摘要:

Inverse analysis utilizing the conjugate gradient method is used to estimate the timewise varying strength of a line heat source placed at a specified location in a rectangular region with insulated boundaries. No prior information was used on the functional form of the source strength with time. Transient temperature recordings taken at the boundaries of the region served as the simulated experimental data needed as input for the inverse analysis. In order to test the accuracy of the inverse analysis under most strict conditions, timewise variation of the source strength is chosen in the form of rectangular, triangular, and sinusoidal functions. Both the regular iterated and modified conjugate gradient method are used for solving the inverse problem. The modified conjugate gradient method often provided the answer with smaller number of iterations when the initial condition for the unknown function was available.

ISSN:0021-8979    

DOI:10.1063/1.350554

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Values of the ratio of the longitudinal diffusion coefficient to mobilityDL/&mgr; for electrons in He, Ar, Kr, and Xe are derived from current waveforms obtained during earlier measurements of electron mobility. The electric field to gas density ratiosE/Ncover the wide range of 10−3to 20 Td, thereby bridging previous experiments at lowE/Nto recent experiments at highE/N. Here 1 Td=1×10−21V m2. The correspondingDL/&mgr; values range from 0.0066 eV for thermal electrons at 77 K to 10 eV. In addition to the well‐known peak inDL/&mgr; for Ar atE/Nbetween 0.01 and 0.1 Td caused by the Ramsauer minimum in the momentum transfer cross section, we find previously unreported low‐energy peaks inDL/&mgr; vsE/Nin Kr and Xe and previously unreported pronounced leveling‐off inDL/&mgr; atE/N≳8 Td in Ar, Kr, and Xe. Calculations of transport coefficients using numerical solutions of the Boltzmann equation and cross section sets in the literature give good agreement with experiment fromE/Nproducing thermal electrons up to average energies ≊10 eV andE/Nup to 100 Td, the upper limit of our calculations. The leveling off ofDL/&mgr; at highE/Nis caused by inelastic collisions.

ISSN:0021-8979    

DOI:10.1063/1.350555

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Neutral CH3radicals in a hollow cathode discharge cell containing methane have been detected with a quadrupole mass spectrometer, using the threshold ionization technique. With the experimental conditions employed in this work the measured efficiency in CH3production has been 0.2%, which is higher than that shown for other types of discharges in the bibliography. On the other hand a high depletion of the CH4density has been observed and the mass spectrum of the discharge shows the appearance of compounds atm=25, 26, and 27, proceeding from recombination processes.

ISSN:0021-8979    

DOI:10.1063/1.350556

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

In the study of the propagation of a high‐power microwave pulse, one of the main concerns is how to minimize the energy loss of the pulse before reaching the destination. In the very high‐power region, one has to prevent the cutoff reflection caused by the excessive ionization in the background air. A frequency autoconversion process that can lead to reflectionless propagation of powerful electromagnetic pulses in self‐generated plasmas is studied. The theory shows that under the proper condition the carrier frequency &ohgr; of the pulse shifts upward during the growth of local plasma frequency &ohgr;pe. Thus, the self‐generated plasma remains underdense to the pulse. A chamber experiment to demonstrate the frequency autoconversion during the pulse propagation through the self‐generated plasma is conducted. The detected frequency shift is compared with the theoretical result calculated by using the measured electron density distribution along the propagation path of the pulse. Good agreement is obtained.

ISSN:0021-8979    

DOI:10.1063/1.350557

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Electromagnetic emission produced by a propagating electron beam in a cylindrical drift chamber can be amplified by axially reflecting screens. Radiation appears at the first and second plasma harmonics with linewidths ∼0.1 &ngr;p. Amplification scales with &ngr;p2and lags electron‐beam voltage by several hundred nanoseconds, implying that electrostatic waves moving at the electron thermal speed must traverse the resonator before amplification begins. Rotating the reflectors beyond 30° lessens amplification, suggesting a broad reflection property.

ISSN:0021-8979    

DOI:10.1063/1.350558

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Quantitative electron channeling and weak‐beam transmission electron microscopy (TEM) measurements of dislocation density have been used to study the formation and annealing of the dislocation network in self‐ion‐implanted aluminum. The channeling linewidth is found to be proportional to the dislocation density. Several mathematical expressions for the dislocation network growth are examined. One by Igataetal. [inEffectsofRadiationonMaterials:Proceedingsofthe10thInternationalSymposium, edited by D. Kramer, H. R. Brager, and J. S. Perrin, STP 725 (ASTM, Philadelphia, 1980), p. 627] is found to describe the implantation data accurately. This model has a linear growth term and terms for dislocation loss due to surface effects and recovery. The fit is insensitive to the relative importance of the two loss terms. Annealing is found to occur in two stages; roughly one third of the dislocation density is gone by 300 °C, corresponding to typical recovery processes, while the remaining network is stable to about 500 °C.

ISSN:0021-8979    

DOI:10.1063/1.350559

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The enthalpy release during the heating of nanocrystalline platinum samples has been measured and at least two different contributions to the excess Gibbs free energy have been identified by controlling the grain size. About one half of the excess enthalpy is released during a first relaxation process without any measurable variation of grain size followed by a second relaxation which can be attributed to grain growth. The isothermal heat release of the grain growth is analyzed to calculate the excess grain boundary enthalpy.

ISSN:0021-8979    

DOI:10.1063/1.350560

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Polycrystalline and amorphous Si‐C films were prepared by rf glow‐discharge decomposition of silane‐methane mixtures at 700 °C. We have demonstrated that polycrystalline SiC films with large grains grow under heavy hydrogen dilution. The bonding properties as a function of film composition and hydrogen dilution were characterized by means of x‐ray diffraction and x‐ray photoelectron spectroscopy. Crystallization takes place at around C contentx=0.5 in Si1−xCx, accompanying some segregation of carbon atoms in grain boundaries, as a result of a preference for heteronuclear bonds. It was shown that C‐C(C3−nSin) (n=0–3) bonds appear in the carbidic phase of C‐rich films, leading to occurrence of compressive strain in the crystalline SiC grains. In addition, effects of hydrogen dilution were discussed in correlation with the strain.

ISSN:0021-8979    

DOI:10.1063/1.350561

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Scanning the scanning tunneling microscopy (STM) tip on a Au(111) (22±1)×&sqrt;3 reconstructed surface causes monatomic stripes to grow preferentially on the fcc portions of the surface. Stripes several 100 nm’s in length, ∼4 nm wide, and separated by spaces ranging from 2 to ∼4 nm (the hcp regions of the reconstructed surface) can be grown. Stripe formation only takes place when the scan direction corresponds, more or less, with the [1,1,−2] primary direction of the reconstruction. Growth occurs much more rapidly than can be accounted for by diffusion alone, leading to the conclusion that the STM tip transports gold, previously picked up from other portions of the surface, to the growing ends of the stripes. Stripes grown in this way may serve as convenient templates for subsequent growth of quantum wire structures by molecular beam epitaxy.

ISSN:0021-8979    

DOI:10.1063/1.350562

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Molecular‐dynamics simulations are employed to study in detail the effects of low‐energy (≤100 eV) bombardment of a Cu (001) surface by Cu atoms. By following the simulation up to 4 ps in real time, the end configuration of defects in the target can be observed. We present results on the vacancy and interstitial distribution in the target, the spontaneous defect recombination, the number of surface vacancies and adatoms produced, and the mixing of target atoms induced by the bombardment. Furthermore, the fate of the projectile atom−backscattering and implantation−and the sputtering behavior are investigated. The relevance of the results on the modelling of ion‐beam (assisted) deposition is discussed.

ISSN:0021-8979    

DOI:10.1063/1.350563

出版商:AIP    

年代:1992

数据来源: AIP