Patent Application: US-19405888-A

Abstract:
two or more superconductors are reacted together to form a non - superconducting phase encapsulating a residual superconducting phase . for example yba 2 cu 3 oxide is reacted with bi 2 sr 2 cacu 2 oxide to form a superconducting product containing a novel non - superconducting oxide of y , bi , and ba . a superconducting yba 2 cu 3 oxide remains as an interconnected phase throughout the product . the new non - superconducting phase largely encapsulates and insulates the superconducting phase .

Description:
as above noted , the invention process is a general one . it will be exemplified using two known superconductors , namely the oxides of yba 2 cu 3 and bi 2 sr 2 cacu 2 . bi 2 sr 2 cacu 2 o 8 + x reacts with yba 2 cu 3 o 7 - x at 950 ° c . to produce a new , face - centered cubic phase with a = 8 . 55 angstroms containing y , bi and ba . this phase appears to be isomorphous with the high - temperature form of cd 3 bi 10 o 18 . reacted samples are superconducting at values of x as high as 0 . 2 in the compositional formula ( yba 2 cu 3 ). sub . ( 1 - x ) ( bi 2 sr 2 cacu 2 ) x o y with a t c of approximately 83k . x = 0 . 2 corresponds to a material with approximately 60 wt . % cubic phase and 40 wt . % residual yba 2 cu 3 o 7 - x . the cubic phase is not superconducting above 77k , but when prepared via a solution - phase route , shows a semiconductor - to - metal transition at approximately 120k . critical temperatures for the 0 & lt ; x ≦ 0 . 2 compositions appear to be independent of x . yba 2 cu 3 o 7 - x (&# 34 ; y - 123 &# 34 ; and bi 2 sr 2 cacu 2 o 8 + x (&# 34 ; bi - 2212 &# 34 ;) are both superconductors with t c & gt ; 77k , but they have quite distinct crystal structures ( 1 - 2 ). ( references collected below .) there is already a considerable amount of literature on substitution of transition metal ions into the y - 123 structure ( 3 - 6 ), but not , so far as i am aware , on the reaction of the two superconducting materials together . this invention involves the products of the reaction between y - 123 and bi - 2212 at a temperature that lies between the melting points of the two materials ( 950 ° c .). the invention also includes materials of similar composition , synthesized by a coprecipitated carbonate process , starting with the salts of the individual component metals . ( the &# 34 ; coprecipitated carbonate &# 34 ; process is described hereinafter .) two sets of materials of general formula ( yba 2 cu 3 ). sub . ( 1 - x ) ( bi 2 sr 2 cacu ) 2 ) x o y were prepared , and are listed in table i . table i______________________________________preparation method , overallcomposition and t . sub . c for ( yba . sub . 2 cu . sub . 3 ). sub . ( 1 - x ) ( bi . sub . 2 sr . sub . 2 cacu . sub . 2 ). sub . xo . sub . y samplessynthetic method x t . sub . c ( zero resistance )/ k______________________________________composite 0 . 053 83composite 0 . 077 83composite 0 . 158 83composite 0 . 200 83composite 0 . 429 semiconductorcoprecipitated 0 . 053 83 ( non - zero ) coprecipitated 0 . 077 83coprecipitated 0 . 158 83 ( non - zero ) coprecipitated 0 . 200 85 ( non - zero ) coprecipitated 0 . 429 semi - conductor to ca . 120k , metal below______________________________________ in one set , composites of y - 123 and bi - 2212 were synthesized by grinding the individual superconductors together in the correct ratios in an agate mortar , pressing them into pellets at 20 , 000 p . s . i . and firing . the starting superconductive ( w . r . grace and co . super - t c - y123 and super t c - b2212 ) had a particle size of 1 - 10 μm and a chemical purity of & gt ; 99 . 9 %. the firing schedule involved heating at 950 ° c . for six hours , cooling to 600 ° over two hours and then cooling to 400 ° c . over 8 hours . in the other set , the samples were prepared by coprecipitation as carbonates from a solution containing the mixed metal nitrates in the correct stoichiometric ratios . the process is described in u . s . ser . no . 155 , 340 filed feb . 12 , 1988 , above referenced . in the process the aqueous solution of the mixed metal nitrates ( y . ba , cu , bi , sr , and ca ) are reacted with an aqueous solution of a quaternary ammonium carbonate , e . g ., tetramethylammonium carbonate , by adding nitrates and quaternary carbonate separately slowly into a reaction vessel , while maintaining the ph in the latter in the range of about 7 . 5 - 10 by drip - wise addition of tetramethylammonium hydroxide . the precipitated carbonates were dried at 110 ° c . and then heated at 540 ° c . in air for 2 hours . the mixed oxides and carbonates produced in this way were then heated in oxygen at 800 ° c . for 12 hours , cooled to 600 ° c . over two hours and to 400 ° c . over eight hours . the gray - black powders were then pressed into pellets and fired , using the same conditions as described for the composite pellets . the metal nitrates ( aldrich chemical co .) were of ≦ 99 . 9 % purity . in both composite and coprecipitated samples , the value of x was deliberately restricted to 0 . 5 in order to avoid melting of the ( bismuth - rich ) samples at the particular processing temperature chosen . x - ray powder diffraction patterns were obtained with a philips apd 3600 diffractometer ( graphite monochrometer and theta - compensating slits ) using cukα radiation and a scan rate of 2 deg 2θ min - 1 . the reported line positions were corrected using an internal silicon standard ( srm - 640b ). y - 123 concentrations were calculated by assuming that the intensity of the lines from the y - 123 phase in the reacted samples was the same as from y - 123 itself , and by constructing a calibration curve based on the x - ray mass absorption coefficients for a binary mixture of y - 123 and bi - 2212 . scanning electron micrographs were taken on a hitachi model 5570 with a kevex 7000 energy dispersive x - ray fluorescence ( edx ) unit attached . all images were recorded with a 20 kv electron beam energy a . c . magnetic susceptibility data were taken using a quantum technology corp . meissner probe operating at a frequency of 20 khz and a maximum a . c . field of 1 oe . resistivity was measured with a four - point resistance meter ( keithley 580 ) using indium contacts to the sample pellet . in fig1 x - ray powder diffraction patterns for the y - 123 starting material and all composite samples are shown . as might be expected , lines characteristic of the y - 123 become less intense as x increases . it is more surprising that by x = 0 . 429 these lines have totally disappeared , having become replaced by a majority phase displaying a cubic pattern , together with traces ( i . e . relative intensities 10 %) of cuo and an unidentified phase . this cubic phase is already clearly visible at x = 0 . 053 . in none of the samples were lines characteristic of the bi - 2212 superconductor to be seen . the cubic pattern indexes as a face - centered cubic structure with a = 8 . 55 angstroms ( see table ii ). table ii______________________________________powder x - ray diffraction linesfor the cubic phase in ( yba . sub . 2 cu . sub . 3 ). sub . ( 1 - x ) ( bi . sub . 2 sr . sub . 2 cacu . sub . 2 ). sub . xo . sub . y obtainedusing cu - kα radiationh k l d ( obs .) d ( calc .) i / i . sub . o______________________________________1 1 1 4 . 934 4 . 936 82 0 0 4 . 276 4 . 275 22 2 0 3 . 027 3 . 023 1003 1 1 2 . 578 2 . 578 84 0 0 2 . 139 2 . 138 333 3 1 1 . 963 1 . 962 54 2 2 1 . 746 1 . 745 514 4 0 1 . 510 1 . 511 225 3 1 1 . 447 1 . 445 46 0 0 1 . 424 1 . 425 26 2 0 1 . 351 1 . 352 184 4 4 1 . 234 1 . 234 56 4 2 1 . 141 1 . 143 23______________________________________ this phase appears to be isomorphous ( fig2 ) with the high - temperature form of cd 3 bi 10 o 18 reported by kutvitskii et al ( 7 ). however , the cadmium compound is reported to have a unit cell dimension of a / 2 , 4 . 24 angstroms . the corresponding x - ray patterns from the coprecipitated set of materials are shown in fig3 . the very strong similarity between the sets suggests that the formation of the cubic phase is a thermodynamically controlled phenomenon rather than a diffusion - controlled process . in the coprecipitated set , small peaks associated with traces of unreacted barium carbonate at 2θ ˜ 24 ° can be seen . also , y - 123 peaks , which persist to x = 0 . 158 in the composite set , appear to collapse to single peaks , even at the lowest level of bi - 2212 incorporation ( x = 0 . 053 ). this indicates that , in addition to the formation of the cubic structure , the y - 123 phase is being transformed to a tetragonal structure in the presence of the bi - 2212 components . from the x - ray powder patterns , it was calculated that the composite sample with x = 0 . 2 has a y - 123 concentration of 40 (± 10 ) wt %. this implies that approximately 1 / 2 of the y - 123 has reacted with the bi to form the cubic phase . fig6 - a shows a scanning electron micrograph of a fracture surface produced by breaking a pellet made from the x = 0 . 158 composite material . two different morphologies are clearly visible : a smooth , dark , sintered region ( a ), and areas consisting of approximately 1 μm - sized discrete , white particles ( b ). bismuth and copper x - ray fluorescence maps of this same region are shown in fig6 - b and 6 - c , respectively . the copper appears to be associated mainly with the smooth regions and the bismuth with the discrete particles . edx spectra of these two regions show that a contains y , ba , cu , with only traces of ca and sr , while b contains y , bi and ba , with traces of ca and cu . these two compositions appear to be associated with the y - 123 and cubic phases respectively . fig6 - d shows the equivalent x = 0 . 158 material , prepared by coprecipitation . in this sample , the large , smooth , dark areas are replaced by 5 - 10 μm &# 34 ; chunks &# 34 ;. these contain y , ba , cu , and traces of ca , and sr , as in the composite sample , but they also contain a trace of bismuth . the presence of bi may induce the y - 123 phase to adopt the tetragonal structure seen in the x - ray pattern of this material . this behavior has been observed in y - 123 that has been doped with iron ( 3 - 6 ): as little as a 2 % substitution of iron for copper in y - 123 can lead to an orthorhombic - to - tetragonal transformation , without loss of superconductivity . the b particles are also present in the coprecipitated sample and have a similar y - bi - ba composition ( with traces of ca and cu ) to those seen in the composite material . at x = 0 . 429 , the b particles corresponding to the cubic phase dominate the structures as seen by s . e . m . as before , for both composite and coprecipitated samples , this phase appears ( by edx ) to consist primarily of y , bi and ba , with traces of ca and cu . the remainder of the copper is mostly present as small , smooth chunks , which , in the composite material , contain only traces of the other elements , and in the coprecipitated sample , contain large concentrations of both strontium and calcium . it appears that both phases in these complex systems exist as solid solutions , and that the exact partition of the elements between the phases is a kinetically controlled phenomenon , determined by the starting materials from which they were synthesized . the results of resistivity vs . temperature measurements are summarized in table i . the starting y - 123 showed zero resistance at 92k . all samples except those with the highest bismuth concentration ( x = 0 . 429 ) show a sudden drop in resistance with an onset temperature of ˜ 88k . all the composite samples that show this drop reach a measured zero resistance at 83k ( see , for example , fig4 ). this indicates the persistence of an interconnected superconducting phase . three of the coprecipitated samples show a resistance vs . temperature curve with a similar shape to the superconducting composite samples . however , in these materials , the sudden resistance drop terminates in a small , finite value at ˜ 83k . this small residual resistance is probably due to a thin coating of baco 3 ( the presence of which was indicated by xrd ) between superconducting grains . both of the two x = 0 . 429 samples show an increasing resistance with temperature down to 120k . in the composite sample , this trend continues down to the lowest temperatures investigated ( 81k ), while in the coprecipitated sample , the temperature coefficient of resistance appears to change sign at ˜ 120k ( see fig5 ). resistance drops in all other samples correlate with a large diamagnetic signal detected in a . c . susceptibility measurements . however , in the x = 0 . 429 coprecipitated material , no such signal was detected , indicating that the resistance drop is due to a semiconductor - to - metal transition . although the corresponding composite sample appears to be semiconducting at all temperatures investigated , fig5 shows that a resistance fluctuation is present at 120k , indicating that a small fraction of this sample may also be undergoing a similar transition . the differences between the samples are probably due to minor variations in the cubic phase solid solution composition . from the above , it is evident that bi - 2212 reacts completely with y - 123 at 950 ° c . to produce a new , face - centered cubic phase of side 8 . 55 angstroms , containing yttrium , bismuth and barium , with traces of copper and calcium in solid solution . xrd analysis of the x = 0 . 2 composite sample showed only the cubic and y - 123 phases to be present , the latter making up ˜ 40 wt . % of the total . it is remarkable that this sample was superconducting at 83k in the presence of ˜ 60 wt . % of non - superconducting material , as shown by both resistivity and magnetic susceptibility measurements . the t c of both composite and coprecipitated superconducting materials is quite constant at 83k , irrespective of bi content . this is surprising , given the extensive solid solution formation and cation substitution by ca and bi , that in the composite case is sufficient to change the y - 123 from an orthorhombic into a tetragonal structure . 1 . siegrist , t . ; 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