STABLE HIGH OXIDATION STATE SUPERCONDUCTING OXIDES

Abstract: High temperature superconducting oxide materials can be taken to a higher, but stable, state of oxidation by removing H-impurities, such as OH<->, using I2/O2 mixtures in a reactive atmosphere process. A higher Tc and a narrower DELTA T-transition result.

Description: STABLE HIGH OXIDATION STATE SUPERCONDUCTING OXIDES BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to high temperature superconducting oxides, and, more particularly, to a pro¬ cess for improving the superconducting properties thereof. 2. Description of Related Art The impurity OH " , in place of 0 ~ (c) , blocks charge (hole) transfer, 0 " (c) to 0 = (c), necessary for superconduc¬ tion in high temperature superconducting oxides. The re¬ moval of OH ~ (c) is a necessary step towards achieving a stable higher state of oxidation. In a paper by D.P. Devor et al, "Hydroxyl impurity effects in YAG(Y 3 A1 = ,0 ;L2 )", Journal of Chemical Physics, Vol. 81 (No. 9), pp. 4104-4117 (1984), it was shown that OH ~ (c) was still present in significant amounts after crys- tal growth at ss2,000°C. High temperature superconducting materials, being processed at a much lower temperature («900 * C) , are expected to have an even higher H-impurity content. In a paper by L.E. Gorre et al, "Hydrogen impurities in oxides and halides". Materials Research Bulletin. Vol. 20, pp. 1441-1451 (1985), it was shown that when the free- OH ~ content in vitreous Si0 2 was reduced, using the proper reactive atmosphere processing (RAP) oxidizing combination, the lattice contracted and the material became extremely radiation hard. It should be recalled that lattice con¬ traction raises T c because that parameter has a negative coefficient with pressure. In a paper by R.C. Pastor et al, "Reactive atmosphere processing of oxides". Materials Research Bulletin. Vol. 16, pp. 1413-1420 (1981), the authors studied degradation of rare-earth oxides, particularly L 2 0 3 , by water vapor. They showed that the oxidizing combination of I 2 /0 2 stabi¬ lized La 2 0 3 , prolonging the onset of H 2 0-uptake by «4 hours (induction period). It is now common knowledge that high temperature superconducting oxides, such as YBa 2 Cu 3 0-7, are easily degraded by water (vapor or liquid) . In a paper by R.C. Pastor et al, "Crystal growth in a reactive atmosphere", Materials Research Bulletin, Vol. 10, pp. 117-124 (1975), the authors drew attention to the reac¬ tivity of the halogen (X) for displacing 0H ~ impurity by electron transfer, X(g) + 0H " (c) ===> X ~ (c) + OH°t (1) A need remains for a process to improve the supercon¬ ducting properties and stability of high temperature super- conducting oxides. SUMMARY OF THE INVENTION In accordance with the invention, a high temperature superconducting oxide is taken to a higher, but stable, state of oxidation by removing H-impurities, such as OH ~ . A higher T c and a narrower ΔT-transition result. Any of the well-known so-called high temperature superconducting oxides may be treated in accordance with the teachings of the invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representation of a chemistry oxidation model of a high temperature superconducting o ^ ide; FIG. 2, on coordinates of weight percent and time, is a thermogravimetric analysis (TGA) thermograms of YBa 2 Cu 3 - 0 7 _ x under I 2 /0 2 and under 0 2 at 400 * C for 64 hours; FIG. 3, on coordinates of weight percent and time, is a TGA thermogram of Bi 4 Ca 3 Sr 3 Cu 4 0 16 . i< . under I 2 /0 2 and under 0 2 at 400°C for 18 hours; FIGS. 4a and 4b, on coordinates of weight percent and temperature, are TGA thermograms of YBa 2 Cu 3 0 7 annealed un¬ der dry 0 2 (FIG. 4a) and wet 0 2 (FIG. 4b); and FIG. 5a, on coordinates of weight percent and tempera¬ ture, is a TGA thermogram of YBa 2 Cu 3 0 7 annealed under I 2 /- 0 2 ; and FIG. 5b is a comparison of the weight difference be¬ tween the heat-up and cool-down curves of YBa 2 Cu 3 0- 7 an- nealed under dry 0 2 , wet 0 2 , and I 2 /0 2 versus temperature. DESCRIPTION OF THE PREFERRED EMBODIMENTS High temperature superconducting oxides, such as (RE) 1 Ba 2 Cu 3 0 v _ x , where RE is a rare earth, the Bi-Ca-Sr-Cu- O family, La z . y Sr y CuO ^ , and Tl 2 Ba 2 CaCu 2 0 β , evidence higher superconducting transition temperature, T c , and narrower ΔT-transition by removal of H-impurities, such as OH-. Current-carrying capability is also increased, along with increased stability toward the degradative action of H 2 0 and increased resistance to damage by ionizing radiation. The following two equations display the one-dimension¬ al movement of a hole in a line of oxide ions, with (Eqn. (3)) and without (Eqn. (2)) an incorporated hydroxide: 0=( C ) <==> o ~ (c) <==> 0=(c) <==> etc. (2) 0=(c) <==> 0 ~ (c) <=/=> OH ~ (c) <=/=> 0=(c) <==> 0 = (c) <==> etc. (3) The movement of electrons is, of course, in the opposite direction to the movement of holes, and is just as applica¬ ble to the model. In Eqn. (2), the hole is spread over the dimension by resonance. It is not so, in Eqn. (3); OH ~ (c) , being a pseudohalide (stable electron configuration) , presents a high energy barrier to the transfer. Therefore, a higher and more stable oxidation state may be achieved if OH ~ (c) is replaced by 0 ~ (c). The two-dimensional movement is illustrated in FIG. 1. Movement occurs where (-) is 0 ~ , but not where (-) is 0H ~ . The higher state of oxidation in the high temperature su- perconducting oxide achieved with the use of I 2 /0 2 over that which made use of.0 2 alone, as practiced conventional¬ ly, is seen in FIG. 2. FIG. 2 stands for two separate runs on the pristine (unoxidized) samples: one under pure 0 2 atmosphere (Curve 10) and the other under I 2 /0 2 (Curve 12); both soaked at 400°C. Weight pickup versus time is the net value between weight loss - removal of 0H ~ and H 2 0 - and the incorpora¬ tion of oxygen as 0 ~ . Since it has been shown that no io¬ dine-pickup occurs at 400°C, the layer net weight pickup under I 2 /0 2 indicates higher content of 0 ~ ; hence, a higher state of oxidation in the solid. That relates to FIG. 5b, discussed in further detail below, which shows that the superoxidized (I 2 /0 2 ) sample suffers the largest weight loss upon thermal cycling to 700°C. The enthalpy change (units: kcal/ ole) for the reac¬ tion of Eqn. (1) in the gas phase is -42.7 for X = F, -48.0 for X = Cl, -41.7 for X = Br, and -36.0 for X = I. Howev¬ er, in an oxide matrix, X ~ incorporation is unacceptable. Thus, it must be displaced to restore the oxide stoichiome- try, χ-(c) + ^o 2 ===> 0~(c) + X°τ. (4) The requirement of Eqn. (4) makes X = F not applica- ble, since forward displacement of Eqn. (4) is not allowed: oxygen cannot replace fluorine. The equilibrium of Eqn. (4) depends on a component reaction, MO ( c ) + X 2 ( g ) <===> MX 2 ( c ) + *jO a ( g ) , ( 5) which, in turn, depends on the electropositive nature of M. Here the reaction has been normalized to one gram-atom ox- ide. Taking into account the electropositive makeup of M in high temperature superconducting oxides eliminates X = Cl. In particular, for YBa 2 Cu 3 0 7 , only X = I can be used and in high dilution by the carrier 0 2 ; hence, the use of I 2 /0 2 , where I 2 is about 1 to 5 mm and the balance is O z (total pressure = 1 atm) . The efficiency of the dilute mixture for achieving a high oxidation state has been shown in FIG. 2. However, where Bi replaces the rare earth as in high temperature superconducting oxides of the Bi-Ca-Sr-Cu- O family, it is possible to use X = Br as well as X = I. Attention has been drawn to the Bi-formulation not requir¬ ing any oxygen annealing; see, e.g., R. Dagani, "New Super¬ conductor", Chemical and Engineering News, page 4 (15 Feb 1988). The effectiveness of I 2 /0 2 in raising the Bi-formu¬ lation to a higher state of oxidation is seen in FIG. 3, where Curve 14 indicates treatment in pure 0 2 and Curve 16 indicates treatment in I 2 /0 2 in accordance with the inven¬ tion. In a paper by R.C. Pastor et al, "Impurity-conditioned solid-solid transition in simple halides", Materials Re- search Bulletin. Vol. 9, pp. 569-578 (1974), the appearance of various solid phases does not always occur sequentially in temperature. When pure orthorhombic RF 3 (R = a rare earth) is heated in an H 2 0-free atmosphere, the only phase transformation seen is melting. The melting-freezing be- havior is reversible. However, under the 25% humidity of air, a solid-solid transition to hexagonal precedes melt¬ ing. The behavior is irreversible, the temperature differ¬ ence between solid-solid transition and melting being de¬ pendent on the previous history. Removing the H-impurity eliminated the hexagonal phase, resulting in an increase in the thermal-stability region of the orthorhombic phase. In view of the high OH ~ (c) content of conventional high tern- perature superconducting oxide materials, the tetragonal- to-orthorhombic transition reported in YBa 2 Cu 3 0 7 is symp¬ tomatic of the impurity influencing the kinetics of two competing transitions. Thus, reactive atmosphere process- ing (RAP) oxidation could even extend the thermal stability existence region of the orthorhombic (the superconducting phase) of YBa 2 Cu 3 0 ? . FIG. 4 compares the weight versus temperature behavior of YBa 2 Cu 3 0 7 in dry and wet 0 2 . "Dry" means as taken di- rectly from the gas cylinder and "wet" refers to the same o 2 that has been equilibrated to »100% humidity at 25 * C. The lower portion of the curve is the heat-up and the upper portion the cool-down. The slightly higher weight pickup by wet 0 2 is evidence that, aside from oxygen (presumably as 0 ~ ) , 0H ~ (c) and interstitial H 2 0 was also added to the material, 0=(c) + H 2 0(g) <===> 20H ~ (c). (6) While X-ray analysis proved that the orthorhombic structure was preserved, the literature shows that the material annealed in wet 0 2 will no longer be superconduct¬ ing; only the "dry" process yields a superconducting mate¬ rial. The sample annealed in dry 0 2 showed a T c of «92K and a ΔT of «10K. FIG. 5 shows the behavior under I 2 /0 2 . It will be noted that in FIG. 5(a) that the heat-up portion registers considerable weight loss. Displacement achieved by means of Eqns. (3) and (4) would not lead to a large weight decrease (mass 17 of OH- replaced by 16 of 0 ~ ) . The reverse of Eqn. (6) is a weight loss. The process is active in heat-up at the surface for dry 0 2 and I 2 /0 2 , but not wet 0 2 . That process produces an anion vacancy []. Build-up of [] increases the escape ten¬ dency of oxide: 0=(c) + [] <===> 2[t] + ^0 2 (g) (7) where [t] is a color center (see, e.g., D.P. Devor et al, supra ) . However, with a low-energy-barrier oxidation available, such as that represented by Eqns. (3) and (4), [t] + 0 2 (g) <===> 20 ~ (c), (8) the material achieves a higher oxidation state. YBa 2 Cu 3 0 7 treated in accordance with the invention (FIG. 5a) was found to have a T c of «101K and a ΔT of «5K, or an increase of about 9K in T 0 and a decrease in ΔT of about.5K. FIG. 5b is a plot of the weight difference between heat-up and cool-down versus temperature for the three cases: dry 0 2 (Curve 18), wet 0 2 (Curve 20), and I 2 /0 2 (Curve 22), in ac¬ cordance with the invention. The weight difference at >350°C cannot be distinguished between the case of dry and wet 0 2 . That would mean that the pick-up of OH ~ by Eqn. (6) from outgas H 2 0, for the case of dry 0 2 , is just as efficient as in the case of wet 0 2 . However, the behavior is different for the case of I 2 /0 2 . In the presence of the same amount of outgas H 2 0, the weight difference is consid¬ erably higher. That can only be attributed to weight pick¬ up as incorporated oxygen (presumably 0 = ) . In further support of the efficacy of treating super¬ conducting oxides with I 2 /0 2 , a single crystal of La 2 Cu0 4 was found to be non-superconducting. The measurement was based on zero-field microwave absorption. When this cry¬ stal was similarly oxidized by I 2 /0 2 , superconduction started at 20K. Based on such evidence, it is possible that the Bednorz-Muller material, La 2 _ sc Sr ϊe Cu0 4 , which was reported to have a T c of «40K, would have a higher value of T,.. and a narrower ΔT by I 2 /0 2 oxidation. As indicated above, the superconducting oxide is treated with a mixture of iodine and oxygen, with the io¬ dine concentration ranging from about 1 to 5 mm in a total of 1 at pressure. Below about 1 mm, the kinetics of the reaction are too slow, while at greater than about 5 mm, the iodine begins to be absorbed by the oxide. The forward speed of Eqn. (1), namely, displacement of OH- by iodine, depends on the concentration [I] which, in turn, depends on [I 2 ] and the working temperature. The latter cannot be in¬ creased "ad infinitum" without sacrificing the oxidation state. However, one cannot make-up by increase in [I 2 ], as that makes it more difficult to remove incorporated iodine by Eqn. (1) by the displacement action of 0 2 , the reverse direction of Eqn. (5) . As indicated above, the use of bromine is thermodynam-r ically possible for bismuth-substituted high temperature superconducting oxides. The bromine concentration is ex¬ pected to be quite less than about 1 mm, with the balance oxygen, in a total of 1 atm pressure. The flow rate of the gas mixture is about 60 cm 3 /min, although this is not very critical. I 2 is condensed after being passed over the superconducting oxide and recovered for recycling. In a furnace tube about 500 cm long, it takes about 10 to 15 minutes to replenish the atmosphere, and it has been found that this is an adequate flow. rate. The treatment is carried out at a temperature of at least about 850°C; less than this temperature is too slow to achieve the desired effect. The treatment must be kept below the melting point of the oxide, which for many of these superconducting oxides is about l,10θ'C. Preferably, the treatment is carried out at a temperature ranging from about 900°C to less than about 1,000°C. The time of treatment is at least a few hours, and may go overnight. Typically, the time of treatment is about 4 to 15 hours. The superconducting oxide may be treated in powder form or as pellets (after sintering) . Thus, there has been disclosed a process for increas¬ ing the superconducting transition temperature and narrow- ing the ΔT-transition in a high temperature superconducting oxide. It will be readily apparent to one skilled in this art that various changes and modifications of an obvious nature may be made without departing from the spirit of the invention, and all such changes and modifications are con¬ sidered to fall within the scope of the invention as de¬ fined by the appended claims.

Claims: CLAIMS What Is Claimed Is: 1. A process for increasing the superconducting tran¬ sition temperature and narrowing the ΔT-transition in a high temperature superconducting oxide comprising: (a) providing said superconducting oxide; and (b) treating said oxide with a gas reactive mix¬ ture of either iodine or bromine and oxygen at an elevated temperature. 2. The process of Claim 1 wherein said gas mixture comprises about 1 to 5 mm iodine and the balance oxygen in a total atmosphere of about one atm. 3. The process of Claim 1 wherein said elevated tem¬ perature ranges from about- 850°C to less than about 1,100°C. . The process of Claim . 3 wherein said elevated tem¬ perature ranges from about 900"C to less than about 1,000°C. 5. The process of Claim 1 wherein said oxide is ex- posed at said elevated temperature for a period of time of at least about 2 hours. 6. The process of Claim 5 wherein said period of time ranges from about 4 to 15 hours. 7. The process of Claim 1 wherein said high tempera¬ ture superconducting oxide includes bismuth and wherein said gas mixture comprises less than about 1 mm bromine and the balance oxygen in a total atmosphere of about one atm. 8. The process of Claim 1 wherein said superconducting oxide is in powder form. 9. The process of Claim 1 wherein said superconducting oxide is in pellet form and said treatment follows a sin¬ tering step to form said pellet. 10. The process of Claim 1 wherein said gas mixture is flowed over said superconducting oxide at a rate of about 60 cm 3 /min. 11. The process of Claim 1 wherein said superconduct- ing oxide is selected from the group consisting of (RE) 3 .- Ba 2 Cu 3 0 7 _ x , where RE is a rare earth, the Bi-Ca-Sr-Cu-O family, La 2 _ y Sr y Cu0 4 , and Tl 2 Ba 2 CaCu 2 O β . 12. The process of Claim 11 wherein said superconduct- ing oxide is selected from the group consisting of YBa 2 - Cu 3 0 7 _ x , Bi 4 Ca 3 Sr 3 Cu 4 O α6^x , and La 2 Cu0 4 .

Inventors: FULLER KEITH C, GORRE LUISA E DI, PASTOR ANTONIO C DI, PASTOR RICARDO C, FULLER, KEITH, C., GORRE, LUISA, E. +DI, PASTOR, ANTONIO, C. +DI, PASTOR, RICARDO, C.
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1992
Main classification: C04B35
IPC codes: C01G1, C04B35, C04B41, H01L39