Abstract:
A low ESR tantalum foil electrolytic capacitor uses a 30-40 wt % sulfuric acid electrolyte and a porous, synthetic spacer material.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to tantalum foil electrolytic capacitors with low equivalent series resistance (ESR). More particularly, it relates to low voltage units which use a strong-acid electrolyte and a spacer fabricated from man-made materials. 
     Tantalum foil electrolytic capacitors offer the advantages over aluminum electrolytics of a high capacitance to volume ratio, operation over an extended temperature range of about -55° C. to 125° C. with a minimum change in capacitance and resistance, and lower equivalent series resistance through the use of lower resistivity electrolytes that are non-corrosive to tantalum. However, tantalum is more expensive than aluminum; thus, a way of reducing the cost of tantalum capacitors would be distinctly advantageous. Current tantalum foil capacitors rated up to 99 VDC utilize kraft paper spacers impregnated with non-strong-acid electrolytes, e.g., ammonium nitrate in DMF or a lithium chloride electrolyte. 
     SUMMARY OF THE INVENTION 
     This invention provides a tantalum foil capacitor with a low equivalent series resistance. 
     The invention also provides a means of reducing foil requirements for a given rating, thus reducing unit size, or of providing units with higher ratings for a given amount of foil (or unit size). 
     It also provides a tantalum foil capacitor with a lower capacity loss at -55° C. than that of standard units. 
     These features are realized by replacing the kraft paper by other spacer materials resistant to strong acids, e.g., sulfuric acid, permitting the use of strong acids, that have low resistivity, as electrolytes. Such acids are not compatible with kraft paper and include sulfuric, nitric, trifluoroacetic, chlorosulfonic, etc., and may be mixed with water to give the desired resistivity and operation over an extended temperature range. 
     Not only is the equivalent series resistance of the capacitors improved, but unexpectedly both a capacity gain is realized and the low-temperature properties are improved. The unexpected gain in capacity provides a means of reducing tantalum foil requirements, and hence capacitor size, for a given rating, or, alternately, providing a capacitor with a higher rating for the same size unit. 
     The case material must, of course, be resistant to the strong acid of the electrolyte. Stainless steel or tantalum tubes with glass to metal hermetic seals have been used satisfactorily, particularly with the 40% sulfuric acid electrolyte. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Referring to the drawing, a capacitor section 10 is shown in a partly unrolled condition. Anode 11 is an etched tantalum foil having an insulating oxide on its surface. Cathode 12 is also a tantalum foil and is preferably etched and has a thin oxide film on its surface for low voltage reversal. Films 13 and 14 are spacers made of porous synthetic material inert to the strong acid electrolyte (not shown). Tabs or wires 15 and 16 are connected to electrodes 11 and 12, respectively, to function as terminals for capacitor section 10 when it is fully assembled. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Low ESR capacitors are preferably made using a woven glass cloth or Celgard 3500 microporous polypropylene film, manufactured by Celanese Corp., as these are not attacked by the electrolyte. Other materials which can be used include porous woven polyolefin cloth, porous perfluoroethylene cloth, polyolefin spun bonded fiber paper, and polypropylene screening. While strong acids such as chlorosulfonic, nitric, trifluoracetic acid, etc., may be used, 30-40 wt % sulfuric acid is preferred, and 40 wt % sulfuric acid most preferred. Although minimum resistivity occurs at 30 wt % concentration for sulfuric acid, the lowest freezing point for aqueous sulfuric acid occurs at 38.58 wt %, namely at -68° C. Since sulfuric, as well as the other strong acids, is hygroscopic, an initial 40 wt % solution is used as both minimums lie below that concentration and any water absorbed will shift the concentration toward those minimums. 
     In the tables below, average life test data are presented for tantalum foil units using 40 wt % sulfuric acid and the following spacers: 1 mil Celgard 3500 microporous polypropylene film, 3 mil glass cloth, and 2 layers of 0.5 mil kraft paper (control). Capacitance is in microfarads and equivalent series resistance (ESR) is in ohms. 
     
                                           TABLE 1__________________________________________________________________________(120 Hz at +85° C.)0 hr       168 hr            500 hr                  1000 hr                        2000 hrSpacerCap   ESR      Cap         ESR            Cap               ESR                  Cap                     ESR                        Cap                           ESR__________________________________________________________________________Celgard46.59   2.60      46.52         2.39            45.72               2.45                  45.43                     2.61                        45.19                           2.73Glass42.19   4.65      42.73         3.41            42.67               3.37                  42.59                     3.30                        41.85                           3.35Kraft38.30   4.51      37.92         4.10            37.38               4.21                  37.00                     4.30                        35.71                           4.81__________________________________________________________________________ 
    
     
                                           TABLE 2__________________________________________________________________________(120 Hz at +125° C.)0 hr       168 hr            500 hr                  1000 hr                        2000 hrSpacerCap   ESR      Cap         ESR            Cap               ESR                  Cap                     ESR                        Cap                           ESR__________________________________________________________________________Celgard43.98   2.59      -- -- 44.62               2.62                  41.09                     3.10                        -- --Glass41.93   4.64      42.01         3.51            41.59               3.69                  41.19                     3.78                        40.83                           4.74Kraft37.54   4.66      36.19         4.71            35.61               5.64                  35.11                     5.64                        33.35                           8.98__________________________________________________________________________  Table 3 below shows room temperature capacitance (μF.) and that at -55° C. for the above capacitors at 120 Hz and 1 kHz. 
    
     
                       TABLE 3______________________________________                       % Change                       (Room Temp/Room Temperature           -55° C.                       -55° C.)120 Hz     1 kHz    120 Hz  1 kHz 120 Hz                                   1 kHz______________________________________Celgard  46.69   41.52    38.10 35.02 -18.4 -15.7Glass  42.44   33.76    --    --    --    --Kraft  38.08   32.75    21.31  9.80 -44.1 -70.3______________________________________