Person: Schmidt, Rainer
Universidad Complutense de Madrid
Faculty / Institute
Física de Materiales
Now showing 1 - 10 of 27
PublicationStudies of the temperature and frequency dependent impedance of an electroceramic functional oxide NTC thermistor(WILEY-V C H VERLAG GMBH, 2007-11-05) Schmidt, RainerThe charge transport mechanism and the macroscopic dielectric constant in polycrystalline device materials commonly exhibit several components such as electrode-sample interface, grain boundary and bulk contributions. In order to gain precise understanding of the functionality of polycrystalline electroceramic device materials it is essential to deconvolute these contributions. The paradigm of functional NTC thermistor ceramics based on thick film spinel manganates has been studied by temperature dependent alternating current impedance spectroscopy. Three typical relaxation phenomena were detected, which all showed a separated temperature dependence of resistivity consistent with thermally activated charge transport. The dominating grain boundary and the interface contributions exhibited distinctively different capacitance allowing clear identification. The composite nature of the dielectric properties in polycrystalline functional ceramics was emphasized, and impedance spectroscopy was shown to be a powerful tool to account for and model such behavior. PublicationImpedance spectroscopy of epitaxial multiferroic thin films(American Physical Society, 2007-06-14) Schmidt, Rainer; Eerenstein, Wilma; Winiecki, Thomas; Morrison, Finlay D.Temperature dependent impedance spectroscopy enables the many contributions to the dielectric and resistive properties of condensed matter to be deconvoluted and characterized separately. We have achieved this for multiferroic epitaxial thin films of BiFeO_(3) (BFO) and BiMnO_(3) (BMO), key examples of materials with strong magnetoelectric coupling. We demonstrate that the true film capacitance of the epitaxial layers is similar to that of the electrode interface, making analysis of capacitance as a function of film thickness necessary to achieve deconvolution. We modeled non-Debye impedance response using Gaussian distributions of relaxation times and reveal that conventional resistivity measurements on multiferroic layers may be dominated by interface effects. Thermally activated charge transport models yielded activation energies of 0.60± 0.05 eV (BFO) and 0.25± 0.03 eV (BMO), which is consistent with conduction dominated by oxygen vacancies (BFO) and electron hopping (BMO). The intrinsic film dielectric constants were determined to be 320± 75 (BFO) and 450± 100 (BMO). PublicationStructural and dielectric properties of ultra-fast microwave-processed La_0.3Ca_0.7Fe_0.7Cr_0.3O3-delta ceramics(Elsevier, 2022-07-13) Sánchez Ahijón, Elena; Schmidt, Rainer; Martínez de Irujo Labalde, Xabier; Haris Masood, Ansari; Fernández Díaz, María Teresa; Morán, Emilio; Molero Sánchez, Beatriz; Prado Gonjal, JesúsPerovskite La_0.3Ca_0.7Fe_0.7Cr_0.3O_(3-delta) (LCFCr) is a mixed ionic and electronic conductor (MIEC) that can be employed as an electrode material in reversible solid oxide fuel cells (RSOFCs). In this work, an ultra-fast (15 min) one-step microwave (MW)-assisted combustion synthesis route has been developed to obtain phase pure and highly crystalline LCFCr powder. The synthesized powders exhibited a sponge-like microstructure with increased electrochemical reaction sites. Neutron thermodiffraction analysis revealed a structural transition above 500 degrees C from the room temperature (RT) orthorhombic Pnma to a rhombohedral R3c perovskite phase. The oxygen vacancy concentration was found to increase from delta = 0.272(7) at RT to delta = 0.333(5) at 900 degrees C. Furthermore, a 3-dimensional G-type antiferromagnetic structure was detected at RT. MW-sintering of pressed green ceramic pellets was carried out at 950 degrees C for 1 h, using a MW-transparent quartz fiber crucible or alternatively a SiC crucible acting as a MW-absorber. Impedance spectroscopy data on sintered ceramic pellets revealed electronic inhomogeneity as demonstrated by the occurrence of three dielectric relaxation processes associated with two grain boundary (GB)-like contributions and one bulk. The dielectric inhomogeneity encountered may be restricted to the extrinsic GB areas, which may be rather thin. More homogeneous dielectric properties of the GBs were found in the pellet that was sintered in the SiC crucible. Publicationac hopping admittance in spinel manganate negative temperature coefficient thermistor electroceramics(American Institute of Physics, 2008-06-01) Schmidt, Rainer; Brinkman, Andrew W.In this work, the ac admittance of a thick film nickel manganate spinel negative temperature coefficient thermistor ceramic system containing a glass phase is investigated. The dominating relaxation process is a grain boundary (GB) effect and has been investigated comprehensively. We present double-logarithmic plots of the specific admittance σ' vs ω and (σ'/σ_(dc)) vs ω, and specific impedance z vs −z"/ω and [(ρ_(dc)/z')−1] vs ω, in order to characterize GB charge transport. Using the complex admittance notation (σ*), an unusually low Jonscher exponent of frequency ~0.007 was obtained and the GB relaxation displayed close to ideal behavior. PublicationMagnetoimpedance spectroscopy of epitaxial multiferroic thin films(American Physical Society, 2012-07) Schmidt, Rainer; Ventura, Jofre; Langenberg, Eric; Nemes, Norbert Marcel; Munuera, Carmen; Varela, Manuel; García-Hernández, Mar; León Yebra, Carlos; Santamaría Sánchez-Barriga, JacoboThe detection of true magnetocapacitance (MC) as a manifestation of magnetoelectric coupling (MEC) in multiferroic materials is a nontrivial task, because pure magnetoresistance (MR) of an extrinsic Maxwell-Wagner-type dielectric relaxation can lead to changes in capacitance [G. Catalan, Appl. Phys. Lett. 88, 102902 (2006)]. In order to clarify such difficulties involved with dielectric spectroscopy on multiferroic materials, we have simulated the dielectric permittivity ε' of two dielectric relaxations in terms of a series of one intrinsic film-type and one extrinsic Maxwell-Wagner-type relaxation. Such a series of two relaxations was represented in the frequency- (f -) and temperature- (T -) dependent notations ε' vs f and ε' vs T by a circuit model consisting in a series of two ideal resistor-capacitor (RC) elements. Such simulations enabled rationalizing experimental f -, T-, and magnetic field- (H-) dependent dielectric spectroscopy data from multiferroic epitaxial thin films of BiMnO3 (BMO) and BiFeO3 (BFO) grown on Nb-doped SrTiO3. Concomitantly, the deconvolution of intrinsic film and extrinsic Maxwell-Wagner relaxations in BMO and BFO films was achieved by fitting f -dependent dielectric data to an adequate equivalent circuit model. Analysis of the H-dependent data in the form of determining the H-dependent values of the equivalent circuit resistors and capacitors then yielded the deconvoluted MC and MR values for the separated intrinsic dielectric relaxations in BMO and BFO thin films. Substantial intrinsic MR effects up to 65% in BMO films below the magnetic transition (TC ≈ 100 K) and perceptible intrinsic MEC up to −1.5% near TC were identified unambiguously. PublicationDiketonylpyridinium cations as a support of new ionic liquid crystals and ion-conductive materials: analysis of counter-ion effects(MDPI AG, 2016-05) Pastor, María Jesús; Cuerva, Cristián; Campo, José A.; Schmidt, Rainer; Torres, María Rosario; Cano, MercedesIonic liquid crystals (ILCs) allow the combination of the high ionic conductivity of ionic liquids (ILs) with the supramolecular organization of liquid crystals (LCs). ILCs salts were obtained by the assembly of long-chained diketonylpyridinium cations of the type [HOO^(R(n)pyH)] + and BF_(4)^(-) , ReO_(4)^(-), NO_(3)^(-), CF_(3)SO_(3)^(-), CuCl_(4)^(2-) counter-ions. We have studied the thermal behavior of five series of compounds by differential scanning calorimetry (DSC) and hot stage polarized light optical microscopy (POM). All materials show thermotropic mesomorphism as well as crystalline polymorphism. X-ray diffraction of the [HOO^(R(12)pyH)][ReO_(4)] crystal reveals a layered structure with alternating polar and apolar sublayers. The mesophases also exhibit a lamellar arrangement detected by variable temperature powder X-ray diffraction. The CuCl_(4)^(2-) salts exhibit the best LC properties followed by the ReO_(4)^(-) ones due to low melting temperature and wide range of existence. The conductivity was probed for the mesophases in one species each from the ReO_(4)^(-) , and CuCl_(4)^(2-) families, and for the solid phase in one of the non-mesomorphic Cl^(-) salts. The highest ionic conductivity was found for the smectic mesophase of the ReO_(4)^(-) containing salt, whereas the solid phases of all salts were dominated by electronic contributions. The ionic conductivity may be favored by the mesophase lamellar structure. PublicationNon-stoichiometry in "CaCu_(3)Ti_(4)O_(12)" (CCTO) ceramics(RSC, 2013) Schmidt, Rainer; Pandey, Shubhra; Fiorenza, Patrick; Sinclair, Darek C.A combined powder X-ray lattice parameter and ceramic impedance spectroscopy study is presented on materials within the CaO–CuO–TiO_(2) ternary phase diagram. Several compositions containing CaCu_(3)Ti_(4)O_(12) (CCTO) and small amounts of secondary phases such as TiO_(2), CaTiO_(3) and CuO are analysed and two different defect mechanisms are identified as the cause of the non-stoichiometry in CCTO. The first mechanism involves a variation in the Cu content, which explains the large differences in the intrinsic bulk and extrinsic grain boundary (GB) resistance, and the formation of the ceramic internal barrier layer capacitor (IBLC) structure. The second mechanism is associated with Ca–Cu anti-site disorder causing an unusually high intrinsic bulk permittivity above that predicted from Clausius–Mossotti calculations. PublicationDielectric response to the low-temperature magnetic defect structure and spin state transition in polycrystalline LaCoO_(3)(American Physical Society, 2009-03) Schmidt, Rainer; Wu, J.; Leighton, C.; Terry, I.The dielectric and magnetic properties and their correlations were investigated in polycrystalline perovskite LaCoO_(3). The intrinsic bulk and grain-boundary (GB) dielectric relaxation processes were deconvoluted using impedance spectroscopy between 20 and 120 K, and resistivity and capacitance were analyzed separately. A thermally induced magnetic transition from a Co^(3+) low-spin (LS) (S=0;t^(6)_(2g)e^(0)_(g)) to a higher spin state occurs at T_(s1) ≈ 80 K, which is controversial in nature and has been suggested to be an intermediate-spin (IS) state (S=1;t^(5)_(2g)e^(1)_(g)) or a high-spin(HS) state (S=2;t^(4)_(2g)e^(2)_(g)) transition. This spin state transition was confirmed by magnetic-susceptibility measurements and was reflected in the impedance by a split of the single GB relaxation process into two coexisting contributions. This apparent electronic phase coexistence at T > 80 K was interpreted as a reflection of the coexistence of magnetic LS and IS/HS states. At lower temperatures (T ≤ 40 K) perceptible variation in bulk dielectric permittivity with temperature appeared to be correlated with the magnetic susceptibility associated with a magnetic defect structure. At 40 K < T < T_(s1), separated GB and bulk resistivity vs T curves were consistent with localized polaron Mott variable-range hopping (VRH) based on impurity conduction. Below 40 K, a crossover from impurity Mott’s VRH to another type of thermally activated charge transport was detected, which was correlated with the appearance of the defect-related magnetism. PublicationEffect of sintering conditions on microstructure and dielectric properties of CaCu3Ti4O12 (CCTO) ceramics(Springer, 2015-06) Löhnert, Romy; Schmidt, Rainer; Töpfer, JörgThe influence of sintering temperature and dwell time on the microstructure formation and dielectric properties of CaCu_(3)Ti_(4)O_(12) ceramics was investigated. For sintering temperatures of 1050 and 1100 °C significant differences in the CaCu_(3)Ti_(4)O_(12) ceramic microstructure and the segregation of a CuO_(x)-rich phase towards the grain boundary (GB) areas were observed with increasing dwell time. In addition to the formation of a semiconducting bulk and insulating grain boundary phase the segregated CuO_(x) forms an intergranular phase, and the effects of this phase on the dielectric properties are rather intriguing. At sintering temperature below 1050 °C only small amounts of CuO_(x) segregate, whereas sintering above 1050 °C (e.g., 1100 °C) leads to increased evaporation of the CuO_(x). Therefore, the effects of the CuOx-rich intergranular phase upon the dielectric properties are felt strongest in samples sintered at 1050 °C. Such effects are discussed in terms of microstructural variations due to liquid phase sintering behavior facilitated by the TiO_(2)-CuO_(x)-eutectic, which appearsto be melted at high sintering temperature prior to evaporation of CuO_(x) at prolonged dwell times at the highest sintering temperatures(1100 °C). PublicationMicrowave-Assisted Routes for the Synthesis of Complex Functional Oxides(MDPI, 2015-05-12) Prado Gonjal, Jesús de la Paz; Schmidt, Rainer; Morán, EmilioThe synthesis of complex functional inorganic materials, such as oxides, can be successfully performed by using microwave irradiation as the source of heat. To achieve this, different routes and set-ups can be used: microwave-assisted synthesis may proceed in the solid state or in solution, aqueous or not, and the set ups may be as simple and accessible as domestic oven or quite sophisticated laboratory equipment. An obvious advantage of this innovative methodology is the considerable reduction in time—minutes rather than hours or days—and, as a consequence, energy saving. No less important is the fact that the particle growth is inhibited and the broad variety of different microwave or microwave-assisted synthesis techniques opens up opportunities for the preparation of inorganic nanoparticles and nanostructures. In this work, various microwave synthesis techniques have been employed: solid-state microwaves, single-mode microwaves using a TE10p cavity and microwave-assisted hydrothermal synthesis. Relevant examples are presented and discussed.