Patent Application
20060237692
Not Applicable
Not Applicable
A portion of the material in this patent document is subject to copyright protection under the copyright laws of the United States and of other countries. The owner of the copyright rights has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the United States Patent and Trademark Office publicly available file or records, but otherwise reserves all copyright rights whatsoever. The copyright owner does not hereby waive any of its rights to have this patent document maintained in secrecy, including without limitation its rights pursuant to 37 C.F.R. § 1.14.
1. Field of the Invention
This invention pertains generally to borocarbide synthesis, and more particularly to methods of synthesizing LiBC and Li1-x BC.
2. Description of Related Art
Layered diboride compounds have been the focus of intense interest since the discovery of superconductivity in MgB2 (see J. Nagamatsu, N. Nakagawa, T. Muranaka, and J. Akimitsu, Nature 410, 63 (2001), incorporated by reference herein). A derived class of materials is the similarly layered borocarbides. Of particular interest are the Li1-xBC compounds where high temperature superconductivity has been predicted for high hole-doping ranges (see, H. Rosner, A. Kitaigorodsky, and W. E. Pickett, Phys. Rev. Lett. 88, 127001 (2002), incorporated herein by reference).
To explore any superconducting behavior in the Li1-xBC compounds, it is essential to achieve clean phases. This is challenging since lithium can be combined with practically every element on the periodic table to form thermodynamically stable compounds (except with the other alkali metals) due to its high electron affinity and strong polarizing power (see, R. Nesper, Prog. Solid. St. Chem. 20, 1 (1990), incorporated herein by reference). The presence of impurity phases, as noted in the early study (see, M. Wörle, R. Nesper, G. Mair, M. Schwarz, and H. G. von Schnering, Z. Anorg. Allg. Chem. 621, 1153 (1995), incorporated herein by reference) can greatly complicate the investigation. Complications increase dramatically in response to magnetism. Another potential complication is the reported weak ferromagnetism in related CaB2C2 (see, J. Akimitsu, K. Takenawa, K. Suzuki, H. Harima, and Y. Kuramoto, Science 293, 1125 (2001), incorporated herein by reference) that could mask or destroy superconductivity. A second key issue is the structural stability of LiBC upon Li de-intercalation, especially at high doping regime. The predicted superconductivity depends sensitively on a minimal lattice distortion (see, H. Rosner, A. Kitaigorodsky, and W. E. Pickett, Phys. Rev. Lett. 88, 127001 (2002), incorporated herein by reference) which is yet to be demonstrated explicitly.
Accordingly, a need exists for a method of synthesizing LiBC and Li1-xBC that is simple and effective while providing clean phases as outlined above. The present invention satisfies those needs, as well as others, and overcomes the deficiencies of previously developed synthesis methods.
The present invention describes the synthesis of stoichiometric LiBC from a heating process such as resulting from either an arc-melting process or a sealed tantalum ampoule process to produce LiBC. The produced LiBC is a parent compound from which hole-doped Li1-xBC may be synthesized. The term stoichiometric in the present invention refers to a compound having a fixed ratio of constituent atoms.
Hole-doped lithium borocarbide Li1-xBC is a promising candidate for phononmediated high-temperature superconductivity (H. Rosner, A. Kitaigorodsky, and W. E. Pickett, Phys. Rev. Lett. 88, 127001 (2002), incorporated herein by reference). A method of synthesizing stoichiometric LiBC by arc-melting is described, wherein a pellet of uniformly-mixed elemental lithium, boron, and graphite is sufficiently heated in an arc-melting process to trigger a self-propagating exothermic reaction.
A second synthesis method is described wherein uniformly-mixed elemental lithium, boron, and graphite (Li—B—C) are sealed in a tantalum ampoule, which is heated, such as in a heating chamber, under sufficient temperature for a sufficient period of time.
Hole-doped compound Li1-xBC (0≦x≦0.37) is described being generated through vacuum de-intercalation from LiBC. It should be appreciated that the hexagonal crystal lattice remains largely intact, with only slight decreases in lattice parameters upon hole-doping. The samples produced are intrinsically diamagnetic and are semiconducting in the 2 K to 300 degree K range studied. Increased hole-doping leads to a color darkening as well as a decrease in resistivity.
An aspect of the invention is the synthesis of stoichiometric LiBC in a more convenient process of arc-melting.
Another aspect of the invention is synthesizing LiBC from a sealed tantalum ampoule.
Another aspect of the invention is de-intercalation of LiBC produced according to the invention to Li1-xBC.
A still further aspect of the invention is to provide synthesized LiBC material with clean phases.
Further aspects of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.
The invention will be more fully understood by reference to the following drawings which are for illustrative purposes only:
Referring more specifically to the drawings, for illustrative purposes the embodiment and results from implementing the present invention are described by the flowcharts and graphs generally shown in
In the present invention, methods are described for synthesizing Li1-xBC (0≦x≦0.37) using both arc-melting and sealed tantalum ampoule methods, in conjunction with vacuum de-intercalation. In the doping range studied, the hexagonal lattice of LiBC has remained largely intact, and the samples are diamagnetic and semiconducting.
1. Method Embodiments.
These above steps may be further generalized as:
(a) mixing, uniformly, elemental mixtures of boron (B) and graphite powders (C) according to a suitable ratio under inert gas, as represented by block group 28;
(b) mixing lithium (Li) with boron (B) and graphite (C) under the inert gas, as represented by block group 30;
(c) pressing sealed mixture Li—B—C into a pellet, as represented by block group 32; and
(d) arc-melting said Li—B—C pellet to trigger a self-propagating exothermic reaction from which LiBC is created, as represented by block group 34.
These above steps may be further generalized as:
It should be appreciated that in general, a number of synthesis variations may be utilized according to the invention. By way of example, the following describes specific synthesis implementations according to the invention.
2. Specific Implementation Example.
The parent compound LiBC was synthesized from elemental mixtures of lithium (ingot, 99.9%), boron (amorphous, 325 mesh powder, 99.99%), and graphite (200 mesh powder, 99.9999%) according to Li:B:C=1.3:1:1. Boron and graphite powders were first ground together in a mortar for thirty (30) minutes to achieve a uniform mixing. They were then loaded into an argon-filled dry box and thoroughly mixed with lithium for another thirty (30) minutes. The lithium pieces had been freshly cut from an ingot inside the dry box.
In this first approach, arc-melting was used to perform synthesis. The mixture (typically 500-800 mg) was sealed into a 6 mm die and transferred outside the dry box to be pressed into a pellet (under a 3000 psi pressure for 10 minutes). The pellet was subsequently loaded under an argon atmosphere into an arc furnace. The furnace was purged a few times and then refilled with argon gas. Prior to melting the pellet, zirconium was first arc melted to purify the argon. The pellet was then heated up to trigger a self-propagating exothermic reaction, turning the black Li—B—C mixture into a golden LiBC pellet. During the reaction, the excess Li, having served as a flux, was vaporized and released from the compound.
In a second approach, the Li—B—C mixture was sealed in a tantalum ampoule, which was in turn sealed in a quartz tube. The assembly was heated to approximately 1000° C. at 3° C./min and annealed for approximately ten (10) hours before cooling down at about 3° C./min. The annealed powders were mostly golden colored LiBC, however, traces of other black borocarbides were also present.
To achieve the hole-doped Li1-xBC, de-intercalation of LiBC was performed under heat in a 10−6 torr vacuum. For comparison, de-intercalation was carried out at three temperatures for approximately twelve (12) hours each at T=600° C., 700° C. and 800° C., respectively. Before and after the de-intercalation, the mass of each sample was monitored. Care was taken to avoid exposures to air at all steps. The percentage mass loss was 4.7% at 600° C., 5.3% at 700° C. and 8.7% at 800° C. This was accompanied by a color darkening from golden to eventually black at increasingly higher annealing temperatures. From the mass loss, the lithium deficiency can be calculated as x=0.20, 0.23, and 0.37 at 600° C., 700° C., and 800° C., respectively.
3. Results and Discussions.
Structural characterizations of the results were carried out using a Siemens D-500 powder X-ray diffractometer. To accurately determine the lattice parameters, an internal reference (Si powder) has been mixed into the Li1-xBC samples to calibrate any peak position shift. Magnetic measurements were performed by a superconducting quantum interference device (SQUID) magnetometer. Electrical transport was measured by a DC resistivity setup.
Stoichiometric LiBC golden powders are formed by both the described arc-melting and sealed tantalum ampoule methods. In comparison, the self-propagating exothermic reaction in the arc furnace is much more efficient and also leads to more homogeneous LiBC being produced.
The lithium-deficient Li1-xBC samples are very difficult to synthesize directly from either arc-melting or sealed tantalum ampoule method. When starting from stoichiometric mixture of Li1-xBC, both methods led to mostly un-reacted B—C and a minor amount of LiBC. Instead, synthesis was started from the parent LiBC compound and used the vacuum de-intercalation method described above.
In the following, the hole-doped samples deintercalated from the arc melted LiBC are examined. The X-ray diffraction patterns are also shown in
Magnetic and transport measurements on the Li1-xBC (0≦x≦0.37) samples have not shown any definitive evidence of superconductivity down to 2 degrees K. Instead, in earlier experiments, a weak ferromagnetism was observed in these samples. The saturation magnetization was up to 5×10−2 emu/g, or about 2 emu/mole, of the same order of magnitude as that seen in CaB2C2 (see, J. Akimitsu, K. Takenawa, K. Suzuki, H. Harima, and Y. Kuramoto, Science 293, 1125 (2001), incorporated herein by reference). Here the magnetic signal was not intrinsic to Li1-xBC, but rather caused by impurities in the starting materials of boron and carbon that were 99.5%-99.9% pure. After replacing them with higher purity ones (99.99% boron and 99.9999% carbon), along with careful sample handling to avoid magnetic contaminations, the saturation magnetization has become routinely below 4×10−3 emu/g, or 0.1 emu/mole. The suppression of the ferromagnetic impurities has revealed clear diamagnetic signatures of the Li1-xBC samples. At 5 K, the diamagnetic susceptibility is about −3×10−6 emu/gOe.
4. Summary of LiBC and hole-doped Li1-xBC Synthesis.
In summary, the synthesis of stoichiometric LiBC has been described by both arc furnace melting and sealed tantalum ampoule methods. It will be appreciated that the arc-melting technique results in a self-propagating exothermal reaction, with excess Li being released. A color change arises in the black pellets of material, wherein the color shifts to a homogenous golden color. The sealed Ta ampoule technique is doubly sealed in quartz and annealed at about 1000° C. for approximately ten hours ramping rate 3° C./minute. The result after annealing is a mostly golden color powder.
It should be recognized that hole-doped Li1-xBC is difficult to achieve directly from these two techniques, but can be realized by vacuum de-intercalation of the parent LiBC compound. With increasing hole-doping (0≦x≦0.37), the LiBC color darkens, the hexagonal lattice has only slight decreases in lattice parameters, and the electrical resistivity decreases. These samples are diamagnetic and semiconducting.
Stoichiometric LiBC has been synthesized using both arc-melting and sealed tantalum ampoule methods. Hole-doped Li1-xBC (0<x≦0.37) compounds have been realized through vacuum de-intercalation from LiBC. The hexagonal crystal lattice has remained largely intact, with only slight decreases in lattice parameters upon hole-doping. The samples are intrinsically diamagnetic and are semiconducting in the 2-300 K range studied. Increased hole-doping leads to a color darkening as well as a decrease in resistivity.
To further adjust the electronic structure of Li1-xBC (0≦x<1), one may grow thin films of Li1-xBC over a substrate and utilize the strain caused by the substrate to induce superconductivity. Typical thin film synthesis techniques, such as evaporation, sputtering, pulsed laser deposition, pulsed electron deposition, molecular beam epitaxy, and electrochemical deposition may be utilized. Different substrates, with a similar crystal structure to that of LiBC, may be used to induce compressive, tensile, or zero strain. These induced strains modify the electronic structure of the Li1-xBC and can in some cases induce superconductivity.
Although the description above contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Therefore, it will be appreciated that the scope of the present invention fully encompasses other embodiments which may become obvious to those skilled in the art, and that the scope of the present invention is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” All structural, chemical, and functional equivalents to the elements of the above-described preferred embodiment that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the present invention, for it to be encompassed by the present claims. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U.S.C. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for”.
This application claims priority from U.S. provisional application Ser. No. 60/428,068 filed on Nov. 20, 2002, incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60428068 | Nov 2002 | US |
