Struktura a termoelektrické vlastnosti vrstevnatých kobaltátů

Předložená práce se zabývá přípravou NaxCoO2 výchozím obsahem sodíku x = 1.0, 0.9, 0.8 a 0.7 pomocí Pechiniho metody. Vzorky byly žíhané v teplotním rozsahu od 550 řC do 750 řC. Připravené vzorky byly strukturně charakterizovány pomoci práškové rentgenové difrakce (PXRD). Prvkové složení bylo určeno pomocí atomové absorpční spektroskopie. Seebeckův koeficient a Weissova konstanta byly určeny pro sérii vzorků obsahujících α- a γ- fázi NaxCoO2. PXRD analýza a měření Seebeckova koeficientu ukázalo, že skutečný obsah sodíku ve vrstevnatém kobaltátu je nezávislý na výchozí stechiometrii reakční směsi a reálná stechiometrie sodíku v NaxCoO2 se pohybovala v rozmezí od x= 0.65 do 0.75. Z měření magnetizace byla zjištěna přítomnost spinelové fáze ve vzorcích α- fáze, která také vykazovala nezvykle vysoké hodnoty Weissovy konstanty v porovnání se strukturně podobnou γ- fázi NaxCoO2. Byly rovněž připraveny tenké vrstvy NaxCoO2 s výchozím obsahem sodíku x = 1.0, 0.52, 0.32 a 0.175 na 00l-orientované podložce α-Al2O3 pomocí CSD metody s využitím spin-coating depozice. Byl studován vznik 00l-orientované vrstvy NaxCoO2 a také nová metoda pro přípravu hhh-orientovaných Co3O4 tenkých vrstev. Orientace připravených vrstev byla charakterizována pomocí Ω- křivek a orientace Co3O4 v rovině substrátu byla studována pomocí měření...The synthesis of NaxCoO2 by Pechini method (with initial sodium stoichiometry x = 1.0, 0.9, 0.8, and 0.7) was studied for the samples heat-treated in the temperature range from 550 řC to 750 řC. The structure characterization and phase composition was carried out by the powder X-ray diffraction (XRD) analysis. The elementary analysis was done using atomic absorption spectroscopy. From the thermoelectric properties, the Seebeck coefficient was measured for both α- and γ- phase. The Weiss constant was determined for α- and γ- phase by linear regression of 1/χ. The real sodium content in sodium cobaltate phase was found to be independent on initial sodium precursor weight and measurement by PXRD and Seebeck coefficient showed that the sodium content in NaxCoO2 falls in the range from x = 0.65 to 0.75. The magnetization measurements showed presence of spinel Co3O4 impurities in the samples and the analysis of magnetic susceptibility showed unusual high values of the Weiss constant for α- phase in comparison with structurally similar γ- NaxCoO2. Thin film of NaxCoO2 with initial sodium stoichiometry x = 1.0, 0.52, 0.32, and 0.175 was deposited by chemical solution deposition method using spin-coating deposition on a 00l-oriented α-Al2O3 single crystal substrate. The formation and structure characterization of...Department of Inorganic ChemistryKatedra anorganické chemiePřírodovědecká fakultaFaculty of Scienc

Preparation of CoCr2O4 nanoparticles by the molecular precursors method

Tato práce se zabývá přípravou a charakterizací nanočástic oxidu kobaltnato- chromitého v matrici oxidu křemičitého. Příprava nanokompozitu sestávala z přípravy molekulárního prekurzoru, který byl následně přeměněn pomocí sol-gel procesu na anorganicko-organický xerogel, jenž byl pyrolýzou zbaven své organické části a nakonec žíhán při teplotách do 1300 řC. Molekulární prekurzor byl připraven reakcí dianhydridu kyseliny ethylendiamintetraoctové s (3-aminopropyl)trimetoxysilanem a následnou komplexací iontů Co2+ a Cr3+ a tedy ve své molekule obsahuje jak prekurzor pro nanočástice tak pro matrici oxidu křemičitého. Na systému s Fe3+ ionty byl studován vliv iontů na rychlost kondenzace gelu. Pro sledování komplexace pomocí Uv-Vis spektroskopie byl připraven speciálně modifikovaný molekulární prekurzor, který neobsahoval -Si(OR)3, čímž se předešlo problémům se vznikem gelu a nehomogenity v absorbovaném prostředí. Distribuce kobaltu a chromu v připraveném xerogelu byla studována EDS a ICP-AES. Prášková rentgenová difrakce ukázala, že během pyrolýzy došlo pouze k částečnému zreagování oxidů kobaltu a chromu, což způsobilo, že nanokompozit obsahoval kromě nanočástic CoCr2O4 s délkou koherentně difraktující domeny menší než 5 nm ještě krystality Cr2O3 s velikostí okolo 100 nm. Průběh reakce těchto dvou oxidů byl...This work is focused on preparation and characterization of cobalt chromite nanoparticles embedded in silica matrix. The preparation of nanocomposites is based on preparation of molecular precursor, which is transformed to inorganic-organic xerogel using sol-gel process and subsequently pyrolysed and annealed at temperatures up to 1300 řC. The molecular precursor was prepared by reaction of dianhydride of ethylenediaminetetraacetic acid with (3-aminopropyl)trimethoxysilane and by subsequent complexation of Co2+ and Cr3+ and therefore, it contains both the precursor for nanoparticles and silica matrix. The system with Fe3+ was used to study an influence of the cations on the rate of condensation of the gel. A specially modified molecular precursor was prepared for the investigations of the complexation using UV-Vis spectroscopy. This precursor does not contain -Si(OR)3 groups to avoid the problems with formation of the gel, which would cause the inhomogeneities in the absorbing medium. Cobalt and chromium distribution in the xerogel was studied by EDS and ICP-AES. The results of powder X-ray diffraction showed that the oxides of cobalt and chromium reacted only partialy during the pyrolysis, which caused that nanocomposite contained except for the CoCr2O4 nanoparticles with a length of coherently diffracting...Department of Inorganic ChemistryKatedra anorganické chemieFaculty of SciencePřírodovědecká fakult

Structure and thermoelectric properties of layered cobaltates

The synthesis of NaxCoO2 by Pechini method (with initial sodium stoichiometry x = 1.0, 0.9, 0.8, and 0.7) was studied for the samples heat-treated in the temperature range from 550 řC to 750 řC. The structure characterization and phase composition was carried out by the powder X-ray diffraction (XRD) analysis. The elementary analysis was done using atomic absorption spectroscopy. From the thermoelectric properties, the Seebeck coefficient was measured for both α- and γ- phase. The Weiss constant was determined for α- and γ- phase by linear regression of 1/χ. The real sodium content in sodium cobaltate phase was found to be independent on initial sodium precursor weight and measurement by PXRD and Seebeck coefficient showed that the sodium content in NaxCoO2 falls in the range from x = 0.65 to 0.75. The magnetization measurements showed presence of spinel Co3O4 impurities in the samples and the analysis of magnetic susceptibility showed unusual high values of the Weiss constant for α- phase in comparison with structurally similar γ- NaxCoO2. Thin film of NaxCoO2 with initial sodium stoichiometry x = 1.0, 0.52, 0.32, and 0.175 was deposited by chemical solution deposition method using spin-coating deposition on a 00l-oriented α-Al2O3 single crystal substrate. The formation and structure characterization of..

Magnetic insulators with hexagonal and cubic structure

Title: Magnetic insulators with hexagonal and cubic structure Author: Mgr. Miroslav Soroka Department: Department of Inorganic Chemistry, Faculty of Science, Charles University of Prague Supervisor: RNDr. Václav Tyrpekl, Ph.D. Consultants: Ing. Josef Buršík, CSc., RNDr. Karel Knížek, Dr. Abstract The presented work deals with the preparation of thin films of transition metal oxides by chemical solution deposition method and the study of their physical properties. Thin films of Z-type (Sr3-xBaxCo2Fe24O41) and W-type (SrCo2-xZnxFe16O27) hexaferrites were prepared with the intention of studying magnetic properties, magneto-electric effect (ME) and spin Seebeck effect (SSE). The study of SSE in hexaferrites as a spin-caloritronic phenomenon was extended by the study of SSE in ε-Fe2-xAlxO3 films and the anomalous Nernst effect (ANE) in La0.7Sr0.3CoO3. Several types of deposition precursors were used for the preparation of thin films: metal alkoxides, 2-ethylhexanoic acid salts and metal citrate complexes. Synthetized materials were structurally characterized by powder X-ray diffraction (pXRD). The texture and orientation relation of the prepared films with substrate were characterized by ω-curves and φ-scan measurements. Scanning transmission electron microscopy (STEM) and atomic force microscopy were used to..

Preparation of CoCr2O4 nanoparticles by the molecular precursors method

This work is focused on preparation and characterization of cobalt chromite nanoparticles embedded in silica matrix. The preparation of nanocomposites is based on preparation of molecular precursor, which is transformed to inorganic-organic xerogel using sol-gel process and subsequently pyrolysed and annealed at temperatures up to 1300 řC. The molecular precursor was prepared by reaction of dianhydride of ethylenediaminetetraacetic acid with (3-aminopropyl)trimethoxysilane and by subsequent complexation of Co2+ and Cr3+ and therefore, it contains both the precursor for nanoparticles and silica matrix. The system with Fe3+ was used to study an influence of the cations on the rate of condensation of the gel. A specially modified molecular precursor was prepared for the investigations of the complexation using UV-Vis spectroscopy. This precursor does not contain -Si(OR)3 groups to avoid the problems with formation of the gel, which would cause the inhomogeneities in the absorbing medium. Cobalt and chromium distribution in the xerogel was studied by EDS and ICP-AES. The results of powder X-ray diffraction showed that the oxides of cobalt and chromium reacted only partialy during the pyrolysis, which caused that nanocomposite contained except for the CoCr2O4 nanoparticles with a length of coherently diffracting..

Magnetické izolátory s hexagonální a kubickou strukturou

Název práce: Magnetické izolátory s hexagonální a kubickou strukturou Autor: Mgr. Miroslav Soroka Katedra: Katedra anorganické chemie, PřF UK, Praha Vedoucí doktorské práce: RNDr. Václav Tyrpekl, Ph.D. Konzultanti: Ing. Josef Buršík, CSc., RNDr. Karel Knížek, Dr. Abstrakt Předložená práce se zabývá přípravou tenkých vrstev magnetických izolátorů s hexagonální a kubickou strukturou na bázi oxidů přechodných kovů pomocí metody depozice z kapalné fáze (CSD) a studiem jejich fyzikálních vlastností. Tenké vrstvy hexagonálních feritů typu Z (Sr3-xBaxCo2Fe24O41) a W (SrCo2-xZnxFe16O27) byly připravené se záměrem studovat magnetické vlastnosti, magneto-elektrický jev (ME) a spinový Seebeckův jev (SSE). Studium SSE, jakožto spin-caloritronického fenoménu, na feritech bylo doplněno studiem SSE ve vrstvách ε-Fe2-xAlxO3 a anomálního Nernstova jevu (ANE) v La0.7Sr0.3CoO3. V rámci optimalizace přípravy tenkých vrstev bylo využito více variant CSD procesů založených na různých typech prekurzorů lišících se svými chemickými vlastnostmi: alkoxidy kovů, soli 2-ethylhexanové kyseliny a citrátové komplexy kovů. Připravené materiály byly strukturně charakterizovány pomoci práškové rentgenové difrakce (pXRD). Orientace připravených vrstev byla charakterizována pomocí ω-křivek a měření φ-skenů. Ke studiu mikrostruktury a...Title: Magnetic insulators with hexagonal and cubic structure Author: Mgr. Miroslav Soroka Department: Department of Inorganic Chemistry, Faculty of Science, Charles University of Prague Supervisor: RNDr. Václav Tyrpekl, Ph.D. Consultants: Ing. Josef Buršík, CSc., RNDr. Karel Knížek, Dr. Abstract The presented work deals with the preparation of thin films of transition metal oxides by chemical solution deposition method and the study of their physical properties. Thin films of Z-type (Sr3-xBaxCo2Fe24O41) and W-type (SrCo2-xZnxFe16O27) hexaferrites were prepared with the intention of studying magnetic properties, magneto-electric effect (ME) and spin Seebeck effect (SSE). The study of SSE in hexaferrites as a spin-caloritronic phenomenon was extended by the study of SSE in ε-Fe2-xAlxO3 films and the anomalous Nernst effect (ANE) in La0.7Sr0.3CoO3. Several types of deposition precursors were used for the preparation of thin films: metal alkoxides, 2-ethylhexanoic acid salts and metal citrate complexes. Synthetized materials were structurally characterized by powder X-ray diffraction (pXRD). The texture and orientation relation of the prepared films with substrate were characterized by ω-curves and φ-scan measurements. Scanning transmission electron microscopy (STEM) and atomic force microscopy were used to...Department of Inorganic ChemistryKatedra anorganické chemieFaculty of SciencePřírodovědecká fakult

Characterization of W-type hexaferrite thin films prepared by chemical solution deposition

Thin films of W-type hexaferrite were grown on SrTiO3(111) single crystals using the chemical solution deposition method and metalorganic precursor solutions prepared using the modified Pechini method. The synthesis of single-phase materials required optimizing the initial stoichiometry and crystallization conditions. The results showed that a concurrent excess of alkaline earth metal and deficiency in divalent metal ion (Zn2+, Co2+) favors the synthesis of pure W hexaferrites. This off-stoichiometry suppressed the formation of an exceedingly stable spinel phase at pyrolysis temperatures. The epitaxial nature of thin films, expressed as the orientation relation (00l)W || (111)SrTiO3 and [110] W || [112] SrTiO3 and hence the pronounced anisotropy of their magnetic properties as well were established by three dimensional x-ray diffraction texture analysis and magnetization measurements, respectively and enabled to identify the temperature interval in which the characteristic spin reorientation transition occurs for a specific degree of cobalt substitution.This work was supported by the Czech Science Foundation, Grant No. 19-06433S, by the Research Infrastructure NanoEnviCz (Ministry of Education, Youth and Sports of the Czech Republic, Projects No. LM2018124), by the “NanoCent” Project No. CZ.02.1.01/0.0/0.0/15_003/0000485, ERDF. Authors also acknowledge financial support of H2020-MSCA-RISE-2016 SPICOLOST Grant No. 734187 (TEM analysis, at LMA-INA, University of Zaragoza).Peer reviewe

Observation of anomalously large magnetoelectric coupling in the hexagonal Z-type ferrite films

Although the bulk hexaferrites are known to exhibit strong magnetoelectric (ME) effects near room temperature, the same effects are not realized in a thin film form. Herein, the growth of nearly epitaxial Co2Z-type hexaferrite Ba0.3Sr2.7Co2Fe24O41 (BSCFO) thin films on a SrTiO3 (111) substrate by a chemical solution deposition method is reported. The reciprocal space mapping on the annealed BSCFO films reveals broadening along the qx directions in the (0014) and (0018) peaks, indicating that the oxygen planes belonging to the off-centered octahedra have randomly distributed out-of-plane tilt to indicate significant disorder in the crystallographic orientation. Quantitative investigations reveal that both ME susceptibility and modulated electric polarization (ΔP) by magnetic field up to 370 K in the BSCFO films are always larger than those of the Co2Z-type single crystal by ≈2–15 times and further increase upon having enhanced qx broadening in the (0014) and (0018) peaks. As the off-centered octahedral sites can be a primary source of ΔP as predicted by the p–d hybridization mechanism, these results indicate that effective off-centering could be further enhanced by the orientational disorder. This study identifies the potential of applying the ME hexaferrite films for multifunctional devices.This work was financially supported by the National Research Foundation of S. Korea through 2019R1A2C2090648, 2019M3E4A1080227, and by the Ministry of Education of S. Korea through 2021R1A6C101B418. The works in Czech were supported by the GACR (Czech Science Foundation) Project Nos. 19-06433S and H2020-MSCA-RISE-2016 SPICOLOST under Grant No. 734187 for TEM analysis at LMA-INA, University of Zaragoza, and by the Nanomaterial Center for Advanced Applications(NANOCENT), No. CZ.02.1.01/0.0/0.0/15_003/0000485.Peer reviewe