Highest Capacitance at higher voltages: Pushing the limits of tantalum high voltage
capacitors
2nd Space Passive Component Days (SPCD), International Symposium
Marcel Hagymási1, Christoph Schnitter2, Rich Reifenheiser3, Oliver Thomas4(1) Tel.: +49 5321 751 53160, e-mail: marcel.hagymasi@hcstarck.com(2) Tel.: +49 5321 751 53939, e-mail: christoph.schnitter@hcstarck.com(3) Tel. +1 617 678 8497, e-mail: rich.reifenheiser@hcstarck.com(4) Tel.: +49 5321 751 5955, e-mail: oliver.thomas@hcstarck.com
H.C. Starck GmbH, Im Schleeke 78-91, 38642 Goslar, Germany

There is an increasing demand for electronic applications using tantalum capacitors operating at high voltages. This
demand is driven by automotive (e.g. driving assistance systems and infosystems), medical devices and aviation, as
well as data storage (SSD). This will require new improved tantalum powders that can meet the specifications ofhigh dielectric strength, improved energy density and excellent reliability at voltages >100V. These applications
must also follow the trend of miniaturization and therefore cannot be satisfied by existing powders since theirvolumetric efficiency is simply too low.
Therefore H.C. Starck has developed a class of new tantalum powders with a tailored microstructure that can provide
higher capacitances than any other tantalum powder and high reliability for anodization voltages from 75 V to 350 V.
In this paper it will be shown that microstructural homogeneity of primary particles as well as pore size distributions
are the key factors for increasing capacitance. Beside superior material properties of these new powders the anodemanufacturing process has to be taken into account in order to achieve capacitors with highest energy density.
Therefore it will be discussed how pressing and sintering conditions influence the performance of anodes made fromthese powders. For anodization of these powders, new organic based electrolytes have to be utilized to prevent
breakdown of the oxide layer at higher voltages > 100 V.

Introduction
As a technology leading company H.C. Starck made considerable efforts to develop new tantalum powders to follow
the trend of miniaturization. In general for tantalum capacitors two different trends can be observed. For lower
application voltages up to 6.3 V HC Starck continues to increase the capacitance of the powder up to 250k µFV/g
and even higher – these powders are often designated as “high CV”. The increase of capacitance ability of suchpowders is achieved by decreasing the primary particle size so that the total surface area is increased.
The second trend which shows a significant growth is to increase the application voltage of tantalum capacitors tohigher values from 25 V to 100 V and even higher. The definition of high voltage range depends on the market andapplication. As a result of intensive research H.C. Starck has recently developed a new class of high voltage powders– called HVMC (high voltage medium capacitance) that can be anodized over wide range from 75 V up to 350 V.
These powders provide the highest capacitance and energy density at such high anodization voltages that cannot be
achieved by other available tantalum powder. The HVMC powders enable the capacitor producers to make newcapacitor types with highest energy densities for new applications that could not be achieved in the past. They aredesigned for different applications such as data storage (solid state drives), battery charger for faster charging aswell as a specific focus on high reliability applications:

Automotive: Assisted driving systems, the change of the board net system of cars to higher voltages andelectromobilityTransportation and railroad applications

Medical application such as implantable defibrillators

Aviation and aerospace, e.g. for satellites



Defense

These powders are used for both solid (MnO2 and Polymer) and wet type capacitors. The anode weight can varyover a wide range up to several grams and also the shape is very different from large flat anode (round, “half moon”,
rectangular) as well as cylindrical ones.
First we will have a look at the microstructure of such a powder in order to explain what makes these powders so
unique and why they are so superior compared to standard powders. Afterwards it will be shown that it is simply notenough to have an excellent high voltage powder – it is also essential to use the right pressing and sintering
conditions to gain the maximum capacitance and good leakage current.
HVMC – new high voltage powder classHigh voltage powders must have a different microstructure than a “high CV” powder used for lower voltages.
Tantalum primary particles require larger sizes so that enough tantalum remains after anodization: too small tantalum
particles will completely converted to Ta2O5. On the other hand too large particles will have a lower surface area
which corresponds to a lower capacitance as well. Also pores has to be large enough so that the formed oxide film
has sufficient space for its growth otherwise they will be closed resulting in lower capacitance. Too large pores canbe regarded as a waste of space and therefore capacitance. Moreover sufficient strong neck sizes as smallest
connection between two adjacent particles are required otherwise the current path will be discontinued. It was
already shown by calculations that each anodization voltage has its own optimum concerning primary particle andpore size distribution [1]. The real structure of tantalum consists not of a single pore or particle but a distribution.
The unique capacitance properties of the HVMC powders are based on the higher homogeneity of both primaryparticle size and its pore size distributions. In contrast to other powders also the naming of the special powder class
was changed – it is not correlated with the expected capacitance (mostly measured at lower voltages) but for thedesignated forming voltage! “HV100” means that this is an HVMC powder with a target formation voltage of 100
V. Through minimal adjustments to the pressing, sintering and forming condition this powder can also be used forlower or higher voltages. The comparison of the cross section of anodes in fig. 1 shows differences in microstructurebetween a standard 12k µFV/g powder and an HV300 powder, both formed at 300 V. Note that this is an SEM inback scattering mode where the level of brightness is correlated to the average atomic weight: heavy
elements/compounds will be bright while lighter elements/compounds are darker.

1
)

2
)

3
)

b
a
)
fig. 1 Microstructureof 300 V formed anode made of a) 12k )µFV/g standard powder that has (1) too large pores, toolarge primary particles and (3) too small primary particle with closing pores b) new HV300 with significantly
improved homogeneity.

The standard powder has a wide primary particle and pore size distribution: beside very large particles (2) thatconsist of sufficient tantalum to be formed to even higher capacitance but having low contribution to capacitancealso too small tantalum particles (3) can be found which are nearly totally converted to oxide while the remaining
pores are already closed. This surface area will also give no contribution to capacitance. Finally too large pores (1)can be regarded as a waste of space that reduces the achievable volumetric capacitance. In contrast to that the HV300
clearly shows the superior microstructure with improved homogeneity: tantalum particles as well as pore size
variation are much smaller so that the provided capacitance is significantly higher due to improved utilization of
tantalum. This is the main reason why the HVMC powders can always provide a higher capacitance than any othercurrent powder. Through our research we have determined the optimum primary particle and pore size distribution at
a given anodization voltage therefore we have developed a different powder for each anodization range that will
provide the highest capacitance [2]. For a range of 100 to 350 V anodization voltage in fig. 2 it is illustrated that overthe entire anodization range HVMC powders are offered providing higher capacitances than the corresponding “bestin class” standard powder available today. To provide a fair competition all anodes have the same press density (6.0
g/cm³) and shrinkage (~10 vol.-%) conditions.

35000

HV100

Capacitance [µFV/g]

32500

30000 + 33 %
27500
25000

HV200

22500
20000
30k µFV/g

17500

HV300

+ 25 %

15000
12500

+ 22 %

17k µFV/k

10000

12k µFV/k

7500
75

100

125

150

175

200
225
250
275
Anodization Voltage [V]

300

325

350

375

fig. 2 Capacitance vs. anodization voltages: HVMC HV100-HV300 vs. standard (Na reduced) powders.

For 100 V the HV100 offer 33 % more capacitance than the 30k µFV/g powder, the HV200 offers more than 25 %capacitance increase at 200 V while the HV300 can provide more than 22 % additional capacitance at 300 V. Thesefindings have been confirmed by capacitor manufactures as well. However to gain the highest capacitance it is notenough to use the right HVMC powder, proper pressing and sintering conditions are required. Next it will be shownhow to find the best anode conditions.

Finding the optimum: Impact of pressing on sintering
As a leading tantalum powder producer it is not enough for H.C. Starck to develop a powder – it has first to be tested
internally to ensure it meets the development requirements such as improve efficiency, increased capacitance orhigher voltage capability. It will also be compared to other new or already existing powder that serves as benchmark.
Based on these results the proper powder will be developed.
While chemical and physical powder properties are only depending on powder production most of the important
capacitor properties like capacitance, leakage current and breakdown behavior are also strongly influenced by theprocessing parameters. Pressing and sintering as well as anodization have a significant impact on such properties andcannot be neglected. It can be compared with fuel consumption of a car: driving mostly high speed with highacceleration will lead to high consumption while a fuel-conversing driving style (lower speed, slower acceleration)with the same car will lead to totally different result. Let’s transfer this picture to tantalum capacitors: Even a perfecthigh voltage powder can provide less than desired results or even fail if the wrong conditions are applied. Moreover
which process parameters are required to find the maximum capacitance and simultaneously an acceptable leakage
current?
Each anodization voltage has its own capacitance optimum – which depends on the powder microstructure. It was
already shown that the HVMC microstructure is unique when compared to existing standard powders [2,3].
Therefore the optimum pressing and sintering conditions can be different from standard powders and has to be
determined.

Experimental Part
Anodes have to be pressed and sintered before anodization. Both steps have an impact on the final anodes. For agiven target sinter density (SD) of e.g. 6.3 g/cm³ different paths are possible to achieve it: a low press density (PD)of 5.0 g/cm³ requires shrinkage of 20.6 vol.-% while a higher press density of 6.0 g/cm³ needs only 4.8 vol.-%. In
both cases the same sinter density will be achieved but the different procedures have a significant impact on the final
capacitance, leakage current, breakdown behavior as well as anode pore structure. We will use HV300 that provides
currently the highest capacitance at 300 V for an extensive investigation to find optimum pressing and sintering
condition. We will look at both weight correlated (“CV/g”) and volume correlated (“CV/cm³”) capacitance, butwhich is the “right” capacitance that should be used to decide which pressing and sintering conditions are ideal?
Finally volume is given for each capacitor type. Powder weight of this volume can be changed by using different
sinter densities which is not included in weight correlated “CV/g” – only the volume correlated capacitance“CV/cm³” takes this into account. This means that a condition that results in lower “CV/g” can provide in the end ahigher “CV/cm³” because more powder is included in the same volume due to higher sinter density that compensate
the lower CV/g.
Round anodes (3.0 mm) were used with press densities from 5.0 up to 6.5 g/cm³ in 0.5 g/cm³ steps. All anodes
were sintered for 20 min at three different temperatures (1380°C, 1435°C, 1490°C). This corresponds to 3 shrinkage
ranges (low, medium and high) of ~5, 10 and 16 vol.-% (1.5 %) that are illustrated in fig. 3. Sinter density varies
over a wide range from 5.2 to 8.0 g/cm³. As described before we achieved comparable SD by different steps: e.g weachieved ~6.0 g/cm³ by applying a PD of 5.5 g/cm³ and low shrinkage of ~6 vol. %- or a PD of 5.0 g/cm³ and highshrinkage of ~16 vol.-%.

9,0

PD 5.0 g/cm³
PD 5.5 g/cm³

8,0Sinter Density [g/cm³]

Sinter Density [g/cm³]

9,0

PD 6.0 g/cm³
PD 6.5 g/cm³

7,0
6,0
5,0
4,0
1360

PD 5.0 g/cm³

PD 5.5 g/cm³

8,0

PD 6.0 g/cm³
PD 6.5 g/cm³

7,0
6,0
5,0
4,0

1410
1460
Ts - 20 min dwell [ C]

1510

0

5

10
Shrinkage [vol.-%]

15

20

a
b
fig. )3 Sinter density of HV300 anodes for all applied press) densities against: a) sinter temperature b) corresponding
shrinkage

All anodes were formed at 60°C in a water/glycol based phosphoric acid electrolyte – it has been shown that glycol
based electrolytes are recommended for high voltage formation >100 V [4-6]. Even if the whole mechanism is notscientifically understood it was observed that such electrolytes reduce the breakdown tendencies and improved theleakage current. It is known that anode size and shape have an impact on forming condition and capacitancebehavior. However, to minimize the effort it was decided that this will be investigated separately. Moreover it isassumed that the general tendencies are comparable.
For all anodes we applied the same constant rate (potentiodynamic) formation program with stepwise decreased rates
until we reached the final voltage (fig. 4). Current is always automatically adapted to provide exactly the required
forming rate (V/min) independent from anode type, amount or even powder type. Standard formation with
galvanostatic formation (current is fixed) has the disadvantage that the area decrease during anodization leads to anincrease of current density (current per area) and therefore to an increase of anodization rate (V/min). Too highforming rate will lead to the increased generation of oxygen [7], increases the breakdown ability and can finallycrack the oxide film. Moreover, if defects such as breakdowns or other structural oxide flaws are generated during
anodization the current will mainly flow over these anodes while the remaining anodes will get much less current
thereby effecting the anodization of good anodes. Using constant rate formation prevents this interaction between“good” and “bad” anodes – in the case of local defects in single anodes the current will be increased to provide thesame oxide growth for all anodes: each anode will get exactly the current that is required! According to many
anodization trials is was found that anodes are much more stable at lower voltages when most of the tantalum is not
yet converted to oxide. With increased voltage the remaining tantalum core is getting smaller while the oxide film is
growing and closes more and more the pores. As a result the tantalum anode gets weaker and anodization rate has tobe decreased. That’s why our anodization profile starts with a fast anodization rate (V/min) when the anode is verystrong and is stepwise decreased when it’s getting weaker. In total our program has 3 different ramp rates until wereach 300 V, in each step the required current is automatically adjusted.

140

300

120

250

100

200

80

150

60

100

40

50

20

0

I [mA/g]

U [V]

350

0
0

7200

14400

21600

28800

36000

t [sec]fig. 4 Potentiodynamic anodization program used for HV300 forming.

Weight correlated capacitance “CV/g”
14000

Capacitance [µFV/g]

Capacitance [µFV/g]

14000

12000

10000
PD 5.0 g/cm³
PD 5.5 g/cm³

8000

PD 6.0 g/cm³

12000

10000
PD 5.0 g/cm³
PD 5.5 g/cm³

8000

PD 6.0 g/cm³
PD 6.5 g/cm³

PD 6.5 g/cm³

6000

6000
5,0

a
)

6,0
7,0
Sinter Density [g/cm³]

8,0

b
)

0

5

10
Shrinkage [vol.-%]15

20

fig. 5 Capacitance of HV300 @ 300 V for different press densities: a) cap vs. sinter density b) cap vs. shrinkage forsame anodes. Apparent cap maximum is encircled with blue dashed line.

Let’s first have a look at weight correlated capacitance. To illustrate the effect of sintering, capacitance is plottedagainst final sinter density as well as to the corresponding shrinkage for all press densities. It is obvious in fig. 5 thatthe combination of the lowest press density of 5.0 g/cm³ and lowest shrinkage of 4 % provides the highest CV/g of~13510 µFV/g. For the lowest PD of 5.0 g/cm³ when increasing shrinkage from 5% to 10 vol.-% had a nominal
impact on capacitance (drops to 13420 µFV/g). At all higher press densities show the same tendency: increasing
shrinkage will decrease weight correlated capacitance. It has to be noticed that the powder has always the highest
surface area and therefore the highest capacitance. Densification of tantalum anodes occurs via solid state sinteringwith the driving force of reducing free surface energy by reducing the power surface area [8]. With increasing
shrinkage surface area is reduced by particle size growth, pore size and volume will also be reduced ultimately
reducing the anode capacitance.

Moreover, increasing press density also results in a decrease of the anode capacitance. Mercury porosity(Micrometritics Instrument) was carried out with all sintered anodes to determine pore size distribution from low
shrinkage anodes. Pore distribution in fig. 6 reveals that with increasing press density the entire pore distribution isshifted to smaller sizes. In general the HV300 and all HVMC powders have two kinds of pores: primary pores are
the smaller pores within the agglomerate structure formed by primary particle structure. They are responsible for the
majority of the total surface area and therefore the capacitance. The larger secondary pores are a result of the empty
space between larger adjacent agglomerates, the contribution to capacitance is negligible but they are important for
impregnation of the cathode material and some electrical properties such as ESR. However, since tantalum itself
cannot be compressed – it has always the same density of 16.6 g/cm³ - only the total pore volume will be decreased
with increasing press density which can be observed in the pore size shift to a smaller size: secondary pore peakdecreases from 2.63 µm (PD 5.0 g/cm³) to 1.44 µm (PD 6.5 g/cm³) while primary pore peaks are shifted from 1.31
µm to 1.12 µm.
5.0 g/cm³

5.5 g/cm³

6.0 g/cm³

6.5 g/cm³

Log Differential Intrusion (mL/g)

primary pores
0.3

0.2

secondary pores

0.1

Pore shift by pressing
0.0
10

5

1
Pore size Diameter (µm)

0.5fig. 6 Sintered anode pore distribution for all investigated press densities (~ 5 % shrinkage) – please not the inverse
logarithmic scale; smaller pores are at the right sight.

In addition higher press density increases the contact areas between adjacent tantalum agglomerates; the remaining
surface area is decreased. Mercury porosimetry can also be used to calculate the pore area which correlates to theanode area and therefore with the capacitance. The weight correlated anode pore area distribution for all press
densities in fig. 7confirmed that increased press density not only shifts the pore sizes to lower values but also thepore area is decreased: the total pore area is decreased from 0.325 m²/g (PD 5.0) to 0.296 m²/g (PD6.5) whichcorresponds to an 8.9 % reduction. The observed capacitance drop with increased press density can be explained by
the changed pore structure and decreased pore area. The observed capacitance decrease with increased press density
for the low shrinkage anodes from 13510 µFV/g (PD 5.0 g/cm³) to 11370 µFV/g (PD 6.5 g/cm³) correlates with 8.4% pore area reduction in fig. 7!

5.0 g/cm³

5.5 g/cm³

6.0 g/cm³

6.5 g/cm³

Cumulative Pore Area (m²/g)

0.3

0.2

0.1

0.0

4

2

1
0.8
0.6
Pore size Diameter (µm)

0.4

0.2

fig. 7 Sintered anode pore area distribution of all press densities (~5 % shrinkage).

Looking only at the weight correlated capacitance, a press density of 5.0 g/cm³ and low shrinkage of 4 % (or even 10%) would provide the highest values of 13511 µFV/g at 300 V – this “apparent” cap maximum is marked in fig. 5
with a blue dashed circle. In contrast to that, anodes with the highest sinter density of 8.0 g/cm³ (PD 6.5 g/cm³) havethe lowest capacitance value of only 8632 µFV/g. Too high pressing and shrinkage conditions leads to smaller
pores that are closed during anodization resulting in additional capacitance loss. The general effect of closing poresand decreasing pore (and therefore anode) area which is valid for all anode conditions can be seen in fig. 8 for an
anode with PD 6.0 g/cm³ and 10 % shrinkage. The pore peak is shifted to smaller sizes while the total intrusion (porevolume) is also reduced since the formed oxide film closes the remaining pore. The formed pore area is significantly
reduced from 0.28 to 0.16 m²/g.
Sintered

300 V formed

0.2

0.1

0.0

5

1
0.5
Pore size Diameter (µm)

Sintered

0.3

Cumulative Pore Area (m²/g)

Log Differential Intrusion (mL/g)

0.3

0.1

300 V formed

0.2

0.1

0.0

5

1
0.5
Pore size Diameter (µm)

0.1fig. 8 Anode pore size and pore area distribution of an HV300 (PD 6.0 g/cm³, 10 % shrinkage) after sintering and
300 V forming.
As already mentioned increasing shrinkage also leads to decreased pore volume as well as pore area that ultimately
reduces the final capacitance. Since all press densities show the same trend we will use anodes with a PD of 5.5

g/cm³ in the following discussion (see fig. 9): Increased sintering temperature and therefore shrinkage reduces
mainly the pore volume (“intrusion”), note how the primary and secondary pore peaks are shifted to smaller sizes.
Finally the weight correlated pore area is reduced which is consistent with the capacitance drop.
1380°C

1435°C

1490°C

1380°C

1435°C

1490°C

Cumulative Pore Area (m²/g)

Log Differential Intrusion (mL/g)

0.3
0.3

0.2

0.1

0.0

5

1
Pore size Diameter (µm)

0.5

0.2

0.1

0.0

5

1
Pore size Diameter (µm)

0.5fig. 9 Effect of sintering on anode pore size and pore area distribution (PD 5.5 g/cm³).

Influence on breakdown stability and leakage current
Beside capacitance the structural strength of the anode has to be taken into account! Anodes with the lowest press
density and shrinkage were very susceptible to oxide breakdown due to the weak connection between adjacentagglomerates and also smaller neck sizes within the agglomerates. Higher shrinkage improves the anode strength andtherefore breakdown probability is reduced. The anode condition that provides the highest “CV/g” (lowest PD of
5.0, lowest shrinkage of 4%) could not be formed without breakdown (fig. 10) – before the final voltage wasreached (~297 V) current was significantly increased which indicates side reaction and local oxide breakdown affected damaged anodes were recognizable by strong gas evolution. The current drop during the final potentiostaticstage at 300 V was very limited, in comparison to a good anodization behavior the final current that was reachedwhen forming is finished was very high (20 mA/g final current instead of 4 mA/g at the end of the anodization,compare fig. 4)

350

160

300

140
120

250

local oxide
breakdown

150

final current 80

I [mA/g]

U [V]

100

200

60

100

40

50

20

0
0

5000

10000

15000

20000

25000

30000

35000

40000

0
45000

t [sec]fig. 10 Constant rate formation curve of low pressed and sintered anode (PD 5.0 g/cm³, 4% shrinkage): Local oxide
breakdown at 298 V causes rapid current increase.

Also the leakage current behavior confirmed that independent from press density the low shrinkage anodes are
susceptible to anode damages that leads to high leakage current values (see fig. 11). For all press densities, increasing
the anode shrinkage from 5 to 10 % results in much lower values due to the stronger anode structure. However,additional shrinkage leads only to an insignificant improvement. That’s why it is recommended to use no more than
~10 vol.-% shrinkage for HV300 anodes formed at 300 V. Even though the capacitance seems to be better for lower
shrinkage anodes, the anode integrity will be jeopardized resulting in higher leakage values. Second tendency thatcan be recognized is that with increased press density (same shrinkage) the leakage current is improved. Similar toshrinkage higher press density improves the structural strength of the anode by improving the contact area between
adjacent agglomerates and particles so that after sintering a stronger connection is realized. The Impact of press
density on leakage current for very high sintered/shrunken anodes is very limited since the anode strength is alreadyvery high at such conditions.
25

25
PD 5.5 g/cm³

20

PD 6.0 g/cm³
PD 6.5 g/cm³

15
10
5
0

PD 5.0 g/cm³

Leakage Current [nA/µFV]Leakage Current [nA/µFV]

PD 5.0 g/cm³

PD 5.5 g/cm³

20

PD 6.0 g/cm³
PD 6.5 g/cm³

15
10

5
0

5,0

6,0
7,0
Sinter Density [g/cm³]

8,0

0

5

10
Shrinkage [vol.-%]15

fig. 11 Leakage current of HV300 at 300 V for all press densities and shrinkages/sinter densities.

20Volume correlated capacitance “CV/cm³”

80000
14000

Capacitance [µFV/cm³]

12000

10000
PD 5.0 g/cm³
PD 5.5 g/cm³

8000

PD 6.0 g/cm³
PD 6.5 g/cm³

6000

0

5

10
Shrinkage [vol.-%]

15

20

75000
72500
70000

PD 5.0 g/cm³
PD 5.5 g/cm³67500

PD 6.0 g/cm³

PD 6.5 g/cm³

65000

5,0

6,0
7,0
Sinter Density [g/cm³]

77500

14000

Capacitance [µFV/g]

77500

Capacitance [µFV/g]

Capacitance [µFV/cm³]

80000

12000

10000
PD 5.0 g/cm³
PD 5.5 g/cm³

8000

PD 6.0 g/cm³
PD 6.5 g/cm³

6000

0

5

10
Shrinkage [vol.-%]

15

20

75000
72500
PD 5.0 g/cm³

70000

PD 5.5 g/cm³
PD 6.0 g/cm³67500

PD 6.5 g/cm³

65000

8,0

0

5

10
Shrinkage [vol.-%]15

20

fig. 12 Volume correlated capacitance of HV300 @ 300 V as a function of press density and shrinkage/sinter density.
Maximum is encircled with blue dashed ellipse.

Only the volume correlated capacitance takes the sinter density into account and therefore it is recommended to use
it to determine the best volumetric pressing and sintering conditions that provides a capacitance maximum. The
appearance of the volume correlated capacitance curves vs. sinter density and shrinkage in fig. 12 differs strongly
from weight correlated capacitance in fig. 5. Using a PD of 6.0 and shrinkage of 5 % provides the highest
capacitance of 78900 µFV/cm³. Increasing shrinkage to 10 % results in very high capacitance values of 78029µFV/cm³ which is in the same range of higher pressed but lower shrunken anodes (78024 µFV/cm³, @ PD 6.5 g/cm³,5 %). This area is regarded as the real maximum capacitance area of HV300. When leakage current behavior (fig.
11) is taken into consideration only one condition is left: PD 6.0 g/cm³ and 10 % shrinkage since the other low
shrinkage anode conditions will lead to significantly worse leakage current. The formerly designated “apparent” CVmaximum anode condition of PD 5.0 g/cm³ and shrinkage 4-10 % has distinctly lower volume correlated
capacitance. In fact: the higher press density of 6.0 g/cm³ leads to lower “CV/g” but the increased powder amount
could compensate for this. Higher press density has even more powder but due to the smaller pore sizes and porevolumes they are closed or filled very quickly at higher voltages that results in a significant capacitance drop.

Conclusion
Due to its structurally superior homogeneity the HVMC power class provides the highest capacitance over a wide
anodization range up to 350 V. Pore size and primary particle size distribution is narrower compared to available
standard high voltage powder. This narrow distribution results in a more efficient use of the Tantalum volume within
the anode resulting in significantly higher capacitance with HVMC powders. It was shown for a 300 V anodizationof HV300 that capacitance, leakage current, and also breakdown behavior is not a pure powder property, pressing
and sintering has a strong impact on the final anode behavior and cannot be neglected. Increasing the press densityimproves the anode strength and decreases the leakage current, this does have a limit as too high of a press density
leads to smaller pores that are closed during anodization, reducing capacitance. Higher shrinkage improves leakage
current but decreases the capacitance due to the reduction in surface area. If only the weight correlated capacitanceis considered the lowest press density of 5.0 g/cm³ with low shrinkage of 4 % will provides the highest capacitanceof 13511 µFV/g. However this condition will also result in higher leakage values due to the poor contact between
agglomerates and the smaller necks and sinter bonds within and between the particles. Therefore this condition
cannot be recommended. It was revealed that only the volume correlated capacitance has to be used to define theoptimum conditions because it takes the increased powder amount of higher press densities into account. Using apress density of 6.0 g/cm³ and shrinkage of 10 % provides the maximum capacitance of 78029 µFV/cm³ and ensures
low leakage current and high breakdown strength. Pressing and sintering conditions must be optimized in order tomaximize the benefits of higher capacitance and lower leakage of the HVMC powders.

[1]

M. Hagymási, H. Haas, C. Schnitter, H. Brumm, ”High Voltage Tantalum Powder – Challenges andOpportunities for new Powder Generation”, CARTS Europe, Nice, France 2011[2]

M. Hagymasi, K.P. Rataij, H. Haas, C. Schnitter, H. Brum, “Tailoring the microstructure: Optimization of
Tantalum powders for highest voltage applications up to 350 V”, QARTS US, Mesa 2015[3]

M. Hagymási, H. Haas, C. Schnitter, H. Brumm, “Novel High Voltage Tantalum for New Applications”, T.I.C.
2013, York, England 2013[4] M. Croset, E. Petreanu, D. Samuel, G. Amsel, J.P. Nadai, “An O 18 Study of the Source of Oxygen in the
Anodic Oxidation of Silicon and Tantalum in Some Organic Solvents”, J. Electrochem. Soc.-1971-Croset-71727[5] N. F. Jackson, “Field crystallization of anodic films on tantalum”, J. Appl. Electrochem. 3 (1973), 91-98[6] Donald H. Stephenson, Martin D. Cymerman, Barry C. Muffoletto, “Using aqueous electrolyte containing
ethylene glycol or polyoxyethylene glycol and phosphoric acid”, US6231993 B1, 29. Sept. 1999
[7] K. P. Rataj, C. Hammer, B. Walther, M. M. Lohrengel “Quantified oxygen evolution at microelectrodes”,
Electrochim. Acta, 90 (2012), p. 12-16[8] R.M. German, G.L. Messing, R.G. Cornwall, “Sintering Theory and Practice Sintering Science Technology”,
ed. T.P.S. University, John Wiley Sons, Ltd., New York (2000)