A high-purity graphite material with high thermal conductivity and low resistivity for photovoltaic applications and its preparation method.

Abstract

This application relates to the field of carbon materials technology, and discloses a high-purity graphite material for photovoltaic applications with high thermal conductivity and low resistivity, and its preparation method. The material is made from graphite pre-impregnated pitch coke micropowder, pitch coke coarse powder, and modified coal tar pitch binder through mixing, molding, and high-temperature treatment. The core of this invention lies in the use of a solvent-dispersed micropowder pre-impregnation process. An interface-modifying liquid containing solvent oil, dissolved medium-temperature coal tar pitch, and surfactants is used to atomize and spray 500-mesh pitch coke micropowder through high-speed shearing. This effectively deagglomerates the micropowder agglomerates, allowing the pitch components to deeply fill the micropores and form a lubricating film on the particle surface. This reduces the internal frictional resistance during powder molding and ensures lossless transmission of isostatic pressure to the center of large-sized green bodies. This invention eliminates the radial density gradient in large-sized products, solves the cracking problem caused by internal stress, and results in products with advantages such as high bulk density, low resistivity, and good structural uniformity.

Description

Technical Field

[0001] This invention relates to the field of carbon materials technology, specifically to a high-purity graphite material for photovoltaic applications with high thermal conductivity and low resistivity, and its preparation method. Background Technology

[0002] As the photovoltaic industry moves towards larger sizes, thinner wafers, and N-type high-efficiency cells, the thermal field system for growing monocrystalline silicon places higher demands on the physicochemical properties of isostatically pressed graphite materials. To meet the requirements for uniform temperature distribution and rapid thermal response during the growth of large-diameter monocrystalline silicon, graphite materials need to possess higher bulk density, higher mechanical strength, and superior electrical and thermal conductivity. Simultaneously, to reduce production costs, the fabrication of large-format (e.g., diameters of 800mm and above) graphite products has become an industry trend.

[0003] In the production process of isostatic graphite, the principle of "aggregate gradation" is usually adopted, that is, introducing a certain proportion of ultrafine powder (such as 500 mesh or finer micro powder) into coarse aggregate to fill the gaps between particles, thereby improving the density of the material. However, as the particle size of the powder decreases, its specific surface area increases sharply, and the surface energy rises. Under the influence of van der Waals forces and electrostatics, ultrafine powder is prone to agglomeration, forming dense secondary agglomerates. In existing dry or wet kneading processes, because the modified coal tar pitch binder still has a high viscosity in the molten state, it is difficult to overcome capillary resistance and penetrate into the internal voids of these micro powder agglomerates. This results in the presence of a large number of dry powder clumps or pseudo-particles in the kneaded paste that are not effectively wetted by the binder. During the subsequent carbonization process, these parts will form structural defects, severely disrupting the continuity of the matrix.

[0004] More critically, in the molding process of large-format graphite products, the agglomeration of micro-powder increases the internal friction angle within the powder. During cold isostatic pressing, the externally applied pressure is significantly attenuated by the friction between powder particles as it is transmitted towards the center of the green body. This pressure transmission loss results in a distinct density gradient in large-format green bodies, with high density at the edges and low density in the center. This heterogeneous structure, with a tight outer layer and a loose inner layer, generates enormous thermal stress during subsequent calcination and high-temperature graphitization processes due to the mismatch between internal and external thermal expansion and contraction behaviors. This can easily induce internal cracks or even explosions in the product, severely restricting the yield and production efficiency of large-format, high-performance graphite products. Furthermore, existing simple physical mixing or solvent wetting methods are insufficient to construct an effective conductive network at the microscale, resulting in high resistivity in the final product, which fails to meet the application requirements of high-end photovoltaic thermal field materials for low energy consumption and high heating efficiency. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a high-purity graphite material for photovoltaic applications with high thermal conductivity and low resistivity, and its preparation method. This solves the problems in the existing large-size isostatic pressing graphite preparation process, where the agglomeration of ultrafine powder leads to high internal friction in the paste, hinders the transmission of molding pressure, and consequently causes uneven radial density distribution, easy cracking during high-temperature treatment, and limited density and conductivity of the final material.

[0006] To achieve the above objectives, the present invention provides the following technical solution: The first aspect of this invention provides a high-purity graphite material for photovoltaic applications with high thermal conductivity and low resistivity. This graphite material is prepared from raw materials comprising the following parts by weight through kneading, molding, calcination, and graphitization: 35 to 45 parts of pre-impregnated pitch coke micropowder; 33 to 40 parts of pitch coke coarse powder; and 20 to 25 parts of modified coal tar pitch binder. The pre-impregnated pitch coke micropowder is obtained by spraying and high-speed mixing 500-mesh pitch coke micropowder with an interface modification liquid. The interface modification liquid comprises the following components: solvent oil, medium-temperature coal tar pitch dissolved in the solvent oil, fatty acid surfactants, and alkylbenzene sulfonates.

[0007] By adopting the above technical solution, the high specific surface area 500-mesh micro powder is pre-wetted with an interface-modifying liquid of specific components, which improves the agglomeration state and interfacial properties of the micro powder. The process includes the following steps: Solvent penetration and micropore filling: Solvent oil, acting as a low-viscosity carrier, carries dissolved medium-temperature coal tar pitch molecules deep into the micropores, blind holes, and cracks on the surface of the pitch coke powder. Utilizing capillary pressure, the modified liquid can physically penetrate and displace air from the pores, achieving liquid-phase filling of internal defects in the powder.

[0008] Depolymerization and Dispersion: The synergistic effect of fatty acid surfactants and alkylbenzene sulfonates adsorbs onto the surface of microparticles, reducing the solid-liquid interfacial tension. This surface modification weakens the van der Waals forces between particles, depolymerizes the hard aggregates formed by electrostatic adsorption, and allows the microparticles to exist in a monodisperse state in the mixed system.

[0009] Lubricated molding and gradient elimination: The solubilized asphalt layer coated on the surface of the micro powder forms an effective lubricating medium film during isostatic pressing. This lubricating film reduces the intermeshing frictional resistance between powder particles, allowing the externally applied molding pressure to be transmitted losslessly to the geometric center of the large-sized green body. This achieves uniformity of the overall density of the green body, eliminates the radial density gradient, and thus solves the problems of residual thermal stress and cracking caused by the externally tight and internally loose structure.

[0010] In-situ carbonization remediation: During the subsequent roasting and graphitization stages, the pitch components that have penetrated into the micropores undergo pyrolysis and condensation reactions, transforming in-situ into solid carbon (coke) with a binding effect. This in-situ generated carbon fills the microstructural defects and constructs continuous electron transport channels and phonon thermal conductivity networks between particle contact points, thereby simultaneously improving the bulk density and compressive strength of the final graphite product and reducing its resistivity.

[0011] Preferably, the matrix aggregates of the pitch coke micro powder and the pitch coke coarse powder are both needle-shaped pitch coke or dense isotropic pitch coke, with a fixed carbon content greater than or equal to 99.0 wt% and an ash content less than or equal to 500 ppm; the softening point of the modified coal tar pitch binder is 105℃ to 115℃, and the coking value is 56% to 60%.

[0012] By adopting the above technical solution and selecting high-purity aggregates and binders with high coking values, the basic carbon content and graphitization degree of the material are guaranteed, the influence of impurity atoms on electron and phonon scattering is reduced, and the thermal and electrical conductivity of the material is further improved.

[0013] Preferably, the interface-modifying liquid is made from the following components in parts by weight: 40 to 65 parts wash oil or anthracene oil, 30 to 40 parts medium-temperature coal tar pitch, 1 to 5 parts stearic acid or oleic acid, and 0.5 to 1.5 parts sodium dodecylbenzene sulfonate; and the viscosity of the interface-modifying liquid at 100°C is 55 mPa·s to 145 mPa·s.

[0014] By employing the above technical solution, wash oil or anthracene oil, as a solvent rich in aromatics, exhibits excellent solubility for medium-temperature coal tar pitch, forming a homogeneous and stable modified liquid system. Controlling the viscosity within the range of 55 mPa·s to 145 mPa·s ensures both the fluidity and penetration depth of the modified liquid on the micronized powder surface, while avoiding film rupture due to excessively low viscosity or uneven distribution due to excessively high viscosity. The combination of stearic acid or oleic acid with sodium dodecylbenzenesulfonate provides chemical adsorption and steric hindrance effects, respectively, ensuring the micronized powder's resistance to re-agglomeration during subsequent kneading.

[0015] Preferably, in the pre-impregnated pitch coke powder, the weight ratio of 500-mesh pitch coke powder to interface modification liquid is 100:(15 to 25); the content of 500-mesh undersize material in the pitch coke powder is greater than or equal to 90%, and the particle size of the pitch coke coarse powder is 200 mesh.

[0016] By adopting the above technical solution, the loading of the modified liquid is precisely controlled, ensuring monomolecular-level coating on the surface of the micro powder while avoiding softening or clumping of the paste due to excessive liquid phase. The gradation of 500-mesh micro powder and 200-mesh coarse powder follows the principle of closest packing, using fine powder to fill the gaps between coarse powder, thus maximizing the packing density of the green body.

[0017] The second aspect of this invention provides a method for preparing high-purity graphite material for photovoltaic applications with high thermal conductivity and low resistivity, specifically including the following steps: The solvent oil is heated, and medium-temperature coal tar pitch is added and stirred to dissolve it. Then fatty acid surfactants and alkylbenzene sulfonates are added, and the interface-modified liquid is obtained by high-shear emulsification. Preheat 500-mesh asphalt coke powder, spray it into the interface modification liquid in the form of atomization, and mix it under high-speed stirring to obtain pre-wetted asphalt coke powder. The pre-impregnated pitch coke micro powder and pitch coke coarse powder are dry-mixed, then molten modified coal tar pitch binder is added for wet mixing, and after cooling, the mixture is crushed to obtain a paste. The paste is ground twice and then pre-pressed in a mold, followed by cold isostatic pressing to obtain a green body. The green blank is subjected to calcination, impregnation and densification cycles, and high-temperature graphitization treatment to obtain the graphite material.

[0018] By adopting the above technical solution, this method systematically solves the problem of agglomeration of ultrafine powder in dry kneading and the densification bottleneck in the molding of large-scale products through a step-by-step interface engineering strategy.

[0019] Preferably, the solvent oil is wash oil, anthracene oil, or a mixture thereof, and the heating temperature is 120°C to 140°C; the high-shear emulsification treatment speed is 2000 rpm to 3000 rpm, and the shearing time is 15 minutes to 20 minutes.

[0020] The above technical solution selects solvents rich in polycyclic aromatic hydrocarbons, such as wash oil or anthracene oil, and utilizes their similar polarity to coal tar pitch, following the principle of "like dissolves like," to ensure the complete dissolution of the asphaltene components. A temperature range of 120℃ to 140℃ ensures suitable fluidity of the system while avoiding excessive volatility of lighter components. The mechanical energy provided by high-speed shearing breaks down the asphalt micelles, ensuring the uniformity of the modified liquid at the microscale.

[0021] Preferably, the preheating temperature of the 500-mesh pitch coke powder is 130°C to 140°C, the spindle speed of the high-speed mixer is 900 rpm to 1200 rpm, and the mixing time is 35 minutes to 45 minutes.

[0022] By employing the above technical solution, the preheating temperature of the powder is controlled to be slightly higher than that of the modifying liquid, preventing a sharp increase in viscosity or localized solidification caused by excessive temperature difference when the modifying liquid contacts the powder, thus ensuring that the liquid phase can be fully wetted and spread. The high rotation speed of 900 rpm to 1200 rpm provides sufficient centrifugal force and shear force, ensuring efficient mixing and momentum transfer of the gas, liquid, and solid phases, and achieving full-coverage modification of the micro powder surface.

[0023] Preferably, the dry mixing temperature is 155°C to 165°C and the time is 10 minutes to 20 minutes; the wet mixing temperature is 165°C to 175°C and the time is 60 minutes to 90 minutes; the weight ratio of the pre-impregnated asphalt coke micro powder to the asphalt coke coarse powder is (35 to 45): (33 to 40).

[0024] By adopting the above technical solution, reasonable dry and wet mixing process parameters ensured sufficient wetting of aggregates and binders and appropriate removal of volatiles. A specific ratio of micro-powder to coarse powder constructed a dense packing skeleton, and the pre-wetted micro-powder filled the voids formed by the coarse powder, improving the packing efficiency of the paste.

[0025] Preferably, the particle size of the secondary grinding is 200 mesh; the pressure of the cold isostatic pressing is 160 MPa to 180 MPa, and the holding time is 100 minutes to 150 minutes.

[0026] By adopting the above technical solution, the high-pressure isostatic pressing process forces the lubricated modified particles to undergo deep plastic deformation and displacement, thereby eliminating bridging gaps between particles to the greatest extent and significantly improving the bulk density and mechanical strength of the green body.

[0027] Preferably, the maximum temperature of the graphitization treatment is 2800℃ to 3000℃, and the holding time is 10 hours to 20 hours.

[0028] By adopting the above technical solution, in a high-temperature environment above 2800℃, amorphous carbon atoms obtain sufficient activation energy, undergo lattice rearrangement and layer stacking, and transform into a three-dimensional ordered graphite microcrystalline structure, thereby endowing the material with excellent electrical and thermal conductivity.

[0029] This invention provides a high-purity graphite material for photovoltaic applications with high thermal conductivity and low resistivity, and its preparation method. It has the following beneficial effects: 1. This invention improves the density of graphite materials through solvent-dispersed micro-powder pre-wetting technology. 500-mesh pitch coke micro-powder is pretreated using an interface-modifying liquid. The surfactant in the modifying liquid reduces the solid-liquid interfacial energy, effectively deagglomerating the hard agglomerates of the micro-powder and making it monodisperse. Simultaneously, the low-viscosity solvent carries coal tar pitch components into the blind pores and microcracks on the surface of the micro-powder, achieving liquid-phase filling of microscopic defects. This treatment improves the filling performance of the powder, resulting in an increase in the bulk density of the final graphite product.

[0030] 2. This invention effectively solves the problems of uneven internal density distribution and easy cracking in large-size graphite products. The solvated asphalt film formed by pre-impregnation on the particle surface plays a lubricating role during cold isostatic pressing, reducing the internal friction coefficient between powder particles, ensuring that the molding pressure can be effectively transmitted to the geometric center of the large-size green body, and eliminating the radial density gradient of the green body. The uniformity of density across the entire cross-section reduces the concentration of thermal stress during firing and cooling, thereby reducing the risk of macroscopic cracks and improving the yield of large-size products.

[0031] 3. This invention optimizes the electrical conductivity and mechanical strength of graphite materials. The asphalt components that penetrate the micropowder and coat the particle surface are transformed in situ into a highly graphitized solid carbon phase during the subsequent high-temperature graphitization process. These in-situ generated carbon phases not only enhance the interfacial bonding force between aggregate particles but also construct continuous electron transport channels between particle contact points. This results in the prepared graphite material exhibiting excellent low resistivity while possessing high compressive and flexural strength. Detailed Implementation

[0032] Preparation Examples 1-4: Preparation Example 1: This preparation example provides a method for preparing an interface modification liquid for pre-wetting photovoltaic graphite material micropowder, including the following steps: 60 parts by weight of wash oil were injected into a reactor with a heating jacket and heated to 130°C.

[0033] At a stirring speed of 300 rpm, slowly add 35 parts by weight of medium-temperature coal tar pitch crushed to less than 5 mm, and stir at a constant temperature for 50 minutes until the pitch is completely dissolved to form a homogeneous black liquid.

[0034] Add 3 parts by weight of stearic acid and 1 part by weight of sodium dodecylbenzenesulfonate to the reactor in sequence, and continue stirring for 10 minutes.

[0035] A high-shear emulsifier was started and the speed was set to 2500 rpm. The mixture was sheared for 15 minutes to obtain interface-modified liquid A. The viscosity of the interface-modified liquid at 100℃ was measured to be 85 mPa·s.

[0036] Preparation Example 2: This preparation example provides a method for preparing an interface modification liquid for pre-wetting photovoltaic graphite material micropowder, including the following steps: 65 parts by weight of wash oil were injected into the reactor and heated to 120°C.

[0037] At a stirring speed of 300 rpm, slowly add 30 parts by weight of medium-temperature coal tar pitch crushed to less than 5 mm, and stir at a constant temperature for 40 minutes until the pitch is completely dissolved.

[0038] Add 1 part by weight of stearic acid and 0.5 parts by weight of sodium dodecylbenzenesulfonate, and continue stirring for 10 minutes.

[0039] A high-shear emulsifier was started, and the speed was set to 2000 rpm. Shearing was performed for 15 minutes to obtain interface-modified liquid B. The viscosity of this interface-modified liquid at 100℃ was measured to be 55 mPa·s.

[0040] Preparation Example 3: This preparation example provides a method for preparing an interface modification liquid for pre-wetting photovoltaic graphite material micropowder, including the following steps: 40 parts by weight of wash oil and 15 parts by weight of anthracene oil were injected into the reactor, mixed evenly, and then heated to 140°C.

[0041] At a stirring speed of 500 rpm, slowly add 40 parts by weight of medium-temperature coal tar pitch and stir at a constant temperature for 60 minutes until the pitch is completely dissolved.

[0042] Add 5 parts by weight of oleic acid and 1.5 parts by weight of sodium dodecylbenzenesulfonate, and continue stirring for 10 minutes.

[0043] A high-shear emulsifier was started, and the rotation speed was set to 3000 rpm. Shearing was performed for 20 minutes to obtain interface-modified liquid C. The viscosity of this interface-modified liquid at 100℃ was measured to be 145 mPa·s.

[0044] Preparation Example 4: This preparation example provides a method for preparing an interface modification liquid for pre-wetting photovoltaic graphite material micropowder, including the following steps: 60 parts by weight of wash oil were injected into the reactor and heated to 130°C.

[0045] At a stirring speed of 400 rpm, slowly add 35 parts by weight of medium-temperature coal tar pitch and stir at a constant temperature for 50 minutes until the pitch is completely dissolved.

[0046] Add 2.5 parts by weight of oleic acid and 1 part by weight of sodium dodecylbenzenesulfonate, and continue stirring for 10 minutes.

[0047] A high-shear emulsifier was started, and the rotation speed was set to 2500 rpm. Shearing was performed for 15 minutes to obtain interface-modified liquid D. The viscosity of this interface-modified liquid at 100℃ was measured to be 82 mPa·s.

[0048] Examples 1-5: Example 1

[0049] This embodiment provides a method for preparing high-purity graphite material for photovoltaic applications with high thermal conductivity and low resistivity, including the following steps: (1) Raw material grinding and classification: The asphalt coke aggregate is ground once by air jet mill, and the particle size of the powder is controlled to be 500 mesh, and the content of the 500 mesh sieve is 92%; the asphalt coke aggregate is ground to 200 mesh, and the content of the 200 mesh sieve is controlled to be 70%.

[0050] (2) Pre-wetting of micro powder in liquid phase: Take 100 parts by weight of the 500 mesh asphalt coke micro powder obtained in step (1) and put it into a high-speed mixer. Preheat it to 140°C and add 20 parts by weight of the interface modification liquid A prepared in Preparation Example 1 (preheated to 120°C) by atomized spraying. Mix and treat for 40 minutes at a spindle speed of 1000 rpm to obtain surface-modified pre-wetting micro powder.

[0051] (3) Mixing: 40 parts by weight of the above pre-impregnated micro powder and 35 parts by weight of 200 mesh pitch coke powder are put into a twin-shaft mixing pot and dry-mixed at 155°C for 15 minutes; then 25 parts by weight of the molten modified coal tar pitch are added, the temperature is raised to 170°C and wet-mixed for 80 minutes, and then crushed after cooling.

[0052] (4) Powder preparation and molding: The crushed paste is ground to 200 mesh (60% pass rate) and loaded into a DLφ980 mold. It is pre-pressed for 10 minutes under 8MPa pressure. Then it is placed in an isostatic press and held under 170MPa pressure for 120 minutes to obtain the green body.

[0053] (5) First firing: Place the green blank in the firing furnace and heat it at a rate of 0.8℃ / h in the range of 350℃-550℃, with the highest temperature reaching 1150℃, and hold it for 50 hours.

[0054] (6) Densification cycle: Preheat a product to 300°C, evacuate to -0.098 MPa, inject impregnated asphalt and pressurize to 1.8 MPa for 100 minutes, then perform a second firing; repeat the above impregnation and firing steps once, for a total of two impregnations and three firings.

[0055] (7) Graphitization: The three baked products are sent into a graphitization furnace, heated to 2900℃, kept at that temperature for 15 hours, and then processed after cooling. Example 2

[0056] This embodiment provides a method for preparing high-purity graphite material for photovoltaic applications with high thermal conductivity and low resistivity, including the following steps: (1) Grinding and grading of raw materials: Same as in Example 1, prepare 500-mesh micro powder (90% undersize) and 200-mesh coarse powder (65% undersize).

[0057] (2) Pre-wetting of micro powder in liquid phase: 100 parts by weight of 500 mesh micro powder were put into a high-speed mixer, preheated to 135°C, and 15 parts by weight of the interface modification liquid A prepared in Preparation Example 1 were sprayed in; the mixture was mixed at 1200 rpm for 35 minutes to obtain pre-wetting micro powder.

[0058] (3) Mixing: Dry mix 45 parts by weight of pre-impregnated micro powder with 35 parts by weight of 200 mesh asphalt coke powder, add 20 parts by weight of modified coal tar pitch, and wet mix at 175°C for 60 minutes.

[0059] (4) Preparation and molding of powder: The paste is made into 200-mesh powder (pass rate 65%), pre-pressed at 10MPa after being put into the mold, and then isostatically pressed at 180MPa for 130 minutes.

[0060] (5) First firing: The heating rate in the range of 350℃-550℃ is controlled at 0.6℃ / h, the maximum temperature is 1120℃, and the holding time is 45 hours.

[0061] (6) Densification cycle: Same as in Example 1, but with two impregnations and three firings.

[0062] (7) Graphitization: Maximum temperature 3000℃, heat preservation for 12 hours. Example 3

[0063] This embodiment provides a method for preparing high-purity graphite material for photovoltaic applications with high thermal conductivity and low resistivity, including the following steps: (1) Grinding and grading of raw materials: Same as in Example 1.

[0064] (2) Pre-wetting of micro powder in liquid phase: Take 100 parts by weight of 500 mesh micro powder and put it into a high-speed mixer, preheat it to 130°C, and spray in 25 parts by weight of the high viscosity interface modified liquid C prepared in Preparation Example 3; mix at 900 rpm for 45 minutes.

[0065] (3) Mixing: Dry mix 35 parts by weight of pre-impregnated micro powder with 40 parts by weight of 200 mesh asphalt coke powder, add 25 parts by weight of modified coal tar pitch, and wet mix at 165°C for 90 minutes.

[0066] (4) Powder preparation and molding: The powder preparation is the same as in Example 1. The isostatic pressing pressure is 160 MPa and the pressure is held for 150 minutes.

[0067] (5) First firing: heating rate of 0.5℃ / h in the range of 350℃-550℃, maximum temperature of 1100℃, and holding time of 60 hours.

[0068] (6) Densification cycle: Same as in Example 1.

[0069] (7) Graphitization: maximum temperature 2850℃, heat preservation for 20 hours. Example 4

[0070] This embodiment provides a method for preparing high-purity graphite material for photovoltaic applications with high thermal conductivity and low resistivity, including the following steps: (1) Grinding and grading of raw materials: Same as in Example 1, but the content of material under 500 mesh sieve is controlled to be 95%.

[0071] (2) Pre-wetting of micro powder in liquid phase: Take 100 parts by weight of 500 mesh micro powder and spray it into 18 parts by weight of the high viscosity interface modification liquid C prepared in Preparation Example 3; mix at 1100 rpm for 40 minutes.

[0072] (3) Mixing: Dry mix 42 parts by weight of pre-impregnated micro powder with 33 parts by weight of 200 mesh asphalt coke powder, add 25 parts by weight of modified coal tar pitch, and wet mix at 175°C for 75 minutes.

[0073] (4) Powder preparation and molding: Isostatic pressing pressure is 165MPa, and pressure is maintained for 120 minutes.

[0074] (5) First roasting: Same as in Example 1.

[0075] (6) Densification cycle: Same as in Example 1.

[0076] (7) Graphitization: maximum temperature 2950℃, heat preservation for 18 hours. Example 5

[0077] This embodiment provides a method for preparing high-purity graphite material for photovoltaic applications with high thermal conductivity and low resistivity, including the following steps: (1) Grinding and grading of raw materials: Same as in Example 1.

[0078] (2) Pre-wetting of micro powder in liquid phase: Take 100 parts by weight of 500 mesh micro powder and spray it into 20 parts by weight of interface modification liquid D prepared in Preparation Example 4; mix at 1000 rpm for 40 minutes.

[0079] (3) Mixing: Dry mix 38 parts by weight of pre-impregnated micro powder with 40 parts by weight of 200 mesh asphalt coke powder, add 22 parts by weight of modified coal tar pitch, and wet mix at 170°C for 80 minutes.

[0080] (4) Powder preparation and molding: Isostatic pressing pressure is 175MPa, and pressure is maintained for 120 minutes.

[0081] (5) First firing: maximum temperature 1150℃, hold for 40 hours.

[0082] (6) Densification cycle: The impregnation and pressure holding time is shortened to 90 minutes, and the rest is the same as in Example 1.

[0083] (7) Graphitization: maximum temperature 2800℃, heat preservation for 10 hours.

[0084] Comparative Examples 1-4: Comparative Example 1: Compared with Example 1, the difference is that the micronized liquid phase pre-wetting step is not performed.

[0085] Specifically, 40 parts by weight of untreated 500-mesh pitch coke powder and 35 parts by weight of 200-mesh pitch coke powder are directly added to a kneading pot with the total binder dosage in Example 1 for kneading. All other steps are the same.

[0086] Comparative Example 2: The difference compared to Example 1 is that a separate micronized liquid phase pre-wetting step is not performed.

[0087] Specifically, the interface-modified liquid A from Preparation Example 1 was directly added into a kneading pot and kneaded simultaneously with 40 parts by weight of 500-mesh pitch coke powder, 35 parts by weight of 200-mesh pitch coke powder, and 25 parts by weight of modified coal tar pitch. That is, all raw materials were physically mixed in one step, and the remaining steps were the same.

[0088] Comparative Example 3: The difference compared to Example 1 is that the liquid medium used in the micronized liquid phase pre-wetting step is different.

[0089] Specifically, an equal part by weight of pure washing oil was used to replace the interface modification liquid A prepared in Example 1 for spraying treatment of 500 mesh powder. That is, the pre-wetting liquid did not contain dissolved coal tar pitch, stearic acid and sodium dodecylbenzene sulfonate, and the other steps were the same.

[0090] Comparative Example 4: The difference compared to Example 1 is that the object being treated by liquid pre-wetting is different.

[0091] Specifically, the interface modification liquid A prepared in Example 1 was sprayed onto the surface of 35 parts by weight of 200-mesh pitch coke powder for pre-wetting treatment, while 40 parts by weight of 500-mesh pitch coke powder was kept in dry powder state and directly added to the mixing pot for mixing. The remaining steps were the same.

[0092] Test Examples 1-4: Test Example 1: Packing characteristics and rheological properties of modified micro powder.

[0093] This test case aims to verify the effect of solvent-dispersed micropowder pre-wetting process on the agglomeration state of ultrafine powder and the pore filling mechanism by comparing the physical packing parameters and surface adsorption characteristics of micropowder raw materials before and after pretreatment.

[0094] Experimental steps: The pre-wetted micro powder prepared in step (2) of Example 1, the pre-wetted micro powder prepared in Example 3, the raw material micro powder without any treatment in Comparative Example 1, and the micro powder treated with pure solvent in Comparative Example 3 were selected as the test samples. All samples were placed in a vacuum drying oven at 60℃ for 2 hours to remove the interference of environmental adsorbed water on the test results, and then cooled to room temperature for later use. The tap density of each sample was determined using an FZS-4 multifunctional powder property tester. 100g of the powder to be tested was placed in a 250ml graduated cylinder, fixed on a tapping table, and the vibration frequency was set to 250 times / min, the vibration amplitude to 3mm, and the total number of vibrations to 3000. After vibration, the powder volume was read, and the tap density was calculated. The angle of repose of powder was determined using the fixed funnel method. The sample was allowed to fall freely through a standard funnel with a 5 mm aperture onto a horizontal disk until a stable cone was formed. The height and base radius of the cone were measured, and the angle of repose was calculated using the arctangent function to characterize the flowability and frictional properties of the powder. The oil absorption value (OAN) of the sample was determined according to GB / T 3780.2 standard. A certain amount of sample was weighed and placed on a glass plate. Dibutyl phthalate (DBP) was added dropwise using a precision burette, and the sample was continuously ground with a glass rod during the addition process until the powder agglomerated into a clump and no excess liquid seeped out. The volume of DBP consumed was recorded.

[0095] The experimental data are shown in Table 1: Table 1: Test results of packing characteristics and surface adsorption properties of micro powder samples

[0096] Results and Conclusions: The pre-impregnated micropowder prepared in Example 1 showed a tap density increase of approximately 47.4% compared to the raw material micropowder in Comparative Example 1, while also reducing the angle of repose by 14.3°. This difference indicates that the hard agglomerates present in the raw material micropowder were effectively deagglomerated under the combined effects of high-speed shearing and solvent penetration. In Comparative Example 1, the 500-mesh ultrafine powder formed a large number of hollow agglomerates due to van der Waals forces, resulting in loose packing and high internal friction. In contrast, the solvated asphalt film formed on the particle surface by the interface-modified liquid in Example 1, on the one hand, prevented the powder from re-agglomerating through steric hindrance, and on the other hand, acted as an interface lubricant, improving the particle rearrangement ability and enabling it to fill the space more tightly during the tapping process.

[0097] Comparing the data from Example 1 and Comparative Example 3, it can be seen that although treatment with pure solvent can improve the powder properties to some extent (the tap density increases to 0.89 g / cm³), the powder's properties are not improved. 3However, the effect was far less than that of the modified liquid system containing asphalt components. This confirms that simple physical wetting cannot provide a lasting agglomeration inhibition effect. In Example 1, the oil absorption value decreased to 28.5 mL / 100g, lower than the 49.2 mL / 100g of Comparative Example 3, indicating that the asphalt components in the modified liquid successfully penetrated and filled the micropores and cracks on the surface of the powder under the drive of capillary force. When subsequent kneading or oil dripping tests were conducted, the amount of external medium required was greatly reduced because the micropores had been pre-occupied.

[0098] Example 3 used a modified liquid with higher viscosity, and its tap density and oil absorption value showed slightly less improvement than those of Example 1. This indicates that the rheological properties of the modified liquid have a direct impact on the penetration efficiency; a lower viscosity medium is more conducive to penetrating deep micropores, thereby obtaining a higher density base.

[0099] Test Example 2: Testing of green body forming quality and density distribution.

[0100] This test case mainly verifies the effect of solvent-dispersed micro powder pre-wetting process on the molding performance of paste by measuring the bulk density distribution and elastic aftereffect data of the green body after isostatic pressing, and evaluates its actual effect in reducing internal stress and improving the uniformity of large-size products.

[0101] Experimental steps: Examples 1 to 3, Comparative Examples 1, 2 and 4 were selected as test objects after the isostatic pressing process in step (4) and before the baking process. All green blanks were DLφ980mm in size. The apparent bulk density of the green billet was determined using a combination of geometric measurement and weighing methods. A large vernier caliper was used to measure the height and diameter of the green billet, the volume was calculated, and the mass was weighed using an industrial electronic crane scale. To assess the internal density uniformity of large-format products, core drilling was used for sampling. Cylindrical core samples with a diameter of 50 mm and a depth of 100 mm were drilled radially from the top surface of the green body at the geometric center and at the edge 50 mm from the outer cylindrical surface. The surfaces of the extracted core samples were cleaned of dust, and their bulk density was measured and recorded as the center density and edge density, respectively. The elastic aftereffect (resilience) of the green compact was measured. After isostatic pressing and allowing it to stand in the mold for 30 minutes, the compact was demolded and removed. The compact diameter was immediately measured and compared with the mold inner diameter. The calculation formula is: (compact compact diameter - mold inner diameter) / mold inner diameter × 100%. This indicator reflects the residual elastic deformation energy and internal stress level of the powder during compression.

[0102] The experimental data are shown in Table 2: Table 2: Test data on uniformity of green body density distribution and elastic aftereffect.

[0103] Results and Conclusions: The average density of the green body in Example 1 reached 1.742 g / cm³. 3 It is higher than the 1.615 g / cm³ of Comparative Example 1. 3 The more crucial difference lies in the uniformity of the density distribution: the density difference between the center and the edge of Example 1 is only 0.007 g / cm³. 3 This indicates that the overall structure is highly uniform; while the density range of Comparative Example 1 is as high as 0.059 g / cm³. 3 The structure exhibits a tight outer layer and a loose inner layer. This confirms that in traditional dry powder mixing processes, the agglomeration of micropowder leads to a large internal friction angle, resulting in significant pressure attenuation when transmitted to the center of large-sized products. Example 1 introduces a solubilized asphalt film onto the particle surface through a pre-impregnation process, effectively reducing the sliding resistance between particles and allowing isostatic pressure to be transmitted to the center of the compact without loss, thereby eliminating the density gradient.

[0104] Regarding the elastic aftereffect data, the rebound rates of Comparative Examples 1 and 4 were as high as 1.84% and 1.71%, respectively, indicating that a large amount of energy was stored at the particle contact points in the form of elastic deformation, which is the main stress source for subsequent calcination cracking. In contrast, the rebound rates of Examples 1 to 3 were all controlled below 0.75%. This indicates that the solvent-dispersed micropowder pre-wetting technology changed the particle contact mode. The lubricating layer made it easier for the particles to undergo plastic displacement and rotational rearrangement under pressure, rather than simple elastic compression. This densification process dominated by plastic deformation not only increased the green body density but also effectively released the internal stress during the molding process.

[0105] Comparing the data of Example 1 and Comparative Example 2 (physical mixing), the density difference of Comparative Example 2 is 0.029 g / cm³. 3 While the yield and rebound rate (1.35%) were better than the blank control, they were still significantly inferior to Example 1. This indicates that simply adding the modifying liquid during the kneading stage cannot achieve uniform coating of the micropowder surface; some micropowder remains in the form of agglomerates, blocking stress transmission channels. Only through a high-speed shear pre-wetting step can an effective lubrication and buffering network be truly constructed.

[0106] Test Example 3: Comparison of the basic physicochemical properties of the final graphite products.

[0107] This test case aims to determine the final physicochemical properties of graphite materials prepared through different process routes after high-temperature graphitization treatment. The focus is on key parameters such as bulk density, resistivity, and mechanical strength to verify the actual effect of solvent-dispersed micropowder pre-wetting technology on improving the overall performance of photovoltaic thermal field materials.

[0108] Experimental steps: The final products after graphitization treatment in step (7) from Examples 1 to 5 and Comparative Examples 1 to 4 were selected as test objects. To eliminate the effects of skin effect and density gradient, all test samples were taken from the geometric center of a large-size graphite block. The center sampling block was processed into standard strips that meet the requirements of each test standard using a diamond wire cutter. According to GB / T 24528-2009 "Method for Determination of Bulk Density of Carbon Materials", the processed regular geometric sample was placed in an oven and dried to constant weight. Its length, width, height and other dimensional parameters and mass were measured, and the bulk density was calculated. According to GB / T 24525-2009 "Method for Determination of Resistivity of Carbon Materials", the resistivity of the sample at room temperature was determined using the four-probe method. During the test, the current direction was ensured to be perpendicular to the molding and pressing direction of the sample. The arithmetic mean of 6 points was taken for each group of samples. According to GB / T 3074.1-2008 "Method for Determination of Flexural Strength of Graphite Electrodes", destructive loading tests were performed on the specimens using a universal testing machine. The loading rate for the compressive strength test was controlled at 0.5 mm / min, and the span for the flexural strength test was set to 100 mm. According to GB / T 1429-2009 "Determination of Ash Content in Carbon Materials", the pulverized sample was placed in a muffle furnace and burned at 850°C with air until constant weight, and the content of residual inorganic matter was calculated.

[0109] The experimental data are shown in Table 3: Table 3: Summary of Physicochemical Properties Test Results of the Final Graphite Products

[0110] Results Analysis and Conclusions: The bulk density of Example 1 reached 1.884 g / cm³. 3 Compared to Comparative Example 1 (blank group), 1.715 g / cm³ 3 The density was increased by approximately 9.8%. This increased density directly led to enhanced mechanical properties; the compressive strength and flexural strength of Example 1 reached 52.4 MPa and 26.8 MPa, respectively, significantly higher than the 34.5 MPa and 16.2 MPa of Comparative Example 1. This indicates that pre-impregnating the micropowder with the interface-modifying liquid allows the solvent components in the liquid to carry asphalt molecules into the blind pores and microcracks within the micropowder. During the subsequent high-temperature carbonization process, these infiltrated components are converted into solid carbon in situ, effectively repairing microscopic defects and thus significantly improving the material's density and overall load-bearing capacity.

[0111] Regarding resistivity, Example 1 showed a low resistivity of 7.65 μΩ·m, while Comparative Example 1 reached 11.45 μΩ·m. This difference confirms the crucial role of interface modification technology in constructing conductive networks. In Example 1, the pitch film coating the micropowder surface formed a highly ordered graphite microcrystalline interface layer during graphitization, reducing the contact resistance between particles and constructing a fast channel for electron transport. In contrast, Comparative Example 2, although introducing the same chemical components, used direct physical mixing, and its resistivity was 9.85 μΩ·m, failing to reach the level of Example 1. This indicates that simply introducing the components is insufficient; a high-speed shear pre-wetting process must be used to achieve uniform monomolecular-level spreading of the modifier on the micropowder surface to effectively improve the interfacial electron transport characteristics.

[0112] Comparative Example 3 was pretreated with pure washing oil, and its bulk density was 1.764 g / cm³. 3 The performance was better than Comparative Example 1 but lower than Example 1, and the improvement in compressive strength was limited. This indicates that while simple solvent wetting helps powder dispersion, the lack of asphalt components as a "carbon source precursor" means that the solvent cannot leave effective bonding carbon bridges at the particle contact points after evaporation, resulting in insufficient interfacial bonding strength. Furthermore, Example 2 achieved the highest bulk density (1.892 g / cm³) while reducing the amount of modified liquid and using high molding pressure. 3 The highest compressive strength (54.1 MPa) indicates that the process has a good synergistic effect with isostatic compaction technology, and high pressure helps to further compress the gaps between particles after solvent lubrication.

[0113] Test Example 4: Uniformity and Simulated Yield Test of Large-Size Products This test case mainly evaluates the cross-sectional density distribution uniformity and thermal shock crack resistance of large-size graphite products with a diameter of 980mm. The aim is to verify the technical effectiveness of solvent-dispersed micro-powder pre-wetting technology in solving the problems of large density gradient and easy cracking in large-size products (center and edge).

[0114] Experimental steps: Five complete cylinders with a diameter of 980 mm (DLφ) were randomly selected from the graphitized products of Examples 1, 3, Comparative Example 1, and 4 as test samples. The cylindrical surfaces and end faces of all samples were inspected under natural light using visual inspection and dye penetrant testing. The number of samples with visible macroscopic cracks was counted, and the yield rate of each group was calculated. Samples without macroscopic cracks from each group were selected for dissection and sampling. The cylinder was cut along its diameter, and sampling points were taken at a cross-section 500 mm from the top surface. Cylindrical specimens with a diameter of 30 mm and a height of 30 mm were drilled at both the geometric center and the edge of the cross-section. After the drilled samples were cleaned and dried, the bulk density was determined using the Archimedes displacement method. The average density at the center and edge of each group of samples was recorded, and the density difference between the edge and the center was calculated to characterize the radial density gradient. The thermal shock resistance of the material was tested using the water quenching method. A rectangular sample measuring 40mm × 40mm × 160mm was cut from the area adjacent to the sampling location. The sample was placed in a heating furnace and heated to 1100℃ for 30 minutes, then immediately removed and immersed in flowing water at 25℃ for rapid cooling. After drying, the surface was inspected for cracks. The heating and rapid cooling process was repeated until fracture or through-cracks appeared on the sample surface, and the average number of cycles before sample failure was recorded.

[0115] The experimental data are shown in Table 4: Table 4: Test data on density uniformity and thermal shock resistance of large-size products

[0116] Results and Conclusions: Example 1 demonstrated extremely high process stability in the fabrication of large-format DLφ980mm products. No macroscopic cracks were found in any of the five samples, the simulated yield reached 100%, and the thermal shock resistance cycle was as high as 28 cycles. In contrast, the yield of Comparative Example 1 was only 20%, with most samples naturally cracking during graphitization cooling due to excessive internal stress, and the thermal shock resistance life of the surviving samples was only 9 cycles. This significant difference mainly stems from the uniformity of the internal structure of the material.

[0117] Density distribution data show that the edge density of Comparative Example 1 is 1.748 g / cm³. 3 The density is much higher than the central density (1.682 g / cm³). 3 The radial density difference is as high as 0.066 g / cm³. 3 This is because the agglomeration of untreated micropowder increases internal friction, hindering pressure transmission during isostatic pressing, resulting in a compacted outer layer and a loose core. During high-temperature graphitization and cooling, this structure, with its tight outer layer and loose inner core, generates enormous tangential tensile stress due to the mismatch in thermal expansion coefficients. Once this stress exceeds the material's strength limit, it leads to cracking.

[0118] The radial density difference in Example 1 was only 0.006 g / cm³. 3 This achieves near-uniform density across the entire cross-section. This is attributed to the lubricating modified layer formed on the surface of the micropowder by the solvent-dispersed micropowder pre-wetting process, which effectively reduces the meshing resistance between powders, allowing the molding pressure to be transmitted to the geometric center of large-size products without loss, and eliminating residual thermal stress caused by the molding density gradient.

[0119] Furthermore, the density difference of Comparative Example 4 (treated only with coarse particles) was 0.038 g / cm³. 3The thermal shock resistance (after 13 cycles) was between that of Example 1 and Comparative Example 1. This indicates that although treating coarse particles can improve flowability to some extent, it fails to solve the problem of the 500-mesh micro powder, which has the largest specific surface area and is most prone to agglomeration in the system. The defects caused by the formation of micro powder agglomerates in the matrix still exist, thus limiting further improvement in product performance and yield. This, in turn, proves the necessity and technical precision of using micro powder as the object of pre-wetting treatment.

Claims

1. A high-purity graphite material for photovoltaic applications with high thermal conductivity and low resistivity, characterized in that, The graphite material is made from raw materials comprising the following parts by weight: 35-45 parts of pre-impregnated pitch coke powder; 33-40 parts of coarse pitch coke; 20-25 parts of modified coal tar pitch binder; The pre-impregnated asphalt coke powder is obtained by spraying and high-speed mixing asphalt coke powder with a particle size of 500 mesh with an interface modification liquid. The interface modification liquid comprises solvent oil, medium-temperature coal tar pitch dissolved in the solvent oil, fatty acid surfactants, and alkylbenzene sulfonate.

2. The high-purity graphite material for photovoltaic applications with high thermal conductivity and low resistivity according to claim 1, characterized in that, The matrix aggregates of both the micro-powder and coarse-powder pitch coke are needle-shaped pitch coke or dense isotropic pitch coke.

3. The high-purity graphite material for photovoltaic applications with high thermal conductivity and low resistivity according to claim 1, characterized in that, The interface-modifying liquid is made from raw materials comprising the following parts by weight: 40-65 parts of wash oil or anthracene oil; 30-40 parts of medium-temperature coal tar pitch; 1-5 parts stearic acid or oleic acid; Sodium dodecylbenzenesulfonate 0.5-1.5 parts; The viscosity of the interface-modified liquid is 55-145 mPa·s at 90℃-100℃.

4. The high-purity graphite material for photovoltaic applications with high thermal conductivity and low resistivity according to claim 1, characterized in that, The content of the 500-mesh sieve underpowder in the asphalt coke fine powder is ≥90%, and the particle size of the asphalt coke coarse powder is 200 mesh.

5. A method for preparing a high-purity graphite material for photovoltaic applications with high thermal conductivity and low resistivity according to any one of claims 1-4, characterized in that, Includes the following steps: The solvent oil is heated, and medium-temperature coal tar pitch is added and stirred to dissolve it. Then stearic acid or oleic acid and dodecylbenzene sulfonate are added. The interface-modified liquid is obtained by high-shear emulsification treatment. Preheat 500-mesh asphalt coke powder, spray it into the interface modification liquid in the form of atomization, and mix it under high-speed stirring to obtain pre-wetted asphalt coke powder. The pre-impregnated pitch coke micro powder and pitch coke coarse powder are dry-mixed, then molten modified coal tar pitch binder is added for wet mixing, and after cooling, the mixture is crushed to obtain a paste. The paste is ground twice and then pre-pressed in a mold, followed by cold isostatic pressing to obtain a green body. The green blank is subjected to calcination, impregnation and densification cycles, and high-temperature graphitization treatment to obtain the graphite material.

6. The method for preparing a high-purity graphite material for photovoltaic applications with high thermal conductivity and low resistivity according to claim 5, characterized in that, The solvent oil is wash oil or anthracene oil, and the heating temperature is 120℃-140℃; the high-shear emulsification treatment speed is 2000rpm-3000rpm, and the shearing time is 15-20 minutes.

7. The method for preparing a high-purity graphite material for photovoltaic applications with high thermal conductivity and low resistivity according to claim 5, characterized in that, The preheating temperature of the 500-mesh asphalt coke powder is 130℃-140℃, the spindle speed of the high-speed mixer is 900rpm-1200rpm, and the mixing time is 35-45 minutes.

8. The method for preparing a high-purity graphite material for photovoltaic applications with high thermal conductivity and low resistivity according to claim 5, characterized in that, The dry mixing temperature is 155℃-165℃, and the time is 10-20 minutes; the wet mixing temperature is 165℃-175℃, and the time is 60-90 minutes.

9. The method for preparing a high-purity graphite material for photovoltaic applications with high thermal conductivity and low resistivity according to claim 5, characterized in that, The particle size of the secondary grinding is 200 mesh; the pressure of the cold isostatic pressing is 160MPa-180MPa, and the holding time is 100-150 minutes.

10. The method for preparing a high-purity graphite material for photovoltaic applications with high thermal conductivity and low resistivity according to claim 5, characterized in that, The maximum temperature for the graphitization treatment is 2800℃-3000℃, and the holding time is 10-20 hours.

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