Elena Koleva

Elena Koleva
Bulgarian Academy of Sciences | BAS · Laboratory for Physical Problems of Electron-Beam Technologies

Prof.

About

171
Publications
28,374
Reads
How we measure 'reads'
A 'read' is counted each time someone views a publication summary (such as the title, abstract, and list of authors), clicks on a figure, or views or downloads the full-text. Learn more
617
Citations

Publications

Publications (171)
Article
Full-text available
The electron beam welding process is analyzed of high-strength steel type 15Cr5Mo samples with deflection oscillations along the zone of interaction. The geometries of the molten and the heat affected zones are studied in order to investigate the influence of the deflection oscillations parameters and to improve the seam quality. A robust engineeri...
Article
Full-text available
Experimental results on the quality of electron beam melting and refining of copper samples by implementing the "method of washers" are analyzed by using an empirical model estimation approach. The influence is studied of the process parameters-melting power and time of refining, on the residual content of impurities, the refining rate, the overall...
Article
Full-text available
“Proceedings International Scientific Conference Industry 4.0”. VI International Scientific Conference “INDUSTRY 4.0”. 08 – 11 Dec. 2021, Borovets, Bulgaria, Vol. 2/12, Year V, 2021, pp. 238-240. ISSN 2535-0153 (Print), ISSN 2535-0161 (Online). http://industry-4.eu/winter/sbornik/2-2021.pdf This work presents experimental and theoretical investiga...
Article
Full-text available
“Proceedings International Scientific Conference Industry 4.0”. VI International Scientific Conference “INDUSTRY 4.0”. 08 – 11 Dec. 2021, Borovets, Bulgaria, Vol. 2/12, Year V, 2021, pp. 241-243. ISSN 2535-0153 (Print), ISSN 2535-0161 (Online). http://industry-4.eu/winter/sbornik/2-2021.pdf Experimental results for the quality of electron beam mel...
Article
Full-text available
“Proceedings International Scientific Conference Industry 4.0”. VI International Scientific Conference “INDUSTRY 4.0”. 08 – 11 Dec. 2021, Borovets, Bulgaria, Vol. 2/12, Year V, 2021, pp. 244-246. ISSN 2535-0153 (Print), ISSN 2535-0161 (Online). http://industry-4.eu/winter/sbornik/2-2021.pdf Grafting is the most effective way of modifying and regul...
Conference Paper
The influence of electron beam lithography parameters (such as electron energy, resist thickness, the exposure dose) on the resist sidewall shape (profile) was studied for the PMMA (polymethyl-methacrylate) positive resist. The profile of the positive tone resist PMMA was investigated depending on varying exposure doses for the resist thicknesses 6...
Conference Paper
The widespread expansion of electronic information technologies in education leads to continuous processes of modernization and sometimes a complete change in the content, methods, organization of training, the learning space, and the goals of the educational system. The ever-increasing amount of information needs to be understood, processed, and r...
Article
Full-text available
International Scientific Journal Mathematical Modeling, Vol. 4 (2020), Issue 3, pp. 79-81, ISSN (print) 2535-0986, ISSN (online): 2603-2929, https://stumejournals.com/journals/mm/2020/3/79.full.pdf The electron beam technological processes like electron beam welding, electron beam additive technologies, etc. depend strongly on the characteristics...
Article
Full-text available
International Scientific Journal Science. Business. Society, Vol. 5 (2020), Issue 3, pp. 112-115, Print ISSN: 2367-8380, Online ISSN: 2534-8485. https://stumejournals.com/journals/sbs/2020/3/112.full.pdf The Smart Homes come rapidly into our everyday lives bringing comfort, security, help and many other benefits. The development of Information and...
Article
Full-text available
International Scientific Journal Machines. Technologies. Materials, Vol. 14 (2020), Issue 8, pp. 352-355, Print ISSN: 1313-0226, Online ISSN: 1314-507X. https://stumejournals.com/journals/mtm/2020/8/352.full.pdf An experimental study of the geometrical characteristics of the cross sections of the thermally affected areas obtained by electron beam...
Article
Full-text available
International Scientific Journal Science. Business. Society, Vol. 5 (2020), Issue 3, pp. 89-91, Print ISSN: 2367-8380, Online ISSN: 2534-8485. https://stumejournals.com/journals/sbs/2020/3/89.full.pdf The rheological non-destructive oscillation testing is conducted in order to predict and quantify the samples' properties of the commercial brand li...
Conference Paper
The process of electron beam grafting is implemented for the production of highly efficient graft copolymers from natural polysaccharides and regulating their properties for application as flocculating agents during the treatment of different wastewaters. In this paper, the electron beam induced modification of corn starch by grafting of acrylamide...
Article
Full-text available
The application of electron beam sweep makes it possible to carry out multifocal and multi-beam welding, as well as combine the welding process with local heating or subsequent heat treatment, which is important when preparing products from thermally-hardened materials. This paper presents a method of electron beam welding (EBW) with dynamic beam p...
Article
Full-text available
Electrotechnica & Electronica (Е+Е), Vol. 55 (3-4), 2020, pp.49-56, ISSN: 0861-4717 (Print), 2603-5421 (Online), https://epluse.ceec.bg/wp-content/uploads/2020/09/20200304-03.pdf The world of technology is changing at an extremely fast pace. Covid-19 has given further impetus to modernization in education, and it seems unlikely that schools and...
Article
Full-text available
A time-dependent heat model is implemented for the simulation of the thermal distribution in stainless steel 316L samples during electron beam irradiation. The influence of following process parameters: the electron beam power and the processing time on the obtained temperature fields in the samples, is investigated. The obtained simulation result...
Article
Full-text available
A time-dependent heat model is implemented for the simulation of the heat distribution in 316L stainless steel samples during electron-beam irradiation. The influence is studied of the electro-beam power and processing time on the temperature field produced in the samples. The simulation results are compared with real experimental data of electron-...
Article
Full-text available
The study reveals the influence of the electron-beam lithography parameters (such as the exposure dose, resist thickness, depth) on the resist profile shape in the case of the PMMA (polymethyl-methacrylate) positive resist. The experiments are performed using an Elphy Quantum (Raith) e-beam lithography control system installed on an Inspect F50 (FE...
Article
One of the difficulties of performing electron beam deposition with wire material is positioning the filler wire relative to the electron beam. The accuracy of the positioning of the beam relative to the wire significantly affects heat input characteristics, the transfer of molten metal from the wire to the substrate, and, as a consequence, the nat...
Article
Full-text available
Additive manufacturing technologies are developing fast world wide. Never the less, in the machine-building industry, manufacturing of especially large products is required, and common processes, for example, selective laser melting, are not able to satisfy this requirement. Multilayer wire-arc deposition, allows to make high-quality large-scale pr...
Article
Full-text available
Industrial application of hollow cathode arc (HCA) discharge necessitates understanding the processes of interaction between the plasma and the cathode and the anode, as well as related processes. The first step is to study the formation process of HCA discharge in vacuum with the micro-flow of plasma-forming gas through the hollow cathode. In this...
Article
Full-text available
Background The actual problem for calculating a shape of free surface of the melt when analyzing the processes of wire-based electron-beam surfacing on the substrate, being introduced into additive manufacturing, is the development of adequate mathematical models of heat and mass transfer. The paper proposed a formulation of the problem of melt mot...
Conference Paper
Full-text available
Abstract — In this paper the simulation of electron beam welding of thin 316L stainless steel thin plates by a moving linear heat source is considered. Regression models for the molten pool geometry characteristics, depending on the electron beam power and the welding velocity are estimated. The thermal histories and the cooling rates defining the...
Article
Full-text available
In this paper the developed software for simulation of the temperature distributions in the cast ingots at electron beam drip melting for different materials is presented. The influence of the change of the process parameters electron beam power, casting velocity and the beam radius on the temperature distributions in the cast ingot and the paramet...
Article
Full-text available
This paper presents the implementation of the overall desirability function-the overall robust optimization approach-for multi-criteria parameter optimization in the robust engineering case, based on estimated models for the means and variances of the quality characteristics. The optimized properties of the synthesised copolymer by electron beam in...
Article
Full-text available
In this article quasi steady-state two-dimensional heat model is implemented for the simulation of the temperature distribution in the cast copper ingots through electron beam drip melting. Regression models are estimated for the dependence of the shape of the crystallization front (molten depth, width, volume of the molten metal) on the variation...
Article
Full-text available
Electron beam irradiation has the ability to modify polymer substrates by the process of graft copolymerization in order to synthesize water-soluble copolymers having flocculating properties. The influence of the variation of the process parameters acrylamide/starch (AMD/St) weight ratio, electron beam irradiation dose and dose rate is investigated...
Article
Prospects of one of unconventional research areas aimed at improving additive technologies have been described. The key aspect of the proposed technology is the formation of multilayered homogeneous materials in vacuum using electric arc as a heating source, as well as using a solid metal wire as a filler metal. It is shown that this approach, comp...
Article
Full-text available
Theoretical and experimental study on developed profiles in the positive tone PMMA (polymethyl-methacrylate) resist and e-beam lithography process parameters was performed. E-beam lithography control system Elphy Quantum (Raith) installed on Scanning Electron Microscope Quanta FEG (FEI) with a field emission cathode and Gaussian intensity distribut...
Article
Full-text available
An experimental investigation of the electron beam irradiation induced grafting of acrylamide onto corn starch was performed. In this way the synthesis of water-soluble copolymers having flocculation abilities is realized. The acrylamide monomer provides a polar graft side chain which turns it into a hydrophilic copolymer. The robust engineering ap...
Article
Full-text available
A normalized processing diagram for powder bed additive manufacturing by concentrated energy beams was constructed on the base of simple analytical model simulating the heating of the powder layers by a moving linear heat source. The distance of the processing regime points below the line of the dependency of the maximal dimensionless temperature (...
Article
Full-text available
In this paper, a study based on the mathematical modelling, applying different process simulation tools (CASINO, TREM, SELID) for characterization of PMMA resist and for the improvement of the resolution concerning the critical dimensions of nano-patterning by electron beam lithography (EBL) is presented. Data for important EBL characteristics (ene...
Article
Full-text available
In this paper, effects of electron beam melting process parameters (e-beam power, refining time, etc.) and the number of melting operations for fulfilling the requirements concerning the composition and the structure of metals obtained after e-beam processing are studied. Results obtained at electron beam melting of molybdenum and tungsten technoge...
Article
Full-text available
The challenge of this paper consisted in optimization application in the case of starch grafting with a vinyl monomer under accelerated electron field. Thus, the robust engineering approach and multi-criteria optimization were considered. Methods based on graphical optimization as well as on overall optimization were implemented. Moreover, a graphi...
Article
Full-text available
Investigation of the variation of the mechanical properties of 316L steel after electron beam surface modification is performed. The samples were processed at an installation in IE BAS - ELIT – 60. The irradiated plates were with thickness of 0.5 mm. The experiments were carried out with a change in the beam power in the range of 0.6 to 0.9 kW and...
Article
Full-text available
This article discusses the experimental results from multi-pool electron beam welding, with dynamic positioning of the electron beam (beam splitting) [1], resulting in the formation of two consecutive welding pools. The 12Cr18Ni10Ti stainless steel samples are welded with a change in the process parameters: the distance between the two electron bea...
Article
Full-text available
This work aims to assess the bremsstrahlung X-Ray signal from the processing zone obtained by the use of a solid wire during electron beam surfacing. Applications of the bremsstrahlung X-Ray signal in the control systems allow positioning of the filler wire. During the experiments, a scintillation detector based on monocrystalline iodine cesium and...
Article
Full-text available
Response Surface Methodology and robust engineering approach are applied for the investigation of preliminary experimental data related to the synthesis of composite biopolymer-based hydrogels via electron beam (e-beam) radiation cross-linking. Two different biopolymers – xanthan gum (XG) and carboxymethylcellulose (CMC) were used for hydrogel prep...
Article
Full-text available
The ISO/IEC 62264 standard is widely used for integration of the business systems of a manufacturer with the corresponding manufacturing control systems based on hierarchical equipment models, functional data and manufacturing operations activity models. In order to achieve the integration of control systems, formal object communication models must...
Article
Full-text available
This paper considers the process electron beam welding of stainless steel type 1H18NT in vacuum. Based on experimental data, the influence of the variations of the following process parameters: electron beam power, welding velocity, the distances from the magnetic lens of the electron gun to the beam focus and to the surface of the treated sample i...
Article
Full-text available
Published by Scientific technical Union of Mechanical Engineering "INDUSTRY 4.0" Abstract: Electron beam (EB) irradiation has the ability to modify polymer substrates by process of graft copolymerization to synthesize water-soluble copolymers having flocculating potential. Models - depicting the dependencies of the described quality characteristic...
Article
В настоящата работа се разглежда електроннолъчево заваряване на неръждаема стомана тип 1H18NT. Оценени са модели за зависимостите на геометричните характеристики на получените заваръчни шевове при вариране на параметрите на процеса – мощност на електронния лъч, скорост на заваряване (на движение на електронния лъч), разстояние от магнитните лещи до...
Data
Publications Assoc. Prof. Elena Koleva
Article
Full-text available
The term "quality" is a wide-ranging concept for which each person or professional sector has its own definition. The most progressive quality point of view is that it is entirely determined by the customer or the end user and is based on the assessment of customer experience with the product or service. The quality of use or ergonomic quality actu...
Article
Електроннолъчевите процеси са процеси, при които се осъществява взаимодействие на лъч от ускорени електрони с висока плътност на мощност с материали във вакуум. Това определя приложението им за повърхността модификация на материали, изпарение на материали и последващото им отлагане на тънки слоеве, заваряване и други. Предимствата на процеса са свъ...
Article
Абстракт: Докладът разглежда процесът на електроннолъчево заваряване във вакуум на неръждаема стомана от тип 1H18NT. Обучени са невронни мрежи, които на базата на параметрите на процеса - мощност на електронния лъч, скорост на заваряване и разстоянията от магнитните лещи на електронната пушка до фокуса на електронния лъч и до повърхността на образе...
Conference Paper
Full-text available
This work considers the electron beam lithography process of exposed and developed polymethyl methacrylate (PMMA) resist profiles under production conditions and in presence of errors in process parameter values. Robust engineering approach is applied and models for the mean values and the variances of the resist profile geometric characteristics d...
Conference Paper
Full-text available
A study about complacency of portable computers. For characterization on complacency were used indicators witch efficiency, usability, effectiveness and quality. Respondents were split into two groups, up to and over 36 years of age.
Article
Full-text available
The optimization of welds obtained by electron beam welding is a multi-criterial task, involving the requirements for the geometrical characteristics of the welds, together with the fulfilment of the requirements for lack of defects. This complex task can be solved through estimation of adequate models for the dependencies of the quality characteri...
Article
Full-text available
This paper considers the process electron beam welding in vacuum of stainless steel 1H18NT. Neural network based models are developed and used for the description of the defectiveness, depending on the process parameters-electron beam power, welding velocity, the distance between the main surface of the magnetic lens of the electron gun and the bea...
Article
Full-text available
We consider questions of building a closed-loop focus control system for electron-beam welding. As a feedback signal, we use the secondary current in the plasma that forms above the welding zone. This article presents a model of a secondary current sensor in plasma during electron-beam welding with focus scanning. A comparison of modeled results wi...
Article
Full-text available
Investigations are carried out and obtained experimental and theoretical data for tantalum scrap recycling by electron beam melting (EBM) is presented in this paper. Different thermal treatment process conditions are realized and results are discussed. A chemical analysis is performed and refining mechanisms for electron beam (EB) refining of Ta ar...
Conference Paper
In this study, a simple patterning method of submicrometer structures is proposed for gas sensor development. Comb-like electrodes patterned in thin Pt layer were proposed to measure gas sensor electrical conductivity. Negative resist SU-8 was used as a masking layer for ion etching of electrodes in 35 nm Pt layer on sapphire substrate. This method...
Chapter
Full-text available
In this chapter, we present a study on three polymer resists that are important for electron beam lithography (EBL) and we demonstrate their successful application for nano-structuring and EBL process optimization. The experiments performed showed that the resists behavior, including their sensitivity and contrast, is influenced by the energy of th...
Article
Full-text available
In this paper the applications of electron beam additive manufacturing systems are reviewed. There by selective electron beam melting of welding wires or of some parts of upper powder layer of a sandwich structure, 2D slices, welded with the others, are built by consequently depositing powder layers. The deposited metal powder is selectively melted...
Article
Full-text available
Experimental investigations of the modification of starch (corn and potato) by grafting of acrylamide using electron beam irradiation in order to synthesize water-soluble copolymers having flocculation abilities is performed. The influence of the variation of the parameters acrylamide/starch weight ratio, electron beam irradiation dose, dose rate a...
Article
Full-text available
The management of the quality during electron beam welding directed to optimization of the process parameters is an important way to improve the use of the expensive equipment and to make the EBW process more efficient in consuming materials, time and energy. The beam emittance as well as the electron beam density distribution are significant and...
Article
Full-text available
Electron beam is at the forefront of welding technology. The choice of optimal welding modes, monitoring the weld quality and/or detecting weld defects in real-time during the electron beam welding process using nondestructive, cost-effective and reliable methods is one of current challenges in this field. The plasma, generated in the keyhole and t...