Mariya Aleksandrova | Technical University - Sofia (original) (raw)

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Papers by Mariya Aleksandrova

Crystals, Sep 20, 2022

This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY

Bulgarian Chemical Communications, 2020

Nanoscience and Nanotechnology, 2016

IOP conference series, Jul 1, 2020

In this paper is presented study of flexible pressure piezoelectric energy harvesting developed w... more In this paper is presented study of flexible pressure piezoelectric energy harvesting developed with an screen-printing of polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) ink on three different metal electrodes (silver, gold and aluminium) with three different topologies – rectangular, meander and side comb. The influence of the electrode type and pattern is studied in terms of polymer coating distribution and morphology, piezoelectric voltage generation and stability at multiple bending, as well as piezoelectric coefficient. The elements are studied at low mass loading of up to 100 g and frequency up to 50 Hz. They produce voltage between 916 and 362 mV from active area of 1 cm2 and piezoelectric polymer film of 3 μm, according to the electrode metal nature and shape. It was found that the silver electrode with meander shape is the optimal for electrical performance and mechanical stability giving superior piezoelectric coefficient of 0.332 V.m/N to the reported values and only 8.6 % decrease of the piezoelectric voltage at 1500 repeating bending cycles.

Journal of Electronic Materials, Nov 7, 2019

Vertically aligned and highly dense Zinc oxide (ZnO) nanorods (NRs) have been successfully synthe... more Vertically aligned and highly dense Zinc oxide (ZnO) nanorods (NRs) have been successfully synthesized by a two-step hydrothermal method and decorated by silver (Ag) nanoparticles (NPs) via a dip coating technique. Absorption spectra indicate the presence of metal Ag NPs. The photoluminescence (PL) spectrum of as-grown ZnO nanorods shows ultra violet (UV) emission centered around 390 nm and a sharp defect-related emission peak around 580 nm. The presence of Ag NPs on the ZnO NRs shows a significant red shift in PL peak position in the visible region and a complete quenching of UV emission. The changes in UV-Vis and PL spectra of ZnO NRs decorated with Ag metal NPs are studied and discussed.

Journal of Fluorescence, Dec 3, 2015

Nanocomposites have gained much importance in different fields, commercially and technologically,... more Nanocomposites have gained much importance in different fields, commercially and technologically, due to the possibilities in tuning the properties of nanocomposites. In the present work, vanadium oxide (V 2 O 5)/tin oxide (SnO 2) nanocomposites were synthesized using simple sol-gel method. Two independent experiments were carried out to synthesis the V 2 O 5 /SnO 2 nanocomposites. The V 2 O 5 /SnO 2 nanocomposites were prepared from the precursors of tin chloride, ammonium metavanadate and ethanol (Experiment 1). A few ml of concentrated hydrochloric acid (HCl) was added in the precursors with and V 2 O 5 /SnO 2 nanocomposites were prepared (Experiment 2) using ethanol as a solvent. Effect of HCl on the modifications of structural, morphological and optical properties of the V 2 O 5 /SnO 2 nanocomposites was studied. Coexistence of V 2 O 5 and SnO 2 phases was confirmed by the X-ray diffraction studies. Thermogravimetric analysis (TGA) shows the stability of the as-synthesized V 2 O 5 /SnO 2 nanocomposites from 350 • C. The crystallinity and the surface morphology of the synthesized V 2 O 5 /SnO 2 nanocomposites were improved effectively due to addition of HCl in the preparation of the precursor solution. UV-DRS absorption spectra showed that the absorbance is modified and the band gap is decreased with increase in crystallite size due to the addition of HCl. The predominant Raman peaks at 140 cm −1 and 991 cm −1 confirm the presence of orthorhombic V 2 O 5 phase. Further, the addition of HCl in the preparation of precursor solution revealed marked difference in the surface morphology of the nanocomposites.

Advanced Materials Letters, Oct 1, 2020

Energy, Aug 1, 2020

Novel potassium niobate (KNbO 3) nanocoatings were sputtered on polyethylene naphthalate (PEN) su... more Novel potassium niobate (KNbO 3) nanocoatings were sputtered on polyethylene naphthalate (PEN) substrates at different sputtering voltages and concentrations of sputtering gas. The relations growth time-thickness and sputtering voltage-deposition speed were established and the vacuum deposition modes were optimized. This was realized, in order to correlate the microstructure, surface morphology and the growing conditions for the nanosized KNbO 3 films. Uniform coatings with variety of thicknesses were obtained without overheating degradation of the substrates at process parameters U s and P Ar varying from 0.6 kV to 0.9 kV and from 1 Â 10 À2 Torr to 9 Â 10 À3 Torr, respectively. New Al/KNbO 3 /Al energy harvesting element was fabricated and tested on tension-compression cycles. For the samples, showing the lowest surface roughness of the KNbO 3 film (3.6%), maximum piezoelectric voltage of 431 mV was measured at loading up to 3 kg. Basic electric parameters, such as capacitance, contact and bulk resistance of the piezoelectric element, involving KNbO 3 film, produced at the optimum sputtering conditions, were measured as a function of the applied mass load. Output electrical power of 2.324 mW was obtained at maximum mechanical loading, which is comparable with the results for lead-containing thin film harvesters. Theoretical analysis of the novel elements was made based on their dielectric behaviour in terms of polarization type. They were connected to passive power management system for operation as an independent power supply for low-power electronics.

International Journal of Polymeric Materials, Nov 1, 2012

AC electrical properties of polymer light-emitting devices with poly(2-methoxy-5-(2′-ethyl-hexylo... more AC electrical properties of polymer light-emitting devices with poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and polyvinylcarbazole (PVK) layers are studied. Polymer nanosized films are deposited by the spin-coating method. Frequency-dependent real and imaginary parts of the impedance are measured and fitted using RC equivalent circuits. Important parameters for such types of structures, like capacitance and contact resistance, are evaluated at different frequencies in the

Sustainability, Aug 13, 2022

Materials Today: Proceedings, 2017

II-VI group quaternary semiconductor alloy thin films of cadmium zinc sulfoselenide (CdZnSSe) wer... more II-VI group quaternary semiconductor alloy thin films of cadmium zinc sulfoselenide (CdZnSSe) were prepared by chemical bath deposition technique on glass substrates. The deposition was carried out with various optimized deposition parameters such as concentration of precursor ions, deposition temperature, time, pH, etc. In order to obtain homogeneous thin films, triethanolamine (TEA) was used as complexing agent. A common cationic surfactant, cetyl-trimethylammoniumbromide (CTAB) was used as a capping agent. The as-deposited thin films were annealed at different temperatures (i.e. 100°, 300° and 500°C) and their structural and optical properties have been investigated by XRD, SEM, EDS and UV-visible optical absorption studies. The optical band gap energy was found to decrease from 2.21 to 2.01 eV, with increase in annealing temperature exhibiting a red shift.

Coatings, Jul 5, 2020

Thin films of ZnO:Ga 2 O 3 (ZGO) were deposited by radio frequency (RF) sputtering at voltages of... more Thin films of ZnO:Ga 2 O 3 (ZGO) were deposited by radio frequency (RF) sputtering at voltages of 0.5, 0.9 and 1.1 kV. The films were studied with respect to their suitability in flexible piezoelectric nanogenerators. The analysis of the spectroscopic and microscopic results showed that piezoelectric features were revealed for the films grown at all sputtering voltages, but the most favorable morphology in terms of low roughness was achieved at 1.1 kV. The effect of the sputtering voltage on the films crystallinity and lattice strain was studied. It was found that the increasing sputtering voltage promoted the films crystallization. Additionally, the presence of oxygen vacancies in the piezoelectric films was negligible as it is not a major factor affecting their performance. The electrical measurements of the Ag/ZnO:Ga 2 O 3 /Ag harvester on a flexible substrate in the low-frequency range showed a piezoelectric voltage of 414 mV, a current of 10.4 µA and an electric power output of 1.4 µW at a mass load of 100 g. These results were achieved by a simple architecture of a single piezoelectric layer with a relatively small size of 3 cm 2 and small piezoelectric film thickness (600 nm) containing lead-free material. It was proven that the sputtered ZGO films are suitable for energy harvesting elements and their performance could be tuned by the sputtering voltage. Another possible application of the proposed device, excluding low-frequency vibrational harvesting, could be a pressure sensor or strain gauge, due to the good linearity of the electrical parameter dependences on the strain.

Modern approaches on material science, Feb 25, 2020

Microsensor design for ammonia (NH 3) gas detection was proposed. The sensor is 4 segmented and c... more Microsensor design for ammonia (NH 3) gas detection was proposed. The sensor is 4 segmented and consists of sensing resistors-interdigitated electrodes and ammonia sensing layer, deposited on thermal oxidized silicon substrate with specific in shape microheater and thermocouple. The zinc oxide (ZnO) layer, grown by RF sputtering method, was used as an ammonia sensing coating. The ammonia sensor, which is of resistive type, changes its resistance, when the sensing film adsorbs or desorbs ammonia gas. The thermal decomposition of ammonium bicarbonate (NH 4 HCO 3) is used to provide constant and stable ratio NH 3 : CO 2 = 2:1, receptivity 66% and 33% [g/mol] in volume. The results obtained from the testing of the ammonia gas are reported for temperature range between 25°C and 50°C. Resulting change in the segments resistance is in the range between 134MΩ to 56MΩ with excellent linearity and short response time.