Foamed graphene with three-dimensional curled porous structure and preparation method thereof

Abstract

The invention provides foamed graphene with a three-dimensional curled porous structure and a preparation method of the foamed graphene, and further relates to application of the foamed graphene in graphene slurry and a piezoresistive sensor. The foamed graphene is prepared by carrying out rapid thermal reduction treatment on graphene oxide powder at the speed of more than 100 DEG C / min, presents an irregular three-dimensional curled hierarchical porous structure, and has the characteristics that the specific surface area is 600-800 m < 2 > / g, the pore wall thickness is less than 5 nm, the pore diameter range is 0-200 nm, and the pore volume is more than 2.2 cc / g. According to the preparation method, instant expansion of the graphene sheet layer is induced through ultrafast thermal reduction, so that a three-dimensional skeleton structure with an ultrathin hole wall and a highly continuous structure which are difficult to obtain by a traditional method is realized, and the conductivity, the mechanical stability and the adsorption capacity of the material are remarkably improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of graphene preparation, and particularly relates to foamed graphene with a three-dimensional crimped porous structure and a preparation method thereof. BACKGROUND

[0002] With the rapid development of flexible electronics, smart wear, health monitoring and human-computer interaction technology, the demand for new flexible conductive materials and high-performance sensitive materials continues to grow. In particular, in applications such as flexible piezoresistive sensors, flexible tactile units, structural health monitoring components, materials not only need to have excellent electrical conductivity, but also must have good mechanical flexibility, stability, and the ability to be coated and processed into films on a large scale.

[0003] Graphene, as a typical two-dimensional carbon material, is considered an ideal basis for constructing the above-mentioned sensitive materials due to its high strength, high electrical conductivity, and chemical stability. However, traditional graphene, especially the solid aggregate structure formed by stacking graphene sheets, is prone to problems such as re-stacking between sheets, limited channels, and reduced specific surface area, which makes it difficult to achieve high sensitivity strain response in practical applications and limits its use in high specific surface area scenarios such as sensing and energy storage.

[0004] Therefore, constructing foamed graphene with a three-dimensional porous structure, thin pore walls, and good connectivity has become one of the key research directions in recent years. In order to introduce a porous structure, one type of existing technology aims to avoid foaming and obtain a dense reinforcement body. For example, in patent CN106966383B, a chemical reducing agent (such as hydrogen iodide, hydrazine hydrate) is used to slowly pre-reduce graphene oxide, and then slowly heated (such as 5℃ / min) to ultra-high temperature (2500-3000℃) treatment, the purpose is to prevent gas from being released quickly, so that graphene sheets are tightly folded and combined, forming high-density, low-specific-surface-area "paper ball" microspheres, which are used to enhance the mechanical properties of composite materials.

[0005] Another type is a method aimed at constructing a porous structure, such as patent CN119176550A, which introduces aminoguanidine carbonate as a reducing agent and foaming agent, and through slow heating (2-5℃ / min) stepwise heat treatment, the reduction reaction and foaming agent decomposition are slow and step-by-step, thus forming a multi-level porous structure. Although this method obtains a porous material, it is complex and requires precise control of multiple steps, and relies on additional chemical foaming agents.

[0006] In summary, in the prior art, whether densification or porosity is pursued, the process involves additional chemical reagents and a slow heating process, which is complex and costly, and it is difficult to design the material structure specifically for the contradictory requirements of high initial resistance change rate and wide range of linear response for piezoresistive sensing. Therefore, developing a method for large-scale controllable preparation of graphene foam with unique foaming structure in one step without additional reagents and for high-performance piezoresistive sensors has become a problem to be solved in the field. SUMMARY

[0007] One object of the present application is to provide a foamed graphene with a three-dimensional curly porous structure and a preparation method thereof. The graphene oxide powder is subjected to ultra-fast thermal reduction treatment, so that the graphite layers are severely expanded under instantaneous high temperature impact, thereby forming an irregular three-dimensional curly structure that is difficult to obtain by traditional processes, so that the material can simultaneously realize a high specific surface area of 600-800 m 2 / g, a multi-level pore distribution of 0-200 nm in pore size, a pore volume of more than 2.2 cc / g, and an ultrathin pore wall of less than 5 nm, to solve the problems of loose porous graphene structure, discontinuous pore channel, insufficient specific surface area, serious stacking of graphene layers, and easy breakage during subsequent pulping process.

[0008] To achieve the above object, the first aspect of the present application provides a foamed graphene with a three-dimensional curly porous structure, which is prepared by subjecting graphene oxide powder to rapid thermal reduction treatment at >100℃ / min. The foamed graphene has an irregular three-dimensional curly porous structure, and the specific surface area of the foamed graphene material is 600-800 m 2 / g, the pore wall thickness is <5 nm, the pore size is 0-200 nm, and the pore volume is >2.2 cc / g.

[0009] Further, the carbon-oxygen ratio of the foamed graphene is 4.8-5.2.

[0010] The present application provides a preparation method of the above-mentioned foamed graphene with a three-dimensional curly porous structure, comprising the following steps: S1: preparing graphene oxide powder; S2: subjecting the graphene oxide powder to rapid thermal reduction treatment at >100℃ / min under a non-oxidizing atmosphere and holding for 1-60 min to obtain the foamed graphene.

[0011] Further, the graphene layers in the graphene oxide powder are 1-2 layers, and the particle size of the graphene oxide powder is 30-300 mesh. The oxygen content in the graphene oxide powder is 50-60wt%, the carbon content is 40-50wt%, and the water content is 5-12wt%.

[0012] Further, in the rapid thermal reduction process, the thermal reduction temperature is 500-800℃.

[0013] The application also provides a graphene slurry, which uses the foamed graphene described above and comprises the following raw materials in mass percentage: foamed graphene 1-8wt%; conductive carbon black 0-5wt%; resin 20-35wt%; solvent 52-79wt%.

[0014] Further, the specific surface area of the conductive carbon black is 200-400m 2 / g, the particle size is 20-50nm, the DBP oil absorption value is >200ml / 100g, and the pH is 8-10.

[0015] Further, the resin is any one or several of polyurethane resin, epoxy resin, acrylic resin, silicone resin, phenolic resin, vinyl resin, and aldehyde ketone resin.

[0016] Further, the solvent is any one or several of ethanol, ethylene glycol, isopropyl alcohol, glycerol, isopropyl propionate, N-methyl pyrrolidone, dimethylformamide, dimethylacetamide, toluene, xylene, isophorone, and n-hexane.

[0017] The application also provides a preparation method of a piezoresistive sensor, which uses the graphene slurry described above and comprises the following steps: The graphene slurry is coated on a flexible substrate by screen printing, blade coating, or spraying to form a sensing film; The sensing film is cured at 80-160℃ to form a stable sensitive layer; Electrodes are made at both ends of the sensitive layer, and the piezoresistive sensor is obtained.

[0018] The application embodiments have the following technical effects: (1) The application forms a three-dimensional foamed structure with random crimped morphology and through multi-level pores by applying a rapid thermal reduction process of more than 100 ℃ / min to graphene oxide powder, so that the layers produce strong gas release and expansion behavior in a very short time, thereby forming a three-dimensional foamed structure with random crimped morphology and through multi-level pores. The structure has a high specific surface area and pore volume, which provides a larger interface contact area for the material than traditional reduced graphene, makes the electron transport path more dense and continuous, and effectively suppresses the re-stacking phenomenon between the layers. The characteristic of the pore wall thickness being less than 5 nm makes the overall skeleton lightweight and has excellent flexibility, which can uniformly buffer stress and maintain the continuity of the conductive path under mechanical deformation, avoiding the instability of the signal caused by the sliding or rupture of the traditional graphene network under bending or compression. At the same time, the distributed pore size of 0-200 nm makes the structure present a highly hierarchical gas permeation channel on the macroscopic level, which improves the infiltration and dispersion behavior of the material in the slurry system and also improves the film uniformity and stability during the subsequent film coating process. More importantly, the three-dimensional porous skeleton enables the electron migration path to form a network in a multidirectional space, which can achieve high conductivity at a lower filling amount, reduce the proportion of invalid carriers in the slurry, and improve the overall functional density of the sensitive film. BRIEF DESCRIPTION OF DRAWINGS

[0019] The accompanying drawings, which are part of this application, serve to further understand this application, the illustrative embodiments of this application and their descriptions serve to explain this application, but do not constitute an improper limitation on this application. Obviously, the drawings in the following description are only some embodiments, and other drawings can be obtained from these drawings by those of ordinary skill in the art without creating labor. In the drawings: Figure 1 TEM image of graphene oxide powder in the application; Figure 2 Scanning electron microscope photo of foamed graphene prepared in Example 1 in the application; Figure 3 Scanning electron microscope photo of foamed graphene prepared in Comparative Example 1 in the application; Figure 4 Scanning electron microscope photo of foamed graphene prepared in Comparative Example 2 in the application; Figure 5 Scanning electron microscope photo of foamed graphene prepared in Comparative Example 3 in the application; Figure 6 Relative resistance change rate-pressure relationship curve of the piezoresistive sensor prepared in Experimental Group 1 and Control Group 1-3 in the application; Figure 7 Relative resistance change rate-pressure relationship curve of the piezoresistive sensor prepared in Experimental Group 1-3 in the application. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments will be clearly and completely described below with reference to the accompanying drawings. The following embodiments are used to illustrate this application, but are not intended to limit the scope of this application.

[0021] In the description of this application, it should be noted that the terms "upper", "lower", "front", "rear", "left", "right", "vertical", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0022] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installation," "connection," and "joining" should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0023] Those skilled in the art should understand that the embodiments described below are merely a part of the embodiments of this application, and not all of the embodiments of this application. These partial embodiments are intended to explain the technical principles of this application and are not intended to limit the scope of protection of this application. Based on the embodiments provided in this application, all other embodiments obtained by those skilled in the art without creative effort should still fall within the scope of protection of this application.

[0024] The foamed graphene with a three-dimensional rolled porous structure, its preparation method, and the graphene slurry and piezoresistive sensor constructed based on the foamed graphene proposed in this application aim to address the long-standing technical bottlenecks of existing graphene materials in terms of structural stability, specific surface area, pore volume, sheet stacking, dispersibility, coating performance, and electrical response stability in flexible sensors. It provides a technical solution that achieves a breakthrough in overall performance from material structure to processing system to device construction, thereby significantly improving the usability and engineering potential of conductive materials in the field of flexible sensing.

[0025] Existing graphene porous materials usually rely on hydrogel freeze-drying method, template method, chemical vapor deposition or traditional slow heating expansion method and other preparation means, but these methods are either uncontrolled structure, thick pore wall, poor connectivity of pore channel, or complex process, high cost, difficult to scale up, at the same time, in the subsequent pulping and film forming process, the key problems such as sheet layer fracture, discontinuous conductive network, uneven film layer, poor repeatability and the like are prone to occur.

[0026] In addition, two-dimensional sheet-shaped graphene is subject to sheet restacking tendency, and in piezoresistive sensing applications, unstable conductive path, low sensitivity or structural damage caused by stress concentration are prone to occur, which is difficult to meet the comprehensive requirements of high sensitivity, high cycle stability and high flexibility of flexible devices, and traditional conductive ink with carbon black as filler is also limited by simple structure, weak piezoresistive response and high filler content dependence, resulting in strong film layer brittleness.

[0027] In view of these long-term unsolved difficulties, the technical scheme proposed by the present application is to implement rapid thermal reduction treatment of graphene oxide powder at more than 100 ℃ / min, so that the oxygen-containing groups are decomposed violently at a high temperature and a large amount of gas is released at a high speed, thereby triggering the micro-explosion and high-speed expansion of the sheet layer, and the sheet layer spontaneously curls to form a foaming structure with high three-dimensionality, interconnected pores and extremely thin pore walls. The specific surface area of the foaming structure can reach 600-800 m 2 / g, the pore wall thickness can be less than 5 nm, the pore volume can be more than 2.2 cc / g, and the pore size range can reach 0-200 nm, which breaks through the limitations of traditional expanded graphene, such as easy collapse, limited specific surface area and discontinuous pore channel, and realizes a leapfrog transformation of the material structure from two-dimensional sheet to lightweight high-strength three-dimensional skeleton.

[0028] This structure not only provides excellent elastic recovery capability in mechanics, ensuring that the conductive network does not fail during deformation, but also forms multiple-point contact and space conductive path in electronic transmission, so that a low filler amount can build a high conductivity network, thereby reducing the film layer cracking or brittleness caused by high filler in the resin system.

[0029] In addition, the large specific surface area and surface roughness of the three-dimensional curled structure make it have stronger wettability and interfacial bonding capacity when mixed with resin and solvent, significantly improve the dispersion stability of the conductive filler in the slurry system, avoid common sheet layer aggregation, sedimentation and abnormal system viscosity caused by interface mismatch, and enable the slurry to realize uniform spreading and form a continuous, dense and stable film layer in the process of screen printing, blade coating and spraying.

[0030] The sensing thin film prepared therefrom can form a sensitive layer showing obvious resistance change under the action of micro pressure after curing. Benefiting from the structural characteristics of thin pore wall, large pore volume and three-dimensional connection of the foamed graphene, the current-carrying path will change controllably during compression, so that the sensor not only has higher sensitivity, but also has fast response, fast recovery and excellent cycle stability, effectively overcoming the problems of easy fatigue, response attenuation, low signal-to-noise ratio and poor device consistency of traditional graphene sensitive layer.

[0031] Specifically, the present application provides a foamed graphene with a three-dimensional curly porous structure, characterized in that the foamed graphene is prepared by rapid thermal reduction treatment of graphene oxide powder at a heating rate of >100℃ / min. The foamed graphene has an irregular three-dimensional curly porous structure, and the specific surface area of the foamed graphene material is 600-800 m 2 / g, the pore wall thickness is <5 nm, the pore size is 0-200 nm, and the pore volume is >2.2 cc / g.

[0032] Specifically, the foamed graphene with a three-dimensional curly porous structure proposed in the present application is prepared by ultra-fast thermal reduction treatment of graphene oxide powder at a heating rate of >100℃ / min.

[0033] Specifically, the performance of the foamed graphene material of the present application depends on the precise balance relationship between the structural parameters. That is, the specific surface area is 600-800 m 2 / g, the pore wall thickness is <5 nm, the pore size is 0-200 nm, and the pore volume is >2.2 cc / g. When these parameters deviate from the range, the physical and chemical properties of the material and its performance in terms of electrical conductivity, mechanics, dispersibility and sensing response will be affected.

[0034] Specifically, from the perspective of specific surface area, when the specific surface area is lower than 600 m 2 / g, the total area of the internal accessible interface of the material decreases significantly, which means that the number of active sites in the material and the density of electron migration channels decrease, resulting in insufficient spatial connectivity of the conductive network.

[0035] Specifically, low specific surface area often stems from excessively thick pore walls or insufficient pore volume. In such cases, the graphene sheets are highly packed, compressing the interlayer space and confining electron transport paths within a two-dimensional plane, resulting in strong interfacial contact resistance and a decrease in overall conductivity. Conversely, while excessively high specific surface area theoretically increases the interfacial contact area, it is often accompanied by ultra-thin pore walls and an extremely loose framework structure, leading to excessively low overall material density. This makes the material prone to collapse under mechanical stress, and the pore structure struggles to maintain stability during compression cycles. This manifests as high sensitivity but poor repeatability and shortened fatigue life. Excessive specific surface area also increases the amount of air and solvent adsorption, significantly affecting the material's electrical response to ambient humidity, thereby reducing the sensor's stability and reproducibility.

[0036] Specifically, the thickness of the pore walls is a key factor determining the mechanical stability and electrical continuity of foamed graphene. When the pore wall thickness exceeds 5 nm, the electron tunneling effect within the sheet is weakened, and the conductive network changes from a quantum leap type to a surface contact type. Electron transport mainly relies on physical contact rather than local tunneling, resulting in decreased sensitivity. Simultaneously, thicker pore walls reduce the material's deformability under external forces, causing its piezoresistive response curve to flatten and signal changes to be insignificant. Conversely, when the pore walls are too thin, although the electron transport distance is shorter, the mechanical strength of the pore walls decreases rapidly, making them prone to cracking under external forces or thermal stress, leading to irreversible interruption of the conductive pathway. Ultrathin pore wall structures may also shrink and collapse during film formation and curing due to solvent evaporation tension, causing the originally three-dimensional interconnected channels to degenerate into a locally stacked structure, ultimately resulting in a high specific surface area but a reduced effective conductive network. Therefore, controlling the pore wall thickness to less than 5 nm while maintaining a certain structural integrity is crucial for achieving both high conductivity and high mechanical stability in the material.

[0037] Specifically, the hierarchical porous structure of 0-200 nm enables foamed graphene to simultaneously possess nanoscale conductive channels and micron-scale stress buffer cavities, achieving a synergy between electron migration and mechanical deformation. If the pore size is too large, the skeletal support decreases, the number of pores per unit volume decreases, and the specific surface area drops sharply. Under pressure, the material is prone to irreversible collapse, exhibiting high initial sensitivity but severe performance degradation after cycling. Large pores also reduce electron contact density, making the conductive path sparser, resulting in a higher initial resistance and increased noise signal. Furthermore, excessively large pore sizes cause significant surface roughness in the slurry during film formation, reducing coating density and interfacial adhesion, which is detrimental to uniform coverage on flexible substrates.

[0038] Specifically, pore volume is a parameter that comprehensively reflects the porosity and overall structural permeability of a material. When the pore volume is below 2.2 cc / g, the internal porosity of the material is insufficient, the deformable space in the three-dimensional framework is limited, and stress concentration occurs at local nodes under external stress, easily causing framework fracture. Conversely, if the pore volume is too large, the material density is too low, and the mechanical support capacity is insufficient. During compression or curing, the material is prone to collapse or compaction, resulting in irreversible structural compression, and the sensitivity decreases with the number of cycles. Excessively high pore volume also means that the distance between the pore walls is too large, reducing the probability of electron tunneling and increasing the initial resistance; in sensing applications, this manifests as large hysteresis and slow recovery.

[0039] Specifically, the carbon-to-oxygen ratio of the foamed graphene is 4.8-5.2.

[0040] This embodiment also provides a method for preparing the above-mentioned foamed graphene with a three-dimensional rolled porous structure, including the following steps: S1: Prepare graphene oxide powder; S2: Under a non-oxidizing atmosphere, the graphene oxide powder is subjected to a rapid thermal reduction treatment at >100℃ / min and held at that temperature for 1-60min to obtain foamed graphene.

[0041] Specifically, the non-oxidizing atmosphere is one or any combination of nitrogen, hydrogen, ammonia, argon, helium, or other rare gases.

[0042] Specifically, this application uses graphene oxide powder as a raw material and applies a rapid thermal reduction treatment to the graphene powder at a rate exceeding 100°C / min. After reaching the target temperature, the graphene oxide powder is then held at that temperature for 1-60 minutes. This process involves, at the molecular level, the drastic removal of oxygen functional groups, the instantaneous reconstruction of the carbon skeleton, and interlayer burst expansion induced by microscopic gas dynamics. It represents a non-equilibrium phase transition behavior that far exceeds that of traditional reduction systems in both energy and time scales.

[0043] Specifically, graphene oxide contains a large number of oxygen-containing functional groups such as carboxyl, hydroxyl, and epoxy groups. These functional groups decompose at high temperatures of 500-800℃ to generate small molecule gases such as CO2, CO, and water vapor. In the traditional slow-heating thermal reduction process, heat is gradually transferred along the thickness of the sheets, the gas generation rate is low, and the release channels are limited. The local pressure is insufficient to significantly pry open the interlayer stacking structure, and ultimately only partially expanded graphene powder with a slightly increased interlayer spacing can be obtained.

[0044] Under the process conditions described in this application, due to the heating rate exceeding 100℃ / min, the heat flow instantaneously acts on the surface and interior of the graphene oxide microsheets, forming an extremely steep temperature gradient. This causes the oxygen-containing groups to decompose violently almost simultaneously and release gases at an explosive rate, generating numerous local high-pressure microcavities between the layers in a short period of time. At this point, the carbon skeleton is not yet fully reduced, and some areas still retain flexible sp... 3 The hybrid structure causes the layers to no longer simply separate under strong pressure, but instead curl, bend and twist together, eventually forming a three-dimensional skeleton system that is macroscopically foamy and microscopically interconnected.

[0045] Because the distribution of oxygen functional groups on the surface of graphene oxide is not completely uniform, the decomposition rates in local areas are asynchronous, resulting in an asymmetric distribution of stress field between the layers. This uneven stress further induces non-uniform curling of the layers, thus forming a "foamed" morphology with highly random and hierarchical porous structure. This uneven curling and interlayer fracture together construct a macroscopically highly interconnected porous network, enabling the material to maintain structural stability and not collapse after annealing and cooling.

[0046] Specifically, during the reduction process of graphene oxide, oxygen atoms are removed, sp... 3 Hybrid carbon is gradually converted into sp 2 Carbon is used to restore the local π-π conjugated system. The instantaneous high temperature of rapid reduction provides sufficient energy for the re-nucleation of this π network, resulting in highly dense conductive islands inside the foamed graphene. Electron migration pathways are shorter and more diverse, avoiding the interlayer bottleneck of electron transport in traditional two-dimensional graphene. Especially when the pore wall thickness is less than 5 nm, charge conduction no longer depends on macroscopic contact surfaces but can achieve electron crossing between adjacent layers through quantum tunneling. This provides the material with more sensitive resistance response characteristics in piezoresistive sensing applications.

[0047] Under compression, even minute changes in the spacing between the pore walls are sufficient to significantly alter the local conductive path, thereby amplifying the output signal. Simultaneously, the multi-point support structure of the three-dimensional framework endows the material with exceptional mechanical resilience, allowing it to maintain the continuity of the conductive network during cyclic deformation. This dual synergy of microscopic electricity and mechanics is unattainable with traditional two-dimensional graphene materials.

[0048] In comparison, existing mainstream processes for preparing porous graphene are difficult to achieve the foamed structure of graphene described in this application, both in principle and in result. First, while template methods (including hard templates such as SiO2, Al2O3, CaCO3, and soft templates such as surfactant self-assembly systems) can form porous structures with certain morphologies, their pore size and pore wall thickness are limited by the template particle size and distribution. Furthermore, the template removal process often requires strong acid etching or high-temperature ablation, which easily introduces impurities and structural collapse, increasing the complexity of the process and making it difficult for the obtained material to maintain a continuous three-dimensional electrical conductivity network.

[0049] Secondly, the hydrogel freeze-drying method relies on the self-assembly behavior of graphene oxide in the aqueous phase and obtains pores through the ice template effect of the gel network. However, due to the thick gel wall and the pore size concentrated in the micrometer range, the mechanical strength of the pore structure is low, and the interfacial tension of the solvent often leads to uneven separation of the sheets during the freeze-drying process. Therefore, it is impossible to obtain a high-level porous structure with a continuous pore size distribution in the range of 0-200 nm and a pore volume greater than 2.2 cc / g as in this application.

[0050] While chemical vapor deposition (CVD) can grow high-quality graphene, it is limited by substrate adhesion and growth rate. The resulting film is a continuous two-dimensional layered structure with almost no three-dimensional porosity, making it difficult to apply to fields with extremely high requirements for deformation response, such as flexible piezoresistive sensing.

[0051] Furthermore, conventional thermal reduction puffing typically employs a slow heating process (10-30℃ / min), gradually removing oxygen functional groups and resulting in a mild gas release rate. The pressure generated between the layers is insufficient to drive complete curling, leading only to partial separation of the layers and a slight increase in interlayer spacing. Therefore, its specific surface area generally does not exceed 300-400 m². 2 The pore volume is only about 0.5-1.0cc / g, which is far lower than the structural parameters of the foamed graphene in this application, and the pore wall thickness is large and the conductive pathway is discontinuous.

[0052] More importantly, in traditional slow-heating systems, the deoxygenation and carbon rearrangement processes are essentially simultaneous, failing to create sufficient non-equilibrium conditions. Consequently, the carbon framework structure tends to be flat and lacks curling, leading to the resulting graphene being extremely prone to collapse under mechanical loading and lacking the self-supporting and elastic recovery properties of the graphene material described in this application. Therefore, it is evident that neither the template method, chemical vapor deposition, hydrogel freeze-drying method, nor conventional thermal reduction expansion method can simultaneously satisfy the conditions of high specific surface area, ultrathin pore walls, high pore volume, and three-dimensional connectivity in terms of structural characteristics and functional properties. Thus, the foamed graphene of the morphology described in this application cannot be prepared.

[0053] The key to this technical solution lies in creating a non-equilibrium reducing environment through an extreme heating rate, causing graphene oxide to undergo a dynamic process of "explosive deoxidation—high-pressure expansion—instantaneous solidification" in an extremely short time. At the molecular level, this process is equivalent to triggering tens of thousands of nanoscale "gas micro-explosions" within the graphene framework. Each micro-explosion corresponds to the decomposition of local oxygen-containing clusters and the rearrangement of the carbon network, and the instantaneous superposition of all micro-explosions constitutes the overall volume expansion. Due to the extremely short thermal shock time, the carbon framework freezes rapidly in a high-energy state, failing to fully recover its planar spline shape. 2 The arrangement forms a three-dimensional distorted network with numerous bends, kinks, and non-six-membered rings.

[0054] Specifically, this distorted network endows the material with higher elastic modulus and surface activity, enabling it to undergo reversible deformation under external stress. At the same time, it provides abundant defect sites and edge states, enhancing the electronic localization and conduction coupling effects, thereby bringing extraordinary piezoresistive sensitivity and controllable conductivity modulation performance.

[0055] Specifically, in step S1 above, the graphene oxide powder contains 1-2 layers of graphite sheets, so the particle size of the graphene powder is 30-300 mesh. The graphene oxide powder contains 50-60 wt% oxygen, 40-50 wt% carbon, and 5-12 wt% water.

[0056] Specifically, in the process of preparing foamed graphene materials with a three-dimensional rolled porous structure, this application uses graphene oxide powder as a precursor.

[0057] Specifically, this application clearly states that the graphene oxide powder used contains 1-2 layers of graphite sheets. This parameter plays a decisive role in the morphology and pore structure of the foamed graphene at the microscopic level.

[0058] Specifically, single-layer or double-layer graphene oxide has a lower interlayer bonding energy, making it more susceptible to being driven by the released gas during thermal reduction to form large-scale interlayer exfoliation.

[0059] Specifically, when the thermal reduction rate exceeds 100℃ / min, the decomposition rate of the oxidized groups is faster than the thermal diffusion rate. The gas does not have enough time to escape uniformly, resulting in instantaneous micro-bursts. This "expands" the carbon layer at the microscale, generating abundant nano- to submicron-sized pores. Single-layer or double-layer structures have higher flexibility and deformability, thus enabling complex three-dimensional curling at the moment of gas release.

[0060] Specifically, if the number of graphene oxide layers increases to more than three, the interlayer van der Waals forces increase, the structural flexibility decreases, and the internal gas cannot diffuse sufficiently during rapid heating, easily leading to insufficient local expansion or incomplete exfoliation, which in turn affects the foaming uniformity and pore connectivity. Therefore, controlling the number of layers to 1-2 not only facilitates the formation of a porous structure with a high specific surface area, but also ensures that the pore wall thickness remains within the range of <5 nm, ensuring a short and continuous electron transport path and improving conductivity and sensitivity.

[0061] Specifically, the particle size of graphene oxide powder is a crucial factor affecting its thermal reaction kinetics and gas diffusion behavior. When the particle size is too large, the internal heat conduction path is long, resulting in a significant temperature gradient. This leads to over-reduction of the outer layer and incomplete reduction of the interior, forming a non-uniform foamed structure. Conversely, when the particle size is too small, although the number of oxygen functional groups per unit volume is high, the gas release is limited. Furthermore, excessively fine particles are prone to agglomeration at high temperatures, causing the pore structure to collapse. Using 30-300 mesh graphene oxide powder ensures good uniformity of thermal conduction and coordinated internal diffusion. During rapid heating, heat energy is quickly transferred throughout the entire particle, causing the decomposition reaction of the oxide groups to occur simultaneously. The generated gas is released uniformly within the particle, resulting in a uniformly distributed pore structure.

[0062] Specifically, the oxygen content in graphene oxide powder is the core parameter that determines the foaming ability of graphene oxide.

[0063] Specifically, the more oxygen-containing functional groups in graphene oxide, the larger its interlayer spacing, and the higher the gas release during thermal decomposition. In this application, controlling the oxygen content at 50-60 wt% ensures a sufficient number of oxygen functional groups to provide the gas pressure required for foaming, while preventing the continuity of the carbon skeleton from being destroyed due to excessive oxidation.

[0064] Specifically, if the oxygen content is less than 50 wt%, the foaming driving force is insufficient, the interlayer separation is not sufficient after thermal reduction, the specific surface area decreases, and the pores are not interconnected; if the oxygen content exceeds 60 wt%, the carbon skeleton is severely damaged, resulting in too many defects in the reduced graphene sheet structure, the pore walls are too thin or even locally fractured, which reduces the mechanical properties.

[0065] Specifically, while the water content of graphene oxide powder is only an auxiliary parameter on a macroscopic level, it plays a crucial role in catalysis and mass transfer during microscopic reactions. Water molecules evaporate first during rapid heating, creating vapor pressure between layers. This additional gas source promotes initial separation between layers. Simultaneously, water vapor can participate in partial reduction reactions under high-temperature conditions, generating gases such as CO and H2, further enhancing the expansion kinetics. Controlling the water content to 5-12 wt% ensures sufficient moisture for the gasification reaction while preventing excessive moisture from causing premature particle aggregation or bursting.

[0066] Specifically, in the above preparation method, the thermal reduction temperature is 500-800℃.

[0067] Specifically, if the reduction temperature is below 500℃, only some low-energy functional groups are removed, resulting in insufficient interlayer gas release and preventing significant foaming behavior. While temperatures exceeding 800℃ lead to more thorough reduction, excessive rearrangement of the carbon skeleton, enhanced defect migration and remelting at high temperatures can cause micropore collapse or pore wall sintering, thus destroying the porous network structure. Therefore, controlling the thermal reduction temperature between 500-800℃ is crucial for achieving uniform foaming. This ensures sufficient decomposition and gas release from functional groups to drive interlayer expansion while preventing secondary densification of the carbon skeleton.

[0068] This application also provides a graphene slurry, which utilizes the foamed graphene provided above, and comprises the following raw materials by weight percentage: Foamed graphene 1-8wt% Conductive carbon black 0-5 wt% Resin 20-35 wt% Solvent 52-79 wt%.

[0069] Specifically, the specific surface area of ​​the conductive carbon black is 200-400 m². 2 / g, particle size 20-50nm, DBP oil absorption value >200ml / 100g, pH 8-10.

[0070] Specifically, the graphene slurry is composed of foamed graphene and conductive carbon black to form a conductive filler system, and is combined with resin and solvent to form a composite slurry with high dispersibility and excellent electrical stability.

[0071] Specifically, the expanded graphene used in this application is a carbon-based material with a three-dimensional rolled porous structure, with a sheet thickness of less than 5 nm, a pore size distribution in the range of 0-200 nm, and a specific surface area as high as 600-800 m². 2 / g. This highly coiled and porous carbon framework provides an ultra-high active interface area and multidimensional channels for electron transport. However, due to the large sheet size of foamed graphene, it is prone to agglomeration in resin solvent systems, leading to local discontinuities in the conductive pathways. In contrast, conductive carbon black has a particle size of only 20-50 nm and a specific surface area of ​​200-400 m². 2 / g, exhibiting spherical or chain-like aggregates, can effectively fill the micro-voids between foamed graphene sheets, forming a continuous "point-surface-volume" conductive network. In this network, foamed graphene provides the main channel for high-speed electron migration, while conductive carbon black acts as a connecting node and an electron hopping bridge, compensating for spatial breaks between graphene sheets and enabling electrons to achieve cross-layer transport through tunneling.

[0072] Furthermore, a small number of oxygen-containing functional groups remain in expanded graphene during the thermal reduction process. These polar groups not only enhance wettability in the resin matrix but also interact with carbon groups on the carbon black surface through hydrogen bonds or van der Waals forces, forming a stable composite interface. When the two carbon materials approach each other in the mixed system, the surface electron cloud of the conductive carbon black overlaps with the π-conjugated electron cloud of the graphene sheets, forming a local bandgap coupling region. In addition, the high DBP oil absorption value of carbon black indicates its extremely strong adsorption capacity, allowing it to firmly adhere to the pore walls and edge regions of expanded graphene, thereby further enhancing the interfacial bonding and electronic coupling stability.

[0073] Example 1: Preparation of foamed graphene 100 mg of pretreated graphene oxide powder (C / O = 1, water content 10 wt%, average number of layers 2) was taken, and the TEM image of the graphene oxide powder is shown below. Figure 1 As shown; Under an argon atmosphere, a crucible containing powder is rapidly pushed into a high-temperature furnace preheated to 700°C to achieve an almost infinite instantaneous heating rate, and then held at that temperature for 5 minutes. After natural cooling, foamed graphene material is obtained, and a scanning electron microscope image of the foamed graphene material is shown below. Figure 2 As shown, by Figure 2 It can be seen that the foamed graphene material has abundant foamed structures.

[0074] Comparative Example 1 Take 100 mg of pretreated graphene oxide powder (C / O=1, water content 10wt%, average number of layers 2). Under an argon atmosphere, a crucible containing powder is placed in a high-temperature furnace, and then heated from room temperature to 700°C at a rate of 10°C / min, and held at that temperature for 5 minutes.

[0075] After cooling, foamed graphene material is obtained, and a scanning electron microscope image of the foamed graphene material is shown below. Figure 3 As shown, by Figure 3 It can be seen that a slow heating rate at a suitable target temperature results in only a small amount of foamed structure in the foamed graphene material.

[0076] Comparative Example 2 Take 100 mg of pretreated graphene oxide powder (C / O=1, water content 10wt%, average number of layers 2). Under an argon atmosphere, the crucible containing the powder is quickly pushed into a high-temperature furnace preheated to 450°C and held at that temperature for 20 minutes.

[0077] After cooling, foamed graphene material is obtained, and a scanning electron microscope image of the foamed graphene material is shown below. Figure 4 As shown, by Figure 4 It can be seen that under rapid heating rate, the low target temperature causes severe layer stacking in the foamed graphene material, with few foamed structures, dense structure, and tending to be flat.

[0078] Comparative Example 3 Take 100 mg of pretreated graphene oxide powder (C / O=1, water content 10wt%, average number of layers 2). Under an argon atmosphere, the crucible containing the powder is quickly pushed into a high-temperature furnace preheated to 950°C and held at that temperature for 20 minutes.

[0079] After cooling, foamed graphene material is obtained, and a scanning electron microscope image of the foamed graphene material is shown below. Figure 5 As shown, by Figure 5 It can be seen that under rapid heating rate conditions, a higher target temperature will result in a significant foaming structure observed in the foamed graphene material. However, some foams tend to collapse and fuse, which may be due to excessive graphitization. Excessively high temperatures may damage the integrity of the foaming structure.

[0080] Experimental Example 1: Fabrication of Graphene Slurry and Piezoresistive Sensor Experimental groups: Graphene slurries were prepared from the foamed graphene obtained in Example 1 to obtain experimental group 1, experimental group 2 and experimental group 3 respectively; The slurry composition of experimental groups 1, 2, and 3, expressed as a percentage by mass, is shown in the table below: Table 1 Raw material composition of the experimental group

[0081] Control groups: The foamed graphene materials prepared by Comparative Examples 1, 2 and 3 were weighed to prepare Control Group 1, Control Group 2 and Control Group 3 respectively; The slurry compositions of control groups 1, 2, and 3, expressed as a percentage by mass, are shown in Table 2 below: Table 2 Raw material composition of the control group

[0082] According to the raw material composition table given above, the above components were stirred in a high-speed dispersion pan at 1500 rpm for 2 hours until they were mixed evenly, and viscous graphene slurries of experimental group 1, experimental group 2, experimental group 3, control group 1, control group 2 and control group 3 were obtained respectively. 1.2 Piezoresistive Sensor Fabrication: Using screen printing technology, the graphene pastes prepared in experimental groups 1, 2, 3, 1, 2, and 3 were printed onto flexible PET films to form patterns. The size of the patterns was 30mm × 30mm × 0.01mm. The printed films were cured in an oven at 110℃ for 1 hour to remove the solvent; Electrodes are made at both ends of the sensitive pattern using silver paste, and wires are connected to them to obtain a piezoresistive sensor.

[0083] Experiment Example 2: Performance Testing of a Piezoresistive Sensor The performance of the piezoresistive sensors prepared in Experimental Group 1, Experimental Group 2, Experimental Group 3, Control Group 1, Control Group 2 and Control Group 3 in Experimental Example 1 were tested respectively. The final test results are shown in Table 3 below. The relationship between the relative resistance change rate and the applied pressure of the piezoresistive sensors prepared in experimental group 1 and control groups 1-3 is shown in Table 3 below. Figure 6 As shown: The curves showing the relationship between the relative resistance change rate and the applied pressure of the piezoresistive sensors prepared in experimental groups 1-3 are as follows: Figure 7 As shown.

[0084] Table 3 Test Results

[0085] As shown in Table 3 above, the piezoresistive sensor prepared in the experimental group exhibits significantly better sensitivity and signal stability than the control group within the same strain range. This performance difference fundamentally stems from the variations in material microstructure, pore morphology, and conductive network morphology resulting from differences in the preparation process of foamed graphene.

[0086] Specifically, the foamed graphene in the experimental group all used the foamed graphene material prepared in Example 1. The preparation route in Example 1 used high-purity graphene oxide powder as a precursor, and subjected it to rapid thermal reduction treatment at a temperature range of 500-800℃ under conditions of rapid heating rates exceeding 100℃ / min. This process causes the functional groups such as hydroxyl, epoxy, and carboxyl groups in the graphene oxide to decompose violently in a short time. The instantaneously released gas forms high pressure between the layers, driving the carbon layers to rapidly peel off and curl, ultimately obtaining a foamed graphene material with a three-dimensional curled porous structure. The specific surface area of ​​this material can reach 600-800 m². 2 The pore volume is greater than 2.2 cc / g, the pore size is uniformly distributed, and the pore wall thickness is less than 5 nm. This three-dimensional coiled porous structure forms a flexible yet stable conductive framework at the microscopic level, with the characteristics of being deformable, recoverable, and having continuous electronic pathways, providing an excellent basis for the strain response of piezoresistive sensors.

[0087] Under the process conditions of Example 1, graphene oxide undergoes high-energy thermal excitation in a short period of time, resulting in the release of a large number of generated CO2, CO, and H2O molecules within an extremely short time, producing violent interlayer microbursts. Because the heating rate exceeds 100°C / min, the gas release rate is much higher than the diffusion rate, creating instantaneous high pressure inside, forcing the carbon layers to separate, curl, and rearrange to form a porous network. During this process, the breaking of carbon-oxygen bonds is accompanied by the recombination of carbon atoms, forming new sp atoms. 2 The hybrid structure and conductive π-network of the foamed graphene create a stable and highly interconnected pore structure, forming continuous electronic pathways. When this foamed graphene is applied to the conductive layer of a piezoresistive sensor, external stress or strain causes minute changes in the pore wall spacing, resulting in significant changes in resistance and achieving a high-sensitivity response. Due to the small pore wall thickness and strong reversible deformation capability of the pores, the destruction and recovery of the conductive pathways during loading and unloading are almost synchronous, thus the sensor exhibits excellent cyclic stability and response recovery.

[0088] In contrast, the control group used foamed graphene materials prepared in Comparative Examples 1-3.

[0089] Due to the lower heating rate in Comparative Example 1, the heat transfer and gas release processes were slower, resulting in the stepwise decomposition of oxygen functional groups over a longer period. The gas gradually escaped without forming sufficient high pressure between the layers, leading to incomplete interlayer delamination and insufficient foaming. This resulted in a material structure closer to partially reduced stacked graphene sheets. The specific surface area and pore volume of the foamed graphene material prepared in Comparative Example 1 were significantly lower than those in Example 1, with uneven pore size distribution and poor pore connectivity. The resulting conductive network was a two-dimensional sheet-like stacked structure, restricting electron migration paths. Under strain, only local contact point changes occurred, resulting in small resistance changes and significantly reduced sensitivity. More importantly, interlayer contacts were prone to slippage and local delamination under stress, leading to signal instability and increased hysteresis. Therefore, the piezoresistive sensor corresponding to Comparative Example 1 exhibited lower sensitivity and poorer linear response in the tests shown in Table 3.

[0090] Under the rapid heating conditions of Comparative Example 2, holding at 450 °C for only 20 minutes, although the temperature could trigger the removal of carboxyl and some hydroxyl groups, the overall energy was insufficient to completely release the binding of epoxy groups and residual oxygen-containing structures, resulting in an oxygen content as high as 17.98 wt% and a carbon content of only 73.51 wt%. At this point, the reduction degree of the graphene sheets was low, and sp 2The carbon framework was not fully recovered, and the degree of electron delocalization was limited, resulting in high internal resistance and a discontinuous conductive network in the foamed graphene material. Simultaneously, the slow and limited gas release at low temperatures led to insufficient interlayer burst expansion, resulting in a dense structure and small pore size, with a pore volume of only 1.446 cc / g. The confined pores and thick, sheet-like structure meant that the resistance change under external force mainly came from minor adjustments at a few contact points, resulting in a flat response curve. While the sensitivity was slightly higher than Comparative Example 1, it was still far lower than Example 1. This indicates that without sufficient foaming, the graphene network cannot form a multidimensional strain transmission pathway, leading to a limited piezoresistive response.

[0091] Comparative Example 3 underwent rapid heating at 950 °C. Although the high temperature led to the complete removal of oxygen-containing groups, increasing the carbon content to 83.61 wt%, the carbon skeleton underwent rearrangement and local graphitization at the ultra-high temperature, resulting in thicker pore walls and locally collapsed pore structures. Due to the excessive reduction in surface energy caused by the high temperature, some curled layers fused or adhered, destroying the independence and elasticity of the pore structure, and reducing the pore volume to 2.032 cc / g. The resulting three-dimensional network could provide some piezoresistive change during the initial loading, but the pore structure was prone to collapse under repeated cycles, and the electron channels were difficult to recover, exhibiting moderate sensitivity but poor stability. Although the graphitized areas at high temperature improved local conductivity, they reduced the overall strain sensitivity because the increased rigidity of the graphitized areas prevented local stress from being effectively transmitted to the conductive path, causing the resistance change to become less sensitive to external pressure.

[0092] Comparing the three comparative examples above, it is clear that the heating rate and reduction temperature play a crucial role in determining the microstructure of foamed graphene. Slow heating leads to gradual gas release and insufficient foaming; medium-low temperature treatment results in incomplete reduction and pore closure; excessively high temperatures cause over-densification or graphitization of the framework. Only under the conditions of ultra-rapid heating as used in Example 1 and temperature control within the temperature range provided in this application can the ideal dynamic process of "burst deoxidation - instantaneous expansion - rapid shaping" be achieved in graphene oxide sheets, forming a three-dimensional coiled porous structure with both high specific surface area and mechanical stability.

[0093] This structure not only exhibits a macroscopic foamed morphology but also forms a uniformly distributed multi-level pore structure ranging from 0 to 200 nm at the microscopic level. This allows for highly interconnected electronic pathways in three-dimensional space, forming a continuous conductive network. This structural feature enables nanoscale changes in the spacing between pore walls under minute pressure to significantly adjust the number of conductive pathways and tunneling distances, thereby amplifying resistance changes and achieving extremely high sensitivity.

[0094] Furthermore, the difference in orifice wall thickness directly affects the reversibility and signal stability of the piezoresistive sensor. In Example 1, the orifice wall thickness is approximately 4 nm, which ensures both the occurrence of electron tunneling and provides sufficient mechanical strength, allowing the conductive network to recover reversibly during compression and release. Comparative Examples 1 and 2 have orifice wall thicknesses of 15.98 nm and 19.32 nm, respectively. The thick carbon layer causes electron transport to rely primarily on the macroscopic contact surface, significantly reducing sensitivity. While the orifice wall in Comparative Example 3 is thinner, it is locally sintered, forming rigid segments, resulting in a nonlinear decay in the strain response. Therefore, an orifice wall thickness of less than 5 nm is a key condition for achieving high sensitivity and high repeatability; this characteristic can only be stably obtained under ultrafast thermal reduction conditions.

[0095] The increased specific surface area and pore volume fundamentally improve the dynamic response characteristics of the conductive network. The specific surface area of ​​657 m² / g and the pore volume of 2.711 cc / g in Example 1 together ensure extremely high interfacial activity and deformable porosity within the material. When external pressure is applied to the sensing membrane, numerous micropores undergo synergistic compression, resulting in a rapid increase in the number of electronic pathways and a rapid decrease in macroscopic resistance due to pore volume reduction. After the pressure is released, the three-dimensional curled skeleton recovers its original shape through elastic restoring force, the pores reopen, the resistance rises, and a stable reversible cycle is formed. This "porous skeleton elastic response—reversible control of conductive pathways" mechanism is unattainable with traditional two-dimensional stacked graphene. In Comparative Examples 1-3, due to the low specific surface area and pore volume, the pores cannot fully recover after closure or collapse, resulting in a significant hysteresis in resistance changes and rapid signal decay with the number of cycles.

[0096] The above description is merely a preferred embodiment of this application and is not intended to limit this application in any way. Although this application has disclosed the preferred embodiment as above, it is not intended to limit this application. Any person skilled in the art can make some modifications or alterations to the above-mentioned technical content to create equivalent embodiments without departing from the scope of the technical solution of this application. The implementation schemes in the above embodiments can be further combined or replaced. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of this application without departing from the content of the technical solution of this application shall still fall within the scope of this application.

Claims

1. A foamed graphene with a three-dimensional rolled porous structure, characterized in that, The foamed graphene is prepared by rapidly reducing graphene oxide powder at a temperature >100℃ / min. The foamed graphene has an irregular three-dimensional coiled porous structure, and the specific surface area of ​​the foamed graphene material is 600-800 m². 2 / g, pore wall thickness <5nm, pore size 0-200nm, pore volume >2.2cc / g.

2. The foamed graphene with a three-dimensional rolled porous structure according to claim 1, characterized in that, The carbon-to-oxygen ratio of the foamed graphene is 4.8-5.

2.

3. A method for preparing foamed graphene with a three-dimensional rolled porous structure as described in claim 1, characterized in that, Includes the following steps: S1: Prepare graphene oxide powder; S2: Under a non-oxidizing atmosphere, the graphene oxide powder is subjected to a rapid thermal reduction treatment at >100℃ / min and held at that temperature for 1-60min to obtain foamed graphene.

4. The method for preparing foamed graphene with a three-dimensional rolled porous structure according to claim 3, characterized in that, The graphene oxide powder contains 1-2 layers of graphite sheets, and the particle size of the graphene oxide powder is 30-300 mesh. The graphene oxide powder contains 50-60 wt% oxygen, 40-50 wt% carbon, and 5-12 wt% water.

5. The method for preparing foamed graphene with a three-dimensional rolled porous structure according to claim 3, characterized in that, In the rapid thermal reduction process, the thermal reduction temperature is 500-800℃.

6. A graphene slurry, using the foamed graphene as described in claim 1, characterized in that, The product comprises the following raw materials by weight percentage: Foamed graphene 1-8wt% Conductive carbon black 0-5 wt% Resin 20-35 wt% Solvent 52-79 wt%.

7. The graphene slurry according to claim 6, characterized in that, The specific surface area of ​​the conductive carbon black is 200-400 m². 2 / g, particle size 20-50nm, DBP oil absorption value > 200ml / 100g, pH 8-10.

8. The graphene slurry according to claim 6, characterized in that, The resin is any one or more of polyurethane resin, epoxy resin, acrylic resin, silicone resin, phenolic resin, vinyl resin, and aldehyde-ketone resin.

9. A graphene slurry according to claim 6, characterized in that, The solvent is any one or more of ethanol, ethylene glycol, isopropanol, glycerol, isopropyl propionate, N-methylpyrrolidone, dimethylformamide, dimethylacetamide, toluene, xylene, isophorone, and n-hexane.

10. A method for fabricating a piezoresistive sensor, using the graphene slurry as described in any one of claims 6-9, characterized in that, Includes the following steps: The graphene slurry is coated onto a flexible substrate by screen printing, scraping, or spraying to form a sensing film; The sensing film is cured at 80-160℃ to form a stable sensitive layer; Electrodes are fabricated at both ends of the sensitive layer to obtain the desired result.

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