Análise e medida de fatores que afetam as curvaturas retardadas em porcelanato

Os revestimentos cerâmicos de porcelanato, tanto os esmaltados como os não esmaltados, apresentam um fenômeno conhecido como “curvaturas retardadas”, que consiste na mudança de curvatura das peças depois da saída do forno. Este fenômeno é mais problemático a medida de aumentam as dimensões das peças. Neste trabalho se quantificou a variação da curvatura ao longo do tempo em peças industriais de porcelanato esmaltado, observando-se que a curvatura parece apresentar uma evolução em um sentido (habitualmente em direção ao sentido côncavo, ou em forma de barco) para, após transcorrido certo tempo, inverter esta tendência. A cinética deste processo foi parametrizada considerando que existem dois mecanismos, simultâneos e antagônicos, com cinéticas diferentes.As análises teóricas apontam que somente existem dois fatores que podem produzir curvaturas retardadas: as tensões residuais e a expansão dos suportes. Em ambos os casos é necessário condições adicionais para que as curvaturas efetivamente se manifestem: Deste modo, por exemplo, a presença de tensões residuais não é sinônimo de curvaturas retardadas, mais sim um mecanismo adicional que permita a liberação progressiva destas tensões; este mecanismo é chamado de fluência. Além disso é necessário que o perfil de tensões residuais não seja simétrico em relação ao plano central da peça. No que diz respeito a expansão dos suportes, também são necessárias condições especiais para que estas possam provocar curvaturas retardadas; em particular, é necessário que estas sejam diferentes nas duas faces da peça (face lisa e tardoz). Uma expansão uniforme provocaria uma leve mudança dimensional, mas não uma curvatura retardada, nem mesmo para peças esmaltadas. A medida dos fatores que influenciam sobre as curvaturas retardadas é complexa pois requer o emprego de técnicas diferentes das habitualmente empregadas para a caracterização de revestimentos cerâmicos. Foram criados, especialmente para este trabalho, procedimentos para a medida dos diferentes fatores que produzem as curvaturas retardadas e se estudou a influencia de algumas variáveis sobre estes fatoresGlazed and unglazed porcelain tiles exhibit a phenomenon known as delayed curvatures, which consists of the change in tile curvature after the tiles leave the kiln. This phenomenon becomes more problematic as tile size increases. In this study, the variation of curvature in time has been quantified in industrial glazed porcelain tiles. It was observed that the curvature usually displays an evolution in one direction (customarily in a concave direction or vessel shape) which, after a certain time has elapsed, reverses this trend. The kinetics of this process has been parameterised, assuming there are two simultaneous, opposing mechanisms, with different kinetics. Theoretical analyses indicate that only two factors can produce delayed curvatures: residual stresses and expansion of the tile body. In both cases, additional circumstances need to occur for delayed curvatures to appear; thus, for example, the presence of residual stresses is not synonymous with delayed curvatures, but an additional mechanism is needed that allows progressive release of these stresses, a mechanism known as creep. In addition, the condition that the stress profile is not symmetrical with respect to the centre plane of the tile also needs to be obeyed. In regard to the expansion of the bodies, special conditions must also occur for these to cause delayed curvatures. In particular, it is necessary for the expansion at the tile fair face and at the rib face to be different. Uniform expansion would cause a slight dimensional change, but not a delayed curvature, not even in the presence of glaze. The measurement of the factors that influence delayed curvatures is complex because different techniques from those typically used in characterising ceramic tiles are required. Procedures have been fine-tuned to measure the different factors that give rise to delayed curvatures, and the influence of certain variables on these factors has been studiedVersió editoria

First-Principles Study of Substitutional Metal Impurities in Graphene: Structural, Electronic and Magnetic Properties

We present a theoretical study using density functional calculations of the structural, electronic and magnetic properties of 3d transition metal, noble metal and Zn atoms interacting with carbon monovacancies in graphene. We pay special attention to the electronic and magnetic properties of these substitutional impurities and found that they can be fully understood using a simple model based on the hybridization between the states of the metal atom, particularly the d shell, and the defect levels associated with an unreconstructed D3h carbon vacancy. We identify three different regimes associated with the occupation of different carbon-metal hybridized electronic levels: (i) bonding states are completely filled for Sc and Ti, and these impurities are non-magnetic; (ii) the non-bonding d shell is partially occupied for V, Cr and Mn and, correspondingly, these impurties present large and localized spin moments; (iii) antibonding states with increasing carbon character are progressively filled for Co, Ni, the noble metals and Zn. The spin moments of these impurities oscillate between 0 and 1 Bohr magnetons and are increasingly delocalized. The substitutional Zn suffers a Jahn-Teller-like distortion from the C3v symmetry and, as a consequence, has a zero spin moment. Fe occupies a distinct position at the border between regimes (ii) and (iii) and shows a more complex behavior: while is non-magnetic at the level of GGA calculations, its spin moment can be switched on using GGA+U calculations with moderate values of the U parameter.Comment: 13 figures, 4 tables. Submitted to Phys. Rev. B on September 26th, 200

Structural models for the Si(553)-Au atomic chain reconstruction

Recent photoemission experiments on the Si(553)-Au reconstruction show a one-dimensional band with a peculiar ~1/4 filling. This band could provide an opportunity for observing large spin-charge separation if electron-electron interactions could be increased. To this end, it is necessary to understand in detail the origin of this surface band. A first step is the determination of the structure of the reconstruction. We present here a study of several structural models using first-principles density functional calculations. Our models are based on a plausible analogy with the similar and better known Si(557)-Au surface, and compared against the sole structure proposed to date for the Si(553)-Au system [Crain JN et al., 2004 Phys. Rev. B 69 125401 ]. Results for the energetics and the band structures are given. Lines for the future investigation are also sketched

Nonlinear stochastic discrete drift-diffusion theory of charge fluctuations and domain relocation times in semiconductor superlattices

A stochastic discrete drift-diffusion model is proposed to account for the effects of shot noise in weakly coupled, highly doped semiconductor superlattices. Their current-voltage characteristics consist of a number stable multistable branches corresponding to electric field profiles displaying two domains separated by a domain wall. If the initial state corresponds to a voltage on the middle of a stable branch and a sudden voltage is switched so that the final voltage corresponds to the next branch, the domains relocate after a certain delay time. Shot noise causes the distribution of delay times to change from a Gaussian to a first passage time distribution as the final voltage approaches that of the end of the first current branch. These results agree qualitatively with experiments by Rogozia {\it et al} (Phys. Rev. B {\bf 64}, 041308(R) (2001)).Comment: 9 pages, 12 figures, 2 column forma

Influence of family-centered goals on dividend policy in family firms: A socioemotional wealth approach

Socioemotional wealth (SEW) preservation is likely to be a key determinant for family firms to shape their dividend policy. This paper analyzes how family-centered goals captured by SEW influence on dividend policy in private family firms, exploring as well the moderating role on these relationships of family involvement in management, generational stage, and firm hazard. Results indicate a negative association between SEW preservation and both the likelihood of giving dividends and the amount of dividend paid. This negative relationship is stronger when the CEO is a family member, in early generational stages and when the firm faces greater performance hazard. The amount of dividend paid is also lower when there are family members in other top management positions beyond the CEO. Thus, the evidence provided suggests that the existing heterogeneity regarding dividend policy in the context of privately held family firms is strongly driven by differences in SEW priorities

Socioemotional wealth and financial decisions in private family SMEs

This study focuses on heterogeneity in family firms by analyzing whether the non-economic aspects that meet the family?s affective needs, or socioemotional wealth (SEW), influence debt financing. In the context of private family small and medium-sized enterprises (SMEs), our results indicate that family firms which are more concerned about preserving their SEW have lower debt levels (total and financial debt) and that CEO gender plays an important moderating role, with female CEOs strengthening the negative effect of SEW preservation on debt financing. Moreover, when family firms are managed by the first generation, the SEW effect on financial debt is even more negative. The findings are consistent with SEW being the point of reference in family SMEs? financial decisions, and highlight the importance of the CEO and family generation in charge of the firm as moderators of the relationship between SEW preservation and debt financing

Electronic structure interpolation via atomic orbitals

We present an efficient scheme for accurate electronic structure interpolations based on the systematically improvable optimized atomic orbitals. The atomic orbitals are generated by minimizing the spillage value between the atomic basis calculations and the converged plane wave basis calculations on some coarse $k$-point grid. They are then used to calculate the band structure of the full Brillouin zone using the linear combination of atomic orbitals (LCAO) algorithms. We find that usually 16 -- 25 orbitals per atom can give an accuracy of about 10 meV compared to the full {\it ab initio} calculations. The current scheme has several advantages over the existing interpolation schemes. The scheme is easy to implement and robust which works equally well for metallic systems and systems with complex band structures. Furthermore, the atomic orbitals have much better transferability than the Shirley's basis and Wannier functions, which is very useful for the perturbation calculations

Injerto autólogo cubital como complemento a una mandibulectomía parcial para el tratamiento de un tumor de células escamosas

Se presenta un caso clínico de un tumor de células escamosas en la cavidad oral del perro, resuelto mediante mandibulectomía parcial combinada con la implantación de un injerto óseo autólogo. En la evolución tras 30 meses no ha presentado recidivas ni metástasis

Universal Magnetic Properties of sp3^3-type Defects in Covalently Functionalized Graphene

Using density-functional calculations, we study the effect of sp$^3$-type defects created by different covalent functionalizations on the electronic and magnetic properties of graphene. We find that the induced magnetic properties are {\it universal}, in the sense that they are largely independent on the particular adsorbates considered. When a weakly-polar single covalent bond is established with the layer, a local spin-moment of 1.0 $\mu_B$ always appears in graphene. This effect is similar to that of H adsorption, which saturates one $p_z$ orbital in the carbon layer. The magnetic couplings between the adsorbates show a strong dependence on the graphene sublattice of chemisorption. Molecules adsorbed at the same sublattice couple ferromagnetically, with an exchange interaction that decays very slowly with distance, while no magnetism is found for adsorbates at opposite sublattices. Similar magnetic properties are obtained if several $p_z$ orbitals are saturated simultaneously by the adsorption of a large molecule. These results might open new routes to engineer the magnetic properties of graphene derivatives by chemical means