摘要:

A thermodynamic model for the description of spherulitic growth in (semi‐) crystalline polymers is presented. Its validity is restricted to the free growth stage of crystallization. The relevance of the role played by surface tension in the early stage of the process is pointed out. With the due approximations the predicted growth rate is found to be consistent with the well‐known Avrami model. As an application, the melting temperaturesTmof poly(ethylene oxide) samples of different molecular weights are derived by fitting the appropriate crystallization thermograms carried out in isothermal conditions. A linear dependence ofT−1mas a function of the inverse polimerization degree is found, which is consistent with a well‐known result of the lattice theory. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360295

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The origin of short‐lived components in the annihilation lifetime spectrum of positronium (Ps) is shown to be due to fast Ps that is collisionally dissociating at the surfaces of the surrounding confinement cavity. The results are consistent with a model of fast (10–100 eV) Ps production by backscattered positrons from the incident beam. It is found that the typical lifetime of dissociating Ps scales with the mean free path of the cavity, and the relative formation intensity depends inversely on the incident positron beam energy. This ubiquitous effect will be present in any Ps formation experiment involving a free surface and can only be eliminated at beam energies less than 10 eV. More practical methods of minimizing the undesirable systematic effects of fast Ps quenching in depth‐profiled positron lifetime spectroscopy and in precision Ps decay rate measurements will be discussed. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360296

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The responsivity and transmission performance of 60‐periodp‐i‐nmultiple‐quantum‐well (MQW) optical modulators having single or double steps in the quantum well (QW) has been experimentally compared to a square‐QW control sample. It has been confirmed that significantly increased shift of the exciton with applied electric field [the quantum confined Stark effect (QCSE)] is obtained in the stepped QWs as compared to a conventional square QW. However, no meaningful increase in optical modulation performance is found, due to broadening of the exciton transition in the stepped QWs. All MQWs contained nominal 45 A˚ Al0.3Ga0.7As barriers. Single‐step QWs consisted of a 15 A˚ GaAs region with an 85 A˚ Al0.1Ga0.9As step. Double‐step QWs consisted of a central Al0.15Ga0.85As plateau, with 20 A˚ GaAs regions on either side. The square QW consisted of 90 A˚ of GaAs. Excellent agreement between measured QCSE and tunneling resonance calculations was found. Our results indicate that stepped MQW devices are intrinsically more susceptible to growth induced degradation than square MQW modulators. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360297

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

How various deviations in perfect photonic crystals, which may arise during fabrication, can affect the size of photonic band gaps is investigated theoretically. The emphasis is on determining the effects of misalignment of basic structural elements and overall surface roughness, because of their general fabrication relevance. As an example, calculations on a newly proposed three‐dimensional photonic crystal are performed. It is shown that the size of the gap is tolerant to significant amounts of deviation from the perfect structure. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360298

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A method for estimating strain in a semiconductor laser chip arising from bonding to foreign materials. The method is based on measurements of dependence of lasing wavelengths on duty ratios of pulsed operation with a constant peak current. The wavelength at zero duty ratio (&lgr;0) is extrapolated from this dependence, from which the strain is estimated. Estimated strain from this method is in good agreement with result obtained from the change of polarization ratio, transverse‐magnetic to transverse‐electric light outputs. The strain arising from packaging affects not only lasing wavelength but also operating characteristics such as threshold current. Moreover, thermal resistance of laser diodes can be obtained from this dependence of lasing wavelengths on duty ratios. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360299

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A continuum model is developed to analyze the heat transfer to a solid particle from continuum plasma. A spherical particle introduced in a quiescent collision‐dominated weakly ionized gas is considered. At steady state, the particle is negatively charged at the floating potential with the flux of ions and the flux of electrons to the particle being equal. A set of continuum conservation equations for ion and electron temperature and densities are solved with Poisson’s equation for the self‐consistent electric field. The variation of ion and electron densities and temperatures is numerically calculated in the entire domain. Thermal ionization and recombination is incorporated in the model. The heat transport to the particle due to the flux of ions and electrons is determined. The results indicate that a thin charge sheath is present near the particle surface with steep gradients in electron and ion number densities. Particle charging significantly affects the amount of heat transferred from the plasma to the particle. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360300

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The thermodynamics of a latent heat storage element, connected to a heat source, periodically varying in time, and to a heat engine, is addressed. Two typical modes of operation, referred to as the series and the parallel setups, are considered. They differ with regard to the active phase of the heat source. For the series mode the entire amount of heat transfer fluid (HTF), coming from the source, is first passed through the thermal storage element (TSE) before entering the engine. For the parallel setup only a fraction of the HTF, supplied by the heat source, is delivered directly to the engine, whereas the remaining fraction of HTF is pumped into the TSE to facilitate the exergy storage. The optimal selection of the freezing point of the phase‐change material (PCM), the stability of operation of the engine, and the entropy production in the TSE during the heat storage‐discharge cycle are considered. The parallel and the series modes of operation are compared for some simplified TSE models. For these models the series setup yields a higher efficiency and stability than the parallel scheme. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360301

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The damage induced by hydrogen and helium plasmas at the surface of crystalline silicon has been monitoredinsituby time‐resolved microwave conductivity and by spectroscopic ellipsometry measurements. Both plasma treatments increase the decay rate of the optically generated excess charge carriers and decrease the amplitude of the microwave reflection transients. While for the helium plasma a high density of electronic defects is created immediately after plasma ignition, a continuously increasing number of recombination centers is observed in the case of the hydrogen plasma exposure. In support of the transient microwave measurements, the analysis of the spectroscopic ellipsometry measurements reveals the creation of a damaged surface layer, which in the case of the helium plasma exposure has a high and in the case of the hydrogen plasma a low fraction of amorphous silicon. This can be explained by the different nature of the processes involved in the interaction of hydrogen (chemical) and helium (physical) plasmas with the silicon surface. After a constant plasma exposure time the damaged surface layer is thicker in the case of the hydrogen plasma exposure. Moreover, the helium plasma treatment produces a more defective overlayer as deduced from the faster decay of the transient microwave signals. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360302

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A simple analytic model valid for all collisionality regimes is developed to describe the power deposition in a cylindrical inductively coupled plasma source with a planar coil. The heating is ohmic at high pressures and remains finite at low pressures. The low‐pressure collisionless heating is due to kinetic nonlocal effects. The model is in good agreement with other calculations of collisionless heating. A diffusion model is then used to determine the plasma density profile and the electron temperature in terms of the gas pressure and the source geometry. The heating and diffusion models are used to determine the scaling of the inductive electric field with applied frequency and input power, and the results are compared with published experimental data to verify the scaling. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360723

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The effect of gas flame annealing on the structure of polycrystalline Si films was studied using transmission electron microscopy and electron spin resonance. This annealing technique involved heating the sample surface to more than 1100 °C using flat gas flames with a scan rate of 1 mm/s and a heating rate of about 260 °C/min. Electron microscopy images revealed that the secondary grain growth proceeded with increasing the number of annealing times (annealing frequency) and that the grain size for samples annealed at 1360 °C was more than 1 &mgr;m whereas secondary grain growth was not significant for samples annealed at 1150 °C. Further, it was found that the spin density in the samples annealed at 1360 °C decreased from 1.5×1018cm−3to 3.8×1017cm−3. It was concluded from the spin resonance results and the electron microscopy images that the secondary grain growth consists of two processes, the initial structural rearrangement of the grain boundaries and the subsequent grain growth. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360303

出版商:AIP    

年代:1995

数据来源: AIP