Method for Continuous Production of Niobium-Germanium Layers on a Substrate

This invention relates generally to methods for producing layers on substrates, and more particularly, to a method for continuously producing layers with intermetallic superconducting A15 phase niobium-germanium (Nb.sub.3 Ge) by the reduction of gaseous halogenides of niobium and germanium.

A method for producing layers of Nb.sub.3 Ge on a heated substrate by supplying a gas mixture containing hydrogen and the niobium and germanium halogenides to a reaction chamber through which an elongated substrate is pulled is described in "Journal of the Less-Common Metals", Vol. 59, 1978, pp. 35-41. A technique for preparing superconducting A15 phase niobium-germanium (Nb.sub.3 Ge) using chemical vapor deposition (CVD) technology is disclosed in "LEEE Transaction on Magnetics", Vol. MAG-14, No. 5, September 1978, pp. 608-610; and Vol. MAG-15, No. 1, January 1979, pp. 516-519. The technique described therein has been used for preparing the intermetallic superconducting compound niobium-tin (Nb.sub.3 Sn) as described in "RCA Review" Vol. XXV, September 1964, pp. 342-365. According to the techniques described in these references, intermetallic A15 phase Nb.sub.3 Ge can be precipitated by reduction of gaseous niobium and germanium halogenides, with hydrogen, on a heated substrate of illustratively copper or steel.

As noted in "Journal of the Less-Common Metals", the known production methods have not permitted the manufacture of superconductors which exceed approximately 20 meters in length. If such known methods are applied to the production of long conductor sections, illustratively, in excess of 100 meters, such long conductors will be characterized by a critical current density which drops steeply with increasing conductor lengths. Thus, although such conductors will have layer thicknesses and critical temperature characteristics which deviate over their length by less than 10%, the critical current density characteristic will decrease so as to be only 25% of the initial value at the end of a conductor ribbon which is 150 meters long. Such a decrease in the critical current density cannot be prevented even if the conductor is carefully prepared, and even if the germanium and niobium are deposited over deposition times of several hours with constant mass flow on a substrate which is transported with constant velocity through an atmosphere of a reaction gas mixture which is held at a constant temperature.

It is, therefore, an object of this invention to provide a method for precipitating superconducting A15 phase Nb.sub.3 Ge from the gaseous phase, onto conductors which exceed 100 meters in length, without incurring a steep drop in the critical current density characteristic as conductor length increases.

The foregoing and other objects are achieved by this invention which provides a method of depositing a layer of superconducting A15 phase Nb.sub.3 Ge onto substrates which exceed 100 meters in length by providing a small amount of water vapor which is admixed to the gas mixture containing the reducing hydrogen and the niobium and germanium halogenides. The total volume percent of water vapor in the gas mixture may be between 0.05 and 0.001 percent, in a reaction chamber.

In addition to containing superconducting intermetallic A15 phase Nb.sub.3 Ge, the layer which is deposited on the substrate also contains tetragonal phase Nb.sub.5 Ge.sub.3. The critical current density of the layer depends on the structure of this two-phase layer. A major advantage of the inventive method described herein over known methods described hereinabove, is that the addition of a small amount of water to the reaction gas atmosphere in the reaction chamber produces a deposition having a uniformly fine crystalline structure. The small amount of water permits the formation of crystallization nuclei, which is required to form and stabilize the fine crystalline structure. This structure insures that long conductor lengths will possess uniform high current density.

Comprehension of the invention is facilitated by reading the following detailed description in conjunction with the annexed drawing in which is shown a schematic and block and line representation of an apparatus for coating substrates with superconducting A15 phase Nb.sub.3 Ge, in accordance with the principles of the invention.

The apparatus shown in the FIGURE for producing superconductors in accordance with the present invention correspond in large measure to the apparatus which is generally used with the CVD technique for producing Nb.sub.3 Ge in accordance with known chemical reactions. As shown in the FIGURE, germanium tetrachloride (GeCl.sub.4) which is required for the reaction is carried by a stream 2 of argon. The argon is saturated with GeCl.sub.4 as it flows through a bath 3 containing liquid GeCl.sub.4. The temperature of bath 3 is held constant within a range between -10.degree. and +25.degree. C., by a thermostat 4. It is essential that the carrier gas argon be saturated with GeCl.sub.4.

Niobium pentachloride (NbCl.sub.5), which is required for the coating process, is formed by the chlorination of solid niobium granulate 5, at a temperature of approximately 920.degree. C. A stream 6 of chlorine and argon gas flows through a tank filled with niobium granulate 5 and is completely reacted to NbCl.sub.5. The composition of the reaction product can be determined from the consumption of niobium and chlorine, which corresponds to niobium tetrachloride (NbCl.sub.4). Additional chlorination of NbCl.sub.4 produces a stable but highly volatile pentachloride of niobium. A mixing chamber 7 is provided which is connected to the tank containing niobium granulate 5. In this chamber, NbCl.sub.4 is subjected to the further chlorination by a stream 8 of chlorine gas, wherein a complete reaction is assured. Mixing chamber 7 further receives argon stream 9 which is saturated with GeCl.sub.4.

Mixing chamber 7 discharges a gas stream 11 comprised of a mixture of Ar, NbCl.sub.5 and GeCl.sub.4. This gas mixture is subsequently admixed with a gas stream 12 of hydrogen (H.sub.2) and Ar as a carrier gas. This produces a gas stream 13 composed of NbCl.sub.5, GeCl.sub.4, H.sub.2 and Ar; which are further admixed, according to the invention, with small amounts of water. The admixing with water is achieved by providing a stream 14 of Ar gas which is fed into a chamber 15 wherein water vapor is added. Argon stream 16 is saturated with water vapor and is combined with gas stream 13 so as to produce a gas stream 17 which is conducted to a coating device 18. Prior to entering coating device 18, gas stream 17 is heated to a temperature T.sub.1 which may be approximately 550.degree. C. Since gas stream 17 flows with a high velocity, there is no danger of a reaction between the components of the reaction gas mixture during the short time prior to entry into coating device 18.

Coating device 18 contains an elongated reaction chamber 19 through which is transported a substrate 20, which may be in the form of a wire or ribbon, which is to be coated. The substrate, which may exceed 100 meters in length, is pulled through reaction chamber 19 as a predetermined velocity v. Substrate 20 may be a ribbon of material known under the trade name of "Hastelloy B" (nickel, alloyed with Mo, Co and Fe) or a steel ribbon.

Substrate 20 enters reaction chamber 19 through an opening 23 of a lock 21, and exits through an opening 24 of a lock 22. Locks 21 and 22 are located on opposite ends of reaction chamber 19. The section of substrate 20 which extends through reaction chamber 19 must be heated to a predetermined temperature T.sub.2 of, for example, 900.degree. C. This heating may be advantageously accomplished by passing a current through the substrate. Contact elements 25 and 26, which may be in the form of roller-shaped graphite contacts, are arranged outside of coating device 18 for conducting a current through substrate 20.

Reaction gas stream 17 which is preheated to the temperature T.sub.1 is introduced into reaction chamber 19 at a high rate of flow of approximately 5 to 13 meters per second. Since the reaction gas mixture cannot be heated sufficiently by the temperature T.sub.2 of substrate 20, to achieve the desired reaction, reaction chamber 19 is additionally heated by a furnace 28 to a temperature T.sub.3. Furnace 28 surrounds reaction chamber 19. Reaction chamber 19 communicates with the outside by means of locks 21 and 22, and an exhaust gas line 29.

The cross-sectional dimensions of the apparatus indicated in the FIGURE are advantageously adapted to the desired flow velocities and throughput of the starting materials. The metal chloride in a reaction gas flow of approximately 20-50 mol per hour, constitute approximately 1 mol percent, approximately 40% of which contributes to the coating. The concentration of the water vapor which is admixed to gas mixture 13, according to the invention, is between 0.05 and 0.001 volume percent, and preferably between 0.003 and 0.03 volume percent of the reaction gas mixture. This corresponds to between 0.3 and 3 mol percent, with respect to the total of the metal chloride in the reaction gas mixture. Even with such a low concentration of water vapor, a constant critical current density for the intermetallic compound Nb.sub.3 Ge may be precipitated on substrate 20 over long conductor lengths.

In a further embodiment of the invention, ribbon conductors having Nb.sub.3 Ge may be produced in apparatus made of quartz glass by CVD technology. In this embodiment, the length of reaction chamber 19 is approximately 75 cm, and its cross-section is approximately 0.85 cm.sup.2. A gas inlet and a gas outlet are provided at opposite ends thereof. Locks 21 and 22, as shown in the FIGURE, having openings 23 and 24, are provided at the respective ends. Openings 23 and 24 are matched to the dimensions of substrate 20 which is to be conducted therethrough. The locks may be in the form of chambers which are flushed with rare gas, and which are closed with graphite. Water-cooled brass cylinders having openings adapted to the dimensions of the substrate may also be provided. The apparatus is heated by a plurality of heated furnaces 28, located in different sections.

In this embodiment, a mixture of Ar, H.sub.2, NbCl.sub.5 and GeCl.sub.4 are throughput at the following rates: Ar, 5.31 mol/h; H.sub.2, 16.85 mol/h; NbCl.sub.5, 0.145 mol/h; and GeCl.sub.4, 0.53 mol/h. The above gas mixture arrives at reaction chamber 19 with a temperature T.sub.1 of approximately 450.degree. C., and flows through it with a velocity of approximately 5 m/sec. In a first embodiment A, the water content of the gas mixture is not controlled, and is less than 0.001 volume percent without the addition of moisture. In a second embodiment B, the reaction gas mixture is humidified in a simple manner by means of a water vapor-saturated argon stream. The additional water throughput in embodiment B is 0.66 mmol/h.

In both embodiments A and B, substrate 20 is a Hastelloy ribbon which is 2 mm wide and 50 microns thick. The ribbon is pulled through the reaction chamber with a velocity v of 30 cm/min and is heated by electric current to a temperature T.sub.2 of approximately 860.degree. C.

The ribbon conductors A and B were each produced at a length of 145 meters under equal conditions, except that ribbon conductor B received additional humidification.

The column identified "Measuring Point [m]", indicates the points along the lengths along the respective conductors at which measurements were made. The columns identified as "T.sub.c,0.5 " refer to critical temperature at the center of the transition from the superconducting to the normal conducting state. The columns identified as "I.sub.c,5T " correspond to the critical current density of the niobium-germanium layers at the magnetic induction of 5 Tesla, and a temperature of 4.2.degree. K.

The Table shows that Conductor B, which is coated by the method in accordance with the invention, maintained a relatively constant critical current density throughout its length, contrary to the Conductor A. In the specific illustrative embodiment, water vapor was introduced into the gas mixture prior to entering the reaction chamber. However, the addition of water vapor may be performed in the reaction chamber.

Although the invention has been described in terms of specific embodiments and applications, it is to be understood that persons of skill in the art can make additional embodiments for other applications, without departing from the scope of the inventions. Thus, although the embodiment example was based upon the formation of superconducting intermetallic compound Nb.sub.3 Ge from specific chlorides of niobium and germanium, other halogenides of these elements known from CVD technology may be used. Accordingly, it is to be understood that the drawings and descriptions of the invention herein are illustrative and are not to be construed in a limiting sense.