Method and device for controlling heat transfer rate of graphene liquid drop printing three-dimensional preform

Abstract

The invention discloses a heat transfer rate control method and device for a graphene liquid drop printing three-dimensional prefabricated body, the heat transfer rate control method is realized by adopting a graphene liquid drop printing three-dimensional prefabricated body device, and the device comprises a printing substrate, a metal enclosing wall, a liftable freezing substrate and a temperature controller, the heat transfer rate control method comprises the steps that the process of printing a three-dimensional prefabricated body is divided into three stages including a low layer, a middle layer and a high layer in sequence according to the number of printing layers, the number of the layers is set to be n1, n2 and n3, and in the layer-by-layer printing process, the temperature of a liftable freezing base plate, the temperature of a metal enclosure and the height of the liftable freezing base plate in each stage are controlled, the control of the ice crystal growth heat transfer rate in the process of printing the three-dimensional preform is realized, and the stability of the ice crystal growth rate is ensured, so that the three-dimensional connected graphene material with uniform axial microstructure and excellent performance is prepared.

Description

Technical Field

[0001] The invention relates to the technical field of graphene preform preparation, and in particular to a method and device for controlling the heat transfer rate of a three-dimensional preform printed by graphene droplets. Background Art

[0002] Three-dimensional connected graphene preforms have excellent properties such as porous structure, ultra-large specific surface area, ultra-high electrical conductivity and rapid mass transfer, and are widely used in the fields of thermal management materials, electromagnetic shielding and absorbing materials, chemical and biological sensing. Combining graphene droplet printing technology with freeze casting process can make full use of the advantages of additive manufacturing technology in design flexibility and rapid realization, as well as the advantages of freeze casting process in microstructure controllability, efficient use of raw materials and environmental friendliness, to achieve precise and controllable preparation of macro and microstructures of three-dimensional connected graphene preforms. In order to achieve stable freezing of graphene droplets during printing, most existing freeze printing methods usually use a single cold source to provide an adjustable freezing temperature for the printing substrate. However, in practical applications, due to the heat accumulation effect, the thermal resistance of the printing layer gradually increases with the process, resulting in inconsistent growth rates of ice crystals in different printing layers. After freeze drying, the formed pore structure has differences in size and morphology, and poor connectivity, which affects the overall performance of the three-dimensional connected graphene preform. In addition, the convective heat transfer between the substrate and the ambient air will produce a vertical temperature gradient around the substrate, resulting in a higher temperature in the area away from the substrate. As printing proceeds layer by layer, the nozzle rises to a point where the temperature is higher than the melting point of the ink, and subsequently deposited droplets melt and cannot maintain their shape, thus interrupting the printing process.

[0003] Therefore, it is necessary to develop a heat transfer rate control method and device suitable for droplet printing to prepare graphene preforms. This method can maintain a low temperature environment in the printing space and regulate the growth rate of ice crystals, thereby ensuring that the macro-microstructure of the three-dimensional interconnected graphene preform can be accurately formed. Summary of the invention

[0004] Based on the above considerations, the present invention introduces a multi-stage temperature-controlled cryo-printing method, which can ensure the stability of the low-temperature environment in the printing space and ensure the continuity and reliability of the cryo-printing process; by adjusting the heat transfer rate of the printing layer and the propagation rate of the solidification interface, the stable growth rate of ice crystals is maintained, which is conducive to the preparation of a three-dimensional connected graphene preform with designable macro- and micro-structures and excellent performance.

[0005] Therefore, the present invention provides a method for controlling the heat transfer rate of a graphene droplet-printed three-dimensional preform, wherein the heat transfer rate control method is implemented by using a graphene droplet-printed three-dimensional preform device, wherein the device comprises a printing substrate, a metal enclosure, a liftable freezing substrate, and a temperature controller;

[0006] The liftable freezing substrate is located below the printing substrate; the metal enclosure is located around the printing substrate, and the temperature controller is used to control the temperature of the metal enclosure and the liftable freezing substrate;

[0007] The heat transfer rate control method includes dividing the three-dimensional preform printing process into three stages according to the number of printing layers, which are respectively low layer, middle layer and high layer, and the number of layers is set to n1, n2, n3 respectively, and the total number of preform layers is n1+n2+n3. During the layer-by-layer printing process, according to the relationship between the heat conduction efficiency of the liftable freezing substrate, the heat radiation efficiency of the metal wall and the heat transfer rate of ice crystal growth, the liftable freezing substrate temperature, the metal wall temperature and the liftable freezing substrate height in each stage are controlled to achieve control of the heat transfer rate of ice crystal growth during the printing of the three-dimensional preform.

[0008] The relationship between the heat conduction efficiency of the liftable refrigeration substrate, the heat radiation efficiency of the metal wall and the heat transfer rate of ice crystal growth is:

[0009] ;

[0010] Among them, T base is the temperature of the cryogenic substrate that can be raised or lowered, T layer is the droplet temperature, T wall is the temperature of the metal fence, R layer is the thermal resistance between printed layers, R0 is the contact thermal resistance between the copper printed substrate and the liftable frozen substrate, n is the number of printed layers, is the emissivity, is the Stefan-Boltzmann constant, r is the inner radius of the metal wall, and h is the height of the cryo-substrate that can be raised and lowered when printing the nth layer;

[0011] As a further illustration of the present invention, when printing the lower preform, the temperature of the droplet and the temperature of the metal wall remain unchanged, and the heat conduction rate of ice crystal growth is increased by lowering the temperature of the liftable freezing substrate. The relationship between the number of printed layers and the temperature of the liftable freezing substrate is:

[0012] ;

[0013] Among them, n1 is the number of low-level printing layers, d is the thickness of a printed single layer, and h0 is the initial height of the liftable freezing substrate;

[0014] During the printing of the lower-layer preform, the printing of the first layer is completed first, and the temperature required for printing the second layer is calculated based on the relationship between the number of printed layers and the temperature of the liftable frozen substrate. The liftable frozen substrate temperature controller controls the liftable frozen substrate to reach the required temperature before starting the second layer printing, and so on, until the temperature of the liftable frozen substrate reaches the lower limit value, and the printing of the lower-layer preform is completed.

[0015] As a further illustration of the present invention, when printing the middle layer preform, the temperature of the droplet, the temperature of the liftable freezing substrate and the temperature of the metal enclosure remain unchanged, the cold radiation of the metal enclosure is used to compensate for the heat accumulation effect, and the effective radiation area for ice crystal growth during printing is increased by lowering the height of the liftable freezing substrate. The relationship between the number of printed layers and the height of the liftable freezing substrate is as follows:

[0016] ;

[0017] During printing of the middle-layer preform, the height of the liftable frozen substrate required for printing the n1+1th layer is calculated first, and the height of the liftable frozen substrate is lowered by the precision lifting axis until it drops to the required height before printing the n1+1th layer, and so on, until the height of the liftable frozen substrate drops to its lower limit, completing printing of the middle-layer preform.

[0018] As a further illustration of the present invention, when printing a high-rise preform, the temperature of the droplet and the temperature of the liftable freezing substrate remain unchanged, and the radiation efficiency of ice crystal growth during printing is increased by lowering the temperature of the metal enclosure. The relationship between the number of printed layers and the temperature of the metal enclosure is:

[0019] ;

[0020] During the printing of the high-rise preform, the metal wall temperature required for printing the n1+n2+1th layer is calculated first, and the temperature of the metal wall is lowered by the metal wall temperature controller. After reaching the required temperature, the printing of the n1+n2+1th layer is started, and so on, to complete the printing of the high-rise preform.

[0021] As a further illustration of the present invention, the device further includes a droplet generating unit, a three-dimensional motion unit and a printing platform;

[0022] The droplet generating unit is installed on the three-dimensional motion unit, located above the printing substrate, and is used to generate graphene droplets; the three-dimensional motion unit is used to realize the movement of the droplet generating unit in three different directions;

[0023] The three-dimensional motion unit includes a horizontal moving module 1, a horizontal moving module 2, and a vertical moving module; the horizontal moving module 1 is installed on the printing platform, and the horizontal moving module 1 is used to realize the longitudinal movement of the droplet generating unit; the vertical moving module is installed on the horizontal moving module 1, and the vertical moving module is used to realize the axial movement of the droplet generating unit; the horizontal moving module 2 is installed on the vertical moving module, and is used to realize the lateral movement of the droplet generating unit.

[0024] As a further illustration of the present invention, the droplet generation unit includes an injection controller, a droplet injection valve, a nozzle temperature control component, a nozzle, a back pressure gas source, and a back pressure gas control valve;

[0025] The droplet generating unit is connected to the three-dimensional motion unit via a droplet ejection valve; one end of the droplet ejection valve close to the printing space temperature control unit is connected to the nozzle and the nozzle temperature control component; the nozzle temperature control component is located at the periphery of the nozzle and is used to heat the nozzle so that the temperature of the ejected droplets is at a set temperature; the other end of the droplet ejection valve is connected to the ejection controller and the back pressure gas source, wherein the back pressure gas source is controlled by the back pressure gas control valve.

[0026] As a further description of the present invention, the liftable frozen substrate includes a frozen substrate body and a lifting shaft, the lifting shaft is installed at the bottom of the printing platform, and the lifting shaft is used to control the lifting of the liftable frozen substrate in the middle of the metal fence.

[0027] As a further explanation of the present invention, a refrigeration medium return channel is provided in both the metal enclosure and the liftable refrigeration base plate, and a refrigeration temperature is generated through the refrigeration medium return channel.

[0028] As a further explanation of the present invention, the temperature controller includes a metal fence temperature controller and a liftable freezing substrate temperature controller. The metal fence temperature controller is used to control the temperature of the metal fence, and the liftable freezing substrate temperature controller is used to control the temperature of the liftable freezing substrate.

[0029] Compared with the prior art, the present invention has the following beneficial technical effects:

[0030] 1. The graphene droplet printing three-dimensional preform device provided by the present invention can solve the technical problems of controlling the cold environment of the printing space and the growth rate of ice crystals; a temperature-controllable and elevating frozen substrate and a metal wall are used to establish a stable and controllable cold environment in the printing space; the temperature of the elevating frozen substrate and the metal wall is lowered as the printing process progresses, and the position of the printing layer is dynamically adjusted using a precise elevating axis to achieve stable and adjustable temperature control in the printing space, which is conducive to the preparation of large-sized, morphology-controllable three-dimensional connected graphene.

[0031] 2. The heat transfer rate control method of the graphene droplet printing three-dimensional preform provided by the present invention solves the technical problem of uncontrollable ice crystal growth rate caused by the heat accumulation effect. By establishing the relationship formula between the number of printing layers and the temperature of the liftable freezing substrate, the number of printing layers and the height of the liftable freezing substrate, and the number of printing layers and the temperature of the metal wall, a mathematical model for heat transfer rate control is obtained to achieve stable control of the heat transfer rate and ensure the stability of the ice crystal growth rate, thereby preparing a three-dimensional connected graphene material with uniform axial microstructure and excellent performance.

[0032] 3. The heat transfer rate control method of the graphene droplet-printed three-dimensional preform provided by the present invention solves the problems of uneven microscopic pore size, different shapes and poor connectivity in the current graphene 3D printing technology. This method deepens the research on the precise and designable preparation of the macro and micro structures of three-dimensional connected graphene preforms, and significantly enhances the application potential of 3D printing technology in the field of porous material preparation. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a schematic diagram of a graphene droplet printing three-dimensional preform device provided by the present invention.

[0034] Figure 2 It is a flow chart of the heat transfer rate control method of a three-dimensional preform printed with graphene droplets provided by the present invention.

[0035] Figure 3 This is the heat transfer rate control result of the graphene droplet printing three-dimensional preform provided by the present invention.

[0036] Figure 4 This is a physical picture of the three-dimensionally connected graphene microstructure morphology before and after the graphene droplet printing three-dimensional preform provided by the present invention regulates the heat transfer rate.

[0037] Reference numerals:

[0038] 1-jet controller, 2-droplet jet valve, 3-nozzle temperature control assembly, 4-nozzle, 5-back pressure gas source, 6-back pressure gas control valve, 7-horizontal moving module one, 8-vertical moving module, 9-horizontal moving module two, 10-printing platform, 11-metal fence, 12-liftable frozen substrate, 13-frozen medium return channel, 14-metal fence temperature controller, 15-printing substrate, 16-lifting axis, 17-liftable frozen substrate temperature controller. DETAILED DESCRIPTION

[0039] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.

[0040] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0041] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0042] In the description of the present invention, it should be noted that the terms "center", "up", "down", "left", "right", "vertical", "horizontal", "inside", "outside", "axis", "longitudinal", "horizontal", etc. indicate positions or positional relationships based on the positions or positional relationships shown in the accompanying drawings, or the positions or positional relationships in which the product of the invention is usually placed when in use. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", "third", etc. are only used to distinguish the description, and cannot be understood as indicating or implying relative importance.

[0043] In addition, the terms "horizontal", "vertical" and the like do not mean that the components are required to be absolutely horizontal or suspended, but can be slightly tilted. For example, "horizontal" only means that its direction is more horizontal than "vertical", and does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0044] In the description of the present invention, it is also necessary to explain that, unless otherwise clearly specified and limited, the terms "set", "install", "connect", and "connect" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0045] The technical solution of the present invention will be explained below in conjunction with specific embodiments.

[0046] like Figure 2 As shown, the heat transfer rate control method for printing a three-dimensional preform with graphene droplets provided by the present invention has a heat accumulation effect during the printing of the three-dimensional preform, and the growth rates of ice crystals in different printing layers are inconsistent. The heat transfer rate is regulated according to the printing requirements, and the ice crystal growth rate can be accurately controlled. The ice crystal growth rate v is:

[0047] ;

[0048] Where L is the latent heat of phase change, A is the solid-liquid interface area, ρ is the liquid density, Q transfer is the heat transfer rate during ice crystal growth;

[0049] The heat conduction efficiency of the liftable frozen substrate 12 and the heat radiation efficiency of the metal enclosure 11 jointly determine the ice crystal growth during the printing process. The relationship between the heat conduction efficiency of the liftable frozen substrate 12 and the heat radiation efficiency of the metal enclosure 11 and the heat transfer rate of ice crystal growth is:

[0050] ;

[0051] Among them, T base is the temperature of the cryogenic substrate that can be raised or lowered, T layer is the droplet temperature, T wall is the temperature of the metal fence, R layer is the thermal resistance between printed layers, R0 is the contact thermal resistance between the copper printed substrate and the liftable frozen substrate, n is the number of printed layers, is the emissivity, is the Stefan-Boltzmann constant, r is the inner radius of the metal wall, and h is the height of the cryo-substrate that can be raised and lowered when printing the nth layer;

[0052] From the above formula, we can see that as the number of printing layers increases, T is controlled layer by layer. base The temperature of the cryostat substrate and T can be raised or lowered wall The temperature of the metal wall can realize the regulation of heat transfer rate during the printing process.

[0053] In some feasible embodiments, the device and heat transfer rate control method provided by the present invention are suitable for printing three-dimensional preforms, and more preferably suitable for multi-layer preforms. As the number of printed layers increases, the heat accumulation effect is multiplied, so that the heat transfer rate of ice crystals during printing is gradually reduced. It is difficult to accurately control the heat transfer rate by single heat conduction or heat radiation. Heat conduction and heat radiation are combined, and the printing process is divided into three stages, namely, low layer, middle layer and high layer, so as to achieve stable control of the heat transfer rate of ice crystals in each stage. The number of layers of the low-layer preform is limited by the minimum temperature of the liftable freezing substrate 12, which is at most n1 layers; the number of layers of the middle-layer preform is limited by the height of the liftable freezing substrate 12, which is at most n2 layers; and the number of layers of the high-layer preform is limited by the minimum temperature of the metal fence 11, which is at most n3 layers. The total number of preform layers is n1+n2+n3.

[0054] When printing the lower preform, the heat conduction rate is increased by gradually reducing the temperature of the liftable frozen substrate 12. At this stage, the focus is on controlling the temperature of the liftable frozen substrate 12 to decrease as the number of printed layers increases. The relationship between the temperature of the liftable frozen substrate 12 and the increase in the number of printed layers is as follows:

[0055] ;

[0056] Among them, n1 is the number of low-level printing layers, d is the thickness of a printed single layer, and h0 is the initial height of the liftable freezing substrate;

[0057] When printing the middle layer preform, the effective radiation area is increased by lowering the height of the liftable freezing substrate 12, and the cold radiation of the metal enclosure 11 is used to compensate for the heat accumulation effect. The number of printing layers printed in this stage is set to n2. At this time, the droplet temperature, the liftable freezing substrate temperature and the metal enclosure temperature remain unchanged. The relationship between the lowering height of the liftable freezing substrate 12 and the number of printing layers is:

[0058] ;

[0059] When printing a high-rise preform, the height of the frozen substrate 12 can be lowered to the lowest position, and the radiation efficiency can be increased by lowering the temperature of the metal enclosure 11. The number of printing layers printed in this stage is set to n3. At this time, the droplet temperature T layer , the temperature T of the freezing substrate 12 can be raised or lowered base is a constant value, and the temperature of the metal enclosure 11 decreases with the increase of the number of printing layers as follows:

[0060] ;

[0061] like Figure 1 As shown, the graphene droplet printing three-dimensional preform device used in the heat transfer rate control method provided by the present invention includes a printing substrate 15, a metal fence 11, a liftable freezing substrate 12, a temperature controller, a droplet generating unit, a three-dimensional motion unit and a printing platform 10; the liftable freezing substrate 12 is located below the printing substrate 15; the metal fence 11 is located around the printing substrate 15, and the temperature controller is used to control the temperature of the metal fence 11 and the liftable freezing substrate 12;

[0062] The droplet generating unit is mounted on the three-dimensional motion unit, located above the printing substrate 15, and is used to generate graphene droplets; the three-dimensional motion unit is used to realize the movement of the droplet generating unit in three different directions;

[0063] The three-dimensional motion unit includes a horizontal moving module 1 7, a horizontal moving module 2 9, and a vertical moving module 8; the horizontal moving module 1 7 is installed on the printing platform 10, and the horizontal moving module 1 7 is used to realize the longitudinal movement of the droplet generating unit; the vertical moving module 8 is installed on the horizontal moving module 1 7, and the vertical moving module 8 is used to realize the axial movement of the droplet generating unit; the horizontal moving module 2 9 is installed on the vertical moving module 8, and is used to realize the lateral movement of the droplet generating unit.

[0064] In some feasible methods, the horizontal moving module 1 7, the horizontal moving module 2 9, and the vertical moving module 8 can all use motor-driven screw transmission components to realize the movement of their carriers in a specified direction. This part belongs to mature existing technology and will not be described in detail here.

[0065] The droplet generating unit includes an injection controller 1, a droplet injection valve 2, a nozzle temperature control component 3, a nozzle 4, a back pressure gas source 5, and a back pressure gas control valve 6;

[0066] The droplet generating unit is connected to the horizontal moving module 2 9 of the three-dimensional motion unit through the droplet ejection valve 2. One end of the droplet ejection valve 2 close to the printing space temperature control unit is connected to the nozzle 4 and the nozzle temperature control component 3. The nozzle temperature control component 3 is located on the periphery of the nozzle 4 and is used to heat the nozzle 4 so that the temperature of the ejected droplets is at a set temperature. The other end of the droplet ejection valve 2 is connected to the ejection controller 1 and the back pressure gas source 5, wherein the back pressure gas source 5 is controlled by the back pressure gas control valve 6.

[0067] In some feasible embodiments, one end of the metal fence 11 is installed on the printing platform, and the liftable freezing substrate 12 includes a freezing substrate body and a lifting shaft 16, the freezing substrate body is connected to one end of the lifting shaft, and the other end of the lifting shaft 16 is installed at the bottom of the printing platform 10, and the up and down movement of the lifting shaft 16 is controlled by a motion controller, thereby driving the liftable freezing substrate 12 to move up and down in the middle of the metal fence 11; a freezing medium return channel 13 is opened in the metal fence 11 and the liftable freezing substrate 12, and a freezing temperature is generated by the freezing medium return channel 13; a metal fence temperature controller 14 is installed on the outside of the metal fence 11; the temperature of the metal fence 11 is controlled by the metal fence temperature controller 14; a liftable freezing substrate temperature controller 17 is installed on one side of the liftable freezing substrate 12, and the temperature of the liftable freezing substrate 12 is controlled by the liftable freezing substrate temperature controller 17.

[0068] In some feasible ways, in order to ensure the normal progress of droplet freezing printing, the printing device needs to be in a dry environment with a humidity value lower than 5%. The size of the metal fence 11 is determined according to the size of the sample to be printed.

[0069] The present invention will be further described below by means of specific examples.

[0070] Example 1

[0071] First, use the lifting shaft 16 to adjust the liftable frozen substrate 12 to the highest position, and record its initial height value. Then, add ink into the ink cartridge, complete the assembly of the various components of the device, and dehumidify the printing environment to ensure that the ambient humidity drops below 5%; when the humidity reaches the predetermined requirement, turn on the liftable frozen substrate temperature controller 17 and the metal enclosure temperature controller 14, and adjust the temperature of the liftable frozen substrate 12 and the metal enclosure 11 to -10°C. Subsequently, open the back pressure gas control solenoid valve 6, set the back pressure value in the solenoid valve to 0.2MPa, and adjust the three-dimensional motion unit to ensure that the distance between the nozzle 4 and the printing substrate 15 is less than 1cm. Next, start the injection controller 1, generate a pulse signal, drive the graphene droplet injection valve 2, form graphene droplets at the nozzle 4, and the droplets are then deposited on the copper printing substrate 15 and begin to solidify, completing the point-by-point printing of the first layer.

[0072] After the first layer is printed, the target temperature (for example -12°C) required for printing the second layer is calculated based on the relationship between the temperature of the liftable frozen substrate 12 and the number of printed layers; the liftable frozen substrate temperature controller 17 immediately starts to lower the temperature of the liftable frozen substrate 12 until the temperature of the liftable frozen substrate 12 reaches the calculated target value; at this time, the printing of the second layer is started, and so on, until the printing of the n1 layer preform is completed; at this time, due to the upper limit of the equipment, the temperature of the liftable frozen substrate 12 cannot continue to decrease.

[0073] After the printing of the n1 layer is completed, the target height of the liftable frozen substrate 12 required for printing the n1+1th layer is calculated based on the relationship formula between the height of the liftable frozen substrate 12 and the number of printed layers; the precision lifting axis 16 begins to adjust the height of the liftable frozen substrate 12 until it drops to the target height, at which time the printing of the n1+1th layer begins; and so on, the printing of the n2 layers of preforms is completed layer by layer; when the printing of the n1+n2 layers is completed, the height of the liftable frozen substrate 12 cannot be further reduced due to the lower height limit of the equipment.

[0074] Next, according to the relationship formula between the temperature of the metal fence 11 and the number of printed layers, the target temperature required for printing the n1+n2+1th layer is calculated; the metal fence temperature controller 14 starts to adjust the temperature of the metal fence 11 until it reaches the target temperature, and then starts to print the n1+n2+1th layer; and so on, the printing of the n3th layer of preform is completed; at this point, the printing process of the n1+n2+n3th layer of preform is finally completed, and the before and after comparison diagram of the printing results is shown in FIG. Figure 4 shown.

[0075] The above embodiments are preferred examples for implementing the present invention, and the present invention is not limited to the above embodiments. Any non-essential additions and substitutions made by those skilled in the art based on the technical features of the technical solution of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for controlling the heat transfer rate of a three-dimensional preform printed with graphene droplets, characterized in that: The heat transfer rate control method is implemented by using a graphene droplet printing three-dimensional preform device, wherein the device comprises a printing substrate (15), a metal enclosure (11), a liftable freezing substrate (12) and a temperature controller; The liftable freezing substrate (12) is located below the printing substrate (15); the metal enclosure (11) is located around the printing substrate (15); and the temperature controller is used to control the temperature of the metal enclosure (11) and the liftable freezing substrate (12); The heat transfer rate control method comprises dividing the three-dimensional preform printing process into three stages according to the number of printing layers, which are respectively a low layer, a middle layer and a high layer, wherein the number of layers is set to n1, n2 and n3 respectively, and the total number of preform layers is n1+n2+n3. During the layer-by-layer printing process, according to the relationship between the heat conduction efficiency of the liftable freezing substrate (12), the heat radiation efficiency of the metal enclosure (11) and the heat transfer rate of ice crystal growth, the temperature of the liftable freezing substrate (12), the temperature of the metal enclosure (11) and the height of the liftable freezing substrate (12) in each stage are controlled to achieve control of the heat transfer rate of ice crystal growth during the printing of the three-dimensional preform.

2. The method for controlling the heat transfer rate of a three-dimensional preform printed with graphene droplets according to claim 1, characterized in that: The relationship between the heat conduction efficiency of the liftable freezing substrate (12), the heat radiation efficiency of the metal enclosure (11) and the heat transfer rate of ice crystal growth is: ； Among them, Q transfer is the heat transfer rate during ice crystal growth, T base is the temperature of the cryogenic substrate (12) which can be raised or lowered, T layer is the droplet temperature, T wall is the temperature of the metal wall (11), R layer is the thermal resistance between the printed layers, R0 is the contact thermal resistance between the printed substrate (15) and the liftable frozen substrate (12), n is the number of printed layers, is the emissivity, is the Stefan-Boltzmann constant, r is the inner radius of the metal wall (11), and h(n) is the height of the cryostat substrate (12) that can be raised or lowered when printing the nth layer.

3. The method for controlling the heat transfer rate of a three-dimensional preform printed with graphene droplets according to claim 2, characterized in that: When printing the lower preform, the temperature of the droplet and the temperature of the metal wall (11) remain unchanged, and the heat conduction rate of ice crystal growth is increased by lowering the temperature of the liftable freezing substrate (12). The relationship between the number of printed layers and the temperature of the liftable freezing substrate (12) is: ； Among them, n1 is the number of low-level printing layers, d is the thickness of a printed single layer, and h0 is the initial height of the liftable freezing substrate; During the printing of the lower preform, the first layer is printed first, and the temperature required for printing the second layer is calculated based on the relationship between the number of printed layers and the temperature of the liftable frozen substrate (12). The liftable frozen substrate temperature controller (14) controls the liftable frozen substrate (12) to reach the required temperature before starting the second layer printing, and so on, until the temperature of the liftable frozen substrate (12) reaches the lower limit value, thereby completing the printing of the lower preform.

4. The method for controlling the heat transfer rate of a three-dimensional preform printed with graphene droplets according to claim 2, characterized in that: When printing the middle layer preform, the temperature of the droplet, the temperature of the elevating freezing substrate (12) and the temperature of the metal enclosure (11) remain unchanged, and the cold radiation of the metal enclosure (11) is used to compensate for the heat accumulation effect. By lowering the height of the elevating freezing substrate (12), the effective radiation area for ice crystal growth during the printing process is increased. The relationship between the number of printed layers and the height of the elevating freezing substrate (12) is as follows: ； During the process of printing the middle-layer preform, the height of the liftable freezing substrate (12) required for printing the n1+1th layer is first calculated, and the height of the liftable freezing substrate (12) is lowered by the precision lifting shaft (16) until it is lowered to the required height, and then the printing of the n1+1th layer is started, and so on, until the height of the liftable freezing substrate (12) is lowered to its lower limit value, and the printing of the middle-layer preform is completed.

5. The method for controlling the heat transfer rate of a three-dimensional preform printed with graphene droplets according to claim 2, characterized in that: When printing a high-layer preform, the temperature of the droplet and the temperature of the elevating freezing substrate (12) remain unchanged. By lowering the temperature of the metal wall (11), the radiation efficiency of ice crystal growth during the printing process is increased. The relationship between the number of printed layers and the temperature of the metal wall (11) is: ； During the process of printing the high-rise preform, the temperature of the metal wall (11) required for printing the n1+n2+1th layer is first calculated, and the temperature of the metal wall (11) is lowered by the metal wall temperature controller (17). After reaching the required temperature, the printing of the n1+n2+1th layer is started, and so on and so forth to complete the printing of the high-rise preform.

6. The method for controlling heat transfer rate of a three-dimensional preform printed with graphene droplets according to claim 1, characterized in that: The device also includes a droplet generating unit, a three-dimensional motion unit and a printing platform (10); The droplet generating unit is mounted on the three-dimensional motion unit and is located above the printing substrate (15), and is used to generate graphene droplets; the three-dimensional motion unit is used to realize the movement of the droplet generating unit in three different directions; The three-dimensional motion unit comprises a horizontal moving module 1 (7), a horizontal moving module 2 (9), and a vertical moving module (8); the horizontal moving module 1 (7) is mounted on the printing platform (10), and the horizontal moving module 1 (7) is used to realize the longitudinal movement of the droplet generating unit; the vertical moving module (8) is mounted on the horizontal moving module 1 (7), and the vertical moving module (8) is used to realize the axial movement of the droplet generating unit; the horizontal moving module 2 (9) is mounted on the vertical moving module (8), and is used to realize the lateral movement of the droplet generating unit.

7. A method for controlling heat transfer rate of a three-dimensional preform printed with graphene droplets according to claim 6, characterized in that: The droplet generating unit comprises an injection controller (1), a droplet injection valve (2), a nozzle temperature control component (3), a nozzle (4), a back pressure gas source (5), and a back pressure gas control valve (6); The droplet generation unit is connected to the three-dimensional motion unit via a droplet ejection valve (2); one end of the droplet ejection valve (2) close to the printing space temperature control unit is connected to a nozzle (4) and a nozzle temperature control component (3); the nozzle temperature control component (3) is located at the periphery of the nozzle (4) and is used to heat the nozzle so that the temperature of the ejected droplets is at a set temperature; the other end of the droplet ejection valve (2) is connected to an ejection controller (1) and a back pressure gas source (5), wherein the back pressure gas source (5) is controlled by a back pressure gas control valve (6).

8. The method for controlling the heat transfer rate of a three-dimensional preform printed with graphene droplets according to claim 6, characterized in that: The liftable freezing substrate (12) comprises a freezing substrate body and a lifting shaft (16); the lifting shaft (16) is installed at the bottom of the printing platform (10); the lifting shaft (16) is used to control the liftable freezing substrate (12) to be lifted or lowered in the middle of the metal enclosure (11).

9. The method for controlling heat transfer rate of a three-dimensional preform printed with graphene droplets according to claim 1, characterized in that: A freezing medium return channel (13) is provided in both the metal enclosure (11) and the liftable freezing base plate (12), and a freezing temperature is generated through the freezing medium return channel (13).

10. The method for controlling heat transfer rate of a three-dimensional preform printed with graphene droplets according to claim 1, characterized in that: The temperature controller comprises a metal enclosure temperature controller (14) and a liftable freezing substrate temperature controller (17); the metal enclosure temperature controller (14) is used to control the temperature of the metal enclosure (11); and the liftable freezing substrate temperature controller (17) is used to control the temperature of the liftable freezing substrate (12).

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