Patent Application: US-68922691-A

Abstract:
a novel method for forming homogeneous silver high temperature superconductor composites . the novel method comprises a chemical coprecipitation of silver , barium , yttrium , and copper salts solutions , followed by calcination and milling processes . the novel method has an advantage of retaining all the virtues immanent in a composite hts , for example , increased critical current density , and improved mechanical properties , while bypassing extant and deficient methodologies for forming a composite , the deficient composites characterized by heterogeneity .

Description:
the novel coprecipitated precursor method of the present invention , summarized above , may be used to prepare a composite powder comprising silver or silver oxide and y - ba - cu oxide high temperature ceramic superconductor . an illustrative flowchart 10 of the overall coprecipitation process is shown in fig1 . we now work through the fig1 flowchart 10 . in fig1 a step 1 ( box 12 ) shows that an aqueous solution comprising the yttrium , barium , copper , and silver ions , in an illustrative ratio 1 : 2 : 3 : 2 ( for 25 % w / w ag in the composite ), may be prepared using hydrated or anhydrous nitrate salts . the total metal ion concentration is preferably adjusted to approximately 1 molar . the metal salt solution ( box 14 ) ( ph : 2 - 3 ) at room temperature is rapidly added , ( step 2 ), with vigorous stirring ( step 3 , arrow 16 ) to a chilled ( preferably 5 °- 10 ° c .) aqueous solution comprising a combination of sodium ( or potassium ) hydroxide and sodium ( or potassium ) carbonate , in amounts adequate to ensure complete precipitation of the metals , and to maintain ph 10 - 11 of the solution after addition is completed . the formed precipitate ( box 18 ) may be collected by either filtration ( step 4 , box 20 ) or centrifugation , and is preferably thoroughly washed ( step 5 , box 22 ) with deionized water to remove all the residual alkali metal ions . the ph of the filtrate is continuously monitored during washing . washing is discontinued when the ph value is approximately 10 . this is necessary in order to minimize the loss of barium , which is partially soluble at greater than 7 ph . the washed coprecipitate is preferably oven dried ( step 5 ) at preferably 80 ° c . in air ( box 24 ). next , the dried coprecipitate is ground to a free - flowing powder ( preferably 100 mesh ), and calcinated ( step 7 , box 26 ) in a stream of dry air or oxygen for approximately 6 hours at 850 °- 900 ° c ., to generate the high temperature superconductor phase . the calcined powder is preferably allowed to cool slowly ( in the furnace ) to room temperature in an air or oxygen stream to ensure that the full oxygen stoichiometry is attained . the black , friable solid product may be removed from the furnace and further ground ( step 8 , box 28 ) to give a free - flowing composite powder ( box 30 ). the final composite material may be characterized using a variety of analytical techniques . phase purity of the ceramic superconductor may be assessed by x - ray powder diffraction analysis . this technique can be supplemented by differential thermal analysis ( dta ), to identify small quantities of low melting point impurity phase ( s ). another thermal analysis technique , thermogravimetric analysis ( tga ), can be used to determine the oxygen stoichiometry of the ceramic superconductor phase . the metals analysis may be performed by either inductively coupled plasma ( icp ) emission spectroscopy , or atomic absorption ( aa ) spectroscopy . the specific surface area of the composite powder may be determined by the b . e . t . multipoint nitrogen adsorption technique . the ceramic microstructure of the sintered composite can be observed by scanning electron microscopy , and the qualitative elemental composition of individual grains or small regions can be determined using energy dispersive x - ray spectroscopy ( edx ). a vibrating sample magnetometer equipped with a liquid helium cryostat may be used to determine the magnetic properties of the superconducting composite . magnetization may be measured as a function of temperature at a low constant applied magnetic field and the critical temperature , the critical temperature transition width , and superconductivity fraction scf ( an estimate of the fraction of the bulk sample that is superconducting ) may be determined for the final composite material . also , magnetization may be measured as a function of the applied magnetic field at a constant temperature and the resulting hysteresis loop . the area enclosed in the loop is indicative of the amount of magnetic flux pinning in the composite material . this also allows an estimate of the intragranular critical current density in the composite material . physical property measurements such as knoop hardness can be measured on a polished section of a pressed pellet of the bulk sintered composite . the coprecipitated precursor for the preparation of a silver / y - ba - cu oxide superconductor composite containing 25 % w / w ag was prepared as follows . the ba ( no 3 ) 2 ( 39 . 22 g ) was dissolved separately with heating and stirring in 0 . 5 l deionized water . the y ( no 3 ) 3 · 6h 2 o ( 28 . 72 g ), cu ( no 3 ) 2 · 2 . 5h 2 o ( 52 . 33 g ), and agno 3 ( 25 . 78 g ) were dissolved in another 0 . 25 l deionized water . the solution containing the y , cu , and ag nitrates was added slowly to the ba nitrate solution at room temperature . the combined metal nitrate solution was added quickly to a chilled ( 5 °- 10 ° c . ), rapidly stirred solution containing na 2 co 3 · h 2 o ( 62 . 00 g ) and naoh ( 27 . 00 g ) in 1 l deionized water . an instantaneous precipitation took place , and the resulting olive - green slurry was stirred for an additional 15 minutes to ensure completeness of the precipitation process . the dark green solid was collected by filtration and washed with 2 l aliquots of deionized water , until the filtrate ph was approximately 10 . 0 . the filtercake was placed in a drying oven at 80 ° c . the resulting dark gray product ( 81 . 8 g ) was ground to a gray - green powder with a mortar and pestle . metals analysis by icp was performed on a sample of the dried coprecipitate : % ba = 24 . 8 ; % cu = 16 / 8 ; % ag = 19 . 6 ( w / w ); y : ba : cu : ag = 1 . 03 : 2 . 00 : 2 . 92 : 2 . 01 ( target = 1 : 2 : 3 : 2 ). a 10 . 6 g sample of the above dried coprecipitate was placed in an alumina ( 99 . 8 %) combustion boat , calcined in static air at 850 ° c . for 6 hours , and allowed to cool slowly to room temperature in the furnace . 8 . 5 g ( 80 % yield ) of a lightly sintered black powder was obtained . the x - ray powder diffraction pattern of the calcined product was measured , and the major peaks characteristic of silver oxide , as well as several weaker peaks corresponding to those expected for bay 2 cu 3 o 7 - x , were observed . a comparison of the x - ray diffraction patterns is shown in fig2 . the specific surface area of the powder was determined to be 0 . 2 m 2 / g . metals analysis by icp was performed on a sample of the composite powder : % ba = 31 . 5 ; % y = 10 . 6 ; % cu = 21 . 5 ; % ag = 24 . 2 ( w / w ); y : ba : cu : ag = 1 . 04 : 2 . 00 : 2 . 95 : 1 . 96 . another 10 . 0 g sample of the dried coprecipitate was calcined at 900 ° c . for 6 hours in flowing oxygen . the analytical results were very similar to those for the sample calcined in air . a small quantity of the calcined powder was pressed at room temperature to form 1 / 2 &# 34 ; diameter and less than 1 / 16 &# 34 ; thick pellets , at about 6000 psi total pressure . the pellet was sintered in a surface for 3 hours at 900 ° c . in flowing oxygen , and allowed to cool in the furnace to room temperature . samples in typical size 6 mm × 3 mm × 1 . 5 mm were cut from these sintered pellets and were used for superconducting and magnetic properties characterization . the magnetization vs . temperature plot for such a sample is shown in fig3 . the tc for this sample was approximately 92 ° k . and the transition width was approximately 3 . 3 °. this is very similar to the values generally seen for pure ybco samples . we also measured the hysteresis loops at two different temperatures ( 77k , b . p . of liquid nitrogen and 5k , near the b . p . of liquid helium ). typical hysteresis loops are shown in fig4 . the area of the loops indicate the flux pinning in the sample . as expected , the amount of flux pinning is much larger at 5k than at 77k . two scanning electron microscope ( sem ) micrographs of the calcined powder samples are shown in fig5 . in fig5 a , micrograph of a 25 % w / w ag20 + ybco prepared by our chemical coprecipitation technique is shown under a magnification of 510 x . in fig5 b , micrograph of a 30 % w / w ag20 + ybco prepared by the conventional mixing and thermal treatment process is shown under the same magnification . it is evident from these figures that the composite sample prepared by our chemical coprecipitation method is much more homogeneous , even at such finer scale as indicated in the micrographs , than a conventionally prepared composite . in addition to the silver / y - ba - cu oxide ceramic superconductor composites , the present coprecipitation method can be used to prepare silver composites with bi - sr - ca - cu oxide and analogous thallium oxide - based families of higher temperature ceramic superconductors as well . the preparation of lead - stabilized bi - sr - ca - cu oxide superconductor powders is also possible using this coprecipitation method . the technique has been demonstrated for several bi - sr - ca - cu oxide superconductor powders ( without lead ). it is expected that this coprecipitation technique is generally useful for preparing other types of metal or metal oxide / ceramic composites which require careful control of both chemical homogeneity and ceramic microstructure . background references for the method of the present invention are now set forth . the disclosures of each of these references are incorporated by reference herein . 1 . lue j . t ., kung , j . h ., yen h . h ., chen . y . c ., wu p . t ., mod . phys . lett ., b , 2 ( 2 ), 589 - 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