摘要:

We review the application of scanning probe microscopes to dimensional measurement of topographic features. Probe microscopes show great promise as metrology tools because they produce three‐dimensional data over almost all solids in a wide range of ambients. Even though these microscopes readily achieve atomic resolution, there are several aspects of their behavior that can cause them to exhibit large measurement errors. The actuators that drive the probe exhibit hysteresis and creep, so they must be independently monitored. In addition, the geometry of an extended probe moving across the surface makes probe‐sample interaction intrinsically nonlinear. Forces on the probe that cause it to flex are another source of inaccuracy. Probe fabrication and characterization are, consequently, important issues. We describe present understanding of these problems and the techniques being developed to solve them.

ISSN:0021-8979    

DOI:10.1063/1.354175

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Chemical engines process mass flows to convert differences in chemical potential into work—the analog of heat engines processing heat flows to generate work from temperature differences. For isothermal endoreversible chemical engines constrained to finite‐time operation—examples of which include mass exchangers, electrochemical, photochemical, and solid‐state devices: (i) optimal cycle types for maximizing average power output, (ii) upper bounds on power production, and (iii) sensitivity of these results to mass‐transfer law, illustrated for linear and diffusive behavior, are derived.

ISSN:0021-8979    

DOI:10.1063/1.354253

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The literature data about finite‐time engines have been reviewed. We have shown that the modern technical thermodynamics largely use the finite‐time engines concept. We use the apparatus of finite‐time thermodynamics to describe a stationary engine—assemblage of multiple thermodynamically identical nonstationary engines working at a fixed phase difference. The author proposes a new model which considers an assemblage of thermodynamically identical engines functioning at different phases. The assemblage is itself a stationary engine. The model allows one to evaluate the internal energy dissipation caused by interaction of the elementary engines that is equivalent to the influence of the environment. We obtained the equations describing the system considered and found the conditions for maximum efficiency.

ISSN:0021-8979    

DOI:10.1063/1.354231

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The effects of computational complexity on the characteristics of a physical signal that is reconstructed from its representation of sampled data are analyzed. It is found that a more complex algorithm does not only require longer time to implement, but also yields an erroneous reconstruction. The reconstruction suffers from contrast degradation, phase shifts, and attenuation of details relative to the true signal. These unwanted effects are caused by the existence of spurious frequencies in the computed spectrum due to rounding‐off errors. The amplitude distribution of the spurious frequencies across the spectral bandwidth strongly depends on the number of data points handled and on the complexity of the particular reconstruction algorithm employed. Since the floating point representation of numbers in a computer is always finite, an upper limit exists in the maximum number of additions or multiplications required to compute a quantity reliably without errors.

ISSN:0021-8979    

DOI:10.1063/1.354232

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Quartz and nickel cylinders were used as grazing incidence x‐ray optics for transition radiation (TR) from Al, Ti, and Cu foil stacks. The cylinders were used to direct the otherwise diverging TR to a small spot (1 mm diameter), with measured fluxes of up to 330 W/A cm2averaged over the spot area. Computer simulations were performed and compared to the measured results. It was found that including surface roughness effects in the form of a Debye–Waller reduction factor on the reflectivity was necessary to get good agreement between the simulations and experimental results in certain cases, but not in others. A simple heuristic model which can be used to determine when roughness effects are important is presented. Use of this model in conjunction with the computer simulations accurately predicts the measured results, and further suggests the use of different optic materials at different x‐ray wavelengths.

ISSN:0021-8979    

DOI:10.1063/1.354233

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Aging of proportional counters in CF4/iC4H10mixtures is studied as a function of gas composition. Anode surfaces are analyzed by Auger electron spectroscopy. Anode‐wire deposits are formed from 95/5 and 90/10 mixtures of CF4/iC4H10; etching of deposits occurs in 50/50 and 80/20 mixtures of CF4/iC4H10and in pure CF4. Gold‐plated wires are resistant to aging resulting from chemical attack by CF4, but non‐gold‐plated wires are too reactive for use in CF4‐containing gases. An apparent cathode aging process resulting in loss of gain rather than in a self‐sustained discharge current is observed in CF4and CF4‐rich gases. Principles of low‐pressure rf plasma chemistry are used to interpret the plasma chemistry in avalanches (≥1 atm, dc). To understand anode aging in CF4/iC4H10gases, a four‐part model is developed considering: (i) plasma polymerization ofiC4H10; (ii) etching of wire deposits by CF4; (iii) deposition that occurs as a result of radical scavenging in strongly etching environments; and (iv) reactivity of the wire surface. Practical guidelines suggested by the model and application of the model to other fluorine‐containing gases are discussed.

ISSN:0021-8979    

DOI:10.1063/1.354234

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Properties of reduced density channels created by energy deposition from electron beams propagating in air are presented. A multi‐beam laser deflection technique used to diagnose the channels is described, as are representative examples of density channels formed in multi‐pulse propagation experiments performed at the Advanced Test Accelerator at the Lawrence Livermore National Laboratory.

ISSN:0021-8979    

DOI:10.1063/1.354235

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Exciton photoluminescence (PL) of quantum wells (QW), affected by the thermal migration and the inherent interface roughness, has been studied by a model which includes exciton transfer between the growth islands with one atomic layer difference in well thickness. Analytical expressions, showing a red shifting and linewidth changing compared to the exciton PL spectra without thermal migration, are obtained. At low temperature, the red shifting and the linewidth changing are uniquely determined by the geometry of the QW. At high temperature, the red shift and the linewidth changing depends on the temperature, structure, and inherent interface fluctuation of the QW. Under the influence of the thermal migration, the inconsistency of the PL and PL excitation spectra has been reexamined and quantitatively obtained as a function of the temperature, quality, and structure of the QW.

ISSN:0021-8979    

DOI:10.1063/1.354236

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The angular distribution of the sputtered Tb, Fe, and Co atoms from the elemental targets could be described by cosn &thgr; distribution withn=1.90, 0.54, and 1.01 for Tb, Fe, and Co, respectively. For TbFeCo composite targets with mixed elemental and intermetallic compound phases, the overall angular distribution of the sputtered Tb, Fe, and Co atoms from the mixed phases could also be described by the cosn &thgr; distribution function, and the values of then’s were related to the volume percentage of the intermetallic compound content in the composite target. This was not true of the Tb distribution for a target containing high intermetallic compound content. In that sample, an off‐normal term has to be added to the cosn &thgr; distribution function. It was found that a composite target with about 23 vol % intermetallic compound would produce a uniform composition distribution of TbFeCo film on the substrate.

ISSN:0021-8979    

DOI:10.1063/1.354237

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Laser resonant shadowgraphy (LRS) and laser nonresonant shadowgraphy (LNRS) are used to monitor the detonation products of lead azide. Photographs of the cloud of products are obtained via illumination with a doubled dye laser tuned on‐resonance to the3P1o‐3P0transition of the Pb atom at 283.31 nm, and off‐resonance at 284.31 nm. The versatility of the diagnostics and its applicability to detonation products expanding into vacuum and into atmospheric pressure air are demonstrated. The LRS monitors the density gradients of both lead atoms and solid particles formed in the detonation, whereas the LNRS detects only the latter. Expansion into vacuum through a nozzle leads to an increase in the velocity (from ∼4.5 to ≳5 km/s) and density of the atoms and to a decrease in the density of the particles. The LRS measurements show that the expansion of both products in air is relatively slow (∼0.75 km/s) and leads to production of shock waves. From the shape of the shock waves created by an obstacle when the products expand into vacuum, the Mach number is estimated to be ≳20 in the outer parts and around 3 in the inner parts of the cloud.

ISSN:0021-8979    

DOI:10.1063/1.354238

出版商:AIP    

年代:1993

数据来源: AIP