Tailoring the microstructure: Optimization of Tantalum powders for highest voltage
applications up to 350 V
Marcel HagymásiTel.: +49 5321 751 3160, e-mail: marcel.hagymasi@hcstarck.com
Kamil Paul Rataj,
Tel.: +49 5321 751 53817, e-mail: kamil-paul.rataj@hcstarck.com
Helmut Haas
Tel.: +49 5321 751 3937, e-mail: helmut.haas@hcstarck.com
Christoph Schnitter
Tel.: +49 5321 751 3939, e-mail: christoph.schnitter@hcstarck.com
Holger Brumm
Tel.: +49 5321 751 3294, e-mail: holger.brumm@hcstarck.com
H.C. Starck GmbH, Im Schleeke 78-91, 38642 Goslar, Germany

The pursuit of powerful electronic applications in medicine, automotive and telecommunication
demands Tantalum capacitors with continuously increasing energy densities and working voltages.
To support this H.C. Starck is currently developing new tantalum powders for anodization voltages>250 V. The key for improvement is to optimize the microstructure of the powder by adjustment of
the primary-particle and pore-sizes. The superior homogeneity and tailor-made size of both – poresand particles - allows achieving higher capacitance densities than with other available tantalum
powder today. In addition, these powders exhibit excellent impregnation properties due to theiroptimized pore structure allowing achievement of low ESR.
Especially for formation voltages above 200 V, a change from aqueous media to organic solvent
containing electrolytes is necessary to achieve high quality dielectric layers. The understanding of theresulting change in formation mechanisms is challenging but essential for powder optimization. The
electrochemical effects and limits that occur at such high voltages and resulting capacitance densities
will be discussed.
Introduction
Pushing the limits of tantalum capacitors by development of new powder grades is one main task of
H.C. Starck as a leading technology company. The increasing request for high voltage tantalum
capacitors in recent years is an ongoing trend that will continue to demand new capacitor designs
and will extend to other application fields. These include high reliability applications such as
automotive and medical devices as well as telecommunications and new data storage devices likeSSDs, all potentially requiring new tantalum powder grades that can meet the new requirements.
In contrast to high CV applications, a proper high voltage powder has to fulfill different needs. Thereis an optimum primary particle size for each anodization voltage which will result in the highest
capacitance. If the primary particles are too large valuable tantalum powder will be wasted, if thepores are too large volume will be jeopardized. On the other hand, if primary particles are too small
they will be totally anodized to Ta2O5, if pores are too small they will be closed during anodization
preventing penetration with the cathode material resulting in lower capacitance densities. To push
the high voltage performance of tantalum capacitors H.C. Starck has recently developed a new classof powders called HVMC grades. This new powder class is produced using our patented Mg vapor
reduction technology which provides superior microstructural homogeneity when compared to Nareduced tantalum powders. In detail this means the primary particle size distribution as well as thepore size distribution of these powders are much narrower than those of conventional Na reduced
powders allowing production of capacitors with higher capacitance densities. Therefore theutilization of tantalum material in these capacitors is significantly increased1,2,3. The latter property is,
aside from strong sinter necks and higher chemical purity, the main reason why the HVMC powders
have drawn such high interest from capacitor makers.
Novel HVMC powder developments
It was previously published that these HVMC powders could be used for anodization ranges of 60200 V3,4. Through extensive research and development during the last year it was possible to extend
this range to even higher voltages of up to 350 V. Moreover, proper production parameter
adjustments allows designing the pore structure of these powders in a broader range as well.
According to different demands H.C. Starck has three formation voltage classifications for these
powders:
1. High Voltage: 40 – 100 V
2. Highest voltage: 100 – 200 V
3. Ultra-high voltage: >250 V
The naming system for this powder class is correlated to customer needs and will be explained fortwo different powders
Powder 1: HV64-4500
Powder 2: HV200-570“HV” refers to “High Voltage“ applications, the first number after that corresponds to therecommended anodization voltage while the second number gives the capacitance at this voltage inµF/cm³ since capacitor makers are more familiar with this unit. For example Powder 1 was developedfor 64 Vf and provides 4500 µF/cm³ while powder 2 is recommended to be used at 200 Vf and will
provide 570 µF/cm³. For the high voltage range anodes (40 – 100 V) a press density of 5.5 g/cm³ wasused and the sinter temperature adjusted to achieve a final shrinkage of 5-7 vol.%. Anodes for thehighest and ultra-high voltage range were produced with higher press density of 6.0 g/cm3 and sintertemperature adjusted to achieve a final shrinkage of 10-12 vol. The higher press density and final
shrinkage conditions of the latter are recommended in order to create a stronger anode body thatcan withstand the significant strain at these high voltages without breakdown. However, these arerecommendations valid for our anode conditions and do not necessarily mean that an HV200 powder
can only be used at 200 V. Depending on the pressing and sintering conditions of anodes it might bepossible to use this powder efficiently even at 250 V and higher.
Tailoring the pore structure
As described above the pore structure of the powder has a significant impact on the impregnation
behavior of the anode with the cathode material and therefore on the electrical characteristics suchas capacitance, ESR and leakage of the finished capacitor. Anodes made from two HVMC powders –one with tailored pores - will be compared in fig. 1 to those made from the best in class 50k µFV/g
standard powder from Na reduction process (same pressing and shrinkage conditions). Both HVMC
powders were developed for a 64 V formation but have slightly different cap behavior which iscorrelated to different pore structures. Both powders have a bimodal anode pore size distribution

that supports the anode infiltration. The larger “secondary” pores act as a highway for the cathodematerial to the interior regions of the anode, from there the material is distributed to the smaller
primary pores. Since the smaller primary pores, which are generated by the primary particles, areresponsible for the majority of the surface area (in other words the majority of capacitance), thisbimodal pore distribution is crucial for optimum utilization of the entire tantalum surface area. The
SEM pictures in fig. 1 (right top and bottom) show case the two different types of pores and thecorresponding microstructure within the anode body. The anode made from the standard 50k
powder (from Na reduction) has a less pronounced bimodal pore size distribution and a broader peakfor the primary pores than those made from the HV64 powders.
primary poressecondary poresfig. 1: Pore size distributions of two anodes made from two HVMC powders in comparison to anodes made from a Na
reduced 50k µFV/g powder. The secondary pores in the SEM picture on the right side are marked with blue circles.

The anodes made from the HV64-4500 powder show a slightly smaller primary pore diameter thanthose from the standard 50k powder but the pore peak is narrower and the peak height is higher.
This means that the anodes made from the HV64-4500 powder have a higher surface area than thosemade from the standard 50k material (compare the pore area diagram). This is confirmed by thecapacitance measurement shown in fig. 2 – the HV64-4500 with the highest primary pore peak also
has the highest capacitance over the entire range.
In contrast, the HV64-4300 powder was designed to allow better pore impregnation to achieve lower
ESR values. To achieve this, the powder production process was modified in a way such that thesecondary powder pores are larger. The pore size distribution for anodes made from the HV64-4300
show clearly that the peak for the secondary pores was shifted from 950 nm to 2 µm. It results in amuch broader bimodal pore distribution compared to those made from the HV64-4500 and thestandard 50k powder. In spite of a smaller primary pore peak height, anodes made from HV64-4300
powder have higher capacitances than those made from the standard 50k powder for the formation
range from 40 to 100 V. This can be explained by the more homogeneous microstructure. Thismeans, compared to the standard 50k powder, it was possible to improve the pore structure andincrease the capacitance at the same time. This clearly shows that the production process for these
HVMC powders can result in tailor-made pore structures: adjusting the bimodality keeping thecapacitance at the same high level!

70000Na 50k
60000

HV64-4500

C [µFV/g]

HV64-4300

50000
40000
30000
20000
0

25

50

75

100

U [V]
fig. 2: Capacitance behavior of anodes made from two different HV64 powders and from a standard 50k powder for a
formation voltage range from 20 – 100 V.

Increasing the capacitance and application range
Also new HVMC powders were developed for the formation range of 100 to 200V which will becompared to standard Na reduced powders (fig 3). The first powder developed in this formation
range, the HV100-1900 powder, has shown at a formation voltage of 100 V a significant increase incapacitance of +14 % in comparison to the best in class 30k powder. By additional processadjustments it was possible to further improve the performance of this powder resulting in the new
HV100-2100 which is recommended for the formation range of 100 V - 150 V providing an additional
increase in capacitance at 100 V formation voltage to + 29 % in total. It has to be noted that the slopeof the capacitance over voltage is a critical factor for the high voltage capability: the lower the slope
the better the high voltage stability! The capacitance over voltage curve for the anodes made fromthe HV100-2100 is nearly parallel to those made from the HV100-1900 powder, and anodes of bothpowders provide significantly higher cap compared to those made from the standard 30k powder
over the entire investigated range.
35000+ 29 %

HV200-570

HV100-1900

Capacitance [µFV/g]

30000

HV100-2100
30k µFV/g

25000

17k µFV+ 14 %

20000
15000+ 25 %
10000
75

100

125

150
175
200
225
Anodization Voltage [V]

250

275

fig. 3: Capacitance behavior of HV100 and HV200 powders in comparison to standard 30k and 17k Na reduced powder.

Capacitance wise HV200-570 is the best in class powder for 200 – 250 V formation range. In
comparison to anodes made from a 17k µFV/g powder (which has been approved for high reliability

application in this range) anodes made from the HV200-570 powder provide a 25% increase incapacitance at 200 V formation. It is also notable that the slope of the capacitance over voltage
curve, as degree of the high voltage capability, is on the same level as for the anodes made from the
17k standard powder. This means anodes made from this powder show both, higher capacitance andexcellent high voltage capability and can even be used for formation voltages up to 250 V!

Highest Voltage: 250 – 350 V
As a result of extensive microstructural investigations and resulting process modifications, it wasrecently possible to extend the application range for the HVMC powder class to formation voltages
up to 350 V. Anodes made from the newly developed HV300 powder show the highest capacitance atformation voltages of 300 V and above. According to our investigations no Na reduced powder candeliver anodes showing such high capacitance values (fig. 4a). In comparison to anodes made froman often used 12k µFV/g powder those from HV300 show 20 – 28 % higher capacitance over thewhole range from 250-350 V formation voltage. The advantage to anodes made from other ultrahigh voltage powders is even more significant (fig. 4a). To explain this significant increase incapacitance it is again essential to investigate the powder and anodes pores as well as their micro
structure. The anodes made from HV300 powder show a very narrow pore size distribution: they
have less large pores ( waste of space) and also less small pores ( which will be closed during
anodization) than anodes made from the standard 12k µFV/g powder (fig. 4b). As a result of this, thepore size distribution for HV300 anodes has a significantly higher peak height. The anodes made from
10k and 9k µFV/g powder, have both too large pores and a too broad pore size distribution whichcorresponds to a low volumetric tantalum usage resulting in much lower capacitance values.
HV300

14000

12k µFV/g
10000+ 20 %

10k µFV/g
8000

9k µFV/g

6000

4000

200

250

300
U [V]

350

TTS-10VR-4

0.3
Log Differential Intrusion (mL/g)

Capacitance [µFV/g]

HV300+ 28 %

12000

TTS-12

400

0.2

0.1

0.0
10

5

1
Pore size Diameter (µm)

0.5fig. 4: Comparison of anodes made from the newly developed HV300 powder with such made from standard Na reduced
ultra-high voltage powders a) Capacitance behavior for the range from 250 – 350 V. b) Anode pore size distribution (samesinter density!)

The effect of the pore size distribution becomes apparent when the anode pore area, which iscorrelated to anode surface area and therefore to a higher capacitance is compared (fig. 5). The
unformed HV300 and 12k µFV/g anode have the same total pore area of 0.254 m²/g but after 300 V
anodization the narrower pore size distribution of HV300 leads, as explained above, to a pore areathat is 18 % higher than 12µFV/g - this is a good correlation to the measured capacitance in fig. 4.

fig. 5 Anode pore area of HV300 and 12µFV/g: unformed and after 300 V anodization.

This is confirmed by investigation of the microstructure of anodes made from standard 12k µFV/g
powder compared to HV300 powder and formed to 300 V.– (fig. 6 is a graph displaying the maindifferences). First the anodes made from the Na reduced 12k µFV/g powder show a less homogenous
anode structure. Large particles (1) with a size of 5-9 µm can be observed having very low surface
area and therefore contribute minimal capacitance while occupying valuable space. This is also true
for pores that are too large (3). Last but not least areas with too small primary particles which aremostly connected with too small pores (2) having a negative impact on capacitance – most of thetantalum core is already formed to oxide and the small pores are nearly blocked with the formedoxide film. These areas are hard to impregnate and give only a low contribution to capacitance as
well. Looking at fig. 5b it is very obvious that the formed HV300 anodes having an improved, much
more homogeneous microstructure – it is superior concerning homogeneity of the primary particle
size distribution (neither too big nor too small particles) and also has a more homogeneous pore
structure without closed areas.

3
)

2
)
1
)

a
b
)
) µFV/g standard powder: (1) too big primary particles, (2)fig. 6: Microstructure of anodes formed to 300 V a) made from 12k
areas with closed pores and too small primary particles (3) big pore area b) made from new HV300 powder with superior
homogeneity

Conclusion
It was shown that using the new HVMC powders significant improvements for tantalum capacitors
are possible. The anode pore size structure can be tailored over a broader range than before and thebimodal pore distribution has a positive impact on electrical properties due to the improved cathodeinfiltration. An additional capacitance increase was realized for the anodization range of 100 – 250 V.
HV300 is a new HVMC powder that extends the capacitance range for ultra-high formation voltages
up to 350 V. The HVMC process based on Mg reduction is the key competence of H.C. Starck andprovides unique homogenous powders for anodization range from 50 to 350 V! These new HVMC
powders with increased capacitance were mainly developed for customers with the following focus:
1. Enable the production of new capacitor types where a standard Na reduced powder does notdeliver enough capacitance.
2. Provide an increase in volumetric efficiency which allows a reduction in the powder volume,thus supporting the trend toward miniaturization.
3. Improve the pore structure which in turn ensures an increase in impregnation efficiency forlower ESR.
Again, the HVMC powder development is not at its limits! H.C. Starck is continuously working on newhigh voltage powder in order to find the maximum capacitance for each anodization voltage.
Formation and Evaluation of high voltage powders

3)

There is not much public knowledge available on anodization of tantalum anodes to very highvoltages above 250 V. However, powder producers, like H.C. Starck need to evaluate theperformance of their new developments like the HVMC powders discussed above at these highest
and ultra-high formation voltages. This performance cannot be simulated by only using simple
powder data like specific surface area or pore size distribution. It is necessary to produce and form
anodes at the required voltage and then measure properties like the real capacitances, leakage
currents and pore size distributions. This poses not only a challenge to the design of the particles, butalso to the formation process and the electrochemical characterization of anodes made from thesenew powders.
Electrolytes for high voltages
As a result of the continuously decreasing pore sizes during anodization it is challenging to form
thick, dense oxide layers with a uniform thickness. With thinner necks, the resistance increases
dramatically during formation of the dielectric layer in liquid electrolytes. Thus, the conductivity ofwater-based electrolytes must be increased via higher acid concentrations or additives5. However,using additives can generate additional side reactions like decomposition/oxidation of the electrolyte
and may reduce the yield of oxide growth during formation. Alternatively the formation program(ramps, constant rate, current limit) can be modified but this will result in disproportionately longer
formation times. Therefore tailored, smart programs with optimized conditions for every powder are
required.
1. Formation in aqueous solutions
In literature it is described that in aqueous solution the main side reaction is oxygen evolution6
according to equation 1, this reaction consumes 4% of the total applied charge.
(1)
For low current densities and anodes having low surface areas the reaction is not critical or
observable (e.g. on tantalum foil or wire) due to a fast diffusion of oxygen from the anode surface

into the electrolyte and low charge volume. But during anodization of pressed pellets with highsurface areas (> 200 cm² per pellet) a strong gas evolution can be observed. Upon first observation
this side reaction seems to have no influence on the oxide growth (equation 2),(2)
but pores within the anode will fill with bubbles from the product of the side reaction and become
electrically disconnected during formation. As a result of this the oxide thickness does not grow inthese pores and the total current decreases in such potentiodynamic experiments. When a criticalsize of the bubble is exceeded it leaves the pore and the blocked area is reconnected again. This
leads to a post-anodization of the surface and thus a higher total current. In cyclo-voltammograms
this is observed as an oscillation (fig. 7).

fig. 7: Cyclovoltammogram of an anode with constant rates of 10 mV/s respectively 50 mV/s in 1.8 mol/L sulfuric acid. The
oscillation at higher potential rates is caused by blocking of the pores with oxygen bubbles and releasing of them by apost-anodization.

If the current density is too high, the dielectric layer can spall off or crack due to the formed oxygen.
One possible solution for this is to decrease the total current in galvanostatic formation or of theconstant rate in potentiodynamic formation to reduce the charge volume for the described side
reaction. By doing this, oxygen can diffuse out of the pores before gas bubbles form and blockage ofactive surface area takes place. Most importantly, anodes made from high capacitance powders haveto be anodized slower otherwise breakdown of the oxide layer is observed at low potentials which, inthe end, result is uncontrolled oxidation of the anodes.
2. Formation using organic and inorganic additives
For very high formation voltages of tantalum anodes, additives are used to increase the conductivityof the electrolyte and the resistance of the dielectric layer. In fact it can be observed that higher
potentials (e.g. 200 V) cannot be reached in common aqueous electrolytes like pure phosphoric acidwithout breakdown of the formed oxide. In literature the prevalent compounds are additives like
ethylene glycol16, phosphate17,18, borate19 or citric acid15,20. The addition of such anions result in
modifications of the oxide layers in structure7,12 or incorporation of those anions into the oxide layer
which can affect the breakdown. The influence of glycol during oxide growth was described on
silicon8-11 and also on tantalum14. M. Corset et. al. have studied the source of oxygen for oxide
formation by 18O marking for different additives13. They described that oxygen is mainly (80 %)provided by water whereas the rest was coming from the additives (dissolved salts). A further

description of the improvement using citric acid is that it can form chelating complexes with metallic
surface impurities. This was also observed with other organic acids like tartaric acid15. Jackson has
shown that additives inhibit the oxide’s electric field induced crystallization14. However a clear model
about the mechanism at higher potentials is still under discussion.
Experimental partIn this work the anodization of high voltage anodes at high potentials (200 V and above) is described.
Effects and limits caused by additives were defined. Aqueous (phosphoric acid in water) and mixed
media (glycol and phosphoric acid in water) were compared concerning their potentiodynamic
formation behavior independent of the concentration of the additive, the conductivity and theconstant rate. The breakdown areas of the anodes have been investigated by SEM pictures and threedifferent stages of cracking have been identified.
Influence of additives
Anodes made from 17k µFV/g tantalum powder that was used as reference for HV200 have beenanodized in common phosphoric acid electrolyte with a conductivity of 350 µS/cm and compared toanodization behavior in the same electrolyte containing ethylene glycol. The constant rate was kept
constant for both experiments (fig. 8). It was observed that in the aqueous electrolyte a complete
breakdown of the samples occurs at about 186 V. In contrast, by adding ethylene glycol all anodes
could be formed up to 250 V without any issues and breakdown of the formed oxide layer. Therefore
further experiments have been performed in electrolytes containing ethylene glycol and phosphoricacid.
Current Density [mA/g]

100

water
breakdown

80
60
40

glycol
20

0
0

50

100
150
Anodization Voltage [V]

200

250

fig. 8: Anodization of 17k µFV/g anodes in water based (red) and glycol based (blue) systems with a constant rate of 0.75
V/min (350 µS/cm). The growth is delayed and smeared out over the whole potential range.

Influence of constant rates during formation
To further improve the oxide quality two different constant rates have been compared. fig. 9 shows
cyclo-voltammograms of 17k µVF/g anodes with two different constant rates. The first plot (upper
curve) shows the anodization using a constant rate of 2 V/min. At about 150 V the current starts tostruggle and oscillates until first breakdown occurs at 300 V followed by an increasing current. The
lower plot shows the same experiment but with a constant rate of only 0.75 V/min. At 0.75 V/min
the current is steady and we only start seeing noise at 200 V, however non –destructive breakdowns
are also observed at 260 V. This means that a repassivation of the cracked area takes place (see also
chapter breakdown stages). Although the first breakdown occurs at the lower constant rate, in
repeating experiments it was observed that the non-destructive breakdown potential is not totallydependent on the constant rate but also the oscillating current.

Current Density [mA/g]

200
350 µS/cm
2 V/min

150

100
350 µS/cm
0.75 V/min50
non destructive breakdown

0
0

100

200
U [V]

300

fig. 9: Cyclo-voltammograms of 17k µFV/g anodes using two different constant rates in ethylene glycol electrolyte.

With increasing voltage it seems that the constant rates have to be adjusted and lowered to reduce
oscillation of the current. Therefore a new ramp program was developed where high constant rates
were used at the beginning with stepwise decreasing of the rates. Therefore, for further experiments
dynamic constant rates have been chosen to slow down the oxide growth at high potentials.
Independent of the powder used the constant ramps were divided into 2 or 3 segments with lower
rates (see next chapter).
Influence of conductivity of the electrolyte
HV200 Anodes were anodized up to 200 V in either 3.5 mS/cm or 0.35 mS/cm conductive electrolyte(both phosphoric acid with ethylene glycol in water). The formation was done with a dynamic
constant rate in three steps shown in fig. 10. The first step was a ramp with a high constant rate of
0.6 V/min. At potentials higher than 100 V the constant rate was decreased to 0.4 V/min and finallyin the potentiostatic phase the potential was kept for about two hours at 200 V. In electrolytes with
3.5 mS/cm conductivity, the peak current is reached after a short time with a fast ramp anddecreases slowly caused by further formation of the oxide layer in pores and necks. During thesecond ramp the current decreases continuously but shows after 20000 s first current peaks whichhave been identified as local breakdowns of the samples with a fast re-passivation.
The red plot shows the same experiment processed in the electrolyte that has one tenth theconductivity of the previous trial. In the first stage the current rises much slower and the peakcurrent is shifted. This means that the formation is decelerated and therefore the oxide has moretime to form. In the second ramp the current decreases slower compared to the previous experimentand no breakdowns are observed. At the end the current reaches the same value in bothexperiments. We can assume that by increasing conductivity of the electrolyte the inner parts of ananode, which are connected via pores, reach their formation potential faster than in low conductive
media. This means that a reduction of the electrolyte conductivity can be compared to a reduction of
the formation rate and therefore a slower oxide growth, which is positive for the formation at highpotentials. The lower current density results in a reduction of thermal stress and crystallization.
However the same oxide thickness can be reached in both electrolytes if the time is adapted and theconsumed charge is the same according to:∫(3)Comparison of the leakage currents after anodization show that the high conductive electrolyte
results in 4 times higher DCL values than the lower conductivity electrolyte.

250

803500 µS/cm
70

200

60

350 µS/cm

U [V]

40

100

30

I [mA/g]

50

150

20

50

10
0

0
0

5000

10000

15000
20000
t [sec]

25000

30000

fig. 10: Formation program of HV200-570 anodes formed in 0.35 mS/cm (red) respectively 3.5 mS/cm (green) conductive
electrolyte containing glycol. The anode formed in the higher conductive electrolyte shows breakdowns withrepassivation during anodization seen as spikes (21.000-24.000 sec).

To exclude aging effects of the electrolyte, two identical solutions were prepared. Several formation
cycles were processed in one of the solutions to mimic a “used” solution. The experiment was thenrepeated in both the fresh and used solution. No increase in leakage current was observed withincreasing anodizing cycles.
Comparison of formation using ethylene glycol vs diethylene glycol
Two different glycols, ethylene glycol and diethylene glycol, with different steric sizes but similar
chemical behavior have been tested (fig. 11). No differences were observed during the formation up
to 320 V. Nevertheless during reformation (> 38000 s) an increasing current was detected in
diethylene glycol. This effect was reproduced several times. It can be assumed that ethylene glycol
inhibits the formation of crystalline tantalum oxide stronger than diethylene glycol and results
therefore in lower leakage currents.

fig. 11: Formation current of 12k µFV/g anode in dependence of time. At potentials higher than 300 V the current increases
in diethylene glycol containing electrolyte.

Investigation of breakdown stages
To characterize the local breakdown of the oxide layer, SEM pictures have been taken from 9k µFV/g
anodes and different states of the breakdown were observed. Results of anodization in water werecompared with those of anodization in ethylene glycol.
The breakdown in water can be separated into three different stages. The first stage is theappearance of dark spots (fig. 12) that cannot be directly chemically identified. These may be
impurities or the effect of the beginning of a morphology change from amorphous to crystalline
oxide state. However they will be interpreted as a source of breakdowns and in fact more
breakdowns occur there than on spot free areas. In contrast to current statements that especiallythe sinter necks are “weak spots” for breakdowns it was observed that mostly plain areas areaffected (fig. 13). Additionally first oxide peel-off is visible with an exposure of the underlying metal.

fig. 12 : First stage with the appearance of dark spots on theoxide (water system).

fig. 13: First stage dark sports that break through on a plainarea (water system).

In the second stage (fig. 14) the oxide forms cracks and reveals the underlying metal until the laststage (fig. 15) is reached where the breakdown proceeds uncontrolled over the whole area and evenover the whole sample. The formed oxide grows in an irregular structure; oxide and metal corecannot be distinguished anymore.

fig. 14: Second stage with oxide forming cracks revealing
the metal underneath with further uncontrolled oxidation(water system).

fig. 15: Last stage of breakdowns with a complete oxidationof metal in an irregular structure (water system).

Comparison of these stages for anodization in pure water to those for anodization in ethylene glycol
similar behavior was observed (compare fig. 12 and fig. 14). However in the last stage in glycol theuncontrolled breakdowns look a bit different with huge cracked holes and less continuous oxidation(fig. 17).

fig. 16: First stage with the presence of dark spots
on the oxide (glycol system).

fig. 17: Oxide structure in last stadium in glycol electrolyte.

Conclusion
The formation of tantalum based anodes to higher potentials above 200 V requires a modification of
the common electrolytes and anodization programs. In galvanostatic anodization mode the current
density increases with decreasing surface area and therefore at potentials >90 V a higher formation
rate occurs. A decrease of the electrolyte conductivity seems to increase the accessible formation
voltage but can be explained as a delayed oxide growth with reduced formation rates. Therefore
lower constant rates lead to similar positive results as well. Additives, especially ethylene glycol,
inhibit the crystallization of tantalum oxide at higher potentials and decrease the leakage current.
From our experiments on these parameters our recommendations for the formation of anodes tohighest voltage are usage of:

1-ethylene glycol containing electrolytes instead of water based-

lower constant rates and a stepwise reduction of the rates or lower conductivity electrolytes-

ethylene glycol is favored compared to diethylene glycol

M. Hagymási, H. Haas, C. Schnitter, H. Brumm, ”High Voltage Tantalum Powder – Challenges andOpportunities for new Powder Generation”, CARTS Europe, Nice, France 2011

2

H. Haas, M. Hagymási, “Tantalum capacitor anodes providing highest capacitances: Where are
the limits?” CARTS Europe, Nice, France 2011

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Applications”, T.I.C. 2013, York, England 2013
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Clara CA, US 2014
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microelectrodes”, Electrochim. Acta, 90 (2012), p. 12-16
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Xiaoge Gregory Zhang, “Electrochemistry of Silicon and Its Oxide”, Springer, 2001

9 J ean-Noël Chazalviel, François Ozanam, “Electrochemically Formed Porous Silica”, Materials 2011,
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