Alexander Tkach

University of Aveiro | UA · Department of Materials and Ceramics Engineering

Oxide materials are of current interest as high-temperature thermoelectrics in the automotive and manufacturing energy-harvesting sectors due to high chemical robustness, low toxicity and low cost. Driven by a need to improve the thermoelectric performance of n-type oxides to the level of p-type analogues, ceramics and composites based on donor-dop...

Strontium titanate (STO) is a classical high-permittivity perovskite material known since the forties of the last century. Although incipient ferroelectric behaviour of SrTiO3 was thoroughly studied during several decades many new physical phenomena in STO and its solid solutions were predicted theoretically and observed experimentally in last seve...

Controlling the sintering and microstructure of lead-free potassium sodium niobate ((K1-xNax)NbO3, KNN) ceramics is of primary importance to optimize its piezoelectric / ferroelectric properties. However, sintering dense and monophasic KNN remains a challenge. Here, we prepare KNN ceramics using spark plasma texturing (SPT), a modified spark plasma...

First prize of the poster contest -------------------------------------------------------------------------------------- FLASH is an electric field-assisted sintering technique recently proposed to densify materials in a more sustainable, energy reductive and cost-effective way than conventional sintering (CS). FLASH sintering promotes the densifi...

Development of giant-permittivity and high-tunability dielectric materials has attracted great interest because of growing demand for smaller and faster energy-storage and electronic devices. Materials such as CaCu3Ti4O12, displaying the giant dielectric permittivity due to extrinsic Maxwell-Wagner interfacial polarization effect, have previously b...

Ultrafast high-temperature sintering (UHS) is a rapidly growing research area of material science and engineering. Herein we present UHS of gadolinia-doped ceria (GDC) powders in single and multi-step approaches. The sintered ceramics were characterised from a physical and electrochemical point of view. When the power is applied gradually during th...

Harvesting ambient mechanical energy at the nanometric scale holds great promise for powering small electronics and achieving self-powered electronic devices. The current review is focused on kinetic energy harvesters, particularly on flexible piezoelectric nanogenerators (p-NGs) based on barium titanate (BaTiO3) nanomaterials. p-NGs based on nanot...

In a view of the research interest in the high-permittivity materials, continuous enhancement of the dielectric permittivity ε with Zn content was reported for conventionally prepared Sr 1-x Zn x TiO 3 ceramics with x up to 0.009, limited by the solubility of Zn on Sr site. Here, we use a sol-gel technique and a relatively low annealing temperature...

This work reports a systematical study highlighting the impact of substrate induced stress as a key parameter for the electrical performance and phase transition temperatures of (K,Na)NbO 3 polycrystalline thin films.

As a perovskite model for fundamental studies, strontium titanate (SrTiO3, ST) have been attracting an enhanced attention. This chapter reviews the importance of Sr and O vacancies in strontium titanate based materials. First, the perovskite crystal structure of SrTiO3 and its antiferrodistortive phase transition from cubic to centrosymmetric tetra...

Both polyaniline (PANI) and graphene are widely studied for their application as capaci-tive electrodes in energy storage devices. However, although PANI can be easy synthesized, is of low cost and has a higher specific capacitance than graphene, pristine PANI electrodes do not present long-term stability due to their large volume changes during re...

Alternative sintering technologies promise to overcome issues associated with conventional ceramic sintering such as high thermal budgets and CO 2 footprint. The sintering process becomes even more relevant for alkali-based piezoelectric ceramics such as K 0.5 Na 0.5 NbO 3 (KNN) typically fired above 1100 • C for several hours that induces secondar...

The current development of clean and high efficiency energy sources such as solar or wind energy sources has to be supported by the design and fabrication of energy storage systems. Electrochemical capacitors (or supercapacitors (SCs)) are promising devices for energy storage thanks to their highly efficient power management and possible small size...

An alternative sintering methodology, Spark Plasma Texturing (SPT), allowed us to prepare 98.0% dense lead free piezoelectric (K1-xNax)NbO3 (KNN) ceramics with a homogenous microstructure of small grains. In this work, Resonant Ultrasound Spectroscopy (RUS) was used to measure bulk, K, and shear, G, moduli of SPT KNN for comparison with conventiona...

Electrode materials for electrochemical capacitors or supercapacitors (SCs) are widely studied, as they are needed for the development of energy storage devices in electrical vehicles and flexible electronics. In the current work, a self-supported paper of reduced graphene oxide (rGO) with polycarbonate (PC) (as rGO-PC composite) was prepared by si...

The voltage dependence of the dielectric permittivity ε’ and the low dielectric loss tanδ of incipient ferroelectrics have drawn vast attention to the use of these materials for the development of tun-ing elements in electronics and telecommunications. Here, we study the DC electric field depend-ence of low-temperature ε’ in ~320 nm thick sol-gel-d...

The vital role of high-quality-factor (Q) high-frequency (f) dielectric resonators in the growing mi-crowave telecommunication, satellite broadcasting and intelligent transport systems has long mo-tivated the search for new, small size, and lightweight integrated components and packages, prepared by low cost and sustainable processes. One approach...

In this article, a novel study of the design and characterization of barium strontium titanate (BST) thick films produced by electrophoretic deposition (EPD) on flexible platinum foils is reported. Intended for sub-6 GHz radio frequency (RF) applications, a study on film and electrode thickness is conducted. First, a comparison of tunability and qu...

Electrostatically coated abrasives have drawn vast attention in many industrial applications. Therefore, influence of humidity on the electrical properties of α-SiC and α-Al2O3 abrasive pow-ders with three μm-range particle sizes are here investigated using electrostatic charge and DC resistivity analysis. From the three particle size ranges used,...

Strontium titanate-based materials with ferroelectric or relaxor-like properties have drawn vast attention as polar dielectrics for electronics and telecommunications. Here, we study the lattice dynamics in sol–gel-derived Sr1−1.5xBixTiO3 thin films with x = 0.0053 and 0.167, deposited on Al2O3 substrates, using a variable-temperature far-infrared...

Alkali tantalates are encouraging functional materials with versatile properties that make them potential players in microelectronics, photocatalytic processes or medicine. Here, KTaO3 single crystals with 0.27% Li and 0.15% or 0.31% Mn contents determined by inductively coupled plas-ma spectroscopy analysis are studied at low temperatures using di...

Alkaline niobate and tantalate perovskites have attracted attention as polar dielectrics for elec-tronics and telecommunications. Here, we studied the polar behaviour, lattice dynamics, and local structure in conventionally processed K0.985Mn0.015TaO3±δ ceramics using a combination of varia-ble-temperature dielectric and Raman spectroscopies, and X...

SrTiO3-based compounds have been attracting considerable interest both for a wide range of applications and from a fundamental point of view. Fundamentally, perovskite-structured SrTiO3 undergoes a cubic (Pm3m) to tetragonal (I4/mcm) phase transition at ~108 K (Ta) associated with rotations of the O octahedra in antiphase around the [001] direction...

In this paper, the samples preparation, experimental details, and characterization of the Electrophoretic Deposition (EPD) of a thick film Barium Strontium Titanate (BST) capacitor intended for radio-frequency applications are reported. With a bias voltage from 0 V to 20 V, the Ba 0.7 Sr 0.3 TiO 3 capacitor fabricated demonstrates 49% of tunability...

Thanks to the advanced technologies for energy generation such as solar cells and thermo- or piezo-generators the amount of electricity transformed from light, heat or mechanical pressure sources can be significantly enhanced. However, there is still a demand for effective storage devices to conserve electrical energy which addresses the wide range...

In recent years, worldwide research has been focused on clean and sustainable energy sources that can respond to the exponentially rising energy demands of humankind. The harvesting of unused heat in relation to automotive exhaustion, industrial processes, and home heating is one possible way of enabling the transformation from a fossil fuel-based...

There is currently a demand for flexible biomedical sensors. Characteristics as soft, thin, elastic, flexible and comfortable to the contact with skin should be matched with performance and ultimately made of sustainable, and biocompatible materials. In this work, chitosan as a flexible biodegradable polymeric matrix was combined with ferroelectric...

The Front Cover shows a flexible functional biocomposite film composed of chitosan biopolymer and BaTiO3 particles, which was prepared by environmentally benign methodologies. This film is flexible, and its elasticity can be tailored by the addition of BaTiO3 particles. The dielectric permittivity increases with the barium titanate content. The evi...

With this contribution, as a comment to the publication in Journal of Materials Science & Technology 44 (2020) 54, reporting giant dielectric response, structural characterization and numerical simulations in Sr1-1.5xBixTiO3 ceramics, we show that the reported results are rather contradicting and not well analysed, while the suggested mechanism for...

With this contribution, as a comment to the publication in Acta Materialia 164 (2019) 76, reporting point defect structure and giant dielectric response in La-doped strontium titanate ceramics, we show that the reported ceramics should have a strong effect of O vacancies instead of possessing displaced Ti, Ti vacancies and Schottky defects within t...

Supercapacitor performance can be enhanced when the specific capacitance of its electrodes is increased by the electrode fabrication parameters, composition, and structure. The electrode application range can be enlarged from supercapacitors to tribo- or piezo- generators as a part of self-charging power cell for one-step energy conversion and stor...

Invited Talk------------------------------------------------------------------------------------------------------------------Development of giant-permittivity and high-tunability dielectric materials has attracted great interest because of growing demand for smaller and faster energy-storage and electronic devices [1-3]. Materials such as CaCu3Ti4...

The purpose of this selective review is primarily to demonstrate the large versatility of the insulating quantum paraelectric perovskite SrTiO3 explained in "Introduction" part, and "Routes of SrTiO3 toward ferroelectricity and other collective states" part. Apart from ferroelectricity under various boundary conditions, it exhibits regular electron...

Extensive research efforts are dedicated to the development of high voltage aqueous supercapacitors to solve their low energy density concern. Whilst the operating voltage window is governed by decomposition of water (i.e. 1.23 V), this can be overcome by levelling potentials of oxygen and hydrogen evolution reactions. Here we report the design of...

With this contribution, as a comment to the publication in Scripta Materialia 170 (2019) 166, reporting giant dielectric response in (Nb + Zn) co-doped strontium titanate ceramics tailored by atmosphere, we show that the reported ceramics should not be called co-doped due to volatilization of Zn and therefore suggested mechanisms for the giant perm...

A indústria cerâmica tradicional e técnica é um dos principais setores com avultadas emissões de gases de carbono, dada a necessidade de etapas de cozedura a temperaturas elevadas, geralmente superiores a 1000 ºC. Diretrizes recentes da união europeia, materializadas no European Green Deal, indicam que a europa deverá ser um continente neutro em em...

If piezoelectric micro-devices based on K0.5Na0.5NbO3 (KNN) thin films are to achieve commercialization, it is critical to optimize the films’ performance using low-cost scalable processing conditions. Here, sol–gel derived KNN thin films are deposited using 0.2 and 0.4 M precursor solutions with 5% solely potassium excess and 20% alkali (both pota...

Among the lead-free perovskite-structure materials, strontium titanate (SrTiO3 - ST) and potassium tantalate (KTaO3 - KT), pure or modified, are of particular importance. They are both quantum paraelectrics with high dielectric permittivity and low losses that can find application in tunable microwave devices due to a dependence of the permittivity...

Currently much worldwide research is focused on clean and sustainable energy sources that can respond to the rising energy demands of the world. Thermoelectric (TE) harvesting of unused heat in automotive exhaustion, industrial processes and home heating will increase the efficiency of energy use, while simultaneously reducing the CO2 emissions. Al...

Among the lead-free titanate-based materials, strontium titanate (SrTiO3), pure or modified, is of particular importance. SrTiO3 is an incipient ferroelectric with high dielectric permittivity and low losses, which finds application in tunable microwave devices due to a dependence of the permittivity on the electric field, or in grain-boundary barr...

Strontium titanate-based materials with ferroelectric or relaxor-like properties have drawn vast attention for research and wide range of applications in different areas. This chapter summarises firstly the electrical behaviour of Bi-doped strontium titanate ceramics, where three relaxation modes, related to individual hopping of off-centre Bi ions...

Invited Talk -------------------------------------------------------------------------------------------- Low cost scalable processing and substrates are critical for optimized polar dielectric performance of functional oxide thin films if they are to achieve commercialization. Here, a comprehensive investigation of the role low-cost MgO, Al2O3, Sr...

Electrical conductivity and Seebeck coefficient of freestanding paper of graphene oxide (GO) and GO reduced by heat treatment at 180 °C (rGO) are tuned by addition of carbon nanotubes (CNT). The electrical conductivity of rGO with 50% of CNT is raised to ~2940 S/m from ~613 S/m for pure rGO or ~34 S/m for GO with 50% of CNT without reduction. Moreo...

Low cost scalable processing and substrates are critical for optimized polar dielectric performance of functional oxide thin films if they are to achieve commercialization. Here, we present a comprehensive investigation of the role low-cost MgO, Al2O3, SrTiO3 and Si substrates on the structural and electrical properties of sol-gel derived SrTiO3 (S...

The effect of nonstoichiometry (Sr/Ti ratio from 0.995 to 1.02) on the bulk and grain boundary contributions to the electrical response of strontium titanate (STO) ceramics is investigated. Nonstoichiometric STO exhibits lower electrical resistivity than its stoichiometric counterpart. This decrease is systematic for both Ti and Sr-excess and with...

Recently we have demonstrated that out-of-plane dielectric properties of polycrystalline SrTiO3 (ST) films depend on the strain/stress induced by the substrates due to thermal expansion misfit. Here, we present a complementary infrared (IR) spectroscopy investigation of the role of Al2O3 and MgO substrates on the lattice dynamics and in-plane diele...

Obtaining high quality BiFeO3 thin-films by low-cost scalable chemical solution deposition (CSD) is still challenging and currently relevant. Here we provide a comprehensive set of experimental evidences on the role of metallic (Pt) and oxide (IrO2 and LaNiO3) electrode interfaces on the texture development and functional properties improvement of...

If piezoelectric thin films sensors based on K0.5Na0.5NbO3 (KNN) are to achieve commercialization, it is critical to optimize the film performance using low-cost scalable processing and substrates. Here, sol–gel derived KNN thin films are deposited using a solution with 5% of potassium excess on Pt/TiO2/SiO2/Si and Pt/SrTiO3 substrates, and rapid t...

Driven by the need for high-temperature n-type thermoelectrics for energy-harvesting sectors, SrTi0.8Nb0.2O3±δ ceramics are prepared here from powders with average particle sizes of 270 and 800 nm by the conventional sintering in air at 1450 °C and further in H2/N2 at 1400 °C. In contrast to coarse particle powders, the use of fine particle powders...

An alternative low cost flexible processing is utilized to fabricate tunable dielectric thick-films with low loss. By using a combination of electrophoretic deposition with chemical solution deposition, 9 μm-thick BaNd2Ti5O14-Ba0.5Sr0.5TiO3 (BNT-BST) composite films are fabricated with a room-temperature relative permittivity of 287 and a loss tang...

Invited Talk ---------------------------------------------------------------------------------------- The increasing need for clean, sustainable energy sources to meet the exponentially rising energy demands of the world has compelled researchers and scientists worldwide to look for new power generation strategies. The capture of unused heat in aut...

Titanium-rich (Sr/Ti = 0.995) strontium titanate (ST) ceramics, air-sintered in a temperature range of 1400–1625 °C, were reported to possess anomalies in the grain growth and analogous anomalies in the grain boundary (GB) conductivity activation energy. However, these two interface-related phenomena, occurring at GBs, could not be associated with...

Optimizing performance using low cost scalable processing and substrates is critical if functional oxide thin films are to achieve commercialisation. Here, we present a comprehensive investigation of the role low cost Al2O3, SrTiO3, MgO substrates on the structure, microstructure and electrical properties of SrTiO3 (ST) thin films, deposited by sol...

Structural, dielectric and magnetic properties of dense Dy-substituted strontium titanate ceramics are investigated. In the Sr1-1.5xDy x TiO3 system, incorporation of Dy onto the Sr site is confirmed by a linear decrease of the lattice parameter up to x =0.05. Dielectric spectroscopy analysis of Sr1-1.5xDy x TiO3 ceramics reveals four relaxations....

Structural, dielectric and magnetic properties of dense Gd-doped strontium titanate ceramics prepared by the conventional mixed oxide method are studied. Lattice parameter is found to decrease linearly in the Sr1-1.5xGd xTiO3 system, confirming the incorporation of Gd onto the Sr site of the perovskite lattice of strontium titanate up to x = 0.05....

The field angle dependence of the magnetization reversal in 20 nm thick polycrystalline Ni films grown on piezoelectric (0 1 1) [PbMg1/3Nb2/3O3]0.68-[PbTiO3]0.32 (PMN-PT) substrates is analysed quantitatively to study the magnetic anisotropy induced in the film by poling the piezosubstrate. While the PMN-PT is in the unpoled state, the magnetizatio...

Due to stability and lack of toxicity metallic tantalum (Ta), its pentavalent oxide (Ta2O5) and tantalate complex oxides become commercially attractive in a number of technologies. Particularly, alkali tantalates with perovskite-like structures, including potassium tantalate, KTaO3, sodium tantalate, NaTaO3, and lithium tantalate, LiTaO3, are encou...

We report fabrication of reduced graphene oxide (rGO) films using chemical reduction by hydrazine hydrate and rGO paper-like samples using low temperature treatment reduction. Structural analysis confirms the formation of the rGO structure for both samples. Current-voltage (I–V) measurements of the rGO film reveal semiconductor behavior with the ma...

Using terahertz spectroscopy we directly resolved the fundamentals of spin-dependent conductivity in ferromagnetic metals. We quantified the differences in conduction by Fermi-level electrons with opposite spins on the sub-100 fs timescale of electron momentum scattering.

Invited Talk -------------------------------------------------------------------------------------------- Multiferroic materials, combining at least two of three properties: ferromagnetism, ferroelectricity and ferroelasticity in the same phase, have been widely studied during the last years and have tremendous potential for multifunctional applica...

This study reports the magnetic and magnetotransport properties of 20 nm thick polycrystalline Ni films deposited by magnetron sputtering on unpoled piezoelectric (011) [PbMg1/3Nb2/3O3]0.68-[PbTiO3]0.32 (PMN-PT) substrates. The magnetoresistance (MR), as well as the magnetization reversal, is found to depend on the polarization state of the piezosu...

The relation between charge transport - defects and ferroelectric response is established for K0.5Na0.5NbO3 (KNN) and Mn-doped KNN ceramics. At room temperature the conduction in KNN is associated with hole transport and can be suppressed by Mn doping. Because of that a less leaky ferroelectric hysteresis loop is obtained for Mn-doped KNN. At high...

Spin-dependent conduction in metals underlies all modern magnetic memory technologies, such as giant magnetoresistance (GMR). The charge current in ferromagnetic transition metals is carried by two non-mixing populations of sp-band Fermi-level electrons: one of majority-spin and one of minority-spin. These electrons experience spin-dependent moment...

Electrical properties and magnetic properties of graphene oxide and reduced graphene oxide films with addition of Ni nanoparticles were investigated. It is found that reduction of GO by Hydrazine hydrate (HH) and subsequent mixing with Ni NPs yields a higher current (10-5A) compared to the samples where GO is first mixed with Ni NPs and further red...

Strong increase in electrical conductivity of graphene oxide (GO) (I ~10^-9 A) is found by addition of Ni nanoparticles (NiNPs) preliminarily solved by HCl (Nisol) (I ~ 10^-4 A) or powder (Nipow) obtained from this solution (I ~ 10^-6 A), while simply mixing GO with NiNPs an insulator similar to pure GO is obtained. Thus, Nisol and Nipow can be use...

Impedance spectroscopy (IS) is used to compare defects and charge transport in (001) oriented K0.5Na0.5NbO3 (KNN) single crystals and its polycrystalline counterpart. KNN single crystals show lower conductivity than polycrystalline counterpart from room temperature up to 200 °C, but above this temperature polycrystalline KNN displays lower conducti...

The structural and low-frequency dielectric properties of Y-doped strontium titanate ceramics prepared by the conventional mixed oxide method are investigated. The lattice parameter is found to decrease linearly in the Sr1 − 1.5xYxTiO3 system, confirming the incorporation of Y onto the Sr site of perovskite lattice of strontium titanate up to the s...

This study reports the magnetotransport and magnetic properties of 20 nm-thick polycrystalline Ni films deposited by magnetron sputtering on unpoled piezoelectric (011) [PbMg1/3Nb2/3O3]0.68-[PbTiO3]0.32 (PMN-PT) substrates. The longitudinal magnetoresistance (MR) of the Ni films on (011) PMN-PT, measured at room temperature in the magnetic field ra...

We determine the magnetic and magnetotransport properties of 33 nm thick Fe 3 O 4 films epitaxially deposited by rf-magnetron sputtering on unpoled (011) [PbMg 1/3 Nb 2/3 O 3 ] 0.68 − [PbTiO 3 ] 0.32 (PMN-PT) substrates. The magnetoresistance (MR), as well as the magnetization reversal, strongly depend on the in-plane crystallographic direction of...

We observe a giant magnetoresistance effect in CoFe/Cu-based multistack using THz time-domain spectroscopy. The magnetic field-dependent dc conductivity, electron scattering time, as well as spin-asymmetry parameter of the structure are successfully determined.

Oxygen deficient KTaO3 thin films were grown by RF magnetron sputtering on Si/SiO2/Ti/Pt substrates. Room temperature X-ray diffraction shows that they are under a compressive strain of 2.3%. Leakage current results reveal the presence of a conductive mechanism, following Poole-Frenkel formalism. The existence of a polar response below T pol ∼ 367°...

A phenomenon that can be exploited for the manipulation of magnetization without the conventional current-generated magnetic fields is magnetoelastic coupling, which might, thus, pave the way for low-power data-storage devices. Here, we report a quantitative analysis of the magnetic uniaxial anisotropy induced by piezoelectric strain in Ni nanostru...

Structure, microstructure and low-temperature dielectric properties of undoped SrTiO3 (STO) thin films prepared by sol-gel and deposited on Si/SiO2/TiO2/Pt substrates are studied. The effect of annealing temperature and of buffer layers on properties of STO films is analysed. Dielectric permittivity ɛ', relative tunability nr and polarization P are...

We investigate the effects of piezoelectric-generated strain on the magnetization configuration of Ni nanostructures fabricated on pre-poled piezoelectric (011) [Pb(Mg0.33Nb0.66)O3] 0.68–[PbTiO3] 0.32 (PMN–PT) by high resolution X-ray microscopy. We observe a strong uniaxial anisotropy in the Ni nanostructures, due to the relaxation of the substrat...

Deliberately oxygen deficient potassium tantalate thin films were grown by RF magnetron sputtering on Si/SiO2/Ti/Pt substrates. Once they were structurally characterized, the effect of oxygen vacancies on their electric properties was addressed by measuring leakage currents, dielectric constant, electric polarization, and thermally stimulated depol...