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Technical Report_PSM_nGimat_April24pdf

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Technical Report_PSM_nGimat_April24pdf

  1. 1.   1   Technical Report Scalable Production of High Temperature Superconductor Bi2212 Precursor Nanopowder nGimat LLC Lexington, KY Supervisor: Prepared By: Stephen Johnson Manasi Chaubal CTO, nGimat LLC M.S. Candidate in Nanoscale Physics
  2. 2.   2   Acknowledgements I would like to thank my supervisor Dr. Stephen Johnson and all my colleagues at nGimat for guiding, training and working with me during my internship at the company. I would also like to thank Dr. Ganesh Venugopal for guiding me during my entire time at nGimat LLC. I am grateful to Dagmar Beck and Emalie Vann Thok for supporting me and helping me secure this internship and all their efforts throughout the degree. I am also grateful to the PSM Program and faculty for their contribution during my degree.
  3. 3.   3   Table of Contents Sr. No. Topic Page No. 1 Abstract 4 2 Project Background, Issues and Solutions 5-20 3 Final Product 20-21 4 Project Merits in terms of Cost Analysis 21-22 5 Conclusions 22 6 References 23
  4. 4.   4   1. Abstract Under the Phase II of the SBIR grant awarded by DoE (Department of Energy) to nGimat LLC, the goal is to optimize the Bi2212 powder composition, achieve carbon content in powder to be less than 100ppm and to scale-up the powder production from 0.5kg/day to 10kg/day to meet the needs of a longer High Temperature Superconductor wire production. The Bi2212 powder produced through the Nanospray CombustionSM process at nGimat LLC shows inconsistent composition for different batches as measured by the in-house XRF instrument. The carbon content measured after calcination process of the as-made Bi2212 powder shows values more than 100ppm. The recommended solutions for these problems are: 1) To correct the errors of the XRF measurements by troubleshooting reaction parameters; 2) To study the rate of carbon uptake by Bi2212 as- made powder in air and design a new furnace and exchange chamber that makes a 2- 3kg/batch powder heat treatment possible, without carbon contamination. 3) To scale up to a 10kg/day level of production, the production rate should be increased by roughly seven-fold. The best option is to produce the powder on the ~100 kg/day production rate system nGimat has developed over the last 3 years. It is estimated to sell the powder at a price 90% lower than the current commercial retail price of Nexans Bi2212 powder. This will help in increasing the Bi2212 market in the U.S. as well as overseas.
  5. 5.   5   2. Project Background 2.1 Company Background nGimat LLC , formed in 2010 is a wholly owned subsidiary of nGimat Co. The company specializes in nanomaterials R&D and manufacturing for the energy, electronics, consumer and biomedical applications. nGimat uses its proprietary NanoSpray CombustionSM processing technology and NanomiserTM device to produce low-cost, high-volume nanoEngineered MaterialsTM with unique performance characteristics. These materials are multi-component metal-oxide nanopowders, which are used in next generation products, particularly targeted for energy storage, conversion and transmission applications. There are currently three major projects at the company: 1) Bi2212 High Temperature Superconductor (HTS) powder production for wire applications funded under the Department of Energy (DOE), 2) Synthesis of YSZ ceramic coatings for Thermal Heat Barrier applications under the Department of Defense (DOD) and 3) Ceramic coated wires for improved insulation under DOE. Among these, my work was primarily focused on the Bi2212 production project and therefore this report hereon discusses the same. Bismuth strontium calcium copper oxide or BSCCO, is a family of HTS having a generalized formula of Bi2Sr2Can-1CunO2n+4+x. Depending on the value of n=1,2 or 3, there are three different phases (2201, 2212, 2223) of the BSCCO system with respect to their chemical stoichiometric compositions. For BSCCO-2201 (also referred to as “Bi2201”) the approximate critical temperature is 20K, for Bi2212 it is 89K and for BSCCO-2223 (Bi-2223) it is 110K. The unit cells of these phases have a layered
  6. 6.   6   structure consisting of bismuth oxide (BiO) double layers that alternate with perovskite- like Sr2Can-1CunO1-2n units having also a layered structure with copper oxide sheets respectively. As the number of copper oxide layers increases, the critical temperature (Tc) increases. But under normal preparatory conditions, the Bi2212 phase is preferred as it is most stable and is often the most dominant phase present even if the starting composition is aimed for Bi2223. nGimat produces this Bi2212 phase nanopowder by the Nanospray CombustionSM process with consistency in composition and carbon content. This powder is then drawn into a wire to test its current carrying capacity. The wire studies are done in collaboration with North Carolina State University (NCSU) and Supermagnetics Inc. This project is funded by US DoE Office of High Energy Physics through SBIR Award No. DE- SC0009705. This is a fast track SBIR grant for a period of two years or phases. During Phase I (January 2013-14), nGimat was successful in investigating the two most suitable compositions of Bi2212 powder showing high-quality performance. Currently in Phase II, the efforts have been to maintain consistency in repeatedly producing Bi2212 of desired compositions and low carbon content (<50ppm). Also, it is aimed to scale-up the powder production from 0.5 kg to 10 kg per day. Thus, the goal is that by the end of Phase II nGimat will have solidified its position in the superconductor community as a quality raw powder provider who is willing to work with each individual customer to tailor the material to their specific needs. These HTS powders have potential future applications in high-field magnets for high- energy physics accelerators and MRI/NMR magnets. Thus, nGimat aims to team with DOE scientists interested in high-field magnets for particle accelerators, wire and magnet
  7. 7.   7   manufacturers, scientists and engineers in the private sector who build magnets for MRI/NMR devices, to supply their raw powder. 2.2 Bi2212 Project Overview and Goals 2.2.1 Bi2212 Phase II Objectives The three main objectives for Phase II are: Phase II Objectives 1. Continue optimization study of Bi2212 nanopowder by varying stoichiometry, particle size and morphology conditions make these variations available to the U.S. superconducting community. 2. Optimize heat treatment conditions and conduct carbon studies to achieve nearly <30ppm carbon 3. Demonstrate >10 kg/day powder production rates of optimized powder 2.2.2 Bi2212 Project Overview nGimat uses its proprietary NanoSpraySM Combustion technology i.e. a Combustion Chemical Vapor Condensation (CCVC) process to produce high purity Bi2212 powders with controllable stoichiometries, ultra-low carbon content and the absence of the Bi2201 phase. A schematic of the process is shown in Figure 1.
  8. 8.   8   Figure 1: Schematic of the NanoSpraySM Combustion process to produce Bi2212 nano- powder (http://ngimatllc.com/technology.html) As described in the flow diagram, the precursors/metal nitrate solutions of Bi, Sr, Ca and Cu are prepared to appropriate concentrations according to the stoichiometries required and they are mixed with alcohols and water solvents in a reactor, which act as combustible fuels. These precursor solutions are then pumped into the NanomiserTM to convert them into ultrafine aerosol. The ultrafine atomization with the NanomiserTM device enables the use of any soluble precursor without concern for its vapor pressure. These controlled droplets are then introduced in the NanoSpraySM Combustion chamber where in the presence of oxygen as a carrier gas they are combusted to form
  9. 9.   9   nanopowders after undergoing nucleation and controlled growth. This powder is then collected in a Bag-house filter separator. The BSCCO nanopowder obtained from this process is called “as-made powder” and its particle size ranges from 50-100nm. This as-made powder contains undesired phases like Bi2201, Bi oxide and Sr-Cu oxide along with the desired Bi2212 phase. Therefore, it is required to heat-treat this powder to oxidize it to optimum levels of oxygen doping that removes the undesired phases at the optimum temperature and time interval. The powder undergoes phase transformation when heat-treated at an optimal temperature (Toptimal) for 72 hours in the presence of oxygen, giving a majority of Bi2212 phase and only 2wt% of Bi2201 and/or strontium cuprates depending on the starting composition. The oxygen is introduced at a temperature at which the Bi2212 is in a stable phase. Figure 2 shows an XRD of the phase transformation of BSCCO during a heat-treatment. Figure 2: XRD of the phase transformation of BSCCO at Toptimal (nGimat, 2014)
  10. 10.   10   It is important that the carbon in the as-made powder is removed while heat-treating it. The presence of carbon contamination during the oxide precursor processing has serious consequences due to the evolution of CO2 during the Bi2212 wire making process. The current density (JC) of the HTS wires is greatly reduced by the presence of carbon, particularly in the form of carbonates. High carbon residue results in poor grain connectivity and increased porosity [Yun, 2014]. Carbon content less than 100 ppm in the powder is considered as feasible. Figure 3 shows a cumulative graphical representation of the removal of carbon in the form of CO2 during a heat treatment process. As the temperature increases, the amount of CO2 released from the powder increases, until it reaches a particular temperature at which the curve plateaus. Above this temperature no more carbon is left to be released in the form of CO2. Figure 3: Carbon removal with increase in temperature during heat treatment process The size of the low carbon content Bi2212 powder after heat treatment grows to about 5- 12µm. This powder is stored immediately in an inert atmosphere (Ar) glove box to
  11. 11.   11   prevent the absorption of carbon or moisture onto it. This powder is then sent to Supermagnetics Inc. to make HTS wire of length greater than 200m.The Bi-2212/Ag round wires are manufactured using a powder-in-tube process of wire fabrication. A silver tube is filled with Bi-2212 precursor powder, drawn to a specified size and cut into pre-determined lengths. During Phase I, nGimat made 37 such monocore elements and bundled and stacked them into another silver tube, drawn to a specified size, and cut once again into predetermined lengths. Seven such lengths were bundled and restacked into a AgMg alloy tube and drawn to a final diameter of 0.84 mm. Figure 4 shows a typical cross-section of the manufactured wire in the as-drawn condition. The ceramic fill factor, which is the ceramic area/entire cross-section of conductor, is 13%. Figure 4: Multifilamentary wire made from nGimat Phase I powder (nGimat 2014) This wire is then sent to NSCU for current capacity measurements using the Partial-Melt process (PMP). The JC measurements on this wire are comparable to those in the literature i.e. Max JC at self-field ~ 3,200 A/mm2 and Max JC at 5T is 1360A/mm2 .
  12. 12.   12   2.2.3 Heat Treatment studies A series of heat treatment studies are done in which the as-made powder is quenched from Toptimal at various times during the 72 hour heat treatment (HT) process. A schematic diagram of the HT experiment is shown in Figure  5. As evident from the XRD shown in Figure  5, a full 72hour heat treatment is required to convert the powder to pure Bi2212 crystal phase. Before this time period, other phases such as 2201 and various sub-oxides still exist in the powder. Figure 5: Heat treatment profile for Bi2212 powder showing the quench experiments (nGimat, 2014)
  13. 13.   13   At one time, the powders heat-treated at Toptimal for 72 hrs showed melting behavior. Bi2212 powder when melts becomes chunky and shinier than the fine powder and has higher particle size than the usual 5-12µm. This was further observed by the SEM images of a melted lot as shown in Figure 6. It was later found that the melting was caused because of a faulty old thermocouple that displayed a lower temperature, an offset of ~10 degrees from the real value. Figure 6: SEM image of slightly melted KYQ-137C powder lot Here any polyhedral particles indicate insufficient transformation to Bi2212 or melt during calcination. Also, the dark particles are Sr-Cu-O or Alkaline Earth Cuprate phases (AEC), which do not participate in the conversion of the precursor into the final oxide
  14. 14.   14   superconductor, and may induce undesired melting during heat treatments. As seen in Figure 7, the actual morphology of optimally heat-treated Bi2212 powder is flake-like. Figure 7: SEM images of Bi2212 heat-treatment powder morphology 2.2.4 Technical issues in Bi2212 powder production The technical issues in the production of the Bi2212 powder are related to the powder morphology, maintaining consistent composition and low carbon content. nGimat has been targeting to produce powder that is at least as good as the only commercial product available on the market- Nexans powder. The drawback with Nexans is that it is a non- US supplier and it produces only a particular composition of powder, limiting the needs of the customers. This particular composition might not be the optimal composition required for certain specific applications. Therefore, nGimat wants to provide powder according to the flexible needs of the customers, at the same time maintaining its high quality and purity. The Nexans powder composition stated in the literature and as measured by nGimat’s XRF instrument shows a significant difference in the atomic % of Bi present in it.
  15. 15.   15   Sample Bi Sr Ca Cu Nexans powder by literature 2.17 1.94 0.93 2.00 Nexans 521 powder by nGimat XRF 2.25 1.89 0.89 2.00 Table 1: Literature and XRF measured composition of Nexans samples. Composition values are in atomic percentages. This can be either due to inaccurate values of nGimat’s XRF standards, or because the published composition was measured with incorrect standards. In either case, these measurements at least serve as a convenient in-house comparison to Nexans powder. nGimat has focused on producing the above two targets of compositions for its own wire production. These powders are as follows: Sample Bi Sr Ca Cu LXA 127A 2.26 1.90 0.90 1.98 LXA 127B 2.15 1.89 0.93 1.93 Table 2: XRF measured compositions for the two nGimat powder lots
  16. 16.   16   Recently, there have been changes in the powder composition when measured by the XRF as shown in the table below. KYP-22B as-made At% Average Composition Target (Nexans) % Deviation Bi 32.33 2.27 2.25 0.92% Sr 25.36 1.79 1.89 -5.13% Ca 13.01 0.92 0.89 3.30% Cu 29.39 2.08 2.00 3.87% Table 3: XRF data of the composition of KYP-22B, a particular batch of powder Such changes have been consistent for three powder runs and their standard deviation from the target is greater than the allowed mark of <3%, creating a huge consistency issue for powder production. This is a concerning problem which needs to be fixed immediately. To address this problem, the first approach assumes that the inconsistency is due to powder contamination from the previous runs. The previous runs might have some lots of inconsistent powder accumulated in the bag house that were mixed with the present batch powder due to insufficient bag house clean-up. To ascertain this, another batch of precursors was run in the NanospraySM Combustion chamber and the XRF compositions were measured thrice, each at two different time intervals, for each sample. But still the compositions showed similar inconsistencies. This then indicated the possibility of inconsistencies in powder morphology/size giving rise to inconsistent XRF measurements because the instrument would absorb X-rays differently if the powder morphology is non-uniform. If this were true, the pressure values of the powder during the run would change significantly in the combustion chamber. On checking the pressure
  17. 17.   17   datalog, a significant change was observed in the pressure values at different time intervals during the run. It was found that this was due to the clogging and eventual breakdown of the orifice, under high production pressure, used in the NanomiserTM device. Thus, this type of systematic troubleshooting approach was helpful in solving the powder composition inconsistency issue and a new quality orifice was employed for consistent output. Another task was to target the carbon content in powder to <30ppm as it is close to the carbon content present in the Nexans powder. The superconductor community agrees that low carbon content is a very important aspect in producing high quality Bi2212 wires. The reason being that excess carbon present in the powder causes evolution of CO2 during wire processing, which creates voids in the wire reducing the JC. The heat treatment and carbon studies were carried out to get an idea of how quickly carbon is absorbed by the powder post heat treatment. Samples heat-treated for 72 hours in flowing oxygen consistently have carbon content below 100 ppm, as shown below in Fig. 8a. The average carbon content of such powders is closer to 60 ppm. The key to achieving consistently low carbon content is in the handling of the powder, as any exposure to air leads to an immediate increase in carbon content. This is illustrated in Fig. 8b, in which samples of heat-treated 2212 powder were deliberately exposed to air for different time periods and then analyzed for carbon content. The data shows that carbon content can climb from 50 ppm to hundreds of ppm in a matter of minutes. Thus, extreme care must be taken to store and handle Bi2212 powder in a carbon free inert environment. Samples for carbon analysis are all sent to Florin Analytical, who measure the carbon using a Leco CS analyzer.
  18. 18.   18   Figure 8: a): Carbon content of Bi2212 powders measured in Phase II. The x-axis is a chronological index of measurements on different samples. b) Carbon content as a function of exposure time to air. LXA 135A and 135B are powder lot numbers. The third task was to scale-up the powder production and collection process from 1 kg/day to 10kg/day to reach production levels matching future DOE and industry demands. The equipment used in Phase I consisted of a small-scale reactor and baghouse system for powder production, combined with a 6” diameter quartz tube furnace for post- processing heat treatments. The former could produce up to 1 kg/day (for 24 hr day) of un-calcined BSCCO nanopowder (single metal oxides and sub-oxides of Bi2212) with typical batch sizes of 300 g (for a 6 hour production run). The tube furnace has been the bottleneck, however, as heat treatment sizes have been limited to ~50 grams with required times of 72 hours. Thus, improved equipment and processes are needed to scale to larger volumes. For higher rates of BSCCO nanopowder production, a transition is needed from the small-scale reactor system to a larger system. The larger reactor and baghouse system has produced nanopowder at a rate near 100 kg/day in the past with a different material. A nanopowder production rate of ~ 10 kg/day of BSCCO on this system is being targeted. In addition to scaled-up nanopowder production equipment, a scaled-up furnace and exchange chamber equipment will also be required for post-
  19. 19.   19   processing heat treatments. A photograph of the furnace and exchange chamber is shown below in Figure  9. The task here was of designing the powder exchange chamber. It is required to install gas ports at various positions on the sides of the exchange chamber to ensure efficient supply of oxygen to the Bi2212 powder collected after a heat-treatment. To do this, the following things are taken into consideration: i. Volume of the exchange chamber (~2m3 ) ii. Flow-rate of the oxygen gas required to fully cover the furnace volume iii. Finding leak points of the furnace (near the edges) Upon completion of the heat-treatment, the powder is inserted into the custom-built exchange chamber via a hydraulic lift, and the powder is then transferred into seal containers. The time taken by the hydraulic lift to lower down the powder and insert it in the exchange chamber is about 30 s. During these 30s, it is necessary that the exchange chamber is fully purged with oxygen and that the pressure of the oxygen is more than the outside atmospheric pressure, to avoid leakage of outside air inside the chamber through the weakly sealed edge points. During this purge, the atmosphere is monitored constantly with the gas analyzer. To date, the best atmosphere achieved in the exchange chamber during powder transfer is ~40 ppm CO2 which is a ~10x reduction from ambient conditions. Powder heat-treated and collected in this manner has exhibited carbon content less than 100ppm for all samples, demonstrating the success of the furnace and exchange chamber operation.
  20. 20.   20   Figure 9: Photograph of the industrial furnace with custom built exchange chamber 3. Final Product: Powder lots for HTS wire Finally, after solving the above technical issues, two wire lots namely LXA 127A and LXA 127B were prepared for final wire shipment to Supermagnetics and then they would be sent to NCSU for magnetization studies. 1kg each of powder lots were shipped. Both the powders have consistent compositions as described earlier and their carbon content is ~40ppm (that is 60% lesser than the carbon for Phase I wire). The LXA 127 powders were heat treated in flowing oxygen to burn out carbon and to calcine the powder into its full crystal phase. The XRD results are shown below in Figure 10. Peak assignments are not given due to the fact that virtually every peak present in the XRD belongs to the 2212 crystal phase. The only exception is the slight bump that appears at 25.9 degrees in the Nexans 521 powder, which corresponds to 2201. As the operation is within the stable
  21. 21.   21   phase region of the Bi2212 phase diagram, the differences in cation stoichiometry in each of these compositions still manifests as nearly pure Bi2212 phase. Figure 10: XRDs of various nGimat powder lots compared with a sample of Nexans 521 granulate 4. Project Merits in terms of cost benefits The manufacturing costs and performance of Bi2212 nanopowders will be directly related to the costs and quality of starting precursor materials. A cursory search through nGimat’s preferred suppliers shows potentially suitable nitrates available for purchase in kg-level quantities, as shown in Table 4. The pricing is shown only to provide estimation for what material costs will be for the SBIR effort and should go lower once Bi2212 is produced in high volumes.
  22. 22.   22   Nitrate ~Cost/kg Bi(NO3)2·5H2O $53 Sr(NO3)2 $9 Ca(NO3)2·4H2O $13 Cu(NO3)2·2.5H2O $60 Table 4: Nitrate precursors from several of nGimat’s suppliers. All materials are >99.5% purity, as required for Bi2212 powder (nGimat, 2014) Using the pricing information in Table 3, it is estimated that nGimat can produce Bi2212 nanopowder at a raw materials cost of ~$110/kg, so in high volumes it can be sold for about $200/kg. Even after factoring in overhead and profit margin, nGimat will be able to market superior Bi2212 powder in kg quantity at a much lower cost than Nexan’s current retail price of $2000/kg, and can also offer stoichiometry variation versions (nGimat, 2014). 5. Conclusion and Future Goals It is concluded that highly pure precursor solutions of the right stoichiometries are needed to produce the right composition of Bi2212. The heat treatment should be done at an optimized rate and held at the right temperature for 72 hours of quenching to get the correct majority 2212 phase and to avoid melting. Heat treatment in proper oxygen purged environment in necessary to obtain carbon content lower than at least 100ppm for favorable superconducting properties in the wire. Lastly, efforts have been made to scale- up the Bi2212 powder production process to produce 10kg/day of powder by improving the reactor and furnace infrastructure. The goal is to make nGimat a world leading U.S. based supplier of Bi2212 precursor powder by the end of Phase II in mid-2015.
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  24. 24.   24   References 1. Phase II Project Report for Award DE-SC0009705, nGimat LLC, August 2014 2. Yun Zhang et. al., Synthesis of Bi2212 superconductors via direct oxidation of metallic precursors, Superconductor Science Technology, March 2014, 27, 005016 3. http://ngimatllc.com/technology.html