Composite plasma electrode with axial gradient rare earth doping structure

Abstract

The invention relates to a composite plasma electrode with an axial gradient rare earth doping structure, a matrix of the composite plasma electrode is tungsten, molybdenum, tungsten-molybdenum alloy or tantalum, rare earth doping components and concentration of the electrode are in continuous or stepped gradient change along the axial direction of the electrode, and the composite plasma electrode is divided into an end electron emission region, a root high-temperature strengthening region and a middle transition region. The electrode has the advantages that the structure and the preparation method are reasonable in design, different types of rare earth elements are distributed in different functional areas in a gradient manner, so that the end part of the electrode has excellent electron emission capability, and meanwhile, the electrode main body has extremely high high-temperature mechanical property, so that the comprehensive performance (arc starting performance, stability and service life) of the electrode is effectively improved, and the service life of the electrode is prolonged. And the preparation method is feasible in process, and a stable and controllable gradient structure can be realized.

Description

Technical Field

[0001] This invention relates to a plasma electrode, specifically a composite plasma electrode with a rare earth doped structure having an axial gradient, used in equipment for arc discharge, plasma spraying, melting, and cutting, and its preparation method, belonging to the field of plasma and high-temperature electrode materials technology. Background Technology

[0002] High-melting-point metals (such as tungsten and molybdenum) and their alloys are key materials for manufacturing electrodes for high-energy-consuming plasma devices. In order to improve the electron emission performance of the electrodes and reduce the work function, the industry generally adopts the method of doping rare earth oxides (such as ThO2, CeO2, La2O3, Y2O3) to form uniformly doped electrodes such as thorium tungsten, cerium tungsten, and lanthanum tungsten.

[0003] However, the physicochemical loads borne by different parts of the plasma electrode vary significantly during operation. First, the electrode tip (arc root) directly bears the arc impact at extremely high temperatures, requiring materials with very low electron work function to facilitate arc initiation, arc stabilization, and reduce cathode voltage drop. Second, the electrode body and root primarily bear enormous thermal loads and high-temperature creep stress, requiring materials with excellent high-temperature creep resistance, recrystallization temperature, and evaporation resistance to ensure the electrode's shape stability and service life during long-term use.

[0004] The commonly used uniform doping technique in existing technologies involves uniformly distributing one or more rare-earth oxides throughout the entire electrode body. This compromise cannot simultaneously and optimally satisfy the nearly contradictory requirements of "high emission" at the electrode tip and "high stability" in the bulk. Uniform doping may result in suboptimal electron emission performance or insufficient high-temperature strength, leading to premature electrode deformation and burn-out, thus limiting further improvements in electrode performance.

[0005] Therefore, there is an urgent need in this field for a novel structural design that can achieve "on-demand allocation" of electrode material properties. Summary of the Invention

[0006] The present invention proposes a composite plasma electrode with an axial gradient rare earth doping structure, which aims to overcome the above-mentioned shortcomings of the prior art, while effectively meeting the requirements of "high emission" at the end and "high stability" of the body.

[0007] The technical solution of the present invention is a composite plasma electrode with an axial gradient rare earth doping structure. The substrate is tungsten, molybdenum, tungsten-molybdenum alloy or tantalum. The rare earth doping composition and concentration of the electrode change continuously or stepwise along its axis, and it is divided into an end electron emission region, a root high-temperature strengthening region and an intermediate transition region.

[0008] Preferably, the end electron emission region is located at the discharge end of the electrode, and this region is doped with one or more of lanthanum oxide, cerium oxide, and praseodymium oxide, with a total rare earth doping concentration of 2.0 wt%-3.5 wt%. This aims to maximize electron emission capability and reduce arc-starting voltage and operating temperature.

[0009] Preferably, the high-temperature strengthening region at the root is located at the root of the electrode, away from the discharge end. This region is doped with one or more of yttrium oxide, gadolinium oxide, and ytterbium oxide, with a total rare earth doping concentration of 1.0 wt%-2.5 wt%. The aim is to significantly improve the electrode's high-temperature creep resistance and recrystallization temperature through mechanisms such as solid solution strengthening and grain refinement, thereby preventing electrode deformation and ablation.

[0010] Preferably, the intermediate transition region connects the end region and the root region. Within this region, the concentration of rare earth elements doped in the end electron emission region gradually decreases, while the concentration of rare earth elements doped in the root high-temperature strengthening region gradually increases. This achieves a smooth performance transition and avoids internal stress concentration and interface failure caused by abrupt performance changes.

[0011] Preferably, the end electron emission region occupies 1 / 5 to 1 / 3 of the total length of the electrode.

[0012] Preferably, the length ratio of the end electron emission region to the root high-temperature strengthening region is (1:4-1:1).

[0013] The electrode is prepared using powder metallurgy. Its gradient structure is formed by layering doped tungsten powders of different compositions in different regions according to the designed gradient, followed by cold isostatic pressing. The formed blank is then sintered at high temperature under hydrogen protection.

[0014] Preferably, the cold isostatic pressing pressure is 150MPa-250MPa, and the high-temperature sintering temperature is 1900℃-2100℃.

[0015] Preferably, the preparation of the doped powders with different components in each region includes: first, preparing a matrix powder; preparing end and root rare earth precursor solutions respectively: dissolving end oxides in nitric acid and root oxides in nitric acid; adding the matrix powder to deionized water and stirring, then adding the prepared rare earth nitrate solution in batches, mixing thoroughly and then heating to dryness to obtain composite precursor powders containing different rare earth components; then performing a two-step reduction in a hydrogen atmosphere to obtain uniformly dispersed rare earth doped tungsten powder; at this point, rare earth matrix powders doped in the end and root regions, as well as several groups of rare earth matrix powders with gradually varying rare earth contents, can be obtained.

[0016] The advantages of this invention are: the structure and preparation method are reasonably designed, and by gradient distribution of different types of rare earth elements in different functional regions, the electrode tip has excellent electron emission capability, while the electrode body has extremely high high-temperature mechanical properties, thereby effectively improving the comprehensive performance of the electrode (arc-starting performance, stability, and lifespan). Moreover, the preparation method is feasible and can achieve a stable and controllable gradient structure. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the composite plasma electrode with an axial gradient rare earth doping structure according to the present invention.

[0018] Figure 2 This is a comparison table of embodiments 1-3 of the present invention. Detailed Implementation

[0019] The present invention will be further described in detail below with reference to embodiments and specific implementation methods.

[0020] like Figure 1 As shown, a composite plasma electrode with an axial gradient rare earth doping structure has a matrix of high-melting-point metals such as tungsten, molybdenum, tungsten-molybdenum alloy, or tantalum. The rare earth doping composition and concentration of the electrode change continuously or in a stepwise gradient along its axial direction (from the end to the root), and is divided into an end electron emission region, a root high-temperature enhancement region, and an intermediate transition region.

[0021] The gradient structure of the three functional zones is as follows: 1. The terminal electron emission region, located at the discharge end of the electrode [occupying approximately 1 / 5 to 1 / 3 of the total length, and with a length ratio of (1:4 to 1:1) to the root region], is preferentially doped with one or more low work function rare earth oxides, such as lanthanum oxide, cerium oxide, and praseodymium oxide. This region has the highest total rare earth doping concentration, preferably 2.0 wt% to 3.5 wt%, more preferably 2.5 wt% to 3.0 wt%, aiming to maximize electron emission capability and reduce arc-starting voltage and operating temperature.

[0022] II. The high-temperature strengthening region at the root is located at the root of the electrode, away from the discharge end. This region is preferentially doped with one or more high-melting-point, high-stability rare earth oxides, such as yttrium oxide, gadolinium oxide, and ytterbium oxide. The total rare earth doping concentration in this region is relatively low, preferably 1.0wt%-2.5wt%, and more preferably 1.2wt%-2.0wt%. The aim is to significantly improve the high-temperature creep resistance and recrystallization temperature of the electrode through solid solution strengthening, grain refinement, and other mechanisms, thereby preventing electrode deformation and ablation.

[0023] Third, the intermediate transition zone connects the end zone and the root zone. In this zone (accounting for about 30%-70% of the total electrode length), the concentration of low work function rare earth elements gradually decreases while the concentration of high melting point rare earth elements gradually increases, achieving a smooth transition in performance and avoiding internal stress concentration and interface failure caused by sudden performance changes.

[0024] During preparation, different rare earth formulations are configured according to the performance requirements of the end, transition zone, and root, and then layered and pressed. A mature industrial powder metallurgy process is employed, specifically including: First, the preparation of rare earth-doped powders: First, high-purity tungsten powder is prepared as the matrix. Separate rare-earth precursor solutions are prepared for the tip and root: the tip uses low-work function rare-earth oxides La₂O₃ or CeO₂ dissolved in nitric acid, and the root uses high-melting-point rare-earth oxide Y₂O₃ dissolved in nitric acid. The tungsten powder is added to deionized water and stirred. Then, the prepared rare-earth nitrate solution is added in batches (according to the designed weight percentage, so that the resulting tip powder contains rare-earth La₂O₃ or CeO₂, the root powder contains Y₂O₃, and there is a transition zone powder with gradually changing rare-earth content). After thorough mixing, the mixture is heated to dryness to obtain composite precursor powders containing different rare-earth components. Next, a two-step reduction is performed in a hydrogen atmosphere (e.g., pre-reduction at 550-650℃, followed by a secondary reduction at 850-1000℃) to obtain uniformly dispersed rare-earth-doped tungsten powder with an average particle size of approximately 1.2-1.4 μm. At this point, rare earth tungsten powders doped in the end region and root region, as well as several groups of rare earth tungsten powders with gradually varying rare earth content, can be obtained.

[0025] Second, cold isostatic pressing and sintering: The prepared powder, consisting of three sections—end, transition zone, and root—is layered and placed into a hollow mold, with each layer approximately 20mm from the end, 20mm from the root, and 60mm from the transition zone, forming a gradient powder layer. The powder is then isostatically pressed at approximately 150-250 MPa to obtain a high-density powder blank. This blank is then sintered at high temperature in a hydrogen atmosphere furnace. The sintering temperature can be raised to approximately 1900-2100℃ and held for a certain time to allow sufficient diffusion and densification of the tungsten powder. After sintering, a rare-earth-containing gradient-doped tungsten rod blank is obtained.

[0026] Third, high-temperature forging results in a dense structure: The sintered tungsten billet is heated to above 1550℃ and then hot-forged. A high-speed forging machine, air hammer, or rotary forging machine can be used to repeatedly forge the billet, continuously reducing the cross-section and increasing the length during the process to further improve density and refine grains. When the forging temperature is above 1550℃, the density of the tungsten rod can approach the theoretical density (≥95%). After forging, the tungsten rod is ground on its outer diameter to remove surface oxide scale and defects, resulting in a bright, dimensionally accurate tungsten rod.

[0027] Fourth, cutting and finishing: Finally, the forged gradient-doped tungsten rod is cut to the required length according to the needs, with a total electrode length of 100mm (discharge end section 0-25mm, root section 75-100mm, intermediate transition section 25-75mm). The ends are then precision-machined by turning or grinding. The tip of the rod is cut into a certain cone angle of 45 degrees and the surface is polished to obtain a gradient rare-earth-doped tungsten electrode that meets the requirements for electrode use.

[0028] The following are three specific preparation examples based on different usage needs. Example 1: Electrode emphasizing high electron emission and rapid arc initiation (suitable for precision welding)

[0029] The gradient structure is designed for the end region (0-25mm): doped with 2.5wt% La2O3. The use of high-concentration lanthanum oxide aims to drastically reduce work function, achieving rapid cold start-up and excellent low-current stability.

[0030] Transition zone (25-75mm): The composition gradually changes linearly from 2.5wt% La2O3 to 1.2wt% Y2O3.

[0031] Root region (75-100mm): Doped with 1.2wt% Y2O3. This ensures the electrode has sufficient high-temperature strength and resistance to deformation.

[0032] Key parameters of the preparation process: Reduction process: Pre-reduction at 650℃ for 1 hour under hydrogen atmosphere, followed by secondary reduction at 900℃ for 2 hours. Sintering process: 1950℃, hydrogen protection, holding for 3 hours.

[0033] Expected performance advantages (vs. uniformly doped 1.5% La2O3 electrode): Critical arc initiation voltage: expected to be reduced by 15-25%, with particularly excellent performance in low current environments. Arc stability: Arc voltage fluctuation range reduced by more than 40%. Lifespan: Lifespan expected to be extended by 50-80% when operating at current specifications below 100A. Example 2: Electrode focusing on ultra-long lifespan and ablation resistance (suitable for high-load plasma cutting / spraying)

[0034] The gradient structure is designed with the end region (0-15mm) doped with 2.0wt% CeO2. CeO2 provides good electron emission while also exhibiting good resistance to ion bombardment.

[0035] Transition zone (15-70mm): The composition gradually changes from 2.0wt% CeO2 to 2.0wt% Y2O3.

[0036] Root region (70-100mm): Doped with 2.0wt% Y₂O₃. High concentration of yttrium oxide is used to maximize the high-temperature strength and recrystallization temperature of the electrode, and to resist high-temperature creep.

[0037] Key parameters of the preparation process: Reduction process: Pre-reduction at 600℃ for 1.5 hours under hydrogen atmosphere, followed by secondary reduction at 950℃ for 2.5 hours. Sintering process: 2050℃, hydrogen protection, holding for 4 hours to ensure sufficient densification of the high Y2O3 region.

[0038] Expected performance advantages (vs. uniformly doped 2.0% CeO2 electrode): Electrode life: Excellent resistance to burn-off during high-load cycling above 250A, with an expected lifespan extension of 100-150% (i.e., a 1 to 1.5-fold increase in lifespan). High-temperature shape stability: Electrode deformation (such as cone passivation) can be reduced by more than 60% after high-intensity operation. Example 3: A general-purpose electrode with balanced overall performance (preferred for industrial use)

[0039] The gradient structure is designed for the end region (0-20mm): doped with 3.0wt% La2O3. This provides optimal arc-starting characteristics.

[0040] Transition zone (20-80mm): The composition gradually changes linearly from 3.0wt% La2O3 to 1.5wt% Y2O3.

[0041] Root region (80-100mm): Doped with 1.5wt% Y2O3. This ensures longevity while also considering material processability and cost.

[0042] Key parameters of the preparation process, reduction and sintering process: same as in Example 1, using mature industrial standard parameters to ensure yield and controllable cost.

[0043] Expected performance advantages (vs. uniformly doped 2.0% CeO2 electrode): Arc start-up voltage: expected to be reduced by 12-18%. Arc stability: voltage fluctuation range reduced by 35-45%. Electrode life: expected to be extended by 70-100% in the common industrial current range of 150-200A. This embodiment achieves the best balance among arc start-up, arc stability, and life.

[0044] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several modifications and improvements without departing from the inventive concept of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A composite plasma electrode with an axially gradient rare-earth doped structure, characterized in that, The matrix is ​​tungsten, molybdenum, tungsten-molybdenum alloy or tantalum. The rare earth doping composition and concentration of the electrode change continuously or stepwise along its axis, and it is divided into the end electron emission region, the root high temperature strengthening region and the intermediate transition region.

2. The composite plasma electrode with an axially gradient rare-earth doped structure as described in claim 1, characterized in that, The terminal electron emission region is located at the discharge end of the electrode. This region is doped with one or more of lanthanum oxide, cerium oxide, and praseodymium oxide, with a total rare earth doping concentration of 2.0wt%-3.5wt%.

3. The composite plasma electrode with an axially gradient rare-earth doped structure as described in claim 1, characterized in that, The aforementioned high-temperature enhanced root region is located at the root of the electrode away from the discharge end. This region is doped with one or more of yttrium oxide, gadolinium oxide, and ytterbium oxide, with a total rare earth doping concentration of 1.0wt%-2.5wt%.

4. A composite plasma electrode with an axially gradient rare-earth doped structure as described in claim 1, characterized in that, The intermediate transition region connects the end region and the root region. In this region, the concentration of rare earth elements doped in the end electron emission region gradually decreases, while the concentration of rare earth elements doped in the root high-temperature strengthening region gradually increases.

5. A composite plasma electrode with an axially gradient rare-earth doped structure as described in claim 1, characterized in that, The terminal electron emission region occupies 1 / 5 to 1 / 3 of the total length of the electrode.

6. A composite plasma electrode with an axially gradient rare-earth doped structure as described in claim 1, characterized in that, The length ratio of the end electron emission region to the root high-temperature strengthening region is (1:4-1:1).

7. A method for preparing a composite plasma electrode with an axially gradient rare-earth doped structure as described in any one of claims 1-6, characterized in that, The gradient structure is formed by using powder metallurgy technology. This is achieved by layering doped tungsten powders of different compositions in different regions according to the designed gradient, followed by cold isostatic pressing. The formed blank is then sintered at high temperature under hydrogen protection.

8. The method for preparing a composite plasma electrode with an axially gradient rare-earth doped structure as described in claim 7, characterized in that, The cold isostatic pressing is performed at a pressure of 150MPa-250MPa, followed by high-temperature sintering at a temperature of 1900℃-2100℃.

9. The method for preparing a composite plasma electrode with an axially gradient rare-earth doped structure as described in claim 7, characterized in that... The preparation of doped powders with different components in each region includes: first, preparing a matrix powder; preparing end and root rare earth precursor solutions separately: dissolving end oxides in nitric acid and root oxides in nitric acid; adding the matrix powder to deionized water and stirring, then adding the prepared rare earth nitrate solution in batches, mixing thoroughly and then heating to dryness to obtain composite precursor powders containing different rare earth components; then performing a two-step reduction in a hydrogen atmosphere to obtain uniformly dispersed rare earth doped tungsten powder; at this point, rare earth matrix powders doped in the end and root regions, as well as several groups of rare earth matrix powders with gradually varying rare earth contents, can be obtained.

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