National Institute Of Materials Physics - Romania

Dielectric spectroscopy is a well-known method to characterize different liquid or solid (nano) materials such as liquid crystals, polymers, composites, ceramics etc. More the material structure is complex, more the molecular dynamics are put in evidence with difficulty. In many such cases, the analysis of the spectra with the classic Havriliak-Negami (H-N) functions is hard to apply or even fails. Therefore, we propose a simple numerical approach (with two different procedures) for the deconvolution of complex spectra in dielectric spectroscopy (DS). The final goal is to obtain the frequency of maximum loss (fmax) depending on the temperatures, Arrhenius diagram, and the activation energy/energies. The developed procedures have as their starting point the "logarithmic derivative of the permittivity". To our knowledge, these procedures have not yet been presented in the literature. The proposed first numerical processing allows a better separation of the different relaxation processes, a decrease in the contribution of the electrical conductivity and a better localization of the frequencies where the dielectric loss has maximum values. The second of the procedures is applied in certain particular situations, and the other procedure is more general. Where the former can be applied, it is equivalent to using the classical H-N functions in terms of obtaining the fmax, but it is simpler to apply. The first procedure uses an "artificial" and simpler H-N function, in which the fewer fitting parameters have no physical meaning, but allow a good localization of the frequency of maximum dielectric loss. The proposed procedure was applied on experimental data of dielectric spectroscopy obtained on pristine materials (nematic mixture E7 and ZnO) and composites E7-ZnO, obtained by a "green technology". The complicated spectra of the dielectric permittivity presented by these samples represent the suitable test to highlight the advantages but also the limitations of our approach. Comparing the values of the activation energy obtained for the two procedures is the criterion for verifying their correctness. The volume of processed experimental data is large, for this reason we present only the results obtained based on numerical approaches. The differences between the results obtained with the two procedures are small. The procedures can be considered as alternatives to the application of fitting with H-N functions. One procedure replaces H-N by eliminating the use of functions with complex variables and the method of least squares and has the advantage that it can apply non-specialized software like Origin. The other allows a better analysis of spectra being an improved-modified H-N approach. Proposed procedures are

The development of new synthetic methods is paramount for the versatile production of complex multicompo-nent electrodes for supercapacitors with enhanced performance. The reactive inverse matrix assisted pulsed laser evaporation (RIMAPLE) technique shows promise for the deposition of complex functional nanocomposites in a facile way. In this work, hybrid supercapacitor electrodes constituted by reduced graphene oxide (rGO) and carbon nanotubes (CNTs) decorated with a myriad of cerium and manganese oxide nanoparticles were obtained by using GO sheets, CNTs, CeO2 nanoparticles and, for the first time, the (NH4)2[Mn2(C6H5O7)2(H2O)2] coor-dination compound as precursors in the RIMAPLE targets. Compositional and structural characterizations revealed that post-annealing treatments at mild conditions (150-250 degrees C) induce a further reduction of the rGO and CNTs, besides an oxidation of the Ce oxide phases (Ce3+ to Ce4+) and a reduction of the Mn oxides (Mn3+ to Mn2+). The substantial change of the carbon and metal oxide nanostructures causes an up to 8-fold increase of the capacitance of the electrodes (63 F/cm3 @ 10 mV/s). Finally, the generation of a high quantity of edge defects at 250 degrees C and long dwell time leads to the drop of the electrode's capacitance.

Hematologic malignancies represent cancer diseases that affect the bone marrow and blood cells and include various subtypes depending mostly on the morphology of the cells. It is well known that an early diagnosis could be very useful for increasing survival rates in cancer, especially in the aggressive forms which can quickly progress to untreatable forms. The development of an easy, fast, and sensitive analytical tool with indicative applications in diagnosis and follow-up care, that could bring benefits in the discovery of new malignant diseases or relapses is reported. Oncofetal antigen/immature laminin receptor protein (OFA/iLRP) is an immunogenic protein found in fetal cells as well as overexpressed on the surface of some malignant tumors, including some hematologic malignancies. An aptamer, AB3, was selected and reported in the literature, having as a target the immature laminin receptor protein. Using the AB3 aptamer and its affinity to immature laminin receptor protein-positive cells an aptasensor was developed and tested on Jurkat cells. For the immobilization of the aptamer, graphene oxide modified screen printed electrodes were used and subjected to an activation procedure. The aptasensor development and cell caption were evaluated using electrochemical methods as well as microscopic techniques. The limit of detection of the developed aptasensor was 3.3 x 10(3) cells mL(-1), meaning 16 cells in the 5 mu L of suspension tested.

Herewith, we investigated the capacitive properties of hybrid nanometric architectures consisting of tungsten oxide nanoparticles deposited on graphite (WOx/G) and graphite coated with highly-porous carbon nanowall layers (WOx/CNW/G). The deposition of the nanostructured oxide was performed by a plasma-assisted sublimation, vapor transport and condensation technique. The samples containing the CNW interlayer show superior areal capacitances making this architecture and its fabrication route promising approaches for supercapacitor applications. Thus, areal capacitances of WOx/CNW/G and WOx/G, estimated from galvanostatic chargedischarge tests (GCD) at an applied density current of 1 mA cm-2, are 1247.2 mF cm-2 and 527.3 mF cm-2, respectively. From SEM and XPS investigations, it was observed that CNW promotes the formation of a high amount of small tungsten oxide particles, homogeneously distributed on electrode surface, and facilitates the formation of highly conductive tungsten nitride species, resulting in electrodes with remarkable charge-storage capacity.

This study focuses on the properties of the BiOi (interstitial Boron-interstitial Oxygen) and CiOi (interstitial Carbon-interstitial Oxygen) defect complexes by 5.5MeV electrons in low resistivity silicon. Two different types of diodes manufactured on p-type epitaxial and Czochralski silicon with a resistivity of about 10 Omega center dot cm were irradiated with fluence values between 1x10(15) cm(-2) and 6x10(15) cm(-2). Such diodes cannot be fully depleted and thus the accurate evaluation of defect concentrations and properties (activation energy, capture cross-section, concentration) from Thermally Stimulated Currents (TSC) experiments alone is not possible. In this study we demonstrate that by performing Thermally Stimulated Capacitance (TS-Cap) experiments in similar conditions to TSC measurements and developing theoretical models for simulating both types of BiOi signals generated in TSC and TS-Cap measurements, accurate evaluations can be performed. The changes of the position-dependent electric field, the effective space charge density N-eff profile as well as the occupation of the BiOi defect during the electric field dependent electron emission, are simulated as a function of temperature. The macroscopic properties (leakage current and..eff) extracted from current-voltage and capacitance-voltage measurements at 20 degrees C are also presented and discussed.

Because of their significant magnetocaloric properties, Ca and Pb-doped LaMnO3 can be employed in magnetic refrigeration systems. The partial substitution effect of Ca2+ by Pb2+ cations on the structural, magnetocaloric properties, and critical behavior of mixed valence perovskites La0.8Ca0.2-xPbxMnO3 (0 <= x <= 0.15), synthesized by the flash combustion route, was studied in this work. The presence of structural transformations of the prepared ceramics from an orthorhombic structure (Pbnm space group) for samples with x <= 0.05 to a rhombohedral system (R (3) over barc space group) for x >= 0.1 was detected by X-ray diffraction analysis, which is related to the A-site disorder. The field-cooled (FC) magnetization indicated a second-order ferromagnetic-paramagnetic transition for the investigated samples when the temperature was increased. Due to the change in the Mn3+/Mn4+ ratio, substituting Ca2+ with larger Pb2+ ions increases the Curie temperature (T-C) by increasing Pb doping content from 90 K for x = 0 to 205 K for x = 0.15. The magnetization isotherms of all samples showing a second-order phase transition were studied, where an applied magnetic field of 5 T, the value of maximum magnetic entropy variation /Delta S-M(max)/ for the sample with x = 0.15 was estimated to be 2.73 Jkg(-1) K-1. In addition, it has a significant relative cooling power of about 276 Jkg(-1) under the same applied magnetic field strength. The critical behavior of the samples was investigated close to T-C. The exponent values were identical to the mean-field method, indicating a large disorder of spin magnetic moments in the perovskite compounds.

Recently, Kittel's theory of ferromagnetic domains in thin films in one dimension, i. e. with the domains extended infinitely over one in-plane direction and with the anisotropy axis oriented perpendicular to the film was revisited by considering the film thickness and unbalanced domains with up and down magnetization, yielding to the computation of magnetic hysteresis [C. M. Teodorescu, Res. Phys. 46 (2023) 106287]. In this work, the above study is extended to samples featuring two-dimensional domain landscapes, for materials with strong magnetic anisotropy, typically characterized by a superunitary ratio between the anisotropy energy and the stray field energy densities. This allows one to compute the most stable structures for vanishing average magnetization < M > together with hysteresis curves for thin films with perpendicular magnetic anisotropy and two-dimensional rectangular domains and also for thin films with in-plane magnetic anisotropy. For two-dimensional films with perpendicular magnetic anisotropy, the most stable structure for < M > = 0 is found to be that with domains infinitely elongated along one in-plane direction, i. e. the one-dimensional case treated in the preceding work. For thin stripes with in-plane magnetization, the domain size l is approximately linear with the stripe lateral size d for low film thickness, while for large film thicknesses it follows a Kittel-like law, but as function of the stripe size l similar to d(1/2). For in-plane magnetized thin films of infinite lateral extent, the most stable structure is the single domain. As for hysteresis curves, the two-dimensional case with perpendicular magnetic anisotropy is shown to evolve from a 2D landscape derived from the checkerboard structure, but with unbalanced domains for magnetization near saturation, towards one-dimensional domain structures for lower magnetization. In some cases and depending also on the demagnetization factor, the one-dimensional case is not reached, and the film exhibit 2D structures on the whole range of the magnetization curve. The hysteresis obtained for thin magnetic stripes with in-plane magnetization also can exhibit a rich structure, with minor cycles on the wings of the magnetization curves evolving towards ,,normal" hysteresis (again, depending on the film thickness, stripe lateral size and demagnetization factor). An infinite thin film with in-plane anisotropy features a steplike magnetization dependence on the applied field.

Fine-tuning of grain sizes can significantly influence the interaction between different dielectric phenomena, allowing the development of materials with tailored dielectric resistivity. By virtue of various synthesis mechanisms, a pathway to manipulate grain sizes and, consequently, tune the material's dielectric response is revealed. Understanding these intricate relationships between granulation and dielectric properties can pave the way for designing and optimizing materials for specific applications where tailored dielectric responses are sought. The experimental part involved the fabrication of dense BCT-BZT ceramics with different grain sizes by varying the synthesis (conventional solid-state reaction route and sol-gel) and consolidation methods. Both consolidation methods produced well-crystallized specimens, with Ba0.85Ca0.15O3Ti0.9Zr0.1 (BCTZ) perovskite as the major phase. Conventional sintering resulted in microstructured and submicron-structured BCT-BZT ceramics, with average grain sizes of 2.35 mu m for the solid-state sample and 0.91 mu m for the sol-gel synthesized ceramic. However, spark plasma sintering produced a nanocrystalline specimen with an average grain size of 67.5 nm. As the grain size decreases, there is a noticeable decrease in the maximum permittivity, a significant reduction in dielectric losses, and a shifting of the Curie temperature towards lower values.

In this study, a sol-gel film based on lead sulfide (PbS) quantum dots incorporated into a host network was synthesized as a special nanostructured composite material with potential applications in temperature sensor systems. This work dealt with the optical, structural, and morphological properties of a representative PbS quantum dot (QD)-containing thin film belonging to the Al2O3-SiO2-P2O5 system. The film was prepared using the sol-gel method combined with the spin coating technique, starting from a precursor solution containing a suspension of PbS QDs in toluene with a narrow size distribution and coated on a glass substrate in a multilayer process, followed by annealing of each deposited layer. The size (approximately 10 nm) of the lead sulfide nanocrystallites was validated by XRD and by the quantum confinement effect based on the band gap value and by TEM results. The photoluminescence peak of 1505 nm was very close to that of the precursor PbS QD solution, which demonstrated that the synthesis route of the film preserved the optical emission characteristic of the PbS QDs. The photoluminescence of the lead sulfide QD-containing film in the near infrared domain demonstrates that this material is a promising candidate for future sensing applications in temperature monitoring.

A novel high-entropy perovskite powder with the composition Bi0.2K0.2Ba0.2Sr0.2Ca0.2TiO3 was successfully synthesized using a modified Pechini method. The precursor powder underwent characterization through Fourier Transform Infrared Spectroscopy and thermal analysis. The resultant Bi0.2K0.2Ba0.2Sr0.2Ca0.2TiO3 powder, obtained post-calcination at 900 C-degrees, was further examined using a variety of techniques including X-ray diffraction, Raman spectroscopy, X-ray fluorescence, scanning electron microscopy, and transmission electron microscopy. Ceramic samples were fabricated by conventional sintering at various temperatures (900, 950, and 1000 C-degrees). The structure, microstructure, and dielectric properties of these ceramics were subsequently analyzed and discussed. The ceramics exhibited a two-phase composition comprising cubic and tetragonal perovskites. The grain size was observed to increase from 35 to 50 nm, contingent on the sintering temperature. All ceramic samples demonstrated relaxor behavior with a dielectric maximum that became more flattened and shifted towards lower temperatures as the grain size decreased.