On the problem of phase transitions in lysozyme crystals

We present experimental evidence of the fact that lysozyme crystals, which are grown from their mother solution and exist in it, dissolve on heating above T=307 K. We argue that the anomaly in the light scattering recently observed at the temperature T=307 K and identified in the reference [Svanidze A. V. et al. 2006. JETP Lett. 84: 551] as a structural crystalline phase transition in the single lysozyme crystals, in fact, corresponds to a temperature limit of the crystal existence.Comment: 6 pages, 1 figur

Growing larger: Scaling up during spark plasma sintering of high– temperature ceramics

Recently, extensive efforts were made towards investigation and development of spark plasma sintering (SPS) as a promising technique for rapid densification of ceramics at relatively low temperatures. In this respect, SPS was used for various processes and materials including fabrication of bulk metals and ceramics and their composites. Furthermore, this ‘low temperature’ processing allowed consolidation of ceramics with strong covalent bonding to the full density. However, significant gap exists between the technological and fabrication achievements to the fundamental understanding of the SPS mechanisms. This gap is due to the complexity of the thermal, electrical and mechanical processes that may be involved during the SPS, in addition to their dependence on the SPS parameters, as well as a reasonable question of possibility of scaling up. The majority of reports provided in peer-reviewed journals usually focuses on the ceramic specimens not larger than 40 mm, which is a reasonable size even for more “conventional” consolidation method that of hot-pressing. Hence, the present work focuses on the natural scaling up processes for fabrication of a large size specimens approaching 100 mm in diameter and 20 mm in height, and provides an analysis for the densification process, structure evolution (including homogeneity), and, crucially, concentrates on the change in the mechanical properties that arises during scaling up process for high-temperature ceramics consolidated at temperatures higher than 1800 °C

Torque-operated gradient-index axicon

We describe a gradient-index axicon based on twisting of crystals. We demonstrate that the focal length of the axicon can be efficiently operated by the torsion moment. The working analytical relations describing the focal length of the axicon and its dependence on different geometrical parameters as well as the torsion moment has been derived.Comment: 13 pages, 5 figure

Engineering of lightweight ceramic composites by spark plasma sintering

It is well recognised that value of porosity and grain size influences mechanical properties such as fracture toughness, elastic modulus and strength dramatically. Boron carbide (B4C), as well as other lightweight ceramics like TiB2, B6O requires high consolidation temperatures owing to its poor sinterability. Therefore such decrease of ceramics properties becomes a materials processing issue since during consolidation process on the final stage of sintering (either pressure-assisted or pressureless one) grain growth starts. To overcome this problem, various metallic and non-metallic binders are used to obtain dense borides. However, the presence of a metallic binder is not desirable for high-temperature structural applications. We propose that reaction-driven consolidation by means of spark plasma sintering (SPS) at temperatures exceeding 1800 °C as an alternative method for fabrication of high-temperature lightweight ceramic composites. This work summarizes recent activity on processing of lightweight ceramics based on boron carbide, boron suboxide and titanium diboride in the respect to mechanical properties: such as hardness, fracture toughness and room and high-temperature strength. Application of various techniques for powders preparation and consolidation by SPS is thoroughly discussed in respect to obtained lightweight ceramic composites properties. A thorough discussion of high-temperature properties for these ceramic composites is also provided

Optical anisotropy induced by torsion stresses in LiNbO3 crystals: appearance of an optical vortex

We report the results of studies of torsion effect on the optical birefringence in LiNbO3 crystals. We have found that twisting of those crystals causes a birefringence distribution revealing non-trivial peculiarities. In particular, it has a special point at the center of cross section perpendicular to the torsion axis where zero birefringence value occurs. It has also been ascertained that the surface of the spatial birefringence distribution has a conical shape, with the cone axis coinciding with the torsion axis. We have revealed that an optical vortex, with the topological charge equal to unity, appears under the torsion of LiNbO3 crystals. It has been shown that, contrary to the q-plate, both the efficiency of spin-orbital coupling and the orbital momentum of the emergent light can be operated by the torque moment.Comment: 28 pages, 8 figure