Pressure-induced electrical and structural anomalies in Pb(1-x)Ca(x)TiO(3) thin films grown at various oxygen pressures by chemical solution route

Lead calcium titanate (Pb(1-x)Ca(x)TiO(3) or PCT) thin films have been thermally treated under different oxygen pressures, 10, 40 and 80 bar, by using the so-called chemical solution deposition method. The structural, morphological, dielectric and ferroelectric properties were characterized by x-ray diffraction, FT-infrared and Raman spectroscopy, atomic force microscopy and polarization-electric-field hysteresis loop measurements. By annealing at a controlled pressure of around 10 and 40 bar, well-crystallized PCT thin films were successfully prepared. For the sample submitted to 80 bar, the x-ray diffraction, Fourier transformed-infrared and Raman data indicated deviation from the tetragonal symmetry. The most interesting feature in the Raman spectra is the occurrence of intense vibrational modes at frequencies of around 747 and 820 cm(-1), whose presence depends strongly on the amount of the pyrochlore phase. In addition, the Raman spectrum indicates the presence of symmetry-breaking disorder, which would be expected for an amorphous (disorder) and mixed pyrochlore-perovskite phase. During the high-pressure annealing process, the crystallinity and the grain size of the annealed film decreased. This process effectively suppressed both the dielectric and ferroelectric behaviour. Ferroelectric hysteresis loop measurements performed on these PCT films exhibited a clear decrease in the remanent polarization with increasing oxygen pressure.

DFT study of the electronic, vibrational, and optical properties of SnO(2)

We report results on the electronic, vibrational, and optical properties of SnO(2) obtained using first-principles calculations performed within the density functional theory. All the calculated phonon frequencies, real and imaginary parts of complex dielectric function, the energy-loss spectrum, the refractive index, the extinction, and the absorption coefficients show good agreement with experimental results. Based on our calculations, the SnO(2) electron and hole effective masses were found to be strongly anisotropic. The lattice contribution to the low-frequency region of the SnO(2) dielectric function arising from optical phonons was also determined resulting the values of E > (1aSyen) (latt) (0) = 14.6 and E > (1ayen) (latt) (0) = 10.7 for directions perpendicular and parallel to the tetragonal c-axis, respectively. This is in excellent agreement with the available experimental data. After adding the electronic contribution to the lattice contribution, a total average value of E >(1)(0) = 18.2 is predicted for the static permittivity constant of SnO(2).

Intergranular properties of uniaxially pressed YBa(2)Cu(3)O(7-delta) ceramic samples

We performed measurements of electrical resistivity as a function of temperature, rho(T), in polycrystalline samples of YBa(2)Cu(3)O(7-delta) (Y-123) subjected to different uniaxial compacting pressures. We observed by using X-ray diffractometry that samples have a very similar composition. Most of the identified peaks are related to the superconducting Y-123 phase. Also, from the X-ray diffraction patterns performed, in powder and pellet samples, we estimated the Lotgering factor along the (00l) direction, F((00l)). The results indicate that F((00l)) increases from 0.13 to 0.16. From electrical resistivity measurements as a function of temperature, we were able to separate contributions arising from both the grain misalignment and microstructural defects. We found appreciable degradation in the normal-state transport properties of samples with an increase in uniaxial compacting pressure. It seems that this type of behavior is associated with an increase in the influence of microstructural defects at the intergranular level. The experimental results are analyzed in the framework of a current conduction model of granular samples.

Electronic, vibrational and related properties of group IV metal oxides by ab initio calculations

We present our theoretical results for the structural, electronic, vibrational and optical properties of MO(2) (M = Sn, Zr, Hf and Ti) obtained by first-principles calculations. Relativistic effects are demonstrated to be important for a realistic description of the detailed structure of the electronic frequency-dependent dielectric function, as well as of the carrier effective masses. Based on our results, we found that the main contribution of the high values calculated for the oxides dielectric constants arises from the vibrational properties of these oxides, and the vibrational static dielectric constant values diminish with increasing pressure. (c) 2008 Elsevier B.V. All rights reserved.

Cobalt-related impurity centers in diamond: electronic properties and hyperfine parameters

Cobalt-related impurity centers in diamond have been studied using first principles calculations. We computed the symmetry, formation and transition energies, and hyperfine parameters of cobalt impurities in isolated configurations and in complexes involving vacancies and nitrogen atoms. We found that the Co impurity in a divacant site is energetically favorable and segregates nitrogen atoms in its neighborhood. Our results are discussed in the context of the recently observed Co-related electrically active centers in synthetic diamond.

Ab initio study of the electronic and optical properties of sillimanite (Al(2)SiO(5)) crystal

Ab initio calculations based on the density functional theory (DFT) are used to investigate the electronic and optical properties of sillimanite. The geometrical parameters of the unit cell, which contain 32 atoms, have been fully optimized and are in good agreement with the experimental data. The electronic structure shows that sillimanite has an indirect band gap of 5.18 eV. The complex dielectric function and optical constants, such as extinction coefficient, refractive index, reflectivity and energy-loss spectrum, are calculated. The optical properties of sillimanite are discussed based on the band structure calculations. It is shown that the O-2p states and Al-3s, Si-3s states play the major role in optical transitions as initial and final states, respectively. (C) 2011 Elsevier B.V. All rights reserved.

Theoretical investigation of electronic and optical properties of andalusite within density functional theory

The electronic and optical properties of andalusite were studied by using quantum-mechanical calculations based on the density functional theory (DFT). The electronic structure shows that andalusite has a direct band gap of 5.01 eV. The complex dielectric function and optical constants, such as extinction coefficient, refractive index, reflectivity and energy-loss spectrum, are calculated. The optical properties of andalusite are discussed based on the band structure calculations. It is shown that the O-2p states and Al-3s states play a major role in optical transitions as initial and final states, respectively. (C) 2010 Elsevier Ltd. All rights reserved.

Properties of lipophilic nucleoside monolayers at the air-water interface

The capability of self-assembly and molecular recognition of biomolecules is essential for many nanotechnological applications, as in the use of alkyl-modified nucleosides and oligonucleotides to increase the cellular uptake of DNA and RNA. In this study, we show that a lipophilic nucleoside, which is an isomer mixture of 2`-palmitoyluridin und 3`-palmitoyluridin, forms Langmuir monolayers and Langmuir-Blodgett films as a typical amphiphile, though with a smaller elasticity. The nucleoside may be incorporated into dipalmitoyl phosphatidyl choline (DPPC) monolayers that serve as a simplified cell membrane model. The molecular-level interactions between the nucleoside and DPPC led to a remarkable condensation of the mixed monolayer, which affected both surface pressure and surface potential isotherms. The morphology of the mixed monolayers was dominated by the small domains of the nucleoside. The mixed monolayers could be deposited onto solid substrates as a one-layer Langmuir Blodgett film that displayed UV-vis absorption spectra typical of aggregated nucleosides owing to the interaction between the nucleoside and DPPC. The formation of solid films with DNA building blocks in the polar heads may open the way for devices and sensors be produced to exploit their molecular recognition properties. (C) 2010 Elsevier B.V. All rights reserved.

Thermoset matrix reinforced with sisal fibers: Effect of the cure cycle on the properties of the biobased composite

Thermoset phenolic composites reinforced with sisal fibers were prepared to optimize the cure step. In the present study, processing parameters such as pressure, temperature, and time interval were varied to control the vaporization of the water generated as a byproduct during the crosslinking reaction. These molecules can vaporize forming voids, which in turn affect the final material properties. The set of results on impact strength revealed that the application of higher pressure before the gel point of the phenolic matrix produced composites with better properties. The SEM images showed that the cure cycle corresponding to the application of higher values of molding pressure at the gel point of the phenolic resin led to the reduction of voids in the matrix. In addition, the increase in the molding pressure during the cure step increased the resin interdiffusion. Better filling of the fiber channels decreased the possibility of water molecules diffusing through the internal spaces of the fibers. These molecules then diffused mainly through the bulk of the thermoset matrix, which led to a decrease in the water diffusion coefficient (D) at all three temperatures (25, 55 and 70 degrees C) considered in the experiments. (C) 2009 Elsevier Ltd. All rights reserved.

Comparison of physical and mechanical properties of microwave-polymerized acrylic resin after disinfection in sodium hypochlorite solutions

This study evaluated the color stability, surface roughness and flexural strength of a microwave-polymerized acrylic resin after immersion in sodium hypochlorite (NaOCl), simulating 20 min of disinfection daily during 180 days. Forty disk-shaped (15 x 4 mm) and 40 rectangular (65 x 10 x 3 mm) specimens were prepared with a microwave-polymerized acrylic resin (Onda-Cryl). Specimens were immersed in either 0.5% NaOCl, 1% NaOCl, Clorox/Calgon and distilled water (control). Color measurements were determined by a portable colorimeter. Three parallel lines, separated by 1.0 mm, were registered on each specimen before and after immersion procedures to analyze the surface roughness. The flexural strength was measured using a 3-point bending test in a universal testing machine with a 50 kgf load cell and a crosshead speed of 1 mm/min. Data were analyzed statistically by ANOVA and Tukey's test (?=0.05). There was no statistically significant differences (p>0.05) among the solutions for color, surface roughness and flexural strength. It may be concluded that immersion in NaOCl solutions simulating short-term daily use during 180 days did not influence the color stability, surface roughness and flexural strength of a microwave-polymerized acrylic resin.

Effect of ageing and immersion in different beverages on properties of denture lining materials

OBJECTIVES: To evaluate the color stability and hardness of two denture liners obtained by direct and indirect techniques, after thermal cycling and immersion in beverages that can cause staining of teeth. MATERIAL AND METHODS: Seventy disc-shaped specimens (18 x 3 mm) processed by direct (DT) and indirect techniques (IT) were made from Elite soft (n=35) and Kooliner (n=35) denture liners. For each material and technique, 10 specimens were subjected to thermal cycling (3,000 cycles) and 25 specimens were stored in water, coffee, tea, soda and red wine for 36 days. The values of color change, Shore A hardness (Elite soft) and Knoop hardness (Kooliner) were obtained. The data were subjected to ANOVA, Tukey's multiple-comparison test, and Kruskal-Wallis test (P<0.05). RESULTS: The thermal cycling promoted a decrease on hardness of Kooliner regardless of the technique used (Initial: 9.09± 1.61; Thermal cycling: 7.77± 1.47) and promoted an increase in the hardness in the DT for Elite Soft (Initial: 40.63± 1.07; Thermal cycling: 43.53± 1.03); hardness of Kooliner (DT: 8.76± 0.95; IT: 7.70± 1.62) and Elite Soft (DT: 42.75± 1.54; IT=39.30± 2.31) from the DT suffered an increase after the immersion in the beverages. The thermal cycling promoted color change only for Kooliner in the IT. Immersion in the beverages did not promote color change for Elite in both techniques. The control group of the DT of Kooliner showed a significant color change. Wine and coffee produced the greatest color change in the DT only for Elite Soft when compared to the other beverages. CONCLUSION: The three variation factors promoted alteration on hardness and color of the tested denture lining materials.

Evaluation of physico-chemical properties of Portland cements and MTA

The purpose of this study was to evaluate the hydrogenionic potential and electrical conductivity of Portland cements and MTA, as well as the amount of arsenic and calcium released from these materials. In Teflon molds, samples of each material were agitated and added to plastic flasks containing distilled water for 3, 24, 72 and 168 h. The results were analyzed with a Kruskal-Wallis non-parametric test for global comparisons and a Dunn-Tukey test for pairwise comparisons. The results revealed no significant differences in the pH of the materials (p > 0.05). The electrical conductivity of the cements were not statistically different (p > 0.05). White non-structural cement and MTA BIO released the largest amount of calcium ions into solution (p < 0.05), while arsenic release was insignificant in all of the materials (p > 0.05). The results indicated that the physico-chemical properties of Portland cements and MTA were similar. Furthermore, all materials produced an alkaline environment and can be considered safe for clinical use because arsenic was not released. The electrical conductivity and the amount of calcium ions released into solution increased over time.

Effect of repeated disinfections by microwave energy on the physical and mechanical properties of denture base acrylic resins

The present study evaluated the effect of repeated simulated microwave disinfection on physical and mechanical properties of Clássico, Onda-Cryl and QC-20 denture base acrylic resins. Aluminum patterns were included in metallic or plastic flasks with dental stone following the traditional packing method. The powder/liquid mixing ratio was established according to the manufacturer's instructions. After water-bath polymerization at 74ºC for 9 h, boiling water for 20 min or microwave energy at 900 W for 10 min, the specimens were deflasked after flask cooling and finished. Each specimen was immersed in 150 mL of distilled water and underwent 5 disinfection cycles in a microwave oven set at 650 W for 3 min. Non-disinfected and disinfected specimens were subjected to the following tets: Knoop hardness test was performed with 25 g load for 10 s, impact strength test was done using the Charpy system with 40 kpcm, and 3-point bending test (flexural strength) was performed at a crosshead speed of 0.5 mm/min until fracture. Data were analyzed statistically by ANOVA and Tukey's test (α= 0.05%). Repeated simulated microwave disinfections decreased the Knoop hardness of Clássico and Onda-Cryl resins and had no effect on the impact strength of QC-20. The flexural strength was similar for all tested resins.

Influence of intrapulpal pressure simulation on the bond strength of adhesive systems to dentin

The purpose of this study was to evaluate the influence of intrapulpal pressure simulation on the bonding effectiveness of etch & rinse and self-etch adhesives to dentin. Eighty sound human molars were distributed into eight groups, according to the permeability level of each sample, measured by an apparatus to assess hydraulic conductance (Lp). Thus, a similar mean permeability was achieved in each group. Three etch & rinse adhesives (Prime & Bond NT - PB, Single Bond -SB, and Excite - EX) and one self-etch system (Clearfil SE Bond - SE) were employed, varying the presence or absence of an intrapulpal pressure (IPP) simulation of 15 cmH2O. After adhesive and restorative procedures were carried out, the samples were stored in distilled water for 24 hours at 37°C, and taken for tensile bond strength (TBS) testing. Fracture analysis was performed using a light microscope at 40 X magnification. The data, obtained in MPa, were then submitted to the Kruskal-Wallis test ( a = 0.05). The results revealed that the TBS of SB and EX was significantly reduced under IPP simulation, differing from the TBS of PB and SE. Moreover, SE obtained the highest bond strength values in the presence of IPP. It could be concluded that IPP simulation can influence the bond strength of certain adhesive systems to dentin and should be considered when in vitro studies are conducted.

Mechanical properties and porosity of dental glass-ceramics hot-pressed at different temperatures

The objective of this work was to evaluate biaxial-flexural-strength (σf), Vickers hardness (HV), fracture toughness (K Ic), Young's modulus (E), Poisson's ratio (ν) and porosity (P) of two commercial glass-ceramics, Empress (E1) and Empress 2 (E2), as a function of the hot-pressing temperature. Ten disks were hot-pressed at 1065, 1070, 1075 and 1080 °C for E1; and at 910, 915, 920 and 925 °C for E2. The porosity was measured by an image analyzer software and s f was determined using the piston-on-three-balls method. K Ic and HV were determined by an indentation method. Elastic constants were determined by the pulse-echo method. For E1 samples treated at different temperatures, there were no statistical differences among the values of all evaluated properties. For E2 samples treated at different temperatures, there were no statistical differences among the values of σf, E, and ν, however HV and K Ic were significantly higher for 910 and 915 °C, respectively. Regarding P, the mean value obtained for E2 for 925 °C was significantly higher compared to other temperatures.