High Voltage Tantalum Powder – Challenges and Opportunities for new Powder Generation
Marcel Hagymási
H.C. Starck GmbH, Im Schleeke 78-91, 38642 Goslar, Germany
Tel.: +49 5321 751 3160, Fax +49 5321 751 4160, e-mail: marcel.hagymasi@hcstarck.com
Helmut Haas
Tel.: +49 5321 751 3937, Fax +49 5321 751 4937, e-mail: helmut.haas@hcstarck.com
Christoph Schnitter
H.C. Starck GmbH, Im Schleeke 78-91, 38642 Goslar, Germany
Tel.: +49 5321 751 3939, Fax +49 5321 751 4939, e-mail: christoph.schnitter@hcstarck.com
Holger Brumm
H.C. Starck GmbH, Im Schleeke 78-91, 38642 Goslar, Germany
Tel.: +49 5321 751 3294, Fax +49 5321 751 4294, e-mail: holger.brumm@hcstarck.com
Introduction
As a leading tantalum producer, H.C. Starck notes an increasing demand for high voltage (HV) powders fordifferent new application fields like flat panel display, automotive as well as medicine devices with highreliability. Any discussion of HV applications must begin with the fundamental question, “what is high voltage
for tantalum capacitors”. This is not an easy question to answer and depends on the application as well as theconsumer. In past, many tantalum makers would answer: capacitors with rated voltages of 50 or 63 V. However,this limit has been already pushed since new techniques of anode production like improved polymer cathodes
have been developed. AVX for example recently reported the development of new tantalum polymer capacitorswith a voltage rating of 125 V, which means a doubling, compared to capacitors available before. Another limit,
too high ESR values, could be solved by using special anode shape1. To enhance the HV ability of tantalum
capacitors it is essential to provide not only new polymer cathode materials or changed anode designs but also
new powders with an adapted microstructure. Improved HV powders could also allow decreasing the forming torated voltage ratio, which would lead to an additional capacitance increase2. This paper gives an overview about
different requirements for such new powders and presents also a new anode characterization method for fastcapacitance evaluation. Finally, new improved tantalum powders for high voltage applications will be shownand discussed.
Demands on Microstructure
To meet the market needs, H.C. Starck is developing new tantalum powders with focus on high formation
voltages ≥ 100 V. A good high voltage powder requires a different microstructure than does a low voltage
powder. The requirements for good high voltage performance include:
1. Suitable big primary particle size
2. Sufficiently strong sinter necks: between primary particles as well as between different agglomerates
3. Sufficient large pore size diameter
4. Good homogeneity of 1. – 3.
5. High purity
HV powders need a proper primary particle size in order to provide enough tantalum that can be converted into
Ta2O5 during anodization. Moreover, remaining tantalum core has to be big enough to ensure a good
conductivity within the anode structure which is also responsible for the resulting capacitance. By using asimple model, the required tantalum primary particle size can be estimated. Therefore, the tantalum structure
can be described as a cylinder with the diameter dTa. The outer diameter dO after formation can be calculated by
the remaining tantalum core diameter dC and oxide thickness which depends on formation voltage U and growthrate (gr 2.3 nm/V)3:

dO  dC  2U * g r

unformed
initial stateanodized
dO
dC

dTa

Ta2O5
Ta coreBy using the different densities of tantalum (16.6 g/cm³) and amorphous Ta 2O5 (8.0 g/cm³)4 it is easy to
calculate back the original tantalum diameter dTa of such a cylinder structure before anodization. Result is theminimal necessary tantalum primary particle size for a given formation voltage and remaining tantalum core
diameter (fig. 1).

Primary Particle Diameter d Ta [nm]

1600
1400
1200

300 nm Ta core

1000

100 nm Ta core

800
600
400
50 nm Ta core

200
0
0

100

200

300

400

500

Formation Voltage [V]fig. 1 Particle diameter dTa for different formation voltages and tantalum core diameters for a cylindricalmicrostructure.
Smaller particles will be totally converted to oxides without remaining tantalum core which is equal to a total
loss for the capacitor. For a 100 V formation with a Ta core diameter of 50 nm for example it is necessary tohave a cylinder diameter of 322 nm before formation, which has to be increased for a 400 V formation to 1188
nm. Even if the real tantalum microstructure has a mono- or bimodal particle size distribution and consists notof uniform cylinder structure, this simple model gives a good hint of the required particle dimension.
Beside primary particle size the size of the sinter necks is one of the most important parameter. A “sinter neck”
is defined as the smallest connection between two adjacent particles which ensure the electrical path through thetantalum structure (fig. 2).
Sinter necks can be distinguished betweena) sinter necks within one and the same powder agglomerateb) sinter necks as interconnection between different agglomerates which are formed during pressing andsintering.

primary
particle

sinter necksinter neckfig. 2 Sinter necks and primary particle within the tantalum structure (150k µFV/g powder).
The latter kind of necks can be improved during pressing the powder to the anodes: higher press densities
increase the cross sectional areas between agglomerates. All sinter necks will then be increased andstrengthened during the anode sintering process. Main driving force for sintering is the reduction of free surface
energy5-7 which is caused by particle curvature: the smaller the particle, the higher the surface energy. Thismeans in case of sinter necks: the smaller the sinter necks, the higher the radius of curvature and therefore thehigher the sinter activity. Material flow by surface diffusion from region with low radius of curvature to necks
with high curvature leads to a neck growth. It was already presented 8 that anode surface area and sinter necksize oppose each other. Increasing sinter neck size caused by shrinkage reduces simultaneously the surface area
and therefore the capacitance of the anode. Similar to primary particles, the sinter necks will be decreased withongoing anodic oxidation. That’s why the calculated minimal diameter seen in fig.1 is not only valid for the“primary particle size” but also for sinter necks: too small neck sizes will be completely formed to oxides
without any conductive connections which mean an additional loss of active capacitor surface.
For every given voltage exist an optimum particle size that ensures the highest capacitance. Too big particles
have low surface area and therefore low capacitance. Too small particles have high initial specific surface but
capacitance will be low due to surface area lost during high voltage formation. Using the simple cylinder modelallows also estimating the maximum capacitance and the correlated unformed surface area for a given voltage.
Therefore, the cylinder capacitor equation was used (l: cylinder length):C  2 o r

l
d
ln O
dC

In case of a 100 V formation, oxide thickness is constant with 230 nm (g r 2.3 nm/V) while the inner tantalum
core diameter is variable. Resulting capacitance and initial surface area can be seen in fig. 3. The highest
possible capacitance was determined with 51.7 kµFV/g from this model and would be reached for a tantalum
core of 125 nm respectively an initial unformed surface area of 0.67 m²/g. According to calculation in fig. 1, thisis correlated to an initial tantalum cylinder diameter of 380 nm. Please note, that this model did not include theinfluence of pores respectively sinter density.

60

1,2

50

1,0

40

0,8

30

0,6

20

0,4

10

0,2

0

unformed surface area [m²/g]

Cap @ 100V [CV/g]

51.7k µFV/g @ 125 nm core diameter

0,0

0

200

400

600

800

1000

tantalum core diameter after 100 V formation [nm]fig. 3 Theoretical capacitance and surface area of 100 V formed tantalum cylinder depending on
remaining tantalum core.
Next important parameter is the pore size distribution. The higher the final formation voltage the higher therequired pore size in which the Ta2O5 can grow. The initial (unformed) pore size distribution affects the poresize distribution after anodization; too small initial pore size will hinder the impregnation after anodization andcan lead to additional capacitance losses. In worst case, the pores will be totally closed and filled with Ta 2O5 sothat additional oxide growth can not occur and no coverage with cathode material will be possible. On the other
hand too big remaining pores can also result in capacitance dissipation since these pores can be regarded asunused space for tantalum surface area. However, currently it is not known which remaining pore size issufficient for a good impregnation with cathode material or low ESR values. Moreover, beside the pure powder
properties also pressing and sintering have a big impact on anode pore size distribution which depends on
producer and their process.
High microstructural homogeneity as well as inter-agglomerate strength are two important but difficult todetermine parameters. Small particles have a higher specific surface area than large particles, but show anincreasing surface area and therefore capacitance loss with increasing formation voltage9. That’s why it can beassumed that powders with a more uniform primary particle size will show a lower capacitance loss duringformation than a heterogeneous powder with comparable initial specific surface.
Impurities like many metallic elements or carbon are often discussed to have a negative impact on wet or solidleakage as well as breakdown behaviour4,10,11. This effect can be magnified as the formation voltage increases12.
Table 1 illustrates the microstructural advantages and disadvantages with respect to HV applications of current
tantalum powders obtained from different production routes, namely from sodium reduction of K2TaF7 and frommagnesium reduction of Ta2O5.
The powder from sodium reduction process reveals large pores with diameters up to 2 µm as well as big primaryparticles with sizes of several µm. The large primary particles provide enough tantalum that can be converted tothick oxide film and the open pore structure ensures enough space for the growing oxide film. Maindisadvantage of this powder is the heterogeneous microstructure: the powder contains not only the desired large
primary particles but also much smaller particles which will be totally converted in tantalum oxide at higher
voltages. This causes two drawbacks: first additional capacitance loss at high voltages and second the “dead”
tantalum oxide will reduce the remaining pore size which will decrease impregnation efficiency.

table 1 Comparison of HV capability of tantalum powders from sodium respectively magnesium
reduction process
Na Reduced Powder
Mg Reduced Powder
Advantages
Big pore size: up to several µm
Very homogenous primary particle structure
high purity
Disadvantages Heterogeneous primary particle structure
Small pores (<400 nm)
higher impurity level
Main Focus
Low CV, high voltage (>50 V)
High CV, low voltage (<50 V)(Past)
Microstructure(showcase)

In contrast, the tantalum powder from magnesium reduction process provides a very homogenous
microstructure with very uniform primary particle size. Main disadvantage here is the comparably small poresize (<400 nm) which is the main reason why this powder is not capable for formation at 100 V because allpores will be closed. Due to this limitation, tantalum powders from magnesium reduction process were mainly
used in the past for high CV applications where small pores and primary particles are necessary.
The simple illustration in table 1 gives the main direction for new developments in HV tantalum powdercapabilities by combination of best properties of both kinds of powders. In case of powder from magnesium
reduction process, the pore size as well as primary particle size has to be increased by keeping its highmicrostructural homogeneity and chemical purity. Therefore, a new production process was developed thatallows to open the pore structure and simultaneously increase the primary particle size, so that high capacitances
at formation voltages of 100 V -150 V can be realised.
In contrast, the powder produced by sodium reduction process has to be optimised with the focus on improved
homogeneity, so that maximum capacitance at formation voltages up to 250 V can be reached.
New fast electrical test for anode characterisation: constant rate formation (CR formation)
Improvement of tantalum powders for HV applications requires a comprehensive evaluation of many anodes forelectrical properties like e.g. capacitance. In the course of this evaluation, formation of anodes needs to be doneat different voltages and shrinkages as well as press densities in order to get detailed information about thecapacitance behaviour of the powder. Subsequently, all anodes have to be washed, dried and measured. Results
are the well-known capacitance vs. voltage curves shown in fig. 4 for different powder types. After reaching amaximum in the range of 12-30 V, the capacitance of all powders drops down due to surface loss during
anodization. As a rule of thumb: the higher the capacitance the higher the capacitance loss with increasing
formation voltage.
This procedure enables a precise capacitance characterization but requires many anodes and measurements. To
ease the powder evaluation, a fast anode characterisation test that provides similar capacitance vs. voltage
information but with less effort was developed. Main focus was not a very precise capacitance determination but
a faster and more easy comparison method for different powders and anode conditions with the goal to find out
which powder / anode condition would show the highest capacitance at a given formation voltage.

160000
140000CV/g [µFV/g]

120000100000
80000
60000
40000
20000
0
0

50

100

150

200

Formation Voltage [V]fig. 4 Capacitance vs. formation voltage for different tantalum powders.
The new electrical test method is called “constant rate formation” since the main idea is to keep the formation
rate Frate (V/min) constant which means a constant tantalum oxide growth (nm/min) during anodization
independent from surface loss during anodization. The formation rate Frate depends on anode surface area A andapplied current I:

Frate IA

(1)

For a constant anode surface area it can be concluded that the higher the current, the higher is the formation rate.
If equation (1) is transposed it is obvious that for a constant F rate the current has to be increased when the anodes
surface area is increased and vice versa. This means: to provide the same formation rate for a high CV anode
like a low CV anode the current has to be increased in the same ratio like the surface ratio of both anodes.

A

I
Frate

(2)

The general capacitance equation of a plate capacitor (3)C   o r

A
d

(3)

is mainly depending on anodes surface area since dielectric constant of Ta2O5 r (~27) as well as the thickness
growth d (~1.8-2.3 nm/V)3,4 is nearly constant for all tantalum anodes. If the surface A in (3) is replaced byequation (2) and both terms are multiplied with U/m (voltage per gram) we get finally equation (4)

C*

U
I
U
  0 r*m
dFrate m(4)

From (4) it follows that in case of a constant rate formation the capacitance “CV/g” can be directly correlatedwith the applied current density “mA/g”. That means also that a surface reduction during formation, which is
responsible for the decreasing capacitance with increasing formation voltage (see fig. 4), can be directly
correlated with the current density. As a result, a current-voltage curve is obtained which is comparable to thecapacitance-voltage curve in fig. 4. To measure this current-voltage behaviour, a power supply with an
intelligent control unit is used which adjusts the required current for a given constant formation rate independentof anode type. The recorded current is always normalised to the used anode weight (mA/g). The adjusted current
depends on different parameter that has been taken into account:Formation rate
Powder typePressing and sintering conditions
Formation temperature
Electrolyte conductivity

o
o
o
o
o

The mechanism of the CR formation test will be demonstrated with two standard powders: A 150 kµFV/g
powder from Mg reduction process and a 40 kµFV/g powder from Na reduction process. The anodes made from150 kµFV/g powder were formed @ 60°C up to 50 V with three different formation rates: 0.15 V/min, 0.25
V/min and 0.77 V/min; the corresponding current voltage curve can be seen in fig. 5a. According to equation(1), the current at a given voltage is increased with increasing formation rate. The first section of all curves,
starting at zero voltage, shows a steep slope which is caused by the native oxide film and can not be correlatedwith capacitance behaviour while the subsequent curve ranges characterize the capacitance progress. For a better
comparison of the different rates all curves have been normalized to the maximum current values (see fig. 5b).
Evaluation of all three curves revealed that the high rate of 0.77 V/min lead to a curve “deformation” and shift
to higher voltages, while 0.25 and 0.15 V/min curve are comparable. The high formation rate increases thedifferences of the oxide thicknesses between the outer and the inner part of the anode; the oxide thickness in theanode centre will be smaller than on the outer anode surface area. This heterogeneous oxide thickness
distribution will be increased with increasing anode size and decreasing pore size. For anodes made from 150
kµFV/g powder, the 0.77 V/min rate can be regarded as too fast, slower rates of 0.25 or 0.15 V/min are morereliable. The comparison of measured capacitance (displayed as single points) to the normalised CR current in
fig. 5b over the range of 16-50 V shows a good agreement with the low Frate curves 0.15 and 0.25 V/min. Using
the low rate of 0.15 V/min, the native oxide layer was determined with ~4 V which is equal to 8 nm oxide
thickness in case of a growth rate of 2 nm/V4.

a)160000600
500
400
rate: 0.25 V/min - current plateau: 233 mA/g
300
200
100
rate 0.15 V/min - current plateau: 148 mA/g

0
0

10

20

30

Formation Voltage [V]400.77 V/min

100

1400000.25 V/min

12000080

10000060

80000
0.15 V/min

40

60000
40000

20

20000

native oxide layer ~4 V

0

50 b)

0
0

10

20

30

40

50

Formation Voltage [V]fig. 5 CR formation of anodes made from 150 kµFV/g powder @ 60°C (sinter density: 6.1 g/cm³) withdifferent rates a) formation current vs. voltage b) normalized current curves in comparison tocapacitance (single points).
However, for anodes made from lower capacitance powders with larger pores 0.77 V/min has been proven as aproper formation rate that provides a good correlation between CR current and real capacitance curves. The CR
formation curve of an anode made from a 40 kµFV/g powder, displayed in fig. 6, differs clearly from thosedescribed above and has only two different stages. The first stage is similar to those described above: a steep
slope caused by native oxide film. A plateau region can not be observed, so that the second section ischaracterised by a continuous slope drop caused by continuous surface area loss. The measured capacitanceshows a good correlation to the CR formation curve over the whole investigated range up to 150 V which meansthat one CR formation can replace 7 single formations including the capacitance measurements.

Cap [µFV/g]

rate: 0.77 V/min -current plateau: 644 mA/g
Normalised Current [%]Formation Current [mA/g]

700

50000

160

45000

140

40000
35000

120

30000

100

25000

80

20000

60

15000

40

10000

20

5000

0
0

25

50

75

100

125

150

Cap [µFV/g]

Formation Current [mA/g]

180

0
175

Formation Voltage [V]fig. 6 CR formation curve of a 40 kµFV/g anode (sinter density 5.7 g/cm³) and corresponding capacitancevalues.
New improved tantalum powders for high voltage applications
As described above, the smaller primary particle size and especially smaller pore sizes were the main reasons
why tantalum powders from Mg reduction process couldn’t be used for high voltage application up to now. To
overcome this limit, new powder types were developed by introducing major process changes allowing toachieve a simultaneous increase of pore size and particle size. This allows combining the high structural
homogeneity and high purity of powder produced by Mg reduction with desired high voltage capability. The
pore size distributions of new tantalum powders HV1-HV4 in comparison to standard 150 kµFV/g powder isillustrated in fig. 7a. The standard 150 kµFV/g powder exhibit a pore size peak at 144 nm while the pore peak of
the newly developed HV powders can be adjusted between 400-870 nm. Using mercury porosimetry allows also
calculating the pore area of the pores size distribution (fig. 7b). Corresponding to the pore size, the surface areaof the HV1 powder which shows the largest pore size maximum at 870 nm, was determined with 0.64 m²/g
while HV4 (pore maximum at 400 nm) has a surface area of 1.13 m²/g.
0.6

HV4

150 kµFV/g: 144 nm
1.0

HV3

HV3
0.4

Pore Area (m²/g)

Log Differential Intrusion (mL/g)

HV4

HV2

HV1
0.2

0.0

a)

1

0.5
Pore size Diameter (µm)

0.1

0.05

HV2

0.0

b)

HV1

0.5

5

1
0.5
Pore size Diameter (µm)

0.1

0.05

fig. 7 Properties of new improved tantalum HV powders: a) Pore size distribution of different HVpowders in comparison to a standard 150 kµFV/g powder b) Pores area of new HV powder.

According to the presented simple cylinder model it was estimated that for a 100 V formation the unformed
particle size should be at least 320 nm in order to provide enough tantalum for anodization, the maximum
capacitance value was calculated for 380 nm particles. SEM investigations reveal that that new powder provides
primary particle structures >350 nm which can also be adjusted over a wide range.
The new CR formation described above was applied for an extensive anode characterization in order to evaluate
the capacitance capability of the new HV1-HV3 powders in comparison to standard 20 kµFV/g and 40 kµFV/g
powders from Na reduction process up to 200 V (fig. 8). Again, the recorded current curve correlates to thecapacitance – the higher the current at a given voltage the higher the capacitance. The anode made from 20
kµFV/g powder has the lowest capacitance up to 165 V. The anode made from 40 kµFV/g powder iscomparable with that made from the HV1 powder up to 75 V. However, at higher voltages the capacitance
performance of the HV1 powder is slightly better since the current curve of the anode made from HV1 powder
is above those of the anode made from the 40 kµFV/g powder. The capacitances of the anodes made from HV2
and HV3 powders are distinctly higher than that of the anode made from 40 kµFV/g powder for a wide
formation range; in case of HV2 the current curve is higher up to 180 V, while the HV3 will perform better up
to 140 V.

200

HV2

b)

HV3

Current [mA/g]

"40k"
150
HV1

100

50
"20k"
0
0

50

100

150

200

Formation Voltage [V]fig. 8 CR formation curves of new HV powder in comparison to conventional 20 kµFV/g and 40 kµFV/g
anodes (formation rate 0.77 V/min, sinter density 7.0 g/cm³).
Using the CR curves allows estimating the capacitance sequence at a given voltage. For a 100 V formation thesequence, in which the capacitance is increasing from left to right, is:
20k < 40k < HV1 < HV2 < HV3.
A 100 V standard formation using anodes made from these powders have been carried out in order to check thevalidity of the CR based sequence with standard measured capacitance. It is already known that the wet
capacitance of high CV anodes is significantly influenced by the measuring frequency due to limited electrolyte
conductivity and small pore sizes13,14. Anodes with small pores require low measuring frequencies (<120 Hz) inorder to allow measurement of the total capacitance. A comparable behaviour was also observed for highvoltage anodes when pores are getting smaller due to increasing oxide film growth. That’s why a measuring
frequency of 20 Hz was used instead of 120 Hz. The determined capacitance is in total agreement to the CR
based sequence and demonstrated that this method is a powerful tool for powder evaluation (Table 2)table 2 Capacitance of standard and new high voltage anodes HV1-HV3
Powder TypeCapacitance @ 100 V [µFV/g]

20 kµFV/g

40 kµFV/g

HV1

HV2

HV3

18.38528.08729.67231.12736.010According to the CR formation test, the HV3 powder will provide the highest capacitance up to 120 V while the
HV1 and HV2 powders should be used for higher voltages up to 150 V. The lower initial current of anodes
made from HV1 powder in comparison to those made from HV2 powder indicates that the HV1 anodes havehigher porosity which should have a positive impact on impregnation and ESR. That’s why the HV1 powder
should be the better choice of a 150 V formation even if the CR current curve, and therefore capacitance, iscomparable at this voltage.
The HV powder shows not only higher capacitance but also improved microstructural homogeneity. Fig. 9
compares the cross section of 100 V formed anodes of a standard 40 kµFV/g powder and new HV2. The
brighter shining tantalum cores are surrounded by the darker Ta2O5 layer. The standard anode structure is moreheterogeneous concerning particle size and pore distribution: apart from open pore regions contains the anodevery dense regions which will be difficult to impregnate. In contrast, HV2 anodes have a more uniform pore size
distribution which allows a better impregnation. Furthermore, also the tantalum particle structure is morehomogeneous concerning particle size and shape. This is the main reason why the HV2 anode capacitance is incomparisons to 40 kµFV/g anode more than 10 % higher.

Ta

Ta2O5

40k

HV2

fig. 9 SEM polished cross section of 100 V formed anodes (sinter density 7.0 g/cm³): The unformed
tantalum core appears bright while the Ta2O5 film is darker, pores are black.
Summary
It was shown that high voltage tantalum capacitors have special requirement on microstructure that has to betaken into account like sufficiently big primary particle size, neck size as well as pore size. For each formation
voltage exist an optimum particle size that provides the highest possible capacitance. By using a simple cylindermodel it was calculated that a 100 V formed anode can provide a capacitance of maximum 51.7 kµFV/g when
the unformed tantalum cylinder has a diameter of 380 nm.
To find best anode conditions, like pressing and sintering, but also to ease the evaluation of different powders, a
new characterization method was presented. It could be shown that the constant rate formation provides currentvoltage curves that can be directly correlated to the capacitance-voltage behaviour with only one single
formation.
Finally, new developments allows to extend the capability of the Mg reduction process to produce excellent
tantalum powders for HV applications which show comparable or even better capacitance behaviour thanstandard sodium reduced powders up to 150 V. Not only capacitance but also the pore structure was improved
which will be an addition benefit for impregnation. The process allowing control of the pore size over aremarkable range is still under development and its limit is not reached. An additional pore expansion > 1 µm
should allow to extent this range up to 200 V by keeping its high homogeneity and improved capacitance.

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