摘要:

The theory of finite‐time thermodynamics for continuous endoreversible cycles gives rise to an optimum efficiency at maximum power output of &eegr;=1−(TL/TH)0.5in contrast to the upper limit of &eegr;=1−(TL/TH) obtained from infinite‐time thermodynamics. It is shown here that, additionally, for continuous Carnot cycles, the finite‐time optimum rate of work output (W˙opt), is exactly half of that obtained for infinite‐time reversible cycles (Carnot work rate,W˙rev) operating between the same temperature limits (i.e.W˙opt=1/2W˙rev). This expression is true for both unoptimized as well as optimized mass flow rate conditions. The formulation used in the analysis is universal for all working substances for cycles using linear heat transfer laws. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363375

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

An atomic force microscope (AFM) design providing a focused spot of order 7 &mgr;m in diameter was used to analyze the motion of vibrating cantilevers in liquid. Picking an operating frequency for tapping mode AFM operation in liquid is complex because there is typically a large number of sharp peaks in the response spectrum of cantilever slope amplitude versus drive frequency. The response spectrum was found to be a product of the cantilever’s broad thermal noise spectrum and an underlying fluid drive spectrum containing the sharp peaks. The geometrical shape of transverse cantilever motion was qualitatively independent of the fluid drive spectrum and could be approximately reproduced by a simple theoretical model. The measurements performed give new insights into the behavior of cantilevers during tapping mode AFM operation in liquid. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363308

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

In order to investigate the feasibility of using Kumakhov capillary x‐ray optics for high energy x‐ray applications, measurements have been performed on the behavior of capillary optics from 10 to 80 keV. Transmission efficiencies of straight polycapillary fibers of different types have been measured as a function of source location and x‐ray energy. The measurements are analyzed using a geometrical optics simulation program, which includes roughness and waviness effects. Despite the low critical angle for total external reflection at high energies, capillary x‐ray optics appear promising for many hard x‐ray applications. Transmission measurements at high energies have also proven to be a very sensitive tool in capillary quality analysis. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363309

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We have examined sputtered metal/Al2O3multilayers (metal: Co, Pt, W) with respect to their suitability as x‐ray mirrors in high temperature environments. The rf‐sputtering technique leads to layered structures with interface roughnesses of only 0.2 nm as confirmed by x‐ray scattering and transmission electron microscopy.Insituresistance measurements characterize the percolation process and indicate the lower thickness limits for the chosen materials which lead to minimum modulation periods of ≊2 nm. The samples were characterized after stepwise annealing up to 1000 °C. Reflectivity values of nearly 100% at the first satellite‐reflection were found in the case of Pt and W (for &lgr;=0.15418 nm). The most stable W/Al2O3multilayer reflects 70% of the primary beam even after annealing at 900 °C. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363310

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A traveling electric potential hill has been used to generate an ion beam with an energy distribution that is mass dependent from a monoenergetic ion beam of mixed masses. This effect can be utilized as a novel method for mass separation applied to identification or enrichment of ions (e.g., of elements, isotopes, or molecules). This theory for mass‐selective acceleration is presented here and is shown to be confirmed by experiment and by a time‐dependent particle‐in‐cell computer simulation. Results show that monoenergetic ions with the particular mass of choice are accelerated by controlling the hill potential and the hill velocity. The hill velocity is typically 20%–30% faster than the ions to be accelerated. The ability of the hill to pickup a particular mass uses the fact that small kinetic energy differences in the lab frame appear much larger in the moving hill frame. Ions will gain energy from the approaching hill if their relative energy in the moving hill frame is less than the peak potential of the hill. The final energy of these accelerated ions can be several times the source energy, which facilitates energy filtering for mass purification or identification. If the hill potential is chosen to accelerate multiple masses, the heaviest mass will have the greatest final energy. Hence, choosing the appropriate hill potential and collector retarding voltage will isolate ions with the lightest, heaviest, or intermediate mass. In the experimental device, called a Solitron, purified20Ne and22Ne are extracted from a ribbon beam of neon that is originally composed of20Ne:22Ne in the natural ratio of 91:9. The isotopic content of the processed beam is determined by measuring the energy distribution of the detected current. These results agree with the theory. In addition to mass selectivity, our theory can also be applied to the filtration of an ion beam according to charge state or energy. Because of this variety of properties, the Solitron is envisioned to have broad applications. The primary application is for the enrichment of stable isotopes for medical and industrial tracers. Other applications include mass analysis of unknown gases (atomic and molecular) and metals, extracting single charge states from a multiply charged beam, accelerating the high energy tail in a beam or plasma with a velocity distribution, and beam bunching.

ISSN:0021-8979    

DOI:10.1063/1.363311

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Piezoelectric elements made of polyvinylidene‐fluoride‐trifluoroethylene copolymer P(VDF/TrFE) have been used to measure the pressure induced on the rear surface of metallic foils irradiated by infra‐red laser pulses of 1.5 ns duration, with intensities up to 3 TW/cm2. The application of such piezoelectric materials under those conditions is new, and a special effort has been made for their improvement and characterization under plate impact loading. Then, the laser experiments have been carried out. First, peak pressures of 0.5–7.5 GPa transmitted in the copolymer have been derived from the peak voltage measured at each shot, using simple assumptions. Next, a more accurate analysis of the data, involving computer simulations, has provided the pressure profiles characterizing the laser shocks driven on the front surface of the targets, over a wide range of laser intensities. Peak pressures of 7–60 GPa on the irradiated surface have been inferred from that analysis. The results have been compared to values obtained from a classical scaling law on one hand, and to predictions of a laser‐matter interaction simulation code on the other hand. An overall coherence has been obtained, despite some discrepancies that have been discussed. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363312

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We report the observation of an anomalously large electro‐optic modulation effect in potassium titanyl phosphate (KTiOPO4) waveguides fabricated under various conditions. An interferometric method was used to measure the effective electro‐optic coefficient in the waveguides by measuring the phase retardation of an optical mode when a voltage was applied across the substrate. We observed that at low modulation frequency the effective electro‐optic coefficient in the waveguides can be higher than the bulk value by a factor up to about 100. A space charge model relating the mobile charge, space charge field, and observed enhancement in the electro‐optic modulation is proposed to explain the mechanism of this effect. Calculation for dc steady state, small signal analysis, and large signal simulation is compared to the experimental observation. Possible device application and long term effects on system operation are also discussed. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363313

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The origin of second harmonic generation (SHG) of Li3VO4was investigated from the viewpoint of the band structure by using the tight‐binding method. The tight‐binding parameters were optimized to reproduce the density of states (DOS) obtained from x ray photoelectron spectroscopy and the optical band gap. Although Li3PO4has the same crystal structure as Li3VO4, it shows no SHG. To explain the difference in optical nonlinearity we compared the electronic structures of Li3VO4and Li3PO4, in particular at the bottom of conduction band (CB) and the top of valence band (VB), since they are known to play a primary role in SHG. In Li3PO4, the bottom of CB consists of P 3sand O 2porbitals and the top of VB is composed of O 2porbitals. These electronic structures result in a relatively low DOS at the bottom of CB and a wide band gap in Li3PO4. On the other hand, in Li3VO4, both bottom of CB and top of VB are composed of V 3dand O 2p. The preferential contribution of V 3dorbitals to the band edge states leads to a high DOS at the bottom of CB, a narrow band gap and delocalization of electrons on V–O bonds. We conclude that these electronic structures are responsible for the high optical‐nonlinearity of Li3VO4. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363314

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We studied the third‐order nonlinear optical properties of phthalocyanine and fullerene in solution and in thin film form through third‐harmonic generation measurement. X‐ray diffraction measurement and scanning electron microscopy observation were also conducted to investigate the relationship between nonlinear optical and structural properties of the thin film. The nonlinearities depend on the &pgr;‐conjugated length and the introduction of a central metal atom. We clarify the factors for forming suitable thin films by organic molecular beam deposition. It is shown that both the crystal structure and molecular alignment can be controlled to enhance the nonlinearities of the phthalocyanine thin film, thereby providing the maximum third‐order susceptibility of 3.0×10−10esu. We show that nonlinearities of the thin films can be estimated experimentally from those of solutions. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363315

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Variable‐dose electron‐beam exposure of thin amorphous As2S3films is studied as a potential fabrication technique of index‐modulated diffractive optical elements for the near infrared. The relationship between the electron dose and the phase delay is determined using a coarse grating structure, which eliminates the influence of volume diffraction and proximity effects. The effective refractive index change is determined by comparing experimental and calculated zeroth‐ and first‐order diffraction efficiencies, taking into account the small but detectable surface modulation. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363316

出版商:AIP    

年代:1996

数据来源: AIP