Magnet teknolojisi için süperiletken şerit ve tellerin sol-jel tekniğiyle yalıtılması

Abstract

ZrC>2 esaslı sol-jel yalıtkan kaplamalar korozyon, aşınma direnci, optik, mikroelektronik ve süperiletkenlik uygulamalan son yıllarda ilgi çekmektedir. Bu kaplamalar yüksek sıcaklık süperiletken (HTS) ve düşük sıcaklık süperiletken (LTS) magnetlerin üretiminde Özellikle Ag ve AgMg esaslı Bi-2212 (Bi2Sr2CaCu20) kompozit şeritler ve Cu-Nb3Sn ve CuSn-Nb3Sn kompozit tellerin yalıtılmasında kullanılması teknik olarak İstenilen özelliklere sahiptir. Süperiletkenleri çevreleyen yalıtkan malzemeler bobinlerin sarımında elektriksel kısa devre oluşumunu engellenmesinde kullanılmaktadır. Bu nedenle, kaplamaların altlığa yapışması çok önemli problemlerdendir. Bu çalışmada, sol-jel tekniğiyle süperiletken malzemelerin üzerine Zr02, MgO-Zr02> Y203-Zr02, Ce02-Zr02, In203-Zr02 ve Sn02-Zr02 yalıtkan kaplamaların üretilmesi incelenmiştir. Ayrıca, bu filmlerin yapışma mukavemeti, mekanik özellikleri ve kaplanmış Ag ve AgMg Bi-2212 malzemelerin ve bobinlerin süperiletkenlik özellikleri tespit edilerek yüksek ve düşük Tc (Kritik sıcaklık) süperiletken yalıtkan kaplamaların 4.2 K ve sıvı He gibi krojenik şartlarında HTS ve LTS magnet teknolojileri için en uygun ve en iyi ince filmlerin belirlenilmesi amaçlanmıştır Zr02 esaslı kaplamalar katkı malzemelerine bağlı olarak 450°C ile 550°C sıcaklıklarında amorf yapılardan kristal yapılara dönüşmüştür. Kaplamaların yüzey morfolojisi ve yapışma özellikleri çözelti hazırlama, geri çekme hızı, ısıl işlem ve tavlama parametrelerine bağlı olarak değişmiştir.. Y203-Zr02, Ce02-Zr02, ln203- Zr02) Sn02-Zr02, MgO-Zr02 ve Zr02 filmler İle birleştirilmiş Ag numunelerinde yapışma mukavemetleri sırasıyla 2.400 MPa, 2.087 MPa, 1.650 MPa, 1.580 MPa, 1.300 MPa ve 0.763 MPa değerleri bulunmuştur. Bununla birlikte, 1 GHz NMR programı için önerilen 3 T HTS bobinler için 13 daldırma sayısı ile kaplanan Zr02'ya katkı maddelerin ilavesiyle 8'e düşürülerek ekonomik olarak çok büyük tasarruf elde edilmiştir. Buna bağlı olarak 3.5 km süperiletken malzeme kaplanmıştır.

INSULATION OF SUPERCONDUCTING TAPES AND WIRES BY SOL-GEL TECHNIQUE FOR MAGNET TECHNOLOGY SUMMARY Keywords: Sol-gel* Insulation, Film Growth, Adhesion Strength, Superconductivity Sol-gel zirconia and zirconia based coatings have attracted growing interest in corrosion, wear resistance, optical, microelectronic and superconductive fields. One of their applications is associated with insulation on Ag and AgMg sheathed Bi-2212 for HTS coils. Insulating materials surrounding the conductors are used to prevent short circuits within the winding of a coil. From a design point of view, the insulation layer must be able to withstand large electric fields (as high as 4xl05 V/cm in some cases) without suffering dielectric breakdown, a phenomenon that leads to electrical short circuit failure between neighboring conductors. In these coatings, one of the most important problems to overcome in the production of coatings is the maximization of coating-to-substrate adhesion. Adhesion is characterized by interfacial bond strength, and is the summation of all intramolecular and interatomic interactions. Adhesion between two materials includes mechanisms based on diffusion, mechanical interlocking, electrostatic attraction, physical adsorption, chemical bonding, and weak boundary layer interaction. Factors affecting adhesion are porosity, micro and macro cracks, residual stresses, and coating parameters. Coating failure can occur by several mechanisms, such as surface damage (e.g. wear, oxidation), elastic or plastic deformation, fracture, etc. The degradation or failure of coatings is fundamentally related to the adhesion and cohesion strength. The failure modes can be described as interfacial, cohesive, or mixed interfacial/cohesive defects. In the present work, we reported the results from Z1O2, MgO-ZrOz, Y2O3-Z1O2, Ce02-Zr02, In203-Zr02, and Sn02-Zr02 coatings produced on Ag and Ag/AgMg sheathed Bi-2212 tapes, and Cu-Nb3Sn wires by the sol-gel technique from solutions of Zr, Mg, Y, Ce, In, and Sn based organometallic compounds. In addition, we noted the mechanical and the adhesion properties of these ceramic insulations on silver tapes as sheathing for HTS conductors. The superconducting properties of the insulated the tapes, the wires and the coils were determined using the IxIO"4 V/m criterion (1 uV/cm). The sols were prepared from Zr, Mg, Y, Ce, In, and Sn based precursor materials. In this procedure, Mg, Y, Ce, In, and Sn based precursor materials were separately dissolved in the isopropanal which İs used as the solvent, by stirring for a 12 h period at 100 rpm. Zirconium tetrabutoxide was then added to each solution. An adequate XXIVamount of acetylacetone (Ac Ac) was also mixed into the solutions in order to act as a stabilizing agent by forming a chelate complex. These solutions were hydrolyzed after stirring for a 3 h period at 100 rpm. The pH values of the solutions were measured by a standard pH meter (Orion 410 Model) and their pH adjusted to produce stable solutions. Commercial Ag tapes with nominal dimensions of 65 mm x 9.62 mm x 0.0125 mm and Ag and AgMg sheathed Bi-2212 tapes with nominal dimensions of 3 mm x0.20 mm x500 mm received from OST (Oxford Superconductivity Technology, Inc.) were used as substrates. The substrate surface was cleaned with acetone. The solutions were used for coatings on the Ag tape, the Ag and AgMg sheathed Bi-2212 tape, and the Cu-Nb3Sn wire conductors by dipping with a withdrawal rate of 2.12 cm/sec at ambient atmosphere in the continuous sol- gel system. The gel layers were transformed to an amorphous layers at about 300°C for 25 sec. The ceramic oxide coatings were subsequently heat treated at various temperatures depending on the coating type. This process was repeated for each deposition in order to obtain thick layers on the substrates. The dipping time was changed from coating to coating. The coatings on Ag and AgMg sheathed Bi-2212 tapes were finally densified by annealing at 862°C for 36 h under an oxygen atmosphere. The coated Cu-Nb3Sn wires were annealed at 800°C for 20 hours under argon atmosphere. The samples were cleaned with acetone in air to remove any dirt, oil, or scales from their surfaces. After the samples were dipped into the solutions, they were prepared in sandwich shape. A few pounds of weight was put on the samples to improve contacting between the coated Ag samples. They were dried at room temperature for one day. The dried samples were then put into the furnace. Oxide films were formed in the interfacial region between two silver tapes by annealing each sample at constant temperature between 500°C and 650°C for 10 min in air. The joint samples were pulled to failure using a mim tensile tester. These samples were prepared such that the failure was a shear mode at the interface of the insulated Ag substrate. Experiments were repeated until an optimum overlap area was obtained in the samples. Real measurements were taken after the optimization. The tests, which showed the shear failure of the joint area, were considered and others discarded. The surface topography of the insulation and the failed surface microstructures of joint samples were examined using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The growth mechanism and kinetics of these coatings were investigated, in-situ, using an Environmental Scanning Electron Microscopy (ESEM) equipped with a hot stage, which can produce a temperature from 25°C to 1200°C at a heating rate of 25°C/min. Differential thermal analysis (DTA) and therraogravimetric analysis (TGA) were made of fresh gels dried at room temperature for a 24 h period with a heating rate of I0°C/min under oxygen atmosphere. The surface topography and elemental distributions of the final heat treated coatings were determined by X-ray mapping at scan rate of 4.6 fr/sec. X-ray diffraction (XRD) analysis of the coatings was carried out by means of a Philips diffractometer, with aCuKa irradition (wavelength, ^.=0. 15418 nm). Specifically, XRD technique was used to determine the structure of the insulated Ag samples to verify that joining did not prevent any phase formation. xxvOur work points out that porosity, coating thickness, and cracks in sol-gel dipped coatings depend on the structure and content of the molecular precursor as well as the relative rates of condensation and evaporation during coating, hi addition, viscosity, withdrawal speed, surface tension, and contact angle with the substrate are very important factors. These factors influence the surface morphology, structure and mechanical properties of the coatings. The typical surface morphology of Zr02, MgO-Zr02, Y203-ZrO2, CeCVZrO^ In203-Zr02, and Sn02-Zr02 is obtained after the heat treatment process. The Zr02 and the Zr02 based coatings exhibit similar surface morphologies, which resemble a mosaic structure. This similarity was generally changed depending on amount of dopant materials. The other parameters influencing the structure of the coatings were heat treatment and annealing temperature and time. It was observed that more compact coatings were obtained on AgMg sheathed Bi- 2212 tapes after the annealing process, which was carried out at a temperature of 862°C for 36 hours under an oxygen atmosphere. The temperature induced a densification of the coatings during this process. According to this result, it can be stated that heat treatment temperatures between 500 and 650°C are necessary to form ceramic oxide films. The samples were also finally annealed to obtain smooth surfaces and compact structures. Furthermore, it was observed that coating islands of dipped tapes did not flake off from tapes with increasing temperatures from 25°C to 900°C after being on the in ESEM expanded hot stage. The regular growth mechanism of coatings was determined on tapes. Coating thickness changed with increasing Zr02 and Zr02 based layers as a parabolic function of the number of dippings. EDS results supported the growth mechanism relationship with increasing elemental atomic percentages of coating materials and decreasing elemental atomic percentages of substrate materials. The dopant materials were homogeneously mixed in Zr02, as determined by X-ray map results. Zr02 coatings with oxide precursors, added as powders such as MgO, Y2O3, Ce02, In203, and Sn02, did not possess the smooth surfaces due to the fact that the solutions were heterogeneously mixed in the solution preparations. Mg, Y, Ce, In, and Sn based organometallic precursors changed the structure of the coatings, making their surfaces very smooth. In this work, an adjustable continuous sol-gel process was also used. The thermal behaviour of Zr02, MgO-Zr02, Y203-Zr02, Ce02-Zr02, In203~Zr02, and Sn02~Zr02 xerogels was determined by DTA and TGA techniques. Theye were dried at room temperature for 24 hours. Exothermic and endotermic peaks were found until 550°C. Then other thermal effects were observed above this temperatures. One was solvent removal at temperatures of about 100°C and 150°C. The second was the combustion of organic groups between 250°C and 400°C. The last stage was the formation of ceramic oxides between 450°C and 550°C depending on coating type. XRD patterns of sol-gel coatings showed the characteristic peaks of insulation materials, and indicated that sol-gel coatings started to crystallize at about 400°C- 550°C. Cubic, orthorombic, and tetragonal phases were clearly indicated on the XRD patterns for Zr02 and Zr02 based coatings prepared from Zr[0(CH2)3CH3]4 and Mg, Y, Ce, In, and Sn based organometallic precursors. On the other hand, cubic Ag xxviphases were determined in all of the samples because coating thicknesses changed between 2 \im and 5 um. The most common phases of the coatings were cubic phases due to the fact that these were produced at low temperatures using wet methods such as sol-gel, chemical synthesis, and precipitation. These coatings contain some residual OH or water, cannot easily be accommodated within the limits shown in the phase diagrams for binary systems. In addition to this factor, the cubic phases present in the fine powders of dopped Zr02 produced by wet methods, may be stabilized by the excess surface energy. Another factor was the chemical structure of precursors. When doping materials such as MgO, Y203, Ce02, In203, and Sn02 were added to Z1O2 solutions as powders, mixing problems occurred during the coating process in the sol-gel method because of settling. However, this effect was not seen in organometallic compound homogeneous suspensions in isoprapanal. Due to this factor, much more chemically homogeneous structures were produced during the coating process. These factors influenced the formation of cubic phases. Although MgO-ZrOi exhibits cubic Z1O2, and MgO, cubic Z1O2, and monolithic Y2O3 phases were found in Yz03-Zr02. Cubic Zr02 phases were also found in Ce02-Zr02- Futhermore, In20rZrO2 exhibited the same structure with Zr02 except for a cubic In203 phase. A different peak was found for tetragonal Sn02 in the Sn02-Zr02 coating. An increase in the crystalline size of coatings was obtained with increasing heat treatment temperatures. The present results are in agreement with earlier reports on producing a cubic phase at low temperatures (450-600°C) by a sol-gel method. The adhesive strength is determined as the shear stress above which the joint abruptly yields. The adhesive strength of a Z1O2 joint was 0.763 MPa. This value increased by dopping with MgO, Y2O3, Ce02, In203, and Sn02 which improve the mechanical properties of Zr02 considerably. The failure was a mixed mode type, as an interfacial/cohesive defect in the coatings produced by Z1O2 solutions with added MgO, Y2O3, Ce02, In203, and Sn02 powders. The average adhesive strength of MgO-Zr02, Y203-Zr02, Ce02-Zr02, In203-Zr02, and Sn02-Zr02 joints were found to be 1.045, 1.150, 0.915, 0.833, and 0.797 MPa, respectively. Adhesion strength of the Ag joints increased by using organometallic compounds instead of oxide powders as a doping material. The average adhesion strength of MgO-Zr02, Y203-Zr02, Ce02-Zr02, bi203-Zr02 and Sn02-Zr02 joints produced from organometallic compounds were determined to be 1.300, 2.400, 2.087, 1.650, and 1.580 MPa, respectively. Y2O3 and Ce02 doped Zr02 showed the best adhesive properties. Especially, these doped materials stabilized Zr02 by toughening and may actually be resisted at high and low temperature because of these properties. These insulation materials are therefore very useful for applications at cryogenic temperatures, e.g. 4.2 K, for HTS magnets. Deformation of the joint samples generally started from one or both sides as peeling. These samples finally failed by shear propagation. The type of deformation changed depending on heat treatment conditions. When the temperature and time increased during sol-gel coating formation, the deformation of the joint samples accelerated with reinforcing to structure formation of AgO in the interfacial area between the thin oxide ceramic coating and the Ag substrate. The SEM results strongly indicated that failure occured at the interface between the Ag substrates and coatings. The Ag substrates possessed micro and macrocracks on the surfaces. In that XX VI tcase, the failure mode of the thin Sims at the interface was the mixed interfacial/cohesive defect type. While interfacial failure was dominated in Z1O2, mixed interfacial/cohesive failure was dominated in other thin films. The cohesive failures which formed only in coatings are clearly seen in Mg0-ZrO2, CeCVZrOa, fci203-Zr02, and Sn02-Zr02. MgO-ZrOî coatings exhibited as cracks on the coating layer. Deformation position of coatings and substrates was at pulling directions. Moreover, EDS results indicated that Ag, Zr, Mg, Y, Ce, In, and Sn were found in the interfacial area. Z1O2 based coatings on Ag and AgMg sheathed Bi-2212 tapes have been in use the insulation process for the 3 T HTS coil insert proposed for the 1 GHz NMR magnet at the NHMFL. Zr02 provides adequate turn-to-tum insulation after 13 dippings. Mg0-ZrO2 does the same with only 8 dippings. In fact, insulation is really complete with only 5-6 dippings but additional dippings are required to provide the stand off for the insulation to survive the Bi-2212 leaks during the partial melt process. The insulation was successfully applied to about 3 km of Ag and AgMg sheathed Bi-2212 tape conductors and then 3 T insert HTS magnets were tested. xxvm