Patent Publication Number: US-2020292246-A1

Title: Devices for heat transfer

Description:
BACKGROUND 
     Heat transfer devices are used to transfer heat between a heat source and a heat sink. Heat transfer devices include two regions, a first region coupled to the heat source and a second region coupled to the heat sink. In the first region, heat is received from the heat source and is then transferred to the second region, for example, by conduction, convection, radiation, phase transition, and the like. Subsequently, heat is transferred from the second region to the heat sink. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The following detailed description references the figures, wherein: 
         FIG. 1  illustrates an example device for heat transfer, according to an example implementation of the present subject matter; 
         FIG. 2  is an example method of heat transfer, according to an example implementation of the present subject matter; 
         FIG. 3( a )  illustrates an example heat pipe, according to an example implementation of the present subject matter; 
         FIG. 3( b )  illustrates an example heat pipe, according to another example implementation of the present subject matter, 
         FIG. 4  illustrates an example vapor chamber, according to an example implementation of the present subject matter; 
         FIG. 5  illustrates an example device for heat transfer, according to an example implementation of the present subject matter; and 
         FIG. 6  illustrates an example method of preparing a device for heat transfer, according to an example implementation of the present subject matter. 
     
    
    
     DETAILED DESCRIPTION 
     Heat transfer devices include devices such as heat pipes and vapor chambers. Heat transfer devices can be used in various systems, such as in spacecraft, electronic devices, solar heat transfer systems, and the like. Generally, heat transfer devices that work on principles of phase transition include a sealed casing enclosing a working fluid of high heat capacity. The working fluid is selected based on compatibility with the casing. For example, when the casing is made of copper, the working fluid can be water. 
     In such a heat transfer device, during operation, working fluid evaporates, for example, in an evaporation area dose to a heat source. The vapors are transferred to a second region where the vapors condense, for example, in a condensation area close to a heat sink. To return the condensed working fluid to the evaporation area for subsequent heat transfer, a fluid transfer mechanism, such as capillary action of a wicking surface, may be used. 
     The present subject matter relates to devices for heat transfer with increased heat dissipation performance, and methods of preparing heat transfer devices. An example device for heat transfer includes a casing. The casing includes a first portion to receive heat from a heat source. A thermo-reversible hydrogel is provided in contact with an inner surface of the first portion and is soaked in a working fluid. A wicking surface is also provided along the inner surface of the casing. The casing further includes a second portion, which is disposed substantially opposite to the first portion. The first portion and second portion are fluidly coupled by the wicking surface and a vapor region. 
     The device of the present subject matter can be prepared by sintering a wicking material, such as copper powder, on an inner surface of the casing followed by coating the thermo-reversible hydrogel in the first portion of the casing and drying the hydrogel. 
     In operation, in a first example, when a first temperature of the first portion is higher than a second temperature of the second portion, vapors of the working fluid are formed at the first portion. This case arises when, for example, the heat source that is in contact with the first portion is switched on. The vapors of the working fluid are transferred to the second portion through the vapor region. At the second portion, the vapors are condensed and the condensed working fluid is transferred to the first portion by the wicking surface. At the first portion, the condensed working fluid is absorbed by the hydrogel, thereby increasing the rate of return of the condensed working fluid and increasing heat dissipation. 
     On the other hand, in a second example, when the second temperature is higher than the first temperature, vapors of the working fluid are formed at the second portion. This case arises when, for example, the heat source is switched off and the first portion cools down faster than the second portion. In this case, the vapors of the working fluid are transferred from the second portion to the first portion, for condensation, through the vapor region, while the condensed working fluid is transferred from the first portion to the second portion by the wicking structure. Further, at the first portion, the vapors are absorbed by the hydrogel, thereby increasing a rate of return of vapors and increasing heat dissipation. 
     The thermo-reversible hydrogel in the device increases rate of dissipation of heat from the working fluid by absorption of the condensed working fluid in the first example and absorption of the vapors in the second example. Therefore, the device of the present subject matter can be used for rapid cooling. 
     The present subject matter provides devices for heat transfer which provide rapid cooling without substantial increase in weight. Therefore, the devices can be used to cool devices, such as Liquid Crystal Display (LCD) panels, Light Emitting Diodes (LEDs), Central Processing Units (CPUs) and the like. Further, the increased heat dissipation performance and rapid cooling helps in increasing power efficiency and reducing risks of explosion due to overheating. 
     The following description refers to the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the following description to refer to the same or similar parts. While several examples are described in the description, modifications, adaptations, and other implementations are possible. Accordingly, the following detailed description does not limit the disclosed examples. Instead, the proper scope of the disclosed examples may be defined by the appended claims. 
     Example implementations of the present subject matter are described with regard to personal computers (PCs) and laptop computers. Although not described, it will be understood that the implementations of the present subject matter can be used with other types of devices as well, such as televisions, tablets, smartphone devices, solar panels, aircraft and the like. 
       FIG. 1  illustrates an example device  100  for heat transfer, according to an example implementation of the present subject matter. The device  100  may be used, for example, in electronic circuitry, spacecraft, and the like. Based on the application, the device  100  may be fabricated as a micro-device or a nano-device. 
     In an example, the device  100  may be one of a heat pipe and a vapor chamber. A heat pipe is a substantially hollow, cylindrical heat transfer device provided between a heat source and a heat sink to transfer heat between the heat source and a heat sink. A vapor chamber may be understood as a flattened heat pipe with substantially planar heat transfer surfaces and a hollow region therebetween. Function of heat pipes and vapor chambers is based on principles of conduction and phase transition. 
     The device  100  includes a casing  102 . In an example, the casing  102  is fabricated from a material which has high thermal conductivity. For example, the casing may be fabricated from a metal, such as copper, aluminum, alloy, and the like. In an example, when the device  100  is a heat pipe, the casing  102  can be fabricated as an elongated hollow cylinder with both ends of the cylinder sealed. In another example, when the device  100  is a vapor chamber, the casing  102  can be fabricated as a hollow, longitudinally flattened structure. 
     The casing  102  includes a first portion  104 . In an example, when the device  100  is a heat pipe, the first portion  104  is present substantially towards a first end of the heat pipe as will be explained later with reference to  FIGS. 3( a ) and 3( b ) . In an example, when the device  100  is a vapor chamber, the first portion  104  is present substantially towards a first flattened surface of the vapor chamber as will be explained later with reference to  FIG. 4 . The first portion  104  is to receive heat from a heat source  106 . The first portion  104  may, therefore, be coupled to the heat source  106 . 
     The heat source  106  is a device from which heat is to be removed, for example, for cooling the device. For this, heat from the heat source  106  is received by the device  100 . For example, in an electronic device, such as a computing device, the heat source  106  may be a Central Processing Unit (CPU) which may get heated during start-up and running of the computing device and hence may have to be cooled by using the device  100 . The casing  102  of the device  100  receives heat from the heat source  106  at the first portion  104 . 
     The casing  102  also includes a working fluid (not shown). The working fluid is selected based on thermal conductivity and compatibility with the material of the casing  102 . For example, when the casing  102  is a copper casing, the working fluid can be water, ethanol, and the like. 
     A thermo-reversible hydrogel  108  is provided in the first portion  104  and is soaked in the working fluid. The thermo-reversible hydrogel  108  may be coated in the first portion  104 . In an example, a thickness of the thermo-reversible hydrogel  108  in the first portion  104  is in a range of 100-800 μm. Thermo-reversible hydrogels are hydrogels which form a gel when cooled and form a viscous fluid state when exposed to heat. Thermo-reversible hydrogels, therefore, do not undergo permanent change. Further, transition from gel to viscous fluid and vice versa can be performed repeatedly based on heat received by the hydrogel. The thermo-reversible hydrogel  108  may be made of polymers selected from ethylene maleic anhydride copolymer, carboxymethyl cellulose, polyvinyl alcohol copolymers, starch grafted copolymer of polyacrylonitrile or polyacrylamide super absorbents, and combinations thereof. 
     In operation, when no heat is received, the thermo-reversible hydrogel  108  retains its gel form. In the gel form, molecules of the thermo-reversible hydrogel  108  form a three-dimensional cross-linked network where the network traps the working fluid. When the thermo-reversible hydrogel  108  receives heat from the heat source  106  through casing  102 , the gel forms a viscous fluid, thereby releasing the working fluid for phase transition and heat transfer. 
     The working fluid evaporates due to the heat and forms vapors which are transferred, for example, by diffusion to a second portion  110  of the casing  102  of the device  100 . The second portion  110  is present substantially opposite to the first portion  104 . In an example, when the device  100  is the heat pipe, the second portion  110  is the second end of the cylinder as will be explained later with reference to  FIGS. 3( a ) and 3( b ) . In an example, when the device  100  is the vapor chamber, the second portion  110  is the second flattened surface of the vapor chamber as will be explained later with reference to  FIG. 4 . 
     In the present description, a temperature of the first portion  104  is referred to as a first temperature and a temperature of the second portion  110  is referred to as a second temperature. 
     At the second portion  110 , as the second temperature of the second portion  110  is lower than the first temperature of the first portion  104 , the vapors condense. The condensed vapors are transferred to the first portion  104  by a wicking surface  112  by capillary forces. The wicking surface  112  is provided along an inner surface of the casing  102  between the first portion  104  and the second portion  110 . In an example, the wicking surface  112  may extend into the first portion  104  and the second portion  110 . The wicking surface  112  may be one of a sintered metal powder, a screen, and a grooved wicking surface. In an example, the wicking surface  112  is fabricated from the material of the casing  102 . For example, when the casing  102  is copper, the wicking surface  112  may be formed from sintered copper powder. 
       FIG. 2  depicts an example method of heat transfer in device  100 , according to an example implementation of the present subject matter. At block  202 , heat is received at the first portion  104  of the casing  102 . On receiving heat, the thermo-reversible hydrogel  108  releases the working fluid. The heat causes the working fluid to form vapors. The vapors diffuse towards the second portion  110 . At the second portion  110 , at block  204 , the vapors of the working fluid are cooled. The vapors, therefore, condense at the second portion  110 . The wicking surface  112  transfers the condensed working fluid from the second portion  110  to the first portion  104  for absorption by the thermo-reversible hydrogel  108 . The method of heat transfer is further explained with respect to  FIGS. 3( a )  and  4 . 
     In an example, the second temperature may be higher than the first portion, for example, when the first portion  104  loses heat faster than the second portion  110 . In this example, vaporization of the working fluid is caused at the second portion  110 . The vapors then diffuse from the second portion  110  to the first portion  104  where the vapors are condensed and absorbed by the thermo-reversible hydrogel  108 . This example is further explained with respect to  FIG. 3( b ) . 
       FIG. 3( a )  depicts operation of an example heat pipe  300  when heat is received from a heat source, according to an example implementation of the present subject matter. Hereinafter, ends of the heat pipe  300  are referred to as bases. The heat pipe  300  includes the casing  102  which is substantially cylindrical and includes a first base  302  and a second base  304 . The first portion  104  of the casing  102  of the heat pipe  300  is present substantially towards the first base  302  and the second portion  110  of the casing  102  of the heat pipe  300  is present substantially towards the second base  304 . In an example, the first base  302  may be rounded to increase surface area available for coating the thermo-reversible hydrogel  108 . 
     A working fluid  306  is provided in the first portion  104 . The thermo-reversible hydrogel  108  is soaked in the working fluid  306 . In an example, when the thermo-reversible hydrogel  108  is saturated with the working fluid  306 , any excess working fluid  306  may be retained unbound in the first portion  104 . The first portion  104  may receive heat from a heat source (not shown). 
     When heat is supplied to the first portion  104  as depicted by arrows  308 , any excess working fluid  306  vaporizes and additionally, the thermo-reversible hydrogel  108  releases the soaked working fluid  306  for vaporization. The vapors  310  are transferred to the second portion  110 , for example, by diffusion. As the second temperature is less than the first temperature, the vapors  310  are condensed. In an example, the second portion  110  may be open to surrounding environment at ambient conditions, and, therefore, may be at a lower temperature than the first portion  104 . During condensation, the vapors  310  reject heat at the second portion  110  as shown by arrows  312 . The condensed working fluid  314  is then transferred by capillary action by the wicking surface  112 . 
     The operation as depicted in  FIG. 3( a )  may occur during start-up and running of the computing device. For example, during start-up, a processing unit of a computing device may generate heat and act as the heat source. Therefore, the heat pipe  300  may be disposed such that the first portion  104  is in close proximity to the processing unit. For example, the heat pipe  300  may be coupled to an enclosure housing the processing unit. The second portion  110  may be disposed such that the second portion  110  is in dose proximity to other components on the computing device. In an example, the second portion  110  may be open to ambient conditions. In another example, the second portion  110  may be disposed in proximity to a fan to increase heat rejection at the second portion  110 . 
     However, when the computing device is switched off, the processing unit may cool down to a temperature lower than that in the second portion, i.e., the second temperature may be higher than the first temperature. In another example, the second portion may be at a higher temperature, for example, due to the ambient temperature bring higher than the temperature in the first portion. This example is as depicted in  FIG. 3( b ) . 
       FIG. 3( b )  depicts operation of the example heat pipe  300  when the second temperature is higher than the first temperature, according to an example implementation of the present subject matter. As shown in  FIG. 3( b ) , the heat pipe  300  receives heat at the second portion  110  as shown by arrows  308 . The received heat causes vaporization of working fluid at the second portion  110 . The vapors  310  of the working fluid are transferred from the second portion  110  to the first portion  104  where the vapors  310  are condensed and absorbed by the hydrogel. The condensed working fluid  314  is then transferred from the first portion  104  to the second portion  110  by the wicking surface  112 . This facilitates further cycles of heat transfer as more of the previously vaporized working fluid returns to the hydrogel. 
     In another example, the device for heat transfer may be a vapor chamber.  FIG. 4  depicts operation of an example vapor chamber  400 , according to an example implementation of the present subject matter. The vapor chamber  400  includes the casing  102  which includes a first flattened surface  402  and a second flattened surface  404  which are substantially opposite to each other. The first portion  104  is present substantially proximate to the first flattened surface  402  and the second portion  110  is present substantially proximate to the second flattened surface  404 . 
     In one example, the first flattened surface  402  includes well structures  406  for holding the thermo-reversible hydrogel  108 . However, the thermo-reversible hydrogel  108  may be provided on the first flattened surface  402  without the well structures  406 . In another example, the thermo-reversible hydrogel  108  may be coated over the wicking surface  112  in the first portion  104 . 
     In operation, when heat is supplied at the first portion  104  as shown by arrows  406 , the thermo-reversible hydrogel  108  releases absorbed working fluid which vaporizes. The heat may be supplied to the first portion  104 , for example, by a heat source (not shown). The vapors as depicted by arrows  408  diffuse within the vapor chamber  400 . Since the second temperature of the second portion is lower than the first temperature of the first portion, the vapors  408  condense. In an example, the second portion  110  may be coupled to a condenser to condense the vapors  408 . The condensed vapors  410  are then transferred to the thermo-reversible hydrogel  108  by the wicking surface  112  provided between the first portion  104  and the second portion  110 . It will be understood that the vapor chamber  400  will also facilitate in removing heat from the second portion  110  and return of the vaporized working fluid back to the hydrogel when the first portion cools to a temperature lower than the second portion, as discussed above with reference to the heat pipe  300 . 
       FIG. 5  illustrates another example device  500  for heat transfer, according to an example implementation of the present subject matter. The device  500  may comprise a casing (not shown). The device  500  includes a first portion  502  and a second portion  504 . In an example, the casing may comprise the first portion  502  and the second portion  504 . The first portion  502  includes a thermo-reversible hydrogel  506  soaked in a working fluid  508 . The thermo-reversible hydrogel  506  may be, for example, coated on a surface of the first portion  502 . In an example, multiple layers of the thermo-reversible hydrogel  506  may be provided in the first portion  502 . Each layer may be a same hydrogel or a different hydrogel. In an example, the thermo-reversible hydrogel  506  can be provided as patterns or grooves to increase surface area of absorption and, thereby, heat dissipation. 
     A vapor region  510  is provided in between and fluidly couples the first portion  502  and the second portion  504 . The vapor region  510  is enclosed by a wicking surface  512 . The wicking surface  512  also fluidly couples the first portion  502  and the second portion  504 . The vapor region  510  and the wicking surface  512  are for heat transfer between the first portion  502  and the second portion  504 . 
     In operation, for example, during start-up of an electronic device, heat is received at the first portion  502 , for example, due to contact with a heat source, such as a processor. When heat is received at the first portion  502 , a first temperature of the first portion  502  becomes higher than a second temperature of the second portion  504 . In this example, the vapor region  510  is to transfer vapors of the working fluid  508  from the first portion  502  to the second portion  504  for condensation and the wicking surface  512  is to transfer condensed working fluid from the second portion  504  to the first portion  502 . At the first portion  502 , the thermo-reversible hydrogel  506  is to absorb the condensed working fluid. 
     Further, for example, when the electronic device is switched off or when an ambient temperature is high, the second temperature of the second portion  504  may be higher than a first temperature of the first portion  502 . Therefore, at the second portion  504 , working fluid vaporizes. In this example, the vapor region  510  is to transfer vapors of the working fluid  508  from the second portion  504  to the first portion  502  for condensation and the wicking surface  512  is to transfer condensed working fluid from the first portion  502  to the second portion  504 . The thermo-reversible hydrogel  506  is to absorb the vapors at the first portion  502 . 
     As explained, the thermo-reversible hydrogel  506  increases rate of dissipation of heat from the working fluid  508  by absorption of the condensed working fluid and absorption of the vapors. Therefore, the heat transfer device of the present subject matter can be used for rapid cooling without substantially increasing weight of the heat transfer devices. 
       FIG. 6  depicts a method  600  of preparing a heat transfer device, according to an example implementation of the present subject matter. The order in which the method  600  is described is not intended to be construed as a limitation, and any number of the described method blocks may be combined in any order to implement the method  600 , or alternative methods. Although the method  600  may be implemented in a variety of systems, the method  600  is explained in relation to the aforementioned devices for heat transfer, for ease of explanation. 
     At block  602 , a wick material powder is sintered along an inner surface of a casing of a device for heat transfer. As will be understood, sintering is a process of forming a solid, porous mass of the wick material without melting the wick material powder to liquefaction. In an example, prior to sintering of the wick material powder, a first portion of the casing may be sealed, for example, by welding. For sintering, the wick material powder is filled into the casing. In an example, the sintering is performed at about 700-1300° C. for about 30-60 minutes. Sintering of the wick material powder helps in forming a wicking surface along an inner surface of the casing. In an example, the device for heat transfer may be device  100 ,  300 ,  400  and the casing may be casing  102 , respectively. In another example, the device may be device  500  which may include a casing (not shown). 
     After sintering, at block  604 , a thermo-reversible hydrogel is coated at a first portion of the casing. In an example, the thermo-reversible hydrogel is coated on top of the wicking surface. In another example, the wicking surface may be scrapped or removed by other techniques and the thermo-reversible hydrogel may be coated directly on the casing. In an example, the first portion may be first portion  104  of the device  100 ,  300 ,  400 . In another example, first portion may be first portion  504  of device  500 . 
     The thermo-reversible hydrogel is coated by spraying a solution comprising the thermo-reversible hydrogel on the inner surface of the casing at the first portion. In an example, the solution includes the thermo-reversible hydrogel in a range of about 0.1-3.0% weight by volume of the solution. The solution may be made with a working fluid as a solvent. The thermo-reversible hydrogel may be sprayed at a pressure in a range of about 0.0005-0.002 Torr. In an example, multiple layers of the thermo-reversible hydrogel may be coated at the first portion. 
     After coating the thermo-reversible hydrogel, at block  606 , the thermo-reversible hydrogel is dried. In an example, the thermo-reversible hydrogel is dried at a temperature of about 105-120° C. for about 15-40 minutes. 
     The drying of the thermo-reversible hydrogel is followed by injecting a working fluid into the first portion of the casing under vacuum. This helps increase the amount of working fluid absorbed by the hydrogel. The amount of working fluid injected may be varied so that the hydrogel is soaked in the working fluid and some excess working fluid remains in contact with the hydrogel. 
     Therefore, the methods and devices of the present subject matter provide an increase in rate of heat dissipation without increasing weight of the device. The increased rate of heat dissipation is due to absorption of condensed working fluid and absorption of vapor by the thermo-reversible hydrogel. Increased heat dissipation reduces chances of damage of the heat source, for example, due to overheating. 
     The preceding description has been presented to illustrate and describe examples of the principles described. This description is not intended to be exhaustive. Many modifications and variations are possible in light of the above teaching.