Generation of amorphous ceramic capacitor coatings on titanium using a continuous sol-gel process
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Thin amorphous films of ceramic capacitor materials were successfully deposited using sol-gel chemistry onto titanium wire using a continuous, computer-controlled process. By repeatedly depositing and calcining very thin layers of material, smooth and even coats can be produced. Surface analyses revealed the layered nature of these thin coats, as well as the amorphous nature of the ceramic. The electrical properties of the better coatings, all composed of niobrium, bismuth, and zinc oxides, were then evaluated.
I. INTRODUCTION Recently we addressed the problem of mass producing high-energy-density ceramic capacitors with dielectric uniformity for use in laser systems. The requirements for high-energy lasers are well beyond the potential of what is currently available. The currents involved at the higher pulse rates are on the order of 100 kA. Thus resistive losses produce very large heat loads. This heat must be removed since most designs are limited by the life of the polymer coating that encapsulates the capacitor. For this reason, maximum surface design temperature is typically +85 °C, although the dielectric materials can withstand temperatures much higher than this. Capacitor design is a mature technology. Improvements in the performance of the size needed can only come from improvements in ceramic formulation and manufacturing technology. What is needed is the difficult combination of much higher dielectric strength to keep the dielectric thickness down, higher dielectric constant to reduce the surface area needed to store 20 J at 50 kV, and very low dissipation factor (Q greater than 1000 at + 80 °C) to keep heat generation low. The properties of primary concern can be realized by using Bi compounds with chemical formulae of the form Bi 2 (ZnNb 2 )O 9 (BZN) and Bi 3 (Ni 2 Nb)O 9 (BNN). Scientists at Bell Laboratories have extensively studied1 this family and recently demonstrated that some family members have Q's of over 10,000, a polycrystalline dielectric constant of over 150, and processing temperatures of about 900 °C. Another plus is that the precise temperature coefficient can be controlled via compositional changes so that capacitance is insensitive to operating temperature over wide ranges of temperature. BZN and BNN have to date been synthesized by conventional powder processing techniques. This leads after processing at 1125 °C to a well-developed
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J. Mater. Res., Vol. 10, No. 10, Oct 1995
http://journals.cambridge.org
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crystal structure of cubic pyrochlore. This work reported here attempted to synthesize BZN, BNN, and mixtures thereof using sol-gel techniques in order to produce an amorphous structure to achieve very high dielectric strength. Synthesis of niobium pentoxide gels has been studied by Alquier et al.2 Niobium (V) compounds are extremely reactive and hydrolyze and polycondense at much higher rates than do bismuth, nickel, and zinc. This can lead to chemical heterogeneities in the final ceramic product.
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