Paul N. Barnes

Effect of precursor calcination temperature on the microstructure and thermoelectric properties of Ca3Co4O9 ceramics

Ca3Co4O9 (CCO) powder precursors were prepared by the chemical sol–gel route and calcined at various temperatures between 923xa0K (CCO-923xa0K) and 1,073xa0K (CCO-1,073xa0K). The calcination temperature was found to be a critical factor affecting the microstructure and thermoelectric properties of CCO ceramic bulk samples. The grain size increases with calcination temperature. The nano-crystals with size about 100xa0nm in the powders calcined at 923xa0K promote large crystal growth and texture development during sintering. Bulk pellets made from CCO-923xa0K powder have large crystal grains, uniform grain size distribution, and a high degree of crystal alignment. By contrast, pellets made from CCO powders at higher calcination temperatures have a bimodal distribution of large and small grains and a large amount of randomly oriented grains. Transmission electron microscopy analysis shows that each crystal grain (identified in SEM images) consists of bundles of CCO nano-lamellas. The nano-lamellas within one bundle share the same c-axis orientation and have fiber texture. The electrical resistivity of CCO-923xa0K is weakly dependent on operating temperature. Compared to the CCO-1,073xa0K sample, the CCO-923xa0K sample has the highest power factor, a lower thermal conductivity, and higher electrical conductivity.

Experiment Setup for Calorimetric Measurements of Losses in HTS Coils Due to AC Current and External Magnetic Fields

We present a design and details of construction of two calorimetric systems that allow us to measure the total loss in high temperature superconducting coils or linear samples carrying alternating current while exposed to a strong alternating magnetic field. This measurement technique is based on the boil-off of liquid nitrogen. The first system is designed to measure ac losses in superconducting coils in self-field generated by AC transport current. The second system contains a permanent magnet rotor and simulates the environment of an electric motor or generator. The sensitivity of the system is such that it can measure low losses from a few milliwatts to several hundred milliwatts, in either a static or dynamic magnetic field.

Thermoelectric Performance Enhancement of Calcium Cobaltite through Barium Grain Boundary Segregation

We report the dramatic increase of the Seebeck coefficient S and thermoelectric performance of calcium cobaltite Ca3Co4O9+δ ceramics through non-stoichiometric addition of minute amount of Ba. The nominal chemistry of polycrystal pellets are Ca3BaxCo4O9+δ (x = 0, 0.01, 0.05, and 0.1). At 323 K, S of Ca3Co4O9+δ is 135 μV K(-1), whereas S of Ba incorporated Ca3Ba0.05Co4O9+δ is 162.5 μV·K(-1), which is the highest S value near room temperature regime reported for calcium cobaltite. The increase of S for Ca3Ba0.05Co4O9+δ sample is accompanied by the decrease of the electrical resistivity ρ, resulting in high power factor S(2)/ρ of 843 μW·m(-1) K(-2) at 1007 K. Moreover, the thermal conductivities κ of Ca3BaxCo4O9+δ decrease with the increase of the Ba addition. The figure-of-merit ZT for Ca3Ba0.05Co4O9+δ reaches 0.52 at 1073 K and a factor of 2.5 increment in comparison with undoped Ca3Co4O9+δ. Nanostructure examinations show that the added Ba segregated at the Ca3Co4O9+δ grain boundaries, while the Ca3Co4O9+δ grain interior is free of Ba. Performance enhancement is attributed to the carrier filtering effect caused by the Ba segregation. In addition, Ba segregation promotes the better crystal alignment and the development of crystal texture.

Superconducting Generators: Enabling Airborne Directed Energy Weapons

Future military weapon systems will rely heavily on both lethal and non-lethal directed energy weapons (DEW), to expand the military’s capabilities to respond to a variety of situations. New systems could feature high power microwaves (HPM), high energy lasers (HEL), or other electromagnetic radiation sources. However, these innovative, complex systems will require significant amounts of reliable electrical power. An application important to the Air Force is that of an airborne high power system. Currently, no multimegawatt-class generator system is flight worthy, keeping possible airborne DEW applications grounded. On the horizon though is a new class of generators— small, compact, and light weight, using superconductor technology. Superconducting generators made of high temperature superconductors (HTS) will enable megawatt-class power systems to take flight. This paper will examine the current state of superconducting generator work in the Air Force and its relation to airborne DEW. Also discussed are new advances in flux pinning and ac losses in YBCO that have greatly increased the conductor’s performance in magnetic fields and will help further the development of HTS generator systems. INTRODUCTION Aircraft electrical generators have been developed and optimized over the years, but these conventional generators cannot provide the high power generation needs in the multimegawatt and 10’s of megawatts range without great size and weight liabilities. There are technologies which can push conventional generators to higher power without dramatic increases in size and weight, but at the same time, efficiency, thermal management, and fatigue life are all sacrificed. Because of this, the Air Force has been researching superconductors for years. A serious look at airborne applications began in the early 1970’s with a program that employed low temperature superconductors (LTS) for the rotor windings. There were many concerns with this design which required cryogenic cooling down to 4.2 K, among these, the integrity of the vacuum and cryogenic system as well as the thermal instability of the LTS material. These concerns led to the development of high-purity “hyperconducting” aluminum. This material is not superconducting, but its electrical resistance is so low at liquid hydrogen temperatures (20 K), it was seen as a possible solution to the problems with LTS at 4.2 K. A 1 MW allcryogenic generator was constructed with this highpurity aluminum that weighed only 100 kg. The next advance was the development of high temperature superconductors (HTS) for use in a generator application. This work was completed by American Superconductor who used Bi2Sr2Ca2Cu3Ox (BSCCO) conductor to create replacement windings for the cryogenic aluminum windings. These windings, when delivered to the prime contractor would create a machine capable of 1 MW of output power and weighed just 90 kg—a 10% savings. However, when the BSCCO windings were placed in the generator, 2 of the coils were destroyed during a welding process by a sub-contractor on the program. Future superconducting generators will be created using what some are calling the Second Generation HTS conductor. The material being developed is YBa2Cu3O7-δ (YBCO). YBCO has a higher current density, greater flux pinning, which yields better performance in magnetic fields, and has a distinct possibility to be up to 5 times cheaper than BSCCO since YBCO coated conductors do not require a silver matrix. Figure 1 below depicts a comparison between BSCCO and YBCO. However, there are still some issues such as the need for biaxially aligned superconducting layer and the low strain tolerance, which is a critical factor when making generator windings. However, possibilities exist to overcome these issues. Figure 1: Comparison of BSCCO and YBCO with respect to current density in applied magnetic field. 1st International Energy Conversion Engineering Conference 17 21 August 2003, Portsmouth, Virginia AIAA 2003-5917 This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. 2 American Institute of Aeronautics and Astronautics Recent advances in BSCCO have further increased its current carrying capability; however, due to its poor performance in magnetic fields, it is relegated to a relatively low operating temperatures to maintain high performace when compared with the HTS material of choice, YBCO. Like BSCCO, YBCO has a critical temperature above that of liquid nitrogen (77 K). BSCCO is a commercial conductor chiefly provided in the U.S. by American Superconductor corporation. It is available in long lengths, with high current density, as well as acceptable mechanical properties. All these factors indicate BSCCO could be used in generator windings in its current technological state, but only at lower temperatures—around 25-30K. The goal is to use YBCO that would only further decrease the size and weight of the system by operating at a higher temperature (60K and higher). An additional reduction would come from in the form of a smaller refrigeration system. AIRBORNE DIRECTED ENERGY WEAPONS The Air Force is considering a variety of energy sources as possible directed energy weapons. However, the power requirements for the new weapons systems far exceed that which is currently available on airborne or mobile platforms. Figure 2 below shows what a “standard” DEW system may look like. Currently, the electrical power generation and thermal management systems need development to meet the required capabilities. HTS generators get to the core of the power generation problem by providing a compact, lightweight, and flight worthy solution. For further discussion, it may be useful to break DEW into three classes which are currently under consideration/development. These classes are based on the electrical output requirements. First, there is the solid-state laser which has a high average radiation output power. An example of this type of weapon is a diode-pumped solid-state high energy laser (HEL). The diodes are the electrical load in the system and will emit infrared radiation which will excite the solid-state laser medium. Another two classes are subsets of high power microwave (HPM) weapons. The first of the HPM classes is one in which the HPM source is required to sustain a long duration burst of radiation. Two possible applications of this technology are large aircraft selfprotect and active denial technology. A final class of DEW is that of pulses of microwave radiation with a high peak power output while the average power remains relatively low. UCAV’s may prove to be a useful platform for these systems. A summary of the classes of DEW can be seen in Figure 3. One of the above technologies that is already in the demonstration phase is the active denial technology (ADT). ADT uses millimeter waves, as as opposed to true microwaves, to heat the skin, causing intense pain without damage. The ADT power sub-system closely resembles that in Figure 2. The goal of ADT is to provide field commanders with a non-lethal option where lethal force may be too excessive. The range of ADT exceeds that of current non-lethal technologies such as rubber bullets as well as small arms fire, keeping its operators out of harm’s way. Ground demonstrations are nearing fruition (Figure 4a) and an airborne system is in the works (Figure 4b) in which a HTS generator will be indispensable due to the confined space requirements, strict weight limitations, and high power needs of the ADT system. Figure 3: System block diagram for a generic electrically powered airborne DEW system. Figure 2: Three classes of electrically powered airborne DEW systems with examples of airborne DEW concepts.

A comparative study of three different chemical vapor deposition techniques of carbon nanotube growth on diamond films

This paper compares between the methods of growing carbon nanotubes (CNTs) on diamond substrates and evaluates the quality of the CNTs and the interfacial strength. One potential application for these materials is a heat sink/spreader for high-power electronic devices. The CNTs and diamond substrates have a significantly higher specific thermal conductivity than traditional heat sink/spreader materials making them good replacement candidates. Only limited research has been performed on these CNT/diamond structures and their suitability of different growth methods.This study investigates three potential chemical vapor deposition (CVD) techniques for growing CNTs on diamond: thermal CVD (T-CVD), microwave plasma-enhanced CVD (MPECVD), and floating catalyst thermal CVD (FCT-CVD). Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (TEM) were used to analyze the morphology and topology of theCNTs. Raman spectroscopy was used to assess the quality of the CNTs by determining the ID/IG peak intensity ratios. Additionally, the CNT/diamond samples were sonicated for qualitative comparisons of the durability of the CNT forests. T-CVD provided the largest diameter tubes, with catalysts residing mainly at the CNT/diamond interface. The MPE-CVD process yielded non uniformdefective CNTs, and FCT-CVD resulted in the smallest diameter CNTs with catalyst particles imbedded throughout the length of the nanotubes.

Investigation of the Relaxation of Persistent Current in Superconducting Closed Loops Made Out of YBCO Coated Conductors

Coated conductors allow the fabrication of closed superconducting loops of arbitrary size. Various mechanisms can play a role in the decay of a persistent current in one such loop and in an assembly of multiple loops magnetically coupled with each other. We report recent experimental results on the relaxation rate of the persistent current in an assembly of closed superconducting loops made out of the currently manufactured coated conductors. One of the main goals of this study is to find the effective ways to control the relaxation rate so as to make it small enough to enable such high temperature persistent magnets to be considered as potential alternatives for energy storage, MRI magnets, and magnetic levitation applications. Here we report the effect of appropriately modified current sweep reversal method on the relaxation rate.