PREPARATION OF SUPERCONDUCTING OXIDES BY ELECTROCHEMICAL OXIDATION: ANODIC SUPERCONDUCTORS

Abstract: Disclosed is a method for preparing superconductive oxides by electrochemical oxidation. An electrochemical cell includes an anode made of an alloy of the metallic precursors of the superconducting oxide. A surface of the anode is in contact with an electrolyte of the cell, and the anode is electrochemically oxidized to form the superconducting oxide.

Description: PREPARATION OF SUPERCONDUCTING OXIDES BY ELECTROCHEMICAL OXIDATION: ANODIC SUPERCONDUCTORS Backg ou d c_f the Inven ion This invention relates to superconducting materials. Superconductors are materials having zero resistance to the flow of electrons below a certain critical temperature, ~2 . It is known that certain 10 metal oxides, e.g., La„_ 3a CuO, , La 7 _ χ Sr χ Cu0 4 , Ba 2 YC 2 0 9 _ y , etc. exhibit superconductivity. It is desirable to provide such oxides in forms, e.g., wires or thin films, that permit practical utilization of their superconductive property. - - Summary of the Invention In general the invention features preparing a superconducting oxide by electrochemical oxidation. According to the invention, an electrochemical cell containing an anode made of an alloy of the metallic 0 precursors of the superconducting oxide is provided. A surface of the anode is in contact with the electrolyte of the cell, and the anode is electrochemically oxidized to form the superconducting oxide. Preferably the metallic precursors are present in the alloy in the 5 stoicherr.etric proportions of the metals in the oxide. In some preferred embodiments, the metallic precursors are La, M, and Cu and the superconducting oxide has the formula a 2 M χ Cu0 4 . M is an alkaline earth element such as Ba, Sr, or Ca. 0 In other preferred err-bcdimer.ts, the metallic precursors are rare earth elements (N), Ba, and Cu and the superconducting oxide has the formula NBa 2 Cu 3 0 ? . Examples of rare earth elements are Y, La, Eu, Gd, Tb, Dy, Hu, Ξr, Tm, Yb, or Lu. In other preferred embodiments the alloy is formed using rapid solidification techniques to provide a hc--.cger.ecus alloy. Formation of superconducting oxides by electrochemical oxidation allows considerable ccntroi over the structure, composition, and thickness of the oxide through adjustment of oxidation conditions, e.g., electrolyte composition, temperature, applied voltage or current and duration of oxidation. For example, because the oxidation proceeds from the outer surface of the anode towards the center cf the anode, the thickness of the superconducting oxide layer formed can be controlled by electrochemically oxidizing until the desired thickness is obtained. Other features and advantages of the invention will be apparent from the description of the preferred embodiments thereof, and from the claims. Description of the Preferred Embodiments The Figure is an electrochemical cell. Referring to the Figure, an electrochemical cell 10 includes a container 12, an anode 14, a reference electrode 16, a counter electrode 17, a power supply 18, and an electrolyte 20. The container 12 is made of an inert material, such as glass. The anode 14 is made of an alloy of the precursor metals of the superconducting oxide. The precursor metals are present in the εtcichic etric proportions of the metals in the target oxide. The alloy is homogeneous, i.e., the chemical composition and microstructure of the alley is substantially uniform throughout. Homogeneity is achieved by using standard rapid solidification techniques such as melt spinning or inert gas atomization. The surface of the anode should be smooth to achieve a reproducible surface condition; the smoothness can be generated through conventional abrasion or polishing (e.g., electropolishing) methods. The alloy may also be degassed in a suitable inert solvent, e.g., cyclohexane. The reference electrode 16 provides a reference potential for the applied voltage (or current) used to form the oxide. The preferred reference electrode is a high impedence glass electrode such as a saturated calomel electrode. The counter electrode completes the electrochemical circuit and allows electric current to pass between itself and the anode. Suitable counter electrodes include standard platinum and graphite electrodes. For potentioεtatic applications, the power supply 18 provides a constant voltage to the cell during the oxidation. Where the oxidation is by galvanostatic techniques, a constant current supply is used. Current and voltage metering instruments are incorporated into the power source configuration. The electrolyte 20, in which the electrodes are immersed, may be aqueous, non-aqueous, or a molten salt, depending upon the alloy composition and desired film characteristics. Non-aqueous electrolytes are most preferred. Examples of suitable aqueous electrolytes include sulphuric acid and sodium hydroxide solutions. Examples of non-aqueous electrolytes, which should be used where oxide formation in the absence of water is desired, includes solvents such as ethanol, amπcr.ia, and acetonitrile. Where ncn-aqueous electrolytes are used, the ionic strength of the electrolyte can be increa-rrd by the addition of, e.g., hydrochloric acid. Examples of molten salt electrolytes are LiCl and Na 2 S0 4 . The oxidations events are carried out using standard methods well known to those skilled in the art. In general, for potentiostatic oxidation, the potential is kept constant (perferably between 2.5 and -2.5 V, with respect to the reference electrode), and the current passed by the anode is monitored to follow the ' rate of oxide formation. For gaivanostatic oxidation, the current is kept constant (preferably between 10 ~ A and 10 A/cm ) and the anode potential is monitored to follow the rate of cxide formation. Other Embodiments Other embodiments are within the following claims. For example, the cell can also contain a means for temperature control of the electrolyte ana a means for regulating the gas content of the eiectrolyte. Moreover, in a limited number of applications, the oxidation can be performed with an open circuit, thereby making a counter electrode unnecessary.

Claims: - - Claims 1. A method of preparing a superconducting oxide comprising the steps of providing an electrochemical cell comprising an anode having a surface in contact with an electrolyte, said anode comprising an alloy of the metallic precursor of the superconducting oxide to be produced, and electrochemically oxidizing said anode to form said superconductin : oxide. 2. The method of claim l wherein said metallic precursors comprises La, , and Cu and said superconducting oxide has the formula La2--xMxCuO4,-y, wherein is an alkaline earth element. 3, The method of claim 1 wherein said metallic presursors comprises N, 3a, and Cu and said superconducting oxide has the formula Ea 2 C _0 7 , wherein N is a rare-earth element. 4. The method of claim 3 wherein said rare-earth element is selected from the group consisting of La, Eu, Gd, Tb, Dy, Ho, Sr, Tm, Yb, or Lu. 5. The method of claim 1 wherein said alloy is formed using rapid solidification processing. 6. The method of claim 1 wherein said metallic precursors are present in said alloy in stoichiorcetric proportions. 7. The method of claim 1 wherein said electrolyte is aqueous, said method further comprising dehydrating said superconducting oxide.

Inventors: LATANISION RONALD M, MOFFAT THOMAS P, NAGARKAR PRADNYA V, SEARSON PETER C, LATANISION, RONALD, M., MOFFAT, THOMAS, P., NAGARKAR, PRADNYA, V., SEARSON, PETER, C.
Applicants: MASSACHUSETTS INST TECHNOLOGY, MASSACHUSETTS INSTITUTE OF TECHNOLOGY
Application year: 1989
Main classification: C25B1
IPC codes: C01G1, C01G3, C25B1, C25B11, C25D9, C25D11, H01L39