Thesis (Ph.D.); Submitted to the University of California at Berkeley, Berkeley, CA (US); 1 Sep 2003. / Published through the Information Bridge: DOE Scientific and Technical Information. "LBNL--55023" Li, Liang-shi. USDOE Director. Office of Science. Office of Basic Energy Sciences (US) 09/01/2003. Report is also available in paper and microfiche from NTIS.

La structure bronze quadratique de tungstène, grâce à sa flexibilité cristallochimique, est une candidate légitime pour le développement de matériaux fonctionnels. L’étude des propriétés diélectriques, pyroélectriques, et ferroélectriques de céramiques de formulation Ba2NdFeNb4-xTaxO15 montre un crossover relaxeur-ferroélectrique-paraélectrique, avec une hystérèse thermique de la transition ferroélectrique. L’étude structurale des monocristaux relaxeurs Ba2LnFeNb4O15 (Ln=La, Pr, Nd, Sm, Eu), obtenus par la méthode du flux, a mis en évidence une structure modulée. L’affinement de la structure de base prouve l’existence de moments dipolaires dans le plan ab (dus à une distorsion des octaèdres [NbO6]). Afin d’établir un lien entre la structure cristalline et les propriétés d’un composé dérivé du multiferroïque MnWO4, la croissance en four à image de Mn0,85Mg0,15WO4 a été entreprise avec succès. / Tetragonal Tungsten Bronze structure, thanks to its compositional flexibility, is a legitimate candidate for the development of functional materials. Study of dielectric, pyroelectric, and ferroelectric properties of Ba2NdFeNb4-xTaxO15 ceramics reveals a relaxor-ferroelectric-paraelectric crossover, with thermal hysteresis of the ferroelectric transition. Structural analysis of Ba2LnFeNb4O15 (Ln=La Pr, Nd, Sm, Eu), relaxor single crystals obtained by flux method, highlighted a modulated structure. Refinement of the basic structure proves the existence of a dipolar moment in the ab plane (induced by a distortion of [NbO6] octahedra). In order to establish a correlation between the crystal structure and the properties of a compound derived from multiferroic MnWO4, the crystal growth in image furnace of Mn0,85Mg0,15WO4 was successfully undertaken.

L'utilisation extensive des combustibles fossiles, l'émission de gaz à effet de serre dans l'atmosphère et les difficultés de recyclage des résidus de consommation incitent au recours accru aux polymères biodégradables. Dans le domaine du génie électrique, de nombreux polymères trouvent des applications dans les systèmes d'isolation des matériels électriques. Ces polymères sont issus du pétrole, peu respectueux de l'environnement et la plupart d'entre eux ne sont pas biodégradables. Dans le but de les remplacer, des polymères biodégradables ont été explorés.Une revue des travaux de recherche sur les polymères biodégradables dans le domaine du génie électrique a été réalisée. Les propriétés thermiques et électriques de polymères biodégradables et conventionnels sont rapportées et comparées.Les polymères biodégradables et / ou biosourcés sélectionnés et mis en œuvre sont le Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), le polycaprolactone (PCL) et l’acide polylactique PLA. Un matériau nanocomposite basé sur PLA a également été préparé. Les propriétés diélectriques pour une large gamme de température et de fréquence ont été mesurées par spectroscopie diélectrique et analysées en s’appuyant sur les propriétés thermiques obtenues par DSC et DMA. La résistivité volumique et la tenue diélectrique ont pu également être mesurées. Ces polyesters biodégradables sont comparés aux polymères conventionnels. / The declining resources of fossil fuels, increase in wide-spread pollution, emission of green-house gases and difficulties in recycling waste materials are pushing biodegradable polymers into prominence. In the domain of electrical engineering, many polymers find applications in various electrical insulation systems. These polymers are petro-based, not eco-friendly and most of them are not biodegradable. With an objective to replace conventional products, biodegradable polymers are explored for their dielectric properties.In this work, a detailed study on the present status in the research work on biodegradable polymers in the electrical engineering domain is presented. Thermal and electrical properties of both biodegradable and classical polymers are compiled and compared.The polymers processed and studied were Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), Polycaprolactone (PCL), Poly(lactic acid) (PLA) and PLA based nanocomposite. Dielectric properties for a wide range of temperature and frequency were measured by dielectric spectroscopy and analyzed with the help of DSC and DMA experiments. Volume resistivity and electrical breakdown were measured on few polymers. These biodegradable polyesters were compared with conventional polymers.

abstract: The optical valley of water, where water is transparent only in the visible range, is a fascinating phenomenon and cannot be modeled by conventional dielectric material modeling. While dielectric properties of materials can be modeled as a sum of Lorentz or Debye simple harmonic oscillators, water is the exception. In 1992 Diaz and Alexopoulos published a causal and passive circuit model that predicted the window of water by adding a “zero shunt” circuit in parallel with every Debye and Lorentz circuit branch. Other than the Diaz model, extensive literature survey yielded no universal dielectric material model that included water or offered an explanation for this window phenomenon. A hybrid phenomenological model of water, proposed by Shubitidze and Osterberg, was the only model other than the Diaz-Alexopoulos model that tried to predict and match the optical valley of water. However, we show that when we apply the requirement that the permittivity function must be a complex analytic function, it fails our test of causality and the model terms lack physical meaning, exhibiting various mathematical and physical contradictions. Left with only the Diaz proposed fundamental model as the only casual model, this dissertation explores its physical implications. Specifically, the theoretical prescription of Kyriazidou et al for creating artificial dielectric materials with a narrow band transparency is experimentally demonstrated for the first time at radiofrequencies. It is proposed that the most general component of the model of the frequency dependent permittivity of materials is not the simple harmonic oscillator but rather the harmonic oscillator augmented by the presence of a zero shunt circuit. The experimental demonstration illustrates the synthesis and design of a new generation of window materials based on that model. Physically realizable Lorentz coatings and RF Debye “molecules” for creating the desired windows material are designed using the full physics computational electromagnetic code. The prescribed material is then implemented in printed circuit board technology combined with composite manufacturing to successfully fabricate a lab demonstrator that exhibits a narrow RF window at a preselected frequency of interest. Demonstrator test data shows good agreement with HFSS predictions. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2017

Submitted by Gabriela Lopes (gmachadolopesufpel@gmail.com) on 2018-07-17T18:53:09Z No. of bitstreams: 1 Tese - Ádamo Araújo.pdf: 1178543 bytes, checksum: 202c2d206529085740d4a2fd31f87d53 (MD5) / Approved for entry into archive by Aline Batista (alinehb.ufpel@gmail.com) on 2018-07-17T21:43:50Z (GMT) No. of bitstreams: 1 Tese - Ádamo Araújo.pdf: 1178543 bytes, checksum: 202c2d206529085740d4a2fd31f87d53 (MD5) / Approved for entry into archive by Aline Batista (alinehb.ufpel@gmail.com) on 2018-07-17T21:46:16Z (GMT) No. of bitstreams: 1 Tese - Ádamo Araújo.pdf: 1178543 bytes, checksum: 202c2d206529085740d4a2fd31f87d53 (MD5) / Made available in DSpace on 2018-07-17T21:49:04Z (GMT). No. of bitstreams: 1 Tese - Ádamo Araújo.pdf: 1178543 bytes, checksum: 202c2d206529085740d4a2fd31f87d53 (MD5) Previous issue date: 2017-03-01 / A automação das operações unitárias de beneficiamento de sementes é uma necessidade real para aperfeiçoar o funcionamento, a operação e o ajuste das máquinas. A massa específica das sementes pode apresentar associação com o vigor, pois sementes com problema de enchimento, atacadas por insetos ou até mesmo quebradas, sofrem redução da massa específica, consequentemente decréscimo de qualidade fisiológica do lote de sementes. O objetivo do trabalho foi desenvolver um sensor capacitivo para medição da massa específica de sementes e analisar a associação entre a permissividade dielétrica relativa e a qualidade fisiológica de sementes beneficiadas em mesa de gravidade. No experimento empregaram-se sementes de soja e de coentro. Foi utilizado a correlação de Pearson para analisar as variáveis de massa específica (), teor de água () e temperatura () com a permissividade dielétrica relativa total (), real (′) e imaginária (′′) nas frequências de 1 kHz, 10 kHz e 100 kHz. A correlação de Pearson foi utilizada para análise da associação entre a permissividade relativa e os atributos fisiológicos e físicos das sementes. A permissividade dielétrica relativa, atributos fisiológicos e físicos foram submetidos a análise de variância e, se significativos, as variáveis foram comparadas pelo teste de Tukey a 5% de probabilidade. Houve alta correlação entre as variáveis com as permissividades relativas analisadas em quase todas as frequências, contudo, há uma melhora na estabilidade dos dados com ao aumento da frequência. O sensor mostrou-se eficiente para a determinação da massa específica das sementes, conhecendo previamente o teor de água e a temperatura das sementes. Pelo do teste de Tukey a permissividade relativa permitiu o ranqueamento das bicas de descarga, semelhante aos atributos fisiológicos e físicos exceto a emergência. A regressão linear indicou que com aumento da frequência melhora na conformidade de valores de permissividade. O sensor mostrou-se relativamente eficiente para a determinação da massa específica. / The automation of unit operations for seed processing is a real necessity to improve the operation, operation and adjustment of the machines. The specific gravity of the seeds may be associated with vigor, because seeds with problems of filling, attacked by insects or even broken, suffer a reduction of the specific gravity, consequently a decrease in the physiological quality of the seed lot. The objective of this work was to develop a capacitive sensor to measure the specific gravity of seeds and analyze the association between the relative dielectric permittivity and the physiological quality of seeds benefited in a gravity table. In the experiment soybean and coriander seeds were used. Pearson correlation was used to analyze the variables of specific gravity (ρ), water content () and temperature () with the total relative (), real (′) and imaginary (ε'') In the frequencies of 1 kHz, 10 kHz and 100 kHz. Pearson correlation was used to analyze the association between relative permittivity and the physiological and physical attributes of the seeds. Relative dielectric permissiveness, physiological and physical attributes were submitted to analysis of variance and, if significant, the variables were compared by the Tukey test at 5% probability. There was a high correlation between the variables with the relative permissivity analyzed at almost all frequencies, however, there is an improvement in the stability of the data with increasing frequency. The sensor was efficient for the determination of the specific gravity of the seeds, previously knowing the water content and the temperature of the seeds. By the Tukey's test the relative permissiveness allowed the rank of discharge nozzles, similar to the physiological and physical attributes except emergency. The linear regression indicated that with increasing frequency, the compliance of permissiveness values improved. The sensor was relatively efficient for determining the specific gravity.

Made available in DSpace on 2016-08-18T18:19:28Z (GMT). No. of bitstreams: 1 FLAVIO MOURA E SILVA JUNIOR.pdf: 1775384 bytes, checksum: b430b5a1969defb4733efc6cf63b21d0 (MD5) Previous issue date: 2010-11-30 / Conselho Nacional de Desenvolvimento Científico e Tecnológico / In this work we investigated the vibrational properties of ceramics Bi2Mn4O10 (BMO) at room temperature by the Raman and Infrared spectroscopies, the phonons were classified by FG matrix method of Wilson. The dielectric properties of the material, in turn, were studied using the impedance spectroscopy in a range of 22 ° C to 275 ° C. Furthermore, static atomistic simulations were performed at BMO in the range 0.0 to 10 GPa with a step of 0.5 GPa. Concerning the Raman modes, ten out forty eight modes predicted by the group theory, were observed in our experimental spectrum while the spectrum of the imaginary part of inverse dielectric constant, we observe 19 LO modes active in infrared. By using the method of FG Wilson saw that the modes in the range of 600-760 cm-1 are stretching into the ab plane to the octahedral chains MnO6. Libration and bending modes were predicted to 400-570 cm-1 and 300-400 cm-1, respectively. It was further observed that the low-frequency modes are mainly due to translations of the Bi ions. The analysis of the impedance spectroscopy measurements led to the conclusion that the relaxing process of BMO are associated with conductive mechanisms, these being due to movement of polarons. The frequencies, for which the imaginary parts of the module (M '' ) and impedance ( Z '' ) exhibit a maximum, obeyed the Arrhenius law in its dependence on the reciprocal temperature, and the calculated activation energies were 0.57 eV and 0.63 eV for each case, respectively. The frequency dependent peaks that appeared in the Bode plot of the real part of dielectric constant were attributed to thermal relaxation processes associated with activated jumps of polarons. The simulations showed that the compound is quite stable, showing no structural phase transition in the pressure range investigated / Neste trabalho foram investigadas as propriedades vibracionais da cerâmica Bi2Mn4O10 (BMO) à temperatura ambiente através das espectroscopias Raman e no Infravermelho. Os fônons foram classificados através do método FG de Wilson. As propriedades dielétricas do material, por sua vez, foram estudadas com o uso da espectroscopia de Impedância num intervalo de 22°C a 275°C. Além disso, simulações atomísticas estáticas foram feitas no BMO no intervalo de 0 a 10 GPa com passo de 0,5 GPa. No que concerne aos modos Raman, dos 48 previstos pela teoria de grupos, apenas dez deles foram observados em nosso espectro experimental enquanto que no espectro da parte imaginária do inverso da constante dielétrica, observamos 19 modos LO ativos no infravermelho. Através do uso do método FG de Wilson vimos que os modos no intervalo de 600-760 cm-1 são estiramento no plano ab para as cadeias octaédricas MnO6. Modos dobramento angular e de libração foram previstos para 400-570 cm-1 e 300-400 cm-1, respectivamente. Constatou-se ainda que os modos de baixa freqüência devem-se principalmente às translações dos íons Bi. As análises das medidas de espectroscopia de Impedância levaram a conclusão de que os processos relaxativos do BMO são associados a mecanismos condutivos, sendo esses devido a movimentos de polarons. As freqüências, para as quais as partes imaginárias do módulo ( M '' ) e da impedância ( Z '' ) exibiram máximos, obedeceram a uma lei tipo Arrhenius em sua dependência com o recíproco da temperatura, sendo que as energias de ativação calculadas foram de 0,57 eV e 0,63 eV para cada caso, respectivamente. Os picos dependentes da freqüência que apareceram no gráfico de Bode da parte real da constante dielétrica foram atribuídos a processos relaxativos termicamente ativados associados a saltos de polarons. As simulações realizadas mostraram que o composto é bastante estável não apresentando transição de fase estrutural no intervalo de pressão investigado.

Made available in DSpace on 2016-08-18T18:19:28Z (GMT). No. of bitstreams: 1 Rafael Mendonca Almeida.pdf: 3974122 bytes, checksum: 09838059334093a825a9917f9ecd4379 (MD5) Previous issue date: 2010-11-26 / Conselho Nacional de Desenvolvimento Científico e Tecnológico / In this work we have investigated the dielectric and ionic properties of the pure Bi2Sn2O7 (BSO) and manganese doped ceramics in the temperature range from 23°C to 326°C. Amongst the main results of the BSO it is worthwhile to point out the PTCR ef-fect when it was analysed the temperature dependence of the electrical resistance. Furthermore, in both samples, the imaginary electrical modulus relaxational peaks are related to conductive processes, but only the BSO shows an anomaly in the tempera-ture range investigated. This anomaly is expressed with a displacement of the peaks to the region of low frequencies, as the temperature rises from the ambient value until 100°C, and after to the region of high frequencies, when the temperature changes from 100°C to 326°C. This dynamic of the data allowed to estimate the phase transition value as being equal to 121.5°C. It was also observed that this transition is possibly ferroic because the capacitance dependence with temperature shows a Curie like behaviour. By other side, the analysis of the real part of the electrical permittivity with temperature highlighted that the BSO phase transition point is located some-where around 125°C, while the one from BSO:Mn is around 120°C. The conductivity dependence with the reciprocal temperature showed an Arrhenius like behaviour and, from the mathematical adjustment at high temperatures, it was possible to estimate an activation energy of 1.26 eV. It was also found that the manganese doping inhibited the effect observed in the graphs of versus frequency, favored the conduction me-chanism of the system and preclude the PTCR effect. Analyzing the temperature de-pendence of the dielectric loss it was realized that this quantity also presents an ano-maly at around 120°C that can be assigned to the structural phase transition. Studying the real part of the admittance as function of the frequency, one could segregate the contributions from the bulk and grain boundary of the system and observe that the response of the last one exhibits a peak at 120°C, probably due to the transi-tion. Regarding the manganese ion substitution in the BSO structure, two hypotheses are proposed. It is believed that in both, the tin ion occurs with an oxidation state equal to 2+ and, but substituting either in the tin ion site or in the bismuth ion site, and releasing oxygen vacancies. / Nesse trabalho, investigamos as propriedades dielétricas e iônicas das cerâmicas Bi2Sn2O7 pura (BSO) e dopado com manganês (BSO:Mn) no intervalo de temperaturas entre 23°C e 326°C. Dentre os principais resultados do BSO, pode-se destacar o efeito PTCR (do inglês, Resistência com Coeficiente Positivo de Temperatura) quando se anali-sou a dependência da resistência elétrica com a temperatura. Além disso, em ambas as amostras, os picos de relaxação do módulo elétrico imaginário ( ) estão relacionados a processos condutivos, mas apenas o BSO exibe uma anomalia no intervalo de tempe-raturas investigadas. Essa anomalia se expressa como o deslocamento dos picos para a região de baixas frequências à medida que a temperatura aumenta do valor ambiente até 100°C e, depois para a região de altas frequências, quando a temperatura varia de 100°C a 326°C. Essa dinâmica dos dados permitiu estimar o valor da transição de fase como sendo igual a 121,5°C. Observou-se também que essa transição é possi-velmente ferróica, pois a dependência da capacitância com a temperatura mostra um comportamento tipo Curie. Por outro lado, a análise da parte real da permissividade elétrica com a temperatura evidenciou que a temperatura de transição do BSO situa-se em algum ponto próximo de 125°C, enquanto que a do BSO:Mn está por volta de 120°C. A dependência da condutividade com a temperatura recíproca mostrou um comportamento tipo Arrhenius e, a partir do ajuste matemático, em altas temperatu-ras, estimou-se uma energia de ativação de 1,26 eV. Ademais se constatou que a do-pagem do BSO com manganês inibiu o efeito observado nos gráficos de versus fre-qüência, facilitou o mecanismo de condução do sistema e impediu o efeito PTCR. Ana-lisando a dependência com a temperatura da perda dielétrica percebe-se que a mesma apresenta uma anomalia por volta de 120°C que pode ser devido à transição de fase estrutural . Estudando a parte real da admitância em função da frequência pu-deram-se separar as contribuições de contorno de grão e bulk do sistema e, observar que a resposta deste último exibe um pico em 120 °C que se deve possivelmente à transição . Quanto à substituição do íon manganês na estrutura do BSO, duas hipóteses são levantadas. Acredita-se que em ambas o íon Mn ocorra no estado de oxidação 2+, mas substituindo ou no sítio do íon de estanho ou no sítio do íon de bis-muto, e liberando vacâncias de oxigênio.

Made available in DSpace on 2016-08-18T18:44:20Z (GMT). No. of bitstreams: 1 Tese-RosivaldoXavierSilva.pdf: 12741226 bytes, checksum: 7bc7743bfa3e2f958b913f2d67969531 (MD5) Previous issue date: 2015-12-11 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The RE2CoMnO6 (RE = La e Y) and Gd(Co0.5Mn0.5)O3 samples were obtained via modified Pechini method (MPM). Raman spectroscopy (RS), Fourier transform infrared (FTIR) spectroscopy, X ray powder diffraction, X ray photoelectron spectroscopy (XPS), SQUID and scanning electron microscopy (SEM) were used to characterize the samples, investigate structural and microstructural evolution, as well as evaluate their vibrational, magnetic and intrinsic dielectric properties. The impact of synthesis conditions on the structural ordering was investigated. We obtained a good control of structural order depending on the annealing temperature for LCMO. We observed an increase in the saturation magnetization, lifetime of the phonons and dielectric constant while suppression of anti-site defects and reduction of dielectric loss. Our investigations on polar phonons by FTIR revealed the extrinsic character of CDC effect on LCMO and clarified the contributions of phonons for dielectric constant in these systems. The temperature dependence Raman spectra of GCMO was investigated between 40 and 300 K and revealed an intriguing spin-phonon coupling, characterized by an increase in the energy of the most intense stretching mode near the magnetic transítion. The correlation between the Raman data and the magnetization suggests that the structure influences the magnitude of the spin-phonon coupling. Correlated analysis of LCMO, GCMO and YCMO systems show that all samples have vibrational properties quite similar. The intrinsic dielectric constants were obtained from the polar phonons dispersive parameters ε intr ̴ 15.8, 17.9 and 16.0, making the contributions to this value explicit, as well as the quality factors, reciprocal of dielectric losses , 𝑄𝑢×𝑓 ≈ 124,74 e 83 THz, extrapolated to microwave region at 10 GHz, to LCMO, GCMO and YCMO, respectively. XPS measures showed that oxidation state for Co and Mn ions are similar each another, being mostly Co2 + and Mn4 + for all investigated systems, however to GCMO, the spin-phonon coupling behavior and losses relatively large indicate that this compound has a high level of structural disordering. / Amostras de RE2CoMnO6 (RE = La e Y) e Gd(Co0.5Mn0.5)O3 foram obtidas pelo método Pechini modificado (MPM). Espectroscopia Raman (RS), espectroscopia no infravermelho por transformada de Fourier (FTIR), difração de raios X (DRX), espectroscopia de fotoelétrons excitados por raios X (XPS), magnetometria e microscopia eletrônica de varredura (MEV) foram utilizados para caracterizar as amostras, investigar evolução estrutural e microestrutural bem como avaliar suas propriedades vibracionais, magnéticas e dielétricas intrínsecas. O efeito da temperatura de tratamento térmico sobre o ordenamento estrutural foi investigado. Um bom controle do ordenamento estrutural em função da temperatura de tratamento térmico para o La2CoMnO6 (LCMO) foi obtido. Observa-se um incremento da magnetização de saturação, aumento do tempo de vida dos fônons, supressão de defeitos de anti-sítio, redução de perdas dielétricas e incremento da constante dielétrica. A investigação dos fônons polares via FTIR revelaram o caráter extrínseco da constante dielétrica colossal (CDC) no LCMO e explicitou as contribuições dos fônons para a constante dielétrica nesses sistemas. A dependência dos espectros Raman com a temperatura do Gd(Co1/2Mn1/2)O3 (GCMO) foi investigada entre 40 e 300 K revelando um acoplamento spin-fônon, caracterizado pelo incremento na energia do modo de estiramento mais intenso próximo à transição magnética. A correlação entre os resultados obtidos pela espectroscopia Raman e a magnetização sugere que a ordem estrutural influencia a magnitude do acoplamento spin-fônon. Análise correlacionada dos sistemas LCMO, GCMO e YCMO, mostram que todas as amostras apresentam características vibracionais bastante semelhantes. A partir dos parâmetros de dispersão dos fônons polares foram obtidas as constantes dielétricas intrínsecas ε intr ̴ 15,8, 17,9 e 16,0, explicitando as contribuições dos fônons para esses valores, e o fator de qualidade, recíproco da perdas dielétricas, 𝑄𝑢×𝑓 ≈ 124,74 e 83 THz, extrapolado para a região de micro-ondas em 10 GHz, para o LCMO, GCMO e YCMO, respectivamente. Medidas de XPS mostraram que os estados de oxidação dos íons de Co e Mn são semelhantes entre si, e principalmente do tipo Co2+/ Mn4+ para todos os sistemas estudados, sendo que para o GCMO, o acoplamento spin-fônon e as perdas dielétricas relativamente maiores indicaram que esse sistema possui elevada desordem estrutural.

Les cristaux liquides sont des matériaux organiques utilisés pour réaliser des dispositifs électroniques ; avant de les intégrer dans des applications, il est nécessaire d'étudier leurs propriétés physico-chimiques et diélectriques pour optimiser leurs performances. Ce travail de thèse est consacré aux nanocolloïdes obtenus par dispersion de nanoparticules ferroélectriques dans un cristal nématique ; il s'agit d'étudier l'influence des inclusions sur les transitions de phases et sur les propriétés diélectriques de la matrice. L'étude des transitions de phases à l'aide de l'Analyse Enthalpique Différentielle (AED) a mis en évidence l'influence des nanoparticules ferroélectriques ; ceci résulte de deux principaux effets ; la polarisation spontanée des nanoparticules et l'ancrage entre les molécules du cristal liquide et les inclusions. La caractérisation diélectrique a révélé le couplage entre la polarisation macroscopique des inclusions et le champ électrique ; ce couplage se manifeste par une augmentation des températures de transitions de phases par rapport à celles déterminées par l'AED. La compétition entre les effets de la polarisation sous champ électrique et de l'ancrage induit une modification des permittivités (parallèle et perpendiculaire) et de l'anisotropie diélectriques. L'utilisation des nanoparticules fortement polaires sélectionnées a confirmé l'importance de la polarisation macroscopique des nanoparticules pour améliorer les propriétés des nanocolloïdes étudiés ; en effet, de très faibles quantités de ces nanoparticules donnent lieu à des améliorations plus significatives que celles obtenues par les nanoparticules brutes. / Liquid crystals are organic materials used to make electronic devices ; before using this material in applications, it is necessary to study their physical-chemical and dielectric properties in order to optimize their performance. This study is devoted to the nanocolloids obtained by dispersing ferroelectric nanoparticles in a nematic liquid crystal ; it means an inclusion influences the phase transitions temperatures and the dielectric properties of the host. The phase transitions measured by using Differential scanning calorimetry (DSC) evidenced the ferroelectric nanoparticles influence ; which is attributed two effects : the nanoparticles spontaneous polarization and the anchoring effect between nanoparticles and liquid crystal. The dielectric characterisation revealed the coupling between the macroscopic polarization of the inclusions and the electric field ; this coupling is manafested by an increase of phase transition temperatures compared to those determined by DSC. The competition between the polarization effect under an electric field and the anchoring effect induces a modification of the permittivities (parallel and perpendicular) and the dielectric anisotropy. Using harvested nanoparticles, the study confirmed the importance of the nanoparticles polarization to increase the properties of the studied nanocolloids. In fact, very small quantity of the harvested nanoparticles presents more significant improvements than those obtained with the non-harvested nanoparticles.

<p style="text-align: justify;">Kroz istoriju, otkrivanje novih materijala i njihovog dizajna dovodilo je do tehnolo&scaron;kih revolucija. U pro&scaron;lom veku, novi materijali naprednih svojstava uveli su elektronske uređaje u svakodnevni život čoveka. Industrija mikročipova predstavlja ogroman deo svetskog trži&scaron;ta, i traži neprestan razvoj da bi pružila bolje proizvode potro&scaron;ačima. Početkom ovog veka, nova grupa materijala, pod nazivom multiferoici, privukla je pažnju naučno-istraživačkog dru&scaron;tva u svetu. Ovi materijali poseduju jedinstvenu karakteristiku da istovremeno ispoljavaju vi&scaron;e od jedne feroične osobine (feroelektričnost, feromagnetizam, feroelastičnost), a &scaron;to je jo&scaron; važnije, mogu da ostvare interakciju među njima. Naime, magnetizacija multiferoika se može postići primenom spolja&scaron;njeg električnog polja, a takođe se mogu i polarisati primenom spolja&scaron;njeg magnetnog polja. Ovo vrlo interesantno svojstvo otvara potencijlanu primenu u oblasti hibridne računarske memorije, senzora, aktuatora, i dr. Sredinom pro&scaron;log veka, jednofazni multiferoici su prvi privukli pažnju, ali poseldnjih godina, kompozitni multiferoici su pokazali bolje rezultate u pogledu funkcionalnih karakteristika. Trend minijaturizacije je takođe prisutan u ovoj oblasti, stoga su multiferoični tanki filmovi vrlo atraktivni u istraživačkih krugovima ne samo zbog niske potro&scaron;nje električne energije, malog utro&scaron;ka meterijala i malih dimenzija, već i zbog dobre magnetoelektrične interakcije.&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Glavni cilj ove doktorske disertacije je bio određivanje optimalnog procesa sinteze/depozicije, i vr&scaron;enje strukturne i funkcionalne karakterizacije multiferoičnih vi&scaron;eslojnih tankih filmova, sačinjenih od naizmenično deponovanih feroelektričnih BaTiO₃ i fero/ferimagnetnih NiFe₂O₄ slojeva (uglavnom na silicijumskim supstratima sa slojem platine). Različite strukture slojeva dizajnirane su u cilju određivanja optimalne, koja bi dala najvi&scaron;e vrednosti magnetoelektričnog efekta.&nbsp;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; U prvom koraku, sintetisani su stabilni solovi/rastvori prekursora, veličine čestica od nekoliko nanometara, reolo&scaron;kih karakteristika pogodnih za depoziciju tehnikama iz tečne faze. Vi&scaron;eslojni filmovi su dobijeni &quot;spin&quot; procesom nano&scaron;enja, pri čemu je termički tretman svakog sloja na 500 &deg;C bio neophodan radi potpunog otparavanja zaostalog rastvarača. Filmovi bez pukotina, ukupne debljine ispod 1 &mu;m, uniformne debljine sloja (60 nm sloj BaTiO₃ i 40 nm sloj NiFe₂O₄) i ravne povr&scaron;ine mogu biti dobijeni sinterovanjem u temperaturnom opsegu od 750 do 900 &deg;C. Strukturna karakterizacija je potvrdila sistem bez prisustva sekundarnih faza, sačinjen od perovskitnog BaTiO₃ i spinelnog NiFe₂O₄. Dielektrična merenja su bila u saglasnosti sa mikrostrukturnom analizom, i vrednostima dielektrične konstante tipične za nanostrukturni sistem, niske vrednosti dielektričnih gubitaka i male provodljivosti. Uticaj međuslojne polarizacije, koja nalikuje Debajeovoj relaksaciji, izražena kroz povećanje dielektrične konstante uspod 100 kHz, bio je jači u nižoj frekventnoj regiji na povi&scaron;enim temperaturama usled termičke aktivacije nosilaca naelektrisanja u feritnoj fazi. Samo čist BaTiO₃ film je pokazao slab feroelektrični histerezis nepotpune saturacije, malo vi&scaron;e polarizacije filma sinterovanog na 900 &deg;C usled ogrubljivanja strukture. Meuđuslojni efekti su takođe primećeni kod feroelektričnih merenja na sobnoj temperaturi, sa izraženijim prisustvom kada se primeni jače električno polje. Na osnovu dielektričnih i feroelektričnih merenja, zaključeno je da film sa debljim titanatnim i tanjim feritnim slojevima ima najverovatnije najbolji dizajn slojeva. Magnetne histerezisne petlje su snimljene na sobnoj temperaturi za čiste NiFe₂O₄ filmove i vi&scaron;eslojne filmove. Analizom vi&scaron;eslojnih filmova različitog dizajna slojeva, pretpostavljeno je da zatezanje nastalo mehaničkom interakcijom između titanatnih i feritnih slojeva jeste prisutno, i da raste sa povećanjem broja kontaktnih povr&scaron;ina, stoga film sa tanjim titanatnim i feritnim slojevima verovatno predstavlja najbolji izbor sa aspekta megnetnih osobina.</p> / <p>Through history, discovery of new materials and material design have led to technological revolutions. In the last century, new materials with advanced properties have introduced electronic devices in our everyday lives. Microchip industry represents one huge part of world market, and needs constant development to provide better products to consumers. In the beginning of this century, a novel group of materials, called multiferroics, have attracted close attention of research society around the world. These materials have a unique property to express more than one ferroic property simultaneously (feroelectricity, ferromagneticity, ferroelasticity), and more important, to achieve coupling between them. Namely, magnetization of multieferroic may be changed with application of external electric field, and they can be polarized with application of the external magnetic field. This is a very interesting property that opens the potential applications in fields of hybrid computer memory, sensors, actuators, etc. In the middle of last century, single phase multiferroics were the first to trigger interest in this special property, but in recent years, composite multiferroics have shown more promising results in terms of functional properties. The trend of miniaturization is also present in this field, so multiferroic thin films are very attractive for research not only because of low power and material consumption or small size, but also because of strong magnetoelectric coupling.<br />The main goal of this thesis was to determine optimal synthesis/deposition process, and perform structural and functional characterization of multiferroic multilayer thin films, composed of ferroelectric BaTiO3 and ferro/ferrimagnetic NiFe2O4 layers in alternating order (mostly on platinum coated silicon substrates). Different layer structures were designed in order to find optimal one which could show the strongest magnetoelectric effect.<br />In the first step, stable precursor sols/solutions were synthesized, with particle size of a few nanometes, and rheological properties suitable for solution deposition. The multilayered thin films were obtained by spin coating and thermal treatment of each layer on 500 &deg;C was necessary in order to completely evaporate traces of residual solvents. Crack free films with overall thickness below 1 &mu;m, uniform single layer thickness (60 nm of BaTiO3 layer and 40 nm of NiFe2O4) and flat surface can be obtained by sintering in temperature range from 750 to 900 &deg;C. Structural characterization confirmed that secondary phase free system with microstructure on nanometer scale was obtained, composed of perovskite BaTiO3 and spinel NiFe2O4. Dielectric measurements were in agreement with microstructural characterization, showing the values of dielectric constant typical for nanostructured system, low values of dielectric losses and low conductivity. The influence of interfacial polarization, resembling Debye behavior, expressed as a rise of dielectric constant below 100 kHz, was stronger in lower frequency range on higher temperatures due to thermal activation of mobile charge carriers in ferrite phase. Only the pure BaTiO3 films showed weak unsaturated ferroelectric hysteresis loops, with slightly higher polarization of films sintered on 900 &deg;C due to coarsening of the structure. The interface effects were also detected in ferroelectric measurements on room temperatures, showing increased presence when higher field is applied. Regarding dielectric and ferroelectric characterization, it was concluded that the multilayered films with thick titanate and thin ferrite layers may probably have the most promising layer design. Magnetic hysteresis loops were recorded on room temperatures for the pure NiFe2O4 and multilayers films. By analysis of different layer design of multilayers, it was assumed that mechanical straining between the ferrite and titanate layers may be present, and increases with the number of contact surfaces, thus the films with thinner titanate and ferrite layers may probably have the best layer design from aspect of magnetic properties.</p>