Preparation of uncladded YBCO wires

PH'YSBCA Physlca C 209 (1993) 273-276 North-Holland Preparation of uncladded Y B C O wires G.Grader, L.Cadoche and G.Shter Chemical Engineering Department, Technion, Haifa 32000, ISRAEL I Wires of YBCO, 1-5 m m in diameter, have been obtained by extrusion of an oxalate derived powder mixed with polyvinyl butyral (PVB) and two phthalate plasticizers.The densification and transport propertiesof the wires were investigatedat various organic loading conditions.For Y B C O powder prepared by oxalate coprecipitationand calcined at 930°C, the maximal strength, relative density and Jc were 20MPa, 92% and 400A/cm 2 ,respectively.Results show that after a 4-hour sintering at 955°C the tensilestrength, density, and Jc are all maximized at a binder content of 3-4 wt.%. The loading of up to 10 wt.% dibutyl phthalate (DBP) plasticizer(in the binder) has littleeffecton the properties.Beyond this loading a drastic drop in Jc is observed. The densificationwas very poor for sinterings below 950oc, which was reflectedby a lower criticalcurrent. 1.INTRODUCTION Fabrication of uncladded H T S C wires by extrusion relays on the preparation of an intimate mixture of the SC powder, binder, plasticizersand solvents[i].Organic solvents must be used here to avoid the degradation of the SC powder [2]. Often, the ceramic and organic components are mixed mechanically by ball compaction or high shear, to form either a paste, or a viscous suspension ~-1,000-10,000 centipoise) called a slip. Alternatively, the mix can be formed by insitu polymerization[3].In the former case the paste can be extruded directly,while in the later case the slip can be tape casted, dried and finally extruded. The advantage of the paste method is that due to the presence of solvent, a proper rheology can be obtained with a low binder content (1-5 wt.%), which leaves a lower porosity at the end of the binder burnout stage. O n the other hand because in the dried tape approach, the solvent is totallyevaporated, the rheology is governed only by the loading and nature of the polymers. This normally requires a higher organic loading (5-20 wt.%). The advantage of the tape approach is the better control over the extrusion process, and an improved dispersionof the ceramic powder. These technique pre date the H T S C era, and therefore have been applied by m a n y research groups since the H T S C field emerged. One of the most c o m m o n slip systems for ceramic powders involves polyvinyl butyral (PVB) as a binder , a phthalate component as a plasticizer and M E K as a solvent[4]. To our knowledge however, the detailed effect of binder and plasticizer content on the densification and transport properties of polycrystaline H T S C wires has not reported. The purpose of this work is therefore to conduct a systematic evaluation of the effect of binder and two phthalate plasticizers contents on the Y B C O superconductor properties.Theeffectof starting powder characteristicson the wire properties and performance is now being studied. The wires produced here are being used by other groups (i.e.current limiting device research[5]). In addition,these wire specimens form the basis for our current work on the melt texturing this material. 1This work was supported by a research grant from Ministry of Science and Technology of Israel 0921-4534/93/506 00 © 1993 - Elsevier Scmnce Pubhshers B V All nghts reserved 274 G Grader et al ~Preparation of uncladded YBCO wtres 2.EXPERIMENTAL 3.RESULTS AND DISCUSSION The YBCO powder for this work was prepared from coprecipitated metal oxalates, which w e r e d e r i v e d from t h e m e t a l carbonates precursors [6]. The raw powder was calcined at 930°C in oxygen for 12 hours, then ball milled in isopropanol for 4 hours, dried and sieved through a 60 mesh screen. T h e a v e r a g e s t a r t i n g particle diameter for the process was about l~tm. The binder used in the process was polyvinyl butyral (PVB, also called butvar or B-76), and MEK was used as a solvent. The two plasticizers tested were butylbenzyl p h t h a l a t e (S-160) and dibutyl phthalate (DPB). The binder was first dissolved in the solvent at a weight ratio of solvent:binder=6:l.An appropriate amount of plasticizer was then added to the organic solution. The range of binder in this study was 1-10 wt.%, while the plasticizer content in the binder was 0-20 wt.%. Depending on the desired binder and plasticizer contents a predetermined a m o u n t of o r g a n i c c o m p o n e n t was m e c h a n i c a l l y blended into the ceramic powder using a roll-mill. Once the paste became homogeneous, it was charged into a ram extruder and forced through an orifice. Wires roughly 1 m e t e r in the length and 1ram in diameter was readily produced at a pressure of 1MPa. The wires were sintered in oxygen, in the 930-975°C range. The sinteriug time varied from 2 to 24 hours. In addition to Jc and Tc measurements on the sintered samples, the tensile strength, density and morphology of the unsintered ("green~) and sintered wires was also evaluated. The tensile strength was measured with a J.J. machine (Lyolds I n s t r u m e n t ) , while the morphology was analyzed with an SEM (Jeol-840).Critical current was measured at zero magnetic field by 4-point probe method using a l~tV*cm "1 electric field criterion. All the data shown r e p r e s e n t s an a v e r a g e of at l e a s t 6 measurements. The tensile strength of green and sintered wires as a function of the binder content, is shown in Figure 1. The strength of the green wire is determined predominately by the strength of the binder, and thereforeremains constant. However the strength of the sintered wires is maximized in the 3-5 wt.% range. This m a x i m u m occurs because at lower binder contents the powder is not dispersed homogeneously in the binder, leading to flaws s t e m m i n g from local sintering effects. At high binder contents, the large porosity left after the binder burnout stage lowers the density of the sintered body, which is reflected in the gradual drop in the tensile s t r e n g t h with increasing binder content. It is also apparent from Figure 1, that raising the sintering time from 4 to 24 hours does not improve the tensile strength. Measurement of the tensile strength vs. sintering temperature is shown in Figure 2. The rise from 10 to 20 MPa from 930°C to 950oc, is consistent with the monotonic rise in the density of the wire as seen in Figure 2. The saturation of the tensile strength beyond 950°C, occurs even though the density increases from 85 to 92% between 950 and 970oc. This effect can be explained by the large increase of grain size that occurs b e t w e e n 950 and 970°C. The l a r g e r randomly oriented grains can allow for larger crack formations which cause the lower strength. The relative density of wires loaded with 1.5-10 wt.% binder and sintered at 955°C for 2-24 hours is shown in Figure 3. A maximnm relative density o f - 8 5 % is obtained at 3.5 wt.% binder, and here again, no improvement is observed when the sintering time is increased beyond 4 hours. These results are consistent with the tensile strength results in Figure 1. The effect of binder content on the Jc of wires sintered at 955°C is shown in Figure 4. Here again, Jc is maximized at _400A/cm 2 with 3.5 wt.% binder, consistent G Grader et al / Preparation of uncladded YBCO wires 20000 "~ ~n~erlng 275 --~I T= 955C ~ 10000 ® :~ 5000 100 t 0 0 4 6 8 10 Wt.% Binder Figure 1. Tensile strength vs. wt.% binder in green and sintering wires. 20.0 I ~, ~,~.o 2 T:955C / o ~ . ~ : oo~ ~:~16.0 ~ 8o~ ~14.0 o ~=,~.o ,o~ 4 A 8 10 Wt. % Binder Figure 4. Jc of wires sintered at 955C vs.binder content. 400~ 100 ~ 2 ~-.~h 7ntering ~ o o ~ \ ~ 2 4 h Sintering'~ ~ 200 T= 955C 1 o o ~ 2 h slntering 10.0 60 920 940 960 980 Sintering Temperature [C] Figure 2. Tensile strength and relative density vs. sintering temperatures, o~'°t ,oo~, ~~,~ - - ~o ,~sntorn0 ~"°I ! . . '1 ~ I ' ' ' ' ' ' .K 1 0 I .K 2 0 Wt % Plasticizer DBP Firure 5. Jc of wires with 4 wt.% binder, vs. DBP plasticizer content. 0 - ~00 100 ~ ~0a~,,.,.,,.,, 0 I 0 2 a ~ A 1 0 12 Wt % Binder Figure 3. Relative density of wire sintered at 955C vs. binder content. I I I 940 950 960 970 Sintering Temperature [ C] Figure 6. Jc of wires sintered for 4 hours at different temperatures. 276 G Grader et al / Preparanon of uncladded YBCO wtres with the density variations in the sample seen in Figure 3. As mentioned earlier, the effect of plasticizer addition to the organic solution was e v a l u a t e d . T h e p u r p o s e of t h e plasticizer is to soften the binder in order to render the ceramic/organic composite more flexible. In the present work, the plasticizer also enabled the extrusion of a smoother wire surface. The addition of either the DBP or the S-160 lowered the strength of the green wires due to the binder softening. Increasing the amounts of either plasticizer also caused a monotonic decrease in the s i n t e r e d t e n s i l e s t r e n g t h of t h e wires. However it was found t h a t DBP was more compatible with t h e b u t v a r b i n d e r and YBCO powder, t h a n S-160. T h e poorer compatibility of S-160 is reflected in smaller s t r e n g t h , densities and Je's t h a n those obtained with DBP. The effect of DBP addition on the Jc of wires with 4% binder is shown in Figure 5. As shown, the addition of up to 10% DBP (relative to the binder), had no effect on the Jc. The drop in Jc at h i g h e r DBP levels is correlated with the corresponding drop in the density of the wires, due to the increased porosity after the organic burnout. In the case of S-160 analog, a drop of Jc from 300A/cm2 to 100A/cm2 was observed after a 5% addition. Finally the Jc of wires with 3.5% binder, which were sintered for 4 hours in oxygen at various temperatures in plotted in Figure 6. The drastic drop in Jc below 955oC is consistent with the poor densification at this t e m p e r a t u r e range. The s a t u r a t i o n of Jc above 955oC reflects the defects introduced to the wires by the large and random grain growth observed in the microstructure at these temperatures. In recent experiments with more reactive YBCO powder, higher relative densities a n d an improved Jc was obtained at significantly lower temperatures. The detailed analysis of these results will be described in a future publication. 4. SUMMARY We have conducted a systematic evaluation of t h e effects of b i n d e r a n d plasticizer contents on the mechanical, morphological, and transport properties of extruded YBCO wires. The binder employed was polyvinyl butyral (butvar), and two phthalate-based plasticizers were tested: dibutyl p h t h a l a t e (DBP) and butylbenzyl phthalate (S-160). It was found t h a t DBP was more compatible t h a n S-160 in the butvarffBCO system. The tensile strength and density are maximized at a binder loading of 3-4 wt.%. A m a x i m u m Jc of ~400A/cm2 was also obtained in this binder content range. ACKNOWLEDGEMENT The authors would like to t h a n k the Crown Center for Superconductivity for continued support. REFERENCES 1. 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