Georg J. Schmitz

A phase field concept for multiphase systems

Abstract The phase field theory describing the evolution of a dual phase boundary is extended to multiphase problems: Each phase is identified with an individual phase field and the transformation between all pairs of phases is treated with its own characteristics. The governing differential equations for the evolution of the multiphase system are derived by minimizing the free energy functional. This free energy functional is expanded in a series over the pair energies between the different phases, where the local fluctuations of one phase are treated with respect to its counter-phase. The proposed generalized multiphase concept reproduces the dual phase system as a limiting case. The relevance of the model for metallic systems is discussed with respect to eutectic and peritectic solidification and grain growth.

Melt-texture joining of YBa2Cu3Oy bulks

Practical applications of high-temperature superconductors require long length and complex shapes of the textured ceramic material. The limitation of processing the REBa2Cu3Oy (RE-123, RE = Y, Sm, Nd, etc) superconductors in desired designs requires the development of joining techniques. In this paper we report on various attempts to join sections cut from single-domain YBa2Cu3Oy (Y-123) monoliths. The lower peritectic temperature rare earth Yb-123 compounds were used as solder for melt texturing at the interface for joining. The various factors influencing the formation of defect structures during the growth of the solder phase have been investigated. Transport current measurements across the joints of the samples and also of a typical configuration of a brick wall structure prepared under optimized conditions are presented.

Mono-domain YBa2Cu3Oy superconductor fabrics prepared by an infiltration process

A novel process for the fabrication of a new form of YBa2Cu3Oy (123) superconducting material, with the dimensions of a thick film and the microstructure of a melt-textured single-domain bulk is described. The process allows the fabrication of 123 as a self-supporting fabric or as a thick film on various substrate materials. The process, which is generic and economical, uses commercially available Y2O3 fabrics as a precursor material. The Y2O3 cloth is infiltrated with barium cuprates and copper oxides from a liquid-phase source, then converted into Y2BaCuO5 (211) phase and eventually to 123. The nucleation and growth of the 123 phase is controlled by seeding the cloth with an oriented heterogeneous MgO or Nd123 seed. Interesting application areas for the new form of the 123 mono-domain fabric are discussed.

Synthesis of polycrystalline BaZrO3 coatings

Barium zirconate was prepared by liquid phase sintering at low temperatures and used as a substrate for a YBCO melt process. The liquid phase was provided by barium cuprate melt, barium peroxide melt or barium zirconate liquid (in case of plasma spraying). The infiltration resistance against the barium cuprate melt could be improved, but is not yet sufficient for application.

ICMEg – the Integrated Computational Materials Engineering expert group – a new European coordination action

Integrated Computational Materials Engineering, or ICME, applies a range of software tools in simultaneous or consecutive combinations. Major successful developments in the area of ICME up to the present have been essentially driven by academic and industrial users of simulation software. Starting with a brief overview of the benefits provided by ICME, the present article will give a short outline of the ICMEg consortium, the I ntegrated C omputational M aterials E ngineering expert g roup, and the related European project. The vision and mission of this project is defined and its practical implementation detailed. The chief aim of ICMEg is to set up a global and open communication standard and protocols on the component scale, on the microstructural scale, as well as basic underlying thermodynamic models and discrete models on the electronic, atomistic and mesoscopic scales.

Towards integrative computational materials engineering of steel components

This article outlines on-going activities at the RWTH Aachen University aiming at a standardized, modular, extendable and open simulation platform for materials processing. This platform on the one hand facilitates the information exchange between different simulation tools and thus strongly reduces the effort to design/re-design production processes. On the other hand, tracking of simulation results along the entire production chain provides new insights into mechanisms, which cannot be explained on the basis of individual simulations. Respective simulation chains provide e.g. the basis for the determination of materials and component properties, like e.g. distortions, for an improved product quality, for more efficient and more reliable production processes and many further aspects. After a short introduction to the platform concept, actual examples for different test case scenarios will be presented and discussed.

Processing of Y2BaCuO5 foams

Foams of conventional ceramics are attractive for a wide range of industrial applications such as filters, insulators, heat exchangers and others. Advanced functional ceramics, e.g., superconductors or piezoceramics profit from a foam structure for a variety of applications. Foams of Y2BaCuO5—being the properitectic phase in the formation of YBa2Cu3O7—find applications as supports for melt processing of bulk YBa2Cu3O7 materials or as preforms for processing of superconducting YBa2Cu3O7 foams. In this paper we discuss the processing of Y2BaCuO5 foams of various porosities and dimensions either as replicas of reticulated polyurethane foams or prepared by burning out of wax spacers embedded into a 211 preform.

Towards a metadata scheme for the description of materials – the description of microstructures

Abstract The property of any material is essentially determined by its microstructure. Numerical models are increasingly the focus of modern engineering as helpful tools for tailoring and optimization of custom-designed microstructures by suitable processing and alloy design. A huge variety of software tools is available to predict various microstructural aspects for different materials. In the general frame of an integrated computational materials engineering (ICME) approach, these microstructure models provide the link between models operating at the atomistic or electronic scales, and models operating on the macroscopic scale of the component and its processing. In view of an improved interoperability of all these different tools it is highly desirable to establish a standardized nomenclature and methodology for the exchange of microstructure data. The scope of this article is to provide a comprehensive system of metadata descriptors for the description of a 3D microstructure. The presented descriptors are limited to a mere geometric description of a static microstructure and have to be complemented by further descriptors, e.g. for properties, numerical representations, kinetic data, and others in the future. Further attributes to each descriptor, e.g. on data origin, data uncertainty, and data validity range are being defined in ongoing work. The proposed descriptors are intended to be independent of any specific numerical representation. The descriptors defined in this article may serve as a first basis for standardization and will simplify the data exchange between different numerical models, as well as promote the integration of experimental data into numerical models of microstructures. An HDF5 template data file for a simple, three phase Al-Cu microstructure being based on the defined descriptors complements this article.

Processing of large grain Y-123 superconductors with pre-defined porous structures

Porous superconductors have inherent cooling advantages over their bulk counterparts and, as a result, are emerging as an important class of materials for practical applications. Single-domain Y–Ba–Cu–O (YBCO) foams processed with a pre-defined, open porous structure, for example, have significant potential for use as elements in resistive superconducting fault current limiters. In this case, the interconnected porosity is ideal for producing reinforced composites with improved mechanical and heat conducting properties. In this paper we describe a few simple methods for fabricating large grain YBCO superconductors with various predefined porous structures via an infiltration process from tailored, porous Y2BaCuO5 (Y-211) pre-forms manufactured by a variety of techniques, including slurry-coating of standard polyurethane foams to replicate their structure. Foams produced by this method typically have a strut thickness of a few hundred µm and pore sizes ranging from 10 to 100 pores per inch (PPI). Foams with increased strut thickness of up to millimetre dimensions can be produced by embedding organic ball spacers within the Y-211 pre-form followed by a burn-out and sintering process. Single-domain YBCO bulk materials with cellular and pre-defined 3D interconnected porosity may be produced by a similar process using tailored wax structures in Y-211 castings.

Magnetic and transport properties of YBa2Cu3Oy superconductor foams

Abstract The recently reported superconducting YBa2Cu3Oy (Y123) foams are highly interesting and promising for variety of applications. In this report we present first magneto-transport measurements of the superconducting properties of these foams. The investigations reveal the superconducting properties being similar to those of bulk melt processed materials. The 123 foams reveal a Tc of 92 K and have a magnetization Jc of 40,000 A/cm2 at 77 K and 0 T. The measurements of magnetic hysteresis versus field show a high anisotropy of the critical current density up to Jcab/Jcc∼7.