Search results for "Impurity"
showing 10 items of 330 documents
Quantum chemical modelling of polarons and perovskite solid solutions
2001
Abstract Following our previous study [J. Phys.: Condens. Matter 10 (1998) 6271] of a single Nb impurity and Nb clusters in KTaO 3 , we present results of the calculations for a series of perovskite KNb x Ta 1− x O 3 (KTN) solid solutions ( x =0, 0.125, 0.25, 0.75, 1). The quantum chemical method of the intermediate neglect of the differential overlap (INDO) combined with the large unit cell (LUC) periodic model is used. According to the INDO calculations, Nb impurity becomes off-center in KTaO 3 already at the lowest studied concentrations ( x =0.125), in a good agreement with XAFS measurements. We compare our results with previous ab initio FP-LMTO calculations. Quantum chemical calculati…
Ab initio calculations of the hydroxyl impurities in BaF2
2011
Abstract OH − impurities in BaF 2 crystal have been studied by using density functional theory (DFT) with hybrid exchange potentials, namely DFT-B3PW. Three different configurations of OH − impurities were investigated and the (1 1 1)-oriented OH − configuration is the most stable one. Our calculations show that OH − as an atomic group has a steady geometrical structure instead of electronic properties in different materials. The studies on band structures and density of states (DOS) of the OH − -impurity systems indicate that there are two defect levels induced by OH − impurities. The two superposed occupied OH − -bands located 1.95 eV above the valance bands (VB) at Γ point mainly consist…
Cleaning sorbents used in matrix solid-phase dispersion with sonication: Application to the estimation of polycyclic aromatic hydrocarbons at ng/g le…
2012
This study shows that ultrasonic assisted extraction is a good practise for removing impurities from sorbents used for matrix-solid phase dispersion (MSPD). A previous washing of the sorbent with an organic solvent or mixture of solvents eliminates part of the interferences, but this treatment is not enough for the quantification of PAHs at ng/g levels. It is demonstrated that the determination of traces of polycyclic aromatic hydrocarbons (PAHs) in solid samples processed by matrix-solid phase dispersion (MSPD) may be severely affected by the presence of sorbent impurities. Different extraction solvents and sonication conditions have been tested, and on the basis of the results obtained a …
Defects in glasses
1995
Abstract The absence of long range order in the glass structure allows to define only point defects in these materials. They are: 1) intrinsic defects—atomic size local deviation from short range order; 2) impurity defects—isolated impurity atoms or ions in the glass network; 3) intrinsic impurity defects—complexes consisting of the impurity atoms chemically bonded to one of the intrinsic defect atoms. The latter defects are characteristic for the doped glasses. Presence of point defects in glasses introduces new spectroscopic properties of these solid materials. Defect generation, interaction and recombination reactions resulting from the external influence causes the glass spectroscopic p…
Solubility and microstructural development of TiO2-containing 3Al2O3·2SiO2 and 2Al2O3·SiO2 mullites obtained from single-phase gels
2007
Abstract The interdependence of the titanium oxide amount and the anisotropic growth of mullites prepared from single-phase gels were investigated. Gels with stoichiometries 3(Al2−xTixO3)·2(SiO2) and 2(Al2−xTixO3)·(SiO2), with 0 ≤ x ≤ 0.15 were prepared by the semialkoxide method. Gels and specimens heated at temperatures between 1200 and 1600 °C were characterized by using infrared spectroscopy (IR), X-ray diffraction (XRD) and transmission and field emission scanning electron microscopies (TEM and FESEM). Al2TiO5 as minor impurity was detected in both series of mullites for gel precursor compositions x = 0.10 and x = 0.15, obtained at temperatures between 1200 and 1600 °C. Variations of l…
Mechanical properties of carbon nanotube fibres: St Venant’s principle at the limit and the role of imperfections
2015
Abstract Carbon nanotube (CNT) fibres, especially if perfect in terms of their purity and alignment, are extremely anisotropic. With their high axial strength but ready slippage between the CNTs, there is utmost difficulty in transferring uniformly any applied force. Finite element analysis is used to predict the stress distribution in CNT fibres loaded by grips attached to their surface, along with the resulting tensile stress–strain curves. This study demonstrates that, in accordance with St Venant’s principle, very considerable length-to-diameter ratios (∼103) are required before the stress becomes uniform across the fibre, even at low strains. It is proposed that lack of perfect orienta…
Influence of impurity scattering on Drude response in heavy-fermion UPd2Al3
2010
The frequency-dependent conductivity of heavy-fermion metals can often be described within the picture of the Drude response: the transport relaxation rate is the only relevant frequency scale and, furthermore, reduced by orders of magnitude compared to normal metals. While the relaxation-time enhancement corresponds to the effective-mass enhancement in these materials, i.e. a fundamental material characteristic, the absolute value of the relaxation time depends on the details of the relevant scattering processes. Here we discuss the influence of impurity scattering on the Drude response of the heavy fermions in UPd2Al3 by comparing different thin film samples.
Transition levels of acceptor impurities in ZnO crystals by DFT-LCAO calculations
2018
This research was partly supported by the Kazakhstan Science Project № AP05134367«Synthesis of nanocrystals in track templates of SiO2/Si for sensory, nano-and optoelectronic applications» and Latvian Super Cluster (LASC), installed in the Institute of Solid State Physics (ISSP) of the University of Latvia. Authors are indebted to D. Gryaznov, A. Popov and A. Dauletbekova for stimulating discussions.
Nonazeotropy in the System Methyl Ethanoate + 1,2-Epoxybutane
1997
New vapor−liquid equilibrium data for the binary system methyl ethanoate + 1,2-epoxybutane are reported at 35 and 101.3 kPa and at 298.15 K. It is shown that no azeotropes are present in the system, in disagreement with data reported by others who claim that two azeotropes are present at 298.15 K. We claim that previously reported data are in error because of impurities in the reagents and experimental errors. According to our results, the system methyl ethanoate + 1,2-epoxybutane behaves like a regular solution and its vapor phase can be considered practically ideal so that it cannot fulfill the necessary conditions for multiple azeotropy. The data were satisfactorily correlated using the …
<title>Annealing of radiation defects in x-irradiated LiBaF<formula><inf><roman>3</roman></inf></formula>&l…
2003
Results of the glow rate technique application for analysis of the activation energy of thermostimulated annealing of X-ray created F-type color centers in pure and containing oxygen centers LiBaF3 crystals are presented. It is shown that depending on the impurity composition two alternative mechanisms could be involved in the annealing of color centers. It is proposed that either the anion vacancy governed migration of F-centers resulting in recombination with complementary defects, or the thermal delocalization of radiation created fluorine (Fi) interstitials captured by anti-structure defects followed by recombination with all kinds of complementary F-type centers are responsible for the…