Technical Capabilities

High Power Capacitor Development

The main advantage of capacitors over batteries and inductors is high power output, useful for pulsed-power, load leveling, and power electronic applications. New dielectric materials and capacitors are under development for hybrid vehicle applications. Large capacitors are being explored as an energy storage source in automobiles. In addition, dielectric materials and capacitor technology are being developed for power-electronic applications. The number of applications for high-power capacitors is expected to grow substantially as the need for portable electric energy increases.

Ultracapacitor Development for High-Power Energy Storage

The Department of Energy is engaged in a program to develop a cost-effective electric vehicle with sufficient power and range to meet our nation's demand for an emission-free consumer automobile. One of the key system elements for this vehicle is an ultracapacitor, which provides high specific power for vehicle acceleration. High energy-storage densities are possible in chemical double layer capacitors. Through the DOE- and NSF-sponsored Graduate Automotive Technology Program (GATE), students will study the fundamental properties of double layer capacitors, model capacitor performance, and test prototype capacitors in a hybrid vehicle.

Figure. 1. A commercial ultracapacitor (Econd 180 Volts, 3.2 Farad)

Dielectric Materials Development for Power-Electronic Applications

A significant effort within the Department of Energy's Office of Transportation Technologies and the U.S. Navy's Advanced Electronic Power Systems (AEPS) program has focused on reducing the size and weight of power electronic devices for electric and hybrid vehicles and ships. Power electronic circuits, which are composed of active switching elements and passive components such as capacitors and inductors, provide motor control, power distribution, and DC/AC conversion functions in electric vehicles.

Progress has been made on reducing the size and weight of active power electronic components such as MOS-controlled thyristors and insulated-gate bipolar transistors. Additional effort on high power capacitors will be needed for load leveling and filter functions. As a subsystem component of electric and hybrid vehicles, the size, cost, and reliability of the high-power capacitor must be addressed. Significant volume savings can be realized if high-dielectric-constant perovskite materials can be substituted for oxides with lower dielectric constants. The objective of this program is to develop new capacitor technologies for integration into power electronic modules. The understanding and optimization of high dielectric constant ferroelectric materials form the central themes of this program. The research and development efforts are highly focused on developing a range of new practical and economical capacitor technologies.

Electrical Measurements

CDS has a comprehensive electrical measurement facility for dielectric materials. Routine automated, multi-sample dielectric tests are carried out at temperatures from that of liquid nitrogen to 200C and for frequencies from 100 Hz to 10 MHz. CDS personnel have written custom data analysis programs to extract dielectric properties from the electrical test data.

Specialized microwave and RF spectrum analyzers available to CDS can measure up to 26 GHz, while a low frequency FFT dielectric spectrometer is capable of measuring to 1.0 mHz. Multi-sample dielectric reliability tests are set up for computer-automated data collection. CDS personnel also designed high-temperature dielectric measuring equipment that operates at temperatures up to 1500K and cryogenic systems down to 10K. A home-built system for quantification of current discharge has been developed to access the energy storage capabilities of materials and multilayer devices.

Other electrical properties can be measured in our laboratory, including pyroelectric current, polarization, hysteresis, piezoelectric coefficients, and electrostrictive strain. We are in the process of expanding our electrical characterization facilities, with conductivity measurements at low loss, new HALT tests, and admittance spectroscopy.

Figure 2. Testing the response time of multilayer ceramic capacitors for power-electronics

Powder Processing. Many synthesis techniques are available to produce micron- to nano-sized particulates of complex chemistries. CDS faculty have a vast knowledge base in solid state calcination with size reduction through ball, vibratory, and attrition molding methods. Sol-gel, coprecipitation, and hydrothermal synthesis are also readily available. There is expertise in dispersion and dissolution. Software packages have been developed to determine both interparticular forces and equilibrium solution chemistry. CDS faculty have developed passivation and chemical coating methods for advance stoichiometry and mixing methods. CDS faculty and company members have access to the particulate characterization laboratory, which has facilities for zeta potential, size distributions, surface area, etc.

Figure 3. An amorphous layer (10 nm) created on
the surface of a BaTiO particle as a result
of incongruent dissolution of Ba into an
aqueous solution at pH=7.0.

Thick film pastes and inks can be manufactured with a three-roll mill. Multilayer capabilities include tape casting, screen printing, and multilayer lamination. Thin/thick films ~0.1 µm to 10 µm can be deposited by electrophoretic deposition in the CDS. Firing of hybrid microelectronic circuits can be done in production sized infrared or resistance heated belt furnace.

For sintering research, CDS has a computer-controlled furnace room with controlled atmosphere firing capabilities and a custom-designed fast fire furnace. A novel rate-controlled sintering system has been designed to establish densification conditions that minimize the transient sintering stresses. This has been successfully demonstrated for integrated passive components. To minimize damage in debinding multilayer structures, a smart binder burnout TGA system has been developed with controlled atmosphere and heating rates.

Thin Film Facilities. Thin film deposition techniques include sol-gel, laser ablation, multilayer magnetron, multi-ion beam reaction sputtering, mist deposition, and molecular beam epitaxies. CDS faculty have experience in the deposition of wide variety of compositions including:

BaTiO₃, (Ba,Sr)TiO₃, SrTiO₃, Pb(Mg_1/3Nb_2/3)O₃,Pb(Zr,Ti)O₃, PbZrO₃, PbTiO₃, YBa₂Cu₃O₇, SrRuO₃, and RuO₂.

Figure 4. Pulse Laser Deposition System - for oxide epitaxial and polycrystalline thin films.

Modeling Capabilities

CDS researchers use electromagnetic modeling to study fundamental phenomena of wave propagation in dielectric materials and dielectric-based devices, as well as for computer aided device design. For this purposes, a powerful electromagnetic field solver based on the Finite Difference Time Domain (FDTD) method is employed, which is one of the most versatile computational techniques presently available. As a time-domain solver of the Maxwell Equations, this method presents a space/time microscope, permitting the designer to visualize, with submicron/subpicosecond resolution, the dynamics of electromagnetic wave propagation. The FDTD technique is well suited for handling complex device configurations because it can conveniently model various inhomogenities encountered in the structure. Simulation analysis is being applied to a wide range of devices, such as multilayer capacitors, microstrip resonators, bandpass filters, patch antennas, leaky wave antennas and dielectric resonator antennas. FDTD simulations are shown to be very efficient for modeling wave processes in dual mode devices and in frequency selective surfaces made of dielectric matrixes with embedded inclusions of different permittivity or conductivity. Such phenomena, as power leakage into space from microstrip lines and surface wave leakage into dielectric substrates that strongly affect microwave devices performance are also being studied. .

Figure 5. 3D pattern of electric field standing wave
at TM241 resonance in rectangular dielectric
resonator simulated using the FDTD method..