Influence of Powder Properties on the Formability of Tantalum Anode Blocks
Release time:
2026-01-04
Tantalum anodes are the core components of tantalum electrolytic capacitors, directly determining their key performance characteristics [11-14].
Traditional tantalum anode blocks are typically formed by first compacting tantalum powder into preforms of specific shapes and dimensions, followed by subsequent processing. The compaction process involves mixing tantalum powder with organic binders to enhance powder flowability, improve density uniformity and compressive strength within the compacted anode, and reduce wear on molds and punches [13-15]. Tantalum leads are typically embedded within the anode during pressing, maintaining close contact with the tantalum powder. Serving as the positive terminal of tantalum capacitors, these leads distribute positive current to each tantalum powder particle [16-18]. Compressed tantalum anodes exhibit varying shapes and dimensions depending on the type of finished tantalum capacitor utilizing these anodes. Following compression molding, vacuum treatment at an appropriate temperature is required prior to formal sintering, based on the binder type, to pre-sinter and remove the binder. This process reduces carbon impurities introduced by the binder, thereby minimizing the impact of impurity carbon on leakage current in tantalum electrolytic capacitors.
Research indicates that low compressive strength in the tantalum powder-binder mixture leads to reduced strength in the pressed tantalum green compact and potential core wire detachment. This weakness further increases susceptibility to cracking during subsequent degreasing and sintering processes. These cracks represent the weakest anodic points and potential failure sites, directly impacting tantalum electrolytic capacitor quality. Therefore, this study investigates the influence of powder physical properties on the compaction of tantalum powder.
1 Experiment
1.1 Experimental Design
In the tantalum powder compaction experiments, three batches of capacitor-grade tantalum powder samples were selected, designated as Sample 1, Sample 2, and Sample 3. Their chemical compositions are shown in Table 1. As indicated in Table 1, Samples 1, 2, and 3 exhibit similar chemical compositions, with oxygen content ranging from 0.16% to 0.17% and comparable levels of other impurities (C, N, H, Fe, Ni, Cr, Si). This excludes differences in chemical composition as a cause for variations in the formability of tantalum anode blocks. The three groups of samples were pressed under identical forming conditions to investigate the influence of physical properties on the formability of tantalum anode blocks.
1.2 Analysis and Testing
In accordance with GB/T 1482-2022 and GB/T3249-2022 standards, physical property testing was conducted using a Hall flowmeter, WLP-202 average particle size analyzer, Scott cup, and DZS-200 test sieve machine. Tantalum powder morphology analysis was performed with a SUPPA-55 scanning electron microscope, while mechanical property testing of tantalum ingots utilized an INSTRON electronic universal testing machine. Infrared absorption spectroscopy was employed to analyze oxygen, carbon, and nitrogen content; direct-reading arc spectrometers analyzed metallic impurity levels in samples.
1.3 Forming and Degreasing/Sintering Conditions
5.62 g of tantalum powder was taken per sample, mixed with 3% binder (stearic acid: alcohol 1:5), evaporated the alcohol in a water bath, dried in an explosion-proof oven at 50°C for 10 minutes, sieved through a 40-mesh screen, and pressed into formed anode blocks with a density of 7.63 g/cm³, diameter of 7 mm, and height of 19.15 mm using an isostatic pressing machine. These were then degreased in an explosion-proof vacuum furnace at 375°C for 180 minutes.
2 Results and Analysis
2.1 Physical Properties
Analysis of the physical properties of the three sample groups (see Table 2) shows that Sample 1 had the smallest average particle size. Sieving results indicate that the -400 mesh fraction accounted for 46.3% of Sample 1, indicating the highest proportion of fine particles among the three samples. A higher proportion of fine particles increases friction between particles, leading to poorer flowability; thus, Sample 1 exhibited almost no flow. In contrast, Samples 2 and 3 exhibited significantly larger average particle sizes (FSSS): 10.8 μm for Sample 2 and 16.4 μm for Sample 3. Similarly, sieve analysis revealed a marked decrease in the proportion of -400 mesh powder for both Samples 2 and 3, accompanied by an increase in the proportion of +200 mesh powder. All test results indicate that Samples 2 and 3 contain a higher proportion of coarse particles and fewer fine particles. Consequently, both samples exhibit good flowability. However, due to Sample 3 having a relatively higher quantity of particles above 200 mesh compared to Sample 2, coupled with a significant reduction in particles below 400 mesh, there is a noticeable difference in flowability between the two samples.
2.2 Particle Size Distribution of Samples
Figure 1 and Table 3 present the particle size comparison diagram and particle size distribution comparison data for the three sample groups, respectively. It can be observed that Sample 1 exhibits the smallest D50 value, consistent with the conclusion that D50 ranges from 10 to 15. Analyzing the concentration of particle size distribution across the three samples, the concentration is typically represented by the span S, defined as S = (D90 - D10) / D50 [19]. Calculations reveal S values of 1.76, 1.25, and 0.96 for the three samples, respectively. This indicates that Sample 1 exhibits a relatively dispersed particle size distribution. Sample 2 exhibits a distinct peak within the 100 μm size range, while Samples 2 and 3 show significantly increased D50 values. Sample 3 has the largest D50 among the three samples, with a markedly reduced peak in the 10–100 μm size range. This aligns with the analysis in Section 2.1, where Sample 1 contained the highest proportion of fine particles and Sample 3 the lowest, indicating the highest concentration. Generally, a broad particle size distribution facilitates segregation of coarse and fine particles during filling, leading to uneven packing. This results in areas of poor density within the formed green compact.
2.3 Press Forming of Samples
Analysis of the formed tantalum anode blocks from the three sample groups (as shown in Figure 2) reveals that Sample 1 exhibited significant breakage during demolding. One block suffered severe deformation, causing the tantalum wire core to detach, indicating the weakest agglomeration strength. Sample 2 also exhibited core detachment due to insufficient mechanical bonding between the tantalum wire core and powder. However, compared to Sample 1, Sample 2 demonstrated superior formability of the tantalum powder-binder mixture, showing only minor edge powder loss. Sample 3 formed blocks without edge powder loss or core detachment during demolding, indicating excellent formability.
2.4 Microscopic Morphology of Samples
Subsequent morphological analysis of the three sample groups and the deburred formed blocks revealed that Figures 3(a) to (c) show SEM images of Samples 1, 2, and 3 at 1000× magnification, respectively. These images indicate that all three samples exhibit similar primary particles, which are irregular in shape. Figures 3(d) to (f) present SEM images of the tantalum anode compacts for Samples 1, 2, and 3 at 70× magnification. Sample 1 exhibits the smallest secondary agglomerates, while Samples 2 and 3 show relatively larger secondary agglomerates. Notably, Sample 3 contains significantly more large secondary agglomerates than Sample 2. According to powder metallurgy forming principles, more irregular powder shapes facilitate better forming and yield higher green compact strength [20-23]. Thus, based solely on primary particle morphology, all three tantalum powder samples theoretically exhibit good powder formability. However, as raw materials for capacitor anodes, all three powder samples underwent subsequent agglomeration treatment. Their formability depends on the size and morphology of the secondary agglomerates. Larger secondary particle sizes result in greater interparticle contact areas, larger necks, and stronger mechanical interlocking during forming, which enhances green compact strength.
2.5 Mechanical Properties of Samples
Mechanical property testing was conducted on degreased ingots from three sample groups, as shown in Table 4. Due to poor forming of Sample 1, edge chipping occurred during compaction. Subsequently, both tantalum anode blocks exhibited significant fractures and core wire detachment during degreasing. Consequently, the tested sample lengths were 5.7 mm and 10.52 mm, significantly shorter than those of degreasing blanks for Samples 2 and 3. This resulted in poor compressive strength, measuring only 4.4 MPa. Sample 2 exhibited loose and detached tantalum wire cores during the preceding hydroforming test. Consequently, one tantalum anode block in the two debinding batches lacked a wire core. However, no fractures occurred in this block post-debinding. The length discrepancy resulted from dimensional control deviations during billet forming. Comparing the mechanical properties of Samples 2 and 3 reveals that the compressive strengths of the two deburred tantalum anode blocks in Sample 2 were 12.5 MPa and 11.7 MPa, respectively, while those in Sample 3 were 13.6 MPa and 14.6 MPa, demonstrating the best mechanical performance among the three groups. Analyzing the physical properties of the samples suggests that the formability of tantalum powder is related to the sample's flowability and the size and distribution of secondary particles: (1) Flowability may affect the strength of the pressed blocks. Poor flowability makes it difficult to fill the corners of the grinding chamber or may cause bridging, leading to poor density in certain areas. (2) A broad particle size distribution may cause segregation of large and small particles during compaction, leading to uneven filling. This results in areas of low density within the compacted green body, creating potential fracture points. (3) Larger secondary particles create greater interparticle contact surfaces during forming, resulting in larger bonding necks and stronger mechanical interlocking. Samples 2 and 3 exhibited superior powder formability during tantalum anode block pressing due to their good flowability, large secondary particles, and narrow particle size distribution. Consequently, the tantalum green compacts demonstrated excellent strength and mechanical properties.
3 Conclusions
(1) The physical properties of tantalum powder directly influence the formability of tantalum anode blocks. Among the three sample groups, the best compressive strength was achieved when the sample exhibited a flow time of 4.1 s, D50 = 331.156 μm, and a particle size distribution span S = 0.96.
(2) As tantalum electrolytic capacitors evolve toward higher voltage and higher specific capacitance, ensuring they meet application demands necessitates continuous exploration to produce tantalum powder with optimal chemical purity, physical properties, and electrical performance to guarantee sufficient pressing strength.
Reference: Powder Metallurgy Industry, Vol. 35, No. 6, December 2025, DOI: 10.13228/j.boyuan.issn1006-6543.20240157. Influence of Powder Properties on the Formability of Tantalum Anode Blocks; Qin Yuanyuan, Li Zhongxiang
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