J.J. Romero

Sources of experimental errors in the observation of nanoscale magnetism

It has been recently reported that some nonmagnetic materials in bulk state exhibit magnetic behavior at the nanoscale due to surface and size effects. The experimental observation of these effects is based on the measurement of very small magnetic signals. Thus, some spurious effects that are not critical for bulk materials with large magnetic signals may become important when measuring small signals (typically below 10−4emu). Here, we summarize some sources of these small magnetic signals that should be considered when studying this new nanomagnetism.

High spatial resolution structure of (K,Na)NbO3 lead-free ferroelectric domains

In this paper, we highlight some practical aspects in the study of ferroelectric domains in lead-free (K,Na)NbO3 piezoceramics. This work presents and discusses the spatial resolved structure of the ferroelectric domain existing in (K,Na)NbO3 based ceramics studied by confocal Raman microscopy (CRM) coupled with atomic force microscopy. In addition to the domain identification, CRM allows a determination of the nature of domain walls and correlation between the structure and piezoelectric properties. The tetragonal constraint for orthorhombic domain formations is demonstrated by this technique. Alternating polarization differences in 180° domain templates are resolved by 60° and 120° domains. The internal stress results in the appearance of an unusual polarization relaxation at the 90° domain wall in (K,Na)NbO3 ceramics. Finally, the implications of the present findings introduce a fruitful discussion in the search for effective structures to improve the piezoelectric response and represent the key point to design new domains that allow engineering the piezoelectric response.

Piezoceramics properties as a function of the structure in the system (K,Na,Li)(Nb,Ta,Sb)O 3

The influence of the sintering conditions and stoichiometry on the crystalline symmetry and electrical properties of the system (K0.44+xNa0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3+x/2, with x = -0.06, 0.00, and 0.04, has been evaluated. By lowering the concentration of K cations, a faster stabilization of the tetragonal phase is reached. Increasing the sintering time also leads to the stabilization of the tetragonal phase at room temperature, as evidenced by Raman spectroscopy, thus improving the piezoelectric properties of these materials. The Raman spectra versus temperature showed 2 anomalies at temperatures of ~70 and ~270 to 330degC, associated with polymorphism and the ferro-paraelectric phase transitions, respectively. Active Raman scattering modes are observed above Curie temperature, indicating that the symmetry is not cubic but pseudocubic. This pseudocubic phase is associated with relaxor diffuseness, attributed to composition fluctuations. In addition, the piezoelectric properties were correlated with the tetragonality of the system, showing a linear dependence between the piezoelectric properties and the tetragonality ratio. The ceramics with c/a = 1.011 ratio exhibit enhanced electrical properties, d33 ~255 pC/N and kp ~0.47.

Control of the Crystalline Structure and Piezoelectric Properties of (K,Na,Li)(Nb,Ta,Sb)O3 Ceramics through Transition Metal Oxide Doping

Divalent transition metal oxide doping of lead-free (K,Na,Li)(Nb,Ta,Sb)O3 piezoceramics is studied. Two different behaviors were observed independently of the doping metal: at low concentrations, the tetragonal structure is preserved, while at a high doping level, the material becomes orthorhombic. For any given doping level, a linear dependence was found between the pseudo-tetragonal lattice distortion and the ionic radii of doping ions. The ferroelectric and piezoelectric properties of the material are reduced by the doping, whereas the mechanical quality factor increases. Thus, the piezoelectric and ferroelectric properties of these lead-free piezoceramics can be easily controlled through metal oxide doping.

Low thermal conductivity and improved thermoelectric performance of nanocrystalline silicon germanium films by sputtering.

Si x Ge1-x alloys are well-known thermoelectric materials with a high figure of merit at high temperatures. In this work, metal-induced crystallization (MIC) has been used to grow Si0.8Ge0.2 films that present improved thermoelectric performance (zT = 5.6 × 10(-4) at room temperature)--according to previously reported values on films--with a relatively large power factor (σ · S (2) = 16 μW · m(-1) · K(-2)). More importantly, a reduction in the thermal conductivity at room temperature (κ = 1.13 ± 0.12 W · m(-1) · K(-1)) compared to other Si-Ge films (∼3 W · m(-1) · K(-1)) has been found. Whereas the usual crystallization of amorphous SiGe (a-SiGe) is achieved at high temperatures and for long times, which triggers dopant loss, MIC reduces the crystallization temperature and the heating time. The associated dopant loss is thus avoided, resulting in a nanostructuration of the film. Using this method, we obtained Si0.8Ge0.2 films (grown by DC plasma sputtering) with appropriate compositional and structural properties. Different thermal treatments were tested in situ (by heating the sample inside the deposition chamber) and ex situ (annealed in an external furnace with controlled conditions). From the studies of the films by: x-ray diffraction (XRD), synchrotron radiation grazing incidence x-ray diffraction (SR-GIXRD), micro Raman, scanning electron microscopy (SEM), x-ray photoemission spectroscopy (XPS), Hall effect, Seebeck coefficient, electrical and thermal conductivity measurements, we observed that the in situ films at 500 °C presented the best zT values with no gold contamination.

Influence of the nanoparticles agglomeration state in the quantum-confinement effects: Experimental evidences

The agglomeration state facilitates particle-particle interaction which produces important effects in the phonon confinement effects at the nanoscale. A partial phonon transmission between close nanoparticles yields a lower momentum conservation relaxation than in a single isolated nanoparticle. It means a larger red shift and broadening of the Raman modes than the expected ones for Raman quantum confinement effects. This particle-particle interaction can drive to error when Raman responses are used to estimate the size of the nanoscaled materials. In this work different corrections are suggested to overtake this source of error.

Temperature dependence of the magnetic properties in LaMnO3+δ

Data are presented on the thermal dependence of the hysteretic properties of cationic vacancies including manganite samples of composition LaMnO3+δ (δ=0.05 and 0.12). Our results evidence the presence in both samples of two magnetic phases having ferro- and antiferromagnetic orders, respectively. The temperature dependence of the coercivity and relaxational properties of the samples is closely linked to the connectivity of the magnetic moment bearing Mn3+–Mn4+ ferromagnetic clusters that demagnetize independently in the case of the δ=0.05 sample and collectively in that of the δ=0.12 one, as evidenced from the activation volume results (δ=0.05) which yielded a size of the same order magnitude as that obtained in previous works for the Mn3+–Mn4+ ferromagnetic cluster size.

Versatile silicon nanoparticles with potential uses as photoluminiscent sensors and photosensitizers

Silicon nanoparticles (including silicon clusters and 1–4 nm size silicon nanocrystals, SiCs) combine size – dependent photoluminescence, the capacity for singlet oxygen and superoxide radical anion generation, and the richness of silicon surface derivatization. Surface modifications as coating/linking with folate, antibodies, adjuvants, and a plethora of other substances may lead to an increased aqueous solubility, stability, biocompatibility, targeting potential, and circulation time in biological systems. Size, synthetic procedures and surface derivatization/oxidation may strongly affect the particles efficiency for reactive oxygen species photosensitization and the interaction with small molecules and biological entities. Thus, through intelligent design it is possible to develop multifunctional nanoparticles with potential applications in imaging, diagnosis, and therapy. Herein, we present and discuss the properties that make SiCs potential photosensitizers for biological uses and describe the most widely used synthesis and surface functionalization procedures in order to help understanding the basics of photoluminescent SiCs and as a guide for researchers aiming to find new applications based on these particles.

Correlation between the structure and the piezoelectric properties of lead-free (K,Na,Li)(Nb,Ta,Sb)O 3 ceramics studied by XRD and Raman spectroscopy

This article reviews on the use of Raman spectroscopy for the study of (K,Na,Li)(Nb,Ta,Sb)O3 lead-free piezoceramics. Currently, this material appears to be one of the most interesting and promising alternatives to the well-known PZT piezoelectric materials. In this work, we prepare piezoceramics with different stoichiometries and study their structural, ferroelectric, and piezoelectric properties. By using both Raman spectroscopy and X-ray diffraction, we establish a direct correlation between the structure and the properties. The results demonstrate that the wavenumber of the A1g vibration is proportional to the tetragonality, the remnant polarization, and the piezoelectric coefficients of these materials. Thus, Raman spectroscopy appears as a very useful technique for a fast evaluation of the crystalline structure and the ferroelectric/ piezoelectric properties.

EXAFS Studies and Magnetic Behavior of FeCuZr Ball-Milled Alloys

Metastable alloys of nominal composition (Fe0.5Cu0.5)100-xZrx (x=0-17 at.%) have been synthesized by high energy ball milling. In spite of the high and positive enthalpy of mixing between Fe and Cu, nanocrystalline or amorphous alloys have been obtained depending on the Zr content. Alloys exhibit a ferromagnetic behavior with a Curie temperature below room temperature. The thermal dependence of the thermorremanence shows a anomalous increase above the Curie temperature. This behavior seems to be related with a magnetovolume effect. Extended X-ray absorption fine structure (EXAFS) measurements have been performed to explain this effect. Preliminary results seem to indicate an almost negligible thermal expansion at temperatures below Tc, while normal thermal expansion takes place at higher temperatures.