A generic process to introduce nanoparticles into powder preforms and its application to Infiltration Growth processing of REBa2Cu3O7 superconductor

Materials Chemistry and Physics 161 (2015) 59e64

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A generic process to introduce nanoparticles into powder preforms and its application to Infiltration Growth processing of REBa2Cu3O7 superconductor P. Missak Swarup Raju a, *, V. Seshubai b, T. Rajasekharan c a b c

GITAM University, Hyderabad Campus, Hyderabad 502329, India School of Physics, University of Hyderabad, Hyderabad 500046, India Rajiv Gandhi University of Knowledge Technologies, Vindhya C4, IIIT Campus, Hyderabad 500032, India

h i g h l i g h t s
Innovative method to introduce nanoparticles uniformly is discussed.
Uniform dispersed solution of ceria nanoparticle and Y-211 is used.
Nanoparticles of CeO2 were distributed in Y-211 matrix.
TEM, FESEM studies were carried out and the results are discussed.
HRTEM and SAED pattern of ceria nanoparticles are provided.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 May 2014 Received in revised form 23 April 2015 Accepted 3 May 2015 Available online 16 May 2015

Performance of oxide superconductors depends strongly on achieving efficient flux pinning leading to enhanced critical current densities from low to high fields. Methods of generating a uniform distribution of nano-metric particles or a network of nano-metric defects, in the superconducting matrix gain importance in this context. We report here a new process, which might be referred to as Nano-Dispersed Sol Casting (NDSC), for introducing nanoparticles, uniformly and without agglomeration, on the surface of Y2BaCuO5 (Y-211) particles. This powder can then be used to fabricate a preform of the desired shape and porosity for use in the Infiltration Growth (IG) process, to produce high current density components. As a specific example, we discuss here the example of CeO2 nanoparticles of ~30 nm diameter, and of CeO2 nano-rods of 300 nm length, which could be deposited by the above process uniformly and without agglomeration. Uniform deposition of individual particles, even when working with relatively high concentrations of the additive, up to 10% by weight, is demonstrated. The process consists of mixing stable Y-211 particle slurry with ceria sol of the required concentration, and then quickly freezing the Y211 and ceria nanoparticle mixture in their fully dispersed state, through polymerization. The process is highly generic in nature; it can be used to dope other ceramic nanoparticles into the RE-123 superconductor. In fact, the NDSC process can be used to dope nanoparticles, in large concentrations if needed, in a matrix of a variety of ceramic powders, with possible advantages in the development of several products. © 2015 Elsevier B.V. All rights reserved.

Keywords: Chemical synthesis Composite materials Nanostructures Superconductors Sintering

1. Introduction The REBa2Cu3O7(RE-123) group of superconductors is the most attractive for applications at the boiling point of liquid nitrogen

* Corresponding author. E-mail address: [email protected] (P.M.S. Raju). http://dx.doi.org/10.1016/j.matchemphys.2015.05.004 0254-0584/© 2015 Elsevier B.V. All rights reserved.

because they are less affected by flux creep problem and also by concerns of toxicity [1e3]. Development of coated conductors using RE-123 superconductors and of bulk superconductor bodies by melt or Infiltration Growth processing techniques has progressed substantially [4e6]. Suitably textured superconductor tapes have been fabricated in long lengths [7]. Complex shaped bodies with microstructures supporting reasonably high current densities have been fabricated using the Infiltration Growth (IG) process [8]. But

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still there are matters of concern. Sharp fall in Jc with increasing magnetic field can be observed in the case of IG processed superconductor bulk bodies as well [9]. The above instances of flux creep can be attributed to the absence of flux pinning centers of appropriate size in the products, active at the high fields involved. H. Suematsu et al. [10] have developed an equation connecting the size of the flux pinning centers and the magnetic fields at which they are active:

2∅ Hp ¼ pffiffiffi 0 2 3 af

(1)

where Hp (T)is the peak field at which the maximum pinning occurs, ∅0 is the flux quantum (∅0 ¼ h/(2e) z 2.067833758(46)  1015 Wb) and af (Å)is the vortex lattice spacing. Various methods have been attempted to introduce fine defects of the order of coherence length, which is a few nanometers (~2e3 nm) in YBCO [11], mostly in melt processed REBCO, and to a much less extent in IG processed materials [12e15]. Substitution of metallic ions in Cu site, thus creating defects in the lattice to pin magnetic flux, has been reported to lower/broaden the superconducting transition temperature/transition width [16]. On the other hand, uniform distribution of non-superconducting and inert inclusions such as of Y2BaCuO5 (Y-211) is known to lead to effective flux pinning, without affecting the transition temperature. Enhanced surface area of fine-sized Y-211 inclusions, arising from large curvatures at their interface with the 123 matrix, gives rise to increased interfacial defect densities due to lattice mismatch [17]. Dopants like Y2O3 [18], CeO2 [19], ZrO2 [13], BaCeO3 [14], Al2O3 [20], NiO [21], etc. have been explored in the literature as fine second phase particles in the 123 matrix; some of such additives had the ability to refine the size of the Y-211 precipitates residual in the RE123 matrix after the process. Alternative methods such as heavy ion irradiation [22] neutron irradiation [23] proton irradiation [24] and electron irradiation [25] have also been used to create controlled fine defects, both columnar and point defects. Though considerable improvement in the critical currents up to moderate magnetic fields has been observed, irradiation methods are not practical when it comes to large scale industrial applications due to cost and the possibility of leaving back radioactive residues. Methods of nanoparticle doping in YBCO(Y123) system reported in the literature [13,14,18e21] involve addition of nanoparticles directly to the precursor Y-123 powder followed by intimate mixing, either dry or wet, in a ball mill. Noninteracting Y2Ba4CuMOy (M ¼ U, Nb, Ta, Mo, W, and Re) [26,27] nanoparticles have been introduced into the Y-123 matrix during melt processing and IG processing. In the IG process, nanoparticles have been introduced into the preform along with the liquid phases [15,27,28]. The Melt Growth process in RE-123 superconductor involves peritectic melting of the superconductor into Y-211 and liquid phases above its peritectic temperature Tp, and forming it back again with a microstructure supporting high current density (Jc) by cooling slowly to a temperature below Tp. The sample shrinks by ~20% and a large amount of porosity is generated due to liquid phase outflow [4]. Hence the melt growth process is not suitable for forming superconductor parts of complex shapes. Also, in melt processing, the introduction of nanometer-sized additions to the superconductor becomes a problem because the additive moves around in the liquids and forms streaks and accumulations in several regions [26,27,29]. In the Infiltration and Growth process, the main advantage is that the RE-211 preform does not melt and flow during the lower temperature treatment. The shrinkage when

near-net-shaped preforms are converted to RE-123 is <0.5%, and thus the IG process is a powerful near-net shaping technique. It is of interest to develop a method of introducing nanoparticles during IG process because the advantage of improved flux pinning can be available in the net-shaped parts produced by the process. IG processed RE-123 produced using optimally prepared preforms, have been demonstrated to show high and flat Jc versus H curves to rather high magnetic fields. The variation of Jc versus H has been correlated, using Eq. (1), to the defects of different size ranges (2e100 nm) occurring in the material. Flux pinning active at very high fields in the material has been attributed to extensive occurrence of nanometer-sized defects associated with twinning in the material. In the case of coated conductors and IG processed material, introduction of nanometer-sized flux pinning sites in the superconductor matrix, uniformly and without agglomeration, can be crucial in addressing the flux creep problem. Obviously, the addition of large amount of nanometer-sized second phase powder directly into the superconductor body will not give the sought after advantages; the particles will just agglomerate in different regions of the material. Hence, in the present work, we have developed a process which is so designed that the particles, in spite of the high concentrations involved, would remain separate from one another, and would form a uniform distribution on the surface of the larger Y-211 particles. The process is highly versatile and can be extended to many nanoadditives to Y-211. Its application is not necessarily limited to superconductor systems only; other ceramic systems can also benefit. An example is cutting tools based on alumina-Zirconia where substantial improvement in performance has been reported by the addition of relatively large percentages of nanoparticles of a different phase [30]. 1.1. Experimental procedure Precursor powders in the required stoichiometry for Y-211 and Y-123 were prepared by the citrate synthesis method [31]. They were heated to 900  C and 950  C for 12 h to obtain Y-123 and Y-211 powders respectively. The powders were nearly spherical with size in the range 1e2 microns. Attaining stable and free flowing slurry of Y-211 involves finding a proper dispersing agent and optimizing the sedimentation conditions. Y-211 powder was tumbled in deionized (DI) water with various dispersants in a mill for 12 h. The stability of the resulting slurries was then tested by pouring them into test-tubes and allowing them to stand for several hours. The dispersant Dolapix 77 (Zchimmer and Schwarz GmbHKG, Germany) was found to be the best; the powder remained well dispersed for more than three hours while those with other dispersants settled down within an hour. Stability of the slurry for three hours is sufficient for the remaining part of the process. Sol containing 5 wt. % of ceria nanoparticles was prepared from cerium nitrate (Ce(NO3)3.6H2O) [32]. Cerium nitrate was dissolved in DI water and an appropriate amount of NH4OH was added to form Ce(OH)4 precipitate which was then dehydrated to form CeO2. The ceria thus formed was washed and dispersed in DI water. A selfstanding ceria sol (Fig. 1(b)) was formed by adjusting the pH and ultrasonicating the mixture using an OSCAR-PR 250 ultrasonic generator and probe. The particles of ceria in the sol were observed on a glass substrate using an FESEM (Zeiss make, model: Ultra 55); they were found to form a network, each particle about 30 nm in size (Fig. 2(a)). These particles were observed under TEM and it was found that the individual particles are of the order of 5 nm (Fig. 2(b) and (c)). SAED pattern was also obtained on these particles and shown in Fig 2(d). The ceria sol was added to the Y-211 slurry. Knowing the solid

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Fig. 1. (a) Sedimentation test with 5 wt. % slurry of Y-211 in water, containing 1 wt. % of various dispersants. A dispersion that did not settle down in three hours was obtained with DOLAPIX 77, and this stability was sufficient to ensure the completion of the process of nanoparticle introduction without segregation. (b) Sol containing 5wt. % of CeO2 nanoparticles.

our earlier experiments to produce the best samples by IG processing [34]. They were then heated at 950  C for 4 h to form Y-211 preforms. The Y-211 preforms thus fabricated containing 2 wt.% and 10 wt.% of ceria nanoparticles, will, hereafter, be referred to as Ce-2 and Ce-10 preforms. Preforms containing zirconium oxide nanoparticles were also fabricated by the above process to demonstrate the versatility of the process. The present paper will examine the characteristics of the Y-211 powder with nanoparticles deposited on their surfaces. Since the porosity in the Y-211 preforms is important from the viewpoint of efficient infiltration of the liquid phases, the characteristics of the preforms obtained by sintering the powder at 950  C will also be examined. An FE-SEM was used to make the observations. The microstructure and superconducting properties of the IG processed materials will be reported elsewhere. 2. Results and discussion

Fig. 2. (a) An FESEM micrograph recorded from a dried droplet of CeO2 sol at 100 kX. CeO2 nanoparticles of 20e40 nm size can be seen. (b) Size of the individual particles is around 5 nm as seen from HRTEM image of the dispersed isolated nanoparticle. (c) HRTEM image shows agglomerated CeO2 nanoparticles. (d) SAED pattern on these particles shows polycrystalline nature.

content per unit volume of the slurry and of the ceria sol, three concentrations of ceria in Y-211 slurry were prepared, viz.2, 5 and 10 wt. %. The mixture was tumbled on a mill for 12 h to distribute the nanoparticles uniformly. In order to freeze the distribution of ceria nanoparticles in the Y-211 slurry, a premix of monomers consisting of methacrylamide (MAM) and methylene bisacrylamide (MBAM) in small percentages in water, was added, and tumbled further. This was followed by polymerization of the slurry in a rectangular mold by the addition of ammonium peroxydisulphate (APS) as initiator and N, N, N0 , N’,- tetramethylethylenediamine (TEMED) as catalyst in small quantities, as described elsewhere by Swaroop et al. [33]. The solid mass thus formed was dried slowly, crushed to powder, and was then heated slowly to 600  C for the removal of organic binders. The powder was uniaxially pressed in a stainless steel die into rectangular pellets of size16 mm  16 mm  5 mm at 460 MPa pressure. The pressure chosen was the one proven to be optimal in

In Fig. 3, we show the FE-SEM image from the Y-211 preform containing 2 wt. % CeO2 nanoparticles. It can be observed from the figure that the present approach has been successful in distributing nanoparticles of CeO2 on the surface of the Y-211 particles. Ceria nanoparticles were distributed uniformly around the Y-211 particles throughout the preform. The particle size is around 60 nm. The nanoparticles are angular with some of them having a hexagonal shape. There are no agglomerates. Agglomeration is a routine problem when dealing with nanoparticles due to their high surface energies, and the present process that avoids it offers a handle to develop products which can derive full benefits from the fine size of the additives. The distribution as observed also offers an opportunity to study without ambiguity the reactions that proceed between the individual nanoparticles and the matrix phase during subsequent processing. The situation changes drastically when the ceria concentration is increased to 10 wt. %., as can be seen from Fig. 4. The ceria nanoparticles now grow into nano-rods of diameter ~ 60 nm and length ~300 nm. Needle-like growth and its direction are usually associated with strong structural anisotropy along any of the coordinate directions with constant rate of feeding of material in a controlled atmosphere [35]. The modern understanding of the needle-like growth of nano structured materials are based on hydrothermal processing, one of the most effective techniques for nano-structure synthesis, where continuous feed of material is

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Fig. 3. Ce-2 preform after heat treating at 950  C for 4 h, as seen in an FESEM. (a) At a magnification of 10,000. Y-211 particles of size around 1-2 microns are observed to be coated almost uniformly with isolated nanoparticles of ceria. (b) At a magnification of 50, 000, the ceria particles anchored on the Y-211 particle surface. (c) Represents the particle size distribution of ceria nanoparticles in Y-211 matrix, it is observed that the size range mostly fall in between 40 and 60 nm.

Fig. 4. Ce-10 preform after heat treating at 950  C for 4 h was observed under FESEM at magnification of (a) 50,000 and (b) at 10,000. Ceria has grown into nano-rods on Y-211 particle surface.

ensured and concentration of starting solution, temperature and duration of reaction can be precisely controlled. Anwar et al. [36] have demonstrated the formation of CeO2 nanorods and nanodumbbells by thermal decomposition of a ceriaeoleate complex at 200  C in a high-boiling organic solvent at extended reaction time. They also observed that nano-rods have a long axis along one of the 〈1 0 0〉 direction. The needle-like growth of ceria particles at higher concentration of ceria in Ce-10 preform in the present work can be attributed to the availability of ceria and its non-reactivity with the Y-211 particles. In order to confirm the non-reactivity of ceria with Y-211, Xray diffractograms were obtained from the Ce-10 preform and Y211 powders, and are compared in Fig. 5. In the XRD, peaks corresponding to ceria were observed with low intensity confirming the presence of low concentrations of independent ceria particles. The additional lines, marked by star in the figure, could be indexed to CeO2-X (JCPDS file no 49-1415). No signature of BaCeO3 or any other secondary phase was observed. The minor uniform shift in Y-211 peak positions observed in XRD pattern compared to ceria-free Y-

211, suggests a mild reaction between ceria nanoparticles and Y-211 which possibly enables the particles to adhere to the Y-211 particles without forming any compounds like BaCeO3. Ceria has been reported to react with Y-211 only when heated at a temperature above 1000  C [37], the preforms were sintered only at 950  C. Fig. 6 (a) and (b) show the preforms of Y-211 produced by the NDSC procedure with 10 wt. % and 2 wt. % of ZrO2 nanoparticles, respectively. The nanoparticles are observed to adhere individually without agglomeration on the Y-211 particle surface. The nanoparticles appear to have reacted strongly with the Y-211, and have anchored themselves on Y-211 at the points of contact. Unlike in ceria-doped preforms, a strong tendency for the Y-211 particles to get rounded and fused together was observed in the preforms fabricated in the presence of ZrO2 nanoparticles. 3. Conclusion A generic process for the introduction of nanoparticles, individually and without agglomeration, into the matrix of a second material is demonstrated. The second phase material is in the form of powder with particle size in the range 1e2 microns. The process is used to introduce up to 10 wt. % of ceria and 10 wt. % Zirconia nanoparticles on the surface of Y-211 particles. 10 wt.% of these additives consist of a very large number of nanoparticles, nevertheless the particles remain separate and without agglomeration on the surface of matrix powder. Preforms made from the coated powder can be used to introduce nanoparticles into the RE-123 superconductor during IG processing. Because of the capability of the IG process to facilitate near-net shaping, this possibility is very relevant and attractive. Another important observation that can be made from the work is that even small amounts of a reactive additive can have the effect of sintering the Y-211 preform, thus having the deleterious effect of

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Fig. 5. An X-ray Diffractogram from Y-211 þ 10 wt. % ceria (Ce-10) is compared with that from pure Y-211 without any additive. The extra peaks in the former could be identified as due to CeO2-X (JCPDS file no 49-1415).

Fig. 6. (a) FE-SEM micrograph obtained on Y-211 preform with 10 wt. % ZrO2 at a magnification of 50,000 X; nanoparticles of ZrO2 can be seen distributed individually and uniformly on Y-211 particles. The reactivity of Zirconia with Y-211 is strong enough to fuse together the Y-211 particles. (b) a micrograph from a Y-211 preform with 2 wt. % ZrO2 shows that the ZrO2 nanoparticles have been absorbed by reaction with Y-211, and that the Y-211 particles have fused together. (c) particle size distribution of Y-211 particle which is around 1-2 micron but most of the got fused and form agglomerates.

reducing liquid phase entry into it during IG process. With ceria addition, because the interaction between Y-211 and ceria is minimal, the preforms remain porous enough for infiltration. With Zirconia, with the present heat treatment of 4 h at 950  C, even with small additions of Zirconia, the preforms lose their porosity. The grain growth in Y-211 is substantial. In the case of Zirconia addition, the optimization of the sintering times and temperatures while fabricating the preforms will be important to optimize the final microstructure of RE-123. The introduction of certain materials have been known to have the effect of refining the Y-211 size in melt processed Y-123. In the earlier days of high temperature superconductivity, it was discovered that the addition of platinum could dramatically reduce the Y211 inclusion size in the melt processed material. Later on,

considerable amount of work has gone into studying the effect of ceria addition on Y-211 refinement. There are two competing effects that can take place: one is the refinement of Y211 size which can increase Jc and the other is the dissolution of ceria in Y-123 and the lowering of Tc, which will result in lowered Jc. So the mechanisms by which the ceria particles refine Y-211 in the final microstructure and the amount of ceria dissolution in the Y-123 matrix are very important. The distribution of ceria as isolated nanoparticles on the surface of the starting powder particles of the Y-211 preform allows unambiguous tracking of the reactions taking place in subsequent IG process steps using these preforms. The NDSC process developed here can be employed to introduce other metal oxide nanoparticles into Y211 preforms, and thus into the YBCO superconducting composites. The formation of ceria

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nano-rods in the case of ceria which interacts only minimally with Y-211 suggests that certain nano-structures of metal oxide nanoparticles can be achieved by suitable choice of a non-reactive matrix on whose surface the metal oxide particles are to be deposited and distributed uniformly. The method can also potentially be very valuable in introducing a variety of nano-powders into powder matrices of materials other than the Y-211.

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