Magnetoelectric metglas/bidomain y + 140°-cut lithium niobate composite for sensing fT magnetic fields

We investigated the magnetoelectric properties of a new laminate composite material based on y+140°-cut congruent lithium niobate piezoelectric plates with an antiparallel polarized “head-to-head” bidomain structure and metglas used as a magnetostrictive layer. A series of bidomain lithium niobate crystals were prepared by annealing under conditions of Li2O outdiffusion from LiNbO3 with a resultant growth of an inversion domain. The measured quasi-static magnetoelectric coupling coefficient achieved |αE31| = 1.9 V·(cm·Oe)–1. At a bending resonance frequency of 6862 Hz, we found a giant |αE31| value up to 1704 V·(cm·Oe)–1. Furthermore, the equivalent magnetic noise spectral density of the investigated composite material was only 92 fT/Hz1/2, a record value for such a low operation frequency. The magnetic-field detection limit of the laminated composite was found to be as low as 200 fT in direct measurements without any additional shielding from external noises.publishe

Low-frequency magnetic sensing by magnetoelectric metglas/bidomain LiNbO3 long bars

We present an investigation into the magnetic sensing performance of magnetoelectric bilayered metglas / bidomain LiNbO3 long thin bars operating in a cantilever or free vibrating regime and under quasi-static and low-frequency resonant conditions. Bidomain single crystals of Y+128o-cut LiNbO3 were engineered by an improved diffusion annealing technique with a polarization macrodomain structure of the “head-to-head” and “tail-to-tail” type. Long composite bars with lengths of 30, 40 and 45 mm, as well as with and without attached small tip proof masses, were studied. ME coefficients as large as 550 V/cm∙Oe, corresponding to a conversion ratio of 27.5 V/Oe, were obtained under resonance conditions at frequencies of the order of 100 Hz in magnetic bias fields as low as 2 Oe. Equivalent magnetic noise spectral densities down to 120 pT/Hz1/2 at 10 Hz and to 68 pT/Hz1/2 at a resonance frequency as low as 81 Hz were obtained for the 45 mm long cantilever bar with a tip proof mass of 1.2 g. In the same composite without any added mass the magnetic noise was shown to be as low as 37 pT/Hz1/2 at a resonance frequency of 244 Hz and 1.2 pT/Hz1/2 at 1335 Hz in a fixed cantilever and free vibrating regimes, respectively. A simple unidimensional dynamic model predicted the possibility to drop the low-frequency magnetic noise by more than one order of magnitude in case all the extrinsic noise sources are suppressed, especially those related to external vibrations, and the thickness ratio of the magnetic-to-piezoelectric phases is optimized. Thus, we have shown that such systems might find use in simple and sensitive room-temperature low-frequency magnetic sensors, e.g., for biomedical applications.publishe

Magnetoelectrics: Three centuries of research heading towards the 4.0 industrial revolution

Magnetoelectric (ME) materials composed of magnetostrictive and piezoelectric phases have been the subject of decades of research due to their versatility and unique capability to couple the magnetic and electric properties of the matter. While these materials are often studied from a fundamental point of view, the 4.0 revolution (automation of traditional manufacturing and industrial practices, using modern smart technology) and the Internet of Things (IoT) context allows the perfect conditions for this type of materials being effectively/finally implemented in a variety of advanced applications. This review starts in the era of Rontgen and Curie and ends up in the present day, highlighting challenges/directions for the time to come. The main materials, configurations, ME coefficients, and processing techniques are reported.This research was funded by FCT—Fundação para a Ciência e Tecnologia: projects UID/FIS/04650/2019, PTDC/EEI-SII/5582/2014, PTDC/BTM-MAT/28237/2017 and PTDC/EMD-EMD/28159/2017 and grants CEECIND/03975/2017, SFRH/BD/132624/2017 and SFRH/BD/131729/2017; the SpanishState Research Agency (AEI) and the European Regional Development Fund (ERFD): project PID2019-106099RB-C43/AEI/10.13039/501100011033; Basque Government Industry and Education Departments:ELKARTEK, HAZITEK and PIBA (PIBA-2018-06) programs.The authors thank the FCT—Fundação para a Ciência e Tecnologia- for financial supportin the framework of the Strategic Funding UID/FIS/04650/2019 and under projects PTDC/EEI-SII/5582/2014, PTDC/BTM-MAT/28237/2017 and PTDC/EMD-EMD/28159/2017. P.M., A.C.L. and N.P. also support from FCT (forthe contract under the Stimulus of Scientific Employment, Individual Support—2017 Call (CEECIND/03975/2017, forthe SFRH/BD/132624/2017 and for the SFRH/BD/131729/2017 grant, respectively). Finally, the authors acknowledgefunding by the Spanish State Research Agency (AEI) and the European Regional Development Fund (ERFD)through the project PID2019-106099RB-C43/AEI/10.13039/501100011033.and from the Basque Government Industryand Education Department under the ELKARTEK, HAZITEK and PIBA (PIBA-2018-06) programs, respectively

Efeito magnetoelétrico em compósitos baseados em piezoelétricos monocristalinos

Doutoramento em Engenharia FísicaEste trabalho expõe um estudo teórico e experimental das propriedades anisotrópicas magnetoelétricas (ME) em diferentes compósitos contendo monocristais piezoelétricos (PE), maioritariamente sem chumbo na sua composição, com vista a diversas aplicações multifuncionais. Uma descrição linear do efeito ME em termos de campos elétricos, magnéticos e elásticos e constantes materiais é apresentada. Um modelo fenomenológico quasi-estático é usado para ilustrar a relação entre as constantes materiais, sua anisotropia e os coeficientes MEs transversais de tensão e carga. Subsequentemente, este modelo é empregue para estimar o máximo coeficiente ME direto de tensão expectável numa série de compósitos tri-camadas de Metglas/Piezocristal/Metglas em função da orientação do cristal PE. Demonstra-se assim como os efeitos MEs são fortemente dependentes da orientação cristalina, o que suporta a possibilidade de se gerarem coeficientes MEs de tensão elevados em compósitos contendo monocristais PEs sem chumbo como o niobato de lítio (LiNbO3; LNO), tantalato de lítio (LiTaO3), ortofosfato de gálio (GaPO4; GPO), quartzo (SiO2), langatato (La3Ga5.5Ta0.5O14) e langasite (La3Ga5SiO14) através da otimização da orientação cristalina. Uma técnica experimental dinâmica de lock-in para a medição da impedância e efeito ME direto é exposta. O formalismo descritivo desta técnica, assim como um arranjo experimental desenvolvido para o efeito são apresentados. O esquema e características deste, assim como diferentes formas de reduzir o ruído e a indesejável indução mútua são exploradas. Um estudo comparativo do efeito ME direto em compósitos tri-camadas de Metglas e monocristais de LNO e PMN-PT conectados de forma simples é exposto. Embora o PMN-PT possua piezocoeficientes de carga muito superiores aos do LNO, o coeficiente ME direto de tensão demonstrou-se comparável entre ambos os compósitos devido a uma muito menor permitividade dielétrica do LNO. Cálculos teóricos indicam aínda que as propriedades MEs poderão ser significativamente melhoradas (até 500 V/(cm.Oe)) através da otimização do ângulo de corte do LNO, espessura relativa entre camadas ferroelétrica/ferromagnética e uma melhor colagem entre o Metglas e o LNO. Vantagens da utilização do material ferroelétrico LNO em compósitos MEs são discutidas. Num estudo subsequente, as propriedades dinâmicas anisotrópicas de impedância e MEs em compósitos tri-camadas de Metglas e monocristais PEs sem chumbo de LNO e GPO são exploradas. Medições foram realizadas em função do corte de cristal, magnitude e orientação do campo magnético de polarização e frequência do campo de modulação. Coeficientes MEs altamente intensos em certos modos de ressonância são explorados, e a sua relação com as propriedades materiais dos cristais e geometria dos compósitos é investigada. Um coeficiente ME de até 249 V/(cm.Oe) foi aqui observado num compósito com um cristal de LNO com corte 41ºY a 323.1 kHz. Mostramos assim que compósitos multicamadas contendo cristais sem chumbo de LNO e GPO podem exibir efeitos MEs anisotrópicos relativamente elevados. Demonstramos também que o controlo da orientação dos cristais PEs pode em princípio ser usado na obtenção de propriedades MEs anisotrópicas desejáveis para qualquer aplicação. Características únicas como elevada estabilidade química, piezoeletricidade linear e robusteza térmica abrem verdadeiras perspetivas para a utilização de compósitos baseados no LNO e GPO em diversas aplicações. Eventualmente, compósitos bi-camadas contendo lâminas PEs com bidomínios de LNO com corte 127ºY foram estudados tanto teoricamente como experimentalmente. Estas lâminas de LNO possuem uma estrutura de bidomínios com vetores de polarização espontânea opostos ao longo da direção da sua espessura (i.e. uma estrutura de macrodomínios ferroelétricos “head-to-head” ou “tail-to-tail”) Medições de impedância, efeito ME e densidade de ruido magnético equivalente foram realizadas nos compósitos operando sob condições quasi-estáticas e de ressonância. Coeficientes MEs de até 578 V/(cm.Oe) foram obtidos a ca. 30 kHz sob ressonâncias de dobramento usando cristais PEs com 0.5 mm de espessura. Medições de densidade de ruído magnético equivalente demosntraram valores de até 153 pT/Hz1/2 a 1 kHz (modo quasi-estático) e 524 fT/Hz1/2 sob condições de ressonância. É de esperar que uma otimização adicional das técnicas de fabrico, geometria dos compósitos e circuitos de detenção possa permitir reduzir estes valores até pelo menos 10 pT/Hz1/2 e 250 fT/Hz1/2, respetivamente, e a frequência de ressonância em pelo menos duas ordens de grandeza. Estes sistemas poderão assim no futuro ser usados em sensores vetoriais de campo magnético simples e sensíveis, passivos e estáveis e operáveis a elevadas temperaturas.This work presents a theoretical and experimental study of the anisotropic magnetoelectric (ME) properties of differently structured composites featuring piezoelectric (PE) single-crystals, mainly lead-free, for diverse multifunctional applications. A linear description of the ME effects in terms of electric, magnetic and elastic fields and material constants is offered. An averaging quasi-static phenomenological model is used to illustrate the relation between the material constants, their anisotropy and the transversal direct ME voltage and charge coefficients. Subsequently, the aforementioned model is employed in the calculation of the maximum expected direct ME voltage coefficient for a series of tri-layered Metglas/Piezocrystal/Metglas composites as a function of the PE crystal orientation. The ME effects are shown to be strongly dependent on the crystal orientation, which supports the possibility of inducing large ME voltage coefficients in composites comprising lead-free PE single crystals such as lithium niobate (LiNbO3; LNO), lithium tantalate LiTaO3, gallium phosphate (GaPO4; GPO), quartz (SiO2), langatate (La3Ga5.5Ta0.5O14) and langasite (La3Ga5SiO14) through the optimization of the crystal orientation. An experimental dynamic lock-in technique for the measurement of the impedance and direct ME effect is presented. The formalism describing this technique and an implemented custom-made setup are introduced. The scheme and characteristics of the latter as well as ways to reduce the noise and the undesirable mutual induction are explored. A comparative study of the direct ME effect in simply bonded tri-layered laminates of Metglas and LNO and PMN-PT crystals is exposed. Though PMN-PT has much larger charge piezocoefficients than LNO, the direct magnetoelectric voltage coefficient is found to be comparable in both trilayers due to the much lower dielectric permittivity of LNO. Calculations show that the ME properties can be significantly improved (up to 500 V/(cm·Oe)) via an optimization of the cut angle of LNO, relative thickness ratio of the ferroelectric/ferromagnetic layers and a better bonding between Metglas and LNO. Advantages of using the LNO ferroelectric in ME composites are discussed. In a subsequent study, the dynamic impedance and ME anisotropic properties of tri-layered composites of Metglas and single-crystalline lead-free PE of LNO and GPO are explored. Measurements have been performed as a function of the crystal-cut, magnitude and orientation of the magnetic bias field and frequency of the modulated field. Greatly enhanced ME coefficients in certain resonance modes are explored, and their relation to the material properties of the crystals and the geometry of the composites is investigated. The largest ME coefficient of up to 249 V/(cm·Oe) was observed for a composite with a 41ºY-cut LNO crystal at 323.1 kHz. We thus show that multilayers comprising lead-free LNO and GPO crystals can exhibit relatively large anisotropic ME effects. We also demonstrate that the control of the PE crystal’s orientation can in principle be used to obtain almost any desired quasi-static and resonant anisotropic ME properties for any given application. Such unique features as chemical stability, linear piezoelectricity and thermal robustness open up a real perspective of using lead-free LNO and GPO based ME tri-layers in various applications. Eventually, bi-layered composites comprising PE bidomain plates of 127ºY-cut LNO were studied both theoretically and experimentally. The LNO plates possessed an engineered bidomain structure with opposite spontaneous polarization vectors along the thickness direction (i.e. a “head-to-head” or “tail-to-tail” ferroelectric macrodomain structure). Impedance, ME effect and equivalent magnetic noise density measurements have been performed on the composites operating under quasi-static and resonant conditions. ME coefficients of up to 578 V/(cm·Oe) were obtained at ca. 30 kHz at the bending resonance using 0.5 mm thick piezoelectric crystals. Equivalent magnetic noise density measurements yielded values down to 153 pT/Hz1/2 at 1 kHz (quasi-static mode) and 524 fT/Hz1/2 under resonant conditions. A further optimization of the fabrication techniques, laminate geometry and detection circuit is expected to allow reducing these values down to at least 10 pT/Hz1/2 and 250 fT/Hz1/2, respectively, and the resonance frequency by at least two orders of magnitude. Such systems may in future thus find use in simple and sensitive, passive and stable, low-frequency and high-temperature vector magnetic field sensors

Magnetoelectric nanocomposites based on electroactive polymers

Tese de doutoramento em Ciências (especialidade de Física)The magnetoelectric (ME) effect is a physical phenomenon with a wide range of device applications such as computer memories, smart sensors, actuators and high frequency microelectronic devices. There are few single-phase ME materials and most of them show weak ME coupling at room temperature. In order overcome this limitation, composite materials with increased ME effect are being developed. Most of the ME investigations have used as piezoelectric matrix ceramic materials, but ceramic composites may become fragile and are limited by deleterious reactions at the interface regions leading to low electrical resistivities and high dielectric losses, making those ceramic composites not attractive for applications. In this way, new multifunctional Poly(vinylidene fluoride) (PVDF) and copolymers based nanocomposites were produced with magnetostrictive NiFe2O4, CoFe2O4 and Ni0.5Zn0.5Fe2O4 nanoparticles. PVDF and copolymers were used due to their flexibility and high piezoelectric coefficient and ferrite nanoparticles due to their good magnetostrictive properties and distinct magnetic response. The piezoelectric, dielectric, ferroelectric, magnetic and ME properties of the resulting nanocomposites were determined and discussed. It was found that the dispersed ferrite nanoparticles strongly enhanced the nucleation of the -phase of the PVDF matrix, essential for the ME response. The origin of such - phase nucleation was attributed to the electrostatic interactions resulting from the presence of negative nanoparticle surfaces that interact with the polymeric CH2 groups that have positive charge density. It was also verified that macroscopic magnetic and dielectric responses of the composites strongly depend on the ferrite nanoparticle content, with both magnetization and dielectric constant increasing for increasing filler content. The -relaxation in the composite samples was similar to the one observed for -PVDF obtained by stretching. A superparamagnetic behaviour was observed for PVDF/NiFe2O4 composites, whereas PVDF/CoFe2O4 samples show a magnetic hysteresis cycle with coercivity of 0.3 T. Ferroelectric and piezoelectric properties were improved when small amount of CoFe2O4 nanoparticles (up to 7% in weight percent (wt.%)) were added to the P(VDFTrFE) matrix. The highest ME response of 41.3 mV/cm.Oe was found in the P(VDFTrFE)/ CoFe2O4 (28/72 wt.%) composite when a HDC=0.25T was transversely applied to the sample surface and a ME voltage coefficient of 5mV/cm.Oe was obtained at a HDC=0.5T for the PVDF/CoFe2O4 (93/7 wt.%) sample. This ME response for the PVDF based composites was possible after stretching of the samples, which also led to the formation of voids. Direct ME effects up to 1.35 mV/cm.Oe were obtained in a HDC =0,5T, for the P(VDFTrFE)/ Ni0.5Zn0.5Fe2O4 (15/85 wt.% ). P(VDF-TrFE)/Ni0.5Zn0.5Fe2O4 nanocomposites show, as compared to P(VDF-TrFE)/CoFe2O4 nanocomposites, linear and nonhysteretic direct magnetoelectric responses up to 0.5 T. It is in this way, novel polymer based ME composites were produced and characterized in such way that it was demonstrated their suitability for sensor applications.O efeito magnetoeléctrico (ME) é um fenómeno físico que tem uma vasta gama de aplicações de que são exemplo as memórias de computador, sensores inteligentes, atuadores e aparelhos microeletrónicos de alta frequência. Existem muito poucos materiais ME de fase única e a maior parte deles exibem um efeito ME muito baixo à temperatura ambiente. Para ultrapassar esta limitação, estão a ser desenvolvidos materiais compósitos com efeito ME melhorado. Contudo, a maior parte das investigações no âmbito dos materiais ME têm usado como matriz piezoelétrica materiais cerâmicos, estes podem-se tornar frágeis e são limitados por reações deletérias nas interfaces levando a resistividades elétricas muito baixas e a elevadas perdas dielétricas, o que faz com que estes compósitos cerâmicos não sejam atrativos do ponto de vista ads aplicações. Desta forma, novos compósitos multifuncionais baseados no Poli(fluoreto de vinilideno) (PVDF) ou nos seus copolímeros foram produzidos através da incorporação de partículas magnetostrictivas de NiFe2O4, CoFe2O4 e Ni0.5Zn0.5Fe2O4. O PVDF e os seus copolímeros foram utilizados devido à sua flexibilidade e alto coeficiente piezoelétrico. Por sua vez, as nanopartículas de ferrites foram usadas devido às suas propriedades magnetostritivas e resposta magnética distinta. As propriedades piezoelétricas, dielétricas, ferroelétricas, magnéticas e ME dos nanocompósitos resultantes foram determinadas e discutidas. Foi descoberto que as nanopartículas de ferrites dispersas no PVDF melhoravam, significativamente a nucleação da fase do polímero, fase essa que é essencial à resposta ME do compósito.A origem desta nucleação foi atribuída às interações eletrostáticas resultantes da presença de nanopartículas com superfícies negativas que interagiam com os grupos CH2 do polímero que possuem densidade de carga negativa. Verificou-se também que a resposta magnética e dielétrica dos compósitos era fortemente dependente da quantidade de ferrites adicionada, com a magnetização e constante dielétrica a aumentarem com o aumento da quantidade de partículas adicionadas. A relaxação nos compósitos foi similar aquela observada no -PVDF obtido através de estiramento.Foi ainda observado um comportamento superparamagético nos compósitos PVDF/NiFe2O4 enquanto que, nas amostras PVDF/CoFe2O4 observou-se um ciclo de histerese magnética com coercividade de 0.3 T. As propriedades piezoelétricas e ferroelétricas também foram melhoradas quando se adicionaram pequenas quantidades de nanopartículas de CoFe2O4 (até 7 % de percentagem em massa (wt.%)) ao P(VDF-TrFE). A maior resposta ME foi verificada na amostra P(VDF-TrFE)/CoFe2O4 (28/72 wt.%) quando um campo magnético HDC=41.3 mV/cm.Oe foi aplicado transversalmente à superfície da amostra, foi também obtido um coeficiente ME de 5mV/cm.Oe na amostra PVDF/CoFe2O4 (93/7 wt.%) quando se aplicou um HDC=0.5T. Esta resposta ME em amostras baseadas em PVDF foi possível graças ao estiramento da amostra, estiramento esse que também deu origem a vazios dentro do compósito. Foram também obtidas respostas ME diretas até 1.35 mV/cm.Oe na amostra P(VDFTrFE)/ Ni0.5Zn0.5Fe2O4 (15/85 wt.% ). Quando sujeitas a HDC até 0.5T estas amostras mostraram um comportamento linear e sem histerese. Desta forma, novos compósitos ME baseados em polímeros foram produzidos e caracterizados de tal forma que foi demonstrada a sua adequação para aplicações na área dos sensores

Investigation of Nonlinear Piezoelectric Energy Harvester for Low-Frequency and Wideband Applications

This paper proposes a monostable nonlinear Piezoelectric Energy Harvester (PEH). The harvester is based on an unconventional exsect-tapered fixed-guided spring design, which introduces nonlinearity into the system due to the bending and stretching of the spring. The physical–mathematical model and finite element simulations were performed to analyze the effects of the stretching-induced nonlinearity on the performance of the energy harvester. The proposed exsect-tapered nonlinear PEH shows a bandwidth and power enhancement of 15.38 and 44.4%, respectively, compared to conventional rectangular nonlinear PEHs. It shows a bandwidth and power enhancement of 11.11 and 26.83%, respectively, compared to a simple, linearly tapered and nonlinear PEH. The exsect-tapered nonlinear PEH improves the power output and operational bandwidth for harvesting low-frequency ambient vibrations

The influence of mechanical activation on the structure and properties of strontium titanate ceramics

Cilj ove doktorske disertacije je bio da se analizira i sagleda uticaj mehaničke aktivacije na strukturu i svojstva nedopiranog i dopiranog SrTiO3 praha, kao i uticaj na strukturu i svojstva keramike dobijene sinterovanjem ispresaka pomenutih prahova. U slučaju sinterovanih nedopiranih SrTiO3 uzoraka dodatno su razmatrane promene u brzini densifikacije i kinetici sinterovanja. Uticaj mehaničke aktivacije i dopanta na električna svojstva stroncijum-titanatne keramike je analiziran merenjem frekventne zavisnosti relativne dielektrične permitivnosti i tangensa ugla dielektričnih gubitaka na sobnoj temperaturi. Magnetnim merenjima, kod dopiranih SrTiO3 uzoraka dobijenih dodavanjem mangan(IV)-oksida, praćen je uticaj mehaničke aktivacije, koncentracije dopanta i jačine magnetnog polja na vrednost specifične magnetizacije. Mehanička aktivacija SrTiO3 praha je vršena u visoko-energetskom planetarnom mlinu sa kuglama, pri čemu je vreme aktivacije nedopiranog praha iznosilo: 5, 10, 30, 60, 90 i 120 minuta, dok je efekat dopiranja razmatran za aktivaciju od 10, 30 i 120 minuta. U cilju dobijanja SrTiO3:Mn sistema, tipa Sr1-xMnxTiO3 (SMnT) ili SrTi1-xMnxO3 (STMn), u polazni SrTiO3 prah je dodat mangan(IV)-oksid (MnO2), pri čemu su izabrane vrednosti za x bile: 0,03, 0,06 i 0,12. Raspodela veličina čestica je ukazala da je mehanička aktivacija dovela do usitnjavanja čestica polaznog praha, uz istovremenu pojavu šire raspodele veličina čestica pri dužim vremenima aktivacije. Skenirajućom i transmisionom elektronskom mikroskopijom (SEM i TEM) analizirana je mikrostruktura i morfologija polaznog i mehanički aktiviranih prahova. BET metodom je utvrđeno da se najviša vrednost specifične površine kod nedopiranih prahova postiže pri aktivaciji u trajanju od 60 minuta, dok pri daljoj aktivaciji specifična površina ostaje približno konstantne vrednosti, usled povećane sekundarne aglomerizacije. Pokazano je da je gustina ispreska kod praha aktiviranog tokom 10-30 minuta najveća. Zapaženo je sniženje intenziteta XRD pikova sa porastom vremena aktivacije, kao i širenje difrakcionih linija, usled smanjenja veličine kristalita i porasta mikronaprezanja. Primenjena mehanička aktivacija je omogućila formiranje kubnog nanokristalnog SrTiO3 praha, pri čemu veličina kristalita opada i do ~20 nm sa uvećanjem vremena aktivacije. Uočena promena u vrednosti parametra kristalne rešetke a je posredno ukazala i na moguće promene u koncentraciji kiseonikovih vakancija. Analizom Ramanovih spektara je praćen uticaj mehaničke aktivacije na promene u fononskom spektru prahova, sa posebnim akcentom na polarne TO mode, čiji oblik i intenzitet bitno zavise od defekata kao što su kiseonikove vakancije. Analiza optičkih svojstava kristalnih materijala pomoću UV-Vis spektroskopije je ukazala na sniženje energije zabranjene zone sa uvećanjem vremena aktivacije. Primenom dilatometrije je ispitivan uticaj mehaničke aktivacije na početni stadijum sinterovanja dvostrano presovanih SrTiO3 prahova. Uočeno je značajno smanjenje temperature početka skupljanja ispreska, kao i uticaj na brzinu skupljanja i na konačne gustine sinterovanih uzoraka. Primena Dornove metode je ukazala na značajno smanjenje efektivne energije aktivacije transporta mase u početnom stadijumu sinterovanja, sa porastom vremena mehaničke aktivacije polaznog praha. Za uzorke dobijene sinterovanjem do 1300 oC, uz izotermsko zadržavanje na maksimalnoj temperaturi u trajanju od 2h, je izvršena korelacija između zapaženih strukturnih promena i promena u dielektričnim svojstvima, nastalim usled mehaničke aktivacije. Posebna pažnja je posvećena uticaju promena: gustine, poroznosti, veličine zrna i kristalita, kao i uticaju promena parametra kristalne rešetke, na dielektrična svojstva. Promene vrednosti relativne dielektrične permitivnosti su razmatrane i sa stanovišta promena u oblasti granice zrna, posebno imajući u vidu rezultate koji su ukazali na promenu koncentracije kiseonikovih vakancija...The aim of this doctoral dissertation is to analyze and evaluate the effect of mechanical activation on the structure and properties of undoped and doped SrTiO3 powders, as well as the effect on the structure and properties of the ceramics obtained by sintering these powders. For the sintered undoped SrTiO3 samples, changes in the densification rate were also analyzed. The effect of mechanical activation and dopant on the electrical properties of strontium titanate ceramics was analyzed by measuring the frequency-dependent relative dielectric permittivity and the loss tangent at room temperature. Magnetic measurements performed on the doped SrTiO3 samples obtained by the addition of manganese(IV) oxide were used to monitor the influence of mechanical activation, dopant concentration and magnetic field strength on magnetization. The mechanical activation of the SrTiO3 powder was performed in a high-energy planetary ball mill; the activation times of the undoped powder were: 5, 10, 30, 60, 90, and 120 minutes, while the doping effect was analyzed for the activation periods of 10, 30, and 120 minutes. In order to obtain SrTiO3:Mn systems belonging to Sr1-xMnxTiO3 (SMnT) or SrTi1-xMnxO3 (STMn) compound types, manganese(IV) oxide (MnO2) was added to the starting SrTiO3 powder, where the values selected for x were: 0.03, 0.06 and 0.12. The particle size distribution indicated that mechanical activation resulted in the comminution of the starting powder particles, accompanied with a wider particle size distribution at longer activation times. The microstructural and morphological analysis of the starting and mechanically activated powders was performed using scanning and transmission electron microscopy (SEM and TEM). The BET method revealed that 60-minute activation resulted in the highest specific surface area in undoped powders, while further activation led to a relatively constant specific surface due to increased secondary agglomeration. The density of the powder activated for 10-30 minutes was the highest. The intensity of XRD peaks decreased with longer activation times and diffraction lines broadened due to decreased crystallite sizes and increased microstrain. Mechanical activation led to the formation of cubic nanocrystalline SrTiO3 powder, with the minimum crystallite size of 20 nm. The observed change in the value of the crystal lattice parameter indirectly indicated possible changes in the concentration of oxygen vacancies. The analysis of Raman spectra revealed the effect of mechanical activation on the phonon spectrum of powders, with particular emphasis on polar TO modes which shape and intensity generally depend on defects such as oxygen vacancies. The analysis of the optical properties of crystalline materials by UV-Vis spectroscopy indicated a correlation between decreased band gap energy and increased activation times. The effect of mechanical activation on the initial stage of sintering of bilaterally pressed SrTiO3 powders was investigated by dilatometry. A significant decrease in the temperature of the onset of densification was observed, as well as the effect of mechanical activation on the densification rate and on the final density of the sintered samples. The application of the Dorn method indicated a significant decrease in the effective activation energy of mass transport in the initial sintering stage, with a longer time of the mechanical activation of the starting powder. For the samples obtained by sintering up to 1300 °C, with isothermal retention at the maximum temperature for 2 h, a correlation between the structural changes and the changes in dielectric properties resulting from mechanical activation was observed. Particular attention was paid to the effect of changes: density, porosity, grain and crystallite size, as well as the effect of crystal lattice parameter changes on the dielectric properties. The changed values of the relative dielectric permittivity were analyzed in the context of the changes in the grainboundary region, with special respect to the results indicating changes in the oxygen vacancy concentration

Smart Materials and Devices for Energy Harvesting

This book is devoted to energy harvesting from smart materials and devices. It focusses on the latest available techniques recently published by researchers all over the world. Energy Harvesting allows otherwise wasted environmental energy to be converted into electric energy, such as vibrations, wind and solar energy. It is a common experience that the limiting factor for wearable electronics, such as smartphones or wearable bands, or for wireless sensors in harsh environments, is the finite energy stored in onboard batteries. Therefore, the answer to the battery “charge or change” issue is energy harvesting because it converts the energy in the precise location where it is needed. In order to achieve this, suitable smart materials are needed, such as piezoelectrics or magnetostrictives. Moreover, energy harvesting may also be exploited for other crucial applications, such as for the powering of implantable medical/sensing devices for humans and animals. Therefore, energy harvesting from smart materials will become increasingly important in the future. This book provides a broad perspective on this topic for researchers and readers with both physics and engineering backgrounds