The present invention relates generally to high temperature superconducting and more particularly to a method of depositing nanoparticles for flux pinning into a superconducting material.
The desirability of providing efficient high temperature superconductors for operation at 20xc2x0 K. and higher is well known. Indeed, there has been an enormous amount of experimental activity in these so called high temperature superconductors since research in the mid 1980s first demonstrated dramatic gains in raising the maximum critical transition temperatures from the 20xc2x0 K. range to the 90xc2x0 K. range.
In general, superconductors and superconducting material exhibit zero resistance when operating at temperatures below their maximum critical transition temperature. This quality of operating at zero resistance facilitates the construction and operation of highly efficient devices such as superconducting magnets, magnetic levitators, propulsion motors and magnetohydronamics, power generators, particle accelerators, microwave and infrared detectors, etc.
In addition to the quality of operating at zero resistance, superconductors display other, unusual characteristics. For example, a surprising effect of superconductivity is that magnetic flux is expelled from superconductors. This is commonly known as the Meissner or Meissner-Ochsenfeld effect. Other unexpected phenomena include current flow via electron pairs rather than individual electrons and large scale quantum behaviors such as flux quantization and flux tubes.
According to current accepted nomenclature, superconducting materials fall within two broad categories, Type I and Type II. The Type I materials are pure metallic elements. Type II materials are alloys or compounds and are characterized by their ability to retain superconductive attributes in the presence of applied magnetic fields. Unlike Type I materials, Type II materials tolerate some degree of applied magnetic flux intrusion into their interiors without destroying the superconducting state. More specifically, as applied flux and/or temperature increase, microscopic flux tubes or fluxons begin to develop within the material. In this mixed state of superconductivity, the material within the flux tubes is in a normal state of resistivity. The material surrounding the flux tubes remains in a state of superconductivity. Since the Type II materials will support mixed superconductivity at elevated temperatures and applied magnetic field intensities, the Type II materials seem to be the best candidates for commercial application and further development.
Under equilibrium conditions, magnetic flux penetrates the bulk of a type II superconductor above the lower critical field. Over most of the available magnetic field-temperature (H-T) space, H greater than Hc1, this magnetic flux exists as a lattice of quantized line vortices or fluxons. Each fluxon is a tube in which superconducting screening currents circulate around a small non-superconducting core. Bulk superconductivity is destroyed when the normal cores overlap at the upper critical field. In isotropic materials such as Nb-Ti and Nb3Sn, vortex lines are continuous, but the weak superconductivity of the blocking layers of High Temperature Superconducting (HTS) compounds produces a stack of weakly coupled xe2x80x98pancakexe2x80x99 vortices whose circulating screening currents are mostly confined within the superconducting CuO2 planes. Superconductors can carry bulk current density only if there is a macroscopic fluxon density gradient. This gradient can be sustained only by pinning the vortices (flux pinning) at microstructural defects. Increasing T and H weaken the potential wells at which vortices are pinned. Flux pinning is determined by spatial perturbations of the free energy of the vortex lines due to local interactions of their normal cores and screening currents with these microstructural imperfections. The critical current density Jc (T, H) is then defined by the balance of the pinning and Lorentz forces. Ideally, a type II superconductor can carry any non-dissipative current density J smaller than Jc. When J exceeds Jc, a superconductor switches into a dissipative, vortex-flow state driven by the Lorentz force.
Various attempts have been made to stabilize or pin the flux vortices. Introducing impurities or defects into the superconducting material is a known way to provide flux pinning. Such flux pins can be holes, nanotubes, particles, grain boundaries or other defects intentionally introduced into the superconducting material.
While the known methods of introducing defects into the superconducting material for flux pinning have achieved some degree of success in operation, these methods usually require an additional processing station, extra processing equipment and/or require the preincorporation of foreign material during processing, potentially inhibiting the proper growth of the superconducting material. As such, these methods are not without the need for improvement.
A need exists therefore for an improved method of depositing nanoparticles for flux pinning into a superconducting material. Such a method would be simple in nature, capable of in-situ completion, and not introduce foreign material or otherwise inhibit the operation of the superconducting material.
Accordingly, it is a primary object of the present invention to provide a method of depositing nanoparticles for flux pinning into a superconducting material.
Another object of the present invention is to provide a method of depositing nanoparticles for flux pinning into a superconducting material that utilizes pulsed laser deposition.
Still another object of the present invention is to provide a method of depositing nanoparticles for flux pinning into a superconducting material that is simple to complete and can be performed in-situ in conjunction with the pulsed laser deposition process for depositing the superconducting material upon the substrate.
It is yet another object of the present invention to provide a method of depositing nanoparticles for flux pinning into a superconducting material that provides for enhanced superconductor operation by introducing flux pin defects into the superconducting material without introducing foreign material.
It is still another object of the present invention to provide a method of depositing nanoparticles for flux pinning into a superconducting material that is simple to perform.
These and other objects of the invention will become apparent as the description of the representative embodiments proceeds.
In accordance with the foregoing principles and objects of the invention, a method of depositing nanoparticles for flux pinning into a superconducting material is described.
In the preferred embodiment, the method of the present invention is performed using the pulsed laser deposition method within a corresponding deposition chamber. Pulsed laser deposition systems are known to those skilled in the art and are commercially available. The method of the present invention utilizes a laser operated in a pulsed mode to irradiate a target. The interaction of the laser and the target causes ejection of atomic and molecular and ionic species from the target. The material expelled by the target is collected on a substrate. Pulsed laser deposition processes are well known in the art and according to the standard practice, the typical deposition process is conducted in a relatively high vacuum, 100-900 mTorr of O2, for example. The high vacuum level is considered necessary for the deposition of a uniform layer of material on the substrate. In this application, O2 is used for YBCO to enhance oxygenation.
According to an important aspect of the present invention, the irradiation of the target is first conducted at a much more moderate vacuum level, in the range of 1-10 Torr of O2, contrary to current, standard practice for HTS deposition. This has the effect of slowing down the ejected material by collisions with the background gas, allowing the ejected atoms and molecules to recombine in the gas phase. This recombination produces nanoparticles of the target material which are then collected on the substrate. The irradiation continues for a time sufficient to collect the desired layer of nanoparticles on the substrate.
After the desired layer of nanoparticles is deposited on the substrate, the pressure in the deposition chamber is, in the range, for example of 100-900 mTorr of O2. The target is again irradiated by the pulsed laser according to the current standard pulsed laser deposition process. These two steps can be repeated as desired to form any number of layers of nanoparticles covered by a uniform layer of material. Advantageously, the method of the present invention can be utilized on all Type II superconducting materials amenable to the pulsed laser deposition process.