CN111457236A - Full-thermal super surface presenting infrared thermal illusion and being invisible under visible light - Google Patents
Full-thermal super surface presenting infrared thermal illusion and being invisible under visible light Download PDFInfo
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Abstract
The invention belongs to the technical field of thermodynamics, and particularly relates to a reconfigurable full-thermal super surface which presents an infrared thermal illusion and is invisible under visible light. The full-thermal super-surface structure unit is formed by periodically extending in the same plane; the heat source is positioned at the bottom of the super-surface structure, heat flow is transmitted along the direction vertical to the plane, the heat flow is transmitted in a heat conduction mode in the structure, the surface of the structure is transmitted to the surrounding environment in a heat convection and heat radiation mode, heat insulation is considered between units and the side surface of the whole super-surface, and the functions of hiding under visible light and presenting a thermal illusion under infrared are realized for an object; the structural unit is a rectangular cylinder, and the central part of the structural unit is a cylindrical open cavity; by using the heat radiation cavity effect, a specific phantom is displayed in the infrared. The invention is suitable for steady state or transient state conditions and is suitable for the condition that the background temperature is uniform or changed. Different from the traditional mechanism for realizing the infrared thermal illusion, the invention greatly expands the application range.
Description
Technical Field
The invention belongs to the technical field of super surfaces, and particularly relates to a reconfigurable full-thermal super surface which presents an infrared thermal illusion and is invisible under visible light, namely the reconfigurable full-thermal super surface presents specific apparent temperature distribution under an infrared field of view and cannot distinguish the difference between an object and a background under the visible light.
Background
All objects with a temperature above absolute zero emit electromagnetic waves, and this macroscopic radiation phenomenon, which is manifested by the thermal motion of microscopic particles, is called thermal radiation. From Wien's law, it is known that the temperature range is wide (10)0~103C), the spectral peak of the thermal radiation is in the infrared wavelength range, so the thermal radiation is in turn referred to as infrared radiation. The infrared radiation characteristic of an object is utilized to detect the surface temperature information of the object in a non-invasive and non-contact mode, and the method is widely applied to the fields of industry, military affairs and the like. As one of the most widely studied technologies in this field, the infrared phantom technology has shown its great value in the fields of military countermeasure and the like. There are two categories of existing infrared phantoms: "true hidden" and "false shown". The expression "invisibility" means that the infrared image of the object is completely merged into the background, and the difference between the object and the background cannot be distinguished from the infrared image. "false" means that the infrared image of the object exhibits characteristics of another object, thereby misleading or spoofing the infrared detector. According to the implementation mode, the existing infrared phantom technology has two main types: regulating the surface temperature and emissivity of the object. In the big background of the explosive development of thermal metamaterials in the last 10 years, researchers have been able to manipulate the surface temperature of objects as desired. However, the conventional research is mainly focused on the field of heat conduction, and the other two heat transfer methods, namely heat convection and heat radiation, are rarely researched. Furthermore, designs based on existing thermal metamaterials cannot hide the target itself in visible light, making this approach limited in applicability. The surface emissivity is mainly regulated and controlled through various coupling effects, such as photothermal effect, magnetocaloric effect, thermoelectric effect and the like. In recent years, phase-change ma based materialstMaterials) has also been extensively studied. But all ofThese emissivity control means all require extra external field to control, which makes the design of the device more complicated and heavy. Although there is a lot of research on the two kinds of control means, the problem is how to realize flexible and reliable infrared phantom under the condition of complicated environment and coexistence of multiple heat transfer modes.
In order to solve the problems, the invention provides a reconfigurable full-thermal super surface which presents an infrared thermal illusion and is invisible under visible light, namely the realized thermal super surface presents a specific thermal illusion graph under an infrared field, and the difference between the graph and the background cannot be distinguished under the visible light. By utilizing the thermal radiation cavity effect, the equivalent emissivity of the surface can be accurately calculated according to the shape, the structure and the area ratio of the surface cavity, and any characteristic graph of apparent temperature distribution of the surface under an infrared view field is realized by the aid of the discretization design and the reconfigurable characteristic of the surface unit. Theoretical derivation was verified by laboratory experiments. The invention is not only suitable for the steady state condition but also suitable for the transient state condition, and is not only suitable for the background with uniform temperature but also suitable for the background with non-uniform temperature. Different from the traditional mechanism for realizing the infrared thermal phantom, the invention has the advantages that the indiscriminability under visible light improves the application range, and the method is simple and easy to prepare.
Disclosure of Invention
The invention aims to provide a reconfigurable full-thermal super surface which presents an infrared thermal illusion and is invisible under visible light.
The full-thermal super-surface provided by the invention is formed by periodically extending structural units with the same material, size and appearance in the same plane (x-y plane); the heat source is positioned at the bottom of the super-surface structure, heat flow is transmitted along the direction (z direction) vertical to the plane, the heat flow is transmitted in a heat conduction mode inside the structure, the surface of the structure is transmitted to the surrounding environment in a heat convection and heat radiation mode, heat insulation is considered between units and the side surface of the whole super-surface, and the functions of hiding under visible light and presenting a thermal illusion under infrared are realized for an object.
The structural unit is a rectangular cylinder, the center part of the structural unit is a cylindrical open cavity, and the whole structure is a complex body with the shape of the rectangular cylinder and the center part of the structural unit is the cylindrical open cavity; setting the side length of the bottom surface (square) of the rectangular cylinder as a, the height as h, the radius of the cylindrical opening cavity as r, the height as h1, 2r < a, h1 < h; the size of h1 can be designed according to needs, h1 can be 0, and at the moment, the whole structural unit is a rectangular column.
The structural units with different cavity structures in the super surface are arranged and combined (namely coded) according to specific requirements, and corresponding specific phantom patterns are displayed (namely decoded) under infrared.
For example, when only the infrared phantom of an object needs to be displayed, the whole super surface only needs to be provided with two types of structural units with different cavity structures, the structural unit(s) with one type of cavity structure are arranged into a pattern corresponding to the object, and the structural unit(s) with the other type of cavity structure are used as the background outside the pattern of the object, so that the phantom graph of the specific object can be displayed under infrared.
In this case, h1 for the structural unit of one of the two types of different cavity structures may be 0.
In the invention, different combination arrangements are carried out on the same super surface according to the display requirements of different objects, namely recombination can be carried out.
The super surface provided by the invention comprehensively considers three heat transfer modes, namely heat conduction, heat convection and heat radiation.
The super-surface provided by the invention is effective to the condition that the temperature distribution of the background is uniform or nonuniform.
The super-surface provided by the invention can realize the effect of thermal illusion under the steady state or transient state condition.
The super-surface provided by the invention can also be expanded to modulate the surface temperature, the modulated parameter freedom degrees are unit height, thermal conductivity, thermal convection coefficient and emissivity, and three heat transfer modes are included.
The thermal infrared imager is a super-surface decoding tool of the present invention. Namely, under the infrared image, the thermal infrared imager can detect the characteristic pattern of the super surface, and the characteristic pattern can reversely deduce the coding mode.
The inventive super-surface and its structural units are further described below by means of specific examples.
The method comprises the steps of firstly, preparing a transparent acrylic thin plate which is used as a base table realized by a super surface, wherein the length of the acrylic thin plate is 22.5cm, the width of the acrylic thin plate is 17 cm, the thickness of the acrylic thin plate is 0.5-1cm, the plane where the acrylic plate is located is defined as an x-y plane, the direction perpendicular to the acrylic plate is a z direction, a square hole array is etched on the plate, and the hole array is carved through in the z direction, the array is formed by translation of a square with the side length of 1cm along the x direction and the y direction, the interval between holes is 0.1 cm, therefore, the lattice length along the x direction and the y direction is 1.1 cm, the x direction period and the y direction period are 20 and 15 respectively, namely, the array of 20 × 15 is arranged on the acrylic thin plate, and the distance from the peripheral edge of the acrylic plate to the nearest square hole is 0.25 cm.
The method comprises the steps of preparing cubic column-shaped structural units for embedding in a hole array of an acrylic sheet, wherein the array period is 20 × 15, so that a total of 300 independent structural units are required to fill the array, 300 copper cubic units with the length and width of 1cm and the height of 2 cm are adopted, the number of the units is 300, the units are divided into two types, one type is 130, no structure is prepared (namely a solid copper column), the other type is 170, a cylindrical cavity structure is prepared at the surface center position, the radius of the cavity is 0.4 cm, the depth of the cavity is 1cm, and two groups of structural units with different surface emissivity can be obtained.
And finally, randomly embedding 300 copper columns into 300 square holes, and keeping the upper surfaces of the copper columns in a horizontal plane to obtain the super-surface of the invention.
The super surface platform provided by the invention has the following coding and decoding modes:
the method includes the steps of providing a thermal infrared imager, providing a set of holes, providing a set of copper pillars, providing a set of holes, providing a set.
The above coding process can be reset, that is, after the coding is completed, all the structural units are taken out and re-coded, which is the reconfigurability of the present invention. The infrared heat illusion patterns of human bodies, machine guns and English letters FD are displayed on the same acrylic sheet and coded by the same 300 structural units, and finally three different heat illusion patterns are formed.
The principle of the present invention is further explained below. Considering the environment of full heat, full heat refers to the simultaneous existence of heat conduction, heat convection and heat radiation. The heat source is disposed on the lower surface of the super surface. Since there is no thermal contact between the structural units, the heat flow is transmitted in the z-direction, inside the structural units in a heat conductive manner, and on the upper surface of the structural units in a heat convective and radiative manner to the surroundings.
The apparent temperature in the infrared field depends on the temperature of the upper surface of the objectT surAnd emissivitysurApparent temperatureT readCan be expressed by the following expression:
wherein the content of the first and second substances,Csignal conversion coefficient (lambda) representing thermal infrared imager1, λ2) For the frequency bands detectable by the thermal infrared imager, these two parameters are both intrinsic to characterizing the thermal imager and are not considered here. u (a) is calculated from the measured values of,T sur) Is the radiation spectrum of the object itself, which can be described by planck's law. Assuming surface temperatureT surKnown, therefore, surface apparent temperatureT readOnly the emissivitysurIt is related. The specific form of the apparent temperature of the surface, estimated by kirchhoff's law of radiation, is as follows:
wherein (x, y) is a position parameter of the unit object on the x-y plane.
The structural design of the surface is guided by the cavity effect of the heat radiation surface. For the convenience of manufacture, the cavity is assumed to be a cylindrical structure, and the area of the circular opening of the cylinder of the cavity is set asS 0(π × r × r) having an inner surface area ofS 1(pi × r × r +2 × pi × r × h1), the intrinsic emissivity of the surface isb. For such an open cavity surface, its equivalent emissivityeCan be obtained from the following formula:
let the area fraction of the opening of the open cavity on the upper surface (a × a) of the cell bef =Pi (r × r)/(a × a), because there is no energy exchange between different cavities, the equivalent emissivity of the whole unit surface according to the superposition of energysurComprises the following steps:
by adjusting the cavity mouth-to-cavity area ratio S of the cylindrical cavity0/S1(aspect ratio) and the ratio of the area to the surface
Different equivalent emissivities can be freely adjusted. It is worth noting that the equivalent emissivity here can only be larger than the intrinsic emissivity of the material, but not smaller than the intrinsic emissivity, which is determined by the principle of the cavity effect.
Any group of unit structures with different emissivities can be designed through the cavity effect, and the units are combined in a specific mode to obtain a specific infrared pattern. The contrast of the infrared pattern is defined as:
then for the above solution, the contrast depends on the maximum and minimum equivalent emissivity of the surface:
given the different groups of cells, the arbitrary permutation and combination thereof does not affect the contrast under the infrared field of view, so that reconstructing the surface does not affect the imaging quality.
The above scheme we assume the surface temperatureT surThe case of no change. Further, the platform can also realize the function ofT surModulation of (3). Different heat transfer links can be obtained by adjusting the heat conductivity, the longitudinal height, the convection heat transfer coefficient, the emissivity and the like of each structural unit.
The invention has the advantages that:
(1) the method provided by the invention comprehensively considers three heat transfer modes, namely heat conduction, heat convection and heat radiation;
(2) the method provided by the invention has a simple structure, is easy to prepare, and only needs to use common materials;
(3) the method provided by the invention is not only suitable for steady state but also suitable for transient state;
(4) the method provided by the invention has expansibility, and can adjust the surface temperature and the emissivity on one platform at the same time.
Drawings
FIG. 1 is a schematic representation of the full thermal super surface structure of the present invention.
FIG. 2 is a pattern of the fully thermal super-surface of the present invention in a visible field of view (looking down from the z-direction). Different arrays are formed by different arrangement modes of the unit structures. Under different arrays, the same pattern was observed.
Fig. 3 is a pattern of the fully thermal super-surface of the present invention under an infrared field of view (looking down from the z-direction). The characteristic phantom pattern under the infrared field of view can be seen, and different unit arrangement forms correspond to different infrared characteristic patterns.
Fig. 4 shows the working conditions under which the present invention works in practice, i.e. conduction heat flow much larger or much smaller than convection superimposed radiation heat flow. In this condition, the temperature of the upper surface is nearly uniform and can be considered as uniform in temperature. The surface temperature of different units is different under the infrared view field through the adjustment of the surface emissivity of different units. Then, different arrays of cells correspond to characteristic infrared patterns (apparent temperature distributions).
FIG. 5 illustrates another condition of the present invention. When the conduction heat flow is comparable to the heat flow radiated by the superposition of convection, adjusting the actual temperature of the surface becomes a more reasonable means of regulation. The adjusting mode comprises four degrees of freedom such as unit thermal conductivity, height, surface convection coefficient and emissivity. The emissivity adjustment will then simultaneously contribute to the true surface temperature and the apparent temperature under the thermal infrared imager.
FIG. 6 is a top view and a front view of the structural design of the present invention, we use a periodic arrangement of 15 × 20 cubes to adjust the emissivity of each cube (structural unit) separately, by making a cylindrical cavity in the center of the top surface of the unitIn a visible light field of view, the whole surface is identical, so that decoding cannot be performed; but in the infrared field of view, the characteristic pattern can be seen, and the decoding operation is realized. Wherein when the conduction heat flow is comparable to the convection radiation heat flow, the surface temperature T can be adjustedsurTo realize the infrared phantom; when the conduction heat flow is far larger or far smaller than the convection radiation heat flow, the surface temperature is close to uniform, and in this case, the surface emissivity needs to be adjustedsurTo realize the infrared illusion. For simplicity, we assume that one of the parameters is a constant value when the other is adjusted. T can be simultaneously paired on such a super surfacesurAndsurthe method is carried out, and the flexibility and the expansibility of the method are embodied.
FIG. 7 is a diagram of experimental principles, devices and final effects of emissivity modulation using cavity effect. Wherein, (a) shows the position and the appearance of the cavity structure on each unit, and copper cubic units with the length and the width of 1cm and the height of 2 cm are adopted in the experiment. The unit is divided into two types, one type does not prepare any structure, and the other type prepares a cylindrical cavity structure at the surface center position to obtain two groups of units with emissivity. The cylindrical cavity has a diameter of 0.8 cm and a height of 1cm, and the theoretical calculated equivalent emissivity is about 0.6 (the intrinsic emissivity of the original copper surface is 0.2).
Fig. 8-10 are experimental setup and effect diagrams for three arrangements, respectively, using high emissivity elements in spatial locations where the surface is humanoid (fig. 8), machine gun (fig. 9) and letters "FD" (fig. 10), and low emissivity elements in surrounding background locations, both elements filling the entire surface to complete the encoding. For coding, the unit structures are arranged on a customized acrylic plate, then the unit structures are placed in constant-temperature hot water at 50 ℃, and shooting is carried out after a stable state is achieved, so that infrared characteristic patterns of human figures, machine guns and letters FD are obtained respectively. The three coding modes under visible light hardly see any difference, but under infrared field of view are completely different. The characteristic patterns are still clear and distinguished by adjusting the observation angles (0 degrees, 30 degrees and 60 degrees). Unit of temperature bar: DEG C.
FIG. 11 is a unit structure grouping of the super-surface extended application designed by the present invention, wherein the unit structure is divided into 6 groups, which correspond to the background and five words of "F", "U", "D", "A" and "N", respectively. Wherein, the unit structures representing five words of 'F', 'U', 'D', 'A' and 'N' are respectively arranged at the positions of the surface space displaying the corresponding letters, and the other positions are filled with units representing the background to form a periodic array.
Fig. 12-14 are surface true temperature simulations of the present invention for regulating thermal conduction, convection and radiation according to the groupings of fig. 11. In FIG. 12, the heat transfer coefficient and emissivity of the fixed unit were constant, the thermal conductivities of the 6 groups were set to 0.5, 1, 2, 3, 4 and 5W/m K, respectively, and the bottoms were placed in thermal reservoirs at temperatures of 350K and 700K, resulting in a surface temperature profile at steady state. Similarly, FIG. 13 shows the surface temperature profile at steady state simulated by modulating the convective heat transfer coefficient and setting the heat transfer coefficients of the 6 groups to 5, 10, 20, 30, 40 and 50W/m 2K, respectively. Fig. 14 shows the surface temperature distribution in the steady state simulated by modulating the emissivity and setting the emissivity of the 6 groups to 0.1, 0.2, 0.4, 0.6, 0.8 and 1, respectively. At lower temperatures (350K), the effect obtained by regulating the thermal convection and radiation is not good. While the modulation of thermal convection and thermal radiation gradually shows effects when the temperature is raised (700K).
Fig. 15-17 are graphs comparing theoretically calculated surface temperatures and simulated data, corresponding to the simulated cases of fig. 12-14, respectively. At low temperatures (350K), the theoretical calculations match well with the simulated data, while at high temperatures (700K), they are slightly off-set, mainly because the cells are in direct thermal contact with the cells in order to better match the real situation during the simulation, so that there is a heat flow in the x-y plane, shunting a portion of the original z-direction heat flow. The comparison of the theoretical calculation value and the simulation data proves that the emissivity and the real surface temperature can be modulated on the super-surface platform.
Detailed Description
The present invention will be described in detail below with reference to specific examples and drawings, but the present invention is not limited thereto.
FIG. 1 is a schematic representation of the reconfigurable fully thermal super-surface structure of the present invention. The heat flow is transferred by means of heat conduction from bottom to top within the super-surface, and dissipated to the environment at the upper surface by thermal convection and thermal radiation mechanisms. The heat flow path of the super surface comprehensively considers three heat transfer modes of heat conduction, heat convection and heat radiation. The thermal infrared imager is positioned right above the upper surface and is used for observing the temperature distribution of the super surface.
First, we have conducted laboratory experiments, see fig. 7-10. surface cells are divided into two classes, one class that produces a cylindrical cavity structure in the center of the upper surface and the other class that maintains the original flat surface without producing additional structures, and two classes of cells with different surface emissivity can be obtained.by means of the radiative cavity effect, the principle is described by equations (3, 4). the cube cell is 1cm long and wide and 2 cm high, and is made of copper (thermal conductivity 397W/m K), and intrinsic emissivity is 0.2. a cylindrical cavity is made in the surface, with a cylinder diameter of 0.8 cm and a cylinder height of 1cm, and the theoretical calculation yields an equivalent of 0.6. an additional acrylic plate is made for fixing the cell structure, with 15 holes 15 and 32 holes, the two groups of cell structures are separately placed on the acrylic plate for encoding operations, three different combinations are compiled, then the entire acrylic plate together with the cell array is placed in a constant temperature of 50 ℃, the water temperature reaches 15 × 10 degrees, the two groups of cell structures are separately placed on the acrylic plate for encoding operations, and if the infrared thermography is adjusted to obtain a clear thermal image of the infrared image, which is a thermal image, which is interpreted by means that the infrared camera, if the infrared camera, the infrared camera is still obtains a clear thermal image with a temperature of 30 degrees, which is interpreted as a thermal image, which is similar to an infrared image, which is clearly shows a thermal effect, and a thermal image, which is clearly shows a thermal effect, and a thermal image with a thermal effect, which is clearly shows a thermal effect, and a thermal effect, which.
The effect of adjusting the convective heat transfer coefficient and emissivity is seen to be much less than that of thermal conductivity, especially at 350K (low temperature), the effect of convection and radiation on surface temperature is shown as well as that of thermal contact, but this thermal contact has little effect on the results, as compared to theoretical calculations and simulation data for surface temperature, respectively, and fig. 15-17 are shown as a comparison of surface temperature (fig. 15), a simulation using commercial software COMSO L, and the thermal contact effect is seen to be very good, but not good, as the thermal contact effect is seen at high temperature, as the thermal conductivity is seen to be very different from that of thermal conductivity, as the thermal contact effect is seen to be very good, as the thermal contact effect is seen to be high temperature, as the thermal contact effect is seen to be very good, as the thermal contact effect is seen to be very good, as the thermal contact effect is seen in fig. 15-17.
Claims (8)
1. A reconfigurable full-thermal super surface which presents an infrared thermal illusion and is invisible under visible light is characterized by being formed by periodically extending structural units with the same material, size and appearance in the same plane; the heat source is positioned at the bottom of the super-surface structure, heat flow is transmitted along the direction vertical to the plane, the heat flow is transmitted in a heat conduction mode in the structure, the surface of the structure is transmitted to the surrounding environment in a heat convection and heat radiation mode, heat insulation is considered between units and the side surface of the whole super-surface, and the functions of hiding under visible light and presenting a thermal illusion under infrared are realized for an object;
the structural unit is a rectangular cylinder, the center of the structural unit is a cylindrical open cavity, and the whole structure is a complex body with the shape of the rectangular cylinder and the center of the cylindrical open cavity; setting the side length of the bottom surface of the rectangular cylinder as a, the height as h, the radius of the cylindrical opening cavity as r, the height as h1, 2r < a, h1 < h; the size of h1 is designed according to needs; when h1 is 0, the whole structural unit is a rectangular cylinder;
the structural units are provided with cavity structures, so that a cavity effect is generated on thermal radiation, different emissivity is formed on the surface of the structural units for thermal radiation for different cavity structures, so that different thermal radiation cavity effects are generated, a specific phantom is displayed under infrared, and corresponding specific phantom patterns are displayed under infrared by arranging and combining the structural units with different cavity structures in the super surface according to specific requirements, namely encoding.
2. The full-thermal super-surface according to claim 1, wherein when only one object needs to be able to display an infrared phantom, the whole super-surface only needs to have two types of structural units with different cavity structures, a plurality of structural units with one type of cavity structures are arranged in a pattern corresponding to the object, and a plurality of structural units with another type of cavity structures are used as a background outside the pattern of the object, so that the phantom image of the specific object can be displayed in the infrared.
3. The full thermal meta-surface of claim 2, wherein h1 for one of the two different types of structural units of cavity structure is 0.
4. The full-thermal super-surface according to claim 2, wherein different combinations of arrangements, i.e. recombinations, are made for the same super-surface according to the display needs of different objects.
5. The total-thermal super-surface according to claim 1, wherein different heat transfer links are obtained by adjusting the heat conductivity, the longitudinal height, the convection heat transfer coefficient and the emissivity of each structural unit, so that the functions of infrared thermal illusion and invisibility under visible light are realized.
6. The full-thermal super-surface according to claim 1, wherein the real surface temperature of each structural unit is regulated and controlled by three regulation and control modes of heat conduction, heat convection and heat radiation, so that the functions of infrared thermal illusion and invisibility under visible light are realized.
7. The full thermal metasurface of claim 1, wherein both infrared thermography and stealth under visible light are achieved under steady state or transient conditions.
8. The full thermal metasurface of claim 1, wherein the background temperature is uniform or non-uniform, and both infrared thermography and stealth under visible light are achieved.
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Cited By (5)
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| CN113191047A (en) * | 2021-04-25 | 2021-07-30 | 华中科技大学 | Regulation and control method for realizing super-structure material soaking substrate |
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| CN115018794B (en) * | 2022-06-09 | 2023-02-07 | 中国矿业大学(北京) | Coal and gangue identification system and method based on visible light/infrared image |
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