Flexible manufacturing of functional ceramic coatings by inkjet printing

Highlights

• High quality epitaxial YBa₂Cu₃O_7 − x films and patterns grown by using inkjet printing

• Film thickness is varied by adjusting drop pitch and concentration of the ink.

• Correlation between drop pitch and film morphology is demonstrated.

• Printing on selected resonant modes allows droplets smaller than nozzle diameter.

Abstract

The implementation of chemical solution deposition methodology with drop on demand inkjet printing technology demonstrates being a highly flexible and low-cost method for functional oxide production. This paper discusses drop formation and drop volume control in terms of mechanical concerns. Printing parameters such as drop pitch and drop volume are adjusted for continuous track formation to establish the limits for bead stability and the minimum width achievable.

YBa₂Cu₃O_7 − x precursor solutions are developed and their rheological and wetting parameters are precisely controlled to obtain high-quality patterned and continuous biaxial textured superconducting coatings. Highly homogeneous tracks, 250 nm thick and 150 μm wide, are obtained to demonstrate the suitability of this methodology to achieve patterned coatings.

Besides, high-quality films with tuneable thickness up to 600 nm are obtained and critical current densities as high as 3 ∙ 10⁶ A cm^− 2 at 77 K and self field are achieved.

Introduction

The unique and high value-added properties of functional ceramic oxides have enabled a significant number of emerging applications leading to a wide spectrum of emergent devices. To fully exploit such innovative and growing areas, increasingly low cost and scalable technologies have to be considered.

Chemical solution deposition (CSD) methodology is being proved as a very efficient alternative for up-scaled and flexible production of functional ceramic oxides [1], [2], [3]. CSD has already been demonstrated as a promising methodology for epitaxial growth of a broad scope of complex ceramic oxides with functional properties such as conductivity, piezoelectricity, ferroelectricity, magnetoresistance, semiconductivity and superconductivity [4], [5], [6], [7], [8]. However, for many applications it is required to structure the films on specific shapes and so, the development of methodologies allowing to print films on specific shapes is worthwhile.

The great potential of high temperature superconductors (HTS) has promoted significant efforts to develop them in long lengths and of quality enough to bring them onto the market. Second generation (2G) HTS coated conductors [9], based on {RE}Ba₂Cu₃O_7 − x ((RE)BCO, being RE a rare earth: Y, Sm, Nd and Gd) are rapidly becoming the most desirable type of HTS wires for power, medical and magnet applications due to their wide temperature and magnetic field working ranges. The need of high quality kilometre-length conductors requires the biaxial textured growth of the superconducting coating on a set of ceramic buffer layers over metallic substrates, achieving the adequate bed for the superconducting layer epitaxial growth. Despite coated conductors already being commercialized, a huge variety of work around the world is being devoted to improve their quality, scalability, cost and production capacity [10].

In this paper, we intend to demonstrate the advantages that can be obtained by merging the low cost CSD based on trifluoroacetates (TFA) [1], [11], [12], [13] with piezoelectric drop on demand (DoD) inkjet printing technology for YBa₂Cu₃O_7 − x (YBCO) coating and patterning [14], [15]. Inkjet printing [16], [17], [18] permits the scalability to large area manufacturing in an easy way as a reel to reel deposition system. Other important advantages of this technology include the low cost and versatility to switch the deposited material and pattern design without printing interruption and the high repeatability of drop ejection mechanism. Furthermore, by using a closed ink reservoir, this methodology ensures a tight control of ink composition and rheological properties during deposition process.

Patterned coatings play a fundamental role in decreasing magnetic losses and are also a way for high current cables in high magnetic field applications [19]. As a remarkable difference with the common deposition methodologies as spin coating, dip coating, slot die techniques, etc. and other currently used full covering techniques such as pulsed laser deposition or chemical vapour deposition, inkjet printing offers the added value of permitting complex patterning in a single-pass deposition, avoiding post treatment process as scratching or laser scripting. Additional attractive features of this technology are the non-contact nature of the process and the reduced material wastage due to the high accuracy of droplet positioning. Implementing effective DoD systems [20], [21], [22] in the fabrication process provides an easy path to deliver very small droplets of functional oxide precursor solutions in well defined positions with the ability to control film thickness by proper settings of drop pitch, drop volume, drop positioning and drop speed [21].

In performing particular inkjet printing applications [23], the entire printing system should be studied, that is to say, the ink that is ejected from the printhead, the printhead itself and the substrate. Specially, inkjet printing is highly sensitive to the physicochemical properties of the ink [24], [25], which means that a high degree of control over the ink characteristics must be accurately tailored to control drop formation and, therefore, drop volume and drop speed [21], [26], [27], spreading on the substrate after impingement [28] and connectivity between drops to, at the end, constrain the morphology of the final dried solid [29]. In this regard, essential parameters such as viscosity, surface tension, density, contact angle, evaporation rate of the solvent, drop volume and drop speed must be strictly met and adjusted.

Over the past few years, inkjet printing has been implemented in an extended range of innovative applications such as organic light emitting diodes [30], solar cells [31], electronic devices [32], [33], [34], [35], polymer displays [36], [37], [38], rapid prototyping [39], biochemical arrays [40], ceramic component manufacturing [41], biotechnology [42], etc. but there have been few reports of functional oxide coatings, particularly of superconductors [43], [44], [45], [46], [47] where biaxial texture and quality of grain boundaries are of great relevance. In this communication, we demonstrate that inkjet printing can be considered a good alternative to the common CSD deposition techniques. We report on our experiments concerning inkjet printing coating and patterning of YBCO on LaAlO₃ (LAO) single crystals using a single nozzle piezoelectric dispenser demonstrating that high critical current structured superconductors can be achieved by this technique.