Preparation and Properties of High-T c Bi-Pb-Sr-Ca-Cu-O Thick Film Superconductors on YSZ Substrates

An evaluation of four firing profiles was performed to determine the optimum processing conditions for producing high-T c Bil.8Pb0.33Srl.87Ca2Cu30 x thick films on yttriastabilized zirconia substrates. Using these four profiles, the effects of sintering temperatures of 830-850°C and soak times of 0.5 to 12 hours were examined. In this study, Tc,zero values of 100K were obtained using a firing profile in which the films were sintered for 1.5 to 2 hours at 840 to 845°C and then quenched to room temperature. X-ray diffraction analyses of these specimens confirmed the presence of the high-Tc phase. Films which were similarly fired and furnace cooled from the peak processing temperature exhibited a two-step superconductive transition to zero resistance, with Tc,z,ro values ranging from 85 to 92K. The other firing profiles evaluated in this investigation yielded specimens which either exhibited critical transition temperatures below 90K or did not exhibit a superconductive transition above 77K. Introduction Shortly after the first report of superconductivity above 77K in YBa_Cu307_ x ceramics [1 ], the Bi-Sr-Ca-Cu-O family of ceramic superconductors was discovered [2]. This new class of materials is unique among high temperature superconductors because it does not contain a rare-earth cation, and several compositions within this material system exhibit superconductivity. The superconductors within this family have the general formula Bi2Sr2Can_lCUnO2n+4, where n=l to 3 and indicates the number of Cu-O layers in the crystal structure. The highest Tc phase has three Cu-O layers (i.e., Bi_Sr2Ca2Cu3010 , or 2223) and exhibits a superconductive transition at 110K. Similarly, the 2212 phase has two Cu-O layers and the 2201 phase has a single copper oxide plane. These latter two compositions exhibit superconductive transition temperatures of 60-85K and 20K respectively [3]. Since the initial discovery of superconductivity in Bi-Sr-Ca-Cu-O ceramics, the preparation of single phase 2223 materials has proven difficult because the 2212 phase is thermodynamically more favorable at elevated temperatures than the 2223 phase [4-5]. The partial substitution of PbO for Bi203 has since been found to help stabilize the high-Tc composition, thereby increasing the volume fraction of this phase [6-7]. By stabilizing the high-Tc phase in Bi-Sr-Ca-Cu-O ceramics, higher process yields have become possible for products utilizing these compositions. Someof theproposedapplicationsof theseuniquematerialsrequirethepreparation of thick film leadassemblies[8] andhybridcircuits[9]. Thickfilm manufacturing[10]is a cost-effectiveprocessfor producingmultilayerhybridcircuitscontainingboth superconductive andconventional electronicstechnologies.In thisreport,thepreparation andpropertiesof screen-printed Bi-Pb-Sr-Ca-Cu-Othickfilms onpolycrystallineyttriastabilizedzirconia(YSZ)substrates is described. Experimental Procedure The superconductive material employed in this work has the nominal chemical composition Bil.8Pb033Sr187Ca2Cu30 x (i.e., Pb-substituted 2223). The powder possessed an average particle size of 12 gm, and ninety percent of the powder possessed an equivalent spherical diameter below 20 gm. To produce thick films, the superconductive powder was blended with an organic vehicle system and deposited onto yttria-stabilized zirconia substrates using a 200 mesh stainless steel screen patterned by photolithography. The dimensions of the printed films were 19.0 mm x 6.4 mm x 25 gm. After deposition, the thick film specimens were fired in air using one of the four f'n-ing profiles described in Table 1. In this report, the four furnace cycles will be referred to as firing profiles 1 through 4. In each case, the specimens were heated to the sintering temperature at a rate of 5°C/min where they were allowed to soak at the peak processing temperature for 0.5 to 12 hours. After the allotted sintering time had expired, the specimens were either furnace cooled at a rate of -5°C/min or quenched to room temperature. Table 1. Firing profiles used to densify Bi-Pb-Sr-Ca-Cu-O thick films. Firing profile Temperature profile 1 2 3 4 Heat to max temp / hold / furnace cool Heat to max temp / hold / quench Heat to max temp / hold / cool to 800°C / hold / furnace cool Heat to max temp / hold / cool to 800°C / hold / quench Firing profiles 1 and 2 use the standard heating rate of 5°C/min to heat the specimens to the peak temperature where they were maintained for the prescribed time. After soaking at the peak temperature, the specimens were either furnace cooled at a rate of -5°/rain or quenched to room temperature depending on the firing profile employed. Profiles 3 and 4 differ from 1 and 2 in that an annealing step at 800°C was introduced into thefiring cycleafterthespecimens wereexposedto thepeaksinteringtemperature. During thisannealingprocess, thespecimens weremaintainedat 800°Cfor 8hoursprior to cooling or quenchingto roomtemperature.In this study,thick film specimenswerefiredat temperaturesof 830,835,840,845,and850°Cusingeachof thefour firing profiles. After firing, thecriticaltransitiontemperature andcritical currentdensityof each specimenweremeasuredusingad.c.four proberesistancetechnique.An appliedcurrent of 10gAwasusedin theresistanceversustemperaturemeasurements, andthecrosssectional areasof thefilms weremeasuredusingaprofilometer. In additionto measuring theelectricalpropertiesof thefilms, Cu-k x-raydiffraction(XRD) andscanningelectron microscopy(SEM)analyseswerealsoperformedonselectedspecimens. Experimental Results The thick film specimens exhibiting the highest superconductive transition temperatures were prepared using firing profile 2. These results were obtained by feting the films for either 2 hours at 840_C or for 1.5 hours at 845_C and quenching the specimens to room temperature. The films prepared in this manner exhibited Tc,o,sct temperatures of approximately 110K, and reached To,zeroat 100K as shown in Figure 1. Although these films exhibited the highest T_,zerovalues obtained in this study, the resistance versus temperature data showed that a small "tail" did exist between the T_,ons_ t and T¢,z_ro temperatures. In these cases, the difference between these two values, or To, is 10K as seen in Figure 1. X-ray diffraction analyses of the 100K films confu'med the existence of the high-T_ phase. The data from the diffraction scan shows a strong diffraction peak at 2 = 4.8_. The presence of an intense peak at this angle is indicative of the high-To phase in Bi-Sr-CaCu-O ceramics as described by Mizuno et al. [7]. Furthermore, the diffraction pattern shown in Figure 2 is in good agreement with the data presented for the 2223 phase in that report. The thick films fired for longer than 2 hours at 840_C (or longer than 1.5 hours at 845_C) using profile 2 exhibited superconductive transitions with more pronounced "tails". The presence of these "tails" is indicative of the coexistance of two superconductive phases in the fired component. In this study, several specimens were produced which possess T_,o,s_t temperatures of 110K but reach zero resistance below 92K. A typical example of this behavior is shown in Figure 3. In this case the Tc,onse t temperature was approximately 110K and the T_,zCrotemperature was 88K. The data shown in this figure is from a specimen fired at 840_C for 4 hours and quenched to room temperature.