Process for producing oxide superconductive material by firing and apparatus therefor

Abstract: In a process or an apparatus for producing an oxide superconductive material by firing, a gas passage is formed in a firing furnace and a material (45) to be fired, consisting of a ceramic superconductive powder and/or a powder expressing superconductivity when heated, is placed in the gas passage. A gas is passed through the gas passage in one direction (A → B → C) to contact the material. For example cylindrical inorganic body (47) open at both ends is provided in a firing zone of a muffle (41) of a bell-shaped furnace (50), and a bottomed cylindrical substrate (44) with an exterior oxide superconductive layer (45) is located inside the inorganic body (47). Gas flow is from outside the body (47) to inside the body (47). Unidirectional gas flow avoids oxygen-lean regions at the surface of the superconductor material.

Description: The present invention relates to a process for producing an oxide superconductive material by firing as well as to an apparatus therefor. More particularly, the present invention relates to a process for producing an oxide superconductive material by firing, which process can suitably carry out complete firing and oxidation of a ceramic superconductive powder, etc., as well as to an apparatus therefor. The present invention further relates to a process for producing a bottomed cylindrical oxide superconductive material by firing, in an atmosphere enabling uniform firing, a bottomed cylindrical material comprising at least two layers of (a) a bottomed cylindrical substrate layer and (b) a layer to become an oxide superconductive layer when fired, formed on the substrate layer (a). Oxide superconductive materials have drawn attention in recent years because they exhibit high critical temperatures, and their application in fields such as electric power, magnetic resonance imaging, magnetic shielding and the like is expected. In magnetic shielding apparatuses, in particular, use of a bottomed (one end-closed) cylindrical superconductive material is being tried for measurement of the very weak magnetism of living body. Cylindrical oxide superconductive materials can generally be obtained in an integral form of (a) a cylindrical substrate and (b) an oxide superconductive layer provided thereon, by forming, on a cylindrical substrate made of a metal or the like, a layer to become an oxide superconductive layer and then subjecting them to firing. For use in production of oxide superconductive material by firing, there are known various types of firing furnaces. An electric furnace is often used for the firing of a special ceramic which can be made at a high temperature and which must be made at a precisely controlled temperature. As the firing furnace used for the above-mentioned purpose, there is known, for example, a firing apparatus [conventional apparatus (1)] as shown in Fig. 8. In Fig. 8, a material 4 to be fired and become an oxide superconductive material is placed on a setter 7 provided in a firing furnace 2; an oxygen-rich gas is introduced into the furnace 2 through a gas inlet 1 to fire the mateiral 4; the gas after firing is exhausted through a gas outlet 8. There is also known a firing apparatus [conventional apparatus (2)] as shown in Fig. 9. In Fig. 9, an oxygen-rich gas is introduced into a firing furnace 2 through a gas passage 9 provided outside a muffle of the furnace 2; the gas after firing is exhausted through a gas outlet 8. In the firing of, in particular, a layer to become an oxide superconductive layer when fired, formed on a cylindrical substrate, it is impossible to use a furnace of hanging bell type, a so-called bell-shaped furnace. This bell-shaped furnace is the same as generally used for uniform heating of wire material (e.g. steel wire) or thin tube coil or for heat treatment of said linear material or thin tube in a particular atmosphere or under vacuum in a state isolated from the outside. In the conventional apparatuses (1) and (2), however, the oxygen-rich gas introduced through the gas inlet 1 gives rise to a stream X which proceeds to the upper portion of the firing furnace 2 owing to the gas speed at the inlet 1 and a stream Y which stays at the lower portion of the firing furnace 2 owing to its temperature lower than that inside the furnace and then is heated by a heating element and rises to the upper portion of the furnace. When the flow rate is increased, the proportion of the stream X increases and, when the flow rate is decreased, the proportion of the stream Y increases. In either case, the stream X and the stream Y collide with each other above the material 4 to be fired, whereby turbulence in gas flow is generated, making it impossible to produce unidirectional gas flow in the vicinity of the material to be fired. When there is generated such a turbulent flow, even when the discharge gas from the gas outlet 8 contains oxygen in a high concentration of 8⊘% or more, an oxygen-lean gas layer is present at the surface of the material 4 to be fired, as a result of an oxygen absorption reaction, as shown in Fig. 10; consequently, there remains, in the sintered material obtained, unreacted carbon derived from binder, etc. which are an organic substance or a carbonate, and the sintered material inevitably has low superconductivity. It is an object of the present invention to reduce or avoid this problem of inadequate or non-uniform firing. In the invention a gas of unidirectional flow is generated in a firing furnace at the material surface, so that no oxygen-lean gas layer is present on the surface of a material to be fired, It is also an object of the present invention to provide a process for producing a bottomed cylindrical oxide superconductive material of desired form by firing, in which process a layer to become an oxide superconductive layer when fired, formed on a bottomed cylindrical substrate, can be subjected to uniform firing. The present invention provides a process for producing a bottomed cylindrical body comprising oxide superconductive material by firing, which comprises (i) providing a cylindrical inorganic material body open at both ends in the firing zone of a muffle of a bell-shaped furnace, (ii) providing, inside the inorganic material body, a bottomed cylindrical body comprising (a) a bottomed cylindrical substrate and (b) a layer adapted to become an oxide superconductive layer when fired, formed on the outer surface of the bottomed cylindrical substrate, with the bottom of the cylindrical body directed upward, and (iii) passing a firing gas first along the outer surface of the inorganic material body and then along its inner surface or first along its inner surface and then along its outer surface to maintain the whole of the layer in an atmosphere enabling uniform firing and effect firing of the layer under this condition. The present invention also provides a process for producing a bottomed cylindrical body comprising oxide superconductive material by firing, which comprises (i) providing a cylindrical inorganic material body open at both ends, (ii) providing, outside and surrounding the inorganic material body, a bottomed cylindrical body comprising (a) a bottomed cylindrical substrate and (b) a layer adapted to become an oxide superconductive layer when fired, formed on the inner surface of the bottomed cylindrical substrate, with the bottom of the cylindrical body directed upward and with the bottomed cylindrical body located in the firing zone of a muffle of a bell-shaped furnace or itself forming the muffle of such a furnace, and (iii) passing a firing gas first along the outer surface of the inorganic material body and then along its inner surface or first along its inner surface and then along its outer surface to maintain the whole of the layer in an atmosphere enabling uniform firing and effect firing of the layer under this condition. Preferably, the average flow rate of gas along the layer is in the range 0.1-3.0 m/sec. Consequently, the layer to become an oxide superconductive layer is fired in a gas atmosphere of given temperature and composition, and the resulting bottomed cylindrical superconductive material can express excellent superconductivity and excellent magnetic shielding property and can be used as a magnetic shielding material enabling measurement of the very weak magnetism of living body, etc. Incidentally, the oxide superconductive material used in the present invention is preferably a Y-Ba-Cu-O type compound or a Bi-Sr-Ca-Cu-O type compound each having a multi-layered perovskite structure. In the drawings: Fig. 1 is a drawing showing an example of the present invention. Fig. 2 is a sectional view showing an example of a bottomed cylindrical superconductive material having an inlet for introducing a sensor therein. Fig. 3 is a drawing showing an other example of the present invention. Fig. 4 is a sectional view showing other example of a bottomed cylindrical superconductive material having an inlet for introducing a sensor therein. Figs. 5 to 7 are each a sectional view showing an example of a bell-shaped furnace wherein a cylindrical material is fired. Figs. 8 to 9 are each a sectional view showing a conventional firing furnace. Fig. 10 is a drawing showing the surface condition of a material to be fired when the material is fired in a conventional firing furnace. The present invention is described in more detail with reference to the embodiments shown in the accompanying drawings. However, the present invention is in no way restricted to these embodiments. Figs. 5 to 7, each shows a sectional view of a firing furnace used for firing of a cylindrical material. In Figs. 5 to 7, a fan 43 is provided at the bottom of a bearer 42 in a muffle 41 of a bell-shaped furnace 5⊘; a both ends-opened cylindrical material 44 to be fired is placed on the bearer 42; and an atmosphere in the muffle 41 is forcibly stirred by heat convection and the action of the fan to uniformize the atmosphere in the muffle 41. When the cylindrical material 44 to be fired is opened at both ends as shown in Fig. 5, even if the material 44 consists of a cylindrical substrate and a layer to become an oxide superconductive material when fired, formed on either surface of the cylindrical material, as shown in Figs. 4 to 6, the atmosphere in the muffle 41 is circulated from the outer surface of the cylindrical material 44 to its inner surface in the order of C → D → E, whereby the firing atmosphere is substantially uniformized and uniform firing can be effected. When the cylindrical material 44 to be fired is closed in one end and consists of a cylindrical substrate 44 and a layer 45 to become an oxide superconductive layer when fired, formed on the outer surface of the substrate 44 as shown in Fig. 6 or consists of a cylindrical substrate 44 and a layer 45 to become an oxide superconductive layer when fired, formed on the inner surface of the substrate 44 as shown in Fig. 7, the atmosphere in the muffle cannot be made substantially uniform even by the action of a fan and becomes turbulent when a conventional bell-shaped furnace is used; therefore, the layer to become an oxide superconductive layer when fired, formed on the cylindrical substrate cannot be fired in a uniform atmosphere and the cylindrical oxide superconductive material obtained cannot express excellent superconductivity. Figs. 1 to 4 show examples of the present invention for firing bottomed cylindrical materials. Fig. 1 shows an example of a firing furnace used for firing a bottomed cylindrical material. In the sectional view and the end view taken at its a-a line both shown in Fig. 1, a bearer 42 consisting of a ring-shaped plate 6⊘ and a circular plate 61 is provided in a muffle 41; a fan 43 is provided below the bearer 42; a both ends-opened cylindrical inorganic material 47 is placed on the ring-shaped plate 6⊘ provided with a gap kept against the inner wall of the muffle 41; a material 46 to be fired, consisting of (a) a bottomed cylindrical substrate 44 and (b) a layer 45 to become an oxide superconductive layer when fired, formed on the outer surface of the substrate 44, is placed on the circular plate 61 provided with a gap kept against the ring-shaped plate 6⊘. After the inside of the muffle 41 of a bell-shaped furnace 1⊘ has been provided as above, the furnace 1⊘ is heated and the fan 43 is actuated to allow the atmosphere gas in the muffle 41 to produce a gas flow of A → B → C in the A, B and C zones defined by the muffle 41, the cylindrical inorganic material 47 and the material 46 to be fired. In this case, the flow rate of the gas flow of A → B → C is preferably ⊘.1-3.⊘ m/sec. If the surface area of the material to be fired or the inside volume of the furnace and the sectional area of the gas passage are large, a large amount of a gas must be introduced in order for the introduced gas to have a sufficiently high velocity; this requires the use of a very large amount of a gas and the installation of a very large preheating chamber, which are not preferable in industrial application. However, when the atmosphere inside the furnace is circulated by the fan provided inside the furnace, as in the case of the bell-shaped furnace, the amount of gas introduced can be small. When the atmosphere inside the furnace is circulated, a gas flow rate smaller than ⊘.1 m/sec incurs the presence of an oxygen-lean layer on the surface of the layer to become an oxide superconductive layer when fired, and a gas flow rate larger than 3.⊘ m/sec invites in some cases the flying of oxide particles from said surface; resultantly, the resulting cylindrical superconductive material has no excellent superconductivity. The distribution of gas flow rate varies depending upon the thickness, composition and shape of the layer to become an oxide superconductive layer when fired and the type of atmosphere used for firing, but it is preferably controlled so as to fall usually in a range of 5⊘-15⊘% of the average flow rate by appropriately selecting the sizes of cylindrical inorganic material 47 and material 46 to be fired, the gaps between muffle 41 and cylindrical inorganic material 47 and between cylindrical inorganic mateiral 47 and material 46 to be fired, the type and gas-blowing capacity of fan 43, etc. When the distribution of gas flow rate deviates from 5⊘-15⊘% of the average flow rate, the atmosphere in the muffle tends to become nonuniform and the distribution of superconductivity becomes nonuniform. The same operation applies also to a bottomed cylindrical material as shown in Fig. 2, having an inlet 48 for introducing a sensor or the like. Next, there is described an example of Fig. 3 for firing a material 46 to be fired, consisting of a bottomed cylindrical substrate 44 and a layer 45 to become an oxide superconductive layer when fired, formed on the inner surface of the substrate 44. In the sectional view and the end view taken at its a'-a' line both shown in Fig. 3, the same operation as in the example of Fig. 1 applies except that a bearer 42 is provided in a muffle 41, the bearer 42 consisting of (a) a ring-shaped plate 62 contacting with the inner wall of the muffle 41 and (b) a ring-shaped plate 63 provided with a gap kept against the ring-shaped plate 62, a fan 43 is provided below the bearer 42, a both ends-opened cylindrical inorganic material 47 is placed on the ring-shaped plate 63 of the bearer 42, and a material 46 to be fired, consisting of a bottomed cylindrical substrate 44 and a layer 45 to become an oxide superconductive layer when fired, formed on the inner surface of the substrate 44 is placed on the ring-shaped plate 62. In this case, also, the gas in the muffle 41 is allowed to produce a gas flow of A' → B' → C' in the zones A', B' and C' defined by the cylindrical inorganic material 47 and the material 46 to be fired, whereby the composition and temperature of the gas contacting with the layer 45 to become an oxide superconductive layer when fired are made uniform. As in the example shown in Fig. 1, the distribution of gas flow rate in the zones A', B' and C' is controlled to be in a range of 5⊘-15⊘% of the average gas flow rate. The same operation applies also to a bottomed cylindrical material as shown in Fig. 4, having an inlet 48 for introducing a sensor or the like. In the firing embodiment of Fig. 3, the muffle 41 may be omitted. In that case, a material 46 to be fired, consisting of a bottomed cylindrical substrate 44 and a layer 45 to become an oxide superconductive layer when fired is allowed to act also as a muffle; a cylindrical inorganic material 47 is provided inside the material 46 to be fired; the gas in the material 46 is allowed to produce a gas flow of A' → B' → C'; whereby the composition and temperature of the gas contacting with the layer 45 to become an oxide superconductive layer when fired are made uniform. In the present invention, the muffle of bell-shaped furnace can be usually made of a stainless steel (e.g. SUS 31⊘S, SUS 43⊘) or a metal (e.g. Inconel). The both ends-opened cylindrical inorganic material is preferably constituted generally by a ceramic (e.g. zirconia ZrO₂, alumina Al₂O₃) , a metal (e.g. Inconel, stainless steel), an emameled metal, an inorganic fiber board or their combination; and there is selected a material which is not deteriorated at the firing temperature used or by the firing gas employed. In the present invention, the layer to become an oxide superconductive layer when fired can be formed using materials selected so as to be able to give an oxide superconductive material, according to a known method such as spray coating, doctor blade coating, dip coating or the like. The oxide superconductive material is not particularly restricted and can be, for example, any of (a) rare earth element-containing oxide superconductive materials of M-Ba-Cu-O type having a perovskite structure wherein M is at least one rare earth element selected from Sc, Y and lanthanides such as La, Eu, Gd, Er, Yb, Lu and the like, and (b) bismuth -containing oxide superconductive materials having a composition represented by Bi₂Sr₂Ca₁Cu₂O x or Bi₂Sr₂ Ca₂Cu₃O x . In the present invention, the substrate can be any as long as it can keep the mechanical strength of the above oxide superconductive material. There can be used, for example, ceramics such as ZrO₂, titania (TiO₂) and the like, as well as metals such as SUS 43⊘, SUS 31⊘, SUS 3⊘4, Inconel, Incolloy, Hastelloy and the like. In the present invention, it is possible to use, between the substrate and the oxide superconductive layer, an intermediate layer acting as a protective material and an adhesive, made of a glass or a nobel metal (e.g. Au, Ag, Pt). The type, thickness, etc. of the intermediate layer can be selected depending upon the use purpose, etc. of the oxide superconductive material obtained. In the present invention, firing is generally effected by using an oxygen-containing gas and allowing the gas to produce a desired gas flow. The firing temperature can be appropriately selected depending upon the type, etc. of the oxide superconductive material used. For example, the firing of a bismuth -containing oxide superconductive material can be generally effected by partially melting the material at 875-9⊘⊘°C in a uniform oxygen-containing atmosphere, annealing the partial melt at a temperature of 85⊘°C or less to give rise to crystallization, converting the atmosphere gas to nitrogen and heat-treating the crystal at a temperature of 7⊘⊘°C or less in a uniform nitrogen atmosphere. The present invention is hereinafter described more specifically, by way of example. Examples 12-20 of the present invention and Comparative Examples 1-11 Powders of Bi₂O₃ , SrCO₃ and CuO each as a starting material for Bi-containing oxide superconductive material, were mixed in amounts giving a composition of Bi₂Sr₂CaCu₂O x and then calcinated. The calcinated product was ground by a trommel together with ZrO₂ flint pebbles using isopropyl alcohol as a solvent, to obtain a starting material powder for Bi-containing oxide superconductive material. A cylindrical substrate having a shape as shown in Table 3 was prepared using Inconel 625, SUS 43⊘ or SUS 31⊘S each of 1.98 mm in thickness. On the outer or inner surface of this cylindrical substrate was formed an intermediate layer consisting of a glass layer and a noble metal (Ag) layer or of the latter layer alone. On the intermediate layer was spray-coated or brush-coated the above-obtained powder, whereby a layer to become a Bi-containing oxide superconductive material when fired was formed so that the layer thickness after firing became about 3⊘⊘ µm. Thus, cylindrical materials were obtained. The details of the constitution and formation method of each cylindrical material are shown in Table 1. Each cylindrical material having, as the outermost layer or the innermost layer, a layer to become a Bi-containing oxide superconductive layer when fired, was fired in one of the firing furnaces of Figs. 1 to 6 indicated in Table 1, using a both ends-opened cylindrical inorganic material made of Inconel 625 and having a shape as shown in Table 1, at 88⊘-9⊘⊘°C for ⊘.5-2.⊘ hours in an oxygen-rich gas. The fired product was annealed to about 8⊘⊘°C at a cooling rate of ⊘.5°C/min to crystallize it. Incidentally, the lengths of the sensor inlets in Examples 16-18 and Comparative Examples 9-11 were 1⊘⊘ mm (Example 16 and Comparative Example 9), 32⊘ mm (Example 17 and Comparative Examples 10 and 11) and 6⊘ mm (Example 18). After the crystallization, the atmosphere gas was converted to an oxygen-lean gas and the crystal was heat-treated at 45⊘-7⊘⊘° C to obtain cylindrical oxide superconductive materials containing Bi. Each of the cylindrical oxide superconductive materials containing Bi was measured for Jc (critical current density) at various points. The average value of the measurements was compared with the minimum value, and each material was rated according to the following criteria ○, ▵ and X. The results are shown in Table 3. ○ : minimum Jc value/average Jc value x 1⊘⊘ ≧ 6⊘ (%) ▵ : 6⊘ ≧ minimum Jc value/average Jc value x 1⊘⊘ ≧ 3⊘ (%) X : minimum Jc value/average Jc value x 1⊘⊘ ≦ 3⊘(%) As is clear from Examples 12-20 and Comparative Examples 1-11, both ends-opened cylindrical materials, even when fired in a conventional bell-shaped furnace, give oxide superconductive materials which are uniform and which have substantially uniform Jc values (Comparative Examples 13-16). Meanwhile, bottomed cylindrical materials consisting of (a) a bottomed cylindrical substrate and (b) a layer to become an oxide superconductive layer when fired, formed on either surface (outer or inner) of the cylindrical substrate, give oxide superconductive materials, having low Jc values locally and are difficult to fire uniformly (Comparative Examples 5-11). On the other hand, when there is used the process of the present invention using a both ends-opened cylindrical inorganic material, bottomed cylindrical mateirals consisting of (a) a bottomed cylindrical substrate and (b) a layer to become a Bi-containing oxide superconductive layer when fired, formed on either surface (outer or inner) of the cylindrical substrate, give uniform oxide superconductive materials.

Claims: A process for producing a bottomed cylindrical body (46) comprising oxide superconductive material by firing, which comprises (i) providing a cylindrical inorganic material body (47) open at both ends in the firing zone of a muffle (41) of a bell-shaped furnace, (ii) providing, inside the inorganic material body (47), a bottomed cylindrical body (46) comprising (a) a bottomed cylindrical substrate (44) and (b) a layer (45) adapted to become an oxide superconductive layer when fired, formed on the outer surface of the bottomed cylindrical substrate (44), with the bottom of the cylindrical body (46) directed upward, and (iii) passing a firing gas first along the outer surface of the inorganic material body (47) and then along its inner surface or first along its inner surface and then along its outer surface to maintain the whole of the layer (45) in an atmosphere enabling uniform firing and effect firing of the layer (45) under this condition. A process for producing a bottomed cylindrical body (46) comprising oxide superconductive material by firing, which comprises (i) providing a cylindrical inorganic material body (47) open at both ends, (ii) providing, outside and surrounding the inorganic material body (47), a bottomed cylindrical body (46) comprising (a) a bottomed cylindrical substrate (44) and (b) a layer (45) adapted to become an oxide superconductive layer when fired, formed on the inner surface of the bottomed cylindrical substrate (44), with the bottom of the cylindrical body (46) directed upward and with the bottomed cylindrical body (46) located in the firing zone of a muffle (41) of a bell-shaped furnace or itself forming the muffle of such a furnace, and (iii) passing a firing gas first along the outer surface of the inorganic material body (47) and then along its inner surface or first along its inner surface and then along its outer surface to maintain the whole of the layer (45) in an atmosphere enabling uniform firing and effect firing of the layer (45) under this condition. A process for producing a bottomed cylindrical oxide superconductive material by firing according to claim 1 or claim 2, wherein the average flow rate of the gas along the layer (45) is in the range 0.1-3.0 m/sec.

Inventors: SHIMIZU HIDEKI NGK, TOGASHI TAKEYOSHI, SHIMIZU, HIDEKI, 205, NGK, TOGASHI, TAKEYOSHI
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1991
Main classification: C04B35
IPC codes: C04B35, H01L39, F27B17