Synthesis of controls for a spacecraft that optimize the pole placement of the closed-loop control system

For a linear time-independent model of spacecraft motion, a method for the optimal pole placement of the spacecraft model transition matrix closed by a feedback is proposed. The method is based on the relationship between the modal and optimization approaches to the synthesis of the feedback ensuring the prescribed quality of the attitude stabilization and stability of the time response of the spacecraft determined by the Kalman-Letov performance index. This relationship is established in the paper. An example of applying the proposed method for controlling the motion of the center of mass and about the center of mass in the stabilization of the program rendezvous trajectory of spacecrafts in the orbital frame of reference is discussed.     

Specific Heat of the Monoclinic Rare Earth Double Tungstates

The specific heat of ARE(WO₄)₂, where A=K, Rb and RE=Y, Nd, Gd, Yb, and Lu single crystals, has been studied. The samples were grown using the Top Seeded Solution Growth – Slow Cooling (TSSG-SC) method, and the resulting crystal structure data was tabulated. The specific heat measurements as a function of temperature in the range from 0.35 to 385 K are presented. Above 2 K, the specific heat behavior can be described by the phonon contribution while below 2 K, an anomalous increase in specific heat, attributed to the proximity of magnetic phase transitions, was observed.     

Assessment of changes in hop resins and polyphenols during long‐term storage

Changes in the content and composition of hop secondary metabolites during storage are reflected in beer quality and in the economics of beer production. A 12‐month storage experiment with T90 pellets of four hop varieties showed different dynamics of hop aging in relation to both storage conditions and hop variety. Negligible effects on the α‐ and β‐acids were detected during storage without air access at +2°C. Storage at +20°C resulted in a final loss of 20–25% α‐acids, but the content of β‐acids did not change significantly. Large decreases in α‐acids (64–88%) and in β‐acids (51–83%) were found in hops stored with access to air at +20°C. The rate of decline accelerated markedly after 6 months of storage. In terms of hop resin changes, Premiant and Sládek were the most and the least stable varieties, respectively. After 12 months, the content of the total polyphenols and flavonoids decreased by 30–40% and by 20–30%, respectively, irrespective of storage conditions. The rate of decline accelerated strongly after 6 months. The DPPH (1,1‐diphenyl‐2‐picrylhydrazyl) antiradical potential decrease was significant only in hops stored under aerobic conditions. The depletion was 9–25% after 1 year; Saaz was the most stable variety. Copyright © 2012 The Institute of Brewing & Distilling     

Effect of an excited-state optical transition on the linewidth enhancement factor of quantum dot lasers

An analytical expression is derived for the linewidth enhancement factor of a quantum-dot laser, which makes it possible to describe its dependence on optical loss and photon density in an explicit form. The model accounts for refractive index variations at the ground-state optical transition due to gain/absorption variations upon the first excited-state transition in quantum dots. It is shown that a decrease in optical loss, an increase in saturated gain, and an increase in the energy separation between the excited-state and ground-state transitions results in a decrease in the α factor both at and above the lasing threshold.     

Study on ablation behavior and mechanism of C/SiC composite irradiated by a laser with various parameters

C/SiC composite has been widely used as a high-temperature material for engineering components due to its excellent thermal properties. Facing the rapid development and threat of high-energy laser, study on the ablation resistance under laser irradiation is strongly required. In this work, a continuous high-energy laser was applied to explore the laser ablation behavior and mechanism of C/SiC composite. From the results, C/SiC composite shows different morphologies when irradiated at various laser power densities for 500 and 700 W/cm². We divided the ablation area into three regions; the central, transition, and edge regions, where the formation of SiO₂, SiO, and the breakage of carbon fiber were observed. The generated highly reflective SiO₂ layer reduces the absorption of laser energy, which is beneficial to lower the back-surface temperature and reduce the damage of composite. In addition, we put forward the ablation physical models and ablation mechanisms irradiated at different power densities. The work provides a basis for the laser ablation resistance of C/SiC composites under different conditions.     

Guiding of emulsion droplets in microfluidic chips along shallow tracks defined by laser ablation

We demonstrate controlled guiding of nanoliter emulsion droplets of polar liquids suspended in oil along shallow hydrophilic tracks fabricated at the base of microchannels located within microfluidic chips. The tracks for droplet guiding are generated by exposing the glass surface of polydimethylsiloxane (PDMS)-coated microscope slides via femtosecond laser ablation. The difference in wettability of glass and PDMS surfaces together with the shallow step-like transverse topographical profile of the ablated tracks allows polar droplets wetting preferentially the glass surface to follow the track. In this study, we investigate guiding of droplets of two different polar liquids (water/ethylene glycol) with and without surfactant suspended in an oil medium along surface tracks of different depths of 1, 1.5, and 2 \(\upmu\)m. The results of experiments are also verified with computational fluid dynamics simulations. Guiding of droplets along the tracks as a function of the droplet composition and size and the surface profile depth is evaluated by analyzing the trajectories of moving droplets with respect to the track central axis, and conditions for stable guiding are identified. The experiments and numerical simulations indicate that while the track topography plays a role in droplet guiding using 1.5- and 2-\(\upmu\)m deep tracks, for the case of the smallest track depth of 1 \(\upmu\)m, droplet guiding is mainly caused by surface energy modification along the track rather than the presence of a topographical step on the surface. Our results can be exploited to sort passively different microdroplets mixed in the same microfluidic chip, based on their inherent wetting properties, and they can also pave the way for guiding of droplets along reconfigurable tracks defined by surface energy modifications obtained using other external control mechanisms such as electric field or light.     

Synthesis and properties of prepolymers and their siloxane analogues by thiol‐ene polyaddition of limonene with dithiols

This work presents a synthesis of monomers from a natural compound (limonene) and dimethylsiloxanes with various chain lengths by hydrosilylation. Based on these monomers, prepolymers with various lengths of the siloxane and methylene parts, as well as their analogues without siloxane moieties, were obtained by thiol‐ene polyaddition with ethanedithiol, butanedithiol and hexanedithiol. Their physicochemical characteristics, namely the thermal stability, phase behaviour and intrinsic viscosity of the polymers obtained, were compared as a function of the length of the methylene spacer and the siloxane part in the polymer chain. © 2019 Society of Chemical Industry