Recently, there has been significant interest toward the development of tunable dielectric materials for voltage-controlled,
frequency-agile phase shifters and filters operating in the microwave regime. The fundamental challenge in designing materials
systems for such tunable devices is the simultaneous requirement of high dielectric tunability (>40%) over a large temperature
interval (−10 °C to +90 °C) coupled with low dielectric losses (between 3.0 dB and 4.0 dB in operational bandwidths ranging
from several hundred MHz up to 30 or more GHz). We show that a high- and temperature-insensitive tunability can be realized
in compositionally graded ferroelectrics and provide a brief review of the results of experimental and theoretical studies
on the dielectric properties of Barium Strontium Titanate (Ba1−xSrxTiO3 or BST) multilayer heterostructures. Theoretically, we discuss the role of thermal stresses on the dielectric properties
using a non-linear thermodynamic model coupled with basic electrostatic considerations to describe the interlayer interactions
between the ferroelectric layers. We show that the thermal strains arising from the thermal expansion coefficient mismatch
between the multilayered film and the substrate may have a significant effect on the dielectric permittivity and tunability
of BST multilayers. Experimentally, compositionally graded BST multilayers (5 mol% MgO doped and undoped) were grown via metallo-organic
solution deposition (MOSD) on Pt–Si substrates and electrically characterized. Optimum conditions were found to exist in BST
multilayers consisting of three distinct layers of ~220 nm nominal thickness with compositions corresponding to Ba0.60Sr0.40TiO3 (BST 60/40), BST 75/25, and BST 90/10. At room temperature, the BST heterostructure has a small-signal dielectric permittivity
of 360 with a dissipation factor of 0.012 and a dielectric tunability of 65% at 444 kV/cm. These properties exhibit minimal
dispersion as a function of temperature ranging from 90 °C to −10 °C. Our results also show that MgO doping improves dielectric
loss (tan δ = 0.008), but results in a moderate dielectric tunability of 29% at 444 kV/cm. Electrical measurements at microwave frequencies
display a decrease in the dielectric permittivity and tunability for both undoped and MgO-doped BST multilayers. At 10 GHz,
the dielectric response, tunability, and the loss characteristics for graded undoped BST are 261, 25% (at 1,778 kV/cm), and
0.078, respectively, and 189 and 15% (at 1,778 kV/cm), and 0.039, respectively, for the MgO-doped graded BST. 相似文献
We here extend the well known positive real lemma (also known as the Kalman-Yakubovich-Popov lemma) to a complex matrix-valued generalized positive rational function, when non-minimal realizations are considered. All state space realizations are partitioned into subsets, each is identified with a set of matrices satisfying the same Lyapunov inclusion. Thus, each subset forms a convex invertible cone, and is in fact is replica of all realizations of positive functions of the same dimensions. We then exploit this result to provide an easy construction procedure of all (not necessarily minimal) state space realizations of generalized positive functions. As a by-product, this approach enables us to characterize systems which can be brought, through a static output feedback, to be generalized positive. 相似文献
A theoretical study of an adsorptive reactor which combines multibed pressure swing adsorption and chemical reaction is presented; such a reactor is referred to as a pressure swing reactor, or PSR. Studies have concentrated on an asymptotic case in which there is the ideal propagation of concentration waves within the reactor beds; the method of characteristics was employed in the solution of the governing PSR equations. The studies assessed the effects of operating conditions, and cycle configurations, on the PSR performance. Calculations indicate enhanced reactant conversion when compared to conventional steady state plug flow operation. In particular, for some reversible reactions, substantial improvements over equilibrium yields have been calculated. For example, for the dissociation reaction 2A B + C, and where B is the only adsorbing component, approximately two-fold improvements over the equilibrium yield of product B have been predicted. Such reaction enhancement can be attributed to the limitation of the backward reaction, which results from the separation of the product species B and C.
In addition to the method of characteristics, a cells-in-series method for the asymptotic case has been developed, and found to yield calculations consistent with the method of characteristics solutions. In a third numerical approach, the spatial discretisation technique of orthogonal collocation on finite elements was applied to the governing PSR equations, and the resulting system of ordinary differential equations solved by a standard integration algorithm. In this case, many of the simplifying model assumptions were relaxed, allowing, for example, the simulation of a non-isothermal PSR with finite mass transfer rates.
One practical significance of reaction enhancement by pressure swing adsorption is a lower temperature of operation than in a conventional reactor. This would lead to savings in the energy requirements of the reactor, and limit the rate and degree of catalyst deactivation due to coke deposition or sintering. 相似文献
The present work dealt with an investigation on tribological behavior of AA5754 matrix reinforced with 10 wt% SiC particles composite by using a pin on disc machine. Sliding tests were conducted under dry and oil lubricated conditions against hardened DIN 100Cr6/EN31 steel counter surface. Wear rates of the matrix alloy and the metal matrix composite (MMC) were measured over load of 100 N under a speed of 4.71 m/s. Detailed scanning electron, optical microscopy analyses were undertaken to clarify the effect of SiC particles on tribological behavior. SiC reinforced AA5754 alloy exhibited lower wear rate than unreinforced alloy. 相似文献
Orange‐emitting phosphorescent copolymers containing iridium complexes and bis(carbazolyl)fluorene groups in their side chains are employed as the emissive layer in multilayer organic light‐emitting diodes (OLEDs). The efficiency of the OLED devices is optimized by varying characteristics of the copolymers: the molecular weight, the iridium loading level, and the nature and length of the linker between the side chains and the polymer backbone. A maximum efficiency of 4.9 ± 0.4%, 8.8 ± 0.7 cd A−1 at 100 cd m−2 is achieved with an optimized copolymer. 相似文献
A non-linear thermodynamic analysis of ferroelectric systems with localized space charges for monolithic and compositionally graded materials is described wherein the electrostatic interlayer interactions are specifically accounted for. The electrostatic coupling is established through the built-in polarization due to the space charges and the intrinsic polarization variations between the ferroelectric layers. The findings show that the polarization hysteresis response of monolithic stress-free barium strontium titanate (BST) ferroelectrics with asymmetrically distributed space charges result in a displacement of the hysteresis loop along the applied electric field axis. In compositionally graded BST multilayers, the hysteresis response is characterized by off-sets along both the polarization and the electric field axes, yet with magnitudes of displacement that are markedly larger than those for monolithic ferroelectrics. 相似文献