SUBATHMOSPHERIC PRESSURE PLASMA SPRAYING OF SUPERCONDUCTIVE CERAMIC

Abstract: A method and a system for producing a superconductive ceramic article. The method comprises maintaining a spray chamber at subatmospheric pressure, operating a plasma spray gun with a plasma-forming gas to produce a plasma spray stream directed towards a substrate in the spray chamber, selecting a partial pressure of oxygen, introducing sufficient oxygen into the spray chamber to maintain the selected partial pressure therein and feeding a superconductive oxide ceramic composition into the spray stream such as to produce a ceramic article in the form of a superconductive oxide coating on the substrate.

Description: The present invention relates to a method of producing a superconductive ceramic article comprising maintaining a spray chamber at subatmospheric pressure, operating a plasma spray gun with a plasma-forming gas and introducing oxygen therein to produce a plasma spray stream directed towards a substrate in the spray chamber, and feeding a superconductive oxide ceramic composition into the spray stream such as to produce a ceramic article in the form of a super conductive oxide coating on the substrate. Recent advances in the field of superconductors have particularly involved certain oxide ceramic compositions that exhibit superconductive properties at temperatures approaching liquid nitrogen temperature (77°K) and higher. The first announcement was by International Business Machines' Zurich Research Laboratory in April 1986 for the oxide composition of barium, lanthanum and copper. Subsequently scientists at the University of Houston discovered a better composition consisting of yttrium oxide, barium oxide, and copper oxide in the atomic proportions 1,2,3 (and thereby known as "1-2-3 composition") with the nominal formula YBa₂Cu₃O 7-X . More broadly, an orthorhombic perovskite crystal structure has been recognized as the basis for superconductive oxides. The problem of preparing shapes and samples of such oxide materials immediately became apparent, and plasma spraying of coatings was recognized as a viable method. There have been numerous publications pertaining to these developments; typical references are as follows: "Superconductor Research Pace Quickens" G. Fisher and M. Schober, Ceramic Bul. 66 , 1087 (1987); "Thermal Spraying Superconducting Oxide Coatings", J. P. Kirkland et al, Advanced Ceramic Materials 2 , No. 3B Special Issue, 401 (1987). However, realization of full potential for superconductive properties has remained elusive. The superconductivity has been incomplete and lacking in reproducibility. The problem of sufficient and reliable superconductivity, for example in plasma sprayed coatings, has been traced broadly to the quality of the superconductive powder utilized, particularly the stoichiometry of constitutents including oxygen and the presence of contaminants. Details are not very well understood. Superconductive ceramics such as the 1-2-3 type are particularly susceptible to reduction of the oxide. A further problem is that superconductivity is detrimentally sensitive to effects in grain boundaries which are inherent from the processing. Subsequent annealing of the material in oxygen helps, but with only some improvement. In U.S. Patent No. 3,576,672 (Harris et al) there is disclosed a method for depositing a ferrite coating by means of an arc plasma torch with an oxygen shroud and/or oxygen carrier gas for the ferrite powder, in ordinary ambient atmosphere. Such a method may be utilized in the production of coatings of the 1-2-3 composition but, it is expected that the aforementioned problems will remain even after subsequent annealing in oxygen. Plasma spraying at subatmospheric pressure in a spray chamber is known, as disclosed, for example, in U.S. Patent No. 3,839,618 (Muehlberger). It is stated therein that the source of plasma gas "is preferably argon or helium, although various other gases may be employed, for example to provide desired environmental conditions in the chamber". However, it is well known in the art of plasma spraying that the plasma-forming gas used for a practical plasma spray gun must be inert or reducing. Such chamber spraying is normally used for spraying metallic coatings without oxygen present in order to obtain oxygen-free coatings. Although suited for spraying metals, low pressure generally has the undesirable effect of reducing the oxygen content of oxide ceramic spray materials, and no real benefit appears to have been recognized. Therefore, oxides have not received significant attention in chamber spraying. In EP-A-0 324 121 a method of producing a superconductive ceramic article as mentioned above has already been described according to Article 54 (3) EPC. According to said application, the oxygen is added to the gas operating the plasma-spray gun and the gas mixture is introduced into a bore of metal metallizing nozzle from a rear end portion thereof. The oxygen gas is added in an amount of at least 30 vol.% into the operating gas. From the publication "New Scientist", no 1595, page 42, 1988, a method for producing a superconductive ceramic article has also been known in which a plasma-spray gun is used to produce a plasma-spray stream into which a superconductive oxide ceramic composition is fed and the spray stream directed towards the substrate in a spray chamber. Said spray chamber is retained at low pressure between 1/40 and 1/4 atmosphere and the chamber contains ambient gas which is admitted to the spray chamber. Therefore, an object of the present invention is to provide a method of producing a superconductive oxide ceramic article with improved superconductive properties. A further object is to provide an improved method of plasma spraying to produce a superconductive ceramic oxide coating. An additional object is to provide an improved method of plasma spraying in a subatmospheric spray chamber. A further object of the present invention is to provide a plasma spraying system for producing superconductive oxide ceramic layers having improved superconducting properties. This will be achieved in accordance with the invention in that a partial pressure of oxygen in the spray chamber is selected, and that sufficient oxygen is introduced substantially at the exit end of the gun to maintain the selected partial pressure in the spray chamber. In one embodiment the oxygen is introduced in the form of a shroud gas enveloping the spray stream. According to another embodiment, the ceramic composition is in the form of a powder and the oxygen is introduced as a carrier gas for the powder. The drawing shows a schematic view of a system for carrying out the present invention. With reference to the drawing, a low pressure plasma spraying system 10 includes a spray chamber 12 and a pumping system 14 for maintaining the spray chamber at subatmospheric pressure. A filter 16 is mounted in ducting 18 between the chamber and the pump for removing dust and free powder from the effluent to protect the pumping system. A plasma spray gun 20 is retained on a mounting 22 in the chamber. The gun, for example, is of the general type described in U.S. Patent No.3,145,287. Operational means for operating plasma spray gun 20 include a conventional power supply 22 to apply a DC arc current via cables 24 to the gun. The operational means also includes a source 26 of a plasma-forming gas which is directed through a tube 28 to gun 20 where the gas is heated by an internal arc to produce a plasma spray stream 30 . Plasma-forming gas is generally inert s u ch as argon, helium and/or nitrogen. Coolant of water or the like is also supplied [by means not shown] to the gun through cables 24. The subatmospheric chamber pressure is generally below about 933·10² Pa (700 torr to about 6,6·10² Pa (5 torr), preferably between 800·10² Pa (600) and 66·10² Pa (50 torr). At the low pressure very high velocity spraying is achieved in order to produce dense coating with minimized boundary effects between particles. Spray stream 30 is directed towards a substrate 32 attached to a mounting 34 that leads slidingly through a sealing port 36 from a drive system 38 for imparting oscillatory motion to the substrate during spraying to obtain coverage over a desired surface area. Other axes of motion (not shown) may also be applied to substrate 32 and/or to plasma gun 20 . Feeding means include a powder feeder 40 for introducing spray powder entrained in a carrier gas from a carrier gas source 42 through tube 44 into spray stream 30 such as to produce a ceramic coating 45 on substrate 32 . The powder is directed into stream 30 just inside a gun nozzle 46 as shown, or outside the gun 20 , in the conventional manner. Substrate temperature is maintained in the range of 300 to 1500°C, preferably 700 to 1000°C, by spray traverse rate, spray rate, water cooling of the mounting (not shown), and/or cooling jets (not shown) of air, carbon dioxide or liquid nitrogen. Substrate 32 is made of a material such as copper, nickel, cobalt and alloys thereof. The substrate is conventionally prepared by light grit blasting or other roughening and cleaning procedure, and further cleaning in a solvent such as acetone or etching with an etchant such as Schantz solution. Alternatively or additionally the substrate is coated with a metallic bond coat such as nickel-aluminum or nickel-chromium-aluminum-yttrium and/or with an oxide coating such as aluminum oxide, prior to the superconductive coating. Such a bond coat may be applied by plasma spraying in open air or in the chamber (without oxygen if appropriate). As a further alternative substrate 32 may be treated before, during, and/or after spraying in the chamber, by conventional transferred arc techniques are suggested in the aforementioned U.S. Patent No. 3,839,618. According to the present invention oxygen is fed into chamber 12 , for example, via line 47 with valve 49 therein for adjusting oxygen pressure in the chamber. Preferably the oxygen is introduced into spray stream 30 at gun 20 . This may be effected through a shroud 48 positioned on the end of the gun coaxially with the nozzle 46 for enveloping the spray stream with oxygen. The shroud receives the oxygen from an oxygen source 50 via a tube 52 . An adjustable valve 54 in the tube line is desirable for adjusting oxygen flow. The shroud 48 is of the known or desired type, for example as described in U.S. Patent No. 3,470,347 (Jackson) for inert gas shrouding. The shrouding also may be effected by a gas ring (not shown) on the nozzle face with oxygen gas jets directed along the spray stream. Alternatively, and more simply, there is no physical shroud and the oxygen is fed into the spray stream by way of utilizing oxygen introduced as the carrier gas via tube 56 and a valve 57 for conveying the powder. The carrier may be all oxygen or may comprise an inert gas to balance the amount of oxygen desired to effect the selected partial pressure as described below. Further according to the present invention a superconductive type of oxide ceramic material in powder form is utilized as the spray powder. This material may be formed of the 1-2-3 composition or other superconductive type of oxide, particularly one that is susceptible to oxygen depletion during exposure to plasma stream temperatures at subatmospheric pressure. Powder is of conventional size, vis. -100 mesh +5 microns (-149 +5 microns), preferably -170 +325 mesh (-88 +44 microns) or, alternatively, -44 +5 microns. Sufficient oxygen is introduced into spray chamber 12 to maintain a selected partial pressure of the oxygen in the chamber during the spraying operation. The selected partial pressure of oxygen is best determined by simple experimentation, because such pressure will depend on the other selected parameters including type of gun and its operating conditions, chamber pressure and the powder composition and size. The experimenting may be effected by selecting successive partial pressures of the oxygen, producing corresponding coating compositions with the successive partial pressures, measuring for a corresponding degree of superconductivity of each of the corresponding coating compositions, determining optimum superconductivity from the corresponding degrees of superconductivity, and selecting an optimum partial pressure of oxygen correlated with the optimum degree of superconductivity. Generally the partial pressure oxygen should be in the range of about 10% to 50% of the total chamber pressure including the plasma gas. Before measuring for superconductivity it may be necessary or at least desirable to anneal the sprayed coating in oxygen. This is done at several temperature levels programmed with time periods to effect the appropriate crystal structure with uniformity and to establish the proper proportion of oxygen content in the crystal lattice. However, it is contemplated that optimum selection of oxygen partial pressure with respect to other parameters may alleviate the need for an annealing step, or at least shorten the time thereof. Annealing, if needed, is effected in an oxygen-containing furnace in the conventional manner. The partial pressure of oxygen is in the range of 0.2 (air) to 1.0 (pure oxygen) and the annealing temperature is 890°C to 950°C. Annealing time is 2 to 45 hours at temperature followed by slow cooling at 1-2°C/min. Specifics are selected so as to form a friable mass of particles of the desired crystal structure and with substantially full oxidation. For example, annealing is done in pure oxygen at 930°C for 5 hours. There are several known methods for measuring for the superconducting property of the coating article, including Meissner effect, and measurements of critical current Jc, temperature Tc and magnetic susceptibility Hc. Alternately, the preferred crystal structure may be determined by X-ray diffraction. A desirable method is simply observation of the Meissner effect. This technique is relatively straightforward in that the coating is observed to levitate above a magnet when cooled in liquid nitrogen. This experiment shows that the coating is, in fact, a high Tc superconductor with an onset temperature above 77K (liquid nitrogen temperature). The measurement is effected at an appropriate temperature for superconductivity, such as with liquid nitrogen, or with a varying temperature to determine superconducting transition point (Tc). Examples of spraying parameters for 1-2-3 composition powder of -44 +5 microns are given in the Table herein. Coatings about 0.5mm thick are sprayed onto a substrate of stainless steel prepared by light grit blasting with fine alumina grit, using Metco plasma guns sold by The Perkin-Elmer Corporation, Westbury, N. Y. In each case excellent coatings are obtained which are very dense with virtually nil discernable particle boundaries due to the spraying at high velocity at low pressure, as well as the elevated substrate temperature and display a high degree of super- conductivity. Generally coatings ranging in thickness from about 25 microns to 0.5 cm or even thicker may be obtained. The coatings may be removed from the substrate for free standing use. Applications are any that may be associated with supercondoctive materials, particularly of the "high temperature" type. These include electrical conductors, magnets and electromagnetic sensors, and may be incorporated into solid state devices soch as Josephson junctions.

Claims: 1. A method of producing a superconductive ceramic article comprising maintaining a spray chamber at subatmospheric pressure, operating a plasma spray gun with a plasma- forming gas and introducing oxygen therein to produce a plasma spray stream directed towards a substrate in the spray chamber, and feeding a superconductive oxide ceramic composition into the spray stream such as to produce a ceramic article in the form of a super conductive oxide coating on the substrate, characterised in that a partial pressure of oxygen in the spray chamber is selected, and that sufficient oxygen is introduced substantially at the exit end of the gun to maintain the selected partial pressure in the spray chamber. 2. A method according to Claim 1 wherein the oxygen is introduced in the form of a shroud gas enveloping the spray stream. 3. A method according to Claim 1 wherein the ceramic composition is in the form of a powder and the oxygen is introduced as a carrier gas for the powder. 4. A method according to Claim 1 wherein the subatmospheric pressure is between about 66.10² Pa and 800.10² Pa (50 and 600 torr). 5. A method according to Claim 4 wherein the partial pressure of the oxygen is between about 10% to 50% of the subatmospheric pressure. 6. A method according to Claim 1 wherein the plasma forming gas is an inert gas. 7. A method according to Claim 1 wherein the ceramic composition is in the form of a powder. 8. A method according to Claim 1 further comprising selecting successive partial pressures of the oxygen, producing corresponding ceramic articles with the successive partial pressures, measuring for a corresponding degree of superconductivity of each of the corresponding ceramic articles, determining optimum superconductivity from the corresponding degrees of superconductivity, and selecting an optimum partial pressure of oxygen correlated with the optimum degree of superconductivity. 9. A method according to Claim 1 wherein the ceramic composition is formed of yttrium oxide, barium oxide, and copper oxide in atomic proportions of 1:2:3.

Inventors: HARRINGTON, JOHN H., RANGASWAMY, SUBRAMANIAM, REARDON, JOSEPH D., SIMOKOS, PAUL D.
Applicants: THE PERKIN-ELMER CORPORATION
Application year: 1989
Main classification: C23C4
IPC codes: C23C4, C04B35, H01L39