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排序方式: 共有1089条查询结果,搜索用时 15 毫秒
1.
Maria Yousaf Lodhi Muhammad Azhar Khan Majid Niaz Akhtar Muhammad Farooq Warsi Asif Mahmood Shahid M. Ramay 《Ceramics International》2018,44(3):2968-2975
The influence of neodymium and nickel substitution on structural and dielectric parameters was investigated in strontium-barium X-type hexagonal ferrites having composition SrBaCu2?xNixNdyFe28?yO46 (x = 0, 0.2, 0.4, 0.6, 0.8, 1 and y = 0, 0.02, 0.04, 0.06, 0.08, 0.1). Sol-gel method was employed for synthesizing these hexagonal ferrites. The XRD plots of all studied materials which were annealed at 1250 °C show single phase characteristics. Lattice parameter ‘c’ increased as a consequence of larger radius of rare earth ion (Nd3+) as compared to (Fe3+), while lattice parameter ‘a’ showed very small variation. The cell volume was obtained in the range 2508.32–2523.75 (Å3). The inclusion of Nd-Ni also affected X-ray density, bulk density and porosity. The FTIR spectroscopy indicated the particular absorption peaks of hexagonal ferrites and it was performed in the range of 500–700 cm?1. On account of Nd-Ni doping, the dielectric constant, dielectric loss and AC-conductivity showed decreasing trend. The occupancy of Nd3+ ions at octahedral site impedes the valence alternation of Fe3+; therefore there was decrease in dielectric permittivity. Ac conductivity has been decreased from 9.14 to 6.49 (Ω cm)?1 at frequency of 2.7 GHz. The Cole-Cole plots of synthesized materials noticeably revealed grain boundary contribution. The appearance of single semi-circle in impedance Cole-Cole graphs confirms the exceptional role of grain boundaries in the conduction process. The considerably lower dielectric parameters of investigated nano X-type ferrites propose their feasibility for high-frequency applications (phase shifters, dielectric resonators, stealth technology etc). 相似文献
2.
During the last few years a number of space-time block codes have been proposed for use in multiple transmit antennas systems. We propose a method to extend any space-time code constructed for m transmit antennas to m p transmit antennas through group-coherent codes (GCCs). GCCs make use of very limited feedback from the receiver (as low as 1 bit). In particular the scheme can be used to extend any orthogonal code (e.g., Alamouti code) to more than two antennas while preserving low decoding complexity, full diversity benefits, and full data rate. 相似文献
3.
Crystallization behaviour of amorphous Ni60 Nb40-x
Cr
x
(x = 0, 5, 10 and 13 at%) alloys was studied by differential Scanning calorimetry and X-ray diffraction measurements. It is shown that the addition of chromium reduces the crystallization temperature, stages of crystallization and activation energies associated with the crystallization sages of the Ni60Nb40 glass. Crystallization of the Ni60Nb40 glass occurred in three stages; in the initial stage a metastable M-phase formed in the amorphous matrix as reported earlier [1] . However, contrary to earlier observation [1], M -phase was not very stable and transformed together with some amorphous phase to the equilibrium Ni3Nb phase in the second stage of crystallization. In the third stage, the remaining amorphous matrix transformed to the equilibrium NiNb phase. On addition of chromium the formation/stability of the M-phase was found to be suppressed and equilibrium NbCr2 phase precipitated preferentially in the first stage. The second stage, corresponding to the formation of Ni3Nb phase, remained almost unaltered. The third stage corresponding to the crystallization of NiNb phase disappeared completely at 13 at% Cr. In the fully crystallized samples the proportion of the NiNb phase decreased and that of NbCr2 phase increased continuously with chromium concentration. 相似文献
4.
Fine particles of cadmium carbonate (CdCO3) were prepared by heating aqueous solutions containing dissolved urea and cadmium sulfate at 85°C for 1 h. Effect of the composition of the reactant mixture on the morphology of the precipitated particles was investigated. It was found that the cadmium sulfate content of the reactant mixture significantly affected the shape and size of the precipitated particles. At certain range of reactant mixture composition, cauliflower-shaped uniform particles were obtained. The later particles were coated with basic copper carbonate (Cu2(OH)2CO3) by heating aqueous dispersions, containing CdCO3 particles, copper (II) nitrate, and urea at 85°C for various periods of time with constant stirring. The coating process was found sensitive to the experimental conditions and in most of the trials; mixtures of the coated and coating precursor particles were obtained. As such, extensive optimizations were carried out and conditions were established for the production of uniformly coated particles.On calcination at 700°C for 1 h, the CdCO3 particles converted into CdO, whereas the coated particles (CdCO3
(core)/Cu2(OH)2CO3
(coating) transformed into CdO(core)/CuO(coating)) without sintering. In the coated particles, the core and coating materials stayed mutually inert during the calcination reaction and independently converted into their respective oxides.All the calcined and uncalcined products were characterized by various physical and chemical methods. 相似文献
5.
The interaction of different metal oxides such as Co3O4, NiO, Al2O3, Cr2O3, Fe2O3 and SiO2 with Na2SO4 at a temperature of 1100 and 1200 K in flowing oxygen has been studied. The thermogravimetric studies for each system were
carried out as a function of Na2SO4 in the mixture. The presence of different constituents in the reaction products were identified by X-ray diffraction analysis
and the morphologies of the reaction products were characterized using metallography and scanning electron microscopy (SEM).
The formation of products was also investigated by thermodynamic computation of free energies of the reactions and the study
of relevant equilibrium phase diagrams. The soluble species in the aqueous solutions of the reaction products were determined
quantitatively using atomic absorption spectrophotometry.
The high temperature interaction products usually contain a 3-phase structure namely, Na2O·M2O
x
, M2O
x
and metal sulphide and/or metal sulphate. The formation of Na2O·M2O
x
depends upon the solid state solubility of metal oxide in the molten salt at high temperatures. Under limited solubility
conditions Na2O·M2O
x
is invariably formed, but as soon as this condition is relaxed the oxide. M2O
x
, precipitates and forms a separate phase. 相似文献
6.
This paper presents the current understanding of the flame retardant mechanism of Casico?. The study includes the flame retardant effect of each individual component: ethylene–acrylate copolymer, chalk and silicone elastomer, as well as the formation of an intumescent structure during heating. The flame retardant properties were investigated by cone calorimetry and oxygen index tests. To obtain insight into the flame retardant mechanism, heat treatment under different conditions has also been performed. The results indicate that the flame retardant mechanism of Casico is complex and is related to a number of reactions, e.g. ester pyrolysis of acrylate groups, formation of carbon dioxide by reaction between carboxylic acid and chalk, ionomer formation and formation of an intumescent structure stabilized by a protecting char. Special emphasis is given to the formation of the intumescent structure and its molecular structure as evaluated from 13C MAS‐NMR and 29Si MAS‐NMR, ESCA and XRD analysis. After treatment at 500°C the intumescent structure consists mainly of silicon oxides and calcium carbonate and after treatment at 1000°C the intumescent structure consists of calcium silicate, calcium oxide and calcium hydroxide. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
7.
Shubneesh Batra Nanseng Jeng Akif Sultan Kyle Picone Surya Bhattacharya Keun-Hyung Park Sanjay Banerjee David Kao Monte Manning Chuck Dennison 《Journal of Electronic Materials》1993,22(5):551-554
When dopants are indiffused from a heavily implanted polycrystalline silicon film deposited on a silicon substrate, high thermal
budget annealing can cause the interfacial “native” oxide at the polycrystalline silicon-single crystal silicon interface
to break up into oxide clusters, causing epitaxial realignment of the polycrystalline silicon layer with respect to the silicon
substrate. Anomalous transient enhanced diffusion occurs during epitaxial realignment and this has adverse effects on the
leakage characteristics of the shallow junctions formed in the silicon substrate using this technique. The degradation in
the leakage current is mainly due to increased generation-recombination in the depletion region because of defect injection
from the interface. 相似文献
8.
J Sra S Bremner D Krum A Dhala Z Blanck S Deshpande M Biehl H Li MR Jazayeri M Akhtar 《Canadian Metallurgical Quarterly》1997,20(6):1613-1618
Atrial defibrillation can be accomplished using low energy shocks and transvenous catheters. The biphasic waveform tilt required to achieve optimal atrial defibrillation thresholds (ADFTs) is, however, not known. The effect of single capacitor biphasic waveform tilt modification on ADFT was assessed in 20 patients. Following AF induction the defibrillation pulses were delivered between the catheters positioned in the coronary sinus and the right atrium. The single capacitor biphasic waveform shocks, delivered over the same pathways, consisted of 65% tilt (65/65 biphasic waveform) to produce an overall tilt of 88%, or 50% tilt (50/50 biphasic waveform) to produce an overall tilt of 75%. Although 65/65 biphasic waveform delivers more energy, the shorter duration 50/50 biphasic waveform reduced stored energy ADFT 21%, from 1.34 +/- 0.82 J with 65/65 biphasic to 1.06 +/- 0.81 J. These differences were not statistically significant. Nine patients had lower ADFT with 50/50 biphasic waveform while five patients had lower ADFT with 65/65 biphasic waveform. Equivalent reduction in ADFT was seen in the remaining six patients. The ADFT was 0.83 +/- 0.65 J when both tilts were considered. In conclusion, biphasic waveform tilt modification may affect the ADFT in an individual patient. The optimal biphasic waveform for ADFT is not known. 相似文献
9.
S. Akhtar D. S. Bernstein 《International Journal of Adaptive Control and Signal Processing》2005,19(10):745-767
Discrete‐time model reference adaptive control (MRAC) is considered with both least squares and projection algorithm parameter identification. For both cases complete Lyapunov proofs are given for stability and convergence. The results extend the approach of Johansson (Int. J. Control 1989; 50 (3):859–869) to include Lyapunov stability for MRAC when the normalized projection algorithm is used for parameter identification. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
10.
Rutherfordα-particles backscattering technique was employed for measurements of diffusion rates in metallic glasses. Effects of relaxation,
crystallization and plastic deformation on diffusion rates were also investigated.
It has been observed that the diffusion rates of a metallic solute are of the same orders of magnitude in both metal-metal
and metal-metalloid glasses. A higher diffusivity is likely if there is a large difference between melting points of the solute
and matrix. Relaxation has no effect on diffusion, however, diffusivity increases on crystallization. An increase in diffusivity
is also observed on plastic deformation of metallic glass. 相似文献