Physica C: Superconductivity

Volume 436, Issue 2, 15 April 2006, Pages 62–67

In situ high temperature optical microscopy study of phase evolution in YBa2Cu3O7−δ films prepared by a fluorine-free sol–gel route

  • a Department of Physics, State Key Laboratory for Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai, China
  • b Department of Nuclear Engineering and Radiological Science, University of Michigan, Ann Arbor, MI 48109, United States
  • c Institute of Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW, Australia
  • d Applied Superconductivity Research Center, Tsinghua University, Beijing, China
  • e Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, China
  • f Department of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, OH 45221-0012, United States
Received 6 September 2005, Revised 4 January 2006, Accepted 12 January 2006, Available online 9 March 2006

Abstract

Phase evolution of YBa2Cu3O7−δ (YBCO) thin films prepared by a newly-developed fluorine-free sol–gel method were studied by combined use of in situ high temperature optical microscope observation and X-ray diffraction. The reaction sequences associated with yttrium, barium and copper in this route were determined respectively. It was found that the reactions that take place were strongly dependent on the experimental conditions such as water partial pressure. The formation of YBCO started around 710 °C and continued up to 800 °C for about 15 min. The formation mechanism of YBCO was found to be dependent on the barium containing phases, which were controlled by the experimental conditions. The formation of a- and c-axes oriented YBCO grains was also found to be governed by the processing reactions.

1. Introduction

As a promising method, trifluoroacetate (TFA) metallorganic deposition (MOD) has successfully fabricated YBa2Cu3O7−δ (YBCO) films with Jc over 1 MA/cm2 at 77 K on both single crystal substrate and buffered metallic substrates of RABiTS [1] and [2]. The TFA-MOD approach has several advantages over vapor deposition methods, particularly in the industrial applications including low cost and continuous process. Trifluoroacetate salts have been found to avoid the formation of stable BaCO3 at the grain boundaries because of its lower stability than barium fluoride [2].

Although fluorine can be removed in the form of HF through heat treatment at high temperatures (>650 °C) in humid, low oxygen partial pressure atmosphere, it is a non-trivial process. Moreover, there are issues related to fluid flow and complicated reactor designs that may be required for scale-up. Recently, a fluorine-free sol–gel approach involving the trimethylacetate salts and propionic acid (TMAP) precursor solution has been developed and a high transport Jc was obtained on the order of 1 MA cm−2 at 77 K [3], [4], [5] and [6]. It provides an alternative method for fabricating long-length conductors in large-scale applications, in which no fluorine-containing compounds including HF, BaF2 are formed. In addition, the films prepared by the TMAP-MOD approach possess much denser microstructures compared to TFA films, and the TMAP precursor solution is stable for several months [4]. The processing route has been established in previous studies and high-quality YBCO films can be synthesized by this newly-developed, fluorine-free TMAP method for coated conductor development. It is believed that carbon could be removed from the materials at low temperature burnout stage (<400 °C) in wet oxygen [4]. The superconducting properties and resultant microstructures of prepared films are found to be strongly correlated with parameters such as oxygen and water partial pressures and resultant microstructures [4], [5], [6] and [7].

In order to control the TMAP-MOD process and to optimize the superconducting properties of the films, it is crucial to understand the detailed mechanism and the related chemistry of the formation of YBCO. In this current work, in situ high temperature optical microscopy and ex situ X-ray diffraction are applied to study the development of the intermediate crystalline phases during the conversions in the TMAP-MOD process. The phase evolution and the mechanism of YBCO formation during the TMAP-MOD approach are discussed.

2. Experimental

The fluorine-free sol–gel solutions were developed in-house. The precursor solutions were prepared by the following procedure: stoichiometric (1:2:3) Y(C4H9COO)3 (yttrium trimethylacetate), Ba(C4H9COO)2, and Cu(C4H9COO)2 were dissolved into propionic acid and amine solvents, forming a dark green solution with an oxide concentration between 0.1 and 0.5 mol l−1. The presence of amine improved the solubility of the precursor powders in propionic acid. After filtration, the precursor solution was deposited onto the substrates by a spin-coater at a speed of 2000–5000 rpm. The multi-coatings with the intermediate baking procedure at 200–250 °C were performed on a hot plate to make thicker films. The samples were burn out in a quartz furnace under humid 200 ppm oxygen at 400 °C for 10 h, and the dew point was 20 °C. Afterward, the intermediate films were subject to a high temperature anneal at 745 °C with a speed of 25 °C min−1, then to 750 °C at the rate of 1 °C min−1, dwelling at 750 °C for 80 min. Ten minutes before the end of the dwelling process, the furnace atmosphere was switched to dry, and the temperature was decreased to 450 °C at the rate of 2.5 °C min−1. After oxygenating for 60 min in flowing oxygen, the samples were furnace-cooled to room temperature. Substrates used in this experiment were commercial LAO and MgO single crystals with the (0 0 1) orientation. The LAO single crystals had dimensions of 12 × 12 × 1 mm3 and were purchased from MTI Company. One side of the single crystal was polished as-purchased. The average thickness of the films was about 80 nm.

The in situ observations were conducted on a high temperature optical microscope (HTOM, OLYMPUS BX51M). The intermediate spin-coated films were used on the observation under the atmosphere of 99%N2 + 1%O2. The heating program was as follows: heating up to 400 °C and dwelling for 10 min, and then to 800 °C with a speed of 100 °C min−1. To prepare the samples for XRD and EDS, the films were cooled to room temperature from a given temperature, such as 550 °C and 650 °C. The cooling rate was set at 130 °C min−1, close to the rate of air-quenching. The phases identified by X-ray diffraction (XRD, D8 Discover GADDS diffractometer, Cr radiation) and EDS detecting unit connected to a PHILIPS515 scanning electric microscope (SEM) were regarded as the intermediate ones at these temperatures during the phase evolution. The high-resolution TEM (HRTEM) experiments were performed on a JEOL JEM 4000EX TEM. Transport resistivity measurements were carried out down to liquid-helium temperature using a standard four-probe method.

3. Results and discussion

As mentioned above, the TMAP-MOD process can be divided into two steps, i.e. burnout and high temperature conversions. The first step of burnout mainly involves the removal of organic materials such as solvents and hydrocarbons in metallorganic salts. The precursor solution is composed of Ba(C2H5COO)2, Y(C2H5COO)3, Cu(C2H5COO)2, C2H5COOH, C4H9COOH, C4H9CONHC5H11 and H2O. The boiling points of propionic acid and amylamine were at 141 °C and 104 °C, respectively [5]. In the baking stage, the extra C2H5COOH, C4H9COOH, and water were evaporated at around 150 °C.

Fig. 1 and Fig. 2 show the intermediate phases and the phase development during the process. The main changes of the microstructures were observed at around 490 °C, 550 °C, and 650 °C through HTOM, and the YBCO began to form at about 710 °C. As can be seen in Fig. 1(a), no noticeable changes can be observed on HTOM until 490 °C.