Silver tungsten carbide graphite material and method for manufacturing the same

Abstract

This invention provides a silver-tungsten carbide graphite material and its preparation method. The preparation method includes the following steps: Step S1: Add ascorbic acid to a graphene oxide dispersion, and then dropwise add AgNO₃ solution to obtain a suspension of graphene oxide and graphene co-loaded with silver; Step S2: Add a mixed complex solution composed of Co(NO₃)₂, ammonium metatungstate, and citric acid to the suspension, centrifuge, wash, and disperse with ethanol to obtain a slurry of graphene oxide and graphene co-loaded with silver and cobalt-tungsten complex; Step S3: Place the slurry in a drying tower, introduce a mixed gas of nitrogen and trace amounts of hydrogen at an inlet temperature of 220-250℃, and perform spray drying and reduction reaction; Step S4: Place in a sintering furnace, heat to 500-600℃ under a N₂ atmosphere, then switch the atmosphere to a mixed gas of H₂ / CH₄ / Ar, heat to 850-950℃, and hold at that temperature. The material prepared by the method of this invention has a highly uniform structure, small reinforcing phase size, clean interface, and strong bonding.

Description

Technical Field

[0001] This invention relates to the field of electrical contact materials technology, specifically to a silver tungsten carbide graphite material and its preparation method. Background Technology

[0002] Silver-tungsten carbide materials are widely used in the contacts of medium and high voltage electrical switches due to their excellent electrical conductivity, wear resistance, and resistance to arc erosion. To further improve their resistance to welding, graphite (C) is often added. However, the addition of graphite significantly reduces the material's strength and conductivity, and the extremely poor wettability between liquid silver and solid graphite or tungsten carbide makes it difficult to prepare dense composite materials with high graphite content using traditional melt infiltration processes. This results in porous materials with weak interfacial bonding.

[0003] In existing technologies, a common method involves mechanically mixing silver powder, tungsten carbide powder, and graphite powder followed by sintering. However, this method struggles to achieve uniform dispersion of the reinforcing phase, leading to graphite agglomeration and significant interface problems. Therefore, developing a preparation method capable of achieving nano-sized and uniformly dispersed reinforcing phases that form a strong interfacial bond with the silver matrix is ​​crucial for improving the overall performance of silver-tungsten carbide-graphite materials. Summary of the Invention

[0004] To address the problem in existing silver-tungsten carbide-graphite materials where uneven dispersion of the reinforcing phase and weak interfacial bonding result in a difficulty in simultaneously achieving strength, conductivity, and weld resistance, this invention provides a novel preparation method and the resulting material. This method aims to achieve uniform distribution of the reinforcing phase and interfacial chemical bonding through molecular-level material design and in-situ reactions, thereby preparing a high-strength, high-conductivity, and highly weld-resistant silver-tungsten carbide-graphite material.

[0005] In a first aspect, the present invention provides a method for preparing silver tungsten carbide graphite material, comprising the following steps: Step S1: Add ascorbic acid to the graphene oxide dispersion, and then add AgNO3 solution dropwise to obtain a suspension of graphene oxide and graphene co-loaded with silver. Step S2: A mixed complex solution consisting of Co(NO3)2, ammonium metatungstate and citric acid is added dropwise to the suspension, centrifuged, washed and dispersed with ethanol to obtain a slurry of graphene oxide and graphene co-loaded with silver and cobalt-tungsten complexes. Step S3: Place the slurry into a drying tower, introduce a mixture of nitrogen and trace amounts of hydrogen, with an inlet temperature of 220-250℃, and carry out spray drying and reduction reaction; Step S4: Press the blank into a rough blank under a pressure of 150MPa, with a density of 6.7-7.0g / cm3. Place the blank into a sintering furnace, heat it to 500-600℃ in a N2 atmosphere, then switch the atmosphere to a mixture of H2 / CH4 / Ar, heat it to 850-950℃, and hold it at that temperature.

[0006] Further, in step S1, the concentration of the graphene oxide dispersion is 1-5 mg / mL; the mass ratio of ascorbic acid to graphene oxide is (1.5-3):1; and the concentration of the silver nitrate solution is 0.05-0.2 mol / L. Ascorbic acid needs to simultaneously perform two tasks: a) partially reducing GO; b) reducing all Ag⁺. If the ratio is too low, the reduction is insufficient, resulting in uneven Ag particle loading; if the ratio is too high, it is uneconomical and may over-reduce GO, destroying its structure as an adsorption platform. If the AgNO₃ concentration is too low, the reaction is slow and inefficient; if the concentration is too high, the Ag⁺ reduction rate is too fast, easily leading to a large number of Ag atoms instantaneously nucleating and agglomerating into coarse particles before being captured by the GO sheets, thus losing the nano-effect.

[0007] Further, in step S2, by mass fraction, the cobalt:tungsten ratio in the mixed complex solution is (0.5-5):(10-35); the ratio of citric acid to the total molar number of metal ions is (1.5-2.5):1. Co, as both a "metal catalyst" and an "interface modifier," exhibits an optimal synergistic range.

[0008] Furthermore, in step S3, the volume percentage of hydrogen in the nitrogen-hydrogen mixture is 3%-10%; the total flow rate of the mixture is 30-60 L / min, and the atomizing gas pressure is 0.4-0.8 MPa. This upgrades the traditional physical drying process to an integrated "drying-activation" process. A highly active, easily formable composite powder is obtained in one step, with significantly improved powder flowability.

[0009] Furthermore, in step S4, the volume percentage of each gas in the mixed gas is as follows: hydrogen: 3% - 10%; methane: 3% - 10%; argon: balance.

[0010] Further, in step S4, the temperature is increased to 500-600℃ at 1-3℃ / min under an N2 atmosphere; the atmosphere is then switched to a mixture of H2 / CH4 / Ar, and the temperature is increased to 850-950℃ at 4-6℃ / min, and held for 1-2 hours. H2 acts as a strong reducing agent, responsible for completely reducing the metal oxide to active metal atoms. CH4 acts as a gaseous carbon source. Under the catalysis of Co, CH4 can be decomposed at a relatively low temperature (850-950℃) to produce active atoms, which react with W to generate WC. If the proportion is too low, carbonization is incomplete, producing W or W2C; if the proportion is too high, free carbon is easily generated, contaminating the interface.

[0011] Slow heating ensures the gradual decomposition and volatilization of organic matter (such as residual citric acid and ethanol), preventing the green body from bubbling and cracking due to instantaneous gas generation. Rapid heating within the reduction-carbonization window reduces the low-temperature dwell time, inhibits premature coarsening of metal particles, and allows the reaction to proceed rapidly and concentrated at higher temperatures, which is beneficial for the formation of fine WC grains.

[0012] Furthermore, the preparation method also includes step S5: stacking the sintered green body obtained in step S4 with silver or silver alloy infiltration sheets; heating to 960-1000℃ under vacuum or protective atmosphere, holding for 30-60 minutes, so that the infiltration material penetrates into the sintered green body.

[0013] Furthermore, the infiltrated sheet is a silver sheet, or a fine-grained silver sheet containing 0.05-0.15wt% nickel (Ni), or a silver alloy sheet containing 0.01-0.1wt% rare earth elements.

[0014] In a second aspect, the present invention provides a silver tungsten carbide graphite material prepared by any of the above preparation methods.

[0015] Beneficial effects of the present invention Compared with the prior art, the present invention has the following significant advantages: By employing a liquid-phase chemical method, silver nanoparticles and cobalt and tungsten precursors are uniformly adsorbed onto graphene oxide / graphene sheets at the molecular / ionic scale, fundamentally solving the aggregation problem of the reinforcing phase (especially nano-carbon materials). The tungsten precursor undergoes an in-situ gas-solid reaction with the carbon source, directly generating nano / submicron tungsten carbide (WC) particles on the graphene sheets. Simultaneously, cobalt acts as a catalyst and surfactant, enriching on the surface and interface of the WC particles, significantly improving the wettability and chemical bonding between the silver matrix and WC / graphene, forming a strong "WC-Co-graphene-silver" interfacial structure.

[0016] The material prepared by the method of this invention has a highly uniform microstructure, small reinforcing phase size, clean interfaces, and strong bonding. Therefore, the material simultaneously possesses high density, high strength, high conductivity, and excellent resistance to arc erosion and welding, making it particularly suitable for high-performance circuit breaker contacts. Attached Figure Description

[0017] Figure 1 This invention provides a schematic flowchart of a method for preparing silver tungsten carbide graphite material. Figure 2 This is a microstructure diagram of the silver tungsten carbide graphite material prepared in Example 1; Figure 3 This is another microstructure diagram of the silver tungsten carbide graphite material prepared in Example 1. Detailed Implementation

[0018] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] like Figure 1 As shown, the present invention provides a method for preparing silver tungsten carbide graphite material, comprising the following steps: Step S1: Add ascorbic acid to the graphene oxide dispersion, and then add AgNO3 solution dropwise to obtain a suspension of graphene oxide and graphene co-loaded with silver. Step S2: A mixed complex solution consisting of Co(NO3)2, ammonium metatungstate and citric acid is added dropwise to the suspension, centrifuged, washed and dispersed with ethanol to obtain a slurry of graphene oxide and graphene co-loaded with silver and cobalt-tungsten complexes. Step S3: Place the slurry into a drying tower, introduce a mixture of nitrogen and trace amounts of hydrogen, with an inlet temperature of 220-250℃, and carry out spray drying and reduction reaction; Step S4: Press the blank into a rough blank under a pressure of 150MPa, with a density of 6.7-7.0g / cm3. Place the blank into a sintering furnace and heat it to 500-600℃ under a N2 atmosphere. Then switch the atmosphere to a mixture of H2 / CH4 / Ar and heat it to 850-950℃. Hold the temperature.

[0020] In step S1, ascorbic acid is first added to pre-reduce graphene oxide (GO), removing some oxygen-containing functional groups and forming sheets where locally electron-rich reduced graphene oxide (rGO) regions coexist with the remaining GO regions. Subsequently, Ag⁺ ions are preferentially adsorbed and reduced in situ in these active regions, generating silver nanoparticles and obtaining an "Ag@rGO / GO" composite suspension with excellent dispersibility.

[0021] In step S2, a solution containing Co²⁺, a W source (ammonium metatungstate), and a complexing agent (citric acid) is added dropwise to the suspension in S1, controlling the pH (approximately 5.0-6.5) to allow the metal-citric acid complex to strongly adsorb onto the negatively charged functional groups on the surface of the GO / rGO-Ag composite sheet. Citric acid forms a stable complex with the metal ions. Under weakly acidic conditions, the carboxyl groups on the GO / rGO surface are negatively ionized and strongly attracted to the positively charged or electrically neutral metal complexes, achieving an atomically uniform distribution of the two key elements, Co and W, on the "Ag@rGO / GO" composite unit.

[0022] Step S3 integrates three processes: rapid solvent evaporation, atmospheric reduction, and thermal decomposition. High temperature and trace amounts of H2 promote the further reduction of GO to rGO; simultaneously, nitrates and some organic matter decompose, and metal complexes transform into oxides or metallic states, yielding a highly chemically active composite powder.

[0023] In step S4, residual organic matter is first removed by slow heating in an inert N2 atmosphere. Then, the temperature is switched to a H2 / CH4 / Ar mixture at a critical temperature window. H2 completely reduces the oxides of W and Co in the precursor to their metallic state. CH4, under the action of a catalyst (Co), cracks to provide active carbon atoms, which react with the newly reduced W atoms to generate nano-tungsten carbide in situ. In this process, Co not only catalyzes the reaction but also forms an alloyed interface layer between the generated WC particles and silver / graphene through surface segregation.

[0024] Example 1 Step S1: Add ascorbic acid to the graphene oxide dispersion, and then add AgNO3 solution dropwise to obtain a suspension of graphene oxide and graphene co-loaded with silver. The concentration of GO dispersion is 1 mg / mL, and 1000 mL (containing 1.0 g of GO) is used. The amount of ascorbic acid (AA) is AA:GO = 1.5:1, which is 1.5 g. The concentration of AgNO3 solution is 0.05 mol / L, the mass of AgNO3 is 102.4 g, and the preparation volume is about 12.0 L (based on 0.05 mol / L).

[0025] Step S2: A mixed complex solution consisting of Co(NO3)2, ammonium metatungstate, and citric acid is added dropwise to the suspension. The mixture is centrifuged, washed, and dispersed with ethanol to obtain a slurry of graphene oxide and graphene co-loaded with silver and cobalt-tungsten complexes. In the mixed complex solution, the ratio of cobalt to tungsten is 0.5:10 (parts by mass), and the molar ratio of citric acid to the total metal ions is 1.5:1.

[0026] Step S3: Place the slurry into a drying tower and introduce a mixture of nitrogen and trace amounts of hydrogen at an inlet temperature of 220-250℃ for spray drying and reduction reaction to obtain active powder; wherein, the spray drying atmosphere is: N2 - 3% H2; the inlet temperature is 220℃.

[0027] Step S4: The active powder is pressed into a rough blank under a pressure of 150 MPa, with a compact density of 6.7 g / cm³. 3 The compact is placed in a sintering furnace and heated to 500-600℃ at a rate of 1℃ / min under N2 atmosphere. Then the atmosphere is switched to a mixture of Ar / 3% H2 / 3% CH4 and heated to 850℃ at a rate of 4℃ / min, and held for 1 hour.

[0028] Example 2 Step S1: Add ascorbic acid to the graphene oxide dispersion, then add AgNO3 solution dropwise to obtain a suspension of graphene oxide and silver co-loaded on graphene. The concentration of the GO dispersion is 5 mg / mL, and 200 mL (containing 1.0 g of GO) is used. The amount of ascorbic acid (AA) is AA:GO = 3:1, i.e., 3.0 g. The concentration of the AgNO3 solution is 0.2 mol / L. The calculated amount of AgNO3 required is approximately 102.4 g, and the preparation volume is approximately 3.0 L.

[0029] Step S2: A mixed complex solution consisting of Co(NO3)2, ammonium metatungstate, and citric acid is added dropwise to the suspension. The mixture is centrifuged, washed, and dispersed with ethanol to obtain a slurry of graphene oxide and graphene co-loaded with silver and cobalt-tungsten complexes. In the mixed complex solution, the cobalt:tungsten ratio is 5:35 (parts by mass). The total molar ratio of citric acid to metal ions is 2.5:1.

[0030] Step S3: Place the slurry into a drying tower and introduce a mixture of nitrogen and trace amounts of hydrogen at an inlet temperature of 220-250℃ for spray drying and reduction reaction to obtain active powder; wherein, the spray drying atmosphere is: N2 - 10% H2; the inlet temperature is 250℃.

[0031] Step S4: The active powder is pressed into a rough blank under a pressure of 150 MPa, with a compact density of 7.0 g / cm³. 3 The pressed blank is placed in a sintering furnace under N2 atmosphere and heated to 600℃ at 1℃ / min; then the atmosphere is switched to a mixture of Ar / 10% H2 / 10% CH4 and heated to 950℃ at 6℃ / min, and held for 2 hours.

[0032] Example 3 The basic parameters are the same as in Example 2, but the parameters in step S3 are changed as follows: Total flow rate of spray drying atmosphere: 30 L / min. Atomizing gas pressure: 0.4 MPa.

[0033] Example 4 The basic parameters are the same as in Example 2, except for step S5, which involves heating the sintered billet to 1000°C under a vacuum or protective atmosphere (by stacking the sintered billet with pure silver infiltration sheets) and holding it at that temperature for 60 minutes, so that the infiltration material can penetrate into the sintered billet.

[0034] Example 5 The basic parameters are the same as in Example 4, except that step S6 is added, in which the billet after melting and infiltration is subjected to re-pressing at a temperature of 400°C and a pressure of 800 MPa for 10 minutes.

[0035] Comparative Example 1 Weigh out 65 wt% Ag, 30 wt% micron WC powder (3-5 μm), and 5.0 wt% graphite powder (5 μm). Dry mix in a planetary ball mill for 12 hours. Press the mixed powder into a compact and sinter at 900°C for 2 hours under a hydrogen atmosphere. After sintering, perform cold pressing at a pressure of 200 MPa.

[0036] Comparative Example 2 After performing only step S1 to obtain the Ag@GO / rGO suspension, it is directly spray-dried (parameters same as in Example 2) to obtain Ag-graphene composite powder. This powder is mechanically mixed with micron-sized WC powder (accounting for 30wt% of the final material), and then steps S4 and S5 are performed (parameters same as in Example 2).

[0037] Test results show that, since WC is an external addition and has no chemical connection with graphene or the silver matrix, the interfacial wettability is poor, the WC distribution is uneven, and it is prone to agglomeration.

[0038] Comparative Example 3 The preceding steps are the same as in Example 2. In step S4, the atmosphere is switched to Ar / 10% H2 (without CH4), and the temperature is raised to 950°C and held for 2 hours.

[0039] Test results show that there is only reduction without carbonization, and the final product mainly contains tungsten metal or tungsten oxide.

[0040] Comparative Example 4 The preceding steps are the same as in Example A2. In step S4, the temperature is raised to 800°C and held for 2 hours under an Ar / H2 / CH4 atmosphere.

[0041] The microstructure of the product obtained in Test Example 1 is shown in [reference needed]. Figure 2 and 3 The surface showed a uniform and dense microstructure.

[0042] The products obtained from Examples 1-5 and Comparative Examples 1-4 were subjected to performance tests, and the results are shown in Table 1.

[0043] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to the embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method of producing a silver tungsten carbide graphite material, characterized by, Includes the following steps: Step S1: Add ascorbic acid to the graphene oxide dispersion, and then add AgNO3 solution dropwise to obtain a suspension of graphene oxide and graphene co-loaded with silver. Step S2: A mixed complex solution consisting of Co(NO3)2, ammonium metatungstate and citric acid is added dropwise to the suspension, centrifuged, washed and dispersed with ethanol to obtain a slurry of graphene oxide and graphene co-loaded with silver and cobalt-tungsten complexes. Step S3: Place the slurry into a drying tower, and introduce a mixture of nitrogen and trace amounts of hydrogen at an inlet temperature of 220-250℃ to carry out spray drying and reduction reaction to obtain active powder. Step S4: The active powder is pressed into a green compact at a pressure of 150 MPa, and the green compact has a density of 6.7-7.0 g / cm 3 The green compact is placed in a sintering furnace, heated to 500-600°C in a N2 atmosphere, the atmosphere is switched to a mixed gas of H2 / CH4 / Ar, heated to 850-950°C, and held.

2. The preparation method according to claim 1, characterized in that, In step S1, the concentration of the graphene oxide dispersion is 1-5 mg / mL; the mass ratio of ascorbic acid to graphene oxide is (1.5-3):1; and the concentration of the silver nitrate solution is 0.05-0.2 mol / L.

3. The preparation method according to claim 1 or 2, characterized in that, In step S2, the cobalt:tungsten ratio in the mixed complex solution is (0.5-5):(10-35) by mass. The ratio of citric acid to the total molar number of metal ions is (1.5-2.5):

1.

4. The preparation method according to claim 1, characterized in that, In step S3, the volume percentage of hydrogen in the nitrogen-hydrogen mixture is 3%-10%; the total flow rate of the mixture is 30-60 L / min; and the atomizing gas pressure is 0.4-0.8 MPa.

5. The preparation method according to claim 1, characterized in that, In step S4, the volume percentage of each gas in the mixed gas is as follows: hydrogen: 3% - 10%; methane: 3% - 10%; argon: balance.

6. The preparation method according to claim 1, characterized in that, In step S4, the temperature is increased to 500-600℃ at 1-3℃ / min under N2 atmosphere; the atmosphere is then switched to a mixture of H2 / CH4 / Ar, and the temperature is increased to 850-950℃ at 4-6℃ / min, and held for 1-2 hours.

7. The preparation method according to claim 1, characterized in that, The preparation method further includes step S5: The sintered blank obtained in step S4 is stacked with silver or silver alloy infiltration sheets. Under vacuum or a protective atmosphere, heat to 960-1000℃ and hold for 30-60 minutes to allow the melt-infiltrating material to penetrate into the sintered green body.

8. The preparation method according to claim 7, characterized in that, The infiltrated sheet is a silver sheet, or a fine-grained silver sheet containing 0.05-0.15wt% nickel (Ni), or a silver alloy sheet containing 0.01-0.1wt% rare earth elements.

9. The preparation method according to claim 7, characterized in that, The preparation method further includes step S6: repressurizing at a temperature of 300-500℃ and a pressure of 500-1000 MPa for 5-15 minutes.

Images