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1.
A complete theory of diffusion of absorbing gases in porous solids is developed. Both ordinary and thermal diffusion are considered. The thermodynamics of irreversible processes is used to derive the general flux equations.A new theoretical model of surface diffusion is presented. Moreover, the proper method of combining surface and gas-phase fluxes is established. The total flux in the pores is represented by a 5 parameter model, which includes the effects of (1) free (gaseous) diffusion, (2) Knudsen diffusion, and (3) surface diffusion.The design of a new diffusion cell is presented along with a stagewise method for analyzing results. The cell is unique in that specimens mounted in copper clad circuit board material permits nonisothermal as well as isothermal operation. Earlier designs were confined to isothermal operation due to mounting specimens in metallic materials such as brass and aluminum.  相似文献   

2.
Assessment of the dielectrophoresis (DEP) cross-over frequency (fxo), cell diameter, and derivative membrane capacitance (Cm) values for a group of undifferentiated human embryonic stem cell (hESC) lines (H1, H9, RCM1, RH1), and for a transgenic subclone of H1 (T8) revealed that hESC lines could not be discriminated on their mean fxo and Cm values, the latter of which ranged from 14 to 20 mF/m2. Differentiation of H1 and H9 to a mesenchymal stem cell-like phenotype resulted in similar significant increases in mean Cm values to 41–49 mF/m2 in both lines (p < 0.0001). BMP4-induced differentiation of RCM1 to a trophoblast cell-like phenotype also resulted in a distinct and significant increase in mean Cm value to 28 mF/m2 (p < 0.0001). The progressive transition to a higher membrane capacitance was also evident after each passage of cell culture as H9 cells transitioned to a mesenchymal stem cell-like state induced by growth on a substrate of hyaluronan. These findings confirm the existence of distinctive parameters between undifferentiated and differentiating cells on which future application of dielectrophoresis in the context of hESC manufacturing can be based.  相似文献   

3.
The rate of evaporation of electrons, νe, from a thoriated tungsten filament depends on the temperature T and on θ, the fraction of the surface covered by thorium atoms. The relation of νe to θ and T has been given by Brattain and Becker. From νe the change in contact potential V of the filament surface produced by the adsorbed thorium can be calculated by the Boltzmann equation. Knowing σ the number of thorium atoms per cm.3, the dipole moment M of each is given by V = 2τσM. By an equation already used for Cs films on tungsten, the 2-dimensional equation of state of the adsorbed film can be calculated from M. Then by Gibbs' adsorption equation the relation of the atom evaporation rate νa to θ and T can be determined. It is thus calculated that in the range from θ = 0.2 to θ = 0.6, νa varies in proportion to ?Hθ where H = 8.1 in good agreement with the value H = 7.8 given by Brattain and Becker's measurements. A recalculation of the data of numerous experiments in 1921–1923, using the new relations of νe to θ1 gives data on νa as a function of T and θ in good agreement with the values of νa calculated from νe.The diffusion coefficients of Th through tungsten crystals, along grain boundaries and over the free filament surface, are calculated. A theory is given for the cause of the variation in the surface diffusion coefficient with σ. The probable mechanism of the production of the metallic thorium within the filament is discussed. At 2400° the thorium which arrives at the surface along grain boundaries, for some unknown reason, does not spread out over the surface as it does at lower temperatures (1900–2100°).  相似文献   

4.
Let us show how property is grasped as an institutional fact. If Jones steals a computer, he does not own it in the sense of property, but only exercises control towards it. If he buys the computer, he controls it too, and moreover owns it in the sense of property. In other words, simply exercising control towards something is a brute fact. This control counts as property only in a certain context: the computer counts as Jones’s property only if he got it through a licit transfer. This is why property is not a brute fact, and is therefore an institutional fact. The same kind of reasoning applies to privacy. When a personal information P about Jones is openly diffused, it seems that P becomes public. From this point of view, a violation of privacy equates to a publication. The problem about this account is the following: who would call “publication of a book” the hacking of it on its author’s computer? No one, because the word “publication” is an institutional word that only refers to a licit diffusion. Considering this answer, we may conclude as follows: if the diffusion of P is illicit, P still counts as private, even if everyone knows about it. If that conclusion is true, privacy is an institutional fact.  相似文献   

5.
A number of numerical codes have been written for the problem of finding the circle of smallest radius in the Euclidean plane that encloses a finite set P of points, but these do not give much insight into the geometry of this circle. We investigate geometric properties of the minimal circle that may be useful in the theoretical analysis of applications. We show that a circle C enclosing P is minimal if and only if it is rigid in the sense that it cannot be translated while still enclosing P. We show that the center of the minimal circle is in the convex hull of P. We use this rigidity result and an analysis of the case of three points to find sharp estimates on the diameter of the minimal circle in terms of the diameter of P.  相似文献   

6.
A droplet-based micro-total-analysis system involving biosensor performance enhancement by integrated surface-acoustic-wave (SAW) microstreaming is shown. The bioreactor consists of an encapsulated droplet with a biosensor on its periphery, with in situ streaming induced by SAW. This paper highlights the characterization by particle image tracking of the speed distribution inside the droplet. The analyte-biosensor interaction is then evaluated by finite element simulation with different streaming conditions. Calculation of the biosensing enhancement shows an optimum in the biosensor response. These results confirm that the evaluation of the Damköhler and Peclet numbers is of primary importance when designing biosensors enhanced by streaming.It has been pointed out that biosensing performances can be limited by the diffusion of the analytes near the sensing surface.1 In the case of low Peclet number hydrodynamic flows, typical of microfluidic systems, molecule displacements are mainly governed by diffusive effects that affect time scales and sensitivity. To overcome this problem, the enhancement of biosensor performance by electrothermal stirring within microchannels was first reported by Meinhart et al.2 Other authors3, 4 numerically studied the analyte transport as a function of the position of a nanowire-based sensor inside a microchannel, stressing on the fact that the challenge for nanobiosensors is not the sensor itself but the fluidic system that delivers the sample. Addressing this problem, Squires et al.5 developed a simple model applicable to biosensors embedded in microchannels. However, the presented model is limited to the case of a steady flow. The use of surface-acoustic waves (SAWs) for stirring in biomicrofluidic and chemical systems is becoming a popular investigation field,6, 7, 8, 9 especially to overcome problems linked to steady flows by enhancing the liquid∕surface interaction.1, 10, 11 The main challenges that need to be addressed when using SAW-induced stirring are the complexity of the flow and its poor reproducibility. However, some technical solutions were proposed to yield a simplified microstreaming. Yeo et al. presented a centrifugation system based on SAW that produces the rotation of the liquid in a droplet in a reproducible way by playing on the configuration of the transducers and reflectors,12 and presented a comprehensive experimental study of the three-dimensional (3D) flow that causes particle concentration in SAW-stirred droplets,13 revealing the presence of an azimuthal secondary flow in addition to the main vortexlike circular flow present in acoustically stirred droplets. The efficiency of SAW stirring in microdroplets to favorably cope with mass transport issues was finally shown by Galopin et al.,14 but the effect of the stirring on the analyte∕biosensor interaction was not studied. It is expected to overcome mass transport limitations by bringing fresh analytes from the bulk solution to the sensing surface.The studied system, described in Fig. ​Fig.1,1, consists of a microliter droplet microchamber squeezed between a hydrophobic piezoelectric substrate and a hydrophobic glass cover. Rayleigh SAWs are generated using interdigitated transducers (interdigital spacing of 50 μm) laid on an X-cut LiNbO3 substrate.1, 15, 16 The hydrophobicity of the substrate and the cover are obtained by grafting octadecyltrichlorosilane (OTS) self-assembled monolayers (contact angle of 108° and hysteresis of 9°). To do so, the surface is first hydroxylized using oxygen plasma (150 W, 100 mT, and 30 sccm3 O2) during 1 min and then immersed for 3 h into a 1 mM OTS solution with n-hexane as a solvent.Open in a separate windowFigure 1(a) General view of the considered system. (b) Mean value of the measured speeds within the droplet as a function of the inlet power before amplification.When Rayleigh waves are radiated toward one-half of the microchamber, a vortex is created in the liquid around an axis orthogonal to the substrate due to the momentum transfer between the solid and the liquid. This wave is generated under the Rayleigh angle into the liquid.Speed cartographies of the flow induced in the droplet are realized using the particle image tracking technique for different SAW generation powers. To do so, instantaneous images of the flow are taken with a high-speed video camera at 200 frames∕s and an aperture time of 500 μs on a 0.25 μl droplet containing 1 μm diameter fluorescent particles. Figure ​Figure11 shows the mean speed measured in the droplet as a function of the inlet power. The great dependence of the induced mean speed with the SAW power enables a large range of flow speeds in the stirred droplet. Moreover, the flow was visualized with a low depth of field objective. It was found to be circular and two dimensional (2D) in a large thickness range of the droplet.The binding of analytes to immobilized ligands on a biosensor is a two step process, including the mass transport of the analyte to the surface, followed by a complexation step,Abulk↔kmAsurface+B↔ka,kdAB(1)with km as the constant rate for mass transport from and to the sensor, and ka and kd as the constant rates of association and dissociation of the complex.At the biosensor surface, the reaction kinetics consumes analytes but their transport is limited by diffusive effects. In this case, the Damköhler number brings valuable information by comparing these two effects. Calling the characteristic time of reaction and diffusion, respectively, τC and τM, the mixing time in diffusion regime can be approximated by τM≈h2∕D with D as the diffusion coefficient and h a characteristic length of the microchannel. Calling RT the ligand concentration on the surface in mole∕m2, the Damköhler number (Da) can be written asDa=τMτC=kaRThD.(2)Depending on the type of reaction, the calculation of Da helps determine if a specific biointeraction will benefit from a mass SAW-based microstreaming. If the Damköhler number is low, the reaction is slow compared to mass transport and the reaction will not significantly benefit from microstirring. For example, the hybridization of 19 base single stranded DNA in a microfluidic system with a characteristic length of 500 μm is characterized by a Damköhler number of 0.07 and is therefore not significantly influenced by mass transport. On the contrary, the binding of biotin to immobilized streptavidin is characterized by a Da number of approximately 104. In this case, the stirring solution will significantly improve the reaction rate.COMSOL numerical simulations were carried out to study the efficiency of the SAW stirring in the case of a droplet-based microbioreactor with a diameter of 1 mm. Assuming a 2D flow, the simulated model takes into account the convective and diffusive effects in the analyte-carrying fluid and the binding kinetics on the biosensor surface. This approach was thoroughly developed by Meinhart et al.2On the biosensor surface, the following equations are solved:∂B∂t=kacs(RT−B)−kdB,(3)∂B∂t=D|∂c∂y|y=0(4)with c as the local concentration of analytes in the droplet and B as the surface concentration of bound analytes on the biosensor surface. Simulation results show that a depleted zone is formed near the biosensor in the case of an interaction without stirring. This zone is characterized by a low concentration of analytes and results from the trapping of analytes on the biosensor surface, thus creating a concentration gradient on the vicinity of the biosensor. When stirring is applied, the geometry of the depleted zone is modified, as it is pushed in the direction of the flow. The geometry of the depleted zone then depends on many parameters, among which the diffusion coefficient D, the speed distribution of the flow (not only near the biosensor but also in the whole microfluidic system), and the reaction kinetics on the biosensor. In our case, which is assimilated to a simple circular flow, the depleted zone reaches a permanent state consisting of an analyte-poor layer situated in the exterior perimeter of the stirred droplet. The diffusion of analytes is then limited again by diffusion from the inner part of the droplet toward its exterior perimeter (see Fig. ​Fig.22).Open in a separate windowFigure 2(a) Mean concentration of bound analytes vs time for different mean flow speeds. (b) The obtained concentration profiles with and without circular stirring, t=10 000 s.The initial analyte and receptor concentrations are, respectively, 0.1 nM in the solution and 3.3×10−3 nM m on the biosensor surface, the diffusion coefficient is D=10−11 m2 s−1, and the reaction constants are ka=106 M−1 s−1 and kd=10−3 s−1. Simulations show that the mean concentration of bound analytes highly increases with the flow speed, improving the efficiency of the biosensing device. To evaluate the benefits of in situ microstreaming with SAW, the same simulations were conducted for Da numbers ranging from 104 to 108 M−1∕s, by ranging the diffusion coefficient from 4×10−12 to 4×10−9 m2∕s, and the association coefficient ka from 104 to 108 M−1∕s. The enhancement factor of analyte capture, defined as the ratio of the binding rate with streaming B and the binding rate without streaming B0, is plotted in Fig. ​Fig.33 for different values of Da. Calculations are done in the case of a mean flow speed of 0.5 mm∕s.Open in a separate windowFigure 3(a) Enhancement factor (defined as the ratio between binding rate with streaming B and binding rate without streaming B0) for different Damkhöler numbers and (b) normalized enhancement factor for different Peclet numbers.One can notice the saturation of the enhancement factor curve for large value of Da to the value of 3.5 for high Da. This can be explained by the fact that for large ka∕Da ratios, the analytes, which normally require penetration in the depleted zone by diffusion, do not have time to interact with the biosensor when they pass in the vicinity of its surface. The efficiency of the streaming is then reduced for large values of Da. In the case of our specific flow configuration, the enhancement factor reaches 3.2 for the interaction of streptavidin on immobilized biotin (Da=103).The reported simulation results can be compared to an experimental value obtained using the droplet-based surface plasmon resonance sensor streamed in situ using SAW reported by Yeo et al.12 By monitoring the streptavidin∕biotin binding interaction on an activated gold slide, they showed that SAW stirring brings an improvement factor of more than 2. This difference can be accounted to the high complexity of the induced 3D flow, which was modeled in a simple manner in our calculations.Other factors must be taken into account when optimizing the improvement factor, such as the flow velocity and the characteristic length of the mixing. To do so, the Peclet number allows the comparison of the convective and diffusive effects.17 For δC a typical variation in concentration on the distance h, the Peclet number is given byPe=UhD.(5)A significantly high Peclet number causes a decrease in biosensing efficiency as the analytes do not have enough time to interact with the biosensing surface by diffusion through the analyte-poor layer. On the contrary, the case of a low Peclet number corresponds to the diffusion-limited problem. Therefore, for each Damköhler number, there is a Peclet number optimizing this factor. To illustrate this fact, Fig. ​Fig.3b3b shows the calculation of the enhancement factor as a function of the Peclet number for a given Da.In this paper, we showed that surface loading of typical analytes on a droplet-based biosensor can be highly increased by SAW microstirring. The system permits the enhancement of the biosensing performances by the continuous renewal of the analyte-carrying fluid near the sensing surface. Thanks to mean flow speeds measured up to 1800 μm∕s, the SAW microstreaming can be beneficial to the biosensing of a large range of analyte∕ligand interactions. In addition to the biosensing performance improvement, such a method can be easily integrated in micro-micro-total-analysis systems, which makes it a convenient tool for liquid handling in future biochips.  相似文献   

7.
The essential content of a recent paper by the present writer comprises a comprehensive discussion of the physical bases underlying derivation of formulas for calculating the temperature distribution T, maximum temperature Tm and average temperature Ta in a toroidal electrical coil of rectangular cross section, internally generated heat and change of wire resistance with temperature being taken into account. Illustratively, the solution for the boundary value condition of constant surface temperature and uniform equivalent thermal conductivity was obtained.For the most part, however, problems that arise in practice are not encompassed in the comparatively simple boundary conditions of constant temperature. Experiment shows that in general the boundary condition is T ? T′ = ? K??n; whereof n denotes the outward drawn normal to the coil surface, K = (knh) the ratio of the equivalent thermal conductivity in the direction of n to the emissivity of the boundary surface, and T and T′ are the corresponding temperatures in the coil surface and the immediately adjacent ambient medium. Again, it frequently ensues in practice that the thermal conductivity is substantially different in the directions of the two principal axes of the cross section.In the present paper formulas for T, Tm, and Ta are obtained for electrical coils of ratio of external to internal radius greater than (roughly) two whereof (i) the thermal conductivity is different in the directions of the two principal axes of the cross section, (ii) K is different on but constant over each of the four faces of the coil, and (iii) no restriction is made as to T′ except that over each face it be expressible in a generalized Fourier series. Determination of T is posed as a boundary-value problem in the mathematical theory of heat; the formal solution of T effected by expansions in orthogonal functions; and Tm and Ta then determined through use of their known relationships with T. The resulting formulas are in the form of rapidly-converging singly-infinite trigonometric-hyperbolic series. Illustrative of application of these general formulas, the maximum temperatures in a coil of given dimensions subject to two different sets of surface conditions are calculated and found to be in excellent agreement with the known measured values.The just-mentioned formulas encompass practically all cases encountered in practice except those coils which do not satisfy the restriction as to ratio of radii. For these latter formulas for T, Tm, and Ta are obtained pursuant to conditions of (i) equivalent thermal conductivity different in the directions of the two principal axes of the cross section, (ii) K, and likewise T′, different on but constant over each of the four faces of the coil. These formulas are in the form of rapidly-converging singly-infinite trigonometric- Bessel function (of zero order) series: Illustratively, the maximum temperature in a coil of given dimensions is calculated and found to be in excellent agreement with the known measured value.  相似文献   

8.
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9.
Mechanically exfoliated two-dimensional ferromagnetic materials (2D FMs) possess long-range ferromagnetic order and topologically nontrivial skyrmions in few layers. However, because of the dimensionality effect, such few-layer systems usually exhibit much lower Curie temperature (TC) compared to their bulk counterparts. It is therefore of great interest to explore effective approaches to enhance their TC, particularly in wafer-scale for practical applications. Here, we report an interfacial proximity-induced high-TC 2D FM Fe3GeTe2 (FGT) via A-type antiferromagnetic material CrSb (CS) which strongly couples to FGT. A superlattice structure of (FGT/CS)n, where n stands for the period of FGT/CS heterostructure, has been successfully produced with sharp interfaces by molecular-beam epitaxy on 2-inch wafers. By performing elemental specific X-ray magnetic circular dichroism (XMCD) measurements, we have unequivocally discovered that TC of 4-layer Fe3GeTe2 can be significantly enhanced from 140 K to 230 K because of the interfacial ferromagnetic coupling. Meanwhile, an inverse proximity effect occurs in the FGT/CS interface, driving the interfacial antiferromagnetic CrSb into a ferrimagnetic state as evidenced by double-switching behavior in hysteresis loops and the XMCD spectra. Density functional theory calculations show that the Fe-Te/Cr-Sb interface is strongly FM coupled and doping of the spin-polarized electrons by the interfacial Cr layer gives rise to the TC enhancement of the Fe3GeTe2 films, in accordance with our XMCD measurements. Strikingly, by introducing rich Fe in a 4-layer FGT/CS superlattice, TC can be further enhanced to near room temperature. Our results provide a feasible approach for enhancing the magnetic order of few-layer 2D FMs in wafer-scale and render opportunities for realizing realistic ultra-thin spintronic devices.  相似文献   

10.
In this paper, we investigate the incremental H∞ performance problem for a class of stochastic switched nonlinear systems by using a state-dependent switching law and the maximum and minimum dwell time approach. By resorting to the state-dependent switching law, some sufficient conditions are provided to cope with the incremental H∞ performance problem, which can be applied even if all subsystems are unstable. Then, based on the maximum and minimum dwell time scheme, the incremental H∞ performance problem to be solvable is derived for two cases: one is all subsystems are incrementally globally asymptotically stable in the mean(IGASiM), another is both IGASiM subsystems and unstable subsystems coexist. When all subsystems are IGASiM, the stochastic switched nonlinear system is IGASiM and possesses a incremental L2-gain under given conditions. When both IGASiM subsystems and unstable subsystems coexist, if the activation time ratio between IGASiM subsystems and unstable ones is not less than a specified constant, the sufficient conditions for the incremental H∞ performance of the stochastic switched nonlinear system are given. Two numerical examples are given to illustrate the validity of methods proposed.  相似文献   

11.
Nanochannels offer a way to align and analyze long biopolymer molecules such as DNA with high precision at potentially single basepair resolution, especially if a means to detect biomolecules in nanochannels electronically can be developed. Integration of nanochannels with electronics will require the development of nanochannel fabrication procedures that will not damage sensitive electronics previously constructed on the device. We present here a near-room-temperature fabrication technology involving parylene-C conformal deposition that is compatible with complementary metal oxide semiconductor electronic devices and present an analysis of the initial impedance measurements of conformally parylene-C coated nanochannels with integrated gold nanoelectrodes.No two cells are exactly alike, either in terms of their genome, the genomic epigenetic modification of the genome, or the expressed proteome.1 The genomic heterogeneity of cells is particularly important from an evolutionary perspective since it represents the stages of evolution of a population of cells under stress.2 Because of the important variances in the genome that occur from cell to cell, it is critical to develop genomic analysis technologies which can do single-cell and single molecule genomic analysis as an electronic “direct read” without intervening amplification steps.3, 4, 5, 6, 7, 8 In this paper, we present a technique which uses conformal coverage of nanochannels containing nanoelectrodes using a room-temperature deposition of parylene-C, a pin-hole-free, excellent electrical insulator with low autofluorescence.9 This procedure should open the door to integration of many kinds of surface electronics with nanochannels. One of the most difficult aspects in introducing electronics into nanochannel technology is the sealing of nanochannel so that the electrodes are not compromised by harsh chemicals or high temperatures. There are various methods to form nanochannels containing nanoelectrodes, including wafer bonding techniques,10 removal of sacrificial materials,11 and nonuniform sputtering deposition.12 Methods employing a sacrificial layer removal show the greater compatibility to electronic integration, but current methods to remove sacrificial materials require either high temperatures11 or harsh chemicals.13, 14The basic device consisted of 12 mm long, 100 nm wide, 100 nm high nanochannels interrogated by 22 pairs of 30 nm wide gold nanoelectrodes. The outline of the fabrication process is shown in Fig. ​Fig.1.1. The fabrication process was carried out on a standard 4 in. single-side polished p-type ⟨100⟩ silicon wafer with 100 nm of dry thermal oxide on the top as an insulating layer, which also helped the wetting of the nanochannels. The first step involved nanofabrication of the 25 nm thick nanoelectrodes on the SiO2 top of the wafer using electron beam lithography (EBL). External gold connection pads were constructed using standard metal lift-off techniques and photolithography to connect to the nanoelectrodes. A Raith E-Line e-beam writing system (Raith USA, Ronkonkoma, NY) was used to expose polymethyl methacrylate (PMMA) for metal lift-off. Figure ​Figure1a1a shows a scanning electron microscopy (SEM) image of the nanoelectrodes. The 100 nm sealed nanochannels were constructed using sacrificial removal techniques. We used EBL to expose a 100 nm thick film of PMMA over the gold nanolines in the region around the nanolines, leaving behind lines of unexposed sacrificial layer of PMMA. We next evaporated 25 nm of SiO2 over the nanolines to improve the surface wetting properties of nanochannel and then conformally coated with 4 μm thick of parylene-C [poly(chloro-p-xylylene)] using a Specialty Coating Systems model PDS 2010 parylene coating system (SCS Systems, Indianapolis, IN). Access holes for the gold electrodes and the feeding channels were etched through by oxygen plasma and 1:10 buffered oxide etchant. To avoid autofluorescence induced in parylene by an active plasma15 and ambient UV radiation,16 it is important not to expose the remaining parylene with plasma and to keep the samples in the dark. The sacrificial removal of PMMA in the nanochannels was done in four steps: (1) soaking the chip in 55 °C 1165 MicroChem resist remover (MicroChem, Newton, MA) for 36 h, (2) room-temperature soaking in 1,2-dichloroethane for 12 h, (3) soaking in room-temperature acetone for 12 h, and (4) drying the nanochannels by critical point drying (CPD-030, BAL-TEC AG, Principality of Liechtenstein), which served to prevent the collapse of the nanochannel resulting from surface tension of the acetone.Open in a separate windowFigure 1(a) SEM image of gold nanoelectrodes; scale bar is 200 nm. (b) 100×100 nm2 PMMA nanoline is written over the gold nanoelectrodes by exposure of the surrounding PMMA. (c) Parylene-C conformal coating over the PMMA nanoline. PMMA is dissolved and parylene-C etched by reactive ion etching.Conductance measurements were done using ac techniques. The ac impedance Ztot of an insulating ionic fluid such as water between electrodes is a complex subject.17 The most general model for the complex impedance of an electrode in ionic solution is typically modeled as the Randle circuit, which is shown in Fig. ​Fig.22.17 There are two major contributions to the imaginary part of the impedance: the capacitance of the double layer (Cdl), which is purely imaginary and has no dc conductance, and the impedance due to charge transfer resulting in electrochemical reactions at the electrode∕electrolyte interface, which can be modeled as a contact resistor (RCT), which is given by the Butler–Volmer equation, which describes the I-V characteristic curve when electrochemical reactions occur at the electrode,18 in series with a complex Warburg impedance (ZW) which represents injected charge transport near the electrode;19 more details can be found in Ref. 20. Since we applied a 10 mV rms ac voltage with no dc offset in our measurements, electrochemical reactions are negligible, which means no electrochemical charge transfer occurred and as a result RCT goes to infinity. We have drawn a gray box around the elements connected to the Warburg impedance branch of the circuit to show that they are negligible in our analysis.Open in a separate windowFigure 2The equivalent circuit of the nanoelectrodes in contact with water lying atop an insulating SiO2 film which covers a silicon substrate. The elements in the gray boxes can be ignored in our measurements since there is no hydrolysis at low voltage, while the elements within the dotted box are coupling reactances to the underlying p-doped silicon wafer.In the case of no direct charge injection, the electrodes are coupled by the purely capacitive dielectric layer impedance Cdl to the solvent and are also coupled capacitively by the dielectric SiO2 film capacitance Cox to the underlying p-doped silicon semiconductor. We model the semiconductor as a purely resistive material with bulk resistivity ρSi. The value of Cdl∕area is on the order of ϵϵoκ, where ϵ is the dielectric constant of water (about 80) and κ is the Debye screening parameter of the counterions in solution: κ=ϵϵokBT∕e2Σici∞zi2,20 where ci∞ is the bulk ion concentration of charge zi. At our salt molarity of 50 mM (1∕2 Tris∕Borate∕EDTA (TBE) buffer), Cdl is approximately 30 μF∕cm2 using 1∕κ∼1 nm.In Fig. ​Fig.3,3, we show the ac impedance measurements between pairs nanoelectrodes for both dry and TBE buffer wet nanochannels. The electrodes are capacitively coupled to the underlying silicon substrate through an oxide capacitor Cox. We model the doped silicon wafer as pure resistors, so there is an R1 that connects both Cox, and each Cox is connected to the ground with an R2. Curve fitting was done by using the 3SPICE circuit emulation code (VAMP Inc., Los Angeles, CA). We therefore obtained the following parameters for the dry curve: Cox=1.32 nF, R1=17.5 μΩ, and R2=32.8 kΩ. R1 is not sensitive in the fit as long as it is smaller than the impedance of Cox. Given ρSi of the wafer of 1–10 Ω cm, R2 should be on the order of 103 Ω, which is slightly smaller than our fitting results. The same parameters for the wafer coupling parameters were then used for fitting the impedance measurements for wet channels. For TBE buffer solution in the nanochannel, curve fitting yields Cdl=50 pF and Rsol=105 Ω. However, given the dimension of our nanochannels, we should get a transverse resistance R∼109 Ω. One possible explanation for this difference is that the evaporated SiO2 film which was put over the PMMA is porous and allows buffer to penetrate the oxide film,21 but given that the film is only 25 nm thick this would at most increase the cross section by one order of magnitude. However, it is known that there is a high fractional presence of mobile counterions associated with the charged channel walls.22 To calculate exact conductance contribution from the surface charges is a tricky business, but since the surface-to-volume ratios in our nanochannels are much greater than the slits, a larger conductance enhancement can be expected, and more work needs to be done.Open in a separate windowFigure 3ac impedance spectra of TBE buffer solution in a transchannel measurement between adjacent pairs of nanoelectrodes separated by 135 μm. The red circles are data for a dry channel and the solid red line is the fit to the model shown in the upper right hand corner. The green squares and dashed green line are for a nanochannel wet with TBE buffer.We have presented a way to fabricate a nanochannel integrated with electrodes. This technology opens up opportunities for electronic detection of charged polymers. With our techniques to fabricate nanoelectrodes with nanochannels, it should be possible to include integrated electronics with nanofludics, allowing the electronic observation of a single DNA molecule at high spatial resolution. However, the present design has problems. Most of the ac went through the silicon wafer instead of the solution. To enhance the sensitivity, we need either to increase the ratio of current going through the liquid to the current going through the wafer or to have a circuit design that picks up the changes in Cdl and Rsol.  相似文献   

12.
This paper studies the problem of composite control for a class of uncertain Markovian jump systems (MJSs) with partial known transition rates, multiple disturbances and actuator saturation. Compared with the existing results, a novel robust composite control scheme is put forward by virtue of adaptive neural network technique. For MJSs, the partial unknown information on transition rates and the actuator saturation influence the design of disturbance observer and the robust H∞ controller. Firstly, without taking account of external disturbances, the network reconstruction error and saturation, a novel robust adaptive control strategy is established to ensure that all the signals of the closed-loop system are asymptotically bounded in mean square. Secondly, the solvability condition for ensuring the robust H∞ performance is given by using a modified adaptive law, where the saturation is treated as a disturbance-like signal. Finally, the simulations for a numerical example and an application example are performed to validate the effectiveness of the proposed results.  相似文献   

13.
In this paper, an analytic solution of nonlinear H∞ robust controller is first proposed and used in a complete six degree-of-freedom nonlinear equations of motion of flight vehicle system with mass and moment inertia uncertainties. A special Lyapunov function with mass and moment inertia uncertainties is considered to solve the associated Hamilton-Jacobi partial differential inequality (HJPDI). The HJPDI is solved analytically, resulting in a nonlinear H∞ robust controller with simple proportional feedback structure. Next, the control surface inverse algorithm (CSIA) is introduced to determine the angles of control surface deflection from the nonlinear H∞ control command. The ranges of prefilter and loss ratio that guarantee stability and robustness of nonlinear H∞ flight control system implemented by CSIA are derived. Real aerodynamic data, engine data and actuator system of F-16 aircraft are carried out in numerical simulations to verify the proposed scheme. The results show that the responses still keep good convergence for large initial perturbation and the robust stability with mass and moment inertia uncertainties in the permissible ranges of the prefilter and loss ratio for which this design guarantees stability give same conclusion.  相似文献   

14.
Although HbA1C is widely accepted as a useful index of mean blood glucose in type 2 diabetic patients its usefulness as screening test for diabetes has been controversial. The present study was undertaken to determine whether the level of HbA1C predicted diabetes in a prediabetic group of subjects. Plasma lipids, oral glucose tolerance, HbA1C was determined in 90 normal control subjects, 57 offspring of one type 2 diabetes mellitus parent and 11 diagnosed type 2 diabetes mellitus individuals. The mean age of participants was 44.5 yrs (not significantly different amongst the three groups) and the mean body mass index was 26.8 (not significantly different amongst the three groups). Two hours after a 75 g glucose challenge, the offspring had a significantly higher plasma glucose level (mean = 7.1 mmol/L, p value = 0.002) than the normals. Similarly the HbA1C values were higher in the offspring than in the normals (mean = 5.78%, p value = 0.016). Besides the significantly higher values for oral glucose tolerance test and HbA1C, the diabetics also were significantly higher for triglycerides (mean = 2.25mmol/L), total cholesterol (mean = 6.24mmol/L) and systolic blood pressure (mean = 138.45mm Hg) than the offspring (P value = 0.031, 0.006, 0.010) and the normals (P value = 0.026, 0.018, 0.002) respectively. The mean values of diastolic blood pressure, LDL cholesterol and HDL cholesterol were not significantly different amongst the three groups.  相似文献   

15.
The quantum Hall effect (QHE) with quantized Hall resistance of h/νe2 started the research on topological quantum states and laid the foundation of topology in physics. Since then, Haldane proposed the QHE without Landau levels, showing nonzero Chern number |C| = 1, which has been experimentally observed at relatively low temperatures. For emerging physics and low-power-consumption electronics, the key issues are how to increase the working temperature and realize high Chern numbers (C > 1). Here, we report the experimental discovery of high-Chern-number QHE (C = 2) without Landau levels and C = 1 Chern insulator state displaying a nearly quantized Hall resistance plateau above the Néel temperature in MnBi2Te4 devices. Our observations provide a new perspective on topological matter and open new avenues for exploration of exotic topological quantum states and topological phase transitions at higher temperatures.  相似文献   

16.
Advancements in the field of electronics during the past few decades have inspired the use of transistors in a diversity of research fields, including biology and medicine. However, signals in living organisms are not only carried by electrons but also through fluxes of ions and biomolecules. Thus, in order to implement the transistor functionality to control biological signals, devices that can modulate currents of ions and biomolecules, i.e., ionic transistors and diodes, are needed. One successful approach for modulation of ionic currents is to use oppositely charged ion-selective membranes to form so called ion bipolar junction transistors (IBJTs). Unfortunately, overall IBJT device performance has been hindered due to the typical low mobility of ions, large geometries of the ion bipolar junction materials, and the possibility of electric field enhanced (EFE) water dissociation in the junction. Here, we introduce a novel polyphosphonium-based anion-selective material into npn-type IBJTs. The new material does not show EFE water dissociation and therefore allows for a reduction of junction length down to 2 μm, which significantly improves the switching performance of the ion transistor to 2 s. The presented improvement in speed as well the simplified design will be useful for future development of advanced iontronic circuits employing IBJTs, for example, addressable drug-delivery devices.There has been a recent interest in developing diodes1–4 and transistors4–8 that conduct and modulate ion currents. Such non-linear iontronic components are, for example, interesting as they allow further control of ions in, for instance, electrophoretic drug delivery devices. A range of microfabricated diodes,9–11 transistors,12,13 and circuits9,14 has been constructed using ion-selective membranes. These membranes contain fixed charges of either polarity, compensated by mobile ions of opposite charge (counter-ions). When immersed in an electrolyte, counter-ions can move through the membrane, while ions with the same charge as the fixed charges (co-ions) are repelled. This renders the membrane selective for the counter-ion and can therefore be considered as p- or n-type ion conductors. By combining two membranes of opposite polarity, a bipolar membrane (BM) configuration is obtained15 (Figure 1(a)). The BM junction can be biased by an ion current in the reverse and forward directions, respectively.16,17 This modulates the ion concentration inside the BM, and thus the ionic conductivity, which then results in an current rectification.2,18 In the three-terminal ion bipolar junction transistor12 (IBJT), an ion-selective base (B) is connected to oppositely selective emitter (E) and collector (C), forming two BM configurations (EB and BC) (Figure 1(b)). pnp- and npn-IBJTs have been constructed14 from photolithography patterned poly(styrene sulfonate) (PSS, p-selective) and quaternized poly(vinylbenzyl chloride) (n-selective) as emitter, collector, and base. In these devices, a neutral poly(ethylene glycol) (PEG) electrolyte is typically inserted into the junction to separate the base from the emitter and collector,12 in order to avoid19 electric field enhanced (EFE) water dissociation16 (Figure 1(a)). EFE water dissociation is typically observed in BMs20 and produces water ions inside the BM under reverse bias, which prevents proper IBJT operation. In PEG-IBJTs, the current between the emitter and collector (IC) is thus modulated by controlling the ion concentration inside the PEG-junction.21 Ions are injected or extracted into the junction depending on the bias of the base (VEB). In a npn-IBJT, a positive bias is typically applied between emitter and collector (VEC), thus allowing anions to migrate from the emitter to the collector. In the cut-off mode (Figure 1(c)), a negative bias VEB is applied, resulting in reverse bias of both EB and BC. Cations in the junction will migrate into the base, while anions will primarily migrate into the collector, due to the higher collector bias. This base current (IB) will extract ions from the junction, which decreases the ionic conductivity in the junction resulting in a low IC. Eventually, the resistive characteristics for ion charge transport, between the emitter and collector, will be entirely dominated by the junction. This gives that most of the applied VEC is consumed across the junction with only a minimal voltage potential drop across the emitter and base terminals.Open in a separate windowFIG. 1.(a) The modes of operation for a BM; forward bias (high conduction and ion accumulation), reverse bias (low conduction and ion depletion), and EFE water dissociation (high conduction, formation of ions). (b) Illustrations of an npn-IBJT, with anion-selective emitter (E) and collector (C) forming a junction with a cation-selective base (B). (c) In cut-off mode, the base and collector extract ions from the junction, prohibiting co-ion migration through the base. (d) In active mode, the forward biased EB injects ions into the base, thus allowing anions from the emitter to migrate as co-ions through the base into the collector.In the active-mode of the npn-IBJT (Figure 1(d)), the VEB bias at the base is reversed (i.e., now positive). This causes injection of cations, from the base, and anions, from the emitter, into the junction. As the ion concentration increases, anions from the emitter can start to drift across the junction to the collector, thus a high IC is obtained. The high concentration of ions inside the junction is reflected in a low resistive value for ion transport. This now causes the voltage to drop over the emitter and collector terminals, thus lowering the EB forward bias and the injection of ions from the base. At the collector-junction interface, the extraction of anions produces an ion depletion zone and a corresponding voltage drop. Thus, in the active-mode, the applied VEC is primarily consumed across the emitter and collector terminals and also at the collector-junction interface.The switching speed of an IBJT should be strongly correlated to the distance separating the emitter and collector,14 as this length determines the volume that needs to be filled or emptied with ions causing modulation of ions in the junction. To achieve a fast-switching IBJT, the junction volume, i.e., the collector-emitter separation, should be as small as possible. However, EFE water dissociation must be avoided since this process ruin the IBJT operation. EFE water dissociation is, in part, driven by the appearance of a large potential drop across a small distance, as occurring at the interface of a BM under reverse bias, producing a high electric field that accelerates the forward reaction rate of water auto-dissociation.16 Miniaturization of the collector-emitter distance is therefore problematic, as the separation inside the EB and BC BMs evidently also mush shrink, resulting in higher reverse bias electric fields across the BMs and thus promoting EFE water dissociation. The problem of EFE water dissociation in an IBJT primarily manifests itself in the cut-off mode, as water ions are generated in the reversed biased EB and BC BMs. These ions produce an elevated cut-off IC, and hence deteriorate the IBJTs on–off performance. Here, we report an IBJT, in which the EFE water dissociation is avoided by the use of a novel polyphosphonium-based anion-selective material, which previously has been shown to prevent EFE water dissociation in BM diodes.11 This allows the collector and emitter to directly contact the base without an intermediate PEG-layer. Without the need for a PEG-separator inside the BMs, the collector-emitter distance is reduced to only 2 μm.Polyphosphonium-based npn-IBJTs were produced following the same manufacturing protocol as was reported for polyphosphonium-based ion diodes.11 Conjugated polymer electrodes and cation-selective base was patterned from ∼200 nm thick poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film on polyethylene terephthalate-sheets using photolithography and dry-etching. The base was rendered electronically insulating by chemical overoxidation via exposure to sodium hypochlorite through a mask. A 2 μm thick SU8-layer was patterned on-top of this configuration, with an opening defining the actual junction. 1 μm thick polyphosphonium-based anion-selective emitter and collector were deposited and patterned using photolithography and dry-etching, to overlap with the base at the opening of the SU8. Finally, a second 10 μm thick layer of SU8 was used to seal the junction. The membranes were hydrated by incubation in dH2O for 24 h before any measurements were carried out. Aqueous 0.1M NaCl electrolytes were used during the measurement. All electrical measurements were performed using a Keithley 2602 source meter.The switching characteristics of the npn-IBJT were obtained by applying VEC of 10 V and alternating VEB at ±3 V for various duration of time, see Figure ​Figure2.2. A periodic 5 s switching with 8 Hz measurement rate was used to record the dynamics of the turn-on/off characteristics of the device. When VEB switches from −3 to +3 V, there is a quick increase in the IB, as ions from the base and emitter migrate into the emitter/base junction. After a delay of ∼0.25 s, IC starts to increase due to the increased ion concentration in the emitter/base junction and the subsequent diffusion of anions into the base. As the IC increases, the IB decreases as the voltage drop between the emitter and base decreases, and after ∼2 s IC reaches 90% of the steady state on-current level. For longer on-switching times, the IB and IC stay stable over 30 s, after which a small increase is observed. This current-drift in both IB and IC is likely due to the contribution of co-ion migration. As cations from the base migrate into the emitter as co-ions, the conductivity in the emitter increases, leading to an increased IC value. This increases the ion concentration at the base, which gives less selective ion injection and thus more cation injection from the base, i.e., a higher IB.Open in a separate windowFIG. 2.Emitter-collector current response as the IBJT is switched between cut-off (VEB = −3 V) and active mode (VEB = 3 V) for VEC = 10 V, at 5 s and 120 s periods.As VEB is switched back to −3 V, there is a sharp negative peak in IE as ions are extracted from the junction, which occur mainly through the base (cations) and collector (anions) terminals. As the ion concentration in the base drops, IC decreases. The transistor turns off to 10% of the value of the steady state on-current within ∼2 s, regardless of the duration of the on-state. The constant turn-off time indicates that ions are not accumulating to a significant extent inside the junction during the on-steady state but are instead constantly transported out of the junction. When all co-ions have been extracted from the junction, the Donnan exclusion prevents subsequent injection of anions into the base, and IC is therefore low. The on/off ratio of IC reaches above 100.A transfer curve was obtained by scanning VEB between −3 and +3 V while keeping VEC at 10 V (Figure 3(a)). As expected, both IC and IB remain low for negative VEB. In this range, both EB and BC are biased in reverse direction. As VEB turns positive, the EB configuration is switched into forward bias and ions are injected into the junction. This leads to a linear increase in IC vs. VEB. For the reverse scan, a minor hysteresis is observed for both the IC and IB scans, again probably due to the contribution of co-ion migration due to long time operation of the device.Open in a separate windowFIG. 3.Transfer and output curves. (a) The transfer curve is low for negative VEB and increases linearly for positive VEB with approximately zero threshold. (b) The output curves show IC saturating with respect of VEC for positive VEB.The transistor output characteristics were obtained by scanning VEC at different VEB values (Figure 3(b)). The saturation regime, i.e., the bias mode was both EB and BC are in forward bias, was avoided as this has negative impact on the stability of the device. As reported for previous IBJT devices, the output characteristics show a clear saturation behaviour of IC across the entire range of VEC. Further, the IC increases linearly with VEB. The increase of both IC and IB when operating for extended periods of time in the active mode is again attributed to the addition and inclusion of co-ions in the junction. The current gain (IC/IB) at VEC = 10 V decreases with VEB and reaches 43.9, 17.9, and 10.7 for VEB = 1 V, 2 V, and 3 V, respectively. For higher base bias voltages, the ion concentration increases in the junction and thus the injection selectivity decreases.In comparison with previously reported IBJTs,12,14,21 the lack of a neutral electrolyte layer in the junction has an overall positive effect on the device characteristics. Main performance improvements are found in a decrease in the turn-on time from 9 s (for npn-IBJT21) to 2 s, for devices with comparable junction widths and heights. The main contribution to the improved switching speed is likely the decreased length between the emitter and collector. Interestingly, simulations have shown that an extended space charge region (ESCR), for a PEG-IBJT in cut-off mode, can extend several micrometers away from the collector.22 Thus, a PEG-IBJT with an emitter-collector separation of single micrometers should show an increased cut-off current due to the ESCR overlapping in the junction. However, by omitting the PEG in the junction, the ESCR is reduced due to screening from the fixed charges in the BM layers. This enables the IBJT, reported here, to operate with retained low cut-off currents. On-off ratios and ion current gains are approximately equal to previous IBJTs,12,14,21 at above 100 and 10, respectively. The on–off ratio and ion current gain are more dependent on the selectivity of the membranes and the charge of the junction.Further, the need to separate the layers in a PEG-IBJT puts high demands on the patterning resolution and alignment accuracy to reduce the separation between emitter/collector and base. As polyphosphonium allows the IBJT to be built without separation of layers, miniaturization of the junction is relatively easier to obtain. The switching speed can potentially be further improved by retaining the base material between the emitter and collector (see Figure 1(b)), thus allowing for a more direct pathway for IC. This design would, however, require a much more accurate layer alignment or that the base patterned on top of the emitter and collector layers. In general, such modifications of device geometry are simpler to accomplish with the non-EFE water dissociating polyphosphonium as fewer active layers are used, suggesting a further use of polyphosphonium to improve switching speed and miniaturization of IBJTs. Such further advancement in IBJT performance would be welcomed, for example, in the continued work towards complex ionic circuits14 to regulate signalling in bioelectronics and in drug delivery applications, in which generation of dynamic and complex gradients, at high spatial resolution, is of generic interest.  相似文献   

17.
Conditions are given for underdamped or overdamped linear dynamical systems in terms of loop matrix parameters, (√(C)R√(C)g,g)2 ? 4(√(C)L√(C)g,g) for all [boxV]g[boxV] = 1, g ∈ H. These criteria are looked upon as natural generalizations of the elementary one-loop RLC series scalar criteria (R/2L)2 ? 1/LC, when written in the more suggestive form: (√(C)R√(C))2 ? 4√(C)L√(C). A simplified test for determining dynamical systems with all complex natural modes or all real modes are presented with additional comments.  相似文献   

18.
If T maps a convex domain DT into itself, and if {ωn} is a real sequence with range in (0, 1] then the recursive averaging process,
Xn+1=(1?omega;n) Xn+ωn+ωnTxn, x0=ξ?DT
generates a sequence {x?n}; with range in DT. Under suitable conditions on DT, T and {ωn} the sequence {x?n} will converge in some sense to a fixed point of T. We prove that if DT is a closed convex subset of a complex Hilbert space H, if Tω = (1 ? ω) I + ωT is a strict contraction for some ω ? (0, 1], and if {ωn} satisfies the conditions,
ωn → 0
and
∑n=0∞ωn=∞
then, for arbitrary ξ ? DT, {x?n} converges strongly to (the unique) fixed point of T. We also prove that if DT and {ωn} satisfy the foregoing conditions, if T has at least one fixed point, and if Tω is non-expansive for some ω ? (0, 1], then for all ξ ? DT, {x?n} converges at least weakly to some fixed point of T. Finally, we apply these results to linear equations involving bounded normal operators and obtain an extension of the classical Neumann operator series.  相似文献   

19.
The first enantioselective assembly of sandwich-shaped organo molecules has been achieved by conducting dual asymmetric Suzuki-Miyaura couplings and nine other reactions. This work also presents the first fully C-C anchored multi-layer 3D chirality with optically pure enantiomers. As confirmed by X-ray diffraction analysis that this chiral framework is featured by a unique C2-symmetry in which a nearly parallel fashion consisting of three layers: top, middle and bottom aromatic rings. Unlike the documented planar or axial chirality, the present chirality shows its top and bottom layers restrict each other from free rotation, i.e., this multi-layer 3D chirality would not exist if either top or bottom layer is removed. Nearly all multi-layered compounds showed strong luminescence of different colors under UV irradiation, and several randomly selected samples displayed aggregation-induced emission (AIE) properties. This work is believed to have broad impacts on chemical, medicinal and material sciences including optoelectronic materials in future.  相似文献   

20.
Graphite and lithium metal are two classic anode materials and their composite has shown promising performance for rechargeable batteries. However, it is generally accepted that Li metal wets graphite poorly, causing its spreading and infiltration difficult. Here we show that graphite can either appear superlithiophilic or lithiophobic, depending on the local redox potential. By comparing the wetting performance of highly ordered pyrolytic graphite, porous carbon paper (PCP), lithiated PCP and graphite powder, we demonstrate that the surface contaminants that pin the contact-line motion and cause contact-angle hysteresis have their own electrochemical-stability windows. The surface contaminants can be either removed or reinforced in a time-dependent manner, depending on whether the reducing agents (C6→LiC6) or the oxidizing agents (air, moisture) dominate in the ambient environment, leading to bifurcating dynamics of either superfast or superslow wetting. Our findings enable new fabrication technology for Li–graphite composite with a controllable Li-metal/graphite ratio and present great promise for the mass production of Li-based anodes for use in high-energy-density batteries.  相似文献   

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