Alexandra Kovalčíková

Terbium-induced phase transitions and weak ferromagnetism in multiferroic bismuth ferrite ceramics

Partial substitution of isovalent rare-earth ions for bismuth is one of the most effective ways to develop room temperature BiFeO3-based multiferroic materials with high resistivity and strong magnetoelectric coupling. However, their structures and properties are composition and processing sensitive, with the underlying mechanisms still far from being completely understood. Here we report on the structural, thermal and magnetic properties of polycrystalline Bi1−xTbxFeO3 (0 ≤ x ≤ 0.30) dense ceramics prepared by spark plasma sintering (SPS). The X-ray diffraction study reveals that increasing terbium content induces a structural transformation from the parental rhombohedral (R3c) polar phase to an orthorhombic (Pnma) non-polar phase at x ≈ 0.20–0.25. Complementary Raman and energy-loss near-edge structure (ELNES) spectroscopy studies indicate that the transition proceeds by the progressive loss of Bi–O hybridization. Suppression of the long-range ferroelectric ordering upon Tb substitution and loss of ferroelectricity at x ≥ 0.25 was also confirmed by differential scanning calorimetry. High-sensitivity magnetic measurements show that the introduction of a small amount of Tb3+ ions at the A-sites of the perovskite structure gives rise to the occurrence of spontaneous magnetization at room temperature. The reduced degree of Fe 3d–4p orbital mixing and the weaker Fe 3d–O 2p hybridization, revealed by ELNES and X-ray near-edge absorption fine structure (NEXAFS) analyses, suggest that the substitution-induced changes in the electronic structure are responsible for the enhanced magnetization in Tb-doped BiFeO3. Among the biphasic (R3c + Pnma) compositions with ferroelectric order, the Bi0.8Tb0.2FeO3 compound shows the highest value of remanent magnetization (Mr ≈ 0.26 emu g−1), which makes this material a potential candidate for magnetoelectric applications.

Local Mechanical Properties of SiC - TiNbC Composite and its Constituents

SiC based composite with 50 % of additives (Ti and NbC with ratio of 9:16) has been prepared. The microstructure, porosity, and chemical composition were studied using SEM equipped with EDS analyser. Local mechanical properties such a hardness and elastic modulus of individual components of the composite were investigated by nanoindentation using Berkovich indenter tip. Hardness and fracture toughness of studied material as a whole was evaluated by means of classic Vickers macroindentation. Indentation cracks were observed and their propagation was analyzed. It was shown that the present phases were distributed uniformly. Moreover, final density was satisfactory with porosity lower than 1 %. The individual constituents shown similar elasticity modulus (550 - 590 GPa). Hardness (HIT) exhibited very pronounced load-size effect. At 10 mN load, hardness was 42.33 GPa ± 1.1 GPa for SiC and 35.73 GPa ± 0.9 GPa for TiNbC, while at 500 mN the composite hardness was 27.61 GPa ± 0.505 GPa. It is in good agreement with macrohardness values, when 27.6 GPa and 25 GPa has been measured for 1 and 10 kg loads, respectively. Indentation fracture toughness was 3.3 MPa.m1/2 ± 0.22 MPa.m1/2. Electrical conductivity was measured by four point probes method and its value was 8.8×104 ± 0.3×104 Sm-1.

Influence of Microstructure on Tribological Properties and Nanohardness of Silicon Carbide Ceramics

The influence of microstructural variations on the tribological properties and nanohardness of liquid phase sintered silicon carbide (LPS SiC) has been observed. In order to modify the microstructures samples were further heat treated at 1650°C and 1850°C for 5 hours to promote grain growth. The depth-sensing indentation tests of SiC materials were performed at several peak loads in the range 10-400 mN. The pin-on-flat dry sliding friction and wear experiments have been made on SiC ceramics in contact with Al2O3 ceramic ball at 10-50 N loads in an ambient environment. The nanohardness of samples with plate-like microstructure was about 34 GPa i.e. 3 GPa higher than nanohardness of SiC with fine globular microstructure. The SiC materials with coarser plate-like microstructure had similar COF (0.4-0.55) and better wear resistance (one order of magnitude at normal forces 10-20N) than SiC materials with fine globular microstructure.

Preparation of Alumina Fibers by Needle-Less Electrospinning

In this study the needle-less electrospinning by means of “NanospiderTM“ (ELMARCO) as technology for the preparation of fine α-Al2O3 fibers with diameters of 0.5 - 1.5 µm is presented. The fabrication consists of three steps: i) preparation of spinning solution, ii) electrospinning of the prepared solution and collection of the composite fibers, iii) calcination of the composite precursor fibers. The electrospun fibers were prepared from polyacrylonitrile/N,N-dimethylformamide (PAN/DMF) polymer solution and Al(NO3)3.9H2O in ratio 1/10/1. Thereafter, the precursor fibers were calcined in the furnace at 900, 1100 and 1200 °C with a rate of 5 °C/min in air. The formation of crystalline phases, surface morphology and diameters of metastable and final alumina fibers were characterized using thermogravimetric analysis, X-ray diffraction analysis, the scanning electron microscopy and transmission electron microscopy. The precursor PAN/Al(NO3)3 fibers were amorphous. The thermal treatment leads to the phase transition from γ-Al2O3 to α-Al2O3 accompanied by removing of polyacrylonitrile (PAN). The fine porous microfibers composed of pure α-Al2O3 phase were prepared after calcinations at 1200 °C.

Indentation Fracture Toughness of Al2O3 + GPLs and Si3N4 + GPLs Systems

The influence of 1 wt.% and 2 wt.% of graphene platelets (GPLs) addition on indentation fracture toughness (IF) of aluminium oxide (Al2O3) and silicon nitride (Si3N4) based composites has been investigated and compared to the monoliths. Ceramic composites reinforced with GPLs were prepared using hot-press processing technology. Microstructures were observed at fracture surfaces by scanning electron microscopy (SEM). Crack type identification was performed by gradually polishing of the indentation surface and mechanical properties of both systems were measured. Indentation fracture toughness was calculated by various methods and R-curves were prepared. The main activated toughening mechanisms, responsible for the increased fracture toughness are crack deflection, crack branching and crack bridging in the forms of graphene sheet pull-out or graphene necking.

Influence of the Microstructure on Macro/Micro versus Nanohardness of SiC Ceramics

The influence of microstructural variations on the macro/microhardness, nanohardness and Young`s modulus of liquid phase sintered silicon carbide (LPS SiC) has been observed. In order to modify the microstructures some samples were further heat treated at 1850°C for 5 hours to promote grain growth. The depth-sensing indentation tests of SiC materials were performed at several peak loads in the range 10-400 mN. For a better assessment, the indentation values of hardness and Young`s modulus modulus of SiC matrix were also compared to the hardness and Elastic modulus of individual SiC grains. The comparison of macro/micro and nanohardness showed that nanohardness was significantly higher, generally by 6-7 GPa. The nanohardness of individual plate-like SiC grains was around 2 GPa higher than nanohardness of SiC matrix.

Changing the Hardness Automotive Steels at Different Strain Rate

Currently, the automotive industry used sheets of different qualities. The most common include IF (inter Interstitial Free) steel and alloyed steel. Use the sheet quality depends on the point of application in the production car. Testing and product testing is a standard part of the process of innovation and production itself. Testing of automotive steels under dynamic conditions is increasingly important. Changing the hardness HV 1 was performed on the fractured bars on the static and dynamic loading conditions. Tests were made on steel IF and S 460.

Contact and Bending Strength Tests of Ceramics: What is the Difference?

The paper deals with the determination of the characteristic strength and the Weibull modulus m of Si3N4 and SiC ceramic materials using conventional four-point bending and unconventional contact tests between opposite rollers and opposite spheres. Ceramographic and fractographic methods were used for the characterization of strength degrading defects represented by processing flaws and by cracks of different types arising during the loading. The processing flaws influenced the Weibull parameters mainly in the bending mode, and the strength and its scatter in contact modes was influenced by lateral, median and contact end cracks, originated during the contact test using rollers, and by cone cracks originated during the contact test using spheres.

Strength Degradation Flaws in the Liquid-Phase-Sintered SiC Based Ceramics

The effect of the heat treatment on the fracture toughness and flexural strength of the silicon carbide – silicon nitride composites prepared by liquid-phase-sintering was investigated. The results were compared to those obtained for a reference silicon carbide material, prepared by the same fabrication route. The fracture toughness increased from 3.19 to 5.15 MPa.m1/2 due to the toughening mechanisms (crack deflection, mechanical interlocking, crack branching) occurring in the heat treated materials during the crack propagation. However, the flexural strength decreased after the heat treatment of the experimental materials. The strength of the investigated materials was degraded by the presence of processing flaws mainly in the form of pores, clusters of pores, and SiC agglomerates.