摘要:

A model was used to compute the performance of an ErNi‐based active magnetic regenerative refrigerator as a function of the hot and cold temperatures, applied magnetic field strength and mass flow rate. A device spanning 4 to 10 K with an applied field of 3 T and a mass flow rate of 1.3 g/s is predicted to have a cooling power of 2.8 W. Approximately 1 W of cooling is predicted with a magnetic field of 1 T. The device performance as a function of the mass flow rate is studied. The performance is not strongly dependent on the hot temperature but is very sensitive to changes in the cold temperature. This is attributed to the material properties of ErNi. Comparisons of the predicted performance of ErNi and three other magnetic refrigerants are also made. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361422

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We analyze a new cold cathode emitter which consists of a thin wide band gap semiconductor material sandwiched between a metallic material, and a low work function semimetallic thin film. We show that under forward bias operation the electrons captured in the low work function material are responsible for an effective reduction of the semimetallic film work function, together with a substantial increase of the cathode emitted current. The dynamic work function shift is shown to increase with the amount of injected current. Potential material candidates are suggested to achieve low‐voltage (<20 V), room‐temperature cold cathode operation with emission currents approaching several hundred A/cm2and large efficiencies. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361105

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The interaction of a high‐power proton beam with 10‐ to 75‐&mgr;m‐thick double layered aluminum targets has been investigated. The experiments were carried out at the Karlsruhe Light Ion Facility using laser Doppler velocimetry with subnanosecond temporal resolution. The results allow us to determine with high accuracy the range of 1.53 MeV protons in the hot aluminum ablation plasma and to reveal the contribution of the thermal conductivity to the state of the target. An analytical model based on an acoustic approximation was established to interpret the ablative acceleration of the target foils during the whole rise time of the accelerator voltage until the maximum ion range is reached. From this model follows that a shock wave is induced into the adjacent material only when the expansion velocity of the ablation zone exactly equals the sound velocity in the target. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361181

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Audible acoustic wave generation during excimer laser interaction with materials has been investigated. It is found that the amplitudes of acoustic waves depend on laser fluence, pulse number, and substrate material characteristics and can be used to determine the nature of laser–material interactions. When laser fluence is below the ablation threshold of the materials, the amplitudes are reduced to zero at large pulse number due to the cleaning of contaminants on the substrate surface. As laser fluence becomes higher than the ablation threshold, the amplitudes of acoustic waves also reduce with increasing pulse number but to a constant level instead of zero due to laser ablation of substrate materials. Since the surface contamination can be completely removed by a few pulses at high laser fluence, the constant level is attributed to the material ablation. It is also found that the constant level increases with laser fluence. By establishing a relationship between the amplitudes and laser parameters, real‐time monitoring of laser–solid interaction can be achieved. Fast Fourier transform analysis of the wave forms shows that there are several frequency components included in the acoustic waves with a peak around 10.9 kHz as the dominant one, which is related to laser material ablation. The monitoring of the acoustic wave emission can, therefore, be used to find the nature of laser–substrate interaction (i.e., surface cleaning or ablation), and to find the ablation threshold. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361182

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We study the basic mechanism for limiting the maximum operating temperature (Tm) of InP‐based multiple‐quantum‐well (MQW) lasers emitting at a wavelength of 1.3 &mgr;m. It is shown that laser operation is characterized in terms of the temperature dependence of threshold gain (gth) by introducing a critical temperature (Tc):gthexhibits a linear relationship with temperature belowTc, while it superlinearly increases with increasing temperature aboveTc. This rapid increase ingthleads to a marked increase in threshold current and a significant reduction in differential quantum efficiency aboveTc. We indicate thatTcexhibits a direct correlation withTm: the higherTc, the higherTm. In the temperature range aboveTc, laser operation moves into a loss‐multiplication regime, where a considerable portion of the injected carriers brings about a significant increase in internal loss rather than gain due to their pileup in the separate confinement heterostructure layers. We demonstrate that an anomalous increase in internal loss, which occurs at the final stage in this loss‐multiplication regime, determinesTmof 1.3 &mgr;m InP‐based MQW lasers. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361183

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The process of cutting metals with a laser beam is described by a mathematical model comprising a set of balance equations for mass, momentum and energy. The molten film at the cutting front is defined as the control volume. The calculation yields average values for the characteristic macroscopic state of the cutting front, such as temperature, melt film velocity and melt film thickness. All relevant parameters of the laser beam and of the process gas flow are considered in the model. Numerical evaluations of the analytical model provide explanations of the process behavior both for inert and for oxygen gas cutting. The narrowing of the kerf width with increasing speed is explained by heat conduction. Furthermore, the model shows that the evaporation temperature is reached at high velocities for thin metal sheets. Using a higher gas pressure increases the diffusion limited oxidation rate and provides higher cutting velocities, while the fraction of oxidized metal decreases substantially with increasing speed. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361098

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Induced noncolinear frequency doubling (INCFD) is presented as a new characterization technique for electro‐optic crystals. Using the interaction of two noncolinear laser beams for the generation of second harmonic light, the method yields a three‐dimensional spatial resolution. First experimental results of INCFD measurements on composition gradients in lithium niobate and on the spatial orientations of 90o‐domain walls in potassium niobate are shown. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361184

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

In previous reports, we predicted with computer simulation that photocarriers from blue bias light can contribute under certain conditions to the quantum efficiency in the red wavelength part of the spectrum for hydrogenated amorphous silicon (a‐Si:H)p‐i‐nand Schottky barrier structures. We termed this phenomenon photogating and subsequently experimentally verified its existence by demonstrating red wavelength quantum efficiency≳1 under blue bias light. In this report, we show how device structure affects this photogating and affects the wavelength ranges where it can appear. We show with computer simulations that certain structures can display a complementary phenomenon of blue quantum efficiencies≳1 under red bias light and discuss supporting experimental data. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361185

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

This article analyzes theoretically the ‘‘in‐line field’’ scheme and the ‘‘crossed‐field’’ scheme for acoustic excitation of an acoustic superlattice for both resonators and transducers. The electrical impedance and the admittance are deduced and the electromechanical coupling coefficients are calculated. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361186

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

An exact asymptotic expression for thermal waves in an inhomogeneous semi‐infinite condensed matter sample at high frequencies is derived. The theory also allows for a fast and accurate retrieval of subsurface thermal conductivity profiles from surface temperature values in response to incident modulated radiation. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361187

出版商:AIP    

年代:1996

数据来源: AIP