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Über dieses Buch

This book provides a comprehensive analysis of the science, technology, and applications of Tantalum and Niobium-based capacitors. The author discusses fundamentals, focusing on thermodynamic stability, major degradation processes and conduction mechanisms in the basic structure of Me-Me2O5-cathode (Me: Ta, Nb). Technology-related coverage includes chapters technology chapters on the major manufacturing steps from capacitor grade powder to the testing of finished capacitors. Applications discussed include high reliability, high charge and energy efficiency, high working voltages, high temperatures, etc. The links between the scientific foundation, breakthrough technologies and outstanding performance and reliability of the capacitors are demonstrated. The theoretical models discussed include the thermodynamics of the amorphous dielectrics, conduction mechanisms in metal-insulator-semiconductor (MIS) structures, band diagrams of the organic semiconductors, etc.

Inhaltsverzeichnis

Frontmatter

Chapter 1. Major Degradation Mechanisms

The basic bilayer of all types of tantalum capacitors, tantalum anode, and anodic oxide film of tantalum as a dielectric is not a thermodynamically stable system. This is demonstrated by the tantalum-oxygen equilibrium diagram that does not contain two-phase equilibrium areas for pure tantalum and tantalum pentoxide]. Relaxation of the Ta-Ta2O5 system into the thermodynamically stable state occurs through oxygen migration from Ta2O5 to Ta, resulting in oxygen vacancies in the tantalum oxide dielectric. Conductivity of the dielectric and, thereby, DCL of tantalum capacitor increase exponentially with the concentration x of oxygen vacancies in the depleted with oxygen Ta2O5–x. Another reason for the thermodynamic instability of tantalum capacitors is the amorphous structure of the anodic oxide film of tantalum formed on crystalline tantalum. Amorphous dielectrics trend to ordering and crystallization spontaneously to reduce their internal energy. Growth of crystalline inclusions in amorphous matrix of the anodic oxide film induces mechanical stress in the film, which results in a disruption of the dielectric and, thereby, in the failure of the capacitor.
Yuri Freeman

Chapter 2. Basic Technology

The manufacturing of tantalum and niobium-based capacitors begins with the pressing and sintering of tantalum, niobium, or NbO anodes and ends with the testing and packaging of the finished capacitors. The major stages of the manufacturing are the making of porous anodes, growing of either Ta2O5 or Nb2O5 dielectric on the surface of porous anodes, forming of either liquid electrolyte, MnO2, or conductive polymer cathode, coating with external layers of carbon and silver in solid capacitors, and testing and packaging of finished capacitors.
Yuri Freeman

Chapter 3. Applications

The results presented in this chapter are dedicated to the specific charge and energy of tantalum capacitors as a function of formation voltage with emphasis on losses at high and low formation voltages; de-rating and high-reliability approach for all types of tantalum capacitors; high working voltage, asymmetric conduction, and anomalous currents with emphasis on polymer tantalum capacitors; and high-temperature applications of MnO2 tantalum capacitors.
Yuri Freeman

Chapter 4. Conclusion

Tantalum capacitors have been on the market for three-quarters of the century. The major advantage of tantalum capacitors in comparison to other major types of capacitors, such as multilayer ceramic capacitors, film capacitors, and aluminum electrolytic capacitors, is their record high volumetric charge and energy efficiency. The high efficiency is provided by the large surface area per unit of volume of tantalum anodes sintered with fine tantalum powder and small submicron thickness of the anodic oxide film of tantalum employed as a dielectric in tantalum capacitors. The other advantage of tantalum capacitors in comparison to other types of the capacitors is capacitance stability with voltage and temperature. There is practically no change in capacitance measured with and without DC bias and small variation in capacitance within normal range of operating temperatures. Besides that, there is no aging of tantalum capacitors; their capacitance remains stable during any practical duration of the testing and field application.
Yuri Freeman

Backmatter

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