ABSTRACT
The Gd2.994Ce0.006Ga3Al2O12 and Gd2.964Ce0.006Dy0.03Ga3Al2O12 nanopowders were prepared using the sol-gel method. The nanocrystalline powders were used for sintering ceramics by low-temperature sintering at high pressure (LTHP). This technique allows maintaining the crystallite size or in some cases even decreases it, leading to interesting physical properties often different compared to the starting material. This study describes the structural and spectroscopic properties of the powders as well as changes in the physical properties of the ceramics induced by the sintering pressure. Particular attention was paid to study the influence of pressure applied during sintering on changes in the persistent luminescence. A mechanism for the persistent luminescence is proposed taking into account the changes induced in ceramics.
ABSTRACT
Graphene, a two-dimensional structure of carbon, due to its structure has unique physico-chemical properties that can be used in numerous research and industry areas. Although this structure is already well known, there are still technological (and cost) barriers which do not allow to produce this material in large quantities and hence prevent its use in various applications. For this reason, many technologies are currently being developed to obtain graphene in forms that would enable its widespread use. The graphene-like ceramics were fabricated by the high isostatic pressure method at different temperatures. This technique allows to obtain dense ceramics with various shapes. The structure and morphology of sintered graphene were investigated by XRD, SEM and the Raman spectroscopy. The hardness, thermal conductivity and electric transport measurements recorded in a wide range of temperatures were used to analyze the physical properties of the obtained ceramics.
ABSTRACT
We report the synthesis, crystal structure, IR, Raman and luminescence studies of four metal-organic frameworks of the following formulas: [CH3CH2NH3]Y1-x-yYbyErx(HCOO)4 (x = 1, y = 0; x = 0.2, y = 0.8; x = 0.02, y = 0.07) and [CH3CH2NH3]Y0.92Eu0.08(HCOO)4. All the compounds are isostructural and crystallize in a polar and non-centrosymmetric monoclinic system (P21 space group). They have been characterized by single crystal and powder X-ray diffraction methods as well as by vibrational spectroscopy (IR and Raman). The assignment of the external and internal modes has been discussed and presented. Furthermore, the optical properties of the Er3+ and Eu3+ ions have been assessed using diffuse absorption, excitation and emission spectra. The site symmetry of the Eu3+ ions has been analyzed using the emission spectrum and the luminescence decay of the red emission.
ABSTRACT
Undoped and Ag-doped TiO2 ceramics have been prepared at temperatures between 500-1000 °C and under pressures up to 8 GPa. Their crystal structures and physical properties were investigated by means of EDX, SEM, TEM, X-ray powder diffraction, and magnetization M, specific heat Cp and electrical resistance ρ measurements. It is found that the anatase-structured As-cast powder transforms into rutile and columbite-type at 500 °C and 5.5 GPa. The stabilization of the latter phase is fulfilled under a pressure of 8 GPa and at temperatures above 800 °C. On the basis of experimental results, we conclude that the physical properties of TiO2 can be tailored along with its crystal structure. In particular, magnetic properties change from paramagnetic in anatase and rutile to magnetic correlations and in all likelihood magnetic-field-induced antiferromagnetic short-range order in columbite-structured TiO2. Contrasting behaviour in the temperature dependences of specific heat between anatase/rutile and columbite-type TiO2 is obvious. Differently from anatase/rutile, the Cp of columbite-type TiO2 exhibits a low-temperature excess, being interpreted as due to magnetic correlations, or else the prevalence of soft modes. An analysis of ρ(T) for columbite-type TiO2 in the temperature range of 280-400 K reveals the presence of a new trapping state at an energy level of â¼28 meV within the originally forbidden gap. Furthermore, thermal fluctuation-induced tunnelling and hopping conductivities are suggested to govern in a lower temperature range. We recognize that the Ag-doped contents do not alter the crystal structure but considerably enhance magnetic correlations, compared to undoped samples.
