During the past decade, computer simulations based on a quantum-mechanical description of the interactions between electrons and between electrons and atomic nuclei have developed an increasingly important impact on solid-state physics and chemistry and on materials science—promoting not only a deeper understanding, but also the possibility to contribute significantly to materials design for future technologies. This development is based on two important columns: (i) The improved description of electronic many-body effects within density-functional theory (DFT) and the upcoming post-DFT methods. (ii) The implementation of the new functionals and many-body techniques within highly efficient, stable, and versatile computer codes, which allow to exploit the potential of modern computer architectures. In this review, I discuss the implementation of various DFT functionals [local-density approximation (LDA), generalized gradient approximation (GGA), meta-GGA, hybrid functional mixing DFT, and exact (Hartree-Fock) exchange] and post-DFT approaches [DFT + U for strong electronic correlations in narrow bands, many-body perturbation theory (GW) for quasiparticle spectra, dynamical correlation effects via the adiabatic-connection fluctuation-dissipation theorem (AC-FDT)] in the Vienna ab initio simulation package VASP. VASP is a plane-wave all-electron code using the projector-augmented wave method to describe the electron-core interaction. The code uses fast iterative techniques for the diagonalization of the DFT Hamiltonian and allows to perform total-energy calculations and structural optimizations for systems with thousands of atoms and ab initio molecular dynamics simulations for ensembles with a few hundred atoms extending over several tens of ps. Applications in many different areas (structure and phase stability, mechanical and dynamical properties, liquids, glasses and quasicrystals, magnetism and magnetic nanostructures, semiconductors and insulators, surfaces, interfaces and thin films, chemical reactions, and catalysis) are reviewed. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2008

Ab-initio simulations of materials using VASP: Density-functional theory and beyond.

Preceramic polymers were proposed over 30 years ago as precursors for the fabrication of mainly Si-based advanced ceramics, generally denoted as polymer-derived ceramics (PDCs). The polymer to ceramic transformation process enabled significant technological breakthroughs in ceramic science and technology, such as the development of ceramic fibers, coatings, or ceramics stable at ultrahigh temperatures (up to 2000°C) with respect to decomposition, crystallization, phase separation, and creep. In recent years, several important advances have been achieved such as the discovery of a variety of functional properties associated with PDCs. Moreover, novel insights into their structure at the nanoscale level have contributed to the fundamental understanding of the various useful and unique features of PDCs related to their high chemical durability or high creep resistance or semiconducting behavior. From the processing point of view, preceramic polymers have been used as reactive binders to produce technical ceramics, they have been manipulated to allow for the formation of ordered pores in the meso-range, they have been tested for joining advanced ceramic components, and have been processed into bulk or macroporous components. Consequently, possible fields of applications of PDCs have been extended significantly by the recent research and development activities. Several key engineering fields suitable for application of PDCs include high-temperature-resistant materials (energy materials, automotive, aerospace, etc.), hard materials, chemical engineering (catalyst support, food- and biotechnology, etc.), or functional materials in electrical engineering as well as in micro/nanoelectronics. The science and technological development of PDCs are highly interdisciplinary, at the forefront of micro- and nanoscience and technology, with expertise provided by chemists, physicists, mineralogists, and materials scientists, and engineers. Moreover, several specialized industries have already commercialized components based on PDCs, and the production and availability of the precursors used has dramatically increased over the past few years. In this feature article, we highlight the following scientific issues related to advanced PDCs research: (1) General synthesis procedures to produce silicon-based preceramic polymers.
(2) Special microstructural features of PDCs.
(3) Unusual materials properties of PDCs, that are related to their unique nanosized microstructure that makes preceramic polymers of great and topical interest to researchers across a wide spectrum of disciplines.
(4) Processing strategies to fabricate ceramic components from preceramic polymers.
(5) Discussion and presentation of several examples of possible real-life applications that take advantage of the special characteristics of preceramic polymers. Note: In the past, a wide range of specialized international symposia have been devoted to PDCs, in particular organized by the American Ceramic Society, the European Materials Society, and the Materials Research Society. Most of the reviews available on PDCs are either not up to date or deal with only a subset of preceramic polymers and ceramics (e.g., silazanes to produce SiCN-based ceramics). Thus, this review is focused on a large number of novel data and developments, and contains materials from the literature but also from sources that are not widely available.

Polymer-Derived Ceramics: 40 Years of Research and Innovation in Advanced Ceramics

Large-Scale Fabrication of Boron Nitride Nanosheets and Their Utilization in Polymeric Composites with Improved Thermal and Mechanical Properties

This review article aims to provide an updated and comprehensive description of the development of the Electric Current Activated/assisted Sintering technique (ECAS) for the obtainment of dense materials including nanostructured ones. The use of ECAS for pure sintering purposes, when starting from already synthesized powders promoters, and to obtain the desired material by simultaneously performing synthesis and consolidation in one-step is reviewed. Specifically, more than a thousand papers published on this subject during the past decades are taken into account. The experimental procedures, formation mechanisms, characteristics, and functionality of a wide spectrum of dense materials fabricated by ECAS are presented. The influence of the most important operating parameters (i.e. current intensity, temperature, processing time, etc.) on product characteristics and process dynamics is reviewed for a large family of materials including ceramics, intermetallics, metal–ceramic and ceramic–ceramic composites. In this review, systems where synthesis and densification stages occur simultaneously, i.e. a fully dense product is formed immediately after reaction completion, as well as those ones for which a satisfactory densification degree is reached only by maintaining the application of the electric current once the full reaction conversion is obtained, are identified. In addition, emphasis is given to the obtainment of nanostructured dense materials due to their rapid progress and wide applications. Specifically, the effect of mechanical activation by ball milling of starting powders on ECAS process dynamics and product characteristics (i.e. density and microstructure) is analysed. The emerging theme from the large majority of the reviewed investigations is the comparison of ECAS over conventional methods including pressureless sintering, hot pressing, and others. Theoretical analysis pertaining to such technique is also proposed following the last results obtained on this topic.

Consolidation/synthesis of materials by electric current activated/assisted sintering

The synthesis of lanthanide-activated phosphors is pertinent to many emerging applications, ranging from high-resolution luminescence imaging to next-generation volumetric full-color display. In particular, the optical processes governed by the 4f-5d transitions of divalent and trivalent lanthanides have been the key to enabling precisely tuned color emission. The fundamental importance of lanthanide-activated phosphors for the physical and biomedical sciences has led to rapid development of novel synthetic methodologies and relevant tools that allow for probing the dynamics of energy transfer processes. Here, we review recent progress in developing methods for preparing lanthanide-activated phosphors, especially those featuring 4f-5d optical transitions. Particular attention will be devoted to two widely studied dopants, Ce3+ and Eu2+. The nature of the 4f-5d transition is examined by combining phenomenological theories with quantum mechanical calculations. An emphasis is placed on the correlation of hos...

Lanthanide-Activated Phosphors Based on 4f-5d Optical Transitions: Theoretical and Experimental Aspects.

This work verifies the applicability of two-stage sintering as a means of suppressing the final stage grain growth of submicrometer alumina. The first heating step should be short at a relatively high-temperature (1400°–1450°C) in order to close porosity without significant grain growth. The second step at temperatures around 1150°C facilitates further densification with limited grain growth. Fine-grained alumina with a relative density of 98.8% and a grain size of 0.9 μm was prepared by two-stage sintering. A standard sintering process resulted in ceramics with identical relative density and a grain size of 1.6 μm.

Two‐Stage Sintering of Alumina with Submicrometer Grain Size

Different microstructures in Si{sub 3}N{sub 4} ceramics containing Y{sub 2}O{sub 3} and Al{sub 2}O{sub 3} as sintering additives were prepared by two-step sintering. Pull-out and elastic bridging were most frequently observed as the toughening mechanisms in samples with fine-grained microstructures having needlelike {beta}-Si{sub 3}N{sub 4} grains with diameters of 1 {micro}m. The values of fracture toughness were varied from 6.1 to 8.2 MPa {center_dot} m{sup 1/2} with respect to the microstructural characteristics, characterized by the volume fraction of needlelike grains and their diameter.

Relationship between Microstructure, Toughening Mechanisms, and Fracture Toughness of Reinforced Silicon Nitride Ceramics

Magnetic properties of Co1−xZnxFe2O4 spinel ferrite nanoparticles synthesized by starch-assisted sol–gel autocombustion method and its ball milling

Ti-doped ZnO (ZnO:Ti) films have been deposited by simultaneous RF magnetron sputtering of ZnO and DC magnetron sputtering of Ti. The advantage of this method is that the Ti content could be independently controlled. TiO2 was favorable to form with decreasing Ti content. The crystallinity was weakened by increasing the Ti contents, which indicated that the increase of Ti contents made the structure of ZnO:Ti film random. The morphology of ZnO:Ti films was also significantly affected by Ti contents. The carriers of ZnO:Ti films may be originated from oxygen vacancies and Ti donors. The variation in carrier mobility could be due to the ionized impurity scattering and surface morphology. The lower obtained resistivity was 9.69×10−3 Ω cm for ZnO:Ti films with 1.1% Ti. With decreasing Ti contents, the films had higher carrier concentrations and lower optical energy gaps.

The properties of Ti-doped ZnO films deposited by simultaneous RF and DC magnetron sputtering

A radio frequency power (r.f.) of 200 W was supplied to ZnO target, and a direct current (d.c.) power of 40 W was supplied to Al target for the preparation of heavily Al-doped ZnO (ZnO:Al) films. The advantage of this kind of deposited method is that the Al content could be changed in a wide range. The ZnO:Al films before and after annealing in N 2 or O 2 all developed a columnar structure. The crystal sizes were nearly the same (approx. 20 nm) for ZnO:Al films before and after annealing in N 2 or O 2 . The as-deposited ZnO:Al films had polycrystalline structure and low resistivity (8.52×10 −3 Ω cm). After annealing in N 2 or O 2 , ZnO:Al films exhibited poor crystallinity, and the resistivity increased substantially. The lower optical energy gap (3.29 eV) could be obtained as ZnO:Al film annealing in O 2 . However, the weak absorption in the visible region of the spectrum terminated at shorter wavelengths with the onset of the ultraviolet absorption edge for all samples. Annealing in N 2 or O 2 did not improve the properties of ZnO:Al films.

Pavol Šajgalík

Papers

Two‐Stage Sintering of Alumina with Submicrometer Grain Size

Relationship between Microstructure, Toughening Mechanisms, and Fracture Toughness of Reinforced Silicon Nitride Ceramics

Magnetic properties of Co1−xZnxFe2O4 spinel ferrite nanoparticles synthesized by starch-assisted sol–gel autocombustion method and its ball milling

The properties of Ti-doped ZnO films deposited by simultaneous RF and DC magnetron sputtering

The properties of heavily Al-doped ZnO films before and after annealing in the different atmosphere