Ana Candida Martins Rodrigues

Impedance spectroscopy study of the electrical conductivity and dielectric constant of polycrystalline LiNbO3

The electrical properties of LiNbO3 ceramic pellets were investigated by impedance spectroscopy in the temperature range of 450–800 °C. Impedance data are presented in the Nyquist plot which, in addition to the separation of grain and grain boundary contributions, allows the determination of the relaxation frequency of samples. The results of bulk electrical conductivity and its activation energy, confirmed by measuring samples with different geometrical parameters, are presented. Relaxation frequencies follow an Arrhenius behavior with the same activation energy found for the electrical conductivity. Relaxation frequencies were also used to calculate bulk dielectric constants and the results are compared to the behavior of the real part of the sample dielectric constant.

Biosilicate®–gelatine bone scaffolds by the foam replica technique: development and characterization

Abstract The development of bioactive glass-ceramic materials has been a topic of great interest aiming at enhancing the mechanical strength of traditional bioactive scaffolds. In the present study, we test and demonstrate the use of Biosilicate® glass-ceramic powder to fabricate bone scaffolds by the foam replica method. Scaffolds possessing the main requirements for use in bone tissue engineering (95% porosity, 200–500 μm pore size) were successfully produced. Gelatine coating was investigated as a simple approach to increase the mechanical competence of the scaffolds. The gelatine coating did not affect the interconnectivity of the pores and did not significantly affect the bioactivity of the Biosilicate® scaffold. The gelatine coating significantly improved the compressive strength (i.e. 0.80 ± 0.05 MPa of coated versus 0.06 ± 0.01 MPa of uncoated scaffolds) of the Biosilicate® scaffold. The combination of Biosilicate® glass-ceramic and gelatine is attractive for producing novel scaffolds for bone tissue engineering.

Characterization and In Vivo Biological Performance of Biosilicate

After an introduction showing the growing interest in glasses and glass-ceramics as biomaterials used for bone healing, we describe a new biomaterial named Biosilicate. Biosilicate is the designation of a group of fully crystallized glass-ceramics of the Na2O-CaO-SiO2-P2O5 system. Several in vitro tests have shown that Biosilicate is a very active biomaterial and that the HCA layer is formed in less than 24 hours of exposure to “simulated body fluid” (SBF) solution. Also, in vitro studies with osteoblastic cells have shown that Biosilicate disks supported significantly larger areas of calcified matrix compared to 45S5 Bioglass, indicating that this bioactive glass-ceramic may promote enhancement of in vitro bone-like tissue formation in osteogenic cell cultures. Finally, due to its special characteristics, Biosilicate has also been successfully tested in several in vivo studies. These studies revealed that the material is biocompatible, presents excellent bioactive properties, and is effective to stimulate the deposition of newly formed bone in animal models. All these data highlight the huge potential of Biosilicate to be used in bone regeneration applications.

Charge carrier concentration and mobility in alkali silicates

The respective contributions of the charge carrier concentration and mobility to the ionic conductivity in glasses remain an open question. In the present work we calculate these two parameters from conductivity data as a function of temperature below and above the glass transition temperature, T(g). The basic hypothesis assumes that ionic displacement results from the migration of cationic pairs formed by a partial dissociation, which is a temperature-activated process. Below T(g) their migration would follow a temperature-activated mechanism, while a free volume mechanism prevails above this temperature, leading to a deviation from the Arrhenius behavior. Expressions are formulated for the variation in ionic conductivity as a function of temperature in the supercooled and glassy states. Fitting the experimental data with the proposed expressions allows for the determination of characteristic parameters such as the charge carrier formation and migration enthalpies. Based on these values, it is then possible to calculate the charge carrier concentration and mobility in the entire temperature range. At room temperature, the mobility of effective charge carriers is estimated close to 10(-4) cm(2) s(-1) V(-1) for alkali disilicates glasses under study, while the ratio between the number of effective charge carriers and the total number of alkali cations is estimated to be from 10(-8) to 10(-10), comparable to the concentration of intrinsic defects in an ionic crystal or dissociated species from a weak electrolyte solution.

Electrical properties along the X and Z axes of LiNbO3 wafers

Electrical conductivity and dielectric relaxation frequency were measured along the X and Z crystallographic axes of a LiNbO3 single-crystal wafer, using the ac impedance spectroscopy technique. The measurements were carried out in the frequency range of 5 Hz–13 MHz and in the temperature range of 460–620 °C. Impedance data were plotted on Nyquist diagrams, which, in addition to the electrical resistance and relaxation frequency of samples, provide information about the charge carriers of the conduction process. Isothermal measurements were also taken as a function of time. Although no difference in the activation energy of conduction was found, a slight but unequivocal difference in electrical conductivity (about 0.16 on a log scale) was established between the two axes. In contrast, a more marked and substantial difference was found between the relaxation frequency of the axes, confirming that differences in the electrical behavior of the X and Z directions of LiNbO3 are more likely due to differences i...

Preparation of LiNbO3 powder from the thermal decomposition of a precursor salt obtained by an evaporative method

The preparation of lithium niobate powder, LiNbO3, using an organic niobium salt and lithium nitrate in a buffer solution of oxalic acid and ammonium oxalate has been investigated. After the evaporation of this starting solution a precursor is obtained, which upon heat treatment, yields LiNbO3. A chemical formula for this precursor is proposed, based on IR and thermal analysis data. The thermal evolution of the precursor powder, to the point of the formation of the LiNbO3 phase, was followed by means of thermal analysis, IR spectroscopy and powder X-ray diffraction (XRD). The results indicate that total crystallization occurs after a 550 °C heat treatment, showing that single-phase LiNbO3 powder, with a crystallite mean size of 88 nm, can be prepared at low processing temperatures.

Characterization of an all solid-state electrochromic window

Sol-gel cerium - titanium oxide layers present potential application as transparent counter-electrode (ion storage layer) in electrochromic windows and mirrors using lithium conducting electrolyte and W03 electrochromic coating. The precursor sol, prepared by mixing Ti(OPri)4 and Ce(N03)6 (NH4)2 in ethanol, is initially dark red and becomes transparent after a few days aging indicating the presence of Ce3 complexes. The layers have been obtained by dip coating technique and heat treated at 4509C during 15 minutes.They have been characterized by XRD, SIMS, optical absorption and electrochemical techniques; it is shown that the electrochemical reaction corresponds to a reversible insertion-extraction oflithium ions within a Ti02 amorphous film containing small Ce02 crystallites. At low sweep frequencies the process is controlled by a diffusion mechanism (DLi ≊ 6.4 lOl2cm2/s at 259C). Characterizations of an all solid state electrochromic window/glass/JTO/ W03/ POE-Li N (502 CF3)2 1 Ti02 - Ce02 I ITO I glass I are also presented.

Charge carrier mobility and concentration as a function of composition in AgPO3-AgI glasses

Conductivity data of the xAgI(1 - x)AgPO(3) system (0 ≤ x ≤ 0.5) were collected in the liquid and glassy states. The difference in the dependence of ionic conductivity on temperature below and above their glass transition temperatures (T(g)) is interpreted by a discontinuity in the charge carriers mobility mechanisms. Charge carrier displacement occurs through an activated mechanism below T(g) and through a Vogel-Fulcher-Tammann-Hesse mechanism above this temperature. Fitting conductivity data with the proposed model allows one to determine separately the enthalpies of charge carrier formation and migration. For the five investigated compositions, the enthalpy of charge carrier formation is found to decrease, with x, from 0.86 to 0.2 eV, while the migration enthalpy remains constant at ≈0.14 eV. Based on these values, the charge carrier mobility and concentration in the glassy state can then be calculated. Mobility values at room temperature (≈10(-4) cm(2) V(-1) s(-1)) do not vary significantly with the AgI content and are in good agreement with those previously measured by the Hall-effect technique. The observed increase in ionic conductivity with x would thus only be due to an increase in the effective charge carrier concentration. Considering AgI as a weak electrolyte, the change in the effective charge carrier concentration is justified and is correlated to the partial free energy of silver iodide forming a regular solution with AgPO(3).

Critical V2O5/TeO2 Ratio Inducing Abrupt Property Changes in Vanadium Tellurite Glasses

Transition metal containing glasses have unique electrical properties and are therefore often used for electrochemical applications, such as in batteries. Among oxide glasses, vanadium tellurite glasses exhibit the highest electronic conductivity and thus the high potential for applications. In this work, we investigate how the dynamic and physical properties vary with composition in the vanadium tellurite system. The results show that there exists a critical V(2)O(5) concentration of 45 mol %, above which the local structure is subjected to a drastic change with increasing V(2)O(5), leading to abrupt changes in both hardness and liquid fragility. Electronic conductivity does not follow the expected correlation to the valence state of the vanadium as predicted by the Mott-Austin equation but shows a linear correlation to the mean distance between vanadium ions. These findings could contribute to designing optimum vanadium tellurite compositions for electrochemical devices. The work gives insight into the mechanism of electron conduction in the vanadium tellurite systems.

Ionic blocking effect in partially crystallized lithium disilicate

A lithium disilicate glass was heat treated at 454°C, the temperature of maximum nucleation rate, up to 25days. At this temperature the growth rate is negligible, and therefore these thermal treatments lead to a glassy matrix in which ellipsoidal crystals of about the same size were randomly dispersed. Crystallized volume fraction (α) of up to 20% was reached. Electrical conductivity of glassy, partially crystallized, and 100% crystallized lithium disilicate was then measured by impedance spectroscopy. The crystallized surface layer was eliminated by polishing in order to solely analyze the effect of bulk crystals. The complex plane plots of impedance data showed one or two semicircles depending on (α). The high frequency semicircle represents the vitreous matrix while the low frequency one was ascribed to the blocking effect imposed to the lithium ions by the existing crystals in the partially crystallized samples. This hypothesis was confirmed by the analysis of the relaxation frequency related to each ...