X Value (x + value)

Distribution by Scientific Domains

Selected Abstracts

Kinetics and mechanism of the reaction of para -chlorophenyl aryl chlorophosphates with anilines in acetonitrile

Hai Whang Lee
The kinetics and mechanism of the nucleophilic substitution reactions of p -chlorophenyl aryl chlorophosphates (2) with anilines are investigated in acetonitrile at 55C. Relatively large magnitudes of ,X and ,X values are indicative of a large degree of bond making in the TS. Smaller magnitudes of ,X (0.20 for X = H) and ,XY (,0.30) than those for the corresponding reactions with phenyl aryl chlorophosphates (1) (,X = 0.54 for X = H and ,XY = ,1.31) are interpreted to indicate partial electron loss, or shunt, towards the electron acceptor equatorial ligand (p -ClC6H4O-) in the bipyramidal pentacoordinated transition state. The inverse secondary kinetic isotope effects (kH/kD = 0.64,0.87) involving deuterated aniline (ND2C6H4X) nucleophiles, and small ,H, and large negative ,S, are obtained. These results are consistent with a concerted nucleophilic substitution mechanism. 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 632,637, 2002 [source]

Influence of post-extrusion parameters on the final morphology of polystyrene/high density polyethylene blends

H. Padilla-Lopez
The deformation of the dispersed phase in polystyrene/high density polyethylene (PS/HDPE) blends produced by ribbon extrusion was studied numerically and experimentally. A mathematical model for the deformation of the dispersed phase in ribbon extrusion processing of polymer blends was developed assuming uniaxial deformation of the ribbon and the equilibrium shapes of the dispersed particles with a pressure balance over a drop. Simulated morphologies as function of the post-extrusion parameters were obtained and compared with experiments. The analysis of the ribbon extrusion process showed that parameters such as draw ratio (DR) and ribbon-water contact length (X) significantly influence the ribbon dimensions, the extensional stress, and the stretching force. The results also showed that deformation and coalescence of the dispersed phase in the ribbon extrusion processing of polymer blends increase at higher DR and/or lower X values. The comparison between the model and the experimental morphologies of PS/HDPE produced a good agreement. [source]

Hydrothermal Synthesis and Characterization of KxNa(1,x)NbO3 Powders

Jun-Hai Lv
NaNbO3, KNbO3, and KxNa(1,x)NbO3 powders were successfully prepared by the hydrothermal method. The phase of the products was identified to be orthorhombic structure by X-ray diffraction (XRD) technique, and the XRD results revealed that the x value of the KxNa(1,x)NbO3 gradually increased with the increase in the ratio of K+ to Na+ in alkaline solution. The morphology and the microstructure were investigated by scanning electron microscopy, energy-dispersive spectroscopy, and transmission electron microscopy, and the results indicated that the ratio of K+ to Na+ in the solution had a great effect on the morphology and the size of products. Na0.5K0.5NbO3 with morphotropic phase boundary composition could be synthesized when the molar ratio of K+ to Na+ was between 4:1 and 6:1 in the solution. A possible formation mechanism of the KxNa(1,x)NbO3 crystal was also proposed based on the experimental results. [source]

Microstructure and Microwave Dielectric Properties of xCa(Al0.5Nb0.5)O3+(1,x)SrTiO3 Solid Solutions

Mingzhe Hu
The crystal structure evolution and microwave (MW) dielectric properties of xCa(Al0.5Nb0.5)O3+(1,x)SrTiO3 (xCAN+(1,x)ST, 0,x,1.0, ) dielectric ceramics were investigated. X-ray diffraction patterns illustrated that a single perovskite solid solution could be formed between the two end members; however, minor amount of pyrochlore phase was also detected in the composition range of 0.5,x,0.9. With the increase of x value, the crystal structure gradually evolved from a simple Pm3m cubic to a monoclinic P21/n space group with the octahedral tilting and the B-site ordering structure transition subsequently developed. The MW dielectric properties of xCAN+(1,x)ST solid solutions were elaborately investigated and correlated to their crystal structures. The microscopic structure-related thermal parameters in the xCAN+(1,x)ST solid solution were analyzed in terms of the Claussius,Mossotti equation to reveal the original contributors in temperature coefficients. Temperature coefficient-compensated ceramic could be obtained in the xCAN+(1,x)ST system in the range of 0.5[source]

Microwave Dielectric Properties of A-Site Modified Ba(Co0.7Zn0.3)1/3Nb2/3O3 by La3+

Jian Jiang Bian
The sintering behavior, ordering state, and microwave dielectric properties of Ba1,xLa2x/3(Zn0.3Co0.7)1/3Nb2/3O3 Ceramics (0,x,0.06) were investigated in this paper. The X-ray diffraction (XRD) results show that all samples exhibit a single perovskite phase except for the sample with x,0.03. The sinterability is slightly improved by La doping. The long range order (LRO) degree on B-site is greatly increased with the increase of x value up to x=0.015 and then slightly decreased with the further increase of x due to the increasing amount of second phases. The dielectric constant at microwave frequency decreases slightly with the increase of x when x<0.015 and increases slightly with further increasing x for the samples sintered at 1375C/10 h. The Qf value increases with x up to x=0.015 and then decreases with further increase of x, which is consistent with the variation trend of LRO degree. The ,f value decreases slightly with the increase of x up to 0.006, then increases greatly with the further increase of x. An optimized dielectric properties of ,r=34, Qf=63 159, GHz and ,f=5.21 ppm/C were obtained for the x=0.01 sample sintered at 1425C/10 h. [source]

Phase Transition and Failure at High Temperature of Bismuth-Layered Piezoelectric Ceramics

Liaoying Zheng
High-temperature bismuth-layered piezoelectric ceramics (CaxSr1,x)Bi4Ti4O15 have been prepared by the conventional solid reaction method. Our results reveal that there is a "phase transition induced by the composition" taking place in the solid solution at around x=0.4. During transition, the crystal structure changes from orthorhombic to pseudo-tetragonal and then back to orthorhombic. Although all (CaxSr1,x)Bi4Ti4O15 (with any x value) undergo a second-order ferroelectric-to-ferroelectric phase transition at about 200C below their Curie temperature, only the ceramic with x=0.4 exhibits a failure in piezoelectric properties during the transition. It is suggested that, on the basis of the X-ray photoelectron spectroscopy results, this material failure is induced by the high concentration of oxygen vacancies in the material during the phase transition. [source]

Characterization and electrical properties of new perovskite films of Ba(Ti,Zr)O3 type doped with lanthanum (BLZT)

E. Delgado
Abstract In the current work, we characterized a new family of compounds with perovskite-type structure with the general formula: Ba1,yLa2y/3Ti1,xZrxO3 (BLZT). Through XRD on crystalline powder we detected a single perovskite cubic phase, Pm3m (221), for the compounds with zirconium content (x) lower than 0.15 and lanthanum content (La) lower than 0.17. Upon increasing the x value in BLZT compounds, the value of La falls to 0.13. Morphology studies through SEM revealed that the incorporation of lanthanum into the composition of Ba(Ti,Zr)O3 (BZT) compounds produces the formation of bigger grains and materials with greater density. The stoichiometry for each compound was corroborated by atomic emission spectroscopy (AES-ICP). The lanthanum-doped compounds demonstrated higher relative permittivity values with respect to the BZT and a decrease of the Curie temperature (Tc) with relation to the amount of La3+ ion present in the structure. Finally, we grew thin films by using a target of the compound Ba0.90La0.067Ti0.91Zr0.09O3 over three different substrates under two atmospheres (Ar and O2), using the Magnetron Sputtering technique by Radio Frequency (RF), showing the best results when growth is over the substrate SrTiO3.Nb 0.1% in the [00l] direction under an O2 atmosphere. ( 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Raman studies and optical properties of some (PbO)x(Bi2O3)0.2(B2O3)0.8,x glasses

Katerina Kotkova
Abstract Five (PbO)x(Bi2O3)0.2(B2O3)0.8,x glasses, where x = 0, 0.2, 0.3, 0.4 and 0.6, were prepared. The dilatometric glass transition temperature (Tg) was found in the region 470 (x = 0), Tg ( C) , 347 (x = 0.6), and the density (,) varied within 4.57 (x = 0) , , (g/cm3) , 8.31 (x = 0.6). Raman spectra indicated the conversion of BO3 to BO4 entities for low x values but for x > 0.3, namely, for x , 0.6, back-conversion occurred, most probably. From the measurements of the optical transmission on very thin bulk samples, the room temperature optical gap values (Eg) were determined to be in the range 4.03 (x = 0), Eg (eV) , 3.08 (x = 0.6). The temperature (T) dependence of the optical gap (Eg(T)) in the region 300 , T(K) , 600 was examined and approximated by a linear relationship of the form of Eg(T) = Eg(0), ,T, where , 10,4(eV/K) varied from 5.1 to 6.8. The non-linear refractive index (n2) was estimated from the optical gap values and it was found to correspond to the n2 values calculated from the experimental third-order non-linear optical susceptibility taken from the literature. Copyright 2008 John Wiley & Sons, Ltd. [source]

Effect of lanthanum substitution on the Raman spectra of barium titanate thin films

P. S. Dobal
Abstract Thin films of Ba1,xLaxTiO3 on platinum substrates were synthesized using the sol,gel method for x values of 0.0, 0.03, 0.05, and 0.10, and the effect of trivalent La3+ substitution on the structural and dielectric properties was studied. Using X-ray diffraction, structural analysis of these compositions revealed a slight increase in the tetragonal distortion of the unit cell with increase in La content. Accordingly, an increase in the tetragonal to cubic transition temperature TT/C was detected by temperature-dependent Raman spectroscopy in the range of 70,500 K. Unlike the results from Raman scattering for the La-doped BaTiO3 films, the dielectric measurements showed broad and diffused dielectric maxima, making the estimation of the transition temperature merely qualitative. Copyright 2006 John Wiley & Sons, Ltd. [source]

Hg(1-x)CdxTe from short to long wave infrared on Si substrates grown by MBE

M. F. Vilela
Abstract In this paper, we show the power of using molecular beam epitaxy (MBE) for the growth of the mercury cadmium telluride (HgCdTe) system. Abrupt composition profiles, changes in doping levels, or switching doping types are easily performed. It is shown that high quality material is achieved with Hg(1- x)CdxTe grown by MBE from a cadmium mole fraction of x =0.15 to x =0.72. Doping elements incorporation as low as 1015 cm -3 for both n-type and p-type material as well as high incorporation levels >1018cm -3 for both carrier types were achieved. Secondary ion mass spectrometry (SIMS) data, x-ray data, Hall data, the influence of doping incorporation with cadmium content and growth rate, etch pit density (EPD), composition uniformity determined from Fourier transform infrared (FTIR) transmission spectroscopy, and surface defect maps from low to high x values are presented to illustrate the versatility and quality of HgCdTe material grown by MBE. ( 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Linear regression analysis for comparing two measurers or methods of measurement: But which regression?

John Ludbrook
Summary 1. There are two reasons for wanting to compare measurers or methods of measurement. One is to calibrate one method or measurer against another; the other is to detect bias. Fixed bias is present when one method gives higher (or lower) values across the whole range of measurement. Proportional bias is present when one method gives values that diverge progressively from those of the other. 2. Linear regression analysis is a popular method for comparing methods of measurement, but the familiar ordinary least squares (OLS) method is rarely acceptable. The OLS method requires that the x values are fixed by the design of the study, whereas it is usual that both y and x values are free to vary and are subject to error. In this case, special regression techniques must be used. 3. Clinical chemists favour techniques such as major axis regression (,Deming's method'), the Passing,Bablok method or the bivariate least median squares method. Other disciplines, such as allometry, astronomy, biology, econometrics, fisheries research, genetics, geology, physics and sports science, have their own preferences. 4. Many Monte Carlo simulations have been performed to try to decide which technique is best, but the results are almost uninterpretable. 5. I suggest that pharmacologists and physiologists should use ordinary least products regression analysis (geometric mean regression, reduced major axis regression): it is versatile, can be used for calibration or to detect bias and can be executed by hand-held calculator or by using the loss function in popular, general-purpose, statistical software. [source]