The Experts below are selected from a list of 102 Experts worldwide ranked by ideXlab platform
Lauro J. Q. Maia - One of the best experts on this subject based on the ideXlab platform.
-
Thermal, structural and optical properties of Al_2CoO_4-Crocoite composite nanoparticles used as pigments
Journal of Thermal Analysis and Calorimetry, 2009Co-Authors: M. Inês B. Bernardi, Vinícius D. Araújo, Alexandre Mesquita, Geraldo J. M. Frigo, Lauro J. Q. MaiaAbstract:Al_2CoO_4–PbCrO_4 and Al_2CoO_4–Pb_2CrO_5 crystalline powders in different proportions were obtained by the polymeric precursor method. Differential scanning calorimetry (DSC) and thermogravimetry (TG) techniques were used to accurately characterize the distinct thermal events occurring during synthesis. The TG and DSC results revealed a series of overlapping decomposition reactions due to different exothermal events, which were identified as H_2O and NO_x elimination and polymer pyrolysis. The X-ray diffraction patterns of the x Al_2CoO_4–(1 − x )PbCrO_4 and x Al_2CoO_4–(1 − x )Pb_2CrO_5 mixed compounds, with x = 1, 0.75, 0.5, 0.25 and 0, were obtained in the crystalline form with their respective phases, and proved consistent with the nominal compositions. The synthesis of these two systems yielded nine different colors and shades.
-
thermal structural and optical properties of al2coo4 Crocoite composite nanoparticles used as pigments
Journal of Thermal Analysis and Calorimetry, 2009Co-Authors: Ines M B Bernardi, Vinícius D. Araújo, Alexandre Mesquita, Geraldo J. M. Frigo, Lauro J. Q. MaiaAbstract:Al2CoO4–PbCrO4 and Al2CoO4–Pb2CrO5 crystalline powders in different proportions were obtained by the polymeric precursor method. Differential scanning calorimetry (DSC) and thermogravimetry (TG) techniques were used to accurately characterize the distinct thermal events occurring during synthesis. The TG and DSC results revealed a series of overlapping decomposition reactions due to different exothermal events, which were identified as H2O and NOx elimination and polymer pyrolysis. The X-ray diffraction patterns of the xAl2CoO4–(1 − x)PbCrO4 and xAl2CoO4–(1 − x)Pb2CrO5 mixed compounds, with x = 1, 0.75, 0.5, 0.25 and 0, were obtained in the crystalline form with their respective phases, and proved consistent with the nominal compositions. The synthesis of these two systems yielded nine different colors and shades.
Kevin S Knight - One of the best experts on this subject based on the ideXlab platform.
-
a neutron powder diffraction determination of the thermal expansion tensor of Crocoite pbcro4 between 60 k and 290 k
Mineralogical Magazine, 1996Co-Authors: Kevin S KnightAbstract:The thermal expansion tensor of Crocoite has been determined from high-resolution neutron time-of-fligh t powder diffraction data . The temperature dependence of the lattice constants between 4 .5 K and 290 K hav e been fitted to a quasi-harmonic Einstein model, and the temperature dependence of the thermal expansio n tensor has been calculated for 60 K T 5 290 K. The magnitudes of the principal expansivities and thei r orientation exhibit saturation behaviour for temperatures above 300 K . The predicted saturated expansio n coefficients are a ll = 33 .1(1) x 10—6 K—1 , a 22 = 15 .72(3) x 10—6 K-1, a33 = 3 .36(1) x 10—6 K—1 , with a 22 parallel tob and a ll lying at an angle of -37 .86(5)° to c for the P2 1/n setting of the crystal structure . The direction of maximum expansion is approximately parallel to both [101] and the least-squares line passin g through the projection of the chromium atoms on (010) . The direction of minimum expansion lie s approximately parallel to [101]. No evidence was found for either a structural or magnetic phase transitio n between 4 .5 K and 300 K .
K S Knight - One of the best experts on this subject based on the ideXlab platform.
-
a high temperature structural phase transition in Crocoite pbcro4 at 1068 k crystal structure refinement at 1073 k and thermal expansion tensor determination at 1000 k
Mineralogical Magazine, 2000Co-Authors: K S KnightAbstract:High-resolution, neutron time-of-flight, powder diffraction data have been collected on natural Crocoite between 873 and 1073 K. Thermal analysis carried out in the 1920s had suggested that chemically pure PbCr04 exhibited two structural phase transitions, at 964 K, to the ~ phase, and at 1056 K, to the y phase. In this study, no evidence was found for the (.(-~ structural phase transition, however a hightemperature phase transition was found at ~1 068 K from the ambient-temperature monazite structure type to the baryte structure type. The phase transition, close to the temperatures reported for the ~ to y phase modifications, is first order and is accompanied by a change in volume of -1.6%. The crystal structure of this phase has been refined using the Rietveld method to agreement factors of Rp = 0.018, Rwp = 0.019, Rp = 0.011. No evidence for premonitory behaviour was found in the temperature dependence of the monoclinic lattice constants from 873 K to 1063 K and these have been used to determine the thermal expansion tensor of Crocoite just below the rhase transition. At 1000 K the magnitudes of the tensor coefficients are (.(11, 2.66(I)x 10-5 K- ; (.(n, 2.04(l)x 10-5 K-I; (.(33, 4.67(4) x 10-5 K-I; and (.(13,-1.80(2) x 10-5 K-I using the IRE convention for the orientation of the tensor basis. The orientation of the principal axes of the thermal expansion tensor are very close to those reported previously for the temperature range 50-300 K.
Akhlesh Lakhtakia - One of the best experts on this subject based on the ideXlab platform.
-
Two Dyakonov–Voigt surface waves guided by a biaxial–isotropic dielectric interface
Scientific Reports, 2020Co-Authors: Chenzhang Zhou, Tom G. Mackay, Akhlesh LakhtakiaAbstract:Electromagnetic surface waves guided by the planar interface of an orthorhombic dielectric material and an isotropic dielectric material were analyzed theoretically and numerically. Both naturally occurring minerals (Crocoite, tellurite, and cerussite) and engineered materials were considered as the orthorhombic partnering material. In addition to conventional Dyakonov surface waves, the analysis revealed that as many as two Dyakonov–Voigt surface waves can propagate in each quadrant of the interface plane, depending upon the birefringence of the orthorhombic partnering material. The coexistence of two Dyakonov–Voigt surface waves marks a fundamental departure from the corresponding case involving the planar interface of a uniaxial dielectric material and an isotropic dielectric material for which only one Dyakonov–Voigt surface wave is possible. The two Dyakonov–Voigt surface waves propagate in different directions in each quadrant of the interface plane, with different relative phase speeds and different penetration depths. Furthermore, the localization characteristics of the two Dyakonov–Voigt surface waves at the planar interface are quite different: the Dyakonov–Voigt surface wave with the higher relative phase speed is much less tightly localized at the interface in the isotropic dielectric partnering material.
-
Review of the role of dielectric anisotropy in Dyakonov surface-wave propagation
Nanostructured Thin Films, 2008Co-Authors: Sudarshan R. Nelatury, John A. Polo, Akhlesh LakhtakiaAbstract:Surface waves (SWs) are localized waves that travel along the planar interface between two different mediums when certain dispersion relations are satisfied. If both mediums have purely dielectric constitutive properties, the characteristics of SW propagation are determined by the anisotropy of both mediums. Surface waves are then called Dyakonov SWs (DSWs), after Dyakonov who theoretically established the possibility of SW propagation at the planar interface of an isotropic dielectric and a positive uniaxial dielectric. Since then, DSW propagation guided by interfaces between a variety of dielectrics has been studied. With an isotropic dielectric on one side, the dielectric on the other side of the interface can be not only positive uniaxial but also biaxial. DSW propagation can also occur along an interface between two uniaxial or biaxial dielectrics that are twisted about a common axis with respect to each other but are otherwise identical. Recently, DSW propagation has been studied taking (i) uniaxial dielectrics such as calomel and dioptase crystals; (ii) biaxial dielectrics such as hemimorphite, Crocoite, tellurite, witherite, and cerussite; and (iii) electro-optic materials such as potassium niobate. With materials that are significantly anisotropic, the angular regime of directions for DSW propagation turns out to be narrow. In the case of naturally occurring crystals, one has to accept the narrow angular existence domain (AED). However, exploiting the Pockels effect not only facilitates dynamic electrical control of DSW propagation, but also widens the AED for DSW propagation.
Veronika Kosařova - One of the best experts on this subject based on the ideXlab platform.
-
Crocoite pbcro4 and mimetite pb5 aso4 3cl rare minerals in highly degraded mediaeval murals in northern bohemia
Journal of Raman Spectroscopy, 2014Co-Authors: David Hradil, Janka Hradilova, Petr Bezdicka, Silvie Svarcova, Zdeňka Cermakova, Veronika KosařovaAbstract:Mural paintings of exceptional quality, which can be discerned in spite of their extensive mechanical damage and colour fading, have been uncovered in the church of St. Gallus in Kuivody, Northern Bohemia, dated to the second half of the 13th century. Materials research with particular use of portable X-ray fluorescence, Raman micro-spectroscopy and powder X-ray micro-diffraction revealed the presence of rare pigments. In Kuivody, it is only a second identification of intentionally used yellow mineral Crocoite (PbCrO4) in European art. Its identification is facilitated by providing a very good Raman scattering, even when present in small amounts in fragmentarily preserved colour layers. Light yellow mimetite (Pb-5(AsO4)(3)Cl) was never before mentioned as intentionally used pigment in Europe. Its finding in Kuivody, however, corresponds more likely with undesirable physical-chemical conditions causing its formation by alteration of orpiment (As2S3) and minium (Pb3O4). Obtained results highlight the importance of Raman spectroscopy for direct identification of mineral pigments in low concentrations, which may be crucial for interpreting cultural heritage objects in historical context. By materials, the almost forgotten paintings in Kuivody can be seen as outstanding and rare example of ancient artistic tradition that has spread to Europe from Mediterranean in early Middle Ages. After all, mineral Crocoite was already used by ancient Egyptians to paint sarcophagi and degraded orpiment decorates the walls of the Nefertari's tomb in Thebes. Copyright (c) 2014 John Wiley & Sons, Ltd.