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Appendix J The annual report on research 070-589 (3rd year, 2004) HTSC ceramic sintering by FEL Radiation Part 2. HTSC material preparation and characterization By Prof. Gideon Grader Dr.Shter Gennady Chemical Engineering Department Technion- Israel Institute of Technology Technion R&D Authority Ltd 10.01.2005 Haifa, Israel 2 Part 2. HTSC material preparation and characterization Prof. Gideon Grader, Dr. Gennady Shter Chemical Engineering Department, Technion Over a period from December 2003 to December 2004, according to schedule of our part in joint project “HTSC ceramic sintering by FEL Radiation” the following work has been done in Technion: 1. Synthesis of high quality YBCO superconducting powder via oxalate coprecipitation method. In general, a procedure of powder synthesis was described earlier in short annual report for 2003 year, (2nd year of project), part 2, Fig.4. At final step of project two main different batches of powder (batches A and B) were prepared and characterized. Here we present the basic results of the powder synthesis and characterization. 1.1.A thermal behavior of synthesized oxalates mix (blue powder) was investigated by TGA/DTA in flowing air in the range of 20-1100oC (Fig.1). 1.2.Based on the TGA/DTA data the mechanism of the blue powder pyrolysis was suggested and described (Table 1). 1.3.Precalcination stage was conducted under air at 700 oC for 7-10 hr. The morphology of precalcined powders is presented in Figure 2 (a, b) by SEM micrographs. Thermogram of blue powder 5 0 TGA DTA -5 -10 -15 %w lost -20 -25 -30 -35 DTA -40 -45 TGA -50 -55 -60 0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1100 Temp (C) Fig.1 . Thermal analysis of synthesized oxalate mix in air (Setaram TG-92 Unit): 3 Stage 1 Table 1. The mechanism of the oxalate mix thermolysis ∆m – ∆m – Temp. REACTIONS Theor exp. Range . (%) (%) (Cº) 9.66 13.7 100 0.5Y2 (C2O4 )3 *9H 2O 0.5Y2 (C2O4 )3 4.5H 2O 235 2BaC2O4 *0.5H 2O 2BaC2O4 H 2O 3CuC2O4 *0.5H 2O 3CuC2O4 1.5H 2O 235 540 3CuC2O4 1.5O2 3CuO 6CO2 540970 540970 Y2 O3 2CuO Y2 Cu 2 O5 5 890920 YBa2Cu3O6.5 BaCuO2 CuO L( BaCuO2 CuO) 6 920970 2 39.55 36.96 3 4 11.8 10.2 2 BaC2O4 O2 2BaCO3 2CO2 0.5Y2 (C2O4 )3 0.75O2 0.5Y2O3 3CO2 BaCO3 CuO BaCuO2 CO2 (Secondary reaction) 0.5Y2O3 2BaCO3 3CuO YBa2Cu3O6.5 2CO2 4BaCO3 Y2Cu2O5 4CuO 2YBa2Cu3O6.5 (Eutectic reaction) 2YBa2Cu3O6.5 BaCuO2 CuO Y2 BaCuO5 L(4 BaCuO2 3CuO) (Peritectic reactions) 7 9701050 A 2YBa2Cu3O6.5 Y2 BaCuO5 L(3BaCuO2 2CuO) (Incongruent reaction) B 1.3 μ 1.3 μ Fig.2. Morphology of the coprecipitated oxalates powder in batches A and B 4 Morphology of precipitated oxalates powder is shown in Fig.2. The effect of synthesis parameters on the particle shapes and size distribution, as well on aggregation is clearly demonstrated. 1.4.A thermal behavior of pre-calcined powder (grey powder) is presented by thermograms (TGA/DTA in flowing air in the range of 20-1100oC) in Figure 3. The composition of this powder was studied by XRD and included BaCO3, CuO and Y2O3 as main components with small amounts of Y2Cu2O5, BaCuO2 and YBCO. The high temperature chemistry of grey powder is described in Table 2. 1.5.The particles shape and size in powders after pre-calcination are evaluated by SEM micrographs that presented in Figure 4. pre-calcined powder thermogram TGA DTA 4.5 %w lost 1.5 -1.5 -4.5 -7.5 -10.5 -13.5 -16.5 0 100 200 300 400 500 600 700 800 900 1000 1100 T (C) Fig.3. TGA/DTA of pore-calcined powder (air flow, Setaram TG-92, alumina crucible) Table 2. Mechanism of the pre-calcined powder thermolysis. Stage ∆m Theo (%) 1 11.8 ∆m Exp (%) 11.02 Temp. Range (Cº) 780-960 REACTIONS 0.5Y2O3 2BaCO3 3CuO YBa2Cu3O6.5 2CO2 4BaCO3 Y2Cu2O5 4CuO 2YBa2Cu3O6.5 2 960-1050 YBa 2 Cu3O6.5 Y2 BaCuO5 BaCuO2 CuO 1.6.Calcinations of grey powder were carried out under heating in oxygen and vacuum in the range of 20-800oC for about 100 hrs. The black soft powder was obtained. The TGA/DTA data is shown in Figure 5 where significant weight loss was observed between 800 and 900 oC associated with reaction of YBCO formation (Fig.4). The composition of calcined powder was investigated by XRD presented in Figure 6. It should be noted, that calcined powder includes along with YBCO significant amount of other phases, such as BaCO3, CuO and Y2Cu2O5. In the framework of presented work we have demonstrated a 5 possibility to obtain the pure YBCO bulk product from the powder not calcined completely too pure (single) YBCO phase. It is important conclusion for development of simplified YBCO synthesis procedure. Morphology of black calcined powders from 2 batches presented in Figure 7. 13 μ (A) 1.7 µ (B) Fig. 4. SEM micrograph of pre-calcined powders. calcined powder thermogram TGA DTA 5 %w lost 0 -5 -10 -15 -20 0 100 200 300 400 500 600 700 800 900 1000 1100 T (C) Fig. 5. TGA/DTA of calcined powder (air flow, Setaram TG-92, alumina crucible) 6 After Calcination powder 1000 900 Intensity (a.u.) 800 700 600 500 400 300 200 100 0 10 15 20 25 30 35 40 45 50 2 Tetha YBa2Cu3O7 BaCO3 Y2Cu2O5 CuO Fig. 6. The phase composition of the powder after calcinations by XRD. B A 5μ 2.5 µ Fig.7. Morphology of the calcined black powder. 1.7.The change of powder morphologies in the sequence blue powder (oxalates) grey powder (pre-calcined ) black powder (calcined superconducting powder) is well corrected with data on Specific Surface Area (SA) measured by BET at temperature of liquid nitrogen (Fig.8). These data also represent an effect of procedure parameters on the properties of final powders. 55 60 7 Surface area vs T surface area (m^2/gr) 40 35 Batch A 30 Batch B 25 20 15 10 5 0 110 700 780 T (C) Fig.8. Specific surface area of coprecipitated oxalates (110oC), pre-calcined (700 oC) and calcined (780 oC) powders. 2. Preparation of the YBCO bulk superconducting samples. The works in this part of project were focused on the preparation of YBCO bulk superconductors: 1st group – the sintered under varied conditions samples; and 2nd group – the melt-textured under special temperature profiles samples. All samples were shaped by pressing of YBCO powders in the form of disc with dimensions of 2550 mm in diameter and 5-15 mm in thickness. The both sintered and met-textured discs were thoroughly polished in organic media to provide the quality of the electrical and magnetic measurements made by another project groups. Below, the main results of sintering and melt-texturing processing and bulk samples characterization are presented in short form. 2.1 Sintering of the YBCO bulk superconducting samples. The principle sequence of discs preparation via sintering is presented by flow chart in short annual report for 2003 year (2nd year of project –part 2, Fig.5). From each synthesized batches of YBCO powder, described above, the different bulk samples were pressed and sintered under varied processing parameters: pressure, heating and cooling rate during sintering, atmosphere and gas pressure in sintering furnace, sintering time and temperature, annealing time and temperature, etc. 2.1.1. A dependence of bulk density, porosity and shrinkage on the sintering temperature was studied for samples pressed from powder batches A and B. Typical examples of these relationships are presented in Table 3 and Fig.9. The sharp change of 8 the macro parameters of sintered bodies was found in the narrow temperature range from 880 to 890 oC, where the formation of YBCO liquid phase is occurred. In this range the mechanism of sintering is changed from solid state to liquid phase. The strong effect of sintering mechanism on the final dimensions of sample is clearly shown in Fig.10, where the samples # is the same with those in Table 3. The effect of temperature and change of sintering mechanism on the microstructure of YBCO bulk are demonstrated by SEM at different magnifications (Figs. 11 and 12). In these figures the drastically change of microstructure at temperature change of only 10 oC is clearly observed: the relatively large particle size and dense morphology of sample shown in Fig. 12 (T=890 oC) is well correlated with liquid phase sintering mechanism in opposite to sample presented in Fig. 11 (T=880 oC, solid phase mechanism). Table 3.Effect of sintering temperature on the macro parameters of YBCO samples (Powder- Batch A, sintering time – 13-14 hrs) Samples T sintering, oC Density, g/cc Porosity , % Shrinkage ,% 1 890 4.497 30.0 24.5 2 890 4.380 32.1 23.8 3 880 2.259 64.6 4.6 4 880 2.211 65.3 4.6 5 870 2.123 66.7 3.6 6 870 2.124 66.7 3.1 Batch A: density vs. sintering max temp. density (gr/cm^3) 4.50 4.00 3.50 3.00 2.50 2.00 1.50 1.00 0.50 0.00 870 870 880 880 890 890 sintering max temp (C) Fig. 9. Effect of sintering temperature on the density of prepared discs (Powder- Batch A). 9 1 2 3 4 5 6 Fig. 10. The effect of sintering temperature on the final dimensions of sintered YBCO discs. 2.5 μ Fig.11. SEM micrographs of YBCO samples sintered at 880 oC (Solid phase sintering). 10 2.5 μ 5μ Fig.12. SEM micrographs of YBCO samples sintered at 890 oC (Liquid phase sintering). 2.1.2. Effect of sintering time on the density, shrinkage and porosity of final bulk was investigated and the results are presented in Table 4. In comparison with sintering temperature, the time is found to have only a slight effect on the macroparameters of YBCO samples (in the sintering range of 10-50 hr). Table 4. Effect of sintering time on the macro parameters of YBCO bulk (Powder batch B, sintering temperature -880 oC) Samples 1 2 3 4 5 6 Sintering time, hr Density, g/cc Porosity , % 13 2.21 64.3 13 2.238 64.9 30 2.250 63.5 30 2.262 63.7 45 2.38 62.7 45 2.386 62.5 Shrinkage ,% 3.7 3.8 3.9 4.1 4.3 4.6 2.1.3. The phase composition of all samples was tested by X-ray powder diffraction. In spite of the fact that YBCO powder was not calcined completely to single phase composition (Fig.6), the final superconducting samples are characterized by pure YBCO phase (Fig. 13). These results represented the finding of optimal technology sequence and parameters that allow to shorter and simplify the general procedure of 11 YBCO sample preparation via coprecipitation. Batch A Disc 4 after Sintering 2500 Intensity (a.u.) 2000 1500 1000 500 0 10 20 30 40 50 60 2 Tetha Fig.13. XRD Patterns for sintered superconducting samples (Pure YBCO phase) Part of the sintered samples was used as starting materials for next step of processing – melt texturing while the second part was polished and passed to other group of project for characterization of electrical and magnetic properties. 2.2. Melt-texturing of the YBCO bulk superconducting samples. In a sintered YBCO samples the crystals are not regularly shaped and also are not ordered in the space of bulk body (Figs. 11, 12). In this case the transport of current is hampered by numerous contacts on grain borders and critical current is limited to small values. One of the routes to avoid this problem is to provide the continuous growth of the crystals in an ordered manner. As result, the large (long) crystals should be grown with the a-b planes oriented in the desired direction. In turn, the crystals are also oriented in parallel each with other and form the domains with textured microstructure. In general, the method used to achieve this type of growth in superconducting materials is Melt-Texturing. The various different sub-techniques of melt-texturing are known: MPMG (Melt Powder Melt Grows), Zone Melting, SLMG (Solid-Liquid Melt Grows), QMG (Quench melt Growth), TSMG (Top Seeded Melt Growth), OCMG (Oxygen Controlled Melt Growth), etc. In all these sub-techniques the crystal growth is achieved by sintering at high temperature in the presence of liquid phase. The main conditions for texturing are as follows: a) the fast heating to partial incongruent melting of material; b) fast cooling to sub-liquidus temperature; c) very slow cooling in the critical temperature range to provide the growth crystals in desired direction. In present work the different temperature profiles were investigated and the materials with textured domains were obtained. The differences in these profiles were the time and temperatures of fast and slow heating-cooling steps. 12 The microstructures of the melt-textured at different conditions samples are presented in Fig 14 A, B, C. These samples are characterized by crystal orientation and formation of the domains with ordered structure. The sample A was melt-textured at temperature lower than needed to form enough amount of liquid for continuous crystal growth with preferable orientation. Results of melt-textured are clearly observed in samples B and C with ordered large and long crystals. A B 50 µ 25 µ C 50 µ Fig. 14. SEM micrographs of the melt-textured YBCO samples: AThe macrostructure of the melt-textured samples is shown in the Fig. 15. The final dimensions and densities of the samples are close each with other, while the cracks were derived in some of them during processing. The samples were very dense with low apparent porosity. 13 Batch A Batch B 1 3 2 4 3 cm Fig. 15. The optical photographs of the melt-textured samples. Batch B Disc 1,Melt Texturing, without polish 800 700 600 500 400 300 200 100 0 10 20 30 40 50 YBCO 2 Tetha BaCuO2, Y2Cu2O5, BaCO3 Fig. 16. XRD data on the phase composition of melt-textured sample before polish. 60 14 batch B Disc 1 Melt Texturing After Polish 2500 intensity (a.u.) 2000 1500 1000 500 0 10 YBCO BaCuO2 20 30 40 50 2 tetha Fig.17. XRD data on phase composition of the melt-textured samples after polishing. The phase composition of the melt –textured samples was investigated by XRD and the formation of secondary phases along with main phase of YBCO was shown. During of incongruently melting the partial decomposition of YBCO is occurred and secondary phases are formed preferably on the surface of discs. Therefore, on XRD of samples before polishing the presence of secondary phases in significant amounts was detected (Fig.16). After polishing, the surface with high concentration of secondary phases was removed and the inner part of sample was found more pure (Fig. 17). The as melt-textured samples will be used as prototype for comparison with samples melttextured by heating combined with FEL irradiation. The sintered samples with low density and high porosity (solid phase sintering), sintered samples with intermediate density and porosity (liquid phase sintering) and melt-textured samples with high density and low porosity were passed to group from College of Judea and Samaria for further millimeter wave measuring. 60