960 A1 WO2017089960 A1 WO 2017089960A1
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WO
WIPO (PCT)
Prior art keywords
substrate
particles
surface layer
voids
solvent
Prior art date
Application number
PCT/IB2016/057042
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English (en)
Inventor
Gideon J. Gouws
Original Assignee
Victoria Link Ltd
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Publication date
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Publication of WO2017089960A1 publication Critical patent/WO2017089960A1/fr

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
    • B22F3/10—Sintering only
    • B22F3/11—Making porous workpieces or articles
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
    • B22F3/22—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces for producing castings from a slip
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F7/00—Manufacture of composite layers, workpieces, or articles, comprising metallic powder, by sintering the powder, with or without compacting wherein at least one part is obtained by sintering or compression
    • B22F7/02—Manufacture of composite layers, workpieces, or articles, comprising metallic powder, by sintering the powder, with or without compacting wherein at least one part is obtained by sintering or compression of composite layers
    • B22F7/04—Manufacture of composite layers, workpieces, or articles, comprising metallic powder, by sintering the powder, with or without compacting wherein at least one part is obtained by sintering or compression of composite layers with one or more layers not made from powder, e.g. made from solid metal
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C24/00—Coating starting from inorganic powder
    • C23C24/08—Coating starting from inorganic powder by application of heat or pressure and heat
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F28—HEAT EXCHANGE IN GENERAL
    • F28D—HEAT-EXCHANGE APPARATUS, NOT PROVIDED FOR IN ANOTHER SUBCLASS, IN WHICH THE HEAT-EXCHANGE MEDIA DO NOT COME INTO DIRECT CONTACT
    • F28D15/00—Heat-exchange apparatus with the intermediate heat-transfer medium in closed tubes passing into or through the conduit walls ; Heat-exchange apparatus employing intermediate heat-transfer medium or bodies
    • F28D15/02—Heat-exchange apparatus with the intermediate heat-transfer medium in closed tubes passing into or through the conduit walls ; Heat-exchange apparatus employing intermediate heat-transfer medium or bodies in which the medium condenses and evaporates, e.g. heat pipes
    • F28D15/0233—Heat-exchange apparatus with the intermediate heat-transfer medium in closed tubes passing into or through the conduit walls ; Heat-exchange apparatus employing intermediate heat-transfer medium or bodies in which the medium condenses and evaporates, e.g. heat pipes the conduits having a particular shape, e.g. non-circular cross-section, annular
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F28—HEAT EXCHANGE IN GENERAL
    • F28D—HEAT-EXCHANGE APPARATUS, NOT PROVIDED FOR IN ANOTHER SUBCLASS, IN WHICH THE HEAT-EXCHANGE MEDIA DO NOT COME INTO DIRECT CONTACT
    • F28D15/00—Heat-exchange apparatus with the intermediate heat-transfer medium in closed tubes passing into or through the conduit walls ; Heat-exchange apparatus employing intermediate heat-transfer medium or bodies
    • F28D15/02—Heat-exchange apparatus with the intermediate heat-transfer medium in closed tubes passing into or through the conduit walls ; Heat-exchange apparatus employing intermediate heat-transfer medium or bodies in which the medium condenses and evaporates, e.g. heat pipes
    • F28D15/04—Heat-exchange apparatus with the intermediate heat-transfer medium in closed tubes passing into or through the conduit walls ; Heat-exchange apparatus employing intermediate heat-transfer medium or bodies in which the medium condenses and evaporates, e.g. heat pipes with tubes having a capillary structure
    • F28D15/046—Heat-exchange apparatus with the intermediate heat-transfer medium in closed tubes passing into or through the conduit walls ; Heat-exchange apparatus employing intermediate heat-transfer medium or bodies in which the medium condenses and evaporates, e.g. heat pipes with tubes having a capillary structure characterised by the material or the construction of the capillary structure
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F28—HEAT EXCHANGE IN GENERAL
    • F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
    • F28F13/00—Arrangements for modifying heat-transfer, e.g. increasing, decreasing
    • F28F13/18—Arrangements for modifying heat-transfer, e.g. increasing, decreasing by applying coatings, e.g. radiation-absorbing, radiation-reflecting; by surface treatment, e.g. polishing
    • F28F13/185—Heat-exchange surfaces provided with microstructures or with porous coatings
    • F28F13/187—Heat-exchange surfaces provided with microstructures or with porous coatings especially adapted for evaporator surfaces or condenser surfaces, e.g. with nucleation sites
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F28—HEAT EXCHANGE IN GENERAL
    • F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
    • F28F2255/00—Heat exchanger elements made of materials having special features or resulting from particular manufacturing processes
    • F28F2255/18—Heat exchanger elements made of materials having special features or resulting from particular manufacturing processes sintered

Definitions

  • the present disclosure relates generally to microstructured material, and more particularly to a thermally conductive substrate having formed thereon a microstructured surface layer of sintered metal surfaced nanoparticles providing a hierarchical void structure for facilitating two-phase heat transfer.
  • the present disclosure provides an article of manufacture including a thermally conductive substrate and a
  • microstructured surface layer of sintered particles on the substrate The sintered particles have outer metal surfaces binding the sintered particles together.
  • the sintered particles have a particle size less than 10 microns.
  • the microstructured surface layer has regions where the particles are concentrated alternated with void regions absent of particles.
  • the void regions have a hierarchical structure including: (i) small voids smaller than one micron; (ii) large voids larger than the small voids; and (iii) intermediate size voids having a size larger than the small voids and smaller than the large voids, the intermediate size voids being elongated and forming channels between walls of the particles.
  • the present disclosure provides a method of making a microstructured surface layer on a thermally conductive substrate.
  • the method includes: (a) making a suspension of particles in a solvent comprising a binder material, the particles having metal surfaces and the particles having a particle size of less than 10 microns; and then (b) coating the substrate with the suspension to form a surface layer on the substrate; and then (c) cooling the coated substrate below a freezing point of the solvent to induce segregation between the particles and the frozen solvent; and then removing the frozen solvent from the surface layer; and then (e) heating the coated substrate to sinter the particles together to form the microstructured surface layer on the substrate.
  • the present disclosure provides a method of two-phase heat transfer including evaporating a liquid at an evaporator to absorb heat from the evaporator and transforming the liquid to a vapor, and condensing the vapor at a condenser to release heat to the condenser and transforming the vapor back to the liquid.
  • the evaporator includes a microstructured surface layer of sintered particles on a thermally conductive substrate, as provided above.
  • the condenser also includes a microstructured surface layer of sintered particles on a thermally conductive substrate, as provided above.
  • FIG. 1 is a pictorial diagram of a microstructured surface layer on a substrate showing a hierarchical void structure at three different length scales.
  • FIGS. 2 to 5 are schematic diagrams showing a series of operations in a freeze casting process.
  • FIG. 6 is a flowchart of a method of fabricating the microstructured surface layer introduced in FIG. 1 .
  • FIGS. 7 to 12 are schematic diagrams showing a series of operations in a lithographic process.
  • FIGS. 13 to 17 are schematic diagrams showing a series of operations in a molding process.
  • FIG. 18 is a cross-section view of a vapor chamber spreading heat from a central processing unit (CPU) to a heat sink.
  • CPU central processing unit
  • FIG.19 is a top view of a microstructured surface on a lower inner wall of the vapor chamber of FIG. 18.
  • FIG. 20 is a cross-section view of a vapor chamber including
  • microstructured surfaces of an evaporator and a condenser having free-standing parallel-spaced interdigitated fins.
  • FIG. 21 is a high magnification scanning electron micrograph of a microstructured surface.
  • FIG. 22 is a medium magnification scanning electron micrograph of the microstructured surface introduced in FIG. 21 .
  • FIG. 23 is a low magnification optical micrograph of a microstructured surface as formed by a molding or templating process in order to create large voids or features in this surface layer.
  • FIG. 24 is a graph of heat flux as a function of excess temperature for a bare copper substrate and for copper substrates having silver microstuctured surface layers of four different silver particle loadings.
  • FIG. 25 is a photograph of boiling on the surface of a copper substrate that does not have a microstructured surface layer.
  • FIG. 26 is a photograph of boiling on a silver microstructured surface layer on a copper substrate.
  • FIG. 27 is a photograph showing the wettability that can be obtained on a strongly hydrophilic surface, which can be used on the evaporator side for evaporation and fluid transport.
  • FIG. 28 is a photograph showing the wettability that can be obtained on a strongly hydrophobic surface, which can be used on the condenser side for condensation and rejection of the droplets back to the condenser side.
  • FIG. 29 is a graph showing the evaporation rate of a water droplet from a microstructured surface according to the instant disclosure compared to the evaporation rate from a conventional copper surface.
  • microstructured material especially adapted to enhance liquid to vapor phase change processes such as boiling, evaporation and condensation.
  • the microstructured material may also be adapted to enhance the movement or transport of liquid on the
  • microstructured material by capillary action.
  • the microstructured material is disposed as a surface layer on a thermally conductive substrate in order to receive heat from or deliver heat to the substrate while containing the liquid and vapor.
  • microstructured surface layer can have a typical thickness of 100 microns to several mm (e.g., up to 1 , 2, 3, 4, 5 mm in thickness) on the substrate.
  • the microstructured material is comprised of sintered particles on the substrate.
  • the sintered particles have outer metal surfaces binding the sintered particles together.
  • the sintered particles are solid metal particles, such as silver particles or copper particles.
  • the sintered particles could also be made of a core of non-metallic material coated with metal.
  • the non-metallic material could be boron nitride or graphite. This form of construction substitutes some of the more expensive metal such as silver or copper with less expensive non-metallic material.
  • the non- metallic material may also reduce the weight of the microstructured material.
  • Non- metallic particles such as boron nitride or graphite also have sufficiently high values of thermal conductivity so as to facilitate efficient heat transfer.
  • the microstructured material may also be constructed of a mixture of metallic and non-metallic particles.
  • the non-metallic particles can be held in the microstructured material by the metallic particles being sintered together.
  • the sintered particles have a particle size less than 10 microns, so that the microstructured material will have small voids smaller than one micron and the microstructured material will provide a large surface area for vapor phase change processes.
  • the microstructured surface layer is fabricated in such a way that it has regions where the particles are concentrated alternated with void regions absent of particles, and the void regions have a hierarchical structure including: (i) the small voids smaller than one micron; (ii) large voids larger than the small voids; and (iii) intermediate size voids having a size larger than the small voids and smaller than the large voids, the intermediate size voids being elongated and forming channels between walls of the particles.
  • the microstructure surface layer can also be shaped by a molding process, or a lithographic or micro-machining process, to have large voids, channels, or ridges. The size, shape and orientation of these voids can be controlled during the fabrication process to enhance phase change or capillary transport.
  • FIG. 1 shows an example of such a hierarchical void structure spanning several length scales.
  • the hierarchical void structure occurs within a microstructured surface layer 22 on a substrate 21 .
  • the hierarchical void structure includes small voids 41 with typical size ⁇ 1 urn between the metallic or nonmetallic particles 42, 43 inside the regions where particles are concentrated.
  • the small voids 41 are largely determined by the packing density of the metal particles and to some extent the degree to which these particles are sintered. These voids (the small voids) are smaller than the typical particle diameter.
  • the hierarchical void structure also includes intermediate sized voids 31 elongated in shape between the regions 32, 33, 34 where the particles are concentrated.
  • These intermediate size voids 31 have a typical width of 5 - 20 urn and can be from 20 to 100 urn in length and form channels between the walls of nanoparticles.
  • these elongated medium voids 31 can be aligned so that their long axes show a preferred orientation to promote a flow of liquid from a condenser to an evaporator in a heat transfer device.
  • the intermediate voids 31 are the result of the ice crystal templating during the freeze process. The rate of freezing has an influence (faster freeze leads to smaller ice crystals) but typically the width of the intermediate voids is in the range of about 5 microns to about 20 microns.
  • the hierarchical void structure also includes larger void regions 23 where no particles are present.
  • the large voids 23 are structures that may be formed mechanically by moulding or templating during the casting process. This provides a more controlled range of dimensions. These voids can have dimensions of 100 urn to several mm (e.g., up to 1 , 2, 3, 4, 5, 10, or 15 mm in size) and can be of random shape or precisely defined shape and can be produced by microfabrication or molding operations during the fabrication. For example, as shown in FIG. 1 , the larger void regions 23 are valleys between parallel-spaced upstanding ridges 24.
  • the microstructured surface layer 22 has an effective surface area that is significantly larger than that of the substrate 21 on which it is disposed.
  • the microstructured surface layer 22 can also be provided with either a hydrophilic surface or a hydrophobic surface, or both a hydrophilic surface and a hydrophobic surface.
  • a hydrophobic surface is desirable for a microstructured surface layer of a condenser, so that the droplets of liquid formed by condensation of vapor will be more easily rejected from the condenser surface.
  • a hydrophilic surface is desirable for a microstructured surface layer of an evaporator, so that liquid will be drawn to the surface where it will evaporate. In either case, the magnification of surface area, together with the void structure in the material and selective wettability of the surface controls the vapor-liquid processes on the surface, leading to an enhanced rate of phase change and heat transfer to or from the surfaces.
  • the article of manufacture including the substrate 21 and the microstructured surface layer 22 on the substrate can be a very useful component of high heat transfer devices such as heat pipes or vapor chambers such as thin vapor chamber heat spreaders used in the cooling of central processing units in thin form factor electronic devices such as tablet computers.
  • the microstructured surface layer 22 can be fabricated on the substrate 21 in such a way as to contain a high density of voids of a desired size and orientation, and to control the size and orientation of these voids during the fabrication process. This is an improvement over current methods of forming such surface by the conventional method of sintering of particles as it provides much greater control over the morphology of the void structure. At the same time it also represents a simpler and more flexible process than using complicated and expensive microfabrication techniques for the formation of such structured surface features.
  • the microstructured surface layer can be fabricated on the substrate by using a freeze casting process. The freeze casting process can be combined with selective molding and microfabrication operations.
  • FIGS. 2 to 5 The idealized formation of a porous structure by freeze casting is illustrated in FIGS. 2 to 5.
  • a suspension 51 of nanoparticles is subjected to a temperature gradient (indicated by the large arrow) to initiate freezing.
  • the entire suspension 51 is not frozen at the same time, and instead the freezing in this example begins on the surface of the substrate 53 at nucleation sites 52 and then the boundary between the frozen suspension and the non-frozen suspension moves along a freeze direction aligned with the temperature gradient.
  • the freeze direction is perpendicular to the substrate.
  • the growing ice crystals 54 reject the nanoparticles and any impurities.
  • the ice is removed by sublimation in a freeze drying operation, leaving walls 55 of nanoparticles.
  • the microstructure in an anneal operation, is heated to sinter the nanoparticles together resulting in a sintered microstructure 56 and improved mechanical strength.
  • FIG. 6 shows a sequence of operations for forming the microstructured surface layer on the thermally conductive substrate using such a freeze casting process.
  • a suspension of the desired metallic and non- metallic nanoparticles is made by suspending the nanoparticles in a solvent that contains a binder. All materials used can be commercially obtained from vendors such as Advanced Material (CT, USA) or Sigma-Aldrich. If necessary a surfactant can further be added to prevent agglomeration of the nanoparticles.
  • the binder should be soluble in the solvent. In the case of using water as the solvent, a water soluble binder polymer such as polyvinyl alcohol or polyethylene oxide can be used.
  • These polymers can serve as both binder and surfactant.
  • the binder is dispersed and dissolved in the solvent after which the nanoparticles are dispersed in this solution. Mixing and dispersion of the nanoparticles can be facilitated by methods such as vortex mixing and ultrasound disruption.
  • a typical mass loading of 1 to 10% of binder in solvent can be used, while typical mass loadings of 10 - 40% for the nanoparticles can be used.
  • the upper limit of nanoparticles concentration is determined by the maximum amount that the solvent can maintained in suspension without particle agglomeration or sedimentation.
  • the thermally conductive substrate is prepared to receive a coating of the suspension.
  • the substrate should typically be a metal with high thermal conductivity such as copper or aluminum. Prior to applying the suspension to the substrate, the substrate should be cleaned of any
  • Adhesion of the microstructured surface layer to the substrate can be enhanced by the deposition of a thin metal surface layer of a material such as silver on the substrate prior to the application of the suspension to the substrate.
  • This thin metal surface layer can be deposited by methods such as vacuum evaporation, sputtering or plating. This thin metal surface layer may have a thickness in the range 10 nm to 500 nm.
  • the adhesion of the suspension to the substrate can be enhanced by the creation of a hydrophilic surface on the substrate. This can be done by the deposition of a compound such as 16- mercaptohexadecanoic acid on the surface. This compound can be deposited from an ethanol solution onto the metallic substrate surface during a self-assembly process.
  • the substrate is coated with the suspension to form a surface layer on the substrate.
  • This coating operation may include lithography or molding to form free-standing structures, as further described below with reference to FIGS. 7 to 12.
  • the suspension can be coated onto the substrate by methods such as dip coating, drop coating, spraying or painting, producing the desired thickness and spread over the substrate.
  • the solvent can now be solidified by lowering the temperature below the freezing point of the solvent. This leads to freezing of the solvent and also induces segregation between the solvent and the nanoparticles plus binder due to impurity rejection by the freeze front of the solidifying solvent.
  • This freezing process can be initiated by cooling of the substrate such as on a cold finger, but it can also be initiated by placing the coated substrate in a cold fluid environment.
  • the temperature can be controlled by methods such the circulation of a cold fluid or by placing the cold finger in contact with a thermoelectric Peltier cooler. In each case the rate of cooling can be controlled so as to control the rate of freezing and so control the size of solvent crystals formed.
  • a fifth operation 65 the solvent crystals are removed by sublimation in a freeze drying process. This removes the solvent phase, revealing the
  • the locations of the nanoparticles in the annealed structure will be substantially the same as the locations of the nanoparticles in the green structure, although the annealed structure may be reduced in size compared to the "green structure” due to some shrinkage during the annealing operation.
  • a sixth operation 66 the green structure is heated in an annealing operation to sinter the nanoparticles together to form the microstructured surface layer on the substrate.
  • the green structure is heated under an inert gas flow or in vacuum in order to sinter the nanoparticles and so provide mechanical strength to the structure.
  • the heating of the annealing operation evaporates the binder polymer. The precise temperature and duration of this annealing operation should be dependent upon the type of particles and the binder used as well as the thickness of the material layer.
  • the annealed microstructured surface layer may undergo further processing, such as creating a hydrophilic surface, a
  • hydrophobic surface or both a hydrophilic surface and a hydrophobic surface.
  • the size, shape and orientation of the different voids present in the material can be controlled by process and material parameters such as nanoparticle size, mass loading of the particles, concentration and type of the binder, and freeze rate.
  • process and material parameters such as nanoparticle size, mass loading of the particles, concentration and type of the binder, and freeze rate.
  • freeze rate should be kept low enough to ensure segregation between the solvent freeze front and the nanoparticle and binder component.
  • the size of small voids between individual nanoparticles is controlled by the size of the particles and the freeze rate, with smaller particles and a lower freeze rate leading to smaller voids.
  • the size and shape of the elongated medium voids is largely determined by the freeze rate but also to a smaller degree by the particle size and by the nature of the binder used. A slow freeze rate will lead to larger voids, while a very fast freeze rate will decrease the size of these voids and also regular elongated shape.
  • the orientation of the elongated voids can be controlled to produce voids with a high degree of alignment of their long axes. This can be achieved by the creation of preferred nucleation sites on the substrate. Such preferred nucleation sites can be achieved by creating mechanical disturbances on the surface, such as by mechanical scratching or etching of the surface. These disturbances should be aligned in the direction desired for the preferred orientation of the voids and should have close to the spacing that the solvent crystals will show at the freeze rate employed.
  • FIGS. 7 to 12 One method to form such a surface is by photolithography and etching as shown in FIGS. 7 to 12.
  • a substrate 71 is cleaned in order to receive a photo-resist coating 72 as shown in FIG. 8.
  • a mask 73 is laid over the photo-resist coating 72 and the mask is illuminated by a light source 74 so that selected areas of the photo-resist are illuminated through holes 75 in the mask 73.
  • the mask has been removed and the photo-resist has been developed and the regions of the photo-resist that were illuminated have been washed away.
  • FIG. 7 One method to form such a surface is by photolithography and etching as shown in FIGS. 7 to 12.
  • FIG. 7 a substrate 71 is cleaned in order to receive a photo-resist coating 72 as shown in FIG. 8.
  • a mask 73 is laid over the photo-resist coating 72 and the mask is illuminated by a light source 74 so that selected areas of the photo-re
  • etchant has been applied over the developed photo-resist so that areas 72 of the substrate where the photo-resist has been washed away have been exposed to the etchant and have been etched away by the etchant. Finally, in FIG. 12, the photo-resist layer 72 has been stripped off of the substrate 71 .
  • photoresist features 77 with the desired size and spacing on the substrate 71 .
  • the substrate can then the etched through the open channels 76 in the pattern 72 using an etchant such as ferric chloride for a copper substrate, leaving a series of aligned channels in the substrate. These channels will act as preferred nucleation sites to the freezing solvent and result in alignment of elongated medium voids where needed in the structure.
  • Other methods that can be used to produce the same effect of mechanical disturbances on the substrate are mechanical abrasion or laser ablation.
  • voids and features with a length scale of typically 0.1 to several mm in size can be created in the material when desired by using photolithographic patterning or molding during the suspension deposition operation.
  • This can be achieved by shaping the suspension on the substrate surface by means of a mold.
  • This mold should be made from a material that does not dissolve in the same solvent as used for the suspension.
  • the mold should be constructed from a material such as acrylonitrile butadiene styrene (ABS) which is not soluble in water but is soluble in acetone.
  • ABS acrylonitrile butadiene styrene
  • Such a mold is used as a sacrificial mold and can be printed on a commercial 3D printer which has a printing resolution good enough to provide the dimensions needed.
  • FIGS. 13 to 17 show such a molding process for obtaining large voids and structures in the material by using a sacrificial mold 82.
  • the sacrificial mold 82 is placed over a substrate 81 before the application of the suspension.
  • the sacrificial mold 82 has regions 83 for displacing the suspension.
  • the suspension 84 is cast into this mold 82 which is in contact with the substrate surface, after which the process operations of freezing and freeze drying are followed, resulting in the green structure 85 in the mold as shown in FIG. 15.
  • the mold 82 is removed by immersing in a solvent that will dissolve the mold, such as acetone for ABS. As shown in FIG. 16, removal of the mold leaves the freeze-dried green structure 85 as shaped by the mold on the substrate 81 , after which this green structure is annealed to remove the binder and sinter of the particles as described above with reference to operation 65 in FIG. 6.
  • thick layers of photoresist can be used to define where the suspension contacts the substrate and pattern the deposited suspension. As with a sacrificial mold, the photoresist can then be removed after the freeze drying operation and then the remaining material is annealed.
  • the wettability of the surface and the subsequent interaction with a liquid like water can further be controlled by the addition of hydrophilic or
  • hydrophobic surface layers on the surface of the material This can be done by the coating of a compound such as 16-mercaptohexadecanoic acid or 1 - hexadecanethiol from an ethanol solution to achieve a hydrophilic or hydrophobic surface respectively.
  • a hydrophobic surface on silver can also be obtained by annealing the material in an oxygen containing atmosphere at temperatures > 100 °C in order to form hydrophobic oxides on the surface of the silver.
  • the photograph in FIG. 27 shows the wettability that can be obtained on a strongly hydrophilic surface, which can be used on the evaporator side for evaporation and fluid transport.
  • FIG. 28 shows the wettability that can be obtained on a strongly hydrophobic surface, which can be used on the condenser side for condensation and rejection of the droplets back to the condenser side.
  • a graph showing the evaporation rate of a water droplet from a microstructured surface according to the instant disclosure compared to the evaporation rate from a conventional copper surface is provided in FIG. 29. The graph shows that the droplet (volume ⁇ 25 ⁇ ) evaporated in ⁇ 1 .2 seconds from a hydrophilic
  • microstructured silver surface prepared as described herein.
  • a similar drop placed on a standard machined copper surface required ⁇ 22 seconds to evaporate.
  • a greater than 10 fold increase (> 10 x) in the rate of heat transfer from the microstructured surface was observed.
  • the article of manufacture comprised of the substrate and the microstructured surface layer on the substrate can be used in the manufacture of two-phase heat transfer devices such as vapor chambers and heat pipes.
  • FIG. 18 shows a typical two-phase vapor chamber heat spreader 91 .
  • This device 91 has a first side or zone 94 where liquid to vapor phase changes take place (the
  • evaporation side or hot side and a second side or zone 95 where vapor to liquid phase changes take place (the condensation side or the cold side).
  • the condensation side or the cold side In the areas where liquid to vapor phase changes takes place, heat will be effectively absorbed by the device 91 , while in the areas where vapor to liquid phase change takes place heat will be effectively released by the device.
  • the evaporation side 94 is then attached to a source of heat 92 and the condensation side 95 is attached to a sink of heat 93, then heat will flow with very low resistance from the evaporation section to the condensation section and draw heat away from the heat source and allow this source to be efficiently cooled.
  • the source of heat may be an electronic component such as a central processing unit (CPU) 92 or graphics processing unit, while the sink of heat 93 may be the enclosure of the electronic system or it may be a further cooling system such as a finned heat sink and fan.
  • CPU central processing unit
  • GPU graphics processing unit
  • the interior of the vapor chamber 96 can be partially evacuated of air and then filled with a working liquid which is evaporated and condensed in the different zones.
  • the liquid that condenses in the condensation section should be returned to the evaporation section to complete the fluid cycle.
  • the return path can be enabled by the gravity return of drops from the condensation cycle or by the capillary transport of the condensed liquid on the inside surface of the vapor chamber or by a combination of these two mechanism.
  • the microstructured surface layer 101 is deposited on the inside wall of such a vapor chamber 96 in order to facilitate the evaporation, condensation, and capillary transport of fluid.
  • the void structure 103 of the microstructured surface layer 101 can be aligned, as shown in FIG. 19, by the methods described above to facilitate capillary movement of the liquid from any part of the section to the hot zone above the CPU 92.
  • the surface of the material in this section can be made hydrophilic by methods such as described above to ensure efficient capillary action as well as evaporation.
  • the material In the condensation zone 95 the material can be made hydrophobic by methods such as described above in order to facilitate the rejection of condensed drops from the condensation zone and their return by gravity to the evaporation section 94.
  • the efficacy of both the evaporation and condensation sections can be further enhanced by shaping of the deposited material to contain structures such as fins that will enhance the surface area. These structures can be fabricated by the use of microfabrication or molding operations such as described above. This can be done on either the evaporation side or on the condensation side or on both.
  • FIG. 20 shows such a finned structure on both the evaporation side 1 12 and the condensation side 1 1 1 of a vapor chamber 1 10, resulting in an interdigitated finned structure.
  • the interdigitation of the evaporator fins with the condenser fins provides close proximity between large surface areas of the evaporator and condenser to facilitate vapor exchange between the evaporator and the condenser.
  • the vapor chambers constructed from these layers can potentially be made very thin with a very low thermal resistance.
  • Such a vapor chamber heat spreader structure would be ideal for the cooling of electronic components in space
  • constrained enclosures such as tablet computers.
  • nanoparticles in a 1 % solution of polyvinyl alcohol in water producing mass loadings of nanoparticles from 10% to 40%.
  • Vortex mixing and ultrasonic disruption were used to prevent particle aggregation and promote dispersion.
  • the detailed microstructure of the silver surface after annealing is shown at different length scales in scanning electron micrographs of FIGS. 21 and 22.
  • the microstructure increased in density from the outside perimeter of the substrate to the center, i.e. in the direction of the freeze front. This is consistent with particles and other impurities being ejected by the advancing ice front and eventually concentrating in the center of the sample. This effect is least noticeable in samples with high Ag particle mass loadings or when very high freeze rates were used and may be due to the fact that in these cases the moving freeze front was unable to reject particles, leading to a more even structure over the substrate.
  • the observed pores are broadly divided into three types based on their shape and size.
  • FIG. 21 Shown in FIG. 21 are the small voids in the structure. These are micro voids formed within the nanoparticle walls, as these walls are not densely packed. These pores have typical dimensions of 0.5— 5 pm, depending on the size of the particles used.
  • FIG. 22 Shown in FIG. 22 are intermediate sized voids. These are elongated porous channels separated by nanoparticle walls, with channels typically 40 to 80 pm in length and approximately 10 pm wide.
  • the walls consisted of sintered nanoparticles that were ejected by the ice crystals and have a typical thickness of 5 to 10 pm. These channels and walls were found to be aligned over small areas (0.2 to 0.5 mm 2 ) of the sample.
  • FIG. 23 Shown in FIG. 23 is a structure of large voids as can be obtained by a molding or templating process. In this case a mold and template process was used to form parallel channels of approximately 5 mm wide in the microstructured surface layer.
  • FIG. 25 shows a mean bubble diameter of 0.41 ⁇ 0.16 mm for bubbles that formed on the machined copper surface at ⁇ ⁇ of ⁇ 2 °C.
  • FIG. 26 shows a mean bubble diameter of 0.28 ⁇ 0.08 mm at similar temperatures on the porous silver surfaces.
  • the density of bubbles was also significantly higher on porous surfaces, with a typical density of 24 bubbles/cm 2 on the copper surface compared to approximately 80 bubbles/cm 2 on the porous surfaces. In the latter case, low contrast between bubbles and surface makes clear identification harder and together with the smaller size of the bubbles it may well lead to an underestimation of bubble density.
  • Significant bubble departure from the microstructured surface also occurs well in advance from that on the copper surface, in agreement with the point of increase in the heat flux observed for the different samples.
  • FIG. 27 is a photograph showing the wettability that can be obtained on a strongly hydrophilic surface, which can be used on the evaporator side for
  • FIG. 28 is a photograph showing the wettability that can be obtained on a strongly hydrophobic surface, which can be used on the condenser side for condensation and rejection of the droplets back to the condenser side.
  • FIG. 29 is a graph showing the evaporation rate of a water droplet from a microstructured surface according to the instant disclosure compared to the evaporation rate from a conventional copper surface. The graph shows that the droplet (volume ⁇ 25 ⁇ ) evaporated in ⁇ 1 .2 seconds from a hydrophilic
  • microstructured silver surface prepared as described herein.
  • a similar drop placed on a standard machined copper surface required ⁇ 22 seconds to evaporate.
  • the different types of voids or pores in the microstructured surface layer may potentially each play a role in contributing to the enhanced heat flux.
  • Both the small and intermediates voids will enhance the number of potential nucleation sites. Due to their channel shape with a high aspect ratio, the intermediate elongated voids may also contribute to an upward squirt or jet effect during bubble departure. This has the potential to further enhance fluid convection during the release of bubbles.
  • the small micro voids in the walls may play a role in feeding liquid to the growing bubbles and sustain the process, while at the same time the high thermal conductivity of the silver nanoparticles as well as the associated high surface area of the porous structure will be effective in transferring heat from the copper surface into the fluid.

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Abstract

Selon l'invention, un substrat conducteur thermique comporte, formée dessus, une couche à surface microstructurée de nanoparticules revêtues de métal frittées fournissant une structure de cavités hiérarchique pour faciliter un transfert de chaleur à deux phases. La structure de cavités hiérarchique comprend des petites cavités de taille inférieure à un micron, des grandes cavités plus grandes que les petites cavités ; et des cavités allongées de taille intermédiaire formant des canaux entre les parois des particules. La microstructure peut être créée en produisant une suspension du solvant de nanoparticules comprenant un matériau liant, en revêtant le substrat avec la suspension pour former une couche de surface sur le substrat, puis en refroidissant le substrat revêtu en dessous d'un point de congélation du solvant pour induire une ségrégation entre les particules et le solvant congelé, puis en enlevant le solvant congelé de la couche de surface, puis en chauffant le substrat revêtu pour fritter les particules ensemble.
PCT/IB2016/057042 2015-11-23 2016-11-22 Surfaces microstructurées pour un meilleur transfert de chaleur par changement de phase WO2017089960A1 (fr)

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US10597286B2 (en) 2017-08-01 2020-03-24 Analog Devices Global Monolithic phase change heat sink
WO2019074845A1 (fr) * 2017-10-13 2019-04-18 Extractcraft, Llc Transfert de chaleur pour distillation d'un extrait
CN111684231A (zh) * 2018-03-19 2020-09-18 保来得株式会社 蕊芯的制造方法
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RU2754127C1 (ru) * 2020-12-23 2021-08-27 Федеральное государственное бюджетное образовательное учреждение высшего образования "Саратовский национальный исследовательский государственный университет имени Н.Г. Чернышевского" Способ переноса нитевидных нанокристаллов на подложку
CN112831185A (zh) * 2021-02-23 2021-05-25 中北大学 梯度导电-均匀导热双功能网络低反射高吸收电磁屏蔽聚合物复合材料
CN112831185B (zh) * 2021-02-23 2022-09-20 中北大学 梯度导电-均匀导热双功能网络低反射高吸收电磁屏蔽聚合物复合材料

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