| High energy density capacitors are supposed to be suitable energy sources in pulse power systems for military applications such as electromagnetic launch system, higher power microwaves, electric armor, directed energy laser, etc. In addition, pulse power capacitors are also needed for other applications, such as defibrillators, X-Ray equipment, pulsed lighting, and power storage modules. Since capacitors occupy more than 30% of the overall volume in conventional power converters and pulse power systems, development of capacitors with decreased volume, weight, cost, high reliability and long life is highly desired.During the past decades, scientists have done lots of research on new type energy storage capacitors. Significant progress was obtained on decreasing the volume and shortening the discharging time. However, as a whole, there is no such a material that has high energy density and power density. Ferroelectric materials have high dielectric constant, high power density and good ferroelectric and piezoelectric properties. As the electric field of bulk materials is low, it usually has low energy density. As a result, ferroelectric film materials become new research direction. In recent years, humans have more powerful cry for environment protection. The production and employment of traditional PZT piezoelectric ceramic is restricted, as it contains large amounts of lead. As a result, BaTiO3 thin film becomes the ideal material for ceramic capacitors because it has high dielectric constant, low dielectric loss and high power density.In order to gain BaTiO3 thin film with good crystallinity, the preparation process involves heat treatment, for example, anneal, or high preparation temperature(higher than 500℃) in both physical methods and chemical methods. This is apparently incompatible with the existing CMOS silicon assembly process. Usually, the estimated ultimate temperature of MOS chip is on the range from 450℃ to 500℃. Moreover, improving the breakdown field strength of BaTiO3 thin film is essential for the application on energy storage capacitors. In conclusion, decreasing the preparation temperature to compatible value of CMOS technology, increasing the storage energy of BaTiO3 thin film is of theoretical significance and practical value.In this research, BaTiO3 thin film was prepared by using multiple target radio frequency magnetron sputtering. On one hand, we applied LaNiO3 as buffer layer to obtain highly c-axis oriented BaTiO3 thin films. Then, reduced the preparation temperature (lower than 500 ℃) and we obtained high quality thin film samples. Studied their electric properties. On the other hand, by changing the sputtering time, we obtained BaTiO3 thin films of different thicknesses. Detected the P-V loops and charging and discharging curves, and then calculated the theory energy storage density and actual discharging energy density. In the end, we got BaTiO3 thin films with higher energy storage density. The main research contents are summarized as follows:1. Preparation of high-quality barium titanate thin films under middle temperature(1) We obtained highly c-axis oriented BaTiO3 thin films by applying LaNiO3 as buffer layer to control the orientation. Results showed that the BaTiO3 thin films had fine grains and dense surface. The dielectric tunability of the film was as high as 42%. The piezoelectric d33 value was 150 pm/V.(2) Reduced the preparation temperature of BaTiO3 thin films to 400 ℃,350℃. The obtained films had decreased crystallinity and good dielectric property. The dielectric constant of BaTiO3 films deposited at 350 ℃ can keep around 110 during the electric field from 0 to 889 kV/cm. The whole preparation process was more compatible with CMOS technology.2. Preparation of high energy storage density BaTiO3 thin film capacitors:(1) Changed the sputtering time to 1hour,2hours and 6hours. The BaTiO3 thin films had good crystalline character and great electrical performance.(2) Measure the hysteresis loops of the BaTiO3 thin films and calculated the theoretical energy storage density. Tested the actual discharging energy density by charging and discharging system. Results showed that the efficiency was around 77%. The efficiency of energy storage density changed from 80% to 93% during 170℃ and 300℃.(3) We can control the discharge time by adjusting the value of load resistance in the charging and discharging system. The shortest discharging time can reach to microsecond level. We can test 10,000 cycles and even 100,000 cycles of charging and discharging which means our BaTiO3 thin films have long charging and discharging life. |
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