Modulated nematic structures induced by chirality and steric polarization

What kind of one-dimensional modulated nematic structures (ODMNS) can form nonchiral and chiral bent-core and dimeric materials? Here, using Landau-deGennes theory of nematics, extended to account for molecular steric polarization, we study a possibility of formation of ODMNS, both in nonchiral and intrinsically chiral liquid crystalline materials. Besides nematic and cholesteric phases, we find four bulk ODMNS for nonchiral materials, two of which have not been reported so far. These new structures are longitudinal ($N_{LP}$) and transverse ($N_{TP}$) periodic waves where the polarization field being periodic in one dimension stays parallel and perpendicular, respectively, to the wave vector. The other two phases have all characteristic features of the twist-bend nematic phase ($N_{TB}$) and the splay-bend nematic phase ($N_{SB}$), but their fine structure appears more complex than that considered so far. The presence of molecular chirality converts nonchiral $N_{TP}$ and $N_{SB}$ into new $N_{TB}$ phases. Interestingly, the nonchiral $N_{LP}$ phase can stay stable even in the presence of intrinsic molecular chirality. Exemplary phase diagrams provide further insights into the relative stability of these new modulated nematic structures.Comment: 5 pages, 5 figure

Shapes for maximal coverage for two-dimensional random sequential adsorption

The random sequential adsorption of various particle shapes is studied in order to determine the influence of particle anisotropy on the saturated random packing. For all tested particles there is an optimal level of anisotropy which maximizes the saturated packing fraction. It is found that a concave shape derived from a dimer of disks gives a packing fraction of 0.5833, which is comparable to the maximum packing fraction of ellipsoids and spherocylinders and higher than any other studied shape. Discussion why this shape is so beneficial for random sequential adsorption is given.Comment: 6 pages, 8 figures, 3 table

Nematic twist--bend phase in an external field

Nematic twist--bend is the fifth nematic phase recognized in nature. This phase exhibits spiral orientational order, thus it is a chiral structure and it can be stabilized in systems composed of achiral molecules. The microscopic origin of this spontaneous chiral symmetry breaking is to a large extent unknown but phenomenologically it appears stabilized by assuming coupling between steric polar and orientational orderings. Understanding of how external fields affect the stability of this phase is of great intellectual interest and of relevance to potential applications. Within mesoscopic Landau--de Gennes theory we find that for compounds with positive anisotropy the helix unwinds to a polar uniaxial nematic, however negative material anisotropy gives rise to a rich sequence of new nematic phases obtained via mechanism of flattening the conical spiral

Stability of Biaxial Nematic Phase for Systems with Variable Molecular Shape Anisotropy

We study the influence of fluctuations in molecular shape on the stability of the biaxial nematic phase by generalizing the mean field model of Mulder and Ruijgrok [Physica A {\bf 113}, 145 (1982)]. We limit ourselves to the case when the molecular shape anisotropy, represented by the alignment tensor, is a random variable of an annealed type. A prototype of such behavior can be found in lyotropic systems - a mixture of potassium laurate, 1-decanol, and $D_2O$, where distribution of the micellar shape adjusts to actual equilibrium conditions. Further examples of materials with the biaxial nematic phase, where molecular shape is subject to fluctuations, are thermotropic materials composed of flexible trimeric- or tetrapod-like molecular units. Our calculations show that the Gaussian equilibrium distribution of the variables describing molecular shape (dispersion force) anisotropy gives rise to new classes of the phase diagrams, absent in the original model. Depending on properties of the shape fluctuations, the stability of the biaxial nematic phase can be either enhanced or depressed, relative to the uniaxial nematic phases. In the former case the splitting of the Landau point into two triple points with a direct phase transition line from isotropic to biaxial phase is observed.Comment: 18 pages containing 6 figure

Microstructure investigations of Co-Si-B alloy after milling and annealing

Purpose: The work presents the microstructure characterization of Co77Si11,5B11,5 metallic glass after high-energy ball milling and heat treatment processes. Design/methodology/approach: The studies were performed on ribbon prepared by melt spinning and this ground in high-energy vibratory ball mill. The tested ribbon and obtained powders were also annealed in specified heat treatment conditions. The morphology of the powder particles of milled ribbon was analyzed by using the confocal laser scanning microscope. The methods of X-ray diffraction were used for the qualitative phase analysis. The parameters of the individual diffraction line profiles were determined by PRO-FIT Toraya procedure. The average crystallite sizes and lattice distortions for Co phase were estimated using Williamson-Hall method. Findings: The studied Co77Si11.5B11.5 metallic glass in annealed state contains hexagonal Co crystalline phases emerged in amorphous matrix. The crystallite size of Co phase in as-cast sample lies in nanoscale. After annealing process the crystallite size increases to 72 nm and diminishes to 46 nm in the powder sample after 30 hours of milling. The milling causes decrease of the crystallite size and increase of lattice distortions of crystalline phase. The powder particles after 30 hours of milling are of spherical shape. Practical implications: The powder particles obtained after milling process of Co-based metallic glass could be suitable components in production of ferromagnetic nanocomposites. Originality/value: The obtained results confirm the utility of applied investigation methods in the microstructure analysis of powder materials with nanocrystalline phases. Keywords: X-ray phase analysis; Toraya procedure; High

Microstructure of composite material with powders of barium ferrite

Purpose: The aim of the present work is the microstructure characterization of commercial powder BaFe12O19 (as-prepared) and composite material with BaFe12O19 powders and polymer matrix, using XRD (X-Ray Diffraction) and SEM (Scanning Electron Microscopy) methods. Design/methodology/approach: The morphology of barium ferrite powders and a fracture surface of the examined composite material was realized by using the scanning electron microscope. The methods of Xray diffraction were used for the qualitative phase analysis. The parameters of diffraction line profiles were determined by PRO-FIT Toraya procedure. Findings: The X-ray diffraction analysis permitted on identification the BaFe12O19 and Fe2O3 phases in an examined material. Basing on Toraya method is determination of: lattice parameters, crystallite size (D) and the lattice distortion (). Distribution of powders of barium ferrite in polymer matrix is irregular and powder particles have irregular shapes and dimensions. Research limitations/implications: For future research the X-ray analysis should be performed by the Rietveld method, which allows to characterization the microstructure of tested material and verification of its qualitative phase composition. Originality/value: The applied Toraya method of structure analysis appeared to be very useful in the microstructure analysis

The influence of manufacturing conditions on microstructure and magnetic properties of BaFe12O19 powders

The aim of the paper was a trial of using mechanical alloying process of mixture iron oxide and barium carbonate to produce BaFe12O 19 powders. The milling process was carried out in a vibratory mill for 10, 20 and 30 hours. The size distributions of powder particles showed that the size of tested particles increases with the increase of milling time indicating the agglomeration process of particles. The milling process of Fe2O3 and BaCO3 mixture for studied milling times causes decrease of the crystallite size of involved phases and leads to increase of Fe2O3 phase content and decrease of BaCO 3 one. The milling process did not lead to formation of BaFe 12O19 phase, thus it probably causes setting of Fe 2O3 on surface layer of BaCO3 powder particles. The XRD investigations of Fe2O3 and BaCO3 mixture milled for 10, 20 and 30 hours and annealed at 1000°C for 1 hour enabled the identification of hard magnetic BaFe12O19 phase. For applied magnetic field of 800 kA/m, the coercive force is equal to 343 kA/m, 358 kA/m and 366 kA/m whereas the remanence is equal to 0.118 T, 0.109 T and 0.127 T for the samples after milling for 10, 20 and 30 hours, respectively

Barium ferrite powders prepared by mechanical alloying and annealing

Purpose: Microstructure and magnetic properties analysis of barium ferrite powder obtained by milling and heat treatment. Design/methodology/approach: The milling process was carried out in a vibratory mill, which generated vibrations of the balls and milled material inside the container during which their collisions occur. After milling process the powders were annealed in electric chamber furnace. The X-ray diffraction methods were used for qualitative phase analysis of studied powder samples. The distribution of powder particles was determined by a laser particle analyzer. The magnetic hysteresis loops of examined powder material were measured by resonance vibrating sample magnetometer (R-VSM). Findings: The milling process of iron oxide and barium carbonate mixture causes decrease of the crystallite size of involved phases. The X-ray vestigations of tested mixture milled for 30 hours and annealed at 950 °C enabled the identification of hard magnetic BaFe12O19 phase and also the presence of Fe2O3 phase in examined material. The Fe2O3 phase is a rest of BaCO3 dissociation in the presence of Fe2O3, which forms a compound of BaFe12O19. The best coercive force (HC) for mixture of powders annealed at 950 °C for 10, 20 and 30 hours is 349 kA/m, 366 kA/m and 364 kA/m, respectively. The arithmetic mean of diameter of Fe2O3 and BaCO3 mixture powders after 30 hours of milling is about 6.0 μm. Practical implications: The barium ferrite powder obtained by milling and annealing can be suitable components to produce sintered and elastic magnets with polymer matrix. Originality/value: The results of tested barium ferrite investigations by different methods confirm their utility in the microstructure and magnetic properties analysis of powder materials

Associations between pharmacotherapy for cardiovascular diseases and periodontitis

The goal of the study was to assess the relationship between cardioprotective medications, i.e., beta-blockers, angiotensin-converting enzyme inhibitors (ACEIs), calcium channel blockers (CCBs), angiotensin II receptor blockers (ARBs), statins, acetylsalicylic acid (ASA), and periodontitis (PD). Background: Xerostomia increases the risk of PD and is a side effect of some pharmacotherapies. Information about the effect of cardioprotective treatment of periodontal status is scarce. Methods: We studied 562 dentate residents of Krakow at the age of 50 to 70 years. Information about treatment was collected using a standardized questionnaire. The pocket depth and clinical attachment level (CAL) were used to ascertain PD. Multivariate logistic regression was applied to assess the relation between cardioprotective medications and PD. Results: PD was found in 74% of participants. The range of cardioprotective drug use among participants was 7% (ARBs) to 32% (beta-blockers). After adjusting for age, sex, number of teeth, smoking, and education, ASA’s use was related to a lower prevalence of PD in all dentate participants (odds ratio (OR) = 0.63, 95% confidence interval (CI): 0.40–0.99). The use of ARBs and statins was found to be associated with a higher prevalence of PD in persons having ≥6 teeth (odds ratio (OR) = 3.57, 95% CI: 1.06–11.99 and OR = 1.81, 95% CI: 1.03–3.16, respectively). Further adjustment for CVD risk factors, history of coronary heart disease, and other chronic diseases did not attenuate the results. There was no significant relation between PD and the use of other cardioprotective drugs