tantalum flakes high reliability electrolytic capacitors

Tantalum Flakes – Powders For High Reliability Electrolytic CapacitorApplicationsJ. Koenitzer, S. Krause, L. Mann, and S. YuanCabot SupermetalsP.O. Box 1608Boyertown, PA 19512Phone: 1-978-671-4075 Email: john_koenitzer@cabot-corp.comT. Izumi and Y. NoguchiCabot Supermetals KK111 Nagayachi, Higashinagahara,Kawahigashi, Fukushima 969-3431, JapanIntroductionTantalum (Ta) powders are used for making highreliability solid electrolytic capacitors. These partsfind applications in various industries, such asautomotive, telecommunications, and computers. Asthe technology in these areas advances, thecharacteristics of the underlying Ta powders mustimprove. It is well known that both the chemicalcomposition and physical structure of Ta powdershave a significant impact on the performance offinished Ta capacitors, including, for example, DCleakage, reliability, and ESR.Today, one challenge is to produce improved Tapowders for high voltage / high temperaturecapactitor applications. Traditionally, the voltagerating (V r ) of a capacitor is defined as the maximumvoltage that can be continuously applied attemperatures up to 85 °C. For use at highertemperatures, an additional voltage de-rating needs tobe applied to assure the same degree of reliability.This additional de-rating is applied linearly above 85°C and reduces the operating voltage to 2/3 V r at 125°C.[1] With the use of Ta capacitors in automotivecontrol systems, the operating temperature range isexpanded to 175 °C with voltage ratings up to 50V.[2] Ta flake powders have the necessarycharacteristics to produce high reliability capacitorsfor this application.Tantalum Flake TechnologyToday, most commercial Ta powders are producedby the chemical reduction of K 2 TaF 7 with Na metal ina molten salt reactor. With this technology, thesurface area of the powder is determined by the ratioof K 2 TaF 7 to diluent salts. Nodular Ta powders havebeen produced that can be utilized over a wide rangeof anode production conditions. For example, lowsurface area, high voltage powders with capacitancevalues in the range 10k to 30k CV/g and formationvoltages up to 200 V have been in production forover 20 years.Tantalum flake powders are produced by mechanicalprocesses that flatten the initial granular powder. Theraw materials for the flaking process can be either Nareduced Ta powders or crushed Ta EB ingots. Thespecific surface area of the flake powder, and hencespecific capacitance (CV/g), is determined by theflake thickness. Thinner flakes result in products withgreater surface area and corresponding higher CV/g.The first step in the flaking process consists offlattening granular Ta powder in a ball mill. In orderto minimize contamination and oxidation, the Taparticles are milled in an organic solvent.Subsequently, an acid leach is performed to purifythe flaked product. Next, the flakes are reduced insize by hydriding and mechanical fragmentation in animpact mill. Heat treatment and de-oxidation processsteps are then used to achieve the proper oxygenconcentration and physical properties, such as flow,Scott density, and particle size distribution, neededfor commercial production of anodes. Themacroscopic and microscopic characteristics offlaked and nodular Ta powders are shown in Figure1.

Figure 1. SEM Photos comparing C350, a highvoltage nodular powder (left), to C275 flake powder(right).The screen cuts of both flake and nodular powdersare similar; however, the underlying particlemorphologies are quite different. This morphologydifference is the fundamental reason flake powdershave superior high formation voltage performance ascompared to nodular powders.Advantages of Flake Capacitor PowdersFlake capacitor powders have the inherent potentialto achieve higher CV/g at a given formation voltageas compared to nodular powders. Anodes made fromnodular powder can be modeled as a series ofinterconnecting spheres or cylinders. Likewise,anodes made from flake powder can be modeled as aseries of interconnecting plates.The theoretical specific capacitance, or CV/g, ofthese different powder structures can be modeledusing the general formula:C = K o K A / twhere C is the capacitance; A is the effective surfacearea; t is dielectric thickness; K o and K are vacuumpermeability and dielectric constant, respectively.The maximum attainable CV/g values of flake andnodular powder (calculated) are shown as a functionof formation voltage in Figure 2. The figure clearlyshows that flake capacitor powders have higher CV/gpotential than nodular powders at any givenformation voltage. The focus of current efforts is toachieve the full capability of both flake and nodularmaterials; however, flake powders inherently havegreater capability and demonstrate improvedperformance in key areas.Capacitance (uFV/g)1,000,000100,00010,00010 Vflake idealnodular ideal0 50 100 150 200 250 300Forming Voltage (V)Figure 2. Maximum CV/g values for flake andnodular Ta powders as a function of V fCurrent nodular and flake product offerings fromCabot Supermetals (CSM) have been evaluated on acomparable basis to understand relative performance.CSM flake products have historically beenmanufactured in Boyertown, PA. Recently, CSM’sflake technology has been successfully implementedon a commercial scale in Aizu, Japan. The Aizu C-275 and the Boyertown C-275 have beendemonstrated to be equivalent by statistical analysisfor virtually all properties. Additionally, CSM Aizuhas developed specific flake products tailored toAsian customers that offer some advantages for thosecustomers’ specific applications. CSM’s current flakeproducts offer the advantages of higher CV/g andlower DC leakage in wet test analysis. Similar resultsare experienced in solid capacitor products based oncustomer feedback.The performance improvement that results fromusing flake powders at high formation voltages isdemonstrated in Figures 3 and 4. These data comparedifferent high voltage nodular powders from theBoyertown and Aizu sites to C-275 (standard flake)and a newly developed C-275A flake product. Figure3 shows the relative CV/g benefit at differentformation voltages. Figure 4 compares the sinteringbehavior of these powders, showing similar shrinkagefor the flake products as compared to the nodularproducts. For higher formation voltages or evenCV/g (uFV/g)lower DC leakage at 150 V f , CSM has the C-255flake product offering.Figure 3. Capacitance vs. Formation Voltage of highvoltage nodular to flake powders.CV/g (uFV/g)CV/g vs. FormationTs=1600C30,00025,00020,00015,00010,00030,00025,00020,00015,00010,000CV/g vs. Ds/DgVf=150 V1.05 1.10 1.15 1.20 1.25Ds/DgC275C275Anodular-1nodular-2nodular-350 100 150 200Formation Voltage.(V)C275C275Anodular-1nodular-2nodular-3

Figure 4. Capacitance vs. Shrinkage of high voltagenodular to flake powders. (Ds/Dg = ratio ofsintered/green anode density)A second key performance benefit of flake powder islower DC leakage. In Figure 5, SEM photos offormed anodes made from high voltage nodular andflake powders are compared. Flake primary particlesare generally much larger than their nodularcounterparts in two dimensions. It is believed thatduring sintering, flake particles form stronger interparticleconnections because of their relatively largecontact area. In addition, the curvature of thedielectric is minimized because of the flat structure. Itis commonly believed that significant stresses buildbetween the particles at high formation voltage as thedielectric layer becomes thicker, thus inducing theformation of dielectric defects. This is especially trueat inter-particle connections or in areas of highsurface curvature. Since flakes are flat, have largerprimary particle size, and are capable of formingstrong inter-particle connections with low curvature,they can handle stress better than their nodularcounterpart. This results in lower DC Leakage, asshown in Figure 6. The data in Figure 6 are from thesame samples as the data in Figures 3 and 4. Thisdata shows the relative benefit of lower DC leakageand higher capacitance for anodes made from flakepowders.Figure 5. SEM of formed high voltage nodular (left)and flake (right) powders.DCL vs. Formation Voltage.Ts=1600CDCL (uA/g)604020050 100 150 200Formation Voltage.(V)C275C275Anodular-1nodular-2nodular-3Figure 6. DC Leakage Current as a function ofFormation VoltageFlake powders are agglomerated, as shown in Figure1, to achieve optimal physical properties, such asflow and anode crush strength, deemed critical toprocessing by capacitor manufacturers. Furthercustomization has been accomplished to matchspecific physical properties (crush, flow, density, andfines) to individual customer’s applications. Howeach capacitor manufacturer utilizes the improvedcapabilities of flake powders in their product designswill depend on their specific anode design,formation/impregnation schedules, and otherprocessing techniques.Future Opportunities for Tantalum FlakePowdersCurrent flake products are able to achieve higherCV/g than their nodular counterparts at highformation voltages, while exhibiting lower DCleakage. There are, however, opportunities forimproving the capabilities of flake products in highformation voltage applications (150 V to 200 V) andfor extending the application of flake products intomoderate formation voltage applications (down to100 V or lower).One focus of improvement is the production of flakepowders with more uniform thickness distribution,and another is developing flake products withthicknesses that are optimized for both higher andlower formation voltages. Current flake productsconsist of individual flakes with a distribution offlake thickness. As shown in Figure 7, the CV/g ofindividual flakes at a given formation voltage is astrong function of flake thickness. At a formationvoltage of 150 V, flakes with thickness less thanabout 250 nm will be formed through, and haveessentially zero CV/g, while flakes with a thicknessof about 300 nm will exhibit a maximum CV/g ofabout 44,000. Flakes that are thicker than 300 nmwill exhibit lower CV/g values, declining to about22,000 CV/g at 700 nm thickness. Of course, theoptimum flake thickness depends on the intendedformation voltage, with higher formation voltageshaving thicker optimum flake thickness.In order to achieve the very highest CV/g for a givenformation voltage, one would like to utilize a flakeproduct with a very narrow distribution of flakethickness, centered just slightly above the optimalthickness for that formation voltage. The effect ofthe distribution of flake thickness on CV/g can beseen in Figures 8 and 9. Figure 8 depicts four normaldistributions of flake thickness, each centered at 500nm (0.5 microns). Figure 9 illustrates the expectedCV/g for different flake thickness distributions whenformed at 150 V. At a formation voltage of 150 V,

flake with a narrow thickness distribution centered on300 nm will result in CV/g approaching 44,000.Flake powder with a similar average thickness, butbroader thickness distribution, will exhibit muchlower CV/g because the individual flake particles thatare thinner than 300 nm will get formed through andnot contribute any capacitance. The observed CV/gof flake samples with larger average thickness willnot display the same sensitivity to flake thicknessdistribution when formed at 150 V, but will exhibitmuch lower CV/g than the potential 44,000 CV/g foran optimized flake product.Maximum CV/g (uFV/g)50,00040,00030,00020,00010,000Calculated CV/g at Vf = 150 V0200 400 600 800Flake Thickness (nm)Figure 9 CV/g as a Function of Average FlakeThickness and Thickness DistributionSummaryIn conclusion, Flake Capacitor Materials offerinherent advantages over nodular powders in highvoltage applications. Specifically, the optimal CV/gand lower DC leakage provide the capacitormanufacturers unique capabilities in their anodedesigns. While existing Flake Capacitor Materialshave enjoyed commercial success, the potential hasnot yet been fully reached. As the basic FlakeTechnology is further extended in Asian markets andfurther researched in CSM Japan, we expect newofferings to extend the product and applicationtechnology.References[1] AVX, TAP Technical Summary and ApplicationGuidelines, p. 112.[2] Tantalum Capacitors - How to Fulfill theSteadily Increasing Reliability Demands ofAutomotive Applications, R. Schuhmann and D.Hahn, CARTS, 2002.Figure 7. CV/g as a function of flake thickness.Flake Thickness Distribution PatternRe1.0lati0.80ve 0.60Fr 0.40eq 0.20ue 0.00nc 0 0 200 400 600 800 1000 1200yFlake Thickness (nm)Figure 8. Model Flake Thickness Distributions.Calculated Maximum CV/gImpact of Thickness Distribution50,00040,00030,00020,00010,000200 300 400 500Average Flake Thicknes (nm)