• Terahertz emission induced by self-oscillations of current in semiconductor resonant tunneling structures

    pg(s) 7-9

    Based on the simultaneous measurements of radio-frequency intrinsic self-oscillations of current and terahertz (THz) light emission from a 30-period GaAs/AlGaAs weakly-coupled superlattice, it was concluded that self-oscillations of current arise due to the cyclic process of the electric-field domain boundary spatial expansion and shrinkage. The domain boundary expands over several SL periods due to the energy dissipation of tunneling electrons, resulting in the carrier trapping in several SL periods behind the leading edge of the domain boundary. In this region, the charge accumulation gives rise to the resonant detuning of subbands in adjacent quantum wells (QWs), creating the population inversion between the two subbands, which ensure the resonant tunneling along the SL axis. The electrons injected from the cathode restore the resonant coupling of subbands, the lower subband is emptied, and the intersubband radiative transitions allowed. The electroluminescence (EL) spectra of the SL demonstrate the main peak with the left and the right sidebands. The main peak is related to the intersubband electron transition energy, while the sidebands are associated with the resonant detuning energy of subbands in adjacent QWs (~ 4 meV). The two-photon pulsed THz emission (~ 4 meV) from a double-barrier GaAs/AlAs resonant tunneling diode biased into self-oscillation regime confirms the last assertion. There is the same cyclic mechanism of relaxation self-oscillations of current (accumulation and drain type), where the first THz pulse is triggered due to the carrier trapping by miniband states, resulting in the miniband energy shift up to higher energies. After the trap release time, the electrons lost their energy via the second THz pulse emission, and the miniband gets back to its steady state.

  • Complexation of metal cations (mono-, di- and trivalent) to cucurbiturils: Insights from a DFT/SMD study

    pg(s) 3-6

    Recently, supramolecular complexes based on synthetic macrocyclic host molecules have received much attention due to their broad applications as drug delivery carriers, biological and chemical sensors, light-emitting materials, bioimaging agents, etc. Cucurbit[n]urils are cavitands known for their high affinity for various guest molecules, although some aspects of their coordination chemistry remain enigmatic. They are still not tested as metalloenzyme models and not much is known about their metal-binding properties. Furthermore, there is no systematic study on the key factors controlling the processes of metal coordination to these systems. In the
    computational study herein, DFT molecular modeling has been employed in order to investigate the interactions of biologically essential mono- (Na+), di- (Mg2+) and some trivalent (La3+, Lu3+) metal cations to cucurbit[n]urils and evaluate the major determinants shaping the process of recognition. The thermodynamic descriptors (Gibbs energies in the gas phase and in a water medium) of the corresponding complexation reactions have been estimated. The results obtained shed light on the mechanism of host–guest recognition and disclose which factors more specifically affect the metal binding process.

  • Abiogenic Metals in Medicine. Insights from Theoretical Studies of the Mechanisms of Action of Silver (I), Strontium (II), and Gallium (III)

    pg(s) 3-6

    Almost half of all known to date proteins contain metal co-factors. Over the course of 3–4 billion years of organism evolution, several metal species of different oxidation state (Na+, K+, Mg2+, Ca2+, Zn2+, Mn2+, Fe2+/3+, Co2+/3+, Ni2+ and Cu+/2+) have been chosen to participate in key biological processes. They are known as “native”, or “biogenic” metals. On the other end stand toxins like mercury and lead, which poison the recipient by competing with the natural co-factors for binding the essential proteins. As a third example, however, should be outlined some abiogenic metal species, which exert a curative effect on the host organism, and are, therefore, applied in medicine as novel drugs. Such are silver (Ag+), strontium (Sr2+), and gallium (Ga3+). The current study investigates their ability to compete with the native cuprous (Cu+), calcium (Ca2+), and ferric (Fe3+) cations, respectively, by exploiting the methods of the computational chemistry. Nowadays, silver finds broad application in many areas of medicinal use, e.g. being added to dressings of burn wounds as a concomitant therapy of skin ulcers, as a potential water disinfectant, or even in ophthalmology as an active component in eye drops. Strontium prevents the destruction of bones and contributes to their restoration. Sr2+ salt of ranelic acid is a medication (under the names Protelos, Protos, Strontium ranelate Aristo) used for treatment of osteoporosis in postmenopausal women and very elderly patients. Gallium, in its cationic form (Ga3+), is well known for its anticancer activity. Nonetheless, recent experimental studies have considered the employment of gallium as a promising “Trojan horse” strategy against pathogenic microorganisms. Herewith, we set on a quest for deciphering the most acclaimed mechanisms of therapeutic action of the aforementioned metal cations at atomic level. The obtained results shed light on the intimate echanisms of metal recognition, thus revealing key factors governing the processes of native/abiogenic metal rivalry. This approach serves not only for explaining already existing experimental findings, but also as a first step in designing/engineering novel drug molecules of potential therapeutic value.

  • High energy formation of active elements: processes of creation and methods of improvement

    pg(s) 42-44

    Thе paper examines the processes associated with the high-energy formation of active elements. A model of the processes is created and certain relations are derived, which are suitable for practical calculations of the parameters of high -energy systems with directional motion. The processes of movement, deformation and penetration of the active element are studied. Methods and means for influencing the processes of creation and improvement of the active elements are proposed.

  • Recovery of sulfur dioxide from flue gas for sulphuric acid production

    pg(s) 40-41

    The paper presents a review of the literature relative to the flue gas desulfurization methods for sulphuric acid (H2SO4) manufacturing. The covered topics include wet and dry methods. Particular attention has been paid to the desulfurization of the flue gas from the copper smelting industry that can lead to a generation of SOx rich streams resulting in high profitability of H2SO4 production.

  • The Picard’s iteration method for determining the critical buckling load of Euler elastic columns

    pg(s) 3-4

    The Picard’s successive iteration method is applied to determine the Euler critical force for a slender column. The initial approximation of the buckling mode of the column is a polynomial function that satisfies the boundary conditions. A numerical example is solved and the obtained result is compared with that obtained by the Euler formula. An assessment of the accuracy of the solution is made at each iteration step. In the paper is shown that the Picard’s iteration method could also be used for obtaining the closed form exact solution of the buckling load. The investigated column is hinged at its upper end and supported by a Q- apparatus at the other. It is loaded with a compressive force at the upper end.

  • Investigation of lipstick properties using statistical methods

    pg(s) 89-91

    The rheological non-destructive oscillation testing is conducted in order to predict and quantify the samples’ properties of the commercial brand lipstick. The influence of the variation of the process parameters temperature, phase angle and strain are investigated. Regression models for the dependencies of the rheological and thermal properties (elastic modulus and shear stress) from the described process parameters are estimated, based on 681 experimental data in the temperature region from 26.9 °C to 74.7 °C.

  • Image segmentation of agricultural products using statistical indicators

    pg(s) 83-88

    Machine inspection is a mandatory technological process in industrial processing agriculture products. The camera detects color and shape based irregularities of the object resulting in a large number of parameters for decision-making and sorting product compliance. The goal was to discover a new criterion for decision-making using only the output signal of the RGB camera. Research employed the digital images of raspberries, blackberries, peas and yellow beans during real processing, obtained from a color sorter machine. The visual texture of the surface of the agricultural products was described via defined statistical indicators of color (color average value (Avg), standard deviation (Stdv), entropy (E), and lacunarity (L) was used from the sphere of image fractal analysis as one of the criteria. By applying the non-parametric tests: Wilcoxon signed rank and Friedman test, statistically significant difference was established for the L and Е criteria between compliant and non-compliant industrial products.

  • Assessment of quality parameters of blueberries and cherry tomatoes treated by micro torch of cold atmospheric pressure plasma

    pg(s) 81-82

    The cold atmospheric pressure operating plasma /CAPP/ offers attractive opportunities for various surface decontamination treatments of both food products and non-food materials. The technology is especially considered as promising mild but efficient surface treatment of fresh food products in order to extend their shelf live and sustain quality. Here we report evaluation of the effect of CAPP micro torch treatment of Canadian blueberries and cherry tomatoes related with the fruit quality and possible cell wall damages and changes of the total anthocyanins content of treated fruits. The problems and opportunities for further application of plasma jet generated CAPP treatment for fresh fruit decontamination are discussed. Thе research is funded by the Bulgarian National Science Fund under Grant DN08/8-2016.

  • Effect of the lactic acid as a substrate on the butyric acid production by Clostridium beijerinckii 4A1 – preliminary data

    pg(s) 79-80

    In recent years, interest in butanol and butyric acid as a potential substitute for traditional fuels has grown. Microbiological production of butanol and butyric acid from carbohydrates is not economically advantageous due to several process defects. The cost reduction potential of the product is related to expanding the substrates used and optimizing the process’ economics. The metabolism of the strains Clostridia is in two stages – acidic and solvatogenic. In the beginning, carboxylic acids are formed, and afterward, these acids induce the preparation of solvents.
    The aim of this work is to study the effect of unusual substrate on butyric acid production by Clostridium beijerinckii 4A1. The experiments performed were based on a Reinforced Clostridial medium, using different concentrations of lactic acid as carbon source, at Т= 37 °C, and anaerobic conditions.

  • Study on the possibilities for the industrial water purification from heavy metal ions

    pg(s) 44-46

    Among the various methods for water purification and treatment from contaminated industrial pollutants, one of the most used is the adsorption methods based on activated carbons. The purpose of our study is to investigate the correlation between the porous texture and chemical nature of the surface and sorption capacity of the activated carbon, obtained based on lignite coal. The object of the study is also to determine its effectiveness as a sorbent for Cd-ions purification of contaminated industrial water. The obtained data from the conducted investigation show that the adsorbent based on such kind of activated carbon has good qualities for water purification from various pollutants and heavy metals