Anodization of aluminum capacitor electrode foil

Current efficiency is increased in the anodization of aluminum foil for electrolytic capacitors by carrying out the anodization below 90.degree. C. in a non-aqueous ethylene glycol-ammonium pentaborate bath containing a small amount of an orthophosphate source, e.g., ammonium dihydrogen phosphate.

BACKGROUND OF THE INVENTION 
This invention relates to the anodization of aluminum foil, particularly 
for use in electrolytic capacitors. Even more particularly, it relates to 
an anodization electrolyte which reduces the electric charge requirements 
compared to conventional formation electrolytes. 
The formation of dielectric barrier oxide films on aluminum foil for 
capacitors is known generally. These films are non-porous and quite thin 
(10.sup.-7 m). Generally, aqueous anodization electrolytes have been used 
for the anodization of aluminum, e.g., aqueous borate, phosphate, or 
citrate electrolytes. The use of non-aqueous electrolytes has been 
restricted usually to post-formation electrolytes, fill electrolytes for 
capacitors, or very specialized processes. Aqueous electrolytes are 
generally preferred since the solvent, water, is so much less expensive. 
Film quality also has been better when aqueous electrolytes have been 
used. 
The industry always has been looking for ways to reduce electrical power 
consumption. A known way of doing this has been to anodize aluminum 
through a porous hydrated layer on its surface. However, hydrate formation 
cannot be used for low-volt anodization as fine pores become blocked. The 
search for means to reduce power consumption has intensified as energy 
costs have increased. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an anodization process having 
increased current efficiency for aluminum electrolytic capacitor foil 
compared to conventional aqueous anodization processes. 
It is another object of the invention to provide an anodization electrolyte 
in which formation rate is increased. 
It is a further object of the invention to provide an electrolyte bath 
containing phosphate that eliminates the formation of aluminum phosphate 
precipitate in the bath. 
The above objects can be achieved by anodizing aluminum in a non-aqueous 
glycol-borate electrolyte to which a small amount of a soluble 
orthophosphate salt or a compound which produces orthophosphate ions in 
the glycol-borate electrolyte solution has been added. This addition gives 
film properties comparable to those obtained with aqueous electrolytes and 
superior to those obtained with a straight glycol-borate electrolyte. The 
anodization is suitably carried out below 90.degree. C. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The anodization process of the present invention uses a glycol-borate-based 
electrolyte which by itself gives a 5-10% increase in charge efficiency, 
as shown by W. J. Bernard and J. W. Cook, J. Electrochem. Soc. 
106:643-6(1959), i.e., a greater amount of oxide film formed per coulomb, 
compared to aqueous electrolytes. With the addition of a small amount of 
an orthophosphate compound, e.g., orthophosphoric acid, mono- or di-basic 
phosphate such as ammonium dihydrogen phosphate or diammonium hydrogen 
phosphate, or other source of orthophosphate ion, to the glycol-borate 
electrolyte, a further 25% increase in charge efficiency is realized. This 
charge saving (30-35%) over conventional aqueous formation electrolytes is 
realized by producing a thicker barrier oxide film for the same amount of 
charge. As will be seen below, the glycol-borate-orthophosphate system 
gave a greater charge efficiency and greater weight gain than did 
glycol-borate alone. However, both systems produced the same CV product. 
Since the dielectric properties appear the same as for pure aluminum 
oxide, Al.sub.2 O.sub.3, known to be formed in pure glycol-borate systems, 
(Bernard et al., op cit), one way of interpreting such results is that the 
dielectric film obtained with the present electrolyte is likely to be an 
aluminum oxyphosphate. This may explain also the fact that aluminum 
phosphate precipitate, common with aqueous phosphate-containing 
electrolytes, did not form. 
In the examples below, formation of various aluminum foils in glycol-borate 
electrolytes, with and without an orthophosphate, is shown. The 
orthophosphate used is ammonium dihydrogen phosphate to permit direct 
comparison to a conventional aqueous ammonium dihydrogen phosphate 
formation electrolyte.

EXAMPLE 1 
In this example, anodization was carried out to the same film voltage as 
obtained with a standard aqueous ammonium dihydrogen phosphate at 
90.degree. C. The experimental formation electrolyte consisted of 17 wt% 
ammonium pentaborate in ethylene glycol (referred to as base) plus the 
addition of 2 wt% ammonium dihydrogen phosphate (ADP). Time is the time to 
reach required voltage, leakage is expressed as .mu.A/1.36 in.sup.2 and 
represents the current passing 5 min after voltage is achieved, and 
capacitance is expressed as .mu.F/1.36 in.sup.2. 
Table 1 
______________________________________ 
Formation Time Capaci- 
Temp. Electrolyte (min) Leakage 
tance 
______________________________________ 
90.degree. C. 
Aqueous ADP 3.0-3.7 35.1 50.9 
25.degree. C. 
Base 2.34 83 56.4 
25.degree. C. 
Base + ADP 2.24 27 55.4 
______________________________________ 
The glycol-borate electrolyte alone gave unacceptable leakage current while 
capacity is acceptable while that with the orthophosphate gave acceptable 
values for both. Thus, the addition of ADP to glycol-borate gave results 
equivalent to conventional aqueous ADP. 
EXAMPLE 2 
This example shows the effect of the presence of water in the formation 
electrolyte. 
Table 2 
______________________________________ 
Formation Time Capaci- 
Temp. Electrolyte (min) Leakage 
tance 
______________________________________ 
90.degree. C. 
Aqueous ADP 1.59 3.1 26.1 
90.degree. C. 
Base + ADP 1.74 28.3 26.9 
90.degree. C. 
Base, ADP + 20% 
1.77 1.0 19.4 
water 
25.degree. C. 
Base + ADP 1.01 17.8 25.9 
25.degree. C. 
Base, ADP + 20% 
1.59 4.2 24.4 
water 
______________________________________ 
The presence of water improved leakage current but at the expense of 
anodization efficiency at both temperatures. The best anodization 
efficiency was obtained at 25.degree. C. with the orthophosphate additive. 
EXAMPLE 3 
Anodization of both etched and unetched foil was investigated using the 
experimental electrolyte of Example 1 at 85.degree. C. Both anodized at 
approximately the same rate to about 200V. At that point, anodization of 
the unetched foil leveled off while the etched foil could be anodized to 
higher voltages. 
By way of comparison, the average formation rate, 0-200V, for two 
glycol-borate electrolytes with and without 0.2 wt% ADP is shown for 
unetched foil at two other current densities, 0.1 mA/cm.sup.2 and 1.0 
mA/cm.sup.2, at 85.degree. C. Electrolyte A is the same as above, 17 wt% 
ammonium pentaborate in ethylene glycol, while electrolyte B is 33 wt% 
ammonium pentaborate in ethylene glycol. 
Table 3 
______________________________________ 
Ave. Formation rate, V/min 
______________________________________ 
Electrolyte 0.1 mA/cm.sup.2 
1.0 mA/cm.sup.2 
A 1.6 19.8 
A + ADP 2.4 25.8 
B 1.8 19.8 
B + ADP 2.3 25.2 
______________________________________ 
This shows the faster formation rate, and hence decreased charge passage 
for a given voltage, for glycolborate electrolytes containing 0.2 wt% 
ammonium dihydrogen phosphate. 
EXAMPLE 4 
This example shows formation voltage and weight gain (mg/100 cm.sup.2) 
versus time using 20 wt% ammonium pentaborate in ethylene glycol with and 
without 0.2% ammonium dihydrogen phosphate on unetched foil at 25.degree. 
C. and 1 mA/cm.sup.2 current density. 
Table 4 
______________________________________ 
Glycol-borate Glycol-borate-ADP 
t(sec) Voltage Wt gain Voltage 
Wt gain 
______________________________________ 
100 54 0.85 62 1.04 
200 102 1.02 122 1.22 
300 152 2.1* 180 2.56* 
400 199 -- 230 -- 
500 244 -- 251 -- 
dV/dt 
(V/sec) 0.49 -- 0.59 -- 
ave wt gain 
(mg/100 sec) 
-- 0.84 -- 1.03 
______________________________________ 
*at 250 sec 
Thus, the presence of an orthophosphate permited anodization to a higher 
voltage than glycol-borate at the same current density and time. To state 
this another way, the average anodization rate is faster at the same 
current density when the orthophosphate is present so the total charge 
passed is less.