2024-12-23
Philip Lessner, James Q. Chen, and Hideaki Sato
This post is based on a presentation given at the Tantalum and Niobium International Study Center (T.I.C) 65th General Assembly in Tokyo, Japan, September 2024. This first part gives a gentle introduction to the working principles of the Tantalum capacitor and covers the invention of the solid Tantalum capacitor in the 1950s and its initial commercialization in the early 1960s.
Introduction
Tantalum capacitors make up one of the five main capacitor dielectric types used in modern electronic systems. The use of tantalum capacitors goes back nearly one hundred years, but since the invention of the solid tantalum capacitor at the beginning of the transistor age, the use of tantalum capacitors in electronics has grown. This is the first in a series of four blog posts that trace the development and application of tantalum capacitors since that invention. Since tantalum capacitors are still under development for future applications, the final post will discuss some of the future directions and challenges for tantalum capacitors.
The Tantalum Capacitor
The use of tantalum capacitors goes back over 90 years when wet tantalum capacitors (Figure 1) were introduced by Fansteel and Tansitor. The tantalum capacitor is a polar device with tantalum metal as the positive electrode (anode) and a wet electrolyte or a solid semiconductor as the negative electrode (cathode) and tantalum pentoxide (Ta2O5) as the dielectric.
Figure 1: Wet Tantalum Capacitor Construction
The wet tantalum capacitor consists of a tantalum anode fabricated either from an etched tantalum foil or sintered tantalum powder to which a tantalum lead wire is attached for the anode contact. A dielectric film of tantalum pentoxide (Ta2O5) is formed on the anode electrochemically from an ionic solution (typically dilute phosphoric acid). The electrolyte is usually sulfuric acid with some additives to increase its viscosity. The cathode can be a high surface area tantalum powder or a noble metal compound such as Pt or RuO2. These are hermetically sealed in a can to which a wire is attached for the cathode contact. In the past, silver cans were used but today the can is tantalum (or sometimes titanium). When they were introduced, wet tantalum capacitors were replacements for aluminum electrolytic capacitors in some demanding applications because of their smaller size and higher reliability. Wet tantalum capacitors are capable of high bulk capacitance and high voltage (125V) operation, but they suffer from high Equivalent Series Resistance (ESR), high dependence of capacitance and ESR on temperature, and are not very volumetrically efficient because of the hardware needed to hermetically seal the capacitor. However, they are still in use today primarily in military and medical applications.
The solid tantalum capacitor (Figure 2) was developed in the mid-1950’s.
Figure 2: Solid Tantalum Capacitor with Manganese Dioxide Cathode
The wet electrolyte is replaced with a conductive manganese dioxide (MnO2) solid cathode (resistivity of about 0.1-1Ω-cm) and contact of the MnO2 cathode is made via a graphite (carbon) and silver layers. The purpose of graphite is to prevent reaction between the MnO2 and silver layers. In the thru-hole hermetic seal version shown in Figure 2, the silver is soldered to a (brass) can and a cathode lead is attached to the can. We’ll see that later developments replaced the solder, can, etc. with thinner layers that made the overall capacitor more volumetrically efficient.
The tantalum capacitor shares the overall capacitor market with several other capacitor types. Other types having significant market share include Film, Multilayer Ceramic (MLCC), Aluminum Electrolytic, and Super (or Ultra) Capacitors. The principal reason to choose a tantalum capacitor is the high capacitance times voltage product (CV, measured in coulombs) that can be realized in a small package size, stable capacitance with voltage and temperature, and high reliability. Equation 1 shows the fundamental relationship between CV and the properties of the Ta2O5 dielectric:
[1]
where K is the relative dielectric constant of Ta2O5, 𝜖0 is the permittivity of free space, A is the area, d is the dielectric thickness, C is the capacitance, and V is the rated voltage. K for Ta is about 27 which is significantly higher than for the Al2O3 dielectric in the aluminum electrolytic capacitor (about 8 to 10) or plastic dielectrics in film capacitors (about 2.5), but much less than the barium titanate dielectrics used in Class II MLCCs which can exceed 1000.
The principal way that high capacitance is achieved in a tantalum capacitor is through the area or A term in Equation 1. Figure 3 show a scanning electron micrograph (SEM) of a capacitor grade tantalum powder. The primary particle size of the powder is less than 1µm and surface area of the powders used in recently introduced tantalum capacitors exceed 2m²/g which gives the tantalum capacitors their high capacitance.
Figure 3: High Surface Area Capacitor Grade Tantalum Powder
For a capacitor to function it also needs to be reliable when energized in a circuit. The Ta2O5 dielectric is an amorphous dielectric capable of withstanding a high electric field (the V/d term in Equation 1). The theoretical breakdown voltage of the Ta2O5 dielectric is about 550 V/µm which is comparable to Al2O3 and plastic film dielectrics and much higher than that crystalline ceramics like barium titanate. In practice, the dielectric strength is usually derated about 2 to 4x to achieve the rated voltage of the capacitor.
Regarding reliability, we’ll return to this topic when we examine the invention and development of the solid tantalum capacitor in the next section as reliability is intimately related to the choice of the cathode semiconductor material.
Invention of the Solid Tantalum Capacitor
The invention of the solid tantalum capacitor with the manganese dioxide cathode has an interesting history as it was invented by two different groups at about the same time. One group was led by Dr. Preston Robinson (Figure 4) at Sprague Electric located in the town of North Adams in western Massachusetts, USA. In US Patent 3,066,2471 (applied for in 1954 and a continuation in part of a 1951 application) he describes a tantalum capacitor with a lead dioxide cathode, but the claim is to a more general use of solid semiconductive material. The key property of this semiconductive material (Claim 1) is “said semiconductive material being reducible in the presence of high fields, said film-forming metal being oxidizable upon reduction of said semiconductor material to heal imperfections in said dielectric film…”.
Figure 4: Dr. Preston Robinson
This “self-healing mechanism” of certain metal oxides is one of the keys to the reliability of the solid tantalum capacitor. No dielectric is perfect: tantalum pentoxide will contain imperfections due to impurities (metals, carbon, etc.), crystalline inclusions, oxygen vacancies, and cracks due to manufacturing stresses. Figure 5 shows one of the healing mechanisms. When the semiconductive MnO2 is heated it transforms to a non-conductive lower oxide (Mn2O3) which forms a non-conductive ‘plug’ at the defect in the dielectric at the defect site. The heating to effect this transformation is provided by the leakage current flowing through the small area of the defect site. When the transformation to the lower manganese oxide occurs, oxygen can also be donated to the dielectric which can neutralize defects such as oxygen vacancies.
Figure 5: Self-Healing Mechanism in Solid Tantalum Capacitors
R.J. Millard of Sprague Electric in US Patent 2,936,5142 described the use of MnO2 in the solid tantalum capacitor. Another key property of MnO2 is that it can be impregnated into the pores of the sintered tantalum anode (Figure 6) via the decomposition of aqueous manganous nitrate at around 300°C:
Mn(NO3)2 → MnO2 + 2NO2 [2]
This impregnation with an aqueous solution of the nitrate salt followed by heating to convert to oxide allows the entire surface of the dielectric to be covered with the cathode ensuring that the ‘A’ term in Equation 1 is fully utilized.
Figure 6: Internal Solid Tantalum Capacitor Construction
At the same time the solid tantalum capacitor was being invented at Sprague Electric, it was also being invented at Bell Telephone Laboratories. The four developers were H.E. Haring, R.L. Taylor, D.A. McLean, and F. Power (Figure 7).
Figure 7: Responsible for the new solid state 'electrolytic capacitor' announced by Bell Telephone Laboratories are, rear: H.E. Haring and R. L. Taylor, seated: D.A. McLean and Mrs. Florence Power
The Bell Labs patent3 (applied for in 1953) like the Sprague patents describes a solid tantalum capacitor with a semiconductive metal oxide (MnO2). It also proposes that ‘reforming’ the dielectric in an electrolyte in-between manganese nitrate impregnation-conversion steps can be done to lower the leakage current and improve the reliability of the capacitor.
Sprague and Bell Labs both asserted to be inventors of the solid tantalum capacitor and while the patents applications were submitted in the mid-1950’s, by the early 1960’s the patents still hadn’t been issued due to the dispute of priority of inventorship. By then the tantalum capacitor market had grown to $29M and other companies, like KEMET (then a division of Union Carbide) were entering the market (Figure 8).
Figure 8: KEMET's Entry in the Solid Tantalum Capacitor Market
In April 1964, Bell Laboratories acknowledged that Sprague Electric’s patents had priority giving Sprague the patent rights to the growing solid tantalum capacitor market. The market began to grow rapidly as solid tantalum capacitors found immediate applications in defense, aerospace, and commercial markets. This led to rapid advances in the performance of these capacitors which we'll explore in the next blog post.
References
1 P. Robinson, “Electrical Capacitors,” US Patent 3,066,247 (November 27, 1962).
2 R.J. Millard, “Electrolytic Device”, US Patent 2,936,514 (May 17, 1960).
3 H.E. Haring and R.L. Taylor, “Dry Electrolytic Device”, US Patent 3,166,693 (January 19, 1965).