Valdek Mikli

• Doctor of Philosophy
• Senio at Tallinn University of Technology

This study investigates the effect of green polar aprotic co-solvents such as N,N-Dimethylpropyleneurea (DMPU), dimethyl isosorbide (DMI), sulfolane (SLF), and γ-valerolactone (GVL) on the properties of regenerated cellulose. Binary solvent systems, containing superbase ionic liquid (SB-IL, 5-methyl-1,5,7-triaza-bicyclo-[4.3.0]-non-6-enium acetate)...

In this study, the impact of Vulcan XC-72R carbon black support pre-treatment on the oxygen reduction reaction performance of the Pt catalyst was investigated. Catalysts were synthesized via the polyol method using ethylene glycol, with different pre-treatment pathways (with H2O2 and HNO3 solutions) applied to the carbon black for surface functiona...

Pnictogen chalcohalides are semiconductors that have emerged as promising materials for energy conversion due to their exceptional optoelectronic properties. Their electronic configuration (ns²), particularly for Bi- and Sb-based compounds, can be a key factor in efficient carrier transport and defect tolerance, similarly, to Pb-perovskites. In the...

In this study, the impact of Vulcan XC-72R carbon black support pre-treatment on the oxygen reduction reaction performance of the Pt catalyst was investigated. Catalysts were synthesized via the polyol method using ethylene glycol, with different pre-treatment pathways (with H 2 O 2 and HNO 3 solutions) applied to the carbon black for surface funct...

With using RF-magnetron sputtering hybrid deposition technique, we achieved the preferred grain orientations closely aligning with the c-axis (002) dominating 63 % in the Sb2Se3 thin film. Fifty days of free access link below: https://authors.elsevier.com/a/1jeG04tTDnpWEJ

The use of precious metal electrocatalysts in clean electrochemical energy conversion and storage applications is widespread, but the sustainability of these materials, in terms of their availability and cost, is constrained. In this research, iron triad-based bimetallic nitrogen-doped carbon (M–N–C) materials were investigated as potential bifunct...

The study proposes using the distillable ionic liquid 5-methyl-1,5,7-triaza-bicyclo-[4.3.0]non-6-enium acetate, [mTBNH][OAc], for cellulose dissolution, making it an environmentally friendly alternative to conventional solvents in the transesterification of cellulose with vinyl esters. This ionic liquid (IL) has high dissolving power for cellulose...

The Bio-active™ archwires are the latest generation multi-force orthodontic archwires made of a Ni-Ti alloy. It is of particular importance to orthodontists to know what their composition and structural characteristics are so that they can determine which one is suitable for a given stage of orthodontic treatment. The aim of this work is to charact...

Monograin powder technology is one possible path to develop the sustainable, lightweight, flexible, and semi-transparent solar cells, which is ideal for integration with various building and product elements. In recent...

One promising and cost-effective method to produce flexible solar panels in the future is monograin layer (MGL) solar cell technology based on monograin powder (MGP) crystals. The results of the present study demonstrate the influence of different alkali salts (LiI, NaI, KI, RbI and CsI) on the properties of Cu2ZnSnS4 (CZTS) MGPs and their effect o...

This article investigates the effects of combining a novel protic ionic liquid-based fire retardant (FR) with alkalized hemp fiber. A pivotal importance of this study refers to the hydrophilic properties and limits regarding poor thermal resistance of green composites where standard guidelines for fire risks are crucial. Although it is well-studied...

A series of heterogeneous catalysts anchored to different polystyrene-based supports has been prepared and applied in an asymmetric [2,3]-Wittig rearrangement reaction of cyclohexanone derivatives. Among them, primary amino acid-derived (aminomethylated)polystyrene-supported catalysts showed excellent reactivity leading to the formation of rearrang...

In this work, the impact of Li and K co-doping on the properties of Cu2ZnSnS4 (CZTS) monograin powders used as absorber materials in monograin layer (MGL) solar cells was investigated. CZTS powders were grown in the liquid phase of flux materials with different LiI-KI ratios by synthesis-growth method. According to Atomic Absorption Spectroscopy, t...

Ternary Zn(O,Se) films were grown onto glass and p-type Si substrates by pulsed laser deposition at substrate temperature of 500 °C, to investigate their optical properties and energy band structure using UV–Vis and UPS spectroscopic methods. It was found that the 500 nm thick Zn(O,Se) film is nearly transparent in the visible region, with an optic...

Currently, the efficiency of Cu2ZnSnS4 (CZTS) monograin layer solar cells is mainly limited by the recombination losses in the bulk and at the surface of absorber/buffer that need to be reduced. One approach to significantly improve the performance of CZTS monograin layer solar cells is to post-treat the surfaces of monograin powder crystals before...

Silicon dioxide thin films are a promising alternative to protective glass in solar cells. In this work, one-layer and four-layer thin films of a perhydropolysilazane (PHPS) were spin-coatedfrom solution in dibutyl ether and cured by exposure to moderate temperatures or ultraviolet (UV) light to obtain glassy SiOx layers. Optical and morphological...

The morphological and structural changes of dual-phase tungsten specimens with high entropy alloy (HEA) as a second phase (95% W, 5% of FeNiCuCrMn) are investigated after deuterium plasma irradiation. The heat flux factor of deuterium plasma pulses is comparable to transient events (ELM-s and VDE-s) in ITER. The study shows that the use of HEA as a...

Graptolite–argillites (black shales) are studied as potential source of different metals. In the processing technologies of graptolite–argillites, a preceding thermal treatment is often applied. In this study, the thermal behavior of Estonian graptolite–argillite (GA) samples from Toolse, Sillamäe and Pakri areas were studied using a Setaram Labsys...

Reliable energy sources are needed in order to keep a Lunar Base on the run, and solar energy is one of the most attractive options. There are two ways to achieve it – to bring necessary solar panels from the Earth or find a way to produce them in-situ on the Moon from local resources. We propose the monograin layer (MGL) solar cell technology, tha...

Battery technology has been recognized as one of the main driving forces and key technologies required to create a carbon-neutral economy and, therefore, batteries are currently one of the most rapidly growing technology sectors due to the rapid rise in the number of electric vehicles (EV´s). Additionally, the employment of Li-ion batteries (LIBs)...

Metal-free catalysts based on nitrogen-doped carbon are among the most promising replacements for Pt-based materials on polymer electrolyte membrane fuel cell (PEMFC) cathodes due to their inherent stability (lack of metal atoms bypasses most of the degradation mechanisms encountered in metal-based non-precious metal catalysts). However, the produc...

Electrooxidation of methanol is an essential topic in developing direct methanol fuel cells, and numerous catalyst materials have been studied for this application. 1–3 Among other materials, the platinum-based composite catalysts containing rare earth metal oxides on various support materials have shown promising properties for methanol oxidation...

Carbon supported platinum catalysts for proton exchange membrane fuel cell (PEMFC) applications have been studied intensively in the scientific community. 1,2 The catalytic activity of the catalyst depends on the characteristics of the carbon support material ³ and on the Pt depositing method 4,5 . The aim of the study was to investigate the oxygen...

Platinum-praseodymium oxide nanocatalyst deposited onto carbon aerogel support was synthesized. The physical properties of the material were characterized by X-ray diffraction, low-temperature nitrogen adsorption, thermogravimetric analysis, and high-resolution scanning electron microscopy with energy dispersive X-ray analysis. The electrochemicall...

Carbon supported platinum catalysts for proton exchange membrane fuel cell (PEMFC) applications have been studied intensively in the scientific community. The aim of this study was to investigate the oxygen reduction reaction (ORR) on platinum nanoparticles deposited onto peat-derived carbon. The Pt nanoparticles were deposited onto the carbon supp...

The power conversion efficiency of CZTS solar cells is still limited by deep defects, low minority carrier lifetime and high recombination rates at the CZTS/CdS interface. The objective of this study was to find an effective method to reduce interface recombination of CZTS monograin layer solar cells. Two-step heterojunction formation process was a...

This research investigates the catalytic performance of a metal−organic framework (MOF) with a functionalized ligand -UiO-66-NH 2- in the oxidative desulfurization of dibenzothiophene (DBT) in n-dodecane as a model fuel mixture (MFM). The solvothermally prepared catalyst was characterized by XRD, FTIR, 1 H NMR, SEM, TGA, and MP-AES analyses. A resp...

Novel composite self-disinfecting films of polylactic acid (PLA) filled with nanosized particles of double sodium–copper(II) paratungstate B Na2Cu3(CuOH)2[W12O40(OH)2]·32H2O (POM) were developed. The solvent casting (POM/PLA film) and solvent-free melt extrusion methods (Extr. POM/PLA film) were applied for film preparation. The copper (II) ion rel...

Ash melting behaviour is an important issue regarding operation of boilers, for instance, to avoid the agglomeration of the fluidized bed. Previous studies have shown that ash melting behaviour of the blends could differ from that of single parent fuels. This study focuses on the melting behaviour of fuel ash blends of reed, pine wood pellets and D...

In this study, four specimens of pure tungsten, which have been irradiated with a high-temperature plasma with 20, 40, 60, and 80 pulses, respectively, are considered. Scanning electron microscopy (SEM) and optical microscope (OM) images of these specimens are used to search for more suitable degrees of darkness for binarizing the images for multif...

Oxidation has been a long sought-after alternative to classical thermal processing of oil shale, in order to obtain valuable raw materials for the chemical industry. A number of different methods have been applied, but thus far, one of the most effective ways to transform oil shale to value added products, such as aliphatic terminal dicarboxylic ac...

Cu10Cd2Sb4S13 micro-crystals were synthesized by molten salt method (so-called monograin powder growth) where the synthesis and growth of formed crystals proceeded in the isothermal solid-liquid equilibrium conditions in molten cadmium iodide at 480 °C. During cooling down process the oversaturated part of material dissolved in molten flux at synth...

Cutting tools containing superhard phases enable efficient dry machining resulting in reduced human and environmental risks. In the current work, spark plasma sintered composites containing up to 20 vol% cBN in Al2O3-based matrix with an addition of 15 vol% ZrO2 or 10 vol% TiN have been tested under dry sliding (with multiple restarting to better s...

In this study, the Cu2Ge(SexS1-x)3 (x=0, 0.2, 0.4, 0.6, 0.8, 1) microcrystalline powders were synthesized from elemental precursors in the liquid phase of lithium iodide (LiI) flux material in evacuated quartz ampoules. The crystal structure and lattice parameters of the Cu2Ge(SexS1-x)3 (CGSSe) were determined by X-ray diffraction measurement. Resu...

FeS2 monograin powders as absorber materials in monograin layer solar cells were grown in the molten phase of two different flux materials - in liquid sulphur (S) and in potassium iodide (KI) at different temperatures - at 500 °C, 550 °C, 600 °C in S and at 740 °C in KI. The cooling temperature profiles were modified to preserve the pyrite phase of...

The application of conventional glass electrodes for pH measurement in food samples has a serious drawback: glass is fragile and should be handled with care in order to prevent breaking and thus contaminating the food with dangerous shattered fragments. The implementation of all-solid-state sensors allows for pH measurements without this contaminat...

Synthesis of Cd-substituted tetrahedrite Cu10Cd2Sb4S13 (TH-Cd) monograin powders are performed by the molten salt synthesis-growth method using two different fluxes: CdI2 and LiI. The X-ray diffraction (XRD) data of the materials indicates that mainly single phase of tetrahedrite Cu10Cd2Sb4S13 compound is formed in both flux salts. XRD pattern of T...

In this research, the irregular surface structure of 15 pure tungsten specimens that are irradiated respectively with 20, 40, 60, 80, and 100 shots of high-temperature deuterium plasma are under observation. The box-counting method is used to evaluate the scanning electron microscope (SEM) images taken from different locations within the same speci...

Heteroatom doped metal-free catalysts are one of the most promising replacements for platinum for the alkaline oxygen reduction reaction (ORR). Due to the lack of metal atoms, they are extremely stable and environmentally friendly. However, production of carbon nanomaterials can have a very high CO2footprint. In this study, we present ORR catalysts...

Sustainable technologies rely on the development of universal catalyst materials. While a lot of the attention has been given to improving the performance of one single catalyst material for one specific application, there is still a need to find ways to develop catalysts that can simultaneously be utilized for several chemo- and electrocatalytic p...

Frost-retted hemp fibers were investigated to assess their suitability for composite applications. Chemical analysis of frost-retted hemp fibers highlighted a high amount of solubles (pectins) at the fibers surface and a low lignin content in the fibers that was attributed to an unfavorable synthesis of lignin in the cell wall due to the particular...

In modern orthodontics, thermally activated archwires are used more widely in clinical practice, because they have unique properties like superelasticity and bio-compatibility. The aim of the present study was to characterize commercial 35° C Cu-NiTi archwires in terms of their phase transition behavior, chemical composition, surface topography pro...

The near‐band‐edge emission of Sb2Se3 microcrystals is studied in detail under high photoluminescence excitation density using a pulsed UV laser (λ = 266 nm, pulse width 0.6 ns). Based on the peak energy positions and the excitation power density and temperature dependencies (T = 3–110 K) of the photoluminescence spectra, the emission is interprete...

The morphological and structural changes of two different dual-phase tungsten alloy (either 3% or 5% of an added mixture of Fe and Ni) are investigated after deuterium plasma irradiation. Heat flux factor of deuterium plasma pulses is comparable to transient events in ITER. This study compares the changes in tungsten alloy materials between two dif...

The formation of a gradient microstructure in pure tantalum under the wear track of dry sliding wear test was investigated. Materials were exposed to different amounts of Severe Plastic Deformations, and the evolution of the microstructure and mechanical properties as well as the wear behavior were studied in this work. Results showed that the fric...

Employing Li‐ion batteries (LIBs) in portable electronics has become a necessity in the modern world but, due to the short application time for any given battery (1–3 years), the quantity of spent LIBs (SLIBs) waste is becoming substantial. Herein, a novel strategy for recycling SLIB graphite and reforming it as a valuable catalyst material for ele...

Metal–air batteries (MABs) and regenerative fuel cells (RFCs) technologies rely on a combination of efficient electrochemical oxygen evolution and oxygen reduction reactions (OER and ORR, respectively). However, such bifunctional oxygen electrocatalysts also rely on the use of readily accessible carbon supports. Herein, we show that shungite, a nat...

This study aimed to characterize and compare the latest generation multi-force Bio-active™ orthodontic archwires to determine if there is a change in their physicochemical and mechanical properties during orthodontic treatment. The physicochemical characterization of the archwires was done by X-ray diffraction (XRD), scanning electron microscopy, e...

We present temperature and laser power dependent photoluminescence (PL) study of Cd substituted tetrahedrite Cu10Cd2Sb4S13 microcrystals. At T = 10 K a single broad, asymmetric and structureless PL band was detected at about 1.08 eV. The temperature and laser power dependencies indicate that the properties of PL emission can be explained by the dis...

Employing Li-ion batteries (LIBs) in portable electronics has become a necessity in the modern world but, due to the short application time for any given battery (1-3 years) [1], the quantity of spent LIBs (SLIBs) waste is becoming substantial. As LIBs contain different valuable materials (the cathode layer holds Li, Co, Ni, Mn, Al and the anode la...

Zinc oxy-selenide (Zn(O,Se)) is a prospective semiconductor that could be applied as a buffer layer in optoelectronic devices as an alternative for toxic CdS layers. A complete investigation of the effect of changing laser fluences from 4 to 6 J cm⁻² on the phase composition,optical and electrical properties of Zn(O,Se) films prepared by pulsed las...

In this study, the average residual stresses were determined in hard PVD nACRo (nc-AlCrN/a-Si3N4), nACo (nc-AlTiN/a-Si3N4), AlCrN, TiAlN, and TiCN commercial coatings through the deflection of the plate substrates and the simultaneous measurement of length variation in thin-walled tubular substrates. The length measuring unit was used for the measu...

Nickel-titanium (NiTi) alloys, due to their properties of superelasticity and shape memory effect, have been extensively used in orthodontic treatment. Their superelasticity allows the orthodontist to apply an almost continuously low force with larger activation that reduces tissue trauma and patient discomfort, and thus facilitates tooth movement....

The morphology, structural and compositional properties of Cu2CdGe(SxSe1−x)4 (x = 0; 0.05; 0.1; 0.15; 0.2; 0.4; 0.6; 0.8; 1) microcrystalline powders and solar cells based on these powders are investigated by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy as well as current-voltage an...

In this study, TiO2 thin films were deposited by ultrasonic spray pyrolysis from solutions with concentrations of 0.1 and 0.2 M. The deposition temperature was adjusted at 350 °C and all samples were annealed at 500 °C for 1 h in air. The thickness of TiO2 films was changed in the range of 50 to ca. 800 nm by varying the number of spray cycles from...

Al0.1CoCrFeNi high-entropy alloy (HEA) was synthesized successfully from elemental powders by mechanical alloying (MA) and subsequent consolidation by spark plasma sintering (SPS). The alloying behavior, microstructure, and mechanical properties of the HEA were assessed using X-ray diffraction, electron microscope, hardness and compression tests. M...

This paper presents the results of the investigation of stress relaxation at the Si-SiO2 interface using electron paramagnetic resonance (EPR) spectra, scanning electron microscopy (SEM) image technique, deflection analysis, X-ray photoelectron spectroscopy (XPS) and C-V characterisation of metal oxide semiconductor (MOS) structures. It has been sh...

Implementation of layered structures with strong nanoscale optimized interfaces, enables engineering of materials with functional properties. In this work, anisotropic functional multi-layered structures are produced by integration of a thin hybrid inter-layer of graphene-augmented-nanofibers/alumina into α-alumina through an ex-situ strategy of pr...

Noble metal free M-N-C oxygen electrocatalysts are generaly synthesized using inorganic salts, organometallic precursors, nanostructured carbon nanotubes, several dopants and carbonization under modifying environments such as NH 3 or H 2 . Reducing synthesis steps or bringing versatility in post product is quite rare. We have introduced a series of...

Purpose: Most of the orthodontic archwires used in the clinical practice nowadays contain nickel (Ni), however many patients and especially kids are allergic to Ni. One possible Ni free alternative is the Titanium-Niobium (Ti-Nb) archwire. Unfortunately, there is not enough information about its mechanical properties in the literature, especially a...

High quality Cu2CdGeSe4 micro-crystalline powder has been synthesized by molten salt method at 700 °C in closed quartz ampoules using elemental Ge, binary CdSe and CuSe as precursor materials and KI as flux material. The effect of initial Cu and Cd content on the bulk composition of grown crystals was investigated. According to energy dispersive X-...

Keywords: Copper zinc tin sulfur selenide Kesterite Cations substituation Monograins Photoluminescence Electron beam induced current A B S T R A C T The open circuit voltage (V OC) deficit of Cu 2 ZnSn(S,Se) 4 (CZTSSe) kesterite solar cells is higher than that of the closely related Cu(InGa)Se 2 solar cells. One of the most promising strategies to...

Cd-substituted tetrahedrite Cu10Cd2Sb4S13 (TH-Cd) monograin powders were synthesized in molten CdI2 (as flux) with the aim to implement these materials as absorbers in monograin layer solar cells. Our study focuses on the influence of technological parameters, like the initial composition of precursors, the amount of CdI2-flux and temperature of sy...

Highlights • Fibrous electrodes were produced from carbon saturated solution by electrospinning. • Densification of fibrous electrodes has great influence on their electrochemical behavior. • The optimum ratio between carbide-derived carbon, carbon black and polymer gives specific capacitance up to 114 F/g.

Preparation of electrocatalysts often relies on the use of multiple starting materials, with examples arising from a single precursor being less common. We have surveyed a series of heterobivalent scaffolds to identify an iron/benzimidazole-based metal–organic framework as a uniform starting material. By merging the catechol and imidazole units tog...

The objective of this study was to find an effective method to improve VOC without JSC loss for Cu2ZnSnSe4 (CZTSe) monograin layer solar cells. Sulfurization of the surface of the kesterite absorber layer may lead to enhanced device efficiency via band gap widening at the surface. Surface sulfurization was carried out in two steps: a CdS layer was...

Cation substitution in Cu2ZnSn(S,Se)4 offers a path to improved solar cell device performance through the modification of the structural, optical and electrical properties of the absorber material. In the present study, the effect of partial substitution of copper by silver in Cu1.85(Zn0.8Cd0.2)1.1SnS4 monograin powders was investigated. (Cu1-xAgx)...

This study is focused on detail research of the structural and morphological changes in cuprate ceramics Bi1.7Pb0.3Sr2CaCu2Ox (B(Pb)SCCO 2212) and Bi1.7Pb0.3Sr2CuOx (B(Pb)SCCO 2201) during the electrochemical treatment. Their electrochemical reduction behavior is investigated by Cyclic Voltammetry (CV) and Chronopotentiometry (CP) in 7 M KOH. The u...

In the present work, we use a modified Equal Channel Angular Pressing technique for structure and properties change of Tantalum and Niobium at room temperature. The main advantage of this modified technique is the possibility to produce relatively large samples with ultrafine-grained microstructure in all volume of the workpiece by reduced deformat...

This work presents an experimental case study of 200 MW low-pressure steam turbine rotor blades damage after 310 thousand hours of operation and 404 start-ups on the power plant. Damaged blade material was analysed by experimental testing of mechanical properties as well as crack initiation and microstructure in the damaged area by using optical an...

Graphene has been employed as electrode materials in various electrochemical biosensors due to its excellent electrical, mechanical, thermal and optical properties. In the present study, Chemical Vapor Deposited (CVD) and epitaxial graphene on SiC were examined as material for electrochemical biosensing application. The surface of both types of gra...

In this study we determined average residual stresses in hard nitride PVD AlCrN, TiAlN and TiCN coatings through simultaneous measurement of length variation in thin-walled tubular substrates and of the curvature of plate substrates. A device for measurement of the length of the tube was developed. Inside the depositing chamber the tube and the pla...

Fibrous mats were prepared by electrospinning of polyacrylamide-graft-polyaniline copolymers (PAAm-g-PANI) in a mixed solvent of water–dimethylformamide. The grafted copolymers were synthesized by oxidative polymerization of aniline in the presence of polyacrylamide; the effect of aniline/polyacrylamide ratio on the morphology of the fibrous mats a...

div> Preparation of electrocatalysts often relies on the use of multiple starting materials – inorganic salts or organometallic precursors, nanostructured carbon supports, organic additives, dopants and carbonization under modifying atmospheres (e.g. NH₃or H₂) – with the examples of electrocatalysts arising from a single pr...

In this work, the influence of partial substitution (up to 10%) of tin (Sn) by germanium (Ge) in Cu2ZnSn(S,Se)4 (CZTSSe) solid solution monograins was studied. The external quantum efficiency measurements of the CZTGSSe monograin layer solar cells showed an increase in the band gap energy of the Ge-substituted CZTGSSe. According to the current-volt...

The Nickel-Zinc (Ni-Zn) alkaline rechargeable batteries have attracted considerable attention and are considered to be one of the most promising candidates for portable electronics and electric vehicles due to their several advantages including high specific energy densities, low cost, nontoxicity and safety. Various electrolyte additives have been...

Monograin layer (MGL) solar cell technology based on CuIn1-xGaxSe2 (CIGSe) monograin powder crystals is a promising approach for the future low-cost production of flexible solar panels. In this study, CuIn1-xGaxSe2 monograin powders (0 ≤ x ≤ 1) were prepared from binary compounds in the liquid phase of potassium iodide as flux material in evacuated...

As yet results of ageing experiments of Kesterite solar cells have not been published. However, at least in crystalsol detailed investigations of monograin-based solar cells have been performed proving very different stabilities depending on the exact composition of the Kesterites. Completely stable solar cells even without any sealing have been de...

The purpose of this study is to X-ray line-profile analysis of the effect of rapid Joule heating and severe plastic deformation concurrently on microstructure and properties evolution in polycrystalline austenitic Fe-balanced superalloy EP718E, which is Inconel 718 derivative. The microstructure of superalloy at different stages of processing was e...